Volatile Evolution of Terrestrial Planets

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Figure 1. During accretion, large bodies are efficiently degassed on impact, yet noble gas measurements suggest that reservoirs within Earth’s mantle remain volatile-rich today. Possible causes include equilibration between a magma ocean and an early massive atmosphere, or incorporation of undegassed material into the mantle, perhaps from an early stage of accretion. Any model describing the evolution of the mantle must account for why different regions in the mantle preserve distinct geochemical signatures in a dynamic convecting regime (From Ballentine, Science 296, 2002, 1247-1248).

Noble gases and the origin of volatile in the Earth's Mantle
Similar to other isotope tracers, the 3He/4He composition of material from mid-ocean ridges is fairly uniform. This is in contrast to the 3He/4He ratio of ocean island volcanics, such as Hawaii and Iceland, which range from mid-ocean ridge values to the highest values found in a mantle source. Unlike other tracers, He cannot be recycled back into the mantle because once degassed from the mantle (and this is a very efficient process) it has a relatively short residence time in the atmosphere before being lost to space. 4He is produced by U and Th decay in the mantle, and provides a proxy for the solid volume sampled. 3He in contrast is only sourced as a trapped component in meteorites or an early massive planetary atmosphere and can only have come from a reservoir in the mantle that has been preserved over the history of the Earth. Because ocean islands tap a source with a high ratio of 3He to 4He, this has been taken as evidence that the source of these ‘hotspots’ or ‘plumes’ in the mantle is volatile (3He) rich, and therefore preserves a remnant of the Earth’s early volatile history. Other noble gases have been investigated in these different mantle sources and have provided a lot of complementary information about volatiles in the different mantle sources. Nevertheless, a lot of detail is compromised by the nature of the samples studied. They are all erupted into either the oceans or the atmosphere that contains relatively large amounts of atmospheric noble gases which, unlike He, are not lost to space. This makes it very hard to distinguish in a sample between components in the mantle that may have an air-like isotope signature (for example a recycled component) and air noble gases that have been trapped during the eruptive process. Resolving the true signature of different mantle heavy noble gas components from simple air contamination provides one of the major research drives in noble gas laboratories around the world.
This information will provide a unique insight into the first 100 Ma of Earth history and tell us where the volatiles now trapped in the Earth came from and how they were incorporated into the Earth’s mantle – questions that to date remain unresolved. In Manchester we use our noble gas laboratories  to address these fundermental science topics.

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