Trace H and F Concentrations in Feldspar as Monitors of Volatile Processes in the Earth and Moon

The advent of ultra-low blank SIMS methods for measuring H and F has opened up new possibilities for exploring nominally H- and F-free minerals as recorders of volatile processing in planetary bodies. Building on our earlier calibration work, we have undertaken a multi-pronged study of H and F incorporation in plagioclase (Pl), with applications to the Earth and Moon. These efforts include:

Solubility experiments. Experiments at 0.5 GPa and 800-850 °C establish that H₂O solubility is a strong function of fO₂ (between the IW and NNO buffers) for An59 and An95. With decreasing fO₂, solubility in An95 increases from 77 μg/g (NNO, 800 °C) to 200 μg/g (IW, 800 °C) or 251 μg/g (IW, 850 °C); a similar trend is found for An59. This suggests a key role for Fe²⁺ (reduced from Fe³⁺ in the starting material) in H incorporation.

Partitioning experiments. Experiments on H and F partitioning between An95 and a lunar basaltic composition have been conducted. Partition coefficients for both H₂O and F between Pl and melt are approximately twice as high at low fO₂ (in Fe capsules) as at higher fO₂ (in AuPd in an outer oxidized Ni capsule), again suggesting an important role for Fe in H incorporation. For otherwise similar Pl H₂O contents, this corresponds to smaller concentrations of H₂O in the melt than inferred previously.

Arc volcanic rocks. We have acquired extensive data on phenocrysts and associated melt inclusions (MI) from Mt. Hood and Crater Lake, Oregon. SIMS allows us to account for diffusional loss with better resolution than previous work using FTIR. Mt. Hood data will be presented by Caseres et al. (this meeting). Results on Pl (with up to 110 μg/g H₂O) from 8 Crater Lake eruptive units spanning a 60 kyr magmatic history will be shown in comparison to both new and previously acquired MI data. Direct measurements of H in Pl phenocrysts containing MI suggest a mineral-melt H₂O partition coefficient of 0.002.

Lunar rocks. H and F have been measured in Pl from 15 Apollo samples. Whereas H is generally below the limit of detection (LOD), F is above LOD in most samples. Relatively high H₂O contents (up to 28 μg/g) in Pl of ferroan anorthosite 60015 are attributed to shock melt inclusions, which may have incorporated volatiles from the lunar regolith. This result contrasts with previous work concluding that pristine H concentrations are retained in lunar Pl.