From deep hydrothermal structure to rock physic models of sulphide deposits at mid-ocean ridges with controlled source electromagnetic data

High-temperature hydrothermal circulation at mid-ocean ridges accounts for 30% of heat exchange between the Earth’s interior and the deep ocean, controls global geochemical cycles, forms valuable seafloor mineral deposits and nourishes chemosynthetic life forms. Penetration of cold seawater into hot crust formed at mid-ocean ridges causes it to heat up to about 500°C, reacts with the magmatic crust extracting metal sulphides and other elements, leaving behind a highly altered reaction zone, and during re-emergence of the fluids causes precipitation of massive sulphides deposits on the seafloor [e.g., 1].

As part of the EU “Blue Mining” project, a variety of geophysical and geological datasets were acquired in summer 2016. Three different kinds of controlled-source electromagnetic (CSEM) experiments were conducted by the University of Southampton (UoS) and partners from Geomar, Germany. Preliminary results have shown that the techniques are sensitive to the contrast in electrical resistivity of the conductive mineral deposits and the resistive oceanic crust. The aim of this project is to use data from ocean bottom instruments (Geomar) and a towed EM source (UoS) to image the deeper hydrothermal structure, and to conduct fine-scale measurements on rock samples, to enhance understanding of the processes controlling the formation of mineral deposits in such settings.

Our methodology is divided in three parts: Firstly, the EM data from ocean bottom instruments (OBEM data) will be analysed and a 2-D sub-seafloor resistivity distribution will be inferred using existing algorithms. A joint inversion of three-component towed-receiver data and two-component OBEM data will be performed to increase the lateral and vertical resolution [2]. Secondly, the student will analyse existing EM measurements on rock samples, with an opportunity to collect further EM data in the NOC rock physics laboratory, and integrate the results into a model of controls on electrical properties at hydrothermal sites [3]. Thirdly, the student will analyse the effect of the 3-D shape of the SMS deposits on data from close source-receiver spacing together with our collaborators at Geomar, who are analysing the shallow resistivity distribution using high-frequency, high-resolution EM data. These data will allow further constraints of the shallow structure to resolve mineral deposition, rock alteration and fluid pathways.

The SPITFIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted at Ocean and Earth Science. Specific training will include: scientific data acquisition at sea and in the laboratory, high-level training in CSEM, active seismology, rock physics and geophysical data inversion techniques. In addition to placement opportunities offered by SPITFIRE, we anticipate that the student will travel to Kiel, Germany to work with our collaborators in this project, and will present his/her work at international conferences.