Model of geomagnetic paleosecular variations for the Northern Hemisphere

Summary A model of radial drifting and oscillating dipoles to represent geomagnetic palaeosecular variations, which showed to be appropriate for the Southern Hemisphere, is evaluated for the Northern Hemisphere. After a new fitting of some involved parameters, its theoretical results are compared with the palaeomagnetic data of Lake de Bouchet (France) and the Black Sea. A time lag between the declination and inclination profiles is observed in both cases. A good cross-correlation coefficient is obtained for the declination and inclination data.Presented at 2nd conference on New Trends in Geomagnetism, Castle of Bechyn, Czechoslovakia, September 24–29, 1990.     

Tidal contribution of the satellites to removing the angular momentum of Jupiter

Summary It has been shown that dynamically, on the basis of the distribution of angular momenta, the Jovian system cannot be considered an analogue within the Solar system. The total tidal decrease in the angular momentum of Jupiter and in its angular velocity of rotation have been estimated, as well as the loss of mechanical energy due to tidal dissipation. It has been concluded that there are no dynamical contradictions with the hypothesis of the common cosmogonic origin of Jupiter and of its eight close satellites.

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Distribution and fluxes of metals in the St. Lawrence River from the outflow of Lake Ontario to Québec City

Six anchor stations in the St. Lawrence River from the outflow of Lake Ontario to Quebec City, were occupied for ca. 24 hours in June 1987 during low flow conditions. Samples of water and suspended particulate matter were separated by continuous-flow centrifugation, and were collected every two hours.During this sampling period, fluxes of dissolved forms of zinc, copper and nickel increased in Lac St. François and downstream relative to the sum of the fluxes for the two upstream stations at the outflow of Lake Ontario. Increases in the flux of dissolved zinc and copper were pronounced below Montreal and above Lac St. Pierre. For particulate forms of metals, all five metals show that there are significant inputs in the section of the St. Lawrence River between Lac St. François and the station just above the entrance to Lac St. Pierre.The average concentrations of dissolved cadmium, lead, zinc, copper and nickel ranged from 7–23 ng/l; 9–35 ng/l; 0.434–0.939 g/l; 0.15–0.89 g/l and 0.58–1.12 g/l respectively.Regression analysis of the dissolved and particulate metal concentrations suggests that the concentration of dissolved cadmium, lead, zinc and nickel can be predicted from the regression equation and the determination of particulate metal concentration. This prediction appears to be independent of the suspended particulate matter concentration which varies from ca. 1 to 10 mg/l from the outflow of Lake Ontario to Québec City.     

Ion microprobe analysis of oxygen isotope ratios in granulite facies magnetites: diffusive exchange as a guide to cooling history

Ion microprobe analysis of magnetites from the Adirondack Mountains, NY, yields oxygen isotope ratios with spatial resolution of 2–8 m and precision in the range of 1 (1 sigma). These analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of ¹⁸O at this scale. High precision micro-analyses of this sort will permit wide-ranging new applications in stable isotope geochemistry. The analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite. Textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite. Magnetites are near to Fe₃O₄ in composition, and optically and chemically homogeneous. A combination of ion probe plus conventional BrF₅ analysis shows that individual grains are homogeneous with ¹⁸O=8.9±1 SMOW from the core to near the rim of 0.1–1.2 mm diameter grains. Depth profiling into crystal growth faces of magnetites shows that rims are 9 depleted in ¹⁸O. These low ¹⁸O values increase in smooth gradients across the outer 10 m of magnetite rims in contact with calcite. These are the sharpest intracrystalline gradients measured to date in geological materials. This discovery is confirmed by bulk analysis of 150–350 m diameter magnetites which average 1.2 lower in ¹⁸O than coarse magnetites due to low ¹⁸O rims. Conventional analysis of coexisting calcite yields °¹⁸O=18.19, suggesting that bulk ¹⁸O (Cc-Mt)=9.3 and yielding an apparent equilibration temperature of 525° C, over 200° C below the temperature of regional metamorphism. Consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories. The data for oxygen in calcite under hydrothermal conditions at high P(H₂O) indicates that diffusion is faster in magnetite and modelling of the low ¹⁸O rims on magnetite would suggest that the Adirondacks experienced slow cooling after Grenville metamorphism, followed by a brief period of rapid cooling, possibly related to uplift. Conversely, the data for calcite at low P(H₂O) show slower oxygen diffusion than in magnetite. Modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals, suggesting that the low ¹⁸O rims form by exchange with late, low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets. In either case, the ion microprobe results indicate that two distinct events are recorded in the post-metamorphic exchange history of these magnetites. Recognition of these events is only possible through microanalysis and has important implications for geothermometry.     

