saava
Our bubbling Earth
R.D.Schuiling
In several places on earth large volumes of gas are seen to escape. These gases are
usually dominated by CO2. The emissions are associated with volcanic activity, and are
attributed to magma degassing. It will be shown that in the case of Milos this explanation
is unacceptable for quantitative reasons. An alternative explanation will be given, and its
consequences for the thermal evolution of certain sections of the crust will be discussed.
Our example
Milos is a volcanic island that forms part of the active volcanic arc in the Aegean. The
geothermal gradient near its surface is extreme, of the order of 8 to 10oC per 10 meter,
i.e. about 30 times the average geothermal gradient on earth ( Fytikas, 1989) In an area of
about 35 km2 on and around Milos 2.2 x 106 tons of CO2 are released annually (Dando et
al, 1995, Dando et al, 2000). This gas release has probably continued for a period of 3
million years (Fytikas et al, 1976), leading to a total gas production so far of 6.6 x 1012
tons. Let us first calculate if magma degassing can be made responsible. Acid and
intermediate magmas like found on Milos can dissolve 1 wt% or less of CO2 (Holloway
and Blank, 1994) so we need 6.6 x 1014 tons of magma to dissolve the CO2 escaped so
far, or approximately 3 x 1014 m3 = 3 x 105 km3. A magma chamber with a cross section
of 35 km2 (the area over which degassing has been observed) should then have a vertical
extension of 8.600 km! It is evident that magma degassing alone cannot offer a
satisfactory explanation for the amount of gas observed.
Carbon isotopic compositions of the emissions around Milos are between +1.1 and –1.0
‰ (Botz et al, 1996), typical of marine carbonates (Keith and Weber, 1964). Milos is
lying over a subducting slab which originates in the Hellenic Trench, South of Crete. In
an earlier episode of subduction, material was subducted in a more northerly zone, and
part of this subducted material has been exhumed again, and is visible as highpressure/low-temperature metamorphic rocks among others on Syros and Sifnos. From
the mineral parageneses it is clear that the material must have been subducted to 90 km
depth or more (Schliestedt et al, 1987). Apart from eclogites, jadeite rocks, glaucophane
schists etc., the subducted formations are rich in carbonate sediments. The subducted slab
dips steeply under the Aegean and even penetrates into the lower mantle (Wortel and
Spakman, 2000).
If the composition of the subducted material is similar to the exhumed part on
Syros/Sifnos, this means that huge volumes of carbonate rocks have been dragged down
to great depth in the mantle, and are now in an almost vertical position. This permits the
generation of very large volumes of CO2 under a limited area (the vertical extent of a
carbonate formation in vertical position becomes very large). The temperature of the
subducted slab lags behind the temperature of the surrounding mantle at any depth, but
both temperatures continuously rise. As a result, carbonate formations will start to
dissociate due to mineral reactions with the surrounding silicate rocks. The enthalpy of
these reactions is dominated by the enthalpy change of CaCO3  CaO + CO2 , and is
rather insensitive to the type of Ca-silicate formed. The amount of heat required to
provide for the CO2-flux is between 1 and 2 orders of magnitude more than can be
provided by normal heat flow in the mantle, but by drawing the required heat from a
much wider zone in the mantle surrounding the subducting slab a sufficient amount of
heat can be provided. From the seismic tomographic sections as shown by Wortel and
Spakman (op.cit.), it seems that the colder part of the mantle is indeed much wider than
the slab itself.
The generated fluids, including water that is produced by dehydration reactions of
hydrous minerals in the slab, are rapidly transported upward. They leave their point of
origin at a lower temperature than the surrounding mantle, but they will reach a point at
which the temperatures of the fluid and the mantle are equal. Farther up, they will start to
exert a heating effect on the surrounding rocks, which can be considerable, provided they
move upward in a somewhat focused manner, possibly along the slab itself.
Effects of fluids on the mantle
At a certain point they have to break through the mantle wedge. Wherever such mixed
CO2-H2O fluids meet dry mantle on their way up, reactions will take place like
2 CaMg(SiO3)2 + 5 MgSiO3 + H2O  Ca2Mg5Si8O22(OH)2 + Mg2SiO4
( in the reaction above, the tremolite proxies for hornblende).
The net result is that water is preferentially retained, whereas the gas phase will become
enriched in CO2. A likely consequence of this change from a dry mantle to a water-rich
mantle, and the temperature rise by the introduction of hot gases is the formation of acid
to intermediate magmas.
Thermal effects of fluids in the crust
The thermal state of the crust is also affected by the passage of such massive, hot,
focused flows of fluids.
Below Milos, and below the Cyclad islands in general, the crust is relatively thin, about
25 km, and the top of the mantle is at high temperatures, around 1300 K (Makris, 1977).
So the gases enter the base of the crust at 1300 K, and bubble out lukewarm or cold at the
earth’s surface, at 298 K. The enthalpy difference between 1 g of CO2 at 1300 K and 298
is about 1.13 kJ (Robie et al, 1978). The total mass of gas passing through the system in 3
million years is 6.6 x 1015 kg, so these gases add 74 x 1017 kJ to a crustal section with a
horizontal extension of 35 km2. This exceeds considerably the contribution from a
normal heat flow over the same surface area and during the same time span, which is
only 2 x 1017kJ. Gas fluxes from the mantle can lead to significant heating of the crustal
sections through which they pass. This heating will result in the formation of relatively
high-temperature/low-pressure metamorphic assemblages, expressed as thermal domes in
metamorphic terrains, like on Naxos, where fluid inclusions are seen to consist mostly of
CO2 (Kreulen, 1977). It has been calculated that 7.5 x 1015kg of fluid have passed through
the metamorphic system at Naxos (Schuiling and Kreulen, 1979), similar to the amount
escaping around Milos. Synmetamorphic quartz veins and pods mark the channels
through which the fluids passed. Naxos is not associated with known volcanic activity,
although there is a strong suspicion that the Stylida peninsula hosts an altered rhyolite
dome capped by silica sinters.
When the fluids finally meet the boiling line of water at a depth of 1 to 2 km, a major part
of the heat transport is taken over by permanently boiling geothermal systems or
intermittent phreatic explosions. Such steam eruptions have been numerous and
sometimes spectacular on Milos, and the resulting depressions have diameters up to 1000
m (Fytikas et al, 1986).
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