Origin of spinel-bearing peridotite from oceanic core complexes
Final Report Abstract
The petrological investigation revealed that the peridotite represent refractory, partial residual mantle material that experienced depletion of incompatible trace elements during upper mantle melting. The degree of partial melting as estimated from spinel compositions was c. 12%. Fractionated middle and heavy rare earth elements imply polybaric melting, with c. 1 - 4% initial melting in the garnet peridotite stability field and subsequent partial melting of ~7 - 10% in the spinel peridotite stability field. Geothermobarometric investigations revealed that the solid-state equilibration of the spinel peridotite occurred at some 1100 - 1150°C and c. 20-23 kbar, corresponding to an equilibration depth of c. 70 ± 5 km. A thermal re-equilibration of the peridotite occurred at ~850 - 1000°C at similar depths. Naturally, the initial mantle melting in the garnet-peridotite stability field must have commenced at depths greater than 70±5 km. It is likely that the residual peridotite rose rapidly through the lithospheric cap towards the ridge axis. The exhumation of the abyssal peridotite occurred, at least in parts, via extensional detachment faulting. Given the shallow to moderate dip angles of the fault surfaces, the exhumation of the peridotite from its equilibration depth would imply an overall ridge-normal horizontal displacement of c. 50 - 160 km if tectonic stretching and detachment faulting were the sole exhumation mechanism.
