A. Stettler

Pre-Imbrian craters and basins: ages, compositions and excavation depths of Apollo 16 breccias

Breccia fragments have been analyzed from the 2–4 mm sieve fraction of three Apollo 16 soils collected in the vicinity of North Ray Crater (63503,17 at Station 13; 67603,1 and 67703,14 at Station 11). Ar39-Ar40 ages, Ar37-Ar38 exposure ages, abundances of major and certain trace elements, and petrographie data relevant to thermal history have been obtained for up to 48 individual fragments. Among the samples. 30 gave Ar39-Ar40 release patterns that allowed the assignment of a high- or intermediate-temperature plateau age and the recognition of three age groups. Group I (10 fragments) are 4.12-4.21 AE, Group 2 (13 fragments) are 3.89-4.02 AE, and Group 3 (6 fragments) are <2.5 AE in age. Only one fragment (3.60 AE) falls outside this grouping and possibly represents Theophilus ejecta. The probability that the gap between 4.12 and 4.02 AE is a statistical fluctuation is only ∼2%. The exposure ages cluster strongly around 50 × 106y. the age of North Ray Crater. The oldest, Group 1 fragments are all anorthositic metamorphosed breccias of light-matrix type. The younger. Group 2 fragments are noritic anorthosite and anorthositic norite breccias with textures indicative of greater annealing (melted matrix), one totally melted sample being of KREEP-basalt texture. The very young. Group 3 fragments are chiefly of glass or devitrified glass. There is a marked distinction between Groups 1 and 2 in compositional as well as textural properties. The Group 2 breccias are generally enriched in Mg, K and REE relative to the aluminous Group I breccias (eg. K ≤ 400 ppm in Group 1 and mostly ≥ 600 ppm in Group 2). This difference is attributed to the introduction of KREEP and mafic ANT components during the formation of the Group 2 breccias. The results are interpreted as reflecting two magnitudes of cratering. The older craters (>4.1 AE) were of medium size (diameters up to a few hundred kilometers), large enough to reset the ages but not capable of excavating deeper than predominantly feldspathic (anorthositic) layers of the crust. The younger craters (∼3.9-4.0 AE) were, in contrast, those ascribed to major basin-forming events and were therefore capable of excavating a deeper and wider spectrum of crustal lithologies. The latter resulted in admixture of KREEP and mafic ANT components with the feldspathic ANT, cover layer. KREEP was thus only excavated in abundance during the basin-forming events, from a sub-crustal layer formed initially at ∼4.4 AE but incorporated in the breccias at ∼4 AE. The KREEP-contaminated. Group 2 breccias have—except two fragments—ages between 3.95 and 4.02 AE. This group includes a crystallized melt (3.97 ± 0.04 AE), close in composition and texture to 14310 (3.87 ± 0.04 AE) which is generally attributed to the Imbrian basin-forming event (∼3.88 AE). The pre-Imbrian. Group 2 breccias of Apollo 16 can best be attributed to the Nectaris basin-forming event, which according to the clustered ages probably occurred at ∼3.98 AE. Our results support a multi-impact lunar cataclysm with the formation of Nectaris (3.98 AE). Humorum. South Serenitatis, Crisium and Imbrium (3.88 AE) within a 0.1 AE time interval.

How old is the crater copernicus

Two KREEP glass concentrates separated from lunar soil 12033 have been dated with the Ar39/Ar40 method. Both samples show low-temperature plateaus in accordance with a major outgassing of the KREEP glasses (800 ± 40) × 106 yr ago. This is the age of Copernicus, provided the identification of KREEP glass as ray material ejected during the Copernican event is true (Hubbardet al., 1971). The exposure age of the two KREEP glass concentrates is 200 × 106 yr and thus distinctly smaller than the ejection age. Possible explanations for this are discussed.

Absolute Time Scale of Lunar Mare Formation and Filling

The high titanium basalts collected in the maria Tranquillitatis and Serenitatis crystallized 3.5-3.9 Ga ago. The ages of the low titanium rocks found in Oceanus Procellarum and on the eastern edge of mare Imbrium are lower, 3.1-3.4 Ga. There is, however, evidence that high-Ti basalts with lower ages and low-Ti basalts with higher ages occur on the Moon. The observed age spread of rocks even in limited areas suggests that lava flow activity in a basin lasted for several 100 Ma. The age variability of Apollo 11 basalts is particularly well documented: there are at least three different times of rock formation, two for the low-K and one for the high-K rocks. The ages of the oldest mare basalts 10003 (high-Ti, low-K rock) and 14053 (an igneous rock with low-Ti, low-K, high-Al mare basalt composition) of 3.91 ± 0.03 Ga and 3.95 ± 0.03 Ga respectively, suggest that mafic basalt flows had already begun to invade the older basins when the last basin-forming impacts occurred.

Noble gas investigations of lunar rocks 10017 and 10071

Abstract The noble gases He, Ne, Ar, Kr and Xe and also K and Ba were measured in the Apollo 11 igneous rocks 10017 and 10071, and in an ilmenite and two feldspar concentrates separated from rock 10071. Whole rock K/Ar ages of rocks 10017 and 10071 are (2350 ± 60) × 106 yr and (2880 ± 60) × 106 yr, respectively. The two feldspar concentrates of rock 10071 have distinctly higher ages: (3260 ± 60) × 106 yr and (3350 ± 70) × 106 yr. These ages are still 10 per cent lower than the Rb/Sr age obtained by Papanastassiou et al. (1970) and some Ar40 diffusion loss must have occurred even in the relatively coarse-grained feldspar. The relative abundance patterns of spallation Ne, Ar, Kr and Xe are in agreement with the ratios predicted from meteoritic production rates. However, diffusion loss of spallation He3 is evident in the whole rock samples, and even more in the feldspar concentrates. The ilmenite shows little or no diffusion loss. The isotopic composition of spallation Kr and Xe is similar to the one observed in meteorites. Small, systematic differences in the spallation Kr spectra of rocks 10017 and 10071 are due to variations in the irradiation hardness (shielding). The Kr spallation spectra in the mineral concentrates are different from the whole rock spectra and also show individual variations, reflecting the differences in target element composition. The relative abundance of cosmic ray produced Xe131 differs by nearly 50 per cent in the two rocks. The other Xe isotopes show no variations of similar magnitude. The origin of the Xe131 yield variability is discussed. Kr81 was measured in all the samples investigated. The Kr81/Kr exposure ages of rocks 10017 and 10071 are (480 ± 25) × 106 yr and (350 ± 15) × 106 yr, respectively. Exposure ages derived from spallation Ne21, Ar38, Kr83 and Xe126 are essentially in agreement with the Kr81/Kr ages. The age of rock 10071 might be somewhat low because of a possible recent exposure of our sample to solar flare particles.

39Ar-40Ar ages of samples from the Apollo 17 station 7 boulder and implications for its formation

Abstract 39 Ar- 40 Ar ages and 37 Ar- 38 Ar exposure ages of samples representing four different lithologies of the Apollo 17 station 7 boulder were measured. The age of the dark veinlet material 77015of3.98 ± 0.04AE is interpreted as representing the time of intrusion of this veinlet into the 77215 clast. The data obtained so far indicate that the vesicular basalt 77135 formed 100–200 m.y. later. However, this has to be confirmed by 39 Ar- 40 Ar investigations on separated mineral and/or grain-size fractions. A small clast enclosed in the 77135 basalt gives a well-defined high temperature age of 3.99 ± 0.02AE . A sample of the noritic clast 77215 gave 4.04 ± 0.03AE , the highest age found so far in this boulder. The 39 Ar- 40 Ar ages obtained are in agreement with the age relationships deduced from the stratigraphic evidence. Taking into account the shielding by the boulder itself, an average 37 Ar- 38 Ar exposure age of (27.5 ± 2.5)m.y. is obtained for the samples collected from the boulder.

