Please contact the correspondence author for reprints of all published articles
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Experimental hydrothermal alteration of a Martian analog basalt: Implications for Martian meteorites
Leslie L. Baker*, Deborah J. Agenbroad and Scott A. Wood
*Correspondence author's address: Department of Geology and Geological Engineering, University of Idaho, Moscow, ID 83844-3022, USA; e-mail address: lbaker@uidaho.edu
Abstract–A number of Martian meteorite samples contain secondary alteration minerals such as Ca-Mg-Fe carbonates, Fe-oxides, and clay minerals. These mineral assemblages hint at hydrothermal processes occurring in the Martian crust, but the alteration conditions are poorly constrained. This study presents the results of experiments which examined the alteration of a high-iron basalt by CO2-saturated, aqueous fluids at 23 and 75 °C and by mixed H2O-CO2 vapors at 200 and 400 °C and water-rock ratios of 1:1 and 1:10. Results indicate that observable alteration of the basalt takes place after runs of only seven days. This alteration includes mobilization of silica into phases such as opal-CT and quartz, as well as the formation of carbonates, oxides, and at some conditions zeolites and hydrous silicates. The degree of alteration increases with run temperature and, in high-temperature vapor experiments, with increasing water content of the vapor. The degree of alteration and the mineralogy observed in the Martian meteorites suggests that none of these samples was exposed to aqueous fluids for long periods of time. Nakhla and Lafayette probably interacted with water for relatively brief periods of time; if so, silica may have been leached from the parent rocks by the altering fluids. ALH84001 shows possible evidence for very limited interaction with an aqueous fluid but the overall slight degree of alteration described for this meteorite strongly suggests that never interacted extensively or at high temperature with any water-bearing fluid. EETA79001 may not have been altered by aqueous fluids at all. The results of this study best support models wherein the meteorite parent rocks were wetted intermittently or for brief periods of time rather than models which invoke long-term reaction with large volumes of water.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Glass-bearing inclusions in olivine of the Chassigny achondrite: Heterogeneous trapping at sub-igneous temperatures
M. E. Varela*, G. Kurat, M. Bonnin-Mosbah, R. Clocchiatti and D. Massare
*Correspondence author's address: Universidad Nacional del Sur-CONICET, Departamento de Geologia, San Juan 670, (8000) Bahia Blanca, Pcia. Buenos Aires, Argentina; e-mail address: evarela@criba.edu.ar
Abstract–Three types of glass-bearing inclusions are present in olivine and chromite of the Chassigny achondrite: Pure glass, monocrystal (glass plus a single mineral grain) and multiphase (glass plus a variety of minerals) inclusions. The occurrence, texture and mineralogy of these inclusions and the chemical composition of the glass suggest an origin by heterogeneous trapping of these phases. The glass is rich in SiO2, Al2O3, Na2O, K2O and poor in MgO, FeO and CaO and contains appreciable amounts of Cl. The compositional variability of the glass is independent of the mineral content of the inclusions. Heating experiments with final temperatures of 900 °C, 1000 °C and 1200 °C were performed with Chassigny inclusions for the first time. The glass of the heated inclusions has a chemical composition similar to that of unheated inclusions. This situation suggests that the glass cannot be a residual melt but rather is an independent component which was trapped with or without mineral phases. The extreme heterogeneity in alkali contents and in particular Rb and Sr contents also suggests precipitation and mixing of solid precursors. The most Rb-rich glasses have near-chondritic Rb/Sr ratios possibly indicating a chondritic source for their precursor(s). None of the inclusions contains a bubble like those of typical melt inclusions in terrestrial igneous minerals. Furthermore, many inclusions are the center of radial cracks in the host olivine indicating development of an overpressure within the inclusions at some time. A volume increase of the inclusions could have been achieved by differential thermal expansion of the content of the inclusion during a heating event. That mechanism requires bubble-free and solid pre-heating inclusion contents. These features are incompatible with an origin of the inclusions by trapping of a silicate melt and point toward heterogeneous trapping of solid phases. The first nitrogen analyses performed in Chassigny glass-bearing inclusions by Nuclear Reaction Analysis (NRA) revealed high and variable N contents of the glass suggesting trapping of a solid precursor presumably at relative low temperatures from a fluid rather than a melt. In conclusion, the glass-bearing inclusions in Chassigny olivine are not a residual after a closed-system evolution of a trapped melt but rather heterogeneously trapped precipitates of a fluid that existed during formation of Chassigny constituents. Consequently, it is very unlikely that the host olivine has an igneous origin.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Spallation recoil and age of presolar grains in meteorites
