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1.
Results of a survey of experimental data on trace element partitioning between crystals and coexisting liquid and between crystals and coexisting aqueous fluid are reported. These studies involve combinations of elements such as Cs, Rb, K, Na, Li, Ba, Sr, Ni and Sm dissolved in minerals such as feldspars, micas, feldspathoids, garnets, pyroxenes, amphibole and olivine. It is shown that the concentration ranges of these elements in minerals with solution behavior according to Henry's law appear to extend to less than 1 wt.% and often to less than 100 ppm of the element in the crystalline phase.The available data indicate that the concentration ranges of solution in minerals according to Henry's law can be positively correlated with the difference of ionic radii between trace element and the host element for which the tracer is assumed to substitute. Furthermore, studies involving crystals and coexisting aqueous fluid have been used to determine whether Henry's law or Raoult's law adequately describes the solution. Based on the assumption that the aqueous fluid solution can be considered ideal, models have been proposed stating that if the difference between the ionic radii of the tracer and the host element in the crystalline phase is less than about 10% (relative to the host element), the solution is ideal (Raoult's law). With larger differences between the ionic radii, there is an excess free energy of mixing, although Henry's law is still obeyed in the concentration ranges where crystalliquid or crystal-fluid partition coefficients are independent of element concentration.  相似文献   

2.
Partitioning of Rb, Sr and Ba between alkali feldspar and a synthetic granitic melt has been determined at 8 kb and 720 to 780°C for a single quaternary granite composition. The results suggest that Henry's law is obeyed by Rb up to ~0.8 wt.% Rb2O in both the liquid and in the alkali feldspar. The measured D values for Rb range from 0.77 to 1.1. For Ba, Henry's Law is obeyed up to ~0.6 wt.% BaO in the liquid and ~5 wt.% BaO in the alkali feldspar. D values for Ba range from 6.4 to 14. For Sr there is only a crude relationship between concentration in the liquid and concentration in the alkali feldspar at concentrations greater than ~0.6 wt.% SrO in the liquid and ~0.4 wt.% SrO in the alkali feldspar. D values for Sr range from 1.2 to 5.0. Partitioning of Sr is apparently sensitive to the concentration of Ba in the system and this partly explains the failure of Sr to obey Henry's Law.Linear least-squares fits to the partitioning data as a function of temperature suggest inverse correlation between temperature and D values. Rb shows only a slight temperature effect whereas Ba and Sr appear to be rather strongly affected by temperature, but the temperature range examined here is small compared to the scatter in the data making these trends relatively uncertain. Other factors that appear to affect partitioning, especially of Sr, are growth rate, development of sector zoning and Or content of the alkali feldspar. These factors severely limit the use of partitioning of these elements in alkali feldspar as geothermometers.The technique for measuring growth rates utilized here combined with measurement of trace element depletion in diffusion boundary layers adjacent to the alkali feldspar crystals makes it possible to estimate diffusivities for Ba and Sr. These estimates suggest a difference of 2 orders of magnitude between diffusivities for Ba and Sr in a vapor-saturated melt and those measured by HOFMANN and MAGARITZ (1976) for a dry obsidian glass.  相似文献   

3.
Although numerous questions still surround the topic of Henry's law (HL) as it applies to trace element partitioning, there now exist sufficient experimental data to make some generalizations regarding HL behavior in minerals. The most important of these is that the commonly-observed failure of HL at low concentration occurs at distinctly different levels even for chemically-similar elements in a single mineral. This observation in turn provides a basis for discerning effects of HL failure in natural systems: through examination of element ratios in minerals and rocks, it is possible, in principle, to distinguish HL effects from changes in partition coefficients due to variations in other magmatic parameters such as temperature and the compositions of phases. Initial applications of this approach to plagioclase/ liquid partitioning of REE and to the general behavior of ZrHf and BaRb during basalt production suggest that HL usually does hold in nature.  相似文献   

