Upper mantle heterogeneities in Africa deduced from Rayleigh wave dispersion

Rayleigh-wave group velocities have been measured along 11 paths on the African continent, north of the equator. The results lead to a division of this part of the continent into three regions: cratonic areas, non-cratonic areas (“mobile zones”), and a region located to the east of the meridian 31°E (Red Sea neighbourhood, Ethiopia, Djibouti). The highest velocities are found beneath the cratonic areas, at ～ 200 km depth (～ 5%). At greater depths, the “mobile zones” exhibit higher velocities than the cratonic regions, but the difference is not as marked (～ 2%) and the resolution is poor. Velocities in the eastern region are similar to those found under young oceanic regions.     

A late quaternary pollen record from the Transvaal bushveld, South Africa

Pollen spectra from cores of organic spring deposits from the Transvaal provide evidence for the climatic evolution of the province during the last 35,000 yr B.P. or more. The past climatic phases are derived from palynological reconstructions of past vegetation types by comparison of fossil pollen data with modern surface pollen spectra from various localities. Evidence is provided for an early moist, cool phase with relatively mesic bushveld and expanded montane forest in the central Transvaal, followed by a drier period with drier bushveld which probably lasted until approximately 25,000 yr B.P. During the next phase, which at the latest ended about 11,000 yr B.P., the temperatures were probably 5°–6°C cooler than at present. At that time bushveld vegetation in the central Transvaal was replaced by open grassland with macchia elements. Climatic amelioration came and semiarid savanna returned to the plains, at first gradually and then developing into a warm Kalahari thornveld-type vegetation. After 6000 yr B.P. it apparently became slightly wetter and a more broad-leafed bushveld developed. About 4000 yr B.P. it again became cooler and slightly wetter and the bushveld vegetation on the central and northern plains was comparable to present open upland types. After 2000 yr B.P. conditions gradually became warner until about 1000 yr B.P., when the modern climate of the central Transvaal bushveld originated.     

Hydration of an active shear zone: interactions between deformation, metasomatism and magmatism—the spinel-lherzolites from the Montferrier (southern France) Oligocene basalts

Geochemical and textural investigations have been simultaneously performed on spinel-lherzolite xenoliths from the Oligo-Miocene alkali basalts of Montferrier (southern France).All the investigated samples have undergone a deformation very particular by intense shearing under high stresses (up to 1.75 kbar), low temperatures ( 900°C) and strain rates of about 10⁻¹⁸ to 10⁻¹⁵ s⁻¹.Mineral chemistry reveals that the Montferrier lherzolites are fragments of an undepleted relatively shallow upper mantle level located at a depth of 50 km (15 kbar). Moreover, Na and Ti enrichment in diopside would reflect a metasomatic event, also emphasized by the common occurrence of pargasite in 50–70% of the investigated samples.Crystallization of this amphibole is attributed to a hydrous infiltration which is related in time and space to the deformation. Indeed, amphibole is preferentially concentrated in strongly deformed zones and in kink-band boundaries of orthopyroxene porphyroclasts. Moreover, the grain boundaries were used by the pervasive agent to percolate into the lherzolite: significant chemical variations (increase in MgO: 15% and decrease in Al₂O₃: 55%) are observed within the range of 7–5 μm adjacent to the grain boundary.Finally, Sr isotopic data (⁸⁷Sr/⁸⁶Sr) demonstrate that the amphibole, i.e. the metasomatic agent, is genetically related to the host lava of the xenoliths. Thus, the hydrous silicate liquid from which the amphibole has crystallized may be an early percolation of the ascending alkali magma.This silicate liquid hydrated the shear zone, located at a depth of 50 km, induced the hydraulic fracturation of the lherzolite and the magmatic conduit opening. Subsequently, the alkali magma sampled some fragments of this strongly deformed and metasomatized undepleted upper mantle level and brought them to the surface.     

10Be accumulation in a soil chronosequence

We have measured the concentration of the cosmogenic isotope¹⁰Be in soil samples from various horizons at six sites, including three independently dated Rappahannock River terraces and a previously undated Piedmont soil to which we have assigned an age. All of the incident¹⁰Be can be accounted for in one of these soils and a second is within a factor of two. In three soils, whose concentrations vary widely with depth, a significant fraction of the incident¹⁰Be cannot be accounted for. Incomplete sampling, and enhanced Be mobility caused by organic components, are the probable reasons for the low inventory of Be from these three soils. Overall, the data from these six sites indicate that¹⁰Be accumulation could be used to assign ages to soils if Be is not mobilized and lost from the soil profile.     

Crustal thickness estimate at AAE (Addis-Ababa, Ethiopia) and NAI (Nairobi, Kenya) using teleseismic P-wave conversions

Long-period teleseismic P waves recorded at AAE (Addis Ababa, Ethiopia) and NAI (Nairobi, Kenya) show comparable P_s conversions on the radial component of ground motion. The timing and amplitude of the P_s conversions are modeled with synthetic seismograms to get an estimate of crustal thickness under both stations. Instrument response and effective source time function are removed from the data using a P-wave equalization procedure. The timing of the P_s conversions, relative to direct P, suggests that both stations have similar crustal thickness. Using constraints on crustal velocities determined by previous surface wave dispersion and travel-time studies, the P_s-P timing suggests a crust of 41 km thickness. This agrees reasonably well with previous crustal estimates. Tangential wave forms exhibit large amplitudes and are consistent at most backazimuths. However, these wave forms could not be explained with models containing simple planar dipping interfaces. The crustal thickness of 41 km taken in conjunction with the close proximity of the stations to the rift zone suggests that crustal thinning is localized to the rift itself.     

Anomalous isotopic concentrations in the sea off Southern California

Measurements are reported of the ¹⁰Be concentrations in 15 sediments from the sea off southern California. The sedimentation patterns are modelled with a two component system, one component having the high ¹⁰Be concentration of pelagic sediment and the other the low concentration of continental runoff. The pelagic component, brought to the region by the California Current with the Pacific Ocean as reservoir, is very likely the source not only of anomalously high ¹⁰Be deposition but also of similar effects with ²¹⁰¹Pb, ^239,240pu and ²⁴¹Pu.