Partitioning of trace elements between carbonate-silicate melts and mantle minerals: Experiment and petrological consequences

The partitioning of a number of trace elements (Ba, Nb, Zr, Y, REE, etc.) between orthopyroxene, garnet, and carbonate-silicate melt was experimentally studied using a belt apparatus at pressures of 3.5–4.2 GPa and temperatures of 1300–1500°C. The experimental products were investigated by electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The experimental melts varied from carbonatitic (～5 wt % SiO₂) at low temperatures (1300–1350°C) to kimberlitic compositions (30 wt % SiO₂) at high temperatures (1500°C). The partition coefficients of most elements between orthopyroxene and melt (D _i ^Opx/L ) and garnet and melt (D _i ^Grt/L ) were almost independent of melt composition (temperature). The D _i ^Opx/L values ranged from <0.01 for the most incompatible Ba and light REE to 0.02–0.08 for moderately incompatible Zr, Y, and heavy REE. The D _i ^Grt/L values were approximately an order of magnitude higher, ～0.07 for light REE, 0.7 for Y, and 1.5 for Yb. The character of D _i ^Grt/L variations in the systems studied is in general similar to that established for silicate melts without volatile components. However, the differences in the behavior of moderately incompatible and compatible elements (e.g., light and heavy REE) in the experimental systems are less pronounced compared with CO₂-free systems. Considering carbonate-silicate and silicate melts as possible agents of mantle metasomatism, it can be concluded that the former can efficiently transport heavy REE, and the latter have a greater affinity for Nb, Ba, and light REE. A characteristic feature of mantle rocks enriched by carbonate-silicate melts is high Ba/La ratio coupled with relatively weakly fractionated REE distribution patterns. It was shown that the high degrees of enrichment observed in natural kimberlites can be explained by a two-stage scenario, including a preliminary invasion of carbonate-silicate melt into depleted harzburgites in the lower parts of the lithosphere and subsequent very low degree melting.     

Partitioning of rare earth and high field strength elements between titanite and phonolitic liquid

We present the results of a LA–ICPMS study of titanites and associated glasses from the mixed-magma phonolitic Fasnia Member of the Diego Hernández Formation, Tenerife, Canary Islands. We employ a method of identifying equilibrium mineral–melt pairs from natural samples using REE contents and a linear form of the lattice strain model equation (Blundy and Wood, 1994), where the Young's modulus (E_M) for the 7-fold coordinated site is an output variable. For felsic magmas that contain crystals potentially derived from a variety of environments within the system, this approach is more rigorous than the use of solely textural criteria such as mineral–glass proximity. We then estimate titanite/melt partition coefficients for Y, Zr, Nb, REE, Hf, Ta, U and Th. In common with prior studies, we find that middle REE partition more strongly into titanite than either light or heavy REE, and that REE partitioning behavior in titanite is reasonably predicted by the lattice strain model. Titanite also fractionates Y from Ho, Zr from Hf, and Nb from Ta. Comparison with experimental data indicates that melt structure effects on partitioning are significant, most particularly in very highly polymerized melts. We use the data to estimate 7-fold coordination radii for trivalent Pr, Nd, Ho, Tm and Lu, and to make approximate predictions of titanite/melt partitioning of Ra, Ac and Pa. Interpolation of data for heavy REE does not predict the behavior of Y, indicating that factors other than charge and radius are involved in partitioning. Variations in Y/Ho induced by magmatic processes appear to be negatively correlated with temperature, and are expected to be greatest in near-minimum melts.     

Rare earth elements in alpine peridotites

The samples from alpine peridotite massifs (Beni Bouchera, Lherz and the Alps) have been analyzed for rare earth elements. The peridotites as a whole are characterized by various degrees of light REE depletion (Ce varies from 1.2 to 0.02 times chondrite) and a small variation in heavy REE (Yb varies about a factor of 2, from 1.3 to 2.2 times chondrite). They show a restricted and regular distribution in a Ce-Yb diagram, giving two types of linear trends for individual massifs (trend A for the Alps and Lizard; trend B for Beni Bouchera and Lherz, branching from trend A). The model calculations of partial melting based on the partition relations of REE among silicate minerals and melts suggest that trend A could represent a series of residua left after partial melting of garnet peridotite despite the fact that there is no garnet observed in the peridotites studied here. It is suggested that trend A would represent a melting event which predated the emplacement of the massifs and occurred at higher pressure (in the presence of garnet) than expected from the present mineralogy. The calculations also suggest that trend B could represent a partial melting event at lower pressures than trend A after the massifs uplifted into spinel peridotite field. It is also suggested that the REE concentrations of the mantle could be estimated as 2–2.5 times chondrite.     

Partitioning of rare earth elements between garnet and andesite melt: an autoradiographic study of P-T-X effects

High-pressure equilibrium studies were conducted in piston-cylinder apparatus to determine rare earth element (REE) partitioning between garnet and H₂O-vapor-saturated liquidus, from 20 kbar/980°C to 30 kbar/1060 °C. Ag capsules were employed to eliminate loss of Fe. Partition coefficients (K_D's) were determinined with autoradiographic techniques employing beta-active isotopes of Ce, Sm, and Tm. Major elements in garnet were determined by microprobe analysis. Synthesis and reversal runs of 24 hr or greater duration were used to bracket values of K_D's within analytical uncertainty.The K_D values for all three REE are constant over the radiogenic concentration range of 1 to 350 ppm, suggesting that the high abundance of natural REE in the starting materials may suppress possible deviations from Henry's Law behavior reported in similar autoradiographic studies of synthetic systems with no natural REE. Changes in K_D with increasing pressure and temperature at near-liquidus conditions suggest that the dominant control of K_D is the average size of cations occupying the 8-fold sites in garnet. Specifically, the substitution of 8-fold Ca for Mg and Fe causes an increase in K_D values greater than that attributable to the coincident effects of pressure and temperature. The inverse correlation of increasing K_D with REE ionic radius supports the interpretation that the average size of the 8-fold cation controls the relative variation of K_D among REE.These experimental K_D values produce less relative fractionation in melts between light REE and heavy REE than do previous K_D's derived from data on coexisting natural garnet phenocryst/rockmatrix pairs. Models for the derivation of orogenic andesites from partial melting of subducted basaltic eclogite are qualitatively improved by these new K_D's. Existing calculations of K_D values necessary for the viability of the eclogite fractionation are also in good agreement with these experimental K_D values.     

Partitioning of iron and magnesium between crystals and partial melts in peridotite upper mantle

The iron-magnesium distribution coefficient, $$K'_D = (X_{\Sigma {\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{\Sigma {\text{FeO}}} )^{{\text{liquid}}} ,$$ has frequently been used as a means of testing whether experimental and natural silicate liquids could have been in equilibrium with olivine of mantle composition. It is shown here that this K′ _D decreases with increasing oxygen fugacity (xxx) for a hydrous partial melt in equilibrium with a natural spinel peridotite assemblage under pressure and temperature conditions corresponding to those of the upper mantle (from 0.52 at the xxx of the iron-wüstite buffer to 0.04 at the xxx of the magnetite-hematite buffer). K′ _D also increases with increasing pressure, with decreasing temperature, and probably with increasing Mg/(Mg+∑ Fe) of the parental peridotite, suggesting that $$K_D = (X_{{\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{{\text{FeO}}} )^{{\text{liquid}}}$$ also increases with increasing pressure and decreasing temperature. Thus, unless these four variables (P, T, xxx, silicate composition) are known for a natural magma, K′ _D and probably K _D are variables, and the Mg/(Mg+∑ Fe) of such a magma cannot be correlated to that of the parent. The K _D determined at 1 atm pressure by Roeder and Emslie has frequently been used to test whether the Mg/(Mg+∑ Fe) ratios of experimentally formed liquids at high pressure in equilibrium with olivine of known Fo content represent the equilibrium Mg/(Mg+Fe²⁺) of this liquid, assuming that ∑Fe=Fe²⁺ and that K′ _D does not vary with P, T, and composition of the system. Published data demonstrate that the oxygen fugacities of the experimental designs employed by different laboratories vary between those of the magnetite-hematite and magnetite-wüstite buffers (6 orders of magnitude), resulting in K′ _D between 0.04 and 0.31 at 1050° C and 15 kbar, for example. Thus, published arguments as to whether the quenched liquids represent equilibrium compositions based on iron-magnesium partitioning are inadequate. The effects of P, T, xxx, and the composition of the starting material must also be considered.     

Mildly incompatible elements in peridotites and the origins of mantle lithosphere

The abundances of the mildly incompatible elements Al, Cr, V, Sc and Yb in more than 1700 mantle peridotite bulk rock analyses are interpreted in the light of a fractional melting model based on experimentally measured partition coefficients (D) and melting reaction stoichiometries. All peridotites examined, irrespective of sample type (abyssal peridotites, orogenic massifs, ophiolites, on/off craton xenoliths), tectonic environment (divergent/convergent/passive margin, intraplate) or the pressure (P) they last equilibrated at in the mantle (plagioclase-, spinel- , or garnet facies), originated as residues at less than 3 GPa, mainly within the spinel-facies. Mantle rocks currently in the garnet facies likely were originally spinel-facies lithosphere underthrust or subducted to greater depths in convergent margins. This view is inescapable even within the widest range of D values employed in the calculations, and is furthermore strengthened when metasomatic effects on the abundances of the mildly incompatible elements in residues are considered. A pressure of origin of below 3 GPa for most mantle lithosphere creates difficulties for any model ascribing a significant volume of deep, cratonic mantle roots to plume sub-cretion or any other vertical tectonic mechanism.     

Partitioning of rare earth elements between an aqueous chloride fluid phase and melt during the decompression-driven degassing of granite magmas

This paper reports the results of numerical simulation for the behavior of rare earth elements (REE) during decompression degassing of H₂O- and Cl-bearing granite melts at pressures decreasing from 3 to 0.5–0.3 kbar under near isothermal conditions (800 ± 25°C). Fluid phase in equilibrium with the melt contains mainly chloride REE complexes, and their behavior during magma degassing is, therefore, intimately related to the behavior of chlorine. It was shown that the contents and distribution patterns of REE in the aqueous chloride fluid phase formed during decompression vary considerably depending on (1) the contents of volatiles (Cl and H₂O) in the initial melt, (2) the redox state of the magma, and (3) the dynamics of fluid phase separation from magmas during their ascent toward the Earth’s surface. During decompressiondriven degassing, the contents of both Cl and REE in the fluid decrease, especially dramatically under opensystem conditions. The REE patterns of the fluid phase compared with those of the melt are characterized by a higher degree of light to heavy REE fractionation. A weak negative Eu anomaly may be present in the REE patterns of Cl-rich fluids formed during the early stages of degassing at relatively high pressures. At a further decrease in pressure and Cl content in the fluid, it is transformed into a positive Eu anomaly increasing during decompression degassing. Such an anomalous behavior of Eu during degassing is related to its occurrence in magmatic melts in two valence states, Eu³⁺ and Eu²⁺, whereas the other REE occur in melts mainly as (REE)³⁺. The Eu³⁺/Eu²⁺ ratio of melt is controlled by the redox state of the magmatic system. The higher the degree of melt reduction, the more pronounced the anomalous behavior of Eu during decompression degassing. The amount of REE extracted by fluid from melt during various stages of degassing does not significantly influence the content and distribution patterns of REE in the melt.     

Partitioning of trace elements between rutile and silicate melts: Implications for subduction zones

Mineral/melt trace element partition coefficients were determined for rutile (TiO₂) for a large number of trace elements (Zr, Hf, Nb, Ta, V, Co, Cu, Zn, Sr, REE, Cr, Sb, W, U, Th). Whilst the high field strength elements (Zr, Hf, Nb, Ta) are compatible in rutile, other studied trace elements are incompatible (Sr, Th, REE). In all experiments we found D_Ta > D_Nb, D_Hf > D_Zr and D_U > D_Th. Partition coefficients for some polyvalent elements (Sb, W, and Co) were sensitive to oxygen fugacity. Melt composition exerts a strong influence on HFSE partition coefficients. With increasing polymerization of the melt, rutile/melt partition coefficients for the high field strength elements Zr, Hf, Nb and Ta increase about an order of magnitude. However, D_Nb/D_Ta and D_Hf/D_Zr are not significantly affected by melt composition. Because D_U ? D_Th, partial melting of rutile-bearing eclogite in subducted lithosphere may cause excesses of ²³⁰Th over ²³⁸U in some island arc lavas, whereas dehydration of subducted lithosphere may cause excesses of ²³⁸U over ²³⁰Th. From our partitioning results we infer partition coefficients for protactinium (Pa) which we predict to be much lower than previously anticipated. Contrary to previous studies, our data imply that rutile should not significantly influence observed ²³¹Pa-²³⁵U disequilibria in certain volcanic rocks.     

Partitioning coefficients between olivine and silicate melts

Variation of Nernst partition coefficients (D) between olivine and silicate melts cannot be neglected when modeling partial melting and fractional crystallization. Published natural and experimental ^{olivine/liquid}D data were examined for covariation with pressure, temperature, olivine forsterite content, and melt SiO₂, H₂O, MgO and MgO/MgO + FeO^total. Values of ^{olivine/liquid}D generally increase with decreasing temperature and melt MgO content, and with increasing melt SiO₂ content, but generally show poor correlations with other variables. Multi-element ^{olivine/liquid}D profiles calculated from regressions of D REE–Sc–Y vs. melt MgO content are compared to results of the Lattice Strain Model to link melt MgO and: D₀ (the strain compensated partition coefficient), E_M³⁺ (Young's Modulus), and r₀ (the size of the M site). Ln D₀ varies linearly with Ln MgO in the melt; E_M³⁺ varies linearly with melt MgO, with a dog-leg at ca. 1.5% MgO; and r₀ remains constant at 0.807 Å. These equations are then used to calculate ^{olivine/liquid}D for these elements using the Lattice Strain Model. These empirical parameterizations of ^{olivine/liquid}D variations yield results comparable to experimental or natural partitioning data, and can easily be integrated into existing trace element modeling algorithms. The ^{olivine/liquid}D data suggest that basaltic melts in equilibrium with pure olivine may acquire small negative Ta–Hf–Zr–Ti anomalies, but that negative Nb anomalies are unlikely to develop. Misfits between results of the Lattice Strain Model and most light rare earth and large ion lithophile partitioning data suggest that kinetic effects may limit the lower value of D for extremely incompatible elements in natural situations characterized by high cooling/crystallization rates.     

Distribution of titanium and the rare earth elements between peridotitic minerals

The concentrations of titanium and rare earth elements (REE) in olivines, orthopyroxenes, clinopyroxenes and spinels from four anhydrous, spinel-bearing peridotite xenoliths have been determined. The distribution of titanium (used as an analogue for the high field strength elements: HFSE) relative to the REE between clinopyroxenes and orthopyroxenes varies as a function of the whole rock composition and modal mineralogy. The distribution coefficients for titanium and the REE in these peridotites do not reflect mineral-melt equilibria. It is believed that subsolidus distribution coefficients for HFSE relative to REE vary with temperature. Ratios of various incompatible elements (e.g., Ti/Eu, Zr/Sm, Hf/Sm and P/Nd) in peridotite minerals differ from those in most primary basalts. However, the abundance ratios of incompatible elements in the bulk peridotite are comparable to those found in modern basalts. Given this and the differing contribution of melt from each phase during melting, near constant ratios of such incompatible elements in primary and primitive basalts and komatiites reflect the buffering of the melt by its residue. These ratios are fixed in the magma during the initial stages of melting because of similar and low distribution coefficients between melt and bulk residue for these element pairs. Differences in the relative abundances of titanium and REE in clinopyroxenes and orthopyroxenes demonstrate that mantle normalized abundance patterns for clinopyroxene are not equivalent to those of the whole rock. Therefore, claims of a widespread HFSE-depleted reservoir in the upper mantle base solely on the relative abundances of incompatible elements in peridotitic clinopyroxenes are invalid.     

深海稀土分布规律与成矿作用

深海稀土是近年发现的一种富集中-重稀土的新型海洋矿产资源,其资源量远超陆地稀土储量,具有重要的潜在应用价值。中国是继日本之后在国际上第二个开展深海稀土调查研究的国家,2011年以来,先后在中印度洋海盆、东南太平洋和西太平洋深海盆地发现了大面积富稀土沉积区,在全球大洋中初步划分出4个深海稀土成矿带：西太平洋深海稀土成矿带、中—东太平洋深海稀土成矿带、东南太平洋深海稀土成矿带和中印度洋海盆-沃顿海盆深海稀土成矿带。深海富稀土沉积主要发育在深海盆地的沸石粘土和远洋粘土中,属于自生成因;部分发育在洋中脊附近的盆地中,受到热液作用的影响。研究发现,深海粘土中稀土元素主要赋存于生物磷灰石中,海水是稀土元素的主要来源;在早期成岩阶段,稀土元素在深海沉积物中发生转移和重新分配,并最终富集于生物磷灰石中;大水深（CCD面之下）、低沉积速率和强底流活动是深海稀土大规模成矿的主要控制因素。今后需要继续加大深海稀土基础调查,加强深海稀土调查探测技术研发,并开展海陆稀土成矿作用对比研究,揭示深海稀土成矿机制和规律。     

