Fractionated highly siderophile element patterns in the forearc mantle peridotites

The New Caledonia ophiolites almost consist solely of mantle peridotites and have been commonly suggested to be generated in a forearc setting. Mantle peridotites in the New Caledonia ophiolites are predominated by refractory harzburgites, which have been subjected to high degrees of partial melting in the presence of slab-derived fluids. A previous Re-Os isotope study on the New Caledonia mantle peridotites has shown that little crustal radiogenic Os was transferred from the slab to the forearc mantle (Liu et al., 2018), suggesting that Os behaves like an immobile element during slab dehydration in the forearc region. Here we present compositions of highly siderophile elements (HSE) of the New Caledonia peridotites. The results indicate that refractory harzburgites from the New Caledonia opholites display non-chondritic, strongly fractionated HSE patterns, which can be classified into two distinct types, i.e.,Group-I and Group-II. The Group-I harzburgites show HSE patterns similar to those of the mantle wedge xenoliths, i.e., they display relative flat patterns from Os to Pt, but are strongly depleted in Pd, yielding suprachondritic Pt/Pd and Pt/Ir ratios. Such patterns can be explained by multi-stage hydrous melting, with the stability of Pt-Fe alloys. In contrast, the Group II harzburgites are strongly depleted in almost all HSEs but Ru, i.e., they display a remarkably positive Ru anomaly. The HSE patterns of the Group-II harzburgites reflect that the sulfides were almost completely removed from the mantle peridotites during hydrous melting. Nevertheless, Ru was plausibly partitioned into chromite and thus preserved in the mantle residues.

Reference

Liu, C.Z., Xu, Y., Wu, F.Y. 2018. Limited recycling of crustal osmium in forearc mantle during slab dehydration. Geology, 46(3), 239-242.