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Lithium nucleosynthesis in the Sun inferred from the solar-wind 7Li/6Li ratio

Abstract

The abundance of lithium measured in meteorites has generally been assumed to be the ‘Solar System value’, which presumably reflects the abundance in the gas cloud out of which the Sun formed1. Lithium is a factor of 140 less abundant in the solar photosphere than in meteorites2; this difference has been attributed to the destruction of lithium (through nuclear reactions) at the base of the Sun's convection zone3,4,5. If this is correct, then the ratio of 7Li/6Li in the Sun's photosphere should be 106 (ref. 6), as 6Li is destroyed much more easily (at a lower temperature) than 7Li: the meteoritic abundance ratio is 7Li/6Li = 12.14 (ref. 7). Here we report that 7Li/6Li = 31 ± 4 for lithium in the solar wind that has been implanted in lunar soil. This low ratio suggests that lithium is produced when energetic protons from solar flares induce spallation reactions with 16O and 12C present in the photosphere.

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Figure 1: The 7Li/6Li isotopic ratio as a function of depth for three lunar soils (labelled by sample number).
Figure 2: The 7Li/6Li isotopic ratio as a function of the atomic Si/Li ratio in lunar soils 10060 and 79035.
Figure 3: Evolution in the Sun of the 7Li/6Li ratio versus the Li/H ratio.

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Acknowledgements

We thank J. Audouze, S. Vauclair and R. Reedy for discussions, and G. Huss for contributions to this Letter. This work was supported by INSU-CNRS, CNES and MNHN.

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Correspondence to Marc Chaussidon.

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Chaussidon, M., Robert, F. Lithium nucleosynthesis in the Sun inferred from the solar-wind 7Li/6Li ratio. Nature 402, 270–273 (1999). https://doi.org/10.1038/46235

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