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Australia: The Land Where Time Began |
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Jack Hills -
Mineral Inclusions in 4 Ga Zircons Provide Constraints on Hadean
Geodynamics In this paper Hopkins, Harrison & Manning present
their findings from a study that looked at the inclusion mineralogy of
1,450 zircons with an age of more than 4 Ga from Jack Hills in Western
Australia, characterising the zircons for composition and phase
assemblage. The results confirmed that the inclusion populations of
these zircons, which were largely of igneous origin, were dominated by
muscovite and quartz (~75 %). Hopkins et
al. suggest that if the
inclusions are original to the igneous zircons this observation, with no
other support, restricts the host melts to their formation at
pressure-temperature (P-T)
conditions of ~650-800oC and greater than 4 kbar. The view
that most of the mineral inclusions are primary is supported by several
lines of evidence, such as their lack of association with cracks,
magmatic crystal forms, and the lack of exchange with fuchsitic
(Cr-rich) micas in the host conglomerates. By applying Ti-in-zircon
thermometry, and phengite, Ti-in-quartz, and Al-in hornblende barometry,
to these inclusion assemblages estimates of magmatic
P-T conditions from 5 to
greater than 12 kbar and 700 ± 40oC resulted. It is indicated by these
data that zircon formation along geotherms of ≤ 60oC/km and
it is also implied by these data that that conductive heat flow near the
surface of <40-85 mW/m2 – a range that is substantially lower
than most global Hadean heat flow estimates. Underthrusting is one of a
number of possible environments that would be capable of generating
melting under conditions of such locally low heat flow early in the
history of the Earth. Possibly this Underthrusting was similar in manner
to modern convergent margins, and this appears to be the most consistent
with the many other geochemical restraints that have been derived from
investigation of Hadean zircons.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |