HR: 0800h
AN: OS21A-1205    [Abstracts]
TI: The Meaning of 87Sr/86Sr Ages of Authigenic Seamount Phosphorite
AU: * Yeh, H
EM: hwyeh@earth.sinica.edu.tw
AF: Hsueh-Wen Yeh, Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, 11529 Taiwan
AU: Chan, L
EM: lui-chan@geol.lsu.edu
AF: Lui-Heung Chan, Department of Geology and Geophysics Louisiana State University, Baton Rouge, LA, 70803 United States
AU: Hein, J R
EM: jhein@usgs.gov
AF: James R. Hein, USGS, 345 Middlefield Road, M.S. 999, Menlo Park, CA, 94025 United States
AB: Phosphorite is a common deposit on many seamounts of world oceans. The phosphate mineral of the deposit is apparently carbonate-fluorapatite (CFA) and contains significant amount of Sr. Evidence shows the mineral to be a marine authigenic phase. The Sr-isotope (i.e. 87Sr/86Sr) ages of the mineral samples from many seamounts of Pacific Ocean show that they were formed episodically. Their presents thus represent periods of unique ocean chemistry in world-ocean history. The implicit assumption of the application of and the episodic conclusion based on the 87Sr/86Sr is rapid formation and insignificant post-formation isotopic alteration of the mineral. This study is positioned to investigate the validity of the assumption to clarify the significance of the episodic conclusion. 87Sr/86Sr of Sr samples extracted sequentially from an essentially pure marine authigenic CFA was measured to determine the isotopic homogeneity in the mineral. The results show that the isotopic composition of first few percents of Sr-extract is significantly different from the rest. This several percents of isotopically different Sr appears to be a mixture of modern seawater and the mineral\-bulk Sr. The contribution of Sr from modern seawater, however, appears to be minor. Thus, the rate of post-formation Sr exchange between the CFA and seawater appears to be slow and shall not affect the strontium-isotope derived age of formation of the CFA significantly. The fact that 87Sr/86Sr of the rest Sr-extracts which constitutes dominant portion of the CFA is homogeneous indicates the rapidity of its formation. The results of this study are consistent with the notion that the authigenic seamount CFA of Pacific basin studied so far were formed rapidly and suffered little post-formation Sr-isotope alteration. This thus renders further support for the proposition that there were episodic genesis of phosphate deposits in Cenozoic world Oceans.
DE: 4267 Paleoceanography
DE: 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting