Radioactivity as a measure of time in gems

Suppose I buy a gem stone (eg a geode or fossilized gem or piece of
fossilized wood) and want to know its age.

Right now I have some nice pieces of fossilized wood (SG 2.0 with
excellent concentric rings) from rock strata identified on geological
maps as metamorphic slate-like stone. If I sell the fossil wood
customers will ask how old it is.

Suppose there is radioactive material in the fossilized wood. The
rate of decay is a clock.

But what I do not understand is how Time 0 for that clock is
determined.

Why should the clock start when that material entered the wood?
Could it not start long before and migrate there in water or carried
by microbes etc?

Peter,

Good question! My training in the Navy taught me that if you know
what element you are looking at and what it’s current radioactivity
level is you can go back in time or project out into the future as to
what it’s radioactive level was or will be. Each element has a
different decay rate or half life.

I think carbon dating is used in many tests to determine age, but I
don’t know how it is determined as to when the carbon was formed to
begin with.

I will be watching this question myself.

Ken Moore
2901 Pleasant

The radioactive material is Carbon 14 ( 6 protons, 8 neutrons ) The
ratio of Carbon 14 to Carbon 12 in any living organism is constant,
because it gets in via joining with Oxygen and forming carbon
dioxide. Carbon dioxide is an integral part of living cycle and is
everywhere.

Once organism dies, the living cycle stops and carbon dioxide does
not enter the organism any longer. Since carbon 14 is radioactive, as
time goes by, the ratio of carbon 12 to carbon 14 increases due to
radioactive decay of carbon 14. Scientist determine age of fossils by
measuring this ratio. The Time 0 is in essence the moment of death.

Leonid Surpin

Hi Peter,

My understanding of radiocarbon dating is sketchy, but what I recall
goes like this:

Living things constantly metabolize carbon. (Being organic
critters…) Some very small percentage of the carbon is actually the
isotope, carbon 14. Apparently this is reasonably constant, and the
percentage level stays at equilibrium with the environment so long as
said critter is alive to continue carbon uptake.

As soon as whatever it was dies, they stop eating. (believe it or
not.) Which means that all the carbon 14 they’ve got at that moment
is all they’ll ever have. Which means the clock starts when they
stop eating. Rough dates can be worked out by calculating the
percentage of carbon 14 in the sample, versus whatever the
equilibrium percentage should have been, and then working
backwards, given carbon 14’s known half-life. (Not known by me, mind
you.)

(In double-checking myself, it turns out that the carbon-14 comes
via plants fixing atmospheric CO2, of which there is a steady
percentage which is C-14. (Steady, because it’s generated by cosmic
ray bombardment of the atmosphere.)) So it’s not eating any old
carbon that does it, it’s plant material, or things that have
recently eaten plant materials.) C-14 having a half life of 5730
years, a year or two between the plant fixing the carbon, and the cow
eating the plant isn’t enough to show up in the results.)

This is why it only works with once-living items. Gems, having never
been great consumers of carbon, don’t date with radiocarbon dating.
(even diamonds.)

Fossilized wood just might date. Dunno. I guess a lot would depend on
how old the sample is, and how precise the lab’s gear is. From my
double checking, it seems that radiocarbon dating is only effective
for samples from within the past 60,000 years or so.

For whatever that’s worth,
Brian.

``````This is why it only works with once-living items. Gems, having
never been great consumers of carbon, don't date with radiocarbon
dating. (even diamonds.)
``````

While this is true, carbon analysis is still useful in dating gems.
It is known that ration of carbon 12 to carbon 13 is indicative of
where substance has originated. Some diamonds are formed from carbon
deep inside Earth mantle. There are other diamonds which were formed
from carbon derived from plate tectonics ( subduction ). Then there
are diamonds formed upon meteorites impact. All of them will have
different carbon 12 to carbon 13 ratios.

Other gems contain two and three phase inclusion. Hydro-carbons are
frequently encountered in liquid phase. These are also useful in gems
age determination.

Leonid Surpin

Thank you. Unfortunately the college system often inhibits good,
honest questions because the student is afraid of being seen as a
"dummy".

I bought my college geology text in high school for different
reasons. My hs friends were from a family whose dad was one of
Ontario’s most eminent geologists and his wife was my hs chem teacher
(later my TA in college chemistry). Also I had discovered two species
of shell fish replacement fossils in the sand stones about 30 miles
SW of Ottawa. The replacing mineral was quartz and the crystals were
small but visible.

ROCK CLOCK

The text is written by a team of ivy league geologists and there is
a section titled “The Radioactive Clock” and “The Scale of Time”. I
will quote it directly and ask the questions I should have asked 40
years ago. The worst that can happen now is that somebody may take
away my rocking chair at the eventide home and declare me “off my
rocker” for a while.

I have sampled, stratum by stratum from 47 strata of a rockface at
about 1,000 foot elevation on a mountain side. I will submit these
samples to the local historic society when I know more. Map 92GE of
the GSC says the rocks of this mountain are Jurassic-Cretaceous so I
guess that means about 100 mya to 160 mya. I have some questionable
fossils/pseudofossils but the fossilized wood is a for sure. I have a
palm-sized piece here and a dozen others in my sampling right here.

Here is what Longwell (Yale) et al say in their text, using uranium
to illustrate the general principle:

Radioactive “decay” proceeds as the U clock runs down.from Time 0
and atoms are lost as they convert to Pb which “changes the atom of
uranium to something different”.

Thus “the ratio of lead to uranium should give the age of the
mineral”.

They caution about the MIGRATION of minerals which invalidates the
clock, and they give the most obvious example of surface weathering.

Here then are the problems I have with being sure this U clock or

(1) The Transformation Problem:

When was the quartz in those Ottawa replacement fossils created? It
migrated into the sandstone and the shells of those shell fish we
assume via rock-borne water. Other than cracks in rocks as conduits
for water, rocks are like sponges I have learned. What the gemologist
needs to know in selling the fossils is when they arrived in the
shells.

______ was transformed into U to start the clock. What is _______?
My rock samples typically contain the whole Periodic Table or close
to it. Were all those elements created at the same time? Did they all
turn to the constituent minerals at the same time? Do we assume that
ALL of the rock clocks in that rock formation with radioactive decay
had the same Time 0? Did the Si and O in the quartz have the same
Time 0? What was Time 0 when they bonded?

(2 The Migration Problem

Parallel this with U. When was the U created? U and other
radioactive mineralization are found in my samples of the 47 strata.
I have done two assays so far. Was U created first in rock A and then
migrated to rock B? Likewise for the Pb of decay. Both can migrate in
and out of rocks given millions and billions of years. How valid can
the ratio then be?

I have run a number of porosity tests on these slate-like strata.
Some samples are so porous (like the fossil wood) that you see many
tiny bubbles form in water and you can hear the bubbling. Minerals
migrate in and out because the strata are profoundly cracked,
fissured, faulted and some narrow strata are crumbled and powdered.
Imagine how much mineralization has been carried in and out over 100
million years.

Also I found a very crumbly and powdery stratum which contains a
large, fairly recent tree root. How recent? When did it get in there?
Maybe it is only.0001% fossilized but it surely has started. Do we
assume that the previous fossil sample is from 100+ mya? In 100 myf,
how will historical geologists know which sample of wood got
fossilized first?

Clocks don’t lie but they have to be read honestly.

Essentially uranium has a number of different isotopes that have
different atomic numbers, masses and decay rates. The dating of
rocks using radioisotopes works by looking at the ratio of these to
each other and to lead, the final product of the radioactive decay.
If we say that U240 has has a decay time of 4 billion years this
means that in that time a lump of U240 will have converted entirely
to lead and will have no radioactivity. A certain mass of U240 will
emit a given number of radioactive particles as it decays to
U239-237-235 etc. This is measurable because different particles are
emitted by the decay. Some are electrons, some protons and some
x-rays as well as whole new atoms of helium. Each step of decay has
its own characteristic energy form and energy. It is the amount of
radioactivity and the type of particle emitted that allows us to know
what step of the decay of the U atom we are measuring. The ratio
between these radiations and the amount of U atoms present give us
the age of the source. So, if the all of earth was created at the
same time the ratio of the U isotopes would be the same everywhere.
When you impart enegy to the U mass then that will change the level
of energy emitted afterwards so roughly speaking the U present in a
rock with have a level of radioactivity and isotope ratios
comensurate with its last energetic activity. Thus, it is easy to
date igneous and metamorphic rocks by radioisotope analysis. When it
comes to sedimentary rocks it is a different thing because the
radioisotopes have come from somewhere else by essentially a
mechanical means. The U and PB are not soluble and are superstable so
again can be used for dating rocks if you know the geologic history
or can compare them to others by using the fossil record.
Sedimentary rocks will contain minerals that have other elements that
are useful in the 0-250 million year range such as Sr/Rb ratio Sr is
found in carbonates and Sr90 is a stable long half life isotope. Rb
is normally found in soluble salts so is useful for sedimentary rock
diagenesis dating (time of turning loose material into rock).

Fossils are formed by replacement of minerals by soluble (at the
time) salts which preserves the external form but not the original
chemical or crystal structure. Again, this meand that radioisotopes
can be used to date the fossilas and hence the strata they are in
because they havent moved.Si an O dont have isotopes that are useful
for dating things. Carbon is only usful for things less than 5000
years old and other elements dont have the chemical stability to be
useful (Cl for example). Murchison got it right back in 1800 by
looking at fossils and pillow lavas and that was when the rest of the
world thought the earth was under 6000 years old.

Study the fossil record. Redeposited fossils are rare and give
themselves away in other ways.

I’ve been holding back on replying to this one, as it’s not
something I have personal expertise in and I was hoping that someone
with practical experience would pipe up first.

Firstly, my understanding is that radiocarbon dating is extremely
expensive (a few \$1000 per sample), and though it has its uses it
would probably be prohibitive if all you wanted was to tell a
customer how old their fossil was.

There are many other forms of radioactive dating - such as looking
at lead/uranium ratios in zircon. I don’t know which (if any) would
be well suited to your purpose. And I doubt they would be much

I would have thought the best way to find out how old the fossil
wood was, would be to work by the age of the strata it was recovered
from, which should probably be known from its depth; patterns in
surrounding strata; surrounding fossils and pollen grains (could
indicate whether the wood was from an ice age or warmer period)
etc…

Kit

Thank you for the reply. I think I came to a similar conclusion that
the U method would work for dating deep, primary igneous rocks
subject to your qualification that all other neighbouring elements in
the deep rock would have been formed at the same time.

When you say U and PB in sedimentary rocks are “superstable” I think
you mean “not migratory”. That being so I must respectfully disagree.
I have several dozen assays of U and Pb before me. All contain enough
U and Pb to be calculated by the assayer. Some are from fine loose
sediments. The highest U value is in a fine loose sediment close to a
major granitic outcropping so I think that may be its source. It
"migrated" in water, air etc. to the new site. I was extra interested
in this one because handling those samples caused my finger tips to
swell as has not happened in handling thousands of other samples. My
MD said he could not tell if the U caused it but the values are at
the level of those granite counter tops which some health experts
have warned about. Two years later and the swelling is less but still
there.

My guess is that if I were to assay these samples of petrified wood
I would get the entire Periodic Table or close to it. All of the
elements migrate. Indeed, that is part of the economic value of this
deposit. It contains many micronutrients with high bioavailability
due to the fact that they were muds, sands, dusts, soils and clays
when dinosaurs and all sorts of other strange creatures were walking
on them and growing in them. The Th values are also high in at least
one stratum and the Longwell et al text says “The chief radioactive
elements are uranium and thorium”.

