Nao Nakanishi’s research while affiliated with Tokyo Institute of Technology and other places

Nucleosynthetic isotope variations are powerful tracers to determine genetic relationships between meteorites and planetary bodies. They can help to link material collected by space missions to known meteorite groups. The Hayabusa 2 mission returned samples from the Cb‐type asteroid (162173) Ryugu. The mineralogical, chemical, and isotopic characteristics of these samples show strong similarities to carbonaceous chondrites and in particular CI chondrites. The nucleosynthetic isotope compositions of Ryugu overlap with CI chondrites for several elements (e.g., Cr, Ti, Fe, and Zn). In contrast to these isotopes, which are of predominately supernovae origin, s ‐process variations in Mo isotope data are similar to those of carbonaceous chondrites, but even more s‐ process depleted. To further constrain the origin of this depletion and test whether this signature is also present for other s ‐process elements, we report Zr isotope compositions for three bulk Ryugu samples (A0106, A0106‐A0107, C0108) collected from the Hayabusa 2 mission. The data are complemented with that of terrestrial rock reference materials, eucrites, and carbonaceous chondrites. The Ryugu samples are characterized by distinct ⁹⁶ Zr enrichment relative to Earth, indicative of a s ‐process depletion. Such depletion is also observed for carbonaceous chondrites and eucrites, in line with previous Zr isotope work, but it is more extreme in Ryugu, as observed for Mo isotopes. Since s ‐process Zr and Mo are coupled in mainstream SiC grains, these distinct s‐ process variations might be due to SiC grain depletion in the analyzed materials, potentially caused by incomplete sample digestion, because the Ryugu samples were dissolved on a hotplate only to avoid high blank levels for other elements (e.g., Cr). However, local depletion of SiC grains cannot be excluded. An alternative, equally possible scenario is that aqueous alteration redistributed anomalous, s ‐process‐depleted, Zr on a local scale, for example, into Ca‐phosphates or phyllosilicates.

Most Eoarchean rocks are characterized by positive μ¹⁸²W anomalies averaging ~ +13 ppm (where μ¹⁸²W values are the part per million difference in ¹⁸²W/¹⁸⁴W between a sample and a laboratory reference material presumed to be representative of the bulk silicate Earth, BSE). Prior studies have concluded that the positive ¹⁸²W anomalies in the upper mantle disappeared by the end of the Archean, yet the timing, nature, and causes of the inferred transition remain poorly understood. In this study, we obtained SmNd mantle extraction model ages (TDM) and μ¹⁸²W values for Neoarchean and Paleoproterozoic granitic and metasedimentary rocks from the Black Hills, South Dakota, USA. The rocks examined have TDM model ages ranging between ~3.6 Ga and 2.3 Ga, permitting the tracing of the evolution of ¹⁸²W in the upper mantle precursors to these rocks, during the purported period of isotopic transition. Of these crustal rocks, the 2.55 Ga Little Elk Granite, with an average TDM age of ~3.2 Ga, is characterized by a well resolved positive anomaly (μ¹⁸²W = +8.2 ± 3.1, 2SE). This observation is consistent with a modest diminution in the upper mantle μ¹⁸²W value relative to the Eoarchean average. By contrast, the 2.60 Ga Bear Mountain Granite, with a slightly younger average TDM age of ~3.0 Ga, exhibits no resolved anomaly. Most other individual rocks examined also lack resolved anomalies, although the group average μ¹⁸²W value of +3.3 ± 1.3 (2SE) for the 1.72 Ga Harney Peak Granite, with the majority of TDM model ages ranging from ~2.7 to 2.3 Ga, may indicate a small positive anomaly for their Neoarchean to Paleoproterozoic upper mantle precursors. The Black Hills rocks provide new evidence for the uneven dissipation during the Mesoarchean through Paleoproterozoic of the positive μ¹⁸²W value that typified the Eoarchean upper mantle. When the new data are combined with data from prior studies, it becomes evident that the transition to a “modern” BSE W isotopic composition was not the result of a smooth linear decrease, but rather an irregular trend. Nevertheless, the collective data indicate that the positive anomalies in the upper mantle were nearly eliminated by the beginning of the Proterozoic, presumably by mantle mixing processes. The diminution in the scale of ¹⁸²W anomalies during the Archean is similar to that of ¹⁴²Nd, and requires a global-scale process that most likely involved vertical and/or lateral mixing within the mantle. The reasons for the delay in the initiation of this mixing process until the end of the Eoarchean, and the completion of mantle mixing by ~2.4 Ga, awaits exploration via geodynamical modeling of different mixing scenarios.

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.

Significance Kimberlites are igneous rocks derived from deep mantle sources. Recent studies have suggested that certain kimberlites originated from a mantle source with relatively primitive chemical composition that was created by early Earth processes. We present W isotope data for a global suite of kimberlites with variable formation ages and find their mantle source(s) to be characterized by ¹⁸² W/ ¹⁸⁴ W averaging ∼6 ppm lower than the upper mantle ratio. This result is consistent with derivation of some kimberlites from one or more early formed mantle reservoirs. The low ¹⁸² W/ ¹⁸⁴ W of these kimberlites is indicative of an ancient mantle source modified by some form of core–mantle interaction, an early silicate fractionation event, an overabundance of late-accreted materials, or a combination of these.

In recent years, the ¹⁸⁷Re‐¹⁸⁷Os isotope system has been increasingly used to study samples containing very small quantities of Os. For such samples, optimisation of measurement procedures is essential to minimise the loss of Os before mass spectrometric measurements. Micro‐distillation is a necessary purification step that is applied after the main Os chemical separation procedure, prior to Os isotope ratio measurements by negative thermal ionisation mass spectrometry (N‐TIMS). However, unlike the other separation steps, this procedure has not yet been optimised for small samples. In this study, we present a refined micro‐distillation method that achieved higher yields and allowed high‐precision ¹⁸⁷Os/¹⁸⁸Os measurements for small‐sized geological samples that contain only a few pg Os. The Os recovery in the micro‐distillation step was tested by changing the operating conditions including heating time and temperature, and amounts of oxidant and reductant. Recoveries were measured by the isotope dilution ICP‐MS method after the addition of ¹⁹⁰Os‐enriched spike solution. We found that the most critical factor controlling the chemical yield of Os during micro‐distillation is the extent of dilution of the reductant (HBr) by H2O evaporated from the oxidant. A refined micro‐distillation method, in which the amount of oxidant solution is reduced from the conventional method, achieved an improved chemical yield of Os (~ 90%). This refined method was applied to the ¹⁸⁷Os/¹⁸⁸Os analysis by N‐TIMS of varying test portions of the geological reference material BIR‐1a. The resulting ¹⁸⁷Os/¹⁸⁸Os ratios of BIR‐1a matched the literature data, with propagated uncertainties of 0.2, 1.1 and 11% digested sample quantities containing 150, 10 and 1 pg of Os, respectively. This article is protected by copyright. All rights reserved.

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.