Abstracts of Papers to be Published in the June 2001 Issue

Please contact the correspondence author for reprints of all published articles

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.

Phase equilibria of the Shergotty meteorite: Constraints on pre-eruptive water contents of martian magmas and fractional crystallization under hydrous conditions

J. C. Dann, A. H. Holzheid, T. L. Grove* and H. Y. McSween, Jr.

*Correspondence Author's address:  Department of Earth, Atmospheric, and Planetary Sciences, 54-1218, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA; e-mail address:  tlgrove@mit.edu

Abstract–An experimental investigation of the Shergotty meteorite was performed at 0.1 MPa under anhydrous conditions at the quartz-fayalite-magnetite buffer and at 100 and 200 MPa under H₂O-saturated conditions at the nickel-nickel oxide buffer. The results of these experiments are used to infer magmatic conditions recorded by co-crystallization of augite and pigeonite phenocrysts found in Shergotty and to investigate the effect of H₂O on fractional crystallization paths followed by shergottite magmas. The phase relations and compositions of the homogeneous magnesian pyroxene cores in Shergotty are most closely approximated by crystallization under H₂O-saturated conditions at 1120 ^oC (~10 ^oC) and 56 MPa (~18 MPa), corresponding to dissolved H₂O contents of 1.8 wt% (~0.6 wt%) and a depth of 5 km (~1.5 km) in the martian crust (uncertainties are 2σ values). The Shergotty magma then lost this water during ascent and eruption. Fractional crystallization of the Shergotty magma under anhydrous conditions produces liquids that follow a strong Fe-enrichment trend at nearly constant SiO₂. Crystallization under H₂O-saturated conditions generates derivative liquids, depleted in FeO and Al₂O₃ and enriched in SiO₂, that are compositionally similar to the Mars Pathfinder andesite rock composition. The presence of ~1.8 wt% water in Shergotty parental magmas could result from assimilation of hydrated crustal materials or from dehydration of hydrous phases in the mantle source region.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.

Vesta, Vestoids, and the HEDs: Relationships and the Origin of Spectral Differences

T. H. Burbine*, P. C. Buchanan, R. P. Binzel, S. J. Bus, T. Hiroi, J. L. Hinrichs, A. Meibom, and T. J. McCoy

*Correspondence Author's address:  Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington DC 20560-0119, USA; e-mail address:  burbine.tom@nmnh.si.edu

Abstract–Spectra of asteroid 4 Vesta and twenty small (estimated diameters less than 10 km) asteroids with Vesta-like spectral properties (Vestoids) were measured at visible and near-infrared wavelengths (~0.44 to ~1.65 µm). All of the measured small asteroids (except for 2579 Spartacus) have reflectance spectra consistent with surface compositions similar to eucrites and howardites and consistent with all being derived from Vesta. None of the observed asteroids have spectra similar to diogenites. We find no spectral distinction between the fifteen objects tabulated as members of a Vesta dynamical family and six of the sampled "non-family" members that reside just outside the semi-major axis (a), eccentricity (e), and inclination (i) region of the family. The spectral consistency and close orbital (a-e-i) match to Vesta and the Vesta family imply that the true bounds of the family extend beyond the subjective cut-off for membership. Asteroid 2579 Spartacus has a spectrum consistent with a mixture of eucritic material and olivine. Spartacus could contain olivine-rich material from Vesta's mantle or may be unrelated to Vesta altogether. Laboratory measurements of the spectra of eucrites show that samples having nearly identical compositions can display a wide range of spectral slopes. Finer particle sizes lead to an increase in the slope, which is usually referred to as reddening. This range of spectral variation for the best-known meteoritic analogs to the Vestoids, regardless of whether they are actually related to each other, suggests that the extremely red spectral slopes for some Vestoids can be explained by very fine-grained eucritic material on their surfaces.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.

