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1.
俯冲带变质过程中的含碳流体   总被引：1，自引：1，他引：0  
刘景波 《岩石学报》2019,35(1):89-98
俯冲带含碳岩石通过俯冲过程的变质反应生成了含碳水流体、富硅酸盐的超临界流体和含碳熔体。不同类型流体的形成与岩石成分和岩石经历的温压条件相关。岩石中碳酸盐矿物脱碳反应的温压条件取决于岩石起初的流体成分:有水存在时,反应发生在低温条件下。在高压条件下,碳酸盐矿物在水或含盐水流体的溶解是生成含碳流体重要的机制,其导致的碳迁移作用可能超过脱碳变质反应的作用。高温条件下,含碳岩石的部分熔融可以生成含碳的熔体,这在热俯冲环境和俯冲带岩石底辟到上覆地幔的情况下是碳迁移重要载体。富硅酸盐的超临界流体可能是在第二临界端点上形成的超临界流体,目前在超高压岩石中观察到的非花岗质成分的多相固体包裹体被认为是这种流体结晶的产物,然而对其理解尚存在很多问题,需要进一步的实验研究。地表含碳岩石在俯冲带被带到深部,俯冲带地温特征的不同导致了不同类型含碳流体的形成,这些流体运移至上覆地幔引起岩石部分熔融产生含碳的岛弧岩浆,岩浆喷出到地表释放了其中的碳,这构成了俯冲带-岛弧系统的碳循环。  相似文献   

2.
新水井铀(钍)矿床位于甘肃省龙首山成矿带,是碱交代型铀矿床的典型代表,其矿体完全产于钠交代蚀变花岗岩中,成矿过程可划分为钠交代蚀变、铀钍矿化和成矿后3个主要阶段。文章对该矿床花岗岩原岩、蚀变岩及矿石开展了系统主微量元素分析,采用Grant等浓度线法探讨了钠交代蚀变和铀钍矿化阶段的元素迁移规律,结果表明:钠交代蚀变阶段为富含Na、Ca、过渡族元素(Sc、V、Cr、Co、Ni)、U、Th及CO2、H2O等挥发分的复杂流体,钠交代过程中原岩中的大离子亲石元素(Rb、Ba)和部分轻稀土元素(LREE)不同程度带出;而铀钍成矿阶段成矿流体则富集重稀土元素(HREE)、U、Th、PO43-等成分,CO2等挥发分大量逸出。结合前人研究,认为新水井矿床成矿流体可能来自地幔流体和大气降水热液的混合;等挥发分CO2的逸出是新水井矿床最重要的矿质沉淀机制,导致了铀钍矿物和磷酸盐矿物(磷灰石)的共沉淀,而磷灰石的沉淀又促进了以磷酸盐形式搬运的Th元素的沉淀。  相似文献   

3.
Migmatites from Cone Peak, California, USA and the Satnur-Sangam road, Southern Karnataka, India contain coarser grained orthopyroxene-bearing leucosomes with subordinate biotite in finer grained hornblende-biotite-pyroxene-bearing hosts. At both localities the leucosomes are enriched in quartz and feldspar and have a higher ratio of pyroxene to hornblende + biotite compared to the host rocks. Biotite grains in leucosomes along the Satnur-Sangam road are concentrated at the margins of orthopyroxene grains and have lower abundances of Ti, Fe, and Cl and a higher abundance of F than biotite grains from the host rock. Fluorapatite grains in all rocks from both localities contain monazite inclusions similar to those produced experimentally by metasomatically induced dissolution and reprecipitation. Some fluorapatite grains at both localities are partially rimmed by allanite. The only compositional differences found between fluorapatite grains in the leucosomes and host rocks were higher concentrations of Cl in grains in leucosomes from Cone Peak. The mineralogies of the rocks suggest that the leucosomes formed by dehydration melting reactions that consumed feldspar, quartz, hornblende, and biotite and produced orthopyroxene. Allanite rims at the margins of fluorapatite grains may have formed by the later retrogression of monazite rims formed by incongruent dissolution of fluorapatite in the melt. Biotite grains at the margins of orthopyroxene crystals in the leucosomes from the Satnur-Sangam road apparently formed by retrogression of orthopyroxene upon the solidification of the anatectic melt. A similar high-grade retrogression did not affect orthopyroxene crystals at Cone Peak, indicating that H2O was removed from the crystallizing leucosomes probably in a low H2O activity fluid. Compositional differences between the paleosome and neosomes at Cone Peak are best explained by metasomatic interaction with concentrated brines while elevated Cl concentrations in fluorapatites in the leucosome suggest interaction with a Cl-bearing fluid. Brines may have been responsible for an exchange of elements between the host rock along the Satnur-Sangam road and zones of melt generation now marked by leucosomes, but fluid flow appears to have been less vigorous than at Cone Peak.  相似文献   

4.
Studies of the mantle-derived iherzolites from Nushan show that in addition to CO2,there were present H2O and small amounts of CO, CH4, SO2,Cl and F in the initial mantlc fluids derived fron the asthenospheric mantlc plumc .The imitial fluids accumulated in some regions of the mantle, resulting in lowering of the mantle solidus (and liquidus) and partial melting of the upper mantle. Melts formed from low-degree of fluid-involved partial melting of the upper mantle would be highly enriched in incompatible elements.Fluies and melts are allthe metasomatic agents for mantle metasomatism, and the interaction between them and the depleted mantle could result in the substan-tial local enrichment of LREE and incompatible elements in the latter.In case that the concentration of H2O in the fluids (and melts) is lower ,only cryptic metasomatism would occur, in case that the concentration of H2O is higher,the degree of partial melting would be higher and hydrous metasomatic phases(e.g. amphiboles )would nucleate. Under such circumstances, there would occur model metasomatism.  相似文献   

5.
Petrochemical characteristics of igneous, sedimentary, and metasomatic rocks; chemical and isotopic compositions of minerals and fluids; and PT parameters of mineral formation at the Nezhdaninsky deposit are reported. A model of hydrothermal system formation is developed on this basis. In addition to decreasing Ba/Rb and Li/Mg ratios in the course of the hydrothermal process, resulting in the formation of ore-bearing metasomatic rocks, increasing K/Ba and diminishing K/Cs ratios indicate the probable participation of magmatic fluid in the ore deposition. The agreement of the K/Rb and K/Ba ratios with the values typical of the main trend of igneous rocks (MT) implies that the K, Rb, and Ba contents were distributed in the ore-forming hydrothermal fluid according to the ratios in the source magmatic chamber. The K/Rb ratios in metasomatic rocks correspond to the MT and approach the pegmatitic-hydrothermal trend and the composition of orthomagmatic fluid of Mo-W greisen. Similar REE patterns of igneous and terrigenous rocks do not allow the REE source to be constrained unequivocally. The lithological control of lithophile element distribution testifies to the supply of host rock components to the hydrothermal system. All studied rocks and minerals are enriched in LREE. The REE total and the contribution of HREE decrease from preore to synore metasomatic rocks, from preore to regenerated carbonates, and from older to younger scheelite. A similar tendency is noted in granitoids of the Kurum pluton. The δ18O values of quartz range from +10.3 to +12.6‰ in Au-Mo-W zones, from +15.9 to +16.4‰ in metasomatic rocks, from +14.8 to +16.6‰ in gold-ore veins, and from +13.5 to +16.9‰ in silver-base-metal ore mineralization. The estimates of \(\delta ^{18} O_{H_2 O} \) suggest that water was supplied from a magmatic source (δ18O = +(5.5?9.0‰)) and as a product of sedimentary rock dehydration. High-temperature (up to 390°C) and highly concentrated (up to 31 wt % NaCl equiv) fluids participated in the mineral formation. The phase separation of the fluid into H2O-CO2 liquid and predominantly carbon dioxide gas was combined with mixing of a high-temperature and relatively highly concentrated chloride solution with a low-temperature and poorly mineralized fluid. The redox conditions varied from equilibrium with CH4-bearing fluid at the gold-molybdenum-tungsten stage to equilibrium with CO2-bearing fluid during the gold-ore stage.  相似文献   

6.
The Okrouhlá Radouň shear zone hosted uranium deposit is developed along the contact of Variscan granites and high-grade metasedimentary rocks of the Moldanubian Zone of the Bohemian Massif. The pre-ore pervasive alteration of wall rocks is characterized by chloritization of mafic minerals, followed by albitization of feldspars and dissolution of quartz giving rise to episyenites. The subsequent fluid circulation led to precipitation of disseminated uraninite and coffinite, and later on, post-ore quartz and carbonate mineralization containing base metal sulfides. The fluid inclusion and stable isotope data suggest low homogenization temperatures (～50–140 °C during pre-ore albitization and post-ore carbonatization, up to 230 °C during pre-ore chloritization), variable fluid salinities (0–25 wt.% NaCl eq.), low fluid δ18O values (?10 to +2 ‰ V-SMOW), low fluid δ13C values (?9 to ?15 ‰ V-PDB), and highly variable ionic composition of the aqueous fluids (especially Na/Ca, Br/Cl, I/Cl, SO4/Cl, NO3/Cl ratios). The available data suggest participation of three fluid endmembers of primarily surficial origin during alteration and mineralization at the deposit: (1) local meteoric water, (2) Na–Ca–Cl basinal brines or shield brines, (3) SO4–NO3–Cl–(H)CO3 playa-like fluids. Pre-ore albitization was caused by circulation of alkaline, oxidized, and Na-rich playa fluids, whereas basinal/shield brines and meteoric water were more important during the post-ore stage of alteration.  相似文献   

7.
Tabular steeply dipping cassiterite‐bearing lodes in the Mount Wells region are hosted by lower greenschist fades metasediment of the Pine Creek Geosyncline within the contact aureole of late orogenic granitoids. The latter are predominantly I‐type, but S‐type phases are developed near the sediment‐granitoid contact.

Quartz, cassiterite, pyrite, arsenopyrite, chalcopyrite and pyrrhotite are the main minerals. Two types of lodes are present: (i) Sn‐quartz lodes containing 5–10 vol% sulphide minerals; and (ii) Sn‐sulphide lodes containing ～ 70 vol% sulphide minerals. At the surface, the former appear as normal quartz veins and the latter as hematite‐quartz breccia resulting from the collapse of original sulphide‐rich lodes as a consequence of volume reduction due to oxidation and leaching.

Two stages of quartz veining are recognized in both types of lodes. Cassiterite is present in stage I while stage II is composed of barren quartz with minor pyrite. Late stage III carbonate veinlets are present in Sn‐sulphide lodes. The lode‐wallrock contact is sharp with weak alteration effects confined to the fringe of the lodes. The alteration minerals include sericite, quartz, tourmaline, chlorite, pyrite and minor K‐feldspar.

Four types of fluid inclusions are present in vein quartz and cassiterite: Type A (CO2 ± H2O ± CH4); Type B (H2O+～ 20% vapour); Type C (H2O+ < 15% vapour) and Type D (H2O+ < 15% vapour + NaCl). Early ‘primary’ inclusions represented by Types A and B are present in stage I only and have a well‐defined temperature mode at ～300°C and a salinity range of 1–20 wt% eq NaCl. Types C and D inclusions are ‘secondary’ in stage I and primary in stage II and have a temperature mode at 120–160°C and salinities from about 1 to more than 26 wt% eq NaCl. Variable H2O‐CO2 ratios of Type A inclusions and homogenization in CO2 or H2O phase at near identical temperature indicate entrapment at the H2O‐CO2 solvus and define a pressure of ～ 100 MPa. The melting sequence of frozen inclusions suggests that the ore fluids were mainly H2O‐CO2‐CH4‐Na‐Ca‐Cl brines. This is also confirmed by Raman Laser Spectrometry.

Oxygen and sulphur isotope data are consistent with a magmatic origin of the ore fluids. The δD values are up to 20%0 higher than those expected for magmatic fluids and probably resulted from interaction of the latter with the carbonaceous strata. This interpretation is supported by δ13C data on the fluid inclusion CO2.

Fluid inclusions, stable isotope and mineralogical data are used to approximate the physico‐chemical parameters of the ore fluids which are as follows: T 300°C, m Cl～2, fO2 ～ 10‐35, mSS ～ 0.01, Sn ～ 1 ppm, Cu ～ 1 ppm and pH ～ 5.5.

It is suggested that fluids of granitic parentage interacted with the enclosing sediment and picked up CO2, CH4 and possibly Ca. The granitic phases became reduced due to this interaction and developed S‐type characteristics. Tin was probably partitioned into the CH4‐bearing reduced fluids. At some stage the fluid overpressure exceeded the lithostatic lode enforcing failure of the carapace and the intruded rocks by hydraulic fracturing causing CH4 and CO2 loss resulting in the precipitation of the ore minerals.  相似文献   

8.
Reactions and partial melting of peraluminous rocks in the presence of H2O-CO2–salt fluids under parameters of granulite-facies metamorphism were modeled in experiments on interaction between orthopyroxene–cordierite–biotite–plagioclase–quartz metapelite with H2O, H2O-CO2, H2O-CO2-NaCl, and H2O-CO2-KCl fluids at 600 MPa and 850°C. Rock melting in the presence of H2O and equimolar H2O-CO2 fluids generates peraluminous (A/CNK1 > 1.1) melts whose composition corresponds to magnesian calcic or calc–alkaline S-type granitoids. The melts are associated with peritectic phases: magnesian spinel and orthopyroxene containing up to 9 wt % Al2O3. In the presence of H2O-CO2-NaCl fluid, cordierite and orthopyroxene are replaced by the association of K-Na biotite, Na-bearing gedrite, spinel, and albite. The Na2O concentrations in the biotite and gedrite are functions of the NaCl concentrations in the starting fluid. Fluids of the composition H2O-CO2-KCl induce cordierite replacement by biotite with corundum and spinel and by these phases in association with potassium feldspar at X KCl = 0.02 in the fluid. When replaced by these phases, cordierite is excluded from the melting reactions, and the overall melting of the metapelite is controlled by peritectic reactions of biotite and orthopyroxene with plagioclase and quartz. These reactions produce such minerals atypical of metapelites as Ca-Na amphibole and clinopyroxene. The compositions of melts derived in the presence of salt-bearing fluids are shifted toward the region with A/CNK < 1.1, as is typical of so-called peraluminous granites of type I. An increase in the concentrations of salts in the fluids leads to depletion of the melts in Al2O3 and CaO and enrichment in alkalis. These relations suggest that the protoliths of I-type peraluminous granites might have been metapelites that were melted when interacting with H2O-CO2-salt fluids. The compositions of the melts can evolve from those with A/CNK > 1.1 (typical of S-type granites) toward those with A/CNK = 1.0–1.1 in response to an increase in the concentrations of alkali salts in the fluids within a few mole percent. Our experiments demonstrate that the origin of new mineral assemblages in metapelite in equilibrium with H2O-CO2-salt fluids is controlled by the activities of alkaline components, while the H2O and CO2 activities play subordinate roles. This conclusion is consistent with the results obtained by simulating metapelite mineral assemblages by Gibbs free energy minimization (using the PERPE_X software), as shown in log(\({a_{{H_2}O}}\))–log(\({a_{N{a_2}O}}\)) and log(\({a_{{H_2}O}}\))–log(\({a_{{K_2}O}}\)) diagrams.  相似文献   

