The magnetic signature of ultramafic-hosted hydrothermal sites

Earth and Planetary Science Letters

Tivey

Kelley

et al. 2017

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25A 2003 high-resolution magnetic survey conducted by the Autonomous Underwater Vehicle ABE 26 over the low-temperature, ultramafic-hosted hydrothermal field Lost City reveals a weak positive 27 magnetic anomaly. This observation is in direct contrast to recent observations of strong positive 28 magnetic anomalies documented over the high-temperature ultramafic-hosted hydrothermal vents 29 fields Rainbow and Ashadze, which indicates that temperature may control the production of 30 magnetization at these sites. The Lost City survey provides a unique opportunity to study a field 31 that is, to date, one of a kind, and is an end member of ultramafic-hosted hydrothermal systems. 32Our results highlight the key contribution of temperature on magnetite production resulting from 33 serpentinization reactions. Whereas high temperature promotes significant production and 34 partitioning of iron into magnetite, low temperature favors iron partitioning into various alteration 35 phases, resulting in a magnetite-poor rock. Moreover, the distribution of magnetic anomalies 36 confirms results of a previous geological survey indicating the progressive migration of 37 hydrothermal activity upslope. These discoveries contribute to the results of 25 years of magnetic 38 exploration of a wide range of hydrothermal sites, from low-to high-temperature and from 39 basalt-to ultramafic-hosted, and thereby validate using high-resolution magnetics as a crucial 40 parameter for locating and characterizing hydrothermal sites hosting unique chemosynthetic-41 based ecosystems and potentially mineral-rich deposits. 42 43 1) Introduction 44The discovery of hydrothermal activity along the Galapagos Rift (Corliss et al., 1979) paved the 45 way for large-scale, deep-sea exploration of oceanic ridges, revealing a myriad of hydrothermal 46 vent fields primarily hosted on basaltic crust, with lesser gabbroic and ultramafic material (Kelley 47 and Shank, 2010). In contrast to intermediate-and fast-spreading systems where basaltic rocks 48 dominate, along slow-to ultraslow-spreading centers, the tectonically-dominated geology 49 (Karson and Elthon, 1987; Tucholke et al., 1998; Escartin et al., 2008) gives rise to a higher 50 abundance of hydrothermal systems hosted by variable amounts of ultramafic and gabbroic 51 material, such as the well-known Rainbow and Ashadze hydrothermal fields (Charlou et al., 52 2002; Charlou et al., 2010; Fouquet et al., 2008). These high-temperature venting systems 53 (>350°C) are characterized by sulfide chimneys emitting low pH fluids rich in carbon dioxide, 54 methane and hydrogen; chemical signatures that are hallmarks of fluid interaction with mafic and 55 ultramafic material in the subsurface (Charlou et al., 2002; Fouquet et al., 2010; Kelley and 56 Shank, 2010;Ohara et al., 2012). 57Although the magnetic signature of basalt-hosted hydrothermal sites is well constrained (Tivey et 58 al., 1993; Tivey and Johnson, 2002; Tivey and Dyment, 2010;Szitkar et al., 2014a; Szitkar et al., 59 2015),...

Section: ) Discussion 158mentioning

confidence: 99%

Section: ) Discussion 158mentioning

confidence: 99%

Section: ) Introduction 44mentioning

confidence: 99%

mentioning

confidence: 99%

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Magnetic exploration of a low-temperature ultramafic-hosted hydrothermal site (Lost City, 30°N, MAR)

Earth and Planetary Science Letters

Tivey

Kelley

et al. 2017

Self Cite

“…The rest of the anomaly is accounted for by a deeper source, probably related to thermal demagnetization of an ascending hydrothermal pipe beneath the active part of the site. The significant contribution of the stockwork zone to the magnetic signature of TAG confirms that it is a common characteristic of all type of hydrothermal sites (Szitkar et al, 2014a(Szitkar et al, , 2014b of potential interest for deep-sea mineral exploration.…”

Section: Discussionmentioning

confidence: 99%

Near-seafloor magnetics reveal tectonic rotation and deep structure at the TAG (Trans-Atlantic Geotraverse) hydrothermal site (Mid-Atlantic Ridge, 26°N)

Dyment

2015

Geology

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We take advantage of geological constraints from Ocean Drilling Program drill holes and high-resolution bathymetry to revisit the near-seafloor magnetic anomaly at the Trans-Atlantic Geotraverse hydrothermal site (Mid-Atlantic Ridge, 26°N). The dipolar anomaly associated with the site is better reduced to the pole if the magnetization vector is tilted by 34°, an observation suggesting that the strongly faulted basalt surrounding the site has been rotated by ~53° along an axis parallel to the Mid-Atlantic Ridge as a probable consequence of the detachment tectonics inferred in this area. The faults and the deeper detachment focus and guide the hot ascending hydrothermal fluid. Magnetic modeling shows that the nonmagnetic stockwork zone is a significant contributor to the observed negative anomaly, the rest being accounted for by a deeper source probably related to thermal demagnetization of an ascending hydrothermal pipe beneath the active part of the site.

Absolute magnetization of the seafloor at a basalt‐hosted hydrothermal site: Insights from a deep‐sea submersible survey

Geophysical Research Letters

Dyment

Fouquet

et al. 2015

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The analysis of high-resolution vector magnetic data acquired by deep-sea submersibles (DSSs) requires the development of specific approaches adapted to their uneven tracks. We present a method that takes advantage of (1) the varying altitude of the DSS above the seafloor and (2) high-resolution multibeam bathymetric data acquired separately, at higher altitude, by an Autonomous Underwater Vehicle, to estimate the absolute magnetization intensity and the magnetic polarity of the shallow subseafloor along the DSS path. We apply this method to data collected by DSS Nautile on a small active basalt-hosted hydrothermal site. The site is associated with a lack of magnetization, in agreement with previous findings at the same kind of sites: the contrast between nonmagnetic sulfide deposits/stockwork zone and strongly magnetized basalt is sufficient to explain the magnetic signal observed at such a low altitude. Both normal and reversed polarities are observed in the lava flows surrounding the site, suggesting complex history of accumulating volcanic flows.