Tracing the Earliest Stages of Planetary Evolution with the Noble Gas Argon


The leading model for the primary differentiation of Earth and other rocky planets and moons holds that these bodies had global oceans of magma that crystallized to form central cores, silicate mantles, and outer crusts. A new experimental study led by Colin Jackson of Tulane University examines the behavior of the noble gas argon during crystallization of Earth’s magma ocean. Noble gases have been considered useful for tracing Earth’s earliest history because they tend to be chemically inert, and thus may not be affected by later chemical processing. The experiments were conducted in multi-anvil and laser-heated diamond anvil presses, which are devices used to subject geological materials to very high pressures and temperatures such as those associated with a deep magma ocean. The authors synthesized high-pressure phases present in the deep interior of the Earth, including the minerals bridgmanite and ferropericlase, and examined the reactivity of argon with the ultramafic melt (i.e., magma ocean) that was crystallizing these minerals. The experimental assemblages were analyzed using electron microprobe and laser ablation techniques. Results showed that the solubility of argon is low in mantle minerals, but relatively high in the ultramafic liquid, implying that argon behaves incompatibly (it is strongly partitioned into the melts rather than the solid minerals) during high-pressure magma-ocean crystallization. This new finding suggests that the budget of noble gases in the solid mantle may be controlled by how much liquid is trapped into the crystallizing mineral piles settling out from the magma ocean. Many of the geochemical signatures of primary mantle sources produced during magma ocean crystallization have been erased due to active plate tectonics on Earth. Thus, this knowledge is critical in interpreting the record of noble gas abundances in mantle derived rocks and using them to trace the earliest processes that have shaped the Earth’s mantle and atmosphere. READ MORE