Physics – Condensed Matter
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.522h&link_type=abstract
Meteoritics, vol. 30, no. 5, page 522
Physics
Condensed Matter
1
Chondrites, Isotopes, Potassium, Models, Nebular, Processes, Volatile Elements, Moderately
Scientific paper
Potassium and other volatile elements are depleted in chondrites, achondrites and planetary bulk compositions to varying degrees relative to Solar or CI compositions [ 1]. The processes responsible involve either incomplete condensation or partial volatilization of elements more volatile than Mg-silicates and Fe-Ni metal. Both processes imply high temperatures of 1200-1400 K in the solar nebula. The survival of residues of partial devolatilization requires that such high temperatures could not have existed for durations long enough to completely devolatilize the residues, estimated to be on the order of a few minutes. Since nebular heating-cooling timescales cannot possibly be so short, this would imply a cool nebula (T(sub)nebula<=700 K) with transient and local heating events. Such partial devolatilization processes leave a distinct isotopic signature: enrichment of the heavy isotopes in residues. Measurements of the isotopic composition of potassium, a moderately volatile element, in bulk chondrites show no evidence for a such devolatilization process [2]. It is presently difficult, if not impossible, to directly measure individual components in chondrites. The chemical effects of volatile element depletion are observed in bulk chondrites, and since we can measure isotopic ratios of potassium to+/-0.5 per mil as little as 2% chemical loss can be resolved. Thus, a mixture of devolatilized grains and volatile-rich matrix(?) will produce recognizable isotopic effects in bulk chondrites. We can thus eliminate the following hypothesis: there exist materials in chondrites that have undergone partial devolatilization either as liquids (e.g. chondrules) or as solid grains, that make up a significant fraction of a chondrite (or derivative achondrite parent bodies). A significant fraction is defined as the amount required to affect the bulk chemical composition of the chondrite. Since this isotopic experiment is more sensitive than a chemical measurement, any observable potassium depletions caused by vaporization will give rise to observable heavy isotope enrichments. A natural analogue is provided by the lunar soils, which have modest chemical depletions of potassium (~15% from K/U or K/La ratios) but large bulk heavy isotope enrichments of 3-13 per mil [3]. Volatile depletion is a feature of the bulk composition of chondrites, achondrites and planets, and so reflects a process that has occurred on a scale of the inner solar system. Since partial devolatilization can be eliminated as a possible cause, it is required that a substantial fraction of presolar condensed matter accreting to the nebular disk must have been heated to temperatures <= 1200-1400 K on comparable scales. This is consistent with models of the nebular disk that attain temperatures "thermostatically regulated" by the condensation of Fe-Ni grains [4]. It should be noted that such models do not destroy all presolar dust, and allow for thermal heterogeneity [5], and the survival of isotopic anomalies and presolar grains. It cannot be claimed that there are no instances of volatilization of moderately volatile elements, just that these cannot constitute a mass fraction of chondrites sufficient to control the chemical composition. Thus, the observed chemical compositions of chondrites and other solar system materials must be attributed to incomplete condensation of volatile elements from an initially hot solar nebula [4,6]. References: [1] Palme H. and Boynton W. V. (1993) in Protostars and Planets III (E. H. Levy and J. I. Lunine, eds.), 979. [2] Humayun M. and Clayton R. N. (1995) GCA, 59, 2131. [3] Humayun M. and Clayton R. N. (1995) GCA, 59, 2115. [4] Boss A. P. (1993) Astrophys. J., 417, 351. [5] Boss A. P., this volume. [6] Cassen P. (1995) LPS XXVI, 219.
Clayton Robert N.
Humayun Munir
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