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Reconstructing the building blocks that made Earth and the Moon is critical to constrain their formation and compositional evolution to the present. Neodymium (Nd) isotopes identify these building blocks by fingerprinting nucleosynthetic components. In addition, the 146 Sm– 142 Nd and 147 Sm– 143 Nd decay systems, with half-lives of 103 million years and 108 billion years, respectively, track potential differences in their samarium (Sm)/Nd ratios. The difference in Earth’s present-day 142 Nd/ 144 Nd ratio compared with chondrites 1 , 2 , and in particular enstatite chondrites, is interpreted as nucleosynthetic isotope variation in the protoplanetary disk. This necessitates that chondrite parent bodies have the same Sm/Nd ratio as Earth’s precursor materials 2 . Here we show that Earth and the Moon instead had a Sm/Nd ratio approximately 2.4 ± 0.5 per cent higher than the average for chondrites and that the initial 142 Nd/ 144 Nd ratio of Earth’s precursor materials is more similar to that of enstatite chondrites than previously proposed 1 , 2 . The difference in the Sm/Nd ratio between Earth and chondrites probably reflects the mineralogical distribution owing to mixing processes within the inner protoplanetary disk. This observation simplifies lunar differentiation to a single stage from formation to solidification of a lunar magma ocean 3 . This also indicates that no Sm/Nd fractionation occurred between the materials that made Earth and the Moon in the Moon-forming giant impact. Isotopic analysis reveals that the samarium/neodymium ratio of the Earth–Moon system is higher than that of chondrites, and that the neodymium composition of Earth is similar to that of enstatite chondrites.

The mantle signatures of elements with distinct affinities for metal isotopically record different stages of Earth’s accretion, revealing that the Moon-forming impactor had a similar composition to the other impactors that made the Earth. The isotopic composition of the Earth's building blocks The bodies that formed the Earth have isotopic natures that have so far remained unclear. Here Nicolas Dauphas shows that elements with differing affinities for metal can be used to decipher the isotopic nature of the Earth's accreting material through time. He finds that the mantle signatures of lithophile, moderately siderophile and highly siderophile elements record different stages of the Earth's accretion, yet all the examined elements point to material that is isotopically most similar to enstatite meteorites. The author concludes that enstatite meteorites and the Earth were formed from the same isotopic reservoir but diverged in their chemical evolution as a result of subsequent fractionation by nebular and planetary processes. The Earth formed by accretion of Moon- to Mars-size embryos coming from various heliocentric distances. The isotopic nature of these bodies is unknown. However, taking meteorites as a guide, most models assume that the Earth must have formed from a heterogeneous assortment of embryos with distinct isotopic compositions 1 , 2 , 3 . High-precision measurements, however, show that the Earth, the Moon and enstatite meteorites have almost indistinguishable isotopic compositions 4 , 5 , 6 , 7 , 8 , 9 , 10 . Models have been proposed that reconcile the Earth–Moon similarity with the inferred heterogeneous nature of Earth-forming material, but these models either require specific geometries for the Moon-forming impact 11 , 12 or can explain only one aspect of the Earth–Moon similarity (that is, 17 O) 1 , 2 , 3 . Here I show that elements with distinct affinities for metal can be used to decipher the isotopic nature of the Earth’s accreting material through time. I find that the mantle signatures of lithophile O, Ca, Ti and Nd, moderately siderophile Cr, Ni and Mo, and highly siderophile Ru record different stages of the Earth’s accretion; yet all those elements point to material that was isotopically most similar to enstatite meteorites. This isotopic similarity indicates that the material accreted by the Earth always comprised a large fraction of enstatite-type impactors (about half were E-type in the first 60 per cent of the accretion and all of the impactors were E-type after that). Accordingly, the giant impactor that formed the Moon probably had an isotopic composition similar to that of the Earth, hence relaxing the constraints on models of lunar formation. Enstatite meteorites and the Earth were formed from the same isotopic reservoir but they diverged in their chemical evolution owing to subsequent fractionation by nebular and planetary processes 13 .

