Abstracts of Papers to be Published in the July 2000 Issue

Please contact the correspondence author for reprints of all published articles

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Numbers, types and compositions of an unbiased collection of cosmic spherules

Susan Taylor*, James H. Lever and Ralph P. Harvey

*Correspondence author's address:  U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire 03755, USA; e-mail address:  staylor@crrel.usace.army.mil

Abstract–Micrometeorites collected from the bottom of the South Pole Water Well (SPWW) may represent a complete, well-preserved sample of the cosmic dust that accreted on Earth from 1100-1500 AD.  We classified 1588 cosmic spherules in the size range 50-800 µm.  The collection has 41% barred olivine spherules, 17% glass spheres, 12% cryptocrystalline spherules, 11% porphyritic olivine spherules, 12% relic grain-bearing spherules, 3% scoriaceous spherules, 2% I-type spherules, 1% Ca-Al-Ti-rich (CAT) spherules and 1% G-type spherules.  We also found bubbly glass spherules, spherules with glass caps, and ones with sulfide coatings, particles that are absent from other collections.  A classification sequence of the stony spherules (scoriaceous, relic grain bearing, porphyritic, barred olivine, cryptocrystalline, glass, and CAT) is consistent with progressive heating and evaporation of Fe from chondritic materials.  The modern-day accretion rate and size distribution measured at the SPWW can account for the stony spherules present in deep-sea collection through preferential dissolution of glass and small stony spherules.  However, weathering alone cannot account for the high accretion rate of I-type spherules determined for two deep-sea collections.  The SPWW collection provides data to constrain models of atmospheric-entry heating and to assess the effects of terrestrial weathering.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Sequential deformation of plains at the margins of Alpha Regio, Venus: Implications for tessera formation

Martha S. Gilmore* and James W. Head

*Correspondence author's address:  Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA; e-mail address:  msg@pop.jpl.nasa.gov

Abstract–The boundaries between the highly deformed tessera terrain and adjacent volcanic plains are primarily those of embayment, where the tessera are stratigraphically older than the plains.  Previous studies show that less than three percent of these boundaries display evidence of tectonic tilting after the emplacement of the plains.  One of these unusual boundaries is the western margin of Alpha Regio tessera, a zone ~100 km in width that separates the plains from the interior structures of Alpha.  This zone is characterized by margin parallel, fine scale (1–5 km) fractures, graben and ridges that truncate and postdate the broad scale (10–30 km) ridges and troughs of the interior of Alpha.  The western margin is embayed by several volcanic plains units which are progressively tilted and deformed by graben with closer proximity to Alpha Regio.  The earliest deformation of the plains consists of NE trending graben ~1 km in width that are similar in morphology and spacing to graben that deform intratessera plains and plains at the eastern boundary of Alpha.  NW-trending graben then formed over an interval marked by the emplacement of two additional plains units; their similarity to NW-trending structures emanating from Eve corona and the Lada Terra rift suggests a possible genetic relationship.  The tilting of the plains adjacent to western Alpha implies relative vertical movement of the margin, either uplift of tessera or downwarping of plains subsequent to the formation and relaxation of the interior of Alpha Regio. Subsidence of plains at this locale is supported by the presence of a basin to the west of Alpha surrounded by a fracture belt contiguous with western Alpha.  Thus the fractures and deformation at the western boundary of Alpha may be related to the formation of a basin to the west of Alpha with some influence from the northernmost extension of the Lada Terra rift.  Such a basin is not present at a section along the eastern boundary of Alpha Regio, where the origin of tilted plains remains equivocal.  We conclude that the deformation along the western margin of Alpha Regio is not directly related to the process of tessera formation, but is rather an example of tessera modification, and is consistent with the stratigraphic position of tessera as the oldest unit observed on Venus.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

The oxygen isotope record in enstatite meteorites

J. Newton*, I.A. Franchi and C.T. Pillinger

*Correspondence author's address:  Planetary Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K.; e-mail address: newton@gpsun01.geoph.s.u-tokyo.ac.jp

Abstract–Oxygen isotope compositions were determined for a suite of enstatite chondrites and aubrites.  In agreement with previous work (Clayton et al. 1984), most samples have oxygen isotope compositions close to the terrestrial fractionation line (TFL), and there appear to be no significant differences in oxygen isotope compositions between individual EH and EL chondrites and aubrites.  Five enstatite meteorites have oxygen isotope compositions that are significantly different from the other samples and >0.2‰ away from the TFL.  Two of these have petrographic evidence of brecciation and interaction between other meteorite types; for the other three, similar scenarios are suggested.  There appears to be a systematic increase in delta¹⁸O from enstatite chondrites (both EH and EL) of petrologic type 3 to those of type 6.  There is also good evidence that the EH meteorites do not fall along a mass fractionation line but along a line slope 0.66.  At the present time, detailed understanding of the origin of these oxygen isotope systematics remain elusive but clearly point to a complex accretion history and/or parent body evolution.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Mineralogical characterization of primitive, type 3 lithologies in Rumuruti chondrites

