Does a Shallow Magma Reservoir Exist in the Wudalianchi Volcanic Field? Constraints From Magnetotelluric Imaging

We obtain new images of the 3D resistivity structure of the Wudalianchi volcanic field (WVF). No low‐resistivity structure is found beneath Laoheishan and Huoshaoshan volcanoes in the WVF, which challenges the hypothesis of one or more magma reservoirs in the upper crust of this region. We observe a low‐resistivity body beneath Weishan volcano; however, estimated magma emplacement rates conflict with the observed volume of that body. Due to the relatively low geothermal gradient and surface heat flow in the area, magma would cool rapidly and could not be sustained for a long time unless new magma was regularly introduced into the system, but our observations do not support this scenario. We suggest that the magma in the WVF mainly originates from the mantle or the base of the crust with little crustal residence, and that the low‐resistivity body in the upper crust is likely due to saline aqueous free fluids. Plain Language Summary The origin of magma, whether it is derived directly from the deep mantle or has previously resided in the crust, plays a crucial role in determining volcanic eruption mechanisms. The presence of a magma reservoir in the shallow crust often signifies an increased risk of volcanic eruptions. The Wudalianchi volcanic field (WVF) is situated in northeastern China. Earlier research suggested the existence of a magma reservoir beneath the Weishan volcano within the WVF. However, our study reveals an absence of magma reservoirs beneath the more recently erupted Laoheishan and Huoshaoshan volcanoes. This raises the question of why a magma reservoir is present beneath Weishan, which experienced an eruption 0.5 Ma ago, rather than beneath Laoheshan and Huoshaoshan, which erupted a mere 300 years ago. Furthermore, the WVF is characterized by an extremely low geothermal gradient, causing magma in the shallow crust to lose substantial heat and cool rapidly, unless replenished by a continuous influx of high‐temperature magma. Nevertheless, the volume of magma injection inferred from the low‐resistivity body beneath Weishan is inconsistent with numerical simulation results. Consequently, we suggest that the low‐resistivity body beneath Weishan volcano is instead a structure containing saline aqueous free fluids, not magma. Key Points Electrical structure of the Wudalianchi volcanic field (WVF) is imaged Subsurface beneath Laoheishan and Huoshaoshan volcanoes is highly resistive Low‐resistivity body beneath Weishan volcano is due to saline aqueo