Unravelling hydrothermal alteration in basaltic ocean crust

Seafloor hydrothermal vents, also known as black chimneys (featured image; Copyright: Woods Hole Oceanographic Institution), produce valuable mineral resources through fluid-rock reactions beneath the seafloor – a process called hydrothermal alteration. On basaltic ocean crust, hydrothermally-altered regions are generally found to be less magnetic, which provides a way to detect seafloor hydrothermal vents. By collaborating with Prof. Chunhui Tao group from the Second Institute of Oceanography, we analyzed a large set of rock samples recovered from the Southwest Indian Ridge. Results were recently published in Geophysical Research Letters (Wang et al., 2020; https://doi.org/10.1029/2020GL087578) that discloses the detailed alteration pathway in hydrothermal vent hosting mid-ocean ridge.

Our microscopic and rock magnetic analyses reveal that fresh basalts were chloritized and brecciated during hydrothermal alteration, where primary titanomagnetites are progressively dissolved (Fig. 1). Titanomagnetite nanoparticle clusters in interstitial glasses were dissolved in the first order, followed by micron-scale dendritic titanomagnetite particles. Fully altered chloritized basalts and chloritized basaltic breccias are almost paramagnetic due to the scarcity of magnetic minerals. Hydrothermal deposits –  produced by hydrothermal circulation at upflow zones – contain intergrowing iron sulfides and oxides, giving a chemical remanent magnetization (Fig. 2).



As a consequence, natural remanent magnetization is reduced by three to four orders-of-magnitude during hydrothermal alteration (Fig. 3). This study establishes the value of rock magnetic proxies for quantifying the alteration degree to trace fluid-rock reactions (Fig. 3). The finding also link direct magneto-mineralogical observations to geophysical interpretations, which is important in understanding seafloor hydrothermal circulation and mid-ocean ridge geodynamics.


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