Partition Coefficients For Sr Research Articles

Temporal variability in seawater Sr/Ca ratios within a coral atoll as an indicator of marine calcifier community diversity

Coral reefs are marine environments where calcium carbonate production and dissolution along with photosynthesis and respiration can have a large impact on the daily biogeochemical cycles of seawater dissolved inorganic carbon (DIC), total alkalinity (TA), oxygen (O2), and major nutrients. Using seawater samples from Tetiaroa Atoll, French Polynesia, we apply isotope dilution thermal ionization mass spectrometry (ID-TIMS) to show that these daily cycles of variability extend to the dissolved seawater Sr/Ca ratio (Sr/Casw). Hourly measurements over a 27+ hour period indicate that Sr/Casw covaries with DIC and TA and exhibits up to a 2% relative difference compared to the regional Pacific Ocean source water Sr/Casw value of 8.53 mmol/mol. We hypothesize that the primary mechanism of hourly variability in Sr/Casw is carbonate precipitation/dissolution by corals and other marine calcifiers on the Tetiaroa reef, while sedimentary redox processes may significantly contribute to an apparent decoupling between net calcification rates estimated from TA compared to dissolved calcium [Ca]sw concentrations. Using a simple Rayleigh distillation model and a two-member mixing equation, we estimate the net partition coefficient of Sr (KD-Sr) for each hourly sample as a mixture of coral (aragonite; KD-Sr ∼ 1.06) and crustose coralline algae (high Mg-calcite; KD-Sr ∼ 0.39) contributions to the measured variability of [Ca]sw and Sr/Casw. As the high Mg-calcites of crustose coralline algae are (1) integral components to the structure and stability of the reef framework and (2) more soluble than aragonite under more acidic conditions, we propose that continued analyses of Sr/Casw within coral reef environments could be a useful tool for monitoring the ecological distribution and contributions of marine calcifiers on reefs as ocean warming and acidification intensify in response to anthropogenic climate change. Additionally, seasonal, interannual, and/or centennial-scale offsets between the Sr/Casw of open ocean source seawater and ambient reef seawater may have significant implications for interpreting past changes in sea surface temperatures derived from the coral Sr/Ca paleothermometer.

Magmatic evolution of the host magma of plutonic rocks in the Procellarum KREEP Terrane

The origin of the KREEP (K, Rare Earth Element and P)-rich component of the Mg-suite rocks of the Procellarum KREEP Terrane (PKT), one of three major lunar crustal terranes, is unclear. In an attempt to determine its origin, we estimated the composition of host magmas of plutonic rocks, including Mg-suite rocks and evolved rocks, from the PKT (PKT-host magmas) by using secondary ion mass spectrometry analyses of plagioclase. Calculated partition coefficients for Sr, Ba and Ti between plagioclase and melts, taking into account the anorthite content of plagioclase, temperature and bulk rock major-element compositions, were applied to determine parental magma compositions. The PKT-host magmas contained 160–360 ppm Sr, 520–7600 ppm Ba and 1.1–7.0 wt.% TiO2; most of them had higher Ti and Ba concentrations than KREEP basalts and high-K KREEP. We used phase relations based on the Rhyolite-MELTS algorithm to explore the evolution of the PKT-host magmas and KREEP basalts from two bulk silicate moon (BSM) starting compositions, a BSM with chondritic ratios of refractory elements, and a crustal-component-enriched BSM with non-chondritic ratios of refractory elements. We propose a three-stage evolution model. Stage-1: polybaric multi-step fractionation from a BSM magma to form ferroan anorthosite (FAN) crust and an evolved magma as the first KREEP-rich component (M0). Stage-2: assimilation and fractional crystallization (AFC) of M0, early cumulate and FAN associated with deep mantle overturn and impact events to form the PKT-host magmas, Mg-suite rocks and an evolved magma as the second KREEP-rich component (K0). Stage-3: further AFC cycles of K0, Mg-suite rocks or FAN associated with shallow mantle overturn and impact events to form KREEP basalts. This three-stage model for a crustal-component-enriched BSM with non-chondritic ratios of refractory elements (e.g., a sub-chondritic Ti/Ba ratio) reproduced the compositions of both the host magmas of FAN and the PKT-host magmas that fractionated to form the Mg-suite rocks and KREEP basalts of the PKT region. In particular, the model reproduced the high Ti and Ba contents of the PKT-host magmas we estimated from plagioclase composition. Variations of TiO2 and Ba contents (and hence Ti/Ba ratios) of the magmas were critical controls on their evolution.

Water strontium concentrations influence strontium concentrations in the pre-hatch otolith region of capelin (Mallotus villosus)

Unique otolith chemistry signatures have been useful in reconstructing environmental histories of teleost fish, including their use as a natural tag of natal habitat. The ability to use otolith chemistry as a chronological measure of life history, however, assumes that the concentrations of trace elements in the ambient environment are taken up and calcified on the otolith in proportion to their environmental availability. Capelin (Mallotus villosus) is a key forage fish species on the Newfoundland Shelf that migrates from offshore to coastal regions during the summer to spawn. Capelin eggs adhere to the sediment at spawning sites, where they remain throughout incubation. As capelin larvae disperse from spawning/rearing sites immediately upon hatch, the pre-hatch region of capelin otoliths, formed during incubation, indicates natal origin. Chemistry in the otolith pre-hatch region of capelin displays high variability, even in environmentally stable lab-rearing experiments and among individuals within the same family, potentially confounding the ability to characterize otolith chemical signatures from different regions. Here, we aimed to determine whether trace element concentrations in the pre-hatch region of capelin otoliths were influenced when naturally fertilized capelin eggs were reared to hatch under varying concentrations of Barium (Ba; 1×, 6×, 7×, 16×, 27×), Strontium (Sr; 1×, 2×, 4×, 5×, 9×), and Ba+Sr (1×, 5×/1.5×, 5×/3×, 10.5×/3.5×, 19×/5× ambient concentration). Naturally fertilized capelin eggs were collected at a beach spawning site in July 2019 and incubated in control or enriched water treatments until hatch. Sagittal otoliths were removed from larvae from each treatment (n = 20) and analysed for their trace element concentrations (e.g., Mg, Mn, Sr, Ba) using Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA ICP-MS). Otolith Sr concentrations from 1 to 2 day old larvae increased significantly with increasing ambient Sr concentrations, and the Sr partition coefficient (0.37 ± 0.01) appeared to be concentration-dependent, decreasing non-linearly with increasing ambient water Sr concentrations. In contrast, otolith Ba concentrations remained consistently high despite environmental enrichment. We estimated high Ba partition coefficients (3.67 ± 0.52), which were calculated using the limits of detection for Ba (i.e., 0.0025 mg/L), due to low ambient Ba concentrations. These high estimated Ba partition coefficients suggested that the pre-hatch otolith Ba concentrations may be primarily maternally derived. Overall, findings suggested that regions varying in ambient water Sr concentrations during capelin egg incubation could result in distinct chemical signatures in the pre-hatch region of capelin otoliths, but variability in water Ba concentrations will not. Future studies could use this variation to investigate population connectivity and productivity, which is currently unknown for capelin.

In situ geochemical composition of apatite in granitoids from the eastern Central Asian Orogenic Belt: A window into petrogenesis

The compositional record of accessory minerals can provide insights into the character and petrogenesis of the parental magma. We demonstrate apatite’s applicability to petrogenetic studies through in situ trace element analysis of apatite in 60 granitoid plutons from the eastern Central Asian Orogenic Belt (CAOB). Rare earth element (REE) patterns of apatites usually mimic the trend of the host rock but are slightly enriched in MREE (middle REE), which is predominantly controlled by parental melt composition and REE partitioning. However, the REE patterns of apatites that are decoupled from those of host rocks, such as the depletions of LREE (light REE) and MREE, as well as the HREE (heavy REE) enrichment, result from the early crystallization of other REE-bearing minerals and late-stage metamorphism. Our data displays no significant relationship between apatite trace elements and highly fractionated granitoids in the eastern CAOB, highlighting the limitation of assessing the degree of magma differentiation through apatite geochemistry. More significantly, the variation of Sr in apatite shows a close correlation with the early crystallization of plagioclase, and the linear correlations of SrPl-SrWR, SrPl-SrAp, and SrAp-SrWR imply constant Sr partition coefficients of plagioclase and apatite. Furthermore, a partition coefficient of Sr between apatite and bulk rock is proposed (DApatite/bulkrockSr=0.69). Our results also imply that apatite crystallizing from adakites inherits the typical trace element signatures of the adakitic melt and is characterized by high Sr, low HREY (Gd ∼ Lu + Y) contents and lack of Eu anomaly. In conclusion, apatite provides a vital bond to understand the petrogenesis of granitoids and shows the potential to track granitic sources in detrital grains.

Temperature Induced Physiological Reaction Norms of the Coccolithophore Gephyrocapsa oceanica and Resulting Coccolith Sr/Ca and Mg/Ca Ratios

Coccolithophores are one of the major contributors to the pelagic production of calcium carbonate and their fossilized remains are a key component of the biogeochemical cycles of calcium (Ca), magnesium (Mg), and other divalent cations present in the intracellular precipitated calcitic structures (coccoliths). The geochemical signature of coccoliths (e.g., Sr/Ca and Mg/Ca ratios) is used as paleoproxy to reconstruct past environmental conditions and to understand the underlying physiological precipitation kinetics. Here, we present the elemental fractionation of Sr and Mg in calcite of the coccolithophore Gephyrocapsa oceanica from controlled laboratory experiments applying an extended temperature gradient (12 to 27°C). The physiological reaction norm of G. oceanica, in terms of growth rate, exhibited optimum behavior while the partition coefficient of Sr (DSr) was linearly correlated with temperature and DMg indicated no specific trend. Our results indicate: (1) a presumably secondary physiological control of DSr, and (2) the importance of calibrating coccolithophore-based proxies using experiments that include the full physiological reaction norms (i.e., a possible non-linear response) to environmental drivers (e.g., temperature, salinity, and pH, etc.). The presented results contribute to an improved understanding of the underlying physiological kinetics involved in regulating coccolith elemental fractionation and give additional implications for designing future laboratory experiments to calibrate and apply coccolithophore based paleoproxies on the fossil sediment record.

Temperature and salinity effects on strontium and barium incorporation into otoliths of flounder Paralichthys olivaceus at early life stages

Using otolith microchemistry as a natural proxy to retrospectively trace the environmental histories of fish is based on the principle that the relationships between otolith elemental incorporation and environmental factors are well understood. In the present study, nine treatments using temperature (16, 19 and 22 ℃) and salinity (22, 26 and 31) as categorical variables in an orthogonal design were established to investigate the relative and interactive effects of temperature and salinity on strontium (88Sr) and barium (138Ba) incorporation into otoliths of flounder Paralichthys olivaceus from larval to juvenile stages (15–93 days post hatching). The results indicated that temperature facilitated the Sr and Ba incorporation into otoliths, while salinity promoted the Sr incorporation but inhibited the Ba incorporation into otoliths of flounder at early life stages. The partition coefficient of Sr was positively related to salinity and temperature, while an opposite salinity effect was found on the partition coefficient of Ba. The partition coefficient of Sr or Ba was not significantly correlated with the somatic growth or otolith precipitation that was promoted by temperature. These findings suggested that the efficiency of elemental incorporation into otoliths from ambient water showed unique responses to temperature or salinity changes, depending on elements and environmental effects. The results in our study did not consistently agree with all previous studies, in which temperature or salinity effects on otolith incorporation of both elements were commonly diverse and species-specific. Nonetheless, this study indicated that Sr:Ca and Ba:Ca ratios in otoliths could provide knowledge for understanding the environmental history of the flounder at early life stages.

Experimental determinations of trace element partitioning between plagioclase, pigeonite, olivine, and lunar basaltic melts and an fO2 dependent model for plagioclase-melt Eu partitioning

We report partition coefficients for Sr, Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Y, and Th for plagioclase, pigeonite, and olivine experimentally grown from a lunar basaltic liquid at 1 bar and two different oxygen fugacities (fO2; fayalite-magnetite-quartz (FMQ) and the iron-wüstite (IW) oxygen buffers) at temperatures (Ts) of 1100–1125 °C. These results expand the partitioning database for plagioclase-silicate melt partitioning at lunar relevant conditions and compositions. Variations of partition coefficients for rare earth elements (REEs) are highly systematic with respect to ionic radii, in agreement with lattice strain theory. We use the lattice strain model to evaluate partition coefficients for highly incompatible elements such as the light (or in the case of plagioclase, heavy) REEs, for which direct determinations are experimentally and analytically difficult. Our experiments suggest that within analytical and experimental uncertainty, over the range of explored fO2 (FMQ to IW), T (1100–1125 °C) and An# conditions (An# >94.5, where An#=Ca/(Ca + Na) × 100, in moles), fO2 exerts an important but secondary control on Eu partitioning in anorthitic plagioclase. In contrast, the global plagioclase-melt partitioning dataset highlights the first-order role of fO2 in determining plagioclase-melt Eu partition coefficients over a wide range of An#s. Using experimental results and predictive models for divalent and trivalent element plagioclase-melt partitioning in the plagioclase ring site, we parameterized an fO2-dependent plagioclase-melt Eu partitioning model. Within a factor of two, the model reproduces measured Eu partition coefficients for a wide range of experimental conditions (750–1400 °C) and compositional systems (basalts to dacites) over > 15 orders of magnitude variation in fO2. When equilibrium partitioning of Eu between plagioclase and melt can be established, the model may be applied as a plagioclase-melt oxybarometer. Preliminary application to plagioclase-hosted melt inclusions in mid-ocean ridge basalts suggests they equilibrated near the FMQ buffer. We used the new Eu partitioning model to evaluate the petrogenesis of a lunar anorthositic impactite that hosts plagioclase with both positive and negative Eu anomalies. Our investigation suggests the distribution of REEs and Eu in this sample is consistent with subsolidus reequilibration after impact-induced heating.

