#85. The Anorthosite Problem [chemistry]

Labradorite facade on Bank Street, Ottawa, ON

CH

Red, theory; black, fact.

The “anorthosite problem” is a problem in geology. Anorthosite is a rock made mostly of a calcium-rich type of feldspar. Labradorite is a well-known example. The moon rocks from the lunar highlands are anorthosites. 

When a magma chamber far below the surface cools and solidifies into rock, it does so very slowly because of the insulation provided by the overlying rocks. Slow cooling leads to big mineral grains in the eventual rock because each grain is a crystal of some mineral, and there is plenty of time for crystal growth.

As the temperature slowly falls, one type of mineral after another comes out of the magma, in a fixed sequence. The iron-rich minerals fall to the bottom of the chamber and collect there, while the calcium and sodium types of feldspar float to the top of the chamber and collect there. 

Anyway, that’s the theory. It predicts that anorthosites will be underlain by iron-rich rocks like gabbro, peridotite, amphibolite, and serpentinite, some of which are called greenstones.

The problem is that immense deposits of anorthosite are not found in association with any iron-rich rocks. 

Interestingly, the biggest of these are the oldest, dating from the Archean and Proterozoic eras of geological history.  And thereon, I believe, hangs a tale.

Radioactive isotopes can be expected to have been much more abundant in the distant past than now if we extrapolate their exponential decay curves backwards in time to those eras. 

Moreover, all these elements (potassium, thorium, and uranium) are incompatible with the crystal lattices of the main rock-forming minerals crystallizing around when anorthosite does. So they will stay in the melt and get concentrated there as the rock-formers leave. The energy of their radiation will get converted into additional heat in the dense magma before the radiation can escape. 

Therefore, the radioactive self-heating of the residual magma will become progressively stronger until the cooling process almost stalls, leading to a long-lasting plateau in the cooling curve of the magma. (I don’t think it actually blows up.)

During the plateau, the loss of the iron-rich minerals has time to run to completion, resulting in a large magma emplacement that forms only granite, a low-iron rock. The iron-rich greenstones will be under that, and nowhere near the anorthosite at the very top. This is the sequence we actually observe.

Therefore, the early anorthosite deposits, however massive, will have no iron-rich rocks in sight.

QED.

P.S. The crystals forming during the plateau phase will be notably large due to the especially slow cooling. This may be the origin of the most amazingly large crystals in geology categorized as “megacrysts.”