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Author Topic:   Geologists and dating (India Basins Half a Billion Years Older Than Thought)
Percy
Member
Posts: 22623
From: New Hampshire
Joined: 12-23-2000
Member Rating: 3.6


Message 16 of 93 (477752)
08-07-2008 8:08 AM
Reply to: Message 15 by peaceharris
08-06-2008 9:06 PM


As Joe said, the Williams reference is in the paper. Just click on the link he provided (Paleomagnetism and Detrital Zircon Geochronology of the Upper Vindhyan Sequence, Son Valley and Rajasthan, India: A ca. 1000Ma Closure age for the Purana Basins?), go to the references section at the end, and you'll find this:
References Section of the Malone/Meert paper writes:
Williams, I.S., 1998. U-Th-Pb geochronology by electron microprobe. In: McKibben,
M.A., et al. (Eds.), Applications of Micro Analytical Techniques to Understanding Mineralizing Processes: Reviews in Economic Geology, vol. 7, pp. 1-35.
I couldn't find the Williams paper online, maybe Joe can provide a link.
--Percy

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Joe Meert
Member (Idle past 5768 days)
Posts: 913
From: Gainesville
Joined: 03-02-2002


Message 17 of 93 (477779)
08-07-2008 3:43 PM
Reply to: Message 15 by peaceharris
08-06-2008 9:06 PM


common Pb corrections
These are fairly standard practice in radiometric dating and you can read about variations of this procedure in an elementary book on radiometric dating. While Williams (1998) procedure is used for these particular samples, it should be noted that most of our zircons contained only small amounts (or none at all) of common lead. If we had not applied the correction, the age profiles would have been slightly skewed towards the older end and would not really have altered our conclusions at all. So, I'm not sure exactly why you require all the raw data and the paper by Williams (I can probably get it scanned in the next few days, but I'm busy with some other writing at the moment). If you want to argue that our estimate of the age of the Vindhyan basin is wrong and think the Ediacaran-Cambrian ages are superior, we present those arguments for you in the paper. I'd be happy to discuss the alternative ages with you; however, you cannot get a lower age range by arguing that we incorrectly applied the common lead correction.
Cheers
Joe Meert

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Replies to this message:
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peaceharris
Member (Idle past 5685 days)
Posts: 128
Joined: 03-28-2005


Message 18 of 93 (477800)
08-07-2008 9:32 PM
Reply to: Message 17 by Joe Meert
08-07-2008 3:43 PM


Re: common Pb corrections
I disagree with the statement "you cannot get a lower age range by arguing that we incorrectly applied the common lead correction."
If you subtracted less than the correct the amount of common present during crystallization, you would obtain an older age.
I can't possibly attack you if I don't understand the justification behind the correction method and if you do not disclose your raw data and your method of subtracting the common Pb.
It's OK if you are busy now, please scan it whenever you are free or maybe someone else here who is in university and has access to this journal, please scan it for us.

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Replies to this message:
 Message 19 by Joe Meert, posted 08-07-2008 9:55 PM peaceharris has replied

  
Joe Meert
Member (Idle past 5768 days)
Posts: 913
From: Gainesville
Joined: 03-02-2002


Message 19 of 93 (477801)
08-07-2008 9:55 PM
Reply to: Message 18 by peaceharris
08-07-2008 9:32 PM


Re: common Pb corrections
The justification is standard and you don't really need the Williams paper to see the basis for it. The Williams paper is simply a method to correct for common Pb in the case of a particular instrumental procedure, it's not some radically strange method. I also think you missed the point that most of our zircons contained very little common lead so the notion that our 1000 Ma minima are closer to 500-600 million years is a moot point. Like I said, if you prefer the Ediacaran-Cambrian range there is still plenty to talk about, but the ages of our detrital zircons won't get you there.
Cheers
Joe Meert

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Replies to this message:
 Message 20 by peaceharris, posted 08-07-2008 11:38 PM Joe Meert has replied

  
peaceharris
Member (Idle past 5685 days)
Posts: 128
Joined: 03-28-2005


Message 20 of 93 (477807)
08-07-2008 11:38 PM
Reply to: Message 19 by Joe Meert
08-07-2008 9:55 PM


Re: common Pb corrections
OK, I don't need the Williams paper since it applies to a particular instrument. What I was looking for is how geologists determine the amount of Pb present initially during crystallization.
On page 13 of your paper, you wrote regarding the grains from the Rewa group "Most of the grains analyzed showed very discordant dispersions between 206Pb/238U and 207Pb/235U ages as well as high common Pb."
Regarding the grains from the Bhander group, you wrote, "The detrital zircon sample processed from the Upper Bhander sandstone (sampled from sites 43, 44 and 45, Rajasthan section) yielded numerous datable grains with varying degrees of concordance."
What is it that you observed in Rewa group that made you conclude that there is high common Pb in those grains?
What makes you believe that the Bhander group contained very little common Pb during crystal formation?

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 Message 19 by Joe Meert, posted 08-07-2008 9:55 PM Joe Meert has replied

Replies to this message:
 Message 21 by Joe Meert, posted 08-08-2008 10:28 AM peaceharris has replied

  
Joe Meert
Member (Idle past 5768 days)
Posts: 913
From: Gainesville
Joined: 03-02-2002


Message 21 of 93 (477844)
08-08-2008 10:28 AM
Reply to: Message 20 by peaceharris
08-07-2008 11:38 PM


Re: common Pb corrections
The degree of initial discordance and the lack of more concordance when the common Pb correction is applied. For the Rewa Group, you should note that the zircons were obtained from a supposed volcanic ash bed. You cannot get a concordant age with a high contamination of common Pb. Spikes in 206Pb during the analysis in the absence of other peaks is a pretty good indicator of excess common Pb. In general, we reject a priori any grains that show more than 10% discordance. That doesn't mean that they are useless, it's simply that at 10% or more discordance, the application of a common lead correction can also be complicated by open system behavior.
Still, like I said, if your preference is for an Ediacaran-Cambrian age for these units, the detrital zircon results are not the best way to go about making your argument.
Cheers
Joe Meert
Edited by Joe Meert, : No reason given.
Edited by Joe Meert, : spelling

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Replies to this message:
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JonF
Member (Idle past 256 days)
Posts: 6174
Joined: 06-23-2003


Message 22 of 93 (477891)
08-08-2008 7:28 PM
Reply to: Message 15 by peaceharris
08-06-2008 9:06 PM


You can have a scanned PDF of any paper you want emailed to you for $12.
Pricing information - Non-MIT Customers
Journal Articles & Conference Papers Order Form

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peaceharris
Member (Idle past 5685 days)
Posts: 128
Joined: 03-28-2005


Message 23 of 93 (477983)
08-10-2008 12:23 PM
Reply to: Message 21 by Joe Meert
08-08-2008 10:28 AM


Re: common Pb corrections
Please give me the spreadsheet containing the raw data. I assume your spreadsheet contains all the formulas used to arrive at your end result from the raw data. This is basically what I want. I want to first understand exactly what you have done, then start the discussion.
What I understand about the Concordia diagram is what is explained at http://www.tulane.edu/...lson/geol212/radiometric_dating.htm
But that website doesn't explain how to calculate (Pb206/Pb204)_o in equation 9. I don't know if geologists apply the isochron method that is used to find out (Sr87/Sr86)_o to find out similarly for (Pb206/Pb204)_o. Or is there some other method?
I will be in a position to start a discussion only if I see your raw data and understand your calculations.

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Replies to this message:
 Message 24 by Percy, posted 08-10-2008 3:21 PM peaceharris has replied
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Percy
Member
Posts: 22623
From: New Hampshire
Joined: 12-23-2000
Member Rating: 3.6


Message 24 of 93 (477988)
08-10-2008 3:21 PM
Reply to: Message 23 by peaceharris
08-10-2008 12:23 PM


Re: common Pb corrections
peaceharris writes:
But that website doesn't explain how to calculate (Pb206/Pb204)_o in equation 9. I don't know if geologists apply the isochron method that is used to find out (Sr87/Sr86)_o to find out similarly for (Pb206/Pb204)_o. Or is there some other method?
Same method. In that Radiometric Dating article, it's the same approach described in the paragraph that begins, "First note that the time..." With time since formation equal to t, you know the (206Pb/204Pb)t ratio by measurement, and you know the (238U/204Pb)t ratio for different minerals in the rock by measurement. Equation (9) has only two unknowns, so as long as the rock has two or more different minerals you can solve for both t and (206Pb/204Pb)0.
--Percy

This message is a reply to:
 Message 23 by peaceharris, posted 08-10-2008 12:23 PM peaceharris has replied

Replies to this message:
 Message 27 by peaceharris, posted 08-10-2008 11:17 PM Percy has replied
 Message 31 by JonF, posted 08-11-2008 9:12 AM Percy has replied

  
Joe Meert
Member (Idle past 5768 days)
Posts: 913
From: Gainesville
Joined: 03-02-2002


Message 25 of 93 (477994)
08-10-2008 5:11 PM
Reply to: Message 23 by peaceharris
08-10-2008 12:23 PM


Re: common Pb corrections
The 'conversation' whatever that means, can be started without the need to go through the common lead correction since there was minimal common Pb (if any) in the zircons we used for the analysis. I have to tell you that I'm a little leery of simply supplying raw data to someone who doesn't know what they are doing with those data since there is a chance they might be abused. Perhaps if you can tell me what 'conversation' you want to have, I can help you. Like I said, if you prefer the Ediacaran age, there are better arguments to be had.
Cheers
Joe MEert

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JonF
Member (Idle past 256 days)
Posts: 6174
Joined: 06-23-2003


Message 26 of 93 (478007)
08-10-2008 8:19 PM
Reply to: Message 23 by peaceharris
08-10-2008 12:23 PM


Re: common Pb corrections
Please give me the spreadsheet containing the raw data. I assume your spreadsheet contains all the formulas used to arrive at your end result from the raw data.
Joe's spreadsheet uses Dr. Ludwig's add-in Isoplot, which in turn contains some extremely sophisticated statistical formulae that are not readable in the spreadsheet (because it's an add-in). You can find a discussion of the formulae in User's Manual for Isoplot 3.00, with gobs of references for you to digest. To critique the math, you will neeed to be an expert in the fitting of straight-line and curve data in cases where both the abscissa and ordinate contain experimental erros (which in this case are coupled) and the errors are not normally distributed ... i.e. not least-squares.
I don't know if Joe is allowed to send you Isoplot even if he wants to. You might have to convince Dr. Ludwig to give it to you.
What I understand about the Concordia diagram is what is explained at http://www.tulane.edu/...lson/geol212/radiometric_dating.htm
IMHO there's no really good online reference on this subject. That one doesn't make the key point strongly enough. The physics of solidification guarantee that zircons strongly reject lead at solidification and relatively easily accept uranium and thorium, so essentially all of the lead one finds in a zircon is due to radioactive decay of one of those elements. Since the age of the Earth was settled long ago, today the action in geochronology is sub-1% accuracy, and to get that you need to do a common lead correction. But that correction is second-order; it pretty much never affects the age much. IIRC a two percent change would be a very large correction for a zircon.
Many correction techniques (I'm not familiar with the particular one Joe used) involve measuring the amount of 204Pb, which is not radiogenic and therefore is common lead, and a model to derive the amounts of other lead isotopes that are common. Ther are techniques that do not depend on measuring 204Pb.
So I'm puzzled, and I think Joe's puzzled, about why you want to discuss a second-order correction.
You might want to look at GEOCHRONOLOGY V: THE U-TH-PB SYSTEM: ZIRCON DATING which begins:
quote:
Zircon (ZrSiO4) is a mineral with a number of properties that make it extremely useful for geochronologists (Figure 1). First of all, it is very hard (hardness 71/2), which means it is extremely resistant to mechanical weathering. Second, it is extremely resistant to chemical weathering and metamorphism. For geochronological purposes, these properties mean it is likely to remain a closed system. Third, it concentrates U (and Th to a lesser extent) and excludes Pb, resulting in typically very high 238U/204Pb ratios. It is quite possibly nature's best clock. Finally, it is reasonably common as an accessory phase in a variety of igneous and metamorphic rocks.
The very high 238U/204Pb ratios in zircon (and similar high minerals such as sphere and apatite) provide some special geochronological opportunities and a special diagram, the concordia diagram, has been developed to take advantage of them. The discussion that follows can be applied to any other system with extremely high 238U/204Pb ratios, but in practice, zircons constitute the principal target for Pb geochronologists.
and goes on to get pretty darned technical. (The entire series of lectures is available at Earth & Atmospheric Sciences 656: Isotope Geochemistry.)
Or Zircon: The Key Mineral:
quote:
Uranium and thorium are found in many minerals, of which, however, only a small number are suitable for dating using the U, Th-Pb approach. This is because only a few are adequately retentive towards them and of these the most retentive is zircon. Other useful minerals include pitchblende (or uraninite, U308), monazite, sphene and apatite. In zircon, the concentrations of U and Th average 1330 and 560 ppm respectively. When in pegmatites, zircons contain more of these elements than they do in normal igneous rocks. Uranium and thorium are found in zircon through the isomorphous replacement of Zr4+ (with an ionic radius of 0.87 ) by U4+ (1.O5 ) and Th4+ (1.10 ) as well as through the presence of thorite inclusions. Such substitution is restricted by the differences in the relevant ionic radii Pb2+ is excluded altogether because its ionic radius is 1.32 and it bears a lower charge. The result is that zircon does not contain much lead at the time of its formation and has very high ratios for U/Pb and Th/Pb, making it a valuable geochronometer (Bowen, 1988). For this reason, this mineral is widely utilised in dating by the U, Th-Pb isotope method, for instance by C. J. Allégre et al. (1974).
Edited by JonF, : No reason given.

