Sorry, I can’t resist a pun.
Kimberlite pipes are – geologically and economically – hugely important things. These weird features appear on maps as simple circular(ish) structures, perhaps up to a few hundred meters across. They are the vent which once supplied a volcano. More importantly, they’re stuffed full* of diamonds.
There’s a number of odd things about kimberlite pipes. Firstly, they are – on the most part – very old. We’ve never seen a kimberlite eruption take place, and they’re not particularly common in the geological past. There have been a few spurts of kimberlitic activity, and periods with none at all. Chemically, they’re very odd indeed. They can be broadly separated into two categories – both ultramafic, but Group 1 kimberlites are CO2 rich potassic, and Group 2 being ultrapotassic (3 times more potassium than sodium) peralkaline (so much potassium and sodium it can’t all go into feldspar minerals as the magma runs out of aluminium).
The source regions for these magmas is also an intereting – and contentious – issue. They’re derived from the mantle (>100 km depth), and make their way through the crust at very high speed (for a magma). The eruptions appear to be fairly explosive, as the pipes we observe often carry pyroclastic materials which fell back into the vent. Due to the general age, and ultramafic nature of these volcanoes, there’s not often** been any of the original volcanic edifice to analyse – all we have left is the sub-surface vent.
The reason for this post is that this week sees the publication of some work I’ve seen presented a couple of times in the last year or so looking at the Igwisi Hills kimberlite volcanoes in Tanzania. These are the youngest known kimberlite volcanoes in the world (Upper Pleistocene/Holocene, 100 – 120 ka). Because they’re so young, they have the surface eruptive structures still preserved. These are able to give us a huge amount of information on how these volcanoes behave.
Prior to the recent study, the Igwisi Hills had only been looked at a couple of times by geologists – most recently in 1966. They’re remote, very low elevation (standing no more than 40 m from the surrounding topography), and generally pretty underwhelming if you don’t know what you’re looking at.
The three vents all demonstrate the explosive eruption style which was assumed from the study of older pipes, but also bear lava flow activity which fills the scoria cone / Maar-like pyroclastic craters. Over a period of a few days to weeks each of these cones formed, developed and finally grew silent.
The bulk of the paper deals with some very careful mapping and logging used to calculate relatively small eruption volumes (~10^5 m^3), and it’s the record of the eruption style and duration of these vents which is valuable. Kimberlite lavas are incredibly rare – now there is perhaps an indication that this is due to the lack of surface preservation, rather than a lack of formation.
To finally have a proper hold of what these volcanoes were doing at the surface is fantastic. however, there’s still a lot of questions unanswered, and if anything, a whole truckload of new questions to ask. We still don’t know how these magmas form, or why they can be so diamond rich. We don’t now why they appear to form such discrete single pulses of activity.
This paper is a beautiful example of how there is still great fieldwork and science to be done even at the surface of our planet. There’s another message it has too I think; after all, if this kimberlite had been one of those laden with diamonds I sincerely doubt we would still have such a pristine site to go and investigate today.
The paper can be found here:
Richard J. Brown, S. Manya, I. Buisman, G. Fontana, M. Field, C. Mac Niocaill, R. S. J. Sparks and F. M. Stuart, 2012. Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania). Bulletin of Volcanology
*Well, maybe 10-15 carats for every 100 tons of rock. A hundred tons of kimberlite would fill about 40 cubic meters, so an olympic sized swimming pool might be expected to have as much as 100 carats (20 g) of diamond in it
**A good paper demonstrating kimberlite pyroclastic density currents was kindly brought to my attention by a colleague who has a near encyclopedic knowledge of the literature.