Apatite Chlorine Concentration Measurements by LA-ICP-MS

Apatite incorporates variable and significant amounts of halogens (mainly F and Cl) in its crystal structure, which can be used to determine the initial F and Cl concentrations of magmas. The amount of chlorine in the apatite lattice also exerts an important compositional control on the degree of fission-track annealing. Chlorine measurements in apatite have conventionally required electron probe microanalysis (EPMA). Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) is increasingly used in apatite fission-track dating to determine U concentrations and also in simultaneous U-Pb dating and trace element measurements of apatite. Apatite Cl measurements by ICP-MS would remove the need for EPMA but the high (12.97 eV) first ionisation potential makes analysis challenging. Apatite Cl data were acquired using two analytical set-ups: a Resonetics M-50 193 nm ArF Excimer laser coupled to an Agilent 7700x quadrupole ICP-MS (using a 26 mu m spot with an 8 Hz repetition rate) and a Photon Machines Analyte Excite 193 nm ArF Excimer laser coupled to a Thermo Scientific iCAP Qc (using a 30 mu m spot with a 4 Hz repetition rate). Chlorine concentrations were determined by LA-ICP-MS (1140 analyses in total) for nineteen apatite occurrences, and there is a comprehensive EPMA Cl and F data set for 13 of the apatite samples. The apatite sample suite includes different compositions representative of the range likely to be encountered in natural apatites, along with extreme variants including two end-member chlorapatites. Between twenty-six and thirty-nine isotopes were determined in each apatite sample corresponding to a typical analytical protocol for integrated apatite fission track (U and Cl contents) and U-Pb dating, along with REE and trace element measurements. Cl-35 backgrounds (present mainly in the argon gas) were ~ 45-65 kcps in the first set-up and ~ 4 kcps in the second set-up. Cl-35 background-corrected signals ranged from ~ 0 cps in end-member fluorapatite to up to ~ 90 kcps in end-member chlorapatite. Use of a collision cell in both analytical set-ups decreased the low mass sensitivity by approximately an order of magnitude without improving the Cl-35 signal-to-background ratio. A minor Ca isotope was used as the internal standard to correct for drift in instrument sensitivity and variations in ablation volume during sessions. The Cl-35/Ca-43 values for each apatite (10-20 analyses each) when plotted against the EPMA Cl concentrations yield excellently constrained calibration relationships, demonstrating the suitability of the analytical protocol and that routine apatite Cl measurements by ICP-MS are achievable.