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Petrology of Inclusion-Rich Lavas at Minna Bluff, Mcmurdo Sound, Antarctica: Implications for Magma Origin, Differentiation, and Eruption Dynamics

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Petrology of Inclusion-Rich Lavas at Minna Bluff, Mcmurdo Sound, Antarctica: Implications for Magma Origin, Differentiation, and Eruption Dynamics PETROLOGY OF INCLUSION-RICH LAVAS AT MINNA BLUFF, MCMURDO SOUND, ANTARCTICA: IMPLICATIONS FOR MAGMA ORIGIN, DIFFERENTIATION, AND ERUPTION DYNAMICS Mary K. Scanlan 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 2008 Committee: Kurt Panter (Ph.D.), Advisor John Farver (Ph.D.) Thom Wilch (Ph.D.) © 2008 Mary Scanlan All Rights Reserved iii ABSTRACT Kurt Panter, Advisor Xeno Ridge, a newly discovered group of inclusion-rich deposits located at the top of the Minna Bluff stratigraphic section, formed as a result of magmatic mixing/mingling and provides insight into magma origins, mixing and eruption dynamics, and the evolution of Minna Bluff. Phonolite to tephriphonolite lavas are contain abundant inclusions which vary in size, shape, and mineralogy. Five inclusion types are identified at Xeno Ridge. The lavas that host the inclusions are dark-gray and porphyritic with a hypocrystalline, vesicular groundmass. Minerals within the host lavas include feldspar (plagioclase = An27–An84, alkali feldspar = Ab41Or57−Ab70Or20), amphibole (kaersutite), diopside (~Wo55En35Fs10), titanomagnetite (Ti# 55), olivine (Fo45, Fo81), and apatite. Type I inclusions are highly vesicular with sinuous forms and crenulate margins with the host indicating magmatic mixing/mingling. The mineral assemblage of Type I includes kaersutite (15-20 vol. %), feldspar (plagioclase = An33−An80, alkali feldspar = ~Ab55Or45), diopside (~Wo55En30Fs6), titanomagnetite (Ti# ~75), olivine (Fo48, Fo85) and apatite. Type II inclusions are kaersutite megacrysts and glomerocrysts dominated by kaersutite with subordinate phenocrysts, microphenocrysts and groundmass composed of plagioclase (An20 – An76), titanomagnetite (Ti# 84), diopside (~Wo50En35Fs15), and apatite. Type III is a single porphyritic inclusion with phenocrysts of anorthoclase and is similar in texture and mineralogy to lavas found within the Minna Bluff stratigraphic section. Type IV are ‘salt and pepper’ colored inclusions with granular textures consisting of kaersutite with interstitial plagioclase (An16 – An55), diopside (~Wo50En36Fs4), titanomagnetite, apatite, sodalite, titanite and olivine. Type V inclusions are similar in appearance to Type IV but have a different mineral assemblage dominated by alkali feldspar (Ab50Or50), nepheline and plagioclase with subordinate, interstitial minerals of Fe-rich clinopyroxene (hedenbergite ~Wo50En17Fs38), titanomagnetite and leucite. Both Types IV and V have sharp contact margins with the host lava indicating that they were fully solidified when entrained by the host magmas, whereas Types I and II were semi-molten. Semi-quantitative geothermobarometric results for kaersutite and clinopyroxene indicate that some crystallized at P-T conditions in the lower crust to upper mantle (5-9 kbars = 15-27 km). High water contents in the magmas induced early crystallization of amphibole and clinopyroxene and suppressed plagioclase, which crystallized along with magnetite at shallow levels within the crust. Four stages are deduced for the mixing and ascent of magma bodies and the incorporation of inclusion types at Xeno Ridge. Stage I: phonolitic magmas that comprise lava compositions prior to mixing with Type I magma ascend into the upper crust. Stage II: replenishing phonotephritic magmas (Type I) ascend from depth within the same conduit system and incorporate partially solidified material (Type II) that originated from sidewall crystallization of previous magmas. Stage III: the mixing event triggered a second episode of magma ascent. Stage IV: the rapid ascent of mixed magmas entrained crystalline selvages of Type IV and V compositions from conduit walls before eruption. iv To my mother, who taught me to always challenge myself and that hard work will result in achieving all I could ever hope. Without your constant love and support, I would not be the person I am today. To my father, who shared a passion for rocks which has inspired and shaped my studies, and will forever influence my life. Thank you for your encouragement, support, and patience. They are and always will be significant, even if you do love soft rocks… And to my two closest friends, Elle Scanlan and Paul Hogan, who each helped me carry on. Thank you so much for being understanding and always ready to offer inspiring words of motivation. Your advice and loving words will never be forgotten. v ACKNOWLEDGMENTS The completion of this project would not have been possible without the encouragement and support of my colleagues, peers, friends, and family. Thank you to my advisor and friend, Kurt Panter, for providing wonderful research and lifetime experiences. I will cherish them all for the rest of my life. I treasure the ability to one day share my Antarctica experience with my children and grandchildren, and for that I am grateful. Thank you for pushing me to think outside of the box, but to keep my feet grounded. I have learned patience and self encouragement during your sabbatical, and the completion of this project was in part driven by my desire to make you proud. Thank you for everything. Thank you to the Minna Bluff team for welcoming me into the Antarctica community and for helping me through my fear of heights. Without constant encouragement on the ice from Thom Wilch (Albion College) and John Smellie (British Antarctica Survey), I would have been lost. Thank you! Thank you to Bill McIntosh and Nelia Dunbar (New Mexico Institute of Mining and Technology) for welcoming me into your home and your lab. Your hospitality will always be appreciated. A special thank you to all in the BGSU geology department. In particular, I would like to thank Shelia Roberts for always offering a supportive ear and words of encouragement. You are a wonderful friend. Thank you Pat Wilhelm and Bill Butcher for attending to my every need while Kurt was away. Your help will always be much appreciated. Thank you to John Farver for answering my silly questions and always being available for counsel. A special thank you goes to Jim Evans for sharing a passion for soft rocks, for which my father will be forever grateful. Thank you to my fellow graduate students who always made me feel welcome and at home. Finally, I would like to thank my family for always being understanding, supportive, and encouraging. Your love and kindness will remain in my heart, and for that I am forever grateful. vi TABLE OF CONTENTS Page INTRODUCTION ................................................................................................................. 1 Geologic Background................................................................................................ 2 FIELD RELATIONSHIPS .................................................................................................... 6 Geology of Minna Bluff............................................................................................. 6 Xeno-Ridge................................................................................................................ 9 ANALYTICAL TECHNIQUES............................................................................................ 13 Mineral Chemistry ..................................................................................................... 13 Whole-Rock Chemistry ............................................................................................. 14 RESULTS ............................................................................................................................. 15 Host Lavas................................................................................................................. 15 Inclusions ................................................................................................................... 23 Type I ............................................................................................................ 23 Type II ............................................................................................................ 32 Type III.......................................................................................................... 38 Type IV.......................................................................................................... 41 Type V........................................................................................................... 46 Comparison of Inclusions and Host Lavas ................................................................ 49 Amphibole...................................................................................................... 52 Clinopyroxene................................................................................................ 52 Feldspar and Feldspathoid............................................................................. 55 Titanomagnetite ............................................................................................. 59 vii Olivine ........................................................................................................... 59 Comparison Summary .................................................................................. 64 Comparison of Whole Rock Compositions ............................................................... 64 Thermobarometry ...................................................................................................... 65 Clinopyroxene Structural Barometer ............................................................
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