Identification of Shocked Quartz by Scanning Cathodoluminescence Imaging

Sam Boggs, Jr.*, David H. Krinsley, Gordon G. Goles, Abbas Seyedolali and Henning Dypvik

*Correspondence Author's address:  Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, USA; e-mail address:  sboggs@oregon.uoregon.edu

Abstract–Quartz grains subjected to high-strain-rate shock waves owing to meteorite or cometary impact on Earth's surface commonly display shock lamellae. These lamellae appear as remarkably straight, thin, planar features (microstructures) in sets within which lamellae are essentially parallel to each other and spaced <20 µm apart. Two or more intersecting sets are typically present. Shock lamellae are commonly recognized and identified by optical methods, by use of the transmission electron microscope (TEM), and by etching polished sections and subsequent examination with a scanning electron microscope (SEM) operated in the secondary-electron mode. We present here a method for observing planar microstructures in shocked quartz by using a cathodoluminescence (CL) detector attached to a SEM. The method relies on the fact that planar microstructures in quartz arising as a result of shock display no CL whatever; thus, they show up as distinct, thin, black lines on otherwise luminescent quartz grains. We used scanning CL imaging to study shocked quartz from the Ries Crater, Germany, a well-known impact crater of Miocene age. We demonstrate that shock-produced planar microstructures are clearly displayed in SEM-CL images and can be distinguished from microfractures generated by tectonism, and subsequently filled with quartz, and other similar features not related to impact events. The SEM-CL method provides a powerful supplement to other methods of identifying shocked quartz. It commonly provides better spatial resolution than does standard optical methods, and does not require etching of quartz grains. Further, it is easier and faster to use than are TEM methods, although it is not capable of the fine-scale defect analysis possible with TEM.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.

Meteorite stranding surfaces and the Greenland icesheet

Ralph P. Harvey*, Anders Meibom and Henning Haack

*Correspondence Author's address:  Department of Geological Sciences, 112 A.W. Smith Building, Case Western Reserve University, Cleveland, OH 44106-7216, USA; e-mail address:  rph@po.cwru.edu

Abstract–30 years of recoveries in East Antarctica have led to significant understanding of the regional characteristics associated with meteorite stranding surfaces. In Antarctica these sites are characterized by patches of snow-free blue ice at high altitude on the icesheet in regions where iceflow is highly restricted. Melting is extremely rare or absent and sublimation rates are high, even though meteorite stranding surfaces are predominantly found within regions where accumulation typically dominates. Localized environmental conditions that persist for thousands of years or longer appear to be the dominant factor rather than shorter-term or seasonal cycles. In this paper we describe our discovery of regions in Northeast Greenland with blue ice areas that exhibit many of the requisite characteristics, suggesting that they are excellent prospects for future meteorite recovery efforts.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.

The Role of Regolith Redistribution in Influencing The Evolution of the Shapes of Asteroids

Lionel Wilson* and Klaus Keil

*Correspondence Author's address:  Hawai'i Institute of Geophysics and Planetology, University of Hawai'i at Manoa, 2525 Correa Road, Honolulu HI 96822, USA; e-mail address:  L.Wilson@Lancaster.ac.uk

Abstract–Major surface fissures and relatively large-scale, angular surface irregularities are expected to have been present on many asteroids at early stages in their histories as a byproduct of at least two processes (impact disruption and reassembly into rubble piles for all classes of asteroid and, for carbonaceous chondrite parent bodies, aqueous alteration) which led to the low bulk densities currently being observed for asteroids. However, in all cases where high-enough resolution images exist, such abrupt, deep irregularities are not observed. We model the spatial redistribution of impact-generated regolith on an asteroid with an idealized irregular shape to show how the complex gravitational field of such a body will lead to the systematic infilling of deep valleys in the surface. Our analysis emphasizes the high efficiency with which regolith redistribution can act to disguise the internal structures of asteroids with sizes in the 20-100 km range.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.

