Collaborative Research: Melting and Mass Transfer in the Central American Arc From Pa-231, Th-230, and Ra-226 Systematics

[9814805 . hirschmann]弧下地幔部分熔融和熔体提取的机制和时间是关于弧岩浆作用尚未解决的关键问题之一。明尼苏达大学和爱荷华大学的研究人员之间的这项合作将利用对年轻火山岩中u系列核素，特别是231Pa的分析，更好地了解中美洲弧下的融化过程，并确定这一过程如何受到俯冲板块成分通量的影响。初步研究表明，231Pa在喷发前从弧次地幔U中强烈分出，时间尺度与231Pa半衰期（33 kyr）相似或小于231Pa半衰期，强烈提示231Pa是熔化和熔体提取的时间和机制的敏感指标。将利用热电离质谱法测量(231Pa)/(235U)和(230Th)/(238U)活度比，利用α计数技术测量(226Ra)/(230Th)活度比，对哥斯达黎加和尼加拉瓜中美洲南部弧形的一组年轻火山岩进行测量。中美洲南部弧是利用u系列同位素研究熔解和传质过程的理想地点，因为影响弧火山作用的许多参数，如地幔的组成和俯冲板块组分的质量通量，在整个地区以系统的、特征明确的方式变化。由此产生的数据集将是我们所知的与弧省相关的u系列同位素的最完整的研究，并与微量元素和长寿命同位素数据相结合，将产生一个独特的数据体，根据这些数据可以测试弧中熔化和传质的假设。

9814805HirschmannThe mechanism and timing of partial melting and melt extraction in the mantle beneath arcs constitutes one of the key unresolved questions regarding arc magmatism. This collaboration between researchers at the Universities of Minnesota and Iowa will use analyses of U-series nuclides and particularly of 231Pa in young volcanic rocks to better understand the melting process beneath the Central American arc and to determine how this process is affected by fluxes of components from the subducting slab. Preliminary studies show that 231Pa is strongly fractionated from U in the sub-arc mantle prior to eruption on a time scale similar to or less than the half life of 231Pa (33 kyr), strongly suggesting that 231Pa is a sensitive indicator of the timing and mechanism of melting and melt extraction. Measurements of (231Pa)/(235U) and (230Th)/(238U) activity ratios using thermal ionization mass spectrometry and (226Ra)/(230Th) ratios using alpha counting techniques will be made for an along-arc suite of young volcanic rocks from the southern Central American arc in Costa Rica and Nicaragua. The southern Central American arc is an ideal location for studying melting and mass transfer processes using U-series isotopes because many of the parameters affecting arc volcanism, such as the composition of the mantle and the mass flux of components from the subducting slab, vary across the region in a systematic and well-characterized fashion. The resulting data set will be to our knowledge the most complete study of U-series isotopes associated with an arc province, and combined with the trace element and long-lived isotopic data will yield a unique body of data against which hypotheses for melting and mass transfer in arcs can be tested.