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peaceharris
Member (Idle past 5685 days)
Posts: 128
Joined: 03-28-2005


Message 27 of 93 (478012)
08-10-2008 11:17 PM
Reply to: Message 24 by Percy
08-10-2008 3:21 PM


Re: common Pb corrections
Percy writes:
Same method.
Joe,
Pls confirm whether the same isochron method is used to determine (Pb206/Pb204)_o.
JonF writes:
The physics of solidification guarantee that zircons strongly reject lead at solidification and relatively easily accept uranium and thorium...
Do you know of any measured data that supports the statement above? I think the statement above should be something that can be proven if true. For example, someone can measure the amount of Pb from a zircon grain from a volcano that just erupted and compare it to the amount of Pb present in the magma of that volcano.
For example, if the Pb content in the zircon is 0.1ppm, and the Pb content in the magma is 10ppm, then the statement above is justified. Then we can also proceed to estimate how much of error results from the common Pb present.
JonF writes:
Joe's spreadsheet uses Dr. Ludwig's add-in Isoplot, which in turn contains some extremely sophisticated statistical formulae that are not readable in the spreadsheet (because it's an add-in).
That's not a problem, as I have Minitab (a statistical software) and I think that I too can analyse the data. Minitab does have many inbuilt statistical tests.
Joe writes:
Perhaps if you can tell me what 'conversation' you want to have, I can help you. Like I said, if you prefer the Ediacaran age, there are better arguments to be had.
The conversation I want to have is the common Pb correction. If there are better arguments to be had, we can consider them later. But first I want to see your raw data and understand how you determined the common Pb.
joe writes:
Spikes in 206Pb during the analysis in the absence of other peaks is a pretty good indicator of excess common Pb.
I don't understand what this means, atleast if you provide the raw data and show me an example of the spike, I might be in a position to understand what you are saying.
Joe writes:
I have to tell you that I'm a little leery of simply supplying raw data to someone who doesn't know what they are doing with those data since there is a chance they might be abused.
OK, it's your wish. If you don't disclose the raw data, it won't be possible for me to apply any statistical test to your data, and I don't see any point in continuing this discussion.
No one here other than you knows how you determined the common Pb correction. Percy thinks you applied the isochron method (something that I doubt). JonF thinks it is not necessary to do any correction because Pb is not incorporated during solidification, but you have admitted that zircons from the Rewa group have high common Pb. Even if I 'abuse' your work, at least at the end of the day other members here would understand your research.

