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Xenon isotope anomalies in the mantle: implications for the origin and evolution of terrestrial volatile elements

The origin of volatile elements such as carbon, nitrogen, water and noble gases on Earth and other terrestrial planets is still poorly understood. Answering this question is crucial to a better understanding of the processes involved in the formation of the solar system.

Xenon isotope anomalies in the mantle: implications for the origin and evolution of terrestrial volatile elements

Publication date: 30/01/2019

Press, Research

Related themes : Origins

Because of their inert nature, rare gases (He, Ne, Ar, Kr, Xe) are ideal tracers of sources of volatile elements. Xenon in particular has nine isotopes with very different characteristics: 124Xe, 126Xe, 128Xe et 130Xe are stable and non-radiogenic, while 129Xe is partly derived from the extinct radioactivity of 129I, and the isotopes 131Xe, 132Xe, 134Xe et 136Xe are partly derived from fission reactions of elements 238U et 244Pu in the mantle. Stable, non-radiogenic xenon isotopes can be used to characterise sources of volatile elements (i.e. their compositions have retained traces of the Earth’s initial composition), while radiogenic isotopes can be used as timekeepers for degassing and recycling processes in the mantle.

However, 1124Xe, 126Xe and 128Xe isotopes are very rare on Earth (<< ppb) and are therefore difficult to measure, particularly in mantle rocks. Analyses of gases fromCO2 wells and thermal springs seem to indicate an excess of these three isotopes in the mantle compared with air. But measurements on basalt glass from ridges and oceanic islands show none. Basalt glass samples are very easily contaminated by air at fractures (Figure 1). When samples are analysed by grinding, the mantle gases contained in the bubbles mix with the air present in these fractures. In addition, atmospheric heavy rare gases (Ar, Kr, Xe) dissolved in seawater (isotopic composition identical to that of air) appear to be recycled into the mantle at subduction zones. All this makes it difficult to determine any xenon anomalies in the mantle.

Figure 1: X-ray microtomography images of the 2πD43 popping rock glass sample: a) 3D reconstruction of the sample volume (3 cm high), b) 2D section of the same sample. This sample has a vesicularity of around 16% (bubble volume vs. total volume), making it one of the most volatile-rich ridge basalts. The grey arrow indicates a fracture in the glass.

To overcome these problems, a new protocol has been put in place, consisting of accumulating heavy rare gases that are as uncontaminated as possible. In practice, a sample is crushed in each stage. For each stage, the neon composition is first measured, enabling the atmospheric contamination to be quantified. If there is very little contamination, the heavy noble gases are stored in a tank, otherwise they are pumped out. This operation is repeated several times to accumulate enough xenon for analysis of the 124, 126 and 128 mass isotopes. To this end, a sample of basaltic glass from the Atlantic ridge, which is very rich in gas, was studied: the popping rock 2πD43 (Figure 1).

The measurements show the highest excesses of 124Xe, 126Xe, 128Xe ever detected in the mantle compared with air. These excesses point to a chondritic origin of xenon in the mantle rather than a solar origin. The composition of these isotopes in the mantle can therefore be explained by a mixture between a chondritic source and atmospheric rare gases recycled at subduction zones. The new, highly accurate data obtained for radiogenic xenon isotopes, coupled with that for stable, non-radiogenic isotopes, have shown that the recycling of xenon and probably other volatile elements in the mantle was not effective before 3 Ga.

Ref: Péron, S., Moreira, M. (2018) Onset of volatile recycling into the mantle determined by xenon anomalies. Geochem. Persp. Let. 9, 21-25.

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