Oxygen-isotope constraints on serpentinization processes in ultramafic rocks from the Mid-Atlantic Ridge (23°N)

Abstract

Früh-Green et al. (1996), we calculate serpentinization temperatures ranging between 430° and 590°C. However, a sensitivity analysis shows that large errors affect these temperatures (about ±100°C). Nevertheless, we think we can determine that the serpentinization occurred at temperatures higher than 350°C.Using the mass-balance equation developed by Taylor (1977), and assuming that seawater (δ 18 θ = Woo) or 18 O hydrothermally enriched seawater (δ 18 θ = +2.4%c) is the serpentinizing fluid, we can determine that the wateπrock ratios (in oxygen atom proportions) were, at 400°C, around 0.55 ± 0.1 and 1.05 ± 0.2, respectively, and at 500°C, 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 Hebert et al., 1990) of cores from Site 670 reported ultramafic rocks that were serpentinized at lower temperature (<200°C). 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 350°C.

Mechanism of magnetic property changes of serpentinites from ODP Holes 897D and 1070A

“…Zhao et al (2006) and Beard et al (2009) investigated the formation of mesh-textured magnetite, and suggested that serpentinization takes places at T<200C based on oxygen-isotope geothermometry results (Agrinier et al, 1996;Bach et al, 2004). However, previous studies on serpentinites recovered from the Kane Fracture Zone, Mid-Atlantic Ridge, proposed a complex metamorphic evolution path including hightemperature (>300-400C) serpentinization (Agrinier et al, 1997) which is in agreement with the results of ODP Sites 670 and 920 (Mével, 2003).…”

“…In our samples, intervals of moderately serpentinized peridotites in the upper part of the hole commonly have a reddish color which reflects later oxidation. Oxygen isotopic compositions of serpentine-magnetite pairs in these intervals correspond to relatively low temperatures (<200°C) (Agrinier et al, 1997). By contrast, serpentinites in the lower parts of the two holes are dark green, and the oxygen isotopic compositions are typical of high-temperature serpentinization (>350°C).…”

Serpentinization and heat generation: constraints from Lost City and Rainbow hydrothermal systems 1 1Associate editor: J. C. Alt

“…Although few data exist to establish unambiguously W/R for oceanic serpentinites, the limited data that do exist suggest values of approximately unity for relatively high-temperature serpentinites from the Mid-Atlantic Ridge (Agrinier and Cannat, 1997), and values considerably higher than this for serpentinites having formed at lower temperatures (Bonatti et al, 1984). According to reaction 1 the minimum W/R for complete serpentinization is approximately 0.3, since values lower than this would fail to provide sufficient water for olivine hydrolysis to proceed to completion, and thus, no fluid would be available for venting.…”

Separation of 3He and CH4 signals on the Mid-Atlantic Ridge at 5°N and 51°N

“…In this regard, our model is diagnostic rather than predictive. A W/R % 1 is indicated by oxygen isotope measurements on serpentine outcropping south of the Kane Fracture Zone (Agrinier and Cannat, 1997). An alternative, prognostic approach would be to incorporate the kinetics of serpentinization as a continuous function of temperature as recently done by Emmanuel and Berkowitz (2006) in a cellular convection model.…”