Irradiation et pré-irradiations de la brèche 14307
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Sample 14307,30, a gas-rich breccia (Group 1 of Warner, 1972) has been studied by coupling track method and light noble gas isotopic analysis. The breccia is made of a glassy dark matrix with embedded millimeter to sub-millimeter sized angular ligth xenoliths. These ones are breccia fragments of higher grade metamorphic facies (Group ⩾ 2). A lighter strata (∼ 0.5 cm thick) intersects the dark matrix.
Noble gas analysis have shown the dark matrix (36Ar = 5.4 × 10−4 cc STP/g) to be enriched in solar type gases with respect to the light fractions (36Ar ⩽ 2.2 × 10−4 cc STP/g). Themean value of the bulk ‘exposure age’ for different samplings is 180 ± 20 × 106 yr, as calculated from spallogenic3He,21Ne and126Xe contents, using our data and those of Bogard and Nyquist (1972). After appropriate correction for radiogenic40Ar, the ratio40Arexc/36Artr is about 5.
72 crystals (70-300µm diam) from one location (No. 12) in the matrix show aminimum track density distribution spreading over 3 orders of magnitude (from 2 × 106 up to 2 × 109 cm−2). The spectrum has at its lower edge a well defined peak (∼ 50% of total crystal number) around 3 × 106 cm−2). Grains with track density variations over a factor of 3 have a low abundance as compared to average lunar soils. Moreover the mineralogy of this location is peculiar due to its large abundance in orthopyroxenes. Considering the lower edge of the track density distribution amaximum surface residence time of 5 × 106 yr can be set for rock 14307 in itspresent shape;
11 feldspars (1-15µm diam) and 22 clinopyroxenes (70-130µm) have been studied in the light xenolith. All crystals have minimum track densities larger than 108 cm−2. No spatial variation of track-densities (2.5 ± 0.5 × 109 cm−2) were found in feldspars inside a millimeter-sized polished section. Clearly these tracks were not acquired by an irradiation of the xenolith as an individual entity, but survived its own formation as a breccia of Group 2. Therefore, solar energetic iron particle tracks have not been erased despite a complex mechanical and thermal history involved by two subsequent brecciation processes;
in the 10 other locations, crystals (70-200µm diam) either from the dark matrix or the lighter strata show a significant departure from the pattern observed in lunar soils; namely:
the minimum track density distribution is strongly peaked at high values (∼ 1-4 × 109 cm−2) for ∼ 95% of the crystals, the remaining ∼ 5% having low-values (0.2-1 × 107 cm−2);
the abundance (2%) of crystals with track density variation over a factor of 3 is about one order of magnitude less than in average lunar soils;
the magnitude of track density gradients within single crystals is small. In fact, thelargest track density variation versus depth found can be described by the relationℚ α D−0.5, in contrast with soil grains which generally exhibit a variation of the formℚ α D−1.1±0.4.
The above observations imply that the peculiar irradiation characteristics of these fossilized soils are more likely to be attributed to some wide scale process rather than to some accidental or local phenomena.
Attempts to account for these findings by present solar VH particle flux and energy distribution (as determined by Crozaz and Walker, 1971; Fleischeret al., 1971b; Priceet al., 1971), current estimates of lunar fine scale erosion, accumulation and turn-over rates, have proven essentially negative. The bulk ‘exposure age’ of the breccia, rather low by lunar soil standards, makes things even worse.
For lack of any better explanation, the above observations could be more easily understood by postulating a higher flux (by factors from ∼ 10 up to 200) and a harder energy spectrum (at least for particles with rigidity less than 0.3 GV) for the solar cosmic rays at the time the constituents of the breccia were part as loose grains of the lunar regolith.
KeywordsBreccia Track Density Lunar Regolith Lunar Soil Dark Matrix
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