SPATIAL ASSOCIATION OF APATITE AND MAFIC MINERALS IN SOME INTERMEDIATE AND FELSIC IGNEOUS ROCKS SUGGESTS AN ORIGIN BY CRYSTALLISATION OF A METASTABLE IMMISCIBLE LIQUID

A spatial association of apatite with pyroxene and amphibole is observed in many felsic to intermediate rocks such as granodiorites, dacites and andesites. Such mineral associations can be produced by destructive, reconstructive or constructive processes. Destructive processes can be ruled out: there is no textural evidence for replacement of mafic minerals by apatite, or the inverse. Reconstruction by exsolution during cooling or other processes can also make spatially associated mineral grains. Although P can substitute for Si in pyroxenes and amphiboles, the amount is generally low and there is no evidence that high-P mafic minerals exist at high temperatures.

A common constructive explication for this association postulates the existence of a zone around a growing crystal that is enriched in elements like P, hence promoting preferential nucleation and growth of that phase. However, such a zone requires that P has a low diffusivity otherwise it is easily lost from the enriched zone. However, if apatite is to be enriched in this zone then it need to draw P from far away and hence have a high diffusivity, so this idea must be rejected. Another possible explanation for plutonic rocks is that apatite and mafic minerals are late-crystallising phases, concentrating in late pockets of liquid. However, such an explanation cannot be applied to volcanic rocks where mafic minerals have enclosed apatite at an early stage of crystallisation.

There is, however, another petrographic environment in which Fe and P are associated: the mafic part of mafic-felsic immiscible liquid pairs. Such immiscibility is commonly seen in volcanic rocks and may be the origin of plutonic nelsonites. I propose that mafic minerals do not crystallise directed from some intermediate and felsic magmas: An immiscible mafic liquid forms, perhaps metastable, and crystallises soon afterwards to pyroxene or amphibole and apatite. This behaviour is not dissimilar to that of sulphides, except that the sulphide liquid exists over a larger temperature interval. Zircon and other minor minerals may also form in the same way in similar magmas, but their lower abundance makes this more difficult to establish.