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1.
Columbite solubility in granitic melts: consequences for the enrichment and fractionation of Nb and Ta in the Earth's crust 总被引:13,自引:0,他引:13
The behaviour of niobium and tantalum in magmatic processes has been investigated by conducting MnNb2O6 and MnTa2O6 solubility experiments in nominally dry to water-saturated peralkaline (aluminium saturation index, A.S.I. 0.64) to peraluminous
(A.S.I. 1.22) granitic melts at 800 to 1035 °C and 800 to 5000 bars. The attainment of equilibrium is demonstrated by the
concurrence of the solubility products from dissolution, crystallization, Mn-doped and Nb- or Ta-doped experiments at the
same pressure and temperature. The solubility products of MnNb2O6 (Ksp
Nb) and MnTa2O6 (Ksp
Ta) at 800 °C and 2 kbar both increase dramatically with alkali contents in water-saturated peralkaline melts. They range from
1.2 × 10−4 and 2.6 × 10−4 mol2/kg2, respectively, in subaluminous melt (A.S.I. 1.02) to 202 × 10−4 and 255 × 10−4 mol2/kg2, respectively, in peralkaline melt (A.S.I. 0.64). This increase from the subaluminous composition can be explained by five
non-bridging oxygens being required for each excess atom of Nb5+ or Ta5+ that is dissolved into the melt. The Ksp
Nb and Ksp
Ta also increase weakly with Al content in peraluminous melts, ranging up to 1.7 × 10−4 and 4.6 × 10−4 mol2/kg2, respectively, in the A.S.I. 1.22 composition. Columbite-tantalite solubilities in subaluminous and peraluminous melts (A.S.I.
1.02 and 1.22) are strongly temperature dependent, increasing by a factor of 10 to 20 from 800 to 1035 °C. By contrast columbite-tantalite
solubility in the peralkaline composition (A.S.I. 0.64) is only weakly temperature dependent, increasing by a factor of less
than 3 over the same temperature range. Similarly, Ksp
Nb and Ksp
Ta increase by more than two orders of magnitude with the first 3 wt% H2O added to the A.S.I. 1.02 and 1.22 compositions, whereas there is no detectable change in solubility for the A.S.I. 0.64
composition over the same range of water contents. Solubilities are only slightly dependent on pressure over the range 800
to 5000 bars. The data for water-saturated sub- and peraluminous granites have been extrapolated to 600 °C, conditions at
which pegmatites and highly evolved granites may crystallize. Using a melt concentration of 0.05 wt% MnO, 70 to 100 ppm Nb
or 500 to 1400 ppm Ta are required for manganocolumbite and manganotantalite saturation, respectively. The solubility data
are also used to model the fractionation of Nb and Ta between rutile and silicate melts. Predicted rutile/melt partition coefficients
increase by about two orders of magnitude from peralkaline to peraluminous granitic compositions. It is demonstrated that
the γNb2O5/γTa2O5 activity coefficient ratio in the melt phase depends on melt composition. This ratio is estimated to decrease by a factor
of 4 to 5 from andesitic to peraluminous granitic melt compositions. Accordingly, all the relevant accessory phases in subaluminous
to peraluminous granites are predicted to incorporate Nb preferentially over Ta. This explains the enrichment of Ta over Nb
observed in highly fractionated granitic rocks, and in the continental crust in general.
Received: 9 August 1996 / Accepted: 26 February 1997 相似文献
2.
Our studies show that the granite bodies (γ
5
2 − 1
and γ
5
3
) which constitute the Huangsha-Tieshanlong composite granitic intrusion in Jiangxi are characterized by their similarities
in mineral assemblage, petrochemistry, trace element and REE distribution pattern. The values of ΣREE, ΣLREE, ΣHREE, ΣCe/ΣY,
δEu and La/Yb apparently decrease from γ
5
2 − 1a
to γ
5
2 − 1b
, γ
5
3
and γ
5
3
. It is shown that the early Yenshanian W(Ta, Nb)-bearing granite (γ
5
2 − 1
) and late Yenshanian Ta, Nb-bearing granite (γ
5
3
) may have been derived from the differentiation and evolution of granitic magmas due to repeated remelting of the crust and
their earlier and later intrusion. Although the earlier (γ
5
2 − 1b
and later (γ
5
3
) albitized Ta, Nb-bearing granites show some obvious differences in REE content, their δEu values and La/Yb ratios are similar
to each other. Therefore, it may be concluded that the early and late Ta, Nb-bearing granites were derived from a congenetic
magma. 相似文献
3.
《Earth》1999,45(3-4):145-165
Ammonium is present as a trace constituent in all granites, with an average concentration of 45 ppm (NH4+), equivalent to 35 ppm of elemental N. It shows wide variations related to petrography and region, but the only significant correlation between ammonium and other geochemical parameters is that it is most abundant in peraluminous granites and least abundant in peralkaline granites. These variations can be related to (a) the amount of sedimentary material in the magmatic source region, and (b) redox conditions in the source region. The ammonium content of granitic magmas can also be modified by fractionation or contamination. Hydrothermal alteration has a major effect on the ammonium content of granitic rocks, and variation due to this cause may exceed the magmatic variation. Most hydrothermally altered granites are enriched in ammonium as a result of the transfer of ammonium from sedimentary country rocks by the hydrothermal fluids. 相似文献
4.
P. B. Tomascak Eirik J. Krogstad Richard J. Walker 《Contributions to Mineralogy and Petrology》1996,125(1):45-59
Neodymium and lead isotope and elemental data are presented for the Sebago batholith (293±2 Ma), the largest exposed granite
in New England. The batholith is lithologically homogeneous, yet internally heterogeneous with respect to rare earth elements
(REE) and Nd isotopic composition. Two-mica granites in the southern/central portion of the batholith (group 1) are characterized
by REE patterns with uniform shapes [CeN/YbN (chondrite normalized) = 9.4–19 and Eu/Eu* (Eu anomaly) = 0.27–0.42] and ɛ
Nd(t) = −3.1 to −2.1. Peripheral two-mica granites (group 2), spatially associated with stromatic and schlieric migmatites,
have a wider range of total REE contents and patterns with variable shapes (CeN/YbN = 6.1–67, Eu/Eu* = 0.20–0.46) and ɛ
Nd(t) = −5.6 to −2.8. The heterogeneous REE character of the group 2 granites records the effects of magmatic differentiation
that involved monazite. Coarse-grained leucogranites and aplites have kinked REE patterns and low total REE, but have Nd isotope
systematics similar to group 2 granites with ɛ
Nd(t) = −5.5 to −4.7. Rare biotite granites have steep REE patterns (CeN/YbN = 51–61, Eu/Eu* = 0.32–0.84) and ɛ
Nd(t) = −4.6 to −3.8. The two-mica granites have a restricted range in initial Pb isotopic composition (206Pb/204Pb = 18.41–18.75; 207Pb/204Pb = 15.60–15.68; 208Pb/204Pb = 38.21–38.55), requiring and old, high U/Pb (but not Th/U) source component. The Nd isotope data are consistent with magma
derivation from two sources: Avalon-like crust (ɛ
Nd>−3), and Central Maine Belt metasedimentary rocks (ɛ
Nd<−4), without material input from the mantle. The variations in isotope systematics and REE patterns are inconsistent with
models of disequilibrium melting which involved monazite.
Received: 8 December 1995 / Accepted: 29 April 1996 相似文献
5.
The Longwangzhuang granite pluton occurs on the southern margin of the North China Craton and consists mainly of biotite syenogranite with aegirine granite being locally distributed.The granites are characterized by high silicon and alkaline contents(SiO2=72.17%-76.82%,K2O+Na2O=8.28%-10.22%,K2O/Na2O>>1),AI(agpaitic index) =0.84-0.95,DI=95-97,ASI(aluminum saturation index)=0.96-1.13,and very high Fe* number(FeO*/(FeO*+Mg)=0.90-0.99),thus the granites are assigned to the metaluminous to weakly peraluminous,alkalic to calc-alkalic ferroan A-type granites.The granites are rich in large ion lithophile elements(LILE),especially high in REE concentrations(REE+Y=854×10-6-1572×10-6);whereas the enrichment of high strength field elements(Nb,Ta,Zr,Hf) is obviously less than that of LILEs,exhibiting mild depletions on trace element spider plots;and the rocks are significantly depleted in Ba,Sr,Ti,and Pb.The low εNd(t) values(-4.5--7.2) and high model ages(2.3-2.5 Ga) of the granites as well as the low εHf(t) values(-1.11--5.26) and high Hf model ages(THf1= 2.1-2.3 Ga,THf2=2.4-2.6 Ga) of zircons from the biotite syenogranite suggest that the granites were probably derived from an enriched mantle source.The zircons from the biotite syenogranite are mainly colorless transparent crystals exhibiting well-developed oscillatory zoning on the cathodoluminescence images with a LA-ICPMS zircon U-Pb age of 1602.1±6.6 Ma(MSWD=0.48).Petrochemical,trace elements,as well as Nd and Hf isotopic compositions of the rocks demonstrate that the granites were formed in a within-plate extensional tectonic regime possibly related to the breakup of the Columbia supercontinent.The granites were most likely formed through extreme fractional crystallization of alkali basaltic magma resulted from partial melting of the mantle,which was fertilized by recycling crustal rocks triggered by the delamination of lithospheric mantle and lower crust following the ~1.8 Ga collision and amalgamation of the North China Craton which is part of the Columbia supercontinent.However,contamination of neo-Archean to Paleoproterozoic crustal rocks during the ascent and emplacement of the magma could not be excluded.Being the youngest known anorogenic magmatism on the southern margin of the North China Craton related to Columbia breakup,it might represent the break off of the North China Craton from Columbia supercontinent at the end of Paleoproterozoic. 相似文献
6.
Olivier Bachmann Paul J. Wallace Julie Bourquin 《Contributions to Mineralogy and Petrology》2010,159(2):187-202
The >60 km3 rhyolitic Kos Plateau Tuff provides an exceptional probe into the behavior of volatile components in highly evolved arc magmas:
it is crystal-rich (30–40 vol% crystals), was rapidly quenched by the explosive eruptive process, and contains abundant homogeneous
melt inclusions in large quartz crystals. Several methods for measuring major, trace and volatile element concentrations (SIMS,
FTIR, Raman spectroscopy, electron microprobe, LA–ICPMS) were applied to these melt inclusions. We found a ~2 wt% range of
H2O contents (4.5–6.5 wt% H2O, measured independently by SIMS, FTIR, and Raman spectroscopy) and relatively low CO2 concentrations (15–140 ppm measured by FTIR, with most analyses <100 ppm). No obvious correlations between H2O, CO2, major and trace elements are observed. These observations require a complex, protracted magma evolution in the upper crust
that included: (1) vapor-saturated crystallization in a chamber located between 1.5 and 2.5 kb pressure, (2) closed-system
degassing (with up to 10 vol% exsolved gas) as melts percolated upwards through a vertically extensive mush zone (2–4 km thick),
and (3) periodic gas fluxing from subjacent, more mafic and more CO2-rich magma, which is preserved as andesite bands in pumices. These processes can account for the range of observed H2O and CO2 values and the lack of correlation between volatiles and trace elements in the melt inclusions. 相似文献
7.
Ni Shijun Luo Yutian Liu Lihua Wang Xuben Li Sue Luo Yangdi Han Dingrong 《中国地球化学学报》1994,13(3):193-201
This study focuses on the thermodynamics of diagenetic fluid from the Eogene Xingouzui Formation which represents the most
important reservoir in Field Oil T in the Jianghan Basin. The measured homogenization temperatures (110–139 °C) of fluid inclusions
in diagenetic minerals fell within the range of 67 –155 °C at the middle diagenetic stage. The pressure of diagenetic fluid
is estimated at 10.2 –56 MPa. The activity of ions in the fluid shows a tendency of Ca2+ > Mg2+ > Na+ > K+ > Fe3+ > Fe2+ for cations, and HCO
3
−
> SO
4
2−
> F− > Cl− > CO
3
2−
for anions. For the gaseous facies, there is a tendency of CO2> CO> H2S> CH4> H2. According to the thermodynamic calculations, the pH and Eh of the fluid are 5.86–6.47 and −0.73–−0.64V, respectively. As
a result of the interaction between such a diagenetic fluid and minerals in the sediments, feldspars were dissolved or alterated
by other minerals. The clay mineral kaolinite was instable and hence was replaced by illite and chloritoid.
This project was jointly funded by the National Natural Science Foundation of China (49133080) and the Open Laboratory of
Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences. 相似文献
8.
Jens Götze Michael Plötze Dieter Klaus Hallbauer 《Geochimica et cosmochimica acta》2004,68(18):3741-3759
Pegmatite quartz from different occurrences in Norway and Namibia was investigated by a combination of ICP-MS, Electron Spin Resonance (ESR), Capillary Ion Analysis (CIA) and Gas Chromatography (GC) to quantify trace elements in very low concentrations and to determine their position in the quartz structure.The studied quartz samples show similar geochemical characteristics with low contents of most trace elements. Remarkable are the elevated concentrations of Al (36-636 ppm), Ti (1.6-25.2 ppm), Ge (1.0-7.1 ppm), Na (5.2 to >50 ppm), K (1.6 to >100 ppm) and Li (2.1-165.6 ppm). These elements are preferentially incorporated into the quartz lattice on substitutional (Al, Ti, Ge) and interstitial (Li, Na, K) positions. Li+ was found to be the main charge compensating ion for Al, Ge and Ti, whereas some ppm of Na and K may also be hosted by fluid inclusions. Ti may be incorporated as substitutional ion for Si or bound on mineral microinclusions (rutile). The results of the ESR measurements show that there may be a redistribution of alkali ions during irradiation. The diamagnetic [AlO4/M+]0 center transforms into the paramagnetic [AlO4]0 center, whilst the compensating ions diffuse away and may be captured by the diamagnetic precursor centers of [GeO4]0 and [TiO4]0 to form paramagnetic centers ([TiO4/Li+]0, [GeO4/Li+]0).In general, fluid inclusions in pegmatite quartz can be classified as H2O-CO2-NaCl type inclusions with water as the predominant volatile. Among the main elements hosted by fluid inclusions in quartz are Na, K, NH4, Ca, Mg and the anionic complexes Cl−, NO3−, HCO3− and SO42−. Gas analysis of trapped fluids shows volatile components in the following order of abundance: H2O > CO2 > N2(+) ≥ CH4 > COS > C2 and C3 hydrocarbons. Additionally, traces of Co, Ni, Zn, Pb, and Cu were detected by CIA in fluid inclusions of some samples. There are indications that the REE and Rb are also bound in fluid inclusions, however, the concentrations of these elements are too low to be measured by CIA. Assuming that the REE preferentially occur in fluid inclusions, the typical chondrite normalized REE distribution patterns with tetrad effects and a distinct negative Eu anomaly would reflect the composition of the mineralizing fluid.For a number of elements, especially those with extremely low concentrations, the “type” of incorporation in quartz could not directly be determined. We conclude that these ions either are too large to substitute for the small Si4+ ion or they are not soluble in the mineralizing fluids to be hosted by fluid inclusions. Some of these elements, which are concentrated in the specific mineralization of certain pegmatites, are not present in elevated concentrations in the paragenetic pegmatite quartz itself. This was observed, for instance, for Be, Cs and Rb in the Li (Be-Cs-Rb) pegmatites of Rubicon or for Nb and Ta for Nb-Ta bearing pegmatites from Norway. It may be concluded that the concentrations of these trace elements in quartz do not reflect the mineralization and that these elements thus, cannot be used as petrogenetic indicator. 相似文献
9.
Gehad M. Saleh 《中国地球化学学报》2006,25(2):97-111
The Zargat Na’ am ring complex crops out 90 km NW of Shalatin City in the Southeastern Desert of Egypt. The ring complex forms
a prominent ridge standing high above the surrounding mafic-ultramafic hills. It is cut by two sets of joints and faults which
strike predominantly NNW-SSE and E-W, and is injected by dikes, porphyritic alkaline syenites, and felsite porphyries. It
consists of alkali syenites, alkali quartz syenites, and peralkaline arfvedsonite-bearing granitic and pegmatitic dikes and
sills. The complex is characterized locally by extreme enrichments in REEs, wolframite and rare, high field strength metals
(HFSM), such as Zr and Nb. The highest concentrations (1.5 wt% Zr, 0.25 wt% Nb, 0.6 wt% Σ REEs) occur in aegirine-albite aplites
that formed around arfvedsonite pegmatites. Quartzhosted melt inclusions in arfvedsonite granite and pegmatite provide unequivocal
evidence that the peralkaline compositions and rare metal enrichments are primary magmatic features. Glass inclusions in quartz
crystals also have high concentrations of incompatible trace elements including Nb (750 ¢ 10−6), Zr (2500 × 10−6) and REEs (1450 × 10−6). The REEs, Nb and Zr compositions of the aegirine-albite aplites plot along the same linear enrichment trends as the melt
inclusions, and Y/Ho ratios mostly display unfractionated, near-chondritic values. The chemical and textural features of the
aegirine-albite aplites are apparently resultant from rapid crystallization after volatile loss from a residual peralkaline
granitic melt similar in composition to the melt inclusions. 相似文献
10.
Maria Rosaria Renna Riccardo Tribuzio Massimo Tiepolo 《Contributions to Mineralogy and Petrology》2007,154(5):493-517
The post-Variscan complex of Porto consists of metaluminous to slightly peraluminous A-type biotite granites mingled with
gabbro-dioritic rocks, and late dykes with basaltic to trachyandesitic composition. U-Pb zircon dating by LA-ICP-MS on two
mafic intrusive samples constrains the time of the gabbro–granite crystallisation at 281 ± 3 Ma and 283 ± 2 Ma. Hornblende 40Ar-39Ar ages from a late trachyandesite dyke date the dyking event at 280 ± 2 Ma, which is within error the U-Pb zircon ages of
the intrusives. Biotite granites show variable major and trace element compositions and similar initial εNd (−0.3 to +0.9). Whole rock chemistry variations and trace element compositions of plagioclase and allanite indicate that
the granites are genetically linked, essentially through fractional crystallisation of feldspars and minor allanite. On the
basis of whole-rock chemistry e.g. initial εNd +4.9 to +1.7 and trace element clinopyroxene compositions, we have ascertained that the mafic intrusives and basic dykes
formed from isotopically depleted mantle source-derived melts with similar trace element signature. These basic melts experienced
slightly different evolutionary histories, controlled by fractional crystallisation and crustal contamination, mainly by the
acid magma that gave rise to the associated biotite granites, but also by the enclosing older Variscan granitoids. U-Pb zircon
data suggest that the Porto complex was affected by hydrothermal fluid circulation at 259 ± 9 Ma. 相似文献
11.
Factors controlling copper solubility and chalcopyrite deposition in the Sungun porphyry copper deposit, Iran 总被引:14,自引:0,他引:14
The Sungun porphyry copper deposit is hosted in a Diorite/granodioritic to quartz-monzonitic stock that intruded Eocene volcanosedimentary
and Cretaceous carbonate rocks. Copper mineralization is associated mainly with potassic alteration and to a lesser extent
with sericitic alteration. Based on previously published fluid inclusion and isotopic data by Hezarkhani and Williams-Jones
most of the copper is interpreted to have deposited during the waning stages of orthomagmatic hydrothermal activity at temperatures
of 400 to 300 °C. These data also indicate that the hydrothermal system involved meteoric waters, and boiled extensively.
In this work, thermodynamic data are used to delineate the stability fields of alteration and ore assemblages as a function
of fS2, fO2 and pH. The solubility of chalcopyrite was evaluated in this range of conditions using recently published experimental data.
