Extinct isotope heterogeneities in the mantles of Earth and Mars: Implications for mantle stirring rates

Abstract

AbstractHeterogeneities in terrestrial samples for 182W/183W and 142Nd/144Nd are only preserved in Hadean and Archean rocks while heterogeneities in 129Xe/130Xe and 136Xe/130Xe persist to very young mantle‐derived rocks. In contrast, meteorites from Mars show that the Martian mantle preserves heterogeneities in 182W/183W and 142Nd/144Nd up to the present. As a consequence of the probable “deep magma ocean” core formation process, we assume that the Earth and Mars both had a very early two‐mantle‐reservoir structure with different initial extinct nuclide isotopic compositions (different 182W/183W, 142Nd/144Nd, 129Xe/130Xe, 136Xe/130Xe ratios). Based on this assumption, we developed a simple stochastic model to trace the evolution of a mantle with two initially distinct layers for the extinct isotopes and its development into a heterogeneous mantle by convective mixing and stretching of these two layers. Using the extinct isotope system 182Hf‐182W, we find that the mantles of Earth and Mars exhibit substantially different mixing or stirring rates. This is consistent with Mars having cooled faster than the Earth due to its smaller size, resulting in less efficient mantle mixing for Mars. Moreover, the mantle stirring rate obtained for Earth using 182Hf‐182W is consistent with the mantle stirring rate of ~500 Myr constrained by the long‐lived isotope system, 87Rb‐87Sr and 147Sm‐143Nd. The apparent absence of 182W/183W isotopic heterogeneity in modern terrestrial rocks is attributed to very active mantle stirring which reduced the 182W/183W isotopic heterogeneity to a relatively small scale (~83 m for a mantle stirring rate of 500 Myr) compared to the common sampling scale of terrestrial basalts (~30 or 100 km). Our results also support the “deep magma ocean” core formation model as being applicable to both Mars and Earth.