40Ar/39Ar In-Situ Dating of Altered Mafic Rocks in the Karoo Large Igneous Provinces.

Dating of the extrusive parts of large igneous provinces has been a challenge because of the lack of mineral phases that can be dated by high-precision techniques. This is the case for the rapidly emplaced Drakensberg lavas, part of the Karoo LIP in South Africa and Lesotho. The circulation of hot fluids through the lava stack during rapid emplacement of continental flood basalts develops relatively high degrees of fracturing and alteration of the rocks, which often results in the re-opening of isotopic systems and inaccurate dates. This alteration occurs on varying length scales, from the outcrop to the micrometric scales, creating Argon loss in minerals of interest for ⁴⁰Ar/³⁹Ar dating (i.e. plagioclase) and making the procedure of separation for step-heating ⁴⁰Ar/³⁹Ar a tedious and sometimes ineffective task. Here, we re-approach measuring ⁴⁰Ar/³⁹Ar by directly analyzing leached and unleached thin sections without having to go through mineral separation, and therefore effectively eliminating the mixing issue of mechanically separating the plagioclase crystals. Half of each plagioclase aliquot was leached in acid, and then irradiated at the TRIGA reactor (Oregon State). We used a 193nm excimer UV-laser attached to a noble gas extraction and purification line, and an Argus VI mass spectrometer at the University of Geneva on thick sections for in-situ analysis. Plagioclase separates from the same Karoo lava flow samples were previously analyzed for ⁴⁰Ar/³⁹Ar geochronology using step heating, on aliquots of both leached and unleached plagioclase separates, using the same noble gas analytical equipment. This allows for a direct comparison of the in-situ­ analysis, testing the potential differences between the two different analytical systems and a potential way of assessing differences in accuracy between the two. Preliminary results show that accurate ages can be achieved by this technique at the cost of a larger precision.