The nature of Earth's building materials as revealed by calcium isotopes

Isotopic ratios have traditionally been used as tracers of the genetic link between meteorites and the Earth. Of the major primitive meteorite groups, enstatite chondrites (EC) are the most similar to Earth with regard to the isotopic composition of most elements (e.g. Javoy et al., 2010). In contrast to many isotope systems, calcium (Ca) exhibits significant mass-dependent variation between Earth and EC, though the magnitude of the difference is debated. Simon and DePaolo (2010) find a 0.4‰ difference between EC and Earth while Huang and Jacobsen (2012) find EC and Earth to be identical within error bars. Here we have developed a new method to resolve the difference in Ca isotopic composition at the 0.1 permil level. The method has been applied to a range of terrestrial and meteoritic samples, including seven EC, 12 carbonaceous chondrites (CC) representing most subgroups (CI, CV, CO, CM, CB, CR), six ordinary chondrites (OC), five lunar basalts, and six terrestrial rock standards. In addition, we measured 13 ocean island basalt (OIB) samples from a series of compositional ranges (EM1, EM2, HIMU) to better estimate the Ca isotopic composition of the mantle. Calcium was purified by a combination of Eichrom DGA and Sr-Spec resins and the isotope ratios 42Ca/44Ca and 43Ca/44Ca were measured by standard bracketing normalized to NIST SRM 915b in medium or high resolution on a Thermo-Fisher Neptune Plus MC-ICP-MS at Washington University in St. Louis. All data reported below follow a mass-dependent fractionation law with δ42/44Ca ≈ 2 × δ43/44Ca. As is convention in Ca isotope studies performed on TIMS, we present the data as δ44/40Ca (calculated as -2 × δ42/44Ca) and renormalize to SRM 915a. We find the δ44/40Ca value of NIST SRM 915b relative to SRM 915a to be 0.69 × 0.01 (2se), which is in excellent agreement with previously reported values. Our results show that geostandards are in good agreement with previous data (e.g. δ44CaBHVO-1 = 0.88 × 0.02‰, 2se). OIBs show variation from 0.8 to 1.1‰ with an average of 0.89 × 0.11‰ (2sd), which is in line with the findings of Huang et al. (2011). This suggests that the mantle is homogenous at the 0.1 permil level with regard to Ca isotopes. Lunar samples are indistinguishable from terrestrial basalt. As observed by both Simon and DePaolo (2010) and Huang and Jacobsen (2012), we find CC to be isotopically light compared to Earth; however, we find a 0.2‰ range and distinct signatures among the different groups. OC are heavier than CC with an average which overlaps with terrestrial samples of 0.90 × 0.09‰ (2sd). Finally, we find that EC are heavier than both OC and CC and display a range from terrestrial up to 1.5‰. One of the main carriers of Ca in EC is Oldhamite (CaS). Hence, we performed a set of leachate experiments which show that CaS is isotopically lighter than the bulk sample. Since CaS is soluble in water and Ca very easily mobile with aqueous fluid, a possible origin for the range observed within EC observed in our study and for the divergent results obtained by Simon and DePaolo and Huang and Jacobsen may be attributed to variation in the amount of CaS in the samples, due to either heterogeneous distribution of CaS or sample alteration. [Huang and Jacobsen (2012), 43rd LPSC #1334; Huang et al. (2011), GCA, 75, 4987-4997; Javoy et al. (2010), EPSL, 293, 259-268; Simon and DePaolo (2010), EPSL, 289, 457-466.]