AN ABSTRACT OF THE THESIS OF Dennis 0. Nelson for the degree of Doctor of Philosophy in Geology presented on June 6, 1979 Title: Strontium Isotopic and Trace Element Geochemistry of the Saddle Mountains and Grande Ronde Basalts of the Columbia River Basalt Group . ft Redacted for Privacy Abstract approved: E. Julius Dasch The Columbia River Basalt (CRB) display significant variations in major and trace element and Sr isotopic compositions. These comp- ositions reflect complex and variable origins for the CRB magmas. Among the most varied is the Saddle Mountains Basalt (SNB) in which Sr ratios vary from 0.7078 to 0.7147 + 0.0002.The higher ratios reflect contamination through consistent correlations with major element compositions. Modeling suggests contamination by assimila- tion of 4.4 to 9.4 wt. percent of radiogenic crustal rocks. High d180values (up to +7.68 per mil) support the model. Age and field relations suggest that the contaminated flowrocks are not the result of progressive contamination of a single magma, but rather reflect the contamination of independent magmas during their ascent. The Pomona and Elephant Mountain Basalts (SMB) (Sr ratio= 0.7078-0.7081) reflect mantle values. These basalts erupted from the Chief Joseph dike swarm along with the Grande Ronde (GRB) and Wanapum Basalts. Together, the GRB and SMB indicate a mantle source with Sr ratios varying from 0.7043 to 0.7081. Disequilibrium fusion as an expla- nation for the high and variable ratios is discounted because the high diffusion rates at mantle temperatures continuously homogenize the Sr ratios of adjacent mineral phases. Further evidence for mantle heterogeneity comes from consideration of the trace element geochemistry of the GRB. This formation is divided into the high- Mg, T-1, T-2, and low-Mg chemical types on the basis of incompatible major and trace element content. Significant variations in trace elements occur, particularily in the LREE ((La/Sm)n = 1.87- 2.31 +0.07). Chondrite-normalized REE plots display uniform slopes and no Eu anomalies. Olivine fractionation of 40% is required to account for the different levels of REE enrichment. Such an amount is inconsistent with small major element variations and large (La/Sm) variation. The latter requires 60+% removal of garnet-clinopyroxene assemblages, inconsistent with the limited range of Cr, Sc, and Yb. Melting models require a LREE-enriched garnet-bearing source. Pro- ducing the (La/Sm)n variation by variable melting of a homogeneous source requires a variation of 15 to 30% melt. This would result in marked differences in (La/Yb) ; this is not observed. n Differences in mineral abundance cannot totally account for observed (La/Sm)n variation; different levels of trace element enrichment in thesource is required. The unfractionated HREE patterns and small variation in (Laftb)nsuggests that garnet is nearly or completely consumed during melting. Melting beyond garnet's disappearence can explain the uniform pattern of the REE and the different levels of REE enrichment because melting beyond garnet uniformly dilutes the REE in the liquid. Modeling using varying mantle mineralogies, levels of source REE enrichment, and differing melting proportions indicates independent magma genesis for the chemical types of the GRB, with the high-Mg and T-1 magmas produced at greater depths through two stage melting (>25%), while the low-Mg and T-2 magmas were produced at shallower depths (< 25%) with garnet still in the residuum. These four magma types ascended independently and erupted contempo- raneously. Strontium Isotopic and Trace Element Geochemistry of the Saddle Mountains and Grande Ronde Basalts ofthe Columbia River Basalt Group by Dennis Oliver Nelson A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy June 1980 APPROVED: Redacted for Privacy Associate Professor oilSeology Redacted for Privacy Chairperson Geology Redacted for Privacy Dean of Gradu te School Date thesis is presented June 6, 1979 Typed by Dennis 0. Nelson for Dennis Oliver Nelson ACKNOWLEDGEMENTS The general thesis topic was initially suggested by my advisor, E. J. Dasch. His humor, enthusiasm, and general support throughout this project is appreciated. A considerable debt of gratitude is owed to Ron Senechal. This project simply could not have been accomplished without his advice and ever-patient guidence in the laboratory. Nor could this thesis have been written without the encouragement, sample powders, and trace and major element analyses provided by Don Swanson and Tom Wright of the U. S. Geological Survey. Additionally, Don spent considerable time in the field, on the phone, and through correspondance with me and greatly con- tributed to my appreciation of the Columbia River