15N-enriched Gondwana lamproites, eastern India: crustal N in the mantle source

A comagmatic suite of lamproites, ultramafic lamprophyres, and minettes was intruded at ∼100 Ma into thick siliciclastic sequences deposited in five intracratonic basins which developed during Gondwana rifting in eastern India. Lamproites are dominated by microphenocrysts of Ti-rich phlogopite, and olivine pseudomorphed by serpentine and talc, with variable modal abundances of aegirine, amphibole, apatite, carbonates, Cr-spinel, fluorite, perovskite, priderite, and titanomagnetite. Compositionally, lamproites are characterized by extremely high contents of TiO2 (4.2–10 wt%), K2O, and P2O5, over a range of Mg# from 80 to 75, in keeping with lamproites worldwide. They have high abundances of Ba, Th, U, and REE (1019–5400 ppm), with pronounced fractionation of REE, where La/Smcn=3.5–5.6, and Gd/Ybcn=7.3–18, consistent with residual garnet in the mantle source. Nitrogen isotope compositions range from +1.6 to +8.7‰ with an average of 3.8‰, and N contents average 107 ppm. Mantle δ15N averages −5‰, with a N content of 2 ppm, whereas continental crust varies from 2 to 6‰ with 30–90 ppm N. Atmospheric N2 is sequestered by N-fixing microorganisms, and stored as kerogen with an average δ15N of +3‰ in sedimentary rocks. Some of the organic N is converted to NH4+ which substitutes for K in crustal K-silicates. The 15N-enriched lamproites are interpreted to have formed from partial melting of enriched harzburgitic mantle lithosphere by decompressional melting accompanying extension. Enrichment resulted from subduction–erosion of continental crust, or subduction of sediments, low-degree partial melting, hybridization of the melts with mantle lithosphere and incubation, prior to decompressional melting. Consequently, some crustal N is recycled to the mantle.