Paper No. 44-7
Presentation Time: 11:05 AM
NEW EVOLUTION MODEL OF ULTRAMAFIC HOSTED HYDROTHERMAL SYSTEMS CONSTRAINED BY NEAR-SEAFLOOR MAGNETICS
Recent advance of near-seafloor magnetic surveys revealed that volcanic lava-hosted hydrothermal fields in various tectonic settings, including volcanic arcs and back-arc regions as well as mid-ocean ridges, yield a common magnetic signature: a magnetic anomaly reflecting a zone of weak or null magnetization [e.g., Tivey et al., 1993, EPSL; Fujii et al., 2015, JGR]. In contrast, ultramafic-hosted hydrothermal sites show completely different signature. Ultramafic-hosted hydrothermal system of the Yokoniwa field developed in a non-transform offset massif of the slow-spreading Central Indian Ridge is characterized by enhanced magnetization [Fujii et al., 2016, EPSL]. Rock magnetic analyses reveal that this enhanced magnetization is due to abundant magnetite in highly serpentinized peridotite [Fujii et al., 2016, G-cubed]. Compared with similar ultramafic-hosted hydrothermal fields of the Rainbow and Ashadze-1 sites along the Mid-Atlantic Ridge [Szitkar et al., 2014, Geology], the following three-stage model for magnetic mineral formation in ultramafic-hosted hydrothermal fields is proposed in our present study [Fujii et al., 2016, EPSL]. During the initial stage of an ultramafic-hosted hydrothermal system, magnetite forms with serpentine and H2 thorough hydrothermal alteration of peridotites. Strongly magnetized pyrrhotite is also created through sulfide mineralization under reductive conditions. This stage occurs at the Ashadze-1 site. Once the serpentinization reaction has progressed, the amount of magnetite creation increases dramatically, strengthening the magnetization. Pyrrhotite creation continues as long as the H2 content of the hydrothermal fluids continues to create a reducing environment in this developing stage, such as that occurring at the Rainbow site. Hot and reduced hydrothermal fluids enable the magnetite and pyrrhotite to maintain stability with no low-temperature oxidation. Finally, once the reaction of ultramafic rocks ceases, H2 is no longer formed in the system, and the conditions become oxidative. This allows the pyrrhotites to convert into nonmagnetic iron sulfide or oxide, reducing their magnetization considerably, such as that occurring at the Yokoniwa site .