Deep carbon cycle and geodynamics: the role of the core and carbonatite melts in the lower mantle

Carbon, though being abundant in the Solar system, barely exceeds 0.01 wt.% in the silicate mantle, whereas it is ~ 3.6 wt.% in primitive chondritic meteorites that most likely formed our planet. This deficit may be due to redistribution of carbon in the liquid metal phase and then in the core at the stage of magma ocean fractionation, because carbon is much more soluble in Fe–Ni ± S melt than in silicate melts. The terrestrial heat and mass transfer are controlled mainly by layered convection and periodic peaks of plume activity as fast mantle jets that rise from the core. Plumes carry significant amounts of CO₂, H₂O, and K₂O (most probably in the form of carbonatite or hydrous carbonatite melts) released by the degassing core on its interaction with oxidized silicate material. There are two mechanisms that may maintain fast plume ascent: (1) local melting at the plume front as a result of doping with volatiles (H₂O, CO₂) as in a gas burner (rise rate 60–110 cm/yr) or (2) flow controlled by diffusion transport of silicate components in carbonatite melt (rise rate ~ 100 cm/yr).     

内蒙古柯单山蛇绿岩地幔橄榄岩铂族元素(PGEs)的分布特征及分异机制探讨

采用Carius管结合MC-ICPMS法分析了内蒙古柯单山蛇绿岩地幔橄榄岩中Ir、Ru、 Pt 和Pd 的含量,与典型的地幔橄榄岩进行对比研究,发现柯单山地幔橄榄岩中Ir和Ru明显亏损,Pt和Pd强烈富集,具有极高的Pd/Ir值,PGEs地幔标准化配分模式具有较陡的正斜率,明显不同于通常观测到的代表部分熔融残留相中铂族元素配分模式(负斜率或平坦型)。柯单山地幔橄榄岩的Ir和Ru与MgO呈正相关关系,表明Ir和Ru的亏损可能与部分熔融过程中硫化物的消耗程度有关,而与PGEs在硫化物/硅酸盐间的能斯特分配系数没有直接关系; Pt、Pd的富集表明本区的地幔橄榄岩不仅仅是经历过部分熔融的残余,而与来自深海的橄榄岩和大陆岩石圈地幔(SCLM)中的方辉橄榄岩相似,因此推测,本区地幔橄榄岩在部分熔融后又经历了富Pd的熔/流体交代,而熔/流体的来源可能是在岩浆分异演化过程中"熔离"出来的硫化物。     

Authigenic associations between selected rare earth elements and trace metals in lacustrine sediments

Surficial sediment samples were collected at 47 stations in Little Traverse Bay, Lake Michigan, to determine the geochemical associations between certain rare earth elements (REE's) and trace metals. Each sample was analyzed for carbonate carbon, organic carbon, grain size, and the elements Al, Ca, Ce, Co, Cr, Eu, Fe, Hf, La, and Mn. Two distinct Ce subpopulatins were identified by graphical analysis, and an R-mode factor analysis was applied to data from the “enriched” Ce subpopulation (18 samples). Results show that the REE's and trace metals are primarily enriched in the authigenic phase of these sediments. Partial correlation analyses indicate that the REE's are primarily associated with hydrous Fe oxides relative to organic matter in this phase. The ratio of Ce/La concentrations increased markedly from the bay margins to the central trough of the bay, indicating that Ce, similar to Fe, exhibits a variable oxidation state in the authigenic phase of nearshore fine-grained sediments. The results of the present study suggest that the REE's and trace metals behave coherently in the authigenic phase of recent lacustrine sediments, and the REE's may be useful as geochemical tracers to differentiate between trace metal enrichments in surface sediments as a result of diagenesis and pollution loadings.     

State of rare earth elements in the rare earth deposits of Northwest Guizhou,China

We studied the states of rare earth elements in ore of the Xianglushan rare earth deposit. Rare earth ore samples were tested and examined by scanning electron microscope, electron probe, and chemical leaching. No independent rare earth minerals were detected by scanning electron microscope. Elements detected by the electronic probe for the in situ micro-zone of the sample included: O, Al, Si, Ca, Mg, Fe, Ti, K, Na, S, Cl, C, Cu, Cr, V, and Pt. Rare earth elements were not detected by electron probe. (NH4)₂SO₄, (NH₄)Cl, NaCl, and H₂SO₄ were used as reagents in chemical leaching experiments that easily leached out rare earth elements under the action of 10% reagent, indicating that the rare earth elements in ore are mainly in the ionic state rather than present as rare earth minerals.     

