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1.
Residence of REE, Y, Th and U in Granites and Crustal Protoliths; Implications for the Chemistry of Crustal Melts 总被引：36，自引：8，他引：28
A systematic study with laser ablation—ICP-MS, scanningelectron microscopy and electron microprobe revealed that 70–95wt% of REE (except Eu), Y, Th and U in granite rocks and crustalprotoliths reside within REEYThU-rich accessories whose nature,composition and associations change with the rock aluminosity.The accessory assemblage of peraluminous granites, migmatitesand high-grade rocks is composed of monazite, xenotime (in low-Cavarieties), apatite, zircon, Thorthosilicate, uraninite andbetafite-pyrochlore. Metaluminous granites have allanite, sphene,apatite, zircon, monazite and Thorthosilicaie. Peralkaline graniteshave aeschinite, fergusonite, samarskite, bastnaesite, fluocerite,allanite, sphene, zircon, monazite, xenotime and Th-orthosilicate.Granulite-grade garnets are enriched in Nd and Sm by no lessthan one order of magnitude with respect to amphibolite-gradegarnets. Granulitegrade feldspars are also enriched in LREEwith respect to amphibolite-grade feldspars. Accessories causenon-Henrian behaviour of REE, Y, Th and U during melt—solidpartitioning. Because elevated fractions of monazite, xenotimeand zircon in common migmatites are included within major minerals,their behaviour during anatexis is controlled by that of theirhost. Settling curves calculated for a convecting magma showthat accessories are too small to settle appreciably, beingseparated from the melt as inclusions within larger minerals.Biotite has the greatest tendency to include accessories, therebyindirectly controlling the geochemistry of REE, Y, Th and U.We conclude that REE, Y, Th and U are unsuitable for petrogeneticalmodelling of granitoids through equilibrium-based trace-elementfractionation equations. KEY WORDS: accessory minerals; geochemical modelling; granitoids; REE, Y, Th, U 相似文献
2.
Daniel E. Harlov Richard Wirth Callum J. Hetherington 《Contributions to Mineralogy and Petrology》2011,162(2):329-348
Monazite [(Ce,LREE,Th,U,Ca)(P,Si)O4], with complex zoning in Th and other elements, is commonly observed in metamorphic and igneous rocks. The hypothesis that
this alteration is a product of fluid-mediated element mass transfer has been tested in the piston-cylinder press (CaF2 assembly, cylindrical graphite oven) at 1,000 MPa and 900°C and in cold seal autoclaves on a hydrothermal line at 500 MPa
and 600°C. Experiments included a relatively homogeneous monazite-(Ce) (7–8 wt% ThO2) from a heavy mineral sand plus a series of alkali-bearing fluids including 2N NaOH, 2N KOH, and Na2Si2O5 + H2O. Experiments were conducted using BSE imaging, EMP analysis, and both TEM and HRTEM. A subset of monazite grains from each
experiment show evidence of partial alteration in the form of areas enriched in Th + Si with sharp curvilinear compositional
boundaries extending from the grain rim into the monazite interior. These ThSiO4-enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic
rocks. In the Na2Si2O5 + H2O experiments, scarce inclusions of britholite formed in the altered monazite. The altered monazite is also characterised
by strong depletion in Pb, Ca, and Y. Thorium and Si mobility, coupled with the formation of britholite inclusions, during
partial alteration in the monazite grain is considered to be the product of fluid-aided coupled dissolution–reprecipitation
as opposed to solid-state diffusion. Since other fluids, including NaCl and KCl brines, do not result in the formation of
these textures, the experimental replication of ThSiO4-enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced,
specifically by alkali-bearing fluids. If true, complex metasomatically induced textures in monazite could yield information
concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event,
presuming that all the original radiogenic Pb has been removed. 相似文献
3.
