Helium isotopes of the Siberian sub-continental lithospheric mantle: Insights from eclogite xenoliths
Abstract:Helium isotopes (3He/4He) have been extensively used to define distinct segments of Earth’s mantle and characterize its chemical structure. Specifically, they have been used to illustrate the long-term isolation and preservation of high-3He/4He (≥50 RA; ) plume-derived materials from the well-mixed and more-extensively degassed depleted MORB mantle (DMM) (8 RA; ). However, the He-isotope signature of the sub-continental lithospheric mantle (SCLM) remains relatively poorly characterized (6.1 RA; ). The Siberian craton hosts >1000 kimberlite intrusions, which carry mantle-derived xenoliths – of varying compositions (i.e., peridotites, dunites, and eclogites) – to the Earth’s surface, making it an ideal setting for investigating the chemical evolution of the SCLM. Here, we report new He-isotope and concentration data for a suite of eclogitic xenoliths (n=10) from the Udachnaya pipe, Siberia.
He-isotopes and [He] contents were determined by crushing garnet and pyroxene mineral separates from 2.7-3.1 Ga Siberian eclogites. 3He/4He values ranged from 0.11 to 1.0 RA, displaying predominantly radiogenic (i.e., low 3He/4He) He-isotope values. In contrast, Siberian flood basalt values extend up to ~13 RA . Helium concentrations span ~4 orders of magnitude from 60 to 569,000 [4He]C ncm3STP/g. The radiogenic nature of Udachnaya eclogites indicate that they have been largely isolated from basaltic metasomatic fluxes over geological time due to position within the lithosphere and/or lithospheric age. Further, low 3He/4He values may reflect the addition of high U-Th material into the lithosphere by accretion of ancient island-arc terrains. These new data add to the growing He-isotope database [5,6] for the Siberian SCLM, and reveal the heterogeneous nature of this region with respect to He-isotopes, as well as the potential importance of crustal recycling and metasomatic processes.
 Stuart et al., 2003. Nature.  Graham, 2002. Reviews in Mineralogy and Geochemistry.  Gautheron and Moreira, 2002. Earth and Planetary Science Letters  Basu et al., 1995. Science  Barry et al., 2013. AGU Abstract.  Day et al., 2012, AGU Abstract.