Major, trace element and noble gas isotope composition of olivine from the alkaline basalts of the Persani Volcanic Field, Romania: constraints on the magma source region

Olivine, as the first crystallization product from basaltic melt, provides important information about the magma origin. Here we provide a detailed textural, major and trace element, and noble gas isotope compositional data for a alkaline basalt suite from the Persani volcanic field (PVF) of the Carpathian-Pannonian Region. This is the youngest monogenetic volcanic field (1,3 Ma - 0,6 Ma) formed in a geodynamically still active zone. A descending, vertical lithospheric slab result in frequent earthquakes, whereas nearby, another young volcanic system is found (Ciomadul). The alkali basalt magmas were formed due to decompression melting in the asthenosphere, at 60–80 km depth.

Thus, olivine composition can be used to characterize the nature of asthenospheric mantle in a postcollisional area. Noble gas isotope ratios, especially the ³He/⁴He, are sensisitve indicators of the mantle composition. There are relatively comprehensive data on mantle xenoliths, however, only sporadic data are from olivine crystals of basalts. This is due the challenge of such studies, because of the need of clean olivine separates and detection of low amount of gases from the primary fluid inclusions.

In the Carpathian-Pannonian Region, we firstly detected noble gas isotopes from phenocrysts of basaltic rocks. We sampled different eruption products of the PVF from different eruption episodes. Following a multi-step sample preparation process, we analysed the olivine separates with noble gas mass spectrometer. Petrographic characteristics and major element composition of most olivine phenocrysts suggest crystallization from primary basaltic magma. Due to fast magma ascent, the olivine crystals preserved the original noble gas isotope ratios in their primary fluid inclusions in most samples.

We got relatively low, ~2-5 R/Ra values (³He/⁴He of the sample divided by ³He/⁴He of the atmosphere) which are lower than the R/Ra values obtained from the olivine and pyroxene crystals of lithospheric mantle xenoliths in the PVF alkaline basalts (~6 R/Ra), suggesting geochemical differences between the local asthenospheric and lithospheric mantle. Our results are also significantly lower than the usual R/Ra of the depleted mantle (~8 R/Ra). The low values can be explained by metasomatism of the asthenospheric magma source region with crustal fluids during former subduction and/or ⁴He addition to the asthenosphere from the radioactive decay of U and Th originated from the subducted lithospheric slab. Another possible explanation could be the lithologic heterogeneity of the magma source region. The Mn, Ca and Zn content of olivine autocrysts also indicate the presence of recycled crustal material in the mantle source, in agreement with the noble gas isotope compositional data. Our results suggest that in a postcollisional setting the asthenosphere is contaminated by recycled crustal material and subduction-related fluids.