Wetting behavior of partial melts during crustal anatexis: the distribution of hydrous silicic melts in polycrystalline aggregates of quartz (original) (raw)
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Chemical Geology, 1995
In partially molten systems, the equilibrium distribution of melt at the grain scale is governed by the principle of interfacial energy minimization. In ideal sources (i.e. partially molten rocks that are monomineralic, have single-valued solid-liquid and solid-solid interfacial energies, and are subject to hydrostatic stress) the wetting angle 0 is known to be a unique characteristic which specifies the melt configuration for a given melt fraction. Crustal rocks cannot be modelled as ideal sources because of their polymineralic nature, the moderate to high anisotropy of interfacial energies which characterizes common refractory minerals, and the possible presence of a crystallographic preferred orientation. That partially molten crustal rocks depart from ideal sources is documented by a series of highP,high-T experiments illustrating the textural relationships of biotite and amphibole with silicic melts. The melt distributions observed in these experiments differ significantly from those expected in ideal sources: (1) crystal-melt interfaces are commonly planar, rational faces rather than smoothly curved, irrational surfaces; and (2) the concept of a unique wetting angle does not hold as shown in the biotite-silicic melt system. These textural features demonstrate that anisot.ropy of crystal-melt interfacial energy is a factor of primary importance in modelling the grain-scale distribution of partial melts. The petrological implications of our study are the following: (1) At high degrees of anisotropy and low melt fractions, melt is predicted to form isolated, plane-faced pockets at grain comers. The overall shape of these pockets, and therefore the value of the connectivity threshold & are expected to be very sensitive to the ratio of solid-solid to solid-liquid interfacial energies, ySS/ yS, (& is the melt fraction at which melt interconnectivity is established). Melt pockets with low volume-to-surface ratio, and low (but non-constant) wetting angles should prevail at high ySyss/-yS,, resulting in very low values of 4c (< 1 to a few vol%). Higher values of &,, a high volume-to-surface ratio of melt pockets, and high wetting angles are expected at low ySS/ yS,. (2) The wetting angle at hornblende-hornblende-melt junctions, at 1200 MPa-975°C is 25". A review of existing data indicates that quartz-melt and feldspar-melt wetting angles are also low to moderate (12-60"). A very low value of 4c should, therefore, be the general rule during crustal anatexis. In particular, a connectivity threshold lower than 34 ~01% is predicted for partially molten amphibolite. (3) In biotite-rich rock-types, such as melanosomes in migmatites, the combination of a pronounced crystalline anisotropy and a marked preferred orientation of mica flakes leads to a very low permeability (normal to layering). Biotite-rich melanosomes should therefore impedt: chemical interactions between neighbouring leucosomes and mesosomes.
Journal of Petrology, 2005
We have conducted experiments on dissolution of quartz, albite, orthoclase, and corundum into H 2 O-saturated haplogranite melt at 800 C and 200 MPa over a duration of 120-1488 h with the aim of ascertaining the diffusive transport properties of granitic melts at crustal anatectic temperatures. Cylinders of anhydrous starting glass and a single mineral phase (quartz or feldspar) were juxtaposed along flat and polished surfaces inside gold or platinum capsules with %10 wt % added H 2 O. Concentration profiles in glass (quenched melt) perpendicular to the mineral-glass interfaces and comparison with relevant phase diagrams suggest that melts at the interface are saturated in the dissolving phases after 384 h, and with longer durations the concentration profiles are controlled only by diffusion of components in the melt. The evolution of the concentration profiles with time indicates that uncoupled diffusion in the melt takes place along the following four linearly independent directions in oxide composition space: SiO 2 , Na 2 O, and K 2 O axes (Si-, Na-, and K-eigenvectors, respectively), and a direction between the Al 2 O 3 , Na 2 O, and K 2 O axes (Al-eigenvector), such that the Al/Na molar ratio is equal to that of the bulk melt and the Al/(Na þ K) molar ratio is equal to the equilibrium ASI (¼ mol. Al 2 O 3 /[Na 2 O þ K 2 O]) of the melt. Experiments in which a glass cylinder was sandwiched between two mineral cylinders-quartz and albite, quartz and K-feldspar, or albite and corundum-tested the validity of the inferred directions of uncoupled diffusion and explored longrange chemical communication in the melt via chemical potential gradients. The application of available solutions to the diffusion equations for the experimental quartz and feldspar dissolution data provides diffusivities along the directions of the Si-eigenvector and Al-eigenvector of %(2Á0-2Á8) · 10 À15 m 2 /s and %(0Á6-2Á4) · 10 À14 m 2 /s, respectively. Minimum diffusivities of alkalis [%(3-9) · 10 À11 m 2 /s] are orders of magnitude greater than the tetrahedral components of the melt. The information provided here determines the rate at which crustal anatexis can occur when sufficient heat is supplied and diffusion is the only mass transport (mixing) process in the melt. The calculated diffusivities imply that a quartzo-feldspathic source rock with initial grain size of 2-3 mm undergoing hydrostatic, H 2 O-saturated melting at 800 C (infinite heat supply) could produce 20-30 vol. % of homogeneous melt in less than 1-10 years. Slower diffusion in H 2 O-undersaturated melts will increase this time frame.
Dry and strong quartz during deformation of the lower crust in the presence of melt
Journal of Geophysical …, 2011
1] Granulite facies migmatitic gneisses from the Seiland Igneous Province (nort hern Norway) were deformed during deep crustal shearing in the presence of melt, whic h formed by dehydration melting of biotite. Partial melting and deformation occurr ed during the intrusion of large gabbroic plutons at the base of the lower crust at 570 to 520 Ma in an intracontinental rift setting. The migmatitic gneisses consist of high-aspec t-ratio leucosome-rich domains and a leucosome-poor, restitic domain of quartzitic compositi on. According to thermodynamic modeling using synkinematic mineral assemblages, deformation occurred at T = 760°C-820°C, P = 0.75-0.95 GPa and in the presence of ≤5 vol % of residual melt. There is direct evidence from microstructural observations, Fourier transform infrared measurements, thermodynamic modeling, a nd titanium-in-quartz thermometry that dry quartz in the leucosome-poor domain deformed a t high differential stress (50-100 MPa) by dislocation creep. High stresses are demo nstrated by the small grain size (11-17 mm) of quartz in localized layers of recrystallized grains, where titanium-in-quartz thermometry yields 770°C-815°C. Dry and strong quartz forms a load-bearing framework in the migmatitic gneisses, where ∼5% melt is present, but does not control the mechanical behavior because it is located in isolated pocke ts. The high stress deformation of quartz overprints an earlier, lower stress deformation, wh ich is preserved particularly in the vicinity of segregated melt pockets. The grain-scale melt distribution, water content and distribution, and the overprinting relationships of quartz microstructures indicate that biotite dehydration melting occurred during deform ation by dislocation creep in quartz. The water partitioned into the segregated melt crys tallizing in isolated pockets, in the vicinity of which quartz shows a higher intracrystal line water content and a large grain size. On the contrary, the leucosome-poor domain of the rock, from which melt was removed, became dry and thereby mechanically stronger. Melt removal at larger scale will result in a lower crust which is dry enough to be mechanically strong. The application of flow laws derived for wet quartz is not appropriate to estimate the behavior of such granulite facies parts of the lower crust. Citation: Menegon, L., P. Nasipuri, H. Stünitz, H. Behrens, and E. Ravna (2011), D ry and strong quartz during deformation of the lower crust in the presence of melt,
An experimental study of grain scale melt segregation mechanisms in two common crustal rock types
Journal of Metamorphic Geology, 2002
Creation of pathways for melt to migrate from its source is the necessary first step for transport of magma to the upper crust. To test the role of different dehydration-melting reactions in the development of permeability during partial melting and deformation in the crust, we experimentally deformed two common crustal rock types. A muscovite-biotite metapelite and a biotite gneiss were deformed at conditions below, at and above their fluid-absent solidus. For the metapelite, temperatures ranged between 650 and 800 uC at P c =700 MPa to investigate the muscovite-dehydration melting reaction. For the biotite gneiss, temperatures ranged between 850 and 950 uC at P c =1000 MPa to explore biotite