Precipitation of pyroxenes and Mg2SiO4 from majoritic garnet: simulation of peridotite exhumation from great depth.

Our experimental simulations of the exhumation path of mantle peridotites show that high-temperature (1400 °C) decompression of lherzolite from 14 to 13 and 12 GPa results in exsolution of interstitial blebs of diopside and Mg₂SiO₄ (wadsleyite) lamellae from majoritic garnet. At lower pressures (from 8 to 5 GPa, atT = 1400 °C) only enstatite exsolves as blebs at garnet boundaries. Continuous high-temperature decompression from 14 to 7 GPa produces zoned majoritic garnet containing blebs of exsolved pyroxenes inside garnet rims. No intracrystalline precipitation of pyroxene was observed in garnet, although such lamellae are found in some natural garnet peridotites. The explanation appears to be the three orders of magnitude difference in grain size between experimental and natural specimens. Our data suggest that Mg₂SiO₄ and diopside exsolutions reflect the deepest point of the exhumation path of garnet peridotites, whereas enstatite precipitation may be restricted to garnets with less majoritic component at shallower depths.Terra Nova, 16, 325–330, 2004 [ABSTRACT FROM AUTHOR]