Experimentally determined mineral-melt partition coefficients for Sc,Y and REE for olivine,orthopyroxene, pigeonite,magnetite and ilmenite

Our current lack of understanding of the partitioning behavior of Sc, Y and the REE (rare-earth elements) can be attributed directly to the lack of a sufficiently large or chemically diverse experimental data set. To address this problem, we conducted a series of experiments using several different natural composition lavas, doped with the elements of interest, as starting compositions. Microprobe analyses of orthopyroxene, pigeonite, olivine, magnetite, ilmenite and co-existing glasses in the experimental charges were used to calculate expressions that describe REE partitioning as a function of a variety of system parameters. Using expressions that represent mineral-melt reactions (versus element ratio distribution coefficients) it is possible to calculate terms that express low-Ca pyroxene-melt partitioning behavior and are independent of both pyroxene and melt composition. Compositional variations suggest that Sc substitution in olivine involves either a paired substitution with Al or, more commonly, with vacancies. The partitioning of Sc is dependent both on melt composition and temperature. Our experimentally determined olivine-melt REE Ds (partition coefficients) are similar to, but slightly higher than those reported by McKay (1986) and support their conclusions that olivines are strongly LREE depleted. Y and REE mineral/melt partition coefficients for magnetite range from 0.003 for La to 0.02 for Lu. Ilmenite partition coefficients range from 0.007 for La to 0.029 for Lu. These experimental values are two orders of magnitude lower than many of the published values determined by phenocryst/matrix separation techniques.     

A parameterized model for REE distribution between low-Ca pyroxene and basaltic melts with applications to REE partitioning in low-Ca pyroxene along a mantle adiabat and during pyroxenite-derived melt and peridotite interaction

Low-Ca pyroxenes play an important role in mantle melting, melt-rock reaction, and magma differentiation processes. In order to better understand REE fractionation during adiabatic mantle melting and pyroxenite-derived melt and peridotite interaction, we developed a parameterized model for REE partitioning between low-Ca pyroxene and basaltic melts. Our parameterization is based on the lattice strain model and a compilation of published experimental data, supplemented by a new set of trace element partitioning experiments for low-Ca pyroxenes produced by pyroxenite-derived melt and peridotite interaction. To test the validity of the assumptions and simplifications used in the model development, we compared model-derived partition coefficients with measured partition coefficients for REE between orthopyroxene and clinopyroxene in well-equilibrated peridotite xenoliths. REE partition coefficients in low-Ca pyroxene correlate negatively with temperature and positively with both calcium content on the M2 site and aluminum content on the tetrahedral site of pyroxene. The strong competing effect between temperature and major element compositions of low-Ca pyroxene results in very small variations in REE partition coefficients in orthopyroxene during adiabatic mantle melting when diopside is in the residue. REE partition coefficients in orthopyroxene can be treated as constants at a given mantle potential temperature during decompression melting of lherzolite and diopside-bearing harzburgite. In the absence of diopside, partition coefficients of light REE in orthopyroxene vary significantly, and such variations should be taken into consideration in geochemical modeling of REE fractionation in clinopyroxene-free harzburgite. Application of the parameterized model to low-Ca pyroxenes produced by reaction between pyroxenite-derived melt and peridotite revealed large variations in the calculated REE partition coefficients in the low-Ca pyroxenes. Temperature and composition of starting pyroxenite must be considered when selecting REE partition coefficients for pyroxenite-derived melt and peridotite interaction.     

Zonation des éléments en traces au cours de la croissance des cristaux dans les bains silicatés: l'exemple de Rb,Cs, Sr et Ba dans le système Qz-Ab-Or-H2O

An indirect method was used to study Na, K, Rb, Cs, Sr and Ba partition coefficients between crystals and silicate melt. Equilibria between a hydrothermal solution and the melt at 800°C and 2 kb and between a hydrothermal solution and crystals at 750°C and 2 kb were separately achieved.For major element partitioning (Na and K), the results obtained here are in good agreement with those of Tuttle and Bowen (1958) which allow us to follow crystal evolution during a fractional crystallization process where the growth of zoned crystals takes place.For minor elements Rb, Cs, Sr, Ba, melt/aqueous solution partition coefficients depend on Na/K as well as the silica content of the melt. These effects are rather small for Rb and Cs, but are much more important for the alkaline earths. The feldspar/aqueous solution partition coefficients also depend on Na/K.The variations of the partition coefficients feldspar/melt are complex in the part of the Qz-Ab-Or diagram located below the cotectic line.During fractional crystallization following the Rayleigh law (assuming that there are no kinetic phenomena) Sr (D > 10) is almost totally removed from the melt in the early stages whereas Cs (D < 0.1) remains in the melt during the whole process. Rb and Ba have partition coefficients closer to unity. The variation of these coefficients, due to changes in bulk composition of liquid and crystals during fractional crystallization, can lead to complex zoning with possible concentration maxima at some stages. Similar phenomena can be expected in non-ideal natural solid solutions, even if no discontinuities can be detected in the physicochemical evolution of the parent magma.     

Effect of melt composition on the partitioning of trace elements between titanite and silicate melt

Isobaric and isothermal experiments were performed to investigate the effect of melt composition on the partitioning of trace elements between titanite (CaTiSiO₅) and a range of different silicate melts. Titanite-melt partition coefficients for 18 trace elements were determined by secondary ion mass spectrometry (SIMS) analyses of experimental run products. The partition coefficients for the rare earth elements and for Th, Nb, and Ta reveal a strong influence of melt composition on partition coefficients, whereas partition coefficients for other studied monovalent, divalent and most quadrivalent (i.e., Zr, Hf) cations are not significantly affected by melt composition. The present data show that the influence of melt composition may not be neglected when modelling trace element partitioning.It is argued that it is mainly the change of coordination number and the regularity of the coordination space of trace elements in the melt structure that controls partition coefficients in our experiments. Furthermore, our data also show that the substitution mechanism by which trace elements are incorporated into titanite crystals may be of additional importance in this context.     

Partitioning of Pb between volcanic glass and coexisting sanidine and plagioclase feldspars

Pb contents were determined by isotope dilution in separated glass, sanidine, and plagioclase from 18 rocks ranging in composition from basalt to rhyolite. These data indicate that Pb is partitioned into silicate melt relative to plagioclase, but is equally distributed between melt and sanidine. Plagioclase/glass distribution coefficients increase from 0.1 to 0.7 in going from basalt to rhyolite. This relationship suggests that the distribution coefficient is dependent upon bulk composition, temperature, or both. Sanidine/glass distribution coefficients are close to unity in rocks ranging in composition from quartz latite to rhyolite. The variation in Pb contents in a natural magma series from Craters of the Moon National Monument, Idaho, indicates that minerals (olivine, plagioclase, magnetite, apatite and clinopyroxene) fractionated from these magmas all have very low crystal/liquid distribution coefficients for Pb.     

Trace element partitioning between type B CAI melts and melilite and spinel: Implications for trace element distribution during CAI formation

