Water Content Of Melt Research Articles

Experimental calibration of a new oxybarometer for silicic magmas based on vanadium partitioning between magnetite and silicate melt

Partition coefficients of vanadium between magnetite and rhyolitic silicate melt, DVmgt/melt, were experimentally determined as a function of oxygen fugacity (0.7–4.0 log units above the fayalite-magnetite-quartz buffer), temperature (800–1000°C), melt alumina saturation index (ASI=0.74–1.14), magnetite composition (0.2–14wt% TiO2) and pressure (1–5kbar; at H2O saturation). Experiments were performed by equilibrating small (≤20µm), V-free magnetite grains in V-doped silicate melts (∼100ppmV) and then analyzing both phases by LA-ICP-MS. Attainment of equilibrium was demonstrated by several reversal experiments. The results suggest that DVmgt/melt depends strongly on fO2, increasing by 1.5–1.7 log units from the MnO-Mn3O4 buffer to the Ni-NiO buffer, and to lesser (but still considerable) extents on melt alumina saturation index (ASI; increasing by 0.3–0.7 log units over 0.4 ASI units) and temperature (increasing by 0.3–0.7 log units over a 200°C interval at a fixed fO2 buffer). Magnetite composition and melt water content seem to have negligible effects. The data were fitted by the following linear regression equation:logDVmgt/melt=0.3726∗10,000T+2.0465∗ASI-0.4773∗ΔFMQ-2.1214,in which temperature is given in K, ASI refers to molar Al2O3/(CaO+Na2O+K2O) and ΔFMQ refers to the deviation of fO2 (in log units) from the fayalite-magnetite-quartz buffer. This equation reproduces all of our data within 0.3 log units, and 89% of them within 0.15 log units. The main advantages of this new oxybarometer over classical magnetite–ilmenite oxybarometry are (1) that it can be applied to rocks that do not contain ilmenite, and (2) that it is easier to apply to slowly-cooled rocks such as granites.

Geochemistry of the Topuk Pluton associated with the Kozbudaklar W-skarn deposit (Western Anatolia, Turkey): Implication for crystallization conditions

The Kozbudaklar scheelite-bearing skarn deposit in the Tavşanlı Zone, western Turkey, occurs at the contact between Eocene Topuk pluton and Triassic İnönü marble of calcic character. The Topuk pluton is medium-coarse grained, granodiorite in composition and has a hypidiomorphic equigranular texture. The host rock contains mafic microgranular enclaves (MME) of monzodiorite–monzogabbro composition and is interrupted by porphyritic granodiorite and granite-aplite vein rocks. The pluton is calk-alkaline, metaluminous and composed of I-type melt character. δ18O and δD compositions of silicate minerals from granodioritic host rock are 5.9–10.6‰ and −77.0 to −71.4‰ and conformable with the range of unaltered I-type granites. Trace element contents indicate that pluton is crystallized from mantle-derived magma interacted with continental crust in a volcanic arc or subduction related setting. Major and trace element concentrations of Topuk pluton are quite consistent with geochemical patterns of Cu-skarn granitoids.Results of mineral chemistry analysis of the pluton yield that plagioclases are of oligoclase–andesine, amphiboles are of magnesio-hornblende and biotites are of ferro-magnesian composition. Amphiboles and biotites of granodioritic host rock are represented by calc-alkaline, I-type melt composition evolved in a subduction environment. Based on the results of plagioclase–Al in hornblende and amphibole chemistry data from the pluton, two different stages are proposed for the magma crystallization. The first stage was developed in a relatively deeper environment (>15 km) under high pressure (>4 kbar) and low log ƒO2 (>−17.6) conditions which reflect fractional crystallization and magma-mixing depth of basaltic magma and these conditions are not correlated with scheelite mineralization. The second crystallization stage of magma which proceeded at shallow depths (<6 km) was also developed in two separate phases with respect to P-T conditions. The first phase associated with scheelite mineralization is characterized by high temperature (788–854 °C), relatively high pressure (1.20–1.62 kbar), shallow depth (5–6 km) and high log ƒO2 (>−12.9 to −11.0) values are accompanied by high H2O contents (5.39–6.88 wt.%). High water content of melt gave rise to magma to ascend to shallower depths (4–3 km) and crystallization to proceed under low pressure (∼1.00 kbar), high temperature (751–859 °C) and log ƒO2 (−13.3 to −11.0) conditions with lower water contents (4.55–5.50 wt.%).

