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821.
The abundance and 13C/12C ratios of carbon were analyzed in basaltic glass from twenty locations along the Juan de Fuca Ridge using a 3-step combustion/extraction technique. Carbon released during the first two combustion steps at 400-500 degrees C and 600-650 degrees C is interpreted to be secondary, and only the carbon recovered during a final combustion step at approximately 1200 degrees C is thought to be indigenous to the samples. For carbon released at approximately 1200 degrees C, glasses analyzed as 1-2 mm chips contained 23-146 ppm C with delta 13C values of -4.8 to -9.3%, whereas samples crushed to 38-63 microns or 63-90 microns yielded 56-103 ppm C with delta 13C values of -6.1 to -9.2%. The concentrations and isotopic compositions of the primary carbon dissolved in the glasses and present in the vesicles are similar to those previously reported for other ocean-ridge basalts. The Juan de Fuca basaltic magmas were not in equilibrium with respect to carbon when they erupted and quenched on the sea floor. Evidence of disequilibrium includes (1) a large range of carbon contents among glasses collected at similar depths, (2) a highly variable calculated carbon isotopic fractionation between melt and vapor determined by comparing crushed and uncrushed splits of the same sample, and (3) a lack of correlation between vesicle abundance, carbon concentration, and depth of eruption. Variations in carbon concentration and delta 13C ratios along the ridge do not correlate with major element chemistry. The observed relationship between carbon concentrations and delta 13C values may be explained by late-stage, variable degrees of open-system (Rayleigh-like) degassing. 相似文献
822.
Multiple Enrichment Processes and their Relationships in the Subcrustal Lithosphere Beneath the Eifel (Germany) 总被引:5,自引:4,他引:1
Three types of enrichment processes may be distinguished inspinel peridotite mantle xenoliths from the West Eifel (Germany):
- Enrichmentof light rare earth elements (LREE) occurs in wholerocks andclinopyroxenes in conjunction with the formation ofTi-pooramphiboles (<05%) in a low-tempetature suite(900C).
- Enrichment in high field strength elements (HFSE; Ti and Hf)is associated with strong zoning of TiO2 in amphibole adjacentto mica hornblendite veins. A reheating from 900 to 1050C associatedwith this vein formation is reflected by the zoning profilesin orthopyroxenes with Ca, Al, and Cr increasing from core torim.
- Moderate enrichment of LREE without amphibole formationin ahigh-temperature suite (1125C) is observed at the contactofperidotites to pyroxenite veins yielding the same temperature.
- Based on geochemical and isotopic evidence, enrichment process(1) is inferred to be due to interaction of peridotites fromthe subcrustal lithosphere with low-density fluids. Process(2) represents a subsequent interaction of amphibole-bearingperidotites formed during the first process with basic meltsmoving through a system of cross-cutting dykes and veins. Processes(2) and (3) are linked to the Quaternary West Eifel volcanism.Geothermobarometry of the xenoliths shows that these processesare related to different depth regimes of the lower lithosphere.
823.
Mafic hypabyssal rocks in the western Triassic and Paleozoicbelt provide important clues to the nature of accretion andarc evolution along this sector of the North American margin.In the east-central part of the belt, near Sawyers Bar, somediabases have been metamorphosed before and accompanying emplacementof the mid-Jurassic English Peak and Russian Peak granitoidswithin the North Fork/Salmon River + Stuart Fork amalgamatedterrane. Certain other dikes/sills, chiefly mafic microdiorites,cut the calc-alkaline plutons but are themselves deutericallyaltered; at least two of these mafic microdiorites near theEnglish Peak body possess hornfelsic textures. Thus, althoughmost mafic microdioritic hypabyssals seem to have been injectedafter granitoid emplacement, a few must have preceded plutonicintrusion. Macroscopic appearances, phase assemblages, mineralcompositions, and textures of the mafic microdioritic and metadiabasicdikes/sills are sufficiently alike to preclude the ready fieldand petrographic distinction of the different magma series.Bulk-rock chemistries fall into two groups, however, with slightlymore porphyritic, altered, synplutonic mafic microdiorite samplesbeing distinctly richer in Si, K, P, Rb, Sr, Zr, and light rareearth elements (LREE) relative to the Mg + Cr + Ni-rich, preplutonicmetadiabases. Analyzed mafic microdiorites have bulk-rock chemicaland isotopic compositions similar to the more ferromagnesianportions of the mid-Jurassic English Peak and Russian Peak plutoniccomplexes, whereas the metadiabases are comparable with theearly Mesozoic Salmon River metabasalts. Although the two groupsof dikes/sills probably overlap in age of emplacement, the maficmicrodiorite group is predominantly younger and uniform in oxygenisotopic composition (bulk-rock 18O 1137, 114 and1146) compared with the older, more intensely metamorphosed,and variably metasomatized Salmon River metadiabases (bulk-rock518 94, 110, and 153). Both types of maficdike/sill locally intrude the more easterly Stuart Fork terrane.Therefore, suturing and regional metamorphism of the outboardNorth Fork/Salmon River oceanic-island arc and inboard StuartFork subduction complex must have occurred during terminal stagesof injection of the pregranitoid diabases into the North Fork(oceanic-island basalts)/Salmon River (island-arc tholeiites)arc + Stuart Fork terrane, but before invasion of the amalgamatedterrane assembly by the calc-alkaline plutons and most compositionallyrelated synplutonic mafic microdiorite dikes/sills. Becauseof their lateral continuation both north and south of the SawyersBar area, the North Fork/Salmon River igneous suite documentsthe construction of an oceanic arc of considerable lateral extentin the central Klamaths before terrane accretion. Suturing wasimmediately followed by the mid-Jurassic intrusion of calc-alkalineplutons + syngranitoid mafic microdioritic hypabyssals. 相似文献
824.
