Technical Comments

Response to Comments on "Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth"

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Science  10 Feb 2006:
Vol. 311, Issue 5762, pp. 779
DOI: 10.1126/science.1121080

Abstract

The mean crystallization temperature of Hadean zircons estimated on the basis of titanium content is ∼680°C. This value corresponds almost uniquely to the temperature of wet minimum melting in present-day crust. The low variance of the temperature distribution (±25°C) also points unequivocally to Hadean zircon growth under conditions that were highly reproducible and thermally regulated. Eutectic-like melting is particularly capable of providing such regulation and is consistent with Hadean zircon growth during wet crustal fusion.

Glikson (1) and Nutman (2) offer intriguing comments and criticism regarding the use of our Ti-in-zircon thermometer to assess conditions on the Hadean Earth (3). There are two broad aspects to their criticism. The first concerns the suitability of the new thermometer for use with Hadean (and all other) zircons whose provenance is unknown (specifically, any zircons whose cocrystallization with rutile cannot be demonstrated). The topic of TiO2 activity was discussed at some length as the focus of figure 2 in (3) and the supporting online material. Because most of the Hadean zircons are widely believed to be of igneous origin, the essence of our argument is that zircon-saturated melts—which, by definition, have high ZrO2 activity—are also likely to have high TiO2 activity. This argument was substantiated by comparing TiO2 contents of siliceous volcanic-rock matrices with the experimental constraints on TiO2 activity then available (4). We have since presented new experimental data confirming the existence of high TiO2 activity in a variety of hydrous, siliceous melts at conditions conducive to zircon crystallization (5). Metaluminous, peraluminous, and trondhjemitic melts saturate in rutile at ∼600 to 700 parts per million (ppm) Ti and ∼2000 ppm Ti at 700°C and 800°C, respectively, implying a TiO2 activity of 1 under these conditions (implicitly referenced to pure liquid TiO2) (6). These modest Ti concentrations are typical [within a small factor; see figure 2 in (3)] of siliceous melts produced by common melting and crystallization processes, and they reaffirm our original statements about the general proximity of siliceous melts to saturation in rutile. Thermodynamics essentially dictates high TiO2 activity (>∼0.5) in the vast majority of magmas capable of crystallizing zircon. Our argument does not apply to unusually alkalic or halogen-rich melts, but we do not consider such melts as plausibly typical of the Hadean.

The remaining criticisms by Glikson (1) and Nutman (2) mostly concern our interpretation of the measured zircon crystallization temperatures, which we ascribed to wet crustal anatexis in environments not unlike those of more recent geologic times. Our argument hinged on the strong clustering of temperatures at ∼700°C and our conviction that this clustering implies a highly regulated process. Indeed, if the eight “high-temperature” outliers in our original database—three of which were obtained from the single zircon pictured in figure 4 in (3)—are discarded as signifying zircons of fundamentally different provenance (e.g., tonalite?), the resulting mean is 688 ± 23°C. The outliers may be important in themselves, but this small variance among 61 of our original 69 temperatures leads inescapably to the conclusion that the circumstances under which the Hadean zircons crystallized were highly reproducible. The one thing Earth can do with high reproducibility is generate liquids by eutectic-like melting, and (near) water-saturated melting in a crust not unlike that of today is the simplest way to reliably generate melts between 650°C and 700°C. Since the publication of our initial report, we have more than doubled the database of Hadean zircon temperatures (7), with the result that the mean is now 680°C ± 25°C after similarly removing from consideration the small number (15%) of thermal outliers at >750°C (Fig. 1). This tight distribution supports not only an anatectic origin for most Hadean zircon-producing magmas but also, indirectly, our contention that TiO2 activity in siliceous melts is highly constrained. If temperature and TiO2 activity were subject to appreciable variation, it is difficult to imagine these two variables conspiring to produce tightly clustered zircon Ti contents. We were drawn to our original conclusion not because it is the only conceivable explanation of our data but because it is the simplest.

Fig. 1.

Updated summary of crystallization temperatures (based on Ti contents) of Hadean zircons from the Jack Hills of Western Australia (6). The pronounced peak at ∼680°C is interpreted as representing zircons crystallized from wet anatectic melts. The few high-temperature outliers may result from eventual zircon saturation during cooling of mafic or tonalitic melts.

Nutman (2) argues that tonalite magmatism could produce zircons at 775°C. Although we accept this to be true, we point out that this is substantially hotter than 680°C [even allowing for subunity TiO2 activity; see figure 3 in (3)] and would result in 300% higher Ti concentrations in the zircons, which is easily detected analytically. Nutman further argues that at the 900° to 950°C temperature of their generation, tonalite melts are grossly undersaturated in zircon “by a factor of 3 or 4” and, therefore, that zircon would not crystallize until significant cooling has occurred. This is also true, but it is worth noting that even a minor difference in initial Zr content would lead to eventual saturation in zircon at substantially different temperatures (up to 25°C), depending on whether the factor is indeed 3 or whether it is 4 (8). In fact, a much larger spread of initial Zr is likely, because the concentration of this element in tonalite melts is not buffered in any way during melt production. The range of resulting Zr contents would lead to eventual saturation in zircon (followed by progressive growth with further cooling) over a range of temperatures, even assuming the major-phase fractionation processes were exactly reproduced in every cooling tonalite magma. Zircon crystallization in this manner is not a regulated process and would not lead to the narrow distribution of low Hadean zircon temperatures we have reported.

The few Hadean zircon temperatures characterized here as thermal outliers may reflect the types of magmas to which Nutman and Glikson refer, and we are uncertain as to why Glikson asserts that rocks of the tonalite-trondhjemite-granodiorite suite would be zircon-poor relative to minimum crustal melts, because they typically contain substantially more Zr [150 ppm (9) versus 50 ppm (10)]. Despite the generally low recorded temperatures, Nutman argues that the apparent scarcity of core-rim relations with pre-4.0 Ga ages suggests that the thermometry records melts generated at higher temperatures. It is extremely rare that an individual grain exhibits more than two generations of crystal growth (11). This means either that zircons virtually never experience more than two generations of igneous-sedimentary cycling or that overgrowths are generally removed during such cycling. We believe the latter explanation is more likely.

Regarding the possibility that surviving Hadean zircon populations are skewed to over-represent granitoids in the provenance crust (2), we suggest that the survivability of low-U zircons formed from mafic magmas would outweigh the relative weathering rates of mafic versus TTG rocks. Indeed, no rock fragments other than quartzite have yet been identified in Jack Hills conglomerates, indicating complete disaggregation of all rock types that contribute zircons. As for the nature of pre-4.0 Ga crust as inferred from isotopic studies of the Archaean mantle, we made no claims about the ultimate fate of Hadean continental crust. In our original report, we quantitatively addressed whether Jack Hills zircons could be derived from impact melt sheets (which would not be subject to shock effects) and rejected that possibility.

We note in conclusion that neither Glikson (1) nor Nutman (2) argues against our view that the data are evidence for near water-saturated melting conditions on Earth during the Hadean. There is surely more to the history of pre-4.0 Ga zircons than we currently understand, and we look forward to seeing what further evidence is revealed in due course.

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