Exhumation of the Western Cyclades:
A Thermochronometric Investigation of Serifos, Aegean Region (Greece)
A thesis presented to
the faculty of
the College of Arts and Sciences of Ohio University
In partial fulfillment
of the requirements for the degree
Master of Science
Heidi A. Vogel
August 2009
© Heidi A. Vogel. All Rights Reserved.
2
This thesis titled
Exhumation of the Western Cyclades:
A Thermochronometric Investigation of Serifos, Aegean Region (Greece)
By
HEIDI A. VOGEL
has been approved for
the Department of Geological Sciences
and the College of Arts and Sciences by
Gregory C. Nadon
Associate Professor of Geological Sciences
Benjamin M. Ogles
Dean, College of Arts and Sciences
3
ABSTRACT
VOGEL, HEIDI A., M.A., August 2009, Geological Sciences
Exhumation of the Western Cyclades: A Thermochronometric Investigation of Serifos,
Aegean Region (Greece) (69 pp.)
Director of Thesis: Gregory C. Nadon
The western Cycladic island of Serifos (Greece) is an exhumed Aegean
metamorphic core complex (MCC) exposing deformed greenschist- to lower
amphibolite-facies metamorphic rocks cross-cut by two distinct magmatic intrusions: an
Eocene S-type mylonitic orthogneiss that syn-kinematically intruded into a mid-crustal
shear zone; and a Miocene undeformed I-type granodiorite that intruded post-
kinematically into shallow crust. Thermochronometric constraints presented here
elucidate the timing of cooling and exhumation of the MCC through mid-crustal levels.
40Ar/39Ar thermochronology performed on phengitic white micas from across the MCC
revealed two distinct cooling age populations: (1) metamorphic rocks of the Cycladic
Blueschist Unit (CBU) yielded 35-28 Ma cooling ages; and (2) rocks from within the
mylonitic orthogneiss yielded 9-8 Ma cooling ages. These results represent episodic
cooling, commencing in the Early Oligocene with extrusion of the CBU-equivalent rocks
and culminating in the Late Miocene with rapid development and exhumation of the
MCC.
Approved:
Gregory C. Nadon
Associate Professor of Geological Sciences 4 ACKNOWLEDGMENTS
David Schneider, my deepest gratitude goes to you. Your name belongs under
"advisor", though technicality prevents it. You provided me with this project and so much more over these past few years: your time and your endless effort; opportunities beyond expectation; challenges that pushed me to my limits and the guidance to see them through. You have given me the invaluable: an unexpected shift in perspective and experiences I will never forget - thank you!
My sincere thanks to Greg Nadon for "adopting" me as his student and all the extras that came with it. I would like to thank Bernhard Grasemann and Christoph
Iglseder, kind and generous colleagues always willing to offer their insight and help.
Lynn and Matt Heizler, thank you for many long hours of lab assistance and making me feel so welcome while away from home in Socorro. Additional thanks to my committee members Doug Green and Keith Milam, and to the faculty of the Ohio University
Geology Department. Funding for this project was provided by the Fonds zur Förderung der wissenschaftlichen Forschung, the University of Ottawa, and an Ohio University
Alumni Graduate Research Grant.
To my family, that unwavering support that requires no prompting- I am so very grateful for you all; I don’t know where I’d be without you. And last, but surely not least, to my children, Audrey and Oscar, for reminding me at the end of every day why it is all worth it. 5
TABLE OF CONTENTS
Page
Abstract...... 3
Acknowledgments...... 4
List of Tables ...... 6
List of Figures...... 7
1. Introduction...... 8
1.1 Extension, Exhumation, and MCC Developme n t ...... 11
2. Tectonic and Geologic Setting...... 14
3. Analytical Methods ...... 21
3.1 Geochemistry...... 24
3.2 40Ar/39Ar Thermochronology...... 26
4. Results...... 28
5. Discussion ...... 49
5.1 40Ar/39Ar Data and Spectra ...... 49
5.2 Thermal & Exhumation History of the Serifos MCC ...... 56
6. Conclusion ...... 59
References...... 62
Appendix: Regression of EMP Data ...... 68 6
LIST OF TABLES
Table Page
1. Electron microprobe analytical data of representative white micas...... 29
2. 40Ar/39Ar analytical data from Serifos...... 36
3. Summary of sample analyses...... 54 7
LIST OF FIGURES
Figure Page
a e s 1. Simplified geodynamics map of the Cyc lad ...... 9
2. Geologic map of Serifos with sample locations...... 16
3. Graphical presentation and classification of white micas separates...... 30
4. Representative backscattered electron images of white mica separates...... 31
5. 40Ar/39Ar age spectra from Serifos ...... 33
6. Map of 40Ar/39Ar age distribution on Serifos...... 55
7. Thermal history diagram of the Serifos metamorphic core complex...... 57
8. Conceptual model for the evolution of the Serifos metamorphic core complex...... 61 8
1. INTRODUCTION
Metamorphic core complexes (MCCs) represent large-scale extension of the
lithosphere and provide unique insight into the mechanisms of crustal flow and the
collapse of orogenic belts. Characterized by the juxtaposition of middle- to lower-crustal
metamorphic and plutonic rocks against low-grade to unmetamorphosed shallow crustal
rocks via a crustal-scale low-angle normal fault (Lister et al., 1984; Baldwin et al., 1993),
MCCs expose once deep-seated rock containing clues to the thermal history of an orogen.
Furthermore, investigations on MCCs yield information about the timing and
mechanisms of exhumation of orogenic crust that has important implications for the
stabilization of mountain belts. The application of thermochronometric techniques, such as the 40Ar/39Ar, U-Th/He, and fission track methods, to extensional terranes provides
temperature-time constraints and cooling age profiles that facilitate the reconstruction of
the thermal histories of MCCs that are critical to the generation of robust models of
exhumation. The following investigation is an application of 40Ar/39Ar thermochronology
on the island of Serifos, Aegean region, that provides key thermal constraints for
resolving the still largely unknown geodynamic history of the western Cycladic
extensional terrane.
The Aegean region of the eastern Mediterranean is well recognized as an area
undergoing active extensional tectonics since the Oligocene (e.g., Jolivet et al., 2004;
Dilek, 2006; Kokkalas et al., 2006). The Cyclades are an archipelago south of the Greek
mainland (Figure 1) that have been a focus of study for Aegean tectonics through which
two significant Cenozoic geodynamic events have been identified (Lister et al., 1984). 9
Evvia Lavrio Andros
Kea Tinos
Amorgos
Ios Milos Sikinos Hellenic slab retreat
Thira
Figure 1. Simplified geodynamics map of the Cyclades (after Iglseder et al., 2009). Pink represents portions of the central/eastern Cylcades and Evvia showing north-directed kinematics. Green represents the western Cyclades islands (including Serios, circled) and Lavrio showing south-directed kinematics. Detachments and associated shear sense shown in red. 10
The first event (M1) is the high-pressure metamorphic episode that generated the
Cycladic blueschist unit (CBU) as a result of subduction and burial of the African-
Arabian plate beneath Europe during Alpine collision (Altherr et al., 1982; Jolivet et al.,
2003). The CBU metamorphic assemblage is strongly overprinted by a second
metamorphic event (M2), which is an Early Miocene tectonothermal episode
characterized by greenschist to lower-amphibolite facies metamorphism and intrusion of primarily I-type granitoid plutons (Altherr et al., 1982; Avigad et al., 1992). This most recent episode was the result of widespread deformation related to syn-convergent, back- arc extension during rollback of the subducting plate (Royden, 1993; Ring et al., 2003).
Current models of the Cenozoic geodynamic history of the Aegean are based on a compilation of geological and geophysical data derived predominantly from the eastern
Cyclades (e.g. Naxos). However, a comprehensive understanding of the tectonometamorphic history of the Hellenic arc is limited by the lack of extensive geological, geophysical, and geochronological constraints from the western Cyclades,
which prevents discrimination among current geodynamic models for the region. Serifos
is a recently recognized metamorphic core complex (Iglseder, 2005; Grasemann and
Petrakakis, 2007) that developed as a result of active north-south extension of the Aegean
lithosphere. Serifos, and its neighbors of Kythnos and Kea to the north, are unique in that,
unlike parts of the eastern Cyclades that record N-directed extension, these islands have
been purportedly exhumed by S-directed extension (Grasemann and Petrakakis, 2007;
Lenauer et al., 2008; Rice et al., 2008). The objective of this project was to constrain the
timing of thermal events and exhumation of Serifos, an island in the western Cyclades
(Figure 1), through the application of 40Ar/39Ar thermochronology. The results presented 11 here provide evidence of possible multi-phase exhumation within a distinct kinematic domain that strongly impacts the current understanding of the geotectonic evolution of the Aegean region.
1.1 Extension, Exhumation, and MCC Development
Metamorphic core complexes form as a result of large-scale continental extension, where the brittle upper crust fractures and extends to expose, through tectonic exhumation, the more ductile middle and lower crust beneath (Davis and Coney, 1979). It is therefore fundamental to the study of an MCC to recognize and understand the relationship between the processes of extension and exhumation, and how they relate to the development of these structures. Extension predominantly occurs within the continental lithosphere of orogens where the vertical strength of the crust, especially where overthickened, is weakened due to complex lithological, structural, and thermal heterogeneities (Dewey, 1988; Whitney et al., 2004). In such a setting, extension drives thinning of the weakened lithosphere at a range of depths, anywhere from shallow surface to mid-crustal levels, and thus creates a high potential for exhumation of deep crustal material and MCC development.
In general there are three main processes by which exhumation occurs: (1) erosion, (2) ductile thinning, and (3) tectonic unroofing through normal faulting. Erosion is the least effective means of exhuming rock, for it does so gradually and over large surface areas. Typically erosion requires an additional mechanism to drive the process of widespread midcrustal exhumation, except in extreme cases (Zeitler et al., 2001), but 12 does make a significant contribution in other examples (Whitney et al., 2004). When intense, erosion has the potential to localize strain and, in doing so, effectively increase rates of unroofing (Dewey, 1988; Zeitler et al., 2001).
The processes of ductile thinning and midcrustal flow, like erosion, help to thin the crust but must also be coeval with an additional exhumation process (tectonic or erosional unroofing) to result in unroofing of deeper material (Dewey, 1988; Ring et al.
1999). This process can contribute significantly to the total exhumation possible by allowing the ductile ascent of high pressure and ultra-high pressure rocks through a thinning overlying crust, as has been modeled for the Himalaya (e.g., Beaumont et al.,
2001).
Tectonic unroofing, by far the most effective exhumation process, is usually manifested as crustal-scale structures with moderate to low-angle extensional shear sense.
In regions of tectonic plate collision, the crust commonly becomes thickened to the point of gravitational instability and as a result it collapses, with the stress being accommodated by the formation of low-angle detachment faults. Collapse of the lithosphere has been widely recognized as evidence that extension is a fundamental process in the tectonic progression of collision belts (e.g., Getty and Gromet, 1992; Ring et al., 1999). Syn-convergent crustal extension can also occur in settings such as the
Aegean region, where slab rollback of a subducting plate weakens the overlying crust to the point of failure. In either scenario, as extension thins the crust it leads to decompression and consequent melting of the lithosphere (Whitney et al., 2004). As a result, migmatites and granitic plutons form in the footwall of the extending shallow crust and provide further positive feedback for extension by localizing strain along a major 13
shear zone. The consequent MCC structure records older (initial) cooling ages in the
hanging wall and younger cooling ages in the footwall as exhumation proceeds. These distinct cooling ages, which can be measured using various thermochronometers such as fission tracks, U-Th/He, and 40Ar/39Ar, are expressed as a sharp break in cooling ages
across the shear zone and constrain the timing of a tectonic unroofing event. 14
2. TECTONIC AND GEOLOGIC SETTING
The western Cycladic archipelago is located in the Aegean Sea, a region noted for
extensional tectonics since the Oligocene (e.g., Lister et al., 1984; Jolivet and Patriat,
1999). Subduction and rollback of the African-Arabian plate and the westward escape of
the Anatolian microplate resulted in north-south to northeast-southwest directed
extension of the Aegean lithosphere (Figure 1). Regional extension produced a corridor
of MCCs north of and parallel to the Hellenic subduction zone that led to significant
exhumation of high-pressure and midcrustal rocks (e.g., the CBU). Within the eastern
Cyclades, exhumation and MCC development took place via localized, north-directed shear along low-angle normal detachment surfaces (e.g., Jolivet et al., 1996; Forster and
Lister, 1999). Within the western Cyclades only the island of Serifos exhibits (most of)
the structural and petrological characteristics typical of an Aegean metamorphic core
complex (Lister et al., 1984) including the presence of low-angle normal (detachment) faults separating low-grade metamorphic rocks in the hanging wall from higher grade metamorphic rocks in the footwall and showing ductile-to-brittle progressive
deformation. Although geologically similar to the eastern Cycladic MCCs, the south-
directed kinematics of the Serifos core complex, as well as neighboring Kea and
Kythnos, are in marked contrast to the typical north-directed shear sense and represent a
newly identified S-directed extensional structural domain in the western Cyclades
(Iglseder, 2005; Grasemann and Petrakakis, 2007). The contrast between kinematics
along the detachment faults preserved throughout the Cyclades may comprise multiple
generations of an anastomosing series of detachment systems that represent distinct 15
exhumation events (Forster and Lister, 1999; Jolivet et al., 2004). Determining the
regional-scale patterns in the thermal history of the Cyclades associated with regional
extension requires an understanding of the time of formation of these detachments.
Petrakakis et al. (2007) over the duration of Project ACCEL (Aegean Core
Complexes along an Extended Lithosphere) produced a geologic map of Serifos (Figure
2). The primary lithostratigraphic unit includes bedrock lithologies of the northern and western portion of Serifos, consisting of basic calc-silicates, graphitic-, ankeritized-, and talc-schists, amphibolites, ortho / paragneisses, and coarse- to ultra fine-grained
(mylonitic) marbles metamorphosed to greenschist- to lower amphibolite-facies conditions (probably during M2). In the west to southwest, calcitic and dolomitic marbles are interbedded and form detachment horizons with structures showing SSW-directed kinematics. Ankeritized dolomite and limonitic iron ore deposits in this unit indicate hydrothermal activity and skarn formation under P-T conditions of 600°C and 2 kbar
(Salamink, 1980). Several meters thickness of amphibolites interbedded with gneissic lamellae form part of a contact metamorphic aureole around the intrusive body. Though compositionally the rocks are relatively consistent, metamorphic grade increases toward the intrusive contact from actinolite-bearing schists to garnet-bearing gneisses (Iglseder,
2005). Poikiloblastic relics of glaucophane preserved within this unit indicate an earlier high-pressure metamorphic event coincident with the CBU formation (M1) on nearby islands. In the south of the island, EW to ENE-WSW striking fold axes are overprinted by SSW-directed kinematic structures. 16
272000 276000 280000
SERF306 N 000214 000214 W E SERF415 SERF312 SERF125 SERF341 S 07SE28 SERF39
07SE35 SERF77
07SE24
SERF358 006 006 10 10 40 40 07SE21
SERF571
SERF567
0002 SERF564 SERF53 002 11 11 1 SERF428 1 4 40
SERF107
Meters
272000 276000 280000
Figure 2. Simplified geologic map of Serifos (after Petrakakis et al., 2007). Stars denote sample locations for white mica 40Ar/39Ar analyses. 17
The bedrock is intruded by a Late Miocene I-type granodiorite pluton (Altherr et
al., 1982; Henjes-Kunst et al., 1988; Iglseder et al., 2009) that dominates the central and
southeastern part of the island, as well as numerous dikes of dacitic composition that cut
both the pluton and surrounding bedrock. The bedrock also contains crosscutting veins of
high-temperature Ca-Fe-Mg skarns that formed as a result of advancing metasomatic
fluid from the granodiorite intrusion (Grasemann and Petrakakis, 2007). The host-rock
immediately adjacent to the intrusion is primarily mylonitic orthogneiss that is highly deformed and intercalated with amphibolites and hydrothermally altered calcite/dolomite marbles. This mylonitized assemblage is thought to represent an earlier S-type granite, distinct from the Miocene granodiorite, which syn-kinematically intruded a mid-crustal ductile shear zone related to Cenozoic extension (Grasemann and Petrakakis, 2007;
Vogel et al., 2007, 2009; Senkowski et al., 2009).
The detailed structural dataset recorded by Project ACCEL for the western
Cyclades revealed the presence of crustal-scale, low-angle normal faults on Serifos.
These features appear in the SW, SE, and NE parts of the island as brittle/ductile to brittle shear zones showing consistent SSW-directed shear. In the SW and SE, ductily deformed
(ultra)mylonites are overlain by thick cataclasites, all bearing pronounced SSW-directed lineation. The cataclasites are cut by a well-defined, SSW-dipping, brittle fault that also shows SSW-directed shear sense. In the NE, the brittle/ductile shear zone contains a N- dipping cataclastic fault plane that also displays top-to-the-south kinematics.
NNE-SSW extension is accompanied by E-W shortening and folding of the mylonites and cataclasites with fold axes parallel to the NNE-SSW stretching lineations.
Evidence of shortening is also recorded for other Cyclades islands (Jolivet et al., 2004), 18
indicating a component of regional E-W compression. The later granodiorite intrusion,
essentially undeformed at lower levels, shows SSW-directed stretching lineations at
structurally higher levels. These structures suggest that the pluton was emplaced into
shallow crust where it was deformed by continued extension during or shortly after
intrusion (Grasemann and Petrakakis, 2007).
Extension is further documented by the intrusion of numerous dikes that crosscut
the detachment and follow a conjugate set of high-angle, brittle normal faults that strike
WNW-ESE. The overprinting of ductile deformation with brittle structures also indicates
continued extension as the detachment was exhumed into shallower crust.
The fabric in the rocks on Serifos coincides with the dip of the detachment zones, gradually changing orientation from N-dipping in the northern parts of the island to S- dipping in the southern parts, and demonstrates an overall domal structure. Similar doming is documented for other Aegean core complexes (e.g., on the islands of Paros,
Naxos, Mykonos; Jolivet et al., 2004), and is attributed to the combined effect of unloading and intrusion into the extending crust as strain was localized along a detachment plane (Block and Royden, 1990). Jolivet et al. (2004) describe the formation of two types of metamorphic domes in the Aegean produced by post-orogenic extension.
The b-type domes (e.g., Andros, Tinos) are elongated perpendicular to the primary extension direction and mostly associated with greenschist-facies recrystallization. The a- type domes (e.g., Paros, Naxos, Mykonos) are elongate parallel to the principal direction of extension and are primarily associated with amphibolite-facies recrystallization as well as magmatism. Geochronological and structural data indicate that exhumation through the brittle-ductile transition occurred at ~19 Ma for b-type domes (Jolivet et al., 2004), 19
but later for a-type domes at ~12-10 Ma (Altherr et al., 1982; Bröcker and Franz, 1998;
Jolivet et al., 2004). Jolivet et al. (2004) suggest that the contrast in structure and timing
of exhumation between these two dome types is related to differences in behavior of
shallower crust (b-type domes) and deeper crust (a-type domes) as boundary conditions
in the Aegean evolved toward an increasingly constrictional regime. Recent studies of the
western Cyclades by Project ACCEL suggest that Serifos is an a-type dome while Kea and Kythnos are b-type domes. Thermochronologic data presented here for Serifos, and in recent work on Kea and Kythnos (Schneider et al., 2008), indicate timing of formation
of the western Cycladic metamorphic domes coeval with exhumation of the respective a-
and b-type domes of other Cycladic islands.
Lithological and structural observations suggest that Serifos represents a previously unrecognized MCC that was formed and subsequently exhumed through
Cenozoic extension of the Aegean region. Previously published geochronological studies focused primarily on the I-type granodiorite pluton that dominates the island geology.
Altherr et al. (1982) and Henjes-Kunst et al. (1988) constrained the timing of intrusion of this granodiorite to between 9.5 to 8.0 Ma based on hornblende K-Ar and apatite fission track dating. Single grain zircon TIMS U-Pb ages indicate crystallization of the granodiorite body and associated dyke generations between 11.6 and 9.5 Ma (Iglseder et al., 2009). Cooling between 8.6 and 6 Ma at rates of > 50ºC/m.y. was also determined for the pluton through apatite fission track thermochronometry and model calculations from the granodiorite (Hejl et al., 2002). In addition, zircon and apatite fission track ages of 11 and 10 Ma, respectively (Brichau et al., 2008), and zircon and apatite (U-Th)/He ages of
~7-5 Ma (Stöckli et al., 2009) further constrain rapid, low-temperature cooling of the 20 granodiorite. Given the observed geological complexity of Serifos, particularly the purported multi-stage plutonism and extensional structures, more detailed thermochronometric work is required to constrain a complete tectonometamorphic history of events, and is thus the focus of this project. The techniques used to do this build upon the framework of geological data previously acquired through detailed study of the eastern Cyclades and preliminary studies of selected islands from the eastern
Cyclades. 21
3. ANALYTICAL METHODS
A total of eighteen rock samples were collected from Serifos for 40Ar/39Ar and
elemental analysis in 2007, supplemented by samples collected by C. Iglseder (University
of Vienna) and D. Schneider (University of Ottawa) in 2006. Sample locations (Figure 2)
were chosen to provide i) timing constraints from both hangingwall and footwall domains of low-angle normal faults, and ii) a geographically distributed set of data covering all the primary rock types, excluding the previously investigated granodiorite intrusion that occupies the SE portion of the island (e.g., Altherr et al., 1982; Henjes-Kunst et al., 1988;
Hejl et al., 2002; Brichau et al., 2008; Iglseder et al., 2009). Rocks were chosen based on unaltered appearance, quantity and quality of visible mica, and distance from existing intrusions to avoid samples in which the radiometric clock may have been reset due to localized heating events. White mica was chosen for 40Ar/39Ar analysis because the
mineral is abundant on the island, is well-behaved in laboratory heating experiments, and
has an accepted isotopic closure temperature of 350°C (McDougall and Harrison 1999).
This closure temperature is particularly useful since it represents the temperature
typically associated with the brittle-ductile transition zone in quartzofeldspathic crust.
Therefore, the cooling history based on white mica will provide information about the passage of rock through the brittle-ductile transition zone and the critical thermal window of MCC development.
White mica size fractions of >150 m were obtained from each sample using standard crushing, magnetic separation, and careful handpicking to ensure >99% purity.
The mica separates were analyzed using 40Ar/39Ar thermochronometry along with 22
electron microprobe (EMP) techniques to determine elemental composition of the
minerals. These data then were combined with existing structural and petrologic results
(Salemink, 1980; Iglseder, 2005) in order to qualitatively assess pressure-temperature
conditions across the island.
The 40Ar/39Ar thermochronologic technique utilizes the decay of the naturally
occurring radioactive isotope 40K into 40Ar to obtain quantitative data on the timing of
cooling of a rock through a given temperature, i.e. a cooling age. According to diffusion
theory, as 40K decays the daughter product (40Ar) either accumulates or diffuses
depending on the diffusivity and temperature of the mineral in which it is present
(McDougall and Harrison, 1999). The maximum temperature at which a mineral can
retain a specific daughter product within its structure is defined as the 'closure
temperature' for that isotope (Dodson, 1973). At higher temperatures the crystal lattice
becomes weak enough to allow the escape of 40Ar by volume diffusion. As the mineral cools, the rate of diffusion of trapped Ar decreases exponentially until the mineral closure temperature is reached. All subsequently produced daughter product is retained within the crystal lattice. The combination of the parent/daughter and decay rate of the isotope analyzed allows calculation of the cooling age (McDougall and Harrison, 1999). If temperatures again rise above the closure temperature due a metamorphic event or if the mineral recrystallizes during deformation, the result is a partial to total loss of the accumulated Ar that essentially resets the radiometric clock. The 'reset' age will record a younger thermal event and can be identified through the shape of the age spectra in combination with petrographic and geochemical analyses (McDougall and Harrison,
1999). 23
A significant advantage of the 40Ar/39Ar method compared to the K/Ar or Rb/Sr systems is that the isotope ratios are measured in a single analysis, thus eliminating the problem of sample heterogeneity often encountered when studying regions with complex thermal histories such as the Cyclades (Wijbrans and McDougall, 1986; Foster et al.,
1990; McDougall and Harrison, 1999). An even greater advantage is the ability to extract argon using the step-heating process, by which a series of apparent ages can be determined for a given sample. The map pattern of ages and often the shape of the age spectrum can be related to the mechanism of resetting. For example, rapid normal faulting often results in a distinct age gradient with younger cooling ages in the footwall, whereas erosional exhumation should show a gradual and slow change in cooling ages
(Ring et al., 1999). A system that was thermally undisturbed since mineral crystallization or cooling produces a flat or plateau age spectrum (McDougall and Harrison, 1999). In addition to apparent age, the ratio of K/Ca, determined from Ca-derived 37Ar and K- derived 39Ar, is measured for each heating step in a corresponding plot and provides information about the mineral phases being degassed at experimental temperatures. The flattest parts of the age spectra generally correspond to relatively constant K/Ca values, whereas the younger apparent ages yield relatively low K/Ca values (Marcoline et al.,
1999). For rocks containing a range of potassium-bearing minerals, this method provides the opportunity to construct relatively complete T-t diagrams illustrating the timing and rates of exhumation.
24
3.1 Geochemistry
The nature of diffusion within a mineral structure determines that retention of
radiogenic argon will vary depending on the mineral phase (Dodson, 1973; Dahl, 1996).
Similarly, the argon release pattern shown by 40Ar/39Ar age spectra varies for different
mineral phases and tends to increase in complexity with mixed phases (McDougall and
Harrison, 1999). Therefore, accurate interpretation of 40Ar/39Ar data requires careful
characterization of the chemical composition of the mineral being analyzed. Detailed
elemental mapping via electron microprobe (EMP) analysis can fulfill this requirement.
Electron microprobe analysis was performed using a Cameca SX-100 operated by
the Bureau of Geology and Mineral Resources, New Mexico Institute of Mining &
Technology (Socorro, NM). The probe is equipped with high-speed, backscatter electron detectors and 3 wavelength dispersive (WD) spectrometers, each outfitted with multiple analyzing crystals, for precise quantitative analysis of a wide range of elemental abundances. Mica separates were prepared in 9-hole grain mounts, filled and covered with epoxy-resin, well polished with a series of coarse to fine abrasives, and treated with
a conductive carbon coating to prevent charging during electron bombardment. During
analysis, heating of a tungsten filament held at negative potential produced and accelerated electrons toward the sample. A series of magnetic electron lenses focused and concentrated a 15kV, 10nA electron beam to a minimum resolution of 1 micron. This
bombardment produced backscattered electrons (BSE), secondary electrons, X-rays, and
cathodoluminescence; the analyses conducted for this study utilized the backscattered 25
electrons and X-rays to acquire both qualitative and quantitative information about
sample composition.
Backscattered electrons (incident electrons deflected back from the surface with
high energy) were used for imaging of samples. The brightness of the image produced is
proportional to the amount of deflected electrons, which is a function of the mean atomic
number (Z) of the mineral. This allows visual discrimination of compositionally distinct
mineral phases within the separates. Interactive BSE images were used during EMP
analysis to assess the purity and homogeneity of sample separates and to assist the
targeting of potassium-rich phases for quantitative analysis of composition. Comparison
of the X-rays generated from the sample during electron bombardment and detected by
the 3 WD spectrometers with those of known standards allowed precise characterization of the mineral phase from each sample.
Using elemental compositions established via EMP analysis, the precise species of K-bearing mica was determined for the sample separates according to the mica classification system of Tischendorf et al. (1997; 2004). The method is based on variation
in the occupancy of octahedrally coordinated cations, specifically elemental differences
VI shown by the expressions (Mg – Li)[=mgli] and (Fetot + Mn + Ti – Al)[=feal], plotted on
a single diagram that provides a straightforward descriptive subdivision of all common
true K-bearing micas. Prior to implementation, the classification scheme required
conversion of the weight % oxide values of the raw EMP data to the proper a.p.f.u.
(atoms per formula unit) values for each elemental parameter. Details of the procedure
and calculations involved in EMP data regression are provided in the Appendix. 26
3.2 40Ar/39Ar Thermochonology
After separation from the whole rock and preparation for 40Ar/39Ar analysis, white mica separates were loaded into machined Al discs and irradiated with flux monitor Fish
Canyon Tuff sanidine (27.84 Ma; Deino and Potts, 1990) for 100 hours in the L-67 position at the 2 MW Ford Reactor at the University of Michigan. Isotopic analyses were conducted at NMT Geochronology Laboratory using a MAP 215-50 mass spectrometer on line with automated all-metal extraction system. The flux monitor crystals (in a copper planchet) were fused with a 10W Synrad CO2 continuous laser within an ultrahigh vacuum argon extraction system. Evolved gases were purified using a SAES GP-50 getter operated at ~450 °C. J-factors were determined to a precision of 0.1% (2 sigma) by analyzing a minimum of four single crystal aliquots from each radial position around the irradiation sample trays. The unknown minerals were step-heated in a double-vacuum Mo resistance furnace. Argon isotopic compositions were determined using an electron multiplier with an overall sensitivity of ~2.66 x 10-16 moles/pA.
Routine measurement of the extraction system, mass spectrometer blanks, and backgrounds took place throughout the course of the analyses. Errors are reported at the
2-sigma confidence level. The results are presented as spectra where the width of each bar (thermal increment) represents the proportion of evolved gas, and the height represents the uncertainty associated with the apparent age. The integrated age is an average cooling age for the sample calculated by summing the isotopic measurements of all steps with an uncertainty calculated by quadratically combining errors of isotopic measurements of all steps. Plateau ages are defined for this investigation as the portion of 27 an age spectrum composed of contiguous increments representing >70% of gas released which result in concordant ages (Mahon, 1996). A preferred age, on the other hand, is calculated as the weighted mean of a selection of mostly contiguous increments which represent >50% of 39Ar gas released and result in concordant ages. The calculated plateau age uncertainties are relatively small because analytical precision in the age of each heating step is high. A mean square weighted deviation (MSWD) below 10 for 40Ar/39Ar data is ideal, whereas higher values indicate scatter in the dataset. Nevertheless, the
MSWD measures the precision of the dataset rather than the accuracy of the resulting age. 28
4. RESULTS
Electron microprobe analysis of white mica separates from 17 of the 18 Serifos
gneiss and schist samples yielded consistent elemental geochemical results (Table 1); this
excludes SERF 428, which was lost during EMP mount production and therefore has
unknown geochemistry. SiO2 content of the white micas is calculated to be between
46.68 and 54.26 wt% (3.2-3.6 a.p.f.u.) with an average value of 52.28 wt%. These values
are high, toward the upper limit of the muscovite compositional range, and suggest a
phengitic component to the white micas (Reider et al., 1998). Other elemental
abundances, including Na2O between 0.01 and 0.67 wt% (0.001-0.09 a.p.f.u.) with an
average value of 0.21 wt%; Al2O3 ranging from 24.04 to 31.13 wt% (1.9-2.5 a.p.f.u.)
with an average value of 26.15 wt%; K2O ranging between 9.4 and 11.4 wt % (0.80-0.96
a.p.f.u.); and Mg/(Mg + Fe) values between 0.23 and 0.80 a.p.f.u., are also consistent
with a muscovitic/phengitic composition. According to the mica classification scheme
presented by Tischendorf et al. (1997; 2004), which was based on cation occupancy of
the octahedral sheet as expressed by the parameters (Mg - Li) [=mgli] and (FeTOT + Mn +
Ti - VIAl) [=feal], all white micas analyzed in the 40Ar/39Ar portion of the investigation
are of a similar phengite composition. Figure 3 is a plot of elemental cation abundances
from representative point EMP analyses of each sample; all samples cluster near the
compositional phengite/muscovite boundary with mgli values between 0.080 and 0.404
and feal values between -1.47 and -1.15. In addition, SEM-BSE imaging of the separates
revealed a small amount of variation in compositional purity and homogeneity of the mica among some of the samples. Visible intergrowth of micron-scale chlorite and white 29 Table 1. Electron microprobe analyical data for representative white micas from Serifos. Sample No. Serf 39 Serf 53 Serf 77 Serf 107 Serf 125 Serf 306 Serf 312 Serf 341 Serf 358 wt% oxide
SiO2 52.23 53.46 52.16 52.29 53.36 54.26 52.53 52.13 51.67 MgO 3.77 3.25 3.82 2.80 3.28 3.78 4.36 3.67 3.80
Na2O 0.11 0.26 0.08 0.28 0.11 0.01 0.17 0.09 0.46
Al2O3 24.46 26.82 24.95 27.58 24.79 23.34 25.44 25.95 27.02
K2O 10.97 9.90 11.16 9.56 11.43 9.44 11.22 11.15 9.82 CaO 0.00 0.01 0.00 0.03 0.07 0.00 0.00 0.02 0.00
TiO 2 0.14 0.05 0.13 0.21 0.10 0.48 0.11 0.19 0.04 FeO 3.74 2.49 3.49 3.18 4.85 4.46 1.91 2.71 2.29 MnO 0.03 0.02 0.06 0.05 0.02 0.02 0.01 0.02 0.01
Cr2O3 0.08 0.03 0.00 0.01 0.00 0.01 0.03 0.00 0.01
Li2O 0.05 0.18 0.03 0.07 0.08 0.03 0.10 0.02 0.09 F 0.00 0.55 0.14 0.28 0.29 0.16 0.36 0.10 0.32 Total 95.59 96.28 95.85 95.99 98.00 95.80 95.76 95.95 95.10 No. of atoms per formula unit Si 3.503 3.493 3.487 3.443 3.509 3.596 3.484 3.466 3.432 Mg 0.377 0.317 0.381 0.275 0.322 0.373 0.431 0.364 0.376 Na 0.014 0.032 0.010 0.036 0.014 0.001 0.021 0.011 0.059 Al 1.934 2.066 1.966 2.140 1.921 1.823 1.988 2.033 2.115 K 0.939 0.825 0.952 0.803 0.958 0.798 0.949 0.946 0.832 Ca 0.000 0.001 0.000 0.002 0.005 0.000 0.000 0.001 0.000 Ti 0.007 0.002 0.007 0.010 0.005 0.024 0.005 0.010 0.002 Fe 0.210 0.136 0.195 0.175 0.267 0.247 0.106 0.151 0.127 Mn 0.002 0.001 0.003 0.003 0.001 0.001 0.000 0.001 0.001 Cr3+ 0.004 0.002 0.000 0.000 0.000 0.000 0.002 0.000 0.000 Li 0.015 0.047 0.008 0.019 0.020 0.009 0.027 0.005 0.023
Sample No. Serf 415 Serf 564 Serf 567 Serf 571 07SE 21 07SE 24 07SE 28 07SE 35 wt% oxide
SiO2 52.41 53.90 53.30 51.28 53.06 50.03 46.68 54.08 MgO 3.59 4.12 3.63 2.71 3.87 2.72 0.81 3.71
Na2O 0.05 0.01 0.43 0.50 0.06 0.32 0.67 0.04
Al2O3 25.05 24.96 27.68 28.15 25.36 27.84 31.13 24.04
K2O 10.71 11.20 9.63 10.08 10.75 10.86 10.22 9.87 CaO 0.00 0.00 0.01 0.01 0.05 0.00 0.00 0.01
TiO 2 0.16 0.15 0.10 0.06 0.10 0.22 0.27 0.23 FeO 3.74 2.65 1.87 3.10 3.17 3.62 4.80 3.81 MnO 0.02 0.00 0.02 0.05 0.07 0.02 0.01 0.00
Cr2O3 0.02 0.02 0.00 0.02 0.00 0.00 0.02 0.04
Li2O 0.02 0.14 0.07 0.34 0.00 0.01 0.01 0.04 F 0.09 0.47 0.29 0.89 0.00 0.08 0.05 0.18 Total 95.74 97.01 96.67 95.96 96.48 95.63 94.60 95.83 No. of atoms per formula unit Si 3.499 3.530 3.461 3.383 3.505 3.356 3.185 3.580 Mg 0.358 0.403 0.351 0.267 0.381 0.272 0.082 0.366 Na 0.006 0.001 0.054 0.064 0.008 0.042 0.088 0.005 Al 1.971 1.927 2.118 2.189 1.974 2.201 2.503 1.875 K 0.912 0.936 0.798 0.849 0.906 0.930 0.890 0.834 Ca 0.000 0.000 0.000 0.001 0.003 0.000 0.000 0.001 Ti 0.008 0.007 0.005 0.003 0.005 0.011 0.014 0.011 Fe 0.209 0.145 0.102 0.171 0.175 0.203 0.274 0.211 Mn 0.001 0.000 0.001 0.003 0.004 0.001 0.000 0.000 Cr3+ 0.001 0.001 0.000 0.001 0.000 0.000 0.001 0.002 Li 0.004 0.038 0.020 0.090 0.000 0.004 0.002 0.011 Notes: Wt% oxide values based on 22 oxygen. 1.326 Li 2O calculated based on positive correlation between F and Li content: Li2O = 0.3935 F (Tischendorf et al., 1997) Number of ions per formula unit calculated on the basis of 11 oxygen. 30
0.00 SERF 39 SERF 53 SERF 77 SERF 107 SERF 125 SERF 306 PHENGITE SERF 312 SERF 341 ] (a.p.f.u.) -1.00 VI SERF 358 SERF 415 SERF 564 +Mn+Ti-Al
tot SERF 567 SERF 571
feal [=Fe 07SE21 MUSCOVITE 07SE24
-2.00 07SE28 07SE35
-1.00 0.00 1.00 mgli [=Mg-Li] (a.p.f.u.)
