Integrated Petrological and Fe-Zn Isotopic Modelling of Plutonic Differentiation

Abstract

The upper continental crust is formed from chemically diverse granitic plutons. Active debate surrounds the range of physical conditions (P-T-X-fO2) and differentiation processes which occur in mush bodies that solidify to form plutons. Transition metal stable isotopes are increasingly employed to trace magmatic processes in both extrusive lavas and intrusive plutonic suites, with a focus on analysis of whole rock powders. However, studies of plutonic suites often overlook the complex textures represented within coarse grained samples, and how these will influence whole rock isotopic compositions. Here we examine the calc-alkaline Boggy Plain Zoned Pluton, SE Australia, which closely approximates closed system behaviour during magmatic differentiation. We combine petrological examination with Fe and Zn isotopic analysis of biotite, hornblende and magnetite mineral separates and whole rock powders. Whole rock Fe isotopic composition (as δ56Fe) increases from 0.038‰ to 0.171‰ with decreasing MgO content, while mineral separates display heavy Fe isotope enrichment in the order magnetite > biotite = hornblende > pyroxene. A lack of correlation between whole rock Fe and Zn isotopic compositions suggests that the Fe isotopic variation is predominantly driven by closed system fractional crystallisation: specifically by the balance between crystallisation of isotopically heavy magnetite, and isotopically light silicates. To demonstrate this quantitatively, temperature dependent mineral-melt fractionation factors were derived from the mineral separate data (Δ56Femag-melt = 0.17x106/T2 and Δ56Febt/hbd-melt = -0.12x106/T2) and used to construct models that successfully reproduce the observed Fe isotopic variation during fractional crystallisation. These fractionation factors are compared to theoretical and empirical estimates from previous studies. We highlight that accurate determinations of temperature and modal mineralogy are critical when modelling Fe isotopic variations in plutonic suites. Successful interpretation of equilibrium Fe isotopic fractionation in a relatively simple calc-alkaline suite like the Boggy Plain Zoned Pluton paves the way for Fe isotopes to be used to investigate more complex mush bodies.