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Why No Granite?
dvandorn
post May 31 2005, 07:22 AM
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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


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edstrick
post May 31 2005, 07:58 AM
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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.
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dvandorn
post May 31 2005, 08:41 AM
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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


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edstrick
post May 31 2005, 10:19 AM
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I think all of those are good arm-waving arguements. I'm not at all sure plate tectonics is required to produce granite, but crustal "cooking" and recycling, probably with a "wet" "juicy" crust, is probably needed.

Io is probably gonna have some spectacular and truely un-earthly minerology, especially sulfide minerals. But Io seems to have cooked all it's water away, more or less. I think there's hydrogen sulfide traces on the surface, but not water. Io may have had a very different minerology, say, 4 billion years ago when I might purely speculate it had maybe mars-like water abundances.

There's other things that can happen. There's recent speculation of the existance of "carbon planets", where the carbon/oxygen ratio is higher than in the solar system and planets have crusts and mantles dominated by things like graphite, diamond, silicon carbide, etc.

On earth, there's rare but geologically fascinating "Feldspathic" igneous rocks, produced by what were 20 years ago (don't know about now) poorly understood mantle process that resulted in aluminum-depleted igneous rocks, with very unfamiliar aluminum-poor minerals like nephelene, sodalite, others I don't remember. <google: feldspathic>. The minerology of the feldspathic rocks is rich and varied, and makes up a minute fraction of earth's total igneous rock output. Such really weird stuff didn't turn up on the moon or <so far> on Mars, but.. who knows. The more a planet cooks with solvents to help things along, the weirder it should get.
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edstrick
post May 31 2005, 10:33 AM
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I don't know what those magnetic stripes are. They're poorly resolved... we can't see substructure in them from orbit worth a damn. I have major doubts they resulted from plate tectonics "as we know it". It's a good arm-waving guess, but it's trying to assume the role of "preferred hypothesis" instead of "none of the above".
A magnetic/gravity anomaly mapping scout class mission that would be in a "powered" ultra-low orbit, possibly using 2 spacecraft like the current Earth-orbiting GRACE mission, would be enormously valuable for martian geophysical studies, probing the earliest history of the planet.

By powered orbit, I mean a vehicle with either ion propulsion or some few hundred kilograms of chemical propulsion. Once aerobraked into low circular orbit, such a mission would descend to an orbit that requires periodic, say weekly or daily orbit reboosts to keep from doing an atmosphere entry. On mission end, as propellant runs low, the spacecraft would need redundant systems to reboost for planetary quarentine reasons.

What really gets me about the stripes and other magnetic anomalies is that there's no correlation with relict ancient topography, other than avoiding the impact basins, and being weak in the northern plains. Look at the MOLA altimetry maps, and the southern highlands are full of scarps and ridges and mountain massifs and other features that don't seem to be the remains of craters or nearly obliterated impact basins. I'd *** LOVE *** to see some researcher use AI aided image processing to progressively intelligently airbrush away craters, starting with small fresh ones and moving to larger and less fresh ones, removing the visual distraction of younger impact dominated geology, and giving a good low-distraction look at the topographic remnant of the oldest topographic landforms on Mars.
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dvandorn
post May 31 2005, 08:01 PM
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I wholeheartedly agree -- there may well be other explanations for the magnetic features that we see (even though they are so very poorly resolved) in the southern highlands. It's interesting, though, that there are *no* anomalies (at the current resolution limits) around the easily identifiable impact basins, such as Hellas and Argyre. There is very little remanent magnetism at all in the basin floors and ringwalls. Which argues that Mars' magnetic field died before they were formed.

I'm just afraid that trying to reconstruct ancient landforms in the southern highlands is akin to observing an omelet and trying to reconstruct the chicken that laid the eggs. Impact modification has been so extensive and so drastic that it's nearly impossible to identify pre-impact landforms in the underlying crust. Add to the impact modification the stretching and cracking of much of the surviving ancient southern highlands crust due to tectonic response to the immense, and far younger, lava structures of the Tharsis Bulge, and it becomes nearly impossible to identify with any certainty *any* topographic features in the highlands that might pre-date both the high-impact-flux era and the Tharsis construction era.

I like your idea of powered orbiters. I despair that we'll never see the funding required to fly them, but they would be invaluable in answering some basic questions about Mars' early history. I think it's important to find out whether or not any other planet in the solar system has ever displayed plate tectonic processes, since those processes are so important to the history and ongoing evolution of our own planet.

-the other Doug


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edstrick
post Jun 1 2005, 10:10 AM
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The ancient highlands are indeed a mess, but by eliminating the distraction of craters, it would be far easier to piece together the remnants of the older landscape. Eyes are very good at that, especially with "noise" reduced.

A low altitude geophysical orbiter might very easily fit in the Scout mission class. Instruments and tech are nothing new.
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deglr6328
post Jun 11 2005, 06:05 PM
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I have an offtopic and inappropriate for this forum geology question.... biggrin.gif Why do granite curbs on sidewalks and roads almost always appear rusty? Surely it must be a property of the rock itself and not residue from rusty cars etc. because I barely ever see it on concrete, for instance. Is there iron METAL in granite?
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abalone
post Jun 12 2005, 01:45 PM
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There is no iron metal in granite. It has mainly quartz, feldspars and mica the latter of which is a potassium, magnesium,aluminium, iron silicate and weathers to clay. In that process the iron turns to iron oxide - rust.
Basalt is similar, it has a lot of olivine and pyroxene conaining magnesium, calcium and iron silicates. In the presence of water and sulfur (or sulfuric acid) they weather to magnesium, calcium and iron sulfates of the type that they had been detecting especially Oppy.
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maryalien
post Dec 7 2005, 12:06 AM
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cool site. i just redid my kitchen and, of course, loving rocks, I got granite (emerald pearl). Hence why i was directed here... magnets are "sticking" to it. in some places i actually feel it slightly repelling the magnet. whats going on here if there is no iron in granite? magnetite?

