Strontium and samarium diffusion in diopside

The volume chemical diffusion of trace amounts of Sr in diopside has been measured as a function of temperature (1100–1300°C), pressure (1 bar–20 kbar), crystallographic direction, and composition. Three experimental/analytical techniques were employed: radiotracer and sectioning; stable tracer and ion microprobe; and Rutherford back-scattering spectroscopy. Comparison of the three yielded excellent agreement. Both natural and synthetic single crystal samples were used with results in the natural diopside giving diffusivities approximately two orders of magnitude greater than those in the man-made crystals. Samarium diffusion in the synthetic crystals was also examined with the ion probe technique with results similar to Sr.Arrhenius relations for diffusion (D = D₀exp[?ΔH_a/RT]) were calculated for different pressures and analytical techniques, and activation volumes (gDV_a) were derived from the equation D = D' exp[?PΔV_a/ RT]. Values of ΔV_a were negative for Sr diffusion. An empirical relation describing the temperature and pressure dependence of D for Sr in the c direction of the synthetic samples is: D(P, T) = 1200 (cm²/sec) exp[?122 (Kcal/mol)/RT (°K)]exp[?P (bar)/(2.94T ? 4640)R]. The expression for D_Sr in the natural samples (c direction) at one atmosphere is: D(0, T) = 54 (cm²/sec) exp[?97 (Kcal/mol)/RT (°K)]. A single compensation trend for all the data was evident for all values of D₀ and ΔH_a in the synthetic crystals.A number of models of geologic processes were investigated in light of the present data. Closure temperatures (T_c) were calculated for examples of mineral-mineral age-dating and trace element geothermometry. High values of T_c indicate that pyroxenes record emplacement events and are generally not disturbed unless a fluid enters the system. Isotopic equilibration times were examined for lower crustal xenoliths and the mantle source region for MORB using the formula of Hofmann and Hart (1978). Equilibration was shown to be too fast for production of isotopic anomalies in MORB via disequilibrium melting. Also. reasonable residence times at lower crustal temperatures were shown to produce the mineralogical-scale isotopic homogeneity observed in a crustal xenolith from Kilbourne Hole, New Mexico.     

The survival of the ‘informal sector’: the shebeens of black Johannesburg

The present paper constitutes a contribution towards an historical geography of the informal sector. A longitudinal analysis is pursued of the survival of shebeens, an institution for the provision of liquor, in the areas of Black settlement around Johannesburg. The concepts conservation and dissolution which derive from the Marxian based literature on petty commodity production afford a theoretical lens for interpreting the survival of the informal sector. Three major phases in the persistence of shebeening in Black Johannesburg are delineated, viz., (1) the era of liqour prohibition extending to 1937, (2) the period 1937–76 when shebeens confront the system of municipal monopoly and a progressive relaxation on liquor controls, and (3) the progressive moves from 1976 towards the official acceptance and legalization of shebeens.     

Simulation of artificial earthquakes

A procedure is developed for the simulation of artificial earthquake accelerograms, The time variation of amplitude and frequency content is preserved in the simulation procedure. Sixteen artificial earthquake accelerograms are simulated and compared with a target accelerogram. The time variation of amplitude and frequency content for 26 historical earthquake accelerograms is characterized.     

Local instability in a periodically forced sliced cylinder

This paper investigates the dynamics of mesoscale eddy generation by instability of time-varying flows. Laboratory experiments on oscillatory motion over topography in a rapidly rotating cylinder have shown that isolated mesoscale eddies, which form in the sidewall boundary layer during certain phases of the forcing cycle, are associated with the onset of chaotic behavior in this system. This paper explores the origin of these eddies by performing computational simulations of the flow, and then interpreting the results of the calculations using spatially localized and quasi-static linear stability theory. For most of the experimental parameter space the quasi-geostrophic simulations are in excellent agreement with the laboratory observations. The eddies arise as a barotropic shear flow instability in regions of space and at times where the inflection points of the instantaneous large-scale flow are farthest from the sidewall, and where Fjortoft's theorem is strongly satisfied. At finite amplitude, advection of the local wavetrains up the bottom slope strengthens the anticyclonic eddies. These then merge, leading in most circumstances to a single strong anticyclonic vortex that can leave the sidewall and penetrate the interior. When parameters are such that the eddy persists all the way around the basin and back to the local instability region, the flow is observed to become chaotic.     

