A study on evolultion of the Earth's atmosphere-mantle system based on nitrogen geochemistry.

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In order to understand the evolition of the Earth's atmosphere-mantle system based on nitrogen, I conducted experiments on pertitioning of nitrogen and noble gases between (A) gas-silicate melt and (B) soild (cpx)-silicate melt. Experiment (A) was conducted under various conditions : T=1300-1600ﾟC,P(N_2)=0.5-1000 atm f(O_2)=IW-2 to IW+10 (logarithmic scale). The results show that (1) concentration of dissolved nitrogen is linearly correlated with P(N_2)(i.e., Henry's Law is obeyed), (2)nitrogen solubility (Henry's constant) has very weak dependence on temperature (3) and on f(O_2), and is approximately equal to that of Ar. All these observations suggest physical dissolution of nitrogen (i.e., dissolution as nitrogen molecules) in basaltic melt in the present experimental conditions. This is also suppoeted by the observation that almost no isotopic exchange occured between ^<15>N^<15>N-enriched nitrogen and normal nitrogen (^<14>N^<14>N-dominated) during dissolution nd extraction of nitrogen in and from the basaltic melt (glass) synthesized under a highly oxydizing condition. In experiment (B), a clinopyroxene (cpx)-basaltic melt pair wes synthesized using a piston-cylinder-type apparatus at 15kbar and 1350-1270ﾟC.The obtainned cpx/melt partition coefficients are 0.06 for nitrogen and 0.11 for argon, not significantly different with each other. The results of the two experiments imply that magmatic processes may not produce large fractionation between nitrogen and argon. Therefore, the two orders of magnitude difference in the N/^<36> Ar ratios of the atmosphere (-10^4) and the mantle (-10^6) must be the result of some processes other than the magmatic ones.