love an answer.

maryalien
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The Messenger
post Dec 7 2005, 05:40 AM
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QUOTE (maryalien @ Dec 6 2005, 05:06 PM)
cool site.  i just redid my kitchen and, of course, loving rocks, I got granite (emerald pearl).  Hence why i was directed here... magnets are "sticking" to it.  in some places i actually feel it slightly repelling the magnet.  whats going on here if there is no iron in granite?    magnetite?

love an answer.

maryalien
*

If it is a powerful enough magnet, it could be attracted to screws underneath, possibly even sunk into the stone. but I would be very surprised if there is actual repulsion...Eddy currents maybe?

Great Essays, Edstrick. I wanted to coment that from your description, the lack of granite shows a limited history of water acting on stone, and this is indirect evidence there has been little chance for life to evolve in the past....O
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edstrick
post Dec 7 2005, 08:52 AM
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It's more (and (literally) deeper) than "a limited history of water acting on stone". Fundamental to the study of "Igneous Petrology" is the concept of "partial melting".

Take more or less original planetary mantle rock, approximately what chondrite meteorites are made of if you melt them enough to settle the iron and iron-sulfide out to forma core and freeze them up again. It's dominated by olivine and pyroxenes (heavy iron and magnesium rich silicate minerals, and smaller amounts of feldspar (lighter calcium+sodium+potasium (mostly calcium) aluminum silicate), plus dribs and drabs of "none of the above".

Heat this under pressure in the presence of of a few percent water trapped in the minerals, and (surprise, surprise) it starts to melt. What melts first includes the water, and a lot of the elements that don't "like" to form solid solutions in the olivine/pyoxene bulk of the mantle. This particularly includes sodium and potassium and somewhat the aluminum. Melt it 5, 8, 10% and it gets all squishy like wet sugar, and you can either squeeze some of the melt out of it, or with more melting, you get the rock-crystal-mush settling and the partial melt separating out of it. The smaller the fraction melted, the more the melt is different in composition from the original. You end up with partial-melt and residual.

Do a lot of this and you get a crust made of first generation partial melt from the mantle.. more or less basalt. Still a lot of olivine and pyroxene, but not totally dominant as in the mantle rocks.

NOW.... partial melt crust... do it more than once... keep it squishy with water under pressure that helps cook out relatively soluble elements and separate them from the source rock.. repeat and after a few times, you have rock with almost no chemical resemblence to the original mantle.

This takes time and repeated geochemical cycling to produce much highly differentiated rock. On the moon, you have small amounts of relatively weird rock, including traces of "lunar granite', which is the result of partial melt from the mantle or first generation differentiated crust, having been trapped in between the floated feldspar crust of the lunar highlands and the dense rocks of the lunar mantle. This trapped relatively large amounts of "incompatible elements" including Potassium, (and uranium and thorium) and Rare Earth Elements and Phosphorus (KREEP), and apparently remelted from it's own heat, intruding higher into the crust as KREEP basalt and other rock types, exposed and ejected during the Imbrium impact. Why the bigger South Pole/Aitken basin didn't dig up the same stuff is a BIG question in lunar geology.

But that stuff is a tiny fraction of lunar rock and crust. The moon ran down and didn't have water to help the crust "cook" intereting recipies. I'm sure Mars did much more and for longer, and under wetter conditions, but it mostly ran down without sustained volcanism and tectonics.

I'm sure Mars has much more crustal variety and differentiation than the Moon, but far less and less complex than the Earth. Most of that history will be in the ancient highlands. The lavas erupted on top will be mostly <not entirely> basaltic, and tend to hide a lot of the most interesting old geology.
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maryalien
post Dec 7 2005, 02:51 PM
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QUOTE (The Messenger @ Dec 7 2005, 05:40 AM)
If it is a powerful enough magnet, it could be attracted to screws underneath, possibly even sunk into the stone. but I would be very surprised if there is actual repulsion...Eddy currents maybe? 

Great Essays, Edstrick. I wanted to coment that from your description, the lack of granite shows a limited history of water acting on stone, and this is indirect evidence there has been little chance for life to evolve in the past....O
*


Throughout all of it there are small chunks of "metal", and there is definately repulsion. so its not the screws underneath, but thanks anyway.
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RNeuhaus
post Dec 7 2005, 04:57 PM
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I found this topic very interesting trying to explain to the "lack or few" granite on Mars surface.

I would like to point out that the Mars northern highlands is of a most recent epochs which corresponds to Amazonia age. That zone is mostly buried by volcan lava, sediments or ashes mixed with water. Hence that zone is not so visible crated since the most craters are buried under the surface because that zone is of low altitude. One important recent discovery done vy Mars Express is that that zone has a very big crater, perhaps 250 kms of diameter (much bigger than ones of Gusev Crater which spans up to 160 km) on the northern lands. I suspect that by the northen lands must have many more hidden craters.

Thus, I assume that the density of craters must be around uniform to both hemisphers.

Rodolfo
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ElkGroveDan
post Dec 7 2005, 06:24 PM
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I'd just like to say that each rock on Mars is unique and important to a full understanding of Martian geology. None of these rocks, no matter how common, should be taken for granite.


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If Occam had heard my theory, things would be very different now.
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