Dislocations in CaTiO3 perovskite deformed at high-temperature: a transmission electron microscopy study

Dislocation configurations in natural single crystals of CaTiO₃ perovskite deformed in high-temperature creep were examined and characterized by transmission electron microscopy. Screw dislocations with Burgers vector [100]_pc and [011]_pc, dissociated on the $(01\bar 1)_{{\text{pc}}} $ plane, form rectangular networks with extended four-fold nodes in the shape of octagons, a configuration never observed in any of the previously investigated perovskites, except CaGeO₃. Screw dislocations with Burgers vector [101]_pc and $(\bar 101)_{{\text{pc}}} $ , on the (010)_pc plane, react to form a twist wall; the dislocations with Burgers vector [002] produced by the reaction decompose into two perfect dislocations [001]_pc. This results in a new configuration, never observed before, with three-fold nodes at the corners of rectangles. Both the octagonal extended nodes and the junctions decomposed into perfect dislocations are seen in samples deformed indifferently by slip on {100}_pc or {110}_pc planes, but they seem to appear only above 1520 K, in the cubic phase.     

Aptian bimodal volcanic associations and granitoids in the northern margin of the Amur microcontinent: Age, sources of material, and geodynamic environments

Data on the composition, age, and source of material of Aptian rocks composing a bimodal volcanic complex and related granitoids in the northern margin of the Amur microcontinent indicate that the granodiorites of the Talalinskii Massif and subalkaline granites of the Dzhiktandiunskii Massif crystallized at 117 ± 2 and 119 ± 2 Ma, respectively (⁴⁰Ar/³⁹Ar method), and their crystallization ages coincide with the age of volcanic rocks of the Gal’kinskii bimodal complex. These data make it possible to combine the rocks within a single volcano-plutonic association. Geochemical and isotopic-geochemical features of trachybasaltic andesites of the Gal’kinskii bimodal complex suggest that the parental melts were derived from such sources as PREMA (or DM) and an enriched source of the EMII type at a subordinate contribution of a crustal source. The parental melts of rhyolites of the Gal’kinskii Complex and granitoids of the Talalinskii and Dzhiktandinskii massifs were derived from crustal material with minor amounts of juvenile material. The bimodal volcanic association and related granitoids dated at 119–115 Ma were most likely formed in geodynamic environments implying the ascent of the asthenospheric mantle.     

CELEBRATING CENTRIFUGAL FORCES IN AMERICAN GEOGRAPHY

GEOGRAPHY IN AMERICA AT THE DAWN OF THE 21^ST CENTURY. Edited by GARY L. GAILE and CORT J. WILLMOTT.     

The Mars oxidant experiment (MOx) for Mars '96

The MOx instrument was developed to characterize the reactive nature of the martian soil. The objectives of MOx were: (1) to measure the rate of degradation of organics in the martian environment; (2) to determine if the reactions seen by the Viking biology experiments were caused by a soil oxidant and measure the reactivity of the soil and atmosphere: (3) to monitor the degradation, when exposed to the martian environment, of materials of potential use in future missions; and, finally, (4) to develop technologies and approaches that can be part of future soil analysis instrumentation. The basic approach taken in the MOx instrument was to place a variety of materials composed as thin films in contact with the soil and monitor the physical and chemical changes that result. The optical reflectance of the thin films was the primary sensing-mode. Thin films of organic materials, metals, and semiconductors were prepared. Laboratory simulations demonstrated the response of thin films to active oxidants.     

Observation of temporal and spatial variations in the Fe/O charge composition of the solar particle event of 4 July, 1974

Comprehensive measurements of the temporal variations of the Z 6 charge composition in the 4 July 1974 solar event made with experiments aboard IMP-7 and 8 show that large variations in the charge composition occur in three-hour-averaged intensities. Hourly averaged fluxes show variations as large as factors of 3 to 4 from one hour to the next and 3 hour averages show nearly a factor of 10 peak to minimum over the event. The precision of the measurements are limited by counting statistics of the Fe-group channel. Iron to oxygen variations are established by both rate channel and pulse height analysis techniques. Comparison of measurements from IMP-7 and 8, separated by about 70 R _E shows that, while significant differences in composition and intensity exist for brief periods, the gross compositional variations are reproduced well at both spacecraft. These observations provide particularly stringent conditions for theories of the acceleration, release, and propagation of solar energetic particles.     

Strain analysis of a shear zone in a granodiorite—discussion

This paper comments about the use of phenomenological relations between stress and strain parameters in the field of natural deformation of rocks. Three criteria are proposed in order to ensure the validity of such an approach.     

Aragonite from deep sea ultramafic rocks

Aragonite mineralization was observed in serpentinized peridotites from the Romanche and Vema Fracture Zones in the Atlantic and the Owen Fracture Zone in the Indian Ocean, either in veins or as radial aggregates in cavities within the serpentinites. Evidence of incipient dissolution of the aragonite crystals was observed in one case. The aragonites tend to have lower Mg content (< 0.03%) and higher Sr content (> 0.95%) relative to other marine aragonites. Their ${}^{18}\text{O}{}^{16}\text{O}$, ${}^{13}\text{C}{}^{12}\text{C}$ and ${}^{87}\text{Sr}{}^{86}\text{Sr}$ isotopic ratios suggest the aragonite was deposited at ocean floor temperatures from solutions derived from sea water circulating in fissures and fractures within the ultramafic rocks. The ${}^{18}\text{O}{}^{16}\text{O}$ ratios of the serpentines indicate serpentinization occurred at higher temperatures, probably deeper in the crust. Low-T reactions between circulating seawater and Mg-silicates (primarily serpentine and pyroxenes) caused high pH and enrichment of Mg and Ca in the solution, conditions favoring carbonate precipitation. Aragonite was formed rather than calcite presumably because the high Mg²⁺ concentration in the solution inhibited calcite precipitation. The high Sr content of the aragonites is probably related, at least in part, to their low temperature of formation. Opaque mineral grains containing over 8% NiO and over 40% MnO were observed concentrated along the margins of some of the aragonite veins, suggesting that Ni is one of the elements mobilized during reactions between ultramafic rocks and circulating seawater.