Abstracts of Papers to be Published in the March 2002 Issue

Please contact the correspondence author for reprints of all published articles

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

The irradiation history of the Ghubara (L5) regolith breccia

T. E. Ferko, M-S. Wang, D. J. Hillegonds, M. E. Lipschutz*, R. Hutchison, L. Franke, P. Scherer, L. Schultz, P. H. Benoit, D. W. G. Sears, A. K. Singhvi and N. Bhandari

*Correspondence author's address:  Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA; e-mail address: rnaapuml@purdue.edu

Abstract–We measured cosmic-ray products—noble gases, radionuclides, thermoluminescence, and nuclear tracks—and trace element contents and mineralogy of samples of three orthogonal and mutually intersecting cores (41–46 cm long) of a 101.6 kg Ghubara individual (1958,805) at The Natural History Museum, London.  The xenoliths, like the host, have high concentrations of trapped solar gases and are heavily shocked.  While contents of noble gases and degree of shock-loading in this individual and three others differ somewhat, the data indicate that Ghubara is a two-generation regolith breccia.  Contents of cosmogenic ²⁶Al and ¹⁰Be and low track densities indicate that the Ghubara individuals were located more than 15 cm below the surface of an 85 cm meteoroid.  Because of its large size, Ghubara's cosmic-ray exposure age is poorly defined to be 15–20 Ma from cosmogenic nuclides.  Ghubara's terrestrial age, based on ¹⁴C data, is 2–3 ka.  Not only is Ghubara the first known case of a two-generation regolith breccia on the macro scale, it also has a complicated thermal and irradiation history.

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Impact processing of chondritic planetesimals:  Siderophile and volatile element fractionation in the Chico L chondrite

Marc D. Norman* and David W. Mittlefehldt

*Correspondence author's address:  Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia; e-mail address: Marc.Norman@anu.edu.au

Abstract–A large impact event 500 million years ago shocked and melted portions of the L chondrite parent body.  Chico is an impact melt breccia produced by this event.  Sawn surfaces of this 105 kg meteorite reveal a dike of fine-grained, clast-poor impact melt cutting shocked host chondrite.  Coarse (1–2 cm diameter) globules of FeNi metal+sulfide are concentrated along the axis of the dike from metal-poor regions toward the margins.  Refractory lithophile element abundance patterns in the melt rock are parallel to average L chondrites, demonstrating near-total fusion of the L chondrite target by the impact and negligible crystal-liquid fractionation during emplacement and cooling of the dike.

Significant geochemical effects of the impact melting event include fractionation of siderophile and chalcophile elements with increasing metal-silicate heterogeneity, and mobilization of moderately to highly volatile elements.  Siderophile and chalcophile elements ratios such as Ni/Co, Cu/Ga, and Ir/Au vary systematically with decreasing metal content of the melt.  Surprisingly small (~10²) effective metal/silicate-melt distribution coefficients for highly siderophile elements probably reflect inefficient segregation of metal despite the large degrees of melting.  Moderately volatile lithophile elements such K and Rb were mobilized and heterogeneously distributed in the L chondrite impact breccias whereas highly volatile elements such as Cs and Pb were profoundly depleted in the region of the parent body sampled by Chico.  Volatile element variations in Chico and other L chondrites are more consistent with a mechanism related to impact heating rather than condensation from a solar nebula.  Impact processing can significantly alter the primary distributions of siderophile and volatile elements in chondritic planetesimals.

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Petrology and geochemistry of D'Orbigny, geochemistry of Sahara 99555, and the origin of angrites

David W. Mittlefehldt*, Marvin Killgore and Michael T. Lee

*Correspondence author's address:  Mail code SN2, NASA Johnson Space Center, Houston, Texas 77058, USA; e-mail address:  david.w.mittlefehldt1@jsc.nasa.gov

Abstract–We have done a detailed petrologic study of the angrite, D'Orbigny, and geochemical study of it and Sahara 99555.  D'Orbigny is an igneous-textured rock composed of Ca-rich olivine, Al-Ti-diopside-hedenbergite, subcalcic kirschsteinite, two generations of hercynitic spinel and anorthite, with the mesostasis phases ulvöspinel, Ca-phosphate, a silico-phosphate phase and Fe-sulfide.  We report an unknown Fe-Ca-Al-Ti-silicate phase in the mesostasis not previously found in angrites.  One hercynitic spinel is a large, rounded homogeneous grain of a different composition than the euhedral and zoned grains.  We believe the former is a xenocryst, the first such described from angrites.  The mafic phases are highly zoned; mg# of cores for olivine are ~64, and for clinopyroxene ~58, and both are zoned to Mg-free rims.  The Ca content of olivine increases with decreasing mg#, until olivine with ~20 mole% Ca is overgrown by subcalcic kirschsteinite with Ca ~30-35 mole%.  Detailed zoning sequences in olivine-subcalcic kirschsteinite and clinopyroxene show slight compositional reversals.  There is no mineralogic control that can explain these reversals, and we believe they were likely caused by local additions of more primitive melt during crystallization of D'Orbigny.

