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

Früh-Green, G. L.; Plas, Alessio; Lécuyer, Christophe

doi:10.2973/odp.proc.sr.147.016.1996

Cited by 73 publications

(130 citation statements)

References 70 publications

(124 reference statements)

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“…Most of the oxygen-isotope studies of oceanic serpentinized peridotites to date have suggested lower temperatures of serpentinization (70°-250°C; Wenner and Taylor, 1971;Sheppard, 1980;Bonatti et al, 1984). However, high temperatures of serpentinization, similar to those calculated in this paper, have recently been reported from the Hess Deep ultramafic exposures (East Pacific Rise; Agrinier et al, 1995;Früh-Green et al, 1996).…”

supporting

confidence: 75%

“…The corresponding oxygen-isotope fractionations, Δ 1 ^serpentine-magnetite), are small (between 5.1%o and 3.8%o), and are lower than the previously published values for oceanic serpentinized peridotites (above 9.5%e; Wenner and Taylor, 1973) but are similar to those measured in Hess Deep serpentinized peridotites (Früh-Green et al, 1996;Agrinier et al, 1995). Although the…”

Section: Oxygen-isotope Fractionation Between Serpentine and Magnetitesupporting

confidence: 54%

“…This range is shifted to higher values (4.3%o to 5.4%o) at 450°C. Again, a considerable uncertainty (± l‰ at least) is associated with these values because the temperature calibration of the (serpentine-water) factor is not well known (Früh-Green et al, 1996).…”

Section: Water-rock Interaction Parameters During Serpentinizationmentioning

confidence: 99%

“…

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).

…”

mentioning

confidence: 99%

“…O fractionation between serpentine and magnetite has not yet been calibrated vs. temperature by laboratory experiments, some natural observations and regularities have led Wenner and Taylor (1971) to propose the following "tentative" geothermometer (revised by Früh-Green et al, 1996):…”

mentioning

confidence: 99%

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

Agrinier¹,

Cannat²

1997

Proceedings of the Ocean Drilling Program

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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.

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supporting

confidence: 75%

Section: Oxygen-isotope Fractionation Between Serpentine and Magnetitesupporting

confidence: 54%

Section: Water-rock Interaction Parameters During Serpentinizationmentioning

confidence: 99%

“…

…”

mentioning

confidence: 99%

mentioning

confidence: 99%

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

Agrinier¹,

Cannat²

1997

Proceedings of the Ocean Drilling Program

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Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid-Atlantic Ridge 36°14′N)

Andréani

Escartı́n

Delacour

et al. 2014

Geochem. Geophys. Geosyst.

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Rainbow is a dome-shaped massif at the 36 14 0 N nontransform offset along the Mid-AtlanticRidge. It hosts three ultramafic-hosted hydrothermal sites: Rainbow is active and high temperature; Clamstone and Ghost City are fossil and low temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W dipping detachment fault, now inactive, uplifted the massif and exhumed these deep-seated rocks. Present-day deformation is accommodated by SSW-NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatiotemporal progression. Markers of high-temperature hydrothermal activity (350 C) are restricted to some samples from the active field while a more diffuse, lower temperature hydrothermal activity (<220 C) is inferred at various locations through anomalously high As, Sb, and Pb contents, attributed to element incorporation in serpentines or microscalesulfide precipitation. Petrographic and geochemical analyses show that the dominant basement alteration is pervasive peridotite serpentinization at 160-260 C, attributed to fluids chemically similar to those venting at Rainbow, and controlled by concomitant alteration of mafic-ultramafic units at depth. Rainbow provides a model for fluid circulation, possibly applicable to hydrothermalism at oceanic detachments elsewhere, where both low-temperature serpentinization and magmatic-driven high-temperature outflow develop contemporaneously, channeled by faults in the footwall and not along the detachment fault.

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Tectonic slicing of subducting oceanic crust along plate interfaces: Numerical modeling

Ruh

Pourhiet

Agard

et al. 2015

Geochem Geophys Geosyst

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Multikilometer‐sized slivers of high‐pressure low‐temperature metamorphic oceanic crust and mantle are observed in many mountain belts. These blueschist and eclogite units were detached from the descending plate during subduction. Large‐scale thermo‐mechanical numerical models based on finite difference marker‐in‐cell staggered grid technique are implemented to investigate slicing processes that lead to the detachment of oceanic slivers and their exhumation before the onset of the continental collision phase. In particular, we investigate the role of the serpentinized subcrustal slab mantle in the mechanisms of shallow and deep crustal slicing. Results show that spatially homogeneous serpentinization of the sub‐Moho slab mantle leads to complete accretion of oceanic crust within the accretionary wedge. Spatially discontinuous serpentinization of the slab mantle in form of unconnected patches can lead to shallow slicing of the oceanic crust below the accretionary wedge and to its deep slicing at mantle depths depending on the patch length, slab angle, convergence velocity and continental geothermal gradient. P‐T paths obtained in this study are compared to natural examples of shallow slicing of the Crescent Terrane below Vancouver Island and deeply sliced crust of the Lago Superiore and Saas‐Zermatt units in the Western Alps.

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

Cited by 73 publications

References 70 publications

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

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

Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid-Atlantic Ridge 36°14′N)

Tectonic slicing of subducting oceanic crust along plate interfaces: Numerical modeling

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