Xenon isotopic anomalies in the mantle: insights into the origin and evolution of terrestrial volatile elements | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  Xenon isotopic anomalies in the mantle: insights into the origin and evolution of terrestrial volatile elements

Origin of volatiles, such as carbon, nitrogen, water and noble gases, on Earth and other terrestrial planets is still misunderstood. However, it is a crucial issue for better understanding processes of solar system formation. Due to their inertness, noble gases (He, Ne, Ar, Kr, Xe) constitute useful tracers of the volatile element sources. In particular, xenon has nine isotopes with very different characteristics: 124Xe, 126Xe, 128Xe and 130Xe are stable, non-radiogenic isotopes, 129Xe is partly derived from the extinct radioactivity of 129I, 131Xe, 132Xe, 134Xe and 136Xe partly come from the fission reactions of still-alive 238U and extinct 244Pu in the mantle. The stable and non-radiogenic xenon isotopes are useful to characterize the volatile sources (i.e., their compositions have kept remnants of the Earth’s initial composition) whereas radiogenic isotopes can be used as chronometers of mantle degassing and regassing.

 

However, 124Xe, 126Xe, 128Xe have very low abundances on Earth (<< ppb) and are thus difficult to measure, in particular in mantle rocks. Analyses of CO2 well gases and gases from thermal springs seem to indicate an excess in 124Xe, 126Xe, 128Xe in the mantle compared to air. But measurements on basaltic glass samples from mid-ocean ridges and oceanic islands show no excess for these isotopes. Glass samples are easily contaminated by air through cracks (figure 1). When crushing the samples for noble gas analyses, mantle gases from the intact bubbles are mixed with atmospheric gases released from the cracks. Moreover, atmospheric heavy noble gases (Ar, Kr, Xe) dissolved in seawater (same isotopic composition as that of air) would be recycled into the mantle at subduction zones. This is why determining any xenon isotopic anomalies in the mantle compared to air is challenging.

 

Figure 1: X-ray microtomography images of the basaltic glass sample popping rock 2πD43: a) 3D reconstruction of the sample volume (height: 3cm), b) one 2D slice of the same sample. This sample has a vesicularity of about 16 % (volume of the vesicles out of the total sample volume), which makes it one of the most volatile-rich mid-ocean ridge basaltic glass samples. The grey arrow indicates one crack in the glass.

 

To overcome these issues, a new protocol was set up, which consisted in accumulating heavy noble gases with the least possible contamination. To do so, a glass sample was step-crushed. For each step, the neon isotopic composition was first measured because it allows quantifying the atmospheric contamination. If the step was slightly contaminated, then heavy noble gases were kept on a charcoal trap, otherwise they were pumped. This was repeated several times to accumulate enough xenon for analyses of 124Xe, 126Xe and 128Xe. A basaltic glass sample from the Mid-Atlantic Ridge very rich in gases was studied, the popping rock 2πD43 (figure 1).

 

The results show the highest excesses in 124Xe, 126Xe and 128Xe ever measured in the mantle compared to air. Theses excesses point towards a chondritic origin for mantle xenon rather than solar. The mantle composition for these isotopes is thus the result of mixing chondritic noble gases with recycled atmospheric noble gases. The new data for the radiogenic xenon isotopes combined with the non-radiogenic isotopes data allowed to demonstrate that recycling of xenon and likely of other volatiles into the mantle was not efficient before 3 Ga.

 

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Date de publication : 
30 January 2019