Hydrothermal Alteration of Peridotite from the Galicia Margin, Iberian Peninsula

Abstract

The Galicia margin lies northwest of the Iberian Peninsula and is a passive ocean margin with thin sedimentary cover. Altered peridotite was recovered from ODP Site 637, on the north-trending ridge at the western edge of the margin, near the oceanic/continental crust boundary.The altered ultramafics were originally clinopyroxene-rich upper mantle harzburgites and are now extensively serpentinized (>85%) and cut by very late-stage carbonate veins. Despite pervasive late, low-temperature alteration, evidence of early, high-temperature alteration remains. Alteration is apparent as (1) amphibole rims on clinopyroxene (>800°C), (2) hornblende + tremolite (450° to 800°C), (3) breakdown of hornblende to form tremolite + chlorite (<450°C), (4) zoned Cr-spinels, (5) hydration of orthopyroxene and olivine to serpentine, (6) serpentine veins, (7) replacement of pyroxene and olivine by calcite, and (8) calcite veins and vugs.Both the relict igneous and the high-temperature alteration minerals (amphiboles) show evidence of brittle deformation. Subsequent low-temperature alteration veins and minerals are deformed only in faulted and brecciated zones. This textural evidence suggests that the low-temperature alteration occurred after emplacement of the ultramafics at the surface. Serpentine fills tension fractures in orthopyroxene, and both serpentine and calcite fill tension cracks in olivine.The high-temperature alterations in these samples are similar to those found in oceanic fracture zone and ophiolite ultramafics. This widespread occurrence of high-temperature alteration suggests that hot fluids were pervasive in these ultramafic blocks.Localization of high-temperature alteration close to large carbonate veins suggests channelization of the late, lowtemperature fluids. Earlier hydrations (e.g., high-temperature alterations and serpentinization) were pervasive.

Serpentine and brucite of ultramafic clasts from the South Chamorro Seamount (Ocean Drilling Program Leg 195, Site 1200): inferences for the serpentinization of the Mariana forearc mantle

“…bastite), and as vein filling (Table 1 Wicks and Plant, 1979;Fig. 3) displays the wide chemical variability of the serpentine minerals from Site 1200 ultramafic clasts, which also matches the wide compositional range of serpentine minerals described in the literature (e.g. Wicks and Plant, 1979;Agrinier et al, 1988;Kimball and Evans, 1988;O'Hanley and Dyar, 1993;Agrinier et al, 1996;Dilek et al, 1997;Viti and Mellini, 1998;Puga et al, 1999). Some of the serpentines analysed have an unusually high FeO content, reaching 14.5 wt.%, and thus fall outside the range reported in the literature for most serpentines in metaperidotites, although similar serpentines, with FeO up to 14.3 wt.%, were reported by Wicks and Plant (1979).…”

“…The serpentine-brucite mixed analyses of Table 3 are excluded. The fields for lizardite, chrysotile and antigorite were drawn using data from Wicks and Plant (1979), Agrinier et al (1988), Kimball and Evans (1988), O'Hanley and Dyar (1993), Agrinier et al (1996), Dilek et al (1997) and Puga et al (1999), excluding analyses with the most extreme FeO and SiO 2 contents. Sample (Leg,Hole,Core, Sample (Leg,- Sample (Leg,-…”

Chemical evidence for near-seafloor precipitation of calcite in serpentinites (Site 897) and serpentinite breccias (Site 899), Iberia Abyssal Plain

“…The relatively simple postrifting burial history at Sites 897 and 899 (Shipboard Scientific Party, 1994a, 1994b provides a relatively well-constrained context for interpretation of chemical data. A close analogy can be drawn between the calcitized serpentinites at Sites 897 and 899 and previously described carbonate-enriched ultramafic rocks at mid-ocean ridges (e.g., Bonatti et al, 1974Bonatti et al, , 1980 and seaward of the Galicia margin (Agrinier et al, 1988;Kimball and Evans, 1988). Samples recovered during Leg 149 span the sediment/basement contact and extend nearly 200 m into serpentinitic materials at both sites, allowing examination of ophicarbonate generation in unprecedented detail.…”

Sulfur geochemistry of peridotite-hosted hydrothermal systems: Comparing the Ligurian ophiolites with oceanic serpentinites

“…During ODP Leg 103 serpentinized ultramafic rocks were recovered from Galicia Bank (Site 637; Fig. 3) (Agrinier and Girardeau, 1988;Kimball and Evans, 1988). Seismic data and further drilling during Leg 149 confirmed the existence of a 300 km long peridotite ridge (Sawyer et al, 1994).…”