Nitrogen and noble gases in micrometeorites

Abstract

Micrometeorites (MMs) currently represent the largest steady-state mass flux of extraterrestrial matter to Earth and may have delivered a significant fraction of volatile elements and organics to the Earths surface. Nitrogen and noble gases contents and isotopic ratios have been measured in a suite of 17 micrometeorites recovered in Antarctica (sampled in blue ice at Cap Prudhomme) and Greenland (separated from cryoconite) that have experienced variable thermal metamorphism during atmospheric entry. MMs were pyrolized using a CO2 laser and the released gases were analyzed for nitrogen and noble gas abundances and isotopic ratios by static mass spectrometry after specific purification. Noble gases are a mixture of cosmogenic, solar, atmospheric, and possibly chondritic components, with atmospheric being predominant in severely heated MMs. delta-15N values vary between -240 +/- 62 ppm and +206 +/- 12 ppm, with most values being within the range of terrestrial and chondritic signatures, given the uncertainties. Crystalline MMs present very high noble gas contents up to two orders of magnitude higher than carbonaceous chondrite concentrations. In contrast, nitrogen contents between 4 ppm and 165 ppm are much lower than those of carbonaceous chondrites, evidencing either initially low N content in MMs and/or degradation of phases hosting nitrogen during atmospheric entry heating and terrestrial weathering. Assuming that the original N content of MMs was comparable to that of carbonaceous chondrites, the contribution of nitrogen delivery by these objects to the terrestrial environment would have been probably marginal from 3.8 Gyr ago to present but could have been significant (~10%) in the Hadean, and even predominant during the latest stages of terrestrial accretion.