Specific structure of convection currents in the system layered intrusion–feeding conduit–parental magma chamber

Russian Geology and Geophysics 48 (2007) 1037–1045 www.elsevier.com/locate/rgg

Specific structure of convection currents in the system layered intrusion–feeding conduit–parental magma chamber L.Sh. Bazarov, V.I. Gordeeva *, E.I. Petrushin Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia Received 27 January 2007

Abstract The structure of convection currents was experimentally studied in the model system layered intrusion–feeding conduit–parental magma chamber. Persistent hydrodynamical and thermophysical interaction between interrelated melts of the parental magma and intrusive body occurs through the feeding conduit. Being associated, they control the structure of convection currents and mechanisms of heat and mass transfer in the intrusive, conduit, and magma chamber. The existence of two convection countercurrents in the conduit has experimentally been established: inner central lifting jet and outer annular downward current along the conduit walls. At the top of the conduit, the downward current has the lowest temperature and appears to be quite in equilibrium with the earlier precipitated crystals. Moving downward along the conduit wall, the annular descending current interacts with the lifting jet and, as a result, becomes hotter and undersaturated relative to the crystals that formed before. Thus, there is no possibility for heterogeneous crystallization to occur on the walls of conduit. The experimentally simulated mechanism of melt interaction in a whole natural system rules out the possibility of formation of a zone of immobile melt with stable steady-state temperature stratification anywhere in the chamber’s volume. © 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Experiment; intrusion; melt; modeling; convection

Introduction The mechanism of formation of layered basic intrusions is one of the most important problems in petrology which remain to be solved. Interpretation of this mechanism is not only of theoretical importance but also can be applied in practice to finding mineral resources genetically related to layered intrusions (platinum, palladium, chrome, vanadium etc.). The late 20th and early 21st centuries are the period when magma formation and differentiation were thoroughly studied both theoretically and experimentally, and the interest to this problem continues to grow. Numerous papers devoted to theoretical and, to a lesser degree, experimental problems of intrusion formation have been published. Zavaritsky and Sobolev (1961), Raguin (1970), Green and Ringwood (1968), Sobolev (1981) and many other Russian and foreign researchers laid the foundation of classical thermodynamical and physicochemical theories of magma evolution. However, the present-day paradigm of magma evolution does not give a clue to understanding of the

* Corresponding author. E-mail address: [email protected] (V.I. Gordeeva)

cause and mechanism of rhythmical layering of igneous complexes (Yaroshevsky, 2007). On the basis of theoretical and experimental modeling, regularities of crystallization of magma were established to be a result of concentrated heat convection (Huppert and Sparks, 1984; Lowell, 1985). Different aspects of crystallization differentiation in layered intrusions are documented in many publications (Ariskin and Barmina, 2000; Ariskin and Frenkel, 1982; Bartlett, 1969; Brandeis and Marsh, 1989; Brandeis and Jaupart, 1986; Campbell, 1978; Elder, 1970; Frenkel et al., 1988; Frenkel, 1995; Jaupart and Tait, 1995; Kushiro, 1979; MacBirney and Noyes, 1979; Mangan and March, 1992; Naslund, 1977; Simakin and Kislov, 1991; Trubitsin and Kharybin, 1997; Wager and Deer, 1939; Wager, 1963; Wager and Brown, 1977). Studying the mechanism of formation of layered intrusion, E.V. Sharkov revealed specific features of cumulate formation during magma crystallization. According to him (Sharkov, 1985, p. 99), “zone of crystallization and pyrocrystalline rock itself are, chiefly, liquidus phases of the initial melt volume (cumulative minerals) but residual melt, filling the space between them (intercumulative material), comprises less than 20–30% of the whole volume. Therefore, the residual liquid

1068-7971/$ - see front matter D 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. 3 doi:10.1016/j.rgg.200 7. 11.0038

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formed in the zone of crystallization was largely squeezed into the main volume of the melt which was constantly mixed to a homogeneous state by convection.” “The section of layered intrusions is a vertically extending sequence of the products of fractional crystallization of the initial magma, varying upward from low- to high-temperature fractions.” (Sharkov, 1985, p. 100). “Layered intrusions were intracrustal intermediate magma chambers, where ascending magmas were accumulated; they were subjected to crystallization differentiation and digestion by new magmas” (Sharkov, 2006, p. 333). He showed that their formation obeyed the laws of solidification theory and was accompanied by intrusion of additional portions of a new magma into solidified intrusive chambers. Data on experimental simulation of dynamics of mass magma crystallization are rather scarce (Bazarov et al., 2001, 2002, 2005; Brandeis and March, 1989; Elder, 1970; Huppert and Turner, 1981; Kaneko and Koyaguchi, 2000). The scientific theories and approaches consider, chiefly, the processes which can proceed only in the chamber volume of crystallizing intrusion but ignore the influence of parental magma chamber. Real natural systems, especially large basic intrusions, consist of the intrusion itself, the parental magma chamber and a feeding conduit enabling the magma to intrude many times into the crystallization chamber of intrusion. There are no publications discussing the convective processes and mechanisms of layering in the natural system intrusion–feeding conduit–parental magma chamber. The aim of our investigations is to study experimentally the influence of parental magma chambers through feeding conduits on the convection proceeding in the volume of magma chambers of intrusions (laccoliths, lopoliths).

Methods Experimental modeling was performed on the equipment designed by the authors for model crystallization. Examples of experimental chambers with proper changes in inner volumes mentioned by Dobretsov and Kirdyashkin (1994), Dobretsov et al. (2001) were a prototype of model chambers. The equipment designed by the authors is a system of interconnected experimental units: crystallization chamber, thermostats, optical system of observation (×112), system of thermostatically controlled heat exchangers in the roof and floor of the chamber, slotted and ordinary light sources, temperature recording system (quick-response thermocouples, digital microvoltmeters, nanovoltmeters). The setup modeling the system intrusion–feeding conduit–parental magma source is shown in Fig. 1. The chamber is a sealed reservoir symmetrical in plan, a rectangular parallelepiped with transparent vertical walls (Fig. 1, position 2). The system NH4Cl– glycerin was used as a model melt. The temperature of melt saturation is 60 °C, the inner sizes of the setup are 240×240×220. Upper (position 1) and lower (position 16) heat exchangers are made of titanium sheets. The flat polished plate of polycrystalline NH4Cl (position 3) modeling the roof of intrusion is safely attached to the polished lower plane of the

roof heat exchanger. The polycrystalline roof plate is 40 mm thick. The floor of intrusive is made of a 80-mm thick flat plate of polycrystalline NH4Cl. A hole 40 mm across drilled in the central part of the floor models a feeding conduit of the intrusion. The conduit is 80 mm high; the area of cross section is 12.6 cm2. The area of the floor is equal to 576 cm2. The area ratio of the floor and conduit is 45.9. The floor rests on a height-calibrated stand made of 2 mm thick sheet of organic glass, which is placed circumferentially near the walls of chamber (position 13). The distance between the roof and floor of intrusion is 40 mm; the thickness of melt layer in the “parental magma chamber” is 60 mm. During experiments, the temperature changes in the roof, floor, main volume of intrusion, feeding conduit and parental magma chamber were recorded by a system of traditional and differential quick-response nichrome-constantan thermocouples. There are 24 of them; their positions are not marked in Fig. 1. The chamber is equipped with two search thermocouples enabling one to measure temperature in nearly every part of the model magma. Required temperatures of heat exchangers (in the roof and in the parental magma chamber) are provided by separate thermostats (U-7, U-10). The temperature measurement inaccuracy is ±0.1 °C. Pipe connections for heat carrier (water) are built into the heat exchangers at the inlet and outlet (position 14). In our experiments the model magmas were prepared according to the same scheme of preliminary operations. In all experiments concentration of NH4Cl was 13.2 g per 100 g glycerin, which corresponds to a saturation temperature of 60 °C. Temperature minimum of “overheating” of the model magma providing a homogeneous state of the system after filling the crystallization chamber was discovered empirically. Filling the crystallization chamber with magma took 8–10 s in all experiments. The use of our crystallization chambers to model the first stages of intrusion formation was supported by the results obtained by Dobretsov and his colleagues (Dobretsov and Kirdyashkin, 1994; Dobretsov et al., 2001). They thoroughly tested the criteria of geometrical and thermal physical resemblance and potential usage of model chambers with vertical transparent walls having a flat layer of viscous liquid (glycerin, eicosane, paraffin) during modeling of natural processes of convection and heat and mass transfer inside the crust and upper mantle. The basis of physical modeling of natural processes, the concept of resemblance, coefficient of resemblance, criteria of resemblance for different conditions of heat exchange and the conditions in which similar movements of magmas occur in geometrically similar systems were considered in detail. The influence of the Prandtl number (Pr) on the structure of currents and heat exchange during the thermal gravity convection was thoroughly examined. The main boundaries of stable convective currents are shown to be independent of the value of the Prandtl number even if Pr > 5 but depend on the value of the Rayleigh number (Ra). Pr = ν/α.

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Fig. 1. Cross-section through the central part of the model chamber. The structure of convective currents during homogeneous magma injection. 1 — heat exchanger of the intrusion roof; 2 — transparent walls of the chamber; 3 — intrusion roof, polycrystalline NH4Cl; 4 — roll convective currents; 5 — central ascending magma current; 6 — reverse horizontal currents from periphery of chamber to feeding conduit; 7 — annular descending magma current; 8 — central axial ascending magma current; 9 — descending annular magma current moving along walls of feeding conduit; 10 — polycrystalline floor (NH4Cl); 11 — reverse magma current in the magma chamber; 12 — ascending magma current in the parental magma chamber; 13 — stand; 14 — pipe connections for heat carrier (water) at inlet and outlet; 15 — descending magma current at periphery of chamber; 16 — heat exchanger of parental magma chamber; 17 — descending bell-shaped magma current moving from feeding conduit; 18 — peripheral magma current in magma chamber.

Here ν — kinematic viscosity (m2/s), which is equal to ratio η/ρ, η — dynamic viscosity (ns/m2), ρ — liquid density (kg/m3), α — thermal conductivity (m2/s). Ra = β⋅g⋅∆T⋅L3/ν⋅α, where β — coefficient of expansion (deg–1); g — gravitational acceleration; ∆T — differential temperature (°C); L — typical size (m). The criteria of geometrical resemblance in experiments are as follows: an average thickness of natural layered intrusion is 4000 m, the model one is 40 mm (ratio 4000:0.04 = 1⋅105); the thickness of intrusion roof is 4000 m, the model one is 40 mm (ratio is equal to 1⋅105). The thickness of intrusion crystalline floor is 8000 m, the model one is 80 mm (ratio is equal to 1⋅105). The distance from a feeding magma

chamber to the earth’s surface (by convention) is 16,000 m, the ratio is 1⋅105. Horizontal sizes of natural symmetrical intrusion are 24,000 m, the model ones are 0.24 mm (ratio is equal to 1⋅105). Given the feeding conduit of the natural intrusion was 400 m across, the model one was 40 mm. The available publications contain no definite evidence about the diameters of feeding conduits of large basic intrusions. We found only one mention (Irvine, 1970) about the sizes of a vertical 150–500 m thick dike relating to the Muskoka basic intrusion and named as “feeding dike”. Initial temperature of slightly “overheated” natural basaltic magma of intrusion, which is equal to 1300 °C with a viscosity of 300 P (Poise), was taken from the works (Persikov, 1984; Sobolev, 1981). The model magma temperature (13.2 g/100 g

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Table 1 Major thermophysical properties of glycerine in the range of temperatures (after Vargaftik (1972)) T, °C

Viscosity

Thermal conductivity (λ), W/(m⋅deg)

Thermal capacity (C0), kJ/(kg⋅deg)

