Chemical composition of hydrous and other minerals and their chromian spinel hosts in chromitite and troctolite from the Hess Deep

Primary hydrous and other minerals enriched with incompatible components were found in ocean-floor ultramafic-mafic plutonic rock suites recovered from two contrasting ridge systems, i.e., the East Pacific Rise (Hess Deep, equatorial Pacific), a typical fast-spreading system, and Mid-Cayman Trough, a typical slow-spreading system. They are characteristically associated with chromian spinel, enriched with Cr, one of compatible elements. The hydrous minerals can be formed through interaction between depleted mid-ocean ridge basalts (MORBs) and oceanic peridotite. Primary MORB produced in the deeper part inevitably react with shallower mantle peridotite; the magma selectively dissolves orthopyroxene with simultaneous olivine precipitation. Chromium is supplied to the melt from orthopyroxene, which is enriched with Cr over Al relative to ordinary basaltic melts. The effects of zone refining are also important for concentrations of the incompatible components, especially H20, Na, and Ti, in the modified magma, which in the extreme case is able to precipitate hydrous minerals. This mechanism is common to both fast- and slow-spreading ridges, and is more effective in stagnant or failed melt conduits. Some ultramafic rocks from upper mantle or transition-zone members of ophiolites have primary hydrous minerals, usually included by chromian spinel. Despite the often a priori assumption that slab-derived components are necessary for the formation of the chromitite with inclusions of primary hydrous minerals, this is clearly not necessary.