Addi Bischoff

Institut für Planetologie and Interdisciplinary Center for Electron Microscopy and Microanalysis, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany; e-mail address:  bischoa@uni-muenster.de

Abstract–Rumuruti chondrites (R-chondrites) constitute a new, well-established, chondrite group different to carbonaceous, ordinary, and enstatite chondrites. Many of these samples are gas-rich regolith breccias showing the typical light/dark structure and consist of abundant fragments of various parent body lithologies embedded in a fine-grained, olivine-rich matrix. Unequilibrated type 3 lithologies among these fragments have frequently been mentioned in various publications. In this study detailed mineralogical data on seven primitive fragments from the R-chondrites Dar al Gani 013 and Hughes 030 are presented. The fragments range from about 300 µm in size up to several millimeters. Generally, the main characteristics can be summarized as follows: a) Unequilibrated type 3 fragments have a well-preserved chondritic texture with a chondrule to matrix ratio of about 1:1. Chondrules and chondrule fragments are embedded in a fine-grained olivine-rich matrix. Thus, the texture is quite similar to that of type 3 carbonaceous chondrites. b) In all cases, matrix olivines in type 3 fragments have a significantly higher Fa-content (44–57 mol%) than olivines in other (equilibrated) lithologies (38–40 mol% Fa). c) Olivines and pyroxenes occurring within chondrules or as fragments are highly variable in composition (Fa_0–65 and Fs_0–33, respectively) and, generally, more magnesian than those found in equilibrated R-chondrites.

Agglomerated material of the R-chondrite parent body (or bodies) was highly unequilibrated. It is suggested that the material that accreted to form the parent body consisted of chondrules and chondrule fragments, mainly having Mg-rich silicate constituents, and Fe-rich highly oxidized fine-grained materials. The dominating phase of this fine-grained material may have been Fa-rich olivine from the beginning. The brecciated whole rocks, the R-chondrite regolith breccias, were not significantly reheated subsequent to brecciation or during lithification, as indicated by negligible degree of equilibration between matrix components and Mg-rich olivines and pyroxenes in primitive, type 3 fragments.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Charlevoix and Sudbury as gravity-readjusted impact structures

J. Rondot

Astroblème Exploration, 1111 rue d'Amiens, Sainte-Foy, Québec, Canada  G1W 4C8; e-mail address:  mrondot@cegep-ste-foy.qc.ca

Abstract–Based on detailed field investigations and the study of impact breccias at the Charlevoix impact structure (Québec) a structural analysis is given. This analysis shows that the annular arrangement of both topography and lithology can be applied to subdivide the two craters discussed in this review. Successive rings characterize the central uplift and its collapsed ring graben in the Charlevoix structure.

Fault breccias show an annular distribution and indicate diverse directions of movement suggesting progressive readjustment. They formed cohesionless products inside fault planes, named mylolisthenite, and are believed to have acted as lubricants during the modification stage of the formation of the structure: listric readjustment by gravity after the formation of the transient crater.

The Sudbury Structure (Ontario) shows fault breccias in a concentric arrangement similar to that of Charlevoix. It may be compared with the Charlevoix structure applying the same mechanical readjustment model with deep listric faults. The only difference is the amount of the rise structural uplift, i.e., height of central uplift, which is less important in the larger impact structure.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Cosmogenic radionuclides and noble gases in Antarctic H chondrites with high and normal natural thermoluminescence levels

Jennifer L. Mokos, Luitgard Franke, Peter Scherer, Ludolf Schultz and Michael E. Lipschutz*

*Correspondence author's address:  Department of Chemistry, Purdue University, West Lafayette, IN 47907-1393; e-mail address: rnaapuml@vm.cc.purdue.edu

Abstract–We report noble gas data for 37 H chondrites collected from the Allan Hills by EUROMET in the 1988/1989 Field Season.  Among these are 16 specimens with high levels (>100 krad) of natural thermoluminescence (NTL), originally interpreted as signaling their derivation from a single meteoroid with an orbit that became Earth-crossing ~100 ka ago.  One of these 16 is an H3 with a cosmic ray exposure age of ~33 Ma and clearly represents a separate fall.  The other 15 H4-6 chondrites derive from 3 separate meteoroids, each of which is represented by a 5- or 6-member group.  These groups have mean exposure ages of 3.7, 4.1 and 6.6 Ma:  the middle-group members all contain solar Ne.  The 2 younger groups also seem to each include a few H chondrites with normal NTL levels.

Measurements of cosmogenic ¹⁰Be (1.5 Ma), ²⁶Al (710 ka) and ³⁶Cl (301 ka) in 14 of the high NTL chondrites indicate that all reflect a simple irradiation history.  In contrast, many of a different, 38-member, randomly-selected suite of Antarctic H chondrites seem to have different cosmic ray irradiation histories.  The 3.7 and 6.6 Ma groups from the 37-member Allan Hills suite come, respectively, from about 5–30 cm and about 5–10 cm depths in 80–125 cm and 60–125 cm-radius meteoroids.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

The Bosumtwi meteorite impact structure, Ghana: A magnetic model

J. Plado*, L. J. Pesonen, C. Koeberl and S. Elo

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

Abstract–A magnetic model is proposed for the Bosumtwi meteorite impact structure in Ghana, Africa.  This relatively young (~1.07 Ma) structure with a diameter of ~10.5 km is exposed within early Proterozoic Birimian-Tarkwaian rocks.  The central part of the structure is buried under post-impact lake sediments, and due to lack of drill cores, geophysics is the only way to reveal its internal structure.  To study the structure below and beyond the lake, a high-resolution low altitude (~70 m) airborne geophysical survey across the structure was conducted, which included measurements of the total magnetic field, electromagnetic data, and gamma radiation. The magnetic data show a circumferential magnetic halo outside the lakeshore, ~12 km in diameter. The central-north part of the lake reveals a central negative magnetic anomaly with smaller positive side-anomalies N and S of it, which is typical for magnetized bodies at shallow latitudes. A few weaker negative magnetic anomalies exist in the eastern and western part of the lake. Together with the northern one they seem to encircle a central uplift. Our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact melt rock or melt-rich suevite bodies.

Petrophysical measurements show a clear difference between the physical properties of pre-impact target rocks and impactites. Suevites have a higher magnetization and have low densities and high porosities compared to the target rocks. In suevites, the remanent magnetization dominates over induced magnetization (Koenigsberger ratio > 3). Preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the Jaramillo normal polarity epoch. This interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake. The reverse polarity remanence component, superimposed to the normal component, is probably a secondary remanence acquired during subsequent reverse polarity events.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Enstatite chemical composition and microstructures in the La Villa H4 chondrite

L. Folco* and M. Mellini

*Correspondence author's address: Museo Nazionale dell'Antartide, Università di Siena, Via Laterina 8, I-53100 Siena, Italy; e-mail address: folco@unisi.it

Abstract–La Villa is an unshocked H4 chondrite. Chemical compositions require crystallization at temperatures higher than 1250 °C for enstatite, and higher than 1211 °C for augite. Widespread (100) polysynthetic twins and (001) contraction cracks in enstatite indicate crystallization as protoenstatite, inverted to ortho- and/or clinoenstatite on cooling. HRTEM shows a range of ortho-clinoenstatite intergrowths: heavily faulted clinoenstatite in radial and poikilitic chondrules, almost regular orthoenstatite in a microgranular chondrule and in the matrix. In the former, the clinoenstatite lamellae are both even or odd multiples of the 9 Å periodicity, a few unit cells thick, twinned and interleaved with minor orthoenstatite. In the latter, orthoenstatite lamellae are regularly stacked for more than 2000 Å. Localized annealing effects, reversing clinoenstatite to orthoenstatite, are revealed by "U-shaped" and "Z-shaped" terminations. The variable microstructures suggest different cooling rates for the different chondrule types, soon after the liquidus-to-solidus transition (1200 to 1300 °C) but prior to accretion. In particular, clinoenstatite-rich crystals from a radial and poikilitic chondrules give cooling rates in the order of 100 and 10 °C/h. Comparisons with previous works on dynamic crystallization experiments and orthopyroxene Fe-Mg cation ordering indicates a nonlinear cooling path from the high chondrule formation temperatures to a post-accretionary low-temperature (340–480 °C) evolution.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Nitrogen isotopic compositions of IIIE iron meteorites

N. Sugiura*, Y. Ikeda, S. Zashu and  J. T. Wasson

*Correspondence author's address:  Department of Earth and Planetary Physics, Univ. of Tokyo, Tokyo, Japan; e-mail address: sugiura@geoph.s.u-tokyo.ac.jp

Abstract–The compositionally closely related iron-meteorite groups IIIE and IIIAB were originally separated based on differences in kamacite bandwidth, the presence of carbides only in IIIE, and marginally resolvable differences on Ga-Ni and Ge-Ni diagrams.  A total of six IIIE iron meteorites have been analyzed for C and N using SIMS and three of these have also been analyzed for N, Ne and Ar by stepped combustion.

