Mineralogy of subducted basaltic crust (MORB) from 25 to 37 GPa, and chemical heterogeneity of the lower mantle

Experimental multi-anvil study of the phase relations in subducted oceanic crust (mid-ocean ridge basalt (MORB)) composition over a pressure range from 25 to 37 GPa, equivalent to depths of 700–950 km in the lower mantle, reveals that at pressures between 25 and 30 GPa majoritic garnet disproportionates into the Mg- and Ca-perovskites and an aluminous calcium ferrite type phase, but no further change in mineralogy takes place at higher pressures up to 37 GPa. The intermittent seismic discontinuity, at least that at 920 km depth reported by Kawakatsu and Niu [H. Kawakatsu, F. Niu, Nature 371 (1994) 301–305], might be attributable to these mineralogical changes in the former basaltic crust of subducted slabs. Moreover, we estimate that a density profile of MORB intersects an average mantle density at around 1500–2000 km depth in the lower mantle. The neutral buoyancy may contribute to the observed transition in seismological heterogeneity at a depth around 1600 km.