Phase relations and solidus temperature of garnet lherzolite at 5 GPa revisited

Partial melting experiments on a mildly depleted peridotite (GKR-001) conducted in a piston-cylinder apparatus provide new insights into the solidus of anhydrous peridotite, melting relations and reactions, and residue compositions at 5 GPa. These have important implications for melting conditions relevant to cratonic lithosphere, the petrogenesis of komatiites and orthopyroxene-enriched peridotites. The solidus of GKR-001 was identified at 1575ºC. As melting progressed, clinopyroxene melted out at ~ 1650ºC, closely followed by garnet at ~ 1670ºC and orthopyroxene at ~ 1725ºC. Orthopyroxene remained stable throughout most of the melting interval. Clinopyroxene's incongruent melting initiated peritectic reactions (Ol + Cpx + Grt = Opx + Melt) at melting degrees ≥ 8%, forming additional orthopyroxene in the residue. Clinopyroxene exhausted at around 24% melting, while garnet-out was encountered at slightly higher melting degree (~ 34%). Orthopyroxene reached its maximum content in the residue after ~ 24% melting and was dissolved back at melting degrees > 45%. The melts formed at ≥ 8% melting are Ti-depleted komatiites (30.7–43.6 Al₂O₃/TiO₂), consistent with melting of a slightly depleted peridotitic source. Melts in equilibrium with a garnet-bearing residue have CaO/Al₂O₃ ratios > 1, while those formed above the garnet-out curve have ratios < 1. Residues from GRK-001 exhibit higher SiO₂ contents than those produced after the partial melting of pyrolytic compositions (KR4003). However, high-pressure partial melting of GKR-001, albeit producing quite orthopyroxene-rich residues at moderate degrees of melting, cannot produce the silica-rich peridotites observed in many cratonic lithospheres worldwide. Comparison between thermodynamic modelling and experimental results reveals inconsistencies in the garnet and pyroxenes stability fields and the absolute temperature of the solidus.