ALBERT

Description: Numerous models have been developed to simulate the reaction of magmas to changes of thermodynamic variables, such as pressure, temperature, oxygen fugacity, and water activity. However, the extensive experimental database still lacks information on the distinct effect of small amounts of H 2 O on olivine + plagioclase + clinopyroxene cotectic crystallization in tholeiitic basalt. We present an experimental study addressing the effects of pressure (at 100, 200, 400, and 700 MPa) and small amounts of H 2 O on phase relations and liquid lines of descent in three tholeiitic basalts representing different evolutionary stages of the Shatsky Rise oceanic plateau magmatic system (compositions AH6, AH3, and AH5 with 8·6, 8·0, and 6·4 wt % MgO, respectively). Two experimental approaches (dry and low H 2 O) are designed to maintain contrasting H 2 O activities during crystallization using (1) graphite–platinum double capsules to perform nearly anhydrous experiments (〈0·15 wt % H 2 O in the melt) and (2) Fe pre-saturated Au 20 Pd 80 capsules to obtain low melt H 2 O contents ranging from 0·4 to 1·1 wt % H 2 O. Under dry conditions, at lower pressures (≤400 MPa), the crystallization in the MgO-rich AH6 and intermediate AH3 basalts follows the typical sequence of tholeiitic differentiation with olivine crystallization at the liquidus followed by olivine + plagioclase and olivine + plagioclase + clinopyroxene. Both basalts are close to multiple saturation at pressures between 400 and 700 MPa. At high pressure (700 MPa) the crystallization sequence is reversed, starting with clinopyroxene at the liquidus. Under low-H 2 O conditions, AH6 and AH3 are very close to multiple saturation, even at the low pressures of 100 and 200 MPa, and the reversed crystallization sequence (clinopyroxene, plagioclase + clinopyroxene, olivine + plagioclase + clinopyroxene) is observed already at 400 MPa. In contrast to the two more MgO-rich basalts, in the most evolved AH5 basalt, clinopyroxene is the liquidus phase at all investigated pressures and under both dry and low-H 2 O conditions, followed by crystallization of plagioclase + clinopyroxene and olivine + plagioclase + clinopyroxene. The most striking observation in our experiments is that the stability of clinopyroxene increases not only with pressure increase but also in the presence of small amounts of H 2 O (when compared with dry counterparts at similar pressures). Small amounts of H 2 O increase the proportion of clinopyroxene in the olivine + plagioclase + clinopyroxene phase assemblage. Our experiments clearly show that the effect of adding 0·4 wt % H 2 O to cotectic melt compositions (e.g. CaO/Al 2 O 3 ratio at a given MgO) is similar to that caused by an increase of pressure from 100 to ~ 300 MPa. This implies that small amounts of H 2 O can lead to significant overestimation of cotectic crystallization pressures (by up to 300 MPa) and that H 2 O contents need to be taken into account in geobarometric models. Our new experiments emphasize the role of low melt H 2 O contents in stabilizing clinopyroxene and provide some new insights into the problem of the ‘pyroxene paradox’. The apparent mantle pressures obtained for some mid-ocean ridge basalts using ‘dry’ geobarometric approaches can actually represent depths within the lower crust, if small amounts of H 2 O are present. The application of our experimental data to natural Shatsky Rise basalts implies that the magmas record partial crystallization processes occurring mainly at low pressure (100 MPa), corresponding to depths of ~3 km beneath the former spreading center, although the more primitive lavas show evidence of differentiation in a deeper reservoir at ~14 km depth (400 MPa).

Description: The effect of the normative anorthite content on the position of cotectic curves in the Qz–Ab–Or–An system has been investigated at 200 MPa and a water activity of 0.5. To simulate compositions as close as possible to those of natural high-silica rhyolites, all investigated compositions also contained ~ 1 wt% FeO and 0.2 wt% TiO 2. The position of the cotectic curves was deduced from crystallization experiments carried out between 790 and 850°C and using fourteen starting glass compositions containing ~ 3 wt% H 2 O. The liquidus phase of the different starting materials was used to constrain the primary fields of quartz, plagioclase and sanidine. The compositions of residual melts coexisting with solid phases were used to define the position of cotectic curves. Compared to the haplogranite system, the eutectic point is shifted away from the Ab apex, and its composition is estimated to be Qz 42 Ab 21 Or 37 when projected onto the haplogranite system. The implications for the estimation of the depth of magma storage conditions are discussed on the basis of an example from the Snake River Plain high-silica rhyolites.

Description: The phase relations have been investigated experimentally at 200 and 500 MPa as a function of water activity for one of the least evolved (Indian Batt Rhyolite) and of a more evolved rhyolite composition (Cougar Point Tuff XV) from the 12·8–8·1 Ma Bruneau–Jarbidge eruptive center of the Yellowstone hotspot. Particular priority was given to accurate determination of the water content of the quenched glasses using infrared spectroscopic techniques. Comparison of the composition of natural and experimentally synthesized phases confirms that high temperatures (〉900°C) and extremely low melt water contents (〈1·5 wt % H 2 O) are required to reproduce the natural mineral assemblages. In melts containing ~0·5–1·5 wt % H 2 O, the liquidus phase is clinopyroxene (excluding Fe–Ti oxides, which are strongly dependent on fO 2 ), and the liquidus temperature of the more evolved Cougar Point Tuff sample (BJR; ~940–1000°C) is at least 30°C lower than that of the Indian Batt Rhyolite lava sample (IBR2; 970–1030°C). For the composition BJR, the comparison of the compositions of the natural and experimental glasses indicates a pre-eruptive temperature of at least 900°C. The composition of clinopyroxene and pigeonite pairs can be reproduced only for water contents below 1·5 wt % H 2 O at 900°C, or lower water contents if the temperature is higher. For the composition IBR2, a minimum temperature of 920°C is necessary to reproduce the main phases at 200 and 500 MPa. At 200 MPa, the pre-eruptive water content of the melt is constrained in the range 0·7–1·3 wt % at 950°C and 0·3–1·0 wt % at 1000°C. At 500 MPa, the pre-eruptive temperatures are slightly higher (by ~30–50°C) for the same ranges of water concentration. The experimental results are used to explore possible proxies to constrain the depth of magma storage. The crystallization sequence of tectosilicates is strongly dependent on pressure between 200 and 500 MPa. In addition, the normative Qtz–Ab–Or contents of glasses quenched from melts coexisting with quartz, sanidine and plagioclase depend on pressure and melt water content, assuming that the normative Qtz and Ab/Or content of such melts is mainly dependent on pressure and water activity, respectively. The combination of results from the phase equilibria and from the composition of glasses indicates that the depth of magma storage for the IBR2 and BJR compositions may be in the range 300–400 MPa (~≤13 km) and 200–300 MPa (~≤10 km), respectively.

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: A rich history of experimental petrology has revealed the paths by which silicic igneous rocks follow mineral–melt equilibria during differentiation. Subdividing these rocks by ‘molar Al versus Ca + Na + K’ illustrates first-order differences in mineralogy and gives insight into formation mechanisms. Peraluminous magmas, formed by partial melting of sediments, largely owe their attributes and compositions to melting reactions in the protoliths, whereas most metaluminous felsic magmas record both continental and mantle inputs. Peralkaline rhyolites are mainly derived from either protracted crystallization or small degrees of partial melting of basalt, with only a marginal crustal contribution. Most silicic magmas hold 3–7 wt% H 2 O melt , which is inversely correlated with pre-eruptive temperature (700 °C to 〉950 °C) but unrelated to their reduced/oxidized state.

Description: One of the various problems faced in experimental petrology is the fact that most experimental products obtained by crystallization experiments are too small, making their accurate identification by electron microprobe and laser ablation analyses very difficult. This problem is magnified when a highly polymerized starting material is used for experiments at low temperature (e.g., 700–800 °C). In this study, we present the results of crystallization experiments performed using a rhyolitic starting glass in which we test the potential of temperature cycling and pre-hydrated starting material to increase crystal size and discuss the effect of those variables on the attainment of chemical equilibrium. Experiments were performed at different temperatures (725 to 815 °C) and pressures (1 and 2 kbar), under water-saturated conditions ( a H 2 O = 1; with a H 2 O being the water activity). During the experiments, temperature was either constant or cycled to ±15 °C around the target temperature during the first half of the runs. We used either a pre-hydrated (7 wt% H 2 O) rhyolitic glass or a dry rhyolitic glass to which 7 wt% H 2 O was added during capsule preparation. Our results differ between 1 and 2 kbar experiments. At 1 kbar, plagioclase and orthopyroxene were the main crystalline phases affected and temperature cycling (±15 °C) did not increase the crystal size of these phases. In contrast, if only the nature of the starting material is considered (dry glass vs. pre-hydrated), the use of a pre-hydrated starting material successfully increased the overall crystal size and decreased the crystal number density. At 2 kbar, plagioclase and amphibole were the main phases and the largest crystals were also obtained when pre-hydrated starting material was used. Contrary to experiments at 1 kbar, temperature cycling also increased the overall crystal size. The different effects of temperature cycling at 1 and 2 kbar are attributed (1) to the different cation diffusivities at 1 and 2 kbar caused by different melt water concentrations and (2) the negligible effect of temperature cycling at 1 kbar (±15 °C) is explained by little dissolution of phases, so that small crystals were already too large to be completely consumed by the dissolution process in the high temperature interval. The results demonstrate that temperature oscillation (depending on the amplitude) and the nature of the starting material (pre-hydrated vs. dry glass + water) are two parameters that can contribute to increase crystal sizes in experiments with rhyolitic melts. However, we also observed that the use of a pre-hydrated starting material increased the occurrence of zoned plagioclase crystals, which may indicate that chemical equilibrium was not perfectly reached.

Description: Bubble formation during continuous decompression from ~400 to ~70 MPa was investigated experimentally in hydrous andesitic melts at T = 1030 °C and at an oxygen fugacity ( f O 2 ) of about log( f O 2 /bar) = QFM+1 (QFM: quartz-fayalite-magnetite buffer). Experiments were carried out at variable decompression rates ( r ), ranging from 0.0005 to 0.1 MPa/s. The samples were directly quenched after decompression, allowing the investigation of the influence of r on the bubble formation. The effect of variable annealing times ( t A ) after decompression was also investigated for experiments performed at a decompression rate of 0.1 MPa/s. These samples were annealed for t A = 0 to 72 h at final pressure (70 MPa) to study changes in vesiculation during magma storage at shallow depths after fast ascent. Backscattered electron (BSE) images of the samples were analyzed to determine bubble number densities (BND). The BND values increase strongly with increasing r and vary from about 10 2.2 mm –3 at 0.0005 MPa/s to about 10 4.6 mm –3 at 0.1 MPa/s. After fast decompression ( r ~ 0.1 MPa/s), the BND decrease significantly with t A , i.e., from ~10 4.6 mm –3 at t A = 0 h to ~10 2.9 mm –3 at t A = 72 h. A comparison of the derived BND values with recently published experimental data demonstrates the essential role of the decompression path on bubble formation. The BND are higher in experiments with multi- or single-step decompression when compared to continuous decompression. The new data show that H 2 O-undersaturated andesitic melts are characterized by 1 to 2 log units higher BND values than H 2 O-saturated rhyolitic melts after decompression with the same rate, indicating a strong influence of melt composition on bubble nucleation. This compositional effect is not predicted accurately by existing models and the interpretation of the vesicularity of dacitic to andesitic melts may lead to overestimations of magma ascent rates by about an order of magnitude.

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.