An ion microprobe study of the partitioning of trace elements between clinopyroxene and liquid in the system diopside-albite-anorthite

The partitioning of trace elements (Sc, Ti, Sr and Sm) between diopsidic clinopyroxene and liquid was studied experimentally in the system diopside-albite-anorthite at 1250°C, 1300°C and 1345°C at 1 atm. Twelve different bulk compositions were selected to study the effects of temperature and chemical composition. A Cameca ion microprobe was used to determine trace element concentrations in both clinopyroxene and liquid. Experiments of different run duration $\text{1}\text{4}\text{8}\text{?}$ days) showed that equilibrium was approached in less than 4 days at 1275°C. Equilibrium was also evaluated by a reversal run. A series of runs of constant bulk composition but with variable trace element contents showed that Henry's Law was obeyed over concentration ranges of the trace elements similar to those encountered in natural systems. The partition coefficients show significant ranges: Sc, 0.345~2.61; Ti, 0.084~0.214; Sr, 0.075~0.136; Sm, 0.054~0.328; the values are comparable with those obtained experimentally by other investigators. The partition coefficients vary as a function of both temperature and chemical composition. The experimental results are discussed in terms of exchange equilibria using the Bottinga-Weill silicate melt model. It is demonstrated that analytical uncertainties of both major and trace elements play an important role in understanding trace element exchange equilibria; propagation of analytical errors in the thermodynamic treatment is equally important.     

Rare earth element partitioning between titanite and silicate melts: Henry’s law revisited

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

锆石微量元素的理论基础及其应用研究进展

锆石是地质学研究中最重要的副矿物,其分布广泛、物理、化学性质稳定,记录了结晶时的年龄、温度、氧逸度以及O-Hf-Si-Zr-Li等多元同位素和微量元素信息,被广泛运用于地球科学的研究中。近年来,随着分析技术的发展,研究者在获取锆石年龄的同时也获取了大量锆石微量元素数据,这些数据的积累推动着研究者对锆石微量元素理论研究的不断深入,并取得了一系列重要进展,如发现锆石微量元素组成受锆石本身的晶格特点主导,符合晶格应变模型和类质同象替代机制;发现锆石微量元素组成受到熔体成分演化影响,锆石结晶时的熔体微量元素组成往往不等同于全岩;发现锆石内部的微量元素不均一特征(矿物包裹体、热点、蜕晶化作用等)可能会严重影响锆石的微量元素组成,继而建立了"干净锆石"的判别指标和筛选机制。此外,锆石微量元素的应用研究也取得了长足进展,研究者们不断尝试通过各类锆石微量元素指标、图解、分配系数,识别母岩浆物理化学性质、反演母岩浆组成,大大推动了锆石微量元素在示踪岩浆源区和岩浆过程中的应用。然而,由于锆石微量元素组成受控于多种因素,使得锆石微量元素在实际应用当中常常面临着多解性问题、重叠问题和分配系数的选择问题,在一定程度上影响了锆石微量元素应用研究的可靠性。未来的锆石微量元素研究将不满足于使用传统的低维指标和图解以及分配系数,而将在充分吸收传统方法精华的基础上,从海量数据与更高的维度中寻找元素之间相关性,基于热力学定律揭示新原理,基于更高空间分辨率揭示动力学因素的影响,从数据驱动和理论驱动的全新视角下深入揭示隐藏在锆石微量元素中的信息。     

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.     

Petrologic implications of trace element variation in clinopyroxene megacrysts from the Nógrád volcanic province, north Hungary: a study by laser ablation microprobe-inductively coupled plasma-mass spectrometry

Clinopyroxene megacrysts in alkali basalts are an important source of information about the evolution of magmatic systems at depth. In this study, we have undertaken a detailed examination of the trace element contents in a suite of megacrysts from 2.5 Ma old alkali basalts in the Nógrád volcanic province of Hungary and Slovakia. The megacrysts range in composition from Mg-rich and in equilibrium with their host magmas, to those that are Fe-rich and must have evolved in more fractionated magmas. The conditions of crystallization of these megacrysts, as calculated from the Al^VI/Al^IV ratios, suggests they all formed at about 30 km, or the crust–mantle boundary in this area. Using the most magnesian megacrysts and compositions of the host lavas, we have calculated the partition coefficients for a range of trace elements. However, the trace element contents in the megacrysts show a systematic variation with major element composition. Moreover, the rate of increase or change in the trace element concentrations is not consistent with models involving constant or steady state partition coefficients. Using a series of assumptions and models, we hypothesize that the partition coefficients between clinopyroxene and melt change substantially during the magmatic evolution of the system. This change is not constant for each element group, with the high field strength elements showing the most substantial increases. Electrostatic charge balance may have been the most important factor in controlling the mineral/melt partitioning.     

Trace element partitioning in the granulite facies

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.     

A quantitative approach to trace element and Sr isotope evolution in the Adamello batholith (northern Italy)

A development of De Paolo's mathematical procedure (1981) for magmatic AFC (Assimilation-Fractional Crystallization) processes is discussed with respect to both trace element and Sr isotopic ratio behaviours during the genesis and evolution of Adamello batholith (northern Italy). Resolution of a two equation-system (one relative to ⁸⁷Sr/⁸⁶Sr ratio variation in a magma generated by an AFC process, the other to its trace element content variations) gives the F (mass of magma at time t/mass of initial magma) and D (bulk partition coefficient) values, by which one can deduce the r (rate of assimilation/rate of crystallization) value during each step of magmatic evolution. This quantitative approach suggests that: 1) there was a common precursor magma for all the Adamello granitoids, with a Mg-rich tholeiitic composition; 2) each intrusive unit appears to have been generated by different extents of AFC; 3) the trace element distribution in the magma seems essentially influenced by mineral fractionation, rather than by the composition of the assimilated crustal material.     

Trace element distribution between orthopyroxene and clinopyroxene in peridotites from the Gakkel Ridge: a SIMS and NanoSIMS study

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.     

A review of experimental studies of crystal/liquid trace element partitioning

Available data on silicate/liquid, phosphate/liquid and oxide/liquid trace element partition coefficients from experimental studies show an encouraging degree of consistency, although much more work is required before a set of coefficients of wide-ranging application to planetary problems can be compiled. The complex dependences of most coefficients on bulk chemical composition (and liquid structure) are important aspects which remain to be fully resolved. Further determinations of coefficients for a number of elements for phosphates, zircon, spinels, amphiboles and garnets are especially desirable and investigations into the effects of pressure and volatiles on the magnitudes of partition coefficients are also needed. There is a major discrepancy among existing data regarding the upper concentration limits of Henry's Law dilute solution behavior under different experimental conditions. Data obtained at 1 atmosphere under dry conditions can apparently be reconciled with data from highpressure, H₂O-saturated experiments only if Henry's Law limits are themselves functions of variables such as pressure, temperature, H₂O activity and chemical composition. Further experiments (including studies bearing on the role of defect substitution at very low trace element concentrations) are required to resolve this question.     

Trace element partitioning between carbonatitic melts and mantle transition zone minerals: Implications for the source of carbonatites

The occurrence of CO₂-rich lavas (carbonatites, kimberlites) and carbonate-rich xenoliths provide evidence for the existence of carbonatitic melts in the mantle. To model the chemical composition of such melts in the deep mantle, we experimentally determined partition coefficients for 23 trace elements (including REE, U-Th, HFSE, LILE) between deep mantle minerals and carbonatite liquids at 20 and 25 GPa and 1600 °C. Under these conditions, majoritic garnet and CaSiO₃ perovskite are the main reservoirs for trace elements. This study used both femtosecond LA-ICP-MS and SIMS techniques to measure reliable trace element concentrations. Comparison of the two techniques shows a general agreement, except for Sc and Ba. Our experimentally determined partition coefficients are consistent with the lattice strain model. The data suggest an effect of melt structure on partition coefficients in this pressure range. For instance, strain-free partition coefficient (D₀) for majorite-carbonatite melts do not follow the order of cation valence, , observed for majorite-CO₂-free silicate melts. The newly determined partition coefficients were combined with trace element composition of majoritic garnets found as inclusions in diamond to model trace element patterns of deep-seated carbonatites. The result compares favorably with natural carbonatites. This suggests that carbonatites can originate from the mantle transition zone.     

Compositional effects on element partitioning between Mg-silicate perovskite and silicate melts

High-pressure melting experiments were performed at ~26 GPa and ~2,200–2,400°C on synthetic peridotite compositions with varying FeO and Al₂O₃ contents and on a synthetic CI chondrite analogue composition. Peridotite liquids show a crystallisation sequence of ferropericlase (Fp) followed down temperature by Mg-silicate perovskite (MgPv) + Fp, which contrasts a sequence of MgPv followed by MgPv + Fp observed in the chondritic composition. The difference in crystallisation sequence is a consequence of the different bulk Mg/Si ratios. MgPv/melt partition coefficients for major, minor and trace elements were determined by electron microprobe and secondary ion mass spectrometry. Partition coefficients of tri- and tetravalent elements increase with increasing Al concentration in MgPv. A lattice strain model indicates that Al³⁺ substitutes predominantly onto the Si-site in MgPv, whereas most elements substitute onto the Mg-site, which is consistent with a charge-compensating coupled substitution mechanism. MgPv/melt partition coefficients for Mg (D_Mg) and Si (D_Si) are related to the melt Mg/Si ratio such that D_Si becomes lower than D_Mg at low Mg/Si melt ratios. We use a crystal fractionation model, based on upper mantle refractory lithophile element ratios, to constrain the amount of MgPv and Ca-silicate perovskite (CaPv) that could have fractionated during a Hadean magma ocean event and could still be present as a chemically distinct heterogeneity in the lower mantle today. We show that a fractionated crystal pile composed of 96% MgPv and 4% CaPv could comprise up to 13 wt% of the entire mantle.     

SIMS analyses on trace and rare earth elements in coexisting clinopyroxene and mica from minette mafic enclaves in potassic syenites crystallized under high pressures

Trace and rare earth element contents were determined by SIMS technique in clinopyroxene and mica crystals from minette lamprophyric enclaves in a potassic syenite host. This co-mingled system was crystallized at high pressures, which varied about 3–5 GPa, as indicated by the presence of K-clinopyroxenes and pyrope-rich garnet with measurable amounts of K₂O and Na₂O, among the near-liquidus phases. Major and trace element composition of these lamprophyric enclaves is quite similar to those observed in silica-rich lamproites, suggesting that similar sources were involved in their origin. In a general view, the concentrations of most trace and rare earth elements in clinopyroxene of the studied enclaves are higher than those referred to by other authors. Clinopyroxene/melt partition coefficient for most trace elements are close to determinations in alkali-basalts and lamproites from Leucite Hills, with considerable differences relative to Gaussberg lamproites. Furthermore, these partition data are completely different from those determined for potassic lavas crystallized under crustal pressures. Spidergrams for clinopyroxenes exhibit negative-Sr anomalies relative to LREE, which have been associated by most authors to crystallization under low-pressures, out of garnet stability field. The presence of pyrope together with K-clinopyroxene excludes such hypotheses for the studied enclaves. Y and HREE are concentrated in clinopyroxene, whilst the other trace elements have Kd<1. LIL elements, except Rb, have incompatible (Kd<1) behavior in phlogopite. The high partition coefficient for Nb (Kd>3) determined in the studied phlogopite is unusual in lamproites, lamprophyres, and basalts, but frequently observed in phlogopite from metasomatic mantle samples, as well as in acid magmas. This partition value may indicate the lack of other mineral phase with high partition for this element during crystallization, and may be enhanced by the liquid composition progressively closer to alkali feldspar, an unsuitable structure for six-coordinated cations. Ce/Yb, Rb/Sr, and Zr/Hf ratios in clinopyroxene and mica suggest that the minettic magma could produce the host Piquiri potassic syenite by fractional crystallization. This hypothesis is not consistent with Ba concentrations in clinopyroxene and mica, which suggest that a Ba-bearing phase (e.g. alkali feldspar) should be among the fractionated phases in order to produce the potassic syenites.     

Equilibrium vs. disequilibrium melting relations in theHigher Himalayan Crystalline (HHC) pelitic migmatites,Sikkim Himalaya

Higher Himalayan Crystalline (HHC) complex of the Sikkim Himalaya predominantly consists of high-grade pelitic migmatites. In this study, reaction textures, mineral/bulk rare earth elements (REE), trace element partition coefficients and trace element zoning profiles in garnet are used to demonstrate a complex petrogenetic process during crustal anatexis. With the help of equilibrium REE and trace element partitioning model, it is shown that strong enrichment of Effective Bulk Composition (EBC) is responsible for the zoning in garnet in these rocks. The data strongly support disequilibrium element partitioning and suggest that the anatectic melts associated with mafic selvedges are likely produced by disequilibrium melting because of fast melt segregation process.     

Controls on the fractionation of isovalent trace elements in magmatic and aqueous systems: evidence from Y/Ho,Zr/Hf,and lanthanide tetrad effect

 The parameters which control the behaviour of isovalent trace elements in magmatic and aqueous systems have been investigated by studying the distribution of yttrium, rare-earth elements (REEs), zirconium, and hafnium. If a geochemical system is characterized by CHArge-and-RAdius-Controlled (CHARAC) trace element behaviour, elements of similar charge and radius, such as the Y-Ho and Zr-Hf twin pairs, should display extremely coherent behaviour, and retain their respective chondritic ratio. Moreover, normalized patterns of REE(III) should be smooth functions of ionic radius and atomic number. Basic to intermediate igneous rocks show Y/Ho and Zr/Hf ratios which are close to the chondritic ratios, indicating CHARAC behaviour of these elements in pure silicate melts. In contrast, aqueous solutions and their precipitates show non-chondritic Y/Ho and Zr/Hf ratios. An important process that causes trace element fractionation in aqueous media is chemical complexation. The complexation behaviour of a trace element, however, does not exclusively depend on its ionic charge and radius, but is additionally controlled by its electron configuration and by the type of complexing ligand, since the latter two determine the character of the chemical bonding (covalent vs electrostatic) in the various complexes. Hence, in contrast to pure melt systems, aqueous systems are characterized by non-CHARAC trace element behaviour, and electron structure must be considered as an important additional parameter. Unlike other magmatic rocks, highly evolved magmas rich in components such as H₂O, Li, B, F, P, and/or Cl often show non-chondritic Y/Ho and Zr/Hf ratios, and “irregular” REE patterns which are sub-divided into four concave-upward segments referred to as “tetrads”. The combination of non-chondritic Y/Ho and Zr/Hf ratios and lanthanide tetrad effect, which cannot be adequately modelled with current mineral/melt partition coefficients which are smooth functions of ionic radius, reveals that non-CHARAC trace element behaviour prevails in highly evolved magmatic systems. The behaviour of high field strength elements in this environment is distinctly different from that in basic to intermediate magmas (i.e. pure silicate melts), but closely resembles trace element behaviour in aqueous media. “Anomalous” behaviour of Y and REEs, and of Zr and Hf, which are hosted by different minerals, and the fact that these minerals show “anomalous” trace element distributions only if they crystallized from highly evolved magmas, indicate that non-CHARAC behaviour is a reflection of specific physicochemical properties of the magma. This supports models which suggest that high-silica magmatic systems which are rich in H₂O, Li, B, F, P, and/or Cl, are transitional between pure silicate melts and hydrothermal fluids. In such a transitional system non-CHARAC behaviour of high field strength elements may be due to chemical complexation with a wide variety of ligands such as non-bridging oxygen, F, B, P, etc., leading to absolute and relative mineral/melt or mineral/aqueous-fluid partition coefficients that are extremely sensitive to the composition and structure of this magma. Hence, any petrogenetic modelling of such magmatic rocks, which utilizes partition coefficients that have not been determined for the specific igneous suite under investigation, may be questionable. But Y/Ho and Zr/Hf ratios provide information on whether or not the evolution of felsic igneous rocks can be quantitatively modelled: samples showing non-chondritic Y/Ho and Zr/Hf ratios or even the lanthanide tetrad effect should not be considered for modelling. However, the most important result of this study is that Y/Ho and Zr/Hf ratios may be used to verify whether Y, REEs, Zr, and Hf in rocks or minerals have been deposited from or modified by silicate melts or aqueous fluids. Received: 4 September 1995 / Accepted: 30 October 1995     

Systematic use of trace elements in igneous processes

Trace element concentration data can be used in a systematic way for the study of igneous processes by means of constructing models of such processes which satisfactorily account for the observations. We propose to treat the problem as an inverse problem. The concept of trace element paths (TEP) is introduced as a representation of the solution to the direct problem. The inverse problem consists of estimating, by a resolution of the equations, the various parameters of a model so as to provide a best fit to observed TEP. A detailed account of the theory is given in the case of equilibrium fractional crystallization. The estimated parameters are then those figuring in the Rayleigh distillation law, namely, 1) the initial concentrations of trace elements in the parental magma, 2) the bulk partition coefficients of the elements, and 3) the degree of crystallization corresponding to each sample of the magmatic suite analyzed.A slightly generalized maximum likelihood method is used to solve the linearized equation by a stable, iterative algorithm. Information theory is then shown to yield an account of the distribution and flow of information during the process of solving the inverse problem. The concept of Data Importances is generalized, and its use in optimizing the study justified. The technique is successfully applied to a synthetic data set, and then illustrated on a data set from Terceira (Azores). The results are used to refine the conclusions reached in part I, and permit a more detailed discussion of the model.Now at Dept. Geological and Planetary Sciences, California Institute of Technology, Pasadena, Calif., USA     

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.     

Aluminum-dependent trace element partitioning in clinopyroxene

As technical advances have dramatically increased our ability to analyze trace elements, the need for more reliable data on the compositional dependence of trace element partitioning between minerals and melt has become increasingly important. The late-Cretaceous Carmacks Group of south central Yukon comprises a succession of primitive high-Mg ankaramitic lavas characterized by shoshonitic chemical affinities and containing large complexly zoned clinopyroxene phenocrysts. The compositional zonation of the clinopyroxene phenocrysts is characterized by relatively Fe-rich (Mg^# = Mg/(Mg + Fe) = 0.85), but mottled, cores surrounded by mantles of cyclically-zoned clinopyroxene whose Mg^# varies repeatedly between 0.9 and 0.80. These cyclically zoned clinopyroxene mantles appear to record the repeated influx and mixing of batches of primitive with more evolved magma in a deep sub-crustal (∼1.2 GPa) magma chamber(s). Laser ablation ICP-MS was used to analyze the trace element variation in these zoned clinopyroxenes. The results indicate more than a threefold variation in the absolute concentrations of Th, Zr, rare earth elements (REE), and Y within individual clinopyroxene phenocrysts, with no apparent change in the degree of REE or high field strength element (HFSE) fractionation. The variation in absolute abundances of trace elements correlates closely with the major element composition of the clinopyroxene, with the most enriched clinopyroxene having the lowest Mg^# and highest Al contents. The problem is that the amount of crystal fractionation required to explain the major element variation (∼20%) in these clinopyroxene phenocrysts cannot explain the increase in the abundance of the incompatible trace elements, which would require more than 70% crystal fractionation, if constant partition coefficients are assumed. The anomalous increase in incompatible trace elements appears to reflect an increase in their partition coefficients with increasing Al^IV in the clinopyroxene; with an increase in Al₂O₃ from 1.5 to 4.0 wt.% during ∼20% crystal fractionation over a temperature decrease of ∼100°C being associated with more that a threefold increase in the partition coefficients of Th, Zr, REE, and Y. The magnitude of these increases may indicate that the substitution of these trace elements into clinopyroxene is better modeled in some natural systems by a local charge balance model, rather than the distributed charge model that better replicates the results of annealed experiments. These findings indicate that the effect of Al on the partition coefficients of incompatible trace elements in clinopyroxene may be under appreciated in natural magmatic systems and that the application of experimentally determined clinopyroxene partition coefficients to natural systems must be done with caution.     

General equations for modeling fluid/rock interaction using trace elements and isotopes

General equations for modeling open and closed system fluid/rock interaction using trace elements and isotopes are presented. Taylor's (1977) open system equation for stable isotopes is extended to account for a change in the Δ value between rock and fluid during interaction. It is shown that hydrothermal systems in which fractionation factors vary during fluid/rock interaction will significantly alter the isotopic ratios of rocks compared to equivalent systems with invariant fractionation factors. The equations for modeling trace element exchange are analogous to the stable isotope equations and permit the calculation of fluid/rock ratios using changes in trace element concentrations in the rock. Similarly to stable isotope exchange, the change of the trace element concentration in a rock during infiltration is a function of the trace element concentration in the fluid, the fluid/rock partition coefficient and the fluid/rock ratio rather than simply the concentration of the trace element in the fluid.     

The effect of sodium and titanium on crystal-melt partitioning of trace elements

Eclogite of recycled slab origin has often been invoked in the source region of mid-ocean ridge and ocean-island basalts. Melts of this eclogitic material are expected to be enriched in incompatible elements including major elements such as Na and Ti. In order to investigate the controls on trace element chemistry of a melt from such a recycled component we have performed trace element partitioning experiments in the simple systems CMAS (CaO-MgO-Al₂O₃-SiO₂), NCMAS (Na₂O-CMAS) and Ti-CMAS (TiO₂-CMAS) at 3 GPa and 1298-1500°C using analogue eclogitic compositions.We show that sodium has a profound effect on clinopyroxene-melt partition coefficients. NCMAS is characterized by elevated partition coefficients, relative to CMAS for all elements except Li. The increase is more pronounced for more highly charged cations, resulting in negative partitioning anomalies for Li, Sr, Ba and Pb. In contrast to sodium, titanium has very little effect on trace element partitioning for all elements except Nb and Ta, which are retained by the rutile that is saturated in this run.     

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.