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 R_A; [1]) plume-derived materials from the well-mixed and more-extensively degassed depleted MORB mantle (DMM) (8 R_A; [2]). However, the He-isotope signature of the sub-continental lithospheric mantle (SCLM) remains relatively poorly characterized (6.1 R_A; [3]). 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 R_A, displaying predominantly radiogenic (i.e., low ³He/⁴He) He-isotope values. In contrast, Siberian flood basalt values extend up to ~13 R_A [4]. 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.

[1] Stuart et al., 2003. Nature. [2] Graham, 2002. Reviews in Mineralogy and Geochemistry. [3] Gautheron and Moreira, 2002. Earth and Planetary Science Letters [4] Basu et al., 1995. Science [5] Barry et al., 2013. AGU Abstract. [6] Day et al., 2012, AGU Abstract.