It is a rare type of sodic carbonatite that erupts from Oldoinyo Lengai in Tanzania. The lavas have very low eruption temperatures (500-600˚C) relative to the three major silicic types (basalts, rhyolites, and andesites). I am not sure that I have seen a single good idea (I apologize to those who may have gotten perfectly valid answers) that satisfies my curiosity about the origin of natrocarbonatite magmas. What kind of source rock(s) do you need to form these beasts? Where do all the Na come from? Could they be generated by immiscible splitting of some weird magma?
Please refer to the series of works by Wylie et al. (Peter) on the evolution of carbonatite. There is a Journal of Petrology volume dedicated to carbonatite, probably vol. 39, 1999 edited by Keith Bell. I will get back to you more on this soon.
Yes, I am aware of that literature. I think we understand carbonatite origin quite well, thanks to other works by Presnall and others. The sodic carbonatite is a different class of problem. I noticed that you requested a reprint of my book (Earth's Materials). Unfortunately, it is out of print. The other papers will be uploaded at some point.
The article on "Petrology and genesis of natrocarbonatite" by
Tony D. Peterson published in Contributions to Mineralogy and Petrology
1990, Volume 105, Issue 2, pp 143-155 provides an explanation of mantle metasomatism if the mechanism of formation of natrocarbonatite from the Mantle Mg-Ca- carbonates.
Hi Guatam, You have all the elements in Martin Menzies's answer. Maybe trace elements will be the most useful to confirm the source of your sodic carbonatite. Also a good article to check:
http://www.sciencedirect.com/science/article/pii/S0012821X12006140
Dear Martin et al.,
Sorry for this delayed response. Thank you all. I guess I am not entirely convinced how you generate such Na-rich character of them. I like shallow immiscibility model but you have got to pump in a lot of Na either during or just prior to the immiscibility strikes. One could appeal to metasomatism but I think we appeal to such a thing too often when we do not have a clear understanding of what reaction/reactions are involved and in what sort of environment. I just WONDER (a wild idea with zero basis) whether the immiscibility could be triggered by the sudden encounter of a deeply buried evaporite (with halite and other nasty things) layer by the magma in the shallow/mid crust! You are probably laughing hard by now....
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