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ELEMENTAL ABUNDANCES in the SILICATE PHASE of PALLASITIC METEORITES Redacted for Privacy Abstract Approved: Roman A

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ELEMENTAL ABUNDANCES in the SILICATE PHASE of PALLASITIC METEORITES Redacted for Privacy Abstract Approved: Roman A AN ABSTRACT OF THE THESIS OF THURMAN DALE COOPER for theMASTER OF SCIENCE (Name) (Degree) in CHEMISTRY presented on June 1, 1973 (Major) (Date) Title: ELEMENTAL ABUNDANCES IN THE SILICATE PHASE OF PALLASITIC METEORITES Redacted for privacy Abstract approved: Roman A. Schmitt The silicate phases of 11 pallasites were analyzed instrumen- tally to determine the concentrations of some major, minor, and trace elements.The silicate phases were found to contain about 98% olivine with 1 to 2% accessory minerals such as lawrencite, schreibersite, troilite, chromite, and farringtonite present.The trace element concentrations, except Sc and Mn, were found to be extremely low and were found primarily in the accessory phases rather than in the pure olivine.An unusual bimodal Mn distribution was noted in the pallasites, and Eagle Station had a chondritic nor- malized REE pattern enrichedin the heavy REE. The silicate phases of pallasites and mesosiderites were shown to be sufficiently diverse in origin such that separate classifications are entirely justified. APPROVED: Redacted for privacy Professor of Chemistry in charge of major Redacted for privacy Chairman of Department of Chemistry Redacted for privacy Dean of Graduate School Date thesis is presented June 1,1973 Typed by Opal Grossnicklaus for Thurman Dale Cooper Elemental Abundances in the Silicate Phase of Pallasitic Meteorites by Thurman Dale Cooper A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science June 1974 ACKNOWLEDGMENTS The author wishes to express his gratitude to Prof. Roman A. Schmitt for his guidance, suggestions, discussions, and thoughtful- ness which have served as an inspiration. The author also wishes to thank Prof. Chih Wang for personal guidance which has been of great value.Drs. Hiroshi Wakita, Plinio Rey, and Donald L. Showalter have contributed helpful methodology and suggestions which have aided this endeavor. Particular thanks is paid to Sharon, the author's wife, for her understanding, patience, and encouragement which have aided the author in all phases of this work. TABLE OF CONTENTS Page I. INTRODUCTION 1 Problems in Obtaining Geochemical Samples 1 Structure 4 s Physical Appearance 5 Physical Appearance of the Iron-Nickel Phase 6 Mineralogical Composition Schreibersite 8 Chromite 8 Lawrencite 8 Far ringtonite 8 II. EXPERIMENTAL 10 Sampling 10 Sample Preparation 10 Activation and Detection Procedures 12 Radiochemical Procedures 14 Accuracy and Precision 16 III, RESULTS AND DISCUSSION 22 Aluminum Analysis 22 Sodium Analysis 22 Manganese Analysis 24 Iron Analysis 27 Nickel Analysis 37 Scandium Analysis 41 Chromium Analysis 45 Cobalt Analysis 47 Zirconium, Hf, Th, and REE Analysis 49 Radiochemical Results 52 Pallasites Compared to Mesosiderites 65 IV. SUMMARY 68 BIBLIOGRAPHY 70 LIST OF FIGURES Figure Page 1. Histogram of Mn Distribution in Pallasitic Olivines. 25 2. Correlation of MnO with FeO. 26 3. Histogram of Fe Distribution in Pallasitic Olivine. 38 4. Chondrite Normalized Rare Earth Pattern for Eagle Station. 51 5. Partition Coefficients for REE of Zircon in a Dacite Matrix. 52 6. Chondrite Normalized Rare Earth Pattern for DTS. 57 7. Chondrite Normalized Pattern for Brenham Olivine. 58 8. Chondrite Normalized Pattern for Ureilites. 59 LIST OF TABLES Table Page 1. Trace Element Concentrations in Troilite. 7 2. Experiment Parameters. 15 3. Standard Rock Analyses. 17 4. Sodium Analysis of Pallasitic Olivine. 23 5. Manganese Analysis of Pallasitic Olivine. 28 6. Iron Analysis of Pallasitic Olivine. 29 7. Iron Abundances for Pallasitic Meteorites ( %), 30 8. Compositional Differences between Olivines (Fe). 33 9. Average Composition of Pallasitic Olivines. 35 10. Nickel Analysis of Pallasitic Olivine. 39 11. Comparison of Calculations and Measured Ni. Contents in Olivines, 42 12. Ni Contents of the Metal Phase. 43 13. Scandium Analysis in Pallasitic Olivine. 44 14, Chromium Analysis in Pallasitic Olivine. 46 15. Cobalt Analysis of Pallasitic Olivine. 48 16. Rare Earth Element Concentrations for Eagle Station Olivine, 50 17. Rare Earth Concentrations for DTS-1. 