Mantle outgassing

In plumes of terrestrial gases from the Earth s mantle, gases with isotopic ratios near 3He/ 4He = 20-30 times the atmospheric ratio are recorded, showing that those mantle gases are ancientand have notmixed with today s atmosphere, which is known to be rich in 4He as a result of uranium decay within the Earth followed by volcanic outgassing. 

The two He isotopes have approximately the same abundance throughout the universe. Some primordial helium was trapped during the accretion of the Earth, with an estimated He/ He ratio of 10 . This is still retained in the mantle and, to a lesser extent, the core, and is now outgassing along deep faults and fissures and during volcanic eruptions. The... 

The simplest case is to consider the steady-state value of the oceanic-atmospheric carbon dioxide reservoir f oc that results from equating mantle outgassing to ingassing by subduction. From equation (7), the condition for steady state is bridge = CdeepThydro- Using equation (9) for ividge and equation (11) for Ti,ydro. we obtain a steady-state reservoir size of... 

A possibly troubling feature of our models is that we implicitly predict a secular trend in the isotopic composition of carbonates. This would occur because the carbonate subducted would presumably have the isotopic composition of sea water, whereas CO2 currently outgassed at midocean ridges is isotopically lighter. One way to avoid this problem is to presume that carbonates and reduced carbon are subducted in the same proportions as they are produced. Because the mantle emits some reduced gases and subducts oxidized ferric iron (Lecuyer Ricard 1999), some reductant must be subducted lest the continents grow secularly more reduced. The natural candidate, perhaps the only viable candidate, is reduced carbon, which is notably refractory and already known to be subducted. This is a reasonable expectation, and it is consistent with what is known of arc volcanoes today (see above), but it is unmotivated by our model. 

This model also works well for the Archaean, with its likely higher mantle heat flow and smaller continental mass and is consistent with calculations which show that there was a much higher flux of hydrothermal fluid into the early oceans compared to the present day (Section 5.4.2). In addition, it is consistent with the many observations of carbonate alteration in Archaean mafic and ultramafic rocks (Rose et al., 1996 Nakamura Kato, 2004). Nakamura and Kato (2004) proposed a global carbon flux of 3.8 x 1013 mol/yr into the ocean floor at 3.46 Ga, an order of magnitude greater than the modern carbon flux of veined MORB (Table 5.3). If these Archaean fluxes are used in Zahnle and Sleep s (2002) ingassing-outgassing... 

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