Mineralogical Controls on the Ti Isotope Composition of Subduction Zone Magmas

The positive Ti isotope versus SiO2-content correlation in igneous rocks reflects the fractional crystallization of Ti-bearing oxide minerals. However, Ti isotope variations of subduction-related igneous rocks indicate that the Ti isotope compositions of their mantle sources are heterogeneous and additional mineral phases may promote Ti isotope fractionation. We have determined the Ti isotope composition of well-characterized subduction-related basalts, andesites and boninites. Samples from the Solomon Islands, the Troodos ophiolite in Cyprus, and Cape Vogel in Papua New Guinea show small but resolvable variations that may be related to differences in their mantle sources. Specifically, the δ49Ti of boninites (+0.109‰ to +0.168‰) is slightly higher than that of tholeiites (−0.027‰ to +0.111‰) from the same localities (Troodos in Cyprus and Cape Vogel in Papua New Guinea). Modeling suggests the partial melting of progressively depleted mantle sources where residual Cr-spinel plays a greater role in controlling the Ti budget during partial melting. More pronounced variations in δ49Ti are clearly linked to the fractional crystallization of Ti-oxides: Samples from Rabaul Volcanic Complex (New Britain, Papua New Guinea) show increasing δ49Ti (up to +0.373‰) with increasing Ti/V and decreasing Dy/Yb. Fractional crystallization models suggest that oxide minerals and amphibole are needed to sufficiently increase the δ49Ti of these magmas. Our study highlights that the combination of diagnostic trace element patterns and Ti isotope compositions in subduction-related igneous rocks can be a powerful tool to constrain petrogenetic processes and to discriminate between different crystallizing mineral phases

Unravelling lunar mantle source processes via the Ti isotope composition of lunar basalts

Formation and crystallisation of the Lunar Magma Ocean (LMO) was one of the most incisive events during the early evolution of the Moon. Lunar Magma Ocean solidification concluded with the coeval formation of K-, REE- and P-rich components (KREEP) and an ilmenite-bearing cumulate (IBC) layer. Gravitational overturn of the lunar mantle generated eruptions of basaltic rocks with variable Ti contents, of which their delta Ti-49 variations may now reflect variable mixtures of ambient lunar mantle and the IBC. To better understand the processes generating the spectrum of lunar low-Ti and high-Ti basalts and the role of Ti-rich phases such as ilmenite, we determined the mass dependent Ti isotope composition of four KREEP-rich samples, 12 low-Ti, and eight high-Ti mare basalts by using a Ti-47 Ti-49 double spike. Our data reveal significant variations in delta Ti-49 for KREEP-rich samples (+0.117 to +0.296 parts per thousand) and intra-group variations in the mare basalts (-0.030 to +0.035 parts per thousand for low-Ti and +0.009 to +0.113 parts per thousand for high-Ti basalts). We modelled the delta Ti-49 of KREEP using previously published HFSE data as well as the b 49 Ti evolution during fractional crystallisation of the LMO. Both approaches yield delta Ti-49(KREEP) similar to measured values and are in excellent agreement with previous studies. The involvement of ilmenite in the petrogenesis of the lunar mare basalts is further evaluated by combining our results with element ratios of HFSE, U and 1h, revealing that partial melting in an overturned lunar mantle and fractional crystallisation of ilmenite must be the main processes accounting for mass dependent Ti isotope variations in lunar basalts. Based on our results we can also exclude formation of high-Ti basalts by simple assimilation of ilmenite by ascending melts from the depleted lunar mantle. Rather, our data are in accord with melting of these basalts from a hybrid mantle source formed in the aftermath of gravitational lunar mantle overturn, which is in good agreement with previous Fe isotope data

Mineralogical Controls on the Ti Isotope Composition of Subduction Zone Magmas

Abstract The positive Ti isotope versus SiO2‐content correlation in igneous rocks reflects the fractional crystallization of Ti‐bearing oxide minerals. However, Ti isotope variations of subduction‐related igneous rocks indicate that the Ti isotope compositions of their mantle sources are heterogeneous and additional mineral phases may promote Ti isotope fractionation. We have determined the Ti isotope composition of well‐characterized subduction‐related basalts, andesites and boninites. Samples from the Solomon Islands, the Troodos ophiolite in Cyprus, and Cape Vogel in Papua New Guinea show small but resolvable variations that may be related to differences in their mantle sources. Specifically, the δ49Ti of boninites (+0.109‰ to +0.168‰) is slightly higher than that of tholeiites (−0.027‰ to +0.111‰) from the same localities (Troodos in Cyprus and Cape Vogel in Papua New Guinea). Modeling suggests the partial melting of progressively depleted mantle sources where residual Cr‐spinel plays a greater role in controlling the Ti budget during partial melting. More pronounced variations in δ49Ti are clearly linked to the fractional crystallization of Ti‐oxides: Samples from Rabaul Volcanic Complex (New Britain, Papua New Guinea) show increasing δ49Ti (up to +0.373‰) with increasing Ti/V and decreasing Dy/Yb. Fractional crystallization models suggest that oxide minerals and amphibole are needed to sufficiently increase the δ49Ti of these magmas. Our study highlights that the combination of diagnostic trace element patterns and Ti isotope compositions in subduction‐related igneous rocks can be a powerful tool to constrain petrogenetic processes and to discriminate between different crystallizing mineral phases

Altering the course of schizophrenia: progress and perspectives

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