Dating of Sirius Group tillites in the Antarctic Dry Valleys with cosmogenic3He and21Ne

We measured in situ produced cosmogenic3He and21Ne in boulder and bedrock surface samples in and near Sirius Group tillites from two localities in Southern Victoria Land, Antarctica. Pyroxene quantitatively retains cosmogenic3He for millions of years. The deposit at Mt. Fleming has a minimum exposure age of ∼ 6.5 Ma, while ∼ 6 Ma is a probable minimum age for the Table Mountain tillite. These lower limits (not taking into account erosion of the sampled surfaces) are based on the samples with the highest concentrations of cosmogenic noble gases and currently accepted production rates. Since the plagioclase-bearing Mt. Fleming samples almost certainly lost part of their cosmogenic Ne, the minimum exposure age at this location is presumably even ≈ 50% higher than the stated value. The concentrations of cosmogenic Ne in our samples constrain uplift rates of the Transantarctic Mountains to < 170 m/Ma. These data contradict the hypothesis of a collapse of the East Antarctic ice sheet in the Pliocene coupled with high uplift rates and support the model of a stable ice sheet since the middle Miocene.