Core within the core?

[Correa Neto]

I had a talk with my mentor today, and yes: resolution turns out to be the problem. I hadn't really appreciated that we only have an angular resolution of about ten degrees, which isn't likely to let us do much more than verify that the core is actually there.

The project itself isn't actually seismology-related. I'm working with the IceCube neutrino telescope, which mainly looks at neutrinos passing through the Earth. (If something hits the telescope from above, it could be any random gunk, but if something has made it through the Earth, we know it's a neutrino). The question turned up whether we could use our data to actually decrease the uncertainties in the model of the Earth, instead of just treating them as sources of error in the results we're actually trying to find. It's looking like the most we'll be able to do is to see that the core-mantle boundary is in fact there, though.

Oh, but this is actually quite interesting. You people will be using a different method to probe, to test the boundary. This is very important in science, just remember how many different tests are made for aspects of relativity and quantum.

I bet a box of good beer your team will be able to publish the results in a good journal.

At last but not least, remember- we can not be sure of the future developments of certain things we start.

 

[Correa Neto]

You still need to address why there's so much at the surface. We know what happens when uranium gets highly concentration--natural nuclear reactors (well, one that I know of for a fact, and I recall hearing about at least one more) have been found. They don't explain why uranium is present in the crust.

For that matter, your mechanisms do not explain why iron is in the crust. Obviously there's a lot more going on then simply "Heavy elements fall inward", whatever definition of "elements" you use.

Enters the Late Heavy Meteor Bombardment. After the crust/core/mantle separation, meteor impacts enriched the crust in heavy elements such as Fe, Au, Pt and Pd. If it weren't for those impacts Earth's crust background values for those elements would be much smaller.

http://www.nature.com/nature/journal/v477/n7363/full/nature10399.html

This put, lots of things happen inside the Earth. There's a lot of heat and mass transfers (check mantle plumes, for example). They may be sloooooooooooow processes in terms of human history (even as species), but they are quite effective in the long term when it comes down to redistribute elements.

 

[Dinwar]

Enters the Late Heavy Meteor Bombardment. After the crust/core/mantle separation, meteor impacts enriched the crust in heavy elements such as Fe, Au, Pt and Pd. If it weren't for those impacts Earth's crust background values for those elements would be much smaller.

This makes a certain amount of sense. Still seems a bit odd to me, though--I mean, you'd still have impacts that punch through the crust, I would think. Chixilub-sized impactors would have been more common, and the crust wouldn't necessarily have been as thick, at least at the beginning, so punching through would require smaller amounts of energy. It must just be that sufficient smaller (not-penetrating) bodies hit the Earth to deposit the materia, and not enough larger (penetrating) bodies hit the Earth to redistribute it very thuroughly.

Hard to believe that most of the water on Earth came from comets.

I don't see why. A comet is just an asteroid where one primary component is ice (astronomers treat ice differently due to some peculiar properties, but it's still a mineral), and we know the Earth formed from smaller bodies being pulled together. EVERYTHING came from such bodies as asteroids, comets, meteors, and the like.

 

[Dorfl]

Oh, but this is actually quite interesting. You people will be using a different method to probe, to test the boundary. This is very important in science, just remember how many different tests are made for aspects of relativity and quantum.

Yeah. I'm still at the stage where my mentor occasionally has to remind me that not everything has to be a groundbreaking discovery: it's fine just to verify existing theories.

I bet a box of good beer your team will be able to publish the results in a good journal.

At last but not least, remember- we can not be sure of the future developments of certain things we start.

Thanks! It will have to wait a few years though. Right now we're not actually at the stage of "Use our detector to gather data and analyse them" as much as "Demonstrate that this analysis could be made if the detector was improved in the following way, so someone pretty please give us the money we need to make those improvements".

 

[Libra]

This makes a certain amount of sense. Still seems a bit odd to me, though--I mean, you'd still have impacts that punch through the crust, I would think. Chixilub-sized impactors would have been more common, and the crust wouldn't necessarily have been as thick, at least at the beginning, so punching through would require smaller amounts of energy. It must just be that sufficient smaller (not-penetrating) bodies hit the Earth to deposit the materia, and not enough larger (penetrating) bodies hit the Earth to redistribute it very thuroughly.

Apologies it this isn't 100% in line, but I've been wondering about this for some time in light of the Vredefort Dome here in our sunny South Africa.

http://en.wikipedia.org/wiki/Vredefort_Dome

With this 300km crater being about 2billion years old and with the sustained damage to the crust, it did take the impact quite well.

The apparent coinciding of the impact and of the initiation of the Platinum-group rich Bushveld Igneous Complex, seems to be more than chance ?

So did the asteroid bring the gold and other heavies or did the impact cause regional volcanism that brought the gold and other heavies from deep down ?

Vredefort Dome Geology

Vredefort Dome Magnetic Image

South African, Gauteng, Gold mines

Very interesting.

 

[Correa Neto]

Interesting questions!

First we must separate some things. The gold deposits are not related to the Vredefort impact. They are Witwatersrand-type mineralizations, whose exact origin is a matter of rather heated debate among ore geologists (paleoplacer vs hydrothermal vs modified paleoplacer).

Now, PGE mineralizations from Sudbury can be interpreted as related somehow to impacts.
http://11ips.laurentian.ca/NR/rdonlyres/F4D1259D-2990-4AFD-8B89-9FEBEED84BAE/0/Hechtetal.pdf
http://econgeol.geoscienceworld.org/content/97/7/1377.abstract
Sure, some will disagree.

A similar thing happens with the relationships between the Bushveld complex (and Merensky reef) and the Vredefort dome. Bushveld, however, can be interpreted as a piece of the mantle which rebounded after the impact.
http://geology.gsapubs.org/content/27/10/923.abstract
These folks say the impact is 30Ma older than Bushveld:
http://geology.gsapubs.org/content/27/10/923.abstract
So, if they are correct, than mineralization in this particular case has no link with the impact. But then, again, some people disagree.

 

[Correa Neto]

This makes a certain amount of sense. Still seems a bit odd to me, though--I mean, you'd still have impacts that punch through the crust, I would think. Chixilub-sized impactors would have been more common, and the crust wouldn't necessarily have been as thick, at least at the beginning, so punching through would require smaller amounts of energy. It must just be that sufficient smaller (not-penetrating) bodies hit the Earth to deposit the materia, and not enough larger (penetrating) bodies hit the Earth to redistribute it very thuroughly.

Remember that the LHMB was not composed just by large impactors. Smaller rocks also felt, and in quite large numbers. The 2.02Ga Vredefort impact (unrelated to and much younger than the LHMB ), for example, was huge and may even cause the mantle to rebound, but the impactor itself has not, as far as we know, punched through the crust.

So, what seems to have happened during the LHMB is that large stretches of the crust were melted by impacts. Heavy elements such as Pt, Au and Pd were added to the crust and mixed with the original crustal materials at these "oceans" of molten rocks, thus building a "thin veneer" (when compared to the whole Earth's radius) enriched in these elements. When it all cooled, plate tectonics kicked in and the fun begun. Heavy precious metals were and are still being concentrated at certain places which humans desperately try to find.

 

[Dinwar]

Remember that the LHMB was not composed just by large impactors. Smaller rocks also felt, and in quite large numbers.

True, but crumbs have callories. What I mean is, even small impactors, if they are large enough in number, can generate a lot of heat (if not enough to melt rock, perhaps enough to enhance mobility for certain elements/minerals; and if not world-wide, at least regionally).

So, what seems to have happened during the LHMB is that large stretches of the crust were melted by impacts.

Would this have remained floating on top of the mantle? If so, that could explain some things--the liquid wouldn't react the way solid crust would (the whole strain rate thing). Bullets react differently to metal than they do to water, for example. If the impactors hit a liquid body they may have been stopped without the pyrotechnics we assocait with impacts, and the body would be at least partially melted before it hit the mantle (then mantle processes and fractional distilation kick in [I'm thinking of mafic subduction zones resulting in intermediate volcanism, by way of analogy]). And if the crust was floating on the surface it may not mix with the mantle (it'd be like a turbidite--some mixing, sure, but the crust would act as essentially a unique fluid body), except for the bits that got dragged down with the impactor.

Yeah, I can see that keeping a lot of the impactor material at the surface. Not all, but excpetions are the rule in biology and geology.

 

[Correa Neto]

The best way I can imagine the situation would be oceans of molten rock and "continents" of solid rock. How deep the molten zones went I don't know, but I guess the depth was variable. So, the molten zone could be sitting over the mantle or the (mostly basaltic) crust. We know by modelling that during the LHMB some wide areas of the crust may have not been melted and were kept cool enough for microbial life, for example. So, it probably was not an homogeneous lava ocean. KABOOOM! KABOOOM! KABOOOM! This whole area melts, but that other one over there not. This place cools down and then KABOOOOOOOOOM! There goes the cool neighborhood. Despite all the heat from the impacts, Earth is no longer as hot as it was back in the times of the core/mantle diferentiation, so downwards migration of the heavy pecious metals is not enough to bring their background values to what it was before the LHMB. Now, this doesn't mean the crust became rich in precious heavy metals. Gold background value, for example, is around 5ppb (parts per billion).

 

[Libra]

A similar thing happens with the relationships between the Bushveld complex (and Merensky reef) and the Vredefort dome. Bushveld, however, can be interpreted as a piece of the mantle which rebounded after the impact.

Thanks for the links and insights Correa Neto.

As far as I could understand, it is the ultramafic rocks found in the centre of the Vredefort structure which is thought to be not related to the Bushveld IC, but rather mantle that rebounded to the crustal levels post impact.

The Bushveld Igneous Complex is igneous and occurs as massive lenticular intrusions brought up through long vertical cracks in the crust. Is this correct ?

http://geology.gsapubs.org/content/27/10/923.abstract
These folks say the impact is 30Ma older than Bushveld:
http://geology.gsapubs.org/content/27/10/923.abstract
So, if they are correct, than mineralization in this particular case has no link with the impact. But then, again, some people disagree.

These two links are the same ?

Is this the one on the 30Ma ?

http://www.sciencedirect.com/science/article/pii/S0012821X9600180X

If so, this portion in the abstract is a weak link imho :

"A concordant 2023 ± 4 Ma (2σ) age for newly crystallized, unshocked zircon grains from recrystallized pseudotachylitic breccia from the central part of the Vredefort Structure provides a good approximation of the time of impact"

A good approximation means it's approximate but not precise, right ? So using an approximation to state an age difference of 30Ma between a +\- 2 billion year old geological formation and a 2.023 billion years (± 4 million years) impact crater is not exactly convincing to me, why is it to others ? Am I missing something ?



90% of the world's Platinum Group Metals occurs in the Merensky and UG2 reefs, part of the BIC formed by huge large-scale igneous intrusions.

This in the SAME PLACE where the largest verified impact crater on earth exists.

Ontop of this, the geological formation's age and the impact's age are virtually identical.

Wow.

So via natural earth-related geological processes a huge igneous intrusion brings 90% of the world's PGMs to the top just just before one of the largest ever asteroids strikes and forms the world's largest verified impact crater in the same place.

Can this really be by chance ?

 

[Correa Neto]

Thanks for the links and insights Correa Neto.

As far as I could understand, it is the ultramafic rocks found in the centre of the Vredefort structure which is thought to be not related to the Bushveld IC, but rather mantle that rebounded to the crustal levels post impact.

The Bushveld Igneous Complex is igneous and occurs as massive lenticular intrusions brought up through long vertical cracks in the crust. Is this correct ?

The Bushveld complex is a mafic/ultramafic layered complex. It also has more silica-rich later phases. Some people think its a lenticular intrusion within the crust while others think its a piece of rebounded mantle. Note that it can also be both (an intrusion within the mantle which was later rebounded).

These two links are the same ?

Nope, cut-and-paste SNAFU

Is this the one on the 30Ma ?

http://www.sciencedirect.com/science/article/pii/S0012821X9600180X

Yes.

If so, this portion in the abstract is a weak link imho :

"A concordant 2023 ± 4 Ma (2σ) age for newly crystallized, unshocked zircon grains from recrystallized pseudotachylitic breccia from the central part of the Vredefort Structure provides a good approximation of the time of impact"

A good approximation means it's approximate but not precise, right ? So using an approximation to state an age difference of 30Ma between a +\- 2 billion year old geological formation and a 2.023 billion years (± 4 million years) impact crater is not exactly convincing to me, why is it to others ? Am I missing something ?

It was published on volume 144 of Earth and Planetary Letters. I checked untill volume 160 for a discussion and a reply. Found none. Since the ages are a key point to the paper, I suppose if there were issues with the error margins, discussions would have been submitted.

These folks also concluded impact postdates Bushveld:
http://link.springer.com/article/10.1007/BF01161720
http://sajg.geoscienceworld.org/content/109/3/279.abstract

But again, as I wrote at my first post, some folks will link the impact with the mineralization. I remember reading about such a proposed link years ago, but it seems its not "fashionable" nowadays.

90% of the world's Platinum Group Metals occurs in the Merensky and UG2 reefs, part of the BIC formed by huge large-scale igneous intrusions.

This in the SAME PLACE where the largest verified impact crater on earth exists.

Ontop of this, the geological formation's age and the impact's age are virtually identical.

Wow.

So via natural earth-related geological processes a huge igneous intrusion brings 90% of the world's PGMs to the top just just before one of the largest ever asteroids strikes and forms the world's largest verified impact crater in the same place.

Can this really be by chance ?

Why not? What were the odds of a meteor striking exactly that shallow reef at the K/Pg boundary? Or of one exploding over Russia at the same time another completely unrelated meteor makes a close fly-by on Earth? We have no idea on how many Merensky reefs are at the upper mantle; the impact may have given us a lucky window or showed us common stuff - if its rebounded mantle.

Or it was a hell of a coincidence: PGEs concentrated within a silica-poor magma chamber located at the continental crust and later a large meteor impacted the area above it.

Remember gut feelings are not reliable and science is a work in progress. We quite often don't have definitive answers.

Back on track, Earth and Planetary Science letters contains two recent papers on the core, mantle and LHB, full contents hidden behind Elsevier' paywall, I guess...

http://ac.els-cdn.com/S0012821X1300...t=1363004644_e0f0f257f16329ff921f820247196ef5
http://ac.els-cdn.com/S0012821X1300...t=1363004670_88180653eaf10fe0b8ace9afdcbf417c
http://ac.els-cdn.com/S0012821X1300...t=1363004906_762e4cb10b21dc2eb22fee4ea99de175

 

[fuelair]

They made one in Florida. Guy's house fell into it.

In all fairness, "They" didn't make it, mild carbonic acid did over time by dissolving limestone. So, it is a natural hole (thus like the one in most people's arses) rather than an artificial/man made one like when someone gets shot.

 

[Subduction Zone]

In answer to the earlier question about uranium. Yes, there probably is a slightly higher concentration of uranium in the cores than there is in the rest of the Earth. Of course there always was not a terribly high concentration of uranium in the Earth as a whole so you will still see only a slight rise in very dense materials in the core.

One interesting side note:

The force of gravity actually goes up as you go deeper into the Earth, at least until you hit the mantle core boundary. Then at that point it will go almost linearly to zero as you approach the center of the Earth. At the core/mantle boundary the acceleration due to gravity would be roughly 10.8 m/s^2.

 

[Libra]

We have no idea on how many Merensky reefs are at the upper mantle; the impact may have given us a lucky window or showed us common stuff - if its rebounded mantle.

Now that wouldn't be good for the gold-price ?

If so, would the Geophysics not have shown this by now ?

Or it was a hell of a coincidence: PGEs concentrated within a silica-poor magma chamber located at the continental crust and later a large meteor impacted the area above it.

Thanks for the links Correa Neto.

Like you say, possibly one hell of a coincidence and definately very interesting.

 

[Correa Neto]

No need to thanks. I can have a lot of funs looking for this type of thing. Part of my insanity, I guess...

Well, even if we have plenty of Merensky reefs in the upper mantle (or middle and lower crust), PGE prices and PGE-mining companies' shares are quite safe. Finding and mining them would be impossible with current technology. You probably would have to dig through some 30 - 40 km if you are in continental crust or 4 - 5 km if you are in oceanic crust. The later figure is feasible in terms of engineering (opening galleries, shafts, raises, etc. and keeping them open and operational), but you would have to start digging your mine in the ocean, some 3km under the sea. Mining costs would be humongous.

Merensky reef is less than 1m-thick. Same is valid for other layers (ex.: UG2 and Bastard reef - I love that name), be them mineralized or not. This means current geophysical methods will most likely not be able to "see" them at the depths we are talking about.

As for hellish coincidences, one way I like to think about mineralizations is that they are formed by series of lucky hits.