On the significance of ultra-magnesian olivines in basaltic rocks

Ultra-magnesian olivines in basaltic rocks or magma are here defined by a forsterite proportion [Fo = Mg/(Mg + Fe) in molar percent] greater than the maximum Fo_91.5 known from Earth's mid-ocean ridge systems. They are relatively common in Archean komatiites and have been also reported from several Phanerozoic large igneous provinces. The presence of ultra-magnesian olivines is generally interpreted as evidence for melts with extremely high MgO concentration (komatiites) and high eruption temperatures. Such melts are considered to be hallmarks of a hotter mantle in the early Earth and of thermal anomalies related to Phanerozoic mantle plumes. Here we show that the link between ultra-magnesian olivines and komatiitic melts is not straightforward. The composition of melt inclusions trapped in ultramagnesian olivine (Fo_93.3) from the Etendeka province of Namibia contradict the predicted 24 wt% MgO for parental melts and 1680 °C mantle potential temperature. Instead, the trapped melts do not exceed 17.5 wt% MgO, and the maximum potential temperature indicated by these compositions is 1520 °C. We show that ultra-magnesian olivines can also be produced by protracted melt extraction from the mantle source, and that this process also leaves a distinctive depletion in incompatible trace elements.