Fluid pathway evolution and mass transfer during Mg-dominated mineral transformations

Abstract

Understanding the alteration of oceanic crust is of fundamental importance to unraveling geochemical and physicochemical processes on Earth. Mineral transformation during geological processes can result in mass transfer, changes in solid mass and volume while generating pathways that enhance fluid access to more remote areas of the crust and mantle. In this study, two hydrothermal experiments were conducted at different conditions, one at 160 °C and ~ 6 bar, and the other at 500 °C and 5 kbar. The reaction products were characterized using XRD and HRTEM. Saponite and talc were identified as products by XRD patterns in low- and high-temperature systems, respectively. This is consistent with HRTEM observations, in which layers with a thickness of ~1.2 and 1.0 nm were observed in products. These results confirm the successful transformation of brucite to saponite at 160 °C, and saponite to talc at 500 °C. HRTEM images clearly show the direct connection between brucite layers and saponite layers and between saponite layers and talc layers, suggesting that solid-state transformation is the main mechanism in both reaction processes. Based on the structural differences, the transformation from brucite to saponite undergoes as much as a 1.6-fold increase in volume and created expandable interlayer spacings of ~0.55 nm, which allows for fluid transport. Further conversion of saponite to talc at 500 °C results in a volume decrease. Inter-particle porosities are created during the progressive reduction of the layer spacing commencing from the outer edges of saponite crystals, potentially enhancing fluid flow. Therefore, the observation that mineral transformations proceed from the edges to interiors of the minerals suggests that fluid pathways at the nanoscale were generated during mineral transformations. The thermostability of the structural water was enhanced through mineral transformations, although water content decreased from ~30.9 wt% to ~4.7 wt%, with implications for water transfer processes in a down-going plate from the surface into Earth's interior. Also, ~2.0 wt% exchangeable cations were sequestered and preserved in the interlayer spaces during the transformation of brucite to saponite. These exchangeable cations, however, were ultimately expelled with the reduction of interlayer spacing during the conversion of saponite to talc. Thus, mineral transformations may have significant effects on mass transfer in various geological processes.