Calcite Mg and Sr partition coefficients in cave environments: implications for interpreting prior calcite precipitation in speleothems

Calcite Mg and Sr partition coefficients in cave environments: implications for interpreting prior calcite precipitation in speleothems

Calcite Mg and Sr partition coefficients in cave environments: Implications for interpreting prior calcite precipitation in speleothems

Trace element to Ca ratios in speleothems have emerged as important proxies that reflect local environmental conditions. However, interpretations of speleothem trace element records can be challenging due to various processes. Positive correlations between speleothem Mg/Ca and Sr/Ca have often been interpreted to reflect prior calcite precipitation (PCP), a process potentially modulated by rainfall variability. For quantitative interpretation of PCP, the distribution coefficients for Mg and Sr (DMg and DSr) are required. Here, we use ten cave monitoring calcite and drip water datasets to investigate the influence of temperature and drip water and calcite Mg/Ca and Sr/Ca ratios on speleothem calcite DMg and DSr. The datasets cover a large range of climatic and geological settings resulting in a large range of drip water Mg/Ca ratios. Speleothem calcite DSr shows a positive correlation with the calcite Mg/Ca ratio. Furthermore, DMg shows a clear temperature dependence (DMg = 0.013*e0.035*T).Previous work proposed that the slope of a trend line through a plot of ln(Sr/Ca) versus ln(Mg/Ca) of a speleothem trace element dataset is between 0.709 and 1.003 if dominated by PCP. However, this only holds true if the initial drip water Mg/Ca and Sr/Ca ratios as well as DSr and DMg are constant for the whole dataset. We use an excel-based PCP model (see Electronic Annex) to assess the potential influence of PCP on drip water and speleothem Mg/Ca and Sr/Ca ratios and simulate different initial drip water Ca, Mg, and Sr concentrations corresponding to limestone, dolostone, and mixed host rock compositions. In the case of enhanced PCP and high Mg/Ca ratios, calcite DSr increases progressively with the mean Mg/Ca ratio of the speleothem time series resulting in steeper slopes of ln(Sr/Ca) versus ln(Mg/Ca) of up to 1.45.We show that PCP can induce slopes ranging from 0.709 (or even shallower) up to 1.45. This large range suggests that the previously applied criteria to detect PCP in speleothem records were too strict and may lead to unjustified exclusion of PCP as a potential interpretation of speleothem and drip water trace element ratios. Thus, the number of speleothem Mg/Ca and Sr/Ca datasets that potentially reflect past changes in effective rainfall may be larger than previously suggested.

On the effect of aqueous strontium on magnesite growth

Strontium was found to be a growth-promoting additive during the precipitation of calcite from aqueous solution. This raises the question of whether growth of the isostructural magnesite is promoted by aqueous strontium ions as well. For this, the influence of Sr2+ on magnesite growth was investigated by macroscopic mixed-flow reactor experiments and hydrothermal atomic force microscopy at 100 °C. It was found that neither growth rates nor molecular step morphologies are affected by the presence of strontium.The partition coefficient of strontium between magnesite and aqueous solution KdSrmgs=XSrXMg/mSrmMg was determined to be <0.21 ± 0.09. In solutions with Sr concentrations > 50 μM and Mg:Sr ratios ≤ 60, strontianite precipitation limited Sr incorporation into growing magnesite to ~0.5 mol%. Similar amounts of Sr incorporation into calcite were reported in literature, although the partition coefficient of Sr between calcite and solution is lower than between magnesite and solution. The slow growth rates of magnesite, however, imply the use of high solution supersaturations. These conditions inevitably promote strontianite precipitation depleting the solution of Sr. In the case of calcite, fast growth rates can be achieved at low supersaturations. Such solutions are permissive of high Sr concentrations without strontianite precipitation.The magnesite growth rates measured here complete the literature data, now spanning a range of supersaturation states of more than one order of magnitude (Ω ≈ 20–240). The completed data allow for a refinement of the kinetic constants of magnesite precipitation. The fit of all rates to the function R = k (Ω − 1)n yielded the reaction order n = 1.76 ± 0.03 and the growth rate constant k = 0.015 ± 0.002 nmol m−2 s−1.

Adsorption and diffusion of strontium in simulated rock fractures quantified via ion beam analysis

AbstractAn experimental technique has been developed and applied to the problem of determining effective diffusion coefficients and partition coefficients of Sr in low permeability geological materials. This technique, the micro-reactor simulated channel method (MRSC), allows rapid determination of contaminant transport parameters with resulting values comparable to those determined by more traditional methods and also creates product surfaces that are amenable for direct chemical analysis. An attempt to further constrain mass flux was completed by detailed ion beam analysis of polished tuff surfaces (tuff is a polycrystalline polyminerallic aggregate dominated by silicate phases) that had been reacted with Sr solutions at concentrations of 10−5, 10−3 and 10−1 mol l−1. Ion beam analysis was carried out using beams of both protons (using particle induced X-ray emission and elastic backscattering spectrometry or EBS) and alpha-particles (using Rutherford backscattering spectrometry). The ion beam analyses showed that increased solution concentrations resulted in increased surface concentrations and that in the highest concentration experiment, Sr penetrated to at least 4 μm below the primary interface. The Sr surface concentrations determined by EBS were 0.06 (±0.05), 0.87 (±0.30) and 2.40 (±1.0) atomic weight % in the experiments with starting solution concentrations of 10−5, 10−3, and 10−1 mol l−1, respectively.

Reactions of strontium anorthite with H2O+CaCl2 fluids at 500 C and high pressure: Kinetic information from in situ synchrotron-radiation XRF analyses of the fluid

The assemblage strontium anorthite, quartz, and kyanite was reacted with H2O+CaCl2 solutions at 500 °C and pressures between 460 and ~1300 MPa using a hydrothermal diamond-anvil cell. Information on the kinetics was obtained in situ based on time-resolved synchrotron-radiation X-ray fluorescence analyses of the Sr concentration in the fluid. The reaction products (anorthite or zoisite) were studied using transmission electron microscopy to obtain information on the reaction mechanism and mineral-fluid partitioning of strontium. The time required for equilibration was primarily controlled by the reaction mechanism, but not discernibly affected by pressure or chloride concentration. Nucleation and growth of zoisite at the expense of strontium anorthite was much faster than the Sr-Ca exchange reaction of strontium anorthite to anorthite, and resulted in chemically homogeneous crystals. The anorthite had developed a high nanoporosity during the reaction, which is indicative of coupled dissolution-precipitation. A zoisite-fluid exchange coefficient K D ( Sr - Ca ) zoisite - fluid = X Sr zoisite X Ca zoisite / X Sr fluid X Ca fluid = 0.42 was obtained for the Sr-Ca fractionation at 500 °C and ~1300 MPa. At low bulk Sr/Ca, this value is in very good agreement with literature data, which are based on zoisite syntheses from oxide and hydroxide mixtures in chloridic fluids at 600 °C, 2 GPa and analyses after quench. This suggests that the Ca-Sr ratios in fluid and zoisite were not affected by back reactions during quenching. The constrained anorthite-fluid Sr partition coefficient for 500 °C, 460 MPa is, likewise, consistent with literature data, but determination of mineral-fluid partition and exchange coefficients can be hampered by quench phases in nanopores if coupled dissolution-precipitation acted as reaction mechanism.

Mg/Ca and Sr/Ca of ostracod valves from living species of Lake Geneva

The Mg/Ca and Sr/Ca ratios of living ostracods belonging to 15 different species and sampled monthly over a one year-cycle at five sites (2, 5, 13, 33, and 70m water depths) in western Lake Geneva (Switzerland) are compared to the oxygen and carbon isotope compositions measured on the same samples as well as to the temperature and chemical composition of the water (δ18OH2O, δ13CDIC, Mg/CaH2O, and Sr/CaH2O) at the time of ostracod calcification. The results indicate that trace element incorporation varied at the species level, mainly because of the ecological and biological differences between the different species (life-cycle, (micro-)habitat preference, biomineralisation processes) and the control thereof on trace element incorporation of the ostracods. In littoral zones, the Mg/Ca and Sr/Ca of ostracod valves increase as temperature and Mg/Ca and Sr/Ca of water increase during spring and summer, hence reflecting mainly seasonal variations. However, given that for Lake Geneva the Mg/Ca and Sr/Ca of water also vary with temperature, it is not possible to distinguish the effects of temperature from those of changes in chemical composition of water on the trace element content in ostracod valves. Results support that both water temperature and water Mg/Ca and Sr/Ca ratios control the final trace element content of Cyprididae valves. In contrast, the trace element content of species living in deeper zones of the basin is influenced by variations in the chemical composition of the pore water for the infaunal species. Trace element content measured for these specimens cannot, therefore, be used to reconstruct the compositions of the water lake bottom. In addition, incorporation of Mg and Sr into the shell differs from one family, sub-family, or even species to the other. This suggests that the distinctive Mg and Sr partition coefficients for the analysed taxa result from different valve calcification strategies that may be phylogenetic.