Density (ρ), kg/m3

Thermal diffusivity (α⋅107), m2/s

Prandtl number Pr = ν/α

Volume expansion (β⋅103), deg–1

dynamic (η⋅103), ns/m2

kinematic (ν⋅104), m2/s

20

1480

11.7

0.2785

2.35

1260

0.940

12.447

0.505

25

1040

8.27

0.2791

2.37

1258

0.936

8835

—

30

600

4.78

0.2799

2.40

1255

0.929

5145

—

35

465

3.71

0.2806

2.43

1252

0.922

4024

—

40

330

2.64

0.2813

2.45

1250

0.918

2876

—

45

255

2.05

0.2820

2.48

1246

0.913

2245

—

50

180

1.45

0.2826

2.51

1244

0.905

1602

—

55

141

1.14

0.2834

2.53

1241

0.902

1264

—

60

102

0.82

0.2840

2.56

1238

0.896

918

—

65

80

0.65

0.2848

2.59

1234

0.891

727

—

70

59

0.48

0.2855

2.61

1231

0.888

539

—

glycerin) is 65 °C (overheat is 5 °C). In all experiments the Prandtl number is more than 100: from Pr = 727 for 65 °C

Convection regime and heat and mass transfer

temperature to Pr = 1.25⋅104 for 20 °C. The main thermophysical characteristics of glycerin in the range of working temperatures are given in Table 1 (Vargaftik, 1972). The chosen system, with glycerin as a model liquid for the experiments, provides the criteria of thermophysical resemblance with a natural magma. Evaluation of resemblance has been discussed by some authors (Dobretsov and Kirdyashkin, 1994; Dobretsov et al., 2001) studying convective processes in the upper mantle.

First run. In the first run, after “intrusion” of the model magma, two zones differing in convective current structure were formed in the volume of the intrusion within 10–20 minutes from the beginning of the test (the period of convective current acceleration): a) a vertical central zone of magma uplift, extending from the parental magma chamber within the feeding conduit (see Fig. 1, positions 5 and 8) to the roof of the intrusion, located directly above the feeding conduit in the volume of intrusion magma, b) a horizontal zone of convective currents of magma, oriented radially from the center to periphery of the intrusion (position 4), and a reverse zone, covering the whole surface of the floor from the periphery of the intrusion to the feeding conduit (position 6). In the lower part of the equipment, in the volume of the parental magma chamber, there are two structurally different convective zones under the floor of the intrusion: the central zone of ascending jet turbulent current (position 12) and the periphery zone of radial convective current, moving from periphery to the central part of the parental magma chamber (position 11). Within the feeding conduit there are two reverse convective currents of the magma: the central ascending jet turbulent current, going from the parental magma chamber through the feeding conduit and reaching the roof of the intrusion (position 8), and periphery descending annular current (position 9). The currents moving horizontally along the upper surface of the floor (positions 6 and 7) from periphery of intrusion chamber to the center (feeding conduit) form a descending annular current within the feeding conduit. The temperature of the main, central and descending currents of the magma near the roof of the intrusion is 52.2 °C 30 minutes after the beginning of the experiment; 4 mm above the heat exchanger surface of the parental magma chamber, the temperature is 60 °C. However, at the level of the upper surface of the floor, the temperature of the descending annular

Results Two runs of experiments were carried out (40 tests) with feeding conduits of different diameters at the base of the model intrusive. In the first run the diameter of the feeding conduit was 40 mm, the initial temperature on the boundaries between the roof and model magma was 20 °C. The temperature of heat carrier (water) in the roof heat exchanger (position 1) was 20 °C. The initial temperature was: 65 °C in the lower heat exchanger of the magma chamber, 60 °C in the lower surface of the floor, and 25 °C in the upper surface of the floor. The floor warmed up by the preliminary filling of the lower volume of the chamber with the 65 °C model magma up to the lower surface of the floor +5 mm. The floor heating took ≈2 h. After the initial temperature on the upper surface of the floor had been raised to 25 °C, the magma chamber was quickly (for 0.5–1 min) emptied and the whole volume of the chamber was filled with the 65 °C model magma. The second run was carried out under similar conditions with the only exception that the diameter of the feeding conduit on the floor was increased to 60 mm.

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current near the inlet into the feeding conduit (position 7) is 46.2 °C but near the outlet of the feeding conduit it is 54.7 °C. The increase in temperature of the descending annular current when flowing through the feeding conduit is due to its thermal interaction with the central ascending jet current moving in the central part of the conduit. In the central ascending jet current, the Rayleigh number for ∆T = 7.5 °C and L = 110 mm is 2.64⋅106; the Prandtl number exceeds 1000. The obtained Rayleigh and Prandtl numbers imply that the regime of convective currents in the central ascending flow from the upper level of the magma chamber to the roof of the intrusion is a jet turbulent current. In the central ascending flow, convective regime of heat and mass transfer remains unchanged during the run (more than 3 h). Radial cooling of the upper magma layers under the roof, directed from the central, vertical, cylindrical “hot” zone to the periphery of the chamber, forms a horizontal temperature gradient (∆Thor = 4.6 °C). Horizontal temperature gradient invokes occurrence of the horizontal radial convective currents directed from center to periphery (position 4). As it is known (Dobretsov et al., 2001), in the presence of horizontal temperature gradient, there is no threshold of stability affecting the liquid motion controlled by the Rayleigh number (Ra = 1.7⋅103). In the presence of horizontal temperature gradient, the fluxed melt moves toward lower temperatures at any low Rayleigh number, which is inferred from the basic laws of hydrodynamics. The Rayleigh numbers obtained for horizontal current in the intrusion magma are 1.22⋅106 with Pr >>102 and horizontal temperature gradient in the upper layer of the melt under the roof ∆Thor = 4.6 °C 30 minutes after the beginning of the experiment, making it possible to characterize the regimes of currents as unsteady three-dimensional (Dobretsov et al., 2001). As a result of difference in magma density, horizontal convective currents transform into sinking, descending and vertical ones near the vertical walls of the chamber. Then they move radially from periphery of the chamber to the central zone (feeding conduit) along the upper surface of the floor of intrusion chamber (position 6). The horizontal currents of melt, moving along the floor from periphery to the center of intrusion chamber, form an annular descending current directed downward along the walls of the feeding conduit. The melt in the descending annular current, interacting with the central ascending jet current within the feeding conduit, warms up (vertical ∆T in the melt in the descending annular current in the feeding conduit is 8.5 °C). Descending annular current of the feeding conduit, interacting with the magma in the lower part of the chamber, spreads radially forming a bell-shaped chamber of parental magma (position 17). As compared with the temperature of the periphery part of the parental magma chamber, a lower temperature of the descending annular current produces reverse temperature gradient relative to the upper volume of the intrusive chamber. Higher temperature of the magma on the periphery of the chamber causes the formation of a reverse

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temperature gradient (∆Trev = 5.3 °C) and convective currents from the periphery to the center of the parental magma chamber (positions 11 and 18). The mechanism of heat and mass transfer between descending annular current of the feeding conduit and the magma of the chamber is convective. When the temperature drops to 55 °C, homogeneous nucleation, i.e. formation of nuclei for NH4Cl crystallization, is initiated. The value of oversaturation triggering the nucleation process is ≈0.4 g NH4Cl per 100 g of glycerin. Under the same conditions, the intensity of nucleation (number of crystallization nuclei per unit volume of the homogeneous magma) depends only on the value of oversaturation. Sizes of crystallization nucleus do not depend on the value of obtained oversaturation (Bazarov et al., 1997; Bazarov et al., 1999). Further rapid growth of crystallization nuclei formed in the homogeneous magma under conditions of significant oversaturation causes the occurrence of the Tyndall effect (light diffraction on individual particles) in the model magma which is precisely registered by the optical system of observation. Further growth of suspended crystals proceeds in the homogeneous magma within the descending branches of convective currents and in zone of decreasing temperature in the area of horizontal reverse currents near the floor (position 6). The initial (inductive) oversaturation (0.4 g/100 g) drops very fast — within 10–20 minutes. Further, the rate of crystal growth decelerates (according to the obtained data up to 0.005–0.01 mm/min for oversaturation up to 0.12 NH4Cl per 100 g glycerin). Entrained by a viscous magma, suspended crystals of NH4Cl growing in a homogeneous medium occur frequently in the region of descending and ascending convective currents of different temperatures. After filling the homogeneous magma chamber, the process of crystal growth begins in the roof and floor in all experiments. As the temperature decreased, the crystals growing in a homogeneous medium within the main volume of the magma and reaching sizes of 5–10 µm begin to cumulate on the floor. Heat and mass transfer onto the newly formed layer of crystals is convective. The density of NH4Cl crystals is 1.526 g/cm3, and melt density at 25 ºC, 1.2 g/cm3. The NH4Cl crystals formed in a homogeneous medium in the main volume of the intrusive magma, which had no time to cumulate on the floor, are entrained by radial convective currents and are transported to the feeding conduit with descending annular current. In the feeding conduit some part of the crystals suspended in the descending annular current begin to dissolve owing to the heating of the central ascending cylindrical current of the magma (position 8). In the magma chamber, the crystals continue to dissolve in the zone of annular bell-shaped tail. The convective currents move from roof center to chamber periphery along the intrusion floor toward the center of the floor and then to the descending annular current in the feeding conduit, bell-like tail of the “chamber” and further in the central ascending current within the conduit up to the roof. A complete cycle takes 15–20 min. The time of one complete cycle was estimated by visualizing currents on microparticles

