TEM Investigation of Nano-Scale Precipitates in Ultrahigh-Pressure Clinopyroxenes

TEM Investigation of Nano-scale Precipitates in Ultrahigh-Pressure Clinopyroxenes by Tina Renee Hill A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Geoscience) at the UNIVERSITY OF WISCONSIN-MADISON 2012 Date of final oral examination: 11/30/12 The dissertation is approved by the following members of the Final Oral Committee: Huifang Xu, Associate Professor, Geoscience John Valley, Professor, Geoscience Phil Brown, Professor, Geoscience Hiromi Konishi, Ph. D., Scientist, Geoscience Paul Voyles, Associate Professor, Materials Science i ABSTRACT This investigation details the TEM characterization of epitaxially-exsolved nano-scale siliceous precipitates in cores of ultrahigh-pressure (UHP) clinopyroxenes from eclogites of the Bohemian Massif and Western Gneiss Region, and a Kokchetav Massif garnet pyroxenite. Siliceous precipitates are observed in cores of UHP clinopyroxenes from multiple UHP metamorphic terranes, and are often used as indicators for UHP metamorphism. Without atomic-scale characterization, these precipitates have been identified only as quartz or coesite. The close structural and orientation relationships of siliceous phases and their host clinopyroxenes are revealed by High Resolution TEM and Scanning TEM (HRTEM/STEM) images and Selected Area Electron Diffraction. Low pressure, low density siliceous phases of keatite, α-cristobalite, tridymite, siliceous glasses, and albite are reported here, not the previously described higher density silica polymorphs. The phase that precipitates may be controlled by a complex combination of factors. Particularly, calculated vacancy content (Ca-Eskola component) in clinopyroxenes may be the overriding factor in creating localized low pressure micro-environments for nano-precipitate exsolution. Higher vacancy content may be linked with the lowest density silica, tridymite; lower vacancy contents may be linked with somewhat higher density polymorphs of silica, including α-cristobalite and the first confirmed natural occurrences of keatite. Mechanisms of exsolution are closely related to the interfaces that develop between phases. The geometry of the substrate in epitaxial mineral growth may cause growth of phases not stable at existing P-T-X conditions. Little strain in HRTEM/STEM images is observed and shows favorable lattice matching between siliceous precipitates/hosts; interface models detailing a calculated 5.4% difference in unit cells at most support this. Nano-scale size effects are also ii important—they stabilize nano-scale and metastable phases such as keatite and α-cristobalite outside of bulk mineral stabilities. In combination with vacancy contents and interfacial effects, localized OH contents within pyroxenes and exsolution temperature may also be important factors. The presence of sheet silicates in all three UHP terranes can likely be attributed to retrograde processes. High tensile strength and low thermal expansivity/compressibility of clinopyroxenes compared to siliceous phases indicates exsolved phases may not affect the rheology of the exhuming rock, but may ultimately affect the interpretation of the P-T path. iii ACKNOWLEDGEMENTS First and foremost, I must thank my advisor, Huifang Xu. Without his academic and logistical support and analytical advice none of this would have been possible. In addition, this dissertation benefited greatly from review by committee members John Valley, Phil Brown, Hiromi Konishi, and Paul Voyles. L. Gordon Medaris provided not only the samples for this study, but many fruitful conversations about them. I am grateful for his supportive collaboration over the past three years. None of the difficult transmission electron microscope data would have been collected without the artistic and precise scientific capabilities of Hiromi Konishi. I would like to thank him for the many hours we spent at the TEM working on my samples, discussing TEM techniques, and buoying my spirits when samples were less than ideal. Data processing would also have been impossible without his guidance. I must also thank John Fournelle for his assistance with my electron microprobe data and many great discussions regarding processing that data. The University of Wisconsin-Madison graduate students are some of the best there are, and they have been excellent colleagues and friends in my time in Madison. In particular, I’d like to thank my good friend and officemate, Chloë Bonamici, for many great conversations about work and life, in general. My funding sources that made this possible are: the University of Wisconsin – Madison Department of Geoscience, NSF, and Sigma Delta Epsilon-Graduate Women in Science. The last group of people I would like to thank is my close family and friends. Without their support and love, this dissertation would not exist. Lastly, I’d like to thank my husband, iv Matthew, for his emotional support through this process. Someday there will be a degree (or support program) for spouses of graduate students and he will certainly earn a degree! I could not have done this without you. v TABLE OF CONTENTS Abstract……………………………………………………………………………………………i Acknowledgements……………………………………………………………………...