New Method for Dating Sedimentary Rocks
Keith Littleton
interested in geochronology. It is:
Neal J. MacNaughton, B. Rassmussen, and I. R. Fletcher.
(1999) SHRIMP uranium-lead dating of diagenetic xenotime
in siliciclastic sedimentary rocks. Science vol. 285. pp. 78-80.
(2 July 1999)
A short section from the article reads:
"Diagenetic xenotime is common in siliciclastic sedimentary
rocks, where it starts to form on detrital zircon shortly
after sediment deposition. It is possible to estimate the
age of sedimentary rocks by in situ uranium-lead analysis
of that xenotime. Two Proterozoic sandstone units from
northwestern Australia, previously constrained to the
age interval of 1790 to 750 million years ago, have
diagenetic xenotime ages of 1704 +/- 7 and 1704+/- 14
million years ago. This method has potential for dating
sedimentary sequences of all ages but should be
especially valuable for refining the Precambrian time
scale."
The xenotime crystals selectively incorporate uranium
into its structure while reject lead as it forms. Thus,
all of the lead found within modern xenotime crystals is
daughter product. This will be a boon to anybody
interested in dating Precambrian sedimentary rocks.
Keith Littleton
litt...@vnet.net
New Orleans, LA
"No man's life, liberty, or property are safe while the
legislature is in session."
- Will Rogers
Jacques Jedwab
<litt...@katie.vnet.net> wrote:
> There is an article that should be of interest to anybody
> interested in geochronology. It is:
>
> Neal J. MacNaughton, B. Rassmussen, and I. R. Fletcher.
> (1999) SHRIMP uranium-lead dating of diagenetic xenotime
> in siliciclastic sedimentary rocks. Science vol. 285. pp. 78-80.
> (2 July 1999)
>
> A short section from the article reads:
>
> "Diagenetic xenotime is common in siliciclastic sedimentary
> rocks ........ This method has potential for dating
> sedimentary sequences of all ages but should be
> especially valuable for refining the Precambrian time
> scale."
>
> The xenotime crystals selectively incorporate uranium
> into its structure while reject lead as it forms. Thus,
> all of the lead found within modern xenotime crystals is
> daughter product. This will be a boon to anybody
> interested in dating Precambrian sedimentary rocks.
>
> Keith Littleton
> litt...@vnet.net
> New Orleans, LA
>
............................
I have indeed observed the presence of high amounts of sedimentary
xenotime in Late Precambrian sedimentary rocks, as examplified by the Roan
formations (Série des Mines) in South West Katanga. The mineralogy was
made under the SEM/EMP.
The concomitant presence/formation of secondary xenotime, and its intimate
association with several other species, potentially containing Pb
(descloizite, plumbojarosite, Pb-REE-phosphates and -sulphophosphates,
like florencite/ gorceixite/goyazite) should be considered. Primary
xenotime as well developed crystals, which is also present, would also be
a difficulty for the proposed method. Other hydrated REE-phosphates, like
churchite, are also present, and not forcibly epitactically deposited on
zircon.
I have no model to date to explain the huge concentrations and variety of
finely dispersed REE-species in the silicified dolostones from Katanga,
and my vision is probably obscured by this mineralogical variety. But
Katanga is also exceptional in many respects, including its geochronology.
J.J.
Steven Kline
Keith Littleton <litt...@katie.vnet.net> wrote in message
news:LBch3.65$ko4....@ralph.vnet.net...
> There is an article that should be of interest to anybody
> interested in geochronology. It is:
>
> Neal J. MacNaughton, B. Rassmussen, and I. R. Fletcher.
> (1999) SHRIMP uranium-lead dating of diagenetic xenotime
> in siliciclastic sedimentary rocks. Science vol. 285. pp. 78-80.
> (2 July 1999)
>
> A short section from the article reads:
>
> "Diagenetic xenotime is common in siliciclastic sedimentary
> rocks, where it starts to form on detrital zircon shortly
> after sediment deposition. It is possible to estimate the
> age of sedimentary rocks by in situ uranium-lead analysis
> of that xenotime. Two Proterozoic sandstone units from
> northwestern Australia, previously constrained to the
> age interval of 1790 to 750 million years ago, have
> diagenetic xenotime ages of 1704 +/- 7 and 1704+/- 14
> million years ago. This method has potential for dating
> sedimentary sequences of all ages but should be
> especially valuable for refining the Precambrian time
> scale."
>
> The xenotime crystals selectively incorporate uranium
> into its structure while reject lead as it forms. Thus,
> all of the lead found within modern xenotime crystals is
> daughter product. This will be a boon to anybody
> interested in dating Precambrian sedimentary rocks.
>
> Keith Littleton
> litt...@vnet.net
> New Orleans, LA
>
mik...@westworld.com
<klin...@earthlink.net>'s laid this one on us:
>does this apply to dating rock art as well?
If the art was created durring the authogenisis of the sediments that
formed the stone that the art appears on.
I think xenotime must be one of these low temperature low pressure
mineral that forms in deep, anoxic basins, perhaps formed during early
diagenesis.
I'm not sure though..
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Today or Tomorrow??TOMATO/TOMOTTO!!
Jimmy Haigh
In article <7m6i1k$7h2$1...@ash.prod.itd.earthlink.net>, Steven Kline
<klin...@earthlink.net> writes
>does this apply to dating rock art as well?
>
--
Jimmy Haigh
Jo Schaper
<ji...@jimmy-haigh.demon.co.uk> wrote:
>Son't know of much Precambrian rock art though.
>
>In article <7m6i1k$7h2$1...@ash.prod.itd.earthlink.net>, Steven Kline
><klin...@earthlink.net> writes
>>does this apply to dating rock art as well?
What about Precambrian rock music? Bum-bum--bum-