Titanite Petrochronology by LA-ICPMS: Method and Significance in Deciphering Igneous Processes

Abstract

We describe an in situ method for simultaneous measurement of U-Th-Pb isotopes and complimentary trace and rare earth elements on titanite using a Thermo Element2 HR ICPMS coupled to Photon Machine Analyte G2 excimer laser. The ablation was carried out with a 25 μm spot size, a laser fluence of 5.56 J/cm2 and a laser repetition rate of 7 Hz. Four standard reference materials (BearLakeRoad titanite, Ecstall titanite, NIST612 and NIST614 glasses) were ablated systematically during the two analytical sessions to ensure data reliability and to correct for downhole elemental fractionation during ablation. Seven unknown granite and granodiorite samples were analyzed in this study to test the stability of the analytical parameters. After correcting for common Pb incorporation (where necessary), all seven samples yielded crystallization ages consistent with previously published data. Trace and rare earth elements determined during the ablation experiments were reduced using three separate reduction schemes in Iolite software. Results were further compared to trace element data measured in solution using a ThermoFisher X-Series II Quadrupole Inductively Coupled Plasma Mass Spectrometer (Q-ICP-MS). The comparison revealed the overall trace elements concentrations retrieved using BLR titanite as a primary reference material and 42Ca (19.8%) as the internal elemental standard (X_Trace_Elements_IS) are closer in value to the concentrations obtained by solution mode Q-ICPMS. We further discussed how temperatures calculated using zirconium concentrations in titanite coupled with total rare earth elements contents and specific elemental ratios can provide essential information about the igneous conditions titanites grew in.