Mineral associations and in-situ major and trace element compositions of dalyite from charoitites, Murun complex, Siberia

Abstract

The rare potassium Zr-silicate dalyite has been found in the charoitites of Murun alkaline complex as an accessory mineral and the main Zr phase. Electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to measure major and trace element compositions of dalyite, respectively. Dalyite from charoitites is defined by significant Ti ↔ Zr substitution and low extent of Na–K solid solution and mineral formula can be presented as K2(Zr1-xTix)Si6O15. The Ti ↔ Zr substitution is a geothermometer in many minerals, however, the presence of zoning with increasing Ti from core towards the rim in dalyite from charoitites might indicate that element activity was more important factor than temperature. The first trace element analysis of dalyite shows that this mineral is enriched in lithophile elements, has strongly fractionated REE patterns but no Eu anomaly. Dalyite shares some common geochemical features with magmatic zircon: it shows HREE-enrichment and positive Ce anomaly relative to chondrite values, which could indicate that rare earth elements occur in similar structural position. Dalyite is an important mineral of peralkaline agpaitic rocks, a typical mineral of magmatic rocks, and likely indicates low-pressure conditions. Our new mineralogical and geochemical results on the dalyite clarify the role of Zr-silicates in agpaitic rocks and provide better understanding of the formation of charoitites.