¹Christopher W. Haberle, ^1,2Laurence A.J. Garvie
The American Mineralogist 102,2415-2421 Link to Article [DOI: https://doi.org/10.2138/am-2017-6180]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, U.S.A.
¹Center for Meteorite Studies, Arizona State University, Arizona 85287-6004, U.S.A
Copyright: The Mineralogical Society of America

The CM and CI carbonaceous chondrites are typically dominated by phyllosilicates with variable proportions of tochilinite, anhydrous silicates, carbonates, sulfides, sulfates, oxides, and organic compounds. During thermal metamorphism the phyllosilicates dehydrate and decompose yielding water and olivine/enstatite. The thermal transformation of carbonate is less well understood, especially in the presence of volatile decomposition products, such as CO, CO2, SO2, H2S, and H2O. Here is described the mineralogical transformation of calcite (CaCO3) to oldhamite (CaS) and portlandite [Ca(OH)2] during extraterrestrial thermal metamorphism on the Sutter’s Mill parent body. Sutter’s Mill is a regolith breccia consisting of at least two lithologic components: phyllosilicate-calcite-bearing and anhydrous olivine-rich. Evidence suggests that the anhydrous stones were derived from extraterrestrial heating of the phyllosilicate-calcite-bearing material. One of only three Sutter’s Mill stones (SM3) collected prior to heavy rainfall over the recovery site is the focus of this study. Its powder X-ray diffraction patterns are dominated by olivine, with lesser enstatite, Fe-sulfides, magnetite, and oldhamite. Oldhamite is absent in the rained-on stones reflecting its water sensitivity and the pristine nature of SM3. Optical micrographs show whitish to bluish grains of oldhamite and portlandite embedded in dark, fine-grained matrix. The presence of abundant olivine and absence of phyllosilicates, tochilinite, and carbonate indicates that SM3 underwent heating to ~750 °C. At this temperature, calcite would have decomposed to lime (CaO). Volatilization experiments show that CO, CO2, SO2, and H2S evolve from CM and CI chondrites heated above 600 °C. Lime that formed through calcite decomposition would have reacted with these gases forming oldhamite under reducing conditions. Residual lime not converted to oldhamite, would have readily hydrated to portlandite, possibly through retrograde reactions during cooling on the parent body. These reactions have parallels to those in coal-fired electricity generating plants and provide an analogous system to draw comparison. Furthermore, the identification of these minerals, which are sensitive to terrestrial alteration, and determination of their formation is enabled only by the rapid collection of samples from an observed fall and their subsequent curation.

¹T. Hopp, ¹M. Fischer-Gödde, ¹T. Kleine
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.11.033]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

Mass-dependent Ru isotope variations (δ102/99Ru) and Ru concentrations were determined for 35 magmatic iron meteorites from the five major chemical groups (IIAB, IID, IIIAB, IVA, IVB). In addition, four equilibrated ordinary chondrites were analyzed. The IIAB, IIIAB and IVB iron meteorites display increasingly heavier Ru isotopic compositions with decreasing Ru content. Modeling demonstrates that the trends for these three iron groups can be reproduced by the incremental extraction of isotopically lighter Ru into solids, which leads to progressively heavier δ102/99Ru in the remaining melt. The modeling further shows that the Ru isotopic variations of the IIAB and IIIAB irons are consistent with derivation from parental melts with an ordinary chondrite-like δ102/99Ru, whereas the IVB irons more likely derive from a melt with heavier δ102/99Ru. This heavy Ru isotopic composition of the IVB parental melt probably results from high-temperature processing of the IVB precursor material. The Ru isotope systematics of the IID and IVA irons are more complex and show no correlation between δ102/99Ru and Ru content. Although most samples exhibit heavy Ru isotopic compositions, especially the late-crystallized irons of these groups deviate from the expected fractional crystallization trends. This deviation most likely results from mixing and re-equilibration of early-crystallized solids and late-stage liquids, followed by further fractional crystallization. The mixing might be related to the migration of liquids through a complex network of dendrites or to the overturn of a cumulate inner core, and bears testimony to the complex solidification history of at least some protoplanetary cores.

