¹Vasilije V.Dobrosavljevic,²Dongzhou Zhang,¹Wolfgang Sturhahn,³Jiyong Zhao,³Thomas S.Toellner,⁴Stella Chariton,⁴Vitali B.Prakapenka,¹Olivia S.Pardo,¹Jennifer M.Jackson
Earth and Planetary Science Letters 584, 117358 Link to Article [https://doi.org/10.1016/j.epsl.2021.117358]
¹Seismological Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI, USA
³Advanced Photon Source, Argonne National Laboratory, Chicago, IL, USA
⁴Center for Advanced Radiation Sources, The University of Chicago, Chicago, IL, USA
Copyright Elsevier

Many studies have suggested silicon as a candidate light element for the cores of Earth and Mercury. However, the effect of silicon on the melting temperatures of core materials and thermal profiles of cores is poorly understood, due to disagreements among melt detection techniques, uncertainties in sample pressure evolution during heating, and sparsity of studies investigating the combined effects of nickel and silicon on the phase diagram of iron. In this study we develop a multi-technique approach for measuring the high-pressure melting and solid phase relations of iron alloys and apply it to Fe0.8Ni0.1Si0.1 (Fe-11wt%Ni-5.3wt%Si), a composition compatible with recent estimates for the cores of Earth and Mercury. This approach combines results (20-83 GPa) from two atomic-level techniques: synchrotron Mössbauer spectroscopy (SMS) and synchrotron x-ray diffraction (XRD). Melting is independently detected by the loss of the Mössbauer signal, produced exclusively by solid-bound iron nuclei, and the onset of a liquid diffuse x-ray scattering signal. The use of a burst heating and background updating method for quantifying changes in the reference background during heating facilitates the determination of liquid diffuse signal onsets and leads to strong reproducibility and excellent agreement in melting temperatures determined separately by the two techniques. XRD measurements additionally constrain the hcp-fcc phase boundary and in-situ pressure evolution of the samples during heating. We apply our updated thermal pressure model to published SMS melting data on fcc-Fe and fcc-Fe0.9Ni0.1 to precisely evaluate the effect of silicon on melting temperatures. We find that the addition of 10 mol% Si to Fe0.9Ni0.1 reduces melting temperatures by ∼250 K at low pressures (<60 GPa) and flattens the hcp-fcc phase boundary. Extrapolating our results, we constrain the location of the hcp-fcc-liquid quasi-triple point at 147±14 GPa and 3140±90 K, which implies a melting temperature reduction of 500 K compared with Fe0.9Ni0.1. The results demonstrate the advantages of combining complementary experimental techniques in investigations of melting under extreme conditions.

¹Yoann Quesnel,²Natalia S. Bezaeva,³Dilyara M. Kuzina,¹Pierre Rochette,¹Jérôme Gattacceca,¹Minoru Uehara,²Dmitry D. Badyukov,³Bulat M. Nasyrtdinov,^4,5,6Dmitry A. Chareev,⁷Cedric Champollion
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13806]
¹Aix-Marseille Université, CNRS, IRD, INRAE, CEREGE, Aix-en-Provence, France
²V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 Kosygin str, 119991 Moscow, Russia
³Institute of Geology and Petroleum Technologies, Kazan Federal University, 4/5 Kremlyovskaya Str, 420008 Kazan, Russia
⁴Institute Experimental Mineralogy, Russian Academy of Science, 4 Academician Osipyan Str, 142432 Chernogolovka, Moscow Region, Russia
⁵Institute of Physics and Technology, Ural Federal University, 19 Mira Str, 620002 Ekaterinburg, Russia
⁶National University of Science and Technology “MISiS”, 4 Leninsky Prospekt, 119049 Moscow, Russia
⁷Géosciences Montpellier, CNRS, Université de Montpellier, Montpellier, France
Published by arrangement with John Wiley & Sons

With no significant topographic expression and limited bedrock exposure, the ~10 km diameter Karla impact structure (Tatarstan, Russia) is poorly known. The age of the impact is also poorly constrained stratigraphically to between 4 and 60 Ma, even if an upper Miocene age is more likely. Targeted gravity and magnetic field surveys were conducted over Karla to explore its size and structure in 2019. Bouguer gravity anomaly data reveal a central positive (+2 mGal) peak ~2 km in diameter surrounded by a concentric negative (−1 mGal) anomaly extending to ~3 km radius; a more irregular, outward-decreasing (+1 to −1 mGal) positive anomaly extends to 6–8 km radius. A complex impact structure with diameter of 8–10 km is consistent with the Bouguer anomalies. Magnetic field data show 1 to several km-wavelength anomalies with amplitude variation from +150 to −150 nT and little concentric structure, although the impact feature broadly corresponds to a magnetic low with a weak central high. A 2-D numerical model of the structure was built using these potential-field data and petrophysical properties measured on collected samples. It confirms a central uplift composed of Paleozoic sediments and Archean crystalline basement up to 1 km of depth. A 500 m deep collapsed disruption cavity filled by breccia and lacustrine deposits accounts for the Bouguer negative ring. The reversely polarized and weak central magnetic anomalies are controlled by the geometry of the crystalline basement associated with the deformation during the central uplift.

¹N.G.Rudraswami,²M.D.Suttle,³Y.Marrocchi,⁴S.Taylor,³J.Villeneuve
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.03.015]
¹National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa 403004, India
²School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
³CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-les-Nancy, F-54501, France
⁴Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, New Hampshire 03755–1290, USA
Copyright Elsevier

We report high-precision secondary ion mass spectrometer triple oxygen isotope systematics (95 individual analyses) from 37 micrometeorites (MMs) collected from South Pole Water Well (SPWW), Antarctica. The study population focuses on unmelted coarse-grained (Cg) MMs (n=23) with both multiple (n=14) and single-mineral (n=9) varieties investigated. We also analysed relict minerals in porphyritic cosmic spherules (n=13) and the relict matrix in a single scoriaceous fine-grained (Fg) MM. The target minerals investigated are primarily olivine (Fo ∼43–99%) and spinel. Textural, chemical and isotopic data confirm that both olivine and spinel grains have retained their pre-atmospheric O-isotope compositions, allowing inferences to be drawn about their formation and parent body affinities. We separate the study population into three groups: spinel-free particles (consisting of the CgMMs and PO cosmic spherules), spinel-bearing MMs and the single FgMM.

Olivine grains in spinel-free MMs vary between δ17O: –12.6‰ and +3.5‰, δ18O: –9.6‰ and +7.5‰, and Δ17O: –9.5‰ and +1.3‰ and define a slope-1 profile in δ18O–δ17O isotope space. They are most likely fragmented chondrules, with both type I and type II varieties represented. Their observed Mg#-Δ17O distribution is best explained by a mixture of CM chondrules and either CR chondrules, Tagish Lake chondrules or WILD2 cometary silicates. One of these chondrule-like MMs has an isotopically heterogeneous composition, characterised by a single olivine grain with a markedly 16O-rich composition (Δ17O: –16.3‰), suggesting it is a relict silicate fragment of AOA material that was incorporated into the chondrule precursor.

We analysed 11 spinel grains in five spinel-bearing MMs. In all instances spinels are nearly pure MgAl2O4 with isotopically light (16O-rich) compositions (ranging from δ17O: –34.4‰ to –0.9‰, δ18O: –30.8‰ to +11.0‰, and Δ17O: –18.3‰ to –4.4‰). They are therefore 16O-poor relative to spinel found in unaltered CAIs, indicating a different origin. Grains with high Cr2O3 contents (>0.5 wt%) are interpreted originating from Al-rich chondrule precursors, while low Cr2O3 spinels (<0.5 wt%) are interpreted as CAI-derived material affected by parent body aqueous alteration. Finally, we report a single FgMM with a 16O-poor composition (Δ17O > 0‰ and δ18O > +15.0‰). This particle adds to our growing inventory of water-rich C-type asteroid samples united by their formation history which is characterised by accretion of abundant heavy water.

Our work strongly supports findings from earlier in-situ O-isotope studies, concluding that small MMs overwhelmingly sample material from CC parent bodies and that CgMMs largely sample chondrules and, to a lesser extent, CAI material. The analysis of CgMMs therefore provides insights into the primitive O-isotope reservoirs that were present in the early solar system and how they interacted.

¹Marina Martínez,¹Adrian J.Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.03.019]
¹Department of Earth & Planetary Sciences, MSC03-2040, 1University of New Mexico, Albuquerque, NM 87131, USA
Copyright Elsevier

Queen Alexandra Range (QUE) 99177 is one of the least altered CR carbonaceous chondrite known, with mineralogical and isotopic characteristics that indicate a high level of pristinity. In this study, we have examined the so-called smooth rims that surround many type I chondrules in QUE 99177, using SEM, EPMA, and FIB-TEM techniques. We have characterized their constituent phases to unravel the precursor material(s) of smooth rims, assess their formation mechanisms, and constrain the conditions of the altering fluid. Smooth rims are the most common type of rims around type I chondrules and exclusively occur around chondrules with Silica-rich Igneous Rims (SIRs). Smooth rims consist of an Fe-rich, hydrous silicate material that is Si- and Fe-rich, with minor Mg, Al, Ca, and Mn, and gives low analytical totals measured by EPMA. TEM observations reveal that the Fe-rich silicate phase is an amorphous gel that contains unaltered crystalline phases and igneous glass. Crystalline phases consist of igneous, unaltered, zoned pyroxenes with compositions consistent with pyroxenes in SIRs, as well as albite and chromite. The amorphous gel preserves previous crystal outlines with morphologies consistent with silica (cristobalite) grains in SIRs and has a composition identical to pseudomorphic silica replacements in SIRs. Based on these observations, we conclude that smooth rims derive from low-temperature aqueous alteration of silica in SIRs by an Fe-rich fluid. We suggest that the Fe was derived by leaching of amorphous silicates in the matrix, which reacted rapidly with melted water ice, although alteration of Fe,Ni metal blebs in SIRs could potentially be an additional source of Fe. Silica underwent dissolution and replacement whereas feldspar and glass remained unaltered because (1) the fluid was slightly alkaline, (2) cristobalite has a reaction rate much higher than quartz and feldspar, and (3) the alteration was very limited and fast, indicating that it was due solely to melting of accreted water ice and there was no introduction of additional fluid from external sources.

¹Steven W.Ruff,²Victoria E.Hamilton,³A. Deanne Rogers,⁴Christopher S.Edwards,⁵Briony H.N.Horgan
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114974]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
²Southwest Research Institute, Boulder, CO, USA
³Department of Geosciences, Stony Brook University, Stony Brook, NY, USA
⁴Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, AZ, USA
⁵Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
Copyright Elsevier

Exposures of bedrock rich in olivine and carbonate link Gusev crater and the Nili Fossae region (NFR), both of which have the highest abundance of olivine yet identified on Mars. They are recognized as possible explosive volcanic tephra deposits, but the nature of their eruption and emplacement is poorly constrained, limiting understanding of what may be a widespread volcanic process on early Mars. The compositional and morphologic similarities of these widely separated olivine and carbonate-rich rocks have not been investigated previously. We examined orbital and in situ thermal infrared spectra and find notably similar compositions between the two locations, with olivine ranging from ~Fo53 to Fo75 and carbonates composed of multiple Mg and Fe-rich phases, although with minimal magnesite in the NFR. A range of morphologic and textural features occur in the deposits of explosive volcanism on Earth that can be used to constrain the origin of those on Mars. We observed features of the olivine-rich bedrock in both locations that resemble those of welded ignimbrite deposits on Earth that formed from pyroclastic density currents in cataclysmic explosive volcanic eruptions. If correct, an ignimbrite interpretation for the olivine-rich bedrock in the NFR and Gusev crater may apply to other occurrences on Mars and indicate a style of volcanism more common in its early history.

¹E.B.Rampe,²B.H.N.Horgan,³R.J.Smith,²N.A.Scudder,⁴E.R.Bamber,⁵A.M.Rutledge,⁶R.Christoffersen
Earth and Planetary Science Letters 584, 117471 Link to Article [https://doi.org/10.1016/j.epsl.2022.117471]
¹Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, TX 77058, USA
²Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907, USA
³Department of Geosciences, SUNY Stony Brook, Stony Brook, NY 11794, USA
⁴Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712, USA
⁵Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, AZ 86001, USA
⁶Jacobs, NASA Johnson Space Center, Mail Code XI3, Houston, TX 77058, USA
Copyright Elsevier

Geomorphic and mineralogical data from the martian surface indicate liquid water was abundant on the martian surface and near subsurface ∼3.5 to 4 Gyr ago, but whether early Mars had a warm and wet climate or whether it was cold and icy with punctuated periods of warmth is still unknown. Mineral assemblages of sedimentary rocks on Mars help determine past aqueous conditions and sediment sources. Here, we report on the primary and secondary mineral and amorphous assemblage of glacial flour from Collier Glacier valley on the northern flank of North Sister in Oregon, U.S.A. to identify mineralogical characteristics of mafic sediments altered under cold, wet conditions. Collier glacial flour is dominated by primary igneous minerals (plagioclase is dominant, with lesser amounts of pyroxene and olivine) and comprises 10-40 wt.% X-ray amorphous materials. Crystalline secondary phases (e.g., phyllosilicates, zeolite) are not significant contributors to the authigenic alteration assemblage. High-resolution transmission electron microscopic observations of the <2 μm size fraction of the flour demonstrate that the X-ray amorphous materials are both primary (i.e., volcanic glass) and secondary in nature. The secondary X-ray amorphous materials are enriched in Si, Al, and Fe, and we observe incipient phyllosilicate formation associated with primary and secondary amorphous materials. Our results indicate chemical weathering on a cold and icy early Mars would have produced X-ray amorphous materials, but not crystalline secondary phases. We suggest that the abundant X-ray amorphous materials recognized from orbit and in situ on Mars could have formed under cold and periodically wet conditions similar to those on North Sister today. Furthermore, the lack of volumetrically significant phyllosilicate formation in Collier Glacier flour indicates phyllosilicates on Mars did not form in a cold and wet climate.

¹Libby D. Tunney,¹Patrick J. A. Hill,¹Christopher D. K. Herd,^1,2Robert W. Hilts,¹Miranda C. Holt
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13803]
¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 Canada
²Department of Physical Sciences, MacEwan University, Edmonton, Alberta, T6J 4S2 Canada
Published by arrangement with John Wiley & Sons

Soluble organic matter analyses of astromaterials can provide valuable information on the chemistry of our solar system and the processes that occur within it. The surface of the Earth, however, is a significant source of organic compounds due to the presence of life; this environment represents a major source of potential contamination for recently fallen meteorites. Here, we analyze select stones of the CM2 Aguas Zarcas carbonaceous chondrite, which fell on April 23, 2019, in Aguas Zarcas, San Carlos county, Alajuela province, Costa Rica, with the goal of determining the complement of intrinsic and contaminant soluble organic matter. The specimens were collected pre- and post-rainfall, days to weeks after the stones fell on the Earth. Through gas chromatography-mass spectrometry analysis of soluble organic matter in dichloromethane and hot water extracts of meteorite powders, we differentiate between extraterrestrial and contaminant sources for each organic compound detected. In this study, N-tert-butyldimethylsilyl- N-methyltrifluoroacetamide (MTBSTFA) was used to derivatize the hot water extracts to test out its “one-pot” extraction capabilities. The majority of the detectable organic compounds are contaminants and can be explained as being sourced from the terrestrial surface onto which the meteorite fell. Our results have implications for how environmental factors, such as land use and rainfall events in this case, can impact the intrinsic organics in carbonaceous chondrites.

¹Randy G. Hopkins,¹John G. Spray
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13799]
¹Planetary and Space Science Centre, University of New Brunswick, 2 Bailey Drive, Fredericton, New Brunswick, E3B 5A3 Canada
Published by arrangement with John Wiley & Sons

Numerical computing software (via MathWorks MATLAB) has been developed to understand the relationship between shock wave passage in geological targets (i.e., heterogeneous media) and the formation of shock veins and associated high-pressure/temperature polymorphs. This approach takes into consideration the pressure due to the passage of the shock front, subsequent rarefaction unloading pressures, and associated heating and cooling rates. The model is applied to calculate pressure–temperature–time conditions for coesite- and stishovite-bearing shock veins within metaquartzites of the Vredefort impact structure of South Africa. To constrain the model, the position of the host metaquartzites at the time of impact is first reconstructed. The developed code then passes the appropriate shock conditions through the target to re-create the shock wave, while simultaneously forming and cooling the shock veins via 2-D steady-state conduction. We have found that (1) at the time of shock vein formation (2.4 s following the initial contact of the projectile), the shock front pressure was 13.8 GPa and the width of the shock wave was of 27 km; (2) the melt within the shock veins initially reached ~3000 °C, which corresponds to the melting temperature of the target rock at 13.8 GPa. Simulation results indicate that conditions reach the stishovite stability field within 2 ms of vein formation (~10–14 GPa; 2000–3000 °C), followed by coesite within 1.29 s (~3–10 GPa; 600–2000 °C). The dwell time of the modeled shock vein system is 4.35 s. The shock vein system is completely solidified 33.4 s after the initial shock front passage. The calculated P–T–t path of the model indicates that the polymorphs within the shock veins of the metaquartzites at Vredefort formed under their normal stability field conditions following rarefaction wave decompression.

^1,2Quinn R.Shollenberger,²Jan Render¹Michelle K.Jordan,¹Kaitlyn A.McCain,²Samuel Ebert,²Addi Bischoff,²Thorsten Kleine,¹Edward D.Young
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.03.001]
¹Department of Earth, Planetary, and Space Sciences, University of California Los Angeles, 595 Charles E Young Dr E, Los Angeles, CA 90095, USA
²Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
Copyright Elsevier

Calcium-aluminum-rich inclusions (CAIs) are highly refractory objects found in different chondrite groups and represent some of the oldest known solids of the Solar System. As such, CAIs provide key information regarding the conditions prevailing in the solar protoplanetary disk as well as subsequent mixing and transport processes. Many studies have investigated CAIs for their isotopic compositions and reported nucleosynthetic isotope anomalies in numerous elements, which are typically explained by the variable incorporation of isotopically highly anomalous presolar phases. However, with the exception of 54Cr-enriched nanospinels, the exact presolar phases responsible for the isotopic heterogeneities are yet to be identified. To address this issue, we here present in-situ Ti isotopic analyses obtained on a diverse set of CAIs from various CV3 chondrites. The in-situ measurements were performed by targeting individual mineral phases of 15 CAIs with laser-ablation mass spectrometry and indicate significant inter- and intra-CAI isotopic heterogeneity in the neutron-rich isotope 50Ti. This is particularly pronounced for primitive fine-grained CAIs, whereas coarse-grained CAIs, which have been subject to melting, exhibit smaller degrees of Ti isotopic heterogeneity.

To further investigate this Ti isotopic heterogeneity, we additionally obtained Ti isotopic compositions of sequential acid leachates from two fine-grained and two coarse-grained CAIs derived from CV3 chondrites. In contrast to potential expectations from the first part of the study, we do not observe any significant intra-CAI Ti isotopic heterogeneity between the different leaching steps. The lack of intra-CAI Ti isotopic heterogeneity in the acid leachate samples of this study likely reflects that the leaching procedure is unable to efficiently separate the carriers of isotopically anomalous Ti in CAIs. By comparing the bulk CAI Ti isotope compositions with Ti isotope data for hibonite-rich objects from the literature, we find that the range of Ti isotope compositions recorded by CAIs from various chondrite groups can be accounted for by the averaging of hibonite grains. In turn, the variable Ti isotope compositions of hibonite grains can be explained by the averaging of isotopically diverse presolar grains present in the Sun’s parental molecular cloud. This effect of averaging is statistically supported by the central limit theorem, and the concept has the potential to be useful for other isotopic systems.

¹Thomas S.Kruijer,²Christoph Burkhardt,¹Lars E.Borg,^2,3Thorsten Kleine
Eaerth and Planetary Science Letters 584, 117440 Link to Article [https://doi.org/10.1016/j.epsl.2022.117440]
¹Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue (L-231), Livermore, CA 94550, USA
²Institut für Planetologie, University of Münster, Wilhelm-Klemm-Strasse 10, 48149, Münster, Germany
³Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
Copyright Elsevier

The origin of pallasites—stony-iron meteorites mainly composed of olivine and Fe-Ni metal—is debated and proposed formation scenarios broadly range from models that explain pallasite formation by internal processes in the mantle of a differentiated planetesimal to those that involve impact–induced mixing of core and mantle materials. Here, the origin of pallasites is examined by studying the nebular source regions of their precursor material using Mo isotopes and their history of metal-silicate segregation using Hf-W chronometry. We report new Mo and W isotopic data for a large suite of pallasite metal samples, alongside Pt isotope data to quantify superimposed cosmic ray exposure effects. Most main-group pallasites exhibit uniform pre-exposure 182W and Mo isotopic compositions that bear an excellent similarity to those of IIIAB iron meteorites. Four main-group pallasites and the IIIAB iron Thunda have more radiogenic pre-exposure 182W compositions, but display the same Mo isotopic composition as other main-group pallasites and IIIAB irons. This strong chronological and genetic link strongly suggests that main-group pallasite metal originated in the IIIAB parent body core. This, combined with prior Pd-Ag chronometric evidence for an early collisional disruption of the IIIAB parent body, implies that main-group pallasites formed by impact–induced mixing of metal and silicates rather than by an internal process on the IIIAB parent body. This mixing led to elevated 182W compositions in some pallasites, which are best accounted for by partial re-equilibration of IIIAB metal with radiogenic 182W from the colliding body. Collectively, our results support models that explain main-group pallasite formation by injection of pallasite metal into the mantle of another differentiated body, implying that pallasite silicates did not primarily derive from the IIIAB mantle, but instead from that of the colliding body.