Mantle metasomatism and rapid ascent of slab components beneath island arcs: Evidence from 238U‐230Th‐226Ra disequilibria of Miyakejima volcano, Izu arc, Japan
Mantle metasomatism and rapid ascent of slab components beneath island arcs: Evidence from 238U‐230Th‐226Ra disequilibria of Miyakejima volcano, Izu arc, Japan
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DOI：
10.1029/2002jb002103
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发表时间：
2003-07
影响因子：
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通讯作者：
T. Yokoyama;K. Kobayashi;T. Kuritani;E. Nakamura
中科院分区：
文献类型：
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作者：
T. Yokoyama;K. Kobayashi;T. Kuritani;E. Nakamura
介绍了日本三宅岛火山熔岩中的²³⁸U - ²³⁰Th - ²²⁶Ra体系，以估算岛弧下岩浆过程的时间尺度。三宅岛火山自公元前7000多年开始有四个近期喷发阶段（1 - 4阶段）。在²³⁸U和²²⁶Ra大量过剩的熔岩中观察到的²³⁸U - ²³⁰Th - ²²⁶Ra不平衡现象，意味着亏损地幔受到与流体相关过程的交代作用。同一熔岩中的微量元素以及Sr - Nd - Pb同位素体系也表明了这种交代作用。在等时线图中，两个岩浆阶段（1阶段和2阶段）的趋势被视为具有共同初始（²³⁰Th/²³²Th）比值的两条不同等时线，尽管3阶段和4阶段的趋势是一条岩浆混合线。我们的模型计算表明，板片来源的流体能够输送一些Th，并且推断出板片组分在地幔楔中的上升时间非常快（<7千年）。这种快速上升可以通过流体的水力破裂模型和熔体的通道流模型来解释地幔楔中近乎瞬时的物质传输。即使考虑到增强²²⁶Ra的熔融过程，这样的时间尺度估算也不会增加。等时线图中的年龄差异对应于单个流体释放事件的间隔（1阶段和2阶段之间为13千年，2阶段和3阶段之间为5千年）。因此，板片的流体释放以及随后的岩浆生成是以数千年时间尺度上的幕式事件发生的。
[1] 238 U- 230 Th- 226 Ra systematics in lavas from Miyakejima volcano, Japan, are presented to estimate the timescale of magmatic processes beneath an island arc. Miyakejima volcano has four recent eruptive stages (Stages 1-4) starting >7000 BP. 238 U- 230 Th- 226 Ra disequilibria observed in lavas with large 238 U and 226 Ra excesses imply metasomatism of depleted mantle by fluid-related processes. This metasomatism is also suggested by trace element and Sr-Nd-Pb isotopic systematics in the same lavas. In the equiline diagram, the trends for two magmatic stages (Stages 1 and 2) are regarded as two different isochrons with a common initial ( 230 Th/ 232 Th) ratio, although the trend for Stages 3 and 4 is a magma mixing line. Our model calculations show that slab-derived fluids can deliver some Th and a very rapid ascent time of the slab components in the mantle wedge (< 7 kyr) is inferred. This rapid ascent can be explained by nearly instantaneous material transport in the mantle wedge by a hydrofracture model for fluid and a channel flow model for melt. Such a timescale estimate is not increased even if melting processes that enhance 226 Ra are taken into account. The age difference in the equiline diagram corresponds to the interval of individual fluid-release events (13 kyr between Stages 1 and 2, and 5 kyr between Stages 2 and 3). Thus fluid release from the slab and subsequent magma generation occur as episodic events on a several-kiloyear timescale.