COMPOSITIONAL VARIATION OF OLIVINE-CHROMIAN SPINEL IN MG-RICH MAGMAS AS A GUIDE TO THEIR RESIDUAL SPINEL PERIDOTITES

Peridotite restites or last equilibrated peridotites for main magma groups are estimated on the basis of compositional relations between olivine (Fo content) and chromian spinel (Cr# = Cr/ (Cr + Al) atomic ratio). A particular magma or magma group develops a particular Fo-Cr# variation trend (= Fo-Cr# fractionation line) during magmatic differentiation. A restite for the magma can be assessed by extrapolating the Fo-Cr# fractionation line back to the olivine-spinel mantle array (= a Fo-Cr# trend for residual mantle peridotites). Cr-#'s of chromian spinel in peridotite restites for magma types are estimated as follows: 0.2-0.6 (mostly 0.4-0.6) for MORB, 0.2-0.5 for intraplate alkali basalts, ca. 0.7 for intraplate tholeiites, ca. 0. 8 for oceanic plateau basalts, 0.3-0.5 (mostly 0.4-0.5) for back-arc basin basalts, > 0.9 for boninites, ca. 0.9 for high-Mg arc tholeiites and high-Mg andesites, 0.1-0.9 (mostly 0.1-0.7) for Quaternary arc basalts of the Northeast Japan arc, and 0.7-0.8 for magmas which produced Precambrian layered intrusions. Arc magmas from a single-arc system can produce various peridotite restites within the upper mantle, which is due to a wide range of melting conditions (depth and degree of melting, hydrous to anhydrous melting) and of source peridotites (fertile to refractory).