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Date Permissions Signed

8-13-2018

Date of Award

Summer 2018

Document Type

Masters Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

DeBari, Susan M., 1962-

Second Advisor

Straub, S. M. (Susanne M.)

Third Advisor

Caplan-Auerbach, Jacqueline

Fourth Advisor

Rusk, Brian G.

Abstract

Magmas from the Izu Bonin rear-arc show a geochemical signature (medium- to high-K and light rare-earth element [LREE] enriched) that resembles the average composition of the continental crust. I investigate that continent-like signature by examining a suite of rear-arc-derived mafic to felsic tephras from International Ocean Discovery Program (IODP) Site U1437, which was drilled as part of IODP Expedition 350. Volcaniclastic glasses from Site U1437 comprise a compositional series from basalt to rhyolite, aged 1.1-4.4 Ma.

Site U1437 tephras record the timespan when the Izu-Bonin arc began rifting and provide a well-dated record of changing magmatic compositions through the transition into arc rifting. Considered with context provided by a comprehensive recontextualization of published analyses ofrocks, magmatic compositions are shown to vary in coherent chronological and spatial trends and can be classified into three series: LREE-depleted volcanic front series; flat pattern REE rift-related series; and LREE-enriched rear-arc seamount chain-type (RASC-type) series which includes Site U1437 glasses. Each series has a distinctive characteristic basalt type whose trace element and radiogenic isotope chemistry matches rhyolite type from the same region.

Geochemical modeling shows that fractional crystallization is a viable mechanism to generate2and oxygen isotope systematics show that some degree of open-system processes are involved in rhyolite petrogenesis; however there is no evidence for large-scale melting of pre-existing crustal sources. The >12.9 Ma crustal sources inferred to exist within the rear-arc region cannot be the source for rear-arc rhyolites because of differing La/Yb and ɛHf signatures. Thus, I favor a model where fractional crystallization is the differentiation mechanism involved in the generation of intraoceanic arc rhyolites in this region.

Type

Text

Keywords

igneous petrology, Izu-Bonin arc, subduction zone, island arc, intracrustal differentiation, rhyolite, IODP, continental crust, fractional crystallization, tephra

DOI

https://doi.org/10.25710/n4qc-ef98

Publisher

Western Washington University

OCLC Number

1050442881

Subject – LCSH

Rhyolite--North Pacific Ocean; Suduction zones--North Pacific Ocean; Borings--North Pacific Ocean; Petrology--North Pacific Ocean; Geochemistry--North Pacific Ocean; Volcanism--North Pacific Ocean

Geographic Coverage

North Pacific Ocean

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.

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