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Silica-bearing chondrules and clasts in ordinary chondrites

Brigham, C. A. and Yabuki, H. and Ouyang, Z. and Murrell, M. T. and El Goresy, A. and Burnett, D. S. (1986) Silica-bearing chondrules and clasts in ordinary chondrites. Geochimica et Cosmochimica Acta, 50 (8). pp. 1655-1666. ISSN 0016-7037. doi:10.1016/0016-7037(86)90128-6.

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Unusual silica-bearing chondrules and clasts have been observed in several unequilibrated ordinary chondrites. There appear to be two distinct types: (I) silica, low-Ca pyroxene assemblages, and (II) silica, fayalite intergrowths coexisting with low-Ca pyroxene. Only the former types are chondrules. The bulk compositions of most of these objects can be expressed in terms of three components—MgO, FeO, and SiO_2—since Ca, Al, and Na are essentially trace elements. Secondary alteration has probably perturbed the Fe/Mg ratios in most objects and produced Ca-rich pyroxenes in some type II clasts, but the silica, low-Ca pyroxene and fayalite are hard to understand as other than primary phases. Such silica-rich bulk compositions can be understood as an intermediate temperature condensate of a solar gas, formed by the loss of refractory solid material by gas-solid separation before the complete condensation of silicon. Alternatively, the silicarich bulk compositions could have been formed by extreme reduction of normal chondritic olivine and orthopyroxene; however, the observed Ca, Al depletion and the lack of Ni-poor metals are not explained by this model. The fayalite-bearing clasts can be explained by a relatively complex two-stage process involving reduction followed by oxidation. Partial reduction of olivine in the presence of S produces Mg-rich pyroxene, silica, and FeFeS; oxidation of metal and sulfide followed by reaction with silica forms fayalite. An alternative is that the fayalite-bearing clasts result from the decomposition of ferrosilite, but this requires the production of metastable ferrosilite during shock or some other high-pressure event. The simplest unflawed alternative is that the fayalite-bearing clasts result from the mixing of a partially molten silica, fayalite liquid with Mg-rich pyroxene.

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Burnett, D. S.0000-0001-9521-8675
Additional Information:© 1986 Pergamon Journals Ltd. Received 11 April 1985, Accepted 6 May 1986, Available online 31 March 2003. The Sharps and Bremervörde samples used in this study were obtained with the cooperation of the U.S. National Museum. We thank D. Lal for the Dhajala specimen. We have benefitted from comments on the manuscript by W. Cassidy, J. Grossman, B. Mason, E. Olsen, and A. Rubin. The Caltech portion of this work was supported by NSF Grant EAR 83-08290. Editorial handling: H. Y. McSween, Jr.
Group:UNSPECIFIED, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
NSFEAR 83-08290
Issue or Number:8
Record Number:CaltechAUTHORS:20150424-145049610
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Official Citation:C.A. Brigham, H. Yabuki, Z. Ouyang, M.T. Murrell, A. El Goresy, D.S. Burnett, Silica-bearing chondrules and clasts in ordinary chondrites, Geochimica et Cosmochimica Acta, Volume 50, Issue 8, August 1986, Pages 1655-1666, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:56970
Deposited By: Ruth Sustaita
Deposited On:24 Apr 2015 22:11
Last Modified:10 Nov 2021 21:06

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