The effect of CaO, Cr 2O 3, Fe 2O 3 and Al 2O 3 on the stability of spinel and garnet peridotites

Abstract

The reaction pyroxene+spinel⇌garnet+olivine defines the boundary between low pressure spinel peridotite and high pressure garnet peridotite. Experimental determination in the four component system MgOCaOAl 2O 3SiO 2 indicates that the CaO content is critical in defining the position of the reaction boundary. Where orthopyroxene is the only pyroxene present, increasing the CaO content decreases the pressure at which the reaction occurs; where both pyroxenes are present changing the CaO content has no effect; and where clinopyroxene is the only pyroxene present, increasing the CaO content increases the pressure of the reaction. Experiments on analyzed clinopyroxenes, orthopyroxenes and spinels from ultramafic nodules, indicate that the reaction boundary moves to higher pressures with increasing Cr 2O 3 and Fe 2O 3, and decreasing Al 2O 3 contents. Variations in the trivalent oxide ratio may result in changes of the equilibrium reaction by as much as 10 kilobars. The results indicate that the composition of the mantle is critical in defining the position of the spinel-to-garnet-peridotite boundary, and leaves open the possibility that spinel and garnet peridotite may coexist throughout a wide depth within the earth's upper mantle.