Abstract Abundant mantle-derived ultramafic xenoliths occur in Cenozoic (7.7–1.5 Ma) mafic alkaline volcanic rocks along the former active margin of West Antarctica, that extends from the northern Antarctic Peninsula to Jones Mountains. The xenoliths are restricted to post-subduction volcanic rocks that were emplaced in fore-arc or back-arc positions relative to the Mesozoic–Cenozoic Antarctic Peninsula volcanic arc. The xenoliths are spinel-bearing, include harzburgites, lherzolites, wehrlites and pyroxenites, and provide the only direct evidence of the composition of the lithospheric mantle underlying most of the margin. The harzburgites may be residues of melt extraction from the upper mantle (in a mid-ocean ridge type setting), that accreted to form oceanic lithosphere, which was then subsequently tectonically emplaced along the active Gondwana margin. An exposed highly depleted dunite–serpentinite upper mantle complex on Gibbs Island, South Shetland Islands, supports this interpretation. In contrast, pyroxenites, wehrlites and lherzolites reflect percolation of mafic alkaline melts through the lithospheric mantle. Volatile and incompatible trace element compositions imply that these interacting melts were related to the post-subduction magmatism which hosts the xenoliths. The scattered distribution of such magmatism and the history of accretion suggest that the dominant composition of sub-Antarctic Peninsula lithospheric mantle is likely to be harzburgitic.

Many archaea and bacteria obtain metabolic energy by catalyzing the oxidation or reduction of sulfur. In marine hydrothermal systems, chemolithoautotrophs that oxidize hydrogen sulfide (H 2 S) or elemental sulfur (S 0) account for much of the primary biomass synthesis. Under reducing conditions in these systems, both S 0 and sulfate can serve as terminal electron acceptors. The energetics of chemolithoautotrophy in marine hydrothermal systems are discussed, focusing principally on sulfur-redox, but also touching on methanogenesis and organic synthesis. Examples are given from deep- and shallow-sea hydrothermal environments, the early Earth, Mars, and Europa. In addition, we present a detailed analysis of the Gibbs free energies (δG r ) of 25 sulfur-redox reactions in a model shallow-marine hydrothermal ecosystem—the seeps, wells, and vents of Vulcano Island (Italy). A number of these reactions represent known metabolisms, but other reactions with no known microbial catalyst are also included to investigate their potential as pos sible energy sources. The reactions considered couple SO 2− 4 S 0 , and H 2 S with a variety of terminal electron acceptors (O 2 , NO − 3 , Fe(III), CO 2) and electron donors (H 2 , CH 4 , formic acid, acetic acid, propanoic acid, NH + 4 , Fe 2+). At all seven study sites on Vulcano, which vary considerably in temperature, pH, and chemical composition, sulfate- and S 0 -reduction reactions are energy-yielding where H 2 , CH 4 , or carboxylic acids serve as the electron donors, but energy-consuming with NH + 4 or Fe 2+ as the reductant. Elemental sulfur- and sulfide-oxidation reactions are energy-yielding at all sites when O 2 , NO − 3 , or Fe(III) are the terminal electron acceptors, but energy-consuming with CO 2 as the oxidant.