Abstracts of Papers to be Published in the September 2000 Issue

Please contact the correspondence author for reprints of all published articles

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

Mineralogy of the sources for magnetic anomalies on Mars

Gunther Kletetschka*, Peter J. Wasilewski and Patrick T. Taylor

*Correspondence author's address:  Mail Code 691, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA; e-mail address:  gunther@denali.gsfc.nasa.gov

Abstract–Recent discovery of intense magnetic anomalies on Mars, which are due to remanent magnetization, requires some explanation for the possible minerals responsible for the anomaly signature.  Thermoremanent magnetization (TRM) in single domain (SD) and multidomain (MD) sized magnetite, hematite and pyrrhotite, all potential minerals, are considered.  The intensity of TRM (in 0.05 mT) is in descending order: SD sized magnetite, SD sized pyrrhotite, MD sized hematite, MD sized pyrrhotite, MD sized magnetite, SD sized hematite.  TRM intensity is <4% of the saturation isothermal remanence (SIRM) for all but the MD hematite, which may have >50% of the SIRM.  Each of these minerals and estimated concentrations of magnetic remanence carriers (assumed to be titanomagnetite) in the SNC meteorites, are used in a thin sheet approximation model to reveal the concentration of each mineral required for the generation of an observed magnetic anomaly (1500 nT at 100 km altitude) assuming TRM acquisition in a 0.05  mT (millitesla) magnetic field.

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

Petrology and geochemistry of the Elephant Moraine A79002 diogenite:  A genomict breccia containing a magnesian harzburgite component

David W. Mittlefehldt

Author's address:  Mail Code C23, Lockheed Martin SO, 2400 Nasa Road 1, Houston, Texas 77058, USA; e-mail address:  david.w.mittlefehldt1@jsc.nasa.gov

Abstract–The Elephant Moraine A79002 (EETA79002) diogenite is a fragmental breccia with a subtle light-dark structure.  It is composed of orthopyroxene, with minor olivine, chromite and ubiquitous, inhomogeneously distributed, ~5–500 µm-sized troilite and metal grains.  These latter are present in the matrix, and as inclusions in and as symplectic intergrowths with orthopyroxene and olivine.  Trace amounts of silica and diopside are also present.  Most orthopyroxene compositions (typical orthopyroxenes) are in the narrow range Wo_2.1–2.7En_74.1–75.6Fs_22.2–23.8 like those of most diogenites.  A few magnesian orthopyroxenes are present with compositions of Wo_1.7–2.5En_77.5–80.2 Fs_18.2–20.3.  These are among the most magnesian orthopyroxenes known from diogenites.  A few ferroan orthopyroxenes have compositions of Wo_2.1–2.9En_71.7–73.7Fs_24.2–25.5.  Differences in Al₂O₃, TiO₂ and Cr₂O₃ between the different orthopyroxene groups are inconsistent with a simple igneous fractionation relationship between them.  Olivine compositions are Fo_75.0–76.9.  The olivines could be in equilibrium with the magnesian orthopyroxenes, but not with the typical or ferroan orthopyroxenes that form the bulk of EETA79002.  Metal grains exhibit a range of Ni and Co contents and Ni/Co ratios; their compositions indicate that they are primary igneous metal.  Metal and troilite grains are more prevalent in the dark samples.

The trace incompatible lithophile element contents of 16 samples are remarkably uniform.  Their Yb concentrations are all within their 2 sigma analytical uncertainties of the mean.  The uniformity and low content of LREE in EETA79002 indicate that negligible amounts of a trapped liquid component, or foreign material mixed in the breccia, could be present.  The siderophile and chalcophile element data show that the light-dark structure is due to the distribution of metal and troilite grains; dark samples contain higher Ni, Co and Se compared to light samples.

EETA79002 appears to contain material from three or more related plutons, a magnesian harzburgite, and two orthopyroxenites, and is a genomict breccia.

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

The Vissannapeta eucrite

S. Ghosh, N. C. Pant, T. K. Rao, C. Rama Mohana, J. B. Ghosh, S. Shome, N. Bhandari*, A. D. Shukla and K. M. Suthar

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

Abstract–A wholly crusted single stone which fell in Vissannapeta, Andhra Pradesh, India has been identified as a cumulate eucrite based on its primary texture and mineral composition: anorthite(An_92.4–94.6),orthopyroxene(En_49.1–51.8 Fs_44.2–49.7 Wo_1.2–4.0) and clinopyroxene (En_38.8–46.8 Fs_14.8–33.6Wo_19.6–46.4).  The stone is pyramidal in shape and the crust shows rib-like flow features indicating that it had an oriented passage through the atmosphere towards the terminal stage of its flight.  Conditions of its fall, mineralogical characteristics and results of measurements of cosmogenic radioactivity (²⁶Al, ²²Na and ⁵⁴Mn) and track density are described.  Compared to Piplia Kalan, another eucrite, which fell about 18 months before Vissannapeta, the observed activity levels of these nuclides are approximately 75% whereas higher activity of ²²Na and ⁵⁴Mn would be expected from solar cycle modulation of galactic cosmic rays.  These results, as well as the track density gradient indicate that Vissannapeta was a small body (equal or less than 120kg) in the interplanetary space wherein the nuclear cascade due to galactic cosmic rays did not develop fully.  Tracks, surface morphology and crustal features indicate at least two fragmentation events in the atmosphere.

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

Crystallization of the basaltic shergottites:  Insights from crystal size distribution (CSD) analysis of pyroxenes

Rachel C.F. Lentz* and Harry Y. McSween, Jr.

*Correspondence author's address:  Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410, USA; e-mail address: rlentz@utk.edu

Abstract–Quantitative petrographic analysis, using the crystal size distribution (CSD) method, provides a novel approach for examining the crystallization histories of basaltic shergottites.  Grain number densities at different sizes are plotted against grain size, and the resulting curve relates to the geologic processes involved with the crystallization of the grain population.  Most basaltic shergottites are dominated by pigeonite and augite, and since plagioclase is primarily interstitial, and therefore constrained in its growth by the surrounding pyroxenes, we limited our size measurements to the pyroxene phases.

The groundmasses of EET A79001 lithology A and DaG 476 are fine-grained with cumulus pyroxene and interstitial plagioclase glass.  Their simple linear CSD plots record a single stage of pyroxene crystallization under steady state conditions of  continuous nucleation and growth.  The textures of QUE 94201 and EET A79001 lithology B are quite different from the other shergottites, with intergrown pyroxene and plagioclase.  Likewise, their CSD plots are also distinct, with curved trends suggesting a lack of large grains, most likely due to interference between simultaneously growing silicate phases.  However, the CSD plot shapes are smooth, also implying a single stage of growth.  Shergotty and Zagami, with coarser cumulus textures, display CSD plots that are generally linear over most grain sizes.  This implies that conditions of nucleation and growth were dominant during formation of the pyroxene populations.  Both plots, however, also display kinks, implying multiple stages of growth.

A similar kink is also visible in a CSD plot of only the Mg-rich cores of Shergotty pyroxenes, suggesting the feature represents changes in conditions during core crystallization, rather than an event coincident with the change in composition to the Fe-rich rims.  The plot may be interpreted as representing two stages of core growth with an intervening short hiatus of nucleation, with continued crystallization associated with ascent of the magma.  Eruption onto the surface probably triggered the compositional change to Fe-rich rims.  CSD analysis of products from a controlled crystallization study agree with experimental and petrologic estimates that cooling rates for Zagami were on the order of a few tenths of a degree per hour.  Growth rates derived from these cooling rates suggest crystallization of Shergotty and Zagami pyroxenes occurred over a period of a few weeks to months.

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

Distant ejecta from the Lockne marine-target impact crater, Sweden

Erik Sturkell, Jens Ormö, Jaak Nõlvak and Åsa Wallin

*Correspondence author's address:  Nordic Volcanological Institute, Grensásvegur 50, IS-108 Reykjavík, Iceland; e-mail address:  erik@norvol.hi.is

Abstract–The Lockne impact event took place in a Middle-Ordovician (455 Ma) epicontinental sea.  The impact resulted in an at least 13.5 km wide, concentric crater in the sea floor.  Lockne is one of very few locations where parts of an ejecta layer have been preserved outside the crater structure.  The ejecta from the Lockne impact rests on progressively higher stratigraphic levels with increasing distance from the crater, hence forming a slightly inclined discontinuity surface in the pre-impact strata.  We report on a ~30 cm thick sandy layer at Hallen, 45 km south of the crater centre.  This layer has a fining upward sequence in its lower part, followed by low-angle cross-laminations indicating two opposite current directions.  It is rich in quartz grains with planar deformation features, and contains numerous, up to 15 cm large, granite clasts from the crystalline basement at the Lockne impact site.  The layer is within a sequence dated to the Baltoniodus gerdae conodont subzone.  The dating is corroborated by chitinozoans indicating the latest Kukruse time below and the late Idavere above the impact layer.  According to the chitinozoans biostratigraphy, some erosion may have occurred due to deposition of the impact layer.  The Hallen outcrop, today 45 km from the centre of the Lockne crater, is at present the most distant accessible occurrence of ejecta from the Lockne impact.  It is also the most distant location so far found where the resurge of water towards the crater has affected the bottom sediments.

A greater crater diameter than hitherto assumed, thus representing greater impact energy, might explain the extent of the ejecta blanket.  Fluidisation of ejecta, to be expected at a marine-target impact, might furthermore have facilitated the wide distribution of ejecta.

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

Symplectic exsolution in olivine from the Nakhla martian meteorite

Takashi Mikouchi*, Ikiko Yamada and Masamichi Miyamoto

*Correspondence author's address:  Mineralogical Institute, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; e-mail address: mikouchi@eps.s.u-tokyo.ac.jp

Abstract–The Nakhla meteorite, commonly accepted to have originated from Mars, is a cumulus clinopyroxenite with ~10 vol.% of Fe-rich olivine.  Almost all olivine grains in Nakhla contain dark lamellar inclusions (>2–3 µm wide).  High-resolution scanning and transmission electron microscopy revealed that the inclusions are complex intergrowth of augite and magnetite.  Such symplectic intergrowth of augite and magnetite in olivine was known in some terrestrial rocks, lunar rocks and a few meteorites.  The inclusion in Nakhla olivine is the first symplectite found in a martian rock.  Apparently, the presence of Fe³⁺ in olivine under an oxidizing condition on Mars caused symplectic exsolution at high temperature (>900 ºC) during cooling.

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

Organosilane occurrence in irghizite samples from the Zhamanshin impact crater, Kazakhstan

Marek Zbik*, Marek Jasieniak and Roger St.C. Smart

*Correspondence author's address:  Ian Wark Research Institute, University of South Australia, Levels Campus, Mawson Lakes, South Australia 5095, Australia; e-mail address:  marek.zbik@unisa.edu.au

Abstract–The composition of surface deposits on vesicle walls in irghizites (i.e., impact glasses at site) from the Zhamanshin meteorite crater were studied using time of flight secondary ion mass spectrometry (ToF-SIMS).  The cavity walls are unique interfaces for condensation of gases from the superheated, high-silica melt during the impact.  Initially, signals from the cavity wall are dominated by hydrocarbon fragments whereas the glass fracture face surrounding the cavity gave only signals corresponding to glass components.  After 12h in UHV, signals from the cavity wall are dominated by peaks corresponding to fragments normally measured from organo-silanes and organo-siloxanes with the majority of the hydrocarbon signals markedly reduced.  Characteristic hydrocarbon fragments are now observed on the glass fracture surface next to the cavity in an annulus around the cavity perimeter.  There are also minor signals in this region from organo-silanes and organo-siloxanes.  In contrast, four tektites (Australites) (i.e., glassy distal ejecta) gave no organo-silane or organo-siloxane signals after the same preparation and vacuum evaporation procedure.  These species appear to be formed only at the impact site where higher levels of organic material are likely to be present in soil and are trapped before evaporation.  This appears to be the first report of naturally occurring silicon-organic compounds.

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

Primordial noble gases in "Phase Q" in carbonaceous and ordinary chondrites studied by closed system stepped etching

Henner Busemann*, Heinrich Baur and Rainer Wieler

*Correspondence author's address:  Physics Institute, University of Bern, Sidlerstr. 5, 3012 Bern, Switzerland; e-mail address: busemann@phim.unibe.ch

