Vesiculation and vesicle loss in mid-ocean ridge basalt glasses: He, Ne, Ar elemental fractionation and pressure influence
Vesiculation and vesicle loss in mid-ocean ridge basalt glasses: He, Ne, Ar elemental fractionation and pressure influence
复制标题
大洋中脊玄武岩玻璃中的囊泡形成和囊泡损失：He、Ne、Ar 元素分馏和压力影响
DOI：
10.1016/s0016-7037(01)00863-8
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发表时间：
2002
影响因子：
5
通讯作者：
M. Moreira
中科院分区：
文献类型：
--
作者：
P. Sarda;M. Moreira
Sarda和Graham（1990）提出，在洋中脊玄武岩（morb）中，脱气通过平衡囊泡作用发生，随后发生不同程度的囊泡损失。该模型预测，在具有囊泡的MORB玻璃的散装样品中，稀有气体代表了囊泡组分和溶解在熔体中的组分之间的二元混合物。由于囊泡作用将产生非常不同的气体和熔体中的稀有气体浓度和元素比例，因此应在MORB稀有气体丰度数据中记录二元混合系统。事实上，已知存在较大范围的4he /40Ar *比值，但这些二元混合系统仍然难以捉摸，因为氦被用作稀有气体丰度的代表，因为氦不受空气添加的影响。在这里，我们表明使用Ar代替He，在Ar浓度较低的地方，4he /40Ar *比值较高，正如简单二元混合系统所期望的那样。利用不断增长的Ne数据库，我们进一步表明，假设He浓度不用于追踪囊泡丰度，则预测的二元混合是由He- ar和He-Ne对记录的。这是因为由于氦的溶解度较高，很大一部分氦留在熔体中。相比之下，Ar或Ne浓度都可以根据空气添加进行校正，可以清楚地追踪囊泡，并产生二元混合模式，适用于全球范围内的脊。从而在稀有气体丰度数据中找到了囊泡和囊泡损失的地球化学支持。He-Ne-Ar浓度数据最好的解释是假设氦与氖或氩的溶解度比在1 bar实验室实验中测量的值高5到15倍，因为He的溶解度更高，而Ne和Ar的溶解度更低。我们认为这是一种压力效应，泡化主要发生在岩浆熔融后地幔上升过程中。
Sarda and Graham (1990) proposed that in mid-ocean ridge basalts (MORBs), degassing occurs through equilibrium vesiculation followed by various extents of vesicle loss. This model predicts that in a bulk sample of MORB glass with vesicles, the rare gases represent a binary mixture between a vesicle component and a component dissolved in the melt. As vesiculation is expected to produce very different rare gas concentrations and elemental ratios in gas and melt, binary mixing systematics should be recorded in the MORB rare gas abundance data. Indeed, a large range of4He/40Ar∗ ratios was known to exist, but these binary mixing systematics remained elusive because helium was used as a proxy for rare gas abundance because helium is not affected by air addition. Here we show that using Ar instead of He, the4He/40Ar∗ ratio is higher where the Ar concentration is lower, as expected from simple binary mixing systematics. Taking advantage of the growing Ne database, we further show that the predicted binary mixing is recorded by the He-Ar and He-Ne couples, provided He concentration is not used to trace vesicle abundance. This is because a significant part of helium remains in the melt due to its higher solubility. In contrast, Ar or Ne concentrations, which can both be corrected for air addition, clearly trace vesicles and yield binary mixing patterns that hold for ridges worldwide. The model of vesiculation and vesicle loss thereby finds geochemical support in the rare gas abundance data. The He-Ne-Ar concentration data is best explained by assuming the ratio of helium to neon or argon solubility is about 5 to 15 times higher than values measured in 1 bar laboratory experiments, due to higher He and lower Ne and Ar solubilities. We propose that this is a pressure effect, and vesiculation mainly occurs during magma ascent in the mantle after melting.