Shock-transformation of whitlockite to merrillite and the implications for meteoritic phosphate

Meteorites represent the only samples available for study on Earth of a number of planetary bodies. The minerals within meteorites therefore hold the key to addressing numerous questions about our solar system. Of particular interest is the Ca-phosphate mineral merrillite, the anhydrous end-member of the merrillite–whitlockite solid solution series. For example, the anhydrous nature of merrillite in Martian meteorites has been interpreted as evidence of water-limited late-stage Martian melts. However, recent research on apatite in the same meteorites suggests higher water content in melts. One complication of using meteorites rather than direct samples is the shock compression all meteorites have experienced, which can alter meteorite mineralogy. Here we show whitlockite transformation into merrillite by shock-compression levels relevant to meteorites, including Martian meteorites. The results open the possibility that at least part of meteoritic merrillite may have originally been H+-bearing whitlockite with implications for interpreting meteorites and the need for future sample return.

Anoxic atmospheres on Mars driven by volcanism: Implications for past environments and life

Mars today has no active volcanism and its atmosphere is oxidizing, dominated by the photochemistry of CO2 and H2O. Mars experienced widespread volcanism in the past and volcanic emissions should have included reducing gases, such as H2 and CO, as well as sulfur-bearing gases. Using a one-dimensional photochemical model, we consider whether plausible volcanic gas fluxes could have switched the redox-state of the past martian atmosphere to reducing conditions. In our model, the total quantity and proportions of volcanic gases depend on the water content, outgassing pressure, and oxygen fugacity of the source melt. We find that, with reasonable melt parameters, the past martian atmosphere (∼3.5Gyr to present) could have easily reached reducing and anoxic conditions with modest levels of volcanism, >0.14km3yr−1, which are well within the range of estimates from thermal evolution models or photogeological studies. Counter-intuitively we also find that more reducing melts with lower oxygen fugacity require greater amounts of volcanism to switch a paleo-atmosphere from oxidizing to reducing. The reason is that sulfur is more stable in such melts and lower absolute fluxes of sulfur-bearing gases more than compensate for increases in the proportions of H2 and CO. These results imply that ancient Mars should have experienced periods with anoxic and reducing atmospheres even through the mid-Amazonian whenever volcanic outgassing was sustained at sufficient levels. Reducing anoxic conditions are potentially conducive to the synthesis of prebiotic organic compounds, such as amino acids, and are therefore relevant to the possibility of life on Mars. Also, anoxic reducing conditions should have influenced the type of minerals that were formed on the surface or deposited from the atmosphere. We suggest looking for elemental polysulfur (S8) as a signature of past reducing atmospheres. Finally, our models allow us to estimate the amount of volcanically sourced atmospheric sulfate deposited over Mars’ history, approximately ∼106–109Tmol, with a spread depending on assumed outgassing rate history and magmatic source conditions.

Influence of Processing Conditions on the Morphological Structure and Ductility of Water-Foamed Injection Molded PP/LDPE Blended Parts

Water-foamed injection molding (WFIM) uses conventional injection molding (CIM) with water as a physical foaming agent. Compared to CIM, WFIM is a much more complicated process. As such, it is critical to determine the processing conditions for fabricating quality parts using WFIM. We used the design of experiment (DOE) method based on the Taguchi method to determine the influence of the processing conditions on the morphological structure and ductility of PP/LDPE WFIM parts, which were investigated by tensile testing and scanning electron microscopy (SEM). Our research suggests that fabricating PP/LDPE super-ductile parts using WFIM is indeed feasible. Our research also indicates that there is a close relationship between the ductility and the foamed structures, both of which are deeply influenced by the processing conditions. The analysis of variance results shows further that the water content had the greatest influence on the ductility, followed by the melt temperature, packing time, packing pressure, and PP/LDPE ratio. However, the ductility was only slightly influenced by the mold temperature, injection pressure, and injection time in WFIM. As to the number of cells, the order of influence was melt temperature, water content, packing time, packing pressure, mold temperature, injection pressure, PP/LDPE ratio, and injection time, in that order. In addition, applying DOE is a quite effective method for deducing the optimal set of effective factors in WFIM to produce super-ductile parts with a maximum number of cells. To our knowledge, this is the first time that the relationship among the processing conditions, ductility, and foamed structure of PP/LDPE WFIM super-ductile parts has been investigated and reported.

