Experimental Studies of the Phase Diagram Leucite – Nepheline – Diopside under 4.0GPa and High Temperatures

Abstract:

One of the most important heat sources for the Earth´s interior is the radioactive decay of radiogenic elements, mainly ²³⁵U, ²³⁸U, ²³²Th and ⁴⁰K radionuclides. However, our planet emits much more heat than that expected for the energy produced by the calculated concentration of these elements in the Earth's Mantle, even if we consider solar radiation and planetary accretion energy too. Such data suggest that the concentration of all these elements, or some of them, is underestimated and several authors suggest that some of these elements could be enriched even in the Earth’s core, despite their lithophyle characteristics. In this study, we focus on the potassium behavior, concentration in the mantle and we aim to find stable mineral phases under high pressure and temperature, able to keep potassium (and by consequence its radioactive ⁴⁰K isotope) and water in their structure in the mantle conditions. In such way, we will be able to better understand the role of potassium in the mantle as a heat source to the Earth’s interior. We conducted experimental runs in which synthetic vitreous samples, stoichiometrically equivalent to different concentrations of leucite, nepheline and diopside, are processed in a 1000 tonf hydraulic press, under 4.0 GPa (equivalent to 120 km Earth deep) and temperatures up to 1400 °C. As run products, we obtained euhedral minerals in equilibrium with a liquid (melt), simulating a potassium enriched mantle environment. The samples are analyzed by XRD, SEM-EDS and EPMA techniques, and the produced data is used to construct the “Leucite-Nepheline-Diopside under 4.0GPa and dry conditions” ternary phase diagram. Preliminary semiquantitative data (EDS), plotted in the diagram, show that clinopyroxene keeps up to 2wt% of K₂O in its structure in absence of potassic phases and in the presence of nepheline. The amount of K₂O decreases to 0,1wt% if kalsilite is present, which is the potassic stable phase in the experiment conditions. Compared to previous studies of this diagram under atmospheric conditions, forsterite field disappear and clinopiroxene stability field show a substantial growth under 4.0 GPa, while nepheline and kalsilite stability fields are partially suppressed. Data also show that wollastonite is well crystallized in the experimental conditions, despite the absence of CO₂.