HR: 0800h
AN: V41E-1503    [Abstracts]
TI: Nitrogen isotopic composition of ultramafic xenoliths
AU: * Marty, B
EM: bmarty@crpg.cnrs-nancy.fr
AF: Centre de Recherches P‚trographiques et G‚ochimiques, 15 rue Notre Dame des Pauvres, Vandoeuvre, 54501 France
AU: * Marty, B
EM: bmarty@crpg.cnrs-nancy.fr
AF: Ecole Nationale Sup‚rieure de G‚ologie, rue du Doyen Marcel Roubault, BP 40, Vandoeuvre, 54501 France
AU: Yokochi, R
EM: yokochi@crpg.cnrs-nancy.fr
AF: Centre de Recherches P‚trographiques et G‚ochimiques, 15 rue Notre Dame des Pauvres, Vandoeuvre, 54501 France
AU: Yokochi, R
EM: yokochi@crpg.cnrs-nancy.fr
AF: Dpt. of Earth and Environmental Sciences, University of Illinois at Chicago, SES2456, 845 W. Taylor St., Chicago, IL 60607-7059 United States
AU: Pik, R
EM: rpik@crpg.cnrs-nancy.fr
AF: Centre de Recherches P‚trographiques et G‚ochimiques, 15 rue Notre Dame des Pauvres, Vandoeuvre, 54501 France
AU: Chazot, G
EM: G.Chazot@opgc.univ-bpclermont.fr
AF: Laboratoire Magmas et Volcans, Universit‚ Blaise Pascal et OPGC, 5 rue Kessler, Clermont-Ferrand, 63038 France
AB: The nitrogen isotopic composition of mantle-derived samples provides important information on the origin of the terrestrial volatile and on the evolution of the mantle through time. Two decades of studies have revealed that there is an isotopic disequilibrium between the atmosphere (δ¹⁵N≡ 0 ‰) and the mantle source of MORB (δ¹⁵N = -3 to -5 ‰). The N isotopic composition of other mantle-derived materials, however, scatters significantly from -15.3 up to +30 ‰ in mantle xenoliths (by melting extraction) and diamonds (Cartigny et al., 1997: Mohapatra and Murty, 2000). Whether this variation reflects real mantle heterogeneity or is due to isotopic fractionation during e.g. magmatic processes is crucial to our understanding of the volatile element evolution of the Earth. N isotope ratios can be sensitive to (i) contamination by surface nitrogen , (ii) kinetic isotope fractionation during gas extraction, and (iii) isotopic fractionation during crystal growth, as documented in the case of diamonds. Peridotitic xenoliths provide a unique opportunity to study the lithospheric mantle. In order to explore the potential role of the above mentioned processes, we have analyzed nine peridotite xenoliths by step-wise heating combustion with a high resolution. Samples were from Yemen (three spinel lherzolites), Eifel, Germany (two wehrlites, two harzburgites and a dunite) and Massif Central, France (one harzburgite).
The extraction of N from refractory minerals was characterized by two release peaks, showing successful discrimination of trapped components from contaminants. We also observed Kinetic isotope fractionation during thermal extraction. Nitrogen in seven peridotite xenoliths is characterized by ¹⁵N enrichments compared to the atmosphere, probably representing metasomatised subcontinental lithospheric mantle. In contrast, the bulk N isotope composition of a mica-bearing wehrlite from Eifel, related to Cenozoic magmatism, is MORB-like (depleted in ¹⁵N by 5 ‰ relative to ATM) . However, an extreme isotopic disequilibrium of Δ¹⁵N = 25 ‰ is observed between olivine and phlogopite in this wehrlite. Nitrogen isotopic disequilibrium within a single ultramafic xenoltih suggests that significant isotopic fractionation may occur during magmatic processes.
Compared to the component trapped in fluid inclusions, bulk mantle xenoliths as well as mineral separates display high N_2/⁴⁰Ar ratios. We suggest that the N₂ enrichment relative to Ar is due either to high nitrogen solubility in refractory minerals, or to chemical speciation of nitrogen in magmas.
DE: 1000 GEOCHEMISTRY
DE: 1030 Geochemical cycles (0330)
DE: 1036 Magma chamber processes (3618)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005