Abstract–HF/HCl-resistant residues of the chondrites CM2 Cold Bokkeveld, CV3 (ox.) Grosnaja, CO3.4 Lancé, CO3.7 Isna, LL3.4 Chainpur and H3.7 Dimmitt have been measured by closed system stepped etching (CSSE) in order to better characterise the noble gases in "phase Q", a major carrier of primordial noble gases.  All isotopic ratios in phase Q of the different meteorites are quite uniform, except for (²⁰Ne/²²Ne)_Q.  As already suggested by precise earlier measurements (Schelhaas et al., 1990; Wieler et al., 1991; 1992), (²⁰Ne/²²Ne)_Q is the least uniform isotopic ratio of the Q noble gases.  The data cluster around 10.1 for Cold Bokkeveld and Lancé and 10.7 for Chainpur, Grosnaja and Dimmitt, respectively.  No correlation of (²⁰Ne/²²Ne)_Q with the classification or the alteration history of the meteorites has been found.  The Ar, Kr and Xe isotopic ratios for all six samples are identical within their uncertainties and similar to earlier Q determinations as well as to Ar-Xe in ureilites.  Thus, an unknown process probably accounts for the alteration of the originally incorporated Ne-Q.  The noble gas elemental compositions provide evidence that Q consists of at least two carbonaceous carrier phases "Q₁" and "Q₂" with slightly distinct chemical properties. (Ar/Xe)_Q and (Kr/Xe)_Q reflect both thermal metamorphism and aqueous alteration.  These parent body processes have led to larger depletions of Ar and Kr relative to Xe.  In contrast, meteorites that suffered severe aqueous alteration, such as the CM chondrites, do not show depletions of He and Ne relative to Ar but rather the highest (He/Ar)_Q and (Ne/Ar)_Q ratios.  This suggests that Q₁ is less susceptible to aqueous alteration than Q₂.  Both sub-phases may well have incorporated noble gases from the same reservoir, as indicated by the nearly constant, though very large depletion of the lighter noble gases relative to solar abundances.  However, the elemental ratios show that Q₁ and Q₂ must have acquired (or lost) noble gases in slightly different element proportions. Cold Bokkeveld suggests that Q₁ may be related to presolar graphite. Q₁ and Q₂ might be related to the sub-phases that have been suggested by Gros and Anders (1977). The distribution of the ²⁰Ne/²²Ne ratios can not be attributed to the carriers Q₁ and Q₂. The residues of Chainpur and Cold Bokkeveld contain significant amounts of Ne-E(L), and the data confirm the suggestion of Huss (1997) that the ²²Ne-E(L) content, and thus the presolar graphite abundances, are correlated with the metamorphic history of the meteorites.

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

Light noble gases and cosmogenic radionuclides in Estherville, Budulan, and other mesosiderites:  Implications for exposure histories and production rates

A. Albrecht, C. Schnabel, S. Vogt, S. Xue, G. F. Herzog, F. Begemann, H. W. Weber,  R. Middleton, D. Fink and J. Klein

Correspondence author's address Department of Chemistry, Rutgers University, New Brunswick, New Jersey 08903, USA; e-mail address:  herzog@rutchem.rutgers.edu

Abstract–We report measurements of ²⁶Al, ¹⁰Be, ⁴¹Ca, and ³⁶Cl in the silicate and metal phases of 11 mesosiderites, including several specimens each of Budulan and Estherville, of the brecciated meteorite Bencubbin, and of the iron meteorite Udei Station.  Average production rate ratios (atom/atom) for metal phase samples from Estherville and Budulan are ²⁶Al/¹⁰Be = 0.79 ± 0.03; ³⁶Cl/¹⁰Be = 5.3 ± 0.2.   For a larger set of meteorites that includes iron meteorites and other mesosiderites, we find ²⁶Al/¹⁰Be = 0.72 ± 0.01 and ³⁶Cl/¹⁰Be = 4.5 ± 0.2.  The average ⁴¹Ca/³⁶Cl production rate ratio is 1.10 ± 0.04 for metal separates from Estherville and four small iron falls.  The ⁴¹Ca activities in dpm/(kg Ca) of various silicate separates from Budulan and Estherville span nearly a factor of 4, from <400 to >1600, indicating preatmospheric radii of >30 cm.  After allowance for composition, the activities of ²⁶Al and ¹⁰Be (dpm/kg silicate) are similar to values measured in most ordinary chondrites and appear to depend only weakly on bulk Fe content.  Unless shielding effects are larger than suggested by the ³⁶Cl and ⁴¹Ca activities of the metal phases, matrix effects are unimportant for ¹⁰Be and minor for ²⁶Al.

Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti.  New estimates of ³⁶Cl/³⁶Ar exposure ages for the metal phases agree well with published values.  Neon-21 production rates for mesosiderite silicates calculated from these ages and from measured ²¹Ne contents are consistently higher than predicted for L chondrites despite the fact that the mesosiderite silicates have lower Mg contents than L chondrites.  We suggest that the elevation of the ²¹Ne production rate in mesosiderite silicates reflects a "matrix effect," that is, the influence of the higher Fe content of mesosiderites, which acts to enhance the flux of low-energy secondary particles and hence the ²¹Ne production from Mg.  As ¹⁰Be production is relatively insensitive to this matrix effect, ¹⁰Be/²¹Ne ages give erroneously low production rates and high exposure ages.  By coincidence, standard ²²Ne/²¹Ne based "shielding" corrections give fairly reliable ²¹Ne production rates in the mesosiderite silicates.

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

A nitrogen isotope study of bencubbinites

N. Sugiura*, S. Zashu, M. Weisberg and M. Prinz

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

Abstract–Nitrogen isotopic compositions  in the bencubbinites (Bencubbin, Hammadah al Hamra 237 (HH 237) and Queen Alexandra Land 94411 (QUE 94411)) and in a petrographically similar chondrite Grosvenor Mountains 95551 (GRO 95551)  were measured by  stepped combustion-static mass spectrometry.  HH 237 and QUE 94411 contain isotopically heavy N, but not as heavy as that in Bencubbin or Weatherford.  GRO 95551 contains isotopically near-normal N and light N and hence it is not related to the bencubbinites, as also indicated by its oxygen isotope composition.  The N carriers in these meteorites were investigated  using secondary ion mass spectrometry (SIMS).  In the bencubbinites, N is mostly located around sulfide in metal clasts and in impact melt areas.  The N carriers in the former are taenite and/or carbide, whereas those in the latter are molten metal, tiny graphitic carbon in metal and/or oxi-nitride glass.  N in the various N carriers is isotopically equilibrated, and therefore the carriers are not pristine presolar grains.  Isotopically near-normal N in GRO 95551 is located in graphite.  The carrier of isotopically light N in GRO 95551 has not been found.

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

Isotopic compositions of different presolar SiC size fractions from the Murchison meteorite

Sachiko Amari*, Ernst Zinner and Roy S. Lewis

*Correspondence author's address:  Laboratory for Space Sciences and the Physics Department, Washington University, One Brookings Drive, St. Louis, Missouri 63130-4899, USA; e-mail address: sa@howdy.wustl.edu

Abstract–We report measurements of isotopic ratios of C, N, Mg, Si, Ca, Ti, Cr and Fe in bulk samples (aggregates of many grains) of up to seven different fractions of SiC, ranging from 0.38 to 3.0 µm in diameter, from the Murchison CM2 carbonaceous chondrite.  ¹²C/¹³C ratios range from 37 to 42 and ¹⁴N/¹⁵N ratios from 370 to 520, within the range of single grain measurements on coarser samples and in agreement with an asymptotic giant branch (AGB) star origin of most of the grains.  Variations among size fractions do not show any simple trend and can be explained by varying contamination with isotopically normal material.  Silicon isotopic ratios vary only little and, with one exception, lie to the right of the single-grain mainstream correlation line.  This might indicate a higher percentage of the minor populations Y and Z among finer grain size fractions.  All bulk samples have large ²⁶Mg excesses attributed to the presence of short-lived ²⁶Al at the time of grain formation.  Inferred ²⁶Al/²⁷Al ratios are much larger than those measured in single larger mainstream grains.  This is probably because of the presence of SiC grains of type X; we obtain an estimate of 0.4 for their ²⁶Al/²⁷Al ratio.  Our Ca isotopic measurements, the first made on presolar SiC grains, show excesses in ⁴²Ca and ⁴³Ca, in general agreement with theoretical expectations for AGB stars.  ⁴⁴Ca excesses are much larger than expected and are probably due to X grains, which have high ⁴⁴Ca excesses due to the decay of short-lived ⁴⁴Ti produced in supernova explosions.  We arrive at an estimate of 0.014 for the initial ⁴⁴Ti/⁴⁸Ti ratio of the X grains, within the range obtained from previous single X grain measurements.  The Ti isotopic ratios of the bulk samples show a V-shaped pattern with excesses of all isotopes relative to ⁴⁸Ti.  ⁴⁶Ti, ⁴⁷Ti and ⁵⁰Ti show excesses relative to the correlation between Ti and Si ratios for single grains and are in general agreement with theoretical models of s-process nucleosynthesis in AGB stars.  In contrast, ⁴⁹Ti does not show any excess relative to the single grain data; it also fails to agree with theory, which predicts much larger excesses than observed.  Measured ⁵³Cr/⁵²Cr and ⁵⁷Fe/⁵⁶Fe ratios are normal within errors.  The first result is expected even for Cr in AGB star envelopes, but the second result suggests that most of the Fe analyzed originates from contamination.  We have found no simple trends in isotopic composition with respect to grain size that can be interpreted in terms of nucleosynthetic origin, unlike the results for Kr, Xe, Ba, and Sr.

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

On the origin of rim textures surrounding anhydrous silicate grains in CM carbonaceous chondrites

Lauren Browning*, Harry Y. McSween, Jr. and Michael E. Zolensky
 
*Correspondence author's address:  Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, San Antonio, Texas 78238-5166, USA; e-mail address:  lbrowning@swri.edu

Abstract–Fine-grained, optically opaque rims coating individual olivine and pyroxene grains in CM matrices and chondrules.  Bulk chemical analyses and observations of these rims indicate the presence of phyllosilicates and disseminated opaques.  Because phyllosilicates could not have survived the chondrule formation process, chondrule silicate rims must have formed entirely by late-state aqueous reactions.  As such, these textures provide a useful benchmark for isolating alteration features from more complex CM matrix materials.  Both chondrule silicate and matrix silicate rims exhibit morphological features commonly associated with advancing stages of replacement reactions in terrestrial serpentinites.  Contacts between many matrix silicate rims and the adjacent matrix materials suggest that these rims formed entirely by aqueous reactions in a parent body setting.  This contrasts with previous assertions that rim textures can only form by the accretion of nebular dust, but does not imply an origin for the rims surrounding other types of CM core components, such as chondrules.

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

Interrelationships among meteoric metals, meteors, interplanetary dust, micrometeorites, and meteorites

Frans J. M. Rietmeijer

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

Abstract–Meteor science, Aeronomy and Meteoritics are different disciplines with natural interfaces.  This paper is an effort to integrate the chemistry and mineralogy of collected interplanetary dust particles (IDPs), micrometeorites and meteorites with meteoric data and with atmospheric metal abundances. Evaporation, ablation and melting of decelerating materials in the Earth's atmosphere are the sources of the observed metal abundances in the upper atmosphere. Many variables ultimately produce the materials and phenomena we can analyze, such as different accretion and parent body histories of incoming extraterrestrial materials, different interactions of meteors with the Earth's middle atmosphere, meteor data reduction and complex chemical interactions of the metals and ions with the ambient atmosphere. IDP-like and UOC matrix materials are reasonable sources for observed meteoric and atmospheric metals. The hypothesis of hierarchical dust accretion predicts that low, correlated refractory element abundances in cometary meteors may be real. It implies that the CI or cosmic standard is not useful to appreciate the chemistry of incoming petrologically heterogeneous cometary matter. The quasi steady state metal abundances in the lower thermosphere and upper mesosphere are derived predominantly from materials with cometary orbital characteristics and velocities such as comets proper and near-earth asteroids. The exact influence of atmospheric chemistry on these abundances still needs further evaluation. Metal abundances in the lower mesosphere and upper stratosphere region are mostly from materials from the asteroidal belt and the Kuiper belt.

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

Evidence for  common break-up events of the acapulcoites/lodranites and chondrites

Dario Terribilini, Otto Eugster*, Gregory F. Herzog and Christoph Schnabel

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

Abstract–Acapulcoites and lodranites are believed to originate on a common parent body and to represent some of the earliest events in the differentiation of the chondritic asteroids.  We have conducted isotopic studies of the noble gases He, Ne, Ar, Kr, and Xe and determinations of the concentrations of the major elements and of the radionuclides ¹⁰Be, ²⁶Al, and ³⁶Cl in an attempt to constrain the cosmic-ray exposure history of two members of the acapulcoite / lodranite clan recovered in Antarctica: Frontier Mountain 95029 and Graves Nunataks 95209.  From cosmic-ray produced ³He, ²¹Ne, and ³⁸Ar and appropriate production rates we derive parent body break-up times of 4.59 ± 0.60 Ma and 6.82 ± 0.60 Ma for FRO95209 and GRA95209, respectively.  These times are consistent with those obtained from the pairs ¹⁰Be-²¹Ne and ²⁶Al-²¹Ne, whereas the times inferred from the pair ³⁶Cl-³⁶Ar are slightly longer, perhaps because the ³⁶Cl activities decreased as a result of decay on Earth.  Terrestrial ages up to ~50 ka for the two meteorites are consistent with the measured ³⁶Cl activities of the metal phases.  All acapulcoites and lodranites dated until now show cosmic-ray exposure ages in the range of 4–10 Ma.  This is the same range as that found for the major exposure age cluster of the H chondrites.  As a common parent body is improbable on the basis of the oxygen isotope systematics, a common set of impactors might have affected the asteroid belt 4–10 Ma ago.

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

Refractory inclusions from the ungrouped carbonaceous chondrites MAC 87300 and MAC 88107

Sara S. Russell, Andrew M. Davis, Glenn J. MacPherson*, Yunbin Guan and Gary R. Huss

*Correspondence author's address:  Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C.  20560-0119, USA; e-mail address:  glenn@glennm.si.edu

Abstract–MAC 87300 and MAC 88107 are two unusual carbonaceous chondrites that are intermediate in chemical composition between the CO3 and CM2 meteorite groups.  Calcium-aluminum-rich inclusions (CAIs) from these two meteorites are mostly spinel-pyroxene and melilite-rich (Type A) varieties.  Spinel-pyroxene inclusions have either a banded or nodular texture, with aluminous diopside rimming iron-poor spinel.  Melilite-rich inclusions (Åk_4–42) are irregular in shape and contain minor spinel (FeO <1 wt%), perovskite and, more rarely, hibonite.  The CAIs in MAC 88107 and MAC 87300 are similar in primary mineralogy to CAIs from low petrologic grade CO3 meteorites, but differ in that they commonly contain phyllosilicates.  The two meteorites also differ somewhat from each other: melilite is more abundant and slightly more aluminum-rich in inclusions from MAC 88107 than in those from MAC 87300, and phyllosilicate is more abundant and magnesium-poor in MAC 87300 CAIs relative to that in MAC 88107.  These differences suggest that the two meteorites are not paired.  CAI sizes and the abundance of melilite-rich CAIs in MAC 88107 and MAC 87300 suggests a genetic relationship to CO3 meteorites, but the CAIs in both have suffered a greater degree of aqueous alteration than is observed in COs.   Al-rich melilite in CAIs from both meteorites generally contains excess ²⁶Mg, presumably from the in situ decay of ²⁶Al.  Although well-defined isochrons are not observed, the ²⁶Mg excesses are consistent with initial ²⁶Al/²⁷Al ~3–5 × 10^–5.  An unusual hibonite-bearing inclusion is isotopically heterogeneous, with two large and abutting hibonite crystals showing significant differences in their degrees of mass-dependent fractionation of ²⁵Mg/²⁴Mg.   The two crystals also show differences in their inferred initial ²⁶Al/²⁷Al, 1 × 10^-5 vs. 3 × 10^-6.

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

The age of the Kaalijärv meteorite craters

Kaare L. Rasmussen*, Bent Aaby and Raymond Gwozdz

*Correspondence author's address: Radiocarbon Laboratory, National Museum of Denmark, Ny Vestergade 11, DK-1471 Copenhagen K, Denmark; e-mail address: kaare.lund.rasmussen@natmus.dk

Abstract–Precise radiometric age determination of the Kaalijärv meteorite craters on the island of Saaremaa in Estonia have so far proved inconclusive.  Here we present trace element analyses of peat cores taken several kilometers away from the Kaalijärv craters that reveal a distinct Ir-enriched layer produced by the meteorite impact.  By radiocarbon dating the peat cores we have determined for the first time the precise age of the impact that generated the Kaalijärv craters.  The calibrated date of the impact is 400–370 BC at ± one standard deviation.

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

Complexities on the acapulcoite-lodranite parent body: Evidence from trace element distributions in silicate minerals

Christine Floss

Author's address:  Laboratory for Space Sciences and the Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA; e-mail address:  floss@howdy.wustl.edu

Abstract–The trace element distributions of individual minerals from seven acapulcoites and lodranites have been studied.  Systematic differences are evident between some members of the two groups.  Specifically, pyroxenes from the lodranites MAC 88177 and LEW 88280 exhibit depletions of the REE and other incompatible trace elements (Ti, Zr, Y), relative to acapulcoite (Acapulco, ALHA81261) pyroxenes, that are consistent with the formation and removal of 15 % or more silicate partial melts from these meteorites.  Phosphate REE patterns in these lodranites also support this scenario.  However, other members of the acapulcoite-lodranite clan exhibit more complex trace element variations.  EET 84302, which has been classified as transitional between the acapulcoites and lodranites generally has trace element characteristics similar to the acapulcoites.  However, its plagioclase REE compositions suggest a somewhat greater degree of metamorphism than that experienced by acapulcoites such as Acapulco and ALHA81261.  Similar and elevated REE abundances in the silicate phases from acapulcoite ALHA81187 and lodranite GRA 95209 suggest that these two meteorites, in fact, experienced similar thermal histories.  This probably included some silicate partial melting, although little melt appears to have been lost from the samples.

