Rutile Saturation 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.

Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation

The surfaces of rocky planets are mostly covered by basaltic crust, but Earth is unique in that it also has extensive regions of felsic crust, manifested in the form of continents. Exactly how felsic crust forms when basaltic magmas are the dominant products of melting the mantles of rocky planets is unclear. A fundamental part of the debate is centered on the low Nb/Ta of Earth’s continental crust (11–13) compared to basalts (15–16). Here, we show that during arc magma differentiation, the extent of Nb/Ta fractionation varies with crustal thickness with the lowest Nb/Ta seen in continental arc magmas. Deep arc cumulates (arclogites) are found to have high Nb/Ta (average ~19) due to the presence of high Nb/Ta magmatic rutiles. We show that the crustal thickness control of Nb/Ta can be explained by rutile saturation being favored at higher pressures. Deep-seated magmatic differentiation, such as in continental arcs and other magmatic orogens, is thus necessary for making continents.

The mobility of Nb in rutile-saturated NaCl- and NaF-bearing aqueous fluids from 1–6.5 GPa and 300–800 °C

Rutile (TiO2) is an important host phase for high field strength elements (HFSE) such as Nb in metamorphic and subduction zone environments. The observed depletion of Nb in arc rocks is often explained by the hypothesis that rutile sequesters HFSE in the subducted slab and overlying sediment, and is chemically inert with respect to aqueous fluids evolved during prograde metamorphism in the forearc to subarc environment. However, field observations of exhumed terranes, and experimental studies, indicate that HFSE may be soluble in complex aqueous fluids at high pressure (i.e., >0.5 GPa) and moderate to high temperature (i.e., >300 degrees C). In this study, we investigated experimentally the mobility of Nb in NaCl- and NaF-bearing aqueous fluids in equilibrium with Nb-bearing rutile at pressure-temperature conditions applicable to fluid evolution in arc environments. Niobium concentrations in aqueous fluid at rutile saturation were measured directly by using a hydrothermal diamond-anvil cell (HDAC) and synchrotron X-ray fluorescence (SXRF) at 2.1 to 6.5 GPa and 300-500 degrees C, and indirectly by performing mass loss experiments in a piston-cylinder (PC) apparatus at similar to 1 GPa and 700-800 degrees C. The concentration of Nb in a 10 wt% NaCl aqueous fluid increases from 6 to 11 mu g/g as temperature increases from 300 to 500 degrees C, over a pressure range from 2.1 to 2.8 GPa, consistent with a positive temperature dependence. The concentration of Nb in a 20 wt% NaCl aqueous fluid varies from 55 to 150 mu g/g at 300 to 500 degrees C, over a pressure range from 1.8 to 6.4 GPa; however, there is no discernible temperature or pressure dependence. The Nb concentration in a 4 wt% NaF-bearing aqueous fluid increases from 180 to 910 mu g/g as temperature increases from 300 to 500 degrees C over the pressure range 2.1 to 6.5 GPa. The data for the F-bearing fluid indicate that the Nb content of the fluid exhibits a dependence on temperature between 300 and 500 degrees C at >= 2 GPa, but there is no observed dependence on pressure. Together, the data demonstrate that the hydrothermal mobility of Nb is strongly controlled by the composition of the fluid, consistent with published data for Ti. At all experimental conditions, however, the concentration of Nb in the fluid is always lower than coexisting rutile, consistent with a role for rutile in moderating the Nb budget of arc rocks.

A method for estimating the activity of titania in magmatic liquids from the compositions of coexisting rhombohedral and cubic iron–titanium oxides

A method is described for estimating the activity of titania (TiO2) in a magmatic liquid from the compositions of coexisting cubic oxide (spinel) and rhombohedral oxide (ilmenite). These estimates are derived from the thermodynamic models of Ghiorso and Evans (Am J Sci 308:957–1039, 2008; see also Sack and Ghiorso in Contrib Mineral Petrol 106:474–505, 1991a; Am Mineral 76:827-847, 1991b) and may be computed self consistently along with temperature and oxygen fugacity for an assumed pressure. The method is applied to a collection of 729 naturally occurring oxide pairs from rhyolites and dacites. For this suite of oxides, values of titania activity relative to rutile saturation range from 0.3 to 0.9. Genetically related groups of oxide pairs display activity–temperature trends with negative slopes at higher activities (0.6–0.9) or positive slopes at lower activities (0.3–0.7). Thermodynamic analysis supports the assumption of two-oxide, liquid equilibrium for the former group, but suggests that such an interpretation for oxide sequences with positive activity–temperature trends may be problematic. Application of the estimation method to oxide pairs from the Shiveluch Volcano and the Bishop Tuff reveals that the former are consistent with having equilibrated with known matrix glass compositions, whereas the latter pairs are inconsistent with equilibration with pre-eruptive liquids trapped in quartz inclusions.

