Abstract
Serpentinization of mantle peridotites has first order effects on the rheology and tectonic behavior of the oceanic lithosphere, on the global water cycle, and on the biosphere at mid-oceanic ridges. Investigating serpentinization of abyssal peridotites is limited by the scarce occurrences of peridotites at or close to the ocean floor at slow and ultra-slow ridge environments where peridotite is exposed by long-lived detachments. The processes controlling hydration of the upper mantle below a thick magmatic crust at fast spreading ridges are poorly constrained. Here we present results based on samples from cores drilled in peridotites from the Samail ophiolite obtained during the Oman Drilling Project. We describe an early generation of highly localized brittle faults ubiquitous through all the peridotite cores and investigate their relation to the main serpentinization event represented by mesh-textured serpentinites. We combine microstructural observations with mineral and bulk chemical analyses as well as oxygen isotope microanalyses obtained by secondary ion mass spectrometry (SIMS). Asymmetric wall rock damage, weakening of crystal preferred orientation (CPO) in small fault clasts, and intense fragmentation within the fault zones even in association with very small displacements suggest that the early stage faults represent seismic events and predate mesh formation. Hydration and mesh texture formation follows in the wake of this faulting. Serpentinization is associated with moderate enrichment of fluid mobile elements including B, Li, Rb and U, indicative of fluid rock interaction characterized by relatively low fluid/rock ratios. This is consistent with a scenario where serpentinization took place below a thick magmatic crust following an earthquake-induced permeability increase. The oxygen isotope compositions of mesh serpentine are consistent with off-axis serpentinization at temperatures in the range 200-250°C
Highlights
Hydrothermal circulation within the oceanic lithosphere has first order effects on its thermal evolution (e.g. Schuiling, 1964; Stein and Stein, 1994), and on the mass-transfer between the oceans and the solid Earth (e.g. Paulick et al, 2006)
We focus on one specific type of fault that has been observed during the description of the BA cores by the Oman Drilling Project (Oman DP) scientific team
It has been shown that formation of lithospheric scale hydrothermal cells can provide enough cooling for local serpenassociated with the lithospheric faults is constrained by the occurrence of hydrated melts reported by Rospabé et al (2019). This demonstrates that the ridge was magmatically active at the onset of hydrothermal alteration. Additional support for this model is the occurrence of anhydrite, typical of alteration in an oceanic environment, reported by Mariani et al (2019) within a core crosscutting the fault studied by Zihlmann et al (2018)
Summary
Hydrothermal circulation within the oceanic lithosphere has first order effects on its thermal evolution (e.g. Schuiling, 1964; Stein and Stein, 1994), and on the mass-transfer between the oceans and the solid Earth (e.g. Paulick et al, 2006). Serpentinization of mantle peridotites is mainly studied at slow to ultra-slow ridge settings where mantle rocks are brought to the seafloor by large scale detachment faults along “magma-starved” portions of ridge (Cannat, 1993). In such settings, the extent of serpentinization is very extensive (Bach et al, 2004) and features formed during the incipient stages of peridotite hydration at depth are largely overprinted. Seismic data from ultra-slow ridges suggest that earthquake-related brittle fracturing may play a key role in the generation of the permeability required to bring ocean water in contact with mantle peridotites (Aupart, 2020). We focus on the earliest stages of faulting, which we interpret to have occurred during the incipient stages of serpentinization and hydrothermal activity
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