# Impact of intragrain chemical heterogeneities on 40Ar/39Ar and 87Rb–87Sr white mica dates

## 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.

## Key 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 phases along cleavage planes and grain boundaries. Re-equilibration involved metasomatic exchange of incompatible elements (Ar, Na, Rb, Sr, Cs, Ba, B, La and Li) during fluid-assisted alteration in a meta-tuff and the Harney Peak Granite. 87Rb–87Sr white mica isochron dates of 1763.24 ± 17.05 Ma and 1677.95 ± 9.99 Ma for the meta-tuff and the Harney Peak Granite, respectively, are a consequence of high-temperature chemical equilibrium. In contrast, 40Ar/39Ar dates of reaction-grown white mica of 1300.36 ± 15.84 Ma and 1284.89 ± 4.87 Ma, record fluid-driven re-equilibration at 364 ± 50 °C (chlorite-thermometry). These results indicate that fluid-assisted processes can reset 40Ar/39Ar dates in white mica, while having a minor effect on the 87Rb–87Sr system, rendering cooling-age interpretations ambiguous when they lack petrological control.

The online version contains supplementary material available at 10.1007/s00410-026-02299-9.