The Precambrian Taratash complex (Middle Urals) is one of the rare windows into the Palaeoproterozoic and earlier history of the eastern margin of the East European Craton. Monazite from intensively deformed rocks within a major amphibolite-facies shear zone in the Taratash complex has been investigated by means of electron-probe microanalysis and laser-ablation SF-ICP-MS.Metamorphic and magmatic cores of monazite from metasedimentary and metagranitoid rocks yield U–Pb ages of 2244 ± 19 and 2230 ± 22 Ma (± 2 σ) and record a previously unknown pre-deformational HT-metamorphic event in the Taratash complex. Subsequent dissolution–reprecipitation of monazite, during shear zone formation under amphibolite-facies conditions, caused patchy zonation and chemical alteration of the recrystallised monazite domains, leading to higher cheralite and huttonite components. This process, which was mediated by a probable (alkali + OH)-bearing metamorphic fluid also caused a total resetting of the U–Pb-system. The patchy domains yield concordant U–Pb-ages between 2052 ± 16 and 2066 ± 22 Ma, interpreted as the age of the shear zone. In line with previously published ages of high grade metamorphism and migmatisation, the data may point to a Palaeoproterozoic orogenic event at the eastern margin of the East European Craton.Post-deformational fluid-induced greenschist-facies retrogression caused partial to complete breakdown of monazite to fluorapatite, REE + Y-rich epidote, allanite and Th-orthosilicate.The retrograde assemblages either form coronas around monazite, or occur as dispersed reaction zones, indicating that the REE, Y, and Th were mobile at least on the thin section scale. The greenschist-facies metamorphic fluid was aqueous and rich in Ca. Monazite affected by advanced breakdown responded to the retrogression by incorporating the cheralite or huttonite components during a fluid-induced dissolution–reprecipitation process. This event did not reset the U–Pb-system but caused partial Pb loss reflected by discordant U–Pb-dates. 相似文献
4.
Paleoproterozoic carbonaceous shales in the Tim-Yastrebovskii ancient rift, which underwent zonal metamorphism at 350–550°C,
contain REE mineralization of silicates (allanite, thorite, and Ce-P huttonite) fluorcarbonates (bastnaesite and synchysite),
phosphates (monazite and xenotime), and REE-bearing apatite. The reason for the wide occurrence of bastnaesite and other REE
minerals is relatively high REE concentrations in the sulfide-bearing carbonaceous shales, with these elements accumulated
in the organic matter in the course of diagenesis. Reaction textures with REE-bearing chlorite, bastnaesite, and allanite
suggest that REE-bearing chlorite and bastnaesite provided REE for the forming of higher temperature allanite and monazite.
This is corroborated by the REE patterns of the monazite, allanite, and bastnaesite, which are almost identical and are characterized
by the strong predominance of LREE. The replacements of REE minerals during metamorphism at 350–550°C took place via a number
successive transitions: (1) Mnz → Aln, Chl
REE → Bst, Chl
REE → Aln, Bst → Aln and (2) Bst → Mnz and Ap
LREE → Mnz. These replacements can be accounted for by prograde metamorphic reactions. 相似文献
5.
6.
Pressure and temperature conditions for crystallization of metamorphic allanite and monazite in metapelites: a case study from the Miyar Valley (high Himalayan Crystalline of Zanskar,NW India) 
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下载免费PDF全文 The textural and chemical evolution of allanite and monazite along a well‐constrained prograde metamorphic suite in the High Himalayan Crystalline of Zanskar was investigated to determine the P–T conditions for the crystallization of these two REE accessory phases. The results of this study reveals that: (i) allanite is the stable REE accessory phase in the biotite and garnet zone and (ii) allanite disappears at the staurolite‐in isograd, simultaneously with the occurrence of the first metamorphic monazite. Both monazite and allanite occur as inclusions in staurolite, indicating that the breakdown of allanite and the formation of monazite proceeded during staurolite crystallization. Staurolite growth modelling indicates that staurolite crystallized between 580 and 610 °C, thus setting the lower temperature limit for the monazite‐forming reaction at ~600 °C. Preservation of allanite and monazite inclusions in garnet (core and rim) constrains the garnet molar composition when the first monazite was overgrown and subsequently encompassed by the garnet crystallization front. Garnet growth modelling and the intersection of isopleths reveal that the monazite closest to the garnet core was overgrown by the garnet advancing crystallization front at 590 °C, which establishes an upper temperature limit for monazite crystallization. Significantly, the substitution of allanite by monazite occurs in close spatial proximity, i.e. at similar P–T conditions, in all rock types investigated, from Al‐rich metapelites to more psammitic metasedimentary rocks. This indicates that major silicate phases, such as staurolite and garnet, do not play a significant role in the monazite‐forming reaction. Our data show that the occurrence of the first metamorphic monazite in these rocks was mainly determined by the P–T conditions, not by bulk chemical composition. In Barrovian terranes, dating prograde monazite in metapelites thus means constraining the time when these rocks reached the 600 °C isotherm. 相似文献
7.
Gehad M. Saleh 《中国地球化学学报》2006,25(1):1-15
1INTRODUCTION THELATEPRECAMBRIANGRANITOIDSOFTHEARABONU BIANSHIELDINEGYPTWEREEXPOSEDBYEARLYTOMIDDLE TERTIARYUPLIFTANDENSUINGEROSIONDURINGTHEREDSEA RIFTINGEVENT(GREENBERG,1981).THEREAREANUMBER OFEFFECTIVEANDRELATIVELYSUCCESSFULSCHEMESFORTHE CLASSIFICATIONOF… 相似文献
8.
Experimental phase equilibrium and trace element partitioningdata are reported for H2O-saturated mid-ocean ridge basalt at2·5 GPa, 750–900°C and oxygen fugacities atthe nickel–nickel oxide buffer. Garnet, omphacite andrutile are present at all temperatures. Amphibole and epidotedisappear as residual phases above 800°C; allanite appearsabove 750°C. The Na–Al-rich silicate glass presentin all run products is likely to have quenched from a supercriticalliquid. Trace element analyses of glasses demonstrate the importantcontrol exerted by residual minerals on liquid chemistry. Inaddition to garnet, which controls heavy rare earth elements(HREE) and Sc, and rutile, which controls Ti, Nb and Ta, allanitebuffers the light REE (LREE; La–Sm) contents of liquidsto relatively low levels and preferentially holds back Th relativeto U. In agreement with previous experimental and metamorphicstudies we propose that residual allanite plays a key role inselectively retaining trace elements in the slab during subduction.Experimental data and analyses of allanite-bearing volcanicrocks are used to derive a model for allanite solubility inliquids as a function of pressure, temperature, anhydrous liquidcomposition and LREE content. The large temperature dependenceof allanite solubility is very similar to that previously determinedfor monazite. Our model, fitted to 48 datapoints, retrievesLREE solubility (in ppm) to within a factor of 1· 40over a pressure range of 0–4 GPa, temperature range of700–1200°C and for liquids with anhydrous SiO2 contentsof 50–84 wt %. This uncertainty in LREE content is equivalentto a temperature uncertainty of only ± 27°C at 1000K, indicating the potential of allanite as a geothermometer.Silicic liquids from either basaltic or sedimentary protolithswill be saturated in allanite except for Ca-poor protolithsor at very high temperatures. For conventional subduction geothermsthe low solubility of LREE (+ Th) in liquids raises questionsabout the mechanism of LREE + Th transport from slab to wedge.It is suggested either that, locally, temperatures experiencedby the slab are high enough to eliminate allanite in the residueor that substantial volumes of H2O-rich fluids must pass throughthe mantle wedge prior to melting. The solubility of accessoryphases in fluids derived from subducted rocks can provide importantconstraints on subduction zone thermal structure. KEY WORDS: subduction; experimental petrology; allanite; solubility; supercritical liquid; eclogite 相似文献
9.
