Some hotspots are hot -evidence from the picrites of West Greenland (Invited)

There is geological and petrological evidence for high temperatures in the mantle source for the 62 Ma picrites of the Vaigat Formation in West Greenland. The formation extends over 250x110 km with thicknesses up to 5 km and a volume of >20,000 km3. The magmas were erupted through the c. 100 km thick stretched continental lithosphere of the Nuussuaq Basin, and the distance to true oceanic lithosphere is at least 200 km. Eruption sites are well known and are often controlled by faults; they extended with time from west to east. Uncontaminated rocks constitute c. 95% of the Vaigat Formation. The volume-weighted average MgO content is 16.6 wt%, a very robust figure; the total spread is 6.5-30.8 wt% MgO, and three fourths of the rocks classify as picrites with MgO ≥12 wt%. The rocks are geochemically depleted and MORB-like, with tightly clustered and well correlated Sr, Nd and Os isotope compositions indicating an asthenospheric mantle source. We know of no other occurrences of similarly voluminous, geochemically coherent, asthenosphere-derived picrite sucessions in the World. On eruption the picritic magmas consisted of melt with ≤14 wt% MgO charged with cognate olivine (+chromite) crystals crystallised in the conduit systems. Olivine continued crystallising post-eruption, and water-quenched picrites consist of ol+crm+glass with up to 9 wt% MgO, quenched at 1180°-1220°C. There is no cpx, and plagioclase only forms tiny quench microlites. Evidence for high temperatures comes from the composition of olivine (up to Mg# 93, with hich Cr contents) and chromite (up to Mg# 77). The high-Mg# crystals are compositionally similar only to minerals from komatiites or produced experimentally from melts at very high temperatures. Back-calculation of melt compositions by stepwise addition of equilibrium olivine to the matrix glasses is justified by the lack of other fractionating phases than olivine and chromite. When olivine is added until the calculated melt is in equilibrium with the most magnesian olivine (Mg# 92.0-92.5) at pressures corresponding to the base of the lithosphere (3GPa), the resulting melts have 21-22 wt% MgO and equilibration temperatures of 1600°-1650°C. The corresponding mantle potential temperatures are around 50 degrees higher, significantly higher than that of the MORB mantle but similar to other hotspot volcanics. If the aggregated melt exciting the melting column was not completely homogenised, some of the most magnesian olivines may have crystallised from the last and very Fe-poor melt fractions. Then a stopping point for the olivine addition process is impossible to identify, and the melt MgO and temperatures should be lower. An absolute minimum content of MgO in the primary melt is the average MgO of the whole picrite succession, unless cumulates have come up and evolved liquids have stayed down. Addition of olivine until the melt has 17.0 wt% MgO results in olivine with Mg# 90-91 and equilibration temperatures close to 1550°C at 3 GPa which is a minimum estimate. The corresponding minimum mantle potential temperature around 1600°C is still significantly higher than that of the MORB mantle.