Bubble formation and decrepitation control the CO 2 content of olivine-hosted melt inclusions
Bubble formation and decrepitation control the CO 2 content of olivine-hosted melt inclusions
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气泡形成和爆裂控制橄榄石熔体包裹体的CO 2 含量
DOI：
10.1002/2016gc006633
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Maclennan J
中科院分区：
文献类型：
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作者：
Maclennan J
橄榄石熔融包裹体的CO2含量以前曾被用来限制玄武岩系统中岩浆房的深度。然而，绝大多数包裹体的CO2含量，这意味着包封压力显着低于那些从独立的岩石气压计。此外，全球数据库的熔体夹杂物组成，从低 环境，表明饱和压力的分布在洋中脊，海洋岛屿和大陆裂谷带之间变化惊人地小。数据库中95%的包裹体的饱和压力为200 MPa或更低，表明熔融包裹体CO2通常不能提供岩浆房深度的准确估计。建立了橄榄石熔体包裹体的P-V-T-X演化模型，从而可以跟踪包裹体系统的性质，因为包裹体遵循P-T路径模型。该模型表明，CO2在夹杂物中的饱和度和蒸汽气泡的形成的主要控制是后捕获结晶的夹杂物的主要元素组成的影响，以及如何将其转化为CO2溶解度的变化。在大多数情况下，外部熔体和夹杂物之间的压力差可能足够高，导致夹杂物爆裂。Decrepitation可以解释基于CO2的气压计和其他岩石学气压计之间的明显不匹配，也可以解释观察到的饱和压力的全球分布。只有当大量的后圈闭结晶发生时，重建的包裹体成分才能提供岩浆房深度的准确估计。
The CO2contents of olivine‐hosted melt inclusions have previously been used to constrain the depth of magma chambers in basaltic systems. However, the vast majority of inclusions have CO2contents which imply entrapment pressures that are significantly lower than those obtained from independent petrological barometers. Furthermore, a global database of melt inclusion compositions from low settings, indicates that the distribution of saturation pressures varies surprisingly little between mid‐ocean ridges, ocean islands, and continental rift zones. 95% of the inclusions in the database have saturation pressures of 200 MPa or less, indicating that melt inclusion CO2does not generally provide an accurate estimate of magma chamber depths. A model of theP‐V‐T‐Xevolution of olivine‐hosted melt inclusions was developed so that the properties of the inclusion system could be tracked as the hosts follow a modelP‐Tpath. The models indicate that the principal control on the saturation of CO2in the inclusion and the formation of vapor bubbles is the effect of postentrapment crystallization on the major element composition of the inclusions and how this translates into variation in CO2solubility. The pressure difference between external melt and the inclusion is likely to be sufficiently high to cause decrepitation of inclusions in most settings. Decrepitation can account for the apparent mismatch between CO2‐based barometry and other petrological barometers, and can also account for the observed global distribution of saturation pressures. Only when substantial postentrapment crystallization occurs can reconstructed inclusion compositions provide an accurate estimate of magma chamber depth.