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1.
Usually it is assumed that the partitioning of trace elements into titanite in metaluminous granitoid plutonic environments takes place under equilibrium conditions and that compositional zoning is due solely to progressive changes in melt chemistry and/or mineral/melt partition coefficients. Examination of titanites from a variety of Caledonian metaluminous granitoids and related rocks has revealed that sector zoning is present, indicating disequilibrium partitioning. The sector zoning in titanites is defined principally by the distribution of the rare earth elements (REE), Y, Nb, Al and Fe. The REE, Y and Nb preferentially occur within the minor (100) sectors relative to the morphologically important (111) sectors. The reverse is true of Al and Fe which preferentially occur within the (111) sectors relative to the (100) sectors. The patterns of sector zoning are complicated by the fact that the relative growth rates of the various crystal faces fluctuated during growth. Sector zoning indicates that crystal-interface kinetics are responsible for the observed patterns of element partitioning. It is concluded that differences in the lateral-layerspreading rates of crystal faces bring about the sector zoning. The results have implications for the use of trace element partition coefficients in the modelling of fractionation processes.  相似文献   

2.
In an investigation of Sm, Sc, Sr, and Ti partitioning between diopside and silicate melt, Rayet al. (1983) showed that the compositional dependence of multicomponent exchange equilibria can not be eliminated by the application of the melt model of Bottinga and Weill (1972), and that the propagation of analytical error is an important contributor in the observed internal error in the calculated equilibria for those exchange reactions. We can demonstrate that the compositional dependence of single component distribution coefficients can be reduced below analytical error by the application of the two-lattice melt model first reported by Nielsenand Drake (1979) in a study of major element partitioning between pyroxene and melt. This model was later used for the calculation of major, minor and trace element partitioning by Nielsen and Dungan (1983). The two-lattice model is a modified version of the Bottinga-Weill model used by Drake (1976) for plagioclase-melt equilibria, and differs from that model in the assumed role of Al. The Nielsen and Dungan (1983) model has been modified here to assume Ti to mix in silicate melts as a network former. Single component distribution coefficients were used in place of multicomponent reactions in order to reduce the propagated analytical and experimental error.  相似文献   

3.
Clinopyroxenes (cpx) in abyssal and ophiolitic peridotites are commonly analyzed for lithophile trace element abundances in order to estimate degrees of melting and porosity conditions during melt extraction, assuming that these data reflect near-solidus conditions. During cooling, however, cpxs always exsolve into parallel lamellae of low-Ca enstatite and high-Ca diopside. This may potentially lead to redistribution of the initial trace element budget. Since orthopyroxene (opx) cannot significantly host most incompatible trace elements, exsolution will lead to an enrichment in the cpx lamellae. In order to address a possibly exsolution-controlled partitioning between cpx and opx, we have obtained major and trace element mineral compositions on 14 plagioclase-free ocean floor mantle rocks. They cover the entire abyssal peridotite compositional spectrum from very fertile to highly depleted compositions. The mean volume proportion of opx lamellae in cpx porphyroclasts lies around 15% of the original cpx. For the light to middle rare earth elements, the enrichment in the measured cpx exsolution is exclusively controlled by these phase proportions. Relative to these highly incompatible trace elements, solely Ti and Yb partition significantly into opx. Lamellar interpyroxene partition coefficients, estimated from NanoSIMS analyses, are around three times as high as the ones for near-solidus bulk pyroxene. The equilibration temperatures for the exsolution lamella are slightly higher than 800°C. The bulk cpx can be reconstructed using the lamellar proportions and their relative partitioning. The implication of such a reconstruction is that the cpx rare earth element patterns shift almost in parallel to lower values. These shifts, however, do not affect mantle melting models proposed thus far for mid-ocean ridges. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.  相似文献   

4.
We propose a theory for crystal-melt trace element partitioning that considers the energetic consequences of crystal-lattice strain, of multi-component major-element silicate liquid mixing, and of trace-element activity coefficients in melts. We demonstrate application of the theory using newly determined partition coefficients for Ca, Mg, Sr, and Ba between pure anorthite and seven CMAS liquid compositions at 1330 °C and 1 atm. By selecting a range of melt compositions in equilibrium with a common crystal composition at equal liquidus temperature and pressure, we have isolated the contribution of melt composition to divalent trace element partitioning in this simple system. The partitioning data are fit to Onuma curves with parameterizations that can be thermodynamically rationalized in terms of the melt major element activity product (aAl2O3)(aSiO2)2 and lattice strain theory modeling. Residuals between observed partition coefficients and the lattice strain plus major oxide melt activity model are then attributed to non-ideality of trace constituents in the liquids. The activity coefficients of the trace species in the melt are found to vary systematically with composition. Accounting for the major and trace element thermodynamics in the melt allows a good fit in which the parameters of the crystal-lattice strain model are independent of melt composition.  相似文献   

5.
We have used trace element partitioning data available in the literature to investigate nonideality of the cations of Yb, Sm, Gd, Ca, Mn, Sc, Ni, and Al in silicate melt, olivine, and low-Ca pyroxene. Results are consistent with ordering of Mg and Fe around trace cations in olivine and pyroxene. On the basis of these data, we suggest there is an increasing tendency for Fe to congregate in the vincinity of the trace cation as the size of the trace cation increases. These results are important both in achieving a better understanding of trace element behavior in crystals and in constraining the temperature and compositional dependence of trace element partitioning.  相似文献   

6.
The ion probe is uniquely suited for measurement of element partitioning between phases in experimental and natural systems. A redetermination of the partitioning of Ni between diopside and quenched silicate melt using samples previously measured by β-track mapping gives 1.87 (weight ratio of 62Ni in diopside/melt), slightly lower than the β-track value of 2.05. Critical to the accurate determination of distribution coefficients are: (1) a secondary ion signal that is linear with concentration in the range measured, and (2) a calibration using known concentrations to correct for differential secondary ion yields from different phases. In the present case the secondary ion signal is linear with Ni concentrations below ~ 1 wt% in both diopside and glass, but nonlinear above. Differential yields were corrected by calibrating the secondary ion signal against compositions determined by electron microprobe.Partition coefficients measured using 58Ni and 60Ni, in contrast to 62Ni, are not constant with concentration in these samples probably due to Ni migration during crystallization. Measurements using these isotopes (or bulk Ni) show a change of partition coefficient with Ni concentration.  相似文献   

7.
《地学前缘(英文版)》2020,11(3):745-763
Tonalite-trondhjemite-granodiorite(TTG) suites constitute a large proportion of the Archean geological record;however,the geodynamic processes that generated them,and Archean continental crust in general,remain a subject of debate.The concentrations and ratios of Sr,Y,La,Yb,Nb,and Ta in TTGs are commonly used to determine the depth of melting of their metabasic sources.The trace element composition of melt produced by metabasic source rocks during anatexis is strongly affected by the presence and abundance of pressure-sensitive minerals,such as plagioclase(Sr-bearing),garnet(Y-and HREE-bearing),and rutile(Nb-and Ta-bearing).Elevated Sr/Y and La/Yb ratios and low concentrations of Nb and Ta in TTGs are generally considered to indicate melting at high pressures(≥2.0 GPa).The depth of melting is a key factor in determining the origin of TTGs as this provides critical information on the tectonic setting of their generation.We use phase equilibrium and trace element modelling to explore the effects of three potential influences on TTG trace element compositions:fractionation of trace elements into peritectic garnet cores,progressive melt loss from the source,and source bulk composition.We model three different compositions of Archean basalts along thermal gradients of 500℃/GPa,750℃/GPa,and 1000℃/GPa.The models produce maj or and trace element melt compositions that are generally consistent with measured compositions of TTGs.Although Sr/Y,La/Yb,Nb,and Ta exhibit pressure-dependent behaviour,other factors also affect these values.Garnet fractionation causes Sr/Y and La/Yb to reach much greater values and in this scenario,the values also increase with increasing temperature.Source bulk composition has an effect in all scenarios and most strongly influences La/Yb,Nb,and Ta.Overall,these results show that Sr/Y,La/Yb,Nb,and Ta can reach values generally considered to be indicative of high pressure melting at a range of P-T conditions including P 2.0 GPa.Consequently,trace element compositions of TTGs alone may provide a misleading impression of the depth of melting of metabasites and the geodynamic environment of Archean crustal growth and reworking.  相似文献   

8.
We experimentally investigate the major and trace elements behavior during the interaction between two partially molten crustal rocks (meta-anorthosite and metapelite) and a basaltic melt at 0.5–0.8 GPa. Results show that a hybrid melt is formed at the basalt-crust contact, where plagioclase crystallizes. This contact layer is enriched in trace elements which are incompatible with plagioclase crystals. Under these conditions, the trace element diffusion coefficients are one order of magnitude larger than those expected. Moreover, the HFSE diffusivity in the hybrid melt is surprisingly higher than the REE one. Such a feature is related to the plagioclase crystallization that changes the trace elements liquid-liquid partitioning (i.e. diffusivity) over a transient equilibrium that will persist as long as the crystal growth proceeds. These experiments suggests that the behaviour of the trace elements is strongly dependent on the crystallization at the magma-crust interface. Diffusive processes like those investigated can be invoked to explain some unusual chemical features of contaminated magmatic suites.  相似文献   

