Alex Ruzicka

Cascadia Meteorite Laboratory, Portland State University, 17 Cramer Hall, 1721 SW Broadway, Portland, OR 97207-0751, United States

Silicate-bearing iron meteorites differ from other iron meteorites in containing variable amounts of silicates, ranging from minor to stony-iron proportions (~50%). These irons provide important constraints on the evolution of planetesimals and asteroids, especially with regard to the nature of metal–silicate separation and mixing. I present a review and synthesis of available data, including a compilation and interpretation of host metal trace-element compositions, oxygen-isotope compositions, textures, mineralogy, phase chemistries, and bulk compositions of silicate portions, ages of silicate and metal portions, and thermal histories. Case studies for the petrogeneses of igneous silicate lithologies from different groups are provided. Silicate-bearing irons were formed on multiple parent bodies under different conditions. The IAB/IIICD irons have silicates that are mainly chondritic in composition, but include some igneous lithologies, and were derived from a volatile-rich asteroid that underwent small amounts of silicate partial melting but larger amounts of metallic melting. A large proportion of IIE irons contain fractionated alkali-silica-rich inclusions formed as partial melts of chondrite, although other IIE irons have silicates of chondritic composition. The IIEs were derived from an H-chondrite-like asteroid that experienced more significant melting than the IAB asteroid. The two stony-iron IVAs were derived from an extensively melted and apparently chemically processed L or LL-like asteroid that also produced a metallic core. Ungrouped silicate-bearing irons were derived from seven additional asteroids. Hf–W age data imply that metal–silicate separation occurred within 0–10 Ma of CAI formation for these irons, suggesting internal heating by ²⁶Al. Chronometers were partly re-set at later times, mainly earlier for the IABs and later for the IIEs, including one late (3.60 ± 0.15 Ga) strong impact that affected the “young silicate” IIEs Watson (unfractionated silicate, and probable impact melt), Netschaëvo (unfractionated, and metamorphosed), and Kodaikanal (fractionated). Kodaikanal probably did not undergo differentiation in this late impact, but the similar ages of the “young silicate” IIEs imply that relatively undifferentiated and differentiated materials co-existed on the same asteroid. The thermal histories and petrogeneses of fractionated IIE irons and IVA stony irons are best accommodated by a model of disruption and reassembly of partly molten asteroids.

Reference
Ruzicka R (in press) Silicate-bearing iron meteorites and their implications for the evolution of asteroidal parent bodies. Chemie der Erde
[doi:10.1016/j.chemer.2013.10.001]
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M. A. Siegler and S. E. Smrekar

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

We reexamine the Apollo Heat Flow Experiment in light of new orbital data. Using three-dimensional thermal conduction models, we examine effects of crustal thickness, density, and radiogenic abundance on measured heat flow values at the Apollo 15 and 17 sites. These models show the importance of regional context on heat flux measurements. We find that measured heat flux can be greatly altered by deep subsurface radiogenic content and crustal density. However, total crustal thickness and the presence of a near-surface radiogenic-rich ejecta provide less leverage, representing only minor (<1.5 mW m⁻²) perturbations on surface heat flux. Using models of the crust implied by Gravity Recovery and Interior Laboratory results, we found that a roughly 9–13 mW m⁻² mantle heat flux best approximate the observed heat flux. This equates to a total mantle heat production of 2.8–4.1 × 10¹¹ W. These heat flow values could imply that the lunar interior is slightly less radiogenic than the Earth’s mantle, perhaps implying that a considerable fraction of terrestrial mantle material was incorporated at the time of formation. These results may also imply that heat flux at the crust-mantle boundary beneath the Procellarum potassium, rare earth element, and phosphorus (KREEP) Terrane (PKT) is anomalously elevated compared to the rest of the Moon. These results also suggest that a limited KREEP-rich layer exists beneath the PKT crust. If a subcrustal KREEP-rich layer extends below the Apollo 17 landing site, required mantle heat flux can drop to roughly 7 mW m⁻², underlining the need for future heat flux measurements outside of the radiogenic-rich PKT region.

