the importance of ultramafic magmatism is often dimin-A parameterization of experimental data in the 0·2-7·0 GPa pressure range constrains both forward models of potential primary ished in models of Phanerozoic hotspot volcanism because magma compositions that exit the melting regime in the mantle and basalt eruptives are more common, and the plume model inverse models for computing the effects of olivine fractionation for is subject to debate (Anderson, 2000). Even where ultraany

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... lava suite. This is used to infer the MgO contents mafic lavas occur, they often have a large olivine phenoof primary magmas from Gorgona, Hawaii, Baffin Island and cryst content and this complicates interpretations of the West Greenland. They typically contain 18-20% MgO for wide MgO content of the primary magma. Indeed, the invariations in assumed peridotite source compositions, but MgO can terpretation that picrites can only form by the acdrop to 14-17% for Fe-enriched sources, and increase to 24-26% cumulation of olivine in basaltic liquids (Bowen, 1928) for fractional melts from Gorgona. Primary magmas with 18-20% went unchallenged until petrographic evidence dem-MgO have potential temperatures of 1520-1570°C. For Gorgona onstrated that skeletal olivine can grow in situ in ultrabasic picrites with 24% MgO, the potential temperature and initial liquids (Drever, 1956; Drever & Johnston, 1957). melting pressure were about 1700°C and 8·0 GPa, respectively; Discussions on primary magma compositions are often melting was hot and deep, consistent with the plume model. There reduced to the following shortlist of frequently asked are important restrictions to magma mixing in mantle plumes. questions: How can the composition of a lava flow be Primary magmas that exit the melting regime are both well-mixed used to derive the composition of its primary magma aggregate fractional melts and isolated fractional melts. The latter extracted from the melting regime in the mantle? How can originate from a hot plume axis and be in equilibrium with can an evaluation be made of the effects of olivine olivines having mg-numbers of 93·0-93·6, but they have MgO fractionation in crust and mantle lithosphere, and the contents and thermal characteristics that are difficult to constrain. possible sampling of wall-rock olivines from the melting regime? How can we best describe potentially complex fractional melting products of the mantle with equilibrium experimental data? Can the physics of the melt collection process be understood by identifying geochemical differ-