Distribution of water masses and meltwater on the continental shelf near the Totten and Moscow University ice shelves

AbstractWarm waters flood the continental shelf of the Amundsen and Bellingshausen seas in West Antarctica, driving rapid basal melt of ice shelves. In contrast, waters on the continental shelf in East Antarctica are cooler and ice shelves experience relatively low rates of basal melt. An exception is provided by the Totten and Moscow University ice shelves on the Sabrina Coast, where satellite‐derived basal melt rates are comparable to West Antarctica. Recent oceanographic observations have revealed that relatively warm (∼−0.4°C) modified Circumpolar Deep Water (mCDW) enters the cavity beneath the Totten Ice Shelf through a 1100 m deep trough, delivering sufficient heat to drive rapid basal melt. Here we use observations from a recent summer survey to show that mCDW is widespread on the continental shelf of the Sabrina Coast, forming a warm (up to 0.3°C) and saline (34.5–34.6) bottom layer overlaid by cold (∼freezing point) and fresh (salinity ∼34.3) Winter Water. Dense Shelf Water is not observed. A 1000 deep m trough allows water at −1.3°C to reach the Moscow University ice‐shelf cavity to drive basal melt. Freshening by addition of glacial meltwater is widespread on the southern shelf at depths above 300–400 m, with maximum meltwater concentrations up to 4–5 ml L−1 observed in outflows from the ice‐shelf cavities. Our observations indicate that the ocean properties on the Sabrina Coast more resemble those found on the continental shelf of the Amundsen and Bellingshausen seas than those typical of East Antarctica.

Diurnal Cycles of Meltwater Percolation, Refreezing, and Drainage in the Supraglacial Snowpack of Haig Glacier, Canadian Rocky Mountains

Meltwater refreezing and storage in the supraglacial snowpack can reduce and delay meltwater runoff from glaciers. These are well-established processes in polar environments, but the importance of meltwater refreezing and the efficiency of meltwater drainage are uncertain on temperate alpine glaciers. To examine these processes and quantify their importance on a mid-latitude mountain glacier, we measured the temperature and meltwater content in the upper 50 cm of the supraglacial snowpack of Haig Glacier in the Canadian Rocky Mountains. Thermistors and TDR probes were installed at 10-cm intervals at two sites in the glacier accumulation area from May to September, 2015. A Denoth meter was used to make point measurements for comparison with the TDR inferences of snowpack dielectric properties. These data are supplemented by automatic weather station data, used to calculate surface melt rates and drive a model of subsurface temperature, refreezing, and drainage. We observed a strong diurnal cycle in snow water content throughout the summer melt season, but subsurface refreezing was only significant in May; after this, overnight refreezing was restricted to a thin surface layer of the snowpack. Overnight decreases in water content after May are associated with meltwater percolation and drainage. There was negligible meltwater retention in the snow on a daily basis, but the refrozen water does represent an ‘energy sink’, with 10-15% of the available melt energy diverted to recycled rather than new meltwater. This reduces the total meltwater runoff from the site, even though no meltwater is retained in the system.

