¹Richard J. Lyons,²Timothy J. Bowling,¹Fred J. Ciesla,³Thomas M. Davison,³Gareth S. Collins
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13301]
¹Department of the Geophysical Sciences, University of Chicago, 5734 S. Ellis Avenue, Chicago, Illinois, 60637 USA
²Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, Colorado, 80302 USA
³Department of Earth Science and Engineering, Imperial College London, London, SW7 2AZ UK
Published by arrangement with John Wiley & Sons

Iron meteorites provide a record of the thermal evolution of their parent bodies, with cooling rates inferred from the structures observed in the Widmanstätten pattern. Traditional planetesimal thermal models suggest that meteorite samples derived from the same iron core would have identical cooling rates, possibly providing constraints on the sizes and structures of their parent bodies. However, some meteorite groups exhibit a range of cooling rates or point to uncomfortably small parent bodies whose survival is difficult to reconcile with dynamical models. Together, these suggest that some meteorites are indicating a more complicated origin. To date, thermal models have largely ignored the effects that impacts would have on the thermal evolution of the iron meteorite parent bodies. Here we report numerical simulations investigating the effects that impacts at different times have on cooling rates of cores of differentiated planetesimals. We find that impacts that occur when the core is near or above its solidus, but the mantle has largely crystallized can expose iron near the surface of the body, leading to rapid and nonuniform cooling. The time period when a planetesimal can be affected in this way can range between 20 and 70 Myr after formation for a typical 100 km radius planetesimal. Collisions during this time would have been common, and thus played an important role in shaping the properties of iron meteorites.

^1,2Marco Langbroek et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13297]
¹Department of Research & Education, Naturalis Biodiversity Center, Darwinweg 2, 2333 CR Leiden, the Netherlands
²Dutch Meteor Society, Leiden, the Netherlands
Published by arrangement with John Wiley & Sons

A carbonaceous chondrite was recovered immediately after the fall near the village of Diepenveen in the Netherlands on October 27, 1873, but came to light only in 2012. Analysis of sodium and poly‐aromatic hydrocarbon content suggests little contamination from handling. Diepenveen is a regolith breccia with an overall petrology consistent with a CM classification. Unlike most other CM chondrites, the bulk oxygen isotopes are extremely ¹⁶O rich, apparently dominated by the signature of anhydrous minerals, distributed on a steep slope pointing to the domain of intrinsic CM water. A small subset plots closer to the normal CM regime, on a parallel line 2 ‰ lower in δ¹⁷O. Different lithologies in Diepenveen experienced varying levels of aqueous alteration processing, being less aqueously altered at places rather than more heated. The presence of an agglutinate grain and the properties of methanol‐soluble organic compounds point to active impact processing of some of the clasts. Diepenveen belongs to a CM clan with ~5 Ma CRE age, longer than most other CM chondrites, and has a relatively young K‐Ar resetting age of ~1.5 Ga. As a CM chondrite, Diepenveen may be representative of samples soon to be returned from the surface of asteroid (162173) Ryugu by the Hayabusa2 spacecraft.

¹A.Néri,¹J.Guignard,¹M.Monnereau,¹M.J.Toplis,¹G.Quitté
Earth and Planetary Science Letters 518, 40-52 Link to Article [https://doi.org/10.1016/j.epsl.2019.04.049]
¹IRAP, Université de Toulouse, CNRS, CNES, UPS, Toulouse, France
Copyright Elsevier

High temperature experiments have been performed to constrain interfacial energies in a three-phase system (metal–forsterite–silicate melt) representative of partially differentiated planetesimals accreted early in the solar system history, with the aim of providing new insights into the factors affecting the interconnection threshold of metal-rich phases. Experiments were run under controlled oxygen fugacity (ΔNi-NiO=−3) at 1440 °C, typically for 24 h. Quantification of the true dihedral angles requires a resolution of at least 30 nm per pixel in order to reveal small-angle wedges of silicate melt at crystal interfaces. At this level of resolution, dihedral angle distributions of silicate melt and olivine appear asymmetric, an observation interpreted in terms of anisotropy of olivine crystals. Based upon the theoretical relation between dihedral angles and interfacial energies in a three-phase system, the relative magnitudes of interfacial energies have been determined to be: γMelt-Ol<γMelt-Ni<γOl-Ni. This order differs from that obtained with experiments using an iron sulfide liquid close to the Fe–FeS eutectic for which γMelt-Sulfide<γMelt-Ol<γOl-Sulfide, implying a lower interconnection threshold for sulfur-rich melts than for pure metallic phases. This dependence of the interconnection threshold on the sulfur content will affect the drainage of metallic phases during melting of small bodies. Assuming a continuous extraction of silicate melt, evolution of the metal volume fraction has been modeled. Several sulfur-rich melts extraction events are possible over a range of temperatures relevant with thermometric data on primitive achondrites (1200–1400 °C and 25% of silicate melt extracted). These successive events provide novel insight into the variability of sulfur content in primitive achondrites, which are either representative of a region that experienced sulfide extraction or from a region that accumulated sulfide melt from overlying parts of the parent body.

