A Hot Source for Picritic Melts

Andy Saunders

Department of Geology, University of Leicester


I briefly review the evidence for elevated potential temperatures in the mantle source of ultramafic liquids. Evidence of the eruption of ultramafic liquids is documented in several large igneous provinces (e.g., North Atlantic Igneous Province1, Caribbean Plateau2 and Etendeka3) and ocean islands (e.g., Kilauea4 and Gorgona5). Such evidence exists in the form of analysed glass, and/or high-Fo olivine phenocrysts from which equilibrium liquid compositions can be calculated6-8. Inverse and forward modelling7,8 indicate high MgO contents of parental and primary melts, and high potential temperatures of the mantle source regions (typically, 1520-1570°C for primary magmas with 18-20% MgO7,8). Even higher source temperatures are predicted for Gorgona komatiites5 and Etendeka picrites3. Such high temperatures are not, however, indicated for the source of mid-ocean ridges9, where primary magmas are predicted to contain lower amounts of MgO (generally < 12%). Icelandic Mg-rich basaltic liquids indicate source temperatures intermediate (~1400-1450°C 8,9) between those of Kilauea (~1550°C7) and MOR (~1250°C)9.

Given the extreme conditions necessary for high-temperature, high-density melts to traverse thick lithosphere with a lower melting point and lower density10, the scarcity of such liquids at the Earth’s surface is not surprising. The absence of abundant picritic melts in places such as Iceland may be a result of such physico-chemical filtering and may not reflect the average composition of the melts crossing the Moho. Similar filtering of dense, magnesian primary liquids may also occur beneath MOR. However, estimates11 of the bulk composition of the ocean crust restrict the MgO content of average Moho-crossing liquids to less than about 12%, consistent with experimental studies9, although some fractionation and Mg-loss may occur within the cool upper mantle below slow-spreading ridges12. Whilst enhanced volatiles (H2O or CO2) in the source can reduce the mantle potential temperatures necessary to produce either magnesian liquids or increased volumes of basaltic melt, near-fractional melting will rapidly remove the volatiles from the source, reducing their efficacy.


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