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首页> 外文学位 >226Radium-230Thorium disequilibrium in plagioclase from recent eruptions at Mount Hood volcano, Oregon: Constraints on crystal residence and magma storage timescales.
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226Radium-230Thorium disequilibrium in plagioclase from recent eruptions at Mount Hood volcano, Oregon: Constraints on crystal residence and magma storage timescales.

机译：226俄勒冈州胡德山火山最近爆发的斜长石中的ase-230 230不平衡：限制了晶体的停留和岩浆的储存时间尺度。

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摘要

Uranium-series crystal dating methods have been used to quantify the temporal and geochemical histories of igneous mineral phases, providing insight into the sub-surface histories of the magmas that produced them. In particular, 226Ra-230Th disequilibrium of bulk plagioclase separates can be used to constrain timescales of crystal residence and magma storage. Crystal size distributions (CSDs) have also been used for this purpose, and combining the two methods can reveal information about magmatic processes that neither can provide in isolation.;Mount Hood is a dominantly andesitic to dacitic stratovolcano characterized by abundant evidence for magma mixing. Two populations of plagioclase have been recognized in nearly all eruptive sequences and have been geochemically-linked to crystallization in mafic and silicic magmas prior to mixing. The two plagioclase populations are distinguishable on the basis of size, providing the opportunity to physically separate and date the populations using uranium-series bulk mineral dating methods.;In this study, we measured uranium-series element concentrations in bulk plagioclase separates of different sizes from the Timberline (eruption age 1,500 a) and Old Maid (eruption age 215 a) eruptive sequences. We calculated 226Ra-230Th bulk plagioclase crystallization ages of eruption age to 3,500 a for plagioclase from the mafic end-member magma, and ages of 8,000 to >10,000 a for plagioclase from the silicic end-member magma. These ages are consistent with a mafic recharge and mixing model for the production of intermediate Mount Hood magmas. The mafic recharge magmas were transient features in the sub-surface, stored in the crust for less than 3 ky before eruption. These magmas interacted with a long-lived silicic magma stored in a shallow-crustal reservoir over timescales greater than 7 ky before eruption.;Comparing plagioclase age data as measured by 226Ra- 230Th disequilibrium and CSD reveals that plagioclase crystals from the silicic reservoir grew for only short periods (maximum of 0.15%) of their overall average histories. These crystals must have experienced episodes of growth hiatus and/or dissolution, consistent with residence in a liquid-poor silicic crystal mush in the shallow crust, which may have spent long periods of its history completely crystalline.
机译：铀系列晶体测年方法已被用来量化火成矿物相的时间和地球化学历史，从而提供了对产生它们的岩浆的地下历史的认识。特别地，大量斜长石分离物的226 Ra-230 Th不平衡可用于限制晶体驻留和岩浆储存的时间尺度。晶体尺寸分布（CSD）也已用于此目的，并且将这两种方法结合使用可以揭示有关岩浆过程的信息，而这两个过程都无法孤立地提供。;胡德山是胶质火山岩的主要安山岩，具有丰富的岩浆混合证据。几乎在所有喷发序列中都可以识别出两个斜长石种群，并且在混合之前，已将这些斜长石种群地球化学连接到铁镁质和硅质岩浆中。这两个斜长石种群在大小上是可区分的，从而提供了使用铀系列大宗矿物定年法对种群进行物理分离和测年的机会。在本研究中，我们测量了不同大小的大长石斜石分离物中铀系列元素的浓度。从林线（喷发年龄1,500 a）和老女仆（喷发年龄215 a）的喷发序列。我们计算了226Ra-230Th的大型斜长石斜长结晶年龄，其中镁铁质末段岩浆的斜长石年龄为3500 a，而硅质长末岩浆的斜长石年龄为8000 a> 10,000 a。这些年龄与生产中级胡德山岩浆的镁铁质补给和混合模型一致。镁铁质补给岩浆是地下的瞬变特征，在喷发前在地壳中的储藏时间少于3 ky。这些岩浆与爆发前大于7 ky的时间尺度内储存在浅壳储层中的长寿命硅质岩浆发生相互作用;;通过226Ra-230Th不平衡和CSD测量的斜长石年龄数据比较显示，硅质储层中的斜长石晶体生长为仅有其总体平均历史的短时间段（最多0.15％）。这些晶体必定经历了裂口和/或溶解的增长，这与滞留在浅层地壳中的液态硅质结晶糊中的现象相符，这可能已经花费了很长一段时间使其完全结晶。

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  • 作者

    Eppich, Gary Richard.;

  • 作者单位

    University of California, Davis.;

  • 授予单位 University of California, Davis.;
  • 学科 Geology.;Mineralogy.;Geochemistry.
  • 学位 M.S.
  • 年度 2010
  • 页码 149 p.
  • 总页数 149
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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