Ferropicrites: geochemical evidence for Fe-rich streaks in upwelling mantle plumes

A comparison of high-MgO magmas from both oceanic and continental settings reveals that they exhibit wide variations in their bulk-rock contents of FeO* (9–16 wt% at 15 wt% MgO). The high-FeO* picrites (ferropicrites) range from Archean to recent in age but are relatively rare at the Earth’s surface, typically forming thin isolated flows near the base of thick lava piles in large igneous provinces. They are characterised by high contents of compatible trace elements (e.g. Cr=400–1650 parts per million (ppm) and Ni=250–1050 ppm) and unaltered samples (Paraná–Etendeka and Madagascar) have relatively smooth, normalised multi-element patterns that lack significant relative enrichments in strongly incompatible elements (e.g. [Ba/La]n=0.5–1.0) and [La/Nb]n=1.2–1.4). The ferropicrites are distinguished from other picritic rocks (e.g. Deccan, Hawaii, West Greenland) by their relatively low abundances of Al2O3 (∼10 wt%) and heavy rare-earth elements (HREEs, e.g. Lu=<10×chondrite). They have 87Sr/86Sri ratios of ∼0.7048 and ϵ Nd values of ∼+4 that are comparable to those of ocean-island basalts. Modelling calculations of combined Sr, Nd and Pb isotopic ratios indicate that some of the ferropicrites may have assimilated upper or lower crust but this appears to have had little effect on major element abundances. The high-FeO* contents of world-wide ferropicrites, relative to ‘normal’ picrites, cannot simply be attributed to variations in degrees of partial melting and/or depth of melt segregation of an anhydrous lherzolite mantle source. Quantitative partition modelling suggests that the contributing parental melts of the ferropicrites were derived by adiabatic decompression melting of a mantle source that was similar in composition to experimentally studied Fe-rich peridotite PHN1611. The parental melts of the Paraná–Etendeka ferropicrites appear to have been generated by ∼10% partial melting, at high pressures (45–35 kbar) and high mantle potential temperatures (T p=1550°C). The relatively low volume of world-wide ferropicrites and their association with igneous rocks of ‘normal’ FeO* contents in mantle plume-related igneous provinces suggest that the former may be derived from Fe-rich streaks in mantle plume starting-heads.