P. F. Schofield C. M. B. Henderson S. A. T. Redfern G. van der Laan 《Physics and Chemistry of Minerals》1993,20(6):375-381
The solid solution sanmartinite (ZnWO4)—cuproscheelite (CuWO4) has been studied using Cu 2p X-ray absorption spectroscopy. While a single L3 absorption peak is observed for CuWO4, two distinct L3 absorption peaks with a separation of ~0.8 eV are observed for the intermediate samples in the solid solution. The two peaks represent distinct Cu sites: one with all CuO6 next nearest neighbours in the (Cu,Zn)O6 chains, another having at least one ZnO6 next nearest neighbour. Both sites show a linear increase in covalency as a function of increasing Cu-content. The relative intensities of the two absorption peaks is dependent upon the Cu-content and has been used to model the site occupancies. The results reveal that the local structural effects can be associated with a composition-dependent structural phase transition from P2/c (ZnWO4) to P $\bar 1$ (CuWO4). Deviations from a single-site model are explained in terms of the local environments, and evidence for site preferences and local clustering are explored. 相似文献
825.
Donald G. Isaak Earl K. Graham Jay D. Bass Hong Wang 《Pure and Applied Geophysics》1993,141(2-4):393-414
We present new elasticity measurements on single-crystal fayalite and combine our results with other data from resonance, pulse superposition interferometry, and Brillouin scattering to provide a set of recommended values for the adiabatic elastic moduliC
ij and their temperature variations. We use a resonance method (RPR) with specimens that were previously investigated by pulse superposition experiments. The nineC
ij of fayalite are determined from three new sets of measurements. One set of our newC
ij data is over the range 300–500 K. We believe that the relatively large discrep ncies found in someC
ij are due in large part to specimen inhomogeneities (chemical and microstructural) coupled with differences in the way various techniques sample, rather than only systematic errors associated with experimental procedures or in the preparations of the specimens.Our recommendeaC
ij's (GPa) and (C
ij/T)
p
(GPa/K) are:
The resulting values for the isotropic bulk and shear moduli,K
s and , and their temperature derivatives are:K
s=134(4) GPa; =50.7(0.3) GPa; (K
s/T)
p
=–0.024(0.005) GPa/K; and (/T)
p
=–0.013(0.001) GPa/K. An important conclusion is thatK
s increases as the Fe/(Fe+Mg) ratio in olivine is increased. 相似文献
826.
M. Varvayanni C. G. Helmis G. T. Amanatidis D. N. Asimakopoulos J. G. Bartzis A. Soilemes K. H. Papadopoulos H. D. Kambezidis 《Pure and Applied Geophysics》1993,140(4):681-720
Field measurements of wind, air temperature and humidity were taken at the eastern part of the Attika district in June 1991, to examine the topographic influences exerted on the local sea breezes. These influences are due either to the elongated Evia island, faced by the northern half of Attica coastline some tens of kilometers offshore, or to the coast-parallel range of Hymettos mountain, rising steeply 12 km onshore. The instrumentation consisted mainly of three tethered meteorological balloons released at characteristic sires (i.e., the coast, a location between shoreline and mountain foot and the mountain top) and three autographic ground-based anemometers operating at selected locations. Data from the ground-based and upper air stations of the Hellenic National Meteorological Service, as well as the diurnal weather maps were also obtained and analyzed. Observations were made under different synoptic wind and the latter was found to determine remarkably the significance of the topographic effects. A preliminary two-dimensional numerical approach was also made concerning the sea breeze capability to reach the Hymettos mountain top in the case of a weak opposing geostrophic flow. 相似文献
827.
