Olivine/melt transition metal partitioning, melt composition, and melt structure—Melt polymerization and Q-speciation in alkaline earth silicate systems

The two most abundant network-modifying cations in magmatic liquids are Ca²⁺ and Mg²⁺. To evaluate the influence of melt structure on exchange of Ca²⁺ and Mg²⁺ with other geochemically important divalent cations (m-cations) between coexisting minerals and melts, high-temperature (1470-1650 °C), ambient-pressure (0.1 MPa) forsterite/melt partitioning experiments were carried out in the system Mg₂SiO₄-CaMgSi₂O₆-SiO₂ with ?1 wt% m-cations (Mn²⁺, Co²⁺, and Ni²⁺) substituting for Ca²⁺ and Mg²⁺. The bulk melt NBO/Si-range (NBO/Si: nonbridging oxygen per silicon) of melt in equilibrium with forsterite was between 1.89 and 2.74. In this NBO/Si-range, the NBO/Si(Ca) (fraction of nonbridging oxygens, NBO, that form bonds with Ca²⁺, Ca²⁺-NBO) is linearly related to NBO/Si, whereas fraction of Mg²⁺-NBO bonds is essentially independent of NBO/Si. For individual m-cations, rate of change of K_D(m−Mg) with NBO/Si(Ca) for the exchange equilibrium, m^melt + Mg^olivine ? m^olivine + Mg^melt, is linear. K_D(m−Mg) decreases as an exponential function of increasing ionic potential, Z/r² (Z: formal electrical charge, r: ionic radius—here calculated with oxygen in sixfold coordination around the divalent cations) of the m-cation. The enthalpy change of the exchange equilibrium, ΔH, decreases linearly with increasing Z/r² [ΔH = 261(9)-81(3)·Z/r² (Å⁻²)]. From existing information on (Ca,Mg)O-SiO₂ melt structure at ambient pressure, these relationships are understood by considering the exchange of divalent cations that form bonds with nonbridging oxygen in individual Qⁿ-species in the melts. The negative ∂K_D(m−Mg)/∂(Z/r²) and ∂(ΔH)/∂(Z/r²) is because increasing Z/r² is because the cations forming bonds with nonbridging oxygen in increasingly depolymerized Qⁿ-species where steric hindrance is decreasingly important. In other words, principles of ionic size/site mismatch commonly observed for trace and minor elements in crystals, also govern their solubility behavior in silicate melts.     

Olivine/silicate melt partitioning of germanium: an example of a nearly constant partition coefficient

The partitioning of germanium between forsterite (Fo) and liquids in the diopside-anorthiteforsterite join was investigated by electron microprobe analysis of Ge-doped samples equilibrated at 1300°–1450°C. Germanium is somewhat incompatible in Fo relative to the haplobasaltic melts, with a grand mean for all simple partition coefficients (D_Fo-l^Ge) of 0.68 ± 0.06. For the melt composition range studied, D_Fo-l^Ge is virtually constant in isothermal series of experiments, and shows only minor overall temperature dependence. The exchange reaction partition coefficient $\text{K}{}_{\mathrm{D}}\text{= (}\text{Mg}{}_2\text{GeO}{}_4\text{)}{}_{\mathrm{Fo}}\text{(SiO}{}_2\text{)}{}_{\mathrm{l}}\text{(Mg}{}_2\text{SiO}{}_4\text{)}{}_{\mathrm{Fo}}\text{(GeO}{}_2\text{)}{}_{\mathrm{l}}$] is near unity in all cases, with a grand mean of 0.93 ± 0.11. One exploratory run at 20 kbar yielded a distinctly lower partition coefficient (D_Fo-l^Ge = 0.54 ± 0.04), which confirms the negative pressure dependence predicted by the thermodynamics of Ge ai Si exchange.These new data indicate that absolute Ge enrichment must occur in terrestrial magmas undergoing olivine fractionation, while $\text{Ge}\text{Si}$ remains nearly constant.     

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.     

