Implications from Uranium-Series Disequilibria in a Bi-lithologic Melt with Varying Lithospheric Caps

Authors

Juliet Messer, University of Nebraska-LincolnFollow

First Advisor

Lynne J. Elkins

Date of this Version

12-3-2021

Document Type

Article

Citation

Messer, J.F.D., 2021, Implications From Uranium-Series Disequilibria In A Bi-lithologic Melt With Varying Lithospheric Caps [M.S. thesis]: University of Nebraska-Lincoln

Comments

A Thesis Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Earth and Atmospheric Sciences, Under the Supervision of Dr. Lynne J. Elkins. Lincoln, Nebraska: December, 2021

Copyright 2021 Juliet F. D. Messer

Abstract

Both spreading rates and local magma supply to mid-ocean ridges affect crustal construction styles and ridge morphology, alternately leading to either asymmetrical (detachment faulting) or symmetrical faulting styles. Uranium-series isotopic disequilibria in mid-ocean ridge basalts (MORB) may provide insight into how melt supply variations relate to ridges’ accretion styles, a processes that are not well understood. I use Reactive Porous Flow (RPF) equilibrium and disequilibrium modeling to simulate U-series disequilibria at mid-ocean ridge (MOR) generated by melt supply variations at both asymmetrical and symmetrical ridge segments.

Guided by my modeling, I predict that enhanced melt contributions from enriched pyroxenitic mantle produce elevated (²³⁰Th/ ²³⁸U) in young basalts due to the higher garnet modes in pyroxenites throughout the melting regime. If symmetrical segments of the Kane-Atlantis Supersegment overlie more pyroxenite-rich mantle zones, as postulated here, I would expect measurably higher (²³⁰Th/ ²³⁸U) in basalts from those areas. Lack of such a systematic signature along symmetrical segments would alternatively suggest that crustal magma pooling patterns dominantly control melt supply variations along slow-spreading ridges without an underlying mantle driving force. The (²²⁶Ra/ ²³⁰Th) and (²³¹Pa/ ²³⁵U) ratios of the RPF equilibrium and disequilibrium models generate results that do not fully explain the global MORB data using a single melting lithology. The model results suggest that mixing melts from multiple mantle sources may be necessary to produce the full global data set. Comparing these preliminary results to traditional dynamic melting models and radiogenic isotopes may provide additional insight into how the mantle melting process truly affects both U-series isotopes and ridge symmetry.

Advisor: Lynne J. Elkins