Experimental investigation of F, Cl, and OH partitioning between apatite and Fe-rich basaltic melt at 1.0–1.2 GPa and 950–1000 °C Article Swipe
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· 2015
· Open Access
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· DOI: https://doi.org/10.2138/am-2015-5233
· OA: W2263035004
Apatite-melt partitioning experiments were conducted in a piston-cylinder press at 1.0-1.2 GPa and 950-1000 °C using an Fe-rich basaltic starting composition and an oxygen fugacity within the range of ΔIW-1 to ΔIW+2. Each experiment had a unique F:Cl:OH ratio to assess the partitioning as a function of the volatile content of apatite and melt. The quenched melt and apatite were analyzed by electron probe microanalysis and secondary ion mass spectrometry techniques. The mineral-melt partition coefficients (D values) determined in this study are as follows: D<sub>F</sub><sup>Ap-Melt</sup> = 4.4-19, D<sub>Cl</sub><sup>Ap-Melt</sup> = 1.1-5, D<sub>OH</sub><sup>Ap-Melt</sup> = 0.07-0.24. This large range in values indicates that a linear relationship does not exist between the concentrations of F, Cl, or OH in apatite and F, Cl, or OH in melt, respectively. This non- Nernstian behavior is a direct consequence of F, Cl, and OH being essential structural constituents in apatite and minor to trace components in the melt. Therefore mineral-melt D values for F, Cl, and OH in apatite should not be used to directly determine the volatile abundances of coexisting silicate melts. However, the apatite-melt D values for F, Cl, and OH are necessarily interdependent given that F, Cl, and OH all mix on the same crystallographic site in apatite. Consequently, we examined the ratio of D values (exchange coefficients) for each volatile pair (OH-F, Cl-F, and OH-Cl) and observed that they display much less variability: K<sub>dCl-F</sub><sup>Ap-Melt</sup> = 0.21± 0.03, K<sub>dOH-F</sub><sup>Ap-Melt</sup> = 0.014 ± 0.002, and K<sub>dOH-Cl</sub><sup>Ap-Melt</sup> = 0.06 ± 0.02 . However, variations with apatite composition, specifically when mole fractions of F in the apatite X-site were low (X<sub>F</sub> < 0.18), were observed and warrant additional study. To implement the exchange coefficient to determine the H<sub>2</sub>O content of a silicate melt at the time of apatite crystallization (apatitebased melt hygrometry), the H<sub>2</sub>O abundance of the apatite, an apatite-melt exchange K<sub>d</sub> that includes OH (either OH-F or OH-Cl), and the abundance of F or Cl in the apatite and F or Cl in the melt at the time of apatite crystallization are needed (F if using the OH-F K<sub>d</sub> and Cl if using the OH-Cl K<sub>d</sub>). To determine the H<sub>2</sub>O content of the parental melt, the F or Cl abundance of the parental melt is needed in place of the F or Cl abundance of the melt at the time of apatite crystallization. Importantly, however, exchange coefficients may vary as a function of temperature, pressure, melt composition, apatite composition, and/or oxygen fugacity, so the combined effects of these parameters must be investigated further before exchange coefficients are applied broadly to determine volatile abundances of coexisting melt from apatite volatile abundances.
