Vesiculation and vesicle loss in mid-ocean ridge basalt glasses: He, Ne, Ar elemental fractionation and pressure influence |
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| Authors: | Philippe Sarda Manuel Moreira |
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| Institution: | 1 Groupe Géochimie des Gaz Rares, Département des Sciences de la Terre et UMR Orsay Terre, Université Paris 11-Orsay, 91405 Orsay Cedex, France
2 Laboratoire de Géochimie et Cosmochimie, Institut de Physique du Globe et Université Paris 7-Denis Diderot, 4, place Jussieu, 75252 Paris Cedex 05, France |
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| Abstract: | 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 of 4He/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, the 4He/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. |
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