Please contact the correspondence author for reprints of all published articles
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
A Model for the Thermal Processing of Particles in Solar Nebula Shocks: Application to Cooling Rates of Chondrules
S.J. Desch* and H. C. Connolly, Jr.
*Correspondence author's address: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd. NW, Washington D.C., 20015, USA; e-mail address: desch@dtm.ciw.edu
Abstract–We present a model for the thermal processing of particles in shock waves typical of the solar nebula. This shock model improves on existing models in that the dissociation and recombination of H2 and the evaporation of particles are accounted for in their effects on the mass, momentum and energy fluxes. Also, besides thermal exchange with the gas and gas-drag heating, particles can be heated by absorbing the thermal radiation emitted by other particles. The flow of radiation is calculated using the equations of radiative transfer in a slab geometry. We compute the thermal histories of particles as they encounter and pass through the shock.
We apply this shock model to the melting and cooling of chondrules in the solar nebula.
We constrain the combinations of shock speed and gas density needed for chondrules to reach melting temperatures, and show that these are consistent with shock waves generated by gravitational instabilities in the protoplanetary disk. After their melting, cooling rates of chondrules in the range 10 - 1000 K\hr-1 are naturally reproduced
by the shock model. Chondrules are kept warm by the reservoir of hot shocked gas, which
cools only as fast as the dust grains and chondrules themselves can radiate away the gas's energy. We predict a positive correlation between the concentration of chondrules in a region and the cooling rates of chondrules in that region. This correlation is supported by the unusually high frequency of (rapidly cooled) barred chondrules among compound chondrules, which must have collided preferentially in regions of high chondrule
density. We discuss these and other compelling consistencies between the meteoritic
record and the shock wave model of chondrule formation.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Campo del Cielo iron meteorite: Sample shielding and meteoroid's preatmospheric size
Liberman R. G.*, Fernández Niello J. O., di Tada M. L., Fifield L. K., Masarik J., and Reedy R. C.
*Correspondence author's address: MIT, 77 Massachusetts Avenue, Room 56-738, Cambridge, MA 02139-4307, USA; e-mail address: liberman@mit.edu
Abstract–Long-lived cosmogenic radioisotopes, 10Be, 26Al, 36Cl, 41Ca and 59Ni, have been measured in five samples from the Campo del Cielo iron meteorite by accelerator mass spectrometry (AMS). The 36Cl activities were significantly above the background. For the concentrations of the other four radioisotopes, only upper limits were obtained that were, however, consistent with the 36Cl result. The measured 36Cl activity allowed an estimate of the meteoroid's preatmospheric size: a radius larger than 300 cm and a mass of at least 840,000 kg. We conclude that this meteorite might be one of the largest meteorites to have been recovered
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Plagioclase-rich chondrules in the reduced CV chondrites: evidence for complex formation history and genetic links between CAIs and ferromagnesian chondrules
Alexander N. Krot*, Ian D. Hutcheon and Klaus Keil
*Correspondence author's address: Hawai'i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai'i at Manoa, Honolulu, Hawaii 96822, USA; e-mail address: sasha@higp.hawaii.edu
Abstract–Plagioclase-rich chondrules (PRCs) in the reduced CV chondrites Efremovka, Leoville, Vigarano and GRO 94329 consist of magnesian low-Ca pyroxene, Al-Ti-Cr-rich pigeonite and augite, forsterite, anorthitic plagioclase, FeNi-metal-sulfide nodules, and crystalline mesostasis composed of silica, anorthitic plagioclase and Al-Ti-Cr-rich augite. The silica grains in the mesostases of the CV PRCs are typically replaced by hedenbergitic pyroxenes, whereas anorthitic plagioclase is replaced by feldspathoids (nepheline and minor sodalite). Some of the PRCs contain regions that are texturally and mineralogically similar to Type I chondrules and consist of forsterite, low-Ca pyroxene and abundant FeNi-metal nodules. Several PRCs are surrounded by igneous rims or form independent compound objects. Twelve PRCs contain relict CAIs composed of anorthite, spinel, high-Ca pyroxene, ±forsterite, and ±Al-rich low-Ca pyroxene. Anorthite of these CAIs is generally more heavily replaced by feldspathoids than anorthitic plagioclase of the host chondrules. This suggests that either the alteration predated formation of the PRCs or that anorthite of the relict CAIs was more susceptible to the alteration than anorthitic plagioclase of the host chondrules. These observations and the presence of igneous rims around PRCs and independent compound PRCs suggest that the CV PRCs may have had a complex, multistage formation history compared to a more simple formation history of the CR PRCs.
Relatively high abundances of moderately-volatile elements such as Cr, Mn and Si in the PRCs suggests that these chondrules could not have been produced by volatilization of the ferromagnesian chondrule precursors or by melting of the refractory materials only. We infer instead that PRCs in carbonaceous chondrites formed by melting of the reduced chondrule precursors (magnesian olivine and pyroxene, FeNi-metal) mixed with the refractory materials (relict CAIs) composed of anorthite, spinel, high-Ca pyroxene, and forsterite. The mineralogical, chemical and textural similarities of the PRCs in several carbonaceous chondrite groups (CV, CO, CH, CR) and common presence of relict CAIs in these chondrules suggest that PRCs may have formed in the region(s) intermediate between the regions where CAIs and ferromagnesian chondrules originated.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
The Frontier Mountain meteorite trap (Antarctica)
Luigi Folco*, Alessandro Capra, Massimo Chiappini, Massimo Frezzotti, Marcello Mellini and Ignazio Ezio Tabacco
*Correspondence author's address: Museo Nazionale dell'Antartide, Università di Siena, via Laterina 8, I-53100 Siena, Italy; e-mail address: folco@unisi.it
Abstract–The Frontier Mountain blue ice field is an important Antarctic meteorite trap which has yielded 472 meteorite specimens since its discovery in 1984. Remote sensing analyses and field campaigns from 1993 to 1999 have furnished new glaciological data on ice flow, ice thickness, bedrock topography, ice ablation and surface mass transport by wind, along with detailed descriptions of the field situation at the trap. This solid set of data combined with an updated meteorite distribution map and terrestrial ages available from literature allows us to better describe the nature of the concentration mechanism. In particular, we observe that the meteorite trap forms in a blue ice field i) located upstream of an absolute and a shallow sub-ice barriers; ii) characterized by compressive ice flow with horizontal velocities decreasing from 100 to <10 cm a-1 on approaching the obstacle; iii) undergoing mean ablation rates of 6.5 cm a-1; iv) nourished by a limited snow accumulation zone extending ~20 km upstream of the blue ice area. We also draw the following conclusions: i) the origin of the meteorite trap can be explained according to the present-day glaciological situation; ii) the meteorite concentration develops according to the general principles of the "ice flow model"; iii) the accumulation model can be described as "stagnant ice or slow-moving ice against an absolute and submerged barriers", according to the descriptive schemes present in literature; iv) the Frontier Mountain ice field is an effective trap for meteorites weighing more than ~200 g; for smaller masses, the combination of wind and glacial drift may remove meteorites in less than a few tens of ka; v) although the activation age of the Frontier Mountain trap is not yet constrained, we infer that one of the most important findsites may be as old as 50 ka, predating the last glacial maximum.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Fine-grained rims in the ALH 81002 and LEW 90500 CM2 meteorites: Their origin and modification
Xin Hua*, Jianhua Wang, and Peter R. Buseck
*Correspondence author's address: Department of Geological Sciences, Arizona State University, Box 871404, Tempe, Arizona 85287-1404, USA; e-mail address: huaxin@asu.edu
Abstract–Antarctic CM meteorites ALH81002 and LEW90500 contain abundant fine-grained rims (FGRs) that surround a variety of coarse-grained objects. FGRs from both meteorites have similar compositions and petrographic features, independent of their enclosed objects. The FGRs are chemically homogeneous at the 10-µm scale for major and minor elements and at the 25-µm scale for trace elements. They display accretionary features and contain large amounts of volatiles, presumably water. They are depleted in Ca, Mn, and S but enriched in P. All FGRs show a slightly fractionated REE pattern, with enrichments of Gd and Yb and depletion of Er. Gd is twice as abundant as Er.