Na-rich carbonate inclusions in perovskite and calzirtite from the Guli intrusive Ca-carbonatite,polar Siberia

Carbonate phases, some rich in Na₂O and comparatively rich in SrO and BaO, occur as inclusions in perovskite and calzirtite (Ca₂Zr₅Ti₂O₁₆) in the carbonatite of the Guli complex, Siberia. This is the first record of alkali carbonates, akin to nyerereite [Na₂Ca(CO₃)₂], in plutonic igneous rocks. The inclusion populations suggest that the parental magma of the complex was Ca-rich but developed Na-rich differentiates during the latest stages. This points to the dominant calcic carbonatites of the complex not being derivatives of alkali-rich parental carbonatites. These alkali-rich carbonate inclusions (and rare inclusions of djerfisherite) have been preserved due to the resistance of perovskite and calzirtite to processes of leaching, hydrothermal alteration and weathering.     

An Example of Island-Arc Petrogenesis: Geochemistry and Petrology of the Southern Luzon Arc, Philippines

Volcanism throughout the Luzon arc is associated with eastwardsubduction of the South China Sea floor along the Manila Trench.The southern section of the arc, the focus of this study, extendsfrom the Lingayen-Dingalan fault to the small islands just southof Luzon. Two segments appear to exist along this section ofthe arc the northern Bataan and southern Mindoro segments whichare separated by the Macolod Corridor. The volcanic rocks have typical arc phenocryst mineralogies:olivine, clinopyroxene, plagioclase, and titanomagnetite inthe most mafic rocks and clinopyroxene, plagioclase, orthopyroxene,titanomagnetite, ? amphibole in the more felsic samples. Complexzoning, sieve textures, and decoupling of incompatible traceelements suggest that processes such as assimilation have takenplace. The rocks from the study area range from basalts to rhyolitesand show typical calc-alkaline features. The rocks of the MacolodCorridor and Mindoro segment are particularly enriched in largeion lithophile elements (LILE), light rare earth elements (LREE),and radiogenic Sr compared with the Bataan segment. The datafall within the mantle array on Sr-Nd isotopic diagrams andgrade toward higher Sr and lower Nd isotopic values from northto south. A likely source for the volcanics of this study is either amid-ocean ridge basalt (MORB)-type mantle that undergoes higherdegrees of partial melting than regions involved in MORB generationor a previously depleted source. We suggest that the high fieldstrength element (HFSE) anomalies have been derived throughdifferential element partitioning during fluid transport fromthe subducted lithosphere to the mantle wedge. Continental crustal material seems to play a significant roleparticularly in the Macolod Corridor and the Mindoro segment,based on the high LILE, La/Sm ratios, radiogenic Sr isotopes,and ¹⁸O values. The Macolod Corridor and the Mindoro segmenthave undergone source contamination by crustal material fromthe North Palawan-Mindoro crustal block either during the collisionof this block with the Manila Trench or by subduction of sedimentsrich in this crustal material. A similar component has alsobeen detected in the Bataan segment but in minor amounts. Thetrace element and isotopic differences between the northernand southern sections of the arc are interpreted in terms ofvariable composition (i. e., variable amounts of a crustal componentintroduced from the Palawan-Mindoro crustal terrain) of themetasomatic fluids released into the source.     

Geochemical specialization of the tin-bearing granitoid massifs of NW Bohemia

The geochemistry of Hercynian tin-bearing granitoid massifs of the Krune hory Mts. (Erzgebirge), Slavkovský les Forest (Kaiserwald) and Smriny (eastern Fichtelgebirge) is compared by statistical processing of 270 analyses including a wide spectrum of major and trace elements. Seven different types of granites are distinguished. Out of these, five types represent the successive differentiation of the largest massif of NW Bohemia: the Karlovy Vary (Karlsbad) massif. This comprises strongly differentiated peraluminous granites evolving towards extreme Li-Rb-Cs-F-and Sn-enrichment in the youngest members, which are albite-topaz-zinwaldite lithium granites. The sixth and seventh types are different from the former by their location in the eastern Krune hory and tectonic setting, and they display geochemical features of anorogenic granites: they are metaluminous albite-zinwaldite granites with marked enrichment of Nb, Y, and HREE in addition to Li, Rb, Cs, F and Sn, indicating contamination by sub-crustal material. Sn-W mineralizations, including flat peri-contact greisen bodies, steep greisen veins and tourmalinized phyllites, are all intimately associated with the most strongly differentiated granites — the Li-granite and the Cinovec-granite respectively.     

Directions and possibilities of mathematical geology

This paper considers the present state of mathematical geology. Three directions are recognized: applied, theoretical, and mathematical. Applied mathematical geology includes formal use of mathematics to solve problems and computer processing of data. Success is achieved by a correspondence of mathematical methods used to the nature of geological data. This correspondence can be demonstrated by purely mathematical means. Theoretical mathematical geology uses mathematics as a language of geology; however, a number of methodological problems must be solved: formalization of initial geological concepts and creation of a strict conceptual basis, substantiation of initial principles of mathematical simulation, creation of theoretical geological models, problems of elementary and coincidence in geology, and methodological substantiations of possibilities of any mathematical model to approximate geological models. The essense and significance of these problems are considered. The main task of mathematical geology is to prove its correspondence to the nature of the geological objects studied, geological data obtained, and geological problems solvable. Finally, the main problems of mathematical geology are not so much mathematical as geological and methodological.     

The effect of bulk rock composition on the stability of amphibole in the upper mantle: Implications for solidus positions and mantle metasomatism

Summary The stability of pargasitic amphibole in the upper mantle is a function of water content and bulk rock composition, and under water-undersaturated conditions, the stability of amphibole controls the solidus position. Experiments in the system Tinaquillo peridotite +0.2% H₂O, a refractory peridotite under water-undersaturated conditions, show that amphibole is stable to 1030°C and 26 kb. In contrast, pargasitic amphibole is stable to 1150°C and 30 kb in Hawaiian pyrolite, a more fertile peridotite composition. This indicates that under water-undersaturated conditions, the most fertile part of a crystallizing mantle diapir with an inhomogeneous composition will solidify first while a more refractory component will contain an alkali-rich melt which will have the ability to metasomatize adjacent regions. The relative stabilities of amphibole in refractory and fertile bulk compositions may result in increasing rather than diminishing chemical contrasts in high temperature lherzolite, i.e. a process of metamorphic differentiation. Ti, Fe, Al and Na metasomatism can therefore be considered a normal occurrence associated with the upward migration and solidification of an H₂O-bearing mantle diapir.

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Der Einfluß der Gesamtgesteins-Zusammensetzung auf die Stabilität von Amphibol im oberen Mantel: Bedeutung für Solidus-Positionen und Mantel-Metasomatose

Zusammenfassung Die Stabilität pargasitischer Amphibole im oberen Mantel ist eine Funktion von Wassergehalt und Gesamtgesteins-Zusammensetzung. Unter wasser-untersättigten Bedingungen, kontrolliert die Stabilität von Amphibol die Solidus-Position. Experimente in dem System Tinaquillo Peridotit +0,2% H₂O, einem refraktären Peridotit unter wasser-untersättigten Bedingungen, zeigen daß Amphibol bis 1030°C und 26 Kb stabil ist. Im Gegensatz dazu ist pargasitische Hornblende in einem Hawaii-Pyrolit, von mehr fertiler Peridotit-Zuammensetzung, bis 1150°C und 30 Kb stabil. Das zeigt, daß bei wasser-untersättigten Bedingungen der am meisten produktive Teil eines kristallisierenden Mantel-Diapirs mit inhomogener Zusammensetzung sich zuerst verfestigen wird, während eine mehr refraktäre Komponente eine alkali-reiche Schmelze enthalten wird, die wiederum die Fähigkeit hat, umliegende Bereiche metasomatisch zu beeinflussen. Die relativen Stabilitäten von Amphibol in refraktären und fertilen Gesamtzusammensetzungen können dazu führen, daß die chemischen Gegensätze in Hochtemperaturlherzoliten eher zunehmen als abnehmen, d. h. ein Prozeß metamorpher Differentiation. Ti, Fe, Al und Na Metasomatose können deshalb als ein verbreiteter Vorgang, der mit der Aufwärtsbewegung und Verfestigung eines H₂O-führenden Mantel-Diapirs assoziiert ist, betrachtet werden.

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With 4 Figures