Ask a Geologist

[razeontherock]

I study the strata deposited adjacent to passive salt diapirs in an effort to understand how diapirs develop, and how diapir growth influences local sedimentation patterns.

Why does salt need diapers?

 

[razeontherock]

This temperature stratification produces very slow convection of the mantle (over the long term, the mantle behaves as a viscous liquid, although it is, in effect, a solid or a very viscous mush).

Is this theoretical, or have we ever actually penetrated the crust to observe the mantle?

 

[Orogeny]

My coal pet scope wasn't specifically adjusted for me, the key difference was that flinc stuff was just so tiny and you almost had to IMAGINE you were able to see the fluid inclusion (IMHO). How she looked at white circles on a white background day after day and inside those find the white bubble and watch it blow up and shrink down was beyond me.

Sounds like a pain. I'm not sure what it's like for others, but I get an almost instant headache if my scope isn't properly adjusted. It took a while to figure out the right settings, and I hated scope work until I did.

Oh wait, no, I meant more exciting and sexy! I mean mud supported wackestone is already about as much excitement as anyone should be allowed to experience, I merely meant that...

Huh huh huh...you said penecontemporaneous. Huh huh huh.

Now I'm a bit rusty on some of the terms (I work almost exclusively with kaolin in paper coatings these days so lemme see if I got this right:

you have an oolitic carbonate in which the central nucleus of the ooid is later dolomitized and well-formed quartz crystals from in the voids?

Is the entire carbonate formation dolomitized or is it just the ooid "nucleus"?

Sorry for the nasty color, our camera seems to want make everything green at the moment. As you can see, the ooid nucleus is dolomitized, the cortices are micritized, and there is an elongate quartz crystal penetrating the ooids on the left side of the picture. Note that the quartz crystal nucleated around a small subangular matrix sand grain. In a couple of my other samples that have larger authigenic quartz, you can actually see remnant fluid inclusions in the quartz that outline the cannibalized ooid cortices. As you can see, quartz formation had almost no detrimental effect on preservation of original fabric. Nice acicular early marine cements preserved on the edges of the ooids.

Sadly the last time I got to play with dolomite was when I was working on SEMO MVT's (Pb-Zn sulfide minerals hosted in a dolomitized reef carbonate in Southeast Missouri).

I was only interested in the organic stuff in the shale above and below it.

I saw some cool carbonates but mostly really cool galenas and sphalerites. The carbonates weren't much to look at.

They usually aren't

 

[Orogeny]

What, in your opinion, is the source of the carbon used to make diamonds found in the earth?

The mantle.

 

[Orogeny]

Why does salt need diapers?

Never heard that one before!

Salt dome - Wikipedia, the free encyclopedia

 

[thaumaturgy]

Is this theoretical, or have we ever actually penetrated the crust to observe the mantle?

We have never drilled into the mantle, however sometimes the mantle comes to us.

Kimberlite pipes (good source of diamonds) are thought to be mantle rocks. Also we see some information on mantle rocks from obducted oceanic plates which show us their bottom parts which include some upper mantle, if I recall correctly.

But here's the really cool thing about geology: the earth makes it's own kind of "x-ray machine" using seismic waves. (NOTE: not actual x-rays!)

When an earthquake occurs it generates waves that travel through the body of the earth and can be detected on the other side.

Two major types of waves of importance here: S and P waves. (There are others but these will be sufficient for this discussion)

S-waves are shear waves. They can't move through liquid. P-waves can.

So when an earthquake occurs we see a couple things happen:

S-waves are "blocked out" where they would have to pass through liquids. This is one of the ways we know the core has a molten outer shell.

P-waves, like light waves can be "bent" or refracted by differences in density, so we learn about the density of the various layers of the earth on gross scales. They can be "slowed down" or sped up based on these differences.

We can get detailed information from p-wave velocities as you go down into the mantle:

(SOURCE)

Couple this "observational data" with what we know about the kind of mineral phases we expect at different pressure and temperature regimes and we have a pretty good picture of the details of the interior of the earth.

 

[thaumaturgy]

Very cool! Thanks for the explanation!

 

[Orogeny]

Is this theoretical, or have we ever actually penetrated the crust to observe the mantle?

We have never directly observed the mantle as such, but we have samples of mantle material brought up as xenoliths in basalt and xenocrysts in kimberlites, as well as mantle tomography that shows subducted plates and convection cells:

(a) subducted plates shown in blue, which indicates high velocity (high density and low temperature):

(b) Annotated mantle tomography image, again with blue being low temperature/high density-velocity and red being high temperature low density-velocity:

 

[Jazer]

Aren't rocks just rocks?

