Volatiles and Gas in Magma
Magma is a viscous mix of molten rocks that rises towards the Earth’s surface.
Magma also contains highly volatile elements such as H 2 O, C, and S, which are
related to the primordial composition of the Mantle (Hart and Zindler 1989). If
magma degases during volcanic eruption, this will give rise to emissions of H 2 ,
H 2 O, CO 2 , SO 2 , H 2 S, S 2 , HCl, CO, HF, He, Ar, N 2 , CH 4 , NH 3 , in decreasing
abundance. The concentration of these volatiles will vary according to the type of
eruption. Satellite measurements have shown that hydrogen sulfide (H 2 S) released
by volcanoes is estimated to be as much as 1–25 Tg (megatons) per year along
with sulfur dioxide (SO 2 ).
Partial melting of mantle peridotite gives rise to the material that supplies the
magma reservoirs. Inside a reservoir, the molten material will subsequently
undergo a process of crystal-liquid fractionation and therefore gas complexes will
be formed (Fig. 5.13). These gaseous phases are either re-dissolved in the melt or
they could escape during volcanic eruptions. The major constituents of volcanic
gases are H, C, O, S, N, Cl, F, Br and rare gases such as He, Ne, and Ar. Hydrogen
Fig. 5.13 Model of a submarine a volcanic edifice at different eruptive stages: a A quiet stage of
eruption gives rise to a lava dome with gentle slopes. The magma column is accompanied by a
small amount of gases and the lava erupted consists essentially of basalt. b An explosive stage is
characterized by the presence of a large amount of gases and volatiles that have segregated at the
top of the magma column. The volatiles consist of H 2 O, CO 2 , sulfuric acid (H 2 S), hydrofluoric
acid (HF) and other trapped volatiles, which enhance bubbles of gas producing explosive
volcanism. During explosive events fragmented rocks from the edifices are extruded on the
surface forming pyroclastic deposits. Explosion will take place when the pressure of the gases has
exceeded that of the surrounding material
124
5 Earth’s Mantle Melting and Volcanism
Magma is a viscous mix of molten rocks that rises towards the Earth’s surface.
Magma also contains highly volatile elements such as H 2 O, C, and S, which are
related to the primordial composition of the Mantle (Hart and Zindler 1989). If
magma degases during volcanic eruption, this will give rise to emissions of H 2 ,
H 2 O, CO 2 , SO 2 , H 2 S, S 2 , HCl, CO, HF, He, Ar, N 2 , CH 4 , NH 3 , in decreasing
abundance. The concentration of these volatiles will vary according to the type of
eruption. Satellite measurements have shown that hydrogen sulfide (H 2 S) released
by volcanoes is estimated to be as much as 1–25 Tg (megatons) per year along
with sulfur dioxide (SO 2 ).
Partial melting of mantle peridotite gives rise to the material that supplies the
magma reservoirs. Inside a reservoir, the molten material will subsequently
undergo a process of crystal-liquid fractionation and therefore gas complexes will
be formed (Fig. 5.13). These gaseous phases are either re-dissolved in the melt or
they could escape during volcanic eruptions. The major constituents of volcanic
gases are H, C, O, S, N, Cl, F, Br and rare gases such as He, Ne, and Ar. Hydrogen
Fig. 5.13 Model of a submarine a volcanic edifice at different eruptive stages: a A quiet stage of
eruption gives rise to a lava dome with gentle slopes. The magma column is accompanied by a
small amount of gases and the lava erupted consists essentially of basalt. b An explosive stage is
characterized by the presence of a large amount of gases and volatiles that have segregated at the
top of the magma column. The volatiles consist of H 2 O, CO 2 , sulfuric acid (H 2 S), hydrofluoric
acid (HF) and other trapped volatiles, which enhance bubbles of gas producing explosive
volcanism. During explosive events fragmented rocks from the edifices are extruded on the
surface forming pyroclastic deposits. Explosion will take place when the pressure of the gases has
exceeded that of the surrounding material
124
5 Earth’s Mantle Melting and Volcanism