georefid:2004-063880SEDIS Publication Catalogueana.macario@awi.dehttp://sedis.iodp.org/pub-catalogue/informationpointOfContact2011-06-23T00:00:00Zhttp://sedis.iodp.org/pub-catalogue/index.php?id=2004-063880<gco:CharacterString>The influence of pressure and mineralogy on seismic velocities in oceanic gabbros; implications for the composition and state of the lower oceanic crust</gco:CharacterString>2004-01-01publicationgeorefid:2004-063880Carlson, R. L.Texas A&M University, Geology and Geophysics, College Station, TX, United StatesauthorMiller, D. J.authorGeological Society of America (GSA), Boulder, CO, United StatespublisherdocumentHardcopyIn: Anonymous, Geological Society of America, South-Central Section, 38th annual meeting36 (1)28We have analyzed the relationship between P-wave velocities, measured at pressures of 40, 100, and 200 MPa, and modal mineralogy in oceanic gabbro samples from ODP Holes 735B, 894G, and 923A. At all pressures, velocities increase with increasing pyroxene content and decreasing alteration (phyllosilicate and amphibole content), but not with increasing olivine content. A Voigt-Reuss-Hill inverse model reveals that the effective bulk densities and elastic moduli of the plagioclase, pyroxene and amphibole phases are near their expected values, but the effective bulk density and elastic moduli of olivine in the gabbros are anomalously low, probably because the olivine is iron rich, and/or because the olivine grains contain ubiquitous networks of cracks. On average, gabbros with velocities typical of seismic layer 3 (6.7 to 7.0 km s-1) contain 5-15% alteration products, including 5-15% amphibole and 0-5% phyllosilicates. These results suggest that the lower crust is slightly to moderately altered. However, total alteration of the lower oceanic crust could be as low as 2%, if the lower crustal gabbros contain abundant olivine grains that have low densities and elastic moduli.completedSolid-earth geophysicsApplied geophysicsalterationbody wavesbulk densitychain silicatescrustelastic constantselastic wavesgabbrosigneous rocksLeg 147Leg 153mineral compositionnesosilicatesOcean Drilling Programoceanic crustODP Site 735ODP Site 894ODP Site 923olivineolivine grouporthosilicatesP-wavesplutonic rockspressurepyroxene groupseismic wavessilicatesvelocity analysisurn:org.iodp:exp:147urnlargerWorkCitationcampaignurn:org.iodp:exp:153urnlargerWorkCitationcampaignurn:org.iodp:exp:118:site:735urnlargerWorkCitationstudyurn:org.iodp:exp:118urnlargerWorkCitationcampaignurn:org.iodp:exp:176:site:735urnlargerWorkCitationstudyurn:org.iodp:exp:176urnlargerWorkCitationcampaignurn:org.iodp:exp:147:site:894urnlargerWorkCitationstudyurn:org.iodp:exp:147urnlargerWorkCitationcampaignurn:org.iodp:exp:153:site:923urnlargerWorkCitationstudyurn:org.iodp:exp:153urnlargerWorkCitationcampaignEnglishgeoscientificInformation-101.313657.1618-32.432723.3233