Rheological Transitions During Partial Melting and Crystallization with Application to Felsic Magma Segregation and Transfer (original) (raw)
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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.
Physics and Chemistry of The Earth Part A-solid Earth and Geodesy, 2001
Structural analysis of migmatites based on the distribution and proportion of the granitic fraction at the outcrop scale is taken as a guide to decipher the behavior of partially molten rocks during orogenesis. This approach evaluates the various mechanisms that control melt segregation and magma mobility from pores to orogens. During partial melting, the first liquid appears at grain interfaces but such textures are rarely preserved in igneous rocks. Mechanisms of melt movement are scale-dependent and it is important to distinguish melt segregation at grain-scale from melt migration which occurs over larger distances. Melt segregation is controlled by melt connectivity and ubiquitous localization of granites in structurally-controlled dilatant sites provides evidence of the efficiency of melt segregation at the outcrop scale, probably achieved by porous flow at the grain scale. Magma mobility is controlled by the continuity of the solid framework which controls the rheologic threshold at the transition from solid-dominated metatexites to liquid-dominated diatexites. The presence of laccoliths of homogeneous leucogranite, emplaced at higher structural levels far from their source, indicates the efficiency of melt migration beyond the grain scale. The transition zone between diatexites (melt source) and granitic laccoliths (melt sink) is characterized by a network of granitic veins centimeter-to meter-thick. The geometric characteristics of this network suggest that, depending on structural level and the competency contrast between liquid and solid, veins propagate by either channeled porous flow, ductile deformation or fracturing. The main driving forces for upward melt migration appear to be buoyancy and dilatancy; the characteristics of local and regional deformation control the patterns of the granitic vein networks. Partial melting and redistribution of melt and magma from segregation by percolation at the grain scale relayed by pervasive migration through vein networks, is associated with chemical differentiation and generation of new rheological layering of the orogenic crust.
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.
1] We present a model of melt segregation in a mush submitted to both compaction and shear. It applies to a granitic melt imbedded within a partially molten continental crust, able to sustain large stress values. The mathematical derivation starts with the equations for melt and plastic flow in a mush. They are manipulated to obtain equations for the mean flow field and for the separation velocity. Assuming that the mean flow field is simple shear, a specific set of equations for the melt flow in a shear field is obtained. After simplifying the equations, they finally reduce to two systems of coupled equations. One is the wellknown equation for compaction. The other is new and describes melt channelling during shear in a mush with a constant viscosity plastic matrix. Three free parameters are observed. One is the usual compaction length, and the other two are functions of the stress and strain amplitude during shear. Compaction instabilities lead to the development of spherical pockets rich in melt while shear channelling instability segregates melt in parallel veins. The size of the pockets and the distance between veins remain close to the compaction length. Actually, the viscosity ratio between the matrix and its melt controls the compaction length L, which is found metric or submetric. The two types of instability segregate melt. However, the compaction process is generally so sluggish that it cannot compete with the channelling one. The channelling time is controlled by the amount of intergranular melt present in the system and of the amplitude of the shear stresses. During each channelling cycle, lasting for about 30 to 300 kyr, the intergranular melt is completely squeezed out from the volume in between veins. As melting progresses, the successive batches of melt, as well as the residual solid matrix, are increasingly more dehydrated. As a result, both phases progressively stiffen without changing their viscosity contrast and the associated compaction length. The segregation process stops when the dehydration process clamps the deformation of the solid matrix.
Feedback between melt percolation and deformation in an exhumed lithosphere–asthenosphere boundary
Earth and Planetary Science Letters, 2008
Keywords: melt percolation strain localization mantle lithosphere olivine pyroxene crystal-preferred orientation dislocation creep diffusion refertilization reactions, lherzolites websterites harzburgites melt segregation Interactions between deformation and melt percolation yield important consequences for the evolution of the mantle lithosphere, controlling its composition and mechanical behavior. In the Lherz massif (Pyrenees, France), the analysis of structural relationships between harzburgites, lherzolites and pyroxenites and of the crystal-preferred orientations (CPO) of olivine and pyroxenes highlights a strong feedback between percolation of basaltic melts and deformation under near-solidus conditions at the lithosphere-asthenosphere boundary. Elongated harzburgite bodies up to tens of meters wide, which are the remnants of an old lithospheric mantle, preserve a constant foliation. This foliation is locally outlined by cm-scale flattened websteritic lenses. At the contact with the enclosing lherzolites, the harzburgite foliation is crosscut by the lherzolites foliation and by cm-wide websterite bands parallel to the contact. Strain intensity in the lherzolites increases with distance to the harzburgites. Based on these observations, we propose that reactive percolation was synchronous to the deformation and propose that variations in instantaneous melt fraction, due to pyroxenes and spinel crystallization during reactive melt transport, guided strain localization. Accordingly, the observed decrease in olivine CPO intensity and change in CPO patterns from harzburgites to distal lherzolites are interpreted as recording changes in the relative contribution of dislocation glide and diffusion processes, which is ruled by a balance between the instantaneous melt fraction and the local strain rate. We also propose that the pervasive websteritic layering in the refertilized lherzolites may result from deformation-assisted melt segregation in a system with decreasing permeability due to refertilization reactions. Finally, we discuss the possible timing and geodynamical context of the refertilization episode.
Geophysical Journal International, 2019
The physics of magmatic systems within continental crust is poorly understood. We developed a thermomechanical compositional two-phase flow formulation based on the conservation equations of mass, momentum and energy for melt and solid, including compaction of the solid matrix, melting, melt segregation, melt ascent and freezing. We use a simplified melting law to track the enrichment or depletion in SiO 2 of the advected silicic melt and solid. The nonlinear viscoplastic rheology includes the effect of melt porosity. 2-D models with different heat input are carried out for cases without and with differential melt-matrix flow. The retention number, R t , as a measure of melt mobility is varied between 1 and infinity. In the case of no melt segregation (large R t) our models show transient oscillatory behaviour followed by stationary convection in the lower crust enforced by a solid-melt phase transition. In the case of two-phase flow (i.e. small R t) melt separates from the solid matrix and accumulates in high melt porosity magma bodies within 10 s ka. We find a new melt ascent mechanism, termed CATMA, for Compaction/decompaction Assisted Two-phase flow Melt Ascent. This is a combination of compaction and decompaction of the contact zones between accumulated magma and solid rock that dislodges solid material from the roof that sinks through and partly dissolves in the magma. This process can be seen as an efficient microstoping mechanism and is associated with the formation of melt rich and chemically enriched channels within the magma body. The emplacement depths of magma change from >20 km for low heat flows to <10 km for high heat flows. In most models with high degrees of melting, two stacked SiO 2-enriched magmatic zones form interpreted as granitic layers. Models with stronger crustal rheology show porosity waves on a few km scale. Deviatoric stresses immediately above the evolving magma bodies are of the order of a few MPa, too small to overcome brittle or plastic yield stresses. The models predict significant chemical separation of depleted versus enriched composition, resulting in significant chemical stratification of the crust with spatial variations in solidus temperatures, and in a dual melt porosity distribution with crystal-poor magma bodies (>60 per cent melt) on top of low melt fraction mushes (<20 per cent). Comparison with the Altiplano-Puna magma body shows that the best agreement with observational data is obtained for a moderate (85-90 mW m −2) heat flux and retention number of the order of 3 to 30.
Journal of Geophysical Research, 1981
The compressibility of basic melt at I atmosphere is about an order of magnitude higher than that of mantle minerals. Consequently, the density contrast between melt and the principal residual crystals in mantle source regions is expected to decrease with increasing source region depth. The increasingly olivine-normative character of primary melts produced at greater depths is also expected to result in a decrease in this density contrast with increasing source region depth. Once vertical permeability is established by melt generated during partial melting, buoyancy-driven melt percolation can under some circumstances segregate melt from the residual crystals in its source region on a geologically rapid time scale. Limits to this process are provided by cooling of the source region (freezing melt in) and rigidity of the crystalline matrix (mechanically trapping melt). Source region size influences these limits strongly: consequently, small, partially molten diapirs (-•km in diameter) may be able to trap large melt fractions (m30%), but larger source regions would be unable to do so. The reduction in density contrast with pressure reduces the buoyant force driving melt percolation and provides another limit to melt segregation. Diapirs at depth may thus stably contain large fractions of melt but may decompress and unload their melt during ascent; this effect would be enhanced in small diapirs and may be relevant to the genesis of komatiitic magma. Melt compression may also be a factor in explaining why the very different maximum depths inferred for typical basic melt segregation from source regions on different planets•-•500 km on the moon, --250 km on Mars, -•100 km on earth--correspond to similar pressures (25-35 kbar); at greater pressures, melt may no longer be capable under ordinary conditions of segregating upwards by buoyancy. This may also help to explain why depleted peridotites overlie more fertile peridotites and how deep regions of the mantle are able to remain fertile over geologic time.
An analogue model of melt segregation and accumulation processes in the Earth's crust
2007
An analogue experiment was carried out to model melt segregation from the solid rock matrix and its subsequent transport. Carbon dioxide gas and sand were used as analogue materials of crustal partial melt and host rock, respectively. The analogue model displays the diffusional transport mode at low flux rates and the transition to the ballistical mode as the response of the system to a higher gas flux. The ballistical mode is characterized by discontinuous transport and extraction of the gas phase in separate batches, which leads to the development of power law batch size distribution in the system. The gas is extracted preferentially in large batches and does not influence the state of the system and size distribution of remaining batches. The implications of the analogue model to real magmatic processes are supported by power law leucosome width distributions measured in several migmatite localities. The emergence of fractality and 1/f power spectrum of system fluctuations provide evidence of possible self-organized critical nature of melt segregation processes.
Melt extraction and accumulation from partially molten rocks
Lithos, 2004
Current models for melt segregation and ascent are not adequate to accurately describe transport and accumulation in combination. We propose that transport is discontinuous and in batches, and that accumulation occurs by stepwise merging of batches. A simple numerical model of jostling spheres that merge when they touch was used to represent stepwise accumulation and transport of batches by propagation of hydrofractures. Results of the numerical model indicate that such a system may quickly develop into a self-organised critical (SOC) state. In this state, the distribution of melt batch volumes can be described by a power law, with an exponent m that lies between 2/3 and 1. Once a self-organised critical state is established, the system is capable of discharging any additional melt without further change to itself. Deformation aids melt extraction efficiency, as it increases the mobility of hydrofractures, enhances accumulation and hence lowers the exponent m. Full connectivity of melt needs never to be reached in the system and melt transport and extraction can occur at very low melt fractions. The chemical evolution of melt from source to emplacement level will be governed by the discontinuous mixing and mingling of batches, each with different histories, and possibly different sources. If no subsequent homogenisation occurs in a magma chamber or the final emplacement structure, the process can be identified by chemical heterogeneity of plutons and volcanic rocks.