Enhanced SCR Proton Flux from the Early Sun, Revisited

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Greater concentrations of [21Ne]c in the dark phase of Kapoeta, compared to the light phase, may represent a large irradiation by solar protons (SCR) in the early solar system or a longer galactic cosmic ray (GCR) irradiation. Lunar rock studies demonstrate that the 21Ne/22Ne ratio can be used to distinguish between components produced by energetic SCR and GCR proton irradiations. The Ne isotopic composition in Kapoeta feldspars suggest the presence of a SCR component. We also have analyzed cosmogenic Xe in the same Kapoeta feldspars and pyroxenes and suggest that these data further support an early SCR irradiation. Cosmogenic 126Xe, hereafter [126Xe]c, is produced only by GCR interactions on Ba and La and not by SCR protons. If dark phase grains of Kapoeta were exposed to long precompaction GCR irradiation in the early solar system, and the light phase only to recent GCR irradiation, then we would expect larger amounts of [126Xe]c in the dark phase. We carried out stepwise extractions of etched pyroxenes and feldspar size separates. The measurement errors are 10-15 % for 132Xe concentrations and 30-35 % and 40-45 % for 126Xe/130Xe ratios in feldspars and pyroxenes, respectively. Ba and La contents determined by INAA of light and dark phases are, respectively, 12 ppm and 1.3 ppm for pyroxenes and 47 ppm and 4.5 ppm for feldspars. Seven out of eight samples, when normalized to their Ba contents, yield similar [126Xe]c concentrations within error limits. Although [126Xe]c in the light phases fall at the lower end of the observed concentration range, there is no tendency for Kapoeta dark phase samples to have higher [126Xe]c concentrations compared to the light phase. Assuming GCR [126Xe]c production rates of 1.3 x 10^(-15) cc/g-ppm Ba-My and 6.1 x 10^(-15) cc/g-ppm La-My, calculated GCR exposure ages are 4.4-6.2 My for pyroxenes and 4.9-6.0 My for feldspars (errors are 50-60 %). These values are similar to GCR ages of 3-4 My determined from 53Mn in light and dark phases of Kapoeta. We also measured [21Ne]c in pyroxenes of Kapoeta dark (five samples) and light (1 sample) of 3.4 x 10^(-8) and 0.8 x 10^(-8) cc/g, respectively, a factor of 4 difference. Previously we reported a factor of 2.5 enrichment of [21Ne]c in feldspars from Kapoeta dark compared to the light. After correcting for the fraction of irradiated grains in our samples, the enrichment factors observed are in agreement with those determined by. If these [21Ne]c excesses are GCR produced, our pyroxene data (where Ne is retentive) suggest that we should also see a [126Xe]c enrichnment factor of ~4 in Kapoeta dark compared to the light. As noted above, [126Xe]c apparently does not show the enrichment expected from an enhanced GCR irradiation. These observations, coupled with the evidence for SCR 21Ne from the 21Ne/22Ne ratio, are all consistent with a significant early irradiation of Kapoeta with energetic solar protons.

Presented at the 58th Annual Meeting of the Meteoritical Society, Washington, D.C.


Physics and Astronomy

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