Lithospheric thickness beneath the southern Kenya Rift: implications from basalt geochemistry

Geochemical data are reported for samples from the flanks and floor of the southern Kenya Rift Valley in the Lake Magadi area, and from two central volcanoes located within the rift valley. Rift lavas include samples of Singaraini and Ol Tepesi basalts on the eastern flank, Kirikiti basalts from the western flank, and plateau trachytes from the rift valley floor. Central volcano samples are from Ol Esayeiti and Lenderut located on the eastern flank. The rift basalts are mildly ne-normative, moderately evolved (Mg#=0.39-0.62) alkali basalts and show an overall range in differentiation. Incompatible trace element abundances are moderately elevated (Nb=17-51; Zr=93-274; La=17-55 ppm) and show strongly coherent variations and constant inter-element ratios (e.g. Zr/Nb=4.2-5.5; Nb/Ta=17.5ǂ.4; (La/Sm)_n=7.3ǃ.1); isotope ratios are restricted in range (⁸⁷Sr/⁸⁶Sr=0.70393-0.70436; ¹⁴³Nd/¹⁴⁴Nd=0.51272-0.51280; ²⁰⁶Pb/²⁰⁴Pb=19.87-19.92; ²⁰⁷Pb/²⁰⁴Pb=15.68-15.70; ²⁰⁸Pb/²⁰⁴Pb=39.56-39.71). Central volcano lavas are more alkaline in character and include basanite (Ol Esayeiti; Mg# >60) and hawaiite to benmoreite (Lenderut; Mg#=0.48-0.38). Incompatible element ratio are similar to those of the rift basalts, although the chondrite normalised REE patterns are steeper (La/Sm)_n=17.4ǃ.2). ⁸⁷Sr/⁸⁶Sr (0.70358, 0.70391), ¹⁴³Nd/¹⁴⁴Nd (0.51280, 0.51267), ²⁰⁶Pb/²⁰⁴Pb (19.96,20.17), ²⁰⁷Pb/²⁰⁴Pb (15.66,15.76) and ²⁰⁸Pb/²⁰⁴Pb (39.80,40.00) ratios of Ol Esayeiti basanites are similar to the rift basalts, whereas the Lenderut lavas have unusually low¹⁴³Nd/¹⁴⁴Nd (0.512388-0.512453) ratios for their ⁸⁷Sr/⁸⁶Sr (0.70370-0.70481) ratios, and distinctly less radiogenic and variable Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb=17.93-19.01; ²⁰⁷Pb/²⁰⁴Pb=15.43-15.58; ²⁰⁸Pb/²⁰⁴Pb=37.91-39.14). An integrated model is developed in which the geochemical signature of the lavas is attributed to variable degrees of melting to depths within the garnet stability field, and in the presence of residual amphibole. The stability fields of these phases in P-T space indicates that the lavas must have formed within the sub-continental lithosphere rather than within the underlying ambient asthenosphere or a rising mantle plume. The subcontinental lithospheric mantle must therefore extend to a depth of at least 75 km beneath the Lake Magadi area, which contrasts with recent gravity models for the area, which infer that lithospheric mantle is absent beneath this section of the southern Kenya Rift.     

Errors due to the anisotropic-common-ray approximation of the coupling ray theory

The common-ray approximation eliminates problems with ray tracing through S-wave singularities and also considerably simplifies the numerical algorithm of the coupling ray theory for S waves, but may introduce errors in travel times due to the perturbation from the common reference ray. These travel-time errors can deteriorate the coupling-ray-theory solution at high frequencies. It is thus of principal importance for numerical applications to estimate the errors due to the common-ray approximation applied. The anisotropic-common-ray approximation of the coupling ray theory is more accurate than the isotropic-common-ray approximation. We derive the equations for estimating the travel-time errors due to the anisotropic-common-ray (and also isotropic-common-ray) approximation of the coupling ray theory. The errors of the common-ray approximations are calculated along the anisotropic common rays in smooth velocity models without interfaces. The derivation is based on the general equations for the second-order perturbations of travel time.     

