Geology Liveblog: Bubblicious Basalt

[wolffyluna]

...I needed a snappy title for this one, and I'm not sure I succeeded.

So, basalt! It's pretty much the most common rock type in the Earth's crust, though there's not necessarily that much where us land-lubbers can get at it, because it's mostly found in the oceanic crust. It's mafic (dark coloured and low silica), fine grained, and pretty good model for/lie to children about a lot of geological stuff. Heck, even other terrestrial bodies, like the Moon and Mars, have basalt!

Generally speaking, it's analysed chemically, not mineralogically, because it's so fine grained. And different sources are chemically distinct, so that's useful.

But that doesn't mean that basalts don't have interesting stuff happening mineralogically.

Basalts are produced by the mantle melting, and then erupting somewhere. (If it melts, doesn't erupt, and then recrystallises, that's a gabbro. Which is just the name for 'a basalt-y thing with big crystals because it cooled slowly.')

So, I mentioned that the fact that things don't melt all at once (fractional melting) makes things get more different from each other. And that's true in some circumstances, but not all. And basalts are one of those cases.

Now, I'm going to try and avoid using phase diagrams here, because a) I have no clue how to source those things, and b) they're not hard to read once you know how to read them, but I don't feel confident teaching people to read them over the internet, and they can be quite confusing if you don't know how to read them.

So, for chemical reaction and phase reasons, when you melt a peridotite-y (eg mantle rocks) thing or a basalt-y thing, that very first melt, when only a little bit has melted, has the same composition no matter what you started with. If it melts more, the melt is going to end up become more like it's parent, yeah, but that first 1% or so? Pretty consistent across rocks.

Which is why mantle melting so consistently makes basalt.

But basalt is sometimes just a first step in a rock's evolution, and even basalts can be different from each other. And the main driver of that fractional crystallisation.

See, there is a distinct order of which things recrystallise first out of a basaltic melt. The order tends to be oxides (eg spinel) --> olivine (generally going from Mg rich to Fe rich) --> clinopyroxene --> plagioclase. And assuming that there is a closed system and plenty of time for things to come to equilibrium, over the long arc of history the crystallised stuff will end up being the same as the original melt--

ppffffFFFFHAHAHA No. That doesn't really happen. Often the system isn't closed. The time it takes for equilibrium to occur is often longer than the geological time this takes place over, not even taking into account that this may erupt and get flash-cooled at any time. But even ignoring that-- for equilibrium to occur, all parts of the system need to be able to interact. And for that, our recrystallised stuff (called cumulates) would have to be this fine suspended... mush in the melts. And it isn't usually. Most often, it sinks or floats (in the case of olivine-y stuff, usually sinks.) So, we've got a melt and a cumulate that have different compositions, because crystallising doesn't happen all at once. And the melt doesn't have access to the cumulate for equilibrising anymore, so when so more of the the melt recrystallises, it recrystallises based on it's new composition, taking it to yet another compositions and making another cumulate with a different composition to the last one-- It's basically a whole new melt, and new fantastic melted rock-- Ahem. Excuse me.

Fractional crystallisation is one of the big drivers of, well, having different rocks. And it's some of the reason that granites exist. (Not all of the reason. There isn't enough basalt to explain all of the granite, and how ~extreme granites are. You don't get them just through fractional crystallisation, but fractional crystallisation plays a role.)

So, I don't know about anyone else, but in first year geology, it was driven into my head that "You will not find olivine and quartz together, unless something really weird has happened. If you find both in a rock, you've probably misidentified one of them." And if pressed for why, the explanation is something about them not tending to occur in the same geological environment.

Which isn't... wrong, necessarily? But it's not the full picture. The real reason is 'because chemistry. And also different geological environments.'

So, we have quartz (SiO2) and we have forsterite, our Mg-rich olivine (Mg2SiO4). And these can react together to form Enstatite, an Mg rich pyroxene (Mg2Si2O6) Quartz + Forsterite --> Enstatite SiO2 + Mg2SiO2 --> Mg2Si2O6

This reaction will occur even if one of these minerals is solid and the other is a liquid.

But, assuming there isn't anything strange going on, one of two things will happen:

1. If there is more quartz than forsterite, all the forsterite will react, leaving quartz and enstatite behind
2. If there is more forsterite than quartz, all the quartz will react, leaving forsterite and enstatite behind.

There is one wrinkle in this however. Only forsterite can react with quartz. Fayalite, the Fe rich olivine end member, can't. So in theory, if you had enough fayalite, you could have both quartz and olivine/fayalite together in the one rock.

It doesn't happen often, though. Quartz tends to show up where there's a lot of silica, olivine when there is very little. So, they do tend not to occur in the same geological environment after all. (And when they do, they react with each other.)