Abstract
Thermal ionisation mass spectrometry (TIMS) of the Cs 2Cl + ion has been used to determine the δ 37Cl value of paleofluids trapped in fluid inclusions. Modifications to the existing published method have lowered the precision by a factor of 2–0.09‰ (1σ) thereby allowing real differences to be measured with greater certainty. The method has been applied to determine δ 37Cl 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 δ 18O signatures are distinctly magmatic. Relative to standard mean ocean chloride (SMOC) the δ 37Cl values of the Capitan samples cluster around 0‰, whereas those from SW-England cluster around +1.8‰. The Br/Cl ratios of the Capitan fluid inclusions are low, indicative of halite dissolution, and are comparable to local high salinity waters derived from leaching Permian evaporites. The Br/Cl ratios of the SW-England fluid inclusions are much higher and within the range of values normally considered to be “magmatic.” We conclude that the δ 37Cl value of the Capitan magmatic fluids confirms the evidence from Br/Cl ratios that Cl was derived from an evaporite source, whereas the δ 37Cl value of the SW-England fluids appears to be more representative of a deep magmatic source.
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