Recent geochemical studies document a wide range of 182W anomalies in mantle-derived rocks, the origin of which is unknown. Here we explore the consequences of basal magma ocean crystallization while 182Hf was extant. We determine the partition coefficient of Hf between bridgmanite and silicate melt DHf throughout the pressure-temperature regime of Earth's lower mantle from first principles molecular dynamics coupled with thermodynamic integration. The calculations are based on Hf4+ entering the Mg-site, which we show is energetically preferable to the Si-site. We find that DHf increases by more than an order of magnitude from 25 GPa to 140 GPa, making Hf much more compatible with increasing pressure. The larger DHf at greater depths produces a strong fractionation of 182Hf between crystallized bridgmanite and silicate melt. We use a simple geochemical model to show that heterogeneous tungsten anomalies are a natural outcome of the formation and crystallization of a basal magma ocean, with the crystallized products enriched in 182W, while the remaining liquid is depleted by a similar amount. Our results show that the observed large variability of 182W/184W in terrestrial samples may be due to the solidification of a basal magma ocean.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science
- ab initio calculation
- silicate melt partitioning
- tungsten anomaly