The near-solidus transition from garnet lherzolite to spinel lherzolite

Abstract

The position of the transition from spinel lherzolite to garnet lherzolite in the system CaO-MgO-Al 2 O 3 -SiO 2 (CMAS) has been determined experimentally at near-solidus temperatures. In reversed experiments, the transition occurs between 18 and 20 kbar at 1200°C and between 26 and 27 kbar at 1500°C, corresponding to higher pressures than previously envisaged. A position for the transition deeper within the Earth further complicates the explanation of the so-called garnet signatures in the trace element and isotope patterns of mid-ocean ridge basalts. If melting during adiabatic upwelling beneath a mid-ocean ridge begins at the depth required for the stability of garnet in peridotitic compositions, simple melting models predict that the amount of melt produced should be much greater than the observed thickness of the oceanic crust. A partial solution to the apparent con¯ict might be that (1) the rather simplistic melting models are in error, (2) that melting begins in garnet pyroxenite veins that are believed to be stable at lower pressures than garnet lherzolite or (3) that melting does not involve garnet at all, but it is clinopyroxene causing the trace element patterns observed in basalts erupted at mid-ocean ridges. A second set of reversal experiments were conducted to investigate the solubility of alumina in both orthopyroxenes and clinopyroxenes at the high temperatures near the solidus in the system CMAS. The results are compatible with most previous studies, and may be used as a starting point to calibrate thermodynamic models for pyroxenes in chemical systems, approximating upper mantle chemistry.

Volatile and trace elements in basaltic glasses from Samoa: Implications for water distribution in the mantle

“…The steepness of the rare-earth element slopes indicates melting within the stability field of garnet lherzolite or melting to small degrees within the stability field of spinel lherzolite (Hauri et al, 1994;Salters et al, 2002), as to be expected to occur beneath an old tectonic plate where the bottom of the lithosphere (e.g. Li et al, 2004) is near the garnet to spinel transition (Kogiso et al, 1998;Robinson and Wood, 1998;Klemme and O'Neill, 2000).…”

Pyroxenite xenoliths from Marsabit (Northern Kenya): evidence for different magmatic events in the lithospheric mantle and interaction between peridotite and pyroxenite

“…With respect to the bulk composition, adding peridotite to pyroxenite would shift the grt-spl transition to higher pressures, given the Cr-rich nature of peridotite when compared to pyroxenite (e.g., Klemme and O'Neill 2000).…”

The geophysical characteristic of the lower lithosphere and asthenosphere in the marginal zone of the East European Craton

“…Since the Scania lavas probably come from asthenosphere at 90-120 km depths (Tappe et al 2016), the Scania xenoliths show that the lithospheric mantle was only in the spinel facies of mantle peridotites in the time of volcanism. The spinel Cr# of 0.1-0.2 (Rehfeldt et al 2007) thus suggests that the LAB was located at 70-85 km (assuming a temperature of 1000-1100 °C; Klemme andO'Neill 2000, andcompilation of Xiong et al 2015). This LAB depth is significantly shallower from that indicated by our seismic data.…”