Lattice Strain Model Research Articles

Uranium partitioning between apatite and hydrothermal fluids at 150–250 °C

This experimental study is focused on the immobilization of dissolved uranium (U) via its entrapment by apatite at hydrothermal conditions. Experiments on apatite crystallization were conducted in 0.5 M sodium chloride (NaCl) solutions at temperatures of 150, 200, and 250 °C. Uranium was introduced into the system as a solid compound (UO3), which controlled the concentration of dissolved U in the hydrothermal fluids. The results showed that U contents in apatite exceed U concentrations in co-existing fluid by 2–3 orders of magnitude. Apparent Nernst partition coefficients (DU) and exchange coefficient (KU/Ca) were evaluated. Experimentally determined DU values were consistent with DU predicted by the Lattice Strain Model at 200 and 250 °C. The obtained KU/Ca values shows dependence on temperature. The results lay the foundation for further assessment of uranium immobilization in apatite and other minerals at repository conditions.

Mineral Textures and Chemistry Trace the Origin and Transcrustal Evolution of the Sanyuangou Syenite in Southern North China Craton

Abstract Alkaline rocks are widely distributed in various geological environments and are important carriers for exploring the formation and compositional diversity of continental crust. Extensive studies have investigated the processes of mantle melting and crustal differentiation that produced such rocks. However, the potential interaction processes between mantle-derived magma and crust during their formation are poorly constrained. In this study, we focus on a Paleoproterozoic garnet-bearing syenite in the Xiong'er large igneous province (LIP) on the southern margin of the North China Craton through detailed whole-rock and mineral analyses. The high K2O (7.4–8.8 wt %) syenite emplaced at ~1772 Ma with ancient inherited zircon (1800–2800 Ma). The dominant mineral assemblage is composed of clinopyroxene, garnet, and K-feldspar with a small amount of titanite. Complex compositional oscillatory zoning of clinopyroxene and garnet indicates that magma mixing played a significant role in the formation of the syenite. We estimated the major elements composition for melts in equilibrium with clinopyroxene and calculated the clinopyroxene/melt partition coefficients by crystal lattice strain model, thus calculating the trace elements of the equilibrated melts. The equilibrium melts of high Mg# (>85) clinopyroxene have high CaO/Al2O3 (>0.6) and low Hf/Sm (<0.4), which suggests a role for carbonatitic metasomatism of the mantle. The variables La/Yb (24.4–56.1), Dy/Yb (0.8–5.9), and Yb (0.6–10.3 μg/g) indicate that the initial melts were formed by 1% to 2% partial melting of spinel-garnet phlogopite lherzolite. An REE-based model for melts in equilibrium with low Mg# (<85) clinopyroxene indicates that 10% to 30% felsic magma from ancient crust participated in hybridization. In addition to well-documented magma mixing, the oscillatory zoning of garnet is also related to competition with titanite. The initial alkaline magmas have a high water content (~4 wt %), which delays the crystallization of K-feldspar, leading to the enrichment of K2O, until the K-feldspar accumulates in the shallow crust to form the syenite. Magma mixing under an open system further leads to alkalinity enrichment. Magma source and crustal evolution jointly determine the potassium-rich characteristics of syenite. Multiple episodes of magma mixing and fractional crystallization occur in the crust (700–300 MPa), suggesting complex and vertically extensive magma chambers. This study represents the first identification of carbonatitic metasomatism as a mantle source in the Xiong'er LIP. Furthermore, it offers a new perspective on magma mixing between the mantle and crust in transcrustal magmatic systems, contributing to the formation of alkaline rocks.

Systematic trends in the substitution mechanisms of minor elements in cassiterite

A dataset of 4751 EPMA point analyses on cassiterite crystals from 18 localities spanning a range of different granite-related Sn-bearing mineral systems was investigated and compared with thermodynamic considerations from speciation and lattice strain models. The existence of previously inferred charge balanced coupled substitution mechanisms of Fe3+, Fe2+, Mn2+, Nb5+, Ta5+ and W6+ for Sn4+ are confirmed. The homo­valent substitution of Sn4+ by Ti4+ and Zr4+ is also confirmed, and the first evidence for homovalent substitution of Nb4+ and W4+ in natural cassiterite is presented. The preferred substitution mechanisms for Ti, Fe, Mn, Nb, Ta, W and Zr vary systematically as a function of the mineralising environment. Cassiterite precipitated from silicate liquid dominated systems (such as in pegmatites) primarily display a coupled Fe–(Nb,Ta) substitution mechanism with a 1:2 stoichiometry, whereas cassiterite precipitated in systems dominated by autometasomatic processes (such as greisens) more closely follow a 1:1 stoichiometry, as well as increased levels of Ti. Cassiterite precipitated solely from hydrothermal fluids (such as Sn–(W) quartz vein systems) exhibit the highest Ti concentrations, with lower contents of Fe–(Nb,Ta) coupled substitutions, instead displaying a switch to the incorporation of Fe3+ via FeO.OH stoichiometry. Hydrothermally precipitated cassiterite from skarn systems display the highest Fe3+ contents, and the lowest Nb, Ta and Ti contents. These systematic changes define a continuous trend in a Ti–(Fe + Mn)–(Nb,Ta) ternary diagram, allowing for classification of the primary source of alluvial cassiterite.

