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The equivalent magnetization (EM) and mantle Bouguer anomaly (MBA) were calculated along the ultraslow-spreading Mohns Ridge axis in the Norwegian-Greenland Sea. The magnetic anomaly and the associated EM were compared with the bathymetry, MBA, seismically determined crustal structure and geochemical data at both the inter-segment scale (>60 km) and the intra-segment scale (20–60 km). At the inter-segment scale, the magnetic highs at the segment centers are independent of the MBA. Of the 13 segments, 9 with magnetic anomalies >700 nT coincide with axial volcanic ridges identified from multibeam bathymetry maps, which suggests that the magnetic highs at the segment centers may be more associated with the extrusive lavas rather than the amount of magma supply. With few exceptions, the magnetic anomaly lows associated with MBA highs at the segment ends increase from south to north. This trend might be explained by thickened extrusive basalts and/or more serpentinized peridotites at the segment ends in the north. At the intra-segment scale, the most prominent features are the decreases in the magnetic anomalies and associated EMs from the segment centers to the ends. The intra-segment magnetic anomalies have positive and negative correlations with the bathymetry and MBA, respectively. The magnetic signal modeled by the seismically determined layer 2A with an assumed constant magnetization is remarkably consistent with the observed magnetic anomaly, which strongly suggests that the thickness of the extrusive basalts dominates the magnetic structure in each segment along the Mohns Ridge. In general, the thickness of the extrusive basalts dominates the magnetic structure along the Mohns Ridge, whereas the contributions from serpentinized peridotites may be significant at the segment ends and may produce long-wavelength magnetic variations. The magnetic data can be used as an indicator of the thickness of the extrusive basalts within segments along the ultraslow-spreading Mohns Ridge.

Although the East China Sea (ECS) is widely accepted as the offshore extension of the South China Block (SCB), an alternate hypothesis is that the ECS basement is a buoyant allochthon that collided with Eurasia in the Cretaceous. In order to constrain the Mesozoic collision position, or the Mesozoic suture zone, wide-angle reflection/refraction seismic profiles (ECS2017 and ECS2019) were carried out in 2017 and 2019 to map the crustal structure of the ECS and clarify the possible Mesozoic suture zone. Crustal structures are generated by both a ray-tracing forward method (RayInvr) and a travel time tomographic inversion (Tomo2D). The resulting velocity models show significant lateral variations along these two seismic profiles. An extended continental crust gradually decreases in thickness from 30 km in the Zhemin Volcanic Belt (ZMVB) to 15 km in the Continental Shelf Basin (CSB) with lateral velocity variations of 4.40–7.15 km/s and 4.30–6.90 km/s, respectively, indicating differences in crustal structure between the ZMVB and the CSB. A 50 km-wide high-velocity anomaly with high density and uplifted Moho is imaged between the ZMVB and the CSB. Combined with the Mesozoic subduction and collision zone of the exotic ECS basement by previous studies, we infer that the high-velocity anomaly marks the position of the Mesozoic suture zone, which is associated with magma intrusion due to upwelling molten through the detached slab with the rollback of the Paleo-Pacific subduction slab.

The basalt-hosted Wocan Hydrothermal Field (WHF), located on the NW slope of an axial volcanic ridge at a depth of ∼3000 m at 6°22′N on the slow-spreading Carlsberg Ridge, northwest Indian Ocean, was discovered in 2013 during Chinese DY28th cruise. Preliminary investigations show that the field consists of two hydrothermal sites: Wocan-1, which shows indications for recent high-temperature hydrothermal activity, is located near the peak of the axial volcanic ridge at a water depth of 2970–2990 m, and Wocan-2 site, located at a water depth of 3100 m, ∼1.7 km to the northwest of Wocan-1. The recovered hydrothermal precipitates can be classified into four groups: (i) Cu-rich chimneys; (ii) Cu-rich massive sulfides; (iii) Fe-rich massive sulfides; and (iv) silicified massive sulfides. We conducted mineral texture and assemblage observation and Laser-ablation ICP-MS analyses of the hydrothermal precipitates to study the mineralization processes. Our results show that there are distinct systematic trace element distributions throughout the different minerals in the four sample groups. In general, chalcopyrite from the group (i) is enriched in Pb, As, Mo, Ga, Ge, V, and Sb, metals that are commonly referred to as medium- to low-temperature elements. In contrast these elements are present in low contents in the chalcopyrite grains from other sample groups. Selenium, a typical high-temperature metal, is enriched in chalcopyrite from groups (ii) and (iv), whereas Ag and Sn are enriched only in some silicified massive sulfides. As with chalcopyrite, pyrite also shows distinct trace element associations in grains with different habitus. The low-temperature association of elements (Pb, Mo, Mn, U, Mg, Ag, and Tl) is typically present in colloform/framboidal pyrite, whereas the high-temperature association (Se, Co, and Bi) is enriched in euhedral pyrite. Sphalerite in the groups (i) and (iii) at Wocan-1 is characterized by high concentrations of Ga, Ge, Pb, Cd, As, and Sb, indicating that sphalerite in these sample groups likely precipitated at intermediate temperatures. Early bornite, which mainly occurs in the central part of the Cu-rich chimney, is typically enriched in Sn and In compared to the other minerals. In contrast, late bornite that likely formed during increasing interaction of hydrothermal fluids with cold, oxygenated seawater has low Sn and In, but significantly higher concentrations of Ag, Au, Mo and U. Digenite, also forming in the exterior parts of the samples during the late stages of hydrothermal fluid venting, is poor in most trace elements, except Ag and U. The notable Ag enrichment in the late-stage mineral assemblages at both Wocan-1 and Wocan-2 may therefore be related to lower temperatures and elevated pH. Our results indicate that Wocan-1 has experienced a cycle of heating with Cu-rich chimney growth and subsequent cooling, followed by late seafloor weathering, while Wocan-2 has seen intermediate- to high-temperature mineralization followed by intense silicification of sulfides. Seafloor weathering processes or mixing of hydrothermal fluids with seawater during the waning stages of hydrothermal fluid flow result in significant redistributions of trace elements in sulfide minerals.

The Tilt-Euler deconvolution based on the first derivatives of the tilt angle is advanced from the routine Euler deconvolution and it is widely used. It estimates both the horizontal location and the depth of magnetic bodies without the known structural index. Since the derivatives of the tilt angle cannot be calculated when the first horizontal derivatives of the magnetic anomaly are equal to zero, there are singularities in the inversion from the Tilt-Euler method. We present an improved method for the Tilt-Euler deconvolution, which avoids the occurrence of singularities in the calculation and reduces the unstable factors in the inversion. Since the Euler methods should be implemented in a window, we apply the standard deviation of the Euler solutions as the measure of dispersion to determine the optimal window size. Both the model test and the field application show that the solutions of the improved Tilt-Euler deconvolution are more concentrated than that of the Tilt-Euler deconvolution and, subsequently, result in a more robust interpretation. In the field application, the results of the proposed method imply that the source body is probably a dike with limited extent.