Timescales of Magma Reservoir Processes Beneath Hawaii

REIDEAR-9980646岩浆滞留和分异的时间尺度难以量化，但对于了解岩浆成藏系统的演化和动力学具有重要意义。目前还没有关于夏威夷莫纳罗亚火山岩浆居住地的定量估计，基拉韦厄火山的估计从数万年到几千年不等。这些估计中的大多数代表平均停留时间，并受制于关于储集层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作用的时间尺度，通过使用Ra-Th不平衡来测量晶体生长速率以来的时间，这是最小停留时间。这样的分析将得出冷却速度，并结合晶体尺寸数据，将得出晶体生长速度，这两个速度都没有直接测量到玄武岩火山的地下水库。1955年基拉韦厄火山东裂谷喷发的初步数据表明，停留时间比大多数先前估计的要长(因此冷却和结晶速度较低)。我们的数据还表明，在晶体生长过程中，Ra和Ba是不同的分馏，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，以检验这些结果的一般性，并确定预测的Ra/Ba分馏是否与年轻熔岩的数据一致。这些结果不仅对研究夏威夷火山的岩浆成藏过程有重要意义，而且对玄武岩分异的时间尺度也有更广泛的意义。

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.