Addition of Fe3+ promoting rapid dehydration for zoisite at subducting plate into the asthenosphere

Zoisite is an important hydrous mineral in subduction oceanic crust. The dehydration effect of Fe3+/Al3+ substitution in hydrous minerals to deep-water transport process remains a globally interesting topic but poorly studied. Using an externally heated diamond anvil cell (DAC) system, the behavior of Fe-bearing zoisite during plate subduction was simulated at high temperature and high pressure (HT/HP). Raman spectroscopy was employed to measure the -OH groups in the zoisite structure. The integral absorbance of the Raman vibrational peaks of -OH under varying temperature and pressure environments is used to indicate the water content of zoisite during subduction. It was found that the incorporation of Fe3+ by substituting Al3+ caused zoisite exhibiting a two-stage dehydration. Under conditions of P/T less than 3.3GPa/823K, H+ migrates within the structure (from the AlM3-M12 environment to the FeM3-M12 environment), resulting in slow dehydration. However, after exceeding this temperature and pressure, H+ rapidly escapes from the structure, leading to rapid dehydration. These temperature and pressure environments correspond to a depth of 120 km in the upper asthenosphere, indicating that Fe3+→Al3+ substitution can reduce the amount of water released from zoisite before it reaches the asthenosphere. Once entering the asthenosphere, zoisite begins to dehydrate rapidly at a rate equivalent to 4.5 times that before. Although zoisite (XFe=0.10) can survive near the mantle transition zone along cold subduction zones, more than 94% of its water will be releases when it leaves the asthenosphere. The amount of water transported into the mantle transition zone is less than 6%. The residual zoisite after undergoing partial melting, may contribute to the formation of low-water silicate melts in the low-velocity layer (LVL).