Material inherited from different nucleosynthesis sources imparts distinct isotopic signatures to meteorites and terrestrial planets. These nucleosynthetic isotope anomalies have been used to constrain the origins of material that formed Earth. However, anomalies have only been identified for elements with high condensation temperatures, leaving the origin of Earth’s volatile elements unconstrained. We determined the isotope composition of the moderately volatile element zinc in 18 bulk meteorites and identified nucleosynthetic zinc isotope anomalies. Using a mass-balance model, we find that carbonaceous bodies, which likely formed beyond the orbit of Jupiter, delivered about half of Earth’s zinc inventory. Combined with previous constraints obtained from studies of other elements, these results indicate that ~10% of Earth’s mass was provided by carbonaceous material.
The difference in the Si isotopic composition between the Earth and primitive meteorites had been used to constrain the amount of Si in the Earth's core. However, there is presently a debate on the magnitude of the isotopic fractionation between metal and silicates as function of temperature based on experimental data. Here, we use a natural sample, an enstatite meteorite, Itqyi, as a natural experiment to determine an independent Si isotopic fractionation factor between metal and silicate. We determined the temperature of equilibrium between metal and silicate as well as the Si isotopic composition between the phases. We find that the dependence of Si isotopes with *Manuscript Click here to view linked References temperature to be : Using this dependence of the 30 Si with temperature we estimate the bulk Earth 30 Si as a function of Si content of the core for different plausible conditions. Even when using the most extreme parameters, we show that the bulk Earth must be isotopically heavier than any chondrites groups. We therefore confirm that core formation alone cannot account for the isotopic difference between the Earth and primitive meteorites. We show that there is no correlation between 30 Si and the Mg/Si ratio suggesting that forsterite fractionation in the solar nebula may have had only a limited effect, if any.Our new results therefore confirm that volatility should have had a fundamental effect in shaping terrestrial planets chemical composition.
As part of an integrated consortium study, we have undertaken O, Cd, Cr, Si, Te, Ti, and Zn whole rock isotopic measurements of the Winchcombe CM2 meteorite. δ66Zn values determined for two Winchcombe aliquots are +0.29 ± 0.05‰ (2SD) and +0.45 ± 0.05‰ (2SD). The difference between these analyses likely reflects sample heterogeneity. Zn isotope compositions for Winchcombe show excellent agreement with published CM2 data. δ114Cd for a single Winchcombe aliquot is +0.29 ± 0.04‰ (2SD), which is close to a previous result for Murchison. δ130Te values for three aliquots gave indistinguishable results, with a mean value of +0.62 ± 0.01‰ (2SD) and are essentially identical to published values for CM2s. ε53Cr and ε54Cr for Winchcombe are 0.319 ± 0.029 (2SE) and 0.775 ± 0.067 (2SE), respectively. Based on its Cr isotopic composition, Winchcombe plots close to other CM2 chondrites. ε50Ti and ε46Ti values for Winchcombe are 3.21 ± 0.09 (2SE) and 0.46 ± 0.08 (2SE), respectively, and are in line with recently published data for CM2s. The δ30Si composition of Winchcombe is −0.50 ± 0.06‰ (2SD, n = 11) and is essentially indistinguishable from measurements obtained on other CM2 chondrites. In conformity with petrographic observations, oxygen isotope analyses of both bulk and micromilled fractions from Winchcombe clearly demonstrate that its parent body experienced extensive aqueous alteration. The style of alteration exhibited by Winchcombe is consistent with relatively closed system processes. Analysis of different fractions within Winchcombe broadly support the view that, while different lithologies within an individual CM2 meteorite can be highly variable, each meteorite is characterized by a predominant alteration type. Mixing of different lithologies within a regolith environment to form cataclastic matrix is supported by oxygen isotope analysis of micromilled fractions from Winchcombe. Previously unpublished bulk oxygen isotope data for 12 CM2 chondrites, when combined with published data, define a well‐constrained regression line with a slope of 0.77. Winchcombe analyses define a more limited linear trend at the isotopically heavy, more aqueously altered, end of the slope 0.77 CM2 array. The CM2 slope 0.77 array intersects the oxygen isotope field of CO3 falls, indicating that the unaltered precursor material to the CMs was essentially identical in oxygen isotope composition to the CO3 falls. Our data are consistent with earlier suggestions that the main differences between the CO3s and CM2s reflect differing amounts of water ice that co‐accreted into their respective parent bodies, being high in the case of CM2s and low in the case of CO3s. The small difference in Si isotope compositions between the CM and CO meteorites can be explained by different proportions of matrix versus refractory silicates. CMs and COs may also be indistinguishable with respect to Ti and Cr isotopes; however, further analysis is required to test this possibility. The close relationship between CO3 and CM2 chondrites revealed by our data supports the emerging view that the snow line within protoplanetary disks marks an important zone of planetesimal accretion.
Primitive chondrites have bulk compositions close to that of the solar photosphere, with however significant variations of elemental ratio relative to the solar composition, depending on the volatility of the elements considered. This is classically understood as indicating a primary geochemical signature due to the formation of the components of chondrites (refractory inclusions, chondrules and matrix), or of their precursors, through condensation of a gas of near solar composition, plus secondary variations due to processes such as (i) incomplete volatilization of presolar components, (ii) complex high-temperature exchanges between condensed phases and the nebular gas, and (iii) sorting and transport of grains in the accretion disk before accretion of chondrite parent bodies. Because most of the mass of chondrites is made by elements which condense at high temperatures, equilibrium condensation produces in general little isotopic fractionation for these elements. Silicon is however an exception with per mil level equilibrium isotopic fractionation at high temperature between the SiO gas and condensed silicates, allowing to use silicon isotopes in chondrites to constrain the origin of their components and to put at test scenarios of condensation. Individual components (chondrule fragments, isolated olivines in the matrix, and matrix fragments) of the carbonaceous chondrite Allende were separated and analysed at high-precision for their silicon isotopic composition. Large variations have been found among chondrules (δ 30 Si from -0.86 ± 0.16 2 s.e. to +0.04 ± 0.03 for 11 chondrules), isolated olivines (δ 30 Si from -0.51 ± 0.12 2 s.e. to +0.20 ± 0.10 for 12 olivines), and matrix (δ 30 Si from -0.95 ± 0.08 2 s.e. to -0.41 ± 0.01 for 17 matrix fragments). These variations distribute on both sides of the bulk δ 30 Si value of Allende (-0.43 ± 0.03 2 s.e., Armytage et al., 2011;Pringle et al., 2013Pringle et al., , 2014Savage and Moynier, 2013). There is a global positive trend between δ 30 Si values and Mg/Fe ratio for chondrules and isolated olivines. This systematics appears in agreement with what can be modeled for producing Allende components, or their precursors, from fractionated condensation of a single gaseous reservoir having initially the silicon isotopic composition of bulk Allende. Mass balance taking into account the mean abundances and δ 30 Si values of Allende components is consistent with their accretion in the Allende parent body in the proportions produced by the condensation of the parent parcel of nebular gas. This supports complementarity between chondrules, olivines and matrix as being a primary feature. However, this conclusion cannot be definitive because of the uncertainties in defining mean δ 30 Si values for Allende components.
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