U. Ott* and F. Begemann
*Correspondence author's address: Max-Planck-Institut für Chemie, Becherweg 27, D-55128 Mainz, Germany; e-mail address: ott@mpch-mainz.mpg.de
Abstract–We have determined the recoil losses from silicon carbide grain size fractions of spallation neon produced by irradiation with 1.6 GeV protons. During the irradiation the SiC grains were dispersed in paraffin wax in order to avoid re-implantation into neighboring grains. Analysis for spallogenic 21Ne of grain size separates in the size range 0.3 µm to 6 µm and comparison with the 22Na activity of the SiC+paraffin mixture indicates an effective recoil range of 2–3 µm with no apparent effect from acid treatments such as routinely used in the isolation of meteoritic SiC grains. Our results indicate that the majority of presolar SiC grains in primitive meteorites, which are about micron-sized, will have lost essentially all spallogenic Ne produced by cosmic ray interaction in the interstellar medium. This argues against the validity of previously published presolar ages of Murchison SiC (~10 to ~130 Ma; increasing with grain size; Lewis et al., 1994), where recoil losses had been based on calculated recoil energies.
It is argued that the observed variations in meteoritic SiC grain size fractions of 21Ne/22Ne ratios are more likely due to the effects of nucleosynthesis in the He burning shell of the parent AGB stars which imposes new boundary conditions on nuclear parameters and stellar models. It is suggested that spallation-Xe produced on the abundant Ba and REE in presolar SiC, rather than spallogenic Ne, may be a promising approach to the presolar age problem. There is a hint in the currently available Xe data (Lewis et al., 1994) that the large (>1 µm) grains may be younger than the smaller (<1µm) ones.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Shock magnetism in fine particle iron
Tamara L. Dickinson and Peter Wasilewski*
*Correspondence author's address: Mail Code 691,Goddard Space Flight, Center Greenbelt, Maryland 20771, USA; e-mail address: ulpjw@lepvax.gsfc.nasa.gov
Abstract–All solid solar system bodies have been affected by impact to varying degrees, and, thus, magnetic records in these bodies may have been modified by shock events. Shock events may have overprinted all primordial magnetic records in meteorites. Shock metamorphism stages ranging from very little to extreme, when melting takes place, have been identified in meteorites. We are examining the creation and destruction of magnetic remanence associated with shock. In this paper, we develop a preliminary framework for understanding the magnetic properties of fine-grained Fe particles (20–110 nm), which carry most of the remanent magnetization in lunar and by extension the kamacite phase in meteorite samples. Initial experiments on shock effects due to a first-order shock induced crystallographic transformation are described. The first characterization of pre- and post-shock magnetic properties for sized iron particles and the first characterization of the transformation remanent magnetization (TMRM) associated with the face-centered-cubic (fcc) to body-centered-cubic (bcc) transformation in fine particle iron spheres are described. This is equivalent to the 13 GPa transitions in bcc iron. We show that the TMRM is in the same direction as the ambient magnetic field present during the shock, but is deflected from the field direction by 30-45°, and that the remanence intensity is 1–2 orders of magnitude less than expected for thermoremanent magnetization (TRM) acquired during cooling through the Curie temperature. Isothermal remanence acquisition curves (RA) reveal the increasing magnetic hardness due to shock. Magnetic hysteresis loops are used to characterize the particle size and the shock induced magnetic anisotropy. Thermal demagnetization experiments describe the probable presence of particle size effects and the effects associated with recovery-recrystallization due to the annealing that takes place during the thermomagnetic experiment. These observations have implications for paleofield determinations and the recognition of thermal unblocking. A TMRM mechanism could produce a shock overprint in a meteorite and might impart a significant directional feature in an asteroid magnetic signature.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Infrared, ultraviolet and electron paramagnetic resonance measurements on presolar diamonds: Implications for optical features and origin
A. Braatz*, U. Ott, Th. Henning, C. Jäger and G. Jeschke
*Correspondence author's address: Max-Planck-Institut für Chemie, Becherweg 27, D-55128 Mainz, Germany; e-mail address: braatz@mpch-mainz.mpg.de
Abstract–Infrared and ultraviolet absorption spectra were obtained for diamonds from the Allende and Murchison meteorites. In addition, and for the first time, electron paramagnetic resonance spectra were measured. The IR and UV data confirm the suspicion of Russell et al. (1996) that nitrogen in presolar diamonds predominantly appears in form of dispersed nitrogen atoms, as it is the case for terrestrial type Ib diamonds.
In accordance with other observations, our electron paramagnetic resonance measurements suggest a high hydrogen content in presolar diamonds. The presolar diamonds most likely originated in a hydrogen-rich region, an environment in which nanometer-sized diamonds may be more stable than graphite (Badziag et al., 1990). This adds to the evidence—previously based mainly on the twin microstructures of presolar diamonds (Daulton et al., 1996) and the absence of graphite with the same isotopic composition as presolar diamonds (Anders and Zinner, 1993)—for a homogeneous nucleation of presolar diamonds from a gas phase.
Based on our results, for detection of diamonds in space, we suggest to search for the nitrogen-induced infrared and ultraviolet absorption features of type Ib diamonds. Other characteristic diamond features which could also be used to detect diamonds in space are: The (–CHn) infrared absorption features due to hydrogen coated diamonds, as they are described by Allamandola et al. (1993) and the infrared multiphonon absorption features of the diamond lattice. The multiphonon features are very weak (Edwards, 1985), but their intensity increases somewhat with increasing temperature (Collins and Fan, 1954), so perhaps a search for them is not totally hopeless.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Petrology and chemistry of the new shergottite Dar al Gani 476
J. Zipfel*, P. Scherer, B. Spettel, G. Dreibus and L.Schultz
*Correspondence author's address: Max-Planck-Institut für Chemie, Abteilung Kosmochemie, Postfach 3060, 55020 Mainz, Germany; e-mail address: zipfel@mpch-mainz.mpg.de
Abstract–In 1998, Dar al Gani 476, was found in the Libyan desert. The meteorite is classified as a basaltic shergottite and is only the thirteenth Martian meteorite known to date. It has a porphyritic texture consisting of a fine-grained groundmass and larger olivines. The groundmass consists of pyroxene and feldspathic glass. Minor phases are oxides and sulfides as well as phosphates. The presence of olivine, orthopyroxene and chromite is a feature DaG 476 has in common with lithology A of EETA79001. However, in DaG 476 these phases appear to be early phenocrysts rather than xenocrysts. Shock features such as twinning, mosaicism and impact melt pockets are ubiquitous. Terrestrial weathering was severe and led to formation of carbonate veins following grain boundaries and cracks.
With a molar MgO/(MgO+FeO) of 0.68, DaG 476 is the most magnesian member among the basaltic shergottites. Compositions of augite and pigeonite and some of the bulk element concentrations are intermediate between those of lherzolitic and basaltic shergottites. However, major elements such as Fe and Ti as well as LREE concentrations are considerably lower than in other shergottites.
Noble gas concentrations are low and dominated by the mantle component previously found in Chassigny. A component, similar to that representing Martian atmosphere, is virtually absent. The ejection age of 1.35 ± 0.10 Ma is older than that of EETA79001 and could possibly mark a distinct ejection.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Noble gases in iddingsite from the Lafayette meteorite: Evidence for liquid water on Mars in the last few hundred million years
T. D. Swindle*, A. H. Treiman, D. J. Lindstrom, M. K. Burkland, B. A. Cohen, J. A. Grier, B. Li and E. K. Olson
*Correspondence author's address: Lunar and Planetary Laboratory, University of Arizona, 1629 East University Boulevard, Tucson, Arizona 85721-0092, USA; e-mail address: tswindle@u.arizona.edu
Abstract–We analyzed noble gases from 18 samples of weathering products ("iddingsite") from the Lafayette meteorite. Potassium-argon ages of 12 samples range from near zero to 670 ± 91 Ma. These ages confirm the martian origin of the iddingsite, but it is not clear whether any or all of the ages represent iddingsite formation, as opposed to later alteration or incorporation of martian atmospheric 40Ar. In any case, since iddingsite formation requires liquid water, this data requires the presence of liquid water near the surface of Mars at least as recently as 1300 Ma ago, and probably as recently as 650 Ma ago. Krypton and xenon analysis of a single 34 (g sample indicates the presence of fractionated martian atmosphere within the iddingsite. This also confirms the martian origin of the iddingsite. The mechanism of incorporation could either be through interaction with liquid water during iddingsite formation, or a result of shock implantation of adsorbed atmospheric gas.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Hydrocode modeling of oblique impacts: The fate of the projectile
E. Pierazzo* and H. J. Melosh
*Correspondence author's address: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA; e-mail address: betty@lpl.arizona.edu
Abstract–All impacts are oblique to some degree. Only rarely do projectiles strike a planetary surface (near) vertically. The effects of an oblique impact event on the target are well known, producing craters that appear circular even for low impact angles (greater than 15° with respect to the surface). However, we still have much to learn about the fate of the projectile, especially in oblique impact events. This work investigates the effect of angle of impact on the projectile.
Sandia's three-dimensional hydrocode CTH was used for a series of high-resolution simulations (50 cells per projectile radius) with varying angle of impact. Simulations were carried out for impacts at 90°, 60°, 45°, 30°, and 15° from the horizontal, while keeping projectile size (5 km in radius), type (dunite), and impact velocity (20 km/s) constant.
The 3D hydrocode simulations presented here show that in oblique impacts the distribution of shock pressure inside the projectile (and in the target as well) is highly complex, possessing only bilateral symmetry, even for a spherical projectile. Available experimental data suggest that only the vertical component of the impact velocity plays a role in an impact. If this were correct, simple theoretical considerations indicate that shock pressure, temperature, and energy would depend on sin2 theta, where theta is the angle of impact (measured from the horizontal). However, our numerical simulations show that the mean shock pressure in the projectile is better fit by a sin( dependence, while shock temperature and energy depend on sin3/2 theta. This demonstrates that in impact events the shock wave is the result of complex processes that cannot be described by simple empirical rules. The mass of shock melt or vapor in the projectile decreases drastically for low impact angles, as a result of the weakening of the shock for decreasing impact angles. In particular, for asteroidal impacts the amount of projectile vaporized is always limited to a small fraction of the projectile mass. In cometary impacts, however, most of the projectile is vaporized even at low impact angles.
In the oblique impact simulations a large fraction of the projectile
material retains a net downrange motion. In agreement with experimental
work, the simulations show that for low impact angles (30° and 15°),
a downrange focusing of projectile material occurs, and a significant amount
of it travels at velocities larger than the escape velocity of Earth.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Light rare earth element-enrichments in ureilites: A detailed ion microprobe study
Yunbin Guan* and Ghislaine Crozaz
*Correspondence author's address: Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119, USA; e-mail address: guany@volcano.si.edu
Abstract–This paper explores the possible origin of the light rare earth (LREE) enrichments observed in some ureilites, a question that has both petrogenetic and chronologic implications for this group of achondritic meteorites. REE and other selected elemental abundances were measured in situ in 14 thin sections representing 11 different ureilites. The spatial microdistributions of REEs in C-rich matrix areas of the three ureilites with the most striking V-shaped whole-rock REE patterns (Kenna, Goalpara, and Novo Urei) were investigated using the ion imaging capability of the ion microprobe.
All olivines and clinopyroxenes measured have LREE-depleted patterns
with little variation in REE abundances, despite large differences in their
major element compositions from ureilite to ureilite. Furthermore,
we searched for but did not find any minor mineral phases that carry LREEs.
The only exception is one Ti-rich area (~20 µm) in LEW85400 with
a major element composition similar to that of titanite; REE abundances
in this area are high, ranging from La ~ 400 x CI to Lu ~ 40 x CI.
In contrast, all ion microprobe analyses of C-rich matrix in Kenna, Goalpara,
and Novo Urei revealed large LREE enrichments. In addition, C-rich
matrix areas in the three polymict ureilites, EET83309, EET87720, and North
Haig, which have less pronounced V-shaped whole-rock REE patterns, show
smaller but distinct LREE-enrichments. C-rich matrix in Antarctic
ureilites tends to have much lower LREE concentrations than the matrix
in non-Antarctic ureilites. There is no obvious association of the LREEs
with other major or minor elements in the C-rich areas. Ion images
further show that the LREE enrichments are homogeneously distributed on
a microscale in most C-rich matrix areas of Kenna, Goalpara, and Novo Urei.
These observations suggest that the LREEs in ureilites most probably are
absorbed on the surface of fine-grained amorphous graphite in the C-rich
matrix. It is unlikely that the LREE-enrichments are due to shock
melts or are the products of metasomatism on the ureilite parent body.
We favor LREE introduction by terrestrial contamination.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Noble gas record, collisional history and pairing of CV, CO, CK and other carbonaceous chondrites
P. Scherer and L. Schultz*
*Correspondence author's address: Max-Planck-Institut für Chemie (Otto-Hahn-Institut), Postfach 3060 ,55020 Mainz, Germany; e-mail address: schultz@mpch-mainz.mpg.de
Abstract–Concentration and isotopic composition of the light noble gases as well as of 84Kr, 129Xe, and 132Xe have been measured in bulk samples of 60 carbonaceous chondrites, 45 were measured for the first time. Solar noble gases were found in 9 specimens (Arch, Acfer 094, Dar al Gani 056, GRA 95229, Grosnaja, Isna, PRE 95404, Y 86009 and Y 86751). These meteorites are thus regolith breccias. CV and CO chondrites contain abundant planetary-type noble gases, but not CK chondrites. A characteristic feature of CK chondrites are high 129Xe/132Xe ratios. The petrologic type of carbonaceous chondrites is correlated with the concentration of trapped heavy noble gases, similar as observations have shown for ordinary chondrites. However, this correlation is disturbed for several meteorites due to a contribution of atmospheric noble gases, an effect correlated to terrestrial weathering effects.
Cosmic-ray exposure ages are calculated from cosmogenic 21Ne.
They range from about 1 Ma to 63.5 Ma for CO, CV and CK classes which is
longer than exposure ages reported for CM and CI chondrites. Only the CO3
chondrite Isna has an exceptionally low exposure age of 0.15 Ma. No dominant
clusters are observed in the cosmic-ray exposure age distribution, only
for CV and CK chondrites potential peaks seem to develop at ~ 9 Ma and
~ 29 Ma. Several pairings among the chondrites from hot deserts are
suggested but 52 of the 60 investigated meteorites are individual falls.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Mössbauer spectroscopic studies of Piplia Kalan (Eucrite) and Lohawat (Howardite) meteorites
R. P. Tripathi, S. K. Sharma, K. L. Shrivastava and H. C. Verma*
*Correspondence author's address: Department of Physics, Indian Institute of Technology, Kanpur 208 010, India; e-mail address: hcverma@iitk.ac.in
Abstract–Two meteorites belonging to the HED group fell recently in Rajasthan, India. One of these, Piplia Kalan was classified as a eucrite and the other, Lohawat as a howardite. In this study we present the results of Mössbauer spectroscopic investigations of these two meteorites. We also compare the results with the Mössbauer experiments reported for Kapoeta howardite and look for systematics in the Mössbauer spectra of HED meteorites.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Magnesium isotopic fractionations in barred olivine chondrules from the Allende meteorite
Keiji Misawa* and Takashi Fujita
*Correspondence author's address: Antarctic Meteorite Research Center, National Institute of Polar Research, Kaga 1-9-10, Tokyo 173-8515, Japan; e-mail address: misawa@nipr.ac.jp
Abstract–The Mg isotope composition in five barred olivine (BO) chondrules, one coarse-grained rim of BO chondrule, a relic spinel in BO chondrule, one skeletal olivine chondrule similar to BO chondrule in mineralogy and composition, and two non-BO chondrules from the Allende meteorite has been measured by thermal ionization mass spectrometry. The Mg isotopes are not fractionated and within terrestrial standard values (±2.0‰ per amu) in seven of the analyzed eight ferromagnesian chondrules. A clump of relic spinel grain and its host BO chondrule R-11 give well resolvable Mg fractionations which show an enrichment of the heavier isotopes, up to +2.5‰ per amu. The Mg isotopic compositions of coarse-grained rim are identical to those of the host chondrule with barred olivine texture. The results imply that ferromagnesian and refractory precursor components of Allende chondrule may have been formed from isotopically heterogeneous reservoirs. In the nebula region where Allende chondrules formed, recycling of chondrules and multiple high-temperature heating did not significantly alter the chemical and isotopic memory of earlier generations.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Invited Review
Small is beautiful: The analysis of nanogram-sized astromaterials
M. E. Zolensky*, C. Pieters, B. Clark and J. J. Papike
*Correspondence author's address: Planetary Science Branch, NASA, Johnson Space Center, Houston, Texas 77058, USA; e-mail address: michael.e.zolensky1@jsc.nasa.gov
Abstract–The capability of modern methods to characterize ultra-small samples is well established from analysis of interplanetary dust particles (IDPs), interstellar grains recovered from meteorites, and other materials requiring ultra-sensitive analytical capabilities. Powerful analytical techniques are available that require, under favorable circumstances, single particles of only a few nanograms for entire suites of fairly comprehensive characterizations. A returned sample of >1,000 particles with total mass of just one microgram permits comprehensive quantitative geochemical measurements that are impractical to carry out in situ by flight instruments. The main goal of this paper is to describe the state-of-the-art in microanalysis of astromaterials.
Given that we can analyze fantastically small quantities of asteroids and comets, etc., we have to ask ourselves how representative are microscopic samples of bodies that measure a few to many km across? With the Galileo flybys of Gaspra and Ida, it is now recognized that even very small airless bodies have indeed developed a particulate regolith. Acquiring a sample of the bulk regolith, a simple sampling strategy, provides two critical pieces of information about the body. Regolith samples are excellent bulk samples since they normally contain all the key components of the local environment, albeit in particulate form. Furthermore, since this fine fraction dominates remote measurements, regolith samples also provide information about surface alteration processes and are a key link to remote sensing of other bodies. Studies indicate that a statistically significant number of nanogram-sized particles should be able to characterize the regolith of a primitive asteroid, although the presence of larger components within even primitive meteorites (e.g.. Murchison), e.g. chondrules, CAI, large crystal fragments, etc., points out the limitations of using data obtained from nanogram-sized samples to characterize entire primitive asteroids. However, most important asteroidal geological processes have left their mark on the matrix, since this is the finest-grained portion and therefore most sensitive to chemical and physical changes. Thus, the following information can be learned from this fine grain size fraction alone: (1) mineral paragenesis; (2) regolith processes, (3) bulk composition; (4) conditions of thermal and aqueous alteration (if any); (5) relationships to planets, comets, meteorites (via isotopic analyses, including oxygen; (6) abundance of water and hydrated material; (7) abundance of organics; (8) history of volatile mobility, (9) presence and origin of presolar and/or interstellar material. Most of this information can even be obtained from dust samples from bodies for which nanogram-sized samples are not truly representative.
Future advances in sensitivity and accuracy of laboratory analytical techniques can be expected to enhance the science value of nano- to microgram sized samples even further. This highlights a key advantage of sample returns - that the most advanced analysis techniques can always be applied in the laboratory, and that well-preserved samples are available for future investigations.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Micro Raman spectroscopy of amphiboles and pyroxenes in the Martian meteorites Zagami and Lewis Cliff 88516
Takashi Mikouchi* and Masamichi Miyamoto
*Correspondence author's address: Mail Code SN2, Planetary Science Branch, NASA Johnson Space Center, Houston, Texas 77058, USA; e-mail address: TMikochi@ems.jsc.nasa.gov
Abstract–We studied micro Raman spectroscopy of amphiboles and pyroxenes in the Martian meteorites Zagami and LEW88516. The obtained Raman spectra of the amphiboles are similar to those of kaersutite, reconfirming the previous studies that they are kaersutitic amphiboles enriched in Ca, Al and Ti. Even though actinolite belongs to the same amphibole group (calcic amphibole) as kaersutite, the Raman spectra of terrestrial actinolite are distinct from those of kaersutite, probably reflecting complex amphibole crystal structures. Al-Ti-rich pyroxene observed in the magmatic inclusions within LEW88516 olivine is compositionally similar to kaersutite, but shows Raman spectra nearly identical to the regular pyroxene rather than amphibole. In contrast to amphibole, this will be due to relatively simple crystal structures of pyroxene. Thus, the Raman spectra of Al-Ti-rich phases in the Martian meteorites are distinct between kaersutite and Al-Ti-rich pyroxene, and this study demonstrates that micro Raman spectroscopy is one of the best tools to perform mineralogical characterization of mineral phases in Martian meteorites.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Refractory forsterite in primitive meteorites: Condensates from the solar nebula?
S. Weinbruch*, H. Palme and B. Spettel
*Correspondence author's address: Institut für Mineralogie, Technische Universität Darmstadt, Schnittspahnstr. 9, D-64287 Darmstadt, Germany; e-mail address: dh6d@hrzpub.tu-darmstadt.de
Abstract–All groups of chondritic meteorites contain discrete grains of forsteritic olivine with FeO contents below 1 wt% and high concentrations of refractory elements, such as Ca, Al and Ti. Ten such grains (52 to 754 µg) with minor amounts of adhering matrix were separated from the Allende meteorite. After bulk chemical analysis by INAA, samples were analyzed with the electron microprobe and some with the ion microprobe.
Matrix that accreted to the forsterite grains has a well-defined, unique composition, different from average Allende matrix in having higher Cr and lower Ni and Co contents, implying limited mixing of Allende matrix.
All samples have approximately chondritic relative abundances of refractory elements: Ca, Al, Sc and REE, although some of these elements, such as Al, do not quantitatively reside in forsterite, while others (e.g., Ca) are intrinsic to forsterite. The chondritic refractory element ratios in bulk samples, the generally high abundance level of refractory elements and the presence of Ca, Al, Ti-rich glass inclusions suggest a genetic relationship of refractory condensates with forsteritic olivine. The Ca, Al, Ti-rich glasses may have acted as nuclei for forsterite condensation.
Arguments are presented that exclude an origin of refractory forsterite by crystallization from melts with compositions characteristic of Allende chondrules: a) All forsterite grains have CaO contents between 0.5 and 0.7 wt% with no apparent zoning, requiring voluminous parental melts with 18 to 20 wt% CaO, far above the average CaO content of Allende chondrules. Similar arguments apply to Al contents. b) The low FeO content of refractory forsterite of 0.2–0.4 wt% imposes an upper limit of about 1 wt% of FeO on the parental melt, too low for ordinary and carbonaceous chondrule melts. c) The Mn contents of refractory forsterites are between 30 to 40 ppm. This is at least one order of magnitude below the Mn content of chondrule olivines in all classes of meteorites. The observed Mn contents of refractory forsterite are much too low for equilibrium between olivine and melts of chondrule composition. d) As shown earlier refractory forsterites have oxygen isotopic compositions different from chondrules (Weinbruch et al., 1993a).
Refractory olivines in carbonaceous chondrites are found in matrix and in chondrules. The compositional similarity of both types was taken to indicate that all refractory forsterites formed inside chondrules (e.g., Jones, 1992). As refractory forsterite cannot have formed by crystallization from chondrule melts, we conclude that refractory forsterite from chondrules are relict grains that survived chondrule melting and probably formed in the same way as refractory forsterite enclosed in matrix. We favor an origin of refractory forsterite by condensation from an oxidized nebular gas.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Crystal-bearing lunar spherules: Impact-melting of the Moon's crust and implications for the origin of meteoritic chondrules
Alex Ruzicka*, Gregory A. Snyder, and Lawrence A. Taylor
*Correspondence author's address: Portland Community College, Sylvania Campus, P.O. Box 19000, Portland, Oregon 97280, USA; e-mail address: aruzicka@uswest.net
Abstract–Crystal-bearing lunar spherules (CLSs) in lunar breccia (14313, 14315, 14318), soil (68001, 24105), and impact-melt-rock (62295) samples can be classified into two types: feldspathic and olivine-rich. Feldspathic CLSs contain equant, tabular, or acicular plagioclase grains set in glass or a pyroxene-olivine mesostasis; the less common olivine-rich CLSs contain euhedral or skeletal olivine set in glass, or possess a barred-olivine texture. Bulk-chemical and mineral-chemical data strongly suggest that feldspathic CLSs formed by impact-melting of mixtures of ferroan anorthosite and Mg-suite rocks that compose the feldspathic crust of the Moon. It is probable that olivine-rich CLSs also formed by impact-melting, but some appear to have been derived from distinctively magnesian lunar materials, atypical of the Moon's crust.
Some CLSs contain reversely-zoned "relict" plagioclase grains that were not entirely melted during CLS formation, thin (less than or equal to 5 µm thick) rims of troilite or phosphate, and chemical gradients in glassy mesostases attributed to metasomatism in a volatile-rich (Na-K-P-rich) environment. CLSs were rimmed and metasomatized prior to brecciation. Compound CLS objects are also present; these formed by low-velocity collisions in an environment, probably an ejecta plume, that contained numerous melt droplets.
Factors other than composition were responsible for producing the crystallinity of the CLSs. We agree with previous workers that relatively slow cooling rates and long ballistic travel times were critical features that enabled these impact-melt droplets to partially or completely crystallize in free-flight. Moreover, incomplete melting of precursor materials formed nucleation sites that aided subsequent crystallization.
Clearly, CLSs do not resemble meteoritic chondrules in all ways. The two types of objects had different precursors and did not experience identical rimming processes, and vapor-fractionation appears to have played a less important role in establishing the compositions of CLSs than of chondrules. However, the many detailed similarities between CLSs and chondrules indicate that it is more difficult to rule out an origin for some chondrules by impact-melting than some have previously argued. Differences between CLSs, chondrules, and their host rocks possibly can be reconciled with an impact-melt origin for some chondrules when different precursors, the higher gravity of the Moon compared to chondrite parent bodies, and the likely presence of nebular gas during chondrule formation are taken into account.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
The titanium contents of lunar mare basalts
Thomas A. Giguere*, G. Jeffrey Taylor, B. Ray Hawke and Paul G. Lucey
*Correspondence author's address: Hawaii Institute of Geophysics and Planetology, University of Hawaii, 2525 Correa Road, Honolulu, Hawaii 96822, USA; e-mail address: giguere@kahana.pgd.hawaii.edu
Abstract–Lunar mare basalt sample data suggest that there is a bimodal
distribution of TiO2 concentrations. Using a refined technique
for remote determination of TiO2, we find that the maria actually
vary continuously from low to high values. The reason for the discrepancy
is that the nine lunar sample return missions were not situated near intermediate
basalt regions. Moreover, maria with 2-4 wt.% TiO2 are
most abundant, and abundance decreases with increasing TiO2.
Maria surfaces with TiO2 >5 wt.% constitute only 20% of the
maria. Although impact mixing of basalts with differing Ti concentrations
may smear out the distribution and decrease the abundance of high-Ti basalts,
the distribution of basalt Ti contents probably reflects both the relative
abundances of ilmenite-free and ilmenite-bearing mantle sources.
This distribution is consistent with models of the formation of mare source
regions as cumulates from the lunar magma ocean.
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