RESUMO
Active volcanic hotspots can tap into domains in Earth's deep interior that were formed more than two billion years ago. High-precision data on variability in tungsten isotopes have shown that some of these domains resulted from differentiation events that occurred within the first fifty million years of Earth history. However, it has not proved easy to resolve analogous variability in neodymium isotope compositions that would track regions of Earth's interior whose composition was established by events occurring within roughly the first five hundred million years of Earth history. Here we report 142Nd/144Nd ratios for Réunion Island igneous rocks, some of which are resolvably either higher or lower than the ratios in modern upper-mantle domains. We also find that Réunion 142Nd/144Nd ratios correlate with helium-isotope ratios (3He/4He), suggesting parallel behaviour of these isotopic systems during very early silicate differentiation, perhaps as early as 4.39 billion years ago. The range of 142Nd/144Nd ratios in Réunion basalts is inconsistent with a single-stage differentiation process, and instead requires mixing of a conjugate melt and residue formed in at least one melting event during the Hadean eon, 4.56 billion to 4 billion years ago. Efficient post-Hadean mixing nearly erased the ancient, anomalous 142Nd/144Nd signatures, and produced the relatively homogeneous 143Nd/144Nd composition that is characteristic of Réunion basalts. Our results show that Réunion magmas tap into a particularly ancient, primitive source compared with other volcanic hotspots, offering insight into the formation and preservation of ancient heterogeneities in Earth's interior.
RESUMO
Globally distributed kimberlites with broadly chondritic initial 143Nd-176Hf isotopic systematics may be derived from a chemically homogenous, relatively primitive mantle source that remained isolated from the convecting mantle for much of the Earth's history. To assess whether this putative reservoir may have preserved remnants of an early Earth process, we report 182W/184W and 142Nd/144Nd data for "primitive" kimberlites from 10 localities worldwide, ranging in age from 1,153 to 89 Ma. Most are characterized by homogeneous µ182W and µ142Nd values averaging -5.9 ± 3.6 ppm (2SD, n = 13) and +2.7 ± 2.9 ppm (2SD, n = 6), respectively. The remarkably uniform yet modestly negative µ182W values, coupled with chondritic to slightly suprachondritic initial 143Nd/144Nd and 176Hf/177Hf ratios over a span of nearly 1,000 Mya, provides permissive evidence that these kimberlites were derived from one or more long-lived, early formed mantle reservoirs. Possible causes for negative µ182W values among these kimberlites include the transfer of W with low µ182W from the core to the mantle source reservoir(s), creation of the source reservoir(s) as a result of early silicate fractionation, or an overabundance of late-accreted materials in the source reservoir(s). By contrast, two younger kimberlites emplaced at 72 and 52 Ma and characterized by distinctly subchondritic initial 176Hf/177Hf and 143Nd/144Nd have µ182W values consistent with the modern upper mantle. These isotopic compositions may reflect contamination of the ancient kimberlite source by recycled crustal components with µ182W ≥ 0.
RESUMO
How much of Earth's compositional variation dates to processes that occurred during planet formation remains an unanswered question. High-precision tungsten isotopic data from rocks from two large igneous provinces, the North Atlantic Igneous Province and the Ontong Java Plateau, reveal preservation to the Phanerozoic of tungsten isotopic heterogeneities in the mantle. These heterogeneities, caused by the decay of hafnium-182 in mantle domains with high hafnium/tungsten ratios, were created during the first ~50 million years of solar system history, indicating that portions of the mantle that formed during Earth's primary accretionary period have survived to the present.