dehydration-melting under lower crustal conditions. Deformation for both sets of experiments was performed at the same strain rate (e . ) 1.37310 x5 s x1 . In the presence of deformation, the positive DV and associated high dilational strain of the muscovite dehydration-melting reaction produces an increase in melt pore pressure with partial melting of the metapelite. In contrast, the biotite dehydration-melting reaction is not associated with a large dilational strain and during deformation and partial melting of the biotite gneiss melt pore pressure builds more gradually. Due to the different rates in pore pressure increase, melt-enhanced deformation microstructures reflect the different dehydration melting reactions themselves. Permeability development in the two rocks differs because grain boundaries control melt distribution to a greater extent in the gneiss. Muscovite-dehydration melting may develop melt pathways at low melt fractions due to a larger volume of melt, in comparison with biotite-dehydration melting, generated at the solidus. This may be a viable physical mechanism in which rapid melt segregation from a metapelitic source rock can occur. Alternatively, the results from the gneiss experiments suggest continual draining of biotite-derived magma from the lower crust with melt migration paths controlled by structural anisotropies in the protolith.
Contributions to Mineralogy and Petrology, 2006
We have experimentally investigated the kinetics of melting of an aplitic leucogranite (quartz+ sodic plagioclase of %Ab 90 +K-feldspar+traces of biotite) at 690, 740, and 800°C, all at 200 MPa H 2 O. Leucogranite cylinders, 3.5 mm in diameter and 7 mm in length, were run in the presence of excess H 2 O using cold-seal pressure vessels for 11-2,925 h. At 690 and 740°C and any experimental time, and 800°C and short run times, silicate glass (melt at run conditions) occurs as interconnected films along most of the mineral boundaries and in fractures, with the predominant volume occurring along quartz/feldspars boundaries and quartz/ plagioclase/K-feldspar triple junctions. Glass film thickness is roughly constant throughout a given experimental charge and increases with experimental temperature and run duration. The results indicate that H 2 O-saturated partial melting of a quartzo-feldspathic protolith will produce an interconnected melt phase even at very low degrees (<5 vol%) of partial melting. Crystal grain boundaries are therefore completely occluded with melt films even at the lowest degrees of partial melting, resulting in a change in the mechanism of mass transport through the rock from advection of aqueous vapor to diffusion through silicate melt. At 690 and 740°C the compositions of glasses are homogeneous and (at both temperatures) close to, but not on, the H 2 O-saturated 200 MPa haplogranite eutectic; glass compositions do not change with run duration. At 800°C glasses are heterogeneous and plot away from the minimum, although their molar ratios ASI (=mol Al 2 O 3 /CaO+Na 2 O+ K 2 O) and Al/Na are constant throughout the entire charge at any experimental time. Glass compositions within individual 800°C experiments form linear trends in (wt%) normative quartz-albite-orthoclase space. The linear trends are oriented perpendicular to the 200 MPa H 2 O haplogranite cotectic line, reflecting nearly constant albite/orthoclase ratio versus variable quartz/feldspar ratio, and have endpoints between the 800°C isotherms on the quartz and feldspar liquidus surfaces. With increasing experimental duration the trends migrate from the potassic side of the minimum toward the bulk rock composition located on the sodic side, due to more rapid (and complete) dissolution of K-feldspar relative to plagioclase. The results indicate that partial melting at or slightly above the solidus (690-740°C) is interface reaction-controlled, and produces disequilibrium melts of near-minimum composition that persist metastably for up to at least 3 months. Relict feldspars show no change in composition or texture, and equilibration between melt and feldspars might take from a few to tens of millions of years. Partial melting at temperatures well above the solidus (800°C) produces heterogeneous, disequilibrium liquids whose compositions are determined by the diffusive transport properties of the melt and local equilibrium with neighboring mineral phases. Feldspars recrystallize and change composition rapidly. Partial melting and equilibration between liquids and feldspars might take from a few to tens of years (H 2 O-saturated conditions) at these temperatures well above the solidus.
Tectonophysics, 2001
The volume change associated with dehydration melting has been investigated experimentally in muscovite and biotitebearing assemblages because it is a possible driving force for melt segregation during orogenesis. Experiments have been performed on cores of a muscovite + biotite-bearing pelite and on a biotite + plagioclase + quartz gneiss. The muscovite + biotitebearing pelite produced a similar set of melt-filled cracks to that observed in muscovite-bearing quartzite under partial melting conditions of 700 MPa, and 850 and 900°C. However, no cracking was observed in the biotite gneiss under a range of temperature conditions (700 MPa, 800 -900°C). The textures of the partially melted rock samples suggest the volume change and associated dilational strain accompanying melting in assemblages with only biotite is insignificant or negative. This is confirmed by calculations of the dilational strain in the biotite gneiss experiments and other experiments in the literature. For example, the dilational strain associated with partial melting of biotite-bearing metagreywacke assemblages ranges from + 1.90% to À 12.24% (given 30% modal biotite, calculated on a 1-oxygen basis), becoming negative when garnet is produced at higher pressure. In contrast, the dilational strain associated with melt-induced cracking in a muscovite-bearing metapelite is higher, + 6.76%, for the same modal abundance. These results suggest that volume change alone is not an important driving force for melt segregation in biotite-only-bearing assemblages, and external deformation at higher melt fractions may be required to segregate melt from the lower crust during partial melting. Reaction-controlled segregation is possible in muscovitebearing rocks and melt will be more easily expelled in the upper to mid levels of the crust because of rapid pore pressure development during early stages of melting. Major element chemistry of melt in the two-mica assemblage is dominated by muscovite melting, even when assemblage contains reacting biotite. Some implications of these results are that: (1) the melt that escapes at low melt volumes from the mid-crust is likely to have a muscovite-melting chemical signature; and (2) in the lower portions of the crust where melting is controlled by biotite stability, melt may become trapped within and along grains and remain distributed, pervasively, at the grain scale until greater melt fractions are generated. Recent modeling of orogenic belts shows that the evolution of collisional belts likely involves the prolonged presence of a weak crustal layer. Melt trapped along grain boundaries from low dilational strain melting reactions may be a mechanism for keeping melt in the crust and weakening it during active orogenesis. D : S 0 0 4 0 -1 9 5 1 ( 0 1 ) 0 0 1 7 2 -X www.elsevier.com/locate/tecto Tectonophysics 342
Journal of Metamorphic Geology, 2013
Melt infiltration into quartzite took place due to generation and migration of partial melts within the high-grade metamorphic rocks of the Big Cottonwood (BC) formation in the Little Cottonwood contact aureole (UT, USA). Melt was produced by muscovite and biotite dehydration melting reactions in the BC formation, which contains pelite and quartzite interlayered on a centimetre to decimetre scale. In the migmatite zone, melt extraction from the pelites resulted in restitic schollen surrounded by K-feldsparenriched quartzite. Melt accumulation occurred in extensional or transpressional domains such as boudin necks, veins and ductile shear zones, during intrusion-related deformation in the contact aureole. The transition between the quartzofeldspathic segregations and quartzite shows a gradual change in texture. Here, thin K-feldspar rims surround single, round quartz grains. The textures are interpreted as melt infiltration texture. Pervasive melt infiltration into the quartzite induced widening of the quartzquartz grain boundaries, and led to progressive isolation of quartz grains. First as clusters of grains, and with increasing infiltration as single quartz grains in the K-feldspar-rich matrix of the melt segregation. A 3D-lCT reconstruction showed that melt formed an interconnected network in the quartzites. Despite abundant macroscopic evidence for deformation in the migmatite zone, individual quartz grains found in quartzofeldspathic segregations have a rounded crystal shape and lack quartz crystallographic orientation, as documented with electron backscatter diffraction (EBSD). Water-rich melts, similar to pegmatitic melts documented in this field study, were able to infiltrate the quartz network and disaggregate grain coherency of the quartzites. The proposed mechanism can serve as a model to explain abundant xenocrysts found in magmatic systems.
Contributions To Mineralogy and Petrology, 1988
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 (T≤500° 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 (T≤550° C), which depletes the melt in P and REE. Below the liquidus, boron increasingly favors the vapor over melt with decreasing temperatures.
Earth and Planetary Science Letters, 1997
We present results of an experimental study to determine the nature of minimum to near-minimum melt compositions in equilibrium with upper mantle peridotite mineralogy at 1 GPa. We confirm earlier conclusions that anhydrous melts of lherzolite at 1 GPa are basaltic with ; 15-20% normative diopside, ) 10% normative olivine and at low degrees of melting are Na O and K O-rich and nepheline-normative in 'fertile' mantle. The most extreme Na O-rich minimum melt 2 2 2 composition is in equilibrium with an albite-bearing harzburgite residue at 12208C. This melt composition is nephelinew x normative with ; 64% SiO and about ; 12% Na O. Our results disagree with recent reports 1,2 that peridotitic 2 2 minimum melt compositions have an 'andesitic' character at 1 GPa. We present reversal experiments showing that these latter melts are not in equilibrium with a spinel or plagioclase lherzolite upper mantle assemblage. We use our new data and Ž data from the literature to define minimum melts i.e. melts in equilibrium with olivineq orthopyroxeneq clinopyroxeneq . plagioclase" spinel for fertile or enriched to refractory lherzolite at 1 GPa. The minimum melt compositions are nephelineq olivine-normative for sodium-rich sources and hyperstheneq olivine-normative for refractory or depleted compositions with very calcic plagioclase or high CarCaq Na ratios in spinel lherzolite. It is not possible to derive quartz-normative basaltic or 'andesitic' melt compositions by partial melting of anhydrous lherzolite at 1 GPa. q 1997 Elsevier Science B.V. during mantle upwelling. However, it is very difficult to determine the chemical compositions of very small melt fractions by direct experiment in largely crystalline silicate systems. The proven approach in simple silicate systems is to use multiple bulk compositions to map out a eutectic or minimum melt w x composition 3,4 , but to do this in complex, natural systems requires a lot of work. Recently, the dia-0012-821Xr97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.
Phase relations and compositional dependence of H2O solubility in quartz-feldspar melts
Chemical Geology, 1992
. Phase relations and compositional dependence of H20 solubility in quartz-feldspar melts, in: Y. Bottinga, D.B. Dingwell and P. Richet (Guest-Editors), Silicate Melts. Chem. Geol., 96: 303-319. Liquidus phase relations are presented for the systems Qz-Ab and Qz-Or at 2 and 5 kbar under both H20-saturated and H20-undersaturated conditions. The data allow to specify the effects on phase equilibria of: ( 1 ) varying the water content of the melt under isobaric conditions; and (2) varying pressure under H20-saturated conditions. The effects are different from each other and they depend on the system investigated. Increasing the water content of the melt at 2 kbar lowers liquidus and eutectic temperatures more in the Qz-Ab than in the Qz-Or system, while eutectic compositions remain unchanged. In contrast, the effect of increasing P (=PH2o) from 2 to 5 kbar is mainly to shift the eutectic composition in the Qz-Ab system towards more albite-rich compositions, whereas no compositional change is observed for the Qz-Or eutectic. The contrast between the two systems when isobarically varying the water content of the melt can be attributed to higher water solubilities in Na-than in K-bearing melt compositions. Liquidus phase relations with varying P ( = PH2o) reflect the combination of the effects of pressure and water content of the melt. The compositional range of water contents in the melt used in this study implies that molecular H20 is the water species involved in the observed changes in phase relations. Current models and recent spectroscopic data for water solubility mechanisms in aluminosilicate glasses and melts are critically examined. The variation in water solubility between aluminosilicate melts is due to the differential incorporation of molecular H20.