2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 96-4
Presentation Time: 8:50 AM

N-ISOTOPE COMPOSITION OF THE PRIMITIVE MANTLE COMPARED TO DIAMONDS


JIA, Yiefei, SRK Consulting China Ltd, 8 Jianguomennei Dajie, Beijing, 100005, China and KERRICH, Robert, Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2, Canada

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.