¹Emily A Worsham ,¹Thorsten Kleine
Science Advances 7, 44 Link to Article [DOI: 10.1126/sciadv.abh2837]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, Münster 48149, Germany

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^1,2,3,4Ye Li,^4,5Alan E. Rubin,^1,2Weibiao Hsu
American Mineralogist 106, 1751–1767 Link to Article [https://doi.org/10.2138/am-2021-7689]
¹CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210034, China 2
²CAS Center for Excellence in Comparative Planetology, Hefei 230026, China 3
³The State Key Laboratory of Planetary Science, Macau University of Science and Technology, Macau 4
⁴Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, California 90095-1567, U.S.A. 5
⁵Maine Mineral and Gem Museum, 99 Main Street, P.O. Box 500, Bethel, Maine 04217, U.S.A.
Copyright: The Mineralogical Society of America

Studies of the new growth and re-distribution of Cu-rich phases in chondrites of different petrologic subtypes can potentially provide insights into post-accretionary parent-body processes. We present a systematic study of the distribution of Cu-rich phases and metallic Cu in Ornans-like carbonaceous chondrites (CO3) that underwent little aqueous alteration or shock (most with shock stages of S1) but exhibit a range of thermal metamorphism (subtype 3.0–3.7). A comparison to ordinary chondrites (OCs), which have undergone a larger range of shock levels, allows us to constrain the relative roles of radiogenic and shock heating in the origin of Cu distribution in chondrites. We found that the Cu content of Ni-rich metal and calculated bulk Cu content of CO3 chondrites (based on mass-balance calculations) show an increase from CO3.0 to CO3.2 chondrites. We speculate that some unidentified phases in the matrix account for a significant portion (nearly ~100 ppm) of the Cu budget in bulk samples of CO3.0 chondrites, while Ni-rich metal is the main Cu-carrier for CO3.2–3.7 chondrites. Within CO3.2–3.7 chondrites, Cu and Ni contents of Ni-rich metal are positively correlated, showing a systematic decrease from lower to higher subtype (~0.41 wt% Cu and ~45.0 wt% Ni in CO3.2 Kainsaz; ~0.28 wt% Cu and ~38.8 wt% Ni in CO3.7 Isna). Metallic Cu grains were found in every sample of CO3.2–3.7 chondrites, but not in any CO3.0–3.1 chondrites. Metallic Cu is: (1) present at metallic-Fe-Ni-pyrrhotite interfaces; (2) associated with fine irregular pyrrhotite grains in Ni-rich-metal-pyrrhotite nodules; (3) associated with fizzed pyrrhotite (fine-grained mixtures of irregularly shaped metal grains surrounded by pyrrhotite); (4) present at the edges of metallic Fe-Ni grains; and (5) present as isolated grains. In some metallic-Cu-bearing mineral assemblages, pyrrhotite has higher Cu concentrations than adjacent Ni-rich metal and shows a drop in Cu concentration at the interface between metallic Cu and Cu-rich pyrrhotite. This implies that the precipitation of metallic Cu grains could be related to the local Cu enrichment of pyrrhotite. We consider that radiogenic heating is mainly responsible for the formation of opaque phases in CO chondrites based on the relatively slow metallographic cooling rate (~0.1–5 °C/Ma), the increasing uniformity of Ni contents in Ni-rich metal with increasing CO subtype (44.3 ± 17.3 wt% in CO3.00 to 38.8 ± 3.4 wt% in CO3.7 chondrite), and the relatively narrow range of pyrrhotite metal/sulfur ratios (~0.976–0.999). Metal/sulfur ratios of pyrrhotite grains in most CO3.2–3.7 chondrites (mean = ~0.986–0.997; except Lancé) are slightly higher than those in CO3.0–3.1 chondrites (mean = ~0.981–0.987; except Y-81020), possibly indicative of a release and re-mobilization of sulfur during progressive heating as previously reported for type-3 chondrites. In this regard, we suggest most metallic Cu grains in CO3 chondrites may have precipitated from Cu-rich pyrrhotite due to sulfidation of Fe-Ni metal during parent-body thermal metamorphism. Locally, a few metallic Cu grains associated with fizzed pyrrhotite could have formed during transient shock-heating. Both thermal and shock metamorphism could be responsible for the formation of metallic Cu.

Although the systematic decrease in the Ni contents of Ni-rich metal from subtype-3.2 to subtype-3.8 also occurs in OCs, the average Cu contents of Ni-rich metal grains are indistinguishable among type-3 OCs of different subtypes. The paucity of metallic Cu in weakly shocked type-3 OCs could be related to: (1) the relatively low-bulk Cu contents of OCs, and/or (2) the relatively rapid metallographic cooling rates at <500–600 °C (~1–10 °C/Ma for LL chondrites), possibly resulting from early disturbance of OC parent bodies. The intergrowth of metallic Cu and irregular pyrrhotite more commonly occurs in shocked type-4 to type-6 OCs than in CO3 chondrites. This could be due to S in type-4 to type-6 OCs being more mobilized due to shock heating than in unshocked CO3 chondrites. We predict that some other groups of carbonaceous chondrites (e.g., CI and CM) are less likely to produce metallic Cu due to the: (1) relatively low amount of metallic Fe-Ni; (2) relatively low parent-body temperatures of ~100–300 °C; (3) high mobility of Cu in solution for aqueously altered samples; and (4) the short heating duration for metamorphosed samples.

¹John J. Donovan,²Julien M. Allaz,³Anette von der Handt,⁴Gareth G.E. Seward,⁵Owen Neill,⁶Karsten Goemann,¹Julie Chouinard,⁷Paul K. Carpenter
American Mineralogist 106, 1717–1735 Link to Article [http://www.minsocam.org/msa/ammin/toc/2021/Abstracts/AM106P1717.pdf]
¹CAMCOR, University of Oregon, Eugene, Oregon, 97403, U.S.A. 2
²Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland 3
³Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A. 4
⁴Department of Earth Science, University of California Santa Barbara, Santa Barbara, California 93101, U.S.A.
⁵Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48013, U.S.A. 6
⁶Central Science Laboratory, University of Tasmania, Hobart, Tasmania 7001, Australia 7
⁷Department of Earth and Planetary Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri 63130, U.S.A.
Copyright: The Mineralogical Society of America

While much progress has been made in electron-probe microanalysis (EPMA) to improve the accuracy of point analysis, the same level of attention has not always been applied to the quantification
of wavelength-dispersive spectrometry (WDS) X-ray intensity maps at the individual pixel level. We
demonstrate that the same level of rigor applied in traditional point analysis can also be applied to the
quantification of pixels in X-ray intensity maps, along with additional acquisition and quantitative
processing procedures to further improve accuracy, precision, and mapping throughput. Accordingly,
X-ray map quantification should include pixel-level corrections for WDS detector deadtime, corrections
for changes in beam current (beam drift), changes in standard intensities (standard drift), high-accuracy
removal of background intensities, quantitative matrix corrections, quantitative correction of spectral
interferences, and, if required, time-dependent corrections (for beam and/or contamination sensitive
materials). The purpose of quantification at the pixel level is to eliminate misinterpretation of intensity
artifacts, inherent in raw X-ray intensity signals, that distort the apparent abundance of an element.
Major and minor element X-ray signals can contain significant artifacts due to absorption and fluorescence effects. Trace element X-ray signals can contain significant artifacts where phases with different
average atomic numbers produce different X-ray continuum (bremsstrahlung) intensities, or where a
spectral interference, even an apparently minor one, can produce a false-positive intensity signal. The
methods we propose for rigorous pixel quantification require calibration of X-ray intensities on the
instrument using standard reference materials, as we already do for point analysis that is then used to
quantify multiple X-ray maps, and thus the relative time overhead associated with such pixel-by-pixel
quantification is small. Moreover, the absolute time overhead associated with this method is usually less
than that required for quantification using manual calibration curve methods while resulting in significantly better accuracy. Applications to geological, synthetic, or engineering materials are numerous as
quantitative maps not only show compositional 2D variation of fine-grained or finely zoned structures
but also provide very accurate quantitative analysis, with precision approaching that of a single point
analysis, when multiple-pixel averaging in compositionally homogeneous domains is utilized.

