Other
Scientific paper
May 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52.1065t&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 52, Issue 5, pp.1065-1075
Other
3
Scientific paper
Fluid inclusions from the tourmaline phase of alteration of the footwall amphibolite from the Tanco zoned granitic pegmatite lie in the system H 2 O-CH 4 -NaCl-CO 2 . These inclusions contain a liquid and vapour phase at room temperature and develop a second liquid phase on cooling in the range -77 to -95°C: the second liquid and the vapour are methane. Isochores for inclusions showing vapour phase (CH 4 ) homogenisation were constructed from the equations of and (1981). The intersections of these isochores with the univariant melting curves for methane clathrate hydrates give estimates of salinity for the inclusions of between 7 and 10 equivalent wt.% NaCl, which are lower than those derived from measurement of the depression of the melting point of ice; a result consistent with salt exclusion by clathrate. The bulk composition of the fluid is estimated to be 91 mol.% H 2 O, 6 mol.% CH 4 , 2 equiv. mol.% NaCl and < 1 mol.% CO 2 . Total homogenisation temperatures have a mean value of 371 ° C ± 36° C (1 ; n = 91). Variation of total homogenisation behaviour between homogenisation into the H 2 O phase and critical homogenisation is compatible with the composition of the inclusions being on the steep H 2 O-rich limb of the H 2 O-CH 4 -NaCl solvus. The data suggest that the consolute temperature for a H 2 O-CH 4 mixture with ~2 mol.% salt should be ~400-420°C. Extrapolation up the bulk isochore to 2700-2900 bars trapping pressure gives a trapping temperature of ~720-780°C; which is an estimate of the intrusion temperature of the Tanco pegmatite. Calculation of f o 2 from the bulk inclusion composition gives values near WI at these conditions which is unreasonably low for fluids derived from the pegmatite, which were probably between QFM and HM. The metasomatic fluids may have resulted from fluid mixing in the wall rock immediately adjacent to the pegmatite. The fluid derived from the pegmatite, a H 2 O-CO 2 fluid, introduced the B necessary to form tourmaline in the amphibolite. On the other hand it is suggested that CH 4 ± H 2 O may have been derived from a metamorphic fluid which formed in equilibrium with graphite at lower oxygen fugacities at ~400°-600°C and which was present in the wall rock at the time of pegmatite emplacement. Supporting evidence is provided by pegmatite wall zone fluids with higher CH 4 (~5 equiv. mol.% in CO 2 ) than the bulk of the pegmatite H 2 O-CO 2 fluids (0.8 mol.% in CO 2 ), suggesting marginal fluid contamination.
Spooner Edward T. C.
Thomas Anne V.
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