Laurence E. Nyquist

Ages of pristine noritic clasts from lunar breccias 15445 and 15455

Abstract Rb-Sr and Sm-Nd isotopic studies were undertaken of two Apollo 15 breccias, 15445 and 15455, collected near Spur Crater on the Apennine Front. Seven pristine lithic clasts including four norites, one anorthositic norite, one troctolitic anorthosite and one spinel troctolite, as well as a matrix sample of 15445, were analysed. In addition, a K-Ar age determination was also made for a plagioclase separate from a different anorthositic norite clast. Sm-Nd isotopic data of a pristine Mg-norite sample, 15445,17, from the large white Clast B of breccia 15445 yielded a precise internal isochron age of 4.46 ± 0.07 Ga for λ( 147 Sm ) = 0.00654 Ga −1 and an initial 143 Nd 144 Nd of 0.506058 ∓ 0.000078 (normalized to 148 Nd 144 Nd = 0.24308) corresponding to an initial ϵNd of +0.71. However, Sm-Nd isotopic results of another Mg-norite sample, 15445,247, chipped from the same Clast B, about 1 cm away from 15445,17, yielded a significantly younger isochron age of 4.28 ± 0.03 Ga and a higher initial 143 Nd 144 Nd of 0.506246 ∓ 0.000037 corresponding to an initial ϵNd of −0.35. The Sm-Nd data indicate that Clast B is heterogeneous and contains at least two similar lithologies. The Rb-Sr isotopic systems were severely disturbed for both norites. No useful isochron ages were observed. The Sm-Nd age and ϵNd data for A-15 Mg-norites 15445,17 and 15445,247, A-17 Mgnorite 78236, and two Mg-gabbronorites 73255,27,45 and 67667 are clearly resolved from each other. Four distinct parental magmas are required for derivation of these two major groups of Mg-suite noritic rocks. The Rb-Sr isotopic system of an anorthositic norite clast, 15455,228, was slightly disturbed; and isotopic data defined an isochron age of 4.59 ± 0.13 Ga for λ( 87 Rb) = 0.0139 Ga −1 or 4.55 ± 0.13 Ga for λ( 87 Rb) = 0.01402 Ga −1 . The corresponding initial 87 Sr 86 Sr for the rock is 0.69899 ∓ 0.00006 and is in satisfactory agreement with the LUNI value of Nyquist et al. (1974). The Sm-Nd system is more disturbed than the Rb-Sr system for this rock. A tie-line between whole-rock samples and a pyroxene separate gives a Sm-Nd age of 4.53 ± 0.29 Ga, which is in nominal agreement with the Rb-Sr isochron age. The 39Ar40Ar age of a plagioclase separate of the rock has a weighted average value of ~ 3.83 Ga, suggesting that the age was reset by the Imbrium impact event. Probably, the Sm-Nd and Rb-Sr systems of the clast were also partly reset by this event. Our age results indicate that some Mg-suite rocks are as old as ferroan-anorthosite-suite rocks. Furthermore, age data of three major crustal rocks (a Mg suite, a ferroan-anorthosite suite, and an evolved suite) show that they all have variable ages. These observations are difficult to explain by the “magma ocean” hypothesis alone. Petrogenetic processes such as production of Mg-suite rocks by “serial magmatism” ( Walker , 1983), of ferroan-anorthosite-suite rocks by “multiple diapiric intrusions” ( Longhi and Ashwal , 1985), and evolved-suite rocks by “continuous magmatism” ( Meyer et al., 1988, 1989) probably all played a role. These processes seem to be able to accommodate the wide ranges of ages and different origins observed for lunar crustal rocks.

Radiometric chronology of the Moon and Mars

“How old is the Earth?” was a topic of scientific inquiry in the late nineteenth century. In the twentieth century the question became “How old are the Earth and other objects in the Solar System?” Related questions are: “How old is the Solar System?”; “How has the Earth changed over geologic time?”; and “How have the planetary bodies in the Solar System changed over time?” Also, “What has made Earth unique in our Solar System?” With the aid of spacecraft-acquired data and samples, and through the study of lunar and Martian meteorites, we are beginning to answer some of these questions as they relate to the Moon and Mars.

Antarctic polymict eucrite Yamato 792769 and the cratering record on the HED parent body

Yamato Y792769 is a polymict breccia containing pyroxenes of a limited range in chemical compositions. Compared to most other Yamato polymict eucrites, Y792769 includes fewer and smaller eucritic clasts with homogenized pyroxenes. Its fine-grained matrix is shock-compacted and sintered. The time of the last thermal event, which produced this texture and formed the breccia, is determined by the well-defined 39Ar-40Ar age of 3.99 ± 0.04Ga. The complete resetting of the 39Ar-40Ar age is consistent with the texture of Y792769, as viewed in the transmission electron microscope (TEM), which suggests shock compaction converted part of the matrix plagioclase to maskelynite. Sm-Nd data define an apparent isochron corresponding to an age of 4.23 ± 0.12Ga. This apparent age probably reflects partial resetting of the Sm-Nd system during breccia formation, but similar ages recorded in the highest-temperature argon releases and in the Rb-Sr model age of relict plagioclases lend some support to its reality. Sm-Nd data for relict pyroxenes fall on a 4.56-Ga Sm-Nd reference isochron and on or above a 4.56-Ga Rb-Sr reference isochron, suggesting slight loss of rubidium during breccia formation. The pyroxene data are most suggestive of a “conventional” ~4.56 Ga age for the protolith rocks. The ~3.99 Ga39Ar-40Ar age predates the accepted age of formation of the lunar Imbrium and Serenitatis basins but probably reflects a period of intense meteoroid bombardment that affected the entire inner solar system.

Issues in dating young rocks from another planet: Martian shergottites

Abstract The 40Ar/39Ar ages of a group of Martian meteorites called shergottites are systematically older by about 25% or more than ages obtained using Sm–Nd and other radiometric dating methods. The older 40Ar/39Ar ages indicate the presence of 40Ar not derived in situ from the radiogenic decay of 40K. The ‘excess’ argon can be associated with several different components, including the Martian atmosphere and mantle. We discuss the sources of Ar in shergottites, and the methods used to separate and identify them.