Geochemistry of back arc basin volcanism in Bransfield Strait, Antarctica: Subducted contributions and along-axis variations
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.