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Date Permissions Signed

7-1-2011

Date of Award

2011

Document Type

Masters Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

DeBari, Susan M., 1962-

Second Advisor

Wallace, Paul Judson

Third Advisor

Sisson, Thomas W.

Fourth Advisor

Schermer, Elizabeth, 1959-

Abstract

The subducting Juan de Fuca plate is the hot endmember of slabs worldwide, and its unique thermal character prompts debate about the role of fluid-flux melting versus decompression melting in the Cascade arc. While slow subduction of this hot slab is expected to result in strong dehydration prior to reaching sub-arc depths, there is no consensus on whether the slab is entirely dehydrated at this point, or whether volcanism is the result of water-poor, decompression melting, or fluid-flux melting. I provide the first measurements of pre-eruptive volatile contents in olivine-hosted melt inclusions from primitive magmas in the northern region of the arc, at Mount Baker and Glacier Peak. These volatile contents and melt inclusion compositions are used to model mantle melting processes. Low-K olivine tholeiite (LKT) and calc-alkaline basalt (CAB) melt inclusions at Glacier Peak have minimum H2O concentrations of 2.0 and 2.2 wt. % and fO2 of ΔQFM +1.1 and +1.5, respectively. The evolved compositions of these melt inclusions in both lava types (host olivine: Fo85-89) are corrected to mantle values by addition of ≤15 wt. % olivine, and the results suggest that the minimum water contents in the parental magmas are 1.7 wt. % and 2.0 wt. %. Measured values themselves may be low due to degassing at crustal depths. The Mount Baker Schreiber's Meadow cinder cone (CAB) has minimum H2O concentrations of 2.3 wt. %, though these contents cannot be adequately disentangled from potential crustal involvement and/or magma mixing/mingling to be corrected to mantle values. Results of modeling indicate that both LKT and CAB magmas at Glacier Peak are the result of 13-15% fluid-fluxed melting of a compositionally heterogeneous mantle source, last equilibrated near the base of the crust. Source regions are interpreted to contain both an ocean island basalt (OIB)-like component and a mid-ocean ridge basalt (MORB)-like component. Minimum H2O contents suggested in the source region are between 0.21 and 0.28 wt %. This is in contrast to southern regions of the Cascade arc, where LKT magmas are considered to be the result of dry decompression melting of a MORB-like source.

Type

Text

DOI

https://doi.org/10.25710/6zmb-5625

Publisher

Western Washington University

OCLC Number

743827008

Subject – LCSH

Olivine--Inclusions--Moisture; Magmas--Moisture

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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