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.
Isotopic analysis of Fe has proved to be a productive tool for the study of the formation and evolution of planetary bodies. The Fe isotopic studies on the Moon revealed that low-Ti and high-Ti ...basalts are positively correlated with TiO2 contents, indicating the occurrence of Fe isotopic fractionation during lunar magma ocean differentiation (Weyer et al., 2005; Liu et al., 2010; Wang et al., 2015; Sossi and Moynier, 2017). If true, very low-Ti basalts may be more representative of the lunar mantle than high-Ti and low-Ti basalts. Thus, an Fe isotopic study of very low-Ti basalts is required to investigate the Fe isotopic evolution scenario during lunar magma ocean differentiation.
In this study, we measured Fe isotopic ratios of soil samples from the Soviet Luna missions, Luna 16, 20, and 24, using MC-ICP-MS. Bulk soil samples without isolation of components and separated monomineralic grains (olivine, pyroxene, and agglutinate) were dedicated for the Fe isotopic study. Among our samples, Luna 24 soil is recognized as very low-Ti basalt. Evidence of non-mass-dependent Fe isotopic fractionation was not found in bulk soils and separated monomineralic olivine, pyroxene, and agglutinate grains. Lunar regolith samples are weathered by meteorite impacts on the Moon’s surface. A mature regolith sample, L1613, showed clearly distinguished Fe isotopic compositions between the fine-grained fraction (δ57Fe = 0.54 ± 0.06‰) and the coarse-grained fraction (δ57Fe = 0.20 ± 0.06‰). Such Fe isotopic differences among different size fractions were smaller in the submature sample (L2001) and the immature sample (L24130.3). The Fe isotopic ratios of separated grains show that agglutinates have higher δ57Fe values than bulk and monomineralic grains, indicating abundant nanophase Fe in agglutinates and partial evaporation of Fe during its melting.
The δ57Fe value for the coarsest-grained fraction of L24130.3 was found to be 0.08 ± 0.04‰, which is the first Fe isotopic composition reported for lunar samples with very low-Ti mare basalt composition. This value is clearly lower than previously reported δ57Fe values for high-Ti and low-Ti basalts and is nearly identical to those of Mg-suite rocks. This observation is consistent with the Fe isotopic evolution model during lunar magma ocean differentiation (Sossi and Moynier, 2017). Our results related to very low-Ti basalt showed that the bulk silicate Moon has lower Fe isotopic composition than the value estimated from averaged low-Ti or high-Ti basalts.
Given the data known to date, including our results, we conclude that it is most likely that Earth and the Moon have the same Fe isotopic composition. We believe that isotopically heavy Fe of silicate Earth is balanced by isotopically light Fe of Earth’s core, as suggested by Galimov’s (2011) model for the formation of the Earth-Moon system.
Microscale isotopic measurements in geological materials are becoming of significant importance in geochemistry. This study evaluated the potential of a new microsampling method called laser ablation ...coupled with filter (LAF) for precise isotopic measurements of small‐sized geological materials, compared with conventional micromilling techniques. We optimised the system to maximise the sample collection rate and minimise the extent of the contamination. Isotope ratios (87Sr/86Sr, 142Nd/144Nd, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) of the glass reference material NIST SRM 610 collected using LAF were measured by thermal ionisation mass spectrometry, and no isotopic fractionation or blank effect was observed. This indicates that LAF can be applied for high‐precision isotope ratio measurement of these elements. Considering the total procedural blanks in LAF sampling, the minimum amounts required for high‐precision isotope measurement were 50, 2 and 20 ng for Sr, Nd and Pb, respectively. The LAF method has advantages over the conventional method. It also causes less damage in the test sample’s depth direction and can be applied to samples with complex shapes. It is expected therefore that the LAF sampling method will be applied to geology and other research fields that require microsampling.
Key Points
A new microsampling method, LAF, was evaluated for precise isotopic measurements of small‐sized geological materials.
Isotope ratios of a glass reference material collected using LAF were measured by TIMS, and no isotopic fractionation was observed.
LAF is applicable to samples with complex shapes.
Nucleosynthetic isotope anomalies in meteorites are useful for investigating the origin of materials in the protoplanetary disk and dynamical processes of planetary formation. In particular, calcium ...and aluminum-rich inclusions (CAIs) found in chondrites are key minerals for decoding the initial conditions of the solar system before the accretion of small planetary bodies. In this study, we report isotopic analyses for three Allende CAIs, fluffy type A (FTA), type B, and fine-grained spinel rich (FS) inclusions, with a specific emphasis on the measurements of 84Sr/86Sr ratios. It was found that the average 84Sr values (106 relative deviations from a standard material) were 175, 129, and 56 ppm for the samples of FTA, type B, and FS inclusions, respectively. Additionally, the FTA samples exhibited heterogeneous 84Sr values, while those for the type B and FS inclusions were homogeneous within individual inclusions. The elevated 84Sr values were most likely explained by the relative enrichment of r-process nuclides in the CAI formation region. The variation of 84Sr values between the FTA and type B inclusions, as well as within the FTA inclusion, suggests the presence of multiple CAI source reservoirs with distinct isotopic compositions, which is either inherited from isotopic heterogeneity in the molecular cloud or caused by the selective destruction of r-process-enriched supernova grains via nebular thermal processing. On the other hand, the reaction between a refractory precursor of the FS inclusion and a gaseous reservoir enriched in Mg, Si, and 16O resulted in the lowest 84Sr values for the FS inclusion.
