IODP Expedition 345: Bulk Mineralogy From Lower Oceanic Crustal Rocks of the Hess Deep

Young lower oceanic crustal rocks produced along the East Pacific Rise are exposed at the sea floor of the Hess Deep by rifting of the Cocos-Nazca ridge. Coring into these rocks (IODP Expedition 345) had the goal of understanding the petrologic processes that produce lower oceanic crust. Variable orientations of magmatic layering and foliation and magnetic remanance directions in the recovered gabbro and troctolite suggest that these samples were extracted from large blocks slumped into the rift. The overall poor recovery of core (~30%) and thick sections of broken rock disaggregated by cataclasis suggest that associated fractures produced the locally intense alteration of the magmatic mineralogy. We characterized this alteration by collecting the cuttings from the rock saw that divided the core into archive and working halves. The resultant ~1 mm wide slot should yield a calculated ~150 g/m of core, constituting the most representative sample of the entire core possible. A plastic housing surrounding the entire saw assembly captured all sedimented cuttings as well as distilled wash and lubricating water. All cuttings and a final wash water (holding suspended clay-size particles) were collected through the single drain at the base of the housing. We recovered 132 g/m of core, in close agreement with the calculations above. Our results show that the suspended material constitutes 10- 30 wt.% of the total sample, demonstrating the need for collection of the rinse water. Modal mineralogy of a few samples was measured by quantitative X-ray powder diffraction and Rietveld methods. These results revealed in descending order of abundance the magmatic plagioclase, augite, and Fe-forsterite identified optically. The higher-temperature alteration mineral actinolite was present at low concentrations. Alteration phyllosilicates included chlorite > prehnite > lizardite > talc. Chrysotile and antigorite were not identified. Rietveld refinements confirmed the presence of low (hydrothermal) albite identified optically and showed that it constitutes 10 to >30% of the total plagioclase. The distinction between magmatic and hydrothermal clinopyroxene identified optically has so far not been achieved. Analcime unidentified optically was found in some samples up to several wt.%. A phyllosilicate with a peak near 15 Å was identified in several samples. Analysis of the drilling mud suggests that this 15 Å phyllosilicate does not represent contamination because drilling mud contains a ~12 Å smectite and is otherwise dominated by barite, with lesser amounts of halite, quartz, and calcite. Comparisons of the amounts of alteration and magmatic minerals provide a quantitative measure of the degree of alteration of the coherent core, which varies from 10 to 50 wt.%. Given a 30% recovery of core, these values provide a minimum estimate of the overall alteration. Comparison of the mineralogy of the sedimented cuttings with the suspended material shows that phyllosilicates are fractionated into the suspended rinse water, confirming the need to collect the suspended material as well as the coarser sedimented cuttings.