44494-GB2
Mantle Generation of Heavy Hydrocarbons
Introduction:
We are interested in hydrocarbon formation from inorganic carbon sources, such as calcite, at the pressure and temperature conditions of Earth's mantle. We address potential reactions experimentally by compressing samples to pressures corresponding to depths of up to 1,000 km and temperatures exceeding 1,000 ºC. By using diamonds as pressure-generating anvils, we are able to observe reactions in situ both visually and using Raman spectroscopy.
Experimental:
We utilize Diamond Anvil Cells (DACs) to experimentally reach high pressures and temperatures. Briefly, DACs use two gem-quality diamonds as pressure-generating anvils, with a sample chamber constructed out of a metal foil between them. Observations (e.g. visual, X-ray or spectroscopic) are made directly through the anvils while samples are subjected to high pressures and temperatures.
High temperatures are produced either by focusing infrared lasers directly on the sample or heating the anvils with resistive wires. For resistively heated experiments, temperature is measured with thermocouples in direct contact with the anvils.
Pressure is measured indirectly in DAC experiments; a common technique utilizes pressure-induced fluorescence shifts in well-studied "calibrant" materials with strong fluorescence, such as Al2O3-ruby.
Progress and Results this Year:
We were joined this summer by Marteve (Marty) Gray, an IUSB physics major, who received a SUMR supplemental grant from the American Chemical Society. Marty was heavily involved our three primary current efforts:
- Analyzing X-ray diffraction data on crystalline reaction products
- Testing new ways to measure sample pressure in the resistively heated DAC
- New experiments in the Fe-CaCO3-H2O system
X-Ray Diffraction Analysis:
We have continued to analyze synchrotron X-ray data collected at the Advanced Photon Source of Argonne National Lab during August 2007. Specifically, we are attempting to identify crystalline phases produced during methane-forming reactions such as the following:
12 FeO + CaCO3 + 3 H2O -> 4 Fe3O4 + CH4 + Ca(OH)2
and
3 Fe + CaCO3 + 3 H2O -> Fe3O4 + CH4 + Ca(OH)2
The above products have all been conclusively identified from experiments conducted at pressures ranging from 2-12 GPa and temperatures from 500 -1500 ºC. However, for experiments at the high end of this temperature range, we know that steep temperature gradients exist across the sample chamber (e.g. from 1500 ºC to <100 ºC over less than 50 microns), and we expect that minor, out-of-equilibrium (or unreacted), phases are produced as well. For example, in the laser-heated experiments we find both unreacted Fe and CaCO3, in addition to FeO and CaO. We observe some diffraction lines that we still cannot identify, even upon pressure and temperature quenching to ambient conditions.
Measuring Sample Pressure:Although pressure measurement at ambient temperature is straightforward, it becomes considerably more challenging at elevated temperatures. Our most-recent experiments involve using the resistively heated DAC for which we measure pressure using fluorescent materials as internal calibrants. At high temperature, however, the peaks for which pressure shifts have been calibrated against pressure both broaden and decrease in temperature. Furthermore, all of our experiments are hydrothermal in nature, and the fluorescent materials (e.g. Al2O3-ruby) become soluble and may act as a contaminant.
Our current approach to minimize this effect has been to line sample chambers with a gold annulus; rather than placing the calibrant directly inside the sample chamber we place it on the gold as close to the sample chamber as possible. Our assumption is that the gold is soft enough, particularly at high temperatures, that the measured pressure will be very close to the actual sample-chamber pressure. Not only does this avoid having the calibrant in contact with the hydrothermal fluid, but it also allows us to use much larger grains of the calibrant material which simplifies fluorescence measurements.
Experiments in the Fe-CaCO3-H2O System:
To increase the methane yield in our experiments, we have conduced a series of runs with native iron, rather than an iron oxide, to further reduce the system. Although this decreases the geological relevance of the experiments, it has proven to increase the methane yield, as reported last year from laser-heating experiments (temperatures > 1000 ºC).
The latest result in this series of experiments is the formation of what we tentatively identify as a methane hydrate phase. We heated this assemblage to 650 ºC at an initial pressure of 2 GPa; during the heating cycle the pressure increased to almost 5 GPa. Upon temperature quench and pressure reduction to 1.8 GPa, we found a doublet in the Raman spectrum near 2900 cm-1. Based on C-H stretching frequencies reported in the literature, this is consistent with a clathrate structure. To our knowledge, this is the first report of a clathrate phase formed upon cooling from high temperatures at high pressures.
