Melt H2O Research Articles

Petrogenesis and structure of oceanic crust in the Lau back-arc basin

Oceanic crust formed along spreading centers in the Lau back-arc basin exhibits a dramatic change in structure and composition with proximity to the nearby Tofua Arc. Results from recent seismic studies in the basin indicate that crust formed near the Tofua Arc is abnormally thick (8–9 km) and compositionally stratified, with a thick low-velocity (3.4–4.5 km/s) upper crust and an abnormally high-velocity (7.2–7.4+ km/s) lower crust (Arai and Dunn, 2014). Lava samples from this area show arc-like compositional enrichments and tend to be more vesicular and differentiated than typical mid-ocean ridge basalts, with an average MgO of ∼3.8 wt.%. We propose that slab-derived water entrained in the near-arc ridge system not only enhances mantle melting, as commonly proposed to explain high crustal production in back-arc environments, but also affects magmatic differentiation and crustal accretion processes. We present a petrologic model of Lau back-arc crustal formation that successfully predicts the unusual crustal stratification imaged in the near-arc regions of the Lau basin, as well as the highly fractionated basaltic andesites and andesites that erupt there. Results from phase equilibria modeling using MELTS indicate that the high water contents found in near-arc parental melts can lead to crystallization of an unusually mafic, high velocity cumulate layer. Best-fit model runs contain initial water contents of ∼0.5–1.0 wt.% H2O in the parental melts, and successfully reproduce geochemical trends of the erupted lavas while crystallizing a cumulate assemblage with calculated seismic velocities consistent with those observed in the near-arc lower crust. Modeled residual melts are also lower density than their dry equivalents, which aids in melt segregation from the cumulate layer. Low-density, water-rich residual melts can lead to the eruption of vesicular lavas that are unusually evolved for an oceanic spreading center.

Partitioning of sulfur and chlorine between aqueous fluid and basaltic melt at 1050°C, 100 and 200 MPa

The partitioning of S and Cl between aqueous fluid and basaltic melt of Mt. Etna has been experimentally investigated at 1050°C, logfO2~FMQ+0.5, FMQ+2.3, and FMQ+2.8 at 200MPa as well as at ~FMQ+2.8 at 100MPa by using HCl(aq) and native S as chlorine and sulfur source. At given P–T–fO2 conditions, the partitioning of S and Cl between fluid and melt typically demonstrates non-linear dependence on S and Cl contents in the system, and only at low concentrations of S (below ~2wt.%) and Cl (below ~1wt.%) in the fluid is the Henrian behavior observed. The partition coefficients of S (DS, wt.% Sfluid/wt.% Smelt) ranged between 3±1 and 96±24 at 200MPa and between 6±2 and 73±18 at 100MPa at logfO2≥FMQ+2.3, and between 164±13 and 236±26 at 200MPa and FMQ+0.5. The partition coefficients of Cl (DCl, wt.% Clfluid/wt.% Clmelt) ranged between ~2±1 and 17±1 at 200MPa and between ~3±2 and 19±1 at 100MPa at logfO2≥FMQ+2.3, and between ~2±1 and 9±1 at 200MPa and at FMQ+0.5.At oxidizing conditions (FMQ+2.3 and FMQ+2.8) sulfur and chlorine demonstrate mutual interaction by favoring partitioning of both volatiles into the fluid relative to melt together with effective extraction of Ca and K from the melt into the fluid. Such interaction indicates that both, S and Cl may change their speciation by effective extraction of Ca and K from the melt and by mutual reactions between species rather than by competitive bonding with extracted cations. These observations are also an indication that bulk activity coefficients of S and Cl in the fluid are decreased, which in turn decrease non-ideality of mixing between interacting fluid species compared to S-deficient aqueous chloridic fluids.The non-linear dependence of Cl fluid–melt partitioning on the concentration of Cl in the fluid can be described by a simple square root function.The data also indicate that Cl may affect the solubility of H2O in basaltic melt which increases by 0.5mol of H2O per 1mol of Cl. It can be presumably attributed to the incorporation mechanism of HCl into the melt structure, i.e. Cl− may complex with network-modifying cation(s) and H+ may bond with half of non-bridging oxygen. The square root behavior of Cl partitioning between many aluminosilicate melts and aqueous fluid may thus be explained by the exchange reaction of one Cl ion from the fluid with two oxygen ions in the melt. Correlations between the mechanisms and the partitioning behavior of S between fluid and melt could not be carved out.At 100MPa and oxidizing conditions (~FMQ+2.3 and FMQ+2.8), the S/Cl ratio in the fluids and S contents in the coexisting melts closely match the compositions of fluids and compositions of olivine hosted melt inclusions from the 2002–2003 eruption of Mt. Etna. At S/Cl fluid ratios of ~5 to 7, measured during passive degassing, experimentally determined melt S contents were similar to primitive magma which contains ~0.27 to 0.33wt.% S. Whereas at S/Cl fluid ratios of ~2 to 4 determined during mild strombolian activity, coexisting melts contained 0.18 to 0.27wt.% S which is similar to partially degassed, slightly evolved melts. This indicates that primitive melts of Mt. Etna might have been more oxidizing than previously determined on melt inclusions.

