Figure 8

Hand specimens of two samples from a serpentinite outcrop. Left - the serpentinite core (dull green) and network of late-stage, low-temperature chrysotile vein filling (shiny fibrous growth) in a serpentinite block. Right - a shear zone that is comprised of later-stage sheared lizardite that bounds a block of massive serpentinite. Both samples collected from a member of a belt of diapiric serpentinite bodies along the San Andreas Fault above the town of Redwood City, California. The presence of chrysotile and lizardite in veins and shear zones that cross cut and bound antigorite blocks supports the hypothesis that vertical mobilization of such serpentinized mantle rocks was enabled by water released from forearc serpentinite that is described in this paper. Consequent partial dissolution of serpentine components occurred into this fluid at mantle depths, and subsequent lizardite and chrysotile serpentine crystallization is thought to have been enabled by this fluid during serpentinite-block ascent to the surface through the depths corresponding to the seismogenic zone of the SAFS.