Epidote – here defined as minerals belonging to the
epidote–clinozoisite solid solution – is a low-μ (μ=238U/204Pb) mineral occurring in a variety of geological
environments and participating in many metamorphic reactions that is stable
throughout a wide range of pressure–temperature conditions. Despite
containing fair amounts of U, its use as a U-Pb geochronometer has been
hindered by the commonly high contents of initial Pb, with isotopic
compositions that cannot be assumed a priori. We present a U-Pb geochronology
of hydrothermal-vein epidote spanning a wide range of Pb (3.9–190 µgg-1), Th (0.01–38 µgg-1), and U (2.6–530 µgg-1) contents and with μ values between 7 and 510 from the Albula area (eastern Swiss Alps), from the Grimsel area (central Swiss Alps), and
from the Heyuan fault (Guangdong Province, China). The investigated epidote
samples show appreciable fractions of initial Pb contents (f206=0.7–1.0) – i.e., relative to radiogenic Pb – that vary to different
extents. A protocol has been developed for in situ U-Pb dating of epidote
by spot-analysis laser ablation inductively coupled plasma mass spectrometry
(LA-ICP-MS) with a magmatic allanite as the primary reference material. The
suitability of the protocol and the reliability of the measured isotopic
ratios have been ascertained by independent measurements of
238U/206Pb and 207Pb/206Pb ratios, respectively, with
quadrupole and multicollector ICP-MS applied to epidote micro-separates
digested and diluted in acids. For age calculation, we used the
Tera–Wasserburg (207Pb/206Pb versus 238U/206Pb)
diagram, which does not require corrections for initial Pb and provides the
initial 207Pb/206Pb ratio. Petrographic and microstructural data
indicate that the calculated ages date the crystallization of vein epidote
from a hydrothermal fluid and that the U-Pb system was not reset to younger
ages by later events. Vein epidote from the Albula area formed in the
Paleocene (62.7±3.0 Ma) and is related to Alpine greenschist-facies
metamorphism. The Miocene (19.2±4.3 and 16.9±3.7 Ma)
epidote veins from the Grimsel area formed during the Handegg deformation
phase (22–17 Ma) of the Alpine evolution of the Aar Massif. Identical
initial 207Pb/206Pb ratios reveal homogeneity in Pb isotopic
compositions of the fluid across ca. 100 m. Vein epidote from the Heyuan
fault is Cretaceous in age (107.2±8.9 Ma) and formed during the
early movements of the fault. In situ U-Pb analyses of epidote returned reliable ages of otherwise undatable epidote–quartz veins. The
Tera–Wasserburg approach has proven pivotal for in situ U-Pb dating of
epidote, and the decisive aspect for low age uncertainties is the variability
in intra-sample initial Pb fractions.
Introduction
Linking petrological and structural information to the timing of geological
events is crucial to better constrain the sequence of geodynamic processes.
In this context, the role of fluids in the continental crust is particularly
relevant because they mediate and influence deformation and metamorphism
(e.g., Wyllie, 1977; Etheridge et al., 1983; Johannes, 1984; Pennacchioni and
Cesare, 1997; Malaspina et al., 2011; Wehrens et al., 2016). The formation
of a hydrothermal vein represents a specific deformation and hydration event
in the geological history of the host rock, during which the vein-filling
minerals record the geochemical signature of the mineralizing fluid (e.g.,
Elburg et al., 2002; Barker et al., 2009; Bons et al., 2012; Parrish et al., 2018; Ricchi et al., 2019, 2020). By combining different geochemical and
geochronological techniques with suitable vein-filling minerals, it is
therefore possible to determine when the vein formed and the isotopic
signature of the fluids for insight into their origin (e.g., Pettke et al., 2000; Barker et al., 2006; Elburg et al., 2002).
The epidote–clinozoisite solid solution
[Ca2Al3Si3O12(OH)-Ca2Al2Fe3+Si3O12(OH)],
hereafter referred to as epidote, produces common rock-forming and
vein-filling minerals (e.g., Bird and Spieler, 2004; Franz and Liebscher,
2004; Guo et al., 2014; Zanoni et al., 2016). Epidote is stable over a wide
range of pressure–temperature conditions and in a multitude of magmatic,
metamorphic, and hydrothermal mineral assemblages (Bird and Spieler, 2004;
Enami et al., 2004; Grapes and Hoskin, 2004; Schmidt and Poli, 2004). Its
complex crystal structure incorporates a large variety of elements, enabling
measurements of trace element (e.g., Frei et al., 2004) and isotopic
(e.g., Guo et al., 2014) signatures. Uranium and thorium are readily accepted
into the epidote structure, with contents that are highly variable but
generally in trace element levels (Frei et al., 2004). Hence, attempts have
been made to use it as a geochronometer by stepwise leaching Pb–Pb
dating (e.g., Buick et al., 1999) and thermal ionization mass spectrometry
(TIMS) U-Pb dating (e.g., Oberli et al., 2004). Buick et al. (1999)
constrained the timing of vein formation and that of subsequent fluid pulses
in garnet–epidote–quartz veins in the Reynolds Range (central Australia).
Oberli et al. (2004; their Sect. 5.3 and their Fig. 5) obtained a U-Pb age of magmatic epidote from the Bergell pluton (eastern central Alps) and
identified epidote formation as a late-stage process during the
solidification of the pluton. However, these techniques allow
microstructural control only if sampling by micro-drilling is viable.
Epidote could provide valuable geochronological and isotopic information
when no other datable minerals are available. Good examples are
epidote–quartz veins that are widespread in the Alps (e.g., Aar Massif and
Albula area). Syn-kinematic epidote in breccias associated with rift-related
faults in the Campos basin (southeastern Brazil) may highlight successive
phases of fault movement (Savastano et al., 2017). In the Zermatt–Saas zone
(western Alps), epidote is a peak-pressure rock-forming mineral in
epidote-bearing rodingites (Zanoni et al., 2016), and it may help to better
constrain the P–T–d–t paths related to pressure-peak metamorphism.
This contribution discusses the applicability of in situ U-Pb dating to minerals compositionally within the epidote–clinozoisite solid solution. To
our knowledge, no analytical protocols have been proposed in this respect.
To fill this gap, we present U-Pb ages measured in hydrothermal-vein
epidote by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) using a magmatic allanite for standardization,
following a protocol similar to that applied to apatite U-Pb dating (e.g., Odlum and Stockli, 2019, 2020). The main issues related to the proposed
geochronometer and addressed in this contribution are (1) the suitability of
magmatic allanite as the most closely matrix-matched reference material for
LA-ICP-MS U-Pb dating of epidote in spot-analysis mode, (2) the
applicability of the protocol with respect to the different contents of
initial Pb fractions (i.e., relative to radiogenic Pb in total Pb) and U in
the studied samples, and (3) the effects on age precision of the interplay
between analyzed volumes and preservation of chemical variability. The
Tera–Wasserburg diagram proves to be the key tool for successful epidote
U-Pb geochronology, allowing for the addition of minerals from the epidote–clinozoisite
solid solution to the list of low-μU-Pb geochronometers. Notably, by
investigating epidote on its own, it is possible to combine U-Pb ages and
isotopic systematics with data from trace element analyses and other
isotopic systems; this may permit us to reconstruct fluid flow and its origin
with information that is all provided by a single mineral.
The challenges of investigating epidote as a geochronometer
Along with relevant amounts of U4+ and Th4+ as Ca substitution in
the A site and of U6+ as Al or Fe substitution in the M site (Frei et
al., 2004), high contents of initial Pb are incorporated by epidote during
crystallization. This causes the dilution of ingrown radiogenic Pb, whose
precise measurements are imperative for U-Pb geochronology, and makes
epidote a low parent-to-daughter or low-μ phase (i.e., μ≲2000; Romer, 2001; Romer and Xiao, 2005). U-Pb dating of
initial Pb-rich minerals can proceed in two ways depending on whether or not
the isotopic composition of the initial Pb is known or can be reasonably
assumed. Assumptions can be based on the modeled evolution of global Pb
isotopic compositions such as those proposed by Cumming and Richards (1975)
and Stacey and Kramers (1975). In the first case, a correction for initial
Pb can be applied, and an initial Pb-corrected U-Pb age can be calculated
from the measured U(±Th)–Pb isotopic ratios of each analysis
(Williams, 1998). However, age inaccuracies due to wrong assumptions
regarding initial Pb isotopic compositions can be significant (see Romer,
2001; Romer and Xiao, 2005). An initial Pb correction can be applied if the
contents of 204Pb – the only non-radiogenic lead isotope – can be
measured precisely, which is not always the case (e.g., because of the
analytical technique employed). Hence, if no other dating method is viable
(e.g., Th contents that are too low, hampering Th-Pb dating), the best solution for
dating low-μ phases is to use a regression through the analyses
uncorrected for initial Pb on the Tera–Wasserburg diagram (Tera and
Wasserburg, 1972), which plots measured 207Pb/206Pb versus
238U/206Pb ratios. Its advantages are that (1) it does not require
corrections for initial Pb isotopic compositions, (2) it provides the initial
207Pb/206Pb ratio itself in addition to an initial Pb-corrected
U-Pb age, and (3) it gives an estimate of the fractions of initial lead
relative to those of radiogenic Pb in each analysis (Tera and Wasserburg,
1972; Ludwig, 1998). This approach is based on the hypothesis that multiple
analyses are performed on material of the same age sharing the same initial
Pb isotopic composition. If these criteria are met, one regression is
defined by the alignment of the measurements of 207Pb/206Pb vs.