During early potassic alteration (>450 °C), Copper solubility is calculated to have been >50 000 ppm, whereas the copper content
of the initial fluid responsible for ore deposition is estimated, from fluid inclusion data, to have been 1200–3800 ppm. This
indicates that initially the fluid was highly undersaturated with respect to chalcopyrite, which agrees with the observation
that veins formed at T > 400 °C contain molybdenite but rarely chalcopyrite. Copper solubility drops rapidly with decreasing temperature, and at
400 °C is approximately 1000 ppm, within the range estimated from fluid inclusion data, whereas at 350 °C it is only 25 ppm.
These calculations are consistent with observations that the bulk of the chalcopyrite deposited at Sungun is hosted by veins
formed at temperatures of 360 ± 60 °C. Other factors that, in principle, may reduce chalcopyrite solubility are increases
in pH, and decreases in fO2 and aCl−. Our analysis shows, however, that most of the change in pH occurred at high temperature when chalcopyrite was grossly undersaturated
in the fluid, and that the direction of change in fO2 increased chalcopyrite solubility. We propose that the Sungun deposit formed mainly in response to the sharp temperature
decrease that accompanied boiling, and partly as a result of the additional heat loss and decrease in aCl−, which occurred as a result of mixing of acidic Cu-bearing magmatic waters with cooler meteoric waters of lower salinity.
Received: 8 July 1998 / Accepted: 8 April 1999 相似文献
12.
Study on the Physical and Chemical Conditions of Ore Formation of Hetai Ductile Shear Zone—Hosted Gold Deposit and Discovery of Melt Inclusions 总被引:1,自引:0,他引:1
The Hetai ductile shear zone-hosted gold deposit occurs in the deep-seated fault mylonite zone of the Sinian-Silurian metamorphic rock series. In this study there have been discovered melt inclusions, fluid-melt inclusions and organic inclusions in ore-bearing quartz veins of the ore deposit and mylonite for the first time. The homogenization temperatures of the various types of inclusions are 160℃, 180 - 350℃, 530℃ and 870℃ for organic inclusions, liquid inclusions, two-phase immiscible liquid inclusions and melt inclusions, respectively. Ore fluid is categorized as the neutral to basic K+ -Ca2+ -Mg2+ -Na+ - SO2- 4-HCO3-Cl- system. The contents of trace gases follow a descending order of H2O>CO2>CH4>(or < ) H2>CO>C2H2>C2I-I6>O2>N2.The concentrations of K , Ca2 + ,SO2-4,HCO3-,Cl- H2O and C2H2 in fluid inclusions are related to the contents of gold and the Au/Ag ratios in ores from different levels of the gold deposit. This is significant for deep ore prospecting in the region. Daughter minerals in melt inclusions were analyzed using SEM. Quartz, orthoclase, wollastonite and other silicate minerals were identified. They were formed in different mineral assemblages.This analysis further proves the existence of melt inclusions in ore veins. Sedimentary metamorphic rocks could form silicate melts during metamorphic anatexis and dynamic metamorphism, which possess melt-solution characteristics. Ore formation is related to the multi-stage forming process of silicate melt and fluid. 相似文献
13.
To interpret the degassing of F-bearing felsic magmas, the solubilities of H2O, NaCl, and KCl in topaz rhyolite liquids have been investigated experimentally at 2000, 500, and ≈1 bar and 700° to 975 °C.
Chloride solubility in these liquids increases with decreasing H2O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio
of ((Al + Na + Ca + Mg)/Si). Small quantities of Cl− exert a strong influence on the exsolution of magmatic volatile phases (MVPs) from F-bearing topaz rhyolite melts at shallow
crustal pressures. Water- and chloride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate
liquids containing as little as 1 wt% H2O and 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H2O required for volatile phase exsolution in chloride-free liquids. The maximum solubility of Cl− in H2O-poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of H2O in chloride-poor liquids by simple linear and negative relationships; there are strong positive deviations from ideality
in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H2O versus Cl− in rhyolite liquids, for experiments conducted at 500 bar and 910°–930 °C, show a distinct 90° break-in-slope pattern that
is indicative of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H2O and Cl− concentrations of the silicate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive
H2O and Cl− concentrations of five North American topaz and tin rhyolite melts, determined from melt inclusion compositions, provides
evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali
chloride-bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior
to eruption.
Received: 6 November 1995 / Accepted: 29 January 1998 相似文献
14.
