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371.
CLARKE D. BARRIE; MACDONALD MICHAEL A.; REYNOLDS PETER H.; LONGSTAFFE FRED J. 《Journal of Petrology》1993,34(4):653-679
The South Mountain Batholith is a peraluminous granitic complexranging in composition from biotite granodiorite to muscovite-topazleucogranite. Leucogranitic rocks (with generally<2% biotite) form a minor part ({small tilde}15%) ofthe batholith, and are of two types: (1) associated leucogranitesoccurring as relatively small zones in fine-grained leucomonzogranites;and (2) independent leucogranites forming generallylarger bodies having no particular spatial association withother rock types. Mean chemical compositions of these two typesof leucogranite are as follows (associated, independent): Na2O(346,383),K2O(440,409),andP2O5 (026, 045)in wt.%;Li(149, 281), F(1199, 2712),Rb (393, 725), U (74, 44), Nb (128, 234),Ta (29, 71), and Zr (52, 31) in ppm. Rare earthelements also differ between the two types (associated, independent):REE (341 ppm, 199 ppm); and in the degree and variabilityof heavy REE fractionation (GdN/YbN=4622, 2007).In addition, associated leucogranite has REE compositions similarto those of its host rocks. Mean 18O values (associated +ll212,independent +ll405; relative to SMOW) are comparablewith the mean for the entire South Mountain Batholith (+l0807).Radiometric dating (40Ar/39Ar on muscovite) shows that bothtypes of leucogranite have identical ages of 3723 Ma, equivalentto ages determined by other techniques for granodiorite andmonzogranite samples elsewhere in the batholith. Field relationsand geochemistry suggest that the associated leucogranite resultsfrom an open-system interaction between a fluid and its hostleucomonzogranite, whereas the independent leucogranite bodiesare discrete intrusions of highly fractionated melts that underwentclosed-system, late-magmatic to post-magmatic fluid alteration.Where mineralized, the associated leucogranite characteristicallyhosts greisen-type or disseminated polymetallic mineralization,whereas the independent leucogranite hosts pegmatitic or disseminatedpolymetallic mineralization. 相似文献
372.
Field evidence from intrusive and effusive carbonatites supportsthe existence of calciocarbonatite magmas. Published experimentalevidence in the model system Na2OCaOAl2O3SiO2CO2indicated the formation of nearly pure (99%) CaCO3 immiscibleliquids from a carbonated silicate liquid. This evidence hasbeen used to support interpretations of extremely CaCO3-richcalciocarbonatite magmas, and immiscible liquids with compositionsof almost pure CaCO3 in metasomatized mantle peridotite andeclogite. Detailed phase relationships are constructed in themodel system, based on phase fields intersected by the joinNaAlSi3O8CaCO3 (Ab-CC) at 1.0, 1.5, and 2.5 GPa between1100 and 1500C, and analyzed immiscible liquids. The miscibilitygap between silicate-rich liquid and carbonate-rich liquid intersectedby the join Ab-CC contracts considerably with decreasing pressure:2.5 GPa, between Ab10CC90 (by wt%) and Ab65CC35 above 1310C;1.5 GPa, betweenAb23CC77 and Ab43CC57 above 1285C; 1.0 GPa,not intersected. The liquidus piercing point between calciteand silicates becomes enriched in CaCO3 with decreasing pressure,from Ab80CC20 at 2.5 GPa to Ab47CC53 at 1.0 GPa. No immiscibleliquid contains more than 80% dissolved CaCO3, and all containat least 5% Na2CO3. A round CaCO3 phase exhibiting morphologysimilar to that displayed by immiscible liquid globules is determinedto be crystalline calcite under experimental conditions. Thetopology of the phase fields and field boundaries illustratesthe kinds of processes and controls existing in magmatic systems.Calciocarbonatite magmas cannot be produced by equilibrium immiscibilityprocess in the mantle. Carbonated silicate magmas in the crustyield residual calciocarbonatite magmas by fractionation alongthe silicate-calcite field boundary, reached either directlyfrom the silicate liquidus or more commonly via the miscibilitygap. Immiscible carbonaterich magmas when freed from the silicateparent cool down a sleep silicate liquidus until they reacha silicate-carbonate field boundary. There is no experimentalevidence for immiscible calciocarbonatite magmas with > 80%CaCO3, and calcite lapilli cannot be formed from 99% CaCO3 magmas.Sovites are surely cumulates. KEY WORDS: carbonatite; join NaAlSi3O8CaCO3; liquid immiscibility; sovite
* Corresponding author. Telephone: (818)-3956239. Fax: (818)-5680935. e-mail: wjl{at}gpi.caltech.edu 相似文献
373.
New Pb, Sr and Nd isotope data from volcaniclastic sedimentsrecovered from the Tonga forearc and Lau Basin permit the isotopicevolution of a section of this arc system, next to the modernisland of Ata, to be traced through the backarc basin riftingprocess from 7.0 Ma to the present. The new data suggest thatthe isotopic character of the mantle wedge remains constant,and of Pacific mid-ocean ridge basalt (MORB) character, duringthe early rifting phase. The isotopic evidence supports traceelement data in showing an increase in the sediment contributionto arc petrogenesis about 23 m.y. after the start ofLau Basin rifting. Since 0.45 Ma the sediment contribution decreasedto pre-rift values with the initiation of spreading in the adjacentbackarc basin, where the high sediment influence is not seenin the isotopes. The isotopes show a relative increase in thevolcaniclastic compared with pelagic sediment involvement duringrifting. The inferred peak in sediment subduction is probablythe result of a decoupling of the two plates owing to roll-backof the Pacific lithosphere at the time of arc rifting. KEY WORDS: isotopes; Pacific; rifting; subduction; volcanism
*Corresponding author. Telephone: (508) 289 3437. Fax: (508) 457 2187. e-mail: pclift{at}whoi.edu 相似文献
374.
The effects of infiltration, diffusion, and local structureon the contact metamorphism of the Cambrian Weeks Formation,a calcareous argillite located in western Utah, were examinedusing stable isotope systematics and mineral mass-balance considerations.The emplacement of the Jurassic Notch Peak granitic stock resultedin metamorphism characterized with increasing proximity to theintrusion by phlogopite, diopside, and wollastonite isograds.The isograds are generally concentric around the stock, withdiopside and wollastonite isograds {small tilde}600 and 400m from the contact, respectively, but are deflected near a pre-intrusionfault where the wollastonite isograd is 1. 5 km from the contact.Where the isograds are concentric, the wollastonite isogradmarks an isotopic front with whole-rock 18O values approachingthe 9.5% value of the stock. In contrast, the 18O values inthe unmetamorphosed to diopside-grade rocks range from 16.3to 20.2%. Near the fault the isotopic front extends throughthe diopside zone, suggesting that the fault was a major conduitfor magmatic water. Waterrock ratios for the diopside- and wollastonite-gradesamples determined from mineralogic mass balance are nearlyone order of magnitude larger than one-box closed-systemratios determined from shifts in stable isotope ratios. Chromatographicmodels for isotopic exchange and propagation of isograds showthat one-dimensional infiltration of magmatic water throughpore spaces would lead to an isotopic front at 50 m from thestock and the wollastonite isograd would be only 8 km from thestock. These distances are significantly smaller than observed.It is suggested that most of the magmatic water flowed throughfractures or thin permeable layers, with the extent of isotopicexchange between the magmatic water in these conduits and theadjacent rocks being controlled by the extent of reaction progressin the rocks. Considerations of CO2H2O interdiffusivitiesin fluids indicate that removal of CO2 from the reaction frontstoward the intrusion or the fault was sufficiently rapid tocontrol the geometry of the isograds. 相似文献
375.
New insight into the evolution of large-volume turbidity currents: comparison of turbidite shape and previous modelling results 总被引:2,自引:0,他引:2
The Marnoso Arenacea Formation provides the most extensive correlation of individual flow deposits (beds) yet documented in an ancient turbidite system. These correlations provide unusually detailed constraints on bed shape, which is used to deduce flow evolution and assess the validity of numerical and laboratory models. Bed volumes have an approximately log‐normal frequency distribution; a small number of flows dominated sediment supply to this non‐channelized basin plain. Turbidite sandstone within small‐volume (<0·7 km3) beds thins downflow in an approximately exponential fashion. This shape is a property of spatially depletive flows, and has been reproduced by previous mathematical models and laboratory experiments. Sandstone intervals in larger‐volume (0·7–7 km3) beds have a broad thickness maximum in their proximal part. Grain‐size trends within this broad thickness maximum indicate spatially near‐uniform flow for distances of ∼30 km, although the flow was temporally unsteady. Previous mathematical models and laboratory experiments have not reproduced this type of deposit shape. This may be because models fail to simulate the way in which near bed sediment concentration tends towards a constant value (saturates) in powerful flows. Alternatively, the discrepancy may be the result of relatively high ratios of flow thickness and sediment settling velocity in submarine flows, together with very gradual changes in sea‐floor gradient. Intra‐bed erosion, temporally varying discharge, and reworking of suspension fallout as bedload could also help to explain the discrepancy in deposit shape. Most large‐volume beds contain an internal erosion surface underlain by inversely graded sandstone, recording waxing and waning flow. It has been inferred previously that these characteristics are diagnostic of turbidites generated by hyperpycnal flood discharge. These turbidites are too voluminous to have been formed by hyperpycnal flows, unless such flows are capable of eroding cubic kilometres of sea‐floor sediment. It is more likely that these flows originated from submarine slope failure. Two beds comprise multiple sandstone intervals separated only by turbidite mudstone. These features suggest that the submarine slope failures occurred as either a waxing and waning event, or in a number of stages. 相似文献
376.
Rates and Processes of Potassic Magma Evolution beneath Sangeang Api Volcano, East Sunda Arc, Indonesia 总被引:11,自引:3,他引:11
TURNER SIMON; FODEN JOHN; GEORGE RHIANNON; EVANS PETER; VARNE RICK; ELBURG MARLINA; JENNER GEORGE 《Journal of Petrology》2003,44(3):491-515
U-series isotopes can provide unique insights into the physicalprocesses of magma evolution by constraining the time scalesover which they operate. This, however, requires rock suitesthat provide a clear and complete record of the liquid lineof descent. Sangeang Api volcano, in the east Sunda arc, providessuch an opportunity because it erupts potassic lavas (SiO2 相似文献
377.
ANSGAR GRESHAKE WOLFGANG KL
CK PETER ARNDT MISCHA MAETZ GEORGE J. FLYNN SASA BAJT ADDI BISCHOFF 《Meteoritics & planetary science》1998,33(2):267-290
Abstract— Depending on their velocity, entry angle and mass, extraterrestrial dust particles suffer certain degrees of heating during entry into Earth's atmosphere, and the mineralogy and chemical composition of these dust particles are significantly changed. In the present study, pulse-heating experiments simulating the atmospheric entry heating of micrometeoroids were carried out in order to understand the mineralogical and chemical changes quantitatively as well as to estimate the peak temperature experienced by the particles during entry heating. Fragments of the CI chondrites Orgueil and Alais as well as pyrrhotites from Orgueil were used as analogue material. The experiments show that the volatile elements S, Zn, Ga, Ge, and Se can be lost from 50 to 100 μm sized CI meteorite fragments at temperatures and heating times applicable to the entry heating of similar sized cosmic dust particles. It is concluded that depletions of these elements relative to CI as observed in micrometeorites are mainly caused by atmospheric entry heating. Besides explaining the element abundances in micrometeorites, the experimentally obtained release patterns can also be used as indicators to estimate the peak heating of dust particles during entry. Using the abundances of Zn and Ge and assuming their original concentrations close to CI, a maximum heating of 1100–1200 °C is obtained for previously analyzed Antarctic micrometeroites. Thermal alteration also strongly influenced the mineralogy of the meteorite fragments. While the unheated samples mainly consisted of phyllosilicates, these phases almost completely transformed into olivine and pyroxene in the fragments heated to ≥800 °C. Therefore, dust particles that still contain hydrous minerals were probably never heated to temperatures ≥800 °C in the atmosphere. During continued heating, the grain size of the newly formed silicates increased and the composition of the olivines equilibrated. Applying these results quantitatively to Antarctic micrometeorites, typical peak temperatures in the range of 1100–1200 °C during atmospheric entry heating are deduced. This temperature range corresponds to the one obtained from the volatile element concentrations measured in these micrometeorites and points to an asteroidal origin of the particles. 相似文献
378.