238U/206Pb ratios, whose lower intercept with the concordia yields
the age of the sample. If the hypothesis proves to be wrong (multiple
mineral generations or co-genetic minerals with different initial Pb
isotopic compositions; e.g., Romer and Siegesmund, 2003), this is highlighted
by the statistical parameters of the regression. The fraction of initial Pb
in each analysis can be estimated from the proximity of individual data
points to the upper 207Pb/206Pb intercept of the regression (Tera and Wasserburg, 1972; Ludwig, 1998), which gives the initial
207Pb/206Pb ratio of the sample. The regression in the
Tera–Wasserburg diagram is better constrained and yields more precise ages
if the variability in initial Pb and U contents is high enough to produce
spread-out data points. Epidote minerals are commonly characterized by
chemical zoning (Franz and Liebscher, 2004), which may also result in
variability in initial Pb fractions and U contents and promote the spread of the
data points along the Tera–Wasserburg regression.
A suitable technique for in situ U–Th-Pb dating is LA-ICP-MS in
spot-analysis mode, provided that U/Pb and Th/Pb elemental fractionation at the ablation site (downhole fractionation, DF) is appropriately corrected for over ablation time by relying on an external reference material (e.g., Sylvester, 2005; Košler, 2007; McFarlane et al., 2016). Since DF is
matrix-dependent (e.g., Sylvester, 2005; Košler, 2007; Sylvester, 2008;
El Korh, 2014), a matrix-matched reference material is most commonly used.
To date, no reference epidote exists, posing the problem of correction for
DF of 238U/206Pb ratios measured in epidote, which is crucial for accurate
age determinations by LA-ICP-MS (Horstwood et al., 2016). One way to date
epidote by LA-ICP-MS would be by dynamic (raster) ablation with a
non-matrix-matched reference material (e.g., Darling et al., 2012). However,
sufficiently large areas within epidote grains are frequently not available for
dynamic ablation. A mineral with a matrix that closely matches that of
epidote is allanite. After early work by ID-TIMS (e.g., Barth et al., 1994),
magmatic allanite has been dated by SIMS (e.g., Catlos et al., 2000), and in
recent years it has successfully been characterized and dated by U–Th-Pb
LA-ICP-MS (e.g., Gregory et al., 2007, 2012; El Korh, 2014; Smye et al., 2014). Several allanite samples have been proposed as suitable primary
reference materials for LA-ICP-MS dating (e.g., Gregory et al., 2007; Smye
et al., 2014). Allanite [(Ca,REE,Th)2(Fe3+,Al)3Si3O12(OH)] is the REE-rich member of the epidote mineral
group, with ThO2 contents of 2 wt %–3 wt % and U concentrations often below 1000 ppm (Gieré and Sorensen, 2004; and references therein), and
it is a promising candidate as a closely matrix-matched reference material
for minerals of the epidote–clinozoisite solid solution. The possible
issues in the use of allanite as a reference material for accurate U-Th-Pb geochronology are mostly related to local isotopic heterogeneity, excess 206Pb due to incorporation of 230Th during crystallization, variable contents of initial Pb, and disturbance of the geochronometer by secondary processes (e.g., hydrothermal alteration; Gregory et al., 2007; Darling et al., 2012; Smye et al., 2014; Burn et al., 2017). Nevertheless,
these issues can be largely avoided by careful selection of spot analyses
referring to backscattered electron (BSE) images and by identifying and
excluding problematic analyses from calculations.
A disadvantage of U-Pb analyses by LA-ICP-MS is the large isobaric
interference on mass 204 by 204Hg of the carrier gas. A correction for
such an interference in order to apply a 204Pb correction – whether
based on measurements of the initial Pb isotopic composition in the same
mineral or in coexisting ones (e.g., Cenki-Tok et al., 2014) – is complex
(e.g., Storey et al., 2006). For this reason, and considering that
epidote-bearing veins may not include other minerals suitable to determine
the initial Pb isotopic composition, the application of the Tera–Wasserburg
approach is preferable. In this study, epidote ages and initial
207Pb/206Pb ratios are assessed from the Tera–Wasserburg diagram.
If initial 207Pb/206Pb ratios are consistent with modeled values
of initial Pb isotopic compositions (e.g., Stacey and Kramers, 1975), then an
accurate 238U/206Pb age can be obtained by averaging single-spot
ages, which are calculated from each analysis corrected for initial Pb by
applying a 207Pb correction (i.e., weighted average 207Pb-corrected
238U/206Pb age; see Williams, 1998).
Geological context and field relations
Hydrothermal epidote veins (Fig. 1) were sampled at Albula Pass (eastern
Swiss Alps), at Grimsel Pass (central Swiss Alps), and at the Heyuan fault
(Guangdong Province, China). Although there are no precise anticipated ages
for the selected epidote samples, the well-constrained tectonic histories of
the sampling areas and of the lithologies hosting the studied
epidote-bearing veins allow us to verify whether or not the obtained ages are
geologically reasonable.
Scans of thin sections of (a) Albula-1, (b) Grimsel-1, (c) Grimsel-2, and (d) Heyuan-1 samples. (a, b) Plane-polarized light on petrographic microscope; (c, d) plane light. Green rectangles indicate the location of the BSE images shown in Fig. 2. bt: biotite; chl: chlorite; ep: epidote; kfs: K-feldspar; plg: plagioclase; qtz: quartz.
BSE images of (a) Albula-1, (b) Grimsel-1, (c) Grimsel-2, and (d) Heyuan-1 epidote. The specific epidote grain shown in panel (a) was not analyzed. The locations of the BSE images are indicated by the green rectangles in Fig. 1. bt: biotite; chl: chlorite; ep: epidote; kfs: K-feldspar; plg: plagioclase; qtz: quartz.
The Albula area is located in the upper Err nappe, close to the tectonic
contact with the Ela nappe. It belongs to the Austroalpine domain, the
basement of the former Adriatic continental margin (e.g., Froitzheim and
Eberli, 1990; Froitzheim et al., 1994). The most common lithology in the Err
basement is the Albula granite, a granodiorite of Variscan to post-Variscan
age (e.g., Manatschal and Nievergelt, 1997; Incerpi et al., 2017) in
which epidote ± quartz veins are widespread. In the late Carboniferous
and early Permian, the Albula granite intruded into the metamorphic basement
of the Err nappe at <3 km of depth (Mohn et al., 2011, and references
therein). Subsequently, the Lower Austroalpine was involved in the Jurassic
rifting that led to the break-up of Pangea (e.g., Manatschal et al., 2000).
During the Alpine orogeny, the Err nappe mainly recorded the deformation
resulting from the W- to NW-directed vergence of the Austroalpine domain
from Cretaceous until early Cenozoic times and was only weakly affected by
the Cenozoic tectonics, when temperatures reached ca. 300 ∘C
(i.e., lower-greenschist facies conditions; e.g., Froitzheim and Manatschal,
1996; Mohn et al., 2011; Epin et al., 2017). Sample Albula-1 was collected
at coordinates 46∘34′36′′ N, 9∘48′06′′ E and has not been described in previous studies.
The Grimsel area is in the Aar Massif, one of the external crystalline
massifs of the Alps (Rolland et al., 2009; Wehrens et al., 2017; Herwegh et
al., 2020). Here, epidote–quartz veins are common in the central Aar granite
and in the Grimsel granodiorite, which during the earliest Permian intruded
into a polycyclic basement bearing evidence of Ordovician metamorphism and
Variscan overprint (Schaltegger and Corfu, 1992; Berger et al., 2017). After
being affected by the Jurassic rifting, the Aar Massif was involved in the
continent–continent collision during the Alpine orogeny, as demonstrated by
the presence of anastomosing high-strain shear zones of Alpine age (e.g.,
Goncalves et al., 2012; Wehrens et al., 2017). Metamorphism at greenschist
facies conditions reached 450±30∘C at 0.6±0.1 kbar in this area (Challandes et al., 2008; Goncalves et al., 2012). The
Alpine history of the Aar Massif is subdivided into three phases: (1) the
Handegg phase (22–17 Ma; Challandes et al., 2008), with stable green
biotite in the shear zones (Challandes et al., 2008; Rolland et al., 2009;
Herwegh et al., 2017; Wehrens et al., 2017); (2) the Oberaar phase in the
southern Aar Massif (14–3.4 Ma; Hofmann et al., 2004; Rolland et al., 2009), with white mica and chlorite stable in the shear zones and
metastable biotite (Herwegh et al., 2017; Wehrens et al., 2017); and (3) the
Pfaffenchopf phase in the northern Aar Massif (<12 Ma; Herwegh et
al., 2020). The epidote–quartz veins analyzed in this study were sampled in
the Nagra Felslabor tunnel at Grimsel Pass. As these veins are only visible
within the tunnel, their relationships with Alpine structures and among
each other are not known. Samples Grimsel-1 and Grimsel-2 have not been
previously described, and they were sampled at a distance of ca. 100 m from
each other, close to the F100 and BK cavern locations in Fig. 1.2 of
Schneeberger et al. (2019), respectively.
The Heyuan fault is a crustal-scale fault that formed in Mesozoic times as a
low-angle normal fault, but it is currently active under a transpressive regime
(Tannock et al., 2020a, b). The footwall of this fault mainly consists
of the Xinfengjiang pluton (the eastern portion of the Fogang batholith), a
late Jurassic biotite granite that intruded into the basement of Proterozoic
to Silurian age during the Yanshanian orogeny (Li et al., 2007; Tannock et
al., 2020a, b). Epidote veins are located in the mylonites at the
transition between undeformed granite and fault zone (Tannock et al., 2020a). The hanging wall is composed of a quartz–sericite
ultracataclasite–phyllonite in contact with a quartz reef and finally
abutted by the sedimentary “red beds” of Cretaceous age (Tannock et al., 2020a). Since the epidote veins are either pre- or syn-kinematic with
respect to the mylonites (Tannock et al., 2020b), we infer that the epidote
veins cannot be older than the pluton itself, but they are also among the
earliest structures related to the early movements of the Heyuan fault
(Tannock et al., 2020a, b). Epidote veins are absent in the footwall
cataclasite and in the quartz reef, which formed after the mylonite.
