Modification of oceanic lithospheric mantle by percolated melts sourced from recycled ancient crust: Evidence from CaOs isotopes of refractory harzburgites

Abstract

Oceanic lithospheric mantle, initially formed beneath the oceanic plate's spreading center, is frequently altered through interaction with mantle-derived mafic melts migrating upward to the crustal regions. Highly refractory harzburgites often retain trace interstitial clinopyroxene and base-metal sulfides (BMS) that precipitate during the interaction with percolating melts. In this study, we present the whole-rock and mineral Ca as well as ReOs isotope compositions of harzburgites from the Moa-Baracoa ophiolite in Cuba, to uncover the melt-peridotite interaction processes and evaluate their impact on the composition of the lithospheric mantle. Thermodynamic modeling indicates that the studied harzburgites have undergone a high degree (20–30 %) of partial melting compared to the depleted mid-ocean ridge basalts mantle, followed by interaction with a very small amount (<2 %) of fore-arc basaltic melts within the lithospheric mantle. The whole-rock δ⁴⁴/⁴⁰Ca values, ranging from 1.06 ‰ to 1.31 ‰, are lower than those calculated for highly refractory harzburgites that have experienced 20–30 % partial melting. This suggests an offset caused by interstitial clinopyroxene with light δ⁴⁴/⁴⁰Ca. Moreover, the elevated ¹⁸⁷Os/¹⁸⁸Os ratios (0.1271–0.1401), combined with low Re concentrations (0.03–0.64) and ¹⁸⁷Re/¹⁸⁸Os ratios (0.08–1.40), imply the addition of radiogenic Os derived either from the subducted slab or ancient crustal components recycled in the mantle. The negative trend of increasing ¹⁸⁷Os/¹⁸⁸Os ratios with decreasing δ⁴⁴/⁴⁰Ca values further validates the co-precipitation of sulfides and clinopyroxene in the Moa-Baracoa harzburgite. Notably, similar mineral assemblages and whole-rock radiogenic Os compositions are observed in certain abyssal peridotites and ophiolitic peridotites. Binary mixing calculations demonstrate that such ReOs characteristics can only occur when rare BMS precipitate from percolating mafic melts with small fractions originating from recycled ancient crust.