Abstract
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 (DMnAp-m) across the range of fO2 conditions studied here. Instead, we find that DMnAp-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 polymerisation. 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 Mn2+ regardless of fO2. Furthermore, XANES spectra from a series of silicate glasses synthesised at various oxygen fugacities demonstrate that Mn2+ is the predominant species, and that the average Mn oxidation state does not vary over a wide range of fO2-T conditions.
Highlights
Apatite [nominally Ca5(PO4)3(F,Cl,OH)] is an accessory mineral found in many igneous, metamorphic, and sedimentary rocks
This suggests that oxygen fugacity is not the main control on Mn apatite-melt partitioning in these compositions, significant variation in Mn partitioning is noted for experiments involving different melt compositions
Apatite-silicate melt partitioning experiments described here demonstrate that fO2 has no discernible influence on Mn or volatile partitioning
Summary
Apatite [nominally Ca5(PO4)3(F,Cl,OH)] is an accessory mineral found in many igneous, metamorphic, and sedimentary rocks. The three primary apatite end-members (fluor-, chlor- and hydroxyapatite) relate to the three anion end members of apatite (F, Cl and OH respectively). The incorporation of these volatiles as major constituents in the apatite crystal structure make it a critical mineral for understanding melt volatile contents in terrestrial (Douce and Roden 2006; Scott et al 2015) and extra-terrestrial systems (Gross et al 2013; McCubbin et al 2016). Recent work (Miles et al 2014; Konecke et al 2017) has suggested that the substitution of redox sensitive elements (e.g. Mn, S, Ce, Eu) into apatite could be used to constrain the oxygen fugacity (fO2) of the melt from which it has crystallized, providing a much needed new oxybarometer. Oxygen fugacity is an important variable when constraining the composition of the earth’s core
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