Distribution of REE between amphibole and pyroxenes in the lithospheric mantle: An assessment from the lattice strain model

Abstract Amphibole and pyroxenes are the main reservoirs of rare earth elements (REEs) in lithospheric mantle that has been affected by hydrous metasomatism. In this study, we developed semi-empirical models for REE partitioning between orthopyroxene and amphibole and between clinopyroxene and amphibole. These models were formulated on the basis of parameterized lattice strain models of mineral–melt REE partitioning for orthopyroxene, clinopyroxene, and amphibole, and they were calibrated using major element and REE data of amphibole and pyroxenes in natural mantle samples from intraplate setting. The mineral–melt REE partitioning models suggest that amphibole is not in equilibrium with coexisting pyroxenes in the mantle samples, and that the amphibole crystallized at a lower temperature than that of the pyroxenes. We estimated the apparent amphibole crystallization temperature using major element compositions of the amphibole and established temperature- and composition-dependent models that can be used to predict apparent pyroxene–amphibole REE partition coefficients for amphibole-bearing peridotite and pyroxenite from intraplate lithospheric mantle. Apparent pyroxene–amphibole REE partition coefficients predicted by the models can be used to infer REE contents of amphibole from REE contents of coexisting pyroxenes. This is especially useful when the grain size of amphibole is too small for trace element analysis.

Trace element partitioning between apatite and silicate melts: Effects of major element composition, temperature, and oxygen fugacity, and implications for the volatile element budget of the lunar magma ocean

Apatite, as an accessory phase in igneous and metamorphic rocks, has important petrological significance due to its capacity to accommodate appreciable amounts of many trace elements in its mineral structure. To better constrain trace element partitioning between apatite and silicate melts, we conducted experiments that produced apatites approaching fluorapatite (FlAp), hydroxylapatite (OHAp) and chlorapatite (ClAp) endmembers separately at 1050 and 1100 °C, 1 GPa pressure, under oxygen fugacity (fO2) about one log unit below iron-wüstite buffer to four log unit above fayalite-magnetite-quartz buffer. We report the results of 12 experiments which demonstrate that ClAp exhibits lower trace element partition coefficients compared with FlAp and OHAp, especially for Rare Earth Elements (REEs) under all run conditions explored, suggesting trace element partitioning is sensitive to anion site occupancy. Divalent cations are less sensitive to anion occupancy. Positive Eu partitioning anomalies (DEu/DEu*, where Eu is the chondrite normalized abundance and Eu* is the interpolated value from neighboring elements ordered by atomic number) are observed in ClAp experiments under relatively low fO2s, whereas negative Eu anomalies are exhibited by FlAp and OHAp under the same fO2 conditions. We infer that anionic occupancies have a direct impact on the substitution mechanisms of trace elements in apatite, thereby influencing their partition coefficients. Beyond the anions, correlations of apatite compositional components (XCa, XNa, XP and XSi) with partition coefficients suggest they exert crystal chemical controls on trace element partitioning. Based on these observations, we developed parameterized lattice strain models to predict the partitioning of divalent and trivalent elements as a function of temperature and apatite composition, and an fO2-dependent apatite-melt Eu partitioning model and oxybarometer. We further developed a Eu in apatite-plagioclase oxybarometer that enables us to calculate the fO2 of apatite and plagioclase-bearing magmatic and subsolidus systems, and evaluated the influence of subsolidus reequilibration on the new oxybarometer. Applied to one of our experiments, winonaite HaH193, and samples from Sept Iles layered intrusion, the oxybarometer recovers their anticipated fO2s, ranging from about two log units below the iron-wüstite buffer to the fayalite-magnetite-quartz buffer. Using the new REE and fO2-dependent Eu partitioning models, we constrained the petrogenesis of lunar KREEP basalt and estimated the relative volatile content in the late lunar magma ocean (LMO) cumulates. The model suggests a relative depletion of Cl in the LMO cumulates, consistent with Cl isotopic analyses and volatile abundance measurements in previous work, suggesting that differential loss of volatiles occurred before or during the late-stage evolution of the LMO.

Sr/Ca in shells of laboratory-grown bivalves (Arctica islandica) serves as a proxy for water temperature – implications for (paleo)environmental research?

Seawater temperature is an essential quantity for paleoclimatological and paleoecological studies. A potential archive that can provide century-long, temporally well-constrained and high-resolution temperature proxy data is available in the form of bivalve shells. However, the number of well-accepted and robust temperature proxies contained in shells is limited to stable oxygen isotopes and carbonate clumped isotopes. Many studies have therefore investigated the possibility to reconstruct temperature from element/Ca properties, specifically Sr/Ca ratios in case of aragonitic shells. As demonstrated here, in agreement with thermodynamic expectations and the lattice strain model, shell Sr/Ca of laboratory-grown Arctica islandica specimens is strongly positively coupled to water temperature. If ultrastructure-related bias is mathematically eliminated, up to 75% of the variability in shell Sr/Ca data can be explained by water temperature. However, in field-grown specimens, this relationship is superimposed by other environmental variables that can hardly be quantified and mathematically eliminated. The explained variability of Sr/Ca is reduced to merely 26% and the prediction uncertainty too large for reliable temperature estimates. Most likely, the equable, less biased conditions in the laboratory resulted in the production of a more uniform shell ultrastructure (with larger and more elongated biomineral units) which in turn was associated with less variable Sr/Ca values and a stronger link to water temperature. Without a detailed understanding and quantification of the factors controlling ultrastructural variations in field-grown bivalves, it remains impossible to employ shell Sr/Ca of wild A. islandica specimens for precise temperature estimates, merely a qualitative temperature reconstruction seems feasible.

Uranium uptake by phosphate minerals at hydrothermal conditions

The goal of this work was to evaluate the immobilization of uranium (U) through crystallization of calcium phosphate minerals (phosphates), which have a strong ability to absorb and retain dissolved uranyl, and therefore, are useful in various geological and environmental applications. To date, most of the experimental studies have been conducted at room temperature and high temperature assessments on uranium immobilization rely on the extrapolation procedure, which is not always accurate. To evaluate uranium partition coefficients between phosphates and hydrothermal fluid, we performed a series of crystallization experiments at 25–350 °C and various aqueous uranium concentrations.Crystallization occurred through the transformation of brushite to monetite or/and apatite in aqueous solutions doped with uranium aliquots. Solid products were extracted and characterized with X-ray diffraction (XRD) and scanning electron microscopy (SEM). The local bonding environment and valence state of uranium in apatite were determined via X-ray absorption spectroscopy (XAS). Uranium concentration in crystals and coexisting solutions were measured with inductively coupled plasma mass spectrometry (ICP-MS). Apparent partition coefficients were calculated as Nernst partition coefficients (DU) and Doener-Hoskins partition coefficients (KD−HU/Ca) (to account for closed reservoir effect). Experimental DU values were compared with those calculated using the lattice strain model. Results showed that >92% of U added to solutions was extracted via this crystallization method and KD−HU/Ca decreases with increasing phosphate crystallization temperature. Thus, phosphates, especially apatite, has a strong potential to immobilize uranium under hydrothermal conditions and can be used in the development of engineering barriers to further improve the efficiency of existing backfill materials in the disposal of nuclear waste.

Compositional variation and Sn isotope fractionation of cassiterite during magmatic-hydrothermal processes

Cassiterite (SnO2) precipitates over a wide physicochemical range during magmatic-hydrothermal processes. Significant variations in chemical and Sn isotopic compositions of cassiterite have been increasingly reported from hydrothermal deposits worldwide. However, the mechanisms responsible for such notable changes in cassiterite geochemistry remain to be fully addressed. Toward this end, we investigated the chemical and Sn isotopic compositions of distinct generations of cassiterite from a highly-evolved magmatic-hydrothermal-metallogenic system (Xianghualing Sn polymetallic deposit, South China). Cassiterite is widespread in the albite granite, greisen, skarn, and sulfide ore of this deposit, covering most of the known cassiterite-bearing rock/ore types. Based on distinct morphologies and mineral paragenesis, three generations of the Xianghualing cassiterite are identified: (1) Cst 1 occurs in albite granite and greisen; (2) Cst 2 occurs in skarn; and (3) Cst 3 is present in sulfide ore. A lattice strain model indicates that inter-element compositional differences of cassiterite are dual-controlled by cassiterite element partition coefficients and melt/fluid compositions for sufficient and deficient elements, respectively. The variations in Zr, Hf, Nb, and Ta elements in cassiterite are recognized as thermo-indicators of the melt/fluid physicochemical conditions. Their concentrations are influenced by pre- and syn-precipitated Zr-Hf-Nb-Ta-enriched minerals that have a wide crystallization temperature range. Due to differentiations of multiple cationic valences, concentrations and interelement ratios of Sb, Fe, V, and U can be utilized to monitor the relative melt/fluid redox state of different cassiterite generations. Tin stable isotopic compositions are notably fractionated in the Xianghualing cassiterite samples, with δamuSn values relative to standard NIST 3161a ranging from −0.16‰ to 0.19‰. By comparison with compiled Sn isotopes of other hydrothermal deposits, kinetic disequilibrium fractionation is proposed as the main control on cassiterite Sn isotopes fractionation. A Rayleigh fractionation model generates fractionation factors of 1.00010–1.00025, consistent with fractionation factors predicted by first principles calculations (1.00013 to 1.00037). Thus, progressive precipitation of Sn-enriched minerals due to the change of physicochemical conditions during magmatic-hydrothermal processes will cause lighter Sn isotopic compositions in residual melt/fluid. This study highlights that cassiterite chemical and Sn stable isotopes are novel indicators to magmatic-hydrothermal processes.

