(2020) Age and Depth of Oceanic Slab-Derived Diamonds and the Formation of Archean Subcontinental Mantle
Shirey S & Smit K
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03g: Room 1, Thursday 25th June 00:30 - 00:33
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Hi Steve, thanks for giving the keynote talk. Although we cannot physically meet, it's still great to discuss in this special way. I agree with your conlcusion as this is also supported by shallow melting scheme of the cratonic mantle keel that needs thickening via slab stacking. For quesitons, regarding the formation of diamons (oxidation from CH4-type fluids or reduction of carbonate-type fluids), as we know cratonic keel can be highly carbonated, how would this relate to the formation of diamonds and/or affect stability of diamonds? For the eclogitic diamonds, its sulfide inclusion abundance seems more than P-type diamond sulfide inclusions. Is there any reason for that? Did subduction bring more reduced fluids along with sulfides to aid diamond formation? Thanks, Jingao
Hi Jingao. Good questions. I'm not sure that the carbonate that we see in the mantle keel is always directly related to the diamonds that we see but rather just good evidence that carbonated fluids get into the mantle during metasomatism. However, you raise a good point and we (as a community) should probably look to see if there are patterns where diamonds are absent that correspond to areas that have been abundantly carbonated. Based on the thermal modeling for basaltic crust showed in the talk, I think the abundance of eclogitic diamonds in the Meso- to Neoarchean is directly related to a thicker basaltic crust then which would give a more efficient mechanism to transport carbonate and water below and into the nascent, thickening mantle keel. This would fit in a general sense with a subduction environment that was less oxidized than today and less able to reach the keel at shallow enough depths for incorporation. I am assuming here that the reduction is happening in the keel where the diamonds are growing, of course, so the transporting fluids out of the eclogite would not be reduced until they got in the keel.
Steve, great talk. I was wondering if you had considered non-subduction models (like those championed by Jean Bedard and others) to bring carbon into the craton keels. It seems to me that it is not necessary to assume that plate tectonics was globally operational at 3 Ga, it very likely started locally by this time, but I think by far the geologic evidence doesn't support global plate tectonics before about 2 Ga.
Hi Kent- I am not sure what geologic evidence you are thinking of to suggest that plate tectonics does not start before 2 Ga. If it's the crustal metamorphic models of Brown and others, I think it is a bit more complicated to use crustal thermal history of the crust to explore what is basically a mantle process. Plate tectonics is a mantle convective process. The strongest evidence therefore must come from mantle derived rocks. When we turn to mantle derived rocks such as TTG's and diamonds (which are minerals of course that grow in the mantle), the evidence is quite strong that plate tectonics starts before 2 Ga –probably before 3.2 Ga and perhaps near 3.5 Ga. Models like those of Bedard and others do a decent job explaining the basic petrologic aspects of the major elements of the rocks but they do not, in my opinion do a very good job localizing and returning to the surface volatiles like C, N, S and assembling them into the pattern that we see on cratons.
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