Impact of intragrain chemical heterogeneities on 40Ar/39Ar and 87Rb–87Sr white mica dates
Vidar Jakobsson, Richard Spikings, Martin Kutzschbach, Bruno Ribeiro, Willis Hames, Alfons Berger, Tobias Erhardt, Wolfgang Müller
TL;DR
This study shows that fluid-driven chemical changes in white mica can reset 40Ar/39Ar dates, making cooling age interpretations unreliable without petrological context.
Contribution
The paper demonstrates that fluid-assisted re-equilibration in white mica can reset 40Ar/39Ar dates, challenging traditional cooling age interpretations.
Findings
Intragrain chemical disequilibrium is common in white mica due to fluid-assisted alteration.
40Ar/39Ar dates in reaction-grown mica record fluid-driven re-equilibration at ~364°C.
87Rb–87Sr isochron dates reflect high-temperature equilibrium, while 40Ar/39Ar dates are reset by lower-temperature processes.
Abstract
White mica commonly yields younger 40Ar/39Ar dates than 87Rb–87Sr isochron dates in greenschist to amphibolite facies rocks, which are often interpreted as 40Ar/39Ar cooling dates. However, recent petrochronological studies showed that younger 40Ar/39Ar dates can result from chemical re-equilibration. We investigate the frequency and conditions under which white mica chemically (re)equilibrates during an orogenic cycle in common upper crustal rocks and its implications for 40Ar-loss and 87Sr/86Sr-(re)equilibration. We have applied double-dating mapping by in-situ 87Rb–87Sr and 40Ar/39Ar geochronology, quantitative chemical mapping, and δ1⁸O analysis to Mesoproterozoic metasedimentary and igneous rocks from the Black Hills, South Dakota. Intragrain chemical disequilibrium is pervasive in white mica, with primary phenocrystic and porphyroblastic compositions overprinted by reaction-grown…
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Taxonomy
TopicsGeological and Geochemical Analysis · Paleontology and Stratigraphy of Fossils · earthquake and tectonic studies