Origin of dacites of Taupo Volcanic Zone, New Zealand

Dacites form a relatively small proportion of lavas in the Taupo Volcanic Zone, New Zealand (5km³), and occur mainly on the eastern side. In this paper their origin is considered in terms of three processes: (a) partial melting of crustal rocks; (b) fractional crystallisation of basalt and andesite; and (c) sub-surface mixing of basic and acid magma. Modelling techniques are used to calculate the most acceptable degree of fractional crystallisation and magma mixing to fit major-element data, and these values are used to compare calculated and observed trace-element values. The success or failure of the model is determined by the closeness of the two sets of values. For partial-melt models, trace-element values alone are calculated by the batch-melting equation.Results indicate that White Island dacite can best be modelled by fractional crystallisation; Manawahe by fractional crystallisation plus limited crustal contamination; Maungaongaonga by partial melting of Western Basement greywacke, and Tauhara by partial melting of this greywacke together with minor mixing with a more basic magma. Results from Parekauau and Horohoro indicate that these lavas are unlikely to have formed by any of the processes examined.     

The formation of small cones in Isidis Planitia,Mars through mobilization of pyroclastic surge deposits

We investigate the distinctive distribution and pattern of subkilometer-sized cones in Isidis Planitia, and their relationship to the material that fills the basin. We observe that: (1) the cones show uniform size, spacing, and morphology across the entire basin; (2) they have large central depressions (relative to cone basal diameters) that are generally filled in and commonly show layering; (3) cone chains form highly organized spatial patterns; cones are also found in isolation and in dense fields; (4) many adjacent chains are parallel, and evenly spaced, creating a pattern that mimics lava flows that have experienced compressional folding; (5) no flows are associated with the cones, even where summit crater rims are breached; (6) the cones are at least temporally related to basin fill; (7) the basin fill material is largely fine-grained, but with locally indurated layers at shallow depth; blocks ejected by small craters from these locally indurated layers are commonly dark in color; and (8) relationships between cones along a chain show no systematic temporal formation sequence along the chain. We conclude that the basin was likely inundated by one or more hot, partially welded pyroclastic flows; devolatilization of these flows remobilized volcaniclastic material to form the cones without associated lava flows. The volume of gas required to account for the observed number of cones is low and does not require the presence of ground water or ice.     

Möglichkeiten der photometrischen Untersuchung von Kugelhaufen mittels photographischer Äquidensiten

A method of estimating the surface-brightness of globular cluster with equidensity curves is described. (Russian abstract)     

Kampfer ein Rauschgift für Fische

Ohne Zusammenfassung     

Intrusion and differentiation of granitic magma at a high level in the crust: the Puscao pluton, Lima Province, Peru

Regression surface techniques have been used on partial whole-rockchemical analyses from the Puscao pluton to establish the three-dimensionalnature of the chemical variations within this body, which wasemplaced at a depth of between 4 km and 8 km within the crust.Block diagrams produced by the computer program are consistentwith the hypothesis that during the period preceding the totalconsolidation of the magma, the pluton has undergone in situdifferentiation to give a layered structure. Statistical analysis confirms that there is a large scale variationwithin the pluton and that local variation, such as might beexpected to result from the contamination of the Puscao magmaby assimilated volcanic material, did not contribute significantlyto the overall variation. Evidence is presented which suggests that the layering withinthe Puscao pluton was disturbed as a result of the emplacementof the much later Cañhas pluton, and that this disturbancehas the form of a simple doming centred upon the site of thepresent Cañas intrusion.     

Coronal disturbances

Time lapse movies acquired with the Sacramento Peak Observatory 6-in filter coronagraph very infrequently show dramatic transient events. Many of these events (at 1.04 < r/r < 1.20) are correlated with H activity at the limb, especially with large surges and eruptive prominences. Metric or centrimetric radio bursts are found associated with certain types of 5303 coronal events. The heights to which such disturbances will propagate can often be estimated from the green line movie data. We report the results of a study of all the examples of such transients observed during the last 16 years.     

An inter-comparison of tidal solutions computed with a range of unstructured grid models of the Irish and Celtic Sea Regions

Three finite element codes, namely TELEMAC, ADCIRC and QUODDY, are used to compute the spatial distributions of the M₂, M₄ and M₆ components of the tide in the sea region off the west coast of Britain. This region is chosen because there is an accurate topographic dataset in the area and detailed open boundary M₂ tidal forcing for driving the model. In addition, accurate solutions (based upon comparisons with extensive observations) using uniform grid finite difference models forced with these open boundary data exist for comparison purposes. By using boundary forcing, bottom topography and bottom drag coefficients identical to those used in an earlier finite difference model, there is no danger of comparing finite element solutions for “untuned unoptimised solutions” with those from a “tuned optimised solution”. In addition, by placing the open boundary in all finite element calculations at the same location as that used in a previous finite difference model and using the same M₂ tidal boundary forcing and water depths, a like with like comparison of solutions derived with the various finite element models was possible. In addition, this open boundary was well removed from the shallow water region, namely the eastern Irish Sea where the higher harmonics were generated. Since these are not included in the open boundary, forcing their generation was determined by physical processes within the models. Consequently, an inter-comparison of these higher harmonics generated by the various finite element codes gives some indication of the degree of variability in the solution particularly in coastal regions from one finite element model to another. Initial calculations using high-resolution near-shore topography in the eastern Irish Sea and including “wetting and drying” showed that M₂ tidal amplitudes and phases in the region computed with TELEMAC were in good agreement with observations. The ADCIRC code gave amplitudes about 30 cm lower and phases about 8° higher. For the M₄ tide, in the eastern Irish Sea amplitudes computed with TELEMAC were about 4 cm higher than ADCIRC on average, with phase differences of order 5°. For the M₆ component, amplitudes and phases showed significant small-scale variability in the eastern Irish Sea, and no clear bias between the models could be found. Although setting a minimum water depth of 5 m in the near-shore region, hence removing wetting and drying, reduced the small-scale variability in the models, the differences in M₂ and M₄ tide between models remained. For M₆, a significant reduction in variability occurred in the eastern Irish Sea when a minimum 5-m water depth was specified. In this case, TELEMAC gave amplitudes that were 1 cm higher and phases 30° lower than ADCIRC on average. For QUODDY in the eastern Irish Sea, average M₂ tidal amplitudes were about 10 cm higher and phase 8° higher than those computed with TELEMAC. For M₄, amplitudes were approximately 2 cm higher with phases of order 15° higher in the northern part of the region and 15° lower in the southern part. For M₆ in the north of the region, amplitudes were 2 cm higher and about 2 cm lower in the south. Very rapid M₆ tidal-phase changes occurred in the near-shore regions. The lessons learned from this model inter-comparison study are summarised in the final section of the paper. In addition, the problems of performing a detailed model–model inter-comparison are discussed, as are the enormous difficulties of conducting a true model skill assessment that would require detailed measurements of tidal boundary forcing, near-shore topography and precise knowledge of bed types and bed forms. Such data are at present not available.