Fluid inclusions in magmatic ilmenite record degassing in basic magmas

Abstract

Magmatic volatile phases play a major role in igneous systems, but indirect sampling of the magmatic fluid, especially for basic magmas, remains challenging to document. Here, we report compelling evidence of primary fluid inclusions trapped within magmatic ilmenite from two different basic intrusive settings: Armorican Massif (France) and Central Iberian Zone (Spain). Fluid inclusions have a solute chemistry dominated by sodium, calcium, chlorine, sulfur and iron, with detectable contents of metals and metalloids and thus likely record the onset of late-stage magmatic volatile saturation in these basic magmas. Hence, we argue the presence of fluid inclusions in ilmenite may be a good indicator for magma degassing in these settings and importantly records the magmatic-hydrothermal transition. Additionally, this study suggests that the trapping of the magmatic volatile phase during ilmenite (and other opaque minerals) crystallization may be more common but at present an underappreciated phenomenon in basic igneous magmatic systems. Fluid inclusions within ilmenite record primary magmatic fluids in intrusive mafic rocks and may work as a proxy for mafic magma degassing, as shown by geochemical data from inclusions in mafic samples from the Armorican Massif and the Central Iberian Zone.