Major Element Partitioning Research Articles

Differentiation of Chemical Components in Basaltic Melts During Eruptions: An Example from Tolbachik–Hawaiian Fissure Zones and Etna Vent

The study on differentiation of chemical components in quenched volcanics from the Etna, Tolbachik and Hawaiian fissure zones was carried out under simulated temperature/pressure (T/P) conditions reflecting real-time conditions in the early stages of magmatic eruptions. The simulation was performed under high-pressure chamber apparatus, and the rocks were then subjected to high T/P conditions resulting in basaltic melts. Various temperature and pressure conditions were adjusted from the apparatus to allow for different rates of magmatic cooling and crystallization. The scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS-microanalysis) was then used to study the quenched glasses with the aim of identifying major element partitioning based on micro-heterogeneity of melts structures. The microscopic study of the experimental melts and their comparisons with natural melts under the electron microscopes indicate the existence of two distinct liquids which fractionated during cooling on the basis of their chemical composition (compositional melts): Fe, Mg liquids concentrated in the poorly polymerized and mobile parts of the melt and K, Na and Al liquids were concentrated in more highly polymerized parts of the melts. Partitioning of Ca and Si appears to be changeable and more dependent on bulk melt composition. Fractionation of non-crystalline melts was evident under post-explosion sharp quenching and high-pressure conditions.

The composition of subduction zone fluids and the origin of the trace element enrichment in arc magmas

The partitioning of major and trace elements between eclogite and aqueous fluids with variable salinity was studied at 700–800 °C and 4–6 GPa in piston cylinder and multi anvil experiments. Fluid compositions were determined using the diamond trap technique combined with laser ablation ICP-MS measurements in the frozen state. In addition to NaCl, SiO2 is the main solute in the fluids. The fluid/eclogite partition coefficients of the large ion lithophile elements (LILE), such as Rb, Cs, Sr, and Ba as well as those of the light rare earths (LREE), of Pb, and of U increase by up to three orders of magnitude with salinity. These elements will therefore be efficiently transported by saline fluids. On the other hand, typical high field strength elements, such as Ti, Nb, and Ta, are not mobilized even at high salinities. Increasing temperature and pressure gradually increases the partitioning into the fluid. In particular, Th is mobilized by silica-rich fluids at 6 GPa already at low salinities. We show that we can fully reproduce the trace element enrichment pattern of primitive arc basalts by adding a few percent of saline fluid (with 5–10 wt% Cl) released from the basaltic slab to the zone of melting in the mantle wedge. Assuming 2 wt% of rutile in the eclogite equilibrated with the saline fluid produces a negative Nb Ta anomaly that is larger than in most primitive arc basalts. Therefore, we conclude that the rutile fraction in the subducted eclogite below most arcs is likely < 1 wt%. In fact, saline fluids would even produce a noticeable negative Nb Ta anomaly without any rutile in the eclogite residue. Metasomatism by sediment melts alone, on the other hand, is unable to produce the enrichment pattern seen in arc basalts. We, therefore, conclude that at least for primitive arc basalts, the release of hydrous fluids from the basaltic part of the subducted slab is the trigger for melting and the main agent of trace element enrichment. The contribution of sediment melts to the petrogenesis of these magmas is likely negligible. In the supplementary material, we provide a “Subduction Calculator” in Excel format, which allows the calculation of the trace element abundance pattern in primitive arc basalts as function of fluid salinity, the amount of fluid released from the basaltic part of the subducted slab, the fluid fraction added to the source, and the degree of melting.

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.

Major and trace element partitioning between majoritic garnet, clinopyroxene, and carbon dioxide-rich liquid in model carbonated peridotite at 10 GPa and interpretations of the element chemistry of majoritic garnet inclusions in diamonds from the subcontinental mantle of Brazil and Guinea

Experimentally determined major and trace element partition coefficients between majoritic garnet, clinopyroxene, and carbon dioxide-rich liquid are reported at 10 GPa and 1800 °C in a model carbonated peridotite composition in the system CaO-MgO-Al2O3-SiO2-CO2. Besides majoritic garnet, the liquid coexists with forsterite, orthopyroxene, and clinopyroxene, making melting phase relations invariant at fixed pressure and temperature conditions. Partition coefficients span a wide range of values - for instance, Sr, Nb, Ba, La, and Ce are highly incompatible in majoritic garnet, while Ca, Y, Nb, and Ho are moderately incompatible, and Lu, Si, Al, and Mg are compatible. Strong fractionation of light rare earth elements (e.g., La, Ce, Nd, Sm) and high field strength elements (e.g., Nb, Ta, Zr, Hf, Th) is seen between majoritic garnet and liquid. The experimentally determined partitioning values are used to calculate compositions of melts in equilibrium with majoritic garnet inclusions in diamonds from select localities in Brazil and Guinea. The calculated melts largely straddle those between documented carbonatites, kimberlites, and alkali basalts, low-degree mantle melting products from carbonated peridotite. This resemblance firmly suggests that majoritic garnet inclusions in diamonds from Brazil and Guinea can simply be interpreted as precipitates from such melts, thereby offering an alternative to the hypothesis that the element chemistry of such inclusions in diamonds can largely, and sometimes only, be ascribed to subducted oceanic crust, and further that, fusion of this crust may limit the terrestrial 'carbon recycling' at depths much beyond corresponding to those of Earth's transition zone.

Chemical weathering of mafic rocks in boreal subarctic environment (northwest Russia) under influence of glacial moraine deposits

Chemical weathering of mafic rocks represents substantial sink of atmospheric CO2 yet the mechanismes of this process in high-latitude regions are poorly understood. This work addresses geochemical migration and partitioning of major and trace elements between rock, soil and surface waters during chemical weathering of mafic rocks and soil formation under the influence of a glacial moraine in the northwestern Russian subarctic. We used multidisciplinary approach which included major and trace element chemical analysis, Sr isotopic measurements, and mineralogical structural and microscopic investigations. Quaternary (Pleistocene) deposits were observed even in the deepest horizons of soil profiles and confirm a strong moraine influence as most soils showed podzolic features. Chemical and Sr isotopic analyses revealed a strong impact of the moraine on soil chemistry and mineralogy. Mineralogical studies showed the presence of non-aeolian quartz and zircon in soils which were not linked to the nature of the parental rocks (i.e. mafic and felsic). At the same time, we observed the presence of etch pit corrosion on the surface of zircons and feldspars, and a newly-formed matter adjacent to the surface of primary phases. The chemical index of alteration (CIA) showed weak or no weathering of rocks within the soil profile and the weathering intensity scale confirmed the “felsic” chemical composition of soils developed over mafic and ultramafic rocks. Analysis of Sr radiogenic isotopes demonstrated preferential removal of easily weatherable and less radiogenic minerals over the full depth of soil profile and a more radiogenic signature of the granitic moraine compared to mafic and felsic bedrocks. The composition of most surface waters reflected the weathering of silicate rocks but did not allow for distinguishing purely mafic source. The chemical weathering rates of ultramafic rocks were calculated to be higher than those for the granitic till, but the moraine depositions which dominate this region hide evidence of the real weathering process. This could be due to retention of Mg and Ca in soil due to precipitation of secondary phases such as Mg-vermiculite or divalent cation adsorption on mineral, organic or organo-mineral phases. As a result, weathering rates estimated for the olivinite rock appear to be lower than the weathering rates of mafic rocks reported in other boreal and subarctic regions. Overall, the felsic moraine deposits are capable to sizably decrease the weathering intensity of mafic rocks which should be taken into account for chemical weathering modelling of high-latitude regions subjected to glaciation in the past.

The effect of diffusive re-equilibration time on trace element partitioning between alkali feldspar and trachytic melts