Trace element partitioning coefficients between terrestrial silicate melts and plagioclase feldspar: Improved and simplified parameters

Nernst partition coefficient data are compiled and parameterized against commonly measured and petrologically significant Input Variables, allowing plagioclase/meltD values for 37 trace elements to be calculated. With few exceptions plagioclase/meltDs increase with decreasing temperature, anorthite-content of plagioclase (An), wt% melt MgO and FeO* (all iron as FeO), molar melt Mg/(Mg + Fetotal); and with increasing wt% melt SiO2 and Na2O + K2O. High field strength cations and rare earth elements (REE) with similar ionic radii have nearly fixed ratios of LnD (e.g. LnDLa/LnDCe). These can be linked as a chain to construct a nearest-neighbour plagioclase/meltDREE (NN-DREE) profile from a point of origin at DCe. Correlations between LnDCe and An content in the partitioning data were regressed to connect these variables algebraically. Analysis of these NN-DREE profiles with the Lattice Strain Model (LSM) requires a temperature value, which was pre-defined by fitting simplified temperature-An paths to experimental data binned for melt H2O content. In the LSM, r3+0 is the size of the site into which REE substitute. Three published r3+0 parameterizations were tested. The Dohmen and Blundy (2014) parameterization of r3+0 [Å] (r3+0 = 1.331 − 0.067An) yielded the best fits to the NN-DREE profiles when values of D3+0 and E3+M were optimized. D3+0 is the strain compensated partition coefficient and E3+M is Young’s Modulus of the M-site hosting REE. Starting from the preferred r3+0 parameterization, NN-DREE profiles at many An contents were fit with a LSM by varying D3+0 and E3+M. These multiple fits yielded a tight linear trend of increasing LnD3+0 with decreasing An content [LnD3+0 = (−2.8323An) – 0.0538] applicable to all three temperature-An functions. Previously proposed parameterizations of Young’s Modulus E3+M can reproduce DREE profiles of very calcic plagioclase (>An0.9), but yield profiles that are too steep to fit DREE profiles of more sodic compositions. The LSM fits for dry melts (H2O < 1%) yield a trend of linearly-decreasing LnE3+M with decreasing An content [LnE3+M = (1.2808An) + 3.4897]. The temperature-An functions for wet (1–5% H2O) and very wet (>5% H2O) melts yield similar LnE3+Mvs An trends, but at lower LnE3+M to compensate for lower temperatures. With these parameterizations of temperature, r3+0, D3+0 and E3+M, the NN-DREE Lattice Strain Model reproduces the increasingly flatter and higher DREE profiles of plagioclase crystallizing from basaltic to rhyolitic melts, with An-content as the sole Input Variable needed.

Reconstruction of hydrothermal fluid composition of the Kamthai carbonatite complex, India using lattice strain model: Implications for LREE/HREE fractionation

The composition of the fluid phase is often a poorly constrained variable in many ore deposits. In this study, we used calcite-fluid partition coefficients extrapolated to the P-T conditions of mineralization using the lattice strain model to reconstruct the composition of the hydrothermal fluids associated with different stages of the hydrothermal alteration and secondary rare earth element (REE) mineralization in the Kamthai carbonatite complex of western India. We identified three generations of calcite including magmatic, two generations of secondary/hydrothermal, as well as partially reequilibrated magmatic grains. The magmatic and partially reequilibrated calcite are unusually enriched in the REE, particularly the LREE, and Sr, which were incorporated via coupled cationic substitutions from highly differentiated brine- and incompatible element-enriched carbonatitic melts. Calcite and carbocernaite were the important primary phases that sequestered the REE from the carbonatite magma. Textural and geochemical evidence suggest that magmatic calcite and carbocernaite underwent extensive partial to complete reequilibration through dissolution and reprecipitation during hydrothermal alteration. The prominent chemical changes during the alteration of calcite involved loss of substantial REE, Sr, Fe, and Mn, and transition from LREE-dominated to LREE-poor compositions. The REE released during hydrothermal alteration were incorporated in secondary REE minerals such as REE-(fluor)carbonates, REE-(hydroxyl)carbonates, ancylite, and cerianite. The reconstructed chemistry of the carbonatite melt and the hydrothermal fluid, together with the trace element chemistry of secondary calcite, point towards progressive decrease in the REE contents. However, the HREE were conserved and ƒO2 increased from the most primitive to the most evolved fluid. The REE were redistributed as alkali‑carbonate-hydroxyl complexes, which can explain the LREE/HREE fractionation observed between the early and the late-stage hydrothermal fluid. Precipitation of the LREE-bearing secondary minerals is attributed to a decrease in temperature of the hydrothermal fluid as a consequence of mixing of the carbonatite-derived fluid with a relatively cooler and oxidized fluid. Such mixing explains the observed negative Ce-anomaly in hydrothermal calcite.

