N. Grögler

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. n nAmong 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. n nThe 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. n nThe 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. n nThe 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.

Über die Verteilung der Uredelgase im Meteoriten Khor Temiki

Die Konzentrationen von He, Ne und Ar und ihre Isotopenzusammensetzung im Aubriten Khor Temiki werden bestimmt. Er enthalt grosere Mengen von leichten Uredelgasen. Diese sind, wie Messungen an separeerten, Strukturelementen zeigen, nur in der feinkornigen Grundmasse (Matrix) enthalten, nicht aber in den groseren Enstatitkristallen. Edelgasmessungen an 6 Korngrosenfraktionen der Matrix ergeben, das die Uredelgaskonzentration mit abnehmender Korngrose zunimmt, wobei die feinste Fraktion (3 μ) sechsmal mehr Uredelgase enthalt als die groseren Enstatitkristallen. Edelgasmessungen an 6 Korngrosenfraktionen der Matrix ergeben, das die Uredelgaskonzentration mit abnehmender Korngrose zunimmt, wobei die feinste Fraktion (3 μ) sechsmal mehr Uredelgase enthalt als die grobste Fraktion (90 μ). Mit zusatzlichen Mineralseparationen wird gezeigt, das es sich nicht um einen verkappten Mineraleffekt handelt. Die experimentell gefundene Antikorrelation zwischen Korngrose und Uredelgasgehalt kann zwanglos als Oberflacheneffekt interpretiert werden. Die Bedeutung dieses Resultates fur die verschiedenen Theorien uber den Einbaumechanismus und die Herkunft der Uredelgase wird diskutiert.

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.

Trapped solar wind noble gases, kr81/kr exposure ages and k/ar ages in apollo 11 lunar material.

Grain size and etching experiments show that the fine lunar material contains large amounts of trapped solar wind particles. Elemental and isotopic compositions of the noble gases in solar material and in the terrestrial atmosphere are significantly different, except for the Ar36/ Ar38 and the Kr isotope ratios. Exposure ages of two rocks and of the fine material are between 380 and 510 x 106 years. Feldspar concentrates give K/Ar ages of 3220 and 3300 x 106 years, significantly higher than the unseparated rock.

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.

Distribution of rare gases in the pyroxene and feldspar of the Khor Temiki meteorite

Abstract From the matrix of the Khor Temiki aubrite three grain size fractions (2.9 μ, 6.2 μ, 11.1 μ) highly enriched in feldspar and three corresponding pyroxene grain size fractions were prepared. The concentrations and isotopic compositions of He, Ne and Ar were measured in these six fractions. Diffusion loss of radiogenic, spallation and trapped rare gases in the feldspar is evident. The pyroxene shows no diffusion loss of spallation produced isotopes, even in micron sized grains. In feldspar38Arspall is not lost; up to 50% of the21Nespall may have been lost; and virtually no3Hespall is retained.40Ar is partially lost in the finest (2.9 μ) feldspar fraction. The trapped gases in the pyroxene show again a strong anti-correlation with the grain size. The feldspar contains less than 10% as much trapped4He as the corresponding pyroxene, approximately 30% trapped20Ne and between 60 to 80% trapped36Ar.

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.

The (78Kr/83Kr)sp(131Xe/126Xe)sp correlation in apollo 12 rocks

Abstract The Apollo 12 rocks show, similar to the Apollo 11 rocks, a good correlation between the 78 Kr/ 83 Krand the 131 Xe/ 126 Xe ratios of the spallation component. The correlation line is distinctly different for Apollo 11 and 12 rocks, reflecting the difference in the Sr/Zr abundance ratio. Correlation lines as function of the Sr/Zr ratios are derived. A resonance integral of the order of 150 barns would be required to explain the excess 131 Xe sp by the capture of epithermal neutrons in 130 Ba.