F. Bella R. Bella P. F. Biagi M. Caputo G. Della Monica A. Ermini E. Pettinelli V. Sgrigna 《Pure and Applied Geophysics》1993,141(1):125-137
Prototype instrumentation, able to automatically measure groundwater radon content variations, is presented. The equipment is made of stainless steel and has spherical valves with automatic and pneumatic control. The deemanation of the gases from the water is obtained by evacuating a suitable expansion chamber. The instrumentation can make discrete sampling ranging from 1 per hour to 1 per 99 hours. The equipment was tested in the laboratory: the efficiency was measured by means of a266Ra solution. A mean value of (0.65±0.07) count/s/Bq was obtained. A calibration test was carried out by comparing countings from the automatic equipment with those obtained by the standard laboratory cell. Results of an operational check over a period of approximately one year indicate that variations in radon at the calibration site are attributable more to meteorological than to tectonic causes. 相似文献
828.
Jörg Overmann Gerhard Sandmann Ken J. Hall Tom G. Northcote 《Aquatic Sciences - Research Across Boundaries》1993,55(1):31-39
Vertical distribution of fossil carotenoids in a sediment core from meromictic Mahoney Lake was studied. Besides okenone and demethylated okenone, lutein and zeaxanthin and-carotene isomers were identified. No carotenoids typical for purple nonsulfur or green sulfur bacteria were detected. The ratio of zeaxanthin to lutein (above 1:1 in all samples) indicates a dominance of cyanobacteria over green algae in the phytoplankton assemblages of the past. Okenone, which is found exclusively in Chromatiaceae, was the dominating carotenoid in all sediment zones.The oldest sediment layers containing okenone were deposited 11 000 years ago. Between 9000 and 7000 and since 3000 years b.p., Chromatiaceae reached a considerable biomass in the lake. Vertical changes in okenone concentration were not related to changes of paleotemperatures. In contrast, okenone concentrations decreased during periods of volcanic ash input. During most of the lake history, however, mean okenone concentrations were positively correlated with sedimentation rates. This indicates that vertical changes of okenone concentration in the sediment reflect past changes of purple sulfur bacterial biomass in the lake.According to these results, the past limnology of Mahoney Lake resembled that of the present with a sulfide-containing monimolimnion and a well-developed population of okenone-bearing purple sulfur bacteria. 相似文献
829.
Summary The crystal-chemistry of clinopyroxene from a suite of Sunda arc volcanic rocks was investigated by X-ray structure refinement and microprobe analysis. Relationships with clinopyroxene from volcanic rocks of the intraplate environment were highlighted.Sunda clinopyroxenes were distinguished into two groups. The first consists of clinopyroxene from SiO2-saturated rock types and crystal rim from SiO2-undersaturated rock types, the second of crystal core from SiO2-undersaturated rock types. The crystal chemical behaviour of the two groups differs significantly, mainly in the geometrical relationships between M2 and T sites. While in the first group M2-O3 and T-O3 bond lengths shorten and lengthen respectively due to Ca and AlIV increase, in the second group in which M2 site is almost fully coordinated by Ca, -O3 bond lengths are forced to lengthen in spite of low AlIV content, in order to mantain the local charge balance around the O3 oxygens.It is noteworthy that Sunda clinopyroxene shows strong analogies with that from basaltic rocks from the Ethiopian plateau and K-rich lavas from the Roman Province respectively, reflecting similarities between the composition and mineralogical assemblages of the host rocks, although they all came from different tectonic environments. A relationship between intracrystalline Mg-Fe 2+ disorder and the explosive character of the volcanism is demonstrated.
