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1.
Perturbation on the simplex is an operation which can be used to numerically describe changes in the composition of, for example, soils, sediments, or rocks. The combination of perturbation and power transformation provides a strong tool for analyzing compositional linear processes in the simplex. When the process is constrained in the sense of a well-known starting (or final) composition, noncentred principal component analysis can be used to estimate the leading perturbation vector of the process. Applying these mathematical tools to chemical major element data from a weathering profile developed on granitoid rocks allows us to model the compositional changes associated with the process of chemical weathering. The comparison of these results with the compositional linear trend defined by erosional products of several of the world's major drainage systems yields close similarities. The latter observation allows for a mathematical formulation of a global mean weathering trend within the system Al2O3–CaO– Na2O– K2O. We further demonstrate the usefulness of the approach for validating processes behind individual trends and for combining the effects of different processes which modify the composition of soils, sediments, and rocks. Alternatives to the Chemical Index of Alteration (CIA) are discussed to obtain a translation-invariant scale for the process of chemical weathering.  相似文献   

2.
本文用电子探针对岩浆和变质重结晶锆石进行了成份分带的研究，结果表明，变质重结晶锆石具有从晶体中心至边缘Ｐ２Ｏ５含量增大的特点，可作为它们的成因标型特征，岩浆锆石则Ｐ２Ｏ５含量变化不稳定，本文论述了它们的成因。  相似文献   

3.
刘鑫  汤艳杰 《岩石学报》2018,34(11):3315-3326
冀西北姚家庄存在一套晚三叠世的超镁铁岩-正长岩杂岩体,岩体内发育具有环带特征的单斜辉石。辉石的环带有两种:简单环带和复杂环带。简单环带一般为正环带,辉石核部的MgO和Cr2O3含量高,Si O2、Fe O和Na2O含量低;边部的主要氧化物含量与核部刚好相反。简单正环带可以分为两类,其中核边接触带平滑、由核到边化学成分具有渐变特征的正环带辉石可能是岩浆在分离结晶或地壳混染过程中形成。而核边接触带有熔蚀结构、由核到边化学成分突变的正环带辉石可能是早期结晶的辉石颗粒受到晚期低镁岩浆的溶蚀改造而成的。复杂环带具有核-幔-边结构,其中,核部低镁高铁、幔部高镁低铁、边部与核部相似,但其Mg#更低,这些特征暗示了岩浆混合作用的存在,形成辉石核部的母岩浆可能来自富集的岩石圈地幔,幔部高Mg#的特征指示了软流圈地幔物质的贡献,其边部低Mg#的特征则指示了地壳物质的加入。具有韵律环带的复杂辉石是在岩浆多期侵入的过程中形成的。辉石环带的组成特征表明,姚家庄杂岩体是由岩浆多期侵位形成的,后期侵入的岩浆与前期就位的岩浆不断反应,形成了具有多种不同环带特征的辉石,并最终形成了空间上由外到内依次为辉石岩、辉石正长岩和正长岩的环状杂岩体。结合前人的研究成果,推测形成姚家庄岩体的岩浆主要来源于富集的岩石圈地幔,并由少量地壳组分和软流圈物质的贡献。  相似文献   

4.
Oscillatory zoning in low δ18O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre‐scale oscillatory zoning of varying grossular–andradite composition (XAdr = 0.13–0.36). The δ18O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with XAdr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ18O indicate that a high Fe3+/Al, high δ18O fluid mixed with a lower Fe3+/Al and δ18O fluid. The high δ18O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ18O fluids were meteoric in origin. These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane.  相似文献   

5.
Study of micro-area chemical compositions indicates that phengite in albite gneiss from hole ZK2304 of the Donghai region has evident compositional zoning. SiO2 and tetrahedrally coordinated Si contents decrease, and Al2O3, AlIV and AlVI contents increase gradually from core to rim. However, K2O, MgO and FeO contents basically remain unchanged from core to rim. According to P-T estimates obtained from geothermometers and barometers, combined with previous experimental data, the core belt (micro-area I) of phengite was formed at T=637-672℃ and P=1.55-1.73 GPa, and the transitional belt (micro-area II) of the phengite were formed at r=594-654℃ and P=1.35-1.45 GPa. Towards the rim belt (micro-area III), the temperature decreased slightly, but the pressure decreased rapidly with r=542-630℃ and P=1.12-1.19 GPa. The P-T evolution path recorded by the compositional zoning of phengite is characterized by significant near-isothermal decompression, revealing that the gneiss has. undergone high-pressure-ultrahi  相似文献   

6.
Wadi Sifein podiform chromite deposits, Central Eastern Desert of Egypt, are hosted by fully serpentinized peridotite that is a part of the dismembered Pan‐African ophiolite complexes. Relics of primary minerals and the chemical characters indicate that the ophiolitic rocks were derived from depleted mantle peridotite of harzburgite and subordinate dunite compositions. The mantle rocks were initially formed at a mid‐oceanic ridge and subsequently thrust at a supra‐subduction zone. The chromite mineralization at Wadi Sifein area displays either pod‐shaped bodies with massive and lumpy chromitite appearance or dissemination of chromian‐spinel in serpentinite matrix. The podiform chromitite exhibits a very limited compositional range in terms of Cr# [Cr/(Cr + Al) atomic ratio] and Mg# [Mg/(Mg + Fe) atomic ratio]. The chromian‐spinel, however, frequently displays optical and geochemical zoning. Four zones can be identified from core to edge: inner core representing the original composition of the chromian‐spinel; narrow Cr‐rich ferritchromit zone; wide ferritchromit zone; and outer Cr‐magnetite/magnetite zone. The zonation of chromian‐spinel is interpreted to be a result of serpentinization rather than magmatic or metamorphic processes. The geochemical data obtained from the chromitite and chromian‐spinel was statistically processed using discriminant and R‐mode factor analyses. Two trends, minor and major, were achieved considering the formation of ferritchromit. The minor trend is controlled by the redistribution of trivalent cations, where Cr2O3 increased on the expense mainly of Al2O3 and to less extent Fe2O3 to form zone II during the peak of serpentinization. The major trend of alteration, however, is explained by the exchange between Mg‐Fe2+ rather than Cr, Al, and Fe3+ to form zone III. Kammererite formation was accompanied the formation of zones III and IV at a 314°C temperature of formation.  相似文献   

7.
《Journal of Structural Geology》2002,24(6-7):1109-1123
This study uses compositional analyses of garnet porphyroblasts to test a previously published microstructure-based model of garnet growth in the Fleur de Lys Supergroup. X-ray maps reveal significant compositional anomalies within garnet, including zoning reversals and steepened compositional gradients. These anomalies occur at the margin of the proposed first stage of garnet growth (G1), and coincide with truncations of inclusion trails and changes in the inclusion assemblage. Intervals of reversed composition zoning and steepened compositional gradients across this boundary are interpreted to represent a hiatus in garnet growth, possibly accompanied by garnet consumption, during which changes in the garnet-forming reaction, P–T conditions and deformation kinematics occurred. The junction of the proposed second and third stages of garnet growth (G2 and G3) coincides with the transition between successive crenulation cleavages, without substantial microstructural truncations or changes in the inclusion assemblage. The G2–G3 boundary is generally marked by uninterrupted normal zoning, with subtle compositional anomalies in some samples. This boundary may in fact record continuous garnet growth, or alternatively mark a relatively short intra-orogenic pause in garnet growth with minimal affect on zoning patterns. Individual porphyroblasts with contrasting inclusion trail microstructures also have different zoning patterns, and this supports the previous recognition of contrasting growth histories between individual porphyroblasts. A combined structural–metamorphic model is presented that integrates the timing of garnet growth and foliation development, reaction history and the P–T–t path in the Fleur de Lys Supergroup.  相似文献   

8.
Geochemical and field data for the Cordillera Paine (CP) pluton of southern Chile, indicate that differentiation took place by closed system in situ fractional crystallization. Minor, local and irregular separation of liquids from crystals led to the formation of evolved granites and aplites which are encountered mostly at the plutons roof and margins. Chemical trends show strong depletions of Sr, Ba, Mg less intense depletions of Ca, La, Ce, Nd, Fe, Ti, Al and enrichment of Nb, Y, Th, Rb and Si with differentiation. Pronounced crystal zoning of Ca, Sr and Ba in plagioclase, Ba in orthoclase and LREE, Y and Th in allanite closely correspond to the whole rock chemical variation. The crystal zoning data suggest that surface equilibrium only was maintained for the zoned elements during crystallization. Thus, continuous separation of liquids from crystals was not necessary to generate the kind highly evolved differentiates whose character reflects fractional crystallization. The schedule of liquid-crystal separation affects mainly the location, degree of dispersion and relative abundance of the differentiates. The homogeneity of the CP pluton and the intense crystal zoning suggest that crystal-liquid separation was inefficient, and that whole rock compositions approach liquid compositions. Assumption of a closed system during crystallization allows estimation of mineral/melt partition coefficients (K d s) using crystal core and whole rock compositions. Crystal zoning and whole rock chemical trends are consistent with models constructed using the K d s thus obtained along with modal abundances from petrographic estimates. Lamont-Doherty Geological Observatory Contribution Number 3701  相似文献   

9.
Fast diffusion along mobile grain boundaries in calcite   总被引：1，自引：0，他引：1  
Experimental measurements of grain boundary diffusion are usually conducted on static boundaries, despite the fact that grain boundaries deep in the Earth are frequently mobile. In order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of 44Ca from a layer of 44Ca-enriched calcite powder during the static recrystallization of a single crystal of calcite at 900°C. A region about 500 μm wide adjacent to the powder layer is heterogeneously enriched in 44Ca, and complex zoning patterns, including sharp steps in composition and continuous increases and decreases in 44Ca content, are developed. In metamorphic rocks, these would normally be interpreted in terms of changes in pressure or temperature, Rayleigh fractionation, or episodic fluid infiltration. These explanations cannot apply to our experiments, and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate. We have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width (D GB δ) to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer. The value of D GB δ in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions. In order to allow the scale of chemical equilibrium (and hence textural evolution) to be predicted under both experimental and geological conditions, we present quantitative diffusion-regime maps for static and mobile boundaries in calcite, using both published values and our new values for grain boundary diffusion in mobile boundaries. Enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of Earth materials, for geochronology, and for interpretations of the length- and time-scales of chemical mass-transport. Moreover, zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones, rather than regions of anomalous mineralogy, water- or melt-content as is generally suggested.  相似文献   

10.
2 O3, TiO2, Zr, and Nb abundances in shales from all the units, strongly suggest a gradual compositional change within this sedimentary succession. Together with the petrological data, the chemical results do not reveal any obvious coeval volcanic contribution to the sediments. On the basis of the chemical data, a comparison is made with other European zones containing detrital sediments composed of reworked crustal components. Received: 4 January 1999 / Accepted: 1 December 1999  相似文献   

11.
The character of oscillatory zoning within a zircon crystal from the syenite Kipawa Complex, Quebec, varies with scale of observation. Analysis of an scanning electron microscopy (SEM) back-scatter gray-scale traverse at a resolution of one pixel = 2.43 μm revealed 145 zones over 5130 μm, whereas a detailed high-resolution (one pixel = 0.195 μm) section near the crystal rim revealed 225 zones over 795 μm. In order to mathematically characterize the zoning pattern, wavelet, Fourier, and nonlinear analysis techniques were used on profiles of the SEM gray-scale data, and a series constructed was from the zone widths. Results demonstrate that the zircon oscillatory zoning preserves nonlinear and periodic components. Secondary ion mass spectrometry, electron microprobe, and SEM analyses of trace elements show the SEM back-scatter bright zones are enriched in U, Th, and rare earth elements (REE) in comparison to the darker zones. REE patterns are sharply heavy REE enriched and have negative Eu anomalies and prominent positive Ce anomalies. We model the oscillatory zoning, including a measure of its chemical variation, by use of a periodically forced nonlinear system. Results of this data-driven model are quantitatively similar to the natural data. We envisage that the small-scale oscillatory zoning was the result of a nonlinear feedback process wherein the crystal growth modified the adjacent melt, which in turn affected the crystal composition. The large-scale harmonic zones likely reflect changes in the bulk geochemistry of the system from which the zircon grew.  相似文献   

12.
Developments in the statistical analysis of compositional data over the last two decades have made possible a much deeper exploration of the nature of variability and the possible processes associated with compositional data sets from many disciplines. In this paper, we concentrate on geochemical data. First, we explain how hypotheses of compositional variability may be formulated within the natural sample space, the unit simplex, including useful hypotheses of sub-compositional discrimination and specific perturbational change. Then we develop through standard methodology, such as generalised likelihood ratio tests, statistical tools to allow the systematic investigation of a lattice of such hypotheses. Some of these tests are simple adaptations of existing multivariate tests but others require special construction. We comment on the use of graphical methods in compositional data analysis and on the ordination of specimens. The recent development of the concept of compositional processes is then explained, together with the necessary tools for a staying-in-the-simplex approach, such as the singular value decomposition of a compositional data set. All these statistical techniques are illustrated for a substantial compositional data set, consisting of 209 major oxide and trace element compositions of metamorphosed limestones from the Grampian Highlands of Scotland. Finally, we discuss some unresolved problems in the statistical analysis of compositional processes.  相似文献   

13.
Miocene Ignimbrite ‘A’ on Gran Canaria containsthree compositional endmember fiamme types(two rhyolites andone trachyte) each of which crystallized distinct feldspar.Various textural and compositional criteria are interpretedas reflecting a complex scenario within the magma chamber inwhich the crystals formed. About 25–30% of the feldsparphenocrysts contain evidence for magma mixing in the form of(1) partial to severe dissolution–resorption rims, (2)distinct zones of drastically different compositions and (3)overgrowth textures on formerly resorbed crystals. Four majortypes of zoning in the oligoclase to anorthoclase feldsparsof ignimbrite ‘A’ include a normal and a reverselyzoned type and two complexly zoned types. The feldspars withnormal and reverse zonation show only minor compositional amplitudesbetween individual zones (  相似文献   

14.
Spectacular reaction textures in poikiloblastic scapolitite boudins, within marbles in the continental crust exposed in the Lützow–Holm Complex, East Antarctica, provide insights into the changing fluid composition and movement of fluid along grain boundaries and fractures. Petrographic and geochemical features indicate scapolite formation under contrasting fluid compositions. Core composition of scapolite poikiloblasts (ScpI) are marialitic (Cl = 0.7 apfu) whereas rims in contact with biotite or clinopyroxene are meionite rich. Fine‐grained recrystallized equigranular scapolite (ScpII) shows prominent chemical zoning, with a marialitic core and a meionitic rim (Cl = 0.36 apfu). Scapolite poikiloblasts are traversed by ScpIII reaction zones along fractures with compositional gradients. Pure CO2 fluid inclusions are observed in healed fractures in scapolite poikiloblasts. These negative crystal‐shaped fluid inclusions are moderately dense, and are believed to be coeval with ScpIII formation at temperatures >600 °C and a minimum pressure of c. 3.8 kbar. Grain‐scale LA‐ICPMS studies on trace and rare earth elements on different textural types of scaplolites and a traverse through scapolite reaction zone with compositional gradient suggest a multistage fluid evolution history. ScpI developed in the presence of an internally buffered, brine‐rich fluid derived probably from an evaporite source during prograde to peak metamorphism. Recrystallization and grain size reduction occurred in the presence of an externally sourced carbonate (CaCO3)‐bearing fluid, resulting in the leaching of Cl, K, Rb and Ba from ScpI along fractures and grain boundaries. Movement of fluids was enhanced by micro‐fracturing during the transformation of ScpI to ScpIII. Fractures in fluorapatite are altered to chlorapatite proving evidence for the pathways of escaping Cl‐bearing fluids released from ScpI. The present study thus provides evidence for the usefulness of scapolite in fingerprinting changing volatile composition and trace element contents of fluids that percolate within the continental crust.  相似文献   

15.
Isotope, trace element, and textural crystal zoning patterns record heterogeneity in magmatic systems not resolved by whole rock analyses. These zoning data are used to infer crystal residence times, magma mixing, and other magmatic processes in many magmatic systems. We present the shared characteristic diagram (SCD) as an organizational framework for crystal zoning data that compares information from different phases and chemical tracers in a common framework. An example from Chaos Crags in the Cascade arc, produces three main results. (1) Anorthite zoning profiles in plagioclase have fewer shared characteristics in mafic inclusions than in the host rhyodacite. (2) Single-crystal 87Sr/86Sr data from previous studies (Tepley et al. 1999) are consistent with more shared history between crystals than in anorthite profiles. This difference reflects a more homogeneous distribution of 87Sr/86Sr than the intensive parameters controlling plagioclase composition. (3) The Chaos Crags system exhibits a layer of heterogeneity in crystal populations that is not represented in whole-rock analyses that indicate only simple binary mixing. The inconsistency between 87Sr/86Sr and anorthite zoning data highlights decoupling between compositionally controlled and temperature/water-controlled zoning in plagioclase from Chaos Crags.  相似文献   

16.
《Applied Geochemistry》2002,17(6):683-698
The compositional variability of the phenocryst-poor member of the 12.8 Ma Topopah Spring Tuff at the potential repository level was assessed by duplicate analysis of 20 core samples from the cross drift at Yucca Mountain, Nevada. Previous analyses of outcrop and core samples of the Topopah Spring Tuff showed that the phenocryst-poor rhyolite, which includes both lithophysal and nonlithophysal zones, is relatively uniform in composition. Analyses of rock samples from the cross drift, the first from the actual potential repository block, also indicate the chemical homogeneity of this unit excluding localized deposits of vapor-phase minerals and low-temperature calcite and opal in fractures, cavities, and faults. The possible influence of vapor-phase minerals and calcite and opal coatings on rock composition at a scale sufficiently large to incorporate these heterogeneously distributed deposits was evaluated and is considered to be relatively minor. Therefore, the composition of the phenocryst-poor member of the Topopah Spring Tuff is considered to be adequately represented by the analyses of samples from the cross drift. The mean composition as represented by the 10 most abundant oxides in wt.% or g/100 g is: SiO2, 76.29; Al2O3, 12.55; FeO, 0.14; Fe2O3, 0.97; MgO, 0.13; CaO, 0.50; Na2O, 3.52; K2O, 4.83; TiO2, 0.11; and MnO, 0.07.  相似文献   

17.
The compositional zoning of almandine-spessartine garnet has been studied in pelitic gneisses from the Kemiö region, SW Finland. Garnets with inverse zoning (outward increase in Mn) abound in the low-grade musc+qz zone. Garnets with inverse zoning, normal zoning (outward decrease in Mn), and with a homogeneous composition occur in the higher-grade sill+ Kfs zone. The formation of garnet with inverse zoning is explained by continuous retrograde reactions in the model system K2O-Al2O3-FeO-MnO-MgO-SiO2-H2O at strongly reduced partial water pressures.A comparison of temperatures based on critical metamorphic reactions and on garnet-biotite thermometry shows the usefulness of geothermometry in rocks with spessartine-rich garnet and Mn-bearing biotite.  相似文献   

18.
Zoning of phosphorus in igneous olivine   总被引：2，自引：2，他引：0  
We describe P zoning in olivines from terrestrial basalts, andesites, dacites, and komatiites and from a martian meteorite. P2O5 contents of olivines vary from below the detection limit (≤0.01 wt%) to 0.2–0.4 wt% over a few microns, with no correlated variations in Fo content. Zoning patterns include P-rich crystal cores with skeletal, hopper, or euhedral shapes; oscillatory zoning; structures suggesting replacement of P-rich zones by P-poor olivine; and sector zoning. Melt inclusions in olivines are usually located near P-rich regions but in direct contact with low-P olivine. Crystallization experiments on basaltic compositions at constant cooling rates (15–30°C/h) reproduce many of these features. We infer that P-rich zones in experimental and natural olivines reflect incorporation of P in excess of equilibrium partitioning during rapid growth, and zoning patterns primarily record crystal-growth-rate variations. Occurrences of high-P phenocryst cores may reflect pulses of rapid crystal growth following delayed nucleation due to undercooling. Most cases of oscillatory zoning in P likely reflect internal factors whereby oscillating growth rates occur without external forcings, but some P zoning in natural olivines may reflect external forcings (e.g., magma mixing events, eruption) that result in variable crystal growth rates and/or P contents in the magma. In experimental and some natural olivines, Al, Cr, and P concentrations are roughly linearly and positively correlated, suggesting coupled substitutions, but in natural phenocrysts, Cr zoning is usually less intense than P zoning, and Al zoning weak to absent. We propose that olivines grow from basic and ultrabasic magmas with correlated zoning in P, Cr, and Al superimposed on normal zoning in Fe/Mg; rapidly diffusing divalent cations homogenize during residence in hot magma; Al and Cr only partially homogenize; and delicate P zoning is preserved because P diffuses very slowly. This interpretation is consistent with the fact that zoning is largely preserved not only in P but also in Al, Cr, and divalent cations in olivines with short residence times at high temperature (e.g., experimentally grown olivines, komatiitic olivines, groundmass olivines, and the rims of olivine phenocrysts grown during eruption). P zoning is widespread in magmatic olivine, revealing details of crystal growth and intra-crystal stratigraphy in what otherwise appear to be relatively featureless crystals. Since it is preserved in early-formed olivines with prolonged residence times in magmas at high temperatures, P zoning has promise as an archive of information about an otherwise largely inaccessible stage of a magma’s history. Study of such features should be a valuable supplement to routine petrographic investigations of basic and ultrabasic rocks, especially because these features can be observed with standard electron microprobe techniques.  相似文献   

19.
Back-scattered electron (BSE)-derived zoning patterns of plagioclase phenocrysts are used to identify magma processes at Bezymianny Volcano, Kamchatka, based on the 2000–2007 sequence of eruptive products. The erupted magmas are two-pyroxene andesites, which last equilibrated at ~915°C temperature, 77–87 MPa pressure, and a water content of ~1.4 wt%. Textural and compositional zoning of individual plagioclase phenocrysts typically includes a repeated core-to-rim sequence of oscillatory zoning (An50–60) truncated by a dissolution surface followed by an abrupt increase in An content (up to An85), which then gradually decreases rimward. This zoning pattern is interpreted to be the result of frequent replenishments of the magma chamber which cause both thermal and chemical interaction between resident and recharge magmas. The outermost 70- to 150-μm-wide zoning patterns of plagioclase phenocrysts are composed of dissolution surface with a subsequent increase in An and Fe contents. Zoning patterns of the rims exhibit correlation among plagioclase phenocrysts within one eruption. Rims are interpreted as a result of crystallization of a batch of magma in the conduit after recharge event.  相似文献   

20.
Metasomatic reaction zones which developed at marble-pelitic schist contacts in a granulite facies terrane in West Greenland contain a consistent sequence of five mineralogical zones. Outward from the carbonates the zones are characterized by the assemblages grossular-diopside-meionite (I), meionite-anorthite-diopside (II), anorthite-diopside-edenitic hornblende (III), anorthite-enstatite (IV), plagioclase-almandine-sillimanite (V). Sphene is superceded by ilmenite between zones (II) and (III); quartz is present in all zones except zone I. Scapolite, plagioclase, clinopyroxene and mica exhibit a small degree of compositional variation which correlates with distance from the carbonate. These small compositional variations are superimposed on a strong CaO chemical potential gradient. Compositional features, zone distributions and CaO activity calculations demonstrate that the zones developed in response to CaO diffusion along a chemical potential gradient of 2 kcal/m. The CaO source appears to be carbonate rocks which release calcium as decarbonation reactions proceed. The maximum volume of CO2 released in this process, and that released during discontinuous reactions in the marbles, will contribute a total volume of CO2 approximately equivalent to the volume of carbonate in the rock. Calculations demonstrate that a terrane consisting of as little as 8% carbonate will release sufficient CO2 to result in complete dehydration of an amphibolite terrane, at deep crustal conditions. Dehydration through CO2 release will be accomplished either through rapid burial, which would prevent both equilibration of mineral assemblages and CO2 release at intermediate crustal levels, or through diffusion-driven metasomatic reactions which would lead to CO2 release primarily at the high temperatures of deep crustal environments. The latter process would be the dominant CO2 source at deep crustal levels if carbonate rocks occur predominately as relatively thin layers.  相似文献   

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