Fast and slow spreading ridges: Structure and hydrothermal activity, ultramafic topographic highs, and CH₄ output

Abstract

Different parts of the world ridge system have quite different morphologies, which reflect different constructional processes. It appears that hydrothermal circulation at all spreading centers is an important exchange process between the ocean and the newly formed oceanic crust. This hydrothermal circulation may vary according to morphology and crustal composition and may also affect ridge constructional processes. The TAG (26°N) and Snake Pit (23°N) hydrothermal sites on the Mid‐Atlantic Ridge (MAR) display Mn/CH4 ratios in overlying seawater similar to those of nonsedimented East Pacific Rise sites, i.e., about 0.2 mol/L. In contrast, large methane anomalies with very low Mn/CH4 ratios of 0.005 mol/L are associated with ultramafic topographic highs near 15°N, close to the axis and on ultramafic walls of the rift valley. The association of ultramafic bodies and CH4 anomalies in seawater indicates active serpentinization processes. CH4 is produced during serpentinization according to the Fisher Tropsch reaction (Charlou et al., 1991). Many ultramafic rock assemblages have been sampled on the MAR away from fracture zones, in particular by the Deep Sea Drilling Project. Changes of mechanical properties and of density of uplifted deep material in the accreting plate boundary zone caused by serpentinization may play an important role in the construction of slow spreading ridges.

Petrologic and Stable Isotope Constraints on Hydrothermal Alteration and Serpentinization of the EPR Shallow Mantle at Hess Deep (Site 895)

“…Thus, the presence of H 2 or CH 4 produced by CO 2 -reduction in the serpentinizing fluid in a rock-dominated system could result in the H-rich serpentine compositions observed in many oceanic serpentinites and ophiolites. The production of H 2 and CH 4 during serpentinization is also consistent with an increasing number of discoveries of methane and hydrogen anomalies associated with ultramafic outcrops on the MAR and gas seeps in ophiolites (e.g., Abrajanoetal., 1988;Charlouetal, 1991aCharlouetal, , 1991bRonaetal., 1992;Bougault, et al, 1993;Charlou and Donval, 1993), and is compatible with the presence of methane in fluid inclusions in gabbroic rocks from ODP Hole 735B at the Southwest Indian Ridge (Kelley, 1994;Kelley et al, 1994;Kelley and Früh-Green, 1995).…”

Geochemical models of metasomatism in ultramafic systems: serpentinization, rodingitization, and sea floor carbonate chimney precipitation

“…In submarine environments CH 4 occurs in hydrothermal plumes, some examples of which have been attributed to serpentinization (Bougault et al, 1993;Fig. 4.…”

“…Oxidation of Fe reduces H in water, producing H 2 , and reduces C in HCO 3 Ϫ producing CH 4 . This process produces the H 2 and light hydrocarbon gases observed in some terrestrial hyperalkaline springs (Abrajano et al, 1990;Coveney et al, 1987;Neal and Stanger, 1983), and in submarine serpentinization (Bougault et al, 1993;Charlou et al, 1991;Charlou and Donval, 1993;Holm and Fig. 2.…”

Importance of water for Archaean granitoid petrology: a comparative study of TTG and potassic granitoids from Barberton Mountain Land, South Africa

“…The initial volatile content of a slab depends on the degree of metasomatic hydration at mid-ocean ridges. The extent of ocean floor alteration is dependent on the spreading rate (Bougalt et al 1993;Gillis 1995) and, correspondingly, on the thickness of oceanic crust. While hydration of oceanic crust might have been more extensive in the Archaean, it is important to note that, even today, oceanic crust (and exposed mantle portions) are heavily hydrated.…”