Thermo-chemical structure of the lithospheric mantle underneath the Siberian craton inferred from long-range seismic profiles

Tectonophysics 615–616 (2014) 154–166

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Thermo-chemical structure of the lithospheric mantle underneath the Siberian craton inferred from long-range seismic profiles O.L. Kuskov a,⁎, V.A. Kronrod a, A.A. Prokofyev a, N.I. Pavlenkova b,1 a b

Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia Shmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow 123995, Russia

a r t i c l e

i n f o

Article history: Received 4 June 2012 Received in revised form 30 December 2013 Accepted 4 January 2014 Available online 15 January 2014 Keywords: Siberian craton Long-range profiles Thermo-chemical structure Xenoliths Mantle composition

a b s t r a c t Based on a self-consistent thermodynamic–geophysical approach and xenolith-based constraints, we map the 2-D seismic, thermal and density structure of the mantle beneath the Siberian craton along the long-range profiles (Craton, Kimberlite, Rift and Meteorite) carried out in Russia with peaceful nuclear explosions. Structural peculiarities of the cratonic mantle are manifested by changes in seismic velocities, the degree and nature of layering and the relief of seismic boundaries. The results predict appreciable lateral temperature variations within the root to a depth of about 200 km, which are the main cause of seismic velocity variations. We find that the cratonic mantle is 300–400 °C colder than the tectonically younger surrounding mantle in this depth range. At greater depths, lateral changes in temperatures have little effect implying that thermal heterogeneity rapidly decreases. The present-day geotherms pass close to the 32.5–35 mW m−2 conductive models and suggest low mantle heat flow. Within the model resolution, the thickness of the thermal boundary layer, TBL (defined as the depth of the 1300 °C adiabat) beneath Siberia does not depend significantly on the composition and can be estimated as 300 ± 30 km; temperature at the base of the TBL is close to the 1450 ± 100 °C isotherm. Changes in the composition from depleted to fertile material reveal a negligible effect on seismic velocities, which are practically unresolved by seismic methods, but remain the most important factor for the density increase of the cratonic root. Density variations in the lower part of the root due to the chemical composition are greater than those caused by temperature. We find that both compositional and thermal anomalies are required to explain the Siberian mantle by a keel model consisting of depleted garnet peridotite at depths of 100 to 180 km and more fertile material at greater depths. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Investigations of the mantle beneath the Siberian craton (SC) have been performed in a number of geochemical, geophysical and thermal studies (Artemieva and Mooney, 2001; Ashchepkov et al., 2010; Boyd et al., 1997; Pavlenkova, 2011; Rosen et al., 2006; Sobolev, 1977; Thybo, 2006). However, temperature–composition–grain size–density– seismic velocity–depth profiles important for the study of evolution and stability of the continents are uncertain and merit further investigation. Seismic, gravity and surface heat-flow data provide only indirect information about the composition and temperature of the deep interior (Artemieva, 2009; Fuchs, 1997; Kaban et al., 2003; Shapiro and Ritzwoller, 2004). Mantle xenoliths are often used to constrain mantle temperatures at the time of kimberlite eruptions and to estimate some petrophysical

⁎ Corresponding author at: Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Kosygin Str. 19, 119991 Moscow B-334, Russia. Tel.: +7 499 1378614; fax: +7 495 9382054. E-mail addresses: [email protected] (O.L. Kuskov), [email protected] (V.A. Kronrod), [email protected] (A.A. Prokofyev), [email protected] (N.I. Pavlenkova). 1 Tel.: +7 495 2542327; fax: +7 495 2556040. 0040-1951/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2014.01.006

properties of cratonic mantle and thickness of the petrologically distinct layer by xenolith P–T arrays (Kobussen et al., 2006; Lee et al., 2005; O'Reilly and Griffin, 2006, 2010). Thermobarometric results for Siberian mantle xenoliths of garnet, garnet–spinel, spinel peridotites, and pyroxenites (Ashchepkov et al., 2010; Boyd et al., 1997; Griffin et al., 2003; Ionov et al., 2010) provide unique information about the compositional heterogeneity and evolution of the cratonic mantle, but do not give direct information about its seismic structure. Combinations of surface heat flow measurements, geophysical data, xenolith thermobarometry and additional thermodynamic principles reduce some of the ambiguity in interpretations of mantle structure and provide the tighter constraints on mantle chemistry and thermal state (Anderson, 1989; Artemieva, 2006; Dalton and Faul, 2010; Deen et al., 2006; Jones et al., 2009; Khan et al., 2008; Kronrod and Kuskov, 2006, 2007; Lebedev et al., 2009; Simmons et al., 2009; Sobolev et al., 1996; Stixrude and Lithgow-Bertelloni, 2011). Seismic studies are probably one of the best tools to infer the thermal state of the upper mantle because seismic velocities are more sensitive to temperature than to composition (e.g., Goes et al., 2000; Poupinet et al., 2003). A set of geophysical data (global P- and S-wave travel times, surface-wave phase velocities, travel time data from the deep seismic sounding) or simply seismic velocity–depth profiles can be

O.L. Kuskov et al. / Tectonophysics 615–616 (2014) 154–166

converted to temperature–depth profiles using petrological constraints on the mantle composition or the composition of xenoliths brought to the surface and a thermodynamic-based inversion scheme (Afonso et al., 2008, 2013; Cammarano et al., 2003, 2009; James et al., 2004; Khan et al., 2011, 2013; Kuskov and Kronrod, 2006, 2007; Röhm et al., 2000; Shapiro and Ritzwoller, 2004; Sobolev et al., 1996). In this work, we present a joint seismic, thermo-chemical and density model for the upper mantle of the Siberian craton. We use the approach of Kuskov et al. (2006, 2011) where isotropic velocities are converted to temperatures and vice versa based on a method of minimization of the Gibbs free energy incorporating equations of state of minerals, phase transformations, anharmonicity and attenuation effects. The major purpose of the present study is to deduce a family of geotherms permitted by absolute velocities and to estimate the thickness of cratonic mantle and its density from long-range seismic profiles (Craton, Kimberlite, Rift and Meteorite) carried out in Russia with peaceful nuclear explosions (Fig. 1). For comparison of the internal structure of a cold cratonic mantle with the surrounding mantle, it is instructive to use the AK135 and PREM reference models (Dziewonski and Anderson, 1981; Kennett et al., 1995). With this in mind, the main objectives of our study are as follows: (1) to map the 2-D seismic, thermal and density state of the Siberian craton upper mantle; (2) to compare the inferred temperatures with heat-flow models and mantle paleotemperatures estimated from thermobarometric results for Siberian xenoliths from kimberlites; (3) to provide the better constraints on the seismic structure, thermal state, composition and density of the mantle in central Siberia. 2. Data and method The thermodynamic basis for modeling phase equilibria and physical properties of the Earth's mantle and various databases have been discussed in a series of papers (e.g., de Capitani and Brown, 1987; Saxena and Eriksson, 1983; Stixrude and Lithgow-Bertelloni, 2011). We basically use the same method as that described in detail in our previous publications (e.g., Kuskov et al., 2006, 2011). Briefly, this is a thermodynamically self-consistent approach based on a method of minimization of the Gibbs free energy in conjunction with the thermal equation of state for solids written in a Mie–Grüneisen–Debye form.

