Serpentinization of olivine–gabbro in Central Indian ridge: Insights into H2 production during alteration in lower oceanic crust and sustenance of life at slow–spreading ridges

Abstract

ABSTRACT We investigated the textural-compositional characteristics across olivine interfaces in variably serpentinized olivine-gabbro samples dredged from the “Vityaz Megamullion” in the northern Central Indian Ridge (CIR). The results are used to understand the role temperature and different chemical properties in infiltrating fluid (e.g. seawater/hydrothermal solution) on the pattern of compositional evolution during progressive alteration in olivine-gabbro. The development of two distinct types of replacement rims (i.e. type-1 and type-2) are conspicuous in our samples. Type-1 pseudomorphic mesh rim, that formed within individual olivine grains, occurs in two different textural modes: (a) as successive layers of Fe-rich (XMg = 0.89 ± 0.02) lizardite ± magnetite, clay and calcite towards the olivine interface, and (b) as homogeneous rim of comparatively Fe-poor lizardite (XMg = 0.93 ± 0.01) ± magnetite. Replacement of olivine by talc and chlorite is evident towards the clinopyroxene and plagioclase grain boundaries, respectively. Local precipitation of clay minerals is also observed at the inner interface of talc layer and within intra-olivine fractures. Type-2 rim is characterized by the development of inter-olivine Mg-rich lizardite (XMg = 0.97 ± 0.01) + magnetite layer cross-cutting type-1 veins; thus, post-dates type-1 replacement. The process of CIR olivine-gabbro alteration records at least two stages of serpentinization. Any presence of brucite has not been observed, suggesting relatively higher silica activity at reacting interfaces during olivine-gabbro alteration than that of peridotite serpentinization at temperature log a SiO 2 aq , log a H 2 O , log a Al 3 + a H + 3 and log a Ca 2 + a H + 2 ) and/or temperature during progressive alteration of olivine. Type-2 replacement attests greater extent of magnetite precipitation as compared to type-1 serpentinization, which may indicate increasing fluid-rock ratio in the system and/or decreasing magnitude of Fe intake into serpentine structure with ongoing alteration. Importantly, it implies release of H2(aq) during olivine-gabbro serpentinization, which is consistent with that observed during peridotite serpentinization. This corroborates that alteration in the deepest part of the oceanic crust may play an important role on the sustenance of biosphere in slow-spreading ridges.