Melting Inside the Tibetan Crust? Constraint From Electrical Conductivity of Peraluminous Granitic Melt

Abstract

Magnetotelluric and seismological studies suggested the presence of partial melts in the middle to lower Himalaya-Tibetan crust. However, the melt fractions inferred by previous work were based on presumed electrical conductivity of melts. We performed measurements on the electrical conductivity of peraluminous granitic melts with 0.16-8.4 wt % H 2 O (the expected compositions in the Tibetan crust) at 600-1,300°C and 0.5-1.0 GPa. Peraluminous melt exhibits lower electrical conductivity than peralkaline melt at dry condition, but this difference diminishes at H 2 O > 2 wt %. With our data, the observed electrical anomalies in the Tibetan crust could be explained by 2-33 vol % of peraluminous granitic melts with H 2 O > 6 wt %. Possible reasons for our inferred melt fractions being higher than seismological constraints include the following: (1) The real melts are more Na and H 2 O rich, (2) the effect of melt reducing seismic velocities was overestimated, and (3) the anomalies at some locations are due to fluids. Plain Language SummaryWhether there are molten zones present in the Tibetan crust is a focus of geophysical and petrological research. Previous interpretations of MT data to infer melt fractions are often based on presumed electrical conductivity values of partial melts. These felsic melts are believed to derive from metapelites and have peraluminous composition, but previous experimental data of electrical conductivity are only for metaluminous and peralkaline melts. In this contribution, we carry out electrical conductivity measurements on anhydrous and hydrous peraluminous granitic melts with 0.16-8.4 wt % of H 2 O at 600-1,300°C and 0.5-1.0 GPa. We find that the electrical conductivity of peraluminous melt is lower than that of peralkaline melt under dry condition, but their difference quickly diminishes at H 2 O greater than 2 wt %. Based on our experimental results, the melt fractions for various regions in the Himalaya-Tibetan crust are inferred and compared with seismological constraints.GUO ET AL. 3906

Shaping the Surface Deformation of Central and South Tibetan Plateau: Insights From Magnetotelluric Array Data

“…Accounting for temperature, pressure, and water content effects altogether, granitic melt conductivity σ f can then be calculated according to a generalized formula constrained by experimental data (X. Guo et al, 2018): where w is the water content (in wt.%), P is the pressure (in GPa), and T is the temperature (in K). J. Chen et al (2018) developed a similar empirical model for the estimation of the melt/fluid conductivity using the Arrhenius equation.…”

“…Accounting for temperature, pressure, and water content effects altogether, granitic melt conductivity σ f can then be calculated according to a generalized formula constrained by experimental data (X. Guo et al, 2018): where w is the water content (in wt.%), P is the pressure (in GPa), and T is the temperature (in K). J.…”

“…Experiments show that water content dissolved in melts can reach as high as at least 10 wt.% at 1 GPa and 900 °C (dacite, Prouteau et al, 1999) or 15 wt.% at 1 GPa and 1000 °C (rhyolite, Duan, 2014). Here we take an upper limit value of 9 wt.% for southern Tibet, as suggested by Guo et al (2018).…”

Electrical Conductivity of Clinopyroxene‐NaCl‐H2O System at High Temperatures and Pressures: Implications for High‐Conductivity Anomalies in the Deep Crust and Subduction Zone

“…In contrast, geophysicists have shown that the high‐conductivity anomalies in the Earth's lithosphere can be caused by the presence of water in nominally anhydrous minerals (Dai & Karato, 2014; Karato, 2019; H. Y. Liu et al, 2019; Wang et al, 2006; Yang et al, 2011), interconnected aqueous fluids (Amiguet et al, 2012; X. Z. Guo et al, 2015; Li et al, 2018; Manthilake et al, 2015, 2016; Shimojuku et al, 2014; Sinmyo & Keppler, 2017), partial melting (Freitas et al, 2019; Gaillard, 2005; X. Guo et al, 2018; Laumonier et al, 2015, 2017; Maumus et al, 2005; H. W. Ni et al, 2011), and interconnected secondary high‐conductivity phases (Bagdassarov et al, 2009; Glover et al, 1996; Kawano et al, 2012; Manthilake et al, 2016; Wang et al, 2013; Zhang & Yoshino, 2016). Despite multiple interpretations for the high‐conductivity anomalies in the deep Earth, the practical origin in a certain geological environment can be constrained based on the MT and seismic data.…”

“…Despite multiple interpretations for the high‐conductivity anomalies in the deep Earth, the practical origin in a certain geological environment can be constrained based on the MT and seismic data. Generally, low seismic waves are also a characteristic of a deep region with a high‐conductivity anomaly (Chen et al, 2009; Gonzalez‐Castillo et al, 2019; X. Guo et al, 2018; Naif et al, 2013). Although aqueous fluids and melts can reconcile the observed anomalous electrical conductivity and seismic waves (Marquis & Hyndman, 1992; Naif et al, 2013), aqueous fluids are more stable than melts in regions with a low geothermal gradient.…”