D'Orbigny is the most ferroan angrite with a bulk rock mg# of 32.  Compositionally, it is virtually identical to Sahara 99555; they are the first set of compositionally identical angrites.  Comparison with the other angrites shows that there is no simple petrogenetic sequence, partial melting with or without fractional crystallization, that can explain the angrite suite.  Angra dos Reis remains an anomalous angrite.

Angrites show no evidence for the brecciation, shock, impact metamorphism, or thermal metamorphism that affected the HED suite and ordinary chondrites.  This suggests that the angrite parent body may have followed a fundamentally different evolutionary path than did these other parent bodies.

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Clearwater East impact structure: A re-interpretation of the projectile type using new platinum-group element data from meteorites

Iain McDonald

Author's address:  Department of Earth Sciences, Cardiff University, P.O. Box 914, Cardiff CF10 3YE, U.K. e-mail address: mcdonaldi1@cardiff.ac.uk

Abstract–Platinum-group element (PGE) concentrations and ratios obtained from samples of the Clearwater East impact melt have been used along with other siderophile element ratios to classify the impacting projectile as a carbonaceous chondrite.  This is at odds with recent chromium isotope analyses that suggest ordinary chondrite-type material is present.  The present study reviews and re-interprets the available PGE data in the light of new PGE data from meteorites and concludes that the PGE ratios in the impact melt are most consistent with ordinary (possibly type-L) chondrite source material, not carbonaceous chondrites.  Therefore the structure was most probably formed by the impact of an asteroid composed of material similar to ordinary chondrites.

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Northwest Africa 032:  Product of lunar volcanism

T. J. Fagan*, G. J. Taylor, K. Keil, T. E. Bunch, J. H. Wittke,  R. L. Korotev, B. L. Jolliff, J. J. Gillis, L. A. Haskin, E. Jarosewich, R. N. Clayton, T. K. Mayeda, V. A. Fernandes, R. Burgess, G. Turner, O. Eugster and S. Lorenzetti

*Correspondence author's address: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo  152-8551, Japan; e-mail address: fagan@geo.titech.ac.jp

Abstract–Mineralogy, major element compositions of minerals, and elemental and oxygen isotopic compositions of the whole rock attest to a lunar origin of the meteorite Northwest Africa 032 (NWA 032), an unbrecciated basalt found in October 1999.  The rock consists predominantly of olivine, pyroxene and chromite phenocrysts, set in a crystalline groundmass of feldspar, pyroxene, ilmenite, troilite and trace metal.  Whole-rock shock veins comprise a minor, but ubiquitous portion of the rock.  Undulatory to mosaic extinction in olivine and pyroxene phenocrysts and micro-faults in groundmass and phenocrysts also are attributed to shock.

Several geochemical signatures taken together indicate unambiguously that NWA 032 originated from the Moon.  The most diagnostic criteria include whole-rock oxygen isotopic composition and ratios of Fe:Mn in the whole rock, olivine, and pyroxene.  A lunar origin is documented further by the presence of Fe-metal, troilite, and ilmenite; zoning to extremely Fe-rich compositions in pyroxene; the ferrous oxidation state of all Fe in pyroxene; and the rare-earth element pattern with a well-defined negative europium anomaly.  This rock is similar in major element chemistry to basalts from Apollo 12 and 15, but is enriched in light rare-earth elements and has an unusually high Th/Sm ratio.  Some Apollo 14 basalts yield a closer match to NWA 032 in rare-earth element patterns, but have higher concentrations of Al₂O₃.  Ar-Ar step release results are complex, but yield a whole-rock age of ca. 2.8 Ga, suggesting that NWA 032 was extruded at 2.8 Ga or earlier.  This rock may be the youngest sample of mare basalt collected to date.  Noble gas concentrations combined with previously collected radionuclide data indicate that the meteorite exposure history is distinct from currently recognized lunar meteorites.  In short, the geochemical and petrographic features of NWA 032 are not matched by Apollo or Luna samples, nor by previously identified lunar meteorites, indicating that it originates from a previously unsampled mare deposit.

Detailed assessment of petrographic features, olivine zoning, and thermodynamic modelling indicate a relatively simple cooling and crystallization history for NWA 032.  Chromite-spinel, olivine, and pyroxene crystallized as phenocrysts while the magma cooled no faster than 2 °C/h based on the polyhedral morphology of olivine.  Comparison of olivine size with crystal growth rates and preserved Fe-Mg diffusion profiles in olivine phenocrysts suggest that olivine was immersed in the melt for no more than 40 days.  Plumose textures in groundmass pyroxene, feldspar, and ilmenite, and Fe-rich rims on the phenocrysts formed during rapid crystallization (cooling rates ~ 20 to 60 °C/h) after eruption.
 

