Evidence of majoritic garnet from the mantle transition zone in the Alpe Arami garnet peridotite

Abstract

ABSTRACT Ultra-high-pressure (UHP) peridotites found along collisional zones record rare information from deep within the Earth. However, the estimation of depth of origin for these UHP rocks has been controversial. A major controversy remains related to the conjectural proposition of mantle transition zone (410–660 km) origin of the Alpe Arami (AA) garnet peridotite massif in the Swiss Alps. In this contribution, we show micro-textural evidence of precursor majoritic garnet by documenting exsolved rutile, high-Al orthopyroxene, jadeite-rich clinopyroxene and olivine within the AA garnets in this peridotite. We also document an unforeseen texture of olivine with ‘necklace’ like enstatite corona in the kelyphite formed after decomposition of relict garnet. These olivines bear FeTiO3 and Cr-spinel exsolution needles indicating retrogression from high-pressure Mg2SiO4. Thus, the occurrence of retrogressed high-pressure Mg2SiO4 with enstatite corona in kelyphite suggests majorite breakdown to precipitate high-pressure Mg2SiO4 near mantle transition zone (MTZ) depth. The SiO2 released during decompression of majoritic garnets reacts with the high-pressure Mg2SiO4 to produce the enstatite corona. Our documented micro-textures show high-pressure Mg2SiO4 are breakdown product of precursor majoritic garnet, indicating that these micro-textures of the AA peridotite massif are sourced from the mantle transition zone (MTZ).