Geochemistry of ODP Hole 135-839B lavas

Four petrographic lava types occur, ranging from aphyric to moderately phyric clinopyroxene-olivine tholeiitic basalts (Unit 1); olivine-clinopyroxene picritic basalts, sparsely to strongly olivine-phyric (Unit 3-type); olivine-clinopyroxene basalts (clinopyroxene dominant) (Unit 4); and moderately to strongly phyric two-pyroxene-plagioclase basaltic andesites (Unit 9-type). The olivine phyric lavas contain forsteritic olivines (extending to Fo92), and very magnesian Cr-rich spinels similar to those occurring in boninitic lavas. The basaltic andesites are mineralogically and petrographically indistinguishable from the modern Tofua Arc basaltic andesites, one notable feature being the highly calcic cores in plagioclase phenocrysts (up to An95). The forsteritic olivines, the Cr-spinels, and the calcic plagioclases are unlikely to have been precipitated in the lava compositions in which they occur, and are thought to have been incorporated from highly primitive melts by way of mixing processes (as advocated by Allan, this volume). Notwithstanding the evidence for mixing, the major element chemistries of the Unit 1- and Unit 9-type lavas are shown to be consistent with the derivation of the Unit 9-type basaltic andesites by means of fractional crystallization, through magmas of similar chemistry to Unit 1. Some trace element discrepancies in the modeling, and the relative volcanic stratigraphy of Site 839, however, preclude a direct liquid line of descent between the actual recovered units. […]