TY - JOUR

T1 - Melt chemistry and redox conditions control titanium isotope fractionation during magmatic differentiation

AU - Hoare, Liam

AU - Klaver, Martijn

AU - Saji, Nikitha S.

AU - Gillies, Jamie

AU - Parkinson, Ian J.

AU - Lissenberg, C. Johan

AU - Millet, Marc-Alban

PY - 2020

Y1 - 2020

N2 - 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 d 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 SiO 2 and decreasing Mg#, our data reveal that each of the three magma series have con- trasting d 49/47 Ti fractionation patterns over comparable ranges of SiO 2 and Mg#. Alkaline differentiation suites from intra- plate settings display the most substantial range of variation ( d 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 TiO 2 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*/TiO 2 vs. SiO 2 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*/TiO 2 vs. SiO 2 during magma differentiation.This indicates that the modal abundance and composition of the Fe -Ti oxide phase assemblage, itself is con- trolled by melt composition, governs Ti isotope fractionation during magma differentiation. In addition to this overall con- trol, hydrous, oxidised calc-alkaline suites display a resolvable increase in d 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 H 2 O content. This indicates the potential of Ti isotopes to be utilised as proxies for geodynamic settings of magma generation. (C) 2020 Elsevier Ltd. All rights reserved.

AB - 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 d 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 SiO 2 and decreasing Mg#, our data reveal that each of the three magma series have con- trasting d 49/47 Ti fractionation patterns over comparable ranges of SiO 2 and Mg#. Alkaline differentiation suites from intra- plate settings display the most substantial range of variation ( d 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 TiO 2 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*/TiO 2 vs. SiO 2 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*/TiO 2 vs. SiO 2 during magma differentiation.This indicates that the modal abundance and composition of the Fe -Ti oxide phase assemblage, itself is con- trolled by melt composition, governs Ti isotope fractionation during magma differentiation. In addition to this overall con- trol, hydrous, oxidised calc-alkaline suites display a resolvable increase in d 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 H 2 O content. This indicates the potential of Ti isotopes to be utilised as proxies for geodynamic settings of magma generation. (C) 2020 Elsevier Ltd. All rights reserved.

KW - Titanium isotopes

KW - Magmatic differentiation

KW - Isotope fractionation

KW - Titanomagnetite

KW - Ilmenite

KW - Water content

KW - CALC-ALKALINE DIFFERENTIATION

KW - TI OXIDE CRYSTALLIZATION

KW - IKI LAVA LAKE

KW - OXYGEN FUGACITY

KW - ASCENSION ISLAND

KW - HIGH-TEMPERATURE

KW - VOLCANIC-ROCKS

KW - AEGEAN SEA

KW - COORDINATION CHEMISTRY

KW - CATION DISTRIBUTION

U2 - 10.1016/j.gca.2020.05.015

DO - 10.1016/j.gca.2020.05.015

M3 - Journal article

VL - 282

SP - 38

EP - 54

JO - Geochimica et Cosmochimica Acta. Supplement

JF - Geochimica et Cosmochimica Acta. Supplement

SN - 0046-564X

ER -