¹E.Nakamura et al. (>10)
Proceedings of the Japan Academy. Series B, Physical and biological sciences 95, 165-177 Link to Article [DOI: 10.2183/pjab.95.013]
¹Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry, Institute for Planetary Materials, Okayama University, Japan

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¹Lidia Pittarello et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.04.033]
¹Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
Copyright Elsevier

Melting experiments attempting to reproduce some of the processes affecting asteroidal and cometary material during atmospheric entry have been performed in a high enthalpy facility. For the first time with the proposed experimental setup, the resulting material has been recovered, studied, and compared with natural analogues, focusing on the thermal and redox reactions triggered by interaction between the melt and the atmospheric gases under high temperature and low pressure conditions. Experimental conditions were tested across a range of parameters, such as heat flux, experiment duration, and pressure, using two types of sample holders materials, namely cork and graphite. A basalt served as asteroidal analog and to calibrate the experiments, before melting a H5 ordinary chondrite meteorite. The quenched melt recovered after the experiments has been analyzed by μ-XRF, EDS-SEM, EMPA, LA-ICP-MS, and XANES spectroscopy.
The glass formed from the basalt is fairly homogeneous, depleted in highly volatile elements (e.g., Na, K), relatively enriched in moderately siderophile elements (e.g., Co, Ni), and has reached an equilibrium redox state with a lower Fe3+/Fetot ratio than that in the starting material. Spherical objects, enriched in SiO2, Na2O and K2O, concentrations, were observed, inferring condensation from the vaporized material. Despite instantaneous quenching, the melt formed from the ordinary chondrite shows extensive crystallization of mostly olivine and magnetite, the latter indicative of oxygen fugacity compatible with presence of both Fe2+ and Fe3+. Similar features have been observed in natural meteorite fusion crusts and in micrometeorites, implying that, at least in terms of maximum temperature reached and chemical reactions, the experiments have successfully reproduced the conditions likely encountered by extraterrestrial material following atmospheric entry.

¹Charity M.Phillips-Lander,¹Andrew S.Elwood Madden,²Elisabeth M.Hausrath,¹Megan Elwood Madden
Geochimica et Cosmochimcia Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.006]
¹School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd Street, Norman, OK 73069, USA
²Department of Geoscience, University of Nevada, Las Vegas 4505 S. Maryland Ave., Las Vegas, NV 89154
Copyright Elsevier

Both high and low calcium pyroxene minerals have been detected over large portions of the martian surface in addition to widespread salts in martian soils and dust. Calcium pyroxenes in martian meteorites are associated with secondary evaporite phases, including sulfates, chlorides, and perchlorates, suggesting the pyroxene may have been altered in saline solutions. Therefore, understanding pyroxene mineral weathering in high salinity brines may provide insight into past aqueous alteration on Mars. This study examines both solute-based dissolution rates and qualitative assessments of weathering textures developed during pyroxene-brine alteration experiments to link dissolution rates and textures and aid in interpreting weathering features observed in Mars meteorites and future pyroxene samples returned from Mars. Batch reactor dissolution experiments were conducted at 298 K to compare diopside (a high Ca-pyroxene) dissolution rates in water (18 MΩcm-1 ultrapure water (UPW); activity of water (ɑH2O) =1.0), 0.35 mol kg-1 NaCl (ɑH2O =0.99), 0.35 mol kg-1 Na2SO4 (ɑH2O =0.98), 2 mol kg-1 NaClO4 (ɑH2O =0.90), 2.5 mol kg-1 Na2SO4 (ɑH2O =0.95), 5.7 mol kg-1 NaCl (ɑH2O =0.75), and 9 mol kg-1 CaCl2 (ɑH2O =0.35) brines at pH 5-6.6 to determine how changing solution chemistry and activity of water influence pyroxene dissolution. Aqueous Si release rates and qualitative textural analyses indicate diopside dissolution rates are influenced by both solution chemistry and activity of water, with diopside weathering increasing along a trend from: 9 mol kg-1 CaCl2 < UPW (-9.82± 0.03 log mol m-2 s-1) ≈ 2 mol kg-1 NaClO4 ≈ 0.35 mol kg-1 Na2SO4 (-9.80± 0.07) ≈ 5.7 mol kg-1 NaCl (-9.69 ± 0.04) < 0.35 mol kg-1 NaCl (-9.45± 0.34) < 2.5 mol kg-1 Na2SO4 (-8.99± 0.09). Dissolution rates increase in sodium sulfate brines with increasing salinity. In contrast, Si-based dissolution rates in 0.35 mol kg-1 NaCl are faster than those measured in 5.7 mol kg-1 NaCl and UPW. However, all of the Si-based rates measured in the chloride and sulfate salt solutions are likely affected by precipitation of Si-rich secondary clay minerals, which removed Si from solution. Qualitative textural analyses indicate similar degrees of dissolution occurred in UPW and 2 M NaClO4; however, no aqueous rate determinations could be made in perchlorate brines due to explosion hazards. Aqueous Si was below detection limits in the 9 mol kg-1 CaCl2 experiments, but textural analysis suggests limited diopside dissolution occurred. Therefore, despite low water activity, diopside dissolution proceeds in both dilute to high salinity brines, readily forming clay minerals under a wide range of conditions. This suggests that outcrops on Mars containing pyroxene preserved with sulfate, chloride, perchlorate, and/or clay minerals likely record relatively short periods (<1 million years) of aqueous alteration. Si-rich spherules similar to those observed in SNC meteorites were also observed in the 5.7 mol kg-1 NaCl brine experiments, indicating that silicate mineral alteration in chloride brines may lead to Si-rich alteration products and coatings.

