Melting of plagioclase + spinel lherzolite at low pressures (0.5 GPa): an experimental approach to the evolution of basaltic melt during mantle refertilisation at shallow depths

The presence of plagioclase + spinel lherzolites among ocean floor samples and in some ophiolite complexes invites speculation on their origin and relationships to processes of magmatism and lithosphere refertilisation beneath mid-ocean ridges. In an experimental approach to their petrogenesis, we have determined the compositions of liquids and co-existing minerals in the six phase assemblage [liquid+olivine+orthopyroxene+clinopyroxene+plagioclase+spinel] at 0.5GPa and 1100°C to 1200°C. In our experimental approach we maintained the olivine Mg# [Mg/(Mg+Fe)] close to 90 (i.e., 88.8-95.5) but varied plagioclase from anorthite to albite. The major variations in liquid compositions are related to plagioclase composition. Liquids have much lower MgO and FeO and higher SiO2 and Al2O3 than liquids in the 6-phase plagioclase+spinel lherzolite at 0.75GPa and 1GPa. Liquids are quartz-normative (silica-oversaturated) for plagioclase that are more calcic than An40 but nepheline-normative (critically silica-undersaturated) for plagioclase that are more sodic than An25. Liquid compositions are quite unlike natural MORB glasses with similar Mg# (i.e., compatible with parental magmas from lherzolitic mantle with Mg#≈90). Our study provides no support for models of MORB petrogenesis which suggest extraction of near-solidus melts from plagioclase lherzolite at low pressure. Similarly, referring to numerical models of melting volumes beneath mid-ocean ridges (Langmuir et al., 1992; McKenzie and Bickle, 1988) in which melt increments are calculated for different sites and these increments pooled to form MORB, our data argue that melts equilibrated with plagioclase±spinel lherzolite at <1GPa cannot be significant components of such 'pooled melt' focussed from within the melting volume. The compositions of minerals from plagioclase±spinel lherzolite at Lanzo (northern Italy; Piccardo et al., 2007) are compared with our experimental assemblages at 0.5, 0.75 and 1GPa, leading to the conclusion that the Lanzo plagioclase±spinel lherzolites equilibrated at pressures between 0.75 and 1GPa, at temperatures ~100-200°C below the solidus. Field, petrological and geochemical studies argue that the Lanzo plagioclase±spinel lherzolites are 'refertilised' by the reaction of residual harzburgite or lherzolite with percolating intergranular basaltic magma (Piccardo et al., 2007). The experimental study suggests that the process of refertilisation took place at depths of 25-30km. Our experimental data also define the co-variance of Na2O in coexisting plagioclase (An25 to An94) and clinopyroxene at 0.5 and 0.75GPa. From these data, the Na2O content of clinopyroxene can be used as a predictor for the co-existing plagioclase composition in the very common occurrences of partially serpentinised peridotite in which plagioclase is completely saussuritised. © 2013 Elsevier B.V..