Christopher D. Wareham Ian L. Millar Alan P. M. Vaughan 《Contributions to Mineralogy and Petrology》1997,128(1):81-96
Subduction-related Mesozoic to Cainozoic granites s.l. in western Palmer Land, Antarctic Peninsula, have similar chemical compositions to Archean tonalite-trondhjemite-granodiorite
(TTG) suites, Phanerozoic slab-melts (adakites), and to experimental partial melts of basaltic material in equilibrium with
amphibole ± pyroxene ± garnet. They are predominantly sodic, metaluminous and most have Al2O3 > 15 wt% and Y < 18 ppm. All are light rare earth element (LREE)-enriched (2 < La/Ybn <30) and most have small Eu anomalies. They have a wide range of initial ɛNd(t) (−6.8 to +4.5) and ɛSr(t) (+293.4 to −3.7), but most Pb isotope compositions deviate by < 0.3% from their mean. The Pb isotope data indicate a crustal
component to all the granites, which Sr and Nd isotope variations suggest is pre-Triassic–Triassic. The 207Pb/204Pb(t) range from 15.602 to 15.666 and appear to preclude a significant Proterozoic, or older, crustal component. The granites have
chemical and isotopic compositions that suggest they are not partial melts of subducted oceanic lithosphere, as has been suggested
for some Archean and Phanerozoic TTG magmas. We conclude that they were produced by mixing between basaltic-andesitic arc
magmas, partial melts of juvenile basaltic lower crust and pre-Triassic crust. The low H(heavy)REE+Y content of some of the
granites requires that garnet was a residual phase in the crust during partial melting, indicating a crustal thickness of
>36 km. Between Triassic and Tertiary times the initial ɛNd(t) of the magmatism increased and ɛSr(t) decreased, suggesting that new continental crust was produced during this period. Underplating by mafic magma was an important
crustal growth mechanism in the arc: the generation of abnormally thick crust, and its subse quent fusion, is considered to
be a consequence of ca. ≥ 180 Ma of subduction and associated magmatism in the region. An implication of the model is that
dense garnet-amphibolite and eclogite residues from partial melting of the lower crust will accumulate. In theory, the setting
was appropriate for such residues to detach from the base of the crust and to sink into the convecting mantle. Such a process
would leave the rest of the crust enriched in large ion lithophile elements/LREE, but depleted in HREE+Y.
Received: 2 October 1995 / Accepted: 5 January 1997 相似文献
15.
Bulk δ
34Srock values, sulfur contents, and magnetic susceptibility were determined for 12 gold-related granitoid intrusions in southwestern
New Brunswick, the Canadian Appalachians. The sulfur isotope compositions of sulfide minerals in some of the granitoid samples
were also analyzed. This new dataset was used to characterize two distinctive groups of granitoids: (1) a Late Devonian granitic
series (GS) and (2) a Late Silurian to Early Devonian granodioritic to monzogranitic series (GMS). The GS rocks have a large
range in δ
34S values of −7.1‰ to +13‰ with an average of 2.2 ± 5.0‰ (1σ), low bulk-S contents (33 to 7,710 ppm) and low magnetic susceptibility
values (<10−4 SI), consistent with reduced ilmenite-series granites. The GMS rocks have a relatively narrower variation in δ
34S values of −4.4‰ to +7.3‰ with an average 1.2 ± 2.9‰ but with larger ranges in bulk-S contents (45 to 11,100 ppm) and high
magnetic susceptibility values (>10−3 SI), indicative of oxidized magnetite-series granites. The exceptions for the GMS rocks are the Lake George granodiorite
and Tower Hill granite that display reduced characteristics, which may have resulted from interaction of the magmas forming
these intrusions with graphite- or organic carbon-bearing sedimentary rocks. The bulk δ
34S values and S contents of the GMS rocks are interpreted in terms of selective assimilation–fractional crystallization (SAFC)
processes. Degassing processes may account for the δ
34S values and S contents of some GS rocks. The characteristics of our sulfur isotope and abundance data suggest that mineralizing
components S and Au in intrusion-related gold systems are dominantly derived from magmatic sources, although minor contaminants
derived from country rocks are evident. In addition, the molar sulfate to sulfide ratio in a granitic rock sample can be calculated
from the δ
34Srock value of the whole-rock sample and the δ
34Ssulfide (or δ
34Ssulfate) value of sulfide and/or sulfate mineral in the sample on the basis of S-isotope fractionation and mass balance under the
condition of magmatic equilibrium. This may be used to predict the speciation of sulfur in granitic rocks, which can be a
potential exploration tool for intrusion-related gold systems. 相似文献
16.
Detailed melt and fluid inclusion studies in quartz hosts from the Variscan Ehrenfriedersdorf complex revealed that ongoing fractional crystallization of the highly evolved H2O-, B-, and F-rich granite magma produced a pegmatite melt, which started to separate into two immiscible phases at about 720°C, 100 MPa. With cooling and further chemical evolution, the immiscibilty field expanded. Two conjugate melts, a peraluminous one and a peralkaline one, coexisted down to temperatures of about 490°C. Additionally, high-salinity brine exsolved throughout the pegmatitic stage, along with low-density vapor. Towards lower temperatures, a hydrothermal system gradually developed. Boiling processes occurred between 450 and 400°C, increasing the salinities of hydrothermal fluids at this stage. Below, the late hydrothermal stage is dominated by low-salinity fluids. Using a combination of synchrotron radiation-induced X-ray fluorescence analysis and Raman spectroscopy, the concentration of trace elements (Mn, Fe, Zn, As, Sb, Rb, Cs, Sr, Zr, Nb, Ta, Ag, Sn, Ta, W, rare earth elements (REE), and Cu) was determined in 52 melt and 8 fluid inclusions that are representative of distinct stages from 720°C down to 380°C. Homogenization temperatures and water contents of both melt and fluid inclusions are used to estimate trapping temperatures, thus revealing the evolutionary stage during the process. Trace elements are partitioned in different proportions between the two pegmatite melts, high-salinity brines and exsolving vapors. Concentrations are strongly shifted by co ncomitant crystallization and precipitation of ore-forming minerals. For example, pegmatite melts at the initial stage (700°C) have about 1,600 ppm of Sn. Concentrations in both melts decrease towards lower temperatures due to the crystallization of cassiterite between 650 and 550°C. Tin is preferentially fractionated into the peralkaline melt by a factor of 2–3. While the last pegmatite melts are low in Sn (64 ppm at 500°C), early hydrothermal fluids become again enriched with about 800 ppm of Sn at the boiling stage. A sudden drop in late hydrothermal fluids (23 ppm of Sn at 370°C) results from precipitation of another cassiterite generation between 400 and 370°C. Zinc concentrations in peraluminous melts are low (some tens of parts per million) and are not correlated with temperature. In coexisting peralkaline melts and high-T brines, they are higher by a factor of 2–3. Zinc continuously increases in hydrothermal fluids (3,000 ppm at 400°C), where the precipitation of sphalerite starts. The main removal of Zn from the fluid system occurs at lower temperatures. Similarly, melt and fluid inclusion concentrations of many other trace elements directly reflect the crystallization and precipitation history of minerals at distinctive temperatures or temperature windows. 相似文献
17.
