GEOCHRONOLOGY OF LARGE IGNEOUS PROVINCES

Geochronology has played a central role in establishing the timescale(s) of LIP magmatism, links between LIP's and other phenomena such as mass extinctions, and to defining the spatial scope of LIP's. One of the most profound revelations from geochronology has been recognition of the brevity (ca. 1 Ma) of LIP emplacement. In some cases (e.g. Karoo-Ferrar) it is now clear that LIP construction occurred in multiple, comparably brief episodes. The most powerful dating tool has been the ⁴⁰Ar/³⁹Ar incremental heating method applied to carefully prepared plagioclase separates, an approach which has successfully cut through many problems encountered by a first generation of dating chiefly by K/Ar. Despite significant progress along these lines, few examples exist in which the duration of individual magma pulses are well-constrained, in part because the unsurpassed resolving power of the ⁴⁰Ar/³⁹Ar method has never been fully achieved in dating the top and bottom of a given stack of lava flows. U/Pb dating has been increasingly applied to dating LIP's, but suffers limited applicability (chiefly to intrusions) due to the paucity of U-rich phases in LIP's. Increasing recognition of LIP's in pre-Mesozoic terranes, where utility of the ⁴⁰Ar/³⁹Ar method often is compromised by alteration, places increasing importance on the recognition of U-bearing and Pb-retentive phases such as zircon and baddelyite. While geochronology of LIP's has clearly made great strides in the past 20 years, many challenges remain. In particular, resolving eruption rates in detail, which is critical to modeling the atmospheric consequences of climate-modifying volcanogenic gases, demands the limits of geochronological precision. Alternatively, cosmogenic nuclide studies may eventually elucidate the distribution of time within lava flow sequences.