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THE PRESENT STATUS OF ZIRCON 总被引：1，自引：0，他引：1
S.K. SAXENA 《Sedimentology》1968,10(3):209-209
The present writer (SAXENA, 1966) does not propose that the use of zircons in problems of stratigraphy, pedogenesis and petrology should be discarded. His conclusions (pp.30–31) make a plea for caution in such use. The writer suggests several possible modes of the origin of Lircons and by no means the authigenic mode of origin is more important than any other. Igneous or detrital zircons could certainly be used provided due consideration is given to other possible modes of origin. The solution does not lie in abandoning the use of zircons but an individual case is to be judged according to the local geologic evidence. It may be possible to find the genctic history imprinted on the morphological features of Zircons. Most of MARSHALL'S (1967) criticism is because of his failure to realize the broader genetic aspects of zircons. 相似文献
2.
S. K. SAXENA 《Sedimentology》1968,10(2):145-146
In a discussion Kalsbeek (1967) suggests that my idea of an authigenic origin of zircons seems to be the result of mis-identification of authigenic titaniferous minerals as zircon. Kalsbeek appears to have formed this opinion firstly because he sees all the crystals (Saxena , 1966a, plate IV) as zircons. I never mentioned this. Since 70% of the matrix in the sandstones consists of such aggregate, the very idea of having so much zircon in the rock would be fantastic and absurd. 相似文献
3.
EVOLUTION OF ZIRCONS IN SEDIMENTARY AND METAMORPHIC ROCKS 总被引：2，自引：0，他引：2
S. K. SAXENA 《Sedimentology》1966,6(1):1-33
Zircons have been studied in the sedimentary rocks of the Sparagmite Group (Eocambrian) and their metamorphic associates from the Trondheim area and the adjacent regions in Norway and Sweden. The majority of zircon population (type B) has been formed in the sediments by authigenesis. Another authigenic associate is rutile. The authigenic zircons have irregular, round and ellipsoidal habits. In metamorphic rocks the same grains show a greater tendency of euhedrism, forming drum-like crystals with acute (331) pyramids and short prisms. Authigenic zircons are usually clear and transparent but some grains may show cloudy appearance. A few other grains (type A) with different habit and many inclusions appear to be of detritai nature. Dissolution of a metamictized zircon population in the source rock is possible in many ways. An important possibility is the dissolution of zircons in alkaline solution. Carbonated lime-rich waters or other acidic solutions could be equally effective. Zirconates, so formed, are transported to the basin of deposition as colloidal particles or as ionic complexes. The authigenic process is visualized as a deposition of the zirconates by adsorption or by precipitation as zirconium hydroxide, possibly due to change in the Eh and pH in the environment. The hydrated oxide subsequently reacts with silica to form metastable hydrozircon. In course of time hydrozircon dehydrates and becomes a normal zircon. Numerous growths have been noted and are classified genetically into (l) late authigenic growths, and (2) growths syngenetic with metamorphism. The shape of the host grain usually controls the final shape of the overgrown grain. There is some effect of metamorphism on the morphology of the authigenic zircons. In view of the characters of the newly generated zircons, the use of shape, size, zoning and such other parameters of zircons in petrogenetic problems, has to be retested to ensure its reliability. Rather than being a mineral of igneous derivation only, zircon is from three parentages: igneous, authigenic and metamorphic. It is argued that the persistence of the authigenic zircons should be greatest because of very low concentration of radioactive elements. 相似文献
4.
ANUBHOOTI SAXENA HIMANSHU K SACHAN PULOK K MUKHERJEE DILIP K MUKHOPADHYA 《Journal of Earth System Science》2012,121(1):29-44
The Malari Leucogranite in the Garhwal Himalaya is cut across by a continental-scale normal fault system called the South
Tibetan Detachment (STD). A mineralogical, geochemical and fluid inclusion study of samples from the fault zone of the Malari
Granite was performed to reveal the imprints of fluid–rock interaction. Fluid inclusion assemblages observed in the alteration
zone indicate the presence of NaCl-dominated aqueous fluids with varied salinity of 6–16 wt.% of NaCl equivalent. Mineralogical
changes include the alteration of feldspar to muscovite and muscovite to chlorite. This alteration took place at temperatures
of 275°–335°C and pressures between 1.9 and 4.2 kbars as revealed by the application of chlorite thermometry, fluid isochores,
and presence of K-feldspar+muscovite+chlorite+quartz mineral assemblage. Geochemical mass-balance estimates predict 32% volume
loss during alteration. An estimated fluid/rock ratio of 82 is based on loss of silica during alteration, and reveals presence
of a moderately low amount of fluid at the time of faulting. 相似文献
5.
R. K. SAXENA 《水文研究》1996,10(10):1273-1281
Lake evaporation has been estimated for a shallow lake using a combination of water and isotope mass balance, accounting for the isotopic non-steady state of lake water. The main feature of the isotope method is that inflows need not be measured. Knowledge of their isotopic content is sufficient. Oxygen-18 content, i.e. (δ18O), of lake water, inflows and outflow was measured on a weekly basis, whereas for precipitation it was monitored daily. The discharge from the lake was also recorded daily. Lake water level, relative humidity, air, and lake water surface temperatures were recorded by a logger. The weather data were recorded on a small island in the lake. It was observed that the lake is isotopically well mixed. Furthermore, the atmospheric moisture was not always in isotopic equilibrium with the precipitation. Daily lake evaporation was estimated as an average of six to eight days depending upon the field logistics. Lake evaporation varied from 0.6 to about 5.4 mm/day during the experimental period. It was found that evaporation estimates are very sensitive to small variations in δ18O of lake evaporate. Induced changes of 10% in δ18O of lake evaporate caused errors in evaporation estimates of 9–31%, while similar induced changes in δ18O of inflows caused errors of 8–18%. Thus, an accurate experimental determination of δ18O of lake evaporate is relatively more important. 相似文献
6.
Thermochemical data on several phases forming in the systemCaO-MgO-Al2O3-SiO2 have been tested for consistency in reproducingexperimental phase equilibrium relationships. Calorimetric dataon enthalpy, entropy and heat capacity have been adjusted withinexperimental errors and new data on some phases have been estimatedusing phase equilibrium data. The consistency of the data inreproducing phase equilibrium relations in the multicomponent-multiphasesystems has been tested by computing phase diagrams using themethod of minimization of total Gibbs free energy. The recentcalorimetric data on most phases can be used without any significantchange except for phases that show cation disorder. Disordercorrections have been added to the heat capacity data on gehlenite,anorthite, spinel and Ca-Tschermak. 相似文献
7.
Experimental phase equilibrium data on compositions of coexistingpyroxenes in the quadrilateral enstatite-diopside-ferrosilite-hedenbergitehave been used to model pyroxene solid solutions and to formulatepyroxene geothermometers. Each pyroxene is treated as a solidsolution of four quad-components using the Kohler formulation
where Gij* is the excess free energy of mixing in a binary solutioncalculated with binary mole fractions (e.g. Xio = Xi/(Xi+Xj))and Xi is the mole fraction in a multicomponent solution. Thefit to the experimental data is achieved by minimizing the totalGibbs free energy of the assemblage. The following set of thermochemicaldata and simple mixture parameters (Wij) are found to be bestsuited. Standard (T = 298?15 K) enthalpy and entropy of formationfrom elements for fictive orthohedenbergite are –1416?8kJ and 84?88 J K–1 mol –1 respectively. The heatcapacity is given by 114?67+17?09E-3T–31?40E5T–2.The Wij data are: Opx: W12 = W21 = 25 W13 = (13?1–0-015T),W31 = (3?37–0?005T), W23 = 20, W32 = 16, W24 = 5, W42= 7, W34 = 15, W43 = 15; Cpx: W12 = (25?484+0?0812P), W21 =(31?216–0?0061P),W31 = W13 = 0W14 = (93?3–0?045T), W41 = (–20?0+0?028T),W23 = 24, W32 = 15, W24 = 12, W42 = 12, W34 = (16?941+0?00592P),W43 = (20?697–0?00235P). Coexisting pyroxene compositionshave been computed in the temperature range of 700 to 1400?C. Two geothermometers have been constructed, one based on atomicfraction of iron (Fe/(Fe + Mg)) in orthopyroxene and the Fe-Mgdistribution coefficient and the other, based on wollastonitecontent of clinopyroxene. The two scales yield different temperatureswhen applied to the same rock. In igneous pyroxenes, the Catransfer ceased at 150 to 200?C above the closure temperatureof the Fe-Mg ion-exchange. In metamorphic rocks an oppositeeffect seems to have prevailed. 相似文献
8.
Phase equilibrium relations are computed in five different chemicalcompositions appropriate for a primitive upper mantle (estimated),a peridotite, an olivine-basalt, an olivine-tholeiite, and aquartz-tholeiite in the pressure range of 1 bar to 100 kb andin the temperature range of 500 to 1500 ?C. The mineralogicalphase assemblages change from peridotite and lherzolite to garnetgranulites, eclogites and kyanite-quartz-eclogites. The smallpressure-temperature field of stability of plagioclase-garnet-granulitein the primitive compositions extends at 600 ?C from less than3 kb to more than 15 kb in the quartz-tholeiite compositionwith intermediate pressure values in other basalts. Continentalcrust with composition similar to quartz-tholeiite remains asplagioclase-garnet-granulite changing to eclogite only belowa depth of 50 km. Calculated adiabats in the peridotite compositions show thatdiapirs must ascend from a depth of at least 300 km to becomepartially molten in the low-velocity zone. Various shallowerdepths of partial melting are possible for material ascendingfrom depths corresponding to the oceanic geothermal gradient. Actual pressure-temperature densities of rocks have been calculatedfrom the mineral densities computed for the equilibrium assemblageson the oceanic geothermal gradient. While there are no importantchanges found in the peridotite density down to a pressure of100 kb, there are important inflections in the pressure-densitycurve in quartz-tholeiite which relate to the increasing conversionof plagjoclase to jadeite component in clinopyroxene and a sharprise in density due to the quartz-coesite phase transformation. 相似文献
9.
Pterospermumocarpon (Type: P. kalviwadiensis), a new morphogenus of fossil fruits showing resemblance with fruits of extant Pterospermum Schreb. (Malvaceae s.l.), is described from the Sindhudurg Formation (Miocene) at the Kalviwadi Village, Sindhudurg District, Maharashtra, India.
Diagnostic feature of the fossil taxon is the dehiscent pentalocular capsule with five distinct sutures and imprints of winged
seed-like structures in the locules. Recent modifications in the systematics of the Malvales, their fossil record, and the
distribution and migration of Pterospermum and other malvalean taxa in the context of the Indian subcontinent are discussed. 相似文献
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