Aleutian tholeiitic and calc-alkaline magma series I: The mafic phenocrysts

Diagnostic mafic silicate assemblages in a continuous spectrum of Aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the Aleutian arc crust. Tectonic segmentation of the arc exerts a primary control on the variable mixing, fractional crystallization and possible assimilation undergone by the magmas. End members of the continuum are termed calc-alkaline (CA) and tholeiitic (TH). CA volcanic rocks (e.g., Buldir and Moffett volcanoes) have low FeO/MgO ratios and contain compositionally diverse phenocryst populations, indicating magma mixing. Their Ni and Cr-rich magnesian olivine and clinopyroxene come from mantle-derived mafic olivine basalts that have mixed with more fractionated magmas at mid-to lower-crustal levels immediately preceding eruption. High-Al amphibole is associated with the mafic end member. In contrast, TH lavas (e.g., Okmok and Westdahl volcanoes) have high FeO/MgO ratios and contain little evidence for mixing. Evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma. These lavas contain groundmass olivine (FO 40–50) and lack Ca-poor pyroxene. Aleutian volcanic rocks with intermediate FeO/MgO ratios are termed transitional tholeiitic (TTH) and calc-alkaline (TCA). TCA magmas are common (e.g., Moffett, Adagdak, Great Sitkin, and Kasatochi volcanoes) and have resulted from mixing of high-Al basalt with more evolved magmas. They contain amphibole (high and low-Al) or orthopyroxene or both and are similar to the Japanese hypersthene-series. TTH magmas (e.g., Okmok and Westdahl) contain orthopyroxene or pigeonite or both, and show some indication of upper crustal mixing. They are mineralogically similar to the Japanese pigeonite-series. High-Al basalt lacks Mg-rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite. The low pressure mineral assemblage of high-Al basalt results from crystallization at higher crustal levels.     

Tidal flushing of an estuarine embayment subject to recurrent dinoflagellate blooms

A rhodamine dye tracer study was conducted over eight tidal cycles to investigate mixing and tidal exchange processes in Perch Pond, a Cape Cod embayment subject to recurrent blooms of the toxic dinoflagellate, Gonyaulax tamarensis. Dye injected at the inlet to Perch Pond during flood tide became well-mixed within the pond in one day and was removed at an effective first order rate of 0.36 d^?1, equivalent to a 70% utilization of the maximum possible tidal exchange. This relatively high flushing efficiency can be attributed to a density-driven circulation within the pond, consisting of a subsurface inflow of high salinity dense water on the flood tide followed by removal of lighter surface layers through the shallow inlet during ebb tide. The formation of a frontal convergence near the inlet on flood tide is consistent with the observed distribution of G. tamarensis cysts and shelifish toxicity. It is also clear that phytoplankton like G. tamarensis, whose maximum growth rates approximate the rate of tidal flushing, can only bloom within the embayment by avoiding the outflowing surface waters. Mixing within the pond is probably less efficient and population losses greater during dry periods when the pond salinity is higher and the stratification weaker.     

Self-similar cataclasis in the formation of fault gouge

Particle-size distributions have been determined for gouge formed by the fresh fracture of granodiorite from the Sierra Nevada batholith, for Pelona schist from the San Andreas fault zone in southern California, and for Berea sandstone from Berea, Ohio, under a variety of triaxial stress states. The finer fractions of the gouge derived from granodiorite and schist are consistent with either a self-similar or a logarithmic normal distribution, whereas the gouge from sandstone is not. Sandstone gouges are texturally similar to the disaggregated protolith, with comminution limited to the polycrystalline fragments and dominantly calcite cement. All three rock types produced significantly less gouge at higher confining pressures, but only the granodiorite showed a significant reduction in particle size with increased confining pressure. Comparison with natural gouges showed that gouges in crystalline rocks from the San Andreas fault zone also tend to be described by either a self-similar or log-normal particle distribution, with a significant reduction in particle size with increased confining pressure (depth). Natural gouges formed in porous sandstone do not follow either a self-similar or a log-normal distribution. Rather, these are represented by mixed log-normal distributions. These textural characteristics are interpreted in terms of the suppression of axial microfracturing by confining pressure and the accommodation of finite strain by scale-independent comminution.     

Isotopic evidence for the origin of sulfur in the Herrin (no. 6) coal member of Illinois

The sulfur isotopic composition of the Herrin (No. 6) Coal from several localities in the Illinois Basin was measured. The sediments immediately overlying these coal beds range from marine shales and limestones to non-marine shales. Organic sulfur, disseminated pyrite, and massive pyrite were extracted from hand samples taken in vertical sections.The $\text{δ}{}^{34}\text{S}$ values from low-sulfur coals (< 0.8% organic sulfur) underlying nonmarine shale were +3.4 to +7.3%₀ for organic sulfur, +1.8 to +16.8%₀ for massive pyrite, and +3.9 to +23.8%₀ for disseminated pyrite. In contrast, the $\text{δ}{}^{34}\text{S}\text{values from high-sulfur coals}$ (> 0.8% organic sulfur) underlying marine sediments were more variable: organic sulfur, ?7.7 to +0.5%₀, pyrites, ?17.8 to +28.5%₀. In both types of coal, organic sulfur is typically enriched in ³⁴S relative to pyritic sulfur.In general, δ ³⁴S values increased from the top to the base of the bed. Vertical and lateral variations in δ ³⁴S are small for organic sulfur but are large for pyritic sulfur. The sulfur content is relatively constant throughout the bed, with organic sulfur content greater than disseminated pyrite content. The results indicate that most of the organic sulfur in high-sulfur coals is derived from post-depositional reactions with a ³⁴S-depleted source. This source is probably related to bacterial reduction of dissolved sulfate in Carboniferous seawater during a marine transgression after peat deposition. The data suggest that sulfate reduction occurred in an open system initially, and then continued in a closed system as sea water penetrated the bed.Organic sulfur in the low-sulfur coals appears to reflect the original plant sulfur, although diagenetic changes in content and isotopic composition of this fraction cannot be ruled out. The wide variability of the δ ³⁴S in pyrite fractions suggests a complex origin involving varying extents of microbial H₂S production from sulfate reservoirs of different isotopic compositions. The precipitation of pyrite may have begun soon after deposition and continued throughout the coalification process.     

Aleutian magnesian andesites: Melts from subducted Pacific ocean crust

Several diagnostic chemical characteristics of an uncommon Aleutian magma type support a proposed origin that involves a small amount of partial melting of subducted Pacific ocean crust (basalt) consisting mainly of garnet and clinopyroxene (eclogite or garnet websterite). Among the characteristics are high La/Yb ratios and Sr contents and low ratios of radiogenic to non-radiogenic Sr and Pb. The major element composition of the andesites resembles that of hydrous melts in equilibrium with peridotite: a low ratio of total Fe to Mg is distinctive. These disparate observations can be reconciled if large ion lithophile (LIL)- element-rich hydrous melt from the subducted oceanic crust equilibrates with olivine and orthopyroxene in overlying LIL-element-depleted mantle and then erupts without interacting with the island are crust. The compositional dissimilarity of the magnesian andesites and most other andesites from the Aleutian island arc precludes application of this model to island are magmatism in general.