Abstract
The mineral composition of oceanic crust is a function of pressure and temperature and thus changes as a slab of crust descends a subduction zone. A knowledge of the mineralogy of subducted material would provide important constraints on its geophysical characteristics. We have sought, by means of high pressure laboratory experiments (in the pressure range 4.6–18 GPa) on a synthetic mix with the composition of mid-ocean ridge basalt (MORB), to determine the sequence of mineralogical changes. We find that at 1,200°C, solid solution of pyroxene in garnet begins around 10 GPa, and increases substantially between 12 and 15 GPa. Complete transformation of eclogite to garnetite is achieved at 15 GPa. The garnetite has a zero-pressure density of 3.76 g cm−3 and accordingly would be denser than the surrounding mantle throughout its stability field, which extends to depths of ˜600 km. If subducted to greater depths, garnetite may transform to perovskitite.
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