Mantle Degassing Mechanism

Even if we accept that noble gases are mostly partitioned in magma and then effectively transported to the surface, we must further examine whether or not noble gases can be readily released from magma into the atmosphere once magma reaches the surface. Effective noble gas degassing mechanism from magma may be due to bubble 

Equation (6.5) was derived for a fractional degassing case in which infinitesimal fraction of vapor separating at a given instant is assumed to be in equilibrium with the melt. However, if the whole vapor phase is assumed to be in equilibrium with the entire melt, which seems to be less realistic in a large-scale magma degassing, Equation (6.5) is modified to 

Determination of absolute loss of noble gases at various shock pressures was attempted by Gazias and Ahrens (1991). For this, they prepared Ar-rich samples vitreous carbon (3-mm diameter cylinder) was equilibrated with Ar atmosphere of 0.25-1.5 kbar at 773-973°C for 13 days so that the samples thus prepared contained a few weight percent of Ar. They found that 28% of the total Ar was released by driving 4-GPa shocks into the Ar-sorbed carbon. Since pulverization of the same sample material to the similar grain size as the shocked one gave much less Ar loss, they concluded that the Ar loss was not a result of the impact-induced 

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Note that if such a scheme is viable, the evolution of mantle noble gases, reflecting degassing and interaction between reservoirs, may still follow those calculated in layered mantle models. An important issue that must be resolved is that there must be a mechanism for the preferential involvement of this previously melted material at OIB. This would link the involvement of these reservoirs in OIB source regions with where such material is stored. In this case, high He/ He ratios at the surface will trace the transfer of material... 

The upper mantle is degassed to the atmosphere according to a rate constant determined by the rate of melting at ridges. Since this is the main mechanism for removing noble gases from the upper mantle, this fixes their upper mantle residence times. 

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