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Australia: The Land Where Time Began |
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Earliest Earth – Minimum Melting
Conditions Revealed by Zircon Thermometer
A record of the conditions that prevailed on Earth at a time soon after
its formation has been found to be preserved in ancient zircons
recovered from Jack Hills, Western Australia. The Hadean Eon, 4.5-4.0
Ga, has been widely considered to have been a period that was uniquely
geodynamically violent, but recent interpretation s by some have
suggested it was, instead, a time that was much more benign, that may
possible been characterised as having oceans like those of the present.
A key to this debate has been knowledge of the temperatures of
crystallisation of zircons dating to the Hadean. It was revealed by a
thermometer based on the content of titanium that at ⁓700oC
these zircons cluster strongly, which cannot be distinguished from
temperatures of granitoid zircon growth of the present, and this
suggests strongly there was a regulated mechanism that was producing
zircon-bearing rocks during the Hadean. The existence of a wet,
minimum-melting conditions within 200 My of the formation of the Solar
System, and these temperatures substantiate the existence these
conditions. It is also suggested that the Earth had also settled into a
pattern of crust formation, erosion and sediment recycling as early as
4.35 Ga.
The Hadean Eon, the first 500 My of Earth history, was the most vigorous
time, dynamically, in the existence of the Earth. It has been speculated
by various authors that the Earth may have:
1)
Collided with another object the size of Mars,
2)
Formed a global magma ocean,
3)
Grown the first continents,
4)
And seen the emergence of life.
Morley suggests that it possible that none of these events actually
occurred, which is consistent with the geochemical record. The
fundamental problem is that there is no known rock on Earth that
contains a record of this interval from which it is possible to discover
these processes as 4.04 Ga is the oldest age of a dated rock (Browning &
Williams, 1999). The problem is how to gain further insights into the
formative stages of the evolution of the Earth.
The geochemical record of the interval between 4.5 Ga and 4.0 Ga has not
entirely been lost, though there are no Hadean rocks that have yet been
discovered. The existence of zircons that have been dated to more than
4.1 Ga that are preserved in the Early Archean metasediments at Mt
Narryer and Jack Hills, Western Australia, has been known of for more
than 20 years (Froude et al., 1983; Compston & Pidgeon, 1986), and
recently more information has begun to be gleaned from them regarding
the nature of the Earth during the Hadean.
An example is that Hf isotope studies have suggested the
existence of the reworking of continental crust prior to 4.1 Ga (Amelin
et al., 1999). The interpretation of the studies of oxygen isotopes has
suggested that protoliths of magmas dating to about 4.3 Ga formed in the
presence of water at the surface of the Earth (Mojzsis, Harrison &
Pidgeon, 2001; Wilde et al., 2001). An estimate of the initial
terrestrial plutonium/uranium ratio, a parameter that is key to
understanding the origin and evolution of the atmosphere was provided
the results of Xenon isotopic studies of these ancient zircons.
The traditional view was that the formation of a continent and
development of a hydrosphere were frustrated by bombardment by
meteorites and basaltic activity until about 4 Ga, and this has been
challenged by these and other results above. It is suggested, instead,
that a surface environment and petrogenetic processes much more similar
to those of the present. In this paper Watson & Harrison exploited a
thermometer that was newly developed, based on the incorporation of Ti
into crystallising zircon. Watson & Harrison conclude, based on these
analyses, that zircons from Jack Hills were sourced predominantly from
crustal melts that had formed at temperatures that ranged from those
characteristic of wet, minimum melting to vapour absent melting under
anatectic conditions.
The content of titanium is suitable, uniquely, as an indicator of zircon
crystallisation temperature. Under all relevant geologic conditions Ti
enters the zircon lattice in homovalent replacement of Zr +4
or Si4+ as Ti is a tetravalent ion. As a consequence the
uptake of Ti does not depend on the availability of other charge
compensating ions. In the case in which TiO2 is saturated,
i.e. rutile is present in the system; the thermodynamic basis of the
thermometer is the simple reaction
TiO2 (rutile) = TiO2
(zircon)
(1)
Summary
Most igneous and metasedimentary rocks of the present αTiO2
is 0.5 or higher. It cannot be assumed a priori that host materials in
the Hadean zircons resemble those that are typical of more recent times,
though their characteristics were governed by the same thermodynamic
considerations. The crystallisation temperatures of most of these
zircons will not be underestimated by more than 50oC to 60o,
given that α TiO2 is ≥0.5.
Watson & Harrison used an ion microprobe (23) to measure concentrations
of Ti in 54 concordant zircons from Jack Hills that ranged in U-Pb age
from 4.0-4.35 Ga. They chose at least 1 analysis spot on each zircon to
coincide with the locations where the ages were determined. Temperatures
that were calculated from 69 spots ranged from 801oC to 644oC.
