A noble gas technique for the identification of mantle and crustal materials and its application to the Kuroko deposits.

Abstract

In this paper, we propose to use noble gas abundance patterns and isotopic ratios to distinguish between mantle and crustal origin of different types of geological samples. The Kuroko deposits were selected as our first application. The mantle, the crust, and the atmosphere have different noble gas abundance patterns and isotopic ratios. The relative abundances of He and Ne are high in the mantle. The heavy noble gases-Ar, Kr and Xe-are enriched in seawater and seawater-related products. The 3He/4He and 129Xe/130Xe isotopic ratios are usually higher for the mantle than for the atmosphere and the crust. These properties may be used to distinguish the source of geological samples. In this work, the abundances of Ne, Ar and Xe and the isotopic ratios of Ar and Xe were determined for 18 Kuroko ore samples. The concentrations of Ne, Ar, and Xe in the Kuroko ores are like those of magmatic rocks. The abundance pattern of noble gases in the Kuroko ores parallels that of mantle-derived samples. Distribution coefficients of noble gases were calculated from two submarine diabase samples that equilibrated with seawater. Assuming these solid-liquid distribution coefficients can be applied to Kuroko and other magmatic rocks, we calculated the noble gas concentrations of their hypothetical source fluids. The results were used to compute the relative contributions of magmatic source fluid and seawater to the Kuroko ore-forming fluid. Based on Xe data, the relative contributions of magmatic source and seawater to the Kuroko ore fluid are calculated to be 75% and 25%, respectively.