Perhaps pre stone age. But at one point in time they found that some rocks were better for cutting with. Usually they were more like a glass that come out of a volcano. Also they use to use flint for arrow heads for hunting. So if you want to take a rock and turn it into a stone you may want to pick the right rock. Then of course it makes a difference for women. If I give my wife a diamond she will be more happy then if she were to get a zircon. Also for tools and industrial use diamonds are a lot better to use.

 

[thaumaturgy]

Then of course it makes a difference for women. If I give my wife a diamond she will be more happy then if she were to get a zircon. Also for tools and industrial use diamonds are a lot better to use.

Well, you lose! You should have done what I did and married a geologist! First she isn't thrilled by diamonds, second she likes colored stones, third since a good emerald free of defect is hard to find (and expensive) I was able to thrill her because the emerald I bought her long ago had a big ol' fluid inclusion (others might call it a flaw) which she just loved! And since it was a "flaw" it was less expensive!

So you see, the real ultimate power of geology is: it's easier to impress a geo-chick!

 

[Orogeny]

Very cool! Thanks for the explanation!

Pleasure. It is thought that coupled dolomitization and authigenic quartz formation occurs in the shallow burial environment (top 2m) in hyperarid tidal flats/sabkhas, as well as in deeper burial environments with evaporite-derived hypersaline pore fluids--exactly what one might expect adjacent to a salt diapir. It's difficult to say which environment this formed in; both may have existed in these rocks at different times. I tend to lean toward the former, simply based on the high-quality preservation of original fabric in these rocks, as it seems to me that a hot, deep-burial environment would wreak havoc on primary textures.

 

[Orogeny]

Well, you lose! You should have done what I did and married a geologist! First she isn't thrilled by diamonds, second she likes colored stones, third since a good emerald free of defect is hard to find (and expensive) I was able to thrill her because the emerald I bought her long ago had a big ol' fluid inclusion (others might call it a flaw) which she just loved! And since it was a "flaw" it was less expensive!

So you see, the real ultimate power of geology is: it's easier to impress a geo-chick!

Oh man, this guy's doing it right.

 

[3rdHeaven]

What happen to Mars? Could that happen to Earth?

 

[Orogeny]

What happen to Mars? Could that happen to Earth?

Haha, that's a pretty broad question, the answer to which is well above my pay grade (which is, at the moment, nothing!). I think that understanding 'what happened to Mars' (if anything) is contingent on understanding what Mars used to be like. We can do that on Earth; we're here and can observe and sample the rock record to our hearts' content. But we're unable to study Mars in as much detail, and I think we won't understand the Red Planet's history until we do.

Short answer:

 

[juvenissun]

Sorry for the nasty color, our camera seems to want make everything green at the moment. As you can see, the ooid nucleus is dolomitized, the cortices are micritized, and there is an elongate quartz crystal penetrating the ooids on the left side of the picture. Note that the quartz crystal nucleated around a small subangular matrix sand grain. In a couple of my other samples that have larger authigenic quartz, you can actually see remnant fluid inclusions in the quartz that outline the cannibalized ooid cortices. As you can see, quartz formation had almost no detrimental effect on preservation of original fabric. Nice acicular early marine cements preserved on the edges of the ooids.

There are A LOT problems with your interpretations on this image.

Start from the main feature, how frequently do you see the "quartz" crystals in your sample? Are they all elongated (or euhedral) like this one? What is the SiO2% of your samples? Do you also see other form of SiO2, such as chert in your samples?

Just curious: what is the structure at the lower right corner of the calcite (?) matrix? Does it cross cut the ooids?

 

[thaumaturgy]

There are A LOT problems with your interpretations on this image.

Start from the main feature, how frequently do you see the "quartz" crystals in your sample? Are they all elongated (or euhedral) like this one?

"euhedral" does not necessarily mean "elongated", Juvenisun. Euhedral merely means "well formed crystal". Since quartz crystals usually take the form of elongated hexagonal prisms longer in the c-axis it would stand to reason that they would be elongate.

And why would you say there are a "LOT" of problems with Orogeny's interpretation? I've seen your version of geology on this forum quite a bit, Juvie and frankly I don't know that you would have much of a leg to stand on in making a claim like that against another geologist.

Just my opinion, though.

 

[Orogeny]

There are A LOT problems with your interpretations on this image.

Yeah? I can't help but notice you didn't list a single one. Would you please?
BTW, there are basically no 'interpretations' in the description you quoted, since it is a PETROGRAPHIC description and not a PETROLOGIC interpretation.

Don't you think its a bit arrogant of you to say there are a LOT of things wrong with my description when I am the one who spent months collecting the samples, mapping the field area, measuring section, doing the background research, and then taking the samples back to the lab, cutting them, thinning them, and then dedicating a few hours of microscopic observation to each of my 130-odd samples and documenting over 40 petrographic characteristics for each sample? Then to top it off, doing geochemical analysis on key samples in order to understand the chemistry of the ocean water they formed in? Don't you think I might know a bit more about the rocks than you, who has seen a plane-polarized light photomicrograph of two and a half ooids, a quartz crystal, and a bit of cement?

