This research involves a study of a 2 km thick volcanic succession which accumulated during the opening stages of the precursor of the Reykjanes-Langjökull axial rift zone in W-Iceland, between 6-3 m.y. Following the initial accumulation of olivine tholeiite lavas, which lie unconformably on an older crustal basement 10-13 m.y.), a central volcano developed in the Hafnarfjall-Skarðsheiði area. It was active for some 1.5 m.y. and consists of four volcanic phases: I. The Brekkufjall phase is characterized by basaltic volcanism followed by voluminous and copious extrusions of differentiated rocks culminating in a sudden caldera collapse (c.5 km wide) in Brekkufjall. II. During the Hafnarfjall phase a thick extrusive sequence of basaltic to rhyolitic compositions accumulated, mainly fed by ENE fissures. During the gradual subsidence of the Hafnarfjall caldera (7 by 5 km) a marked decrease occurred in lava accumulation rate outside the caldera. Epicentres of three cone sheet swarms coincide in time and space with three basinal structures of this caldera. III. The Skarðsheiði phase is characterized by N-S fissuring and a marked bimodal basalt-rhyolite lava accumulation. IV. Remnants of the Heioarhorn phase include compositions ranging from basalts to rhyolites. The western boundary of the axial rift zone is marked by large intrusives, basalt flexuring, a sheet swarm and the disappearance of dyke swarms. The lenticular unit was later buried by lavas of the Hvalfjörður lenticular unit. Rocks of the central volcano follow the Þingrmúli trend, but is discontinuous in the basal tic andesite range. Basalts (frequently porphyritic) with relatively monotonous compositions and low LIL abundancies predominate during episodes of low extrusion rate whereas high elemental dispersion characterizes basalts of high extrusion rate episodes. The basalt compositions are believed to be controlled more by partial melting processes rather than by episodes of low-pressure fractionation. The differentiated rocks are considered to have predominantly formed by partial melting of the lower crust.

The extent of weathering incongruency during soil formation from rock controls local carbon and nutrient cycling in ecosystems, as well as the evolution of hydrologic flow paths. Prior studies of basalt weathering, including those that have quantified the dynamics of well-mixed, bench-scale laboratory reactors or characterized the structure and integrated response of field systems, indicate a strong influence of system scale on weathering rate and trajectory. For example, integrated catchment response tends to produce lower weathering rates than do well mixed reactors, but the mechanisms underlying these disparities remain unclear. Here we present pore water geochemistry and physical sensor data gathered during two controlled rainfall-runoff events on a large-scale convergent model hillslope mantled with 1 m uniform depth of granular basaltic porous media. The dense sampler and sensor array (1488 samplers and sensors embedded in 330 m(3) of basalt) showed that rainfall-induced dissolution of basaltic glass produced supersaturation of pore waters with respect to multiple secondary solids including allophane, gibbsite, ferrihydrite, birnessite and calcite. The spatial distribution of saturation state was heterogeneous, suggesting an accumulation of solutes leading to precipitation of secondary solids along hydrologic flow paths. Rapid dissolution of primary silicates was widespread throughout the entire hillslope, irrespective of up-gradient flowpath length. However, coherent spatial variations in solution chemistry and saturation indices were observed in depth profiles and between distinct topographic regions of the hillslope. Colloids (110-2000 nm) enriched in iron (Fe), aluminum (Al), and phosphorus (P) were mobile in soil pore waters.

No description available.

