Anhydrous Melts Research Articles

Granulite/charnockite from the Juiz de Fora domain, central segment of the Brasiliano Ribeira Belt

The Ribeira Belt is the product of the last stage of Brasiliano collage in southeastern Brazil. Its Ocidental Terrane comprises the reworked margin of the San Francisco Craton, which is composed by two thrust sheets: the Andrelandia and the Juiz de Fora domains (Heilbron et al. 1998). The latter includes three different tectonically bounded litho-stratigraphic units: pre-1.8 Ga. granulite facies orthogneisses; post-1.8 Ga. metasedimentary rocks; and Brasiliano-Pan-African granitoids and charnockitoids. Each unit comprises a characteristic type of granulite/charnockite. The pre-1.8 Ga. unit is composed of basic to acid orthopyroxene-bearing orthogneisses with a granoblastic fabric. Field and petrological data indicate that granulite metamorphism is the result of infiltration of CO2-rich fluids. A late mylonitic foliation and retrogressive mineral products are contemporaneous to the Brasiliano-Pan-African tectonic stacking. The paragneisses display granulite facies parageneses within a foliated and/or mylonitic structure and show features of in situ migmatization caused by the breakdown of biotite. Charnockite bodies occur as garnet-(orthopyroxene)-bearing leucosomes within the paragneisses and also as isotropic to slightly foliated individualized igneous bodies with enclaves of the other units. They are interpreted as the product of anatexis of the paragneisses with minor contributions of the prthogranulites. Granulite facies metamorphism of the paragneisses, and anhydrous melting were caused by tectonic stacking related to the Brasiliano collage.

GRANULITE/CHARNOCKITE FROM THE JUIZ DE FORA DOMAIN, CENTRAL SEGMENT OF THE BRASILIANO RIBEIRA BELT

The Ribeira Belt is the product of the last stage of Brasiliano collage in southeastern Brazil. Its Ocidental Terrane comprises the reworked margin of the San Francisco Craton, which is composed by two thrust sheets: the Andrelandia and the Juiz de Fora domains (Heilbron et al. 1998). The latter includes three different tectonically bounded litho-stratigraphic units: pre-1.8 Ga. granulite facies orthogneisses; post-1.8 Ga. metasedimentary rocks; and Brasiliano-Pan-African granitoids and charnockitoids. Each unit comprises a characteristic type of granulite/charnockite. The pre-1.8 Ga. unit is composed of basic to acid orthopyroxene-bearing orthogneisses with a granoblastic fabric. Field and petrological data indicate that granulite metamorphism is the result of infiltration of CO2-rich fluids. A late mylonitic foliation and retrogressive mineral products are contemporaneous to the Brasiliano-Pan-African tectonic stacking. The paragneisses display granulite facies parageneses within a foliated and/or mylonitic structure and show features of in situ migmatization caused by the breakdown of biotite. Charnockite bodies occur as garnet-(orthopyroxene)-bearing leucosomes within the paragneisses and also as isotropic to slightly foliated individualized igneous bodies with enclaves of the other units. They are interpreted as the product of anatexis of the paragneisses with minor contributions of the prthogranulites. Granulite facies metamorphism of the paragneisses, and anhydrous melting were caused by tectonic stacking related to the Brasiliano collage.

Effect of Fe 2+ on garnet–melt trace element partitioning: experiments in FCMAS and quantification of crystal-chemical controls in natural systems