Experimental studies were carried out to evaluate phase relationsinvolving titaniteFAl-titanite solid solutionin the system CaSiO3Al2SiO5TiO2CaF2. Theexperiments were conducted at 9001000°C and 1·14·0GPa. The average F/Al ratio in titanite solid solution in theexperimental run products is 1·01 ± 0·06,and XAl ranges from 0·33 ± 0·02 to 0·91± 0·05, consistent with the substitution [TiO2+]1[AlF2+]1.Analysis of the phase relations indicates that titanite solidsolutions coexisting with rutile are always low in XAl, whereasthe maximum XAl of titanite solid solution occurs with fluoriteand either anorthite or Al2SiO5. Reaction displacement experimentswere performed by adding fluorite to the assemblage anorthite+ rutile = titanite + kyanite. The reaction shifts from 1·60GPa to 1·15 ± 0·05 GPa at 900°C, from1·79 GPa to 1·375 ± 0·025 GPa at1000°C, and from 1·98 GPa to 1·575 ±0·025 GPa at 1100°C. The data show that the activityof CaTiSiO4O is very close to the ideal molecular activity model(XTi) at 1100°C, but shows a negative deviation at 1000°Cand 900°C. The results constrain 相似文献
379.
The formation of ooids 总被引:5,自引:0,他引:5
Field and laboratory studies suggest that different types of ooids form during quiet and agitated water conditions. Both types have been synthesized in the laboratory. Quiet water types exhibit a radial orientation of carbonate crystals, whereas in those formed in agitated conditions, a tangential orientation is prevalent. Successful laboratory formation of quiet water ooids was accomplished in supersaturated seawater solutions containing humic acids. Negative results were obtained from strictly inorganic solutions, and from those containing simple amino acids, single proteins, mixtures of proteins or mucopolysaccharides, soil and sediment extracts. Partly successful results were obtained using an organic extract from Bahamian ooids. The organic parameters most important in quiet water ooid formation are molecular weight, the presence of carboxyl groups and an ability to participate in hydrophobic/hydrophilic interactions, all of which are critical to membrane formation. Membranes form concentric shells which act as growth surfaces for carbonate and also induce the periodicity in carbonate precipitation. Ooids exhibiting a tangential orientation of batten-like crystals have been synthesized under conditions of agitation, supersaturation and without the intervention of organic processes during the precipitation. Complete growth may be divided into agitation, resting and sleeping stages In the agitation stage, quartz nuclei induce an inorganic, heterogeneous nucleation from a supersaturated solution, which finally ceases as a result of Mg2+ and possibly H+ poisoning of the carbonate surfaces. No further precipitation occurs until the crystal surfaces are reactivated by removal of Mg2+ and H+ during the resting stage. Following a series of agitation and resting stages, precipitation is inhibited by a degree of poisoning which is not totally removed during the resting stage. For further growth, a new substrate is required and is provided by the development of organic membranes around the grains. This occurs when the grains are buried in the subsurface, the period of organic growth constituting the sleeping stage. Only 2% of an ooid's life is spent growing in the agitated environment, while 95% of its life is spent accreting organic membranes in the subsurface. Our experimental work indicates that ooids of Bahamian type are inorganic precipitates. The tangential arrangement of battens is the result of suspension in an environment where the degree of turbulence is sufficient to induce grain to grain contact of sufficient strength and frequency to inhibit any crystal growth other than tangential. The role of organics is to provide a substrate for further growth after precipitation has slowed to a point when no further accretion is occurring. 相似文献
380.
PETER A. RIEZEBOS RUDOLF T. SLOTBOOM 《Boreas: An International Journal of Quaternary Research》1978,7(2):75-82
Pollen analysis and radiocarbon dating of a core section from the valley head of the Husterbaach near Hosingen (Oesling) provides a very detailed record of the successive vegetations from 1200 A. D. until today. This palynological record agrees largely with the consecutive events known from the agricultural history of that area. Particularly significant are the curves obtained of several cultivated and ruderal plants. In this section it could be demonstrated that the interruption of peat growth by deposition of minerogenic material corresponds with a comparatively high arable land/pasture ratio. As well as being intrinsically interesting for assessing which features of the present landscape are due to former human activities, a detailed knowledge of the Subatlantic evolution of the vegetation and land use in the Oesling is indispensable, as its geomorphological development in the immediate past has largely been influenced by man. 相似文献