Early Archean serpentine mud volcanoes at Isua, Greenland, as a niche for early life

Abstract

The Isua Supracrustal Belt, Greenland, of Early Archean age (3.81-3.70 Ga) represents the oldest crustal segment on Earth. Its complex lithology comprises an ophiolite-like unit and volcanic rocks reminiscent of boninites, which tie Isua supracrustals to an island arc environment. We here present zinc (Zn) isotope compositions measured on serpentinites and other rocks from the Isua supracrustal sequence and on serpentinites from modern ophiolites, midocean ridges, and the Mariana forearc. In stark contrast to modern midocean ridge and ophiolite serpentinites, Zn in Isua and Mariana serpentinites is markedly depleted in heavy isotopes with respect to the igneous average. Based on recent results of Zn isotope fractionation between coexisting species in solution, the Isua serpentinites were permeated by carbonate-rich, high-pH hydrothermal solutions at medium temperature (100-300 °C). Zinc isotopes therefore stand out as a pH meter for fossil hydrothermal solutions. The geochemical features of the Isua fluids resemble the interstitial fluids sampled in the mud volcano serpentinites of the Mariana forearc. The reduced character and the high pH inferred for these fluids make Archean serpentine mud volcanoes a particularly favorable setting for the early stabilization of amino acids.

Fig. 1.

Zinc isotope compositions of Isua supracrustal rocks and serpentinites from the Mariana forearc, Gakkel Ridge, and Alpine ophiolites, compared with δ⁶⁶Zn data in marine shales (this study and data from ref. 48), deep-sea carbonates (data from refs.  and 48), and FeMn nodules (data from ref. 31). The gray field represents the worldwide igneous average (20).

Fig. 2.

Theoretical isotope fractionation factors (β) between isotopomers as a function of temperature for different Zn species. Values are inferred from ab initio calculations for Zn in sulfate-rich (dashed lines, data from ref. 12), sulfide-rich (solid lines, data from ref. 13), and carbonate-rich fluids (dotted lines, data from ref. 13).

Fig. 3.

SEM images of a section of Isua peridotite 8c. atg: antigorite; ol: olivine; Co-ptl: cobaltian pentlandite; ox: oxide (Fe-Ni-Cr). (A) Only one sulfide (black circle) is present. (B) The sulfide is cobaltian-pentlandite (Fe₁₉Ni₂₉Co₄S₄₈). (C) Other white spots in A are not sulfides but oxides as analyzed by EDS.