40Ar/39Ar analyses of artificially mixed micas and the treatment of complex age spectra from samples with multiple mica populations

Kula, Joseph; Spell, Terry L.; Zanetti, Kathleen A.

doi:10.1016/j.chemgeo.2010.04.015

Cited by 30 publications

(48 citation statements)

References 41 publications

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“…In this context it is worth noting that Kula et al . () concluded that this method may incorrectly assign geological relevance to age values representing mixtures, due to similar degassing behaviour of the contributing end‐members.…”

Section: Discussionmentioning

confidence: 97%

The geological significance of ⁴⁰Ar/³⁹Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

Bröcker

Baldwin

Arkudas

2013

Journal Metamorphic Geology

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The Attic‐Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic subduction zone processes and exhumation. A prerequisite to develop an improved tectono‐metamorphic understanding of this area is dating of distinct P–T–D stages. To evaluate the geological significance of phengite ages of variably overprinted rocks, 40Ar/39Ar and Rb–Sr analyses were undertaken on transitional blueschist–greenschist and greenschist facies samples from the islands of Syros and Sifnos. White mica geochronology indicates a large age variability (40Ar/39Ar: 41–27 Ma; Rb–Sr: 34–20 Ma). Petrologically similar samples have either experienced greenschist facies overprinting at different times or variations in ages record variable degrees of greenschist facies retrogression and incomplete resetting of isotopic systematics. The 40Ar/39Ar and Rb–Sr data for metamorphic rocks from both islands record only minor, localized evidence for Miocene ages (c. 21 Ma) that are well documented elsewhere in the Cyclades and interpreted to result from retrogression of high‐pressure mineral assemblages during lower pressure metamorphism. Field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and/or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and/or foliation as a result of, possibly deformation‐enhanced, channelized fluid ingress. Published and new 40Ar/39Ar and Rb–Sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist‐to‐greenschist‐ and/or greenschist facies metamorphism. The data do not record the timing of peak HP metamorphism, but may accurately record continuous (partial) resetting of isotopic systematics and/or (re)crystallization of white mica during exhumation and greenschist facies retrogression. The form of 40Ar/39Ar phengite age spectra are complex with the lowest temperature steps yielding Middle to Late Miocene ages. The youngest Rb–Sr ages suggest maximum ages of 20.6 ± 0.8 Ma (Syros) and 22.5 ± 0.6 Ma (Sifnos) for the timing of greenschist facies overprinting. The results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes, particularly when mixing of populations and/or incomplete resetting of isotopic systematics occurs during exhumation. These data capture the full range of isotopic age variations in retrogressed HP rocks documented in previous isotopic studies, and can be interpreted in terms of the geodynamic evolution of the Aegean.

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Section: Discussionmentioning

confidence: 97%

The geological significance of ⁴⁰Ar/³⁹Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

Bröcker

Baldwin

Arkudas

2013

Journal Metamorphic Geology

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“…In particular, the work we present here is of interest because mixtures of phengitic white mica with muscovite have been thought to behave in a manner that is incompatible with the diffusion hypothesis (Wijbrans & McDougall 1986, 1988Kula et al 2010). The apparent age spectra imply that argon is released, first from muscovite, then phengite and then back to muscovite again, producing characteristic 'hump-backs' in the apparent age release spectra (Fig.…”

Section: Introductionmentioning

confidence: 88%

“…Kula et al 2010), thereby ensuring chaotic mixing. It is also possible to obtain confusing data if sufficient 'cleaning' time is not allowed in order that a sample loses 'volatiles' from high-diffusivity pathways before the step-heating experiment is commenced.…”

Section: Introductionmentioning

confidence: 98%

“…Step-heating experiments are routinely conducted for the purposes of 40 Ar/ 39 Ar geochronology, but only rarely has diffusional modelling been undertaken in an effort to extract information from white mica as to the duration of heating events or as to the nature of the pressure -temperature-time path to which the minerals were subjected in the natural environment (Wijbrans & McDougall 1986, 1988Hames & Bowring 1994;Kirschner et al 1996;Lister & Baldwin 1996;Lister & Raouzaios 1996;Baldwin & Lister 1998;Forster & Lister 2004;Mulch et al 2005;Harrison et al 2009;Kula et al 2010;Forster et al 2011;Viete et al 2011). This is because, for the last few decades, the consensus has been that hydrous minerals break down in vacuo (e.g.…”

