V52A-01 10:20h
Aluminum coordination and density in high-pressure aluminosilicate glasses: Significance for the structural changes and densification in basaltic magmas
Densities of aluminosilicate melts are strongly pressure-dependent, owing in part to atomic-scale structural changes. Glasses are commonly used as a first approximation for melts because the structure of a glass is that of the liquid at the glass transition temperature. This study used $^{27}$Al MAS NMR to determine the speciation of aluminum ions in glasses quenched from simple basalt-like melts at pressures up to 10 GPa. The spectra establish that high field strength (i.e., Ca) modifier cations induce more high-coordinated Al than lower field strength (i.e., Na and K) cations. The amount of high-coordinated Al gradually increases with increasing synthesis pressure to percentages as high as 81 % for the Ca-aluminosilicate glass at 10 GPa, which yields an average Al-coordination of 5.25. Densities of glasses synthesized at 10 GPa are 8 % (K-aluminosilicate) to 16 % (Ca-aluminosilicate) higher than those quenched at ambient pressure. Additionally, a rapidly decompressed ($\sim$1 second) Ca-aluminosilicate glass (5GPa) was densified by about 14 % relative to the ambient pressure sample, compared to 11 % for the conventionally decompressed 5 GPa glass ($\sim$14 hours). Previous density measurements show that MORB melts at pressures between 5 and 10 GPa are roughly 20 to 30 percent more dense than the ambient pressure melt, which suggests that rapidly decompressed glasses retain much or even most of the total densification and structural changes that occur in the high-pressure melts. The observed increases in Al coordination and density show linear trends, which suggests that this structural change is a major part of the densification mechanism of high-pressure melts. In detail, the data reveal that other changes, such as the compression of modifier cation sites and/or decreased network bond angles, must also contribute to the densification of the melt, especially at low pressure ($<$ 3 GPa).
V52A-02 10:35h
Experimental Determination of the K2CO3 Fusion Curve to 3 GPa and Constraints on the Liquid Equation of State
An equation of state (P-V-T relation) for carbonate liquids is of considerable geological interest despite the fact that carbonate magmas are volumetrically sparse compared to other igneous rocks. They are efficient agents of mantle metasomatism because of their high mobility and high concentrations of incompatible trace elements. They are also frequently associated with alkaline silicate magmas and may play a role in their genesis. Information on the density of carbonate liquids to high pressure is needed for an accurate assessment of their buoyancy in the deep mantle, as well as their thermodynamic stability to high pressure. Although the one-bar density and compressibility of alkaline carbonate liquids are well known (Zhu et al., 1991; Liu and Lange, 2003), the pressure dependence of the liquid compressibility (K?͈? = dK/dP) is not well constrained. For example, Dobson et al. (1996) report a density for K2CO3 liquid at 4 GPa and 1500\??C, obtained by the falling sphere method, of 3.10 g/cm3. Liu and Lange (2003) used the 3rd-order Birch-Murnaghan equation of state to show that this liquid density value is consistent with a liquid K?͈? value of 2.5, which in turn will lead to a strongly negative dT/dP slope in the K2CO3 fusion curve at pressures $>$ 1.0 GPa. Thus the implications of this 4 GPa liquid density measurement are that alkaline carbonate liquids are stable to remarkably low temperatures at high pressure and that they are expected to become more dense than silicate melts (and possibly mantle minerals) at deep mantle depth, which has profound implications for the migration of carbonate melts in the mantle. Owing to the significance of these inferences, it is important to independently verify the density of high-pressure carbonate liquids. In this study, we provide this test by experimentally determining the fusion curve of K2CO3 to 3 GPa and comparing our results with the calculated melting reaction. The requisite thermodynamic data needed to calculate the fusion curve are available from the literature, with the only unknown being the pressure dependence of the liquid compressibility, namely K?͈?. At pressures $<$= 0.5 GPa, the effect of not including a liquid K?͈? value is negligible, and the calculated fusion curve is in excellent agreement with that determined experimentally to 0.45 GPa in an internally heated pressure vessel by differential thermal analysis (Klement and Cohen, 1975). In order to constrain the value of liquid K?͈? from fusion curve analysis, the melting reaction must be determined to higher pressure. In our experiments to 3 GPa in a piston-cylinder apparatus, crystalline K2CO3 powder is preheated to 400 C to eliminate the water and tightly packed in a platinum capsule whose top is marked carefully. A small Pt ball is put at the top of the capsule, covered by a thin layer of K2CO3 powder to prevent it from sticking to the top of the capsule. The Pt ball falls to the bottom when the carbonate becomes liquid and remains at the top of the capsule when the carbonate does not melt. Our results show that K2CO3 remains crystalline up to 1300??C at 3 GPa, which indicates an exceptionally high liquid K?͈? value that is $>$= 18 (based on use of the 3rd-order Birch-Murnaghan EOS). These data indicate that alkaline carbonate liquids are not stable at exceptionally low temperatures at high pressure and that they remain strongly buoyant relative to silicate melts in the deep mantle. These results also support the hypothesis that highly compressible liquids at one bar have correspondingly high K?͈? values (Lange, 2002).
