Diverse origins of fluid in magmatic inclusions at Bingham (Utah, USA), Butte (Montana, USA), St. Austell (Cornwall, UK), and Ascension Island (mid-Atlantic, UK), indicated by laser microprobe analysis of Cl, K, Br, I, Ba + Te, U, Ar, Kr, and Xe

Abstract

Saline fluid inclusions (FI) trapped at close to the temperature of final solidification of a granitic melt occur in rocks from Bingham, Utah, Ascension Island, mid-Atlantic Ocean, and St. Austell, Cornwall. Slightly lower temperature FI occur at Butte, Montana. Argon, Kr, and Xe were extracted from FI by laser microprobe decrepitation of minute portions of the samples after neutron irradiation (along with synthetic FI of known composition), and measured in a low blank, high sensitivity, pulse-counting mass spectrometer. Results enable measurement of Cl, K, Br, and I simultaneously with 36r, 40Ar, 84Kr, and 129Xe, in ∼10 −7 cc of fluid, the contents of a single spherical inclusion ∼57 μm in diameter. Average K Cl in Bingham and St. Austell FI are ∼0.25 and ∼0.15, respectively, broadly consistent with the composition of fluids equilibrated with rocks at temperatures close to the eutectic in the granite system (400–600°C and 0.5 – 2 Kb). Within-sample variations in K Cl are significant and may be a result of exsolution of fluids from magmas over a range of temperatures and/or pressures. Halogen ratios are confined to a narrow range, with I Cl and Br Cl in Bingham, Ascension, and some Butte FI ∼ 1 − 8 × 10 −5 and 1 − 3 × 10 −3, respectively, probably evidence of a common source of salinity, presumably part of the Earth's mantle. A component of salinity derived from continental crust may be indicated by higher I Cl and lower Br Cl in St. Austell Fl. Radiogenic 40Ar produced in situ from K in FI after trapping is usually insignificant. 40Ar e is defined as 40Ar in excess of the amounts attributable to atmospheric gases and that produced by decay of K in FI. Variations in 40Ar e Cl in Bingham, Ascension, and St. Austell FI are greater than can be explained by just different Cl concentrations in FI, (typically between ∼5 × 10 −7 and 8 × 10 −6 ), probably because Ar and Cl have been fractionated within these systems by outgassing of a magma in the interval between exsolution of different FI generations. Concentrations of Ar and 84Kr in most Bingham, Ascension, and St. Austell FI are ∼2 orders of magnitude greater than in most lavas and granites, with 84Kr 36Ar ranging between ∼0.015 and ∼0.08, grossly consistent with the composition of fluids exsolved from magmas. Several distinct fluid types are present at Butte: (1) rare “magmatic” FI having halogen and noble gas abundances similar to Bingham and Ascension and (2) abundant FI having 36Ar and 84Kr concentrations similar to air-saturated fresh waters, with slightly lower Br Cl than FI at Bingham and Ascension (∼5–10 × 10 −4) and relatively high 40Ar e Cl (∼1 × 10 −5 and 2.8 × 10 −5). Mixing prior to trapping between fluid derived from magmas and meteoric water that acquired 40Ar e plus some halogens by interaction with country rocks is the most likely origin of most Butte FI.