HR: 15:10h
AN: V52D-07    [PDF]
TI: Global similarity in oxygen-isotope composition of MORB
AU: * Cooper, K M
EM: cooper@ess.washington.edu
AF: Dept of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 United States
AU: Eiler, J M
EM: eiler@gps.caltech.edu
AF: Geological & Planetary Sci Caltech, 1200 E. California Blvd, Pasadena, CA 91125 United States
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, Dept. Earth & Planetary Sci., 20 Oxford St., Cambridge, MA 02138 United States
AU: Sims, K W
EM: ksims@whoi.edu
AF: WHOI, Dept. Geology & Geophysics, Woods Hole, MA 02543 United States
AB: Primitive lavas from volcanic arcs and ocean islands vary in $\delta^{18}$O. In some instances, these variations can be attributed to the presence of recycled crustal materials in their mantle source. In these cases, oxygen isotope data provide unique constraints on amounts and origins of those components. Radiogenic isotope data suggest mid-ocean-ridge basalts (MORBs) also contain variable amounts of recycled crustal materials within their mantle sources, suggesting that oxygen isotope data might be similarly useful in constraining their abundances and origins. We present new high-precision laser-fluorination analyses of fresh MORB glasses for the Australian-Antarctic discordance (AAD; n=11) and the 9-$10\deg$ N region of the East Pacific Rise (EPR; n=23). When combined with previous measurements in the same lab, we now have a dataset of 130 geographically and geochemically diverse MORBs, representative of much of the global range of physical and chemical properties of mid ocean ridge spreading centers. Within a given geographically (e.g., mid-Atlantic ridge) or compositionally (e.g., NMORB) restricted subset, there are relationships between $\delta^{18}$O and other geochemical properties that can be attributed to the varying influence of recycled crustal materials. However, as a whole, MORB from all different areas spans a restricted range in $\delta^{18}$O (5.2-5.8$\permil$), and subsets of the data each have a similar range and similar mean composition (of 5.45-5.54 $\permil$). This low variance in $\delta^{18}$O places upper bounds on the influence of recycled crust in producing the other geochemical distinctions among MORBs. For example, there is no systematic difference in the amount of recycled crust sampled during different degrees of partial melting (e.g., low-degree melting of anomalously cold mantle beneath the AAD compared to other locations), different spreading rates, or within the different ocean basins (Indian vs. Pacific vs. Atlantic). These observations suggest that 1) at the scale sampled by MORB melting, all MORB source regions preserve oxygen-isotope heterogeneity, and 2) this heterogeneity is due to the presence of similar and small ($<$10%) percentages of recycled crustal material. In particular, the fact that $\delta^{18}$O across the AAD shows no difference between Indian and Pacific MORB suggests that the MORB source in both ocean basins contains a similar average amount of recycled crust, and if the distinction in radiogenic-isotope composition (e.g., Sr, Pb) is due to the presence of pelagic sediment in the Indian mantle, it cannot be present in abundances greater than 1-2%
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting