Acid-susceptive material as a host phase of argon-rich noble gas in the carbonaceous chondrite Ningqiang

Abstract

A fine-grained dark inclusion in the Ningqiang carbonaceous chondrite consists of relatively pristine solar nebular materials and has high concentrations of heavy primordial rare gases. Trapped 36Ar concentration amounts to 6 x 10^(-6) cc STP/g, which is higher than that of Ningqiang host by a factor of three. Light HF-HCl etching of the dark inclusion removed 86, 73, and 64% of the primordial 36Ar, 84Kr, and 132Xe, respectively. Thus, the majority of the noble gases in this inclusion are located in very acid-susceptive material. Based on the elemental composition, the noble gases lost from the dark inclusion during the acid-treatments are Ar-rich, and the noble gases remaining in the inclusion are Q and HL gases. Transmission electron microscopy showed that the acid treatments removed thin Si, Mg, and Fe-rich amorphous rims present around small olivine and pyroxene grains in the dark inclusion, suggesting that the Ar-rich gases reside in the amorphous layers. A possible origin of the Ar-rich gases is the acquisition of noble-gas ions with a composition fractionated relative to solar abundance favoring the heavy elements by the effect of incomplete ionization under plasma conditions at 8000 K electron temperature.