Isua (Greenland) ∼3700 Ma meta-serpentinite olivine Mg# and δ18O signatures show connection between the early mantle and hydrosphere: Geodynamic implications

The similar to 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 similar to 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 Fo(91-92) olivine with delta O-18(VSMOW) of + 5.4 +/- 0.1 parts per thousand (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 Fo(90)(.5). The northern lens 'B' contains coarser-grained Fo(96-98) olivine which encloses magnetite as clusters and trails, some arranged with 60 degrees-120 degrees conjugate intersections. The Fo(96-98) olivine has delta O-18(VSMOW) of + 3.2 +/- 0.2 parts per thousand and is in equilibrium with high-Al antigorite and the accessory UHP Ti-rich minerals. We interpret that the Fo(90-92) versus the Fo(96-98) olivine assemblage variants are caused by a varying degree of fluid influx during ca. 3700 Ma serpentinisation, prior to peak UHP metamorphism. The Fo(90-92) variant was a rock-dominated system with a postulated early serpentine + ferroan brucite alteration assemblage, with abundant relict mantle-like delta O-18(VSMOW )+ 5.4 parts per thousand olivine. The Fo(96-98) 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 delta(18)O(VSMOW )Fo(96-98) 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 Fo(96-98) 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.

Automated summary

The study reveals that the ultramafic schists in the Isua supracrustal belt exhibit complex rock characteristics, containing ancient ultra-high-pressure components. These variations may have been caused by varying degrees of fluid influx and metamorphism during serpentinization around 3.7 billion years ago. These findings contribute to understanding mantle evolution and geological processes in the early Earth.