HR: 0800h
AN: U41A-0730    [Abstracts]
TI: Cryptic Striations in the Indian Ocean Mantle Revealed by Hf Isotopes in Southeast Indian Ridge MORB Glasses
AU: * Graham, D W
EM: dgraham@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR 97331 United States
AU: Blichert-Toft, J
EM: jblicher@ens-lyon.fr
AF: Ecole Normale Superieure, Laboratoire des Sciences de la Terre, Lyon, 69364 France
AU: Russo, C J
EM: crusso@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences, Corvallis, OR 97331 United States
AU: Rubin, K H
EM: krubin@hawaii.edu
AF: University of Hawaii, SOEST, Honolulu, HI 96822 United States
AU: Albarede, F
EM: albarede@ens-lyon.fr
AF: Ecole Normale Superieure, Laboratoire des Sciences de la Terre, Lyon, 69364 France
AB: The Southeast Indian Ridge (SEIR) provides a unique opportunity to study the consequences of along-axis variation in upper mantle temperature and flow. Over a distance of 2500 km, between $86\deg$E and $120\deg$E, axial depth increases from 2300m to 5000m. The axial depth gradient occurs at intermediate and constant spreading rate (70-75 mm/y full rate) and in the absence of large transform offsets and nearby hotspots. There is also a morphologic transition from axial high to axial valley going eastward, due to the decrease in melt production rate and crustal thickness. The range in axial depth and ridge morphology is similar to the global range for spreading ridges away from hotspots, making the SEIR a regional-scale analogue of the 50,000 km-long global ocean ridge system. Previous work has established that He, Pb, Sr and Nd isotope variations along the SEIR are primarily controlled by changes in the depth of melting of isotopically heterogeneous mantle, and that all SEIR lavas west of the Australian-Antarctic Discordance are true Indian-type as indicated by their high $^{208}$Pb/$^{206}$Pb ratios. New Hf isotope results have been obtained for 48 MORB glasses recovered from the SEIR between the Amsterdam-St. Paul Plateau and the Australian-Antarctic Discordance. $\epsilon$$_{Hf}$ ranges from +5.5 to +17.8. The extreme values occur on the ASP Plateau (5.5) and in the westernmost AAD (17.8), and are similar to values measured previously in those regions. Overall, Hf and Nd isotopes along the SEIR show the typical positive correlation seen in oceanic basalts. However, over a stretch of 2000 km, between $88\deg$E and $110\deg$E, the Hf isotope compositions are strikingly bimodal, with a "gap" of more than 1 epsilon unit between the two groupings ($\epsilon$$_{Hf}$ = 9.5 to 11.5 and $\epsilon$$_{Hf}$ = 12.5 to 14.6, respectively). Although this Hf isotope bimodality occurs within the band of globally correlated $\epsilon$$_{Hf}$ - $\epsilon$$_{Nd}$ data, our sample suite has a strong spatial resolution, suggesting the presence of mantle striations (streaks) beneath the SEIR. The bimodality in Hf isotopes is not observed in other geochemical parameters, indicating that the streaks carry a cryptic memory of augmented Lu/Hf fractionation at some time in the past. The streaks defined by the Hf isotope bimodality display a Poisson spatial distribution, (i.e., number of streaks observed is proportional to the length of ridge sampled) with an average computed streak width of 50-100 km. This is the expected spatial distribution for a well-stirred upper mantle in which geochemical heterogeneity is continually created by tectonic recycling and diminished by convective refolding and stretching.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3035 Midocean ridge processes
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Union [U]
MN: 2004 AGU Fall Meeting