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Hosts of hydrogen in ALH 84001:  Evidence for hydrous martian salts in the oldest martian meteorite?

John M. Eiler*, Nami Kitchen, Lauri Leshin and Melissa Strausberg

*Correspondence author's address:  Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA; e-mail address:  eiler@gps.caltech.edu

Abstract–The Martian meteorite, ALH84001, contains D-rich hydrogen of plausible Martian origin (Leshin et al. 1996).  The phase identity of the host(s) of this hydrogen are not well known and could include organic matter (McKay et al., 1996), phlogopite (Brearley 2000), glass (Mittlefehldt 1994) and/or other, unidentified components of this rock.  Previous ion microprobe studies indicate that much of the hydrogen in ALH84001 as texturally associated with concretions of nominally anhydrous carbonates, glass and oxides (Boctor et al., 1998; Sugiura and Hoshino, 2000).  We examined the physical and chemical properties of the host(s) of this hydrogen by stepped pyrolysis of variously pre-treated sub-samples.  A continuous-flow method of water reduction and mass spectrometry (Eiler and Kitchen 2001) was used to permit detailed study of the small amounts of this hydrogen-poor sample available for study.  We find that the host(s) of D-rich hydrogen released from ALH84001 at relatively low temperatures (~500 °C) is soluble in orthophosphoric and dilute hydrochloric acids and undergoes near-complete isotopic exchange with water within hours at temperatures of 200 to 300 °C.  These characteristics are most consistent with the carrier phase(s) being a hydrous salt (e.g., carbonate, sulfate or halide); the thermal stability of this material is inconsistent with many examples of such minerals (e.g., gypsum) and instead suggests one or more relatively refractory hydrous carbonates (e.g., hydromagnesite).  Hydrous salts (particularly hydrous carbonates) are common on the earth only in evaporite, sabkha, and hydrocryogenic-weathering environments; we suggest that much (if not all) of the 'Martian' hydrogen in ALH84001 was introduced in analogous environments on or near the martian surface rather than through biological activity or hydrothermal alteration of silicates in the crust.
 

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Space shuttle observations of terrestrial impact structures using SIR-C and X-SAR radars

John F. McHone, Ronald Greeley, Kevin K. Williams*, Dan G. Blumberg and Ruslan O. Kuzmin

*Correspondence author's address:  Department of Geological Sciences, Box 871404, Arizona State University, Tempe, Arizona 85287-1404, USA; e-mail address:  kkw@asu.edu

Abstract–Ten terrestrial impact structures were imaged during two flights of the 1994 Space Radar Laboratory (SRL) experiment.  These craters include Wolf Creek, Australia; Roter Kamm, Namibia; Zhamanshin, Kazakhstan; B.P. and Oasis, Libya; Aorounga, Chad; Amguid, Algeria; and Spider, Connolly Basin and Henbury, Australia.  SRL contained two co-registered instruments; the United States SIR-C polarimetric radar system operating in L-band (lambda = 24 cm) and C-band (lambda = 5.6 cm), and the joint German/Italian X-SAR vertically-polarized radar operating in X-band (lambda = 3 cm).
 
Comparisons show SRL images to be complementary to, or in some cases superior to, corresponding optical images for evaluating size, location, and relative age of impact features.  Regardless of wavelength or polarization, craters with significant relief appear prominently on radar as a result of slope and roughness effects.  In desert regions, longer wavelengths penetrate dry sand mantles to reveal hidden crater dimensions or associated buried landforms.  Radar polarities and wavelengths are particularly sensitive to vegetation, surface roughness, and soil moisture or electrical properties.  In the more temperate environments of Kazakhstan and Australia, SRL images show detailed stream patterns that reveal the location and structure of otherwise obscured impact features.
 

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

The relative formation ages of ferromagnesian chondrules inferred from their initial aluminum-26/aluminum-27 ratios

Smail Mostefaoui*, Noriko T. Kita, Shigeko Togashi, Shogo Tachibana, Hiroko Nagahara and Yuichi Morishita

*Correspondence author's address:  Max Planck Institute for Chemistry, D-55020 Mainz, Germany; e-mail address: smail@mpch-mainz.mpg.de

