High nitrogen solubility in stishovite (SiO2) under lower mantle conditions

Nitrogen is a crucial volatile element in the early Earth’s evolution and the origin of life. Despite its importance, nitrogen’s behavior in the Earth's interior remains poorly understood. Compared to other volatile elements, nitrogen is depleted in the Earth’s atmosphere (the so-called “missing nitrogen”), calling for a hidden deep reservoir. To investigate nitrogen’s behavior in the deep Earth including how the reservoir formed, high-pressure and high-temperature experiments were conducted at 28 GPa and 1,400–1,700 °C. To reproduce the conditions in the lower mantle, the redox was controlled using a Fe–FeO buffer. We observed that depending on the temperature conditions, stishovite can incorporate up to 90–404 ppm nitrogen, experimentally demonstrating that stishovite has the highest nitrogen solubility among the deep mantle minerals. Stishovite is the main mineral component of subducted nitrogen-rich sedimentary rocks and eroded continental crust that are eventually transported down to the lower mantle. Our results suggest that nitrogen could have been continuously transported into the lower mantle via subduction, ever since plate tectonics began.

Scientific RepoRtS | (2020) 10:10897 | https://doi.org/10.1038/s41598-020-67621-2 www.nature.com/scientificreports/ in BSE, which conflicts with the nitrogen depletion compared to carbon in the BSE, as reported by Marty 5 . First principle molecular dynamic simulations clarified that nitrogen solely cannot explain both the density and the bulk sound velocity of the outer core 24 . However, there is a possibility that the Earth's core plays an important role in reserving nitrogen. Li et al. 13 conducted high-pressure experiments under Ni-NiO and Fe-FeO buffers, reporting that the deeper part of the upper mantle can become a nitrogen reservoir through the solidification of a magma ocean because the maximum nitrogen solubilities in forsterite and enstatite were determined as ca. 10 ppm and 100 ppm 13 , respectively. In addition, Yoshioka et al. 15 reported a 21.5 ± 18.1 ppm nitrogen solubility in bridgmanite, in high-pressure experiments with a multi-anvil apparatus and under reduced conditions close to the Fe-FeO buffer. This is the only study of the nitrogen solubility of bridgmanite which, occupies approximately 80 vol% of the lower mantle 25 . A nitrogen reservoir might have also been formed in the mantle by subducting slabs 26 , because the inward nitrogen flux of subduction is one order of magnitude higher than that of the outgassing flux [27][28][29][30] . The Earth is the only planet in the Solar system with active plate tectonics 31 . Such volatile cycle via subduction has remained a very important research topic in geochemistry. To date, various NH 4 -bearing silicate minerals have been synthesized under conditions corresponding to cold subducting slabs in the deep upper mantle 4,18,20,21 . Sediments and eroded continental crust in subducting slabs can be nitrogen-rich. Average concentrations of nitrogen in sediments and upper crusts are, respectively, 560 ppm and 150 ppm 32 . However, experimental conditions reported in earlier studies 4,18,20,21 are limited to the deep upper mantle (< 13 GPa). Consequently, nitrogen behavior in the mantle transition zone (> 13 GPa) and in the lower mantle (> 24 GPa) are still unknown. Therefore, we conducted high-pressure experiments at lower mantle conditions to investigate nitrogen incorporation into deep mantle minerals such as stishovite and bridgmanite.

Results
Description of the run products. The experimental conditions are presented in Table 1 and Supplementary Fig. S1. Figure 1 illustrates the cell assembly used for the high-pressure experiments. Figure 2 and Supplementary Figs. S3-S8 present the backscattered electron (BSE) images of recovered cell assemblies and recovered samples. Each outer gold (Au) capsule included an inner platinum (Pt) capsule surrounded by a Fe-FeO buffer. The two inner Pt capsules of the cell assembly accommodated the Al-free and Al-bearing samples, respectively. The outer Au and the inner Pt capsule partially melted during the runs, because of iron and water coexistence ( Fig. 2 and Supplementary Figs. S3-S8). However, no Fe-FeO leaked out of the Au capsules. After all the runs were completed, a subsequent EDS analysis revealed that pure metallic iron was still present and FeO contained no Mg. The Fe and FeO coexistence after the runs indicated that the experiments' oxygen fugacity conditions were that of the lower mantle. Therefore, we inferred that hydrogen was generated in the outer Au capsule and permeated the inner capsule. The MgO-rich hydrous melt in the inner Pt capsule was expected to contain 15 NH 3 under a hydrogen-coexisting environment. Run products consisted of stishovite, bridgmanite, and a hydrous melt. The resulting single crystals of stishovite and bridgmanite were larger than 40 μm and 80 μm, respectively. These grain sizes were sufficiently large to be analyzed by nanoscale secondary ion mass spectrometry  29 SiH 2 − ion counts. These latter have been estimated from the 28 SiH 2 − ion counts based on the 29 Si − / 28 Si − isotopic ratio (5.06 × 10 -2 ). The Supplementary Figure S9 shows the time-dependence of the representative ion counts. In stishovite, the 15 N 16 O − ion counts were higher than 28 S9b). Therefore, in bridgmanite, we were able to estimate only the maximum possible concentration. Figure 3 and Supplementary Fig. S10 present nitrogen incorporation rates in stishovite and bridgmanite, respectively, in both the Al-free and Al-bearing systems. These results suggest that the nitrogen solubility of stishovite is markedly higher than that of bridgmanite, for both systems. Although nitrogen solubilities of different silicate minerals from 15 N 16 O − / 30 Si − cannot be compared directly because of a matrix effect, it is noteworthy that the ion intensity ratio of stishovite was one to two orders of magnitude larger than that of bridgmanite, in all recovered samples ( Fig. 3 and Supplementary Fig. S10). The nitrogen solubility in stishovite in the Al-free system can be higher than that in the Al-bearing system (see Supplementary Fig. S11). Due to the low counts of 15 N 16 O ions, the nitrogen solubility in bridgmanite between the Al-free and Al-bearing systems was difficult to evaluate. Figure 4 presents the nitrogen solubility in stishovite, estimated using nitrogen-implanted quartz glass as standard samples. The nitrogen solubility in stishovite varied from 90 ppm (μg/g) to 404 ppm (μg/g). Stishovite can incorporate higher concentrations of nitrogen than bridgmanite (21.5 ± 18.1 ppm) 15 . In addition, the nitrogen solubility in stishovite increases with temperature. This implies that nitrogen might be incorporated in the stishovite crystal structure as N 3− into oxygen vacancies or as N 2 into Schottky defects 13 (lattice point defects formed by unoccupied cation and anion sites in a stoichiometric ratio). Although oxygen vacancies might be more numerous in crystal structures at higher temperatures, substitutions between N 3− and O 2− require charge compensation. In stishovite, nitrogen is not expected to exist as NH 4 + because NH 4 -bearing silicate minerals release nitrogen at temperatures of 750-1,000 °C 20,33,34 much lower than those of our experimental conditions. Nitrogen solubilities in mantle minerals were investigated from high-pressure and high-temperature experiments: e.g. forsterite (7.1 ± 1.6 ppm), Al-bearing enstatite (50-100 ppm), wadsleyite (188.8 ± 15.6 ppm), ringwoodite (110.4 ± 109.8 ppm), bridgmanite (21.5 ± 18.1 ppm), and Ca-perovskite (28.3 ± 23.6 ppm) 13,15 . Nitrogen solubility in stishovite can be higher than in these minerals.

Discussion
Metamorphism of SiO 2 -rich sediments and nitrogen-containing continental crust form stishovite in subducting slabs. Moreover, Lee et al. 35 reported that subduction erosion plays an important role in nitrogen transport into the mantle based on the nitrogen isotope data. The subduction erosion invokes the removal of continental material at the frontal or basal areas of continental margins. The mineral proportion of clinopyroxene, which can incorporate a significant quantity of nitrogen 4,13 in subducted sediments and eroded continental crust, decreases with increasing depth 36 . In addition, NH 4 -bearing silicate minerals are unstable at high temperatures 20,33,34 . Therefore, stishovite, which can incorporate nitrogen at higher temperatures up to 1,700 °C, is the major mineral in the subducted sediment and, presumably, in the subducted eroded continental crust. Ingalls et al. 37 reported that continental crusts can subduct into the mid upper mantle in spite of its high buoyancy. Moreover, stishovite has the highest nitrogen solubility in major minerals formed by the phase transition in subducting slabs. We propose stishovite as the major nitrogen carrier down to the transition zone and the lower mantle, if stishovite can inherit nitrogen from clinopyroxene 4,13 or some NH 4 -bearing high-pressure minerals 18 which are stable up to 10 GPa and 700 °C 18,36 . This P-T condition corresponds to a depth of 300 km where stishovite forms from metamorphism of the subducted sediments. Based on the mass of subducted material, we estimated the mass of nitrogen exclusively transported by stishovite. Current subduction rates derived from sediments and continental crusts are, respectively, 1.65 km 3 / year and 2.1 km 3 /year, consisting of > 1.7 km 3 /year from subduction erosion and 0.4 km 3 /year from crustal subduction during continental collision 38 . Stishovite accounts for about 40 vol% of the subducted sediment and about 30 vol% of the continental crust, at pressures higher than 10 GPa or depths greater than 300 km 36 . Stishovite can incorporate up to 90 μg/g nitrogen at 28 GPa and 1,400 °C (see Fig. 4), corresponding to the present cold slab geotherm 39 . From these parameters, we estimated that stishovite can transport a maximum nitrogen quantity of 5.0 × 10 8 kg/year into the mantle transition zone and the lower mantle. If stishovite can transport nitrogen into the lower mantle, then almost all the transported nitrogen can be retained there. This is because the current nitrogen outgassing from the lower mantle [OIB (Ocean island basalt): 1.15 × 10 5 kg/year] is three orders of magnitude lower than the nitrogen amount transported into the lower mantle by stishovite 28 . Thus, the maximum estimation of nitrogen transported into the lower mantle ranged from 0.46 to 0.53 PAN (PAN: Present Atmospheric Nitrogen) since 3.6-4.2 billion years ago, when plate tectonics started [40][41][42][43] although these start times of plate tectonics are controversial. Furthermore, outgassing C/N ratios (mol) from subduction (C/N = 5.2 × 10 3 ) and OIB (C/N = 7.1 × 10 3 ) are approximately one order of magnitude higher than that from MORB (Mid-ocean ridge basalt, C/N = 6.8 × 10 2 ) 28,44 . Bridgmanite and metallic iron in the lower mantle incorporate nitrogen 15,23 and can store a maximum of 25 PAN 15 ; this nitrogen storage capacity is greater than that of either the upper mantle or the transition zone 15 . Subduction is expected to play an essential role in the co-evolution between the atmosphere and the deep mantle, especially in the lower mantle. While metallic iron in the reduced lower mantle is expected to play a crucial role in reserving much nitrogen 23 , silicate minerals are the only nitrogen reservoir in subduction zones under oxidized conditions. K-hollandite and garnet are formed by the transition of the subducted sediments and continental crust in the mantle. These minerals can also incorporate nitrogen, as Watenphul et al. 18 and Li et al. 13 reported based on the experiments carried under lower pressures corresponding to those of the upper mantle. Although nitrogen solubilities in K-hollandite and garnet have not been estimated under the high-pressure corresponding to the lower mantle, the maximum of the nitrogen mass transported into the lower mantle via subducting slabs might be greater than the estimates solely based on stishovite transport. Compared to carbon, the formation of nitrogen reservoir in the lower mantle can deplete nitrogen effectively and might explain the "missing" nitrogen.
Nitrogen concentration in the Archean atmosphere remains controversial. Some researchers have reported that the partial pressure of nitrogen (pN 2 ) was half as low as that of the present atmosphere [45][46][47] . Others suggested that the partial pressure of nitrogen (pN 2 ) in the Archean atmosphere might have been 1.4-3 times higher than that at present. The whole Earth was not frozen in the Archean Earth despite the fainter Sun 48 . The higher pN 2 in the atmosphere might reconcile the faint young Sun paradox 2 : greater nitrogen contents in the early Earth's atmosphere might have enhanced the greenhouse effect of CO 2 2 . Goldblatt et al. 2 suggested that nitrogen was transported into the deep Earth via subduction zones lowering the higher initial concentration in the atmosphere down to the present level. These results are supported by high-pressure experiments of Mallik et al. reporting that the Archean atmosphere had a 1.4-1.6 times 19 . Barry and Hilton 49 also reported that the early atmospheric N content was approximately 50% higher than it is today based on a model using N-He-Ne-Ar isotope data Our results are consistent with the previous studies 2, 19,49 reporting that the mass of nitrogen in the ancient atmosphere was greater than that at present if the fluxes of nitrogen outgassing from the ancient and the present OIB were similar. Nitrogen transport into the lower mantle by stishovite and the formation of a deep nitrogen reservoir might have affected the global nitrogen cycle and the history of the Earth's atmosphere. If the lower mantle could capture the nitrogen transported by stishovite, then a deep "hidden" nitrogen reservoir has formed in the lower mantle since the plate tectonics started (Fig. 5). Stishovite might have played a key role in the co-evolution of the atmosphere and the deep mantle.

Methods
High-pressure experiments using a multi-anvil apparatus. High-pressure and high-temperature experiments were conducted at the Geodynamics Research Center, Ehime University, Japan. We used a Kawai-type 2000 tons multi-anvil apparatus (Orange-2000) and a Kawai-type 3,000 tons multi-anvil apparatus (Orange-3000) installed at the Geodynamics Research Center, Ehime University, Japan. All experiments were conducted at 28 GPa and 1,400-1,700 °C, for a 2 h heating duration.  [50][51][52] and enclosed in an Au outer capsule. The two Au capsules of the cell assembly were insulated from the Re heater by a 25 μm thick magnesia sleeve. We used 150 mesh iron powders and iron oxide (FeO) powders with particle sizes smaller than 200 mesh for the Fe-FeO buffer (Fe:FeO = 2:1, wt%); 20-50 μl of water were added per each 0.5 g Fe-FeO. As previously described, Fig. 1 depicts the cell assembly used for this study. Thermal insulation was provided by a LaCrO 3 sleeve. The Pt sample capsule was produced combining two Pt tubes, with 0.1 mm thick walls and outer diameters of 1.3 and 1.5 mm, by welding each end of the capsules 53 . An Au capsule was made from an Au tube with 0.1 mm wall thickness and 2.5 mm outer diameter. The temperature was measured by a W-Re (W3%Re-W25%Re) thermocouple inserted in the octahedron, attached to the Au capsules. Hydrogen fugacity in the inner and outer capsules was assumed to be equal because Pt has a high hydrogen permeability compared to Au. At high temperatures, 15  nitrogen analysis of silicate minerals using nanoSiMS. A quantitative nitrogen analysis was conducted using a NanoSIMS 50 (Cameca) installed at the Atmosphere and Ocean Research Institute, The University of Tokyo (AORI), Japan. This NanoSIMS has a magnetic sector mass analyzer with seven parallel detection systems to simultaneously detect up to seven ion species as secondary ions. We employed a Cs + primary ion beam with a 2 nA current, 5 μm diameter, and a 5 μm × 5 μm or 10 μm × 10 μm raster. We detected nitrogen as 15 N 16 O − , as described by Li et al. 11 To avoid charge-up, an e − gun was used at 500-800 nA. Count times for the detection of 30 Si − , 30 SiH − , 28   www.nature.com/scientificreports/