Mineral composition of mid-ocean-ridge basalts from ODP Leg 187 sites

The chemical compositions of olivine, plagioclase, pyroxene, and spinel in lavas collected during Ocean Drilling Program Leg 187 in the Australian Antarctic Discordance, Southeast Indian Ridge (41°-46°S, 126°-135°E) were analyzed, and modeling of the theoretical equilibrium petrogenetic conditions between olivine and melt was conducted. The cores of larger olivine phenocrysts, particularly in the isotopic Indian-type mid-ocean-ridge basalt (MORB), are not equilibrated with melt compositions and are considered to be xenocrystic. Larger plagioclase phenocrysts with compositionally reversed zonation are also xenocrystic. The compositions of primary magma were calculated using a "maximum olivine fractionation" model for primitive MORB that should fractionate only olivine. Olivine compositions equilibrated with calculated primary magma and compositions of calculated primary magma suggest that (1) isotopic Pacific-type MORB is more fractionated than Indian-type MORB, (2) Pacific-type MORB was produced by higher degrees of partial melting than Indian-type MORB, and (3) primary magma for Indian-type MORB was segregated from mantle at 10 kbar (~30 km depth), whereas that for Pacific-type MORB was segregated at 15 kbar (~45 km depth).

Grain sizes and mineral composition of the basement of ODP Site 206-1256

Hole 1256C was cored 88.5 m into basement, and Hole 1256D, the deep reentry hole, was cored 502 m into basement during Ocean Drilling Program Leg 206. Hole 1256D is located ~30 m south of Hole 1256C (Wilson, Teagle, Acton, et al., 2003, doi:10.2973/odp.proc.ir.206.2003). A thick massive flow drilled in both holes, Units 1256C-18 and 1256D-1, consists of a single cooling unit of cryptocrystalline to fine-grained basalt, interpreted as a ponded lava, 32 m and at least 74.2 m thick, respectively. This ponded flow gives us a unique opportunity to examine textural variations from the glassy, folded crust of the lava pond recovered from the top of Unit 1256C-18 through the coarse-grained, thick massive lava body to the unusually recrystallized and deformed base cored in Unit 1256C-18. Some detailed descriptions of the textures and grain size variations through the lava pond (Units 1256C-18 and 1256D-1), with special reference to the recrystallization of the base of Unit 1256C-18, are presented here.

Chemical composition of rocks and minerals from the Doldrums and Arkhangelsky Fracture Zones

The book deals with results of complex geological and geophysical studies in the Doldrums and Arkhangelsky Fracture Zones of the Central Atlantic. Description of the main features of bottom relief, sediments and crustal structure, geomagnetic field, composition of igneous and sedimentary rocks are given in the book. The authors made conclusions on tectonic delamination of the oceanic crust and existence of specific rock complexes forming non-spreading blocks