From: cmhall@umich.edu (Chris M. Hall)
Subject: Re: Fission-track dating .. help!!??
Date: Dec 20 1995
Newsgroups: sci.geo.geology

In article <4b7jms$jba$1@mhadf.production.compuserve.com> Griff Phillips <100533.3211@CompuServe.COM> writes:
>From: Griff Phillips <100533.3211@CompuServe.COM>
>Subject: Fission-track dating .. help!!??
>Date: 20 Dec 1995 00:00:28 GMT

>Hi there... can anybody tell me some basics about this mysterious 
>process called "fission-track dating" ? I'm not a geologist; my 
>degree is in Electronics, so I have some background in college 
>physics but not a whole lot. Basically if someone could answer 
>the following questions I'd be very grateful. It's for a 
>(vaguely) science-related publishing project I'm working on part 
>time. I'd look it up in a textbook, but my local library doesn't 
>have any.


I'll take a swing at this. Try to track down a book called Isotope Geology by 
Gunter Faure. This has info on all the widely used techniques.

>1) What are the basic principles behind fission-track ?

Natural uranium comes in 2 main flavours, 235U and 238U. Both alpha decay via 
long paths to Pb and this is the basis of U-Pb dating. However, 238U also 
spontaneously fissions (splits into 2 roughly equal fragments) with a very 
long half life. The recoiling fission fragments cause crytal damage in the 
mineral that hosts them, and this damage can remain unhealed for millions of 
years. By cutting, polishing and acid etching the sample, it's possible to 
open up these fission tracks to the point where they are visible with an 
optical microscope. Count the tracks per unit area; the more tracks the older 
the sample.

>2) What equipment do you need to carry out fission-track ? 

Not much. Some simple chemicals, some standard mineral samples, maybe some 
muscovite for the external detector method (look this one up), access to a 
nuclear reactor, and a high quality gelogical microscope. All and all, it's 
the cheapest dating method in terms of initial investment.

The reactor is needed because you need to calibrate your sample's U 
concentration, which is usually done by irradiating part of it with neutrons, 
causing 235U to fission and produce artificial (as opposed to natural) tracks. 
The age is actually derived from the ratio of natural to induced tracks.

>3) Is it expensive ? Dangerous ? Fun ? Profitable ? Unusual ?

Expensive: no, not compared to anything else. The only other low cost methods 
are K-Ar with used equipment and 14C using beta counting.

Dangerous: not really. Just need to be a little careful around the acids and 
the low level radioactivity you produce. And there's the eye strain from 
counting tracks :)

Fun: don't know, never did it. I've heard from friends who have that it is 
pretty tedious.

Profitable? Doubt it.

Unusual? It's a pretty standard technique, but not as widely used as Ar, Sr or 
Pb based methods.

>4) What's the most practical use of fission-track ?

It's useful for relatively young samples and in cases where you want to work 
out the timing of tectonic events.

>5) What are its main limitations ?

The tracks anneal at relatively low temperatures, which will tend to "reset 
the clock". This however can be used to advantage if you want to date low T 
events.

>OK, enough questions. Feel free to redirect me to a more 
>appropriate site if you know of one. In the meantime, if you need 
>to know anything about IBM S/370 Assembler language, don't 
>hesitate to ask...

>Cheers - Griff (England) 

Haven't used a 370 in over 20 years. How are you with OS/2?

Chris Hall
cmhall@umich.edu

From: fcrary@rintintin.Colorado.EDU (Frank Crary)
Newsgroups: sci.space.science
Subject: Re: 3 ages of meteorites
Date: 15 Aug 1996 16:01:29 GMT

In article <4utff9$p5@news.fsu.edu>,
Joseph Cain <cain@quartz.gly.fsu.edu> wrote:
>>   ALH84001 hit Antarctica 13,000 years
>>ago, 
>>  The rock came off Mars' surface a couple of billion
>>years ago.  It landed 13,000 years ago. 

Actually, it left the surface of Mars about 15 million years ago.

>Does anyone have a thumbnail summary of how the three ages were
>determined? 
>i.e.	age of formation of the rock from magma, 
>	time in space       (I understand that this has to do with cosmic ray
>				exposure but I am fuzzy on the details)
>	time since landing on Earth

I'm not sure about the time since landing on Earth, but the time
in space if from cosmic ray exposure. Basically, when a cosmic
ray goes through a rock, it leaves a trail of altered minerals.
It's fairly easy to count the density of such trails and, from
an estimate of the cosmic ray flux, get a date. As for the
age of formation, as well as the age of the carbonates and
the date of shock events, that's all from isotope dating.
There are several radioactives that decay into convenient
elements. That is, elements that are removed from the rock
when something happens to it. That "something" depends on
the radioactive, which lets you use different radioactives 
to date different events (e.g. the last time the rock was
melted, the last time it was shocked, etc.) Basically, you
know that, just after the last event, the abundance of the 
decay product was zero. By measuring the current abundance of
the radioactive and the decay product, and knowing the 
half-life of the radioactive, you can get the age of the 
event. (Sometimes it's more complex, i.e. when the decay
product is itself radioactive, but the basic idea is still
similar.)

                                                      Frank Crary
                                                      CU Boulder