Pathways of Meltwater Export from Petermann Glacier, Greenland

AbstractIntrusions of Atlantic Water cause basal melting of Greenland’s marine-terminating glaciers and ice shelves, such as that of Petermann Glacier, in northwest Greenland. The fate of the resulting glacial meltwater is largely unknown. It is investigated here, using hydrographic observations collected during a research cruise in Petermann Fjord and adjacent Nares Strait onboard icebreaker (I/B) Oden in August 2015. A three end-member mixing method provides the concentration of Petermann ice shelf meltwater. Meltwater from Petermann is found in all of the casts in adjacent Nares Strait, with the highest concentration along the Greenland coast in the direction of Kelvin wave phase propagation. The meltwater from Petermann mostly flows out on the northeast side of the fjord as a baroclinic boundary current, with the depth of maximum meltwater concentrations approximately 150 m and shoaling along its pathway. At the outer sill, which separates the fjord from the ambient ocean, approximately 0.3 mSv (1 Sv ≡ 106 m3 s−1) of basal meltwater leaves the fjord at depths between 100 and 300 m. The total geostrophic heat and freshwater fluxes close to the glacier’s terminus in August 2015 were similar to those estimated in August 2009, before the two major calving events that reduced the length of Petermann’s ice tongue by nearly a third and despite warmer inflowing Atlantic Water. These results provide a baseline but also highlight what is needed to assess properly the impact on ocean circulation and sea level of Greenland’s mass loss as the Atlantic Water warms up.

Characterization of Hydrogeochemical Processes Controlling Major Ion Chemistry of the Batal Glacier Meltwater, Chandra Basin, Himachal Pradesh, India

This manuscript discusses about the solute acquisition processes controlling hydrogeochemistry and suspended sediment characteristics of meltwater of the Batal glacier, Western Himalaya, India. The predominance of anions and cations in meltwater follows the order: SO42− > HCO3− > Cl− > NO3− and Ca2+> Mg2+> Na+> K+, respectively. High excess of (Ca + Mg) over (Na + K), high input of (Ca + Mg) to the TZ+ (total cations) and very low input of (Na + K) to the total cations (TZ+) demonstrate that meltwater chemistry of Batal glacier is predominantly regulated by carbonate type weathering along with minor input from silicate type weathering. Sulphide oxidation is the dominant mechanism responsible for delivery of hydrogen ions (protons) for weathering in the glacier environment, which is revealed by C-ratio of meltwater. The Piper trilinear plot indicates that Ca–SO4 is the prevalent water type in the study area with dominancy of (Ca + Mg) (alkaline earth metals) over (Na + K) (alkali metals) and dominancy of (SO4 + Cl) (strong acid) over (HCO3) (weak acid). Interrelation among the different chemical constituents of meltwater and various factor regulating meltwater chemistry of the Batal glacier are evaluated by multivariate statistical analysis. Average suspended sediment concentrations in meltwater of the study area for August and September 2014 were computed to be 706 and 98.3 mg/l, respectively. These results show that in August suspended sediment concentrations is high and in September its concentration is low.

Silica-rich lavas in the oceanic crust: experimental evidence for fractional crystallization under low water activity

We experimentally investigated phase relations and phase compositions as well as the influence of water activity (aH2O) and redox conditions on the equilibrium crystallization path within an oceanic dacitic potassium-depleted system at shallow pressure (200 MPa). Moreover, we measured the partitioning of trace elements between melt and plagioclase via secondary ion mass spectrometry for a highly evolved experiment (SiO2 = 74.6 wt%). As starting material, we used a dacitic glass dredged at the Pacific-Antarctic Rise. Phase assemblages in natural high-silica systems reported from different locations of fast-spreading oceanic crust could be experimentally reproduced only in a relatively small range of temperature and melt-water content (T ~950 °C; melt H2O < 1.5 wt%) at redox conditions slightly below the quartz–fayalite–magnetite buffer. The relatively low water content is remarkable, because distinct hydrothermal influence is generally regarded as key for producing silica-rich rocks in an oceanic environment. However, our conclusion is also supported by mineral and melt chemistry of natural evolved rocks; these rocks are only congruent to the composition of those experimental phases that are produced under low aH2O. Low FeO contents under water-saturated conditions and the characteristic enrichment of Al2O3 in high aH2O experiments, in particular, contradict natural observations, while experiments with low aH2O match the natural trend. Moreover, the observation that highly evolved experimental melts remain H2O-poor while they are relatively enriched in chlorine implies a decoupling between these two volatiles during crustal contamination.

