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Serpentinization at the Rainbow and Saldanha sites, Mid-Atlantic Ridge; mineralogical, geochemical, and isotopic features

Isabel Ribeiro da Costa, Frederick Joseph Wicks and Fernando J. A. S. Barriga
Serpentinization at the Rainbow and Saldanha sites, Mid-Atlantic Ridge; mineralogical, geochemical, and isotopic features
The Canadian Mineralogist (September 2019) 57 (5): 677-706

Abstract

The Rainbow hydrothermal field (36 degrees 14'N) and the Saldanha seamount (36 degrees 34'N), in the Mid-Atlantic Ridge (MAR), are tectonic exposures of serpentinized upper mantle peridotites, both associated with significant hydrothermal activity. On the basis of detailed mineralogical and geochemical characterization of serpentinites from both sites, several serpentinization-related issues are discussed in the present work. As expected in oceanic environments, most of the sampled rocks are lizardite-chrysotile serpentinites exhibiting a variety of pseudomorphic through non-pseudomorphic textures, such textural evolution probably being related to changing water/rock ratios during this retrograde process. Oxygen isotope temperatures indicate that the serpentinization took place at 300-200 degrees C; on the other hand, isotopic data suggest that replacement of early pseudomorphic lizardite by lizardite + or - chrysotile non-pseudomorphic textures requires that temperatures and/or water/rock ratios are high enough to promote the necessary dissolution-recrystallization processes. Mass-balance calculations for olivine-serpentine and orthopyroxene-serpentine pairs provided a basis for establishing serpentinization reactions likely to have produced the present rocks. Moreover, these calculations also showed that, notwithstanding some noticeable loss of MgO from olivine and of SiO2 from orthopyroxene, serpentinization of both minerals implies volume increases on the order of 26-27%, therefore potentially promoting the overall expansion of the rock. The geochemical and isotopic features of the studied rocks indicate that unmodified seawater was responsible for the serpentinization of the MAR peridotites. However, the mineralogy and REE patterns of some of these serpentinites indicate occasional subsequent interaction of the serpentinized rocks with seawater at much lower temperatures (seafloor alteration, characterized by carbonate deposition and negative Ce anomalies), or with high-temperature ore-forming hydrothermal fluids (ore-forming alteration, characterized by sulfide precipitation and steep positive Eu anomalies).


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 57
Serial Issue: 5
Title: Serpentinization at the Rainbow and Saldanha sites, Mid-Atlantic Ridge; mineralogical, geochemical, and isotopic features
Affiliation: Universidade de Lisboa, Departamento de Geologia, Lisbon, Portugal
Pages: 677-706
Published: 20190904
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 115
Accession Number: 2019-081257
Categories: Isotope geochemistryIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables, sketch map
N35°00'00" - N38°00'00", W34°00'00" - W31°30'00"
Secondary Affiliation: Royal Ontario Museum, CAN, Canada
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201943
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