Comment on “Systematic variations of argon diffusion in feldspars and implications for thermochronometry” by Cassata and Renne

Lovera, Oscar M.; Harrison, T. Mark; Boehnke, P.

doi:10.1016/j.gca.2014.11.016

Cited by 7 publications

(4 citation statements)

References 18 publications

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“…Complications ensue if the "dominant" activation energy varies between one part of an Arrhenius plot to another; e.g. see Cassata and Renne (2013) and the subsequent discussion by Lovera et al (2015), but this can be dealt with by local application of the FAP (e.g. as in Forster et al, 2015).…”

Section: Comparison Of Different Arrhenius Data Used To Estimate Argo...mentioning

confidence: 99%

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Direct dating of overprinting fluid systems in the Martabe epithermal gold deposit using highly retentive alunite

Muston¹,

Forster

Vasegh

et al. 2023

Geochronology

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Abstract. The Martabe gold deposits in Sumatra formed in a shallow crustal epithermal environment associated with intermediate mafic intrusions adjacent to an active right-lateral wrench system. Gas/fluid temperatures reached 200–350 ∘C. The structural geology suggests episodic switches in stress orientations during a Plio-Pleistocene seismotectonic evolution. Different mineralisation events may have been associated with oscillations in this earthquake cycle, so samples containing alunite were collected for 40Ar / 39Ar geochronology to constrain the timing. 39Ar diffusion experiments were performed to constrain variation in argon retentivity. The age spectra were produced by incremental step-heating with heating times chosen so similar percentages of 39Ar gas release occurred during as many steps as possible. This ensured the detail necessary for analysis of the complex morphology of these spectra by applying the method of asymptotes and limits, which enabled recognition of different growth events of alunite in overprinting fluid systems. It was possible to provide estimates as to the frequency of individual events and their duration. The heating schedule also ensured that Arrhenius data populated the inverse temperature axis with sufficient detail to allow modelling. Activation energies were between 370–660 kJ mol−1. Application of Dodson's recursion determined closure temperatures that range from 400–560 ∘C for a cooling rate of 100 ∘C Ma−1. Such estimates are higher than any temperature to be expected in the natural system, giving confidence that the ages represent the timing of growth during periods of active fluid movement and alteration: a hypothesis confirmed by modelling age spectra using the MacArgon program. We conclude that gold in the Purnama pit resulted from overprinting fluid rock interactions during very short mineralisation episodes at ∼2.25 and ∼2.00 Ma.

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Section: Comparison Of Different Arrhenius Data Used To Estimate Argo...mentioning

confidence: 99%

“…Age variation can only reflect processes that occurred in the natural environment. Since there are variations in the age spectrum that correspond to the observed changes in the Arrhenius plot, Cassata et al (2013) cannot be correct (see Lovera et al, 2015).…”

Section: Fundamental Limitations Of Uhv 39 Ar Diffusion Experimentsmentioning

confidence: 99%

Direct dating of overprinting fluid systems in the Martabe epithermal gold deposit using highly retentive alunite

Muston¹,

Forster

Vasegh

et al. 2023

Geochronology

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“…The claim was made that following MDD theory, these arrays require that smaller grain fragments have smaller sized domains, which led those authors to conclude that the non-linear Arrhenius trajectories must be caused by changes in diffusivity. However, as pointed out by Lovera et al, (2015) [99], these arrays can be explained within the framework of MDD theory if the largest domain is assumed to occupy most of the analysed fragment (see also [17]). Finally, Kung and Villa (2021) [58] observed that the rate of Ar release from a cleavage fragment of orthoclase from Itrongay, Madagascar, varied during laboratory degassing at a constant temperature of 888 ± 2 • C (Figure 8A), which they correlated with heating-induced changes in vibrational modes documented by Raman spectroscopy.…”

Section: Alkali Feldsparmentioning

confidence: 99%

Thermochronology of Alkali Feldspar and Muscovite at T > 150 °C Using the 40Ar/39Ar Method: A Review

Spikings

Popov

2021

Minerals

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The 40Ar/39Ar method applied to K-feldspars and muscovite has been often used to construct continuous thermal history paths between ~150–600 °C, which are usually applied to structural and tectonic questions in many varied geological settings. However, other authors contest the use of 40Ar/39Ar thermochronology because they argue that the assumptions are rarely valid. Here we review and evaluate the key assumptions, which are that (i) 40Ar is dominantly redistributed in K-feldspars and muscovite by thermally-driven volume diffusion, and (ii) laboratory experiments (high temperatures and short time scales) can accurately recover intrinsic diffusion parameters that apply to geological settings (lower temperatures over longer time scales). Studies do not entirely negate the application of diffusion theory to recover thermal histories, although they reveal the paramount importance of first accounting for fluid interaction and secondary reaction products via a detailed textural study of single crystals. Furthermore, an expanding database of experimental evidence shows that laboratory step-heating can induce structural and textural changes, and thus extreme caution must be made when extrapolating laboratory derived rate loss constants to the geological past. We conclude with a set of recommendations that include minimum sample characterisation prior to degassing, an assessment of mineralogical transformations during degassing and the use of in situ dating.

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“…We recognize that many experimental data sets of diffusion in feldspars and glasses, including those published by Cassata and Renne [2013] and Gombosi et al [2015], exhibit nonlinearity in Arrhenius space above experimental temperatures of ∼ 600-700 ∘ C but below the respective melting temperatures of the phases. While the precise causes are debated, Arrhenius nonlinearity in feldspars may arise from the progressive exhaustion of multiple-diffusion domains, changes in mineral structure induced by laboratory heating, variations in the dominant mechanism(s) of diffusion, or some combination of the three (see, e.g., the discussion and references in Cassata and Renne [2013], Lovera et al [2015], and Cassata and Renne [2015]). For the Apollo 16 glasses and other synthetic polymers, Arrhenius nonlinearity at elevated experimental temperatures has been attributed to changes in the state of polymerization preceding melting that alters the diffusion kinetics [Gombosi et al, 2015, and references therein].…”

Section: A2 Diffusive Propertiesmentioning

confidence: 99%

Diffusive loss of argon in response to melt vein formation in polygenetic impact melt breccias

Mercer

Hodges

2017

JGR Planets

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Many planetary surfaces in the solar system have experienced prolonged bombardment. With each impact, new rocks can be assembled that incorporate freshly generated impact melts with fragments of older rocks. Some breccias can become polygenetic, containing multiple generations of impact melt products, and can potentially provide important insights into the extensive bombardment history of a region. However, the amount of chronological information that can be extracted from such samples depends on how well the mineral isotopic systems of geochronometers can preserve the ages of individual melt generations without being disturbed by younger events. We model the thermal evolution of impact melt veins and the resulting loss of Ar from K‐bearing phases common in impact melt breccias to assess the potential for preserving the 40Ar/39Ar ages of individual melt generations. Our model results demonstrate that millimeter‐scale, clast‐free melt veins cause significant heating of adjacent host rock minerals and can cause detectable Ar loss in contact zones that are generally thinner than, and at most about the same thickness as, the vein width. The incorporation of cold clasts in melt veins reduces the magnitudes of heating and Ar loss in the host rocks, and Ar loss can be virtually undetectable for sufficiently clast‐rich veins. Quantitative evidence of the timing of impacts, as measured with the 40Ar/39Ar method, can be preserved in polygenetic impact melt breccias, particularly for those containing millimeter‐scale bodies of clast‐bearing melt products.

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Comment on “Systematic variations of argon diffusion in feldspars and implications for thermochronometry” by Cassata and Renne

Cited by 7 publications

References 18 publications

Direct dating of overprinting fluid systems in the Martabe epithermal gold deposit using highly retentive alunite

Direct dating of overprinting fluid systems in the Martabe epithermal gold deposit using highly retentive alunite

Thermochronology of Alkali Feldspar and Muscovite at T > 150 °C Using the 40Ar/39Ar Method: A Review

Diffusive loss of argon in response to melt vein formation in polygenetic impact melt breccias

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