OXYGEN-ISOTOPE CONSTRAINTS ON SERPENTINIZATION PROCESSES IN ULTRAMAFIC ROCKS FROM THE MID-ATLANTIC RIDGE (23N)

Pierre Agrinier and Mathilde Cannat

ABSTRACT

Extensively serpentinized peridotites, drilled at the Mid-Atlantic Ridge south of the Kane Fracture Zone (Ocean Drilling Program Holes 920B, 920D, and 670A), have been analyzed for the 18O/16O ratios of their serpentine and magnetite to derive temperature and water: rock ratio information about the hydrothermal process that produced their serpentinization.

These peridotites have low delta18O values (3.72.6), and show low 18O/16O fractionation factors between coexisting serpentine and magnetite; D(serpentine-magnetite) values are between 5.1 and 3.8. By applying the serpentine-magnetite geothermometer of Wenner and Taylor (1971) as revised by Frh-Green et al. (1996), we calculate serpentinization temperatures ranging between 430 and 590C. However, a sensitivity analysis shows that large errors affect these temperatures (about 100C). Nevertheless, we think we can determine that the serpentinization occurred at temperatures higher than 350C.

Using the mass-balance equation developed by Taylor (1977), and assuming that seawater (delta18O = 0) or 18O hydrothermally enriched seawater (delta18O = +2.4) is the serpentinizing fluid, we can determine that the water:rock ratios (in oxygen atom proportions) were, at 400C, around 0.55 0.1 and 1.05 0.2, respectively, and at 500C, 0.4 0.1 and 0.8 0.1, respectively.

The relatively high serpentinization temperatures derived from this study imply that serpentinization has occurred at significant depth in the axial lithosphere. This has potentially important consequences, which we discuss, on the amount of tectonic rotation during the uplift of the ultramafic rocks to the seafloor, on the orientation of principal stresses at depth beneath the axial valley, and on the seismic velocity and density structure of the oceanic lithosphere.

All types of serpentine (meshwork and vein) from Site 920 appear to have formed at these relatively high temperatures. However, previous oxygen-isotope studies (Komor et al., 1990; Hbert et al., 1990) of cores from Site 670 reported ultramafic rocks that were serpentinized at lower temperature (<200C). This indicates that the ultramafic component of the axial lithosphere in the area did not become entirely serpentinized at depths corresponding to rock temperatures of at least 350C.

Date of initial receipt: 7 August 1995
Date of acceptance: 1 February 1996


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