Timescales of Magma Reservoir Processes Beneath Hawaii

岩浆驻留和分异的时间尺度难以量化，但对于理解岩浆储集系统的演化和动力学非常重要。夏威夷莫纳罗亚火山的岩浆居住目前没有定量的估计，基拉韦厄火山的估计从几十年到几千年不等。 大多数这些估计代表平均停留时间，并受到有关水库的大小和岩浆供应率的假设。 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不平衡来测量晶体生长速率，这是一个最小的停留时间。这种分析将得出冷却速率，并与晶体尺寸数据相结合，将得出晶体生长速率，这两种速率都没有直接测量玄武岩火山的地下储层。 1955年基拉韦厄东部裂谷喷发的初步数据表明，它的停留时间比大多数以前的估计要长（因此冷却和结晶速率要低）。 我们的数据还表明，Ra和Ba的差异分馏在晶体生长过程中，需要分馏校正时，使用Ba作为化学类似物Ra。 我们建议分析额外的样本，以测试这些结果的一般性，并确定是否预测Ra/Ba分馏与年轻熔岩的数据是一致的。 这些结果将有重要的影响，不仅在夏威夷火山岩浆库过程的研究，而且更普遍的玄武岩分异的时间尺度。

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.