The geochemical characteristics of olivine-hosted melt inclusions in Quaternary potassic basalts around Wudalianchi area, North-East China

Sporadic intraplate volcanism occurred around the Wudalianchi, northeastern China, in the Miocene (9.6 - 7.0 Ma), middle Pleistocene (0.56 - 0.13 Ma), and recent (1719 - 1721 AD) periods. Whole-rock chemical compositions of the volcanic rocks range from alkali basalt to trachyandesite characterized by high K₂O content (>4.4 wt%) and by EM-1 like Sr - Nd - Pb isotopic signature (Basu et al., 1991). Many recent geochemical and petrological studies suggest considerable contributions to the origin of the volcanism from the stagnant Pacific slab lying beneath northeastern China. In this study, we obtained 47 basaltic samples from the Quaternary volcanoes in this volcanic field and analyzed whole-rock and mineral chemical compositions as well as volatile contents of olivine-hosted melt inclusions (MIs) in order to clarify the origin of the volcanism particularly in terms of volatiles such as H₂O, CO₂, S, Cl, and F.

The whole-rock compositions of these basalts are enriched in K₂O contents (4.4 - 6.0 wt%) and depleted in FeO contents (7.1 - 9.6 wt%) showing tight liner compositional trends in SiO₂ contents from 45.0 to 54.0 wt%. Olivine phenocrysts containing melt inclusions show Fo# (=100Mg/(Mg+Fe)_mol) from 83.7 to 91.2, suggesting that these basaltic samples were derived from nearly primitive magma generated in the upper mantle. Despite the limited range of whole-rock compositions, olivine-hosted MIs possess wide range of compositions with K₂O contents from 2.7 to 6.7 wt% and with FeO contents from 4.4 to 7.0 wt% but limited range of TiO₂ contents from 2.4 to 2.9 wt%. This characteristic relationship among whole-rock, olivine, and MI compositions cannot be explained simply by fractionation of olivines, but suggests that pre-eruptive magma heterogeneities are preserved in MIs. Volatile contents in MIs were measured by Secondary Ion Mass Spectrometry. Although most samples are degassed in terms of H₂O and CO₂, the highest H₂O content of 1.33 wt% shows good agreement with previous estimation (H₂O content 1.1 - 1.4 wt%; Fo#=85.2 - 86.1) by Kuritani et al. (2013). Fluorine contents in MIs are >1000 ppm for the given H₂O content of 1.33 wt% and are clearly on the dehydration side from the DMM - FOZO trend proposed by Shimizu et al. (2019) suggesting the influence of subducted slab materials from mantle transition zone.