Terrestrial Applications of the Triple Oxygen Isotope Geothermometer

点击翻译按钮获取中文摘要

The oxygen isotope compositions of natural materials have been used for over 60 years as a quantitative 'thermometer' for ancient rock and mineral samples. The difference in the oxygen isotope composition (the 18O/16O ratio) of a mineral and the water from which it formed provides a temperature estimate that has a precision of ~1°C. The problem with application of this method to ancient samples is that the water - usually ocean water - is not preserved in the rock and needs to be inferred. This introduces some ambiguity to the derived paleotemperature estimate. Namely, was the oxygen isotope composition of the ocean water at the time the mineral formed the same as it is today? Researchers have been tackling this question for decades, and it is still unresolved. In this proposal, the team will use a novel, previously untried, technique in which all three naturally occurring oxygen isotopes are used for temperature determinations. It has long been thought that the rare isotope 17O provides no additional information to the commonly measured 18O/16O ratios. More recently, this team and other research groups have shown that there are subtle differences in the way temperature affects the 18O/16O and 17O/16O ratios. By accurately measuring all three isotopes, 16O, 17O and 18O, it is proposed to eliminate the uncertainties inherent in using the 18O/16O ratios alone. This award will help support the graduate theses of two students, one of whom is a woman, the other a minority. It will also include research opportunities for an undergraduate student, allowing them hands-on access to sophisticated laboratory equipment. Such an opportunity is invaluable for young potential scientists. Both the investigator and the graduate students will work closely with undergraduate students recruited over the three year time frame of this project. This is a new research avenue and there are many outstanding questions. As part of our research, they will calibrate the temperature dependence on the 18O/16O and 17O/16O ratios of quartz and water by experimentally equilibrating a quartz-water mixture at high temperatures and pressures. They will expand the preliminary tests of the technique to the very important mineral calcite. Marine carbonates are ubiquitous in the rock record, and if the approach works for calcite it will vastly expand the research potential of this new technique. Unfortunately, the analysis of calcite for 17O/16O ratios has proven to be very difficult. This research group will initially spend time conducting analytical tests to quantifying this measurement and reproduce it for a standard. Once the methodology is further established, they will proceed to analyze some of the most pristine, ancient samples in order to test the age-old question 'Is the oxygen isotope composition of ancient seawater the same as it is today?' At the same time, they will apply this work to high temperature systems as a way of retrieving both temperatures and fluid compositions (sources) in hydrothermal systems. This work will have important applications to the economic geology community which strives to understand how mineral deposits form.