Large scale isotopic Sr, Nd and O isotopic anatomy of altered oceanic crust: DSDP/ODP sites417/418

Large-scale compositional domains at DSDP/ODP drill sites 417A, 417D and 418A were analyzed for O, Sr and Nd isotope ratios, and REE, U, K, Rb and Sr abundances, to constrain the bulk chemical composition of the oceanic crust that is recycled at subduction zones. The combination of the three sites gives the composition of the upper oceanic crust in this region over a distance of about 8 km. The δ¹⁸O_SMOW and⁸⁷Sr/⁸⁶Sr_meas of compositional domains 10–100 m in size correlate well, with a range of 7.7–19.2 and 0.70364–0.70744, and mean of 9.96 and 0.70475, respectively. The Rb inventory of the upper crust increases by about an order of magnitude, while Sr contents remain constant. U abundances increase moderately under oxidizing alteration conditions and nearly triple in the commonly reducing alteration environments of the upper oceanic crust. REEs are influenced by alteration only to a small extent, and recycled oceanic crust is similar to MORB with respect to¹⁴³Nd/¹⁴⁴Nd. Even though the average composition of the upper oceanic crust is well defined, the large scale composition varies widely. Highly altered compositional domains may not have a large impact on the average composition of the oceanic crust, but they may preferentially contribute to fluids or partial melts derived from the crust by prograde metamorphic reactions.     

Cretaceous foraminifera from the Turonian succession at Beer, southeastern Devon, England

During Winter 2000/2001 a major cliff fall occurred at the back of the bay in Beer, southeastern Devon. In the subsequent remedial works the cliff was cleared of loose material prior to stabilisation works. This process provided a suite of samples from previously inaccessible parts of the cliff succession. By adding these samples to earlier suites of samples collected over the last 30 years it has been possible to improve our knowledge of the foraminifera of Turonian age in southwestern England.The planktic foraminifera are, perhaps, the most significant component of the assemblages as many of the taxa recorded in Devon are of southern aspect and are relatively rare in others parts of the U.K. The planktic zonation recognised in this section is, therefore, almost identical with that recorded in Tethyan areas and a precise correlation can be generated. There are relatively large numbers of important taxa such as Helvetoglobotruncana helvetica, Dicarinella imbricata, Marginotruncana sigali, M. pseudolinneiana, M. coronata and M. schneegansi. The benthic foraminifera, by comparison, are relatively rare and are represented by a low diversity assemblage. This is typical of northwestern European chalk successions of Turonian age and is coincident with the highest sea levels of the Cretaceous. In the middle to late Turonian there is a dramatic shallowing event that is recorded world-wide at this level. The assemblage changes towards the top of the accessible succession at Beer record this significant, world-wide event.     

Interaction between subducted continental crust and the mantle

The syncollisional mafic-ultramafic rocks with Nb, Zr, Ti negative anomalies in the North Dabie terrane have Sr and Nd isotopic compositions with EMI features. Their low and variable initial e_Nd values ranging from ?2 to ?18 are similar to those of their gneissic country rocks and the ultrahigh-pressure metamorphic rocks in the South Dabie terrane. These Sr and Nd isotopic features are difficult to be interpreted by mantle metasomatism related to oceanic subduction or crust assimilation, but is best explained by the mantle metasomatism related to continental subduction.     

A possible atmosphere for Pluto

At the temperature of Pluto (～43°K) the only gas which would neither condense nor escape is neon. Since neon is cosmically abundant it is suggested that Pluto may have a fairly extensive atmosphere consisting of almost pure neon. The possibility that such an atmosphere exists is analyzed, along with the possibility that oceans of liquid neon may exist at the surface.A neon atmosphere would not produce any observable absorption lines. However, if it were very thick, then Rayleigh scattering would result in Pluto having a much higher albedo in the ultraviolet than in the visual, which is not observed to be the case. This enables us to set an upper limit on the mass of Pluto's atmosphere.