Further characterizing Eppawala-AP as a potential Cl-rich apatite Sr isotope reference material

Abstract

Strontium isotope composition is a powerful proxy for tracking the source and evolution of a range of geological processes in magmatic, sedimentary and metamorphic environments. Owing to the mobility of Sr with fluids, Sr isotope is also an ideal proxy to investigate, and has been extensively used to tackle the history of magmatic–hydrothermal systems and associated water–rock interactions. Apatite is an excellent choice for such a purpose as it is ubiquitous in almost all these environments, and contains abundant Sr but exceptionally limited Rb. Microanalytical techniques are preferred for apatite Sr isotope studies since most apatites are small in size and may have a complicated growth history to be decoded. Matrix-matched reference materials are a prerequisite for microanalysis, and although variable F- and OH-rich apatite Sr isotope reference materials have been developed, chlorine-rich ones are lacking. Here, we further evaluate Eppawala-AP, a Cl-rich apatite, as a potential Sr isotope reference material. Our extensive Sr isotope microanalysis at three laboratories confirmed that the Eppawala-AP was homogeneous in terms of Sr isotope composition, with a recommended ⁸⁷Sr/⁸⁶Sr ratio of 0.704989 ± 0.000017 (2 SD, n = 9) determined by thermal ionization mass spectrometry from another two laboratories. The Eppawala-AP also had homogeneous trace element contents and Nd isotope composition. Therefore, Eppawala-AP can be used as a Sr isotope reference material, and also as a working reference to monitor the Nd isotope and trace element analysis, and thus could be expected to fill the much-needed gap for Cl-rich apatite reference materials and facilitate more precise and accurate analysis.