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

Earth’s mantle contains solar-type helium and neon, unlike the atmosphere which is enriched in heavier, planetary-type noble gases. High 3He/4He and solar-like 20Ne/22Ne in plume-derived ocean island basalts imply the presence of a primitive reservoir, but its precise location (deep mantle vs. core) remains unresolved. Models invoke either incorporation of solar wind-irradiated solids or dissolution of nebular gases into a magma ocean during accretion, with the latter likely of limited efficiency. Differences between OIB and MORB noble gas signatures require at least two mantle reservoirs, and some studies propose Earth’s core as a contributor to mantle solar-type He and Ne. Iron meteorites represent analogues of planetary core materials, yet cosmogenic noble gases typically obscure primordial signatures. The Washington County iron meteorite has been reported to contain excess non-cosmogenic He and 20Ne, but prior studies lacked well-constrained sample locations and may have included surface-implanted solar wind. This study tests whether interior metal from Washington County contains solar noble gases, bearing on whether cores of differentiated bodies (and Earth’s core) can host and supply solar noble gases to the mantle.

• Mantle plumes exhibit high 3He/4He and solar-like 20Ne/22Ne, in some cases reaching or exceeding solar wind Ne-B values, suggesting a primitive reservoir possibly linked to nebular gas capture, though capture efficiency is debated.
• MORB sources show lower 3He/4He and 20Ne/22Ne, implying a degassed, homogenized upper mantle distinct from a deep, gas-rich reservoir.
• Proposed sources for mantle solar noble gases include solar wind implantation into building blocks and magma ocean dissolution; some models allow a core contribution to mantle noble gas fluxes (total or OIB-only).
• Iron meteorites are core analogues formed by early differentiation; cosmogenic noble gases from long space exposure complicate detection of primordial components.
• Washington County previously showed excess non-cosmogenic He and Ne; a claim of unfractionated solar He–Ne–Ar was later retracted due to potential recent surface implantation. Other iron meteorites also show light solar noble gases, but their origin (parent-body vs. transit irradiation) is unclear.

• Samples: A 3-cm slab of Washington County (WC_3078A) was cut into 15 aliquots spanning 0.2–2.8 cm from the fusion crust to interior. Four aliquots (WC_2, WC_5, WC_11, WC_14) were selected for high-resolution stepwise heating. Additional splits (WC_g, WC_s) and a schreibersite-rich acid residue (WC_r; 14.1 mg residue from 1.03 g dissolved material yielding ~3–5 µm schreibersite) were analyzed at a second laboratory.
• Mineralogy: SEM re-examined primary phases; focus on identifying noble gas carriers. Degassing patterns identified two main hosts: schreibersite ((Fe,Ni)3P) and metal (kamacite–taenite).
• Gas extraction: Stepwise heating from 600 to 1800 °C (up to 25 steps for WC_5) in a resistance-heated furnace (Ta tube, Mo crucible); Mainz splits to 2000 °C. Active gases removed via cold SAES Al–Zr and Ti getters; Ar separated by LN2-cooled charcoal traps; He/Ne separated cryogenically (He at 48 K; Ne released at 120 K). For some runs Al–Zr getters were omitted after leak detection to minimize air contamination.
• Measurements: VG 3600 noble gas mass spectrometer; He and 40Ar on Faraday cups; others by single-ion counting. Interference checks and corrections (masses 18, 40, 42, 44) applied. Calibration gases bracketing samples corrected mass fractionation; He standard 4He/3He = 40183 ± 87. Blanks determined between 800–1800 °C; blank compositions assumed air-like within stated uncertainties; blank contributions small for cosmogenic nuclides.
• Data treatment: He–Ne isotope ratios and concentrations plotted in 4He/21Ne–4He/3He and 20Ne/22Ne–21Ne/22Ne spaces to assess mixing between cosmogenic (GCR) and solar components (SW or Ne-B). Ar data were not further interpreted due to low SW abundance and high cosmogenic production.
• Temperature-release association: Major degassing peaks at ~1100 °C (schreibersite) and ≥1400 °C (kamacite–taenite) allowed phase-specific interpretation using selected extraction steps and mineral separates.

