[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]

Granite is a rock made of two feldspars: a Sodium/Calcium silicate a Potassium silicate, plus quartz (silicon dioxide), plus amounts of other silicates containing iron and magnesium (Mafic minerals). Granite is the result of repeated "fractional crystallization" and "fractional melting" in reasonably water rich conditinos that progressively separate out elements that form low melting point "Felsic" minerals, starting from far more iron/magnesium rich "mafic" mantle rocks.

The earth's continental crust, which is dominantly granitic and metamorphosed granite together with somewhate more mafic rock, was produced by multigeneration refining of rock in and above subduction zones to produce granite intrusions like the Sierra Nevada which were progressively "accreted" together in plate tectonics to form continental crust.

You can produce very small amounts of granite-ish material as the last stage bit of melt left as a partial-melt (already enriched in felsic minerals) mantle rock, but it's trivial in volume. The "KREEP" melt on the moon, "Potassium, Rare Earth Elements and Phosphorus" melt is something like that. Some of the most extreme types are called "lunar granite" but the moon has no true granite. Mars may have gone much further to producing a complex differentiated crust than the moon did in it's early history.. first few hundred million years, but that history is still almost entirely blank for us.

[dvandorn]

So, what Mars (and the Moon, for that matter) seem to lack, in the process of granite development, is plate tectonics, eh?

Now, while Mars lacks any obvious signs of plate tectonics in its current landforms, there is magnetic "striping" observed in some of the ancient cratered southern hemisphere, particularly in the Sirenum region. While it's not as distinct as the striping in, say, the Atlantic Ocean floor, there are definite regions that feature alternating remanent magnetic fields with reversed north/south orientations. That would argue, at least with some force, that crustal spreading occurred in Mars' ancient past. You would think that crustal spreading would require at least two floating plates and a subduction zone somewhere along a plate boundary -- unless you subduct somewhere, you can't spread for very long anywhere else.

On Earth, subduction seems to require an oceanic overlay to work properly. Now, back when Mars had a molten core and generated a global magnetic field (a requirement for the creation of the remanent magnetism observed by orbital sensors), and when this crustal spreading would have been occurring, Mars might well have had a fair amount of standing water. Perhaps even oceans. And since subduction zones naturally form rifts that would collect standing water, you could argue that Mars may well have had all of the processes in place required to make granite.

Perhaps what Mars did *not* have was enough time during which active plate tectonic prcoesses were occurring to form significant amounts of granite. Earth's oldest granites are, what, 2 to 3 billion years old? If it took a billion years of plate tectonics on Earth to build up significant granitic deposits, maybe that tells us that Mars only sported plate tectonics for a relatively brief time, on the order of a half-billion years or less.

Of course, that still doesn't explain why we don't see more quartz on Mars. Since quartz is one of the ingredients of granite, maybe some other factor (possibly simply a compositional difference) limiting quartz formation is also partially responsible for the lack of granite on Mars...

-the other Doug