Evidence from olivine/melt element partitioning that nonbridging oxygen in silicate melts are not equivalent

Partitioning of Ca, Mn, Mg, and Fe²⁺ between olivine and melt has been used to examine the influence of energetically nonequivalent nonbridging oxygen in silicate melts. Partitioning experiments were conducted at ambient pressure in air and 1400°C with melts in equilibrium with forsterite-rich olivine (Fo >95 mol%). The main compositional variables of the melts were NBO/T and Na/(Na+Ca). In all melts, the main structural units were of Q⁴, Q³, and Q² type with nonbridging oxygen, therefore, in the Q³ and Q² units.For melts with high Q³/Q²-abundance ratio (corresponding to NBO/T near 1), increasing Na/(Na+Ca) [and Na/(Na+Ca+Mn+Mg+Fe²⁺)] results in a systematic decrease of the partition coefficients, K_Ca^ol/melt, K_Mn^ol/melt, K_Mg^ol/melt, and K_Fe2+^ol/melt, because of ordering of the network-modifying Ca, Mn, Mg, and Fe²⁺ among nonbridging oxygen in Q³ and Q² structural units. This decrease is more pronounced the smaller the ionic radius of the cation. With decreasing Q³/Q² abundance ratio (less-polymerized melts) this effect becomes less pronounced.Activity-composition relations among network-modifying cations in silicate melts are, therefore, governed by availability of energetically nonequivalent nonbridging oxygen in individual Qⁿ-species in the melt. As a result, any composition change that enhances abundance of highly depolymerized Qⁿ-species will cause partition coefficients to decrease.     

The effect of melt composition on the wetting angle between silicate melts and olivine

The wetting angle between silicate melts containing Ca, Li, Na, or K and olivine single crystals have been measured as part of an investigation of the dependence of the solid-liquid interfacial energy on melt composition and olivine orientation. The wetting angle increases with increasing silica content of the melt on (100) surfaces, but decreases with increasing silica content on (010) and (001) surfaces. For a given silica content, the wetting angle on (100) decreases in going from Ca to Li to Na to K, while the wetting angle on (010) and (001) increases in going from Ca to K-bearing melts. Based on published values for liquid-vapor interfacial energies, the observed changes in wetting angle with changes in melt composition indicate that the solid-liquid interfacial energy increases with increasing silica content of the melt for the (100) surface. However, for (010) and (001) surfaces, the variation of the solid-liquid interfacial energy with silica content depends upon whether Ca or K is present in the melt. In addition, the solid-liquid interfacial energy depends upon the orientation of the olivine in the following manner: _sl ⁽⁰¹⁰⁾ _sl ⁽⁰⁰¹⁾ _sl ⁽¹⁰⁰⁾ .     

Redox equilibria of iron and silicate melt structure: Implications for olivine/melt element partitioning

Olivine/melt partitioning of ΣFe, Fe²⁺, Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, and Ni²⁺ has been determined in the systems CaO-MgO-FeO-Fe₂O₃-SiO₂ (FD) and CaO-MgO-FeO-Fe₂O₃-Al₂O₃-SiO₂ (FDA3) as a function of oxygen fugacity (f_O2) at 0.1 MPa pressure. Total iron oxide content of the starting materials was ∼20 wt%. The f_O2 was to used to control the Fe³⁺/ΣFe (ΣFe: total iron) of the melts. The Fe³⁺/ΣFe and structural roles of Fe²⁺ and Fe³⁺ were determined with ⁵⁷Fe resonant absorption Mössbauer spectroscopy. Changes in melt polymerization, NBO/T, as a function of f_O2 was estimated from the Mössbauer data and existing melt structure information. It varies by ∼100% in melts coexisting with olivine in the FDA3 system and by about 300% in the FD system in the Fe³⁺/ΣFe range of the experiments (0.805-0.092). The partition coefficients ( in olivine/wt% in melt) are systematic functions of f_O2 and, therefore, NBO/T of the melt. There is a -minimum in the FDA3 system at NBO/T-values corresponding to intermediate Fe³⁺/ΣFe (0.34-0.44). In the Al-free system, FD, where the NBO/T values of melts range between ∼1 and ∼2.9, the partition coefficients are positively correlated with NBO/T (decreasing Fe³⁺/ΣFe). These relationships are explained by consideration of solution behavior in the melts governed by Qⁿ-unit distribution and structural changes of the divalent cations in the melts (coordination number, complexing with Fe³⁺, and distortion of the polyhedra).     

Thermodynamics of Al/Al avoidance in the ordering of Al/Si tetrahedral framework structures

The main driving force behind Al/Si ordering in tetrahedral framework aluminosilicates is nearest-neighbour Al/Al avoidance. Computer simulation is used to explore the direct consequences of such Al/Al avoidance. The main result is that the order-disorder transition temperature T _c falls dramatically as the concentration x of Al in the structure is reduced, and if the only interactions are those associated with nearest-neighbour Al/Al avoidance, T _c becomes zero for x less than some critical value x _c , where x _c =0.31 for the feldspar framework and x _c =0.34 for cordierite. Also a large degree of short range order is found above T _c . Both results differ radically from the standard Bragg-Williams model. Plots of entropy and enthalpy of ordering are given as functions of x and T, which may be used to interpret experimental data or for extrapolation into ranges of x and T inaccessible to experiment. Received: 14 May 1997 / Revised, accepted: 2 June 1997     

The effect of liquid composition on the partitioning of Ni between olivine and silicate melt

We report the results of experiments designed to separate the effects of temperature and pressure from liquid composition on the partitioning of Ni between olivine and liquid, \(D_{\text{Ni}}^{\text{ol/liq}}\). Experiments were performed from 1300 to 1600 °C and 1 atm to 3.0 GPa, using mid-ocean ridge basalt (MORB) glass surrounded by powdered olivine in graphite–Pt double capsules at high pressure and powdered MORB in crucibles fabricated from single crystals of San Carlos olivine at one atmosphere. In these experiments, pressure and temperature were varied in such a way that we produced a series of liquids, each with an approximately constant composition (~12, ~15, and ~21 wt% MgO). Previously, we used a similar approach to show that \(D_{\text{Ni}}^{\text{ol/liq}}\) for a liquid with ~18 wt% MgO is a strong function of temperature. Combining the new data presented here with our previous results allows us to separate the effects of temperature from composition. We fit our data based on a Ni–Mg exchange reaction, which yields \(\ln \left( {D_{\text{Ni}}^{\text{molar}} } \right) = \frac{{ -\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } }}{RT} + \frac{{\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } }}{R} - \ln \left( {\frac{{X_{\text{MgO}}^{\text{liq}} }}{{X_{{{\text{MgSi}}_{ 0. 5} {\text{O}}_{ 2} }}^{\text{ol}} }}} \right).\) Each subset of constant composition experiments displays roughly the same temperature dependence of \(D_{\text{Ni}}^{\text{ol/liq}}\) (i.e.,\(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\)) as previously reported for liquids with ~18 wt% MgO. Fitting new data presented here (15 experiments) in conjunction with our 13 previously published experiments (those with ~18 wt% MgO in the silicate liquid) to the above expression gives \(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = 3641 ± 396 (K) and \(\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = ? 1.597 ± 0.229. Adding data from the literature yields \(-\Delta _{r(1)} H_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = 4505 ± 196 (K) and \(\Delta _{r(1)} S_{{T_{\text{ref}} ,P_{\text{ref}} }}^{ \circ } /R\) = ? 2.075 ± 0.120, a set of coefficients that leads to a predictive equation for \(D_{\text{Ni}}^{\text{ol/liq}}\) applicable to a wide range of melt compositions. We use the results of our work to model the melting of peridotite beneath lithosphere of varying thickness and show that: (1) a positive correlation between NiO in magnesian olivine phenocrysts and lithospheric thickness is expected given a temperature-dependent \(D_{\text{Ni}}^{\text{ol/liq}} ,\) and (2) the magnitude of the slope for natural samples is consistent with our experimentally determined temperature dependence. Alternative processes to generate the positive correlation between NiO in magnesian olivines and lithospheric thickness, such as the melting of olivine-free pyroxenite, are possible, but they are not required to explain the observed correlation of NiO concentration in initially crystallizing olivine with lithospheric thickness.     

LA-ICPMS analyses of silicate melt inclusions in co-precipitated minerals: Quantification, data analysis and mineral/melt partitioning

We present a new approach to determine the composition of silicate melt inclusions (SMI) using LA-ICPMS. In this study, we take advantage of the occurrence of SMI in co-precipitated mineral phases to quantify their composition without depending on additional sources of information. Quantitative SMI analyses are obtained by assuming that the ratio of selected elements in SMI trapped in different phases are identical. In addition Fe/Mg exchange equilibrium between olivine and melt was successfully used to quantify LA-ICPMS analyses of SMI in olivine. Results show that compositions of SMI from the different host minerals are identical within their uncertainty. Thus (1) the quantification approach is valid; (2) analyses are not affected by the composition of the host phase; (3) the derived melt compositions are representative of the original melt, excluding significant syn- or postentrapment modification such as boundary layer effects or diffusive reequilibration with the host mineral. With this data we established a large dataset of mineral/melt partition coefficients for the investigated mineral phases in hydrous calc-alkaline basaltic-andesitic melts. The clinopyroxene/melt and plagioclase/melt partition coefficients are consistent with the lattice strain model of Blundy and Wood [Blundy, J., Wood B., 1994. Prediction of crystal-melt partition-coefficients from elastic-moduli. Nature372, 452-454].     

The influence of melt composition on the partitioning of REEs, Y, Sc, Zr and Al between forsterite and melt in the system CMAS

Partition coefficients for a range of Rare Earth Elements (REEs), Y, Sc, Al and Zr were determined between forsteritic olivine (nearly end-member Mg₂SiO₄) and ten melt compositions in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) at 1 bar and 1400 °C, with concentrations of the trace elements in the olivine and the melt measured by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The REEs and Sc were added at levels sufficient to ensure that concentrations in the olivine were well above the detection limits. The REE partition coefficients decrease with increasing silica in the melt, indicating strong bonding between REEO_1.5 and SiO₂ in the melt. The variation of as a function of ionic radius is well described by the Brice equation for each composition, although a small proportion of this variation is due to the increase in the strength of the REEO_1.5-SiO₂ interactions in the melt with ionic radius. Scandium behaves very similarly to the REEs, but a global fit of the data from all ten melt compositions suggests that deviates somewhat from the parabolas established by the REE and Y, implying that Sc may substitute into olivine differently to that of the REEs. In contrast to the behaviour of the large trivalent cations, the concentration of Al in olivine is proportional to the square root of its concentration in the melt, indicating a coupled substitution in olivine with a high degree of short-range order. The lack of any correlation of REE partition coefficients with Al in olivine or melt suggests that the REE substitution in olivine is charge-balanced by cation vacancies. The partition coefficient of the tetravalent trace element Zr, which is highly incompatible in olivine, depends on the CaO content of the melt.     

Competing effects of crystal chemistry and silicate melt composition on trace element behavior in magmatic systems: insights from crystal/silicate melt partitioning of the REE,HFSE, Sn,In, Ga,Ba, Pt and Rh

We present new partition coefficients for the REE, HFSE, Sn, In, Ga, Ba, Pt and Rh between clinopyroxene, olivine and basaltic melt as a function of crystal chemistry and melt composition at temperatures of 1190–1300 °C and 1-bar pressure. Two components, namely \(\mathrm {Al_2O_3}\) and \(\mathrm {Na_2O}\), were chosen to be investigated since they are known to affect the structure of silicate melts and especially clinopyroxene crystal chemistry. The amount of \(^{[4]}\mathrm{Al}\) in clinopyroxene will result in an increase of \(D_i^\mathrm{{cpx/melt}}\) even after applying a correction factor to account for the effect of melt polymerization. Moreover, the positive correlation between \(^{[4]}\mathrm{Al}\) and \(D_i^\mathrm{{cpx/melt}}\) is not restricted to the REE, but also applies for Sn, Ga, In, and Ba. The addition of up to 2.6 wt% \(\mathrm {Na_2O}\) to the silicate melt universally increases the \(D_i^\mathrm{{cpx/melt}}\) without any concomitant change in crystal chemistry or a significant effect in melt polymerization. This compositional effect is likely due to the ability of Na to break REE–Al complexes in the melt. Our results emphasize the importance of considering all variables that affect the behavior of trace elements in magmatic systems before applying the lattice strain model and derive meaningful results for the changes in the parameters of the crystallographic sites.     

The effect of sulfur on the partitioning of Ni and other first-row transition elements between olivine and silicate melt

The effect of sulfur dissolved as sulfide (S²⁻) in silicate melts on the activity coefficients of NiO and some other oxides of divalent cations (Ca, Cr, Mn, Fe and Co) has been determined from olivine/melt partitioning experiments at 1400 °C in six melt compositions in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS), and in derivatives of these compositions at 1370 °C, obtained from the six CMAS compositions by substituting Fe for Mg (FeCMAS). Amounts of S²⁻ were varied from zero to sulfide saturation, reaching 4100 μg g⁻¹ S in the most sulfur-rich silicate melt. The sulfide solubilities compare reasonably well with those predicted from the parameterization of the sulfide capacity of silicate melts at 1400 °C of O’Neill and Mavrogenes (2002), although in detail systematic deviations indicate that a more sophisticated model may improve the prediction of sulfide capacities.The results show a barely discernible effect of S²⁻ in the silicate melt on Fe, Co and Ni partition coefficients, and also surprisingly, a tiny but resolvable effect on Ca partitioning, but no detectable effect on Cr, Mn or some other lithophile incompatible elements (Sc, Ti, V, Y, Zr and Hf). Decreasing Mg# of olivine (reflecting increasing FeO in the system) has a significant influence on the partitioning of several of the divalent cations, particularly Ca and Ni. We find a remarkably systematic correlation between and the ionic radius of M²⁺, where M = Ca, Cr, Mn, Fe, Co or Ni, which is attributable to a simple relationship between size mismatch and excess free energies of mixing in Mg-rich olivine solid solutions.Neither the effect of S²⁻ nor of Mg#^ol is large enough by an order of magnitude to account for the reported variations of obtained from electron microprobe analyses of olivine/glass pairs from mid-ocean ridge basalts (MORBs). Comparing these MORB glass analyses with the Ni-MgO systematics of MORB from other studies in the literature, which were obtained using a variety of analytical techniques, shows that these electron microprobe analyses are anomalous. We suggest that the reported variation of with S content in MORB is an analytical artifact.Mass balance of melt and olivine compositions with the starting compositions shows that dissolved S²⁻ depresses the olivine liquidus of haplobasaltic silicate melts by 5.8 × 10⁻³ (±1.3 × 10⁻³) K per μg g⁻¹ of S²⁻, which is negligible in most contexts. We also present data for the partitioning of some incompatible trace elements (Sc, Ti, Y, Zr and Hf) between olivine and melt. The data for Sc and Y confirm previous results showing that and decrease with increasing SiO₂ content of the melt. Values of average 0.01 with most falling in the range 0.005-0.015. Zr and Hf are considerably more incompatible than Ti in olivine, with and about 10⁻³. The ratio / is well constrained at 0.611 ± 0.016.     

Equilibrium density and expansivity of silicate melts in the glass transition range

Equilibrium volumes and expansivities of three liquids in the system anorthite (CaAl₂Si₂O₈)–diopside (CaMgSi₂O₆) have been derived from dilatometric measurements of the equilibrium length of samples in the glass transition range. The typical temperature range of 40 K for the measurements is limited at low temperature by the very long times necessary to reach structural equilibrium and at high temperature by the penetration of the rod used to measure sample dilatation. Despite such narrow intervals, the expansivities are determined to better than 3% thanks to the high precision with which length changes are measured. The coefficient of volume thermal expansion (1/V dV/dT) of the fully relaxed liquid just above the glass transition is found to decrease linearly from diopside composition (139 ± 4 × 10⁻⁶ K⁻¹) to anorthite composition (59 ± 2 × 10⁻⁶ K⁻¹). These values are greater than those determined for the same liquids at superliquidus temperatures, demonstrating that expansivities of silicate melts may decrease markedly with increasing temperature. A predictive model based upon partial molar volumes which vary as a linear function of the logarithm of temperature is proposed. Received: 25 February 2000 / Accepted: 29 May 2000     

Distribution of tetrahedral Al and Si in coexisting biotite and Ca-amphibole

The distribution of tetrahedrally coordinated ions between coexisting biotite and Ca-amphibole is found to follow a regular trend of approximately Al _Bi ^IV =1.95+0.35 Al _A ^IV . It is concluded that deviations permit the recognition of disequilibrium mineral associations. The potential influence of crystal-chemical factors is considered.     

Systematics of calcium partitioning between olivine and silicate melt: implications for melt structure and calcium content of magmatic olivines

A systematic characterization of the chemical factors that control calcium partitioning between olivine and melt in a magmatic environment was undertaken using experiments performed on compositionally simple systems (CaO-MgO-SiO₂, CaO-MgO-Al₂O₃-SiO₂, CaO-MgO-Al₂O₃-SiO₂-Cr₂O₃, CaO-MgO-Al₂O₃-SiO₂-TiO₂, CaO-MgO-Al₂O₃-SiO₂-Na₂O, CaO-MgO-Al₂O₃-SiO₂-FeO, CaO-MgO-Al₂O₃-SiO₂-FeO-Na₂O) over a wide range of temperature (1050–1530 °C) at one bar pressure. The calcium concentration of olivines is shown to be dependent not only on the forsterite content of the olivine but to a large extent on melt composition. For a fixed CaO content of the melt, these results show that the CaO concentration of olivine is strongly sensitive to the amount of alumina, alkali and ferrous iron present in the coexisting melt. Oxygen fugacity and temperature are not found directly to affect Ca partitioning. It is thus proposed that the systematic variations of the calcium content of olivine may be used as an “in-situ chemical potentiometer” of the lime activity of the melt. Based upon these data in synthetic systems, an empirical model describing Ca partitioning between olivine and melt is developed. When applied to natural olivines this model reproduces their Ca content, where melt composition is known, to within ±10% relative. The model may therefore be used to predict changes in melt composition during olivine crystallization and/or to assess whether an olivine is in equilibrium with its host magma. Finally, the wide range of Ca partitioning observed at fixed crystal composition confirms that minor element partitioning between crystal and melt cannot be predicted from the physical characteristics of the crystal alone, and that the non-ideality of the melt has to be taken into account. Received: 12 June 1998 / Accepted: 1 February 1999     

Computational study of tetrahedral Al–Si and octahedral Al–Mg ordering in phengite

 As part of a wider study of the nature and origins of cation order–disorder in micas, a variety of computational techniques have been used to investigate the nature of tetrahedral and octahedral ordering in phengite, K₂ ^[6](Al₃Mg)^[4](Si₇Al)O₂₀(OH)₄. Values of the atomic exchange interaction parameters J _n used to model the energies of order–disorder were calculated. Both tetrahedral Al–Si and octahedral Al–Mg ordering were studied and hence three types of interaction parameter were necessary: for T–T, O–O and T–O interactions (where T denotes tetrahedral sites and O denotes octahedral sites). Values for the T–T and O–O interactions were taken from results on other systems, whilst we calculated new values for the T–O interactions. We have demonstrated that modelling the octahedral and tetrahedral sheets alone and independently produces different results from modelling a whole T–O–T layer, hence justifying the inclusion of the T–O interactions. Simulations of a whole T–O–T layer of phengite indicated the presence of short-range order, but no long-range order was observed. Received: 8 August 2002 / Accepted: 14 February 2003 Acknowledgements The authors are grateful to EPSRC (EJP) and the Royal Society (CIS) for financial support. Monte Carlo simulations were performed on the Mineral Physics Group's Beowulf cluster and the University of Cambridge's High Performance Computing Facility.     

Vanadium partitioning between orthopyroxene,spinel and silicate melt and the redox states of mantle source regions for primary magmas

The partitioning of V between orthopyroxene-liquid and spinel-liquid has been investigated in synthetic and natural mafic and ultramafic compositions as a function of temperature and oxygen fugacity (fO₂) at 100 kPa and in one experiment at higher pressure. The purpose of the experiments was to understand redox relationships for V in silicate melts with a view to deriving an empirical oxygen barometer for geochemically altered mafic and ultramafic magmas in the geologic record. Partitioning data for both orthopyroxene-liquid and spinel-liquid show profound changes at an fO₂ approximately 3 orders of magnitude below the nickel-nickel oxide (NNO) buffer, suggesting changes in the dominant valence state of V in silicate liquids from V³⁺ to V⁴⁺, near this fO₂.The results of the experiments on orthopyroxene-liquid are combined with published data for olivine-liquid and are applied to suites of mafic and ultramafic magmas that have equilibrated with a harzburgite residue in the mantle. The results show that Archean alumina-undepleted komatiites could have formed at fairly high oxygen fugacities, near ΔNNO ∼ 0, somewhat higher than Cretaceous komatiites and related picrites in the Caribbean region (between ΔNNO ∼ −1 to −3), and plume-related picrites from West Greenland (ΔNNO ∼ − 3). Picrites and boninites from convergent margins record the highest fO₂’s by this method, (ΔNNO = +1 to +2), consistent with other petrological estimates of their redox states. The approach developed in this study can thus provide estimates for the redox states of altered, mantle-derived magmas in the geological record, to which more conventional methods of oxygen barometry cannot be applied.     

Effect of water on the fluorine and chlorine partitioning behavior between olivine and silicate melt

Halogens show a range from moderate (F) to highly (Cl, Br, I) volatile and incompatible behavior, which makes them excellent tracers for volatile transport processes in the Earth’s mantle. Experimentally determined fluorine and chlorine partitioning data between mantle minerals and silicate melt enable us to estimate Mid Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB) source region concentrations for these elements. This study investigates the effect of varying small amounts of water on the fluorine and chlorine partitioning behavior at 1280?°C and 0.3 GPa between olivine and silicate melt in the Fe-free CMAS+F–Cl–Br–I–H₂O model system. Results show that, within the uncertainty of the analyses, water has no effect on the chlorine partitioning behavior for bulk water contents ranging from 0.03 (2) wt% H₂O (D_Cl ^ol/melt = 1.6?±?0.9 × 10^?4) to 0.33 (6) wt% H₂O (D_Cl ^ol/melt = 2.2?±?1.1 × 10^?4). Consequently, with the effect of pressure being negligible in the uppermost mantle (Joachim et al. Chem Geol 416:65–78, 2015), temperature is the only parameter that needs to be considered for the determination of chlorine partition coefficients between olivine and melt at least in the simplified iron-free CMAS+F–Cl–Br–I–H₂O system. In contrast, the fluorine partition coefficient increases linearly in this range and may be described at 1280?°C and 0.3 GPa with (R ²?=?0.99): \(D_{F}^{\text{ol/melt}}\ =\ 3.6\pm 0.4\ \times \ {{10}^{-3}}\ \times \ {{X}_{{{\text{H}}_{\text{2}}}\text{O}}}\left( \text{wt }\!\!\%\!\!\text{ } \right)\ +\ 6\ \pm \ 0.4\times \,{{10}^{-4}}\). The observed fluorine partitioning behavior supports the theory suggested by Crépisson et al. (Earth Planet Sci Lett 390:287–295, 2014) that fluorine and water are incorporated as clumped OH/F defects in the olivine structure. Results of this study further suggest that fluorine concentration estimates in OIB source regions are at least 10% lower than previously expected (Joachim et al. Chem Geol 416:65–78, 2015), implying that consideration of the effect of water on the fluorine partitioning behavior between Earth’s mantle minerals and silicate melt is vital for a correct estimation of fluorine abundances in OIB source regions. Estimates for MORB source fluorine concentrations as well as chlorine abundances in both mantle source regions are within uncertainty not affected by the presence of water.     

Determination of fluid/melt partition coefficients by LA-ICPMS analysis of co-existing fluid and silicate melt inclusions: Controls on element partitioning

Analyses of co-existing silicate melt and fluid inclusions, entrapped in quartz crystals in volatile saturated magmatic systems, allowed direct quantitative determination of fluid/melt partition coefficients. Investigations of various granitic systems (peralkaline to peraluminous in composition, log fO₂ = NNO−1.7 to NNO+4.5) exsolving fluids with various chlorinities (1-14 mol/kg) allowed us to assess the effect of these variables on the fluid/melt partition coefficients (D). Partition coefficients for Pb, Zn, Ag and Fe show a nearly linear increase with the chlorinity of these fluid (D_Pb ∼ 6 ∗ m_Cl, D_Zn ∼ 8 ∗ m_Cl, D_Ag ∼ 4 ∗ m_Cl, D_Fe ∼ 1.4 ∗ m_Cl, where m_Cl is the molinity of Cl). This suggests that these metals are dissolved primarily as Cl-complexes and neither oxygen fugacity nor the composition of the melt affects significantly their fluid/melt partitioning. By contrast, partition coefficients for Mo, B, As, Sb and Bi are highest in low salinity (1-2 mol/kg Cl) fluids with maximum values of D_Mo ∼ 20, D_B ∼ 15, D_As ∼ 13, D_Sb ∼ 8, D_Bi ∼ 15 indicating dissolution as non-chloride (e.g., hydroxy) complexes. Fluid/melt partition coefficients of copper are highly variable, but highest between vapor like fluids and silicate melt (D_Cu ? 2700), indicating an important role for ligands other than Cl. Partition coefficients for W generally increase with increasing chlorinity, but are exceptionally low in some of the studied brines which may indicate an effect of other parameters. Fluid/melt partition coefficients of Sn show a high variability but likely increase with the chlorinity of the fluid (D_Sn = 0.3-42, D_W = 0.8-60), and decrease with decreasing oxygen fugacity or melt peraluminosity.