PGE and Au Abundances in MORB Glasses from Depleted Mantle Melting and in Alkali Basalts from Metasomatized Mantle Melting.

摘要

MORB glasses from the East Pacific Rise and the Lau Basin are distinguished from alkali basalts from Germany by low absolute concentrations of all PGEs and an overall unfractionated to moderately fractionated PGE distribution pattern, with a ratio (PPGE/ IPGE)mn of about 1 (to 10). The whole MORB data set indicates a relatively narrow range of PGE contents (Ir 0.01-0.09 ppb, Ru 0.02-0.1 ppb, Rh 0.01-0.03 ppb, Pt 0.05-0.4 ppb, Pd 0.03-0.4 ppb) with Au varying from 0.08 to 10 ppb. In contrast, all alkali basalts display a fractionated PGE-pattern, (PPGE/ IPGE)mn > 1. The absolute contents of all PGEs and Au increase from continental quartz tholeiites, which are similar to the low MORB values, alkali ollvine basalts to olivine nephelinites. The non-metasomatized and on the other side the metasomatized and probably more oxidized source mantle region have two fundamentally contrasting patterns of PGE behaviour and fractionation of the basalts. It is assumed that the MORB glasses were generated by the separation of a sulphur-saturated melt from a non-metasomatized MORB-source peridotite. A residual sulphide phase was retained in the mantle source during partial melting and these sulphides have mainly retained the precious metals. Because sulphide-melt/ silicate-melt partition coefficients for the different PGEs are not significantly different from one another, unfractionated PGE-pattern and low concentrations were consequently determined in the MORB glasses. In some cases, subsequent post-melting segregation and assimilation of chromites and olivines can lead to a progressively PGE fractionation in MORB. In marked contrast to ocean floor tholeiites, oxidizing metasomatic fluids that have conditioned the lithospheric upper mantle for the formation of alkali basaltic magmas have also oxidized and dissolved the PGE-concentrating sulphides in the source mantle. The oxidation of the mantle sulphides by metasomatic processes gave rise to a redistribution between the residual mantle minerals of the PGEs and a change in their geochemical behaviour. On account of their higher solubility in silicate melts PPGEs and Au are favourably transported along with sulphur into the melt, while the IPGEs are retained in the residue probably incorporated into spinels or precipitated as alloys, possibly engulfed by recrystallizing spinels. Therefore mantle metasomatism is one of the most essential processes which indirectly dominate the PGE distribution between magma and peridotitic residue. The sulphide oxidizing and dissolving character of the mantle metasomatism is the triggering factor, ultimately responsible for inter-element fractionation and (PPGE/ IPGE)mn > I in products of e. g. the continental intra-plate volcanism of the Hessian Depression. The influence of the degree of partial melting on the PGE distribution is of minor importance, which is demonstrated by the fact that the absolute concentrations of all precious metals in the investigated set of basalts decrease with increasing degree of melting.