Geochemical and petrological studies of lavas, pyroclastica and associated xenoliths from the Christiana Islands, Aegean Sea

Geochemical and petrological investigations of the volcanic islands of Christiana proved their genetic connection with the calc-alkaline volcanism of the Santorini group and the submarine Kolombos volcano. The lavas of Christiana range from andesite to dacite while pumice is of rhyolithic chemistry. Rather constant K/Rb ratios indicate consanguine magmas. Increased Ni, Cr and Mg concentrations in several lavas indicate a certain admixture of peridotite material to the calc-alkaline magmas. Xenoliths found below an autochthonous pumice layer prove penetration of granites, phyllites and limestone by the rising magma. The geochemical investigations support a subduction type origin of the volcanic material of the Christiana Islands

Platinum-group element concentrations in mafic and ultramafic lithologies drilled from the Hess Deep.

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 fastspreading 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