Basalt. Also, the Survey provided financial support of field and laboratory studies during two summers. Additional financial support was provided by grants from the GSA Penrose Foundation and the Sigma Xi Society. Bill Taubeneck's encouragements and interests were always welcomed and Josh Cocker's oxygen isotopic data and moral support was appreciated. Several individuals at Portland State University deserve thanks. Marvin Beeson and Michael Moran taught me a considerable amount regarding the Columbia River Basalt west of the Cascades and regarding the application of trace element geochemistry to stratigraphy. I appreciate their openess and respect their logic; their contributions to the Columbia River Basalt have been significant. I also wish to thank Virginia Pfaff and Barbara Portwood, both of whom helped with the early typing andalways had encouraging words, and Gene Pierson, Dave Matty, Fred Gullixson, and other students and faculty of the Earth Sciencesdeparment at Portland State who were always willing to help. Finally, I must thank my wife, Barbara, and daughters,Michelle and Mindy, who put up with a lot since I began this in 1973,and whose patience, particularily during the last months, have been incredible. TABLE OF CONTENTS Introduction 1 Presentation of Data and Discussion of Errors 14 The Application of Strontium Isotopic Composition to Problems of Correlation of Volcanic Rocks 31 Interlayering of Picture Gorge and Grande Ronde Basalts 35 Saddle Mountains Intracanyon Basalts 37 Evaluation of Isotopic Equilibrium and Disequilibrium Hypotheses as Applied to Magma Genesis 40 Implications for a Heterogeneous Mantle 43 Diffusion Considerations: Mechanisms and Pressure 44 Diffusion Considerations: Temperature 51 Contamination Models for the Columbia River Basalt 60 Application to the Grande Ronde Basalt of Central Washington 67 Application to the Saddle Mountains Basalt 76 Physical Models of Contamination 87 Trace Element Modeling of the Grande Ronde Basalt 96 Crystal Fractionation Models 105 Variable Melting of a Homogeneous Mantle 116 Heterogeneity in the Mantle Source Region 121 Model 1 - Mantle of Uniform Mineralogy 145 Model 2 - Mantle Heterogeneities in Modal Mineral- ogy with Melting at the Same Depth 149 Model 3 - Heterogeneous Mantle with Melting at Different Depths 150 Application of Strontium Isotopes to Mantle Events 159 Conclusions 164 References Cited 171 Appendices 193 I. Analytical Procedures 193 II. Assumptions and Calculations Pertaining to Contamination Models 199 III. Trace Element Modeling 203 Partition Coefficients 203 Crystal Fractionation 207 Melting Models 212 Zone Refining Calculations 219 LIST OF ILLUSTRATIONS Figure Page 1 Index Map 1 2 Temperature dependence of the diffusion coefficient 53 3 Equilibrium time for Sr isotopes 57 4a. Initial Sr ratio vs. Sr content for selected Grande Ronde Basalts 63 b. Initial Sr ratio vs. 1/Sr for selected Grande Ronde Basalts 64 5a. Initial Sr ratio vs. wt. % Si02 Grande Ronde Basalts of northeast Oregon-southeast Washington 68 b. Initial Sr ratio vs. 1/Sr - Grande Ronde Basalt of northeast Oregon-southeast Washington 69 6 a. Initial Srratio vs. wt % K- Grande Ronde Basalt of centralWashington 71 b. Initial Srratio vs. Rb/Sr- Grande Ronde Basalt of centralWashington 72 c. Initial Srratio vs. 1/Sr -Grande Ronde Basalt of centralWashington 73 7 Trace elements in contaminated flowrocks 86 8 Areal extent and feeder dikes of Grande Ronde Basalt 99 9Rare earth patterns of the Grande Ronde Basalt 104 10 (La/Sm) as a function of mineral fractionation 108 n 11 (la/Sm) as a function of mineral assemblage fractionation 110 n 12 (La/Sm) vs. Sc content - Grande Ronde Basalt 111 n 13 (La/Sm)n vs. Cr content - Grande Ronde Basalt 113 14 (La/Sm) vs. Yb content - Grande Ronde Basalt 114 n n 15 AFM diagram - Grande Ronde Basalt 115 16 Ratio of (La /Sm) in liquid relative to source as a function of degree of melt 118 17Melting model for chondritic garnet lherzolite 120 18 Melting model for chondritic spinel lherzolite 122 19 Melting model for a LREE-enriched spinel lherzolite 126 20 Melting model for a LREE-enriched garnet lherzolite 127 21 a. (La/Sm)n as a function of degree of melt 129 b. (La/Sm)n as a function of mineralogy at 20% melt 130 22Melt model for LREE-enriched garnet lherzolite with 20% garnet 131 23 Melt model for LREE-enriched garnet lherzolite with 14% garnet 133 24Melt model for LREE-enriched garnet lherzolite with 12% garnet 134 25 (La/Yb) as a function of degree of melt 136 26 Two stage melting of garnet lherzolite 138 27Three stage melting of phlogopite-garnet lherzolite 139 28 Rare earth patterns of high-Mg and T-1 compositions 147 29 Rare earth patterns of low-Mg and T-2 compositions 148 30 Laand Ybfor