Trace Element Partition Coefficients Research Articles

Trace element partitioning in the lunar magma ocean: an experimental study

Modeling the behavior of trace elements during lunar magma ocean solidification is important to further our understanding of the chemical evolution of the Moon. Lunar magma ocean evolution models rely on consistent datasets on how trace elements partition between a lunar silicate melt and coexisting minerals at different pressures, temperatures, and redox conditions. Here we report new experimental trace element partition coefficients (D) between clinopyroxene (cpx), pigeonite, orthopyroxene, plagioclase, olivine (ol), and silicate melt at conditions relevant for the lunar magma ocean. The data include Dcpx−melt at ambient and high pressures (1.5 GPa and 1310 °C), and partition coefficients at ambient pressure for pig, opx, ol, and pl. Overall, clinopyroxene is a phase that may control the fractionation of key geochemical trace element ratios, such as Lu/Hf and Sm/Nd, during the evolution of the lunar magma ocean. We explore the impact of the new silicate Dmineral−melt on the trace element evolution of the lunar magma ocean and we find that accessory phosphate minerals, such as apatite or whitlockite are of critical importance to explain the observed trace element and isotopic signature of the KREEP reservoir on the Moon. The new partition coefficients were applied to calculate the trace element evolution of the residual melts of the crystallizing lunar magma ocean and we propose a new trace element composition for the urKREEP reservoir. The new data will be useful for future thermo-chemical models in order to adequately predict the duration of the lunar magma ocean and the age of the Moon.

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.

Fingerprinting crustal anatexis with apatite trace element, halogen, and Sr isotope data

The geochemistry of apatite is a valuable tool for granite petrogenetic study. Here we explore the capability of apatite for fingerprinting anatectic reactions during crustal melting. We conducted a detailed petrographic, trace element, volatile, and Sr isotopic study of the apatites from four groups of leucosomes and leucogranites that formed by amphibole dehydration melting (group I), hydration melting (group II), muscovite dehydration melting (group III), and feldspar accumulation (Group IV) from the central Himalayan orogen. The coexistence of primary CO2-rich fluid and multiphase solid inclusions in apatite indicates fluid–melt immiscibility and suggests that C–O–H fluids were present during hydration melting. We observe marked differences in major and trace element, as well as halogen (F and Cl) contents in apatite from different groups, corresponding to different melting reactions. Apatite REE, Y, Mn, F, and F/Cl exhibit a decrease, whereas Sr/Y and Cl increase, from muscovite dehydration melting to hydration melting, and to amphibole dehydration melting. Amphibole dehydration melting results in similar CaO, Sr, and Eu/Eu* in apatite to hydration melting, but the latter leads to lower (La/Yb)N in apatite. These geochemical differences in apatite can be explained by different minerals participating in the melting reactions and varying melt polymerization resulting from the different melting reactions. The degree of melt polymerization decreases in the sequence from muscovite dehydration melting to hydration melting, and to amphibole dehydration melting. A higher degree of melt polymerization results in higher values for trace element partition coefficients between apatite and melt, and therefore the enhanced incorporation of REE, Y and Mn in apatite. Higher Sr and Eu/Eu* in apatite reflect the consumption of a larger amount of plagioclase in the melting reaction. The F and Cl contents in partial melts were calculated based on apatite composition, which exhibit an increasing trend from hydration melting to muscovite hydration melting, and to amphibole hydration melting. Such a trend can be explained by different proportions of hydrous minerals (muscovite and amphibole) participating in the melting reactions, with amphibole responsible for the high Cl content in melt and apatite. In addition, the apatite Sr isotopic compositions record the Sr isotopic disequilibrium during partial melting and exhibit distinct evolutional trends resulting from different anatectic reactions. Muscovite dehydration melting forms melt with higher 87Sr/86Sr than the restite due to radiogenic Sr released by muscovite. Apatite therefore records progressively decreasing 87Sr/86Sr during melt evolution before the isotopic homogenization between the partial melt and the restite. In contrast, apatites crystallized from the melt of hydration melting show an evolutional trend of increasing 87Sr/86Sr. In summary, the elemental and isotopic compositions of apatite can be related to different types of melting reactions, and can therefore be used as an indicator of the mechanisms of melt formation during crustal anatexis. We demonstrate such an application through the case study of group IV apatite from feldspar accumulation, and also explore its utility in granite petrogenetic studies.

Multivariate statistical analysis of trace elements in apatite: Discrimination of apatite with different origins

Partial least squares-discriminant analysis (PLS-DA) was used to construct the link between trace element contents of magmatic or hydrothermal apatite and deposit and rock types. Apatite with different origins is discriminated by characteristic chemical composition. In average, ore magmatic apatite has higher light REE and Th, and lower Sr than barren rock apatite, independent of deposit types. Hydrothermal apatite has lower La, Ce, Pr, and Nd contents than magmatic apatite, probably because fluids prefer migrating light rare earth element out of apatite. Hydrothermal apatite from iron oxide copper gold (IOCG) and iron oxide apatite (IOA) deposits is discriminated by high Mn and Sr relative to magmatic apatite, probably because these elements are fluid mobile. Barren magmatic apatite from granitoid-related deposits is well separated from ore magmatic apatite by higher Sr, Eu, Gd, Tb, Dy, and lower Mn contents, which are likely related to the relatively low degree of differentiation and high aluminum saturation index of parental magma.Magmatic apatite from IOA deposits is discriminated by relatively high Nd, Sm, Gd, Tb, Dy, and low Mn and U contents, which are possibly related to the crystallization of Mn-compatible magnetite microlites and peraluminous magmas. Magmatic apatite from granitoid-related W deposits is relatively rich in Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The relative enrichment of Sr in magmatic apatite from granitoid-related Cu-Pb-Zn and Pb-Zn deposits is likely due to less fractionation of Sr-compatible plagioclase. Melt of sediment component accounts for high Mn and Th in magmatic apatite from granitoid-related Mo deposits. Magmatic apatite from porphyry deposits is discriminated by relatively high V, Sr, and Eu contents. Magmatic apatite from porphyry Mo deposits has relatively high Pr, Nd, and Sm and low Sr contents compared to other types of porphyry deposits due to the more evolved magma. Hydrothermal apatite from IOA deposits is discriminated by high V and Sr, whereas those from IOCG deposits have high Sm, Eu, Gd, Tb, and U contents, because the lower temperature of IOCG cause precipitation of REE in Cl-rich fluid. Relatively low Gd and Tb in hydrothermal apatite from skarn deposits are possibly due to the crystallization of amphibole.Apatite in carbonatite has high contents of trace elements possibly because high contents of Ca and P in melts cause high apatite-melt partition coefficients of trace elements. Apatite from sedimentary and metamorphic rocks is discriminated by relative enrichment of Sr and Eu. Compared to igneous rocks, apatite from sedimentary rocks has low REE contents, reflecting low REE budget in surface waters. Breakdown of some minerals to release and remobilize REE and U during metamorphism can interpret high heavy REE and U contents in apatite from high-grade metamorphic rocks relative to those from low- and medium-grade metamorphic rocks. The ability to discriminate apatite from different types of deposits and rocks indicates the application potential of apatite chemistry in mineral exploration. A flowchart was proposed to identify apatite with unknown origins.

Spinel Harzburgite-Derived Silicate Melts Forming Sulfide-Bearing Orthopyroxenite in the Lithosphere. Part 1: Partition Coefficients and Volatile Evolution Accompanying Fluid- and Redox-Induced Sulfide Formation

We report abundances of major trace and volatile elements in an orthopyroxenite vein cutting a sub-arc, mantle-derived, spinel harzburgite xenolith from Kamchatka. The orthopyroxenite contains abundant sulfides and is characterized by the presence of glass (formerly melt) both interstitially and as inclusions in minerals, comparable with similar veins from the West Bismarck arc. The glass formed by quenching of residual melts following crystallization of abundant orthopyroxene, amphibole, and minor olivine and spinel. The interstitial glass has a low-Ti, high-Mg# andesite composition, with a wide range of H2O and S contents but more limited F and Cl variations. We calculate trace element partition coefficients using mineral and glass data, including those for halogens in amphibole, which agree with experimental results from the literature. Despite having a similar, high-Mg# andesite composition, the orthopyroxene-hosted glass inclusions usually contain much more H2O and S than the interstitial glass (4–7 wt% and ∼2,600 ppm, respectively). The initial vein-forming melts were oxidized, recording oxygen fugacity conditions up to ∼1.5 log units above the fayalite–magnetite–quartz oxygen buffer. They intruded the sub-arc mantle lithosphere at ≥1,300°C, where they partially crystallized to form high-Mg# andesitic derivative melts at ca. 1,050–1,100°C. Comparison with literature data on glass-free orthopyroxenite veins from Kamchatka and the glass-bearing ones from West Bismarck reveals fundamental similarities indicating common parental melts, which were originally produced by low-degree melting (≤5%) of spinel harzburgite at ≥1,360°C and ≤1.5 GPa. This harzburgite source likely contained ≤0.05 wt% H2O and a few ppm of halogens. Volatile evolution inferred from glass compositions shows that (i) redox exchange between S6+ in the original melt and Fe2+ in the host mantle minerals, together with (ii) the formation of an S-bearing, (H2O, Cl)-rich hydrothermal fluid from the original melt, provides the conditions for the formation of abundant sulfides in the orthopyroxenites during cooling. During this process, up to 85% of the original melt S content (∼2,600 ppm) is locally precipitated as magmatic and hydrothermal sulfides. As such, melts derived from spinel harzburgite sources can concentrate chalcophile and highly siderophile metals in orthopyroxenite dykes and sills in the lithosphere.

Trace element partitioning during incipient melting of phlogopite-peridotite in the spinel and garnet stability fields

Potassium-rich magmatism represents subordinate magma volumes worldwide but has been observed in many geodynamic settings. Most potassic magmas are thought to derive from very-low degrees of melting of metasomatized mantle lithologies. We performed piston-cylinder experiments to determine trace element partition coefficients between incipient potassic silicate melts and phlogopite ± pargasite peridotite in the spinel (1 GPa) and garnet stability fields (3 GPa). Most of the rare Earth elements (REEs) are compatible in pargasite but incompatible in phlogopite. Although garnet remains the mineral phase most efficiently fractionating heavy from light REEs (DYbgrt/melt/DLagrt/melt > 750), orthopyroxene can also significantly fractionate REEs, e.g., achieving DYbopx/melt/DLaopx/melt > 100 at 3 GPa. Mineral-liquid partition coefficients vary by about one order of magnitude between incipient melts derived from the spinel and garnet stability fields. We thus show that trace element partitioning at the onset of melting is controlled more by pressure (through melt composition) than by the extent of melting. With increasing pressure, Rb and Ba exhibit different behaviors in phlogopite, with DBaphl/melt > DRbphl/meltat 1 GPa and the opposite at 3 GPa. At 1 GPa, a decrease of melt polymerization (lower NBO/T) with increasing melt fraction translates into a significant decrease of most phlogopite partition coefficients. Finally, we show that resolvable inter-element fractionations do occur when phlogopite- (and pargasite)-bearing peridotite are melted, indicating that trace element ratios are not always faithfully representative of that of their sources but bear the imprint of varied P–T conditions of melting and contrasted pre-metasomatic histories. This self-consistent partition coefficient dataset thus gives a new scope to understand the complex petrogenesis of K-rich magmas in orogenic settings.

Sectoral and growth rate control on elemental uptake by individual calcite crystals

Heterogeneous distribution of trace elements (impurities) within individual calcite crystals is a phenomenon commonly observed in natural and laboratory systems. Changes in thermodynamic intensive parameters (mostly chemical potential and temperature) cannot always explain the inhomogeneous impurity patterns in calcite crystals and it has been suggested that growth rate and crystallographic orientation may exert strong effects on elemental incorporation into calcite. In addition, there are a number of experimental studies on micro-scale element (E) distribution between non-equivalent pairs of calcite vicinal faces (known as sectoral zoning); however, the variability of partition coefficients (e.g., KE = (E/Ca)calcite/(E/Ca)fluid) within individual crystals remains undetermined.In this study, we have extended the work on elemental distribution between crystal sectors to evaluation of partition coefficients of trace and minor elements (Li, B, Mg, and Sr) in calcite crystal faces (10–14) and (01−12), whose growth rates were assessed. Growth entrapment model (GEM) and lattice strain theory were applied to explain KE heterogeneity by varying near-surface diffusivity of Mg and Sr and by varying surface enrichment factor for Li, B, Mg, and Sr. Decoupling of sectoral and growth rate effects reveals that sectoral zoning plays a key role in elemental distribution. More specifically, KLi and KB vary by more than one order of magnitude and KMg varies by a factor of two within individual crystal faces. Strontium sectoral distribution is different from those of Li, B, and Mg and KSr varies by up to a factor of two. These behaviors likely reflect different mechanisms of incorporation of Li, B, Mg, and Sr into calcite.

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.

The physical and chemical evolution of magmatic fluids in near-solidus silicic magma reservoirs: Implications for the formation of pegmatites

AbstractAs ascending magmas undergo cooling and crystallization, water and fluid-mobile elements (e.g., Li, B, C, F, S, Cl) become increasingly enriched in the residual melt until fluid saturation is reached. The consequential exsolution of a fluid phase dominated by H2O (magmatic volatile phase or MVP) is predicted to occur early in the evolution of long-lived crystal-rich “mushy” magma reservoirs and can be simulated by tracking the chemical and physical evolution of these reservoirs in thermomechanical numerical models. Pegmatites are commonly interpreted as the products of crystallization of late-stage volatile-rich liquids sourced from granitic igneous bodies. However, little is known about the timing and mechanism of extraction of pegmatitic liquids from their source. In this study, we review findings from thermomechanical models on the physical and chemical evolution of melt and MVP in near-solidus magma reservoirs and apply these to textural and chemical observations from pegmatites. As an example, we use a three-phase compaction model of a section of a mushy reservoir and couple this to fluid-melt and mineral-melt partition coefficients of volatile trace elements (Li, Cl, S, F, B). We track various physical parameters of melt, crystals, and MVP, such as volume fractions, densities, velocities, as well as the content in the volatile trace elements mentioned above. The results suggest that typical pegmatite-like compositions (i.e., enriched in incompatible elements) require high crystallinities (>70–75 vol% crystals) in the magma reservoir, at which MVP is efficiently trapped in the crystal network. Fluid-mobile trace elements can become enriched beyond contents expected from closed-system equilibrium crystallization by transport of MVP from more-evolved mush domains. From a thermomechanical perspective, these observations indicate that, rather than from melt, pegmatites may more likely be generated from pressurized, solute-rich MVP with high concentrations of dissolved silicate melt and fluid-mobile elements. Hydraulic fracturing provides a mechanism for the extraction and emplacement of such pegmatite-generating liquids in and around the main parental near-solidus mush as pockets, dikes, and small intrusive bodies. This thermomechanical framework for the extraction of MVP from mushes and associated formation of pegmatites integrates both igneous and hydrothermal realms into the concept of transcrustal magmatic distillation columns.

Pegmatitic granite fluid compositions and thermochronometry in the Seridó Belt, Borborema Province, Brazil: Insights from trace element advection-diffusion-partitioning halos in host schist and gneiss

At the Seridó Belt pegmatitic province, fluid advection from dykes and sills led to trace element diffusion and partitioning in biotite, producing sub metric concentration halos in host mica schist and gneiss. We analyzed trace element concentration vs. distance profiles in the host rock of pegmatitic granite intrusions showing enrichments in Li, F, Cl, Zn, Rb, Nb, Sn, Cs, Ta, U and depletions in Sr and Ba. We estimated fluid temperatures in the range 600 to 700 °C using methods of Ti in biotite, F-OH in biotite-apatite and Tschermak component in biotite-muscovite. We analytically modeled thermal profiles around pegmatitic planar intrusions and found that cooling by thermal flats in the first meters into the host rock is reached in 100 years for a 10 m thick sill. The model constraints sustained heat flow during pegmatitic melt emplacement into a colder host rock. The measured trace element profiles were fitted with solutions of differential equations modeling advection-diffusion-partitioning in porous media under isothermal and cooling conditions. Each trace element profile is equally fitted by values of pegmatitic fluid concentration vs. fluid flux duration plotting as a straight line. We obtained a theorical framework combining pegmatitic fluid trace element composition, fluid flux duration and velocity, rock/fluid trace element partition coefficient, and cooling rate. Using published fluid inclusion trace element composition, we obtained fluid flux duration between tens of years to several centuries for the studied profiles. In the case of low temperature fluid flux at 450 °C the modeled flux duration is increased by less than one order of magnitude. The results allow us to interpret the trace element halos as sub millennial hot isothermal fluid influx from the pegmatites into their host rock. Trace element partitioning from the fluid into host rock falls mostly within the range of 0.01 to 25 and decreases in the order Nb, Cu, Rb, Zn-Li, Cs-Sn. The model constraints fluid advection velocity between 1 × 10−11 m/s and 1 × 10−9 m/s, typical of regional metamorphism to shear focused fluid flow regimes. The trace element enrichment of biotite along the profiles reproduces the observed pattern for whole rock, indicating that biotite is their main sink during pegmatitic fluid influx. Using experimental F in biotite diffusivity we obtained a minimal planar diffusion distance, or the effective porosity, between 7 and 15 μm for the total F re-homogenization of metamorphic biotite by pegmatitic fluid. As shown from others fluid-present petrogenetic systems, the trace element contents of biotite in host rock at the contact with pegmatitic intrusions can be a potential tool for the assessment of their rare metal potential.

High pressure trace element partitioning between clinopyroxene and alkali basaltic melts

We present new experimental data on major and trace element partition coefficients (D) between clinopyroxene and a K-basaltic melt from Procida Island (Campi Flegrei Volcanic District, south Italy). Time-series experiments were conducted at 0.8 GPa and 1080–1250 °C aiming to investigate the role of the crystallization kinetics on trace elements partitioning behaviour at a pressure relevant for deep magmatic reservoirs. Results indicate that large ion lithophile elements (LILE) are incompatible (e.g., DSr ≤ 0.15), light rare elements (LREE; e.g., DLa ≤ 0.20) are always more incompatible than heavy rare elements (HREE), which in some cases result to be compatible with clinopyroxene (e.g., DDy = 1.40); high field strength elements (HFSE) are generally incompatible (DHFSE ≤ 0.8), while transition elements (TE) range from slightly incompatible (e.g., DV = 0.6) to highly compatible (e.g., DCr = 63). The calculated D values for LILEs, REEs, HFSEs, and TEs tend to decrease with the increase of temperature and to increase with increasing tetrahedrally-coordinated aluminium content, in agreement with the previous studies. Moreover, we observed the influence of the growth rate on the partition coefficients, with the highest DREE values calculated in the runs with the highest growth rate (~10−7 cm s−1), due to the less efficient rejection of incompatible elements during rapid crystal growth, that in this study is not linked to disequilibrium conditions, but to the presence of pre-existing nuclei. Additionally, the apparent increase in DREE values with time observed in some runs is not referable to a change in time but rather to the different degrees of polymerization, expressed as the ratios NBO/T of these melts, strictly related to a loss of Fe occurred during the experiments, and thus to a different melt viscosity. Finally, the application of the experimental clinopyroxene-melt partition coefficients highlights that the deepest step of the magmatic differentiation in the Campi Flegrei Volcanic District is represented by the fractionation of about 20–30% of a clinopyroxenitic mineral assemblage from a basaltic parental magma.

A systematic assessment of the diamond trap method for measuring fluid compositions in high-pressure experiments

Abstract A variety of experimental techniques have been proposed to measure the composition of aqueous fluids in high-pressure experiments. In particular, the “diamond trap method,” where the fluid is sampled in the pore space of diamond powder and analyzed by laser-ablation ICP-MS after the experiment, has become a popular tool. Here, we carried out several tests to assess the reliability of this method. (1) We prepared several capsules loaded with fluid of known composition and analyzed the fluid by laser-ablation ICP-MS, either (a) after drying the diamond trap at ambient condition; (b) after freezing and subsequent freeze-drying; and (c) after freezing and by analyzing a frozen state. Of these methods, the analysis in the frozen state (c) was most accurate, while the results from the other two methods were poorly reproducible, and the averages sometimes deviated from the expected composition by more than a factor of 2. (2) We tested the reliability of the diamond trap method by using it to measure mineral solubilities in some well-studied systems at high pressure and high temperature in piston-cylinder runs. In the systems quartz-H2O, forsterite-enstatite-H2O, and albite-H2O, the results from analyzing the diamond trap in a frozen state by laser-ablation ICP-MS generally agreed well with the expected compositions according to literature data. However, in the systems corundum-H2O and rutile-H2O, the data from the analysis of the diamond trap were poorly reproducible and appeared to indicate much higher solubilities than expected. We attribute this not to some unreliability of the analytical method, but instead to the fact that in these systems, minor temperature gradients along the capsule may induce the dissolution and re-precipitation of material during the run, which causes a contamination of the diamond trap by solid phases. (3) We carried out several tests on the reliability of the diamond trap to measure fluid compositions and trace element partition coefficients in the eclogite-fluid system at 4 GPa and 800 °C using piston-cylinder experiments. The good agreement between “forward” and “reversed” experiments—with trace elements initially either doped in the solid starting material or the fluid—as well as the independence of partition coefficients on bulk concentrations suggests that the data obtained are reliable in most cases. We also show that the rate of quenching/cooling has little effect on the analytical results, that temperature oscillations during the run can be used to enhance grain growth, and that well-equilibrated samples can be obtained in conventional piston-cylinder runs. Overall, our results suggest that the diamond trap method combined with laser-ablation ICP-MS in frozen state yields reliable results accurate within a factor of two in most cases; however, the precipitation of accessory minerals in the diamond trap during the run may severely affect the data in some systems and may lead to a gross overestimation of fluid concentrations.

Behavior of apatite in granitic melts derived from partial melting of muscovite in metasedimentary sources

Fluid-absent and fluid-fluxed melting of muscovite in metasedimentary sources are two types of crustal anatexis to produce the Himalaya Cenozoic leucogranites. Apatite grains separated from melts derived from the two types of parting melting have different geochemical compositions. The leucogranites derived from fluid-fluxed melting have relict apatite grains and magmatic crystallized apatite grains, by contrast, there are only crystallized apatite grains in the leucogranites derived from fluid-absent melting. Moreover, apatite grains crystallized from fluid-fluxed melting of muscovite contain higher Sr, but lower Th and LREE than those from fluid-absent melting of muscovite, which could be controlled by the distribution of partitioning coefficient (<i>D</i>_Ap/Melt) between apatite and leucogranite. <i>D</i>_Ap/Melt in granites derived from fluid-absent melting is higher than those from fluid-fluxed melting. So, not only SiO₂ and A/CNK, but also types of crustal anatexis are sensitive to trace element partition coefficients for apatite. In addition, due to being not susceptible to alteration, apatite has a high potential to yield information about petrogenetic processes that are invisible at the whole-rock scale and thus is a useful tool as a petrogenetic indicator.

The influence of fractionation of REE-enriched minerals on the zircon partition coefficients

Zircon is widely used to simulate melt generation, migration and evolution within the crust and mantle. The achievable performance of melt modelling generally depends on the availability of reliable trace element partition coefficients (D). However, a large range of DREE values for zircon from natural samples and experimental studies has been reported, with values spanning up to 3 orders of magnitude. Unfortunately, a gap of knowledge on this variability is evident. In this study we model the crystallization processes of common REE-bearing minerals from granitic melts and show that the measured zircon DREE would be elevated if there is crystallization of REE-enriched minerals subsequent to zircon. Nevertheless, compared to zircon DREE values measured from experimental studies, this mechanism appears to have a less significant influence on those from natural granite samples since the quantity of crystallized REE-enriched minerals is very low in natural magmatic systems and/or most of them crystallize prior to zircon. Combined with recently published studies, this work supports that analysis of natural zircon/host groundmass pairs provides more robust DREE values applicable to natural systems than those measured from experimental studies, which can be used to constrain the provenance of detrital zircons.

The Fate of Sulfur and Chalcophile Elements During Crystallization of the Lunar Magma Ocean

AbstractTo assess the viability of sulfide liquid saturation during crystallization of the lunar magma ocean (LMO), we present a new data set describing both the sulfur (S) concentration at sulfide liquid saturation (SCSS) and sulfide liquid‐silicate melt partition coefficients of many trace elements for various differentiated lunar magmas at lunar‐relevant conditions. Using these parameterizations, we model the SCSS and the distribution of the most chalcophile elements with progressive LMO crystallization in the absence and presence of sulfide liquids. Modeling results for different modes of LMO crystallization show that for proposed lunar mantle S abundances FeS sulfide liquid saturation is expected to occur between 96% and 98% of LMO crystallization. This is decreased to &gt;91% for Fe‐S liquids with 30% Ni or Cu. Saturation of S‐poor sulfide liquids can occur at &gt;75% of LMO crystallization. The timing of sulfide liquid saturation depends most strongly on the assumed S content of the lunar mantle following formation of the lunar core and on the sulfide liquid composition. Modeled abundances of chalcophile elements indicate that sulfide‐liquid saturation during late‐stage LMO crystallization would yield much lower abundances of Ni and Cu than observed in KREEP basalts and estimated for the urKREEP reservoir, as well as lower Ni/Co than observed in the latter. Sulfide liquids therefore did not affect moderately siderophile and chalcophile element fractionation within the LMO, supporting the hypothesis that the nonvolatile, siderophile element abundances of the lunar mantle reflect a phase of core formation and/or the addition of a meteoritic late veneer.