Garnet–melt trace element partitioning experiments were performed in the system FeO–CaO–MgO–Al 2O 3–SiO 2 (FCMAS) at 3 GPa and 1540°C, aimed specifically at studying the effect of garnet Fe 2+ content on partition coefficients ( D Grt/Melt). D Grt/Melt, measured by SIMS, for trivalent elements entering the garnet X-site show a small but significant dependence on garnet almandine content. This dependence is rationalised using the lattice strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454], which describes partitioning of an element i with radius r i and valency Z in terms of three parameters: the effective radius of the site r 0( Z), the strain-free partition coefficient D 0( Z) for a cation with radius r 0( Z), and the apparent compressibility of the garnet X-site given by its Young's modulus E X( Z). Combination of these results with data in Fe-free systems [Van Westrenen, W., Blundy, J.D., Wood, B.J., 1999. Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt. Am. Mineral. 84, 838–847] and crystal structure data for spessartine, andradite, and uvarovite, leads to the following equations for r 0(3+) and E X(3+) as a function of garnet composition ( X) and pressure ( P): r 0(3+) [ A ̊ ]=0.930X Py +0.993X Gr +0.916X Alm +0.946X Spes +1.05(X And +X Uv )−0.005(P [ GPa]−3.0)(±0.005 A ̊ ) E X (3+) [ GPa]=3.5×10 12(1.38+r 0(3+) [ A ̊ ]) −26.7(±30 GPa) Accuracy of these equations is shown by application to the existing garnet–melt partitioning database, covering a wide range of P and T conditions (1.8 GPa< P<5.0 GPa; 975°C< T<1640°C). D Grt/Melt for all 3+ elements entering the X-site (REE, Sc and Y) are predicted to within 10–40% at given P, T, and X, when D Grt/Melt for just one of these elements is known. In the absence of such knowledge, relative element fractionation (e.g. D Sm Grt/Melt/ D Nd Grt/Melt) can be predicted. As an example, we predict that during partial melting of garnet peridotite, group A eclogite, and garnet pyroxenite, r 0(3+) for garnets ranges from 0.939±0.005 to 0.953±0.009 Å. These values are consistently smaller than the ionic radius of the heaviest REE, Lu. The above equations quantify the crystal-chemical controls on garnet–melt partitioning for the REE, Y and Sc. As such, they represent a major advance en route to predicting D Grt/Melt for these elements as a function of P, T and X.

Partition coefficients of Nb and Ta between rutile and anhydrous haplogranite melts

Partition coefficients (D) for Nb and Ta between rutile and haplogranite melts in the K2O-Al2O3-SiO2 system have been measured as functions of the K2O/Al2O3 ratio, the concentrations of Nb2O5 and Ta2O5, the temperature, in air and at 1 atmosphere pressure. The Ds increase in value as the K* [K2O/(K2O + Al2O3)] molar ratio continuously decreases from highly peralkaline [K* ∼ 0.9] to highly peraluminous [K* ∼ 0.35] melts. The D values increase more dramatically with a unit decrease in K* in peraluminous melts than in peralkaline melts. This compositional dependence of Ds can be explained by the high activity of NbAlO4 species in peraluminous melts and the high activity of KONb species (or low activity of NbAlO4 species) in peralkaline melts. A coupled substitution, Al+3 + Nb+5 (or Ta+5) = 2Ti+4, accounts for the Ds of Nb (Ta) being much greater in peraluminous melts than in peralkaline melts because this substitution allows Nb (Ta) to enter into the rutile structure more easily. The Ds of Ta between rutile and melt are greater than those of Nb at comparable concentrations because the molecular electronic polarizability of Ta is weaker than that of Nb. The Nb+5 with a large polarizing power forms a stronger covalent bond with oxygen than Ta+5 with a small polarizing power. The formation of the strong bond, Nb-O, distorts the rutile structure more severely than the weak bond, Ta-O; therefore, it is easier for Ta to partition into rutile than for Nb. These results imply that the utilization of the Nb/Ta ratio in liquid as a petrogenetic indicator in granitic melts must be done with caution if rutile (or other TiO2-rich phases) is a liquidus phase. The crystallization of rutile will increase the Nb/Ta ratio of the residual liquid because the Ds of Ta between rutile and melts are greater than those of Nb.

Melting of Refractory Mantle at 1middle dot5, 2 and 2middle dot5 GPa under Anhydrous and H2O-undersaturated Conditions: Implications for the Petrogenesis of High-Ca Boninites and the Influence of Subduction Components on Mantle Melting

Boninites are an important ‘end-member’ supra-subduction zone magmatic suite as they have the highest H2O contents and require the most refractory of mantle wedge sources. The pressure–temperature conditions of boninite origins in the mantle wedge are important to understanding subduction zone initiation and subsequent evolution. Reaction experiments at 1·5 GPa (1350–1530°C), 2 GPa (1400–1600°C) and 2·5 GPa (1450–1530°C) between a model primary high-Ca boninite magma composition and a refractory harzburgite under anhydrous and H2O-undersaturated conditions (2–3 wt % H2O in the melt) have been completed. The boninite composition was modelled on melt inclusions occurring in the most magnesian olivine phenocrysts in high-Ca boninites from the Northern Tongan forearc and the Upper Pillow Lavas of the Troodos ophiolite. Direct melting experiments on a model refractory lherzolite and a harzburgite composition at 1·5 GPa under anhydrous conditions (1400–1600°C) have also been completed. Experiments establish a P, T ‘melting grid’ for refractory harzburgite at 1·5, 2 and 2·5 GPa and in the presence of 2–3 wt % H2O. The effect of 2–3 wt % dissolved H2O produces a liquidus depression in primary boninite of ∼112 ± 19°C at a given temperature. The H2O-bearing melts, recalculated to 100 wt % anhydrous, are ∼2–6 wt % higher in MgO, ∼1–2 wt % higher in SiO2 and ∼1–1·5 wt % lower in FeO, compared with nominally anhydrous melts at the same P and T. These differences are consistent with a change in the melting reaction, resulting in a higher contribution of orthopyroxene to the melt phase, compared with anhydrous conditions. We conclude that high-Ca boninite petrogenesis requires temperatures as high as ∼1480°C at depths of ∼45 km in the mantle wedge; these are constraints for any proposed model of intra-oceanic subduction zones. A comparison of the results from the boninite–harzburgite reaction experiments with the direct melting experiments on refractory lherzolite and harzburgite indicates that the influence of subduction components (included in the composition of the added model boninite) is to cause high-pressure melting cotectics to move towards the olivine apex (i.e. to relatively higher pressures) of the molecular normative projection from diopside onto the base of the ‘basalt tetrahedron’ [Jd + CaTs + Lc–Qz–Ol] compared with anhydrous melting of normal mantle in the absence of a subduction component. The subduction component involved in high-Ca boninite petrogenesis in addition to H2O has relatively high Al2O3 and Na2O contents. The experimental data from this and other studies empirically quantify the absolute effect of dissolved H2O (0·2–21 wt %) on the liquidus depression of olivine-saturated basaltic melts with ∼<3 wt % total alkalis as follows: \[\ olivine\ liquidus\ depression\ ({^\circ}C)\ =\ 74.403\ {\times}\ (H_{2}O\ wt\ \%)^{0{\cdot}352}.\ \] The equation that describes this empirical relationship is non-linear with an error of ∼9 relative percent.

On the stability relations of hydrous minerals in water-undersaturated magmas

Reaction of a hydrous mineral with a water-bearing silicate melt is analyzed in terms of the sto- ichiometry of (in)congruent dissolution. Application of the law of mass action provides an explana- tion for the presence of an isobaric thermal maximum that manifests itself in terms of enhanced stability of the phase under conditions of water undersaturation. It is shown that the water content of the melt at which the maximum develops is independent of pressure and water speciation, but is strongly dependent on the identity of other (anhydrous) melt species and upon the stoichiometric numbers of additional solid phases involved in the reaction. Utilizing MELTS, the analysis is applied to the cummingtonite-bearing rhyolitic magmas of the Taupo volcanic zone of New Zealand.

Chemical diffusivities of 18 trace elements in granitoid melts

We have measured effective binary chemical diffusion coefficients for Be, B, Mg, Ca, Ti, Ge, Sr, Y, Zr, Nb, Cs, Ba, Nd, Tb, Lu, Hf, Ta, and W at concentrations below 1000 ppm in synthetic granitoid melts by performing infinite couple diffusion anneals. By imposing systematic variations in the viscosity of a granitic melt through changes in temperature and composition, we have created an internally consistent set of diffusivities that can be used to assess quantitatively the relationship between melt viscosity, melt composition, and tracer diffusivities. One set of experiments were performed at 1 atm in an anhydrous haplogranitic melt near to the eutectic composition under air at temperatures of 1600, 1400, and 1137°C. A second set of experiments investigated the independent effects of the addition of approximately 3.7 wt.% H2O at 1600 and 1300C at 1.0 GPa pressure, and of 20 wt.% Na2O at 1 atm at 1200, 960, and 810°C. Within experimental error the measured diffusivities are Arrhenian, and show clear dependencies on ionic charge and radius, as well as on the viscosity of the melt.Monovalent, divalent, and trivalent cations show increasing activation energies with increasing field strength, whereas cations with higher charges have activation energies subequal to that of viscous flow. The Eyring equation is moderately successful in relating diffusivities of high field strength cations to melt viscosity, but underestimates the diffusivities of other cations by up to 4 orders of magnitude. The database of diffusivities reported in this contribution can serve to calibrate empirical models for the prediction of tracer diffusivities over a large range in melt composition, temperature, and viscosity.

