Hydrothermal Mn-deposits of the Franciscan Assemblage, II. Isotope and trace element geochemistry, and implications for hydrothermal convection at spreading centers

Details Hydrothermal Mn-deposits of the Franciscan Assemblage, II. Isotope and trace element geochemistry, and implications for hydrothermal convection at spreading centers Hydrothermal Mn-deposits of the Franciscan Assemblage, II. Isotope and trace element geochemistry, and implications for hydrothermal convection at spreading centers

Nov 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984e%26psl..71...31c&link_type=abstract

Earth and Planetary Science Letters, Volume 71, Issue 1, p. 31-45.

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Major element, trace element and Sr, Nd, Pb and O isotopic data for a Franciscan Mn-deposit suggest an origin by seafloor hydrothermal circulation. Based on Q-mode factor analysis the cherts and Mn-lenses of the Blue Jay mine formed from a combination of 4 components representing 1 biogenic, 1 hydrothermal, and 2 detrital sources. Rb-Sr, U-Th-Pb and O isotopic systematics in the Mn-lenses were affected by input from the hydrothermal circulation of material leached from the underlying basalts. Nd isotopic compositions in both cherts and Mn-lenses are identical and within the range measured for Pacific Ocean water suggesting the REE were not mobilized by hydrothermal activity. Correlation of δ18O with SiO2 and MnO2 in the Mn-lenses implies the lenses formed by simple mixing of hydrothermally derived Mn-oxides with seawater and biogenic silica. δ18O of the cherts is both uniform and depleted relative to DSDP Jurassic cherts but similar to microquartz-bearing cherts of the Monterey Formation: this suggests that diagenetic activity exerted more control on oxygen isotope compositions then hydrothermal alteration or metamorphism. Finally, a well defined Rb-Sr isochron of 158 +/- 5 Myr was obtained for these cherts and opens the possibility of determining absolute radiometric ages for similar cherts throughout the geologic record.

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