Equilibrium Transition Metal Stable Isotope Fractionation observed in the Boggy Plain Pluton, SE Australia

Abstract:

The continental crust is built from chemically diverse granitic plutons, but the precise differentiation mechanism(s) in the crystal mush bodies which form these plutons are still debated. Variation in transition metal stable isotopes have been successfully modelled as an equilibrium process during fractional crystallisation in genetically related, extrusive lavas. It is unclear how/if the drivers of isotopic fractionation differ in mush regions, where petrogenesis is assumed to be more complex.

The well characterised Boggy Plain pluton, SE Australia, is concentrically zoned as a result of fractional crystallisation of a calc-alkaline magma [1]. Textural examination of samples is used to assess crystallisation histories and mineral (dis)equilibrium. Redox sensitive (Fe, V) and insensitive (Zn) transition metal isotopes in silicate and oxide mineral separates are analysed to assess each system's sensitivity and response to changing parameters (e.g. P-T-X-fO₂) during differentiation.

Mineral separates have Zn isotopic compositions within the BSE range, except one isotopically heavy biotite in the most evolve sample. This difference may be caused by late stage fluid circulation, which has no significant effect on Fe or V isotopes. For V isotopes, δ⁵¹V_biotite > δ⁵¹V_hornblende and there is no resolvable inter-mineral Fe isotope fractionation between biotite and hornblende. The consistent direction and magnitude of offset between phases suggests isotopic equilibrium. A Rayleigh fractionation model using the Fe and V mineral separate isotope data shows that the balance between crystallisation of isotopically light silicates and heavy magnetite prevents an increase in whole rock δ⁵⁶Fe as has been observed in other evolved granites [2]. Magnetite hosts 40-70% of V throughout the differentiation sequence with fractionation of isotopically light magnetite throughout. The contrasting behaviour of two redox sensitive elements highlights the importance of phase control on overall isotopic evolution of magmatic systems. Demonstration of equilibrium fractionation amongst the main rock-forming phases will allow further interrogation of changing physical parameters during mush processes.

[1] Wyborn 1983 [2] Foden et al. 2015