Calcium- and aluminum-rich inclusions (CAIs), occurring in chondritic meteorites and considered the oldest materials in the solar system, can provide critical information about the environment and time scale of creation of planetary materials. However, interpretation of the trace element and isotope compositions of CAIs, particularly the light elements Li, Be, and B, is hampered by the lack of constraint on melilite-melt and spinel-melt partition coefficients. We determined melilite-melt and spinel-melt partition coefficients for 21 elements by performing controlled cooling rate (2 °C/h) experiments at 1 atmosphere pressure in sealed platinum capsules using a synthetic type B CAI melt. Trace element concentrations were measured by secondary ion mass spectrometry (SIMS) and/or laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Melilites vary only slightly in composition, ranging from Åk_31-43. Results for the partitioning of trace elements between melilite and melt in three experiments and between spinel and melt in two experiments show that partition coefficients are independent of trace element concentration, are in good agreement for different analytical techniques (SIMS and LA-ICP-MS), and are in agreement with previous measurements in the literature. Partition coefficients between intermediate composition melilites and CAI melt are the following: Li, 0.5; Be, 1.0; B, 0.22; Rb, 0.012; Sr, 0.68; Zr, 0.004; Nb, 0.003; Cs, 0.002; Ba, 0.018; La, 0.056; Nd, 0.065; Sm, 0.073; Eu, 0.67; Er, 0.037; Yb, 0.018; Hf, 0.001; Ta, 0.003; Pb, 0.15; U, 0.001; Th, 0.002. Site size energetics analysis is used to assess isovalent partitioning into the different cation sites. The Young’s modulus deduced from +2 cations partitioning into the melilite X site agrees well with the bulk modulus of melilite based on X-ray diffraction methods. The changes in light element partitioning as melilite composition varies are predicted and used in several models of fractional crystallization to evaluate if the observed Li, Be, and B systematics in Allende CAI 3529-41 are consistent with crystallization from a melt. Models of crystallization agree reasonably well with observed light element variations in areas previously interpreted to be unperturbed by secondary processes [Chaussidon, M., Robert, F., McKeegan, K.D., 2006. Li and B isotopic variations in an Allende CAI: Evidence for the in situ decay of short-lived ¹⁰Be and for the possible presence of the short-lived nuclide ⁷Be in the early solar system. Geochim. Cosmochim. Acta70, 224-245], indicating that the trends of light elements could reflect fractional crystallization of a melt. In contrast, areas interpreted to have been affected by alteration processes are not consistent with crystallization models.     

Timescales of disequilibrium melting in the crust: constraints from modelling the distribution of multiple trace elements and a case study from the Lesser Himalayan rocks of Sikkim

A numerical code has been developed to track the distribution of trace elements in crustal rocks undergoing melting. The model handles diffusion with moving boundaries and accounts for the processes of diffusion, dissolution and precipitation in a partially molten system. Among the various input parameters for modelling, source composition (i.e. modal abundance) variations, diffusion coefficients and partition coefficients are found to exert a significant control on the melt chemistry. The other inputs such as melt reaction stoichiometry, kinetics of melting and grain size of protolith have lesser influence. Exploration of the general behaviour indicates that for systems in which disequilibrium melting of the kind considered in this paper occurs, trace element concentrations may be used to constrain the composition of the protolith or the timescales of melting, depending on the specific circumstances. After exploring some general features of melting in a pelitic system, the model is applied to calculate trace element distributions in migmatites from the Lesser Himalayan rocks in Sikkim, India. We focus on the distribution of trace elements during the initial stages of melt formation. These partially molten rocks show disequilibrium distribution of trace elements, and the numerical code is capable of quantitatively reproducing many of the observed patterns. The results of the modelling indicate that the observed melts in this zone were formed within 50,000 years and that segregation of melts (into leucosome and restite) was complete between 50,000 and 250,000 years. These short timescales may point to deformation-enhanced melt segregation at least on a hand specimen scale. It is important to distinguish between timescales of segregation over these scales and timescales of removal of melt on an outcrop scale to form plutons—the latter, requiring higher degrees of melting and larger distances of migration, take longer.     

LA-ICPMS analyses of silicate melt inclusions in co-precipitated minerals: Quantification, data analysis and mineral/melt partitioning

We present a new approach to determine the composition of silicate melt inclusions (SMI) using LA-ICPMS. In this study, we take advantage of the occurrence of SMI in co-precipitated mineral phases to quantify their composition without depending on additional sources of information. Quantitative SMI analyses are obtained by assuming that the ratio of selected elements in SMI trapped in different phases are identical. In addition Fe/Mg exchange equilibrium between olivine and melt was successfully used to quantify LA-ICPMS analyses of SMI in olivine. Results show that compositions of SMI from the different host minerals are identical within their uncertainty. Thus (1) the quantification approach is valid; (2) analyses are not affected by the composition of the host phase; (3) the derived melt compositions are representative of the original melt, excluding significant syn- or postentrapment modification such as boundary layer effects or diffusive reequilibration with the host mineral. With this data we established a large dataset of mineral/melt partition coefficients for the investigated mineral phases in hydrous calc-alkaline basaltic-andesitic melts. The clinopyroxene/melt and plagioclase/melt partition coefficients are consistent with the lattice strain model of Blundy and Wood [Blundy, J., Wood B., 1994. Prediction of crystal-melt partition-coefficients from elastic-moduli. Nature372, 452-454].     

Effect of melt composition on basalt and peridotite interaction: laboratory dissolution experiments with applications to mineral compositional variations in mantle xenoliths from the North China Craton

Interaction between basaltic melts and peridotites has played an important role in modifying the lithospheric and asthenospheric mantle during magma genesis in a number of tectonic settings. Compositions of basaltic melts vary considerably and may play an important role in controlling the kinetics of melt–peridotite interaction. To better understand the effect of melt composition on melt–peridotite interaction, we conducted spinel lherzolite dissolution experiments at 2 GPa and 1,425 °C using the dissolution couple method. The reacting melts include a basaltic andesite, a ferro-basalt, and an alkali basalt. Dissolution of lherzolite in the basaltic andesite and the ferro-basalt produced harzburgite–lherzolite sequences with a thin orthopyroxenite layer at the melt–harzburgite interface, whereas dissolution of lherzolite in the alkali basalt produced a dunite–harzburgite–lherzolite sequence. Systematic variations in mineral compositions across the lithological units are observed. These mineral compositional variations are attributed to grain-scale processes that involve dissolution, precipitation, and reprecipitation and depend strongly on reacting melt composition. Comparison of mineral compositional variations across the dissolution couples with those observed in mantle xenoliths from the North China Craton (NCC) helps to assess the spatial and temporal variations in the extent of siliceous melt and peridotite interaction in modifying the lithospheric mantle beneath the NCC. We found that such melt–rock interaction mainly took place in Early Cretaceous, and is responsible for the enrichment of pyroxene in the lithospheric mantle. Spatially, siliceous melt–peridotite interaction took place in the ancient orogens with thickened lower crust.     

100MPa、850℃和800℃条件下锡在流体与富磷过铝质熔体相间分配的实验研究

利用江西宜春414岩体中的钠长石花岗岩作为实验初始物,制备含不同1〉20s含量（0．27％－7．71％）的实验玻璃,本次实验研究了100MPa、850℃和800℃条件下Sn在流体与富磷过铝质熔体相间的分配。实验结果显示,Sn在流体与熔体相间的分配系数（Dsofluid/mclt）变化于2．10×10^-4－1．36×10^3之间,指示Sn强烈趋向于在富磷过铝质熔体中富集。随体系中P2Os含量从0．27％增至1．91％,Sn在流体与熔体相间的分配系数逐渐增加,当体系中R2O5含量进一步增加时,Sn在两相间的分配系数呈降低的趋势。本次实验结果表明,P可能不是Sn以流体相形式进行搬运的主要络合剂。     

Determination of melt influence on divalent element partitioning between anorthite and CMAS melts

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 (a_Al2O3)(a_SiO2)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.     

Lead isotope variability in olivine-hosted melt inclusions from Iceland

The lead isotope and trace element compositions of a suite of olivine-hosted melt inclusions in primitive lava flows from the Reykjanes Peninsula in Iceland show extreme variability. Much of this variability is present in the composition of inclusions from one hand specimen of Háleyjabunga, a depleted picrite lava shield that erupted 13 ka. ²⁰⁸Pb/²⁰⁶Pb compositions in this sample span 50-90% of the total range found in Atlantic MORB, indicating that high-amplitude compositional heterogeneity is present in the mantle source of melts that aggregated to form a single eruption. The trace element and isotopic trends in the melt inclusions are coincident with those in whole rock samples from young lava flows of the Reykjanes Peninsula, and extend the total range of variation towards more depleted compositions. The incompatible trace element and lead isotope compositions of the inclusions are strongly coupled and lie close to binary mixing trends between the extreme melt inclusion compositions. These relationships indicate that the trace element variation in the melt inclusions reflects heterogeneity in the composition of the mantle source entering the melting region under the Reykjanes Peninsula. Large positive Sr concentration anomalies are present in three of the inclusions, but do not correlate with indicators of mantle melting or source variations and are likely to arise by reaction with plagioclase during crustal storage. Fractional melting of heterogeneous mantle is predicted to generate melts with a wide range of compositions, filling a large volume in trace element-isotope space. However, the compositional variations observed in the melt inclusions lie close to binary mixing curves. These observations may be accounted for by a two-stage model of melt mixing. The first stage occurs in porous channels that transport melt in the mantle and takes place before inclusion entrapment. This mixing stage generates a bimodal distribution of melt compositions that is supplied from the channels to sub-Moho and lower crustal magma lenses. The second stage of mixing occurs in these chambers, producing the binary mixing trends recorded in the inclusion compositions. The distribution of isotopic compositions observed in the melt inclusions and whole rock samples from the Reykjanes Peninsula is therefore controlled by melt mixing. These results have important implications for the interpretation of basalt composition in terms of distinct compositional entities within the upwelling solid mantle under mid-ocean ridges and ocean islands.     

