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Project Progress

Ce and Eu anomalies in zircon (ZrSiO₄): indicators of oxygen fugacity and melt composition.

The first phase of this project has come to a completion. We demonstrated that Hadean magmas have redox states that are broadly similar to Archean and modern times, and that they are more oxidized than lunar magmas. We predicted that volatile gases entering the Hadean atmosphere from volcanic eruptions would have been dominated by neutral species such as H₂O, SO₂, and CO₂. The results were published in Nature in December of 2011. The paper was accompanied by a “news and views” and a Nature ‘podcast’ (an internet radio interview) available for download on the Nature website. We followed up our initial study with a more comprehensive data set, with an additional ~30 experiments. These two papers lay the foundational groundwork for scientists to: (1) utilize the methodology developed for additional experimental studies, (2) apply our calibration to other natural settings. The follow-up paper was published in 2012.

We have now completed the early phases of a study that explores a means to obtain more precise estimates of magma oxidation states. These studies move into completely uncharted territory because no previous study has attempted to elucidate the oxidation state of a magma using Ce⁴⁺/Ce³⁺ ratios in zircon. We are directly measuring Ce⁴⁺/Ce³⁺ ratios in the zircon crystal, with a spatial resolution of ~2×5 μm by XANES (X-ray Absorption Near Edge Structure). We have measured ratios in Hadean zircons separated from their original host rocks, and modern samples that serve as stepping stones to interpret ancient zircons.

After a request for an invited review, we wrote a very comprehensive paper (~20,000 word, 19 figures), discussing our knowledge of the Hadean Earth, and how experimental geochemistry has helped us understand it. We have now submitted our revisions, and we anticipate that this paper will be accepted for publication soon.

Biosignature mimics – mass dependent carbon isotope fractionation.

The abundances and isotopic compositions of C, S, N, H are used to trace biological activity in ancient sedimentary rocks. We undertook an experimental study of carbon diffusion in the grain boundaries of rocks and in Fe metal, in order determine the effect of isotopic mass on diffusion. Our ion probe work on these experiments demonstrated that C isotopes are fractionated as they diffuse through an Fe polycrystalline rod. In particular, ¹²C diffuses faster through the rod than ¹³C. We are in the final stages of preparing a manuscript for submission to PNAS which demonstrates that C isotopes can be fractionated by diffusion. As far as we are aware, this is the first example demonstrating this phenomenon for carbon. The study lays the framework for more complex carbon isotope studies.

Apatite thermometer and oxygen fugacity sensor

Apatite is an important mineral that may form by biological processes. Some of the oldest known isotopically light carbon is associated with apatite crystals in ancient 3.8 Ga sediments. We have been experimentally calibrating an apatite thermometer and oxygen fugacity sensor to apply to natural settings. Previous research (2011, NAI report) indicates that Eu partitioning between basalt-apatite is highly T-_f_O₂ dependent, though no direct measurement of Eu valence was then available. Subsequent Eu XANES results define the Eu²⁺/Eu³⁺ of basalts at different temperature conditions, with results defining the sinusoidal valence transition for alkali basalts (e.g., KREEP). While one goal is to constrain the abundance and nature of votatiles on small solar system bodies such as the Moon with these new tools, we anticipate that our calibration may have other applications

Trace elements in quartz

A number of Hadean and Archaen sediments (e.g., Jack Hills, Mt Narryer, Wittswatersraand, Mt Alfred, etc) are dominated by quartz (quartzites or quartz sandstones/conglomerates/siltstones). The dominance of quartz in ancient sediments makes the development of new environmental indicators within the mineral highly advantageous to deciphering early terrestrial conditions. A total of 49 experiments in the Al₂O₃-SiO₂-H₂O (/- TiO₂) system, have been carried out to constrain Al-solubility in quartz at different P-T conditions – i.e., this research represents a new thermobarometer. In addition to constraining P-T-X conditions for Al-in-quartz, 19 ‘infinite-source’, Al diffusion experiments have been carried out within the phase (including sections cut parallel and perpendicular to the c-axis). These experiments define Al diffusivity (Arrhenius line) and demonstrate that Al³ is among the slowest diffusing species in quartz (with only Si and “dry” O reporting lower values) – making the mineral ideal for thermobarometry purposes.

To test the Al-in-quartz system, with comparison to other thermometers known from quartz (e.g., Ti-in-quartz), >1,100 natural quartz analyses (~50% from values reported within the literature, and ~50% acquired in-house by EPMA) to characterize quartz geochemical trends from different crystallization environments. This quartz “database” suggests that magmatic (including intrusive and extrusive varities), metamorphic, hydrothermal and high pH/sulfidation environments all define compositional fields with regard to Al- and Ti-contents.