ALBERT

Notes: Abstract The Austurhorn intrusive complex in southeastern Iceland represents an exhumed Tertiary central volcano. The geometry of the intrusion and geochemistry of the mafic and felsic rocks indicate Austurhorn was a volcanic center analogous to Eyjafjallajökull and Torfajökull in Iceland's eastern neovolcanic zone (EVZ). Early transitional tholeiitic basalt magmatism at Austurhorn formed a shallow crustal chamber ∼5 km in diameter. Apparent rhythmic modal layering of, and intrusive contacts within, the gabbro indicate the mafic chamber was replenished frequently as it cooled and crystallized. Felsic activity postdated near-solidification of the gabbro; numerous granitic magmas intruded along gabbro margins and within the adjacent crust. Field relations indicate that infrequent felsic replenishment prevented convective mixing of the Austurhorn chamber during this time, although commingled mafic and felsic magmas are observed in an extensive net veined complex. Late stage mafic dikes intrude the entire complex, suggesting that magmatic heat was abundantly available throughout the evolution of the Austurhorn system. Plagioclase and clinopyroxene compositions in mafic through felsic rocks, including gabbros, support a model of progressive differentiation. Field relations constrain the felsic magmas to originate at P≥1 kbar, presumably by fractional crystallization. The structure and geochemistry of the Austurhorn intrusive complex suggest formation in an immature rift environment similar to the modern EVZ. The proposed rift segment was parallel to the western and eastern neovolcanic zones, and probably resulted from a reorganization of plate boundaries ∼7 Ma (Saemundsson 1979; Helgason 1985; Jancin et al. 1985) triggered by activity of the Iceland mantle plume.

Notes: Summary A Newton-Kantorowitsch-analysis of the modified Newton's method in generalized Banach-spaces is given. The application of generalized norms — mappings from a linear space into an ordered Banach-space — improves convergence- and existence results as well as error estimates compared with the real-norm theory.

Notes: Abstract A diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the Southwest Indian Ridge (54°S, 7°16′ E) between the Islas Orcadas and Shaka Fracture Zones. The cumulus nature of the gabbroic rocks in the suite is indicated by phase, modal and cryptic layering, igneous lamination, and low incompatible element abundances. We present a mass-balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk-rock major element compositions. The model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid. Calculated proportions of trapped liquid range from 3%–15%; values that are characteristic of adcumulates to mesocumulates. Models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids, thus explaining the high TiO2 contents observed in magnesian clinopyroxenes. Cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers, but disequilibrium plagioclase compositions suggest some crystallization at greater depth. Furthermore, basalt compositions projected onto the olivine-clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures (from less than 10 to greater than 20 kb) as well as polybaric fractional crystallization. We suggest that the Southwest Indian Ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros. The adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large, but ephemeral magma chambers below segments of the Southwest Indian Ridge.

Notes: Abstract About 12.3 km3 of basaltic magma were erupted from the Lakagigar fissure in Iceland in 1783, which may have been derived from the high-level reservoir of Grimsvotn central volcano, by lateral flow within the rifted crust. We have studied the petrology of quenched, glassy tephra from sections through pyroclastic cones along the fissure. The chemical composition of matrix glass of the 1783 tephra is heterogeneous and ranges from olivine tholeiite to Fe−Ti rich basalt, but the most common magma erupted is quartz tholeiite (Mg#43.6 to 37.2). The tephra are characterized by low crystal content (5 to 9 vol%). Glass inclusions trapped in plagioclase and Fo86 to Fo75 olivine phenocrysts show a large range of compositions, from primitive olivine tholeiite (Mg#64.3), quartz tholeiite (Mg#43–37), to Fe−Ti basalts (Mg#33.5) which represent the most differentiated liquids and are trapped as rare melt inclusions in clinopyroxene. Both matrix glass and melt inclusion data indicate a chemically heterogeneous magma reservoir, with quartz tholeiite dominant. LREE-depleted olivine-tholeiite melt-inclusions in Mg-rich olivine and anorthitic-plagioclase phenocrysts may represent primitive magma batches ascending into the reservoir at the time of the eruption. Vesicularity of matrix glasses correlates with differentiation, ranging from 10 to 60 vol.% in evolved quartz-tholeiite glasses, whereas olivine-tholeiite glasses contain less than 10 vol.% vesicles. FTIR analyses of olivine-tholeiite melt-inclusions indicate concentrations of 0.47 wt% H2O and 430 to 510 ppm for CO2. Chlorine in glass inclusions and matrix glasses increases from 50 ppm in primitive tholeiite to 230 ppm in Fe−Ti basalts, without clear evidence of degassing. Melt inclusion analyses show that sulfur varies from 915 ppm to 1970 ppm, as total FeO* increases from 9 to 13.5 wt%. Sulfur degassing correlates both with vesicularity and magma composition. Thus sulfur in matrix glasses decreases from 1490 ppm to 500 ppm, as Mg # decreases from 47 to 37 and vesicularity of the magma strongly increases. These results indicate loss of at least 75% of sulfur during the eruption. The correlation of low sulfur content in matrix glasses with high vesicularity is regarded as evidence of the control of a major exsolving volatile phase on the degassing efficiency of the magma. Our model is consistent a quasi-permanent CO2 flux through the shallow-level magmatic reservoir of Grimsvotn. Following magma withdrawal from the reservoir and during eruption from the Lakagigar fissure, sulfur degassing was controlled by inherent CO2-induced vesicularity of the magma.

Notes: Abstract Rural development and economic change has generally been associated with growth and the in-migration of nonlocal firms or their branch plants and offices. Such change has been critiqued and at times resisted because of its implicit “urbanism” and conflict with rural values and modes of social interaction. The inevitability of the conflict has always been assumed, given the perspectives of development groups and many rural residents. This paper examines the apparent conflicts between the rural ethos and the “growth ethos,” and the considers the necessity for the pursuit of the forms of growth that tend to undermine rural values. The severely limited set of changes in the local economy considered by the common forms of growth-sponsoring economic development groups is then examined. Finally, the existence of alternative forms of economic change are hypothesized and their viability demonstrated. We conclude that improved economic well-being for rural residents need not sacrifice their values and lifestyles on the altar of urban-influenced “economic growth.”