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Magmatic Evolution of Early Subduction Zones: Geochemical Modeling and Chemical Stratigraphy of Boninite and Fore Arc Basalt from the Bonin Fore Arc

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Magmatic Evolution of Early Subduction Zones: Geochemical Modeling and Chemical Stratigraphy of Boninite and Fore Arc Basalt from the Bonin Fore Arc Utah State University DigitalCommons@USU All Graduate Theses and Dissertations Graduate Studies 5-2017 Magmatic Evolution of Early Subduction Zones: Geochemical Modeling and Chemical Stratigraphy of Boninite and Fore Arc Basalt from the Bonin Fore Arc Emily A. Haugen Utah State University Follow this and additional works at: https://digitalcommons.usu.edu/etd Part of the Geology Commons Recommended Citation Haugen, Emily A., "Magmatic Evolution of Early Subduction Zones: Geochemical Modeling and Chemical Stratigraphy of Boninite and Fore Arc Basalt from the Bonin Fore Arc" (2017). All Graduate Theses and Dissertations. 5934. https://digitalcommons.usu.edu/etd/5934 This Thesis is brought to you for free and open access by the Graduate Studies at DigitalCommons@USU. It has been accepted for inclusion in All Graduate Theses and Dissertations by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. MAGMATIC EVOLUTION OF EARLY SUBDUCTION ZONES: GEOCHEMICAL MODELING AND CHEMICAL STRATIGRAPHY OF BONINITE AND FORE ARC BASALT FROM THE BONIN FORE ARC by Emily A. Haugen A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in Geology Approved: ________________________ ________________________ John Shervais, Ph.D. Alexis Ault, Ph.D. Major Professor Committee Member ________________________ ________________________ Anthony Lowry, Ph.D. Mark R. McLellan, Ph.D. Committee Member Vice President for Research and Dean of the School of Graduate Studies UTAH STATE UNIVERSITY Logan, Utah 2017 ii Copyright Emily A. Haugen 2017 All Rights Reserved iii ABSTRACT Magmatic Evolution of Early Subduction Zones: Geochemical Modeling and Chemical Stratigraphy of Boninite and Fore Arc Basalt from the Bonin Fore Arc by Emily A. Haugen, Master of Science Utah State University, 2017 Major Professor: Dr. John Shervais Department: Geology IODP expedition 352 drilled four core in the Izu-Bonin fore arc in the Western Pacific. The Izu-Bonin fore arc records early subduction processes in the form of volcanics, notably Fore Arc Basalt (FAB) and boninite. FAB are Mid- Ocean Ridge Basalt (MORB)-like tholeiites with variable fluid mobile element enrichment and lower Ti/V ratio than MORB. Boninites are hydrous high-Mg andesite low in TiO2 with a distinct subduction zone character and extreme depletion of Rare Earth Elements (REE). These volcanics are used to track the evolution of a subduction zone from initiation to maturity with FAB produced first and boninite produced later. Forward modeling of mantle-derived melts from a Deplete MORB Mantle source composition produces FAB from ~20% melt in the spinel lherzolite field and 1% in the garnet lherzolite field. Boninite is modeled from high degrees of melt the FAB residue and requires an addition of melt to iv match the model with observed samples. Three types of boninite have been sampled, Basaltic Boninite (BB), Low-Silica Boninite (LSB), and High-Silica Boninite (HSB). Within the core, LSB is produced first with HSB modeled from the LSB residue. BB is generated throughout the core. The geochemistry of the four core analyzed records period of recharge and fraction of the magma chamber. The base of Core U1442A and U1439C record a transition from more FAB-like samples to boninite samples from low Cr and high Al concentrations. Few of the shipboard-defined units record magma mixing as seen by the scatter in the depth plots and petrographically. Enrichment of Fluid Mobile Elements and High-Field Strength incompatible Elements indicate a subduction input in the form of fluids and melt. (190 pages) v PUBLIC ABSTRACT Magmatic Evolution of Early Subduction Zones: Geochemical Modeling and Chemical Stratigraphy of Boninite and Fore Arc Basalt from the Bonin Fore Arc Emily A. Haugen The Izu-Bonin-Mariana arc stretches south from Japan to Guam in the Western Pacific. International Ocean Discovery Project Expedition 352 drilled four core in the fore arc of the Izu-Bonin arc east of the Bonin Islands: U1439C, U1440B, U1441A, and U1442A. From the four core, 124 samples were retrieved and analyzed for major and trace elements. Two main rock types were identified: FAB and boninite. FAB is a Mid-Ocean Ridge Basalt (MORB)-like tholeiite with variable fluid mobile element enrichment such as Rb, Ba, and Sr, and low Ti/V ratios more similar to an island arc volcanic than a mid-ocean ridge volcanic. Boninite is a hydrous high-Mg andesite with low TiO2 and distinctive subduction zone characteristics in the form of elevated fluid mobile elements and melt mobile elements. FAB was assumed to be formed from a Depleted MORB- Mantle (DMM) source and the boninite was formed from a depleted mantle source, presumably the mantle after FAB melt was extracted. Here, we used the Rare Earth Elements (REE) of the samples to model melt scenarios for the FAB and boninite in order to better understand the initial volcanic product of subduction zones. This research was funded by in joint by the National Science Foundation, vi Consortium for Ocean Leadership, and International Ocean Discovery Program. 