Estimating the recharge properties of the deep ocean using noble gases and helium isotopes

AbstractThe distribution of noble gases and helium isotopes in the dense shelf waters of Antarctica reflects the boundary conditions near the ocean surface: air‐sea exchange, sea ice formation, and subsurface ice melt. We use a nonlinear least squares solution to determine the value of the recharge temperature and salinity, as well as the excess air injection and glacial meltwater content throughout the water column and in the precursor to Antarctic Bottom Water. The noble gas‐derived recharge temperature and salinity in the Weddell Gyre are −1.95°C and 34.95 psu near 5500 m; these cold, salty recharge values are a result of surface cooling as well as brine rejection during sea ice formation in Antarctic polynyas. In comparison, the global value for deep water recharge temperature is −0.44°C at 5500 m, which is 1.5°C warmer than the southern hemisphere deep water recharge temperature, reflecting a distinct contribution from the north Atlantic. The contrast between northern and southern hemisphere recharge properties highlights the impact of sea ice formation on setting the gas properties in southern sourced deep water. Below 1000 m, glacial meltwater averages 3.5‰ by volume and represents greater than 50% of the excess neon and argon found in the water column. These results indicate glacial melt has a nonnegligible impact on the atmospheric gas content of Antarctic Bottom Water.

Effect of H2O on metal–silicate partitioning of Ni, Co, V, Cr, Mn and Fe: Implications for the oxidation state of the Earth and Mars

This study investigates the metal–silicate partitioning of Ni, Co, V, Cr, Mn and Fe during core mantle differentiation of terrestrial planets under hydrous conditions. For this, we equilibrated a molten hydrous CI chondrite model composition with various Fe-rich alloys in the system Fe–C–Ni–Co–Si–S in a multi-anvil over a range of P, T, fO2 and water content (5–20GPa, 2073–2500K, from 1 to 5 log units below the iron–wüstite (IW) buffer and for XH2O varying from 500ppm to 1.5wt%). By comparing the present experiments with the available data sets on dry systems, we observes that the effect of water on the partition coefficients of moderately siderophile elements is only moderate. For example, for iron we observed a decrease in the partition coefficient of Fe (Dmet/silFe) from 9.5 to 4.3, with increasing water content of the silicate melt, from 0 to 1.44wt%, respectively. The evolution of metal–silicate partition coefficients of Ni, Co, V, Cr, Mn and Fe are modelled based on sets of empirical parameters. These empirical models are then used to refine the process of core segregation during accretion of Mars and the Earth. It appears that the likely presence of 3.5wt% water on Mars during the core–mantle segregation could account for ∼74% of the FeO content of the Martian mantle. In contrast, water does not play such an important role for the Earth; only 4–6% of the FeO content of its mantle could be due to the water-induced Fe-oxidation, for a likely initial water concentration of 1.8wt%. Thus, in order to reproduce the present-day FeO content of 8wt% in the mantle, the Earth could initially have been accreted from a large fraction (between 85% and 90%) of reducing bodies (similar to EH chondrites), with 10–15% of the Earth’s mass likely made of more oxidized components that introduced the major part of water and FeO to the Earth. This high proportion of enstatite chondrites in the original constitution of the Earth is consistent with the 17O,48Ca,50Ti,62Ni and 90Mo isotopic study by Dauphas et al. (2014). If we assume that the CI-chondrite was oxidized during accretion, its intrinsically high water content suggests a maximum initial water concentration in the range of 1.2–1.8wt% for the Earth, and 2.5–3.5wt% for Mars.

Variable H2O content in magmas from the Tongariro Volcanic Centre and its relation to crustal storage and magma ascent

The water content of crystal-hosted glass inclusions from Mt. Ruapehu has been determined by Fourier transform infrared spectroscopy (FTIR) at the IR beamline of the Australian Synchrotron. The results are compared with those from previous investigations as well as with calculated melt water concentrations in other magmas from the Tongariro Volcannic Center (TgVC). It is shown that low and high water content in different magmas can be related to distinct styles of magma ascent and intermittent crustal storage. The first style is related to frequent small magma batches erupted from the central volcanoes of Mt. Tongariro and Mt. Ruapehu. It produces highly porphyritic two-pyroxene-plagioclase andesites which generally show water contents below 3wt%. The second style is sourced from mid-crustal intrusions which are characterized by highly differentiated hornblende dacites with dissolved water concentrations of up to 6wt% H2O.

A model for polythermal ice incorporating gravity-driven moisture transport

The flow of ice sheets and glaciers dissipates significant amounts of heat, which can result in the formation of ‘temperate ice’, a binary mixture of ice and small amounts of melt water that exists at the melting point. Many ice masses are polythermal, in the sense that they contain cold ice, below the melting point, as well as temperate ice. Temperature and melt water (or moisture) content conversely affect the flow of these ice masses through their effect on ice viscosity and sliding behaviour. Ice flow models therefore require a component that can solve for temperature and moisture content, and determine the free boundary between the cold and temperate subdomains. We present such a model, based on the theory of compacting partial melts. By contrast with other models, we describe gravity- and pressure-gradient-driven drainage of moisture, while maintaining a divergence-free ice flow at leading order. We also derive the relevant boundary conditions at the free cold–temperate boundary, and find that the boundary behaves differently depending on whether ice enters or exits the temperate region. The paper also describes a number of test cases used to compare with a numerical solution, and investigates asymptotic solutions applicable to the limit of small compaction pressure gradients in the temperate ice regions. A simplified enthalpy-gradient model is finally proposed, which captures most of the behaviour of the full model in this limit.

Density and seismic velocity of hydrous melts under crustal and upper mantle conditions

We present a new model for calculating the density of hydrous silicate melts as a function of P, T, H2O concentration, and melt composition. We optimize VPr,Tr, ∂V/∂T, ∂V/∂P, ∂V2/∂T∂P, and K′ of H2O end-member components in hydrous silicate melts, as well as K′ of anhydrous silicate melts, using previously reported experimental results. The parameter set for H2O end-member component in silicate melt optimized in this study is internally consistent with the parameter values for the properties of anhydrous silicate melt reported by Lange and Carmichael (1987, 1990). The model calculation developed in this study reproduces the experimentally determined densities of various hydrous melts, and can be used to calculate the relationships between pressures, temperatures, and H2O concentrations of various hydrous melts from ultramafic to felsic compositions at pressures of 0–4.29 GPa. Using the new parameter set, we investigate the effects of H2O content on the seismic velocity of hydrous melts, as well as seismic velocities in partially molten regions of subduction zones. The results show that water content in silicate melt plays a key role in determining seismic velocity structure, and therefore must be taken into account when interpreting seismic tomography.

Seasonal Changes in Fe along a Glaciated Greenlandic Fjord

Greenland’s ice sheet is the second largest on Earth, and is under threat from a warming Arctic climate. An increase in freshwater discharge from Greenland has the potential to strongly influence the composition of adjacent water masses with the largest impact on marine ecosystems likely to be found within the glaciated fjords. Here we demonstrate that physical and chemical estuarine processes within a large Greenlandic fjord are critical factors in determining the fate of meltwater derived nutrients and particles, especially for non-conservative elements such as Fe. Concentrations of Fe and macronutrients in surface waters along Godthabsfjord, a southwest Greenlandic fjord with freshwater input from 6 glaciers, changed markedly between the onset and peak of the meltwater season due to the development of a thin (<10 m), outflowing, low-salinity surface layer. Dissolved (<0.2 µm) Fe concentrations in meltwater entering Godthabsfjord (200 nM), in freshly melted glacial ice (mean 38 nM) and in surface waters close to a land terminating glacial system (80 nM) all indicated high Fe inputs into the fjord in summer. Total dissolvable (unfiltered at pH <2.0) Fe was similarly high with concentrations always in excess of 100 nM throughout the fjord and reaching up to 5.0 µM close to glacial outflows in summer. Yet, despite the large seasonal freshwater influx into the fjord, Fe concentrations near the fjord mouth in the out-flowing surface layer were similar in summer to those measured before the meltwater season. Furthermore, turbidity profiles indicated that sub-glacial particulate Fe inputs may not actually mix into the outflowing surface layer of this fjord. Emphasis has previously been placed on the possibility of increased Fe export from Greenland as meltwater fluxes increase. Here we suggest that in-fjord processes may be effective at removing Fe from surface waters before it can be exported to coastal seas.

