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Eruptive history and petrology of Mount Drum volcano, Wrangell Mountains, Alaska

Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80x200 km) of southcentral Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to rhyolite. The last constructional activity occured in the vicinity of Snider Peak, on the south flank of the volcano, where extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top and a part of the south flank of the volcano, produced more than 7 km3 of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and 72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks, but have higher MgO contents at a given SiO2, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small, but significant, range in 87Sr/86Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages. In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas. Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks.

Chemical composition and evolution of the upper continental crust: Contrasting results from surface samples and shales

The average chemical composition of juvenile upper continental crust (UC) as a function of age is estimated from chemical analyses, geologic maps, stratigraphic sections and isotopic ages. High plutonic/supracrustal ratios in Archean UC may reflect in part a different mode of crustal formation prior to 2.5 Ga. Greenstones show an increase in felsic volcanics and graywackes in post-Archean successions, a drop almost to zero in the proportion of komatiites at the Archean-Proterozoic (A/P) boundary, and an increase in the proportion of andesites in the Phanerozoic. Compared to the Early Archean, Late and post-Archean UC are depleted in Mg, Cr, Ni and Co, and post-Archean UC is enriched in LILE (large-ion lithophile elements; K, Rb, Ba, Th, U), HFSE (high field strength elements; Ti, P, Zr, Hf, Nb, Ta, Y), and HREE (heavy rare-earth elements). Negative Eu anomalies characterize UC of all ages, although they are relatively small in Archean UC. Cratonic shales show the same secular compositional changes as UC with the following important exceptions: At the A/P boundary, only shales show a decrease in Fe, V and Sc and an increase in Na, Ca and Sr, and only UC shows a significant increase in HREE and Y. Although post-Archean UC has a larger Eu anomaly than Archean UC, it is clear that both Archean UC and Archean shales have negative Eu anomalies, and that shales give only a weak indication of an increasing Eu anomaly in UC formed after the Archean. In both UC and shales at the A/P boundary, Cr/Th, Ni/Co and Co/Th ratios decrease, whereas the Th/U ratio increases only in shales and not in UC. The lack of HREE depletion and the high Fe, V and Sc contents of Archean shales indicate these shales were not derived from sources similar in composition to exposed Archean UC. The shale sources, which are now removed by erosion, must have been composed largely of basalt (± komatiite) (60%) and granite (40%), with little contribution from HREE-depleted TTG (tonalite-trondhjemite-granodiorite), which now dominates in Archean exposures. This suggests strong vertical zonation in the Archean continents with granites intruded into greenstones at very shallow levels. Compositional changes in UC or/and shales at the A/P boundary can be explained in one of the four ways: (1) a greater amount of basalt and komatiite in Archean UC (high Cr and Ni in Archean UC and shales); (2) garnet/amphibole fractionation during production of TTG by partial melting of hydrous basaltic crust (low HREE and Y in Archean UC); (3) change in TTG magma source from descending slab or thickened crust to metasomatized mantle wedge (increase in LILE and LREE in post-Archean UC and shales); and (4) a decrease in the intensity of chemical weathering after the Archean (increase in Na, Ca and Sr in post-Archean shales).

Sialyl cholesterol enhances the development of grafted neurons and motor recovery

Trophic actions of α-sialyl cholesterol (SC) and its sialidase-tolerant derivative, α-(3β-hydroxysialyl) cholesterol (SCt), were carried out on the development of midbrain neurons both in vitro and in vivo transplantation studies. Low to moderate concentrations of SC (0.01 to 0.05 μg/ml) facilitated neurite extension but had no effects on cell survival of primary cultured midbrain neurons. However, high concentration of SC (0.1 Mg/ml) disturbed both neunte genesis and cell survival. SCt had a similar effect on midbrain neurons. At higher concentrations, SC and SCt induced concentration-dependent morphological changes in astrocytes from flat to fibrous. The effect on astrocytes was stronger in SCt than SC. At highest concentration tested (20 μg/ml), the proliferation of astrocytes was completely blocked, cells became detached and finally died. This effect of SC and SCt was partially blocked by simultaneous application of aFGF. Following dopaminergic cell grafting in vivo, SC and SCt had biphasic effects: a low dose (0.2 mg/kg, SC) enhanced motor recovery at 4 and 6 weeks after transplantation, while the highest dose (20 mg/kg, SC) disturbed motor recovery at all periods tested. These effects on motor recovery were paralleled by an effect on neurite genesis as studied by tyrosine hydroxylase immunostaining. Thus, at low concentrations, SC and SCt are neurotrophic agents that stimulate the development and differentiation of dopaminergic neurons.

