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A model of electrical conductivity in the mid-mantle transition zone was obtained with improved constraints. An L 1-norm regularization inversion algorithm is proposed here that reduces the influence of noisy data on three-dimensional geomagnetic depth sounding inversion from C-responses. Here the regularization is implemented by using an L 1-norm to measure the predicted data error, which is normalized by the C-response covariance, but an L 2-norm is used to measure the regularization term associated with model parameters. The limited-memory quasi-Newton method (L-BFGS) is used to invert for the three-dimensional electrical conductivity model. The model is discretized by curved rectangular prisms in spherical coordinates. Sensitivity tests show that for good-quality data contaminated by Gaussian noise, L 1 inversion, which could perform as well as L 2 inversion, can adequately recover the main features of the electrical conductivity structure within the region of data coverage. When data errors are drawn from an exponential distribution, L 1 inversion obtains relatively reliable reconstruction of the electrical structure, even when the noise level is comparable to that of actual C-responses. C-responses from 129 low-latitude and midlatitude geomagnetic observatories are inverted using L 1-norm minimization of the data error. The resulting model reveals an electrically conductive feature in the lower mantle transition zone and upper lower mantle that is broadly coincident with that found in previous studies. The reduced influence of data with large variances on L 1-norm misfits, along with inclusion of responses estimated from more observatories, makes L 1 inversion more clearly identify these deep conductive features while identifying previously obscured anoconductive zones. A feature of particular interest is the high electrical conductivity anomaly beneath the Bermuda-Sargasso Sea region in the mid-mantle transition zone and the uppermost lower mantle. Rock physics analysis indicates that the anomaly is most possibly caused by the wet upwelling material with excessive~650 K higher temperature, suggesting a narrow tail with a broad head. Plain-Language Summary There remains much to discover regarding the geological characteristics of the Bermuda zone. Specialized scientific instruments deployed in various parts of the world, including the Bermuda zone, have been used to measure geomagnetic and electrical conductivity data for several decades. We used customized computer codes to analyze and model the data recorded by these instruments. We detected a region of very high electrical conductivity in the depth range of 410 to 1,200 km beneath the Bermuda-Sargasso Sea area. To understand why the electrical conductivity is so high in this region, we also considered recent results from laboratory experiments of the conductivity of materials subjected to high temperature and high pressure. We find that between depths of 520 and 670 km, the studied region is hydrous and also very hot, with temperatures as high as 1,700°C...

The geochemistry of Cenozoic intracontinental high-Mg andesites (HMAs) in northeast Asia, together with regional geophysical data, offers an opportunity to explore the genetic relationship between the formation of intracontinental HMAs and subduction of the Pacific plate. Compared with primary HMAs in arcs, Cenozoic intracontinental HMAs in northeast Asia have lower Mg# [100 × Mg/(Mg + Fe2+)] values (53–56) and CaO contents (5.8–6.6 wt%), higher alkali (Na2O + K2O) contents (5.15–6.45 wt%), and enriched Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.7056–0.7059; εNd = –4.9 to –3.4; εHf = –4.7 to –2.6) as well as lower Pb isotope ratios (206Pb/204Pb = 16.76–19.19; 207Pb/204Pb = 15.42–15.45; 208/Pb/204Pb = 36.71–37.11). These Cenozoic intracontinental HMAs are similar to Cenozoic potassic basalts in northeast China with respect to their Sr-Nd-Pb-Hf isotopic compositions but have higher SiO2 and Al2O3 contents and lower K2O, MgO, and light rare earth element contents. These features indicate that these Cenozoic intracontinental HMAs originated from the mantle, where recycled ancient sediments and water contributed to partial melting of peridotite. Combined with the presence of a large low-resistivity anomaly derived from the mantle transition zone (MTZ) near these intracontinental HMAs, and their occurrence above the stagnant slab front within the MTZ (at 600 km depth) in northeast Asia, we conclude that the stagnant slab front, with high contents of recycled ancient sediments and water, has controlled the formation of Cenozoic intracontinental HMAs in northeast Asia.

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