Geoscience Reference
In-Depth Information
0.829
207
Pb/
206
Pb
Piton de la Fournaise volcano
(Réunion Island, Indian Ocean)
0.828
0.827
0.826
0.825
0.824
0.823
0
200
400
600
800
1000
1200
1400
1600
Cumulated days of eruption since November 4, 1975
Figure 6.4
207
Pb/
206
Pb ratios in recent lavas of the Piton de la Fournaise volcano (Réunion Island, Indian
Ocean). The rapid fluctuation of this ratio over a time scale of a small number of years shows that
the magmatic reservoir holds a volume of lava equivalent to just a few years of eruption. There is
therefore no magma chamber larger than a cubic kilometer beneath this volcano (after Vlastelic
nuclide
86
Sr) corresponds to their introduction into the ocean by rivers or into the man-
tle by subduction zones, while their “death” corresponds to sedimentation (ocean) and
extraction into mid-ocean ridges (mantle). Radioactivity corresponds to the death of a
radioactive nuclide (e.g.
87
Rb) and to the birth of a radiogenic isotope (e.g.
87
Sr). These
concepts will also be used to explain mixing in the mantle or the ocean at the end of this
chapter.
The concept of residence time requires that fluctuations in the input of an element of
duration shorter than its residence time do not show up in the output. It has very diverse
applications in oceanography and limnology as well as in magmatic petrology or man-
tle geochemistry. Let us take an example from the magmatic field. An erupting magma
reservoir that is continuously replenished can be thought of in the same way as a lake,
the upstream being the inflow of fresh magma, the downstream the eruptive activity, and
the cumulates the equivalent of sediments. In this context, the compatible elements that
are entrained by crystallization are reactive elements and their residence time in the reser-
voir is shorter than that of the incompatible and therefore inert elements. A reservoir like
this will therefore damp out all fluctuations in the concentration of an element whose
time characteristic is less than its residence time. The characteristic period of fluctuation
of the
207
Pb/
206
Pb ratio in the lava of the active volcano Piton de la Fournaise (on the
island of Réunion) is of a few years (
Fig. 6.4
). As Pb is incompatible (
i
β
≈
0), we thus
τ
H
of magma in the reservoir supply-
have an important constraint on the renewal time
ing the volcano. This time
τ
H
is of the order of a few years. Given the eruption rate
Q
,
the mass
M
×
τ
H
of the reservoir can be evaluated. These simple observations limit
the volume of the magma reservoir to less than one cubic kilometer. The concept of res-
idence time has greatly altered the way we see volcanic systems: until a few years ago,
most volcanoes were once thought to tap immense magma chambers, which, thanks to
such observations, now turn out to be much smaller systems of feeder dikes filled with
crystal mush.
=
Q
