Effects of F, B2O3 and P2O5 on the solubility of water in haplogranite melts compared to natural silicate melts

The effects of F, B2O3 and P2O5 on the H2O solubility in a haplogranite liquid (36 wt. % SiO2, 39 wt. % NaAlSi3O8, 25 wt. % KAlSi3O8) have been determined at 0.5, 1, 2, and 3 kb and 800, 850, and 900°C. The H2O solubility increases with increasing F and B content of the melt. The H2O solubility increase in more important at high pressure (2 and 3 kb) than at low pressure (0.5 kb). At 2 kb and 800°C, the H2O solubility increases from 5.94 to 8.22 wt. % H2O with increasing F content in the melt from 0 to 4.55 wt. %, corresponding to a linear H2O solubility increase of 0.53 mol H2O/mol F. With addition of 4.35 wt. % B2O3, the H2O solubility increases up to 6.86 wt. % H2O at 2 kb and 800°C, corresponding to a linear increase of 1.05 mol H2O/mol B2O3. The results allow to define the individual effects of fluorine and boron on H2O solubility in haplogranitic melts with compositions close to that of H2O-saturated thermal minima (at 0.5–3 kb). Although P has a dramatic effect on the phase relations in the haplogranite system, its effect on the H2O solubility was found to be negligible in natural melt compositions. The concominant increase in H2O solubility and F can not be interpreted on the basis of the available spectroscopic data (existence of hydrated aluminofluoride complexes or not). In contrast, hydrated borates or more probably boroxol complexes have been demonstrated in B-bearing hydrous melts

Incorporation of H_2 in vitreous silica, qualitative and quantitative determination from Raman and infrared spectroscopy

Incorporation mechanisms of H_2 in silica glass were studied with Raman and infrared (IR) microspectroscopy. Hydrogenated samples were prepared at temperatures between 800 deg C and 955 deg C at 2 kbar total pressure. Hydrogen fugacities (f_{H_2}) were controlled using the double capsule technique with the iron-w\"ustite (IW) buffer assemblage generating f_{H_2} of 1290-1370 bars corresponding to H_2 partial pressures (P_{H_2}) of 960-975 bars. We found that silica glass hydrogenated under such conditions contains molecular hydrogen (H_2) in addition to SiH and SiOH groups. H_2 molecules dissolved in the quenched glasses introduce a band at 4136 cm^{-1} in the Raman spectra which in comparison to that of gaseous H_2 is wider and is shifted to lower frequency. IR spectra of hydrogenated samples contain a band at 4138 cm^{-1} which we assign to the stretching vibration of H_2 molecules located in non-centrosymmetric sites. The Raman and IR spectra indicate that the dissolved H_2 molecules interact with the silicate network. We suggest that the H_2 band is the envelope of at least three components due to the occupation of at least three different interstitial sites by H_2 molecules. Both, Raman and IR spectra of hydrogenated glasses contain bands at ~2255 cm^{-1} which may be due to the vibration of SiH groups

Water solubility in silica and quartzofeldspathic melts

Water solubility in silica melts was determined at 100–600 MPa, 1200–1350 °C, and at each temperature (T) was found to increase with pressure (P). At P £ 250 MPa, the effect of T on water solubility in silica melts is small and within analytical precision (±0.15 wt% H2O). A positive correlation with T was observed at 400 MPa. Increasing solubility of water with increasing T was observed when large amounts of water are dissolved in silica and quartzofeldspathic melts (i.e., when molecular water is the dominant species in the glasses at room temperature), as already observed for feldspar melts. Change in water solubility (expressed in mol%) with decreasing SiO2 content of the melt is nonlinear along the silica-albite join. In the compositional range Ab100 to Ab25 (100 to 25 mol% albite, respectively, compositions calculated on an eight-oxygen basis), the solubility of water at 200 MPa decreases only slightly with decreasing Ab content (–0.1 ± 0.01 mol% H2O per mol% albite). However, at Ab contents less than 25 mol%, water solubility decreases sharply with increasing Qz content. Similar behavior was observed at 500 MPa. These results suggest that two different incorporation mechanisms of water in quartzofeldspathic melts must be considered: one corresponding to an NaAlSi3O8-H2O mechanism, the other to an SiO2-H2O mechanism

