HR: 0830h
AN: V11E-0534    [PDF]
TI: Mont Albert to Buck Mountain: Provenance of Appalachian Ophiolite Chromites Using Osmium Isotopes
AU: * Minarik, W G
EM: minarik@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Gale, A
EM: agale@wam.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Booker, C
EM: rina1511@excite.com
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AB: Osmium $^{187}$Os/$^{188}$Os isotopic ratios have been determined for chrome-rich spinels from a suite of Appalachian ophiolites thought to represent Iapetus margin mantle formed and emplaced during the Ordovician. Because Re is incompatible during mantle melting while Os is compatible, non-radiogenic initial $^{187}$Os/$^{188}$Os can constrain the average source and the timing of melt extraction, especially as Os is concentrated in chromite. Radiogenic ratios indicate contamination from aged sources with high Re/Os, such as mafic or continental crust. In rocks where spinel is the only remaining primary mineral, these properties can constrain the tectonic environment of formation as well as active-margin Os transport. There is little correction for $^{187}$Os in-growth since the Ordovician due to very low sample Re. Each ultramafic unit (from Mont Albert on the Gasp\'{e} Peninsula of Qu\'{e}bec down to the Blue Ridge of North Carolina) forms a unique cluster of $^{187}$Os/$^{188}$Os ratios, spanning 1 to 3%, but the whole range is about 10%. This corresponds to a range of initial $\gamma$Os of -1 to +9, where $\gamma$Os is the percent deviation from a chondritic source at the age of formation (roughly 500 Ma). Within ophiolites where detailed mapping and other geochemical information are available, there is a correlation between mantle-like Os and tholeiitic basalts; radiogenic Os and boninites (Thetford Mines). Continental arc-related mantle chromites (Baltimore Mafic Complex; $\gamma$Os +4 to +7) are the most radiogenic. The least radiogenic are chromites from the Staten Island serpentinite and Mont Albert ($\gamma$Os -1 and 0, respectively), either indicating formation from a previously depleted source or that they predate the other Taconic ophiolites. The restricted range of each ophiolite, compared to the whole of the data set, allow provenance links to be made between isolated bodies. For example, the Buck Creek, NC ultramafic complex, which has undergone granulite facies metamorphism, (Tenthorey et al., 1996) has a similar $^{187}$Os/$^{188}$Os to other Ordovician NC Blue Ridge dunites ($\gamma$Os = +6.5 to +8.5), but distinctly different from a NC Piedmont chromitite ($\gamma$Os = +2, Falls Lake m\'{e}lange, Stoddard et al, 1989) that is inferred to be Neoproterozoic in age.
DE: 1040 Isotopic composition/chemistry
DE: 3035 Midocean ridge processes
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 9350 North America
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting