Does a Heavy Fe-Isotope Composition of Akilia Quartz-Amphibole-Pyroxene Rocks Necessitate a BIF Origin?

Our plain-English summary of Does a Heavy Fe-Isotope Composition of Akilia Quartz-Amphibole-Pyroxene Rocks Necessitate a BIF Origin?, by M.J. Whitehouse, R. Schoenberg, C.M. Fedo, et al (Astrobiology, 2015).

The study

• Authors: M.J. Whitehouse, R. Schoenberg, C.M. Fedo, B.S. Kamber
• Journal: Astrobiology
• Published: 2015;15(10):816-24
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The age and origin of the quartz-amphibole-pyroxene (qap) gneiss from the island of Akilia, southern West Greenland, have been the subject of intense debate since the light C-isotope composition of graphite inclusions in apatite was interpreted to indicate the presence of Earth's earliest biological activity. Although this claim for biogenic relicts has been vigorously challenged, the possibility that the rocks might represent some of Earth's earliest water-lain sediments and, hence, a suitable repository for life remains an open question. While some workers have suggested that the entire sequence represents an originally mafic-ultramafic igneous precursor subsequently modified by metasomatism, quartz injection, high-grade metamorphism, and extreme ductile deformation, others maintain that at least a small part of the sequence retains geochemical characteristics indicative of a chemical sedimentary origin. Fractionated Fe isotopes with δ(56)Fe values similar to those observed in Isua BIF have been reported from high-SiO2 units of qap and used to support a chemical sedimentary protolith for the qap unit. Here, we present new Fe isotope data from all lithologic variants in the qap gneiss on Akilia, including layers of undisputed ultramafic igneous origin. Since the latter require introduction of fractionated Fe into at least part of the qap unit, we argue that Fe isotopes must therefore be treated with considerable caution when used to infer BIF for part or all of the qap protolith.

In plain English

This was a laboratory geochemistry study of iron isotopes in ancient rocks from the island of Akilia, southern West Greenland. Earlier workers had argued that heavy iron isotope values in a thin quartz-amphibole-pyroxene (qap) gneiss showed it began as banded iron formation (BIF), a marine chemical sediment, which matters to claims that the rocks hold traces of Earth's earliest life. The authors measured bulk-rock iron isotopes, in duplicate, in samples covering all rock types in the unit. These included ultramafic boudins and layers, host ultramafic rocks and amphibolites. Combined with earlier published data, the quartz-bearing rocks had delta-56Fe values from +0.17 to +1.1 per mil. The ultramafic layers and boudins, considered igneous in origin, had a similarly wide range, from +0.23 to +0.96 per mil. The host rocks were relatively unfractionated. The authors concluded that the heavy iron in the ultramafic rocks cannot be an original igneous feature. They also concluded that the iron isotope signature of all qap rocks may be secondary, so iron isotopes should be treated with considerable caution when used to infer a BIF origin. They noted that repeated metamorphism obscured the original mineralogy, which limited modeling.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: laboratory geochemical study: bulk-rock iron isotope measurements (solution multicollector ICP-MS, samples analyzed in duplicate) on field-collected rock samples
• Who was studied: 10 rock samples (AK-prefix samples listed in Table 1) from the quartz-amphibole-pyroxene gneiss unit and its host rocks on the southwest tip of Akilia, southern West Greenland. They comprised quartz-pyroxene rocks, ultramafic boudins, a vein and a layer, and host ultramafic rocks, hornblendite and amphibolite.
• Main results:
  • Quartz-bearing rocks in the 5 m wide zone (this study's samples plotted together with the data of Dauphas et al., 2004) had delta-56Fe values ranging from +0.17 to +1.1 per mil, with five of the six analyzed samples above +0.75 per mil.
  • The quartz-bearing rocks had Fe/Ti ratios from about 3,300 to 13,000.
  • Ultramafic layers and boudins had delta-56Fe values from +0.23 to +0.96 per mil. Their Fe/Ti ratios were 92 to 1150, lower than those of the quartz-bearing rocks but higher than the bulk silicate Earth value of 45.
  • One of the highest delta-56Fe values, similar to that of the most quartz-rich rocks, was found in a thin ultramafic layer.
  • Host ultramafic rocks and amphibolites were relatively unfractionated, with delta-56Fe values slightly above zero. One host sample (AK-02) had +0.14 per mil, overlapping the least-fractionated quartz-pyroxene rock reported by Dauphas et al. (2004).
• The authors' conclusion: The authors concluded that the fractionated iron in the ultramafic rocks must have been introduced during alteration. They concluded that the iron isotope signature of all qap rocks may be secondary and have no direct bearing on their origin, and that iron isotopes must be treated with considerable caution when used to infer a BIF origin.
• Limitations the authors state:
  • The authors state that repeated metamorphic events obscured the likely original mineralogy of the rocks, which seriously limited attempts at modeling, because iron solubility and isotope fractionation depend on mineralogy and fluid composition.

Topics

Medical terms in this study, explained in our Lexicon: Amphibole Group, Pyroxene, Metamorphism, Protolith.

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