Fig. 1. Schematic location of the long-range seismic profiles carried out in the Siberian Craton with peaceful nuclear explosions (after Egorkin, 2001, 2004; Pavlenkova and Pavlenkova, 2006). Letters indicate location of the shots.

155

The approach relates the equilibrium mineral assemblage for an assumed mantle composition and equations of state (EOS) of minerals with seismic properties. The phase composition and physical properties of the mantle were modeled within the dry Na2O–TiO2–CaO–FeO– MgO–Al2O3–SiO2 (NaTiCFMAS) system including the non-ideal solid solution phases (Table 1). The pressure–depth correlation was taken from the PREM model. Input data for the thermodynamic quantities are summarized in the THERMOSEISM database. The database was established by supplementing the calorimetric data for low-pressure phases and the EOS for low- and high-pressure phases with data calculated from high-P–T experiments (Fabrichnaya and Kuskov, 1994; Kuskov, 1997). The output P–T results contain the self-consistent information on phase assemblage (the mineral phases, their proportions and individual chemical compositions), the total density and seismic velocities. The

Table 1 Bulk composition models (wt.%), phase composition (mol%) and physical properties of garnet harzburgite (Hzb), garnet lherzolite (Lh), average garnet peridotite (GP) and primitive mantle (PM) composition in the NaTiCFMAS system a. Composition

GP

PM

Hzb

Lh

SiO2 TiO2 Al2O3 FeO MgO CaO Na2O Total MG#

45.42 0.08 1.32 7.03 45.28 0.78 0.09 100.0 92.00

45.25 0.21 4.50 8.48 37.58 3.64 0.34 100.0 88.80

45.7 0.02 0.40 6.14 47.51 0.20 0.03 100.0 93.20

46.15 0.05 1.21 6.55 45.25 0.71 0.08 100.0 92.5

Phase composition, physical properties 100 km (Р = 2.9 GPa, 600 °С) Ol 65.8(Fo92.8) 55.8(Fo92.5) Gar 1.5 5.4 Opx 27.0 10.0 Cpx 5.6 28.4 Ilm 0.1 0.4 ρ, g cm−3 3.334 3.403 −1 Vp, km s 8.320 8.332 −1 Vs, km s 4.724 4.695 KS, GPa 131.58 136.25 G, GPa 74.40 75.02

67.3(Fo93.4) 0.37 30.9 1.4 0.03 3.309 8.323 4.739 130.13 74.31

61.7(Fo93.2) 1.3 32.0 4.9 0.1 3.325 8.314 4.730 130.64 74.40

310 km (Р = 10.25 GPa, 1450 °С) Ol 65.8(Fo92.4) Gar 1.5 Opx 25.7 Cpx 6.9 Ilm 0.1 ρ, g cm−3 3.416 −1 Vp, km s 8.610 −1 Vs, km s 4.671 KS, GPa 153.86 G, GPa 74.53

67.47(Fo93.2) 0.40 31.0 1.1 0.03 3.391 8.612 4.684 152.31 74.40

61.9(Fo92.8) 1.4 31.1 5.5 0.1 3.409 8.609 4.679 153.12 74.65

56.1(Fo91.2) 5.6 0.0 37.9 0.4 3.488 8.633 4.657 159.10 75.70

GP: (100 km/2.9 GPa/600 °С). 65.8% Ol (Fo92.8) + 27% Opx (En92.2OrthoDi0.4oFs7OrthoHed0.2OrthoCor0.2) + 1.5% Gar (Py70Alm24Gros6) + 5.6% Cpx (ClEn24Di42ClFs6.2Hed13Jd14ClCor0.8). GP: (310 km/10.25 GPa/1450 °С). 65.8% Ol (Fo92.4) + 25.7% Opx (En89OrthoDi2.7OrthoFs7OrthoHed1.2OrthoCor0.1) + 1.5% Gar (Py85Alm12Gros3) + 6.9% Cpx (ClEn46Di25ClFs5Hed12Jd11.8ClCor0.2). PM: (100 km/2.9 GPa/600 °С). 55.8% Ol (Fo92.5) + 5.4% Gar (Py68Alm25Gros7) + 10% Opx (En92OrthoDi0.4.OrthoFs7.2 OrthoHed0.2OrthoCor0.2) + 28.4% Cpx (ClEn23Di44.5ClFs6.5Hed14Jd11.3ClCor0.7). PM: (310 km/10.25 GPa/1450 °С). 56.1% Ol (Fo91.2) + 5.6% Gar (Py82Alm14Gros4) + 37.9% Cpx (ClEn38.7Di31.8ClFs6 Hed14.8Jd8.5ClCor0.2). a The NaTiCFMAS system includes the following solid solution phases: olivine (Ol), spinel (Sp), plagioclase (Pl) and ilmenite (Ilm) – binary solutions; garnet (Gar: almandine, pyrope, grossular); orthopyroxene (Opx: MgSiO3, FeSiO3, Ca0.5 Mg0.5SiO3, Ca0.5Fe0.5SiO3, Al2O3); clinopyroxene (Cpx: same components as in Opx plus jadeite end-member). Bulk compositions normalized to 100% were taken from Griffin et al. (2003) for garnet harzburgite and garnet lherzolite (Daldyn Field, Siberia, Archon) and from McDonough (1990) for average garnet peridotite and primitive mantle composition. Total Ti is included in ilmenite. The compositions of phase assemblages (mol%) are given as an example.

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3. Siberian craton 3.1. Composition and physical properties Compositional spectrum of cratonic peridotites, which display a range of major element compositions as well as the thickness of thermal boundary layer vary from craton to craton, depending on their stabilization age, tectonic history and thermal regime (Artemieva, 2009; King, 2005; Kopylova and Russell, 2000; Lee et al., 2005). The study of xenoliths in the diamond-bearing kimberlites indicates that a large portion of the thick cratonic keel is largely composed of peridotites depleted in basaltic components, which is associated with the early stage of the crustal development. They consist of olivine and orthopyroxene with subordinate garnet and clinopyroxene

Vp, km s-1 8,3

8,4

8,5

100

a) H, km

150

200

250

300

PM Lherzolite Harzburgite

VS, km s-1 4,60

4,65

4,70

4,75

100

b)

150

H, km

aggregate elastic properties were estimated by Voigt–Reuss–Hill averaging. The effect of uncertainties in the computation procedure depends on the thermodynamic database used (Cobden et al., 2008; Kuskov et al., 2006; Stixrude and Lithgow-Bertelloni, 2011). The uncertainties in the input thermodynamic quantities are common for all compositions and are less significant than those in the seismic models. Discrepancies in the calculations can be associated with different input parameters such as key values, EOS and form of presentation of EOS, interaction parameters W = WU + PWV − TWS. An evaluation of uncertainties has shown that seismic velocities can provide temperature estimates within ±100 °C. For a given chemical composition, the effect of temperature variations of ±100 °C causes less than ±0.015 g cm−3 (±0.4%) density variations and ±0.04 km s−1 (± 0.5% for V P and ± 0.8% for VS ) P- and S-velocity variations. Effect of pressure variations of ± 1 GPa (± 30 km) causes less than ± 0.025 g cm−3 (± 0.7%) density variations and ± 0.08 km s−1 (±1%) P-velocity variations, that is less or close to the estimated uncertainties in the velocity models (Pavlenkova and Pavlenkova, 2006). Thus, the uncertainty in the thermodynamic parameters and modeled velocities do not allow us to constrain temperature and the thickness of the thermal boundary layer from seismic models any tighter than ±100 °C and ±30 km. Effect of composition on the physical properties is shown in Fig. 2a–c. We find that P-velocities for the fertile (basalt rich) primitive mantle (PM) composition are slightly greater (≤0.3%) than those for the depleted rocks, whereas S-velocities are slightly less (≤0.7%); an increase in fertility produces opposite effect on seismic velocities (Kuskov et al., 2006). However these differences fall within the estimated uncertainties and can be explained by the thermodynamic model used. Such differences cannot be resolved by seismic studies. Garnet harzburgite composition has a lower density with respect to the composition of garnet lherzolite and PM (Fig. 2c), but similar seismic velocities (Fig. 2a, b). In the estimates of the P–T points from thermobarometry of garnets no corrections have been made for the effect of Cr owing to the lack of adequate data. Our estimates show that for the bulk mantle composition with ~ 0.4 wt.% of Cr2O3 the influence of Cr on aggregate's bulk properties of Cr-bearing assemblages shows only a minor effect (Kuskov et al., 2006). The effect of water and partial melt on seismic velocities of cold cratonic mantle is not considered here. We ignore also the effect of grain size (Dalton and Faul, 2010) on anelasticity due to the lack of experimental data on multi-component solid solutions. A correct thermal interpretation must account for the dissipative effects due to anelasticity especially at depths where temperature approaches the solidus; this procedure is described in Appendix A.

200 250 GP PM Hzb

300

Density, g cm-3 3,30

3,35

3,40

3,45

3,50

100

c) 150

H, km

156

200 250 300

Hzb PM Lh

Fig. 2. Comparison of P-velocities (a) and S-velocities (b), and densities (c) for garnet harzburgite (Hzb), garnet lherzolite (Lh), average garnet peridotite (GP) and primitive mantle (PM) calculated along conductive geotherms 35 mW m−2 (dashed lines) and 40 mW m−2 (solid lines). For Archean geotherms, VP increases with depth while VS decreases. Under cratonic geotherm, an increase in fertility produces the opposite effect on seismic velocities.

(Gaul et al., 2000; Griffin et al., 1996, 2003; Solovyeva et al., 1994). Other lithologies that occur in the mantle are significantly less abundant than peridotites. The spinel-bearing lherzolite varieties prevail at shallower depths. Despite the fact that these rocks exhibit no strict succession as to the extent of their depletion, it can probably be assumed that a cratonic mantle has a strongly depleted composition down to depths of 150–180 km implying an increase in the fertility and density of the lower parts of Archean mantle in agreement with the geophysical– petrological modeling (Afonso et al., 2008; Arndt et al., 2009; Boyd

Fig. 3. Velocity cross sections along the profiles Craton (a), Kimberlite (b), Meteorite (c) and Rift (d) (modified from Pavlenkova and Pavlenkova, 2006). 1 — Seismic boundaries with constant velocity (the velocities increase linearly between the boundaries), 2 — reflector with high amplitude reflections, 3 — low velocity layer, 4 — high reflectivity zone. Seismic boundaries: M is the bottom of the crust (Moho), N1, N2, L, H and T are the boundaries in the upper mantle. Letters + digits indicate location of the shots: C1, C2, C3 and C4 – along the profile Craton; K1, K2 and K3 along the profile Kimberlite; R1, R2 and R3 along the profile Rift; M1, M2, M3 and M4 along the profile Meteorite. The Craton profile crosses the Daldyn–Alakit kimberlite field over about 250 km (2100–2350 km).

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a)

b)

c)

d)

CRATON

KIMBERLITE

METEORITE

RIFT

157

158

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et al., 1997; Kopylova and Russell, 2000; Kuskov et al., 2006; Lee et al., 2005; O'Reilly and Griffin, 2006, 2010). This implies that the mantle may be composed of a mixture of depleted and metasomatically enriched garnet lherzolites; i.e., the mantle material becomes gradually enriched with basaltic components with depth, probably up to samples from the top of the fertile convecting mantle. Table 1 presents some model compositions of the Siberian mantle. We are aware that petrological data obtained on xenoliths from a small number of kimberlite pipes may not be representative of the mantle beneath the whole Siberian craton. Most of the sampled xenoliths are common peridotites such as harzburgite and lherzolite, and only a few have more unusual compositions (Ashchepkov et al., 2010; Sobolev, 1977; Solovyeva et al., 1994). Here we restrict our analysis to some of xenoliths (Boyd et al., 1997; Griffin et al., 2003; Ionov et al., 2010; Shimizu et al., 1997) and use the average composition of garnet peridotite (hereafter GP model) from McDonough (1990). The asthenospheric mantle is assumed to be composed of fertile material of primitive mantle (PM model) close to a pyrolite model (Jordan, 1978; McDonough, 1990). We consider four petrologic models, which span a large range of Al2O3 (0.4–4.5%), CaO (0.2–3.6%) and FeO/MgO concentrations (Table 1, Fig. 2): (1) a strongly depleted garnet harzburgite, (2) a somewhat depleted garnet lherzolite, (3) GP model, (4) PM model. Other mantle rocks such as amphibole, phlogopite or carbonatebearing assemblages are unlikely to determine the velocities averaged over ~3000 km length scales. With petrological point of view, it is likely that compositional variations exist in the lithospheric mantle. However seismic data on long profiles should not be sensitive to local inhomogeneities. Cratonic mantle temperatures and densities are calculated (unless noted otherwise) for depleted compositions at depths of 100–180 km and for fertile PM composition at greater depths. Our results indicate an upper mantle mineralogy as consisting chiefly of olivine, two aluminous pyroxenes and garnet down to about 300 km depth, where the two pyroxenes are replaced by a single high-pressure Cpx. The expected location of this phase transition probably coincides with a small magnitude seismic discontinuity, the “X-discontinuity” (e.g., Matsukage et al., 2005). This phase transition is not modeled in the present study because of the lack of adequate thermodynamic data in the NaTiCFMAS system and because high-pressure experiments have shown that the amount of Opx is small at ~ 300 km depth due to the pyroxene/garnet phase transformation in a primitive mantle composition (e.g., Akashi et al., 2009). Changes in the composition of the depleted garnet peridotite (on-cratonic mantle) to off-cratonic mantle rock show minor effect on velocities (Fig. 2a, b). These forward calculations indicate that temperature is the main parameter affecting seismic velocities in agreement with previous findings (e.g., Goes et al., 2000). Therefore, seismic velocities can be used to directly invert for mantle temperatures. Composition, however, still remains the most important factor for the density of the continental roots, for understanding the nature of the mantle discontinuities and for the dynamical behavior of the mantle (Fabrichnaya and Kuskov, 1991; James et al., 2004; Jordan, 1978; King, 2005). Moreover, composition has a complex effect on velocities and densities: an increase in the FeO content and a decrease in the Al2O3 content lead both to a velocity decrease but can compensate a change in density (Kuskov et al., 2006). Compositional gradients from depleted to fertile material lead to a significant change in density (Δρ/ρ ~2–3%, Fig. 2c, Table 1), resulting in only minor changes in both velocities (Fig. 2a, b). Change in density by 2% is equivalent to the change in temperature by 500 °С. This is explained by the higher amount of garnet in the fertile mantle (5–6 mol% or ~15–18 wt.%). On the whole, the compositional changes from strongly depleted material to fertile primitive mantle have an insignificant impact on the seismic velocities that is practically unresolved by seismic methods (Afonso et al., 2008; Jones et al., 2009; Kuskov et al., 2006, 2011), although they are accompanied by noticeable changes in the rock's density.

