P. Schmoldt, PhD - MTNet - DIAS
P. Schmoldt, PhD - MTNet - DIAS P. Schmoldt, PhD - MTNet - DIAS
5. Earth’s properties observable with magnetotellurics Fig. 5.5.: Typical electric conductivity structures below a continental shield (solid line) and oceanic lithosphere (dashed line); from Heinson [1999] Earth’s conductivity structures, different models have been proposed (e.g. Figs. 5.5 and 5.6). The different models are based on (long-term) Earth bound MT and GDS measurements [e.g. Bahr et al., 1993; Schultz et al., 1993; Olsen, 1998; Heinson, 1999; Neal et al., 2000; Utada et al., 2003; Tarits et al., 2004; Kuvshinov et al., 2005], satellite supported geomagnetic experiments [e.g. Kuvshinov and Olsen, 2006, 2008], laboratory studies on Earth’s materials [e.g. Xu et al., 1998a, 2000a; Nover, 2005; Yoshino et al., 2008], and theoretical considerations [e.g. Ledo and Jones, 2005; Jones et al., 2009]. More recent models, incorporating step like changes of conductivity, are favourable over previous smooth models as they have been proven to demonstrate better agreement with assumed phase changes within the Earth. Phase changes of Earth materials are a consequence of moving across boundaries in the P-T space. Lab studies (Sec. 5.3) aim to derive the conditions at which these phase changes occur for certain materials and their results can be used to guide interpretations of magnetotelluric and seismic investigations. The depth of the different step changes for certain regions of the Earth is strongly dependent on the geological history of the region; today it is commonly assumed that interfaces of the mantle transition zone (MTZ) and below are comparatively flat, whereas shallower interfaces are thought to exhibit a more pronounced topography. This conclusion, however, is certainly biased by the reduced resolution of deep-seated features. 88
5.2. Variation of electric conductivity with depth Fig. 5.6.: Collection of electric conductivity-depth profiles; from Yoshino et al. [2008]. Orange and blue regions represent geophysically observed conductivity profiles in the Pacific (Ref. A) [Kuvshinov et al., 2005], and the continental mantle (Ref. B) [Olsen, 1998], (Ref. C) [Tarits et al., 2004], and (Ref D.) [Neal et al., 2000], respectively. The thick solid line represents the electric conductivity of olivine, wadsleyite and ringwoodite without water, whereas dashed lines indicate the electric conductivity of hydrous olivine, wadsleyite and ringwoodite as a function of water content (red: 1.0 wt%; green: 0.5 wt%; blue: 0.1 wt%), all derived through lab experiments by Yoshino et al. [2008]. The light green solid line denotes the results of previous experimental studies by Xu et al. [1998a]. 5.2.1. The Earth’s crust The Earth’s crust is petrological defined as the area above the peridotitic mantle usually exhibiting a thickness of 5 – 7 km (oceanic crust) or 30 – 50 km (continental crust). In regions with basaltic or non-existing underplating the crust–mantle boundary coincides with the Mohorovičić discontinuity (commonly referred to as Moho). The Moho was first identified by Mohorovičić [1910] using refracted waves from the 1909 shallow-focus earthquake near Zagreb to determine the existence of a medium with higher velocity at depth. Today, the Moho is seismologically defined by a significant increase in velocity, with values changing usually from approximately 6 km/s to 8 km/s (P-waves) and from approximately 3.5 km/s to 4.5 km/s (S-waves); however, velocity values may vary for certain regions of the Earth. Chemically the Moho is defined by a change from felsic (continental crust) or mafic (oceanic crust) to ultra-mafic materials. For Archean regions with komatiitic (ultramafic) underplating the Moho can be sensed at significantly shallower depth than the petrologically defined crust–mantle boundary. The Moho usually coincides with a change in density from around 2.7 g/cm 3 (continental crust) or 2.9 g/cm 3 (oceanic crust) to around 3.2 g/cm 3 . An identification of the Moho using electric methods is more difficult, since the changes from felsic or mafic material to ultra-mafic material 89
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5.2. Variation of electric conductivity with depth<br />
Fig. 5.6.: Collection of electric conductivity-depth profiles; from Yoshino et al. [2008]. Orange and blue regions represent geophysically<br />
observed conductivity profiles in the Pacific (Ref. A) [Kuvshinov et al., 2005], and the continental mantle (Ref. B) [Olsen,<br />
1998], (Ref. C) [Tarits et al., 2004], and (Ref D.) [Neal et al., 2000], respectively. The thick solid line represents the electric conductivity<br />
of olivine, wadsleyite and ringwoodite without water, whereas dashed lines indicate the electric conductivity of hydrous<br />
olivine, wadsleyite and ringwoodite as a function of water content (red: 1.0 wt%; green: 0.5 wt%; blue: 0.1 wt%), all derived through<br />
lab experiments by Yoshino et al. [2008]. The light green solid line denotes the results of previous experimental studies by Xu et al.<br />
[1998a].<br />
5.2.1. The Earth’s crust<br />
The Earth’s crust is petrological defined as the area above the peridotitic mantle usually<br />
exhibiting a thickness of 5 – 7 km (oceanic crust) or 30 – 50 km (continental crust). In<br />
regions with basaltic or non-existing underplating the crust–mantle boundary coincides<br />
with the Mohorovičić discontinuity (commonly referred to as Moho). The Moho was<br />
first identified by Mohorovičić [1910] using refracted waves from the 1909 shallow-focus<br />
earthquake near Zagreb to determine the existence of a medium with higher velocity at<br />
depth. Today, the Moho is seismologically defined by a significant increase in velocity,<br />
with values changing usually from approximately 6 km/s to 8 km/s (P-waves) and from<br />
approximately 3.5 km/s to 4.5 km/s (S-waves); however, velocity values may vary for<br />
certain regions of the Earth. Chemically the Moho is defined by a change from felsic<br />
(continental crust) or mafic (oceanic crust) to ultra-mafic materials. For Archean regions<br />
with komatiitic (ultramafic) underplating the Moho can be sensed at significantly shallower<br />
depth than the petrologically defined crust–mantle boundary. The Moho usually<br />
coincides with a change in density from around 2.7 g/cm 3 (continental crust) or 2.9 g/cm 3<br />
(oceanic crust) to around 3.2 g/cm 3 . An identification of the Moho using electric methods<br />
is more difficult, since the changes from felsic or mafic material to ultra-mafic material<br />
89
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