Solar and galactic cosmic ray records of the Fermo (H) chondrite regolith breccia

G.Bonino, N. Bhandari*, S.V.S. Murty, R.R. Mahajan, K.M.Suthar, A.D.Shukla, P.N.Shukla, G.Cini Castagnoli and C.Taricco

*Correspondence Author's address:  Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India; e-mail address:  bhandari@prl.ernet.in

Abstract–We demonstrate the presence of solar flare as well as neutron capture effects in the isotopic composition of rare gases in the Fermo regolith breccia acquired on its parent body based on the measurements of tracks, rare gases and radionuclides. The track density along a 3.2 cm long core decreases by a factor of about 6 and by more than a factor of 13 within the meteorite, indicating small (2-9 cm) and asymmetrical ablation. Rare gases show a large trapped component; the isotopic ratios, particularly ²⁰Ne/²²Ne ~ 11 and ²⁰Ne/³⁶Ar = 10 are indicative of solar component. The galactic cosmic ray exposure age is determined to be 8.8 Ma. Activities of a dozen radionuclides ranging in half life from 16 day ⁴⁸V to 0.73 Ma ²⁶Al are consistent with their expected production rates. Track, rare gas and radionuclide data show that the meteoroid was a small body (<120 kg) and had a simple, one stage exposure history to cosmic rays in the interplanetary space. However, ⁸²Kr and ¹²⁸Xe show an excess due to neutron irradiation on the parent body of the meteorite. The presence of solar gases and the neutron capture effects indicate several stages of irradiation on the parent asteroid. The chemical composition of Fermo confirms that it belongs to the H group of ordinary chondrites with lithic clasts having varying compositions. δ¹⁵N is found to be 8.3 + 1.2%o, close to the typical values observed in H chondrites.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.

FeO in Mercury's crust and mantle

M.S. Robinson* and G.J. Taylor

*Correspondence Author's address:  Department of Geological Sciences, Northwestern University, Evanston, IL, USA; e-mail address:  robinson@eros.earth.northwestern.edu

Abstract–Mercury has widespread plains deposits proposed to be volcanic in origin. In a Mariner 10 color derived parameter image, sensitive to FeO and maturity, these volcanic plains have a value equivalent to, or slightly elevated above, the hemispheric average, thus implying FeO equivalent to, or slightly less than, the hemispheric average (~3 wt% FeO). Since FeO has a solid/liquid distribution coefficient ~1 during partial melting, we estimate the mantle of Mercury to have an FeO abundance equal to the lava flows. This is consistent with models that predict Mercury was assembled from planetesimals formed near the planet's current position. This new estimate of Mercury's bulk FeO (~3 wt%) is consistent with data for the other terrestrial planets that suggest there was a radial gradient in FeO in the solar nebula.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.

Isotope anomalies in tellurium and palladium from allende nanodiamonds

R. Maas, R.D. Loss, K.J.R. Rosman, J.R. De Laeter*, R. S. Lewis, G.R. Huss, and G.W. Lugmair

*Correspondence Author's address:  Department of Applied Physics, Curtin University of Technology, Western Australia; e-mail address:  rdelaete@cc.curtin.edu.au

Abstract–The elemental and isotopic abundances of Te and Pd have been measured by thermal ionization mass spectrometry in a purified sample of interstellar nanodiamonds from the Allende meteorite, after combustion of the diamonds in an oxygen plasma. Small positive anomalies were found in ¹²⁸Te (4.0 ± 1.5 %o) and ¹³⁰Te (9.3 ± 2.8 %o) from three analyses of the Allende nanodiamond sample EB, and in ¹¹⁰Pd (9.4 ± 5.7 %o) from two analyses of the same sample. No other anomalies of a significant nature were found in either Te or Pd. These results are consistent with the neutron-burst model (Meyer et al. 2000), and the r-process based rapid separation model of Ott (1996) in that ^128,130Te and ¹¹⁰Pd are enhanced relative to their solar abundances. However, in the framework of the neutron burst model, some separation between stable products and radioactive precursors may be required in order to be consistent with the full data set of Te isotopes. In the framework of the rapid separation scenario, our data suggests a separation time of approximately 4000 sec., based on the magnitudes of the precursor life-times for ¹²⁸Te and ¹³⁰Te. The elemental abundance ratio Te-H/Xe-H agrees with the prediction of the rapid separation model, provided little fractionation occurred during trapping of the exotic components by the diamonds, but differs significantly from expectations based on the neutron burst model. Differences in the inferred ¹²⁸Te/¹³⁰Te ratio between our work and that of Richter et al. (1998), point to the need for further investigations.

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