THE ORIGIN OF BASALT AND CAUSE OF MELTING BENEATH EAST ANTARCTICA AS REVEALED BY THE SOUTHERNMOST VOLCANOES ON EARTH Jenna L. Reindel 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 December 2018 Committee: Kurt S. Panter, Advisor Peter Gorsevski Daniel F. Kelley © 2018 Jenna L. Reindel All Rights Reserved iii ABSTRACT Kurt S. Panter, Advisor The southernmost volcanoes on Earth, Mt. Early and Sheridan Bluff, are two basaltic monogenetic volcanoes located 87°S at the head of the Scott Glacier, in the southern Transantarctic Mountains. The Early Miocene volcanoes lie ~1000 km from any other volcano and ~200 km from the shoulder of the West Antarctic Rift System (WARS), which is the foci of most Cenozoic alkaline volcanism in Antarctica. WARS is part of a larger diffuse alkaline magmatic province (DAMP) including volcanism in New Zealand and Australia. Dating by the 40Ar/39Ar method indicates that Mt. Early is older than previously determined and closer in age to Sheridan Bluff (~19 Ma). Basalts range in composition from alkaline (~6 wt. % Ne- normative) to subalkaline tholeiite (~6 wt. % Hy-normative). Tholeiite have higher ratios of Zr/Nb (9) than alkaline basalts (4) and have lower ratios of La/Yb (5), La/Lu (<50), and Gd/Yb (2) than alkaline basalts (20, 100-150, and ~3 respectively). Fractional crystallization, assimilation-fractional crystallization (AFC), and partial melting are tested as mechanisms to explain compositional variations. Crystal fractionation alone cannot explain the difference in composition. Modelling AFC on plots of Rb, Ba, and La/Nd versus TiO2 show that unrealistic bulk partition coefficients are required to explain the array of compositions using contaminates from the upper crust. I conclude that the coexistence of tholeiite and alkaline basalt is likely due to different degrees of partial melting. The basalts mirror partial melting trends for a common garnet lherzolite source on plots of La/Lu versus Nb/Yb. These models also suggest that tholeiite was produced by higher degrees of melting at shallower depths. Mt. Early and Sheridan Bluff iv basalt, especially tholeiite, are distinctive from DAMP basalt, having lower Ce/Pb (≤ 20), Gd/Yb (≤ 3) and lack K and Pb anomalies on normalized multi-element plots. Negative K anomalies are a prominent feature of DAMP basalt and are used to support amphibole-rich lithospheric sources. It is likely that the basalts from Mt. Early and Sheridan Bluff were derived from asthenosphere with little or no input from mantle lithosphere. Causes for volcanism in DAMP include passive extension, metasomatism and plumes. A recent viable mechanism for triggering volcanism in this region is lithospheric delamination and has been proposed based on geophysical evidence. v ACKNOWLEDGMENTS This research is funded by NSF Polar Programs grant #1443576. I would like to thank my advisor, Kurt Panter, for making this research possible and enjoyable. The knowledge you’ve instilled in me is priceless, and I’m grateful for your continued support and guidance. Thank you for giving me the opportunity to experience field work in Antarctica, for mentoring and believing in me as a scholar, and providing me with professional development. Thank you to my committee members for your valuable input and guidance. Thank you to John Smellie (University of Leicester, UK) for your knowledge and support during our time in Antarctica. Thanks to Tim Burton (UK), for being the best mountaineering guide and friend, for teaching me valuable mountaineering skills, and for always making me laugh and feel safe. Thanks to Kenn Borek Air Ltd for the field support to and from our remote field location. Thanks to Gordon Moore (University of Michigan), for your expertise and guidance during my microprobe sessions at the Electron Microprobe Analysis Laboratory (EMAL). Thanks to Charlie Onasch and John Farver for assistance and direction in making thin sections. Finally, thank you to my family, friends, colleagues and teachers for helping me along the way and for making graduate school the best it could be. Talons up! vi TABLE OF CONTENTS Page CHAPTER 1. INTRODUCTION. ......................................................................................... 1 CHAPTER 2. GEOLOGIC SETTING .................................................................................. 4 West Antarctic Rift System ....................................................................................... 4 Geology of Sheridan Bluff and Mt. Early .................................................................. 6 CHAPTER 3. FIELD OBSERVATIONS AND METHODS OF STUDY ........................... 8 This Study: Field Logistics & Observations .............................................................. 8 Sample Preparation for Petrography and Mineral Chemistry .................................... 9 Sample Preparation for Dating................................................................................... 9 Mineral Chemistry ..................................................................................................... 10 Whole Rock Chemistry .............................................................................................. 11 CHAPTER 4. RESULTS ....................................................................................................... 13 Petrography ................................................................................................................ 13 Age ........................................................................................................................... 14 Mineral Chemistry ..................................................................................................... 14 Whole Rock Chemistry .............................................................................................. 17 Thermobarometry ...................................................................................................... 20 CHAPTER 5. DISCUSSION ................................................................................................. 23 Evolution by Crystal Fractionation ............................................................................ 24 Crustal Contamination ............................................................................................... 26 Partial Melting ........................................................................................................... 28 Mantle Sources for Basalt .......................................................................................... 29 vii Causes for Volcanism ................................................................................................ 31 CHAPTER 6. CONCLUSION............................................................................................... 34 REFERENCES… .................................................................................................................. 36 APPENDIX A. TABLES…………………………………………………………… .......... 48 APPENDIX B. FIGURES……………………………………………………… ................ 79 1 CHAPTER 1. INTRODUCTION A major objective of this study is to explain the petrogenesis of the magmas erupted at Mt. Early and Sheridan Bluff, upper Scott Glacier, Antarctica, and whether their source and melt generation is associated with Cenozoic volcanism in the West Antarctic Rift System (WARS). Basaltic magmas have distinct tectonic origins and melting regimes that can be distinguished from each other both mineralogically and geochemically. The difference in compositions for primitive magmas erupted at different plate boundaries has to do with the depth of melting, amount of melting and mantle source composition. Different lithologies and mantle source compositions have been proposed to explain major element contents and enrichments in incompatible trace elements (Pilet, 2015 and references therein). Alkaline volcanism associated with continental rifts like the WARS is generally considered to be the result of small degrees of partial melting of mantle peridotite at great depths. The peridotite is often considered to contain components of recycled oceanic crust and mantle lithosphere. This means the material was brought down by subduction into the mantle and brought back up by large scale upwelling in the convecting mantle and explains the ideas behind mantle plumes. The long period of time it takes for this process can explain the radiogenic isotope signatures as well. This recycled peridotite material is often considered to have been altered (i.e. metasomatized) and CO2 -enriched in the subduction process to produce low SiO2, CaO, and Al2O3 contents found in alkaline magmas (Pilet et al., 2011). Another proposed model is the melting of metasomatized hydrous cumulates at the base of the lithosphere that occur in the form of amphibole-bearing metasomatic veins. High degrees of melting of these amphibole-bearing veins along with interaction of the resultant melts with the surrounding mantle as it rises towards the surface could account for the major and trace elements observed in alkali basalts (Pilet, 2015; Pilet et al., 2008; 2011). Metasomatism in 2 the shallow mantle may also help explain radiogenic isotope signatures in shorter periods of time due to the higher amount of isotope fractionation between the parent and daughter that takes place. The driving force for late Cenozoic magmatism in West Antarctica has been a topic of considerable debate. Geochemical investigations,