Nitrogen solubility in the deep mantle and the origin of Earth's primordial nitrogen budget
Nitrogen solubility in the deep mantle and the origin of Earth's primordial nitrogen budget
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DOI：
10.1016/j.epsl.2018.02.021
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发表时间：
2018-04
影响因子：
5.3
通讯作者：
T. Yoshioka;M. Wiedenbeck;S. Shcheka;H. Keppler
中科院分区：
文献类型：
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作者：
T. Yoshioka;M. Wiedenbeck;S. Shcheka;H. Keppler
测量了与还原的富氮流体相共存的地球过渡带和下地幔主要矿物（瓦兹利石、尖橄榄石、桥锰矿和钙硅酸盐钙钛矿）中氮的溶解度。实验在接近 Fe-FeO 缓冲液的多砧压机中进行，压力为 14 至 24 GPa，温度为 1100 至 1800°C。起始材料富含 15 N，并且通过二次离子质谱法测量运行产物中的氮浓度。在瓦兹利石和尖伍德石中观察到的氮 (15 N) 溶解度通常在 10 至 250 μg/g 范围内，并且随着温度的升高而急剧增加。氮在桥锰矿中的溶解度约为 20 μg/g，而钙硅酸盐钙钛矿在可比条件下的溶解度约为 30 μg/g。源自共存相的氮的分配系数为D N 瓦兹利石/橄榄石＝5.1±2.1，DN 尖菱石/瓦兹利石＝0.49±0.29，以及D N 菱锰矿/尖伍德石＝0.24(+0.30/-0.19)。还测量了与硅酸盐共存的固体富铁金属相中的氮溶解度，在 23 GPa 和 1400°C 下达到了接近 1 wt.% 15 N 的最大值。这些数据得出铁金属和桥锰矿之间的氮分配系数 D N 金属/桥锰矿~ 98，这意味着在含有约 1% 铁金属的下地幔中，约一半的氮仍然存在于铁金属和桥锰矿之间。硅酸盐。瓦兹利石和尖角橄榄石中氮的高溶解度可能是过渡区超深钻石中经常观察到的低氮浓度的原因。总体而言，溶解度数据表明过渡带和下地幔的储存能力至少是目前大气中氮质量的 33 倍。通过将矿物中的氮溶解度数据与硅酸盐熔体中的氮溶解度数据相结合，可以估计氮的矿物/熔体分配系数，从而可以模拟岩浆海洋结晶过程中氮的行为。这些模型表明，如果岩浆海洋与氮分压仅为几巴的原始大气共存，那么深部地幔中一定会捕获数倍于当前大气质量的氮。因此，近地表储层中氮相对于其他挥发物的明显消耗似乎反映了固体地球中储存了更大的氮储层。这些储层之间的动态交换可能引起了地球历史上大气压的重大波动。
The solubility of nitrogen in the major minerals of the Earth's transition zone and lower mantle (wadsleyite, ringwoodite, bridgmanite, and Ca-silicate perovskite) coexisting with a reduced, nitrogen-rich fluid phase was measured. Experiments were carried out in multi-anvil presses at 14 to 24 GPa and 1100 to 1800° C close to the Fe–FeO buffer. Starting materials were enriched in 15 N and the nitrogen concentrations in run products were measured by secondary ion mass spectrometry. Observed nitrogen (15 N) solubilities in wadsleyite and ringwoodite typically range from 10 to 250 μg/g and strongly increase with temperature. Nitrogen solubility in bridgmanite is about 20 μg/g, while Ca-silicate perovskite incorporates about 30 μg/g under comparable conditions. Partition coefficients of nitrogen derived from coexisting phases are D N wadsleyite/olivine= 5.1±2.1, D N ringwoodite/wadsleyite= 0.49±0.29, and D N bridgmanite/ringwoodite= 0.24 (+ 0.30/− 0.19). Nitrogen solubility in the solid, iron-rich metal phase coexisting with the silicates was also measured and reached a maximum of nearly 1 wt.% 15 N at 23 GPa and 1400° C. These data yield a partition coefficient of nitrogen between iron metal and bridgmanite of D N metal/bridgmanite∼ 98, implying that in a lower mantle containing about 1% of iron metal, about half of the nitrogen still resides in the silicates. The high nitrogen solubility in wadsleyite and ringwoodite may be responsible for the low nitrogen concentrations often observed in ultradeep diamonds from the transition zone. Overall, the solubility data suggest that the transition zone and the lower mantle have the capacity to store at least 33 times the mass of nitrogen presently residing in the atmosphere. By combining the nitrogen solubility data in minerals with data on nitrogen solubility in silicate melts, mineral/melt partition coefficients of nitrogen can be estimated, from which the behavior of nitrogen during magma ocean crystallization can be modeled. Such models show that if the magma ocean coexisted with a primordial atmosphere having a nitrogen partial pressure of just a few bars, several times the current atmospheric mass of nitrogen must have been trapped in the deep mantle. It is therefore plausible that the apparent depletion of nitrogen relative to other volatiles in the near-surface reservoirs reflects the storage of a larger reservoir of nitrogen in the solid Earth. Dynamic exchange between these reservoirs may have induced major fluctuations of bulk atmospheric pressure over Earth's history.