Other
Scientific paper
Nov 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.4583g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 61, Issue 21, pp.4583-4597
Other
3
Scientific paper
The distribution of elements in the hydrothermal Mn crusts from the Pitcairn Island hotspot can be explained by considering processes occurring within the hotspot volcano. These include the role of boiling of the ascending hydrothermal fluid which leads to sulphide deposition and seawater intrusion into the upper part of the volcano. This in turn results in the development of an oxycline above which FeOOH can precipitate. The high Mn/Fe ratios and low contents of Ni and Zn in the massive crusts are thought to reflect the rapid incorporation of the ore elements into sulphide minerals such that only a small proportion of these elements are available for incorporation in the crusts. The small positive Eu anomaly in the crusts on a NASC-normalized basis reflects the lower temperatures in the hydrothermal fluids at the Pitcairn Island hotspot (<250°C) compared to those at mid-ocean ridges (c.350°C). The oxyanions (CrO 2- 4 , VO 3- 4 , AsO 3- 4 , and PO 3- 4 ), the rare earth elements (REE), and Th are scavenged by FeOOH particles within the upper part of the seamount and can, therefore, be incorporated into the crusts associated with the Fe-rich phase. Molybdenum and uranium are scavenged in only negligible amounts by the FeOOH. Molybdenum is, therefore, most probably scavenged from seawater by manganese oxides during the actual formation of the Mn crusts. The alkali metals, Li, Rb, and Cs, remain in solution in the hydrothermal fluids and are selectively incorporated into the buserite structure of the Mn crusts in the absence of strong competition from the divalent transition metal ions. The compositional characteristics of the hydrothermal Mn crusts can, therefore, be explained in terms of a simple model of element scavenging and uptake occurring mainly within the seamount. A laser-ablation ICPMS profile in one of the hydrothermal crusts (DS 63-4) revealed varying REE concentrations and patterns in the different layers of the crust. These data indicate that the hydrothermal component was variable during the formation of the crust. It was calculated to be almost 100% in the upper 18.0 mm of this crust (apart from the samples at 6.0-8.0 mm) and about 80% in the lower layers.
Garbe-Schonberg C.-D.
Glasby G. P.
Jeschke Gunnar
Stoffers Peter
Stüben Doris
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