Ancient melt percolation in forearc mantle pyroxenites: Evidence from highly siderophile elements and Os isotope ratios

Pyroxenites, comprising only approximately 2 %-10 % by volume of the upper mantle, are among the most important lithological heterogeneity in the mantle. The formation of pyroxenite veins within peridotites is often attributed to migrating melts which provides an important mechanism for crust-mantle interaction. The Re-Os isotope system provides valuable insight into crust-mantle interaction, as there is a significant contrast in Os-187/Os-188 between crustal and mantle rocks. Previous Os isotope studies have shown that the forearc mantle wedge mainly consists of highly heterogeneous ancient peridotites. However, limited Re-Os isotope data of forearc mantle pyroxenites exist. Here, two types of mantle pyroxenites from the New Caledonia forearc ophiolite were selected for a comparative study. Contrasting microtextures and geochemical compositions support that these pyroxenites were formed by different processes. The Massif du Sud pyroxenites, hosted in harzburgites, have a cumulate origin and crystallized from boninitic melts generated by remelting of depleted mantle during subduction initiation, whereas the Tiebaghi pyroxenites within the host lherzolites are the products of melt-peridotite reactions. Both types of pyroxenites display highly fractionated highly siderophile element (HSE) patterns. The Massif du Sud pyroxenites are strongly enriched in palladium group platinum-group elements (PPGE) relative to iridium group platinum-group elements (IPGE) with remarkable positive Pt anomalies, which are similar to those of primitive arc lavas and mantle wedge pyroxenite xenoliths. In contrast, the Tiebaghi pyroxenites have flat IPGE but variable PPGE and Re patterns. Compared to the forearc mantle peridotites, the Tiebaghi pyroxenites have higher Os isotope ratios. Ancient basaltic melt percolation followed by long-term (>1 Ga) radiogenic ingrowth can successfully produce the observed correlations of Os-187/Os-188 with Os or Al2O3 contents. Besides, the oldest Re-depletion model age (T-RD) of similar to 1.35 Ga has been obtained for the host lherzolites. Therefore, the Tiebaghi pyroxenites and their host lherzolites probably represent a stranded old mantle relict that has undergone ancient partial melting and melt percolation events. Our study supports the presence of highly radiogenic mantle domains in the forearc and provides significant constraints on the role of ancient basaltic melt percolation for generating mantle heterogeneity.