Anomalous pressure effect on superconductivity in (Lu1−xScx)5Ir4Si10

Abstract Results of pressure effect on the superconducting transition temperature (T c ) with hydrostatic pressure up to 21 kbar are reported for (Lu 1−x Sc x ) 5 Ir 4 Si 10 (0 ≤ x ≤ 1.0). A nonlinear increase in T c with increasing Sc substitution is observed while the room-temperature lattice parameters and unit-cell volume follow the Vegard's Law. The compounds can be separated into two groups. With low Sc content (x=0.05, 0.1, and 0.2) the T c 's are lower and enhanced by external pressure. With high Sc content (x = 0.3, 0.5, 0.95, and 1.0) the T c 's are higher and depressed by external pressure. It is interpreted that phases below a critical value x = 0.3 exhibit an electronic instability and are very sensitive to external pressure. These results can be understood on the basis that the charge-density waves occur in the superconducting compound Lu 5 Ir 4 Si 10 .

Geochemistry and provenance of sediments from the Pongola Supergroup, South Africa: Evidence for a 3.0-Ga-old continental craton

The ~3.0 Ga Pongola Supergroup of South Africa is one of the oldest well-preserved supracrustal successions. The sequence is divided into the Nsuze (chiefly volcanics, quartzites, and pelites) and the overlying Mozaan Groups (chiefly quartzites and pelites).Relative to North American Shale Composite (NASC), most Pongola pelites have similar La/Yb and Eu/Eu∗ ratios and Large Ion Lithophile Elements (LILE), High Field Strength Elements (HFSE), REE, V and Sc contents. Ni (30–300 ppm) and Cr (100–800 ppm) contents are greater than those of NASC, while Cr/Ni ratios are generally higher than shales and pelites of all ages. These trends may reflect residual Cr enrichment during intense chemical weathering of komatiitic source rocks. Local populations of Nsuze pelites, based on major and trace element data, may result from source heterogeneities and/or contemporaneous volcanic input into the sedimentary basin. Pongola quartzite compositions and detrital modes are similar to those of sandstones from Phanerozoic passive-margin and basement uplift tectonic settings.REE, LILE and HFSE contents in pelites suggest that Pongola source rocks were remarkably similar in composition to average Phanerozoic upper-continental crust. Trace element, geographic and isotopic constraints favor 3.2 to 3.0 Ga hood granites as a major source for Pongola detritus. Moderate contributions from mafic sources are also required, whereas relatively minor contributions from tonalite and komatiite are allowed. These results indicate the existence of a chemically evolved (felsic) craton in southern Africa by 3.0 Ga. This craton appears to have been more evolved than sources for either older Moodies (~3.3 Ga) or most of the younger Witwatersrand sediments (2.8-2.7 Ga). Pelites from the Kaapvaal craton record at least two mafic to felsic source-area compositional cycles in pelites deposited from the mid to late Archean. Additional cycles may be masked by a depositional or erosional hiatus between the Moodies and Pongola sequences. These sedimentary patterns suggest that crustal development in southern Africa was episodic in nature during the early Precambrian.

Neutron activation-induced beta autoradiography as a technique for locating minor phases in thin section; application to rare earth element and platinum-group element mineral analysis

Samples which have been irradiated in a thermal neutron flux are first allowed to decay for an appropriate period. The distribution of isotopes which decay by beta emission is then detected using a specific nuclear track emulsion. Data are presented to show the relative sensitivities of all elements which emit beta particles after thermal neutron capture and to demonstrate their relative rates of decay. The technique may be used most sensitively to locate minerals containing high concentrations of Sc, Co, Y, Sb, Cs, rare earth elements, Ta, Re, Ir, Au, Th, and U. Potential applications in the systematic search for rare earth element-bearing minerals in granitic rocks and platinum-group element minerals in chromitites.--Modified journal abstract.

Neutron activation analysis of some North Adriatic organisms

The present work was undertaken to contribute additional information on the concentration of microelements in marine organisms, and especially on the ecological characteristics of the North Adriatic Sea. Samples of biota were collected during 1976 and 1980 from various parts of the west Istrian coast. All samples (Mytilus galloprovincialis Lam. and Zostera marina L.) were dried at 110°C to a constant weight, homogenized and powdered and subjected to instrumental neutron activation analysis. The biota in North Adriatic contain a relatively high concentration of microelements, mainly Fe, Co, Se, Sr, Eu, Sc and Cs. The results indicated that Zostera marina L. accumulates most of the investigated elements and that concentration factors especially for europium, iron, scandium and selenium are very high.