Abstracts of Papers to be Published in the July 2001 Issue

Please contact the correspondence author for reprints of all published articles

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

Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid

Edward R.D. Scott*, Henning Haack and Stanley G. Love

*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, HI 96822, USA; e-mail address:  escott@higp.hawaii.edu

Abstract–We propose that mesosiderites formed when a 200–400 km diameter asteroid with a molten core was disrupted by a 50–150 km diameter projectile.  To test whether impacts can excavate core iron and mix it with crustal material, we used a low-resolution, smoothed-particle hydrodynamics computer simulation. For 50–300 km diameter differentiated targets, we found that significant proportions of scrambled core material (and hence potential mesosiderite metal material) could be generated.  For near-catastrophic impacts that reduce the target to 80% of its original diameter and about half of its original mass, the proportion of scrambled core material would be about 5 vol.%, equivalent to ~10 vol.% of mesosiderite-like material.  The paucity of olivine in mesosiderites and the lack of metal-poor or troilite-rich meteorites from the mesosiderite body probably reflect biased sampling.  Mesosiderites may be olivine-poor because mantle material was preferentially excluded from the metal-rich regions of the reaccreted body.  Molten metal globules probably crystallized around small, cool fragments of crust hindering migration of metal to the core.  If mantle fragments were much hotter and larger than crustal fragments, little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally.  The rapid cooling rates of mesosiderites above 850 °C can be attributed to local thermal equilibration between hot and cold ejecta.  Very slow cooling below 400°C probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris.  We infer that our model is more plausible than an earlier model that invoked an impact at ~1 km/s to mix projectile metal with target silicates.  If large impacts cannot effectively strip mantles from asteroidal cores, as we infer, we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material.  This may help to explain why relatively few olivine-rich (A-type) and metal-rich asteroids (M-type) are known.  Some S-type asteroids may be scrambled differentiated bodies.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
 
Molecular and chiral analyses of some protein amino acid derivatives in the Murchison and Murray meteorite

Sandra Pizzarello* and George W. Cooper

*Correspondence author's address:  Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, USA; e-mail address:  pizzar@asu.edu

Abstract–The varied organic suite extracted from the Murchison meteorite contains several amino acids that are common to the biosphere.  Some of these have been found to be non-racemic, but the indigenous nature of their L-enantiomeric excesses has been subject to debate in view of possible terrestrial contamination.  We have investigated two amino acids of common terrestrial and meteoritic occurrence, alanine and glutamic acid, and assessed their indigenous enantiomeric ratios in the  Murchison and Murray meteorites through the ratios of some of their derivatives.   Analyzed were: N-acetyl alanine, alpha-imino propioacetic acid, N-acetyl glutamic acid and pyroglutamic acid.

Both alanine derivatives were found to be racemic, while those of glutamic acid showed L-enantiomeric excesses varying from 16% to 47.2% for pyroglutamic acid, and from 8.6% to 41% for N-acetyl glutamic acid.  The ¹³C was determined for the two enantiomers of Murchison pyroglutamic acid both before and after acid hydrolysis of the lactam to glutamic acid.  The values of +27.7‰ (D-pyro), +10.0‰ (L-pyro), +32.2‰ (D-glu) and +14.6‰ (L-glu) were obtained.

The racemic nature of alanine derivatives strongly suggests that alanine itself, as  indigenous to the meteorite, is racemic.  The explanation of the L-enantiomeric excesses found for glutamic acid derivatives is less direct; however, the variability of the enantiomeric ratios for these compounds and the distinctly lower ¹³C values determined for pyroglutamic L-enantiomer point to a terrestrial contamination, possibly dating to the time of fall.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
 
Metallographic cooling rates of group IIF iron meteorites

Kaare L. Rasmussen*, Henning Haack and Finn Ulff-Møller

*Correspondence author's address: Carbon-14 Laboratory, National Museum of Denmark, Ny Vestergade 11, DK-1471 Copenhagen K, Denmark; e-mail address:  kaare.lund.rasmussen@natmus.dk

Abstract–Metallographic cooling rates have been calculated for all five members of group IIF using two different techniques. We have determined cooling rates of ~5 °C/Ma based on Ni profiles through the taenite rim enclosing kamacite spindles.  Ni profiles through the kamacite phase are less precise cooling rate indicators, but suggest a cooling rate of ~1 °C/Ma within an order of magnitude at lower temperatures (360–400 °C).  Based on the kamacite bandwidth and the Ni profiles through the taenite we estimate that the kamacite nucleated 130–200 °C below the temperature predicted from the phase diagram.  The size of and the distance between large kamacite spindles is found to be consistent with the thermal history that we have determined on the basis of Ni profiles in kamacite and taenite.  We find that previously published kamacite bandwidth cooling rates for the five group IIF members are most likely in error because of the presence of large schreibersite spindles in some kamacite spindles and because undercooling of kamacite were ignored.  Contrary to previous workers we find that the metallographic cooling rates are consistent with cooling in a common core.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
 
Manganese-chromium formation intervals for chondrules from the Bishunpur and Chainpur meteorites

L. Nyquist*, D. Lindstrom, D. Mittlefehldt, C-Y. Shih,  H. Wiesmann, S. Wentworth and R. Martinez

*Correspondence author's address:  Mail Code SN2, NASA Johnson Space Center, Houston, Texas 77058, USA; e-mail address:  laurence.e.nyquist1@jsc.nasa.gov

Abstract–Whole-chondrule Mn-Cr isochrons are presented for chondrules separated from the Chainpur (LL3.4) and Bishunpur (LL3.1) meteorites.  The chondrules were initially surveyed by instrumental neutron activation analysis (INAA).  LL chondrite-normalized Mn/Cr, Mn/Fe, and Sc/Fe served to identify chondrules with unusually high or low Mn/Cr ratios, and to correlate the abundances of other elements to Sc, the most refractory element measured.  A subset of chondrules from each chondrite was chosen for analysis by a scanning electron microscope equipped with an energy dispersive X-ray (EDX) spectrometer prior to high precision Cr-isotopic analyses.  ⁵³Cr/⁵²Cr correlates with ⁵⁵Mn/⁵²Cr to give initial (⁵³Mn/⁵⁵Mn)_I = 9.4 ± 1.7 × 10^–6 for Chainpur chondrules and (⁵³Mn/⁵⁵Mn)_I = 9.5 ± 3.1 × 10^–6 for Bishunpur chondrules.  The corresponding chondrule formation intervals are, respectively, t_LEW = –10 ± 1 Ma for Chainpur and –10 ± 2 Ma for Bishunpur relative to the time of igneous crystallization of the LEW86010 angrite.  Because Mn/Sc correlates positively with Mn/Cr for both the Chainpur and Bishunpur chondrules, indicating dependence of the Mn/Cr ratio on the relative volatility of the elements, we identify the event dated by the isochrons as volatility-driven elemental fractionation for chondrule precursors in the solar nebula.  Thus, our data suggest that the precursors to LL chondrules condensed from the nebula 5.8 ± 2.7 Ma after the time when initial (⁵³Mn/⁵⁵Mn)_I = 2.8 ± 0.3 × 10^–5 for CAIs, our preferred value, determined from data for (a) mineral separates of Type B Allende CAI BR1, (b) spinels from Efremovka CAI E38, and (c) bulk chondrites.

Mn-Cr formation intervals for meteorites are presented relative to average I(Mn) = (⁵³Mn/⁵⁵Mn)_Ch = 9.46 × 10^–6 for chondrules.  Mn/Cr ratios for radiogenic growth of ⁵³Cr in the solar nebula and later reservoirs are calculated relative to average (I(Mn), (⁵³Cr)_I) = (9.46 ± 0.08 × 10^–6, –0.23 ± 0.08) for chondrules.  Inferred values of Mn/Cr lie within expected ranges.  Thus, it appears that evolution of the Cr isotopic composition can be traced from condensation of CAI via condensation of the ferromagnesian precursors of chondrules to basalt generation on differentiated asteroids.  Measured values of (⁵³Cr) for individual chondrules exhibit the entire range of values that has been observed as initial (⁵³Cr) values for samples from various planetary objects, and which has been attributed to radial heterogeneity in initial ⁵³Mn/⁵⁵Mn in the early solar system.  Estimated ⁵⁵Mn/⁵²Cr = 0.42 ± 0.05 for the bulk earth, combined with (⁵³Cr) = 0 for the Earth, plots very close to the chondrule isochrons, so that the Earth appears to have the Mn-Cr systematics of a refractory chondrule.  Thus, the Earth apparently formed from material that had been depleted in Mn relative to Cr contemporaneously with condensation of chondrule precursors.  If, as seems likely, the Earth's core formed after complete decay of ⁵³Mn, there must have been little differential partitioning of Mn and Cr at that time.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
 
Exposure age, terrestrial age and pre-atmospheric radius of the Chinguetti mesosiderite:  Not part of a much larger mass

K. C. Welten*, P. A. Bland, S. S. Russell, M. M. Grady, M. W. Caffee, J. Masarik, A. J. T. Jull, H. W. Weber and L. Schultz

*Correspondence author's address: Space Sciences Laboratory, University of California, Berkeley, California 94720-7450, USA; e-mail address: kcwelten@uclink4.berkeley.edu

Abstract–We measured the concentrations of the cosmogenic radionuclides ¹⁴C (half-life = 5.73 × 10³ years) in the bulk and of ¹⁰Be (1.5 × 10⁶ years), ²⁶Al (7.05 × 10⁵ years), ³⁶Cl (3.01 × 10⁵ years) and the light noble gases in metal and stone fractions of the Chinguetti meteorite to investigate the controversial claim that the 4.5 kg mesosiderite is part of a much larger mass in the Mauritanian desert.  Based on the ³⁶Cl-³⁶Ar, ¹⁰Be-²¹Ne and ²⁶Al-²¹Ne pairs in the metal fraction, we derive an average cosmic-ray exposure age of 66 ± 7 million years (Ma).  Chinguetti is now the third out of 20 mesosiderites with an exposure age between 60 and 70 Ma.  This may be the first hint of a major impact on the parent body of the mesosiderites, which show ages ranging from 10–300 Ma (Terribilini et al., 2000).  From the ¹⁴C-¹⁰Be pair we derive a terrestrial age of 18 ± 1 ka, which seems too recent to be consistent with the original description of the main mass having a heavily wind eroded base, overhung by the upper part of the meteorite.  Finally, from the radionuclide concentrations in combination with Monte Carlo based calculations, we conclude that our sample of Chinguetti was irradiated at a depth of ~15 cm in an object not larger than 80 cm in radius.  This is the most compelling evidence against the reports that the Chinguetti mesosiderite is a small fragment of a mass 100 m long and 40 m high.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
 
Noble gases in enstatite chondrites I: Exposure ages, pairing, and weathering effects

Andrea Patzer* and Ludolf Schultz

*Correspondence author's address:  Lunar and Planetary Laboratory, University of Arizona, Tucson Arizona 85721, USA; e-mail address: apatzer@lpl.arizona.edu

Abstract–Cosmic ray exposure ages (CREAs) calculated from cosmogenic noble gas nuclides are reported for 57 enstatite chondrites (E-chondrites), 43 of them were measured for the first time.  With a total of 62 individual E-chondrites (literature and this data, corrected for pairing) the observed spectrum of ages ranges between 0.07 and 66 Ma.  Three clusters seem to develop at about 3.5, 8, and 25 Ma, respectively.  Since the uncertainty of ages is estimated to be about 20% (in contrast to 10% to 15% for ordinary chondrites) and the number of examined samples is still comparatively small these peaks have to be confirmed by more measurements.  Regarding the two subgroups, EH- and EL-chondrites, no systematic trend is apparent in the distribution of CREAs.

Several E-chondrites yield significantly lower ³⁸Ar-ages compared to those calculated from cosmogenic ³He and ²¹Ne.  For these E-chondrites, we suggest a reduction of cosmogenic ³⁸Ar as a result of weathering.  In order to prove the possible influence of terrestrial alteration on the cosmogenic noble gas record of E-chondritic material we simulated terrestrial weathering in an experiment of 12 weeks duration.  The treatment showed that a significant amount of cosmogenic ³⁸Ar is lost on Earth by the influence of water.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
 
Cosmic-ray production rates of He-, Ne- and Ar-isotopes in H-chondrites based on ³⁶Cl-³⁶Ar-ages

I. Leya*, Th. Graf, K. Nishiizumi and R. Wieler

*Correspondence author's address:  ETH Zürich, Isotope Geology and Mineral Resources, NOC61, CH-8092 Zürich, Switzerland; e-mail address:  leya@erdw.ethz.ch

