Earth's oldest mantle fabrics indicate Eoarchaean subduction

Area of Science:

• Geology
• Geophysics
• Mineralogy

Background:

• The operation of subduction processes in the Eoarchaean era (4.0–3.6 billion years ago) remains a subject of scientific debate.
• The oldest terrestrial olivine, found in the Isua supracrustal belt, Greenland (∼3,720 million years ago), exhibits microstructural fabrics.
• These fabrics include a shape-preferred orientation of olivine crystals and a well-defined lattice-preferred orientation (LPO).

Purpose of the Study:

• To investigate the origin and tectonic setting of Eoarchaean olivine fabrics.
• To determine if microstructural data from ancient ultramafic rocks can provide insights into early Earth geodynamics.
• To test the hypothesis that B-type LPO in Isua dunites indicates a supra-subduction mantle wedge setting.

Main Methods:

• Microstructural analysis of olivine crystals from Isua dunites.
• Characterization of shape-preferred orientation and lattice-preferred orientation (LPO).
• Comparison of observed fabrics with experimental data and modern subduction zone settings.
• Integration of compositional data from associated mafic rocks.

Main Results:

• The Isua dunites exhibit a B-type LPO, with [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane.
• This B-type fabric is consistent with deformation in a mantle wedge above a subduction zone, mirroring fabrics observed in modern Earth settings.
• Compositional data from nearby mafic rocks support a supra-subduction mantle wedge interpretation.

Conclusions:

• The microstructural features of the Isua dunites provide compelling evidence for Eoarchaean subduction.
• Ancient ultramafic rocks are valuable archives for reconstructing early Earth geodynamics.
• This study supports the operation of plate tectonics, including subduction, during the Eoarchaean era.