Primitive meteorites preserve the chemical and isotopic composition of the first aggregates that formed from dust and gas in the solar nebula during the earliest stages of solar system evolution. ...Gradual increase in the size of solid bodies from dust to aggregates and then to planetesimals finally led to the formation of planets within a few to tens of million years after the start of condensation. Thus the rocky planets of the inner solar system are likely the result of the accumulation of numerous smaller primitive as well as differentiated bodies. The chemically most primitive known meteorites are chondrites and they consist mostly of metal and silicates. Chondritic meteorites are derived from distinct primitive planetary bodies that experienced only limited element fractionation during formation and subsequent differentiation. Different chondrite classes show distinct chemical and isotopic characteristics, which may reflect heterogeneities in the solar nebula and the slightly different pathways of their formation. To a first approximation the chemical composition of the bulk Earth bears great similarities to primitive meteorites. However, for some elements there are striking and significant differences. The Earth shows a much stronger depletion of the moderate to highly volatile elements compared to chondrites. In addition, mixing trends of specific isotopes reveal that the Earth is most enriched in
s
-process isotopes compared to all other analysed bulk solar system materials. It is currently not possible to fully define and quantify the different chemical and isotopic materials that formed the Earth, because a major component seems missing in the extant collections of extraterrestrial samples. Variations in nucleosynthetic isotope compositions as well as the strong depletion of moderately and strongly volatile elements points towards a source in the inner solar system for this missing material. It is conceivable that Venus and Mercury contain a much larger fraction of this missing component. Thus, for a complete reconstruction of the conditions that led to the formation of the inner solar system planets (Mercury to Mars) samples from the inner planets Venus and Mercury are of great interest and importance. High precision chemical and isotopic analyses in the laboratory of rocky material from inner solar system bodies could complete the knowledge on the chemical, isotopic and mineralogical make-up of the solar nebula just prior to planet formation and enhance our understanding of the evolution of the solar nebula in general and the formation of the rocky planets in particular.
Several models exist to describe the growth and evolution of Earth; however, variables such as the type of precursor materials, extent of mixing, and material loss during accretion are poorly ...constrained. High-precision palladium-silver isotope data show that Earth's mantle is similar in ¹⁰⁷Ag/¹⁰⁹Ag to primitive, volatile-rich chondrites, suggesting that Earth accreted a considerable amount of material with high contents of moderately volatile elements. Contradictory evidence from terrestrial chromium and strontium isotope data are reconciled by heterogeneous accretion, which includes a transition from dominantly volatile-depleted to volatile-rich materials with possibly high water contents. The Moon-forming giant impact probably involved the collision with a Mars-like protoplanet that had an oxidized mantle, enriched in moderately volatile elements.
Recent work based on analyses of meteorite and terrestrial whole-rock samples showed that the r- and s- process isotopes of Hf were homogeneously distributed throughout the inner solar system. We ...report new Hf isotope data for Calcium-Aluminum-rich inclusions (CAIs) of the CV3 carbonaceous chondrite Allende, and novel high-precision Zr isotope data for these CAIs and three carbonaceous chondrites (CM, CO, CK). Our Zr data reveal enrichments in the neutron-rich isotope super(96)Zr (< or =, slant1 epsilon in super(96)Zr/ super(90)Zr) for bulk chondrites and CAIs (~2 epsilon ). Potential isotope effects due to incomplete sample dissolution, galactic and cosmic ray spallation, and the nuclear field shift are assessed and excluded, leading to the conclusion that the super(96)Zr isotope variations are of nucleosynthetic origin. The super(96)Zr enrichments are coupled with super(50)Ti excesses suggesting that both nuclides were produced in the same astrophysical environment. The same CAIs also exhibit deficits in r-process Hf isotopes, which provides strong evidence for a decoupling between the nucleosynthetic processes that produce the light (A < or =, slant 130) and heavy (A > 130) neutron-rich isotopes. We propose that the light neutron-capture isotopes largely formed in Type II supernovae (SNeII) with higher mass progenitors than the supernovae that produced the heavy r-process isotopes. In the context of our model, the light isotopes (e.g. super(96)Zr) are predominantly synthesized via charged-particle reactions in a high entropy wind environment, in which Hf isotopes are not produced. Collectively, our data indicates that CAIs sampled an excess of materials produced in a normal mass (12-25 M sub(middot in circle)) SNII.
The geochemistry of Cd in seawater has attracted significant attention owing to the nutrient‐like properties of this element. Recent culturing studies have demonstrated that Cd is a biologically ...important trace metal that plays a role in the sequestration of inorganic carbon. This conclusion is supported by recent isotope data for Cd dissolved in seawater and incorporated in cultured phytoplankton. These results show that plankton features isotopically light Cd while Cd‐depleted surface waters typically exhibit complimentary heavy Cd isotope compositions. Seawater samples from below 900 m depth display a uniform and intermediate isotope composition of ε114/110Cd = +3.3 ± 0.5. This study investigates whether ferromanganese (Fe‐Mn) crusts are robust archives of deep water Cd isotope compositions. To this end, Cd isotope data were obtained for the recent growth surfaces of 15 Fe‐Mn crusts from the Atlantic, Pacific, Indian, and Southern oceans and two USGS Fe‐Mn reference nodules using double spike multiple collector inductively coupled plasma mass spectrometry. The Fe‐Mn crusts yield a mean ε114/110Cd of +3.2 ± 0.4 (2 SE, n = 14). Data for all but one of the samples are identical, within the analytical uncertainty of ±1.1ε114/110Cd (2 SD), to the mean deep water Cd isotope value. This indicates that Fe‐Mn crusts record seawater Cd isotope compositions without significant isotope fractionation. A single sample from the Southern Ocean exhibits a light Cd isotope composition of ε114/110Cd = 0.2 ± 1.1. The origin of this signature is unclear, but it may reflect variations in deep water Cd isotope compositions related to differences in surface water Cd utilization or long‐term changes in seawater ε114/110Cd. The results suggest that time series analyses of Fe‐Mn crusts may be utilized to study changes in marine Cd utilization.
Zinc isotope compositions (δ66Zn) and concentrations were determined for metal samples of 15 iron meteorites across groups IAB, IIAB, and IIIAB. Also analyzed were troilite and other inclusions from ...the IAB iron Toluca. Furthermore, the first Zn isotope data are presented for metal–silicate partitioning experiments that were conducted at 1.5 GPa and 1650 K. Three partitioning experiments with run durations of between 10 and 60 min provide consistent Zn metal–silicate partition coefficients of ∼0.7 and indicate that Zn isotope fractionation between molten metal and silicate is either small (at less than about ±0.2‰) or absent. Metals from the different iron meteorite groups display distinct ranges in Zn contents, with concentrations of 0.08–0.24 μg/g for IIABs, 0.8–2.5 μg/g for IIIABs, and 12–40 μg/g for IABs. In contrast, all three groups show a similar range of δ66Zn values (reported relative to ‘JMC Lyon Zn’) from +0.5‰ to +3.0‰, with no clear systematic differences between groups. However, distinct linear trends are defined by samples from each group in plots of δ66Zn vs. 1/Zn, and these correlations are supported by literature data. Based on the high Zn concentration and δ66Zn ≈ 0 determined for a chromite-rich inclusion of Toluca, modeling is employed to demonstrate that the Zn trends are best explained by segregation of chromite from the metal phase. This process can account for the observed Zn–δ66Zn–Cr systematics of iron meteorite metals, if Zn is highly compatible in chromite and Zn partitioning is accompanied by isotope fractionation with Δ66Znchr-met≈−1.5‰. Based on these findings, it is likely that the parent bodies of the IAB complex, IIAB and IIIAB iron meteorites featured δ66Zn values of about −1.0 to +0.5‰, similar to the Zn isotope composition inferred for the bulk silicate Earth and results obtained for chondritic meteorites. Together, this implies that most solar system bodies formed with similar bulk Zn isotope compositions despite large differences in Zn contents.
•Experimental investigation of Zn isotope fractionation between metal and silicate.•Zn isotope fractionation during metal–silicate partitioning small or negligible.•Iron meteorites of each group display positive correlation of δ66Zn with 1/Zn.•Trends due to segregation of Zn-rich chromite from the metal phase.•Meteorite parent bodies and the Earth have similar δ66Zn values.
The extinct 107Pd–107Ag decay system (half-life ∼6.5Ma) is a useful chronometer to constrain the thermal evolution of the IAB parent body. To this end, Pd/Ag concentrations and the Ag isotope ...compositions of metals separated from 6 different IAB iron meteorites were determined. The samples show ε107Ag variations between +0.1 and +15.8 with 108Pd/109Ag ratios between 38 and 200. The data can be divided into two groups based on their petrology, each defining an isochron: a graphite and troilite rich inclusion bearing group (A), with the IAB meteorites Toluca, Odessa and Canyon Diablo and a more silicate rich group (B), which includes Campo Del Cielo, Caddo County and Goose Lake. Using the initial abundance of 107Pd derived from carbonaceous chondrites, the corresponding age for the group (A) is 18.7 (+3.6/−5.0)Ma after the start of the solar system and 14.9 (+2.5/−4.9)Ma for the group (B). This suggests that the last thermal event to reach high enough temperatures to melt metal on the IAB parent body occurred within the first 15Ma of our solar system.
► The extinct 107Pd/107Ag chronometer constrains the IAB parent body evolution. ► Metal from 6 IAB meteorites define 2 isochrons corresponding to inclusion mineralogy. ► Group A rich in graphite and troilite inclusions—defines age of 18.7Ma after CAI's. ► Group B rich in silicate rich inclusions—defines an age of 14.9Ma after CAI's. ► Disruption and reassembly of IAB parent body at 10–12Ma.
The giant impact theory is the most widely recognized formation scenario of the Earth's Moon. Giant impact models based on dynamical simulations predict that the Moon acquired a significant amount of ...impactor (Theia) material, which is challenging to reconcile with geochemical data for O, Si, Cr, Ti and W isotopes in the Earth and Moon. Three new giant impact scenarios have been proposed to account for this discrepancy – hit-and-run impact, impact with a fast-spinning protoEarth and massive impactors – each one reducing the proportion of the impactor in the Moon compared to the original canonical giant impact model. The validity of each theory and their different dynamical varieties are evaluated here using an integrated approach that considers new high-precision Zr isotope measurements of lunar rocks, and quantitative geochemical modelling of the isotopic composition of the impactor Theia. All analysed lunar samples (whole-rock, ilmenite and pyroxene separates) display identical Zr isotope compositions to that of the Earth within the uncertainty of 13 ppm for 96Zr/90Zr (2σ weighted average). This 13 ppm upper limit is used to infer the most extreme isotopic composition that Theia could have possessed, relative to the Earth, for each of the proposed giant impact theories. The calculated Theian composition is compared with the Zr isotope compositions of different solar system materials in order to constrain the source region of the impactor. As a first order approximation, we show that all considered models (including the canonical) are plausible, alleviating the initial requirement for the new giant impact models. Albeit, the canonical and hit-and-run models are the most restrictive, suggesting that the impactor originated from a region close to the Earth. The fast-spinning protoEarth and massive impactor models are more relaxed and increase the allowed impactor distance from the Earth. Similar calculations carried out for O, Cr, Ti and Si isotope data support these conclusions but exclude a CI- and enstatite chondrite-like composition for Theia. Thus, the impactor Theia most likely had a Zr isotope composition close to that of the Earth, and this suggests that a large part of the inner solar system (or accretion region of the Earth, Theia and enstatite chondrites) had a uniform Zr isotope composition.
•Mass-dependent Zr isotope fractionation in synthetic standards.•Earth and Moon have identical Zr isotope compositions.•Giant impact theories are constrained.•The impactor Theia was volatile-depleted.
