Noble gas variation during partial crustal melting and magma ascent processes

Abstract

Noble gas isotopes, although present in trace amounts, are generally more reliable and less ambiguous recorders of their source than the major volatile species. In volcanic settings in particular, this advantage derives from their chemical inertness, as noble gas isotopic and elemental fractionations are strongly coupled to their source and modified only by physical processes during magma ascent and eruption. The Neogene volcano El Hoyazo (Betic Cordillera, SE Spain) is a highly favourable natural laboratory to study the links between partial crustal melting processes occurring at depth and the eruptive products at the surface, because partially melted crustal xenoliths are preserved in silicic lavas. Comparing the noble gas isotopic compositions of xenoliths and lavas has the potential to yield new insights into volatile behaviour during melting processes at inaccessible depths in the crust.At El Hoyazo, noble gases trapped in lava glasses, and the fluid/melt inclusions within xeno- and phenocrysts, provide novel information on: (i) their response to the crustal melting process including mechanisms such as magma mixing (and crustal assimilation) of two endmembers: i.e. the extracted felsic melt from the country metapelitic crust, and the basic-intermediate magma from the underplating in the region. The results reveal significant modification of magmatic noble gases by the interaction with the partially melted crust; (ii) noble gas variations during degassing and magma ascent, showing higher atmospheric influence in the lava samples from shallower depths than in the deeper lavas and minerals; and (iii) higher magmatic influence in crystals of garnet from deeper lava than in both shallower crystals of amphibole, and garnet crystals within the crustal xenoliths. In addition, we find that noble gases in melt inclusions are also likely accumulating in their shrinkage bubbles, and not only remaining dissolved in the melt.