Diffusion vs. fluid alteration in alkali feldspar 40Ar/39Ar thermochronology: Does cross-correlation of log(r/r0) and age spectra validate thermal histories?

Daniil V Popov,Richard A Spikings

doi:10.1016/j.chemgeo.2020.119506

Daniil V Popov, Richard A Spikings

Open Access

https://doi.org/10.1016/j.chemgeo.2020.119506

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Abstract

For six decades geoscientists have been trying to quantitatively understand the nature of radiogenic Ar loss from alkali feldspar. Some researchers suggest that volume diffusion is the dominant mechanism, and they use conventional step-heating 40Ar/39Ar data from alkali feldspar to recover the thermal histories of rocks. They argue that a high degree of correlation between log(r/r0) and 40Ar/39Ar age spectra, which is observed in a number of natural examples, justifies this hypothesis. In contrast, other investigators suggest that fluid-mediated recrystallisation and alteration control the radiogenic Ar redistribution, hence rendering alkali feldspar useless as a thermochronometer. By means of numerical modelling, we found that the latter mechanism as well is able to produce samples with highly correlated log(r/r0) and 40Ar/39Ar age spectra. In addition, we show that apparent thermal histories recovered for altered alkali feldspar crystals by interpreting step-heating 40Ar/39Ar data may be grossly inaccurate, and yet seemingly fit the prevailing understanding of regional geology. Such inaccurate apparent thermal histories can be obtained even from alkali feldspar crystals that underwent volumetrically low degrees of alteration. Therefore, we conclude that conventional step-heating 40Ar/39Ar data are insufficient to support the assumption that radiogenic Ar loss from alkali feldspar occurred solely by volume diffusion and validate the constrained thermal histories, even if upheld by a priori knowledge of regional tectonics. We further suggest that all thermochronological constraints obtained using such data should be supported by detailed petrological characterisation of alkali feldspar.

Highlights

A very wide range of natural phenomena result in temperature changes in rocks, and thermochronological methods are used to investigate them
multi-diffusion domain (MDD) theory has been used in many studies to reconstruct the thermal histories of intrusive and high-grade metamorphic rocks by inversion modelling of step-heating 40Ar/39Ar data obtained from alkali feldspar from these rocks (e.g. Dunlap and Fossen, 1998; Foster and John, 1999; Spikings et al, 2002; Reiners et al, 2004; McLaren et al, 2007; Sanders, 2008; Metcalf et al, 2009; Wong et al, 2010)
An essential consequence of fluid-induced dissolution-reprecipitation of alkali feldspar is that it can remove 40Ar from the affected zones and reset their age to the time of interaction (e.g. Lee et al, 1995; Parsons et al, 1999; Villa, 2006; Chafe et al, 2014; consider Ar partitioning behaviour between minerals and fluids, e.g. Kelley, 2002). How would such non-diffusive loss of 40Ar affect correlations between the log(r/r0) and 40Ar/39Ar age spectra? Can we ignore volumetrically low degrees of fluid-mediated recrystallisation and alteration of alkali feldspar (≤6 vol%; see Lovera et al, 2002) when constraining thermal histories of rocks using MDD theory? Can we find a geologically reasonable scenario of fluid-mediated recrystallisation that would result in an MDD best-fit t-T path that seems reasonable when correlated with independent knowledge of the regional tectonic history?

Summary

Introduction

A very wide range of natural phenomena result in temperature changes in rocks, and thermochronological methods are used to investigate them. The temperature gap between these techniques (~180–350 °C) may be accommodated by interpreting 40Ar/39Ar data acquired from alkali feldspars by step-heating using the multi-diffusion domain (MDD) theory of Lovera et al (1989). The primary aim of this work is to test the hypothesis that volumetrically low degrees of monomineralic alteration of alkali feldspar have little effect on the accuracy of continuous t-T solutions obtained by interpreting step-heating 40Ar/39Ar data derived from it, and that the same 40Ar/39Ar data can be used to validate the accuracy of these t-T solutions. Similar to most thermochronological methods, MDD theory is founded on the assumption that the daughter isotope (40Ar) is redistributed and lost from crystals by volume diffusion. While multiple studies suggest that stepheating 40Ar/39Ar data can be used to obtain sensible Ar diffusion parameters and thermal histories for alkali feldspar crystals While multiple studies suggest that stepheating 40Ar/39Ar data can be used to obtain sensible Ar diffusion parameters and thermal histories for alkali feldspar crystals (e.g. Lovera et al, 1989, 1997, 2002; Dunlap and Fossen, 1998; Foster and John, 1999; Spikings et al, 2002; Reiners et al, 2004; McLaren et al, 2007; Sanders, 2008; Metcalf et al, 2009; Wong et al, 2010), some

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