2013 Annual Science Report
Rensselaer Polytechnic Institute Reporting | SEP 2012 – AUG 2013
Project 6: The Environment of the Early Earth
This project involves the development of capabilities that will allow scientists to obtain information about the conditions on early Earth (3.0 to 4.5 billion years ago) by conducting chemical analyzes of crystals (minerals) that have survived since that time. Minerals incorporate trace concentrations of ions and gaseous molecules from the local environment. We are conducting experiments to calibrate the uptake of these “impurities” that we expect to serve as indicators of temperature, moisture, oxidation state and atmosphere composition. Our focus has been mainly on zircon, quartz, and apatite.
We are conducting additional studies to investigate the redox environment of the Early Earth though measurements of impurities in zircon. We have recently expanded our studies to include direct measurements (by XANES) of Ce4+/Ce3+ ratios in zircon in modern, experimental, and ancient zircons. The former two set the stage for the interpretation of ancient Ce4+/Ce3+ ratios recorded in Hadean zircon. In order to test the robustness of this ratio, we have also been conducting experiments that will enable us to evaluate the likelihood of redox preservation in zircon crystals. We submitted the our first paper on our XANES work related to zircon in September of 2013 (Trail et al.).
We recently published 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 also started to investigate how the processes of biology influence the geochemistry of rocks and accessory minerals in particular. For example, sedimentary -type granites are more reducing than igneous-types; this difference has been attributed to the presence of carbonaceous material in the sedimentary source rocks. We are exploring whether this holds true for the sedimentary- and igneous-type rocks of the very well studied Lachlan fold belt. Our findings indicate that indeed zircons found in sedimentary -type rocks (on average) crystallized under more reduced conditions than those found in igneous-type rocks. We intend to explore whether this simple observation might help us understand how biology influences the redox states of even more ancient rocks, through crustal recycling.
Biosignature mimics – mass dependent carbon isotope fractionation.
The abundances and isotopic compositions of C, S, N, H are commonly used to identify biological activity in ancient sedimentary rocks. Key to this strategy is the assumption that certain isotopic fractionations (e.g., 13C depletion in carbonaceous material) are uniquely biological in origin. As part of our investigation of abiotic phenomena that might impersonate biosignatures, 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, resulting in 13C depletion near the distal end of a diffusion profile. Our manuscript on this topic was recently accepted for publication and is now in press (Mueller et al).
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 experimentally calibrated an apatite thermometer and oxygen fugacity sensor to apply to natural settings. We have also applied this calibration to apatites from lunar mare basalts. Surprisingly, our initial data suggest relatively oxidizing conditions, most likely attributable to disturbed apatite chemistry from the time of original crystallization.
Trace elements in quartz
We have been investigating the geochemistry of quartz from well-constrained environments throughout the world by LA-ICP-MS (e.g., Lachlan fold belt, granitoids of the northeast United States , volcanic rocks from the US and Australia). With these ~200 samples, we have built up a geochemical database of many trace elements in quartz, and observe systematic differences in trace element chemistry depending on the formation environment. Along with experiments, these data are being used as stepping stone to interpret and understand the geochemistry of quartz from ancient sedimentary environments (e.g., the Jack Hills and Mt. Alfred locations in Western Australia and the Witwatersrand in South Africa).
PROJECT INVESTIGATORS:Bruce Watson
PROJECT MEMBERS:Suzanne Baldwin
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Earth's early biosphere.
Effects of extraterrestrial events upon the biosphere