Bleached chondrules: Evidence for widespread aqueous processes on the parent asteroids of ordinary chondrites

• DOI: 10.1111/j.1945-5100.2000.tb01429.x
• Publication Date: 2010-02-04T11:09:09Z
• Authors: Jeffrey N. GROSSMAN; Conel M. O'D. ALE...; Jianhua WANG; Adrian J. BREARLEY
• Keywords: Chondrule; Pyroxene; Troilite; Parent body; Enstatite; Ordinary chondrite

Abstract— We present the first detailed study of a population texturally distinct chondrules previously described by Kurat (1969), Christophe Michel‐Lévy (1976), and Skinner et al. (1989) that are sharply depleted in alkalis Al their outer portions. These “bleached” chondrules, which exclusively radial pyroxene cryptocrystalline texture, have porous zones where mesostasis has been lost. Bleached all type 3 ordinary chondrites lower abundances types 4–6. They most abundant L LL groups, apparently less common H chondrites, absent enstatite chondrites. used x‐ray mapping traditional electron microprobe techniques to characterize bleached cross section studied from Semarkona ion for trace elements isotopes, transmission microscopy. Chondrule bleaching was result low‐temperature alteration aqueous fluids flowing through finegrained chondrite matrix prior thermal metamorphism. During alteration, interstitial glass dissolved partially replaced phyllosilicates, troilite altered pentlandite, but completely unaffected. Calcium‐rich formed at inner margins zones, either as early stages metamorphism or because fluid‐chondrule reaction. The mineralogy is extremely sensitive provide favorable location growth metamorphic minerals higher petrologic types. ubiquitous presence implies they experienced asteroidal histories, there no relationship between degree grade. A correlation oxidation state groups consistent with source water being accreted ices released during organic matter. Ordinary were probably open systems after accretion, may carried volatile them dehydration. Individual certainly leading bleaching.