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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Cooling rate responsiveness of pyroxene geothermometry

Junji Yamamoto, Hidemi Ishibashi, Koshi Nishimura
Geochemical Journal, Vol. 51, No. , P. 457-467, 2017

ABSTRACT

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.

KEYWORDS

two-pyroxene thermometer, mantle xenolith, chemical zoning, cooling-rate responsiveness, numerical simulation

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