Rare-earth elements as source indicators of Pan-African granites from Obudu Plateau,Southeastern Nigeria 总被引:1,自引:1,他引:0
The rare-earth element (REE) concentrations of representative granite samples from the southeast of the Obudu Plateau, Nigeria, were analyzed with an attempt to determine the signatures of their source, evolutionary history and tectonic setting. Results indicated that the granites have high absolute REE concentrations (190×10^-6-1191×10^-6; av.=549×10^-6) with the chondrite-normalized REE patterns characterized by steep negative slopes and prominent to slight or no negative Eu anomalies. All the samples are also characterized by high and variable concentrations of the LREE (151×10^-6-1169×10^-6; av.= 466×10^-6), while the HREE show low abundance (4×10^-6-107×10^-6; av.=28×10^-6). These are consistent with the variable levels of REE fractionation, and differentiation of the granites. This is further supported by the range of REE contents, the chondrite-normalized patterns and the ratios of LaN/YbN (2.30-343.37), CeN/YbN (5.94-716.87), LaN/SmN (3.14-11.68) and TbN/YbN (0.58-1.65). The general parallelism of the REE patterns, suggest that all the granites were comagmatic in origin, while the high Eu/Eu* ratios (0.085-2.807; av.=0.9398) indicate high fo2 at the source. Similarly, irregular variations in LaN/YbN, CeN/YbN and Eu/Eu* ratios and REE abundances among the samples suggest behaviors that are related to mantle and crustal sources. 相似文献
18.
High PT experiments were performed in the range 2.5–19 GPa and 800–1,500°C using a synthetic peridotite doped with trace elements
and OH-apatite or with Cl-apatite + phlogopite. The aim of the study was (1) to investigate the stability and phase relations
of apatite and its high PT breakdown products, (2) to study the compositional evolution with P and T of phosphate and coexisting
silicate phases and (3) to measure the Cl-OH partitioning between apatite and coexisting calcic amphibole, phlogopite and
K-richterite. Apatite is stable in a garnet-lherzolite assemblage in the range 2.5–8.7 GPa and 800–1,100°C. The high-P breakdown
product of apatite is tuite γ-Ca3 (PO4)2, which is stable in the range 8–15 GPa and 1,100–1,300°C. Coexisting apatite and tuite were observed at 8 GPa/1,050°C and
8.7 GPa/1,000°C. MgO in apatite increases with P from 0.8 wt% at 2.5 GPa to 3.2 wt% at 8.7 GPa. Both apatite and tuite may
contain significant Na, Sr and REE with a correlation indicating 2 Ca2+=Na+ + REE3+. Tuite has always higher Sr and REE and lower Fe and Mg than apatite. Phosphorus in the peridotite phases decreases in the
order Pmelt ≫ Pgrt ≫ PMg2SiO4 > Pcpx > Popx. The phosphate-saturated P2O5 content of garnet increases from 0.07 wt% at 2.5 GPa to 1.5 wt% at 12.8 GPa. Due to the low bulk Na content of the peridotite,
[8]Na[4]P[8]M2+
−1
[4]Si−1 only plays a minor role in controlling the phosphorus content of garnet. Instead, element correlations indicate a major contribution
of [6]M2+[4]P[6]M3+
−1
[4]Si−1. Pyroxenes contain ~200–500 ppm P and olivine has 0.14–0.23 wt% P2O5 in the P range 4–8.7 GPa without correlation with P, T or XMg. At ≥12.7 GPa, all Mg2SiO4 polymorphs have <200 ppm P. Coexisting olivine and wadsleyite show an equal preference for phosphorus. In case of coexisting
wadsleyite and ringwoodite, the latter fractionates phosphorus. Although garnet shows by far the highest phosphorus concentrations
of any peridotite silicate phase, olivine is no less important as phosphorus carrier and could store the entire bulk phosphorus
budget of primitive mantle. In the Cl-apatite + phlogopite-doped peridotite, apatite contains 0.65–1.35 wt% Cl in the PT range
2.5–8.7 GPa/800–1,000°C. Apatite coexists with calcic amphibole at 2.5 GPa, phlogopite at 2.5–5 GPa and K-richterite at 7 GPa,
and all silicates contain between 0.2 and 0.6 wt% Cl. No solid potassic phase is stable between 5 and 8.7 GPa. Cl strongly
increases the solubility of K in hydrous fluids. This may lead to the breakdown of phlogopite and give rise to the local presence
in the mantle of fluids strongly enriched in K, Cl, P and incompatible trace elements. Such fluids may get trapped as micro-inclusions
in diamonds and provide bulk compositions suitable for the formation of unusual phases such as KCl or hypersilicic Cl-rich
mica. 相似文献
19.