V52A-03 10:50h
Temperature Dependent Thermal Expansivities for Volcanic Melts
The thermal expansivities of three volcanic melts (Etna 1992, Vesuvius 1631 and Slapany) have been obtained from room temperature up to 1800K. The compositions are tephriphonolite, trachybasalt and basanite. The low temperature volumes were derived from measurements of the glass density of each sample having 5-5 K.min-1 cooling-heating history at 298 K, followed by measurements of the glass thermal expansion coefficient from 298 K to the respective glass transition interval. Supercooled liquid volumes and molar thermal expansivities were indirectly determined by combining scanning calorimetric and dilatometric measurements assuming that kinetics of enthalpy and shear relaxation are equivalent (Webb et al. 1992). High temperature densities were measured using Pt double bob Archimedean densitometry. In addition, oxidation state of iron has been analyzed using wet chemistry method. Small amount of samples has been taken from the liquids using "dip" technique at regular temperature step during high temperature densitometry. The measured high temperature densities have been compared with calculated densities at same temperature interval using Lange & Carmichael model (1987) and multicomponent density model of Lange (1997), respectively, incorporating the necessary correction for oxidation state. The resulting data for liquid volumes near glass transition temperature (950 - 990 K) and at super-liquidus temperature (1512 - 1803 K) are combined to yield thermal expansivities over the entire supercooled to stable liquid range. This study confirms that the temperature-dependent thermal expansivities previously obtained for synthetic aluminosilicate melts are also observed for natural compositions. The temperature-dependence increases from basanite to tephriphonolite composition.
V52A-04 11:05h
A Model For the Viscosity of Melts in the System Ab-An-Dp
The prediction of viscosity in natural silicate melts remains one of the most challenging and elusive goals in Earth Sciences. We present a strategy for fitting non-Arrhenian models to viscosity data which can be employed towards a full multicomponent model for melt viscosity. Here, we have adopted the Vogel-Tammann-Fulcher equation (VTF): log $\eta$ = A + B $/$ ( T $-$ C ) which is an effective descriptor of viscosity for most geochemically-important melts. The parameters A, B and C show strong correlations that are a reflection of: i) the nonlinear nature of the VTF equation, ii) the distribution and quality of the experimental data, and iii) co-dependence on composition. Our analysis has shown that the value of A is relatively insensitive to composition implying that the viscosities of silicate melts may converge to a single common value. The corollary to this is that the number of compositionally dependent parameters is reduced by a third; all compositional dependencies of melt viscosity are confined to the parameters B and C. We use this strategy to model the non-Arrhenian viscous properties of silicate melts in the chemical system: NaAlSi$_{3}$O$_{8}$ (Ab) : CaAl$_{2}$Si$_{2}$O$_{8}$ (An) : CaMgSi$_{2}$O$_{6}$ (Dp). This system is important because these melts show near-Arrhenian to strongly non-Arrhenian properties. Our modelling is based on experimental data compiled from the literature; the database comprises more than 500 measurements of viscosity on 50 different melt compositions and spanning a temperature range of 960 to 2400 K.The majority of data derive from binary melt compositions. Our optimization fits the full dataset simultaneously based on $\chi$$^{2}$ minimization techniques and returns values of B and C for each melt and a single estimate of A. The optimized value for A deriving from this dataset is $-$4.69 $\pm$ 0.5 (e.g., $\eta$$_{o}$ = 10$^{-4.7}$ Pa s). The original data are reproduced to within error ($\pm$ 0.2 log units) which strongly corroborates our assertion of a common high-T limit for all silicate melts. In conclusion, we will present a complete parameterization for the compositional dependence of the values of B and C.
V52A-05 11:20h
Rheology and Structure of Iron-Bearing Na$_{2}$O-Al$_{2}$O$_{3}$-SiO$_{2}$ Melts
The viscosity of a series of Na$_{2}$O-FeO-Fe$_{2}$O$_{3}$-Al$_{2}$O$_{3}$-SiO$_{2}$ melts has been measured by micropenetration methods in the range 10$^{8.5}$-10$^{13}$ Pa s in argon. The viscosity of the Fe-bearing melts follows the same compositional trend as the Fe-free Na$_{2}$O-Al$_{2}$O$_{3}$-SiO$_{2}$ melts as a function of Na/Al at constant SiO$_{2}$ content: at a constant temperature, viscosity increases with decreasing peralkalinity and remains almost constant for all investigated compositions in the peraluminous field. The addition of 1 mol% Fe$_{2}$O$_{3}$ to the Fe-free peraluminous compositions causes ~1 log unit decrease in viscosity, while ~3 mol% Fe$_{2}$O$_{3}$ needs to be added to the peralkaline compositions to produce a similar decrease in viscosity. The need for more charge-balancing cations in the peraluminous melt structure appears to control the Fe$^{2+}$/Fe$^{total}$, with the peraluminous compositions having a large Fe$^{2+}$/Fe$^{total}$ (0.6) compared with that for the peralkaline compositions (0.3). There exist different structures and flow mechanisms in the peralkaline and peraluminous Na$_{2}$O-FeO-Fe$_{2}$O$_{3}$-Al$_{2}$O$_{3}$-SiO$_{2}$ melts. The structure in these melts changes when there are no longer enough charge-balancing Na$^{+}$ and Fe$^{2+}$ for the tetrahedrally co-ordinated Al$^{3+}$ and Fe$^{3+}$. As the melts become peraluminous the structure changes to probably include tri-clusters in which one oxygen is the apex of three tetrahedra. Although tri-clusters probably exist in peraluminous composition melts, the configurational entropy term at a viscosity of 10$^{12}$ Pa s [B$_{e}$/S$_{conf}$(T$_{g}$)] calculated for these melts suggests that the flow process is not initiated at a tri-cluster site. Flow appears to begin with the movement of an Al$^{Na}$-, Fe$^{Na}$- or Al$^{Fe}$ tetrahedron, with a tricluster being formed during the flow process. The general structure and flow mechanism of these Na$_{2}$O-FeO-Fe$_{2}$O$_{3}$-Al$_{2}$O$_{3}$-SiO$_{2}$ melts appear to be the same as those expected in peralkaline and peraluminous Na$_{2}$O-Al$_{2}$O$_{3}$-SiO$_{2}$ melts. It was not possible to vary the compositions of these melts sufficiently to determine the whether Fe$^{2+}$ and Na$^{+}$ have a preference for the type of tetrahedron (Al$^{3+}$ or Fe$^{3+}$) they charge-balance.
V52A-06 11:35h
Dynamic Reduction of FeO-Bearing, Anhydrous Aluminosilicate Melts
We have studied the reduction dynamics of FeO-bearing aluminosilicate melts at oxygen activities sufficiently low to form metallic iron. The experiments involved reacting droplets, suspended from refractory metal wires, with a high-temperature ($\sim$1400$^{o}$C), controlled-oxygen-activity environment maintained by a dynamic CO:CO$_{2}$ buffer. In the case of an FeO-doped magnesium aluminosilicate ("Fe-MAS") melt ($\sim$5 mol% FeO) exposed to an oxygen activity of 2x10$^{-13}$ ("QIF-2"; CO:CO$_{2}$=240:1), the reduction dynamic is rate-limited by chemical diffusion of Mg$^{2+}$: oxygen chemically ablates from the free surface and the network-modifying cations diffuse inward, charge-compensated by a counterflux of electron holes (the "semiconductor condition" holds for diffusion dynamics in this melt [e.g., Cook and Cooper, 2000]); nm-scale crystals of pure $\alpha$-Fe nucleate at an internal front. Diffusion of an oxygen species is not involved. In the case of FeO-doped calcium-magnesium aluminosilicate (Fe-CMAS) melt ($\sim$8 mol% FeO) exposed to an oxygen activity of 2x10$^{-15}$ (QIF-4; CO:CO$_{2}$=1750:1), the dynamic changes: molten Fe-C-Si alloy droplets form near the surface, and bubbles are seen to form internally, truncating at an internal front. Further, the reaction occurs more slowly than that seen for the similarly polymerized Fe-MAS melt. The results suggests that molecular CO diffuses inward, consuming electron holes so as to form carbonate ion species in the melt [cf. Brooker et al., 2001]. Quenching produces a driving potential to reverse the internal reduction reaction, so creating the bubbles. Consumption of the electron holes by reaction with the carbon species dramatically reduces the reduction-to-metal kinetics. Brooker RA, Kohn SC, Holloway JR, McMillan PF (2001) Chem Geol 174:241-254; Cook GB, Cooper RF (2000) Am Mineral 85:397-406
V52A-07 11:50h
Rheological properties of explosively erupted magmas at the Campi Flegrei caldera (Italy) through experimental studies.
