Petrology of selected leg 147 basaltic lavas and dikes

Abstract

Sites 894 and 895 of Ocean Drilling Program Leg 147 recovered a variety of basaltic material from the Hess Deep intrarift ridge, including cores of dikes that cut gabbroic and peridotitic wall rock and fragments of pillowed, surficial lava flows. Site 894 samples include highly primitive Plagioclase, olivine, and Cr-spinel phyric basaltic dikes, with melt Mg/(Mg+Fe2+) (Mg#) as high as 0.70; a basaltic picrite, derived from a melt with an Mg# > 0.7; and more evolved, relatively aphyric basalts. The Site 894 basalts are more depleted than East Pacific Rise (EPR) N-MORB in incompatible elements, with extreme depletions in both the light rare earth elements (LREE) and Ta and Nb. These depletions are illustrated by their chondrite-normalized La/Sm and Ta/Hf ratios, which vary from 0.30 to 0.51 and from 0.12 to 0.28, respectively, and demand an unusually depleted mantle source. The intrusive relations, mineralogy, and geochemistry of these samples support an off-axis origin of these samples during opening of the Hess Deep. The Site 895 basalts, sampled from both dikes and pillowed flows, are aphyric and more evolved and less depleted in LREE and Ta and Nb than the Site 894 samples, with (La/Sm)N and (Ta/Hf)N ratios varying from 0.60 to 0.71 and from 0.46 to 0.53, respectively. Unlike the Site 894 samples, these characteristics allow derivation from a mantle similar in composition to that underlying the EPR. Intrusive relations and geochemical characteristics support a mixed origin for the Site 895 samples, with some originating at or near the EPR ridge crest, and others representing volcanism associated with Hess Deep opening.