^1,4T. J. Tenner, ^2,3M. Kimura,¹ N. T. Kita
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12791]
¹WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin, USA
²Faculty of Science, Ibaraki University, Mito, Japan
³National Institute of Polar Research, Tokyo, Japan
⁴Chemistry Division, Nuclear and Radiochemistry, Los Alamos National Laboratory, Los Alamos, New Mexico, USA
Published by agreement with John Wiley & Sons

High-precision secondary ion mass spectrometry (SIMS) was employed to investigate oxygen three isotopes of phenocrysts in 35 chondrules from the Yamato (Y) 82094 ungrouped 3.2 carbonaceous chondrite. Twenty-one of 21 chondrules have multiple homogeneous pyroxene data (∆17O 3SD analytical uncertainty: 0.7‰); 17 of 17 chondrules have multiple homogeneous pyroxene and plagioclase data. Twenty-one of 25 chondrules have one or more olivine data matching coexisting pyroxene data. Such homogeneous phenocrysts (1) are interpreted to have crystallized from the final chondrule melt, defining host O-isotope ratios; and (2) suggest efficient O-isotope exchange between ambient gas and chondrule melt during formation. Host values plot within 0.7‰ of the primitive chondrule mineral (PCM) line. Seventeen chondrules have relict olivine and/or spinel, with some δ17O and δ18O values approaching −40‰, similar to CAI or AOA-like precursors. Regarding host chondrule data, 22 of 34 have Mg#s of 98.8–99.5 and ∆17O of −3.9‰ to −6.1‰, consistent with most Acfer 094, CO, CR, and CV chondrite chondrules, and suggesting a common reduced O-isotope reservoir devoid of 16O-poor H2O. Six Y-82094 chondrules have ∆17O near −2.5‰, with Mg#s of 64–97, consistent with lower Mg# chondrules from Acfer 094, CO, CR, and CV chondrites; their signatures suggest precursors consisting of those forming Mg# ~99, ∆17O: −5‰ ± 1‰ chondrules plus 16O-poor H2O, at high dust enrichments. Three type II chondrules plot slightly above the PCM line, near the terrestrial fractionation line (∆17O: ~+0.1‰). Their O-isotopes and olivine chemistry are like LL3 type II chondrules, suggesting they sampled ordinary chondrite-like chondrule precursors. Finally, three Mg# >99 chondrules have ∆17O of −6.7‰ to −8.1‰, potentially due to 16O-rich refractory precursor components. The predominance of Mg# ~99, ∆17O: −5‰ ± 1‰ chondrules and a high chondrule-to-matrix ratio suggests bulk Y-82094 characteristics are closely related to anhydrous dust sampled by most carbonaceous chondrite chondrules.

¹Terrence Blackburn, ²Conel M. O’D. Alexander, ²Richard Carlson, ³Linda T. Elkins-Tanton
Geochmica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.11.038]
¹Earth and Planetary Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064
²Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015
³School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287
Copyright Elsevier

A working timeline for the history of ordinary chondrites includes chondrule formation as early as 0-2 Ma after our Solar System’s earliest forming solids (CAIs), followed by rapid accretion into undifferentiated planetesimals that were heated internally by 26Al decay and cooled over a period of tens of millions of years. There remains conflict, however, between metallographic cooling rate (Ni-metal) and radioisotopic thermochronometric data over the sizes and lifetimes of the chondrite parent bodies, as well as the timing of impact related disruptions. The importance of establishing the timing of parent body disruption is heightened by the use of meteorites as recorders of asteroid belt wide disruption events and their use to interpret Solar System dynamical models. Here we attempt to resolve these records by contributing new 207Pb-206Pb data obtained on phosphates isolated from nine previously unstudied ordinary chondrites. These new results, along with previously published Pb-phosphate, Ni-metal and thermometry data, are interpreted with a series of numerical models designed to simulate the thermal evolution for a chondrite parent body that either remains intact or is disrupted by impact prior to forming smaller unsorted “rubble piles”.

Our thermal model and previously published thermometry data limit accretion time to 2.05-2.25 Ma after CAIs. Measured Pb-phosphate data place minimum estimates on parent body diameters of ∼260-280 km for both the L and H chondrite parent bodies. They also consistently show that petrologic Type 6 (highest thermal metamorphism) chondrites from both the H and L bodies have younger ages and, therefore, cooled more slowly than Type 5 (lesser metamorphism) chondrites. This is interpreted as evidence for Type 5 chondrite origination from shallower depths than Type 6 chondrites within initially concentrically zoned bodies. This contrasts metallographic cooling rate data that are inconsistent with such a simple onion shell scenario. One model that can reconcile these two data sets takes into account subtle differences in temperature to which each system responds. This working model requires that disruption occur early enough such that the Ni-metal system can record the cooling rate associated with a rubble pile (30 Ma). For this 30-70 Ma timeline, reaccretion into smaller rubble piles will ensure that the originally deeply buried and hot Type 6 samples will always cool faster as a result of disruption, yielding nearly uniform ages that record the time of parent body disruption. This is consistent with the available Pb-phosphate data, where all but one Type 6 chondrite (H, n=3; L, n=4) yields a cooling age within a narrow 4505 ± 5 Ma timeframe. These data collectively imply that both the H and L chondrite parent bodies were catastrophically disrupted at ∼60 Ma. In addition, combined Ni-metal and Pb-phosphate models confirm that a subset of Type 4 chondrites record early rapid cooling likely associated with erosional impacting of the H and L parent bodies on ∼5 Ma timescales.

¹D. E. Brownlee, ¹D. J. Joswiak
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12804]
¹Department of Astronomy, University of Washington, Seattle, Washington, USA
Published by arrangement with John Wiley & Sons

Asteroids and comets are surviving members of the vast planetesimal population that was distributed across the early solar system. They appear to be a diverse set of bodies but we present evidence from comet samples that the body-to-body diversity of the initial rocky component mix in planetesimals may have declined with distance from the Sun. Laboratory measurements of the minor element Mn in olivine collected from Comet Wild 2 suggests that the micron-sized rocky crystalline contents of this comet formed in numerous inner solar system environments. The results are consistent with a scenario where silicates such as olivine form at incandescent temperatures in multiple environments and then mix as they are transported to distant cold regions where silicates could accrete with ice and organics to form comets. Accreting far from silicate formation regions, many ice-rich planetesimals are likely to have started with similar complex mixtures of diverse rocky components formed in various high-temperature environments. This contrasts with asteroidal meteorite parent bodies whose silicates retain regional properties that give different chondrite classes their distinctive properties.

¹Patricia Craig, ²Vincent Chevrier,³M.R.G. Sayyed, ⁴R. Islam
Planetary and Space Science (in Press) Link to Article [http://dx.doi.org/10.1016/j.pss.2016.11.008]
¹Lunar and Planetary Institute, 3600 Bay Area Blvd, Houston, TX 77058USA
²Arkansas Center for Space and Planetary Sciences, STON F47, 332 N. Arkansas Ave, University of Arkansas, Fayetteville, AR 72701 USA
³Department of Geology, Poona College (Affiliated to Savitribai Phule Pune University) Camp, Pune411001, India
⁴Wadia Institute of Himalayan Geology, 33 General Mahadeo Singh Road, Dehradun, 248001, India

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^1,2,3Tatsunori Yokoyama, ^1,2,4Keiji Misawa, ⁵Osamu Okano, ⁶Chi-Yu Shih, ⁷Laurence E. Nyquist, ⁸Justin I. Simon, ^6,7,8Michael J. Tappa, ³Shigekazu Yoneda
Earth and Planetary Science Letters 458, 233–240 Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.10.037]
¹Department of Polar Science, SOKENDAI (The Graduate University for Advanced Studies), 10-3 Midoricho, Tachikawa, 190-8518, Japan
²Universities Space Research Association–Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, USA
³National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, 305-0005, Japan
⁴National Institute of Polar Research, 10-3 Midoricho, Tachikawa, 190-8518, Japan
⁵Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Okayama, 700-8530, Japan
⁶Jacobs, NASA Johnson Space Center, Mail Code XI3, Houston, TX 77058, USA
⁷Center for Isotope Cosmochemistry and Geochronology, Astromaterials Research and Exploration Science, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058-3696, USA
⁸Aerodyne Industries, Jacobs JETS Contract, NASA Johnson Space Center, Houston, TX 77058, USA
Copyright Elsevier

New K–Ca and Rb–Sr isotopic analyses have been performed on alkali-rich igneous rock fragments in the Yamato (Y)-74442 and Bhola LL-chondritic breccias to better understand the extent and timing of alkali enrichments in the early solar system. The Y-74442 fragments yield a K–Ca age of 4.41±0.28 Ga4.41±0.28 Ga for λ(40K) = 0.5543 Ga−1 with an initial 40Ca/44Ca ratio of 47.1618±0.003247.1618±0.0032. Studying the same fragments with the Rb–Sr isotope system yields an age of 4.420±0.031 Ga4.420±0.031 Ga for λ(87Rb) = 0.01402 Ga−1 with an initial ratio of 87Sr/86Sr = 0.7203 ± 0.0044. An igneous rock fragment contained in Bhola shows a similar alkali fractionation pattern to those of Y-74442 fragments but does not plot on the K–Ca or Rb–Sr isochron of the Y-74442 fragments. Calcium isotopic compositions of whole-rock samples of angrite and chondrites are primordial, indistinguishable from mantle-derived terrestrial rocks, and here considered to represent the initial composition of bulk silicate Earth. The initial ε40Ca value determined for the source of the alkali clasts in Y-74442 that is ∼0.5 ε-units higher than the solar system value implies an early alkali enrichment.

