Rutile Solubility Research Articles

Rutile solubility and TiO2 activity in silicate melts: An experimental study

Sixty-eight new experiments on rutile solubility in synthetic silicate melts of a wide compositional range have been conducted at ambient pressure (P) at temperature (T) from 1230 to 1500 °C. The results support the previous findings, namely: (1) at fixed T addition of silica and alumina results in essential decrease of rutile solubility; (2) at fixed composition increasing T results in essential increase in rutile solubility. Based on the new and literature experimental data (350 data points) new equations describing rutile solubility in silicate melts as a function of T, P, and melt composition, both empirical and consistent with the thermodynamic model of MELTS (Ghiorso and Sack, 1995), are derived. The new equation predicts log XTiO2 (TiO2 mole fraction) in the melts with a standard error of 0.066, which is equivalent to ca 15% error, in a wide range of melts major element composition and the XTiO2 value in the range from 0.0008 to 0.39 (TiO2 from 0.1 to 44.8 wt%) covered by the experiments. The equations are applied to calculate the TiO2 activity values relative to rutile saturation (aTiO2, m/rut) in a variety of well documented natural magmatic rocks. The resulting aTiO2, m/rut values for different tested samples vary from 0.1 to close to 1. Simultaneously solving the new equations and the equation for Ti-in-Zircon geothermometer (Ferry and Watson, 2007) gives more reliable T estimates than those based on the assumption of constant aTiO2, m/rut = 0.6 or 1.

Temperature-dependent Rutile Solubility in Garnet and Clinopyroxene from Mantle Eclogite: Implications for Continental Crust Formation and V-based Oxybarometry

Abstract Despite its accessory mineral status in metabasaltic rocks, rutile controls the whole-rock Ti, Nb and Ta budget. These are key elements used to trace fluid- and melt-mediated mass transfer across the mantle–crust boundary. Rutile also contains significant amounts of the redox-sensitive element V, which is increasingly used to estimate oxygen fugacity. Kimberlite-borne mantle eclogite xenoliths, which are frequently rutile-bearing, have been interpreted as residues from the extraction of silicic partial melt similar in composition to the average continental crust. Published mineral compositions for eclogite xenoliths from various cratons combined with geothermobarometrical calculations show that TiO2 contents in garnet and clinopyroxene increase with increasing temperature of last residence in the lithospheric mantle, whereas apparent clinopyroxene–garnet distribution coefficients decrease. This implies that (1) increasing TiO2 contents in eclogitic garnet or clinopyroxene are not a signature of increasing metasomatism with depth, (2) whole-rock eclogites reconstructed without rutile will increasingly underestimate TiO2, Nb and Ta contents with decreasing temperature, and (3) low-temperature eclogites are more likely to contain free rutile. Only about a third of the ∼250 samples considered here would have whole-rock TiO2 contents (reconstructed with calculated rutile modes) required for rutile saturation during subduction and partial melting. If there is a role for subducting oceanic crust now sampled as mantle eclogite, the characteristic Ti–Nb–Ta depletion in continental crust may require fluid-dominated processes, where these elements are not efficiently mobilised. In garnet, Ti uptake on the octahedral site is accommodated primarily by coupled substitution with Na and subordinately with a divalent metal cation, and there is no evidence for substitution on the tetrahedral site. For samples equilibrated to the conductive geotherm, Ti in addition to Na enrichment may be indicative of equilibration in the diamond stability field. The jadeite component in clinopyroxene as a function of temperature is a good indicator of the geotherm to which the various samples equilibrated, and can be used to reveal samples within each suite that have been affected by isobaric heating. The distribution of V in eclogitic garnet, clinopyroxene and rutile is affected by bulk composition, temperature and oxygen fugacity. In carefully vetted, low-temperature samples with TiO2 contents >0·8 wt%, V-based oxybarometry may monitor redox conditions prevailing during metamorphism of oceanic crust or, at lower TiO2, during (secular) cooling-related exsolution of rutile from garnet or clinopyroxene, whereas in higher-temperature ilmenite-bearing samples metasomatic conditions may be recorded.

Ti-in-quartz: Evaluating the role of kinetics in high temperature crystal growth experiments

We present results from 25 hydrothermal quartz growth experiments, all conducted at 800 °C and 1 kbar but with varying starting materials and run times, to address discrepancies between calibrations of the titanium-in-quartz (TitaniQ) thermobarometer. In our experiments, a gold capsule is loaded with silica glass, water, and either rutile or anatase as the TiO2 source. In most experiments, there is also a large quartz seed crystal contained in an open inner capsule. The use of rutile versus anatase has a significant influence on the (re)crystallization pathways of the SiO2 and TiO2 components. When rutile is used, quartz overgrowths have abundant open cavities and complex zonations. The rutile does not completely dissolve because rutile is the stable TiO2 polymorph, and yet, new rutile forms at the quartz seed-overgrowth interface and on the outer surface of quartz crystals. This suggests crystallization of quartz near Ωrut ∼ 1, but wide-ranging Ti concentrations and zonations in quartz are indicative of kinetic effects. When powdered anatase is used, the quartz overgrowths look markedly different, lacking the open cavities and instead exhibiting step edges and terraces. The Ti concentrations in quartz from these experiments are also wide-ranging but reach larger values.Our results span the range of previous calibrations and indicate that Ti concentrations in quartz are sensitive to the TiO2/SiO2 ratio of the fluid as opposed to the absolute concentration (or activity) of dissolved TiO2. We present a kinetic model for quartz and rutile growth from a fluid where the input parameters are the initial degrees of supersaturation with respect to quartz and rutile, the total reactive surface area, and rate constants that link the degree of supersaturation to net precipitation rates. The model can explain many of the salient features of our experimental results, as well as those from previous studies, but requires that the rate constant multiplied by the reactive surface area for rutile is less than that of quartz, and that rutile solubility depends on the SiO2 concentration of the fluid, as documented in the recent literature. Complete quartz-rutile equilibrium may not have been established in any of the experimental studies, but low-pressure experiments with slowly grown quartz seem to be more reliable than extrapolations from high-pressure experiments for thermobarometry of shallow natural systems.

TiO2 Solubility and Nb and Ta Partitioning in Rutile‐Silica‐Rich Supercritical Fluid Systems: Implications for Subduction Zone Processes

AbstractTo understand Ti, Nb, and Ta mobility in supercritical fluids and Nb/Ta fractionation in subduction zones, we conducted piston cylinder experiments to determine rutile (TiO2) solubility and Nb and Ta partition coefficients (Dru/sf Nb and Dru/sf Ta) in rutile (ru)‐silica‐rich supercritical fluid (sf) systems at 1.5–2.5 GPa and 920–1150 °C over variable fluid chemistries including solute (SiO2 ± albite component), H2O, Cl, and F contents. Under the investigated conditions, TiO2 solubility in the supercritical fluids varies from 761 ± 107 to 9795 ± 448 ppm; Dru/sf Nb and Dru/sf Ta vary from 12 ± 1 to 208 ± 30 and 34 ± 5 to 2464 ± 140, respectively. Higher solute, Cl, and F contents in the systems and higher temperatures result in higher TiO2 solubilities and lower Dru/sf Nb and Dru/sf Ta, and thus, fluid chemistry and temperature exert main controls on Ti, Nb, and Ta mobility. In all cases, Dru/sf Nb/Dru/sf Ta < 0.70, suggesting that supercritical fluids released from subducting slabs are higher in Nb/Ta relative to their protoliths. Therefore, such fluids are expected to result in higher Nb/Ta ratios in mantle wedges and arc magmas. Although Ti, Nb, and Ta could be mobile in supercritical fluids, Nb/Ta ratios in primitive arc basalts compared to those in mid‐ocean ridge basalts show that only ~10% of arc basalts are possibly disturbed by solute‐rich supercritical fluids during their generation. Therefore, if the fluids released from subducting slabs are dominantly supercritical, then most of them must be stable only in narrow P‐T spaces and hence too short‐lived to ascend very far into mantle wedges.

Rutile solubility in NaF–NaCl–KCl-bearing aqueous fluids at 0.5–2.79 GPa and 250–650 °C

The complex nature of trace element mobility in subduction zone environments is thought to be primarily controlled by fluid–rock interactions, episodic behavior of fluids released, mineral assemblages, and element partitioning during phase transformations and mineral breakdown throughout the transition from hydrated basalt to blueschist to eclogite. Quantitative data that constrain the partitioning of trace elements between fluid(s) and mineral(s) are required in order to model trace element mobility during prograde and retrograde metamorphic fluid evolution in subduction environments. The stability of rutile has been proposed to control the mobility of HFSE during subduction, accounting for the observed depletion of Nb and Ta in arc magmas. Recent experimental studies demonstrate that the solubility of rutile in aqueous fluids at temperatures >700°C and pressures <2GPa increases by several orders of magnitude relative to pure H2O as the concentrations of ligands (e.g., F and Cl) in the fluid increase. Considering that prograde devolatilization in arcs begins at ∼300°C, there is a need for quantitative constraints on rutile solubility and the partitioning of HFSE between rutile and aqueous fluid over a wider range of temperature and pressure than is currently available. In this study, new experimental data are presented that quantify the solubility of rutile in aqueous fluids from 0.5 to 2.79GPa and 250 to 650°C. Rutile solubility was determined by using synchrotron X-ray fluorescence to measure the concentration of Zr in an aqueous fluid saturated with a Zr-bearing rutile crystal within a hydrothermal diamond anvil cell. At the PT conditions of the experiments, published diffusion data indicate that Zr is effectively immobile (logDZr ∼10−25m2/s at 650°C and ∼10−30m2/s at 250°C) with diffusion length-scales of <0.2μm in rutile for our run durations (<10h). Hence, the Zr/Ti ratio of the starting rutile, which was quantified, does not change during the experiment, and the measured concentration of Zr in the fluid was used to calculate the concentration of Ti (i.e., the solubility of rutile) in the fluid. The salts NaF, NaCl, and KCl were systematically added to the aqueous fluid, and the relative effects of fluid composition, pressure, and temperature on rutile solubility were quantified. The results indicate that fluid composition exerts the greatest control on rutile solubility in aqueous fluid, consistent with previous studies, and that increasing temperature has a positive, albeit less pronounced, effect. The solubility of Zr-rutile in aqueous fluid increases with the addition of halides in the following order: 2wt% NaF<30wt% KCl<30wt% NaCl<3wt% NaF<(10wt% NaCl+2wt% NaF)<4wt% NaF. The solubility of rutile in the fluid increases with the 2nd to 3rd power of the Cl− concentration, and the 3rd to 4th power of the F− concentration. These new data are consistent with observations from field studies of exhumed terranes that indicate that rutile is soluble in complex aqueous fluids, and that fluid composition is the primary control on rutile solubility and HFSE mobility.

Rutile solubility in hydrous rhyolite melts at 750–900 °C and 2 kbar, with application to titanium-in-quartz (TitaniQ) thermobarometry

The solubility of rutile in water-saturated haplogranite melts with molar Al/(Na+K)-ratios ranging from 0.84 to 1.25 was determined at 750–900°C and 2kbar in cold-seal pressure vessel experiments. Due to the low diffusivity of Ti at these conditions a new method was developed to determine TiO2 solubility. In this method, glasses with TiO2 gradients were used as starting material, and after the experiments the TiO2 content was measured at the contact between rutile-bearing and rutile-free glass. The glasses were either directly equilibrated at the desired P–T conditions (i.e., crystallization experiments), or first treated at 50–150°C lower temperatures and then subjected to the desired conditions (i.e., dissolution experiments). The results obtained in crystallization and dissolution experiments agree well with each other, suggesting that equilibrium was attained.Rutile solubility in peralkaline melts strongly increases with temperature and the amount of excess alkalies according to the relation: logTiO2(wt%)=(1.8∗ΔANK-0.53)∗10,000/T-(12.8∗ΔANK-4.3)in which ΔANK is the deviation of the molar Al/(Na+K)-ratio from unity and T is given in Kelvin. Excess alumina does not seem to promote TiO2 solubility. For natural melt compositions it was found that if Ca, Mg and Fe are assumed to be 10 times less effective in promoting TiO2 solubility than excess Na and K, good fits to previous solubility data in silicic (⩾70wt% SiO2) melts at upper crustal pressures (⩽10kbar) are obtained.Application of this extended TiO2 solubility model to natural melt inclusions in quartz phenocrysts from five silicic volcanic systems returns TiO2 activities that are similar to those calculated with a previous experiment-based model and to those calculated from pairs of coexisting Fe–Ti-oxides, but are up to 2.9 times higher than those calculated with MELTS and rhyolite-MELTS. Pressures calculated from Ti concentrations in the host quartz using the most recent calibration of the TitaniQ thermobarometer are in good agreement with independent pressure estimates, suggesting that at upper crustal conditions this calibration is valid.

Rutile solubility in supercritical NaAlSi 3O 8–H 2O fluids

Rutile solubility was measured in the supercritical NaAlSi 3O 8–H 2O system using hydrothermal piston–cylinder methods at 900 °C. Solubility in intermediate albite–H 2O fluids varies as a function of both pressure ( P) and fluid composition ( X). Rutile solubility increases with increased albite content of the fluid, and is on the order of ~1000 to ~5000 ppm for all P–X conditions investigated here. The relationship between solubility and pressure is complex, and solubility decreases with increasing pressure for intermediate fluid compositions (35–75 wt.% H 2O). Ti contents of glasses indicate that, with rising pressure at 900 °C, the liquid–vapor miscibility gap in the albite–H 2O system closes at 0.9–1.0 GPa. A single supercritical fluid is stable above this pressure, to 2.0–2.2 GPa when further compression induces a return to a subcritical state. Supercritical fluids could be plausible transfer agents for Ti and other HFSE in high- P metasomatic environments, but the results show that their stability may be more limited than previously recognized.

Extremely high solubility of rutile in chloride and fluoride-bearing metamorphic fluids: An experimental investigation

Rutile (TiO 2 ) is an important high field strength element sequestering phase, and thus understanding its solubility in metamorphic fluids is important if we are to understand terrestrial trace element cycling. Titanium is often assumed to be fluid immobile, and previous experimental data suggest that rutile has very low solubility in H 2 O. However, fluids in high-temperature metamorphic rocks are known to be more complex in composition. The solubility of rutile in chloride and fluoride-rich hydrous fluids was determined experimentally at 0.5 GPa and high temperatures. The experimental results indicate much higher rutile solubility in brines compared to pure H 2 O. In chloride brines solubility was 2–4 times higher than in H 2 O, and in fluoride brines solubility was elevated 20–100 times. The results imply that titanium may be much more mobile in metamorphic fluids than previously anticipated, as solubility is critically dependent on fluid chemistry. We also present a possible mechanism for rutile precipitation in metamorphic veins due to a change in fluid chemistry, following the precipitation of halogen-rich minerals such as apatite.

Rutile solubility in H 2O, H 2O–SiO 2, and H 2O–NaAlSi 3O 8 fluids at 0.7–2.0 GPa and 700–1000 °C: Implications for mobility of nominally insoluble elements

The solubility of rutile was measured in H 2O, H 2O–SiO 2 and H 2O–NaAlSi 3O 8 fluids at 700–1000 °C, 0.7–2.0 GPa, in a piston-cylinder apparatus. Solubility was determined by weight loss using a double-capsule method. Rutile solubility in pure H 2O shows isothermal increase with pressure ( P), isobaric increase with temperature ( T), and is low at all conditions investigated (6–118 ppm Ti). Rutile solubility in H 2O is given by log c Ti ° = 6.173 − 5425/ T + 178.4 P/ T, where c Ti ° is Ti concentration in ppm, T is in K, and P in GPa. This leads to thermodynamic properties of the reaction rutile = TiO 2,aq of Δ S r ° = 28.6 J/mol K, Δ H r ° = 104 kJ/mol, and Δ V r ° = − 3.4 cm 3/mol. At 800 °C and 1 GPa, addition of SiO 2 (up to quartz saturation) did not change rutile solubility relative to that in pure H 2O. Determination of rutile solubility in H 2O–NaAlSi 3O 8 fluids was complicated by incongruent dissolution of albite to paragonite or corundum + fluid; however, fluid compositions could be estimated within narrow limits using a mass-balance scheme. The solubility of rutile increases linearly with dissolved Na–Al silicate at fixed P and T, as described by c Ti = c Ti ° + Bw s where c Ti is ppm Ti, w s is wt.% dissolved silicate and B is given by log B 6.512 − 1.665 P − 6224/ T + 2215 P/ T, with T and P again in K and GPa. The results help explain discrepancies among previous studies of rutile solubility in H 2O at similar P and T. The new data agree within error with those of Tropper and Manning [Tropper, P., Manning, C.E., 2005. Very low solubility of rutile in H 2O at high pressure and temperature, and its implications for Ti mobility in subduction zones. American Mineralogist 90, 502–505.], but give lower solubility than earlier piston-cylinder-based determinations due to suppression of new crystal growth in the present experiments. However, the new data yield higher solubilities than are predicted from a hydrothermal diamond-anvil study, probably because of our longer run times and more complete equilibration. Combination of predicted Ti concentrations in melt-saturated H 2O with H 2O-saturated albite melts suggests that the melt–vapor partition coefficient for Ti is constant at 9.5 ± 1.5 from 700 to 900 °C at 1 GPa and rutile saturation, implying that an H 2O-rich magmatic vapor phase can transport significant Ti in mid- to deep-crustal settings. Because crustal and mantle fluids will contain alkalis, Al and Si, the results in H 2O–NaAlSi 3O 8 fluids provide a better foundation for modeling high- P metasomatic processes than pure H 2O values. The strong increase in rutile solubility with dissolved Na–Al silicate suggests that complexing with these constituents promotes Ti mobility and transport during fluid–rock interaction in the lower crust and upper mantle.

Rutile solubility in albite-H 2O and Na 2Si 3O 7-H 2O at high temperatures and pressures by in-situ synchrotron radiation micro-XRF

Rutile is an important mineral host for high-field strength elements, so its solubility in geologic fluids at high pressure and temperature plays an important role in the crustal and mantle processes that control the terrestrial cycling of these elements. However, experimental measurements of rutile solubility are in conflict by a factor of more than 100 at most studied conditions. We performed new measurements of rutile solubility in H 2O-albite and H 2O-Na 2Si 3O 7 (NS3) fluids by in-situ synchrotron radiation X-ray fluorescence spectroscopy using modified Bassett-type hydrothermal diamond-anvil cells. Minimum detection limits were 1.9 and 2.3 ppm Ti by weight for the two cells. Three albite-H 2O experiments at starting bulk compositions of 2.7, 6.7 and 10.3 wt.% albite involved spectral acquisition at rutile saturation in the presence of albite crystals, melt, or a single homogeneous fluid phase; after accounting for the additional phases, corrected fluid compositions were 0.6 to 7.5 wt.% dissolved silicate over the run conditions. At ≤ 2.7 wt.% albite, rutile dissolution rate was slow and steady state was not achieved at 600–800 °C; however, at higher dissolved albite contents, constant solubility with time was observed. Rutile solubilities in the presence of a single fluid phase at 700 °C, 0.79 GPa, 5.4 wt.% albite, and at 800 °C, 1.10 GPa, 6.7 wt.% albite, were 37 ± 2 and 156 ± 6 ppm, respectively. These data agree with results acquired using hydrothermal piston-cylinder methods with long run times and suppression of new crystal growth, but not with data derived from visual observation in hydrothermal diamond-anvil cells. This discrepancy is likely due to lack of equilibrium in the latter approach. Two experiments in 10 and 30 wt.% NS3 at 660–800 °C, 0.5 ± 0.1 GPa, show extreme concentration-dependent enhancement of rutile solubility to ∼ 4500 ppm. The data indicate a strong positive correlation between rutile solubility and Na/Al. Because (Na + K)/Al is likely to be greater than unity in aqueous fluids at high pressure and temperature due to incongruent dissolution of albite and micas, the increase in rutile solubility along the albite-NS3 join points to the possibility of significant Ti transport by silicate-bearing aqueous fluids in the lower crust and upper mantle.

A model for rutile saturation in silicate melts with applications to eclogite partial melting in subduction zones and mantle plumes

This experimental study examines the solubility of rutile in silicate melts and presents a model for rutile saturation as a function of temperature, pressure and melt composition. Rutile saturation experiments were carried out in the system SiO 2–TiO 2–Al 2O 3–MgO–CaO–Na 2O–K 2O at 1 bar to 35 kbar and 1150 to 1450 °C on model rhyodacite (∼ 69 wt.% SiO 2) and haplobasalt (∼ 54 wt.% SiO 2) base melt compositions. At rutile saturation, the concentration of TiO 2 in the model rhyodacite base melt increases at 1 bar from 3.27 ± 0.03 wt.% at 1150 °C to 13.88 ± 0.04 wt.% at 1450 °C. At 1350 °C it decreases from 8.89 ± 0.08 wt.% at 1 bar to 2.06 ± 0.13 wt.% at 35 kbar. Rutile solubility is significantly higher in the haplobasalt base melt at a given pressure and temperature. The concentration of TiO 2 in rutile-saturated haplobasalt increases at 1 bar from 20.9 ± 0.3 wt.% at 1300 °C to 39.0 ± 0.3 wt.% at 1450 °C. At 1350 °C it decreases from 25.8 ± 0.3 wt.% at 1 bar to 15.67 ± 0.16 wt.% at 15 kbar. Results from these experiments were combined with data from the literature to formulate a model for rutile saturation in silicate melts. Application of this model to partial melting of MORB-type eclogite indicates that beneath volcanic arcs low-degree, hydrous partial melts of rutile-bearing subducted oceanic crust contain only ∼ 600 ppm TiO 2. Therefore, rutile will remain as a residual phase in the eclogite and the amount of TiO 2 that will be transferred to the mantle wedge will be small because so little TiO 2 is dissolved in the melt. Partial melting of recycled oceanic crust in an upwelling mantle plume can exhaust rutile from the residual solid at moderate degrees of partial melting (∼ 22%). The retention of rutile in subducted oceanic lithosphere during dehydration and/or partial melting, combined with exhaustion of rutile during partial melting of eclogite in mantle plumes suggests that HFSE enrichments in recycled crust that were established during subduction may be detectible in OIB.

Rutile saturation in hydrous siliceous melts and its bearing on Ti-thermometry of quartz and zircon

The TiO 2 solubility of rutile-saturated hydrous siliceous melts has been investigated at P = 1 GPa and T = 650–1000 °C for several representative felsic compositions. The dissolution of a rutile crystal into a TiO 2 undersaturated melt provides information on both TiO 2 solubility and Ti diffusion. Results of this study confirm that TiO 2 solubility is strongly dependent on both temperature and melt composition, but not on the amount of H 2O present. For a given T, TiO 2 content decreases as the melts become more felsic. The solubility of TiO 2 is given by: log ⁡ ( Ti , ppm ) = 7.95 − 5305 T + 0.124 FM where T is in K and FM is a melt composition parameter, FM = 1 Si . Na + K + 2 ( Ca + Mg + Fe ) Al in which the chemical symbols represent cation fractions. Results of dissolution experiments yield an activation energy ( E) for Ti transport in a hydrous felsic melt of 186 ± 27 kJ/mol and a frequency factor, D o, of 3.6 ± 1.2 m 2/s. These results suggest an activation energy similar to that established for Zr diffusion in similar melts, but with Ti diffusion rates 2–3 orders of magnitude faster. Both TiO 2 solubility and Ti diffusion have important applications in geothermometry, particularly in light of new thermometers calibrated for the incorporation of Ti into quartz and zircon. Rutile saturation is improbable in the types of melts where these thermometers are most likely to be useful, and therefore it is important that rutile solubility behavior in these melts to be well-constrained. TiO 2 activities in silicic melts at typical magmatic temperatures are generally 0.6 or higher, implying that Ti thermometry of out-of-context zircons will rarely underestimate zircon crystallization temperature by more than ∼ 50 °C.

Solubility of rutile in subduction zone fluids, as determined by experiments in the hydrothermal diamond anvil cell

The solubility of rutile in H2O (±NaCl, albite)-fluids was measured by direct observation of dissolving mineral grains within the hydrothermal diamond anvil cell. This new approach for studying mineral solubilities at high p/T has the advantage that the sample eventually gets completely dissolved and can be visually observed during this process, which eliminates the risk of overestimating true solubilities due to dissolution and reprecipitation during the run. Our measured rutile solubilities in pure H2O are up to 1000 times lower than those reported in previous studies, ranging from 8 ppm to 26 ppm at 850–1000 °C/1.2–2.3 GPa. Nonlinear least square fitting of the data set resulted in the following equation for rutile solubility in pure water:cTiO2,fluid=A·exp(−Δ⁢H°R⁢T)·exp(−Δ⁢V°R⁢T·(p−1bar)),where A=8728.7 ppm, ΔH°=74,671 J·mol−1, ΔV°=−0.61275 J·mol−1 K−1, and p, T are given in bar and Kelvin, respectively. The presence of 9 wt.% dissolved albite leads to an increase in the TiO2-solubility by a factor of 10, whereas addition of 15 wt.% NaCl results in a 2–3 times higher solubility than in pure water. Our results suggest that the solubility of TiO2 (and, by inference, high field strength elements in general) in common mantle or subduction zone fluids is very low. In particular, the solubility of TiO2 in these fluids is 2 to 3 orders of magnitude lower than the solubility of TiO2 in clinopyroxene, implying a fluid/clinopyroxene partition coefficient of Ti in the order of 10−2 to 10−3. This suggests that the presence of accessory phases such as rutile is not required to explain the high field strength element depletion in arc magmas.

Liquidus karrooite stability and composition at reducing conditions

We have determined the stability of rutile and karrooite on the liquidus of pseudobinary silicate melts of anorthite–diopside eutectic composition with, in addition, of up to 32 wt% TiO2 at one atm total pressure and at wide range of oxygen fugacities. At 1,300°C and at an fO2 below 10−11.2 atm rutile (TiO2) is replaced as liquidus phase by the pseudobrookite-type MgTi2O5–Ti3O5 solid solution with some Al in the crystal structure. The composition of karrooite was found to be strongly dependent on oxygen fugacity. Crystalline phases were identified by transmission electron microscopy (TEM). The results obtained in this study are relevant for understanding the chemistry of lunar armalcolites. Using “excess” rutile solubility at reducing conditions, we estimated the Ti4+ /Ti3+ ratio in silicate melts at 1,300°C as function of fO2.

Ti(IV) hydrolysis constants derived from rutile solubility measurements made from 100 to 300°C

Using a Au–Ir hydrothermal reaction cell and dilute buffer solutions to control pH, the isobaric solubility of rutile (TiO 2) was measured over a broad pH range from pH 1 to 13 between 100 and 300°C. The solubility data were regressed to derive the cumulative and stepwise hydrolysis constants for Ti(IV) in aqueous solution. The data were accurately modeled using the hydrolysis species: Ti(OH) + 3, Ti(OH) 4(aq) and Ti(OH) − 5. The following cumulative ( K s3, K s4, K s5) hydrolysis constants (log K and error, reactions written with respect to the solid oxide) and stepwise ( K 34, K 45) hydrolysis constants (log K and error) were determined by regression: 100°C 150°C 200°C 250°C 300°C log K s3 −5.44(0.032) −5.42(0.106) −5.28(0.055) −5.03(0.046) −4.61(0.112) log K s4 −7.72(0.150) −7.66(0.202) −7.57(0.153) −7.93(0.202) −7.92(1.11) log K s5 −17.8(0.068) −17.1(0.138) −16.4(0.132) −15.9(0.018) −15.4(0.057) log K 34 −2.28(0.153) −2.24(0.228) −2.29(0.162) −2.90(0.207) −3.31(1.12) log K 45 −10.1(0.165) −9.42(0.245) −8.86(0.202) −7.95(0.203) −7.43(1.11) These data are required to model the chemical behavior of Ti(IV) in aqueous solutions and provide a baseline for the Ti-solubility-limited dissolution of titanates.

Dissolution of primary minerals of basalt in natural waters: I. Calculation of mineral solubilities from 0°C to 350°C

The solubilities of forsterite, fayalite, enstatite, ferrosilite, hedenbergite, diopside, anorthite, high-albite, magnetite, hematite, ulvöspinel, ilmenite, F-apatite and OH-apatite and olivine, plagioclase, orthopyroxene, clinopyroxene and Fe–Ti oxide solid solutions of fixed composition were calculated in the temperature range 0–350°C at saturated water vapour pressure. The thermodynamic database used for end-member minerals was that of Robie and Hemingway [Robie, R.A., Hemingway, B.S., 1995. Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (105 Pascals) pressures and at higher temperatures. U.S. Geol. Surv. Bull. 2131, 461 pp.] except for plagioclases [Arnórsson, S., Stefánsson, A., 1999. Assessment of feldspar solubility constants in water in the range of 0° to 350°C at vapor saturation pressures. Am. J. Sci. 299, 173–209.] and that of Shock and Helgeson [Shock, E.L., Helgeson, H.C., 1988. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: correlation algorithms for ionic species and equation-of-state predictions to 5 kb and 1000°C. Geochim. Cosmochim. Acta 53, 2009–2036.] and Shock et al. [Shock, E.L., Oelkers, E.H., Johnson, J.W., Sverjensky, D.A., Helgeson, H.C., 1992. Calculation of the thermodynamic properties of aqueous species at high pressures and temperatures: effective electrostatic radii, dissociation constants, and standard partial molal properties to 1000°C and 5 kbar. J. Chem. Soc., Faraday Trans. 88, 803–826.] for most aqueous species. For aqueous Fe(OH) 4 − and Al(OH) 4 −, the thermodynamic properties reported by Diakonov et al. and Pokrovskii and Helgeson [Pokrovskii, V.A., Helgeson, H.C., 1995. Thermodynamic properties of aqueous species and the solubilities of minerals at high pressures and temperatures: the system Al 2O 3–H 2O–NaCl. Am. J. Sci. 295, 1255–1342], respectively, were used. In the present study, the standard partial molal properties and the HKF equation-of-state parameters for aqueous H 4SiO 4 0 were revised to better describe the recent experimental results at low temperatures. For H 4SiO 4 0, the new parameters are also consistent with quartz solubility experiments up to 900°C and 5 kbar. Further, the HKF equation-of-state parameters for aqueous Ti(OH) 4 0 were estimated from rutile solubility [Ziemniak, S.E., Jones, M.E., Combs, K.E.S., 1993. Solubility behaviour of titanium (IV) oxide in alkaline media at elevated temperatures. J. Sol. Chem. 22, 601–623.], which enables the calculations of the solubility of Ti-bearing minerals at elevated temperatures and pressures. Much higher solubilities were found for the silicate minerals below 100°C than previously reported, which is related to higher quartz solubility at low temperature and correspondingly new data on the thermodynamic properties of H 4SiO 4 0. The present results are particularly important for the stabilities of primary basaltic minerals of natural composition under weathering conditions. They are also of importance for the study of equilibrium/dis-equilibrium conditions in active geothermal systems.

The interactions between M +5 cations (Nb +5, Ta +5, or P +5) and anhydrous haplogranite melts

The interactions of high-field-strength cations with haplogranite melts of peraluminous to peralkaline compositions were examined by determining the saturation of rutile as functions of the concentrations of the oxides of the pentavalent cations Nb, Ta, and P. The phase equilibria combined with existing spectroscopic data show that the competition of the high-field-strength cations for the alkali aluminosilicate components varies systematically from peraluminous to peralkaline melts. The data also show that the melt species of Nb +5, Ta +5, and P +5 are very similar. In peraluminous melts, these pentavalent cations have a strong affinity for Al and form very stable M +5AIO 4 complexes; in peralkaline melts, the governing species are characterized by M +5OK bond types, whereas both M +5OAI and M +5OK species exist in the transition between peraluminous to peralkaline melts. The solubility of rutile is significantly affected by the hierarchy of interactions that exist among M +5 cations, Ti +4, and the aluminosilicate network. The composition of rutile in peraluminous melts is a solid solution along the join Ti 2O 4-M +5AIO 4 where M +5 is Ta and/or Nb. Nb +5 and Ta +5 partition strongly into rutile with Al +3 playing the role of a charge balancing cation, Nb +5 + Al +3 = 2Ti +4. Rutile in peralkaline melts contains significantly smaller concentrations of the M +5AlO 4 component, reflecting the much lower activity of M +5AlO 4 species. P +5 does not enter into solid solution in rutile because the ionic radius of P +5 is much smaller than Ti. Nb/Ta in peraluminous pegmatites also enter into solid solution in rutile by a coupled mechanism except that Fe +3 assumes the role of Al +3. The main geochemical conclusions are that Nb, Ta, and P have similar speciations in peraluminous and peralkaline melts. It follows that fractionation of these cations, particularly the perturbation of the Nb/Ta ratios in silicic magmas, can be brought about only by interactions with a crystalline phase. The Nb/Ta ratio can be substantially changed only through the crystallization of Ti-rich crystalline phases and/or Nb/Ta oxides such as columbite.

