N-isotope composition of the primitive mantle compared to diamonds

The nitrogen isotopic composition of the Earth's primitive mantle is controversial. Chromium-isotopic ratios of various terrestrial minerals and rocks, and chondritic meteorites are consistent with the silicate Earth being a mixture of enstatite and carbonaceous chondrites. From their relative proportions and N-isotope compositions we estimate that the bulk primitive mantle δ15N is − 7 ± 3‰. The negative value, as also evidenced by mantle-derived oceanic basalts and diamonds, is an intrinsic long-term feature of Earth's mantle. Some enstatite chondrite-like δ15N values down to − 24‰ measured in very rare diamonds could be interpreted as a heterogeneous mantle. δ15N values in oceanic island basalts derived from the deep mantle have three components: deep mantle of ~− 9‰ consistent with estimate, recycled sediments of about 15‰, and atmospheric N incorporated from groundwater and/or subducted atmospheric N (Mohapatra and Murty, 2000a; Chem. Geol. 164, 305-320). Some enriched δ15N values in MORB and OIB result from degassing fractionation. Shift of the upper mantle from an initial − 7‰ to − 5‰ by the Neoarchean can be explained by a combination of sediment recycling through subduction and upper mantle magma degassing processes.