A New Method for Quantitative Estimation of Volume Change in Metamorphic Rocks Using Deformed Mineral Vein Sets

Abstract:

The flux of water-rich fluids in subduction environments is an essential aspect of research into seismic and volcanic activities of the same regions and is an important but largely unquantified part of the global water cycle.

Water in the subducting plate is released when the water-bearing hydrous minerals break down. This released fluid will be unsaturated in quartz and as it rises through overlying subducted sediments, it will leach quartz from these quartz-bearing rocks and has the potential to cause volume changes in the rock. Therefore, with information on the solubility of quartz and P­­–T conditions, the volume change of the metamorphic rocks exhumed from the deep subduction zone can be used as an important constraint on the time-integrated fluid flux passing through subduction channels.

Several methods have been proposed to estimate the volume change of rocks, but they are associated with large uncertainties. In this study, we take an approach to estimating volume change based on geometric analyses of deformed mineral vein sets. With sufficient data, this method can, in principle, be used to estimate all components of finite deformation: volume change, strain, and rotation. Although the theoretical basis for this analysis has already been established, there are no examples where this method has been specifically applied to estimating volume change. In this study we developed a new approach that combines this method with a statistical analysis. Our new method allows both the parameters of finite deformation including volume change and the associated uncertainties to be estimated from field data. Deformation analysis of metagraywacke in the Del Puerto canyon in the Franciscan belt was performed using this new method, and finite strain, mean vorticity number and volume change were estimated. The results and observations of the rock structure suggest that consideration of vorticity change during progressive deformation and this needs to be taken into account when estimating rock volume change using deformed mineral vein sets.