Depth of formation of subcontinental off-craton peridotites

The subcontinental lithosphere is thought to be a mantle region from which melts have been extracted. This process lowers Ca and Al contents, thus making the lithosphere more refractory, but little is known about the depth at which those melts are extracted. Experimental evidence suggests that iron might be used as a pressure gauge. At high pressures, melting depletes the residue in iron, but at lower pressures (P < 2 GPa) and low degrees of melting, the residue must be enriched in iron. However, such Fe-enrichments have never been documented in mantle peridotites. Here we show that Fe–Al relationships in mantle xenoliths from central Asia match those predicted by melting experiments both at low and moderate pressures. We find that Fe is negatively correlated with Al in fertile (> 3.5% Al2O3) peridotites consistent with 0–6% melting at 1 GPa. In contrast, Fe is positively correlated with Al in more refractory peridotites produced by higher degrees (up to 35%) of partial melting at 2–4 GPa. The current position of these rocks in the lithosphere estimated from thermo-barometry may be quite different from the depth of melt extraction. We attribute this to tectonic processes during the transformation of an initial assemblage of melting residues into mature continental lithosphere.