ENERGETIC PROTON IRRADIATION HISTORY OF THE HOWARDITE PARENT BODY REGOLITH AND IMPLICATIONS FOR ANCIENT SOLAR-ACTIVITY

Previous studies have shown that the Kapoeta howardite, as well as sev eral other meteorites, contains excess concentrations of cosmogenic Ne in the darkened, solar-irradiated phase compared to the light, nonirr adiated phase. The two explanations offered for the nuclear production of these Ne excesses in the parent body regolith are either from gala ctic cosmic-ray proton (GCR) irradiation or from a greatly enhanced fl ux of energetic solar ''cosmic-ray'' protons (SCR), as compared to the recent solar flux. Combining new isotopic data we obtained on acid-et ched, separated feldspar from Kapoeta light and dark phases with liter ature data, we show that the cosmogenic Ne-21/Ne-22 ratio of light pha se feldspar (0.80) is consistent with only GCR irradiation in space fo r similar to 3 Ma. However, the Ne-21/Ne-22 ratio (0.68) derived for i rradiation of dark phase feldspar in the Kapoeta regolith indicates th at cosmogenic Ne was produced in roughly equal proportions from galact ic and solar protons. Considering a simple model of an immature Kapoet a parent body regolith, the duration of this early galactic exposure w as only similar to 3-6 Ma, which would be an upper limit to the solar exposure time of individual grains. Concentrations of cosmogenic Ne-21 in pyroxene separates and of cosmogenic Xe-126 in both feldspar and p yroxene are consistent with this interpretation. The near-surface irra diation time of individual grains in the Kapoeta regolith probably var ied considerably due to regolith mixing to an average GCR irradiation depth of similar to 10 cm. Because of the very different depth scales for production of solar similar to Fe tracks, SCR Ne, and GCR Ne, the actual regolith exposure times for average grains probably differed co rrespondingly. However, both the SCR Ne-21 and solar track ages appear to be longer because of enhanced production by early solar activity. The SCR/GCR production ratio of Ne-21 inferred from the Kapoeta data i s larger by a at least a factor of 10 and possibly as much as a factor of similar to 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.