Cumulus and postcumulus crystallization in the oceanic crust: major- and trace-element geochemistry of Leg 153 gabbroic rocks

Abstract

Major-and minor-element mineral compositions were determined by electron microprobe analysis. These data were collected using the JEOL JXA-8600 Superprobe at the University of Houston. Polished thin sections were prepared from 55 gabbroic rock samples from Sites 921,922, and 923. X-ray intensity data were reduced using an on-line NORAN ZAF routine. The electron probe was operated using an accelerating voltage of 15 kV and a beam current of 30 nA. Six to nine spots on several grains for each major constituent phase were analyzed. Pyroxene analyses were obtained using a defocused beam with a diameter of 25 µm, to attempt to determine magmatic (pre-exsolution) pyroxene compositions. Exsolution lamellae of low-calcium pyroxene in host augites are ubiquitous. Plagioclase was analyzed using a 10-µm defocused beam to prevent sodium migration during exposure to the electron beam. A 10-µm spot diameter was also used for analysis of apatites. Oxides and olivine were analyzed using 1-µm spot diameter. Elements were calibrated using natural and synthetic mineral and glass standards. Counting times were generally set at 100 s for the peak, with 50-s counting times on background positions on either side of the peak. Instrumental neutron activation analysis (INAA) of whole-rock powders and mineral separates from selected samples have been obtained using the laboratory at the NASA Johnson Space Center. Gamma ray spectra were processed using the TEABAGS program (Lind-strom and Korotev, 1982). Whole-rock powders were prepared by choosing a sawn piece of sample adjacent to the material from which mineral separates were obtained. The sample was jawcrushed using hardened steel plates and powdered in a SPEX mill using hardened steel container and balls. Plagioclase and clinopyroxene mineral separates were obtained by jawcrushing ultrasonically cleaned pieces of fresh sample, sieving the resulting crumbled rock, recovering the 250-to 500-µm-sized fraction, magnetically separating initial plagio-clase, clinopyroxene, and olivine fractions, and finally careful hand-picking under a stereoscopic microscope. Separates of 50-100 mg were obtained by this procedure and this material was then ultrason-ically cleaned in 1-N HCL for 5-10 min. The resulting mineral separates were >99% pure. Rare-earth element (REE) and trace-element compositions of cli-nopyroxene were obtained by secondary ion mass spectrometry at the Woods Hole Oceanographic Institution in Dr. N. Shimizu's laboratory. Round, 1-in-diameter, thin-section mounts of selected gabbroic rocks were prepared for ion probe studies, so that clinopyroxene grains could be analyzed in situ to permit evaluation of trace-element zoning. Energy filtering suppressed molecular interferences (Shimi-zu and Hart, 1982). The ion probe excavates a pit that is about 20-25 µm in diameter and about 25 µm deep. Given the scale of exsolution in clinopyroxene common in these rocks, with exsolution lamellae that are 1-3 µm wide of orthopyroxene in clinopyroxene hosts, it is unlikely that exsolution phenomena produce the scatter in ion probe data observed. The instrument collects ion intensity ratios repeatedly, over a period of about 25 min during REE analyses and 20 min for other trace-element analyses. The data presented are average results from these repeated measurements.