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Mineral Sciences Investigations 1976-1977

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Mineral Sciences Investigations 1976-1977 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES • NUMBER 22 Mineral Sciences Investigations 1976-1977 Robert F. Fudali EDITOR SMITHSONIAN INSTITUTION PRESS City of Washington 1979 ABSTRACT Fudali, Robert F., editor. Mineral Sciences Investigations 1976-1977. Smithsonian Contributions to the Earth Sciences, number 22, 73 pages, 22 figures, 20 tables, 1979.—This volume is comprised of six short contributions reporting the results of some of the research carried out by the Department of Mineral Sciences, Smithsonian Institution, during the period 1976-1977. Included are: a comparison of impact breccias and glasses from Lonar Crater (India) with very similar speci- mens from the moon; petrographic descriptions and chemical analyses of virtually all the known pyroxene-plagioclase achondrite meteorites and a discussion of the relationships within this class; a comparative chemical study of sixty Australian tektites from widely separated localities; a description of a new, rapid technique of sample preparation for whole-rock analyses using the electron microprobe; an interlaboratory comparison of the precision and accuracy of electron microprobe analyses; and a tabulation of the chemical compositions of some electron micro- probe reference samples. OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: Aerial view of Ulawan Volcano, New Britain. Library of Congress Cataloging in Publication Data Main entry under title: Mineral sciences investigations, 1976-1977. (Smithsonian contributions to the earth sciences ; no. 22) "Six short contributions reporting the results of some the research carried out by the Smithsonian's Department of Mineral Sciences." Bibliography: p. 1. Mineralogy—Addresses, essays, lectures. I. Fudali, Robert F. II. National Museum of Natural History. Dept. of Mineral Sciences. III. Series: Smithsonian Institution. Smithsonian contribu- tions to the earth sciences ; no. 22. QE1.S227 no. 22 [Q364] 560'.943 [549] 78-24474 Contents Page METEORITES PETROLOGY, MINERALOGY, AND DISTRIBUTION OF LONAR (INDIA) AND LUNAR IMPACT BRECCIAS AND GLASSES, by Kurt Fredriksson, Phyllis Brenner, Ananda Dube, Daniel Milton, Carol Mooring, and Joseph A. Nelen ... 1 CHEMICAL VARIATION AMONG AUSTRALIAN TEKTITES, by Brian Mason 14 THE PYROXENE-PLAGIOCLASE ACHONDRITES, by Brian Mason, Eugene Jarosewich, and Joseph A. Nelen 27 ANALYTICAL LABORATORY FUSION OF ROCK AND MINERAL POWDERS FOR ELECTRON MICROPROBE ANALY- SIS, by Peter A. Jezek, John M. Sinton, Eugene Jarosewich, and Charles R. Obermeyer 46 MICROPROBE ANALYSES OF FOUR NATURAL GLASSES AND ONE MINERAL: AN INTERLABORATORY STUDY OF PRECISION AND ACCURACY, by Eugene Jarosewich, Alan S. Parkes, and Lovell B. Wiggins 53 ELECTRON MICROPROBE REFERENCE SAMPLES FOR MINERAL ANALYSES, by Eugene Jarosewich, Joseph A. Nelen, and Julie A. Norberg 68 in Mineral Sciences Investigations 1976-1977 Petrology, Mineralogy, and Distribution of Lonar (India) and Lunar Impact Breccias and Glasses Kurt Fredriksson, Phyllis Brenner, Ananda Dube, Daniel Milton, Carol Mooring, and Joseph A. Nelen soils is more clearly recognized, the existence of ABSTRACT Lonar Crater, India, is coming to be seen as one of Chemically and mineralogically, the shock meta- earth's more fortuitous catastrophes. This impact morphosed breccias and melt rocks at Lonar Crater crater is located in the Deccan Trap basalts of (India) are the closest terrestrial analogs to the India (19°58'N, 76°31'E), and affords unique op- impact-generated rocks and soils of the moon. Com- portunities for earthbound scientists to study ana- parative studies reveal a wide range of virtually logs to lunar processes. We report herein a com- identical forms and textures between Lonar and lunar samples. Thus, the advantage of knowing the parative study of the petrography, morphology, and unshocked target rocks at Lonar, especially with re- chemistry of shocked Lonar and lunar materials. gards to alternating layers of different competency, ACKNOWLEDGMENTS.