RE: dsel - eclogite xenolithsAbout 1.5 years ago, there was a lot of discussion on the SGF bb about eclogite xenoliths and their importance in the STar kimberlite.
I've copied a few exerpt from dsel's posts from back then for you to peruse. Some of the info will be too technical, but I think if you read through all of it you will get the gist of why finding ecologite xenoliths is so important!
One definition first:
XENOLILTHS: these are chunks of rock that have been incorporated into a rising molten material. They are foreign rock that have been picked up along the way. In this case, ecologite and peridotite are names for different types of mantle rock that have been picked up by rising kimberlite magma (which is a different chemical composition from the ecologite and peridotite). The xenoliths can occur as chunks that remain relatively intact during transport, or they can crumble and fall apart along the way (disaggregate), which then causes the individual minerals (including diamonds) to be released into the kimberlite magma.
So, on to dsel's posts:
Listed below is an excerpt of the Schulze paper that I mentioned a few posts ago.
I think it’s interesting to note the following items:
1. The first sentence “Diamond-bearing xenoliths are rare and, in contrast to the peridotite-suite dominated diamond inclusion population, most diamondiferous xenoliths are eclogites”
This is the same thing that was mentioned in the Anand et al paper (ie eclogite xenoliths are more resistant than peridotite xenoliths):
On p.1 https://web.utk.edu/~anandm/FLA_0084.PDF
“However, due to the friable nature of peridotites, during
their transport in the kimberlitic magma, they are
mostly all disaggregated and their diamonds are re-
leased into the kimberlite. In contrast, due to greater
resistance, eclogite xenoliths sometimes remain intact,
thereby preserving the textural context in which their
diamonds originally formed”
2. Eclogites have an average diamond grade of 23,000 carats/tonne vs peridotite’s 125 carats/tonne. I think I now understand why Shore was so encouraged in finding the 4 cm eclogite nodules/xenoliths.
3. I too wonder how many of these golf ball sized 23,000 carat/tonne eclogite xeonliths are floating around in the Star pipe
Schulze paper extract
https://www.rimfire.com.au/PDF/Schultz_paper.pdf
The Significance of Eclogite and Cr-poor Megacryst Garnets in Diamond Exploration DANIEL J. SCHULZE Department of Geology, University of Toronto Erindale College Mississauga, Ontario, Canada L5L 1C6
On p. 14
Abundance of Diamonds in Diamond-bearing Eclogites
Diamond-bearing xenoliths are rare and, in contrast to the peridotite-suite dominated diamond inclusion population, most diamondiferous xenoliths are eclogites. Many diamond eclogites are extraordinarily rich in diamonds (Robinson, 1979; Robinson et al., 1984; SchuIze, 1992).
A compilation (SchuIze, 1992) showed that the average diamond grade of such eclogites was 14 000 carats per ton, approximately two orders of magnitude greater than known diamond-bearing peridotites (125 carats per ton). New data for diamond grades of 13 diamondiferous eclogites from the Udachnaya Mine are presented in Table 2. These eclogites have an average diamond grade of approximately 28 000 carats per ton, increasing the world average for diamond eclogites (21 samples) to a grade of approximately 23 000 carats per ton (data from Robinson, 1979; Robinson et al., 1984; SchuIze, 1992; Schuize et al., 1996; Table 2).
Disaggregation of relatively small quantities of diamond-rich eclogite could easily provide economic quantities of diamonds in a kimberlite, yet such a process would also provide relatively small quantities of xenocrystal garnet (and clinopyroxene) indicative of the origin of the diamonds.
It only takes 4300 tonnes of 23,000 carat/tonne (100,000,000 carats) of eclogite to be dispersed in 500,000,000 tonnes of kimberlite to produce a grade of 0.2 carats/tonne.
Wouldn’t it be interesting if Shore’s shaft program ran into a few tonnes of the 23,000 carat/tonne eclogite?
They might have to add a zero to the 3000-5000 carats they were hoping to recover!