Noble gases provide unique clues to the structure of the Earth and the degassing of volatiles into the atmosphere. Since the noble gases are highly depleted in the Earth, their isotopic compositions are prone to substantial changes due to radiogenic additions, even from scarce parent elements and low-yield nuclear processes. On a global scale, noble gas isotopic variations reflect planetary differentiation processes that generate fractionations between these volatiles and parent elements.

It has long been recognized that the atmosphere is not primary (i.e., solar) and so is not simply a remnant of the solar nebula. From similarities with volcanic emanations,...

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