The heat capacity (C.sub.p) of synthetic members of the orthopyroxene solid solution enstatite (Mg.sub.2 Si.sub.2 O.sub.6 : En)-ferrosilite (Fe.sub.2 Si.sub.2 O.sub.6 : Fs) was measured at between 2 to 300 K using relaxation calorimetry and between 340 to 823 K using differential scanning calorimetry. The samples (nine solid-solution members plus En and Fs) were characterised by optical microscopy and by X-ray powder diffraction methods. The actual compositions of the solid-solution members were checked by comparing the cell volumes determined in this work with corresponding data in the literature which provide a correlation between cell volume and composition.

High-pressure experiments explore what it might take to make exoplanets habitable

High-pressure experiments explore what it might take to make exoplanets habitable

High-pressure experiments explore what it might take to make exoplanets habitable

High-pressure experiments explore what it might take to make exoplanets habitable

Neodynium isotope data reveal that the Earth is enriched in material from red giant stars relative to its presumed meteoritic building blocks, refuting models of a hidden reservoir of 142 Nd-depleted material or a ‘super-chondritic’ Earth. Chondritic meteorites as proxies for Earth's composition Christoph Burkhardt et al . show that, compared to chondritic meteorites, the Earth's precursor bodies were enriched in neodymium produced by the slow neutron capture 's-process' of nucleosynthesis. This s-process excess leads to a higher 142 Nd/ 144 Nd ratio and, after correction for this effect, the 142 Nd/ 144 Nd ratio of chondritic meteorites and the accessible Earth are indistinguishable within five parts per million. The 142 Nd offset between the accessible silicate Earth and chondritic meteorites therefore reflects a higher proportion of s-process neodymium in the Earth, and not early differentiation processes. The authors conclude that there is no need for hidden-reservoir or 'super-chondritic' Earth models, as previously proposed, and that although chondritic meteorites formed at a greater heliocentric distance and contain a different mix of presolar components than the Earth, they nevertheless may be suitable proxies for the Earth's bulk chemical composition. A long-standing paradigm assumes that the chemical and isotopic compositions of many elements in the bulk silicate Earth are the same as in chondrites 1 , 2 , 3 , 4 . However, the accessible Earth has a greater 142 Nd/ 144 Nd ratio than do chondrites. Because 142 Nd is the decay product of the now-extinct 146 Sm (which has a half-life of 103 million years 5 ), this 142 Nd difference seems to require a higher-than-chondritic Sm/Nd ratio for the accessible Earth. This must have been acquired during global silicate differentiation within the first 30 million years of Solar System formation 6 and implies the formation of a complementary 142 Nd-depleted reservoir that either is hidden in the deep Earth 6 , or lost to space by impact erosion 3 , 7 . Whether this complementary reservoir existed, and whether or not it has been lost from Earth, is a matter of debate 3 , 8 , 9 , and has implications for determining the bulk composition of Earth, its heat content and structure, as well as for constraining the modes and timescales of its geodynamical evolution 3 , 7 , 9 , 10 . Here we show that, compared with chondrites, Earth’s precursor bodies were enriched in neodymium that was produced by the slow neutron capture process (s-process) of nucleosynthesis. This s-process excess leads to higher 142 Nd/ 144 Nd ratios; after correction for this effect, the 142 Nd/ 144 Nd ratios of chondrites and the accessible Earth are indistinguishable within five parts per million. The 142 Nd offset between the accessible silicate Earth and chondrites therefore reflects a higher proportion of s-process neodymium in the Earth, and not early differentiation processes. As such, our results obviate the need for hidden-reservoir or super-chondritic Earth models and imply a chondritic Sm/Nd ratio for the bulk Earth. Although chondrites formed at greater heliocentric distances and contain a different mix of presolar components than Earth, they nevertheless are suitable proxies for Earth’s bulk chemical composition.

