Distribution of REE between clinopyroxene and basaltic melt along a mantle adiabat: effects of major element composition, water, and temperature

The distribution of rare earth elements (REE) between clinopyroxene (cpx) and basaltic melt is important in deciphering the processes of mantle melting. REE and Y partition coefficients from a given cpx-melt partitioning experiment can be quantitatively described by the lattice strain model. We analyzed published REE and Y partitioning data between cpx and basaltic melts using the nonlinear regression method and parameterized key partitioning parameters in the lattice strain model (D ₀, r ₀ and E) as functions of pressure, temperature, and compositions of cpx and melt. D ₀ is found to positively correlate with Al in tetrahedral site (Al ^T ) and Mg in the M2 site (Mg^M2) of cpx and negatively correlate with temperature and water content in the melt. r ₀ is negatively correlated with Al in M1 site (Al^M1) and Mg^M2 in cpx. And E is positively correlated with r ₀. During adiabatic melting of spinel lherzolite, temperature, Al ^T , and Mg^M2 in cpx all decrease systematically as a function of pressure or degree of melting. The competing effects between temperature and cpx composition result in very small variations in REE partition coefficients along a mantle adiabat. A higher potential temperature (1,400°C) gives rise to REE partition coefficients slightly lower than those at a lower potential temperature (1,300°C) because the temperature effect overwhelms the compositional effect. A set of constant REE partition coefficients therefore may be used to accurately model REE fractionation during partial melting of spinel lherzolite along a mantle adiabat. As cpx has low Al and Mg abundances at high temperature during melting in the garnet stability field, REE are more incompatible in cpx. Heavy REE depletion in the melt may imply deep melting of a hydrous garnet lherzolite. Water-dependent cpx partition coefficients need to be considered for modeling low-degree hydrous melting.     

Experimental study of interaction between fluid-bearing basaltic melts and peridotite: A mantle-crustal source of trap magmas in the Norilsk area

The paper reports data on the geology and tectono-magmatic reactivation of the Norilsk area and on the stratigraphy and geochemistry of its volcanic sequence, with the discussion of the sources and genesis of the ore magmas and the scale of the ore-forming process. According to the geochemistry of the lavas and intrusive rocks (Ti concentration and the La/Sm and Gd/Yb ratios), two types of the parental magmas are recognized: high-Ti magmas of the OIB type (from bottom to top, suites iv, sv, and gd of phase 1) and low-Ti magmas (suites hk, tk, and nd of phase 2 and suites mr-mk of phase 3), which were derived from the lithospheric mantle. The magmatic differentiation of the parental low-Ti magma of the tk type into a magma of the nd type was associated with the derivation of an evolved magma of the nd type, which was depleted in ore elements, and an ore magma, which was a mixture of silicate and sulfide melts, protocrysts of silicate minerals, and chromite. Judging from their geochemical parameters, the intrusions of the lower Norilsk type were comagmatic with the lavas of the upper part of the nd suite, and the ore-bearing intrusions of the upper Norilsk type were comagmatic with the lavas of the mr-mk suites. When the ore-bearing intrusions were emplaced, their magmas entrained droplets of sulfide melt and protocrysts of olivine and chromite and brought them to the modern magmatic chamber. These protocrysts are xenogenic with respect to the magma that formed the intrusions. In certain instances (Talnakh and Kharaelakh intrusions), the moving magma entrained single portions of sulfide magma, which were emplaced as individual subphases. The experimental study of the peridotite-basalt-fluid system shows that mantle reservoirs with protoliths of subducted oceanic crustal material could serve as sources of relatively low-temperature (1250–1350°C) high-Ti magnesian magmas of the rifting stage from an olivine-free source.     

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.     

Non-modal melting in an upwelling mantle column: Steady-state models with applications to REE depletion in abyssal peridotites and the dynamics of melt migration in the mantle

Simple models for trace element fractionation during concurrent melting and melt migration in an upwelling steady-state mantle were developed. Based on petrologic considerations, we divided the mantle column into two regions: a single-lithology lower region that consists of partially molten garnet and spinel lherzolites and a double-lithology upper region where high-porosity dunite channels or melt-filled fractures are embedded in a porous lherzolite/harzburgite matrix. Analytical solutions for the case of a constant and uniform relative melting suction rate and a linearly variable relative melt suction rate were obtained. Key parameters and the first order characteristics of melting and melt migration in a 1-D steady-state mantle column were examined through forward calculations and Monte Carlo simulations. Melting in the upwelling single-lithology column is equivalent to non-modal batch melting, whereas melting and melt migration in the double-lithology region can be viewed as a nonlinear combination of batch melting and fractional melting, depending on the amount of melt extracted to the channel. The degree of melting (F), the degree of melting at the depth of melt-channel initiation (F_d) and the relative rate of melt suction (R) are important in controlling the extent of depletion of the incompatible trace element in the matrix. Spatially variable R affects the abundance of an incompatible trace element in the melt and residual solid the most in near fractional melting. There is a strong nonlinear trade off among the three parameters. Given F_d, it is possible to constrain F and R from incompatible trace element abundances in residual peridotite.To explore the dynamics of melt migration in the mantle, we used the two melting models developed in this study and published REE and Y abundances in diopside in abyssal peridotites from the Central Indian Ridge to infer their melting and melt migration history. Overall, the degrees of melting inferred from the trace element data are not sensitive to the value of F_d used in the inversion and ranges from 10% to 15%. The relative rate of melt suction depends slightly on the choice of F_d and ranges from 0.85 to 1.0 for F_d = 0.05 and 0.75 to 0.97 for F_d = 0. Further, the estimated R is inversely correlated with F, a robust feature independent of the choice of F_d. The upward decrease of R in an upwelling mantle column can be understood in terms of melt focusing in the lower part of the double-lithology region. And finally, given F and R, we found that the permeability and porosity of the lherzolite/harzburgite matrix also increase as a function of F in the melting column, with melt fractions ranging from 0.2% to 0.7% for a grain size of 5 mm.     

Constraints on clinopyroxene/melt partitioning of REE, Rb, Sr, Ti, Cr, Zr, and Nb during mantle melting: First insights from direct peridotite melting experiments at 1.0 GPa

Earlier piston-cylinder experiments in our laboratory produced a collection of mantle melting run products at 1.0 GPa that have now been analyzed by ion probe for selected REE, Ti, Cr, Rb, Sr, Y, Zr, and Nb. Natural starting materials were used and experiments were run in graphite-lined Pt capsules with the melt separated from the residual minerals into a layer of vitreous carbon spheres (VCS) to circumvent quench modification. The glass phase in 18 run products, representing melt percentages of ∼2-20 wt%, yielded excellent data that were inverted to yield the first estimates ever of clinopyroxene/melt distribution coefficients, Ds, derived from direct peridotite partial melting experiments. Uncertainties were estimated with a Monte Carlo method.For the REE and Y, these Ds were then compared to Ds calculated with the widely-used model of Wood and Blundy (1997) and the two sets overlap at the ±2σ level in 123 of 128 cases (∼96%). This indicates to us that: 1) the experiments analyzed here are well equilibrated with respect to major and trace element distributions, thus supporting the efficacy of the VCS technique and its variation involving diamond (e.g., Baker and Stolper, “Determining the composition of high-pressure mantle melts using diamond aggregates” [1994], Geochim. Cosmochim. Acta58, 2811-2827); 2) the model of Wood and Blundy (1997), calibrated largely on the basis of large melt fraction, inverse- or sandwich-type experiments, describes REE and Y partitioning during peridotite melting well, even very near the solidus; and it suggests that the cpx/melt Ds derived here for other elements, not modeled by the Wood and Blundy formulation, are probably also correct for peridotite melting to within their ±2σ uncertainties. D^sp/liq and D^cpx/liq values for Cr calculated directly from electron microprobe data decrease by about a factor of five with increasing temperature and melt percentage.The degree to which our experiments appear to have equilibrated seems at odds with recent measurements of the diffusivities of REE in diopside which suggest that relatively small percentages of our starting mineral grains should have equilibrated diffusively. Instead, we suggest that equilibration occurs much more rapidly through the processes of recrystallization and grain coarsening, accomplished through dissolution and reprecipitation. This suggestion is supported by the observation that our final grain sizes are typically 5-10 times larger than the ∼10 μm starting sizes, indicating that substantial mass transfer occurred in our experiments, probably mediated by the melt phase in which diffusion is faster.     

Numerical modeling of major element distribution between chromian spinel and basaltic melt, with application to chromian spinel in MORBs

A set of empirical equations is developed which allows calculation of chromian spinel composition in equilibrium with a basaltic melt under a known set of conditions. These equations are calibrated with published experimental data for the temperature range from 1,093 to 1,490 °C and a pressure of up to 20 kbar. It is demonstrated that the composition of chromian spinel from the experiments, which crystallizes from basaltic, boninitic, and komatiitic melt and varies from high-Al to high-Cr, and high-Fe³⁺ can be successfully reproduced using the suggested model. The composition of chromian spinel has been calculated using the glass composition for a set of primitive basaltic lavas using the suggested set of empirical equations. Good agreement between the calculated composition and composition of rims of chromian spinel included in glass is achieved for compositionally diverse spinel from a Hawaiian sample and 15 MORB samples. The Fe₂O₃ content in the chromian spinels is, however, often variable and higher than the calculated Fe₂O₃ content. Additional calculations using 76 published MORB glass analyses reproduced most of the MORB chromian spinel range, except for the most Cr- and Mg-rich. The crystallization of these Cr-rich chromian spinels likely occurred from a more primitive Al-poor melt than that of the 76 MORB glasses. An example of a more primitive glass can occasionally be found as glass inclusions in these Cr- and Mg-rich chromian spinel microphenocrysts.     

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.     

Experimental comparison of trace element partitioning between clinopyroxene and melt in carbonate and silicate systems, and implications for mantle metasomatism

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     

Pyroxenite xenoliths from Marsabit (Northern Kenya): evidence for different magmatic events in the lithospheric mantle and interaction between peridotite and pyroxenite

Garnet-bearing and garnet-free pyroxenite xenoliths from Quaternary basanites of Marsabit, northern Kenya, were analysed for microstructures and mineral compositions (major and trace elements) to constrain the thermal and compositional evolution of the lithospheric mantle in this region. Garnet-bearing rocks are amphibole-bearing websterite with ~5–10 vol% orthopyroxene. Clinopyroxene is LREE-depleted and garnet has high HREE contents, in agreement with an origin as cumulates from basaltic mantle melts. Primary orthopyroxene inclusions in garnet suggest that the parental melts were orthopyroxene-saturated. Rock fabrics vary from weakly to strongly deformed. Thermobarometry indicates extensive decompression and cooling (~970–1,100°C at ~2.3–2.6 GPa to ~700–800°C at ~0.5–1.0 GPa) during deformation, best interpreted as pyroxenite intrusion into thick Paleozoic continental lithosphere subsequently followed by continental rifting (i.e., formation of the Mesozoic Anza Graben). During continental rifting, garnet websterites were decompressed (garnet-to-spinel transition) and experienced the same P–T evolution as their host peridotites. Strongly deformed samples show compositional overlaps with cpx-rich, initially garnet-bearing lherzolite, best explained by partial re-equilibration of peridotite and pyroxenite during deformation and mechanical mingling. In contrast, garnet-free pyroxenites include undeformed, cumulate-like samples, indicating that they are younger than the garnet websterites. Major and trace element compositions of clinopyroxene and calculated equilibrium melts suggest crystallisation from alkaline basaltic melt similar to the host basanite, which suggests formation in the context of alkaline magmatism during the development of the Kenya rift. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Phase relationships in the system KAlSiO4-Mg2SiO4-SiO2-H2O as a model for hybridization between hydrous siliceous melts and peridotite

The system KAlSiO₄-Mg₂SiO₄-SiO₂-H₂O includes model representatives of (1) hydrous siliceous magma from subducted oceanic crust — the eutectic liquid in KAlSi₃O₈-SiO₂-H₂O, and (2) the overlying mantle peridotite — the assemblage forsterite+enstatite (Fo+En). In a series of partly schematic isobaric isothermal sections, the products of hybridization between the model materials at pressures between 20 and 30 kbar have been determined. The liquid dissolves peridotite components with little change in composition. Hybridization is not a simple mixing process, because of the incongruent melting of peridotitic assemblages with phlogopite (Ph). Hybridization causes solidification of the liquid, with products a sequence of three mineral assemblages: Ph, Ph+quartz (Qz), and Ph+En. The products represent an absolute geochemical separation and local concentration of all potassium from the liquid. Hybridization is accompanied by H₂O-saturation of melts, and evolution of aqueous fluid. Although there are significant differences between the melt composition and that of the magma rising from subducted oceanic slab, and between Fo+En and the mantle rock, extrapolation of the results suggests that the conclusions can probably be extended to mantle conditions with sodium in the melt, and jadeitic clinopyroxene included in the hybrid products.     

