The effects of melt depletion and metasomatism on highly siderophile and strongly chalcophile elements: S–Se–Te–Re–PGE systematics of peridotite xenoliths from Kilbourne Hole, New Mexico

摘要

The composition of the Earth’s upper mantle is a function of melt depletion and subsequent metasomatism; the latter obscuring many of the key characteristics of the former, and potentially making predictions of Primitive Upper Mantle (PUM) composition problematic. To date, estimates of PUM abundances of highly siderophile element (HSE = platinum group elements (PGE) and Re) and the strongly chalcophile elements Se and Te, have been the subject of less scrutiny than the lithophile elements. Critically, estimates of HSE and strongly chalcophile element abundances in PUM may have been derived by including a large number of metasomatized and refertilized samples whose HSE and chalcophile element abundances may not be representative of melt depletion alone. Unravelling the effects of metasomatism on the S–Se–Te–HSE abundances in peridotite xenoliths from Kilbourne Hole, New Mexico, USA, potentially provides valuable insights into the abundances of HSE and strongly chalcophile element abundances in PUM. Superimposed upon the effects of melt depletion is the addition of metasomatic sulfide in approximately half of the xenoliths from this study, while the remaining half have lost sulfide to a late S-undersaturated melt. Despite these observations, the Kilbourne Hole peridotite xenoliths have HSE systematics that are, in general, indistinguishable from orogenic peridotites and peridotite xenoliths used for determination of PUM HSE abundances. This study represents the first instance where Se-Te-HSE systematics in peridotite xenoliths are scrutinized in detail in order to test their usefulness for PUM estimates. Despite earlier studies attesting to the relative immobility of Se during supergene weathering, low S, Se, Os and Se/Te in peridotite xenoliths suggests that Se may be more mobile than originally thought, and for this reason, peridotite xenoliths may not be suitable for making predictions of the abundance of these elements in PUM. Removal of Se, in turn, lowers the Se/Te in basalt-borne xenolithic peridotites to subchondritic values. This is in contrast to what has been recently reported in kimberlite-borne peridotite xenoliths. Moreover, Te added to melt depleted peridotite in metasomatic sulfide is more difficult to remove in a S-undersaturated melt than the HSE and Se due to the generation of micron-scale tellurides. The effects of these processes in orogenic peridotites and xenoliths, from which PUM abundances have been calculated, require further scrutiny before unequivocal Se-Te-Re-PGE values for PUM can be derived.