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Upper Mantle Temperature Determined from Combining Mineral Earth and Planetary Science Letters 236 (2005) 258–268 www.elsevier.com/locate/epsl Upper mantle temperature determined from combining mineral composition, electrical conductivity laboratory studies and magnetotelluric field observations: Application to the intermontane belt, Northern Canadian Cordillera Juanjo Ledoa,b,T, Alan G. Jonesa,c aGeological Survey of Canada, 615 Booth St., Ottawa, Ontario, Canada, K1A 0E9 bDepartament de Geodina`mica i Geofı´sica, Universitat de Barcelona. Martı´ i Franque`s s/n 08028 Barcelona, Spain cDublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland Received 6 July 2004; received in revised form 26 November 2004; accepted 14 January 2005 Available online 27 June 2005 Editor: V. Courtillot Abstract Using laboratory-derived temperature dependences of the electrical conductivity of mantle minerals coupled with appropriate mixing laws, we determine the bulk conductivity of mantle mineral assemblages for the ternary olivine– orthopyroxene–clinopyroxene (Ol–Opx–Cpx) system. We calculate physical property bounds (Hashin–Shtrikman bounds) as a function of the fraction of different phases present; these limits correspond to the extreme situations where the most conducting phase is either fully interconnected or fully disconnected. The relationships we present between temperature, mineral composition and bulk electrical conductivity allow constraining one of them given the other two. We apply this approach to an area of the North American Cordilleran Intermontane Belt in the Yukon Territory, northern Canada, where xenolith evidence indicates bimodal upper mantle mineral assemblages (harzburgite and lherzolite). This locality coincides spatially with an upper mantle region of low electrical conductivity determined by long period magnetotelluric data. Given the mantle mineral composition and the maximum and minimum bounds on the electrical conductivity, deduced by non-linear model appraisal, we determine the permitted extremal temperature bounds of the Intermontane belt mantle rocks directly below the Moho to a depth of some 80 km. We show that the mantle in this region is at a minimum temperature of 820 8C and a maximum temperature of 1020 8C; the latter is some 200 8C colder than that suggested in a recent interpretation of an observed collocated low velocity zone from a teleseismic survey. D 2005 Elsevier B.V. All rights reserved. Keywords: magnetotellurics; electrical conductivity; rock physics; mantle properties; northern Canadian Cordillera T Corresponding author. Departament de Geodina`mica i Geofı´sica, Universitat de Barcelona. Martı´ i Franque`s s/n 08028 Barcelona, Spain. E-mail addresses: [email protected] (J. Ledo), [email protected] (A.G. Jones). 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.01.044 J. Ledo, A.G. Jones / Earth and Planetary Science Letters 236 (2005) 258–268 259 1. Introduction Various mixing relationships can be used to quantify the bulk conductivity of a multiphase rock, The electrical conductivity of the continental and work in the past has primarily concentrated on mantle is a physical parameter that allows constraints two-phase assemblages, e.g., [19] Schmeling (1986). to be placed on mantle properties and geometries, For our purposes we need to quantify a three-phase thereby aiding deduction of formation processes and assemblage, namely Ol–Opx–Cpx rocks. There are evolutionary development ([1] and [2]). Regional and many mixing laws possible to describe the intercon- global studies of the mantle’s electrical conductivity nectivity, but with the use of the Hashin–Shtrikman have been undertaken since Lahiri and Price’s (1939) [20] bounds one can constrain the electrical conduc- [3] groundbreaking work, and since then several tivity limits of the assemblage for a given specific radially symmetric global models had been presented. petrology and temperature. Commonly, the Hashin– Early work primarily used geomagnetic arrays (e.g., Shtrikman upper and lower bounds vary by orders of [4]), but since the 1980s, and particularly through the magnitude, as one is usually considering a resistive 1990s, EM studies are now almost universally under- phase or phases with a highly conductive phase, such taken using the magnetotelluric method (see, e.g., [1]). as partial melt (e.g., [19]) or conducting minerals Reviews of regional and global studies of the mantle’s (e.g., sulphides, [21]). In our case the three phases are electrical conductivity can be found in [1,4-8]. all resistive that do not vary significantly between Electrically, the mantle is considered to be formed them—the electrical conductivity ratio among these mainly by silicates that behave as semiconductors at mantle minerals is never higher than six. Thus, the mantle temperatures below the adiabat. In the upper consequence is that the upper and lower Hashin– mantle olivine is the dominant mineral, and is Shtrikman bounds are close to one another, and all assumed to control the conductivity of impurity-free, other possible mixing relationships lie in-between. solid mantle rocks. The presence of impurities or Accordingly, if the bulk electrical conductivity of a defects, with a valency misfit in the crystal lattice, was region of lithospheric mantle can be determined by the long thought to be the dominant semiconduction magnetotelluric (MT) method with a high degree of mechanism [5]. More recently, the main defect reliability and precision, the use of laboratory responsible for electrical conduction in the temper- electrical conductivity measurements and the ature range 700–1300 8C has been associated with the Hashin–Shtrikman bounds for a three component polaron Fe3+ [9]. Support for this mechanism is system can constrain the temperature of that region. provided by the experiments carried out by several The relationships presented here between temperature, authors [9–13]. mineral composition and electrical conductivity The electrical conductivity of mantle-forming allows limiting one of them given the other two. A minerals can thus be described by the solid-state caveat of the previous reasoning is that a more Arrhenius equation, depending on the appropriate conductive phase, e.g., partial melt [19,22–24], hydro- activation energies, Boltzmann’s constant and abso- gen diffusion [25,2], interconnected carbon [26] or lute temperature. Considering a pyrolitic bulk com- sulphides [21] cannot be present. position for the upper mantle [14] the amount and The region discussed in this study (Intermontane distribution of the minerals olivine (Ol), orthopyrox- Belt, Northern Canadian Cordillera) is a special case ene (Opx) and clinopyroxene (Cpx) controls the bulk because of two important and unusual factors: 1) the conductivity of the mantle. Early laboratory work on lower crust is not conductive, and 2) the upper mantle the electrical conductivity of mantle minerals was is not conductive, i.e., in both regions there are no inconsistent, leading to questions as to its utility [15]. interconnected conductive phases. The existence of a However, over the last decade and a half the temper- conducting lower crust produces a shadow zone ature dependences of the electrical conductivities of beneath it preventing determination of the true these mantle minerals, and others appropriate for electrical conductivity [1]. Thus, as a conducting deeper mantle rocks, have been determined by careful lower crust exists in almost all continental regions laboratory measurements [9,11–13,16–18], and are [27], usually only a lower bound can be set on the summarized in [8]. uppermost mantle’s resistivity [1]. In locations where 260 J. Ledo, A.G. Jones / Earth and Planetary Science Letters 236 (2005) 258–268 the lower crust is resistive, such as the southern part of solid state Arrhenius relationship with up to three the Slave craton [28], the southern part of the Rae branches with different activation energies associated province [29], and the location discussed herein, then to different conduction mechanisms. Thus, the elec- the resistivity of the mantle directly below the Moho trical conductivity can be expressed as the sum of can be determined reliably and, given high quality three thermally activated processes: data, with precision. X3 The electrical conductivity model of the region ÀAi=kT r ¼ rie ð1Þ obtained by Ledo et al. (2004) [30],showsa i¼1 spatially defined zone of low electrical conductivity r at mantle depths in the Intermontane belt that where is the conductivity of olivine in Siemens per coincides with a zone of xenoliths having bimodal metre (S/m), k is Boltzmann’s constant, A is the lherzolitic and harzburgitic composition [31]. In this activation energy, T is the temperature in Kelvin, and paper, we take that mantle mineral composition and subscript i refers to the three branches or conduction mechanisms. The main conclusion from the results the observed electrical conductivity, combine them obtained from Constable and Duba (1990) [13] with laboratory studies of the temperature depend- combining different datasets was that the electrical ence of conductivity of mantle minerals and assumed mixing laws, to determine temperature bounds of the conductivity of olivine rocks between 640 and 1500 8 Intermontane belt mantle rocks. We demonstrate that C can be described with three different conduction b 8 the mantle in this region is at a minimum temper- mechanisms. For low temperatures ( 720 C) the ature of 820 8C and a maximum temperature of 1020 conduction mechanism has
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