Extension of lattice strain theory to mineral/mineral rare-earth element partitioning: An approach for assessing disequilibrium and developing internally consistent partition coefficients between olivine, orthopyroxene, clinopyroxene and basaltic melt

Olivine/melt and orthopyroxene/melt rare-earth element (REE) partition coefficients consistent with clinopyroxene/melt partition coefficients were determined indirectly from subsolidus partitioning between olivine, orthopyroxene, and clinopyroxene after suitable correction for temperature. Heavy- and middle-REE ratios for olivine/clinopyroxene and orthopyroxene/clinopyroxene pairs correlate negatively with effective cationic radius, whereas those for the light REEs correlate positively with cationic radius, generating a U-shaped pattern in apparent mineral/clinopyroxene partition coefficients versus cationic radius. Lattice strain models of partitioning modified for subsolidus conditions yield negative correlations of olivine/clinopyroxene and orthopyroxene/clinopyroxene with respect to cationic radii, predicting well the measured partitioning behaviors of the heavy and middle REEs but not that of the light REEs. The light-REE systematics cannot be explained with lattice strain theory and, instead, can be explained by disequilibrium enrichment of the light REEs in melt inclusions or on the rims of olivine and orthopyroxene. Realistic light-REE partition coefficients were thus extrapolated from the measured heavy- and middle-REE partition coefficients using the lattice strain model. Light REE olivine/melt and orthopyroxene/melt partition coefficients calculated in this manner are lower than most published values, but agree reasonably well with partitioning experiments using the most recent in situ analytical techniques (secondary-ionization mass spectrometry and laser ablation inductively coupled plasma mass spectrometry). These new olivine/melt and orthopyroxene/melt partition coefficients are useful for accurate modeling of the REE contents of clinopyroxene-poor to -free lithologies, such as harzburgitic residues of melting. Finally, the application of the lattice strain theory to subsolidus conditions represents a framework for assessing the degree of REE disequilibrium in a rock.     

Fluid+melt intrusions in lechatelierite from the Popigai suevites: A product of dynamic interaction melts and fluids during the shock melting of the target gneiss

The paper presents data on lechatelierite form suevites of the Daldyn Formation in the Popigai astrobleme. Some of the lechatelierite samples show a complicated structure and contain block of diaplectic quartz glass and dynamic “intrusions” of glasses of types I, II, and III. The glasses of types I and II abound in fluid inclusions and display evidence of partial homogenization with lechatelierite. The glasses of type III are clearly separated from all other glasses but show evidence of dynamic interaction with them in the molten state. Fluid inclusions in the glasses of types I and II are syngenetic but have notably different densities from those of completely liquid or gaseous inclusions at 20°C. As is indicated by cryometric data, the liquid phase of the inclusions is aqueous solution of low salinity (5–8 wt % NaCl_equiv). The bulk petrochemistry of the glasses of type I characterizes them as highly silicic (96.04 wt % SiO₂ on average), with elevated K and Na concentrations (Na₂O + K₂O = 0.72 wt % on average), with 0.73 wt % Al₂O₃ (on average) and analytical totals 1.97 wt % less than 100%. The glasses of type II are also rich in SiO₂ (91.51 wt % SiO₂ on average) but contain a broader spectrum of concentrations of major oxides (totaling 5.53 wt % on average) and deficient analytical totals (by 2.96 wt % on average). The glasses of type III are completely equal to impactites produced by melting gneisses of the Popigai astrobleme. The glasses of type I are interpreted to be the intrusion products of the “early” highly mobile and H₂O-rich fluid+melt mixtures, whose protolithic material was K-Na feldspars of the target rocks. The derivation of these melts was associated with the capturing of much silica and water at a highly mobile behavior of K and Na and an inert behavior of Al. The glasses of type II were produced by the extensive mixing of silica and water at the limited involvement of apogneiss melts, and these glasses are sometimes deficient in Al. The glasses of type III are usual mixed apogneiss melts. Excess silica in the glasses of types I and II and their richness in water and deficiency in Al suggest impact anatexis and the selective separation of components during their derivation; the parental fluid-melt mixtures of these glasses were derived from such “hydrous” varieties of the target gneisses as diaphthorized and fractured rocks. The evolution and partial vitrification of lechatelierite and the glasses of types I and II proceeded under residual shock pressures, as follows from data on the dense (from ∼0.5 to 1 g/cm³) aqueous inclusions in these glasses, which suggest that the inclusions were captured in the glasses under pressures from ∼0.8 to 3.3 GPa. It follows that our lechatelierite samples have a complex multistage genesis, and their quenching facilitated the preservation of “intrusions” of various stages of shock melting, including the products of the “early” impact anatexis of the gneisses with the selective separation of components at the active participation of water.     

A model for coupled fluid-flow and mixed-volatile mineral reactions with applications to regional metamorphism

The effect of fluid flow on mixed-volatile reactions in metamorphic rocks is described by an expression derived from the standard equation for coupled chemical-reaction and fluid-flow in porous media. If local mineral-fluid equilibrium is assumed, the expression quantitatively relates the time-integrated flux at any point in a flow-system to the progress of devolatilization reactions and the temperature- and pressure-gradients along the direction of flow. Model calculations indicate that rocks are generally devolatilized by fluids flowing uptemperature and/or down-pressure. Flow down-temperature typically results in hydration and carbonation of rocks. Time-integrated fluid fluxes implied by visible amounts of mineral products of devolatilization reactions are on the order of 5·10²–5·10⁴ mol/cm². The model was applied to regionally metamorphosed impure carbonate rocks from south-central Maine, USA, to obtain estimates of fluid flux, flow-direction, and in-situ metamorphic-rock permeability from petrologic data. Calculated time-integrated fluxes are 10⁴–10⁶ cm³/cm² at 400°–450° C, 3,500 bars. Fluid flowed from regions of low temperature to regions of high temperature at the peak of the metamorphic event. Using Darcy's Law and estimates for the duration of metamorphism and hydrologic head, calculated fluxes are 0.1–20·10^-4 m/year and minimum permeabilities are 10^-10–10^-6 Darcy. The range of inferred permeability is in good agreement with published laboratory measurements of the permeability of metamorphic rocks.     

东波超镁铁岩体:西藏雅鲁藏布江缝合带西段一个甚具铬铁矿前景的地幔橄榄岩体

东波超镁铁岩体产在雅鲁藏布江缝合带的西段,与周边白垩纪沉积岩地层和火山岩以断层接触.航磁资料显示该岩体约400km2规模,地表出露连续,地下有一定延深.超镁铁岩体由亏损的地幔橄榄岩组成,主要有高镁的方辉橄榄岩、纯橄岩和少量二辉橄榄岩.方辉橄榄岩和二辉橄榄岩中橄榄石和斜方辉石属高镁型,分别为Fo=89.5～91.5和Mg#=90～91.5.但二辉橄榄岩中的Al2O3和CaO含量明显高于方辉橄榄岩.方辉橄榄岩中单斜辉石Mg#=92～95,二辉橄榄岩的Mg#=92～93,两者的值也重叠.二辉橄榄岩中的Al2O3和CaO含量要明显高于方辉橄榄岩.这些均为阿尔卑斯型地幔橄榄岩的典型特征.纯橄岩中的橄榄石Fo=92～93.2,其斜方辉石和单斜辉石的Mg#=～93,但Al2O3和CaO的含量比方辉橄榄岩和二辉橄榄岩的低.三种岩石的成分变化规律,反映了地幔部分熔融程度的差异.二辉橄榄岩铬尖晶石的Cr#值20～30,反映为典型深海橄榄岩特征,指示MOR环境.与其不同的是,方辉橄榄岩的铬尖晶石的Cr#=20～75,指示MOR和SSZ两者兼有环境.岩石的原始地幔标准化的REE和微量元素蛛网图模式支持了上述的认识.东波地幔橄榄岩中的岩石学特征与产有大型铬铁矿床的罗布莎地幔橄榄岩可对比,岩体中已多处发现块状铬铁矿石,其铬铁矿的Cr2O3含量56％～59％,表明东波是寻找铬铁矿大矿和富矿甚具前景的一个超镁铁岩体.     

Interpretation of light curves of the cataclysmic variable OY Car in a model with shockless interaction between a gaseous stream and the disk

To determine the parameters of the accretion disk and shock-wave region responsible for the formation of the orbital peak in the light curve of the binary system OY Car (an SU UMa-type variable), we have analyzed its U BV R and JK light curves using two gas-dynamical models with different regions of shock interaction: one with a hot line along the stream from the Lagrange point L1 and one with a hot spot on the accretion disk. The hot-line model can better describe the quiescent state of the system: the maximum X² for the optical light curves does not exceed 207, whereas the minimum residual for the hot-spot model is X²>290. The shape of the eclipse is almost identical in both models; the main differences are in interpreting out-of-eclipse portions of the light curves, whose shape can varyin the transition from one orbital cycle to another. The hot-spot model is not able to describe variations of the system’s brightness at orbital phases ?～0.1–0.6. The rather complex behavior of the observed flux in this phase interval can be explained in the hot-line model as being due to variations of the temperature and size of the system. Based on the analysis of a sequence of 20 B curves of OY Car, we conclude that the flux variations in the primary minimum are due to variations of the luminosity of the accretion disk, whereas the flux variability in the vicinity of the orbital peak is due to the combined effect of the radiation of the disk and hot line. The JK light curves of OY Car in the quiescent state and during a small flare also indicate preference for the hot-line model, since the primaryminimum and the flux near quadratures calculated using the hot-spot model are not consistent with the observations.