Multi-isotopic studies on these alkali-rich fragments reveal that the source material of Y-74442 fragments had elemental ratios of K/Ca = 0.43 ± 0.18, Rb/Sr = 3.45 ± 0.66 and K/Rb ∼ 170, that may have formed from mixtures of an alkali-rich component (possibly an alkali-enriched gaseous reservoir produced by fractionation of early nebular condensates) and chondritic components that were flash-heated during an impact event on the LL-chondrite parent body ∼4.42 Ga ago. Further enrichments of potassium and rubidium relative to calcium and strontium as well as a mutual alkali-fractionation (K/Rb ∼ 50 and heavier alkali-enrichment) would have likely occurred during subsequent cooling and differentiation of this melt. Alkali fragments in Bhola might have undergone similar solid–vapor fractionation processes to those of Y-74442 fragments but appear to have formed via a distinct impact melting event.

^1,2Alan E. Rubin, ³John P. Breen, ¹Junko Isa, ⁴Sean Tutorow
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12797]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California 90095–1567, USA
²Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095–1567, USA
³Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095–1567, USA
⁴eegooblago meteorites, Greeley, Colorado 80634, USA
Published by arrangement with John Wiley & Sons

NWA 10214 is an LL3-6 breccia containing ~8 vol% clasts including LL5, LL6, and shocked-darkened LL fragments as well as matrix-rich Clast 6 (a new kind of chondrite). This clast is a dark-colored, subrounded, 6.1 × 7.0 mm inclusion, consisting of 60 vol% fine-grained matrix, 32 vol% coarse silicate grains, and 8 vol% coarse opaque grains. The large chondrules and chondrule fragments are mainly Type IB; one small chondrule is Type IIA. Also present are one 450 × 600 μm spinel-pyroxene-olivine CAI and one 85 × 110 μm AOI. Clast 6 possesses a unique set of properties. (1) It resembles carbonaceous chondrites in having relatively abundant matrix, CAIs, and AOIs; the clast’s matrix composition is close to that in CV3 Vigarano. (2) It resembles type-3 OC in its olivine and low-Ca pyroxene compositional distributions, and in the Fe/Mn ratio of ferroan olivine grains. Its mean chondrule size is within 1σ of that of H chondrites. The O-isotopic compositions of the chondrules are in the ordinary- and R-chondrite ranges. (3) It resembles type-3 enstatite chondrites in the minor element concentrations in low-Ca pyroxene grains and in having a high low-Ca pyroxene/olivine ratio in chondrules. Clast 6 is a new variety of type-3 OC, somewhat more reduced than H chondrites or chondritic clasts in the Netschaevo IIE iron; the clast formed in a nebular region where aerodynamic radial drift processes deposited a high abundance of matrix material and CAIs. A chunk of this chondrite was ejected from its parent asteroid and later impacted the LL body at low relative velocity.

¹Geoffrey H. Howarth, ²Arya Udry
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12799]
¹Department of Geological Sciences, University of Cape Town, Rondebosch, South Africa
²Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, Nevada, USA
Published by arrangement with John Wiley & Sons

Olivine-phyric shergottites represent primitive basaltic to picritic rocks, spanning a large range of Mg# and olivine abundances. As primitive olivine-bearing magmas are commonly representative of their mantle source on Earth, understanding the petrology and evolution of olivine-phyric shergottites is critical in our understanding of Martian mantle compositions. We present data for the olivine-phyric shergottite Northwest Africa (NWA) 10170 to constrain the petrology with specific implications for magma plumbing-system dynamics. The calculated oxygen fugacity and bulk-rock REE concentrations (based on modal abundance) are consistent with a geochemically intermediate classification for NWA 10170, and overall similarity with NWA 6234. In addition, we present trace element data using laser ablation ICP-MS for coarse-grained olivine cores, and compare these data with terrestrial and Martian data sets. The olivines in NWA 10170 contain cores with compositions of Fo77 that evolve to rims with composition of Fo58, and are characterized by cores with low Ni contents (400–600 ppm). Nickel is compatible in olivine and such low Ni content for olivine cores in NWA 10170 suggests either early-stage fractionation and loss of olivine from the magma in a staging chamber at depth, or that Martian magmas have lower Ni than terrestrial magmas. We suggest that both are true in this case. Therefore, the magma does not represent a primary mantle melt, but rather has undergone 10–15% fractionation in a staging chamber prior to extrusion/intrusion at the surface of Mars. This further implies that careful evaluation of not only the Mg# but also the trace element concentrations of olivine needs to be conducted to evaluate pristine mantle melts versus those that have fractionated olivine (±pyroxene and oxide minerals) in staging chambers.

