Zonation des éléments en traces au cours de la croissance des cristaux dans les bains silicatés: l'exemple de Rb,Cs, Sr et Ba dans le système Qz-Ab-Or-H2O

An indirect method was used to study Na, K, Rb, Cs, Sr and Ba partition coefficients between crystals and silicate melt. Equilibria between a hydrothermal solution and the melt at 800°C and 2 kb and between a hydrothermal solution and crystals at 750°C and 2 kb were separately achieved.For major element partitioning (Na and K), the results obtained here are in good agreement with those of Tuttle and Bowen (1958) which allow us to follow crystal evolution during a fractional crystallization process where the growth of zoned crystals takes place.For minor elements Rb, Cs, Sr, Ba, melt/aqueous solution partition coefficients depend on Na/K as well as the silica content of the melt. These effects are rather small for Rb and Cs, but are much more important for the alkaline earths. The feldspar/aqueous solution partition coefficients also depend on Na/K.The variations of the partition coefficients feldspar/melt are complex in the part of the Qz-Ab-Or diagram located below the cotectic line.During fractional crystallization following the Rayleigh law (assuming that there are no kinetic phenomena) Sr (D > 10) is almost totally removed from the melt in the early stages whereas Cs (D < 0.1) remains in the melt during the whole process. Rb and Ba have partition coefficients closer to unity. The variation of these coefficients, due to changes in bulk composition of liquid and crystals during fractional crystallization, can lead to complex zoning with possible concentration maxima at some stages. Similar phenomena can be expected in non-ideal natural solid solutions, even if no discontinuities can be detected in the physicochemical evolution of the parent magma.     

The influence of melt structure on trace element partitioning near the peridotite solidus

This experimental study examines the mineral/melt partitioning of Na, Ti, La, Sm, Ho, and Lu among high-Ca clinopyroxene, plagioclase, and silicate melts analogous to varying degrees of peridotite partial melting. Experiments performed at a pressure of 1.5 GPa and temperatures of 1,285 to 1,345 °C produced silicate melts saturated with high-Ca clinopyroxene, plagioclase and/or spinel, and, in one case, orthopyroxene and garnet. Partition coefficients measured in experiments in which clinopyroxene coexists with basaltic melt containing ~18 to 19 wt% Al₂O₃ and up to ~3 wt% Na₂O are consistent with those determined experimentally in a majority of the previous studies, with values of ~0.05 for the light rare earths and of ~0.70 for the heavy rare earths. The magnitudes of clinopyroxene/melt partition coefficients for the rare earth elements correlate with pyroxene composition in these experiments, and relative compatibilities are consistent with the effects of lattice strain energy. Clinopyroxene/melt partition coefficients measured in experiments in which the melt contains ~20 wt% Al₂O₃ and ~4 to 8 wt% Na₂O are unusually large (e.g., values for Lu of up to 1.33±0.05) and are not consistent with the dependence on pyroxene composition found in previous studies. The magnitudes of the partition coefficients measured in these experiments correlate with the degree of polymerization of the melt, rather than with crystal composition, indicating a significant melt structural influence on trace element partitioning. The ratio of non-bridging oxygens to tetrahedrally coordinated cations (NBO/T) in the melt provides a measure of this effect; melt structure has a significant influence on trace element compatibility only for values of NBO/T less than ~0.49. This result suggests that when ascending peridotite intersects the solidus at relatively low pressures (~1.5 GPa or less), the compatibility of trace elements in the residual solid varies significantly during the initial stages of partial melting in response to the changing liquid composition. It is unlikely that this effect is important at higher pressures due to the increased compatibility of SiO₂, Na₂O, and Al₂O₃ in the residual peridotite, and correspondingly larger values of NBO/T for small degree partial melts.Editorial responsibility: T.L. Grove     

The distribution of rare earth elements between monzogranitic melt and the aqueous volatile phase in experimental investigations at 800 °C and 200 MPa

The partitioning of the rare earth elements between a peraluminous monzogranitic melt and a chloride-bearing, sulfur- and carbon dioxide-free, aqueous volatile phase was examined experimentally as a function of chloride and major element concentrations at 800 °C and 200 MPa. The light rare earth elements (e.g. La, Ce) partition into the aqueous volatile phase to a greater extent than the heavy rare earth elements (e.g. Yb, Lu). Distribution of the rare earth elements and the major elements H, Na, K, Ca, and Al between the melt phase (mp) and aqueous volatile phase (aq) is a function of the chlorine concentration in the system, and our data are consistent with the rare earth and major elements occurring as chloride complexes in the aqueous volatile phase. Apparent equilibrium constants for experiments at 800 °C and 200 MPa, K ^′ _REE,Na ^aq/mp , expressed as the ratio of the concentration of a given rare earth element in the aqueous volatile phase to the concentration of the same element in the melt phase, divided by the cubed ratio of sodium in the aqueous volatile phase to the concentration of sodium in the melt phase, decrease systematically with increasing atomic number from K ^′ _La,Na ^aq/mp = 0.41(±0.03) to K ^′ _Lu,Na ^aq/mp =0.11(±0.01), except for Eu. These experimentally derived apparent equilibrium constants for the rare earth elements can be used in a numerical simulation of magmatic volatile exsolution. The simulation gave results consistent with the elemental distribution in the potassic alteration zone of a deep porphyry copper deposit, but higher concentrations of heavy rare earth elements are released into the magmatic aqueous solution than are captured in the secondary mineralization. Received: 1 November 1999 / Accepted: 7 June 2000     

Determination of melt influence on divalent element partitioning between anorthite and CMAS melts

We propose a theory for crystal-melt trace element partitioning that considers the energetic consequences of crystal-lattice strain, of multi-component major-element silicate liquid mixing, and of trace-element activity coefficients in melts. We demonstrate application of the theory using newly determined partition coefficients for Ca, Mg, Sr, and Ba between pure anorthite and seven CMAS liquid compositions at 1330 °C and 1 atm. By selecting a range of melt compositions in equilibrium with a common crystal composition at equal liquidus temperature and pressure, we have isolated the contribution of melt composition to divalent trace element partitioning in this simple system. The partitioning data are fit to Onuma curves with parameterizations that can be thermodynamically rationalized in terms of the melt major element activity product (a_Al2O3)(a_SiO2)² and lattice strain theory modeling. Residuals between observed partition coefficients and the lattice strain plus major oxide melt activity model are then attributed to non-ideality of trace constituents in the liquids. The activity coefficients of the trace species in the melt are found to vary systematically with composition. Accounting for the major and trace element thermodynamics in the melt allows a good fit in which the parameters of the crystal-lattice strain model are independent of melt composition.     

Gold solubility, speciation, and partitioning as a function of HCl in the brine-silicate melt-metallic gold system at 800°C and 100 MPa

A vapor-undersaturated synthetic brine was equilibrated with metallic gold and a haplogranitic melt at 800°C and 100 MPa to examine the solubility, speciation and partitioning of gold in the silicate melt-brine-metallic gold system. The starting composition of the NaCl-KCl-HCl-H₂O brine was 70 wt.% NaCl (equivalent) with starting KCl/NaCl ranging from 0.5 to 1. KCl/HCl was varied from 3.2 to 104 to evaluate the solubility and partitioning of gold as a function of the concentration of HCl in the brine. Inclusions of brine were trapped in a silicate glass during quench. Inclusion-poor and inclusion-rich portions of glass were analyzed for gold and chloride by using neutron activation analysis. The inclusion-poor glass yielded an estimate of the solubility of gold and chloride in the silicate melt. The solubility of gold in the melt, at gold metal saturation, was estimated as ≈1 ppm. The solubility of gold in the brine was estimated by mass balance, given the concentration of gold and chloride in the inclusion-poor and inclusion-rich glasses. The solubility of gold metal at low-HCl concentrations in the brine, C_HCl^b, (3 × 10³ to 1.1 × 10⁴ ppm) is ≈40 ppm (by weight) and is independent of the HCl concentration under those conditions. For C_HCl^b of 1.1 × 10⁴ to 4.0 × 10⁴ ppm, the solubility of gold increased from 40 to 840 ppm, and the solubility is given by: log C_Au^b = [2.2 · log C_HCl^b] − 7.2(1) These data suggest that a significant amount of gold is not chloride complexed in brines at low-HCl concentrations (< 1.1 × 10⁴ ppm), but that gold-chloride complexes, possibly AuCl₂H, are important at elevated concentrations of HCl (> 1.1 × 10⁴ ppm). The calculated Nernst partition coefficient (D_Au^b/m) for gold between a brine and melt varied from 40 to 830 over a range of brine HCl concentrations of 3 × 10³ to 1.1 × 10⁴ ppm. Our results indicate a significant amount of gold can be transported by a brine in the magmatic-hydrothermal environment independent of the fugacity of sulfur in the system. Thus brines provide an effective mechanism for the scavenging of gold from a crystallizing melt and transport into an associated magmatic-hydrothermal system, regardless of their sulfur contents.     

