Determination of the origin of salinity in granite-related fluids: evidence from chlorine isotopes in fluid inclusions

Abstract

Thermal ionisation mass spectrometry of the Cs2Cl+ ion has been used to determine the δ37Cl value of two sets of magmatic fluids trapped in fluid inclusions. Samples were studied from the Capitan pluton in New Mexico and the SW-England batholith, both of which have high temperature fluids whose δD and δ18O values are distinctly magmatic. Relative to standard mean ocean chloride, the δ37Cl values of the Capitan fluid inclusions cluster around 0‰, while those from SW-England cluster around +1.9‰. We conclude that Cl in the Capitan magmatic fluids was derived from local evaporite sequences whereas in SW-England the Cl appears to be more representative of a deep magmatic source. In SW-England fluid inclusions of lower temperature and salinity, in E–W veins, have δ37Cl values that also appear to be magmatic. The younger N–S veins, that contain high salinity fluids, have δ37Cl values that indicate a seawater origin for the Cl.