The implications of muscovite sub-spectra in phengitic white mica on the theory and practice of argon geochronology

Forster, Marnie1, Lister, Prof. Gordon1

1Research School of Earth Sciences, Australian National University, Canberra, Australia

This study illustrates a new method for the quantitative determination of the timing of movement in ductile shear bands formed in mylonites, or in strongly stretched metamorphic tectonites. The method is of particular use where phengitic white mica is involved, since interlaying in this mineral is usually so fine as to preclude the application of laser methods. In any case, laser methods as they are currently applied, do not have the capability of extracting exact and detailed age-temperature spectra. Laser methods also fail to achieve the multitudinous steps of the age spectrum evident from our high-definition UHV diffusion experiments. Laser methods also lose all information in terms of the low-volume early release of argon that is essential for the recognition of sub-spectra. Computer modelling and simulation shows that such detail in the age spectrum is essential in terms of being able to accurately infer the timing and duration of metamorphic events,

Here we show that high-definition ultra-high-vacuum (UHV) 39Ar diffusion experiments using phengitic white mica are routinely able to extract muscovite sub-spectra in the first 10-30% of 39Ar gas release during 40Ar/39Ar geochronology. A critical factor is that the recognition of muscovite sub-spectra requires Arrhenius data in order to recognise the steps dominated by release of 39Ar from muscovite. In turn this requires precise measurement of temperature during each heating step. The muscovite sub-spectrum is distinct and separate to the main spectrum, which is itself dominated by mixing of gas released from phengite as well as muscovite. The muscovite sub-spectra allow consistent estimates of the timing of the formation of microstructural shear bands in various mylonites, as well as allowing quantitative estimates of temperature variation with time during the tectonic history of shear zones.

Our new data reveals hitherto unsuspected variation in the timing of exhumation of individual slices of the eclogite-blueschist belt, caused by Eocene and Miocene detachment-related shear zones. With excellent outcrop, the eclogite-blueschist belt exposed in the Cycladic archipelago in the Aegean Sea, Greece, offers a spectacular natural laboratory in which to decipher the structural geology of a highly extended orogenic belt and to ascertain the history of the different fabrics and microstructures that can be observed. Using phengitic white mica we demonstrate a robust correlation of age with microstructure, once again dispelling the myth that 40Ar/39Ar geochronology using this mineral, produces cooling ages alone. Previous work in the Cycladic eclogite-blueschist belt has incorrectly assumed that the diffusion parameters for phengitic white mica were the same as for muscovite. Arrhenius data suggest this is not the case, and that phengitic white mica is considerably more retentive of argon than muscovite. Previous workers have also erred in dismissing microstructural variation in age as an artefact, supposedly as the result of the incorporation of excess argon. This has led to inconsistencies in interpretation, because phengite is able to retain argon at temperatures that exceed those estimated using metamorphic mineral parageneses.

The argon system has been treated as a thermochronometer. However, we demonstrate a robust correlation between microstructure and age, down to the detail present in complex tectonic sequence diagrams produced during fabric and microstructural analysis of individual thin-sections. This points to new strategies being required in terms of the theory and practice of argon geochronology.


Dr Marnie Forster is following in the footsteps of Prof Ian McDougall running the ANU Argon Laboratory  with the assistance of Davood Vasegh and Agnes the argon robot.

High resolution 40Ar/39Ar geochronology in continental margin settings – the Aegean plate margin as a natural laboratory for subduction processes.

Wijbrans, Jan1, Uunk, Bertrum1, de Paz Alvarez,Manuel1, Huybens, Rosanne1, Brouwer, Fraukje1

1Department of Earth Sciences, Vrije Universiteit Amsterdam, the Netherlands

Time scales of tectonic and metamorphic processes in the greenschist-blueschist domain are essential to better understand the dynamics of accretionary wedges. However, such time scales are difficult to obtain because the number of geochronometers available to us is limited. For these geochronometers only partial setting, or resetting, of the isotopic clocks can be expected at temperatures reached in the blueschist and greenschist facies, whilst shear zone deformation may not cause full recrystallization. New, or more refined geochronological approaches, can thus shed additional light on the processes in accretionary wedges.

The Aegean subduction complex has emerged over the years as a key natural laboratory to study subduction processes. Our focus in recent years has been on the subduction related processes as experienced by the rocks of Syros and Sifnos islands. Here, we report (1) our efforts to further refine dating approaches:  40Ar/39Ar by dating of suites of single phengite crystals on an outcrop to section scale, and (2) our work exploring new approaches of dating of minerals free of lattice-bound potassium such as garnet, amphiboles and epidotes by dating the signal derived from the fluid inclusions by stepwise crushing.