^1,2Milligan, J.N.,¹Royer, D.L.,³Franks, P.J.,⁴Upchurch, G.R.,¹McKee, M.L.
Geophysical Research Letters 46, 3462-3472 Link to Article [DOI: 10.1029/2018GL081215]
¹Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT, United States
²Department of Geology, Baylor University, Waco, TX, United States
³School of Life and Environmental Sciences, University of Sydney, Sydney, NSW, Australia
⁴Department of Biology, Texas State University, San Marcos, TX, United States

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¹Petrova, E.V.,¹Maksimova, A.A.,¹Chukin, A.V.,¹Oshtrakh, M.I.
Hyperfine Interactions 240, 42 Link to Article [DOI: 10.1007/s10751-019-1592-9]
¹Department of Experimental Physics, Institute of Physics and Technology, Ural Federal University, Ekaterinburg, 620002, Russian Federation

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¹Yoko Kebukawa,²Conel M. O’D. Alexander,¹George D. Cody
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13302]
¹Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, Washington, District of Columbia, 20015 USA
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, District of Columbia, 20015 USA
Published by arrangement with John Wiley & Sons

Past studies of the various separable carbonaceous fractions have been unable to account for all of C in primitive chondrites. In particular, up to 20–50% of the C is lost during acid leaching of bulk samples even after the C in carbonates and soluble organic matter is accounted for. To try to better characterize the nature of this “missing C,” we have compared the bulk infrared (IR) absorption spectra of a number of primitive chondrites with those of their previously reported insoluble organic matter (IOM). The aliphatic C–H stretching bands, in particular, allow us to compare the molecular structures of bulk C with that of IOM. The spectral differences between bulk C and IOM reflect “missing C” phases that were lost during acid leaching, although we cannot completely exclude the possibility that the OM was modified after demineralization. Comparing IR spectra of bulk meteorite powder and IOM suggests that the missing C varies in its molecular structure, and that mildly thermally metamorphosed type 3 chondrites tend to be richer in an aliphatic fraction with lower CH₂/CH₃ ratios, relative to IOM, compared to aqueously altered carbonaceous chondrites (CI/CM/CR). The missing C is most likely released from acid‐labile functional groups, such as esters, acetals, and amides, during demineralization, although it cannot be ruled out that some fraction of the missing C is in small grains that are difficult to recover from suspension, or in water‐soluble compounds trapped in phyllosilicates.

¹Anthony Gargano,²Zachary Sharp
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13303]
¹Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, 87131‐0001 USA
²Center for Stable Isotopes, University of New Mexico, Albuquerque, New Mexico, 87131‐0001 USA
Published by arrangement with John Wiley & Sons

The bulk chlorine concentrations and isotopic compositions of a suite of non‐carbonaceous (NC) and carbonaceous (CC) iron meteorites were measured using gas source mass spectrometry. The δ37Cl values of magmatic irons range from −7.2 to 18.0‰ versus standard mean ocean chloride and are unrelated to their chlorine concentrations, which range from 0.3 to 161 ppm. Nonmagmatic IAB irons are comparatively Cl‐rich containing >161 ppm with δ37Cl values ranging from −6.1 to −3.2‰. The anomalously high and low δ37Cl values are inconsistent with a terrestrial source, and as Cl contents in magmatic irons are largely consistent with derivation from a chondrite‐like silicate complement, we suggest that Cl is indigenous to iron meteorites. Two NC irons, Cape York and Gibeon, have high cooling rates with anomalously high δ37Cl values of 13.4 and 18.0‰. We interpret these high isotopic compositions to result from Cl degassing during the disruption of their parent bodies, consistent with their low volatile contents (Ga, Ge, Ag). As no relevant mechanisms in iron meteorite parent bodies are expected to decrease δ37Cl values, whereas volatilization is known to increase δ37Cl values by the preferential loss of light isotopes, we interpret the low isotope values of <−5‰ and down to −7.2‰ to most closely represent the primordial isotopic composition of Cl in the solar nebula. Similar conclusions have been derived from low δ37Cl values down to −6, and −3.8‰ measured in Martian and Vestan meteorites, respectively. These low δ37Cl values are in contrast to those of chondrites which average around 0‰ previously explained by the incorporation of isotopically heavy HCl clathrate into chondrite parent bodies. The poor retention of low δ37Cl values in many differentiated planetary materials suggest that extensive devolatilization occurred during planet formation, which can explain Earth’s high δ37Cl value by the loss of approximately 60% of the initial Cl content.