Syn-kinematic epidote veins formed at a temperature of ca. 330 ∘C,
as indicated by the white mica composition in the mylonites (Tannock et al., 2020a). Sample Heyuan-1 is discussed in Tannock et al. (2020a, b), and
its sampling location is shown in Fig. 1 of Tannock et al. (2020a; their
sample HY17-5).
Methods
Except where stated, sample preparation and measurements were carried out in
the laboratory facilities at the Institute of Geological Sciences,
University of Bern, Switzerland.
Imaging and screening methods for sample selection
Thin (30 µm) and thick (50–60 µm) sections were inspected by
petrographic and electron microscopy, respectively, on a Zeiss Axioplan
microscope and on a Zeiss EVO50 SEM using BSE imaging (ca. 1 nA beam
current, 20 kV accelerating voltage, working distance 8.5–10.0 mm). BSE
images were used to plan analysis spots – all of the same size – within
epidote grains so as to avoid mixing of different zonings in each single
measurement, as well as mineral and fluid inclusions. Major element contents
were acquired by an electron probe micro-analyzer (EPMA), and REE, U, Th, and Pb
contents were measured by LA-ICP-MS upon screening many samples by employing
methods presented elsewhere (Pettke et al., 2012). The details of the EPMA
and LA-ICP-MS setups are reported in Appendix A.
U-Pb geochronology by LA-ICP-MS
Isotopic measurements of U, Th, and Pb were performed on thin and thick sections
for epidote and on acrylic grain mounts for allanite. To minimize surface
contamination, the thin and thick sections were cleaned with ethanol, and the
grain mounts were cleaned with ethanol and 5 % HNO3. Measurements of U, Th, and Pb isotopic ratios were performed with a Resonetics RESOlutionSE 193 nm excimer
laser system (Applied Spectra, USA) equipped with an S-155 large-volume
constant-geometry chamber (Laurin Technic, Australia) coupled with an
Agilent 7900 ICP-QMS. The suitability of analytical conditions (Table 1)
was checked in each session by performing preliminary analyses on secondary
reference materials of known ages – namely CAPb (for details see Burn et al., 2017), CAP, and AVC allanite (for details see Barth et al., 1994; Gregory et al., 2007) – and comparing them to their published U-Pb ages (see Table 2). Low fluence of 3 J s-2, a low repetition rate
of 5 Hz, and a large spot size of 50 µm were combined to ensure a slow increase in the depth to diameter ratio of the laser crater during a 40 s
ablation time to minimize elemental (U/Th/Pb) DF. An additional
session was carried out with a laser spot of 30 µm and the same laser conditions. The aim was to assess the effects of using a smaller spot size on the correction for DF (see Chew et al., 2014) and to explore whether the use of allanite as a primary reference material can still provide accurate
data at these conditions. If so, this would extend the applicability of the
present protocol to smaller epidote grains. For this test, we selected two
samples: the one also used for solution ICP-MS measurements (sample
Albula-1; see Sect. 3.3) and the one with the smallest averaged analytical
errors in 238U/206Pb and 207Pb/206Pb ratios (sample
Grimsel-1). The 30 µm analyses were done in the same areas with
crystals analyzed in the previous sessions after polishing the thin and thick
sections to remove the condensation blankets around the ablation craters.
Measurement conditions of the Agilent 7900 for U-Th-Pb isotopic data by LA-ICP-MS.
RF power 14 June 201923 July 201916 January 20201280 W1320 W1380 WFluence (all sessions) 3 J cm-2Repetition rate (all sessions) 5 Hz Cell gas flow 14 June and 23 July 2019 16 January 20203.0 mL min-1 N2 and 350 mL min-1 He 3.0 mL min-1 N2 and 400 mL min-1 HeSensitivity on mass 232 measured on NIST SRM612 by dynamic ablation (beam size, fluence, repetition rate, scan rate) 14 June 201923 July 201916 January 20204150 cps ppm-1 (50 µm,4410 cps ppm-1 (50 µm,3590 cps ppm-1 (50 µm,2.5 J cm-2, 5 Hz, 5 µms-1)2.5 J cm-2, 5 Hz, 5 µms-1)2.5 J cm-2, 5 Hz, 5 µms-1)232/238 ratio (all sessions) >0.97248/232 ratio (all sessions) <0.002Background (all sessions) 30 s Pre-cleaning (beam size in µm) 14 June and 23 July 2019 16 January 202010 pulses (64); followed by wait time of 10 s before ablation 10 pulses (30); followed by wait time of 10 s before ablationAblation time (beam size in µm) 14 June and 23 July 2019 16 January 202040 s (50) 30 s (30)Measured masses (dwell times in ms) 14 June and 23 July 2019 16 January 2020204 (40), 206 (40), 207 (40), 208 (40), 232 (40), 238 (40) 206 (40), 207 (40), 208 (40), 232 (40), 238 (40)Primary reference material (all sessions) Tara allanite Secondary reference materials 14 June 201923 July 2019 and 16 January 2020 CAPb allaniteCAP and AVC allanites
Reference data of Tara allanite for normalization of U-Th-Pb
isotopic data by LA-ICP-MS and published U-Pb Tera–Wasserburg ages
of CAPb, CAP, and AVC allanite secondary reference materials. The ratios for
Tara allanite are averages calculated from the measurements by Smye et
al. (2014) by ID-TIMS; one measurement was excluded (see text). Uncertainties
are given in brackets and are calculated as 2 standard errors. The subscript
r indicates the radiogenic ratio, and the subscript i indicates initial.
1U-Pb age used as a reference in this contribution (see Sect. 5.1 for details).
2 Calculated from five ID-TIMS data points from Smye et al. (2014); unanchored regression.
3 From Burn et al. (2017).
4 From Gregory et al. (2007).
In all sessions, Tara allanite (see Gregory et al., 2007; Smye et al., 2014)
is chosen as the primary reference material because it is the most homogenous
allanite in terms of U-Th-Pb isotopes and the most promising reference
material for U-Pb geochronology (Gregory et al., 2007; Smye et al., 2014; Burn et al., 2017; Liao et al., 2020). Tara allanite reference isotopic
ratios and their uncertainties (Table 2) were calculated by averaging five
of the six ID-TIMS measurements reported by Smye et al. (2014), excluding
the measurement that yielded the youngest U-Pb age outside uncertainty. The
analytical sequence involved measurements of the reference Tara allanite
separated by blocks of three to nine sample measurements including allanite secondary
reference materials for quality control. Analysis spots in allanite were
planned based on BSE images to avoid chemical and isotopic heterogeneity
(i.e., mixing of zoning) within each single analysis and inclusions (e.g.,
rare <1µm sized thorite; Smye et al., 2014). A few analyses, however,
were intentionally placed on fluid inclusions and across zoning in
sample Albula-1 to determine that these features would not compromise the
use of this sample for solution ICP-MS (see Sects. 3.3 and 4.3).
Raw data were treated in the software Iolite (version 7.08) by the
VisualAge_UcomPbine data reduction scheme (Chew et al., 2014), and the correction for DF was carried out by selecting an exponential function. Iolite fits this function to model the measured DF on the analyses
of the primary reference material and then applies it to all unknown
analyses to correct them for DF. The quality of signals and that of the
correction for DF were considered to determine the validity of each
measurement. Assessing the quality of signals implies inspection of the
laser signal of each isotope across each measurement to discard – partially
or entirely – those that are contaminated by impurities, such as mineral or
fluid inclusions, or that show isotopic heterogeneity during ablation. The
accuracy of DF correction depends on the ablation behavior being the same
between the primary reference material and the sample, and it is assessed by the
unknowns giving flat 206Pb/238U ratios across the ablation once
corrected for DF. Although we do not apply an initial Pb correction for age
calculation, correcting the time-resolved DF-corrected 206Pb/238U
ratios for initial Pb ensures that no sloping results from zoning in initial
Pb contents, which can be prominent in minerals with high initial Pb
contents. A 208Pb correction is therefore applied to each analysis of
the epidote unknowns using initial 207Pb/206Pb and
208Pb/206Pb ratios obtained from preliminary Tera–Wasserburg
diagrams. This and the subsequent normalization of the measured ratios based
on the reference values of the primary reference material ensure that the
238U/206Pb and 207Pb/206Pb ratios used in the
Tera–Wasserburg diagrams are true values and that the U-Pb age calculated
with these ratios is accurate. Since the isotopic fractionation between
207Pb and 206Pb is negligible (e.g., Burn et al., 2017), we address
the suitability of the DF correction based on allanite as a primary reference
material only for 206Pb/238U ratios. A 207Pb correction was
applied to the primary reference material (i.e., Tara allanite) by the
VisualAge_UcomPbine data reduction scheme (DRS) before it was
used for normalization (Chew et al., 2014) with an initial
207Pb/206Pb value of 0.866±0.079 obtained from a
Tera–Wasserburg diagram plotting five ID-TIMS analyses by Smye et al. (2014), since it has been shown that initial 207Pb/206Pb ratios of
allanite can deviate from model values (e.g., Cenki-Tok et al., 2014). The
uncertainties in the 238U/206Pb and 207Pb/206Pb ratios
obtained from the VisualAge_UcomPbine DRS (Chew et al., 2014)
include an overall propagated uncertainty coming from the reproducibility of
the primary reference material. Isoplot 3.7.5 (Ludwig, 2012) was used for
age calculations. Age determination of epidote samples and allanite
secondary reference materials relies on the Tera–Wasserburg approach (Tera
and Wasserburg, 1972; Ludwig, 1998). Since the initial Pb isotopic
composition of CAP, CAPb, and AVC allanite is known and consistent with
a modeled two-stage evolution of initial Pb isotopic compositions (Barth et
al., 1994; Gregory et al., 2007; Burn et al., 2017), we ensured better age
precision by anchoring the Tera–Wasserburg regressions of these allanite
samples to an initial 207Pb/206Pb ratio of 0.854±0.015
(275 Ma; Stacey and Kramers, 1975) and calculated their weighted average
207Pb-corrected 238U/206Pb ages using the same value.
