4
Peter Stille and Graham Shields
~_ ......... ___ m~ = eXt- I
~.~
( 6~Sr/NSr )i
A
8
|
I
I
I
I
I
I
I
~Rb / e~Sr
Fig. 1.I. Schematic Rb-Sr isochron diagram. Cogenetic mineral phases A and B formed at
the same time (t=0) and integrated the same initial (STSr/86Sr)i ratio but different quantities
of Rb and St. Since the time of crystallization, the radioactive decay of 8TRb has produced
radiogenic 87Sr leading to A' and B' (now). Both points define a straight line (isochron)
whose intercept with the 87Sr/O6Sr axi.~ corresponds to the initial Sr ratio. The slope of the
Hne (eAt-!) ~s equal to the time elapsed since the formation of mineral phases A and B.
This equation corresponds to a straight line of the general form :Y= B+X*m. Axes
Y and X represent 875r/86Sr and 87Rb/86Sr, respectively. The factor (e~a-1)
corresponds to the steepness or gradient of the straight line in Fig. 1.1. The
gradient is thus dependent on the time passed since the beginning of the
chronometer. At time "t=0", the gradient is zero. The older the system, the steeper
the straight line. Therefore, this gradient allows us to determine the age directly.
Equation (VII) can be reduced to the genera/equation for radioactive age dating:
t = ~/~ ln[('S7Sr/S6 Sr)'~ -(STSr/S6 Sr)i
)
+ I
(VIii)
If the inital strontium isotopic composition of the system is unknown, we can only
determine the age if there are several components of this system present, which
formed in equilibrium with each other and had different inital Rb/Sr ratios. If
these mineral components formed at the same time, then we can define a so-called
isochron, which would have been a straight, horizontal line at the time of
formation (isotopic equilibrium, isotopic homogenization). After time "t>O" a
straight line will be formed which is also termed an isochron but whose gradient
Peter Stille and Graham Shields
~_ ......... ___ m~ = eXt- I
~.~
( 6~Sr/NSr )i
A
8
|
I
I
I
I
I
I
I
~Rb / e~Sr
Fig. 1.I. Schematic Rb-Sr isochron diagram. Cogenetic mineral phases A and B formed at
the same time (t=0) and integrated the same initial (STSr/86Sr)i ratio but different quantities
of Rb and St. Since the time of crystallization, the radioactive decay of 8TRb has produced
radiogenic 87Sr leading to A' and B' (now). Both points define a straight line (isochron)
whose intercept with the 87Sr/O6Sr axi.~ corresponds to the initial Sr ratio. The slope of the
Hne (eAt-!) ~s equal to the time elapsed since the formation of mineral phases A and B.
This equation corresponds to a straight line of the general form :Y= B+X*m. Axes
Y and X represent 875r/86Sr and 87Rb/86Sr, respectively. The factor (e~a-1)
corresponds to the steepness or gradient of the straight line in Fig. 1.1. The
gradient is thus dependent on the time passed since the beginning of the
chronometer. At time "t=0", the gradient is zero. The older the system, the steeper
the straight line. Therefore, this gradient allows us to determine the age directly.
Equation (VII) can be reduced to the genera/equation for radioactive age dating:
t = ~/~ ln[('S7Sr/S6 Sr)'~ -(STSr/S6 Sr)i
)
+ I
(VIii)
If the inital strontium isotopic composition of the system is unknown, we can only
determine the age if there are several components of this system present, which
formed in equilibrium with each other and had different inital Rb/Sr ratios. If
these mineral components formed at the same time, then we can define a so-called
isochron, which would have been a straight, horizontal line at the time of
formation (isotopic equilibrium, isotopic homogenization). After time "t>O" a
straight line will be formed which is also termed an isochron but whose gradient