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[1] The effects of low-temperature alteration on the Rb-Sr and Sm-Nd isotope systems were investigated in 14-28 Ma mid-ocean ridge basalts recovered during Ocean Drilling Program (ODP) Leg 187 from the Australian Antarctic Discordance through comparison of pristine glass and associated variably altered basalts. Both Nd and Sm are immobile during low-temperature alteration, and 143 Nd/ 144 Nd displays mantle values even in heavily altered samples. In contrast, 87 Sr/ 86 Sr and Rb concentrations increase during seawater-rock interaction, which is especially apparent in single samples with macroscopically zoned alteration domains. The increase in 87 Sr/ 86 Sr roughly correlates with the visible degree of alteration, indicating a higher seawater/rock ratio in the more altered samples. Sr concentrations, however, do not systematically increase with increasing degree of alteration, most likely reflecting exchange of Sr in smectite interlayer sites. The degree of alteration in the uppermost oceanic crust of the Australian Antarctic Discordance is independent of crustal age. A comparison with literature data for young and old altered oceanic crust suggests that most low-temperature alteration occurs within a few million years after formation of the oceanic crust, probably reflecting greater fluid flux through the crust during its early history as a result of higher permeability and increased fluid circulation near the ridge.

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