Petrophysics

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Petrologic interpretation of geoelectric models

       Recent active development of methods of geoelectrics and petrophysics provides new opportunities for interpretation of deep geoelectric models. Petrologic interpretation of geoelectric models contemplates the “filling” of deep horizons of models with low-crust and upper-mantle rocks by selecting rocks with necessary dependence of electroconductivity on temperatures and by fulfilling the following conditions:

  1. Distribution of electroconductivity depending on modern thermal field in deep horizons of model should correspond to results of laboratory measurements of electroconductivity depending on temperature on examined deep-seated rock samples.
  2. The depth of interpreted horizons in model and their position with regard to the regional tectonic structures should correspond to facies of metamorphism for the analyzed deep-seated rocks.
  3. Examined rocks should be representative, i.e. they should represent significant volumes of the low crust and lithosphere mantle.
  4. During interpretation of any part of the profile it is preferable in the first instance to analyze deep-seated rocks whose xenoliths outcrop to the surface in this part. If not, then data from the books should be used.

        The proposed method of interpretation of deep geoelectric models is based on laboratory measurements of electroconductivity of deep-seated rock samples under high pressures and temperatures and on petrologic analysis of deep-seated rock samples (xenoliths). The method includes construction of a paleogeotherm at the time of outcropping of xenoliths and a modern geotherm. It gives possibility of conversion of results of laboratory measurements “electroconductivity from temperature” for different values of geotherms which enables to compare these data with the data on vertical profiles of electroconductivity from depth for geoelectric models of magnetotelluric soundings.

 

      First results:

1. We have compared the values of electroconductivity from the models of lithosphere mantle of the suture zone of Tarim and Tien Shan received within the frameworks of “Geodynamics of Tien Shan” project (Bielinski et al, 2003) with the results of laboratory measurements of electroconductivity at lower-crust and upper-mantle xenoliths from this region carried out in the Goethe Universitaet Frankfurt am Main under the leadership of N.Bagdassarov (Batalev et al., 2009). We have contoured the massifs of spinel lherzolites, eclogites and granulites.

 

Геоэлектрическая структура земной коры

Fig. 1. Geoelectric structure of the Earth’s crust along МТ-profile at longitude 76º east across Tien Shan [Bielinski, et al 2003], (right panel), resistance is shown in color, deep faults according to [Thompson, et al 2002]. Distribution of deep temperatures is shown in the form of isolines. Dotted contours show the areas of supposed location of deep-seated massifs of eclogites, spinel lherzolites and granulites according to data of petrologic interpretation; left and bottom panels – results of laboratory measurements of electroconductivity of deep-seated rocks in comparison with the data from other researchers and results of MT-inversion.

 

2. Having compared the modern geotherm with the paleogeotherm (~70 million years ago) estimated according to the results of thermobarometry of lherzolite and granulite xenoliths, we ascertained that the temperature at the depth of 35-40 km decreased from 750-800ºС to 750-700ºС.

Термобарометрия эклогитов

Fig. 2. Thermobarometry of eclogites [Simonov et al. 2008], spinel lherzolites and granulites, modern and paleo-geotherms (~70 million years ago) with the values of thermal flow in mW/m2 and location of Moho discontinuity for the region of Ak-Sai depression, Kyrgyzstan.

 

3. Using the results of thermobarometry we have compared the boundary between granulites and spinel lherzolites (which usually corresponds to geophysical Moho discontinuity) with modern Moho discontinuity, and have determined that for ~70 million years the estimates of depths up to Moho discontinuity increased from 35-40 km to 55-60 km in the investigated section of lithosphere.

 

Положение границы между шпинелевыми лерцолитами и гранулитами

Fig. 3. Position of boundary between spinel lherzolites and granulites ~70 million years ago according to the data of thermobarometry of xenoliths, left panel, and the modern position of Moho discontinuity – according to the data of magnetotellurics and the thermal model, right panel.

