VOLCANIC GLASS IN NAZCA PLATE SEDIMENTS AT ODP SITE 1232: ORIGIN AND TRANSPORT

Along the coast of Chile, Pleistocene-recent marine sediments reflect complex sources associated with transport by riverine input, coastal runoff, glacial meltwater and turbidite plumes. Turbidites are uncommon on the seaward side of a deep-sea trench, however, a 300-meter sequence of repetitive Pleistocene turbidites was drilled at ODP Site 1232 on the Nazca Plate across the Peru-Chile Trench from South America. Coarsest sediments (sandy silt, 30-300 microns) contain anomalously large amounts of fresh, angular volcanic glass. This study aims to investigate the origin and sources of coarse sediments at IODP Site 1232 using mineralogy, chemical and textural analysis at the particle scale, and microfossil identification. A special objective is to interpret how the large quantities of fragile glass were able to survive transportation and reaction with seawater. To date we have investigated the variation in proportions of glass, minerals and microfossils downcore with the goal of discerning temporal changes. Initial petrographic and geochemical analyses show that all samples contain >45% fresh angular glass; other phases found are devitrified glass, lithic fragments, quartz, feldspar, pyroxene, zircon, opaques, chlorite and biotite mica. Based on optical and SEM imaging, glass grains are fresh and angular with little rounding. Four distinct textural varieties of glasses are present: 1) dark brown non-vesicular blocky shards; 2) poorly vesicular shards; 3) highly vesicular shards with spherical vesicles; and 4) shards with pipe vesicles. Energy Dispersive X-ray major element analysis of glass grains reveals compositions ranging from basalt to dacite. A subset of grains was analyzed by LA-ICPMS yielding broadly similar trace element abundance patterns. All glasses show enrichment in Rb, Ba, Th, Pb, and depletion in Nb and Ta compared with rare earth elements, a classic feature of volcanic rocks from continental arcs. An origin by erosion, transportation and deposition of volcanic debris from the adjacent Chilean Southern Volcanic Zone is likely, although the details of that pathway are yet to be interpreted and understood. Future work will address the specific sources of glass and mineral particles in the turbidites, their age, as well as temporal changes which may be related to changing conditions on the adjacent continent.