Dating complete 100+ m sections by single grain phengite dating has the added benefit that all available lithologies in the section contribute to a histogram or PD- plot. Where previously a multigrain average from a single rock specimen was obtained, now is revealed that different units record an age range often as much as 10 Myrs wide, the oldest ages recording crystallization and the younger resetting following crystallization. Dating fluid inclusions by stepwise crushing allows the identification of multiple fluid reservoirs that contribute to the obtained signal sequentially. Typically, the first crushing steps reveal a reservoir that contains large amounts of excess 40Ar, whereas following exhaustion of this reservoir other distinctly different sources of argon are revealed. Age signals thus obtained are interpreted as documenting periods of increased fluid mobility during and following mineral crystallisation.

Combination of these two approaches provides new insights into the tectonic processes during deep subduction and subsequent exhumation to shallower depths.


Jan Wijbrans is currently professor of Argon Geochronometry at the Vrije Universiteit Amsterdam. Jan was an ANU postgraduate research scholar at the Research School of Earth Sciences, ANU from 1981 until 1985, when he worked under the guidance of prof. Ian McDougall on timescales of metamorphic processes using the 40Ar/39Ar method.

40Ar/39Ar geochronology of syn-kinematic phengite reveals the tectonic history of underthrusted European crust (W Alps): a synthesis

Rolland, Yann1

1EDYTEM, Université de Savoie Mont Blanc, La Bourget du Lac, France

The major problem of 40Ar/39Ar geochronology applied to deformation is often to find sufficiently large crystals that grew in a clear relationship to the deformation context, and that have been preserved in the following tectonic history.

The dating of white mica syn-kinematic fibres (phengite) in complement to their PT conditions (obtained by combined mineralogical mapping for instance) allows the dating of deformation stages at a depth constrained by PT calculations and along certain geothermal conditions. Kinematics of related shear zones provide the polarity of tectonic motions. The integration of such data into the tectonic framework of a geodynamic zone allow deciphering along-belt variations, which relate to strain propagation and changes in the stress field through time and space.

The External Crystalline Massifs (ECM) of the Alps provide a nice case example of a crustal domain in which this technique was successfully applied to the dating of brittle-ductile motions at mid-crustal levels (10-15 km). The Western Alps are a curved mountain belt that accommodated tectonic motions related to subduction dynamics in the Mediterranean domain after a brief period of continent-continent collision of the Apulian microblock with Eurasia. Subsequently, the timing and the along-belt variations of deformations has been progressive through time.

In this presentation, I propose a synthesis of analyses conducted at the External Alps scale to investigate the timing of shear zone deformation since the onset of Eocene-Oligocene collision. The data include 40Ar/39Ar stepwise dating of synkinematic phengites, the associated shear zone kinematics and thermobarometric constraints on the depth and temperature of deformation. The advent of new dating approaches, based on U-Pb dating of syn-kinematic allanite, of hydrothermal vein monazite, and more recently, of calcite from veins and fault gouges, do confirm the timing of deformation suggested by the obtained 40Ar/39Ar ages on micas.

These dating constraints are used to discuss the timing of deformation of underthrusted European crust related to changes in tectonic modes during the Alps’ tectonic history. These changes are related, at the larger scale, to the evolution of the Mediterranean domain, i.e., to the rapid underthrusting of the external part of European plate, dated in the ECM at 35-32 Ma, followed by the onset a mainly dextral strike-slip context. This strike-slip context is controlled by the anticlockwise rotation of Apulia, notably driven by the Apulian roll-back.


Yann has worked on many orogen examples, starting from his PhD work on the Himalaya-Karakoram, to the Alps, Caucasus, Tien Shan… He develops approaches coupling geochronology and geodynamics. He has used the Ar/Ar chronometer applied to dating shear zone activity since his post-doc at the ANU in 2001.

Bitterroot and Anaconda Core Complexes: Cretaceous Ductile Flow and Eocene Detachment Faulting in the Northern U.S. Rocky Mountains Defined by Ar/Ar Thermochronology

Foster, David1

1Department of Geological Sciences, University of Florida, Gainesville, United States

The Bitterroot and Anaconda metamorphic core complexes of western Montana and central Idaho, U.S.A. were exhumed by Eocene extensional detachment faulting between about 53 and 38 Ma.  The rocks in the lower plates of these core complexes include highly sheared Proterozoic to Palaeozoic metasediments, Cretaceous granitoids, and Eocene felsic plutonic rocks. Early exhumation of the core rocks occurred after crust of in the Montana hinterland was dramatically thickened (forming the “Montanaplano”) between about 130 and 80 Ma. By Late Cretaceous time the middle crust of the orogenic pile was plastically deforming, resulting in large-scale nappes and shear zones. Voluminous intermediate to felsic igneous rocks intruded the shear zones and formed sheet-like plutons at thrust ramps.  Plastic flow led to ductile thinning of the middle crust coincident with out-wedging in the Montana fold and thrust belt. Rocks exposed in these two core complexes experienced about 5-10 km of exhumation in the Late Cretaceous on the basis of metamorphic assemblages, reconstruction of sections, and thermochronology of upper plate rocks. Eocene extension began in the Northern Rockies about 53 Ma and resulted in linked extensional detachments and magmatism in both core complexes. The Bitterroot complex underwent as much as 15 km of additional exhumation along a ductile to brittle detachment system that initiated with amphibolite facies mylonite. The Anaconda complex underwent about 10-12 km in the deepest part and records green-schist facies mylonite overprinted by transitional brittle-ductile fabrics, and brittle deformation.  Ar/Ar mica cooling ages from both detachments decrease from west to east and constrain the rates of fault slip and unroofing. Exhumation of the middle crustal rocks was at least a two-stage process with significant Cretaceous-Paleogene thinning by ductile flow, followed by unroofing beneath the east-rooted detachment systems. Thermochronology data indicate that both detachment systems developed as composite structures with shallower crustal levels and less exhumation in the west rooting to about 10-km deeper to the east. The hanging wall of the Anaconda complex includes the less-deformed Cretaceous Boulder batholith and related volcanic rocks, while hanging wall rocks of the Bitterroot complex, in the Sapphire Range, record Cretaceous exhumation and more limited Eocene unroofing.


Professor David A. Foster is Chair of the Department of Geological Sciences at the University of Florida.  His research is focussed on applications of geochronology and thermochronology to Precambrian to Recent tectonics.

Timescale of events around the Cretaceous-Paleogene Boundary: Links between the Chicxulub impact, Deccan volcanism, and the Cretaceous-Paleogene mass extinction

Sprain, Courtney J.1, Renne, Paul R.2,3, Clemens, William A.3, Wilson, Gregory P.4, Self, Steve3, Vanderkluysen, Loyc5, Pande, Kanchan6, Fendley, Isabel7, and Mittal, Tushar8 

1University of Florida, Gainesville, United States, 2Berkeley Geochronology Center, Berkeley, USA, 3University of California-Berkeley, Berkeley, USA, 4University of Washington, Seattle, USA, 5Drexel University, Philadelphia, USA, 6Indian Institute of Technology, Mumbai, India, 7University of Oxford, Oxford, United Kingdom, 8Massachusetts Institute of Technology, Cambridge, USA

The Cretaceous-Paleogene boundary (KPB) mass extinction is one of the most important biotic turnover events in Earth history. This event is important to study for several reasons, the most relevant being its implications on our understanding of the effects of abrupt climate change. Although the temporal coincidence between the Chicxulub crater and the KPB has strongly implicated the impact as the main player in the mass extinction, the eruption of the Deccan Traps (DT) cannot be dismissed as a possible contributor. The timing of DT eruptions spans the KPB and, furthermore, the onset of DT volcanism roughly coincides with Late Cretaceous records of environmental change. Both the Chicxulub impact and DT volcanism have similar environmental forcing mechanisms, albeit acting on different timescales. Until recently, insufficient geochronology has made it difficult to tease apart effects from either agent. 

To better understand the effects of both the Chicxulub impact and the DT in the KPB crises, we developed a high-precision chronologic framework that outlines the temporal sequence of biotic and climatic changes, and proposed perturbations, around the KPB using 40Ar/39Ar geochronology and paleomagnetism. This work was primarily conducted in two areas: the Hell Creek region of NE Montana, USA and the Deccan Traps, India. The Hell Creek region is one of the best-studied terrestrial KPB sites in the world. We developed a high-precision chronostratigraphic framework for fluvial sediments within the Hell Creek, using 40Ar/39Ar dating, magnetostratigraphy, and chemical fingerprinting. This work constrained the timing of terrestrial faunal decline and recovery while calibrating North American Land Mammal Ages biostratigraphy. The coupling of our magnetostratigraphic sections and high-precision 40Ar/39Ar ages further allowed for calibration of the circum-KPB polarity chron (C29r) at unprecedented precision, enabling correlation of our record to other KPB records around the globe. To better understand the role of the DT in the KPB extinction, we developed high precision 40Ar/39Ar ages for >20 lavas ranging the entire DT stratigraphy.

Tying all of this work together, we are able to determine: 1) the decline in terrestrial faunas began between 400 ka and 150 ka pre-KPB, 2) terrestrial disaster faunas are constrained to the first ~25 ka of the Paleogene, and recovery occurred gradually over the next 850 ka, 3) over 90% of the DT volume was erupted in < 1 Ma, with 50-75% emplaced post-KPB, 4) the onset of volcanism is approximately coincident with the onset of pre-KPB warming, but despite this 5) pre-KPB records of climate change coincide temporally with the eruption of the smallest DT phases, suggesting that if the DT are the source of pre-KPB climate change, the release of climate-modifying gases cannot be directly related to eruptive volume as previously assumed. Overall, our new work highlights the close temporal relationship between the Chicxulub impact, Deccan volcanism, and the KPB. But more work is needed, specifically addressing Deccan volatile release and eruption tempo, in order to fully understand the impact of the DT on the Earth system and its role in the mass extinction.


Courtney Sprain is an Assistant Professor at the University of Florida. She received her Ph.D. in Earth and Planetary Science in 2017 from the University of California, Berkeley and her B.S. degrees in Geology and Geophysics from the University of Minnesota in 2012. She specializes in 40Ar/39Ar Geochronology and Paleomagnetism

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