CHEMICAL CHARACTERIZATION OF SAFOD PHASE 3 CORE

We report new X-Ray Fluorescence (XRF) compositions of 25 SAFOD Phase 3 core samples from Hole G, which nearly doubles the whole-rock chemical dataset for the core. The cores contain two foliated gouges where creep is localized, the central deforming zone (CDZ) and southwest deforming zone (SDZ), which consist of porphyroclasts of serpentinite and quartzofeldspathic crustal rocks in a matrix of Mg-rich clays. The CDZ and SDZ are bounded on either side by variably deformed structural units of sedimentary rock. Sampling focused on a) the foliated gouge-wall rock boundaries and b) meters-long sections of the wall rocks that lacked chemical data. Preliminary study shows that the XRF dataset supports many of the conclusions derived from previous structural and petrographic investigations. Consistent with the abundance of serpentinite in the foliated gouges, the major, minor, and trace element chemistry of the CDZ and SDZ are all characterized by a large ultramafic component (≥50%, higher for the CDZ than the SDZ). Combined chemical and mineralogical data indicate a generally sharp transition at the gouge-wall rock boundaries, without an intermediate zone of alteration resulting from diffusive mass transfer across the contact. However, some infiltration of pore fluids from the foliated gouge into the wall rocks has occurred along fractures, with accompanying local enrichment in Mg, Cr, and Ni and depletion in Si. The different rock units that were identified on either side of the CDZ and SDZ based on sedimentary, structural and deformational features can also be distinguished based on their chemistry. In addition, the new data suggest that the cataclasite to ultracataclasite unit on the southwest side of the SDZ is composed of two, compositionally distinct subunits of approximately equal width. The expanded XRF dataset for SAFOD will be a valuable tool for characterizing the various rock units and understanding the patterns of fluid flow in an active fault zone.