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of porous gum and aluminum, 0.1–0.2 mm in size, preliminarily introduced into the model magma. The feeding conduit was not obliterated by probable heterogeneous crystallization on its inner walls. The reason is that at the entry in the descending annular current within the feeding conduit the cooling melt (moving along the floor of the intrusion) is undersaturated with NH4Cl owing to the heating of the central ascending cylindrical current. Within the feeding conduit crystals of NH4Cl situated in the magma of the descending annular current are partly dissolved owing to an increase in magma temperature as a result of heat exchange between two reverse currents: descending annular and central ascending jet cylindrical one. The second run was carried out using a similar model of crystallization chamber intrusion–feeding conduit–parental magma chamber. Increased diameter of the feeding conduit to 60 mm was the only difference. The other parameters of processes (initial temperatures of the roof and floor, “intrusion” of homogeneous and heterogeneous magmas etc.) remain unchanged. In comparison with the first run there were no essential differences in the structure of convective processes, crystallization on the roof and floor of the intrusion, conceptual changes in the mechanisms of heat and mass transfer, processes of nucleation and growth of crystals in homogeneous magmas. Insignificant changes were due to the increase in diameter (area of cross section) of central ascending jet current from the feeding conduit and of descending annular current within the feeding conduit as well as to an increase in differential temperature (∆T) in the central part of the roof and on the periphery near the roof. The Rayleigh number in horizontal convective currents of the intrusion also augments with an increase in temperature gradient. The main features of convective processes, mechanisms of homogeneous nucleation and crystal growth in the magma, on the roof and floor, occurrence of two different convective zones within the magma (central vertical descending cylindrical under the feeding conduit and peripheral horizontal zones), similarity of convective currents near the roof and floor, mechanisms of heat and mass transfer in different zones of crystallization including the magma in the parental chamber remain stable. The full run takes more time. Induced periodical vibrations with a frequency of 1–5 Hz that model seismic activity in the intrusive zone lead to some contraction of cumulus and filling floor hollows with crystals (Likhachev, 2000).

Discussion Analyzed literature evidences that layering of the main volume of rocks is a characteristic feature of large intrusions formed due to initial basaltic magma. Wager and Braun (1967) believe that large layered intrusions, being closed systems, occur as a result of intrusion of a great amount of magma (hundreds of cubic kilometers).

