Melt chemistry and redox conditions control titanium isotope fractionation during magmatic differentiation. (1st August 2020)

• Record Type: Journal Article
• Authors: Hoare, Liam; Klaver, Martijn; Saji, Nikitha S.; Gillies, Jamie; Parkinson, Ian J.; Lissenberg, C. Johan; Millet, Marc-Alban
• Is Part Of: Geochimica et cosmochimica acta. Volume 282(2020)
• Page Start: 38
• Page End: 54
• Publication Date: 2020-08-01
• Subjects: Titanium isotopes -- Magmatic differentiation -- Isotope fractionation -- Titanomagnetite -- Ilmenite -- Water content; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2020.05.015
• Languages: English
• ISSNs: 0016-7037

Abstract

Highlights: Magma series show distinct Ti isotope fractionation patterns during differentiation. Fe and Ti content of parental melts is the main control on Ti isotope fractionation. Redox state and water content are secondary controls for arc magmas. 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 δ 49/47 Ti (deviation in 49 Ti/ 47 Ti 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 δ 49/47 Ti fractionation patterns over comparable ranges of SiO2 and Mg#. Alkaline differentiation suites from intraplate settings display the most substantial range of variation (δ 49/47 Ti = +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 δ 49/47 Ti 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 H2 O content. This indicates the potential of Ti isotopes to be utilised as proxies for geodynamic settings of magma generation.