Effects of dehydration on pore water chemistry due to mineral phase changes simulating the burial diagenesis of biogenic silica

Abstract

In the deep subsurface of thick marine sedimentary formations, there may be groundwater that is thought to have been altered during the burial diagenesis from seawater taken into the pore during deposition. Such a field is considered to be one where groundwater flow is slow because it is thought to be unaffected by freshwater seepage flow, even through uplift and erosion of the stratum. Although dehydration from minerals and upward migration of pore water due to burial compaction are thought to be the main causes of changes in Cl concentration in deep groundwater that does not mix with freshwater, quantitative understanding is insufficient. In this study, we focused on the dehydration reaction associated with the phase change of biogenic silica (Opal A). We simulated a test to evaluate the related dehydration and oxygen-hydrogen isotope exchange reaction with the surrounding water. As a result, the phase change from opal A to quartz was confirmed, and the water content derived from the silanol group of opal A was 2.7 wt%. Consistent with the water content of diatomaceous fossils in the stratum, the dehydration associated with the phase change from opal A to quartz due to diagenesis could be quantitatively evaluated.