………iii Table of Contents……………………………………………………………………………..…v Introduction………………………………………………………………………………..…….1 Chapter 1. Natural occurrence of keatite precipitates in UHP clinopyroxene from the Kokchetav Massif: A TEM investigation………………………..………………………….…..12 Chapter 2. Low pressure α-cristobalite and Na-Al silicate glass in UHP clinopyroxene from a Bohemian Massif eclogite: An HRTEM study……………………………………………….…46 Chapter 3. Tridymite, keatite, and α-cristobalite revealed by HRTEM in UHP clinopyroxene from the Western Gneiss Region Hareitland eclogite………………………………………..….89 Chapter 4: Thesis Conclusions………………………………………………………………..140 Appendix 1. Table of standardized k-factors for the FEI aberration-corrected FEG-STEM at the University of Wisconsin – Madison…………………………………………………...……….148 Appendix 2: TEM sample preparation: the difficulties of natural samples….....................................................................................................................................150 Appendix 3: University of Wisconsin – Madison sample numbering………………….……..153 1 INTRODUCTION This thesis describes the results of high resolution transmission electron microscope (HRTEM) characterization of the interface-controlled exsolution of micro- and nano-scale siliceous precipitates within ultrahigh-pressure (UHP) clinopyroxenes. These precipitates noted in many UHP metamorphic clinopyroxenes were previously thought to be either coesite or quartz, and only nano-scale characterization has been able to shed light on exact mineralogic phases. Metamorphic petrologists regard high pressure (HP) conditions as the P-T field in which lithostatic pressures exceed those defined by the aragonite-calcite polymorphic transition curve. In this HP field, a jadeite-rich clinopyroxene solid solution is stable. UHP conditions occupy the P-T field at pressures greater than the coesite-quartz boundary. Diamond is well within the UHP stability field except at low temperatures (Ernst and Liou 2008). Geologic processes occurring under UHP conditions can be difficult to recognize due to intense retrogressive reactions and/or fluid and deformation activities experienced by rocks during exhumation. However, studies of these rocks in laboratories with the aid of advanced, state-of-the-art analytical instruments and techniques such as the high resolution transmission electron microscope (HRTEM) utilized in this investigation, help to shed light on processes operating in the deep Earth at convergent plate boundaries. The long-term goal of UHP metamorphism (UHPM) research is in its significance for understanding large-scale mantle dynamics, major elements of plate tectonics such as continental collisions, deep subduction and exhumation, mountain building, and geochemical recycling from surface to great depths. 2 UHPM has now been recognized as an important feature of the major Phanerozoic continental plate collisions zones, represented by the Caledonian, Variscan/Hercynian, Alpine and Himalayan orogenic belts (Carswell and Compagnoni 2003). Recent progress in experimental determination of P-T stability fields of new phases, phase transformations of minerals which were known before to be stable only at shallow depths of the continental crust such as some siliceous phases observed in this investigation, and newly-reported exsolution products of UHPM minerals, indicate that continental rocks were subducted to depths exceeding 100-250 km (Hacker 2007). Extensive research in UHP metamorphic rocks has recently led to new constraints and the improvement of geotectonic models developed for the now well-known extremely deep subduction of continental rocks and their exhumation within many well-known orogenic belts (i.e. those of the Bohemian Massif, Czech Republic, Kokchetav Massif, Kazahkstan, and the Western Gneiss Region (WGR), Norway; the regions chosen for this dissertation). In the majority of well-established occurrences of UHPM rocks, mineral indicators of UHPM are best preserved in metabasic rocks, therefore, eclogitic rocks have been chosen for this study. In most instances it has been the positive identification of preserved inclusions of coesite within robust minerals such as garnet and zircon that has led to the recognition that rocks have experienced UHPM conditions. The three regions chosen for this thesis were chosen

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