[meteorite-list] Ordovician Meteorites...was New or maybe oldQUESTION???

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Tue, 6 May 2008 01:40:37 -0500
Message-ID: <13ea01c8af44$18766b70$db45e146_at_ATARIENGINE>

Hi, Eric, List,

    I didn't explain that well.

    The Sr87/Sr86 ratio dates one of the following:

    a) the original formation date of the material,
like an undifferentiated meteorite.

    b) the date of a primary process like differentiation,
as the separation of mantle rock in the Earth from
the original protoplanet or the differentiation of a
large asteroid.

    c) the complete metamorphosis of a whole new
crustal rock, like a granite, from older rocks.

    I made one mistatement; Sr87 doesn't decay,
only Rb87. The half life of Rb87 is very, very long:
48.80 BILLION years, so the increase in Sr87
in a rock from decaying Rb87 is very small and
requires very sensitive measurement. The half life
is so long that the change in 488 million years is
only a dimishment of Rb87 by one part in 1000!

    Rb is more volatile than Sr. It can be driven out
by high and prolonged heat. The effect of an impact
could reset the Rb content, but not the Sr content,
which is very refractory. The shocked L chondrites
have their Rb/Sr ratios reset, but NOT their Sr87/Sr86
ratios, which correspond to their formation age.
Dating that shock to 465 +/-15 million years is done
by treating the shock as a "metamorphic" event.

    Since tektites have a respectable Rb content (100 to
200 ppm), they didn't get hot enough to be completely
transformed by any impact (in case you were thinking
that). And their Sr87/Sr86 ratio wouldn't have changed
a whit.

    The solar nebular value is calculated to be 0.698955
to 0.698985. After the Sun "lights," the ratios begin to
diverge as material nearer the Sun loses Rb faster. This
gives the material from which each planet forms a
different range of Sr87/Sr86 initial ratios, depending on
how much Rb is lost. Then, the differentiation into core,
mantle, crust, produce distinct ratios in each. Crustal
rocks (usually granites) are the newest and have the
highest ratios, but not as high as tektites.

    The Sr87/Sr86 values of tektites are very limited in
range: 0.7121 to 0.7223. You won't find these values
in the surface rocks of the Earth. That Sr87/Sr86 ratio
just does not correspond to any Earthly rocks.

    Here's a site that has a nice presentation of how the
ratios of terrestrial materials evolve. Note that nowhere
will you see any material with ratios as high as tektites
mentioned; the graphs don't go that high. There isn't
any such material on Earth... except tektites.
http://www.ig.uit.no/studier/GEO-3102/rbsr.swf
(The Flash presentation is nice, but they slipped
a decimal point on the half life...)

    Another excellent site:
http://www.fsteiger.com/radioact.html

    From the standpoint of Sr87/Sr86, the rocks of
a planet are essentially as old as the planet unless
some kind of fundamental geological process like
differentiation or metamorphosis has re-worked them.
Fresh lava from the mantle, for example, is a "new"
rock on the surface, but its ratio is just the same as
the mantle rock it came from: around 0.7040 --
it's as old as the planet. Well, as old as its mantle,
anyway.

    It's hard to find strontium data on tektites:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19640001430_1964001430.pdf
"Variation of Strontium Isotopes in Tektites,"
by C. C. Schnetzler and W. E. Pinson, Jr.,
Department of Geology and Geophysics, MIT.

    Yes, it's old, but the "newest" strontium data
on tektites is 25 years old; this is 20 years older.
They miscalculated the date because the value
of the half life of Rb87 was not well constrained;
they say 400-425 million and they're off by 50
million years. (They thought the half life was
<47 billion. Recent measurements say more like
50 billion. It's hard to measure, counting the atoms
as they fall off any process so slow.)

This report was ordered up by NASA because
they wanted to know if tektites came from the
Moon (a place they were going to)! Schnetzler
and Pinson pointed out the isochron and asked
if NASA thought the Moon was less than a half
billion years old? Although not asked to, they
evaluated the question of whether tektites could
be terrestrial in origin? Again, they pointed out
that the Sr87/Sr86 ratios could hardly be derived
from terrestrial materials.

    I said:

> They are all erosion products and, when mixed
> together, they have roughly similar ages to the very
> age suggested by the initial Sr87/Sr86 ratio of tektites

    Well, that too was a poor explanation. An "eroded"
product doesn't have it's Sr87/Sr86 ratio changed, nor
a sedimentary material, nor a melted one. What I meant
was that the geological "age" of the Earth's surface was
similar to the Sr87/Sr86 age of the tektites, NOT that
the mixed material at the Earth's surface would have that
Sr87/Sr86 age -- it wouldn't! It would be made up of
much older material.

    And in so doing, I misled you into saying:

> which indicates the Sr87/Sr86 ratio is the average of
> the terrestrial material that was melted.

