Metamorphism disturbs the Re-Os signatures of platinum-group minerals in ophiolite chromitites

Abstract

The Os-isotope compositions of platinum-group minerals (PGMs) in ophiolite chromitites are commonly regarded as resistant to fluid-related processes, and have been used to track the evolution of Earth’s convecting mantle. However, we have found significant differences in 187Os/188Os between primary and secondary PGMs from metamorphosed ophiolite chromitites of the Dobromirtsi Ultramafic Massif, in the Central Rhodope Metamorphic Core Complex of southeastern Bulgaria. Primary (magmatic) PGMs hosted in unaltered chromite cores have 187Os/188Os from 0.1231 to 0.1270, and 187Re/188Os ≤ 0.002. T MA and T RD model ages, calculated relative to the Enstatite Chondrite Reservoir, cluster around three main peaks: ca. 0.3, 0.4, and 0.6 Ga. Secondary PGMs, produced by alteration of magmatic PGMs, have a wider range of variation (187Os/188Os = 0.1124–0.1398, 187Re/188Os ≤ 0.024); these grains yield T MA and T RD model ages from –1.7 Ga up to 2.2 Ga. The larger range in 187Os/188Os in the secondary PGMs is interpreted as due to reactions between the primary PGMs and infiltrating metamorphic-hydrothermal fluids with a range of Os-isotope compositions. This redistribution of Os in PGMs during metamorphism has significant implications for the interpretation of both whole-rock and in situ Os-isotope data in mantle-derived rocks.