Our results indicate that those FGRs are not genetically related to their enclosed cores. They were sampled from a reservoir of homogeneously mixed dust, prior to accretion to their parent body. The rim materials subsequently experienced aqueous alteration under identical conditions. Based on their mineral, textural, and especially chemical similarities, we conclude that ALH81002 and LEW90500 likely have a similar or identical source.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Application of MELTS to kinetic evaporation models of FeO-bearing silicate melts
C.M.O'D. Alexander
*Correspondence author's address: Department Terrestrial Magnetism, The Carnegie Institution of Washington, 5241 Broad Branch Road, Washington DC 20015, USA; e-mail address: alexande@dtm.ciw.edu
Abstract–Incorporation of the MELTS silicate melt solution model into models of evaporation successfully reproduces the evaporation behavior of alkali-free, FeO-bearing (>=2 mole%) chondritic melts at temperatures between 1700°C and 2000°C. In conjunction with the Berman CMAS melt solution model for FeO-poor melts, evaporation of alkali-free melts can now be modeled over a very wide range of conditions. MELTS-based evaporation models can also quite successfully reproduce the evaporation behavior of K when Al/(Na+K)>1. However, reproduction of Na evaporation experiments is much poorer.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Active Capture and Anomalous Adsorption: New Mechanisms for the Incorporation of Heavy Noble Gases
C. M. Hohenberg*, N. Thonnard and A. Meshik
*Correspondence author's address: McDonnell Center for Space Sciences and Department of Physics, Washington University, St. Louis, Mo 63130, USA; e-mail address: cmh@wuphys.wustl.edu
Abstract–Active capture is a new process for the incorporation of large quantities of heavy noble gases into growing surfaces. Adsorption in the conventional sense involves surface bonding by polarization (Van der Waals forces). What is referred to as "anomalous adsorption" of heavy noble gases involves chemical bonds and can occur when other (more chemically active) species are not available to pre-empt sites with unfilled bonds. Anomalous adsorption has been observed under conditions of fracture, vacuum deposition and ionizing radiation. Active capture depends upon anomalous adsorption to retain noble gases on a surface long enough to be captured in a growing surface film as it is deposited. The fundamental principle may be the impingement onto the growing film with sufficient energy to liberate surface electrons (work function energy of a few eV) so that they are retained by anomalous adsorption long enough to be entrapped in the growing surface. Trapping efficiencies of ~ 1 percent have been observed for Kr and Xe in laboratory experiments, implying a fundamentally new mechanism for the incorporation of heavy noble gases onto surfaces. It may play a role in explaining the large concentrations of planetary noble gases contained in phase-Q.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
The Foelsche structure, Northern Territory, Australia: An impact crater of probable Neoproterozoic age
Peter W. Haines* and David J. Rawlings
*Correspondence author's address: School of Earth Sciences, University of Tasmania, GPO Box 252-97, Hobart, TAS 7001, Australia; e-mail address: Peter.Haines@utas.edu.au
Abstract–The Foelsche structure is situated in the McArthur Basin of northern Australia (lat. 16o 40' S, long. 136o 47' E). It comprises a roughly circular outcrop of flat-lying Neoproterozoic Bukalara Sandstone, overlying and partly rimmed by tangentially striking, discontinuous outcrops of dipping, fractured and brecciated Mesoproterozoic Limmen Sandstone. The outcrop expression coincides with a prominent circular aeromagnetic anomaly, which can be explained in terms of the local disruption and removal or displacement of a regional mafic igneous layer within a circular area at depth. Samples of red, lithic, pebbly sandstone from the stratigraphically lowest exposed levels of the Bukalara Sandstone within the Foelsche structure contain detrital quartz grains displaying mosaicism, planar fractures (PFs) and planar deformation features (PDFs). PFs and PDFs occur in multiple intersecting sets with orientations consistent with a shock metamorphic origin. The abundance and angular nature of the shocked grains indicates a nearby provenance. Surface expression and geophysical data are consistent with a partly buried complex impact crater of ca. 6 km diameter with an obscured central uplift ca. 2 km in diameter. The deformed outcrops of Limmen Sandstone are interpreted as relics of the original crater rim, but the central region of the crater, from which the shocked grains were likely derived, remains buried. From the best available age constraints the Foelsche structure is most likely of Neoproterozoic age.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Heavily-hydrated lithic clasts in CH chondrites and the related, metal-rich chondrites QUE 94411 and Hammadah al Hamra 237
Ansgar Greshake*, Alexander N. Krot, Anders Meibom, Michael K. Weisberg, Michael E. Zolensky, and Klaus Keil
*Correspondence author's address: Museum für Naturkunde, Institut für Mineralogie, Humboldt-Universität zu Berlin, Invalidenstrasse 43, 10115 Berlin, Germany; e-mail address: ansgar.greshake@rz.hu-berlin.de
Abstract–Fine-grained, heavily-hydrated lithic clasts in the metal-rich (CB) chondrites QUE 94411 and Hammadah al Hamra 237 and CH chondrites, such as PAT 91546 and ALH85085, are mineralogically similar suggesting genetic relationship between these meteorites. These clasts contain no anhydrous silicates and consist of framboidal and platelet magnetite, prismatic sulfides (pentlandite and pyrrhotite), and Fe-Mn-Mg-bearing Ca-carbonates set in a phyllosilicate-rich matrix. Two types of phyllosilicates were identified: serpentine, with basal spacing of ~0.73 nm, and saponite, with basal spacings of ~1.1-1.2 nm. Chondrules and FeNi-metal grains in CB and CH chondrites are believed to have formed at high temperature (>1300 K) by condensation in a solar nebula region that experienced complete vaporization. The absence of aqueous alteration of chondrules and metal grains in CB and CH chondrites indicates that the clasts experienced hydration in an asteroidal setting prior to incorporation into the CH and CB parent bodies. The hydrated clasts were either incorporated during regolith gardening or accreted together with chondrules and FeNi-metal grains after these high-temperature components had been transported from their hot formation region to a much colder region of the solar nebula.
|
|
|