Crystal chemistry of accessory minerals as a probe of magmatic oxygen fugacity: an experimental study

Authors: Thomas Stokes (University of Edinburgh)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: July 2019

Abstract: It is well established that oxygen fugacity, fO₂ , is one of the key parameters that needs to be quantified in order to understand igneous processes, model the geophysical behaviour of the core and mantle, to understand the exchange of C-O-H-S gases between the atmosphere and the interior of the Earth, and to further our understanding of other terrestrial planets. Despite this it remains one of the most poorly constrained geochemical variables, limiting our understanding of terrestrial systems. Recent work has focused on using accessory minerals for determining magmatic fO₂ , as a probe to constraining conditions in planetary interiors. Accessory minerals are already important petrological tools for providing insight into magmatic conditions. These minerals may concentrate a variety of trace elements, and hence are crucial in understanding the elemental budget of magmas. Accessory minerals such as zircon and apatite are also some of the hardier minerals found in igneous rocks and are, therefore, less likely to be altered by processes such as chemical weathering, metasomatism or crustal anatexis. Furthermore, study of detrital accessory minerals in ancient sedimentary rocks could provide much needed insight into the evolution of the oxidation state of the early Earth. This work aims to assess how the compositions and structures of two accessory minerals, spinel and apatite, respond to variations in magmatic fO₂ and to determine whether these minerals could act as probes of fO₂ in planetary interiors. Focus has been concentrated on the element manganese, as (1) it is a relatively abundant trace element, (2) it can exist in valence states from Mn²⁺ to Mn⁵⁺ in nature, and (3) recent work has suggested that Mn may become preferentially concentrated in apatite under reduced conditions. In an initial investigation, large single crystals of Mn-rich spinel were synthesised under a variety of fO₂ conditions. X-ray absorption near edge structure (XANES) spectroscopy and structural refinements of single crystal X-ray diffraction data were used to determine Mn valence state and coordination. Results show that Mn is present in spinel as both Mn²⁺ and Mn³⁺, distributed over both octahedral and tetrahedral cation sites. However, in contrast to the Fe⁺²/Fe³⁺ ratio, little variation in Mn valence as a function of fO₂ was observed. Results were, however, useful in testing and refining protocols for modelling Mn XANES data in a simple, model system. In contrast to results from spinel, previous studies have indicated that Mn valence may change significantly in the accessory mineral apatite due to variations in magmatic fO₂ .

Subject Areas: Earth Science, Chemistry

Instruments: I18-Microfocus Spectroscopy