Partitioning of lanthanides and Y between immiscible silicate and fluoride melts, fluorite and cryolite and the origin of the lanthanide tetrad effect in igneous rocks

Some F-rich granitic rocks show anomalous, nonchondritic ratios of Y/Ho, extreme negative Eu anomalies, and unusual, discontinuous, segmented chondrite-normalised plots of rare earth elements (REE). The effects of F-rich fluids have been proposed as one of the explanations for the geochemical anomalies in the evolved granitic systems, as the stability of nonsilicate complexes of individual rare earths may affect the fluid-melt element partitioning. The lanthanide tetrad effect, related to different configurations of 4f-electron subshells of the lanthanide elements, is one of the factors affecting such complexing behaviour. We present the first experimental demonstration of the decoupling of Y and Ho, and the tetrad effect in the partitioning of rare earths between immiscible silicate and fluoride melts. Two types of experiments were performed: dry runs at atmospheric pressure in a high-temperature centrifuge at 1100 to 1200°C, and experiments with the addition of H₂O at 700 to 800°C and 100 MPa in rapid-quench cold-seal pressure vessels. Run products were analysed by electron microprobe (major components), solution-based inductively coupled plasma mass spectrometry (ICP-MS) (REE in the centrifuged runs), and laser ablation ICP-MS (REE and Li in the products of rapid-quench runs). All the dry centrifuge runs were performed at super-liquidus, two-phase conditions. In the experiments with water-bearing mixtures, minor amounts of aqueous vapour were present in addition to the melts. We found that lanthanides and Y concentrated strongly in the fluoride liquids, with two-melt partition coefficients reaching values as high as 100-220 in water-bearing compositions. In all the experimental samples, two-melt partition coefficients of lanthanides show subtle periodicity consistent with the tetrad effect, and the partition coefficient of Y is greater than that of Ho. One of the mixtures also produced abundant fluorite (CaF₂) and cryolite (Na₃AlF₆) crystals, which enabled us to study fluorite-melt and cryolite-melt REE partitioning. REE concentrations in fluorite are high and comparable to those in the fluoride melt. However, fluorite-melt partition coefficients appear to depend mostly on ionic radii and show neither significant tetrad anomalies, nor differences in Y and Ho partitioning. In contrast, REE concentrations in cryolite are low (∼5-10 times lower than in the silicate melt), and cryolite-melt REE partitioning shows very strong tetrad and Y-Ho anomalies. Our results imply that Y-Ho and lanthanide tetrad anomalies are likely to be caused mainly by aluminofluoride complexes, and the tetrad REE patterns in natural igneous rocks can result from fractionation of F-rich magmatic fluids.     

The partitioning of copper between silicate melts and two-phase aqueous fluids: An experimental investigation at 1 kbar, 800° C and 0.5 kbar, 850° C

 Experiments were performed in the three phase system high-silica rhyolite melt+low-salinity aqueous vapor+hydrosaline brine, to investigate the partitioning equilibria for copper in magmatic-hydrothermal systems at 800° C and 1 kbar, and 850° C and 0.5 kbar. D^aqm/mlt _Cu and apparent equilibrium constants, K^aqm/mlt _Cu,Na, between the aqueous mixture (aqm=quenched vapor+brine) and the silicate melt (mlt) are calculated. D^aqm/mlt _Cu increases with increasing aqueous chloride concentration and is a function of pressure. K^aqm/mlt _Cu,Na=215(±73) at 1 kbar and 800° C and K^aqm/mlt _Cu,Na=11(±6) at 0.5 kbar and 850°C. Decreasing pressure from 1 to 0.5 kbar lowers K^aqm/mlt _Cu,Na by a factor of approximately 20. Data revealed no difference in K^aqm/mlt _Cu,Na or D^aqm/mlt _Cu as a function of the melt aluminium saturation index. Within the 2-phase field the K^aqm/mlt _Cu,Na show no variation with total aqueous chloride, indicating that copper-sodium exchange between the vapor, brine and silicate melt is independent of the mass proportion of vapor and brine. Model copper-sodium apparent equilibrium constants for the hydrosaline brine and the silicate melt revealed a negative dependence on pressure. Model apparent equilibrium constants for copper-sodium exchange between the brine and vapor were close to unity at 1 kbar and 800° C. Received: 27 June 1994/Accepted: 30 March 1995     

The Partitioning of Tungsten bwtween Aqueous Fluids and Silicate Melts

An experimental study has been carried out to determine the partition coefficients of tungsten between aqueous fluids and granitic melts at 800 °C and 1.5 kb with natural granite as the starting material. The effects of the solutions on the partition coefficients of tungsten show a sequence of P > CO ₃ ²⁻ > B > H₂O. The effects are limited (generallyK _D < 0.3) and the tungsten shows a preferential trend toward the melt over the aqueous fluid. The value ofK _D increases with increasing concentration of phosphorus; theK _D increases first and then reduces with the concentration of CO ₃ ²⁻ when temperature decreases, theK _D between the solution of CO ₃ ²⁻ and the silicate melt increases, and that between the solution of B₄O ₇ ²⁻ and the silicate melt decreases. The partition coefficients of phosphorus and sodium between fluids and silicate melts have been calculated from the concentrations of the elements in the melts. TheK _D value for phosphorus is 0.38 and that for sodium is 0.56. Evidence shows that the elements tend to become richer and richer in the melts.     

Partitioning of F between aqueous fluids and albite granite melt and its petrogenetic and metallogenetic significance

The fluid/melt partitioning experiments on fluorine were carried out in the system albite-H₂O-HF atP = 100 MPa, 770°C ≤T≤800°C: and wt = 2% −6% conditions. The concentrations of fluorine in quenched glasses (melt) were determined by electron microprobe and those of fluorine in the coexisting aqueous fluid were calculated by the method of mass balance. The result shows that the fluorine was concentrated in granitic melt relative to the coexisting fluid. The partition coefficient D_F(wt _F ^F1 /wt _F ^Mt ) ranges from 0.35 to 0.89. It increases with increasing fluorine content in the system. This means that there is not just one single value of partition coefficient for fluorine in the granitic melt-fluid system. The partitioning behavior of fluorine in this system depends critically on fluorine and proton (H⁺) concentrations. Our data suggest that F-rich granitic melts exist in nature and that fluorine may not be an important complexing agent of metal elements in F-bearing fluids. The project was financially supported by both the National Natural Science Foundation of China (No. 49603048) and the State Key Laboratory of Mineral Deposit Research, Nanjing University.     

Significance of Europium anomalies in silicate melts and crystal-melt equilibria: a re-evaluation

Although europium speciation in silicate melts partly reflects prevailing oxygen fugacities, melt composition and structure play the major role in determining Eu²⁺/ Eu³⁺ ratios and europium partitioning into mineral phases. Experimental evidence by different investigators on the magnitude of the compositional effect on Eu²⁺/Eu³⁺ provides consistent results only if account is taken of the oxygen buffer system employed in the experiments. The medium-dependent reduction of europium can be understood in terms of the preferential stabilization of Eu²⁺ by a strong aluminosilicate complex in the melt phase, and to a much lesser degree by metasilicate complexes. The stability of these complexes increases as the field strength of the associated cation species decreases. Hence Eu²⁺-aluminosilicate complexes are preferentially stabilized relative to trivalent lanthanides in melts of appropriate composition and their presence minimizes the enthalpy of the melt. The influence of these complexes is particularly pronounced in melts with a high feldspathic component and a strongly polymerized structure. Their petrogenetic influence is best documented through the behaviour of europium relative to the other lanthanides during anhydrous anatexis in high-grade metamorphic terrains and in the anomalous europium partition coefficients of phenocryst phases which formed during the crystallization of highly silicic magmas.     

