Timescales of Magma Reservoir Processes Beneath Hawaii

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Reid EAR-9980646 Time scales of magma residence and differentiation are difficult to quantify bur are important in understanding the evolution and dynamics of the magmatic reservoir system. No quantitative estimates for magma residence in Mauna Loa Volcano in Hawaii are currently available, and estimates for Kilauea Volcano range from tens to thousands of years. Most of these estimates represent average residence times, and are subject to assumptions about the size of the reservoir and the magma supply rate. We propose a different approach to determining timescales of magmatic processes by using Ra-Th disequilibria to measure the time since crystal growth rates, which is a minimum residence time. Such analysis will yield cooling rates, and combined with crystal size data, will yield crystal growth rates, both of which have not been measured directly for sub-surface reservoirs in basaltic volcanoes. Preliminary data from the 1955 east rift eruption of Kilauea suggest that residence is longer (and therefore that cooling and crystallization rates are lower) than most previous estimates. Our data also suggest that Ra and Ba are differentially fractionated during crystal growth, requiring fractionation correction when using Ba as a chemical analog for Ra. We propose to analyze additional samples to test the generality of these results and to determine whether the predicted Ra/Ba fractionations are consistent with data for young lavas. These results will have important implications not only for studies of magma reservoir processes in Hawaiian volcanoes, but also more generally to timescales of basaltic differentiation.