Microstructure and Microwave Dielectric Properties of Modified Zinc Titanates (II)

Zinc titanates ZnO–TiO 2 (Zn/Ti = 0.67–2.0) and Zn 1−xMg xTiO 3 (x = 0–0.4) were prepared by a conventional mixed-oxide method, and the microstructure and dielectric properties in the microwave range were investigated. In the ZnO–TiO 2 system, zinc orthotitanate, Zn 2TiO 4, had rutile solubility up to 0.33 mol followed by a decrease in the cubic lattice parameters as the amount of rutile increased. The zero τ f was obtained at near Zn/Ti = 1.15 composition. The ϵ r and Q∗f of the composition were 25 and 23,000, respectively. In the Zn 1−xMg xTiO 3 system, the composite structure with zinc orthotitanate and rutile of 1ZnO·1TiO 2 transformed into the (Zn, Mg)TiO 3 hexagonal solid solution at x = 0.3–0.4. However, phase decomposition occurred in this range at a temperature above 1160°C, which induced microcracks and resulted in a decrease in Q factors. A range of dielectric resonators with ϵ r = 20–30, Q = 2,500–13,000 at 10 GHz, and τ f = −70 to +50 ppm/°C can be obtained in this system at a sintering temperature as low as 1100°C.

Ti4+ in silicate melts: Energetics from high-temperature calorimetric studies and implications for melt structure

The drop solution calorimetric method was used to determine the enthalpy of solution of rutile (ΔHsol) in silicate melts. High-temperature (1760 K) in situ calorimetric data show that ΔHsol is a strong function of melt composition. For potassium endmember melts, potassium disilicate, and trisilicate, ΔHsol increases (from 28–48 kJ/mol) as TiO2 concentration increases. For calcium disilicate melts ΔHsol is constant (69 kJ/mol). For mixed potassium-calcium compositions, ΔHsol is more exothermic than for the calcium endmember but remains constant at 43.9 kJ/mol. The enthalpy of solution of rutile at 1760 K and the enthalpy of mixing at 978 K derived from lead borate solution calorimetry for Ti-bearing potassium aluminosilicate glasses have been used to model the homogeneous equilibria among Ti species in the potassium-bearing melts. The energetics of Ti speciation were used to predict quantitatively the excess heat capacity of titanium-bearing silicate melts previously observed by Lange and Navrotsky (1993), suggesting that heat capacities, mixing properties, and rutile solubility are all controlled by the temperature and composition dependence of the same set of homogenous equilibria among titanium species in the melts. Though knowing the exact microscopic nature of these species is not necessary for macroscopic thermodynamic modeling, the model is consistent with a gradual variation with composition and temperature in mid-range order involving five-coordinated titanyl groups and alkali atoms as proposed by Farges et al. (1996a,b,c).

Rutile solubility and mobility in supercritical aqueous fluids

Experimental and thermodynamic data and the apparent immobility of Ti under metamorphic conditions suggest that rutile is very insoluble in aqueous fluids at upper crustal conditions. New solubility measurements at 1.0–2.93 GPa and 800–1200°C show, however, that under certain pressure and temperature conditions rutile is quite soluble in H2O. Solubilities were estimated from the measured weight loss of a single crystal equilibrated with a known mass of fluid in a piston cylinder apparatus. Measured solubilities in H2O range from 0.15 wt% (wt loss crystal/wt fluid) at 2.93 GPa and 1000°C to 1.9% at 1.0 GPa and 1100°C. Solubility increases with increasing temperature and with decreasing pressure in a manner given by the following fit to the experimental data: $$\log _{10} m_{Ti} = - 7049/T - (0.589* P)/T + 5.14$$ wherem Ti is the molality of Ti in the fluid,T is in degress Kelvin andP is in MPa. The effect of fluid composition on rutile solubility was also examined at 1.0 GPa and 1000°C for H2O-CO2, 1m NaCl, and 1m HF fluids. Kesults suggest that solubility depends on the mole fraction of H2O in the fluid but is independent of ionic strength and fluid pH. This behavior implies that Ti dissolves as the neutrally-charged hydrolysis product Ti(OH)4. The free energy of this species was calculated for each set of experimental conditions. TheP-T dependence of rutile solubility suggests that aqueous fluids derived from subducted oceanic lithosphere would dissolve rutile or other Ti-rich minerals from the deepest portion of the mantle wedge and precipitate them at higher levels. Subsequent melting of the base of the mantle wedge would form HFSE-depleted IAB.