Isotopes as tracers of magmatic sources
I. Rb-Sr isotopes and evolution in course of time
A. Basic equation of the Rb-Sr system
The Rb-Sr (and Sm-Nd) system is somehow more complicated
than the U-Pb (on zircon) system, because real rocks and minerals always
contain some of the daughter nuclide (D, 87Sr in this case). This
problem is overcome by the use of the “isochron” technique that involves the
non-radiogenic isotope of Sr, 86Sr (see G214).
87Rb → 87Sr (l= 1.42 10-11
yr-1)
86Sr is stable.
Establish the basic
isochron equation, which is a relation between 87Sr/86Sr,
(87Sr/86Sr)0 and 87Rb/86Sr
– in yesterday’s notation, it’s a relation involving N, D and D0.
This equation can be used in several ways:
-
By building an isochron diagram
(as you did last year in G214), i.e. 87Sr/86Sr = f (87Rb/86Sr).
In this diagram, a suite of cogenetic samples plot along a line, whose slope is
a function of the time.
-
By building “isotopic evolution
diagrams”, 87Sr/86Sr = f(t). In this diagram, a sample
evolves along a line of slope = 87Rb/86Sr.
B. Using isotopic evolution diagrams
1. Forward evolution
The primitive mantle has the following isotopic
characteristics: its 87Rb/86Sr ratio is 0.027, and its
original (at T=4.5 Ga) 87Sr/86Sr value was 0.699.
Draw the line of
evolution of this mantle (“CHondritic Uniform Mantle”, or CHUR) from past to
present.
At time T = 3.65 Ga (that’s yesterday’s sample…), the mantle
melted and produced a magma which eventually cooled into a rock of 87Rb/86Sr
= 0.15.
Draw its line of
evolution. What is the present-day 87Sr/86Sr of this
sample?
Now consider a sediment formed at 3.2 Ga from this rock,
with a 87Rb/86Sr of 0.25.
Draw its line of
evolution.
2. From present to past
Now plot on your
previous diagram a granitic sample with present-day values as follows, and draw
its evolution line back into the past.
87Rb/86Sr
|
87Sr/86Sr
|
0.35
|
0.7145
|
This rock formed at 2.0 Ga.
- What was its 87Sr/86Sr
at this time? (i.e., original Sr value).
- Among the 3 rocks plotted above, what
is its most likely source (i.e., the rock that melted to form the granitic
magma)
- If that rock was a pure product from
the mantle, what would be its age?
This “pseudo-age” is often referred to as a “TCHUR”.
Note that, as rock “evolves” by successive melting or
erosion event, they “move” to higher 87Rb/86Sr ratios –
i.e. to rock that are richer in Rb. This is because Rb is, in general, a more
incompatible elements than Sr and therefore it is more readily concentrated in
the melts.
II. Various isotopic tracers
A. Sm-Nd system
The isochron method (and, consequently, the use of initial
ratios) can be used for many other systems, e.g.
144Nd is stable; the ratio 143Nd/144Nd
can be used to interpret rock sources.
This ratio has a very restricted range of variation;
therefore, the “e” notation is commonly
used. It works just like the d for O
isotopes (G214), by calculating the deviation from a standard, except that an e unit is 10-4, whereas a d unit is 10-3 difference with the
standard. The standard for Nd isotopes is commonly the “CHUR”, or CHondritic
Uniform Reservoir:
Typical variations for eNd
are between -10 and +10.
Note that, for Sm-Nd system, more “evolved” rocks have lower
147Sm/144Nd values than more “primitive” rocks: in other
terms, they evolve “under” the CHUR line instead of above as previously.
- Plot a (quantitative) Sm-Nd evolution
diagram for the mantle, and for a more evolved sample (as we did above).
- Indicate on the graph what eNd represents.
B. Pb isotopic systems
238U → (…) → 206Pb (l = 1.5512 10-10 yr-1)
235U → (…) → 207Pb (l = 9.8485 10-10 yr-1)
(yes, the same systems we looked at yesterday –but they’re
not used in the same way here ! And yes, it is possible to get isochron ages
from any of the two systems, without needing the Concordia method).
204Pb is stable in both case; 206Pb/204Pb
and 207Pb/204Pb are therefore used as tracers.
232Th → (…) → 208Pb (l = 4.4975 10-11 yr-1)
204Pb is stable again; 208Pb/204Pb
is yet another tracer.
C.
Other systems
176Lu →176Hf (l = 1.94 10-11
yr-1)
(178Hf stable, 176Hf/178Hf
or eHf)
D. Isotopic characteristic of Earth’s reservoirs
Empirically determined. Different part of the Earth appear
to have different characteristics. Two example:
-
Mantle/crust difference. Clearly
seen using Sr-Nd diagrams.
-
Differences within the mantle
itself, reflected in the chemistry of erupted lava. Mostly seen with the
various Pb isotopes. It appears that there is a geographic logic behind this
(why??)