To investigate the formation processes of metal grains in chondrites, we measured the abundances of highly siderophile elements (HSEs: Re, Os, Ir, Ru, Pt, Rh, Pd, and Au) using laser ...ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on individual Fe-Ni metal grains from four petrologic type 3 ordinary chondrites: NWA 6910 (L3.3), NWA 4910 (LL3.1), Richfield (LL3.7), and SAH 97210 (L/LL3.2). Among HSEs, the abundances of Pd and Au in the metal grains had positive correlations with the measured Ni abundances, indicating equilibrium partitioning of Pd and Au between kamacite and taenite via thermal metamorphism. In contrast, the other HSEs (Re, Os, Ir, Ru, Pt, Rh) showed large variations in concentrations spanning nearly three orders of magnitude without evidence of redistribution between kamacite and taenite, suggesting that these elements preserved the initial compositions before kamacite-taenite segregation. The CI-normalized HSE patterns presented large depletions in Os and Ir with relatively large Os/Ir variations (0.29–3.2) and the Ru/Ir ratios also varied significantly (0.27–40). In addition, HSE abundances in fine metal grains showed wide variations compared to those of coarse metal grains. We suggest that the variation of refractory HSE compositions in Fe-Ni metal grains with characteristic Os-Ir depletions were most likely caused by solid metal-liquid metal partitioning during crystallization of a Fe-Ni metal melt containing 2 wt.% of C. The liquid metal is considered to be generated during multiple heating events related to chondrule formation. The lack of Fe-Ni metal grains exhibiting coexistence of liquid metal and solid metal composition within a single metal grain would suggest that solid metal grains were physically segregated from the liquid metal during the crystallization of Fe-Ni metals. Droplets of the segregated liquid metal collided and merged with other liquid metal droplets and solid metal grains to form coarser metal grains. The resultant larger metal grains have relatively homogeneous HSE abundances that are close to the bulk metal composition as a result of the mixing of liquid metal with solid metal. In contrast, molten metal droplets and solid metal grains that did not collide and merge formed finer metal grains formed finer metal grains with more variable HSE abundances.
Zircon is resistant to alteration over a wide range of geological environments, and isotopic ratios within the mineral provide constraints on ages and their parental magmas. Trace element ...compositions in zircon are also expected to reflect those of their parent magmas, and have a potential as essential indicators for their host rocks. Because most detrital zircons that accumulate at river mouths are derived primarily from granitoids, the classification of zircon within granitoids is potentially meaningful. This study employs the conventional classification scheme of granites (I‐, S‐, M‐, and A‐types). To clarify geochemical characteristics of zircons in A‐type granites, trace element compositions of zircons extracted from the A‐type Ashizuri granitoids were examined. Zircons from the Ashizuri granitoids commonly show enrichments of heavy rare earth elements and positive Ce anomalies, indicating that these zircons were igneous in origin. In addition, zircons in these A‐type granites are characterized by enrichments of Nb, Y, Ta, Th, and U and strong negative Eu anomalies, which exhibit good positive correlations with those in their whole rocks. This fact indicates that these signatures in zircons reflect well those in their parental bodies and are useful in identifying zircons derived from A‐type granite. Based on compilations of available data, zircons from A‐type granites can be clearly discriminated from other‐types of granites within Nb/Sr–Eu anomaly, U/Sr–Eu anomaly, Nb/Sr–U/Sr, and Nb/Sr–Ta/Sr cross‐plots. All indices used in these diagrams were selected based on the geochemical features of both zircon and whole rock of A‐type granites. Application of these discrimination diagrams to detrital zircons will likely provide further insights. For example, some Hadean detrital zircons plot in similar fields to A‐type granites, implying the existence of A‐type magmatism in the Earth's earliest history.
