Eppawala-AP, Sri Lanka, an Apatite Reference Material for High Precision Chlorine Isotope Analysis

In this study, an apatite reference material (RM), Eppawala-AP, was obtained and calibrated from a mega-crystal of Eppawala carbonatite (collected in northwestern region of Sri Lanka) to standardize chlorine isotope analysis. A homogeneity test and chlorine isotope composition measurement of the RM were performed using secondary ion mass spectrometry (SIMS) and continuous gas flow isotope ratio mass spectrometry (CF-IRMS), respectively. The RM, with a recommended delta Cl-37 value of -0.74 +/- 0.15 parts per thousand, can be utilized as a matrix-matched N standard (similar to 1.55 wt. % Cl) to correct the instrumental mass fractionation (IMF) during in situ analysis, or as a quality control material for both in situ and bulk analysis, to facilitate high quality measurement and inter-laboratory data comparison. This RM is available to the scientific community by contacting the corresponding author. Our homogeneity test indicates an absence of orientation effect during SIMS analysis for the specific apatite grain investigated here, and suggests a repeatability of 0.13 parts per thousand for SIMS apatite chlorine isotope analysis. The matrix effect of minerals with complex chemistry predicts that the chlorine content is a controlling factor of IMF during in situ apatite chlorine isotope analysis; hence, additional RMs with variable concentrations of chlorine are required for routine delta Cl-37 measurement.