Vanadium isotope fractionation during plutonic differentiation and implications for the isotopic composition of the upper continental crust

Le résumé fourni par la source

The analysis of emerging stable isotopic systems in clastic sedimentary rocks is increasingly used to determine the average composition of the upper continental crust through geological time. Any temporal variations can then be linked to global-scale processes such as the oxygenation of the atmosphere or onset of plate tectonics. Given that clastic sediments are ultimately eroded from the upper continental crust, knowledge of the potential isotopic variability in the plutonic rocks which make up the crust is vital for interpreting these sedimentary records. Here we focus on the multi-valent transition metal element vanadium (V) and present the first investigation of the V isotopic composition of an upper crustal granitic pluton and its mineral separates. We use well-characterised samples from the calc-alkaline Boggy Plain Zoned Pluton, Australia. Whole rock samples and mineral separates show increases in δ51V during magmatic differentiation, similar to what has been documented for extrusive differentiation suites. However, whole rock δ51V is scattered, reflecting variations in the modal mineralogy and demonstrating the typical heterogeneity generated when dealing with coarse grained igneous rocks. In contrast, mineral separates show well-defined trends in δ51V, where mineral-melt fractionation factors are largely controlled by bonding environment rather than direct redox variations. We interpret the increase in δ51V during magmatic differentiation to be driven by crystallisation of isotopically light magnetite, biotite and hornblende, in contrast with previous interpretations from extrusive lavas that oxide crystallisation alone is the main driver of V isotopic fractionation. The overall range of whole rock samples and their mineral separates is > 0.6 ‰ within this single plutonic body. The range highlights that the upper continental crust can have extremely heterogeneous V isotopic composition over small geographic areas. This detailed examination of V isotopes in a simple system may shed light on the discrepancy between interpretations of the timing of felsic crust formation derived from the V and Ti isotopic compositions of glacial diamictites.

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