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

Abstract

Titanium offers a burgeoning isotope system that has shown significant promise as a tracer of magmatic processes. Recent studies have shown that Ti displays significant massdependent 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 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 contrasting 49/47 Ti fractionation patterns over comparable ranges of SiO 2 and Mg#. Alkaline differentiation suites from intraplate settings display the most substantial range of variation ( 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 less oxide 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 controlled by melt composition, governs Ti isotope fractionation during magma evolution. In addition to this overall control, hydrous, oxidised calc-alkaline suites display a resolvable increase in 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.

Titanium isotopes constrain a magmatic transition at the Hadean-Archean boundary in the Acasta Gneiss Complex

“…We assume a Ti isotopic fractionation of +0.344‰ between melt and residue, which is the fractionation factor between residue (E-MORB) and felsic melt (TTGs) ( 49 ). This Ti isotope fractionation factor between melt and solid residue is also consistent with that estimated using the method of ( 32 ), which is based on the correlation between FeO/TiO 2 and SiO 2 concentrations (i.e., ~0.32‰ at 1000°C and ~0.42‰ at 800°C). For low-degree partial melting, the calculated Ti isotopic fractionation is, at most, +0.34‰ (at negligible melt fraction), which is significantly lower than the δ 49 Ti values of +0.54 to +0.95‰ observed in the ITG samples.…”

“…The δ 49 Ti compositions of rocks shown in (B) are from calc-alkaline (blue circles), intraplate tholeiitic (red circles), and modern MORB tholeiitic (gray circles) settings. The rock samples are Kos in the Aegean Arc ( 36 ), Agung volcano in the Sunda Arc ( 29 ), Santorini ( 32 ), Paleozoic granitoids ( 36 ), Archean TTGs ( 36 ), I- and S-type granitoids ( 36 ), Hekla/Afar ( 30 ), Kīlauea Iki lava lake ( 31 ), Ko’olau ( 38 ), komatiites ( 49 , 51 ), Monowai ( 32 ), and MORB tholeiitic rocks from Alarcon Rise (gray circles). A-type granitoids (this study, white triangles; see table S1) from Lachlan Fold Belt in Australia and from Hailar basin ( 38 ) are shown.…”

“… ( A ) Relationship between δ 49 Ti and Log(SiO 2 /TiO 2 ) for calc-alkaline rocks (blue circles) from Agung, Kos ( 29 , 36 ), and Santorini ( 32 ) and for intraplate tholeiitic rocks (red circles) from Hekla, Afar, and Hawaii ( 24 , 25 , 38 ), Ascension, and Heard ( 32 ). Rocks from Alarcon Rise and Monowai (gray circles) ( 32 ), Hailar basin (purple circles) ( 38 ), AGC, and calc-alkaline and tholeiitic MELTS model output are shown. ( B ) Calculated TIMI for diverse igneous rocks.…”

“…Six of the seven analyzed rocks from the unique setting of Alarcon Rise specified as a mid-ocean ridge basalt (MORB) differentiation series have TIMIs between 0.89 and 1.08 (average of 0.97), similar to other tholeiitic rocks. Subduction-related samples from Santorini (Greece, Aegean Arc) that are calc-alkaline to mildly tholeiitic (37) and tholeiites from the Monowai seamount in the Tonga-Kermadec arc (32) have TIMIs from 0.68 to 0.88 (average of 0.77). One basalt (SiO 2 = 52 wt %) from Monowai with a TIMI of 0.57 has been excluded, as its  49 Ti composition of +0.01 indicates that the magma is undersaturated in Fe-Ti oxides.…”

“…Recent work has shown that Ti isotopes can distinguish between rocks that underwent magmatic differentiation along the calc-alkaline trend and those that fall on the intraplate tholeiitic trend (29)(30)(31)(32). The 49 Ti/ 47 Ti isotopic ratio (reported in  49 Ti notation; the per mil deviation relative to the 49 Ti/ 47 Ti ratio of the OL-Ti reference material) increases with SiO 2 content in both calc-alkaline and tholeiitic rocks (29)(30)(31)(32), but with the latter showing a much more rapid increase in  49 Ti with SiO 2 . Titanium is highly fluid immobile and unlikely to be substantially mobilized by metamorphic disturbance.…”