Enstatite-jadeite join and its role in the Earth's mantle

Phase relations on the enstatite-jadeite join were experimentally determined at solidus temperatures and 90–152 kb, and at 1400–2050°C/175–219 kb, with a split-sphere anvil apparatus (USSA-2000). New findings include immiscibility in garnet and determination of the stability of NaAlSiO₄ (calcium ferrite structure) with stishovite. A thermodynamic model for the enstatite-jadeite join was developed to calculate a complete phase diagram for the join at 500–2500°C and 0–270 kb. The results indicate that the two major discontinuities in the Earth's mantle at 400 and 670 km depths could correspond respectively to the formation and the breakdown of garnet with a pyroxene composition. A model for a chondritic upper mantle is proposed in which large-scale chemical and mineral layering was produced by fractionation of liquidus phases in a magma ocean. Solidification was completed at 400 km depth by crystallization of sodium-enriched residual melts, which produced a pyroxene layer at 300–400 km depths.     

Carbonatite melt inclusions in coexisting magnetite,apatite and monticellite in Kerimasi calciocarbonatite,Tanzania: melt evolution and petrogenesis

Kerimasi calciocarbonatite consists principally of calcite together with lesser apatite, magnetite, and monticellite. Calcite hosts fluid and S-bearing Na–K–Ca-carbonate inclusions. Carbonatite melt and fluid inclusions occur in apatite and magnetite, and silicate melt inclusions in magnetite. This study presents statistically significant compositional data for quenched S- and P-bearing, Ca-alkali-rich carbonatite melt inclusions in magnetite and apatite. Magnetite-hosted silicate melts are peralkaline with normative sodium-metasilicate. On the basis of our microthermometric results on apatite-hosted melt inclusions and forsterite–monticellite phase relationships, temperatures of the early stage of magma evolution are estimated to be 900–1,000°C. At this time three immiscible liquid phases coexisted: (1) a Ca-rich, P-, S- and alkali-bearing carbonatite melt, (2) a Mg- and Fe-rich, peralkaline silicate melt, and (3) a C–O–H–S-alkali fluid. During the development of coexisting carbonatite and silicate melts, the Si/Al and Mg/Fe ratio of the silicate melt decreased with contemporaneous increase in alkalis due to olivine fractionation, whereas the alkali content of the carbonatite melt increased with concomitant decrease in CaO resulting from calcite fractionation. Overall the peralkalinity of the bulk composition of the immiscible melts increased, resulting in a decrease in the size of the miscibility gap in the pseudoquaternary system studied. Inclusion data indicate the formation of a carbonatite magma that is extremely enriched in alkalis with a composition similar to that of Oldoinyo Lengai natrocarbonatite. In contrast to the bulk compositions of calciocarbonatite rocks, the melt inclusions investigated contain significant amount of alkalis (Na₂O + K₂O) that is at least 5–10 wt%. The compositions of carbonatite melt inclusions are considered as being better representatives of parental magma composition than those of any bulk rock.     

On one method of solving the three-dimensional direct magnetic problem

Summary The solution of the direct magnetic problem is derived for three-dimensional bodies under the assumption that the components of the magnetization vector are analytical functions of the co-ordinates within the body being considered. The solution to the problem is given in the form of Green-type integrals and, much like with gravitational fields of inhomogeneous bodies, it may serve the purpose of solving the problem of the analytical continuation of the external field into the body.     

Middle to late Pleistocene uplift rate of the Hungarian Mountain Range at the Danube Bend, (Pannonian Basin) using in situ produced He

Topography of the terraced Danube Bend area indicates fast incision of the Danube River, which was followed by its tributaries dissecting deeply the former terrace levels. These surfaces are vertically bended along the river course, indicating antecedent incision of the Danube into the SW–NE trending Hungarian Mountain Range (HMR). Timing and rate of the incision of the Danube into the HMR and consequently, the rate of vertical motions have remained unresolved so far. This study aims at quantifying the landscape evolution and neotectonic deformation of the central part of the HMR. We used terrace levels along the antecedent section of the Danube River to constrain its incision rate, which is a measure for the uplift rate of the HMR.

