HR: 1340h
AN: MR43A-0883    [Abstracts]
TI: Fe/Mn Ratios in Ocean Island Basalts: a Tracer for Outer Core-Lower Mantle Interaction?
AU: * Qin, L
EM: qinlp@uchicago.edu
AF: University of Chicago, Dept. of the Geophysical Sciences, 5734 S. Ellis Ave., Chicago, IL 60637 United States
AU: * Qin, L
EM: qinlp@uchicago.edu
AF: National High Magnetic Field Laboratory & Dept. of Geological Sciences, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310 United States
AU: Humayun, M
EM: humayun@magnet.fsu.edu
AF: National High Magnetic Field Laboratory & Dept. of Geological Sciences, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310 United States
AB: Fe/Mn ratio can be used as a tracer to test the Outer Core-Lower Mantle interaction hypothesis about the source characteristics of some ocean island basalts (e. g. Hawaiian plume). Literature data for Fe/Mn can not resolve a difference in Fe/Mn of less than 15%. We developed an ICP-MS method to precisely measure Fe/Mn to better than 0.5%. The intensities of $^{57}$Fe and $^{55}$Mn peaks were monitored by ICP-MS, and corrected for background and interferences ($<$2%). Fe/Mn ratios were obtained by standardization against gravimetrically prepared standards. For the standards, a 0.5% difference in Fe/Mn ratio is resolvable at 2$\sigma$ level. Analysis of individual olivine and orthopyroxene grains picked from a single Kilbourne Hole peridotite xenolith showed that all five olivine and five orthopyroxene grains had uniform Fe/Mn ratio of 69.83$\pm$0.40 (2$\sigma$) and 44.27$\pm$0.16, respectively. We have recently reported precise Fe/Mn ratios for Hawaiian lavas, which showed they had resolvably higher Fe/Mn ratios (66-71) than Pacific MORBs (55-58) and Icelandic picrites. Here we report new results for OIBs, MORBs and komatiites. To avoid ubiquitous secondary Fe-Mn oxides, Fe/Mn ratios in Pacific, Atlantic and Indian MORBs were determined by Laser Ablation ICP-MS and the result showed ratios of 53 to 56, consistent with SN-ICP-MS results for fresh Pacific MORB. Icelandic picrites (MgO: 10-29%) had Fe/Mn ratios from 54 to 64. Six relatively fresh komatiites from Belingwe (MgO: 20-29%) yielded a Fe/Mn of 58.33$\pm$0.44. Seven Tahiti basalts (MgO: 8-21%) had Fe/Mn mostly between 64-70, overlapping the range for Hawaiian picrites. At the same MgO content, Tahitian and Hawaiian lavas exhibited higher Fe/Mn than MORBs, Icelandic and Belingwe lavas. This implies that the source regions of Tahiti lavas were enriched in Fe relative to other mantle sources (e. g. MORBs, Iceland, Belingwe). Together with previous results for Hawaii, this implies that high Fe/Mn is characteristic of at least 2 plumes from the Pacific Superswell. It is conceivable that this is evidence for excess Fe due to Core-Mantle interaction at the base of the Pacific Superswell.
DE: 1025 Composition of the mantle
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting