Meteoritics & Planetary Science, Volume 39, Number 5 (2004)

Book Review: Cometography: A Catalog of Comets, Volume 2: 1800-1899, Gary W. Kronk

2021-02-20T01:30:51Z

Book Review: Cometography: A Catalog of Comets, Volume 2: 1800-1899, Gary W. Kronk Marsden, B. G. Book Review: Cometography: A Catalog of Comets, Volume 2: 1800-1899, Gary W. Kronk. Cambridge University Press (2003).

40Ar-39Ar studies of whole rock nakhlites: Evidence for the timing of formation and aqueous alteration on Mars

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40Ar-39Ar studies of whole rock nakhlites: Evidence for the timing of formation and aqueous alteration on Mars Swindle, T. D.; Olson, E. K. 20-25 mg whole rock samples of the nakhlites Lafayette and Nakhla have been analyzed via the 40Ar-39Ar technique, in part to verify their formation ages, but primarily, in an attempt to determine the timing of aqueous alteration in these martian meteorites. As in previous studies, plateaus in apparent age are observed at about 1300 Ma (1322 +/- 10 for Lafayette, 1332 +/- 10 and 1323 +/- 11 for Nakhla), presumably corresponding to crystallization ages. The plateaus are not entirely flat, perhaps reflecting the effects of recoil during creation of 39Ar in the nuclear irradiation. The first 5-20% of the K-derived Ar released from all three samples give apparent ages <1300 Ma. Coupled with the fact that chronometric isotopic studies of nakhlites typically show some disturbance, we believe the low temperature pattern represents more recent (than 1300 Ma) formation of martian aqueous alteration products such as iddingsite. No low temperature plateaus are observed. This is consistent with petrographic evidence for multiple formation events, although the lack of low temperature plateaus is far from conclusive. On the other hand, if there was a single time of alteration, we believe that it will be difficult, if not impossible, to determine it using the K-Ar system.

Noble gas studies in CAIs from CV3 chondrites: No evidence for primordial noble gases

2021-02-20T01:30:45Z

Noble gas studies in CAIs from CV3 chondrites: No evidence for primordial noble gases Vogel, N.; Bauer, H.; Bischoff, A.; Leya, I.; Wieler, R. Calcium-aluminum-rich inclusions (CAIs) were among the first solids in the solar system and were, similar to chondrules, created at very high temperatures. While in chondrules, trapped noble gases have recently been detected, the presence of trapped gases in CAIs is unclear but could have important implications for CAI formation and for early solar system evolution in general. To reassess this question, He, Ne, and Ar isotopes were measured in small, carefully separated and, thus, uncontaminated samples of CAIs from the CV3 chondrites Allende, Axtell, and Efremovka. The 20Ne/22Ne ratios of all CAIs studied here are <0.9, indicating the absence of trapped Ne as, e.g., Ne-HL, Ne-Q, or solar wind Ne. The 21Ne/22Ne ratios range from 0.86 to 0.72, with fine-grained, more altered CAIs usually showing lower values than coarse-grained, less altered CAIs. This is attributed to variable amounts of cosmogenic Ne produced from Na-rich alteration phases rather than to the presence of Ne-G or Ne-R (essentially pure 22Ne) in the samples. Our interpretation is supported by model calculations of the isotopic composition of cosmogenic Ne in minerals common in CAIs. The 36Ar/38Ar ratios are between 0.7 and 4.8, with fine-grained CAIs within one meteorite showing higher ratios than the coarse-grained ones. This agrees with higher concentrations of cosmogenic 36Ar produced by neutron capture on 35Cl with subsequent Beta- -decay in finer-grained, more altered, and thus, more Cl-rich CAIs than in coarser-grained, less altered ones. Although our data do not strictly contradict the presence of small amounts of Ne-G, Ne-R, or trapped Ar in the CAIs, our noble gas signatures are most simply explained by cosmogenic production, mainly from Na-, Ca-, and Cl-rich minerals.

The preservation state of organic matter in micrometeorites in Antarctica

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The preservation state of organic matter in micrometeorites in Antarctica Sephton, M. A.; Bland, P. A.; Pillinger, C. T.; Gilmour, I. The recovery of large numbers of meteorites from Antarctica has dramatically increased the amount of extraterrestrial material available for laboratory studies of solar system origin and evolution. Yet, the great age of Antarctic meteorites raises the concern that significant amounts of terrestrial weathering has corrupted their pre-terrestrial record. Organic matter found in carbonaceous chondrites is one of the components most susceptible to alteration by terrestrial processes. To assess the effects of Antarctic weathering on both non-Antarctic and Antarctic chondritic organic matter, a number of CM chondrites have been analyzed. Mössbauer spectroscopy has been used to ascertain pre-terrestrial and terrestrial oxidation levels, while pyrolysis-gas chromatography-mass spectrometry was used to determine the constitution of any organic matter present. Increased oxidation levels for iron bearing minerals within the non-Antarctic chondrites are likely to be a response to increased amounts of parent body aqueous alteration. Parent body processing also appears to remove ether bonds from organic material and alkyl side chains from its constituent units. The iron in Antarctic chondrites is generally more oxidized than that in their non- Antarctic counterparts, reflecting terrestrial weathering. Antarctic weathering of chondritic organic matter appears to proceed in a similar way to parent body aqueous alteration and simply enhances the organic responses observed in the non-Antarctic data set. Degradation of the record of preterrestrial processes in Antarctic chondrites should be taken into account when interpreting data from these meteorites.