Abstracts of Papers to be Published in the May 2000 Issue

Please contact the correspondence author for reprints of all published articles

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

The iodine-xenon system in clasts and chondrules from ordinary chondrites:  Implications for early solar system chronology

J. D. Gilmour*, J. A. Whitby, G. Turner, J. C. Bridges and R. Hutchison

*Correspondence author's address: Department of Earth Sciences, University of Manchester, Manchester M13 9PL, United Kingdom; e-mail address: Jamie.Gilmour@man.ac.uk

Abstract–We have studied the iodine-xenon system in chondrules and clasts from ordinary chondrites.  Cristobalite bearing clasts from Parnallee (LL3.6) closed to xenon loss 1-4 Ma after Bjurböle. Feline (a feldspar and nepheline rich clast also from Parnallee) closed at 7.04 ± 0.15 Ma.  Two out of three chondrules from Parnallee that yielded well defined initial iodine ratios gave ages identical to Bjurböle's within error.  A clast from Barwell (L5) has a well-defined initial iodine ratio corresponding to closure 3.62 ± 0.60 Ma before Bjurböle.

Partial disturbance and complete obliteration of the I-Xe system by shock are revealed in clasts from Julesburg (L3.6) and Quenggouk (H4) respectively.  Partial disturbance by shock is capable of generating anomalously high initial iodine ratios.  In some cases these could be misinterpreted, yielding erroneous ages.  A macrochondrule from Isoulane-n-Amahar contains concentrations of iodine similar to 'ordinary' chondrules but, unlike most ordinary chondrules, contains no radiogenic ¹²⁹Xe.  This requires resetting 50 Ma or more later than most chondrules.

The earliest chondrule ages in the I-Xe, Mn-Cr and Al-Mg systems are in reasonable agreement.  This, and the frequent lack of evidence for metamorphism capable of resetting the I-Xe chronometer, leads us to conclude that (at least) the earliest chondrule I-Xe ages represent formation.  If so, chondrule formation took place at a time when sizeable parent bodies were present in the solar system.

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

An experimental study on kinetically-driven precipitation of Ca-Mg-Fe carbonates from solution:  Implications for the low temperature formation of carbonates in martian meteorite Allan Hills 84001

D. C. Golden, D. W. Ming*, C. S. Schwandt, R. V. Morris, S. V. Yang and G. E. Lofgren

*Correspondence author's address: Mail Code SN2, NASA Johnson Space Center, Houston, Texas  77058, USA; e-mail address: douglas.w.ming1@jsc.nasa.gov

Abstract–Spherical carbonate globules of similar composition, size, and radial Ca-, Mg- and Fe-zonation to those in Martian meteorite ALH84001 were precipitated from Mg-rich, supersaturated solutions of Ca-Mg-Fe-CO₂-H₂O at 150 °C.  The supersaturated solutions (pH = 6-7) were prepared at room temperature and contained in Teflon^TM-lined stainless steel vessels, which were sealed and heated to 150 °C for 24 hr.  Experiments were also conducted at 25 °C and no globules comparable to those of ALH84001 were precipitated.  Instead, amorphous Fe-rich carbonates were formed after 24 hours and Mg-Fe calcites formed after 96 hours. These experiments suggest a possible low-temperature, inorganic origin for the carbonates in Martian meteorite ALH84001.

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

Bleached chondrules:  Evidence for widespread aqueous processes on the parent asteroids of ordinary chondrites

Jeffrey N. Grossman*, Conel M. O'D. Alexander, Jianhua Wang and Adrian J. Brearley

*Correspondence author's address:  US Geological Survey, 954 National Center, Reston, Virginia 20192, USA; e-mail address:  jgrossman@usgs.gov

Abstract–We present the first detailed study of a population of texturally distinct chondrules previously described by Kurat (1969), Christophe Michel-Lévy (1976) and Skinner et al. (1989) that are sharply depleted in alkalis and Al in their outer portions. These "bleached" chondrules, which are exclusively radial pyroxene (RP) and cryptocrystalline (C) in texture, have porous outer zones where mesostasis has been lost. Bleached chondrules are present in all type-3 ordinary chondrites, and are present in lower abundances in types 4-6. They are most abundant in the L and LL groups, apparently less common in H chondrites, and absent in enstatite chondrites.  We used x-ray mapping and traditional electron microprobe techniques to characterize bleached chondrules in a cross-section of ordinary chondrites.  We studied bleached chondrules from Semarkona by ion microprobe for trace elements and hydrogen isotopes, and by transmission electron microscopy.  Chondrule bleaching was the result of low-temperature alteration by aqueous fluids flowing through fine-grained chondrite matrix prior to thermal metamorphism.  During aqueous alteration, interstitial glass dissolved and was partially replaced by phyllosilicates, troilite was altered to pentlandite, but pyroxene was completely unaffected.  Ca-rich zones formed at the inner margins of the bleached zones, either as the result of the early stages of metamorphism or due to fluid-chondrule reaction.  The mineralogy of bleached chondrules is extremely sensitive to thermal metamorphism in type 3 ordinary chondrites, and bleached zones provide a favorable location for the growth of metamorphic minerals in higher petrologic types.  The ubiquitous presence of bleached chondrules in ordinary chondrites implies that they all experienced aqueous alteration early in their asteroidal histories, but there is no relationship between the degree of alteration and metamorphic grade.  A correlation between the oxidation state of chondrite groups and their degree of aqueous alteration is consistent with the source of water being either accreted ices or water released during oxidation of organic matter.  Ordinary chondrites were probably open systems after accretion, and aqueous fluids may have carried volatile elements with them during dehydration.  Individual radial pyroxene and cryptocrystalline chondrules were certainly open systems in all chondrites that experienced aqueous alteration leading to bleaching.

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

Mineralogical evidence for the origin of diamond in ureilites

Y. Nakamuta* and Y. Aoki

*Correspondence author's address:  Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Fukuoka, 812-8581 Japan; e-mail address:  nakamuta@geo.kyushu-u.ac.jp

Abstract–The X-ray powder diffraction patterns of 50–100 µm carbon-rich grains from five ureilitic meteorites, Kenna, ALH-78019, Y-82100, Y-791538 and ALH-77257, were obtained by using a Gandolfi camera.   The results reveal that the basal spacing of part of the graphite coexisting with diamond is slightly smaller compared to the normal spacing.   Compressed graphite is experimentally known to occur at the initial stage of the direct transformation from graphite to diamond structures at high pressures and temperatures.   The presence of the compressed graphite in ureilites, therefore, gives clear evidence that the diamond formed by high-pressure conversion of graphite.   The modes of occurrence of carbon minerals observed with reflected light through an optical microscope reveal that graphite coexisted with olivine and pyroxene during igneous or metamorphic processes and, furthermore, that part of the graphite was converted to diamond by impact.   The relative X-ray intensity of diamond to graphite increases in the following order: ALH-78019 and Y-82100 < Y-791538 < Kenna < ALH-77257, and correlates with the shock level which is estimated mainly based on the shock features of silicates.   Therefore, the relative amounts of diamond to graphite suggested by X-ray intensities may be useful as a measure of the degree of shock.

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

Noble gases in interplanetary dust particles, I: The excess ³He problem, and estimates of the relative fluxes of solar wind and solar energetic particles in interplanetary space

R. O. Pepin*, R. L. Palma and D. J. Schlutter

*Correspondence author's address:  School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA; e-mail address:  pepin001@maroon.tc.umn.edu

Abstract–We report mass-spectrometric measurements of light noble gases pyrolytically extracted from 28 interplanetary dust particles (IDPs) and discuss these new data in the context of earlier analyses of 44 IDPs at Minnesota. The noble gas data base for IDPs is still very sparse, especially given their wide mineralogic and chemical variability, but two intriguing differences from isotopic distributions observed in lunar and meteoritic regolith grains are already apparent. First are puzzling overabundances of ³He, manifested as often strikingly elevated ³He/⁴He ratios—up to >40 times the solar wind value—and found primarily but not exclusively in shards of some of the larger IDPs ("cluster particles") that fragmented on impact with the collectors carried by high-altitude aircraft. It is difficult to attribute these high ratios to ³He production by cosmic ray-induced spallation during estimated space residence times of IDPs, or by direct implantation of solar-flare He. Minimum exposure ages inferred from the ³He excesses range from ~50 Ma to an impossible >10 Ga, compared to Poynting-Robertson drag lifetimes for low-density 20–30 µm particles on the order of ~0.1 Ma for an asteroidal source and ~10 Ma for origin in the Kuiper belt. The second difference is a dominant contribution of solar-energetic-particle (SEP) gases, to the virtual exclusion of solar wind (SW) components, in several particles scattered throughout the various data sets but most clearly and consistently observed in recent measurements of a group of individual and cluster IDPs from three different collectors. Values of the SEP/SW fluence ratio in interplanetary space from a simple model utilizing these data are ~1% of the relative SEP/SW abundances observed in lunar regolith grains, but still factors of ~10–100 above estimates for this ratio in low-energy solar particle emission.

