[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Dating Methods/redy to read explanation

Hello all,

Here is the text sent encoded by Kasia. One of the tables came apart a
little, but the rest seems intact.

Thanks Kasia.

Happy reading,

Martin




The age of the Solar system has been determined to be about 4.6 billion
years.  This age was derived from radioisotopic dating of meteorites and has
been reconfirmed by dating other solar system material (moon, mars, earth).
Radioactivity is a natural process through which the unstable isotopes of
certain elements spontaneously change into other, more stable isotopes.
Since the radioactive decay process is continuous, the ratio of radiogenic
(daughter) to radioactive (parent) isotopes  in minerals increases with time
and becomes a "clock" determining the age of the rock.  If we know the rate
of decay, and we can measure how many radiogenic atoms are now present in the
sample, then we can calculate how long the sample has been "making" the
daughter atoms, therefore the age of the sample.  This process, however, is
not easy and requires some assumptions to be used.  Firstly, the system is
assumed to be a closed one.  This means that no parent or daughter atoms are
lost or gained except by decay.  In meteorites this can be very easily
affected, since they land on the surface of the Earth where there are many
potential contaminates.  Secondly, the amount of  radiogenic isotope
originally present in the sample has to be known.  Estimating for this amount
is very important, as inaccurate estimates will affect the calculated age of
the specimen.  For example if the original amount of the daughter atoms was
in fact higher then estimated, the meteorite will appear older than it is
actually.  Another important assumption to be made is that the isotopic
composition of the sample is representative and accurate.  Dealing with
meteorites, we do not always have luxury of  working with large pieces, since
meteorites are rare and usually not very large themselves (and very
expensive!) so making sure that sampling is representative becomes
difficult.  As we can see dating meteorites is a science that requires a
large number of estimates to be made and so the age inferred may not be a
hundred per cent accurate.

Utilizing the four major types of ages considered for meteorites (formation
age, gas retention age, cosmic ray exposure age, and terrestrial age) we can
try to determine the rocks history.  The formation age is probably the most
important, because in agreement with its name it marks the time of the
formation of the meteoroids (crystallization of their minerals) when they
were still a part of their parent body.  Events like collisions are common
throughout the life of the parent bodies and can cause fracturing and
heating, this allows gas to escape.  Since the last degassing event, new gas
(39Ar, which is the daughter isotope of 39K) will begin to accumulate.
Through the assessment of its abundance scientists can calculate the gas
retention age.  Usually an impact is the cause of  the meteoroids separation
from its parent body.  The time of this event could be represented by the gas
retention age.  However, if the meteoroid has not been shocked sufficiently
to affect gas retention, this age might be the same as solidification age .
References: