Apatite stability under different oxygen fugacities relevant to planetary bodies
Apatite is widely distributed in terrestrial and extraterrestrial environments and may therefore crystallize in relatively oxidized environments found here on Earth, and in reduced settings such as the Moon. We present a series of oxygen fugacity-buffered apatite stability and apatite-fluid solubility experiments conducted at 1 atm and in a piston cylinder, respectively. The first style of experiments involved enclosing polished slabs of Durango apatite in evacuated silica tubes with a solid-state oxygen fugacity buffer, followed by heating to ~1100 °C for 65 or 90 h. At oxygen fugacities equal to and lower than the Fe-FeO equilibrium, crystals revealed alteration often in the form of convoluted features, which may to be related to the stability of the apatite component P2O5 under reducing conditions. Preferential evaporation of P2O5 from a haplobasalt heated to 1350 °C under reducing conditions – compared to similar experiments conducted under oxidizing conditions – also supports this interpretation. Fifteen solubility experiments were conducted in a piston cylinder device at 900 or 925 °C and 1 GPa, in either ~3.4 N NaCl or 2 N NaOH fluids. The oxygen fugacity was buffered at about 4 log units below the fayalite magnetite quartz equilibrium (FMQ-4) to 8 log units above this buffer (FMQ + 8). Apatite solubilities were determined by crystal weight loss. There is no systematic sensitivity to solubility vs. oxygen fugacity in H2O-NaOH fluid. In the H2O-NaCl fluid, apatite is less soluble by about a factor of 2 under the most oxidizing experimental conditions. This change in solubility is relatively subtle when compared to intensive variables explored in other studies, such as temperature, pressure, and XNaCl in the fluid. Overall, the apatite crystal structure is resilient across a range of different imposed oxygen fugacities, which contrasts with experimental results for another phosphate, monazite.
KeywordsApatite Oxygen fugacity Stability Planetary Solubility
We thank Jacob Buettner for assistance. Rosario Esposito is thanked for assistance with the EPMA at UCLA. Haplobasalt glasses were characterized by EPMA at Rensselaer Polytechnic Institute. This work was supported by EAR-1447404 and EAR-1751903. The LA-ICP-MS instrument is partially supported by a grant from the Instrumentation and Facilities Program, Division of Earth Sciences, NSF (EAR-1545637). Dan Harlov, Peter Tropper, James Webster and an anonymous reviewer are thanked for comments and suggestions that greatly improved the manuscript.
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