HR: 11:20h
AN: T12A-05    [Abstracts]
TI: Mantle Heterogeneity and Melting Along a Regional Axial Depth Gradient: Th-U Disequilibria Along the Southeast Indian Ridge
AU: * Russo, C J
EM: crusso@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Administration Building, Corvallis, OR 97331 United States
AU: Rubin, K H
EM: krubin@hawaii.edu
AF: School of Ocean and Earth Science and Technology, University of Hawaii, Dept. of Geology and Geophysics 1680 East-West Road, Honolulu, HI 96822 United States
AU: Graham, D W
EM: dgraham@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Administration Building, Corvallis, OR 97331 United States
AB: Lateral variation in mantle temperature is generally considered to be the main cause for observed global correlations between regionally averaged mid-ocean ridge basalt (MORB) chemistry and ridge axis depth (Klein and Langmuir, 1987). Axial depth should be shallower above hotter mantle because the underlying mantle crosses its solidus at greater depth compared to cooler regions, and leads to greater crustal production. One expectation of this global model is that melting beneath ridges should involve progressively more (deep) garnet peridotite as ridge depth shallows. A negative correlation between ($^{230}$Th/$^{238}$U) and axial depth in a global dataset (largely dominated by eastern Pacific and Atlantic MORB) generally supports this notion, because garnet is known to fractionate Th from U during melting (Bourdon et al., 1996). The extent to which such global variations reflect only variations in melting conditions of passively upwelling mantle versus other conditions related to mantle heterogeneity or actively fed melting anomalies is presently unknown. To further address this question on the regional scale, we have measured Th-U isotopes on 11 basaltic glasses collected along the Southeast Indian Ridge (SEIR) from $\sim$90$\deg$ E to $\sim$117$\deg$ E. This $\sim$2600 km section of ridge is characterized by a west to east gradient in axial depth from $\sim$2300 m to $>$4500 m, similar to that of the global ridge system away from the influence of hotspots. However, unlike the global ($^{230}$Th/$^{238}$U)-axial depth correlation which uses regionally averaged data for ridge segments from a range of spreading rates, the SEIR is spreading at a nearly constant rate of 70-75 mm/yr and is devoid of large transform offsets. These first 11 SEIR glasses were selected to span the geographic range and to be representative of the elemental and isotopic compositions. Th and U concentrations range from 130-940 ppb and 55-267 ppb, respectively, with Th/U values ranging from 2.36 to 3.77. All samples show $^{230}$Th excesses, with ($^{230}$Th/$^{238}$U) ranging from 1.01 to 1.24. There is no correlation between ($^{230}$Th/$^{238}$U) and axial depth, in contrast to the simple prediction from the global scale model. On a ($^{238}$U/$^{232}$Th)-($^{230}$Th/$^{232}$Th) equiline diagram the SEIR data form 3 geographical, non-collinear groupings: three basalts from the westernmost portion of our study area have the highest ($^{230}$Th/$^{232}$Th) and ($^{238}$U/$^{232}$Th), the easternmost basalt has significantly lower values, and basalts from a central region (101$\deg$E to 114$\deg$E) have intermediate values. The 7 central region basalts form a well-correlated positively sloping ($^{230}$Th/$^{232}$Th) vs. ($^{238}$U/$^{232}$Th) array which is shallower than the equiline and extends from 7% to 24% $^{230}$Th-excess. Overall, ($^{230}$Th/$^{232}$Th) shows a strong negative correlation with axial depth and, along with Th/U, correlates well with other isotopic tracers such as $^{3}$He/$^{4}$He and $^{208}$Pb/$^{206}$Pb, which vary systematically along axis. In contrast, ($^{230}$Th/$^{238}$U) shows no systematic variations with these isotopic parameters or axial depth. Unlike the global dataset, the Th-U disequilibria in SEIR MORB suggests that, at a regional scale, melting in the Indian Ocean mantle is primarily controlled by variations in mantle composition.
DE: 3640 Igneous petrology
DE: 3035 Midocean ridge processes
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting