Hot primary melts and mantle source for the Parana-Etendeka flood basalt province: New constraints from Al-in-olivine thermometry

Continental flood basalts (CFB) are amongst the most voluminous volcanic eruptions in Earth's history. They are rapidly emplaced, and in rare cases the thick lava piles are associated with primitive magmas that have high MgO contents. The compositions of these primitive melts are consistent with a deep-sourced, high-temperature mantle plume origin. Whilst the association of CFBs with impacting mantle plumes is widely accepted, the magnitude of the thermal anomaly is not yet resolved. The development of Al-in-olivine thermometry, however, allows the crystallisation temperature of (near-)liquidus olivine to be determined without knowing the composition of the co-existing melt. This provides both a robust minimum estimate of mantle temperature and a value from which potential temperature (T-P) can be back-calculated. This technique has previously confirmed that crystallisation temperatures in CFB settings can be a few hundred degrees greater than those estimated for MORB, and the results hint at a diversity in crystallisation temperatures between different CFB settings. In this study, we re-assess the T-P of the mantle source of the Parana-Etendeka CFB province by applying the Al-in-olivine thermometer to olivine-spinel pairs from picrites and ferropicrites. We show that the mean crystallisation temperature of olivine with Fo(> 90) in the picrites is 1458 degrees C, with a maximum temperature of 1511 degrees C. Using the mean value, we calculate a preferred T-P of 1623 degrees C, for an assumed lithospheric thickness of 50 km and magma emplacement pressure of 0.5 GPa. This represents a thermal anomaly of around +300 degrees C relative to ambient mantle, and confirms that the mantle source of the Parana-Etendeka CFB is the second hottest known from Phanerozoic Large Igneous Provinces, after the Caribbean Large Igneous Province. The ferropicrites record a cooler mean olivine crystallisation temperature of 1296 degrees C. Given that these low-volume melts derive from deeper and earlier melting of mantle pyroxenite, their temperature is not directly comparable to that of the picrites but they appear to require a somewhat cooler mantle source - perhaps found at the front or edges of a rising plume head.