Investigation of inclusions in 3.0-4.4 Gyr old zircon from the Mt. Narryer terrane, Western Australia
Final Report Abstract
In this study, the detailed analysis of not only separated detrital zircons, but in situ zircons inside the Jack Hills conglomerate as well as the conglomerate itself sheds new light on the potential formation conditions of Hadean zircon grains. Estimates on formation conditions have been made on the basis of (1) the occurrence and characteristics of three types of carbon inclusions (graphite, carbonates, and CO2), (2) a so far overlooked zircon population of small, extremely radiation damaged zircon grains, as well as (3) quartz inclusions in zircon and the quartz conglomerates itself. The identification of graphite and CO2 along with the Ti-in-zircon temperature obtained from the host zircons place formation conditions close to the QFM buffer at a about 700°C and at least 6.7 kbar for zircons older than about 3.5 Gyrs, and to about 700°C and 4.7 kbar with oxidising conditions at a later stage. The close resemblance between “young” and “old” zircon grains and small in situ and larger separated zircons indicates similar conditions throughout the whole time span between 4.4 and 3.3 Ga. Continuous ages of zircons of both types imply recurring conditions, which require an isolated scenario, in contrast to a model that is based on modern plate tectonics, where continuous transport of material in and out of the system would be involved. The host rock of zircons is possibly of felsic composition, as at least some quartz inclusions appear to be primary. The presence of Th and U enriched zircons, found in situ, indicate a crystallisation from an evolved melt. However, separated and in situ zircons appear to be derived from the same rock, but just represent different stages of a recurring differentiation process. The presence of non-formula cations such as Fe, P, Y, and Ca in the U-and Th-rich zircons indicates that these zircons were recently altered, likely during weathering. The initial discovery of diamond inclusions (Menneken et al., 2007) must be regarded with caution, since the TEM study of JH15-419 revealed that this authentic appearing diamond inclusion is most likely the result of polishing paste being pressed into a cavity. The identification of CO2 along with graphite in this study explains how such cavities might have come to exist in the first place and how in earlier studies the diamond inclusion could have been come into place along with graphite. This study has shown that characteristics of detrital zircons from Jack Hills, Western Australia cannot be simply interpreted to represent the conditions at the time given by their Pb-Pb age. Many zircons have experienced some recrystallisation, as evidenced by complex internal textures and heterogeneous Pb-Pb ages. But also the discrepancy observed in formation temperatures between inclusions (quartz and graphite) and their host zircon might indicate a change in zircon properties from their original formation. Conclusions have to be drawn carefully including all available relations and characteristics. Jack Hills zircons pose a special case after all, as they are the oldest known preserved pieces of early Earth, but they are not necessarily a typical representation of conditions prevailed on the entire Earth at the time of their formation. Future work should involve extended analyses of in-situ zircon grains to close the gap on the potential sample bias as well as a closer examination on characteristics, indicating processes that might have changed inclusion paragenesis or isotope characteristics.
Publications
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(2009) An evaluation of the potential of using Raman spectroscopy to determine the carbon isotope composition of CO2 inclusions. J. Geochem. Exploration, 101, p. 70
Menneken M., Geisler T.
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(2009) Raman spectroscopic determination of the isotope composition of CO2 inclusions. Geochim. Cosmochim. Acta, 73, A869
Menneken M., Geisler T., Nemchin A.A., and Strauss H.
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(2010) Implications of carbon dioxide inclusions in Jack Hills zircons. In: Tyler, I.M. and Knox-Robinson, C.M. (Eds.), Fifth International Archean Symposium Abstracts. Geological Survey of Western Australia, Record 2010
Menneken M., Geisler T., Nemchin A.A, Grange M, Gasharova B.
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(2011) Thermometry of quartz from the metaconglomerate of Jack Hills, Western Australia. Mineralogical Magazine, 75 (3), 1455
Menneken M., Nemchin A.A., Geisler T.
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(2012) Insights on early Earth crustal evolution from zircon and quartz from Jack Hills, Western Australia. PhD thesis, University of Münster, Germany
Menneken M.
