Green spherules from Apollo 15: Inferences about their origin from inert gas measurements

Green spherules from the clod 15426 and from fines 15421 contain about 100 times less trapped inert gases than normal bulk fines from Apollo 15. These spherules have apparently never been directly exposed to the solar wind. Spherules from other fines contain about 10 times more trapped gas than those from the clod. The gas in the former is surface correlated. However, spherules from fines 15401 are exceptionally gas-poor. (He⁴/Ne²⁰) _T in all spherules is commonly less than 10, which implies severe He⁴ losses. (Ne²⁰/Ar³⁶) _T on the other hand is nearly always greater than 10; and ranges up to 20.3.The Ne²¹ _C and Ar³⁸ _C radiation ages vary from 22 to 750 × 10⁶ yr, but most of them lie in the range 200–400 × 10⁶ yr. He³ _C ages are always much younger, owing to He³ _C losses.The trapped gases can be of solar-wind origin, but this origin requires a two-stage model for the spherules from the clods. First, solar wind was trapped in a parent material, from which the spherules were formed, presumably by impact melting. When the spherules were formed, some fraction of the original gas was retained by them. Another possibility is that the gases were absorbed from an ambient gas phase.The trapped gases may also be assumed to represent primordial lunar gas. The composition of this gas is then similar to the solar or unfractionated component of gas-rich meteorites, but unlike that in most of the carbonaceous chondrites.The Ar⁴⁰-Ar³⁶ systematics show two families of spherules: those from 15426 and 15421 which define a line with slope of about 4–5; and those from the fines which fall near a line with slope of about 1.4–1.9. Both lines have similar Ar⁴⁰-intercept-values of about 3–9 × 10^–6 cm³ STP g^–1 of Ar⁴⁰. The corresponding K-Ar⁴⁰ age can be as old as 4300 or as young as 2500 × 10⁶.The gas content of the spherules from fines suggests strongly that all spherules were at one time in clod-like material. This, in turn, seems to imply that a body or layer of cloddy material like 15426 was, and perhaps still is present in the Apollo 15 landing area. Cone Crater impact has tapped this body, but has probably not produced the clods. The green material may have been transported to the Apollo 15 site from elsewhere either as impact-ejecta or by a volcanic eruption. Our results do not permit a choice between the two possibilities.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April, 1973.On leave of absence at the Max-Planck-Institut für Kernphysik, Heidelberg, Germany.