The physicochemical conditions of hydrothermal ore-forming systems of “black smokers” associated with mantle ultrabasites in the Central Atlantic region

Abstract

Study of fluid inclusions in minerals from sulfide ores sampled in the Ashadze and Logatchev hydrothermal fields during the Serpentine cruise of the R/V Pourquoi Pas? in 2007 gave an insight into the physicochemical conditions of ore-forming systems of “black smokers” associated with mantle ultrabasites in the Central Atlantic region. It was established that the studied hydrothermal systems differ in mineral formation temperatures, fluid salinity, and hydrothermal processes running within basaltic complexes in both low-spreading (Mid-Atlantic Ridge) and high-spreading (East Pacific Rise) mid-ocean ridges. It was shown for the Ashadze and Logatchev hydrothermal fields that the temperatures and salinity of inclusion solutions in the minerals vary over a wider range of values than those of fluids flowing out from the ridge vents onto the ocean floor. Study of fluid inclusions showed that the Ashadze and Logatchev sulfide structures resulted from the action of both high-temperature (up to 355 °C) and low-temperature (no lower than 170 °C) solutions. The salinity of fluid in the inclusions (mainly 8 wt.%) is more than twice as high as the salinity of seawater. The data obtained substantiate the phase separation of fluid in the deepest-water submarine hydrothermal Ashadze system and give an idea of the chemical composition and temperature of fluids forming sulfide deposits in the Ashadze and Logatchev hydrothermal fields in the Central Atlantic region.

Fluid Circulation Along an Oceanic Detachment Fault: Insights From Fluid Inclusions in Silicified Brecciated Fault Rocks (Mid‐Atlantic Ridge at 13°20′N)

“…NaCl). This range is comparable to ranges of salinity (and Th) measured in fluid inclusions (Figure 8a) in rocks associated with hydrothermal fields both of mafic (TAG: Petersen et al, 1998;MARK: Delaney et al, 1987;Saccocia & Gillis, 1995) and ultramafic nature (Rainbow, Logatchev, Ashadze, Semenov, Irinovskoe; Bortnikov et al, 2011Bortnikov et al, , 2014Bortnikov et al, , 2015Simonov et al, 2015). Fluid inclusions from the highly silicified sample ODM218 display a large salinity range (2.1-10 wt.% eq.…”

“…NaCl; Figure 8a). (Delaney et al, 1987;Saccocia & Gillis, 1995) and TAG (Petersen et al, 1998), detachment plane (ultra)mafic rocks hosting hydrothermal vents (Rainbow, Logatchev, Ashadze, Semenov, Irinovskoe; Bortnikov et al, 2011Bortnikov et al, , 2014Bortnikov et al, , 2015Simonov et al, 2015); (b) Heterogeneity of inclusion Th and salinity at the quartz grain scale for the primary inclusions only, for clarity issues, using a similar color and symbol for inclusions in each grain. Highly silicified mafic breccias (ODM155 and ODM218) present higher homogenization temperatures (180°C-350°C) than moderately silicified ones, either mafic (ODM195) or mixed mafic-ultramafic (ODM173; 150°C-220°C; Figure 8a; Table 3).…”

“…Figure 8. Homogenization temperatures (Th) against salinity for (a) all primary and secondary fluid inclusions; data are compared to Th and salinity ranges measured in fluid inclusions from oceanic gabbros (MARK:Kelley & Delaney, 1987), diabases (Atlantis Massif: Castelain et al, 2014), mafic silicified breccias from MARK(Delaney et al, 1987;Saccocia & Gillis, 1995) and TAG(Petersen et al, 1998), detachment plane (ultra)mafic rocks hosting hydrothermal vents (Rainbow, Logatchev, Ashadze, Semenov, Irinovskoe;Bortnikov et al, 2011Bortnikov et al, , 2014Bortnikov et al, , 2015Simonov et al, 2015); (b) Heterogeneity of inclusion Th and salinity at the quartz grain scale for the primary inclusions only, for clarity issues, using a similar color and symbol for inclusions in each grain. Seawater salinity is indicated with a blue line.…”