Partial melting below the magmatic arc in the central Andes deduced from geoelectromagnetic field experiments and laboratory data

PHYSICS O F T H E EARTH ANDPLANETARY INTERIORS

ELSEVIER

Physics of the Earth and PlanetaryInteriors 103 (1997) 17-31

Partial melting below the magmatic arc in the central Andes deduced from geoelectromagnetic field experiments and laboratory data Frank R. Schilling a,*, Georg M. Partzsch a, Heinrich Brasse b, Gerhard Schwarz b,1 a Freie Universiti~t Berlin, Institutfiir Mineralogie, Takustr. 6, 14195 Berlin, Germany b Freie Universitiit Berlin, Fachrichtung Geophysik, Malteserstr. 74-100, 12249 Berlin, Germany

Received 11 October 1996;received in revised form 24 February 1997; accepted 14 March 1997

Abstract Magnetotelluric and geomagnetic deep soundings in northern Chile revealed a pronounced high conductivity zone (HCZ). Below the Western Cordillera, which constitutes the present magmatic arc with active volcanism of the South American continental margin, conductivities in the range of 1 S / m are observed. The anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km, are interpreted in terms of partial melting. Other geophysical observations, such as a zone of low seismic velocities (LVZ) at similar depths, high heat flow values ( > 100 m W / m 2) and a pronounced negative anomaly in the residual gravity field, are also considered. Impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities. At sub-solidus temperatures, electrical behavior is described by defect electrons with an activation energy of 1.34 eV and a conductivity of 2.5 m S / m at 900°C. Model calculations using a modified-brick-layer model (MBL) were compared with experimental observations. A good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 S / m at 1250°C assuming an activation energy of 1 eV. The same MBL model is used to calculate melt proportions beneath the Western Cordillera. Between 14 and 27 vol.% of interconnected melt are necessary to explain the observed HCZ. The stability of the melt rich crust is explained by a dynamic melting-crystallisation behavior during crustal anatexis and by magma filled dikes. © 1997 Elsevier Science B.V. Keywords: Northern Chile; High conductivityzone; Magmatic arc; Magnetotelluric;Geomagneticdeep sounding

1. Introduction The central Andes are located in the convergence zone between the oceanic Nazca plate and the South

* Corresponding author. mPresentaddress: Sveriges GeologiskaUndersiSkning,Box 670, 75128 Uppsala, Sweden.

American plate. In the study area (Fig. l), between 20 ° and 26°S, the topographic relief rises from approximately 8000 m below sea level in the P e r u Chile trench to more than 6000 m in the Western Cordillera (WC) on the border of Chile, Bolivia and Argentina. The WC constitutes the present magmatic arc with recent volcanism. Mainly andesitic lava and acidic ignimbrites are extruded. The back-arc to the

0031-9201/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. P11S0031-9201(97)00011-3

F.R. Set illi,~ et al. / t~ltyxic.s o / t h e Earth a , d Plam, larv lnterior.~ 103 (1997) /7 31

18 71°W

70 ° W

69 ° W

68" W

67 ° W

66 ° W

65 ° W

64 ° W

20 ° S

20 ° S

21 ° S

21 ° S

22 ° S '

22" S

23 ° S

23 ° S

24 ° S

24" S

25 ° S

25" S

71 ° W

70 ° W

69 ° W

68 ° W

67 ° W

66" W

65" W

64" W

Fig. l. Location of magnetotelluric sites in the magmatic arc/forearc regions in northern Chile. A and B indicate profiles of two-dimensionalmodels mentioned in the text. Abbreviationsfor morphostructural units according to Reutter et al. (1994): CC Coastal Cordillera, LV LongitudinalValley, PC Precordillera, PD Preandean Depression, WC Western Cordillera, AP Altiplano, PU Puna, EC Eastern Cordillera, SA SubandeanRanges, APVC Altiplano-PunaVolcanicComplex. AF, WF: schematicalrun of Atacama Fault and West Fissure.

east commences with the high plateaus of the Altip l a n o / P u n a , followed by the Eastern Cordillera, the Subandean Ranges and the Andean foreland of the Chaco. Since the Jurassic the magmatic arc has migrated from the area of the Coastal Cordillera to the present position in the Western Cordillera (cf.Coira et al., 1982; Scheuber, 1994). A similar crustal composition as expected below the WC is now exposed in the Coastal Cordillera. Mainly pyroxene gneisses, amphibolites and gabbros with local migmatization have formed the intermediate crustal level at about 15 km depth of the Jurassic arc (Lucassen and Franz, 1994). These rock types - - which may correspond

to the Preandean basement - - are expected beneath the Western Cordillera as well. Volcanic rock samples show no indications of anomalous composition of the crust. The widely distributed ignimbrites indicate the presence of acidic magmas and fluids. The central Andes are characterized by several geophysical anomalies. A crustal thickness of up to 70 km below the Altiplano and the Western Cordillera has been deduced from seismic and gravimetric measurements (refer, e.g., to James, 1971; Ocola and Meyer, 1972; Chinn et al., 1980; Wigger et al., 1994). Below the WC the Moho is not clearly resolved, whilst a distinct low velocity zone (LVZ) and a zone of high seismic attenuation are observed. The

F.R. Schilling et al. / Physics of the Earth and Planetary Interiors 103 (1997) 17-31

Bouguer anomaly reaches - 4 5 0 mGal in the magmatic arc and even the residual field amounts to approx. 40 mGal (Gtitze et al., 1994). Also, high heat flow values of > 100 m W / m 2 have been measured in the WC (Henry and Pollack, 1988; Giese, 1994; Hamza and Mufioz, 1996). An extensive zone of high electrical conductivity (HCZ) below the Western Cordillera has been detected by magnetotelluric and geomagnetic deep sounding experiments (Schwarz et al., 1994). Commencing at a depth of approximately 20 km, the thickness of this HCZ is not well resolved due to the inherent limitations of the electromagnetic method: a period length of more than 1 day would be necessary to penetrate through this conductor with modelled resistivities in the range of 0.5-2 rim. A minimum thickness of 40 km must be assumed, however. The actual structure of the central Andean crust is the result of different deformation processes and magmatic evolutions. The extreme thickening of the crust has been attributed to magmatic addition (James, 1971; Thorpe et al., 1981), crustal shortening (Reutter et al., 1988; Roeder, 1988; Sheffels, 1990) or thermal uplift in combination with shortening (Isacks, 1988). Crustal balancing carried out by Schmitz (1994) indicates that only a combination of different mechanisms may explain the development of the Andean crust. High conductivity zones in the middle and lower crust can be explained by saline fluids, graphite or partial melts. However, seismic data, heat flow and geological observations constrain the interpretation of the electromagnetic anomalies and it will be shown in this contribution that partial melts are most likely the cause of the observed conductivities below the magmatic arc. Laboratory measurements and model calculations have thus been performed in order to get more information about the behavior of partially molten rocks. The aim of this paper is providing a more detailed insight into the conductivity structure of the Andean magmatic arc.

2. Electrical resistivity models derived from magnetotelluric data

In the 1980s, several magnetotelluric (MT) and geomagnetic deep sounding (GDS) experiments were

19

performed in northern Chile, northwestern Argentina and southern Bolivia together with seismic and gravimetric investigations by the research group 'Mobility of Active Continental Margins' at the Free University of Berlin (GiStze et al., 1994; Giese, 1994; Schwarz et al., 1994; Wigger et al., 1994). These M T / G D S profiles have been recently complemented by measurements further north (Echternacht et al., 1997). Fig. 1 outlines the main morphostructural units of the Central Andes and shows the location of the M T / G D S sites. In the framework of this paper, we will concentrate mainly on the present magmatic arc (the Western Cordillera) and the fore-arc (Coastal Cordillera, Longitudinal Valley and Precordillera) in Northern Chile. The magnetotelluric method consists of the measurement and analysis of the horizontal components of natural electromagnetic field variations, yielding an impedance tensor as a complex transfer function of period. Apparent resistivities and phases are derived from this function. Two-dimensional conductivity modelling has been carried out to explain the data. Additional information is obtained by analyzing the vertical magnetic field component. Resultant transfer functions are conventionally displayed as induction arrows, indicating strike directions and the location of conductivity contrasts. Fig. 2 shows the results of 2-D modelling of the electromagnetic data along two profiles (A, B in Fig. 1) in northern Chile (cf. Massow, 1994; Krtiger, 1994; Schwarz and Kriiger, 1997). The most striking feature of models A and B (Fig. 2a, b) is a high conductivity zone (HCZ) with resistivities as low as 0.5-2 l ) m below the Western Cordillera, commencing at a depth of approximately 20 km. The depth extent of these HCZs is not clearly resolved due to the lack of appropriate signal strength at long periods (the overall conductance is higher than 20 kS!). A similarly large conductivity anomaly below the volcanic arc in southern Peru and central Bolivia, which lies much closer to the coast, had previously been detected by Schmucker et al. (1966) during early investigations in the adjacent areas to the north and described as the 'Andean anomaly'. These data were later reexamined by Tarits and Menvielle (1986), who carried out 3-D thin sheet model calculations and concluded that a zone of partial melting is present in the deeper crust.

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F.R. Schilling et al. / Physics q/'the Earth and Phmeta(~' Interiors 103 (1997) 17 31

West B

East 0

CoastalRanae

Western Cordillera

100 km

o 2o 40 6o 8o km

A

Preandean

W. Cordillera

Depression 0 o-,

50 .............

100 km L_.

20. 40-

~

60-

100 km Fig. 2. Two-dimensional models of the electrical conductivity distribution beneath the magmatic arc and forearc in northern Chile. See locations in Fig. 1. Resisitivity values in l~m.

Although the conductivity structure of the fore-arc will not be discussed in detail here, the relatively low average resistivity of the upper and lower crust of 200 r i m in both models (Fig. 2) should be mentioned. 'Normal' crusts exhibit values of more than

Trench

Forearc

1000 ~ m (Haak and Hutton, 1986). This may indicate a 'wet' crust in large parts of the fore-arc - possibly stemming from fluids released in the down-going slab and dehydration mineral reactions although this needs further investigation. -

-

Magmatic Arc

0 20 40 60 80 100

Fig. 3. Regions of high conductivity, low seismic velocity and high seismic absorption below the magmatic arc in northern Chile. The zone of absorption is not accessible by electromagnetic methods unless observatory, i.e. extremely long period, data would be analyzed.

F.R. Schilling et al. / Physics of the Earth and Planetary Interiors 103 (1997) 17-31

The region of enhanced conductivity beneath the Western Cordillera is sketched in Fig. 3 together with the observed zones of low seismic velocities and high seismic absorption. They overlap in a depth range of approximately 20-50 km and put an important constraint on the interpretation of the magmatic arc.

3. Possible causes of enhanced conductivity

Common candidates to explain high conductivity zones in the lower crust are (for an overview see Jones, 1992): • graphite, • ore minerals, • saline fluids, • partial melting. These candidates will be discussed in this section - with further regard to the other geophysical observations. There is increasing evidence that interconnected graphite films play a major role in explaining the observed high conductivity zones in the lower crust (cf. Duba and Shankland, 1982; Frost et al., 1989; Haak et al., 1991; JSdicke, 1992). Only small quantities of graphite - - which e.g. may precipitate along shear zones - - are necessary to reduce the overall resistivity of rocks significantly. However, graphite is only stable at low oxygen fugacities (Frost et al., 1989). All volcanic rocks show idiomorph magnetite (higher oxygen fugacity), whilst no wiJstite (lower oxygen fugacity) or graphite are observed. Additionally, the required interconnection (or at least reconnection) of the graphite film is unlikely to be stable under an active orogenic regime due to the continual magmatically induced tectonics. Graphite at grain boundaries does not reduce the seismic velocities to the values observed in the LVZ below the Western Cordillera, however, and other mechanisms must be assumed to explain the observed LVZ, e.g. a deeply fractured crust or a felsic crustal composition (Schmitz et al., 1997). Because of the ductile behavior of the lower crust, a deeply fractured crust can be excluded from being responsible for reducing the seismic velocities (no earthquakes, temperatures above 600°C, see Giese, 1994) of the lower crust. If, on the other hand, felsic crustal

21

compositions are assumed to explain the observed LVZ (e.g., Beck et al., 1996), their lower densities would overcompensate the observed gravity anomaly (Heinsohn, 1993). Thus it seems unreasonable that the HCZ of this magmatic arc can be explained by interconnected graphite since this would not lead to the observed seismic and gravity anomalies. Connected ore minerals like ilmenite, magnetite, pyrrhotite (Duba et al., 1994; Mareschal et al., 1992) or other accessory minerals lead to an increase of electrical conductivity. Ore minerals tend to accumulate. In comparison to a graphitic network, a much higher content of ore minerals is necessary to achieve an electrical conductivity of 0.5-1 S / m . It seems unreasonable to assume conducting ore bodies of several hundreds km 3 extent in the lower crust. The necessary fractional quantities cannot be obtained by any known crustal or mantle rock composition and fractionation mechanism. Additionally, the negative residual gravity anomaly of approximately - 4 0 mGal in the Western Cordillera could not be achieved when assuming an ore complex with its high density. This should result in a positive anomaly instead of the observed negative one. The assumption of fluids in the crust is the common interpretation of the observed conductivities of about 0.005 S / m in the Andean fore-arc (Schwarz et al., 1994). The HCZ beneath the active volcanic arc with a conductivity of 1 S / m extends to a depth of at least 60 km. If brines were the cause of this HCZs an interconnected network of veins of at least 5 vol.% would be required (Quist et al., 1970). The necessary high amount of interconnected fluids would not be stable in rocks due to their different densities, respectively. In a very short period of time, most of the fluids would rise to lower crustal levels or to the surface (Frost and Bucher, 1994). Even if a very low thermal gradient is assumed, solidus temperatures of fluid bearing crustal rocks would be reached and the fluid would be dissolved in partial melt. If excess water is assumed beneath the Western Cordillera, partial melting would already be expected at lower crustal levels (20-30 km) of the thickened crust (.-~ 70 km) if the observed heat flow values ( > 100 m W / m 2) are taken into account (Giese, 1994). Interconnected melt would increase electrical conductivity significantly. As discussed below, large

