High-resolution geochronology is essential for determining the growth rate of volcanoes, which is one of the key factors for establishing the
periodicity of volcanic eruptions. However, there are less high-resolution eruptive histories (
Short-term eruptive histories and compositional variations in lavas and pyroclastic deposits of many arc volcanic fields are well
established. However, high-resolution eruptive histories that extend back
The Milos volcanic field (VF) is a long-lived volcanic complex that has been active for over 3
The Milos VF is part of the South Aegean Volcanic Arc (SAVA), an arc which was formed in the eastern Mediterranean by subduction of the African plate
beneath the Aegean microplate (Fig. 1; Nicholls, 1971; Spakman et al., 1988; Duermeijer et al., 2000; Pe-Piper and Piper, 2007; Rontogianni et al.,
2011). The present-day Benioff zone is located approximately 90
Map of the South Aegean Volcanic Arc (SAVA). Red triangles indicate volcanic fields (VFs): the Susaki, Methana and Milos VFs in the western SAVA, Santorini VF in the centre, and Nisyros VF in the eastern SAVA. Red contour lines show the depth to the Benioff zone (Hayes et al., 2018). The white arrow represents the GPS-determined plate velocity of the Aegean microplate relative to the African plate from Doglioni et al. (2002).
Distribution of the proximal and medial facies of the submarine pumice-cone/crypto-dome volcanoes, submarine, submarine–subaerial and subaerial domes, and rhyolitic complexes (tuff cone and associated lava) of Milos, modified after Fytikas et al. (1986) and Stewart and McPhie (2006). The distal facies of Stewart and McPhie (2006) is not shown.
The Milos VF is exposed on the islands of the Milos archipelago: Milos, Antimilos, Kimolos and Polyegos. The focus of this study is Milos, which has a
surface area of 151
Simplified geological map of Milos with
The geology of Milos can be divided into four main units: (1) metamorphic basement, (2) Neogene sedimentary rocks, (3) volcanic sequences and (4) the alluvial cover. The metamorphic basement crops out at the south-west, south and south-east of Milos (Fig. 3) and is also found as clasts in many volcanic units. The metamorphic rocks include lawsonite-free jadeite eclogite, lawsonite eclogite, glaucophane schist, quartz–muscovite–chlorite and chlorite–amphibole schist (Fytikas et al., 1976, 1986; Grasemann et al., 2018; Kornprobst et al., 1979). The exposed units belong to the Cycladic Blueschist Unit (Lower Cycladic nappe), whereas eclogite pebbles in the phreatic eruption products called “green lahar” by Fytikas (1977) are derived from the Upper Cycladic nappe (Grasemann et al., 2018).
On top of this metamorphic basement, Neogene fossiliferous marine sedimentary rocks were deposited (e.g. Van Hinsbergen et al. 2004). This sedimentary
sequence can be divided into a lower unit A and upper unit B that is unconformably overlain by volcaniclastic sediments (Van Hinsbergen et al.,
2004). Unit A is 80
The Pliocene–recent volcanic sequence of Milos has been subdivided into different units by Angelier et al. (1977) and Fytikas et al. (1986). In addition, Stewart and McPhie (2006) provided a detailed facies analysis of the different volcanic units. The subdivision by Angelier et al. (1977) is not constrained well due to their limited amount of age data. The subdivision of volcanic units by Fytikas et al. (1986) and facies descriptions of Stewart and McPhie (2006) are summarized below. It is important to note that according to Stewart and McPhie (2006), the five volcanic cycles described by Fytikas et al. (1986) are difficult to match with existing age data and the continuous progression in volcanic construction (Fig. 4). For example, the first phase of Fytikas et al. (1986), the Basal Pyroclastic Series, contains the large pumice-cone/crypto-dome volcanoes according to Stewart and McPhie (2006). Two of these pumice-cone/crypto-dome volcanoes are much younger and intercalated between the Complex of Domes and Lava Flows (CDLF) of Fytikas et al. (1986).
Previous proposed stratigraphic frameworks for Milos by Angelier et al. (1977), Fytikas et al. (1986), and Stewart and McPhie (2006). Volcanic unit II of Angelier et al. (1977) contains unit I. Stewart and McPhie (2006) described the volcanic facies of Milos mainly based on the geochronological studies of Angelier et al. (1977) and Fytikas et al. (1986). Abbreviation: SFCPCV – submarine felsic pumice-cone/crypto-dome volcanoes.
The first volcanic unit deposited in the Milos area is the Basal Pyroclastic Series (BPS) (Fytikas et al., 1986) or submarine felsic pumice-cone/crypto-dome volcanoes (Stewart and McPhie, 2006, Figs. 2–4). This unit consists of thickly bedded pumice breccia with a rhyolitic–dacitic composition. These
rhyolites–dacites are aphyric or contain quartz–feldspar
The second volcanic unit was named the Complex of Domes and Lava Flows (Fytikas et al., 1986), and the volcanic facies of this unit are described
as submarine dacitic and andesitic domes by Stewart and McPhie (2006). This phase of effusive submarine volcanism was predominantly
andesitic and dacitic in composition and produced microcrystalline rocks with phenocrysts of pyroxene, amphibole, biotite and plagioclase. The eruption
centres were mainly located along NNE faults and formed up to 300
The third volcanic unit is called the Pyroclastic Series and Lava Domes (PSLD) by Fytikas et al. (1986) and belongs to the submarine-to-subaerial dacitic
and andesitic lava domes of Stewart and McPhie (2006). This highly variable group is dominated by rhyolitic, dacitic and andesitic lavas, domes,
pyroclastic deposits and felsic pumiceous sediments (Stewart and McPhie, 2006). Thickness varies between 50–200
The fourth unit consists of the subaerially constructed rhyolitic complexes of Trachilas and Fyriplaka (CTF) (Fytikas et al., 1986), which Stewart and
McPhie (2006) interpreted as subaerial rhyolitic lava–pumice cones. These two volcanic complexes are built from rhyolitic pumice deposits and lavas
that contain quartz and biotite phenocrysts (10
The fifth volcanic unit comprises deposits from phreatic activity, especially in the northern part of the Zefiria Graben and near Agia Kiriaki (Fig. 2
of Stewart and McPhie, 2006). Many overlapping craters are surrounded by lithic breccias that are composed of variably altered metamorphic basement
clasts and volcanic clasts. This phreatic activity has continued into historic times (Traineau and Dalabakis, 1989). Fytikas et al. (1986) referred to
this unit as “green lahar”, although it is indicated that this deposit is not a lahar but the product of phreatic eruptions in the last 0.2
Previous geochronological work is summarized in Table 1. Angelier et al. (1977) reported six K–Ar ages (0.95–2.50
Published eruption ages of stratigraphic units of the island of Milos.
Angelier et al. (1977) do not provide sample names but only numbers for the sample locations. Here the location is given after “Angelier_” (Angelier et al. 1977, their Fig. 3).Abbreviations are as follows: BPS – Basal Pyroclastic Series; CDLF – Complex of Domes and Lava Flows; PSLD – pyroclastic series and lava domes; CTF – complexes ofTrachilas and Fyriplaka. See more details in Fig. 4.
The previous geochronological work for the Milos VF is mainly based on K–Ar ages. However, K–Ar ages may show undesirable and unresolvable scatter due to
various problems: (1) inaccurate determination of radiogenic argon due to either incorporation of excess argon or incomplete degassing of argon during
the experiments; (2) inclusion of cumulate or wall rock phenocrysts in bulk analyses; (3) disturbance of a variety of geological processes such as
slow cooling or thermal reheating; (4) unrecognized heterogeneities due to separate measurements of potassium and argon content by different methods;
(5) requirement of relatively large quantities (milligrams) of pure sample (e.g. Lee, 2015). In addition to these methodological issues, in the case
of Milos we observe that hydrothermal alteration caused substantial kaolinitization, in particular of the felsic volcanic samples, that most likely has
affected the K–Ar systematics. Some of these issues are also valid for the
Samples were collected from all major volcanic units on Milos island based on the studies of Fytikas et al. (1986), Stewart and McPhie (2006), and our
own observations in the field. Photos of the sample locations and thin sections can be found in Supplement file I. Approximately 2
Heavy-liquid density separation techniques (IJlst, 1973) were used to purify mineral separates (groundmass, biotite, amphibole) required for the
The mineral and groundmass samples were wrapped in either 6 or 9 mm aluminium foil and packed in 20
In total, 24 samples (8 groundmasses, 15 biotites and 2 amphiboles; for sample G15M0026 both biotite and amphibole were analysed) were measured by
either
Samples and standards were heated with a focused laser beam at 8 % power using a 50
A reliable plateau age is defined as experiments with at least three consecutive steps overlapping at 2
Major-element concentrations were measured by X-ray fluorescence spectroscopy (XRF) on a Panalytical AxiosMax. A Panalytical Eagon2 was used to create
40
The minimum and/or maximum eruption volume of each volcano during each eruption period is derived from the ranges of thickness and surface areas that
are reported in Campos Venuti and Rossi (1996) and Stewart and McPhie (2006). We converted these volumes to
dense rock equivalent (DRE) based on the magma type of different deposits. This analysis only includes the onshore deposits and results in a smaller
estimate for larger pyroclastic volumes. The DRE volume is calculated using the equation of Crosweller et al. (2012):
Tephra density is assumed to be 1000
In this section, we present our groundmass, biotite and amphibole
Incremental heating
The age in bold is regarded as the best estimate of the eruptive age. The
The age in bold is regarded as the best estimate of the eruptive age. The
All groundmass samples yielding
Groundmass
Two lava samples, G15M0019 and G15M0020, were collected from Kontaro in north-eastern Milos (Fig. 2). Three replicate incremental heating step
experiments on groundmass from sample G15M0019 (VU108-Z6a_4; VU108-Z6a_5 and VU108-Z6b_1; Fig. 5b) were performed that are not reproducible. Their
plateau ages range from 1.55
Sample G15M0032B (obsidian) was collected from a pumice-cone volcano at Dhemeneghaki (Fig. 2). One incremental heating experiment on this sample
(VU108-Z18, Fig. 5d) yielded a plateau age of 1.825
The results shown in Fig. 5 did not yield weighted mean plateau ages according to standard criteria including
Groundmass
Sample G15M0015 is also a crypto-dome breccia from Profitis Illias (Fig. 2). Two replicate incremental-step heating experiments were performed on the
groundmass of this sample (VU108-Z9a and VU108-Z9b_1, Fig. 6b). The experiment VU108-Z9a groundmass shows a disturbed age spectrum and ages increase
from
Sample G15M0029 is an andesite collected from Korakia in the north-east of Milos (Fig. 2). Two incremental heating experiments (VU108-Z16a and
VU108-Z16b_1, Fig. 6c) were performed on this sample. The two experiments are remarkably similar and show a decreasing age from
Results of nine single-fusion experiments are given in Fig. 7. Nine or ten replicate single-fusion experiments were conducted on 5–10
Biotite
Sample G15M0025 was collected from the Mavros Kavos lava dome located in the west of Milos (Fig. 2). The biotite of this sample (VU108-Z2, Fig. 7b)
shows a weighted mean age of 2.36
Samples G15M0023 and G15M0024 are from the Triades lava dome north-east of Milos (Fig. 2). A mafic enclave G15M0022 (host rock G15M0021) was collected
from a lava near Cape Vani (Fig. 2). The total fusion experiments of the biotites show that their initial
Sample G15M0013 is from the rhyolitic Halepa lava dome in the south of Milos (Fig. 2). The total fusion experiment (VU108-Z13, Fig. 7c) on biotite of
this sample produced a weighted mean age of 1.04
Samples G15M0034 and G15M0035 were collected from a lava dome located south-east of the Trachilas cone (Fig. 2). Nine total fusion experiments
(VU108-Z21, Fig. 7c) were performed on biotite of sample G15M0035 and yielded the age of 0.63
Sample G15M0033 was collected from the Kalamos lava along the coast of the south-west of the Fyriplaka rhyolitic complex (Fig. 2). Biotite of this
sample (VU108-Z19, Fig. 7c) yielded 0.412
Figure 8 displays the biotite
Biotite
Sample G15M0007 was collected from the rhyolitic Trachilas complex in the north of Milos (Fig. 2). Twenty-two total fusion (VU110-Z12, Table 3) and
two incremental heating experiments (VU110-Z12a and 12b, Fig. 8b) were performed on biotite of this sample. The total fusion experiments did not
result in a reliable age due to the large errors of single steps (
Three pumice clasts (G15M0008-9 and G15M0012) were sampled from different layers of the Fyriplaka complex (Fig. 2). The first incremental-step heating
experiment on biotite from sample G15M0009 (VU110-Z23a, Fig. 8c) gave negative ages at the lower-temperature heating steps. Four consecutive higher-temperature heating steps seem to define a plateau of 0.11
For biotite of sample G15M0012, both incremental-step heating experiments are comparable. Both of them yielded plateau ages of
0.05
Biotite of sample G15M0008 did not result in a reliable plateau in the first incremental-step heating experiment (VU110-Z22a, Fig. 8c) but shows a
very disturbed age spectrum. The second experiment (VU110-Z22b) yielded 0.062
There are only two amphibole samples that yielded
Amphibole
Sample G15M0026 is from the same location as sample G15M0025, which gives us the opportunity to compare the biotite age with the amphibole age. One
total fusion experiment on biotite (VU108-Z1b) yielded a weighted mean age of 2.35
Major-element results are given in Table 4. The
Major-element composition of volcanic samples from the Milos volcanic field.
The classification of rock type for each sample is on the basis of field observation and the
Although some samples of Antimilos are tholeiitic, all of the Milos volcanic units belong to the calc-alkaline and medium- to high-K series
(Fig. 10b). A mafic inclusion, sample G15M0022, has high
Eruption age vs.
Figure 11a shows the cumulative volcanic output volume of the Milos VF over time. This diagram shows that the Milos VF can be separated into three
periods: periods I (
More than half of our
The MSWD value, as a measure of the scatter of the individual step ages, is based on the error enveloping around the data point. The decrease in error
will automatically cause an increase in MSWD (e.g. York, 1968; Wendt and Carl, 1991). The MSWD values reported in this study are relatively high. In
part this is caused by the fact that modern multi-collector mass spectrometers used for
Potential drawbacks of the
In this section, our
The
Figure 12 compares previous published K–Ar, U–Pb zircon and fission track ages from the same volcanic units with the new
The obsidian fission track ages (Bigazzi and Radi, 1981; Arias et al., 2006) for the Dhemeneghaki volcano are 0.25
Angelier et al. (1977) reported one dacite sample in the north-west of Milos with an age of 1.71
The amphibole of sample G15M0004 of the Adamas dacitic lava dome, located
The Korakia andesite has an age of 1.59
Nine selected stratigraphic columns covering the
We obtained
A basaltic andesite dyke near Kleftiko on the south-western coast of Milos has a K–Ar age of 3.50
Unfortunately, we were not able to date all key volcanic units of the Milos VF. This was due to three factors: (1) we did not collect samples from all
units; (2) some of the collected samples were not fresh enough after inspection of thin sections; and (3) some of the
The published volcanic units that we include are the Profitis Illias volcano (3.08
Fytikas et al. (1986) also analysed a pumice coming from the Sarakiniko deposits east of Adamas (1.85
Figures 13 and 14 summarize our new
Diagram presenting three periods of different long-term volumetric volcanic output rate of the Milos volcanic field based on the new
Biostratigraphy shows that the youngest layer with dateable fossils (bio-event, the last common occurrence of
The volume estimates of the Milos VF are hampered by limited exposure of several volcanic units and unknown age relationships. Therefore, not all
units can be attributed to a certain volcano. Furthermore, we also do not know how much of the volcanic products was lost through transport by air, sea
currents and erosion. Therefore, the discussion here only provides a first-order estimate of the onshore extruded magma volume. Taking into account all
these limitations, our age data and the volume estimates by Stewart and McPhie (2006) indicate at least three periods of different long-term
volumetric volcanic output rates (
Summary of the eruption ages of the Milos volcanic field.
The lower boundary of Period I is based on our estimate of the oldest volcanic units of Milos at
The change from periods I to II is based on the sharp increase in the
Period III began with a time interval of 0.4
The youngest volcanic activity of Milos (0.11
Figure 11 shows temporal major-element variations during the evolution of the Milos VF. The volcanic units of Period III are dominantly rhyolitic in
composition, whereas during periods I and II the compositions of volcanic units range between basaltic andesite to rhyolite. However, the
It is noteworthy that the value of the
Milos is approximately 15
This study reports 21 new
In combination with previously published age data, geochemistry and facies analysis the following points can be made.
The exact age of the start of volcanism in the Milos VF is still unclear due to the high degree of alteration of the oldest deposits. The best
estimate based on our new Based on the long-term volumetric volcanic output rate, the volcanic history of the Milos VF can be divided into two slow growth periods – periods I ( Periods I and II are characterized by andesitic to rhyolitic lavas and pyroclastic units, whereas those of Period III are dominantly
rhyolitic. The The long-term volumetric volcanic output rate of Milos is 0.2–4.7
All data are included in the tables of this paper and the Supplement.
The supplement related to this article is available online at:
XZ did the sample preparation, laboratory experiment, data evaluation and paper preparation. JW laid out the project. KK helped with the
The authors declare that they have no conflict of interest.
We would like to thank Roel van Elsas for assistance with rock crushing and mineral separation. Kiki Dings helped with the XRF bead preparation and measurements. Lara Borst and Onno Postma assisted with the
This research has been supported by the China Scholarship Council (CSC, grant no. 201506400055), NWO (grant no. 834.09.004), and the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC (grant agreement no. 319209).
This paper was edited by Peter Abbott and reviewed by Jocelyn McPhie and Jörn-Frederik Wotzlaw.