We investigated the incorporation of hydrogen into zircon at 1650 and 1550 °C, and pressures of 2.5 and 1.5 GPa under water-saturated conditions in a piston-cylinder apparatus. Concentrations were determined by polarized Fourier transform infrared spectroscopy using the zircon absorption coefficient εi = 36 241 cm-2 per mol H2O/L and range from ~90 to 200 ppm H2O by weight. Crystals grown in the presence of Ti4+ or Th4+ do not differ significantly in their H2O content. We also synthesized zircons with various concentrations of Lu2O3 and Al2O3 to characterize changes in band positions and hydrogen concentrations related to coupled substitutions in zircon. Trivalent cations correlate in a nearly 1:1 molar fashion with hydrogen highlighting a potentially important coupled substitution in high water activity environments. Bands from undoped and doped zircons in the OH stretching region of the infrared spectrum show broad agreement when compared to spectra from natural samples. Heating experiments at 1 atm and 1000 °C produce a decrease in the integrated area; while some bands disappeared entirely, others are particularly stable with little decrease in integrated area after 128 h at 1000 °C. Results presented here help eliminate uncertainties that arose from Fourier transform infrared studies of natural zircons and provide further clarification for the origin of band positions in natural samples. In addition to the water activity of the crystallizing medium, the H2O content of natural grains will likely be significantly influenced by trivalent cation concentrations. In crustal zircons especially, trivalent atomic contents generally exceed those of phosphorus, meaning that hydrogen may be particularly important for trivalent cation charge compensation. An unanticipated result of this study was the development of a reasonably effective technique that produces relatively homogenous zircons doped with minor impurities. This technique could potentially be utilized in studies aimed at developing zircon standards, because it yields crystals that appear to be more homogenous than those produced by the flux method, and are generally free of inclusions.
Because physical and chemical processes of the past are determined from analysis of a preserved geologic record, little is known about terrestrial crustal processes of the first 500 Ma during the so-called Hadean Eon. What is known from direct measurements has been derived almost exclusively from the study of greater than 4.0 Ga detrital zircons from the Jack Hills, Western Australia. The geochemistry of these zircons has direct application to understanding the origin and evolution of the rocks during the Hadean because: (i) U-Th-Pb age determinations by ion microprobe suggests the presence of crust as early as 4.37 Ga, or shortly after lunar formation; (ii) high-resolution U-Th-Pb zircon depth profiles reported here reveal several episodes of zircon growth in the Hadean previously unrecognized; (iii) core regions of pre-4.0 Ga zircons with igneous compositions are enriched in O-18 and contain metaluminous and peraluminous mineral inclusions, both features indicative of S-type grainitod protoliths. Study of these ancient zircons provides a unique window into the first half billion years that permits assessment of the potential of the Hadean Earth to host an emergent biosphere.
This study is focused on mineralogical and chemical characterization of an authigenic carbonate rock (crust) collected at a recently discovered cold seep on the US North Atlantic continental margin. X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicate that the carbonate rock is composed of microcrystalline aragonite cement, white acicular aragonite crystals (AcAr), equant quartz crystals, small microcrystalline aluminosilicates, and trace amounts of iron sulfide microcrystals. Element/calcium ratios were measured with laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) using a calcite standard, which was prepared by annealing USGS certified carbonate powder (MACS-3). The occurrence of microscopic, non-carbonate inclusions precluded evaluation of trace elements in the aragonite cement, but allowed for in situ analysis of AcAr crystals. Carbon and oxygen isotopes were analyzed via isotope ratio mass spectrometry (IRMS) and expressed as δ13C and δ18O. Low δ13C values suggest that aragonite grew as a result of anaerobic oxidation of methane and observed δ18O values indicate that the temperature of aragonite crystallization was 1.7–1.9 °C.
Granitoids are silicic rocks that make up the majority of the continental crust, but different models arise for the origins of these rocks. One classification scheme defines different granitoid types on the basis of materials involved in the melting/crystallization process. In this end-member case, granitoids may be derived from melting of a preexisting igneous rock, while other granitoids, by contrast, are formed or influenced by melting of buried sedimentary material. In the latter case, assimilated sedimentary material altered by chemical processes occurring at the near surface of Earth—including biological activity—could influence magma chemical properties. Here, we apply a redox-sensitive calibration based on the incorporation of Ce into zircon crystals found in these two rock types, termed sedimentary-type (S-type) and igneous-type (I-type) granitoids. The ∼400 Ma Lachlan Fold Belt rocks of southeastern Australia were chosen for investigation here; these rocks have been a key target used to describe and explore granitoid genesis for close to 50 years. We observe that zircons found in S-type granitoids formed under more reducing conditions than those formed from I-type granitoids from the same terrain. This observation, while reflecting 9 granitoids and 289 analyses of zircons from a region where over 400 different plutons have been identified, is consistent with the incorporation of (reduced) organic matter in the former and highlights one possible manner in which life may modify the composition of igneous minerals. The chemical properties of rocks or igneous minerals may extend the search for ancient biological activity to the earliest period of known igneous activity, which dates back to ∼4.4 billion years ago. If organic matter was incorporated into Hadean sediments that were buried and melted, then these biological remnants could imprint a chemical signature within the subsequent melt and the resulting crystal assemblage, including zircon. Key Words: Hadean Earth—Biological activity—Peraluminous granites—Lachlan—Sediments—Ce anomaly—Zircon. Astrobiology 15, 575–586.
