Graduation Year

2019

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Geology

Major Professor

Jeffrey Ryan, Ph.D.

Committee Member

Zachary Atlas, Ph.D.

Committee Member

Aurelie Germa, Ph.D.

Keywords

mass spectrometry, Subduction, ultramafic, fluid-rock partition coefficient, International Ocean Discovery Program

Abstract

In the Mariana subduction system, active serpentinite mud volcanoes are associated with the subduction of the Pacific plate beneath the Philippine plate in a non-accretionary convergent plate margin. This location offers a unique opportunity to study the subduction zone interface with little crustal contamination. The systematics of fluid-mobile trace elements (FME) (As, Cs, Rb, Sb, Tl, Pb, and Sr) in erupted serpentinite muds and entrained serpentinized ultramafic and mafic clasts can place constraints on the release of slab-derived fluids from the downgoing plate, and ultimately the pressure/temperature (P/TC°) conditions at which these fluids are mobilized.

The samples analyzed in this study were recovered during International Ocean Discovery Program (IODP) Expedition 366, Mariana Convergent Margin and South Chamorro Seamount, and expand on existing data from Ocean Drilling Program (ODP) Legs 125 and 195 (Conical and South Chamorro Seamounts, respectively). Samples included ultramafic muds and clasts from the shallow subduction channel as well as mafic clasts and carbonates from subducted Pacific seamounts.

Key findings of this study are that:

(1) releases of FME from the subducting plate enrich the relatively depleted overlying mantle wedge,

(2) elements with an affinity to be fluid-mobile appear to be enriched in distinct patterns, with As, Cs, and Rb increasing and Sr and, to a lesser extent, Pb decreasing with increasing depth-to-slab, possibly indicating an enrichment source from the downgoing plate,

(3) concentrations of FME in serpentinites vary with depth-to-subducting slab due to P/TC° conditions, and may be used as a tracer for dehydration of the subducting slab,

(4) precipitation of minerals (aragonite, brucite, gypsum) strongly controls Ca, Mg, and S, respectively, and thus likely influence some FME concentrations when present,

(5) pH changes with the introduction of seawater near the seafloor alter fluid-mobile element concentrations deposited into shallow serpentine muds, altering the pore fluid signature found within the serpentinized muds, and

(6) progressive dehydration of the slab with increasing P/TC° conditions occurs as a result of at least two separate diagenetic regimes, opal dehydration and the conversion of smectite to illite in shallow-sourced seamounts, and the breakdown of clays at deeper-sourced seamounts.

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