The conventional K-Ar dating method was applied to the 1986 dacite flow from the new lava dome at Mount St. Helens, Washington. Porphyritic dacite which solidified on the surface of the lava dome in 1986 gives a whole rock K-Ar “age” of 0.35±0.05 million years (Ma). Mineral concentrates from the dacite which formed ins 1986 give K-Ar “ages” from 0.34±0.06 Ma (feldspar-glass concentrate) to 2.8±...

U.S. Department of the Interior U.S. Geological Survey Wheeler Geologic Monument (Half Moon Pass quadrangle) provides exceptional exposures of three outflow tuff sheets erupted from the San Luis caldera complex. Lowest sheet is Rat Creek Tuff, which is nonwelded throughout but grades upward from light-tan rhyolite (~74% SiO 2 ) into pale brown dacite (~66% SiO 2 ) that contains sparse dark-brow...

Sharply defined veins in :stockworks and siliceous breccia matrix contain metallic mineral assembhges and follow a number of subparallel trends within and marginal to a ] p tassium-feldspar porphyritic dacite. The .porphyritic dacite and metal-bearing veins and breccias exhibit both con-. formable and crosscutting relationships with andesite and dacite country rocks of the Hazelton Group. Vein ...

We present an experimental and petrological study aimed at estimating the pre-eruptive conditions of a Holocene dacitic lava from Volcán San Pedro (36 S, Chilean Andes). Phase-equilibrium experiments were performed at temperatures (T) from 800 to 950 C, and mainly at 200MPa, but also at 55, 150, and 406MPa. Oxygen fugacity ( fO2) ranged from the Ni---NiO buffer (NNO) to 3 5 log units above (NNO...

An Eocene submarine boninite series volcanic center is exposed on the island of Chichi-jima, Bonin Islands, Japan. Five rock types, boninite, bronzite andesite, dacite, quartz dacite, and rhyolite, were distinguished within the boninite volcanic sequence on the basis of petrographic and geochemical observations. Boninite lavas contain high magnesium, nickel, and chromium contents indicative of ...

H2O diffusion in dacitic melt was investigated at 0.48–0.95 GPa and 786–893 K in a piston-cylinder apparatus. The diffusion couple design was used, in which a nominally dry dacitic glass makes one half and is juxtaposed with a hydrous dacitic glass containing up to 8 wt.% total water (H2Ot). H2O concentration profiles were measured on quenched glasses with infrared microspectroscopy. The H2O di...

We report the first experimental study of the diffusion of H2O in dacitic melts. Dehydration experiments were conducted at 551–637 8C and 1–1450 bars for samples with ~0.8 to ~2.5 wt.% total dissolved H2O. The H2O diffusivity is proportional to total H2O contents at V0.8 wt.%, but increases exponentially with total H2O at higher H2O contents. The diffusivity of the total dissolved H2O (in Am /s...

User studies have revealed that programmers face several obstacles when learning application programming interfaces (APIs). A considerable part of such difficulties relate to discovery of API elements and the relationships among them. To address discoverability problems, we show how to complement APIs with design annotations, which document design decisions in a program-processable form for typ...

Precise Fe/Mn ratios and MnO contents have been determined for basalts from the Hawaiian shields of Ko’olau and Kilauea by inductively coupled plasma mass spectrometry. It is well known that the youngest Ko’olau (Makapu’u-stage) shield lavas define a geochemical endmember for Hawaiian lavas in terms of CaO and SiO2 contents and isotopic ratios of O, Sr, Nd, Hf, Pb, and Os. We find that their Mn...

A significant tin deposit has been identified in the northeast part of the Late Proterozoic Arabian Shield. Cassiterite-topaz-quartz greisens are distributed over 16 km2 in flat-topped cupolas of a highly evolved, zinnwaldite-bearing alkali-feldspar granite which composes part of the silsilah ring complex (lat 26"06'N, long 4240'E). The 587 m.y. old alkali-feldspar granite is overlain by a cara...