CORRELATION BETWEEN ROCK TYPE AND IRRADIATION HISTORY OF APOLLO 11 IGNEOUS ROCKS.

Abstract High-K and low-K Apollo 11 lunar rocks show different distributions of exposure ages T e . The low-K rocks group around 100 × 10 6 y(65 × 10 6 ≤T e ≤ 130 × 10 6 y; except one rock out of9) whereas the high-K rocks have either ages between 30 and 55 × 10 6 y or between 240 and 450 × 10 6 y. From the ( 78 K/ 83 Kr) sp versus ( 131 Xe/ 126 Xe) sp correlation diagram it is concluded that the low-K rocks were systematically exposed to a harder irradiation than the high-K rocks. The observed grouping of exposure ages is essentially in agreement with only three impacts originally ejecting the Apollo 11 rocks analyzed so far. Three models explaining the systematically higher shielding of the high-K rocks are discussed. Model A suggests that high-K rocks were initially one (or several) large boulder(s), reduced to present size by space erosion and break-up on the lunar surface. Low-K rocks were directly ejected as small rocks. Model B requires a stratigraphy of the lunar bedrock. High-K material must be closer to the surface than low-K material and has received a substantial preirradiation prior to the ejection. Model C requires that the high-K rocks came from small local craters, whereas the low-K rocks were ejected from a larger, more distant crater. Independent of any model, the exposure age group found for the low-K rocks corresponds to the time of formation of the crater or craters which produced these rocks.

He, Ne and Ar composition in a neutron activated sea-floor basalt glass

Abstract We have measured the composition of He, Ne and Ar in the glassy rim of a fresh oceanic basalt from the East Pacific Rise. Three splits of the sample were subjected to neutron irradiation prior to analysis to investigate the noble gas release as a function of different neutron doses. The shapes of the observed 39 Ar- 40 Ar degassing profiles depend critically on the irradiation dose due to a severe redistribution of excess 40 Ar. This shows that the application of the 39 Ar- 40 Ar method for dating of glassy samples requires the utmost care. Including the He and Ne results from the irradiated specimens, it was possible to determine the concentrations of Li and Mg and to estimate upper limits for Na and F in addition to the concentrations of K, Ca and Cl which are measured routinely as a byproduct of the 39 Ar- 40 Ar dating method. This provides a useful extension of the 39 Ar- 40 Ar technique since it may help to identify the outgassing sites during stepwise heating experiments. Furthermore it allows one to study the diffusion behavior of He and Ne isotopes artificially produced during irradiation. In our samples the activation energies of 3 He and 21 Ne tend to increase with growing neutron doses.

39Ar-40Ar systematics of two millimeter-sized rock fragments from Mare Crisium

Abstract Two small fragments, L24B, a glass-rich agglutinate (1.9 mg) and L24A, a fine-grained lithic fragment (9.4 mg), from the Luna 24 landing site have been neutron irradiated for the purpose of 39 Ar- 40 Ar dating. A fairly well-defined 39 Ar- 40 Ar plateau age of 3.65 ± 0.12 AE was found for the larger fragment. After appropriate corrections the composition of the trapped and spallogenic Ar could be deciphered. The evolution of 38 Ar sp / 37 Ar showed that 660 m.y. and 500 m.y. were the most reliable exposure ages for L24A and L24B, respectively. The Ti contents of ≤0.6% determined by gamma-counting prior to the Ar analysis indicate both fragments being associated with the group of low-Ti or even very low-Ti basalts.

Potassium-argon age of Apollo 11 rock 10003

Abstract The K-Ar ages of a whole rock sample and a feldspar concentrate from lunar rock 10003 were determined as (3.74 ± 0.06) × 10 9 y and (3.82 ± 0.05) × 10 9 y respectively. The K Ar age of the feldspar concentrate is in essential agreement with the high temperature K Ar age of (3.92 ± 0.07) × 10 9 y obtained by Turner [1] with the 40 Ar/ 39 Ar dating technique. Our results thus confirm the relatively high age of the low-K Apollo 11 rock 10003.