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3.2 When did Earth become suitable for habitation? (Harrison, McKeegan, Mojzsis)

	Figure 3.2.4. Historgram of lead-lead ages for ancient zircons from Jack Hills sample 634. Two peaks are evident at ~3.1 Gyr and ~4.0 Gyr, tailing to ages as old as 4.37 Gyr.

The earliest evidence of life on Earth comes from graphitic inclusions within >3.83 Gyr marine sediments from West Greenland that were found by ion microprobe analysis to contain isotopically light carbon (Mojzsis et al. 1996; cf. Fedo and Whitehouse 2002). As this places the emergence of life prior to the end of a period of intense bombardment in the inner Solar System (3.8-3.9; Ryder 1990), this result raises the possibility that life originated during the Hadean Eon (4.5-4.0 Gyr) – a period of Earth history for which there is no known rock record. That being the case, how can we determine whether the criteria thought necessary for biopoesis (i.e., energy source, organic molecules, liquid water) were extant during this time? Since the necessary energy sources and molecular building blocks for life were surely available during the formative stages of planetary evolution (Chyba and McDonald 1995), our question reduces to: When did suitably quiescent conditions and liquid water first appear at the Earth’s surface?

Attempts to trace Earth history back in time via the rock record reach an impasse around 4 billion years ago. However, the discovery of detrital zircons (Compston and Pidgeon 1986) from the Jack Hills, West Australia, as old as 4.3 to 4.4 Gyr offers the prospect of gaining unprecedented insights into surface environmental conditions during the earliest phase of Earth evolution (Mojzsis et al. 2001, Wilde et al. 2001; Peck et al. 2001). For example, oxygen isotope compositions of these ancient zircons suggest the presence of a terrestrial hydrosphere and stable continents only 200 Myr after accretion (Mojzsis et al. 2001). These preliminary results challenge the traditional view that continental formation and development of a hydrosphere were frustrated by meteorite bombardment and basaltic igneous activity until ~4.0 Gyr. We intend to utilize these ancient zircons – the only tangible record of the Hadean Eon – to seek insights into the origin of the atmosphere, hydrosphere, and the geodynamo during the earliest stages of Earth evolution.

3.2.1 The age of the atmosphere

The Earth’s atmosphere is thought to have been derived from mantle degassing (Brown, 1952). Although cometary water has more recently been suggested as a possible source (e.g., Delsemme 2001), D/H measurements of comets Halley, Hyakutake, and Hale-Bopp are inconsistent with this hypothesis (Bockelee-Morvan et al., 2000). While comets formed in the Jovian region might have a similarly low D/H ratio to the Earth’s oceans (Mumma et al. 2001), this idea remains untested.

Excesses of ¹²⁹Xe in mantle-derived samples relative to the atmosphere have been interpreted to indicate the presence of live parent ¹²⁹I in the deep Earth following early degassing of the atmosphere (e.g., Staudacher and Allegre 1982; Allegre et al. 1983) since ¹²⁹I decays to ¹²⁹Xe with a half life (t_1/2) of only 16 million years. If true, then it is argued that the present atmosphere and hydrosphere must have formed by ~4.4 Gyr (Podosek 1970).

However, the isotopes of Xe offer additional constraints on the age of the atmosphere. The plutonium isotope ²⁴⁴Pu has a short half life of 82 million years (t_1/2=82 Myr). Spontaneous fission of ²⁴⁴Pu produces ¹³¹Xe, ¹³²Xe, ¹³⁴Xe, and ¹³⁶Xe in characteristic relative abundances. The component of mid-ocean ridge basalts associated with fission decay of ²⁴⁴Pu implies a source that is 100 times lower than the chondritic plutonium/uranium ratio of 0.007 (it is assumed that Pu/U ratios equivalent to chondrites should be "primordial", i.e. solar) (Kunz et al. 1998). This appears paradoxical as other noble gas isotopic data are interpreted to indicate the presence in the deep Earth of a primitive reservoir of solar composition (e.g., Honda et al. 1991) that presumably would be associated with chondritic Pu/U (Azbel and Tolstikhin 1993). In the absence of a mechanism to fractionate Pu from U during planetary formation, the possibility remains that ¹²⁹Xe differences simply reflect inherited heterogeneities in the I/Xe ratio of terrestrial reservoirs. If this is the case, then the atmosphere and hydrosphere could be substantially younger than ~4.4 Gyr (Caffee et al. 1999). Alternatively, since the absolute amounts of ¹²⁹Xe excess to fissiogenic ¹³⁶Xe (fission of both ²⁴⁴Pu and ²³⁸U produces ¹³⁶Xe) in the atmosphere and their relative proportions are comparable to what would be left behind in the Earth after an early outgassing, such as that resulting from a Giant Impactor event, xenon isotope systematics may carry little information regarding the age of the atmosphere.

The place to begin to address this issue is to determine the terrestrial Pu/U. There are no firm constraints on this parameter, but existing data are suggestive of a chondritic Pu/U for the Earth (e.g., Caffee et al., 1999; Honda et al., 2000). The fundamental problem in elucidating this parameter is illustrated in Figure 7.2.1, which shows the expected ¹³¹Xe/¹³⁶Xe due to Pu and U fission during the first 900 Myr of Earth history. Clearly, zircons <4 Gyr in age contain very little signal of ²⁴⁴Pu (i.e., ¹³¹Xe). Materials formed between 4.4-4.1 Gyr provide the best opportunity to characterize terrestrial Pu/U and thus better utilize Xe isotope studies to understand the evolution of the atmosphere.

Harrison and his colleagues in Manchester, England (outside collaborators I. Gilmour and G. Turner) have recently discovered the very first evidence for extinct ²⁴⁴Pu in individual 4,150 Myr old zircons from Jack Hills and propose to carry out a research program to determine the Pu/U ratio in the Earth and the early crust and investigate the implications for the earliest differentiation of crust and atmosphere. Their ability to analyze the isotopic composition of xenon in individual zircons relies on the development in Manchester of a uniquely sensitive mass spectrometer based on the principle of laser resonance ionization (Gilmour et al. 1994). The instrument, RELAX (Refrigerator Enhanced Laser Analyser for Xenon), is capable of analyzing samples of only a few thousand atoms, some two orders of magnitude smaller than conventional noble gas mass spectrometers, permitting single zircons to be measured. In the first four RELAX analyses one has a ¹³¹Xe/¹³⁶Xe composition consistent with a chondritic Pu/U ratio, but three others have lower values. The preliminary interpretation of this result is that the lower ratios reflect partial xenon degassing subsequent to the extinction of ²⁴⁴Pu. What this means is twofold: analysis of an aggregate of old zircons would lead to a highly ambiguous result due to mixing of degassed and un-degassed populations, and assessment of the terrestrial Pu/U will require a large number of measurements using single crystals varying in age from 4.1 to 4.4 Gyr to define the upper bound of the population thus far discovered.

3.2.2 The age of the hydrosphere (and redox state of Earth’s surface)

Oxygen isotopes in rocks and minerals provide a means to discriminate among possible sources (i.e., mantle, metasedimentary, hybrid) of granitoids (Taylor and Sheppard 1986). Zircon appears promising in this role as exchange rates for O are very slow (Watson and Cherniak 1997), potentially permitting preservation of the protolith signature through high-grade metamorphism (Valley et al. 1994). For this reason Mojzsis et al. (2001) and Wilde et al. (2001) used the ¹⁸O values of Jack Hills zircons to infer crust-hydrosphere interactions at ca. 4.3 Gyr. These preliminary results warrant further study because of the surprising conclusion that a liquid hydrosphere (as opposed to steam atmosphere) was in place by ~4.3 Gyr. We propose to expand the approach to focus also on the "microrock" (i.e. polymineralic inclusion) environments encapsulated within the zircons.

Igneous zircons typically contain inclusions that reflect the parent melt (Chupin et al. 1988). 10-100 mm-sized inclusions have been found in >4 Gyr zircons from Jack Hills quartzites (Maas et al. 1992; Wilde et al. 2001), including assemblages of quartz, K-feldspar, biotite, chlorite and muscovite. One inclusion containing quartz, feldspar, apatite, and monazite (Maas et al. 1992) is significant as it is typical of S-type granitoids (Rapp and Watson 1986) where S-type refers to granitic rocks derived ultimately from sediments. Mojzsis et al. (2001) noted that peraluminous inclusions (those with high Al concentrations relative to alkali elements) encapsulated in zircons enriched in ¹⁸O are suggestive of the melt having originated from rocks at the Earth’s surface. This is because Al-rich compositions occur today by virtue of weathering and so indicate early development of a sedimentary cycling environment, and because high ¹⁸O (relative to mantle rocks, for example) is also an indication of interactions with a hydrosphere. While specific circumstances could be convolved to create this signal in the absence of surficial processes, they would be exceptional.

In rare cases, melt inclusions are also preserved in zircon. If the encapsulating zircon shielded a melt inclusion from changing external conditions since formation, then it may be feasible to extract information regarding the ƒO₂ (fugacity of oxygen) of the melt from which the zircon crystallized. If the melt protolith originated at the Earth’s surface (e.g., normative corundum, meaning Al-rich and so having experienced a weathering cycle, and heavy ¹⁸O), then it remains possible that a faint record of life at the Earth’s surface might be detectable. As an example of how this might be inferred, consider why I-type granites are pink in color and garnet-bearing S-type granites are white. In a subduction environment where I-type (I for igneous) granitic rocks are made beneath island arcs, ƒO₂ is maintained above the hematite-magnetite buffer (Zen 1988) and Fe³⁺ is soluble in K-feldspar, leading to a pink coloration (Zen 1985; Zen 1988). In S-type granitoids (again, S for sedimentary) of SE Australia, the rocks are white because graphite in the Paleozoic source rocks controlled the ƒO₂, producing more reducing conditions that stabilize Fe²⁺ in the evolved magmas that derive from them. The ultimate source of reduced C responsible for lowering ƒO₂ in S-type granitoids is fossil biomass within sediments that formed at the rock-hydrosphere or atmosphere interface. While a surprising concept, the color of a granitoid can relate to the presence or absence of reduced C at the Earth’s surface!

