Magmatic flare-up recorded in Rb-Sr reset-ages during Gondwana break-up, Kohler Range, Marie Byrd Land, Antarctica 

The Rb-Sr dating system has long been recognised as being susceptible to thermal perturbation. Its so-called closure temperature (TC) in biotite, the lowermost temperature at which interstitial diffusion of radiogenic 87Sr is no longer facilitated, is ca. 400 ˚C, much lower than e.g., the commonly used U-Pb system in zircon in excess of 900 ˚C. Newly developed in-situ analyses or Rb-Sr isotope systematics in biotite using a collision cell mass spectrometer (ICP-MS/MS) have now enabled rapid dating using multiple grains and with high spatial resolution (on a single grain scale). This now allows making use of the thermal characteristics of this dating tool. However, the modes and degrees of isotope reset are still poorly understood. Here, we analyse rocks from the Kohler Range in Antarctica for their Rb-Sr systematics in biotite. The plutonic rocks formed as part of the active Permian Gondwana margin (ca. 280-260 Ma). Whilst two samples record their original Permian age, two other samples yield cretaceous ages (~100 Ma), consistent with regional magmatic activity during the breakup of Gondwana. Two samples with mixed populations of micas confirmed a partial reset in this rock, and thereby the full reset for the Cretaceous ages. An intriguing aspect is that reset is predominant in spot analyses with higher Rb/Sr, making these sections apparently more prone to thermal reset, likely through higher diffusion potentials. We interpret the resetting of the system as a response to thermal perturbation of the igneous rocks with no apparent petrographic footprint. It is demonstrated that in-situ Rb-Sr dating of igneous micas in plutons can be used to date magmatic flare-ups in rocks that can be hundreds of millions of tears older. The technique thus holds immense potential to decipher the thermal histories of terranes that otherwise remain undetected and may be used to shed new light on a large number of suspect Rb-Sr (mixed?) ages with no apparent geologic meaning.