Investigations of Southwest Pacific Intraplate and Backarc Volcanism using Traditional and Non-Traditional Isotopes

Abstract

Volcanic activity in the southwest Pacific Ocean provides ample opportunity to study various tectonic settings spanning as much as ~140 million years. Highprecision stable and radiogenic isotopes are powerful geochemical tools capable of identifying melt source contributions and petrological processes in mantle-derived melts in various tectonic settings, expanding our understanding of the dynamics of underlying Pacific mantle domains. In this dissertation, I present methodological improvements to Fe isotopic measurement methods capable of achieving an analytical precision of ±0.046‰. An accompanying secondary statistical correction can account for instrumental drift between analyses. At such high precision, potential exists to resolve Fe isotope fractionation behavior in response to processes such as melting, fractionation, and metasomatism in ultramafic materials. Predictable Fe isotope behavior can also be used to identify isotopically distinct source compositions contributing to mantlederived melts. Here, Fe isotope behavior is characterized in a suite of young boninitic melts from the Mata Volcanic Field, located in the northeastern Lau Basin. The combination of δ56Fe and melt compositions suggest that Mata volcanoes see variable input from a subduction fluid-like component, and the degree of its contribution may be influenced by proximity to the Tonga forearc versus nearby backarc basin spreading centers. The isotopically distinct, long-lived (120+ Myr) Rurutu hotspot track is a third potential constraint to hotspot-based Pacific absolute plate motion models, which are subject to large model uncertainty, particularly after ~47-50 Ma. Located in a geologically complex region of the southwest Pacific known as the “Hotspot Highway”, other hotspot tracks overlap with the Rurutu track, requiring use of radiogenic (Sr-Pb-Nd-Hf) isotope ratios to identify Rurutu-origin seamounts. Paired with 40Ar-39Ar age data, I present evidence of age-progressive (~75-42 Ma), dual track Rurutu volcanism in the Tuvalu chain. Additionally, I present a computational method that identifies the highest-probability location and age of the Rurutu Bend, analogous to the ~47-50 Ma Hawaiian-Emperor Bend. Identification of this ~50-49 Ma Bend in the Rurutu hotspot track confirms the continuity of Rurutu volcanic activity throughout the Pacific ocean basin, establishing its importance as a third major, singly-sourced constraint on Pacific absolute plate motion.