Calcium-aluminum-rich inclusions (CAIs) in chondrite meteorites are the oldest rocks in the Solar System and were formed by condensation from nebular gas. Recent mass spectrometric measurements have ...revealed that CAIs possess nucleosynthetic isotopic compositions different from those of terrestrial materials for various elements, indicating a heterogeneous distribution of nuclides from various stellar sources in the early Solar System. CAIs are classified into coarse-grained (CGs) and fine-grained (FGs) inclusions. The former have experienced secondary melting through thermal events after their formation, while the latter evidently avoided the remelting. Thus, FGs are considered to be direct condensates from a high-temperature gas, making them ideal for the investigation of the origin and formation process of CAIs. In this study, the elemental abundances and Sr isotopic compositions in eight FGs from a carbonaceous chondrite Allende were analyzed by utilizing a micromilling technique. These FG samples were found to have rare-earth element (REE) patterns reflecting various degrees of elemental fractionation and variable µ84Sr values ranging from 61 to 844 ppm. It cannot be ruled out that matrix contamination during micromilling or secondary alteration on the Allende parent body has affected the elemental abundances and µ84Sr values observed in FGs to some extent; however, the large variation in µ84Sr values could reflect the variability in the FG formation processes. Importantly, REE-fractionated FGs, which were depleted in heavy REEs relative to light REEs, had relatively high µ84Sr values. This suggests that the formation of REE-fractionated FGs was triggered by rapid heating events, such as FU Orionis that occurred periodically in the early Solar System, and that at least two different heating events probably formed FGs with two different µ84Sr values.
We conducted high-precision Sr and Nd isotopic measurements in bulk chondrites using a complete sample digestion technique. Our new data indicate that enstatite and ordinary chondrites possess ...uniform and small, but resolvable, Sr and Nd isotopic deviations from terrestrial rocks. In contrast, the Sr isotope ratios varied across different classes of carbonaceous chondrites (CM, CO, and CV). The deviation of data from the s-process mixing line in Sr-Nd isotopic space likely resulted from the incorporation of calcium-aluminum-rich inclusions (CAIs) into carbonaceous chondrite parent bodies. Planetary-scale Sr and Nd isotopic heterogeneities among terrestrial rocks, enstatite, ordinary chondrites, and CAI-subtracted carbonaceous chondrites suggest a heterogeneous distribution of s-process-enriched materials in the early solar system, probably caused by nebular thermal processing. The observed Sr and Nd isotopic variation across the CAI-subtracted carbonaceous chondrites cannot be explained solely by nebular thermal processing, but is likely attributable to s-process-depleted silicate grains that repeatedly circulated among the early solar system. These grains were transferred and incorporated at varying degrees into the formation region of the parent bodies of individual carbonaceous chondrites.
CR chondrites are suitable for understanding the genetic linkage between metals and chondrules due to the unique characteristics of the coexisting metal phases with chondrules. Metal grains are found ...in three different locations of CR chondrites; chondrule interior (“interior grain”), chondrule surficial shells (“margin grain”), and the matrix (“isolated grain”). Here we report the abundances of highly siderophile elements (HSEs) and major elements in three types of metals (interior, margin, and isolated grains) from three CR chondrites (NWA 801, NWA 7184, and Dhofar 1432) by using femtosecond LA-ICP-MS (fs LA-ICP-MS) and EPMA. Additionally, we report the isotopic compositions of Os and Fe in the metals by using micro-milling sampling coupled with N-TIMS and MC-ICP-MS. The CR metals have variations in 187Os/188Os and δ57Fe values ranging from 0.1193 to 0.1314 and from −1.05 to +0.25, respectively. HSE abundances, except for Pd and Au, in the three types of metals increase as the abundance of Ir increases. A possible explanation for the variations of HSE abundances within and among grains, 187Os/188Os values within each grain, and δ57Fe values among grains, is the condensation of liquid metal from a gaseous reservoir followed by fractional crystallization. Most of the CR metals have negative δ57Fe values, suggesting that Fe in metal phases might have formed by condensation prior to Fe condensation in silicate phases. The chondrules and three types of metal grains in CR chondrites are believed to have formed contemporaneously in the same region. The existence of large isolated metals in matrix and compound chondrules might be the result of collision and merging of the metal and silicate droplets.
The Os isotopic composition and abundances of highly siderophile elements (HSE; Re, Os, Ir, Ru, Pt, Rh, Pd, and Au) and volatile chalcophile elements (VCE; S, Ge, As, Se, Sn, Sb, and Te) in ...chondrites show considerable variabilities across different chondrite groups, which provide essential insights into understanding the elemental fractionation during the formation of chondrite parent bodies and their precursor materials. Here, we have developed a method using a sealed glass tube digestion to simultaneously measure the 187Os/188Os ratios and the abundances of HSEs, VCEs, and other elements including Mg, Al, Ca, and Mo in chondrites. This study evaluated the potential blank increase from the quartz tube digestion and subsequent chemical process and the loss of other elements from the Os extraction processes. The new method was validated by the repeated measurements of the Allende meteorite reference sample provided by the Smithsonian Institution (Smithsonian Allende). The elemental abundances were determined by the isotope dilution (ID) coupled with ICP-MS/MS, excluding those of Mg, Al, Ca, As, Mo, Rh, and Au that were determined by a combination of ID and internal standardization (IS). The reproducibility of ten replicate analyses generally yielded less than 10%, except for Sn, Sb, and Mo, which is consistent with existing literature data. The analysis of the Re-Os systematics of minute sample powders revealed insights into their chemical heterogeneity. In particular, this study gives compelling evidence that the chemical heterogeneity observed in the Smithsonian Allende is closely related to sample size.
Variations in Mo isotopic ratios (δ98/95Mo values) in volcanic arc lavas reflect various processes occurring beneath island arcs, including slab dehydration, mantle melting, and crystal ...fractionation, making this tracer important in recent geochemistry. Previous studies of Mo isotopes have focused primarily on samples from the volcanic front (VF) of oceanic island arcs, and only a limited number of studies have been conducted on volcanoes in the rear arc (RA) regions. To better understand the behavior of Mo isotopes in subduction zones, we focused on the Northern Izu (N-Izu) arc and determined major and trace element abundances and δ98/95Mo values in basaltic samples from one VF (Izu-Oshima) and two RA (Niijima and Kozushima) islands. The N-Izu basalts are characterized by relatively uniform Zr/Hf ratios with systematic decrease in Ba/Th and increase in Ce/Mo ratios with increasing depth of the Wadati-Benioff zone, indicating a gradual decrease in the contribution of aqueous fluids from the subducting slab to a chemically homogeneous mantle wedge. Similarly, the δ98/95Mo values in the N-Izu basalts decrease with increasing slab depth from Izu Oshima (+0.11 ± 0.09‰) to Niijima (–0.14 ± 0.05‰) and Kozushima (–0.20 ± 0.21‰). The most straightforward interpretation for the elevated δ98/95Mo values in the N-Izu VF basalts is the contribution of slab-derived aqueous fluid enriched in heavy Mo isotopes to the source mantle of the VF volcanoes, due to the presence of residual minerals in the slab in which light Mo isotopes are preferentially distributed. In contrast, the low δ98/95Mo values in the RA basalts, which are as low as the mean MORB value (–0.18‰), suggest that the slab-derived fluid in the RA region is depleted in heavy Mo isotopes as a result of the continuous loss of heavy Mo isotopes via slab dehydration in the VF region, while other possibilities, including the incorporation of slab melts containing sedimentary components, cannot be ruled out.
We present high-precision Nd isotope compositions for ordinary and carbonaceous chondrites determined using thermal ionization mass spectrometry with dynamic and multistatic methods. The ordinary ...chondrites had uniform and non-terrestrial μ142Nd, μ148Nd, and μ150Nd values, with data that plot along the mixing line between s-process and terrestrial components in μ150Nd versus μ148Nd and μ142Nd versus μ148,150Nd diagrams. In contrast, the carbonaceous chondrites were characterized by larger anomalies in their μ142Nd, μ148Nd, and μ150Nd values compared to ordinary chondrites. Importantly, the data for carbonaceous chondrites plot along the s-process and terrestrial mixing line in a μ150Nd versus μ148Nd diagram, whereas they have systematically lower μ142Nd values than the s-process and terrestrial mixing line in μ142Nd versus μ148,150Nd diagrams. This shift likely results from the incorporation of calcium- and aluminum-rich inclusions (CAIs), indicating that the Nd isotopic variability in the ordinary chondrites and CAI-free carbonaceous chondrites was caused solely by the heterogeneous distribution of s-process nuclides. The isotopic variation most likely results from nebular thermal processing that caused selective destruction of s-process-depleted (or r-process-enriched) dust grains in the inner Solar System where the parent bodies of ordinary chondrites formed, whereas such grains were preserved in the region of carbonaceous chondrite parent body formation. The Nd isotope dichotomy between ordinary and bulk aliquots of carbonaceous chondrites can be related to the presence of Jupiter, which may have separated two isotopically distinct reservoirs that were present in the solar nebula. After correcting for s-process anomalies and CAI contributions to the Nd isotopes observed in the chondrites, we obtained a μ142Nd value (−2.4±4.8 ppm) that was indistinguishable from the terrestrial value. Our results corroborate the interpretation that a missing reservoir (e.g., a hidden enriched reservoir, erosional loss of crust) is not required to explain the observed differences in 142Nd/144Nd ratios between chondrites and terrestrial materials.
•Ordinary and carbonaceous chondrites possess non-radiogenic Nd isotope anomalies.•Nd isotope variability was caused by the heterogeneous distribution of s-nuclides.•Nebular thermal processing possible cause of planetary-scale Nd isotope variability.•Excess 142Nd of silicate Earth does not require missing enriched reservoirs.
Felsic magmas produced at subduction zones have played an important role in the generation and evolution of the continental crust. For the origin of felsic magmas, processes such as fractional ...crystallization of mafic magmas, partial melting of crustal materials, partial melting of subducting slabs, and partial melting of pyroxenitic mantle wedge components have been proposed. Recent experimental studies have predicted that felsic melt can also be produced in the mantle wedge by the separation of slab-derived supercritical liquid beyond depths corresponding to the critical point. To date, however, the presence of felsic magma of this origin has not yet been reported. In this study, we investigated dacitic lavas and preceding calc-alkaline andesite lavas from the Rishiri Volcano, located at the rear of the Kuril arc. We show that hydrous felsic melt and aqueous fluid were separated from slab-derived supercritical liquid in the mantle wedge. The former erupted as dacitic magma whilst the aqueous fluid induced the generation of primary basaltic magma involved in creating calc-alkaline andesite magma. We infer that slab-derived supercritical liquid is an efficient transport medium for moving silicate-rich components from subducting slabs to the Earth's surface, and that this process may have contributed to the growth of the continental crust.
New high-precision Mo isotopic data were obtained for 10 iron meteorites and two carbonaceous, five ordinary, and two rumuruti chondrites. A clear isotopic dichotomy is observed in iMo− 94Mo diagrams ...between the CC meteorites (carbonaceous chondrites and IVB irons) and other noncarbonaceous (NC) meteorites. The Mo isotope variabilities within the CC meteorites can indicate either s-process matter distributed heterogeneously throughout various chondritic components in the different outer solar system materials or that generated by a local parent-body processing. In contrast, the presence of two end-member components for the Mo isotope composition, that is, NC-A and NC-B, was suggested in the NC reservoir. The NC-B component represents the remaining counterpart of the gaseous source reservoir for type B calcium-aluminum-rich inclusions, which was presumably formed via thermal processing that destroyed r-process-rich carriers. Two models were proposed to consider the observed Mo isotope variability among the NCs. In model 1, the NC-A reservoir was formed closer to the Sun than the NC-B reservoir by another thermal processing that destroyed s-process-depleted phases. The Mo isotopic composition of the NC region changed via outward motion of particles from the two reservoirs, resulting in a gradual change from NC-A- to NC-B-like components as a function of the heliocentric distance. In model 2, the Mo isotopic composition in individual NCs is controlled by the amount of metal and matrix-like material that is removed from and added to the NC-B reservoir. Such a fractionation process most likely occurred locally in time and/or space in the inner solar system.
The surface of Mars has experienced progressive oxidation, resulting in the formation of sulfate minerals as evidenced from surface exploration missions. However, no clear evidence for the presence ...of sulfate minerals has been reported within Martian meteorites. This study examined sulfur speciation in impact glasses of three basaltic shergottites, Elephant Moraine (EETA) 79001, Larkman Nunatak (LAR) 06319, and Dhofar 019, using X-ray absorption near-edge structure (XANES) spectroscopy. The measured XANES spectra were classified into four types: (1) sulfide, (2) highly reduced sulfide glass (∼IW+1), (3) mixture of sulfide and sulfate, and (4) sulfate. The sulfate spectra observed from EETA79001 and LAR 06319 were mixed with sulfide from the reduced igneous host rock as impact glasses were formed by shock on the surface of Mars, both sulfide and sulfate would have possibly originated on Mars. Besides, highly reduced sulfide present in the same impact glasses is inconsistent with secondary alteration on the oxic Earth’s environment. In contrast to EETA79001 and LAR 06319, all of the XANES spectra from Dhofar 019 showed the only sulfate, whose origin is most likely from terrestrial alteration. Combining with the geochemical signatures of volatile elements (e.g., D/H, C, and halogens) in impact glasses of EETA79001 and LAR 06319, we propose two possible scenarios for the formation of sulfate species to the shergottite host-rocks: (i) oxidation of sulfide minerals by subsurface oxic water in Mars, or (ii) precipitation of sulfate mineral derived from Martian subsurface water. The difference between the two models is the source of S(VI) species, whether it originated from (i) magmatic sulfide in shergottite or (ii) sulfate ion in the subsurface water/ice. Both models indicate that the ancient (∼4 Ga) water reservoir might have already been oxic, and it requires post-magmatic water–rock interaction that formed sulfate minerals whose oxidized signatures were incorporated into impact glass.
We report the results of a comprehensive, high precision survey of the Cr isotopic compositions of primitive chondrites, along with some differentiated meteorites. To ensure complete dissolution of ...our samples, they were first fused with lithium borate–tetraborate at 1050–1000
°C. Relative to the NIST Cr standard SRM 3112a, carbonaceous chondrites exhibit excesses in
54Cr/
52Cr from 0.4 to 1.6
ε (1
ε
=
1 part in 10,000), and ordinary chondrites display a common
54Cr/
52Cr deficit of ∼0.4
ε. Analyses of acid-digestion residues of chondrites show that carbonaceous and ordinary chondrites share a common
54Cr-enriched carrier, which is characterized by a large excess in
54Cr/
52Cr (up to 200
ε) associated with a very small deficit in
53Cr/
52Cr (<2
ε). We did not find
54Cr anomalies in either bulk enstatite chondrites or in leachates of their acid-digestion residues. This either requires that the enstatite chondrite parent bodies did not incorporate the
54Cr anomaly carrier phase during their accretion, or the phase was destroyed by parent body metamorphism. Chromium in the terrestrial rocks and lunar samples analyzed here show no deviation from the NIST SRM 3112a Cr standard. The eucrite and Martian meteorites studied exhibit small deficits in
54Cr/
52Cr. The
54Cr/
52Cr variations among different meteorite classes suggest that there was a spatial and/or temporal heterogeneity in the distribution of a
54Cr-rich component in the inner Solar System.
We confirm the correlated excesses in
54Cr/
52Cr and
53Cr/
52Cr for bulk carbonaceous chondrites, but the new data yield a steeper slope (∼6.6) than that reported in
Shukolyukov and Lugmair (2006). The correlated excesses may affect the use of the Mn–Cr chronometer in carbonaceous chondrites. We could not confirm the bulk carbonaceous chondrite Mn–Cr isochron reported by
Shukolyukov and Lugmair (2006) and
Moynier et al. (2007), mostly because we find much smaller total variations in
ε
53Cr (∼0.2). All bulk chondrites have small
ε
53Cr excesses (up to 0.3) relative to the Earth, most likely reflecting the sub-chondritic Mn/Cr ratio of the Earth. The
ε
53Cr variations in chondrites do seem to grossly correlate with Mn/Cr and yield an initial Solar System
53Mn/
55Mn value of 5.4(±2.4)
×
10
−6, corresponding to an absolute age of 4566.4 (±2.2)
Ma.
Nuclear interactions with cosmic rays result in coupled excesses in
ε
54Cr and
ε
53Cr with a ∼4:1 ratio in phases with high Fe/Cr. These are most dramatically demonstrated in the iron meteorite Carbo, showing excesses in
ε
54Cr of up to 140
ε. These new results show that the Mn–Cr chronometer should be used with caution in samples/minerals with high Fe/Cr and long cosmic ray exposure ages.
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