Oxygen fugacity and melt composition are both known to have a strong influence on the partitioning of trace elements between coexisting minerals and melt. Previous work has suggested that Mn partitioning between apatite and silicate melt may be strongly affected by oxygen fugacity and could, therefore, act as an oxybarometer. Here, we present a new study on the partitioning of Mn between apatite and melt at high temperature (1400-1250 °C) and 1 GPa pressure, for various melt compositions and oxygen fugacities (NNO +4.7 to NNO-10). We find that there is no demonstrable variation in the partition coefficient for Mn between apatite and silicate melt (DMn Ap-m) across the range of fO2 conditions studied here. Instead, we find that DMn Ap-m varies significantly with melt composition and that in particular, the proportion of non-bridging oxygens strongly influences partitioning of Mn between apatite and melt. We propose that variations in the Mn content of natural apatite, previously thought to reflect variations in fO2, are instead related to the degree of melt polymerization. These findings are consistent with the results of Mn K-edge XANES spectroscopy, which demonstrate that Mn in coexisting apatite and silicate glass is present predominantly as Mn 2+ regardless of fO2. Furthermore, XANES spectra from a series of silicate glasses synthesised at various oxygen fugacities demonstrate that Mn 2+ is the predominant species, and that the average Mn oxidation state does not vary over a wide range of fO2-T conditions.
Manganese incorporation in synthetic hercynite, and partitioning between hercynite and silicate melt synthesised at 1.0 GPa, 1250°C, and at a fO2 buffered by Fe-FeO, has been studied by X-ray absorption spectroscopy and single-crystal X-ray structure refinement. Spectra indicate the presence of both Mn 2+ and Mn 3+ (and possibly also Mn 4+ ) in synthetic hercynite and partitioning of Mn 2+ into the melt phase, and Mn 3+ into hercynite, respectively, under run conditions. X-ray refinement is consistent with partial disorder of Fe and Al across tetrahedral and octahedral sites. A higher than expected degree of Fe-Al disorder in the Mn-bearing hercynite can be explained by preferential incorporation of Mn 2+ onto the tetrahedral site, and indicates that Fe-Al disorder in pure, stoichiometric hercynite cannot necessarily be used to determine closure temperatures in natural spinel. However, partitioning of Mn 2+ and Mn 3+ between melt and hercynite suggests that Mn incorporation in hercynite could be used as a measure of fO2 conditions in magmas during spinel crystallisation. 2 Running title: Mn incorporation in hercynite
The spinel-group minerals, found in a range of igneous rocks, are resistant to weathering and can incorporate several multivalent elements, meaning they have the potential to provide insight into the redox conditions of parental magmas. Naturally occurring spinel can contain varying quantities of Mn, an element which occurs terrestrially and extra-terrestrially as Mn 2+ , Mn 3+ , Mn 4+ and Mn 5+ . However, a lack of information on the effects of oxygen fugacity (f O 2 ) on: (1) Mn valence state and cation distribution; and (2) on spinel-melt partitioning means that the potential for a Mn-in-spinel oxy-barometer remains largely untested. Here, we use electron probe microanalysis, micro-focus X-ray Absorption Near Edge Structure (XANES) spectroscopy and single-crystal X-ray diffraction (SC-XRD) to investigate cation distribution and valence state in spinels in the Al-Mn-O and Fe-Mn-O systems synthesized at ambient pressure under varying f O 2 conditions. In contrast to previous studies, we find that the spectral resolution of the Mn K-edge XANES spectra is insufficient to provide quantitative data on Mn valence state and site occupancy, although it does verify that Mn is incorporated as both Mn 2+ and Mn 3+ , distributed over tetrahedral and octahedral sites. Combination of data from XANES and SC-XRD refinements can, however, be used to model Mn, Al and Fe valence and site occupancy. It would be expected that Mn-Fe spinels have the potential to record f O 2 conditions in parental melts due to changes to the octahedral site under conditions that were more reducing. However, decoupling the effects of temperature and oxygen fugacity on the T Fe 3+ -T Mn 2+ exchange in the Mn-Fe spinels remains challenging. In contrast, little variation is noted in Mn-Al spinels as a function of f O 2 , implying that crystal chemistry and cation site geometry may significantly influence cation distribution, and by inference, crystal-melt partitioning, in spinel-group minerals.K E Y WO R D S : spinel, manganese, XANES, oxygen fugacity, jacobsite, galaxite.
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