Thermodynamics of Al/Al avoidance in the ordering of Al/Si tetrahedral framework structures

The main driving force behind Al/Si ordering in tetrahedral framework aluminosilicates is nearest-neighbour Al/Al avoidance. Computer simulation is used to explore the direct consequences of such Al/Al avoidance. The main result is that the order-disorder transition temperature T _c falls dramatically as the concentration x of Al in the structure is reduced, and if the only interactions are those associated with nearest-neighbour Al/Al avoidance, T _c becomes zero for x less than some critical value x _c , where x _c =0.31 for the feldspar framework and x _c =0.34 for cordierite. Also a large degree of short range order is found above T _c . Both results differ radically from the standard Bragg-Williams model. Plots of entropy and enthalpy of ordering are given as functions of x and T, which may be used to interpret experimental data or for extrapolation into ranges of x and T inaccessible to experiment. Received: 14 May 1997 / Revised, accepted: 2 June 1997     

The transition from amphibolite-facies mylonite to chloritic breccia and role of the mylonite in formation of Eocene epizonal plutons,Bitterroot dome,Montana

The Bitterroot mylonite is a ductile-deformed amphibolitefacies mylonite (A-mylonite), abruptly capped by ductile- to brittle-deformed greenschist-facies mylonite (G-mylonite). The movement picture of the A-mylonite from its lineation and S-C-surfaces is strongly focused; the average orientation for the G-mylonite is similar but much more diffuse.Regional metamorphism, and intrusion of quartz diorite orthogneiss, granodiorite, and granite of the Idaho-Bitterroot batholith between 105 and less than 60 m.y. ago was followed by regional extension, formation of the A-mylonite, and rapid drop in temperature from about 700 °C to about 280 °C, at a time inferred to be about 57 to 53 m.y. ago. Pressure-relief melting of water-undersaturated rocks at deeper crustal levels, in response to the rapid decrease in pressure, may have produced the nearly dry magmas emplaced as very shallow plutons or erupted as the Challis rhyolitic volcanics 52 or 53 m.y. ago.The greenschist-facies mylonite/chloritic breccia formed during further cooling to about 100 °C, during listric normal faulting on the eastern flank of the Bitterroot dome about 40 m.y. ago.

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Zusammenfassung Der Bitterroot-Mylonit ist ein duktil deformierter amphibolitfazieller Mylonit (A-Mylonit), der von einem duktil bis spröd deformierten grünschieferfaziellen Mylonit (G-Mylonit) scharf getrennt wird. Das Bewegungsbild des A-Mylonites ist anhand seiner Lineationen und seiner S-C-Gefüge stark ausgerichtet; die Durchschnittsorientierung für die G-Mylonite ist ähnlich, streut aber breiter. Der Regionalmetamorphose, den Intrusionen von Quarzdiorit, Orthogneis, Granodiorit und dem Granitbatholithen von Idaho-Bitterroot zwischen 105 und mindestens 60 Ma folgte eine regionale Dehnungsphase, dann die Bildung der A-Mylonite, danach ein rascher Temperaturabfall von ungefähr 700 °C auf ca. 280 °C, wahrscheinlich im Zeitraum vor 57 bis 53 Ma. Druckabhängiges Schmelzen wasseruntersättigter Gesteine in tieferen Krustenniveaus könnte als Reaktion auf den rapiden Druckabfall zur Bildung der nahezu trockenen Magmen geführt haben, die dann vor ca. 52 oder 53 Ma als sehr flachliegende Plutone intrudierten oder wie der Challis-Rhyolith eruptiert wurden.Der grünschieferfazielle Mylonit und die Chloritbrekzien bildeten sich während der weiteren Abkühlung bis auf ca. 100 °C mit der Entwicklung von listrisch geformten Abschiebungen auf der Ostflanke des Bitterroot-Domes vor ca. 40 Ma.

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Résumé L'ensemble mylonitique de Bitterroot comporte une mylonite engendrée par déformation ductile dans le facies des amphibolites (mylonite A), en contact brusque avec une mylonite, ductile à cassante, appartenant au facies des schistes verts (mylonite G). L'image cinématique de la mylonite A est définie de manière très précise par sa linéation et la disposition des plans s et c; l'image de la mylonite G est semblabe, mais plus diffuse.Le métamorphisme régional et l'intrusion de méta-diorite quarzique, de granodiorite et de granite formant le batholite de l'Idaho-Bitterroot, entre 105 Ma et moins de 60 Ma, ont été suivis d'une extension régionale accompagnée de la formation de la mylonite A et d'une chute rapide de la température depuis 700 °C jusqu' à ±280 °C, probablement entre 57 et 53 Ma. La fusion de roches sous-saturées en eau, provoquée dans des niveaux crustaux plus profonds par la baisse rapide de la pression peut être à l'origine des magmas presque secs responsables de la mise en place de plutons peu profonds et d'éruptions comme celle des volcanites rhyolitiques de Challis il y a 52 ou 53 Ma.La mylonite de facies schistes verts et la brèche chloritique se sont formées au cours du refroidissement ultérieur, jusqu'à ±100 °C, à l'occasion de la production de failles normales listriques sur le flanc est du dôme de Bitterroot, il y a environ 40 Ma.

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, Bitterroot (-), (G-), . - , Bitterroot, ; , G- , . , -, , Idaho-Bitterroot 105 60 : , — -, — 700 ° 280 °, , , 57 53 . , , , , 52–53 , , Challis. 100 °, Bitterroot 40 .

     

To be or not to be... stationary? That is the question

Stationarity in one form or another is an essential characteristic of the random function in the practice of geostatistics. Unfortunately it is a term that is both misunderstood and misused. While this presentation will not lay to rest all ambiguities or disagreements, it provides an overview and attempts to set a standard terminology so that all practitioners may communicate from a common basis. The importance of stationarity is reviewed and examples are given to illustrate the distinctions between the different forms of stationarity.     

Strontium isotopic and selected trace element variations between two Aleutian volcanic centers (Adak and Atka): implications for the development of arc volcanic plumbing systems

Major and trace element concentrations and initial ⁸⁷Sr/⁸⁶Sr ratios of lavas from the Aleutian volcanic centers of Adak and Atka have been used to study the evolution of their respective lithospheric plumbing systems. The centers are within 150 km of one another and show similar overall silica ranges (47–67%), but Adak (40 km³) is smaller than Atka (200 km³). Adak's lavas are chemically and isotopically heterogeneous (⁸⁷Sr/⁸⁶Sr:0.70285–0.70330) and two units contain lithospheric xenoliths. The lavas of the much larger Atka, on the other hand, have much less variability in major and trace elements as well as ⁸⁷Sr/⁸⁶Sr (0.70320–0.70345). We suggest that these characteristics are a measure of the relative maturity and cleanliness of the lithospheric plumbing systems that supply magma to these centers. Because Aleutian volcanic centers often remain fixed for relatively long periods of time (5 m.y.), once established, magmatic passageways are repeatedly used. Young plumbing systems are relatively cool and contain large amounts of wallrock contaminant, and ascending magmas undergo contamination as well as concurrent crystallization and fractionation. With time, however, heat and mass transfer between ascending magmas and wallrock produce thermal and chemical boundary layers that insulate subsequent magmas. In effect, the plumbing system matures. The chemical heterogeneity displayed by young, dirty systems (like Adak) reflects not only the magma source but also the wallrock encountered during ascent and possibly the effects of extensive crystal fractionation. Thus, it is the petrologic data of mature, clean systems, like Atka, that yield the most direct and unambiguous information on the ultimate origin of the lavas and their near surface evolution.     

Book Review: Spatial Statistics: Methodological Aspects and Applications. By Marc Moore (ed.). Lecture Notes in Statistics 159. Springer,New York, 2001, 282 p., $69.95 (U.S.), paperback,ISBN 0-387-95240-3

Mathematical Geosciences -     

CLASS B2108+213: a new wide-separation gravitational lens system

We present observations of CLASS B2108+213, the widest separation gravitational lens system discovered by the Cosmic Lens All-Sky Survey. Radio imaging using the VLA at 8.46 GHz and MERLIN at 5 GHz shows two compact components separated by 4.56 arcsec with a faint third component in between which we believe is emission from a lensing galaxy. 5-GHz VLBA observations reveal milliarcsecond-scale structure in the two lensed images that is consistent with gravitational lensing. Optical emission from the two lensed images and two lensing galaxies within the Einstein radius is detected in Hubble Space Telescope imaging. Furthermore, an optical gravitational arc, associated with the strongest lensed component, has been detected. Surrounding the system is a number of faint galaxies which may help explain the wide image separation. A plausible mass distribution model for CLASS B2108+213 is also presented.