Characteristics of platinum-group elements in basalts from spreading axis of Mariana Trough

Total platinum-group elements ( PGEs) abundances in basalts from the spreading axis of Mariana Trough ranged from 0.418 x 10(9) to 1.022 x 10(-9), and primitive mantle-normalized PGE patterns are of positive slope showing the relative enrichment of PPGE ( platinum, palladium, rhodium) and gold relative to IPGE. Compared with other mantle-originated rocks, these basalts have lower PGE contents and wider ranges of primitive mantle-normalized ratios of palladium content to iridium one, palladium content to platinum one and palladium content to gold one exhibiting relative platinum and iridium depletion. Characteristics of PGE patterns indicated that the studied Mariana Trough basalts originated from low partial melting, and the MORB mantle beneath the spreading center had been contaminated by the arc-island mantle. In the aspect of trace elements, Mariana Trough basalts showed the enrichment of LILE, lead and LREE, indicating that they had been influenced by subduction compositions. All these demonstrated that Mariana Trough basalts are products of partial melting from a mixed mantle (the contamination of MORB mantle by arc-island mantle).