Volatile generation and release from continental large igneous provinces - Volcanism and Global Environmental Change - page 187

estimated from field and seismic observations or the aureole total organic carbon

(TOC) and vitrinite re

ectivity data. The total mass of carbon produced, W C , can
be converted to equivalents of CH 4 (

¼ W C ¼ W C

3.66).
If only 1 wt.% of the organic carbon in shale or siltstone is transformed into
gaseous carbon compounds,

1.34) and CO 2 (

the gas production potential associated with a

20,000 km 3 sill intrusion is 230 5,000 1200 Gt

CH 4 ). More gas may be generated if the shales have higher TOC contents and if
the sills are emplaced within hydrocarbon-bearing lithologies. This means that a
single melt batch injected into an organic-bearing sedimentary basin can generate
suf

- - 920 Gt C (corresponding to 310 -

cient CH 4 , rapidly enough, to cause global warming (Aarnes et al ., 2010 ).
Since the inner contact aureoles reach peak metamorphic conditions (typically
400 500 C) shortly after sill emplacement (10 - - 500 years), the metamorphic

reactions and associated fluid production are also very fast.

In addition to the heated sedimentary rocks, the igneous sub-volcanic and the
volcanic parts of LIPs also release volatiles. Basaltic melts contain H 2 O, CO 2 and
SO 2 that is released during magma ascent and decompression. The carbon content
of un-degassed basaltic magmas is poorly constrained, but may be on the order of
13 Mt CO 2 /km 3 (Self et al ., 2005 ). Studies from sills in the Siberian Traps have
suggested that high-temperature interactions between dolerite and evaporites
can explain high halogen concentrations in melt inclusions (Black et al ., 2012 ).

12.6 Gas-release mechanisms
When sedimentary host rocks are heated by magma intrusion, the resulting over-
pressure may lead to hydrofracturing and the formation of vertical pipes.
Both dehydration reactions and pore- uid boiling are important processes for a

rapid increase in the local fluid pressure in contact aureoles (Jamtveit et al ., 2004 ).

Additional processes that can increase the local fluid pressure and lead to venting

include volatile exsolution from the melt and interactions between near-surface
water and the melt. Vent structures and breccia pipes are characteristic features of
the three volcanic basins presented here, although they vary greatly in formation,
size and degassing style. The dehydration-related hydrofracturing was recently
explored by Aarnes et al . ( 2012 ) , showing that the organic content in a shale is
important for determining if the aureole fractures during heating. Increasing
the organic carbon content from 1 to 10 wt.% has an effect comparable to a three
orders of magnitude decrease in permeability. In cases where the permeability of
the sedimentary rocks is relatively high, metamorphic and igneous gases may be
released pervasively and seep out at the surface without any vent being formed.
A modern analogy of such seep activity can be found in the Salton Sea area in
California (Mazzini et al ., 2011 ).