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New Mineral Chemistry and Oxygen Isotopes

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New Mineral Chemistry and Oxygen Isotopes NEW MINERAL CHEMISTRY AND OXYGEN ISOTOPES FROM ALKALINE BASALTS IN THE NORTHWEST ROSS SEA, ANTARCTICA: INSIGHTS ON MAGMA GENESIS ACROSS RIFTED CONTINENTAL AND OCEANIC LITHOSPHERE Susan R. Krans A Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE August 2013 Committee: Kurt Panter, Advisor John Farver Chad Deering © 2013 Susan Krans All Rights Reserved iii ABSTRACT Kurt Panter, Advisor The West Antarctic Rift system hosts one of the world’s most extensive alkaline igneous provinces. Rifting that lead to the breakup of the proto-Pacific margin of Gondwana in the Late Cretaceous was initially amagmatic and not related to mantle plume activity. Mantle heterogeneities and magmatic processes along a continent-ocean transect in the Antarctic Northwest Ross Sea region are deduced from geochemical and isotopic study of alkaline basalts. Specifically, new mineral chemistry and oxygen isotopes from the least differentiated basalts expand upon previous studies that have focused primarily on whole-rock data. Alkali olivine basalt and basanite represent two mafic end-members in the Northwest Ross Sea and are characterized by high whole rock Mg# (59 ± 9), Ni + Cr (> 200 ppm), highly variable CIPW normative (> 20% hypersthene to > 20% nepheline) and trace element contents (e.g., Sr = 400 to 1100 ppm, La/Yb = 11 to 28, Nb/Y = 1.2 to 3.6). Phenocryst phases are primarily olivine (Fo66 to Fo91) and clinopyroxene (diopside) with rare amphibole (kaersutite) and exhibit varying degrees of compositional zoning. Temperature and pressure estimates based on mineral-liquid equilibrium range from 1206-1331°C (olivine), 1220-1284°C and 0.9-1.3 GPa (clinopyoxene), indicating that early crystallization occurred at or below the Moho. Olivine oxygen isotopes measured by SIMS range from 4.71 to 5.44‰ and average 5.15 ± 0.52‰ and encompass values for clinopyroxene (4.97 ± 0.36 ‰) and (5.17 ± 0.10 ‰) measured by laser fluorination. iv Correlation between oxygen isotopes and degree of partial melting (i.e. Nb/Y) suggests that lower degree melts preferentially consume a lower δ18O source interpreted as metasomatic veins in the lithospheric mantle. Temperature and pressure estimates across the continent-ocean transect indicate a region of lithospheric necking previously identified for Northern Victoria Land. Evidence for disequilibrium observed texturally and compositionally in minerals suggests complex crustal processes, mainly in the ocean- continent transition zone. The results of this study support previous suggestions that late Cenozoic alkaline magmatism in the West Antarctic Rift System is controlled by variations in partial melting of a heterogeneous mantle source and highlights future potential to investigate physical controls on volcanism at magma-poor rift margins. v To my father, who instilled in me a sense of adventure, a love for science, and an eagerness to learn. To my mother, who taught me to question everything and never be satisfied with “good enough”. To my mentors and peers, whose endless support and guidance has been a guiding light in the dark allies of the unknown. I would not be where I am today if not for your wisdom and comradery. vi ACKNOWLEDGMENTS I would like to thank Noriko Kita and Jim Kern at the University of Wisconsin- Madison SIMS laboratory, and Gordon Moore at the University of Michigan EMAL laboratory for technical support. I would also like to thank my committee members Dr. Chad Deering (University of Wisconsin-Oshkosh, Dr. John Farver and especially my advisor Dr. Kurt Panter for all their guidance and support throughout this project. This research was funded by a collaborative grant NSF ANT-0943274. vii TABLE OF CONTENTS Page 1. INTRODUCTION ............................................................................................................. 1 2. GEOLOGIC BACKGROUND .......................................................................................... 5 3. METHODS ........................................................................................................................ 7 3.1. Sample Summary ................................................................................................ 7 3.2. Sample Preparation ............................................................................................. 9 3.3. Mineral Chemistry .............................................................................................. 10 3.4. Oxygen Isotopes.................................................................................................. 11 4. RESULTS .......................................................................................................................... 12 4.1. Petrography ......................................................................................................... 12 4.2. Mineral Chemistry ............................................................................................. 14 4.3. Oxygen Isotopes ................................................................................................. 17 4.4. Thermobarometry and Water Estimates ............................................................ 20 5. DISCUSSION .................................................................................................................... 25 5.1. Magma Origin ..................................................................................................... 25 5.2. Variations Across the Ocean-Continent Transect ............................................... 30 5.3. Crustal Influences on Magmas ............................................................................ 33 6. CONCLUDING STATEMENTS ...................................................................................... 37 REFERENCES ...................................................................................................................... 39 viii LIST OF FIGURES Figure Page 1 Map of NWRS samples ............................................................................................. 49 2 TAS Diagram ............................................................................................................ 50 3 Glomercrysts and xenoliths in thin section ................................................................ 51 4 Disequilibrium textures in thin section ...................................................................... 52 5 Quench textures in thin section .................................................................................. 53 6 Mineral chemistry by zone ........................................................................................ 53 7 Olivine chemical transects ......................................................................................... 54 8 δ18O olivine vs. Fo ..................................................................................................... 56 9 Δ18O fractionation for NWRS mineral pairs .............................................................. 57 10 Core-rim variation in calculated KD(Fe-Mg) exchange equilibrium ......................... 58 11 Mineral-liquid tests for equilibrium ........................................................................... 59 12 Al-, Ti-Tschermak substitution in amphibole ............................................................ 60 13 Amphibole P-T-H2O and thermal stability ................................................................ 61 14 Differentiation trends ................................................................................................. 62 15 Ni/Mn ratios for high Fo olivine ................................................................................ 63 16 Partial melting trends ................................................................................................. 64 17 Degree of partial melting (Nb/Y) vs. δ18O olivine..................................................... 65 18 T-P-Depth estimates for NWRS minerals .................................................................. 66 19 T-olivine with distance from the CRS ....................................................................... 67 20 Clinopyroxene T-P with distance from the CRS ....................................................... 68 21 Geochemical variations with distance from the CRS ................................................ 69 ix 22 Assessment of contamination from continental lithosphere ...................................... 70 23 Conceptual model for NWRS magmatism from continent to ocean ......................... 71 x LIST OF TABLES Table Page 1 Whole-rock major and trace elements, and radiogenic isotope compositions ........... 73 2 Petrographic description of samples .......................................................................... 76 3 Representative olivine chemistry ............................................................................... 78 4 Representative clinopyroxene chemistry ................................................................... 83 5 Representative amphibole chemistry ......................................................................... 86 6 Representative plagioclase chemistry ........................................................................ 87 7 Oxygen isotope composition of minerals .................................................................. 88 8 T-P-H2O estimates ..................................................................................................... 89 9 Comparison of least
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