Regression and weighted average 207Pb-corrected 238U/206Pb
ages of allanite secondary reference materials are summarized in Table 3,
and their Tera–Wasserburg diagrams are presented in Fig. B1 (Appendix B).
U-Pb LA-ICP-MS ages of allanite secondary reference materials measured in three analytical sessions in this study. Age uncertainties are 95 % confidence.
Sample14 June 2019 23 July 2019 16 January 2020 RegressionWeighted averageRegressionWeighted averageRegressionWeighted averageU-Pb age [Ma]U-Pb age [Ma]U-Pb age [Ma]U-Pb age [Ma]U-Pb age [Ma]U-Pb age [Ma]CAPb284.2±2.61284.2±2.0––––MSWD=0.34MSWD=0.34274±292MSWD=0.33CAP––288.5±2.91288.6±2.3283.0±3.41282.5±3.2MSWD=1.04MSWD=1.03MSWD=1.2MSWD=1.2286.9±5.22299±202MSWD=1.11MSWD = 1.14AVC––292.4±3.71292.2±2.3285.2±4.51285.1±3.5MSWD=0.49MSWD=0.48MSWD=0.70MSWD=0.69293.2±7.42283±162MSWD=0.53MSWD=0.78
1 Regression anchored to a 207Pb/206Pb value of 0.854±0.015 (275 Ma; Stacey and Kramers, 1975). 2 Unanchored regression.
Solution ICP-MS
Independent measurements of 238U/206Pb and 207Pb/206Pb
ratios were performed on two epidote micro-separates to check their
consistency with U-Pb isotopic data measured by LA-ICP-MS and hence the
reliability of the latter data. The material was separated from the
epidote–quartz vein of sample Albula-1, which is the one with the lowest
degree of deformation and largest epidote crystals (see Sect. 4.1). Clean
and pure epidote grains were handpicked under a binocular microscope. The
epidote separates were pre-cleaned with MilliQ™ water. Based on
LA-ICP-MS U and Pb concentration data, four sample aliquots – two from
each epidote micro-separate and each corresponding to ca. 300 ng of total Pb
– were weighed in acid-cleaned Teflon beakers and dissolved following the
procedure of Nägler and Kamber (1996). Samples were leached with aqua
regia at 120 ∘C for 2 d. The leachate was transferred into a
second pre-cleaned Teflon beaker. To ensure complete dissolution a
concentrated HF:HNO3 (3:1 by volume) was added to the supernatant, and
the beakers were placed on a heating plate at 90 ∘C for 2 d.
After drying, 2 mL of 6.4 M HCl was added, and the beakers were placed on a heating plate at 150 ∘C for 2 d. The same procedure was applied to standard AGV-2 (Weis et al., 2006) and to two blanks, and
complete dissolution was achieved for all samples and standards. Finally,
the samples were dissolved in 1 mL of 0.5 M HNO3.
To determine 238U/206Pb ratios, a 10 % aliquot of digested
samples and standards was further diluted with 0.5 M HNO3 up to a final
volume of 10 mL. Two solutions with two different dilution factors were
prepared from each sample aliquot and were analyzed on a 7700x Agilent
quadrupole ICP-MS at the Department of Geography, University of Bern,
Switzerland. Standard AGV-2 (Weis et al., 2006) was used to correct for
instrumental fractionation and to check the accuracy of measurements. Final
sample concentrations of 206Pb, 207Pb, and 238U (for both
dilution factors of each sample aliquot) and their corresponding analytical
uncertainties as relative standard deviations – solely based on counting
statistics – were calculated by referring to a calibration curve based on
three dilution factors of AGV-2 standard. The 238U/206Pb ratio and
uncertainty as 2 SE of each sample aliquot were calculated with Isoplot
3.7.5 (Ludwig, 2012) as weighted average values between the
238U/206Pb ratios calculated from the measurements of both
dilution factors, which were the same within uncertainty for all sample
aliquots. The remaining sample material was dried and redissolved in 0.5 mL
of 1 M HNO3 for Sr-Pb column chemistry using a pre-cleaned Sr-spec™ resin (Horwitz et al., 1992). After loading, the sample matrix was eluted from the column with 1.5 mL of 1 M HNO3, while Sr and Pb were retained on the column. The Sr and Pb fractions were eluted with 1 mL of 0.01 M
HNO3 and 8 mL of 0.01 M HCl, respectively, following Villa (2009) and
Quistini et al. (2017). After drying, the Pb fraction was dissolved and
further diluted in 0.5 M HNO3 for measurement of Pb isotopes on a
Thermo Fisher Neptune Plus MC-ICP mass spectrometer. Measurements were carried out in dry
plasma mode using a CETAC Aridus 2 desolvating system. Thallium was added to
samples and standards to correct for instrumental mass fractionation with
repeated measurements of NIST SRM 981 to quantify the external
reproducibility of the measurements (Villa, 2009); the measured Pb isotopic
composition was indistinguishable from those reported by Rehkämper and
Mezger (2000). The four pairs of isotopic ratios measured by solution
ICP-MS are only compared to the Tera–Wasserburg diagram based on
LA-ICP-MS data (50 µm spot size) and are not used to calculate an
age because the statistical robustness of a regression based on only four
data points is limited.
ResultsPetrography and U–Th-Pb contents of samples selected for U-Pb geochronology
Four samples were selected for this contribution mainly based on (1) the size of
epidote grains in order to use the largest laser beam possible for
LA-ICP-MS and (2) U contents that are both as high and as variable as
possible within the sample. Larger laser beams maximize the precision of
U-Pb geochronology measurements. High U contents ensure higher contents of
uranogenic Pb isotopes and therefore improve the precision of U and Pb
isotopic measurements; their variability contributes to a larger spread of
the analyses in Tera–Wasserburg diagrams for well-constrained regressions.
The studied samples have epidote components (XEpi) between 0.52 and 0.98 (calculated as Fe3+/(Al+Fe3+-2); Cr< limit of detection) and ΣREE between 3.3 and 210 µgg-1.
One sample from the Albula area, sample Albula-1, was selected for U-Pb geochronology. Two veins can be recognized (Fig. 1a), both crosscutting the host rock with sharp boundaries.
The first is a 2–3 cm wide epidote–quartz–plagioclase vein (Vein1). Epidote grains are elongated, with lengths between ca. 0.5 mm along the vein boundaries and ca. 1 cm towards the center of the vein, with an aspect ratio up to ca. 7:1. Fractures are common and grains are euhedral to subhedral. Quartz is fractured and plagioclase is limited to a ca. 2 mm wide portion along the vein boundaries, associated with the smallest epidote grains. U contents of epidote range between 3.7 and 89 µgg-1 (Table 4). Th contents are 0.01–0.05 µgg-1 (19 out of 25 measurements are below the limits of detection of 0.03–0.07 µgg-1 with a spot size of 24 µm and 0.003 µgg-1 with a 60 µm spot size). Pb contents are 3.9–62 µgg-1, total Pb/U ratios 0.14–10, and μ values 7–510.
The second is a ca. 1 mm wide epidote–quartz–plagioclase vein (Vein2). Epidote grains range between a few micrometers (µm) and 2 mm in diameter, most being fractured and euhedral to subhedral. Epidote grains of ca. 1–2 mm in diameter are mantled by thin layers of micrometer-sized anhedral epidote grains. Quartz subgrains resulting from recrystallization and plagioclase wrap the epidote grains. U contents of epidote are 26–140 µgg-1 (Table 4), and Th contents and 0.67–14 µgg-1. Pb contents range from 24–64 µgg-1, Pb/U ratios from 0.46–1.7, and μ values from 43–160.
Concentrations of Pb, Th, and U, as well as the Th/U and Pb/U ratios and μ values measured by laser ablation ICP-MS with the trace element protocol in Appendix A. The symbol < is followed by limits of detection (calculated for each element in each measurement individually following the formulation in Pettke et al., 2012). μ values are calculated from total Pb and total U contents by considering an isotopic abundance of 1.4 % for 204Pb and 93 % for 238U.
BSE images of epidote (Fig. 2a) reveal growth zoning and intra-grain
veinlets resulting from interaction with a secondary fluid. Sample Albula-1
was selected for solution ICP-MS given the large size of epidote grains.
Sample Grimsel-1 (Fig. 1b) displays a folded epidote–quartz vein
crosscutting a weakly deformed portion of the host rock. Epidote grains are
generally prismatic and range between a few micrometers and ca. 2 mm in size. They are mostly subhedral to anhedral and cracked, and they form clusters
with no preferential grain orientation. Quartz subgrains indicate dynamic
recrystallization via subgrain rotation. Green biotite and rare chlorite are
associated with the epidote-bearing vein. Epidote in BSE images (Fig. 2b)
exhibits weak patchy zonation towards the rims and the presence of porosity.
K-feldspar is recognized within epidote cracks. U contents are 54–350 µgg-1 (Table 4), and Th contents are 0.04–4.9 µgg-1. Pb contents range between 79 and 190 µgg-1, with Pb/U ratios from 0.45–1.7 and μ values between 41 and 160.
Sample Grimsel-2 (Fig. 1c) consists of an epidote–quartz–biotite vein
cutting through a weakly deformed sector of the host rock. The vein
boundaries are sharp and nonlinear. Euhedral to subhedral epidote grains
are cracked by stretching-induced fracturing, with single fragments ranging
from a few micrometers to ca. 3 mm in size. Epidote grains can be estimated to have had an aspect ratio up to ca. 6:1 before fracturing. Quartz is recrystallized by subgrain rotation. Biotite grain sizes range between ca. 100 and 500 µm. BSE images (Fig. 2c) show that epidote exhibits regular growth zoning. Epidote contains 109–535 µgg-1 of U and 0.07–0.53 µgg-1 of Th (Table 4). Pb contents range from 51–97 µgg-1, Pb/U ratios are 0.20–0.86, and μ values are 83–510.
