Elucidating Igneous and Ore-Forming Processes with Iron Isotopes by using Experimental and Field-Based Methods.

Abstract

Iron (Fe) is a vital resource and the fourth most common element in the Earth’s crust, but variations in the Fe isotope composition of igneous rocks were only recently identified. This dissertation uses experimental and field-based methods to demonstrate the utility of Fe isotopes in tracking igneous and ore-forming processes. Chapter II presents the first experimental data that measure directly Fe isotope fractionation among phases in a fluid-bearing magmatic assemblage. The results, some of which contradict theoretical predictions, indicate that Fe isotopes fractionate during crystallization of magnetite from a melt and that Fe isotope fractionation between melt—fluid is influenced by the Cl content of the fluid. This is important considering the frequent extrapolation of data obtained from Fe-Cl complexes that are unrealistic for magmatic systems. Chapter III applies Fe isotopes to natural ore samples since Fe is globally mined from the rocks of iron oxide—apatite (IOA) deposits, which are a globally important source of Fe and other elements such as the rare earths but lack a genetic model. I focus on the world-class Los Colorados IOA, Chile as a case study and combine the Fe and O isotope composition of magnetite to investigate their formation. The data are consistent with a high-temperature (i.e., magmatic/magmatic-hydrothermal) origin for IOA deposits, and contributed to the development of a published novel IOA model. Iron is also abundant in layered mafic intrusions, and Chapter IV focuses on the uppermost portion of the world’s largest exposed mafic magma chamber, the Bushveld Complex, South Africa. These Fe isotope data demonstrate that fractional crystallization is reflected in the Fe isotope signature of the uppermost Bushveld. Stratigraphically, over the top ~2.5 km of this 9 km-thick intrusion, there is little variation in both whole rock and magnetite Fe isotope compositions, revealing that, despite theoretical predictions for the crystallization of magnetite to shift the isotopic composition of the whole rock, the presence of other Fe-bearing phases can buffer that effect. By incorporating published fractionation factors to model the measured data, this study provides the first benchmark for Fe isotope evolution during the crystallization of a large magma chamber.