Figure 3. Graphical presentation and classification of white micas using representative EMP analysis from each sample. Geochemistry is consistent throughout; all white mica separates plot as phengite. (classification scheme after Tischendorf et al., 1997, 2004) A B
WT. MICA
WT. MICA
C D
CHLORITE CHLORITE WT. MICA
WT. MICA
Figure 4. Representative electron backscatter images of sample separates revealing general purity and homogeneity of the white mica phase. A and B (SERF 571-mylonite, and SERF 125-meta-arentite, respec- tively) are examples of generally homogeneous white mica ; C and D (SERF 77-gneiss, and 07SE 35-qtz. 31 schist, respectively) illustrate visible intergrowth of white mica with chlorite as seen in some of the samples. 32
mica was found in separates collected from several samples in the north and west
portions of the island (Figure 4c and d); separates from the remaining samples contained
relatively homogeneous white mica with very little visible impurities (Figure 4a and b).
The confirmation of white mica separates of phengitic composition allowed interpretation
of 40Ar/39Ar data and spectra based on a 350˚C closure temperature (Dodson, 1973;
McDougall and Harrison, 1999).
Incremental heating of white mica during 40Ar/39Ar analysis yielded various types
of apparent age spectra. The assignment of an age to each spectrum was based foremost
on the presence of a plateau. If no plateau was present, a preferred age was determined.
In the absence of either a calculable plateau or preferred age, the integrated age was used
as the best estimate age. Two distinct populations of apparent ages and spectra shape were observed: (1) disturbed spectra with apparent 40Ar/39Ar ages of 35-28 Ma; and (2)
relatively well-behaved, flat spectra with apparent 40Ar/39Ar ages of 9-8 Ma (age spectra
are shown in Figure 5; analytical results are presented in Table 2).
Eleven samples from CBU-equivalent units in the northern and western portions
of the island yielded disturbed spectra of type (1) that exhibited a slight convex upward
shape and Early Oligocene apparent ages (Figure 5a, c, e-h, j, l, o-r). The general hump-
shape of these spectra is defined by heating increments that initially monotonically
increase in age (present in all samples of this type), approach a plateau state during
moderate heating stages, and decline again as the final heating stages are approached.
K/Ca ratios associated with these increments closely mimic the initial rising trend while
maintaining reasonably high values for the remainder of gas release. The final heating 1000
a C / K 100/ C a 10 40 1
E G F 30 F H G M D E C H I J L K B I J K SERF 53 quartzite D 20 C 8.93 ± 0.05 Ma (MSWD = 8.28) B 10 A SERF 39 schist C D E F G H I J K L M A SERF 77 gneiss
Apparent age (Ma) B Integrated Age = 30.01 ± 0.04 Ma Integrated Age = 8.84 ± 0.03 Ma Integrated Age = 29.38 ± 0.05 Ma 0 A 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 39 39 39 a) Cumulative % Ar Released b) Cumulative % Ar Released c) Cumulative % Ar Released
1000
a C / K 100/ C a 10 40 1
F E E G K L 30 F G L M H M H K J D J D SERF 107 mylonite I C I C 20 9.33 ± 0.16 Ma* (MSWD = 22) B B A 10 L E F G M A SERF 306 meta-arenite Apparent age (Ma) B C D H I J K SERF 125 meta-arenite Integrated Age = 9.17 ± 0.06 Ma Integrated Age = 31.45 ± 0.05 Ma Integrated Age = 32.64 ± 0.05 Ma 0 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 39 39 39 d) Cumulative % Ar Released e) Cumulative % Ar Released f) Cumulative % Ar Released
Figure 5. 40Ar/39Ar age spectra from Serifos. ( * denotes preferred age.) 33 1000
a C / K 100/ C a 10 40 1
E E F L F G H K 30 I J K M D J D G H L I C C SERF 358 schist B 20 B 8.89 ± 0.05 Ma (MSWD = 5.28)
10 A A L SERF 312 schist SERF 341 calc-silc mylonite B C D E F G H I J K Apparent age (Ma) M Integrated Age = 32.71 ± 0.05 Ma Integrated Age = 32.92 ± 0.05 Ma A Integrated Age = 8.86 ± 0.04 Ma 0 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 39 39 39 g) Cumulative % Ar Released h) Cumulative % Ar Released i) Cumulative % Ar Released
1000
a C / K 100/ C a 10 40 1 E E F G L F G L H K D H K 30 J I J SERF 428 schist M D L C B I 12.9 ± 0.42 Ma* (MSWD = 30) C 38.51 ± 0.44 Ma* (MSWD = 13) 20 B A F K D E G J 10 SERF 415 schist B C H I A SERF 564 schist
Apparent age (Ma) A Integrated Age = 34.06 ± 0.05 Ma Integrated Age = 15.01 ± 0.04 Ma Integrated Age = 35.96 ± 0.06 Ma 0 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 39 39 39 j) Cumulative % Ar Released k) Cumulative % Ar Released l) Cumulative % Ar Released
Figure 5 (continued). 40Ar/39Ar age spectra from Serifos. ( * denotes preferred age.) 34 1000
a C / K 100/ C a 10 40 1
C D 30 G F H SERF 571 mylonite B E 20 SERF 567 mylonite 8.43 ± 0.04 Ma (MSWD = 8.44) 8.452 ± 0.018 Ma (MSWD = 1.34) 36.30 ± 4.7 Ma* (MSWD = 17) 10 A 07SE21 gneiss E F K M D E F G H I K J Apparent age (Ma) B C D G H I J L A Integrated Age = 8.31 ± 0.03 Ma Integrated Age = 8.40 ± 0.03 Ma Integrated Age = 32.22 ± 0.09 Ma 0 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 39 39 39 m) Cumulative % Ar Released n) Cumulative % Ar Released o) Cumulative % Ar Released
1000
a C / K 100/ C a 10 40 1
J D E C D 30 K I F J C D E F E H B H I G H F G 20 C B A 35.30 ± 4.2 Ma* (MSWD = 19) A 10 07SE 24 gneiss 07SE 28 orthogneiss 07SE 35 quartz schist Apparent age (Ma) Integrated Age = 32.24 ± 0.09 Ma Integrated Age = 28.21 ± 0.07 Ma Integrated Age = 31.13 ± 0.07 Ma 0 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 39 39 39 p) Cumulative % Ar Released q) Cumulative % Ar Released r) Cumulative % Ar Released
Figure 5 (continued). 40Ar/39Ar age spectra from Serifos. ( * denotes preferred age.) 35 36 σ ±1 0.0 0.00 0.00 94.7 8.93 0.05 Ar Age 39 Ar* (%) (%) (Ma) (Ma) 40 K/Ca 286.774±189.952 1780.751±471.512 K mol) Ar -15 39 Ar 39 ) (x 10 -3 Ar/ 36 (x 10 Ar 39 Ar/ 37 n=11 343.23 677.4 K2O=10.79% 30.01n=13 0.05 115.03 92.0 K2O=4.18% 8.84 0.03 Ar 39 Ar/ 40 steps C-M n=11 MSWD=8.28 108.97 no plateau n=0 MSWD=0.00 0.00 σ σ Ar analytical data from Serifos analytical data from Ar 39 (°C) σ σ Ar/ Muscovite, 8.8 mg, J=0.0013884±0.11% Muscovite, 7.6 mg, J=0.0013919±0.09% 40 Plateau ± 1 Plateau ± 1 Integrated age ± 1 CDEF 790G 840H 865I 890J 915 4.41K 940 4.54LM 4.08 990 1040 0.0170 4.13Integrated age ± 1 1140 0.0052 4.13 1290 0.0019 4.09 1640 0.0016 4.06 3.11 0.0009 4.02 4.55 3.57 0.0011 4.20 2.05 0.0012 4.92 7.97 0.0039 1.69 11.26 0.0088 1.54 10.64 0.0036 1.81 0.0063 9.65 30.0 1.77 14.22 97.8 1.63 270.4 12.29 3.42 79.2 76.7 315.8 1.52 16.50 586.4 12.65 85.2 4.22 474.1 12.2 6.44 87.9 22.0 89.0 427.7 2.01 31.2 131.8 86.9 5.33 39.6 52.0 87.1 57.7 8.74 88.0 141.3 8.73 62.7 8.71 81.4 77.8 77.0 89.3 88.0 9.09 0.09 9.21 0.07 74.7 93.6 0.06 8.91 95.4 8.87 100.0 0.06 0.05 8.86 0.05 8.87 9.38 0.04 0.05 9.21 0.09 0.28 0.11 ID 39, SERF Temp SERF 53, SERF Table 2. Table ## A# B# C# D 640# E 715# F 790# G 840# H 12.30 865# I 12.63 890# J 12.67 915 K 0.0134 13.31 940 0.0054 14.40 990 0.0017 13.56 1040 1140 0.0007 14.02 22.19 0.0003 13.72# 7.74 0.0003# A 10.76 4.08 10.62 0.0002 5.54 B 10.41 3.14 0.0002 9.90 2.79 20.76 0.0003 0.0001 2.23 640 25.74 38.2 0.0003 2.64 715 71.56 2.53 95.3 302.8 57.82 46.7 698.6 1.88 37.58 1498.0 1.78 16.87 81.9 90.5 31.01 1.92 1524.4 93.0 1.6 5.73 2481.5 31.12 94.3 30.68 10.5 2062.3 4.5 0.0475 95.1 21.52 18.0 94.4 1908.7 38.9 0.0301 14.33 4836.1 94.6 55.7 1572.3 28.50 25.72 66.7 94.8 50.22 30.75 95.0 75.7 33.68 0.23 94.5 8.20 32.03 84.8 0.08 0.13 93.7 32.86 1.91 100.0 0.07 0.05 32.21 4.15 0.06 25.39 25.12 0.07 10.8 24.47 0.08 16.9 12.1 0.05 0.07 0.08 57.8 1.7 5.3 5.11 8.30 0.52 0.15 Table 2 (continued). 40Ar/39Ar analytical data from Serifos
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (°C) (x 10-3 ) (x 10-15 mol) (%) (%) (Ma) (Ma) SERF 77, Muscovite, 5.5 mg, J=0.0013914±1.08% # A 640 14.08 0.0670 33.97 2.79 7.6 28.8 1.3 10.14 0.42 # B 715 8.03 0.0593 7.03 5.81 8.6 74.2 3.9 14.89 0.19 # C 790 9.98 0.0327 4.89 8.10 15.6 85.6 7.6 21.32 0.12 # D 840 11.59 0.0157 4.89 7.53 32.6 87.5 11.1 25.29 0.14 # E 865 13.48 0.0054 5.52 22.65 93.7 87.9 21.5 29.51 0.09 # F 890 13.89 0.0016 2.80 56.37 320.7 94.0 47.3 32.49 0.06 # G 915 13.35 0.0014 2.51 51.50 377.1 94.5 70.9 31.38 0.06 # H 940 12.12 0.0025 2.95 24.47 200.5 92.8 82.1 28.01 0.09 # I 990 12.14 0.0073 2.77 18.37 70.0 93.3 90.5 28.19 0.09 # J 1040 12.28 0.0151 3.49 10.42 33.8 91.6 95.3 28.01 0.14 # K 1140 13.93 0.0226 4.99 6.18 22.6 89.4 98.1 31.00 0.18 # L 1290 15.08 0.0573 8.82 1.42 8.9 82.7 98.8 31.05 0.66 # M 1640 17.05 0.0180 13.05 2.71 28.3 77.4 100.0 32.83 0.35 Integrated age ± 1 σ n=13 218.32 61.2 K2O=10.96% 29.38 0.32 Plateau ± 1σ no plateau n=0 MSWD=0.00 0.00 0.000±0.000 0.0 0.00 0.00
SERF 107, Muscovite, 6.8 mg, J=0.0013965±0.06% # A 640 94.33 0.0403 312.77 4.99 12.7 2.0 2.2 4.80 1.16 B 715 11.47 0.0340 27.20 10.31 15.0 29.9 6.9 8.62 0.19 C 790 10.41 0.0267 23.49 18.52 19.1 33.4 15.2 8.73 0.13 D 840 6.80 0.0114 11.22 26.13 44.9 51.3 27.0 8.76 0.08 # E 865 5.02 0.0050 3.97 25.18 101.0 76.7 38.4 9.67 0.05 # F 890 4.82 0.0029 2.78 21.93 176.1 83.0 48.3 10.05 0.06 G 915 4.66 0.0028 2.99 25.15 182.3 81.1 59.6 9.49 0.04 H 940 4.60 0.0022 3.25 22.56 229.9 79.1 69.8 9.15 0.04 I 990 4.68 0.0026 3.65 24.84 197.9 76.9 81.0 9.05 0.04 J 1040 4.83 0.0046 4.40 17.70 111.3 73.1 88.9 8.88 0.06 K 1140 5.40 0.0181 6.41 12.58 28.1 65.0 94.6 8.82 0.08 # L 1290 7.16 0.0187 7.78 5.16 27.3 67.9 96.9 12.20 0.14 # M 1640 6.85 0.0081 9.96 6.77 62.7 57.1 100.0 9.83 0.12 Integrated age ± 1 σ n=13 221.84 52.0 K2O=8.97% 9.17 0.06 σ Plateau ± 1 steps B-D;G-K n=8 MSWD=22 157.80 118.358±78.507 71.1 9.12* 0.23 37 Table 2 (continued). 40Ar/39Ar analytical data from Serifos
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (°C) (x 10-3 ) (x 10-15 mol) (%) (%) (Ma) (Ma) SERF 125, Muscovite, 6.4 mg, J=0.0013972±0.06% # A 640 27.21 0.5527 70.82 2.05 0.9 23.3 1.0 15.88 0.60 # B 715 12.31 1.9333 14.67 2.69 0.3 66.1 2.3 20.41 0.37 # C 790 12.25 0.1630 8.42 4.96 3.1 79.8 4.7 24.49 0.19 # D 840 14.73 0.0076 11.07 6.89 67.4 77.8 8.0 28.67 0.18 # E 865 15.99 0.0018 7.22 31.64 281.0 86.7 23.3 34.59 0.08 # F 890 14.52 0.0010 3.46 42.53 493.0 93.0 43.8 33.72 0.06 # G 915 13.92 0.0012 3.34 37.05 417.0 92.9 61.6 32.32 0.06 # H 940 13.08 0.0014 3.18 23.08 370.8 92.8 72.7 30.35 0.06 # I 990 11.31 0.0019 3.06 19.76 265.2 92.0 82.3 26.04 0.06 # J 1040 12.12 0.0027 3.20 8.27 187.1 92.2 86.2 27.94 0.10 # K 1140 13.29 0.0066 4.07 5.81 77.7 91.0 89.0 30.23 0.13 # L 1290 14.13 0.0047 2.74 19.33 108.2 94.3 98.4 33.27 0.07 # M 1640 17.13 0.0136 7.19 3.38 37.4 87.6 100.0 37.44 0.25 Integrated age ± 1 σ n=13 207.45 13.9 K2O=8.91% 31.45 0.05 Plateau ± 1σ no plateau n=0 MSWD=0.00 0.00 0.000±0.000 0.0 0.00 0.00
SERF 306, Muscovite, 5.9 mg, J=0.0013913±0.07% # A 640 24.94 0.1674 68.79 1.81 3.0 18.6 0.8 11.59 0.68 # B 715 12.49 0.2628 10.81 3.75 1.9 74.6 2.3 23.24 0.19 # C 790 12.75 0.0078 7.26 7.45 65.0 83.2 5.5 26.43 0.13 # D 840 13.79 0.0044 6.98 9.35 114.7 85.1 9.4 29.20 0.11 # E 865 16.44 0.0019 8.07 21.53 275.5 85.5 18.5 34.94 0.10 # F 890 15.65 0.0006 4.17 44.15 799.9 92.1 37.1 35.84 0.07 # G 915 15.01 0.0005 3.78 45.25 1062.0 92.6 56.1 34.53 0.07 # H 940 14.53 0.0006 3.84 32.47 891.8 92.2 69.8 33.32 0.07 # I 990 11.92 0.0016 2.93 31.28 311.8 92.7 83.0 27.53 0.06 # J 1040 13.05 0.0017 3.01 14.48 305.5 93.2 89.1 30.25 0.07 # K 1140 14.92 0.0026 3.89 18.35 197.1 92.3 96.8 34.24 0.08 # L 1290 15.08 0.0137 3.60 6.88 37.1 93.0 99.7 34.84 0.12 # M 1640 21.87 0.0258 23.29 0.73 19.8 68.5 100.0 37.23 0.77 Integrated age ± 1 σ n=13 237.52 69.8 K2O=11.11% 32.64 0.06 σ Plateau ± 1 no plateau n=0 MSWD=0.00 0.00 824.718±339.049 0.0 0.00 0.00 38 Table 2 (continued). 40Ar/39Ar analytical data from Serifos
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (°C) (x 10-3 ) (x 10-15 mol) (%) (%) (Ma) (Ma) SERF 312, Muscovite, 8.1 mg, J=0.0013945±0.06% # A 640 30.53 0.8310 86.23 2.26 0.6 16.8 0.7 12.84 0.61 # B 715 15.77 3.1153 21.63 3.76 0.2 61.1 1.9 24.14 0.24 # C 790 13.56 0.2892 10.42 7.64 1.8 77.5 4.4 26.26 0.13 # D 840 14.78 0.0118 10.86 12.06 43.2 78.3 8.3 28.88 0.12 # E 890 15.78 0.0022 5.58 97.31 232.3 89.6 39.8 35.20 0.06 # F 940 14.85 0.0016 3.33 94.79 310.6 93.4 70.5 34.54 0.05 # G 990 12.45 0.0035 3.39 30.45 144.9 92.0 80.3 28.57 0.06 # H 1040 12.00 0.0035 2.70 21.84 145.3 93.3 87.4 27.95 0.06 # I 1090 13.80 0.0025 2.88 28.68 203.3 93.8 96.7 32.28 0.07 # J 1140 14.29 0.0056 4.21 5.85 91.4 91.3 98.6 32.52 0.13 # K 1190 14.02 0.0148 3.47 2.43 34.4 92.7 99.4 32.40 0.26 # L 1290 14.69 0.1366 6.36 1.96 3.7 87.3 100.0 31.97 0.30 Integrated age ± 1 σ n=12 309.05 9.3 K2O=10.51% 32.71 0.05 Plateau ± 1σ no plateau n=0 MSWD=0.00 0.00 0.000±0.000 0.0 0.00 0.00
SERF 341, Muscovite, 7.4 mg, J=0.0013899±0.07% # A 640 24.82 0.1587 68.55 2.07 3.2 18.4 0.7 11.44 0.55 # B 715 12.48 0.2245 10.61 4.23 2.3 75.0 2.3 23.34 0.17 # C 790 12.62 0.0082 6.70 8.19 61.9 84.3 5.2 26.48 0.11 # D 840 14.11 0.0041 7.57 10.15 124.7 84.1 8.8 29.51 0.12 # E 865 16.56 0.0016 8.18 24.80 311.6 85.4 17.7 35.11 0.08 # F 890 15.73 0.0005 4.11 60.60 941.9 92.3 39.5 36.02 0.06 # G 915 15.06 0.0006 3.76 42.10 806.6 92.6 54.6 34.65 0.07 # H 940 14.72 0.0006 3.86 38.38 840.7 92.2 68.4 33.73 0.06 # I 990 11.98 0.0009 2.91 39.08 562.4 92.8 82.4 27.67 0.05 # J 1040 13.21 0.0013 2.81 16.54 389.3 93.7 88.4 30.79 0.08 # K 1140 14.98 0.0022 3.41 23.16 227.5 93.3 96.7 34.70 0.07 # L 1290 15.29 0.0087 3.58 8.65 58.8 93.1 99.8 35.35 0.10 # M 1640 24.83 0.0391 32.68 0.59 13.1 61.1 100.0 37.66 0.97 Integrated age ± 1 σ n=13 278.53 82.8 K2O=10.40% 32.92 0.06 Plateau ± 1σ no plateau n=0 MSWD=0.00 0.00 0.000±0.000 0.0 0.00 0.00 39 Table 2 (continued). 40Ar/39Ar analytical data from Serifos
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1 σ (°C) (x 10 -3 ) (x 10 -15 mol) (%) (%) (Ma) (Ma) SERF 358, Muscovite, 6.8 mg, J=0.0013855±1.08% # A 640 22.83 0.0156 69.32 4.62 32.8 10.3 1.9 5.87 0.47 B 715 8.78 0.0048 17.77 10.22 106.0 40.2 6.0 8.81 0.19 C 790 8.09 0.0012 15.48 22.63 414.5 43.5 15.2 8.76 0.10 D 840 6.37 0.0004 9.06 25.69 1174.5 57.9 25.6 9.19 0.07 E 865 4.61 0.0004 3.54 42.53 1326.9 77.3 42.9 8.88 0.04 F 890 4.44 0.0002 3.01 37.61 2662.9 79.9 58.1 8.85 0.04 G 915 4.64 0.0003 3.77 31.51 1957.8 76.0 70.9 8.79 0.05 H 940 5.15 0.0001 5.58 21.62 5884.7 68.0 79.6 8.73 0.07 I 990 5.71 0.0009 7.06 16.49 537.1 63.4 86.3 9.03 0.07 J 1040 5.08 0.0004 5.06 15.88 1364.9 70.6 92.8 8.93 0.08 K 1140 5.27 0.0005 5.47 10.06 1027.0 69.3 96.8 9.10 0.11 L 1290 6.04 0.0000 5.35 2.02 12949.3 73.9 97.7 11.12 0.51 M 1640 6.28 0.0001 8.88 5.75 4020.5 58.2 100.0 9.12 0.15 Integrated age ± 1 σ n=13 246.64 568.8 K2O=10.06% 8.86 0.10 Plateau ± 1 σ steps B-M n=12 MSWD=5.28 242.01 1968.0 ±3603.3 98.1 8.89 0.11
SERF 415, Muscovite, 9.1 mg, J=0.0013929±0.11% # A 640 16.02 0.1986 32.09 4.84 2.6 40.9 1.3 16.40 0.26 # B 715 12.48 0.0840 6.89 7.82 6.1 83.7 3.3 26.07 0.17 # C 790 12.79 0.0050 5.50 14.67 102.1 87.3 7.2 27.85 0.11 # D 840 16.35 0.0022 9.08 18.63 229.9 83.6 12.1 34.03 0.12 # E 865 16.94 0.0006 5.06 83.41 789.3 91.2 34.1 38.39 0.06 # F 890 16.09 0.0004 4.31 60.92 1196.0 92.1 50.2 36.84 0.07 # G 915 16.31 0.0006 4.30 54.85 857.6 92.2 64.7 37.40 0.07 # H 940 14.76 0.0007 3.78 35.14 766.0 92.4 74.0 33.95 0.08 # I 990 10.81 0.0005 2.30 51.24 1030.9 93.7 87.5 25.27 0.06 # J 1040 13.98 0.0006 3.32 32.77 788.0 93.0 96.2 32.37 0.08 # K 1140 16.04 0.0042 5.41 10.20 122.2 90.0 98.8 35.94 0.14 # L 1290 17.13 0.0217 7.38 4.24 23.6 87.3 100.0 37.18 0.26 # M 1640 104.70 0.1026 277.51 0.14 5.0 21.7 100.0 56.18 6.31 Integrated age ± 1 σ n=13 378.88 93.7 K2O=11.48% 34.06 0.06
Plateau ± 1 σ no plateau n=0 MSWD=0.00 0.00 0.000±0.000 0.0 0.00 0.00 40 Table 2 (continued). 40Ar/39Ar analytical data from Serifos
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (°C) (x 10-3 ) (x 10-15 mol) (%) (%) (Ma) (Ma) SERF 428, Muscovite, 5 mg, J=0.0013899±1.08% # A 640 40.68 0.0251 124.87 2.83 20.3 9.3 1.6 9.45 0.64 B 715 7.82 0.0109 10.01 7.04 46.6 62.2 5.6 12.16 0.14 C 790 6.26 0.0068 3.96 9.80 74.8 81.3 11.1 12.73 0.10 D 840 6.78 0.0041 4.82 11.09 123.1 79.0 17.4 13.38 0.09 E 865 5.98 0.0021 2.64 20.20 246.8 86.9 28.9 12.98 0.07 # F 890 7.16 0.0012 1.66 19.55 425.3 93.1 39.9 16.65 0.07 # G 915 5.99 0.0011 1.54 15.09 479.7 92.4 48.5 13.83 0.07 H 940 5.73 0.0009 2.29 13.49 585.6 88.2 56.1 12.63 0.08 I 990 5.70 0.0005 2.46 18.93 1101.0 87.3 66.9 12.42 0.08 J 1040 6.27 0.0010 2.91 15.85 504.0 86.3 75.8 13.52 0.07 # K 1140 6.95 0.0013 2.36 29.94 394.7 90.0 92.8 15.61 0.05 # L 1290 12.59 0.0022 2.99 9.45 232.4 93.0 98.2 29.13 0.13 # M 1640 16.41 0.0029 11.90 3.24 178.3 78.6 100.0 32.04 0.30 Integrated age ± 1 σ n=13 176.48 201.3 K2O=9.75% 15.01 0.17 Plateau ± 1σ steps B-E;H-J n=7 MSWD=30 96.38 513.418±311.060 54.6 12.9* 0.42
