Uranium series and beryllium isotope evidence for an extended history of subduction modification of the mantle below Nicaragua
Uranium series and beryllium isotope evidence for an extended history of subduction modification of the mantle below Nicaragua
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铀系列和铍同位素证据证明尼加拉瓜下方地幔俯冲改造的悠久历史
DOI：
10.1016/0016-7037(94)90273-9
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发表时间：
1994
影响因子：
5
通讯作者：
M. Murrell
中科院分区：
文献类型：
--
作者：
M. Reagan;J. Morris;E. A. Herrstrom;M. Murrell
在尼加拉瓜的八座火山的历史熔岩中测量了铀系核素、铍和铅同位素。尼加拉瓜北方低钛样品的（230 Th）（232 Th）比值为2.23 ~ 2.56，富集238 U和234 U，比230 Th高1- 16%。尼加拉瓜南部的（230 Th）（232 Th）≈ 2.1，（238 U）（230 Th）= 1.0-0.9。高钛样品具有中等的（230 Th）（232 Th）比值，并强烈富集230 Th。所有样品的铅同位素数据在地幔阵列与206铅204铅= 18.50-18.63。所有样品的Th同位素比值意味着源U/Th比值显着高于铅同位素比值所暗示的。所有历史样品的（210 Po）（230 Th）和（226 Ra）（230 Th）都大于1，表明熔岩在生成后不到8,000年就喷发了。所有火山岩样品的钍同位素比值与文献中的10 Be、9 Be和B/Be以及87 Sr、86 Sr和Ba/La比值具有良好的相关性。(238 U）（230 Th）比值与这些比值中的任何一个都没有很好的相关性，而是与Th浓度和公布的La Yb比值反相关。这些观察和由此得出的推论，导致了下面的首选，虽然不是完全独特的解释。钍，铍，锶同位素比值的Cocos板块沉积物在DSDP 495与低钛火山的回归趋势的比较表明，俯冲成分转移到地幔具有类似的同位素组成的散装沉积物。火山岩样品的B/Be和Ba/La比值与同位素有很好的相关性，但俯冲成分中的推断比值分别比散装沉积物中的值高至少10倍和5倍。这些关系表明，俯冲组件相结合的熔体和含水流体的元素输运特性。（230 Th）（232 Th）和87 Sr 86 Sr比值（对应于10 Be 9 Be比值为0）分别约为2.0和0.704，明显高于假设的熔岩MORB源的预期值。我们将这些特征归因于俯冲相关的锶，铀，加上或减去钍，以及相关的示踪剂的地幔前约4.5马，这样的10 Be信号的这个事件已经衰减了。观测到的良好相关性主要是由4.5Ma之间10 Be、U、Th、B和Ba的加入产生的。和约200-300 ka。最近的事件发生在200-300 ka，可能比8 ka更近。它只输送了一小部分来自板片的10 Be、B、Ba和可能储存在地幔中的钍的总通量，但增加了U> Th，并可能提高了铀对钍的分配系数。因此，铀过剩的幅度与该事件期间地幔源部分熔融的程度有关。高钛熔岩的铀系和铍同位素体系与其来源于低钛熔体萃取后的地幔残留物相一致。
U-series nuclides, beryllium, and lead isotopes have been measured on historic lavas from eight volcanoes in Nicaragua. Low-Ti samples from northern Nicaragua have (230 Th)(232 Th) ratios from 2.23 to 2.56 and are enriched in 238 U and 234 U over 230 Th by 1–16%. Those from southern Nicaragua have (230 Th)(232 Th)≈ 2.1 and have (238 U)(230 Th)= 1.0–0.9. High-Ti samples have intermediate (230 Th)(232 Th) ratios and are strongly enriched in 230 Th. Lead isotopic data for all samples plot in the mantle array with 206 Pb 204 Pb= 18.50–18.63. Th isotopic ratios for all samples imply source U/Th ratios that are significantly higher than those implied by lead isotopic ratios. All historic samples have (210 Po)(230 Th) and thus (226 Ra)(230 Th)> 1, suggesting that lavas erupt less than 8,000 y after generation. Thorium isotopic ratios for all volcanic samples correlate well with 10 Be 9 Be and B/Be as well as with 87 Sr 86 Sr and Ba/La ratios from the literature.(238 U)(230 Th) ratios do not correlate well with any of these ratios but rather anti-correlate with Th concentrations and published La Yb ratios. These observations, and inferences therefrom, lead to the following preferred, although not completely unique, interpretations. Comparison of thorium, beryllium, and strontium isotopic ratios of the Cocos plate sediments at DSDP 495 with the low-Ti volcanic regression trends shows that the subducted component transferred to the mantle has isotopic compositions similar to the bulk sediments. B/Be and Ba/La ratios of the volcanic samples are well correlated with the isotopes, but the inferred ratios in the subduction component are at least 10 and 5 times higher, respectively, than values in the bulk sediment. These relationships suggest that the subduction component combines the element-transport properties of both a melt and a hydrous fluid. The (230 Th)(232 Th) and 87 Sr 86 Sr ratios corresponding to a 10 Be 9 Be ratio of zero are about 2.0 and 0.704, significantly higher than expected in the postulated MORB source for the lavas. We attribute these characteristics to subduction related addition of strontium, uranium, plus or minus thorium, and related tracers to the mantle prior to about 4.5 Ma, such that the 10 Be signal of this event has decayed away. The observed good correlations are generated primarily by addition of 10 Be, U> Th, B and Ba between 4.5 Ma. and ca. 200–300 ka. The most recent event occurred since 200–300 ka and perhaps more recently than 8 ka. It transported only a fraction of the total slab-derived flux of 10 Be, B, Ba, and probably thorium stored within the mantle, but added U> Th and may have raised the partition coefficient of uranium over thorium. The magnitude of the uranium excess thus relates to the extent of partial melting of the mantle source during this event. The U-series and beryllium isotope systematics of the high-Ti lavas are consistent with their derivation from the mantle residue after extraction of low-Ti melts.