Other
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...15f&link_type=abstract
Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 15
Other
Argon Isotopes, Chemical Composition, Feldspars, Lunar Rocks, Meteorites, Meteoritic Composition, Moon, Infrared Lasers, Laser Heating, Ultraviolet Lasers
Scientific paper
The lunar meteorite Dar al Gani 262 (DAG 262) was found in the Sahara Desert in Libya on March 23,1997. This was the first lunar meteorite found in a desert and is the thirteenth lunar meteorite discovered. DAG 262 is a polymict anorthositic lunar highland breccia. The Ar-40-Ar-39 dating technique has been applied to DAG 262 in an attempt to determine the crystallization age and shock events experienced by this meteorite. Previous studies have indicated that this meteorite may have suffered up to four shock events. Due to the brecciated nature of the rock and the likelihood of multiple shock events, a laser-probe technique has been used to analyze individual components (minerals and clasts) of the meteorite. The sample supplied to us ( about 1.5 g) shows two distinct clast types (1) feldspathic and (2) basaltic, the latter not having been previously described. Plagioclases show fractures and undulatory extinction as the result of shock events. The feldspathic clast has small, round low-Ca pyroxenes distributed within it. Electron-microprobe analyses of feldspar and pyroxene are given. The bulk composition as determined by suggests that the source of this clast is the ferroan-anorthositic suite. The basaltic clast shows a very fine intergranular texture with some larger plagioclase grains. The two clasts are separated by a melt-glass matrix that contains plagioclase with the same chemical composition as those plagioclases within the feldspathic clast. A devitrified mafic-glass spherule of - 150 mm diameter was identified within the matrix as well as other small fragments of possible glass spheres. Some dendritic veins can be observed on the outer part of the section, which formed after breccia lithification. Two laser Ar-40-Ar-39 experiments are being carried out: (1) infrared laser-stepped heating of feldspathic fragments that have been separated from the meteorite; and (2) ultraviolet laser spot fusion (50 micron) of a slice of meteorite (about 1.0 x 0.5 cm) showing both clast types. Preliminary results from experiments (1) have been obtained so far. Three feldspathic fragments (DAG 1-3, each weighing about 1 mg) have been heated using between 20-30 steps, of increasing laser output power, each of 1 min duration. A plot of Ar-36/Ar-40 VS. Ar-39/Ar-40 for 20 extractions obtained from DAG3. Overall, the data show increasing Ar-36/Ar-40 and decreasing Ar-39/Ar-40 as heating proceeds. In detail the release is more complex; the plot shows a significant decrease in Ar-39/Ar-40 corresponding to an increase in apparent age from 1.3 to 5.1 Ga. This variation may be attributed to the combined effects of terrestrial alteration that result in loss of radiogenic Ar-40 and implantation of lunar Ar-40 in grain boundaries that lead to an increase in apparent age. Points 5-20 define a linear correlation interpreted to represent mixing between radiogenic Ar-40, released at intermediate temperature, and trapped Ar that dominates the high temperature release. The intercept of the correlation on the Ar-39/Ar-40 axis corresponds to an age of 1.95 Ga. This age is low compared with other lunar meteorites and is most likely related to a shock event. The trapped component released at high-temperature has Ar-36/Ar-40 of about 2.8, which is close to the value of about 2.5 reported previously for the bulk meteorite. The increasing proportion of trapped Ar released at high temperature is contrary to the usual release pattern obtained for lunar soils. Two possible explanations are given: (1) Ar loss from grain boundaries occurred at depth within the lunar regolith, while preserving trapped Ar with higher Ar-36/Ar-40 in the center of grains; or (2) release of trapped Ar from a low-K-bearing mineral present in feldspathic fragments. Anomalously high apparent ages (>4.5 Ga) obtained at low temperature from steps 3 and 4 indicate that DAG3 contains implanted lunar atmosphere Ar-40 presumably acquired near the surface of the lunar regolith. This is inconsistent with the explanation (1) that loss occurred at depth. Fine-grained pyroxene is a common component in the matrix of DAG 262 [21, has low K content, and will retain Ar to a higher temperature than plagioclase making explanation (2) the most likely. Release of Ar from fragment DAG1 was similar to that of DAG3 giving an indistinguishable intercept age of 2.0 Ga. However, DAG2 contained much lower levels of trapped Ar and 19% of the total 39Ar released at high temperature gave an age of 3.05 Ga. This age is slightly below previously inferred total gas ages for lunar meteorites and may be related to the crystallization event. These are based on plots of 37Ar/36Ar VS Ar-38/Ar-36 and give ages of 185 Ma, 252 Ma and 122 Ma for DAG1, DAG2 and DAG3 respectively. The cosmic-ray exposure ages indicate the components of DAG 262 had different exposure histories, an expected result given that this meteorite is a breccia. The K/Ca weight ratio calculated from Ar-39/Ar-37 of 3.6 x 10 -3 is similar to the bulk ratio obtained of 3.8 x 10-3. A much lower K/Ca of 0.6 x 10-3 obtained from feldspar by electron microprobe is evidence for the impurity of the feldspar separates analyzed for the age study. The low K/Ca ratio of the feldspar compared to the bulk indicates that there is an additional K-bearing phase in DAG 262 that remains to be identified. Current investigations are aimed at using the UV laser to determine Ar-40/Ar-39 ages from individual minerals within DAG 262. (Additional information contained in original.)
Burgess Ray
Fernandes Vera Assis
Turner Gary
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