Extreme positive Ce-anomalies in a 3.0 Ga submarine volcanic sequence,Murchison Province: Oxygenated marine bottom waters |
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| Authors: | Robert Kerrich Nuru Said |
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| Institution: | 1. Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada;2. Faculty of Geology and Geography, Tomsk State University, 36 Lenin Ave, Tomsk 634050, Russia;3. University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1, Canada;4. Geological Survey of Canada, Ottawa, ON K1A 0E8, Canada;5. Department of Earth & Ecosystem Sciences, Lund U,Sweden;6. Texas Tech University, 125 Science Building, Lubbock, TX79409-1053, USA;1. Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, United States;2. Department of Earth and Planetary Sciences/GEOTOP, McGill University, Montreal, QC H3A 2T5, Canada;3. Department of Geological Sciences and Geological Engineering, Queen''s University, Kingston, Ontario K7L 3N6, Canada;4. Princeton University, Guyot Hall, Princeton, NJ 08544, United States;1. IFREMER, Centre de Brest, 29280 Plouzané, France;2. Université de Brest, IUEM, UMR 6538, 29280 Plouzané, France;3. Origins Laboratory, Department of the Geophysical Sciences & Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, United States;4. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, United States;5. Department of Earth Sciences, University of California, Riverside, CA 92521-0423, United States |
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| Abstract: | Systematic positive anomalies of Ce, where Ce/Ce* spans 2.1 to 11.4, are present in basalts and rhyolites of a 2.96 Ga submarine volcanic sequence of the Murchison Province, Western Australia. This volcanic sequence is host to a stratabound Cu–Zn deposit that formed on the seafloor from a seawater hydrothermal system. These are true Ce anomalies as Pr/Pr* < 1. In modern oxygenated marine water Ce is sequestered by Mn-oxides and hydroxides, which coprecipitate with Fe-oxides and hydroxides as nodules and crusts on the ocean floor, as well as Fe–Mn chemical sediments from hydrothermal systems at ocean spreading centers. Fe–Mn sediments have positive Ce anomalies and marine water complementary negative anomalies. Such Ce anomalies have not formerly been reported for Archean hydrothermally altered volcanic rocks. These extreme anomalies are attributed to Mn-transport in shallow-circulating oxygenated marine bottom waters peripheral to the deeper, hotter, hydrothermal system from which the Cu–Zn deposit formed, and record an oxygenated marine environment ~ 500 Ma before the so-called great oxidation event at ~ 2.4 Ga. Results for positive Ce anomalies in the Golden Grove volcanic sequence are complementary to negative anomalies in Archean BIF, collectively stemming from particulate scavenging of Ce+ 3 in an oxic water column. |
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