High sensitivity mapping of Ti distributions in Hadean zircons

doi:10.1016/j.epsl.2007.05.016

Earth and Planetary Science Letters

Volume 261, Issues 1–2, 15 September 2007, Pages 9-19

https://doi.org/10.1016/j.epsl.2007.05.016 Get rights and content

Abstract

Detrital zircons as old as nearly 4.4 Ga from the Jack Hills, Western Australia, offer possible insights into a phase of Earth history for which there exists no known rock record. Ti concentrations of Hadean zircons indicate a spectrum of crystallization temperatures that range from a cluster at ca. 680 °C to apparent values exceeding 1200 °C. The low temperature peak has been interpreted to indicate the existence of ‘wet’ melting conditions during the Hadean, but alternate views have been advanced. We have developed methods for quantitative ion imaging of titanium in zircons using positive and negative secondary ions, produced respectively under bombardment of O⁻ and Cs⁺, that permit detailed insights regarding Ti concentration distributions. Each approach has particular advantages that tradeoff in terms of sensitivity, ultimate lateral resolution, and reproducibility. Coupled with high resolution spot analyses, these ion images show that Ti contents greater than about 20 ppm in the Jack Hills zircons are associated with cracks or other crystal imperfections and that virtually all of the high apparent temperatures (i.e., > 800 °C) yet obtained are suspect for contamination by Ti extraneous to the zircon. Removing these from consideration strengthens the previous conclusion that the Hadean Jack Hills population is dominated by zircons crystallized at 680–720 °C.

Introduction

Due to their inherent resistance to alteration by weathering, dissolution, shock, and diffusive exchange, and their enrichment in U and Th relative to daughter product Pb, zircon is widely recognized as the premiere U–Pb geochronometer (Hanchar and Hoskin, 2003, Cherniak and Watson, 2003). While highly valued in that role, the trace element and isotopic chemistry of zircon is also increasingly recognized as a valuable probe of environmental conditions that existed during crystallization (Hoskin and Schaltegger, 2003). Even in cases where zircon has been removed from its original rock context, such as detrital grains in clastic rocks, trace elements, isotopic signatures, and mineral inclusions can yield important information regarding source conditions [e.g., (Watson and Harrison, 2005, Maas et al., 1992, Harrison et al., 2005)], provided that the record has been faithfully preserved.

Using the Ti concentration in zircon as a crystallization thermometer, Watson and Harrison (Watson and Harrison, 2005, Watson and Harrison, 2006) reported 136 ion microprobe Ti measurements on zircons ranging in age from 3.91–4.35 Ga. The vast majority of Hadean zircons plot in a normal distribution at an average model temperature of 683 ± 27 °C, with a scattered tail defined by apparent values up to ∼ 1200 °C. A similar cluster is also present in 36 > 4 Ga zircons reported by Valley et al. (2006). This temperature spectrum was interpreted by Watson and Harrison, 2005, Watson and Harrison, 2006 as indicating that the Hadean zircons were dominantly sourced from wet, minimum melts under prograde conditions. Others subsequently argued that the low temperature Hadean peak could reflect derivation from zircons that formed during cooling of mafic (Valley et al., 2006, Coogan and Hinton, 2006) or intermediate (Nutman, 2006, Glikson, 2006) magmas.

Conclusions regarding provenance that can be derived from individual detrital zircons depend upon the preservation of their original chemical signatures. For example, Hoskin (2005) concluded that some Jack Hills zircons have undergone hydrothermal alteration that has modified their original composition. However, in the absence of recrystallization or complete loss of crystalline integrity, the extremely low diffusivity of Ti in zircon (Cherniak and Watson, 2006) assures that the distributions imposed during crystallization will be retained indefinitely under virtually all crustal conditions.

Cathodoluminescence (CL) and other electron-based imaging of zircon carried out in conjunction with ion microprobe U–Pb dating has proved very useful in improving the accuracy of the age interpretation (see examples in (Hanchar and Hoskin, 2003)). While valuable for revealing precursor grain nuclei (i.e., restitic cores) and/or compositional zoning and other growth features in zircons, CL imaging is incapable of revealing coherent thermochemical domains (i.e., crystal chemistry associated with growth during a specific thermal history episode) or identifying nanoscale Ti-rich inclusions. Without additional information, the structures typically observed in CL imaging could represent time gaps ranging from hours to hundreds of millions of years. However, complementing CL imaging with ion imaging of Ti in zircon has the potential to identify chemical-isotopic regions associated with discrete crystallization events. This approach can furthermore identify anomalously high Ti values due to sub-micron Ti-rich phases that could otherwise be incorrectly interpreted in terms of anomalously high temperatures.

In this paper, we develop techniques for ion imaging Ti distributions in zircon and apply both these methods and spot analysis to > 4 Ga zircons from Jack Hills, Western Australia. We find that grains yielding high (> 800 °C) apparent crystallization temperatures from conventional spot analyses are contaminated by elevated amounts of Ti contained along cracks. Furthermore, high resolution imaging of Ti within Hadean zircons tends to reveal a narrow temperature range of crystallization temperatures in individual grains. While as yet a limited data set, these data are more consistent with zircon forming in low temperature partial melts relative to crystallization during cooling mafic or intermediate magmas.

Section snippets

Analytical details

The high-spatial resolution ion microprobe is the preferred tool to measure low levels of Ti in zircon. Titanium concentrations of ca.1–10 ppm (Watson and Harrison, 2005, Watson and Harrison, 2006) in most Hadean Jack Hills zircons are below the detection limit of the electron microprobe and close to that of laser ablation ICPMS analysis. However, at low mass resolution using an O⁻ primary beam, doubly-charged ions of ⁹²Zr, ⁹⁴Zr and ⁹⁶Zr interfere with singly-charged ions of ⁴⁶Ti (8.25%), ⁴⁷Ti

Prior Ti analysis of Jack Hills zircons

Zircons from the Narryer Gneiss Complex of Western Australia, including the Jack Hills (Compston and Pidgeon, 1986), yield ages as old as nearly 4.4 Ga (Compston and Pidgeon, 1986, Harrison et al., 2005). As the only known remnants of the Hadean Eon, > 4 Ga zircons currently represent our best opportunity to gather chemical information regarding the history of differentiation and crust formation on early Earth. Their very existence extends the history of crust formation to ∼400 Ma earlier than

Results

We report 158 Ti concentration measurements on 34 Jack Hills zircons ranging in age from 4.11 to 4.38 Ga (Fig. 2A; Table 1 in Appendix). Calculated temperatures using the calibration of Watson and Harrison (2005) range from 590–1380 °C (assuming a_TiO2 = a_SiO2 = 1). On the basis of these analyses, we selected seven zircons for further characterization by Ti ion imaging as being representative of different emergent patterns. For example, apparent temperatures for FC27 3–11 are highly variable,

Discussion

It is clear from inspection of the ion images in Fig. 3A–G that Ti concentration distributions in the seven imaged zircons do not generally exhibit patterns expected for zircon growth under either a prograde or retrograde crystallization history (i.e., the patterns tend not to correlate with crystal morphology or CL growth zones). Instead, they reveal Ti ‘hot spots’ that appear to be associated with cracks. In virtually all cases, spot analysis concentrations are consistent with ion image data

Conclusions

The potential of the Ti-in-zircon thermometer as a probe to identify coherent thermochemical domains is significantly enhanced by utilization of ion imaging. We investigated the use of rastering both O⁻ and Cs⁺ ion beams across extended portions of zircon crystals to reveal complex Ti distributions at lateral resolutions of ∼ 4 and ∼ 1 μm, respectively. In both cases, non-uniform charging across the investigated area can be compensated for by normalization to a SiO species. These applications are

Acknowledgements

This work was supported by NSF grant EAR-0635969, ARC grant DP0666497, and a grant from the UCRP program of Lawrence Livermore National Laboratory. We acknowledge facility support from the Instrumentation and Facilities Program of the National Science Foundation. We thank Bruce Watson, Kevin McKeegan, and Rick Ryerson for helpful discussions, Bruce again for supplying the synthetic zircons, Janet Harvey and Gilles Peltzer for assistance with image manipulation, Michelle Hopkins for SEM imaging,

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High sensitivity mapping of Ti distributions in Hadean zircons

Abstract

Introduction

Section snippets

Analytical details

Prior Ti analysis of Jack Hills zircons

Results

Discussion

Conclusions

Acknowledgements

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Priscoan (4.00–4.03 Ga) orthogneisses from northwestern Canada

Contrib. Mineral. Petrol.

Diffusion in zircon

Ti diffusion in zircon

EOS (Trans., Am. Geophys. Union)

Jack Hills, evidence of more very old detrital zircons in Western Australia

Nature

Do the trace element compositions of detrital zircons require Hadean continental crust?

Geology

Comment on “Zircon thermometer reveals minimum melting conditions on earliest Earth”

Science