Abstract:
Bransfield Strait is a Quaternary, ensialic back arc basin at the transition from rifting
to spreading. Fresh volcanic rocks occur on numerous submarine features distributed
along the rift axis, including a discontinuous neovolcanic ridge similar to the nascent
spreading centers seen in some other back arc basins. Smaller edifices near the northeast
end of the rift yielded basalts with the most arc-like compositions (e.g., high large-ion
lithophile element/high field strength element and ⁸⁷Sr/⁸⁶Sr). The most mid-ocean ridge
basalt (MORB)-like basalts are from a large, caldera-topped seamount and a 30-km-long
axial neovolcanic ridge toward the southwest end of the rift, but these two features also
yielded andesite and rhyolite, respectively. The volcanic and geochemical variations are
not systematic along axis and do not reflect the unidirectional propagation of rifting
suggested by geophysical data. The most depleted basalts have major and trace element
characteristics indistinguishable from MORB except for slightly higher Cs and Pb
concentrations. Pb isotopic ratios show little variation compared to Sr and Nd isotopic
ratios and do not extend to the depleted Pb isotopic ratios found in other back arc basins.
Either the depleted mantle beneath Bransfield Strait has higher than normal Pb isotopic
ratios or the subducted component beneath Bransfield Strait has such high Pb
concentrations that it dominates the Pb isotopic composition of the Bransfield Strait
mantle without significantly affecting the Sr and Nd isotopic compositions. Metalliferous
sediments and fluids extracted from a subducting slab may have the necessary high
concentrations of Pb.
