Nitrogen and light noble gases in Shergotty

Two samples of Shergotty and one of EETA 79001 lithology A have been analyzed for nitrogen and light noble gases. Nitrogen yields are of the order of one ppm, with isotopic ratios within a few per mil of the terrestrial atmospheric composition after correction for spallogenic nitrogen. The nitrogen data do not indicate the presence of any significant amounts of the high-δ15N component seen in EETA 79001 glass, nor do they show the isotopically light component, with δ15N of about −35%o, reported previously for Shergotty. The possibility that these components are present but masked by variations in the ArN ratio of the light component cannot be excluded. The spallogenic noble gas data are consistent with cosmic ray irradiation of both Shergotty and EETA 79001 at shallow shielding depths. Cosmic-ray exposure ages are in the range 0.5 to 0.8 My for EETA 79001 and 2.0 to 5.2 My for Shergotty, depending on the choice of object size and shielding. Interpretation of the spallation data is complicated by uncertainties in production rates, unknown contributions from solar cosmic rays, and the possibility that Shergotty has suffered some He loss. In the absence of He loss, the exposure age of Shergotty would lie in the range ~2.0 to ~3.5 My. Radiogenic argon appears to be very heterogeneously distributed on a small scale in Shergotty, due either to small scale mineralogical inhomogeneity or to significant variation in the degree of degassing of minerals during shock, although the presence of trapped argon with a high 40Ar36Ar ratio and heterogeneous distribution cannot be ruled out. The present study does not provide evidence for or against an origin of Shergotty on Mars.