The behaviour of tungsten in granitic melt-vapour systems

An experimental study has been carried out to determine the effect of solution composition on the partitioning behaviour of tungsten in granitic melt-vapour systems at 800° C and 1 kbar. With chloride and phosphate solutions, tungsten partitions strongly into the aqueous phase, whereas with fluoride, carbonate and borate solutions, and water alone, tungsten partitions in favour of the melt. With chloride solutions, the fluid/melt partition coefficients (K _D) for W show a marked positive correlation with chloride concentration, and suggest that at low chloride concentrations W-Cl complexes with low Cl∶W ratios (such as associated equivalents of (WO₃)₂C1⁻) may be present. In contrast, at higher chloride concentrations complexes with high Cl∶W ratios (such as WOCl₄, WCl₆ and associated ionic equivalents) may predominate. With phosphate solutions, K _D shows little variation with phosphate concentration, and phosphorus heteropolytungstates (such as H₃[PW₁₂O₄₀]) may be present. There is no evidence to suggest that fluoride, carbonate or borate complexes of tungsten are important under the experimental conditions: the data for these compositions can be interpreted assuming that isopolytungstates (such as H₆[H₂W₁₂O₄₀]) are present. Within high temperature hydrothermal solutions tungsten may be transported principally as isopolytungstates and heteropolytungstates in addition to chloride complexes, and this may, in part, account for the common association of apatite and arsenopyrite with scheelite and wolframite in tungsten deposits.     

Mg-Fe partitioning between olivine and ultramafic melts at high pressures

Precise determination of the partitioning of Mg and Fe²⁺ between olivine and ultramafic melt has been made at pressures from 5 to 13 GPa using a MA-8 type multi-anvil high-pressure apparatus (PREM) installed at Earthquake Research Institute, University of Tokyo. A very short rhenium capsule (<100 μm sample thickness) was adopted to minimize temperature variation within the sample container. Synthetic gels with the composition of the upper mantle peridotite were used as starting materials to promote the homogeneity. Analyses of quenched melts and coexisting olivines were made with an electron probe microanalyzer. The obtained partition coefficient, K_D [=(FeO/MgO)^ol/(FeO/MgO)^melt], decreases from 0.35 to 0.25 with increasing pressure from 5 to 13 GPa, suggesting a negative correlation between pressure and K_D above 5 GPa. Our result is consistent with a parabolic relationship between K_D and degree of polymerization (NBO/T) of melts reported by previous studies at lower pressures. The negative correlation between pressure and K_D suggests that olivine crystallizing in a magma ocean becomes more Mg-rich with depth and that primary magmas generated in the upper mantle become more Fe-rich with depth than previously estimated.     

Effect of melt composition on the partitioning of trace elements between titanite and silicate melt

Isobaric and isothermal experiments were performed to investigate the effect of melt composition on the partitioning of trace elements between titanite (CaTiSiO₅) and a range of different silicate melts. Titanite-melt partition coefficients for 18 trace elements were determined by secondary ion mass spectrometry (SIMS) analyses of experimental run products. The partition coefficients for the rare earth elements and for Th, Nb, and Ta reveal a strong influence of melt composition on partition coefficients, whereas partition coefficients for other studied monovalent, divalent and most quadrivalent (i.e., Zr, Hf) cations are not significantly affected by melt composition. The present data show that the influence of melt composition may not be neglected when modelling trace element partitioning.It is argued that it is mainly the change of coordination number and the regularity of the coordination space of trace elements in the melt structure that controls partition coefficients in our experiments. Furthermore, our data also show that the substitution mechanism by which trace elements are incorporated into titanite crystals may be of additional importance in this context.     

Distribution of REE between clinopyroxene and basaltic melt along a mantle adiabat: effects of major element composition, water, and temperature

The distribution of rare earth elements (REE) between clinopyroxene (cpx) and basaltic melt is important in deciphering the processes of mantle melting. REE and Y partition coefficients from a given cpx-melt partitioning experiment can be quantitatively described by the lattice strain model. We analyzed published REE and Y partitioning data between cpx and basaltic melts using the nonlinear regression method and parameterized key partitioning parameters in the lattice strain model (D ₀, r ₀ and E) as functions of pressure, temperature, and compositions of cpx and melt. D ₀ is found to positively correlate with Al in tetrahedral site (Al ^T ) and Mg in the M2 site (Mg^M2) of cpx and negatively correlate with temperature and water content in the melt. r ₀ is negatively correlated with Al in M1 site (Al^M1) and Mg^M2 in cpx. And E is positively correlated with r ₀. During adiabatic melting of spinel lherzolite, temperature, Al ^T , and Mg^M2 in cpx all decrease systematically as a function of pressure or degree of melting. The competing effects between temperature and cpx composition result in very small variations in REE partition coefficients along a mantle adiabat. A higher potential temperature (1,400°C) gives rise to REE partition coefficients slightly lower than those at a lower potential temperature (1,300°C) because the temperature effect overwhelms the compositional effect. A set of constant REE partition coefficients therefore may be used to accurately model REE fractionation during partial melting of spinel lherzolite along a mantle adiabat. As cpx has low Al and Mg abundances at high temperature during melting in the garnet stability field, REE are more incompatible in cpx. Heavy REE depletion in the melt may imply deep melting of a hydrous garnet lherzolite. Water-dependent cpx partition coefficients need to be considered for modeling low-degree hydrous melting.     

Experimental study of fluorine and chlorine contents in mica (biotite) and their partitioning between mica,phonolite melt,and fluid

This paper reports the results of a study of the composition of mica (biotite) crystallizing in the system of phonolite melt-Cl- and F-bearing aqueous fluid at T ～ 850°C, P = 200 MPa, and \(f_{O_2 } \) = Ni-NiO, as well as data on F and Cl partitioning between coexisting phases. It was established that Cl content in mica is significantly lower than in phonolite melt and, especially, in fluid. Fluorine shows a different behavior in this system: its content in mica is always higher than in phonolite melt but lower than in fluid. The mica-melt partition coefficients of Cl and F also behave differently. The Cl partition coefficient gradually increases from 0.17 to 0.33 with increasing Cl content in the system, whereas the partition coefficient of F sharply decreases from 3.0 to 1.0 with increasing total F content. The apparent partition coefficients of F between biotite and groundmass (melt) in various magmatic rocks are usually significantly higher than the experimental values. It was supposed that the higher ^Bt/glassD_F values in natural samples could be related to the influence of later oxidation reactions, reequilibration of biotite at continuously decreasing \(f_{H_2 O} \)/f _HF ratio, and an increase in this coefficients with decreasing total F content in the system.     

Trace element partitioning between apatite and silicate melts

We present new experimental apatite/melt trace element partition coefficients for a large number of trace elements (Cs, Rb, Ba, La, Ce, Pr, Sm, Gd, Lu, Y, Sr, Zr, Hf, Nb, Ta, U, Pb, and Th). The experiments were conducted at pressures of 1.0 GPa and temperatures of 1250 °C. The rare earth elements (La, Ce, Pr, Sm, Gd, and Lu), Y, and Sr are compatible in apatite, whereas the larger lithophile elements (Cs, Rb, and Ba) are strongly incompatible. Other trace elements such as U, Th, and Pb have partition coefficients close to unity. In all experiments we found D_Hf > D_Zr, D_Ta ≈ D_Nb, and D_Ba > D_Rb > D_Cs. The experiments reveal a strong influence of melt composition on REE partition coefficients. With increasing polymerisation of the melt, apatite/melt partition coefficients for the rare earth elements increase for about an order of magnitude. We also present some results in fluorine-rich and water-rich systems, respectively, but no significant influence of either H₂O or F on the partitioning was found. Furthermore, we also present experimentally determined partition coefficients in close-to natural compositions which should be directly applicable to magmatic processes.     

Melt structural control on olivine/melt element partitioning of Ca and Mn

Relationships between mineral/silicate melt partition coefficients and melt structure have been examined by combining Ca and Mn olivine/melt partitioning data with available melt structure information. Compositions were chosen so that melts with olivine on their liquidii range in degree of polymerization, NBO/T, from ∼0.5 to ∼2.5 under near isothermal conditions (1350-1400°C). Olivine/melt Ca-Mn exchange coefficients, Ca(olivine)/CaO(melt)/MnO(olivine)/MnO(melt) (K_{D Ca-Mn}^olivine/melt), as a function of melt NBO/T have a parabolic shape with a minimum K_{D Ca-Mn}^olivine/melt-value at NBO/T near 1. Notably, published K_{D Fe2⁺-Mg}^olivine/melt versus NBO/T functions are also parabolic with a maximum in K_{D Fe2⁺-Mg}^olivine/melt near 1 (Kushiro and Mysen, 2002).The olivine/melt partitioning data are modeled in terms of structural units (Qⁿ-species) in the melt. The NBO/T-value corresponding to the minimum K_{D Ca-Mn}^olivine/melt is near that where the abundance ratio of Qⁿ-species, X_Q3/X_Q2, has its largest value. Therefore, the activity coefficient ratio in the melt, γ_Ca2⁺(melt)/γ_Mn2⁺(melt), attains a minimum where the abundance ratio of X_Q3/X_Q2 is at maximum. It is inferred from this relationship that Ca²⁺ in the melts is dominantly bonded to nonbridging oxygen (Ca-NBO) in Q³-species, whereas Mn²⁺ is bonded to nonbridging oxygen (Mn-NBO) in less polymerized Qⁿ-species such as Q².     