9.
The fidelity of melt inclusions as records of melt composition   总被引：5，自引：5，他引：0  
A series of experiments created melt inclusions in plagioclase and pyroxene crystals grown from a basaltic melt at 1,150°C, 1.0 GPa to investigate diffusive fractionation during melt inclusion formation; additionally, P diffusion in a basaltic melt was measured at 1.0 GPa. Melt inclusions and melts within a few 100 microns of plagioclase–melt interfaces were analyzed for comparison with melt compositions far from the crystals. Melt inclusions and melt compositions in the boundary layer close to the crystal–melt interface were similar, but both differ significantly in incompatible element concentrations from melt found greater than approximately 200 microns away from the crystals. The compositional profiles of S, Cl, P, Fe, and Al in the boundary layers were successfully reproduced by a two-step model of rapid crystal growth followed by diffusive relaxation toward equilibrium after termination of crystal growth. Applying this model to investigate possible incompatible element enrichment in natural melt inclusions demonstrated that at growth rates high enough to create the conditions for melt inclusion formation, ∼10−9–10−8 m s−1, the concentration of water in the boundary layer near the crystal was similar to that of the bulk melt because of its high diffusion coefficient, but sulfur, with a diffusivity similar to major elements and CO2, was somewhat enriched in the boundary layer melt, and phosphorus, with its low diffusion coefficient similar to other high-field strength elements and rare earth elements, was significantly enriched. Thus, the concentrations of sulfur and phosphorus in melt inclusions may over-estimate their values in the bulk melt, and other elements with similar diffusion coefficients may also be enriched in melt inclusions relative to the bulk melt. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

10.
Each of two calcitic stalagmites from Grotte de Clamouse, Herault, southern France, displays a discrete aragonite layer dated at around 1100 yr BP. The layer of fanning aragonite ray crystals is immediately preceded by calcite with Mg and Sr compositions that are uniquely high for the past 3 kyr. Trace element compositions close to the boundary between original aragonite and calcite are consistent with quasi‐equilibrium partitioning of trace elements between the phases. Study of modern dripwaters demonstrates that pronounced covariation of Mg/Ca and Sr/Ca ratios in dripwater occurs owing to large amounts of calcite precipitation upflow of the drips that fed the stalagmites. Trace element to Ca ratios are enhanced during seasonally dry periods. Ion microprobe data demonstrate a pronounced covariation of trace elements, including Mg and Sr in calcite, and Sr, U and Ba in aragonite. The mean peak spacing is close to the long‐term mean of annual growth rates determined by differences in U‐series ages and so the trace element peaks are interpreted as annual. The trace element chemistry of the stalagmites on annual to inter‐annual scales thus directly reflects the amounts of prior calcite precipitation, interpreted as an index of aridity. The longer‐term context is a multi‐decadal period of aridity (1200–1100 yr BP) possibly correlated with an analogous episode in Central America. The arid period culminated in the nucleation of aragonite, but within a decade was followed by a return to precursor conditions. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

11.
盘石山幔源透辉石微量元素地球化学   总被引：4，自引：0，他引：4       下载免费PDF全文
支霞臣  李彬贤 《地质科学》1995,30(4):384-392
用质子探针和电子探针分别测定盘石山幔源橄揽岩包体中透辉石、橄榄石的微量元素和主量元素丰度;用X-射线荧光光谱法测定包体全岩的主量和微量元素丰度。定量讨论微量元素的分布和分配。其中绝大部分Sr、Zr、Ti、Na赋存在透辉石中,绝大部分Mn、Zn、Ni赋存在橄揽石中,在T=1194-1435K范围,分配系数变化如下:InDOl/CpxZn=1.44-2.19;InDOl/CpxMn=0.21-0.40,InDOl/CpxTi=4.76-5.61,InDOl/CpxNi(平均值)=2.20.透辉石中Ti/Zr=42-103,低于原始地幔值,由地幔隐性交代作用造成。  相似文献   

12.
Within individual plutons, the trace element concentrations in S-type granites generally increase with maficity (total iron and magnesium content and expressed as atomic Fe + Mg in this study); the degree of variability in trace element concentration also expands markedly with the same parameter. The strongly peraluminous, high-level S-type granites of the Peninsular Pluton (Cape Granite Suite, South Africa) are the product of biotite incongruent melting of a metasedimentary source near the base of the crust. Leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite. Trace elements such as Rb, Ba, Sr and Eu, that are concentrated in reactant minerals in the melting process, show considerable scatter within the granites. This is interpreted to reflect compositional variation in the source. In contrast, elements such as LREE, Zr and Hf, which are concentrated within refractory accessory phases (zircon and monazite), show well-defined negative correlations with increasing SiO2 and increase linearly with increasing maficity. This is interpreted to reflect coupled co-entrainment of accessory minerals and peritectic phases to the melt: leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon-saturated character of this hypothetical earlier magma. Trace element behaviour of granites from the Peninsular Pluton has been modelled via both equilibrium and disequilibrium trace element melting. In the disequilibrium case, melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals, but before the melt has dissolved any accessory minerals. Thus, the trace element signature of the melt is largely inherited from the reactants in the melting reaction, with no contribution from zircon and monazite dissolution. In the equilibrium case, melt leaves the source with entrained crystals, after reaching zircon and monazite saturation. A significant proportion of the rocks of the Peninsular Pluton have trace element concentrations below those predicted by zircon and monazite saturation. In the case of the most leucocratic rocks all compositions are zircon undersaturated; whilst the majority of the most mafic compositions are zircon oversaturated. However, in both cases, zircon is commonly xenocrystic. Thus, the leucocratic rocks represent close to pure melts, which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study. Zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years.  相似文献   

13.
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14.
Compositional zoning of plagioclase is useful as a recorder of dynamic geological conditions if the mechanisms of crystal growth are known. Although the present lack of quantitative information on specific kinetic processes limits their accuracy, numerical simulations of phenoycryst plagioclase growth are useful both for identifying the most influential kinetic processes (for example, diffusion) that should receive priority in experimental measurements and for designing informative growth experiments. The interaction of kinetic processes at a crystal face is so complex that the overall result cannot be assessed intuitively. A primary purpose of these papers is to explore this interaction in the plagioclase system as quantitatively as data permit.The growth of a single face of a plagioclase crystal in an infinite melt was simulated in computer models for: (1) anhydrous and hydrous plagioclase melts; (2) for different undercoolings; and (3) for both interface-controlled and melt-transport controlled growth. Major uncertainties include the velocity and nonequilibrium partitioning laws in the interface-controlled model, and transport coefficients for melt components. Comparison of computed models with published growth velocity data for anhydrous melts was used to estimate a transport coefficient (with the form for diffusion), and that coefficient was extrapolated to hydrous melts on the basis of the Stokes-Einstein relationship.The results of simulations suggest that undercoolings reasonable for plutonic systems could result in deviations of crystal composition from that in equilibrium with the melt of several mole % An; geothermometers based on the assumption of equilibrium partitioning will be in error significantly. Similarly, the volatile content and composition of melt trapped during growth would deviate significantly from bulk melt properties. The velocity of crystal growth and deviation of crystal composition from equilibrium show low sensitivity to water content because larger water contents result in greater accumulation of water at the interface and a consequent depression of effective undercooling.The large magnitude of the derived transport parameter suggests that local convection as well as diffusion may occur during growth in the anhydrous system. The addition of water to the system reduces viscosity and increases the density gradients near the crystal, making local convection even more probable. Our meagre knowledge of transport by diffusion and convection in the melt is probably the most important factor limiting the accuracy of growth simulations.  相似文献   

15.
Trace element concentrations of peridotitic garnet inclusions in diamonds from two Chinese kimberlite pipes were determined using the ion microprobe. Garnet xenocrysts from the same two kimberlite pipes were also analyzed for comparison. In contrast to their extremely refractory major element compositions, all harzburgitic garnets showed enrichment in light rare earth elements (REE) relative to chondrite, resulting in sinuous REE patterns. Both normal and sinuous REE patterns were observed from the lherzolitic garnets. Concentrations of REE in garnets changed significantly from diamond to diamond and no specific correlations were observed with their major element compositions. Analyses of randomly selected two to three points within every grain of a large number of garnet inclusions by the ion microprobe demonstrated that there was no evident compositional heterogeneity, and multiple grains of one phase from a single diamond host also exhibit very similar compositions. This implies that the trace element heterogeneity within one grain or among multiple inclusions from the same diamond host, as reported from Siberian diamonds, is not a common feature for these Chinese diamonds. Concentrations of Na, Ti, and Zr tend to decrease when garnets become more refractory, but variations of Sr and Li are more complex. Compositions rich in light REE and relatively poor in high field strength elements (HFSE) of the harzburgitic garnet inclusions in diamonds are generally consistent with metasomatism by carbonatite melts. The trace element features observed from the garnet inclusions in Chinese diamonds may be caused by carbonatite melt infiltration and partial melt extraction. Spatial and temporal gradients in melt/rock ratio and temperature are the main reasons for the large variations of REE patterns and other trace element concentrations. Received: 27 April 1999 / Accepted: 1 March 2000  相似文献   

16.
Textural and compositional zoning in plagioclase phenocrysts in a sample from Parinacota volcano (Chile) was investigated using backscattered electron images and electron microprobe analysis of major and trace elements. Large (2 mm) oscillatory zoned crystals (type I) with resorption surfaces of moderate An discontinuities (Ⲓ% An) and decreasing trace-element contents (Sr, Mg, Ti) towards the rim reflect melt differentiation and turbulent convection in the main magma body. Early recharge with a low-Sr mafic magma is seen in the core. Small-scale Sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years. Smaller crystals (type II) with low trace-element/An ratio reflect the influence of an H2O-rich melt probably from a differentiated boundary layer. Closed-system in-situ crystallisation, mafic magma recharge and the role of a water-rich differentiated boundary layer can be distinguished from the An-trace element relationships. Crystals apparently move relatively freely between different parts and regimes in the magma chamber, evidence for "convective crystal dispersion". High-Sr type II crystals indicate an earlier input of Sr-rich mafic magma. Recharge of two distinct mafic magma types is thus identified (high-Sr and low-Sr), which must have been present - at increasing recharge rates with time - in the plumbing system throughout the volcano's history.  相似文献   

17.
Understanding the mechanisms responsible for the generation of chemical gradients in high-volume ignimbrites is key to retrieve information on the processes that control the maturation and eruption of large silicic magmatic reservoirs. Over the last 60 ky, two large ignimbrites showing remarkable zoning were emplaced during caldera-forming eruptions at Campi Flegrei (i.e., Campanian Ignimbrite, CI, ~?39 ka and Neapolitan Yellow Tuff, NYT, ~?15 ka). While the CI displays linear compositional, thermal and crystallinity gradients, the NYT is a more complex ignimbrite characterized by crystal-poor magmas ranging in composition from trachy-andesites to phonolites. By combining major and trace element compositions of matrix glasses and mineral phases from juvenile clasts located at different stratigraphic heights along the NYT pyroclastic sequence, we interpret such compositional gradients as the result of mixing/mingling between three different magmas: (1) a resident evolved magma showing geochemical characteristics of a melt extracted from a cumulate mush dominated by clinopyroxene, plagioclase and oxides with minor sanidine and biotite; (2) a hotter and more mafic magma from recharge providing high-An plagioclase and high-Mg clinopyroxene crystals and (3) a compositionally intermediate magma derived from remelting of low temperature mineral phases (i.e., sanidine and biotite) within the cumulate crystal mush. We suggest that the presence of a refractory crystal mush, as documented by the occurrence of abundant crystal clots containing clinopyroxene, plagioclase and oxides, is the main reason for the lack of erupted crystal-rich material in the NYT. A comparison between the NYT and the CI, characterized by both crystal-poor extracted melts and crystal-rich magmas representing remobilized portions of a “mature” (i.e., sanidine dominated) cumulate residue, allows evaluation of the capability of crystal mushes of becoming eruptible upon recharge.  相似文献   

18.
Analyses of trace elements in the mineral phases of granulites provide important information about the trace element distribution in the lower crust. Since granulites are often considered residues of partial melting processes, trace element characteristics of their mineral phases may record mineral/melt equilibria thus giving an opportunity to understand the nature and composition of melts in the lower continental crust. This study provides an extensive set of mineral trace element data obtained by LA-ICP-MS analyses of mafic and intermediate granulites from Central Finland. Mass balance calculations using the analytical data indicate a pronounced contribution of the accessory minerals apatite for the REE and ilmenite for the HFSE. Coherent mineral/mineral ratios between samples point to a close approach to equilibrium except for minerals intergrown with garnet porphyroblasts. Mineral trace element data were used for the formulation of a set of D mineral/melt partition coefficients that is applicable for trace element modelling under lower crustal conditions. D mineral/melt were derived by the application of predictive models and using observed constant mineral/mineral ratios. The comparison of the calculated D mineral/melt with experimental data as well as the relationship between mineral trace element contents and a leucosome with a composition close to an equilibrium melt provides additional constraints on mineral/melt partitioning. The D values derived in this study are broadly similar to magmatic partition coefficients for intermediate melt compositions. They provide a first coherent set of D values for Sc, V, Cr and Ni between clinopyroxene, amphibole, garnet, orthopyroxene, ilmenite and melt. In addition, they emphasize the strong impact that ilmenite exerts on the distribution of Nb and Ta.  相似文献   

19.
Experiments in the systems diopside-albite (Di-Ab) and diopside-albite-dolomite (Di-Ab-Dmt), doped with a wide range of trace elements, have been used to characterise the difference between clinopyroxene-silicate melt and clinopyroxene-carbonate melt partitioning. Experiments in Di-Ab-Dmt yielded clinopyroxene and olivine in equilibrium with CO2-saturated dolomitic carbonate melt at 3 GPa, 1375 °C. The experiments in Di-Ab were designed to bracket those conditions (3 GPa, 1640 °C and 0.8 GPa, 1375 °C), and so minimise the contribution of differential temperature and pressure to partitioning. Partition coefficients, determined by SIMS analysis of run products, differ markedly for some elements between Di-Ab and Di-Ab-Dmt systems. Notably, in the carbonate system clinopyroxene-melt partition coefficients for Si, Al, Ga, heavy REE, Ti and Zr are higher by factors of 5 to 200 than in the silicate system. Conversely, partition coefficients for Nb, light REE, alkali metals and alkaline earths show much less fractionation (<3). The observed differences compare quantitatively with experimental data on partitioning between immiscible carbonate and silicate melts, indicating that changes in melt chemistry provide the dominant control on variation in partition coefficients in this case. The importance of melt chemistry in controlling several aspects of element partitioning is discussed in light of the energetics of the partitioning process. The compositions of clinopyroxene and carbonate melt in our experiments closely match those of near-solidus melts and crystals in CMAS-CO2 at 3 GPa, suggesting that our partition coefficients have direct relevance to melting of carbonated mantle lherzolite. Melts so produced will be characterised by elevated incompatible trace element concentrations, due to the low degrees of melting involved, but marked depletions of Ti and Zr, and fractionated REE patterns. These are common features of natural carbonatites. The different behaviour of trace elements in carbonate and silicate systems will lead to contrasted styles of trace element metasomatism in the mantle. Received: 15 July 1999 / Accepted: 18 February 2000  相似文献   

20.
Compositional and thermal convection in magma chambers   总被引：7，自引：1，他引：7  
Magma chambers cool and crystallize at a rate determined by the heat flux from the chamber. The heat is lost predominantly through the roof, whereas crystallization takes place mainly at the floor. Both processes provide destabilizing buoyancy fluxes which drive highly unsteady, chaotic convection in the magma. Even at the lowest cooling rates the thermal Rayleigh number Ra is found to be extremely large for both mafic and granitic magmas. Since the compositional and thermal buoyancy fluxes are directly related it can be shown that the compositional Rayleigh number Rs (and therefore a total Rayleigh number) is very much greater than Ra. In the case of basaltic melt crystallizing olivine Rs is up to 106 times greater than Ra. However compositional and thermal buoyancy fluxes are roughly equal. Therefore thermal and compositional density gradients contribute equally to convection velocities in the interior of the magma. Effects of thermal buoyancy generated by latent heat release at the floor are included.The latent heat boundary layer at the floor of a basaltic chamber is shown to be of the order of 1 m thick with very low thermal gradients whereas the compositional boundary layer is about 1 cm thick with large compositional gradients. As a consequence, the variation in the degree of supercooling in front of the crystal-liquid interface is dominated by compositional effects. The habit and composition of the growing crystals is also controlled by the nature of the compositional boundary layer. Elongate crystals are predicted to form when the thickness of the compositional boundary layer is small compared with the crystal size (as in laboratory experiments with aqueous solutions). In contrast, equant crystals form when the boundary layer is thicker than the crystals (as in most magma chambers). Instability of the boundary layer in the latter case gives rise to zoning within crystals. Diffusion of compatible trace elements through the boundary layer can also explain an inverse correlation, observed in layered intrusions, between Ni concentration in olivine and the proportion of Ni-bearing phases in the crystallizing assemblage.  相似文献   

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