Progress of hydration reactions in olivine–H 2O and orthopyroxenite–H 2O systems at 250 °C and vapor-saturated pressure

Abstract

We conducted hydrothermal experiments in olivine (Ol; Fo 91)–H 2O and orthopyroxenite (Opx; composed of 95% of orthopyroxene, En 66)–H 2O systems under conditions of 250 °C and vapor-saturated pressure (P sat) to examine the temporal evolution of the solution chemistry and products in runs of up to 1008 h in duration. The maximal degree of hydration (i.e., H 2O content in the solid sample) in the Ol–H 2O experiments (3.6 wt.%) was much higher than that in the Opx–H 2O experiments (0.4 wt.%). In the Ol–H 2O experiments, Mg and Si in solution showed an initial increase (stage I) before decreasing (stage II) and finally attaining a steady state after 504 h (stage III). Following a drop in silica activity toward the level of brucite stability field, the products also changed from serpentine + magnetite (stages I and II) to serpentine + brucite + magnetite (stage III). Serpentine minerals also changed from lizardite (stages I and II) to lizardite + chrysotile (stage III). The zoning pattern of the products around olivine at 1008 h (brucite + serpentine at the olivine contact, and serpentine at the outermost rim) is consistent with the temporal changes in the mineralogy of the products, and is similar to the pattern observed in the mesh rims in partly serpentinized dunites. In the Opx–H 2O experiments, chlorite formed after orthopyroxene grains, which differs from the formation of talc and serpentine after orthopyroxene (bastite), as observed in natural hydrated harzburgites. The Opx–H 2O system maintained 10–10 3 times higher silica activity than Ol–H 2O system, suggesting that brucite does not form after olivine during hydration of peridotites when the Ol–H 2O system is linked to the Opx–H 2O system. The progress of hydration reactions is affected by mechanical properties of host rocks. The hydration reactions observed in this study produced hierarchical fractures in the reactants, which became filled with reaction products, similar to mesh textures after olivine in natural peridotites. This reaction-induced fracturing produced new reaction surfaces and fluid pathways that enhanced the hydration reactions. The overall reaction producing serpentine + brucite in the Ol–H 2O experiments showed the large volume expansion ( V/V 0 = 1.43 at stage III), whereas that producing only serpentine proceeded with near constant volume ( V/V 0 = 1.19 at stage I). The volume expansion is more difficult to occur in the oceanic lithosphere than in our experiments during serpentinization. Thus, in the case that volume expansion is prevented at reaction sites, one of the following outcomes occurs: (1) the hydration reaction stops until new fractures form, or (2) the reaction proceeds with low volume expansion (absence of brucite) by removing Mg from the system. These two outcomes would produce contrasting distributions or extent of hydration in oceanic lithosphere.