Modeling of retrograde diffusion zoning in garnet: evidence for slow cooling of granulites from the Highland Complex of Sri Lanka   总被引：2，自引：0，他引：2  

G. W. A. R. Fernando  C. A. Hauzenberger  L. P. Baumgartner  W. Hofmeister 《Mineralogy and Petrology》2003,78(1-2):53-71

Summary ?Diffusion modeling of zoning profiles in garnet rims from mafic granulites is used to estimate cooling rates in the Proterozoic basement of Sri Lanka, which represents a small, but important fragment of the Gondwana super-continent. Metamorphic peak temperatures and pressures, estimated with two-pyroxene thermometry and garnet–clinopyroxene–plagioclase–quartz (GADS) barometry, yield 875±20 °C and 9.0±0.1 kbar. These peak metamorphic conditions are slightly higher than results obtained by garnet-biotite Fe–Mg exchange thermometry of 820±20 °C. Reset flat zoning profiles were observed in most garnets. Only narrow garnet rims touching biotite exhibit retrograde zoning in terms of Fe and Mg exchange. The garnet zoning observed requires a slow cooling history. Equilibrium was achieved along grain boundaries during or close to peak metamorphism. During subsequent cooling to lower temperatures, only local exchange between garnet and biotite occurred. A cooling rate of 1–5 °C/Ma is estimated. The estimated temperature-time history from garnet profiles is in good agreement with the cooling history inferred from mineral radiogenic ages in the literature. Received December 11, 2001; revised version accepted August 28, 2002  相似文献   

pH changes in peralkaline late-magmatic fluids     

Gregor Markl  Lukas Baumgartner 《Contributions to Mineralogy and Petrology》2002,144(3):331-346

The 1.15-Ga-old Ilimaussaq intrusive complex in South Greenland shows an extensive fractionation trend from alkaline augite syenite to various varieties of strongly peralkaline, agpaitic nepheline syenites. The peralkaline nepheline-bearing syenites crystallized between ca. 900 and 450 °C at 1 kbar and they are cut by late-magmatic hydrothermal veins with nepheline-absent assemblages of albite + aegirine + analcime - sodalite - Na-Be-silicates (tugtupite, chkalovite, sørensenite) - ussingite (NaAlSi₃O₈*NaOH). Based on fluid inclusions and phase equilibria, these veins crystallized between 300 and 500 °C at 1 kbar. Textures indicate that the hydrothermal veins at least partly replaced earlier Ilimaussaq rocks. The occurrence of ussingite and tugtupite suggests that the late-magmatic fluids had strongly basic pH values. Speciation calculations show that the pH in fluids of the system Na-Al-Si-O-H-Cl mainly depends on the Na/Cl ratio and, to a lesser degree, on salinity and temperature. If the Na/Cl ratio is greater than 1, pH (at 400 °C and 1 kbar, where neutrality is about at pH 5) lies between 7 and 12. Because Na/Cl tends to decrease in the final stages of magmatism and during crystallization of the vein assemblage, pH of late-magmatic fluids generally should become more acidic, and only two processes can increase Na/Cl and, thus, pH: dissolution of a Cl-poor or Cl-free Na silicate or unmixing of an HCl-enriched vapour phase. Field observations and microtextures suggest that replacement reactions are responsible for the change to basic pH at least in some alteration assemblages. While replacement of 1 mol nepheline by 1 mol analcime would not alter the pH, the volume-conserving reaction 1.85  Ne + 2.3  H₂ O + 0.19  H₄ SiO₄ = 1.02  Anl + 0.83   Na⁺ + 0.83  Al(OH)₄^-1.85\;{\rm Ne} + 2.3\;{\rm H}_{\rm 2} {\rm O} + 0.19\;{\rm H}_{\rm 4} {\rm SiO}_4 = 1.02\;{\rm Anl} + 0.83\,\;{\rm Na}^ + + 0.83\;{\rm Al(OH)}_4^ - can be used to model the replacement process quantitatively, provided it occurred in a more or less closed system. Progress of this reaction leads to successively increasing pH of the fluid during fluid-rock interaction and stabilizes minerals such as ussingite and tugtupite. Transferring the two processes to a larger scale, it is proposed that the extreme 'hyper-agpaitic' assemblages at Ilimaussaq or at the Kola peninsula, which include copious amounts of very basic, water-soluble minerals such as trona, villiaumite or thermonatrite, are formed either in this way by autometasomatic reactions of late-magmatic fluids or melts (or supercritical fluid-melt-mixtures) with earlier crystallized rocks of the same plutonic complex or by large-scale vapour unmixing in the very final stages of magmatism.  相似文献   

Solubility of the assemblage albite+K-feldspar+andalusite+quartz in supercritical aqueous chloride solutions at 650 °C and 2 kbar     

Tatyana M. Pak  Christoph A. Hauzenberger  Lukas P. Baumgartner   《Chemical Geology》2003,200(3-4):377-393

The solubility of the high grade pelite assemblage albite+K-feldspar+andalusite+quartz at 650 °C and 2 kbar was determined in aqueous solutions over a total chloride range of 0.01–3 m_Cl^tot using rapid-quench hydrothermal technique. The concentration of Na, K, Si, and Al was determined in the fluid phase after quench. The K/Na ratio was determined by approaching the equilibrium from below and above. It is 0.34 at low chloride concentrations and decreases slightly to 0.31 with increasing total chloride. Silica and aluminum concentrations were determined only from undersaturation. The silica solubility is found to be independent of chloride concentration and is 0.13 molal. Aluminum is nearly independent of chloride concentration decreasing only slightly from 0.0015 to 0.0007 molal. Comparison of the experimental data with thermodynamic model calculations demonstrates that the silica concentrations are well predicted, while significant differences exist between individual databases for Al speciation and its total concentration. Al concentrations are underestimated by up to 10 to 15 orders of magnitude using the SUPCRT92 database. Predicted K/Na ratios are underestimated by up to 30%. The best predictions achieved for this simplified high-grade pelite assemblage are those using the SUPCRT92 database with revised thermodynamic data for feldspars and K- and Na-species (J. Phys. Chem. Ref. Data 24 (1995) 1401) and additional Al-species (Am. J. Sci. 295 (1995) 1255; Geochim. Cosmochim. Acta 61 (1997) 2175). The use of ideal mixing for neutral complexes in combination with the extended Debye–Hückel activity model for the charged species yields the most compatible speciation model.  相似文献   

Factors controlling carbonate dissolution rates quantified in a field test in the Austrian alps   总被引：4，自引：0，他引：4  

Lukas Plan   《Geomorphology》2005,68(3-4):201-212

Carbonate dissolution rates were investigated by measuring the mass difference of carbonate tablets exposed to natural dissolution for 1 year. 70 tablets were distributed over 13 test sites on the north slope of the Hochschwab Massif in the Austrian Alps. The influences of altitude, subsoil vs. sub-aerial exposure, vegetation, karst morphology, soil humidity, sample lithology, and sample surface morphology were investigated. The observed dissolution rates varied between 13 and 40 μm/a for subcutaneous samples and about 11 μm/a for sub-aerial exposure. Outstandingly high rates of 48 μm/a were observed in a doline and nearly zero rates were measured at a site influenced by seeping spring water.A mass balance, using high-resolution hydrological data, was calculated for the Kläffer Spring, which has an average outflow of 4.8 m³/s. It indicated a loss of 21×10⁶ kg of carbonate rock per year which gives an average dissolution rate of 95 μm/a for the catchment area of 83 km². The dissolution rates of 10 μm/a from the sub-aerially exposed samples are comparable to values from limestone pedestals, which were protected from dissolution by glacial erratics for the past 15 ka.  相似文献   

Drought 2002 in Colorado: An Unprecedented Drought or a Routine Drought?   总被引：1，自引：0，他引：1  

Roger A. Pielke Sr.  Nolan Doesken  Odilia Bliss  Tara Green  Clara Chaffin  Jose D. Salas  Connie A. Woodhouse  Jeffrey J. Lukas  Klaus Wolter 《Pure and Applied Geophysics》2005,162(8-9):1455-1479

The 2002 drought in Colorado was reported by the media and by public figures, and even by a national drought-monitoring agency, as an exceptionally severe drought. In this paper we examine evidence for this claim. Our study shows that, while the impacts of water shortages were exceptional everywhere, the observed precipitation deficit was less than extreme over a good fraction of the state. A likely explanation of this discrepancy is the imbalance between water supply and water demand over time. For a given level of water supply, water shortages become intensified as water demands increase over time. The sobering conclusion is that Colorado is more vulnerable to drought today than under similar precipitation deficits in the past.  相似文献   