An experimental study of Sr, Ba and Rb partitioning between alkali feldspar and silicate liquid in the system nepheline–kalsilite–quartz at 0.1 GPa P(H2O): a revisitation and reassessment

AbstractThe partitioning of Sr, Ba and Rb between alkali feldspar and melt has been determined at 0.1 GPa water vapour pressure, mainly on one Na-rich series and one K-rich series within the system nepheline-kalsilite-quartz. Experiments were also carried out with small amounts of the anorthite molecule or peralkaline components (Na, K metasilicates). The compositions of the alkali feldspars and coexisting quenched glasses were determined by electron microprobe analysis. Except for some peralkaline compositions, the crystal/liquid partition coefficients for Sr and Ba are always &gt;1; the crystal/liquid partition coefficient for Rb is always &lt;1. For sodic alkali feldspars DSr &gt; DBa and for potassic feldspars DSr &lt; DBa. Partition coefficients for Sr and Ba increase: (1) with decreasing temperature; (2) with increasing Or content of feldspar; (3) with increasing silica-undersaturation of the melt; (4) with decreasing peralkalinity. The variation in the value of DRb is less clear, but it is higher for K-rich feldspars. Multiple linear regression equations are fitted to correlate ln(D) with independent compositional and physical variables. Where rock/groundmass major-element data are available for felsic natural rocks, equations are recommended for obtaining reliable alkali feldspar partition coefficients for modelling fractional crystallization processes. The structural properties of silicate melts and crystal chemical relations are used to rationalize trends in partition coefficients.

A micro-analytical approach to partition coeffi cients in plagioclase and clinopyroxene of basaltic sills in Serra Geral Formation, Paraná Basin, Brazil

In the present study, detailed micro-analytical data (major and trace elements) in minerals from sills of Serra Geral Formation represents the fi rst attempt to evaluate the partition coeffi cient of Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Cs, Ba, REE, Hf, Ta, Pb, Th and U among coexisting plagioclase, augite and pigeonite from 12 tholeiitic basaltic sills along the eastern border of the Parana Basin, Brazil. These 12 samples come from Rio Grande do Sul, Parana, Sao Paulo and Goias states. Plagioclase/melt partition coeffi cients K D were determined in core, intermediate and rim zones using LA-ICP-MS and the results were compared with the variations of the elements Ca, Al, Na and Si in those areas. Partition coeffi cients of Sr and Eu decrease with the increase of Ca concentration in plagioclase. A reduction in temperature and consequently a rise in Na and Si levels increase the partition coeffi cients of Sr and Eu at the rim of mineral The K D of Ni and V in clinopyroxene has positive correlations with Mg 2+ , Ca 2+ and Al 3+ due to higher concentrations of those elements at the core and lower concentration towards the rim and negative correlations of K D of Sc, Cr, Co, Pb and Lu, due to preferential substitutions by Fe 2+ in sixfold coordination. The use of LA-ICP-MS and EPMA made possible the chemical quantifi cation of major and trace elements and the determination of partition coeffi cients in coexisting plagioclase and clinopyroxene in basaltic melts.

Response of the coccolithophores Emiliania huxleyi and Coccolithus braarudii to changing seawater Mg 2+ and Ca 2+ concentrations: Mg/Ca, Sr/Ca ratios and δ 44/40Ca, δ 26/24Mg of coccolith calcite