The titanium-in-quartz (TitaniQ) thermobarometer: A critical examination and re-calibration

Synthetic quartz was grown in rutile-bearing H2O (±NaCl) fluids at 600–800°C and 1–10kbar by dissolution and reprecipitation of quartz in a small thermal gradient. Rapid attainment of rutile saturation is proved by the formation of rutile crystals at the contact between old quartz substrate and new quartz overgrowth in several experiments. Titanium concentrations in new quartz correlate positively with the concentrations of Li and Al and depend strongly on quartz growth rate, increasing by up to a factor of 2.5 as the growth rate varied from ∼4 to ∼110μm/day. Considering the composition of the most slowly grown quartz samples as most representative we obtain Ti concentrations that are about three times lower than those obtained during earlier calibrations of the TitaniQ thermobarometer. Our data can be fitted by the equation logTi(ppm)=-0.27943·104/T-660.53·(P0.35/T)+5.6459 where T is given in Kelvin and P in kbar.An independent test was made by analyzing igneous quartz from five intrusive and three volcanic magma systems that crystallized at known pressures (0.8–2.7kbar) and temperatures (675–780°C). The activity of TiO2 was constrained from the composition of melt inclusions hosted in the analyzed quartz. Although the results depend on the model chosen to calculate aTiO2, they agree much better with our calibration than with previous TitaniQ calibrations. Crystallization pressures calculated based on our calibration agree within 0.1–1.0kbar with independent pressure estimates, whereas those calculated based on previous calibrations are consistently higher by 3–9kbar. Slight deviations between our calibration and the natural data are either due to uncertainties in the calculation of aTiO2 of the natural melts or due to failure to synthesize structurally perfect quartz even at our slowest growth rates. Our findings imply that TitaniQ should not be applied to quartz grown from hydrothermal fluids, because growth rates in these environments can be very variable. TitaniQ is more likely to work in igneous quartz, although the present models for TiO2 solubility in quartz and silicate melts may still need to be refined.

Experimental constraints on rutile saturation during partial melting of metabasalt at the amphibolite to eclogite transition, with applications to TTG genesis

TiO2 solubility in rutile-saturated felsic melts and coexisting minerals was determined at 1.5-3.5 GPa, 750-1250 °C, and 5-30 wt% H2O. TiO2 solubility in the melt primarily increases with temperature and melt basicity; it increases slightly with water content in the melt, and it decreases with pressure. A general TiO2 solubility model was obtained and is expressed as: ln(TiO2)melt = ln(TiO2)rutile + 1.701 - (9041/T) - 0.173P + 0.348FM + 0.016H2O, where TiO2 and H2O are in wt%, T is in Kelvin, P in GPa, and FM is the melt composition parameter given by FM = (1/Si)·[Na + K + 2(Ca + Fe + Mn + Mg)]/ Al, in which the chemical symbols represent cation fractions. TiO2 solubility in amphibole, garnet, and clinopyroxene also increases with temperature and empirical equations describing this temperature dependence were derived. These data were used to assess the protolith TiO2 content required for rutile saturation during partial melting of hydrous metabasalt at the amphibolite to eclogite transition. The results show that only 0.8-1.0 wt% TiO2 is required for rutile saturation during low-degree (<20%) melting. Rutile is stable up to ~1150 °C with 1.6 wt% TiO2 in the protolith and 30-40% melting for dehydration melting and up to ~1050 °C and 50-60% melting for fluid-present melting. The data also show that 0.7-0.8 wt% TiO2 in the protolith is needed for rutile saturation during subsolidus dehydration. Therefore, nearly all basaltic protoliths in deep-crustal settings and subduction zones will be saturated with rutile during subsolidus dehydration and low-degree melting at hydrous conditions.

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.

Anhydrous Partial Melting Experiments on MORB-like Eclogite: Phase Relations, Phase Compositions and Mineral-Melt Partitioning of Major Elements at 2-3 GPa

We present melt and mineral compositions from nominally anhydrous partial melting experiments at 2---3 GPa on a quartz eclogite composition (G2) similar to average oceanic crust. Near-solidus partial melts at 3 GPa, determined with melt traps of vitreous carbon spheres, have 55---57 wt % SiO2, rather less silica than the dacitic compositions that are generally assumed for near-solidus eclogite partial melts. At 2 GPa, equivalent near-solidus partial melts are less silicic ( 52 wt % SiO2). The 3 GPa near-solidus partial melts (up to melt fractions of 3%) are saturated in rutile and have 5 7---6 7 wt % TiO2. The G2 composition is K2O-poor (0 03 wt%), but a modified composition with 0 26 wt%K2O (G2K) produces dacitic near-solidus melts with 61---64 wt % SiO2. Rutile saturation for G2K extends to higher melt fraction ( 13%) and occurs at lower TiO2 melt contents (3 3 wt %) than for G2. These results can be understood in terms of a simplified thermodynamic model in which alkalis increase the SiO2 content of liquids saturated in quartz, which in turn diminishes the TiO2 concentrations required to maintain rutile saturation. Additionally, the mode of residual garnet and generation of silicic liquids by partial melting of anhydrous eclogite are linked, as garnet is required to mass-balance formation of appreciable SiO2-rich melt. Partitioning of Na between clinopyroxene and melt shows significant increases with pressure, but only modest shifts with changing temperature. In contrast, partitioning of Ti between cpx and melt, as well as between cpx and garnet, shows pronounced dependence on temperature for compositions relevant to anhydrous partial melting of eclogite. Mixtures between partial melts of eclogite and primitive picritic Hawaiian magmas are similar to magnesian, SiO2-rich compositions inferred from melt inclusions from the Koolau volcano. However, in detail, no eclogitic partial melt has been identified that is capable of explaining all of the compositional features of the exotic Koolau component. Based on phase compositions in our experiments, the calculated density of near-solidus eclogite is 3440 kg/m, notably less than commonly assumed. Therefore, the excess temperature required for a plume to support a given proportion of eclogite in the upper mantle may be less than previously assumed.

Rutile saturation in magmas: implications for Ti [sbnd]Nb [sbnd]Ta depletion in island-arc basalts

The TiO 2 contents of rutile-saturated melts ranging from basalt to rhyodacite have been investigated at P = 8–30 kbar and T = 1000–1300°C under hydrous, CO 2-saturated, and volatile-absent conditions. Dissolved TiO 2 is positively correlated with T and not strongly dependent on P total. For fixed P and T, TiO 2 content decreases markedly as the melts become more felsic. The distribution of TiO 2 between rutile and liquid, expressed as a wt.% concentration ratio, D (rut/liq), is given by: In D = −3.16 + (9373T) + 0.026P − 0.152FM where T is in Kelvins, P in kbar and FM is a melt composition parameter, FM = [Na+K+ 2(Ca+Fe+Mg)]/Al· 1/Si in which the chemical symbols represent cation fractions. The first term expresses the competition of aluminate and titanate anions for charge-compensating cations, and the second term expresses the inverse dependence of dissolved TiO 2 on SiO 2 content. There is no apparent dependence of rutile solubility on water content. For ranges of probable solidus conditions, rutile saturation in basaltic, andesitic, and dacitic liquids requires 7–9, 5–7, and 1–3 wt.% TiO 2, respectively. These concentrations are well in excess of those found in the respective rock types, so depletion in Nb, Ta, and Ti and reduced Nb/U and Nb/Th ratios in volcanic rocks erupted at convergent plate margins cannot be attributed to residual rutile in their source regions. Thus, Nb, Ta and Ti depletion must be an inherent property of the source region. We suggest that the island-arc source region has been depleted in Nb and Ta by a previous episode of melt extraction (MORB), zoning refining, or equilibration with a percolating melt or fluid. Such a process markedly depletes the LILE and HFSE element concentrations in the residuum, but ratios such as Nb/U, Nb/Th and U/Th remain relatively constant due to similar solid-melt partition coefficients. The depletion of Nb relative to Th in the source regions of island-arc magmas occurs during hybridization of the source by rutile-saturated (Nb/Ta-depleted) melts or aqueous fluids. If the hybridizing agent is a melt, a relatively felsic composition, produced under low T (900°) hydrous conditions, is required.

Rutile solubility and titanium coordination in silicate melts

The solubility of rutile has been determined in a series of compositions in the K 2O-Al 2O 3-SiO 2 system ( K ∗ = K 2O (K 2O + Al 2O 3) = 0.38–0.90), and the CaO-Al 2O 3-SiO 2 system ( C ∗ = CaO (CaO + Al 2O 3) = 0.47–0.59 ). Isothermal results in the KAS system at 1325°C, 1400°C, and 1475°C show rutile solubility to be a strong function of the K ∗ ratio. For example, at 1475°C the amount of TiO 2 required for rutile saturation varies from 9.5 wt% ( K ∗ = 0.38 ) to 11.5 wt% ( K ∗ = 0.48 ) to 41.2 wt% ( K ∗ = 0.90 ). In the CAS system at 1475°C, rutile solubility is not a strong function of C ∗ . The amount of TiO 2 required for saturation varies from 14 wt% ( C ∗ = 0.48 ) to 16.2 wt% ( C ∗ = 0.59 ). The solubility changes in KAS melts are interpreted to be due to the formation of strong complexes between Ti and K + in excess of that needed to charge balance Al 3+. The suggested stoichiometry of this complex is K 2Ti 2O 5 or K 2Ti 3O 7. In CAS melts, the data suggest that Ca 2+ in excess of A1 3+ is not as effective at complexing with Ti as is K +. The greater solubility of rutile in CAS melts when C ∗ is less than 0.54 compared to KAS melts of equal K ∗ ratio results primarily from competition between Ti and Al for complexing cations (Ca vs. K). TiK β x-ray emission spectra of KAS glasses ( K ∗ = 0.43–0.60 ) with 7 mole% added TiO 2, rutile, and Ba 2TiO 4, demonstrate that the average Ti-O bond length in these glasses is equal to that of rutile rather than Ba 2TiO 4, implying that Ti in these compositions is 6-fold rather than 4-fold coordinated. Re-examination of published spectroscopic data in light of these results and the solubility data, suggests that the 6-fold coordination polyhedron of Ti is highly distorted, with at least one Ti-O bond grossly undersatisfied in terms of Pauling's rules.

TiO2 activity in metamorphosed pelitic and basic rocks: principles and applications to metamorphism in southeastern Canadian Cordillera

The activity of TiO2 can be precisely defined as a function of pressure, temperature and activities of other components for common mineral assemblages in metapelites (ilmenite-quartz-garnet-plagioclase-Al2SiO5) and in metabasites (plagioclase-sphene-ilmenite-quartzgarnet). These mineral assemblages can be modelled by the equilibria: 1) 3ilmenite+Al2SiO5+2quartz=almandine+3TiO2 2) anorthite + 2sphene = grossular + 2TiO2 + quartz 3) 3anorthite+3quartz+6ilmenite = grossular+ 6TiO2+2almandine. These mineral assemblages can be used at \(a_{{\text{Tio}}_{\text{2}} } = 1\) (rutile saturation) and a given T to get maximum pressure limits of some metapelites and metabasites. When electron microprobe analyses of mineral grains adjacent to Ti-bearing phases are made, these data give maximum pressure estimates in reasonable agreement with other geobarometers. The activity of TiO2 in many metapelites is very near rutile saturation, but for metabasites the activity of TiO2 in some sillimanite zone rocks is as low as 0.6. The solubility of TiO2 in biotite, hornblende and garnet is a complex function of T, P, the activities of components in coexisting minerals and crystal chemical constraints in these minerals. At a given P and T the solubility of TiO2 in biotite and hornblende does not appear to be strongly dependent upon \(a_{{\text{Tio}}_{\text{2}} } = 1\) for sphene and ilmenite versus rutile-bearing assemblages.