Transfer of Os isotopic signatures from peridotite to chromitite in the subcontinental mantle: Insights from in situ analysis of platinum-group and base-metal minerals (Ojén peridotite massif, southern Spain)

Small (< 5 m) chromitite bodies in the Ojén peridotite massif (southern Spain) were formed when small fractions of volatile-rich melts, residual after kilometric‐scale infiltration through the subcontinental mantle, reacted with Cr-rich pyroxenite enclosed in dunite. The Os isotopic compositions of accessory platinum-group minerals (PGM) and base-metal minerals (BMM, including sulfides and arsenides) in chromitite and the host peridotite have similar values and ranges of heterogeneity. The formation of chromitite, which involved partial melting, melt transport/pooling and melt–rock reactions, did not erase the original 187Os/188Os heterogeneity of the peridotite source. The distribution of Os model ages calculated for chromitite-hosted PGM–BMM mainly clusters at ~ 0.3 Ga; minor peaks extend back to ~ 1.4 Ga. Base-metal sulfides (BMS) hosted in peridotite show a relatively minor age peak at ~ 0.2 Ga and two prominent peaks at ~ 0.6 and 1.2 Ga. Independent geochronological data constrain the formation of chromitites at 21 Ma ago, suggesting that these older model ages reflect the sourcing of Os from volumes of older subcontinental lithospheric mantle that underwent several episodes of melting and metasomatism since the Mesoproterozoic. We interpret the different distributions of 187Os/188Os and model ages in chromitite and peridotite as due to the combined control that mantle source heterogeneity, melting and melt/rock interaction processes exert on the Os-isotope composition of the parental melts of chromitite.

► Chromitites are hosted in subcontinental lithospheric mantle.
► Chromitites are precipitated from very residual small-volume melts.
► Peridotite-hosted BMS and chromitite-hosted PGM have similar 187Os/188Os.
► 187Os/188Os variability is not erased by partial melting or percolation reactions.