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71.
72.
The South Kawishiwi intrusion, located along the western marginof the Duluth Complex, Minnesota, is one of several compositeintrusions that are found in the Complex. The Duluth Complexis the principal exposed plutonic portion of the 1.1 Ga MidcontineniRift system. In the Spruce Road area the South Kawishiwi intrusionis divided into seven distinct units that are part of the broaderSouth Kawishiwi Troctolite Series defined by Severson (Tech.Rep. NRRI/TR-91/13a, Natural Resources Research Institute, Universityof Minnesota, Duluth, 1994). Units may be characterized as follows:Unit Ibasal accumulation of heterogeneous gabbro, troctolite,and norite; Unit IInorite with abundant inverted pigeonite;Unit IIItroctolite and olivine gabbro with local oxide-richlayers; Unit IVmlatroctolite, troctolite, olivine gabbro;Unit VIincreased plagioclase abundance in troctolitesand leucocratic troctolites; Unit VIstrongly alteredtroctolite; Unit VIIsimilar to Unit V, troctolite andleucocratic troctolite. Country rocks in the Spruce Road areaare granodiorite to quartz monzonite of the Archean Giants RangeBatholith. Sutfide mineralization, consisting of 15 vol.% of disseminated pyrrhotitt, cubanite, chalcopyrite and pentlandite,occurs in Units I, II, III, and VI. Oxygen isotopic analysesindicate that Unit II has experienced extensive crustal contamination.18O values of Unit II range from 6.9 to 7.1% and are 18O enrichedcompared with values of 5.16.8% found in other units.Silica contamination is indicated based not only on 18O values,but also by the predominance of orthopyroxene in the unit. Possiblehigh-18O contaminant rocks include the Giants Range Batholithand pelitic rocks of the Lower Proterozoic Virginia Formationor Biwabik Iron Formation. Mass balance computations suggestthat units in the Spruce Road area may be related through varyingdegrees of fractionation of a high-Al, olivine tholeiite magma.Modeling of trace element concentrations and variations in mineralchemistry suggest that discontinuities within the major unitsdeveloped by in situ boundary-layer equilibrium crystallizationof solidification zones 2050 m in thickness, followedby recharge of fresh magma. Upward enrichment of incompatibleelements, olivine Fa content, and plagioclase Ab content maybe effectively explained by this process. 18O values of uncontaminatedrock types are strongly correlative with modal mineralogy, andcan also be modeled by boundary-layer fractionation, A parentalmagma 18 O value of 6.3% is calculated for Unit VII based onolivine and plagioclase values, and is similar to that of severalother large, layered mafic intrusives. KEY WORDS: Duluth Complex; South Kawishiwi Intrusion; high-Al olivine tholeiite; open system crystallization; oxygen isotopes
* Present address: Korea Basic Science Center, Isotope Research Group, Yeocun Dong 2241, Yusung Ku, Yusung P.O. Box 41, Taejean 305333, Korea 相似文献
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On the meltwater genesis of drumlins 总被引:1,自引:0,他引:1
Large subglacial cavities in basal ice. the presumed precursors of depositional drumlins, can be created by corrasion from suspended sediment during a large water sheet outburst flood. The cavities are primarily the result of vortex action. For a given flood discharge the corrasion rate of the ice roof increases with water sheet velocity, V, to a power greater than V16/3 , Because of ice displacment during a flood, V can vary spatially along a flow line by up to an order of magnitude. The highest velocities normally occur downstream from major bed depressions or near the ice margin. Fields of large drumlins frequently occur at such sites. The process of formation of large subglacial cavities is predicted to be more velocity-sensitive than the erosion of bedrock. No accurate estimate of water sheet flood volumes can be made at this time but they could be at least an order of magnitude less than the 80 000 km3 , which was previously estimated. 相似文献
75.
S. P. SCHWENZER J. FRITZ D. STÖFFLER M. TRIELOFF M. AMINI A. GRESHAKE S. HERRMANN K. HERWIG K. P. JOCHUM R. K. MOHAPATRA B. STOLL U. OTT 《Meteoritics & planetary science》2008,43(11):1841-1859
Abstract— Noble gas data from Martian meteorites have provided key constraints about their origin and evolution, and their parent body. These meteorites have witnessed varying shock metamorphic overprinting (at least 5 to 14 GPa for the nakhlites and up to 45–55 GPa (e.g., the lherzolitic shergottite Allan Hills [ALH] A77005), solar heating, cosmic‐ray exposure, and weathering both on Mars and Earth. Influences on the helium budgets of Martian meteorites were evaluated by using a new data set and literature data. Concentrations of 3He, 4He, U, and Th are measured and shock pressures for same sample aliquots of 13 Martian meteorites were determined to asses a possible relationship between shock pressure and helium concentration. Partitioning of 4He into cosmogenic and radiogenic components was performed using the lowest 4He/3He ratio we measured on mineral separates (4He/3He = 4.1, pyroxene of ALHA77005). Our study revealed significant losses of radiogenic 4He. Systematics of cosmogenic 3He and neon led to the conclusion that solar radiation heating during transfer from Mars to Earth and terrestrial weathering can be ruled out as major causes of the observed losses of radiogenic helium in bulk meteorites. For bulk rock we observed a correlation of shock pressure and radiogenic 4He loss, ranging between ?20% for Chassigny and other moderately shocked Martian meteorites up to total loss for meteorites shocked above 40 GPa. A steep increase of loss occurs around 30 GPa, the pressure at which plagioclase transforms to maskelynite. This correlation suggests significant 4He loss induced by shock metamorphism. Noble gas loss in rocks is seen as diffusion due to (1) the temperature increase during shock loading (shock temperature) and (2) the remaining waste heat after adiabatic unloading (post shock temperature). Modeling of 4He diffusion in the main U, Th carrier phase apatite showed that post‐shock temperatures of ?300 °C are necessary to explain observed losses. This temperature corresponds to the post‐shock temperature calculated for bulk rocks shocked at about 40 GPa. From our investigation, data survey, and modeling, we conclude that the shock event during launch of the meteorites is the principal cause for 4He loss. 相似文献
76.
An atmosphere–ocean climate box model is used to examine the influence of cloud feedback on the equilibria of the climate system. The model consists of three non-linear ordinary differential equations, which are simplified forms of the first law of thermodynamics for the atmosphere and ocean and the continuity equation for the atmospheric component of the hydrological cycle. The mass continuity equation expresses the cloud liquid water content as a function of the evaporation rate from the ocean surface and the precipitation rate. Cloud formation releases latent heat. The model clouds also absorb solar energy at a rate consistent with recent findings. The model simulates snow–ice albedo feedback, water vapour feedback and cloud feedback. The global mean precipitation and surface temperature are analysed as they respond to enhanced greenhouse warming. Model results show that cloud feedback can lead to the occurrence of multiple climate equilibria. Some of these are warmer than the present equilibrium, with increased precipitation, while others are colder, with reduced precipitation. If the cloud feedback is weak, enhanced greenhouse forcing leads to a small alteration of the present equilibrium. If the cloud feedback is strong enough, the climate system can be forced into a warmer and wetter equilibrium. 相似文献
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F. A. PODOSEK U. OTT J. C. BRANNON C. R. NEAL T. J. BERNATOWICZ P. SWAN S. E. MAHAN 《Meteoritics & planetary science》1997,32(5):617-627
Abstract— Stepwise dissolution of bulk Orgueil reveals that all of the Cr in the whole rock is isotopically anomalous, with an anomaly pattern that is thus far unique. Most of the Cr (along with other major and minor cations) is dissolved by acetic and nitric acids; it is deficient in 54Cr by ~5 ?. Subsequent treatment with hydrochloric acid dissolves a small fraction of the Cr with positive 54Cr anomalies, up to ~210 ?. Mass balance indicates that whole rock Cr is isotopically normal within analytical uncertainties. The least extravagant interpretation of these results is that some mineral phase is enriched in a heavy-Cr nucleosynthetic component, while most of the Cr is a homogenized mixture of diverse nucleosynthetic components that would be normal except for lack of the postulated heavy Cr carrier. The carrier is likely, but not necessarily, presolar interstellar grains. Its identity is unknown and constrained only circumstantially: it must be relatively rich in Cr, it is substantially soluble in hydrochloric acid, and it is not magnetite or spinel/chromite. Scanning electron microscope (SEM) examination of Orgueil reveals candidate Cr-rich oxides, silicates, sulfides and phosphides, but none of these can be identified yet as the heavy Cr carrier. Whether presolar or not, the carrier is not chemically resistant and likely not thermally refractory, thereby differing from most other phases known to host isotopic anomalies. Its survival (or production) thus establishes constraints on a different regime of nebular history. 相似文献
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