Abstract
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.
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