What is Under the Antarctic Ice: An Integrated Study of U-Pb, O and Lu-Hf Isotopes

Miss Bei Chen1, Mr Ian Campbell1

1Research School of Earth Sciences, Australian National University, Canberra, Australia

Antarctica is the central piece in the Gondwana jigsaw, connecting Australia, India and Africa. Little bedrock is exposed in Antarctica with over 98 % of the continent covered by ice. Its geology can provide new insights into the relationship between Antarctica and its neighbours and elucidate its role in the amalgamation and breakup of the Gondwana supercontinent.

Detrital zircons separated from IODP holes drilled around Antarctica have been analysed for U-Pb, O and Lu-Hf isotopes. U-Pb results show major detrital zircon crystallization peaks at ca. 250, 550, 950 and 1200 Ma, with a minor peak at 1600 Ma. They broadly correlate with younger peaks in the Australian detrital zircon population, but the older Australian peaks are missing. By far the largest peak, at ca. 550 Ma, is interpreted to represent zircons derived from the Transgondwana Supermountain formed by the collision between East and West Gondwana. Unlike previous studies, based on 40Ar/39Ar dating from hornblende and biotite, our data show a significant ca. 250 Ma peak, indicating that Antarctica was affected by an event of Pangaea age.

Oxygen isotope in zircons display a step increase at the end of the Archean, consistent with the temporal evolution of zircon δ18O as recognized by Valley et al. (2005). δ18O values of ca. 500 Ma group from Antarctica cover a large range (4.9-11‰), similar to the range of δ18O in ca. 500 Ma detrital zircons from Australia. Interestingly, zircons of ca. 100 Ma from West Antarctica are unusual in having δ18O less than the mantle value, implying crystallization from a felsic magma produced by melting wet basalt. Lu-Hf isotopes of these detrital zircons show that they were derived from juvenile crust, which formed around 100 Ma. εHf values of the ca. 550 Ma zircons show the largest variation (-18 to 10), and in this respect are similar to zircons of the same age from Australia. Arc Mantle Hf model ages reveal three major periods of growth of Antarctic continental crust: 300-500, 1200-1600 and 2000-2300 Ma, and indicate that the growth of the Antarctic continental started to form after the Archean. An unexpected outcome of this study is that it showed that Antarctica is younger than the other continents we have investigated both in terms of its model ages, and the orogenic events that have affected it.

Reference: Valley, J. W., Lackey, J. S., Cavosie, A. J., Clechenko, C. C., Spicuzza, M. J., Basei, M. A. S., … & Peck, W. H. (2005). 4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon. Contributions to Mineralogy and Petrology, 150(6), 561-580.


Biography

2016 – Present: Ph.D. student Research School of Earth Science, Australian National University

2013 – 2016: M.S. in Geochemistry State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences

2009 – 2013: B.S. in Geology College of Earth Sciences, Jilin University, China

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