SERF 564, Muscovite, 10.2 mg, J=0.0013869±0.10% # AA 990 109.50 1.3004 1010.59 0.00 0.4 -172.6 0.0 -549.06 740.47 # A 640 17.11 0.9917 41.03 6.55 0.5 29.6 1.5 12.66 0.29 # B 715 10.28 0.3705 9.37 10.69 1.4 73.4 3.8 18.79 0.12 # C 790 12.62 0.0189 7.77 19.52 26.9 81.8 8.1 25.66 0.12 # D 840 14.93 0.0123 7.14 14.37 41.4 85.9 11.3 31.81 0.12 E 865 17.87 0.0044 8.34 63.52 115.5 86.2 25.4 38.15 0.08 F 890 16.96 0.0022 4.65 84.98 227.6 91.9 44.2 38.57 0.07 G 915 16.86 0.0017 4.48 78.49 307.3 92.2 61.6 38.47 0.07 # H 940 16.45 0.0016 4.60 59.80 314.1 91.7 74.9 37.37 0.07 # I 990 14.60 0.0023 4.26 39.12 220.3 91.4 83.6 33.08 0.08 # J 1040 15.01 0.0041 5.50 16.87 124.5 89.2 87.3 33.18 0.13 # K 1140 16.22 0.0037 5.49 22.37 138.1 90.0 92.2 36.15 0.11 L 1290 16.92 0.0068 4.07 32.49 74.7 92.9 99.4 38.90 0.09 # M 1640 26.22 0.0075 32.31 2.49 68.5 63.6 100.0 41.24 0.52 Integrated age ± 1
σ n=14 451.28 18.9 K2O=12.25% 35.96 0.07 41 Plateau ± 1σ steps E-G;L n=4 MSWD=13 289.50 0.000±0.000 57.5 38.51* 0.44 Table 2 (continued). 40Ar/39Ar analytical data from Serifos
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (°C) (x 10-3 ) (x 10-15 mol) (%) (%) (Ma) (Ma) SERF 567, Muscovite, 9.4 mg, J=0.0013945±0.08% # A 640 14.79 0.0320 41.93 6.98 16.0 16.2 2.4 6.02 0.27 # B 715 5.95 0.0103 9.79 12.62 49.6 51.4 6.8 7.69 0.08 # C 790 5.67 0.0034 8.23 21.60 149.0 57.1 14.3 8.12 0.06 D 840 4.91 0.0013 5.26 37.98 381.6 68.3 27.5 8.43 0.04 E 865 4.03 0.0009 2.14 36.83 572.8 84.3 40.4 8.53 0.03 F 890 4.01 0.0006 2.14 30.21 817.8 84.2 50.9 8.47 0.03 G 915 4.16 0.0006 2.87 26.00 868.0 79.6 59.9 8.32 0.04 H 940 4.48 0.0006 4.03 21.60 830.3 73.4 67.4 8.26 0.05 I 990 4.47 0.0006 3.85 31.44 820.8 74.6 78.4 8.37 0.04 J 1040 4.23 0.0005 2.96 26.67 963.7 79.3 87.6 8.42 0.04 K 1140 4.19 0.0008 2.80 17.36 649.9 80.2 93.7 8.44 0.05 L 1290 4.12 0.0005 2.88 6.88 1121.1 79.3 96.1 8.20 0.10 M 1640 4.34 0.0016 2.80 11.26 312.1 81.0 100.0 8.82 0.07 Integrated age ± 1 σ n=13 287.43 234.0 K2O=8.42% 8.31 0.03 Plateau ± 1σ steps D-M n=10 MSWD=8.44 246.23 710.0 ±253.8 85.7 8.43 0.04
SERF 571, Muscovite, 7.1 mg, J=0.0013966±0.07% # A 640 27.90 0.0643 85.70 4.68 7.9 9.3 1.8 6.49 0.47 # B 715 8.33 0.0362 17.06 6.32 14.1 39.5 4.2 8.27 0.16 # C 790 6.74 0.0146 11.89 11.18 34.8 47.8 8.5 8.10 0.09 D 840 6.45 0.0061 10.43 22.18 83.3 52.2 17.1 8.47 0.09 E 890 4.55 0.0019 3.970 72.37 266.7 74.2 45.0 8.49 0.03 F 940 4.38 0.0012 3.439 77.85 433.7 76.8 75.0 8.44 0.03 G 990 4.55 0.0031 4.055 33.64 164.8 73.7 87.9 8.43 0.04 H 1040 4.30 0.0054 3.321 19.09 93.8 77.2 95.3 8.35 0.05 I 1090 4.46 0.0119 3.649 7.63 42.9 75.8 98.2 8.50 0.09 J 1140 4.22 0.0121 2.494 2.91 42.0 82.6 99.3 8.76 0.21 K 1190 4.23 0.0077 2.681 1.74 65.9 81.3 100.0 8.65 0.34 Integrated age ± 1 σ n=11 259.59 94.2 K2O=10.06% 8.40 0.03 Plateau ± 1σ steps D-K n=8 MSWD=1.34 237.42 264.6 ±137.4 91.5 8.45 0.02 42 Table 2 (continued). 40Ar/39Ar analytical data from Serifos
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (°C) (x 10-3 ) (x 10-15 mol) (%) (%) (Ma) (Ma) 07SE21, Mica, 5.64 mg, J=0.0007047±0.10% # A 600 32.13 0.0311 67.06 1.13 16.4 38.3 1.4 15.59 0.95 # B 750 25.54 0.0027 15.29 6.59 186.3 82.3 9.4 26.53 0.20 C 800 33.80 0.0011 15.39 15.53 479.7 86.5 28.4 36.81 0.15 D 850 31.36 0.0005 9.24 27.34 973.5 91.3 61.7 36.04 0.11 # E 900 23.45 0.0008 6.77 18.47 660.8 91.5 84.2 27.06 0.11 # F 950 24.49 -0.0024 6.94 5.52 - 91.6 91.0 28.30 0.20 # G 1000 27.76 -0.0031 5.79 2.88 - 93.8 94.5 32.81 0.40 # H 1100 29.33 0.0091 10.89 1.70 55.8 89.0 96.6 32.90 0.64 # I 1150 32.89 -0.0314 22.66 0.64 - 79.6 97.3 33.00 1.47 # J 1600 95.43 0.0025 244.22 2.17 203.5 24.4 100.0 29.33 0.97 Integrated age ± 1 σ n=10 81.97 542.6 K2O=7.92% 32.22 0.10 Plateau ± 1σ steps C-D n=2 MSWD=17 42.90 52.3 36.30* 4.70
07SE 24, Mica, 8 mg, J=0.0007156±0.16% # A 600 64.70 0.0294 183.27 1.23 17.4 16.3 1.0 13.57 0.70 # C 750 72.44 0.0039 183.67 7.93 131.1 25.1 7.6 23.30 0.46 # D 800 30.91 0.0009 11.44 15.02 562.1 89.1 20.0 35.19 0.12 # E 850 29.64 0.0003 4.79 41.27 1472.5 95.2 54.1 36.08 0.07 # F 900 25.58 0.0009 4.07 27.00 591.1 95.3 76.5 31.20 0.06 # H 950 22.93 0.0009 3.53 13.33 560.1 95.5 87.5 28.03 0.07 # I 1000 21.71 0.0003 3.38 6.94 1533.1 95.4 93.3 26.54 0.08 # J 1100 27.46 0.0095 10.73 2.65 53.7 88.5 95.4 31.08 0.18 # L 1150 29.55 0.0143 17.28 1.50 35.8 82.7 96.7 31.28 0.31 # M 1250 57.92 0.0798 105.98 0.94 6.4 45.9 97.5 34.03 0.69 # N 1600 109.90 0.0021 271.53 3.06 248.0 27.0 100.0 37.89 0.73 Integrated age ± 1 σ n=11 120.88 236.4 K2O=8.11% 32.24 0.11 Plateau ± 1σ no plateau n=0 MSWD=0.00 0.00 0.0 0.00 0.00 43 Table 2 (continued). 40Ar/39Ar analytical data from Serifos
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (°C) (x 10-3 ) (x 10-15 mol) (%) (%) (Ma) (Ma) 07SE 28, Mica, 8 mg, J=0.0007149±0.15% # A 600 28.22 0.0195 48.74 2.71 26.1 49.0 2.5 17.73 0.30 # B 750 24.66 0.0037 18.01 11.81 137.8 78.4 13.1 24.77 0.11 # C 800 25.22 0.0008 7.38 16.59 613.1 91.4 28.1 29.48 0.09 # D 850 24.03 0.0010 3.57 24.10 496.5 95.6 49.9 29.39 0.06 # E 900 23.42 0.0011 3.90 19.62 448.8 95.1 67.7 28.49 0.08 # F 950 23.83 0.0015 5.34 16.19 340.0 93.4 82.3 28.48 0.08 # G 1000 21.06 0.0009 3.95 9.35 573.3 94.5 90.8 25.48 0.08 # H 1100 22.41 0.0028 5.79 4.59 183.5 92.4 94.9 26.50 0.13 # I 1150 31.37 0.0090 14.54 1.46 56.6 86.3 96.3 34.59 0.37 # J 1600 84.72 0.0034 190.11 4.14 151.2 33.7 100.0 36.44 0.54 Integrated age ± 1 σ n=10 110.57 245.1 K2O=7.43% 28.22 0.08 Plateau ± 1σ no plateau n=0 MSWD=0.00 0.00 0.0 0.00 0.00
07SE 35, Mica, 5.8 mg, J=0.0007122±0.15% # A 600 30.94 0.0313 50.10 1.27 16.3 52.2 1.3 20.61 0.75 # B 750 25.92 0.0043 13.60 11.39 118.4 84.5 13.1 27.92 0.14 C 800 31.09 0.0006 10.38 23.30 893.1 90.1 37.1 35.64 0.12 D 850 29.78 0.0005 7.78 25.82 1001.6 92.3 63.8 34.97 0.10 # E 900 23.27 0.0002 4.39 12.93 2044.1 94.4 77.1 28.01 0.11 # F 950 21.04 0.0008 4.06 12.20 630.0 94.3 89.7 25.31 0.11 # G 1000 21.74 0.0006 3.00 8.14 865.5 95.9 98.2 26.59 0.15 # H 1100 24.38 0.0127 8.79 1.79 40.3 89.3 100.0 27.78 0.62 Integrated age ± 1 σ n=8 96.84 317.1 K2O=9.00% 31.14 0.08 Plateau ± 1σ steps C-D n=2 MSWD=19 49.10 50.7 35.30* 4.20 44 Table 2 (continued). 40Ar/39Ar analytical data from Serifos
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (°C) (x 10-3 ) (x 10-15 mol) (%) (%) (Ma) (Ma)
Notes: Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions. Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties. Integrated age calculated by summing isotopic measurements of all steps. Integrated age error calculated by quadratically combining errors of isotopic measurements of all steps. Plateau age is inverse-variance-weighted mean of selected steps. Plateau age error is inverse-variance-weighted mean error (Taylor, 1982) times root MSWD where MSWD>1. Plateau error is weighted error of Taylor (1982). Decay constants and isotopic abundances after Steiger and Jäger (1977). # symbol preceding sample ID denotes analyses excluded from plateau age calculations. * symbol denotes preferred ages. 39 Weight percent K 2O calculated from Ar signal, sample weight, and instrument sensitivity. Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 28.02 Ma Decay Constant (LambdaK (total)) = 5.543e-10/a Correction factors: 39 37 ( Ar/ Ar) Ca = 0.0007 ± 5e-05 36 37 ( Ar/ Ar) Ca = 0.00028 ± 2e-05 38 39 ( Ar/ Ar) K = 0.0129 40 39 ( Ar/ Ar) K = 0 ± 0.002 45 46
stages of all type 1 spectra sharply increase in apparent age, with the exception of SERF
39 (Figure 5a), a schist from the northeast coast of Serifos. Accompanying this age
gradient is an antithetical decrease in the K/Ca ratio that is markedly distinct from the
trend of preceding increments, suggesting a change in the diffusional domain being
degassed to a lower-K, less retentive mineral phase. No definable plateaus were present
in the type 1 spectra, however a few of them yielded a calculable preferred age (Table 2)
including the two southernmost samples from the CBU units (SERF 564 and 07SE21)
and one from the central portion of the island (07SE35). The southernmost sample, SERF
564 (Figure 5l, a schist near the detachment surface south of Meghalo Livadhi), produced
a preferred age of 38.51 ± 0.44 Ma (MSWD=13) over 57.5% of the spectrum; 07SE21
(Figure 5o, a gneiss from the western coast of Serifos) yielded a preferred age of 36.30 ±
4.7 Ma (MSWD=17) over 52.3% of the spectrum; and 07SE35 (Figure 5r, a quartz schist
from north-central Serifos) yielded a preferred age of 35.30 ± 4.2 Ma (MSWD=19).