We show that these groups cannot be resolved on the basis of N abundances or isotopic compositions, but that they are marginally different in C isotopic composition and  nitride occurrence. Cosmic-ray exposure age distributions of IIIE and IIIAB irons seem to be significantly different.

There is a significant N isotopic range among the IIIE irons.  A negative correlation between delta¹⁵N and N concentration suggests that the increase in delta¹⁵N resulted from diffusional loss of N.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Water soluble ions in the Nakhla martian metorite

Douglas J. Sawyer, Michael D. McGehee, Julie Canepa and Carleton B. Moore*

*Correspondence author's address:  Center for Meteorite Studies, Arizona State University, Tempe, AZ  85287-2504, USA; e-mail address:  cmoore@asu.edu
 
Abstract–Water soluble ion concentrations from the Martian achondrite Nakhla and three asteroidal achondrites are reported.  The Nakhla sample contains significant concentrations of chloride, sulfate, magnesium, sodium, calcium, and potassium ions.  The results are interpreted to indicate that this rock has been in contact with an seawater-like brine on the Martian surface.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

The pyroxene pallasites:  Vermillion and Yamato 8451

Joseph S. Boesenberg*, Andrew M. Davis, Martin Prinz, Michael K. Weisberg, Robert N. Clayton and Toshiko K. Mayeda

*Correspondence author's address:  Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York 10024, USA; e-mail address:  bosenbrg@amnh.org

Abstract–Two pallasites, Vermillion and Yamato 8451, have been studied to obtain petrologic, trace element and oxygen isotopic data.  Both meteorites contain low-Ca and high-Ca pyroxenes (<2% by volume), and have been dubbed "pyroxene pallasites".  Pyroxene occurs as large individual grains, as inclusions in olivine and in other pyroxene, and as grains along the edges of olivine.  Symplectic overgrowths, sometimes found in Main Group and Eagle Station pallasites, are not seen in the pyroxene pallasites.  Olivine compositions are Fa_10-12, similar to those of Main Group pallasites.  Siderophile trace element data show that metal in the two meteorites has significantly differing compositions that are, for many elements, outside the range of the Main Group and Eagle Station pallasites.  These compositions also differ from those of IAB and IIIAB irons.  Rare earth element (REE) patterns in merrillite are similar to those seen in other pallasites, indicating formation by subsolidus reaction between metal and silicate, with the merrillite inheriting its pattern from the surrounding silicates.  The oxygen isotopic compositions of Vermillion and Y8451 are similar, but differ from Main Group or Eagle Station pallasites, as well as other achondrite and primitive achondrite groups.  Although Vermillion and Y8451 have similar mineralogy, pyroxene compositions, REE patterns, and oxygen isotopic compositions, there is sufficient evidence to resist formally grouping these two meteorites.  This evidence includes the texture of Vermillion, siderophile trace element data, and the presence of cohenite in Vermillion.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Calcium, aluminum-rich inclusions in enstatite chondrites

Timothy J. Fagan*, Alexander N. Krot and Klaus Keil

*Correspondence author's address:  Hawai'i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai'i at Manoa, Honolulu, Hawaii 96822, USA; e-mail address:  fagan@pgd.hawaii.edu

Abstract–Like Ca,Al-rich inclusions (CAIs) from carbonaceous and ordinary chondrites, enstatite chondrite (EC) CAIs are composed of refractory minerals such as spinel, perovskite, Al,Ti-diopside, melilite, hibonite, and anorthitic plagioclase, which may be partially to completely surrounded by halos of Na,(±Cl)-rich minerals.  Porous, aggregate, and compact textures of the refractory cores in EC CAIs and rare Wark-Lovering rims are also similar to CAIs from other chondrite groups.  However, the small size (<100 µm), low abundance (<1% by mode in thin section), occurrence of only spinel or hibonite-rich types, and presence of primary Ti(±V)-oxides, and secondary geikelite and Ti,Fe-sulfides distinguish the assemblage of EC CAIs from other groups.

