A new method for the generation of N2-containing fluids in high-pressure experiments

A new method is described which permits the generation of a pure Nz fluid phase in high-pressure experiments. The method is based on the thermal decomposition of silver azide (AgN3) to metallic silver (Ag) and molecular nitrogen (N2). Buffering of oxygen fugacities is not required to obtain the Nz fluid. By the introduction of suitable amounts of silver azide, water, silver oxalate and silver oxide, any mixture of N2, H20, COz, and O2 can be generated in the sampie capsules. Key-words : nitrogen, high-pressure, experimental petrology, fluid phase, silver azide. Numerous investigations of fluid inclusions during the last twenty years have revealed a remarkably wide variety of chemical composi­ tions of fluid phases in the Earth's crust and mantle. Some of these phases cannot be fully represented in the C-H-O system, but contain molecular nitrogen as an additional component or even as nearly pure Nz. Therefore, in some cases it may be necessary to investigate petro­ logical phase equilibria in the presence of fluid phases which contain Nz. Hitherto, no method was available for the generation of a pure Nz fluid inside the inert met al capsules conventionally used in high­ pressure experiments. Holloway and Reese (1974) obtained ternary Nz-COz-HzO fluids by the decomposition of organic nitrogen com­ pounds such as guanidine nitrate or ammonium oxalate in the presence of oxygen buffers. In addition, Nz-Oz mixtures can be generated by heating several nitrates. However, the fluid phase compositions accessible by these * Present address: California Institute of Techno­ logy, Division of Geological and Planetary Sciences 170-25, Pasadena CA 91125, U.S.A. 001:1 0.1127/ejm/01 /1 /0135 methods are very restricted by the stoichiome­ try of the fluid-gene rating compounds. In contrast, the method described here allows the generation of a pure Nz atmosphere in the respective samples. Beyond that, by addition of water, silver oxalate or silver oxide, respectively, any mixture of Nz with HzO, COz and Oz can be obtained. For the generation of these fluid phases, no extern al oxygen buffer is required as in the method of Holloway and Reese (1974). Only in the case of long-term runs, or if platinum capsules are used, is buffe­ ring necessary to prevent changes of the fluid phase composition which can arise from hydro­ gen diffusion through the walls of the sample capsule. Therefore, the new technique descri­ bed he re is generally superior to the older method of Holloway and Reese.