Alexandre Corgne

Trace-element fractionation in Hadean mantle generated by melt segregation from a magma ocean

Calculations of the energetics of terrestrial accretion indicate that the Earth was extensively molten in its early history. Examination of early Archaean rocks from West Greenland (3.6–3.8 Gyr old) using short-lived 146Sm–142Nd chronometry indicates that an episode of mantle differentiation took place close to the end of accretion (4.46 ± 0.11 Gyr ago). This has produced a chemically depleted mantle with an Sm/Nd ratio higher than the chondritic value. In contrast, application of 176Lu–176Hf systematics to 3.6–3.8-Gyr-old zircons from West Greenland indicates derivation from a mantle source with a chondritic Lu/Hf ratio. Although an early Sm/Nd fractionation could be explained by basaltic crust formation, magma ocean crystallization or formation of continental crust, the absence of coeval Lu/Hf fractionation is in sharp contrast with the well-known covariant behaviour of Sm/Nd and Lu/Hf ratios in crustal formation processes. Here we show using mineral–melt partitioning data for high-pressure mantle minerals that the observed Nd and Hf signatures could have been produced by segregation of melt from a crystallizing magma ocean at upper-mantle pressures early in Earths history. This residual melt would have risen buoyantly and ultimately formed the earliest terrestrial protocrust.

Atomistic simulations of trace element incorporation into the large site of MgSiO3 and CaSiO3 perovskites

We have used computer simulation techniques to study the mechanisms of trace element incorporation into the large sites of MgSiO3 and CaSiO3 perovskites on the atomistic level. Both relaxation and solution energies corresponding to the incorporation of isovalent and heterovalent defects can be fitted using the ‘lattice strain’ model. This underlines the importance of crystal chemistry in the partitioning of trace elements between perovskite and melt. As expected, solution energies, which take some account of possible melt components, approximate more closely experimental perovskite–melt partitioning observations than do relaxation energies. With calculated solution energies we find that the optimum site radius r0 decreases with increasing defect charge while the apparent Young’s modulus Eα of the large site increases with charge in both perovskites, consistent with experimental perovskite–melt partitioning data. For a given trace element charge r0 is smaller for MgSiO3 than for CaSiO3, as observed experimentally. For a given trace element the difference in calculated solution energy between the two perovskites is reflected in the difference in experimental partition coefficients. For all the charge balancing mechanisms we have considered the solution energies for REE 3+ , and 4+ cations (including U 4+ and Th 4+ ) are much lower in CaSiO3 perovskite than in MgSiO3 perovskite, which provides an explanation for the observed ease with which CaSiO3 perovskite accommodates 3+ and 4+ cations.

La diversité des basaltes de Patagonie à la latitude du point triple du Chili (46°-47° lat. S): Données complémentaires et implications sur les conditions de la subduction

Abstract The basalts exposed around the Lago General Carrera–Buenos Aires display a wide range of chemical compositions. These include: Cretaceous calc-alkaline magmas typical of the Andean arc, Eocene to Quaternary intraplate alkali magmas and Quaternary magmas, the geochemical signature of which shows similarities with those of Chile ridge MORB. Such diversity is related to the evolution of the subduction processes below Patagonia, starting with a ‘normal’ subduction regime, followed by collision–subduction of the active Farallon–Aluk and Chile ridges together with the development of ‘asthenospheric windows’ during the Eocene and since the Upper Miocene respectively.