Melting the hydrous, subarc mantle: the origin of primitive andesites

This experimental study is the first comprehensive investigation of the melting behavior of an olivine + orthopyroxene ± spinel—bearing fertile mantle (FM) composition as a function of variable pressure and water content. The fertile composition was enriched with a metasomatic slab component of ≤0.5 % alkalis and investigated from 1135 to 1470 °C at 1.0–2.0 GPa. A depleted lherzolite with 0.4 % alkali addition was also studied from 1225 to 1240 °C at 1.2 GPa. Melts of both compositions were water-undersaturated: fertile lherzolite melts contained 0–6.4 wt% H2O, and depleted lherzolite melts contained ~2.5 wt% H2O. H2O contents of experimental glasses are measured using electron microprobe, secondary ion mass spectrometry, and synchrotron-source reflection Fourier transform infrared spectroscopy, a novel technique for analyzing H2O in petrologic experiments. Using this new dataset in conjunction with results from previous hydrous experimental studies, a thermobarometer and a hygrometer–thermometer are presented to determine the conditions under which primitive lavas were last in equilibration with the mantle. These predictive models are functions of H2O content and pressure, respectively. A predictive melting model is also presented that calculates melt compositions in equilibrium with an olivine + orthopyroxene ± spinel residual assemblage (harzburgite). This model quantitatively predicts the following influences of H2O on mantle lherzolite melting: (1) As melting pressure increases, melt compositions become more olivine-normative, (2) as melting extent increases, melt compositions become depleted in the normative plagioclase component, and (3) as melt H2O content increases, melts become more quartz-normative. Natural high-Mg# [molar Mg/(Mg + Fe2+)], high-MgO basaltic andesite and andesite lavas—or primitive andesites (PAs)—contain high SiO2 contents at mantle-equilibrated Mg#s. Their compositional characteristics cannot be readily explained by melting of mantle lherzolite under anhydrous conditions. This study shows that experimental melts of a FM peridotite plus the addition of alkalis reproduce the compositions of natural PAs in SiO2, Al2O3, TiO2, Cr2O3, MgO, and Na2O at 1.0–1.2 GPa and H2O contents of 0–7 wt%. Our results also suggest that PAs form under a maximum range of extents of melting from F = 0.2–0.3. The CaO contents of the melts produced are 1–5 wt% higher than the natural samples. This is not a result of a depleted source composition or of extremely high extents of melt but is potentially caused by a very low CaO content contribution from deeper in the mantle wedge.

Experimental phase equilibria of a Mount St. Helens rhyodacite: a framework for interpreting crystallization paths in degassing silicic magmas

We present isothermal (885 °C) phase equilibrium experiments for a rhyodacite from Mount St. Helens (USA) at variable total pressure (25–457 MPa) and fluid composition (XH2Ofl = 0.6–1.0) under relatively oxidizing conditions (NNO to NNO + 3). Run products were characterized by SEM, electron microprobe, and SIMS. Experimental phase assemblages and phase chemistry are consistent with those of natural samples from Mount St. Helens from the last 4000 years. Our results emphasize the importance of pressure and melt H2O content in controlling phase proportions and compositions, showing how significant textural and compositional variability may be generated in the absence of mixing, cooling, or even decompression. Rather, variations in the bulk volatile content of magmas, and the potential for fluid migration relative to surrounding melts, mean that magmas may take varied trajectories through pressure–fluid composition space during storage, transport, and eruption. We introduce a novel method for projecting isothermal phase equilibria into CO2–H2O space (as conventionally done for melt inclusions) and use this projection to interpret petrological data from Mount St. Helens dacites. By fitting the experimental data as empirical functions of melt water content, we show how different scenarios of isothermal magma degassing (e.g., water-saturated ascent, vapor-buffered ascent, and vapor fluxing) can have quite different textural and chemical consequences. We explore how petrological data might be used to infer degassing paths of natural magmas and conclude that melt CO2 content is a much more useful parameter in this regard than melt H2O.

An H2O–CO2 mixed fluid saturation model compatible with rhyolite-MELTS

A thermodynamic model for estimating the saturation conditions of H2O–CO2 mixed fluids in multicomponent silicate liquids is described. The model extends the capabilities of rhyolite-MELTS (Gualda et al. in J Petrol 53:875–890, 2012a) and augments the water saturation model in MELTS (Ghiorso and Sack in Contrib Mineral Petrol 119:197–212, 1995). The model is internally consistent with the fluid-phase thermodynamic model of Duan and Zhang (Geochim Cosmochim Acta 70:2311–2324, 2006). It may be used independently of rhyolite-MELTS to estimate intensive variables and fluid saturation conditions from glass inclusions trapped in phenocrysts. The model is calibrated from published experimental data on water and carbon dioxide solubility, and mixed fluid saturation in silicate liquids. The model is constructed on the assumption that water dissolves to form a hydroxyl melt species, and that carbon dioxide both a molecular species and a carbonate ion, the latter complexed with calcium. Excess enthalpy interaction terms in part compensate for these simplistic assumptions regarding speciation. The model is restricted to natural composition liquids over the pressure range 0–3 GPa. One characteristic of the model is that fluid saturation isobars at pressures greater than ~100 MPa always display a maximum in melt CO2 at nonzero H2O melt concentrations, regardless of bulk composition. This feature is universal and can be attributed to the dominance of hydroxyl speciation at low water concentrations. The model is applied to four examples. The first involves estimation of pressures from H2O–CO2-bearing glass inclusions found in quartz phenocrysts of the Bishop Tuff. The second illustrates H2O and CO2 partitioning between melt and fluid during fluid-saturated equilibrium and fractional crystallization of MORB. The third example demonstrates that the position of the quartz–feldspar cotectic surface is insensitive to melt CO2 contents, which facilitates geobarometry using phase equilibria. The final example shows the effect of H2O and CO2 on the crystallization paths of a high-silica rhyolite composition representative of the late-erupted Bishop Tuff. Software that implements the model is available at ofm-research.org, and the model is incorporated into the latest version (1.1+) of rhyolite-MELTS.

Preserved near ultrahigh-pressure melt from continental crust subducted to mantle depths

Remnants of hydrous melt formed at mantle depths have been identified and characterized within high-pressure leucogranulites of the Orlica-Snieznik Dome (Bohemian Massif, central Europe). They occur as nanogranites in garnet formed via partial melting of granitoids during the Variscan orogeny. Melt composition and H2O content have been investigated in situ after experimental re-homogenization of the nanogranites, and are consistent with melts produced experimentally from crustal lithologies at mantle depths. This is the first geochemical study of melt inclusions from natural crustal rocks equilibrated close to the stability field of coesite, shedding light on how continental crust melts during deep subduction. Whereas decompressional melting is commonly invoked for deeply subducted crustal lithologies, melting occurred near or at the metamorphic peak pressure in the Orlica-Snieznik granulites. Melting of deeply subducted crustal rocks significantly modifies the rheology and thus promotes fast exhumation: this process has a critical influence on the geodynamic evolution of subduction-collision orogens as well as crustal differentiation at depth.

Partial melting of fertile peridotite fluxed by hydrous rhyolitic melt at 2–3 GPa: implications for mantle wedge hybridization by sediment melt and generation of ultrapotassic magmas in convergent margins

We investigated the melting behavior of peridotite fluxed with 25 wt% of H2O-bearing rhyolitic sediment melt (1.8 wt% bulk H2O), by performing experiments from 1100 to 1300 °C at 2 GPa and 1050–1350 °C at 3 GPa. The apparent solidus of our bulk composition lies between 1100 and 1150 °C at both pressures, which is at a higher temperature than the vapor-saturated solidus and close to the pargasite dehydration solidus of peridotite. With increasing temperature, reacted melt fraction increases from 20 to 36 wt% from 1200 to 1300 °C at 2 GPa and 7 to 24 wt% from 1225 to 1350 °C at 3 GPa. Orthopyroxene is present as a residual phase in all the experiments, while olivine is present as a residual phase in all the experiments at 2 GPa only. Amphibole is absent above 1100 °C at both pressures, clinopyroxene disappears above 1200 and 1300 °C at 2 and 3 GPa, respectively, and garnet (only present at 3 GPa) melts out above 1300 °C. Upon reaction with the mantle wedge and subsequent melting of the hybrid rock, subducted sediment-derived rhyolites evolve in composition to a nepheline-normative ultrapotassic leucitite, similar in major element composition to ultrapotassic lavas from active arcs such as Sunda and inactive arcs such as in the Roman Magmatic Province. Fluxing peridotite with H2O versus H2O-bearing sediment melt at similar pressures does not appear to have an effect on isobaric melt productivity, but does have significant effect on melting reactions and resultant melt composition, with influx of sediment melt adding K2O to the system, thereby stabilizing phlogopite, which in turn buffers the reacted melt to ultrapotassic compositions. Previous experimental studies, along with this study, find that phlogopite can be stable near the hotter core of the mantle wedge and, hence, is likely to be subducted to deeper mantle, thereby influencing deeper cycling of volatiles and large ion lithophile elements. Also, because D Rb phl/melt ≫ D Sr phl/melt and D Nd phl/melt , D Sm phl/melt ≪ 1, long-term stability of phlogopite in the mantle can create ‘enriched mantle’ domains (eSr and eNd ≥ 0).

Rapid Crystallization of Plagioclase Phenocrysts in Silicic Melts during Fluid-saturated Ascent: Phase Equilibrium and Decompression Experiments

Three crystal-poor obsidian samples (one dacite, 67 wt % SiO2; two rhyolites, 73 and 75 wt % SiO2), which erupted effusively from monogenetic vents, contain sparse (<2%) plagioclase phenocrysts that span a remarkably wide and continuous range in composition ( 30 mol % An). Many, but not all, of the plagioclase crystals display diffusion-limited growth textures (e.g. swallow-tails, skeletal, vermiform). Hypotheses to explain the paradox of a wide compositional range despite a low abundance of plagioclase include (1) incorporation of xenocrysts and/or magma mingling, (2) slow crystallization of plagioclase driven by slow cooling in a magma chamber, (3) slow crystallization of plagioclase followed by a resorption (e.g. heating) event, and (4) crystallization driven by rapid degassing (i.e. loss of melt H2O)6 rapid cooling during ascent. To test these hypotheses, a series of phase equilibrium experiments were conducted under pure-H2O fluid-saturated conditions in a cold-seal pressure vessel between 30 and 300 MPa and 750 and 950 C. The results show that the plagioclase population in each obsidian sample could have grown from their respective melts, with the exception of a single calcic core (An60–63) in one sample. The results additionally rule out slow cooling in a magma chamber, because this would lead to equilibrium abundances of plagioclase ( 20%), which are far higher than what is observed in the samples (<2%). Nor can resorption (i.e. heating) explain the low abundance of plagioclase, because this would eliminate the more sodic plagioclase crystals and hence the wide compositional range of plagioclase that is observed. The most viable hypothesis is that the sparse plagioclase phenocrysts grew relatively rapidly during magma ascent to the surface; this is consistent with the results of isothermal (850 C) continuous decompression experiments (2 9, 1 0, 0 8, and 0 1 MPa h), under pure-H2O fluid-saturated conditions, which were performed on one of the rhyolites (MLV-36; 73 wt % SiO2) and quenched at PH2O1⁄489, 58 and 40 MPa. The four decompression rates correspond to degassing rates of 1 6, 0 56, 0 45 and 0 06 wt % H2O per day. Decompressions 1 0 MPa(PH2O) h, initiated above the liquidus, quenched to 100% glass at all final PH2O. Decompressions at 0 8 MPa(PH2O) h, also initiated above the liquidus, led to plagioclase crystals nearly five times larger than those grown in runs decompressed at the same rate, but initiated just below the plagioclase-in curve. It is the kinetic hindrance to nucleation that permits crystal growth to be concentrated on relatively few crystals, leading to larger crystals. Plagioclase growth rates from these experiments show that the largest phenocrysts ( 1 mm) in the MLV-36 obsidian could have grown in <42 h. A cooling rate of 1 2 C h closely matches both the increase in melt viscosity with time and the effective undercooling with time that occurs during the 0 8 MPa(PH2O) h decompression over the first 50 h. The combined results point to crystallization of sparse plagioclase driven by relatively rapid rates of degassing6 cooling VC The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 981 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2015, Vol. 56, No. 5, 981–1006 doi: 10.1093/petrology/egv025

Bubbles matter: An assessment of the contribution of vapor bubbles to melt inclusion volatile budgets

Melt inclusions (MI) are considered the best tool available for determining the pre-eruptive volatile contents of magmas. H 2 O and CO 2 concentrations of the glass phase in MI are commonly used both as a barometer and to track magma degassing behavior during ascent due to the strong pressure dependence of H 2 O and CO 2 solubilities in silicate melts. The often unstated and sometimes overlooked requirement for this method to be valid is that the glass phase in the MI must represent the composition of the melt that was trapped at depth in the volcanic plumbing system. However, melt inclusions commonly contain a vapor bubble that formed after trapping owing to differential shrinkage of the melt compared to the host crystal, and/or crystallization at the inclusion-host interface. Such bubbles may contain a substantial portion of volatiles, such as CO 2 , that were originally dissolved in the melt. In this study, we determined the contribution of CO 2 in the vapor bubble to the overall CO 2 content of MI based on quantitative Raman analysis of the vapor bubbles in MI from the 1959 Kilauea Iki (Hawaii), 1960 Kapoho (Hawaii), 1974 Fuego volcano (Guatemala), and 1977 Seguam Island (Alaska) eruptions. We found that the bubbles typically contain 40 to 90% of the total CO 2 in the MI. Reconstructing the original CO 2 content by adding the CO 2 in the bubble back into the melt results in an increase in CO 2 concentration by as much as an order of magnitude (thousands of parts per million). Reconstructed CO 2 concentrations correspond to trapping pressures that are significantly greater than one would predict based on analysis of the volatiles in the glass alone. Trapping depths can be as much as 10 km deeper than estimates that ignore the CO 2 in the bubble. In addition to CO 2 in the vapor bubbles, many MI showed the presence of a carbonate mineral phase. Failure to recognize the carbonate during petrographic examination or analysis of the glass and to include its contained CO 2 when reconstructing the CO 2 content of the originally trapped melt will introduce additional errors into the calculated volatile budget. Our results emphasize that accurate determination of the pre-eruptive volatile content of melts based on analysis of melt inclusions must consider the volatiles contained in the bubble (and carbonates, if present). This can be accomplished either by analysis of the bubble and the glass followed by mass-balance reconstruction of the original volatile content of the melt, or by re-homogenization of the MI prior to conducting microanalysis of the quenched, glassy MI.

Degassing of H2O in a phonolitic melt: A closer look at decompression experiments

Melt degassing during magma ascent is controlled by the decompression rate and can be simulated in decompression experiments. H2O-bearing phonolitic melts were decompressed at a super-liquidus T of 1323K in an internally heated argon pressure vessel, applying continuous decompression (CD) as well as to date commonly used step-wise decompression (SD) techniques to investigate the effect of decompression method on melt degassing. The hydrous melts were decompressed from 200MPa at nominal decompression rates of 0.0028–1.7MPa·s−1. At final pressure (Pfinal), the samples were quenched rapidly at isobaric conditions with ~150K·s−1.The bubbles in the quenched samples are often deformed and dented. Flow textures in the glass indicate melt transport at high viscosity. We suggest that this observation is due to bubble shrinkage during quench. This general problem was mostly overlooked in the interpretation of experimentally degassed samples to date. Bubble shrinkage due to decreasing molar volume (Vm) of the exsolved H2O in the bubbles occurs during isobaric rapid quench until the melt is too viscous too relax. The decrease of Vm(H2O) during cooling at Pfinal of the experiments results in a decrease of the bubble volume by a shrinking factor Bs:Bs=VmH2OPfinalTrqVmH2OPfinalTfwhere Trq is the run temperature of the sample prior to isobaric rapid quench and Tf is the fictive temperature where the bubbles stop to shrink. In a first approximation, the glass transition temperature (Tg) of the melt can be used for Tf. At a Pfinal of 75MPa and a Tg of ~840K in our experiments, the bubble volume decreases by a Bs of ~2.5. Thus, bubble textures, size distributions and porosities derived from the vitrified samples do not correspond to the conditions prior to isobaric rapid quench. Nevertheless, the volume fraction of bubbles (Xb) in the melt can be calculated by:Xbmelt=Bs⋅Xbglass1+Bs⋅Xbglass−Xbglassusing the measured volume fraction of bubbles in the glass in consideration of Bs. The corrected porosities are within a relative error of <10% equal to the expected porosities at Trq that can be easily calculated from the residual H2O content in the glass.At nominal decompression rates >0.17MPa·s−1 and a Pfinal of 75MPa, the decompression method has only minor influence on melt degassing. SD and CD result in high bubble number densities of 104–105mm−3. Fast P drop leads to immediate supersaturation with H2O in the melt. At such high nominal decompression rates, the diffusional transport of H2O is limited and therefore bubble nucleation is the predominant degassing process. The residual H2O contents in the melts decompressed to 75MPa increase with nominal decompression rate. After homogeneous nucleation is triggered, CD rates ≤0.024MPa·s−1 facilitate continuous reduction of the supersaturation by H2O diffusion into previously nucleated bubbles. Bubble number densities of CD samples with low nominal decompression rates are several orders of magnitude lower than for SD experiments and the bubble diameters are larger. The reproducibility of MSD experiments with low nominal decompression rates is worse than for CD runs. Commonly used SD techniques are therefore not suitable to simulate melt degassing during continuous magma ascent with low ascent rates.

Experimental Evidence for Polybaric Differentiation of Primitive Arc Basalt beneath St. Vincent, Lesser Antilles

Equilibrium crystallization experiments have been performed on a primitive high-MgO basalt (HMB) from Soufriere, St. Vincent, with three initial H2O contents (0·6, 2·3 and 4·5 wt %), at pressures of 0·4, 0·7, 1·0 and 1·3 GPa and temperatures from 1350 to 950°C. Redox conditions, as determined by µXANES analysis of Fe3+ in experimental glasses, were 1–4 log units above the nickel–nickel oxide (NNO) buffer. The aim of the study was to explore the differentiation conditions that gave rise to the observed geochemical variation in lavas and plutonic (cumulate) xenoliths from St. Vincent. An experiment with 4·5 wt % initial H2O is multiply saturated close to its liquidus (1180°C and 1·3 GPa) with a spinel lherzolite assemblage, which is consistent with a primary origin for HMB in the mantle wedge. Multiple saturation of HMB with 2·3 wt % H2O was not observed, but is inferred to occur at pressures >1·3 GPa. The experimental results show that initial H2O content has significant influence on differentiation paths of primary HMB magma, with different lava varieties generated under discrete, well-constrained P–T–H2O conditions. Low-magnesian basalts (LMB) can be generated from HMB with 2·3–4·5 wt % H2O at pressures of 1·0–1·3 GPa, corresponding to Moho depths beneath St. Vincent. The CaO contents of LMB are sensitive to differentiation pressure: high-CaO LMB are produced at pressures >0·5 GPa. Basaltic andesites (BA) can be generated at 0·7–1·0 GPa from HMB with 0·6–2·3 wt % H2O. High-alumina basalts (HAB) are produced at mid- to upper-crustal conditions (≤0·4 GPa) by differentiation of HMB with high initial H2O (≥4 wt %) through delay of plagioclase crystallization and dominant fractionation of olivine, clinopyroxene and spinel. St. Vincent andesites could be produced from relatively dry (≤0·6 wt % H2O) HMB only at lower-crustal conditions. This is suggestive of a partial melting origin from precursor HMB that had solidified at depth to produce gabbros with ∼30% hornblende (i.e. ∼0·6 wt % structurally bound H2O). The experimentally determined differentiation conditions are consistent with polybaric differentiation within a hot zone that extends from the Moho and uppermost mantle to the mid- or upper crust. Within the hot zone differentiation occurs by a combination of crystallization of HMB with 2–5 wt % H2O and partial melting of ancestral HMB gabbros. Although the experimental melts provide an excellent match to erupted lava compositions, experimental crystal compositions do not match either phenocrysts or cumulate crystals, as preserved in xenoliths. The failure to reproduce natural crystal compositions suggests that these are formed as differentiated magmas ascend and attain their H2O-saturated liquidi at shallower pressures. Thus there is a disconnect between the high-pressure phase compositions and assemblages that generate liquid compositional diversity and the low-pressure composition and assemblages that occur as phenocrysts and in cumulate xenoliths. This finding lends support to the idea of cryptic fractionation in the generation of arc magmas.

Partitioning of light lithophile elements during basalt eruptions on Earth and application to Martian shergottites

An enigmatic record of light lithophile element (LLE) zoning in pyroxenes in basaltic shergottite meteorites, whereby LLE concentrations decrease dramatically from the cores to the rims, has been interpreted as being due to partitioning of LLE into a hydrous vapor during magma ascent to the surface on Mars. These trends are used as evidence that Martian basaltic melts are water-rich (McSween et al., 2001). Lithium and boron are light lithophile elements (LLE) that partition into volcanic minerals and into vapor from silicate melts, making them potential tracers of degassing processes during magma ascent to the surface of Earth and of other planets. While LLE degassing behavior is relatively well understood for silica-rich melts, where water and LLE concentrations are relatively high, very little data exists for LLE abundance, heterogeneity and degassing in basaltic melts. The lack of data hampers interpretation of the trends in the shergottite meteorites. Through a geochemical study of LLE, volatile and trace elements in olivine-hosted melt inclusions from Kilauea Volcano, Hawaii, it can be demonstrated that lithium behaves similarly to the light to middle rare Earth elements during melting, magma mixing and fractionation. Considerable heterogeneity in lithium and boron is inherited from mantle-derived primary melts, which is dominant over the fractionation and degassing signal. Lithium and boron are only very weakly volatile in basaltic melt erupted from Kilauea Volcano, with vapor-melt partition coefficients <0.1. Degassing of LLE is further inhibited at high temperatures. Pyroxene and associated melt inclusion LLE concentrations from a range of volcanoes are used to quantify lithium pyroxene-melt partition coefficients, which correlate negatively with melt H2O content, ranging from 0.13 at low water contents to <0.08 at H2O contents >4 wt%. The observed terrestrial LLE partitioning behavior is extrapolated to Martian primitive melts through modeling. The zoning observed in the shergottite pyroxenes is only consistent with degassing of LLE from a Martian melt near its liquidus temperature if the vapor-melt partition coefficient was an order of magnitude larger than observed on Earth. The range in LLE and trace elements observed in shergottite pyroxenes are instead consistent with concurrent mixing and fractionation of heterogeneous melts from the mantle.

A K-feldspar–liquid hygrometer specific to alkaline differentiated magmas

We present a K-feldspar–liquid hygrometer specific to alkaline differentiated magmas that is calibrated through the regression analysis of sanidine and anorthoclase crystals coexisting with trachyte and phonolite melts. Partial-regression leverage plots were used to determine the minimum number of regression parameters that closely describe the variance of the dataset. The derived model was tested using K-feldspar–liquid pairs not included into the calibration dataset in order to address issues of systematic errors. When K-feldspar and plagioclase crystals coprecipitate from the same alkaline liquid under identical P-T-X-fO2-H2O conditions, the ability prediction of the new hygrometer is comparable to that of previous plagioclase–liquid models. To minimize the error of H2O estimate caused by the inadvertent use of disequilibrium data in natural samples, we have also calibrated a new test for equilibrium based on Or–Ab exchange between K-feldspar and coexisting melt. As an immediate application for both equilibrium and hygrometer models, we used as input data K-feldspar–liquid pairs from alkaline explosive eruptions at the Phlegrean Fields. The estimates of H2O dissolved in natural trachyte and phonolite magmas closely match those determined by melt inclusion analysis and H2O solubility modeling. This leads to the conclusion that our new models can significantly contribute to a better quantitative characterization of the H2O content in differentiated alkaline magmas feeding large-volume explosive eruptions.

Nature and Evolution of Primitive Vesuvius Magmas: an Experimental Study

Two mafic eruptive products from Vesuvius, a tephrite and a trachybasalt, have been crystallized in the laboratory to constrain the nature of primitive Vesuvius magmas and their crustal evolution. Experiments were performed at high temperatures (from 1000 to ≥1200°C) and both at 0·1 MPa and at high pressures (from 50 to 200 MPa) under H2O-bearing fluid-absent and H2O- and CO2-bearing fluid-present conditions. Experiments started from glass except for a few that started from glass plus San Carlos olivine crystals to force olivine saturation. Melt H2O concentrations reached a maximum of 6·0 wt % and experimental fO2 ranged from NNO – 0·1 to NNO + 3·4 (where NNO is nickel–nickel oxide buffer). Clinopyroxene (Mg# up to 93) is the liquidus phase for the two investigated samples; it is followed by leucite for H2O in melt <3 wt %, and by phlogopite (Mg# up to 81) for H2O in melt >3 wt %. Olivine (Fo85) crystallized spontaneously in only one experimental charge. Plagioclase was not found. Upon progressive crystallization of clinopyroxene, glass K2O and Al2O3 contents strongly increase whereas MgO, CaO and CaO/Al2O3 decrease; the residual melts follow the evolution of Vesuvius whole-rocks from trachybasalt to tephrite, phonotephrite and to tephriphonolite. Concentrations of H2O and CO2 in near-liquidus 200 MPa glasses and primitive melt inclusions from the literature overlap. The earliest evolutionary stage, corresponding to the crystallization of Fo-rich olivine, was reconstructed by the olivine-added experiments. They show that the primitive Vesuvius melts are trachybasalts (K2O ∼ 4·5–5·5 wt %, MgO = 8–9 wt %, Mg# = 75–80, CaO/Al2O3 = 0·9–0·95) that crystallize Fo-rich olivine (90–91) as the liquidus phase between 1150 and 1200°C and from 300 to <200 MPa. Primitive Vesuvius melts are volatile-rich (1·5–4·5 wt % H2O and 600–4500 ppm CO2 in primitive melt inclusions) and oxidized (from NNO + 0·4 to NNO + 1·2). Assimilation of carbonate wall-rocks by ascending primitive magmas can account for the disappearance of olivine from crystallization sequences and explains the lack of rocks representative of olivine-crystallizing magmas. A correlation between carbonate assimilation and the type of feeding system is proposed: carbonate assimilation is promoted for primitive magma batches of small volumes. In contrast, for longer-lived, large-volume, less frequently recharged, hence more evolved, cooler reservoirs, magma–carbonate interaction is limited. Primitive magmas from Vesuvius and other Campanian volcanoes have similar redox states. However, the Cr# of Vesuvius spinels is distinctive and therefore the peridotitic component in the mantle source of Vesuvius differs from that of the other Campanian magmas.

C-O-H-Cl-S-F Volatile Solubilities, Partitioning, and Mixing in Phonolitic-Trachytic Melts and Aqueous-Carbonic Vapor Saline Liquid at 200 MPa

Hydrothermal experiments were conducted at 200MPa and 900^10188C to determine the solubilities, fluid(s)^melt partitioning, and mixing properties of H2O, CO2, S, Cl, and F in phonolitic^trachytic melts saturated in vapor, vapor plus saline liquid, or saline liquid.The bulk compositions and S, Cl, and F concentrations of the run-product glasses were determined by electron microprobe and the H2O and CO2 contents by Fourier-transform infrared spectroscopy (FTIR). A new parameterization was developed to calculate molar absorption coefficients for FTIR analysis of carbonate in glasses and applied to the run-product glasses.The concentrations of volatiles in the fluid(s) were determined by mass-balance calculations and checked with chloridometer analysis and gravimetry.The range in oxygen fugacity of these experiments is NNO to NNOþ 2 (where NNO is nickel^nickel oxide buffer).The phonolitic^trachytic melts dissolved up to 7 5 wt % H2O, 0 94 wt % Cl, 0 73 wt % CO2, 0 75 wt % F, and 0 16 wt % S, and the integrated bulk fluid(s) contained up to 99 mol % H2O, 34 mol % Cl, 82 mol % CO2, 1 7 mol % F, and 3 7 mol % S.The mixing relationships of H2O, CO2, and Cl in melt versus fluid(s) are complex and strongly non-ideal at these pressure^temperature conditions, particularly with two fluid phases stable. The concentrations of H2O and CO2 in melt change with the addition of Cl S to the system, and the solubility of Cl in melt varies with S.The reductions in H2O and CO2 solubility in melt exceed those resulting from simple dilution of the coexisting fluid(s) owing to addition of other volatiles.The partitioning of H2O and CO2 between fluid(s) and melt varies as a function of fluid(s) and melt composition.The experimental data are applied to phonolitic and related magmas of Mt. Somma^ Vesuvius, Italy, Mt. Erebus, Antarctica, and Cripple Creek, USA, to better interpret processes of fluid(s) exsolution in eruptive and mineralizing systems. Application of the experimental results also provides constraints on eruptive and mineralizing fluid(s) compositions.

The Composition of Hydrous Partial Melts of Garnet Peridotite at 6 GPa: Implications for the Origin of Group II Kimberlites

Chemical compositions of hydrous melts, compatible with those that would form by incipient melting of upper mantle peridotite at 180 km depth, have been determined using a series of iterative crystallization experiments. Experiments were performed in a multianvil apparatus at 6 GPa and 14008C and a melt was ultimately produced that was saturated in a residual peridotite assemblage (olivineþ clinopyroxeneþgarnet orthopyroxene). The multiply saturated hydrous melts have higher (MgþFe)/Si and Al/Ca compared with hydrous melts produced at lower pressures.The melt compositions are similar to those determined near the dry peridotite solidus at 17008C, when compared on an H2O-free basis. Melt H2O contents were determined to be 11wt % using mass balance, and these estimates were made more accurate by maintaining a large proportion of melt (470 wt %) in each experiment. If the geophysically inferred seismic low-velocity zone is caused by the presence of H2O-rich melt then at the base of this zone, at 220 km, these results imply that the melt must contain 15^16 wt % H2O.The hydrous melt compositions, when compared on a volatile-free basis, are found to be similar to those of group II kimberlites (orangeites). The low FeO and Na2O but enriched K2O concentrations in group II magmas imply their derivation from melt-depleted cratonic lithosphere enriched by the metasomatic addition principally of K2O and H2O. A simple model is proposed in which this enrichment occurs by the addition of phlogopite to the source peridotite. Using determined K2O and H2O partition coefficients and assuming that the ratio of both components in the source is controlled by their ratio in phlogopite, group II kimberlite magmas can be constrained as being the product of 0 2 wt % melting of a garnet peridotite source rock enriched with 1 7 wt % phlogopite, undergoing melting at near-adiabatic temperatures.

Experimental reactions between olivine and orthopyroxene with phonolite melt: implications for the origins of hydrous amphibole + phlogopite + diopside bearing metasomatic veins

Amphibole + phlogopite + diopside bearing veins are observed in a large number of upper mantle xenoliths, but the composition of the melt that forms them is poorly constrained. Recent data from the Heldburg Phonolite, Central Germany, has shown that phonolite melt will react with olivine and orthopyroxene xenocrysts to form reaction rims of amphibole + phlogopite + diopside at mid-lower crustal pressures. This is the first example of where a melt has reacted with peridotite to form the mineralogy of the metasomatic veins. It is therefore necessary to explore whether a phonolite melt could be the parent melt that forms amphibole + phlogopite + diopside metasomatic veins. Experimental reactions between single crystals of olivine and orthopyroxene with phonolite melt were conducted at upper mantle conditions of 1.0–1.5 GPa and 900–1,000 °C. Melt water contents were varied from anhydrous to >12 wt. H2O. Olivine reacts to form phlogopite reaction rims with overgrowths of diopside 1,000 °C. Orthopyroxene reacts to form amphibole with epitaxial diopside overgrowths <1,000 °C. No reaction rims form when the bulk melt H2O is lower than ~3.8 wt%. Pressure has little effect over the small range tested. These experiments reproduce reaction rims on olivine and orthopyroxene observed in the Heldburg Phonolite, Central Germany, and suggest that a relatively narrow range of temperatures and melt water contents is required for rim formation. The compositions of rim amphibole, phlogopite and diopside from the experiments have very similar compositions to those from Heldburg but do not match those from metasomatic veins. Phenocrysts from Heldburg are similar to the metasomatic veins, suggesting that a phonolite could potentially form the veins if vein formation is dominated by crystallization rather than reaction and replacement of wall rock phases.

