MASS TRANSFER OF CO2, CA AND NA DRIVEN BY H2O INFILTRATION DURING SUBDUCTION ZONE METAMORPHISM OF THE FRANCISCAN COMPLEX, CALIFORNIA (Invited Presentation)

The release and transport of fluids in subduction zones affects many fundamental processes, including the transfer of volatile elements from earth's surface to the mantle. Recent work has focused on the behavior of CO₂ during subduction, and there is debate over the relative importance of devolatilization reactions compared to reactions in which carbonate minerals are completely dissolved in fluid. Studies of rocks over a range of metamorphic grades and compositions are needed to better identify and quantify the mechanisms of CO₂ release during subduction. Metagraywacke rocks of the Franciscan Complex provide a natural laboratory in which processes of CO₂ release can be evaluated. The high-pressure/low-temperature index minerals jadeite and lawsonite are distributed unevenly in the metagraywacke exposures, in some cases in oval map patterns. Surrounding unreacted graywacke contains albite, chlorite, phengite, quartz, calcite, titanite, epidote, and minor (<1%) lawsonite. Jadeite-bearing rocks contain more abundant lawsonite (>10%), quartz, chlorite, phengite, trace albite (<2%), rutile instead of titanite, and are devoid of calcite and epidote. Transformation of unreacted greywacke to jadeite-bearing rock can be completely described by 6 net-transfer reactions that produce the jadeite, acmite and diopside components of pyroxene, lawsonite, and rutile by consuming albite, calcite, epidote, titanite, and chlorite. Determination of the progress of the reactions from measured modes reveals a release of CO₂ and consumption of H₂O during the jadeite-forming reaction. Phase equilibria indicate decarbonation occurred in the presence of fluids with XCO₂<0.39 (for any P≤2 GPa). Abundant lawsonite suggests that jadeite-bearing rocks lost Na and gained Ca during reaction. These observations point to a jadeite-forming reaction driven by infiltration of aqueous fluid flowing in the direction of decreasing T. The map distribution of jadeite indicates that infiltration occurred in vertical 'pipes' with diameters of several 100 m. The study suggests a strong link between H₂O release and CO₂ release during subduction zone metamorphism.