Timescales of Magma Reservoir Processes Beneath Hawaii

岩浆居住和分异的时间尺度很难量化，但对认识岩浆储层系统的演化和动力学具有重要意义。夏威夷莫纳罗亚火山的岩浆存在时间目前尚无定量估计，而基拉韦厄火山的岩浆存在时间则在数万年至数千年之间。这些估计大多代表了平均停留时间，并受到有关储层大小和岩浆供应速率的假设的影响。我们提出了一种不同的方法来确定岩浆过程的时间尺度，通过使用Ra-Th不平衡来测量自晶体生长速率以来的时间，这是一个最小停留时间。这样的分析将得出冷却速率，结合晶体尺寸数据，将得出晶体生长速率，这两者都没有直接测量过玄武岩火山地下储层。1955年基拉韦厄火山东裂谷喷发的初步数据表明，与大多数先前的估计相比，其停留时间更长（因此冷却和结晶速率更低）。我们的数据还表明，Ra和Ba在晶体生长过程中会发生不同的分馏，当使用Ba作为Ra的化学类似物时，需要进行分馏校正。我们建议分析更多的样品来测试这些结果的普遍性，并确定预测的Ra/Ba分馏是否与年轻熔岩的数据一致。这些结果不仅对夏威夷火山岩浆储层过程的研究具有重要意义，而且对玄武岩分异的时间尺度具有更广泛的意义。

ReidEAR-9980646Time 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.