Bubble-enhanced basanite–tephrite mixing in the early stages of the Cumbre Vieja 2021 eruption, La Palma, Canary Islands

Syneruptive magma mixing is widespread in volcanic eruptions, affecting explosivity and composition of products, but its evidence in basaltic systems is usually cryptic. Here we report direct evidence of mixing between basanitic and tephritic magmas in the first days of the 2021 Tajogaite eruption of Cumbre Vieja, La Palma. Groundmass glass in tephritic tephra from the fifth day of the eruption is locally inhomogeneous, showing micron-scale filamentary structures of Si-poor and Fe-, Mg-rich melt, forming complex filaments attached to bubbles. Their compositional distribution attests the presence of primitive basanitic magma, with compositions similar to late-erupted melts, interacting with an evolved tephritic melt during the first week of the event. From filament morphology, we suggest their generation by dragging and folding of basanitic melt during bubble migration through melt interfaces. Semi-quantitative diffusion modelling indicates that the filamentary structures are short-lived, dissipating in timescales of tens of seconds. In combination with thermobarometric constraints, we suggest a mixing onset by sub-Moho remobilization of a tephritic reservoir by basanite input, followed by turbulent ascent of a mingled magma. In the shallow conduit or lava fountain, bubble nucleation and migration triggered further mingling of the distinct melt-phases. This phenomenon might have enhanced the explosive behaviour of the eruption in such period, where violent strombolian explosions were common

Phase equilibrium constraints on the viscosity of silicic magmas with implications for mafic-silicic mixing processes

Isobaric crystallization paths obtained from phase equilibrium experiments show that, whereas in rhyolitic compositions melt fraction trends are distinctly eutectic-like, dacitic and more mafic compositions have their crystallinities linearly correlated with temperature. As a consequence, the viscosities of the latter continuously increase on cooling, whereas for the former they remain constant or even decrease during 80% of the crystallisation interval, which opens new perspectives for the fluid dynamical modeling of felsic magma chambers. Given the typical dike widths observed for basaltic magmas, results of analogue modelling predict that injection of mafic magmas into crystallizing intermediate to silicic plutons under pre-eruption conditions cannot yield homogeneous composition. Homogenization can occur, however, if injection takes place in the early stages of magmatic evolution (i.e. at near liquidus conditions) but only in magmas of dacitic or more mafic composition. More generally, the potential for efficient mixing between silicic and mafic magmas sharing large interfaces at upper crustal levels is greater for dry basalts than for wet ones. At the other extreme, small mafic enclaves found in many granitoids behave essentially as rigid objects during a substantial part of the crystallization interval of the host magmas which implies that finite strain analyses carried out on such markers can give only a minimum estimate of the total amount of strain experienced by the host pluton. Mafic enclaves carried by granitic magmas behave as passive markers only at near solidus conditions, typically when the host granitic magma shows near-solid behavior. Thus they cannot be used as fossil indicators of direction of magmatic flow

Quantification of the CO2 budget and H2O–CO2 systematics in subduction-zone magmas through the experimental hydration of melt inclusions in olivine at high H2O pressure

Highlights • Melt inclusions from Klyuchevskoy were homogenized at 1150 °C and PH2O=500 MPaPH2O=500 MPa. • High-P experiments can recover initial H2O and CO2 contents in dehydrated inclusions. • Isothermal (de)hydration results in linear trend of CO2 and H2O in inclusion glasses. • Parental Klyuchevskoy magmas contain ∼3800 ppm CO2 and 4–5 wt.% H2O. • At least 80% of CO2 is slab-derived in the Klyuchevskoy magmas with CO2/Nb ∼3000. Abstract Reliable evaluation of CO2 contents in parental arc magmas, which can be preserved in melt inclusions in phenocrysts, is required to verify the proposed efficiency of CO2 recycling at convergent margins. Quantification of bulk CO2 concentration in melt inclusions requires their complete homogenization. Using samples from lavas from the Bulochka vent of Klyuchevskoy Volcano (Kamchatka), we applied a novel experimental approach to homogenize and re-equilibrate naturally dehydrated (<1 wt.% H2O) melt inclusions from high-Fo (85–91 mol.%) olivine. The experiments were performed at temperatures of 1150–1400 °C, pressures of up to 500 MPa, under dry to H2O-saturated conditions and with oxygen fugacity ranging from CCO to QFM+3.3. No homogenization was achieved at dry conditions. Complete dissolution of fluid bubbles (homogenization) in the melt inclusions was achieved at H2O pressures of 500 MPa and temperature of 1150 °C, when water content in the melt inclusions reached 4–5 wt.% H2O. The CO2 content in the homogenized inclusions is 3800±140 ppm3800±140 ppm and CO2/Nb = 3000 ± 420, which are the highest values reported so far for the typical middle-K primitive arc melts and fall within the range of values inferred from the magmatic flux and volcanic gas data for primary arc magma compositions. About 83% of the CO2 in Klyuchevskoy magmas is likely to be derived from the subducting slab and can be attributed to flux melting with a fluid having a CO2/H2O ratio of ∼0.06. The H2O and CO2 contents in the melt inclusions after hydrous experiments were found to correlate positively with each other and negatively with the volume of fluid bubble, reflecting increasing internal pressure in melt inclusions with increasing melt hydration. Therefore, similar trends observed in some natural sets of melt inclusions can be attributed to a partial dehydration of melts after entrapment, operating simultaneously with or following post-entrapment crystallization. Our study implies that the process of post-entrapment dehydration can be completely reversed under high pressure experimental conditions. If temperature, redox conditions and pressure of melt inclusion entrapment can be independently estimated, then our novel experimental approach (homogenization at high H2O pressure) can be used to reconstruct the initial CO2 content and also the entire composition of melt inclusions in olivine, including their initial H2O content, from any type of volcanic rock. With this approach volatiles in ancient lavas can also be determined, expanding our knowledge of volatile recycling further back in Earth history

The effect of temperature and bulk composition on the solution mechanism of phosphorus in peraluminous haplogranitic magma.

Solution mechanisms of P in peraluminous glasses and melts in the system CaO-Na2O-K2O-Al2O3- SiO2-P2O5 have been examined with in-situ microRaman spectroscopy from ambient temperature to near 1200 °C. The principal aim was to examine the relative stabilities of phosphate complexes as functions of P content, peraluminosity, and temperature. Increasing peraluminosity was accomplished by increasing the proportions of Al3+ and Ca2+ of constant SiO2 content. The molar ratio Al2O3/ (CaO+Na2O+K2O) (A/CNK) ranged from ~1 to ~1.3. In all compositions, P5+ is bonded to Al3+ to form AlPO4 complexes. In addition, there is evidence for pyrophosphate complexing (P2O7). In melts with the highest (Ca+Na+K)/P, there is probably also a small fraction of orthophosphate complexes present. The relative importance of AlPO4-like complexes is correlated positively with peraluminosity (A/CNK), P2O5 content, and increasing temperature at temperatures above that of the glass transition. These structural relationships among phosphate complexes are coupled with decreasing polymerization of the aluminosilicate melts

Effects of fluorine on the solubilities of Nb, Ta, Zr and Hf minerals in highly fluxed water-saturated haplogranitic melts

