Isua (Greenland) ~3700 Ma meta-serpentinite olivine Mg# and δ 18 O signatures show connection between the early mantle and hydrosphere: Geodynamic implications

Abstract

The ~ 3700 Ma Inner Arc Group of the Isua supracrustal belt (Greenland) contains a 10 km long strip of ultramafic schists with two ≤ 1 km long meta-dunite lenses, preserving relict olivine + antigorite + titano-clinohumite and titano-chondrodite ~ 2.6 GPa ultra-high-pressure (UHP) assemblages. There are two distinct relict meta-peridotite variants: The southern lens ‘A’ variant is dominated by an aggregate of Fo91-92 olivine with δ18OVSMOW of + 5.4 ± 0.1‰ (95% confidence) in which are rare small clinopyroxene inclusions, plus rare interstitial accessory chrome spinel partly altered to magnetite. The olivine grains are bounded by serpentine domains, which are intergrown with the olivine margins with a marginally more iron-rich composition of Fo90.5. The northern lens ‘B’ contains coarser-grained Fo96-98 olivine which encloses magnetite as clusters and trails, some arranged with 60°-120° conjugate intersections. The Fo96-98 olivine has δ18OVSMOW of + 3.2 ± 0.2‰ and is in equilibrium with high-Al antigorite and the accessory UHP Ti-rich minerals. We interpret that the Fo90-92 versus the Fo96-98 olivine assemblage variants are caused by a varying degree of fluid influx during ca. 3700 Ma serpentinisation, prior to peak UHP metamorphism. The Fo90-92 variant was a rock-dominated system with a postulated early serpentine + ferroan brucite alteration assemblage, with abundant relict mantle-like δ18OVSMOW + 5.4‰ olivine. The Fo96-98 variant was a fluid-dominated system, where obliteration of the protolith olivine formed serpentine + magnetite + brucite ± magnesite. During prograde UHP metamorphism consumption of brucite by reaction with serpentine gave rise to a lower δ18OVSMOW Fo96-98 olivine + serpentine assemblage. Both varieties have similar bulk chemistry, interpreted as depleted mantle that was enriched in LILE and LREE by a melt or fluid, prior to varying degrees of serpentinisation and superimposed UHP metamorphism. In Phanerozoic abyssal peridotites similar textural, chemical and isotopic variations are the result of rock versus fluid-dominated serpentinisation and variable alteration temperatures. Analogously, the Fo96-98 Isua peridotites are interpreted as mantle peridotite entirely serpentinised by high temperature seawater in a fluid-dominated environment and then metamorphosed under UHP conditions at a convergent plate boundary. This indicates lateral lithosphere motions facilitating hydrosphere - mantle communication early in Earth's history.