The origin of gases that caused the Permian–Triassic extinction - Volcanism and Global Environmental Change - page 157

and contributed to mass extinctions. The rst paper (Svensen et al ., 2004 ) focused

on the relationship between the North Atlantic Magmatic Province and Eocene
global warming, but subsequent papers dealt with other LIPs, including the Siber-
ian Traps (Svensen et al ., 2007 , 2009 ). This work, summarized in Chapter 12 ,
deals mainly with processes that take place at relatively low temperatures, mainly
by the destabilization of hydrocarbons. Iacono-Marziano et al . ( 2012a ) subse-
quently showed that high-temperature interaction between magma and organic
matter (hydrocarbons or coal) can lead to a dramatic reduction of the magma redox
state, and signi cant release of reduced CO-dominated gas mixtures. The impact

on the environment of these toxic gases was explored in climate models developed
by Iacono-Marziano et al . ( 2012b ).
Ganino et al .( 2008 ) described how the amount of gas released by high-
temperature contact metamorphism could be quanti ed, using the aureole sur-

rounding the Panzhihua intrusion in China as an example. This intrusion is part
of the Emeishan LIP and its emplacement coincides with the end-Guadalupian
mass extinction, which occurred at 261 Ma, only about 10 Myr before the
Permian Triassic extinction. Ganino et al .( 2008 ) used two approaches to quantify

gas release: (1) the stoichiometry of metamorphic reactions and (2) differences
in the measured volatile contents of the metamorphosed rocks and their protoliths.
A large proportion of the Panzhihua aureole consists of brucite marble, pro-
duced by metamorphism of Proterozoic dolostones. Although Ganino et al .( 2008 )
assumed that the brucite formed through retrograde hydration of periclase, the
petrographic and geochemical observations summarized by Ganino et al .( 2013 )
demonstrate that it resulted from a prograde alteration of dolomite to brucite:

-

MgCa(CO 3 ) 2 (dolomite) þ H 2 O ! Mg(OH) 2 (brucite)

þ CaCO 3 (calcite) þ CO 2 .

Assuming an excess of water, which is justi ed by the presence of abundant

amphibole and biotite in metamorphosed ma c dykes intruding the aureole (Ganino

et al ., 2013 ), the proportion of CO 2 released by this reaction is 23.8% of the mass of
the original dolostone. The measured loss on ignition measured in brucite marbles is
about 38% (Ganino et al ., 2008 ), which, when the 12% water in brucite is subtracted,
gives 25% CO 2 , a value very similar to that calculated from the stoichiometry.
Pang et al .( 2012 ) applied the same approach to impure limestones, pelites
and evaporites in the aureoles surrounding intrusions of the Siberian LIP. The
calculated proportion of released CO 2 varied from negligible amounts in the pelitic
rocks to values like those of Ganino et al .( 2008 ) in the carbonates. Pang et al .
( 2012 ) also evaluated the amount of SO 2 that could potentially be released from
anhydrite in the evaporites. They noted that this mineral melts only at temperatures
well above those monitored from metamorphic assemblages in the aureoles and