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The distribution of nitrogen in geologic systems is modulated by its partitioning between silicate (mineral and melt) and fluid phases. Under geologically applicable oxygen fugacity, pressure, and temperatures, nitrogen can be multiply-speciated, with N2 coexisting with reduced nitride (N 3) species. Non-polar, neutral species, including N2, tend to concentrate in fluids, while charged nitride species have a greater propensity to concentrate in silicate phases. The stoichiometry of converting N2 to single N atom nitride species implies that nitrogen speciation may depend on its concentration, and this leads to the hypothesis that the partitioning of nitrogen between silicate and fluid phases also depends on concentration, potentially biasing prior experimental work in doped systems and influencing the behavior of nitrogen in geologic systems. To test this hypothesis, we have completed a series high pressure (~1.75 GPa, 800 ◦C) experiments that react minerals, melts, and fluids with variable nitrogen concentrations (3.1–17.1 wt% N). Our results imply order-of-magnitude-scale increases in mineral/melt and melt/fluid partitioning as nitrogen concentrations decrease within natural ranges. For example, decreasing the N concentration from 2500 to 2 ppm increases predicted DN melt/fluid values by over an order of magnitude at constant PT conditions. This means that loss of nitrogen from a degassing magma or dehydrating slab is a self-limiting process that becomes increasingly inefficient as nitrogen concentration falls. Despite this, nitrogen remains highly concentrated in the atmosphere, which receives N from fluids exsolved from slabs and magmas. To maintain a nitrogen-rich atmosphere we therefore suggest that warm and oxidizing conditions have prevailed over subduction zones because warm slabs dehydrate under lower pressures where nitrogen is more easily partitioned into fluids, and oxidizing conditions also promote nitrogen partitioning into fluids. Concentration-dependent partitioning of nitrogen will also serve to moderate any initial variations of N/K in slab materials upon dehydration, and this may help to explain the relatively uniform N/K ratio of MORB mantle. We supplement our nitrogen concentration experiments with a temperature series (1.5–2 GPa, 750–950 ◦C). Our temperature series data reveal that at high temperature nitrogen favors melts over fluids, while temperature has no resolvable effect of biotite-fluid partitioning.
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The partitioning of H between olivine and melt at low pressures (10–200 MPa)
The concentration of H2OT (i.e., total H; assumed here to be the sum of hydroxyl and molecular water) in silicate minerals and melts exerts a primary control on the physicochemical properties of Earth’s crust and mantle. The partitioning of H2OT between minerals and melts is a key parameter used to model the H2OT contents of magmatic source regions in terrestrial and planetary systems. In mafic systems, olivine is frequently the first crystallizing phase, which makes it a useful tracer of primitive melt composition. Previously, the partitioning of H2OT between olivine and melt was only experimentally constrained at H2O-undersaturated conditions and pressures ≥500 MPa, which are broadly applicable to Earth’s mid- to lower-crust and uppermost mantle. However, we have few constraints at upper crustal pressures, limiting our ability to model pre-eruptive H2OT contents of magmas, decompression rates, and ultimately volcanic hazards. Here we present the first experimental determination of the partitioning of H2OT between olivine and melt at pressures relevant to Earth’s upper crust (10–200 MPa). Contrary to predictions based upon the extrapolation of experimental results from pressures ≥500 MPa, we find that the olivine-melt H2OT partition coefficient (DH2OTol/melt ranges from 0.0011 to 0.00033) decreases with increasing melt H2OT and increasing pressure from 10–200 MPa. One explanation for the observed relationship between DH2OTol/melt and melt H2OT concentration is that DH2OTol/melt is controlled by the speciation of H (e.g., hydroxyl and molecular water) in silicate melts. We calculate the concentration of hydroxyl (OH) dissolved in the melt, and assuming H is only incorporated into olivine as OH, calculate an olivine-melt OH partition coefficient (DOHOl/melt). Our data indicate that DOHOl/melt is constant (DOHOl/melt=0.0011±0.0002; 1 St.dev.) and that the proportion of molecular water (H2Om) to OH in the melt controls the variation of DH2OTOl/melt with melt H2OT concentration for our experiments. We also compared San Carlos olivine seed crystals to olivine crystallized in the same experimental charges. Our data indicate that San Carlos olivine seed crystals have lower H2OT concentrations than olivine crystallized during the experiments, which may be explained by differences in their compositions and, therefore, extrinsic point defect populations. Our results demonstrate that at low pressures (≤200 MPa), the partitioning of H2OT between olivine and melt is primarily dependent upon the speciation of H in silicate melts. Similarly, in combination with prior experimental work on clinopyroxene and plagioclase, our results suggest that the speciation of H in silicate melts may be a primary control on the partitioning of H2OT between all nominally anhydrous minerals and melts, at least at H2OT contents up to ∼5 wt%. We apply our results to prior estimates for magma decompression rates from the 1977 fire fountain eruption at Seguam volcano and find that, for the variable DH2OTOl/melt determined in our study, median magma decompression rates are slightly slower (a factor of ∼2.5) but within the uncertainty of models using a constant DH2OTol/melt=0.0009±0.0003 (1 St.dev.) (Towbin et al. 2023). Therefore, prior estimates of magma decompression rates based upon H+ diffusion in olivine may be slightly overestimated if the dependence of DH2OTol/melt on the speciation of H in the melt is unaccounted for.
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- Award ID(s):
- 2017897
- PAR ID:
- 10679963
- Editor(s):
- Baker, Don R
- Publisher / Repository:
- Mineralogical Society of America
- Date Published:
- Journal Name:
- American Mineralogist
- Volume:
- 111
- Issue:
- 1
- ISSN:
- 0003-004X
- Page Range / eLocation ID:
- 29 to 46
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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