Cosmic ray spallation and the special anomaly in achondrites

Cosmic ray spallation and the special anomaly in achondrites


Examination of the isotopic composition of xenon from achondrites reveals that the124Xe, 126Xe, 128Xe and130Xe are all linearly correlated indicating only two principal components for this isotopes-trapped and cosmic ray-produced xenon. No evidence is found for the reaction 127I(n, γβ)128Xe in the achondrites. We thus place approximate limits on the neutron flux relative to the flux in the Abee chondrite. A correlation in the plot of124Xe/130Xe versus129Xe/130Xe points to an a129Xe/126Xe ratio of ≤ 0.84 in xenon from cosmic ray spallation and allows us to set a lower limit on the amount of radiogenic129Xe from the decay of extinct129I in the achondrites. The average value of ≳4.4 × 10−12 cm3/g excess129I for achondrites contrasts with typical values of ∼ 10−10 cm3/g for ordinary chondrites and ∼ 10−9 cm3/g for enstatite chondrites.

We have presented Rb-S, ~ ages on several igneous rocks from the Apollo 12 mission [6,7,10]. With the exception of rock 12013, which is quite distinctive, all of the basaltic rocks studied by us have yielded ages between 3.16 and 3.36 AE.

The high I values for some Apollo 14 rocks are indicative either of magma sources with comparatively large differences in Rb/Sr in the mantle of tile moon (although the average Rb/Sr in such a mantle is still very low) or of possible contamination of upwelling low Rb/Sr magmas with an early formed lunar crust with high Rb/Sr. Tile existence of extremely high initial values had been found previously for 12013 [10] so some source regions with extremely high Rb, U, K, and Th enrichments must exist. The observations on 12013, in conjunction with other lines of evidence, indicate that the moon formed a crust rich in U, Th, K, and Rb near the tile time of its formation.



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