CAPÍTULO II MARCO TEÓRICO
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The relative proportions of the three main phases (water, carbon dioxide and non-condensables) are shown in this diagram. Full analyses as expressed both as moles per gram of calcite and relative percentages can be found in Appendix 5.2,
It can be seen from the diagram that the fluids present during the precipitation of calcite within the metadomains appear to be fairly uniform in composition as expressed in terms of the relative proportions of HjO, CO^ and non condensibles. Furthermore, all of the samples analysed are water rich with the relative proprtion of HjO ranging from approximately 70 to >95 %. However, some samples from widely spaced localities appear to contain significant quantities of non-condensible gases which may reflect input from organic matter within the sedimentary succession which was undergoing thermal maturation.
Using the data obtained from fluid extraction analysis it is possible to express CO; concentrations within the fluid in terms of moles CO; per kg of water. Several studies includng that of Arnorsson et al. (1983) have proposed gas thermometers on the basis that gas concentrations in geothermal fluids are controlled by temperature dependent equilibria with alteration minerals present within the reservoir rock. In Table 5.2 below compositional data is recalculated and expressed as moles CO; per kg of water is presented for each analysis.
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Fluid inclusion evidence presented above indicates that fluid temperatures ranged from approximately 50 ”C up to 300 °C. Therefore, it is possible to express fluids present during calcite precipitation in terms of compositional and temperature variations as shown in Figure 5.15.
Sample No. log molCOj/kg H2O Sample No. log molCOj/kg H2O
MglC 1.07 4A -0.04 C3 0.32 2A 1.07 2A 0.68 3B -0.17 3A 0.54 B5 0.43 C l 0.90 C6 0.33 4B 0.84 C7 1.14 X3 0.99 XI 0.73 Mug 2 0.81 MglA 0.24 Mug 3 1.10 HC5 0.12 HC 10 0.88 0.47
Table 5.2. Fluid compositions from metadomain calcites expressed as mol CQo/kg HoQ Unfortunately, paired temperature-composition data was not available so only the range is shown. Also shown is data from Arnorsson and Gunnlaugsson (1985) which shows concentrations of CO; in geothermal reservoir waters. It can be seen that the CO; contents of fluids present during the precipitation of calcite within the metadomains are significantly higher than those predicted using the data of Arnorsson and Gunnlaugsson (1985), often by a factor of up to 10^. This suggests that CO; concentrations within the geothermal fluid were not controlled by fluid-mineral equilibria but that an additional source of CO; was available. This might include CO; generated during the thennal maturation of organic matter within the sediments underlying the lavas.
-2 8-4: -6 102 o <b 100 ■^0 0“ 300 Aquifer tem p. ®C
• Iceland S v«rt»«ngi R eykfanes. w ater* ab o v e 300"C * C erro Prieto. M exico
0 Ic elan d ' o th e r w a te r* a O tkeria. K enya * B foadland* New Z e a la n d c O num a. Japan a W atrakei. New Z ealan d T Torrgonan. P hilippine* e Kaw erau. New Z ealand a P alinpinon. P hilippines Figure 5.15 TcmDcraiure-C02 relationships in hydrothermal waters from a range in environments (Arnorsson and Gunnlaugsson. 1985)
Also shown is the approximate lemperature-compQsitiQnal range of metadomain fluids (stiPDled areal as determined from combined
fluid inclusion-extraction analysis of calcites.
Figure 5.16 shows how the ratio between CO; and non condensibles varies within fluids present at the time of calcite precipitation.
-5 -6 - i ■’ -8
8" ,
bO -9 o□ High Craigton samples
# South Clyde localities values expressed in moles
per gram of calcite
-10 • 10 -9 “ T “ -8 " T " -7 T “-6 -6 -1
log non condensibles (mol "‘10 g )
Figure 5.16 Compositional variation in the gas content of metadomaifl fluids present at time of calcite precipitation
It can be seen that at even at widely spaced localities the ratio CO; / non-condensible gases remains relatively constant. It is also apparent that the absolute concentration of both of these
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parameters differs markedly in that samples from High Craigton Quaiiy are more depleted in both CO; and non-condensible gases than samples collected south of the Clyde. It was stated earlier in this Chapter that fluid boiling is thought to have occurred at High Craigton Quarry on the basis of fluid inclusion evidence. Such a process would lead to an overall decrease in the concentration of dissolved gases in the residual fluid because of partitioning of gases into the escaping vapour phase as discussed by Giggenbach (1981). Such a process would also lead to calcite precipitation as discussed in Chapter 2 and also provides a possible explanation for observed variations in the absolute concentration of gaseous phases within metadomain fluids.
5.7 Conclusions
a. Analcime
Fluids present during the formation of analcime are typically of very low salinity and show a restricted range in temperature (180 to 220 °C). Mean homogenisation temperature is relatively high (203 °C). This fluid is unusual in that it appears to have transported significant quantités of hydrocarbon rich material, inferring that it came into contact with organic rich sediments during its history. If metadomains developed prior to burial, then a possible source rock may be the Ballagan Formation of the Inverclyde Group. Within the Campsie Fells these beds reach a maximum thickness of 500 metres (Francis, 1983). Upon heating the hydrocarbon material begins to change colour at approximately 230”C, which is similar to results obtained by Hall et al.(1989). This probably represents a maximum trapping temperature for the hydrocarbons. If the fluid did interact with the Ballagan Formation it would be expected that salinity would be markedly higher than that which is observed. This might not be the case if fluid residence time within the sediments was short.
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b. Calcite
Fluids present during deposition of calcite within the metadomains across the lava field typically apear to be Ca^^, N a\ Cl' dominated brines which showing a very wide range of salinités (ranging from approximately 0 to 25 equiv. wt.
%
NaCl). These brines also show a wide range in temperatures, ranging from approximately 50 ”C up to 300 "C. It appears that these brines are actually a hybrid formed during the interaction and various degrees of mixing between two "end-member" fluids. One fluid appears to have been a connate brine at high temperature whilst the other appears to have been a low salinity, surface derived fluid which was progressively heated during advection through the lavas. The situation is further complicated since it appears that as weU as mixing there have been various degrees of boiling of the hybrid, possibly followed by further mixing. Such processes are found to occur throughout the shallow parts of geothermal systems worldwide. Following initial precipitation of calcite during metadomain activity there appears to have been a later fluid event, possibly related to burial of the lavas. This fluid is characteristically of low salinity and low temperature (estimates range from 80 to 110 °C).Fluid extraction analysis provides a means of determining the absolute concentrations of gaseous components within fluids trapped at the time of calcite precipitation. CO; concentrations witliin fluids present during calcite precipitation aie higher than predicted using information from Arnorsson and Gunnlaugsson (1985) by up to a factor of 10^. This suggests that CO; concentrations were not controlled by mineral-fluid equilibria but that an independent reservoir was involved. One possible source may have organic matter undergoing thermal decomposition within the underlying sediments which also appeal* to have supplied hydrocarbon material to the metadomain fluid. It also appears that physical processes such as fluid boiling also exert an influence on absolute gas concentiations within the hydrothermal fluid.
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