Geochemical Journal, Vol. 51 (No. 6), pp. 457-467, 2017
Junji Yamamoto,1* Hidemi Ishibashi2 and Koshi Nishimura3
1The Hokkaido University Museum, Nishi 8, Kita 10, Kita-ku, Sapporo 060-0810, Japan
2Department of Geoscience, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ward, Shizuoka 422-8529, Japan
3Natural Science Laboratory, Toyo University, 5-28-20 Hakusan, Bunkyo-ku, Tokyo 112-8606, Japan
(Received December 10, 2016; Accepted March 2, 2017)
Geothermometry is fundamentally important to explore thermal processes within the Earth. An extremely popular geothermometer is the two-pyroxene thermometer, which is based on the temperature dependence of elemental partitioning between pyroxenes in a rock. This technique is ambiguous in terms of its responsiveness to change in the temperature of the system. We performed a numerical simulation of one-dimensional calcium diffusion in a clinopyroxene using Ca-Mg inter-diffusion coefficients. While applying the simulation to rock bodies with various temperature conditions and both heating and cooling rates, we investigated the time scale for the re-equilibration of elemental partitioning between enstatite and diopside. Those results enable us to evaluate the responsiveness of the two-pyroxene thermometer to change in temperature. For heating processes up to 1300°C, chemical zoning is not well developed at a heating rate faster than 10°C/yr because the duration for the diffusion is insufficient. In addition, at a heating rate 10−4°C/yr and >1200°C, the simulated diffusion profiles show no chemical zoning. This occurs because the chemical equilibrium between the pyroxenes is achieved via elemental diffusion. For cooling processes, a rock body will cool down to closure temperature before making an observable zoning at a high cooling rate such as 100°C/yr. In addition, no detectable zoning of Ca in clinopyroxene developed under high temperature (>∼1100°C) and a slow cooling rate (<∼10−4°C/yr) down to 700°C, properly reflecting temperature without detectable zoning of Ca. In contrast, for a rock body with detectable chemical zoning, it is difficult to ascertain the appropriate pyroxene temperature. Model diffusion profiles reflect conditions of changing temperature. Therefore, a graphic representation of diffusion profiles under various initial temperatures and different rates of temperature change would be useful to estimate the thermal history of rock bodies.
Key words: two-pyroxene thermometer, mantle xenolith, chemical zoning, cooling-rate responsiveness, numerical simulation