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40Ar

Argon-40 (40Ar) is a stable isotope of the noble gas argon (Ar). It has 18 protons and 22 neutrons, giving a mass number of 40. It is the most abundant isotope of natural argon, accounting for about 99.6% of argon found in nature.

In natural argon, the isotopic composition is approximately 36Ar 0.34%, 38Ar 0.06%, and 40Ar 99.6%. Argon makes

Origin and production: The primary source of radiogenic 40Ar in the Earth is the decay of potassium-40

Applications: 40Ar is central to potassium-argon dating and argon-argon dating used in geology and archaeology. In

See also: 40K, 39Ar, 36Ar, 38Ar.

up
about
0.93%
of
the
Earth's
atmosphere
by
volume,
and
nearly
all
atmospheric
argon
is
40Ar.
The
high
abundance
of
40Ar
in
argon
stems
from
its
stability
and
from
its
formation
through
natural
processes
over
geological
timescales.
(40K)
to
40Ar.
As
rocks
weather
and
degas,
radiogenic
40Ar
is
released
and
accumulates
in
gas
reservoirs
and
mineral
inclusions,
contributing
to
the
argon
inventory
observed
in
geology
and
atmospheric
samples.
40Ar/39Ar
dating,
a
sample
is
irradiated
to
produce
a
known
amount
of
39Ar,
allowing
measurement
of
the
40Ar*/39Ar
ratio
to
determine
ages
of
volcanic
rocks
and
minerals.
The
method
provides
precise
ages
for
events
ranging
from
millions
to
hundreds
of
millions
of
years
and
is
widely
used
in
geochronology
and
thermochronology.