Calcium and magnesium concentrations in seawater have varied over geological time scales. On short time scales, variations in the major ion composition of seawater influences coccolithophorid physiology and the chemistry of biogenically produced coccoliths. Validation of those changes via controlled laboratory experiments is a crucial step in applying coccolithophorid based paleoproxies for the reconstruction of past environmental conditions. Therefore, we examined the response of two species of coccolithophores, Emiliania huxleyi and Coccolithus braarudii, to changes in the seawater Mg/Ca ratio (≈0.5 to 10 mol/mol) by either manipulating the magnesium or calcium concentration under controlled laboratory conditions. Concurrently, seawater Sr/Ca ratios were also modified (≈2 to 40 mmol/mol), while keeping salinity constant at 35. The physiological response was monitored by measurements of the cell growth rate as well as the production rates of particulate inorganic and organic carbon, and chlorophyll a. Additionally, coccolithophorid calcite was analyzed for its elemental composition (Sr/Ca and Mg/Ca) as well as isotope fractionation of calcium and magnesium (Δ 44/40Ca and Δ 26/24Mg). Our results reveal that physiological rates were substantially influenced by changes in seawater calcium rather than magnesium concentration within the range estimated to have occurred over the past 250 million years when coccolithophores appear in the fossil record. All physiological rates of E. huxleyi decreased at a calcium concentration above 25 mmol L −1, whereas C. braarudii displayed a higher tolerance to increased seawater calcium concentrations. Partition coefficient of Sr was calculated as 0.36 ± 0.04 (±2 σ) independent of species. Partition coefficient of Mg 2+ increased with increasing seawater Ca 2+ concentrations in both coccolithophore species. Calcium isotope fractionation was constant at 1.1 ± 0.1‰ (±2 σ) and not altered by changes in seawater Mg/Ca ratio. There is a well-defined inverse linear relationship between calcium isotope fractionation and partition coefficient of Sr 2+ in all experiments, suggesting similar controls on both proxies in the investigated species. Magnesium isotope ratios were relatively stable for seawater Mg/Ca ratios ranging from 1 to 5, with a higher degree of fractionation in Emiliania huxleyi (by ≈0.2‰ in Δ 26/24Mg). Although Mg/Ca ratios in the calcite of coccolithophores and foraminifera are similar, the former have considerably higher Δ 26/24Mg (by >+3‰), presumably due to differences in calcification mechanisms between the two taxa. These observations suggest, a physiological control over magnesium elemental and isotopic fractionation during the process of calcification in coccolithophores.

Experimentally determined biomediated Sr partition coefficient for dolomite: Significance and implication for natural dolomite

Two strains of moderately halophilic bacteria were grown in aerobic culture experiments containing gel medium to determine the Sr partition coefficient between dolomite and the medium from which it precipitates at 15 to 45 °C. The results demonstrate that Sr incorporation in dolomite does occur not by the substitution of Ca, but rather by Mg. They also suggest that Sr partitioning between the culture medium and the minerals is better described by the Nernst equation ( D Sr dol = Sr dol/Sr bmi), instead of the Henderson and Kracek equation ( D Sr dol = (Sr/Ca) dol/(Sr/Ca) solution. The maximum value for D Sr dol occurs at 15 °C in cultures with and without sulfate, while the minimum values occur at 35 °C, where the bacteria exhibit optimal growth. For experiments at 25, 35 and 45 °C, we observed that D Sr dol values are greater in cultures with sulfate than in cultures without sulfate, whereas D Sr dol values are smaller in cultures with sulfate than in cultures without sulfate at 15 °C. Together, our observations suggest that D Sr dol is apparently related to microbial activity, temperature and sulfate concentration, regardless of the convention used to assess the D Sr dol. These results have implications for the interpretation of depositional environments of ancient dolomite. The results of our culture experiments show that higher Sr concentrations in ancient dolomite could reflect microbial mediated primary precipitation. In contrast, previous interpretations concluded that high Sr concentrations in ancient dolomites are an indication of secondary replacement of aragonite, which incorporates high Sr concentrations in its crystal lattice, reflecting a diagenetic process.

Rb and Sr partitioning between haplogranitic melts and aqueous solutions

Rubidium and strontium partitioning experiments between haplogranitic melts and aqueous fluids (water or 1.16–3.56 m (NaCl + KCl) ± HCl) were conducted at 750–950 °C and 0.2–1.4 GPa to investigate the effects of melt and fluid composition, pressure, and temperature. In addition, we studied if the applied technique (rapid and slow quench, and in-situ determination of trace element concentration in the fluid) has a bearing on the obtained data. There is good agreement of the data from different techniques for chloridic solutions, whereas back reactions between fluid and melt upon cooling have a significant effect on results from the experiments with water. The Rb fluid–melt partition coefficient shows no recognizable dependence on melt composition and temperature. For chloridic solutions, it is ∼0.4, independent of pressure. In experiments with water, it is one to two orders of magnitude lower and increases with pressure. The strontium fluid–melt partition coefficient does not depend on temperature. It increases slightly with pressure in Cl free experiments. In chloridic fluids, there is a sharp increase in the Sr partition coefficient with the alumina saturation index (ASI) from 0.003 at an ASI of 0.8 to a maximum of 0.3 at an ASI of 1.05. At higher ASI, it decreases slightly to 0.2 at an ASI of 1.6. It is one to two orders of magnitude higher in chloridic fluids compared to those found in H 2O experiments. The Rb/Sr ratio in non-chloridic solutions in equilibrium with metaluminous melts increases with pressure, whereas the Rb/Sr ratio in chloridic fluids is independent of pressure and decreases with fluid salinity. The obtained fluid–melt partition coefficients are in good agreement with data from natural cogenetic fluid and melt inclusions. Numerical modeling shows that although the Rb/Sr ratio in the residual melt is particularly sensitive to the degree of fractional crystallization, exsolution of a fluid phase, and associated fluid–melt partitioning is not a significant factor controlling Rb and Sr concentrations in the residual melt during crystallization of most granitoids.

Mechanisms of formation of barium-rich phlogopite and strontium-rich apatite during the final stages of alkaline magma evolution

Carbonatite veinlets in fergusite from the Dunkeldyk potassium-rich basaltoid complex (southeastern Pamirs) are composed of clinopyroxene, phlogopite, and apatite phenocrysts embedded in a crystallized calcite-bearing groundmass. The examination of back-scattered electron images revealed areas of significantly different compositions in fluorapatite and fluorphlogopite. The content of BaO in the phlogopite ranges from 0.68 to 10.9 wt %. There are also variations in MgO and F contents. The maximum BaO content corresponds to high mole fractions of the Ba end member kinoshitalite (up to 0.24) in the phlogopite. The zoned fluorapatite phenocrysts are rich in SrO (0.77–25.4 wt %). An increase in SrO content is accompanied by an increase in Ce2O3, La2O3, and BaO and a distinct decrease in CaO. Most of the apatite grains are rimmed by elongated colorless crystals showing the maximum SrO contents. Based on the experimentally determined Ba and Sr partition coefficients between these minerals, silicate and carbonate melts, and fluid, a model was proposed for the enrichment of phases in these trace elements. It was shown that the mineral-forming media of the Ba-rich phlogopites was a residual melt enriched in volatiles (including F) and fluid-mobile elements. During that stage, the decomposition reactions of early Ba-bearing feldspars with subsequent incorporation of BaO in Ba-rich phlogopites played an important role. The mechanism of formation of Sr-rich apatites is fundamentally different: early apatite grains with moderate Sr contents recrystallized under the influence of Sr-rich fluids released during the late magmatic stage. Thus, despite their close association in a single rock, the Ba-bearing phlogopite and Sr-rich apatite were formed by significantly different mechanisms. Our previous investigations of melt and fluid inclusions in minerals from the rocks of the Dunkeldyk complex and the results obtained in this study allowed us to suggest that the barium, fluorite-carbonatite, and rare metal mineralization occurring in the region developed owing to the prolonged evolution of primary magmas, resulting in the formation of melt-solutions (brines) and hydrothermal systems.

Garnet-bearing tonalitic porphyry from East Kunlun, Northeast Tibetan Plateau: implications for adakite and magmas from the MASH Zone

A garnet-bearing tonalitic porphyry from the Achiq Kol area, northeast Tibetan Plateau has been dated by SHRIMP U-Pb zircon techniques and gives a Late Triassic age of 213 ± 3 Ma. The porphyry contains phenocrysts of Ca-rich, Mn-poor garnet (CaO > 5 wt%; MnO 17 wt%) contents, and is metaluminous to slightly peraluminous (ACNK = 0.89–1.05). The rock samples are enriched in LILE and LREE but depleted in Nb and Ti, showing typical features of subduction-related magmas. The relatively high Sr/Y (~38) ratios and low HREE (Yb < 0.8 ppm) contents suggest that garnet is a residual phase, while suppressed crystallization of plagioclase and lack of negative Eu anomalies indicate a high water fugacity in the magma. Nd–Sr isotope compositions of the rock (eNdT = −1.38 to −2.33; 87Sr/86Sri = 0.7065–0.7067) suggest that both mantle- and crust-derived materials were involved in the petrogenesis, which is consistent with the reverse compositional zoning of plagioclase, interpreted to indicate magma mixing. Both garnet phenocrysts and their ilmenite inclusions contain low MgO contents which, in combination with the oxygen isotope composition of garnet separates (+6.23‰), suggests that these minerals formed in a lower crust-derived felsic melt probably in the MASH zone. Although the rock samples are similar to adakitic rocks in many aspects, their moderate Sr contents (<260 ppm) and La/Yb ratios (mostly 16–21) are significantly lower than those of adakitic rocks. Because of high partition coefficients for Sr and LREE, fractionation of apatite at an early stage in the evolution of the magma may have effectively decreased both Sr and LREE in the residual melt. It is suggested that extensive crystallization of apatite as an early phase may prevent some arc magmas from evolving into adakitic rocks even under high water fugacity.