3.2. Seismic velocity models The crustal and mantle structure under the Siberian Platform was studied by the GEON Center of the USSR with chemical and peaceful nuclear explosions (PNE), Fig. 1. It is the unique network of long-range profiles, where P waves from PNEs were recorded at epicentral distances reaching 3000 km; no reliable data on the S-velocities are available. It is assumed that the old and cold sub-crustal mantle is characterized by low heat flow (Duchkov and Sokolova, 1997) and low/high-velocity anomalies (Egorkin, 2004; Thybo, 2006; Thybo and Perchuć, 1997), and lacks zones of partial melting (Egorkin, 2001). Tomographic velocity inversion of first arrivals along the Craton profile has shown evidence for the existence of the LVZ in eastern and central Siberia (Nielsen et al., 2002; Thybo, 2006). Seismic data for profiles Craton, Kimberlite, Rift and Meteorite were interpreted by several Russian and international groups (Fuchs, 1997). They used various methods of analysis of experimental data and velocity modeling (Egorkin, 2001, 2004; Oreshin et al., 2002; Pavlenkova et al., 2002; Suvorov et al., 2010). These models are similar in average changes of the velocity with depth (within 0.1–0.2 km s−1), although they differ in the profile cross points and contain different thin layers with higher and lower velocity. The dissimilarity between various models may be generally due to inversion methodology. We map the average thermal and density structure of the Siberian mantle at depths of 100 to 300 km from the seismic models presented by Pavlenkova and Pavlenkova (2006) and partly modified in this study (Fig. 3a–d). We are aware of the limitations of using P-velocities rather than the original data of the deep seismic sounding from which the models are constructed. Although it is difficult to expect that in this case we obtain a model, that will be better than previously published (e.g., Fuchs, 1997). 2-D velocity models of the crust and upper mantle were constructed for all profiles using both PNE and chemical explosion records. The latter gave possibility to correct the mantle travel times for the crustal inhomogeneity and increase data on the uppermost mantle structure, which was difficult to get from the PNE records alone. Before the velocity modeling the wave analysis was made for the determination of regular waves and their travel times changes along the profiles. The intercept-time method (Pavlenkova, 1982) was used for the construction of the time cross sections and for the determination of reliable starting velocity models. The resulting velocity cross sections along the profiles Craton, Kimberlite, Meteorite and Rift are presented in Fig. 3. Their reliability was tested by their comparison in the cross points of profiles (Fig. 1); they show good agreement in the velocities at the level of 0.05 km s−1 at depths between 50 and 200 km and 0.1 km s−1 in the deeper part. The upper mantle is shown to be of layered structure, with reflecting boundaries at depths of about 100, 150, 240, and 320 km and velocities changing from 8.2 to 8.7 km s−1. The boundaries are not simple discontinuities, but heterogeneous (thin layering) zones, which can be associated with rheological boundaries (Pavlenkova, 2011). Solovyeva et al. (1994) note that xenoliths taken from the depths of some seismic boundaries have indications of film melting. We use only average mantle velocity structure. No low velocity zone has been detected in the central part of the SC at depths greater than 100 km. Crossed seismic profiles show the lack of velocity anisotropy in the upper mantle. Fig. 4 shows some examples of P-velocity sections from the 2-D models (Fig. 3) compared with the AK135 reference model and velocities calculated here for garnet peridotite xenoliths from the Devonian Udachnaya pipe (Ionov et al., 2010). The regional velocities are faster than in the standard model up to 250–300 km depth. In general, velocities for xenoliths are located between the global and regional velocities. Seismic velocities increase both lateral and vertical from the west to the east for the Craton and Kimberlite profile (Figs. 3a, b and 4a) and from the NW to the SE for the Meteorite and Rift profiles (Figs. 3c, d and 4b). Cross-cutting profiles Kimberlite and Meteorite suggest that the observed velocities have a significant isotropic component since

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VP, km s-1 8,00

100

4.1. 1-D temperature profiles Let us consider first the Oreshin et al. (2002) seismic model (Fig. 5a). The ТР profiles deduced for garnet harzburgite and lherzolite are shown in Fig. 5b–d. The physically unfeasible drop in temperature (Fig. 5b, c) is associated with a high-velocity layer (Fig. 5a). If we combine the Craton and Kimberlite models, we get the monotonous velocity profile from 100 to 250 km depth because the high-velocity layer at depths between 120 and 150 km would be eliminated. As a result, we will get the monotonous T profile (Fig. 5d). The ТР profiles (Fig. 5b–d) pass at substantially lower temperatures compared to the average continental geotherm Т Р (AK135) but close to the 40 mW m − 2 conductive

a)

1100 km 1900 km

H, km

2300 km

200

300

U283 U260 U501 U1147 U64 U85 U148 U183 U503 U50

AK 1

35

400

VP, km s-1 8,00

100

8,20

8,40

8,60

b) AK 1

35

200 Meteorite

300 800 km 1200 km 2000 km

400

VP, km s-1 8,00

4. Results: Thermal and density structure of the Siberian craton lithospheric mantle

8,20

100

H, km

We converted to temperature and density P velocity models (Figs. 3 and 5a) together with the compositional models listed in Table 1. Temperature for the surrounding mantle (TP(AK135)) is inferred from the AK135 model using the PM composition; TP(AK135) for depleted GP and fertile PM compositions are similar because the compositional effect on the derived temperatures plays a secondary role (Fig. 5b). Since the AK135 model shows low wavespeeds compared with regional seismic models (Figs. 4 and 5a), temperatures derived for the surrounding mantle would be much higher than those from the regional models. Figs. 3–10 show how differences in the seismic structure and composition affect the thermal state and density of the mantle. Basic limitations imposed by the seismic data are due to the small number of PNEs, covering a small area of the region (Fig. 1). The present-day thermal regime of cratonic mantle is compared with the Н(Р)–Т parameters of garnet peridotite xenoliths from the Mir, Udachnaya and Obnazhennaya pipes. For reference, we show a potential 1300 °C adiabat, which is thought to be a reasonable estimate of temperature of the asthenosphere (Poudjom Djomani et al., 2001; Sleep, 2005). The thickness of the thermal boundary layer (TBL), containing conductive lid and transition layer, is defined by the depth of the intersection of the cratonic geotherm with the mantle adiabat (Eaton et al., 2009).

8,50 Craton

H, km

the shot points M2 and K2 located in the neighborhood (see Fig. 1) reveal similar velocities at depths of 100 to 140 km (Fig. 4c). For comparison, we show a model of Oreshin et al. (2002), who produced a combined analysis of SKS splitting and regional P travel times at several stations in the Siberian platform (Fig. 5). Oreshin et al. (2002) found that the upper mantle between the Moho and 150 km depth is responsible for not more than about 30% of the large-scale effect in the SKS phase. The major effect is accumulated in a broad low-velocity zone, the top of which is found at a depth of 150 km. Their Craton and Kimberlite profiles (Fig. 5a), unlike the models shown in Fig. 4, contain a high-velocity layer at depths of ~ 120–150 km where the fastest P velocities must be close to the isotropic values and a low-velocity zone at depths of 148–248 km where the observed Р-velocities (8.30–8.35 km s−1) are identical for both profiles; according to the Oreshin et al. anisotropy within this zone can be caused by recent mantle flow. This creates a paradox because at depths of 148–248 km we can see that seismic observations are in good agreement with calculated isotropic velocities of garnet peridotite xenoliths. On the contrary, at depths of 120–150 km the Oreshin et al. isotropic velocities (8.48–8.53 km s−1) are inconsistent with isotropic velocities calculated for garnet peridotite xenoliths from the Udachnaya pipe and with ultrasonic measured velocity and anisotropy in garnet peridotite xenoliths by Long and Christensen (2000), who found VP of 8.3 km s− 1 (~ 5% anisotropy) at ~ 130 km (4 GPa, 950 °C). As it is seen from Fig. 5a, there is no evidence in the xenolith results to support such a large velocity increase. Reaching isotropic velocities in this depth range requires a temperature of about 400–500 °C (Fig. 5b, c), that is too low to be realistic.

159

200

8,40

8,60

c) AK 1

35

300 Meteorite, M2 Kimberlite, K2

400 Fig. 4. P-velocity sections in the mantle of the Siberian Craton (see Fig. 3) compared with the AK135 model. (a) The Berezovo–Ust'-Maya (Craton) profile at the distances of 1100 km (the shot C2, the west edge of the SC), 1900 km (the central part) and 2300 km (the shot C3, the Daldyn–Alakit kimberlite field); (b) the Meteorite model at the distances of 800 km (close to the shot M1, the NW edge of the SC), 1200 km (the shot M2) and 2000 km (the shot M3, pre-Baikal region, the South East of SC); (c) the Meteorite model (the shot M2) and the Kimberlite model (the shot K2). Effect of the specific composition of granular and sheared garnet peridotite xenoliths from the Udachnaya pipe on the velocities is shown by the symbols; equilibration P–T parameters for garnet-bearing samples were estimated using Opx–Gar barometer and Cpx–Opx thermometer by Ionov et al. (2010).

geotherm through the domain of Н(Р)–Т parameters for xenoliths from the Udachnaya pipe. Note that geotherms calculated using a strongly depleted composition (Hzb) and a somewhat depleted composition (Lh) show a difference of less than ~30 °C (Fig. 5d). The intersection of a cratonic geotherm for any of the depleted composition with the mantle adiabat occurs at ~ 1400 °C and ~ 230 km (Fig. 5b–d). As it is seen, the ТР profiles support the conclusion of the existence of cratonic mantle with a characteristic thickness ca. 230 km, which is consistent with the observations from surface waves (Pasyanos, 2010; Priestley et al., 2006). However at these Н(Р)–Т conditions, the cratonic mantle is 2.8–3.3% (ρ(Hzb) = 3.328 and ρ(Lh) = 3.345 g cm−3 at 7.45 GPa and 1400 °C) less dense than the AK135/PREM mantle density at the same depth. These calculations show that the cratonic mantle would be significantly buoyant relative to the surrounding mantle.

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a)

VP, km s 8,20

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H, km

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Oreshin et al./Kimberlite

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a)

)

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Low-T xenoliths High-T xenoliths Harzburgite AK135_PM AK135_GP

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1400 t o adiaba 1300

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32

TP, oC

TP,oC

b)

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AK

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100

)

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H, km

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o C adiabat 1300

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Fig. 6. Upper mantle temperatures derived from the P-velocity models shown in Figs. 3 and 4. (a) TP along the Craton profile at different distances, (b) TP along the Meteorite profile (close to the shot M1) and 2000 km (the shot M3), (c) TP along the cross-cutting Meteorite (the shot M2) and Kimberlite (the shot K2) profiles. H(P)–T parameters for low- and high-temperature xenoliths (stars) from the Mir, Udachnaya and Obnazhennaya kimberlite pipes (Glebovitsky et al., 2001; Ionov et al., 2010; Solovyeva et al., 1994) are shown for comparison. For other definitions see legend in Figs. 4, and 5. Intersection of the geotherms with the 1300 °C adiabat corresponds to a depth of the TBL ca. 300 km.

200 250 300

Craton+Kimberlite profiles

35

40

Harzburgite Lherzolite

Fig. 5. Craton and Kimberlite models; the Oreshin et al. (2002) P-velocity models (a) and predicted temperatures along the Craton (b), Kimberlite (c) and Craton + Kimberlite (d) profiles for the garnet harzburgite and lherzolite compositions (marked by the triangles) from the Yakutsk kimberlite province (Table 1). Open and filled stars are the P–T parameters for low- and high-temperature xenoliths of garnet peridotites from Udachnaya (Boyd et al., 1997; Ionov et al., 2010; Shimizu et al., 1997; Solovyeva et al., 1994). The dashed lines are the continental geotherms corresponding to a surface heat flow of 35, 40, and 50 mW m−2 (Deen et al., 2006). The range of temperatures at 100 and 150 km depth (Artemieva and Mooney, 2001) is marked by the squares in (d). Upper mantle velocities beneath southern Africa (SATZ, Zhao et al., 1999) and velocities of xenoliths (see Fig. 4) are shown for comparison in (a).

In Fig. 6a–c we demonstrate some examples of selected geotherms derived from seismic models (Figs. 3 and 4) and calculated for the depleted GP model at depths of 100–180 km, and for the fertile PM composition at greater depths. It is likely, that the kink in the geotherm (Fig. 6a) is due to the uncertainty in the velocity–depth gradient for the Craton model at a distance of 1100 km (Fig. 4a). Converting mantle P-velocity along the Meteorite and Kimberlite profiles for shot points M2 and K2 located in the neighborhood and close to the craton's center (Fig. 1) reveals the same temperatures at depths between 100 (TP ~ 560 °С) and 140 km (TP ~ 700 °С), Fig. 6c. Similar temperatures were obtained from surface heat-flow data (Artemieva, 2006; Artemieva and Mooney, 2001). The average values of the thermal gradient (3.5–4.7 °C km−1) are 1.5 times lower than the paleotemperature gradients for ancient cratons (Glebovitsky et al., 2001). The TP profiles lie below the Н(Р)–Т estimates for the low- and high-temperature garnet peridotites from Yakutian kimberlite pipes and substantially

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Fig. 8. Compositional effects on the density distribution along the Meteorite profile based on temperatures shown in Fig. 7. Upper panel is the density values calculated for average garnet peridotite (GP) at depths between 100 and 300 km; this model assumes a single composition for the entire mantle. Lower panel is the density values calculated for the GP composition at depths between 100 and 180 km and for the PM composition at greater depths. The increase in density at 180 km corresponds to a change in composition.

4.2. 2-D temperature and density profiles As was discussed above, fertility can increase gradually or in a stepwise manner with depth. Because the composition has a secondary effect on the seismic velocities (Fig. 2a, b) and the estimated temperatures (Fig. 5b–d), we approximate here a multi-layer structure of cratonic mantle (Ashchepkov et al., 2010) by the depleted GP composition at depths of 100–180 km and, assuming a chemical (petrological) boundary layer (Afonso et al., 2008; Forte and Perry, 2000; King, 2005; Lee et al., 2005; O'Reilly and Griffin, 2010; Poudjom Djomani et al., 2001), by the fertile PM composition at greater depths. 2-D upper mantle velocity models along the Craton, Kimberlite, Rift and Meteorite profiles (Fig. 3) were converted to the 2-D temperature (Fig. 7) and 2-D density profiles (Figs. 8 and 9). Thickness of the thermal boundary layer is estimated to be 300 ± 30 km; temperature at this depth is close to the 1450 °С isotherm (Fig. 7). Density variations in the mantle are due to changes in temperature and composition; an increase in density at a depth of 180 km corresponds to a change in composition from the GP to PM. Fig. 7. 2-D temperature models in the mantle beneath the Siberian Craton along the profiles Craton (a), Kimberlite (b), Meteorite (c) and Rift (d) based on the conversion of seismic models shown in Fig. 3. The composition of the mantle is approximated by the GP composition at depths between 100 and 180 km and by the PM composition at greater depths. Letters indicate location of the shots. The black dots indicate the intersection of temperature profiles with the potential 1300 °C adiabat. The depth of the TBL is close to the ~1450 °C isotherm and is estimated as 300 ± 30 km for all profiles.

lower than the ТР(AK135). The present-day seismic geotherms pass close to the 32.5–35 mW m−2 conductive models and intersect mantle adiabat at ca. 300 km depth and ~ 1450 °C (Fig. 6). From garnet thermobarometry (Ashchepkov et al., 2010; Griffin et al., 1996) and shear wave seismic tomography (Deen et al., 2006), the temperatures under the SC can also be estimated close to the 32.5–35 mW m− 2 geotherms.

5. Discussion 5.1. Thermal structure Differences between seismic models (Figs. 4 and 5a), caused by processing technique translate into large temperature variations and different thickness of the TBL. It is seen that the low-temperature anomalies in the deep and cold lithospheric root penetrate at least to a depth of 230 km (Fig. 5b–d), and possibly as much as 300 km (Figs. 6 and 7). It should be emphasized that the geotherms calculated for various petrological models (Hzb, Lh, GP and PM) differ from each other by less than 50 °С (Figs. 5 and 6), which is associated with a minor impact of the composition on the seismic velocities (Fig. 2). This means that the discrimination of fine differences in the composition of the mantle by seismic methods only does not seem possible. Jones et al. (2009) based on mineral physics data found that a contrast between a

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Density, g cm-3 3,30

3,35

3,40

3,45

100

3,50

PM

H, km

150 200 PM _1

250 300

GP (35 mW) PM (35 mW) PM (40 mW) PREM AK135

30

0 oC

GP

Fig. 10. Comparison of the AK135 and PREM model density with densities for the GP and PM compositions calculated along the conductive geotherms of 35 and 40 mW m−2. Effect of specific composition of garnet peridotite xenoliths from the Udachnaya pipe (symbols are shown in Fig. 4) on the evaluated densities is shown for comparison. At about 300 km depth, the density of the PM composition calculated along the 1300 °C adiabat with the gradient of 0.465 °C km−1 (PM_1300 °C) and the AK135/PREM model density converge.

Fig. 9. Modeled 2-D density distribution in the Siberian mantle along the profiles Craton (a), Kimberlite (b), Meteorite (c) and Rift (d) based on temperatures shown in Fig. 7; see text for details. At the base of the TBL, the PM density ρ(10.2 GPa/1450 °С) = 3.49 g cm−3 is consistent with the AK135 and PREM model density (ρ(310 km) = 3.49 g cm−3).

harzburghitic and lherzolitic mantle should not be detectable seismically, and only marginally in conductivity. This implies that the thickness of the cratonic mantle does not depend significantly on the composition. On average, there is a systematic decrease in temperature from the west to the east for the Craton and Kimberlite profiles and from the NW to the SE for the Meteorite and Rift profiles (Fig. 7). Lateral temperatures within the root vary appreciably at depths up to ca. 200 km reflecting somewhat different thermal state along all profiles. At greater depths, lateral changes in temperatures carry an insignificant effect implying that the inferred thermal heterogeneity diminishes rapidly below 250 km. At a depth ca. 300 km, the derived temperatures provide similar estimates. Within the uncertainty of the analysis the craton's center is somewhat colder than its marginal parts. The temperature profiles exhibit a substantial decrease in temperature beneath the SC as compared with the average temperature in the surrounding mantle. For example, the 900 °С isotherm under the SC lies at depths of

150–200 km (Fig. 7), while according to the AK135 model, this temperature corresponds to a depth of ~90 km (Fig. 6). Temperatures inferred from the AK135 model for an average mantle are ~300–400 °C higher than temperatures beneath the ancient craton (Figs. 5 and 6). AK135 model producing maximal TP at ~220 km reveals inflection with a negative gradient at depths below ~220 km, leading to non-physical behavior of the temperature. This can be explained by the fact that at depths between 210 and 300 km seismic gradient in the AK135 model is two times greater than P-velocity gradient (ΔVP/ΔH ~ 0.0017 s−1) in the regional models. Such a rapid growth of velocities in the global model results in a decrease of temperature with depth. The difficulties in interpreting the reference model in terms of temperature and composition has been pointed out (Cammarano et al., 2003; Kuskov et al., 2006). Temperature at the base of the TBL (300 ± 30 km) is close to the 1450 °С isotherm for all profiles (Fig. 7). Assuming the adiabatic gradient in the asthenosphere, this temperature estimate is consistent within uncertainty with temperatures at the 410-km discontinuity found by Khan et al. (2013) beneath Australia and by Katsura et al. (2010) from experimental phase transitions in the (Mg,Fe)2SiO4 system. These results are in accord with the values of the cratonic root thickness estimated from heat flow observations (Artemieva, 2009), thermobarometry data (Ashchepkov et al., 2010), tomographic model (Bushenkova et al., 2002) and Q reduction in the depth range of 250 to 300 km depth, determined from the mantle wave spectra (Egorkin and Kun, 1978). The lowest part of the cratonic root must not differ strongly in physical and chemical characteristics from the adjacent convecting mantle. Temperatures of the cratonic mantle are much lower than the average temperature for the surrounding mantle and pass near the geotherms of 32.5–35.0 mW m−2 (Fig. 6), that results in the increase in the thickness of the mantle beneath the Siberian craton up to 300 km (Fig. 7). Ashchepkov et al. (2010) found that the Yakutsk kimberlite field has temperatures that are close to a 35 mW m− 2 geotherm, which intersects the mantle adiabat at ca. 300 km depth. A similar conclusion may be inferred from Deen et al. (2006), where the inversion of seismotomographic data yielded estimated geotherms of 32.5–35 mW m− 2 over most of the central Archean parts of the SC. This means that although the applied approaches and methods are different, the results obtained are mutually consistent. Correspondingly, the present-day cratonic mantle is characterized by lower temperatures than the ancient mantle (e.g., Arndt et al., 2009; Jaupart and Mareschal, 2007). For the Yakutsk kimberlite province, the heat flow values are estimated at the level of 20–30 mW m−2 (Duchkov and Sokolova, 1997), which indicate that temperatures under this part of the craton are the

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lowest among Precambrian provinces. The present-day thermal regime of the mantle can be estimated from the temperature slopes shown in Fig. 6. With the average T gradient of 3.5–4.7 K km−1 at depths of 100–300 km and with an average thermal conductivity of 3–4 W m−1 K−1, we get an upper mantle heat flow value of 11–17 mW m−2. For major Precambrian provinces, the typical estimates of mantle heat flow based on various assumptions are within 11–20 mW m−2 (Artemieva and Mooney, 2001; Jaupart and Mareschal, 2007; Kronrod and Kuskov, 2007; Rosen et al., 2009; Roy and Mareschal, 2011; Rudnick et al., 1998). 5.2. Density structure Effect of the composition on the 2-D density distribution along the long-range profiles based on the derived temperatures (Fig. 7) is shown in Figs. 8 and 9. Note that lateral density variations in Figs. 8 and 9 are due to thermal rather than compositional anomalies. Fig. 8 (upper panel) demonstrates that at the base of the TBL density of the depleted GP composition (ρ(310 km, 1450 °С) ~ 3.42 g cm−3) is 2% less dense than the AK135/PREM model density at the same depth. In other words, density of the depleted cratonic mantle is globally reduced up to 300 km. The density curves show that if the Siberian mantle consisted of highly depleted composition, it would be too buoyant to represent the composition of the whole cratonic mantle and satisfy the isostasy and geoid constraints (Forte and Perry, 2000; Kaban et al., 2003; Mooney and Vidale, 2003; Poudjom Djomani et al., 2001; Sleep, 2005). On the other hand, Fig. 8 (lower panel) shows that density of the fertile PM composition at the base of the TBL (ρ(310 km, 1450 °С) ~ 3.49 g cm − 3 ) is consistent with the AK135/PREM model density. This density contrast of 2% is equivalent to temperature contrast of ~500 °С. If cratonic mantle would have had a depleted composition with a density of 3.49 g cm− 3, the temperature at the base of the TBL would be ~950 °С. Such a cold mantle does not correspond to either thermal models (Artemieva and Mooney, 2001; Duchkov and Sokolova, 1997) or thermobarometry estimates for peridotite xenoliths (Ashchepkov et al., 2010; Ionov et al., 2010). Moreover, a temperature contrast of 500 °С must lead to a significant increase in P-velocity (VP(GP, 310 km/950 °С) ~ 8.73 km s−1) that is not observed in global and regional seismic models, according to which VP(310 km) ~ 8.5– 8.6 km s−1 (Figs. 3 and 4). Thus, the cratonic mantle is chemically stratified and becomes more fertile in composition with depth. Our analysis shows that the density variations in the lower part of the root due to the chemical composition are greater than those caused by temperature (Figs. 7–9). Such a cratonic keel model reconciles petrological and geophysical evidence and is in qualitative agreement with other studies (Afonso et al., 2008; Arndt et al., 2009; King, 2005). Pronounced variations in density both lateral and vertical (Figs. 8 and 9) show that the PM density at the base of cratonic mantle (~300 km) is consistent with the average density of the ambient mantle in accordance with the AK135/PREM models at the same depth. However as noted by O'Reilly and Griffin (2010), the refertilized zone may still constitute an intact lithospheric root, cooler and somewhat less fertile than the surrounding mantle. Nonetheless, all of the density and temperature profiles are fairly similar at about 300 km (Figs. 7 and 9), assuming that at this depth the mantle under Siberia is not appreciably distinct from the underlying asthenosphere. Fig. 9 shows the mantle density models beneath the Siberian craton and West-Siberian Plate (see Fig. 3). The lateral changes of density are due to thermal anomalies derived from the seismic cross sections (Fig. 3). In reality, the cratonic mantle differs in density from surrounding geostructures. It follows from the 100 mGal negative gravity anomaly observed in the craton area. The gravity modeling (Kaban et al., 2003; Pavlenkova and Romanyuk, 1991) shows that the average upper mantle density should be lower beneath the Siberian craton than beneath the West-Siberian Plate, assuming either thermal or compositional lateral heterogeneity,

