Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v53b2155w&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V53B-2155
Mathematics
Logic
1115 Radioisotope Geochronology, 1160 Planetary And Lunar Geochronology, 3662 Meteorite Mineralogy And Petrology (1028, 6240), 6022 Impact Phenomena (5420, 8136), 6205 Asteroids
Scientific paper
Impact cratering is the dominant geologic process affecting the surfaces of small asteroids and, due to the relative ease of resetting the 39Ar-40Ar (Ar-Ar) system, heavily shocked and/or melted meteorites can be used to reconstruct the impact history of the asteroid belt. MIL05029 is an L-chondrite melt rock with an Ar-Ar age greater than 4.4 Ga, making it the third such meteorite known (the other two being Shaw and PAT91501). Moreover, while Shaw and PAT91501 have Ar-Ar ages between 4.4-4.5 Ga, three whole-rock samples of MIL05029 give well-defined plateau ages of 4.53±0.02 Ga, the oldest ever measured in an L-chondrite melt rock. As a result of this older age, the event that created MIL05029 could have occurred while metamorphism was still occurring on the parent body. Both Shaw and PAT91501 contain shocked relic material, indicating impact as the heat source. However, MIL05029 is a complete melt without this tell-tale signature, making definitive identification of the melt-forming event more complicated. MIL05029 is depleted in metal and sulfide relative to unshocked L-chondrites (<2% vs. ~14%), indicating a loss of denser phases after melting of the asteroid. Because the canonical L-chondrite parent body was not sufficiently hot for differentiation, the metal depletion in MIL05029 is either the product of shock-melting on that body (the more probable solution), or indicates MIL05069 formed on a previously unknown parent body where endogenous melting occurred. Petrologically, all three meteorites are grossly similar. For example, all three contain large pyroxene grains that poikilitically enclose olivine, but melt in both Shaw and PAT9150 contains vesicles, which is not true for MIL05029. The Ar-Ar data reveals two main sources of 39Ar (a proxy for K), both with similar activation energies, but with grain sizes different by about a factor of 5. Microprobe data also reveal two sources of K, melt inclusions and albitic feldspar, whose K mass balance and grain size match well with that determined from the Ar-Ar data. A small amount of 39Ar (~5%) released at high temperature (>1100°C) is unaccounted for, but is most likely due to recoil or melted feldspar.
Kring David A.
Swindle Timothy D.
Weirich John R.
Wittmann Axel
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