Abstract–We performed a systematic high precision SIMS 26Al-26Mg isotopic study for 11 ferromagnesian chondrules from the highly unequilibrated ordinary chondrite Bishunpur (LL3.1).  The chondrules are porphyritic and contain various amounts of olivine and pyroxene and interstitial plagioclase and/or glass. The chemical compositions of the chondrules vary from FeO-poor to FeO-rich.  Eight chondrules show resolvable ²⁶Mg-excesses with a maximum ²⁶Mg of ~1% in two chondrules.  The initial ²⁶Al/²⁷Al ratios inferred for these chondrules range between (2.28 ± 0.73) × 10^–5 to (0.45 ± 0.21) × 10^–5.  Assuming a homogeneous distribution of Al isotopes in the early solar system, this range corresponds to ages relative to CAIs between 0.7 ± 0.2 Ma and 2.4-0.4/+0.7 Ma.  The inferred total span of the chondrule formation ages is at least 1 Ma, which is too long to form chondrules by the X-wind.  The initial ²⁶Al/²⁷Al ratios of the chondrules are found to correlate with the proportion of olivine to pyroxene suggesting that olivine-rich chondrules formed earlier than pyroxene-rich chondrules.  Though we do not have a completely satisfactory explanation of this correlation we tentatively interpret it as a result of evaporative loss of Si from earlier generations of chondrules followed by addition of Si to the precursors of later generation chondrules.
 

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Fall, classification and cosmogenic records of the Sabrum (LL6) chondrite

S. Ghosh, S. V. S. Murty, P. N. Shukla, A. D. Shukla, R. R Mahajan, N. Bhandari*, N. C. Pant, J. B. Ghosh and S. Shome

*Correspondence author's address:  Physical Research Laboratory, Navrangpura, Ahmedabad 380009, India; e-mail address: bhandari@prl.ernet.in

Abstract–The petrographic and chemical characteristics of a fresh Indian meteorite fall at Sabrum are described.  Its mean mineral composition is defined by olivine (Fa_31.4), orthopyroxene (Fs_25.1,Wo_2.0), clinopyroxene (Wo₄₅En_45.6Fs_9.4) and plagioclase (An_10.6Ab_83.6Or_5.8).  The meteorite shows moderate shock features, which indicate that it, belong to the S4 category.  Based on mineralogical and chemical criteria the meteorite is classified as LL6 brecciated veined chondrite.  Several cosmogenic radioisotopes (⁴⁶Sc, ⁷Be, ⁵⁴Mn, ²²Na and ²⁶Al), noble gas (He, Ne, Ar, Kr and Xe), nitrogen isotopes and particle tracks density have been measured.  Concentrations of cosmogenic ²¹Ne and ³⁸Ar indicate that its cosmic ray exposure is 24.8 Ma.  Small amounts of trapped Kr and Xe, consistent with petrologic class 5/6, are present.  The track density in olivines is found to be (1.3 ± 0.3) × 10⁶/cm².  Activities of most of the short lived isotopes are lower than those expected from solar cycle variation.  ²²Na/²⁶Al (1.12 ± 0.02) is found to be significantly anomalous, being approximately 25% lower than expected from the climax neutron monitor data.  These results indicate that the cosmic ray flux during the terminal segment of the meteoroid orbit was low.  The activities of ²⁶Al and ⁶⁰Co and the track density indicate small meteoroid size with radius ~15 cm.

Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.

Hornblende alteration and fluid inclusions in Kärdla impact crater, Estonia - evidence for impact-induced hydrothermal activity

Kalle Kirsimäe*, Sten Suuroja, Juho Kirs, Aulis Kärki, Maile Polikarpus, Väino Puura and Kalle Suuroja

*Correspondence author's address:  Institute of Geology, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia; e-mail address: arps@ut.ee

Abstract–The well-preserved Kärdla impact crater, on Hiiumaa Island, Estonia, is a 4-km diameter structure formed in a shallow Ordovician sea about 455 Ma ago into a target composed of thin (~150 m) unconsolidated sedimentary layer above a crystalline basement composed of migmatite granites, amphibolites and gneisses.  The fractured and crushed amphibolites in the crater area are strongly altered and replaced with secondary chloritic minerals.  The most intensive chloritization is found in permeable breccias and heavily shattered basement around and above the central uplift.  Alteration is believed to have resulted from  convective flow of hydrothermal fluids through the central areas of the crater.  Chloritic mineral associations suggest formation temperatures of 100–300 °C, in agreement with the most frequent quartz fluid inclusion homogenization temperatures of 150–300 °C in allochthonous breccia.  The rather low salinity of fluids in Kärdla crater (<13 wt% NaCl_eq) suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the cater rim walls raised above sea level after the impact, or by invasion of sea water through the disturbed sedimentary cover and fractured crystalline basement.  The well developed hydrothermal system in Kärdla crater shows that the thermal history of the shock heated and uplifted rocks in the central crater area, rather than cooling of impact-melt or suevite sheets, controlled the distribution and intensity of the impact-induced hydrothermal processes.
 

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