Abstract not available

Volcanism associated with the onset of Karoo flood basalt eruptions (c. 180 Ma) at Sterkspruit, South Africa, began with emplacement of thin lava flows before abruptly switching to phreatomagmatic and magmatic activity that formed a nest of craters, spatter and tuff rings and cones that collectively comprise a crater complex >40 km� filled by 9-18 km� of volcaniclastic debris. Phreatomagmatic activity driven by interaction of Karoo magma with groundwater hosted in country rock and crater-filling debris quarried broad, mainly shallow craters (hundreds of metres, but not kilometers deep) into wall-rock. Closely spaced individual vents, the consequence of magma emplaced over a broad area through a network of feeder dikes and stocks, were active at the same time or over short periods of time. Highly ephemeral access of external water to vents drove repeated and reversible switches between explosive to effusive magmatic and explosive phreatomagmatic activity, resulting in vents and craters that grew laterally and vertically into adjacent ones through quarrying and vent migration. Deposits within the Sterkspruit crater complex are dominated by 7-15 km� of massive, unsorted polymict lapilli tuff and tuff breccia juxtaposed with localised fountain-fed lava and strombolian spatter deposits. Transport within the complex was dominated by jets and fountains of volcaniclastic debris and by mass movement. Country-rock breccias indicate that craters grew via a combination of mechanical fragmentation, granulation and mass-movement of 7-12 km� of wall-rock, adding mass and previously locked-up pore-water to the volcanic system. Ash and lapilli, the deposits of plumes 5-15 km high, form a 50-110 m-thick ejecta blanket mantling Clarens Formation country rock that thins gradually away from the crater-complex margins. Explosive volcanism was succeeded by brief fluvial and eolian reworking of volcaniclastic debris and formation of a shallow crater lake 12 km� in extent, and then by voluminous effusion of flood basalt that inundated the Sterkspruit crater complex with lava. Flood basalt magmas involved in Sterkspruit eruptions were chemically heterogenous. This study documents the rapid (perhaps simultaneous) eruption of multiple, chemically distinct basaltic magmas, which cannot be simply related to one another, from one vent site, and possibly many others, within the Sterkspruit crater complex. Five distinct magma types were involved in eruptions at Sterkspruit, indicating that in the early stages of flood basalt eruption (i) magma batches may be small and not simply related to one another, (ii) heterogeneities in the magma source region may be close to each other in time and space, and (iii) eruptions of chemically distinct magmas may take place over short intervals of space and time without significant hybridisation. Formation of the Sterkspruit Complex, and many others like it in South Africa, confirms that the opening phases of Karoo flood basalt volcanism were explosive, and that the volume of the products of explosive volcanism may have important implications for climate change and landscape development associated with the emplacement of large igneous provinces.

Thesis (Ph. D.)--Massachusetts Institute of Technology and woods Hole Oceanographic Institution, 1982. / Bibliography: p. 265-290.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1982. / Supervised by William J. Jenkins. Includes bibliographical references (leaves 265-290).

¡@¡@Igneous activity of Taiwan in Miocene mainly were concentrated in the area north of Miaoli. Nanman is one of the basalt in southern of Taiwan that had been studied by OM, XRD, SEM/EDS and TEM. The rock type is alkali basalt with phenocryst of augite and olivine. The primary minerals include olivine + augite + plagioclase + Anorthoclase + apatite +titanomagnetite + illmenite, and secondary minerals are chlorite + smectite + calcite ¡Ó gypsum.Alteration of Nanman basalt was take placed in mixture and crack of olivine, other minerals are almost fresh. The main replacement are chloritic minerals shown five occurrence types : (1) replaced mixture, (2) vein center of olivine crack, (3) vien of olivine, (4) fibrous and (5) mica-like clay between vein and residual olivine. Each types has clear grain boundary. It¡¦s mean they are not gradual. (4) and (5) are smectite with anomaly composititon. It cause by lack of Al resource, and smectite occurred far from the crack that element do not easily move. Thus, smectite replaced olivine often showed Al-poor and excess Mg. Si/(Si+Al) and Fe/(Fe+Mg) in smectite is positive correlation. This may be considered the moving ability of elements. Composition of (1), (2) and (3) are chlorite/smectite. However there is no observed mixed-layer minerals in XRD. According to TEM , they are chlorite + smectite. Chlorite within abundant of dehydrated cracks could be a evidence of smectite. Because of mixed chlorite and smectite, the correlation between Si/(Si+Al) and Fe/(Fe+Mg) is negative. Different occurrences lead to different mineral, suggest water/rock ratio is an important control factor. High rock/water ratio trend to form chlorite, and low rock/water ratio prefer to form smectite. According to all the results, the steps of altered olivine are first forming chlorite at mixture and olivine cracks. Then, the residual olivine becomes smectite with poor Al and excess Mg, when the rock was cooling down.

This study addresses the influences on ridge basalt chemistry, through analysis of their major and trace element and isotopic composition at scales ranging from individual ridge segments to the entire length of the ridge system. Local-scale studies of basalts along the Mid-Atlantic Ridge shed light on crustal accretion at slow-spreading ridges, and on the nature of plume-ridge interaction in this region. We show that segments must have multiple supplies of magma delivered along their length, but with preferential delivery of magma to segment centers. Plume-ridge interaction near the Azores is not simple two- component mixing between “plume” mantle and “depleted” mantle as previously argued. The elevated highly incompatible trace element ratios possessed by basalts well south of the plume are the definitive sign of a low-degree melt, which can fractionate highly incompatible element ratios. We show that a low-degree melt of plume mantle acts to metasomatize ambient depleted mantle, creating a mixed source that melts to produce the enriched basalts south of the Azores. This metasomatized source is the enriched mixing component that produces the observed geochemical gradient, rather than bulk plume mantle. The latter half of this study is global in scope, involving a carefully compiled ridge basalt geochemical database. This database is unparalleled in size and coverage – including data from portions of the Gakkel and Southwest Indian Ridges and Lau basin that were unavailable in prior data compilations. It includes a catalog of 771 global ridge segments, enabling the calculation of mean MORB by averaging the “segment means”, including weighting on segment length and spreading rate and a quantitative treatment of errors. We show that the mean composition of ocean ridge basalts is more enriched than previously suggested, and argue for a re-definition of “normal MORB”. Segment basalt compositions are individually corrected for crystal fractionation, arriving at parental magma compositions that can be interpreted in terms of mantle processes. The fractionation-corrected mean segment compositions correlate with ridge depth, and with each other, in a manner that is consistent with control by mantle temperature variations. Mantle compositional heterogeneity is also seen, but appears to be a second-order effect. / Earth and Planetary Sciences

The Kemess North porphyry Au-Cu-Mo deposit (300 Mt resource @ 0.30 g/t Au and 0.16% Cu)i s situated in the Toodoggone district, along the eastern margin of the Stikinia terrane in British Columbia. Mineralization is genetically related to the ca. 202 Ma, moderately SE-plunging, Kemess North diorite and is also hosted by proximal Takla Group basalt country rock. The nearby 202.7 ± 1.9 Ma Sovereign diorite has a comparable emplacement age, mineralogy, and chemistry to the Kemess North diorite, but is unmineralized. Toodoggone Formation volcaniclastic rocks (199.1 ± 0.3 Ma) crop out as prominent N-trending ridges or as isolated, fault-bounded blocks within Takla Group basalt. The unmineralized, (197.3 + 1.1/0.9 Ma) Duncan pluton intrudes Takla Group basalt. Seven vein types are separated into four stages of formation with respect to Au-Cu-Mo mineralization. Early-stage veins include magnetite stringer veins and later quartz-magnetite-pyrite + chalcopyrite + molybdenite veins. These veins are restricted mainly to the diorite, are associated with locally preserved potassic (biotite) alteration, and resulted in most of the Au-Cu-Mo mineralization at Kemess North. Main-stage quartz-pyrite + chalcopyrite ± molybdenite veins are the most abundant vein type and are present in the diorite and proximal Takla Group basalt. The veins are associated with phyllic (sericite-quartz) alteration and have a Re-Os molybdenite age of 201.8 ± 1.2 Ma. Late-stage pyrite-chalcopyrite and anhydrite ± pyrite ± chalcopyrite veins and associate phyllic (sericite-chlorite-pyrite) alteration occur in diorite and Takla Group country rocks. Lastly, post-mineralizationan hydrite and carbonate-zeolite veins cut all rocks. Fluid inclusion studies indicate that early-and main-stage ore fluids deposited Au-Cu-Mo at similar temperatures (about 400°C to 375°C) and pressures (0.9 to 3.0 kbar), corresponding to crustal depths of 3 to 10 km. Sulfur and Pb isotope compositions suggest that metals from the early-stage fluid were derived from the Kemess North diorite; metals in the main-stage fluid were derived from the diorite and probably Takla Group country rock and meteoric fluids. An E-striking, steeply S-dipping fault truncates the northern extremity of the ore body. Late NW- to NE-striking normal faults vertically displace the deposit resulting in graben-and-horst block shuffling of the stratigraphy.