In recent years, new methods to evaluate the rheological properties of silicate melts have been developed. Nevertheless, we are still far away from fully satisfactory modelling of complex eruptive processes due to lack of data on evolved magmas. The Campi Flegrei caldera (CFc), a restless nested structure in Southern Italy, is an ideal natural laboratory for this purpose. The erupted magmas have a broad spectrum of compositions, from trachybasalt to alkali trachyte. Analysing the deposits of the Campanian Ignimbrite, Neapolitan Yellow Tuff, Minopoli 2, Fondo Riccio, Agnano-Monte Spina and Astroni eruptions, we have completed the description of the temperature-dependent Newtonian viscosities for the dry compositional spectrum of the erupted magmas that was initiated by the Munich, Rome and Pisa groups. Viscosities were investigated at high (1323-1873 K) and low (889-1134 K) temperature using the concentric cylinder and the micropenetration techniques, respectively. In addition, complementary hydrous viscosity measurements have been performed on trachybasaltic and latitic compositions. We synthesised samples with different water content (0.5- 2wt%) using an internally heated pressure vessel at 200 MPa and 1473 K. Water content was checked using FTIR spectroscopy. For this the molar absorptivity has been calibrated for both compositions. Hydrous viscosity measurements using the micropenetration technique were performed between 883 and 893 K, depending on the glass transition temperature Tg of each sample. In order to evaluate the Tg we performed calorimetric measurements using differential scanning calorimetry. For each sample the cooling rate was unknown and the heating rate 10 K/min. Tg was defined as the onset of the glass transition. The obtained results provide the first rheological properties dataset for the whole CFc compositional range, helping to constrain the modelling of magma chamber processes and conduit dynamics.
V52A-08 12:05h
The Influence of Volatiles on the Glass Transition
The glass transition is the temperature interval across which melt relaxation times change appreciably, from instantaneous at high temperatures (super-cooled liquid) to infinite at low temperatures (glass). It has several important implications, being where the speciation of water is locked into the melt structure, viscous flow and welding stop and elastic response to mechanical stress commences. It depends on composition, thermal history and the timescale of the experimental method being used. In this study we use the variation of heat capacity with temperature measured by differential scanning calorimetry to define glass transition temperatures ({\it T$_{g}$}) for a series of synthetic and natural samples with a range of volatile contents. Two methods have been used to define {\it T$_{g}$}: a) the temperature of the onset of the glass transition, and b) the temperature of the peak in heat capacity across the glass transition. For each sample the thermal history (cooling and heating at 10 K/min) is the same, allowing direct comparison of {\it T$_{g}$}. The samples include natural glasses with a wide range of compositions (trachyte, dacite, phonolite, basalt) that have been doped with varying amounts of water, basaltic glasses from the Hawaiian drill core that contain naturally occurring amounts of water and jadeite glasses synthesised with varying amounts of water and carbon dioxide. We will show that for each natural composition water affects {\it T$_{g}$} with the same overall pattern. Increasing water contents cause {\it T$_{g}$} to decrease (2 wt.% water causes drops of 190 to $280\deg$C), with the largest effect occurring with the addition of the first 1 wt.% of water (drops of 110 to $200\deg$C). The drop varies depending on the glass composition. The effect of carbon dioxide on the {\it T$_{g}$} of jadeite glass is less clear, no systematic variation with concentration occurs. Water affects the {\it T$_{g}$} of jadeite glass in exactly the same way as in the natural compositions. We will also examine how the width of the glass transition (in terms of temperature) and the height of the heat capacity peak are affected by composition and volatile content.