The Lunar Breccia Dhofar 1442: Noble Gases, Nitrogen, and Carbon Released by Stepwise Combustion and Crushing

Abstract

AbstractStepwise crushing and combustion methods were applied to study the KREEP-rich lunar breccia Dhofar 1442, the clastic material of which is cemented by porous matrix. The stepwise crushing released significant amount of gases of extraterrestrial origin from gas voids. Argon, nitrogen, and carbon are simultaneously released by stepwise combustion at 1100°С. The simultaneous high-temperature degassing of these gases, as well as the coincidence of С/N ratio and nitrogen and carbon contents in high-temperature combustion steps with those of crushing indicate that the gas carriers are voids in high-temperature phases (in particular, minerals, glasses), which are decomposed/melted at these temperatures. Helium and neon are released from the same positions at lower temperatures. The isotopic composition of neon obtained by stepwise combustion and crushing corresponds to the composition of fractionated solar wind. The fraction of argon in the first crushing steps is higher than that of any other of studied gases. The 40Ar/36Ar in the trapped lunar argon is ~18, which is not consistent with empirical model implying that 40Аr is implanted from lunar atmosphere (McKay et al., 1986; Eugster et al., 2001; Joy et al., 2011). We believe that the entrapment of volatile elements in gas voids of the meteorite Dhofar 1442 was caused by the redistribution of gases from one structural sites into others during impact events that accompanied the cratering, in particular, leading to the formation of the impact melt breccia Dhofar 1442. The trapped gases of the meteorite Dhofar 1442 contain not only typical volatile components (solar, radiogenic, cosmogenic, re-implanted 40Ar) of lunar breccias, but also nitrogen and carbon formed through the oxidation of organic matter of metamorphosed chondrites, which are present in the breccia. With increasing number of strokes and, correspondingly, a degree of crushing, the elemental ratios change. A slight decrease of 4He/20Ne ratio during crushing is likely related to the different diffusion ability and permeability of helium relative to neon under temperature influence and/or to the heterogeneous distribution of these gases in voids of different size. The 4He/36Ar, 20Ne/36Ar, 14N/36Ar, and 12С/36Ar ratios increase by factors of 10–100 during crushing. This can be explained by the combination of dynamically different processes leading to the argon fractionation relative to other gases and uneven redistribution of gases from different positions in voids of different sizes during impact metamorphism.