CARBON, NITROGEN, ARGON AND HELIUM STUDY OF IMPACT DIAMONDS FROM EBELIAKH ALLUVIAL DEPOSITS AND POPIGAI CRATER

Nineteen diamond aggregate specimens (1-2 mm in size) from impactites of Popigai crater and five diamond samples (5-7 mm in size) from Ebeli akh river placers were studied. Our investigations indicate that sampl es fi om Ebeliakh were formed in an impact event with the exception of one specimen (Y7). The carbon isotopic composition of diamonds from P opigai varies within the previously reported limits (delta(13)C, -8 to -22 parts per thousand); whereas, diamonds from Ebeliakh placers show heavier values of delta(13)C (-7 to -10 parts per thousand). All the specimens studied contain very low amounts of N, mostly <20 ppm, but a few contained up to 60 ppm. For specimens, where the quantity of N al lowed reliable analysis, delta(15)N values were found to be in the ran ge of -3.9 to +11.9 parts per thousand. On the basis of combined Ar an d N study, it was concluded that impact diamonds studied here can be a mixture of at least two types of gas carriers (e.g., different diamon d components). A possible explanation would be involvement of a carbon vapour deposition (CVD) process or diamond growth in the impact melt in addition to the direct graphite-diamond shock transformation. The d elta(13)C distributions and different N/Ar-36 correlations have indica ted a difference between impact diamonds from Ebeliakh and diamonds ex tracted from Popigai crater. This could be explained by the existence of different diamond populations formed during the Popigai impact even t. On the other hand, Ebeliakh diamonds could have resulted from a sep arate impact event to Popigai and an alternative crater is yet to be f ound.