ps418

I went and got a paper dealing with convective versus conductive cooling of igneous bodies. Since it bears on Snelling and Woodmorappe's claims for rapid cooling via hydrothermal convection, I thought I'd post some excerpts from the paper. If anyone knows of similar research, especially review articles, please post some references.

Parmentier and Schedl (1981) argued that the size and shape of the metamorphic aureole surrounding intrusive igneous bodies can be used as an indicator of convective versus conductive cooling, and show that this evidence is generally consistent with oxygen and hydrogen isotopic evidence indicating convective or conductive cooling regimes. Examples of both convectively and conductively cooled intrusions are discussed in the paper. While mainstream geology has no problem with convectively cooled intrusions, of course, young-earthism would have a major problem explaining the cooling of large igneous intrusions by conduction in only a gew thousand years.

"For an impermeable intrusion, as for purely conductive cooling, the thermal aureole widens with depth. For a permeable intrusion, the thermal aureole narrows with depth. The width of the aureole above the intrusion is strongly influenced by near surface hydrologic conditions" (p. 18).

". . . in all of the examples considered there is a general consistency between the form of the metamorphic aureole and oxygen isotopic sampling and other geologic data" (p. 19)

One examples of a conductively cooled intrusion is the Santa Rosa stock, a roughly 8x16km granodiorite intrusion intruding metapeletic country rock.

"Oxygen and hydrogen isotope studies by Shieh and Taylor (1970) show that the intrusion and country rock have normal isotopic ratios suggesting that convective cooling by meteoric waters did not occur, probably due to the low permeability of the country rocks" (p. 8).

"The Ardara intrusion in DOnegal, northwest Ireland (Pitcher and Read 1960) and Toulumne intrusion in the Sierra Nevada (Kerrick 1970) both of which, like the Santa Rosa stock, are dioritic to granodioritic intrusions into sedimentary and metasedimentary rocks, also appear to have aureoles consistent with conductive cooling" (p. 9).

". . . the metamorphic aureole of a conductively cooled intrusion should widen with depth below the top of the intrusion. In the examples discussed above, the topographic relief is not sufficient to determine the variation of aureole with depth. An aureole which can be seen to widen with depth, however, is that of the Alta stock (Moore and Kerrick 1976). This stock is dioritic to granodioritic in composition and intrudes a layered sedimentary sequence of carbonates, sandstones and pelitic rocks. The presence of roof pendants indicates that the top of the intrusion is preserved at the highest level of exposure. On the southern side of the intrusion, where the contact is nearly vertical, a zoned metamorphic aureole is exposed with a relief of about 350m. The width of the tremolite zone of this aureole increases from about 850m to 1150m over the depth range exposed. Over the same depth range the width of the forsterite zone increases from 450 to 650m. Although there have been no hydrogen and oxygen isotope studies to indicate the role of meteoric hydrothermal circulation, the high CO2 content of volatiles present . . . argues against the presence of significant quantities of meteoric water" (p. 10).

Another example is the Lilloise intrusion, southeast Greenland:

"In contrast to that on Skye, the high temperature aureole of the Lilloise intrusion has a width consistent with conductive cooling. The aureole width as well as the lack on an 18O depletion indicate that convective cooling did not occur" (p. 19).

Parmentier, E.M. and Schedl, A., 1981. Thermal Aureoles of Igneous Intrusions: Some Possible Indications of Hydrothermal Convective Cooling, Journal of Geology 89, pp. 1-22.


Patrick

ps418

I found a pretty interesting paper by accident that deals with the use of mineral thermochronometers to deduce cooling histories. Most of this is a bit above my head, but it is fascinating nonetheless:

B. J. A. WILLIGERS, E. J. KROGSTAD and J. R. WIJBRANS, 2001. Comparison of Thermochronometers in a Slowly Cooled Granulite Terrain: Nagssugtoqidian Orogen, West Greenland. Journal of Petrology 42(9), pp. 1729-1749.

Uranium–Pb sphene and apatite, and 40Ar/39Ar hornblende, muscovite and K-feldspar ages from the core of the Proterozoic Nagssugtoqidian orogen, West Greenland, are used to constrain the timing of granulite-facies metamorphism and the subsequent cooling history. Metamorphic monazite growth occurred at 1858 ± 2, 1830 ± 1 and 1807 ± 2 Ma and defines the peak of metamorphism. The uncertainty in the cooling rates has to include the error in the decay constants of the systems used. This source of uncertainty is, however, negligible if a single decay scheme is used or when the age difference between the chronometers is large (>100 m.y.). Over the last two decades increasingly higher closure temperatures have been proposed. This trend reflects the difficulty of determining ‘absolute’ closure temperatures and in using a limited number of closure temperature estimates to infer closure temperatures of other geochronometers. Cooling rates at Ussuit were 2·9 ± 1·7°C/m.y. from 1762 Ma (670°C) to 1705 Ma (500°C), 1·5 ± 1·1°C/m.y. from 1705 Ma to 1640 Ma (410°C), and 0·9 ± 0·4°C/m.y. between 1640 and 1416 Ma (200°C). Between 1720 and 1645 Ma cooling rates in Lersletten, 60 km north of Ussuit, are indistinguishable from those at Ussuit. After 1645 Ma, however, the area cooled to 200°C at a slightly faster rate of 2·6 ± 1·2°C/m.y.

John Solum

Hi Patrick,

This is more of the type of application of cooling histories I'm familiar with since my office mate did something similar for her PhD (but with the Grenville Orogen). Converting the closure temperatures to depths (using the geothermal gradient), rates of uplift can be calculated. Also, variations in the cooling histories of rocks on either side of a fault can be used to determine the age of the fault (when the cooling histories are the same on either side, that indicates that motion along the fault had stopped since the area was uplifting uniformly). It's definitely cool stuff.