WATER AND OXYGEN FUGACITY IN SUBDUCTION ZONE MAGMATISM

Author

Janine Louise Andrys, University of Rhode IslandFollow

Date of Award

2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Specialization

Marine Geology and Geophysics

Department

Oceanography

First Advisor

Katherine Kelley

Abstract

The water content and oxygen fugacity (fO₂) of magmas is particularly influential for the behavior of iron during magmatic evolution. At mid-ocean ridges, magmas typically contain low water contents and relatively reduced oxygen fugacities and nearly universally display trends in Fe-enrichment with progressive magmatic evolution. Arc volcanoes, on the other hand, display a wide range in water contents, oxygen fugacities, and varying levels of Fe-enrichment (tholeiitic behavior) and depletion (calc-alkaline behavior). On average, however, arc volcanoes produce magmas that have both elevated water contents and elevated oxygen fugacities compared to mid-ocean ridge basalts. The higher fO₂ and H₂O of some arc volcanoes allows early Fe-depletion to occur during differentiation. This is an important phenomenon because bulk continental crust displays similarly low concentrations of FeO* to modern calc-alkaline arc magmas. The specific processes that drive early Fe-depletion trends, however, are unresolved. Systematics between Fe-depletion/enrichment and H₂O and fO₂ indicate that these magmatic variables are important in driving Fe-depletion trends, but because H2O and fO₂ commonly co-vary in natural samples, their respective roles have remained elusive. This thesis addresses the roles of H₂O and fO₂ in producing the magmatic trends of arc magmas by (1) conducting equilibrium crystallization experiments to investigate the phase equilibria and resulting melt compositions produced during crystallization at high water pressures and oxidized conditions, (2) evaluating western Aleutian volcanoes with varying iron trends and oxygen fugacities, but similar water contents to assess the magmatic conditions necessary to promote Fe-depletion, and (3) establishing systematics between redox and various geochemical proxies amongst back-arc basin basalts to explore the origin of elevated oxygen fugacities at arcs.

By combining these varied approaches, the roles of H₂O and fO₂ during magmatic evolution at arcs become more distinct. Crystallization experiments demonstrate that mild calc-alkaline magmatic trends can be produced through crystallization of a basaltic parent magma at high water pressure and oxidized conditions, but that strongly calc-alkaline trends with continuous Si-enrichment and Fe-depletion are not achievable through simple crystallization. In our comparison between western Aleutian volcanoes, we analyze melt inclusions to constrain magmatic water contents and oxygen fugacities at two contrasting volcanoes and use these constraints to evaluate differences in their major element chemistry and petrography. We show that, despite a distinct difference in fO₂, these two volcanoes achieve similar liquid lines of descent. We assess the origin of elevated fO₂ at arcs by analyzing ratios of Fe³⁺/Fe²⁺ in a global suite of back-arc basin basalts and show that the majority of oxidation at arcs can be attributed to contributions of an oxidizing agent from the subducting slab.

Recommended Citation

Andrys, Janine Louise, "WATER AND OXYGEN FUGACITY IN SUBDUCTION ZONE MAGMATISM" (2023). Open Access Dissertations. Paper 1566.
https://digitalcommons.uri.edu/oa_diss/1566