M. Bizzarro

Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites

Long- and short-lived radioactive isotopes and their daughter products in meteorites are chronometers that can test models for Solar System formation. Differentiated meteorites come from parent bodies that were once molten and separated into metal cores and silicate mantles. Mineral ages for these meteorites, however, are typically younger than age constraints for planetesimal differentiation. Such young ages indicate that the energy required to melt their parent bodies could not have come from the most likely heat source—radioactive decay of short-lived nuclides (26Al and 60Fe) injected from a nearby supernova—because these would have largely decayed by the time of melting. Here we report an age of 4.5662 ± 0.0001u2009billion years (based on Pb–Pb dating) for basaltic angrites, which is only 1u2009Myr younger than the currently accepted minimum age of the Solar System and corresponds to a time when 26Al and 60Fe decay could have triggered planetesimal melting. Small 26Mg excesses in bulk angrite samples confirm that 26Al decay contributed to the melting of their parent body. These results indicate that the accretion of differentiated planetesimals pre-dated that of undifferentiated planetesimals, and reveals the minimum Solar System age to be 4.5695 ± 0.0002u2009billion years.

Hf isotope evidence for a hidden mantle reservoir

High-precision Hf isotopic analyses and U-Pb ages of carbonatites and kimberlites from Greenland and eastern North America, including Earth9s oldest known carbonatite (3 Ga), indicate derivation from an enriched mantle source. This previously unidentified mantle reservoir—marked by an unradiogenic Hf isotopic composition and preserved in the deep mantle for at least 3 b.y.—may account for the mass imbalance in Earth9s Hf-Nd budget. The Hf isotopic data presented here support a common mantle source region and genetic link between carbonatite and some oceanic- island basalt volcanoes.

In situ 87Sr/86Sr investigation of igneous apatites and carbonates using laser-ablation MC-ICP-MS

Abstract In situ Sr isotopic compositions of coexisting apatite and carbonate for carbonatites from the Sarfartoq alkaline complex, Greenland, have been determined by laser-ablation multicollector inductively coupled plasma mass spectrometry. This study is the first to examine the extent of Sr isotopic homogeneity among coexisting igneous minerals containing high Sr (>3000 ppm) and low Rb (≪1 ppm) contents within a single ∼50-μm-thick thin-section mount. This technique is capable of producing measured 87 Sr/ 86 Sr values with analytical precision (∼0.005%, 2σ) approaching those obtained by thermal ionization mass spectrometry but in a much shorter interval of time (100 s vs. >1 h, respectively). The combined total analyses ( n = 107) of apatite and carbonate yield 87 Sr/ 86 Sr compositions ranging from ∼0.7025 to ∼0.7031. This relatively large variation in Sr isotopic compositions (∼0.0006) is ∼1 order of magnitude larger than the estimated external reproducibility (∼0.00005, 2σ) of the method. The large range in 87 Sr/ 86 Sr values suggests that apatite and carbonate precipitated predominantly under nonequilibrium conditions. The isotopic variations observed within individual hand specimens may therefore reflect larger (regional) scale open-system processes, possibly involving mixing of carbonatitic melts derived from distinct mantle sources or from a common isotopically heterogeneous mantle.