Partition Coefficients For Rare Earth Elements Research Articles

Trace-element modeling of the magmatic evolution of rare-earth-rich carbonatite from the Miaoya deposit, Central China

Carbonatites are known to contain the highest concentrations of rare-earth elements (REE) among all igneous rocks. The REE distribution of carbonatites is commonly believed to be controlled by that of the rock-forming Ca minerals (i.e., calcite, dolomite, and ankerite) and apatite because of their high modal content and tolerance for the substitution of Ca by light REE (LREE). Contrary to this conjecture, calcite from the Miaoya carbonatite (China), analyzed in situ by laser-ablation inductively-coupled-plasma mass-spectrometry, is characterized by low REE contents (100–260 ppm) and relatively flat chondrite-normalized REE distribution patterns [average (La/Yb) CN = 1.6]. The carbonatite contains abundant REE-rich minerals, including monazite and fluorapatite, both precipitated earlier than the REE-poor calcite, and REE-fluorocarbonates that postdated the calcite. Hydrothermal REE-bearing fluorite and barite veins are not observed at Miaoya. The textural and analytical evidence indicates that the initially high concentrations of REE and P in the carbonatitic magma facilitated early precipitation of REE-rich phosphates. Subsequent crystallization of REE-poor calcite led to enrichment of the residual liquid in REE, particularly LREE. This implies that REE are generally incompatible with respect to calcite and the calcite/melt partition coefficients for heavy REE (HREE) are significantly greater than those for LREE. Precipitation of REE-fluorocarbonates late in the evolutionary history resulted in depletion of the residual liquid in LREE, as manifested by the development of HREE-enriched late-stage calcite [(La/Yb) CN ≈ 0.7] in syenites associated with the carbonatite. The observed variations of REE distribution between calcite and whole rocks are interpreted to arise from multistage fractional crystallization (phosphates ⇒ calcite ⇒ REE-fluorocarbonates) from an initially REE-rich carbonatitic liquid.

The influence of melt composition on the partitioning of REEs, Y, Sc, Zr and Al between forsterite and melt in the system CMAS

Partition coefficients for a range of Rare Earth Elements (REEs), Y, Sc, Al and Zr were determined between forsteritic olivine (nearly end-member Mg 2SiO 4) and ten melt compositions in the system CaO–MgO–Al 2O 3–SiO 2 (CMAS) at 1 bar and 1400 °C, with concentrations of the trace elements in the olivine and the melt measured by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The REEs and Sc were added at levels sufficient to ensure that concentrations in the olivine were well above the detection limits. The REE partition coefficients ( D REE ol / melt ) decrease with increasing silica in the melt, indicating strong bonding between REEO 1.5 and SiO 2 in the melt. The variation of D REE ol / melt as a function of ionic radius is well described by the Brice equation for each composition, although a small proportion of this variation is due to the increase in the strength of the REEO 1.5–SiO 2 interactions in the melt with ionic radius. Scandium behaves very similarly to the REEs, but a global fit of the data from all ten melt compositions suggests that D Sc ol / melt deviates somewhat from the parabolas established by the REE and Y, implying that Sc may substitute into olivine differently to that of the REEs. In contrast to the behaviour of the large trivalent cations, the concentration of Al in olivine is proportional to the square root of its concentration in the melt, indicating a coupled substitution in olivine with a high degree of short-range order. The lack of any correlation of REE partition coefficients with Al in olivine or melt suggests that the REE substitution in olivine is charge-balanced by cation vacancies. The partition coefficient of the tetravalent trace element Zr, which is highly incompatible in olivine, depends on the CaO content of the melt.

Late Miocene–Pliocene eclogite facies metamorphism, D'Entrecasteaux Islands, SE Papua New Guinea

AbstractThe D'Entrecasteaux Islands of south‐eastern Papua New Guinea are active metamorphic core complexes that formed within a region where the plate tectonic regime has transitioned from subduction to rifting. While rapid, post 4 Myr exhumation and cooling of amphibolite and greenschist facies rocks that constitute the footwall of the crustal scale detachment fault system have been previously documented on Fergusson and Goodenough Islands of the D'Entrecasteaux chain, the timing of eclogite facies metamorphism in rocks of the footwall was unknown. Recent work revealed that at least one of the eclogite bodies formed during the Pliocene. We present combined in situ ion microprobe U–Pb age analyses of zircon from five variably retrogressed eclogite samples from Fergusson and Goodenough Islands that document Late Miocene–Pliocene (8–2 Ma) eclogite formation on these islands. Textural relationships and zircon–garnet rare earth element partition coefficients indicate that U–Pb ages constrain zircon crystallization under eclogite facies conditions in all samples. Results suggest westward younging of eclogite facies metamorphism from Fergusson to Goodenough Island. Present‐day exposure of Late Miocene–Pliocene eclogites requires exhumation rates > 2.5 cm yr−1.

Effect of melt structure on trace-element partitioning between clinopyroxene and silicic, alkaline, aluminous melts

We have investigated trace-element partitioning between clinopyroxene and silicic melts ranging from 62 to 69 wt% SiO 2 in piston-cylinder experiments at 1.1 GPa and 1145-1155 °C. Run products consist of diopsidic clinopyroxene (with 3-5 wt% Al 2 O 3 and ∼1 wt% Na 2 O) coexisting with silicic, aluminous, and alkaline melts. Clinopyroxene-melt partition coefficients for rare earth elements (REE) are significantly higher (e.g., D Sm = 0.76 to 3.1) than previous studies of clinopyroxene-basaltic melt partitioning. The maximum D REE is also shifted toward middle REE (e.g., Tb). Although the partition coefficients of Sr and Ti (D Sr = 0.23-0.39, D Ti = 0.28-1.0) are also elevated relative to clinopyroxene-basaltic melt, partition coefficients for Zr and Li (0.14-0.23 and 0.16-0.25, respectively) are within the range of literature values. The high partition coefficients for REE, Sr, and Ti in this study do not reflect crystal-compositional effects. For instance, no relationship is observed between IV Al and these partition coefficients. REE and Ti partition coefficients correlate poorly with jadeite component of the clinopyroxene; however, the elevated Na 2 O contents of both the melt and clinopyroxene make it difficult to discriminate fully the effect of jadeite component from the effect of changes in melt structure. The non-bridging oxygen to tetrahedral cation ratio (NBO/T; Mysen et al. 1985) varies from 0.08 to 0.17 and roughly correlates with epx/melt D REE , consistent with a recent study (Gaetani 2004) that concluded that melt structure significantly affects partitioning when NBO/T < 0.49. Notably, we find a strong correlation between the molar Ca 2+ /(M + + M 2+ ) of the melt and the optimum D (D 0 ) for the REE, indicating that melt structure, and not jadeite content, is the dominant control on clinopyroxene-melt partitioning in these experiments. The partition coefficients for the REE, Ti, Zr, and Sr are in good agreement with observed clino-pyroxene-silicic glass partitioning within mantle xenoliths (Chazot et al. 1996; Vannucci et al. 1998), indicating these results may apply to lithospheric melting. Furthermore, the origin of hump-shaped REE patterns observed in residual clinopyroxene from abyssal peridotites could be explained if shallow depleted melts of peridotite are silica-rich. A two-stage fractional melting model assuming clinopy-roxene-silicic melt partitioning at shallow mantle depths from 10 to 30 km beneath a mid-ocean ridge can reproduce the observed hump-shaped REE patterns. If so, hump-shaped REE patterns may provide evidence for the existence and extraction of silica-rich melt created in the shallow mantle.

Trace element partitioning and substitution mechanisms in calcium perovskites

We have determined the partition coefficients of a large number of trace elements between CaTiO3 perovskite and anhydrous silicate melts at atmospheric pressure and 3 GPa. Determination of the concentration limits of Henry’s law behaviour in the CaO-Al2O3–SiO2–TiO2 system reveals that the incorporation of rare earth elements (REE) and tetravalent large ion lithophile elements (LILE4+ such as U and Th) at the Ca-site of CaTiO3 perovskite occurs with charge compensation through Ca-vacancy formation rather than by coupled substitution of Al for Ti. When melt composition is varied, we find that partition coefficients for REE and Th are strong functions of the CaO content of the melt. The observed trends are in excellent agreement with those predicted from the Ca-vacancy model. Given that they adopt the same crystal structure and have similar trace element partitioning behaviour, CaTiO3 perovskite and the deep mantle phase CaSiO3 perovskite can be considered analogous to one another. When the analogy is pursued in detail, we find that partitioning into both phases follows the composition-dependence predicted by the Ca-vacancy model. Thus, substitution of REE, U4+ and Th into CaSiO3 in the lower mantle also occurs with Ca-vacancy formation to balance charge. Furthermore when 2+, 3+ and 4+ partition coefficients for both phases are plotted as functions of CaO melt content, the trends for CaSiO3 and CaTiO3 appear to be continuous. This surprising result means that partitioning into Ca-perovskite is independent of pressure and temperature and also of whether or not the host is CaSiO3 or CaTiO3. One implication is that CaSiO3 crystallising from a peridotitic magma ocean may have partition coefficients for Th and U up to about 400. Crystallisation and sequestration of as little as 0.25 volume% of this phase in the lower mantle early in earth history would make a significant contribution to current mantle heat production.

Experimental REE partitioning between calcite and aqueous solution at 25.DEG.C. and 1atm: Constraints on the incorporation of seawater REE into seamount-type limestones

Partitioning experiments of rare earth elements (REEs) between calcite and aqueous (CaCl2 + NaCl) solution at 25°C and 1 atm have been made in order to elucidate the incorporation process of seawater REE into marine limestones. Calcite-supersaturated solution doped with REE was constantly pumped into a reactor, in which calcite seeds and the solution were mixed and CO2 + N2 gas was bubbled to allow calcite overgrowths. Absolute values of REE partition coefficients, Kd(REE) = (XREE/XCa)calcite/([REE]total/[Ca])solution, could not be determined because of very small calcite overgrowths and difficulty in evaluating XCa. The relative values of Kd(REE) like Kd(REE)/Kd(Gd), however, were determined successfully. Kd(REE) given by [REE]total depends on REE(III)-carbonate complexation in solution, and does not correspond to a reaction incorporating a specific dissolved REE species into calcite. Hence, here we used Kd(REE)ω = (XREE/XCa)calcite/([REECO3+]/[Ca])solution, which is given by Kd(REE) and REE(III)-carbonate complexation constants. Kd(REE)ω relates directly to ΔGr. for the reaction incorporating REECO3+(aq) into calcite. The series variation of experimental logKd(REE)ω shows a convex tetrad effect, suggesting larger Racah parameters of REECO3+(aq) relative to REE(III) in calcite. The variation also shows a kink at around Pm, Sm and Eu, possibly reflecting a coordination change of REE(III) in calcite across the series. Yttrium is definitely enriched in solution relative to heavy REEs. Using the relative values of experimental logKd(REE)ω and REE analyses of upper Paleozoic Japanese Ishimaki and Tahara limestones, REE abundance patterns for seawater coexistent with the marine limestones have been calculated provided that pH, ΣCO2 and salinity are the same in the ancient and present times. The calculated patterns are quite similar to those for relatively deep waters at around 400 to 1000 m in the modern Pacific. Ishimaki and Tahara limestones are of the seamount-type. Hence, the calculated seawater REE patterns with large negative Ce anomalies suggest that the incorporation of seawater REE into the limestones occurred in moderately deep water, probably because of subsidence of volcanic seamounts that were capped by Ishimaki and Tahara limestones.

Determination of zircon/melt trace element partition coefficients from SIMS analysis of melt inclusions in zircon

