THE EFFECTS OF INITIAL BULK ROCK AND VOLATILE CHEMISTRY ON SUBDUCTION ZONE PROCESSES

Quantifying the nature of fluids in subduction zones is crucial to understanding tectonic processes that occur at destructive plate boundaries such as slab metamorphism, volatile release, mantle wedge dynamics and metasomatism. A recent study (Baxter & Caddick, 2013) examined the duration and amount of fluid released along three subduction zone profiles for simple initial bulk rock compositions (pelite and altered MORB), with results showing a strong correlation between garnet growth and fluid release. This study focused on H2O in the fluid phase and did not explore the potential effects of CO2 on fluid chemistry and mineral stabilities. Recent findings (e.g. Ague & Nicolescu, 2014), however, suggest that CO2 plays an important role in subduction zone processes.

Here we aim to better understand (i) how specific compositional factors in a rock control the retention or release of fluids, (ii) how CO2 affects the chemistry and the dynamics of the subducting system, and (iii) what proportion of C entering a subduction zone is devolatilized before or at the sub-arc. Numerical models were built to predict the effect of initial CO2 and H20 content on the volatile release from the subducting rock and the mineral assemblages that may be diagnostic of specific volatile contents along a subduction P-T trajectory. This study focuses on results for a Nicaraguan-type subduction path for altered mid-ocean ridge basalts and sediments. In addition to predicting the direct effects of CO2 on subduction zone mineral assemblages, we will attempt to quantify how fluid chemistry and volatile release is affected by subtle changes in the initial bulk rock chemistry.

Initial results suggest that, as long as volatiles are initially present, the P-T path controls the chemistry of the evolving fluid phase (modeled as a simple H2O-CO2 fluid) much more strongly than initial H2O and CO2 contents, within certain protolith composition bounds. However, the stable mineral assemblage is strongly affected by the initial H2O and CO2 contents and ratio, particularly at greater temperatures and depths. Models predict that the initial amount of fluid released is correlated most strongly to the amount of H2O (rather than CO2) in the rock and that the fluid chemistry typically ranges from 100% H20 (mol %) at the top of the slab to 90% H20 (mol %) at pressures above 2.5 GPa.