^1,2,3Achen Duan,^1,4Yunzhao Wu,⁵Edward A. Cloutis,^1,2Jinfei Yu,¹Shaolin Li,^1,2Yun Jiang
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13750]
¹Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210023 China
²School of Astronomy & Space Sciences, University of Science and Technology of China, Hefei, 230026 China
³Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing, 210023 China
⁴CAS Center for Excellence in Comparative Planetology, China
⁵Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, R3B 2E9 Manitoba, Canada
⁶State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, PR China
⁷CAS Center for Excellence in Comparative Planetology, China
Published by arrangement with John Wiley & Sons

Carbonaceous chondrites (CCs) are important materials for understanding the early evolution of the solar system and delivery of organic material to the early Earth. Spectral analysis of CCs can establish the relationship between them and their possible parent asteroids, which helps to determine the surface composition of the asteroid. In this paper, the 0.3–26 μm reflectance spectra of a series of coals ranging from lignite to anthracite (Earth analogs of organic matter contained in CCs), a coal heated to various durations and temperatures, and reflectance spectra of CM2 meteorites were analyzed in conjunction with compositional information to derive spectral–compositional relationships. All types of coals have strong aromatic absorptions (3.28 and 5–6.5 μm) and aliphatic “triplet” absorptions (3.38, 3.41, and 3.48 μm). In contrast, CM2 meteorites have obvious aliphatic absorptions and lack aromatic absorptions. The reason is the weak absorption coefficients of aromatic materials and the overlap with strong OH/H₂O absorption. Absorptions in the coal spectra are strongly related to elemental H/C ratio. When the H/C ratio is >0.55, the absorption intensity of an aliphatic increases linearly with the increase of H/C. For heated coal, increasing heating time above 1 h at 450 °C causes the disappearance of the aliphatic “triplet” absorptions. Similarly, heating Murchison meteorite to 400 °C for 1 week causes all the organic absorptions to disappear. This implies that in remote sensing detections, only asteroids (e.g., with CM and CI carbonaceous chondrites compositions) that experienced low thermal metamorphism (<400 °C) are suitable as potential targets for detecting organic compounds using features in the 3–4 µm region.

¹Edward D. Young
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13752]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, 90095 USA
Published by arrangement with John Wiley & Sons

The trapped melt fractional crystallization model for the IIIAB iron meteorites put forward by J. T. Wasson two decades prior is revisited. The basic precepts upon which the model was based remain true, and the model can be implemented using Ir and Au solid/liquid distribution coefficients that are broadly consistent with experimental data. For this reason, the difference between the Wasson model and some more recent trapped melt models lies mainly with inferences about the S concentrations of the core of the IIIAB iron meteorite parent body. For the Wasson model, S bulk concentrations of about 2 wt% are implied. For the more recent model, much greater concentrations of between about 12–15 wt% are indicated. The two different trapped melt models profoundly influence the interpretation of high δ⁵⁷Fe values relative to chondrites in the IIIAB irons. The Wasson model suggests that there should be more variations in δ⁵⁷Fe than are observed among these meteorites, while the more recent trapped melt model relies on the crystallization of FeS from the trapped melt to raise the δ⁵⁷Fe of the latter, thus minimizing the variability. The interpretation of Fe isotope ratios in the IIIAB meteorites therefore depends critically on the S concentration of the parent body core.

¹Putirka, K.D.,¹Xu, S.
Nature Communications 12, 6168 Link to Article [DOI https://doi.org/10.1038/s41467-021-26403-8]
¹Department of Earth and Environmental Sciences, California State University, 2576 E. San Ramon Ave, MS/ST 24, Fresno, CA, 93740, USA
²Gemini Observatory/NSF’s NOIR Lab, #314, 670N. A’ohoku Place, Hilo, HI, 96720, USA

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