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Author Topic:   World's Oldest Rock?
petrophysics1
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Posts: 343
From: Boulder, Wy
Joined: 04-05-2006


(1)
Message 16 of 18 (688271)
01-21-2013 2:28 PM
Reply to: Message 14 by RAZD
01-19-2013 4:18 PM


Re: data, method
I didn't realize that this was just a useless NPR article about some work T. Mark Harrison did back in 2001. (My tax dollars at work.)

You can read about Mark Harrison here and down in paragraph 6 about this work on zircons.

http://www.geosociety.org/awards/09speeches/day.htm

There is however a little problem with these zircons not being the closed system Mr. Harrison thought they were back in 2001. You can read about it in this GSA abstract from 2011 here.

http://geology.gsapubs.org/content/39/12/1143.abstract

If that went over your head it's explained a bit clearer here:

http://earth-pages.co.uk/tag/jack-hills/

So basically like most stuff on NPR the article in the OP is mostly bullshit.

P.S. in my last link at the bottom you might like reading the first link there.ĦEarthfs eTime Capsulesf May Be Flawed (

Edited by petrophysics1, : Add PS

Edited by petrophysics1, : No reason given.


This message is a reply to:
 Message 14 by RAZD, posted 01-19-2013 4:18 PM RAZD has acknowledged this reply

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Taq
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Posts: 6098
Joined: 03-06-2009
Member Rating: 4.2


Message 17 of 18 (688402)
01-22-2013 10:59 AM
Reply to: Message 16 by petrophysics1
01-21-2013 2:28 PM


Re: data, method
So basically like most stuff on NPR the article in the OP is mostly bullshit.

So the dates for the 4.4 billion year ages for the zircons, once the younger contaminants and portions that lost Pb are removed, are still solid. What is not supported are the claims about the Earth environment 4.4 billion years ago. Is that correct?


This message is a reply to:
 Message 16 by petrophysics1, posted 01-21-2013 2:28 PM petrophysics1 has not yet responded

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Pressie
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Posts: 1577
From: Pretoria, SA
Joined: 06-18-2010
Member Rating: 2.1


(1)
Message 18 of 18 (688753)
01-25-2013 3:33 AM
Reply to: Message 17 by Taq
01-22-2013 10:59 AM


Re: data, method
Taq writes:

So the dates for the 4.4 billion year ages for the zircons, once the younger contaminants and portions that lost Pb are removed, are still solid. What is not supported are the claims about the Earth environment 4.4 billion years ago. Is that correct?


That’s exactly what I got from those references as well. They basically mean you have to be careful when dating zircons and stay well clear of those inclusions when doing it, while certain conclusions made from the inclusions were incorrect in the past.

The first link provided by petrophysicist1 dates from 2009: http://www.geosociety.org/awards/09speeches/day.htm

Ryerson writes:

Mark’s efforts in developing accessory mineral parameters and secondary ion microscopy collided in 2001 when his group co-discovered heavy oxygen isotopic compositions in >4.0 Ga zircons from the Jack Hills of Western Australia, a result that strongly suggested the presence of a liquid water hydrosphere on the earliest Earth.


Ass well as
Ryerson writes:

The accomplishments to date include vastly expanded databases of Hadean ages, oxygen and Lu-Hf and Sm-Nd isotope systems, as well as detection of fission-Xe related to the U/Pu ratio of the early Earth. With Bruce Watson he also developed and applied a novel Ti-in-zircon geothermometer to obtain crystallization temperatures from these detritral zircon grains, and continuing analysis of mineral inclusions is providing P-T data to constrain a Hadean geotherm. Taken together, the results argue strongly for the existence of continents as early as 4.4 Ga (perhaps even 4.5 Ga), and crustal cycling and plate boundary interactions much like those of today.


The second reference provided, http://www.geosociety.org/awards/09speeches/day.htm, dating from 2009:
Rasmussen writes:

The Hadean (before 4.0 Ga) crust has long been considered to comprise mainly primitive mafic and ultramafic rocks. However, mineral inclusions in detrital zircons as old as 4.4 Ga from Jack Hills, Australia, have been interpreted to be magmatic and to provide evidence for extensive granitic crust. In situ U-Pb dating of monazite and xenotime inclusions in 4.25–3.35 Ga detrital zircons from Jack Hills shows that these inclusions are not magmatic, but formed during metamorphism at either 2.68 Ga or 0.8 Ga.


Ass well as:
Rasmussen writes:

These results call for a reassessment of the source melts of the Hadean zircons and the composition of the earliest crust.

The last reference, dating from 2013, http://earth-pages.co.uk/tag/jack-hills/

Jack Hills writes:

While the Hadean zircon dates remain robust, a closer look at their inclusions shows that they did not remain geochemically closed systems thereafter. It was on the assumption of zircons being geological ‘time capsules’ that much of the excitement rested. Even using the presence of zircons from 4.4 Ga – they are most common in granites but do occur in mafic and intermediate igneous rocks – to suggest early ‘sialic’ continental crust is suspect. Despite having some tiny bits from Earth’s early days, it seems we are none the wiser.


Lastly, from the reference within the reference: http://news.sciencemag.org/...me-capsules-may-be-flaw-1.html:
Perkins writes:

But a new study suggests that these so-called inclusions are not as pristine as scientists thought, raising doubts about conclusions that researchers have drawn from them, from the rise of early oceans to the movements of the ancient continents.

Also

Perkins writes:

A high proportion of zircon inclusions are made up of phosphate minerals called apatite, Williams says. If that type of mineral is missing from all of the zircons in a particular sample, he notes, that's a clue that the inclusions may have been chemically altered


The influence of inclusions on dating zircons was already general knowledge, communicated to everyone, in for example, a textbook written in 1992 and used at the Uni I studied at then.

An introduction to the Rock-Forming Minerals, Deer, Howie & Zussman, Second Edition, 1992, under the heading Zircon (pages 22 to 26):

DHZ writes:

…For age determinations the Th/U ratio or the ratios of the various lead isotopes to each other or to isotopes of uranium or thorium are determined. There are many problems connected woth the interpretation of discordant U/Pb data, one of them being chemical reactions involving migration of Pb and U. The capability of the ion microprobe to obtain age data from micrometer-size areas of a single grain of zircon in a thin section or grain mount can avoid areas with cracks or inclusions and make interpretation of the results more useful…..


So, yes, what I get from these references, is that the conclusions the authors made about the environment from the inclusions in zircon, could have been inaccurate, but that the dates obtained for those zircons are solid.

Edited by Pressie, : Spelling mistake

Edited by Pressie, : Spelling

Edited by Pressie, : Clarified last sentence


This message is a reply to:
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