Meteoroid ablation spheres from deep-sea sediments

We present analyses of spheres magnetically extracted from mid-Pacific abyssal clays 0–500,000 years old. The concentration of spheres >200 μm is a few times 10 ppb. The spheres were divided into three groups using their dominant mineralogy, and are named iron, glassy, and silicate. Most spheres were formed from particles that completely melted as they separated from their parent meteoroids during the ablation process. However, some of the silicate spheres contain relict grains of the parent meteoroids that did not experience any melting. Typically, these relict grains are olivine crystals whose cores are Mg-rich (Fo_89–99). Commonly the outer rims of these grains were altered during heating. Other relict mineral grains include enstatite, ferrous spinel, chromite, and pentlandite.The three groups of spheres may possibly indicate some genetic significance. It seems reasonable to expect iron-rich spheres to be produced during ablation of iron and metal-rich silicate meteoroids. Metal spheres are probably not produced by ablation of predominantly silicate meteoroids because studies of fusion crusts and laboratory ablated silicate materials have never yielded separate metal spheres, but rather have produced spheres with intergrown iron oxide and silicate phases. The iron spheres possess identical mineralogy with the fusion crusts of Boguslavka, Norfork, and N'Kandhla iron meteorites as well as with the ablation debris created in the laboratory using iron and nickel-iron samples.The glassy spheres are considerably more Fe-rich than the silicate spheres. They consist of magnetite and a Fe-rich glass which is relatively low in Si. Some of these spheres may have experienced pronounced volatile depletion during the ablation process and could have been derived from silicate or metal-rich silicate meteoroids.The silicate spheres are undoubtedly derived from ablation of stony meteoroids. Two of the mineral assemblages occurring in these spheres (olivine-magnetite-glass and sulfide) are identical to those described in the natural fusion crusts of Allende, Orgueil, and Murchison meteorites, laboratory-made ablation debris, and melted interplanetary dust collected from the stratosphere. Bulk compositions and relict grains are useful for determining the parent meteoroid types for the silicate spheres. Bulk analyses of spheres have non-volatile elemental abundances similar to chondritic abundances. Analyses of relict grains identified high-temperature minerals which often occur as larger crystals in a fine-grained matrix that is characterized by voids. These voids were caused by escaping volatiles as minerals decomposed during ablation. Because larger crystals of higher-temperature minerals are associated with fine-grained, low-temperature, volatile-rich matrix, the obvious candidates for parent meteoroids of the silicate spheres containing relict grains are carbonaceous chondrites.     

Kr and Xe isotopes from spontaneous fission of 248Cm and 250Cf: Comparison with noble gases in carbonaceous chondrites

Relative yields of Kr and Xe isotopes from the spontaneous fission of ²⁴⁸Cm and ²⁵⁰Cf have been determined mass spectrometrically. The yields are as follows: ⁸³Kr/⁸⁴Kr/⁸⁵Kr/⁸⁶Kr = 0.223/0.458/0.596/ ≡ 1.00 and 0.306/0.582/0.793/ ≡ 1.00;¹³¹Xe/¹³²Xe/¹³⁴Xe/¹³⁶Xe = 0.486/0.819/1.075/ ≡ 1.00 and 0.343/0.506/0.851/ ≡ 1.00 from ²⁴⁸Cm and ²⁵⁰Cf, respectively. The Xe yields from ²⁴⁸Cm agree with an earlier determination by Leich et al. [24]. Neither of these yield patterns matches that of “fissiogenic” Kr and Xe in carbonaceous chondrites and hence ²⁴⁸Cm and ²⁵⁰Cf are ruled out as progenitors of the meteoritic Kr and Xe. In general, none of the spontaneously fissioning nuclides of actinide elements can be identified as a possible progenitor. Even the mixtures of actinides, including a combination of ²⁴⁸Cm and ²⁵⁰Cm, are unsuitable. The origin of “anomalous” Kr and Xe in carbonaceous chondrites must then be traced either to the spontaneous fission of a superheavy element or to peculiarities in specific nucleosynthetic reactions.     

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.     

The Uranium-lead isotopic dating of South African acid lavas

Bulletin of Volcanology -     

Vibration of cylindrical shells with clamped ends

A general method is outlined for the determination of natural frequencies of cylindrical shells with any boundary conditions when the effects of rotatory inertia and transverse shear deformation are included in the analysis. This is applied to cylindrical shells with both ends clamped. It is shown that the inclusion of these effects tends to have a greater effect upon frequencies of cylindrical shells with clamped ends than it does for corresponding shells with simply supported ends, for which numerical results are available. The authors suggest an empirical relation, which together with the latter results enables rapid estimates to be made of the effects of rotatory inertia and shear deformation on the frequencies of a wide range of cylindrical shells with clamped ends. An assessment of the accuracy of the theory with these effects included is made by comparing frequencies with values from a three-dimensional elasticity theory, but this comparison has to be restricted to cylindrical shells with simply supported ends.