We present new experimental data on major and trace element partition coefficients between alkali feldspar and trachytic melt. Experiments were conducted at 500 MPa, 870–890 °C to investigate through short disequilibrium and long near-equilibrium experiments the influence of diffusive re-equilibration on trace element partitioning during crystallization. Our data show that Ba and Sr behave compatibly, and their partition coefficients are influenced by re-equilibration time, orthoclase (Or) content, growth rate and cation order-disorder. High field strength elements (HFSE) and rare earth elements (except Eu) are strongly incompatible, but alkali feldspar efficiently fractionates light (LREE) from heavy rare earth elements (HREE).Our crystallization experiments reveal a strong influence of disequilibrium crystal growth on the partitioning of Ba and Sr. In particular, short-duration experiments show that rapid alkali feldspar crystal growth after nucleation, promotes disordered growth and less selectivity in the partitioning of compatible trace elements that easily enter the crystal lattice (e.g., Ba and Sr). This produces partition coefficients of compatible elements higher than those obtained through long-duration experiments, in which growth is slower and more selective. On approach to equilibrium, decreasing growth rate and timescales of diffusive re-equilibration facilitates the incorporation of compatible elements within the crystal lattice (through ordered growth), on the basis of the charge and size of the octahedral site, resulting in lower partition coefficients of Ba and Sr. Therefore, our results indicate that besides crystal-chemical effects, i.e. lattice strain, the substitution mechanisms during growth (with different degrees of ordering) of Ba and Sr in alkali feldspar are strongly influenced by diffusive re-equilibration and crystal growth kinetics. This implies that partition coefficients of Ba and Sr can be used to establish timescales of crystallization processes under pre- and syn-eruptive conditions. The application of our results to alkali feldspar in rocks from Campi Flegrei, constrain the magma residence time at subliquidus conditions in a reservoir to a maximum of 6 days under disequilibrium conditions and to a minimum of 9 days upon approaching near-equilibrium conditions.

Silicate liquid immiscibility in impact melts

AbstractWe have investigated silicate emulsions in impact glasses and impact melt rocks from the Wabar (Saudi Arabia), Kamil (Egypt), Barringer (USA), and Tenoumer (Mauritania) impact structures, and in experimentally generated impact glasses and laser‐generated glasses (MEMIN research unit) by scanning electron microscopy, electron microprobe analysis, and transmission electron microscopy. Textural evidence of silicate liquid immiscibility includes droplets of one glass disseminated in a chemically distinct glassy matrix; sharp phase boundaries (menisci) between the two glasses; deformation and coalescence of droplets; and occurrence of secondary, nanometer‐sized quench droplets in Si‐rich glasses. The compositions of the conjugate immiscible liquids (Si‐rich and Fe‐rich) are consistent with phase separation in two‐liquid fields in the general system Fe2SiO4–KAlSi3O8–SiO2–CaO–MgO–TiO2–P2O5. Major‐element partition coefficients are well correlated with the degree of polymerization (NBO/T) of the Si‐rich melt: Fe, Ca, Mg, and Ti are concentrated in the poorly polymerized, Fe‐rich melt, whereas K, Na, and Si prefer the highly polymerized, Si‐rich melt. Partitioning of Al is less pronounced and depends on bulk melt composition. Thus, major element partitioning between the conjugate liquids closely follows trends known from tholeiitic basalts, lunar basalts, and experimental analogs. The characteristics of impact melt inhomogeneity produced by melt unmixing in a miscibility gap are then compared to impact melt inhomogeneity caused by incomplete homogenization of different (miscible or immiscible) impact melts that result from shock melting of different target lithologies from the crater's melt zone, which do not fully homogenize and equilibrate due to rapid quenching. By taking previous reports on silicate emulsions in impact glasses into account, it follows that silicate impact melts of variable composition, cooling rate, and crystallization history might readily unmix during cooling, thereby rendering silicate liquid immiscibility a much more common process in the evolution of impact melts than previously recognized.

Clinopyroxene-melt element partitioning during interaction between trachybasaltic magma and siliceous crust: Clues from quartzite enclaves at Mt. Etna volcano

