The origin of gases that caused the Permian–Triassic extinction - Volcanism and Global Environmental Change - page 159

extracted during its interaction with the lithosphere before the main phase of
magmatism. The new data on the melt inclusions in olivine from Ayan River
picrites fully agree with this prediction. Compared to inclusions in olivine from
earlier uncontaminated Gudchikhinsky picrites the Cl/K ratio of melt of Ayan
River picrites falls by 70%. The same low Cl/K ratio is recorded in later picritic
magmas from the Maymecha Kotuy province.

From the modeling and melt-inclusion data, we infer that the early magmas were
rich in Cl and would have liberated this phase at an early stage of the emplacement
of the Siberian province. The amounts of CO 2 and Cl emitted at this early stage
may have been 100 000

-

20 000 Gt respectively.
These huge amounts of isotopically light carbon from recycled altered oceanic
crust are quite enough to explain the carbon isotope excursion and signi

- 200 000 Gt and 10 000 -

cant

depletion of the ozone layer (Sobolev et al ., 2011 ). We thus associate the rst

extinction pulse identi ed by Song et al .( 2012 ) with the degassing of these

magmatic gases. These magmas contained low SO 2 contents and this species did
not contribute much to the early extinction event ( Figure 10.6 ).
The eruption of early magmas including Gudchikhinsky picrites, and a major
degassing event, was followed by eruption of the much more voluminous
main series of magmas. However, the estimate of mantle-sourced Cl released by
the main series of magmas is only around 2000 Gt. This number is three to four
times less than the estimate of Black et al . ( 2012 ) . The likely reason for this
difference is that our estimate does not include Cl assimilated in the crust, while
Black et al . ( 2012 ) use the average concentrations of contaminated and uncontam-
inated melts. In any case, the estimated Cl emissions are at least two to three times
lower than those predicted for the degassing before the main magmatic events.
On the other hand, the main series eruptions coincided with the emplacement of
the intrusive complex beneath the flood basalts and therefore with the release of
thermogenic gases from the metamorphic aureoles surrounding the intrusions. As
described by Svensen et al .( 2009 ), Pang et al .( 2012 ) and Iacono- Marziano et al .
( 2012 ), contact metamorphism of the sedimentary rocks released large quantities of
a toxic cocktail of gases; not only CO 2 and halocarbons from the carbonates, but
also SO 2 (when evaporites were assimilated), and reduced species such as CH 4 and
CO (when hydrocarbons or coal were assimilated). We thus associate Song et al . ' s
( 2012 ) second pulse of extinction with the release of these gases ( Figure 10.6 ).

10.5 Conclusions
1. The mantle source of magmas during the initial stages of the Siberian LIP
was dominated by pyroxenite formed from altered recycled oceanic crust with
an unusually high Cl content, a relatively low content of S and elevated