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.