Mafic layered intrusions, formed by the slow cooling and differentiation of basaltic magmas within the crust often contain enormous metallic ore resources, of vital importance for industrial and societal applications. These precious metals, including chromium (Cr) and Platinum-Group Elements (PGE), are often hosted within continuous, nearly monomineralic dense oxide layers. Despite many decades of scientific research, the processes that resulted in the formation of these spectacular layering features and the accumulation of precious metals to such high concentrations are heavily debated. Part of the problem arises from the fact that compositional data suggests the influence of post-magmatic processes and fluid circulation operating in the magma chamber. Such processes are likely to have modified the primary cumulate pile and influenced precious metal distribution, but the timing and extent to which these rocks have been subjected to subsolidus modification is not known. The ca. 2.0 Ga Bushveld Complex in South Africa and the ca. 60 Ma Rum layered intrusion in Scotland both contain PGE-rich mineralized layers, and have been cornerstones for understanding open-system magmatic processes for decades.
In this work I developed geochemical tools to quantify the influence of post-magmatic processes, using high-precision iron (Fe) isotope measurements on ferromagnesian sililcates (olivine and pyroxene) and oxides (Cr-spinel).
In particular, I address the following questions:
(i) what is the extent of inter-mineral Fe isotope variations?
(ii) are minerals in isotopic equilibrium or disequilibrium with each other?
(iii) what are the potential mechanisms that control Fe isotope fractionation?
(iv) what is the potential of Fe isotopes as tracers for magmatic processes involved in the formation of mafic layered intrusions?
The results show that most mineral pairs in our samples are in isotopic equilibrium. The magnitude of inter-mineral Fe isotope fractionation indicate that primary mineral compositions have been extensively modified by iron-magnesium exchange occurring down to subsolidus temperatures (~ 800ºC). However despite evidence for isotopic equilibrium, the sign of Fe isotope partitioning between Cr-spinel and silicates (olivine and pyroxenes) does not agree with that predicted by consideration of charge and coordination environment. In particular, This can be attributed to strong compositional effects on equilibrium partitioning of Fe in Cr-rich spinel specifically, as this is the case in particular in highly magnesian lavas and layered intrusions. This work thus highlights the importance of mineral compositional effects in controlling Fe isotope behaviour between minerals in mantle-derived rocks, an aspect that is not well understood and often not taken into consideration. Because Fe isotopes are routinely used as a fingerprinting tool to investigate magmatic processes, the conclusions of this work have far-reaching implications for the application of high-temperature stable isotope geochemistry across the fields of igneous and metamorphic petrology as well as ore geology and cosmochemistry.
