Environmental Engineering Reference
In-Depth Information
rst
model we assume that CO
2
and HCl are fully extracted from the plume if the
temperature approaches that of the carbonatite solidus (Dasgupta and Hirschmann,
2007
). This model gives an upper bound for melt mobility, assuming that
melts produced by an infinitely low degree of melting can move out of the plume.
In the second model we assume that CO
2
and HCl are fully extracted from both
peridotitic and pyroxenitic components only if 1% melting is achieved. This model
gives a lower boundary for melt mobility, assuming that only 1% carbonate
We consider two end-member models for the extraction of volatiles. In the
-
silicate melts can move out of the plume.
According to the thermomechanical model (Sobolev
et al
.,
2011
) the CO
2
,Cl
and S in the plume are fully extracted during its interaction with the lithosphere,
and a major part is extracted before the main phase of magmatism. The total
amount of released volatiles is proportional to the volume of the plume, which
is poorly constrained. Nevertheless, using a 2.5% scaling between the amount of
magma intruded into the crust and plume volume (Sobolev
et al
.,
2011
), and a total
volume of Siberian magmas in the crust (including deep portions of the crust) of
4 to 8 million km
3
, we estimate the volume of the Siberian plume to be 160 to 320
million km
3
. This volume is equivalent to a sphere with a radius of 340
420 km.
The mass of extracted CO
2
for such a plume, which comes mostly from the
recycled component of the plume, is more than 100 000
-
-
200 000 Gt. We also
estimate that such a plume generates 10 000
-
20 000 Gt of Cl and about the same
amount of S.
The ratio of Cl/K in uncontaminated melts could be used as a proxy of mantle
source degassing. Reported here are data for Ayan River picrites (middle of the
Norilsk section) and alkaline picrites and meimechites of the Maymecha
-
Kotuy
province (latest stage of Siberian LIP), which indicate almost 70% degassing of the
mantle source since the beginning of the Norilsk section (the Gudchikhinsky suite).
This is consistent with the major conclusion of the model explained above
that
the majority of volatile components from the plume should have been extracted
before the main magmatic stage. This also allows us to estimate chlorine emission
from the lavas (excluding crustal volatiles) as a function of their potassium content.
For an average K
2
O between 0.5 and 1.0 wt% (Fedorenko
et al
.,
1996
) and
Cl/K
2
O
-
10
6
km
3
of Siberian
¼
0.03 (
Figure 10.4
)a4
flood volcanics contains
1500
3000 Gt of Cl. This estimate does not include Cl from crustal sources and
thus could be considered as a minimum amount.
-
10.3 Thermogenic gases
Svensen
et al
.(
2004
) advanced the hypothesis that heating of sedimentary rocks
adjacent to intrusions in LIPs released the gases that changed global climate