巴尔哲超大型稀有稀土矿床成矿机制研究

巴尔哲矿床中的矿化和非矿化碱性花岗岩主要造岩矿物均为微斜长石、石英、钠闪石和钠长石,但其相对含量及颗粒大小明显不同,且两类岩石中包裹体的组成特征及锆石的结晶习性也有显著差异.主量元素分析显示,矿化与非矿化碱性花岗岩均以富硅、富碱、贫镁和钙为特征,为较典型的非造山A型花岗岩.尽管矿化碱性花岗岩中K_2O和Na_2O的含量均没有明显的增加,但其Na+K/Al、Na_2O+K_2O/CaO、FeO~*/MgO及K_2O/MgO等岩石化学参数与非矿化碱性花岗岩明显不同.在矿化碱性花岗岩中除了矿化的稀土元素及Nb、Zr强烈富集外,U、Th及Y也明显富集,而Ba、Sr、P、Eu和Ti表现为强烈的亏损.在非矿化碱性花岗岩中除了大离子亲石元素Rb略有富集外,稀土元素、Nb、Zr、U、Th、Ta及Y并无明显富集,虽然Sr、P、Eu和Ti也表现为亏损,但与矿化碱性花岗岩相比其亏损程度明显降低.岩相学、岩石化学及微量元素地球化学特征显示,矿化碱性花岗岩不可能是非矿化碱性花岗岩硅化和钠长石化作用的产物,二者应是同一岩浆体系不同演化阶段熔体固结的产物.K/Rb、Rb/Sr及δEu等地球化学参数显示,矿化碱性花岗岩是高演化A型花岗质熔体固结的产物;而岩石学、包裹体及地球化学特征则显示,这种高演化的A型花岗质熔体已经进入了岩浆一热液过渡阶段.巴尔哲矿床稀有稀土元素的超常富集和成矿与A型花岗岩的高演化过程密切相关.     

An experimental determination of rare earth partition coefficients between a chloride containing vapor phase and silicate melts

The partitioning behavior of cerium, europium, gadolinium and ytterbium between an aqueous “vapor” phase and water saturated silicate melt have been experimentally examined using a new experimental approach employing radioactive tracers and a double-capsule technique. Equilibrium was established by reversing the partition coefficient¹ and by betatrack autoradiography. Aqueous solution compositions were varied by adding different amounts of chloride and in some cases fluoride or carbon dioxide. The H₂O contents of the Spruce Pine pegmatite melts were varied by conducting experiments at 4.0 kb, 800°C and at 1.25 kb, 800°C. A jadeite-nepheline composition (75 wt% Jadeite) also was employed at 4.0 kb, 800°C.The chloride experiments (Spruce Pine 4 kb, 800°C) show a linear relationship between the cube of the chloride molality and the partition coefficients of the trivalent rare earths. Europium, under the experimental fO₂ conditions (quartz-fayalite-magnetite buffer), varied linearly as the fifth power of the chloride molality. At the chloride molalities examined (<1.1 mC1), all the rare earths partitioned preferentially into the melt phase (K_P^RE <1). Relative to pure water, the presence of chloride and fluoride fon increased the partitioning of the individual rare earths into the vapor phase, while carbon dioxide did not. Europium anomalies were recorded 1n all experiments, particularly those involving the Spruce P1ne melt at 4.0 kb and 800°C which displayed a large positive europium anomaly at all chloride molalities. Furthermore, a relative fractionation of the trivalent rare earths was also observed in these experiments, such that K_P^Ce>K_P^Gd>K_P^Yb. The smaller ytterbium ion was consistently concentrated in the melt phase relative to the other rare earths in all experiments on the Spruce Pine composition. Experiments on the jadeite-nepheline composition showed no relative fractionation and a positive europium anomaly. The 1.25 kb experiment on the Spruce Pine composition showed a negative europium anomaly in plots of K_p^RE vs. REE.The overall rare earth partitioning at a constant chloride molality (mCl = .914) was such that K_P^SP(1.25 kb) > K_P^SP(4.0 kb) > K_P^Jd-Ne(4.0 kb), where SP = Spruce Pine, Jd-Ne = jadeitenepheli Using the model of Burnnam (1975), It is suggested that the trivalent rare earth partitioning is related to the cube of the melt octahedral site concentration; a property which 1n hydrous melts 1s dependent on melt composition and hydroxyl molality. Excellent agreement was found for the Spruce Pine melt, whereas the jadeite-nepheline melt gave apparent hydroxyl molalities which were too high for the measured partition coefficient. Additional octahedral sites are proposed for this unusual composition perhaps due to some aluminum in 6-fold coordination. The apparent compositional variation of europium partitioning at a constant oxygen fugacity is believed to be related to both the octahedral melt site concentration for trlvalent europium and an 8-coordinated site concentration for divalent europium. Any parameter which affects the numbers of these sites (_PH₂O, melt composition) will affect the rare earth partitioning. The observed dependency of the partition coefficient on the structural state of the melt could be as significant as its dependency on crystalline structural constraints. Furthermore, since _PH₂O can drastically effect the melt structural state, its effects could be reflected in melt/crystal partition coefficients.     

