Serpentinization of the oceanic lithosphere and some geochemical consequences: Constraints from the Leka Ophiolite Complex, Norway

摘要

Serpentinization of ultrainafic rocks is an important process which modifies the petrophysical and geochemical properties of the affected rock. The Leka Ophiolite Complex (LOC) represents a part of the oceanic lithosphere which has been extensively serpentinized at the ocean floor. The ultramafic lithologies of the LOC preserve the history of hydration processes that took place during ocean-floor metamorphism over a wide range of decreasing temperatures. Serpentinization leads to the formation of less dense phases which results in a significant increase in bulk volume of the affected rocks. This change in density and volume of the ultramafic lithologies does not occur simultaneously in all lithologies and results in deformation of the surrounding rocks. The deformation is particularly evident in mm to dm thick, fractured and altered orthopyroxenite dykes in a dunite matrix. The fracturing process of the altered orthopyroxenite is driven by the reaction-assisted volume changes occurring in the dunites. Chemical and textural evidence show that major elements like Mg, Si and Al are essentially redistributed within the rock during serpentinization on an outcrop scale. However, there is abundant evidence for the mobility of Ca, Na, Fe and Mn at grain-size to regional scale. Alteration of primary clinopyroxenes to serpentine and clinochlore is a Ca-releasing reaction and contributes significantly to the mobilization of Ca in the fluids. This results in rodingitization of the crustal layer and also in the replacement of primary clinopyroxene, orthopyroxene and olivine by secondary diopside. Fe and Mn are mobilized simultaneously during the dissolution and subsequent precipitation of minerals throughout the serpentinization process. Olivine, in particular, is affected during the transport of iron and manganese. Metamorphic olivine has wide range of chemical compositions with Mg# as low as 0.68 and MnO contents of up to 1.5 wt.%. The fluid chemistry from high-temperature, ultramafic-hosted vent sites like Rainbow and Logatchev has elevated amounts of Fe and Mn suggesting that Fe and Mn could be transported out of the system as the degree of serpentinization increases. Therefore, serpentinization of the ultramafic part of the oceanic lithosphere may play an important role in constraining the global ocean chemical budget as well as ocean-floor mineral deposits.