This paper reports the first concentrations of platinum-group minerals (PGM) and platinum-group elements (PGE), of tens of parts per billion, to be described from the lower oceanic crust exposed in Hess Deep, if not the first concentrations reported from modern-day unsubducted lower oceanic crust. The results indicate that detectable PGE concentrations occur in fast-spreading oceanic crust. The PGE-bearing melt extracted from the mantle provides the potential for PGE to be concentrated in lower oceanic crustal ultramafic rocks, and this study has identified such concentrations in the magmatic plutonic lithologies from Hess Deep. However, PGE concentrations calculated in 100% of sulfide reveal that sulfides in the residual mantle harzburgite at Hess Deep are enriched in PGE relative to lower crustal dunite. Therefore, the harzburgites retain some PGE and remain fertile for the extraction of more PGE during subduction.
Of the high-level gabbros from Site 894 and gabbro, troctolite, dunite, and harzburgite from Site 895, the troctolite from Holes 895D and 895E was found to have the maximum values of Pt (36 ppb) and Pd (54 ppb), together with a dunite from Hole 895E, which had maximum values of Pt (24 ppb) and Pd (46 ppb). In both lithologies, the greatest values of PGE coincide with the greatest modal abundance of base-metal sulfide and chrome-spinel. Detectable values of PGE were observed also in gabbro and harzburgite and the slightly more PGE-enriched gabbros are also those with relatively depleted concentrations of rare earth elements (REE).
PGM have been located in both dunite and troctolite. In the troctolite, the most common PGM are alloys of precious and base metals, specifically Pd-Sn-Cu, Cu-Pt-Au, Pt-Fe-Ni-Cu, and Ni-Pt-Fe. One Pd-Bi-Te mineral was observed. These PGM are associated with chrome-spinel and pentlandite, which is altering to magnetite, millerite, Ni-Fe alloys, and oxides. Cu-bearing sulfides and native Cu are present. In the dunite, an Ir-Pt alloy and a Pd-Sn alloy occur close together within a Ni-Fe alloy.
Date of initial receipt: 2 August 1994
Date of acceptance: 2 February 1995
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