Composition of melts and minerals from volcanic rocks of the Kuril-Kamchatka Arc

In this paper allivalites, coarse- and giant-textured olivine-anorthite rocks occurring as separate blocks in the eruption products of many volcanoes from the frontal part of the Kuril-Kamchatka Arc are under consideration. New data are reported on petrography, mineralogy, and composition of melt inclusions in minerals from ten allivalite samples collected at Ksudach, Ilinsky, Zavaritsky, Kudryavy, and Golovnin Volcanoes. Crystallization temperatures of allivalite minerals were estimated as 970-1080°C at melt water content of 3.0-3.5 wt % and oxygen fugacity NNO=1-2. Genetic connection was established between compositions of melt inclusions and interstitial glasses in allivalites and volcanic rocks. Cumulate nature of allivalites was demonstrated. Using mass balance calculations, degree of fractionation of primary melts during formation of cumulate layers was estimated for various volcanoes as 22-46%.

Chemical and isotopic compositions of sediments and some minerals from the Derugin Basin, Sea of Okhotsk

The paper reports specific mineralogical and geochemical characteristics of deposits from local depressions of the Derugin Basin. They were formed in an environment with periodic changes from oxic to anoxic conditions and show evidence for presence of hydrogen sulfide in bottom waters. Deposits of this type can be considered as a modern model for ancient ore-bearing black shale associations. Compared with typical metalliferous black shale sequences, which are characterized by high contents of organic matter, the sediments described here are depleted in elements of the organophilic association (Mo, Ni, Cu, Zn, V, and U), but have higher Mn contents.

Geochemistry of selected minerals at DSDP Leg 65 Holes

The majority of the basalts drilled on Leg 65 in the Gulf of California are aphyric to sparsely phyric massive flows ranging in average thickness between 5 meters in the upper part of the sections in Holes 483 and 483B, where they are interlayered with sediment, and 14 meters in Hole 485A, where interlayered sediments constitute more than half of the section. Massive flows interlayered with pillows are generally less than 4 meters thick. The pillow lavas recovered are more phyric (up to 15 modal%) and contain two to three generations of plagioclase and olivine ± clinopyroxene. Plagioclase generally exceeds 60% of any given phenocryst assemblage. Resorbed olivine, clinopyroxene, and plagioclase megacrysts may reflect a high-pressure stage, the phenocrysts crystallizing in the main magma chamber and the skeletal microphenocrysts in dikes. Precise measurements of length/width ratios of different phenocryst types and compositions show low aspect ratios and large crystal volumes for early crystals and high ratios and low volumes for late crystals grown under strong undercooling conditions. The minerals examined show wide ranges in composition: in particular, plagioclase ranges from An92 to An36; clinopyroxene ranges from Ca41Mg51Fe8 in the cores of phenocrysts to Ca40**36 Mg45**49Fe15**20 in the groundmass; and olivine ranges from Fo86 to Fo81. The wide range in mineral compositions, together with evidence of disequilibrium based on textures and comparisons of glass and mineral compositions, indicate complex crystallization histories involving both polybaric crystal fractionation and magma mixing.

(Table 1) Percentage of non-opaque heavy minerals, heavy residue, magnetite, and opaque heavy minerals, DSDP Sites 58-445 and 58-446

Heavy-mineral analyses were made for 39 samples, 27 from DSDP Site 445 and 12 from Site 446. About one-fourth of the samples were so loose that they were easily disaggregated in water. The amount of heavy residue and the magnetite content of the heavy fraction were very high, 0.2 to 44 per cent and (on the average) more than 20 per cent, respectively. Among the non-opaque heavy minerals, common hornblende (0 to 80%) and augite (0 to 98%) are most abundant. Pale-green and bluish-green amphiboles (around 10%) and the epidote group (a few to 48%) are next in abundance. Euhedral apatite and biotite and irregularly shaped chromite are not abundant, but are present throughout the sequence. Hacksaw structure is developed in pale-green amphibole and augite. At Site 445, a fair amount of chlorite and a few glauconite(?) grains are present from Core 445-81 downward. The content of common hornblende and opaque minerals also changes from Core 445-81 downward. A geological boundary may exist between Cores 445-77 and 445-81. Source rocks of the sediments at both sites were basaltic volcanic rocks (possibly alkali suite), schists, and ultramafic rocks. The degree of lithification and amount of heavy residue, and the content of magnetite, non-opaque heavy minerals (excluding mafic minerals), and mafic minerals in the cores were compared with Eocene, Oligocene, and Miocene sandstones of southwest Japan. In many respects, the sediments at Sites 445 and 446 are quite different from those of southwest Japan. From the early Eocene to the early Miocene, the area of these sites belonged to a different geologic province than southwest Japan.