ジルコンは変質等に強く, その同位体比組成や微量元素組成は年代や親マグマ, 母岩の情報を制約しうる. 河口に堆積する砕屑性ジルコンの多くが花崗岩質岩由来であるとみなし, 各種花崗岩中のジルコンを微量元素組成を用いて分類することを試みた. 本研究では高知県足摺岬に露出する A 型花崗岩からジルコンを分離し, その微量元素組成を決定し, 他のI‐, S‐, M型花崗岩中のジルコンとの組成を比較した. 分析に用いたジルコンの多くが重希土類元素の濃集とCe正異常を示すことからジルコンの火成起源が示唆される. これに加え, A型花崗岩中のジルコンはNb, Y, Ta, Th, Uを高濃度で含み, 強いEu負異常を示す. これらが母岩の化学的特徴とも良く一致することから, 母岩の情報はジルコンに引き継がれていると言える. 先行研究を含め既存のデータを集約すると, A型花崗岩中のジルコンはNb/Sr–Eu異常, U/Sr–Eu異常, Nb/Sr–U/Sr, Nb/Sr–Ta/Sr図上で他のI‐, S‐, M型花崗岩中のジルコンと区別できる.
We examined trace element compositions of zircons extracted from the A‐type Ashizuri granitoids. Based on compilations of available data, zircons from A‐type granites can be clearly discriminated from other‐types of granites within Nb/Sr–Eu anomaly, U/Sr–Eu anomaly, Nb/Sr–U/Sr, and Nb/Sr–Ta/Sr cross‐plots.
We report Os isotope compositions of metal grains in two CBa chondrites (Bencubbin and Gujba) determined using a micromilling sampling coupled with thermal ionization mass spectrometry, together with ...the abundances of major and trace siderophile elements obtained by electron probe microanalysis and femtosecond laser ablation inductively coupled plasma–mass spectrometry. The CBa metal grains presented 187Os/188Os ratios akin to carbonaceous chondrites with limited variations (0.1257–0.1270). Most of the CBa metal grains were scattered along a 187Re‐187Os reference isochron of IIIAB iron meteorites, indicating that the CBa metals experienced limited Re‐Os fractionation at the time of their formation. The Re/Os ratios of sampling spots for the CBa metals, recast from the observed 187Os/188Os ratios, had a positive correlation with their Os/Ir ratios. In addition, the metal grains showed a positive correlation in a Pd/Fe versus Ni/Fe diagram. These correlations suggest that the CBa metal grains have formed via equilibrium condensation or evaporation from a gaseous reservoir at ~10−4 bar with enhanced metal abundances. Compared to the Bencubbin metals, the Gujba metals are characterized by having systematically lower Pd/Fe and Ni/Fe ratios that span subchondritic values. Such a difference was most likely induced by the compositionally heterogeneous impact plume from which the metals were condensed.
A new chromatographic separation technique was developed for the measurement of the strontium (Sr) isotope abundance ratio using inductively coupled plasma mass spectrometry (ICP-MS). This technique, ...LC-ICP-MS, utilizes the addition of 18-crown-the eluent and an ODS column. The technique completely eliminates the spectral interference of
87
Rb (an isobar of
87
Sr) with
87
Sr during the ICP-MS measurement. 18-Crown-6 selectively forms a clathrate compound with some metals. When the Sr isotope abundances are measured using LC-ICP-MS with an ODS column, the addition of a specific amount of 18-crown-6 to the HNO
3
/0.1% methanol eluent elongates the retention time of Rb. Therefore, Rb can be completely separated from Sr using LC because Rb is captured into the 18-crown-6 molecule and forms a clathrate compound. The technique enables the measurement of Sr isotope abundance ratios easily, even in the presence of a large amount of Rb. The proposed technique was applied to the measurement of Sr isotope abundance ratios in rice samples. To validate the LC-ICP-MS technique, a fraction containing Sr was collected, and the
87
Sr/
86
Sr isotope ratio was precisely measured using ICP-MS.
The chromatographic separation technique was developed for the measurement of the strontium isotope abundance ratio by LC-ICP-MS with ODS column. The technique eliminates a spectral interference of
87
Rb with
87
Sr in ICP-MS measurement.
A new chromatographic separation technique was developed for the measurement of the strontium (Sr) isotope abundance ratio using inductively coupled plasma mass spectrometry (ICP-MS). This technique, ...LC-ICP-MS, utilizes the addition of 18-crown-the eluent and an ODS column. The technique completely eliminates the spectral interference of 87Rb (an isobar of 87Sr) with 87Sr during the ICP-MS measurement. 18-Crown-6 selectively forms a clathrate compound with some metals. When the Sr isotope abundances are measured using LC-ICP-MS with an ODS column, the addition of a specific amount of 18-crown-6 to the HNO3/0.1% methanol eluent elongates the retention time of Rb. Therefore, Rb can be completely separated from Sr using LC because Rb is captured into the 18-crown-6 molecule and forms a clathrate compound. The technique enables the measurement of Sr isotope abundance ratios easily, even in the presence of a large amount of Rb. The proposed technique was applied to the measurement of Sr isotope abundance ratios in rice samples. To validate the LC-ICP-MS technique, a fraction containing Sr was collected, and the 87Sr/86Sr isotope ratio was precisely measured using ICP-MS.
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