Hydrothermal influences on magnetic mineral assemblages in marine sediments (Guaymas Basin, Gulf of California, IODP Expedition 385)

A long-term aim of scientific ocean drilling is to understand the interactions and exchanges between volcanic and magmatic processes that take place during the formation of the Earth's solid crust and the circulation of oceanic water through the sub- seafloor environment. These control the fundamental chemical exchanges between oceanic crust, marine sediments and seawater in the Earth system. International Ocean Discovery Program Expedition 385 aims to sample a sequence of marine sediments in the Guaymas Basin (Gulf of California) that have been intruded at various depths by sub-horizontal sheets of magma known as sills. The transfer of heat into the host rocks during cooling and crystallization of such sills provides energy that drives the hydrothermal circulation of seawater. This results in the exchange of chemical elements between the hot fluids and the sediments and rocks they pass through, leading to alteration of existing minerals in the rocks and the formation of new minerals by precipitation. An important aspect of this poorly understood system is the alteration and growth of iron-bearing phases (oxides, hydroxide and sulphides). Since these are naturally magnetic, their nature and distribution across zones of alteration associated with sill intrusions may be investigated using rock magnetic techniques that are highly sensitive to the composition, grain-size and concentration of magnetic minerals.

This project, therefore, will test the hypothesis that the intrusion of magmatic sills into marine sediments results in distinctive changes to their primary magnetic mineralogy (via the production of new magnetic phases during hydrothermal fluid migration) that may be characterized and distinguished using sensitive rock magnetic experiments. To achieve this, core samples recovered during drilling in the Guaymas Basin will be sub-sampled in a systematic manner at progressively increasing distances from contacts between igneous sills and their host sedimentary rocks. State-of-the-art experimental rock magnetic techniques will then be used to fully characterise the changes in magnetic mineralogy associated with sill-driven hydrothermal circulation in marine sediments. This will include analysis of experimental data on the growth of laboratory-imparted magnetizations known as an isothermal remanent magnetizations (IRMs). The shape of IRM acquisition curves has long been used as an indicator of the presence of different magnetic minerals such as magnetite and hematite. If these experiments are performed at a sufficiently high resolution, however, then almost imperceptible changes in the shape of the acquisition curves may be quantified using numerical "end member modelling" of the data. This approach allows very subtle differences in magnetic mineralogy between specimens to be determined along with the relative contributions of different phases to the overall magnetic signal in each sample. These novel analyses will be augmented by experiments involving measuring the magnetic hysteresis properties of the specimens (hysteresis is a phenomenon by which changes in the magnetization of samples lag behind those of magnetic fields applied to them in the laboratory). These data may then be analysed to produce so-called "first order reversal curves" or "FORCs" that provide information on the magnetic grain sizes present in a sample and the degree to which these grains interact with other magnetically.

Once the magnetic experiments on samples collected in profiles away from intrusive contacts are completed, the rock magnetic data will be combined with results from complementary petrological, geochemical and thermal modelling analyses that are planned by other members of the Expedition 385 Science Party. This will provide a comprehensive physical and chemical characterization of hydrothermal processes associated with sill emplacement that will advance our understanding of these dynamic systems.

The topic addressed by this project will contribute to outstanding questions in the plate tectonic processes of magmatism and hydrothermal circulation that inspire tremendous public interest in Earth Sciences. Therefore, in addition to the benefits of the project to a range of academic disciplines, the main impact of the project lies in its potential to engage members of the public and school/college students in IODP science. The potential for impact in this area will be enhanced by:

(1) Shipboard outreach in collaboration with the education team on IODP Expedition 385, who will deliver a series of ship-to-shore interactive broadcasts to schools and colleges around the world. This will allow discussion of shipboard science and life with hundreds of young people, hopefully inspiring some of these to take up scientific careers.

(2) Shore-based practical engagement of young scientists in post-cruise laboratory analyses, via engagement with work experience opportunities. Such hands-on experiences have real potential to motivate and inspire, while providing insights into scientific research as a career.

(3) Contributing to public awareness of science via social media, using the vibrant Twitter account of Earth Sciences at Plymouth University (@EarthSciPlymUni), which currently reaches over nearly 7000 followers, including teachers and prospective Earth Science students.

(4) Engagement of the public in scientific discussions via Plymouth's SciBar, a programme of public events organized by the local branch of the British Science Association. This provides opportunities to talk directly to the general public about research in a way that is approachable and easy to understand, and to engage in open discussion about research issues.