¹Zhou Zhang,¹Anne Pommier
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005390]
¹University of California San Diego, Scripps Institution of Oceanography, Institute of Geophysics and Planetary Physics, La Jolla, CA, USA
Published by arrangement with John Wiley & Sons

We report electrical conductivity measurements on metal–olivine systems at 5 and 7 GPa and up to 1675°C in order to investigate the electrical properties of Core-Mantle Boundary (CMB) systems. Electrical experiments were conducted in the multi-anvil apparatus using the impedance spectroscopy technique. The samples are composed of one metal layer (Fe, FeS, FeSi2, or Fe-Ni-S-Si) and one polycrystalline olivine layer, with the metal:olivine ratio ranging from 1:0.7 to 1:9.2. For all samples, we observe that the bulk electrical conductivity increases with temperature from 10-2.5 to 101.8 S/m, which is higher than the conductivity of polycrystalline olivine but lower than the conductivity of the pure metal phase at similar conditions. In some experiments, a conductivity jump is observed at the temperature corresponding to the melting temperature of the metallic phase. Both the metal:olivine ratio and the metal phase geometry control the electrical conductivity of the two-layer samples. By combining electrical results, textural analyses of the samples, and previous studies of the structure and composition of Mercury’s interior, we propose an electrical profile of the deep interior of the planet that accounts for a layered CMB-outer core structure. The electrical model agrees with existing conductivity estimates of Mercury’s lower mantle and CMB using magnetic observations and thermodynamic calculations, and thus, supports the hypothesis of a layered CMB-outermost core structure in the present-day interior of Mercury. We propose that the layered CMB-outer core structure is possibly electrically insulating, which may influence the planet’s structure and cooling history.

¹Bérengère Mougel, ^1,2Frédéric Moynier, ¹Christa Göpel
Earth and Planetary Science Letters 481, 1-8 Link to Article [https://doi.org/10.1016/j.epsl.2017.10.018]
¹Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, CNRS UMR 7154, Paris, France
²Institut Universitaire de France and Université Paris Diderot, Paris, France
Copyright Elsevier

Among the elements exhibiting non-mass dependent isotopic variations in meteorites, chromium (Cr) has been central in arguing for an isotopic homogeneity between the Earth and the Moon, thus questioning physical models of Moon formation. However, the Cr isotopic composition of the Moon relies on two samples only, which define an average value that is slightly different from the terrestrial standard. Here, by determining the Cr isotopic composition of 17 lunar, 9 terrestrial and 5 enstatite chondrite samples, we re-assess the isotopic similarity between these different planetary bodies, and provide the first robust estimate for the Moon. In average, terrestrial and enstatite samples show similar ε54Cr. On the other hand, lunar samples show variables excesses of 53Cr and 54Cr compared to terrestrial and enstatite chondrites samples with correlated ε53Cr and ε54Cr (per 10,000 deviation of the 53Cr/52Cr and 54Cr/52Cr ratios normalized to the 50Cr/52Cr ratio from the NIST SRM 3112a Cr standard). Unlike previous suggestions, we show for the first time that cosmic irradiation can affect significantly the Cr isotopic composition of lunar materials. Moreover, we also suggest that rather than spallation reactions, neutron capture effects are the dominant process controlling the Cr isotope composition of lunar igneous rocks. This is supported by the correlation between ε53Cr and ε54Cr, and 150Sm/152Sm ratios. After correction of these effects, the average ε54Cr of the Moon is indistinguishable from the terrestrial and enstatite chondrite materials reinforcing the idea of an Earth–Moon–enstatite chondrite system homogeneity. This is compatible with the most recent scenarios of Moon formation suggesting an efficient physical homogenization after a high-energy impact on a fast spinning Earth, and/or with an impactor originating from the same reservoir in the inner proto-planetary disk as the Earth and enstatite chondrites and having similar composition.

¹K. Righter,²K. Pando,³N. Marin,^2,1,4D. K. Ross,⁵M. Righter,²L. Danielson,⁵T. J. Lapen,⁶C. Lee
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13005]
¹Mailcode XI2, NASA Johnson Space Center, Houston, Texas, 77058, USA
²Jacobs JETS, NASA Johnson Space Center, Houston, Texas, USA
³School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
⁴University of Texas El Paso, Houston, Texas, USA
⁵Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, USA
⁶Department of Earth Science, Rice University, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Volatile element concentrations in planets are controlled by many factors such as precursor material composition, core formation, differentiation, magma ocean and magmatic degassing, and late accretionary processes. To better constrain the role of core formation, we report new experiments defining the effect of temperature, and metallic S and C content on the metal-silicate partition coefficient (or D(i) metal/silicate) of the volatile siderophile elements (VSE) Bi, Cd, In, and Sn. Additionally, the effect of pressure on metal-silicate partitioning between 1 and 3 GPa, and olivine-melt partitioning at 1 GPa have been studied for Bi, Cd, In, Sn, As, Sb, and Ge. Temperature clearly causes a decrease in D(i) metal/silicate for all elements. Sulfur and C have a large influence on activity coefficients in metallic Fe liquids, with C causing a decrease in D(i) metal/silicate, and S causing an increase. Pressure has only a small effect on D(Cd), D(In), and D(Ge) metal/silicate. Depletions of Bi, Cd, In, and Sn in the terrestrial and Martian mantles are consistent with high PTcore formation and metal-silicate equilibrium at the high temperatures indicated by previous studies. A late Hadean matte would influence Bi the most, due to its high D(sulfide/silicate) ~2000, but segregation of a matte would only reduce the mantle Bi content by 50%; all other less chalcophile elements (e.g., Sn, In, and Cd) would be minimally affected. The lunar depletions of highly VSE require a combination of core formation and an additional depletion mechanism—most likely the Moon-forming giant impact, or lunar magma ocean degassing.

¹Seann J. McKibbin, ¹Hugh St. C. O’Neill
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13004]
¹Research School of Earth Sciences, Australian National University, Acton, Australian Capital Territory, Australia
Published by arrangement with John Wiley & Sons

Angrite meteorites are samples of early planetesimal magmatic rocks, distinguished from more typical “basaltic eucrites” by compositions that are silica undersaturated, relatively oxidized, and with high CaO/Al2O3. The latter is not expected from nebular, chondritic materials that might form a primitive mantle, such as a source enriched in refractory inclusions with fixed CaO/Al2O3 (e.g., CV chondrite). Here we present results of “reversal” crystallization experiments for two possible parental angrite compositions (approximating the D’Orbigny meteorite) to investigate the role of spinel as a sink for Al2O3. This mineral has previously been produced with angritic melts during “forward” melting of CV chondrite and may be abundant in the angrite source. At oxidizing conditions, we confirm that spinel is a liquidus phase and that angritic magmas form near the olivine-anorthite-spinel-liquid peritectic. A stability gap separates Al-rich liquidus spinels and lower temperature spinels, the latter of which are similar to those in basaltic eucrites. Al-rich spinel is likely more abundant in the angritic source than other Fe-rich core-forming components such as metal or sulfide, and a CV chondrite-like composition generates most features of angrite magmas by fractionation of observed olivine and liquidus spinel. Direct CaO excess, via carbonate addition, is therefore limited. In this model, discrepancies remain for Li, Sc, Cr(-Al), and Ba, which may record local accretion conditions or early processing. The possible role of spinel as a sink for 26Al may have strong influence on the thermal evolution of the angrite parent body.

^1,2E.L. Walton,³T. G. Sharp,³J. Hu,⁴O. Tschauner
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13011]
¹Department of Physical Sciences, MacEwan University, Edmonton, Alberta, Canada
²Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
³Arizona State University, Tempe, Arizona, USA
⁴Department of Geoscience, University of Nevada, Las Vegas, Nevada, USA
Published by arrangement with John Wiley & Sons

Impact metamorphic effects from quartz and feldspar and to a lesser extent olivine and pyroxene have been studied in detail. Comparatively, studies documenting shock effects in other minerals, such as double chain inosilicates, phyllosilicates, carbonates, and sulfates, are lacking. In this study, we investigate impact metamorphism recorded in crystalline basement rocks from the Steen River impact structure (SRIS), a 25 km diameter complex crater in NW Alberta, Canada. An array of advanced analytical techniques was used to characterize the breakdown of biotite in two distinct settings: along the margins of localized regions of shock melting and within granitic target rocks entrained as clasts in a breccia. In response to elevated temperature gradients along shock vein margins, biotite transformed at high pressure to an almandine-Ca/Fe majorite-rich garnet with a density of 4.2 g cm−3. The shock-produced garnets are poikilitic, with oxide and silicate glass inclusions. Areas interstitial to garnets are vesiculated, in support of models for the formation of shock veins via oscillatory slip, with deformation continuing during pressure release. Biotite within granitic clasts entrained within the hot breccia matrix thermally decomposed at ambient pressure to produce a fine-grained mineral assemblage of orthopyroxene + sanidine + titanomagnetite. These minerals are aligned to the (001) cleavage plane of the original crystal. In this and previous work, the transformation of an inosilicate (pargasite) and a phyllosilicate (biotite) to form garnet, an easily identifiable, robust mineral, has been documented. We contend that in deeply eroded astroblemes, high-pressure minerals that form within or in the environs of shock veins may serve as one of the possibly few surviving indicators of impact metamorphism.

¹A. Zucchini,¹M. Petrelli,¹F. Frondini,²C. M. Petrone,³P. Sassi,¹A. Di Michele,¹S. Palmerini,¹O. Trippella,¹M. Busso
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13002]
¹Department of Physics and Geology, University of Perugia, Perugia, Italy
Section of Perugia, INFN, Perugia, Italy
²Department of Earth Sciences, The Natural History Museum, London, UK
³Department of Chemistry, Biology and Biotechnology, University of Perugia, Perugia, Italy
Published by arrangement with John Wiley & Sons

The Mineo pallasite is characterized here for the first time. The only 42 g still available worldwide is part of the collection of the Department of Physics and Geology, University of Perugia. A multianalytical approach was used, joining field-emission scanning electron microscopy, Raman analysis, X-ray powder diffraction, electron-probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry. Results highlighted that (1) the Mineo pallasite belongs to the Main Group pallasites; (2) the silicate component is essentially olivine, with no pyroxene component; (3) the olivine chemical composition varies in terms of both iron and trace elements; (4) the metal phase is essentially kamacite with the taenite mainly found in the plessite structure; (5) phosphide phases are present as schreibersite and barringerite. The observed compositional variability in olivines as well as their occurrence as both angular and rounded crystals suggest that the Mineo pallasite could have been derived from a large impact of a differentiated parent body with a larger solid body. The resulting pallasite conglomerate consists of the compositionally different olivines, likely coming from different areas of the same differentiated parent body, and the residual molten Fe-Ni.

¹John G. Spray, ¹Suporn Boonsue
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12997]
¹Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, Canada
Published by arrangement with John Wiley & Sons

Coesite and stishovite are developed in shock veins within metaquartzites beyond a radius of ~30 km from the center of the 2.02 Ga Vredefort impact structure. This work focuses on deploying analytical field emission scanning electron microscopy, electron backscattered diffraction, and Raman spectrometry to better understand the temporal and spatial relations of these silica polymorphs. α-Quartz in the host metaquartzites, away from shock veins, exhibits planar features, Brazil twins, and decorated planar deformation features, indicating a primary (bulk) shock loading of >5 < 35 GPa. Within the shock veins, coesite forms anhedral grains, ranging in size from 0.5 to 4 μm, with an average of 1.25 μm. It occurs in clasts, where it displays a distinct jigsaw texture, indicative of partial reversion to a less dense SiO2 phase, now represented by microcrystalline quartz. It is also developed in the matrix of the shock veins, where it is typically of smaller size (<1 μm). Stishovite occurs as euhedral acicular crystals, typically <0.5 μm wide and up to 15 μm in length, associated with clast–matrix or shock vein margin–matrix interfaces. In this context, the needles occur as radiating or subparallel clusters, which grow into/over both coesite and what is now microcrystalline quartz. Stishovite also occurs as more blebby, subhedral to anhedral grains in the vein matrix (typically <1 μm). We propose a model for the evolution of the veins (1) precursory frictional melting in a microfault (~1 mm wide) generates a molten matrix containing quartz clasts. This is followed by (2) arrival of the main shock front, which shocks to 35 GPa. This generates coesite in the clasts and in the matrix. (3) On initial shock release, the coesite partly reverts to a less dense SiO2 phase, which is now represented by microcrystalline quartz. (4) With continued release, stishovite forms euhedral needle clusters at solid–liquid interfaces and as anhedral crystals in the matrix. (5) With decreasing pressure–temperature, the matrix completes crystallization to yield a microcrystalline quasi-igneous texture comprising quartz–coesite–stishovite–kyanite–biotite–alkali feldspar and accessory phases. It is possible that the shock vein represents the locus of a thermal spike within the bulk shock, in which case there is no requirement for additional pressure (i.e., the bulk shock was ≃35 GPa). However, if that pressure was not realized from the main shock, then supplementary pressure excursions within the vein would have been required. These could have taken the form of localized reverberations from wave trapping, or implosion processes, including pore collapse, phase change–initiated volume reduction, and melt cavitation.

¹Alison C. Hunt, ¹David L. Cook, ^2,3Tim Lichtenberg, ¹Philip M. Reger,¹ Mattias Ek, ⁴Gregor J. Golabek, ¹Maria Schönbächler
Earth and Planetary Science Letters 482, 490-500 Link to Article [https://doi.org/10.1016/j.epsl.2017.11.034]
¹Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
²Institute of Geophysics, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland
³Institute for Astronomy, ETH Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
⁴Bayerisches Geoinstitut, University of Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany
Copyright Elsevier

The short-lived ¹⁸²Hf–¹⁸²W decay system is a powerful chronometer for constraining the timing of metal–silicate separation and core formation in planetesimals and planets. Neutron capture effects on W isotopes, however, significantly hamper the application of this tool. In order to correct for neutron capture effects, Pt isotopes have emerged as a reliable in-situ neutron dosimeter. This study applies this method to IAB iron meteorites, in order to constrain the timing of metal segregation on the IAB parent body.

The ε¹⁸²W values obtained for the IAB iron meteorites range from −3.61 ± 0.10 to −2.73 ± 0.09. Correlating εⁱPt with ε¹⁸²W data yields a pre-neutron capture ε¹⁸²W of −2.90 ± 0.06. This corresponds to a metal–silicate separation age of 6.0 ± 0.8 Ma after CAI for the IAB parent body, and is interpreted to represent a body-wide melting event. Later, between 10 and 14 Ma after CAI, an impact led to a catastrophic break-up and subsequent reassembly of the parent body. Thermal models of the interior evolution that are consistent with these estimates suggest that the IAB parent body underwent metal–silicate separation as a result of internal heating by short-lived radionuclides and accreted at around 1.4±0.1 Ma after CAIs with a radius of greater than 60 km.