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 Message 24 by Percy, posted 08-10-2008 3:21 PM Percy has replied

Replies to this message:
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 Message 30 by Joe Meert, posted 08-11-2008 8:52 AM peaceharris has replied

  
Percy
Member
Posts: 22623
From: New Hampshire
Joined: 12-23-2000
Member Rating: 3.6


Message 28 of 93 (478035)
08-11-2008 7:54 AM
Reply to: Message 27 by peaceharris
08-10-2008 11:17 PM


Re: common Pb corrections
peaceharris writes:
Percy thinks you applied the isochron method (something that I doubt).
I didn't comment one way or the other on what method Joe used. You asked, "how to calculate (Pb206/Pb204)_o in equation 9" from the Radiometric Dating article, and whether they used the same method as for 87Sr/86Sr, and I explained that it was. It's all right there in the article.
--Percy

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JonF
Member (Idle past 256 days)
Posts: 6174
Joined: 06-23-2003


Message 29 of 93 (478040)
08-11-2008 8:50 AM
Reply to: Message 27 by peaceharris
08-10-2008 11:17 PM


Re: common Pb corrections
The physics of solidification guarantee that zircons strongly reject lead at solidification and relatively easily accept uranium and thorium...
Do you know of any measured data that supports the statement above? I think the statement above should be something that can be proven if true. For example, someone can measure the amount of Pb from a zircon grain from a volcano that just erupted and compare it to the amount of Pb present in the magma of that volcano.
For example, if the Pb content in the zircon is 0.1ppm, and the Pb content in the magma is 10ppm, then the statement above is justified. Then we can also proceed to estimate how much of error results from the common Pb present.
I don't know offhand of any measured data that proves this statement; I'm not a crystallographer, and it's one of those things that's so well established that few if any people bother to trace it down. But you could start with the papers referenced in my second link.
{ABE: I draw your attention to that which I posted already:
quote:
Such substitution is restricted by the differences in the relevant ionic radii. Pb2+ is excluded altogether because its ionic radius is 1.32 and it bears a lower charge. The result is that zircon does not contain much lead at the time of its formation and has very high ratios for U/Pb and Th/Pb, making it a valuable geochronometer (Bowen, 1988). For this reason, this mineral is widely utilised in dating by the U, Th-Pb isotope method, for instance by C. J. Allégre et al. (1974).
{emphasis added}
So you can go dig up "Bowen, R., 1988. Isotopes in the Earth Sciences. Elsevier Applied Publishers Ltd. Press" to start with.
end ABE}
FWIW, the RATE group (Baumgardner, Snelling, Austin, and Vardiman: arguably the YECs who are most knowledgable about radiometric dating, and who are definitely interested in discrediting it) accept this fact. From HELIUM DIFFUSION RATES SUPPORT ACCELERATED NUCLEAR DECAY:
quote:
Samples 1 through 3 had helium retentions of 58, 27, and 17 percent. The fact that these percentages are high confirms that a large amount of nuclear decay did indeed occur in the zircons. Other evidence strongly supports much nuclear decay having occurred in the past [14, pp. 335-337]. We emphasize this point because many creationists have assumed that "old" radioisotopic ages are merely an artifact of analysis, not really indicating the occurrence of large amounts of nuclear decay. But according to the measured amount of lead physically present in the zircons, approximately 1.5 billion years worth ” at today’s rates ” of nuclear decay occurred. Supporting that, sample 1 still retains 58% of all the alpha particles (the helium) that would have been emitted during this decay of uranium and thorium to lead.
{emphasis in original}
Note that these knowledgable people, frantically trying to discredit radiometric dating, did not even bother to try to argue that common lead is significant in zircons. Instead they said exactly the opposite; significant amounts of lead in a zircon is prima facie evidence of large amounts of nuclear decay since the zircon formed.
But it can easily be proved beyond reasonable doubt that Joe's samples (and almost all the tens of thousands of samples for which this kind of analysis has been carried out) had low common lead. He's already briefly alluded to it, apparently assuming that you know enough to figure it out.
If we start with a freshly-formed zircon and plot its position on the concordia diagram, it will plot at (or indistinguishably close to) (0,0) if and only if it has zero (or essentially zero) common lead. As it ages, the point representing it will move to other points on the concordia diagram, tracing a path. Assuming for the moment that that no relevant material is removed or added, those other points will lie on the concordia curve if and only if the point representing the zircon originally plotted at (0,0) and the zircon started with essentially zero common lead.
So, when we plot the point representing a zircon todary and observe that it lies on the concordia curve, there are two possibilities:
  1. The zircon started with zero (or near-zero) common lead when it formed.
  2. The zircon started with noticably non-zero common lead, and some event added or removed relevant material in exactly the proportions required to move its point back onto the concordia curve.
If you investigate possibility 2 mathematically, you will find that the postulated event will have to be significantly sensitive to different isotopes of the same element, ruling out pretty much all chemical and mechanical processes. And any process has to occur in just the right amount to get the point back onto the curve.
Tain't gonna happen. Maybe once in a few tens of thousands of tries, but not several times in one study, and not over and over and over again.
QED. Zircons start with essentially zero common lead. You can dig up the reasons if you are interested, but the points representing them consistently plotting on the concordia curve proves it.
That's not a problem, as I have Minitab (a statistical software) and I think that I too can analyse the data. Minitab does have many inbuilt statistical tests.
These are very specialized statistical tests, and I wouldn't assume Minitab does them.
JonF thinks it is not necessary to do any correction because Pb is not incorporated during solidification, but you have admitted that zircons from the Rewa group have high common Pb.
In this context, "high common Pb" is not very much at all in absolute terms. And did you notice his reason for deducing that they have high common Pb? They didn't plot close enough to the concordia curve.
I understand his research and the method well enough to know that the common Pb correction is a tiny fraction of the determined ages.
Edited by JonF, : Add marked material

