Noble gases, nitrogen, cosmic ray exposure history and mineralogy of Beni M’hira (L6) chondrite

Ramakant R. Mahajan, Laridhi Ouazaa Nejia, Dwijesh Ray, Sekhar Naik

Planetary and Space Science
In Press, Accepted Manuscript, Available online 3 January 2018


• Beni M’hira belongs to Ordovician impact event, 4He gas retention age is 485 Ma.
• Cosmic ray exposure age of Beni M’hira is 15.6 Ma.
• Trapped noble gas composition (Ar, Kr, Xe) is Q type.
• Nitrogen isotopic composition in Beni M’hira is 14.6‰.”

“The concentrations and isotopic composition of noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon(Xe) and nitrogen were measured in the Beni M’hira L6 chondrite. The cosmic ray exposure age of Beni M’hira is estimated of 15.6 ± 3.7 (Ma). The radiogenic age, of around 485 ± 64 Ma, derived from 4He, and of around 504 ± 51 Ma from 40Ar, suggests an age resetting indicating the event impact. The heavy noble gases (Ar, Kr and Xe) concentrations imply that the gas is a mixture of trapped component Q and solar wind. The measured nitrogen abundance of 0.74 ppm and the isotopic signature of δ15N = 14.6‰ are within the range of ordinary chondrites. The homogeneous chemical composition of olivine (Fa:26 ± 0.25) and low-Ca pyroxene (Fs:22.4 ± 0.29) suggest that the Beni M’hira meteorite is an equilibrated chondrite. This is further corroborated by strong chondrule-matrix textural integration (lack of chondrules, except a few relict clast). Shock metamorphism generally corresponds to S5 (>45 GPa), however, locally disequilibrium melting (shock-melt veins) suggests, that the peak shock metamorphism was at ∼75 GPa, 950 °C.”