Sample Heyuan-1 (Fig. 1d) is characterized by an epidote–quartz–K-feldspar–chlorite assemblage that fills pockets cutting
through the granite-forming minerals or interstitial among the magmatic
minerals. The sample is crosscut by quartz ± hematite veins (see Fig. 4c in Tannock et al., 2020b). Epidote is variably shaped, from elongated
without preferential orientation to prismatic. Epidote ranges between tens
of micrometers and ca. 2 mm in length and forms clusters of euhedral to
anhedral crystals. Quartz associated with epidote is mostly recrystallized,
as indicated by the presence of quartz subgrains. Some millimeter-sized quartz
grains, however, display undulose extinction. Chlorite associated with
epidote forms interstitial aggregates of ca. 500–1000 µm in size.
Growth zoning of epidote is recognized from BSE images (Fig. 2d), and
K-feldspar is intertwined with smaller-sized epidote grains along the
boundaries of larger ones, as well as with quartz filling epidote fractures.
The measured U content of Heyuan-1 epidote is 2.6–34 µgg-1
(Table 4). Th contents range between 0.04 and 38 µgg-1, with most analyses below 10 µgg-1. Pb contents are 9.4–27 µgg-1, Pb/U ratios range between 0.42 and 4.4, and μ values range between 16 and 190.
In summary, among the four samples selected for U-Pb geochronology,
measured U contents of epidote are highly variable (2.6–530 µgg-1; n=80; Table 4), and the intra-sample variability in U concentrations is ca. 1 order of magnitude (Fig. 3). Samples Albula-1 and Heyuan-1 both contain a few to tens of micrograms per gram (µgg-1) of U, whereas samples Grimsel-1 and Grimsel-2 have higher U contents of hundreds of micrograms per gram (µgg-1). Thorium concentrations span 4 orders of magnitude (0.01–38 µgg-1; n=56). Samples with similar U
concentrations display different Th contents, creating variability in Th/U ratios. Pb contents are 3.9–190 µgg-1, and Pb/U ratios span between 0.14 and 10, with each sample varying to different extents. With only 4 out of 80 Th measurements above 10 µgg-1 (one in sample Albula-1 and three in sample Heyuan-1), Th-Pb geochronology is not viable. All epidote samples exhibit μ values well below 2000.
Th and U contents of the analyzed epidote samples.
Testing Tara allanite as a reference material for epidote U-Pb geochronology
To assess the validity of allanite as a primary reference material for epidote
dating, we compared the DF correction of 206Pb/238U ratios during
the ablation time using Tara allanite as a reference for CAPb allanite (Burn et al., 2017) as matrix-matched (Fig. 4a), for Plešovice zircon (Sláma et al., 2008) as non-matrix-matched (Fig. 4b), and for epidote as closely matrix-matched (Fig. 5). An accurate correction for DF produces flat time-resolved lines of DF-corrected 206Pb/238U ratios for unknowns: sloping or more complex-shaped curves indicate either zoning in initial Pb contents or that the DF correction is not compensating for the difference in matrix. As expected, CAPb allanite has DF-corrected 206Pb/238U ratios that are flat when standardized to Tara allanite (Fig. 4a, both measured with a 50 µm spot). Some of the DF-corrected 238U/206Pb ratios measured in Albula-1 and Grimsel-1 epidotes did not display a flat trend, and we therefore applied a 208Pb correction to verify if this was due to zoning in initial Pb contents. The fact that the majority (122 out of 127 analyses) of 208Pb- and DF-corrected
time-resolved 206Pb/238U ratios are flat (Fig. 5) indicates
similar ablation behavior and downhole fractionation of U from Pb between
epidote and allanite for our analytical setup, as well as accurate correction for
DF in epidote by using Tara allanite as the primary reference material with both
50 µm (Fig. 5a–d) and 30 µm (Fig. 5e–f) spot sizes. The
analyses whose 208Pb- and DF-corrected time-resolved
206Pb/238U ratios are not flat or do not overlap with all other
analyses were excluded from age calculation (see Sect. 4.3) as they could
indicate either analytical instability during ablation or a different
initial Pb isotopic composition. In contrast, the distinct ablation behavior
of (Plešovice) zircon is revealed by complex-shaped DF-corrected
238U/206Pb ratios even with a 50 µm spot (Fig. 4b; also see Fig. 6 of Burn et al., 2017).
206Pb/238U ratios measured by LA-ICP-MS corrected for
downhole fractionation (DF) of (a) CAPb allanite and (b) Plešovice zircon with Tara allanite as the primary reference material. Each individual line represents one analysis. Measurements are with a 50 µm spot size. The DF-corrected 206Pb/238U ratios include both initial and radiogenic Pb.
208Pb-corrected 206Pb/238U ratios measured by
LA-ICP-MS corrected for downhole fractionation (DF) of (a) Albula-1, (b) Grimsel-1, (c) Grimsel-2, (d) Heyuan-1, (e) Albula-1, and (f) Grimsel-1
epidote samples with Tara allanite as the primary reference material. Each
individual line represents one analysis. Measurements are with spot sizes of
50 µm(a–d) and of 30 µm(e–f).
Laser ablation ICP-MS U-Pb data of unknown samples
Uncertainties in the LA-ICP-MS 238U/206Pb and 207Pb/206Pb
ratios are 2 standard errors (2 SE), and all age uncertainties calculated
with Isoplot are 95 % confidence.
Tera–Wasserburg diagrams of (a) Albula-1, (b) Grimsel-1, (c) Grimsel-2, and (d) Heyuan-1 epidote samples with 50 µm measurements. All ratios are uncorrected for initial Pb. Ages are calculated from the lower intercept of the regressions through the analyses with the concordia, whereas initial 207Pb/206Pb ratios are calculated from the upper intercept of the regressions with the y axis. Data-point error ellipses are 2σ, and age uncertainties are 95 % confidence. Error envelopes are plotted with Isoplot 3.7.5 (Ludwig, 2012).
A total of 22 spot analyses were measured in sample Albula-1 with a
50 µm spot (Table 5). 238U/206Pb ratios are 0.535–33.2 with uncertainties of 2 %–4 %. 207Pb/206Pb ratios are 0.586–0.837 ± 0.8–2 % (Table 5). No analyses were excluded as all 208Pb- and DF-corrected time-resolved 206Pb/238U ratios are flat and overlap (Fig. 5a). A Tera–Wasserburg regression based on all data
points (Fig. 6a) yields an intercept age of 62.7±3.0 Ma (MSWD=1.6) with a 207Pb/206Pb intercept of 0.8334±0.0043. The
good spread of the data points along the regression line reflects variable
fractions of initial Pb (f206=0.69–1.0; Table 5). The small mean squared weighted deviation (MSWD)
value indicates that there is no resolvable age difference between the two
veins at the stated analytical precision. In fact, if the analyses from the
two veins are considered separately, the Tera–Wasserburg ages of Vein1 and
Vein2 are 67.6±5.0 Ma (n=18) and 58.9±3.8 Ma (n=4),
which overlap within uncertainty. The initial 207Pb/206Pb ratio of Albula-1 epidote indicated by the Tera–Wasserburg diagram is within
the uncertainty of the model value of 0.840±0.015 at 63 Ma (Stacey and Kramers, 1975). By using this model initial ratio for a
207Pb correction, the weighted average 207Pb-corrected
238U/206Pb age is 65.0±2.5 Ma (MSWD=0.91). A total of 18 additional measurements were carried out in sample Albula-1 (10 in Vein1 and 8 in Vein2) with a spot size of 30 µm. Analysis 8 (Table 5) was
excluded from age calculation because it was considered unreliable based on
its 208Pb- and DF-corrected time-resolved 206Pb/238U ratio
(Fig. 5e). The Tera–Wasserburg diagram based on 17 analyses with a 30 µm spot yields an intercept age of 65.9±4.6 Ma and a
207Pb/206Pb intercept of 0.8308±0.0087 (MSWD=1.4). Both values are within the uncertainty of those obtained with a spot size of 50 µm. By combining the 50 and 30 µm datasets, an intercept age of 63.9±2.6 Ma is obtained with an initial 207Pb/206Pb
ratio of 0.8331±0.0039 (MSWD=1.6). Given the consistency of the
measured U-Pb data (i.e., no isotopic heterogeneity given by fluid
inclusions and zonation even with different spot sizes), sample Albula-1 is
considered suitable for solution ICP-MS (see Sect. 4.4).
238U/206Pb and 207Pb/206Pb ratios and
their uncertainties as 2 standard errors (2 SE) measured by LA-ICP-MS.
f206 is calculated using the initial 207Pb/206Pb indicated by the Tera–Wasserburg diagrams. (a) 50 µm spot size; (b) 30 µm spot size.
A total of 23 50 µm analyses were carried out in sample Grimsel-1.
