Ar-Ar studies of two lunar mare rocks: LAP02205 and EET96008

Introduction: Since 1999, the lunar sample collection has been supplemented with about 50 meteorites from the Moon. Many of these stones are paired, and the total number of different lunar meteorites in the present collection is 37. Of these 37, five are lunar mare basalts, which represents ~14% of the lunar meteorite collection. Considering that the maria represent ~17% of the lunar surface, the lunar mare basalts are a good representative of the lunar maria. When major and trace element composition of these lunar mare basalt meteorites are compared with mare basalts collected by the Apollo and Luna missions, it is clear that the basaltic meteorites represent basalt flows different from those the missions do. In the present work, we present new chemical and Ar-Ar age data for two lunar mare basalt meteorites LAP02205,12 and EET96008,45. Method: Prior to irradiation, meteorites LAP (2.36 mg) and EET (4.42 mg) were each split into two approximately equal halves: one for bulk IR-laser step heating, and the other was used for sample characterization using SEM and EMPA. The latter portion was then used for UV-laser spot analyses on individual minerals in an attempt to determine the crystallisation age and timing of any shock events experienced by the meteorites. Each of the fragments contained differing proportions of plagioclase and pyroxene, thus providing the possibility of extracting different geologic events from the Ar-release pattern at different temperatures during step heating. Further Ar-Ar analyses are currently being carried out on the fragments obtained from the meteorites sections. Samples LAP02205,12 This sample is a low-Ti lunar mare basalt, showing a coarse-grained subophitic texture [1-7]. The mineral composition of the section we have studied is dominated by relatively large crystals of plagioclase (100-600 μm) and pyroxene (100-500 μm). Previous work [1-7] has also indicated the presence of minor olivines. The plagioclase crystals are elongated subhedral to anhedral, the An content varies from 89 to 93, slightly higher than that reported by [7] but within range to that reported by [2], with K contents of 500 to >1000 ppm. The pyroxenes are anhedral showing magmatic zoning with a core richer in Mg than the rim (typical core has 8.5 wt% Mg and typical rim has 0.4 wt% Mg). The K content in pyroxenes is about 10 times lower to that of plagioclases suggesting that most of the 39Ar release will be released from the plagioclase. Maskelynite was found adjacent to an area of mesostasis and has a K content similar to that of plagioclase. Present wthin the mesostasis area are pure silica veins, FeSi, FeS, together with ilmenite (<5 μm) and glass with a chemical composition between that of plagioclase and pyroxene. The plagioclase crystals are highly fractured by shock disturbance. The pyroxenes do not present such shock features, although [1,2 & 7] report undulatory to mosaic extinction in pyroxene, and [7] suggest that LAP has experienced shock related pressures of <30 to >75 GPa. EET96008,45 This sample is a fragmented breccia dominated by basaltic phases and minor highland material and other clasts [8,9]. Warren and Ulff-Møller [10] suggested that the mare component of this meteorite originated either as a shallow intrusion or as an unusually deep-ponded flow. The section used for the Ar-Ar studies comprised two main regions: (1) having brecciated appearance, and (2) a gabbroic texture comprised mainly of plagioclase (~400 μm) and pyroxene (~400 μm), but also minor olivines. The large pyroxene crystals show exsolution lamellae superposed by shock related cracks. These lamellae are atypical for mare basalts [10]. The mare component of this meteorite show textural, mineral and geochemical similarities to low-Ti and VLT basalts [9&10]. The K concentration of plagioclase varies from 100-900 ppm, and pyroxene between 0-100 ppm, so that during Ar-Ar step heating experiments, 39Ar release will be exected to be dominated by plagioclase. U-Pb isochron ages of 3.53±0.27 Ga for apattite and 3.52±0.10 Ga for whitlockite were reported previously for this meteorite {9}. This meteorite has an ejection depth of 540600 g/cm2 from the lunar surface, a Moon-Earth transfer time of <<10 ky, and terrestrial age 80 ± 30 ky [11]. Ar-Ar Results: LAP02205 The Ar-Ar age spectrum and K/Ca results for LAP are shown in Fig. 1. The spectrum is relatively straightforward to interpret: the initial step, corresponding to ~8% of the Ar shows a high apparent age (~6.49 Ga) and has no geological significance, and is mostly due to surface related Ar. The following 28% of the 39Ar release is marked by a relatively high K/Ca values. Comparing with EMPA K/Ca, the likely origin of this release is a combination of phases such as K-rich glass and plagioclase. The remaining ~92% give fairly consistent apparent ages between 2.864±0.016 and 3.056±0.011 Ga with an integrated age of 2.915±0.010 Ga. Both of these ages are within error similar to those determined by Sm-Nd, Rb-Sr and Ar-Ar by [12], and the Pb-Pb age reported by [13].