Базальт-риолитовая серия островов Галиндез и Уругвай (Антарктический полуостров, Западная Антарктида) как возможный объект геохимического моделирования

Представлены предварительные результаты всех этапов геохимического моделирования процесса эволюции эффузивного магматизма островов Галиндез и Уругвай (Антарктический полуостров, Западная Ан­тарктида). Охарактеризированы возникшие сложности и способы их преодоления. На основе построенной мо­дели сделана попытка оценки температурного, барического и флюидного режимов функционирования магма­тической системы, а также ее рудогенерирующей способности. Установлены: близкое соответствие магматиче­ской эволюции рассмотренной системы принятой модели фракционной кристаллизации; температурный ре­жим (-1130 ... ~ 880 °С) и глубина функционирования магматической камеры (9-12 км, литостатическое дав­ление 3-4 кбар); концентрация воды в исходном и остаточном расплаве на всех этапах эволюции системы; массовая доля жидкой фазы в системе на момент отделения флюида (f = 0,27) и его температура (Т = ~880 °С). Полученные результаты следует считать предварительными. Создание более достоверной модели требует рез­кого улучшения качества исходных геохимических данных.

Experimental evidence that microbial activity lowers the albedo of glaciers

doi: 10.7185/geochemlet.1611 Darkening of glacier and ice sheet surfaces is an important positive feedback to increasing global temperatures. Deposition of impurities on glaciers is primarily believed to reduce surface albedo, resulting in greater melt and mass loss. However, no study has yet included the effects of biological activity in albedo reduction models. Here, we provide the first experimental evidence that microbial activity can significantly decrease glacier surface albedo. Indeed, the addition of nutrients at ice meltwater concentrations to microbe-impurity mixtures resulted in extensive microbial organic carbon fixation and accumulation in Green land Ice Sheet surface debris. Accumulated organic carbon, over the period of a melt season, darkened the glacial debris in our experiments from 31.1 % to 15.6 % surface reflectivity (used as an analogue for albedo in our calculations), generating a strongly absorbing surface. Our experiments are the first to quantify the microbially-induced potential melt increase for the Greenland Ice Sheet (up to an average of 17.3 ± 2.5 Gt yr-1 at present and up to ~85 Gt yr-1 by 2100, based on our first order calculations). Mass loss from glaciers will conceivably intensify through enhanced microbial activity, resulting from longer melt seasons and fertilisation from anthropogenic sources.

Experimental Constraints on Plagioclase Crystallization during H2O- and H2O–CO2-Saturated Magma Decompression

Experiments simulating magma decompression allow the textures of volcanic rocks to be calibrated against known eruptive conditions. Interpretation of natural samples may be complicated, however, by both the decompression path and the composition of exsolving volatiles, which affect the time evolution of crystal textures. Here we present the results of decompression experiments at elevated temperature and pressure designed to assess the effects of degassing path on crystallization of Mount St. Helens rhyodacite. Three families of experiments were employed to simulate varied PH2O–t trajectories: single-step, H2O-saturated decompression (SSD); continuous, H2O-saturated decompression (CD); continuous, H2O–CO2-saturated decompression. Quantitative textural data (crystal abundance, number density, and size) are used to calculate plagioclase nucleation and growth rates and assess deviations from equilibrium in run products. These are the first experiments to quantify feldspar nucleation and growth rates during H2O–CO2-saturated decompression. We find that reducing the initial melt water content through addition of CO2 increases nucleation rates relative to the pure water case, an effect most pronounced at low dP/dt. Moreover, these early formed textural distinctions persist at the lowest pressures examined, suggesting that deep H2O– CO2 fluids could leave a lasting textural ‘fingerprint’ on erupted magmas. Crystals formed prior to decompression during the annealing process also modulate sample textures, and growth on pre-existing crystals contributes significantly to added crystallinity at a wide range of experimental conditions. The phase assemblage itself is a dynamic variable that can be used in conjunction with textural data to infer conditions of magma ascent and eruption. Finally, quantitative textural data from experimental samples are compared with those of natural pyroclasts erupted during the summer 1980 explosive–effusive transition at Mount St. Helens. This comparison supports a model of magma ejected from multiple storage regions present in the upper crust following the May 18 Plinian eruption, such that subsequent eruptions tapped magmas that experienced varied decompression and degassing histories.

Constraints on the Nature and Evolution of the Magma Plumbing System of Mt. Etna Volcano (1991–2008) from a Combined Thermodynamic and Kinetic Modelling of the Compositional Record of Minerals

Deciphering the evolution of the internal dynamics of magmatic plumbing systems and identifying the key parameters that drive such dynamics are major goals of modern volcanology. Here we present a novel petrological approach that combines kinetic modelling of the diffusive relaxation of chemical zoning patterns in olivine crystals with thermodynamic modelling (MELTS) to constrain the nature and evolution of the plumbing system of Mt. Etna and the processes governing its internal dynamics. We investigated the compositional and temporal record preserved in 180 olivine crystals that were erupted between 1991 and 2008. Detailed systemization of the information stored in the sequential zoning record of the olivines reveals the existence of at least five compositionally different magmatic environments (MEs), characterized by different olivine compositions: M 0 (Fo79–83), M 1 (Fo75–78), M 2 (Fo70–72), M 3 (Fo65–69) and mm 1 (Fo73–75). Several routes of magma transfer connect these environments. We identified three prominent magma passageways between the environments M 0 :M 1 , M 2 :mm 1 and M 1 :M 2 that were active during the entire period of observation between 1991 and 2008. Modelling the diffusive relaxation of the olivine zoning patterns reveals that the transfer of magma along such routes can occur over fairly heterogeneous timescales ranging from days to 2 years. Although some of the passageways have been sporadically active in the months and sometimes years before an eruption, the magma migration activity increases clearly in the weeks and days prior to an eruptive event. In this context, major transfer routes such as M 2 :mm 1 might represent temporary passageways that are activated only shortly before eruptive events. A forward modelling approach was developed using thermodynamic calculations with the MELTS software to identify the key intensive variables associated with the different magmatic environments. In this approach the observed populations of mineral compositions (e.g. Fo79–83), rather than single compositions, are associated with thermodynamic parameters [pressure, temperature, water content, oxygen fugacity ( f O 2 ) and bulk composition of the melt] to identify the most plausible set corresponding to each ME. We found that temperature, water content and possibly oxidation state are the main distinguishing features of the different magmatic environments. The most primitive olivine population M 0 (Fo79–83) and some of its associated clinopyroxenes formed at high melt water contents (3·5–5·2 wt %), at f O 2 conditions buffered at quartz–fayalite–magnetite (QFM) or Ni–NiO (NNO), at temperatures ≥1110°C and at pressures ranging between 1·5 and 3·0 kbar (or higher). The intermediate population M 1 (Fo75–78) can be produced over a broad spectrum of conditions, but all require similar temperatures and lower water contents (0·1–1·4 wt %). The most evolved, more Fe-rich olivines of M 2 and M 3 are products of melts with much lower water contants (0·2–1·1 wt % H 2 O for M 2 , <0·5 wt % H 2 O for M 3 ), and at probably somewhat more reducing (QFM) conditions at somewhat lower temperatures (∼1080°C). The M 3 environment characterized by very low water contents and reducing conditions could be related to an enhanced flux of CO 2 . Combination of the characteristics of the various magmatic environments with temporal information (residence time in different environments and timing of magma transfer between these environments) allows a dynamic model of the plumbing system beneath Mt. Etna to be constructed.