Experimental investigation on the exsolution of pyroxene from a polycrystalline majoritic garnet at high pressure and temperature

Garnet containing exsolved pyroxene is generally recognized as the breakdown product of former majoritic garnet formed in the deep upper mantle. In the last two decades, former majoritic garnet has been recognized in various natural ultrahigh-pressure metamorphic rocks representing the exhumed remnants of past subduction zones or mantle from depth greater than 200-300 km. The most convincing evidence for former majoritic garnet has been only the strong crystallographic topotaxy between oriented pyroxene rods and garnet host. However, such a crystallographic topotaxy is not clear for interstitial pyroxene, which is more important for the determination of depth origin of former majortic garnet due to its relatively large volume comparing to exsolved intracrystalline pyroxene. To characterize quantitatively the decompression of majortic garnet, we have conducted systematic annealing experiments at 5-12 GPa and 1000-1400 C with a polycrystalline majoritic garnet (Si=3.21, grain size = 20-30 um) synthesized at 15 GPa and 1400 C . The experimental results show that interstitial pyroxene precipitation is an earlier stage exsolution from majoritic garnet at higher pressures comparing to intracrystalline precipitation. As revealed by EBSD analyses, there is no crystallographic topotaxy between the interstitial pyroxene precipitations and the garnet grains around it and between the intracrystalline pyroxene precipitations and the host garnet. However, many intracrystalline pyroxene precipitations have the same crystallographic orientation, indicating a possible exsolution origin. The similarity in exsolution microstructure between our experimental specimens and natural majoritic garnet samples suggests that interstitial precipitation and intracrystalline precipitation are two different stages of exsolution. The lack of a clear crystallographic topotaxy in experimental specimens between the pyroxene precipitations and the garnets is ascribed to the recrystalization of pyroxene under high temperature conditions.