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F.R. Schilling et al. / Physics o[the Earth and Planetary lnterior~ 103 (19971 17-31

melt fractions are necessary to form an extensive HCZ like the one observed below the Western Cordillera. Partial melts would also reduce seismic velocity and - - to a lesser degree - - the density of the rocks (Schmitz et al., 1997). This is in accordance with the observed LVZ and a negative residual gravity anomaly. Neither graphite at grain boundaries nor ore minerals may explain the joint geoelectric, seismic and gravimetric observations. The density difference between fluid and rock, as well as the influence of fluids on the solidus temperature of rocks, exclude fluids as an explanation of the observed HCZ. The best interpretation of the observed petrophysical anomalies seems to be the existence of partial melts beneath the Western Cordillera.

4. Electrical properties of partial melts The electrical conductivity of rocks primarily depends on mineralogical composition, temperature, fluid content, and oxygen fugacity. Dry rocks can be described as semi conductors with increasing conductivity due to increasing temperature. The number and mobility of electrons and electron holes determine the electronic conduction (Macdonald, 1987). The generation (activation) of defects can often be described by an Arrhenius equation (electronic conductivity): ~ = O-oeeXp(- E a e ] k;r 1'

(1)

where o'e denotes electrical conductivity, O-o~ a constant, Ea~ the activation energy for electronic transport, k the Boltzmann constant and T the temperature in K. The oxidation state of different minerals influences the number and species of defects and hence the conductivity. The most pronounced effect on the conductivity below the melting point arises from the content of water. Only a small percentage of water is sufficient to increase the electrical conductivity of a cold and completely dry basalt by several orders of magnitude. Increasing temperature reduces the difference in electrical conductivity between dry and fluid-bearing rocks (Olhoeft, 1981). The conductivity of silicate melts is about two orders of magnitude higher than the conductivity of rocks near their solidus temperature (Presnall et al., 1972).

The main ionic conductivity of melts can be attributed to alkaline ions and their interactions and is described by another Arrhenius equation with similar activation energies as for rocks (0.8-1.2 eV. see e.g., Tyburczy and Waft, 1983). The Arrhenius equation for ionic conductivity cr~ with ~,~ as a constant and E~ as the activation energy for ionic transport is expressed by: o-~ = ~-[exp( - E " ' " -k-~)

(2)

The electrical conductivity of silicate melts varies only in the range of half an order of magnitude for geologically reasonable compositions (Waft and Weill, 1975; Murase and McBirney, 1973; Rai and Manghnani, 1978a). A change in oxygen fugacity of several orders of magnitude changes the conductivity of a silicate melt by less then 30% (Waft and Weill, 1975). The pressure dependence of the conductivity of silicate melts is probably small (Tyburczy and Waft, 1983; Satherley and Smedley, 1985) and may be neglected. The electrical conductivity of partial melt is influenced mainly by the presence or absence of an interconnected network of melt. The connectivity of melt mainly depends on solid-solid and solid-liquid interfacial energies. At laboratory scales equilibrium melt distributions at hydrostatic stress conditions are assumed (Beer~, 1975a,b; Bulau et al., 1979; Bargen van and Waft, 1986). Most of the observed melt distributions can be described by the wetting angle (dihedral angle): 0 2cos 2

"y~ ~1

(3)

where "y,~ is the solid-solid interfacial energy or grain boundary energy, and T,~ is the solid-liquid interfacial energy or surface energy. At low melt fractions an interconnected network of melt will be formed at grain edges if 0 ° < ~ < 60 ° (Fig. 4b). Wetting angles same as in Eq. (3) in of water bearing silicate melt in contact to quartz or feldspar are less then 60 ° (Laporte, 1994). Hence most of the crustal rock compositions should lead to an interconnected melt network at grain edges. With increasing melt portions, melt is observed at grain boundaries (Fig. 4a, cf. Mehnert et al., 1973; BiJsch et al., 1974). In