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

The South Range Breccia Belt of the Sudbury Impact Structure:  A possible terrace collapse feature

Ronald G. Scott* and John G. Spray

*Correspondence author's address: Ottawa-Carleton Geoscience Center and Dept. of Earth Sciences, University of Ottawa, 140 Louis Pasteur, Ottawa, Ontario, K1N 6N5, Canada; e-mail address: rscott@science.uottawa.ca

Abstract–The South Range Breccia Belt (SRBB) is an arcuate, 45 km long zone of Sudbury Breccia in the South Range of the 1.85 Ga Sudbury Impact Structure. The belt varies in thickness between tens of meters to hundreds of meters, and is composed of a polymict assemblage of Huronian Supergroup (2.49–2.20 Ga), Nipissing Diabase (2.2 Ga) and Proterozoic granitoid  breccia fragments ranging in size from a few millimeters to tens of meters. The SRBB matrix is composed of a fine-grained (~100 µm) assemblage of biotite, quartz and ilmenite, with trace amounts of plagioclase, zircon, titanite, epidote, pyrite, chalcopyrite, pyrrhotite and occasionally chlorite. The SRBB  hosts the Frood-Stobie, Vermilion and Kirkwood quartz diorite Offset Dykes, the former being associated with one of the largest Ni-Cu-PGE sulphide deposits in the world.

Optical petrography and whole rock geochemistry concur with previous studies that have suggested that the matrix of the SRBB is derived from comminution and at least partial frictional melting of the wall rock Huronian Supergroup lithologies. REE data from all sampled lithologies associated with the SRBB exhibit crustal signatures when normalized to C1 chondrite values. Additionally, REE data from the quartz diorites, disseminated sulphides in Sudbury Breccia and a sample of an aphanitic biotite-hornblende tonalite dyke exhibit flat slopes when compared to the mafic and felsic norites, quartz gabbro and granophyre units of the Sudbury Igneous Complex (SIC), suggesting that these lithologies are representative of bulk SIC melt.

We suggest that the SRBB was formed by high strain-rate (> 1 m/s), gravity driven seismogenic slip of the inner ring of the Sudbury Impact Structure during post-impact crustal re-adjustment (crater modification stage). Failure of the hanging wall may have facilitated the injection of bulk SIC melt into the SRBB, along with the Ni-Cu-PGE sulfides of the Frood-Stobie deposit. Post-impact Penokean (1.9-1.7 Ga) tectonism, particularly north-west directed shearing along the South Range Shear Zone (SRSZ) and associated thrust faulting could account for the present sub-vertical orientation of the SRBB, and the apparent lack of a connection at depth with the SIC.

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

Aspects of the validation of magnetic remanence in meteorites

Peter Wasilewski* and Tamara Dickinson

*Correspondence author's address: Code 691 NASA Goddard Space Flight Center, Greenbelt, Maryland 20771

Abstract–Meteorite magnetic records constitute physical evidence of processes acting during early Solar System evolution.  Consequently the validation of these records is important in meteorite research.  The first step in the validation process should be the REM value.  The REM value is the ratio of NRM (natural remanence) to SIRM (saturation remanent magnetization imparted by a 1 Tesla magnetic field).  The REM values range  over 3 ~ 4 orders of magnitude for stony meteorites and for chondrules from Allende(C3V-S1), Bjurbole (L4-S1) and Chainpur (LL3-S1) meteorites.
 REM values computed from published NRM and SIRM data, identify many orders of magnitude range in the REM values including REM values > 100 e-3.  These data suggest a dependence for the NRM intensity on the curatorial location from which the sample was obtained.  Any Earth rock acquiring TRM in the geomagnetic field has a restricted range in REM mostly between 5 and 50 e-3, the exception  being the mineral hematite in the multidomain size range.  The only terrestrial samples with REM much greater than 100 e-3 are those struck by lightning.
 The REM value provides a physical basis for recognition between valid records and those that "might be contaminated".  The isothermal remanence acquisition (RA) curve is presented as a contamination curve which allows an indication of the level of magnetic field contamination required to give the computed "REM" (RM/SIRM) value.  In the case of the Bjurbole and Chainpur chondrules, with REM values >100 e-3, the RA curve indicates that unrealistically large contamination magnetic fields would be required to give REM values >100 e-3.  This would suggest other than contamination by a hand magnet that is normally available to the experimenter.  This would require an explanation that would involve large magnetic fields during chondrule formation, or some extraordinary remanence acquisition mechanism that remains to be described.
 Magnetic contamination experiments, using ~80 mT and ~40 mT magnets, demonstrate that the "REM" values and extent of modification of the magnetic vector record are mineralogy dependent and this is mostly related to the amount and characteristics of the mineral tetrataenite.  The complexity of the meteorite records suggest validation of the record as a first step.  The REM value is the first physical statement that can be made in this validation

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

Shock experiments with the H6 chondrite Kernouvé:  Pressure calibration of microscopic shock effects

R. T. Schmitt

Author's address:  Institut für Mineralogie, Museum für Naturkunde, Humboldt-Universität zu Berlin, Invalidenstr. 43, D-10115 Berlin, Germany; e-mail address: ralf-thomas.schmitt@rz.hu-berlin.de

Abstract–Shock recovery experiments were carried out on samples of the H6 chondrite Kernouvé at shock pressures of 10, 15, 20, 25, 30, 35, 45 and 60 GPa and preheating temperatures of 293 K (low-temperature experiments) and 920 K (high-temperature experiments), respectively. Using a calculated equation of state of Kernouvé, pressure pulse durations of 0.3 to 1.2 µs were estimated. The shocked samples were investigated by optical microscopy to calibrate the various shock effects in olivine, orthopyroxene, oligoclase and troilite.

 The following pressure calibration is proposed for silicates: (1) undulatory extinction of olivine <15 GPa, (2) weak mosaicism of olivine from 10–15 GPa to 20–25 GPa, (3) onset of strong mosaicism of olivine at 20–25 GPa, (4) transformation of oligoclase to diaplectic glass completed at 25–30 GPa (low-temperature experiments) and at 20–25 GPa (high-temperature experiments), respectively, (5) onset of weak mosaicism in orthopyroxene is at 30–35 GPa (low-temperature experiments) and at 25–30 GPa (high-temperature experiments), respectively, and (6) recrystallization or melting of olivine starting at 45–60 GPa (low-temperature experiments) and at 35–45 GPa (high-temperature experiments), respectively, and completed above 45–60 GPa in the high-temperature experiments.

 Troilite displays distinct differences between the samples shocked at low and high temperatures. In the low-temperature experiments the following effects can be observed in troilite: (1) undulatory extinction up to 25 GPa, (2) twinning up to 45 GPa, (3) partial recrystallization from 30 GPa to 60 GPa and (4) complete recrystallization above 35 GPa, whereas in the high-temperature experiments troilite shows (1) complete recrystallization from 10 GPa up to 45 GPa and (2) melting and crystallization above 45 GPa.

 Localized shock-induced melting is observed in samples shocked to pressures above 15 GPa in the high-temperature experiments and above 30 GPa for the low-temperature experiments, respectively, in the form of FeNi metal and/or troilite melt injections and intergrowths, and as pockets and veins of whole rock melt. Obviously, the onset and abundance of shock-induced localized melting strongly depends on the initial temperature of the sample.

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

Petrology of unique achondrite Queen Alexandra Range 93148:  A piece of the pallasite (HED?) parent body?

Cyrena Anne Goodrich* and Kevin Righter

*Correspondence author's address:  Max-Planck-Institut für Chemie, Abteilung Kosmochemie, PO 3060, 55020 Mainz, Germany; e-mail address:  goodrich@mpch-mainz.mpg.de

Abstract–QUE 93148 is a small (1.1 g) olivine-rich achondrite (mg 86) that contains variable amounts of orthopyroxene (mg 87) and kamacite (6.7 wt.% Ni), with minor augite.  Olivine in QUE 93148 contains an unusual suite of inclusions:  1) 5 x 100 µm-sized lamellae with a CaO- and Cr₂O₃-rich (~10 and 22 wt.%, respectively) composition that may represent a submicrometer-scale intergrowth of chromite and pyroxene(s);  2) 75 x 500 µm-sized lamellar symplectites comprised of chromite and two pyroxenes, with minor metal;  3) 15-20 µm-sized, irregularly-shaped symplectites comprised of chromite and pyroxene(s);  4) 100-150 µm-sized, elliptical inclusions comprised of chromite, two pyroxenes, metal, troilite, and rare whitlockite.  Type 1, 2 and 3 inclusions probably formed by exsolution from the host olivine during slow cooling, whereas type 4 more likely resulted from early entrapment of silicate and metallic melts followed by closed-system oxidation.

QUE 93148 can be distinguished from most other olivine-rich achondrites (ureilites, winonaites, lodranites, acapulcoites, brachinites, Eagle Station-type pallasites and pyroxene pallasites), as well as from mesosiderites, by some or all of the following properties:  oxygen isotopic composition, Fe-Mn-Mg relations of olivine, CaO and Cr₂O₃ contents of olivine, orthopyroxene compositions, molar Cr/(Cr+Al) ratios of chromite, metal composition, texture, and the presence of the inclusions.  In terms of many of these properties, it shows an affinity to main group pallasites.  Nevertheless, it cannot be identified as belonging to this group.  QUE 93148 appears to be a unique achondrite.  Possibly it should be considered to be a pyroxene pallasite that is genetically related to main group pallasites.  Alternatively, it may be derived from the mantle of the pallasite (HED?) parent body.

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

Bo Xian (LL3.9):  Oxygen isotopic and mineralogical characterisation of separated chondrules

Chunlai Li, John C. Bridges*, Robert Hutchison, Ian A. Franchi, Arabelle S. Sexton, Ziyuan Ouyang and Colin T. Pillinger

*Correspondence author's address: Department of Mineralogy, Natural History Museum, Cromwell Road, London, SW7 5BD, UK; e-mail address:  jcb@nhm.ac.uk

Abstract–We report the results of a mineralogical and oxygen isotopic study of 362 chondrules disaggregated from the Bo Xian chondrite.  The range of mineral compositions (Fa 0.8–31.2%, mean = 23.5%, mode = 27–28%) are consistent with a reclassification of this meteorite from LL4 to LL3.9.  Chondrule diameters range from 0.20 to 3.40 mm (mean = 0.74 mm) in the disaggregated population.  A lower mean diameter (0.64 mm) calculated from thin section measurements partly reflects the high proportion of chondrule fragments.  The chondrule size distribution, which is approximately log-normal, is consistent with size sorting mechanisms.  This sorting could be linked to the fragmentation of many chondrules on the parent body.  However, in detail, the variation in diameter of different chondrule types and a hiatus in the size distribution at 0.6 mm indicate that there may have been complex controls perhaps partly being determined by the chondrule formation mechanism.  Seven percent of the sectioned chondrules (102) contain chemically fractionated mineral assemblages - cristobalite-bearing and Al-rich.  This significant degree of chemical heterogeneity probably resulted from both igneous and volatility controls.

Oxygen isotope compositions were determined on mineral separates and 16 of the sectioned chondrules.  Three separate isotopic exchange events have been identified.  The dominant one is a low temperature hydrous gas-solid exchange event between ¹⁶O-rich solid and ¹⁶O-poor gas reservoirs which lay along a slope 1.0 line on three-isotope plots.  Partial equilibration with the gas by feldspar and cristobalite, which exchanged more rapidly than olivine or pyroxene, led to formation of a slope 0.77 mixing line for Bo Xian and other LL chondrites.  Mineralogy is the dominant control on the extent of this exchange, no relationship between isotopic composition and chondrule texture or size was identified.  The feldspar separate and cristobalite-rich chondrules have the most ¹⁶O-poor compositions.  Subsequently, thermal metamorphism in the parent body led to partial isotopic equilibration between the different mineral phases.   A third exchange event, predating the other 2, is probably shown by one of the Al-rich chondrules.  This has an ¹⁶O-rich composition, lying below the terrestrial fractionation line.  Another Al-rich chondrule has a normal OC isotopic composition.  It is not clear whether the isotopic fractionation recorded in some Al-rich chondrules can be achieved by the dominant gas-solid exchange.  Instead, the precursor oxygen to the mineral phases may have become ¹⁶O-rich during an earlier phase of mass independent fractionation.

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

Microstructures of metal grains in ordinary chondrites: Implications for their thermal histories

Hugues Leroux*, Jean-Claude Doukhan and Claude Perron

*Correspondence author's address: Laboratoire Structure et Propriétés de l'Etat Solide, Université Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq-Cedex  France; e-mail address:  hugues.leroux@univ-lille1.fr

Abstract–This paper reports one of the first attempts to investigate by analytical transmission electron microscopy the microstructures and compositions of Fe-Ni metal grains in ordinary chondrites. Three ordinary chondrites, Saint Séverin, LL6, Agen H5 and Tsarev L6, were selected because they display contrasting microstructures, reflecting different thermal histories.

In Saint Séverin, the microstructure of the Ni-rich metal grains is due to slow cooling. It consists of a two phase assemblage with a honeycomb structure resulting from spinodal decomposition similar to the cloudy zone of iron meteorites. Microanalyses show that the Ni-rich phase is tetrataenite (Ni = 47 wt.%) and the Ni-poor one, with a composition of about 25% Ni, is either martensite or taenite, these two occurring adjacent to each other. The observation that the Ni-poor phase is partly fcc resolves the disagreement between previous TEM and Mössbauer studies on iron meteorites and ordinary chondrite metal. The Ni content of the honeycomb phase is much higher than in mesosiderites, confirming that mesosiderites cooled much more slowly. The high-Ni tetrataenite rim in contact with the cloudy zone displays high Ni compositional variability on a very fine scale, suggesting that the corresponding area was destabilized and partially decomposed at low temperature.

Both Agen and Tsarev display evidence of reheating and subsequent fast cooling obviously related to shock events. Their metallic particles mostly consist of martensite, the micro-structure of which depends on local Ni content. Microstructures are controlled by both the temperature at which martensite forms and that at which it possibly decomposes. In high Ni zones (>15 wt%), martensitic transformation started at low temperature (<300 °C). As no further recovery occurred, these zones contain a high density of lattice defects. In low Ni zones (<15 wt%), martensite grains formed at higher temperature and their lattice defects recovered. These martensite grains present a lath texture with numerous tiny precipitates of Ni-rich taenite (Ni = 50 wt%) at lath boundaries. Ni composition profiles across precipitate/matrix interfaces show that the growth of these precipitates was controlled by preferential diffusion of Ni along lattice defects. The cooling rates deduced from Ni concentration profiles and precipitate sizes are within the range 1–10 °C/year for Tsarev, and 10–100 °C/year for Agen.

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

Neutron capture effects on Sm, Eu and Gd in Apollo 15 deep drill core samples

Hiroshi Hidaka*, Mitsuru Ebihara and Shigekazu Yoneda

*Correspondence author's address: Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan; e-mail address: hidaka@ue.ipc.hiroshima-u.ac.jp

Abstract–The isotopic compositions of Sm and Gd in seven lunar samples from the Apollo 15 deep drill core were determined to discuss the effects of neutron capture near the lunar surface.  Large isotopic deviations of ¹⁵⁰Sm/¹⁴⁹Sm, ¹⁵⁶Gd/¹⁵⁵Gd and ¹⁵⁸Gd/¹⁵⁷Gd derived from neutron capture effects were observed in all samples.  Although neutron capture products in lunar samples were extensively in the 1970s, our precise isotopic measurements resulted in several new findings.  The neutron fluence in the Apollo 15 drill core is a function of depth with a symmetric peak at 190 g/cm² depth from the surface, confirming the results of earlier investigations.  Neutron fluence values calculated from the isotopic shifts by comparison to artificially irradiated standard reagents were (5.16–7.49) × 10¹⁶ n/cm².  These values are 1.3 to 1.4 times larger than those previously reported.  Variations of epsilon_Sm/epsilon_Gd with depth are interpreted as due to variations in the neutron energy spectrum.  Here epsilon_Sm and epsilon_Gd are defined as in previous studies of lunar neutron stratigraphy.  Our data suggest that the neutron is more thermalized at the lower layers than it is at the upper layers.  In addition to large isotopic shifts for ¹⁴⁹Sm, ¹⁵⁰Sm, ¹⁵⁵Gd, ¹⁵⁶Gd, ¹⁵⁷Gd and ¹⁵⁸Gd, isotopic enrichments of ¹⁵²Gd and ¹⁵⁴Gd derived from neutron capture for ¹⁵¹Eu and ¹⁵³Eu, respectively, were also observed in all samples.

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

⁴⁰Ar/³⁹Ar age of the El'gygytgyn impact event, Chukotka, Russia

Paul W. Layer

Author's address:  Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska 99775, USA; e-mail address:  player@gi.alaska.edu

Abstract–Lake El'gygytgyn, Chukotka Russia, lies in a ~18 km crater of presumably impact origin.  The crater is sited in Cretaceous volcanic rocks of the Okhotsk Chukotka volcanic belt.  Laser ⁴⁰Ar/³⁹Ar dating of impact-melted volcanic rocks from the rim of Lake El'gygytgyn yields a 10 sample weighted plateau age of 3.58 ± 0.04 Ma.  The argon step-heating method was critical in this study in identifying inherited argon in the samples due to incomplete degassing of the Cretaceous volcanic rocks during impact melting.  This age is consistent with, but more precise than, previous K-Ar and fission track ages and indicates an "instantaneous" formation of the crater.  This tight age control, in conjunction with the presence of impactites, shocked quartz and other features, is consistent with an impact origin for the structure and seems to discount internal (volcanogenic) origin models.

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

Mineralogical and chemical composition and cosmic-ray exposure history of two mesosiderites and two iron meteorites

Dario Terribilini, Otto Eugster*, David W. Mittlefehldt, Larryn W. Diamond, Stephan Vogt and Daode Wang

*Correspondence author's address:  Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland; e-mail address:  eugster@phim.unibe.ch

Abstract–We performed a comprehensive study of the noble gas isotopic abundances, radionuclide activities, and mineralogical and chemical composition of two mesosiderites and two iron meteorites.  For the mesosiderites Dong Ujimqin Qi and Weiyuan, the silicate and the metal phases were studied.  The anomalous ataxite Rafrüti is not chemically related to any other meteorite class, whereas Ningbo is a type IVA octahedrite.  The mineralogy, and major and trace  element abundances of the silicate phases of  Dong Ujimqin Qi and Weiyuan are similar to those of other mesosiderites and distinct from those of  the howardites.  The cosmic-ray exposure history was studied based on the concentrations of the cosmogenic noble gas nuclei and radionuclide activities.  For the iron meteorites cosmic-ray exposure ages were calculated from the pairs ¹⁰Be-²¹Ne, ²⁶Al-²¹Ne, and ³⁶Cl-³⁶Ar.  Rafrüti yields the youngest exposure age of all ataxites (6.8 ± 1.7 Ma) whereas that of Ningbo with 107 ± 15 Ma falls within the range observed for the other octahedrites.  The parent body break-up times of the mesosiderites Dong Ujimqin Qi and Weiyuan are 252 ±  50 Ma and 25.9 ± 5.0 Ma, respectively.  We find no evidence for a common break-up event for the mesosiderites and the howardites.

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

Petrology of the Indian eucrite Piplia Kalan

P. C. Buchanan*, D. W. Mittlefehldt, R. Hutchison, C. Koeberl, D. J. Lindstrom and M. K. Pandit

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

Abstract–Piplia Kalan is an equilibrated eucrite consisting of 60–80 vol% lithic clasts in a subordinate brecciated matrix.  Ophitic/subophitic clasts fall into two groups: finer-grained lithology A and coarser-grained lithology B.  Very fine-grained clasts with equigranular textures (lithology C) also occur and originally were hypocrystalline in texture.  The variety of materials represented in Piplia Kalan suggests cooling histories ranging from quenching to slower crystallization.  Despite textural differences, clasts and matrix have similar mineral and bulk compositions.  Thus, Piplia Kalan is probably best classified as a genomict breccia that could represent fragments of a single lava flow or shallow intrusive body, including fine-grained or glassy outer margin and more slowly cooled coarser-grained interior.  Bulk composition suggests that the meteorite is most closely related to the main group eucrites, but it probably was affected by minor amounts of fractional crystallization.  Piplia Kalan displays evidence of an early shock event, including brecciated matrix and areas of lithic clasts that contain very fine-grained, granular pyroxene between deformed feldspar laths.  The meteorite also displays evidence of at least one episode of extensive thermal metamorphism: hypocrystalline materials are recrystallized to hornfelsic textures and minerals throughout the meteorite contain abundant inclusions that are relatively large in size.  Veins of brown glass transect both clasts and matrix and indicate a second, post-metamorphism shock event.

