WARC DISCOVERIES

Baked BIFs: Can isotopic biosignatures survive the effects of high-grade metamorphism in the Earth’s earliest sediments?

The evidence for the earliest life on Earth is contained in sedimentary rocks. Unfortunately, these have been metamorphosed, sometimes to very high temperatures. Can the isotopic fingerprints of ancient life be preserved in such altered rocks? WARC researchers John Valley, Clark Johnson, and Brian Beard sought to test the “survivability” of isotopic biosignatures in chemical sediments (Banded Iron Formations, or “BIFs”) that have been subjected to high-grade metamorphism through a O and Fe isotope study of the 1.9 b.y. old Biwabik iron formation, Minnesota, which has been subjected to heating through contact metamorphism by the 1.1 b.y. old Duluth intrusive complex. Their results, published in Contributions to Mineralogy and Petrology, demonstrate that O and Fe isotopes equilibrate among minerals within the rocks during metamorphism, but, in the absence of extensive interactions with fluids, the O and Fe isotopes in bulk samples may reflect their original, primary sedimentary compositions.

Citation:
Valaas-Hyslop, E, Valley, JW, Johnson, CM, and Beard, BL (2007) The effects of metamorphism on O and Fe isotope compositions in the Biwabik Iron Formation, northern Minnesota. Contributions to Mineralogy and Petrology. DOI 10.1007/s00410-007-0244-2. A PDF copy of the paper can be found here.

Posted January 11, 2008