A field and chemical study of serpentinization - Stonyford, California: Chemical flux and mass balance

Serpentinized harzburgites and dunites in the Coast Range ophiolite near Stonyford, California, form massive, decameter- to kilometer-scale blocks in serpentinite schist; together these form serpentinite broken formation that grades into melange where exotic blocks have been incorporated into the serpeninite schist. Whole rock geochemical data and modal reconstruction of protolith compositions show that serpentinization here proceeded essentially isochemically for Si, Mg, and Fe, whereas other elements (Ca, Al, Cr) were lost to an aqueous flux. Mass balance calculations based on actual primary and secondary mineral compositions show that significant Fe, Al, and Cr may be accommodated in serpentine. The transformation of orthopyroxene to serpentine (bastite) releases significant amounts of silica, which forms additional serpentine when it reacts with MgO released by the serpentinization olivine; this reaction suppressed brucite formation and accounts in part for the conservation of silica and magnesia documented by whole rock geochemistry. For normal harzburgites (20-25% modal orthopyroxene), approximately half of the potential brucite was suppressed. Volume expansion was considerable: 25-30% for pseudomorphic replacement of orthopyroxene, 50-60% for replacement of olivine. The increase in volume resulted primarily from the addition of water to hydrate the primary silicate assemblage; loss of Al and Ca to aqueous solutions results in a slight loss of mass. Expansion is accommodated by orthogonal fractures at both the microscopic and macroscopic scales. Subsequent movement along the macroscopic serpentinized fractures led to the formation of serpentinite broken formation, with a matrix of sheared and foliated serpentinite, and relict blocks of massive, less serpentinized peridotite. This movement may have resulted in part from the volume expansion of the peridotite, as rigid, less serpentinized blocks were forced to adjust to increased volumes in their totally serpentinized selvages by differential movements that forced the blocks to move in the direction of least principal stress.