Partial Melting of Garnet Lherzolite with H2o and CO2 at 3 GPa: Implications for Intraplate Magmatism.

Abstract:

The origin and source rock of alkali-rich and SiO₂-undersatured magmas in the Earth`s upper mantle have been under debate for a long time. The garnet signature in rare earth element patterns of such magmas suggest a garnet-bearing source rock, which could be garnet lherzolite or garnet pyroxenite.

Partial melting experiments were performed at 3 GPa and 1345-1445 °C in a piston-cylinder apparatus using mixtures of natural lherzolite with 0.4-0.7 wt% H₂O and 0.4-0.7 wt% CO₂ as starting materials. Different designs of AuPd capsules were used for melt extraction. Mineral and melt phases were analysed with electron microprobe and laser ablation inductively coupled plasma mass spectrometry.

The degree of partial melting in the experiments ranges from ~20% to ~4% and decreases with decreasing temperature and decreasing volatile content in the starting material. All samples contain olivine and orthopyroxene. Garnet is present in experiments performed below 1420 °C. The amount of clinopyroxene decreases with increasing degree of partial melting and volatile concentration in the starting material. Depending on the capsule design the melts quenched to glasses or to a mixture of quench crystals and residual glass.

The composition of the partial melts ranges from basalts through picrobasalts to foidites. The alkali concentration increases and the SiO₂ concentration decreases with decreasing degree of partial melting and increasing volatile concentration in the starting material. The partial melts are similar in many aspects to alkali intraplate magmas (basanites to melilitites), although they are richer in MgO. Compositions closer to natural basanites could be obtained either at lower degree of melting (and lower volatile contents) or through olivine fractionation. Our results strongly suggests that.

SiO₂-undersaturated intraplate magmas can be generated by mantle melting of garnet-lherzolite in the presence of H₂O and CO₂ in the Earth`s upper mantle at 3 GPa (~100 km depth).