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Joe Meert
Member (Idle past 5768 days)
Posts: 913
From: Gainesville
Joined: 03-02-2002


Message 30 of 93 (478041)
08-11-2008 8:52 AM
Reply to: Message 27 by peaceharris
08-10-2008 11:17 PM


Re: common Pb corrections
Peaceharris,
I believe I mentioned (at least) twice, that the common Pb correction is very small (to non-existent) for many of the grains used in this study. Basically, it's application of the Stacey and Kramers (1975) model showing the evolution of common Pb on earth over time. It's incorporated into secure macros on the spreadsheet. The spreadsheet is not my intellectual property so I'm not free to distribute it. Furthermore, the article supplies all the data in an appendix which you are free to obtain through Precambrian Research. I think I understand what you are getting at. You basically think the earth is 6000 years old (or so) and wish to reject all geochronology. To answer some of your other questions:
(1) The spike I was speaking of is observed in the display as the zircon is being ionized.
(2) Common lead corrections vary in zircon geochronology (a point I've made several times here). The reason we mentioned the high common lead in the Rewa was because the data were very wonky and made no sense (both in terms of their degree of discordance and in their elemental abundances). The consistency in the Bhander/Marwar data showed very little evidence of any common lead and the 'corrections' where applied, changed the ages by less than 2%. 2% won't get you to 6000 years.
(3) The estimate for the age of the Vindhyan basin was determined through 4 observations (a) The similarity in paleomagnetic directions between the Bhander-Rewa Group and the Majhgawan kimberlite (b) The Ar-Ar age of 1073 Ma determined for the Majhgawan kimberlite (c) the lack of any zircons younger than 1020 Ma in the Bhander-Rewa and (d) the presence of young zircons in the supposedly correlative Marwar supergroup.
So let me ask you a question, you've apparently concluded that zircon geochronology is incredibly flawed and the age of the earth is far younger than modern science so why do you need any data at all? Is your faith so weak that you require experimental verification of your biblical interpretation? Doesn't requiring scientific verification put science ahead of faith? I can send you a word document of the raw data, you can put it into a spreadsheet and abuse it however you want. However, since you need data to verify your faith, why not do the research yourself?
Cheers
Joe Meert
Edited by Joe Meert, : No reason given.

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