The primary mineral assemblage in EC CAIs is devoid of indicators (e.g. oldhamite, osbornite) of low oxygen fugacities.  Thus, high-temperature processing of the CAIs did not occur under the reducing conditions characteristic of ECs, implying that either:  (1) the CAIs are foreign to EC-forming regions, or (2) oxygen fugacities fluctuated within the EC-forming region.  In contrast, secondary geikelite and Ti,Fe-sulfide, which replace perovskite, indicate that alteration of perovskite occurred under reducing conditions distinct from CAIs in the other chondrite groups.  We have not ascertained whether the reduced alteration of EC CAIs occurred in a nebular or parent body setting.  We conclude that each chondrite group is correlated with a unique assemblage of CAIs, indicating spatial or temporal variations in physical conditions during production and/or dispersal of CAIs.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Origin of lunar high-Ti ultramafic glasses: Constraints from phase relations and dissolution kinetics of clinopyroxene-ilmenite cumulates

James A. Van Orman* and Timothy L. Grove

*Correspondence author's address:  Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA; e-mail address:  javanorm@mit.edu

Abstract–Phase equilibrium and dissolution kinetics experiments on synthetic late stage magma ocean cumulates are used to place constraints on hypotheses for the origin of lunar high-Ti ultramafic glasses. Models for the production of high-Ti lunar magmas have called for either (1) assimilation of late stage clinopyroxene-ilmenite cumulates at shallow levels or (2) sinking of cpx-ilmenite cumulates to form a hybrid mantle source. To satisfy the constraints of our experiments, we propose an alternative model that involves shallow level reaction and mixing of cumulates, followed by sinking of hybrid high-Ti materials. This model can fulfill compositional requirements imposed by the pristine lunar glass suite that are difficult to satisfy in assimilation models. It also avoids difficulties that arise in overturn models from the low solidus temperatures of cpx-ilmenite cumulates. Partially molten cpx-ilmenite cumulates become gravitationally unstable with respect to underlying mafic cumulates only when they have cooled to within about 30 °C of their solidus (~1125 °C at 100 km depth), and at these temperatures the viscosity of mafic cumulates is too high to allow for growth and descent of cpx-ilmenite diapirs on the appropriate time scale.  Reaction and mixing between late stage liquids and mafic cumulates at shallow levels would produce a refractory hybrid material that is negatively buoyant at higher temperatures, and could sink more efficiently to the depths inferred for production of high-Ti ultramafic glasses.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Metamorphic diogenite GRO 95555:  Mineral chemistry of orthopyroxene and spinel and comparisons to the diogenite suite

J. J. Papike*, C. K. Shearer, M. N. Spilde and J. M. Karner

*Correspondence author's address:  Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-1126, USA; e-mail address:

Abstract–GRO 95555 is a relatively newly discovered and unique metamorphic diogenite.  It does not show the usual brecciated appearance of other diogenites or wide compositional variability of orthopyroxene or spinel.  EMP and SIMS analysis of orthopyroxene and EMP analysis of spinel show limited compositional variability and compositions for orthopyroxene for Fe/(Fe+Mg) atomic, Al, Zr, Y, and Yb fall in the middle of the compositional ranges of the diogenite suite.  Apparently GRO 95555 formed at sufficient depth and/or at a location to escape brecciation by meteorite bombardment and in a thermal regime that led to high-grade metamorphism resulting in homogeneous mineral compositions.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Major and trace element compositions of georgiaites:  Clues to the source of North American tektites

Edward F. Albin, Marc D. Norman and Michael Roden*

*Correspondence author's address:  Department of Geology, University of Georgia, Athens Georgia 30602, USA; e-mail address: mroden@uga.edu

Abstract–Electron microprobe and laser ablation ICPMS analyses of 24 georgiaites show that these tektites are all Si-rich (79–83% wt. SiO₂) glasses with variable major and trace element abundances  (e.g., FeO varies from 2.1 to 3.7 wt%). Glass compositions are similar to, but not identical with average upper continental crust.  For example, georgiaites are LREE-enriched with small negative Eu anomalies (Eu/Eu* = 0.73–0.86) and La-Th-Sc systematics are intermediate between that of Archean and post-Archean continental crust. When the georgiaite data are placed in the context of data for all North American tektites, triangular arrays appear on some oxide-oxide plots (e.g., FeO-MgO).  Large variations in refractory element abundances and  ratios compared to the variation in SiO₂ favors mixing over volatilization as a cause of the compositional variation.  If all the tektites formed as a result of a single impact, then triangular arrays in oxide-oxide variation diagrams require at least three source components.  These components include a Si-rich material, probably a quartz-rich sand which was predominant in the formation of georgiaites.  Two relatively silica-poor and iron-rich components have compositional characteristics similar to shales and greywackes.  The La-Th-Sc systematics of the georgiaites and most other North American tektites are distinctive and could potentially be used to link the tektites to Eocene sediments at the Chesapeake Bay impact structure.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Crystallization of magmatic iron meteorites: The effects of phosphorus and liquid immiscibility

Nancy L. Chabot* and Michael J. Drake

*Correspondence author's address:  Mail Code SN2, NASA Johnson Space Center, Houston, Texas 77058, USA; e-mail address:  nchabot@ems.jsc.nasa.gov

Abstract–Magmatic iron meteorites are commonly thought to have formed by fractional crystallization of the metallic cores of asteroid-sized bodies. As fractional crystallization proceeds, light elements such as P and S become enriched in the molten portion of the core. The light element content of the metallic liquid influences the partitioning behavior of trace elements, and may cause liquid immiscibility to occur. The elemental trends observed in magmatic iron meteorites may have been affected by both of these processes.