124 samples were analyzed using an X-Ray Fluorescence (XRF) and Inductively Coupled Plasma - Mass Spectrometer (ICP-MS) to determine the major and trace elements. These analyses were then used to recreate the chemostratigraphy defined by the shipboard crew and determine variations within the core. We found that there was variability as the magma evolved over time and mixed with other melts, seen in magma mixing horizons. Boninite samples were separated based on their SiO2 and MgO concentrations into Basaltic Boninite (BB), Low-Silica Boninite (LSB), and High-Silica Boninite (HSB) with BB being more primitive and HSB being more evolved. These volcanics are the first known products of the subduction zone and were used to model the early evolution of the subduction zone. FAB was the first product due to its proximity to the trench and greater age than the boninite. Assumed to be generated from DMM, FAB was modeled with a total melt extraction of ~20% spinel lherzolite and 1% garnet lherzolite. Boninite was assumed to be generated from the FAB residue because it requires a depleted source and because the FAB residue was within the hydrous flux melt zone of the subduction factory. Boninite was modeled at high degree of melt from the FAB residue, however an additional melt must be added to the model to match the observed samples. We proposed a small fraction of FAB melt mixed with the models because it is still present in the subduction factory, observed in core U1439C with a FAB sample in the HSB regime. vii ACKNOWLEDGMENTS I would like to thank Dr. John Shervais for making available to me the core he collected on IODP Expedition 352 to the Bonin Fore Arc and data therein, and my committee members Drs. Alexis Ault and Anthony Lowry for their support and assistance throughout the entire process. I would also like to thank the IODP 352 Researchers for their input at meetings and sharing of data. Emily A. Haugen viii CONTENTS Page ABSTRACT .............................................................................................................. iii PUBLIC ABSTRACT ............................................................................................... iv ACKNOWLEDGMENTS ........................................................................................... v LIST OF TABLES .................................................................................................. viii LIST OF FIGURES .................................................................................................. ix CHAPTER I. Introduction ........................................................................................................ 1 Background ........................................................................................................ 5 Core Descriptions ............................................................................................ 12 U1440B ............................................................................................. 12 U1441A ............................................................................................. 14 U1442A ............................................................................................. 15 U1439C ............................................................................................. 17 II. Methods ........................................................................................................... 19 Sample Preparation and Analysis.................................................... 19 Forward Modeling of Mantle-derived Melts ..................................... 21 III. Results ............................................................................................................. 25 Petrology .......................................................................................................... 26 FAB ................................................................................................... 26 Boninite ............................................................................................. 27 Other ................................................................................................. 27 Geochemistry of FAB and Boninite ................................................................. 28 Major Elements................................................................................. 28 Trace Elements ...............................................................................
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