Solar wind signatures from the core of a planetary body

It is well known that Earth’s interior hosts light solar noble gases, contrasting to planetary signatures in the atmosphere, while the exact source location of primordial gases within Earth is enigmatic. Our study finds that the terrestrial core itself might solve this core issue in geochemistry.

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The acquisition of solar-type helium and neon within Earth is either ascribed to incorporation of solar wind irradiated solids or solar nebula gas dissolution into a magma ocean during early stages of terrestrial accretion ~4.5 billion years ago. The most diagnostic and distinct isotopic fingerprint is provided by the isotopic ratio of neon. 20Ne/22Ne end-member ratios for (deep) mantle derived samples cluster around the canonical “solar” 12.5 value compared to a “planetary” value of 9.8 in the atmosphere (Figure 1, [1]). Since decades, the research group for geo- and cosmochemistry at the Institute of Earth Sciences of the Heidelberg University, Germany, measures He and Ne isotopes in samples from plume-derived oceanic island basalts (i.e., the lower mantle) and mid oceanic ridge basalts (i.e., the upper mantle). It was the group leader, Prof. Dr. Mario Trieloff – my co-author and PhD supervisor – who identified that the isotopic ratios of neon in mantle plumes are indistinguishable from the solar wind implanted Ne-B component in meteorites with a 20Ne/22Ne ratio of ~12.5 [2].

Ne isotopic signatures of Earth's mantle

Figure 1: Neon 3-isotope plot showing different components and ratios observed in terrestrial mantle samples from OIBs and MORB (from my publication on the acquisition of terrestrial neon [1]).

When I started my PhD in 2015, I was highly interested in meteorites and motivated to deciphering unresolved mysteries in geosciences using isotopic tracers. I was amazed by the immense geo- and cosmochemical potential provided by studying noble gases, in particular by the (solar) isotopic ratios of neon. Despite the ubiquitous nature of implanted solar wind neon in the surface of extraterrestrial materials, I needed quite a while during my initial research ambitions to realize that the implanted component was generally considered insufficient to significantly contribute to the solar noble gas budget during terrestrial accretion. My detailed investigations proved that black is white, and by showing that accretion of solar wind irradiated cosmic dust, on its own, might have provided sufficient quantities of solar neon to Earth’s mantle to explain the observed signatures, I published the first part of my PhD studies [1].

The consecutive part of my PhD focused on the imprecisely known source location of the solar signatures within Earth (Figure 2, [3]) – which was an unsatisfactory state of knowledge for me. Traditionally, the source reservoir for solar noble gases is assumed to be isolated in the lower mantle, a domain that is sampled by deep-rooted mantle plumes. Although such mantle upwellings are possibly ascending from the core-mantle-boundary and previous hypotheses suggested that the Earth’s core might be a potential reservoir for solar He and Ne, there was no hope to test these models by sampling the terrestrial core.

Earth's possible interior reservoirs for primordial gases
Figure 2: Schematic models showing possible reservoirs within Earth (orange) for solar-type He and Ne and fluxes as arrows (redrawn from [3] and modified for my PhD thesis).

For the benefit of my studies, I was permitted to investigate iron meteorites, fragments of the cores of protoplanets and our only available analogue to materials from the deep interior of Earth. It was my privilege to conduct the most detailed noble gas study [4] of the iron meteorite Washington County (Figure 3), which was prominent for keeping a “noble” secret even after about 60 years of noble gas studies on this specimen.

Washington County iron meteorite study
Figure 3: The iron meteorite Washington County (photo in the upper left corner from 1928 [5], with permission of the Mineralogical Society of America). The BSE-image in the background shows the main mineralogical constituents of Washington County identified on a polished sample surface with SEM-EDX. Labels alongside the polished sample slab in the lower left corner indicate the locations of interior aliquots that were analysed for noble gases [4]. The fractional degassing patterns (right-hand side) of measured He and Ne isotopes during the high-resolution stepwise heating extractions of sample WC_5 show major gas releases from schreibersite at ~1100 °C and kamacite-taenite at ~1400 °C.

To pursue the educated guess to reveal something spectacular, I faced the remarkably intricate measurements and stood for the first time in my career vis-à-vis with a noble gas mass spectrometer. I finally understood why a fellow scientist during one of my first conferences once called noble gas geo- and cosmochemistry the “black magic” of geochemistry. Nevertheless, the great effort paid off. I was astonished by the results of my measurements, which unambiguously indicate the presence of solar wind signatures in the interior of Washington County (Figure 4). In this regard, I found the first solid proof – otherwise only provided by experimental and modelling approaches – for light solar noble gases within the metal of a planetary body, including far-reaching implications.

He and Ne isotopic composition of Washington County

Figure 4: Both, a) He and Ne and b) Ne-3-isotope plots indicate the presence of solar wind derived noble gases in the metal of Washington County [4].

Most intriguingly, metallic cores of other planetary bodies, including the Earth might host solar wind derived helium and neon that partitioned into segregating metal melts during core formation. Recently determined partition data [6] for He and Ne between liquid metal and silicate are in favour for such scenarios. Furthermore, a closer look at available noble gas data until the year 2004 [7] points towards hitherto unreported (or unrecognized?) evidence for light solar noble gases in several other iron meteorites. The direct implication of the results of our study [4] is that the terrestrial core is a realistic source reservoir for light solar noble gas signatures observed in Earth’s mantle. Earth’s core might play an active, but until now neglected, role in the noble gas geochemistry of the terrestrial mantle. This can possibly alter our understanding of volatile geodynamics.

References:

[1] Vogt, M., Hopp, J., Gail, H.-P., Ott, U. & Trieloff, M. Acquisition of terrestrial neon during accretion – A mixture of solar wind and planetary components. Geochimica et Cosmochimica Acta 264, 141-164 (2019).

[2] Trieloff, M., Kunz, J., Clague, D. A., Harrison, D. & Allègre, C. J. The Nature of Pristine Noble Gases in Mantle Plumes. Science 288, 1036–1038 (2000).

[3] Porcelli, D. & Ballentine, C. J. Models for the Distribution of Terrestrial Noble Gases and Evolution of the Atmosphere. Reviews in Mineralogy and Geochemistry 47, 411–480 (2002).

[4] Vogt, M., Trieloff, M., Ott, U., Hopp, J. & Schwarz, W. H. Solar noble gases in an iron meteorite indicate terrestrial mantle signatures derive from Earth’s core. Communications Earth & Environment 2, 92, doi:10.1038/s43247-021-00162-2 (2021).

[5] Palache, C. & Shannon, E. V. A new meteorite from Washington County, Colorado. American Mineralogist 13, 406–409 (1928).

[6] Bouhifd, M. A., Jephcoat, A. P., Porcelli, D., Kelley, S. P. & Marty, B. Potential of Earth’s core as a reservoir for noble gases: Case for helium and neon. Geochemical Perspectives Letters 15, 15-18 (2020)

[7] Schultz, L. & Franke, L. Helium, neon, and argon in meteorites: A data collection. Meteoritics & Planetary Science 39, 1889–1890 (2004).

Manfred Vogt

Postdoctoral Researcher, Ruprecht-Karls-Universität Heidelberg

Geoscientist in Geo- and Cosmochemistry