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How Well Can We Constrain When Life Began on Earth?
Life is fundamentally based on the chemistry of carbon-bearing compounds. In addition to boasting an impressively diverse chemistry, carbon also has three naturally occuring isotopes. The isotopes C-12 (~99% of carbon) and C-13 (~1%) are stable, meaning they undergo no radioactive decays. The radioactive isotope C-14 is well-known for its use in radiocarbon dating, in which its ~5700 year half-life allows for the dating of many archeological and recent biological materials. However, the stable isotopes C-12 and C-13 are also quite useful for tracing the origins of carbon in the geologic record.
Photosynthesis, the process by which plants synthesize sugars, involves the uptake of carbon from the environment -- but plants preferentially take in C-12. Because of this, biogenic matter on Earth is notably enriched in C-12 compared to abiotic sources of carbon (e.g., volcanic outgassing from the mantle). This isotopic fingerprint for life was exploited by Mojzsis et al. (1996), who found C-12-enriched graphite trapped within apatite in >3.8 billion year old (Ga) rocks in Akilia, West Greenland. This discovery provided evidence that life on Earth was at least as ancient as 3.8 Ga.
Rock and mineral records become much more sparse before 3.8 Ga. However, >4 Ga zircons from Jack Hills, Western Australia include very rare carbon-bearing materials. Unfortunately, past studies of these materials largely turned up contamination (as shown by Dobrzhinetskaya et al., 2014). I am currently identifying uncontaminated carbonaceous materials in the zircons and analyzing their isotopic composition via ion microprobe. This project will help to constrain the timing of the origin of life in the period before the earliest known rock record.
We have successfully found one 4.10 Ga zircon containing primary graphite, and its light isotopic signature suggests that it may be biogenic (although abiotic mechanisms to produce such a signature cannot be definitively ruled out with only one datapoint). This sample may represent the earliest evidence yet found for terrestrial life!
Paper on potentially biogenic 4.1 Ga carbon
Our search continues for more of the Hadean carbon isotope record! More samples, especially spanning a large range in ages, may allow us to better constrain both when life began on Earth and the response of the early carbon cycle to major events in the first few hundred million years of our planet and solar system's history (e.g., meteorite bombardment).
Preservation vs. Alteration of Mineral Inclusions in Magmatic Zircon
In order to determine the significance of carbonaceous inclusions' isotopic compositions, we need to understand how those inclusions formed and whether they were subsequently altered. This is especially important for the Jack Hills zircons, which were deposited in a sandstone at ca. 3 Ga and have experienced several metamorphic episodes since.
Left: Transmitted light image of a Hadean Jack Hills zircon with two visible subsurface inclusions.
Right: Secondary quartz has infiltrated cracks in this ca. 3 Ga detrital zircon from Jack Hills. Zircon in blue, quartz in red. Color cathodoluminescence image.
As zircon grows in either a magma or a metamorphic environment, it often encloses other mineral phases or packets of melt. Mineral inclusions are useful in showing what other materials were present as the zircon was forming and may give clues to P-T conditions during formation (e.g., Hopkins et al., 2010 for >4 Ga magmatic zircons; Liu and Liou, 2011 for investigating ultra-high pressure metamorphism). Many questions have been raised in recent years about the fidelity with which magmatic zircon mineral inclusions reflect their parent magma's composition. Problems might arise due to alteration of the inclusions by later metamorphism or due to zircons potentially capturing a biased inclusion population during their growth (e.g., Darling et al., 2009; Jennings et al., 2011).
It is important to develop a more systematic understanding of the information contained in magmatic mineral inclusions and of the conditions for their preservation (or otherwise). As part of this effort I have investigated the role of cracks and other alteration features in Jack Hills zircons as pathways for fluid alteration of the various inclusions. I am also examining zircons from several other locations with differing metamorphic histories and inclusion mineralogies in order to better understand inclusion preservation and alteration.
Paper on inclusion preservation among Jack Hills zircons
Paper on inclusion preservation in an amphibolite to lower granulite facies Grenville orthogneiss
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