PhD defense: Experimental study of sulfur distribution between apatite and silicate melts: application to volcanic degassing.
Amphitheatre
Abstract
Magmatic degassing is a universal process that shapes our atmosphere as well as those of other planets, and also causes many mineral concentrations of economic interest. Sulphur is a volatile element that fundamentally influences magmatic degassing. Among the minerals capable of recording the activity of volatile elements in magmas, apatite is certainly one of the most promising: although scarce, this mineral is ubiquitous in magmatic rocks.
This experimental thesis studies the behaviour of sulphur in two main aspects. The first is the solubility of sulphur in basanites, a composition that has been little studied, with a focus on the La Palma 2021 eruption. The experiments were conducted at fixed temperature and pressure (1040°C-200MPa) and oxygen fugacity between NNO -0.47 and NNO+4.10.
In the second approach, the distribution of sulphur between apatite and silicate melt was studied in relation to the fugacity of sulphur species. A study was carried out on various compositions at 200MPa, with temperatures between 1000 and 1040°C and oxygen fugacity between NNO-0.82 and NNO+4.17.
The results showed that the solubility of sulphur in the silicate melt is primarily driven by oxygen fugacity. A positive correlation was found between the S content in the melt and in apatite, strongly influenced by oxygen fugacity. A detailed study on the fugacity of sulphur species allowed for the identification of a positive trend between the sulphur content and the sum of the fugacities. From this pattern, it was possible to create models, both for general compositions, to reconstruct the sulphur content in apatites from the fugacities of sulphur species and vice versa.