4.
Interrelationships between chemical compositions and crystal structures of minerals pioneered by Goldschmidt have been overlooked by modern geochemists. While analytical techniques and data for trace elements have become more sophisticated, progress has been slow in relating abundance data of individual elements to their relative enrichments in specific sites in mineral structures. The concept of diadochy has degenerated into an analytical relationship between major and trace elements, instead of its original crystallographic basis of replacement of one atom by another at a specific site in a crystal structure. Future interpretations of trace element data must consider the partitioning of atoms between different coordination sites in multisite mineral structures.The well-known partitioning of nickel and chromium into the earliest minerals during magmatic crystallization is extended to magma evolution in the mantle. The strong preference of Ni2+ and Cr3+ for octahedral sites in minerals leads to their enrichment in certain Iherzolites which are refractory residua during partial fusion of the mantle.  相似文献   

5.
We present detailed experimental results on the partitioning of rare earth elements (REE) between titanite and a range of different silicate melts. Our results show that Henry’s law of trace element partitioning depends on bulk composition, the available partners for heterovalent substitution, crystal composition, and melt composition. We illustrate that the partition coefficients for Sm depend very strongly on the bulk concentration of Sm in the system. The substitution mechanism, by which rare earth elements are incorporated into the crystal structure, plays an important role for trace element partitioning and also for the onset of Henry’s law. Our data show that there are clear differences between substitution mechanisms of major elements compared to elements which are present only as traces. Our experiments also clearly show that the onset of Henry’s law depends on the concentrations of the sum of all trace elements which are incorporated into the crystal by the same substitution mechanism. For geochemical modelling of magmatic processes involving titanite, and indeed other accessory phases, it is of crucial importance to first evaluate whether the REE, and other trace elements, are present as traces or as major elements, only then appropriate D values may be chosen.  相似文献   

6.
To better understand the partitioning behavior of elements during the formation and evolution of iron meteorites, two sets of experiments were conducted at 1 atm in the Fe-Ni-P system. The first set examined the effect of P on solid metal/liquid metal partitioning behavior of 22 elements, while the other set explored the effect of the crystal structures of body-centered cubic (α)- and face-centered cubic (γ)-solid Fe alloys on partitioning behavior. Overall, the effect of P on the partition coefficients for the majority of the elements was minimal. As, Au, Ga, Ge, Ir, Os, Pt, Re, and Sb showed slightly increasing partition coefficients with increasing P-content of the metallic liquid. Co, Cu, Pd, and Sn showed constant partition coefficients. Rh, Ru, W, and Mo showed phosphorophile (P-loving) tendencies. Parameterization models were applied to solid metal/liquid metal results for 12 elements. As, Au, Pt, and Re failed to match previous parameterization models, requiring the determination of separate parameters for the Fe-Ni-S and Fe-Ni-P systems.Experiments with coexisting α and γ Fe alloy solids produced partitioning ratios close to unity, indicating that an α versus γ Fe alloy crystal structure has only a minor influence on the partitioning behaviors of the trace element studied. A simple relationship between an element’s natural crystal structure and its α/γ partitioning ratio was not observed. If an iron meteorite crystallizes from a single metallic liquid that contains both S and P, the effect of P on the distribution of elements between the crystallizing solids and the residual liquid will be minor in comparison to the effect of S. This indicates that to a first order, fractional crystallization models of the Fe-Ni-S-P system that do not take into account P are appropriate for interpreting the evolution of iron meteorites if the effects of S are appropriately included in the effort.  相似文献   

7.
微量元素在幔源矿物与热液之间分配系数的差异是造成地幔不均一的一个重要因素，对于认识地球演化、元素的分异和板块构造具有重要的意义。热液的组成、矿物的结构、温度、压力以及氧逸度都与分配系数密切相关。不同微量元素在相同矿物或热液中的分配系数存在差别，而相同的元素在不同矿物中的分配系数也可能出现很大的变化，这是研究微量元素分异和不同地幔端员形成的理论基础。在较低的温度和压力下，热液的组成对分配系数的影响很大，随着温度和压力的升高，热液组成的影响逐渐减弱，而矿物的组成与结构的影响逐渐增大。由于分配系数影响因素的复杂性，因此在考虑地球深部微量元素的迁移和分异时需慎重对待。  相似文献   

8.
Hydrous high-pressure veins formed during dehydration of eclogites in two paleo-subduction zones (Trescolmen locality in the Adula nappe, central Alps and Münchberg Gneiss Massif, Variscan fold belt, Germany) constrain the major and trace element composition of solutes in fluids liberated during dehydration of eclogites. Similar initial isotopic compositions of veins and host eclogites at the time of metamorphism indicate that the fluids were derived predominantly from the host rocks. Quartz, kyanite, paragonite, phengite, zoisite and omphacite are the dominant minerals in the veins. The major element compositions of the veins are in agreement with experimental evidence indicating that the composition of solutes in such fluids is dominated by SiO2 and Al2O3. Relative to N-MORB, the veins show enrichments of Cs, Rb, Ba, Pb, and K, comparable or slightly lower abundances of Sr, U, and Th, and very low abundances of Nd, Sm, Zr, Nb, Ti and Y. The differential fractionation of highly incompatible elements such as K, U and Th in the veins, as well as the presence of hydrous minerals in the eclogites rule out partial melting as a cause for vein formation. These results confirm previous suggestions that fluids derived from subducted basalt may have low abundances of high field strength elements, rare earth elements and Y. Variable vein-eclogite enrichment factors of incompatible alkalis and to a lesser extent Pb appear to reflect mineralogical controls (phengite, epidote-group minerals) on partitioning of these elements during dehydration of eclogite in subduction zones. However, abundance variations of incompatible elements in minerals from eclogites suggest that the composition of fluids released from eclogites at temperatures <700°C may not reflect true equilibrium partitioning during dehydration. Simple models for the trace elements U and Th indicate the relative importance of the basaltic and sedimentary portions of subducted oceanic crust in producing the characteristic chemical signatures of these elements in convergent plate margin volcanism.  相似文献   

9.
Surface enrichment and trace-element uptake during crystal growth   总被引：2，自引：0，他引：2  
Adsorption or enrichment of trace elements in the near-surface or interfacial regions of minerals has been documented in systems ranging from low-temperature aqueous environments to magmas. Under static conditions, this surface enrichment results from chemical equilibrium between the host medium of the crystal and the surface layer, which may exhibit diversity or flexibility in the types of atomic sites present. If the crystal is growing, any trace elements bound in the surface layer may be buried and trapped in the newly-formed lattice, resulting in lattice concentrations that deviate substantially from those predicted by equilibrium partitioning between the crystal and its growth medium. The effectiveness of this growth entrapment process depends upon the interplay between the growth rate, V, of the crystal (which can be thought of as the burial rate of surface-enriched element i) and the diffusivity, Di , in the near-surface region of the crystal (which determines how efficiently i can escape to the surface by diffusion). The competition can be quantified in terms of the dimensionless number Vl/Di; (termed the growth Péclet number, Pe), where l is the half-thickness of the surface-enriched layer. Depending upon the degree of surface enrichment, some growth entrapment is possible if Pe > 0.1, and the process is highly efficient if Pe > 10. An assessment of Pe for a wide variety of geological situations suggests that growth entrapment is common in diagenetic and metamorphic environments but limited to slow lattice diffusants in silicic igneous systems. It is probably rare to nonexistent in basaltic systems except at laboratory growth rates.  相似文献   

10.
The distribution of Mg, Mn, Fe, Co and Ni among olivine, orthopyroxene, calcic clinopyroxene and liquid can be described by exchange reactions of the form: M (phase A) + Mg (phase B) = M (phase B) + Mg (phase A). The thermochemical data predict the observed partitioning within the limits of error of both sets of data when assumptions of ideal solid solution are used, except for Mn-Mg exchange. Because ΔS and ΔV for these exchange reactions are generally small, KD varies relatively little with temperature and pressure, although individual ion distribution coefficients (D values) are more sensitive to these variables and to changing liquid composition.Trace elements present at the ppm level can enter crystals in normal lattice sites, can participate in point defect equilibria and can enter various defect sites present metastably in the crystal as a result of its P,T history. Metastable defect equilibria, in which the number of sites (grain boundaries, surfaces, intergrowths, etc.) is fixed, can lead to enhanced trace element solubility in the crystal at very low concentrations and thus to apparent deviations from Henry's law.  相似文献   

11.
In the Earth's upper crust, where aqueous fluids can circulate freely, most mineral transformations are controlled by the coupling between the dissolution of a mineral that releases chemical species into the fluid and precipitation of new minerals that contain some of the released species in their crystal structure, the coupled process being driven by a reduction of the total free-energy of the system. Such coupled dissolution-precipitation processes occur at the fluid-mineral interface where the chemical gradients are highest and heterogeneous nucleation can be promoted, therefore controlling the growth kinetics of the new minerals. Time-lapse nanoscale imaging using Atomic Force Microscopy (AFM) can monitor the whole coupled process under in situ conditions and allow identifying the time scales involved and the controlling parameters. We have performed a series of experiments on carbonate minerals (calcite, siderite, dolomite and magnesite) where dissolution of the carbonate and precipitation of a new mineral was imaged and followed through time. In the presence of various species in the reacting fluid (e. g. antimony, selenium, arsenic, phosphate), the calcium released during calcite dissolution binds with these species to form new minerals that sequester these hazardous species in the form of a stable solid phase. For siderite, the coupling involves the release of Fe2+ ions that subsequently become oxidized and then precipitate in the form of FeIII oxyhydroxides. For dolomite and magnesite, dissolution in the presence of pure water (undersaturated with any possible phase) results in the immediate precipitation of hydrated Mg-carbonate phases. In all these systems, dissolution and precipitation are coupled and occur directly in a boundary layer at the carbonate surface. Scaling arguments demonstrate that the thickness of this boundary layer is controlled by the rate of carbonate dissolution, the equilibrium concentration of the precipitates and the kinetics of diffusion of species in a boundary layer. From these parameters a characteristic time scale and a characteristic length scale of the boundary layer can be derived. This boundary layer grows with time and never reaches a steady state thickness as long as dissolution of the carbonate is faster than precipitation of the new mineral. At ambient temperature, the surface reactions of these dissolving carbonates occur on time-scales of the order of seconds to minutes, indicating the rapid surface rearrangement of carbonates in the presence of aqueous fluids. As a consequence, many carbonate-fluid reactions in low temperature environments are controlled by local thermodynamic equilibria rather than by the global equilibrium in the whole system.  相似文献   

12.
Intergrowth defects (intercalation of different structure types) are common in some rock-forming minerals and can be demonstrated to affect trace element distributions, especially when new crystallographic site types are introduced by the intercalated structure. Atoms that do not readily substitute into the host crystal may be incorporated within the intercalated structure. Trace element partitioning patterns, EPR data and mineral dislocation densities suggest that dislocations do not exercise primary control on the partitioning of compatible trace elements, but the effects of dislocations on incompatible elements may be important.Intergrowth defects in geologically important materials include disordered intergrowth structures in Sulfides and the stacking and chain-width disorder that is found in pyroxenes and amphiboles from a range of occurrences. Intergrowth defects may range from a few Ångstroms wide to optically resolvable size. It is important to evaluate their effects when interpreting geological trace element data.  相似文献   

13.
对松多榴辉岩中单矿物进行的LA-ICP-MS原位微区微量元素分析研究结果表明,石榴石主要富集中、重稀土元素和Y,同时具有高丰度的Sc、V、Cr和Co等元素;绿辉石中的微量元素以中稀土元素、Sr、Sc、V、Cr、Co、Ni和Ti为主,含有一定量的Zr、Hf等。石榴石、绿辉石、角闪石和绿帘石中均显示轻稀土元素亏损的特点,表明在退变质过程中没有发生明显的富轻稀土元素的外来流体交代作用,因而其微量元素矿物地球化学的某些特点不同于苏鲁地区的榴辉岩。石榴石变斑晶中某些元素(如Ti、Zr)的分带性暗示了榴辉岩在紧随峰期变质之后的折返过程中发生了降压增温过程。榴辉岩主要变质矿物中微量元素的分配显然受到矿物主量元素的分配所控制,如MgO在石榴石和绿辉石之间的分配对Ni、Co、Ti分配的控制以及CaO的分配对Sr、Y、REE分配的控制等。退变质过程中矿物的形成或分解以及物理化学条件的改变都可以引起矿物间微量元素的重新分配。由绿辉石退变质而形成的角闪石,较之原先的绿辉石,其微量元素配分曲线总体特征会发生变化,但元素总体丰度相近,某些元素特点相似,又反映了绿辉石和角闪石之间的成生联系。金红石是Ti、Nb、Ta、Zr、Hf的主要赋存矿物,而与之共生的绿帘石所表现出来的高场强元素的亏损特征表明了金红石的存在所带来的影响。  相似文献   

14.
Carbon has been proposed as a potential light element in planetary cores, included in models of planetary core formation, and found in meteoritic samples and minerals. To better understand the effect of C on the partitioning behavior of elements, solid/liquid partition coefficients (D = (solid metal)/(liquid metal)) were determined for 17 elements (As, Au, Co, Cr, Cu, Ga, Ge, Ir, Ni, Os, Pd, Pt, Re, Ru, Sb, Sn, and W) over a range of C contents in the Fe-Ni-C system at 1 atm. The partition coefficients for the majority of the elements increased as the C content of the liquid increased, an effect analogous to that of S for many of the elements. In contrast, three of the elements, Cr, Re, and W, were found to have anthracophile (C-loving) preferences, partitioning more strongly into the metallic liquid as the C content increased, resulting in decreases to their partition coefficients. For half of the elements examined, the prediction that partitioning in the Fe-Ni-S and Fe-Ni-C systems could be parameterized using a single set of variables was not supported. The effects of S and C on elemental partitioning behavior can be quite different; consequently, the presence of different non-metals can result in different fractionation patterns, and that uniqueness offers the opportunity to gain insight into the evolution of planetary bodies.  相似文献   

15.
The mantles of the Earth and Moon are similarly depleted in V, Cr, and Mn relative to the concentrations of these elements in chondritic meteorites. The similar depletions have been used as evidence that the Moon inherited its mantle from the Earth after a giant impact event. We have conducted liquid metal-liquid silicate partitioning experiments for V, Cr, and Mn from 3 to 14 GPa and 1723 to 2573 K to understand the behavior of these elements during planetary core formation. Our experiments have included systematic studies of the effects of temperature, silicate composition, metallic S-content, metallic C-content, and pressure. Temperature has a significant effect on the partitioning of V, Cr, Mn, with all three elements increasing their partitioning into the metallic liquid with increasing temperature. In contrast, pressure is not observed to affect the partitioning behavior. The experimental results show the partitioning of Cr and Mn are hardly dependent on the silicate composition, whereas V partitions more strongly into depolymerized silicate melts. The addition of either S or C to the metallic liquid causes increased metal-silicate partition coefficients for all three elements. Parameterizing and applying the experimental data, we find that the Earth’s mantle depletions of V, Cr, and possibly Mn can be explained by core formation in a high-temperature magma ocean under oxygen fugacity conditions about two log units below the iron-wüstite buffer, though the depletion of Mn may be due entirely to its volatility. However, more oxidizing conditions proposed in recent core formation models for the Earth cannot account for any of the depletions. Additionally, because we observe no pressure effect on the partitioning behavior, the data do not require the mantle of the Moon to be derived from the Earth’s mantle, although this is not ruled out. All that is required to create depletions of V, Cr, and Mn in a mantle is a planetary body that is hot enough and reducing enough during its core formation. Such conditions could have existed on the Moon-forming impactor.  相似文献   

16.
The partitioning of trace elements (Sc, Ti, Sr and Sm) between diopsidic clinopyroxene and liquid was studied experimentally in the system diopside-albite-anorthite at 1250°C, 1300°C and 1345°C at 1 atm. Twelve different bulk compositions were selected to study the effects of temperature and chemical composition. A Cameca ion microprobe was used to determine trace element concentrations in both clinopyroxene and liquid. Experiments of different run duration 148? days) showed that equilibrium was approached in less than 4 days at 1275°C. Equilibrium was also evaluated by a reversal run. A series of runs of constant bulk composition but with variable trace element contents showed that Henry's Law was obeyed over concentration ranges of the trace elements similar to those encountered in natural systems. The partition coefficients show significant ranges: Sc, 0.345~2.61; Ti, 0.084~0.214; Sr, 0.075~0.136; Sm, 0.054~0.328; the values are comparable with those obtained experimentally by other investigators. The partition coefficients vary as a function of both temperature and chemical composition. The experimental results are discussed in terms of exchange equilibria using the Bottinga-Weill silicate melt model. It is demonstrated that analytical uncertainties of both major and trace elements play an important role in understanding trace element exchange equilibria; propagation of analytical errors in the thermodynamic treatment is equally important.  相似文献   

17.
In this paper, we examine the distribution of incompatible elements in Earth’s mantle based on data reported for 20 mantle xenoliths collected from 5 localities worldwide. A structural model combined with an element partitioning model forms the basis for our analyses. The former separates a bulk peridotite into mineral crystal lattices, interfaces (grain and interphase boundaries), and intra- and inter-granular inclusions as sites for incompatible elements. The latter relates the distribution of elements among these sites based on lattice strain theory. By treating both intra- and inter-granular inclusions as a melt-like phase, the combined models successfully reproduce the relative concentrations of incompatible elements among minerals, clean rock (reconstituted from mineral compositions and mineral mode), and whole rock. The analyses reveal common signatures in the rocks: (1) incompatible elements in the crystal lattices of olivine, orthopyroxene and clinopyroxene achieved chemical equilibrium. (2) Olivine, orthopyroxene and clinopyroxene grains contain similar amounts of an intra-granular, melt-like component possibly in the form of sub-micron inclusions with weight (≈volume) fractions between 5 × 10−5 and 1 × 10−2. (3) All rocks contain an inter-granular melt-like component with a fraction between 10−4 and 10−2, well above the amount expected to be stored along interfaces. (4) Fractions of the inter- and intra-granular components are positively correlated, indicating that they were originated from the same process. (5) The inter- and intra-granular melt-like phases are chemically equilibrated with other structural components. Based on plausible upwelling rates for mantle xenoliths, it is unlikely that the melt-like component formed during ascent. Instead, its ubiquitous appearance, its invisibility to optical microscopy, and its absorption of the incompatible elements in a manner similar to a melt phase even at sub-solidus condition, all might be explained by the presence of amorphous silica precipitates such as those observed previously in naturally occurring and experimentally annealed mantle composites. From the mineral mode, grain size, and mineral plus whole rock concentrations of incompatible elements, we can ascribe the chemical signatures of xenoliths to achievement of chemical equilibrium at mantle conditions rather than to a consequence of some disequilibrium (metasomatic) effect as has been done previously. Although it should be tested by additional analytical studies, our model will make it possible to determine whether or not a rock is chemically equilibrated in terms of the distribution of incompatible elements or if a metasomatic (disequilibrium) event is required.  相似文献   

18.
The transfer of fluid and elements from subducting crust to the overlying mantle wedge is a fundamental process affecting arc magmatism and the chemical differentiation of the Earth. While the production of fluid by breakdown of hydrous minerals is well understood, the liberation of trace elements remains generally unconstrained. In this paper, we evaluate the behaviour of trace elements during prograde metamorphism and dehydration using samples of high-pressure, low-temperature metamorphic rocks from New Caledonia. Samples examined include mafic and pelitic rock-types that range in grade from lawsonite blueschist to eclogite facies, and represent typical lithologies of subducting crust. Under lawsonite blueschist facies conditions, the low temperatures of metamorphism inhibit equilibrium partitioning between metamorphic minerals and allow for the persistence of igneous and detrital minerals. Despite this, the most important hosts for trace-elements include lawsonite, (REE, Pb, Sr), titanite (REE, Nb, Ta), allanite (LREE, U, Th), phengite (LILE) and zircon (Zr, Hf). At epidote blueschist to eclogite facies conditions, trace-element equilibrium may be attained and epidote (REE, Sr, Th, U, Pb), garnet (HREE), rutile (Nb, Ta), phengite (LILE) and zircon (Zr, Hf) are the major trace-element hosts. Chlorite, albite, amphibole and omphacite contain very low concentrations of the investigated trace elements. The comparison of mineral trace-element data and bulk-rock data at different metamorphic grades indicates that trace elements are not liberated in significant quantities by prograde metamorphism up to eclogite facies. Combining our mineral trace-element data with established phase equilibria, we show that the trace elements considered are retained by newly-formed major and accessory minerals during mineral breakdown reactions to depths of up to 150 km. In contrast, significant volumes of fluid are released by dehydration reactions. Therefore, there is a decoupling of fluid release and trace element release in subducting slabs. We suggest that the flux of trace elements from the slab is not simply linked to mineral breakdown, but results from complex fluid-rock interactions and fluid-assisted partial melting in the slab.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00410-003-0495-5.Editorial responsibility: J. Hoefs  相似文献   

19.
A mantle value of 17.5 for Nb/Ta appears well established; less well established are crustal values of 11–12, although it appears that Nb/Ta for crustal-derived melts is less than mantle Nb/Ta, demonstrating fractionation of these two elements during crustal evolution, and suggesting that Nb/Ta variation may be indicative of a particular chemical process within the crust-mantle system.

Experimental studies on silicate and carbonatitic liquids at high pressure indicate that, although silicate minerals such as garnet, amphibole and clinopyroxene do fractionate Nb and Ta, the partition coefficients (D's) for both elements are very low. Thus involvement of these minerals may explain relatively small changes in Nb/Ta, but appears inadequate to explain the crust-mantle variation. However, high-quality data for Nb, Ta may be used to provide information on mantle melting or metasomatic processes (e.g., amphibole in the source region decreases Nb/Ta in derived melts, while carbonatitic metasomatism will increase Nb/Ta in affected mantle). Titanate minerals have high D's for Nb and Ta, and do fractionate these elements (e.g., DNb/DTa rutile/liquid of 0.5–0.8), and their involvement in crystal fractionation would increase Nb/Ta in derivative liquids. In contrast, DNb/DTa for rutile/fluid is 1.25, so that rocks affected by fluid equilibrated with residual rutile will show a decrease in Nb/Ta

Some Archaean gneisses appear to have high Nb/Ta, and may be a complementary component to that part of the crust which has a relatively low Nb/Ta, such as crustal-derived magmas (e.g., A- ad I-type granites and silicic volcanics). Within the crustal system pegmatites are known to have extremely high and variable Nb, Ta contents, often with low Nb/Ta. A fluid is generally considered to be involved in the generation of these rocks. Thus it is possible that fluid/melt partitioning may be the key to fractionating Nb and Ta, with preference for Ta in the fluid, and enrichment of Ta relative to Nb into the mid-upper crustal system, as the crust evolved, through upward movement of fluid.  相似文献   


20.
Kinetic effects on trace element partitioning have been measured for anorthite, forsterite, and diopside grown from synthetic compositions doped with REE. A seeding technique allowed determination of crystal growth rates and partitioning information was obtained from electron microprobe analyses. Compositional deviations from equilibrium values were sought in the crystals and as gradients in the quenched liquids adjacent to the crystals. The principal result is that large deviations in trace element distribution coefficients from equilibrium values do not occur because of a compensating effect. Rapid growth depletes the melt adjacent to the crystal in the elements of which the crystal is composed, leading to different values for apparent distribution coefficients. However, as the boundary layer melt becomes depleted in the components of the crystal, growth slows and the size of the compositional perturbations decreases. Crystals grown at very high rates (e.g., > 0.2 μm/sec for diopside) tended to be too small for accurate microprobe analyses, but are probably not compositionally extreme since the melts adjacent to the crystals did not acquire sizable compositional gradients. At moderately high growth rates (e.g., 0.02 μm/sec), crystals form in the presence of boundary layer compositions perturbed by as much as 10% from bulk melt values and, in diopside, attain concentrations for excluded trace elements about 70% higher than equilibrium values for crystals plus bulk melt. At the slower growth rates typical of igneous systems, kinetic effects on trace element partitioning are probably negligible.  相似文献   

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