 

Petrologic interpretation is aimed at studying questions related to electroconductivity of low-crust and upper-mantle horizons. The average value of electrical resistivity at the depths of 100-140 km is ~100 Оhm×m, which is higher than that of partial melt [Park, et al, 1996]; thus, the melt can be present only in small quantities (<1 %) or in the isolated cells. On the contrary, the water contained in the crystal lattice of olivine can decrease the electrical resistivity to 100 Оhm*m [Karato, 1990], however, the recent measurements of water content in the crystal lattice of olivine from xenoliths of spinel lherzolites of Orto-Suu by the method of infrared spectrometry in the Institute of Geology and Mineralogy SB RAS have shown its almost complete absence. That’s why the low values of electrical resistivity in the upper mantle at the depths of 100-140 km (~100 Оhm×m) should be explained by other reasons.

It is assumed that in the suture zone of Tarim and Tien Shan these horizons do not contain the quantity of free fluid enough for the formation of anomalously conductive objects.

Сопоставление результатов лабораторных экспериментов

The distinctive features of electroconductivity of these horizons are mainly determined by their material composition and temperature distribution. This assumption is based on the agreement of results of “dry experiments” of laboratory measurements of electroconductivity of samples of lherzolite and granulite xenoliths and eclogites [Batalev et al., 2008; 2009] with the values of electroconductivity of low-crust and upper-mantle horizons in the models of magnetotellutic soundings. Fig.4 shows the comparison of results received during laboratory experiments on samples of eclogites and spinel lherzolites with the values of electroconductivity on the vertical profiles of geoelectric model taken near the At-Bashy and Kok-Shaal Too mountain ranges. 

Fig. 4 Comparison of results received during laboratory experiments on samples of eclogites and spinel lherzolites with the values of electroconductivity on the vertical profiles of geoelectric model.

 

 In the temperature range from ~700-750°C to ~950-1000°C which corresponds to the depths of the lithosphere mantle, we observe the agreement of results of “dry experiments” of laboratory measurements of electroconductivity of eclogites and spinel lherzolites samples with the values of electroconductivity on the investigated sections of the vertical profiles. Thereby, the assumption about the absence of free water in this temperature (depth) range is confirmed. The correspondence of electroconductivity values of deep-seated rock samples received in laboratory conditions with electroconductivity values for the corresponding deep-seated massifs means with high probability that the massifs contoured in fig. 9 have eclogite, lherzolite and granulite composition. The additional arguments for such interpretation are the location of deep-seated eclogite massif as the continuation of the low-angle underthrust of Tarim platform under the Tien Shan crust, the location of the lherzolite massif directly under this underthrust, and the general morphology of geoelectric structures which corresponds to subduction-collision scenario (fig. 9).

      In the temperature range from ~300°C to ~700°C which approximately corresponds to the depths of the crustal conductive layer, the values of electroconductivity on the vertical profiles are almost ten times higher than the data of laboratory measurements of electroconductivity. This is clearly shown in the diagram in the form of the local maximum. This phenomenon is probably related to filling of the porous space in the rocks of the crustal conductive layer with the fluid which becomes empty during the dehydration of water-containing rocks [Vanyan, Gliko, 1999].

      Under the temperatures higher than 1000°C, the data of laboratory measurements of electroconductivity exceed the values of electroconductivity on the vertical profile, which is probably related to the phase changes occurring in the substance of the upper mantle.

5. According to the results of interpretation of magnetotelluric soundings, laboratory measurements of electroconductivity under high temperatures and pressures, elastic waves velocities and thermobarometry of the low-crust and upper-mantle xenoliths, we have ascertained that the depth of occurrence of Moho discontinuity in the southern part of the Central Tien Shan had increased from 35 to 55 km over 70 million years. And along with this, the value of thermal flow at the surface had decreased from 80 mW/m2 to 60 mW/m2 over the same period which indicates the cooling of the region’s lithosphere during this period of time.

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Fig.5. Results of estimation of geotherms, Sounthern Tien Shan, longitude 76º east; thermobarometry of granulites, lherzolites and eclogites with positioning of the Moho surface and the boundary between granulites and spinel lherzolites.

 

Photogallery

Geographic location

40 km. from Bishkek