TY - JOUR

T1 - Heterogeneous kimberlite metasomatism revealed from a combined He-Os isotope study of Siberian megacrystalline dunite xenoliths

AU - Pernet-Fisher, John F.

AU - Barry, Peter H.

AU - Day, James M.D.

AU - Pearson, D. Graham

AU - Woodland, Sarah

AU - Agashev, Aleksey M.

AU - Pokhilenko, Lyudmila N.

AU - Pokhilenko, Nikolay P.

PY - 2019/12/1

Y1 - 2019/12/1

N2 - The Siberian sub-continental lithospheric mantle (SCLM) is regionally heterogeneous due to the complex modification of ancient cratonic material by various metasomatic fluids and interaction with the Siberian plume at ∼250 Ma. Here, we assess the extent and origin of this heterogeneity by analysis of helium isotopes, rhenium-osmium isotopes, trace-element, and highly siderophile element [HSE: Os, Ir, Ru, Pt, Pd, Re] abundances in a suite of mantle-derived megacrystalline dunite xenoliths from the 360 Ma Udachnaya East kimberlite pipe, Siberia. This enables assessment of the style and extent of metasomatism acting to modify the volatile budget of the Siberian SCLM. The olivine megacrysts have 3He/4He values that range from 2.3 to 7.4 RA (where RA = the 3He/4He of air), outside the canonical range for the lithospheric mantle (6.1 ± 2.1 RA; Day et al., 2015). High [He] and low U + Th concentrations in these olivine megacrysts indicate that the Udachnaya megacrystalline dunite xenolith suite has undergone minimal post-eruptive modification of He isotopes by 4He recoil. We instead interpret He isotope variations to reflect pre- or syn-eruptive metasomatic signatures during kimberlite emplacement. The dunites can be divided into two groups, ultimately reflecting modification by two distinct metasomatic styles. Group 1 dunites are characterized by highly unradiogenic Os isotope compositions (γOs360Ma −16 to −13) and ancient melt depletion ages (∼3 Ga) typical of ancient cratonic lithospheric mantle; yet display a range of He isotope ratios (2.3–5.9 RA). This indicates that group 1 dunites have been modified by gas-rich metasomatic fluids acting to change He abundances and isotope ratios, while preserving unradiogenic Os isotope and HSE abundance systematics. Group 2 dunites extend to significantly more radiogenic Os isotope signatures and higher He isotope values (γOs360Ma > −5 to +53; 3He/4He from 3.7 to 7.4 RA). These two groups define a negative hyperbolic array between Os and He isotopes which we interpret to result from metasomatism by variable mixtures between: (1) a gas-rich and HSE-poor fluid (similar to the fluids acting to modify Group 1 dunites) with helium isotope compositions above the MORB range (>8 RA) derived from the asthenosphere, and: (2) strongly radiogenic fluids (∼2 RA; 187Os/188Os > 0.25) likely sourced from within the SCLM. The Udachnaya megacrystalline dunite xenolith suite provides insight into how mantle-plume derived fluids interact with metasomatic fluids within the SCLM.

AB - The Siberian sub-continental lithospheric mantle (SCLM) is regionally heterogeneous due to the complex modification of ancient cratonic material by various metasomatic fluids and interaction with the Siberian plume at ∼250 Ma. Here, we assess the extent and origin of this heterogeneity by analysis of helium isotopes, rhenium-osmium isotopes, trace-element, and highly siderophile element [HSE: Os, Ir, Ru, Pt, Pd, Re] abundances in a suite of mantle-derived megacrystalline dunite xenoliths from the 360 Ma Udachnaya East kimberlite pipe, Siberia. This enables assessment of the style and extent of metasomatism acting to modify the volatile budget of the Siberian SCLM. The olivine megacrysts have 3He/4He values that range from 2.3 to 7.4 RA (where RA = the 3He/4He of air), outside the canonical range for the lithospheric mantle (6.1 ± 2.1 RA; Day et al., 2015). High [He] and low U + Th concentrations in these olivine megacrysts indicate that the Udachnaya megacrystalline dunite xenolith suite has undergone minimal post-eruptive modification of He isotopes by 4He recoil. We instead interpret He isotope variations to reflect pre- or syn-eruptive metasomatic signatures during kimberlite emplacement. The dunites can be divided into two groups, ultimately reflecting modification by two distinct metasomatic styles. Group 1 dunites are characterized by highly unradiogenic Os isotope compositions (γOs360Ma −16 to −13) and ancient melt depletion ages (∼3 Ga) typical of ancient cratonic lithospheric mantle; yet display a range of He isotope ratios (2.3–5.9 RA). This indicates that group 1 dunites have been modified by gas-rich metasomatic fluids acting to change He abundances and isotope ratios, while preserving unradiogenic Os isotope and HSE abundance systematics. Group 2 dunites extend to significantly more radiogenic Os isotope signatures and higher He isotope values (γOs360Ma > −5 to +53; 3He/4He from 3.7 to 7.4 RA). These two groups define a negative hyperbolic array between Os and He isotopes which we interpret to result from metasomatism by variable mixtures between: (1) a gas-rich and HSE-poor fluid (similar to the fluids acting to modify Group 1 dunites) with helium isotope compositions above the MORB range (>8 RA) derived from the asthenosphere, and: (2) strongly radiogenic fluids (∼2 RA; 187Os/188Os > 0.25) likely sourced from within the SCLM. The Udachnaya megacrystalline dunite xenolith suite provides insight into how mantle-plume derived fluids interact with metasomatic fluids within the SCLM.

KW - Helium isotopes

KW - Mantle dunites

KW - Metasomatism

KW - Osmium isotopes

KW - Peridotites

KW - SCLM

KW - NOBLE-GASES

KW - COSMOGENIC HE-3

KW - ULTRAMAFIC XENOLITHS

KW - PERIDOTITE XENOLITHS

KW - CARBONATITE METASOMATISM

KW - UDACHNAYA KIMBERLITE

KW - LITHOSPHERIC MANTLE BENEATH

KW - HIGH HE-3/HE-4 RATIOS

KW - RE-OS

KW - HELIUM-ISOTOPES

UR - http://www.scopus.com/inward/record.url?scp=85071434734&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2019.07.054

DO - 10.1016/j.gca.2019.07.054

M3 - Article

AN - SCOPUS:85071434734

VL - 266

SP - 220

EP - 236

JO - Geochmica et Cosmochimica Acta

JF - Geochmica et Cosmochimica Acta

SN - 0016-7037

ER -