Metamorphic and Diagenetic CO Degassing

Often the discussion of global CO2 degassing has concentrated only on volcanic processes. This neglects additional processes that may be just as important globally. For example, the range shown in table 4.1 for estimated total mantle degassing can be distinctly lower than that estimated for CO2 consumption by silicate weathering (6 ± 3 x 1018 mol/ my). The remainder must be metamorphic or diagenetic in origin (Kerrick, 2001). The pathfinding study of Barnes et al. (1978) pointed to the importance of nonvolcanic degassing in the circum-Pacific and Mediterranean-Tethyan orogenic belt. They suggested that CO2 discharges from hot springs, gas vents, and so on, are aided by the high seismicity in these zones. The study of Kerrick (2001) emphasizes the importance of areas of high heat flow overlying subsurface magmas which includes many of the world's geothermal systems. Morner and Etiope (2002) have summarized data for measured fluxes of metamor-phic and diagenetic CO2 and CH4 degassing from terrestrial environments and estimate large global fluxes from the land. However, their total land flux (14 x 1018mol/my) is probably too high for application over extended geologic time because it exceeds the maximum total global degassing based on silicate weathering (6 ± 3 x 1018 mol/my).

Kerrick and Caldeira (1998) and Kerrick (2001) have examined in detail the problem of metamorphic degassing. They conclude that (1) much metamorphic degassing occurs as a result of continent-continent and arc-

continent collisions; (2) extensional regimes, such as rift zones, with high heat flow are also excellent environments for degassing; (3) major faults may provide conduits for significant release to the atmosphere of deeply buried CO2; (4) synmetamorphic intrusions may cause significant regional fluid flow, which aids decarbonation and transport of CO2 to the surface; and (5) high CO2 levels during the Eocene were more likely caused by degassing from the American Cordilleran belt than from the Himalaya-Karakoram belt as previously believed. In an earlier study based on measurements of degassing at the Salton Trough (California) and Taupo Volcanic Zone (New Zealand), Kerrick et al. (1995) concluded that present global metamorphic degassing could be as much as 1 x 1018 mol/my, which is the value shown in table 4.1.

During subduction at active plate boundaries, the extent of metamorphic decomposition and degassing, based on phase equilibrium arguments, depends on the thermal gradient and the ability of water to reach great depths (Kerrick and Connolly, 2001). Kerrick and Connolly conclude that little metamorphic degassing occurs at these plate boundaries, but there is not general agreement on this subject. The isotopic analysis of volcanic gases (13C/12C, 3He) sampled at arc volcanoes indicates a considerable flux of CO2 from subducted carbonates of 2.3 x 1018 mol/ my (Sano and Williams, 1996), whereas high-pressure experiments and modeling of metamorphism suggest that most of the down-going CaCO3 is subducted deeply into the mantle without metamorphic decarbonation (Molina and Poli, 2000).

Another, relatively neglected, degassing process is that accompanying both shallow and deep diagenesis. It is well known that the maturation of buried organic matter involves the loss of CO2 (and CH4; see below; Durand, 1980). In addition, there is the globally important process of the diagenetic conversion of smectite to illite (Hower et al., 1976). This involves the release of acid and the dissolution and removal of calcium carbonate, and it must be an additional source of low-temperature CO2. A large variety of measurements of fluxes of CO2 and CH4, emanating from diagenetic processes below the land surface (including petroleum maturation), is summarized by Morner and Etiope (2002). The gases are emitted to the surface in cool seeps, springs, and so on, and the CH4 becomes rapidly oxidized to CO2 in the atmosphere.

Organic Gardening

Organic Gardening

Gardening is also a great way to provide healthy food for you and your loved ones. When you buy produce from the store, it just isnt the same as presenting a salad to your family that came exclusively from your garden worked by your own two hands.

Get My Free Ebook


Post a comment