A peculiar characteristic of the paroxysmal sequence that occurred on March 16, 2013 at the New South East Crater of Mt. Etna volcano (eastern Sicily, Italy) was the eruption of siliceous crustal xenoliths representative of the sedimentary basement beneath the volcanic edifice. These xenoliths are quartzites that occur as subspherical bombs enclosed in a thin trachybasaltic lava envelope. At the quartzite-magma interface a reaction corona develops due to the interaction between the Etnean trachybasaltic magma and the partially melted quartzite. Three distinct domains are observed: (i) the trachybasaltic lava itself (Zone 1), including Al-rich clinopyroxene phenocrysts dispersed in a matrix glass, (ii) the hybrid melt (Zone 2), developing at the quartzite-magma interface and feeding the growth of newly-formed Al-poor clinopyroxenes, and (iii) the partially melted quartzite (Zone 3), producing abundant siliceous melt. These features makes it possible to quantify the effect of magma contamination by siliceous crust in terms of clinopyroxene-melt element partitioning. Major and trace element partition coefficients have been calculated using the compositions of clinopyroxene rims and glasses next to the crystal surface. Zone 1 and Zone 2 partition coefficients correspond to, respectively, the chemical analyses of Al-rich phenocrysts and matrix glasses, and the chemical analyses of newly-formed Al-poor crystals and hybrid glasses. For clinopyroxenes from both the hybrid layer and the lava flow expected relationships are observed between the partition coefficient, the valence of the element, and the ionic radius. However, with respect to Zone 1 partition coefficients, values of Zone 2 partition coefficients show a net decrease for transition metals (TE), high-field strength elements (HFSE) and rare earth elements including yttrium (REE+Y), and an increase for large ion lithophile elements (LILE). This variation is associated with coupled substitutions on the M1, M2 and T sites of the type M1(Al, Fe3+)+TAl=M2(Mg, Fe2+)+TSi. The different incorporation of trace elements into clinopyroxenes of hybrid origin is controlled by cation substitution reactions reflecting local charge-balance requirements. According to the lattice strain theory, simultaneous cation exchanges across the M1, M2, and T sites have profound effects on REE+Y and HFSE partitioning. Conversely, both temperature and melt composition have only a minor effect when the thermal path of magma is restricted to ~70°C and the value of non-bridging oxygens per tetrahedral cations (NBO/T) shifts moderately from 0.31 to 0.43. As a consequence, Zone 2 partition coefficients for REE+Y and HFSE diverge significantly from those derived for Zone 1, accounting for limited cation incorporation into the newly-formed clinopyroxenes at the quartzite-magma interface.

The solubility of rocks in metamorphic fluids: A model for rock-dominated conditions to upper mantle pressure and temperature

Fluids exert a key control on the mobility of elements at high pressure and temperature in the crust and mantle. However, the prediction of fluid composition and speciation in compositionally complex fluid–rock systems, typically present in subduction zones, has been hampered by multiple challenges. We develop a computational framework to study the role of phase equilibria and complex solid-solutions on aqueous fluid speciation in equilibrium with rocks to 900 °C and 3 GPa. This is accomplished by merging conventional phase-equilibrium modeling involving electrolyte-free molecular fluids, with an electrostatic approach to model solute–solute and solute–solvent interactions in the fluid phase. This framework is applied to constrain the activity ratios, composition of aqueous solutes, and pH of a fluid in equilibrium with a pelite lithology. Two solvent compositions are considered: pure H2O, and a COH fluid generated by equilibration of H2O and graphite. In both cases, we find that the pH is alkaline. Disparities between the predicted peralkalinity of our fluid ([Na]+[K])/[Al]∼6 to 12 and results from independent mineral solubility experiments (∼2) point to the presence of Na–K–Al–Si polymers representing ca. 60 to 85% of the total K and Al content of the fluid at 600 °C and 2.2 GPa, and to an important fraction of dissolved Ca and Mg not accounted for in present speciation models. The addition of graphite to the system reduces the relative permittivity by ca. 40% at elevated T and low P, triggers the formation of C-bearing anions, and brings the pH closer to neutrality by up to 0.6 units at low T. This ionic C pool represents up to 45 mol% of the fluid ligands at elevated P, and is dominant at low P despite the low ionic strength of the fluid (<0.05). The present study offers new possibilities for exploring redox–pH dependent processes that govern volatile, major and trace element partitioning between rocks and fluids in experimental or natural systems.

Distribution and partitioning of major and trace elements in pyrite-bearing sediments of a Mediterranean coastal lagoon

The formation of iron sulphide minerals exerts significant control on the behaviour of trace elements in sediments. In this study, three short sediment cores, retrieved from the remote Antinioti lagoon (N. Kerkyra Island, NW Greece), are investigated concerning the solid phase composition, distribution, and partitioning of major (Al, Fe) and trace elements (Cd, Cu, Mn, Pb, and Zn). According to 210Pb, the sediments sampled correspond to depositions of the last 120 years. The high amounts of organic carbon (4.1–27.5%) result in the formation of Fe sulphides, predominantly pyrite, already at the surface sediment layers. Pyrite morphologies include monocrystals, polyframboids, and complex FeS–FeS2 aggregates. According to synchrotron-generated micro X-ray fluorescence and X-ray absorption near-edge structure spectra, authigenically formed, Mn-containing, Fe(III) oxyhydroxides (goethite type) co-exist with pyrite in the sediments studied. Microscopic techniques evidence the formation of galena, sphalerite and CuS, whereas sequential extractions show that carbonates are important hosts for Mn, Cd, and Zn. However, significant percentages of non-lattice held elements are bound to Fe/Mn oxyhydroxides that resist reductive dissolution (on average 60% of Pb, 46% of Cd, 43% of Zn and 9% of Cu). The partitioning pattern changes drastically in the deeper part of the core that is influenced by freshwater inputs. In these sediments, the post-depositional pyritization mechanism, illustrated by overgrowths of Fe monosulphides on pre-existing pyrite grains, results in relatively high degree of pyritization that reaches 49% for Cd, 66% for Cu, 32% for Zn and 7% for Pb.

A geochemical evaluation of potential magma ocean dynamics using a parameterized model for perovskite crystallization

Magnesium perovskite (MgPv) is likely the first phase to crystallize from a deep magma ocean. Consequently, MgPv crystallization has a strong control on the dynamics and chemical evolution associated with the earliest stages of silicate Earth differentiation. In order to better understand the chemical evolution associated with MgPv crystallization during a magma ocean, a parameterized model for major and trace element partitioning by MgPv has been developed. The parameterization is based on a compilation of published experimental data and is applied to batch and near-fractional crystallization scenarios of ultramafic liquids, allowing for a more complete analysis of the geochemical implications for magma ocean crystallization. The chemical signatures associated with modeled MgPv fractionation are evaluated in the context of possible dynamical outcomes to a magma ocean (e.g. basal magma ocean (BMO) or crystal settling). It is shown that fractionating MgPv from ultramafic liquids imparts diagnostic signatures (e.g. Ca/Al, HFSE anomalies, ε176Hf–ε143Nd) in both the liquid and solid phases. These signatures are not currently observed in the accessible Earth, suggesting that either early-fractionating MgPv was subsequently homogenized or crystal suspension was dominant during the earliest stages of magma ocean crystallization. A BMO that fractionates CaPv and MgPv is also considered and shown to mute many of unobserved geochemical effects associated with a MgPv-only fractionation, offering an alternative possibility for the evolution of a BMO depleted in heat producing elements.

Silicate liquid immiscibility in the ~ 2.5 Ga Fe-rich andesite at the top of the Dongargarh large igneous province (India)

Microstructures indicative of silicate liquid immiscibility were identified in the groundmass of plagioclase–pyroxene-phyric andesites from the Mangikhuta Volcanics at the top of the ~2.5Ga Dongargarh large igneous province (LIP), central India. The bulk rock is an Fe-rich, Al-poor andesite (SiO2 57–58wt.%, FeO≤9.86wt.%, Al2O3 ~13wt.%) compositionally similar to icelandite. The Mangikhuta andesites are residual liquids after extensive fractional crystallization along an iron depletion and silica enrichment trend of the magma that produced underlying iron-rich tholeiitic basalts. Evidence for melt unmixing includes devitrified iron-rich glassy globules (10–20μm) and numerous iron-rich, crystalline relict melt structures (up to 50μm) with ferroan augite composition and occasionally with attached tiny magnetite crystals. Further evidence includes Si-rich ocelli and Fe-rich melt structures with Si-rich cores. The formation of immiscible globules and melt structures took place after 75% of the bulk melt had crystallized. This is interpreted to mark the onset of liquid immiscibility.Major element concentrations of the conjugate melts (Fe-rich and Si-rich) closely match compositions measured in natural samples and in experiments. However, it appears that bulk melt compositions influence major element partitioning (Di) between co-existing Fe-rich (LFe) and Si-rich (LSi) melts (Di=CiLFe/CiLSi). DFe, DMg and DCa increase, and DAl decreases in the conjugate melts when the liquid evolves along iron depletion and silica enrichment trends. In contrast, DFe, DMg and DCa decrease, and DAl increases when the bulk liquid evolves along iron enrichment and silica depletion trends.The melt unmixing in the Dongargarh andesite has begun at ~1000°C and an oxygen fugacity between quartz-fayalite–magnetite (QFM) and magnetite–wustite (MW). Late-stage evolution of the Dongargarh large igneous province (LIP) has led the conditions that give rise to liquid immiscibility.

