Next Section | Table of Contents


During Expedition 305, a total of 145 discrete samples were analyzed for major and trace element concentrations by inductively coupled plasma–atomic emission spectrometry. Hole U1309D gabbroic rocks have compositions that are among the most primitive sampled along the MAR (e.g., Casey, 1997; Kelemen, Kikawa, Miller, et al., 2004), as indicated by high Mg# (67–87 excluding oxide gabbros), low TiO2 (<0.72 wt%), and low trace element contents (Fig. F13). Among all other DSDP/ODP/IODP drill holes, Hole U1309D is unique in that it represents a section of primitive to slightly evolved gabbroic rocks and includes intrusive diabase intervals as well as olivine-rich rocks, which may represent primitive cumulates.

Gabbroic rocks from Hole U1309D are divided into two geochemical units based on chemical trends and range of Mg# (~10) (Fig. F14). Unit I includes the section from 20 to ~600 mbsf, coinciding upcore with a general decrease in the presence of olivine-rich rock types and a general increase in the thickness of relatively thick oxide gabbro, although the actual lithostratigraphy is in detail more complicated with interfingering oxide gabbros, gabbros, olivine gabbros, troctolites, and dunitic troctolites. Unit II extends from ~600 mbsf to the bottom of the hole, coinciding downcore with a general increase in the volume percent of olivine gabbro. Although four fault zones are identified in Unit II (695, 746, 785, and 1107 mbsf), systematic geochemical changes are not consistently apparent across these fault zones.

All diabase intervals have basaltic compositions (Fig. F15) but show significant variation in incompatible trace elements like Y and Zr. Compared to the diabases from Expedition 304 in the upper part of Hole U1309D, the samples from Expedition 305 are somewhat less evolved in composition but still overlap the compositional field defined by glasses from the MAR.

Water samples collected at the beginning of operations during Expedition 305 (~395 mbsf) and at ~1215 mbsf were both essentially indistinguishable from seawater in terms of pH (7.4–7.7) and salinity (30‰).

Next Section | Table of Contents