Most of the platinum in the Earth’s mantle was brought by meteorites after the formation of the core

By analyzing, for the first time, the natural isotopic composition of platinum in meteorites as well as in ancient (over 3.85 billion years old) and modern terrestrial rocks, an international team including IPGP researchers has established the origin of terrestrial platinum. While the platinum contained in modern rocks was brought by meteorites after the formation of the Earth’s core, a fraction of the platinum contained in the oldest terrestrial rocks is primordial platinum, dating from before the formation of the Earth’s core. This suggests that modern-style plate tectonics began more than 3.85 billion years ago.

Ref : Creech, J.B., Baker, J.A., Handler, M.R., Lorand, J.-P., Storey, M., Wainwright, A.N., Luguet, A., Moynier, F., Bizzarro, M. (2017) Late accretion history of the terrestrial planets inferred from platinum stable isotopes. Geochem. Persp. Let. 3, 94-104.