Date of Award

Spring 2021

Document Type

Open Access Thesis

First Advisor

Michael Bizimis

Abstract

Water in Earth’s mantle, present as structurally bound hydrogen in minerals, influences large scale processes like mantle deformation, plate tectonics, and melting. To better understand the processes controlling water concentrations in the Earth’s upper mantle, we analyzed hydrogen (calculated as ppm wt. H2O), major, and trace element concentrations in minerals from peridotite mantle xenoliths of Lanzarote, Canaries. The Canaries Islands are thought to be the surface expression of a deep-seated mantle plume erupting through the Atlantic Ocean lithosphere. Carbonatite magmas have erupted in the Canaries and previous studies have shown evidence for carbonatite metasomatism in Lanzarote peridotites. Thus, the Canaries offer a rare opportunity to examine the role of carbonatite metasomatism as a mechanism in rehydrating the oceanic lithosphere during plume-lithosphere interaction.

Peridotites in this study are found as xenoliths in Quaternary to recent basaltic lavas of the Timanfaya volcanic series on the island of Lanzarote. Major element data show that these peridotites are highly depleted compared to the primitive mantle, with a high average olivine Mg# (91.1 ± 0.5), low bulk rock Al2O3 (0.4 to 1.4 wt.%), high spinel Cr# (40 to 72) from which high degrees of melting were estimated (15 to 21%). Trace element data in orthopyroxene, clinopyroxene, and bulk rock frequently show high La/Yb ratios (up to 12.2, 71.5, 45.2; respectively) and strong negative Zr, Hf, and Ti anomalies relative to REE, consistent with carbonatite metasomatism. There is no evidence of hydrous phases (e.g., amphibole and phlogopite), thus water in these peridotites is thought to be stored only in nominally anhydrous minerals and melt inclusions. Water concentrations (determined by polarized FTIR spectroscopy), in the peridotite silicate minerals range from 2.7 to 13 ppm in olivine, 42 to 265 ppm in orthopyroxene (Opx), and 76 to 470 ppm in clinopyroxene (Cpx). Reconstructed bulk rock water concentrations range from 5 to 115 ppm, generally lower than estimates for the depleted mantle (DM, 50-200 ppm), but significantly higher than expected from the degree of depletion. With decreasing Ti/Eucpx ratios, a proxy for carbonatite metasomatism, the bulk water and clinopyroxene water increases. The calculated trace element compositions of melts in equilibrium with Cpx strongly resemble those of carbonatites. However, the bulk rock water concentrations are two orders of magnitude lower than what is expected by equilibrium with a carbonatite melt. We conclude that while carbonatitic metasomatism did rehydrate the Lanzarote mantle lithosphere it did so to a much lesser extent than expected, implying lower partitioning of H in the presence of CO2 than a silicate melt. Therefore, the inference for volatile-rich metasomatism from carbonatitic melts does not translate to excess water addition in the oceanic lithosphere.

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© 2021, Sierra N. Patterson

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