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Vanadium and niobium behavior in rutile as a function of oxygen fugacity: evidence from natural samples

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Liu, Lei; Xiao, Yilin; Aulbach, Sonja; Li, Dongyong; Hou, Zhenhui
Contributions to Mineralogy and Petrology
Vanadium occurs in multiple valence states in nature, whereas Nb is exclusively pentavalent. Both are compatible in rutile, but the relationship of V--Nb partitioning and dependence on oxygen fugacity (expressed as fO2) has not yet been systematically investigated. We acquired trace-element concentrations on rutile grains (n = 86) in nine eclogitic samples from the Dabie-Sulu orogenic belt by laser ablation inductively coupled plasma mass spectrometry (LA--ICP--MS) and combined them with published results in order to assess the direct and indirect effects of oxygen fugacity on the partitioning of V and Nb into rutile. A well-defined negative correlation between Nb (7--1,200 ppm) and V concentrations (50--3,200 ppm) was found, documenting a competitive relationship in the rutile crystal that does not appear to be controlled by bulk rock or mineral compositions. Based on the published relationship of RtDV and V valence with ∆QFM, we suggest that the priority order of V incorporation into rutile is V4+ > V3+ > V5+. The inferred Nb--V competitive relationship in rutile from the Dabie-Sulu orogenic belt could be explained by decreasing fO2 due to dehydration reactions involving loss of oxidizing fluids during continental subduction: The increased proportion of V3+ (expressed as V3+/∑V) and attendant decrease in RtDV is suggested to lead to an increase in rutile lattice sites available for Nb5+. A similar effect may be observed under more oxidizing conditions. When V5+/∑V increases, RtDV shows a dramatic decline and Nb concentration increases considerably. This is possibly documented by rutile in highly metasomatized and oxidized MARID-type (MARID: mica--amphibole--rutile--ilmenite--diopside) mantle xenoliths from the Kaapvaal craton, which also show a negative V--Nb covariation. In addition, their Nb/Ta covaries with V concentrations: For V concentrations <1,250 ppm, Nb/Ta ranges between 35 and 45, whereas for V > 1,250 ppm, Nb/Ta is considerably lower (5--15). This relationship is mainly controlled by a change in Nb concentrations, suggesting that the indirect dependence of RtDNb on fO2, which is not mirrored in RtDTa, can exert considerable influence on rutile Nb--Ta fractionation.
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