Solar noble gases in an iron meteorite indicate terrestrial mantle signatures derive from Earth's core

The presence of solar-type noble gases (He and Ne) in Earth's mantle is a significant puzzle in geochemistry. Unlike the atmosphere, which is dominated by planetary-type noble gases depleted in He and Ne, mantle plumes exhibit high ³He/⁴He ratios and solar-like ²⁰Ne/²²Ne ratios, sometimes indistinguishable from solar wind (SW)-implanted Ne in meteorites. Two main hypotheses attempt to explain the origin of these light solar noble gases: (1) solar wind implantation into Earth's building blocks before accretion, and (2) incorporation of captured nebular gases into an early magma ocean. However, the exact source remains uncertain. Oceanic island basalts (OIBs) show high contributions of light solar noble gases, indicating a pristine, gas-rich source deep within the Earth, potentially distinct from the degassed upper mantle source for mid-ocean ridge basalts (MORBs). Some models propose Earth's core as a possible source for these solar noble gases, either for the entire mantle flux or at least for the OIB flux. Iron meteorites, as analogues to core material, offer insights into this question. While the Washington County iron meteorite has previously shown evidence of excess light noble gases, the exact location of analyzed samples was unclear. This study aims to clarify this by analyzing interior samples of Washington County to investigate the presence and distribution of solar noble gases, providing a direct test of the core-as-source hypothesis.

Previous studies have investigated the origin of light noble gases in Earth's mantle, proposing solar wind implantation or nebular gas incorporation as potential sources. Analyses of OIBs and MORBs revealed distinct reservoirs within the Earth's interior—a gas-rich, pristine source sampled by plumes and a well-homogenized, degassed reservoir in the upper mantle. The core has been suggested as a possible contributor to the solar noble gas signature in the mantle. Iron meteorites, being analogs to planetary core material, have been studied for their noble gas content. While Washington County showed prior evidence of solar gases, the location of the analyzed samples and the interpretation of these findings have been debated in the literature.

This study employed high-resolution stepwise heating gas extraction experiments on four aliquots of a 3-cm-long metal slab from the Washington County iron meteorite. Samples were taken from varying distances from the fusion crust to examine both near-surface and interior compositions. Helium, neon, and argon analyses were performed using up to 25 gas extraction steps between 600 and 1800 °C to identify distinct gas release phases and host minerals (schreibersite and kamacite-taenite). A schreibersite etch residue and two bulk samples were also analyzed. Data was analyzed using ⁴He/²¹Ne-⁴He/³He and ²⁰Ne/²²Ne-²¹Ne/²²Ne diagrams to investigate the relative contributions of cosmogenic and solar components. The study used a VG 3600 noble gas mass spectrometer and meticulous blank correction procedures to account for potential contaminants. Instrumental mass fractionation was corrected using calibration gases. Blank contributions were considered, particularly for ³He, ²⁰Ne, ²²Ne, ³⁶Ar, and ⁴⁰Ar, applying corrections where necessary. Data is presented in tables and figures, detailing concentrations, isotopic ratios, and gas extraction profiles.

The high-resolution stepwise heating extractions revealed two major gas release peaks, attributed to schreibersite (at ~1100 °C) and kamacite-taenite (at ≥1400 °C). Helium and neon data show striking excesses of solar gases across all interior samples analyzed in both minerals (schreibersite and kamacite-taenite). The data plots along a mixing line between cosmogenic and solar wind compositions in both ⁴He/²¹Ne-⁴He/³He and ²⁰Ne/²²Ne-²¹Ne/²²Ne diagrams. Only high-temperature extractions deviate from this trend, indicating mixing with air. The presence of the solar component in interior samples, several centimeters away from the fusion crust, demonstrates that these gases are not recent solar wind implantation but represent a primordial component incorporated during the formation of the meteorite's parent body. The near-unfractionated He/²⁰Ne ratios (~0.26) are similar to solar wind values. Calculations using various chondrite types as possible precursors (CV, E, CI, CM) revealed partition coefficients (DNe) for Ne between metal and silicate in the range of 10⁻²–10⁻¹. These values are consistent with previous experimental estimates under relevant pressure conditions.

The findings demonstrate that solar noble gases can be incorporated into the cores of differentiated planetesimals, even in small bodies. This supports the hypothesis that Earth's core could be a significant reservoir of solar noble gases. Calculations estimate that only a minor fraction (1-2%, or even lower if considering only OIB flux) of Washington County-like metal is needed to explain the solar noble gas inventory in the Earth's mantle. However, a larger proportion of solar gas-bearing iron cores might have existed closer to the sun. Additional evidence of solar gases in other iron meteorites suggests that Washington County may not be unique. The core's role in supplying solar gases to the mantle can explain the different noble gas signatures in various mantle regimes. The model presented suggests higher partition coefficients at low pressures during metal segregation and lower coefficients at high pressures explain gas transfer between core and mantle. Notably, no noble gas partition data for core-mantle boundary conditions exists, although a single value of DHe ≈ 9 × 10⁻³ at 40 GPa suggests consistency with low partition coefficients at such conditions.

This study provides compelling evidence for the presence of solar noble gases within the core of a small differentiated planetesimal, a crucial missing link in understanding planetary core formation. The findings strongly support the hypothesis that Earth's core is a significant reservoir of solar noble gases, contributing to the mantle's solar-type signatures. Future research should focus on investigating other iron meteorites and improving models of core-mantle interaction to refine our understanding of the Earth's volatile geodynamics.

The study focuses on a single iron meteorite, and while similar findings in other iron meteorites suggest generalizability, further analysis is needed to confirm the ubiquity of this phenomenon. The exact conditions of solar wind irradiation of the protolith remain uncertain. While partition coefficients have been estimated, direct measurements under core-mantle boundary conditions are needed for more precise quantification of gas exchange.