Reference
Siegler MA and Smrekar SE (in press) Lunar heat flow: Regional prospective of the Apollo landing sites. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004453]
Published by arrangement with John Wiley & Sons

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Elizabeth Gibney

We still seek a copyright agreement with Nature to display abstracts of their cosmochemistry related publications.

Reference
Gibney E (2014) Comet craft ready to wake. Nature 505:269–270.
[doi:10.1038/505269a]

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Sune G. Nielsen^a,b, Julie Prytulak^a,c, Bernard J. Wood^a and Alex N. Halliday^a

^aDepartment of Earth Sciences, University of Oxford, South Parks Road, OX1 3AN, Oxford, UK
^bDepartment of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
^cDepartment of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK

It has been argued that the stable isotopic composition of the element vanadium (V) provides a potential indicator of the effects high-energy irradiation early in Solar System development. Such irradiation would produce enrichment in the minor isotope, ⁵⁰V compared with the 400 times more abundant ⁵¹V (Gounelle et al., 2001 and Lee et al., 1998). Here we show that the vanadium isotopic composition of the silicate Earth is enriched in ⁵¹V by ~0.8‰ compared with carbonaceous and ordinary chondrites as well as achondrites from Mars and the asteroid 4 Vesta. Although V is depleted by core formation, experiments reveal no isotopic fractionation between metal and silicate that could account for the observed difference in V isotope composition between terrestrial and extraterrestrial materials. Nucleosynthetic provenance of the terrestrial vanadium isotope offset is inconsistent with anomalies of other nucleosynthetically produced isotopes in bulk meteorites, which are more variable than vanadium (Burkhardt et al., 2011, Carlson et al., 2007 and Trinquier et al., 2009). Furthermore, V isotopes are unlikely to have been affected by volatilization, parent body alteration or impact erosion of Earthʼs surface. Therefore, the cause of the isotopic difference is unclear. One possibility is that Earthʼs isotopically heavier V reflects a deficit in material irradiated during the initial stages of Solar System formation. Whatever the cause, the terrestrial deficit in ⁵⁰V implies that bulk Earth cannot be entirely reconstructed by mixtures of different meteorites.

Reference
Nielsen SG, Prytulak J, Wood BJ and Halliday AN (2014) Vanadium isotopic difference between the silicate Earth and meteorites. Earth and Planetary Science Letters 389:167–175.
[doi:10.1016/j.epsl.2013.12.030]
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David A. Minton^a and Harold F. Levison^b

^aPurdue University Department of Earth, Atmospheric, & Planetary Sciences, 550 Stadium Mall Drive, West Lafayette, IN 47907
^bSouthwest Research Institute and NASA Lunar Science Institute, 1050 Walnut St. Suite 300, Boulder, CO 80302

We develop a model for planetesimal-driven migration (PDM) in the context of rocky planetary embryos in the terrestrial planet region during the runaway and oligarchic growth phases of inner planet formation. We develop this model by first showing that there are five necessary and sufficient criteria that must be simultaneously satisfied in order for a rocky inner solar system embryo to migrate via PDM. To investigate which embryos within a given disk satisfy the five criteria, we have developed a Monte Carlo planetesimal merger code that simulates the growth of embryos from a planetesimal disk with nebular gas. The results of our Monte Carlo planetesimal merger code suggest that, for typical values of the minimum mass solar nebula for the inner solar system, an average of 0.2 embryos capable of PDM emerge over the lifetime of the disk. Many disks in our simulations produce no migration candidates, but some produced as many as 3. The number of embryos that experience PDM in a disk increases with increasing disk mass and decreasing il planetesimal mass, although we were not able to simulate disks where the average initial planetesimal size was smaller than 50 km. For disks 4× more massive than the standard minimum mass solar nebula, we estimate that an average of 1.5 embryos capable of PDM emerge, with some producing as many as 7.

