40AR/39AR THERMOCHRONOLOGY OF AUSTRALIA'S CONJUGATE MARGIN IN ANTARCTICA FROM ICE-RAFTED HORNBLENDE, BIOTITE AND FELDSPAR GRAINS

Characterizing the subglacial geology of Antarctica is an important step towards identifying dynamical changes in the ice sheet from their contributions to ice rafted detritus (IRD) in the Southern Ocean. A circum-Antarctic survey of ⁴⁰Ar/³⁹Ar ages of detrital hornblende from glaciogenic sediments has demonstrated that the geological variations are sufficiently systematic as to yield a powerful tracer of ice sheet dynamics [1]. A downcore study of IRD peak layers at ODP Site 1165 off Prydz Bay has revealed two different populations of Proterozoic ⁴⁰Ar/³⁹Ar ages (1100-1300 Ma and 1500-1600 Ma) suggesting transport of icebergs from the Wilkes Land and Adélie/George V Land coasts, respectively, to Site 1165 [2], based upon on-shore data and the results of [1].

We have performed ⁴⁰Ar/³⁹Ar dating on detrital hornblende, biotite and feldspar from 9 marine sediment cores along the coasts of George V Land, Adélie Land, and Wilkes Land (95˚ to 165˚ E) in an attempt to better refine the thermochronologic history of this sector. Hornblende has a closure temperature to diffusion of Ar of ~ 500°C, while biotite (~300°C) and feldspar (~200°C) have lower closure temperatures, thus a comparison of their age populations yields insights into the erosion history of their sources. ⁴⁰Ar/³⁹Ar hornblende ages are consistent with both [1] and on-land age distributions, showing dominant populations of 400-600 Ma (Ross orogen ages) near the Ross Sea and George V Land, 1500-1700 Ma, with several grains older than 2000 Ma near Adélie Land, and 1100-1300 Ma (Grenville ages) near Wilkes land, reflecting major, distinct tectonothermal events in Adélie and George V Land. ⁴⁰Ar/³⁹Ar biotite ages also reflect the proximal onland geology, with ages generally agreeing with the hornblende ages, indicating rapid cooling following the last major tectonothermal event. Preliminary ⁴⁰Ar/³⁹Ar feldspar ages are not as well constrained to age populations as the hornblende and biotite, but they are generally offset below the biotite ages. While both hornblende and feldspar are rock-forming minerals, and thus ideal as provenance tracers for IRD, it may be that the lower closure temperature of the feldspar make it less viable than the hornblende and biotite in this area.

[1] Roy et al. (2007) Chem. Geol. 244, 507-519

[2] Williams et al. (2007) ISAES Ext. Abst. 037