Serpentinization at Isua, a forearc environment identified by Zn isotopes
Abstract
The present study deals with the isotope compositions of Zn in the 3.8 Ga old serpentinites from the Archean complex of Isua, Greenland. The sample set includes igneous, metamorphic, and sedimentary rocks such as serpentinites, metagabbros, amphibolites, talc schists, and metagreywackes. For comparison, modern serpentines from Mexico and the Alps were also analyzed together with deep-sea clays from the Pacific, and metalliferous sediments from the East Pacific Rise and from the TAG area. The modern serpentinites show the enrichment in heavy zinc, with δ66/64Zn values from +0.21 to +0.52‰, typical of the modern igneous rocks. In contrast, the δ66/64Zn values of the Isua serpentines are conspicuously negative and range from -0.48 ‰ to +0.04 ‰ and can be clearly distinguished from other both igneous and sedimentary lithologies from the same area, in which the δ66/64Zn values range from -0.08 to 0.36‰ and overlap with the range of modern igneous rocks. Along mid-oceanic ridges, serpentinization takes place in high-temperature hydrothermal fields (typically > 250°C). The modern serpentinites analyzed seem to have formed in that way and formed by interaction between peridotites and low-pH, high-temperature, high-salinity seawater. Under such conditions, zinc is leached from rock by hydrothermal fluids as Zn2+ [1]. During this reaction, Zn from the rocks becomes depleted in light isotopes and its δ66/64Zn rises. The higher the temperature of the fluid, the lighter its isotope Zn composition becomes [2]. The remarkably low δ66/64Zn observed in Isua serpentinites indicates a different context of alteration and suggests reaction of peridotites with a low-temperature and high-pH fluid, which stabilizes Zn-sulfide complexes in solution. This is consistent with the arc subduction zone model of Isua [3]. Our δ66/64Zn data suggest that the closest equivalents of Isua serpentinites are the serpentinite mud volcanoes from the Mariana forearc [4]. This arc is partially emerged, and serpentinization takes place with low temperature, low-chlorinity, high-pH freshwater. The low chlorinity of the fluid inhibited the formation of protons during serpentinization and instead favored the release of hydrogen, methane and ammonia gas. As observed in modern ophiolitic terranes, subaerial igneous terranes favour the influx of freshwater, limit the temperature of water-rock interaction, produce slightly alkaline hydrothermal fluids, and cause hydrogen degassing. For all these conditions plus the constant supply of nutrients by runoff, the rim of the 3.8 Ga Archean forearc of Isua might have been a suitable environment for early life. [1] Tagirov, B.R., Seward, T.M., 2010. Chem. Geol. 269, 301-311 [2] John, S.G. et al., 2007. EPSL 269 17-28 [3] Furnes, H., et al., 2009. Lithos [4] Fryer, P. et al., 2006. Gcubed 8, DOI 2005GC001201.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.B41B0306P
- Keywords:
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- 0400 BIOGEOSCIENCES;
- 1000 GEOCHEMISTRY