Analysis 14 (Table 5) was excluded as its 208Pb- and DF-corrected
time-resolved 206Pb/238U ratio does not overlap with all other
analyses (Fig. 5b), possibly indicating a different initial Pb isotopic
composition; 22 analyses define a regression in a Tera–Wasserburg
diagram (Fig. 6b) with an intercept age of 19.2±4.3 Ma (MSWD=0.79) and a 207Pb/206Pb intercept of 0.7964±0.0027. All
data points plot close to the y axis (238U/206Pb ratios =1.10–10.3 ± 2–4 %; 207Pb/206Pb ratios =0.774–0.794 ± 0.5–0.8 %; Table 5), indicating high and nearly invariable initial Pb fractions (f206=0.97–1.0; Table 5) in all measurements. The initial 207Pb/206Pb value is outside the uncertainty of the model initial 207Pb/206Pb ratio of 0.837±0.015 at 19 Ma of Stacey and Kramers (1975). Hence, a 207Pb correction with the modeled ratio would yield an inaccurate weighted average 207Pb-corrected 238U/206Pb age; thus, this sample can only be dated by the Tera–Wasserburg approach. A total of 25 additional measurements were made with a spot size of 30 µm. Analysis 13 (Table 5) is excluded based on
its 208Pb- and DF-corrected time-resolved 206Pb/238U ratio
not overlapping with the other analyses (Fig. 5f). The Tera–Wasserburg age
based on 22 analyses is 17±10 Ma with an MSWD of 1.3. The initial
207Pb/206Pb ratio of 0.7863±0.0049 is slightly outside
the uncertainty of that obtained with a spot size of 50 µm. The age
obtained by combining the analyses with 50 and 30 µm spot sizes is 15.2±6.5 Ma with an initial 207Pb/206Pb ratio of 0.7918±0.0037. However, the MSWD of the combined dataset is 2.6, which is much higher
than those obtained from the analyses with constant spot size, indicating
that the two datasets should be treated separately.
A total of 16 50 µm analyses of sample Grimsel-2 define a regression in the Tera–Wasserburg diagram (Fig. 6c), yielding an age of 16.9±3.7 Ma
(MSWD=0.40). No analyses were excluded before age calculation (Fig. 5c). The spread along the regression is limited and the data points are close to the y axis (238U/206Pb ratios =8.35–20.1 ± 2–3 %; 207Pb/206Pb ratios =0.759–0.784 ± 0.7–0.9 %; Table 5), indicating high and similar fractions of initial Pb in all analyses (f206=0.95–0.98; Table 5). The initial 207Pb/206Pb ratio of 0.7998±0.0054 is outside the uncertainty of the model initial 207Pb/206Pb value of 0.837±0.015 at 17 Ma (Stacey and Kramers, 1975), which cannot be used for a 207Pb correction. The Tera–Wasserburg approach is therefore the only viable method to date this sample.
A total of 23 analyses were carried out in sample Heyuan-1 with a spot size
of 50 µm, and analyses 22 and 23 (Table 5) were excluded based on the anomalous 208Pb- and DF-corrected time-resolved 206Pb/238U ratios (Fig. 5d); 21 analyses define a regression (Fig. 6d) with an age of 107.2±8.9 Ma (MSWD=0.91). Initial Pb fractions are highly
variable in the different measurements (f206=0.71–0.99; Table 5),
yielding an appreciable spread along the regression (238U/206Pb
ratios =0.753–18.2 ± 3–4 %; 207Pb/206Pb ratios =0.596–0.809 ± 2–3 %; Table 5). The initial 207Pb/206Pb ratio of 0.8161±0.0061 indicated by the upper intercept is outside the uncertainty of the modeled initial 207Pb/206Pb value of 0.843±0.015 at 107 Ma (Stacey and Kramers, 1975). Thus, the age of this
sample is best determined from the Tera–Wasserburg regression.
On a final note, we have calculated Tera–Wasserburg regression ages of the
presented epidote samples by using a different primary reference material:
CAPb (June 2019; reference values from Burn et al., 2017) and CAP (July
2019 and January 2020; reference values from Barth et al., 1994) allanite.
The resulting epidote U-Pb ages remain (within uncertainty) identical to
those calculated with Tara allanite as the primary reference material (Appendix C).
Solution ICP-MS U-Pb data
Uncertainties in the solution ICP-MS 238U/206Pb and
207Pb/206Pb ratios are 2 standard errors (2 SE).
For measurements by solution ICP-MS of 238U/206Pb and
207Pb/206Pb ratios in Albula-1 epidote, ca. 30 mg of material was necessary to ensure ca. 300 ng of total Pb. 238U/206Pb ratios range between 3.04 and 3.67, with uncertainties between 0.8 % and 1.6 %, and 207Pb/206Pb ratios range between 0.81319 and 0.81674 ± 0.03–0.04 ‰ (Table 6). The uncertainties in the solution ICP-MS
238U/206Pb ratios are lower than those measured by LA-ICP-MS
with a 50 µm spot size by a factor of 2.5, and a decrease by a factor
of 13–100 occurs in analytical uncertainties in 207Pb/206Pb
ratios. The two aliquots from each epidote micro-separate (A–B and C–D in
Table 6) yield identical ratios within uncertainty. 238U/206Pb and
207Pb/206Pb ratios display minor spread in a Tera–Wasserburg
diagram (Fig. 7). In comparison to LA-ICP-MS data, the intra-sample
variability of the solution ICP-MS 238U/206Pb and
207Pb/206Pb ratios is only 2 % and 14 %, respectively,
attesting to the homogenization of initial Pb fractions in the micro-separates.
This confirms that no statistically robust Tera–Wasserburg regression can be
calculated from the solution ICP-MS data alone. The 238U/206Pb
and 207Pb/206Pb ratios measured by solution ICP-MS overlap with
individual LA-ICP-MS data points (50 µm spot; Fig. 7).
238U/206Pb and 207Pb/206Pb ratios measured by
solution ICP-MS. Uncertainties are given as 2 standard errors
(2 SE).
Tera–Wasserburg diagram showing the comparison between laser ablation ICP-MS and solution ICP-MS data points. The error envelope of LA-ICP-MS data points is plotted with Isoplot 3.7.5 (Ludwig, 2012). All
ratios are uncorrected for initial Pb. Data-point error ellipses are
2σ.
DiscussionCAPb, CAP, and AVC allanite as quality control
Although allanite geochronology is beyond the scope of this contribution,
analyses of CAPb, CAP, and AVC allanite provide a quality control on
U-Pb measurements during the analytical sessions. The epidote samples
analyzed in this study contain too little Th for Th-Pb geochronology, and we only explore epidote as a U-Pb geochronometer. For this reason, although the usual reference ages for allanite are Th-Pb ages, we check the reliability of our U-Pb measurements of allanite secondary reference materials by comparing them to published U-Pb ages (see Table 2) rather than their Th-Pb reference ages. In fact, ages from the two systems differ for these allanite samples, likely due to excess 206Pb and the open-system behavior of the U-Pb system as opposed to the Th-Pb one (Barth et al., 1994; Oberli et al., 2004; Gregory et al., 2007, 2012; Darling et al., 2012; El Korh, 2014; Smye et al., 2014; Burn et al., 2017). Although this is a
relevant issue in allanite geochronology per se, it does not impact the
usefulness of allanite as a secondary reference material for U-Pb dating in
that it is the reproducibility of measured U-Pb ages in comparison with
published values that confirms the reliability of U-Pb measurements.
Tera–Wasserburg and weighted average 207Pb-corrected
238U/206Pb ages determined from our analyses of CAPb, CAP, and AVC allanite (Table 3) in all analytical sessions are overall consistent with their published U-Pb ages, attesting to the reliability of our U-Pb measurements. AVC allanite analyzed in two sessions provided Tera–Wasserburg ages of 292.4±3.7 and 285.2±4.5 Ma and identical weighted average 207Pb-corrected 238U/206Pb ages. Our AVC allanite ages are within the uncertainty of the Tera–Wasserburg age of 289.6±5.6 Ma of Gregory et al. (2007). CAPb allanite (Burn et
al., 2017) was used in one session, yielding a Tera–Wasserburg age of 284.2±2.6 Ma and a weighted average 207Pb-corrected
238U/206Pb age of 284.2±2.0 Ma, both within the uncertainty of
those calculated by Burn et al. (2017) of 284.9±2.8
and 283.8±2.8 Ma, respectively. CAP allanite was used in two sessions, returning
intercept ages of 288.5±2.9 and 283.0±3.4 Ma, with
comparable weighted average 207Pb-corrected 238U/206Pb ages.
Our CAP U-Pb ages are within the uncertainty of or close to the U-Pb age of
275.0±4.7 Ma determined by SHRIMP by Gregory et al. (2007). The ages
of CAPb, CAP, and AVC allanite obtained from unanchored Tera–Wasserburg
regressions through our data are identical to those obtained from the
anchored regressions but less precise (Table 3). We acknowledge that the
Tera–Wasserburg age of 288.5±2.9 Ma obtained from CAP allanite in
the July 2019 session is slightly outside the uncertainty of that reported by
Gregory et al. (2007). However, we do not regard this difference outside
the uncertainty of ca. 2 % as significant given the complications of U-Pb
age determinations of CAP allanite (e.g., Barth et al., 1994; Burn et al., 2017) and given the fact that AVC allanite used in the same session yielded
an age within the uncertainty of that measured by Gregory et al. (2007),
attesting to reliable U-Pb measurements.
The values of excess variance calculated by Isoplot 3.7.5 (Ludwig, 2012)
from the 207Pb-corrected ages of CAP and AVC allanite samples in all analytical sessions conducted between July 2019 and October 2020 are
respectively 1.9 % and 2.2 % (2σ). These values are considered
reasonable estimates of the long-term reproducibility, but they are not
propagated onto our epidote ages because (1) they are not directly applicable
to Tera–Wasserburg regression ages, and (2) the effect of the propagation
would be negligible.