A. N. Zaitsev J. Keller J. Spratt T. E. Jeffries V. V. Sharygin 《Geology of Ore Deposits》2009,51(7):608-616
Alkali carbonates nyerereite, ideally Na2Ca(CO3)2 and gregoryite, ideally Na2CO3, are the major minerals in natrocarbonatite lavas from Oldoinyo Lengai volcano, northern Tanzania. They occur as pheno- and
microphenocrysts in groundmass consisting of fluorite and sylvite; nyerereite typically forms prismatic crystals and gregoryite
occurs as round, oval crystals. Both minerals are characterized by relatively high contents of various minor elements. Raman
spectroscopy data indicate the presence of sulfur and phosphorous as (SO4)2− and (PO4)3− groups. Microprobe analyses show variable composition of both nyerereite and gregoryite. Nyerereite contains 6.1–8.7 wt %
K2O, with subordinate amounts of SrO (1.7–3.3 wt %), BaO (0.3–1.6 wt %), SO3 (0.8–1.5 wt %), P2O5 (0.2–0.8 wt %) and Cl (0.1–0.35 wt %). Gregoryite contains 5.0–11.9 wt % CaO, 3.4–5.8 wt % SO3, 1.3–4.6 wt % P2O5, 0.6–1.0 wt % SrO, 0.1–0.6 wt % BaO and 0.3–0.7 wt % Cl. The content of F is below detection limits in nyerereite and gregoryite.
Laser ablation ICP-MS analyses show that REE, Mn, Mg, Rb and Li are typical trace elements in these minerals. Nyerereite is
enriched in REE (up to 1080 ppm) and Rb (up to 140 ppm), while gregoryite contains more Mg (up to 367 ppm) and Li (up to 241
ppm) as compared with nyerereite. 相似文献
20.
The data obtained on melt and fluid inclusions in minerals of granites, metasomatic rocks, and veins with tin ore mineralization
at the Industrial’noe deposit in the southern part of the Omsukchan trough, northeastern Russia, indicate that the melt from
which the quartz of the granites crystallized contained globules of salt melts. Silicate melt inclusions were used to determine
the principal parameters of the magmatic melts that formed the granites, which had temperatures at 760–1020°C, were under
pressures of 0.3–3.6 kbar, and had densities of 2.11–2.60 g/cm3 and water concentrations of 1.7–7.0 wt %. The results obtained on the fluid inclusions testify that the parameters of the
mineral-forming fluids broadly varied and corresponded to temperatures at 920–275°C, pressures 0.1–3.1 kbar, densities of
0.70–1.90 g/cm3, and salinities of 4.0–75.0 wt % equiv. NaCl. Electron microprobe analyses of the glasses of twelve homogenized inclusions
show concentrations of major components typical of an acid magmatic melt (wt %, average): 73.2% SiO2, 15.3% Al2O3, 1.3% FeO, 0.6% CaO, 3.1% Na2O, and 4.5% K2O at elevated concentrations of Cl (up to 0.51 wt %, average 0.31 wt %). The concentrations and distribution of some elements
(Cl, K, Ca, Mn, Fe, Cu, Zn, Pb, As, Br, Rb, Sr, and Sn) in polyphase salt globules in quartz from both the granites and a
mineralized miarolitic cavity in granite were assayed by micro-PIXE (proton-induced X-ray emission). Analyses of eight salt
globules in quartz from the granites point to high concentrations (average, wt %) of Cl (27.5), Fe (9.7), Cu (7.2), Mn (1.1),
Zn (0.66), Pb (0.37) and (average, ppm) As (2020), Rb (1850), Sr (1090), and Br (990). The salt globules in the miarolitic
quartz are rich in (average of 29 globules, wt %) Cl (25.0), Fe (5.4), Mn (1.0), Zn (0.50), Pb (0.24) and (ppm) Rb (810),
Sn (540), and Br (470). The synthesis of all data obtained on melt and fluid inclusions in minerals from the Industrial’noe
deposit suggest that the genesis of the tin ore mineralization was related to the crystallization of acid magmatic melts.
Original Russian Text@ V.B. Naumov, V.S. Kamenetsky, 2006, published in Geokhimiya, 2006, No. 12, pp. 1279–1289. 相似文献