Trace element partitioning between pyrochlore, microlite, fersmite and silicate melts

We present experimentally determined trace element partition coefficients (D) between pyrochlore-group minerals (Ca2(Nb,Ta)2O6(O,F)), Ca fersmite (CaNb2O6), and silicate melts. Our data indicate that pyrochlores and fersmite are able to strongly fractionate trace elements during the evolution of SiO2-undersaturated magmas. Pyrochlore efficiently fractionates Zr and Hf from Nb and Ta, with DZr and DHf below or equal to unity, and DNb and DTa significantly above unity. We find that DTa pyrochlore-group mineral/silicate melt is always higher than DNb, which agrees with the HFSE partitioning of all other Ti–rich minerals such as perovskite, rutile, ilmenite or Fe-Ti spinel. Our experimental partition coefficients also show that, under oxidizing conditions, DTh is higher than corresponding DU and this implies that pyrochlore-group minerals may fractionate U and Th in silicate magmas. The rare earth element (REE) partition coefficients are around unity, only the light REE are compatible in pyrochlore-group minerals, which explains the high rare earth element concentrations in naturally occurring magmatic pyrochlores.

On a synthesis of crystal population dynamics and trace element partitioning models: A mechanism for zoning in minerals

Crystal zoning is considered in the context of the evolution of a population of crystals. Crystal size distribution (CSD) theory provides an important framework and constraints that are applied in a numerical model of crystal nucleation and growth in an infinite half-sheet which proxies for a sill. The CSD/crystal population approach is combined with a relatively simple principle of trace element (TE) partitioning for batch crystallization. A model of crystal nucleation and growth that is applied to a constrained volume of magma is modified by the addition of a co-precipitating phase and TE modeling. The coprecipitating phase does not consume the TE and so the TE's concentration in the residual liquid increases. Thus the primary phase for which the TE is compatible incorporates more as it grows. A single partition coefficient value is used and is held constant. One crystal in the much larger, evolving population of crystals is tracked and TE concentration in the crystal vs. time is recorded as the crystal grows. In general, TE concentration within the crystal initially decreases while only the primary phase is present and then begins to increase in that crystal when the second/coprecipitating phase appears. For the relatively short solidification interval utilized in the modeling, one half-cycle of oscillation: high concentration to low to high, or normal to reverse zoning, is demonstrated. Beyond the TE's partition coefficient, the presence and magnitude of zoning is dependent upon the time the second phase begins within the solidification interval and the mass proportion of crystallization of primary phase to second phase—which is held constant throughout the remainder of solidification once the second phase appears. The model, as currently implemented, is based solely on thermal and mass balances. A multi-faceted crystallization history, one involving, e.g., more complicated phase equilibria, crystal fractionation, convection, and magma mixing would expose the tracked crystal to changing surroundings such that the mass balance mechanism that yielded the half-cycle of zoning obtained here would perhaps continue to yield oscillatory zoning.

Partition coefficients of trace elements between carbonates and melt and suprasolidus phase relation of Ca-Mg-carbonates at 6 GPa

Abstract The presence of Ca-Mg-carbonates affects the melting and phase relations of peridotites and eclogites in the mantle, and (partial) melting of carbonates liberates carbon from the mantle to shallower depths. The onset and composition of incipient melting of carbonated peridotites and carbonated eclogites are influenced by the pure CaCO3-MgCO3-system making the understanding of the phase relations of Ca-Mg-carbonates fundamental in assessing carbon fluxes in the mantle. By performing high-pressure and high-temperature experiments, this study clarifies the suprasolidus phase relations of the nominally anhydrous CaCO3-MgCO3-system at 6 GPa showing that Ca-Mg-carbonates will (partially) melt for temperatures above ~1300 °C. A comparison with data from thermodynamic modeling confirms the experimental results. Furthermore, partition coefficients for Li, Na, K, Sr, Ba, Nb, Y, and rare earth elements between calcite and dolomitic melt, Ca-magnesite and dolomitic melt, and magnesite and dolomitic melt are established. Experiments were performed at 6 GPa and between 1350 to 1600 °C utilizing a rotating multi-anvil press. Rotation of the multi-anvil press is indispensable to establish equilibrium between solids and carbonate liquid. Major and trace elements were quantified with EPMA and LA-ICP-MS, respectively. The melting temperature and phase relations of Ca-Mg-carbonates depend on the Mg/Ca-ratio. For instance, Ca-rich carbonates with a molar Mg/(Mg+Ca)-ratio (XMg) of 0.2 will transform into a dolomitic melt (XMg = 0.33–0.31) and calcite crystals (XMg = 0.19–0.14) at 1350–1440 °C. Partial melting of Mg-rich carbonates (XMg = 0.85) will produce a dolomitic melt (XMg = 0.5–0.8) and Ca-bearing magnesite (XMg = 0.89–0.96) at 1400–1600 °C. Trace element distribution into calcite and magnesite seems to follow lattice constraints for divalent cations. For instance, the compatibility of calcite (XMg = 0.14–0.19) for Sr and Ba decreases as the cation radii increases. Ca-Mg-carbonates are incompatible for rare earth elements (REEs), whereby the distribution between carbonates and dolomitic melt depends on the Mg/Ca ratio and temperature. For instance, at 1600 °C, partition coefficients between magnesite (XMg = 0.96) and dolomitic melt (XMg = 0.8) vary by two orders of magnitudes from 0.001 to 0.1 for light-REEs to heavy-REEs. In contrast, partition coefficients of REEs (and Sr, Ba, Nb, and Y) between magnesite (XMg = 0.89) and dolomitic melt (XMg = 0.5) are more uniform scattering marginal between ~0.1–0.2 at 1400 °C.

Isotopic analyses of clinopyroxenes demonstrate the effects of kimberlite melt metasomatism upon the lithospheric mantle

The trace element and radiogenic isotope systematics of clinopyroxene have frequently been used to characterise mantle metasomatic processes, because it is the main host of most lithophile elements in the lithospheric mantle. To further our understanding of mantle metasomatism, both solution-mode Sr-Nd-Hf-Pb and in situ trace element and Sr isotopic data have been acquired for clinopyroxene grains from a suite of peridotite (lherzolites and wehrlites), MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside), and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks from the Kimberley kimberlites (South Africa). The studied mantle samples can be divided into two groups on the basis of their clinopyroxene trace element compositions, and this subdivision is reinforced by their isotopic ratios. Type 1 clinopyroxene, which comprises PIC, wehrlite, and some sheared lherzolite samples, is characterised by low Sr (~100–200 ppm) and LREE concentrations, moderate HFSE contents (e.g., ~40–75 ppm Zr; La/Zr < 0.04), and restricted isotopic compositions (e.g., 87Sr/86Sri = 0.70369–0.70383; εNdi = +3.1 to +3.6) resembling those of their host kimberlite magmas. Available trace element partition coefficients can be used to show that Type 1 clinopyroxenes are close to being in equilibrium with kimberlite melt compositions, supporting a genetic link between kimberlites and these metasomatised lithologies. Thermobarometric estimates for Type 1 samples in this study indicate equilibration depths of 135–160 km within the lithosphere, thus showing that kimberlite melt metasomatism is prevalent in the deeper part of the lithosphere beneath Kimberley. In contrast, Type 2 clinopyroxenes occur in MARID rocks and coarse granular lherzolites in this study, which derive from shallower depths (<135 km), and have higher Sr (~350–1000 ppm) and LREE contents, corresponding to higher La/Zr of > ~ 0.05. The isotopic compositions of Type 2 clinopyroxenes are more variable and extend from compositions resembling the “enriched mantle” towards those of Type 1 rocks (e.g., εNdi = −12.7 to −4.4). To constrain the source of these variations, in situ Sr isotope analyses of clinopyroxene were undertaken, including zoned grains in Type 2 samples. MARID and lherzolite clinopyroxene cores display generally radiogenic but variable 87Sr/86Sri values (0.70526–0.71177), which are correlated with Sr contents and La/Zr ratios, and which might be explained by the interaction between peridotite and melts from different enriched sources within the lithospheric mantle. Most notably, the rims of these Type 2 clinopyroxenes trend towards compositions similar to those of the host kimberlite and Type 1 clinopyroxene from PIC and wehrlites. These results are interpreted to represent clinopyroxene overgrowth during late-stage (shortly before/during entrainment) metasomatism by kimberlite magmas. Our study shows that a pervasive, alkaline metasomatic event caused MARID to be generated and harzburgites to be converted to lherzolite in the lithospheric mantle beneath the Kimberley area, which was followed by kimberlite metasomatism during Cretaceous magmatism. This latter event is the time at which discrete PIC, wehrlite, and sheared lherzolite lithologies were formed, and MARID and granular lherzolites were partly modified.