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1.
The mechanisms and kinetics of equilibration between peraluminousminerals and granitic melt were investigated experimentallyby the dissolution of corundum and andalusite into H2O-saturatedmetaluminous haplogranitic melt at 800°C and 200 MPa. Mineraland haplogranitic glass rods were juxtaposed inside platinumcapsules, and then subjected to experimental conditions fortimes ranging from 12 to 2900 h. Upon melting, the mineral –meltinterface retreats with the square root of time. The compositionof the melt at the interface changes with time, but its ASI[aluminum saturation index = molar Al2O3/(CaO + Na2O + K2O)]remains constant at  相似文献   
2.
This study examines hydration–diffusion in the metaluminous haplogranite system at 200 MPa H2O and 800–300°C. At 800°C hydration is accompanied by melting and uphill diffusion of sodium from anhydrous glass toward the region of hydration and melting, whereas potassium diffuses away from the hydration front and into anhydrous glass. Silicon and aluminum are simply diluted upon hydration. There is no change in molecular Al/(Na + K) throughout the entire hydration-diffusion aureole and, therefore, (1) there is no loss of alkalis to the vapor, and (2) K migrates to replace Na in order to maintain local charge balance required by IVAl. Alkali diffusion occurs over a viscosity contrast from 104.1 Pa s in hydrous liquid to 1011.8–1013.5 Pa s in anhydrous glass. From these results, we interpret that: (1) Na is structurally or energetically favored over K as a charge-balancing cation for IVAl in hydrous granitic liquids, whereas the opposite behavior has been observed for anhydrous melts, and (2) the diffusion of alkalis through silicate melts is largely independent of viscosity. Results from 600°C are similar to those at 800°C, but hydration at 300°C involves a loss of Na and concomitant increase in molar Al/(Na + K) in the hydration zone due to hydrogen-alkali exchange between fluid and glass. Hydration behavior at 400°C is transitional between those at 300°C and 600°C, suggesting that the change in hydration mechanism occurs near the glass transition.  相似文献   
3.
The addition of phosphorus to H2O-saturated and initially subaluminous haplogranitic (Qz–Ab–Or) compositions at 200 MPa(H2O) promotes expansion of the liquidus field of quartz, a marked decrease of the solidus temperature, increased solubility limits of H2O in melt at low phosphorus concentrations, and fractionation of melt out of the haplogranite plane (projected along an Or28 isopleth) toward a peralkaline, silica-poor but quartz-saturated minimum composition. The partition coefficient for P2O5 between aqueous vapor and melt with an ASI (aluminum saturation index, mol Al/[mol Na+K])=1 is negligible (0.06), and consequently so are the effects of phosphorus on other melt-vapor relations involving major components. Phosphorus becomes more soluble in vapor, however, as the concentration of a NaPO3 component increases via the fractionation of melt by crystallization of quartz and feldspar. The experimental results here corroborate existing concepts regarding the interaction of phosphorus with alkali aluminosilicate melt: phosphorus has an affinity for alkalis and Al, but not Si. Phosphorus is incorporated into alkali feldspars by the exchange component AlPSi-2. For subaluminous compositions (ASI=1), the distribution coefficient of phosphorus between alkali feldspar and melt, D[P]Af/m, is 0.3. This value increases to D[P]Af/m=1.0 at a melt ASI value of 1.3. The increase in D[P]Af/m with ASI is expected from the fact that excess Al promotes the AlPSi-2 exchange. With this experimental data, the P2O5 content of feldspars and whole rocks can reveal important facets of crystallization and phosphorus geochemistry in subaluminous to peraluminous granitic systems.  相似文献   
4.
Subhorizontally layered pegmatite-aplite bodies are characterized by fine-grained, sodic to granitic aplite that is usually juxtaposed abruptly above by much coarser-grained, commonly graphic potassic pegmatite. Although well studied, there currently is little concensus as to how such dikes form. The Little Three dike near Ramona, California, is representative of such zoned bodies in this and other regions, and contains discontinuous miarolitic pockets near the base of the graphic pegmatite zone. Tourmaline, garnet, biotite, and muscovite show no changes in major- or minor-element compositions indicative of progressive magmatic fractionation until the immediate vicinity of the main miarolitic zone, where they record abrupt and extreme enrichments in Li, F, and Mn. There is no correlation of chemical changes in the dike with the appearance of small miarolitic vugs well below the main miarolitic zone, nor is there any indication that the aplite, graphic pegmatite, or miarolitic pockets represent separate magma injections. The chemistries of tourmaline, garnet, and micas, however, preclude conventional models of Rayleigh fractionation or traditional zone refining. Textural features and modeled cooling histories indicate that the dike cooled quickly and might have solidified partially or totally to glass before crystallization commenced. Geothermometry based on the compositions of coexisting plagioclase and homogeneous, nonperthitic K-feldspar indicates inward crystallization of the dike, from ∼400–435 °C at the margins to ∼350–390 °C within 20–30 cm of the pocket horizon, then a sharp decrease to 240–275 °C in the pockets where K-feldspar is perthitic. We interpret the feldspar geothermometry (except perhaps in the miarolitic cavities) to reflect the temperatures at crystallization fronts that advanced into the pegmatite, first from the foot wall and eventually joined by a similar front downward from the hanging wall. Crystallization down from the hanging wall may have commenced after ∼70–80% of the foot wall aplite had crystallized. The very abrupt increases of Li, Mn, and F in tourmaline and garnet near the miarolitic zone appear to be explained best by the process of constitutional zone refining, in which a fluxed crystallization front sweeps an incompatible element-enriched boundary layer through a solid or semi-solid. After these two highly fluxed boundary layers merged near the main miarolitic zone, compositional evolution could have proceeded by crystal-melt fractionation. Received: 24 March 1998 / Accepted: 10 March 1999  相似文献   
5.
Vapor-saturated experiments at 200 MPa with peraluminous, lithophile-element-rich rhyolite obsidian from Macusani, Peru, reveal high miscibility of H2O and silicate melt components. The H2O content of melt at saturation (11.5+-0.5 wt.%) is almost twice that predicted by existing melt speciation models. The corresponding solubility of melt components in vapor decreases from 15 wt.% dissolved solids (750°–775° C) to 9 wt.% at 600° C. With regard to major and most minor components, macusanite melt dissolves congruently in vapor. Among the elements studied (B, P, F, Li, Rb, Cs, Be, Sr, Ba, Nb, Zr, Hf, Y, Pb, Th, U, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm), only boron has a vapor/melt partition coefficient (D[B]) consistently 1 at superliquidus temperatures (>645° C). Phosphorus and fluorine behave similarly, with D[P] and D[F]<0.5. Little or no significant vapor/melt fractionation is evident among most periodic groups (alkalis, alkaline earths, Zr/Hf, or the REE). The temperature dependence of vapor/melt partition coefficients is generally greatest for cations with charge +3 (except Nb and U); most vapor/melt partition coefficients for trace elements increase with decreasing temperature to the liquidus. Crystallization proceeds by condensation of crystalline phases from vapor; most coexisting melts are aphyric. Changes in the major element content of melt are dominated by the mineral assemblage crystallized from vapor, which includes subequal proportions of white mica, quartz, albite, and orthoclase. The volumetric proportion of (mica + or-thoclase)/albite increases slightly with decreasing T, creating a sodic, alkaline vapor. Vapor deposition of topaz (T500° C), which consumes F from melt, returns K/Na ratios of melt to near unity with the vapor-deposition of albite. The abundances of most trace elements in residual melt change little with the crystallization of major phases, but in some cases are strongly controlled by the deposition of accessory phases including apatite (T550° C), which depletes the melt in P and REE. Below the liquidus, boron increasingly favors the vapor over melt with decreasing temperatures.  相似文献   
6.
We have conducted experiments on dissolution of quartz, albite,orthoclase, and corundum into H2O-saturated haplogranite meltat 800°C and 200 MPa over a duration of 120–1488 hwith the aim of ascertaining the diffusive transport propertiesof granitic melts at crustal anatectic temperatures. Cylindersof anhydrous starting glass and a single mineral phase (quartzor feldspar) were juxtaposed along flat and polished surfacesinside gold or platinum capsules with 10 wt % added H2O. Concentrationprofiles in glass (quenched melt) perpendicular to the mineral–glassinterfaces and comparison with relevant phase diagrams suggestthat melts at the interface are saturated in the dissolvingphases after 384 h, and with longer durations the concentrationprofiles are controlled only by diffusion of components in themelt. The evolution of the concentration profiles with timeindicates that uncoupled diffusion in the melt takes place alongthe following four linearly independent directions in oxidecomposition space: SiO2, Na2O, and K2O axes (Si-, Na-, and K-eigenvectors,respectively), and a direction between the Al2O3, Na2O, andK2O axes (Al-eigenvector), such that the Al/Na molar ratio isequal to that of the bulk melt and the Al/(Na + K) molar ratiois equal to the equilibrium ASI (= mol. Al2O3/[Na2O + K2O])of the melt. Experiments in which a glass cylinder was sandwichedbetween two mineral cylinders—quartz and albite, quartzand K-feldspar, or albite and corundum—tested the validityof the inferred directions of uncoupled diffusion and exploredlong-range chemical communication in the melt via chemical potentialgradients. The application of available solutions to the diffusionequations for the experimental quartz and feldspar dissolutiondata provides diffusivities along the directions of the Si-eigenvectorand Al-eigenvector of (2·0–2·8) x 10–15m2/s and (0·6–2·4) x 10–14 m2/s, respectively.Minimum diffusivities of alkalis [(3–9) x 10–11m2/s] are orders of magnitude greater than the tetrahedral componentsof the melt. The information provided here determines the rateat which crustal anatexis can occur when sufficient heat issupplied and diffusion is the only mass transport (mixing) processin the melt. The calculated diffusivities imply that a quartzo-feldspathicsource rock with initial grain size of 2–3 mm undergoinghydrostatic, H2O-saturated melting at 800°C (infinite heatsupply) could produce 20–30 vol. % of homogeneous meltin less than 1–10 years. Slower diffusion in H2O-undersaturatedmelts will increase this time frame. KEY WORDS: chemical diffusion; haplogranite; mineral dissolution experiments; crustal anatexis  相似文献   
7.
Abstract Natural, pure CO2 inclusions in quartz and olivine (c. Fo90) were exposed to controlled fH2 conditions at T= 718–728°C and Ptotal= 2 kbar; their compositions were monitored (before and after exposures) by microsampling Raman spectroscopy (MRS) and microthermometry. In both minerals exposed at the graphite–methane buffer (fH2= 73 bar), fluid speciations record the diffusion of hydrogen into the inclusions. In quartz, room-temperature products in euhedral isolated (EI type) inclusions are carbonic phases with molar compositions of c. CO2(60) + CH4(40) plus graphite (Gr) and H2O, whereas anhedral inclusions along secondary fractures (AS type) are Gr-free and contain H2O plus carbonic phases with compositions in the range c. CO2(60) + CH4(40) to CO2(10) + CH4(90). EI type inclusions in olivine evolved to c. CO2(90–95) + CH4(5–10) without Gr, whereas AS type inclusions have a range of compositions from CO2(90) + CH4(10) ± Gr to CH4(50) + H2(50) ± Gr; neither H2O nor any hydrous species was detected by optical microscopy or MRS in the olivine-hosted products. Differences in composition between and among the texturally distinct populations of inclusions in both minerals probably arise from variations in initial fluid densities, as all inclusions apparently equilibrated with the ambient fH2. These relations suggest that compositional variability among inclusions in a given natural sample does not require the entrapment of multiple generations of fluids. In addition, the absence of H2O in the olivine-hosted inclusions would require the extraction of oxygen from the fluids, in which case re-equilibration mechanisms may be dependent on the composition and structure of the host mineral. Many of the same samples were re-exposed to identical P–T conditions using Ar as the pressure medium, yielding ambient fH2= 0.06 bar. In most inclusions, the carbonic fluids returned to pure CO2 and graphite persisted in the products. Reversal of the mechanisms from the prior exposure at fH2= 73 bar did not occur in any inclusions but the AS types in olivine, in which minor CO2 was produced at the expense of CH4 and/or graphite. The observed non-reversibility of previous mechanisms may be attributed to: (1) slower fluid–solid reactions compared to reactions in the homogeneous fluid phase; (2) depressed activities of graphite due to poor ordering; and/or (3) low ambient fO2 at the conditions of the second run.  相似文献   
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