Cosmogenic neon produced from sodium in meteoritic minerals

Cosmogenic neon in sodium-rich oligoclase feldspar from the ordinary chondrites St. Severin and Guaren?a is characterized by an unusually high²²Ne/²¹Ne = 1.50 ± 0.02. This high ratio is due to the cosmogenic²²Ne/²¹Ne production ratio in sodium which is 2.9 ± 0.3, two to three times the production ratio in any other target element. The relative production rate of²¹Ne per gram sodium is one quarter the production rate per gram magnesium. The striking enrichment of²²Ne relative to²¹Ne in sodium arises from enhanced indirect production from²³Na via²²Na.The unusual composition of cosmogenic neon in sodium and sodium-rich minerals explains the high²²Ne/²¹Ne ratios observed in inclusions of the Allende carbonaceous chondrite, and observed during low-temperature extraction of neon from ordinary chondrites. The isotopic composition of cosmogenic neon released during the stepwise heating of a trapped gas-rich meteorite containing sodium-rich phases can be expected to vary, and use of a constant cosmogenic neon composition to derive the composition of the trapped gas may not be justified. Preferential loss of this²²Ne-enriched cosmogenic neon from meteoritic feldspar can result in a 2–3% drop in the measured cosmogenic²²Ne/²¹Ne ratio in a bulk meteorite sample. This apparent change in composition can lead to overestimation of the minimum pre-atmospheric mass of the meteorite by a factor of two.     

40Ar-39Ar chronology of isolated phases from Bununu and Malvern howardites

High resolution⁴⁰Ar-³⁹Ar age spectra have been measured on plagioclase and glass from two howardites. Both the plagioclase and glass from the gas-rich Bununu howardite show well-defined age plateaux, yielding distinct ages of 4.42 ± 0.04 and 4.24 ± 0.05 AE, respectively. These age patterns are rather well behaved and are interpreted as representing the distinct times of formation of plagioclase from igneous processes and of glass fragments produced by impact on the meteorite body. The release pattern for the glass from the heavily shocked Malvern howardite is undulating at low and intermediate temperatures but does have a high-temperature plateau. Its age spectrum indicates little apparent diffusion loss, but rather an extensive redistribution of either⁴⁰Ar during the shock event or of³⁹Ar during the neutron irradiation or both. The total K-Ar age of Malvern glass is 3.64 ± 0.04 AE and the high-temperature plateau is 3.73 ± 0.05 AE. The age spectrum of the Malvern plagioclase has an intermediate temperature “plateau” at 3.80 AE that represents 20% of the total⁴⁰Ar content and increases towards a high-temperature plateau at 4.29 ± 0.04 AE containing 26% of the total gas release. It seems likely that the event which formed the Malvern glass also reset part of the plagioclase. The distinct histories observed for the different phases of these howardites are consistent with their formation from a regolith. The present results along with similar young ages for igneous clasts from Kapoeta clearly show that the regoliths were extant on the parent bodies of howardites and that they were subjected to violent impact events at least as recently as 3.7 AE ago.     

A parameterization of the boundary layer of a tropical cyclone

In an earlier paper by one of the authors (Smith, 1968), a momentum integral method was developed to parameterize the gross constraint imposed by the surface boundary layer of a steady, axisymmetric, tropical cyclone on the meridional circulation within the vortex itself. Specifically, the method provides an effective means of estimating the radial variation of mean upflow/downflow induced by the boundary layer, compatible with a prescribed radial variation of azimuthal velocity just above the boundary layer,V _gr. However, it relies on a judicious choice of vertical profiles of radial and azimuthal velocity components within the boundary layer. An especially suitable set of profiles is discussed herein; these are Ekman-like profiles in which turbulent mixing is characterized by a vertically constant eddy diffusivityK _M , matched to a constant stress sublayer just above the sea surface. An attractive feature of the formulation is that a suitable value forK _M as a function of radius, which is extremely difficult to extract from observational data, can be calculated when the state of the sea surface, described by a roughness lengthZ ₀, is prescribed. Although observations ofZ ₀ at high wind speeds are not yet available, the effect of radial variations in sea surface roughness can be assessed and it is shown that these affect the upflow to a significant degree.     

Lunar polymict breccia 14321: a petrographic study

Seven petrographic thin sections of lunar rock sample 14321, ‘Big Bertha’, have been examined. It is a complex rock incorporating diverse lithic and single crystal fragments and represents a sampling of the heterogeneous Fra Mauro formation, considered by the writers to be lithified debris from the Imbrium impact event. Electron probe microanalysis and microscopic study of textures reveal the assembly history of this breccia which in turn allows some interpretation of the nature of the pre-Imbrium crust and the effect of the Imbrium impact and the subsequent transportation to the Apollo 14 site. The present-day polymict breccia 14321 is composed of basaltic clasts originating from the fragmentation of a single or closely related set of lava cooling units, a set of fragmental clasts designated as microbreccia 3 (themselves polymict microbreccias), and a light colored matrix which formed rock 14321 by cementing the two major groups of clasts. The light colored matrix material is derived from the fragmentation and mutual abrasion of the basalt and microbreccia 3. On the basis of consistent textural relations two older sets of microbreccias have been identified within microbreccia 3. Microbreccia 1 clasts are well-rounded, relatively light colored, and noritic. They are always completely enclosed within microbreccia 3, most often forming the central cores of rounded accretionary lapilli structures which we have designated as microbreccia 2. Microbreccias 1, 2, 3, and macrobreccia 14321 represent a chronological series of fragmentation and lithification events. Each of these events involved some thermal and/or shock metamorphism as evidenced by mineralogical and textural criteria, and the chronological order of formation of the breccias also corresponds to a decreasing intensity of associated thermal effects. The petrology and mineralogy of 14321 are described in detail in this paper. A more general interpretation of the combined petrographic and chemical data is given in Duncanet al. (1975a).     

Inner-city deprivation: Problems and policies in advanced capitalist countries

The problems of the inner city are a subject of growing concern in the advanced capitalist world. The level of social deprivation may be identified on a territorial basis, by mapping numerical indicators relating to various relevant conditions. However, the application of conventional geographical methodology with its emphasis on pattern identification and ecological analysis provides an inadequate foundation for the explanation of localized deprivation and for the design of remedial policy. Inner-city deprivation arises from the operation of the contemporary capitalist system, the understanding of which requires a holistic social-science perspective.     

Performance analysis of the spaceborne laser ranging system

The Spaceborne Laser Ranging System is a proposed short pulse laser on board an orbiting spacecraft.^1,2,3,4 It measures the distance between the spacecraft and many laser retroreflectors (targets) deployed on the Earth’s surface. The precision of these range measurements is assumed to be about ±2 cm (M. W. Fitzmaurice, private communication). These measurements are then used together with the orbital dynamics of the spacecraft, to derive the relative position of the laser ground targets. Assuming a six day observing period with 50% cloud cover, uncertainties in the baseline for target separations of 50 km to 1200 km were estimated to be on the order of 1 to 3 cm and the corresponding values in the vertical direction, ranged from 1 cm to 12 cm. By redetermining the measurements of the relative target positions, the estimated precision in the baseline for a target separation of 50 km is less than 0.3 cm and for a separation of 1200 km is less than 1 cm. In the vertical direction, the estimated precision ranged from 0.4 cm to 4.0 cm respectively. As a result of the repeated estimation of the relative laser target positions, most of the non-temporal effects of error sources as exemplified by the errors in geopotential are reduced. The Spaceborne Laser Ranging System’s capability of determining baselines to a high degree of precision provides a measure of strain and strain rate as shown byCohen, 1979. These quantities are essential for crustal dynamic studies which include determination and monitoring of strain near seismic zones, land subsidence, and edifice building preceding volcanic eruptions. It is evident that such a system can also be used for geodetic surveys where such precisions are more than adquate.     

Fertile and barren AlCr-spinel harzburgites from the upper mantle: Ion and electron probe analyses of trace elements in olivine and orthopyroxene: Relation to lherzolites

Ion and electron microprobe analyses of twenty-one CrAl-spinel harzburgite xenoliths from southern African kimberlites show two chemical groups. Orthopyroxenes from “fertile” harzburgites have higher CaO (mean of 11, 0.95 wt.%), Al₂O₃ (3.05 wt.%), Cr₂O₃ (0.85 wt.%) and Li (0.8 ppmw) than those from “barren” harzburgites (mean of 10, CaO 0.24 wt.%, Al₂O₃ 1.10 wt.%, Cr₂O₃ 0.35 wt.%, Li 0.3 ppmw). Olivines from all harzburgites have similar chemistry except that mean values of Li and Na are higher for barren than fertile harzburgites (Li 0.9 vs. 0.4 ppmw; Na₂O 16 vs. 7 ppmw). Orthopyroxenes from fertile harzburgites are chemically distinct from those in garne lherzolites from southern Africa and spinel lherzolites from southwest U.S.A., but orthopyroxenes from barren harzburgites are indistinguishable from those in many coarse garnet lherzolites.Chromium, Ca, Ni, Na and Li in coexisting olivines and orthopyroxenes from the above rock types show complex patterns, which for Ca, Cr and Ni can be related to pressure and temperature. Temperatures from an empirically calibrated thermometer based on Ni-Mg exchange between olivine and orthopyroxene, measured modes of harzburgites (fertile, mean of 10: ol 68, opx 31, spinel-silicate intergrowth <0.5; barren, mean of 8: ol 76, opx 23, spinel and spinel-silicate intergrowth 1), and high-pressure experimental studies suggest (a) that harzburgites are residues of partial melting, (b) that barren harzburgites were melted to a greater extent at a higher temperature (though probably at a similar depth) than fertile harzburgites, and (c) that incomplete reequilibration during retrograde metamorphism has led to development of complex inter- and intragranular textures, probably in the range ～700–900°C.