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 det ermine the delta(37)Cl value of two sets of magmatic fluids trapped in flui d inclusions. Samples were studied from the Capitan pluton in New Mexico an d the SW-England batholith, both of which have high temperature fluids whos e delta D and delta(18)O values are distinctly magmatic. Relative to standa rd mean ocean chloride, the delta(37)Cl values of the Capitan fluid inclusi ons cluster around 0 parts per thousand, while those from SW-England cluste r around + 1.9 parts per thousand. We conclude that Cl in the Capitan magma tic 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, h ave delta(37)Cl values that also appear to be magmatic. The younger N-S vei ns, that contain high salinity fluids, have delta(37)Cl values that indicat e a seawater origin for the Cl. (C) 2000 Elsevier Science B.V. All rights r eserved.