Mechanics and applications of titanium isotope fractionation in terrestrial magmas

Hoare, Liam 2021. Mechanics and applications of titanium isotope fractionation in terrestrial magmas. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

This thesis investigates the systematics of mass dependent titanium (Ti) isotope variation in magmatic processes on Earth. Titanium is a relatively abundant minor element that has five stable isotopes and possess a single valence state under terrestrial conditions (Ti4+). The most common Ti-bearing minerals are oxides such as ilmenite, rutile and titanomagnetite. As a refractory lithophile element, Ti is generally considered to be incompatible and fluid-immobile during magmatic processes. All these properties make Ti an attractive prospect to be utilised as a novel isotopic tool to investigate a range of magmatic processes. During fractional crystallisation of a magma, Ti is enriched in the remaining melt until the onset of Fe-Ti oxide crystallisation. The crystallisation of Fe-Ti oxides produces Ti isotope fractionation due to the contrast in co-ordination state of Ti in Fe-Ti oxides relative to silicate melt. Titanium occupies a lower spatial coordination in silicate melts relative to Fe-Ti oxides, which results in lighter isotopes of Ti being preferentially partitioned into Fe Ti oxides. Consequently, the remaining melt is progressively enriched in heavy isotopes of Ti during fractional crystallisation. This observation enables Ti isotopes to be utilised as a novel tracer of magmatic evolution, particularly Fe-Ti oxide-melt equilibria. High precision measurements of Ti isotope ratios are performed using a double spike technique, which involves doping the sample solution with a synthetic solution of a known isotope composition to account for instrumental isotope fractionation during analysis via mass spectrometry. The Ti isotope measurements are performed using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), with Ti isotope compositions of analysed samples reported in the delta notiation relative to a reference standard; δ49/47TiOL-Ti (where OL-Ti is the Origins Lab Ti reference material). This thesis reports the Ti isotope compositions of igneous rocks and their mineral constituents to better constrain the mechanics of Ti isotope fractionation in magmatic v systems with the aim of developing Ti isotopes as a novel isotopic tool that can be applied to investigating processes occurring in the Earth’s mantle and crust. Chapter 3 presents a comprehensive set of Ti isotope data for a range of well characterised magmatic differentiation suites from different tectonic settings; a tholeiitic mid oceanic ridge and subduction zone magmas, calc-alkaline subduction zone, and intraplate magma suites. Evolved magmas from intraplate settings display significantly more variation in their Ti isotope compositions relative to magmas from other tectonic settings at similar SiO2 contents. This is due to high initial TiO2 concentrations in their parental magmas which enables high degrees of Fe-Ti crystallisation, both ilmenite and titanomagnetite, which produces a greater magnitude of Ti isotope fractionation. In addition, calc-alkaline subduction zone magmas exhibit higher δ49/47Ti values at a given Mg# relative to tholeiitic magmas. Calc-alkaline magma possess higher water contents which enables Fe-Ti oxides to crystallise earlier as the crystallisation of other minerals such as plagioclase is suppressed in the presence of water. Thus calc-alkaline magmas display increased δ49/47Ti at higher Mg# relative to tholeiitic magmas due to the earlier onset of Fe-Ti oxide crystallisation. Chapter 4 presents Ti isotope fractionation factors for different Fe-Ti oxide minerals, namely titanomagnetite, ilmenite and rutile, which have been derived from crystal-groundmass pairs in lavas from Santorini (calc-alkaline subduction zone) and Heard Island (intraplate), and from rutile-melt experiments. Selected oxides and groundmass/glass were extracted via micro-mill for MC-ICP-MS analysis. The results show that titanomagnetites display the largest mineral/melt Ti isotope fractionation, followed by ilmenite, then rutile. This is consistent with stable isotope theory which dictates that lighter isotopes prefer longer bonds, with Ti-O bond length decreasing from titanomagnetite to ilmenite and rutile. Furthermore, the titanomagnetite-melt fractionation factor also increases as a function of titanomagnetite TiO2 content, with magnetite from Heard Island (>20 wt% TiO2) consistently displaying a greater fractionation factor in comparison to Santorini (≤15 wt% TiO2). This data enables the vi calculation of oxide-melt Ti isotope fractionation factors as a function of temperature, and TiO2 content in the case of titanomagnetite. These fractionation factors are then applied to successfully model melting of the Earth’s mantle and to reproduce the δ49/47Ti evolution the magmatic differentiation suites from Chapter 3 using constraints from mineral compositions and modal proportions in lavas from the differentiation suites. Chapter 5 combines the enhanced quantitative understanding of Ti isotope fractionation in magmatic systems gained from the previous chapter to apply Ti isotopes as a novel tool to probe the geodynamic origin of The Oman ophiolite. Primitive (Mg# >60) lavas and dikes from Oman, filtered from an overall database of >1200 samples, show large variations in concentrations and ratios of redox-sensitive elements. Importantly, the range of variation in these elements and ratios increases both spatially and temporally throughout the ophiolite. Mantle melting models considering varying source redox state indicate that the trace element systematics of the lavas and sheeted dikes can be replicated by melting of a progressively oxidised source. The trace element systematics are complemented by the first Ti isotope study of Oman lavas and dikes, to evaluate the redox state of mantle melts. The earliest phase of Oman magmatism display Ti isotope evolution pathways that require the presence of elevated water contents and are distinct from tholeiitic suites typical of mid oceanic ridge settings. The second phase of magmatism possess Ti isotope signatures that are comparable to hydrous arc magmas from mature subduction zones. Overall, the trace element and Ti isotope systematic of Oman ophiolite magmas suggest the progressive introduction of water in the active melting column under the Oman ophiolite and thus its likely formation in close proximity to a nascent subduction zone.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Funders:	NERC
Date of First Compliant Deposit:	17 February 2022
Last Modified:	11 Mar 2023 02:59
URI:	https://orca.cardiff.ac.uk/id/eprint/147587

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