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Rheological Transitions During Partial Melting and Crystallization With JOURNAL OF PETROLOGY VOLUME 37 NUMBER 6 PAGES 1579-1600 1996 JEAN LOUIS VIGNERESSE1*, PIERRE BARBEY2 AND MICHEL CUNEY5 "CREGU, BP 2J, 54501 VANDOEUVRE, FRANCE; ALSO ECOLE NAT. SUP. GEOLOGIE, NANCY, FRANCE *LABO. PETROLOGIE, UNIVERSITE NANCY I, BP 239, 54506 VANDOEUVRE, FRANCE "CREGU (ALSO GDR 77), CNRS-CREGU, BP 23, 54501 VANDOEUVRB, FRANCE Rheological Transitions During Partial Melting and Crystallization with Downloaded from https://academic.oup.com/petrology/article/37/6/1579/1406720 by guest on 28 September 2021 Application to Felsic Magma Segregation and Transfer We consider the rheological behaviour of felsic magma in the corresponds to the rigid percolation threshold (RPT). Above the zone of partial melting and during subsequent crystallization. RPT, clusters of particles can sustain stress, and the liquid We also introduce and combine concepts (mushy zone, percola- fraction can still flow. The only remaining possibilities for tion theory, granular flow, shear localization) derived from the rearranging particles are local shear zones, often within the non-geological literature and apply them to field observations on intrusion rim, which, as a consequence, develops dilatancy. This migmatiUs and granites. Segregation and transportation of stage of segregation during crystallization is totally different felsic magmas is commonly observed in association with non- from that of magma segregation during incipient melting. coaxial deformation, suggesting that gravity forces have limited (4) Finally, the system becomes totally locked when random influence during magma segregation. Solid to liquid and liquid close packing is reached, at ~ 72-75% solidification; this is the to solid transitions are shown to be rheologically different, which particle locking threshold (PLT). infirms the concept of a unique rheological critical melt The introduction of four thresholds must be viewed in the percentage for both transitions. Four stages are examined, which context of a twofold division of the cycle that generates igneous depend on the melt fraction present. rocks, first involving a transition from solid to liquid (i.e. (1) A minimum of 8% melt by volume must first be produced partial melting) and then a transition from liquid to solid (i.e. to overcome the liquid percolation threshold (LPT) above crystallization). Neither transition is simply the reverse of the which melt pockets can connect, thus allowing local magma other. In the case of melting, pockets of melt have to be connected displacement. Transport of the liquid phase is amplified by to afford a path to escaping magma. This is a bond-percolation, deformation toward dilatant sinks and is restricted to a very in the sense of physical percolation theory. In the case of crys- local scale. This corresponds to partially molten domains tallization, randomly distributed solid particles mechanically illustrated by incipient migmatites. interact, and contacts between them can propagate forces. (2) When more melt (20-25%) is present, a melt escape Building a crystal framework is a site-percolation, for which threshold (MET) allows segregation and transport of the melt the threshold is higher than that of bond-percolation. For each and part of the residual solid phase, over large distances. This transition two thresholds are applicable. The present approach, corresponds to segregation and transfer of magma towards the which basically differs from that based on a unique critical melt upper crust fraction, expands and clarifies the idea of a first and a second (3) Segregation of magma also occurs during granite percolation threshold One threshold in each transition (LPT emplacement and crystallization. In a flowing magma contain- and RPT, respectively) corresponds to a percolation threshold in ing few particles (^20%), particles rotate independently the sense of physical percolation theory. Its value is independent within the flow, defining a fabric. As soon as sufficient crystals of external forces, but relies on the type and abundance of are formed, they interact to construct a rigid skeleton. Such a minerals forming the matrix within which melt connectivity is random loose packed framework involves ~55% solids and developing. The exact value of the second threshold (METor •Corresponding authoT. Telephone 33 03 83 44 19 00. Euc 33 03 83 44 00 29. e-mail: [email protected] © Oxford Univenity Pros 1996 JOURNAL OF PETROLOGY VOLUME 37 NUMBER 6 DECEMBER 1996 PUT) will vary according to external forces, such as deforma- studies have tried to model two-phase materials, tion and the particle shape. using porous flow equations, slurry or debris flow models (McKenzie, 1984; Fowler, 1987; Bergantz, KEY WORDS: migmatites; partial melting; granites; magma segrega-1992; Worster, 1992; Major & Pierson, 1992; tion; magma solidification Coussot, 1994). Granular flow theory has been applied to sedimentation, flowing sands, or ava- lanches (Drake, 1990; Baxter & Behringer, 1991; Hutter & Koch, 1991). This represents a very large INTRODUCTION volume of literature, from which we have only cited The continental crust is a complex mixture of het- a few key references, but investigated many more. erogeneous materials, but despite the differences in Typically, the papers from the non-geological lit- Downloaded from https://academic.oup.com/petrology/article/37/6/1579/1406720 by guest on 28 September 2021 the mechanical and chemical properties of the erature are very specialized and relate to very spe- various rocks, it can be considered on the whole to cific materials; therefore many must be read to behave Theologically as a monophase. Under certain obtain an overview of the ideas expressed. The other temperature and pressure conditions, however, this major problem comes from poorly illustrated papers, heterogeneous material may become polyphase as a which makes the transferability of ideas, concepts consequence of partial melting. In this case, the and results to other fields difficult. rheological contrast between the molten and the We attempt to clarify and rationalize these ideas solid phases leads to segregation processes that to both the transition from solid to melt (i.e. partial operate at all scales. melting) and the transition from melt to solid (i.e. The present paper briefly describes the behaviour crystallization). Reference is made mostly to felsic of partially molten systems and examines the segre- magma behaviour, but the application of these con- gation of liquid from solid phases as a function of the cepts to mafic or ultramafic magmas is briefly dis- liquid percentage and the degree of deformation. We cussed. In subsequent discussions, we restrict the have attempted to compare field observations with word 'melt' to crystal-free silicate liquids, whereas theoretical or experimental studies (Van der Molen 'magma' refers to silicate melts which contain sus- & Paterson, 1979; Hibbard & Watters, 1985; pended crystals. We also systematically use the Wickham, 1987; Dell'Angelo & Tullis, 1988; Miller volume fraction of melt (P) or the volume fraction of et al., 1988; Hand & Dirks, 1992). Recent papers first formed crystals ((f>), and when explicitly needed, dealing with melt generation, segregation and crys- the degree of partial melting (F), which is a weight tallization include experimental studies, mostly on fraction. mafic and ultramafic rocks (Maaloe & Scheie, 1982; Marsh, 1989; Cooper, 1990; Waff & Faul, 1992; Hirth & Kohlstedt, 1995; Lejeune & Richet, 1995) but also on felsic rocks (Jurewicz & Watson, 1985; RHEOLOGICAL TRANSITIONS Johannes, 1988; Laporte, 1994). Numerical mod- IN MAGMAS elling has also resulted in important publications The generic cycle of felsic magmas starts from a (e.g. McKenzie, 1984; Ribe, 1987). Recently, there source rock undergoing partial melting and ends has been renewed interest in this topic, based upon with a fully crystallized magma. Migmatites are a experimental (Rushmer, 1995; Laporte & Watson, good example of partially molten source rocks, 1995; Rutter & Neumann, 1995) and field observa- whereas granitic intrusions display structures formed tions (Brown et al., 1995; John & Stunitz, 1997). when magma stopped moving and crystallized. Because the processes (wetting of grain bound- The presence of a liquid phase in a partly molten aries, segregation of a liquid phase, flow with sus- rock severely modifies the bulk rheological properties pended particles, shear localization) are not specific of the whole rock. For instance, bulk viscosity varies to magma flow, we have also considered theoretical by about six orders of magnitude from the solidus up studies of percolation and mechanical or chemical to the liquidus. Consequently, the whole material non-linear instabilities (Stauffer, 1985; Guyon et al., behaves as a single phase at the very onset of 1990; Chvoj, 1993; Grinfeld, 1993). Studies from melting, as two separate phases when melt segregates other fields which relate to similar phenomena from the matrix, as a liquid with solid suspensions encompass metallurgy, granular flows, non-New- when magma is transported and ponds in the upper tonian flows or liquid crystals and packing of par- crust, or as a very viscous body at the end of its ticles (Wildenmuth & Williams, 1985; Carter, 1988; crystallization history. To model such large vari- Campbell, 1990; Drake, 1990; Cazabat, 1991; Dlu- ations in the physical properties of the multiphase gogorski et al., 1994; Rogers et al., 1994). Previous material, power law functions have been introduced, 1580 J.
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