The stability and equation of state of majoritic garnet synthesized from natural basalt at mantle conditions

The high-pressure stability and equation of state of majoritic garnet synthesized from natural basalt have been determined using the laser-heated diamond cell and x-ray diffraction. Natural mid-ocean ridge basalt (MORB) and tholeiitic basalt transform from a garnet-dominated mineral assemblage to a silicate perovskite-dominated mineralogy near 42 (±3) GPa. Static compression data (at 300 K) for the majoritic garnet synthesized from the natural tholeiitic basalt yields an isothermal bulk modulus, K0T, of 226.2 (±9.3) GPa with dK0T/dP constrained to be 4. An equally acceptable fit to the data is obtained if K0T is constrained to be 180 GPa and dK0T/dP is 7 (±1). Using our new equation of state and phase stability results for basaltic (majorite-structured) garnet, we calculate density models for the subducting oceanic crust at mantle pressures and temperatures, and find that the basaltic crust is 0.4 g/cm³ (9%) less dense than the lower mantle at 660 km depth. If we assume that the subducted crust can delaminate from the underlying lithosphere, a simple buoyancy model for the crustal portion of subducting slabs indicates that basaltic crust may not sink below 660 km in depth.