Other
Scientific paper
Mar 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54..559t&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 54, Issue 3, pp.559-573
Other
2
Scientific paper
Fluid inclusions in tourmaline and quartz from the footwall contact of the Tanco granitic pegmatite, S.E. Manitoba were studied using microthermometry (MT), laser Raman spectroscopy (LRS) and gas chromatography (GC). CH 4 -bearing, aqueous inclusions occur in metasomatic tourmaline of the footwall amphibolite contact. The internal pressures estimated from MT are lower than those obtained from LRS (mean difference = 54 ± 19 bars). The difference is probably due to errors in the measurement of Th CH 4 (V) and to the presence of clathrate at Th CH 4 (V) into which CO 2 had been preferentially partitioned. LRS estimates of pressure (125-184 bars) are believed to be more accurate. Aqueous phase salinities based on LRS estimates of pressure are higher than those derived using the data from MT: 10-20 eq. wt% NaCl. The composition of the inclusions determined by GC bulk analysis is 97.3 mol% H 2 O, 2.2 mol% CH 4 , 0.4 mol% CO 2 , 250 ppm C 2 H 6 , 130 ppm N 2 , 33 ppm C 3 H 8 , 11 ppm C 2 H 4 , and 3 ppm C 3 H 6 , plus trace amounts of C 4 hydrocarbons. The composition is broadly similar to that calculated from MT (92% H 2 O and 8% CH 4 , with 7 eq. wt% NaCl dissolved in the aqueous phase and 2 mol% CO 2 dissolved in the CH 4 phase), as expected due to the dominance of a single generation of inclusions in the tourmaline. However, two important differences in composition are: 1. (i) the CH 4 to CO 2 ratio of this fluid determined by GC is 5.33, which is significantly lower than that indicated by MT (49.0); and 2. (ii) the H 2 O content estimated from MT is 92 mol% compared to 98 mol% from GC. GC analyses may have been contaminated by the presence of secondary inclusions in the tourmaline. However, the rarity of the latter suggests that they cannot be completely responsible for the discrepancy. The differences may be accounted for by the presence of clathrate during measurement of Th CH 4 (critical), which would reduce CO 2 relative to CH 4 in the residual fluid, and by errors in visually estimating vol% H 2 O. The compositions of the primary inclusions in tourmaline are unlike any of those found within the pegmatite and indicate that the fluid was externally derived, probably of metamorphic origin. Inclusions in quartz of the border unit of the pegmatite are secondary and are either aqueous (18 to 30 eq. wt% CaCl 2 ; Th total = 184 ± 14° C ) or carbonic. Tm CO 2 for the carbonic inclusions ranges from -57.5 to -65.4°C and is positively correlated with Th CO 2 . Analyses of X CH 4 based on LRS agree within 5 mol% of those derived from MT and together indicate a range of compositions from 5 to 50 mol% CH 4 in the CO 2 phase. Bulk analysis by GC gives 99.0 mol% H 2 O, 0.6 mol% CO 2 , 0.4 mol% CH 4 , 160 ppm N 2 , 7 ppm C 2 H 6 , 4 ppm C 3 H 8 , and 2 ppm C 2 H 4 , with trace amounts of COS (carbonyl sulphide) and C 3 H 6 . The level of H 2 O in the analysis is consistent with the dominance of the aqueous inclusions in these samples, and the CH 4 : CO 2 ratios are consistent with estimates from MT and LRS. The preservation of variable ratios of CH 4 :CO 2 in inclusions < 50 m apart indicates that neither H 2 diffusion out of the inclusions nor reduction of fluids leaving the pegmatite were responsible for the more oxidized chemistries of the border unit inclusions relative to those in the tourmaline of the metasomatised amphibolite. The compositions of the inclusions in the quartz lie between those of the fluid trapped by the tourmaline (externally derived) and the measured composition of a CO 2 -bearing pegmatitic fluid, which indicates that the secondary fluids trapped in the border unit quartz were produced by late mixing.
Bray Colin J.
Dill Pasteris Jill
Spooner Edward T. C.
Thomas Anne V.
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