Compositional and pressure effects on the solubility of H2O and CO2 in mafic melts

The effect of the anhydrous composition on the solubilities of H2O and CO2 in mafic melts varying from MORB to nephelinite was investigated experimentally between 50 and 500MPa at 1200 to 1250°C. In all compositions, CO2 is only present as carbonate species in the quenched glasses. The concentrations of dissolved H2O and CO2 have been analyzed by KFT (Karl–Fischer titration) and FTIR (Fourier-transform infrared spectroscopy). The Mid-Infrared (MIR) absorption coefficients for the H2O band at 3500cm−1 are identical within error for all investigated melt compositions and equal to 59.2±4L/(mol∗cm). The absorption coefficients for the carbonate bands vary in the range 306±32 to 360±24L/(mol∗cm) for the 1430cm−1 band and in the range 349±25 to 394±27L/(mol∗cm) for the 1520cm−1 band. However, a simple correlation with the melt composition could not be determined.Water solubility in mafic to intermediate melts increases slightly with the total alkali content and the effect of composition is more pronounced at higher pressures. At 500MPa, the solubility of H2O in melts coexisting with nearly pure H2O fluids varies from 8.8 to 9.5wt.% H2O. A strong effect of melt composition on the solubility of CO2 is observed at all investigated pressures. For instance, at 500MPa, mafic melts coexisting with nearly pure CO2 fluids can dissolve from around 0.32 to more than 1.30wt.% CO2 as melt composition changes from tholeiite to nephelinite. The compositional effect on the solubility of CO2 in melts coexisting with pure CO2 fluid is best described by non-linear (exponential) correlations with compositional parameters such as the parameter Π proposed by Dixon (1997; American Mineralogist, 82: 368–378) or structural parameters (e.g., nonbridging oxygen per tetrahedrally coordinated cation). The obtained relationships are used to propose empirically derived equations of the form ln(CO2)=1.150·lnP+6.71·Π*−1.345, where CO2 is the solubility of CO2 in silicate melts in wt.% (at 1200 to 1250°C), P is pressure in MPa and Π* is a compositional parameter (Π*=Ca2++0.8K++0.7Na++0.4Mg2++0.4Fe2+)/(Si4++Al3+) with cations expressed as cation fractions in melt.

High P–T Brillouin scattering study of H2O melting to 26 GPa

The properties of solid and liquid phases of H2O at high pressure and temperature remain an active area of research. In this study, Brillouin spectroscopy has been used to determine the temperature dependence of sound velocities in H2O as a function of pressure up to 26 GPa through the phase field of ice VII and into the liquid to a maximum temperature of 1200 K. The Brillouin shift of the quasi-longitudinal acoustic mode moves to lower frequencies upon melting at each pressure. As a test of the method, measurements of the melting of Ar by Brillouin scattering at several pressures show a similar behavior for the acoustic mode, and measured melting points are consistent with previous results. The results of H2O melting are consistent with previously reported melting curves below 20 GPa. The data at higher pressure indicate that ice melts at a higher temperature than a number of previous studies have indicated.

Amphibole as an archivist of magmatic crystallization conditions: problems, potential, and implications for inferring magma storage prior to the paroxysmal 2010 eruption of Mount Merapi, Indonesia

Amphibole is widely employed to calculate crystallization temperature and pressure, although its potential as a geobarometer has always been debated. Recently, Ridolfi et al. (Contrib Mineral Petrol 160:45–66, 2010) and Ridolfi and Renzulli (Contrib Mineral Petrol 163:877–895, 2012) have presented calibrations for calculating temperature, pressure, fO2, melt H2O, and melt major and minor oxide composition from amphibole with a large compositional range. Using their calibrations, we have (i) calculated crystallization conditions for amphibole from eleven published experimental studies to examine the problems and the potential of the new calibrations; and (ii) calculated crystallization conditions for amphibole from basaltic–andesitic pyroclasts erupted during the paroxysmal 2010 eruption of Mount Merapi in Java, Indonesia, to infer pre-eruptive conditions. Our comparison of experimental and calculated values shows that calculated crystallization temperatures are reasonable estimates. Calculated fO2 and melt SiO2 content yields potentially useful estimates at moderately reduced to moderately oxidized conditions and intermediate to felsic melt compositions. However, calculated crystallization pressure and melt H2O content are untenable estimates that largely reflect compositional variation in the crystallizing magmas and crystallization temperature and not the calculated parameters. Amphibole from Merapi’s pyroclasts yields calculated conditions of ~200–800 MPa, ~900–1,050 °C, ~NNO + 0.3–NNO + 1.1, ~3.7–7.2 wt% melt H2O, and ~58–71 wt% melt SiO2. We interpret the variations in calculated temperature, fO2, and melt SiO2 content as reasonable estimates, but conclude that the large calculated pressure variation for amphibole from Merapi and many other arc volcanoes is evidence for thorough mixing of mafic to felsic magmas and not necessarily evidence for crystallization over a large depth range. In contrast, bimodal pressure estimates obtained for other arc magmas reflect amphibole crystallization from mafic and more evolved magmas, respectively, and should not necessarily be taken as evidence for crystallization in two reservoirs at variable depth.