Diamond in the lower mantle

Abstract

Diamond contains mineral inclusions of all three lower-mantle associations, juvenile ultramafic, mafic and carbonatitic; it is also an accessory mineral in all these associations. While the first two associations coexist with diamond, the carbonatitic association is a parental medium for the lower-mantle diamond. Physical and chemical characteristics of lower-mantle diamond differ from ones of lithospheric origin. Most of the lower-mantle diamonds are ‘nitrogen -free’ Type II variety. The others are usually low-nitrogen stones with the average nitrogen aggregation rate of 94%. The high proportion of nitrogen-aggregated diamonds suggests that they had a prolonged residence in the lower mantle under high-T conditions, which resulted in an almost complete transformation of single-atomic and paired nitrogen centers into polyatomic complexes. In contrast to lithospheric diamonds, almost all analyzed lower-mantle ones (70–89%) have noticeable levels of hydrogen centers (up to 4–6 cm−1). The isotopic compositions of lower-mantle diamonds are located within a narrow range: from −5.45 to −1.26‰ δ 13C VPDB, with an average value of −4.36‰Â ±Â 2.28‰ (2σ). It may be considered as the juvenile lower-mantle carbon isotopic composition. The isotopic composition of nitrogen for lower-mantle diamonds is located within a close range, from −5.2 to −1.0‰ δ 15Natm, with an average value of δ 15Natm = −3.00‰Â ±Â 2.37‰ δ 15Natm. Lower-mantle diamond was formed in carbonate-oxide parental melts and fluids, which experienced fractional crystallization with the decrease of temperature and changes in the melt composition. The most important role in this process belongs to the carbonate component in the parental melt.