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Energetic proton irradiation history of the howardite parent body regolith and implications for ancient solar activity
Author(s) -
RAO M. N.,
GARRISON D. H.,
PALMA R. L.,
BOGARD D. D.
Publication year - 1997
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1997.tb01298.x
Subject(s) - regolith , meteorite , irradiation , cosmic ray , feldspar , parent body , pyroxene , astrobiology , astrophysics , physics , mineralogy , geology , quartz , olivine , chondrite , nuclear physics , paleontology
— Previous studies have shown that the Kapoeta howardite, as well as several other meteorites, contains excess concentrations of cosmogenic Ne in the darkened, solar‐irradiated phase compared to the light, non‐irradiated phase. The two explanations offered for the nuclear production of these Ne excesses in the parent body regolith are either from galactic cosmic‐ray proton (GCR) irradiation or from a greatly enhanced flux of energetic solar “cosmic‐ray” protons (SCR), as compared to the recent solar flux. Combining new isotopic data we obtained on acid‐etched, separated feldspar from Kapoeta light and dark phases with literature data, we show that the cosmogenic 21 Ne/ 22 Ne ratio of light phase feldspar (0.80) is consistent with only GCR irradiation in space for ∼3 Ma. However, the 21 Ne/ 22 Ne ratio (0.68) derived for irradiation of dark phase feldspar in the Kapoeta regolith indicates that cosmogenic Ne was produced in roughly equal proportions from galactic and solar protons. Considering a simple model of an immature Kapoeta parent body regolith, the duration of this early galactic exposure was only ∼3–6 Ma, which would be an upper limit to the solar exposure time of individual grains. Concentrations of cosmogenic 21 Ne in pyroxene separates and of cosmogenic 126 Xe in both feldspar and pyroxene are consistent with this interpretation. The near‐surface irradiation time of individual grains in the Kapoeta regolith probably varied considerably due to regolith mixing to an average GCR irradiation depth of ∼10 cm. Because of the very different depth scales for production of solar ∼Fe tracks, SCR Ne, and GCR Ne, the actual regolith exposure times for average grains probably differed correspondingly. However, both the SCR 21 Ne and solar track ages appear to be longer because of enhanced production by early solar activity. The SCR/GCR production ratio of 21 Ne inferred from the Kapoeta data is larger by a at least a factor of 10 and possibly as much as a factor of ∼50 compared to recent solar particle fluxes. Thus, this study indicates that our early Sun was much more active and emitted a substantially higher flux of energetic (>10 MeV/nucleon) protons.