Stable isotope analysis of carbon and nitrogen in angrites

Angrites are a small group of ancient basaltic achondrites, notable for their unusual chemistry and extreme volatile depletion. No comprehensive study of indigenous light elements currently exists for the group. Measurement of the abundances and isotopic composition of carbon and nitrogen could provide information pertaining to the evolution of the angrite parent body. Bulk-sample stepped combustion analyses of five angrites and a glass separate from D'Orbigny were combined with earlier data and acid dissolution experiments of carbonates found in D'Orbigny to compile an inventory of indigenous carbon and nitrogen. Indigenous carbon combusted between 700 degrees C and 1200 degrees C, with abundances of 10-140ppm and a mass-weighted C-13 of -25 to -20 parts per thousand with the exception of D'Orbigny (C-13 approximately -5 parts per thousand). Nitrogen was released at 850-1200 oC, 1-20ppm with a N-15 -3 parts per thousand to +4 parts per thousand; again, D'Orbigny (N-15 approximately +20 to +25 parts per thousand) was an exception. We interpret these components as largely indigenous and decoupled; the carbon in graphitic or amorphous form, while the nitrogen is present as a dissolved component in the silicates. No relationship with the textural sub-classification of angrites is apparent. We suggest that the angrite parent body contains a reservoir of reduced carbon and thus may have undergone a change in redox conditions, although the timing and mechanism for this remain unclear.