Formation of chlorite rims and the impact of pore-lining chlorite on reservoir quality: a case study from Shiqianfeng sandstones in upper Permian of Dongpu Depression,Bohai Bay Basin,eastern China

The source material, precursor and formation processes of chlorite rims, and impact of pore-lining chlorite on reservoir quality of the Shiqianfeng sandstones, Dongpu Depression, Bohai Bay Basin, China, are studied using an integrated approach, including core observation, point-count analysis of thin-sections, scanning electron microscopy, electron microprobe analysis and cathode luminescence. The petrographic analysis shows that chlorite rims consist of grain-coating chlorite, poorly crystalline pore-lining chlorite and euhedral-crystallised pore-lining chlorite. The chemical composition shows that pore-lining chlorite is mainly Fe-rich with an average of 0.785 for Fe/(Fe+Mg) ratio. Petrographic analysis shows a large amount of volcanic dust (3.0～16.0 vol%, average of 7.93 vol%) in Shiqianfeng sandstones, which determines the formation of poorly crystalline pore-lining chlorite. Transformation of volcanic dust to smectite rims started with shallow burial depth at an early diagenetic stage, followed by in situ alteration of the smectite rims to poorly crystalline chlorite rims. Euhedral-crystallised chlorite mainly develops in sandstones with high porosity, high permeability and open flow systems. Pore-lining chlorite can inhibit quartz overgrowth but cannot effectively prevent pore-filling of authigenic quartz, carbonate and kaolinite cements, and therefore cannot prevent porosity destruction. However, the occurrence of pore-lining chlorite is a barometer of good reservoir quality and intense hydrodynamic conditions.     

Discrete element modelling of fragmentable geomaterials with size dependent strength

This paper presents the application of a discrete element technique to the analysis of the dynamic indentation of either a purely brittle or a brittle viscoplastic geomaterial which can experience fragmentation resulting in fragments with size dependent strength characteristics.     

Composition of hydrous melts in equilibrium with quartz eclogites

Summary Compositions of the hydrous melts in equilibrium with garnet, omphacitic clinopyroxene and quartz have been investigated experimentally at 28.5 and 35 kbar. They are represented by silica-rich liquids (> 70% SiO₂) with low MgO, FeO and CaO contents. The removal of ca 10–15% of the magma of this composition may be sufficient to convert quartz eclogite formed after subduction of altered MORB into a quartz-free bimineralic eclogite assemblage, which is a common type of xenoliths in kimberlites.At 28.5 kbar the solidus temperature is between 700 and 750° C in the system quartz eclogite—water, and the high pressure amphibole-out boundary lies at ca 25 kbar in accord with the previous studies.

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Die Zusammensetzung wasserhältiger Schmelzen im Gleichgewicht mit Quarz-Eklogiten

Zusammenfassung Um Prozesse zu simulieren, die bei der Subduktion von Ozeanbodenbasalten durch partielle Anatexis im Stabilitätsfeld von Eklogiten ablaufen, wurde die Zusammensetzung wasserhältiger Schmelzen in Gleichgewicht mit Granat, Omphacit und Quarz bei 28.5 und 35 Kbar experimentell untersucht. Diese Schmelzen sind reich an SiO₂ (> 70 Gew%) und arm an Mg0, Fe0 and CaO. Die Extraktion von ca. 10–15% derartiger Schmelzen würde genügen, um quarzführende Eklogite, die durch die Subduktion von alteriertem MORB Material entstanden sind, in quarzfreie bimineralische Eklogite umzuwandeln wie sie häufig als Xenolithe in Kimberliten beobachtet werden.Im System Quarz-Eklogit-Wasser liegt die Solidustemperatur bei 28.5 Kbar zwischen 700 und 750°C. Die obere Stabilitätsgrenze von Amphibol liegt in diesem Temperaturbereich bei ca. 25 Kbar.

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With 1 Figures