Here we use ³He, a terrestrial in situ produced cosmogenic nuclide (TCN), to date uplifted geomorphologic levels along in the Danube Bend gorge. This method, first applied in the Carpathian–Pannonian system in the framework of present study, proved to be suitable for the quantification of landscape evolution in this area. Our ³He exposure age data suggest a maximum incision rate of 2.7 ± 0.1 mm/y for the last 170 ky. Considering likely effect of erosion a more conservative value of 1.6 mm/y for the last 270 ky, was obtained. Both rates are significantly higher than the incision rate of 0.41 mm/y of the Danube derived from previous geologic and geomorphic data for the last 360 ky. The formation of the terrace levels in the Danube Bend probably occurred during the last two glacial cycles (OIS 1–8). According to the exposure age data, there is no direct relationship between the terrace formation and climate in the Danube Bend. Incision of the Danube appears to be connected to the uplift of the HMR, obtained incision rate values can be taken as valid approximations of the uplift rate in the Danube Bend area.     

Two-pyroxene thermobarometry with new experimental data in the system CaO-MgO-Al2O3-SiO2

In the system CaO-MgO-Al₂O₃-SiO₂ (CMAS), equilibrium alumina contents of orthopyroxene and clinopyroxene, both coexisting with spinel + forsterite or spinel + anorthite, have been reversed in 16 runs at 1,300–1,400°C and 10.2–20.8 kbar, using PbO flux. The present data and the data of Perkins and Newton (1980) have been modeled using the Redlich-Kister equation. The resulting model satisfies most of the reversed data in the CMAS system, agrees very well with thermochemical measurements, and is consistent with the model for the enstatite-diopside join of Lindsley et al. (1981) and with the system MgO-Al₂O₃-SiO₂ of Gasparik and Newton (1984). The present data, however, do not confirm the negative slopes of Al-isopleths in the spinel lherzolite field suggested by Dixon and Presnall (1980). The new model has been used to calculate a graphical two-pyroxene thermobarometer applicable to natural two-pyroxene assemblages closely approaching in composition the CMAS system.     

Experimental study of subsolidus phase relations and mixing properties of pyroxene in the system CaO-Al2O3-SiO2

Subsolidus phase relations in the system CaO-Al₂O₃-SiO₂ (CAS) were experimentally determined with tight reversals of several univariant curves and with 14 equilibration experiments containing the assemblage pyroxene + anorthite, where pyroxene is a binary solid solution of Ca-Tschermak (CaTs-CaAl₂SiO₆) and Ca-Eskola (CaEs-Ca_0.5AlSi₂O₆) endmembers.Reversals were obtained on the following reactions (bar, °C): 3An = Gr + 2Ky + Q (P = 22T ? 700), 3An + Cor = Gr + 3Ky (P = 21.8T ? 950), 3CaTs= Gr + 2Cor(P = 55T ? 53900), and 6CaTs_{(1 ? x)}CaEs_x = 2(1 ? 2x)Gr + 4(1 ? 2x)Cor + 9xAn. Observed slopes indicate 9.8 J/mol · K of Al-Si disorder in Ca-Tschermak pyroxene and 5.3 J/mol·K of Al-Si disorder in anorthite, at 1300°C. It is suggested that Al-Si disorder in anorthite increases by 1.9 J/mol · K from 700°C to 1300°C.Compositions of CaTs-CaEs pyroxene in equilibrium with anorthite and PbO-rich liquid were experimentally determined at 1400–1430°C and 22.7–30.8 kbar. Microprobe measurements gave compositions which are consistent with an ideal pyroxene solution and the following parameters for the reaction 3An = 2CaTs + 2CaEs (J, bar, K): 2RTln(X_CaTs · X_CaEs) + 60200 + 86.4T ? (5.06 + 13 × 10^?7P)P = 0, resulting in ΔH⁰_j = ?39.8 kJ/mol and S⁰ = 461.8 J/mol · K for the Ca-Eskola endmember at 1300°C. The obtained properties of the Ca-Eskola component are necessary for thermobarometry based on pyroxene bearing assemblages containing plagioclase, quartz, or kyanite.     

Spatial organization of a shredder guild of caddisflies (Trichoptera) in a riffle - Searching for the effect of competition

The shredder guild plays an outstanding role in the functioning of headwater stream ecosystems by processing allochthonous leaf litter. Traditionally, the abiotic habitat template is regarded as the major determinant of its organization, and only a limited number of studies support the importance of biotic interactions. The aim of the present study was to examine whether competition plays a significant role in organizing the shredder guild of caddisflies in a riffle. Null-model based co-occurrence, co-existence and guild variation analyses were used in the study of guild organization. In addition, the traditional variance to mean ratio was applied for measuring the intraspecific aggregation of guild members. The non-significant metric values of co-occurrence and co-existence analyses predicted that competition was of limited importance in structuring the spatial organization of the shredder guild. The observed aggregated spatial distribution of species, suggests that besides stochastic events, deterministic forces should also contribute to the organization of the shredder guild of caddisflies.