Chemical composition of spinel from Uralian-Alaskan-type Mafic–Ultramafic complexes and its petrogenetic significance

Uralian-Alaskan-type mafic–ultramafic complexes are recognized as a distinct class of intrusions regarding lithologic assemblage, mineral chemistry and petrogenetic setting. In the present study, we discuss new data on the distribution of major elements in minerals of the spinel group in rocks from Uralian-Alaskan-type complexes in the Ural Mountains, Russia. Cr-rich spinel (Cr₂O₃ = 20–53 wt%) in dunite with interstitial clinopyroxene and in wehrlite cumulates indicate that it reacted with interstitial liquid resulting in the progressive substitution of Al₂O₂ and Cr₂O₃ by Fe₂O₃ and TiO₂. A distinct change in the spinel chemistry in dunite (Cr₂O₃ = 47–53 wt%), towards Al₂O₃- and Cr₂O₃-poor but Fe₂O₃-rich compositions monitors the onset of clinopyroxene fractionation in wehrlite (Cr₂O₃ = 15–35 wt%, Al₂O₃ = 1–8 wt%, Fe₂O₃ = 25–55 wt%). In more fractionated mafic rocks, the calculated initial composition of exsolved spinel traces the sustained crystallization of clinopyroxene by decreasing Cr₂O₃ and increasing FeO, Fe₂O₃ and fO₂. Finally, the initiation of feldspar crystallization buffers the Al₂O₃ content in most of the spinels in mafic rocks at very low Cr₂O₃ contents (<5 wt%). The fractionation path all along and the reaction with interstitial liquid are accompanied by increasing Fe₂O₃ contents in the spinel. This likely is caused by a significant increase in the oxygen fugacity, which suggests closed system fractionation processes. Spinel with Cr₂O₃ < 27 wt% is exsolved into a Fe₂O₃-rich and an Al₂O₃-rich phase forming a variety of textures. Remarkably, exsolved spinel in different lithologies from complexes 200 km apart follows one distinct solvus line defining a temperature of ca. 600°C. This indicates that the parental magmas were emplaced and eventually cooled at similar levels in the lithosphere, likely near the crust–mantle boundary. Eventually, these 600°C hot bodies were rapidly transported into colder regions of the upper crust during a regional tectonic event, probably during the major active phase of the Main Uralian Fault.     

Paläomagnetische Untersuchungen im Schilfsandstein (Trias,Km2) Westeuropas

Zusammenfassung Von insgesamt 46 beprobten Profilen aus dem Schilfsandstein (Trias, Km2) Westeuropas wurden bislang 41 Profile mit 343 Einheiten und 870 Handstücken mit 1897 Einzelkernen auf ihren paläomagnetischen Inhalt untersucht.Thermische Entmagnetisierungsversuche an 277 Proben zeigen, daß die natürliche remanente Magnetisierung (NRM) von mehreren, an verschiedene Magnetisierungsträger gebundenen Komponenten getragen wird.Als charakteristische remanente Magnetisierung (CARM), von der angenommen wird, daß sie das zur Ablagerungszeit herrschende Paläofeld abbildet, kristallisiert sich für die Sedimentationsräume Weserbergland und Franken eine Deklination (DR=Deklination rotiert) von 60° und eine Inklination (IR=Inklination rotiert) von 30° (DR/IR 60/30) und für Schwaben und die Nordschweiz DR/IR 240/–30 heraus.Neben der stabilen Paläofeldrichtung werden auch sedimentäre und tektonische Gefüge als weitere stabile Magnetisierungsrichtungen abgebildet. Diese Richtungen liegen der Achse maximaler Suszeptibilität (K1 max) parallel.Unter Annahme einer normalen Polarität des Erdmagnetfeldes zur Zeit der Schilfsandsteinablagerung im Weserbergland und Franken und einer reversen Polarität zur Zeit der Ablagerung in Schwaben und der Nordschweiz errechnet sich ein Paläonordpol bei 111 E 28 N; er fällt damit in die Nähe des Triaspoles der Europäischen Polwanderungskurve (McElhinny, 1973).Für den Ablagerungszeitraum des Raibler Sandsteins in den Südalpen wird ein Paläonordpol bei 128 W 48 N errechnet.Die magnetostratigraphische Zeitskala der Trias (Burek, 1970) kann um die Einstufung des Schilfsandsteins in das höhere obere Karn mit beginnender normaler Polarität und dann reverser Polarität ergänzt werden.

