U–Pb dating and Sm–Nd isotopic measurement of Ti–bearing andradite: Reference material development and in situ applications by LA-(MC)-ICP-MS

 Ti-bearing andradite [Ca₃Fe²⁺Ti⁴⁺Si₃O₁₂] is generally widespread, a diagnostic accessory phase of silica undersaturated alkalic plutonic and volcanic rocks but also occurs in skarn and hydrothermal alteration assemblages, which is associated with the major economic critical metals (Nb, LREE) mineral in deposits related to carbonatite and alkaline intrusions, although it never occurs in large quantities. Because of their enrichment in incompatible elements, carbonatites and alkaline rocks have also been increasingly used by isotope geochemists to study mantle evolution processes leading to continental magmatism and to track the pathways of lithospheric recycling. For both practical and academic applications, it is important to understand the timing of carbonatite and alkaline magmatism relative to such geodynamic processes as plate collisions, rifting, crustal upcoming.

Considering its potential U-Pb dating and perseverance later alteration as well LREE-enrichment, in situ laser ablation is considered as the most suitable method to measure their U-Pb and Sm-Nd isotopic compositions. Twenty-seven Ti-bearing andradite samples from thirteen typical carbonatite and/or alkaline intrusions in Prairie Lake and Ice River, Canada, Magnet Cove, USA, Alnö, Sweden, Fen, Norway, Ozernaya and Ural, Russia, Zijinshan and Fanshan, China, Tamazert, Morocco, San Ieo, Italy, Kaiserstuhl, Germany, Schaffhausen, Switzerland, were conducted for major, trace, U-Pb and Sm-Nd measurement using Electron microprobe (EPMA) and laser ablation (multicollector) inductively coupled plasma mass spectrometer (LA-(MC)-ICP-MS). Based on development of several in-house Ti-bearing andradite reference materials (PL34, IR18, MC15) demonstrates that precise and accurate U-Pb ages can be obtained after common Pb correction. Moreover, we obtained a reliable in situ Sm-Nd isotopic data because of the relatively moderate LREE content in our samples. These results will have significant implications for understanding the genesis of carbonatite or alkaline intrusion related to metallogenic geochronology and ore deposits research.