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

Abstract White mica commonly yields younger 40 Ar/ 39 Ar dates than 87 Rb– 87 Sr isochron dates in greenschist to amphibolite facies rocks, which are often interpreted as 40 Ar/ 39 Ar cooling dates. However, recent petrochronological studies showed that younger 40 Ar/ 39 Ar 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 40 Ar-loss and 87 Sr/ 86 Sr-(re)equilibration. We have applied double-dating mapping by in-situ 87 Rb– 87 Sr and 40 Ar/ 39 Ar geochronology, quantitative chemical mapping, and δ 1 ⁸O 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. 87 Rb– 87 Sr 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, 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar dates in white mica, while having a minor effect on the 87 Rb– 87 Sr system, rendering cooling-age interpretations ambiguous when they lack petrological control.