ALBERT

Description: Hydrothermal experiments were conducted at ca. 1 to 7000 bars and 700 to 1250 °C in 121 rhyolitic to basaltic systems to determine Cl solubility in silicate melts, i.e., the maximum Cl concentration in melts that are saturated in a hydrosaline liquid with or without an aqueous or aqueous-carbonic vapor. The Cl concentration of melts increases with the Cl contents of the fluid unless the melt coexists with vapor plus hydrosaline liquid at fixed pressure and temperature; this phase assemblage buffers the Cl content of each phase with increasing Cl in the system. The Cl content of fluid(s)-saturated melts is independent of the CO 2 concentration of the saline liquid ± vapor with up to 21 wt% CO 2 in the fluid(s). The experiments show that Cl dissolution in aluminosilicate melts increases with temperature and pressure. Chlorine solubility is also a function of melt composition; it increases with the molar ([Al 1/2 +Ca 1/2 +Mg 1/2 +Na]/Si) of the melt. These experimental data have been integrated with results involving 41 other experiments ( Webster and De Vivo 2002 ) to develop a broadly expanded model that supports calculation of Cl solubility in 163 aluminosilicate melts. This empirical model applies to Cl dissolution in melts of most silicate magmas at depths as great as 25 km. It determines the exsolution of hydrosaline liquid, with or without a coexisting vapor, as magmas ascend from depth, cool, crystallize, and differentiate from mafic to felsic compositions. In combination with H 2 O solubility models, our model supports determination of H 2 O-Cl solubility relations for most aluminosilicate magmas and is useful for barometric estimations based on silicate melt inclusions containing low CO 2 and moderate to high-Cl concentrations. The model is applied to the phase relations of fluids in volatile-enriched magmas of Augustine volcano, Alaska. The Cl and H 2 O concentrations of melt inclusions from 14, basaltic to dacitic eruptive units are compared with modeled solubilities of Cl and H 2 O in Augustine melts. The majority of these eruptions involved magmas that first exsolved aqueous to aqueous-carbonic vapors when the melts were dacitic in composition (i.e., before the residual melts in these magmas had evolved to felsic compositions) and well prior to the eruptions. Hydrosaline liquid with or without a vapor phase exsolved from other, more-felsic fractions of Augustine melts at low, near-surface pressures of several tens of bars.

Description: The solubility of H 2 O- and CO 2 -bearing fluids in trachytic and trachybasaltic melts from erupted magmas of the Campi Flegrei Volcanic District has been investigated experimentally at 1100 and 1200 °C, respectively, and at 100, 200, 300, 400, and 500 MPa. The solubility of H 2 O in the investigated melts varies between 3.48 ± 0.07 wt% at 100 MPa to 10.76 ± 0.12 wt% at 500 MPa in trachytic melts and from 3.49 ± 0.07 wt% at 100 MPa to 9.10 ± 0.11 wt% at 500 MPa in trachybasaltic melts. The content of dissolved CO 2 in melts coexisting with the most CO 2 -rich fluid phase increases from 281 ± 24 ppm at 100 MPa to 2710 ± 99 ppm at 500 MPa in trachyte, and from 727 ± 102 ppm at 100 MPa to 3565 ± 111 ppm at 500 MPa in trachybasalt. Natural samples from the Campanian Ignimbrite eruption (trachyte) and from the Solchiaro eruption (trachybasalt) were collected around the city of Naples and on Procida Island. Deuterium/hydrogen (D/H) ratios were analyzed in natural pumices pre-heated at different temperatures to remove water adsorbed and/or imprinted by glass alteration processes. It has been determined that heating of the glass to 350 °C efficiently removes most of secondary water and the remaining concentrations represent primary magmatic water preserved in the erupted material. Hydrogen isotope composition (with D values ranging between –70 and –110) and its correlation with bulk water content in selected pumice samples of the Campanian Ignimbrite eruption are consistent with isotopic fractionation between magmatic fluid and melt during degassing of erupting magma. Hence, the H 2 O and CO 2 contents in natural glasses from pumice samples are considered as minimum estimates on volatile concentrations in the melt just prior to the eruption or at the fragmentation event. The water contents in natural glasses vary from 0.83 ± 0.07 to 3.74 ± 0.06 wt% for trachytes from the Campanian Ignimbrite eruption and from 1.96 ± 0.06 to 3.47 ± 0.07 wt% for trachybasalts from the Solchiaro eruption. The CO 2 contents vary from 78 ± 120 ppm CO 2 to 1743 ± 274 ppm for trachytes from the Campanian Ignimbrite eruption and from 240 ± 293 to 1213 ± 250 ppm for trachybasalts from the Solchiaro eruption. A combination of natural and experimental data provides minimum pressure estimates for the storage and ascent conditions of magmas. The Campanian Ignimbrite magma could have been stored or ponded during its rising path at two different levels: a deeper one corresponding to depth of about 8 to 15 km and a shallower one at about 1 to 8 km. Trachybasalts from Solchiaro erupted from the deepest level of about 11 km with a storage or ponding level at around 2 to 8 km depth. Although an uncertainty of at least a kilometer has to be considered in estimating storage or ponding depths, these estimates point to significantly deeper magmatic sources for both eruptions as those considered previously.

Description: Isothermal decompression experiments were performed to simulate magma ascent at Unzen volcano from the depths of magma storage to shallow crustal levels, corresponding to pressure decrease from 300 to 50 MPa. A partially crystallized synthetic rhyodacitic magma (representing equilibrium conditions at 850°C and 300 MPa) was used as a starting material; this has a composition identical to the groundmass of Unzen rocks erupted in 1991–1995. Decompression rates were varied from 0·0002 to 20 MPa s –1 . Experiments conducted with decompression rates ≥0·1 MPa s –1 were decompressed continuously; a multi-step decompression approach was used at decompression rates ≤0·1 MPa s –1 . The experiments were fluid-saturated, either containing only water as a fluid component (H 2 O-bearing) or containing a water and carbon dioxide mixture (H 2 O + CO 2 ; initial mole fraction of H 2 O in the fluid ~0·6). The experimental products of the H 2 O-bearing experiments consist of amphibole, pyroxenes, oxides and glass. Plagioclase microlites nucleated and grew only in experiments with the two lowest decompression rates of 0·0005 and 0·0002 MPa s –1 . The length of those plagioclases is up to 200–250 µm, which is consistent with the size of plagioclase microlites observed in the natural samples. The experimental products of the H 2 O + CO 2 -bearing system are composed of pyroxenes, oxides, glass and plagioclase. Plagioclase microlites in the H 2 O + CO 2 -system were already present in the starting assemblage and grew to a maximum size of ~80 µm. Equilibrium concentrations of water in the residual glasses at the final pressure of 50 MPa are reached at decompression rates ≤1 MPa s –1 for the H 2 O + CO 2 -bearing system and ≤0·1 MPa s –1 for the H 2 O-bearing system. The bubble number density (BND) values range from 10 3·7 to 10 5·6 mm –3 in the H 2 O-bearing system and from 10 4·6 to 10 6·4 mm –3 in the H 2 O + CO 2 -bearing systems. In both systems, BND values decrease with decreasing decompression rate from 20 to 0·01 MPa s –1 , and increase with decreasing decompression rates 〈 0·01 MPa s –1 , which is interpreted to reflect predominant bubble growth and bubble nucleation, respectively. The onset of crystallization, observed from changes in the chemical composition of the residual melt, occurs at decompression rates 〈 0·1 MPa s –1 . At the lowest decompression rate (0·0002 MPa s –1 ) the chemical composition of the residual melt in the H 2 O + CO 2 -bearing system becomes similar to the natural matrix glass composition. There is no significant variation of the microlite number density (MND) value as a function of the decompression rate. The MND values for plagioclases-only range from 10 5·4 to 10 5·7 mm –3 , whereas the MND values for the other phases range from 10 5·3 to 10 5·9 mm –3 . Our experimental MND Pl values are in the range of those from natural samples (10 5 –10 6 mm –3 ). We show that the size of microlites nucleating and crystallizing during decompression (plagioclase in our experimental dataset) is useful to constrain magma ascent rates at the onset of the crystallization of the corresponding phase. Based on the size of plagioclase microlites and on the composition of the residual melts, the average magma ascent rates of Unzen magmas in the pressure range 200 to 50 MPa is estimated to be 10–50 m h –1 .

Description: Magma degassing processes are commonly elucidated by studies of melt inclusions in erupted phenocrysts and measurements of gas discharge at volcanic vents, allied to experimentally constrained models of volatile solubility. Here we develop an alternative experimental approach aimed at directly simulating decompression-driven, closed-system degassing of basaltic magma in equilibrium with an H–C–O–S–Cl fluid under oxidized conditions (f O2 of 1·0–2·4 log units above the Ni–NiO buffer). Synthetic experimental starting materials were based on basaltic magmas erupted at the persistently degassing volcanoes of Stromboli (Italy) and Masaya (Nicaragua) with an initial volatile inventory matched to the most undegassed melt inclusions from each volcano. Experiments were run at 25–400 MPa under super-liquidus conditions (1150°C). Run product glasses and starting materials were analysed by electron microprobe, secondary ion mass spectrometry, Fourier transform infrared spectroscopy, Karl-Fischer titration, Fe 2+ /Fe 3+ colorimetry and CS analyser. The composition of the exsolved vapour in each run was determined by mass balance. Our results show that H 2 O/CO 2 ratios increase systematically with decreasing pressure, whereas CO 2 /S ratios attain a maximum at pressures of 100–300 MPa. S is preferentially released over Cl at low pressures, leading to a sharp increase in vapour S/Cl ratios and a sharp drop in the S/Cl ratios of glasses. This accords with published measurements of volatile concentrations in melt inclusion and groundmass glasses at Stromboli (and Etna). Experiments with different S abundances show that the H 2 O and CO 2 contents of the melt at fluid saturation are not affected. The CO 2 solubility in experiments using both sets of starting materials is well matched to calculated solubilities using published models. Models consistently overestimate H 2 O solubilities for the Stromboli-like composition, leading to calculated vapour compositions that are more CO 2 -rich and calculated degassing trajectories that are more strongly curved than observed in experiments. The difference is less acute for the Masaya-like composition, emphasizing the important compositional dependence of solubility and melt–vapour partitioning. Our novel experimental method can be readily extended to other bulk compositions.

Description: We have investigated experimentally the partitioning of Au between solid and liquid sulfide phases and basaltic melts at 200 MPa, at redox conditions close to the sulfide-sulfate transition, over temperatures between 1050 and 1200 °C, which span the monosulfide solid solution (MSS) - sulfide liquid (SuL) solidus. The measured MSS/basalt partition coefficient of Au ( D Au MSS-sil ) is about 100–200, whereas the partition coefficient of sulfide liquid/basalt ( D Au SuL-sil ) is approximately 10 times larger at 2200. Although we find that temperature, pressure, and oxygen fugacity ( f O 2 ) exert relatively weak controls on Au partitioning, they exert major indirect influences on Au behavior by controlling the identity of the condensed sulfide phase and by affecting S solubility. These observations have important implications for the behavior of Au in the processes of partial melting in the mantle and magma crystallization in the crust. The occurrence of natural magmas with elevated concentrations of Au and presumably other highly siderophile and chalcophile elements requires predominance of MSS over SuL in the source or/and oxidizing conditions close to or above the sulfide-sulfate transition in the magma.

Description: Synthetic fluid inclusions formed in high P-T experiments, which are subsequently analyzed with LA-ICP-MS, enable us to collect thermodynamic data to constrain metal transport in aqueous fluids as well as partitioning of metals between coexisting phases. The most essential prerequisite for such studies is to ensure that equilibrium conditions between liquid and solid phases are reached prior to the formation of synthetic fluid inclusions in the host mineral. Various methods have been proposed by different authors to achieve this goal, but to this point our knowledge on the best approach to synthesize equilibrated fluid inclusions under constrained pressure, temperature, and compositional ( P , T , and X ) conditions remains poor. In addition, information on the time needed to reach equilibrium metal concentrations in the fluid as well as on the timing of the onset of fluid inclusion formation in the host mineral are scarce. The latter has been tested in a series of time-dependent experiments at 800 °C and 200 MPa using scheelite (CaWO 4 ), molybdenite (MoS 2 ) and metallic gold as dissolving phases and using different approaches to optimize the formation of equilibrated fluid inclusions. Both $${f}_{{\mathrm{O}}_{2}}$$ and $${f}_{{\mathrm{s}}_{2}}$$ were fixed during all experiments using the pyrite-pyrrhotite-magnetite buffer (PPM). As an intermediate in situ quenching of the sample charge plays an important role in the synthesis of fluid inclusions, we further tested the efficiency of such an intermediate quench for re-opening fluid inclusions formed at 600 °C and 200 MPa. Our results reveal that fluid inclusions start forming almost instantaneously and that equilibrium between fluid and solid phases occurs in the timescale of less than two hours for molybdenite and gold up to ca. 10 h for scheelite. The best approach to synthesize equilibrated fluid inclusions at 800 °C was obtained by using an intermediate quench on a previously unfractured quartz host. Experiments at 600 °C showed similar results and illustrate that this should be the method of choice down to this temperature. Below 600 °C pre-treatment of the quartz host (HF etching and/or thermal fracturing) becomes important to produce large enough fluid inclusions for the analyses via LA-ICP-MS and special care must be taken to prevent premature entrapment of the fluid. Fluids with 8 wt% NaCl in equilibrium with scheelite, molybdenite and gold at 800 °C and 200 MPa have concentrations of ca. 7300 ppm W, 1300 ppm Mo, and 300 ppm Au, respectively, which is in good agreement with results from other studies or extrapolation from lower temperatures. It can be concluded that the formation of synthetic fluid inclusions from an equilibrated fluid is possible, but different experimental designs are required, depending on the investigated temperature. In general, dissolution of solid phases seems to be much faster than previously assumed, so that experimental run durations can be designed considerably shorter, which is of great advantage when using fast-consuming mineral buffers.