Environmental Engineering Reference
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individual sills is dif
cult to measure due to limited three-dimensional seismic
imaging of stacked sill re
field exposures and boreholes.
Estimating the maximum individual sill volume in a basin is important for under-
standing the basin devolatilization reactions during contact metamorphism. Using the
well-constrained Karoo Basin as an example, 100-m-thick sills commonly cover
100
ections, and few continuous
100 km 2 . This gives a volume of 1,000 km 3 , which is considerable and even
comparable in volume to the largest silicic volcanic eruptions and single basaltic
ows (Bryan et al ., 2010 ). At the extreme, but still realistic end, single sill intrusions
may have volumes up to 18,000 km 3 (300
200 m).
The thicknesses of metamorphic aureoles commonly extend about the same
distance as the sill thickness, both below and above the sill ( Figure 12.4c ). This
means that a single sill emplacement event may heat up to 36,000 km 3 of sediment-
ary rocks in the end-member example given above. The aureole size ultimately
depends on both the geological setting and the measuring methods (Aarnes et al .,
2010 ). Sills emplaced at deep levels in a basin will have thicker aureoles compared
to a shallower sill because the sill cooling is less ef
300 km
flow in the aureole
may lead to a more widespread aureole and lower maximum temperatures in cases
where the host sedimentary rocks have high permeability (Iyer et al ., 2013 ).
The timescale of metamorphism around a sill of that size will depend on the
emplacement time of the sill, depth of emplacement, the geothermal gradient, the
magma temperature and the host-rock composition and water content (Aarnes et al .,
2011 ). Although metamorphism in the aureole may last for > 1,000 years, meta-
morphism is a rapid process and will start immediately following emplacement.
cient. Fluid
12.4 Gas production mechanisms
The chemical composition of the sedimentary rocks heated by igneous intrusions
has a profound in
fluid composition (e.g. Ganino and
Arndt, 2009 ; Svensen et al ., 2004 ; Svensen et al ., 2009 ). For instance, organic-rich
shale generates CH 4 during contact metamorphism, whereas coal generates CO 2 -
dominated
uence on the metamorphic
fluids. In addition, water is generated. Since many sedimentary basins
contained hydrogen-rich kerogen and oil and gas accumulations at the time of sill
emplacement, gases like methane (CH 4 ) and ethane (C 2 H 6 ) may have dominated
the
fluid will be
dominated by 13 C-enriched CO 2 ,or 13 C-depleted CH 4 if organic matter or graphite
is present. Some of the key metamorphic reactions taking place when carbonates,
marls and shales are heated are presented below (Tracy and Frost, 1991 ):
fluid. If limestones or dolostones are heated, the generated
5 dolomite
þ
8 quartz
þ
H 2 O
¼
tremolite
þ
3 calcite
þ
7CO 2
(12.1)
þ
þ
¼
þ
þ
tremolite
3 calcite
2 quartz
5 diopside
3CO 2
H 2 O
(12.2)
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