Pumpellyite from the oceanic crust, DSDP/ODP Hole 504B

Ishizuka, Hiroki

doi:10.1180/minmag.1999.063.6.08

Cited by 7 publications

(4 citation statements)

References 18 publications

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“…Hyaloclastite clasts display pumpellyite aggregates in an assemblage of chlorite and quartz. Similar assemblages have been described in hydrothermally altered oceanic basalts [e.g., M evel, 1981; Ishizuka, 1999] and ascribed to seawater/rock interactions at temperatures consistent with prehnite-pumpellyite conditions (at 2508C), which are somewhat cooler than greenschist conditions. The remaining three breccias show textures that are not clearly fault-related, and are referred to as undetermined in Table 2 (see also Figure 1b).…”

Section: Deformed Samples From the Detachment Rubble Blanketsupporting

confidence: 74%

Pervasive silicification and hanging wall overplating along the 13°20′N oceanic detachment fault (Mid‐Atlantic Ridge)

Bonnemains

Escartı́n

Mével

et al. 2017

Geochem Geophys Geosyst

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The corrugated detachment fault zone of the active 13°20′N oceanic core complex (Mid‐Atlantic Ridge) was investigated with a deep‐sea vehicle to assess the links between deformation, alteration, and magmatism at detachment fault zones. We present a study of 18 in situ fault rock samples from striated fault outcrops on the flanks of microbathymetric corrugations. All the samples are mafic breccias that are mostly derived from a diabase protolith, with two of them also showing mixing with ultramafic clasts. Breccias are cataclastic and display variable deformation textures, recording numerous slip events, and showing pervasive silicification throughout the fault zone. Deformation‐silicification relationships are also complex, showing both static and syntectonic quartz precipitation; undeformed quartz overprints the fault breccia textures, and reflective and striated fault surfaces cross‐cut silicified rocks. In situ detachment fault rocks are mainly fault breccias with almost exclusively basaltic clasts, with rare ultramafic ones, a lithology and texture never observed previously at other oceanic detachment fault zones. We propose the lower dyke complex in the hanging wall crust at the volcanic rift valley floor is the most plausible diabase source. Mechanical mixing of predominantly mafic and rare ultramafic clasts suggests an underlying ultramafic footwall and that mafic accretion operates in the shallowest crust (1–2 km), at the base of the dyke complex at temperatures >400°C. Silicification is produced by silica‐rich fluids syntectonically channeled along the fault zone, and likely derived from hydrothermal alteration of basaltic rocks, likely mixed with serpentinization‐derived fluids.

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Section: Deformed Samples From the Detachment Rubble Blanketsupporting

confidence: 74%

Pervasive silicification and hanging wall overplating along the 13°20′N oceanic detachment fault (Mid‐Atlantic Ridge)

Bonnemains

Escartı́n

Mével

et al. 2017

Geochem Geophys Geosyst

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“…The presence of pumpellyite (rather uncommon in the oceanic environment; Me´vel, 1981), in corona with chlorite and goethite around chalcopyrite in a calcitebearing gabbro also suggest a low-temperature, fluidcontrolled, hydrothermal reaction. The same feature has been observed by Ishizuka (1999): the pumpellyite and the associated chlorite and goethite are the products of the reaction between plagioclase and an iron oxide with a fluid bringing Cu and S.…”

Section: Origin and Evolution Of Paragenesessupporting

confidence: 72%

Ocean‐floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker

Berger

Féménias

Mercier

et al. 2005

Journal Metamorphic Geology

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The Limousin ophiolite is located at the suture zone between two major thrust sheets in the western French Massif Central. This ophiolitic section comprises mantle-harzburgite, mantle-dunite, wehrlites, troctolites and layered gabbros. It has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and/or pargasite-bearing chloritites. With various thermobarometric methods, it is possible to show that the different varieties of amphibole have registered low-P (c. 0.2 GPa) conditions with temperature ranging from high-T, latemagmatic conditions to greenschist-zeolite metamorphic facies. The abundance of undeformed metamorphic rocks (which is typical of the lower oceanic crust), the occurrence of Ca-Al (-Mg) metasomatism illustrated by the growth of Ca-Al silicates in veins or replacing the primary magmatic minerals, the P-T conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from Ocean Drilling Program and worldwide ophiolites are the main arguments for an ocean-floor hydrothermal metamorphism in the vicinity of a palaeo-ridge. Among the West-European Variscan ophiolites, the Limousin ophiolites constitute an extremely rare occurrence that has not been involved in any HP (subduction-related) or MP (orogenic) metamorphism as observed in other ophiolite occurrences (i.e. France, Spain and Germany).

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“…It was also observed that basalts displaying the mineralogical assemblages characteristic of the prehnite-pumpellyite facies were rarely found in the seafloor, but were more frequent in ophiolites. Prehnite alone is common in oceanic metagabbros (e.g., [78,129]), but pumpellyite has only been observed three times in ocean-floor metabasalts [93,120,128]. Pumpellyite was associated with prehnite in two of these three oceanic occurrences.…”

Section: Misusing Eskola's Concept Of Metamorphic Faciesmentioning

confidence: 99%

Hydrothermal alteration vs. ocean-floor metamorphism. A comparison between two case histories: the TAG hydrothermal mound (Mid-Atlantic Ridge) vs. DSDP/ODP Hole 504B (Equatorial East Pacific)

Honnorez

2003

Comptes Rendus. Géoscience

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This article presents a review of hydrothermal alteration of the various basaltic rocks forming the oceanic crust, emphasizing especially on the water-rock interaction processes, the petrography and secondary mineralogy of hydrothermally altered rocks from the present-day ocean, hence excluding the ophiolitic complexes. A brief summary of the history of the first studies of hydrothermally altered oceanic rocks leads to a comparison between Miyashiro's concept of 'ocean-floor metamorphism' and that of hydrothermal alteration that warrant caution when applying Eskola's metamorphic facies of regional metamorphism to hydrothermally altered oceanic rocks. The functioning of mid-oceanic ridge axial hydrothermal systems, and the role of oceanic Layer-3 gabbros are discussed in detail. The mechanisms of the various alteration processes of the rocks forming the oceanic crust are presented. The case histories of the two examples more particularly studied by the author and his collaborators, are compared, i.e., the DSDP-ODP Hole 504B reference section South of the Costa Rica Ridge, and the TAG active mound at the Mid-Atlantic Ridge. The significance of the paragonitic phyllosilicate in highly altered rocks form the TAG Mound is compared to the other known occurrences of 'white micas' in the oceanic crust. The importance of metabasites as major components of the oceanic crust is emphasized. The elemental fluxes resulting from hydrothermal alteration of oceanic rocks in regulating seawater chemistry are only briefly alluded, because this topic is covered by the last article of this thematic issue by J.C. Alt, who assesses the chemical budgets resulting from ocean hydrothermal activity.

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Pumpellyite from the oceanic crust, DSDP/ODP Hole 504B

Cited by 7 publications

References 18 publications

Pervasive silicification and hanging wall overplating along the 13°20′N oceanic detachment fault (Mid‐Atlantic Ridge)

Pervasive silicification and hanging wall overplating along the 13°20′N oceanic detachment fault (Mid‐Atlantic Ridge)

Ocean‐floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker

Hydrothermal alteration vs. ocean-floor metamorphism. A comparison between two case histories: the TAG hydrothermal mound (Mid-Atlantic Ridge) vs. DSDP/ODP Hole 504B (Equatorial East Pacific)

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