Australia: The Land Where Time Began |
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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).
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |