The Role of Volatiles in the Thermal History of Metamorphic Terranes |
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| Authors: | BRADY JOHN B |
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| Institution: | Geology Department, Smith College Northampton, MA 01063 |
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| Abstract: | Analytical and numerical solutions to the differential equationsfor the conduction of heat with heat production or with fluidflow have been used to evaluate the role of volatiles in thethermal history of regional metamorphic terranes. The maximumthermal effect from pervasive, single-pass, regional volatileflow may be predicted from a steady-state solution given byBredehoeft & Papadopoulos (1965). For fluid velocity vF(m/s) and connected porosity  , combinations of volatile fluxvF (m3 of fluid/m2s) and transport distance L(m) such that vLis greater than 3?6?10–7 should produce regional temperatureincreases due to fluid flow, if the flow persists for l05–106a (depending on the transport distance L). The absolute valueof the temperature increase due to volatile flow will be greaterin regions with higher ambient geothermal gradients. For L=20km, a volatile flux of 1?8 ? 10–11 (m3 of fluid/m2s) orgreater is required to achieve a temperature effect. Few geologicprocesses release volatiles at this rate for extended periodsof time, so regional thermal effects from the single-pass, pervasiveflow of volatiles are unlikely. A new analytical solution forthe steady state temperature distribution between idealizedparallel channels of fluid flow is presented along with theresults of two-dimensional numerical models of channelized fluidflow. Both approaches show that little temperature increaseis expected near channels of fluid flow relative to the rocksbetween the channels, unless the channels exceed 100 m in widthor unless the fluid fluxes are very large and transient. A possiblethermal effect of volatile flow in metamorphic terranes is theproduction of metamorphic hot spots due to focusing of volatilesinto widely spaced channels or conduits exceeding 1 km in width.Given a sufficient fluid flux (exceeding 10–10 m3 of fluid/m2s),thermal gradients of over 100K from center to edge may be producedin such channels during relatively short time intervals (105–106a). |
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