F.R. Schilling et aL / Physics of the Earth and Planetary Interiors 103 (1997) 17-31

23

a~

b~

solid grain

melt

Fig. 4. Model assumptions and simplified structural equivalents for simple meltingmodels with isolated melt pockets/magmachambers (a), connected melt at grain edges/magma filled dikes (b), and melt connected at grain boundaries (c).

many investigations deviations from the ideal behavior are observed (Waft and Faul, 1992; Laporte, 1994). They describe additional melt pools and melt at grain boundaries. The latter will lead to a higher connectivity of melts and hence to a small increase of the electrical conductivity. Deviatoric stress in the lithosphere will facilitate grain boundary connectivity (Jin et al., 1994). In order to describe the measured conductivity with the aid of models, we need detailed information

about the electrical conductivity of partial melts, melt distributions and melt fractions at different temperatures measured for at least one specific sample.

5. Laboratory experiments The available laboratory data for electrical conductivities of silicate melts cannot easily be compared. Earlier studies were carried out at a single

24

F.R. Schilling et al. / Physics of the Earth and Planetary Interiors 103 (1997) 17 31

frequency (e.g., Waft and Weill, 1975; Presnall et al., 1972), and neglected the frequency dispersion of conductivity (Macdonald, 1987). To investigate the influence of partial melting on electrical conductivity, laboratory experiments were carried out as a function of temperature at normal total pressure and at different oxygen fugacities (Partzsch, 1997). Different Hz/CO2-mixtures were used to get constant oxygen fugacities in the stability field of haematite, wiistite and magnetite. The oxygen fugacity was controlled and monitored through a zirconia gas sensor close to the sample. A two-electrode arrangement was used for the conductivity experiments (Fig. 5). Conductivity was measured between 0.1 and 106 Hz with an impedance apparatus (Zahner Electric). The sample used was a fresh, fine crystalline pyroxene granulite stemming from the Jurassic magmatic arc of the Coastal Cordillera (Lucassen, pers. comm.). It did not contain any quartz, in order to avoid cracking during the quartz a / / 3 transition at 573°C (Table 1). The sample shows a very low crack density and appears homogeneous and isotropic in thin sections. The deviation from ideal homogeneity and isotropy was measured ultrasonically and was found to be smaller than the experimental error ( < 1%).

Table 1 Mineralogical composition of the rock sample Oxide

Weight %

SiO~ TiO~ AI203 F%O~ MnO MgO CaO Na,O K20 P205

52.3 0.9 18.3 9.5 0.2 6.1 9.4 3.4 0.4 0.1 100.6

X

Mineralogical composition (point counter) plagioclase pyroxene ore biotite

Volume ~

59.2 34.3 4.2 2.3 100.0

E

In the temperature range below 1000°C there is a linear increase of conductivity, whereas between 1000°C and 1070°C a steepening of the slope is observed (Fig. 6). At 1070°C a conductivity increase of nearly two orders of magnitude is detected within a narrow temperature range. The small electrode impedance in the low temperature region can be attributed to electronic conductivity (Macdonald, 1987). At sub-solidus temperatures electric conductivity increases according to the

shielding

alumina platinum

I lOmm thermocouple

Fig. 5. Apparatus to measure electrical conductivity up to 1400°C under controlled oxygen fugacities.

F.R. Schilling et aL / Physics of the Earth and Planeta O' Interiors 103 (1997) 17-31 temperature [°C] ]

g

1150 i

1100 I

1050 i

1000 i

950 I

900 i

850 i

o

~ -0.5 -1 -1.5

i -2.5 N -3 -3.5 . . . . . . . . . . . . . . . . . . . 0.7 0.75 0.8 0.85 temperature [100011<]

0.9

Fig. 6. Electrical conductivity vs. temperature in the stability range of magnetite. + represents measured values whereas bold lines are calculated values using the MBL-model at measured melt fractions, indicated by .. At temperatures above 1040°C, melt is observed in thin sections of quenched samples.

Arrhenius equation (Eq. (1)), cf. Fig. 6. The mineral content of the sample indicates that the oxygen fugacity is in the stability field of magnetite. To avoid mineral reactions and to get similar oxygen fugacity conditions to those observed in samples from the Andean crust (magnetite bearing rocks), the electrical conductivity experiments were performed in the stability field of magnetite. The activation energy of the solid rock was calculated by regression analysis as 1.34 eV (+0.01 eV). At more elevated temperatures, the higher conductivity is caused by the ionic conductivity mechanism of a silicate melt (Waft and Weill, 1975). Even for very small melt portions, interconnected melt is observed at grain edges and grain boundaries in thin sections of quenched samples. The interconnected melt causes a marked increase in conductivity (Fig. 6). It takes up to 200 h to obtain constant conductivity at l l00°C within the melting range. This amount of time is required to get close to the morphological equilibrium and to distribute the melt along grain edges.

