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.     

Oligocene to Present kilometres scale subsidence and exhumation of the Ligurian Alps and the Tertiary Piedmont Basin (NW Italy) revealed by apatite (U–Th)/He thermochronology: correlation with regional tectonics

The Ligurian Alps segment of the Alpine–Apennine orogen in NW Italy is unconformably covered by Upper Eocene to Holocene sediments in the Tertiary Piedmont Basin (TPB) and Po Plain. These sediments dip towards the north demonstrating the erosional nature of the southern border of the succession and implying that the adjacent orogenic belt formed the substratum rather than the margin of the sedimentary basin. Apatite (U–Th)/He and fission track thermochronology shows that the orogen first subsided and was buried at >4 km from 30 to 26 Ma and began its exhumation thereafter. From 26 Ma to present, this upward movement was contemporaneous with subsidence in the northern TPB. The couple exhumation in the S and subsidence in the N migrated northwards through time. Vertical movements in the area are similar to those reconstructed in Corsica. In both cases, the onset of exhumation becomes younger away from the Ligurian‐Provençal basin and has little correlation with the opening of the surrounding oceanic basins.     

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.     

Reporting of cosmogenic nuclide data for exposure age and erosion rate determinations

We propose guidelines for the reporting of in situ cosmogenic nuclide data for exposure age and erosion rate determinations. This is motivated by the need to maintain the utility of such data in the future, and to delineate best scientific practice. These guidelines will allow published exposure ages and erosion rates to be recalculated with confidence by others in the future, if, as is likely, procedures to calculate cosmogenic nuclide production rates are modified in the meantime.     

A complete and easily accessible means of calculating surface exposure ages or erosion rates from Be and Al measurements

We codify previously published means of calculating exposure ages and erosion rates from ¹⁰Be and ²⁶Al concentrations in rock surfaces, and present a single complete and straightforward method that reflects currently accepted practices and is consistent with existing production rate calibration measurements. It is intended to enable geoscientists, who wish to use cosmogenic-nuclide exposure age or erosion rate measurements in their work to: (a) calculate exposure ages and erosion rates; (b) compare previously published exposure ages or erosion rate measurements on a common basis; (c) evaluate the sensitivity of their results to differences between published production rate scaling schemes. The method is available online at http://hess.ess.washington.edu.     

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.     

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.     

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.