TO: Tom Davies
FM: Jay Miller
JA Daily Science Report for Expedition 305, 13 February 2005.
SCIENCE UPDATE: Cores 305-U1309D-229R to -237R (1101.6 to 1143.9 mbsf, 69% recovery) continue the striking change in lithologies encountered at the end of our last bit run. Core 305-U1309D-229R starts in medium-grained, leucocratic troctolitic gabbro, that in places has >50% olivine. Down section we pass through coarse-grained gabbro to olivine gabbro that includes a couple of thin shear zones that are steeply dipping in the core reference frame. We also recovered intervals of gabbro with steeply dipping fractures with cm wide alteration halos. The fractured, altered olivine gabbro continues through Core 305-U1039D-231R to troctolite, where the abundance of olivine increases gradationally but rapidly over a few cm, then back to gabbro. The gabbro has a sharp lower contact with olivine-rich troctolite (40-60% olivine). Continuing down section we see troctolite with more and more abundant olivine, as much as 80-90% in some intervals and plagioclase has a distinct interstitial character. There are gradational transitions to more plagioclase-rich troctolite, with coarser grained plagioclase and olivine, to leucocratic troctolite with hints of serpentine foliation. The subvertical serpentine foliation increases in intensity down section, but translates to steeply dipping (40-60░ in the core reference frame) in some intervals as the core becomes much more olivine rich (> 80%) with interstitial plagioclase. The foliation appears to become more suppressed as rock becomes less altered. In Core 305-U1309D-234R to -236R serpentine foliation returns first as chaotic, subhorizontal anastomosing veins, translating over a few m to a strong subvertical orientation. The bottom of Core 305-U1309D-236Rá and the top of Core 305-U1309D-237R contain coarse-grained olivine gabbro, with a sharp lower contact into plagioclase-poor troctolite with a strong serpentine foliation.