U-Th-Pb systematics in three Apollo 14 basalts and the problem of initial Pb in lunar rocks

Abstract

The isotopic composition of Pb and the elemental concentration of U, Th and Pb were measured on 'total' rock samples 14053, 14073 and 14310 and on mineral separates of 14310 and 14053. Sample #73 appears to be quite similar to #310. Sample #310 yields total rock model ages of T(²⁰⁶Pb/²³⁸U) = 4.24AE, T(²⁰⁷Pb/²³⁵U) = 4.27AE, and T(²⁰⁸Pb/²³²Th) = 4.13AE. These are nearly concordant and distinct from the Rb-Sr and K-Ar crystallization ages of 3.88 AE. Mineral separates from 14310 show a wide spread in ²⁰⁷Pb/²⁰⁶Pb ranging from 0.483 to 0.995. The data points define a reasonable linear array on the coupled Pb-U evolution diagram. Similar analyses of 14053 give high, discordant total rock model ages of T(²⁰⁶Pb/²³⁸U) = 5.60AE,T(²⁰⁷Pb/²³⁵U) = 5.18AE, and T(²⁰⁸Pb/²³²Th) = 5.48AE. Mineral separates show a range of ²⁰⁷Pb/²⁰⁶Pb from 0.716 to 1.209. These data also define a reasonable linear array on the coupled Pb-U evolution diagram. These are the first Pb-U isochrons obtained for lunar basalts and indicate a reasonable solution to the previous discrepancy between the different methods of 'absolute' age determination. The resulting U-Pb isochron ages are compatible with the Rb-Sr and K-Ar ages on the same rocks. However, it is not possible to establish a precise time of 'crystallization' from the Pb-U data because of the small angle of intersection between the linear arrays and the concordia curve. These data show that total rock model ages do not in general yield crystallization ages. The data on #310 and #053 show that these rocks were formed containing a highly radiogenic initial lead (²⁰⁷Pb/²⁰⁶Pb)I0 ≈ 1.46 which accounts for the excessively high total rock model ages by the U-Th-Pb method. The only significant discrepancy in the data is the apparent variability of (²⁰⁸Pb/²⁰⁶Pb)I0 in #053 which remains to be resolved. The (²⁰⁷Pb/²⁰⁶Pb)I0 in these rocks corresponds to the radiogenic lead evolved between 4.51 and 3.88 AE in a U-rich environment. Such data from initial Pb may provide a new chronometer for early lunar evolution. The high ²⁰⁷Pb/²⁰⁶Pb ages in some total lunar soils as well as in treated fractions may be partly explained as a consequence of the contribution of lunar basalts with radiogenic initial Pb. The data prove that at the time of extrusion of some basalts, unsupported lead with extremely high ²⁰⁷Pb/²⁰⁶Pb ratios was added to the lunar surface.

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