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

Thermal ionisation mass spectrometry of the Cs2Cl+ ion has been used to determine the delta(37)Cl 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 delta D and delta(18)O values are distinctly magmatic. Relative to standard mean ocean chloride, the delta(37)Cl values of the Capitan fluid inclusions cluster around 0 parts per thousand, while those from SW-England cluster around + 1.9 parts per thousand. 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 delta(37)Cl values that also appear to be magmatic. The younger N-S veins, that contain high salinity fluids, have delta(37)Cl values that indicate a seawater origin for the Cl. (C) 2000 Elsevier Science B.V. All rights reserved.