Geochemical Partitioning of Major Elements in Brine Impacted Coal Fly Ash Residues

Fly ash-brine co-disposal technique has been considered as a way of disposing fly ash and brine (hyper-saline water) by some power stations in South Africa. This practice was aimed at using the fly ash to capture most of the elements in brine. However, the geochemical partitioning of the major elements in the waste materials after the fly ashbrine interaction has not been fully understood. This study focuses on understanding the geochemical partitioning of the major elements captured in the fly ash solid residues after the fly ash-brine interaction experiment. XRF and sequential extraction procedure were respectively applied to determine the chemical composition and partitioning of the major elements in fresh fly ash and the solid residues recovered after fly ash-brine interaction. The comparison of the results of the XRF analysis carried out on the fresh fly ash and the solid residues showed that the major elements such as Si, Ca, Mg and Na increased in the solid residues after the fly ash-brine interaction. This indicates that Ca, Mg and Na in the brine solution were captured by the fly ash during the interaction. However, the sequential extraction results showed that significant concentrations of Ca, Na and Mg were released into the water soluble, exchangeable and carbonate fractions. The results show that significant amounts of the elements captured in the fly ash solid residues during fly ash-brine interaction exist in the form which can be easily leached out when in contact with aqueous solution.

The importance of melt TiO2 in affecting major and trace element partitioning between Fe–Ti oxides and lunar picritic glass melts

Lunar mare basalts and picritic glasses have TiO2 abundances ranging from less than 1wt.% to over 16%. Any high-Ti mare basalt or picritic glass petrogenetic model must include Fe–Ti oxides in the mantle source or invoke Fe–Ti oxide assimilation during magma ascent to the lunar surface. We conducted partitioning experiments to investigate high field strength element (HFSE), rare earth element (REE), and transition metal distribution between Fe–Ti oxides and lunar picritic glass melts over a range of melt compositions. Our results suggest that ilmenite–melt and armalcolite–melt HFSE, Cr, and V partition coefficients (DHFSE, DCr, DV) are strongly dependent on melt TiO2 content, whereas ilmenite–melt REE partition coefficients appear to be insensitive to melt composition. As TiO2 increases in picritic glass melts, HFSE, Cr, and V activities in melt also increase and Fe–Ti oxide–melt DHFSE, DCr and DV decrease. The effect of Ti on partitioning behavior can be attributed to the formation of Fe–O–Ti melt species in high-Ti melts. Ilmenite DHFSE range from compatible in oxides in equilibrium with low-Ti melts to incompatible in oxides in equilibrium with depolymerized high-Ti picritic glass melts. DHFSE are inversely correlated with TiO2 abundance in the melt and become nearly constant for melts with more than 6.8% TiO2. We present simple partitioning models that utilize the solubility of ilmenite and armalcolite in melt to effectively predict HFSE partition coefficients across a wide range of picritic glass melt compositions. The HFSE budget of ilmenite cumulates that crystallize from the lunar magma ocean strongly depends on the composition of the magma ocean. Low-Ti and high-Ti lunar basalts can be produced by an ilmenite or armalcolite bearing hybridized mantle source, or by assimilation of late-stage magma ocean cumulates. The dependence of DHFSE and DCr on melt TiO2 is consistent with the formation of lunar Type 1 armalcolite from high TiO2 picritic glass melt, however, lunar Type 2 and 3 armalcolite have a more complicated (yet undetermined) history of formation.

Eclogitic Metasediments from Central Qiangtang, Northern Tibet: Evidence for Continental Subduction during the Eastern and Western Qiangtang Collision

Abstract Eclogitic metasediments from the central Qiangtang metamorphic terrane provide new insights into the continental subduction during the eastern and western Qiangtang collision. Petrologic observations show that the metasediments correspond to meta-sandstones of a continental margin. It is characterized by the garnet + omphacite + rutile + phengite + quartz assemblage, and the peak metamorphic temperature and pressure were estimated to be 535 ± 40°C and ∼27 kbar, respectively, by major element partitioning thermobarometry. Subsequent retrogression occurred at ∼472 ± 30°C and ∼6-9 kbar. The occurrence of eclogitized central Qiangtang terrane indicates the subduction of the western Qiangtang passive continental margin beneath eastern Qiangtang when eastern and western Qiangtang collided along the Shuanghu suture.

Pressure effect on element partitioning between minerals and silicate melt: Melting experiments on basalt up to 20 GPa

High pressure melting experiments of alkali basalt were performed from 3 to 20GPa, and the partitioning of major and trace elements between silicate melt and coexisting minerals was investigated using EPMA and LA-ICP-MS. Garnet existed at around the liquidus at all pressures, while clinopyroxene (cpx) was observed between 3 and 12GPa. K-hollandite was identified at 20GPa. The calculated partition coefficients (D∗-values) for garnet/melt and cpx/melt showed a similar pressure dependence: DNa∗ and DK∗ increase with pressure, while D∗-values of rare earth elements decreased. Apparent differences were found only in the behavior of DSr∗, which increased with pressure for garnet/melt but remained almost constant for cpx/melt. Partition coefficient profiles on PC-IR diagrams display a remarkable increase in peak position with increasing pressure for monovalent ions. However, the peak positions of divalent ions remained nearly constant and a slight decrease was found for the partition curve of trivalent ions. Under high pressure, both the crystal structure and ions are compressed. The observed pressure dependences in the peak positions of partition coefficient curves can be qualitatively explained by the difference in compressibility of monovalent and trivalent cations: the former being more compressible than the latter.

A Geochemical Analytical Scheme for the Appraisal of Partitioning and Mobility of Major elements in Weathered Dry Disposed Coal Fly Ash

South Africa is endowed with significant deposits of coal which is utilized in electricity generation to meet the nation’s energy demand. A large volume of waste solid residue from the combustion of pulverized feed coal in power stations is dry disposed in stock piles or dumps. Chemical interactions of dry disposed fly ash with ingressed CO 2 from the atmosphere and infiltrating rain water would cause dissolution of the soluble components in the fly ash matrix. Chemical partitioning and mobility of major elements in samples from cores drilled into serially stacked weathered dry disposed fly ash were investigated using a modified five steps sequential extraction scheme. A total acid digestion was carried out on the original ash core samples prior to extraction to validate the extraction procedure. The geochemical distribution of the investigated major elements in 59 drilled core samples was determined by x-ray fluorescence and inductively coupled plasma mass spectrometry. The relationship between SiO 2 and chemical index of alteration (CIA) showed 8 year and 20-year-old core samples have a moderate to high degree of weathering. Conversely, 1-year-old cores samples showed characteristics between low and moderate-high degrees of weathering. A cluster and discriminant analysis of the major elements was also able to reveal the subtle chemical alteration differences of the core samples. Functional analysis revealed the disparities in the dissolution patterns of major soluble components in the matrix of the drilled core samples. Modified sequential extractions revealed high concentration of the major species in the leachates for every mineralogical fraction; although the bulk of the major elements are locked up in the insoluble phase of the core samples (i.e. residual fraction) which would not be released under normal environmental conditions. It is noteworthy that the concentration of major elements in the labile fractions (water soluble + exchangeable + carbonate) was high and this has implications for the long-term durability of residual mineral phases. Relative enrichment and depletion trends of major elements are promoted by heterogeneity in the ash dump (i.e. moisture content), gradual reduction of pore water pH and continuous brine and water irrigation. Key words: Coal fly ash; Weathering; Sequential extraction scheme; Cluster analysis; Factor Analysis

Plagioclase–melt (dis)equilibrium due to cooling dynamics: Implications for thermometry, barometry and hygrometry

The compositional variation of plagioclase and the partitioning of major elements between plagioclase and melt have been experimentally measured as a function of the cooling rate. Crystals were grown from a basaltic melt at a pressure of 500MPa under (i) variable cooling rates of 0.5, 2.1, 3, 9.4, and 15°C/min from 1250°C down to 1000°C, (ii) quenching temperatures of 1025, 1050, 1075, 1090, and 1100°C at the fixed cooling rate of 0.5°C/min, and (iii) isothermal temperatures of 1000, 1025, 1050, 1075, 1090, and 1100°C. Our results show that euhedral, faceted plagioclases form during isothermal and slower cooling experiments exhibiting idiomorphic tabular shapes. In contrast, dendritic shapes are observed from faster cooled charges. As the cooling rate is increased, concentrations of Al+Ca+Fe+Mg increase and Si+Na+K decrease in plagioclase favoring higher An and lower Ab+Or contents. Significant variations of pl–liqKd are also observed by the comparison between isothermal and cooled charges; notably, pl–liqKdAb–An, pl–liqKdCa–Na and pl–liqKdFe–Mg progressively change with increasing cooling rate. Therefore, crystal–melt exchange reactions have the potential to reveal the departure from equilibrium for plagioclase-bearing cooling magmas. Finally, thermometers, barometers, and hygrometers derived through the plagioclase–liquid equilibria have been tested at these non-equilibrium experimental conditions. Since such models are based on assumption of equilibrium, any form of disequilibrium will yield errors. Results show that errors on estimates of temperature, pressure, and melt-water content increase systematically with increasing cooling rate (i.e. disequilibrium condition) depicting monotonic trends towards drastic overestimates. These trends are perfectly correlated with those of pl–liqKdCa–Na, pl–liqKdAb–An, and pl–liqKdFe–Mg, thus demonstrating their ability to test (dis)equilibrium conditions.

Major and trace element partition and fluxes in the Rhône River

Dissolved and particulate major and trace element concentrations and suspended particulate contents were determined during three years on the lower Rhône River (France) from subsurface water samples collected about twice a month and more regularly during flood events. Dissolved elements can be grouped into two sets showing similar relationship between concentration and water discharge related to dilution process and to local and diffuse inputs of dissolved elements in the Rhône basin. The variation of NO 3/Na and Cl/Na ratios and the absence of correlation between NO 3 and the enrichment in Cl show that Rhône River is not highly affected by fertilizer applications compared to other large French rivers (Seine and Loire rivers). In particulate phase, Ca, Sr, U, Pb, Bi, Cd, Sb and Zn displayed high enrichment factors over shale composition. Lead, Bi, Cd and Zn enrichment could be related to the anthropogenic inputs, while Ca, Sr and U enrichment was explained by lithological composition of the Rhône basin dominated by carbonate rocks. Distribution coefficients of elements (Kd) are relatively stable with however some variations with increasing SPM concentrations. These variations were attributed to local and diffuse inputs and to a higher contribution of carbonate bedrock in the Rhône basin during flood events. Dissolved and particulate major and trace elements, and suspended particulate matter (SPM) fluxes of the Rhône River to the Mediterranean Sea were estimated for the period 2001–2003. SPM and particulate element fluxes were highly dependent on flood events: 69 to 91% of the annual fluxes were exported in less than 12% of the time. Rhône River inputs were the principal source of particulate elements (except for Cd and Zn) in the north-western Mediterranean Sea, while the major part of dissolved elements were carried out by atmospheric inputs except dissolved As, for which Rhône River contributed to more than 50% of the input to the marine system.

Major and trace element partitioning between dissolved and particulate phases in Antarctic surface snow

In order to provide a new insight into the Antarctic snow chemistry, partitioning of major and trace elements between dissolved and particulate (i.e. insoluble particles, >0.45 μm) phases have been investigated in a number of coastal and inland snow samples, along with their total and acid-dissolvable (0.5% nitric acid) concentrations. Alkaline and alkaline-earth elements (Na, K, Ca, Mg, Sr) were mainly present in the dissolved phase, while Fe and Al were predominantly associated with the particulate matter, without any significant difference between inland and coastal samples. On the other hand, partitioning of trace elements depended on the sampling site position, showing a general decrease of the particulate fraction by moving from the coast to the plateau. Cd, Cu, Pb and Zn were for the most part in the dissolved phase, while Cr was mainly associated with the particulate fraction. Co, Mn and V were equally distributed between dissolved and particulate phases in the samples collected from the plateau and preferentially associated with the particulate in the coastal samples. The correlation between the elements and the inter-sample variability of their concentration significantly decreased for the plateau samples compared to the coastal ones, according to a change in the relative contribution of the metal sources and in good agreement with the estimated marine and crustal enrichment factors. In addition, samples from the plateau were characterised by higher enrichment factors of anthropogenic elements (Cd, Cr, Cu, Pb and Zn), compared to the coastal area. Finally, it was observed that the acid-dissolvable metal concentrations were generally lower than the total concentration values, showing that the acid treatment can dissolve only a given fraction of the metal associated with the particulate (<20% for iron and aluminium).