Abstract:
One of the goals of igneous petrology is to use the subtle and more obvious differences in the geochemistry of primitive basalts to place constraints on mantle composition, melting conditions and dynamics of mantle upwelling and melt extraction. For this goal to be achieved, our first-order understanding of mantle melting must be refined by high-quality, systematic data on correlated melt and residual phase compositions under known pressures and temperatures. Discrepancies in earlier data on melt compositions from a fertile mantle composition [MORB (mid-ocean ridge basalt) Pyrolite mg-number 87] and refractory lherzolite (Tinaquillo Lherzolite mg-number 90) are resolved here. Errors in earlier data resulted from drift of W/Re thermocouples at 1 GPa and access of water, lowering liquidus temperatures by 30-80 degrees C. We demonstrate the suitability of the 'sandwich' technique for determining the compositions of multiphase-saturated liquids in lherzolite, provided fine-grained sintered oxide mixes are used as the peridotite starting materials, and the changes in bulk composition are considered. Compositions of liquids in equilibrium with lherzolitic to harzburgitic residue at 1 GPa, 1300-1450degrees C in the two lherzolite compositions are reported. Melt compositions are olivine + hypersthene-normative (olivine tholeiites) with the more refractory composition producing a lower melt fraction (7-8% at 1300 degrees C) compared with the model MORB source (18-20% at 1300 degrees C).
