| Abstract |
Mantle heterogeneity is chiefly caused by input of Earth’s surface material into the mantle via slab subdution. HIMU ocean basalts are defined by their unique geochemical characteristics derived from ancient subudcted slabs, and therefore it is essential to explore the origin of HIMU to better understand the differentiation and evolution in the mantle. We conducted high-precision isotope measurements on HIMU basalts from the Pacific and Atlantic. While they exhibit similar Sr, Nd, and Hf isotopic compositions, the HIMU basalts in the Atlantic show systematically higher 207Pb/204Pb for a given 206Pb/204Pb than the HIMU basalts in the Pacific. The Pb isotope evolution model suggests that both HIMU reservoirs formed around 2 Ga; however, the HIMU reservoir beneath the Pacific is about 0.3 Ga younger than that beneath the Atlantic. We interpret the age difference as that the HIMU reservoir beneath the Pacific involved younger subducted materials more abundant than that beneath the Atlantic, followed by the long-term isolation of the two reservoirs in the convecting mantle. Using the same Pb evolution model, Th/U of the HIMU reservoir deduced from the 206Pb/204Pb?208Pb/204Pb relationship is higher than that of present-day altered MORB, Th/U of which is lowered by addition of U from oxic hydrothermal fluids. This suggests that the precursor of the HIMU reservoir was hydrothermally altered MORB with minimal U enrichment in the less oxic environment in the Archean or early Proterozoic. Consequently, the formation age of the HIMU reservoir should predate the Great Oxidation Event. |
