Slab-mantle interactions: 1. Sheared and refertilised garnet peridotite xenoliths — samples of Wadati-Benioff zones?

Abstract

Most sheared and deformed garnet peridotite-facies xenoliths found in kimberlites worldwide have been formed from initially depleted peridotite protoliths which have subsequently experienced varying degrees of refertilisation. Ti, Fe, Ca, Al, Na, P, Zr and Y have been introduced to various degrees by silicate melts and the protoliths thereby refertilised. These melts were not kimberlitic — they possessed “basaltic” 100 Mg ( Mg + Fe) mole ratios of ∼65–70 and contained several wt.% TiO 2, and were often capable of precipitating ilmenite. Equilibration temperatures recorded by sheared, refertilised garnet periodtites are usually above those of a steady-state shield geotherm. It is proposed that these rocks acquired their high equilibration temperatures, refertilised character and highly-deformed fabrics along Wadati-Benioff zones in regions of active subduction beneath cratons. Bodies of former serpentinite in the deeper layers of the basaltic crust of the subducted lithosphere or in underlying harzburgite, may transport water to depths of several hundred kilometers. Water released by the breakdown of subducted former serpentinite migrates upwards into hotter regions of the (eclogitic) former oceanic crust, causing partial melting to occur, particularly in the depth interval between 150 and 300 km. These slab-derived melts are episodically injected into peridotite immediately overlying the former oceanic crust. Hybridisation occurs in this boundary layer, with Ti, Fe, Ca, Al, Na, P, Zr and Y being transferred from the slab into this region, thereby rendering it fertile. Episodic and repeated injection of small volumes of slab-derived melts into adjacent pre-stressed peridotite would also contribute to local weakening, accompanied by catastrophic shear failure at high strain rates. The deformed fabrics of most sheared peridotites may have been produced in this manner. Local transient heating may also accompany displacements of this kind, thus accounting for the seemingly elevated equilibration temperatures recorded by these rocks.