11:45 AM - 12:00 PM
[SMP36-05] Origin of Moho
Keywords:Moho, ophiolite, dunite, hydrous melting of mantle, mid ocean ridge, subduction zone
The DTZ is considered either as a pile of cumulates or as residual peridotite leached by melts (e.g., Abily & Ceuleneer, 2012). However, we present here the third hypothesis, which should be examined by the Oman Drilling Project and ChikyuOman campaign that describe the cores on board RV Chikyu, that is, the unusually thick dunite is produced by hydrous melting of mantle peridotite at low pressures in the divergent plate boundaries. At lower pressure and in hydrous conditions, the liquidus field of forsterite expands relative to that of enstatite and enstatite melts incongruently to produce dunites. The in-situ melt should be magnesian andesitic in composition, which will mix with dominant high pressure basaltic melts coming below before reaching to the seafloor. Rospabe et al. (2017, Geology 45) suggested involvement of a hydrous melt in the genesis of the DTZ, and synmagmatic faults (Abily, Ceuleneer, & Launeau, 2011, Geology 39) could be main avenues for seawater penetration down to the Moho. Koepke et al. (2005, Terra Nova, 17; 2007, CMP, 153) suggested that seawater caused H2O-saturated melting of lower crustal gabbro. Koepke & Feig (2006, abstract of 11th EMPG, Bristol) showed that chromitites could be also produced by a fluid-induced partial melting of harzburgite at MOHO level. The availability of water should be variable along the mid-ocean ridges, resulting in the variable thickness of the DTZ.
Shallow melting of hydrous mantle could be responsible for both the advent of continents in the subduction zones (Tamura et al., 2016, Scientific Reports 6) and the genesis of Moho in the oceans.