Seafloor hydrothermal clay alteration at Jade in the back-arc Okinawa Trough: Mineralogy, geochemistry and isotope characteristics

Journal Article · Wed Sep 01 04:00:00 EDT 1999 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/S0016-7037(99)00158-1· OSTI ID:20000936

Marumo, Katsumi; Hattori, K H

Seafloor hydrothermal activity at Jade has resulted in extensive alteration of the host epiclastic sediments and pumiceous tuffs, forming mica, kaolins (kaolinite and halloysite), Mg-rich chlorite, talc, montmorillonite, and a mixed-layer mineral of dioctahedral chlorite and montmorillonite (Chl/Mont). Clay mineral assemblages show a vertical variation, which reflects variable amounts of cold seawater incorporated into hot hydrothermal fluids in subsurface sediments and tuff. However, mixing alone cannot explain the occurrence of abundant kaolin minerals at Jade. The formation of kaolin minerals requires much more acidic fluid than expected from simple mixing of hydrothermal fluids and cold seawater. Low pH values are likely attained by oxidation of H{sub 2}S either dissolved in the hydrothermal fluid or released from the fluid during decompression. The fluid reaching the seafloor is discharged into cold seawater, which caused precipitation of sulfides close to vents and native sulfur and barite at the margins of the vent areas. Halloysite, barite and anhydrite show Sr isotope compositions similar to marine Sr, indicating the derivation of marine Sr directly from seawater or by the dissolution of calcareous nannoplanktons. At Jade, there is only one black smoker actively discharging high temperature ({approximately}320 C) fluid, but there are many fossil sulfide chimneys and mounds in the area. The mineralogy and high Au and Cu in these precipitates suggest highly metalliferous hydrothermal activity in the past. These activities likely resulted in discharge of hydrothermal plumes and fall-outs of sulfides and sulfates on the seafloor. These fall-outs were incorporated in sediments far from the vent areas. They are now recorded as high metal contents in sediments with no petrographic and mineralogical evidence of in-situ hydrothermal activity. Some are high as 8,100 ppm for Cu, 12,500 ppm for Zn, 1,000 ppm for As, 100 ppm for Ag and 21,000 ppm for Pb. Detrital grains of montmorillonite in such sediments are coated with Fe-oxyhydroxides during the suspension in seawater before settling on the seafloor. The depths of such metal anomalies in sediments suggest high levels of metalliferous hydrothermal activities from 1,800 to 300 ybp.

• Research Organization: Geological Survey of Japan, Tsukuba, Ibaraki (JP)
• OSTI ID: 20000936
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 18 Vol. 63; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
CLAYS
GEOCHEMISTRY
GEOLOGIC HISTORY
HYDROTHERMAL ALTERATION
HYDROTHERMAL SYSTEMS
ISOTOPE RATIO
MINERALOGY
SEA BED