Primitive neon from the center of the Galápagos hotspot
Primitive neon from the center of the Galápagos hotspot
复制标题
加拉帕戈斯热点中心的原始霓虹灯
DOI：
10.1016/j.epsl.2009.06.008
复制
发表时间：
2009
影响因子：
5.3
通讯作者：
M. Moreira
中科院分区：
文献类型：
--
作者：
M. D. Kurz;J. Curtice;D. Fornari;D. Geist;M. Moreira
我们提出了新的氦和氖测量疏浚玄武岩玻璃从西加拉帕戈斯，重点是海底侧翼的费尔南迪纳火山，但包括相邻的侧翼的达尔文，厄瓜多尔，沃尔夫和罗卡雷东达。来自费尔南迪纳火山海底侧翼的样品具有最少的放射性氦和氖; 3 He/4 He比从18到29倍大气压（Ra）不等，这跨越了以前在费尔南迪纳观测到的范围。来自费尔南迪纳北部的样品具有接近大洋中脊玄武岩（MORB）值（7.6至11.8 Ra）的3 He/4 He比值，并与来自达尔文、厄瓜多尔、沃尔夫和罗卡雷东达火山的陆上熔岩流相似。在三同位素氖图上，新的海底费尔南迪纳数据定义了一条比夏威夷和冰岛的数据更接近“太阳”的线，因此是地球上发现的放射性最小，最原始的氖同位素组成。相比之下，北方挖泥船的氖同位素组成与MORB相似。费尔南迪纳火山和邻近火山之间的同位素对比非常明显，它们的火山口距离只有35到40公里，这与费尔南迪纳火山位于热点中心上方的假设是一致的。测量的3 He/22 Ne比值，再加上氦和氖同位素系统表明，3 He/22 Ne值是非常低的费尔南迪纳样品相比，北方疏浚。耦合破碎和熔化实验表明，囊泡往往有较低的20 Ne/22 Ne和较高的3 He/22 Ne，这表明囊泡的形成不一定是在溶解度平衡，和囊泡优先受大气污染（相比，主机玻璃）。Mg#和总氦含量的玻璃之间有一个粗略的相关性，表明分馏/脱气在一个浅岩浆库是一个主要的控制惰性气体含量。由于加拉帕戈斯火山之间的火山管道以及主要和微量元素的相似性，费尔南迪纳（1.5）和北方疏浚物（15）之间的3 He/22 Ne变化（外推到太阳氖）可能与地幔源特征有关，而不是最近的脱气或熔融的影响。简单的封闭系统演化模型表明，自地球历史的最初几亿年以来，非放射成因的氖同位素组成需要保存，这与下地幔中的未脱气物质是一致的。
We present new helium and neon measurements in dredged basaltic glasses from the Western Galápagos, with an emphasis on the submarine flanks of Fernandina volcano, but including the adjacent flanks of Darwin, Ecuador, Wolf, and Roca Redonda. The samples from the submarine flanks of Fernandina volcano have the least radiogenic helium and neon;3He/4He ratios vary from 18 to 29 times atmospheric (Ra), which spans the range previously observed at subaerial Fernandina. Samples from north of Fernandina have3He/4He ratios closer to Mid-ocean ridge basalt (MORB) values (7.6 to 11.8 Ra), and are similar to subaerial lava flows from Darwin, Ecuador, Wolf, and Roca Redonda volcanoes. On a three-isotope neon diagram, the new submarine Fernandina data define a line that is closer to “solar” than data from Hawaii and Iceland, and therefore are among the least radiogenic, most primitive, neon isotopic compositions found on Earth. In contrast, the northern dredges have neon isotopic compositions similar to MORB. This sharp isotopic contrast between Fernandina and the adjacent volcanoes, whose summit calderas are only 35 to 40 km apart, is consistent with the hypothesis that Fernandina lies over the hotspot center. Measured3He/22Ne ratios, coupled with helium and neon isotopic systematics show that3He/22Ne values are extremely low in the Fernandina samples compared to the northern dredges. Coupled crushing and melting experiments show that vesicles often have lower20Ne/22Ne and higher3He/22Ne, suggesting that vesicle formation is not necessarily in solubility equilibrium, and that vesicles are preferentially affected by atmospheric contamination (as compared to host glass). There is a crude correlation between Mg# and total helium content in the glasses, suggesting that fractionation/degassing in a shallow magma reservoir is a primary control on noble gas contents. Due to similarities in volcanic plumbing between the Galápagos volcanoes, and in major and trace elements, the large3He/22Ne variations (extrapolated to solar neon) between Fernandina (1.5) and the northern dredges (15) may be related to mantle source characteristics rather than the effects of recent degassing or melting. Simple closed-system evolution models show that the unradiogenic neon isotopic compositions require preservation since the first few hundred million years of Earth history, which is consistent with undegassed material in the lower mantle.