A REE-based zircon geothermometer based on improved lattice strain modeling of zircon-melt REE partition coefficients

Several methods for tracing geological processes based on the rare earth element (REE) concentrations in zircon have been proposed in recent years. Application of these methods requires quantitative knowledge of the partition coefficients (D) for the REE between zircon and co-existing phases, which may be melt, fluid, or other minerals. Precisely determining the DREE of zircon, however, is challenging. This is especially true for the light rare earth elements, which generally occur in very low concentrations in zircon. This difficulty can be overcome by modeling DREE using a prediction function based on the crystal lattice strain model. The prediction function is defined by three empirically derived lattice strain parameters: the apparent Young's Modulus E of the Zr site in which the REE resides; the ionic radius r0 of a (fictive) REE optimally fitting into this site; and the maximum partition coefficient D0 of this optimally fitting REE. In principle, these three parameters can be determined by fitting the prediction function to sets of measured DREE. The ionic radii of the REE, however, are all larger than r0 for zircon, commonly resulting in strain model fits yielding physically unrealistic values of r0, in particular. Here, we use a compilation of zircon-melt REE partition coefficients from the literature to establish a new empirical relationship between r0 and E, which is used as an additional constraint during lattice strain model curve fitting. Lattice strain parameters for literature DREE data sets of natural and synthetic zircon samples are calculated using our constraint resulting in a new parametrization of the inverse temperature dependence of D0. This parametrization enables quantitative estimates of REE-based crystallization temperatures of magmatic zircons, complementing estimates based on Ti-in-zircon geothermometry.

Crystal vs. melt compositional effects on the partitioning of the first-row transition and high field strength elements between clinopyroxene and silicic, alkaline, aluminous melts

Abstract The first-row transition element (FRTE) and high field strength element (HFSE) systematics are powerful tools for tracking the source and evolution of mantle-derived magmas. Clinopyroxene is generally considered a key fractionating mineral controlling the partitioning of trace elements between melt and residual solid during mantle melting. Although partitioning of FRTE and HFSE between clinopyroxene and basaltic melts has been well-studied, experimental constraints on their partitioning behavior in the presence of siliceous, aluminous, and alkali-rich melts are still lacking. Here we present clinopyroxene-silicic melt (67–69 wt% SiO2) partitioning experiments at 1 bar pressure and 1070–1100 °C for Co, Mn, Ni, Cu, Zn, Fe, Sc, Cr, V, Ti, Zr, Hf, Nb, and Ta. Run products consist of diopsidic clinopyroxene coexisting with various melt compositions with non-bridging oxygen to tetrahedral cation ratio (NBO/T) ranging from 0.10 to 0.22. Using our new partition coefficients (Ds) and combined with literature data, we assess some of the effects of crystal chemistry and the melt composition on the partitioning of FRTE and HFSE in this simple system. We show that partitioning of FRTE varies from mildly incompatible (e.g., D = ~0.1−1 for V, Cu, and Zn) to highly compatible (e.g., D &amp;gt; 10 for Cr and Ni), with the highest compatibilities observed for Ni (DNi = 13−34). The partitioning of HFSE varies from highly incompatible (D = 0.01−0.08) for Nb and Ta to mildly incompatible (D = 0.18−0.82) for Zr, Hf, and Ti. Our measured clinopyroxene-melt Ds are consistent with the theoretical predictions of the lattice strain model. Ds data for most tri-, tetra-, and pentavalent elements tend to increase with increasing tetrahedrally coordinated Al content, in agreement with those anticipated from crystal-chemical considerations. In contrast to ivAl concentrations, the clinopyroxene Na concentration has very little effect on trace element partitioning due to its low concentrations in clinopyroxene at relatively low-pressure conditions. These data further support a significant control of melt composition/structure on partitioning for highly polymerized melts. In general, measured Ds roughly increase to different extents with increasing polymerization of the melt (i.e., lower NBO/T or higher ASI). For our equilibrium melt compositions, Ds for several FRTE, such as Co and Ni, correlate well with the melt molar Mg2+/(M+ + M2+), whereas Ds for HFSE vary as a function of the melt alkali concentration. These well-defined trends support the role of melt NBO species (e.g., Mg2+) or complexing ligands (e.g., Na+ and K+) in controlling the partitioning of these elements. Overall, our new Ds data demonstrate that even very small changes in melt major-element compositions can greatly affect element partitioning in strongly polymerized silicic systems. These findings have important implications relevant to petrogenetic studies of the interaction between silicic melt and peridotite that occurs at shallow mantle conditions in various tectonic settings.

Unleashing alkali feldspar: Ra/Th ages and chemical and isotopic constraints on Holocene phonolite magmatism, Canary Islands

Abstract Accurately dating phenocrysts in Holocene volcanic rocks poses many challenges but is critical to placing magmatic processes that occur prior to eruption into a temporal frame-work. We dated alkali feldspar (i.e., orthoclase Or10 to Or46) crystals in four young phonolites from the Teide–Pico Viejo volcanic complex, Tenerife (Spain), using (226Ra)/(230Th) isotopes. Partition coefficients of Ra (DRa) and DRa/DBa of feldspars were predicted using an approach based on the lattice strain model, which yielded crystallization ages that overlap or predate known eruption ages for the Lavas Negras (ca. 1 ka), Montaña Blanca (ca. 2 ka), Arenas Blancas (ca. 2–4 ka), and Teide H (ca. 6 ka) phonolites. Crystallization of feldspar may occur up to the time of eruption, with &amp;gt;8 ka crystals also present, possibly suggesting extended magma differentiation times. However, feldspars yielding finite (226Ra)/(230Th) ages are mostly in equilibrium with the groundmass, unlike &amp;gt;8 ka crystals, which were therefore identified as antecrysts/xenocrysts. The 87Sr/86Sr ratios of feldspars indicate that crystallization predated late-stage assimilation, affecting 87Sr/86Sr ratios of some melts. The (226Ra)/(230Th) ages also constrain the tempo of phonolite magma evolution on Tenerife. Integration of (226Ra)/(230Th) ages with feldspar major elements, trace elements, and isotopes provides a powerful means for investigating crystallization histories using a dominant mineral that controls the overall magmatic evolution of phonolites on thousand-year time scales.

Eu speciation in apatite at 1 bar: An experimental study of valence-state partitioning by XANES, lattice strain, and Eu/Eu* in basaltic systems

Abstract Partition coefficients for rare earth elements (REEs) between apatite and basaltic melt were determined as a function of oxygen fugacity (fO2; iron-wüstite to hematite-magnetite buffers) at 1 bar and between 1110 and 1175 °C. Apatite-melt partitioning data for REE3+ (La, Sm, Gd, Lu) show near constant values at all experimental conditions, while bulk Eu becomes more incompatible (with an increasing negative anomaly) with decreasing fO2. Experiments define three apatite calibrations that can theoretically be used as redox sensors. The first, a XANES calibration that directly measures Eu valence in apatite, requires saturation at similar temperature-composition conditions to experiments and is defined by: ( E u 3 + ∑ E u ) Apatite = 1 1 + 10 - 0.10 ± 0.01 × l o g ⁡ ( f o 2 ) - 1.63 ± 0.16 . The second technique involves analysis of Sm, Eu, and Gd in both apatite and coexisting basaltic melt (glass), and is defined by: ( Eu E u * ) D Sm × Gd = 1 1 + 10 - 0.15 ± 0.03 × l o g ⁡ ( f o 2 ) - 2.46 ± 0.41 . The third technique is based on the lattice strain model and also requires analysis of REE in both apatite and basalt. This calibration is defined by ( Eu E u * ) D lattice strain = 1 1 + 10 - 0.20 ± 0.03 × l o g ⁡ ( f o 2 ) - 3.03 ± 0.42 . The Eu valence-state partitioning techniques based on (Sm×Gd) and lattice strain are virtually indistinguishable, such that either methodology is valid. Application of any of these calibrations is best carried out in systems where both apatite and coexisting glass are present and in direct contact with one another. In holocrystalline rocks, whole rock analyses can be used as a guide to melt composition, but considerations and corrections must be made to either the lattice strain or Sm×Gd techniques to ensure that the effect of plagioclase crystallization either prior to or during apatite growth can be removed. Similarly, if the melt source has an inherited either a positive or negative Eu anomaly, appropriate corrections must also be made to lattice strain or Sm×Gd techniques that are based on whole rock analyses. This being the case, if apatite is primary and saturates from the parent melt early during the crystallization sequence, these corrections may be minimal. The partition coefficients for the REE between apatite and melt range from a maximum DEu3+ = 1.67 ± 0.25 (as determined by lattice strain) to DLu3+ = 0.69 ± 0.10. The REE partition coefficient pattern, as observed in the Onuma diagram, is in a fortuitous situation where the most compatible REE (Eu3+) is also the polyvalent element used to monitor fO2. These experiments provide a quantitative means of assessing Eu anomalies in apatite and how they be used to constrain the oxygen fugacity of silicate melts.

Trace element partitioning between anhydrite, sulfate melt, and silicate melt

AbstractAnhydrite has become increasingly recognized as a primary igneous phase since its discovery in pumices from the 1982 eruption of El Chichón, Mexico. Recent work has provided evidence that immiscible sulfate melts may also be present in high-temperature, sulfur-rich, arc magmas. In this study we present partition coefficients for 37 trace elements between anhydrite, sulfate melt and silicate melt based on experiments at 0.2–1 GPa, 800–1200 °C, and fO2 &amp;gt; NNO+2.5.Sulfate melt–silicate melt partition coefficients are shown to vary consistently with ionic potential (the ratio of nominal charge to ionic radius, Z/r) and show peaks in compatibility close to the ionic potential of Ca and S. Partition coefficients for many elements, particularly REE, are more than an order of magnitude lower than previously published data, likely related to differences in silicate melt composition between the studies. Several highly charged cations, including V, W, and Mo are somewhat compatible in sulfate melt but are strongly incompatible in anhydrite. Their concentrations in quench material from natural samples may help to fingerprint the original presence of sulfate melt.Partition coefficients for 2+ and 3+ cations between anhydrite and silicate melt vary primarily as a function of the calcium partition coefficients (DCaAnh−Sil) and can be described in terms of exchange reactions involving the Ca2+ site in anhydrite. Trivalent cations are dominantly charge-balanced by Na1+. Most data are well fit using a simple lattice-strain model, although some features of the partitioning data, including DLaAnh−Sil&amp;gt;DCeAnh−Sil, suggest the occurrence of two distinct anhydrite Ca-sites with slightly different optimum radii at the experimental conditions.The ratio DSrAnh−Sil&amp;gt;DCaAnh−Sil is shown to be relatively insensitive to silicate melt composition and should vary from 0.63–0.53 between 1200–800 °C, based on a simple, “one-site” lattice strain model. Comparison to DSrAnh−Sil and DCaAnh−Sil calculated for natural anhydrite suggests that in most cases, including the S-rich eruptions of Pinatubo and El Chichón, the composition of anhydrite is consistent with early crystallization of anhydrite close to the liquidus of silicate melt with a composition approximately that of the bulk erupted material. This illustrates how anhydrite (and perhaps sulfate melt) provides a mechanism to transport large quantities of sulfur from significant depth to the eruptive environment.

An Experimental Study of Trace Element Partitioning between Peridotite Minerals and Alkaline Basaltic Melts at 1250°C and 1 GPa: Crystal and Melt Composition Impacts on Partition Coefficients

Abstract Peridotite–magma interaction is important in establishing magma pathways through the mantle and in metasomatism of the lithospheric mantle. Reactions that consume orthopyroxene and produce olivine and clinopyroxene are of particular interest because these reactions should lead to a redistribution of trace elements between the solid and melt phases at equilibrium. This study examines interaction of a silica-undersaturated alkaline basalt (basanite) with a range of peridotite compositions from dunite, through harzburgite to wehrlite at 1250°C and 1 GPa. Our experiments used the natural concentration of trace elements in the starting materials which allowed us to measure mineral—olivine partition coefficients for Rb, Ca, Co, Sr, Sc, Ct, Y, Ti, V and Zr. For orthopyroxene—and clinopyroxene—melt we additionally measured partitioning of Cs, Ba, all rare earth elements (REE; except Pm), Hf, Th, U, Nb and Ta. We show that there are subtle variations in the partition coefficients, particularly of the REEs that are related to the bulk composition of the system. We also show that with the exception of cations that can have multiple valence states, e.g. vanadium, the lattice strain model and in particular the double fit routine gives excellent agreement between the calculated and experimentally determined partition coefficients. The double fit model allows us to examine the effect of mineral composition on partitioning such that we can show preference of trace elements for the M1 and M2 sites in the pyroxenes. Although our results are similar to those of previous studies, there are two main differences: first we have a complete set of partition coefficients for every trace element that is measurable by LA-ICPMS in our starting material, where previous studies may be missing one or more elements in particular one or more of the middle REE in the pyroxenes Second, we show that although partition coefficients for trace elements in orthopyroxene are comparable between this and previous studies, the REE in clinopyroxene are typically a factor of 2–3 lower in this study. We also note that are correlations between partition coefficient and the composition of olivine, orthopyroxene, clinopyroxene and glass (melt). The relation of partitioning to melt composition suggests that some further development of the lattice strain model is needed. Finally, we show that there is agreement between our measured partition coefficients and those predicted from parameterized models of clinopyroxene–melt partitioning, however, there are unresolved differences that may result from differences in the substitution mechanisms of trace elements in M1 vs. M2 sites in clinopyroxene that are in part related to the composition of the coexisting melt.

Nature of the mineralizing fluids in the Balda and Motiya W-prospects, western India: Constraints from chemical and B-isotope composition of tourmaline

In the Proterozoic tungsten belts of Balda and Motiya in western India, tungsten mineralization is hosted in tourmaline-bearing quartz veins intrusive into pelitic schists and granites. Tourmaline is a ubiquitous phase in all rock types, and in this study, we use its major, trace element and B-isotope composition to constrain the nature of the tungsten (W)-bearing hydrothermal fluid and the processes involved in the precipitation of wolframite. We have also reconstructed the compositions of the W-precipitating fluids from mineral-fluid trace element partition coefficients that were extrapolated using the Lattice Strain Model. The tourmalines from both belts are of schorl composition and have high alkali, low Ca content, and moderate X-site vacancies. High Li, Mn, Zn, and Sn in the fluid in Motiya is suggestive of relatively saline fluid possibly derived from a granitic source. The high V/Sc ratios of tourmalines in the mineralized veins and wall-rock tourmalinites indicate an important role of biotite dissolution and fluid-rock interaction that contributed Fe–Mn for the precipitation of tourmaline and wolframite. The tourmalines in the mineralized veins at Balda (δ11Btur = −10.9 ± 0.7‰, 2σ; n = 10) and those in the associated granites (δ11Btur = −11.5 ± 0.7‰, 2σ; n = 6) have similar B-isotope composition, while those of the associated topaz granites and pegmatites (−13.9 ± 0.7‰, 2σ; n = 19) are isotopically lighter than the granites. The B-isotopic variation in the granite-pegmatite-vein system can be explained by fluid exsolution with the mineralized vein forming from exsolved fluid and the topaz-bearing granites and the pegmatites crystallizing from the residual melts. The δ11B of the mineralizing fluid is estimated to be ca. −7.4‰ at Balda and ca. −7.6‰ at Motiya, and were possibly derived from granites, consistent with extensive tourmalinization and muscovitization of the adjacent wall rocks, and high concentration of elements such as F, Li, B, Sn, Mn in the tourmalines and the reconstructed fluid. The chemistry and B-isotope composition of tourmalines in both the belts support the hypothesis that W-bearing hydrothermal fluid was primarily derived from a fractionated granitic source, and that precipitation of wolframite and tourmaline involved interaction of the granitic fluid with surrounding pelitic rocks.

Distribution coefficient of rare-earth dopants in Y3Al5O12 garnet

We present the distribution coefficient for the entire series of rare-earth dopants (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) in flux-grown Y3Al5O12 garnet crystals. When the distribution coefficient is plotted in a logarithmic scale as a function of dopant radius, it shows a smooth parabolic drop from the smallest dopant (Lu3+) at 1.5 to the largest dopant (La3+) at 0.004. This behavior can be interpreted by the lattice strain model, with Lu3+ having close to the ideal radius for the 8-fold coordination site of the aluminum garnet structure.

Quantification of excess 231Pa in late Quaternary igneous baddeleyite

AbstractInitial excess protactinium (231Pa) is a frequently suspected source of discordance in baddeleyite (ZrO2) geochronology, which limits accurate U/Pb dating, but such excesses have never been directly demonstrated. In this study, Pa incorporation in late Holocene baddeleyite from Somma-Vesuvius (Campanian Volcanic Province, central Italy) and Laacher See (East Eifel Volcanic Field, western Germany) was quantified by U-Th-Pa measurements using a large-geometry ion microprobe. Baddeleyite crystals isolated from subvolcanic syenites have average U concentrations of ~200 ppm and are largely stoichiometric with minor abundances of Nb, Hf, Ti, and Fe up to a few weight percent. Measured (231Pa)/(235U) activity ratios are significantly above the secular equilibrium value of unity and range from 3.4(8) to 14.9(2.6) in Vesuvius baddeleyite and from 3.6(9) to 8.9(1.4) in Laacher See baddeleyite (values within parentheses represent uncertainties in the last significant figures reported as 1σ throughout the text). Crystallization ages of 5.12(56) ka (Vesuvius; MSWD = 0.96, n = 12) and 15.6(2.0) ka (Laacher See; MSWD = 0.91, n = 10) were obtained from (230Th)/(238U) disequilibria for the same crystals, which are close to the respective eruption ages. Applying a corresponding age correction indicates average initial (231Pa)/(235U)0 of 8.8(1.0) (Vesuvius) and 7.9(5) (Laacher See). For reasonable melt activities, model baddeleyite-melt distribution coefficients of DPa/DU = 5.8(2) and 4.1(2) are obtained for Vesuvius and Laacher See, respectively. Speciation-dependent (Pa4+ vs. Pa5+) partitioning coefficients (D values) from crystal lattice strain models for tetra- and pentavalent proxy ions significantly exceed DPa/DU inferred from direct analysis of 231Pa for Pa5+. This is consistent with predominantly reduced Pa4+ in the melt, for which D values similar to U4+ are expected. Contrary to common assumptions, baddeleyite-crystallizing melts from Vesuvius and Laacher See appear to be dominated by Pa4+ rather than Pa5+. An initial disequilibrium correction for baddeleyite geochronology using DPa/DU = 5 ± 1 is recommended for oxidized phonolitic melt compositions.

Effect of fO2 on Eu partitioning between clinopyroxene, orthopyroxene and basaltic melt: Development of a Eu3+/Eu2+ oxybarometer

We present partition coefficients for Cs, Li, Sr, Ba, Zn, Mn, Co, Ni, Sc, Ga, Y, La, Nd, Sm, Eu, Tb, Yb, Lu, Ti, Zr, Hf, Ta, Nb and P for synthetic clinopyroxene and orthopyroxene crystallized from a Fe-free basalt at 1.5 GPa and four different oxygen fugacities (fO2) ranging from 6 log units above the quartz-fayalite-magnetite oxygen buffer (QFM + 6) to QFM-5 at temperatures of 1275–1300 °C. Partition coefficients for the rare earth elements (REE) vary as a function of their ionic radii, in agreement with lattice strain theory. We use the lattice strain model to evaluate partition coefficients for Eu2+ and Eu3+. Our results suggest that fO2 exerts a primary and important control on Eu, Ni and Co partitioning in clinopyroxene whereas fO2 seemingly has no recognizable effect on the partitioning behavior of these elements in orthopyroxene. However Eu anomalies in orthopyroxene, orthopyroxene Onuma diagrams and calculated proportions of Eu2+ in melts all show evidence that fO2 influence Eu partitioning also in orthopyroxene. We combined our results, available experimental data and predictive models for divalent and trivalent Eu clinopyroxene-melt and orthopyroxene-melt partitioning to parameterize fO2-dependent Eu partitioning models. The reported models reproduce measured Eu partition coefficients within a factor of two. Our model can be applied as clinopyroxene-melt and orthopyroxene-melt oxybarometers if Eu equilibrium partitioning between melt and pyroxene can be demonstrated.