Reference
Minton DA and Levison HF (2014) Planetesimal-driven migration of terrestrial planet embryos. Icarus
[doi:10.1016/j.icarus.2014.01.001]
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Y.-M. Lim¹, K.-W. Min¹, P. D. Feldman², W. Han³, and J. Edelstein⁴

¹Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea
²Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
³Korea Astronomy and Space Science Institute (KASI), 776 Daedeokdae-ro, Yuseong-gu, Daejeon 305-348, Korea
⁴Space Sciences Laboratory, University of California, Berkeley, 7 Gauss Way, Berkeley, CA 94720, USA

We present the results of far-ultraviolet observations of comet C/2001 Q4 (NEAT) that were made with the Far-Ultraviolet Imaging Spectrograph on board the Korean satellite STSAT-1. The observations were conducted in two campaigns during its perihelion approach between 2004 May 8 and 15. Based on the scanning mode observations in the wavelength band of 1400-1700 Å, we have constructed an image of the comet with an angular size of 5°×5°, which corresponds to the central coma region. Several important fluorescence emission lines were detected including S I multiplets at 1429 and 1479 Å, C I multiplets at 1561 and 1657 Å, and the CO A¹Π-X¹Σ⁺ Fourth Positive system; we have estimated the production rates of the corresponding species from the fluxes of these emission lines. The estimated production rate of CO was Q_CO = (2.65 ± 0.63) × 10²⁸ s^-1, which is 6.2%-7.4% of the water production rate and is consistent with earlier predictions. The average carbon production rate was estimated to be QC  = ~1.59 × 10²⁸ s^-1, which is ~60% of the CO production rate. However, the observed carbon profile was steeper than that predicted using the two-component Haser model in the inner coma region, while it was consistent with the model in the outer region. The average sulfur production rate was QS  = (4.03±1.03) × 10²⁷ s^-1, which corresponds to ~1% of the water production rate.

Reference
Lim Y-M, Min K-W, Feldman PD, Han W and Edelstein J (2014) Far-ultraviolet Observations of Comet C/2001 Q4 (NEAT) with FIMS/SPEAR. The Astrophysical Journal 781:80.
[doi:10.1088/0004-637X/781/2/80]

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Marc D. Norman^a,b and Alexander A. Nemchin^c

^aLunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, USA
^bResearch School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT 0200, Australia
^cDepartment of Applied Geology, Curtin University of Technology, Perth, WA 6845, Australia

A sharp rise in the flux of asteroid-size bodies traversing the inner Solar System at 3.9 Ga has become a central tenet of recent models describing planetary dynamics and the potential habitability of early terrestrial environments. The prevalence of ~3.9 Ga crystallization ages for lunar impact-melt breccias and U–Pb isotopic compositions of lunar crustal rocks provide the primary evidence for a short-lived, cataclysmic episode of late heavy bombardment at that time. Here we report U–Pb isotopic compositions of zirconolite and apatite in coarse-grained lunar melt rock 67955, measured by ion microprobe, that date a basin-scale impact melting event on the Moon at 4.22±0.01 Ga followed by entrainment within lower grade ejecta from a younger basin approximately 300 million yr later. Significant impacts prior to 3.9 Ga are also recorded by lunar zircons although the magnitudes of those events are difficult to establish. Other isotopic evidence such as ⁴⁰Ar–³⁹Ar ages of granulitic lunar breccias, regolith fragments, and clasts extracted from fragmental breccias, and Re–Os isotopic compositions of lunar metal is also suggestive of impact-related thermal events in the lunar crust during the period 4.1–4.3 Ga. We conclude that numerous large impactors hit the Moon prior to the canonical 3.9 Ga cataclysm, that some of those pre-cataclysm impacts were similar in size to the younger lunar basins, and that the oldest preserved lunar basins are likely to be significantly older than 3.9 Ga. This provides sample-based support for dynamical models capable of producing older basins on the Moon and discrete populations of impactors. An extended period of basin formation implies a less intense cataclysm at 3.9 Ga, and therefore a better opportunity for preservation of early habitable niches and Hadean crust on the Earth. A diminished cataclysm at 3.9 Ga suggests that the similarity in the age of the oldest terrestrial continental crust with the canonical lunar cataclysm is likely to be coincidental with no genetic significance.

