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Petrology of eucrites, howardites and mesosiderites

Duke, Michael B. and Silver, Leon T. (1967) Petrology of eucrites, howardites and mesosiderites. Geochimica et Cosmochimica Acta, 31 (10). pp. 1637-1665. ISSN 0016-7037.

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The eucrite and howardite calcium-rich achondrites and many mesosiderites are considered as a coherent meteorite assemblage, their silicates consisting essentially of calciumpoor monoclinic and orthorhombic pyroxenes and calcium-rich plagioclase feldspar. The achondrites can be grouped according to their brecciated structure as follows: eucrites—unbrecciated and monomict brecciated achondrites; howardites—polymict brecciated achondrites. Many mesosiderites contain brecciated structures; they are distinguished from the achondrites by their large metallic fraction. The structure and composition of rock fragments in the breccias indicate a complicated sequence of events including magmatic differentiation, brecciation, recrystallization and refragmentation, and ejection from the parent body. Detailed mineralogical and chemical data suggest that the magmatic differentiation proceeded primarily by the separation of pyroxene from an ultrabasic parent material that had a much lower alkali content than ordinary chondrites. Magmatic crystallization took place in environments ranging from extrusive to deep-seated intrusive. Polymict breccias contain fragments with a wide variety of magmatic and recrystallization textures, which suggests that the breccias were formed either in very large or repeated fragmentation events. Monomict breccias contain fragments with a small range of similar magmatic textures, which suggests that these breccias were formed by small or single events. Petrographic evidence suggests that many of the breccias are impact breccias. Either in their original magmatic crystallization sites or in the sites of breccia accumulation, most of these meteorites apparently had a near-surface location prior to ejection from the parent body. Evidence obtained from eucrites, howardites and mesosiderites forms an important part of our understanding of the early evolution of the surface regions of their parent body. Chemical and oxidation conditions were different from those presently found in the Earth's crust and upper mantle, but the necessary conditions may have been present in the early history of the Earth. A lunar origin for eucrites, howardites and mesosiderites is proposed, but an asteroidal origin can not be presently excluded.

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Additional Information:© 1967 Pergamon Press Ltd. Received 8 May 1967; accepted in revised form 13 June 1967. We wish to acknowledge the kind co-operation of the following people who generously supplied specimens and thin sections for this work: M. H. HEY; C. B. MOORE; W. V. ENGELHARDT; B. MASON; E. P. HENDERSON; J. F. LOVERING; P. W. GAST; E. ANDERS; K. KEIL; K. TUREKIAN; E. OLSEN. A. CHODOS, A. D. MAYNES and E. BINGHAM, staff analysts of the Division of Geological Sciences, California Institute of Technology, were extremely helpful in discussing analytical techniques. R. V. HUENE made the excellent thin and polished thin-sections and assisted on several problems. Professor HARRISON BROWN gave his continuing interest and co-operation to the project and provided the analysis of the Pasamonte meteorite which had been made by Dr. MAYNES. E. M. SHOEMAKER provided stimulating ideas and discussion in regard to the postmagmatic brecciation of the meteorites. Financial support for materials, chemical analyses and photographic equipment and materials was provided from the research program of L. T. SILVER under the U.S. Atomic Energy Commission contract At(04-3)-427. Travel funds were provided from National Aeronautics and Space Administration contract NSG 56-60. A grant from the Geological Society of America covered the cost of several chemical analyses. The senior author had a National Science Foundation Graduate Fellowship from 1960-1962. A critical review by T. L. WRIGHT, U.S. Geological Survey, is gratefully acknowledged.
Funding AgencyGrant Number
Atomic Energy CommissionAT(04-3)-427
NASANsG 56-60
Geological Society of AmericaUNSPECIFIED
NSF Graduate FellowshipUNSPECIFIED
Issue or Number:10
Record Number:CaltechAUTHORS:20151021-104842616
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Official Citation:Michael B. Duke, Leon T. Silver, Petrology of eucrites, howardites and mesosiderites, Geochimica et Cosmochimica Acta, Volume 31, Issue 10, October 1967, Pages 1637-1665, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:61373
Deposited By: Tony Diaz
Deposited On:21 Oct 2015 18:51
Last Modified:03 Oct 2019 09:07

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