61 18. Rare Earth Concentrations in Olivine from Brenham, 62 19. Rare Earth Concentrations in Hot Acid Washed Olivine from Brenham, 63 ELEMENTAL ABUNDANCES IN THE SILICATE PHASE OF PALLASITIC METEORITES I.INTRODUCTION Problems in Obtaining Geochemical Samples Cosmochemists are interested in studying the processes which transform an accumulation of primordial material intoa layered dif- ferentiated planet.Even in the present age information about many planetary processes is admittedlyscarce because of the physical difficulties inherent in sampling. Many portions of the earth's crust are difficult to reach and sample properly such as wild uncharted areas, polar regions where ice layers and extreme temperature con- ditions make sampling difficult, and deep ocean sites where water pressure presents difficult technical problems; however, these prob- lems are not remotely comparable to the extreme difficultiesencoun- tered in sampling deep within the earth's interior. If one envisions the difficulties present in properly sampling this planet today, then the problems of obtaining information about the primeval processes which occurred during the birth ofour solar sys- tem become much more complex.Planetary differentiation processes have occurred on the earth which have drastically altered the initial physical properties and chemical environment of this planet.The protoplanets are viewed as having accreted from the material which 2 also formed the sun and therefore they should have been extremely close in chemical composition to the sun itself.On the earth's crust, magmatic and metamorphic processes have occurred which have obliterated this record; however, certain meteorites called chondrites have arrived on the earth which do possess a composition extremely similar to solar abundance data in the class of non-gaseous elements. These meteorites are viewed as primeval material which has not undergone whole planet melting.Probably they are fragments of small bodies such as asteroids which were so small that their heat radiation rate was somewhat comparable to their heat formation rate. Other meteorites called achondrites are not so primitive in their composition and show evidence of planetary melting and metamorphic processes.Indeed, for many samples of this group, their composi- tion is extremely similar to basalts which are common on the earth's crust.Because of their crustal similarity, these meteorites are studied in the hope they will shed some light on crustal processes here on this earth. A third class of meteorites are the pallasites or stony-irons. These meteorites are a near equal mixture of iron-nickel and olivine. Most hypotheses about their origin point to an origin at or near a planet' s core. A fourth class of meteorites are the iron meteorites which are composed of an iron-nickel mixture with very small amounts of 3 silicate inclusions. Again, these meteorites are postulated to have an origin that was well below the original planet' s surface. All of this brief discussion is intended to illustrate the concept that meteorites may be fragments from many different sites in one or more planets or planetesimals.Furthermore, since our sampling techniques are extremely limited on the earth, these missiles from space may represent the only samples man will ever obtain that reflect geological processes which occur well below a planet' s surface. Viewed in this perspective, one may readily appreciate their value and interest to the geologist and cosmochemist. Pallasites, or stony-irons, will be the only class of meteorites discussed in depth in this thesis since this class was the only one studied by the author. Historically, the view that meteorites could fall from the sky was not always accepted in scientific circles.President Thomas Jefferson is quoted as saying that he would rather believe Yankee professors would lie than to believe their report of a meteorite fall. Most meteorites are not easily distinguishable from earth rocks to the untrained observer, so such skepticism is understandable. How- ever, the pallasites are remarkable in their unique structure which is quite different from any earthly analog.The Krasnojarsk pallasite was discovered in 1749 and moved to the Academy of Sciences in 1772 4 by Pallas.Chladni examined it in 1794 and concluded it must have possessed an extraterrestrial origin. The pallasites are composed of approximately near-equal amounts of iron-nickel and olivine.Olivine is a silicate mineral [(Mg, Fe)SiO 4] which is a solid solution of the divalent metal sili- 2 cates.Olivine is a dense, close-packed structure which does not tolerate the substitution of other ions whose ionic radii differ appreci- ably from the hexacoordinated Fe++ or Mg++ ionic radii of 0.69 and 0.80 A,respectively (Whittaker and Muntus, 1970).For this reason, the trace element content of olivine is usually much lower than that of other silicates.Because terrestrial olivines exist in equilibrium with other silicates, some trace element concentrations are three orders of magnitude higher than those found in pallasitic olivine which has established an equilibrium with a pure iron-nickel phase. Structure The structure of pallasites resembles a three dimensional web of iron-nickel metal containing dispersed individual crystals of olivine, The metal phase has been shown to be continuous through conductivity tests.This continuity may
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