The ability to constrain the age of calcite formation is of great utility to the Earth science community, due to the ubiquity of calcite across a wide spectrum of geological systems. Here, we present the first in situ laser ablation inductively coupled tandem quadrupole mass
spectrometry (LA-ICP-MS/MS) Lu–Hf ages for calcite, demonstrating
geologically meaningful ages for iron oxide copper gold (IOCG) and skarn mineralisation, carbonatite intrusion, and low-grade metamorphism. The analysed samples range in age
between ca. 0.9 and ca. 2 Ga with uncertainties between 1.7 % and
0.6 % obtained from calcite with Lu concentrations as low as ca.
0.5 ppm. The Lu–Hf system in calcite appears to be able to preserve primary
precipitation ages over a significant amount of geological time, although
further research is required to constrain the closure temperature. The
in situ approach allows calcite to be rapidly dated while maintaining its
petrogenetic context with mineralisation and other associated mineral
processes. Therefore, LA-ICP-MS/MS Lu–Hf dating of calcite can be used to
resolve the timing of complex mineral paragenetic sequences that are a
feature of many ancient rock systems.
Introduction
Calcite (CaCO3) is the main mineral phase of most carbonate sedimentary
rocks and their metamorphic equivalents. Calcite is also a common diagenetic
phase and is a major component of carbonatites. Calcite is also a common
product of hydrothermal alteration and constituent of mineralising systems
where it may precipitate from fluids during pre-ore, ore-stage, and post-ore
forming processes (Debruyne et al., 2016). The ability to directly date
calcite unlocks the possibility of constraining the timing of a vast array of
geological processes that can be difficult to date using conventional
methods.
Accurate in situ U–Pb geochronology of calcite has been applied to a variety of geological systems (e.g. Li et al., 2014; Roberts and Walker, 2016; Ring and Gerdes, 2016). However, calcite often incorporates significant quantities of Pb during crystallisation (i.e. “initial” or “common” Pb), which can limit the utility of U–Pb geochronology (Rasbury and Cole, 2009). Moreover, Pb is highly fluid mobile (Brugger et al., 2016), so it can be difficult to obtain primary age information with the U–Pb method in hydrothermal or
strongly altered systems (Roberts et al., 2020; Simpson et al., 2021b).
Further, given the propensity for calcite to undergo recrystallisation,
calcite U–Pb geochronology is rarely applicable to Precambrian systems as
the calcite U–Pb system invariably does not remain closed over long
timescales (Whitehouse and Russell, 1997).
Alternative dating systems involving the radioisotopic decay of rare earth
elements (REEs) such as Sm–Nd and Lu–Hf, have previously been applied to
calcite (e.g. Maas et al., 2020; Barker et al., 2009; Peng et al., 2003; Nie et al., 1999), based on the moderate to strong compatibility of REEs in
carbonates in many systems (Debruyne et al., 2016; Zhong and Mucci,
1995; Terakado and Masuda, 1988; Elzinga et al., 2002). However, it should be
noted that REE compatibility will be dependent on the conditions of calcite
formation and can vary. Importantly for geochronology, experimental
evidence indicates that Lu and Hf are highly immobile in many hydrothermal
fluids (Migdisov et al., 2016; Brugger et al., 2016), meaning that the Lu–Hf
system is potentially preserved relative to the U–Pb system during post-formation processes. However, concentrations of Lu and Hf are generally low (ppm to ppt
range) in calcite, necessitating the dissolution of large quantities of
material (up to 2 g) per sample for conventional Lu–Hf geochronology (Maas
et al., 2020). These large quantities significantly reduce the spatial
resolution of the technique and have the additional problem of potential
contamination from inclusions. Furthermore, age variation is difficult to
detect, and bulk samples may produce a meaningless average age derived from
mixing of age domains. The dissolution process also removes calcite from
its petrological context. The recent development of in situ Lu–Hf geochronology of individual minerals by laser ablation inductively coupled tandem quadrupole mass
spectrometry (LA-ICP-MS/MS) allows for rapid acquisition of spatially resolved data, and has been demonstrated for garnet (Ribeiro et al., 2021; Tamblyn et al., 2021) and apatite (Glorie et al., 2022).
In this study, we present the first in situ Lu–Hf dating of calcite from a variety of geological environments. We demonstrate the that in situ calcite Lu–Hf geochronology can produce meaningful ages for complexly deformed and
hydrothermally altered systems, such as mineral deposits, as well as
carbonatite intrusions and low-grade metamorphism.
Geological background of samples
The analysed samples were selected (1) to demonstrate that calcite Lu–Hf
can date primary calcite formation in carbonatites, (2) to reveal the
potential of the method to unravel complex ore systems or later events, and (3) to characterise large calcite samples that would make suitable reference
materials for in situ analysis.
Phalaborwa carbonatite, South Africa
The Phalaborwa Igneous Complex is located ∼ 450 km northeast
of Johannesburg, in Limpopo Province, South Africa. The igneous complex
is the result of several distinct pulses of alkaline intrusions that were
emplaced into Archean granitic gneiss (Staff, 1976). The Loolekop pipe is
located in the centre of the Phalaborwa Igneous Complex and was intruded by
two episodes of carbonatite emplaced at the intersection of five major
faults and shear zones (Staff, 1976; Basson et al., 2017). The oldest
carbonatite is termed the “transgressive banded” carbonatite and has an
emplacement age of 2060.0 ± 2.2 Ma (baddeleyite secondary ion mass spectrometry (SIMS) U–Pb; Wu et al.,
2011). This is intruded by a slightly younger carbonatite termed the
“banded” carbonatite and has an emplacement age of 2059.8 ± 1.3 Ma
(baddeleyite SIMS U–Pb; Wu et al., 2011). The Phalaborwa carbonatite is
unique as it is the only known example of a carbonatite containing economic
Cu mineralisation (Groves and Vielreicher, 2001). In the banded
carbonatite–phoscorite, Cu mineralisation is primarily in the form of
bornite inter-grown with valleriite with minor chalcopyrite (Staff, 1976).
In the transgressive carbonatite, Cu mineralisation is present as
chalcopyrite inter-grown with cubanite and valleriite (Staff, 1976). Cu
mineralisation is interpreted as being magmatic–hydrothermal in origin, with Cu
leached by high-temperature hydrothermal fluids at depth, precipitating
along fractures within the hosting carbonatite (Le Bras et al., 2021). The
sample used in this study (P01) is representative of carbonatite-hosted
Cu-mineralisation from within the Loolekop pipe (Fig. 1). The sample is
mineralogically composed of chalcopyrite inter-grown with cubanite and
pyrrhotite alongside an assemblage of magnetite, dolomite, calcite, biotite,
pyroxene, and valleriite. As the Phalaborwa carbonatite has a well-constrained crystallisation age, it provides an ideal case study to
demonstrate the utility of the in situ Lu–Hf method for dating igneous calcite directly associated with Cu mineralisation.
A combination of SEM mineral maps (a, c, d, e) and photos of analysed samples. (a) P01 (Phalaborwa carbonatite) shows calcite in petrogenetic context to chalcopyrite. (b) Photo of LC 1 hand sample. (c) Photo of the ME 1 sample in outcrop, with inset showing the mineralogy of the analysed sample. (d) Calcite from ME 2 (Mt Isa) in contact with hematite, pyrrhotite, and andradite, with inset showing hand sample. (e) OL-MB (Otter Lake), showing analysed calcite with associated minerals, with inset showing relationship between apatite (Ap) and calcite (Cal) in hand sample. (f) FF014 (Flin Flon deposit) shows calcite vein in chlorite matrix with disseminated pyrite, with inset showing analysed block (dark coloured matrix is composed of chlorite). Black circles represent laser spot locations. Mineral abbreviations: Cal – calcite; Cpy – chalcopyrite; Py – pyrite; Mag – magnetite; Cu – cubanite; Di – diopside; Scp – scapolite; An – andradite. Larger-size sample images are included in Appendix C.
The Eastern Fold Belt, Mt Isa Block, Queensland, Australia
The Eastern Fold Belt of the Mount Isa Domain has experienced multiple
episodes of deformation, magmatism, metamorphism, mineralisation, and
pervasive hydrothermal alteration across the Paleo- to Mesoproterozoic and
hence represents one of the most metasomatised crustal blocks on Earth
(Oliver et al., 2008). Hydrothermal calcite is common across the Mount Isa
region, in the Mary Kathleen Domain (Oliver et al., 1993) and in many of the
IOCG deposits of the Cloncurry District. For this study we have selected
calcite samples from the Lime Creek calcite quarry and the Mt Elliott IOCG
deposit for Lu–Hf analysis. The Mt Isa Domain has both regional and deposit-level age constraints, making it a good area to demonstrate the technique.
The Lime Creek quarry is one of a number of large calcite pods or veins that
are exposed in Mary Kathleen Domain. The Lime Creek quarry is hosted within
the ca. 1760 Ma Argylla Formation and lies along the steeply dipping
NNW-trending Tribulation–Lime Creek Fault, which offsets regional-scale
“D2” folds (Marshall, 2003). Breccias along this fault contain clasts
of calc–silicate rocks and metadiorite with a matrix consisting of
albite–actinolite–diopside–biotite–titanite–apatite that are subsequently
overprinted by the undeformed Lime Creek calcite-dominated veins (Marshall,
2003). These veins are extremely coarse-grained with calcite crystals larger
than 1 m3, actinolite crystals over 1 m in length, and apatite,
biotite, diopside, and titanite grains over 20 cm in diameter (Oliver et al.,
1993; Marshall, 2003). Based on cross-cutting relationships, it is
interpreted as the Lime Creek vein system and other calcite pods and veins of
this style precipitating post-faulting during late-“D3” deformation (ca. 1550–1500 Ma) of the Isan Orogeny (Giles and Nutman, 2002; Marshall, 2003).
This style of veining is common throughout the Mary Kathleen Domain and
provides evidence of kilometre-scale fluid transport during late-stage metamorphism
(Oliver et al., 1993). Based on C and O isotope analysis of calcite from
these veins, they are interpreted as having formed from hydrothermal fluids
likely associated with the intrusion of the ca. 1530 to 1500 Ma
Williams–Naraku batholiths (Oliver et al., 1993). Although no direct dating
has been completed on the Lime Creek quarry, titanite from the nearby and
cognate Knobby Quarry has produced three titanite U–Pb ages of 1521 ± 5, 1527 ± 7, and 1555 ± 5 Ma (Oliver et al.,
2004). The sample analysed in this study (LC1) consists of very
coarse-grained calcite with coarse-grained diopside collected from a large
calcite pod in the Lime Creek quarry (Fig. 1).
Mt Elliott is an IOCG deposit located in the Eastern Fold Belt of the Mount
Isa Inlier (Duncan et al., 2011). The deposit is situated within northwest-striking splays of the Mount Dore Fault (Wang and Williams, 2001; Duncan et
al., 2011) and is hosted within skarn-altered and deformed phyllites and
schists (Garrett, 1992; Wang and Williams, 2001). The host rocks were
metamorphosed to lower amphibolite facies during the ca. 1600–1580 Ma
D2 deformation of the Isan Orogeny (Wang and Williams, 2001; Garrett,
1992). The formation of early albite–hematite (red rock) alteration enhanced
brittle fracturing and brecciation of the shale (Garrett, 1992). This was
infilled by two stages of open-space skarn development: (1) diopside–magnetite–hematite–calcite–titanite–allanite–phlogopite and (2) actinolite–scapolite–magnetite–andradite–calcite–epidote–allanite–chlorite
and biotite (Garrett, 1992; Wang and Williams, 2001). Sulfides in the
second stage include chalcopyrite, pyrrhotite, and pyrite (Garrett, 1992).
Although the two skarn assemblages are difficult to distinguish
mineralogically, the second episode is the most widespread and represents
the major Cu–Au event (Garrett, 1992; Wang and Williams, 2001). A variety
of geochronological techniques have been applied to constrain the age of
Cu–Au mineralisation. The earliest phase of skarn development has been
dated to 1530 ± 11 Ma (U–Pb titanite; Duncan et al., 2011). The
second stage of skarn development associated with Cu–Au mineralisation has
been dated to 1513 ± 5 Ma (molybdenite Re–Os; Duncan et al., 2011)
and 1510 ± 3 Ma (actinolite Ar–Ar; Wang and Williams, 2001). Two
outcrop samples from the Mount Elliott Cu–Au deposit were selected for Lu–Hf geochronology. Mt Elliott 1 (ME 1) consists of coarse-grained pink-coloured
calcite that is coeval with the formation of diopside, scapolite, and
magnetite (Fig. 1). Although the paragenesis of this sample is relatively
unconstrained, the lack of sulfides may indicate that this sample belongs
to the early pre-mineralisation skarn assemblage. Calcite from sample Mt Elliott 2 (ME 2) is coeval with the formation of andradite, pyrite,
chalcopyrite, pyrrhotite, and magnetite (Fig. 1). The close relationship
between calcite and chalcopyrite in this sample indicates that it is
associated with the main Cu–Au-bearing skarn assemblage.
