This post will tell a (hopefully nonfictional, rather than fictional) interesting story that starts with some observations of carbonatite samples. I’ve already posted images of a few of these interesting rocks, here are 2 more examples:

Carbonatites are mantle-derived igneous rocks. Carbonatite petrogenesis is still actively researched and current interest in carbonatites rests on mining and extraction enterprises: carbonatites are enriched in Nb and rare earth elements.

Since time immemorable, if I showed you a rock primarily made of either calcite or dolomite (or a mix of the two), you called it ‘marble’. AFAIK, the first people to claim that they found a “limestone rock of magmatic origin” was Högbom in 1895 and Brøgger in 1921, who published his findings regarding the petrology at the Fen Complex. Their claims were disputed for decades until the 1960 eruption of Ol Doinyo Lengai in Tanzania, confirming the igneous origin of carbonatites.

Being curious, I wondered what Brøgger presented as evidence- and I couldn’t find anything other than language like “carbonatite appears in intrusion-like formations usually associated with magmatic activity”…. pretty weak evidence. To be fair, the report is in German, of which I am not fluent. But I really couldn’t find any published evidence to support published claims of magmatic origin (before 1960 or so).

So, I wondered ‘what kind of evidence would demonstrate an igneous origin?’ I wondered if any accessory minerals (mostly apatite, but some clinopyroxene and mica is also present) could be considered evidence. In my (limited) readings, I recall that of all the Fen rock samples that I have, the only ones that are definitively magmatic in origin are the damtjerites and phonolites- I’ve posted examples of those earlier. Damtjerite has characteristic ‘ocelli’ or “pelletal lapilli”, ovoid-shaped features, and these appear in several of the Fen samples. Looking carefully at Fen 3 ii, I found one along the bottom edge:

A closer view with epi-darkifield illumination:

Zooming in further to the reaction rim surrounding (I think) biotite (augite? unclear.) shows some of the crystallites:

The ovoid structure would not have been preserved during a metamorphic process. Therefore, the carbonatite and ovoid formed at the same time, so the carbonatite is of magmatic origin. Being largely ignorant of petrology, this seems to me to be strong evidence.

An earlier post described the trouble I had identifying apatite, because most descriptions of apatite in thin section go like this: “Apatite is obnoxious in thin section. It often plucks out in grinding, leaving a little hexagonal “apatite wuz here” hole in your section.” Not the case with carbonatites- apatite is a major accessory, typically forming anhedral crystals (which is something I still don’t understand- how can a crystal be anhedral?).

That got me thinking “What is different about apatite in carbonatite compared to apatite in other rocks?” What if the apatite crystal was not wetted by silicate melt, and so is not tightly bound to the rest of the rock; grinding could indeed pluck out the weakly-attached apatite crystal. But if apatite is wetted by the carbonate melt, it would remain in place after grinding.

That implies that the carbonatite melt and silicate magma are also immiscible. I then wondered if I can find evidence of that in a sample? Sure enough, looking closely at Fen 1 b (image reposted here for continuity):

Reveals, at the light-dark boundary- first the XPL transmitted, then the epi-darkfield reflected:

In transmitted light, it’s hard to make sense of what we see- two different microcrystalline (cumulate?) masses. The more I examine this sample, the more interesting it becomes… [continued on next post]