Transitions in the style of mantle convection at high Rayleigh numbers

The pattern and style of mantle convection govern the thermal evolution, internal dynamics, and large-scale surface deformation of the terrestrial planets. In order to characterize the nature of heat transport and convective behaviour at Rayleigh numbers, Ra, appropriate for planetary mantles (between 10⁴ and 10⁸), we perform a set of laboratory experiments. Convection is driven by a temperature gradient imposed between two rigid surfaces, and there is no internal heating. As the Rayleigh number is increased, two transitions in convective behaviour occur. First we observe a change from steady to time-dependent convection at Ra≈10⁵. A second transition occurs at higher Rayleigh numbers, Ra≈5×10⁶, with large-scale time-dependent flow being replaced by isolated rising and sinking plumes. Corresponding to the latter transition, the exponent β in the power law relating the Nusselt number Nu to the Rayleigh number (NuRa^β) is reduced. Both rising and sinking plumes always consist of plume heads followed by tails. There is no characteristic frequency for the formation of plumes.     

Relation of lava compositions to volcano size and structure in El Salvador

Most of the lavas at the nine volcanic centers along the volcanic front of El Salvador are basalts, basaltic andesites and andesites. The compositional variation within and among these centers can be explained by fractionation processes within the crust. Cognate gabbroic inclusions found in the lavas have appropriate mineralogy (plagioclase, olivine, magnetite and augite) to be cumulates formed by fractional crystallization. Two main variation trends occur, depending on the proportion of plagioclase removal. The more common, or normal, trend has a high (> 55%) proportion of plagioclase being removed. A less common, Al-rich, trend has a low (40%) proportion of plagioclase being removed. The Al-rich trend is found only at volcanoes that lack large negative Bouguer gravity anomalies. These volcanoes are unlikely to have large shallow magma chambers and fractionation probably occurs deeper in the crust where plagioclase removal is inhibited.The incompatible element (Na₂O, K₂O, Rb, Ba) contents of lavas vary systematically with the volume of the volcanic centers. At the same level of SiO₂, large volcanic centers have higher incompatible element contents than small volcanic centers. This suggests that open system fractionation in a periodically refilled chamber is the controlling factor. The large difference in Ba contents of lavas between eastern (low) and western (high) El Salvador suggests a difference in the mantle source region.     

Binary coefficients: A theoretical and empirical study

Binary coefficients can be assigned to several categories on the basis of algebraic and conceptual properties. The phi coefficient of association is related algebraically to the chi-square statistic for 2-by-2 contingency tables, and use of this coefficient in cluster analysis permits the objective, nonarbitrary partitioning of objects among groups on the basis of previously selected levels of significant, positive association. Similarity, matching, and distance coefficients possess neither conceptual nor operational statistical meaning for many geological data sets. The weighted pair group method and flexible clustering strategy may give an overly conservative partitioning of objects among groups. Clustering by the unweighted pair group method, using the phi coefficient, is recommended for the analysis of biostratigraphic and paleoecologic presence—absence data.     

The Al-Fe(III)epidote miscibility gap in a metamorphic profile through the penninic series of the Tauern window,Austria