6. Model calculations

Laboratory measurements are often described by using two phase models. The first exact effective

25

medium model for the conductivity of dispersed spheres in a continuous medium was put forward by Maxwell (1881). Wagner (1914) and more recently Hashin and Shtrikman (1962) and Waft (1974) extended this model to describe complex conductivity. Series and parallel layer models have been used since Maxwell (1881). A third geometric model - the brick-layer model (BL model) - - was suggested by Beekmans and Heyne (1976). This model describes in a more realistic way the geometry of grains (minerals) and boundaries (melt) (Macdonald, 1987). The brick-layer model is valid for only up to several percent boundary phase (Macdonald, 1987); therefore, it was modified (see Eq. (4) below) to calculate electrical conductivities for melt fractions between 0 and 100%. The measured conductivity behavior is simulated by a modified-brick-layer model (MBL) using the electrical conductivities of rock and melt as a function of temperature. The MBL model describes an interconnected melt distribution (Fig. 4c) as it is observed in thin sections of quenched samples. The conductivity behavior of solid rock is calculated by an Arrhenius equation (Eq. (1)) using the fit parameters from Fig. 6 in the low temperature region below 1000°C. The conductivity of melt is set to 7 S / m at 1250°C with an activation energy of 1 eV from Eq. (2). The conductivity value of the melt is in good accordance with published data (e.g., Presnall et al., 1972; Murase and McBirney, 1973; Rai and Manghnani, 1978a; Satherley and Smedley, 1985; Tyburczy and Waft, 1983; Waft and Weill, 1975). The circles and bold line in Fig. 6 were calculated using the modified brick-layer model with experimentally observed melt volume fractions x and f = 1 - x (Partzsch, 1997). 1) -

o" =

0~ ( f _ f 2 / 3 ) + o. (f2/3 _ f _

1)

(4)

where o-l is the conductivity of the liquid (melt), is the conductivity of the solid (rock) and o- is the resulting conductivity of a partially molten rock. For the MBL model no dimensional (mm or km) information is required. The model may thus be used scale-independently for micro- and macro-structures. To simulate the conductivity behavior of the conductivity experiments in the range of partial melting,

26

F.R. Schilling et al. / Physics
experimentally obtained melt quantities were used to calculate the electrical conductivity of the examined rock sample. Model calculations and conductivities obtained from experiments are in good accordance and demonstrate the applicability of the model for partially molten rocks (Fig. 4). The same scale-independent Modified-BrickLayer Model (MBL model) was used to calculate melt fractions in the HCZ. A highly conducting melt (10 S / m ) and a low conducting melt (5 S / m ) were assumed (e.g., Presnall et al., 1972; Murase and McBirney, 1973; Rai and Manghnani, 1978a,b; Satherley and Smedley, 1985; Tyburczy and Waft, 1983; Waft and Weill, 1975) in the model calculation (Eq. (4)). The melt quantities of the proposed crustal melts are calculated using the conductivity values obtained from MT measurements of the HCZ (1 S / m ) and the conductivity of a dry rock (0.5 × 10 -B S / m ) (Shankland and Ander, 1983; Krop~cek et al., 1989). The model conductivities obtained for partially molten rocks are shown in Fig. 7 as a function of melt fraction and melt conductivity and compared with the Hashin-Shtrikman (HS) upper and lower bounds (Hashin and Shtrikman, 1962; see also Schmeling, 1984). The MBL model conductivities almost coincide with the HS upper bound. At

melt fractions above 3 vol.% conductivity of the partially molten rock is nearly independent of the conductivity of the insulating solid phase (Mac:donald, 1987). To explain the observed conductivity behavior in the magmatic arc region a melt fraction between 14 and 27 vol.% is necessary (Fig. 7). In the continental crust a network of magmatic dikes, partially molten rocks (anatexis) and magma chambers is assumed. Magma filled dikes and partially molten rocks (Fig. 4b) are expected to dominate the melt structure in the H C Z / L V Z , rather than melt segregation at grain boundaries (Fig. 4c). Melt at grain boundaries would not be stable across a large area (cf. Stolper et al., 1981) or - - in the case of isolated pockets - - the velocity reduction would be too small to explain the observed LVZ (Fig. 4a, cf. Schmitz et al., 1997) and HCZ. There is a fundamental disagreement concerning the scale of the laboratory samples and the continental crust. In the laboratories samples of a few mm in diameter are usually used, whereas in nature structures of hundreds of km exist. In the laboratory mineral grains with melt interconnected at grain boundaries and grain laces are considered, whereas in the magmatic arc large scale structures, e.g. dikes are relevant. Because the arrangement of interconnected melt

10

E 0.1

®

"6 tO

0.01

(.P

0.001

1E-4 0

10

20

30

40

50

Xmelt [Vol.%] Fig. 7. Calculated electrical conductivity of a partially molten rock as a function of melt traction (MBL-model). A high conducting (10 S / m , curve 1) and low conducting (5 S / m , curve 2) melt are assumed. Conductivity of the solid rock is set to 0.5 m S / m . Between 14 and 27 vol.% melt are necessary to obtain the 1 S / m observed beneath the Western Cordillera. For comparison the Hashin-Shtrikman upper and lower bounds (melt conductivity 10 S / m ) are indicated by the gray shaded areaO). The upper bound almost coincides with I.

F.R. Schilling et aL / Physics of the Earth and Planetary. Interiors 103 (1997) 17-31

(grain edges, grain boundaries or dikes, Fig. 4b, c) has only a small influence on the electrical conductivity, the results are independent of the geometry (Waff, 1974) of the interconnected melt distribution. 7. A model for the Andean arc: discussion and synthesis

Geoelectric and seismic observations together with model calculations lead to the assumption that the deeper Andean crust below the Western Cordillera consists of partially molten rocks. The main problem is whether a crust with high melt portions in the order of 20 vol.% would disturb the mechanical stability of the arrangement. The density difference between solid rock and melt may lead to a mobilization of melt if we assume either huge magma chambers (Stolper et al., 1981) or melt at grain boundaries (mineral fractionation). Melt interconnected through dikes would lead to a more stable configuration if the density difference is small. Due to their different compressibilities, only small density differences between gabbroitic rocks and basic melt phases are observed at a depth of about 35 km. At 1 GPa and 800°C a gabbro has a density

Trench

Forearc

27

between 2.89 and 2.91 g / c m 3 (Olhoeft and Johnson, 1990), whereas a basic melt with 1 vol.% water has a density of about 2.88 g / c m 3 (Lange, 1994). Due to this small density difference, no immediate mobilisation of the melt fraction is required (Stolper et al., 1981). The amount of melt is triggered by the water content and transferred heat. In the Western Cordillera large areas are covered by ignimbrites erupted from a water over-saturated magma. From this petrological observation and the geoelectrically estimated high conductivity of the upper crust (0.05 S/m), a fluid rich crust in the whole Andean range can be deduced. The temperature distribution, based on model calculations (heat flow 100 mW/m2), also gives indications for partial melting in the deeper crust of the Central Andes (Giese, 1994). Seismic investigations have been conducted in the same region (Schmitz et al., 1997). Beneath the Western Cordillera, between 20 and 70 km, p-wave velocities of 5.9 to 6.3 k m / s were observed, while gravimetric measurements in this area have revealed a negative Bouguer anomaly ( - 4 5 0 mGal). Calculated low velocity and a small density decrease by partial melts are in accordance with observed low velocity zones and the Bouguer anomaly (Heinsohn, 1993; Schmitz

Magmatic Arc

Fig. 8. Model to explain the high conductivities beneath the Andean magmatic arc. The subducting slab is defined by the Wadati-Benioff zone. The crustal velocities in the forearc down to 70 km can be attributed to a gabbroidic to tonalitic crust and a hydrated mantle with crustal densities and velocities (Giese, pers. comm.). Melts derived from the mantle wedge are trapped at the lower boundary of the crust (temperature --- 1200°C). Only a small percentage of these magmas is penetrating the crust through dikes and reaches magma chambers in higher crustal levels or is even erupted at the surface. The transferred heat together with conductive heat transport will suffice to produce crustal anatexis of water bearing rocks or dehydration melting. The minimum temperature at the top of the observed HCZ will be about 650°C. Draft after Echternacht (pers. comm.).

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F.R. Schilling et al. / Physics of the Earth and Planetary Interiors 103 (1997) 17 31

et al., 1997), whereas details of the gravity residual field are still not well understood. A simplified model of the Andean crust is given in Fig. 8. The subducting slab is defined by the Wadati-Benioff zone. The crustal velocities in the fore-arc down to 70 km can be attributed to a gabbroitic to tonalitic crust and a hydrated mantle with crustal densities and velocities (Giese, pers. comm.). Mantle wedge derived melts are trapped at the lower boundary of the crust. Only a small percentage of these magmas penetrate the crust through dikes and reach magma chambers in higher crustal levels or are erupted at the surface. The heat which is transferred in this way and by conductive heat transport will suffice to produce crustal anatexis of water bearing rocks or dehydration melting. The minimum temperature at the top of the observed HCZ will be about 650°C, the eutectic temperature of granitic, water saturated magmas at 0.6 GPa. This is in good accordance with geothermal gradients larger than 30 K / k m assumed in active magmatic arcs (Gill, 1981). The eutectic composition in anatectic rocks will have a lower density than the surrounding rocks (restite) and tend to rise to the surface. The decrease in pressure due to the mobilization increases solidus temperatures which in turn causes the melt phase to recrystallize. This leads to a partial melt which cannot reach the surface owing to the described dynamic melting and crystallisation behavior. 8. Conclusion The Western Cordillera (WC) on the border of Chile, Bolivia and Argentina constitutes a present magmatic arc with recent volcanism. Different mechanisms are discussed to interpret the observed very highly conducting zones (HCZs). The observed petrophysical anomalies are most likely a consequence of partial melts beneath the Western Cordillera. To get more insight into the electrical behavior of partial melts, impedance spectroscopic experiments at different temperatures and oxygen fugacities were carried out. In the temperature range below 1000°C there is a linear increase of conductivity, whereas between 1000°C and 1070°C a steepening of the slope is observed. In the low temperature region the electrical conductivity can be attributed to electronic

conductivity by way of grain boundary defects. At sub-solidus temperatures electric conductivity increases in accordance with an Arrhenius equation with an activation energy E A of 1.34 eV (0.01 eV). At these elevated temperatures high conductivity is caused by the ionic conductivity mechanism of a silicate melt. Even for very small melt portions, interconnected melt at grain-boundaries is observed in thin sections of quenched samples. This causes a marked increase in conductivity. Information about electrical conductivity of partial melt, melt distributions and melt fractions at different temperatures measured on one specific sample is presented. The electrical conductivity behavior of the sample was simulated using the modified-brick-layer model (MBL). Simulated and measured conductivities are in good accordance. The conductivity measured in the laboratory is simulated by a MBL using the electrical conductivities of rock and melt as a function of temperature. Model calculations and conductivities obtained from experiments are in good accordance with one another and demonstrate the applicability of the MBL model for partially molten rocks. The same scale independent MBL model is used to calculate melt fractions in the HCZ. The melt quantities of the assumed crustal melts are calculated using the conductivity values obtained from MT measurements of the HCZ. At melt fractions above 3 vol.%, the conductivity of the partially molten rock is nearly independent of the conductivity of the solid phase. A melt fraction between 14 and 27 vol.CA is necessary to explain the observed conductivities in the magmatic arc. The arrangement of interconnected melt (grain edges or grain boundaries) has only a small influence on the observed electrical conductivity data. Geoelectric and seismic observations, together with model calculations, suggest that the deeper Andean crust below the Western Cordillera consists of partially molten rocks. In the continental crust a network of magmatic dikes, partially molten rocks (anatexis) and magma chambers is relevant. The minimum temperature at the top of the observed HCZ is about 650°C, the eutectic temperature of granitic rocks (Bowen, 1956). A simplified model of the Andean crust is presented and a dynamic melting-crystallisation behav-

F.R. Schilling et al. / Physics of the Earth and Planetary Interiors 103 (1997) 17-31 ior o f partial m e l t s is p r o p o s e d . F u r t h e r i n v e s t i g a t i o n s are n e c e s s a r y to o b t a i n m o r e i n f o r m a t i o n a b o u t m e l t d i s t r i b u t i o n s , a n d the N o r t h - S o u t h e l o n g a t i o n o f the o b s e r v e d H C Z . F u r t h e r m o r e , r e f i n e m e n t o f the data m a y lead to a n a r r o w e r r a n g e o f p o s s i b l e m e l t fractions.

Acknowledgements

T h i s s t u d y w a s c a r r i e d o u t w i t h i n the f r a m e w o r k o f the Special R e s e a r c h P r o j e c t ' D e f o r m a t i o n Proc e s s e s in the A n d e s ' , f u n d e d b y the D e u t s c h e Forschungsgemeinschaft. The authors express their g r a t i t u d e to P. Giese, V. H a a k a n d J. A r n d t for m a n y s t i m u l a t i n g d i s c u s s i o n s a n d to F. S i m p s o n a n d F. H u n t e r for critically r e a d i n g the m a n u s c r i p t . T h e c o n s t r u c t i v e c o m m e n t s o f R.H.S. H u t t o n a n d a n a n o n y m o u s r e v i e w e r are g r a t e f u l l y a c k n o w l e d g e d .

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