First active hydrothermal vents on an ultraslow-spreading center: Southwest Email alerting services articles cite this article to receive free e-mail alerts when new www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geology www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click official positions of the Society. citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect presentation of diverse opinions and positions by scientists worldwide, regardless of their race, includes a reference to the article's full citation. GSA provides this and other forums for the the abstracts only of their articles on their own or their organization's Web site providing the posting to further education and science. This file may not be posted to any Web site, but authors may post works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent their employment. Individual scientists are hereby granted permission, without fees or further

Coupled magmatic and tectonic activity plays an important role in high-temperature hydrothermal circulation at mid-ocean ridges. The circulation patterns for such systems have been elucidated by microearthquakes and geochemical data over a broad spectrum of spreading rates, but such data have not been generally available for ultra-slow spreading ridges. Here we report new geophysical and fluid geochemical data for high-temperature active hydrothermal venting at Dragon Horn area (49.7°E) on the Southwest Indian Ridge. Twin detachment faults penetrating to the depth of 13 ± 2 km below the seafloor were identified based on the microearthquakes. The geochemical composition of the hydrothermal fluids suggests a long reaction path involving both mafic and ultramafic lithologies. Combined with numerical simulations, our results demonstrate that these hydrothermal fluids could circulate~6 km deeper than the Moho boundary and to much greater depths than those at TransAtlantic Geotraverse and Logachev-1 hydrothermal fields on the Mid-Atlantic Ridge.

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Inversion of near‐bottom magnetic data reveals a well‐defined low crustal magnetization zone (LMZ) near a local topographic high (37°47′S, 49°39′E) on the ultraslow‐spreading Southwest Indian Ridge (SWIR). The magnetic data were collected by the autonomous underwater vehicle ABE on board R/V DaYangYiHao in February‐March 2007. The first active hydrothermal vent field observed on the SWIR is located in Area A within and adjacent to the LMZ at the local topographic high, implying that this LMZ may be the result of hydrothermal alteration of magnetic minerals. The maximum reduction in crustal magnetization is 3 A/M. The spatial extent of the LMZ is estimated to be at least 6.7 × 104 m2, which is larger than that of the LMZs at the TAG vent field on the Mid‐Atlantic Ridge (MAR), as well as the Relict Field, Bastille, Dante‐Grotto, and New Field vent‐sites on the Juan de Fuca Ridge (JdF). The calculated magnetic moment, i.e., the product of the spatial extent and amplitude of crustal magnetization reduction is at least −3 × 107 Am2 for the LMZ on the SWIR, while that for the TAG field on the MAR is −8 × 107 Am2 and that for the four individual vent fields on the JdF range from −5 × 107 to −3 × 107 Am2. Together these results indicate that crustal demagnetization is a common feature of basalt‐hosted hydrothermal vent fields at mid‐ocean ridges of all spreading rates. Furthermore, the crustal demagnetization of the Area A on the ultraslow‐spreading SWIR is comparable in strength to that of the TAG area on the slow‐spreading MAR.

Hydrothermal alteration at high-temperature vents near mid-ocean ridge is thought to produce pervasive magnetization lows on basaltic ocean crust, but the detailed alteration process is insufficiently documented. Here, we performed microscopic and magnetic analyses on a large set of hydrothermal-related basaltic samples from the Southwest Indian Ridge. Fresh basalts were chloritized and brecciated during hydrothermal alteration, where titanomagnetite nanoparticle clusters hosted in interstitial glasses were dissolved in the first order, followed by large micron-scale dendritic particles. Natural remanent magnetization was reduced from 10 0 -10 1 A/m for fresh basalts to 10 −3 A/m for fully altered basalts. Hydrothermal deposits acquired a chemical remanent magnetization of 10 −2 A/m. Our results link direct magneto-mineralogical observations to geophysical interpretations, which is important in understanding seafloor hydrothermal circulation and mid-ocean ridge geodynamics.Plain Language Summary Seafloor hydrothermal vents, also known as black chimneys, 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. To understand the cause of this phenomenon, we studied how fluid-rock reactions modify magnetic minerals on basaltic ocean crust in the Southwest Indian Ridge. We have established a detailed alteration pathway that the primary magnetic minerals were progressively replaced by nonmagnetic minerals with increasing alteration degree and the strongly magnetized nanoparticles were preferentially consumed at the very beginning of the reaction. This finding directly contributes to magnetic surveying the seafloor hydrothermal vents and establishes the value of rock magnetic proxies for quantifying the alteration degree to trace fluid-rock reactions.

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