163

or both. A more thorough analysis requires additional data on seismic and gravity modeling. Densities for depleted and fertile compositions, calculated within a typical range of cratonic geotherms in comparison with the AK135/ PREM density are illustrated in Fig. 10, which shows that the depleted compositions are less dense while the fertile PM composition is denser than the “seismic” density at least at depths of 100–200 km. At these depths, the Siberian mantle has lower temperatures, higher velocities, and lower densities than the surrounding mantle. The PM density calculated along the mantle 1300 °C adiabat gradually increases and at about 300 km depth approaches the AK135/PREM model densities. The basic conclusion arising from Fig. 10 is that the results seem unable to explain the reasonable density distribution for any uniform composition (either depleted or fertile) throughout the entire cratonic mantle. A comparison of densities for granular and sheared garnet peridotites from the Udachnaya pipe shows that seismic density is more than 2% denser than that of garnet peridotites. It is likely that these results admit the compositional variations and an appreciable increase in fertility with depth and assume that the mantle beneath the Siberian craton is chemically stratified. Note that a similar layered structure has been proposed in xenolith studies from some Archean cratons (Bizzaro and Stevenson, 2003; Bruneton et al., 2004; Kopylova and Russell, 2000; Kuskov et al., 2006; Lee and Rudnick, 1999). The more fertile material at the base of the root, where the lithosphere and the asthenosphere have about the same temperature, must not differ strongly in physical and chemical characteristics from that of adjacent convecting mantle. As it is seen from Figs. 9 and 10, at the base of the TBL density of the PM composition (ρ ~ 3.49 g cm−3) is consistent with the AK135 and PREM density (ρ(310 km) ~3.49 g cm−3). Note however, that seismic densities are poorly constrained. 6. Conclusions The internal structure of the non-convecting mantle must be highly heterogeneous, because the ancient cratons were formed as a result of the collision and accretion of numerous micro-continents transformed into deformed terranes. Its structure keeps a memory about essential vertical and lateral contrasts in thermal regime, composition and physical properties (density, viscosity, seismic velocities, etc.). Using a thermodynamic-geophysical approach, we map the 2-D seismic, thermal and density structure of the mantle beneath the Siberian craton along the long-range seismic profiles Craton, Kimberlite, Rift and Meteorite. The approach yields the self-consistent information on temperature–composition–density–velocity–depth profiles and provides the more reliable constraints on the thermo-chemical structure of cratonic mantle from combining seismic models with mineral physics data. However interpretation of a multi-layer structure of the mantle keel will always remain somewhat speculative. On the basis of a joint analysis of regional seismic models, xenolith-based constraints and a thermodynamic-based inversion scheme the following conclusions can be made. (1.) Structural peculiarities of the cratonic mantle are manifested by changes in seismic velocities, the degree and nature of layering and the relief of seismic boundaries. Our models predict appreciable lateral temperature variations to a depth of about 200 km, which are the main cause of seismic velocity variations; the craton's center is somewhat colder than its marginal parts. We find that the cratonic mantle is 300–400 °C colder than the tectonically younger average mantle in this depth range. At greater depths, lateral changes in temperatures carry an insignificant effect implying that the derived thermal heterogeneity decreases rapidly. The present-day geotherms pass close to the 32.5–35 mW m−2 conductive models, are substantially lower than the average continental geotherm ТР(AK135), and suggest a low mantle heat flow, 11–17 mW m−2.

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(2.) Within the model resolution, the thickness of the thermal boundary layer beneath Siberia does not depend significantly on the composition and can be estimated as 300 ± 30 km; temperature at the base of the TBL is close to the 1450 ± 100 °C isotherm. At a depth of ~300 km, lateral changes in velocities, temperatures and densities have little effect, assuming that the cratonic mantle is not appreciably distinct from the underlying convective mantle. At the base of the TBL, the calculated density is consistent with the PREM model density. (3.) Changes in the composition from depleted to fertile material reveal a negligible effect on seismic velocities, but remain the most important factor for the density increase of the cratonic root. Discrimination of fine differences in the composition of cratonic mantle as well as a location of the petrologically distinct layers (the chemical boundary layer and the lithosphere– asthenosphere boundary) by seismic methods only does not seem possible. Density variations in the lower part of the root due to the chemical composition are greater than those caused by temperature. We find that both compositional and thermal anomalies are required to explain the Siberian mantle by a keel model consisting of depleted garnet peridotite at depths of 100 to 180 km and more fertile material at greater depths. The results suggest that the mantle beneath the Siberian craton is chemically stratified. Acknowledgments Reviews by Nicholas Arndt, Amir Khan, and the anonymous reviewers helped improve the presentation and clarify the discussion. We thank Hans Thybo for the helpful comments. This research was supported by the Russian Academy of Sciences under Programs 22 and 28 and by RFBR grant 12-05-00033. Appendix A A correct thermal interpretation must account for the dissipative effects due to anelasticity especially at depths of the mantle where temperature approaches the solidus. Following many authors (e.g., Anderson, 1989; Cammarano et al., 2003; Sobolev et al., 1996), the quality factor Q S, that is the inverse of seismic attenuation, can be calculated from α

Q S ðP; T; ωÞ ¼ A1 ω expðαgT m ðР Þ=T Þ;

ð1Þ

where А1, α, g are dimensionless parameters, ω is the seismic frequency fixed at 1 Hz; α = 0.2 and g = 30. Velocity, with account for both anharmonic and anelastic effects, can be expressed as (Goes et al., 2000; Sobolev et al., 1996): h i V anel ðP; T; X; ωÞ ¼ V anh ðP; T; X Þ 1–1=2Q P;S ðP; T; ωÞtanðπα=2Þ ;

ð2Þ

where X stands for the bulk composition model and Vanh(P, T, X) are self-consistent anharmonic velocities. The solidus temperature (Tm) of peridotite up to 10 GPa was taken from Hirschmann (2000). Despite large uncertainties exist in anelasticity model (Cobden et al., 2008; Durek and Ekström, 1996; Matas and Bukowinski, 2007; Montagner and Kennett, 1996) based on current knowledge, this procedure is justified. The A1 value has a significant effect on the results of retrieving the temperature from seismic models (Cammarano et al., 2003; Kronrod and Kuskov, 2007; Shapiro and Ritzwoller, 2004; Sobolev et al., 1996). The value of QS calculated for given А1 and g should be qualitatively consistent with the seismic models, which prescribe QS to be less than 100 at depths of 100–200 km, and less than 200 at depths of 200–400 km (Durek and Ekström, 1996; Dziewonski and Anderson, 1981; Kennett et al., 1995). Based on these data and assuming that the profile of the AK135 reference model and an average continental

geotherm of 50–55 mW m− 2 (Artemieva, 2009; Deen et al., 2006) should be mutually consistent, we apply the depth-dependent values of the parameter А1 with an exponential depth dependency up to the lower boundary of cratonic mantle (Kronrod and Kuskov, 2007): А1 ¼ m1 exp½m2 ðH‐80Þ=ðHlit ‐80Þ ðH ≤H lit Þ; А1 ¼ Alit ðH NHlit Þ;

ð3Þ

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