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Melt chemistry and redox conditions control titanium isotope fractionation during magmatic differentiation

Hoare, Liam, Klaver, Martijn, Saji, Nikitha S., Gillies, Jamie, Parkinson, Ian J., Lissenberg, C. Johan and Millet, Marc-Alban 2020. Melt chemistry and redox conditions control titanium isotope fractionation during magmatic differentiation. Geochimica et Cosmochimica Acta 282 , pp. 38-54. 10.1016/j.gca.2020.05.015
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Abstract

Titanium offers a burgeoning isotope system that has shown significant promise as a tracer of magmatic processes. Recent studies have shown that Ti isotopes display significant mass-dependent variations linked to the crystallisation of Fe-Ti oxides during magma differentiation. We present a comprehensive set of Ti isotope data for a range of differentiation suites from alkaline (Ascension Island, Afar and Heard Island), calc-alkaline (Santorini) and tholeiitic (Monowai seamount and Alarcon Rise) magma series to further explore the mechanics of Ti isotope fractionation in magmas. Whilst all suites display an increase in δ49/47Ti (deviation in 49Ti/47Ti of a sample relative to the OL-Ti reference material) during magma differentiation relative to indices such as increasing SiO2 and decreasing Mg#, our data reveal that each of the three magma series have contrasting δ49/47Ti fractionation patterns over comparable ranges of SiO2 and Mg#. Alkaline differentiation suites from intraplate settings display the most substantial range of variation (δ49/47Ti = +0.01 to +2.32‰), followed by tholeiites (−0.01 to +1.06‰) and calc-alkaline magmas (+0.06 to +0.64‰). Alkaline magmas possess high initial melt TiO2 contents which enables early saturation of ilmenite + titanomagnetite and a substantial degree of oxide crystallisation, whereas tholeiitic and calc-alkaline suites crystallise fewer oxides and have titanomagnetite as the dominant oxide phase. Positive slopes of FeO*/TiO2 vs. SiO2 during magma differentiation are related to high degrees of crystallisation of Ti-rich oxides (i.e. ilmenite). Bulk solid-melt Ti isotope fractionation factors co-vary with the magnitude of the slope of FeO*/TiO2 vs. SiO2 during magma differentiation.This indicates that the modal abundance and composition of the Fe-Ti oxide phase assemblage, itself is controlled by melt composition, governs Ti isotope fractionation during magma differentiation. In addition to this overall control, hydrous, oxidised calc-alkaline suites display a resolvable increase in δ49/47Ti at higher Mg# relative to drier and more reduced tholeiitic arc suites. These subparallel Ti isotope fractionation patterns are best explained by the earlier onset of oxide segregation in arc magmas with a higher oxidation state and H2O content. This indicates the potential of Ti isotopes to be utilised as proxies for geodynamic settings of magma generation.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Publisher: Elsevier
ISSN: 0016-7037
Date of First Compliant Deposit: 20 May 2020
Date of Acceptance: 13 May 2020
Last Modified: 31 Jul 2020 15:52
URI: http://orca.cf.ac.uk/id/eprint/131848

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