Over the past week I have learned a lot about garnet and amphibole. Garnet is one of those minerals I tend to associate with metamorphic activity, as I’m sure many of you do. But after travelling around the Auvergne region of France, I had to open up to the idea that garnet could be an inclusive mineral in erupted igneous rock. Who would have thought!
The clue lies in the presence of amphibole in these lava samples. Now, initially the presence of amphibole brought its own problems for me, as it is the resultant mineral from hydration of pyroxene. Certainly this is to be expected when you’re at a destructive margin, where hydration of magmas can occur, but here’s the thing – the Auvergne volcanoes are all intra-plate. So how would water get in?
Lava can be made hydrous via mechanisms other than physical water input from subduction zones or recycling at spreading ridges. In an intra-continental setting, the occurrence of hotspots can cause continental minerals to melt and release oxygen and hydrogen locked up in their structures, which then regroups to form H2O and voilà – you have water!
And if you then learn that garnet can form from a hydrous mantle source (provided there is also some spare aluminium) it all makes perfect sense. The fact that garnets are present in the Auvergne in small amounts, and that amphiboles are present in vast swathes point to the fact that the Auvergne volcanics formed from a hotspot – a hotspot which triggered dehydration of continental material and subsequent hydration of the eruptive magmas.