Exploring the melting behaviour of the Earth's heterogeneous upper mantle

Abstract

The existence, creation and modification of compositional heterogeneities within the Earth's upper mantle and their influences on the petrogenesis of primitive magmas have been highly debated for decades. The possible origins of mantle heterogeneities within the Earth's peridotite-dominated mantle are numerous and complex. One major mechanism invokes an important role for recycling of oceanic crust at subduction zones, and the storage of these recycled mafic (and sedimentary) materials in the deep mantle, before it is thought to be incorporated in the adiabatically upwelling upper mantle sources of some primitive mantle-derived magmas. This data is then combined with the results of earlier experimental investigations to constrain the complex melting behaviours of (residual) eclogite bodies in upwelling mantle, the nature of the interactions of their partial melts with ambient peridotitic mantle, and the nature of re-melting of such metasomatised mantle assemblages, and a brief evaluation of the contributions of these processes to the genesis of some primitive mantle-derived magmas. This study highlights that mantle heterogeneites caused by upwelling of subducted oceanic crust and its residues 'breed' an infinite number of various heterogeneities, ranging from Si02-oversaturated to Si02-undersaturated. To assess the influence of recycled oceanic crust on the creation of mantle heterogeneities and on the petrogenesis of primitive magmas in homogeneous, buoyant, upwelling peridotite-dominated mantle, high pressure experimental investigations of the melting behaviour of subducted oceanic crust and its residues will be presented. I track a sequential process in which melts are redistributed from the (initially) low temperature melting of average oceanic crust (Spandler et al., 2008), and then from the residues (nominally anhydrous eclogitic compositions) of such subducted oceanic crust (Res2 and REC, this study). This study establishes the phase and melting relations and compositions of reasonable melting residues, which are themselves derived directly from subducted oceanic crust at high pressures and temperatures (GA2; Spandler et al., 2008). In particular, this study has systematically determined the solidus temperatures, phase relations, and partial melt compositions xv during upwelling of both, a residual quartz/coesite-bearing eclogite (Res2) and a residual quartz/coesite-free eclogite (REC) from 5 to 3 GPa and 1500-1200{u00B0}C.