In conclusion I have to think that because radioactive elements and
their decay products migrate so readily and given that they have
millions and billions of years in which to do so, the rock clock can
only be valid for deep, primary igneous rock.

``````Essentially uranium has a number of different isotopes that have
different atomic numbers, masses and decay rates. The dating of
rocks using radioisotopes works by looking at the ratio of these
to each other and to lead, the final product of the radioactive
decay. If we say that U240 has has a decay time of 4 billion years
this means that in that time a lump of U240 will have converted
``````

That’s not quite correct. Normally we talk about half-lives rather
than ‘decay times’. After a single half life, half the uranium atoms
will have decayed; after two half lives, three quarters will have
decayed (the original half, then half of the remaining half)… and
so on. However, assuming there are a large number of uranium atoms in
our sample, it will take practically forever for the uranium to have
changed entirely.

``````A certain mass of U240 will emit a given number of radioactive
particles as it decays to U239-237-235 etc. This is measurable
because different particles are emitted by the decay. Some are
electrons, some protons and some x-rays as well as whole new atoms
of helium. Each step of decay has its own characteristic energy
form and energy. It is the amount of radioactivity and the type of
particle emitted that allows us to know what step of the decay of
the U atom we are measuring. The ratio between these radiations and
the amount of U atoms present give us the age of the source.
``````

You can also put the substance in a mass spectrometer and work out
the relative proportions of the various isotopes.

``````So, if the all of earth was created at the same time the ratio of
the U isotopes would be the same everywhere. When you impart enegy
to the U mass then that will change the level of energy emitted
afterwards so roughly speaking the U present in a rock with have a
level of radioactivity and isotope ratios comensurate with its last
energetic activity.
``````

The normal method of uranium dating depends on the fact that uranium
is readily incorporated into of the crystal lattice of zircon as the
formed from the decay of uranium it remains trapped within the
crystal. Therefore it’s possible to use the uranium:lead ratio to
date these crystals within the rock.

``````Fossils are formed by replacement of minerals by soluble (at the
time) salts which preserves the external form but not the original
chemical or crystal structure. Again, this meand that
radioisotopes can be used to date the fossilas and hence the strata
they are in because they havent moved.Si an O dont have isotopes
that are useful for dating things. Carbon is only usful for things
less than 5000 years old
``````

Actually… Considerably older than that, because of the difference
between ‘half-life’ and ‘decay time’. Carbon dating can deal with
objects up to about 50,000 years or so despite the half life around
6k years.

Right,

superstable means that it doesnt alter or weather easily to nother
mineral. Quartz will always be quartz, it doesnt weather to
something else. Feldspar, for example eathers to a number of
minerals, kaolinite and sericite being amongst the most common.
Uranium Oxide stays as that in rocks and soils. This is not the same
as transportation, which is why most sedimentary rocks exist.

The critical thing about U/Th radiocarbon dating is the ratio of one
to another, it doesnt matter a jot how much of it you have, that
doesnt tell you anything useful as far as age goes. Assaying your
samples for elements wont tell you anything useful about its
petrogenesis. Calcium carbonate is just that but there is a big
difference between limescale in your kettle, marble, a fossil and a
giant stalactite in a cave. They are all formed by different
processes and tell very different srories if you can read them.

Finally I said that radioisotope dating works for telling the time
of its last high energy activity. All of matter in the universe is
the same age, some of it has been recycled more often than other
bits. Your sedimentaries are (and have been since they became rocks)
at their lowest activity and it is a certainty that the only elements
present that are active are the least interesting either
commercially or analytically.

I have a collection of coins, some are easy to date because they
have a date on them. Others are possible to date easily because they
belong to a series of kings or emperors whose history and sequence is
known. If you dont know the history then you are lost as they are all
made of the same metals. The fossil and stratigraphic record is your
history, not the elements.

Nick Royall