The effect of fluorine on the solubilities of Mn-columbite (MnNb2O6), Mn-tantalite (MnTa2O6), zircon (ZrSiO4) and hafnon (HfSiO4) were determined in highly fluxed, water-saturated haplogranitic melts at 800 to 1000°C and 2kbar. The melt composition corresponds to the intersection of the granite minimum with the albite-orthoclase tieline (Ab72Or28) in the quartz-albite-orthoclase system (Q-Ab-Or), which is representative of a highly fluxed melt, from which high field strength element minerals may crystallize. The melt contains 1.7wt.% P2O5, 1.05wt.% Li2O and 1.83wt.% B2O3. The main purpose of this study is to examine the effect of F on columbite, tantalite, zircon and hafnon solubility for a melt with this composition. Up to 6wt.% fluorine was added as AgF in order to keep the aluminum saturation index (ASI, molar Al/[Na+K]) of the melt constant. In an additional experiment F was added as AlF3 to make a glass peraluminous. The nominal ASI of the melts are close to 1 for the minimum composition and approximately 1.32 in peraluminous glasses, but if Li is considered as an alkali, the molar ratio Al/[Na+K+Li] of the melts are alkaline (0.87) and subaluminous (1.09), respectively. The molar solubility products [MnO]*[Nb2O5] and [MnO]*[Ta2O5] are nearly independent of the F content of the melt, at approximately 18.19±1.2 and 43.65±2.5×10-4 (mol2/kg2), respectively for the minimum composition. By contrast, there is a positive dependence of zircon and hafnon solubilities on the fluorine content in the minimum composition, which increases from 2.03±0.03×10-4 (mol/kg) ZrO2 and 4.04±0.2×10-4 (mol/kg) HfO2 for melts with 0wt.%F to 3.81±0.3×10-4 (mol/kg) ZrO2 and 6.18±0.04×10-4 (mol/kg) HfO2 for melts with 8wt.%F. Comparison of the data from this work and previous studies indicates that ASI of the melt seems to have a stronger effect than the contents of fluxing elements in the melt and the overall conclusion is that fluorine is less important (relative to melt compositions) than previously thought for the control on the behavior of high field strength elements in highly evolved granitic melts. Moreover, this study confirms that although Nb, Ta, Zr and Hf are all high field strength elements, Nb-Ta and Zr-Hf are complexed differently in the melt

Water solubility in aluminosilicate melts of haplogranite composition at 2 kbar

The compositional dependence of H2O solubility was investigated at 2 kbar and 800°C in haplogranite melts (system SiO2---1bNaAlSi3O8---1bKAlSi3O8 or Qz---1bAb---1bOr). The sixteen investigated compositions contained 25, 35 or 45 wt.% normative Qz and various Ab/(Ab+Or) ratios (0.15–0.92). Starting solid materials were anhydrous bubble-free glasses to which 10 wt.% H2O was added. The H2O contents of the isobarically quenched melts (glasses) were measured by Karl-Fischer titration. The results show that H2O solubility in aluminosilicate melts depends significantly upon anhydrous composition. The highest solubility values are obtained for the most Ab-rich melts. At a constant normative quartz content, the solubility of water decreases from 6.49 ± 0.20 wt.% H2O for a composition Qz35Ab60Or05 (normative composition expressed in wt.%) to 5.50 ± 0.15 wt.% H2O for a composition Qz35Ab10Or55. Along this join, the most significant changes are observed for Ab-rich melts whereas H2O solubility in Or-rich melts remains almost constant. The H2O solubility data imply that H2O is preferentially associated with the Ab component in aluminosilicate melts. Application of the results to natural granitic melts suggests that Na-rich, H2O-saturated melts may be significantly less viscous than H2O-saturated, K-rich melts. The temperature dependence of H2O solubility, investigated for composition Qz28Ab38Or34 at 2 kbar, is low. Increasing temperature from 750° to 1150°C only causes a decrease in H2O solubility from 6.00 to 5.41 wt.% H2O. These data are in agreement with previous data obtained for albite melts