Reference
Norman MD and Alexander A. Nemchin AA (in press) A 4.2 billion year old impact basin on the Moon: U–Pb dating of zirconolite and apatite in lunar melt rock 67955. Earth and Planetary Science Letters
[doi:10.1016/j.epsl.2013.11.040]
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Romain Tartèse^a and Mahesh Anand^b

^aPlanetary and Space Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
^bDepartment of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom

Refers To
Romain Tartèse, Mahesh Anand
Late delivery of chondritic hydrogen into the lunar mantle: Insights from mare basalts
Earth and Planetary Science Letters, Volume 361, 1 January 2013, Pages 480-486

The authors regret for an error which was introduced in an intermediate step involved in calculating the amount of chondritic material added to the lunar interior to account for the estimated H content of the mare basalt source regions (Section 6, lines 12–25). The revised text is as follows:
Assuming a 400 km deep solidified mantle with 25 ppm H and a density of 3300 kg m⁻³ implies that ~1×10¹⁸ kg of H has been added by chondrite-type impactors and efficiently mixed in the upper lunar mantle. Taking the measured H content of ~5000–15 000 ppm in CI chondrites (Alexander et al., 2012 and Kerridge, 1985), this represents a mass of about 6.2×10¹⁹ to 1.8×10²⁰ kg of CI-type material accreted to the lunar upper mantle. This corresponds to 0.2–0.5 wt.% of the 400 km deep upper mantle considered here. By comparison, HSE abundances in lunar basalts require an amount of ~1.6×10¹⁹ kgof chondritic material to have been accreted and mixed into the lunar upper mantle (Bottke et al., 2010 and Day et al., 2007), around 8±4 times less than that required for a lunar mantle with 25 ppm H.

Reference
Tartèse R and Mahesh Anand M (in press) Corrigendum to “Late delivery of chondritic hydrogen into the lunar mantle: Insights from mare basalts” [Earth Planet. Sci. Lett. 361 (2013) 480–486]. Earth and Planetary Science Letters
[doi:10.1016/j.epsl.2014.01.002]
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Pavlo P. Korsun^a, Philippe Rousselot^b, Irina V. Kulyk^a, Viktor L. Afanasiev^c, Oleksandra V. Ivanova^a

^aMain Astronomical Observatory of NAS of Ukraine, Akademika Zabolotnoho 27, 03680 Kyiv, Ukraine
^bUniversity of Franche-Comté, Observatoire des Sciences de l’Univers THETA, Institut UTINAM – UMR CNRS 6213, BP 1615, 25010 Besançon Cedex, France
^cSpecial Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz, 369167, Russia

