Decompression of majoritic garnet: an experimental investigation of mantle peridotite exhumation

Garnet peridotites containing relict majoritic garnet with abundant lamellae and interstitial blebs of pyroxene were recently discovered within mantle xenoliths and ultra high-pressure metamorphic terranes, but experimental reproduction of such microstructures has not yet been carried out. This chapter presents preliminary experimental results of our studies of the microstructures associated with supersilicic (majoritic) garnet high-temperature decompression (T = 1400°C)from 14 to 13, 12 and 7 GPa, and from 8 to 5 GPa. Experiments were performed in a Walker-style multi-anvil apparatus on a natural mineral mix corresponding to the bulk chemistry of garnet peridotite. We produced exsolution lamellae of Mg2SiO4 and interstitial blebs of diopside in the run products decompressed from 14 to 13 GPa and from 14 to 12 GPa at T = 1400°C. Blebs of both interstitial diopside and enstatite were formed during a slow high-temperature decompression from 14 to 7 GPa. Only blebs of interstitial enstatite were exsolved from majoritic garnet during decompression from 8 to 5 GPa at the same temperature. Mg2SiO4 has not been reported as an exsolution product in natural ultra high-pressure garnet peridotite, nor did we observe it in our experimental charges at pressures consistent with those inferred from studies of mantle xenoliths and orogenic belt garnet peridotites (7–10 GPa). The observation that Mg2SiO4 and diopside may be exsolved during decompression at higher pressures is consistent with expansion of the garnet field at the expense of wadsleyite at P > 13 GPa in Ringwood's diagram. The new experimental results may serve as a template for interpretation of microstructures in natural garnet peridotites, providing a better understanding of the depth from which such rocks have been exhumed.