Noble gases in carbonaceous chondrites

Details Noble gases in carbonaceous chondrites Noble gases in carbonaceous chondrites

Jul 1970

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1970gecoa..34..781m&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 34, Issue 7, pp.781-824

Computer Science

57

Scientific paper

He, Ne, Ar, Kr and Xe were measured in 73 samples of 32 carbonaceous chondrites and 3 ureilites. All carbonaceous chondrites contain "planetary" primordial gas; many also contain "solar" gas. Types I, II and III V (= Vigarano subclass) always have sizable contents of He and Ne, with mean He 4 / Ne 20 = 340, 220 and 290; Ne 20 / Ar 36 0.2. Types III O (= Ornans subclass), C4, and ureilites have little or no primordial He and Ne; Ne 20 / Ar 36 0.03. These trends were probably established during accretion rather than during metamorphism. Primordial He shows a more than threefold variation in He 3 / He 4 ratio, from ~ 1,2 × 10 -4 to ~ 4 × 10 -4 . Much smaller variations, from 5.3 to 5.5, were found for Ar 36 / Ar 38 . Both correlate with the well-known variation in Ne 20 / Ne 22 . Radiation ages for Types I and II are always less than 15 m.y., while those for Type III extend to 40 m.y. This trend correlates with crushing strength and may imply that the age distribution is governed by the meteorites' lifetime against collisional destruction. K-Ar ages increase in the order I < II < III, presumably reflecting increasing grain size and gas retentivity. Radiogenic Xe 129 contents for most meteorites lie between 2 and 10 × 10 -10 ccSTP/g, although Xe r 129 / Xe 132 ratios vary by a factor of 400. Most carbonaceous chondrites also contain a fission component; it is more abundant in low-temperature meteorites or structural components (I's, II's, III V's, matrix) than in those showing evidence of higher temperatures (III O's, C4's, chondrules). Apparently it is derived from a volatile progenitor, perhaps a super-heavy element of Z = 112-119.

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