Spectrophotometric monitoring of distant comet C/2002 VQ94 (LINEAR) was performed with the 6-m telescope of SAO RAS (Special Astrophysical Observatory of Russian Academy of Sciences) and with the 2.5-m Nordic Optical Telescope (Observatory del Roque de los Muchachos, Canarias, Spain) between 2008 and 2013. During this period the comet was on the outbound segment of its orbit, between heliocentric distances of 8.36 au and 16.84 au. Analysis of the spectra revealed the presence of the CO⁺ and N₂⁺emissions in the cometary coma at a distance of 8.36 au from the Sun. This distance is larger than ionic emissions have been detected in any previous objects. Only continuum, with no traces of emissions, was detected in the spectrum obtained in 2009 when the comet was at a distance of 9.86 au. From the spectra obtained in 2008, average column densities of 2.04×10⁹ mol cm^-2 for N₂⁺ and 3.26×10¹⁰ mol cm^-2 for CO⁺were measured in the cometary coma. The derived values correspond to N₂⁺/CO⁺=0.06 within the projected slit. Images obtained through a red continuum filter in 2008 showed a bright, dust coma, indicating a high level of physical activity. A considerably lower level of activity was observed in 2009 and 2011 at distances of 9.86 au and 13.40 au respectively. No noticeable activity was detected in 2013 at a heliocentric distance of 16.84 au.The Afρ parameter, which is used as an indicator of cometary activity, was measured as 2000 cm in 2008, and 800 cm in 2009 and 2011. The Afρ values correspond to dust production rates between 10-20 kg s^-1, 4-6 kg s^-1 and 3-5 kg s^-1 at 8.36, 9.86, and 13.40 au respectively. There is an obvious correlation between the decrease of the dust production rate of the nucleus and the disappearance of the emissions in the spectrum of C/2002 VQ94 (LINEAR) at heliocentric distances greater than 9 au. The colors and size of the nucleus of C/2002 VQ94 (LINEAR) were estimated from the images obtained during the late stage at a heliocentric distance of 16.84 au, when the activity had probable ceased. The B-V and V-R colors were estimated to be 1.07±0.05 and 0.54±0.03 respectively. The effective nucleus radius of 482 km is in agreement with the previously published results, obtained from the observations of the comet during its early inactive stage ( and ).

Reference
Korsun PP, Philippe Rousselot P, Kulyk IV, Afanasiev VL and Ivanova OV (in press) Distant activity of comet C/2002 VQ94 (LINEAR): optical spectrophotometric monitoring between 8.4 and 16.8 au from the Sun. Icarus
[doi:10.1016/j.icarus.2014.01.006]
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G.J. Archer^a, R.D. Ash^a, E.S. Bullock^b, R.J. Walker^a

^aDepartment of Geology, University of Maryland, College Park, MD 20742
^bDepartment of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560

The abundances of highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, and Pd, as well as ¹⁸⁷Re-¹⁸⁷Os isotopic systematics were determined for calcium-aluminum-rich inclusions (CAIs), chondrules, and matrix, separated from the CV3 carbonaceous chondrite Allende. Consistent with prior studies, CAIs are characterized by significant depletions in Pd relative to the other HSE, while the other HSE are in generally bulk chondritic relative abundances. The depletions in Pd can be linked with initial formation of CAIs via condensation, or subsequent processing by evaporative processes. Chondrules generally have relative HSE patterns similar to CAIs, although they have lower absolute abundances. Palladium depletions in chondrules may reflect solid metal-liquid metal fractionation at the time of formation, or alternatively, be the result of processes that acted on precursor materials. Matrix samples have nearly chondritic absolute abundances of all HSE measured. Consequently, matrix is the only major chondritic component examined here that shows no relative depletion in Pd. Mass balance suggests the existence of an unidentified Pd-rich carrier, although it is possible that the dataset presented here is too limited to represent typical HSE abundances of some chondritic components (e.g., chondrules).
The ¹⁸⁷Re-¹⁸⁷Os isotopic systematics of only six out of twenty-four Allende chondritic components analyzed plot within uncertainties of a 4568 Ma primordial reference isochron. The deviations from the expected isochron most likely reflect late-stage, open-system behavior within the last 2 billion years, and, in some cases, could even have resulted from terrestrial alteration. The open-system behavior is most readily observed in small, millimeter-size sub-samples of Allende, consistent with Re and/or Os mobility on that scale.

Reference
Archer GJ, Ash RD, Bullock ES and Walker RJ (in press) Highly Siderophile Elements and 187Re-187Os Isotopic Systematics of the Allende Meteorite: Evidence for Primary Nebular Processes and Late-stage Alteration. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.12.032]
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