^1,2R. Liu,^1,3L. Hu,¹M. Humayun
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12803]
¹National High Magnetic Field Laboratory and Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida, USA
²Department of Geosciences, Texas Tech University, Lubbock, Texas, USA
³Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah, USA
Published by arrangement with John Wiley & Sons

Rhenium is an important element with which to test hypotheses of isotope variation. Historically, it has been difficult to precisely correct the instrumental mass bias in thermal ionization mass spectrometry. We used W as an internal standard to correct mass bias on the MC-ICP-MS, and obtained the first precise δ187Re values (~±0.02‰, 2SE) for iron meteorites and chondritic metal. Relative to metal from H chondrites, IVB irons are systematically higher in δ187Re by ~0.14 ‰. δ187Re for other irons are similar to H chondritic metal, although some individual samples show significant isotope fractionation. Since 185Re has a high neutron capture cross section, the effect of galactic cosmic-ray (GCR) irradiation on δ187Re was examined using correlations with Pt isotopes. The pre-GCR irradiation δ187Re for IVB irons is lower, but the difference in δ187Re between IVB irons and other meteoritic metal remains. Nuclear volume-dependent fractionation for Re is about the right magnitude near the melting point of iron, but because of the refractory and compatible character of Re, a compelling explanation in terms of mass-dependent fractionation is elusive. The magnitude of a nucleosynthetic s-process deficit for Re estimated from Mo and Ru isotopes is essentially unresolvable. Since thermal processing reduced nucleosynthetic effects in Pd, it is conceivable that Re isotopic variations larger than those in Mo and Ru may be present in IVBs since Re is more refractory than Mo and Ru. Thus, the Re isotopic difference between IVBs and other irons or chondritic metal remains unexplained.

^1,2,3Kathleen E. Vander Kaaden, ^1,3Francis M. McCubbin, ⁴Larry R. Nittler, ⁵Patrick N. Peplowski, ⁴Shoshana Z. Weider, ⁴Elizabeth A. Frank,⁶Timothy J. McCoy
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.11.041]
¹Institute of Meteoritics, Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA.
²Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, USA
³NASA Johnson Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, TX 77058, USA.
⁴Department of Terrestrial Magnetism, Carnegie Institution of Washington, DC 20015, USA.
⁵The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
⁶Department of Mineral Sciences, National Museum of Natural History, 10th and Constitution Aves. NW, Smithsonian Institution, Washington, DC 20560, USA.
Copyright Elsevier

Orbital data from the MESSENGER mission to Mercury have facilitated a new view of the planet’s structure, chemical makeup, and diverse surface, and have confirmed Mercury’s status as a geochemical endmember among the terrestrial planets. In this work, the most recent results from MESSENGER’s X-Ray Spectrometer, Gamma-Ray Spectrometer, and Neutron Spectrometer have been used to identify nine distinct geochemical regions on Mercury. Using a variation on the classical CIPW normative mineralogy calculation, elemental composition data is used to constrain the potential mineralogy of Mercury’s surface; the calculated silicate mineralogy is dominated by plagioclase, pyroxene (both orthopyroxene and clinopyroxene), and olivine, with lesser amounts of quartz. Petrologically, the rocks on the surface of Mercury are highly diverse and vary from komatiitic to boninitic. The high abundance of alkalis on Mercury’s surface results in several of the nine regions being classified as alkali-rich komatiites and/or boninites. In addition, Mercury’s surface terranes span a wide range of SiO2 values that encompass crustal compositions that are more silica-rich than geochemical terranes on the Moon, Mars, and Vesta, but the range is similar to that of Earth. Although the composition of Mercury’s surface appears to be chemically evolved, the high SiO2 content is a primitive feature and a direct result of the planet’s low oxygen fugacity.

^1,2M.J. Genge, ³J. Larsen, ⁴M. Van Ginneken,^1,2M.D. Suttle
Geology (in Press) Link to Article [doi: 10.1130/G38352.1]
¹Department of Earth Sciences and Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK
²Department of Earth Science, Natural History Museum, Cromwell Road, London SW7 2BT, UK
³Project Stardust, Oslo, Norway
⁴Département des Géosciences, Université Libre de Bruxelles, Avenue FD. Roosevelt, 2 B-1050 Bruxelles, Belgium

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