The duplicate Ti determinations in most zircons yielded, in most cases,
similar temperatures; however, there was 1 zircon fragment with CLA
zoning suggesting a simple magmatic history that shows systematic
diminution in the temperature of crystallisation from 778oC
near the core to 751oC near the rim. This pattern is
consistent with cooling as the host magma cooled progressively.
The low, restricted temperature range which, if taken at face value,
implies water saturated conditions of melting. There are 2 alternatives
scenarios that were examined prior to exploring this possibility.
First, could the distribution of zircon temperature result from cooling
melts that derived from the expected high flux of impacting bolides?
Watson & Harrison ruled this
possibility out for the following reason:
i)
In the Qz-Ab-Or-H2O system the melting temperature exceeds
800oC (Holtz, Becker & Freise, 2001), even in the presence of
270-bar steam atmosphere that resulted=d from complete evaporation of
the ocean (24);
ii)
There is a low temperature distribution dispersion, which implies there
is a dominant, regulated melting mechanism (this is especially true if
the 8 outliers in the distribution at
T >750oC have been
attributed to an alternative mechanism);
iii)
Temperature of saturation of zircons that are calculated for magmas that
are produced by wholesale melting of average crust (26) exceeds the
average temperature observed in zircons from the Hadean.
Second, Could the temperature distribution of zircons from the Hadean
reflect residual liquids that might have fractionated from magmas of
higher temperature and are mafic? Watson & Harrison suggest this
scenario is ruled out by the expectation that in a mafic complex
late-stage crystallisation would yield average temperatures that are
appreciably higher for the formation of zircon. Zircon saturation
temperatures that have been calculated for late differentiates of the
Skaergaard intrusion, e.g., yield an average temperature of 870oC
(Brooks, 1969; Wager & Brown, 1968). Temperatures of 787oC
to 806oC were given by the thermometer used by Watson &
Harrison for 3 Skaergaard zircons that were analysed in this study
(According to Watson & Harrison these are probably biased slightly
downwards by undersaturation in rutile). Also, this mechanism doesn’t
seem to meet the requirement of a regulated mechanism by the data of
Watson & Harrison, though fractionation of mafic magma can indeed yield
felsic melts and zircons of intermediate temperatures.
Watson & Harrison believe, therefore, that their crystallisation
temperatures for zircons dating to the Hadean provide evidence that is
definitive, about the state of the early Earth, especially when
considered in the context of the inclusion assemblage that has been
documented by other researchers. Quartz, K-feldspar, chloritized
biotite, chlorite, amphibole, muscovite, albite, Ca-Al silicate
(plagioclase), rutile, apatite, FeOOH, Ni-rich pyrite, thorite (ThO2),
and monazite (Trail, Mojzsis & Harrison, 2004; Maas et
al., 1992), are all minerals
that are contained within zircons that date to the Hadean. These
inclusions are characteristic of granite assemblages or their hydrated
and/or oxidised equivalents (Trail, Mojzsis & Harrison, 2004). The
possibility has remained until now that the inclusion assemblage may
represent siliceous, felspathic, material of anhydrous character that
was altered by later exposure of the zircons to crustal metamorphic
fluids. The thermometer of Watson & Harrison provided strong evidence
against this possibility: even when allowance was made for subunity TiO2
activity, they are too low for zircons to have crystallised from dry
siliceous melts (31). It is suggested by the restricted range of
temperatures, also, that a set of circumstances that are highly
reproducible removed melt fertility from rocks under prograde
conditions, which is consistent with crustal anatexis throughout the
Hadean. There are few temperatures older than 4.2 Ga, but a slight
down-temperature “focusing” of typical magmatic conditions between 4.35
and 4.0 Ga is hinted at in the present database.
Melting in an ensemble of crustal environments, not unlike that of the
present, under conditions of
water saturation is, according to Watson & Harrison, the simplest
scenario. The zircon crystallisation temperatures of Watson & Harrison,
when taken collectively, mimic expectations of “modern day” igneous
zircons, which most pointing towards a crystal anatectic origin.
The existence of wet, minimum melting conditions at 4.35 to 4.0 Ga that
is inferred from mineral inclusion studies is substantiated by the
present results and are consistent with the early Hadean hydrosphere
hypothesis (Mojzsis, Harrison & Pidgeon, 2001; Wilde et
al. 2001). Also, it is
strongly suggested by the present results that within about 100 My of
formation, the Earth had settled into a pattern of the formation of the
crust, erosion, and the recycling of sediment that is similar to that
has been produced during the known plate tectonic era. It is implied by
the rapid establishment of this cycle, also, that the pace of geological
activity in general (driven by rapid mantle convection) was much more
rapid in the Hadean than in more recent times.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |