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1.
The low-angle dip schistosity zones of the Belomorian mobile belt of northern Karelia are zones of plastic flow of thrust origin. They were formed from 1.85 to 1.90 Ga: 1879 ± 21 Ma according to 40Ar/39Ar for amphibole from amphibolites and 1857 ± 13 Ma according to the Sm–Nd isochron in amphibolites. The P–T parameters of rock metamorphism in low-angle dip schistosity zones correspond to the boundary of amphibolite and granulite facies of metamorphism: T = 640–765°C, rarely rising to 826°C; P = 8.0–11.7 kbar. The hypothesis of the two-stage Paleoproterozoic metamorphism of rocks of the Belomorian mobile belt was introduced.  相似文献   

2.
Garnet in metapelites from the Wölz Complex of the Austroalpine crystalline basement east of the Tauern Window characteristically consists of two growth phases, which preserve a comprehensive record of the geothermal history during polymetamorphism. From numerical modelling of garnet formation, detailed information on the pressure–temperature–time (P–T–t) evolution during prograde metamorphism is obtained. In that respect, the combined influences of chemical fractionation associated with garnet growth, modification of the original growth zoning through intragranular diffusion and the nucleation history on the chemical zoning of garnet as P and T change during growth are considered. The concentric chemical zoning observed in garnet and the homogenous rock matrix, which is devoid of chemical segregation, render the simulation of garnet growth through successive equilibrium states reliable. Whereas the first growth phase of garnet was formed at isobaric conditions of ～3.8 kbar at low heating/cooling rates, the second growth phase grew along a Barrovian P–T path marked with a thermal peak of ～625°C at ～10 kbar and a maximum in P of ～10.4 kbar at ～610°C. For the heating rate during the growth of the second phase of garnet, average rates faster than 50°C Ma?1 are obtained. From geochronological investigations the first growth phase of garnet from the Wölz Complex pertains to the Permian metamorphic event. The second growth phase grew in the course of Eo-Alpine metamorphism during the Cretaceous.  相似文献   

3.
The Belomorian Mobile Belt (BMB) in northern Karelia mostly consists of gently sloping shear zones, whose gneisses and migmatized amphibolites and blastomylonites are typically thinly banded, with their banding consistently dipping north- and northeastward. These gently sloping shear zones were not affected by folding after they were produced and are not cut by Paleoproterozoic metabasite dikes. Intrusive metabasites in the gently sloping shear zones make up relatively small (usually <5 m) equant or elongate bodies and occur as fragments of larger bodies. These fragments are often concentrated in stripes. Metabasites in the gently sloping shear zone are sometimes also found as lenses and tabular bodies of relatively small thickness, which are conformable with the foliation of the host rocks. The gently sloping shear zones cut across older domains of more complicated structure, which suggests that these zones are gently sloping ductile shear zones. Along these zones, the nappes were thrust south- and southwestward, and this process was the last in the origin of major structural features of BMB when the Paleoproterozoic Lapland–Kola orogen was formed. Practically identical age values were obtained for the gently sloping shear zone in the two widely separated Engonozero and Chupa segments of BMB: 1879 ± 21 Ma (40Ar/39Ar amphibole age of amphibolite whose protolith was mafic rock) and 1857 ± 13 Ma (Sm–Nd mineral isochron age of garnet amphibolites after gabbronorite). The P–T metamorphic parameters in these gently sloping shear zones are remarkably different from the metamorphic parameters outside these zones: the pressure is 3–4 kbar lower and the temperature is 60–100°C lower. Thrusting-related decompression triggered the transition from the older high-pressure episode of Paleoproterozoic metamorphism to a younger syn-thrusting higher temperature metamorphic episode. The peak metamorphic parameters corresponding to the boundary between the amphibolite and granulite facies were reached only in the central portions of the shear zones: T= 680–760°C, P = 8.0–11.9 kbar. In areas of the most intense migmatization, temperature estimates in the central portions of the shear are as high as 810–830°C. The marginal portions of the shear zones were formed at lower temperatures of 610–630°C. The temperature heterogeneous and rock heating in the gently sloping shear zones may have resulted from flows of high-temperature metamorphic fluid that were focused to the central portions of the zones.  相似文献   

4.
Phase equilibria modeling of the pressure–temperature (P–T) path of regional metamorphism and associated fluid expulsion, combined with constraints on the timescale of garnet growth by Sm–Nd geochronology, elucidates the fluid production rate and fluid flux during Barrovian metamorphism of pelitic rocks from Townshend Dam, VT, USA. This modeling builds on a published companion study that utilized Sm–Nd geochronology of concentric growth zones in multiple garnet grains, to constrain the duration of garnet growth in a large sample of schist at Townshend Dam to 3.8?±?2.2 million years (Gatewood et al., Chem Geol 401:151–168, 2015). P–T pseudosections combined with observed mineral compositions constrain garnet growth conditions, and were utilized to construct P–T path-dependent thermodynamic forward models. These models determine that garnet growth was initiated at ~?0.6 GPa and ~?525 °C, with a roughly linear loading and heating P–T trajectory to >?0.8 GPa and ~?610 °C. Loading and heating rates of 2.4 km·Myear?1 (with a range of 1.6 to 5.8 km·million year?1) and 23 °C·million year?1 (with a range of 14 to 54 °C·million year?1), respectively, are consistent with model estimates and chronologic constraints for tectono-metamorphic rates during orogenesis. Phase equilibria modeling also constrains the amount of water release during garnet growth to be ~?0.7 wt% (or >?2 vol%), largely resulting from the complete consumption of chlorite. Coupling this estimate with calculated garnet growth durations provides a fluid production rate of 5.2 kg·m?3·million year?1 (with a range of 3.2 to 12.2 kg·m?3·million year?1) and when integrated over the overlying crustal column, a regional-scale fluid flux of 0.07–0.37 kg·m?2·million year?1. This range of values is consistent with those derived by numerical models and theory for regional-scale, pervasive fluid flow. This study signifies the first derivation of a fluid production rate and fluid flux in regional metamorphism using a direct chronology of water-producing (garnet-forming) reactions and can provide a framework for future studies on elucidating the nature and timescales of fluid release.  相似文献   

5.
The paper presents original authors’ data on aluminous schists in the Tsogt tectonic plate in the Southern Altai Metamorphic Belt. The nappe includes a medium-temperature/medium-pressure zonal metamorphic complex, whose metamorphic grade varies from the greenschist to epidote-amphibolite facies. The garnet and garnet–staurolite schists contain three garnet generations of different composition and morphology. The P–T metamorphic parameters estimated by mineralogical geothermometers and geobarometers and by numerical modeling with the PERPLEX 668 software provide evidence of two successive metamorphic episodes: high-gradient (of the andalusite–sillimanite type, geothermal gradient approximately 40–50°/km) and low-gradient (kyanite–sillimanite type, geothermal gradient approximately 27°/km). The P-T parameters of the older episode are T = 545–575°C and P = 3.1–3.7 kbar. Metamorphism during the younger episode was zonal, and its peak parameters were T = 560–565°C, P = 6.4–7.2 kbar for the garnet zone and T = 585–615°C, P = 7.1–7.8 kbar for the staurolite zone. The metamorphism evolved according to a clockwise P–T path: the pressure increased during the first episode at a practically constant temperature, and then during the second episode, the temperature increased at a nearly constant pressure. Such trends are typical of metamorphism related to collisional tectonic settings and may be explained by crustal thickening due to overthrusting. The regional crustal thickening reached at least 15–18 km.  相似文献   

6.
The P–T conditions of Early Paleozoic metamorphism in the Sangilen block of the Tuvino-Mongolian Massif (southeastern part of the Central Asian Mobile Belt) achieved a value of 910–950°C and 3–4 kbar, which corresponded to the conditions of ultrahigh temperature–low pressure (UHT/LP) metamorphism. During retrograde metamorphism, cooling down to 850°C was accompanied by compression (up to 5.5–7 kbar), and then cooling down to 580–650°C took place at nearly the same pressure (5.5–6.5 kbar). UHT metamorphism was related to the elevated heat flow from the mantle, leading also to an intensive basite magmatism. The “counter-clockwise” P–T evolution was evoked by underthrusting of the hot tectonic slab (Erzin complex) beneath the colder one (Moren complex).  相似文献   

7.
Experimental studies in the system Fe,Ni–olivine–carbonate–S (P = 6.3 GPa, T = 1050–1550°C, t = 40–60 h) aimed at modeling of the interaction of subducted carbonates and sulfur with rocks of the silicate mantle and at investigation of the likely mechanism of the formation of mantle sulfides were performed. It is shown that an association of olivine + orthopyroxene + magnesite + pyrite coexisting with a sulfur melt/fluid with dissolved Fe, Ni, and O is formed at T ≤ 1250°C. An association of low-Fe olivine, orthopyroxene, and magnesite and two immiscible melts of the carbonate and S–Fe–Ni–O compositions are formed at T ≥ 1350°C. It is shown that the reduced S-bearing fluids may transform silicates and carbonates, extract metals from the solid-phase matrix, and provide conditions for generation of sulfide melts.  相似文献   

8.
The metabasites of Chadegan, including eclogite, garnet amphibolite and amphibolite, are forming a part of Sanandaj–Sirjan Zone. These rocks have formed during the subduction of the Neo–Tethys ocean crust under Iranian plate. This subduction resulted in a subduction metamorphism under high pressuremedium temperature of eclogite and amphibolites facies condition. Then the metamorphic rocks were exhumed during the continental collision between the Afro–Arabian continent and the Iranian microcontinent. In the metabasite rocks, with typical MORB composition, garnet preserved a compositional zoning occurred during metamorphism. The magnesium (XMg) gradually increases from core to rim of garnets, while the manganese (XMn) decreases towards the rim. Chondrite–normalized Rare Earth Element patterns for these garnets exhibit core–to–rim increases in Light Rare Earth Elements. The chondrite–normalized REE patterns of garnets, amphiboles and pyroxenes display positive trend from LREEs to Heavy Rare Earth Elements (especially in garnet), which suggests the role of these minerals as the major controller of HREE distribution. The geochemical features show that the studied eclogite and associated rocks have a MORB origin, and probably formed in a deep–seated subduction channel environment. The geothermometry estimation yields average pressure of ~22 kbar and temperature of 470–520°C for eclogite fomation. The thermobarometry results gave T = 650–700°C and P ≈ 10–11 kbar for amphibolite facies.  相似文献   

9.
The results of thermobarometry yielded the P–T parameters of formation and evolution of sapphirine- bearing granulites in the Anabar shield with peak values of UHT metamorphism in the range of T = 920–1000°C at P = 9–11 kbar. Isotope–geochronological data indicate a polymetamorphic evolution of these rocks. Detrital zircon cores in the center of crystals yielded ages of 3.36, 2.75, 2.6, and 2.5 Ga. Later, superimposed metamorphic transformations of the detrital zircon formed rims dated to 2.4, 2.3, 2.2, and 1.83 Ga. A potential provenance source of the detrital zircons could be hypersthene plagiogneisses and metabasics of the Daldyn Group with a premetamorphic age no less than 3.32 Ga and products of their metamorphism of about 2.7 Ga old.  相似文献   

10.
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11.
Pyroxenes of general stoichiometry Mg(Ge x Si1?x )O3 were encountered in attempts to synthesise Ge-substituted talcs at 0.2 GPa, 650–700 °C. Orthopyroxenes (Pbca) of compositions x = 0.21, 0.30, and 0.34 were identified, and also a P21/c clinopyroxene of composition x = 0.63, and C2/c clinopyroxenes of compositions x = 0.91 and 1. End-member clinoenstatite MgSiO3-P21/c synthesised at 16 GPa, 1300 °C and transformed from C2/c was also included in the study. Crystal structure refinements using single-crystal XRD data showed that unit-cell parameters vary linearly with Si–Ge for the Pbca and P21/c pyroxenes, both of which have two symmetrically non-equivalent tetrahedral chains. Refinement of Si–Ge occupancies at tetrahedral sites showed that the two chains of all primitive pyroxenes have very different compositions, with XGe(TB) ? XGe(TA). This difference arises from the greater flexibility of the B-chain to rotate in response to tetrahedral expansion due to increasing Ge content. The TA-M2 shared polyhedral edge imposes significant constraints on the flexibility of the A-chain, which can accommodate much less Ge than the B-chain. Linear trends of cell parameters, site occupancies, and structural parameters for the primitive pyroxenes, when extrapolated to published data for MgGeO3–Pbca, extend across the entire Si–Ge join.  相似文献   

12.
Mubarak shear belt provides an opportunity to investigate quantitative finite strain (Rs), proportions of pure shear and simple shear components, sense of shear indicators, subhorizontal to steeply plunging mineral lineations, in a dextral transpressional zone. The structural style of the Mubarak shear belt is consistent with dextral transpression within the Central Eastern Desert where dextral and reverse shear have developed simultaneously with the regional foliation. The high strain zone of the Mubarak shear belt is characterized by steeply dipping foliation with sub-horizontal stretching lineation (simple shear) surrounded by thrust imbrications with slightly plunging stretching lineations. Strain estimates from the Mubarak shear belt are used to determine how pure and simple shear components of deformation are partitioned. The axial ratios in XZ sections range from 1.16 to 2.33 with the maximum stretch, S X , ranges from 1.06 to 1.48. The minimum stretch, S Z , ranges from 0.65 to 0.92 indicating a moderate variation in vertical shortening. Volcaniclastic metasediments and metagabbros were subjected to prograde low-grade regional metamorphism in the range of greenschist to lower amphibolite facies (450–650°C at 2–4 kbar). Medium pressure (6–8 kbar at 530°C) was estimated from the high strain zone within the dextral strike-slip shear zones. Retrograde metamorphism occurred at a temperature range of 250–280°C. There is a trend towards decreasing the ratio of 100Mg/(Mg + Fetot + Mn) away from the high strain zone of the Mubarak shear belt. Integrated strain and temperature estimates indicate that the simple shear (non-coaxial) components of deformation played a significant role in formation and exhumation of the Mubarak shear belt during the accumulation of finite strain and consequently during progressive transpression and thrusting.  相似文献   

13.
Experimental studies in the Fe3C–SiO2–MgO system (P = 6.3 GPa, T = 1100–1500°C, t = 20–40 h) have been carried out. It has been established that carbide-oxide interaction resulted in the formation of Fe-orthopyroxene, graphite, wustite, and cohenite (1100 and 1200°C), as well as a Fe–C–O melt (1300–1500°C). The main processes occurring in the system at 1100 and 1200°C are the oxidation of cohenite, the extraction of carbon from carbide, and the crystallization of metastable graphite, as well as the formation of ferrosilicates. At T ≥ 1300°C, graphite crystallization and diamond growth occur as a result of the redox interaction of a predominantly metallic melt (Fe–C–O) with oxides and silicates. The carbide–oxide interaction studied can be considered as the basis for modeling a number of carbon-producing processes in the lithospheric mantle at fO2 values near the iron–wustite buffer.  相似文献   

14.
High pressure in situ synchrotron X-ray diffraction experiment of strontium orthophosphate Sr3(PO4)2 has been carried out to 20.0 GPa at room temperature using multianvil apparatus. Fitting a third-order Birch–Murnaghan equation of state to the P–V data yields a volume of V 0 = 498.0 ± 0.1 Å3, an isothermal bulk modulus of K T  = 89.5 ± 1.7 GPa, and first pressure derivative of K T ′ = 6.57 ± 0.34. If K T ′ is fixed at 4, K T is obtained as 104.4 ± 1.2 GPa. Analysis of axial compressible modulus shows that the a-axis (K a  = 79.6 ± 3.2 GPa) is more compressible than the c-axis (K c  = 116.4 ± 4.3 GPa). Based on the high pressure Raman spectroscopic results, the mode Grüneisen parameters are determined and the average mode Grüneisen parameter of PO4 vibrations of Sr3(PO4)2 is calculated to be 0.30(2).  相似文献   

15.
Omphacite is an important mineral component of eclogite. Single-crystal synchrotron X-ray diffraction data on natural (Ca, Na) (Mg, Fe, Al)Si2O6 omphacite have been collected at the Advanced Photon Source beamlines 13-BM-C and 13-ID-D up to 47 GPa at ambient temperature. Unit cell parameter and crystal structure refinements were carried out to constrain the isothermal equation of state and compression mechanism. The third-order Birch–Murnaghan equation of state (BM3) fit of all data gives V 0 = 423.9(3) Å3, K T0 = 116(2) GPa and K T0′ = 4.3(2). These elastic parameters are consistent with the general trend of the diopside–jadeite join. The eight-coordinated polyhedra (M2 and M21) are the most compressible and contribute to majority of the unit cell compression, while the SiO4 tetrahedra (Si1 and Si2) behave as rigid structural units and are the most incompressible. Axial compressibilities are determined by fitting linearized BM3 equation of state to pressure dependences of unit cell parameters. Throughout the investigated pressure range, the b-axis is more compressible than the c-axis. The axial compressibility of the a-axis is the largest among the three axes at 0 GPa, yet it quickly drops to the smallest at pressures above 5 GPa, which is explained by the rotation of the stiffest major compression axis toward the a-axis with the increase in pressure.  相似文献   

16.
The highly calcic anorthosite (An>95) from the Sittampundi Layered Complex (SLC) develops corundum, spinel and sapphirine that are hitherto not reported from any anorthositic rocks in the world. Petrological observations indicate the following sequence of mineral growth: plagioclasematrix → corundum; clinopyroxene → amphibole; corundum + amphibole → plagioclasecorona + spinel; and spinel + corundum → coronitic sapphirine. Phase relations in the CaO–Na2O–Al2O3–SiO2–H2O (CNASH) system suggest that corundum was presumably developed through vapour present incongruent melting of the highly calcic plagioclase during ultra-high temperature (UHT) metamorphism (T ≥ 1000 °C, P ≥ 9 kbar). Topological constraints in parts of the Na2O–CaO–MgO–Al2O3–SiO2–H2O (NCMASH) system suggest that subsequent to the UHT metamorphism, aqueous fluid(s) permeated the rock and the assemblage corundum + amphibole + anorthite + clinozoisite was stabilized during high-pressure (HP) metamorphism (11 ± 2 kbar, 750 ± 50 °C). Constraints of the NCMASH topology and thermodynamic and textural modeling study suggest that coronitic plagioclase and spinel formed at the expense of corundum + amphibole during a steeply decompressive retrograde P–T path (7–8 kbar and 700–800 °C) in an open system. Textural modeling studies combined with chemical potential diagrams (μSiO2–μMgO) in the MASH system support the view that sapphirine also formed from due to silica and Mg metasomatism of the precursor spinel ± corundum, on the steeply decompressive retrograde P–T path, prior to onset of significant cooling of the SLC. Extremely channelized fluid flow and large positive solid volume change of the stoichiometrically balanced sapphirine forming reaction explains the localized growth of sapphirine.  相似文献   

17.
The first results of experimental study of diamond dissolution in a S-bearing Fe melt at high P–T parameters are reported and the morphology of partially dissolved crystals is compared with that of natural diamonds. Our results show that under the experimental conditions (4 GPa, 1400°C), flat-faced octahedral diamond crystals are transformed into curve-faced octahedroids with morphological features similar to those of natural diamonds.  相似文献   

18.
The liquidus water content of a haplogranite melt at high pressure (P) and temperature (T) is important, because it is a key parameter for constraining the volume of granite that could be produced by melting of the deep crust. Previous estimates based on melting experiments at low P (≤0.5 GPa) show substantial scatter when extrapolated to deep crustal P and T (700–1000 °C, 0.6–1.5 GPa). To improve the high-P constraints on H2O concentration at the granite liquidus, we performed experiments in a piston–cylinder apparatus at 1.0 GPa using a range of haplogranite compositions in the albite (Ab: NaAlSi3O8)—orthoclase (Or: KAlSi3O8)—quartz (Qz: SiO2)—H2O system. We used equal weight fractions of the feldspar components and varied the Qz between 20 and 30 wt%. In each experiment, synthetic granitic composition glass + H2O was homogenized well above the liquidus T, and T was lowered by increments until quartz and alkali feldspar crystalized from the liquid. To establish reversed equilibrium, we crystallized the homogenized melt at the lower T and then raised T until we found that the crystalline phases were completely resorbed into the liquid. The reversed liquidus minimum temperatures at 3.0, 4.1, 5.8, 8.0, and 12.0 wt% H2O are 935–985, 875–900, 775–800, 725–775, and 650–675 °C, respectively. Quenched charges were analyzed by petrographic microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), and electron microprobe analysis (EMPA). The equation for the reversed haplogranite liquidus minimum curve for Ab36.25Or36.25Qz27.5 (wt% basis) at 1.0 GPa is \(T = - 0.0995 w_{{{\text{H}}_{ 2} {\text{O}}}}^{ 3} + 5.0242w_{{{\text{H}}_{ 2} {\text{O}}}}^{ 2} - 88.183 w_{{{\text{H}}_{ 2} {\text{O}}}} + 1171.0\) for \(0 \le w_{{{\text{H}}_{ 2} {\text{O}}}} \le 17\) wt% and \(T\) is in °C. We present a revised \(P - T\) diagram of liquidus minimum H2O isopleths which integrates data from previous determinations of vapor-saturated melting and the lower pressure vapor-undersaturated melting studies conducted by other workers on the haplogranite system. For lower H2O (<5.8 wt%) and higher temperature, our results plot on the high end of the extrapolated water contents at liquidus minima when compared to the previous estimates. As a consequence, amounts of metaluminous granites that can be produced from lower crustal biotite–amphibole gneisses by dehydration melting are more restricted than previously thought.  相似文献   

19.
The origin and sources of the Il’mensky-Vishnevogorsky miaskite-carbonatite complex, one of the world’s largest alkaline complexes, with unique rare-metal and colored-stone mineralization and Nb, Zr, and REE deposits, are discussed in this paper. Geochemical and isotopic studies, including of Nd, Sr, C, and O isotopes, as well as estimation of PT formation conditions, of miaskites and carbonatites from various deposits of the Il’mensky-Vishnevogorsky Complex have been carried out. The Vishnevogorsky, Potaninsky, and Buldym Nb-REE deposits and the Il’mensky, Baidashevo, and Uvil’dy occurrences related to carbonatites were investigated. Their geological setting, composition, and ore resource potential are characterized. The genetic models and typical features of the Il’mensky-Vishnevogorsky Complex are considered. The rocks of the Il’mensky-Vishnevogorsky Complex were formed at T = 1000?230°C and P = 2–5 kbar. Carbonated miaskite melt was divided into immiscible silicate and carbonate liquids at T = 1000°C and P = 5 kbar. Miaskite crystallized at T = 850?700°C and P = 3.5–2.5 kbar. The formation temperature of carbonatite I of the Vishnevogorsky pluton was close to the temperature of miaskite crystallization (700–900°C). The crystallization temperature of carbonate-silicate rock and carbonatite I in the Central alkaline tract was 650–600°C. The formation temperature of carbonatite II varied from 590 to 490°C. Dolomite-calcite carbonatite III and dolomite carbonatite IV of the Buldym massif were formed at T = 575?410°C and T = 315?230°C, respectively. The geochemical features of carbonatites belonging to the Il’mensky-Vishnevogorsky Complex differ from those of carbonatites related to alkaline ultramafic rocks and are close to those of carbonatites related to nepheline syenite or carbonatites localized in linear fracture zones. A high Sr content in early carbonatites along with relatively low Ba, Nb, Ta, Ti, Zr, and Hf contents and a certain enrichment in HREE (a low La/Yb ratio) in comparison with carbonatites of the alkaline ultramafic association are typical. The geochemistry of carbonatites of the Il’mensky-Vishnevogorsky Complex corresponds to the trend of geochemical evolution of carbonatitic melts and their fluid derivatives. The Sr, Nd, C, and O isotopic compositions indicate a mantle magmatic source of the Il’mensky-Vishnevogorsky Complex and participation of moderately depleted mantle (DM) and enriched mantle EM1 in magma generation. Carbonatite and miaskite of the Vishnevogorsky pluton are related to the DM magma source, and carbonatite of the Buldym massif, to the EM1 source, probably, involved in the plume ascent.  相似文献   

20.
The estimation of the fluid/rock (W/R) ratio during serpentinization on the basis of oxygen isotope characteristics is peculiar, because this process is accompanied by not only changes in the stoichiometric proportions of oxygen in fluid and rock, but also by the formation of associated minerals. These factors should be taken into account for environments when the volume of aqueous fluid is limited, for instance, for serpentinization of the deep-seated rocks of oceanic lithosphere under low spreading rates. We studied isotope characteristics of samples collected in dives of submersible MIR during Cruise 50 of the R/V Akademik Mstislav Keldysh along vertical profile on the southern slope of the Atlantis Massif, which hosts the Lost City hydrothermal field. Almost all studied serpentinites have homogenous strontium isotope composition corresponding to the composition of the modern seawater. Oxygen isotope composition of these serpentinites shows systematic variations from 2. 6 to 6.1‰ with sampling depth, which indicates the preservation of stratigraphic position of samples in the sequence of the Atlantis Massif and the global serpentinization of the entire plutonic sequence. The value of the fluid–rock ratio during serpentinization in a system closed to fluid was estimated using the dissolution–crystallization model. This model takes into account the variable stoichiometry of oxygen and the effect of the simultaneous crystallization of brucite on the oxygen isotope composition of newly formed serpentine. The results show that at moderately elevated temperatures (≈300°C) and 0.1 < W/R < 5, fluid, crystallizing serpentine, and brucite are characterized by sharp variations in oxygen isotope composition: 1.3–7.8, 2.5–8.9, and 4.5–1.9‰, respectively. The model explains the observed range of δ18O in the serpentinized harzburgites of the Atlantis Massif. According to our estimates, the rocks of the studied sequence of the Atlantis Massif were serpentinized at 270–350°C and W/R = 0.7–3. For lower temperature serpentinization, for instance, at T = 250°C, the W/R ratio can be as high as 6. The present-day serpentinization of the deepseated zones of the Atlantis Massif with the Lost City fluid participance proceeds at T > 270°C and W/R ratio <1. These conditions are similar to those of serpentinization of harzburgites from the lower parts of the studied sequence of the Atlantis Massif.  相似文献   

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