The oxidation states of iron and volatile compositions of Expedition 352 glasses

M Brounce; Mark K Reagan; Daniel A Coulthard; Katherine A Kelley; Elizabeth Cottrell

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Abstract

The oxidation states of iron and volatile compositions of Expedition 352 glasses

M Brounce, Mark K Reagan, Daniel A Coulthard, Katherine A Kelley and Elizabeth Cottrell

Abstracts with programs - Geological Society of America, Vol.51(5)

Geological Society of America, 2019 annual meeting & exposition

09/2019

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Abstract

The subduction of oceanic lithosphere is linked to the production of hydrous arc basalts that are enriched in key trace elements (e.g., Ba) and have higher proportions of oxidized iron than mid-ocean ridge basalts. To constrain the timing and mechanisms of the initiation and maturation of subduction processes on petrogenesis, we analyzed volcanic glasses recovered from IODP Expedition 352 to the Izu-Bonin-Mariana forearc for Fe (super 3+) /Fe* ratios (XANES) and major, trace and volatile element contents. The oldest recovered lavas are forearc basalt (FAB) with ages of 51.9 to >51.3 Ma. The FAB glasses (3.34-8.74 wt% MgO) have H (sub 2) O from 0.18-0.85 wt%, CO (sub 2) from 75-233 ppm, S contents controlled by saturation with a sulfide phase (602-1386 ppm), Ba/La ratios from 3.9-10, and Fe (super 3+) /Fe* ratios from 0.136-0.177. These H (sub 2) O-CO (sub 2) -S contents suggest eruption on the seafloor in deep water ( approximately 3700 mbsl), and CO (sub 2) -only degassing during crystallization. These compositions are similar to that of MORB, and suggest that decompression melting of dry, depleted, and reduced mantle dominates the earliest stages of subduction initiation. These rocks are followed stratigraphically by boninitic lavas (>51.3 Ma to 50.3 Ma; 0.23-7.75 wt% MgO), with H (sub 2) O from 1.51-3.19 wt%, CO (sub 2) below detection via FTIR, S contents below that required for sulfide saturation (5-235 ppm), Ba/La ratios from 11-29, and Fe (super 3+) /Fe* ratios from 0.181-0.225. These H (sub 2) O-CO (sub 2) -S also suggest eruption on the seafloor at approximately 3800 mbsl, and degassing of a COHS fluid that may have reduced magmatic Fe (super 3+) /Fe* to some degree (dependent on the proportion of H (sub 2) O to S in the fluid, e.g.,). These data demonstrate that, once initiated, subduction proceeds rapidly, and many of the characteristic features of arc volcanism (hydrous, trace element enriched, and oxidized) are established via fluid-fluxed melting of the mantle in just 0.6-1.2 my. The production of different Fe (super 3+) /Fe* ratios in FAB and boninites over a relatively short time span, as well as the coherence of high Fe (super 3+) /Fe* and Ba/La ratios with high H (sub 2) O contents, suggest the production of oxidized arc basalts is linked to signatures of slab dehydration.

Glasses

Iron

Pacific Ocean

basalts

Cenozoic

chemical composition

Eocene

Expedition 352

Geochemistry of rocks, soils, and sediments

Igneous and metamorphic petrology

igneous rocks

International Ocean Discovery Program

metals

North Pacific

Northwest Pacific

oxidation

Paleogene

plate tectonics

subduction

Tertiary

volatiles

volcanic glass

volcanic rocks

West Pacific

Details

Title: Subtitle: The oxidation states of iron and volatile compositions of Expedition 352 glasses
Creators: M Brounce
Mark K Reagan - University of Iowa
Daniel A Coulthard - Massey University
Katherine A Kelley - University of Rhode Island
Elizabeth Cottrell - Smithsonian Institution
Resource Type: Abstract
Publication Details: Abstracts with programs - Geological Society of America, Vol.51(5)
Conference: Geological Society of America, 2019 annual meeting & exposition
Publisher: Geological Society of America (GSA)
ISSN: 0016-7592
Alternative title: Geological Society of America, 2019 annual meeting & exposition
Language: English
Date published: 09/2019
Academic Unit: Earth and Environmental Sciences
Record Identifier: 9984229965402771

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