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 = 0.17 × 106/T2 and Δ56Febt/hbd-melt = −0.12 × 106/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.
Highlights
It is generally accepted that many crustal magma reservoirs are not melt-dominated systems, but instead are crystal-rich mushy regions where high melt fractions are present only transiently (e.g. Bachmann and Bergantz, 2004; Hildreth, 2004; Cashman et al, 2017; Sparks et al, 2019)
The whole rock δ56Fe values vary with whole rock MgO content. δ56Fe values increase from 0.038 ± 0.024‰ in the most mafic samples to a maximum value of 0.171 ± 0.032‰ at 0.52 wt.% MgO / 73.07 wt.% SiO2 (Figure 6). This range is similar to values previously measured in other I-type granitoids, which typically show maximum δ56Fe ~ 0.21‰ with differentiation (Foden et al, 2015)
Magma fO2 will control the fractionating mineral assemblage, leading to distinct trends in whole rock δ56Fe for I, A- and S-type granites (Foden et al, 2015), this study has demonstrated that variation in mineral Fe3+/ΣFe does not have a resolvable effect on Fe isotope fractionation factors between specific mineral pairs, suggesting limited use of Fe isotopes in mineral phases as a direct fO2 proxy
Summary
It is generally accepted that many crustal magma reservoirs are not melt-dominated systems, but instead are crystal-rich mushy regions where high melt fractions are present only transiently (e.g. Bachmann and Bergantz, 2004; Hildreth, 2004; Cashman et al, 2017; Sparks et al, 2019). There is debate about how efficient crystal-liquid segregation processes are in evolved crystal mushes with a more viscous, low-density melt (Holness, 2018; Bachmann and Huber, 2019). Other processes such as magma recharge, mixing, assimilation and/or reactive porous flow (Jackson et al, 2018; Weinberg et al, 2021) may be important controls on chemical variability. Chemical trends in plutonic suites are commonly modelled as the liquid line of descent of a liquid magma body undergoing fractional crystallisation and/or assimilation (e.g. Burton-Johnson et al, 2019), even though this may not be realistic
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