Among enstatite-rich meteorites are included enstatite chondrites and enstatite achondrites (aubrites). The reducing conditions of origin are reflected in their mineralogy. Due to the lack of oxygen-bearing mineral assemblages allowing the application of traditional geothermometers, sulfides are used as a tool to constrain the conditions of their origin. In general, sulfide-based geothermometers rely on the contents of major or minor elements traditionally determined by electron probe microanalysis. This method requires the analyzed material to be a homogenous single phase in the analytical volume. However, sulfides of enstatite-rich meteorites frequently contain tiny lamellar inclusions of different phases, and therefore, the inclusions might affect the overall composition of sulfides. Consequently, the results of such analyses might influence the estimates of the conditions under which the given meteorite formed. This study discusses the effect of using the low-overvoltage approach to analyze iron and nickel (10 kV) in the primary sulfides of enstatiterich meteorites and how results compare to those obtained with the traditional analytical protocol (20 kV). The sulfides analyzed included Cr-Ti-bearing troilite, daubréelite (FeCr,S,), and (Mg,Fe,Mn)S-monosulfide. Unfortunately, troilite often contains lamellar inclusion of daubréelite. Moreover, troilite inclusions are occasionally also included in (Mg,Fe,Mn) S-monosulfide. Therefore, obtaining an unbiased analysis of these minerals is intricate. Due to this, the main objective of using a lower accelerating voltage is to reduce the analytical volume to the minimum to increase the probability of avoiding tiny inclusions. Even if the analytical volume is inclusion-free, another complication might occur as the analysis of troilite may be affected by the neighboring daubréelite due to boundary fluorescence. Consequently, both phenomena bias the Cr content measured in troilite similarly, and due to the complexity of troilite-daubréelite assemblage, it is nearly impossible to quantify the amount of Cr content unbiased. Subsequently, to obtain the best possible dataset, precise sample screening and careful analytical point location setting are required in general. Using lower accelerating voltage brings many advantages as it allows better observation of the inclusions, and due to reducing the analytical volume, it reduces the chance of the presence of inclusions and suppresses the bias in Cr from boundary fluorescence. However, it also has disadvantages as the analysis is not trivial and does not favor trace elements analysis in general. Results demonstrate the importance of point-by-point inspection of the acquired data and subsequent elimination of biased analyses from the final datasets.

X-ray absorption near-edge structure (XANES) spectroscopy is a powerful technique to quantitatively investigate sulfur speciation in geologically complex materials such as minerals, glasses, soils, organic compounds, industrial slags, and extraterrestrial materials. This technique allows non-destructive investigation of the coordination chemistry and oxidation state of sulfur species ranging from sulfide (2-oxidation state) to sulfate (6+ oxidation state). Each sulfur species has a unique spectral shape with a characteristic K-edge representing the s→p and d hybridization photoelectron transitions. As such, sulfur speciation is used to measure the oxidation state of samples by comparing the overall XANES spectra to that of reference compounds. Although many S XANES spectral standards exist for terrestrial applications under oxidized conditions, new sulfide standards are needed to investigate reduced (oxygen fugacity, fO2, below IW) silicate systems relevant for studies of extraterrestrial materials and systems. Sulfides found in certain meteorites (e.g., enstatite chondrites and aubrites) and predicted to exist on Mercury, such as CaS (oldhamite), MgS (niningerite), and FeCr2S4 (daubreelite), are stable at fO2 below IW-3 but rapidly oxidize to sulfate and/or produce sulfurous gases under terrestrial surface conditions. XANES spectra of these compounds collected to date have been of variable quality, possibly due to the unstable nature of certain sulfides under typical (e.g., oxidizing) laboratory conditions. A new set of compounds was prepared for this study and their XANES spectra are analyzed for comparison with potential extraterrestrial analogs. S K-edge XANES spectra were collected at Argonne National Lab for FeS (troilite), MnS (alabandite), CaS (oldhamite), MgS (niningerite), Ni1-xS, NiS2, CaSO4 (anhydrite), MgSO4, FeSO4, Fe2(SO4)3, FeCr2S4 (daubreelite), Na2S, Al2S3, Ni7S6, and Ni3S2; the latter five were analyzed for the first time using XANES. These standards expand upon the existing S XANES end-member libraries at a higher spectral resolution (0.25 eV steps) near the S K-edge. Processed spectra, those that have been normalized and \"flattened,\" are compared to quantify uncertainties due to data processing methods. Future investigations that require well-characterized sulfide standards, such as the ones presented here, may have important implications for understanding sulfur speciation in reduced silicate glasses and minerals with applications for the early Earth, Moon, Mercury, and enstatite chondrites.