The available data on the velocity of convective currents within a magma chamber are very scarce. According to Wager and Braun (1967), rapid convective currents move in the chamber of the Skaergaard intrusion with a velocity of 3 km/day. In the exposed part of the intrusion, suspended crystals in a 2.3-km thick layered sequence may have gone through the full cycle of moving in the chamber for 3 days. Wager and Braun (1967) calculated that if the rate of cumulus accumulation was about 20 cm/year, a 3-mm thick layer would accumulate. Unfortunately, Wager and Braun (1967) considered convection in the volume of intrusion only as convective cells with horizontal sizes lesser than vertical. According to Hess (1960), the velocities of heat loss imply that the Stillwater intrusion crystallization (precisely, its studied exposed part) proceeded for 50,000 years, with the drop of temperature being 125 °C (from 1225 to 1100 °C). The rate of cumulus accumulation was about 10 cm/year. In recent decades the mechanisms of repeated injection and their mixing with the parental magma have been used to explain layering and ore content of igneous complexes (Sharkov, 1980, 2006; Sharkov and Bogatikov, 1985). Our experimental studies of the mechanisms of layered intrusion formation using the model intrusion-feeding conduitparental magmatic chamber revealed specific features of convective processes in the volume of intrusion as well as in the volume of feeding conduit and parental magma chamber. The processes of heterogeneous nucleation and crystal growth on the roof of the bodies of magma intrusions (Bazarov et al., 2001, 2002) and on the roof of model laccoliths and lopoliths are close, as inferred from heat and mass transfer mechanisms. The only differences are various structures of convective currents (vertical cellular currents in flat bodies and horizontal, in laccoliths and lopoliths). Significant differences are observed in mechanisms of nucleation and further crystal growth in the main volume of magma. In the flat bodies nucleation occurs only in the upper horizontal layer of the magma, and in the models of laccoliths and lopoliths, in the volume of the magma. At the earliest stages of crystallization in the system intrusion–feeding conduit–parental magma chamber the following heat and mass transference mechanisms were established experimentally.

Intrusion of homogeneous magmas On the roof of the intrusion (just after the chamber volume has been filled) heterogeneous nucleation and directed crystal growth on the polycrystalline base proceed by the convective mechanism of heat and mass transfer to the newly formed layer. The growth of NH4Cl crystals on the roof continues throughout the experiment. In the intrusion magma the mechanism of homogeneous nucleation and further crystal growth is triggered by the convective heat and mass transfer. Released in the process of nucleation and further homogeneous crystal growth, the heat dissipates in the magma volume.

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On the floor of intrusion crystals grow on the polycrystalline base, followed by settling under the action of gravity. Heat and mass transfer between the magma and young layer is provided by horizontal “reverse” flat currents. In the feeding conduit, after the volume of crystallization chamber has been filled with a homogeneous model magma, two counter currents form: inner, central ascending jet current achieving the roof of the intrusion and outer descending annular current achieving the heat exchanger of the parental magma chamber. In the feeding conduit another two currents are preserved throughout the run (for 3–4 hours): central ascending cylindrical jet turbulent current and outer descending annular current moving along the walls of the feeding conduit. Heterogeneous crystal growth on the walls of the feeding conduit proceeds only within 10 minutes after filling the chamber with the model magma (sticking). Further crystal growth on the walls of the feeding conduit almost deceases in all tests up to the end of the experiment. In the melt of “parental magma chamber”, immediately after the volume of crystallization chamber has been filled, two zones of different convective currents appear. In the central part of the parental magma chamber two counter currents form: descending bell-shaped annular current widening downward in the chamber moving from the feeding conduit and central ascending jet current achieving the roof of the intrusion (positions 5 and 12). Reverse centerward radial convective currents interact with the outer part of descending annular bell-like current from the feeding conduit of the floor, which form in the peripheral part of the parental magma chamber (position 11).

Mechanism of cumulus formation on the floor of the intrusion Injection of initial homogeneous magma, when the temperature in the magma volume decreases to 55 °C and the magma become oversaturated (≈0.4 NH4Cl per 100 g glycerin), triggers nucleation and promotes further NH4Cl crystal growth. The growth of suspended crystals in the magma proceeds almost in the whole volume of the intrusion. As crystals growing in the intrusion magma reach ≈5−10 µm in size, they begin to settle down onto the surface of the intrusion floor by gravity and by the transport effect of convective currents. The descending annular current in the feeding conduit containing crystals formed in the main volume of the homogeneous intrusion warms up owing to its interaction with the inner central ascending current of the conduit, which leads to undersaturation of the magma and dissolving of crystals. Thus, an undersaturated melt, which lost some of its material cumulated on the intrusive’s floor, is transported by the descending annular current from the feeding conduit into the volume of the parental magma chamber. Further convective interaction of the descending annular bell-like current leaking out from the feeding conduit of the intrusion with the parental magma of the chamber leads to

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renewal, replenishing with initial constituents, and return of the current as a central vertical ascending current of the feeding conduit into the volume of the intrusion. The full cycle of the differential magma volume in the interconnected convective current in the system intrusion– feeding conduit–parental magma chamber takes about 18– 20 minutes. The studied mechanism of thermal interaction of the whole system implies the exclusive influence of the feeding conduit and magma chamber on the duration of intrusion crystallization. From the most conservative estimate, this time increases, at least, by a factor of 3–5 in comparison with the time of crystallization of magmatic bodies determined by many authors who ignore the effect of feeding conduits and magma chambers. A difference in concentration of the constituent to be crystallized in the magma between the descending and ascending branches of convective currents, especially, in the zone of feeding conduits, intrusion magma and parental magma chamber, with crystals periodically present in the zones of saturated and undersaturated magmas leads to periodic dissolving and growth of the outer zone of crystals, drastic change in growth rate on moving in the descending and ascending branches of convective currents, zoning in crystals, capture of inclusions of mineral-bearing environment, habit change, and so on. For convective current forming in the model melt of intrusion it is very important to have a feeding conduit providing a horizontal temperature gradient and, consequently, formation of horizontal convective currents within the intrusion. Periodic renewal of magma in the chamber of intrusion owing to interaction of parental magma of the chamber and injection of new magma portions from the volume of the parental chamber through nonobliterated feeding conduit into the intrusion chamber leads to rhythmic generation of new layers of cumulus on the floor. Our calculations show that formation of one cumulus layer corresponds to one full cycle of convective currents in the system intrusion–feeding conduit–parental magma chamber. We have carried out two tentative experiments using two feeding conduits, 25 mm in diameter, in the floor of intrusive body, with injection of initial homogeneous magmas. Irrespective of the number of feeding conduits, the formation mechanism of horizontal cumulus layers on the floor is unchangeable. In the volume of the intrusion there are two systems of horizontal currents, two central ascending and descending annular currents within the feeding conduits. In a single parental magma chamber there are two descending bell-like currents, two central ascending and two systems of reverse convective currents. No obliteration of the feeding conduits is observed.

Conclusions Our experimental study of the structure of convective currents in the system intrusion-feeding conduit-parental magma chamber allows the following conclusions:

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1. Layering of large basic intrusions is a result of hydrodynamic and thermophysical interaction of magmas (either homogeneous or heterogeneous) of the entire natural system intrusion-feeding conduit-parental magma chamber. The magma interaction in the intrusion chamber, feeding conduit and parental magma chamber controls the structure of convective currents and mechanisms of heat and mass transfer in the intrusion chamber, feeding conduit and parental magma chamber, the features of crystallization processes on the roof and floor of the intrusion, complete or partial dissolving of crystals in the feeding conduit and parental magma chamber and, consequently, the mechanism features of formation of rhythmic layering (cumulus) on the bottom. 2. Horizontal convective currents, moving under the top of the intrusion from center to periphery, and reverse horizontal currents, moving from periphery of the chamber along the bottom surface to the feeding conduit and further going as a descending annular current through the feeding conduit into the volume of parental magma chamber, entrain the whole magma volume of the system intrusion–feeding conduit–parental magma chamber into convection and, accordingly, heat and mass transfer. Experimentally established mechanism of melt interaction in the entire system excludes the possibility of formation of a stagnant magma zone with stable temperature stratification throughout the intrusion chamber. 3. Descending annular current in the lower part of the feeding conduit has the lowest temperature in the magma volume and is virtually in equilibrium with crystals in the magma. On heating, during the motion in the feeding conduit by interaction (heat exchange) with the central ascending cylindrical “hot” current, the magma of the descending annular current becomes undersaturated relative to new crystals. As a result, they partly or completely dissolve at the lower outlet of the feeding conduit into the parental magma chamber. In the absence of agencies causing a decrease in the cross section area of the feeding conduit the intrusion emplacement takes much more time as calculated without regard to influence of feeding conduits and interaction with the parental chamber. We would like to thank Professor A.G. Kirdyashkin for his advice and consultations during experiment preparations. We also acknowledge the useful recommendations of Professor E.V. Sharkov and Professor A.A. Ariskin. The work was supported by grant 04-05-64358 from the Russian Foundation for Basic Research.

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