Zonal and sectorial monazite-(Ce) crystals from ceramic granite pegmatites of the Adui pluton are enriched in Ce and La, whereas
monazite from the miarolitic gemstone pegmatites at the western contact of the pluton are enriched in Nd, Sm, Gd, and Y. This
difference is caused by earlier crystallization of ceramic pegmatites and higher temperatures of their formation (650–600°C
for ceramic pegmatites and 550–300°C for miarolitic pegmatites). Monazites from ceramic and miarolitic pegmatites of the Adui
pluton differ in La and Nd contents, but their compositional trend in La-Nd coordinates is similar to the variation in monazite
composition from the early to late granite pegmatites in the Ilmeny Mountains. It is suggested that decrease in temperature
is a factor controlling REE contents in monazite. Heterovalent P ↔ Si and REE ↔ Th, Ca isomorphism in the consecutive zones
of growth pyramids in monazite is both unidirectional and wave-like. Monazite from granite pegmatites of the Adui pluton and
Ilmeny Mountains crystallized mainly under slightly alkaline conditions. 相似文献
10.
Paleoproterozoic metapelites of the Vorontsovskaya structure contain accessory REE phosphates (monazite, xenotime, and REE-apatite),
fluorine-carbonates (bastnaesite and synchysite), and silicate (allanite). Analysis of phase equilibria involving REE-bearing
minerals indicates that bastnaesite is stable only in the greenschist facies and decomposes with the synthesis of monazite
at temperatures below the staurolite isograde (490–500°C) at a pressure of 3 kbar. Monazite first appears in the greenschist
facies, and its stability expands with increasing temperature, including the granulite facies. A diversity of reaction textures
suggests that the mineral is formed in the garnet zone by a reaction of bastnaesite with apatite and by the partial decomposition
of REE-bearing chlorite. Monazite is produced in the garnet and staurolite zones by a reaction of allanite with apatite and
by a decomposition reaction of REE-bearing apatite. 相似文献
11.
Allanite from the Tertiary Rieserferner pluton (Austrian Alps) is texturally and chemically heterogeneous. Continuous covariation trends reflect coupled substitution of Ca+Al vs. Fe+REE+Th in allanite, whereas systematic variations in La/Nd demonstrate the increasingly stronger depletion of LREE in the melt during its crystallization. Allanite samples (corrected for 206Pbexcess) from two rocks scatter in the concordia diagram and define discordias from 31.8±0.4 Ma and 32.2±0.4 Ma to ca. 540 Ma. The apparent inheritance does not originate from the inclusion of older allanite or a high- phase, such as monazite, xenotime, or zircon, but from the incorporation of radiogenic Pb originating from a precursor. Since allanite requires a high enrichment of Th, (U), and LREE, it may form at the expense of a Th-LREE-rich precursor in metamorphic rocks or where such a phase had dissolved in melts. Likely precursors acquire with time radiogenic Pb isotopic compositions. This Pb, if incorporated in the product mineral, may give the illusion of inheritance. The allanite samples from the Rieserferner pluton show a tendency from high Th/Ucalc (30–50) in samples with an Alpine age to low Th/Ucalc (4–12) in samples with distinct inheritance. This relation between extent of inheritance and apparent Th/Ucalc indicates a lower Th/Ucalc for a possible precursor, falling into the Th/U range commonly encountered for monazite. Precursor monazite would have originated from assimilated Palaeozoic rocks and give rise to localized enrichments of Th and LREE in the melt, thus eventually enabling the growth of allanite.Electronic Supplementary Material Supplementary material is available in the online version of this article at .Editorial responsibility: J. Hoefs 相似文献
12.
The North Qilian Orogen (NQO) in northwest China underwent oceanic subduction and subsequent continental collision. Metasedimentary rocks from a deep borehole in the Dingxi Basin, NQO, contain garnet, biotite, plagioclase, quartz and minor cummingtonite and chlorite in the matrix, with inclusions of kyanite and staurolite in garnet. The mineral textures and compositions define clockwise pressure–temperature evolution with peak conditions of ~10.5 kbar and ~670°C, followed by isothermal decompression down to ~6.5 kbar. Age and Hf isotope data of detrital zircon support the formation of the sedimentary protolith in an arc setting at ~460 Ma, and the age and rare earth element characteristics of metamorphic monazite reflect exhumation at ~425 Ma. These results indicate a complete cycle of deposition–burial–exhumation for the sedimentary rocks, and directly constrain the continental collision process in NQO to yield a geotherm of ~21°C/km and to culminate before 425 Ma. 相似文献
13.
E. JANOTS M. ENGI A. BERGER J. ALLAZ J.-O. SCHWARZ C. SPANDLER 《Journal of Metamorphic Geology》2008,26(5):509-526
The distribution of REE minerals in metasedimentary rocks was investigated to gain insight into the stability of allanite, monazite and xenotime in metapelites. Samples were collected in the central Swiss Alps, along a well‐established metamorphic field gradient that record conditions from very low grade metamorphism (250 °C) to the lower amphibolite facies (～600 °C). In the Alpine metapelites investigated, mass balance calculations show that LREE are mainly transferred between monazite and allanite during the course of prograde metamorphism. At very low grade metamorphism, detrital monazite grains (mostly Variscan in age) have two distinct populations in terms of LREE and MREE compositions. Newly formed monazite crystallized during low‐grade metamorphism (<440 °C); these are enriched in La, but depleted in Th and Y, compared with inherited grains. Upon the appearance of chloritoid (～440–450 °C, thermometry based on chlorite–choritoid and carbonaceous material), monazite is consumed, and MREE and LREE are taken up preferentially in two distinct zones of allanite distinguishable by EMPA and X‐ray mapping. Prior to garnet growth, allanite acquires two growth zones of clinozoisite: a first one rich in HREE + Y and a second one containing low REE contents. Following garnet growth, close to the chloritoid–out zone boundary (～556–580 °C, based on phase equilibrium calculations), allanite and its rims are partially to totally replaced by monazite and xenotime, both associated with plagioclase (± biotite ± staurolite ± kyanite ± quartz). In these samples, epidote relics are located in the matrix or as inclusions in garnet, and these preserve their characteristic chemical and textural growth zoning, indicating that they did not experience re‐equilibration following their prograde formation. Hence, the partial breakdown of allanite to monazite offers the attractive possibility to obtain in situ ages, representing two distinct crystallization stages. In addition, the complex REE + Y and Th zoning pattern of allanite and monazite are essential monitors of crystallization conditions at relatively low metamorphic grade. 相似文献
14.
Whole-rock, Phosphate, and Silicate Compositional Trends across an Amphibolite- to Granulite-facies Transition, Tamil Nadu, India 总被引：2，自引：0，他引：2
Chemical trends from north (amphibolite facies) to south (granulitefacies) along a 95 km traverse in Tamil Nadu, Southern India,include: whole-rock depletion of Rb, Cs, Th and U, enrichmentin Ti and F, and depletion in Fe and Mn in biotite and amphibole;increases in Al and decreases in Mn in orthopyroxene; enrichmentof fluorapatite in F. K-feldspar blebs are widespread alongquartz–plagioclase grain boundaries, and could indicateeither partial melting or metasomatism. In the northernmostportion of the traverse the principal rare earth element (REE)-bearingminerals are allanite and titanite. South of a clinopyroxeneisograd, monazite grains independent of fluorapatite are themajor REE- and Th-bearing phase. Further south independent monaziteis rare but Th-free monazite inclusions are common in fluorapatite.During prograde metamorphism, independent monazite was replacedby REE-rich fluorapatite in which the monazite inclusions laterformed. The loss of independent monazite was accompanied bya loss of whole-rock Th and possibly a small depletion in lightREE. Most mineralogical features along the traverse can be accountedfor by progressive dehydration and oxidation reactions. Trace-elementdepletion is best explained by the action of an externally derivedlow H2O activity brine migrating from a source at greater depth,possibly preceded or accompanied by partial melting. KEY WORDS: granulite facies; charnockite; metasomatism; Archean; Tamil Nadu, India; fluorapatite; monazite; allanite; titanite; biotite 相似文献
15.
Michel Corsini V. Bosse G. Féraud A. Demoux G. Crevola 《International Journal of Earth Sciences》2010,99(2):327-341
Detailed 40Ar/39Ar geochronology on single grains of muscovite was performed in the Variscan Tanneron Massif (SE France) to determine the
precise timing of the post-collisional exhumation processes. Thirty-two plateau ages, obtained on metamorphic and magmatic
rocks sampled along an east–west transect through the massif, vary from 302 ± 2 to 321 ± 2 Ma, and reveal a heterogeneous
exhumation of the lower crust that lasted about 20 Ma during late Carboniferous. In the eastern part of the massif, the closure
of the K–Ar isotopic system is at 311–315 Ma, whereas in the middle part of the massif it closes earlier at 317–321 Ma. These
cooling paths are likely to be the result of differential exhumation processes of distinct crustal blocks controlled by a
major ductile fault, the La Moure fault that separates both domains. In the western part of the massif, the ages decrease
from 318 to 303 Ma approaching the Rouet granite, which provides the youngest age at 303.6 ± 1.2 Ma. This age distribution
can be explained by the occurrence of a thermal structure spatially associated to the magmatic complex. These ages argue in
favour of a cooling of the magmatic body at around 15 Ma after the country rocks in the western Tanneron. The emplacement
of the Rouet granite in the core of an antiform is responsible for recrystallization and post-isotopic closure disturbances
of the K–Ar chronometer in the muscovite from the host rocks. These new 40Ar/39Ar ages clearly outline that at least two different processes may contribute to the exhumation of the lower crust in the later
stage of collision. During the first stage between 320 and 310 Ma, the differential motion of tectonic blocks limited by ductile
shear zones controls the post-collisional exhumation. This event could be related to orogen parallel shearing associated with
crustal-scale strike-slip faults and regional folding. The final exhumation stages at around 300 Ma take place within the
tectonic doming associated to magmatic intrusions in the core of antiformal structures. Local ductile to brittle normal faulting
is coeval to Upper Carboniferous intracontinental basins opening. 相似文献
16.
The composition of accessory REE minerals (allanite, chevkinite, fergusonite, and REE carbonates) in alkaline metasomatic rocks of the Main Sayan Fault (quartz-albite-microcline-riebeckite-aegirine, quartzalbite-microcline-magnetite, and clinopyroxene-albite) was studied using back-scattered scanning electron microscopy. Chevkinite occurs only in quartz-albite-microline metasomatic rock. The paragenesis of allanite and titanite is stable in clinopyroxene-albite metasomatic rocks. Allanite and fergusonite are typical of all zones of the metasomatic column. Chevkinite and allanite are often altered due to interaction with hydrothermal fluid and lose some amount of LREE. Secondary bastnaesite, synchysite, and ancylite are formed after allanite, while secondary monazite is developed after chevkinite. Presumably, the low-temperature alteration of allanite and chevkinite under effect of F?, CO 3 2? , and P 4 3? -bearing fluids had not any significant manifestation in the total REE content in metasomatic rocks. 相似文献
17.
The chemical Th–U total Pb isochron method (CHIME) of dating was carried out on accessory minerals in samples from the Okcheon metamorphic belt in Korea. Dated minerals include xenotime and monazite with overgrown mantles in a granitic gneiss clast from the Hwanggangri Formation, metamorphic allanite in garnet-bearing muscovite–chlorite schist of the Munjuri Formation, and polycrase and monazite in post-tectonic granite from the Hwanggangri area. Overgrowth of mantles took place at 369 ± 10 Ma on c. 1750 Ma cores of xenotime and monazite in the granitic gneiss. Allanite, occurring in textural equilibrium with peak metamorphic minerals, yields a CHIME age of 246 ± 15 Ma that is discriminably older than the polycrase (170 ± 6 Ma) and monazite (170 ± 3 Ma) ages of the post-tectonic granite. These chronological data suggest that some of the metasedimentary rocks in the belt formed through a single stage of metamorphism at c. 250 Ma from post-370 Ma sediments. Late Permian age signatures have also been reported from the Precambrian Gyeonggi and Yeongnam massifs that border the Okcheon metamorphic belt, and indicate that parts of the basement massifs and the metamorphic belt were affected by the same regional metamorphic event. 相似文献
18.