9.
《Precambrian Research》2001,105(2-4):371-383
It has been well established that fluids played an important part in determining chemical characteristics in many crustal terranes. Studies of fluid inclusions in eclogites have established that brines coexisted with the primary mineral assemblages during their metamorphic crystallization. These brines are currently multiply saturated in halide salts, carbonates, oxides, and sulfides. As a first step in quantitatively bounding the composition of the brines during metamorphism, the equilibrium compositions of the brines at room temperature were computed using the aqueous speciation codes EQ3/6. The results demonstrate that the brines are high density solutions (ca. 1.4 g/cm3) that have ionic strengths of approximately 8 mol, and are approximately 40% dissolved solids, by weight. These are predominately Na- and K-rich brines, with subordinate Ca and Mg. The approximate Na:K:Ca:Mg molar ratios are 4:2:0.5:0.2, but are sensitive to the assumed HCO3− concentrations. Charge balance is primarily maintained by the very high Cl concentrations. These brines bear resemblance to brines analyzed from fluid inclusions in metamorphic rocks reported by Roedder (Roedder, E., 1972. Composition of fluid inclusions. US Geol. Surv. Prof. Paper 440JJ, p. 164). Although these fluids have the potential of acting as significant metasomatic agents in subduction zones and deep crustal environments, their impact will be mineralogically discernible only if the fluid release and movement is channelized.  相似文献   

10.
Mantle xenoliths from Tenerife show evidence of metasomatismand recrystallization overprinting the effects of extensivepartial melting. The evidence includes: recrystallization ofexsolved orthopyroxene porphyroclasts highly depleted in incompatibletrace elements into incompatible-trace-element-enriched, poikiliticorthopyroxene with no visible exsolution lamellae; formationof olivine and REE–Cr-rich, strongly Zr–Hf–Ti-depletedclinopyroxene at the expense of orthopyroxene; the presenceof phlogopite; whole-rock CaO/Al2O3 >> 1 (Ca metasomatism) inrecrystallized rocks; and enrichment in incompatible elementsin recrystallized rocks, relative to rocks showing little evidenceof recrystallization. The ‘higher-than-normal’ degreeof partial melting that preceded the metasomatism probably resultsfrom plume activity during the opening of the Central AtlanticOcean. Sr–Nd isotopic compositions are closely similarto those of Tenerife basalts, indicating resetting from theexpected original mid-ocean ridge basalt composition by themetasomatizing fluids. Metasomatism was caused by silicic carbonatitemelts, and involved open-system processes, such as trappingof elements compatible with newly formed acceptor minerals,leaving residual fluids moving to shallower levels. The compositionsof the metasomatizing fluids changed with time, probably asa result of changing compositions of the melts produced in theCanary Islands plume. Spinel dunites and wehrlites representrocks where all, or most, orthopyroxene has been consumed throughthe metasomatic reactions. KEY WORDS: Canary Islands; Tenerife; mantle xenoliths; geochemistry; Ca metasomatism; open-system processes; lithosphere; ocean islands  相似文献   

11.
 The melting reaction: albite(solid)+ H2O(fluid) =albite-H2O(melt) has been determined in the presence of H2O–NaCl fluids at 5 and 9.2 kbar, and results compared with those obtained in presence of H2O–CO2 fluids. To a good approximation, albite melts congruently at 9 kbar, indicating that the melting temperature at constant pressure is principally determined by water activity. At 5 kbar, the temperature (T)- mole fraction (X (H2O) ) melting relations in the two systems are almost coincident. By contrast, H2O–NaCl mixing at 9 kbar is quite non-ideal; albite melts ∼70 °C higher in H2O–NaCl brines than in H2O–CO2 fluids for X (H2O) =0.8 and ∼100 °C higher for X (H2O) =0.5. The melting temperature of albite in H2O–NaCl fluids of X (H2O)=0.8 is ∼100 °C higher than in pure water. The P–T curves for albite melting at constant H2O–NaCl show a temperature minimum at about 5 kbar. Water activities in H2O–NaCl fluids calculated from these results, from new experimental data on the dehydration of brucite in presence of H2O–NaCl fluid at 9 kbar, and from previously published experimental data, indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar. Aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction-related metamorphism. Low water activity in high pressure-temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting, but may be due to the presence of low a (H2O) brines. Received: 17 March 1995/Accepted: 9 April 1996  相似文献   

12.
The Orot (Or) and Ermakovka (Er) intrusions of aegirine granites with various resources of accompanying Be mineralization in Transbaikalia were studied to reproduce Be behavior during the crystallization and degassing of alkaline granite magma. Data on the petrography and geochemistry of the rocks and the microthermomety and microprobe analysis of fluid and melt inclusions in them indicate that the intrusions were formed by discrete magma portions derived from a single magmatic source during successive stages of its differentiation. The intrusions crystallized at temperatures higher than 1030–1070°C (Or) and 840–640°C (Er), and the melts contained elevated concentrations of H2O and F: from 2.1 to 3.5% F and from <1 to 1% H2O for the former intrusion and from 3.9 to 6.7% F and from <2.6 to 4.1% H2O for the latter. Fluids were released from the magma during a late crystallization stage for the Orot intrusion and an intermediate stage for the Ermakovka intrusion. Early in the course of this process, the fluids were halide-sulfate brines with the Cl: F ratio higher for the Orot intrusion and lower for the Ermakovka intrusion. A temperature decrease resulted in the exsolution of the fluids into two immiscible phases, one of which contained low and the other high concentrations of salts. The magmatic brines and low-salinity solutions of both intrusions were enriched in Be (up to 1.1 g/kg), which is comparable with the concentration of this element in the emanations of Be-bearing pegmatites in the Pamirs and is manyfold higher than C Be in magmatic fluids related to granite intrusions with W-Sn mineralization. The Be ore mineralization of the Orot and Ermakovka deposits was produced by solutions whose composition and Be concentrations were analogous to those in the low-salinity phase of the corresponding magmatic fluids. The brines of the Ermakovka intrusion were enriched in Mo (up to 17 g/kg) and, to a lesser extent, Mn, Ce, and La and produced uneconomic monazite-molybdenite ore mineralization. Based on available data and results of our calculations, we arrived at the conclusion that the very high alkali concentrations in the melts of both intrusions (ASI < 1), their high F concentrations (up to 4.1%), and the absence of magmatic Be-bearing minerals facilitated Be selective extraction by the separated fluids in the form of its most soluble F-complexes. The high oxidation of the melt predetermined the predominance of hexavalent S and Mo compounds, which could be efficiently extracted by the fluid phase in the form of sulfates and molybdates of alkali metals. The differences in the ore potentials of the Orot and Ermakovka intrusions were caused by the different H2O, F, and perhaps, also Be concentrations in the parental melts, which was, in turn, caused by their different degrees of differentiation during the preintrusive stages of their magmatic evolution.  相似文献   

13.
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   

14.
Phase equilibria in the system CaO–MgO–SiO2–CO2–H2O–NaCl are calculated to illustrate phase relations in metacarbonates over a wide-range of P–T–X[H2O–CO2–NaCl] conditions. Calculations are performed using the equation of state of Duan et al. (Geochim Cosmochim Acta 59:2869–2882, 1995) for H2O–CO2–NaCl fluids and the internally consistent data set of Gottschalk (Eur J Mineral 9:175–223, 1997) for thermodynamic properties of solids. Results are presented in isothermal-isobarical plots showing stable mineral assemblages as a function of fluid composition. It is shown that in contact-metamorphic P–T regimes the presence of very small concentrations of NaCl in the fluid causes almost all decarbonation reactions to proceed within the two fluid solvus of the H2O–CO2–NaCl system. Substantial flow of magma-derived fluids into marbles has been documented for many contact aureoles by shifts in stable isotope geochemistry of the host rocks and by the progress of volatile-producing mineral reactions controlled by fluid compositions. Time-integrated fluid fluxes have been estimated by combining fluid advection/dispersion models with the spatial arrangement of mineral reactions and isotopic resetting. All existing models assume that minerals react in the presence of a single phase H2O–CO2 fluid and do not allow for the effect that fluid immiscibility has on the flow patterns. It is shown that fluids emanating from calc-alkaline melts that crystallize at shallow depths are brines. Their salinity may vary depending mainly on pressure and fraction of crystallized melt. Infiltration-driven decarbonation reactions in the host rocks inevitably proceed at the boundaries of the two fluid solvus where the produced CO2 is immiscible and may separate from the brine as a low salinity, low density H2O–CO2 fluid. Most parameters of fluid–rock interaction in contact aureoles that are derived from progress of mineral reactions and stable isotope resetting are probably incorrect because fluid phase separation is disregarded.  相似文献   

15.
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16.
Experiments on partial melting of mantle lherzolite have been realized at 0.6 and 1.0 GPa and the chemical compositional variations of melts during different melting stages have been first discussed. The results show that the trends of variations in SiO2, CaO, Al2O3, Na2O and TiO2 are different at different melting stages. The melts produced at lower pressure are richer in SiO2 than those at higher pressure. The mantle-derived silica-rich fluids (silicate melts) are polygenetic, but the basic and intermediate-acid silicate melts in mantle peridotite xenoliths from the same host rocks, which have equivalent contents of volatile and alkali components and different contents of other components, should result from in-situ (low-degree) partial melting of mantle peridotite under different conditions (e.g. at different depths, with introduction of C-O-H fluids or in the presence of metasomatic minerals). The intermediate-acid melts may be the result of partial melting (at lower pressure) Opx + Sp + K-Na-rich fluid±(Amphi)±(Phlog)= Ol+melt.But the intermediate-acid magmas cannot be produced from the partial melting of normal mantle peridotite unless the crustal materials are introduced to some extent.  相似文献   

17.
Simple one-dimensional numerical models are presented for coupled advection-hydrodynamic dispersion and kinetically controlled oxidatioin-reduction reactions in graphite-free porous media containing magnetite coexisting with silicate assemblages. Fluid-solid interactions involving either OH (O2-H2O-H2) or COH (O2-H2O-H2-CO2-CO-CH4) fluids are considered at ∼500 ∘C and 5 kbar. The major implications of the modeling are as follows: (1) Regional (km scale) reduction of typical magnetite-bearing rocks originally at f O2 near NNO may be possible during long-term metamorphic fluid flow if the infiltrating fluids have sufficiently low f O2 and sufficiently large concentrations of CH4 and/or H2. Regional oxidation of such rocks by highly oxidized OH or COH fluids appears to be difficult to achieve. (2) Nearly identical mineral assemblages and modes may be produced by very different kinetic reaction pathways. The model implies that “equilibrium” assemblages preserved in rocks may not always reflect the true kinetic reaction path that evolved during fluid flow, and highlights the need for quantitative measurements of metamorphic reaction rates. (3) Preservation of sharp lithologic contacts between rocks of very different redox states containing accessory amounts of oxides may be unlikely if fluid-rock interaction times exceed 103104 years. Substantial contact disruption over these times scales is predicted even for oxide-rich rocks if redox contrasts between layers are large. Flow across lithologic contacts may produce asymmetric patterns of metasomatic mineral zonation that may prove useful for mapping flow directions in metamorphic sequences. (4) For fluid flow in typical T gradients through originally homogeneous rock, significant major element metasomatism (e.g., K, Na, Ca) may be possible without producing large changes in oxide abundances. Received:12 November 1997 / Accepted: 9 March 1998  相似文献   

18.
Whole rock trace element and isotopic compositions of different HP–LT metamorphic rocks of the Ile de Groix were analysed to characterise geochemical fingerprints during subduction and exhumation in a late Palaeozoic HP metamorphic terrain. Massive metabasites of hydrothermally altered enriched mid-ocean ridge basalt (E-MORB) origin are in association with banded metabasic rocks of volcano-sedimentary origin and metapelites. Fluid-rock interactions that likely occurred during seafloor hydrothermal alteration and early subduction metasomatism increased δ18O values, as well as K2O, Na2O, MgO, and LILE contents and decreased CaO contents of metabasites. Most metabasites have retained their early-subduction and pre-HP trace element and isotopic composition, even for rocks metamorphosed to lower eclogite-facies P–T conditions. Micaschists also preserved apparent pelitic protolith trace element values and oxygen isotopic compositions. During retrograde metamorphism related to the exhumation, metabasites were rehydrated by fluids in equilibrium with the host rock compositions, which were likely derived from the basic rocks. This style of fluid–rock interaction formed a greenschist facies mineral assemblage. Metabasites that underwent pervasive alteration by seafloor hydrothermal and metasomatism processes prior to peak metamorphism, show greater effects of retrogression and albitisation, probably because they were richer in H2O and Na2O. The variety of metamorphic assemblages on the Ile de Groix is thus directly related to the pre-HP rock composition. The extent of retrogression in the western part of the Ile de Groix primarily reflects stronger metasomatic intensities than in the eastern part.  相似文献   

19.
Highly concentrated NaCl brines are important geothermal fluids; chloride complexation of metals in such brines increases the solubility of minerals and plays a fundamental role in the genesis of hydrothermal ore deposits. There is experimental evidence that the molecular nature of the NaCl–water system changes over the pressure–temperature range of the Earth's crust. A transition of concentrated NaCl–H2O brines to a "hydrous molten salt" at high P and T has been argued to stabilize an aqueous fluid phase in the deep crust.  相似文献   

20.
The ultrahigh‐pressure pyrope whiteschists from the Brossasco‐Isasca Unit of the Southern Dora‐Maira Massif represent metasomatic rocks originated at the expense of post‐Variscan granitoids by the influx of fluids along shear zones. In this study, geochemical, petrological and fluid‐inclusion data, correlated with different generations of pyrope‐rich garnet (from medium, to very‐coarse‐grained in size) allow constraints to be placed on the relative timing of metasomatism and sources of the metasomatic fluid. Geochemical investigations reveal that whiteschists are strongly enriched in Mg and depleted in Na, K, Ca and LILE (Cs, Pb, Rb, Sr, Ba) with respect to the metagranite. Three generations of pyrope, with different composition and mineral inclusions, have been distinguished: (i) the prograde Prp I, which constitutes the large core of megablasts and the small core of porphyroblasts; (ii) the peak Prp II, which constitutes the inner rim of megablasts and porphyroblasts and the core of small neoblasts; and (iii) the early retrograde Prp III, which locally constitutes an outer rim. Two generations of fluid inclusions have been recognized: (i) primary fluid inclusions in prograde kyanite that represent a NaCl‐MgCl2‐rich brine (6–28 wt% NaCleq with Si and Al as other dissolved cations) trapped during growth of Prp I (type‐I fluid); (ii) primary multiphase‐solid inclusions in Prp II that are remnants of an alumino‐silicate aqueous solution, containing Mg, Fe, alkalies, Ca and subordinate P, Cl, S, CO32‐, LILE (Pb, Cs, Sr, Rb, K, LREE, Ba), U and Th (type‐II fluid), at the peak pressure stage. We propose a model that illustrates the prograde metasomatic and metamorphic evolution of the whiteschists and that could also explain the genesis of other Mg‐rich, alkali‐poor schists of the Alps. During Alpine metamorphism, the post‐Variscan metagranite of the Brossasco‐Isasca Unit experienced a prograde metamorphism at HP conditions (stage A: ～1.6 GPa and ≤ 600 °C), as indicated by the growth of an almandine‐rich garnet in some xenoliths. At stage B (1.7–2.1 GPa and 560–590 °C), the influx of external fluids, originated from antigorite breakdown in subducting oceanic serpentinites, promoted the increase in Mg and the decrease of alkalies and Ca in the orthogneiss toward a whiteschist composition. During stage C (2.1 < P < 2.8 GPa and 590 < T < 650 °C), the metasomatic fluid influx coupled with internal dehydration reactions involving Mg‐chlorite promoted the growth of Prp I in the presence of the type‐I MgCl2‐brine. At the metamorphic peak (stage D: 4.0–4.3 GPa and 730 °C), Prp II growth occurred in the presence of a type–II alumino‐silicate aqueous solution, mostly generated by internal dehydration reactions involving phlogopite and talc. The contribution of metasomatic external brines at the metamorphic climax appears negligible. This fluid, showing enrichment in LILE and depletion in HFSE, could represent a metasomatic agent for the supra‐subduction mantle wedge.  相似文献   

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