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doi:10.2204/iodp.proc.335.203.2016 Data report: lithologic diversity, mineral composition, and temperature and redox conditions in ODP/IODP Hole 1256D—a study of sand and gravels recovered during IODP Expedition 3351Bénédicte Abily2, 3 and Marie Python4AbstractThis data report presents an uncommon study of fine drill cuttings (sand and gravels) recovered from hole maintenance operations in Ocean Drilling Program/Integrated Ocean Drilling Program (IODP) Hole 1256D (East Pacific Rise) during Expedition 335. Drill cuttings represent an integrated sampling of the entire hole and offer the opportunity to study the petrological characteristics of the various lithologies making up the upper oceanic crust. We selected the fine-grained fraction of cuttings coming from 12 pipe deployments (runs) and conducted microscopic observations (mineralogy and texture), image processing, electron microprobe analyses (~4400 analyses), and oxythermobarometry calculations on 16–25 grain-mount thin sections. This study shows that drill cuttings sample all of the lithologies in the hole (glass, aphyric and phyric basalts, dolerite, gabbro, diorite, gabbronorite, albitite, and three kinds of granoblastic basalt) but overrepresent non- or poorly recovered materials like albitites and granoblastic basalts. Drill cuttings are thus a good alternative to study these “rare” lithologies. Based on the composition of the main minerals and the oxythermobarometry calculations, two major lithologic groups were defined in the cuttings: a basaltic lavas-dolerites group and a gabbroic lithologies-granoblastic basalts group. Basaltic lavas (glass and basalts) and dolerites are characterized by Mg-augites rich in Al2O3, Cr2O3, and TiO2; MgO-rich plagioclases with a mean composition of An60–70; and titanomagnetites. Their Fe-Ti oxides (magnetite and ilmenite) indicate relatively reducing conditions (ΔNNO = –1.26 to –0.80), and clinopyroxenes in dolerites give a mean equilibrium temperature of 1017°C. Gabbroic lithologies and granoblastic basalts are characterized by Ca-rich clinopyroxenes (augites and diopsides) depleted in Al2O3, Cr2O3, and TiO2; MgO-poor plagioclases with a mean composition of An50–60; and Ti-poor magnetites. Oxybarometers give oxidizing (ΔNNO = –0.08 to +0.58) to very oxidizing (ΔNNO = +1.97 to +2.38) conditions for these lithologies, and their pyroxenes indicate high (909°C–985°C) to relatively high (783°C–888°C) mean temperatures according to the thermometer considered. Albitites show equivalent mineral compositions and temperature and redox conditions to gabbroic lithologies and granoblastic basalts. They are, however, characterized by albitic plagioclases (An0–20) and usually display slightly lower temperatures (780°C) and higher ΔNNO values (+2.66). One grain of gabbro (Run14EXJB-T-G15) strongly differs from the other gabbroic grains (~40 grains) in terms of clinopyroxene composition, temperature, and oxidation state. This gabbro has thus probably suffered from peculiar processes compared to the other gabbroic lithologies. |
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