Low-temperature hydrothermal alteration of mid-oceanic ridge basalt from the Southwest Indian Ridge in the Indian Ocean

Author(s): Nakamura, Kentaro
Author Affiliation(s): Primary:
University of Tokyo, Department of Geosystem Engineering, Tokyo, Japan
Volume Title: Shigen Chishitsu
Source: Shigen Chishitsu, 51(2), p.121-132. Publisher: Shigen Chishitsu Gakkai - Society of Resource Geologists of Japan, Tokyo, Japan. ISSN: 0918-2454
Note: In Japanese with English summary. 50 refs.; illus., incl. 3 tables, sketch map
Summary: Low-temperature hydrothermally altered basalts dredged from Southwest Indian Ridge (SWIR) near the Rodriguez Triple Junction in the Indian Ocean have been studied to document the petrological and geochemical characteristics and to compare these features with those in the upper part of IPOD/ODP Hole 504B. Dredged samples were classified into two groups of B1-group and B2-group on the basis of the degree of alteration. B1-group samples are characterized by the presence of brownish to greenish clay minerals which fill veins and/or vesicles. On the other hand, B2-group samples are characterized by the presence of yellowish clay minerals which partly to totally replace olivine phenocrysts. Electron microprobe analyses of these clay minerals indicate that brownish to greenish clay minerals in B1-group samples are composed of celadonite, saponite, Fe-oxyhydroxide, and mixed layer of saponite-celadonite and Fe-oxyhydroxide-celadonite with various proportions and yellowish clay minerals in B2-group samples are pure saponite. Compared to the relatively fresh samples, whole-rock chemical compositions of B1-group and B2-group samples are enriched in K2O, Rb, and U. The enrichment of K2O and Rb suggests that these elements were added to the altered basalts from low-temperature hydrothermal solutions during the formation of the K-rich celadonite. On the other hand, MgO concentration of B1-group and B2-group are about the same as that of the relatively fresh samples, indicating that MgO is relatively immobile during the formation of the Mg-rich saponite at low-temperature. This result suggests that the low-temperature hydrothermal alteration at mid-oceanic ridge does not play an important role as an Mg sink. These petrological and geochemical features of B1-group and B2-group samples can be comparable to those of the upper and lower pillow zone in IPOD/ODP Hole 504B, respectively. However, the altered basalts from SWIR contain few carbonate and zeolite minerals, implying that these altered basalts dredged from the ridge axis of SWIR have not undergone the off-axis hydrothermal alteration reported by Alt et al. (1986) from IPOD/ODP Hole 504B.
Year of Publication: 2001
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 05 Petrology, Igneous and Metamorphic; Basalts; Celadonite; Clay minerals; Deep Sea Drilling Project; Hydrothermal alteration; IPOD; Igneous rocks; Indian Ocean; Low temperature; Metasomatism; Mica group; Mid-ocean ridge basalts; Mineral composition; Ocean Drilling Program; Saponite; Sheet silicates; Silicates; Southwest Indian Ridge; Temperature; Volcanic rocks
Coordinates: S300000 S300000 E0600000 E0600000
Record ID: 2002036625
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