In the microrock environment, the intrinsic ƒO₂ of melt inclusions can be determined via knowledge of the Fe³⁺/Fe²⁺ ratio (Delaney et al., 1998) and a thermodynamic model (Kilinc et al. 1983). The µXANES (X-ray absorption near edge structure) method is capable of determining Fe³⁺/Fe²⁺ on a 3 mm spot with ±5% accuracy (Bajt et al. 1994). Because zircon solubility in melts (i.e., melt inclusions trapped within zircon are zircon saturated) is both a function of Fe³⁺/Fe²⁺ (i.e., network former vs. network modifier) and temperature (Baker et al. 2002), both ƒO₂ and peak melting temperature can in theory be determined by knowing Fe³⁺/Fe²⁺ and dissolved Zr. Thus it may be possible to assess whether conditions consistent with the presence of reduced carbon at the Earth’s surface – potentially bearing on the presence of life – were extant between 4.4 to 4.0 Gyr.

3.2.3 The initiation age of the geodynamo

The earliest evidence for geodynamo activity comes from the intrinsic magnetism of 3.5 Gyr volcanics in southern Africa. Because Jack Hills zircons are hosted in low grade sediments, they could retain the oldest record of the Earth’s magnetic field. Preliminary measurements using an ultra-sensitive SQUID magnetometer (Baudenbacher et al., 2002) show that a Jack Hills zircon carries a weak (10^-13 Am^-2) intrinsic remanent magnetism (J.L. Kirschvink pers. comm.). Provided the detrital grains were not re-magnetized, the presence or absence of intrinsic magnetism could be our only signal of geodynamo activity during the earliest period of Earth history. For example, should Harrison discover that all zircons younger than, say, 4.2 Gyr are magnetic but that all older zircons are not, this could be evidence that the terrestrial geodynamo initiated at ca. 4.2-4.3 Gyr. While admittedly a long shot, a positive result could potentially place a profound constraint on the dynamical evolution of the planet.

Harrison and colleagues will provide an outside paleomagnetics laboratory (Kirschvink’s laboratory at Caltech is the lead candidate at this writing) 4.0-4.4 Gyr zircons that have been dated without exposure to strong magnetic fields. If age analysis is found to remagnetize the grains, the grains will be characterized before dating (this is feasible as ~100 grains can be measured in a 24-hour period). It may be necessary to undertake limited drilling in the Jack Hills quartzites to obtain unweathered cores. ⁴⁰Ar/³⁹Ar dating of inclusions and single crystal xenon isotopic may analyses may be useful in establishing zircon thermal histories and thus age of magnetization.

3.2.4 Measurements of ancient zircons- building the database

	Figure 3.2.1. The changing 131Xe/136Xe due to fission of 244Pu and 238U over the first 900 Myr of Earth history. >4.1 Gyr zircons are ideal to assess the initial terrestrial Pu/U ratio.

While the discovery of >4.3 Gyr Jack Hills zircons provides unique new opportunities to gain insights into the earliest evolution of the atmosphere, hydrosphere, and continents, zircons older than 4.2 Gyr make up only ~0.5% of the detrital population (Amelin 1998). To overcome this hurdle, Harrison and coworkers have refined a method to survey ²⁰⁷Pb/²⁰⁶Pb ages of large numbers of zircons. Using a high-resolution ion microprobe in multi-collector mode, an age with ±1% precision can be obtained in less than one minute. In this way, these workers have thus far analyzed a total of 15,000 grains thereby increasing the number of dated Jack Hill zircons by a factor of 15 (Compston and Pidgeon 1986; Maas et al. 1992; Amelin 1998; Mojzsis et al. 2001; Wilde et al. 2001). Two clear age peaks are evident at ~3.3 Gyr and ~4.0 Gyr (Figure 7.2.4) tailing off at older ages. From this population, Harrison and others have thus far identified 105 zircons in the age range 4.1 to 4.2 Gyr and 31 >4.2 Gyr including three that are >4.35 Gyr. One ~8 µg zircon is 4.37 Gyr. We are confident that we know the yield of the most ancient zircons and therefore can predict the quantities required for future experiments from the database statistics.

Harrison estimates that the experiments proposed above require about three times the present complement of >4.1 Gyr zircons and thus he will need to age characterize another 30,000 zircons. The approach will be to rapidly survey large numbers of zircons using our established multi-collector approach at UCLA. However, this effort is coordinated with the group at the Research School of Earth Sciences at the Australian National University who will undertake about half the analyses. When sufficient quantities of the old zircons have been acquired, they will be disbursed to our collaborators for xenon isotopic and magnetic measurements. Oxygen isotopic and inclusion studies will commence at UCLA immediately with material already in hand.

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