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Gold/Mining/Energy : ISLAND-ARC RESOURCES IAR-VSE
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To: Dave R. Webb who wrote (133)1/21/1999 3:25:00 PM
From: VAUGHN  Read Replies (1) of 186
 
Hello Dave

So the sampling and application of this system focuses exclusively on the peridotitic pyropes?

While I can appreciate how it would then give an indication of economic potential in so far as peridotitic sourced diamonds are concerned, by ignoring potential eclogitic diamond economics it would seem to only look at part of the picture.

If I recall correctly, my reading suggests that it is not unusual for and some might suggest typical for eclogitic diamonds to constitute 60 to 80% of the stones by quantity and in many cases quality and value in the majority of economic kimberlites.

There must be more to the system than this Dave? It doesn't make sense that the system would ignore potentially economic geochemistry because it isn't peridotitic.

Regarding your Dawson Plot reference, that was my point. I referenced plotting Cr+Al against Si in garnets. “P” type garnets crystallize from 900 to 1300C at pressures of 45 to 60 Kbars (100 to 200km in the mantle) “P” type diamonds are believed to be formed between 150km and 200km. But this is where it gets confusing in light of the “J” system you explained, as you will see below. You suggest that the “J” system proposes that quality increases with Cr but the 50km difference (100 to 150km) lower pressure corresponds to the higher Cr+Al content on this plot. Based on geobareometry (Moore and Gurney) have shown that ”E” type diamonds have origins that may be deeper than 300km and form under pressures of up to roughly 130Kbars. When such stones are analyzed and Al+Cr are plotted against Si, the Al+Cr content % drops and Si increases with pressure. That is to say that a diamond with a higher Cr content than another was formed at a shallower depth.

I have no data to suggest one way or the other whether you can correlate shallower depth with diamond quality? However, one would think that up to a point, the deeper into the mantle you go where diamonds are formed under higher pressure and temperature, you should get a harder (denser) stone (then you get graphite). Whether the fact that “P” diamonds are formed out of carbon-12 and “E” type out of carbon-13 enters in any way into the hardness issue my reading does not say. Simply that associated geobareometric signatures suggest the sources of the respective carbon.

I look forward to reading those papers, as I would like to understand and get to the bottom of the contradictions that these conflicting issues raise.

Regards
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