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Titel |
Is there life left in Akilia - novel approaches to an old controversy |
VerfasserIn |
Martin Whitehouse, Chris Fedo, Tony Kemp, Balz Kamber, Ronny Schoenberg, Jeff Vervoort |
Konferenz |
EGU General Assembly 2014
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 16 (2014) |
Datensatznummer |
250097957
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Publikation (Nr.) |
EGU/EGU2014-13587.pdf |
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Zusammenfassung |
For almost two decades, an enigmatic iron-rich quartz-amphibole-pyroxene (QAP) rock unit
hosted in granulite-facies TTG gneisses on Akilia, SW Greenland has been claimed to be
either a chemical sediment containing >3.84 Ga biogenic remnants [1] or a younger product
of polyphase metasomatism and metamorphism of an ultramafic protolith [2,3]. Although in
recent years Akilia has therefore been largely ignored in the investigation of early Earth
environments, constraining the true protolith of these rocks remains an important goal
because the very earliest (known) rock record on Earth is typified by poly-metamorphic,
poly-deformed lithologies, some of which may indeed preserve evidence of early
biological activity. We present new whole-rock Fe-isotope data and zircon Lu-Hf data
from the Akilia QAP unit, additional isotopic tools that may be used to constrain
protolith.
The ~5m-wide QAP unit on Akilia exhibits lithological variation from pure
quartzite to ultramafic rocks, of which the ~3 cm thin, finely banded unit that yielded
purported C-isotope evidence for biological activity, has been considered by some
as the most likely candidate for a chemical sediment. Previous Fe isotope studies
of QAP have yielded δ56FeIRMM up to c. +1o[4], at one end of the -3oto
+1orange typically exhibited by chemical sediments [5]. Our Fe-isotope analyses reveal
similar, positively fractionated Fe-isotopes, but importantly also from ultramafic
layers within the QAP unit that are apparently of igneous origin. Finely banded
QAP has yielded metamorphic zircon, predominantly ~2.7 Ga in age, but with
rare cores of ~3.6 Ga. Hf isotopes in these zircons are exceedingly radiogenic and
heterogeneous, ranging in ÉHf(2.7) from +38 to +231, while the single ~3.6 Ga zircon has
ÉHf(3.6) of +18.7. Whole-rock ÉHf(2.7) for the QAP rocks are also very radiogenic,
ÉHf(2.7) ranging from +39 to +103, while a thin ultramafic layer has ÉHf(2.7) of
+37. In contrast, ultramafic and amphibolite rocks adjacent to the QAP unit are
near-chondritic.
Models for the origin of QAP require an explanation for both the highly fractionated
Fe-isotopes and Lu/Hf, not just in the finely-banded “BIF candidate” but also in igneous
ultramafic components of QAP. These models include extreme metasomatic redistribution of
elements during two (3.6 and 2.7 Ga) high-grade events in an originally igneous protolith, or
wholesale chemical and isotopic overprint of an originally mixed lithology whose protoliths
are beyond recognition.
[1] Mojzsis, S.J. et al. (1997), Nature 384, 55-59; [2] Fedo, C.M., Whitehouse, M.J.
(2002), Science 296, 1448-1452; [3] Whitehouse, M.J. et al. (2009), J.Geol. Soc. London,
166, 333-344; [4] Dauphas, N. et al. (2004) Science, 306, 2077-80; [5] Johnson, C.M., Beard,
B.L. (2006), GSA Today 16, 4–10. |
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