Integrated ages of all other type 1 samples ranged between 28 and 35 Ma with errors of ±
0.5-1.0 Ma.
Type 2 spectra included samples from the southeastern portion of the island,
within the mylonitic orthogneiss and adjacent to the granodiorite pluton. This type generally yielded flat age spectra with well-defined plateaus (MSWD < 10) that reveal
Late Miocene cooling between 8 and 9 Ma (Figure 5b, d, i, m, n). In the initial and final heating increments, apparent ages increase very slightly but otherwise the spectra remain flat. K/Ca ratios follow a pattern similar to the type 1 spectra with a rise in the initial
segments that mimics the apparent age trend, relatively constant values across the plateau
segments, and a change in the final steps to a decrease in K/Ca ratio antithetical to the 47 apparent age trend. Four of the five type 2 spectra yielded well-defined plateaus ages between 8.43 and 8.93 Ma: SERF 567 (Figure 5m, a mylonite at the southwestern margin of the granodiorite pluton) produced a plateau across 85.7% of the spectrum with an age of 8.43 ± 0.04 Ma (MSWD=8.44); SERF 571 (Figure 5n, a mylonite slightly north of
SERF 567) yielded a plateau age of 8.45 ± 0.018 (MSWD=1.34) over 97.1% of gas release; SERF 358 (Figure 5i, a schist from within the orthogneiss along the northern margin of the pluton) yielded a plateau across 97.2% of the spectrum with an age of 8.89
± 0.05 Ma (MSWD=5.28). SERF 53 (Figure 5b, a quartzite from within the orthogneiss unit at the far western coast of Serifos) produced the oldest of the plateaus, yielding an age of 8.93 ± 0.05 Ma (MSWD=8.28) that covered 94.7% of the spectrum. SERF 107
(Figure 5d, a mylonite near the southern intrusive margin) contained no plateau but did yield a reasonable preferred age of 9.33 ± 0.16 (MSWD=22).
One sample, SERF 428 (Figure 5k, a schist located north of SERF 107) did not strictly conform to the pattern of other spectra from the orthogneiss unit. This apparent age spectrum contained no plateau and only a poorly defined preferred age of 12.9 ± 0.42
Ma (MSWD=30). The integrated age was slightly older at 15.01 ± 0.04 Ma due to a dramatic increase in apparent age in the final heating increments. Interestingly, this spectrum seems to show properties of both type 1 and type 2 spectra. The overall appearance is relatively flat and yields a Late Miocene integrated age similar to the type 2 spectra. However, on careful inspection the hump-shape centered on increment F of the type 1 spectra is also observed in the SERF 428 spectrum, although significantly muted, as well as the sharp rise in the final increments. The lack of geochemical data for this sample and anomalous spectra shape make accurate interpretation of the age difficult, 48 however, given the relative similarity to the assigned ages of type 2 spectra, the calculated preferred age seems a reasonable estimate for cooling of this sample. 49
5. DISCUSSION
5.1 40Ar/39Ar Data and Spectra
To interpret 40Ar/39Ar thermochronometric data for the purpose of obtaining information about the thermal history of a rock, it is necessary to have an understanding of the concept of mineral closure temperature, the role it plays in Ar retention and loss, and its affect on argon release within the 40Ar/39Ar step-heating spectrum. The shape of the age spectrum can be indicative of the nature and rate of cooling of the mineral. For example, samples that exhibit flat age spectra are typically interpreted to have cooled rapidly and remained thermally undisturbed since cooling (e.g., Wijbrans and
McDougall, 1986; Baldwin et al., 1993; McDougall and Harrison, 1999), having quickly passed through and remained below the temperature zone of partial argon retention. In the case where multiple thermochronometers are utilized, close agreement among apparent ages from a range of minerals with different closure temperatures can be used to provide further evidence of rapid cooling. In contrast to flat spectra, a gradient in apparent age across an age spectrum implies that either (1) cooling rates were slow, or (2) the isotopic clock of the mineral was partially reset due to a subsequent thermal or recrystallization (deformation) event (McDougall and Harrison, 1999). In the case of partial resetting, variation in the age spectrum has been ascribed to loss of argon through diffusion from either existing microstructural and/or microchemical domains or new domains produced through subsequent deformation and/or metamorphism (Forster and
Lister, 2004). In either case, the diffusion domains act as distinct reservoirs that independently release gas during 40Ar/39Ar step-heating. The manner of release and 50 mixing of gas from different reservoirs, whether progressive or episodic, affects the shape of the resulting age spectrum (e.g., Wijbrans and McDougall, 1986, 1988; Heizler et al.,
1997; Baldwin and Lister, 1998; Forster and Lister, 2004).
Forster and Lister (2004) describe various spectra produced from different scenarios of reservoir mixing and illustrate that an increase (or decrease) in the apparent age gradient is related to the release of gas from a more (or less) retentive domain.
Progressive mixing of reservoirs produces a more gradual transition in the age gradient that obscures the data and makes interpretation more difficult. Nevertheless, aside from the case of complete progressive mixing of reservoirs, obtaining meaningful age constraints is possible particularly where age gradients can be shown to follow a systematic (non-random) pattern throughout a dataset. Upper and lower age limits within an apparent age spectrum may represent distinct thermal or deformation events. An age limit may only reflect a lower (or upper) bound for the actual timing of an event, however if this were the case one might expect variation in ages among samples. Therefore, an age limit that is consistent within a dataset and region may be a relatively accurate estimate of timing of a thermal or deformation event. Although this method requires a dataset larger than that presented here to determine statistical significance, the general concept of age limits may be broadly applied to the more disturbed spectra as a guide for interpretation.
All of the 40Ar/39Ar spectra derived from CBU-equivalent units on Serifos exhibit a disturbed (type 1) pattern (Figure 5). Throughout these spectra three distinct age limits consistently appear. The first of these is derived from the lower age limit of the initial heating increments that ranges 11.4-20.6 Ma and averages to 14.9 Ma. The sharp rise of apparent age seen in the initial heating steps is similar to predicted age gradients from 51
models of diffusion loss of argon during thermal overprinting (Turner et al., 1966;
Turner, 1968). Wijbrans and McDougall (1986) attributed a similar spike in apparent age
of white mica spectra from Naxos to partial argon loss from a minor, rapid thermal event.
The consistency in age of the first heating step (~11-20 Ma) among these samples
suggests that the steep stair-step pattern may be the result of argon loss due to M2
overprinting, regionally constrained to 12-30 Ma (Altherr et al., 1982; Avigad et al.,
1992). However, most of the initial steps have low radiogenic argon content (<50%) that
precludes any useful interpretation of these increments.
A second age limit coincides with both peak ages of segments in the hump-shaped
portion of the spectrum and peak ages of the final heating steps, ranging from 32 to 41
Ma with an average value of 36 Ma. The third age limit corresponds to the low value
segments at the upper end of the hump-shaped portion of the spectra and includes ages
from 24-33 Ma with an average of 27 Ma. 40Ar/39Ar dating of white micas on Naxos
(Wijbrans and McDougall, 1986) revealed complex spectra with a similar pattern that
were interpreted to be the product of multiple mineral phases within the same mica grain showing varying degrees of daughter retention. The gradient of apparent age in the
complex spectra from Serifos closely mimics the observed K/Ca spectra of each sample,
an indicator that the change in age within the samples is related to changes in the mineral
phase. Given the observed phase intergrowth within some of the mica grains, differential
release of argon due to the presence of multiple diffusion domains within the micas likely
explains the age gradient of Oligocene 40Ar/39Ar age spectra on Serifos. The two age
limits may be the result of partial recrystallization during a Late Oligocene
thermal/deformation event (represented by the ~27 Ma age limit) of an existing white 52 mica phase, formed in the Late Eocene (represented by the ~36 Ma age limit), producing two separate phases of white mica growth within the analyzed samples. Despite complexity and a lack of well-defined plateau ages, the combination of integrated ages and observed age limits are remarkably consistent among these spectra. Concordance with regional trends, including initial exhumation of the CBU following M1 conditions at
40-35 Ma (e.g., Altherr et al., 1979; Maluski et al., 1987; Jolivet et al., 2003) and M2 overprinting at ~30-12 Ma (Altherr et al., 1982; Avigad et al., 1992), suggests that the ages may be reasonable estimates of the timing of thermal or deformation events.
Samples from within the mylonitic orthogneiss unit on Serifos record similar flat age spectra with relatively well-defined plateau ages of 8-9 Ma and are thus interpreted to have cooled through the 350-500°C closure temperature of white mica during Late
Miocene exhumation and to have subsequently remained thermally undisturbed. A combination of recent thermochronological data provide further evidence for rapid low- temperature cooling on Serifos at that time: single grain zircon TIMS U-Pb ages show crystallization of the granodiorite body and associated dike generations between 11.6 and
9.5 Ma, and cooling ages from Rb-Sr on biotite between 8.5 and 7.7 Ma (Iglseder et al.,
2009). In addition, zircon and apatite fission track ages of 11 and 10 Ma, respectively
(Brichau et al., 2008), and zircon and apatite (U-Th)/He ages between 8 and 5 Ma
(Stöckli et al., 2009) constrain island-wide, low-temperature cooling on Serifos.
There is notable consistency in the results of 40Ar/39Ar thermochronology presented here; the data are summarized in Table 3. All 40Ar/39Ar age spectra exhibit one of two distinct patterns: either simple and flat, or disturbed with a significant apparent age gradient. Moreover, spectra of each type show both a strikingly similar pattern of 53
apparent age across the spectrum as well as concordant integrated or plateau ages, thus
forming two distinct populations. Significantly, the two age populations are derived from
locations that are geographically distinct from one another (Figure 6); flat spectra of Late
Miocene age are limited to the central and southeast regions of the island within the mylonitic orthogniess, whereas Oligocene age spectra exhibiting an apparent age gradient are seen only for samples from the northern and western portions of the island within
CBU-equivalent rocks. The differences in age are sharp and coincide with the contact between the highly sheared zone of mylonitic orthogneiss and the adjacent, structurally higher but lower metamorphic grade CBU-equivalent units as well as proximity to the pluton. This pattern of 40Ar/39Ar age distribution seen on Serifos is also seen within other
recognized MCCs of the Cycladic islands (e.g., Jolivet et al., 2004). The age distribution
on Serifos, marked by a sharp gradient in age coincident with the intrusive margin and
separating 34-28 Ma Early Oligocene ages from 9-8 Ma Late Miocene ages, is attributed
to cooling and exhumation associated with progressive Oligo-Miocene extension and
Late Miocene development of a MCC along a mid-crustal shear zone (detachment). Table 3. Summary of sample analyses. Total gas Sample UTM coordinates Rock type Mineral Type age (Ma) Plateau age MSWD % 39 Ar SERF 39 0281042 4118310 schist phengite 30.01 - - - SERF 53 0280402 4112509 quartzite phengite 8.84 8.93 ± 0.05 8.28 94.7 SERF 77 0278564 4118014 gneiss phengite 29.38 - - - SERF 107 0279232 4111096 mylonite phengite 9.17 9.12 ± 0.23* 22 71.1 SERF 125 0273745 4119518 meta-arenite phengite 31.45 - - - SERF 306 0279495 4120458 meta-arentite phengite 32.64 - - - SERF 312 0279456 4120004 schist phengite 32.71 - - - SERF 341 0278309 4119059 calc-silc mylonite phengite 32.92 - - - SERF 358 0275077 4115227 schist phengite 8.86 8.89 ± 0.11 5.28 97.2 SERF 415 0277254 4120106 schist phengite 34.07 - - - SERF 428 0279584 4111346 schist phengite 15.01 12.90 ± 0.42* 30.00 54.6 SERF 564 0270970 4112038 schist phengite 35.96 38.51 ± 0.44* 13.00 57.5 SERF 567 0273534 4113237 mylonite phengite 8.31 8.45 ± 0.04 8.44 85.7 SERF 571 0273846 4114398 mylonite phengite 8.40 8.45 ± 0.02 1.34 97.1 07SE 21 0272021 4115481 gneiss phengite 32.22 36.30 ± 4.7* 17.00 52.3 07SE 24 0273784 4116990 gneiss phengite 32.24 - - - 07SE 28 0276984 4118606 orthogneiss phengite 28.22 - - - 07SE 35 0275446 4117903 quartz schist phengite 31.14 35.30 ± 4.2* 19.00 50.7 * preferred age 54 55
272000 276000 280000
SERF306 - 33 Ma (f) Wt. mica 40Ar/39Ar: SERF415 - 34 Ma (j) 000214 000214 28-34 Ma SERF312 - 33 Ma (g) SERF125 - 32 Ma (e) 8-9 Ma SERF341 - 33 Ma (h) SERF39 - 30 Ma (a) 07SE28 - 28 Ma (q) N SERF77 - 29 Ma (c) 07SE35 - 31 Ma (r)
07SE24 - 32 Ma (p) SERF358 - 8.8 Ma (i) 006 006 10 10 40 40 07SE21 - 32 Ma (o)
SERF571 - 8.5 Ma (n)
SERF567 - 8.4 Ma (m)
SERF53 - 8.9 Ma (b)
SERF564 - 36 Ma (l) 002 002 11 11 1 SERF428 - 15 Ma (k) 1 40 40
SERF107 - 9.3 Ma (d)
Meters
272000 276000 280000
Figure 6. Distribution of white mica 40Ar/39Ar ages across Serifos. Note the geographic distinction between the two age populations and coincidence of the age-break with the intrusive margin. (Letters in parentheses refer to corresponding spectra in Figure 5.) 56
5.2 Thermal & Exhumation History of the Serifos MCC
Based on its closure temperature, the white mica 40Ar/39Ar thermochronometer records cooling through 350°C, a nominal temperature for the brittle-ductile transition zone in quartzofeldspathic rocks. Therefore, cooling ages recorded are representative of the passage of rock through this mid-crustal temperature zone. The 40Ar/39Ar ages presented here record two distinct periods of cooling through mid-crustal levels for the island of Serifos: one event at 34-28 Ma, and another at 9-8 Ma. The rocks in which these two cooling events are recorded are also structurally distinct from one another, separated by an intensely mylonitized, ductile shear zone recording SSW-directed kinematics.
Through integration of these results with that of high-temperature U-Pb and low- temperature (U-Th)/He thermochronometers a temperature-time curve for the evolution of the Serifos MCC was constructed (Figure 7). The figure reveals two distinct periods of rapid cooling that coincide with the two primary metamorphic events of the Cycladic region. Initial exhumation (decompression) following the Eocene subduction-related high-pressure metamorphism (M1) documented for much of the CBU is regionally constrained to 40-35 Ma (e.g., Altherr et al., 1979; Maluski et al., 1987; Jolivet et al.,
2003). A preliminary crystallization age of 40 Ma for the mylonitic orthogneiss (Vogel et al., 2007, 2009; Senkowski et al., 2009) is consistent with this timing, suggesting magmatism possibly coeval with exhumation. White mica 40Ar/39Ar ages from the CBU- equivalent units record cooling of 34-28 Ma, broadly coeval with regional exhumation trends of the CBU. 57
1000° M1 zircon U-Pb
g
n M2 i a f )C( erutarepmeT )C(
o
o
r
n
u
500° M22b U B
C
a-type dome wt. mica Ar-Ar
msitam
msita
b-type dome
mgam
gam apatite&zircon 0° (U-Th)/He 02550 Age (Ma)
Figure 7. Schematic T-t diagram illustrating the thermal history of the Serifos MCC within the western Aegean regional context as constrained by white mica 40Ar/39Ar cool- ing ages in combination with various additional geochronometers (Altherr et al., 1979; Maluski et al., 1987; Hejl et al., 2002; Brichau et al., 2008; Iglseder et al., 2009; Stockli et al., 2009). On Serifos, crystallization of the mylonitic orthogneiss at 37 Ma (zircon U-Pb: Vogel et al., 2007, 2009; Senkowski et al., 2009) and mid-crustal cooling of CBU units at 35-30 Ma (wt. mica Ar-Ar) is consistent with initial exhumation (decompression) of the CBU following M1 HP conditions, regionally constrained to 40-35 Ma. Similar to other Aegean a-type domes, mid-Miocene heating and magmatism on Serifos is succeeded by rapid cooling through mid-crustal levels as illustrated by wt.mica 40Ar/39Ar ages of 9-8 Ma (M2a). In contrast, regional b-type domes illustrate ductile-brittle deformation and cool- ing at 19-17 Ma (M2b). 58
For Aegean a-type domes and on Serifos, mid-Miocene heating and magmatism is succeeded by rapid cooling through the 350°C isotherm as illustrated by the 9-8 Ma white mica 40Ar/39Ar cooling ages presented here. (U-Th)/He zircon and apatite ages constrain island-wide low-temperature cooling to shallow crustal levels to 8-5 Ma. These results demonstrate the rapid and protracted nature of extension and exhumation within the western Cyclades from the Early Oligocene to the Late Miocene. It is interesting to note the difference in cooling trend between a- and b-type domes within the Cyclades islands and the Aegean in general. Kea and Kithnos, b-type domes of the western
Cyclades, illustrate ductile-brittle deformation and cooling at 20-15 Ma and low-
temperature cooling from ~14-8 Ma (Schneider et al., 2008, 2009; Stöckli et al., 2009),
with no accompanying magmatism. In contrast, Serifos, similar to other a-type domes of
this region, shows slightly later, more rapid cooling from depth at 9-8 Ma to shallow
crust at 8-6 Ma, accompanied by an intrusive magmatic body. 59
6. CONCLUSIONS
New white mica 40Ar/39Ar data constrain the timing of mid-crustal exhumation and
evolution of the Serifos metamorphic core complex (Figure 8). Initial extension of the
CBU shortly following M1 conditions occurs along discontinuous faults in the brittle
crust and continuous shear zones in the ductile crust, leading to rotation of crustal fault blocks. Continued extension leads to continuous faulting of the brittle crust and intrusion of a subduction-related S-type granitoid at ~40 Ma and high-temperature skarn formation associated with metasomatic fluids. The orthogniess pluton intrudes syn-kinematically
where it is intensely mylonitized along with adjacent country rock in a mid-crustal shear
zone, and CBU-equivalent rocks cool by ~30 Ma. As a result of unloading (unroofing)
and a second magmatic episode (intrusion of the I-type grandodiorite) the crust domes
upward. A new low angle detachment forms at structurally higher levels and
accommodates rapid mid-crustal exhumation and cooling by 8 Ma. Continued extension
and doming of the detachment result in full exhumation of the MCC and island-wide low-
temperature cooling.
The thermochronometric constraints presented here are interpreted as representing
two episodes of exhumation and rapid cooling, commencing in the Late Eocene to Early
Oligocene with extrusion of the Cycladic Blueschist Unit-equivalent rocks and
culminating in the Late Miocene with development of a metamorphic core complex. The
concurrence of crystallization and cooling ages for the intrusive bodies suggests that both
episodes of exhumation were coeval with magmatic activity. These results indicate rapid
multistage exhumation of the Western Cyclades during south-directed kinematics and 60 illustrate the notably protracted nature of extensional deformation in this region since at least the Early Oligocene. 61
NNE SSW AB
37-30 Ma
CD
11-8 Ma ~8-5 Ma Figure 8. Conceptual model for the evolution of the Serifos metamorphic core complex. (modified after Grasemann & Petrakakis, 2007) A) Initial CBU extension along discon- tinuous faults in the brittle crust and continuous shear zones in the ductile crust leading to rotation of fault blocks. The timing of this is shortly following M1 conditions. B) Continu- ous faulting of the brittle crust and synkinematic intrusion of the ca. 37 Ma orthogneiss unit plus HT skarn formation culminating in cooling of CBU-equivelant rocks through mid- crustal levels by 30 Ma. C) As a result of unloading (unroofing) and a second magmatic episode (intrusion of the granodiorite) the crust domes upward. A new low angle detach- ment is formed at higher structural levels and accomodates rapid mid-crustal exhumation and cooling. D) Low-grade deformation of the granodiorite and its country rocks. Further doming of the detachment due to continuous exhumation of the metamorphic core com- plex; results in island-wide low-temperature cooling (constrained by U-Th/He). 62
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APPENDIX: REGRESSION OF EMP DATA
• Raw EMP data was initially sorted using the criteria of Guidotti & Sassi (1998). Points were discarded if they did not meet the following criteria:
1. Total oxide > 90% 2. Si content > 6.00 a.p.f.u. 3. Total (Na+Ca+K) > 1.60 a.p.f.u. 4. Ca < 0.1 a.p.f.u. 5. Si content < 60% oxide (raw EMP data and all criteria above based on 22 oxygen)
[12] [6] [6] [4] • Using general white mica formula: A1 M10-1 M22 T4 O10 (OH, F, Cl)2, calculated formula units in a.p.f.u., based on 12 oxygen or 22 positive charges, for each analyzed point of each sample.
• Used values of the following wt% oxides from raw EMP data for input in formula unit calculations: SiO2, MgO, Na2O, Al2O3, K2O, CaO, TiO2, FeO,, MnO, Cr2O3
• Calculated formula units of each element above using a white mica formula calculation spreadsheet based on the following steps:
1. Calculate the Mol Number by dividing the weight percentage of each oxide by the formula weight of that oxide: Mol Number = (wt% Ox)/(Mol wt Oxide)
2. Calculate the Oxygen Number (X) by multiplying the Mol Number of each oxide by the number of oxygen in the wt% oxide formula: Oxygen Number (X) = (Mol Number) x (No of O)
3. Calculate the Normalized Oxygen Number (Z) by multiplying the Oxygen Number (X) of each oxide by a normalization constant (equal to the number of oxygen per formula unit divided by the sum of the Oxygen Numbers): Normalized Oxygen Number (Z) = (X) x (F); F = (No of O per FU) / (Y) = 12 / (Y); Y = Sum of Oxygen Numbers from step 2 above.
4. Calculate the Formula Unit (FU) by multiplying the Normalized Oxygen Number (Z) of each oxide by the number of cations per oxygen in the oxide formula: Formula Unit (FU) = (Z) x (# of cations per O) / (No of O)
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[12] [6] [6] [4] VI • Given the formula A1 M10-1 M22 T4 O10 (OH, F, Cl)2, calculated Al as follows: T = 4 = Si + AlIV FU of Al = Altotal VI IV Al = Altotal - Al = Altotal - 4 + (FU of Si)
• Determined Li2O (not included in raw EMP data) based on positive correlation between fluorine and lithium for dioctahedral micas (Tischendorf et al., 1997): 1.326 Formula: Li2O = 0.3935 F Range of validity (wt%): F = 0.01 to 8
• Calculated Li in a.p.f.u. following the formula unit calculation procedure above.
• White mica samples were classified according to Tischendorf et al. (2004): each analyzed point of each sample was plotted on an mgli-feal diagram as [Mg-Li] (x-axis) VI vs. [Fetotal+Mn+Ti- Al ] (y-axis) in units of a.p.f.u. based on 12 oxygen or 22 positive charges.
• A representative point was taken from each plotted sample set and re-plotted on a new mgli-feal diagram to maximize the clarity of the results.