• Interior metal samples of Washington County contain significant excess solar-type He and Ne. Bulk and stepwise-heating data plot along mixing lines between GCR and solar wind (SW) or Ne-B in both 4He/21Ne–4He/3He and 20Ne/22Ne–21Ne/22Ne diagrams, indicating a solar component in both schreibersite and kamacite–taenite hosts.
• Two principal gas-release peaks correspond to schreibersite (~1100 °C) and kamacite–taenite (≥1400 °C). Except for certain high-T steps (e.g., WC_s at 2000 °C and WC_14 at 1480 °C) showing minor air mixing, most extractions conform to GCR–solar mixing.
• Quantified solar components: ~4.8 × 10^−9 cm^3 STP g^−1 3He_solar and ~2.1 × 10^−8 cm^3 STP g^−1 20Ne_solar with a near-unfractionated He/20Ne ≈ 0.26 (SW ≈ 0.3).
• Partitioning constraints: Assuming SW-irradiated chondritic protoliths: required D_Ne between metal and bulk protolith to match Washington County abundances is ~4.6 × 10^−2 (CV) or ~3.4 × 10^−2 (E); for CI and CM, ~1.6 × 10^−1 and ~9.6 × 10^−2, respectively. Experimental constraints show D_He ≈ 11.8 ± 1.8 at ≤1 GPa (percolative core formation), implying efficient noble gas transfer into metal; at higher pressures, D_He ~10^−3–10^−2 and D_Ne ~10^−2–10^−1 (liquid metal–liquid silicate up to 16 GPa), enabling back-transfer at the CMB.
• Core contribution estimates: Using present mantle He flux 267–1070 mol yr^−1 (800 ± 170 mol yr^−1 recent estimate) and a post-moon-forming degassing interval of ~4.4 Ga, the required initial core 3He concentration is ~5.5 × 10^−11 cm^3 g^−1 to supply all mantle flux, corresponding to ~1% contribution of Washington County–type metal to Earth’s core. With higher Hadean fluxes (10×), ~2% would suffice. If only the OIB flux is core-sourced, required contribution drops by 10–100× to ~0.02–0.2%.
• Implications: Solar wind-irradiated noble gases were incorporated into planetesimal metal during segregation, demonstrating that solar signatures can enter and reside in planetary cores. Washington County provides the first definitive evidence of solar noble gases in small-body metal, bridging to planetary core formation processes.

The presence of solar-type He and Ne in interior phases of an iron meteorite demonstrates that solar wind–implanted noble gases can be carried into segregating metal during early differentiation. This directly supports the hypothesis that Earth’s core could have incorporated solar noble gases and could act as a long-term reservoir supplying solar signatures observed in mantle-derived basalts, especially OIBs. The phase-specific degassing peaks and isotopic mixing relationships in both schreibersite and kamacite–taenite confirm widespread distribution of the solar component unrelated to recent surface implantation. Partition coefficients consistent with experimental constraints indicate efficient uptake of noble gases into metal during low-pressure percolative core formation in planetesimals, and the lower metal–silicate partitioning at higher pressures provides a mechanism for noble gas return from the core to the mantle at the CMB. Quantitative flux calculations show that only a small fraction (≈1–2% for total mantle flux; ≈0.02–0.2% for OIB-only flux) of Washington County–like material contributing to Earth’s core is sufficient to explain present mantle solar noble gas signatures. These results suggest Earth’s core has played an active role in mantle noble gas geochemistry and may help maintain distinct mantle reservoirs by differential fluxing from below.

This study provides robust evidence that interior metal of the Washington County iron meteorite contains solar wind–derived helium and neon, distributed in both schreibersite and Fe–Ni metal. The data show mixing between cosmogenic and solar components and quantify near-solar He/Ne ratios, demonstrating that solar noble gases can enter and be retained in metallic cores of differentiated small bodies. Extrapolated to Earth, only a small contribution of such solar gas–bearing metal to the core is needed to account for mantle solar noble gas signatures, particularly for OIB sources. The work advances a coherent framework in which early solar wind irradiation of precursors, low-pressure metal segregation, and later core–mantle boundary exchange together shape Earth’s noble gas inventory. Future research should: (1) expand surveys of iron meteorites across groups to assess the prevalence and origins of solar noble gases; (2) determine noble gas partitioning under true CMB conditions; (3) model coupled core–mantle volatile exchange through time; and (4) refine constraints on the irradiation histories of planetary building blocks.

• Argon data were not informative due to low SW abundance and high cosmogenic production, limiting multi-element confirmation to He and Ne.
• Some high-temperature steps (e.g., WC_14 at 1480 °C; WC_s at 2000 °C) show deviations toward air mixing, indicating minor contamination in specific extractions.
• Lack of direct experimental noble gas partitioning data at core–mantle boundary pressures and temperatures introduces uncertainty in back-transfer efficiency estimates.
• Assumptions about protolith compositions (e.g., CV or enstatite chondrites) and irradiation levels affect inferred partition coefficients and inventories.
• Heterogeneous distribution of trapped solar gases at millimeter scales within the meteorite complicates bulk extrapolation.
• For other iron meteorites with light solar gases, the irradiation epoch (parent body vs. transit) remains unresolved, limiting broader generalization.