A possible effect of melt structure on the Mg-Fe partitioning between olivine and melt

Partitioning of Mg and Fe²⁺ between olivine and mafic melts has been determined experimentally for eight different synthetic compositions in the temperature range between 1335 and 1425°C at 0.1 MPa pressure and at fo₂ ∼1 log unit below the quartz-fayalite-magnetite buffer. The partition coefficient [K_D = (Fe²⁺/Mg)^ol/(Fe²⁺/Mg)^melt] increases from 0.25 to 0.34 with increasing depolymerization of melt (NBO/T of melt from 0.25-1.2), and then decreases with further depolymerization of melt (NBO/T from 1.2-2.8). These variations are similar to those observed in natural basalt-peridotite systems. In particular, the variation in NBO/T ranges for basaltic-picritic melts (0.4-1.5) is nearly identical to that obtained in the present experiments. Because the present experiments were carried out at constant pressure (0.1 MPa) and in a relatively small temperature range (90°C), the observed variations of Mg and Fe²⁺ partitioning between olivine and melt must depend primarily on the composition or structure of melt. Such variations of K_D may depend on the relative proportions of four-, five-, and six-coordinated Mg²⁺ and Fe²⁺ in melt as a function of degree of NBO/T.     

Partitioning of manganese between forsterite and silicate liquid

Partition coefficients for Mn between forsterite and liquid in the system MgO-CaO-Na₂O-Al₂O₃-SiO₂ (+ about 0.2% Mn) were measured by electron microprobe for a variety of melt compositions over the temperature range 1250–1450°C at one atm pressure. The forsterite-liquid partition coefficient of Mn (mole ratio, MnO in Fo/MnO in liquid, designated D_mn^fo?Liq) depends on liquid composition as well as temperature: at 1350°C, D_Mn^Fo?Liqranges from 0.60 (basic melt, SiO₂ = 47wt%) to 1.24 (acidic melt, SiO₂ = 65wt%). At lower temperatures, the partition coefficient is more strongly dependent on melt composition.The effects of melt composition and temperature on D_Mn^fo?Liq can be separately evaluated by use of the Si:O atomic ratio of the melts. A plot of D_mn^Fo?Liq measured at various temperatures vs melt Si:O for numerous liquid compositions reveals discrete, constant-temperature curves that are not well defined by plotting D_Mn^Fo?Liq against other melt composition parameters such as melt basicity or MgO content. For constant Si:O in the melt, In D_Mn^Fo?Liq vs reciprocal absolute temperature is linear; however, the slope of the plot becomes more positive for higher values of Si:O, indicating a higher energy state for Mn²⁺ ions in acidic melts than in basic melts.Comparison of Mn partitioning data for the iron-free system used in this study with data of other workers on iron-bearing compositions suggests that the effect of iron on Mn partitioning between olivine and melt is small over the range of basalt liquidus temperatures.     

Thermodynamic controls on element partitioning between titanomagnetite and andesitic–dacitic silicate melts

Titanomagnetite–melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity (fO₂) and temperature (T) in an andesitic–dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite–melt partitioning of the divalent cations (Mg²⁺, Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺) and Cu²⁺/Cu⁺ with increasing fO₂ between 0.2 and 3.7 log units above the fayalite–magnetite–quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral–melt partitioning of divalent cations, a more rigorous justification of magnetite–melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO₂ implies a reciprocal relationship between a(Fe²⁺O) and a(Fe³⁺O_1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with fO₂ in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in fO₂ in silicic, but less so in mafic bulk systems.