We have examined experimentally the effect of P on the solid metal/liquid metal partitioning behavior of Ag and Pd, Re and Os, two element pairs used to date iron meteorite processes. Phosphorus has no effect on the partition coefficient of either Ag or Pd, which are incompatible and identical within experimental error. Compatible Re and Os also have identical partitioning behavior, within experimental error, and show increasing compatibility in the solid metal with increasing P-content of the metallic liquid. Including the effects of both S and P on the partitioning behavior of Re and Os, simple fractional crystallization calculations can reproduce the large variation of Re and Os concentrations observed in four magmatic iron meteorite groups but have difficulty matching the later crystallizing portions of the trends.

We have also conducted experiments with three phases, solid metal and two immiscible metallic liquids, to determine the location of the liquid immiscibility field near conditions thought to be relevant to magmatic iron meteorites. Our results show a significantly smaller liquid immiscibility field as compared to the previously published Fe-P-S phase diagram. Our revised phase diagram suggests that liquid immiscibility was encountered during the crystallization of asteroidal cores, but much later during the crystallization process than predicted by the previously published diagram.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

A clast of Bali-like oxidized CV material in the reduced CV chondrite breccia Vigarano

Alexander N. Krot*, Anders Meibom and Klaus Keil

*Correspondence author's address:  Hawai'i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai'i at Manoa, Honolulu, Hawai'i 96822, USA; e-mail address:  sasha@pgd.hawaii.edu

Abstract–The CV (Vigarano-type) chondrites are a petrologically diverse group of meteorites that are divided into the reduced and the Bali-like and Allende-like oxidized subgroups largely based on secondary mineralogy (Weisberg et al., 1997; Krot et al., 1998b).  Some chondrules and CAIs in the reduced CV chondrite Vigarano show alteration features similar to those in Allende:  metal is oxidized to magnetite; low-Ca pyroxene, forsterite, and magnetite are rimmed and veined by ferrous olivine (Fs_40–50), and plagioclase mesostases and melilite are replaced by nepheline and sodalite (Sylvester et al., 1993; Kimura and Ikeda, 1996, 1997, 1998).

Our petrographic observations indicate that Vigarano also contains individual chondrules, chondrule fragments and lithic clasts of the Bali-like oxidized CV materials. The largest lithic clast, ~1 x 2 cm in size, is composed of opaque matrix, type I chondrules (400–2000 µm in apparent diameter) surrounded by coarse-grained and fine-grained rims, and rare CAIs.  The matrix/chondrule ratio is ~1.1.  Opaque nodules in chondrules in the clast consist of Cr-poor and Cr-rich magnetite, Ni- and Co-rich metal, Ni-poor and Ni-rich sulfide; low-Ni metal nodules occur only inside chondrule phenocrysts.  Cr-poor magnetite is preferentially replaced by fayalite.  Chondrule mesostases are replaced by phyllosilicates; low-Ca pyroxene and olivine phenocrysts appear to be unaltered.  Matrix in the clast consists of very fine-grained (<1 µm) ferrous olivine, anhedral fayalite grains (Fa_80–100), rounded objects of porous Ca-Fe-rich pyroxenes (Fs_10–50Wo₅₀), Ni-poor sulfide, Ni- and Co-rich metal, and phyllosilicates; magnetite is rare.

Based on the presence of the Bali-like lithified chondritic clast, in addition to individual chondrules and CAIs of both Bali-like and Allende-like materials, in the reduced CV chondrite Vigarano, we infer that (1) all three types of materials were mixed during regolith gardening on the CV asteroidal body, and (2) the reduced and oxidized CV materials may have originated from a single, heterogeneously-altered asteroid.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

A new Martian meteorite from the Sahara:  The shergottite Dar al Gani 489

L. Folco*, I. A. Franchi, M. D'Orazio, S. Rocchi and L. Schultz

*Correspondence author's address:  Museo Nazionale Antartide, Via Laterina 8, I-53100 Siena, Italy; e-mail address: folco@unisi.it

