Noble gas geochemistry of phenocrysts from the Ciomadul volcanic dome field (Eastern Carpathians)

Abstract Noble gas isotopic composition of fluid inclusions was analyzed in amphibole, plagioclase and clinopyroxene phenocrysts from the shoshonitic and dacitic volcanic products of the Ciomadul volcanic dome field, the youngest volcanic system within the Carpathian-Pannonian Region. The highest Rc/RA ratios (3.0–3.8 RA) were obtained for high-mg clinopyroxene of the Malnas shoshonite. High-Al amphiboles from the Bixad dacitic pumices have Rc/RA ratios between 1.16 and 2.11 RA. These values overlap with the noble gas signature of the present-day CO2 emission. Thus, our new results reinforce the conclusion that the mantle component of the Ciomadul primitive magmas has relatively lower Rc/RA signature compared to the nearby Persani lithospheric mantle. This is likely due to the thorough metasomatic nature resulting in elevated large ion lithophile elements and high water content of the Ciomadul magmas. On the other hand, the Rc/RA ratios from plagioclase-hosted fluid inclusions and those from low-Al amphiboles are 0.06–0.12 RA and 0.39–0.77 RA, respectively defining a dominant crustal origin (>90%) for the trapped fluids. Noteworthy, these minerals represent a low-temperature crystal mush assemblage that existed for protracted time in the magma reservoir. We tested the fraction of mantle contribution for different mantle end-member values, considering also the effect of magma aging on the R/RA ratios due to longer (up to 50 kyr) residence time. This resulted in a maximum of ~50–60% mantle fluid contribution for the high-Al amphibole-hosted fluid inclusions and a lower, ~25% mantle fluid contribution, for the low-Al amphiboles. The elevated mantle fluid contribution in the case of the high-Al amphiboles can be explained by a fresh magma recharge event and shorter residence time before the eruption. The results of this study imply that fluid inclusion in primitive clinopyroxene and amphibole phenocrysts could reflect the magmatic end-member, which is in the case of Ciomadul a strongly metasomatized lithospheric mantle with relatively low Rc/RA values. Thus, the noble gas signature of the lithospheric mantle could be heterogeneous even in a restricted area.