The Central Indian Ridge between 21°- 24°S, near the
RodriguezT riple Junction, was investigatedb y RV SONNE
during the 1983 and 1986 cruises of the GEMINO project
(Geothermal Metallogenesis Indian Ocean) for the purpose
of locating sites of hydrothermal activity. In response to
reconnaissance data of conductive heat flow, sediment
cores, and hydrocasts, two intermediate spreading ridge segments
at 2l.5°S (area EX,/FX) and 23°S (area JX) were
selected for detailed exploration including mapping, sampling
and visual seafloor observation. Hydrothermally
influenced sediment cores and altered basalts were found
in both study areas. Total dissolvable Mn concentrations
up to 27.5 nmol/kg and CHa values up to 45.6 nL/L occur
within 450 m above bottom in the northern and southern
overlapping propagators in area EX/FX. In addition,
63He values up to 18.6% and a potential temperature
anomaly (0.01°C) were found. In area JX, maximum concentrations
of 36.1 nL/L CHa and 10.7 nmol,/kg total dissolvable
Mn in the water column seem to be related to a
central neovolcanic ridge close to a ridge-transform intersection.
At the eastern flank of this ridge, a significant
potential temperature anomaly of 0.04°C was measured.
The results obtained in areas EX/FX and JX provide evidence
for hydrothermal activity. However, in comparison
to the TAG hydrothermal field at the Mid-Atlantic Ridge
at 26°N, the data indicate a more restricted and probably
lower temperature hydrothermal discharge.
Forty‐four sediment samples and 23 Mn crusts and Mn nodules have been analyzed for several elements in order to characterize their geochemical nature. Most of the Mn crusts and Mn nodules are of hydrogenous origin, although one crust (U375) from the flanks of the Rarotonga volcano is thought to be of hydrothermal origin. δ‐MnO2 is the principal Mn mineral in the crusts, whereas todorokite and birnessite occur in addition to δ‐MnO2 in the nodules. The sediments can be divided into carbonates, with calcite as the dominant phase, and deep‐sea clay, consisting mainly of illite and smectite.
Abstract Sediment cores have been taken from Granite Harbour (3) and New Harbour (1)in water From 254 to 550 m deep.The cores, which range m length from 36 to 56 cm, are muddy sand and sandy mud with a few scattered pebbles, and have been analysed in detail for size, mineralogy and composition. Thirty-two chemical analyses are presen ted. Each core varies little throughout its length, but there is a substantial difference between cores from Granite Harbour and New Harbour.Petrographic study shows that Granite Harbour cores are dominated by granitic material from the local basement, whereas the New Harbour core has mainly debris from the McMurdo Volcanics, probably windblown and ice-rafted from the south.The difference in source is the main influence on the major and trace element chemistry.
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