Section: Introductionmentioning

confidence: 99%

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⁴⁰ Ar/ ³⁹ Ar geochronology and the diffusion of ³⁹ Ar in phengite–muscovite intergrowths during step-heating experiments in vacuo

Forster

Lister

2013

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Step-heating experiments in vacuo are routine when conducting 40 Ar/ 39 Ar geochronology, including for white mica. White mica can break down, due to dehydroxylation and delamination, so experiments involving mica are often conducted in relative haste, and not with the care and precision necessary when intending to apply multi-diffusion-domain theory to model the results. Here we show, however, that carefully managed step-heating experiments appear to allow release of argon through solid-state diffusion processes alone. We analysed phengite-muscovite intergrowths in high-pressure metamorphic rocks exhumed in and beneath extensional ductile shear zones during continental extension. Such materials often yield Arrhenius plots in which there is a distinct steepening of slope mid-way through the step-heating sequence. This steepening appears to correspond with steps in which release of argon from phengite components dominate. We analysed the data using a computer program (eArgon) and numerically simulated mixing of gas released from multiple diffusion domains. The results suggest that diffusion of 39 Ar in phengitic white mica involves radically different diffusion parameters in comparison with muscovite. If these results extrapolate to nature then 40 Ar/ 39 Ar geochronology may allow direct dating of white mica mineral growth during metamorphism.Supplementary material: Data files A, B and C are available at www.geolsoc.org.uk/SUP18619. Data file A C++ computer code used to infer data for an Arrhenius plot, assuming different diffusion geometries. These methods are excerpted from the eArgon computer program used to analyse these data. Data file B Analytical methods and procedures used in the laboratory for 40 Ar/ 39 Ar geochronology performed on the samples reported. Data file C XML formatted data tables for the step-heating experiments reported in this study, in a form that can be read by the eArgon computer program.Step-heating experiments are routinely conducted for the purposes of 40 Ar/ 39 Ar geochronology, but only rarely has diffusional modelling been undertaken in an effort to extract information from white mica as to the duration of heating events or as to the nature of the pressure -temperature-time path to which the minerals were subjected in the natural environment (

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“…The similarity of the 39 Ar-40 Ar ages of muscovite from Variscan late-and post-D 3 granites, independent of their emplacement age, indicate that the Gouveia area was heated above 350°C during the emplacement of the youngest granites and the Asturian and Saalian movements, causing degassing (Kula et al, 2010), with very rapid cooling thereafter. 302.6 ± 6.7 288.7 ± 2.6…”

Section: Muscovitementioning

confidence: 88%

U–Pb and 39Ar/40Ar data constraining the ages of the source, emplacement and recrystallization/cooling events from late- to post-D3 Variscan granites of the Gouveia area, central Portugal

Neiva

Williams

Lima

et al. 2012

Lithos

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40Ar/39Ar analyses of artificially mixed micas and the treatment of complex age spectra from samples with multiple mica populations

Cited by 30 publications

References 41 publications

The geological significance of ⁴⁰Ar/³⁹Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

The geological significance of ⁴⁰Ar/³⁹Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

⁴⁰ Ar/ ³⁹ Ar geochronology and the diffusion of ³⁹ Ar in phengite–muscovite intergrowths during step-heating experiments in vacuo

U–Pb and 39Ar/40Ar data constraining the ages of the source, emplacement and recrystallization/cooling events from late- to post-D3 Variscan granites of the Gouveia area, central Portugal

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40Ar/39Ar analyses of artificially mixed micas and the treatment of complex age spectra from samples with multiple mica populations

Cited by 30 publications

References 41 publications

The geological significance of 40Ar/39Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

The geological significance of 40Ar/39Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

40 Ar/ 39 Ar geochronology and the diffusion of 39 Ar in phengite–muscovite intergrowths during step-heating experiments in vacuo

U–Pb and 39Ar/40Ar data constraining the ages of the source, emplacement and recrystallization/cooling events from late- to post-D3 Variscan granites of the Gouveia area, central Portugal

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The geological significance of ⁴⁰Ar/³⁹Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

The geological significance of ⁴⁰Ar/³⁹Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

⁴⁰ Ar/ ³⁹ Ar geochronology and the diffusion of ³⁹ Ar in phengite–muscovite intergrowths during step-heating experiments in vacuo