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 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. 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.