Australia: The Land Where Time Began
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)
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.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|