Abstract–Dar al Gani 489 (DaG 489) is a meteorite fragment of 2146 g found in the Libyan Sahara by a meteorite finder during one of his search campaigns in 1997–98.  It is a porphyritic rock with mm-sized olivine crystals (Fo_79–59) set in a fine-grained groundmass (avg. grain size 0.1 mm) consisting of pigeonite (En_75–57 Wo_5–15) crystals and interstitial feldspathic glass (An_67–56 Or_0–1).  Minor phases include enstatite (En_82–71 Wo_2–4), augite (En_48–52 Wo_29–32), chromite, Ti-chromite, ilmenite, pyrrhotite, merrillite and secondary calcite and iron oxides.  Based on mineralogical, petrographic, bulk chemical, oxygen isotope and noble gases data, DaG 489 can be classified as a highly shocked Martian meteorite (e.g., Fe/Mn_(bulk) = 42.1, Ni/Mg_(bulk) = 0.002; delta ¹⁷O = 2.89, delta ¹⁸O = 4.98 and DELTA ¹⁷O = 0.305), belonging to the basaltic shergottite subgroup.

The texture and modal composition of DaG 489 are indeed those of basalts; nonetheless, the bulk chemistry, the abundance of large olivine and chromite crystals, and enstatitic pyroxene suggest some relationship with lherzolitic shergottites. As such, DaG 489 is similar to the hybrid shergottite EETA79001 lithology A; however there are some relevant differences including a higher olivine content (20 vol%), the lack of orthopyroxene megacrysts, a higher molar Mg/(Mg+Fe)_(molar) = 0.68 and a lower REE content in the bulk sample. Therefore DaG 489 has the potential of providing us with a further petrogenetic link between basaltic and lherzolitic shergottites.

Noble gases data show that DaG 489 has an ejection age of ~1.3 Ma. This young age lends support to the requirement of several ejection events to produce the current population of SNC meteorites.

In terms of texture, mineral and bulk compositions, shock level and weathering features DaG 489 is essentially identical to Dar al Gani 476 (DaG 476), another basaltic shergottite independently found about 25 km due NNE of DaG 489. Since DaG 489 also has the same exposure history as DaG 476, it is very likely that both meteorites are fragments of the same fall.

In addition to the existing hypotheses on the petrogenesis of the similar EETA79001 lithology A and the identical DaG 476, we propose that DaG 489 could have formed through high-degree partial melting of a lherzolite-like material.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Reaction between H₂, CO and H₂S over Fe,Ni metal in the solar nebula:  Experimental evidence for the formation of sulfur-bearing organic molecules and sulfides

Jordi Llorca* and Ignasi Casanova

*Correspondence author's address: Institut d'Estudis Espacials de Catalunya, Edifici Nexus, Gran Capità, 2-4, E-08034 Barcelona, Spain; e-mail address: jllorca@kripto.qui.ub.es

Abstract–Detailed laboratory studies have been carried out in order to simulate the interaction between nanometer-sized kamacite metal particles and different gas mixtures consisting of H₂:H₂S (250:0.1), H₂:CO (250:1) and H₂:CO:H₂S (250:1:0.1) under nebular-type conditions (5 x 10^–4 atm and 473 K). Reaction of H₂ + H₂S with kamacite particles for 1000 h leads to the formation of pyrrhotite. Incorporation of CO into the gaseous reactant mixture results in the formation of both sulfide and carbide phases. At the same time, amorphous carbon is deposited onto the metal particles and organic molecules are evolved, namely hydrocarbons and thiols in the C₁-C₅ and C₁-C₂ range, respectively. Carbon deposition and production of organics are enhanced with respect to experiments performed with H₂ + CO, where a carbide phase is formed. There is no evidence for the existence of sulfur poisoning effects on the metal-catalysed hydrogenation of CO through Fischer-Tropsch-type reactions in nebular environments. In fact, it is experimentally demonstrated that sulfur-containing organic species could be synthesized by such reactions from nebular gas.

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

The lack of potassium isotopic fractionation in Bishunpur chondrules

C. M. O'D. Alexander*, J. N. Grossman, J. Wang, B. Zanda, M. Bourot-Denise and R. H. Hewins

*Correspondence author's address: Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, D. C. 20015-1305, USA; e-mail address: alexande@dtm.ciw.edu

Abstract–In a search for evidence of evaporation during chondrule formation, the mesostases of eleven Bishunpur chondrules, and melt inclusions in olivine phenocrysts in seven of them have been analyzed for their alkali element abundances and K isotopic compositions.  Except for six points, all areas of the chondrules that were analyzed had delta⁴¹K compositions that were normal within error (typically ±3‰, 2 sigma).  The six 'anomalous' points are probably all artifacts. Experiments have shown that free evaporation of K leads to large ⁴¹K enrichments in the evaporation residues, consistent with Rayleigh fractionation.  Under Rayleigh conditions, a 3‰ enrichment in delta⁴¹K is produced by about a 12% loss of K.  The range of L-chondrite normalized K/Al ratios (a measure of the K elemental fractionation) in the areas analyzed vary by almost three orders of magnitude. If all chondrules started out with L-chondrite-like K abundances and the K loss occurred via Rayleigh fractionation, the most K depleted chondrules would have had compositions of up to delta⁴¹K ~ 200‰. Clearly, K fractionation did not occur by evaporation under Rayleigh conditions. Yet experiments and modeling indicate that K should have been lost during chondrule formation under currently accepted formation conditions (peak temperature, cooling rate, etc.). Invoking precursors with variable alkali abundances to produce the range of K/Al fractionation in chondrules does not explain the K isotopic data because any K that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation. If K loss and isotopic fractionation was inevitable during chondrule formation, the absence of K isotopic fractionation in Bishunpur chondrules requires that they exchanged K with an isotopically normal reservoir during or after formation. There is evidence for alkali exchange between chondrules and rim/matrix in all UOCs. However, melt inclusions can have alkali abundances that are much lower than the mesostases of the host chondrules, suggesting that they at least remained closed since formation. If it is correct that some or all melt inclusions remained closed since formation, the absence of K isotopic fractionation in them requires that the K isotopic exchange took place during chondrule formation, which would probably require gas-chondrule exchange. Potassium evaporated from fine grained dust and chondrules during chondrule formation may have produced sufficient K vapor pressure for gas-chondrule isotopic exchange to be complete on the timescales of chondrule formation. Alternatively, our understanding of chondrule formation conditions based on synthesis experiments needs some re-evaluation.
 

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Oxygen isotopes in chondrule olivine and isolated olivine grains from the CO3 chondrite, ALHA77307

Rhian H. Jones*, John M. Saxton, Ian C. Lyon and Grenville Turner

*Correspondence author's address:  Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA; e-mail address: rjones@unm.edu

Abstract–We have measured oxygen isotope ratios in a variety of olivine grains in the CO3 chondrite, ALHA77307, using secondary ion mass spectrometry, in order to study the chondrule formation process and the origin of isolated olivine grains in unequilibrated chondrites.  Oxygen isotope ratios of olivines in this chondrite are variable, from delta¹⁷O = –15.5‰ to +4.5‰ and delta¹⁸O = –11.5‰ to +3.9‰, with delta¹⁷O varying from –10.4 to +3.5‰. Forsteritic olivines, Fa<1, are enriched in ¹⁶O relative to the bulk chondrite, while more FeO-rich olivines are more depleted in ¹⁶O. Most ratios lie close to the CCAM line with negative values of DELTA¹⁷O, although one grain of composition Fa₄ has a mean DELTA¹⁷O of +1.6‰. Marked oxygen isotopic heterogeneity within one FeO-rich chondrule is the result of incorporation of relict, ¹⁶O-rich, Mg-rich grains into a more ¹⁶O-depleted host. Isolated olivine grains, including isolated forsterites, have similar O isotope ratios to olivine in chondrules of corresponding chemical composition. This is consistent with derivation of isolated olivine from chondrules, as well as the possibility that isolated grains are chondrule precursors. The high ¹⁶O in forsteritic olivine is similar to that observed in forsterite in CV and CI chondrites and the ordinary chondrite Julesburg, and suggests nebula-wide processes for the origin of forsterite which appears to be a primitive nebular component.
 

Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.

Terrestrial age measurements using natural thermoluminescence of a drained zone under the fusion crust of Antarctic ordinary chondrites

Jannette M. C. Akridge, Paul H. Benoit* and Derek W. G. Sears

*Correspondence author's address:  Cosmochemistry Group, Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, USA; e-mail address: pbenoit@comp.uark.edu

Abstract–Miono et al. (1990) and Miono and Nakanishi, (1994) have proposed that the build-up of natural TL in a drained layer directly below the meteorite fusion crust can be used to determine terrestrial ages of meteorites in the 40 to 200 ka range. We have measured the natural TL of the drained layer of 15 meteorites.  The data indicate that this technique could be used to determine terrestrial ages of meteorites with ages <200 ka, after which TL equilibrium is reached.  Comparison of TL build-up with terrestrial ages for a suite of Antarctic meteorites suggests that the meteorites have been exposed to temperatures of 0 to 5 °C.  The close correspondence between natural TL levels and surface exposure TL growth curves suggest that Allan Hills meteorites with ages < 200 ka have spent a significant portion of their terrestrial history exposed on the ice surface, rather than being buried in the ice sheet.  The technique is, however, sensitive to thermal history and, for Antarctic meteorites with terrestrial ages <200 ka, natural TL of the drained zone largely reflects exposure on the ice surface.
 

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