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.     

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.     

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.     

Weather modification in arid regions: The Israeli experience

One of the suggestions for modifying weather in semi-arid zones is to alter the albedo over large geographical areas. Work on exploration of this suggestion has been going on for a number of years. Most of the effort along these lines has been to use numerical models, in which it is possible to alter the albedo. One of the questions asked is: What is the minimum size area over which the albedo should be altered to create a noticeable effect in the desert atmospheric flow? In this review, we show, by means of numerical experimentation, using Negev data, that the minimum size desert area which should be affected is 250 km. Experiments to enhance rainfall by cloud seeding in Israel have been carried out for about 26 years. Most of the experimentation has been taking place in the northern part of the country. There are two methods for seeding—“static”, in which seeding causes changes in the cloud's microphysics, and “dynamic”, in which seeding causes changes in the cloud's dynamics. Results of both types of experiments in Israel are discussed. Experiments are now under way to apply these techniques to desert clouds.     

Glacier variations in the Sierra Nevada, California, as related to a 1200-year tree-ring chronology

A time-series of tree-ringwidth indices for alpine timberline foxtail pine (Pinus balfouriana) from the Sierra Nevada of California shows a growth response to summer, late fall, and early winter temperatures that is the inverse of that resulting in the expansion of alpine glaciers. These are correlated with lichen-dated moraines and avalanche deposits that accumulated during subsequent decades. Minima in the ringwidth record, reflecting marked temperature declines occurred at 810, 1470, 1610, 1700 and 1810. Cold periods of lesser extent are also indicated between 1190 to 1400 and suggest that the initial pulses of the Sierran Matthes advances may have begun as early as 1190, or 150 yr earlier than previously dated. The Matthes advances were preceded by a period of pronounced warmth from 900 to 1190 during which timberline rose 10 m to its present elevation. A warm period is also indicated between 1500 and 1580. Recent work extending the tree-ring chronology to 3031 yr B.P. and radiocarbon dating of weathered samples to 6300 yr B.P. suggests that the chronology may ultimately be applicable to the dating of earlier Sierran glacier advances.     

Chromium and phosphorus enrichment in the metal of type II (C2) carbonaceous chondrites

Electron microprobe analyses of metal grains in nine C2 meteorites show consistently high Cr and P contents, with large grain to grain variations within individual meteorites. Cr ranges from 0.16 to 1.0 wt.% (average 0.6 per cent for 43 analyses). P ranges from 0.00 to 3.2 wt.% (average 0.42 per cent). In addition, metal grains in seven C3 meteorites show a lesser enrichment of Cr, 0.00 to 0.7 per cent (average 0.13 per cent for 59 analyses), with no P present. Both of these elements, Cr and P, are generally below detection in the metal of other chondrite groups, equilibrated and unequilibrated. C2 metal grains occur typically as spherical to ovate blebs contained within single, euhedral forsterite crystals or crystal fragments that are either isolated in the black C2 matrix or are in clusters making up white inclusions. Metal is rare within true chondrules. Calculations of the expected Cr contents in metal condensing directly from a solar nebular gas agree remarkably well with the observed values. Unfortunately, no calculation is possible for P because of insufficient data at the present time. The composition and the textural relationships indicate this Cr, P rich metal, and the enclosing forsterite are direct condensates from a cooling solar nebula. Cr and P, in the quantities reported here, characterize C2 metal.     

Adjustment by least squares in Geodesy and photogrammetry

Sans résumé

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Adjustment by least squares in Geodesy and photogrammetry