Geochemistry of rare earth elements in oceanic phillipsites

The behavior of rare earth elements (REE) was examined in oceanic phillipsites collected from four horizons of eupelagic clay in the Southern Basin of the Pacific. The REE concentrations were determined in the >50-μm-fraction phillipsite samples by the ICP-MS method. The composition of separate phillipsite accretions was studied using the electron microprobe and secondary ion mass-spectrometry. Rare earth elements in phillipsite-only samples are related to the admixture of ferrocalcium hydroxophosphates. The analysis of separate phillipsite accretions reveals low (<0.1–18.1 ppm) REE (III) concentrations. The Ce concentration varies between 2.7 and 140 ppm. The correlation analysis shows that REE (III) are present as an admixture of iron oxyhydroxides in separate phillipsite accretions. Based on the REE (III) concentration in iron oxyhydroxides, we can identify two generations of phillipsite accretions. Massive rounded accretions (phillipsite I) are depleted in REE, while pseudorhombic (phillipsite II) accretions are enriched in REE and marked by a positive Ce anomaly. Oceanic phillipsites do not accumulate REE or inherit the REE signature of the volcaniclastic material and oceanic deep water. Hence, the REE distribution in phillipsites does not depend on the sedimentation rate and host sediment composition.     

Li isotope fractionation in peridotites and mafic melts

We have measured the Li isotope ratios of a range of co-existing phases from peridotites and mafic magmas to investigate high-temperature fractionations of ⁷Li/⁶Li. The Li isotopic compositions of seven mantle peridotites, reconstructed from analyses of mineral separates, show little variation (δ⁷Li 3.2-4.9‰) despite a wide range in fertility and radiogenic isotopic compositions. The most fertile samples yield a best estimate of δ⁷Li ∼ 3.5‰ for the upper mantle. Bulk analyses of olivine separates from the xenoliths are typically ∼1.5‰ isotopically lighter than co-existing orthopyroxenes, suggestive of a small, high-temperature equilibrium isotope fractionation. On the other hand, bulk analyses of olivine phenocrysts and their host melts are isotopically indistinguishable. Given these observations, equilibrium mantle melting should generate melts with δ⁷Li little different from their sources (<0.5‰ lighter). In contrast to olivine and orthopyroxene, that dominate peridotite Li budgets, bulk clinopyroxene analyses are highly variable (δ⁷Li = 6.6‰ to −8.1‰). Phlogopite separated from a modally metasomatised xenolith yielded an extreme δ⁷Li of −18.9‰. Such large Li isotope variability is indicative of isotopic disequilibrium. This inference is strongly reinforced by in situ, secondary ion mass-spectrometry analyses which show Li isotope zonation in peridotite minerals. The simplest zoning patterns show isotopically light rims. This style of zoning is also observed in the phenocrysts of holocrystalline Hawaiian lavas. More dramatically, a single orthopyroxene crystal from a San Carlos xenolith shows a W-shaped Li isotope profile with a 40‰ range in δ⁷Li, close to the isotope variability seen in all terrestrial whole rock analyses. We attribute Li isotope zonation in mineral phases to diffusive fractionation of Li isotopes, within mineral phases and along melt pathways that pervade xenoliths. Given the high diffusivity of Li, the Li isotope profiles we observe can persist, at most, only a few years at magmatic temperatures. Our results thus highlight the potential of Li isotopes as a high-resolution geospeedometer of the final phases of magmatic activity and cooling.