Kristallchemie von Klinopyroxenen aus Gesteinen des Vulkanbogens der Sunda-Inseln
Zusammenfassung Die Kristallchemie von Klinopyroxenen aus einer Serie von vulkanischen Gesteinen des Sunda-Bogens wurde mit Röntgen-Strukturverfeinerungen und Mikrosonden-analysen untersucht. Beziehungen zu Klinopyroxenen vulkanischer Gesteine aus intra-Plattenpositionen werden beleuchtet. Die Sunda-Klinopyroxene wurden in zwei Gruppen eingeteilt. Die erste umfaßt Klinopyroxene SiO2-gesättigter Gesteinstypen und Kristallsäume SiO2-untersättigter Gesteinstypen, die zweite Kristallkerne SiO2-untersättigter Gesteinstypen. Das kristallchemische Verhalten der beiden Gruppen unterscheidet sich beträchtlich, hauptsächlich in den geometrischen Beziehungen zwischen den M2 und T Positionen. Während in der ersten Gruppe die M2-O3 und T-O3 Bindungsldngen durch Zunahme von Ca und AlIV kleiner bzw. größer werden, sind in der zweiten Gruppe die T-O3 Bindungslängen trotz niedrigen AlIV Gehaltes zu einer Vergrößerung gezwungen, damit um die O3 Atome der lokale Ladungsausgleich erhalten bleibt.Es ist bemerkenswert, daß die Sunda-Klinopyroxene große Analogien zu denen aus basaltischen Gesteinen des Abessinischen Plateaus und aus K-reichen Laven sowohl der Römischen Provinz als auch des Westafrikanischen Grabens zeigen. Dies spiegelt Ähnlichkeiten zwischen Zusammensetzung und Mineralbestand der Muttergesteine wider, obwohl sie all aus tektonisch unterschiedlichen Positionen kommen.Eine Beziehung zwischen intrakristalliner Mg-Fe2+ Unordnung und dem explosiven Charakter des Vulkanismus wird aufgezeigt.相似文献
830.
Dr. G. H. Grantham Dr. R. J. Thomas Dr. B. M. Eglington Dr. D. de Bruin Dr. A. Atanasov Dr. M. J. Evans 《Mineralogy and Petrology》1993,49(1-2):91-102
Summary Olivine-plagioclase and phlogopite-plagioclase coronas have been identified from olivine melanorites of the Mid- to Late Proterozoic Equeefa Suite in southern Natal, South Africa. Olivine, in contact with plagioclase, is mantled by a shell of clear orthopyroxene, in turn rimmed by pale green (pargasitic) clinoamphibole. Locally a third rim, composed of a fine pargasite-spinel symplectite is developed adjacent to the plagioclase. The second corona reaction has produced greenish-brown pargasite at phlogopite-plagioclase interfaces. A third, less obvious reaction, between olivine and phlogopite is also noted. Analytical data of all the mineral phases present, along with the coronas, are given. Two-pyroxene thermometry yields magmatic core temperatures ( 1120°C), with rim compositions indicating equilibration at 850°C. Consistent with this, the modelled olivine-plagioclase reaction occurs between 830–1050°C with awaterbetween 0.1 and 1.0 at 7 kbar. The three reactions took place during a prolonged history of cooling and partial hydration of the magmatic olivine melanorites from over 1000°C down to 600°C. The P-T conditions indicated by the reactions suggest this cooling process was essentially isobaric, indicating that the area was not subjected to rapid uplift or burial throughout this entire period.
Korona-Texturen in proterozoischen Olivin-Melanoriten der Equeefa Suite, Natal Metamorphic Province, Südafrika
Zusammenfassung Aus Olivin-Melanoriten der mittel- bis spätproterozoischen Equeefa Suite im südlichen Natal, Südafrika, wurden Olivin-Plagioklas und Phlogopit-Plagioklas-Koronartexturen beobachtet. Olivin, der mit Plagioklas im Kontakt steht, wird von einem klaren Saum vom Orthopyroxen ummantelt, der seinerseits von blaßgrünem (pargasitischem) Klinoamphibol umsäumt wird. Stellenweise ist ein dritter Saum, bestehend aus feinkörnigem symplektitischem Pargasit-Spinell im Kontakt mit Plagioklas ausgebildet. Die zweite koronabildende Reaktion resultiert in Bildung eines grünbraunen Pargasites an Phlogopit-Plagioklas Kornkontakten. Eine dritte, weniger auffällige Reaktion zwischen Olivin und Phlogopit wurde ebenfalls beobachtet. Zwei-Pyroxen-Thermometrie ergab magmatische Temperaturen der Kernbereiche ( 1120°C) und belegt eine Gleichgewichtseinstellung in den Randzonen bei ca. 850°C. Olivin-Plagioklas-Modellreaktionen liegen ebenfalls in einem Temperaturbereich von 830–1050°C bei Wasseraktivitäten von 0.1 bis 1.0 und einem Druck von 7 kbar. Die drei Reaktionen liefen im Zuge einer länger andauernden Abkühlung unter teilweiser Hydratisierung der magmatischen Melanorite in einem Temperaturbereich von 1000°C bis ca. 600°C ab. Die aus den Reaktionen ableitbaren P-T-Bedingungen sprechen für eine im wesentlichen isobare Abkhlungsgeschichte und zeigen, daß dieses Gebiet wáhrend dieser gesamten Periode keiner raschen Hebung bzw. keiner Versenkung unterworfen worden ist.相似文献