The Flin Flon Greenstone Belt stretches across central Manitoba through to
east central Saskatchewan and hosts several world-class Zn–Cu VMS deposits
including the Flin Flon, Callinan, and 777 deposits (Koo and Mossman, 1975).
Zn–Cu mineralisation is interpreted as having formed contemporaneously with
deposition of the 1888.9 ± 1.6 Ma Millrock Member during the
Trans-Hudson Orogeny (Koo and Mossman, 1975; Rayner, 2010; Gibson et al.,
2012). The Flin Flon Zn–Cu orebody is recognised as having undergone six
distinct deformation events that have affected the shape of the deposit
(Lafrance et al., 2016; Schetselaar et al., 2017). “D1” and D2
were associated with the intra-oceanic accretion of the Flin Flon Arc to
other volcanic terrains before ca. 1872 Ma (Lafrance et al., 2016).
D3 occurred from 1847–1842 Ma as a response to the final accretion
of the Flin Flon Terrane to the Glennie Terrane, producing west-verging
folds within stacked, east-dipping thrust sheets of basement and cover rocks
bounded by NNW-striking thrust faults (Lafrance et al., 2016). “D4”
resulted from the collision between the Flin Flon and Glennie Complex with
the Sask Craton and is broadly coeval with the ca. 1840 Ma Phantom Lakes
dyke (Gibson et al., 2012; Lafrance et al., 2016). “D5” deformation
produced a penetrative regional cleavage (S5) that is defined by a
continuous chloritic foliation ubiquitous in the volcanic basement rocks
(Gibson et al., 2012; Lafrance et al., 2016). EWE–WNW-directed compression
during “D6” deformation produced a second regional penetrative cleavage and reactivated a variety of regional-scale faults (Gibson et al., 2012; Lafrance et al., 2016). Regional greenschist to granulite facies
metamorphism is associated with D5–6 deformation at ca. 1820–1790 Ma
(Schneider et al., 2007). The Flin Flon mine horizon was imbricated during
D3 thrusting with the shape of the ore lenses moulded during
D4 and D5 deformation (Schetselaar et al., 2017). Regional
greenschist to amphibolite grade metamorphism occurred between 1820–1790 Ma (U–Pb monazite; Schneider et al., 2007), with rocks in the Flin Flon
deposit reaching greenschist facies (Koo and Mossman, 1975). The sample
selected for this study is from the hydrothermally altered and sheared
footwall of the Flin Flon VMS deposit. This sample is composed of highly
foliated chlorite and calcite with disseminated pyrite and residual
titanomagnetite. A band of highly foliated calcite was selected for Lu–Hf
analysis (sample FF014; Fig. 1) to constrain the age of syn-metamorphic
shearing of the deposit.
Yates U–Th prospect, Otter Lake area, Grenville Province, Canada
The Otter Lake area is located in SE Ontario within the Grenville Province.
The Grenville Province can be distinguished from surrounding provinces based
on various structural, metamorphic, and isotopic signatures attributed to the
overprinting ca. 1080–980 Ma Grenvillian Orogeny (Rivers, 2015). This
orogenic event produced widespread metamorphism from granulite to
amphibolite facies (van Breemen and Corriveau, 2005) accompanied by widespread
hydrothermal alteration in the Otter Lake area (Kretz et al., 1999). The Yates U–Th prospect is located approximately 100 km northwest of Ottawa and
is renowned for the occurrence of pegmatites that contain large euhedral
crystals of apatite set within a matrix of predominantly orange–pink
calcite, with diopside, allanite, titanite, fluorite, thorite, and phlogopite
(Schumann et al., 2019). A wide range of dates have been produced from the
Yates mine, including titanite Pb–Pb and U–Pb ages between ca. 1020 and
998 Ma (Frei et al., 1997; Kennedy et al., 2011); apatite Pb–Pb and U–Pb
ages of 913 ± 7 Ma (Barfod et al., 2005), 933 ± 12, and 920–850 Ma (Chew et al., 2011; Xiang et al., 2021); and an apatite Lu–Hf age of 1031 ± 6 Ma (Barfod et al., 2005). In addition, Simpson et al. (2021a) obtained an in situ Lu–Hf apatite age of 1000 ± 11 Ma (when corrected for laser-induced elemental fractionation). Importantly, the
apatite Lu–Hf and Pb–Pb ages were obtained from the same large
apatite crystal, indicating that the Lu–Hf and U–Pb systems were decoupled (as opposed to multiple generations of apatite growth). Barfod et
al. (2005) argued that late-stage fluid interactions may have affected Pb
retentivity in the apatite, as the apatite was unlikely to be above the
apatite Pb closure temperature at ca. 913 Ma. Calcite from a specimen
containing coarse-grained euhedral apatite with pink calcite, quartz, and
diopside was selected for calcite Lu–Hf analysis (OL-MB, Fig. 1). The
apatite is enclosed in the sampled calcite and is interpreted as having crystallised just prior to the calcite but during the same hydrothermal
event.
Method
The samples were mounted in 2.5 cm diameter epoxy mounts and screened for Lu
concentration by LA-ICP-MS to determine suitability for Lu–Hf analysis.
Mineral liberation analysis (MLA) maps were obtained using a Hitachi SU3800
scanning electron microscope (SEM) to reveal the petrogenetic context of the
analysed calcite.
In situ Lu–Hf dating method
Analysis was conducted at Adelaide Microscopy, the University of Adelaide.
Calcite samples were analysed using a RESOlution 193 nm laser ablation
system (Applied Spectra) with a S155 sample chamber (Laurin Technic). The
laser ablation system was coupled to an Agilent 8900 tandem mass
spectrometer (ICP-MS/MS). The methodology largely follows that of Simpson et
al. (2021a) including an initial instrument tune conducted with no NH3
in the reaction cell to achieve robust plasma conditions (U / Th = 1.00–1.05) and minimal oxide interferences
(ThO / Th < 0.2 %). A carrier gas of 3.5 mL min-1 N2 was added after the sample cell in order to increase sensitivity (Hu et al., 2008). Analytical conditions are included in Table D1.
Methods for the separation of 176Hf from 176Lu and 176Yb follow
that of Simpson et al. (2021a). In more detail, the Agilent 8900x utilises a
reaction cell between two quadrupole mass analysers, which can be used to
separate isobaric interferences. The first quadrupole is used as a mass
filter (e.g. when set to mass 176, only 176Lu, 176Yb, and
176Hf can pass), thereby minimising potential background interferences
and other, unwanted reactions. Following this, a mixture of 10 % NH3 and 90 % He is added to the reaction cell (at a rate of 3 mL min-1). This
mixture is optimised to promote formation of the Hf reaction product
Hf((NH)(NH2)(NH3)3)+, and the second quadrupole is set to
82 amu higher than the first (e.g. Q1 = 176 amu and Q2 = 258 amu). This method minimises the equivalent Lu and Yb reaction products (∼ 0.03 % for Lu and below detection for Yb), such that the isobaric interferences on 176Hf are negligible (Simpson et al., 2021a). Lens voltages were tuned to increase sensitivity on the Hf reaction product (Simpson et al., 2021a). In order to calculate Lu / Hf ratios, 176Hf (+82) was measured directly, 175Lu was measured as a proxy for 176Lu, and 178Hf (+82) was measured as a proxy for 177Hf
(Simpson et al., 2021a). 176Hf /176Lu, 176Lu /177Hf, and 176Hf /177Hf ratios were calculated as part of the normalisation to
NIST610, as opposed to separately converting measured 175Lu and
178Hf into 176Lu and 178Hf. In more detail, if we assume that
the 176Lu /175Lu ratio (or 177HF /178Hf ratio) is identical between NIST SRM 610 and all analysed samples, a correction factor calculated from the percentage difference between the 175Lu /178Hf ratio
measured in NIST SRM 610 and the published 176Lu /177Hf will
correct the unknowns for matrix-independent fractionation and differences in
isotopic abundance. 43Ca was measured for internal normalisation of
trace element abundances, and the following isotopes were measured to
monitor for inclusions: 27Al, 47Ti, 89Y, 90Zr,
140Ce, and 172Yb.
Lutetium abundances in most calcite samples are low (< 6 ppm), so we
employed a large laser diameter of 257 µm and a repetition rate of
10 Hz to maximise sensitivity. High sensitivity is important in order to
either measure common Hf (in this case 178Hf) or demonstrate that
176Hf is sufficiently above detection limits that the effects of common
Hf are negligible. Smaller spot sizes could be employed for higher-Lu and/or
higher-Hf samples. An extra 20 s delay was added after each interval
of sample ablation in order to ensure the washout had reached background
levels. NIST SRM 610 glass (176Lu /177Hf: 0.1379 ± 0.005; 176Hf /177Hf: 0.282122 ± 0.000009; Nebel et al., 2009) was used as the primary
reference material and was analysed using a spot size of 43 µm. The
smaller spot size was required to ensure that 175Lu was measured in
pulse counting mode (< 4 Mcps). Consistent with observations in
Simpson et al. (2021a), Lu and Hf showed no measurable downhole
fractionation in the analysed carbonates (Fig. 2); as such, no downhole
correction was applied to the data.
Time-resolved signals for 175Lu /(176)Hf (+82),
43Ca, 175Lu, and (176)Hf (+82) demonstrating the effects
of plasma loading on the signal intensities (i.e. dip in signal intensities
at ∼ 10–15 s ablation) but not for the 176Hf /175Lu
ratio, which remains constant downhole. The time-resolved intensity of each
analyte has been offset in the graph for better comparison; therefore the y axis scale is not continuous. Green horizontal lines show the scale for
176Hf (+82), blue horizontal lines show the scale for 175Lu, red
horizontal lines show the scale for 43Ca, and black horizontal lines
show the scale for the 176Hf (+82) /175Lu ratio. Presented data are from an analysis of MKED calcite.
A side effect of the use of large ablation spots is “plasma loading”, for
which the introduction of a large amount of material reduces the ionising
efficiency of the plasma (Kroslakova and Günther, 2007). Plasma loading
was observed in the time-resolved signals, with a reduction in signal
intensity for all isotopes after ∼ 10 to 15 s of
ablation. Following this, the signal stabilised after ∼ 18 s of ablation (Fig. 2). Importantly, this variation in signal
intensity was not observed in the calculated time-resolved isotope ratios
(Fig. 2), which means that identical ratios were calculated whether this
decrease in signal intensity was included in the ratio calculation or not.
Importantly, plasma loading can be affected by sample matrix (Kroslakova and
Günther, 2007), especially for minerals containing easily ionised
elements such as Ca. This necessitates matrix-matched calibration, despite
the observed lack of downhole changes in Lu–Hf ratios (Simpson et al.,
2021a).
The large ablation volume caused accumulation of ablated material in the
tubing and on the interface cones during the first analytical session, which
coincided with a decrease in signal intensity over time. Consequently,
session 1 records slightly more signal drift compared to session 2. However,
there was no measurable corresponding drift observed in the calculated
isotopic ratios, apart from a slight decrease in precision due to the lower
sensitivity toward the end of the run. Therefore, we recommend that cones
are cleaned prior to analysis, and suggest a maximum session duration of
approximately 7 h when using spot diameters of > 200 µm. In addition to this, the accumulated material was sometimes mobilised in
later analyses, potentially contaminating data. This was observed by
increases in Al during the start of ablation that decayed down to background
levels. Importantly, similar Al spikes were not observed during background
measurement, indicating that contamination due to material remobilised during
ablation is likely; hence the additional 20 s of washout did not
fix this. This contamination did not generally produce a measurable effect
on calculated Lu / Hf ratios. However, we stress that this contamination is important to monitor as Hf concentrations are sometimes in the parts per trillion level. As such we recommend close monitoring of signals, particularly Al
concentrations, and the removal of 1–3 s of each analysis after signal
stabilisation if necessary.
Data processing
For both LA-ICP-MS and LA-ICP-MS/MS analysis, a stoichiometric Ca
concentration of 40.04 wt % for calcite was used for internal
normalisation of trace element concentrations. Although the high Ca counts per second for
all analysed samples indicate that they are close to stoichiometric calcite,
there may be slight inaccuracies in calculated element concentrations due to
major element substitutions from Mg, Fe, and Mn that are common in
carbonates. However, element concentrations were largely used as relative
proxies to monitor for inclusions.
Ages and Lu and Hf concentration information for the analysed
samples.
Note: 95 % CI refers to the 95 % confidence interval uncertainty in the calculated age. n refers to the number of analyses used for the age calculation. % Hf corr refers to the average percentage decrease in age due to the common-Hf correction. Hf∗ concentrations have been calculated from 178Hf and assume no radiogenic ingrowth of 176Hf and thus represent the “common” Hf concentration for each sample.
Background subtractions, element concentrations, and ratio calculations were
performed using the LADR software (Norris and Danyushevsky, 2018). Where
178Hf was measured above detection limits (∼ 2 ppt for
178Hf), common-Hf corrections were applied to the data after background
subtractions but prior to normalisation to the standard. In more detail,
the 178Hf counts per second measurement for each sampling cycle of the analysis
period of each laser spot was used to calculate the common-Hf component of
the corresponding 176Hf cycles per second measurement, using the following equation:
176Hfr=176Hfm-176Hf178Hfc×178Hfm,
where 176Hfr is radiogenic 176Hf, 176Hfm is measured 176Hf, 178Hfm is the measured 177Hf, and
176Hf178Hfc is the initial or common-176Hf /178Hf ratio. These corrections were applied using an assumed initial 176Hf /178Hf ratio of 0.192 ± 0.004, which is equivalent to a 176Hf /177Hf ratio of 0.2816 ± 0.006. This
value is based on the Hf evolution of the crust, with an uncertainty that
comfortably covers likely natural variation. The uncertainty in the initial
176Hf /178Hf ratio used for the common-Hf corrections has been
propagated to the final ages, in order to account for any inaccuracies
introduced by the value used. However, as most analyses have < 1 %
common Hf (Table 1), any inaccuracy related to the initial
176Hf /178Hf ratio is negligible compared to the total uncertainty estimates, given Hf isotopes do not vary significantly with time (Vervoort, 2014; Fisher and Vervoort, 2018). Such corrections, however, should be used with caution for samples with higher common Hf, although the dataset presented in this study is not sufficient to determine what an appropriate cutoff should be.
Subsequent to this correction, isotopic ratios were corrected using an
external reference material bracketing approach (commonly used in LA-ICP-MS
geochronology), with primary and secondary reference materials interspaced
with unknowns through each analytical session. The data were normalised to
NIST SRM 610 glass to correct for drift and matrix-independent
fractionation. The Lu–Hf isotopic ratios published in Nebel et al. (2009)
were used for the NIST610 SRM normalisation. Following this,
176Hf /176Lu, 176Lu /177Hf, and 176Lu /176Hf ratios were corrected to MKED calcite. Although the age of MKED calcite is
currently not independently constrained, calcite is interpreted from
textural evidence to have formed with the MKED titanite reference material,
and therefore the titanite thermal ionising mass spectrometry (TIMS) U–Pb age was used (1517.32 ± 0.32 Ma;
Spandler et al., 2016). Further details are outlined in Appendix A. This
correction method is similar to that used by Roberts et al. (2017) for
calcite U–Pb, where the observed analytical offset between the measured and
expected Lu–Hf ratio in the standard is applied (as a percentage correction
factor) to the ratios of the unknowns. This offset is inferred to be due to
a combination of laser-induced (matrix-dependent) elemental fractionation
and plasma loading. The uncorrected ages for MKED calcite as well as for ME 1 across four analytical sessions are constant within uncertainty,
indicating that the age offset is a systematic analytical bias that is applicable
to the calcite samples of unknown age (Fig. A2). Weighted average ages were
calculated using ISOPLOTR (Vermeesch, 2018), using the 176Lu decay
constant determined by Söderlund et al. (2004): 0.00001867 ± 0.00000008 Myr-1.
Correct handling of uncertainties in geochronology is important in order to
draw accurate conclusions about the resulting ages. As per the
recommendations for LA-ICP-MS U–Pb uncertainty propagation in Horstwood et
al. (2016), uncertainties are categorised as random, in which case they are
propagated to individual analyses, or systematic, in which case they are
propagated to the final calculated age. As such, the uncertainties
associated with the measurement of the primary standard (NIST SRM 610) have
been propagated to the uncertainties of individual analyses. The following
systematic uncertainties have been propagated to the final ages: measurement
uncertainty in the secondary standard (MKED C), uncertainty in the titanite
U–Pb age used as the reference age for MKED C, uncertainties associated with
the 176Lu decay constant, and the reference 176Hf /177Hf ratios for NIST SRM 610. Although for completeness it would be good to propagate uncertainty relating to potential differences in 175Lu /176Lu and
177Hf /178Hf between NIST SRM 610 and samples (i.e. natural
variation in these ratios), currently there appear to be no data on this.
These uncertainties are likely to be negligibly small relative to the
overall uncertainty estimates for the analyses. The uncertainty associated
with the reference 176Lu /177Hf, 176Lu /176Hf, and 176Hf /176Lu ratios of NIST SRM 610 is not propagated, as the correction factor associated with NIST610 SRM is cancelled during the
correction to MKED calcite (as the NIST610 SRM correction factor is applied
equally to MKED calcite and the unknowns samples and thus becomes
redundant). Uncertainty relating to long-term reproducibility of the
standards has not been propagated, as the standard data for all sessions
do not show scatter outside of what would be expected from a single
population. More data, however, are required to fully constrain this.
Lu–Hf results
The analysed calcite generally contains < 1 % common Hf, apart from
sample P01, which contains up to 13 % common Hf in individual analyses
(Table 1). Consequently, the common-Hf corrections are small (or effectively
non-existent), and the resultant ages are not significantly affected by the
assumed initial 176Hf /177Hf ratio. Corrected and uncorrected data are included in the supplementary data set by Simpson (2021). The inverse isochron and weighted mean single-spot Lu–Hf ages, reported below, are corrected against MKED calcite for matrix-dependent fractionation and are common-Hf corrected (where relevant) (Fig. 3). For analyses with inclusions, the signals have been cropped to remove inclusions or, in the case of more significant signal disturbances, excluded from age calculations. Inclusions were detected in the following samples: MKED calcite (6), LC1 (1), P01 (19), and FF014 (6). Excluded data points are included in Simpson (2021). Due to the large number of inclusions, P01 was analysed over two sessions. Data are presented as inverse isochrons (Li and Vermeesch, 2021) and as common-Hf-corrected weighted average ages (Fig. 3).
Anchored inverse isochron and weighted average “single-spot” ages
for analysed samples, corrected for matrix-induced fractionation against
MKED1 calcite. Isochrons have been anchored to an initial
177Hf /176Hf ratio of 3.55 ± 0.07. Ellipses represent data points and 2σ uncertainty. Weighted average ages are corrected for common-Hf where relevant (see Table 1 and text). Blue bars represent
2σ uncertainties. Black lines represent weighted average ages, with
grey boxes representing the 95 % confidence interval uncertainty.
Discussion
The Phalaborwa carbonatite sample produced a Hf-corrected weighted average
Lu–Hf age of 2050 ± 30 Ma (Fig. 3), consistent with previous
baddeleyite U–Pb SIMS ages (∼ 2060 Ma; Wu et al., 2011).
Importantly, the consistency between the calcite Lu–Hf age and existing
constraints on carbonatite formation demonstrates that calcite Lu–Hf dating
can produce primary age information for early Paleoproterozoic calcite. This
result also demonstrates that calcite Lu–Hf geochronology is a viable
technique to directly date carbonatite magmatism and associated
mineralisation, even in the case of old calcite samples with only
∼ 0.5 ppm Lu.
The weighted average Lu–Hf ages for samples ME 1 and ME 2 are 1538 ± 9 and 1504 ± 13 Ma respectively (Fig. 3). The ages of these samples are consistent with the paragenetic timing of alteration at Mt Elliott, providing evidence for calcite precipitation during at least two
temporally distinct alteration events. Sample ME 1 is from a coarse
calcite–diopside–scapolite–magnetite vein that does not contain sulfides
(Fig. 1); the age is, therefore, consistent with formation prior to the
major ∼ 1510 Ma Cu–Au mineralisation event (Duncan et al.,
2011; Wang and Williams, 2001). In addition, this age overlaps with a
titanite U–Pb age from the Mt Elliott deposit (1530 ± 11 Ma; Duncan
et al., 2011) and is potentially related to regional Na–Ca alteration
between ca. 1555 and ca. 1521 Ma (Oliver et al., 2004). The 1504 ± 13 Ma age obtained from sample ME 2 that has an ore-stage paragenesis conforms with the 207Pb /206Pb age of cogenetic andradite (1507 ± 35 Ma; Appendix B) and overlaps with the ca. 1510 Ma main mineralisation event
(Wang and Williams, 2001; Duncan et al., 2011). Additionally, data for ME1 were pooled from all four analytical sessions in order to test reproducibility. Similar to the standard (MKED1; Appendix A), ME1 does not
show excess scatter between sessions (Fig. 3).
Sample LC1, from the Lime Creek quarry, Eastern Fold Belt, Mt Isa Inlier
produced an age of 1513 ± 26 Ma, consistent with published titanite
U–Pb ages (1521 ± 5, 1527 ± 7 Ma) from the nearby Knobby
Quarry (Oliver et al., 2004). Additionally, this age is consistent with the
intrusion of the ca. 1530–1500 Ma Williams–Naraku batholiths, which are interpreted as being the source of the fluids from which the calcite
precipitated (Page and Sun, 1998; Oliver et al., 1993). Our results for this
sample further demonstrate that calcite Lu–Hf geochronology is an effective
technique for constraining the age of calcite mineralisation.
Sample OL-MB from Otter Lake produced a Lu–Hf age of 892 ± 12 Ma
(Fig. 3). This age is significantly younger than the apatite solution Lu–Hf
age of 1030 ± 6 Ma (Barfod et al., 2005) and the in situ apatite Lu–Hf age of 1000 ± 11 Ma (Simpson et al., 2021a) but is similar to the apatite Pb–Pb age of 913 ± 7 Ma (Barfod et al., 2005) and the latest stage of extensional activity on the nearby Bancroft Shear Zone (1045–893 Ma, Ar–Ar
phlogopite; Cosca et al., 1995). Given the similarity between the ca. 0.9 Ga
ages, obtained by different methods, it seems likely that the calcite either
grew or records Lu–Hf isotopic resetting during the same event that induced
resetting of the apatite Pb–Pb system. The slight difference between the
calcite Lu–Hf age (894 ± 12 Ma) and apatite Pb–Pb age (913 ± 7 Ma) may be due to analytical (i.e. mixing of age domains in the solution
Pb–Pb age) rather than geological reasons, particularly given an individual
crystal of apatite from the Yates mine produced a U–Pb age range of 920–850 Ma (Xiang et al., 2021). The age difference may also be due to
the underestimation of uncertainties. Large (∼ 3 cm) apatite
crystals such as the one analysed by Barfod et al. (2005) are expected to
have Pb closure temperatures of up to 600 ∘C (Krogstad and Walker,
1994; Barfod et al., 2005), giving a possible upper limit to Lu–Hf closure
in calcite. We note that this is significantly higher than the closure
temperature of Ar–Ar in phlogopite (ca. 400 ∘C), indicating that the Otter Lake area potentially had a different thermal history and/or that
isotopic resetting in the apatite and calcite was aided by late fluid
interactions, as hypothesised by Barfod et al. (2005). As such, further
work is required to constrain the Lu–Hf closure temperature in calcite.
The in situ Lu–Hf age of 1810 ± 18 Ma for the cleavage-hosted calcite vein from the Flin Flon VMS deposit (FF14; Fig. 3), as expected, is younger than the timing of initial mineralisation at the deposit (Rayner, 2010; Koo and Mossman, 1975; Stern et al., 1995). Instead, the age is in excellent
agreement with ca. 1820–1790 Ma regional peak greenschist to amphibolite
grade metamorphism (Schneider et al., 2007), suggesting the calcite
precipitated during metamorphism related to deformation stage D5 or
D6, associated with the final collision between the Flin Flon–Glennie
Complex and the Sask Craton (Lafrance et al., 2016). This regional event
locally reached maximum greenschist-facies metamorphism (Koo and Mossman,
1975), suggesting the calcite grew under low-grade metamorphic conditions.
Sample FF014, therefore, demonstrates that calcite Lu–Hf geochronology has
the potential to date low-grade metamorphism, which has been difficult using
traditional dating methods (e.g. Henrichs et al., 2018).
In summary, we demonstrate that in situ Lu–Hf geochronology can produce geologically
meaningful ages for calcite from a variety of mineralisation styles (e.g.
IOCG, carbonatite, and skarn alteration) as well as greenschist-facies
metamorphism. The technique also has great potential to date a range of
other geological settings and processes (e.g. chemical sedimentation,
carbonation reactions) provided calcite contains sufficient Lu for analysis.
Limitations
The success rate of the in situ Lu–Hf dating approach in calcite is intrinsically related to (1) the concentration of Lu and (2) the ingrowth time for radiogenic Hf (Fig. 4). Generally, the method is more suitable for REE-rich calcite typically observed in mineral deposits and carbonatites, and/or for Precambrian samples. In addition, the currently available mass spectrometers require large laser beam diameters (257 µm) for successful calcite Lu–Hf dating, limiting spatial resolution compared to most laser ablation dating techniques. We note that for high-Lu samples, such as ME 1 (or samples that incorporate common Hf), smaller spot sizes are feasible. Additionally, particularly in hydrothermal settings, calcite often forms large, millimetre- to centimetre-scale crystals, reducing the need for small ablation
volumes. While individual calcite crystals in other settings can sometimes
be < 260 µm, the total amount of calcite is often large enough
that aggregates of pure (or close to pure) calcite can be ablated. Caution
should be used with such analysis, however, as this may affect laser-induced
fractionation, individual crystals may be of different ages, and there may
be micro-inclusions of other minerals.
Lu parts per million vs. 2σ uncertainty for each calcite analysis. The
grey curve shows a function fitted to the data from samples with ages
between 1500 and 1540 Ma (samples ME 1, ME 2, and MKED, with symbols outlined in black). Only data points with similar ages were used to construct this guiding curve as the obtained precision is age-dependent. The Lu–Hf ages for older samples (e.g. P01 and FF014) are more precise relative to younger
samples for a given Lu concentration (assuming no common Hf). Note: MKED is
the calcite Lu–Hf standard used to correct the analysed samples. All data
for MKED are included in Simpson (2021).
Advantages of in situ Lu–Hf dating of calcite compared to other
geochronological methods
The previous dissolution-based Lu–Hf geochronology has produced scattered
isochrons, indicative of isotopic disturbances (Maas et al., 2020). While
individual data points are significantly less precise than dissolution-based
methods, the ability to gain spatially resolved data on a much smaller scale
(> 260 µm) ass well as to obtain a large number of analyses in
a single session can make data interpretation easier (Simpson et al.,
2021a). Importantly, trace element data can be obtained simultaneously to
interrogate each data point for inclusions or age zonation. Furthermore,
calcite Lu–Hf dating can overcome two issues often encountered during U–Pb
dating: (1) in contrast to Pb, calcite does not incorporate significant
concentrations of common Hf, and (2) Lu is comparatively resistant to
thermal diffusion in calcite (Cherniak, 1997), increasing the likelihood of
primary precipitation ages being preserved. However, it should be
acknowledged that fluid mobility and recrystallisation of the calcite may
affect Lu–Hf ages and are difficult to predict. This opens the possibility
that time constraints can be obtained for carbonates from the first
three-quarters of Earth history that are generally difficult to date by
other methods. Importantly, calcite is commonly associated with ore
formation, meaning in situ Lu–Hf dating affords the possibility to directly
constrain the age of mineralising events and the temporal evolution of
mineral deposit systems.
From our work, we suggest samples ME1 and OL-MB calcite could be developed
as primary reference materials due to being (1) common-Hf free, (2) homogenous in age across crystals up ∼ 1 cm in size, and (3) available in large quantities. We aim to characterise such reference
materials and make them available to the wider geochronology community.
Conclusions and future directions
Calcite is among the most common of rock-forming minerals, meaning that
in situ Lu–Hf geochronology of calcite has enormous potential to constrain the age of formation and/or alteration of a range of igneous, sedimentary,
metamorphic, and hydrothermal rock systems, including rock types that are
considered very difficult to date (e.g. marbles). This technique has
particular application to mineral deposits as it allows for the ability to
constrain the age of pre-ore, ore-stage, and post-ore events (e.g. Fig. 3).
Furthermore, given the successful dating of old (∼ 2 Ga)
calcite with < 1 ppm Lu (e.g. sample P01; Table 1), this technique
has the potential to date old calcite from a variety of settings with
relatively low heavy rare earth element (HREE) concentrations. In situ Lu–Hf dating of calcite can be
regarded as a complementary, and in some cases alternative, technique to
carbonate U–Pb dating, where Lu–Hf dating is well suited for older
samples, or to obtain primary precipitation ages for systems affected by Pb
mobility. Coupling in situ Lu–Hf dating with other isotopic systems (U–Th–Pb, C, O, Sr, Nd) may be particularly powerful for constraining the origin, nature, and redox conditions of the fluids or melts from which the calcite precipitated.
MKED calcite sample description
A sample of orange–pink calcite associated with the MKED1 titanite U–Pb
standard (1517.3 ± 0.3 Ma, U–Pb TIMS; Spandler et al., 2016) was
analysed as a matrix-matched secondary standard in order to correct unknown
samples for matrix-related analytical offsets, such as laser-induced
elemental fractionation and plasma loading effects. The calcite was sampled
from the same drill core from which the titanite standard was taken (full
details can be found in Spandler et al., 2016). The sample consists of
massive calcite surrounding large (∼ 8 cm) euhedral titanite
crystals. The titanite is interpreted as having grown in the same fluid as the calcite but just prior to calcite crystallisation. The average age across
all four analytic sessions is 1560 ± 10 Ma (Fig. A1), suggesting that
matrix fractionation during laser ablation produces ages that are
systematically approximately 3 % too old.
Images of MKED calcite. Panel (a) shows calcite chip from where the analysed sample was taken. Panel (b) shows underside of the same chip, where a large titanite crystal has been removed. Red box shows remnant fragments of titanite.
Demonstration of the systematic analytical offset observed for
calcite Lu–Hf ages. Green rectangles are 95 % confidence intervals around
weighted mean ages for each session, with the session number in the bottom right
corner of each rectangle. Horizontal black line shows the weighted average
age of all analytical sessions, with the grey rectangle showing 95 %
confidence interval uncertainty. The combined weighted average age for all
analytical sessions is shown in the top right corner. The expected age is
from Spandler et al. (2016). Weighted mean ages were calculated using
ISOPLOTR (Vermeesch, 2018).
Mt Elliott andradite U–Pb data
Cogenetic andradite was analysed from the Mt Elliott 2 calcite sample (Fig. 1). The sample was analysed using the same laser system as used for Lu–Hf analysis but coupled with an Agilent 7900 quadrupole mass spectrometer. As
the University of Adelaide does not currently possess an andradite U–Pb
standard, U–Pb and Pb–Pb ratios were corrected to NIST610 SRM, using ratios
from Stern and Amelin (2003). A large aspect ratio ablation spot (120 µm in diameter, drilling approximately 30 µm deep) was used to
minimise the effects of downhole fractionation (Sylvester, 2008); however, it
is possible that calculated U–Pb ages are inaccurate due to the lack of
matrix-matched primary standard. As the data appear to be concordant,
however, a weighted average age can be calculated from the
207Pb /206Pb ratios (Fig. B1), which should not be significantly affected by laser-induced matrix fractionation. As such, the calculated age is considered accurate within uncertainty.
U–Pb age of ME 2 andradite. Panel (a) shows the weighted average
207Pb /206Pb age, and panel (b) shows the concordia age on a Tera–Wasserburg concordia plot. For the weighted average, vertical
rectangles are 2σ uncertainties around calculated single-spot ages,
with the black bar showing calculated weighted mean age and the grey
rectangle showing associated 95 % confidence interval uncertainty. For the concordia plot, each ellipse shows the 2σ uncertainty around each
analysis, with the white ellipse representing 95 % confidence interval
uncertainty around the calculated concordia age. Weighted mean age and
concordia age were calculated using ISOPLOTR (Vermeesch, 2018).
Large sample images
The following are larger versions of the sample images from Fig. 1.
Mineral abbreviations are as follows: Cal – calcite; Cpy – chalcopyrite; Py – pyrite; Mag – magnetite; Cu – cubanite; Di – diopside; Scp – scapolite; An – andradite.
Large image of sample P01 from Phalaborwa carbonatite, South
Africa. Large images shows SEM mineral map. Inset shows hand sample photo.
Cpy: chalcopyrite; Cu: cubanite; Cal: calcite; Mag: magnetite.
Large image of sample LC1 from Lime Creek, Mt Isa region,
Australia. Image shows hand sample. Di: diopside; Cal: calcite.
Large image of sample ME1 from Mt Elliott, Mt Isa region,
Australia. Large images shows sample location and inset shows hand sample.
Di: diopside; Scp: scapolite; Cal: calcite; Mag: magnetite.
Large image of sample ME 2 from Mt Elliott, Mt Isa region,
Australia. Large images shows SEM mineral map with black circles showing
laser spot locations. Inset shows hand sample photo. Cpy: chalcopyrite; Py:
pyrite; Cal: calcite; An: andradite.
Large image of sample OL-MB from the Yates mine, Canada. Large
images shows SEM mineral map with black circles showing laser spot
locations. Inset shows hand sample photo. Ap: apatite; Cal: calcite.
Large image of sample FF014 from Flin Flon, Manitoba and
Saskatchewan, Canada. Large images shows SEM mineral map with black circles
showing laser spot locations. Inset shows hand sample photo. Py: pyrite;
Cal: calcite.
Analysis and LA-ICP-MS/MS tuning parameters.
Plasma parameters Radio frequency (RF) power1350 WSample depth4 mmAr carrier gas0.94 L min-1He carrier gas0.38 L min-1N2 addition3.5 mL min-1Lens parameters Extract 1-1.5 VExtract 2-140 VOmega bias-70 VOmega lens8.0 VQ1 entrance-45 VQ1 exit1.0 VCell focus1.0 VCell entrance-120 VCell exit-100 VDeflect10.0 VPlate bias-60 VQ1 parameters Q1 bias-1.0 VQ1 pre-filter bias-10.0 VQ1 post-filter bias-10.0 VCell parameters He flow1.0 mL min-110 % HN3+ 90 % He gas flow3 mL min-1Octopole bias-2.0 VAxial acceleration2.0 VOctopole RF180 VEnergy discrimination-13.0 VQ2 parameters Q2 bias-15 VWait time offset5 msAnalysis parameters Laser wavelength193 nmLaser fluence10 J cm-2Laser spot diameter257 µm (43 µm; NIST610 glass)Laser repetition rate10 HzWashout30 s (post-cleaning pulse) + 20 s (post analysis)Background30 sAnalysis time40 sIsotopes measured/dwell times (ms)27Al (2), 43Ca (2), 47Ti (2), 89Y (2), 90Zr (2), 140Ce (2), 172Yb (10),175Lu (10),175+82Lu (100), 176+82Hf (150), 178+82Hf (150)Data availability
The Lu–Hf and trace element dataset can be found at
10.25909/17425541.v1 (Simpson, 2021).
Author contributions
AS contributed the following: conceptualisation, method development, experimentation, and paper drafting. StG contributed the following: conceptualisation, paper drafting, and primary supervision. MH contributed the following: conceptualisation, paper drafting, and secondary supervision. CS contributed the following: conceptualisation, sampling, and paper drafting. SaG contributed the following: method development, experimentation, and paper drafting. BC contributed the following: experimentation and paper drafting.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
The authors would like to thank the MinEx CRC for funding this research. The
initial method development and apatite dating were supported by the
Australian Research Council. Morgan Blades is thanked for
supplying a sample of Otter Lake calcite. Anthony Milnes from the Tate
Museum at the University of Adelaide is acknowledged for help during
sampling and Aoife McFadden is acknowledged for assistance in operating the
SEM at Adelaide Microscopy. Nick Roberts and Donald Davis are thanked for constructive comments during review.
Financial support
This research has been supported by the Australian Research Council (grant no. DP200101881).
Review statement
This paper was edited by Sandra Kamo and reviewed by Nick Roberts and Donald Davis.
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