Back to Ganoksin | FAQ | Contact

Puzzling over diamond origins


#1

I am puzzling over something in diamond origins and was wondering if
an Orchidian might help. First, diamonds in ore bodies are said to be
younger than the host rock. Second, the mantle is mostly solid.

Would you not then expect kimberlite diamonds sort of encased in a
shell of host rock within the larger body of host rock? The "shell"
would represent chunks of solid diamond-carrying mantle rock picked
up by molten kimberlite lava as it rushes to the surface. Even if it
is melted by the kimberlite, would you not expect it to be
geologically differentiated from the greater host rock?


#2

Do a little research into the geochemistry and geophysics of the
mantle as it applies to the petrology of kimberlite and you will
find the errors to your original statement for the parameters for the
formation of diamonds. This forum is too limited in available time
and space to adequately explain the formation processes involved.
Your initial statement of conditions is incorrect.

John


#3

You might find this recent article in Science News of interest…

Bill


#4
Your initial statement of conditions is incorrect. 

Maybe so. Perhaps I should have quoted the primary source which
presented those conditions. Hausel’s paper “Diamonds and Mantle
Source Rocks in the Wyoming Craton” can be found with a WWW search.
He cites Helmstaedt in saying “… most lamproite and kimberlite host
rocks yield much younger ages than the diamonds themselves”. That
sounds to me like the diamonds are older and formed in an older
mantle host rock and after that a younger lava moves upward to
capture them, eg a kimberlite pipe.

If that is so would one not expect diamonds to be found in “a host
rock within a host rock”? The proximal host rock would be the older
host rock and the distal host rock would be the younger host rock.
But then if I had clarity on that I would no longer be puzzling over
it.


#5

Thanks Bill. My guess is that those researchers would answer by
saying that the older host rock is mixed into the melt of the new
host rock so completely that they cannot be differentiated (otherwise
the ore body would be conglomerate-like or breccia-like).

How much detail do jewellers think their customers would like to
know about the origins of stone which they are buying in general?

The Xinjiang book on gems and jades makes me wonder if the Chinese
jade market has more interest in this matter than we find in the
West. After all, they have thousands of years behind them as jade
sellers and buyers. Xinjiang provides three major descriptions with
maps, petrographic analyses and assay profiles to tell the customer
where these stones originate. Perhaps the food and wine market is
analogous. Most people do not want to know very much about what they
eat and drink. The gourmet wants to know quite a lot more. The
Kelowna million dollar emerald would not have earned one line of
newsprint if gem buyers here had higher disclosure expectations.
Perhaps the promoters would have been disappointed.