Experimental study of the phase and melting relations of homogeneous basalt plus peridotite mixtures and implications for the petrogenesis of flood basalts

Flood basalt provinces may constitute some of the most catastrophic volcani c events in the Earths history. A popular model to explain them involves ad iabatic ascent of plumes of anomalously hot peridotite from a thermal bound ary layer deep in the mantle, across the peridotite solidus. However, perid otitic plumes probably require unreasonably high potential temperatures to generate sufficient volumes of magma and high enough melting rates to produ ce flood volcanism. This lead to the suggestion that low melting eclogitic or pyroxenitic heterogeneities may be present in the source regions of the flood basalts. In order to constrain petrogenetic models for flood basalts generated in this way, an experimental investigation of the melting relatio ns of homogeneous peridotite + oceanic basalt mixtures has been performed. Experiments were conducted at 3.5 GPa on a fertile peridotite (MPY90)-ocean ic basalt (GA1) compositional join. The hybrid basalt + peridotite composit ions crystallised garnet Iherzolite at subsolidus temperatures plus quenche d ne-normative picritic liquids at temperatures just above the solidus, ove r the compositional range MPY90 to GAl(50)MPY90(50). The solidus temperatur e decreased slightly from similar to 1500 degrees C for MPY90(-) to similar to 1450 degrees C for GA1(50)MPY90(50). Compositions similar to GA1(30)MPY 90(70) have 100% melting compressed into a melting interval which is approx imately 50-60% smaller than that for pure MPY90, due to a liquidus minimum. During adiabatic ascent of hybrid source material containing a few lens of percent basalt in peridotite, the lower solidus and compressed solidus-liq uidus temperature interval may conspire to substantially enhance melt produ ctivity. Mixtures of recycled oceanic crust and peridotite in mantle plumes may therefore provide a viable sourer for some flood volcanics. Evidence f or this would include higher than normal Fe/Mg values in natural primary li quids, consistent with equilibration with more Fe-rich olivine than normal pyrolitic olivine (i.e. <Fo(89-92)) Modelling of fractionation trends in We st Greenland picrites is presented to demonstrate that melts parental to th e Greenland picrites were in equilibrium at mantle P-T conditions with oliv ine with Fo(84-86), suggesting an Fe-enriched source compared with normal p eridotite, and consistent with the presence of a basaltic component in the source.