TYPES OF FOLDING OBSERVED IN COAL DEPOSITS OF THE SOUTH URALIAN BASIN

If temperature and composition dependencies of partitioning are taken into account in modeling equilibrium processes, unexpected chemical trends are derived. Failure to consider variations in partition coefficients with temperature and composition in modeling equilibrium melting or crystallization processes leads to wildly erroneous results. Because of changes in partition coefficients with temperature and composition, incompatible elements can exhibit apparently compatible behavior, complex parallel or perpendicular trends can develop from a single process and bulk composition, and trends that appear to require several controlling phases can be explained with only one phase of variable composition. The exercises illustrated here demonstrate that complex trends can be misinterpreted if the variations in partition coefficients are not taken into account, and the “average” phase proportions and compositions controlling an evolutionary trend can be erroneously estimated. Complex trends, such as those in Apollo 15 green glass beads, can possibly be understood in terms of simple partial equilibrium melting of one or two phases.     

北京周口店岩体中钾长石巨晶的特征及成因

花岗岩类岩石中的钾长石巨晶,有的认为是斑晶,有的认为是变斑晶。本文通过对北京周口店岩体中钾长石巨晶的野外产状、显微结构、矿物化学等特征研究,证明巨晶是在岩浆结晶作用的早—中期阶段,直接从不饱和水的熔体中生长而成的斑晶。钾长石巨晶多见于较贫钾长石组分的花岗岩类岩石(石英二长岩-花岗闪长岩)中,其原因可能是,与花岗岩和白岗岩岩浆相比,这种成分的岩浆温度较高,粘度较小,组分扩散较快,钾长石的成核速率较小,而生长速度较大。周口店岩体中钾长石巨晶的大小和含量的变异,可能主要受冷凝梯度dT/dt的控制。     

Trace element composition of igneous zircon: a thermal and compositional record of the accumulation and evolution of a large silicic batholith, Spirit Mountain, Nevada

Hafnium, U, Th, and REE content of zircons from the Spirit Mountain batholith in southern Nevada correlate with calculated temperatures from the Ti-in-zircon thermometer to support field and petrologic evidence of rejuvenation of crystal mush and melt extraction events during the 2-million year accumulation of the granitoid batholith. Marked variation in zircon composition from sample to sample, from grain to grain within individual samples, and from zone to zone within individual grains documents in detail a history of fluctuating conditions with repeated episodes of replenishment, reheating, crystal mush rejuvenation, fractional crystallization, and melt segregation. The zircons exhibit compositional and thermal variability indicative of variations in host melt composition due to (1) melt rejuvenation, mixing, and fractionation (2) coeval growth of other REE-rich accessory minerals, and possibly (3) fluctuation in fO₂.     

The influence of melt structure on trace element partitioning near the peridotite solidus

This experimental study examines the mineral/melt partitioning of Na, Ti, La, Sm, Ho, and Lu among high-Ca clinopyroxene, plagioclase, and silicate melts analogous to varying degrees of peridotite partial melting. Experiments performed at a pressure of 1.5 GPa and temperatures of 1,285 to 1,345 °C produced silicate melts saturated with high-Ca clinopyroxene, plagioclase and/or spinel, and, in one case, orthopyroxene and garnet. Partition coefficients measured in experiments in which clinopyroxene coexists with basaltic melt containing ~18 to 19 wt% Al₂O₃ and up to ~3 wt% Na₂O are consistent with those determined experimentally in a majority of the previous studies, with values of ~0.05 for the light rare earths and of ~0.70 for the heavy rare earths. The magnitudes of clinopyroxene/melt partition coefficients for the rare earth elements correlate with pyroxene composition in these experiments, and relative compatibilities are consistent with the effects of lattice strain energy. Clinopyroxene/melt partition coefficients measured in experiments in which the melt contains ~20 wt% Al₂O₃ and ~4 to 8 wt% Na₂O are unusually large (e.g., values for Lu of up to 1.33±0.05) and are not consistent with the dependence on pyroxene composition found in previous studies. The magnitudes of the partition coefficients measured in these experiments correlate with the degree of polymerization of the melt, rather than with crystal composition, indicating a significant melt structural influence on trace element partitioning. The ratio of non-bridging oxygens to tetrahedrally coordinated cations (NBO/T) in the melt provides a measure of this effect; melt structure has a significant influence on trace element compatibility only for values of NBO/T less than ~0.49. This result suggests that when ascending peridotite intersects the solidus at relatively low pressures (~1.5 GPa or less), the compatibility of trace elements in the residual solid varies significantly during the initial stages of partial melting in response to the changing liquid composition. It is unlikely that this effect is important at higher pressures due to the increased compatibility of SiO₂, Na₂O, and Al₂O₃ in the residual peridotite, and correspondingly larger values of NBO/T for small degree partial melts.Editorial responsibility: T.L. Grove     

Melt composition control of Zr/Hf fractionation in magmatic processes

Zircon (ZrSiO₄) and hafnon (HfSiO₄) solubilities in water-saturated granitic melts have been determined as a function of melt composition at 800° and 1035°C at 200 MPa. The solubilities of zircon and hafnon in metaluminous or peraluminous melts are orders of magnitude lower than in strongly peralkaline melt. Moreover, the molar ratio of zircon and hafnon solubility is a function of melt composition. Although the solubilities are nearly identical in peralkaline melts, zircon on a molar basis is up to five times more soluble than hafnon in peraluminous melts. Accordingly, calculated partition coefficients of Zr and Hf between zircon and melt are nearly equal for the peralkaline melts, whereas for metaluminous and peraluminous melts D_Hf/D_Zr for zircon is 0.5 to 0.2. Consequently, zircon fractionation will strongly decrease Zr/Hf in some granites, whereas it has little effect on the Zr/Hf ratio in alkaline melts or similar depolymerized melt compositions.The ratio of the molar solubilities of zircon and hafnon for a given melt composition, temperature, and pressure is proportional to the Hf/Zr activity coefficient ratio in the melt. The data imply that this ratio is nearly constant and probably close to unity for a wide range of peralkaline and similar depolymerized melts. However, it changes by a factor of two to five over a relatively small interval of melt compositions when a nearly fully polymerized melt structure is approached. For most ferromagnesian minerals in equilibrium with a depolymerized melt, D_Hf > D_Zr. Typical values of D_Hf/D_Zr range from 1.5 to 2.5 for clinopyroxene, amphibole, and titanite. Because of the change in the Hf/Zr activity ratio in the melt, the relative fractionation of Zr and Hf by these minerals will disappear or even be reversed when the melt composition approaches that of a metaluminous or peraluminous granite. It is thus not surprising that fractional crystallization of such granitic magmas leads to a decrease in Zr/Hf, whereas fractional crystallization of depolymerized melts tends to increase Zr/Hf. There is no need to invoke fluid metasomatism to explain these effects. Results demonstrate that for ions with identical charge and nearly identical radius, crystal chemistry does not alone determine relative compatibilities. Rather, the effect of changing activity coefficients in the melt may be comparable to or even larger than elastic strain effects in the crystal lattice.     

Experimental Studies on Partial Melting of Massive Samples of Granite

As a basis of modern petrology,the equilibrium relations describing the melting of granite were established mainly on melting experiments of Powder samples.Such experiments,however,have serious limitations in providing information about the variations in compositional and fabric features of the minerals and in the composition and distribution of the melt.Our experiments using massive samples indicate that melt occure mainly at the quartz-plagioclase and quartz-potash feldspar boundaries and the composition of the melt is dependent on local characteristics in the melting system,showing no correlation with the bulk composition of the rock samples.At lower temperatures(740-760℃,0.2GPa),the melt plots at or near the eutectic point in Q-Ab-Or-An-H2O diagram,indicating equilibrium melting.At higher temperatures(790-800℃，0.2GPa)the melt becomes lower in SiO2 and higher in Na2O,deviating makedly from the eutectic line but without disappearance of any mineral phase,suggesting a non-equilibrium process.It is obvious that the phase-equilibrium relations in natural massive granites may be greatly different from those deduced from powder experiments.     

A comparative study of melt-rock reactions in the mantle: laboratory dissolution experiments and geological field observations

Systematic variations in mineralogy and chemical composition across dunite-harzburgite (DH) and dunite-harzburgite-lherzolite (DHL) sequences in the mantle sections of ophiolites have been widely observed. The compositional variations are due to melt-rock reactions as basaltic melts travel through mantle peridotite, and may be key attributes to understanding melting and melt transport processes in the mantle. In order to better understand melt-rock reactions in the mantle, we conducted laboratory dissolution experiments by juxtaposing a spinel lherzolite against an alkali basalt or a mid-ocean ridge basalt. The charges were run at 1 GPa and either 1,300°C or 1,320°C for 8–28 h. Afterward, the charges were slowly cooled to 1,200°C and 1 GPa, which was maintained for at least 24 h to promote in situ crystallization of interstitial melts. Cooling allowed for better characterization of the mineralogy and mineral compositional trends produced and observed from melt-rock reactions. Dissolution of lherzolite in basaltic melts with cooling results in a clinopyroxene-bearing DHL sequence, in contrast to sequences observed in previously reported isothermal-isobaric dissolution experiments, but similar to those observed in the mantle sections of ophiolites. Compositional variations in minerals in the experimental charges follow similar melt-rock trends suggested by the field observations, including traverses across DH and DHL sequences from mantle sections of ophiolites as well as clinopyroxene and olivine from clinopyroxenite, dunite, and wehrlite dikes and xenoliths. These chemical variations are controlled by the composition of reacting melt, mineralogy and composition of host peridotite, and grain-scale processes that occur at various stages of melt-peridotite reaction. We suggest that laboratory dissolution experiments are a robust model to natural melt-rock reaction processes and that clinopyroxene in replacive dunites in the mantle sections of ophiolites is genetically linked to clinopyroxene in cumulate dunite and pyroxenites through melt transport and melt-rock reaction processes in the mantle.     

熔体包裹体的形成、改造和分析方法及其矿床学应用

熔体包裹体研究不仅广泛应用于火山岩和部分侵入岩系统,而且因其具有可以保存岩浆初始挥发分和金属组成的优势,近来也逐步应用于矿床学领域.在介绍熔体包裹体形成机制和捕获后成分改造的基础上,简要归纳了目前常用的熔体包裹体分析方法,以斑岩型Cu-(Mo-Au)和斑岩型Mo成矿系统为例,重点介绍熔体包裹体在矿床学领域的应用,包括成矿金属和挥发分含量的测定,以及熔体-流体分配系数测定等方面.然而,熔体包裹体在捕获后均会受到不同程度的成分改造,且对于大多数造岩矿物内的熔体包裹体,其成分改造的具体机制仍不明了,因此在实际应用过程中,需要对其组成进行具体分析和甄别.随着分析技术的改善和提高,熔体包裹体捕获后具体成分改造机制有待进一步查明,进而推动熔体包裹体的应用.现阶段熔体包裹体在斑岩型Cu-(Mo-Au)和斑岩型Mo成矿岩浆系统的成功应用表明,相比全岩地球化学研究,熔体包裹体已成为研究成矿岩浆体系内成矿金属和挥发分演化的重要手段.