Phase Relations in Spinel Lherzolite KLB-1 According to Results of Thermodynamic Modeling up to 30 GPa: Peculiarities of Mineral Assemblages and Geodynamic Effects

Abstract

Abstract: The composition of spinel lherzolite xenolith KLB-1 from the Kilbourne Hole volcanic crater, United States, which is close to the composition of the Earth’s primitive mantle, was used for thermodynamic modeling of phase relations in the Na2O–CaO–FeO–MgO–Al2O3–SiO2 system (NCFMAS) using the Perple_X software package in the temperature range of 900–2000 °C and pressures of 0.0001–30 GPa. The calculated phase diagram is in good agreement with published thermodynamic data on KLB-1 composition and reveals the peculiarities of mantle mineral assemblages at P–T parameters on which experimental data are insufficient or absent. The results showed that the mineral assemblage of garnet wehrlite (garnet + olivine + clinopyroxene), the least common type of mantle peridotite on the Earth’s surface, prevails in the upper mantle since the Archean. Mineral assemblage of garnet lherzolite (garnet + olivine + clinopyroxene + orthopyroxene), which is a variety of mantle peridotites most widely found on the Earth’s surface, is formed in the lithospheric mantle because its temperatures are lower than those of the convecting mantle. Thermodynamic modeling reveals a ringwoodite-free field in the P–T diagram (located at the bottom of the mantle transition zone), which is crosscut by Archean adiabats and by the geotherms of Archean and the hottest Phanerozoic plumes. This area causes a change, from negative to positive, in the slope of the boundary between the lower mantle and the mantle transition zone. A positive slope of the boundary in the Archean should have stimulated the ascend of lower mantle plumes through the transition zone. Conversely, this boundary has a negative slope for most Phanerozoic plumes, rising from the lower mantle, and as a result, the plumes either slow down or stop.