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

We present the first detailed study of a population of texturally distinct chondrules previously described by Kurat (1969), Christophe Michel-Levy (19 76), and Skinner et al. (1989) that are sharply depleted in alkalis and Al in their enter portions. These "bleached" chondrules, which are exclusively radial pyroxene and cryptocrystalline in texture, have porous outer zones where mesostasis has been lost. Bleached chondrules are present in all type 3 ordinary chondrites and are present in lower abundances in types 4-6. Th ey are most abundant in the L and LL groups, apparently less common in H ch ondrites, and absent in enstatite chondrites. We used x-ray mapping and tra ditional electron microprobe techniques to characterize bleached chondrules in a cross section of ordinary chondrites. We studied bleached chondrules from Semarkona by ion microprobe for trace elements and H isotopes, and by transmission electron microscopy. Chondrule bleaching was the result of low -temperature alteration by aqueous fluids flowing through fine-grained chon drite matrix prior to thermal metamorphism. During aqueous alteration, inte rstitial glass dissolved and was partially replaced by phyllosilicates, tro ilite was altered to pentlandite, but pyroxene was completely unaffected. C alcium-rich zones formed at the inner margins of the bleached zones, either as the result of the early stages of metamorphism or because of fluid-chon drule reaction. The mineralogy of bleached chondrules is extremely sensitiv e to thermal metamorphism in type 3 ordinary chondrites, and bleached zones provide a favorable location for the growth of metamorphic minerals in hig her petrologic types. The ubiquitous presence of bleached chondrules in ord inary chondrites implies that they all experienced aqueous alteration early in their asteroidal histories, but there is no relationship between the de gree of alteration and metamorphic grade. A correlation between the oxidati on state of chondrite groups and their degree of aqueous alteration is cons istent with the source of water being either accreted ices or water release d during oxidation of organic matter. Ordinary chondrites were probably ope n systems after accretion, and aqueous fluids may have carried volatile ele ments with them during dehydration. Individual radial pyroxene and cryptocr ystalline chondrules were certainly open systems in all chondrites that exp erienced aqueous alteration leading to bleaching.