Atsushi Okamoto Taketo Kikuchi Noriyoshi Tsuchiya 《Contributions to Mineralogy and Petrology》2008,156(3):323-336
Pelitic schists of the Sanbagawa metamorphic belt contain several types of polymineralic veins that formed during the late
stages of exhumation. The vein mineral assemblages are quartz + albite + K-feldspar + chlorite ± calcite (Type I, II) and
quartz + albite + calcite (Type III). Type I and II veins contain quartz and albite with stretched-crystal and elongate-blocky
textures, respectively. The mineral species within Type I veins vary with compositional bands within the host rocks. Type
III veins are characterized by euhedral to subhedral quartz grains with concentric zoning and a homogeneous distribution along
the vein length. The vein textures vary depending on the crack aperture during multiple crack-seal events: <0.08 mm for Type
I, and 0.5–10 mm for Type III. Type II veins show intermediate features between Type I and III veins in terms of mineral distribution
(weak dependence on the host rock composition) and apparent crack aperture (less than 1–15 mm). These observations suggest
a transition in the dominant transport mechanism of vein components with increasing crack aperture, from diffusion from host
rocks to fluid advection along cracks. 相似文献
19.
Humberto Terrazas Salas Hermínio Arias Nalini Jr. Júlio César Mendes 《Environmental Geology》2006,49(4):520-526
One hundred samples of granitic rock were collected from granite traders in Belo Horizonte. Autoradiography, optical microscopy,
diffractometry, and chemical analysis (X-ray spectrometry, X-ray fluorescence, neutron activation, gravimetry, and electron
probe microanalysis) were used to determine the mineral assemblages and lithotypes. Autoradiographic results for several samples
showed the presence of monazite, allanite, and zircon. Chemical analysis revealed uranium concentrations ≤30 ppm and thorium
≤130 ppm. Higher concentrations generally correlated with high concentrations of light rare earths in silica-rich rocks of
granitic composition. Calculations were made of radioactive doses for samples, of floor tiles in a standard room, with total
concentration of uranium and thorium greater than 60 ppm. On the basis of calculations of 232 Th, 40 K, and 226 Ra from analysis of Th, K, and U, the doses calculated were between 0.11 and 0.34 mSv/year, which are much lower than the
acceptable international exposure standard of 1.0 mSv/year. 相似文献
20.
Ajay Kumar S. N. S. Birua Dheeraj Pande A. R. Nath P. V. Ramesh Babu S. A. Pandit 《Journal of the Geological Society of India》2009,73(4):537-542
Several radioactive quartz-pebble conglomerate (QPC) occurrences at the western margin of Archaean Bonai granite and overlying
Iron Ore Group (IOG) rocks have recently been located over a total strike length of 8–10 km intermittently in a NE-SW to E-W
trend with steep dips due north-west to north in parts of Sundargarh district of Orissa. The QPC samples have analysed up
to 0.039% U3O8 and 0.035% ThO2 with high concentration of Y (74 to 518 ppm), La(<100 to 880 ppm), Cr ( 126 to 633 ppm), Zr (137 to 1250 ppm) and Pb (31
to 581 ppm). Cellulose Nitrate (CN) film studies of few QPC samples indicated adsorbed uranium over goethite and infiltrated
ferruginous material (limonite), secondary uranium as encrustation and fracture filling and discrete sub-rounded grains of
monazite, zircon, allanite and rare xenotime in the matrix of QPC as radioactive phases. Higher content of Th over U, elevated
concentration of Y and La in QPC eliminates the possibility of its low temperature product by epigenetic processes. Poor correlation
of U with elements like Pb, Y, Zr, La and Cr can be explained due to surficial leaching of uranium from QPC after its deposition
as reflected by adsorbed U over iron-oxides and low U/Th ratio in QPC in the area. 相似文献