Tara allanite as a reference material for LA-ICP-MS dating of epidote
The presented data confirm that Tara allanite is an appropriate primary
reference material for U-Pb dating of epidote by LA-ICP-MS in
spot-analysis mode. The primary reference material is used to correct the
measured isotopic ratios for DF, which is crucial to obtain a reliable U-Pb
geochronology by LA-ICP-MS (Horstwood et al., 2016). In this respect, 122
out of 127 analyses of epidote yield 208Pb- and DF-corrected
206Pb/238U ratios that are flat and that overlap throughout
ablation time, demonstrating that the correction for DF is accurately
carried out by using Tara allanite with a spot size of 50 and even
30 µm (Fig. 5). This is corroborated by the fact that the
Tera–Wasserburg ages and initial 207Pb/206Pb ratios of Albula-1
and Grimsel-1 epidotes with a 30 µm spot remain consistent with the dataset with a 50 µm spot size. However, in both epidote samples age precision decreases with a spot of 30 µm. In the case of Grimsel-1 epidote, this is expected because the poor spread in 238U/206Pb and 207Pb/206Pb ratios, combined with larger analytical uncertainties of the 30 µm measurements, leads to an even more poorly constrained regression than that with a 50 µm spot size. Thanks to the larger spread in plotted ratios of Albula-1 epidote, the effects of the lower analytical precision with a 30 µm spot size are less dramatic but still noticeable. The flat and overlapping trends of most 208Pb- and DF-corrected time-resolved 206Pb/238U ratios of sample Grimsel-1 (Fig. 5b and f) indicate that the fact that its initial 207Pb/206Pb ratios are slightly outside uncertainty when measured with different spot sizes cannot be due to inaccurate correction for DF.
Validation of 238U/206Pb and 207Pb/206Pb ratios by solution ICP-MS and considerations of analyzed volumes versus age precision
The consistency between the datasets acquired by different techniques lends
support to the accuracy of our LA-ICP-MS data and of the calculated
Tera–Wasserburg ages. The minor variability of 238U/206Pb ratios measured by solution ICP-MS data despite the high analytical precision with respect to LA-ICP-MS acquisitions is expected because sample preparation for solution ICP-MS requires homogenization of ca. 9×109µm3 (ca. 30 mg of material). By comparison, with LA-ICP-MS homogenization occurred over a volume of ca. 20–24×103µm3, as the measured depth of the LA-ICP-MS craters is between 10 and 12 µm. With decreasing spread in the plotted LA-ICP-MS 238U/206Pb and 207Pb/206Pb ratios, the regression becomes less constrained. Epidote samples Grimsel-1 and Grimsel-2 give larger uncertainties than Albula-1 and Heyuan-1 epidote because of the more limited spread in their LA-ICP-MS data points despite having higher U and Pb contents and hence better counting statistics. This suggests that higher analytical precision alone does not ensure better age precision unless it is accompanied by a sufficiently large data-point spread on the Tera–Wasserburg
diagram. It also confirms that epidote samples with relatively low U and Pb
contents should not be automatically considered unsuitable for U-Pb
geochronology.
It is therefore crucial that the spot size of LA-ICP-MS analyses represents
a compromise ensuring sufficient analytical precision and the smallest
extent possible of sample homogenization in order to preserve the
variability in initial Pb contents. It should be noted that the analyzed
samples are characterized by variable Th/U ratios (Fig. 3). The
fractionation of Th from U is commonly attributed to oxidizing conditions
(e.g., Frei et al., 2004). It is thus possible that the variability in
the initial Pb fractions of each individual epidote vein might be determined by
physicochemical conditions upon epidote crystallization, such as oxidizing
conditions or re-equilibration along fluid pathways.
Isotopic composition of initial Pb
Among the epidote samples analyzed in this study, only Albula-1 epidote
gives an initial 207Pb/206Pb ratio that overlaps within
uncertainty with the model value of Stacey and Kramers (1975). Accordingly,
the weighted average 207Pb-corrected 238U/206Pb age of sample
Albula-1 is within the uncertainty of its Tera–Wasserburg age. Epidotes from
all other samples yielded Tera–Wasserburg initial 207Pb/206Pb
ratios that deviate from model values, indicating non-negligible additions
of inherited radiogenic components to the initial Pb. Radiogenic Pb
components can be inherited by the fluid at its source and/or during
circulation and re-equilibration with rocks along its pathway containing
U-Th-bearing minerals (e.g., Romer, 2001). The weighted average
207Pb-corrected 238U/206Pb ages of epidote Grimsel-1,
Grimsel-2, and Heyuan-1 calculated by assuming a model initial
207Pb/206Pb ratio would be grossly inaccurate, implying that these
three samples can only be dated with a Tera–Wasserburg regression. These
considerations confirm the Tera–Wasserburg approach as the most suitable –
and often the only viable – method for accurate U-Pb dating of low-μ phases
such as epidote (see Romer, 2001; Romer and Xiao, 2005; Odlum and Stockli,
2019, 2020).
Geological constraints on epidote U-Pb ages
To evaluate the geological accuracy of the U-Pb ages calculated from
epidote in the veins presented above we consider other geochronological
constraints on the deformation history of their respective host rocks.
Albula-1 epidote gives a Paleocene age of 62.7±3.0 Ma. Although, to
our knowledge, no isotope geochronology is available in the Albula region,
our epidote U-Pb age is consistent with geodynamic events taking place in
its surroundings. For example, the Err–Platta system was investigated by
Handy et al. (1996), and their D2 – in the stability field of epidote – is
dated 80–67 Ma (K-Ar on white mica); epidote growth is also observed in the post-D2 deformation (Handy et al., 1996). A rutile U-Pb age of 63.0±3.0 Ma was calculated by Picazo et al. (2019) from the
Malenco–Margna boundary (Passo d'Ur, ca. 90 km south–southeast of the
Albula area; Fig. 1a in Picazo et al., 2019), dating the stacking of the
nappes associated with metamorphism at high pressure.
Sample Grimsel-1 yielded initial 207Pb/206Pb ratios that differ
outside uncertainty when measured with different spot sizes (50 vs. 30 µm). Although minimal, the difference in initial
207Pb/206Pb ratios given by the 50 and 30 µm measurements might indicate isotopic heterogeneity or unreliability of the 30 µm dataset. The 30 µm analyses were carried out in the same epidote grains as the 50 µm ones, and no textural evidence (e.g., zoning)
supports isotopic heterogeneity. Therefore, we regard the 30 µm
dataset as questionable from a technical standpoint and only discuss the
results obtained from the 50 µm dataset. Epidote U-Pb ages in samples Grimsel-1 and Grimsel-2 yield (early) Miocene ages of 19.2±4.3 and 16.9±3.7 Ma, respectively. These ages are within the
uncertainty of each other and can be attributed to the early deformation in
the area between 22 and 17 Ma (Handegg phase; Challandes et al., 2008; Rolland
et al., 2009). This is corroborated by the presence of green biotite
associated with epidote in the epidote-bearing veins (Challandes et al., 2008; Rolland et al., 2009; Herwegh et al., 2017; Wehrens et al., 2017).
Notably, Grimsel-1 and Grimsel-2 epidote samples yield initial
207Pb/206Pb ratios that are identical within uncertainty and
indicate an inherited radiogenic component. This implies that the Pb
isotopic signature of the circulating fluid(s) was homogenously
re-equilibrated over a ca. 100 m distance.
Epidote of sample Heyuan-1 yields an age of 107.2±8.9 Ma, which is
(early) Cretaceous and consistent with the earliest movements of the Heyuan
fault. This is supported by the sample being crosscut by an
earliest-generation quartz vein associated with hematite, as the formation
of quartz veins post-dates that of epidote veins across the Heyuan fault but
is related to the early phases of faulting (Tannock et al., 2020a, b).
Epidote ages as time of crystallization in low-temperature veins
Having established that the calculated epidote U-Pb ages are consistent
with geological events that affected the host rocks, we now discuss whether
these ages can be truly considered representative of epidote
crystallization. The highest temperatures recorded by the deformation events
that affected the meta-granitoid rocks hosting the analyzed epidote veins at
Albula Pass, at Grimsel Pass, and at the Heyuan fault are respectively 300 ∘C (Mohn et al., 2011), 450±30∘C (Challandes
et al., 2008; Goncalves et al., 2012), and 330 ∘C (Tannock et al., 2020a). All these temperatures are well below 685–750 ∘C,
which was proposed by Dahl (1997) as the range for the closure temperature of Pb
diffusion in epidote. Nevertheless, resetting of the U-Pb geochronometer can occur independently of temperature-controlled diffusion via fluid-mediated dissolution–precipitation processes, which can be assessed
with BSE imaging. Albula-1 and Grimsel-2 epidotes display growth zoning,
which is regarded as primary zoning, and thus lack significant elemental
diffusion (Franz and Liebscher, 2004). Since care was taken to avoid mixing
of different zoning domains in each single analysis – including those
associated with secondary veinlets – and considering that the MSWDs of the
calculated epidote ages are all close to or below 1 (i.e., only one epidote
generation can be distinguished at the stated analytical precision), we can
conclude that the ages of Albula-1 and Grimsel-2 epidote date their
crystallization and therefore the formation of the epidote-bearing veins.
The epidote-bearing vein in sample Grimsel-1 is folded; epidote does not
display prominent zoning, and it is fractured and porous. This may raise
questions as to whether the age of 19.2±4.3 Ma dates the formation
of Grimsel-1 epidote or the resetting of the U-Pb geochronometer by
interaction with a fluid assisting the Alpine deformation. However, epidote
is associated with green biotite, hinting that the vein epidote formed
during the Handegg phase (22–17 Ma) in the stability field of green biotite
(Challandes et al., 2008; Rolland et al., 2009; Herwegh et al., 2017;
Wehrens et al., 2017). Therefore, the age of 19.2±4.3 Ma is
interpreted as the crystallization age of epidote. The minor presence of
chlorite may indicate that the subsequent folding of the vein
occurred at the end of the Handegg phase or at the beginning of the Oberaar
phase with the onset of chlorite crystallization (Herwegh et al., 2017;
Wehrens et al., 2017).
In sample Heyuan-1, epidote is present in pockets filled by an
epidote–quartz(–chlorite) assemblage. This microstructure might suggest a
magmatic origin of epidote, and consequently it might suggest that epidote formed
in the Jurassic as a magmatic mineral and that the U-Pb system was reset by
ingression of fluids related to the first movements of the Heyuan fault.
