"Oxygen and hydrogen isotope studies provide the main evidence for the
existence of meteoric hydrothermal circulation systems associated with shallow
intrusions in permeable country rock. A number of such intrusions, now exposed
at the surface by erosion, are found to be depleted in 18-O relative to normal
igneous rock. The waters discharged from modern geothermal systems are also
found to be enriched in 18-O relative to local meteoric waters. both of these
observations are explained by the interaction of hot intrusive rock with lower
18O meteoric waters and demonstrate significant geochemical transport by such
hydrothermal circulation systems."~E. M. Parmentier and A. Schedl, "Thermal
Aureoles of Igneous Intrusions: Some Possible Indications of Hydrothermal
Convective Cooling," The Journal of Geology, 89:1(Jan. 1981):1-22, p. 1-2
Intrusions which did not undergo a hydrothermal cooling are also found.
They have a normal isotopic signature because the O-18 couldn't escape. one
such conductively cooled instrusion is the Santa Rosa intrusion.
"The aureole of the Santa Rosa intrusion in the Santa Rosa range of Nevada has
been described by Compton. The Santa Rosa is a grandioritic stock intruded
into metapelitic country rocks. Oxygen and hydrogen isotope studies by Shieh
and Taylor show that the intrusion and country rock have normal oxygen isotopic
ratios suggesting that convective cooling by meteoric waters did not occur
probably due to the low permeability of the country rocks."~E. M. Parmentier
and A. Schedl, "Thermal Aureoles of Igneous Intrusions: Some Possible
Indications of Hydrothermal Convective Cooling," The Journal of Geology,
89:1(Jan. 1981):1-22, p. 8
This is an 8 x 16 km wide intrusion so conductive cooling would take a long
long time.
While I know that you, David, have stated that you don't need to have all
the intrusions cooled within a eyar, there are those on the list who need to
have everything cool within a 4-6000 year period. Intrustions lacking
hydrothermal activity present a greater problem for their chronology since
these things remain hot for hundreds of thousands of years.
glenn
Foundation, Fall and Flood
http://www.isource.net/~grmorton/dmd.htm