Australia: The Land Where Time Began

A biography of the Australian continent 

Hadean Granites Formation by Melting of Igneous Crust

Detrital zircon grains, that have been dated up to 4.4 Ga, that were recovered from meta-sedimentary rocks of Jack Hills, Australia, are the oldest known samples of Earth. Insights have been provided by these zircons into the magmas from which the zircons crystallised, by implication, igneous activity and tectonics in the first 500 My of the history of the Earth, the Hadean Eon. The composition of these magmas and the relative contributions of igneous and sedimentary components to their sources have, however, not yet been resolved. In this paper Burnham & Berry trace the element concentrations in the zircons from Jack Hills to those of the zircons from the locality where igneous (I-) and sedimentary (S-) type granites were first distinguished. It was shown by this study that the Hadean zircons had predominantly crystallised from I-type magmas formed by melting of a reduced, garnet-bearing igneous crust. They further propose that phosphorous content of zircon and the phosphorous to rare earth elements ratio can be used to distinguish between the sources of I- and S- type. A new geochemical tool has been provided by these elemental discriminants to assess the relative contributions of primeval magmatism and melting of sediments that have been recycled to the continents over geological time.

Other Hadean Zircons

There are only scarce comprehensive data sets for other ancient zircons. Data for 2 relevant comparators are shown in this study. Throughout the Archaean tonalite-trondhjemite-granodiorite (TTG) suites were abundant, and by extension may also have been present during the Hadean. Concentrations of trace elements in the TTG zircon differ greatly from those of zircons from S-type granites, as would be expected for the melting of a basaltic source. They display a greater degree of similarity to zircons from I-type granites, the P concentrations in particular, though the concentrations of the HREE are lower and more depleted than in the JHZs, which is consistent with the widely accepted derivation of TTG from a source that is garnet-bearing (Rapp & Watson, 1995). In the JHZs the HREE concentrations are intermediate between those of zircons and from TTG and I-type granites, suggesting a source that has an intermediate amount of garnet. Differences in pressure or composition (garnet is stabilised by higher pressure and mafic components) (46), possibly resulting in this difference in mineralogy, though it is not easy to deconvolve the 2 effects. There is a striking similarity between the concentrations of Hf in TTG zircon and to those in the JHZs and more variable than those of zircons from I- or S-type granites; the significance of this result requires further study.

The Idiwhaa tonolitic gneiss (ITG) of the Acasta Gneiss Complex, Canada (Reimink et al., 2013), is the oldest intact terrestrial rock unit; the composition of this rock unit is similar to Icelandic rocks that are more evolved, and therefore an Iceland-like setting has been suggested as the source of the earliest-formed continental crust. According to Brenham & Berry they note that the concentrations of P and the U/Th of ITG zircons display a striking similarity to the JHZs and I-type LFB zircons, and this is consistent with the metaluminous character of the ITG bulk rock, though there are not enough published data for a robust evaluation of similarity of ITG zircons to JHZs. ITG zircons have concentrations of Y and Yb that are similar to LFB I-type zircon, though without the strong REE depletion that is evident in TTG zircon, which is consistent with the flat chondrite-normalised bulk rock REE patterns of the ITG. It was concluded by Burnham & Berry that it is likely the ITG formed in a different setting than the granites from which the JHZs crystallised.

It is inevitable that any analogue study will be imperfect, given that there is so little knowledge of the Earth in the Hadean and early Archaean, which includes the nature of the processes of plate tectonics, as well as the average composition of sediments. There are, for example, differences in the zircon content of Ti and O isotope compositions of ITG and Icelandic zircon (3) despite other geochemical similarities (47). Nevertheless by examination of other rock suites that are well understood it is possible to identify geochemical trends and attempt to relate these to older, more enigmatic samples such as the JHZs. A model in which the JHZs formed, the melting of reduced , garnet-bearing lower crust, which is consistent with Hf and O isotope data (7,8) and suggests a TTG-like environment, is supported by the majority of trace element data. Several other hypotheses for the formation for these ancient grains are ruled out by the data from this study and this study demonstrates that concentrations of trace elements in zircon can be useful in provenance analysis, in spite of claims to the contrary (2,48).

Sources & Further reading

  1. Burnham, A. D. and A. J. Berry (2017). "Formation of Hadean granites by melting of igneous crust." Nature Geosci 10(6): 457-461.



Author: M. H. Monroe
Last Updated 31/07/2017
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