Conveniently enough, there’s a Scientific American article about the use of basalt as a CO2 sink, which was posted yesterday. I suppose that using basalt for its CO2 sponging abilities isn’t a bad second option; if nothing else, there’s a lot more basalt in the world than there is easily available ultramafic rocks. Basalt is being produced every day from volcanoes, while ultramafic melts would be very uncommon in this day and age. To get an ultramafic rock, you need a much higher degree of melting of the mantle peridotite than you’d normally get, now that the Earth has cooled off a bit.
Depending on the type of basalt, you’ll also get olivine in it, which is what I talked about yesterday as the main constituent of ultramafic rocks, the thing which weathers so nicely once you add a little carbonic acid. I doubt that you could go much less mafic1 than basalt and still get much bang for your buck.
The reason for this comes down to Bowen’s Reaction Series, the terror of all first year students of geology. The reaction series is really just a simplified description of how magmas crystallize, because different minerals are stable at different temperatures and pressures. We’re most concerned with the left side of the series, in this case.
So let’s pretend we’ve got some mafic (but not ultramafic) magma, which spews to the surface and becomes lava. The first thing that will crystallize in it as it starts to cool is olivine. As the lava continues to cool, some of the olivine (not very stable at these low pressures) will react with the remaining melt and begin forming pyroxene. More cooling, and the pyroxene starts converting over to amphibole. Melt composition also plays a big role, but that’s getting a little too complicated for a Tuesday before I’ve had lunch, I think. By the time all your lava has cooled down, you’re going to end up with a mixture of what’s more stable at the surface, such as pyroxene and amphibole.
That’s generally how the reaction series works. The important thing to keep in mind is that the higher you are in that reaction series, the less stable the mineral is at the surface. And the less stable it is, the easier it is for carbonic acid to come along and work its magic. Ultramafic rocks are ideal for this because they’re mostly olivine. Depending on the type of basalt, there are still a lot of minerals that break down very easily, such as pyroxene – and some basalts do have significant amounts of olivine in them still.
This still has the same pitfalls and questions as using the ultramafic rocks, I think. The biggest being, of course, that if you think it takes a long time for an ultramafic rock to weather, it’s going to take even longer for basalt.
I’m also really wondering about the one sort of throw-away statement at the end of the article:
Already, a proposed coal-fired power plant proposed in Linden, N.J. includes plans to pump captured CO2 emissions into an offshore sediment, albeit not a basalt one.
Putting aside the the cringe-inducing phrase “an offshore sediment,” I’m wondering what exactly the goal is, there. Are the sediments in question ones that they expect the CO2 to react with? Are they just hoping the sediments are going to hold on to the CO2 long enough that it’ll be someone else’s problem, which is often the goal when we’re talking about injecting carbon down somewhere deep in the ocean? That’s a little worrying.
1 – Just in case you didn’t know, all this “mafic” business is just a reference to the major non-silica components of the rock. Mafic is shorthand for magnesium/ferric (ferric meaning iron) since there’s a lot of those elements in this sort of rock. You’ll also hear “felsic” which is shorthand for feldspar/silicate, which you find in abundance in rocks like granite.