The Earth's timing of accretion and acquisition of moderately volatile compounds is uncertain. Hafnium-W and Mn-Cr isotopic data can bracket the timing of early planetary differentiation and core ...formation. The Ag-Pd system has also been utilized but its application has been limited by a lack of high pressure and temperature metal-silicate partitioning for Pd and Ag. Because Ag (and Bi) are volatile chalcophile siderophile elements, understanding their early distribution can constrain the origin of volatile elements in differentiated bodies and planets. Unfortunately, neither Ag or Bi have been studied across the wide range of pressure and temperature conditions that are relevant to accretion and core-mantle differentiation. Here, new high-pressure and temperature multi-anvil metal-silicate equilibrium experiments for Bi and Ag have been carried out at conditions relevant to planetary accretion and metal silicate differentiation that allow a more refined and complete understanding of element partitioning during core formation. The new metal-silicate partitioning data are combined with previously reported data, and utilized to predict the distributions of Bi, Pd, and Ag at conditions of accretion relevant for Earth and Mars. Application of the new partitioning results to Earth shows that D(Bi) and D(Ag) (D = metal/silicate concentration ratio) are lowered due to the effect of pressure and Si alloyed in the metallic liquid, resulting in higher predicted mantle Bi and Ag abundances than in the bulk silicate Earth (BSE), as well as high and variable Pd/Ag. The unradiogenic Ag isotopic composition of the BSE could have been generated by early accretion of volatile-poor (high Pd/Ag) pre-cursors, followed by later accretion of volatile–rich (low Pd/Ag) material, in agreement with earlier studies of Pd-Ag and Mn-Cr (Schönbächler et al., 2010). However, these main accretion phases would have to be followed by segregation of a sulfide liquid (at least 1.5% of magma ocean) at high pressures (>30 GPa), to explain the primitive upper mantle (PUM) Bi, Pd, and Ag, as well as Au, Pt, Cu and Ni concentrations as proposed previously. If the early accreted bulk Earth was volatile depleted with high Pd/Ag ratios, portions of the mantle may contain ancient domains that developed positive 107Ag isotopic anomalies (as also argued by noble gases, Nd, W, and Os isotopes). In comparison, Bi, Pd, and Ag concentrations in the martian mantle could have been set by simple metal-silicate equilibrium. Mars accreted and differentiated relatively rapidly, while also developing a deep magma ocean with a high Pd/Ag ratio that could have evolved positive 107Ag anomalies, in contrast to Earth. Measurements on shergottites may reveal these predicted Ag isotopic anomalies.
•Pressure affects lnKd (Fe-Bi), but only minimally lnKd (Fe-P) and (Fe-Ag).Biand Ag in Earth's mantle are due to core formation and sulfide removal.Biand Ag in Mars' mantle are due to core formation, not a late veneer.•ε107Ag anomalies may have formed in early Earth and been preserved in the mantle.•Rapid accretion and high Pd/Ag in Mars may have produced even higher ε107Ag anomaly.
A growing number of elements show well-resolved nucleosynthetic isotope anomalies in bulk-rock samples of solar system materials. In order to establish the occurrence and extent of such isotopic ...heterogeneities in Zr, and to investigate the origin of the widespread heterogeneities in our solar system, new high-precision Zr isotope data are reported for a range of primitive and differentiated meteorites. The majority of the carbonaceous chondrites (CV, CM, CO, CK) display variable ε96Zr values (⩽1.4) relative to the Earth. The data indicate the heterogeneous distribution of 96Zr-rich CAIs in these meteorites, which sampled supernova (SN) material that was likely synthesized by charged-particle reactions or neutron-captures. Other carbonaceous chondrites (CI, CB, CR), ordinary chondrites and eucrites display variable, well-resolved 96Zr excesses correlated with potential, not clearly resolved variations in 91Zr relative to the bulk–Earth and enstatite chondrites. This tentative correlation is supported by nucleosynthetic models and provides evidence for variable contributions of average solar system s-process material to different regions of the solar system, with the Earth representing the most s-process enriched material. New s-process model calculations indicate that this s-process component was produced in both low and intermediate mass asymptotic giant branch (AGB) stars. The isotopic heterogeneity pattern is different to the s-process signature resolved in a previous Zr leaching experiment, which was attributed to low mass AGB stars. The bulk-rock heterogeneity requires several nucleosynthetic sources, and therefore opposes the theory of the injection of material from a single source (e.g., supernova, AGB star) and argues for a selective dust-sorting mechanism within the solar nebula. Thermal processing of labile carrier phases is considered and, if correct, necessitates the destruction and removal of non-s-process material from the innermost solar system. New Zr isotope data on mineral separates and a fusion crust sample from chondrites indicate that this non-s-process material could be silicates.
PDF
