Fluid inclusion volatile analysis by heated crushing, on-line gas chromatography; applications to Archean fluids

Abstract

Eighteen fluid inclusion volatile peaks have been detected and identified from 1–2-g samples (quartz) by gas chromatography using heated (∼ 105°C) on-line crushing, helium carrier gas, a single porous polymer column (HayeSep R; 10′ × 1/8″; 100/120#; Ni alloy tubing), two temperature programme conditions for separate sample aliquots, micro-thermal conductivity (TCD) and photoionization detectors (PID; 11.7 eV lamp) and off-line digital peak processing. In order of retention time these volatile peaks are: N 2, Ar, CO, CH 4, CO 2, C 2H 4, C 2H 6, C 2H 2, COS, C 3H 6, C 3H 6, C 3H 4 (propyne), H 2O (22.7 mins at 80°C), SO 2, ±iso-C 4H 10±C 4H 8 (1-butene) ± CH 3SH, C 4H 8 (iso-butylene), (?) C 4H 6 (1,3 butadiene), and ± n-C 4H 10 ±C 4H 8 (trans-2-butene) (80°C and −70°C temperature programme conditions combined). H 2O is analysed directly. O 2 can be analysed cryogenically between N 2 and Ar, but has not been detected in natural samples to date. H 2S, SO 2, NH 3, HCl, HCN and H 2 cannot be analysed at present. Blanks determined by crushing heat-treated Brazilian quartz (800–900°C/4hrs) are zero for 80°C temperature programme conditions, except for a large, unidentified peak at ∼ 64 mins, but contain H 2O, CO 2 and some low molecular weight hydrocarbons at −70°C temperature conditions due to cryogenic accumulation from the carrier gas and subsequent elution. TCD detection limits are ∼30 ppm molar in inclusions; PID detection limits are ∼ 1 ppm molar in inclusions and lower for unsaturated hydrocarbons (e.g. ∼ 0.2 ppm for C 2H 2; ∼ 0.3 ppb for C 3H 6). Precisions are ∼±1–2%, except for H 2O (∼±13%). Major fluid inclusion volatile species have been successfully analysed on a ∼50 mg fluid inclusion section chip (∼ 7 m × ∼ 10 m × ∼ 100 μm). Two distinct end-member Archean fluids, one internal and one external, have been found related to the Tanco zoned, granitic pegmatite, SE Manitoba. The former is an H 2O (∼96%)-CO 2 (∼40%)-CH 4-N 2 fluid (S species not included) with a moderate salinity of 6.6 ± 1.3 eq. wt.% NaCl which is interpreted to be magmatic in origin, whereas the latter is an H 2O (∼97%)-CH 4 (∼2%)-CO 2 (∼0.4%)-C 2H 6-N 2 fluid with a distinctly higher salinity of ∼10–20 eq. wt% NaCl which is interpreted to be of metamorphic/lower crustal (∼2,900 bar/∼10km) origin. The volatile compositions of H 2O (∼87–94%)-CO 2 (∼6–13%)-CH 4-N 2 fluids with ∼5–6 eq. wt.% NaCl from primary inclusions from three structurally controlled, mafic-ultramafic rock hosted Archean Au-quartz vein deposits in the Barberton greenstone belt, southern Africa ( n=9) are distinctly different from the composition of the Tanco external fluid, but similar to the composition of primary fluids of interpreted