The observed variations in the trace element abundances of these samples from the acapulcoite-lodranite clan emphasize the complex and varied processes that have acted on their parent body.  The simple bimodal classification of these meteorites based primarily on petrographic criteria, which has been used to date, appears to be inadequate to describe this diverse group of samples, as they represent a range of degrees of partial melting, both with and without accompanying melt migration.  In some instances, secondary processes on the parent body, such as cryptic metasomatism, have further modified sample compositions.

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

The differentiation of eucrites:  The role of in situ crystallization

J. A. Barrat*, J. Blichert-Toft, Ph. Gillet and F. Keller

*Correspondence author's address:  Université d'Angers, Faculté des Sciences, 2 Bd Lavoisier, 49045 Angers Cedex, France; e-mail address: barrat@univ-angers.fr

Abstract–We report on major and trace element analyses of 17 eucrites, including three cumulate eucrites (Binda, Moore County, and Serra de Magé), determined by, respectively, ICP-AES and ICP-MS.  The results obtained for Binda and Moore County are consistent with the model of Treiman (1997) for the formation of cumulate eucrites, which holds that these meteorites were produced from a eucritic melt.  Our sample of Serra de Magé contains unusually large amounts of pyroxene and probably an accessory phase rich in HREEs and is therefore not representative of this eucrite as known from literature data.

Our results for the noncumulate eucrites Bereba, Bouvante, Cachari, Caldera, Camel Donga, Ibitira, Jonzac, Juvinas, Lakangaon, Millbillillie, Padvarninkai, Pasamonte, Sioux County and Stannern are in good agreement with literature data.  The observed decoupling between major and trace elements for noncumulate eucrites can be explained by in-situ crystallization during the differentiation of an asteroidal magma ocean.  This model can further account for both the Nuevo Laredo and the Stannern trends but has as a consequence that none of the analyzed eucrites represents a primary melt.

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

Space weathering on airless bodies:  Resolving a mystery with lunar samples

Carlé M. Pieters*, Larry A. Taylor, Sarah K. Noble, Lindsay P. Keller, Bruce Hapke, Richard V. Morris, Carl C. Allen, David S. McKay and Susan Wentworth

*Correspondence author's address:  Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA; e-mail address: pieters@mare.geo.brown.edu

Abstract–Using new techniques to examine the products of space weathering of lunar soils, we demonstrate that nanophase reduced iron (npFe⁰) is produced on the surface of grains by a combination of vapor deposition and irradiation effects.  The optical properties of soils (both measured and modeled) are shown to be highly dependent on the cumulative amount of npFe⁰, which varies with different starting materials and the energetics of different parts of the solar system.  The measured properties of intermediate albedo asteroids, the abundant S-type in particular, are shown to directly mimic the effects predicted for small amounts of npFe⁰ on grains of an ordinary chondrite regolith.  This measurement and characterization of space weathering products seems to remove a final obstacle hindering a link between the abundant ordinary chondrite meteorites and common asteroids.

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

New type of radiation of bright Leonid meteors above 130 km

Pavel Spurný* , Hans Betlem,  Klaas Jobse,  Pavel Koten and Jaap Van't Leven

*Correspondence author's address:  Astronomical Institute, Ondrejov Observatory, 251 65 Ondrejov, Czech Republic; e-mail address: spurny@asu.cas.cz

Abstract–In this paper we study the extremely high beginning parts of atmospheric trajectories of seven Leonid meteors recorded by sensitive TV systems equipped with image intensifiers up to apparent magnitude +6.5.  For all the seven cases we observed comet-like diffuse structures with sizes of the order of kilometers and quickly developing during the meteoroid penetration through the atmosphere.  For the brightest event with maximum absolute magnitude of -12.5, we observed an arc looking like a solar protuberance and producing a jet detectable several kilometers sideways from the brightest parts of the meteor head, and moving with velocities over 100 km/s.  These jets are common features for the seven studied meteors.  Precise position in trajectory, velocity and brightness at each point is available for all seven meteors, because of double-station records on 85 km base-line.  When these meteoroids reached 130 km height, this diffuse structure of the radiation quickly transformed to usual meteor appearance resembling moving droplet, and a meteor train started to develop.  These meteor phenomena above 130 km were not recognized before our observations, and they cannot be explained by standard ablation theory.

 

Top	Previous page