¹Philipp Gleißner, ¹Harry Becker
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.12.017]
¹Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstr. 74-100, 12249 Berlin, Germany
Copyright Elsevier

Fe-Ni metal-schreibersite-troilite intergrowths in Apollo 16 impact melt rocks and new highly siderophile element (HSE) and S abundance data indicate that millimeter-scale closed-system fractional crystallization processes during cooling of impactor-derived metal melt droplets in impact-melts are the main reason for compositional variations and strong differences in abundances and ratios of HSE in multiple aliquots from Apollo 16 impact melt rocks. Element ratios obtained from linear regression of such data are therefore prone to error, but weighted averages take into account full element budgets in the samples and thus represent a more accurate estimate of their impactor contributions. Modeling of solid metal-liquid metal partitioning in the Fe-Ni-S-P system and HSE patterns in impactites from different landing sites suggest that bulk compositions of ancient lunar impactites should be representative of impact melt compositions and that large-scale fractionation of the HSE by in situ segregation of solid metal or sulfide liquid in impact melt sheets most likely did not occur. The compositional record of lunar impactites indicates accretion of variable amounts of chondritic and non-chondritic impactor material and the mixing of these components during remelting of earlier ejecta deposits. The non-chondritic composition appears most prominently in some Apollo 16 impactites and is characterized by suprachondritic HSE/Ir ratios which increase from refractory to moderately volatile HSE and exhibit a characteristic enrichment of Ru relative to Pt. Large-scale fractional crystallization of solid metal from sulfur and phosphorous rich metallic melt with high P/S in planetesimal or embryo cores is currently the most likely process that may have produced these compositions. Similar materials or processes may have contributed to the HSE signature of the bulk silicate Earth (BSE).

¹G.J. MacPherson, ^1,2E.S. Bullock, ^3,4T.J. Tenner, ^3,5D. Nakashima, ³N.T. Kita, ^1,6M.A. Ivanova, ⁷A.N. Krot, ⁸M.I. Petaev, ⁸S.B. Jacobsen
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.12.006]
¹Dept. of Mineral Sciences, Museum of Natural History, Smithsonian Institution, Washington, DC, USA, 20560
²Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Rd., N.W., Washington, DC 20015
³WiscSIMS, University of Wisconsin, Madison, WI 53706, USA
⁴Los Alamos National Laboratory, Los Alamos, NM, 87545
⁵Tohoku University, Miyagi 980-8578, Japan
⁶Vernadsky Institute, Moscow, Kosygin St. 119991, Russia
⁷University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822, USA
⁸Harvard University, Cambridge, Massachusetts 02138, USA
Copyright Elsevier

In order to further elucidate possible temporal relationships between different varieties of calcium-, aluminum-rich inclusions (CAIs), we measured the aluminum-magnesium isotopic systematics of seven examples of the rare type known as forsterite-bearing Type B (FoB) inclusions from four different CV3 carbonaceous chondrites: Allende, Efremovka, NWA 3118, and Vigarano. The primary phases (forsterite, Al-Ti-rich diopside, spinel, melilite, and anorthite) in each inclusion were analyzed in situ using high-precision secondary ion mass-spectrometry (SIMS). In all cases, minerals with low Al/Mg ratios (all except anorthite) yield well-defined internal Al-Mg isochrons, with a range of initial 26Al/27Al ratios [(26Al/27Al)0] ranging from (5.30±0.22)×10−5 down to (4.17±0.43)×10−5. Anorthite in all cases is significantly disturbed relative to the isochrons defined by the other phases in the same CAIs, and in several cases contains no resolved excesses of radiogenic 26Mg (δ26Mg∗) even at 27Al/24Mg ratios greater than 1000. The fact that some FoBs preserve (26Al/27Al)0 of ∼ 5.2×10−5, close to the canonical value of (5.23±0.13)×10−5 inferred from bulk magnesium-isotope measurements of CV CAIs (Jacobsen et al., 2008), demonstrates that FoBs began forming very early, contemporaneous with other more-refractory CAIs. The range of (26Al/27Al)0 values further shows that FoBs continued to be reprocessed over ∼200,000 years of nebular history, consistent with results obtained for other types of igneous CAIs in CV chondrites. The absence of any correlation between of CAI+FoB formation or reprocessing times with bulk composition or CAI type means that there is no temporal evolutionary sequence between the diverse CAI types. The initial δ26Mg∗ value in the most primitive FoB (SJ101) is significantly lower than the canonical solar system value of −0.040±0.029‰.

^1,2Michelle S. Thompson,¹Thomas J. Zega,^3,4Jane Y. Howe
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12798]
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
²NASA Johnson Space Center, Houston, Texas, USA
³Hitachi High-Technologies Canada Inc., Rexdale, Ontario, Canada
⁴Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario, Canada
Published by arrangement with John Wiley & Sons

We report the results of the first dynamic, in situ heating of lunar soils to simulate micrometeorite impacts on the lunar surface. We performed slow- and rapid-heating experiments inside the transmission electron microscope to understand the chemical and microstructural changes in surface soils resulting from space-weathering processes. Our slow-heating experiments show that the formation of Fe nanoparticles begins at ~575 °C. These nanoparticles also form as a result of rapid-heating experiments, and electron energy-loss spectroscopy measurements indicate the Fe nanoparticles are composed entirely of Fe0, suggesting this simulation accurately mimics micrometeorite space-weathering processes occurring on airless body surfaces. In addition to Fe nanoparticles, rapid-heating experiments also formed vesiculated textures in the samples. Several grains were subjected to repeated thermal shocks, and the measured size distribution and number of Fe nanoparticles evolved with each subsequent heating event. These results provide insight into the formation and growth mechanisms for Fe nanoparticles in space-weathered soils and could provide a new methodology for relative age dating of individual soil grains from within a sample population.

¹Heather Charles, ²Timothy Titus, ²Rosalyn Hayward, ²Christopher Edwards, ³Caitlin Ahrens
Earth and Planetary Science Letters 458, 152-160 Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.10.022]
¹USGS/NAU, United States
²USGS, United States
³University of Arkansas, United States
Copyright Elsevier

The composition of two dune fields, Ogygis Undae and the NE–SW trending dune field in Gale crater (the “Bagnold Dune Field” and “Western Dune Field”), were analyzed using thermal emission spectra from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) and the Mars Odyssey Thermal Emission Imaging System (THEMIS). The Gale crater dune field was used as a baseline as other orbital compositional analyses have been conducted, and in situ sampling results will soon be available.

Results from unmixing thermal emission spectra showed a spatial variation between feldspar mineral abundances and pyroxene mineral abundances in Ogygis Undae. Other datasets, including nighttime thermal inertia values, also showed variation throughout the dune field. One explanation proposed for this variation is a bimodal distribution of two sand populations. This distribution is seen in some terrestrial dune fields.

The two dune fields varied in both mineral types present and in uniformity of composition. These differences point to different source lithologies and different distances travelled from source material. Examining these differences further will allow for a greater understanding of aeolian processes on Mars.

^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

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

^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.