Start from the main feature, how frequently do you see the "quartz" crystals in your sample?

Very. And there is no reason to call it "quartz". It is quartz. It has low birefringence, is uniaxial positive, forms elongate hexagonal crystals with hexagonal bipyramidal termination, has a length-oriented c-axis, and is more durable under compaction than the surrounding calcite.

Are they all elongated (or euhedral) like this one? What is the SiO2% of your samples? Do you also see other form of SiO2, such as chert in your samples?

Elongate=/=euhedral. You may have the rest of the information you've requested when you explain to me exactly how you think my observations are faulty.

Just curious: what is the structure at the lower right corner of the calcite (?) matrix? Does it cross cut the ooids?

You seem to think you're the expert, why don't you tell me?

 

[thaumaturgy]

Yeah? I can't help but notice you didn't list a single one. Would you please?
BTW, there are basically no 'interpretations' in the description you quoted, since it is a PETROGRAPHIC description and not a PETROLOGIC interpretation.

Don't you think its a bit arrogant of you to say there are a LOT of things wrong with my description when I am the one who spent months collecting the samples, mapping the field area, measuring section, doing the background research, and then taking the samples back to the lab, cutting them, thinning them, and then dedicating a few hours of microscopic observation to each of my 130-odd samples and documenting over 40 petrographic characteristics for each sample? Then to top it off, doing geochemical analysis on key samples in order to understand the chemistry of the ocean water they formed in? Don't you think I might know a bit more about the rocks than you, who has seen a plane-polarized light photomicrograph of two and a half ooids, a quartz crystal, and a bit of cement?

Orogeny, not sure if you've met Juvenissun. He has some claims to geologic background, but after quite a bit of time engaging him on it I have found little worth actually noting from his comments.

He will, after a while, he'll roll out great lines like these:

It is VERY EASY to explain that what you said above does not apply to the situation of a global flood. But I am NOT going to tell you why this time. Why should I educate you if you do not appreciate anything I have said? I have posted many idea along this thread. However, to you, it seemed I have said nothing. If so, why should I continue?

and

To your benefit, if you do not understand, then humble yourself and start to ask some basic questions. One of the purpose for me to hang around here is to educate.

Be prepared.

Very. And there is no reason to call it "quartz". It is quartz. It has low birefringence, is uniaxial positive, forms elongate hexagonal crystals with hexagonal bipyramidal termination, has a length-oriented c-axis, and is more durable under compaction than the surrounding calcite.

Well, I suspect in the fourth sentence you overwhelmed his geology background.

You seem to think you're the expert, why don't you tell me?

Remember: if you aren't going to humble yourself before his greatness he doesn't need to teach you anything.

That's too bad for you, though. I bet your work would be made a lot easier if you had the geologic insights of Juvie.

Enjoy!

 

[Orogeny]

Orogeny, not sure if you've met Juvenissun. He has some claims to geologic background, but after quite a bit of time engaging him on it I have found little worth actually noting from his comments.

Oh my yes, I've absolutely met Juve. He's been nothing but giggles so far, and based on this last comment, it sure doesn't seem like that's gonna change!

He will, after a while, he'll roll out great lines like these:
and
Be prepared.

Been there done that. He only says those things when he hasn't got an answer.

Well, I suspect in the fourth sentence you overwhelmed his geology background.

That couldn't have been intentional...

That's too bad for you, though. I bet your work would be made a lot easier if you had the geologic insights of Juvie.

No doubt. After all, I've got A LOT of problems!

 

[juvenissun]

Sorry for the nasty color, our camera seems to want make everything green at the moment. As you can see, the ooid nucleus is dolomitized, the cortices are micritized, and there is an elongate quartz crystal penetrating the ooids on the left side of the picture. Note that the quartz crystal nucleated around a small subangular matrix sand grain. In a couple of my other samples that have larger authigenic quartz, you can actually see remnant fluid inclusions in the quartz that outline the cannibalized ooid cortices. As you can see, quartz formation had almost no detrimental effect on preservation of original fabric. Nice acicular early marine cements preserved on the edges of the ooids.

OK, a lot of these elongated quartz. I assume this image may sort of represent the textural relationship between the quartz and the ooids. Right?

How do you know it is the quartz which "penetrated" the ooids, rather than the ooids which enclosed the quartz?

The interpretation by the green text should be wrong. I do not see any nuclei of this authigenic quartz.

-------

I am trying to polish your interpretation, which can only be good to you. If you don't like it, then I will more than happy to stop (there is a long list to go). Be humble, kid.

By the way, if you don't have the information related to my previous questions, I suggest you to get them one way or another. They would be important for a good interpretation. You may even encounter those questions in your defense.