Sulphur Isotope Fractionation During Degassing of Canary Island Magmas

English Abstract:

Isotopic fractionation of sulphur in volcanic systems is controlled by the speciation of sulphur in the silicate melt, degassing of sulphur-rich volatiles, and saturation with sulphur-rich melts or minerals. These factors are in turn tightly linked to the oxidation state of the melt, as well as temperature and pressure. The complexity of such multi parameter systems results in a wide array of sulphur behaviours in magmas, the understanding of which is still limited. In this work, we present triple S-isotope ion microprobe measurements of crystal-hosted melt and sulphide inclusions as well as matrix glasses from the 2011-2012 submarine eruption at El Hierro, Canary Islands, which involved volatile-rich basanite magma. Comparison of δ³⁴S measurements on matrix glasses (457 ± 80 ppm S) and S-rich melt inclusions (up to 5000 ppm S) indicate that degassing results in the preferential loss of heavy sulphur isotopes (up to 10 permil relative). This can only occur in a narrow oxygen fugacity range. Isotopic and compositional differences among olivine-, pyroxene- and magnetite-hosted inclusions appear to describe the evolution of the oxidation state of the magmatic system, with consideration to the order of appearance of phenocryst phases. The isotopic characteristics of El Hierro products are consistent with bulk rock multiple S isotope compositions from nearly all other historical eruptions of the Canary Archipelago, measured in both sulphide and sulphate fractions by gas-source isotope-ratio mass spectrometry. Variations between and within islands are highlighted in view of their other compositional particularities and lateral configuration. This analysis further provides insight into the degassing history of the notable 1730-36 basaltic fissure eruption of Lanzarote. Our work implies that volatile-rich magmas in the Canary Islands are subject to open-system degassing processes that effectively fractionate sulphur isotopes.