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1.
The upper greenschist - lower amphibolite facies, argillaceous to chemical-exhalative metasedimentary sequence of the Mesoarchaean Ghattihosahalli Schist Belt (GHSB), southern India, has been examined with a special focus on the paragenesis and solid solution characteristics of barian feldspars and associated dioctahedral Ba-Cr-bearing micas. Barian feldspars occur as untwinned porphyroblasts in a recrystallized finely banded matrix of barite, quartz and minor white mica. Idioblastic celsian (Cls 98-76Or 2-20Ab 1-8) and hyalophane (Cls 55-39Or 35-51Ab 10) predate the greenschist-facies foliation, whereas xenoblastic hyalophane (Cls 44-35Or 45-59Ab 8–17) and mantles on celsian (Cls 45-35Or 42-60Ab 13-5) as well as xenoblastic barian K-feldspar (Cls 6Or 90Ab 2) postdate the last fabric-defining event. The preservation of extremely complex zoning patterns down to the micron-scale shows that diffusional homogenization did not operate at fluid-present low to medium-grade conditions (350–550 °C, 3–5 kb). Microstructures indicate that at these conditions barian feldspars deform exclusively by brittle fracturing and do not undergo recrystallization. Barian feldspar compositions confirm the positive correlation of Na-content with temperature and the existence of a narrow asymmetric compositional gap (Cls 90-85?Cls 55, ~350 °C) which probably closes at lower amphibolite facies conditions (X c ~Cls 75; T c ~550 °C). White micas are solid solutions of the end-members muscovite, ganterite (Ba 0.5?K 0.5)Al 2(Al 1.5Si 2.5)O 10(OH) 2, paragonite, celadonite with a significant substitution of [VI]Al by Cr. Zoning is a common feature with cores being enriched in Ba. The data document extensive Ba substitution for K from muscovite to ganterite, exclusively controlled by the coupled substitution [XII]K + [IV]Si ? [XII]Ba + [IV]Al and strongly dependent on bulk composition. The extent of solid solution from (Ms+Gnt) towards paragonite and celadonite end-members is controlled by the miscibility gap in the (Ms+Gnt)–Pg–Cel pseudoternary, with the Pg-substitution depending on temperature and the Cel-substitution on pressure. [IV]Si values between 3.1 and 3.3 in Ba-poor micas indicate minimum pressures of chemical equilibration in the order of 3–5 kbar, while the most sodian compositions of low-celadonite micas provide an upper temperature estimate of ~550 °C, consistent with P-T estimates for assemblages of metapelites (500–550 °C, 4–5 kb). 相似文献
2.
Phase relations for the bulk compositions of the celadonites between the MgAl, MgFe 3+ and Fe 2+Fe 3+ types (celadonite = KR 2+R 3+ Si 4O 10 (OH) 2) under magnetite-iron and nickel-nickel oxide solid-fluid buffers indicate the extent of solid solution possible in this potassic mica series at temperatures between 300° and 430° C at 2 Kb total pressure. Other possible combinations of Mg, Al, Fe ions in octahedrally coordinated sites did not produce single-phase mica products. The ferrous celadonite micas are stable only under oxygen fugacities where magnetite is the stable oxide—where both Fe 2+ and Fe 3+ can coexist. However the celadonite with the highest thermal stability at 2 Kb total pressure, nickel-nickel oxide buffer conditions is the KMgFe 3+Si 4O 10(OH) 2 phase which is stable up to 420°C, well into low grade metamorphic conditions. It is thus apparent that the presence of celadonite or glauconite mica will not be indicative of changing diagenetic conditions. 相似文献
3.
Subsolidus phase relations for a K-doped lherzolite are investigated in the model system K 2O–Na 2O–CaO–FeO–MgO–Al 2O 3–SiO 2–H 2O at 1.5–6.0 GPa and 680–1,000°C. Phlogopite is ubiquitous and coexists with Ca-amphibole up to 3.2 GPa and 900°C. High-pressure phlogopites show a peculiar mineral chemistry dependent on pressure: e.g., at 5.5 GPa and 680°C, excess of Si (up to 3.4 apfu) coupled with deficiency in Al (as low as 0.58 apfu) and K + Na (as low as 0.97 apfu), suggest a significant amount of a talc/10 Å phase component ([ v] XIISi 1K ?1Al ?1 IV , where [ v] XII is interlayer vacancy). Mixed layering or solid solution relations between high-pressure phlogopites and the 10 Å phase, Mg 3Si 4O 10(OH) 2 nH 2O, are envisaged. Phlogopite modal abundance, derived by weighted least squares, is maximum at high-pressure and relative low-temperature conditions and therefore along the slab–mantle interface (10.3 ± 0.7 wt.%, at 4.8 GPa, 680°C). In phlogopite-bearing systems, Ca-amphibole breaks down between 2.5 and 3.0 GPa, and 1,000°C, through the water conservative reaction 5(pa + 0.2 KNa ?1) + 17en + 15phl = (10di + 4jd) + 5py + 12fo + 20(phl + 0.2 talc), governed by bulk composition and pressure-dependent variations of K/OH in K-bearing phases and as a result, it does not necessarily imply a release of fluid. 相似文献
4.
Ultrahigh‐pressure (UHP) rocks from the Western Gneiss Region (WGR) of Norway record subduction of Baltican continental crust during the Silurian to Devonian Scandian continental collision. Here, we report a new coesite locality from the island of Harøya in the Nordøyane UHP domain, the most northerly yet documented in the WGR, and reconstruct the P–T history of the host eclogite. The coesite–eclogite lies within migmatitic orthogneiss, interpreted as Baltica basement, that underwent multiple stages of deformation and partial melting during exhumation. Two stages of metamorphism have been deduced from petrography and mineral chemistry. The early (M1) assemblage comprises garnet (Pyr 38–41Alm 35–37Grs 23–26Spss 1) and omphacite (Na 0.35–0.40Ca 0.57–0.60Fe 2+0.08–0.10Mg 0.53Fe 3+0.01Al VI0.40–0.42) 2(Al IV0.03–0.06Si 1.94–1.97) 2O 6, with subordinate phengite, kyanite, rutile, coesite and apatite, all present as inclusions in garnet. The later (M2) assemblage comprises retrograde rims on garnet (Pyr 38–40Alm 40–44Grs 16–21Spss 1), diopside rims on omphacite (Na 0.04–0.06Ca 0.88–0.91Fe 2+0.09–0.13Mg 0.81–83Fe 3+0.08Al VI0.03) 2(Al IV0.07–0.08Si 1.92–1.93) 2O 6, plagioclase, biotite, pargasite, orthopyroxene and ilmenite. Metamorphic P–T conditions estimated using thermocalc are ~3 GPa and 760 °C for M1, consistent with the presence of coesite, and ~1 GPa and 813 °C for M2, consistent with possible phengite dehydration melting during decompression. Comparison with other WGR eclogites containing the same assemblage shows a broad similarity in peak (M1) P–T conditions, confirming suggestions that large portions of the WGR were buried to depths of ~100 km during Scandian subduction. Field relations suggest that exhumation, accompanied by widespread partial melting, involved an early phase of top‐northwest shearing, followed by subhorizontal sinistral shearing along northwest‐dipping foliations, related to regional transtension. The present results add to the growing body of data on the distribution, maximum P–T conditions, and exhumation paths of WGR coesite–eclogites and their host rocks that is required to constrain quantitative models for the formation and exhumation of UHP metamorphic rocks during the Scandian collision. 相似文献
5.
