The Lohawat howardite: Mineralogy, chemistry and cosmogenic effects

Abstract

Abstract— The Lohawat meteorite is a texturally heterogeneous breccia having a variety of mineral and lithic fragments. Among mineral fragments, pyroxenes show a wide range of composition (Wo 0.011 –0.17En 0.37–0.78 Fs 0.21–0.60 ) whereas plagioclase is anorthitic (An 0.92 Ab 0.07 Or 0.007 ). Abundant rounded “chondrule‐like” objects ranging in size up to ∼7 mm, some with concentric layering, have been observed. Petrographic features, trace element composition and rare earth element patterns show the presence of eucritic and diogenitic components confirming that it is a typical howardite. Cosmogenic tracks, rare gases (He, Ne, and Ar) and radionuclides ( 22 Na and 26 Al) were measured. Track density in olivine and plagioclase varies between 0.7 to 6 times 10 6 /cm 2 . 38 Ar exposure age is estimated to be ∼110 Ma, being the highest among howardites. The track production rates correspond to ablation of 9 to 15 cm, implying a radius for its preatmospheric size of ∼27 cm. 22 Na/ 26 Al ∼ 1, as expected from the production models and solar modulation of galactic cosmic‐ray fluxes before its fall, suggesting that the meteoroid did not undergo any fragmentation during the past ∼2 Ma in interplanetary space. The radiogenic age based on K‐Ar method is 4.3 Ga while the U‐Th‐ 4 He age is 3.3 Ga indicating partial loss of He.