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In the Schilfsandstein (Triassic, Km2) of Western Europe 41 sections with 343 units and 1897 cores were sampled for palaeomagnetic investigations. By thermal demagnetization experiments different magnetic minerals were determined as carriers of the natural rémanent magnetization (NRM).The characteristic remanent magnetization (CARM) reflecting the palaeofield direction is described in the Weserbergland and Franken by DR/IR 60/30 (DR = declination after tectonic correction/IR=inclination after tectonic correction) and in Schwaben and Northern Switzerland by DR/IR 240/–30.In addition stable directions are reported which are related to sedimentary and tectonic fabric. These directions are parallel to the axis of maximum susceptibility (Klmax).Assuming normal polarity for the magnetic field during deposition of the Schilfsandstein in the Weserbergland and Franken and assuming reversed polarity in Schwaben and Northern Switzerland a palaeonorthpole is estimated at 111 E 28 N. This poleposition fits near the Triassic poleposition of the European polar wandering curve (McElhinny, 1973).The poleposition of the Bergamasker Alps (Raibler Sandstein, Southern Alps) is figured out at 128 W 48 N.The triassic magnetostratigraphic time scale ofBurek (1970) can be completed as far as the Schilfsandstein is to arrange into the highest Upper Carnian starting with normal polarity going on with reversed polarity.

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Résumé Dans le Grès à roseaux (Trias, Km 2) de l'Europe occidentale, 343 prélèvements provenant de 41 profils et 1897 carrottes extraites de 870 échantillons ont fait l'objet de recherches paléomagnétiques. — Des essais de démagnétisation thermique sur 277 échantillons ont révélé différents minéraux magnétiques comme étant les porteurs de l'aimantation rémanente naturelle (NRM). — L'aimentation rémanente caractéristique (CARM) reflétant la direction du paléochamp magnétique au moment de la sédimentation donne des valeurs de 60/30 pour DR/IR (DR=déclinaison après correction tectonique, — IR=inclinaison après correction tectonique) dans le Weserbergland et en Franconie et 240/– 30 pour DR/IR en Souabe et dans la Suisse septentrionale. — En plus de la direction stable du paléo champ, on a trouvé d'autres directions d'aimentation stables reliées aux structures sédimentaires et tectoniques. Ces directions sont parallèles à l'axe de la susceptibilité magnétique maximale (K1 max). — En supposant une polarité normale du champ magnétique terrestre au moment de la sédimentation du Grès à roseaux dans le Weserbergland et la Franconie, et une polarité inversée pendant sa formation en Souabe et en Suisse septentrionale, il en résulte une position géographique de 111 E et 28° N pour le paléopôle Nord, ce qui coïncide à peu près avec la position du pôle triasique sur la courbe polaire de l'Europe (McElhinny, 1973). Pendant la sédimentation du Grès de Raibler dans les Alpes bergamasques, le paléopôle Nord se trouvait à 128° O 48° N. — L'echelle magnétostratigraphique pour le Trias (Burek, 1970) peut maintenant être complétée en datant le Grès à roseaux dans la partie supérieure du Carnien supérieur, débutant par une aimantation normale et continuant par une aimantation inversée.

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343 41 ( ) 1897 870 . 277 , — NRM — . — CARM —, , , , , (DR = Deklination rotiert) 60°, (IR = Inklination rotiert) 30° (DR/IR 60/30), DR/IR 240/-30. , . /_1makc. , 111 E 28 N, . ., . 128 W 48 N. (Burek, 1970) . . , .

     

Geochemical evidence (C,N and Pb isotopes) of recent anthropogenic impact in south‐central Chile from two environmentally distinct lake sediment records

In this paper, we compare the elemental and isotopic (C, N, Pb) geochemistry of lake sediments from two contrasted environments in south‐central Chile. The first lake, Laguna Chica de San Pedro (LCSP), is situated in the urbanised area of the Biobio Region (36°S). The second lake, Lago Puyehue (40° S), is located 400 km to the southeast of LCSP and within an Andean national park. Our aim is to identify environmental impacts associated with increasing industrial activities and land degradation during the last 150 a. In LCSP, shifts in C/N atomic ratios, δ¹³C and δ¹⁵N from 1915–1937 to the late 1980s are attributed to successive land degradation episodes in the lake watershed. Based on a Pb isotopic mixing model, we estimate that up to 20% of lead in LCSP sediments is supplied from urban atmospheric pollution. By contrast, human impact in the watershed of Lago Puyehue is very limited. We observe no change in organic geochemistry during the last 150 a and lead contamination remains lower than 5%, even during the last decades. Although contamination levels are much higher in LCSP than in Lago Puyehue, a peak in anthropogenic Pb is recorded during the same period (1974–1976) at both sites. This maximum contamination level is consistent with increased industrial activity in the vicinity of Concepción. Copyright © 2010 John Wiley & Sons, Ltd.     

Solar magnetic and bolometric cycles recorded in sea sediments

The total carbonate and thermoluminescence (TL) profiles of the GT89-3 Ionian sea sediment core have been measured in the upper 200 cm of the core spanning the last 3100 years in order to test the presence of the Gleissberg (80–90 yr) cycle in the two different time series recorded in the same archive. Two different sampling intervals respectively of 2.5 mm and 2 mm have been chosen for the measurements in order to obtain results independent from sampling effects in the time series. We have revealed the Gleissberg cycle at 83 and 92 yr in both records.     

Additional Horizontal Traces of Ground Reflections on Ionograms from the Interkosmos-19 Satellite in the Night Trough

Geomagnetism and Aeronomy - Additional horizontal traces of ground reflections were observed in a studied series of high-latitude, Kp sounding ionograms from the Interkosmos-19 satellite. The...     

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Profondità ipocentrali e coefficienti di assorbimento per zone Etnee

Riassunto Si discutono le diverse formule che permettono il calcolo, con i dati macrosismici, delle profondità ipocentrali dei terremoti e dei coefficienti di assorbimento delle zone interessate. Si indicano delle modifiche, nella conduzione dei calcoli richiesti dalla formula delKövesligethy, apportanti notevoli semplificazioni ai calcoli stessi. In base ai metodi indicati vengono studiati i terremoti della regione etnea e particolarmente di quella orientale. Per le profondità ipocentrali i valori ottenuti, compresi fra 0.1 e 2.5 Km, confermano le idee che si hanno sulla genesi dei terremoti della zona. Per i coefficienti di assorbimento invece i risultati, compresi fra 0.03 e 0.35, hanno permesso, di trovare una legge di variazione del coefficiente stesso con la profondità, legge che per la bassa regione orientale ha l'espressione seguente: (x)=0.27e ^–3.5x.

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Summary They discuss the various formulae which enable to calculate, by means of the macrosismical data, the ipocentral earthquake depths and the absorption coefficients. They propose some modifications in the calculus required by theKövesligethy formula, which simplify considerably these calculations. On the basis of the indicated methods, the earthquakes of the Aetna area, especially those of the eastern side, are examined. The values obtained for the ipocentral depths, comprised between 0.1 and 2.5 Km, confirm the opinion they have on the origin of the earthquakes of this area. On the contrary the results for the absorption coefficients comprised between 0.03 and 0.35 enabled to find a variation law of this coefficient with the depth. For the low eastern region this law has the following form: (x)=0.27e ^–3.5x.

     

马鞍桥金矿床中香沟岩体锆石U-Pb定年、地球化学及其与成矿关系研究

对出露马鞍桥金矿床中香沟二长花岗斑岩进行了单颗粒锆石U-Pb定年和岩石地球化学研究.结果表明,锆石的LA-ICPMS U-Pb年龄值为(242.0±0.8)Ma,与前人确定的秦岭造山带的主造山时间((242±21)Ma)一致,显示香沟岩体可能和印支期华北与扬子板块的碰撞事件有关.香沟岩体以高硅富碱为特征,属于高钾钙碱性系列花岗岩类.香沟岩体高Al(Al2O3=14.49~15.61)和Sr(457.10~630.82μg/g)、亏损Y(<16μg/g)和HREE(Yb<0.45μg/g),并具有较高的Sr/Y(76.24~97.34)和(La/Yb)N(29.65~46.10)比值及强分异的稀土元素组成模式,其地球化学特征显示香沟岩体花岗岩类属于C型埃达克质(adakitic)岩石.岩石初始Sr同位素比值ISr=0.70642~0.70668,εNd(t)=-4.5~-4.0,TDM=1152~1220Ma.香沟岩体具有较低的εNd(t),ISr值和较高的TDM值,同时其Na2O/K2O接近1(Na2O/K2O=0.95~1.10),显示香沟花岗岩不是俯冲洋壳部分熔融形成的Ⅰ型埃达岩或底侵玄武质下地壳熔融所产...     

The impact of vertebrate herbivores on wetland vegetation in Atchafalaya Bay, Louisiana

Delta islands in the Atchafalaya and Wax Lake deltas in Atchafalaya Bay, Louisiana, are in an extremely dynamic successional phase. These islands initially supported large marshes dominated by the pioneering plant species Sagittaria latifolia and Sagittaria platyphylla. A general decrease in vegetated areas has occurred in the delta island marshes in the Atchafalaya Delta since about 1980, while in the Wax Lake Delta portion of the complex the vegetation still flourished. The Atchafalaya Delta provides an interesting setting for the study of herbivory because of the complex interaction of biotic and physical factors operating in this delta. We hypothesized that grazing by herbivores has a marked effect on vegetation in these developing marshes. To test this hypothesis, exclosures were erected on islands in both deltas in September 1985 and January 1986. Each set of exclosure treatments included an openly-grazed control area, an ungrazed area, an area allowing nutria grazing, and one allowing waterfowl grazing in each site. Results of the experiment, based on field sampling of vegetation, indicated decreases in plant biomass and changes in plant species composition in grazed treatments. Waterfowl and nutria reduced biomass aboul equally, but there was a more marked effect in the openly grazed areas. These findings may be extrapolated to sediment diversion areas along the Mississippi River.     

ON THE OCCURRENCE OF SPHAEROSIDERITE IN A SUBBITUMINOUS COAL FROM HSIAN COAL MINE,LIAONING PROVINCE

Mr.T.F.Hou,geologist of the National Geological Survey of Chinabrought back in 1931 from Liaoning province,a specimen of subbituminous coalin which are included patches or aggregates of small oolites.This interestingspecimen was given to the writer for microscopical investigation.Both polished