Abstract–We present the concentrations and isotopic compositions of He, Ne, and Ar for non-magnetic fractions and bulk samples of 17 H-chondrites which were recently investigated for their ³⁶Cl-³⁶Ar cosmic-ray exposure ages (Graf et al., 2001).  All selected meteorites are observed falls with cosmic-ray exposure ages close to the 7 Ma peak.  The rare gas data are consistent with ¹⁰Be and ³⁶Cl production rates in the metal phase.  Remarkably, only one out of the 17 H-chondrites, Bath, shows clear indications for a complex exposure history.  Based on rare gas concentrations and ³⁶Cl-³⁶Ar exposure ages, ²¹Ne production rates as a function of ²²Ne/²¹Ne and a mean ³⁸Ar production rate are determined.  The results confirm model calculations which predict that the relationship between ²¹Ne production rates and ²²Ne/²¹Ne is ambiguous for high shielding.  Besides the mean ³⁸Ar production rate we also give production rate ratios P(³⁸Ar from Ca) / P(³⁸Ar from Fe).  They vary between 10 and 77, showing no significant correlation with ³⁸Ar-concentrations or ²²Ne/²¹Ne.  By investigating the metal-separates, Graf et al. (2001) found significant ³He deficits for six out of the 17 meteorites.  For the non-magnetic fractions and bulk samples investigated here the data points in a ³He/²¹Ne versus ²²Ne/²¹Ne diagram plot in the area defined by most of the H-chondrites.  This means that ³He deficits in the metal phase are much more pronounced than in silicate minerals and we will argue that ³H diffusive losses in meteorites should be the rule rather than the exception.  The ²¹Ne exposure ages, calculated on the basis of modeled ²¹Ne production rates, confirm the assumption by Graf et al. (2001) that the H5-chondrites with low ³He/³⁸Ar in the metal formed in a separate event than those with normal ³He/³⁸Ar ratios.  The data can best be interpreted by assuming that the prominent 7 Ma exposure age peak of the H-chondrites is due to at least two events about 7.0 and 7.6 Ma ago.

Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
 
²⁶Al in CAIs and chondrules from unequilibrated ordinary chondrites

Gary R. Huss*, Glenn J. MacPherson, G. J. Wasserburg, Sara S. Russell and Gopalan Srinivasan

*Correspondence author's address:  Department of Geological Sciences and Center for Meteorite Studies, Arizona State University, P.O. Box 871404, Tempe, Arizona 85287-1404, USA; e-mail address:  gary.huss@asu.edu

Abstract–In order to investigate the distribution of ²⁶Al in chondrites, we measured aluminum-magnesium systematics in four calcium-aluminum-rich inclusions (CAIs) and eleven aluminum-rich chondrules from unequilibrated ordinary chondrites (UOCs).  All four CAIs were found to contain radiogenic ²⁶Mg (²⁶Mg*) from the decay of ²⁶Al.  The inferred initial ²⁶Al/²⁷Al ratios for these objects ((²⁶Al/²⁷Al)_o ~ 5 × 10^–5) are indistinguishable from the (²⁶Al/²⁷Al)_o ratios found in most CAIs from carbonaceous chondrites.  These observations, together with the similarities in mineralogy and oxygen isotopic compositions of the two sets of CAIs, imply that CAIs in UOCs and carbonaceous chondrites formed by similar processes from similar (or the same) isotopic reservoirs, or perhaps in a single location in the solar system.  We also found ²⁶Mg* in two of eleven aluminum-rich chondrules.  The (²⁶Al/²⁷Al)_o ratio inferred for both of these chondrules is ~1 × 10^–5, clearly distinct from most CAIs but consistent with the values found in chondrules from type 3.0–3.1 UOCs and for aluminum-rich chondrules from lightly metamorphosed carbonaceous chondrites (~0.5 × 10^–5 to ~2 x 10^–5).  The consistency of the (²⁶Al/²⁷Al)o ratios for CAIs and chondrules in primitive chondrites, independent of meteorite class, implies broad-scale nebular homogeneity with respect to ²⁶Al and indicates that the differences in initial ratios can be interpreted in terms of formation time.  A timeline based on ²⁶Al indicates that chondrules began to form one to two million years after most CAIs formed, that accretion of meteorite parent bodies was essentially complete by four million years after CAIs, and that metamorphism was essentially over in type 4 chondrite parent bodies by five to six million years after CAIs formed.  Type 6 chondrites apparently did not cool until more than seven million years after CAIs formed.  This timeline is consistent with ²⁶Al as a principal heat source for melting and metamorphism.

 

Top	Previous page