The compositional variations in epidotes, Ca₂Al₂(Fe³⁺, Al)-Si₃O₁₂(OH), from a prograde Mesozoic rock series in the Eastern Alps, Austria, are systematically related to metamorphic grade and the oxidation state of the rock. With increasing metamorphic grade the average composition of the zoned epidotes shifts to Fe³⁺-poorer compositions reflecting not only the effect of temperature and total pressure but also the concomitant decrease of the oxidation state of the rocks. Oxidized hematite-bearing assemblages are: 90 mole % Al₂FeEp (greenschists) 70 mole % Al₂FeEp (garnet amphibolites) 58 mole % Al₂FeEp (eclogites); reduced sulfidebearing assemblages are: 42 mole % Al₂FeEp (greenschists) 24 mole % Al₂FeEp (garnet amphibolites) 23 mole % Al₂FeEp (eclogites).A similar compositional evolution of the epidotes as in the spatial sequence of the samples can be observed within the single zoned crystals, reflecting the temporal changes of temperature, total pressure and oxygen fugacity during the prograde crystallization. The Fe³⁺-contents of core and rim decrease with increasing metamorphic grade and decreasing oxidation state. Generally the zoned epidotes consist of a Fe³⁺-rich core (90 to 63 mole % Al₂FeEp) and a Fe³⁺-poorer rim (55 to 23 mole % Al₂FeEp). Core and rim of the epidote crystals are separated by a compositional gap the extension of which is independent of the bulk rock composition, the oxidation state, and the mineralogical composition of the assemblages but becomes smaller with increasing metamorphic grade: 7253 mole % Al₂FeEp (low grade greenschists, 400° C) 6355 mole % Al₂FeEp (higher grade greenschists, 500° C), and 6055 mole % Al₂FeEp (garnet amphibolites, 500–550° C). At the temperature conditions of the highest grade garnet amphibolites and eclogites (550° C) the compositional gap closes at a composition of 58 mole % Al₂FeEp.The data presented thus confirm clearly the existence of an asymmetric miscibility gap in the monoclinic Al-Fe(III)-epidote solid solution series, which, for the first time, has been assumed by Strens (1964, 1965).A model is proposed that describes the prograde compositional evolution of the epidotes studied through the competing mechanisms of growth and diffusional Al-Fe³⁺ exchange and their dependence on metamorphic grade and oxidation state.     

Size and shape variation in the planktonic foraminiferHeterohelix striata (late cretaceous,southern Scandinavia)

The biserial planktonic foraminiferal species Heterohelix striata (Ehrenberg)has been analyzed by morphometric techniques from six sequential late Maestrichtian core samples from southern Sweden and one sample from the “Dania”quarry of Denmark. The study is based on measurements of number of chambers, size of test (length, width, and thickness),marginal angle, and chamber sizes. In addition, prolocular and apertural sizes were analyzed in the “Dania”quarry sample. The study includes analyses of the ontogenetic development of the tests, stratigraphic variation in size and shape, intrasample variability, and relationship between gross morphology of the test and prolocular and apertural dimensions.     

Plagioclase—melt equilibria

The crystallization of plagioclase feldspar from magmatic liquid has been investigated experimentally under equilibrium conditions at 1 atm total pressure in the temperature range 1400-1095°C. Natural and synthetic melts of composition basalt to rhyolite were used, crystallizing plagioclase of composition An₈₉-An₃₂.The experimental results are analyzed initially in terms of elemental plagioclase/melt partition coefficients (D). D_Si is always less than unity and is invariant with temperature. D_A1 is always greater than unity and is relatively insensitive to temperature. D_Na is less than unity above 1200°C and is strongly dependent upon temperature. D_Ca is greater than unity below 1430°C and is strongly dependent upon temperature.Analysis of the temperature-dependence of equilibrium constants for plagioclase-melt formation and exchange reactions in which several mixing models for the melt are considered, leads to the conclusion that, with appropriate choice of melt-components, the melt-components mix quasi-ideally. At fixed temperature in the absence of H₂O, the equilibrium constant for the equilibrium of albite with the melt is insensitive to changes in melt-composition, and is insensitive to changes in pressure up to at least 10 kbars. As a consequence the composition of plagioclase crystallizing at known temperature and at low total pressure from a dry melt of known composition may be predicted []. However, the equilibrium constant is sensitive to changes in water pressure.The analysis further suggests that Na is intimately associated with tetrahedrally-coordinated Al in the melt, while Ca appears to be partitioned between at least two distinct melt-sites.     

Determination of vertical temperature profiles for the atmospheric boundary layer by ground-based microwave radiometry

A useful method for remote sensing of vertical temperature profiles in the atmospheric boundary layer is described. From angular measurements of brightness temperature at 58 GHz, profiles have been inferred up to an altitude of 700 m. Calculations were done with an iterative inversion procedure (Smith et al., 1972) using Twomey-type smoothing (Twomey, 1963). It is shown how an initial-guess profile can be directly derived from the radiation measurements using, a nomogram.