Volatile-rich silicate melts from Oldoinyo Lengai volcano (Tanzania): Implications for carbonatite genesis and eruptive behavior
Volatile-rich silicate melts from Oldoinyo Lengai volcano (Tanzania): Implications for carbonatite genesis and eruptive behavior
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DOI：
10.1016/j.epsl.2012.11.006
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发表时间：
2013-01-01
影响因子：
5.3
通讯作者：
Ramirez, Carlos
中科院分区：
文献类型：
--
作者：
de Moor, J. Maarten;Fischer, Tobias P.;Ramirez, Carlos
本文研究了2007-2008年坦桑尼亚Oldoinyo Lengai火山爆发中硅酸盐玻璃（含蓝宝石的熔融包裹体和基质玻璃）的挥发性、痕量和主要元素组成。2007-2008年喷发的非均质火山灰的主体成分与之前25年的碳酸盐喷涌喷发过程中形成的幼稚云母岩浆与钠碳酸岩的物理混合相一致。熔融包裹体和基体玻璃的硅不饱和成分范围很广，从演化程度最低的熔融包裹体中SiO2和(Na+K)/Al的比值接近46 wt%到基体玻璃中SiO2和(Na+K)/Al的比值高达12 wt%。熔体包裹体与基体玻璃之间SiO2的消耗伴随着所有不相容微量元素（Ba、Nb、La、Ce、Sr、Zr、Y）的强富集，这与SiO2含量高于熔体包裹体的大块矿物组合的分晶化一致，但与碳酸盐同化的硅酸盐熔体演化不一致。熔体包裹体挥发物丰富，含有2.7 wt% ~ 8.7 wt%的CO2和0.7 wt% ~ 10.1 wt%的H2O，表明Oldoinyo Lengai是一个含水体系。这与长期以来认为Oldoinyo Lengai相对无水的假设相反，这种假设是基于对钠碳酸岩熔岩缺水的观察。我们认为钠碳酸岩是由含水碳酸盐液体在低压下脱气形成的。硅酸盐玻璃的数据表明，水浓度与不相容元素富集呈负相关，我们认为这是由于水的减压脱气引起的熔体结晶。Oldoinyo Lengai的喷发旋回反映了由于广泛的H2O驱动结晶而引起的整体硅酸盐岩浆黏度的变化，当挥发物（即H2O > CO2气体和碳酸盐液体）无法与富含晶体的铅锌矿岩浆分离时发生爆发。熔体水含量随压力的增大而减小；然而，熔体包裹体中的二氧化碳浓度被不混溶的碳酸盐液体所缓冲。CO2含量随着熔体演化参数（如(Na+K)/Al）的增加而增加，这是由于熔体中碱富集和SiO2损耗增强了溶解度。另一方面，基体玻璃和演化出的熔融包裹体经历了低压（< 50 MPa） CO2脱气，并且没有被共存的碳酸盐液体缓冲。虽然熔体包裹体是目前发现的最富CO2的，但它们的CO2/Nb比值无一例外地低于MORE，这表明Oldoinyo Lengai不需要富含挥金物的地幔源。(C) 2012 Elsevier B.V.版权所有
This study presents volatile, trace, and major element compositions of silicate glasses (nepheline-hosted melt inclusions and matrix glass) from the 2007-2008 explosive eruption at Oldoinyo Lengai volcano, Tanzania. The bulk compositions of the heterogeneous ash erupted in 2007-2008 are consistent with physical mixing between juvenile nephelinite magma and natrocarbonatite emplaced during the preceding similar to 25 years of effusive carbonatite eruption. The melt inclusions and matrix glasses span a wide range of silica-undersaturated compositions, from similar to 46 wt% SiO2 and (Na+K)/Al similar to 3 in the least evolved melt inclusions to 38 wt% SiO2 and (Na+K)/Al up to 12 in the matrix glass. The depletion in SiO2 between melt inclusions and matrix glass is accompanied by strong enrichment in all of the incompatible trace elements measured (Ba, Nb, La, Ce, Sr, Zr, Y), which is consistent with fractional crystallization of a bulk mineral assemblage with SiO2 higher than that of the melt inclusions but inconsistent with silicate melt evolution by assimilation of carbonatite. The melt inclusions are volatile-rich with 2.7 wt% to 8.7 wt% CO2 and 0.7 wt% to 10.1 wt% H2O, indicating that Oldoinyo Lengai is a hydrous system. This is contrary to the long-held assumption that Oldoinyo Lengai is relatively anhydrous, which is based on the observation that natrocarbonatite lavas are water-poor. We argue that natrocarbonatites are derived from hydrous carbonate liquid that degas H2O at low pressure. The silicate glass data show that H2O concentration is negatively correlated with incompatible element enrichment, which we attribute to crystallization of the melt in response to decompression degassing of H2O. The eruptive cycle at Oldoinyo Lengai reflects changes in bulk silicate magma viscosity due to extensive H2O-driven crystallization and explosive eruptions occur when volatiles (i.e. H2O > CO2 gas, and carbonate liquid) cannot separate from the crystal-rich nephelinite magma. Melt H2O content decreases as a function of pressure; however CO2 concentration in the melt inclusions is buffered by the presence of immiscible carbonate liquid. CO2 content increases with melt evolution parameters (e.g. increasing (Na+K)/Al) due to enhanced solubility with alkali enrichment and SiO2 depletion in the melt. The matrix glasses and evolved melt inclusions, on the other hand, experienced low pressure (< 50 MPa) CO2 degassing and were not buffered by a coexisting carbonate liquid. Whereas the melt inclusions are the most CO2-rich yet identified, their CO2/Nb ratios are without exception lower than that in MORE, indicating that a volatile-rich mantle source is not required for Oldoinyo Lengai. (C) 2012 Elsevier B.V. All rights reserved.