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

A multi-step model for the origin of E3 (enstatite) chondrites

Melinda Hutson* and Alex Ruzicka

*Correspondence author's address: Portland Community College, P.O. Box 19000, SYL ST312, Portland, Oregon 97221, USA; e-mail address:  mhutson@pcc.edu

Abstract–It appears that the mineralogy and chemical properties of type 3 enstatite chondrites could have been established by fractionation processes (removal of a refractory component, and depletion of water) in the solar nebula, and by equilibration with nebular gas at low-to-intermediate temperatures (~700–950 K).  We describe a model for the origin of type 3 enstatite chondrites that for the first time can simultaneously account for the mineral abundances, bulk-chemistry, and phase compositions of these chondrites, by the operation of plausible processes in the solar nebula.  This model, which assumes a representative nebular gas pressure of 10^–5 bar, entails three steps: (1) initial removal of 56% of the equilibrium condensed phases in a system of solar composition at 1270 K; (2) an average loss of 80–85% water vapor in the remaining gas; and (3) two different closure temperatures for the condensed phases.  The first step involves a "refractory-element fractionation" and is needed to account for the overall major-element composition of enstatite chondrites, assuming an initial system with a solar composition.  The second step, water-vapor depletion, is needed to stabilize Si-bearing metal, oldhamite, and niningerite, which are characteristic minerals of the enstatite chondrites.  Variations in closure temperatures are suggested by the way in which the bulk chemistry and mineral assemblages of predicted condensates change with temperature, and how these parameters correlate with the observations of enstatite chondrites.  In general, most phases in type 3 enstatite chondrites appear to have ceased equilibrating with nebular gas at ~900–950 K, except for Fe-metal, which continued to partially react with nebular gas to temperatures as low as ~700 K.

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

An abiotic origin for hydrocarbons in the ALH 84001 martian meteorite through cooling of magmatic and impact-generated gases

Mikhail Yu. Zolotov* and Everett L. Shock

*Correspondence author's address:  Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130-4899, USA; e-mail address:  zolotov@zonvark.wustl.edu

Abstract–Thermodynamic calculations of metastable equilibria were used to evaluate the potential for abiotic synthesis of aliphatic and polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite ALH 84001. The calculations show that PAHs and normal alkanes could form metastably from CO, CO₂, and H₂ below ~250–300 °C during rapid cooling of trapped magmatic and/or impact-generated gases. Depending on temperature, bulk composition, and oxidation-reduction conditions, PAHs and normal alkanes can form simultaneously or separately. Moreover, PAHs can form at lower H/C ratios, higher CO/CO₂ ratios, and higher temperatures than normal alkanes. Dry conditions with H/C ratios less than ~0.01–0.001 together with high CO/CO₂ ratios also favor the formation of unalkylated PAHs. The observed abundance of PAHs, their low alkylation, and a variable but high aromatic to aliphatic ratio in ALH 84001 all correspond to low H/C and high CO/CO₂ ratios in magmatic/impact gases, and can be used to deduce spatial variations of these ratios. Some hydrocarbons could have been formed from trapped magmatic gases, especially if the cooling was fast enough to prevent reequilibration. We propose that subsequent impact heating(s) in ALH 84001 could have led to dissociation of ferrous carbonates to yield fine-gain magnetite, formation of a CO-rich local gas phase, reduction of water vapor to hydrogen, reequilibration of the trapped magmatic gases, aromatization of hydrocarbons formed previously, and overprinting of the synthesis from magmatic gases, if any. Rapid cooling and high-temperature quenching of CO-, H₂-rich impact gases could have led to magnetite-catalyzed hydrocarbon synthesis.
 

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

Classification of Didwana-Rajod meteorite:  A Mössbauer spectroscopic study

B.S.Paliwal, R.P.Tripathi, H.C. Verma* and S.K.Sharma

*Correspondence author's address:  Department of Physics, I.I.T. Kanpur, 208016, India; e-mail address:  hcverma@iitk.ac.in

Abstract–Mössbauer spectroscopic studies of Didwana-Rajod chondrite which fell on August 12, 1991 in western Rajasthan, India, are presented.  The results are compared with the Mössbauer data of several enstatite and ordinary chondrites including Dhajala chondrite for which Mössbauer data were recorded during the present work.  The iron phases and its oxidation states strongly suggest that it should be classified as an H-type ordinary chondrite as against the earlier suggestion based on petrographic studies that it could be an enstatite chondrite.  The present study demonstrates that Mössbauer spectroscopy is a very powerful technique to assist in classification of meteorites.

 

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