Site U1531

Abstract

Background and objectives 3 Operations 6 Igneous petrology and volcanology 9 Alteration 10 Structural geology 10 Geochemistry 11 Paleomagnetism 13 Physical properties 15 Downhole measurements 15 Microbiology 16 References C.E.J. de Ronde et al. Site U1531 IODP Proceedings 2 V o l u m e 3 7 6 Hole U1531E. We could not advance any deeper because the TDCS core barrel got stuck in the bottom-hole assembly (BHA) and could not be retrieved. After the TDCS test, we cored with the RCB system from 17.9 to 39.6 mbsf and recovered only 0.8 m (4%). At this point, the blower motor in the top drive failed. We decided not to repair it at this time because of our remaining scientific priorities and the impending end of the expedition. Our final operation at Site U1531 consisted of a downhole temperature measurement with the Elevated Borehole Temperature Sensor (ETBS) memory tool. The tool recorded a maximum temperature of ~5°C at 20 mbsf. In total, 264.25 h, or 11.01 days, were spent at Site U1531. Principal results Igneous petrology and volcanology At Site U1531, one igneous unit was observed in Holes U1531A (0-1.14 mbsf), U1531B (0-21.97 mbsf), U1531C (0-23.75 mbsf), and U1531E (17.90-34.95 mbsf) as well as in several "ghost cores" from Hole U1531E. Igneous Unit 1 consists of unaltered to slightly altered plagioclase-pyroxene-phyric dacite lava intercalated with unconsolidated ash, ash with lapilli, and lapilli tephra with ash. The moderately to highly vesicular dacite lava contains glomerocrysts and phenocrysts of plagioclase, orthopyroxene, and clinopyroxene and Fe-Ti oxides in a cryptocrystalline and hypocrystalline groundmass. Volcaniclastic material is mainly composed of unaltered dacite clasts, crystals, and subordinate lithic components. The composition of the dacite lava and pyroclastic rocks from Site U1531 is similar to that of dacite cored in the upper sections at Sites U1527 and U1529. Alteration One alteration type was identified at Site U1531 based on an alteration assemblage identified using a combination of macroscopic and microscopic descriptions and X-ray diffraction (XRD) analyses. Alteration Type I is classified as slightly altered and has an alteration mineral assemblage of smectite, Fe oxyhydroxide, zeolite, pyrite, and rare native sulfur. Alteration occurs as infilling and lining of vesicles and smectite replacing the glassy matrix.