Platinum Concentrations and Tungsten Isotope Ratios of Earth’s Mantle as Tracers for Late Veneer Mixing into the Early Mantle

Friday, 19 December 2014
Li Zeng, Harvard Coll Observatory, Cambridge, MA, United States, Stein B Jacobsen, Harvard University, Earth and Planetary Sciences, Cambridge, MA, United States and Dimitar D. Sasselov, Harvard University, Astronomy, Cambridge, MA, United States
Platinum (Pt) and tungsten (W) are both depleted in Earth’s mantle. Due to their siderophile nature, they were both strongly partitioned into the Earth’s core during its formation. However, in particular the Pt concentration in the Earth’s mantle is much higher than expected from experimental data on metal-silicate partitioning appropriate for the conditions of core formation. A plausible explanation for this high Pt concentration is the late veneer hypothesis, where volatile-rich chondritic type material was delivered to Earth’s surface after core formation. This can in principle explain both the volatiles in the Earth’s ocean-atmosphere as well as the high Pt concentrations in the Earth’s mantle. There are tungsten isotopic heterogeneities (182W/183W variations) in the early Earth that have been explained as being due to late veneer addition, as this material would have lower 182W/183W than the post-core formation mantle. There is also the gradual increase of Pt abundance through history in mantle as measured in mantle-derived rocks of various old ages. Both observations are thought to be caused by gradual mixing of late veneer material into Earth’s mantle through plate subduction and mantle convection through geologic time. This would increase the Pt concentration and decrease the 182W/183W ratio in the mantle with time, as observed. Here we model the late veneer material as a thin sheet which is subducted into the mantle, and get stretched and mixed with the mantle material gradually. The stretching is assumed to follow a simple exponential law of decrease of the characteristic size of heterogeneity regions. The melting events that produce the rock samples of various ages measured on the surface are modeled as random geometric sampling of a sampling box with a certain length-scale. We are testing various scenarios of this mathematical model to see if both the variations in W isotopic ratio and the Pt concentration in Earth’s history can be made consistent with each other within our model.