However, a magmatic origin of the epidote can be ruled out based on the
association of epidote with chlorite instead of biotite (the magmatic
phyllosilicate stable in the Xinfengjiang pluton; Li et al., 2007; Tannock
et al., 2020a, b). Coexistence with chlorite is consistent with the
temperature of mylonitization during Heyuan normal faulting (330 ∘C; Tannock et al., 2020a). Furthermore, the Th/U ratios measured in Heyuan-1 epidote are ≪1, whereas the Fogang batholith – which
comprises the Xinfengjiang pluton – has Th/U ratios ≫1 (Li et al., 2007). This lends support to a non-magmatic origin of the studied epidote because epidote generally reflects the trace
element composition of its host rock (Frei et al., 2004). We thus conclude
that all epidote U-Pb ages presented in this study date the crystallization of the epidote grains that formed during low-temperature fluid circulation.
Concluding remarks and future prospects
This contribution presents a protocol to determine U-Pb ages and initial
207Pb/206Pb compositions of epidote (i.e., epidote–clinozoisite
solid solution), which is highly but variably enriched in initial Pb. This
includes preliminary screening of the material to verify the presence of
sufficiently high U contents (mainly between 7 and 310 µgg-1 in
our samples) and intra-sample chemical variability. If these geochemical
characteristics are ascertained, measurements by spot-analysis LA-ICP-MS
using a quadrupole mass spectrometer can allow U-Pb ages to be determined
with uncertainties between ca. 5 % and 20 %, with the lowest precision being
related to poor variability in initial Pb fractions. It is demonstrated that
epidote and allanite have similar downhole fractionation of Pb from U during
LA-ICP-MS spot analysis, and the consistency between the data measured by
LA-ICP-MS and solution ICP-MS corroborates the accuracy of
238U/206Pb and 207Pb/206Pb ratios for epidote samples determined by using Tara allanite as the primary reference material. We have shown that all effects due to downhole fractionation are accurately
corrected for, even with a spot size as small as 30 µm, by using Tara allanite as a primary reference material. Therefore, the lack of a standard that is perfectly matrix-matched to epidote does not prevent U-Pb dating of the epidote–clinozoisite solid solution by spot-analysis LA-ICP-MS with precision between ca. 5 % and 20 %. The geological significance of the ages is verified against the geological evolution of the areas of origin of the epidote samples. In each sample, the obtained ages are interpreted as dating epidote crystallization during hydrothermal-vein formation.
The key strategy for U-Pb dating of epidote is the Tera–Wasserburg
regression with data uncorrected for initial Pb. U-Pb geochronology of
epidote is most successful when the epidote samples display sufficiently large
variability in initial Pb fractions, even when high analytical precision
is achieved, which may be related to variable physicochemical conditions
during the crystallization of vein-filling epidote. Although it is
recommended that the largest spot size possible be used to ensure good
counting statistics, it is imperative that geochemical heterogeneity be
preserved among the different analyses in order to obtain a well-constrained
Tera–Wasserburg regression and a small age uncertainty. An unexpected perk
highlighted by the present study is that relatively low U contents (i.e.,
tens of µgg-1) do not necessarily hamper age determinations at a geologically useful precision provided that the spread of
238U/206Pb and 207Pb/206Pb ratios is large enough.
This study presents a protocol that can be readily applied to date
epidote-bearing hydrothermal veins and to assess the initial Pb isotopic
variability of the epidote-forming fluid. Better insight can now be gained
from the application of epidote U-Pb dating into the mechanisms that led to the hydration of the continental crust in the Aar Massif and in the Err nappe. This study represents a basis from which further developments may
allow researchers to date high-P epidote-bearing veins in subducted oceanic units and to
determine at the same time where the vein-forming fluid originated thanks to
the combination of trace element, age, and isotopic data measured in epidote.
Multiple phases of fault reactivation may be identified in fault-plane
epidote. Whether or not plagioclase recrystallization in metamorphosed
granitoid rocks is linked to the formation of epidote-bearing veins may be
solved by measuring U-Pb ages and initial Pb isotopic compositions in
epidote, which has proven its potential to become an invaluable geochemical,
isotopic, and geochronological tool.
Contents of major elements were measured by an electron probe micro-analyzer
(EPMA) using a JEOL-8200 microprobe at the Institute of Geological Sciences
(University of Bern) with 15 KeV accelerating voltage, 10 nA specimen
current and 2 µm electron beam diameter. The following natural and synthetic standards were used for calibration: wollastonite (SiO2), olivine (MgO), anorthite (CaO, Al2O3), garnet (FeO), Topaz (F), tephroite (MnO), tugtupite (Cl), rutile (TiO2), and celestite (SrO).
The structural formula was calculated based on 12.5 oxygen cations.
REE, U, Th, and Pb contents of epidote samples were measured on thin (30 µm) or thick (50–60 µm) sections pre-cleaned with ethanol.
Concentrations in sample Albula-1 were measured on a Geolas Pro 193 nm ArF
excimer laser (Coherent, USA) coupled with an ELAN DRCe quadrupole ICP-MS
(QMS; Perkin Elmer, USA) at the Institute of Geological Sciences (University
of Bern). Instrument optimization and measurement procedures (similar to
those reported in Pettke et al., 2012) employed an ablation rate of ca. 0.1 µm per laser pulse, 10 Hz, and beam sizes between 24 and 60 µm, the largest possible to minimize limits of detection and to avoid inclusions and fractures. Ablation was done in a 1 L min-1 He–0.008 L min-1 H2 atmosphere. Concentrations in samples Grimsel-1, Grimsel-2, and Heyuan-1 were measured on a RESOlutionSE 193 nm excimer laser system (Applied Spectra, USA) equipped with an S-155 large-volume constant-geometry chamber (Laurin Technic, Australia) coupled with an Agilent 7900 ICP-QMS. Ablation was carried out in a He atmosphere, which was allowed to mix with Ar carrier gas for transport to the ICP-MS. The repetition rate was 5 Hz at spot sizes between 20 and 50 µm.
On both systems, analytical conditions were optimized with NIST SRM612 so as
to keep the ThO production rate <0.2 % and a Th/U sensitivity ratio of
0.97–1.0, the latter indicative of robust plasma conditions. GSD-1G from the
USGS was employed as the external standard, whereas quality control was
monitored by measuring SRM612 from NIST measured as an unknown. A true-time
linear drift correction was applied by bracketing standardization. Data
acquired on both systems were reduced offline using SILLS (Guillong et al., 2008), with the sum of measured total oxides (98.3 % for epidote and 100 % for SRM-NIST 612) used for internal standardization (compare Halter et al., 2002). Limits of detection were rigorously calculated for each element in each analysis by employing the formulation detailed in Pettke et al. (2012).
Tera–Wasserburg diagrams were produced for allanite secondary reference materials. All
regressions are anchored to a 207Pb/206Pb intercept of 0.854±0.015 at 275 Ma following Stacey and Kramers (1975), and all age
uncertainties are 95 % confidence.
Tera–Wasserburg diagrams of allanite secondary standards. (a) CAPb allanite, 14 June 2019 session; (b) CAP allanite, 23 July 2019 session; (c) CAP allanite, 16 January 2020 session. (d) AVC allanite, 23 July 2019 session; (e) AVC allanite, 16 January 2020 session. All ratios are
uncorrected for initial Pb. Data-point error ellipses are 2σ, and age
uncertainties are 95 % confidence.
U-Pb ages of epidote samples with
CAPb (14 June 2019) and CAP (23 July 2019) allanites
as primary reference materials. Measurements are with a 50 µm spot size. Age uncertainties are 95 % confidence.
Sample14 June 201923 July 2019Tera–Wasserburg U-Pb age [Ma] Albula-161.1±2.8–MSWD=1.6n=22Grimsel-118.8±4.3–MSWD=0.78n=22Grimsel-216.5±3.5–MSWD=0.39n=16Heyuan-1–101.7±8.9MSWD=0.87n=21Code and data availability
All data are included in the paper (see tables).
Author contributions
VP prepared the samples, ran the measurements, reduced and evaluated the data, and prepared the paper. TE, DR, AB, and MH laid out the project. MW and IMV supervised the work in the clean lab for digestion of epidote micro-separates, as well as the measurements and data reduction of solution ICP-MS data. PL helped with the EPMA measurements,
contributed to the discussions on the analytical LA-ICP-MS setup, and
provided fundamental insight into LA-ICP-MS data evaluation and
processing. TP enabled the measurements of REE, Pb, Th, and U
contents by LA-ICP-MS. AB and MH helped with the geological interpretation of the data. All authors were involved in data evaluation and interpretation, as well as in the writing of the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors thank Elizabeth Catlos, James Darling, and Axel Schmitt for their
constructive reviews and suggestions. Veronica Peverelli would like to thank
Francesca Piccoli, Gabriela Baltzer, and Daniel Rufer for help in the
laboratories, Lisa Tannock for providing samples, and Patrick Neuhaus from
the Geography Department of the University of Bern for carrying out the solution
ICP-MS measurements of 238U/206Pb ratios. We acknowledge funding of our new LA-ICP-MS facility through the Swiss National Science Foundation, project 206021_170722, to Daniela Rubatto and Thomas Pettke.
The solution ICP-MS isotope data were obtained on a Neptune MC-ICP mass
spectrometer acquired with funds from the NCCR PlanetS supported by the
Swiss National Science Foundation under grant no. 51NF40-141881. This work is part
of the PhD thesis of Veronica Peverelli, who acknowledges SNF funding
(project no. 178785) granted to Alfons Berger.
Financial support
This research has been supported by the Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (SNF) (grant no. 178785).
Review statement
This paper was edited by Axel Schmitt and reviewed by James Darling and Elizabeth Catlos.
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