The large-scale context for the TOGA Coupled Ocean-Atmosphere Response Experiment     

R. Lukas  P. J. Webster  M. Ji  A. Leetmaa 《Meteorology and Atmospheric Physics》1995,56(1-2):3-16

Summary The TOGA Coupled Ocean-Atmosphere Response Experiment (COARE) concentrated a variety of observational systems in the warm pool of the western equatorial Pacific for an Intensive Observation Period (IOP) November 1992 through February 1993. In this paper, aspects of the largescale variations of the tropical atmosphere and Pacific Ocean surrounding the observations of air-sea interaction in the Intensive Flux Array (IFA) during the IOP are described, with the objective of providing a context for the future analyses of these observations.The evolution of the 1991–1992 El Niño/Southern Oscillation event was unusual: Warm SST anomalies in the equatorial cold tongue region switched to colder than climatology in the last half of 1992, but waters warmer than 30°C remained displaced eastward just west of the dateline, coninuing to fuel anomalous convection there during the IOP. Fortunately, SST in the IFA remained warmer than 29°C during most of the IOP, and convective activity was observed over the IFA. The Southern Oscillation Index, which had relaxed to near zero prior to the experiment, decreased during the IOP, reflecting sea level presure changes associated with renewed westerly wind activity. In response to these westerly wind events, the warm pool migrated back into the central equatorial Pacific, leading to a reintensification of the ENSO warm SST anomalies east of the dateline.With 10 Figures  相似文献   

Multi-Century Tree-Ring Reconstructions of Colorado Streamflow for Water Resource Planning   总被引：3，自引：0，他引：3  

Connie A. Woodhouse  Jeffrey J. Lukas 《Climatic change》2006,78(2-4):293-315

Water resource management requires knowledge of the natural variability in streamflow over multiple time scales. Reconstructions of streamflow derived from moisture-sensitive trees extend, in both time and magnitude, the variability provided by relatively short gage records. In this study, we present a network of 14 annual streamflow reconstructions, 300–600 years long, for gages in the Upper Colorado and South Platte River basins in Colorado generated from new and existing tree-ring chronologies. Gages for the reconstruction were selected on the basis of their importance to two of the largest Colorado Front Range water providers, who provided the natural flow data for the calibration with tree-ring data. The reconstruction models explain 63–76% of the variance in the gage records and capture low flows particularly well. Analyses of the reconstructions indicate that the 20th century gage record does not fully represent the range of streamflow characteristics seen in the prior two to five centuries. Multi-year drought events more severe than the 1950s drought have occurred, notably in the 19th century, and the distribution of extreme low flow years is markedly uneven over the past three centuries. When the 14 reconstructions are grouped into Upper Colorado, northern South Platte, and southern South Platte regional flow reconstructions, the three time series show a high degree of coherence, but also time-varying divergences that may reflect the differential influence of climatic features operating in the western U.S. These reconstructions are currently being used by water managers to assess the reliability of water supply systems under a broader range of conditions than indicated by the gage records alone.  相似文献   

Turonian Radiolarians from Karnezeika,Argolis Peninsula,Peloponnesus (Greece)     

Alexandre N. Bandini  Peter O. Baumgartner  Michéle Caron 《Swiss Journal of Geoscience》2006,99(1):S1-S20

Near Karnezeika a roughly 140 m thick Upper Cretaceous section consists of interbedded pelagic limestones, cherts and coarse polymict breccias including ophiolites and shallow water limestones. At the base, pink pelagic limestones rest on deeply altered and fractured Lower Jurassic Pantokrator Limestone. This first pelagic facies is dated as middle Turonian, based on planktonic Foraminifera. Over 100 m of coarse ophiolite-carbonate breccias, interpreted as a channel or canyon fill in a pelagic environment, document the erosion of the Late Jurassic nappe edifice along the Cretaceous Pelagonian margin. Above these breccias, we mesured 16 m of principally pink and red pelagic limestones and radiolarian cherts, in which we recovered well-preserved radiolarians discussed here. In this interval, the presence of planktonic Foraminfera allows to state a late Turonian to Coniacian age. More than 40 radiolarian species are described and figured in this work. The radiolarian chronostratigraphy established by 10 different authors in 11 publications was compared for this study and used to establish radiolarian ranges. This exercise shows major discrepancies between authors for the radiolarian ranges of the studied assemblage. Nevertheless, a Turonian age can be stated based on a synthesis of cited radiolarian ranges. This age is consistent with the age based on planktonic foraminifera. In combining the ages of both Radiolaria and planktonic Foraminifera, the studied samples can be restricted to the late Turonian. However, the discrepancies of published radiolarian ranges call for an urgent, major revision of the Late Cretaceous radiolarian biochronology. The integration of planktonic foraminifera with radiolarians may greatly enhance biochronologic resolution in sections where both groups occur.  相似文献   

Metamorphic history of riebeckite- and aegirine-augite-bearing high-pressure–low-temperature blocks within the Siuna Serpentinite Mélange,northeastern Nicaragua     

Kennet E. Flores  Susanne Skora  Celine Martin  George E. Harlow  Dionisio Rodríguez  Peter O. Baumgartner 《International Geology Review》2015,57(5-8):943-977

The Siuna Serpentinite Mélange (SSM) is a subduction-zone-related complex that contains diverse blocks of igneous and sedimentary origin, overprinted by various metamorphic conditions. The SSM is located at the southern border of the Chortís block and marks the boundary between continental and oceanic crusts in the western margin of the Caribbean Plate. The serpentinite matrix mainly consists of lizardite/chrysotile, Cr-rich spinel, and relict orthopyroxene that suggest a harzburgitic protolith and an upper mantle supra-subduction zone origin. Blocks within the southern and central regions range from Jurassic pelagic sediments to mafic/intermediate igneous rocks that are metamorphosed to various degrees, ranging from prehnite-pumpellyite/greenschist to likely blueschist facies (e.g. riebeckite-bearing metashale) conditions. In contrast, the northern section encloses almost exclusively epidote-amphibolite facies metabasite blocks, and minor mica- and chlorite-rich rocks of metasomatic origin, respectively. Some of the epidote-amphibolite blocks contain relic garnet-rich zones embedded in an amphibole-rich matrix. The garnets appear to record two generations of growth and contain mineral inclusions such as amphibole, apatite, titanite, aegirine-augite, and quartz. Thermobarometric estimates for the garnet-rich zones and epidote-amphibolite-rich matrix suggest a prograde blueschist facies at ~1.2 GPa and 400–450°C, an eclogite facies metamorphic peak at 1.5–1.7 GPa and 565–614°C, and a post-peak epidote-amphibolite facies metamorphism. These pressure and temperature estimates indicate a classical clockwise PT path that has been observed in many palaeo-subduction zone environments worldwide. Phengite Ar–Ar dating of mica-rich rock yields 140 Ma and suggests an Early Cretaceous exhumation along the southern edge of the continental Chortís block.  相似文献   

Statistically Extrapolated Nowcasting of Summertime Precipitation over the Eastern Alps     

Min CHEN  Benedikt BICA  Lukas TÜ  CHLER  Alexander KANN  Yong WANG 《大气科学进展》2017,34(7):925-938

This paper presents a new multiple linear regression(MLR) approach to updating the hourly, extrapolated precipitation forecasts generated by the INCA(Integrated Nowcasting through Comprehensive Analysis) system for the Eastern Alps.The generalized form of the model approximates the updated precipitation forecast as a linear response to combinations of predictors selected through a backward elimination algorithm from a pool of predictors. The predictors comprise the raw output of the extrapolated precipitation forecast, the latest radar observations, the convective analysis, and the precipitation analysis. For every MLR model, bias and distribution correction procedures are designed to further correct the systematic regression errors. Applications of the MLR models to a verification dataset containing two months of qualified samples,and to one-month gridded data, are performed and evaluated. Generally, MLR yields slight, but definite, improvements in the intensity accuracy of forecasts during the late evening to morning period, and significantly improves the forecasts for large thresholds. The structure–amplitude–location scores, used to evaluate the performance of the MLR approach,based on its simulation of morphological features, indicate that MLR typically reduces the overestimation of amplitudes and generates similar horizontal structures in precipitation patterns and slightly degraded location forecasts, when compared with the extrapolated nowcasting.  相似文献