cpxwing log entry 001

Mushrooms and clinopyroxene –
a good combination?

This is Simon’s blog reporting on the development of a new single-clinopyroxene HPx-EOS.

The new model is designed to replace pre-existing versions that do not currently overlap in P—T—x space.

So far three clinopyroxene EOS exist. Ordered diopside—omphacite—jadeite (Green et al., 2007) is for coexisting sodic—calcic clinopyroxenes. It accounts for order—disorder on the octahedral M1 and M2 sites, but not on the tetrahedral T sites. It is appropriate for high pressure but unsuitable for high temperatures.

Crystallography of diopside
(from Wikimedia commons)

The high-temperature augite model of Green et al. (2016) takes care of ordering on the T sites, but not on the M sites. It cannot reproduce coexisting pyroxenes that are separated by a solvus. It should only be used where ordering on the M sites is subordinated, at high temperature.

The latest model of this family is part of the ‘igneous set’ (Holland et al., 2018) with a similar structure to the augite model. It is applicable over a large P—T—x range, including upper mantle conditions. Minor components such as K, Ti and Cr are also considered in this model. However, it is not able to reproduce solvi.

We are trying to take the strengths of these three sets of a—x relationships and combine them into a single monster-model taking care of ordering on both the M and T sites, including components that are important at mantle conditions. Our new model will be capable of simulating coexisting clinopyroxenes at low temperature and to incorporate tetrahedral Al. Consideration of K, Ti and Cr also make the model appropriate to simulate mantle-related equilibria.

Subsets of the full model are going to replace the current sets to allow for consistent and thermodynamically-equivalent calculations.

The new model (project name ‘cpxwing’) will therefore allow to explore phase relations in a variety of geodynamic settings. Calculations for peridotites, subduction zone metamorphism, high-pressure mafic granulites and eclogites up to upper mantle conditions will be possible with cpxwing.