An experimental investigation of the stability of majoritic garnet in the Earth’s mantle and an improved majorite geobarometer

Abstract

The stability of the majorite component in garnet has been experimentally investigated at high pressure and high temperature, focusing on the effect of bulk composition and temperature. High-pressure experiments were performed in a multi-anvil apparatus, at pressures ranging from 6 to 14.5 GPa, and temperatures between 1400 and 1700 °C. Experiments were performed in a range of bulk compositions in the system SiO2–Al2O3–Cr2O3–CaO–MgO with varying Cr/(Cr + Al) ratios. The majorite content of garnet gradually increases with pressure, and the composition of the garnet, specifically the Cr/(Cr + Al) ratio, exerts a significant effect on the majorite substitution. We found no significant effect of temperature. We use the experimental results in combination with the literature data to derive two empirical geobarometers, which can be used to determine the equilibration pressure of natural majoritic garnets of peridotitic and eclogitic bulk compositions. The barometer for peridotitic compositions is $${\text{P}} = - 77.1 + 27.6 \times {\text{Si}} + 1.67 \times {\text{Cr}}$$ And the barometer for eclogitic compositions is $${\text{P}} = - 29.6 + 11.8 \times {\text{Si}} + 7.81 \times {\text{Na}} + 4.49 \times {\text{Ca}}.$$