Vanadium Isotopic Compositions of the Panzhihua Layered Intrusion Associated with Giant Fe-Ti-V Oxide Deposit, SW China

The Panzhihua layered intrusion is one of the largest intrusions that strikes NE-SW and hosts significant magmatic Fe-Ti-V oxide deposit in the central part of the Emeishan large igneous province, SW China^[1]. Vanadium (V) is the major element of the Fe-Ti-V oxide, and has multiple oxidation states that promotes fractionation of its isotopes in the redox processes at high temperatures^[2]. In order to investigate the fractionation mechanism of V isotopes during magmatic differentiation, we systematically analyzed V isotopic compositions (reported as δ⁵¹V = [(⁵¹V/⁵⁰V)_sample / (⁵¹V/⁵⁰V)_AA-1] × 1000(‰)) of whole-rocks and mineral separates (clinopyroxene and magnetite) collected from a profile of 1.6 km in thickness that cuts through the intrusion.

Whole rocks have a large δ⁵¹V ranging from -0.96‰ to +1.70‰ and most of the magnetite separates have slightly lighter δ⁵¹V than whole rocks; the δ⁵¹V values of a portion of clinopyroxene separates are slightly heavier than whole rocks and magnetite separates. Furthermore, the δ⁵¹V values of magnetite separates (-1.01‰ to +1.68‰) have the same variation tendency with whole rocks, indicating that magnetite separates control δ⁵¹V values of whole rocks, which was accorded with the conjectures by Prytulak et al. (2017) and Wu et al. (2018). Throughout the entire profile, δ⁵¹V values of whole rocks and mineral separates from marginal zone (MGZ), lower zone (LZ) and upper zone (UZ) are close to the BSE (-0.89 ± 0.10‰)^[3], while δ⁵¹V values of whole rocks and mineral separates from middle zone (sub-zones MZa and MZb) are obviously higher than the BSE, likely suggesting the multiple replenishment of magma in the late stage. Such a large variation of δ⁵¹V may result from the differences of partitioning and isotope fractionation factors of V between (silicate/oxide) mineral separates and melt. In summary, our results exhibit that V isotopes could be significantly fractionated ( 2.66‰) during magmatic evolution, and the V isotopic compositions of whole rocks are controlled by fractional crystallization of magnetite separates.

[1] X. Y. Song. et al. (2013) G3 14, 712-732

[2] P. Sossi et al. (2018) CMP 173, 27.

[3] Qi et al. (2018) submitted to EPSL.

[4] Prytulak et al. (2017) GPL 3, 75-84

[5] Wu et al. (2018) EPSL 493, 128-139