Volatile generation and release from continental large igneous provinces - Volcanism and Global Environmental Change - page 186

3 dolomite þ diopside ¼ 4 calcite þ 2 forsterite þ 2CO 2 (12.3)

dolomite ¼ periclase þ calcite þ CO 2 (12.4)

2CH 2 O ¼ CH 4 þ CO 2 (12.5)

fluid composition
is CO 2 -rich. Reaction (12.5) is a generalized way of describing devolatilization of
organic matter.
Evaporites with anhydrite and rock salt may generate SO 2 and HCl and, if
organic matter or petroleum is present, CH 4 and halocarbons such as methyl
chloride (CH 3 Cl) and methyl bromide (CH 3 Br) may also form. This was recently
con

Reaction (12.3) will produce olivine (forsterite) when the pore

rmed by experiments with natural rock salt from the Tunguska Basin in
East Siberia (Svensen et al ., 2009 ). The samples were heated to 275 Cinagas
chromatography mass spectrometer (Svensen et al ., 2009 ) without crushing,
to simulate contact metamorphism. The results show that at room temperature,
following crushing, the released gas contained butane, benzene, and sulfur
dioxide. No halocarbons were detected. When the samples were heated, the
concentrations of sulfur dioxide had increased signi cantly(upto130times),

and the concentration of other sulfur gases and hydrocarbons decreased. Interest-
ingly, halocarbons CH 3 Cl and CH 3 Br were identi ed in all heating runs.

Previous experiments have solely focussed on biological samples (e.g. Keppler
et al ., 2000 ). Even though the details about halocarbon formation during our
experiments remain poorly understood we attribute the formation of these mol-
ecules to reactions between hydrocarbon compounds within the fluid inclusions
and dissolved chloride from the rock salt.

12.5 Constraining gas generation in volcanic basins
Numerous approaches can be used for constraining gas generation in contact aureoles,
including thermodynamic calculations based on mineral-water equilibrium, rock
heating and pyrolysis, numerical simulations and estimates based on aureole data.
Here we present an approach utilizing geochemical analysis of aureole rocks.
Based on metamorphism of carbon-rich sediments, it is possible to estimate
carbon production potential in an intruded sedimentary basin. This method requires
the contact aureole volume (area A thickness h ) and the amount of carbon in

wt.% converted to CH 4 or CO 2 , F C . The total mass of carbon, W C , produced in
contact aureoles is:

ρ ,
where ρ is the rock density (2,400 kg/m 3 ). The area A is measured from seismic
or

¼ F C A h

W C

field data. The thickness h of the metamorphosed sedimentary layer can be