Isotope tracers of core formation

1Bernard Bourdon, 2Mathieu Roskosz, 3Remco C.Hin
Earth-Science Reviews 181, 61-81 Link to Article []
1Laboratoire de Géologie de Lyon, ENS de Lyon, UCBL and CNRS, 46 Allée d’Italie, Lyon, France
2IMPMC, MNHN, UPMC, UMR CNRS 7590, 61 rue Buffon, 75005 Paris, France
3School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK

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Analysis of meteoritic activity in the Vth-XVth centuries using an extended survey of European Medieval Sources

1María José Martíneza, 2Francisco J. Marcob
Icarus (in Press) Link to Article []
1Universidad Politécnica de Valencia. Dept. Matemàtica Aplicada. IUMPA. Camino de Vera SN. 46022. Valencia. Spain
2Universitat Jaume I. Dep. Matemàtiques. IMAC. Campus de Riu Sec. 12071 Castellón. Spain
Copyright Elsevier

The distribution of meteors streams throughout a year from the 5th to the 15th century is investigated, based on a new compilation of meteor records in the diverse European Medieval sources. The records of meteor showers and storms in the chronicles of Korea, Japan, China and Arab have also been considered, and compare their appearance dates with those of showers obtained above, as well as with modern observations. We found that the three sets of data are in agreement with each other, the Perseids, Leonids, and Lyrids being best represented.

The origin of the unique achondrite Northwest Africa 6704: Constraints from petrology, chemistry and Re–Os, O and Ti isotope systematics

1Yuki Hibiya et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article[]
1Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
Copyright Elsevier

Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and 187Re–187Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En55–57Wo3–4Fs40–42; Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa50–53; Fe/Mn = 1.1–2.1), chromite (Cr# ∼ 0.94), awaruite, sulfides, plagioclase (Ab92An5Or3) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1–102 °C/h), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high super-saturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re–Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O–Ti isotope systematics (Δ17O = −1.052 ± 0.004, 2 SD; ε50Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = −2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

Can Formulas Derived from Pyroxenes and/or HEDs be Used to Determine the Mineralogies of V‐type Asteroids?

1T. H. Burbine,2P. C. Buchanan,3R. L. Klima,4R. P. Binzel
Journal of Geophysical Research, Planets (in Press) Link to Article []
1Department of Astronomy, Mount Holyoke College, South Hadley, MA, USA
2Department of Geology, Kilgore College, Kilgore, TX, USA
3Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
4Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
Published by arrangement with John Wiley & Sons

We compare methods for determining the pyroxene mineralogies of V‐type near‐Earth asteroids from their reflectance spectra. We evaluate whether band centers derived from the spectra of synthetic pyroxenes can be used to achieve greater analytical accuracy than is achieved through the use of band centers derived from the spectra of basaltic achondrites (howardites, eucrites, diogenites or HEDs). We conclude that band centers derived from the reflectance spectra of synthetic pyroxenes with known mineralogies do not provide useful diagnostic information to derive equations for determining accurate pyroxene compositions of V‐type asteroids. Band centers from the reflectance spectra of HEDs with known pyroxene mineralogies can be used to derive equations for determining accurate pyroxene compositions of V‐type asteroids. HEDs are physical mixtures of a number of different types of pyroxenes and best simulate the surfaces of V‐type asteroids. Formulas using the Band I center appear best for determining asteroid pyroxene mineralogies for V‐type asteroids due to the current difficulty in doing accurate temperature corrections to the Band II center. Most of thes observed V‐type near‐Earth asteroids have interpreted mineralogies similar to eucrites or howardites. One of the observed near‐Earth asteroids could possibly have a surface mineralogy similar to diogenites.

Image Reconstruction Techniques in Neutron and Gamma‐Ray Spectroscopy: Improving Lunar Prospector Data

1J. T. Wilson,1D. J. Lawrence,1P. N. Peplowski,1J. T. S. Cahill,2V. R. Eke,2R. J. Massey,3L. F. A. Teodoro
Journal of Geophysical Research, Planets (in Press) Link to Article []
1The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
2Institute for Computational Cosmology, Department of Physics, Durham University, Science Laboratories, Durham, UK
3BAER, Planetary Systems Branch, Space Sciences and Astrobiology Division, MS 245‐3, NASA Ames Research CenterMoffett Field, CA, USA
Published by arrangement with John Wiley & Sons

We present improved resolution maps of the Lunar Prospector Neutron Spectrometer thermal, epithermal and fast neutron data and Gamma‐Ray Spectrometer Th‐line fluxes via global application of pixon image reconstruction techniques. With the use of mock data sets, we show that the pixon image reconstruction method compares favorably with other methods that have been used in planetary neutron and gamma‐ray spectroscopy. The improved thermal neutron maps are able to clearly distinguish variations in composition across the lunar surface, including within the lunar basins of Hertzsprung and Schrödinger. The improvement in resolution reveals a correlation between albedo and thermal neutron flux within the basins. The consequent increase in dynamic range confirms that Hertzsprung basin contains one of the most anorthositic parts of the lunar crust, including nearly pure anorthite over a region tens of km in diameter. At Orientale, the reconstructed epithermal neutron data show broad overlap with cpr but there remains a mismatch between measures of regolith maturity that sample the surface and those that probe the near‐subsurface, which is consistent with a complex layering scenario.

Shergottite Northwest Africa 6963: A Pyroxene‐Cumulate Martian Gabbro

1Justin Filiberto et al. (>10)
Journal of Geophysical Research, Planets (in Press) Link to Article []
1Southern Illinois University, Department of GeologyCarbondale, IL, USA
2School of Environment, Earth and Ecosystem Sciences, Walton Hall, The Open UniversityMilton Keynes, MK, UK
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 6963 was found in Guelmim‐Es‐Semara, Morocco, and based on its bulk chemistry and oxygen isotopes, it was classified as a Martian meteorite. On the basis of a preliminary study of the textures and crystal sizes, it was re‐subclassified as a gabbroic shergottite because of the similarity with terrestrial and lunar gabbros. However, the previous work was not a quantitative investigation of NWA 6963; to supplement the original re‐subclassification and enable full comparison between this and other Martian samples, here we investigate the mineralogy, petrology, geochemistry, quantitative textural analyses, and spectral properties of gabbroic shergottite NWA 6963 to constrain its petrogenesis, including the depth of emplacement (i.e., base of a flow versus crustal intrusion).
NWA 6963 is an enriched shergottite with similar mineralogy to the basaltic shergottites, but importantly, does not contain any fine‐grained mesostasis. Consistent with the mineralogy, the reflectance (VNIR and TIR) spectrum of powdered NWA 6963 is similar to other shergottites because they are all dominated by pyroxene, but its reflectance is distinct in terms of albedo and spectral contrast due to its gabbroic texture. NWA 6963 represents a partial cumulate gabbro that is associated with the basaltic shergottites. Therefore, NWA 6963 could represent a hypabyssal intrusive feeder dike system for the basaltic shergottites that erupted on the surface.