Partition coefficients ( zircon/meltD M) for rare earth elements (REE) (La, Ce, Nd, Sm, Dy, Er and Yb) and other trace elements (Ba, Rb, B, Sr, Ti, Y and Nb) between zircon and melt have been calculated from secondary ion mass spectrometric (SIMS) analyses of zircon/melt inclusion pairs. The melt inclusion-mineral (MIM) technique shows that D REE increase in compatibility with increasing atomic number, similar to results of previous studies. However, D REE determined using the MIM technique are, in general, lower than previously reported values. Calculated D REE indicate that light REE with atomic numbers less than Sm are incompatible in zircon and become more incompatible with decreasing atomic number. This behavior is in contrast to most previously published results which indicate D > 1 and define a flat partitioning pattern for elements from La through Sm. The partition coefficients for the heavy REE determined using the MIM technique are lower than previously published results by factors of ≈15 to 20 but follow a similar trend. These differences are thought to reflect the effects of mineral and/or glass contaminants in samples from earlier studies which employed bulk analysis techniques. D REE determined using the MIM technique agree well with values predicted using the equations of Brice (1975), which are based on the size and elasticity of crystallographic sites. The presence of Ce 4+ in the melt results in elevated D Ce compared to neighboring REE due to the similar valence and size of Ce 4+ and Zr 4+. Predicted zircon/meltD values for Ce 4+ and Ce 3+ indicate that the Ce 4+/Ce 3+ ratios of the melt ranged from about 10 −3 to 10 −2. Partition coefficients for other trace elements determined in this study increase in compatibility in the order Ba < Rb < B < Sr < Ti < Y < Nb, with Ba, Rb, B and Sr showing incompatible behavior (D M < 1.0), and Ti, Y and Nb showing compatible behavior (D M > 1.0). The effect of partition coefficients on melt evolution during petrogenetic modeling was examined using partition coefficients determined in this study and compared to trends obtained using published partition coefficients. The lower D REE determined in this study result in smaller REE bulk distribution coefficients, for a given mineral assemblage, compared to those calculated using previously reported values. As an example, fractional crystallization of an assemblage composed of 35% hornblende, 64.5% plagioclase and 0.5% zircon produces a melt that becomes increasingly more enriched in Yb using the D Yb from this study. Using D Yb from Fujimaki (1986) results in a melt that becomes progressively depleted in Yb during crystallization.

High field strength element/rare earth element fractionation during partial melting in the presence of garnet: Implications for identification of mantle heterogeneities

A synthesis of recent garnet‐melt trace element partitioning data for key trace elements (Ti, Hf, Zr, U, Th, Sm, and Yb) is used to compare and contrast the trace element signatures imparted on mantle melts by garnets from peridotitic and eclogitic source rocks. Garnet‐melt partition coefficients DGrt/Melt are very sensitive to changes in garnet major element composition. Specifically, high‐pressure, high‐temperature experimental studies show that high field strength elements (HFSE) Zr, Hf, and Ti are incompatible in garnets with &lt;19 ± 1 mol% Ca on their X site, with DTi &lt; DZr ≤ DHf &lt; 1, while at higher Ca levels, all three become compatible with DZr &gt; DHf &gt; DTi &gt; 1. U and Th also have higher partition coefficients at higher garnet Ca contents, while the amount of fractionation between the two decreases. In contrast, the heavy rare earth element partition coefficients DHREE are hardly affected by a change in garnet Ca content. We provide a semiquantitative explanation for the behavior of the high field strength elements based on a crystal lattice strain model in which Zr and Hf are split between the X and Y sites in Ca‐rich garnet and in which significant changes in garnet elasticity occur as a function of garnet composition. The large variations in both absolute DGrt/Melt values and DGrt/Melt ratios (e.g., DZr/DYb), in conjunction with compositional differences between natural peridotitic (Ca poor) and eclogitic (Ca richer) garnets, allow identification of trace element ratios that may best serve as a fingerprint for the presence of eclogitic garnet. We present simple batch melting calculations for two end‐member melting scenarios (anhydrous garnet peridotite melting and anhydrous bimineralic eclogite melting). Our calculations show that near‐uniform Zr/HREE and Hf/HREE as a function of melt fraction, in combination with Hf/Sm and Zr/Sm ratios that are smaller than the source ratio, could serve as fingerprints for the presence of Ca‐rich garnet in the source of mantle melts. Our calculations show that it is impossible to define one unique “garnet signature” to determine the presence or absence of garnet in basalt sources but rather that different garnet‐bearing sources are likely to produce distinctly different “garnet signatures.”

Experimental determination of aluminous clinopyroxene–melt partition coefficients for potassic liquids, with application to the evolution of the Roman province potassic magmas

We have experimentally determined the partition coefficients of rare earth elements (REE), HFSE (Ti, Zr, Hf, Nb, Ta) and other trace elements between clinopyroxene and potassic silicate melts at 0.1–200 MPa and 1042–1140°C. The major and trace element contents of clinopyroxene and liquid were determined by electron probe and ion microprobe, respectively. The liquidus clinopyroxenes are extremely rich in Al 2O 3 (up to 12%) and Fe 2O 3, which enables us to test suggestions that partition coefficients of HFSE and REE depend on the Al content of the clinopyroxene (e.g., Lundstrom, C.C., Shaw, H.F., Ryerson, F.J., Williams, Q., Gill, J., 1998. Crystal chemical control of clinopyroxene–melt partitioning in the Di–Ab–An system: implications for elemental fractionations in the depleted mantle. Geochim. Cosmochim. Acta 62, 2849–2862.). When compared with previous data for low-alumina clinopyroxene crystallised under similar P– T conditions, the increase in weight partition coefficient D i (=[ I] xtl/[ I] liq) for all the HFSE and REE is dramatic. For Ti and the REE, partition coefficients increase by about one order of magnitude as Al iv concentration of clinopyroxene increases from 0.02 to 0.5. For Nb, Ta and Zr, the effect is even more pronounced, a two order of magnitude increase being found to correspond to this increase in Al iv. We applied the data to model the fractionation of historic lavas from Vesuvius. Previous work using the major elements (Belkin, H.E., Kilburn, C.R.J., DeVivo, B., 1993. Sampling and major element chemistry of the recent (AD 1631–1944) Vesuvius activity. J. Volcanol. Geotherm. Res. 58, 273–290.) indicates a liquid line of descent from trachybasalt to leucite tephrite dominated by clinopyroxene crystallisation. The REE and HFSE concentrations in the lavas are consistent with 40–50% fractional crystallisation of clinopyroxene (accompanied by minor leucite) between the two magmatic endmembers, with partition coefficients for the REE appropriate for clinopyroxenes containing ≤7.5 wt.% Al 2O 3. Pyroxenes of the latter composition are frequent phenocrysts in the lavas and previous experimental work (Dolfi, D., Trigila, R., 1988. Chemical relations between clinopyroxenes and coexisting glasses obtained from melting experiments on alkaline basic lavas. Rend. Soc. Ital. Mineral. Petrol. 43, 1101–1110; Trigila, R., De Benedetti, A.A., 1993. Petrogenesis of Vesuvius historical lavas constrained by Pearce element ratio analysis and experimental phase equilibria. J. Volcanol. Geotherm. Res. 58, 315–343.) implies that they precipitate under very low pressure low water activity conditions, i.e. within the magma chamber at <200 MPa.

Experimental determination of the partitioning behavior of rare earth and high field strength elements between pargasitic amphibole and natural silicate melts

The primary goal of this investigation was to derive a set of expressions that can be used to calculate the amphibole-melt partitioning behavior of the rare earth elements (REE) and the high field strength elements (HFSE) in natural systems. To supplement the existing data set on basaltic systems, we conducted experiments on systems where amphibole was in equilibrium with dacitic, tonalitic and low Si rhyolitic melts. These experiments, doped with La, Sm, Gd, Lu, Ta, Nb, Y, Zr, and Hf, were run at pressures of 2 and 5 kbar, temperatures between 900°C and 945°C, and oxidation conditions ranging from QFM-1 to NiNiO+1. The partitioning data obtained in this study were combined with published data to calculate two sets of expressions describing trace element partitioning. The first set models the partitioning of trace elements into amphibole using temperature, pressure and several compositional parameters, including the compositionally-compensated partition coefficients of Ti, Al, Caand SiO 2, and the exchange of Fe and Mg between the crystal and the melt (D Mg/D Fe). The second set of expressions are slightly less precise, but require no specific knowledge of P, T, or f O2 and, for application to natural systems, can be constructed solely on the basis of information available from standard electron microprobe analyses. These expressions predict amphibole-melt partition coefficients for REE and HFSE within an internal precision of 14–40% (relative) for alkali basalt to low Si rhyolite, from 850°C to 1100°C, 2–20 kbar and oxygen fugacity from QFM-1 to NiNiO+1. Partition coefficients calculated from the expressions derived in this study were used to model the partial melting and fractional crystallization of a hypothetical amphibolite and hydrous melt, respectively. Fractionation and/or melting in amphibole-bearing systems produces a magma with a convex upward REE pattern, a characteristic common to many hornblende-bearing dacites. However, the removal or addition of an amphibole component cannot produce the strong HFSE depletion relative to the REE observed in many arc magmas.

Experimental partitioning of high field strength and rare earth elements between clinopyroxene and garnet in andesitic to tonalitic systems

Partition coefficients of the rare earth elements (REE) and the high field strength elements (HFSE: Ti, Zr, Hf, Nb, Ta) among clinopyroxenes, garnets, and andesitic to granodioritic melts were experimentally determined at pressures ranging from 1 to 3 GPa and temperatures between 900 and 1150°C. Natural rocks of quartz–dioritic and basaltic composition were used as starting materials. Melt compositions covered a range from dioritic to quartz–dioritic and granodioritic. Partition coefficients obtained from experiments with different doping levels at the same run condition indicate that Henry’s Law is fulfilled. Partition coefficients were investigated as a function of temperature and phase compositions. Apparent correlations between the clinopyroxene partition coefficients of the REE and degree of depolymerization expressed as nonbridging oxygens per tetrahedron (NBO/T) are due to variations in the Na contents of the clinopyroxenes which are controlled by pressure or NBO/T. Based on the model of Blundy and Wood (1994), which accounts for the strain associated with placing a cation on a particular crystallographic site when the radius of the cation differs from the optimal radius for that site, significant differences between Zr− and Hf partition coefficients as well as correlations between Ti and REE partition coefficients can be explained in terms of differences of their ionic radii. For garnets D La/D Yb ratios change as a function of temperature much more than for clinopyroxenes. This is also consistent with the Blundy and Wood (1994) model.

Distinct site preferences for heavy and light REE in amphibole and the prediction of Amph/L D REE

New experimental amphibole/melt partition coefficients from a variety of geologically relevant amphibole (pargasite, kaersutite, and K-richterite) and melt compositions obtained under conditions of interest to upper-mantle studies are combined with the results of X-ray single-crystal structure refinement. The ideal cation radii (r0), calculated using the lattice-site elastic-strain model of Blundy and Wood (1994) under the hypothesis of complete REE (rare earth elements) ordering at [8]M4, mostly differ significantly from those obtained from both the structure refinement and the ionic radius of [8]Ca2+. Heavier REE may also strongly deviate from the parabolic trends defined by the other REE. On the basis of the crystal-chemical knowledge of major-element site-preference in amphibole and the occurrence of two sites with different co-ordination within the M4 cavity (M4 for Ca and Na, M4′ for Fe2+ and Mg), we propose a new model for REE incorporation. LREE order at the [8]M4 site, whereas HREE prefer the M4′ site with lower co-ordination in amphiboles with a significant cummingtonite component, and may also enter the M2 octahedron, at least in richterite. This more complex model is consistent with the observed Amph/LD, and drops the usual assumption that REE behave as a homogeneous group and order at the M4 site. The availability of multiple crystal-chemical mechanisms for REE3+ incorporation explains why measured and estimated Amph/LDHREE may differ by up to one order of magnitude. When REE enter two different sites within the same cavity, a fit performed on the basis of a single curve may appear correct, but the values obtained for r0 are biased towards those of the dominant site, and the Young's modulus is underestimated. When REE are incorporated in multiple sites in different cavities, the observed pattern cannot be reduced to a single curve, and the partition coefficients of heavy REE would be strongly underestimated by a single-site fit. The simplistic assumption that REE occupy a single site within the amphibole structure can thus substantially bias predictive models based on the elastic-strain theory. Our combined approach allows linkage between fine-scale site preference and the macroscopic properties of minerals and provides more reliable predictive models for mineral/melt partitioning. After the possible site-assignments have been identified, the shape of the Onuma curves constructed from accurately determined Amph/LDREE now allows the active mechanisms for REE incorporation in amphiboles to be recognised even where site populations are not available. The REE preference for polyhedra with smaller size and lower co-ordination than those occupied by Ca invalidates the general idea that Ca acts as a “carrier” for REE.

Distribution of trace elements between amphibole and clinopyroxene from mantle peridotites of the Eifel (western Germany): An ion-microprobe study

The pargasite/diopside partition coefficients ( D) for the rare-earth elements (REE), Sr, Y, Zr and Hf, determined with ion-microprobe analysis on low-temperature West Eifel mantle peridotites, have values of between 1 and 2. In contrast, Nb and Ba are strongly enriched in amphibole with partition coefficients of D Nb = 100–200 and D Ba > 1000. Small sample-to-sample variations in the REE partition coefficients in the West Eifel samples appear to be related to compositional factors, specifically the Na content of clinopyroxene. The REE partition coefficients of edenitic-pargasitic hornblende/diopside pairs from East Eifel xenoliths have higher values ( D = 3–8) than those from the West Eifel and these differences may be attributable to the lower Na and K contents of the East Eifel xenoliths. The partition coefficients for amphibole/clinopyroxene pairs of natural peridotites differ from the literature values for those of high-temperature melts. Compared to the West Eifel xenoliths most of the melt-related partition coefficients are higher for LREE ( D = 2–4), but decrease and become similar for HREE ( D = 1). It is unclear to what extent these differences are the result of temperature, pressure and composition, or the result of lack of equilibrium.

Hornblende-melt trace-element partitioning measured by ion microprobe

Trace-element abundances were measured in situ by ion microprobe in five samples of hornblende and melt ranging from basaltic andesite to high-silica rhyolite. Except for one sample, for which quench overgrowth or disequilibrium is suspected, the abundance ratios show systematic inter-element and inter-sample variations, and probably approach true partition coefficients. Apparent partition coefficients are reported for La, Ce, Nd, Sm, Dy, Er, Yb, Sc, Ti, V, Cr, Sr, Y and Zr. Rare-earth elements (REE) and Y form smooth convex-upward partitioning patterns that rise to higher D-values and become increasingly convex in more evolved samples. Apparent partition coefficients for REE, Y, Ti, V and Cr can be parameterized as functions of the distribution of Ca between hornblende and melt, giving expressions to predict hornblende-melt trace-element partitioning values. These expressions are used to show that heavy REE-enriched hornblende/whole-rock REE abundance patterns in granitoids may result from partial re-equilibration of hornblende and late-stage residual liquids rather than from anomalous partitioning values.

The partitioning of Sc, Y, and the rare earth elements between high-Ca pyroxene and natural mafic to intermediate lavas at 1 atmosphere

The effects of composition and temperature on the partitioning behavior of Sc, Y, and the rare earth elements (REEs) between high-Ca clinopyroxene and natural silicate melts were evaluated from doped experiments on natural mafic to intermediate composition lavas at 1 atmosphere pressure. Partition coefficients for these elements were found to be dependent on temperature and composition. The most important compositional parameters controlling clinopyroxene-melt partitioning for Y, Sc, and REEs are Al content of the liquid and pyroxene Ca content. Towards the goal of deriving expressions describing partitioning behavior, approximations were made of equilibrium constants for reactions involving a REE-Al component in the pyroxene. Regression of these equilibrium constants over the experimental temperature range (1180-1050°C) produced expressions which, when applied to the experimental glasses, reproduced the clinopyroxene trace element contents with precisions between 9 and 32% (1σ). The most important conclusion of this work is that pyroxene-melt partition coefficients for trivalent cations have different compositional dependencies than divalent cations because of the participation of Al in paired substitution. Values for high-Ca pyroxene Sc, Y, and REE partition coefficients have a range of over a factor of two between alkali basalts, andesites, and tholeiitic basalts (e.g., 0.2-0.7 for Sm at 1100°C). This represents a large proportion of the total range for D values from all mafic and intermediate magmas. Our contribution is to describe the parameters that control partitioning behavior. This will allow us to more accurately determine REE partitioning for specific systems.

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.

Solution properties of rare earth elements in silicate melts: Inferences from immiscible liquids

The partitioning behavior of rare earth elements (REE) between immiscible silicate liquids was investigated as a function of temperature and total REE oxide concentration. REE preferentially partition into the end-member liquids that are rich in network-modifying cations, with two-liquid partition coefficients greater than any other cation. The two-liquid partition coefficients of REE in a given experiment are essentially identical to one another. No evidence was seen to suggest that the solution mechanisms of REE in high-silica liquids vary with differences in their ionic radii. “Two-lattice” models of trace element solution behavior fail to remove the compositional dependence of REE two-liquid partition coefficients due to the assumption that all network-forming and network-modifying cations mix ideally with one another. As a result, it is probably incorrect to derive physical interpretations from the results of the application of two-lattice models. The results are applied to the origins of the LREE-enriched phenocryst-matrix partition coefficients seen for some ferromagnesian minerals in high-silica rhyolites. Since liquids with extremely high concentrations of T-O-T linkages do not strongly discriminate between the REE on the basis of their ionic radii, the anomalous REE crystal-matrix partition coefficient patterns are best explained as the result of contamination of these minerals by LREE accessory phases.

The coprecipitation of rare-earth elements with calcite and aragonite

Apparent partition coefficients of rare-earth elements (REE's) between calcium carbonates and aqueous solution were experimentally examined. Calcite or aragonite crystals were synthesized by gradual release of CO 2 gas from calcium bicarbonate solution doped with REE's at room temperature and pressure. The apparent partition coefficients for five calcite runs range from ∼ 2.5 to ∼ 10, and those for the aragonite runs fall in a range from ∼ 2.5 to ∼ 5. The mutual REE fractionation for a set of partition coefficients is relatively small. The pattern shapes of apparent partition coefficients for calcite obviously change with varying concentration level of the REE's doped in the initial solution. Such concentration dependence reveals that crystal structure-ionic radius control is not a major factor under the present experimental conditions, but other mechanisms such as the effect of REE complexation in the solution seem to be in operation.

Partition coefficients for rare earth elements in mafic minerals of high silica rhyolites: The importance of accessory mineral inclusions

REE concentrations of mafic mineral separates from high-silica rhyolites measured by INAA are high and variable compared to electron microprobe analyses of the minerals themselves. The mafic phases commonly contain inclusions or have adhering grains of accessory rare earth element (REE)-rich minerals. Optical and electron microscopic observation revealed discrete grains of chevkinite (rare earth titano-silicate) included within clinopyroxenes from the Sierra La Primavera (Mexico) rhyolite, and monazite grains adhering to ortho- and clinopyroxenes from the Bishop Tuff (California). During hand-picking of mineral separates, inclusions are only partly removed. As a result, the magnitude and variability of true mineral-melt partition coefficients for light REE have been overestimated. The true REE partition coefficients of La Primavera and Bishop Tuff pyroxenes obtained by microprobe are only slightly higher than they are in lower-silica rhyolites, and are not as variable as previously thought. The partitioning slope is positive, as in less silicic systems. The relative partitioning behavior of REE in high-silica rhyolites is dominated by crystal-chemical controls and not by liquid structural effects. When a partition coefficient is used for crystallization calculations, adhering phases and inclusions should be retained or the minor phases must be accounted for separately. Alternatively, a bulk distribution coefficient can be calculated using whole rock and glass compositions and the glass mode.

Zirconium, hafnium, and rare earth element partition coefficients for ilmenite and other minerals in high‐Ti lunar mare basalts: An experimental study

Partition coefficients were determined for Gd, Lu, Hf, and Zr among ilmenite, armalcolite, and synthetic high‐Ti mare basaltic melts at temperatures from 1122° to 1150°C, and at oxygen fugacities of IW × 100.5, by in situ analysis with the electron microprobe, using samples doped to percent concentration levels. Coefficients for Zr were also measured for samples containing 600–1600 ppm Zr, using a special high‐sensitivity electron microprobe technique. In addition, coefficients were determined for Hf and Zr between chromian ulvospinel and melt, for Hf between pigeonite and melt, and for Lu between olivine and melt by microprobe analysis of samples doped to percent levels. Recommended values for ilmenite are D(Gd) = 0.006, D(Lu) = 0.09, D(Hf) = 0.42, and D(Zr) = 0.33. Recommended values for armalcolite are D(Gd) = 0.006, D(Lu) = 0.046, D(Hf) = 1.5, and D(Zr) = 0.9. Values measured using the microprobe are in general agreement with values measured by analyzing mineral separates from the sanle run products by isotope dilution at the USGS Isotope Lab in Denver. Agreement between microprobe and ID is within 3% for Hf, and within 10% for Zr. For Lu, the ID results were 15–35% lower than the microprobe values. The source of this disagreement has not been identified. Parition coefficients measured in this study and the collaborative ID study confirm the general results of earlier measurements. D values for ilmenite are less than 0.01 for the LREE (light rare earth element), are around 0.1 for the HREE (heavy rare earth elements), and are several times greater than this for Zr and Hf. These results constitute the first simultaneous experimental measurement of ilmenite distribution coefficients for the three important elements Lu, Hr, arid Zr in mare basaltic systems.