    If you mixed and melted terrestrial materials, you would
have an age much older than tektites:
http://www.sciencemag.org/cgi/content/abstract/140/3572/1214-a
    "The abundance of radiogenic Sr87 relative to Sr86
at the time of crystallization has been determined for
45 rocks. The total range in the ratio Sr87/Sr86 is less
than 2 percent. Ratios for recent lavas range from
0.702 to 0.711. Oceanic basalts are closely grouped
at 0.703, whereas ratios for continental volcanic rocks
spread from 0.702 to 0.711. Among the volcanic
rocks, ranging from basalt to rhyolite, no correlation was
found between original ratio and rock type. Older mafic
and felsic rocks that include both plutonic and extrusive
types also cover this same range in original Sr87/Sr86 ratios;
however, there is a definite trend with geologic time.
Precambrian rocks give values as low as 0.700. The data
indicate that Sr87/Sr86 of the weathering crust has changed
1.1 percent in 3000 million years, while the ratio in the mantle
has changed no more than 0.5 percent."

    The slight decreases in Earthly values over long
geological periods is probably due to the fact that Sr is
very soluble in water. You can measure the Sr87/Sr86 ratio
of seawater and tell where the Sr is coming from. In times
of sea floor spreading, the Sr values are mantle-like. In
times of erosion of continental rocks, the Sr values are
younger, as the Sr is leached out of those youngest rocks.

    "Lower" numerical values correspond to older rocks;
"higher" numerical values correspond to younger materials.
Earth rocks are 0.702 to 0.711; tektites are 0.7121 to 0.7232.
These Sr87/Sr86 values of tektites are outside the terrestrial
range. Tektites cannot be formed from local surface materials,
nor any materials, of the Earth.

    Too tired to tackle the Hf/Sm system... but it's a mess too.
We'll just stick to one point per post.


Sterling K. Webb
---------------------------------------------------------------------------
----- Original Message -----
From: <star-bits at tx.rr.com>
To: "Meteorite List" <meteorite-list at meteoritecentral.com>
Sent: Monday, May 05, 2008 7:40 PM
Subject: Re: [meteorite-list] Ordovician Meteorites...was New or maybe
oldQUESTION???


---- "Sterling K. Webb" <sterling_k_webb at sbcglobal.net> wrote:

<Whoa, Eric! You're confusing two sets of dating. The dates you're talking
about are the K/Ar dates.>

Except that I never mentioned or implied K/Ar dating which does reset from
shock or melting unlike Rb/Sr.

<.) You get two pieces of information from the Rb/Sr isochron. One is an age
of igneous formation ("How old are these mountains?"), set by the zero point
of the isochron. The other is that you can determine the initial Sr87/Sr86
ratio of the material at the zero point of the isochron.>

Yes I agree. All the isotope dating schemes use isochrons and they provide
valid data if interpreted correctly.

< Tektites have a very tight specific cluster of initial Sr87/Sr86 ratios
which is not like any known terrestrial material. Does it tell us something
about the material of the impactor?>

Probably not. A major impact melts significantly more terrestrial material
than there is impactor material. So unless the impactor has a ratio that
is significantly different than terrestrial, the ratios change very little
from the average terrestrial ratio.

< Whether a tektite is formed depends on the impacted material... The
problem is that tektites have been widely suggested to derive from sandy
surficial materials, or from loess, or off-shore sediments -- you know the
list. They are all erosion products and, when mixed together, they have
roughly similar ages to the very age suggested by the initial Sr87/Sr86
ratio of tektites, just less precise, so the Hf/Sm dates are similar to the
Rb/Sr ones. >

Which indicates the Sr87/Sr86 ratio is the average of the terrestrial
material that was melted.

< Only small selected portions of the Earth's surface are very, very old,
and the surface is always littered with the more recent debris. It's cruder
data, but it fits the current opinion better.>

Which also fits with a young terrestrial age.

< So, why isn't the Sr87/86 value point of tektites better known? Well, it
strongly suggests that tektites are not formed from local surface materials
of the Earth, for one thing, but from a single material with a single unique
and non-terrestrial origin, and not one of the ones we know about or have
sampled yet. There simply is no explanation for that, not even a bad one.>

Unless the explanation is the Sr87/86 measures the average age of the
terrestrial melt material which seems like an explanation, even a good one,
to me.

< BTW, I never said nor even implied tektites formed from L chondrites; you
just jumped there.>

?Formed from L chondrites?, no you never said or implied that, nor did I.
You also never said that L chondrites were the impactors which caused the
formation of tektites however that seemed to be infered from the following:

< A one-hundred-fold increase in
meteorites, five ice ages in 100 million years, one of them
the worst in Earth history ("Snowball Earth"), massive
breakups of major bodies, the complete re-surfacing of
Venus (surface age 480 million years). And all tektites
have an original Rb/Sr melt date of 440-480 million years
ago. All just a coincidence, of course... >

All a coincidence? No. The average terrestrial melt/tektite Rb/Sr date?
Yep.

--
Eric Olson
7682 Firethorn Dr
Fayetteville, NC 28311
http://www.star-bits.com
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Received on Tue 06 May 2008 02:40:37 AM PDT