The solubility of the albite-paragonite-quartz mineral assemblage was measured as a function of NaCl and fluorine concentration at 400°C, 500 bars and at 450°C, 500 and 1000 bars. Decreasing Al concentrations with increasing NaCl molality in F-free fluids of low salinity (mNaCl < 0.01) demonstrates that Al(OH) 4− dominates Al speciation and is formed according to the reaction 0.5 NaAl 3Si 3O 12H 2(cr)+2 H 2O = 0.5 NaAlSi 3O 8(cr)+Al(OH) 4−+H +. Log K results for this reaction are −11.28 ± 0.10 and −10.59 ± 0.10 at 400°C, 500 bars and 450°C, 1000 bars, respectively. Upon further salinity increase, Al concentration becomes constant (at 400°C, 500 bars) or even rises (at 450°C, 1000 bars). The observed Al behavior can be explained by the formation of NaAl(OH) 40(aq) or NaAl(OH) 3Cl (aq)0. The calculated constant for the reaction Al(OH) 4−+Na +=NaAl(OH) 40(aq) expressed in log units is equal to 2.46 and 2.04 at 400°C, 500 bars and 450°C, 1000 bars, respectively. These values are in good agreement with the predictions given in Diakonov et al. (1996). Addition of fluoride at m(NaCl) = const = 0.5 caused a sharp increase in Al concentration in equilibrium with the albite-paragonite-quartz mineral assemblage. As fluid pH was also constant, this solubility increase indicates strong aluminum-fluoride complexation with the formation of NaAl(OH) 3F (aq)0 and NaAl(OH) 2F 20(aq), according to 0.5 NaAl 3Si 3O 12H 2(cr)+Na ++HF (aq)0+H 2O = 0.5 NaAlSi 3O 8(cr)+ NaAl(OH) 3F (aq)0+H +, log K = −5.17 and −5.23 at 400°C and 450°C, 500 bars, respectively, and 0.5 NaAl 3Si 3O 12H 2(cr)+Na ++2 HF (aq)0 = 0.5 NaAlSi 3O 8(cr)+NaAl(OH) 2F 20(aq)+H +, log K = −2.19 and −1.64 at the same P-T conditions. It was found that temperature increase and pressure decrease promote the formation of Na-Al-OH-F species. Stability of NaAl(OH) 2F 20(aq) in low-density fluids also increases relative to NaAl(OH) 3F (aq)0. These complexes, together with Al(OH) 2F (aq)0 and AlOHF 20(aq), whose stability constants were calculated from the corundum solubility measured by Soboleva and Zaraisky (1990) and Zaraisky (1994), are likely to dominate Al speciation in metamorphic fluids containing several ppm of fluorine. 相似文献
6.
In the oxidation zone of the Berezovskoe gold deposit in the middle Urals, Russia, minerals of the beudantite–segnitite series (idealized formulas PbFe 3 3+ AsO 4)(SO 4)(OH) 6 and PbFe 3 3+ AsO 4)(AsO 3OH)(OH) 6, respectively) form a multicomponent solid solution system with wide variations in the As, S, Fe, Cu, and Sb contents and less variable P, Cr, Zn, Pb, and contents K. The found minerals of this system correspond to series from beudantite with 1.25 S apfu to S-free segnitite, with segnitite lacking between 1.57 and 1.79 As apfu. Segnitite at the Berezovskoe deposit contains presumably pentavalent Sb (up to 15.2 wt % Sb 2O 5 = 0.76 Sb apfu, the highest Sb content in the alunite supergroup minerals), which replaces Fe 3+. The Sb content increases with increasing As/S value. On the contrary, beudantite is free of or very poor in Sb (0.00–0.03 Sb apfu). Many samples of segnitite are enriched in Cu (up to 8.2 wt% CuO = 0.83 Cu apfu, uncommonly high Cu content for this mineral) and/or in Zn (up to 2.0 wt% ZnO = 0.19 Zn apfu). Both Cu and Zn replace Fe. The generalized formula of a hypothetic end member of the segnitite series with 1 Sb apfu is Pb(Fe 3+ M 2+Sb 5+)(AsO 4) 2(OH) 6, where M = Cu, Zn, Fe 2+. The chemical evolution of beudantite–segnitite series minerals at the Berezovskoe deposit is characterized by an increase in the S/As value with a decrease in the Sb content from early to late generations. 相似文献
7.
Garnet peridotites occur in quartzofeldspathic gneisses in the Northern Qaidam Mountains, western China. They are rich in Mg and Cr, with mineral compositions similar to those in mantle peridotites found in other orogenic belts and as xenoliths in kimberlite. Garnet‐bearing lherzolites interlayered with dunite display oriented ilmenite and chromite lamellae in olivine and pyroxene lamellae in garnet that have been interpreted to indicate pressures in excess of 6 GPa. However, some garnet porphyroblasts include hornblende, chlorite and spinel + orthopyroxene symplectite after garnet; some clinopyroxene porphyroblasts include abundant actinolite/edenite, calcite and lizardite in the lherzolite; some olivine porphyroblasts (Fo 92) include an earlier generation Mg‐rich olivine (Fo 95–99), F‐rich clinohumite, pyroxene, chromite, anthophyllite/cummingtonite, Cl‐rich lizardite, carbonates and a new type of brittle mica, here termed ‘Ca‐phlogopite’, in the associated dunite. The pyrope content of garnet increases from core to rim, reaching the pyrope content (72 mol.%) of garnet typically found in the xenoliths in kimberlite. The simplest interpretation of these observations is that the rock association was formerly mantle peridotite emplaced into the oceanic crust that was subjected to serpentinization by seawater‐derived fluids near the sea floor. Dehydration during subduction to 3.0–3.5 GPa and 700 °C transformed these serpentinites into garnet lherzolite and dunite, depending on their Al and Ca contents. Pseudosection modelling using thermocalc shows that dehydration of the serpentinites is progressive, and involved three stages for Al‐rich and two stages for Al‐poor serpentinites, corresponding to the breakdown of the key hydrous minerals. Static burial and exhumation make olivine a pressure vessel for the pre‐subduction mineral inclusions during ultrahigh‐pressure (UHP) metamorphism. The time span of the UHP event is constrained by the clear interface between the two generations of olivine to be very short, implying rapid subduction and exhumation. 相似文献
8.
Amphiboles were synthesized from bulk compositions prepared along the join Ca 1.8Mg 5.2Si 8O 22(OH) 2–Ca 1.8Mg 3Ga 4Si 6O 22(OH) 2 hydrothermally at 750–850 °C and 1.0–1.8 GPa, and along the join Ca 2Mg 5Si 8O 22F 2–Ca 2Mg 3Ga 4Si 6O 22F 2, anhydrously at 1000 °C and 0.7 GPa to document how closely the tschermak-type substitution is obeyed in these analogues
of aluminous amphiboles. Electron-microprobe analyses and Rietveld X-ray diffraction structure refinements were performed
to determine cation site occupancies. The extent of Ga substitution was found to be limited in both joins, but with the fluorine
series having about twice the Ga content (0.6 atoms per formula unit, apfu) of the hydroxyl-series amphiboles (0.3 apfu).
The tschermak-type substitution was followed very closely in the hydroxyl series with essentially equal partitioning of Ga
between tetrahedral and octahedral sites. The fluorine-series amphiboles deviated significantly from the tschermak-type substitution
and, instead, appeared to follow a substitution that is close to a Ca-pargasite substitution of the type: [6]Ga 3++2 [4]Ga 3++1/2 [A] Ca 2+ = [6]Mg 2++2 [4]Si 4++1/2 [A]□. Infrared spectroscopy revealed an inverse correlation between the intensity of the OH-stretching bands and the Ga content
for the hydroxyl- and fluorine-series amphiboles. The direct correlation between the Ga and F content and inverse relationship between the Ga and
OH content may be a general phenomenon present in other minerals and suggests, for example, that high F contents in titanite
are controlled by the Al content of the host rock and that there may be similar direct Al–F correlations in tschermakitic
amphiboles. Evidence for the possibility that Al (Ga) might substitute onto only a subset of the tetrahedral sites in tschermakitic
amphiboles was sought but not observed in this study.
Received: 5 March 2001 / Accepted: 31 July 2001 相似文献
9.
The solubility of Tio 2 in phlogopites has been experimentally determined in the system K 2Mg 6Al 2Si 6O 20(OH) 4-K 2Mg 4TiAl 2Si 6O 20(OH) 4-K 2Mg 5TiAl 4Si 4O 20(OH) 4 between 825–1300°C and 10–30 kbar under vapour absent conditions. Starting compositions lie along the join K 2Mg 6Al 2Si 6O 20(OH) 4-K 2Mg 4.5TiAl 3Si 5O 20(OH) 4 which represents a combination of the Mg[VI]2 Si[IV] = Ti[VI]2 Al[VI] and substitution mechanisms for Ti in phlogopites. The results of the experiments indicate a systematic increase in solubility of Ti with increasing temperature and decreasing pressure for given bulk Tio 2 content. Under isobaric conditions high temperature Ti-saturated phlogopite breaks down to Ti-deficient phlogopite + rutile + vapour. Mass balance calculations suggest that the vapour phase may contain K 2O dissolved in H 2O and that the reaction is controlled by the vapour phase. Analyses of phlogopites coexisting with rutile and vapour can be represented in terms of the end-member components phlogopite [K 2Mg 6Al 2Si 6O 20(OH) 4], eastonite [K 2Mg 5Al 4Si 5O 20(OH) 4], an octahedral site deficient Ti-phlogopite (Ti-OSD) of composition K 2(Mg 4Ti□)Al 2Si 6)O 20(OH) 4, and Ti-eastonite [K 2Mg 5TiAl 4Si 4O 20(OH) 4]. With decreasing amounts of Ti in these phlogopites there is a decrease in the Ti-eastonite component and increase in the eastonite component.The general equation for the breakdown of Ti-phlogopite solid solution to Ti-free phlogopite + rutile + vapour is: 14 Ti-eastonite + 7 Ti- OSD ? 16 eastonite + 3 phlogopite + 21 rutile + 4 H2O + 2 K2O. Lack of knowledge of H 2O and K 2O activities in the vapour phase does not permit evaluation of thermodynamic constants for this reaction. The Ti solubility in phlogopites and hence its potential as a geothermobarometer under lower crustal to upper mantle conditions is likely controlled by common mantle minerals such as forsterite. 相似文献
10.
The chemical compositions of white micas separated from adjacent rocks of glaucophane and greenschist facies are compared with respect to their Si +4 content. The micas are predominantly phengitic, i.e. between muscovite, K[Al 2Si 3AlO 10(OH) 2] and celadonite, K[(R +2R +3)Si 4O 10(OH) 2] in composition. Constancy of Si content in micas coming from rocks of different bulk chemical composition but closely similar physical conditions indicates that the silica content of a potassic dioctahedral mica can be used to indicate the pressure and temperature conditions of its formation. This conclusion is in part based upon previous experimental data obtained for synthetic phengites. 相似文献
11.
Germanate garnets are often used as isostructural analogues of silicate garnets to provide insight into the crystal chemistry and symmetry of the less accessible natural garnet solid solutions. We synthesised two series of germanate garnets at 3 GPa along the join VIIICa 3VI(CaGe) IVGe 3O 12– VIIICa 3VIFe 2IVGe 3O 12 at 900 °C and 1,100 °C. Samples with compositions close to the CaGeO 3 end-member consist of tetragonal garnet with a small amount of triclinic CaGe 2O 5. Samples with nominal compositions between XFe=0.4 and 1.0 consist of a mixture of tetragonal and cubic garnets; whereas, single-phase cubic garnets were obtained for compositions with XFe>1.2 ( XFe gives the iron content expressed in atoms per formula unit, and varies between 0 and 2 along the join). Run products which were primarily single-phase garnet were investigated using Mössbauer spectroscopy. Spectra from samples synthesised at 1,100°C consist of one well-resolved doublet that can be assigned to Fe 3+ in the octahedral site of the garnet structure. A second doublet, present primarily in samples synthesised at 900°C, can be assigned to Fe 2+ at the octahedral sites of the garnet structure. The relative abundance of Fe 2+ decreases with increasing iron content. Transmission electron microscopy analyses confirm this tendency and show that the garnets are essentially defect-free. The unit-cell parameters of tetragonal VIIICa 3VI(CaGe) IVGe 3O 3 garnet decrease with increasing synthesis temperature, and the deviation from cubic symmetry becomes smaller. Cubic garnets show a linear decrease of unit-cell parameter with increasing iron content. The results are discussed in the context of iron incorporation into VIIIMg 3VI(MgSi) IVSi 3O 3 majorite. 相似文献
12.
Garnets in UHP eclogites from Bixiling in Dabieshan were investigated by Fourier transform infrared spectroscopy (FTIR). The results indicate that all garnets contain structural water that occurs as hydroxyl (OH −) and non-structural molecular water (H 2O) possibly in the form of sub-microscopic fluid inclusions. The structural hydroxyl contents range from 92 to 1735 ppm (H 2O wt.) and most are between 200 and 1000 ppm. Therefore, garnet in eclogite can recycle surface water into the mantle. Various water contents were observed among different samples of the same outcrop (∼150 m) and in different domains of the same sample (∼1 cm). This variability in structural H 2O contents suggests that the mobility of fluids during UHP metamorphism was very limited, and that both subduction and exhumation processes of UHP rocks occurred in a short time interval. 相似文献
13.
Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated
in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diffusion
halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr 2O 3, and 20 to 220 ppm H 2O. Thermobarometric calculations show that pyrope crystals with different Cr contents formed at different depths ranging from
50 km (where T ≈ 600 °C and P = 15 kbar) to 95 km (where T ≈ 800 °C and P = 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered
crichtonite series minerals (AM 21O 38, where A = Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi 2O 6), and a new oxide mineral, carmichaelite (MO 2−x(OH) x, where M = Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb 2O 5), Cr (up to ∼6 wt% Cr 2O 3), and OH (up to ∼0.9 wt% H 2O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and
blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole,
phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in
composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin,
such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength
elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and
exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau,
contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in
metasomatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc basalts,
providing a possible mechanism for HFSE depletion in these rocks.
Received: 20 December 1997 / Accepted: 15 October 1998 相似文献
14.
Summary The phase relations of K-richterite, KNaCaMg 5Si 8O 22(OH) 2, and phlogopite, K 3Mg 6 Al 2Si 6O 20(OH) 2, have been investigated at pressures of 5–15 GPa and temperatures of 1000–1500 °C. K-richterite is stable to about 1450 °C
at 9–10 GPa, where the dp/dT-slope of the decomposition curve changes from positive to negative. At 1000 °C the alkali-rich,
low-Al amphibole is stable to more than 14 GPa. Phlogopite has a more limited stability range with a maximum thermal stability
limit of 1350 °C at 4–5 GPa and a pressure stability limit of 9–10 GPa at 1000 °C. The high-pressure decomposition reactions
for both of the phases produce relatively small amounts of highly alkaline water-dominated fluids, in combination with mineral
assemblages that are relatively close to the decomposing hydrous phase in bulk composition. In contrast, the incongruent melting
of K-richterite and phlogopite in the 1–3 GPa range involves a larger proportion of hydrous silicate melts.
The K-richterite breakdown produces high-Ca pyroxene and orthoenstatite or clinoenstatite at all pressures above 4 GPa. At
higher pressures additional phases are: wadeite-structured K 2Si VISi IV
3O 9 at 10 GPa and 1500 °C, wadeite-structured K 2Si VISi IV
3O 9 and phase X at 15 GPa and 1500 °C, and stishovite at 15 GPa and 1100 °C. The solid breakdown phases of phlogopite are dominated
by pyrope and forsterite. At 9–10 GPa and 1100–1400 °C phase X is an additional phase, partly accompanied by clinoenstatite
close to the decomposition curve. Phase X has variable composition. In the KCMSH-system (K 2CaMg 5Si 8O 22(OH) 2) investigated by Inoue et al. (1998) and in the KMASH-system investigated in this report the compositions are approximately K 4Mg 8Si 8O 25(OH) 2 and K 3.7Mg 7.4Al 0.6Si 8.0O 25(OH) 2, respectively.
Observations from natural compositions and from the phlogopite-diopside system indicate that phlogopite-clinopyroxene assemblages
are stable along common geothermal gradients (including subduction zones) to 8–9 GPa and are replaced by K-richterite at higher
pressures. The stability relations of the pure end member phases of K-richterite and phlogopite are consistent with these
observations, suggesting that K-richterite may be stable into the mantle transition zone, at least along colder slab geotherms.
The breakdown of moderate proportions of K-richterite in peridotite in the upper part of the transition zone may be accompanied
by the formation of the potassic and hydrous phase X. Additional hydrogen released by this breakdown may dissolve in wadsleyite.
Therefore, very small amounts of hydrous fluids may be released during such a decomposition.
Received April 10, 2000; revised version accepted November 6, 2000 相似文献
15.
Some of the most vanadium-rich silicate minerals known are present in green mica schist from the Hemlo gold deposit, Ontario, Canada. Vanadium-rich silicate minerals include green mica (up to 17.6 wt. % V 2O 3), phlogopite (10.1 wt. % V 2O 3), pumpellyite (25.7 wt. % V 2O 3), garnet (18.5 wt. % V 2O 3), epidote-group minerals (9.1 wt. % V 2O 3), antimonian vesuvianite (4.3 wt. % V 2O 3), and titanite (18.5 wt. % V 2O 5). In addition, minor amounts of V (<2 wt. % V 2O 3) are present in tourmaline, chlorite, talc and tremolite in other lithologies of the Hemlo deposit. The principal substitution that incorporates V into most of these silicate minerals is Al 3+=V 3+ in octahedral positions. Vanadium is incorporated into phlogopite mainly by the two substitutions: 3Mg 2+ =2V 3++ and VIMg 2++ IVSi 4+= VIV 3+ + IVAl 3+, and all of the three substitutions Ti 4++O 2- =V 3++(OH,F) -, Ti 4+=V 4+, and 5Ti 4+=4V 5+ + may have operated in titanite.Vanadium-enriched green mica schist from the Hemlo gold deposit is characterized by uniform Ti/Zr ratios, systematically low Ti, Ni, Co and Sc abundances, and low levels of incompatible elements Th, U, Hf and Zr and is distinct in these respects from its Cr-enriched counterpart. These geochemical features, along with textural evidence (relict quartz and oligoclase phenocrysts), indicate that the V-enriched green mica schist from Hemlo was most likely derived mainly from quartz-oligoclase porphyry. However, its anomalously high V and Cr contents were probably introduced metasomatically from local maficultramafic sources and were fixed in green mica and oxides during the waning of a second regional metamorphism. Vanadium was further remobilized, and its concentration probably enhanced, during the late hydrothermal alteration, which resulted in the formation of the characteristic V-rich calc-silicate minerals. 相似文献
16.
A geochemical study of pegmatitic micas from Minas Gerais State in Brazil was performed with an electron microprobe, in order to examine the variations of Rb, K, Al and F contents. It is observed a linear decreasing relationship between the [Rb/K] ratio of the micas and their contents in Al VI. The interpretation is based on the hypothesis that the partition coefficient CRb/Kmica/fl between fluid and mineral does not vary significantly as a function of temperature and pressure in the narrow conditions of crystallization of pegmatites. It is suggested that the relation: CRb/Kmica/fl=0.55×(5?[Al VI]) is of crystallochemical order. Micas with low contents in Al take higher contents in Rb because the potassic sites where Rb enters are larger. This relation gives another way of calculating the values [Rb/K] of the fluids knowing Rb, K, Al and Si in the micas. This crystallochemistry also allows us to foresee a direct correlation between Rb and F in the pegmatitic micas. To cite this article: J. Quéméneur et al., C. R. Geoscience 336 (2004). 相似文献
17.
Sodic amphiboles in high pressure and ultra-high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na 2O–MgO–Al 2O 3–SiO 2–H 2O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experiments at 20–30?kbar and 700–800?°C to investigate the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane=2jadeite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeite–talc?±?quartz does not fix its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nyböite (Nyb), Al–Na-cummingtonite (Al–Na-Cm) and sodium anthophyllite (Na-Anth) are also buffered via the model reactions: 3cummingtonite?+?4quartz?+?4H 2O=7talc, nyböite?+?3quartz=3jadeite?+?talc, 3Al–Na-cummingtonite + 11quartz + 2H 2O=6jadeite + 5talc, and 3 sodium anthophyllite?+?13quartz?+?4H 2O=3 jadeite + 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of Al IV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: [Na AAl IV]<=>[□ ASi] (edenite) toward nyböite, and [Na (M4)Al VI]<=>[Mg (M4)Mg VI] toward cummingtonite. The extent of nyböite and cummingtonite substitution increases with temperature and decreases with pressure in the experiments. Similar compositional variations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications. 相似文献
18.
We report the first finding of diamond and moissanite in metasedimentary crustal rocks of Pohorje Mountains (Slovenia) in the Austroalpine ultrahigh‐pressure (UHP) metamorphic terrane of the Eastern Alps. Microscopic observations and Raman spectroscopy show that diamond occurs in situ as inclusions in garnet, being heterogeneously distributed. Under the optical microscope, diamond‐bearing inclusions are of cuboidal to rounded shape and of pinkish, yellow to brownish colour. The Raman spectra of the investigated diamond show a sharp, first order peak of sp 3‐bonded carbon, in most cases centred between 1332 and 1330 cm ?1, with a full width at half maximum between 3 and 5 cm ?1. Several spectra show Raman bands typical for disordered graphitic (sp 2‐bonded) carbon. Detailed observations show that diamond occurs either as a monomineralic, single‐crystal inclusion or it is associated with SiC (moissanite), CO 2 and CH 4 in polyphase inclusions . This rare record of diamond occurring with moissanite as fluid‐inclusion daughter minerals implies the crystallization of diamond and moissanite from a supercritical fluid at reducing conditions. Thermodynamic modelling suggests that diamond‐bearing gneisses attained P–T conditions of ≥3.5 GPa and 800–850 °C, similar to eclogites and garnet peridotites. We argue that diamond formed when carbonaceous sediment underwent UHP metamorphism at mantle depth exceeding 100 km during continental subduction in the Late Cretaceous ( c. 95–92 Ma). The finding of diamond confirms UHP metamorphism in the Pohorje Mountains, the most deeply subducted part of Austroalpine units. 相似文献
19.
Several types of multiphase solid (MS) inclusions are identified in garnet from ultrahigh‐pressure (UHP) eclogite in the Dabie orogen. The mineralogy of MS inclusions ranges from pure K‐feldspar to pure quartz, with predominance of intermediate types consisting of K‐feldspar + quartz ± silicate (plagioclase or epidote) ± barite. The typical MS inclusions are usually surrounded with radial cracks in the host garnet, similar to where garnet contains relict coesite. Barite aggregates display significant heterogeneity in major element composition, with total contents of only 57–73% and highly variable SiO 2 contents of 0.32–25.85% that are positively correlated with BaO and SO 3 contents. The occurrence of MS inclusions provides petrographic evidence for partial melting in the UHP metamorphic rock. The occurrence of barite aggregates with variably high SiO 2 contents suggests the coexistence of aqueous fluid with hydrous melt under HP eclogite facies conditions. Thus, local dehydration melting is inferred to take place inside the UHP metamorphic slice during continental collision. This is ascribed to phengite breakdown during ‘hot’ exhumation of the deeply subducted continental crust. As a consequence, the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction‐zone metamorphism. 相似文献
20.
Minor granulites (believed to be pre-Triassic), surrounded by abundant amphibolite-facies orthogneiss, occur in the same region as the well-documented Triassic high- and ultrahigh-pressure (HP and UHP) eclogites in the Dabie–Sulu terranes, eastern China. Moreover, some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation. Granulitized HP eclogites/garnet clinopyroxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775–805 °C and ≥15 kbar, followed by granulite- to amphibolite-facies overprint of ca. 750–800 °C and 6–11 kbar. The presence of (Na, Ca, Ba, Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions. Metamorphic mineral assemblages and P– T estimates for felsic granulite at Huangtuling and mafic granulite at Huilanshan indicate peak conditions of 850 °C and 12 kbar for the granulite-facies metamorphism and 700 °C and 6 kbar for amphibolite-facies retrograde metamorphism. Cordierite–orthopyroxene and ferropargasite–plagioclase coronas and symplectites around garnet record a strong, rapid decompression, possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites. Carbonic fluid (CO2-rich) inclusions are predominant in both HP granulites and granulitized HP/UHP eclogites/garnet clinopyroxenites. They have low densities, having been reset during decompression. Minor amounts of CH4 and/or N2 as well as carbonate are present. In the granulitized HP/UHP eclogites/garnet clinopyroxenites, early fluids are high-salinity brines with minor N2, whereas low-salinity fluids formed during retrogression. Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz matrices (UHP eclogite). For HP granulites, a limited number of primary CO2 and mixed H2O–CO2(liquid) inclusions have also been observed in undeformed quartz inclusions within garnet, orthopyroxene, and plagioclase which contain abundant, low-density CO2±carbonate inclusions. It is suggested that the primary fluid in the HP granulites was high-density CO2, mixed with a significant quantity of water. The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions (“giant myrmekites”) occurring at quartz–feldspar boundaries. Compared with the UHP eclogites in this region, the granulites were exhumed in the presence of massive, externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids, probably derived from the surrounding gneisses. 相似文献
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