Experimental Study of the Effect of Temperature on Water Solubility in Natural Rhyolite Melt to 100 MPa

The effect of temperature on water solubility in rhyolite melt was experimentally determined at 850–1200°C and 22–100 MPa. A natural high-silica rhyolite glass was equilibrated with pure water vapor, and the water content in the quenched glass was determined by IR spectroscopy. The results demonstrate that water solubility in rhyolite melt has a negative temperature dependence, which becomes weaker at high temperatures and low water contents. This temperature dependence can be modeled adequately on the basis of ideal mixing of water species and anhydrous rhyolite melt components. The model reproduces both the present and previously published solubility data for water in rhyolitic melts to 100 MPa and over a wide range of temperature from near the solidus to 1200°C, thereby permitting calculation of water saturation under varying temperature conditions. At 50–100 MPa, an increase in the fraction of excess water as a result of a rise in temperature can cause a four- to eight-fold increase in the fractional amount of volumetric expansion above that caused by pure thermal expansion, per unit temperature rise. Thus, the negative temperature dependence of water solubility could be of fundamental importance in the development of gravitational instability in shallow, water-saturated silicic magma chambers.

Determination of the influence of pressure and dissolved water on the viscosity of highly viscous melts; application of a new parallel-plate viscometer

A parallel-plate viscometer has been designed for use in an internally heated pressure vessel (IHPV) at pressures up to 350 MPa and at temperatures up to 900 ℃. The viscosity of a melt is determined by measuring the rate of deformation of a cylindrical sample as a function of an applied, constant stress at a fixed temperature. The viscometer consists of a small furnace with two independent heating resistors, a moveable load by which the stress is applied to the sample, and a pressure- resistant transducer (LVDT) that measures the deformation of the sample. The accessible viscosity range covers three orders of magnitude from 10 8.5 Pa · s to 10 11.5 Pa · s. Calibration measurements on the standard melt DGG1 at 0.1MPa demonstrated the precision of the viscometer to be within ±0.08 log units. Subsequent measurements at elevated pressure on DGG1- melt, Di 100 -melt (Di = CaMgSi 2 O 6 ), and Ab 55 Di 45 -melt (Ab = NaAlSi 3 O 8 , composition in weight per­cent) showed a pronounced increase of viscosity with pressure. Comparison with literature data on the pressure dependence of the transformation temperature of Di 100 -melt (Rosenhauer et al. 1979) confirmed the reliability of these findings. The dependence on pressure becomes smaller with in­creasing temperature for these depolymerized melts; e.g., in the case of Di 100 -melt (NBO/T = 2) from dη/dP = +0.23 log units/100 MPa at 751 ℃ to dη/dP = +0.18 log units/100 MPa at 770 ℃. In contrast to the depolymerized melts, a polymerized melt of haplotonalitic composition (NBO/T = 0) shows a decrease by -0.12 log units/100 MPa in the pressure range 50-350 MPa at 889 ℃. Possible application of the new viscometer to study rheological properties of volatile-bearing melts was tested successfully with a hydrous haplotonalitic melt. Addition of 3.80 wt% of water to the anhydrous melt strongly shifts the viscosity-temperature relationship toward lower temperatures; e.g., at a viscosity of 10 10.5 Pa · s from 883 to 515 ℃. The measured viscosities did not drift during the run, indicating that water loss is negligible within the time scale of the experiments, as confirmed by IR-microspectroscopic analysis.

The interactions between M +5 cations (Nb +5, Ta +5, or P +5) and anhydrous haplogranite melts

The interactions of high-field-strength cations with haplogranite melts of peraluminous to peralkaline compositions were examined by determining the saturation of rutile as functions of the concentrations of the oxides of the pentavalent cations Nb, Ta, and P. The phase equilibria combined with existing spectroscopic data show that the competition of the high-field-strength cations for the alkali aluminosilicate components varies systematically from peraluminous to peralkaline melts. The data also show that the melt species of Nb +5, Ta +5, and P +5 are very similar. In peraluminous melts, these pentavalent cations have a strong affinity for Al and form very stable M +5AIO 4 complexes; in peralkaline melts, the governing species are characterized by M +5OK bond types, whereas both M +5OAI and M +5OK species exist in the transition between peraluminous to peralkaline melts. The solubility of rutile is significantly affected by the hierarchy of interactions that exist among M +5 cations, Ti +4, and the aluminosilicate network. The composition of rutile in peraluminous melts is a solid solution along the join Ti 2O 4-M +5AIO 4 where M +5 is Ta and/or Nb. Nb +5 and Ta +5 partition strongly into rutile with Al +3 playing the role of a charge balancing cation, Nb +5 + Al +3 = 2Ti +4. Rutile in peralkaline melts contains significantly smaller concentrations of the M +5AlO 4 component, reflecting the much lower activity of M +5AlO 4 species. P +5 does not enter into solid solution in rutile because the ionic radius of P +5 is much smaller than Ti. Nb/Ta in peraluminous pegmatites also enter into solid solution in rutile by a coupled mechanism except that Fe +3 assumes the role of Al +3. The main geochemical conclusions are that Nb, Ta, and P have similar speciations in peraluminous and peralkaline melts. It follows that fractionation of these cations, particularly the perturbation of the Nb/Ta ratios in silicic magmas, can be brought about only by interactions with a crystalline phase. The Nb/Ta ratio can be substantially changed only through the crystallization of Ti-rich crystalline phases and/or Nb/Ta oxides such as columbite.

Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt

We performed experiments at 3.0 GPa and 1530‐1565 °C to investigate the effects of crystal composition on trace element partitioning between garnet and anhydrous silicate melt. Bulk compositions along the pyrope (Py: Mg 3Al2Si3O12)-grossular (Gr: Ca 3Al2Si3O12) join, doped with a suite of trace elements (Li, B, K, Sc, Ti, Sr, Y, Zr, Nb, Cd, In, REE, Hf, Ta, Th, and U) produced homogeneous garnets, ranging in composition from Py 84Gr16 to Py 9Gr91, in equilibrium with melt. Trace element partition coefficients ( D-values), measured by SIMS, depend greatly on the Mg/ (Mg + Ca) of garnet. For example, from Py84 to Py9, DLa increases from 0.004 to 0.2, whereas DU increases from 0.029 to 0.42. These variations can be explained by the lattice strain model of Blundy and Wood (1994), which describes trace element partitioning of an element i in terms of the ionic radius of i (ri), the size of the lattice site on which i partitions (r0), the Young’s modulus of the site (), and the (theoretical) partition coefficient D0 for an ion of radius r0. For trivalent cations substituting in the garnet X-site (Y, REE, Sc, and In), apparent values of r0 fitted to our data vary systematically from 0.935 ± 0.004 A (Py 84) to 0.99 ± 0.01 A (Py 9), a trend consistent with variations in the size of the X-site. Values of D0 show an increase from Py9 (D0 = 2.8 ± 0.1) to Py 84 (4.8 ± 0.1) and Young’s modulus E varies from 257 ± 20 GPa for Py 60 to 590 ± 40 GPa for Py 84. These results allow a quantitative assessment of the influence of crystal chemistry on garnet-melt D-values, thereby forming the basis for a predictive model similar to that recently developed for clinopyroxene-melt partitioning by Wood and Blundy (1997). Our new data emphasize the importance of taking into account crystal composition when modeling trace element behavior in natural systems.

An experimental investigation of radon diffusion in an anhydrous andesitic melt at atmospheric pressure: implications for radon degassing from erupting magmas

We have measured the diffusivity of radon in a dry natural andesitic melt at atmospheric pressure and at temperatures ranging between 1350°C and 1500°C. The procedure is based on the determination by gamma-ray spectrometry of radon losses undergone by a disk-shaped melted sample during heating. A theoretical treatment of radon diffusion allowed us to relate these losses to diffusivities and values of D Rn can be described by the following Arrhenius equation: D Rn = D 0 · exp(− E a / RT), with D 0 = 1.5 10 4 m 2 · s −1 and E a = 466 kJ · mol −1. Our results confirm that the larger the ionic radius of the noble gas, the higher the frequency factor D 0 and the activation energy E a . At a temperature of 1225 ± 25°C the diffusivity of radon approaches that of other heavy noble gases (log D Rn = −12.0 ± 0.1 where D Rn is in m 2 · s −1). From the present study it is demonstrated that neither diffusion processes, which are too slow and balanced by radioactive ingrowth of radon from its parent, nor exsolution of pure radon bubbles, because of its exceedingly low abundance in the magma, can be responsible for the complete radon depletion observed in erupted lavas. The exsolution of water and other major gas phases acting as carriers of trace gaseous species appears to be the main cause for radon as well as other noble gases degassing.

高圧地球科学 地球マントル中の水

High-pressure experiments on the stability of hydrous minerals likely to be present in the Earth's mantle provide constraints on the distribution of water in the mantle, and the form in which it is stored. However, there has been confusion about compositions and structures between hydrous minerals. In regions of elevated mantle temperature, water may be stored not in hydrous minerals but in anhydrous minerals, melts, and fluids. Such water is important to understand the dynamics and evolution of the mantle, volcanism, and metasomatism. In this article, the state and migration of water in the mantle are reviewed.

Migration of the Upper Cretaceous subduction-related volcanism towards the back-arc basin of the eastern Pontide magmatic arc (NE Turkey)

The Eastern Pontide active continental margin extends along the eastern part of the Black Sea with an E–W trend and comprises the parallel oriented northern, southern and axial zones. Each zone is fault bound and distinguished by distinctive magmatism, lithofacies and stratigraphy. In the northern zone, from the coast of the Black Sea to Torul, the cessation of Liassic bimodal volcanism and accumulation of anoxic carbonates of the Valanginian–Barremian was followed by bimodal volcanism which evolved from a tholeiitic (TH)–calc-alkaline (CA) to high K–CA composition. These volcanics erupted sporadically in asymmetric deep basins until the end of the Senonian. In the southern zone, around Torul and Gümüşhane, Liassic bimodal volcanism and the accumulation of the Malm–Cenomanian platform carbonate was succeeded by Campanian volcanism, i.e. much later than volcanism in the northern zone. A second epoch of CA–high K–CA andesitic volcanism occurred in the southern zone and is intercalated with a Campanian red pelagic limestone which is also widespread in the northern zone, and known as the marker lithologic unit in the eastern Pontides. South of the southern zone, in the Bayburt–Maden area, the pelagic limestone of the Early Cretaceous and overlying ophiolitic olistostromal mélange (which contains Cenomanian MOR, WP and IA basaltic pillow lavas) are overlain by high–K calc-alkaline Campanian andesitic volcanic rocks, implying that the ophiolitic mélange was formed in a back-arc environment. The major and trace element geochemistry and REE patterns of the Upper Cretaceous volcanic rocks suggest a variation from hydrous IA to anhydrous back-arc melting conditions on the subduction zone and the migration of arc magmatism laterally from the northern to the southern zones during the Senonian. This change to dehydration melting and the migration of the subduction-related Late Cretaceous volcanism towards the south in the eastern Pontide Magmatic arc, requires a south-dipping subduction polarity during the Senonian. Copyright © 1999 John Wiley & Sons, Ltd.

The depth of the spinel to garnet transition at the peridotite solidus

We have determined the pressure and temperature of the spinel–garnet transition close to the solidus of fertile and depleted peridotite from a combination of synthesis, sandwich and garnet-seeded experiments. Garnet is unstable in MORB-Pyrolite below 2.8 GPa (depths <85 km) at a solidus temperature of 1470°C. In Tinaquillo Lherzolite, which is related to MORB-Pyrolite by about 7% fractional melting, the transformation occurs at 3.1 GPa at a solidus temperature of 1520°C. In both cases the transformation interval is narrow, approximately 0.1–0.2 GPa. Because the solidus temperature increases by 7°C and the spinel–garnet transformation pressure by 0.05 GPa for every 1% melt extracted, extremely high potential temperatures ( T p > 1450°C) are required to generate the 3–5% melting in the garnet field implied by recent trace element and isotope studies. It appears likely, therefore, that the `garnet signature' arises from some mechanism other than anhydrous melting of garnet peridotite.

Na-K interdiffusion in alkali feldspar melts

Na-K chemical interdiffusion between albite and orthoclase melts has been measured at 1.0 GPa between 800 and 1600°C and at 2.0 GPa, 1400°C, in anhydrous melts and in hydrous, 5.5 wt% H 2O, melts at 1.0 GPa, 1200 and 1400°C. Anhydrous Na-K diffusivities at X or = 0.5 display Arrhenian behavior even through the inferred glass transition. After correction for diffusion occurring during heating Na-K interdiffusion can be described by D=2.39(+1.7,−0.98)×10 −5 exp(−145.8±6.8/ RT) where D is the diffusion coefficient in m 2s −1, 145.8 is the activation energy in kJ mol −1, R is the gas constant, and T is the temperature in kelvins. These diffusivities are four decades greater than estimated Si-Al diffusivities in the same melts due to the low alkali-oxygen bond valence, or cation field strength, and the abundance of appropriate charge-balanced locations for alkalis in alkali feldspar melts. Extrapolation of diffusion coefficients to 600–650°C for comparison with Na-K interdiffusion in alkali feldspar crystals demonstrates that melting increases diffusion by six orders of magnitude. The effect of pressure on Na-K diffusion at 1400°C is not measurable in this system and suggests that only minor dilation of the coordinating oxygen polyhedra which enclose alkalis in the melt is necessary for diffusion. A preliminary Arrhenius equation for alkali diffusion in silicic melts with 6 ± 0.5 wt% H 2O was calculated using results of this and previous studies D=4.97(+0.85,−0.73)×10 −6 exp(−115.8±1.8/ RT). The effect of H 2O only increases diffusivities by approximately a factor of four, distinctly different from the three orders of magnitude differences between Si-Al, Zr, and P diffusion in anhydrous and hydrous granitic melts. The differing behavior of alkalis and these other cations in hydrous silicic melts is associated with differences in the type and abundance of coordination polyhedra and the low bond valences and cation field strengths of alkalis.

Energy barriers to viscous flow and the prediction of glass transition temperatures of molten silicates

The results of multi-anvil melting experiments are reported for a range of compositions in the system CaO-MgO-Al2O3-SiO2. The liquidus crystallization fields for forsterite, or­thopyroxene. clinopyroxene. and garnet have been mapped out at 10 GPa. as have their intersections at various cotectics. The composition of the liquid that is multiply saturated in forsterite. orthopyroxene, clinopyroxene. and garnet has been also determined to within ±0.5-1.0 wt% (2σ). and the result is in excellent agreement with a previous estimate (Herzberg 1992). These experiments confirm that the effect of pressure is to reduce Al2O3 and increase MgO and SiO2 in magmas formed by the melting of garnet lherzolite with increasing pressure (Herzberg 1992). Melting experiments in the system MgO-SiO2 also have been performed to constrain how pressure affects the compositions of liquids that are saturated in harzbiugite [L + Ol + Opx]. Experiments in both CaO-MgO-Al2O3-SiO2 and MgO-SiO2 demonstrate that there is a maximum normative olivine content to liquids formed by initial or advanced melting of peridotite in the upper mantle, and this occurs at 7 to 8 GPa. For most peridotites that undergo decompression melting in a plume, clino­pyroxene and garnet are the first crystalline phases to melt out and. with a few important but rare exceptions, the experimentally constrained liquids are unlike most volcanic rocks. Advanced anhydrous melting will yield liquids with a residual harzburgite mineralogy [L + Ol + Opx]. and these liquids are similar in composition to most komatiites with Cre­taceous and 2700 Myr Archaean ages.

Melting experiments in the systems CaO-MgO-Al2O3-SiO2and MgO-SiO2at 3 to 15 GPa

The results of multi-anvil melting experiments are reported for a range of compositions in the system CaO-MgO-Al 2 O 3 -SiO 2 . The liquidus crystallization fields for forsterite, orthopyroxene, clinopyroxene, and garnet have been mapped out at 10 GPa, as have their intersections at various cotectics. The composition of the liquid that is multiply saturated in forsterite, orthopyroxene, clinopyroxene, and garnet has been also determined to within + or -0.5-1.0 wt% (2sigma ), and the result is in excellent agreement with a previous estimate (Herzberg 1992). These experiments confirm that the effect of pressure is to reduce Al 2 O 3 and increase MgO and SiO 2 in magmas formed by the melting of garnet lherzolite with increasing pressure (Herzberg 1992). Melting experiments in the system MgO-SiO 2 also have been performed to constrain how pressure affects the compositions of liquids that are saturated in harzburgite [L+Ol+Opx]. Experiments in both CaO-MgO-Al 2 O 3 -SiO 2 and MgO-SiO 2 demonstrate that there is a maximum normative olivine content to liquids formed by initial or advanced melting of peridotite in the upper mantle, and this occurs at 7 to 8 GPa. For most peridotite that undergo decompression melting in a plume, clinopyroxene and garnet are the first crystalline phases to melt out and, with a few important but rare exceptions, the experimentally constrained liquids are unlike most volcanic rocks. Advanced anhydrous melting will yield liquids with a residual harzburgite mineralogy [L+Ol+Opx], and these liquids are similar in composition to most komatiites with Cretaceous and 2700 Myr Archaean ages.

The influence of water on melting of mantle peridotite

This experimental study examines the effects of variable concentrations of dissolved H2O on the compositions of silicate melts and their coexisting mineral assemblage of olivine + orthopyroxene ± clinopyroxene ± spinel ± garnet. Experiments were performed at pressures of 1.2 to 2.0 GPa and temperatures of 1100 to 1345 °C, with up to ∼12 wt% H2O dissolved in the liquid. The effects of increasing the concentration of dissolved H2O on the major element compositions of melts in equilibrium with a spinel lherzolite mineral assemblage are to decrease the concentrations of SiO2, FeO, MgO, and CaO. The concentration of Al2O3 is unaffected. The lower SiO2 contents of the hydrous melts result from an increase in the activity coefficient for SiO2 with increasing dissolved H2O. The lower concentrations of FeO and MgO result from the lower temperatures at which H2O-bearing melts coexist with mantle minerals as compared to anhydrous melts. These compositional changes produce an elevated SiO2/(MgO + FeO) ratio in hydrous peridotite partial melts, making them relatively SiO2 rich when compared to anhydrous melts on a volatile-free basis. Hydrous peridotite melting reactions are affected primarily by the lowered mantle solidus. Temperature-induced compositional variations in coexisting pyroxenes lower the proportion of clinopyroxene entering the melt relative to orthopyroxene. Isobaric batch melting calculations indicate that fluid-undersaturated peridotite melting is characterized by significantly lower melt productivity than anhydrous peridotite melting, and that the peridotite melting process in subduction zones is strongly influenced by the composition of the H2O-rich component introduced into the mantle wedge from the subducted slab.

Petrogenesis of Cenozoic Basalts from Vietnam: Implication for Origins of a `Diffuse Igneous Province'

Basalt magmatism occurred throughout east and southeast Asia after the early Tertiary India–Asia collision. This activity does not conform to the ‘Large Igneous Province’ model in view of lower eruption and melt production rates, wide dispersal of centres and the apparent absence of deep mantle upwelling. Age data for Vietnamese plateau basalts reflect spatial–temporal patterns consistent with a rotating stress field rather than supra-hotspot lithosphere migration. For most of the volcanic centres there are two eruptive episodes: an early series formed by high-SiO2, low-FeO* quartz and olivine tholeiites—large melt fractions of refractory (lithosphere-like) mantle—and a later series made up of low-SiO2, high-FeO* olivine tholeiites, alkali basalts and basanites—smaller melt fractions of more fertile (asthenosphere-like) mantle. Comparison of Mg-15 normalized basalt compositions with parameterized anhydrous and hydrous experimental melt compositions allowed calculation of melt segregation pressures and temperatures. Computed for anhydrous conditions these range from <4 GPa and ∼1470°C (for alkali basalts) to <0.5 GPa and ∼1400°C (quartz tholeiites), and for H2O-undersaturated conditions, from <3.5 GPa and ∼1450°C to ∼1.5 GPa and 1350–1400°C, respectively. Hydrous conditions are more realistic in view of high measured basalt H2O+ contents, pressure estimates consistent with melting below a thinned mechanical boundary layer (MBL) and interpolated mantle adiabats of 2–3°C/km (compared with <1°C/km for anhydrous conditions), consistent with fluid dynamic constraints and a 1440°C potential temperature. After collision-induced ‘extrusion’ of east and southeast Asia, the lithosphere was probably thinned during heating and transtension; this converted refractory MBL into a low-viscosity thermal boundary layer (TBL), and caused upward penetration and polybaric melting of TBL–asthenosphere columns.