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

Abstract

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),...

Near-seafloor magnetic signatures unveil serpentinization dynamics at ultramafic-hosted hydrothermal sites

“…The positive magnetic anomaly observed over UMHSs results from several factors, including (1) an accumulation of strongly magnetized magnetite produced by serpentinization reactions within the hydrothermal plumbing and stockwork zones; (2) the dimensions of the hydrothermal system at depth, i.e., the volume of its deep parts; and (3) the presence of reducing hydrothermal fluids that prevent later oxidation of serpentinitehosted magnetite by contact with seawater (Szitkar et al, 2014b). In addition, it has also been shown that the amount of magnetite produced by serpentinization reactions is influenced by hydrothermal fluid temperature (Szitkar et al, 2017), in accordance with laboratory experiments (Klein et al, 2014). Indeed, high temperatures (>200 °C) favor iron partitioning into magnetite, whereas low temperatures preferentially partition it into less-magnetic brucite (Früh-Green et al, 1996, 2004Klein et al, 2014).…”

“…Indeed, high temperatures (>200 °C) favor iron partitioning into magnetite, whereas low temperatures preferentially partition it into less-magnetic brucite (Früh-Green et al, 1996, 2004Klein et al, 2014). This latter case is likely to result in the formation of a limited amount of magnetite within the stockwork zones beneath UMHSs, and hence a weak positive magnetic anomaly (Szitkar et al, 2017).…”

“…At first order, the magnetite magnetization is subject to the intensity of the magnetizing field, grain size, and domain state (Dunlop and Prévot, 1982;Cullity and Graham, 2009); however, serpentinization does not have any significant impact on the magnetic grain size (Oufi et al, 2002;Malvoisin et al, 2012). Moreover, on a first order, these three hydrothermal vent fields have comparable dimensions (e.g., Szitkar et al, 2014bSzitkar et al, , 2017; i.e., the main observed differences are likely to be dominated by variations in the magnetite concentrations within the host rocks (see the Data Repository).…”

“…DR1 in the Data Repository). This inversion was performed using the same parameters as for Rainbow and Lost City to ensure a proper comparison (Szitkar et al, 2014b(Szitkar et al, , 2017; see also the Data Repository). Away from the vent field, equivalent magnetization reaches an average of 1.2 A/m.…”

“…Although the magnetic footprint of basalt-hosted hydrothermal systems is well constrained (e.g., Tivey et al, 1993;Honsho et al, 2013;Szitkar et al, 2014a), that of ultramafic-hosted hydrothermal sites (UMHSs) remains poorly understood. Two recent studies over highand low-temperature UMHSs, the Rainbow and Lost City vent fields (MidAtlantic Ridge), respectively (Szitkar et al, 2014b(Szitkar et al, , 2017, have emphasized the relation between fluid temperature and amount of magnetite generated by serpentinization reactions within the sites' stockwork zones or plumbing systems. Nevertheless, the precise evolution of magnetite production with temperature remains unknown.…”