The Lohawat Howardite: mineralogy, chemistry and cosmogenic effects

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_0.37-0.78Fs_0.21-0.60) whereas plagioclase is anorthitic (An_0.92Ab_0.07Or_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 (²²Na and ²⁶Al) were measured. Track density in olivine and plagioclase varies between 0.7 to 6 times 10⁶/cm². ³⁸Ar 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. ²²Na/²⁶Al ∼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-⁴He age is 3.3 Ga indicating partial loss of He.