Mineral chemistry of the chromian spinels and chromitites in mafic and ultramafic rocks sampled from oceanic floor
Abstract:Chromite or chromian spinel (simply called “spinel” after this) is common a mineral in mafic and ultramafic deep-seated rocks. The chemistry, especially Cr/(Cr+Al) ratio and Mg/(Me+Fe2+) ratio, is well known index of degree of melting of the mantle source. The chromitite composed of high density of spinel is explained as a reaction product between different degrees of fractionation of basaltic melts (Arai and Yurimoto, 1994). Therefore, tracing spinel chemistry helps to reveal complex processes of the petrogenesis.
Contrary to its common occurrence in ophiolites, there are few chromitite samples reported from the ocean floor, although spinel are very common mineral in the mafic and ultramafic samples. A chromitite “minipod” included in a troctolite with a hybrid origin of melt-impregnated dunite was reported from Hess Deep at the junction of East Pacific Rise and Galapagos Rift (Site 895, ODP Leg 147; Arai and Matsukage, 1996). A Spinel-rich seam (2-3 cm thick) in dunite at Atlantis Bank, Southwestern Indian Ridge was sampled during a dive investigation using by the manned submersible Shinkai 6500 (Morishita et al., 2007) and some dredged samples (Payot et al., 2014). A few samples are reported from Mid-Atlantic Ridge, Site 1271 during ODP Leg 209 (Abe, 2011). Some of them show high temperature metamorphism up to the mid-amphibolite facies. Here shows the comparison of chromitites and spinel in the different ocean and will be assumed the petrogenesis.