Hello Chuck
To my knowledge, no North American kimberlites have been age dated beyond 200Ma and most that have been found up here are approximately 50Ma. If Alberta's have been emplaced in Devonian sendiments and limestones, that would make them much younger still.
As I understand it, if emplaced, and the craton is subject subsequently to tectonic forces, diamonds in emplaced pipes near the surface should not be affected unless they are directly exposed to heat sources which they probably have not been in Alberta.
If exposed to tectonic forces prior to emplacement they most probably are turned to graphite and/or CO2.
The key as I understand it, is the existance and preservation of the diamond stability field under the craton which only exists under very old tectonically quite cratons, and its sampling by eclogitic or peridotic sourced kimberlites that errupt through it. The impregnated kimberlites must reach the surface as quickly as possible as it is the rate of their decent that is the key to diamond preservation. The kimberlites heat must be dissapated, and only rapid decent allows this through pressure induced expansion. As crustal pressures reduce near the surface the contained CO2 expands and cools the kimberlite magma. Hitting a water table within 30km of the surface is also theorized to assist in this explosive expansion and erruption, cooling the kimberlite.
You may have noted in Walt's summary of the Geoscience Forum that many kimberlites are actually reconsolidated colapsed sedimentary deposits resulting from the erruptive magma and surface rock, trees and animals collapsing back into what is essentially an open pit after the initial erruption.
Again, the heat must be dissapated quickly. During decent, any peridotic or eclogitic scoured clasts (xenoliths) or diamonds (xenocrysts)are being reabsorbed. If the incubated diamonds within the xenoliths do not reach the surface where they will be free of the corrosive effects of the kimberlites heat and chemistry, they will be heat damaged and/or reabsorbed (turned into graphite or CO2).
The poor Alberta geochemistry statements I have referred to are, I believe based both on the limited number and/or ratio of subcalcic (G10) garnets, their poor grain size, poor size ratios, and the apparent chemistry and limited quantity of clinopyroxene, olivine and chromite all of which tend to be sourced from peridotitic rock and the diamond stability field.
This is obviously a superficial summary of a very complex subject but that is the general gist of it.
Hope this answers your questions.
Regards |
