[X] My Assistant

[dvandorn]

This is more of a general question/comment about Martian geology, but since Oppy is in such a unique environment (unlike any of the other landing sites we've visited), I figure it belongs here more than anywhere else.

As far as I've been able to tell in my reading of the results of the various Mars probes, there doesn't seem to be much in the way of granite on the surface of Mars. There's a lot of basaltic and andesitic lava-rock (with a lot of olivine, pyroxene and even ilmenite), some ancient feldspathic/anorthositic rock, a lot of sulfates and other salts, a lot of rusted iron -- but little to no granite.

There's almost no granite in any of the lunar samples, either. Quartz also seems to be very rare, both in lunar samples and in what we see on Mars.

Has anyone here heard any good theories on why Earth produced so much granite and neither of the other rocky bodies we've looked at in detail seem to have much, if any?

-the other Doug

[edstrick]

Earth has generated a bogglaceous variety of "weird rocks". That magnetic "granite" is one of them, it seems.

Some of them are the result of reasonably normal magmas eating (assimilating) another rock and ending up with very weird chemistry. You can get carbonatite volcanic rocks, at least some of which form when magma eats a lot of limestone and you actually have black lava flows with carbonate minerals in them that turn white (on the surface) as they cool. There's one active carbonatite volcano in Africa, I don't recall if it's carbonates come from assimilated limestone or what.

Others are the result of poorly understood processes in the mantle, maybe or maybe not involving subducted crust. One thing I learned about in Igneous Petrology back around 1979 were the existance of "Feldspathoid" igneous rocks. They make up a teeny tiny fraction of all igneous rocks, but a good fraction of the different petrologic types. Sometimes, for reasons that were not understood worth diddly back then (I have no idea about now), igneous melts are produced that are aluminum-deficient. Instead of producing normal feldspar minerals in the calcium/sodium/potassium feldspar mixing series, you get "weird" silicates that have fewer aluminum atoms than normal igneous minerals, things like nephelinite, sodalite, i-cant-remember-ite. Some of the minerals normally only show up in metamorphic rocks where the sediments that got cooked were considerably changed from original weathered igneous rock in composition. Here they're showing up in rocks formed from melt. What makes such weird melt.. I don't know. I don't know if anybody knows yet.

The weirdness of the feldspathoid igneous rocks came to mind when there was a recent discussion on exoplanets that might be more carbon-rich than our solar system's planets, ending up perhaps with silicon-carbide mantles and asphalt covered crusts. Gonna be a lot of surprises.