—We are indebted to Walter may yield invaluable insights into lunar impact Brown, Andrea Eddy, Becky Fredriksson, and Julie processes. Also, chemical differences reported herein Norberg for technical and/or editing assistance. between Lonar basalts and impact glasses suggest some probable trends in lunar analogs. Conversely, Support from the Geological Survey of India, the the isochemical nature of much of the Lonar glass Smithsonian Research Foundation and Foreign Cur- and basalt demonstrates that lunar glasses may often rency Program is gratefully acknowledged. One of be chemically equated to their parent rocks. us (DM) received partial support under a NASA contract. We also thank Laurel Wilkening and Hans Suess for access to their unpublished radio- Introduction metric data from Lonar. As the importance of meteorite impacts as a mechanism producing lunar land forms, rocks, and General Geology Kurt Fredriksson, Phyllis Brenner, Joseph Nelen, Department The Lonar crater is an almost circular depression, of Mineral Sciences, National Museum of Natural History, 1830 m in diameter, approximately 150 m deep. Smithsonian Institution, Washington, D.C. 20560. Ananda Lonar Lake, a shallow saline lake, occupies most Dube, Geological Survey of India, Calcutta 13, India. Daniel of the crater floor. The rim of the crater is raised Milton, United States Geological Survey, National Center, Reston, Virginia 22092. Carol Mooring, Department of Ge- approximately 20 m above the surrounding plain. ology, University of Wisconsin, Madison, Wisconsin 53706. A smaller circular depression in the traps, 300 m in SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES diameter and about 700 m north of Lonar Crater, grained, slightly to moderately vesicular or amygda- apparently is a second crater, probably excavated loidal basalt which has weathered to "soft" trap. after throwout from the main crater landed, but Major minerals in both textural types are plagio- during settling of fallout. Various data on general clase (An 50-70) 33%-48%, clinopyroxene (augite morphology, conclusive evidence for the impact ± pigeonite) 17%—34%, and opaques (ilmenite origin, and preliminary studies of petrography, and/or magnetite) 2%-12%. Olivine may be pres- stratigraphy, and the chemistry of the impact rocks ent in small amounts. Alteration minerals include have been published elsewhere (Fredriksson et al., chlorite minerals, iron oxides, serpentine, and epi- 1973). Recent fission track dating of Lonar shock- dote. In addition, within both groups more coarse- melted glasses indicates an age less than ~ 50,000 grained sub-ophitic basalt may contain 20%-30% years (Wilkening, pers. comm.). Carbon-14 dating interstitial fresh glass or palagonite. shows an age of more than 30,000 years for organic In contrast, lunar basalts range in grain size from material recovered 50 m down in one of the ~90 m coarse to fine to vitrophyric, and are often vesicular. thick sediment columns in the lake (Suess, pers. In coarse lunar basalts pyroxene may occur as poiki- comm.). The crater's age then lies between 30 and litic grains in plagioclase. Plagioclase constitutes 50 thousand years. 25%-3O%, clinopyroxene 45%-55%, and opaques The Geological Survey of India has drilled a 15%-2O% (including ilmenite, armalocolite, chrome- series of five boreholes approximately on a line spinel, ulvospinel, rutile, metallic iron, nickel-iron, trending NE-SW in the floor of the crater. Each and troilite). Minor olivine may also be present. drillhole encountered 90 to 100 m of lake sediment, Many of the observed differences between Lonar which was found to contain small amounts of im- and lunar impact-produced rocks and glasses dis- pact glass in the form of fragments and spherules, cussed below may be explained by the chemical and also rounded lithic fragments exhibiting vary- differences between the target basalts (see Table 1). ing degrees of shock metamorphism. This material Shockwaves generated by a hypervelocity impact was probably eroded from the crater rim. After cause a variety of deformations and changes in the penetrating the sediment, each of the drillings re- targets. Thus, slight shocking of Lonar basalt turned cores of coarser breccia apparently composed (<250 kb) produces minor cataclasis in plagioclase of blocks up to meters in size which are unshocked and pyroxene, wavy extinction in pyroxene, and or slightly shocked and contain crude shatter cones. shock-induced twinning in plagioclase. Increasing Beneath the coarse breccia, the first four drillings pressures within this range produce closely spaced encountered a layer that yielded essentially no core twins in pyroxene (this twinning seems to be related recovery, apparently because it was composed of to possible strong shear loci). Large plagioclase and unconsolidated to extremely friable microbreccia. pyroxene crystals also tend to recrystallize or granu- The assumption was confirmed in the fifth hole for late into smaller domains, optically independent. In which an improved core catcher was used, resulting some cases planar elements in plagioclase are also in approximately 100% core recovery. In addition, produced, often accompanied by strong cataclasis, one deep (~100 m) and several shallower holes were warping of the grain, and reduced birefringence. drilled in the smaller crater and on its rim. Current In "moderately" shocked basalt (~250-450 kb), evidence indicates that the structure, although arti- plagioclase
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