HR: 17:35h
AN: V44B-07    [Abstracts]
TI: Oxygen isotopes in MORB revisited: insights into the nature of the Indian MORB source and implications for crustal recycling
AU: * Cooper, K M
EM: kmcooper@geology.ucdavis.edu
AF: Geology Dept., University of California, 1 Shields Ave., Davis, CA 95616, United States
AU: Eiler, J M
EM: eiler@gps.caltech.edu
AF: GPS, Caltech, Pasadena, CA 91125, United States
AU: Sims, K W
EM: ksims@whoi.edu
AF: Geol.&Geophysics, WHOI, Woods Hole, MA 02543, United States
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: EPS, Harvard, Cambridge, MA 02138, United States
AB: Geochemical differences between Indian ocean MORB (Mid Ocean Ridge Basalt) and Pacific or Atlantic MORB have been attributed to addition of one or more of the following enriched materials to the depleted mantle beneath the Indian ridge: 1) local plume material; 2) recycled pelagic sediment; 3) subduction-modified mantle wedge material; or 4) recycled continental lithospheric mantle or lower crust. We present high-precision laser- fluorination analyses of δ¹⁸O in fresh MORB glasses from the Australian-Antarctic discordance (AAD) and use these data to evaluate models for the distinctive character of Indian MORB mantle. Our samples include both Indian- and Pacific-type AAD MORB, and also include samples representative of the most enriched and most depleted (including 'ultra-depleted') AAD MORB. Based on these data, we found that the addition of lower crust to the upper mantle is most consistent with the combination of δ¹⁸O and radiogenic-isotope data for the AAD Indian MORB. In addition, there is no significant difference between mean (5.49-5.52‰) or standard deviation (0.1‰) of δ¹⁸O of Indian and Pacific MORB within the AAD. More broadly, a compilation of high-precision MORB data which includes data for the 9-10° N region of the East Pacific Rise and previously- published data for MORB globally shows no distinction in range (~5.25-5.80) or mean (5.5) of δ¹⁸O between Indian, Pacific or Atlantic MORB globally. Although some of the range in data reflects scatter due to analytical uncertainty (0.05‰ 1 s.d.), the larger standard deviation of 0.1‰ for data for each ocean basin indicates that there is oxygen-isotope heterogeneity preserved in the upper mantle on the scale of sampling during MORB melting. In subsets of the global data (e.g., 'Normal' MORB: Eiler et al., 2000, Nature, v.403 p. 530; or data for the north Atlantic: Cooper et al., 2004, EPSL v. 220, p.297), this heterogeneity in oxygen correlates with trace-element and radiogenic isotopic signatures of enrichment. Furthermore, data for each ocean basin can be fit by a normal distribution. The final distribution of data reflects the average percentage and physical distribution of enriched material in the upper mantle, potentially modified by sampling during melting, and with some scatter introduced through measurement errors. However, Monte Carlo simulations show that if significant differences in the skewness or mean of δ¹⁸O sampled during melting (which would be expected if the percentage or distribution of enriched material sampled differed significantly between ocean basins), we would expect these differences to be apparent in the data sets even with the limited number of samples (134 total). Therefore, either the upper mantle globally contains a similar percentage of crustal material which has a consistent oxygen-isotope composition, or there is a fortuitous trade-off between the percentage and δ¹⁸O of enriched material in the different ocean basins. This in turn implies that crustal materials with the most extreme oxygen-isotope compositions (e.g., pelagic sediments) are present only in very minor abundances in the upper mantle. In addition, it also implies that differences in radiogenic-isotope data between the different ocean basins are due primarily to the age and nature (e.g., altered oceanic crust vs. lower continental crust) of the enriched component, rather than due to large differences in abundance of enriched material.
DE: 1025 Composition of the mantle
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3621 Mantle processes (1038)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting