Computer Science
Scientific paper
Oct 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003e%26psl.215...43j&link_type=abstract
Earth and Planetary Science Letters, Volume 215, Issue 1-2, p. 43-56.
Computer Science
2
Nitrogen Isotopes, Lamproites, Crustal Recycling, Mantle, Eastern India
Scientific paper
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.
Fyfe William S.
Gupta Akash Kumar
Jia Yu
Kerrich Rob
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