APPLICATION OF THE 40AR/39AR AGES OF HORNBLENDE FOR TRACKING ICE RAFTED DETRITUS PROVENANCE AROUND ANTARCTICA (Invited Presentation)

The approximately radial pattern of ice sheet drainage divides around most of East Antarctica, combined with the strong contrast in geology and geologic ages between East and West Antarctica and the general trends of geological variability around East Antarctica contribute to making the application of radiogenic isotope tracers and detrital geochronometers useful tools for studying the history of past ice sheets. Antarctica has been substantially glaciated since approximately the Eocene-Oligocene boundary, but with some important changes in Cenozoic climate that are associated with significant retreats and advances of the ice margins. Where the glaciers enter the ocean, icebergs calve off and carry pieces of the subglacial geology from where they were derived. As they are moved with the currents around the continent, their melting causes them to drop debris (ice rafted debris or detritus, IRD) into the ocean. Although the entire suite of grain sizes of proximal terrigenous sediments around Antarctica is likely brought to the ocean by glacial and glacial-marine processes, the sand and coarser sediment fractions in the pelagic realm must have been rafted to the site because surface and bottom currents would not be sufficiently energetic to carry these particles. The 40Ar/39Ar system in hornblende is a medium-high temperature thermochronometer with a closure temperature that is controlled by composition and cooling rate, in the general range of 500-600C. Thus the 40Ar/39Ar ages of hornblendes represent the most recent major tectonothermal event of the geological source terrain. We will present an overview of the background data on 40Ar/39Ar from detrital hornblende around Antarctica, and will review examples of the application to understanding dynamic behavior of past ice sheet sectors.