Thermodynamic forward modeling of progressive dehydration reactions during subduction of oceanic crust under greenschist facies conditions

摘要

We developed a new approach to the study of dehydration reactions using thermodynamic forward modeling based on the differential thermodynamic approach (the Gibbs' method) and the use of mass balance equations. We applied the modeling to mafic systems under conditions of the greenschist facies in order to investigate continuous, progressive dehydration reactions during subduction of oceanic crust. By specifying the changes in P-T, the model enables us to predict changes in modal mineralogy and composition as well as the incremental amount of water produced. Calculations for rocks of MORB (mid-ocean ridge basalt) bulk composition subducting along several typical P-T paths show that dehydration proceeds rapidly near the boundary between the greenschist and (epidote-) amphibolite facies, where chlorite is reacted out. The calculated depths for peak rates of dehydration correlate well with peaks in clusters of seismic activity and the depth ranges of low-frequency tremors and high-Poisson's ratio in the subduction zones of SW Japan and Cape Mendocino. This finding indicates a strong causal linkage between seismic phenomena and dehydration reactions in subducting oceanic crust.