The Controls on Vanadium, Iron and Zinc Stable Isotope Fractionation in Upper Crustal Plutons

Abstract

Chemically diverse plutonic rocks make up most of the upper continental crust, but the exact crystal-liquid segregation processes and physical conditions in solidifying mush bodies are debated. Transition metal stable isotopes are increasingly used to investigate magmatic processes, but knowledge of their specific behaviour in plutonic settings is lacking. Better understanding of the cause(s), magnitudes and directions of isotope fractionation in relatively simple, closed system plutons is required to assess the efficacy of transition metal stable isotopes at investigating mush processes.
This thesis investigated the calc-alkaline Boggy Plain Zoned Pluton, SE Australia, and the tholeiitic Red Hill Intrusion, Tasmania. This was the first time that the Fe-V-Zn isotopic composition of whole rock powders and mineral separates from closed system suites have been interpreted in combination with detailed textural observations, which provided a more comprehensive understanding of the controls of isotope fractionation in plutonic settings.
Fractional crystallisation was the major control on Fe and V isotopic trends, whereas Zn isotopic fractionation was mostly driven by fluids. It was observed that the isotopic composition of powders from coarse-grained, cumulative rocks does not necessarily represent a true melt composition, questioning the assumptions and modelling approaches of many previous studies of intrusive rocks. Instead, a focus on mineral separates was more informative, and enabled derivation of the first temperature-dependent mineral-melt fractionation factors for calc-alkaline magmas, and development of a new method for calculating melt composition from mineral separate data, which will be useful for future studies. The magnitudes of fractionation factors were overwhelmingly influenced by temperature, whereas changing fO2 had no resolvable effect. Hence, accurate temperature estimates and temperature-dependent fractionation factors are vital for future work on plutonic settings. The isotopic variability observed on the pluton-scale highlights the significant isotopic heterogeneity of the upper crust in general. This has implications for how accurately clastic sediments record changes in ‘average’ crustal isotopic composition over geological timescales, and thus understanding temporal changes in the isotopic compositions of the crust and oceans.