"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."
Laboratory Measurements of Trace Element Partition Coefficients Between Zircon, Aqueous Fluid and Silicate Melt at High and Ultrahigh Pressures
Intellectual Merit. Although zircon is widely used for dating geologic events and identifying the source (mantle versus recycled crust) of crustal rocks, it has not been developed as a tool for characterizing fluid-rock interaction. Zircon could potentially be used to date fluid infiltration events and estimate the trace element composition of fluids. This would be particularly useful for evaluating the role of fluids in the formation of High- and Ultrahigh-Pressure (HP and UHP) rocks that typically form in subduction zones. Laboratory experiments are proposed to characterize the behavior of zircon in the presence of aqueous fluids and silicate melts. D(zircon/fluid) partition coefficients will be measured for P, Na, Ca, Mg, Sc, Sr, Y, Nb, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb, Lu, Hf, Pb, Th and U at 1.5-3.5 GPa. In subsolidus synthesis (forward) experiments at 800-1100°C the assemblages zircon + quartz and zircon + baddeleyite will be equilibrated with various fluid compositions (H2O, 1m NaOH, 1m HCl) at fO2 = NNO to allow measurement of zircon aqueous solubility and D(zircon/fluid) values. D(zircon/fluid) values measured in recrystallization (reversal) experiments at similar conditions will be used to test for equilibrium. Measured D(zircon/fluid) values will be compared with D(zircon/melt) values recently measured at the same conditions to identify the distinctive features of the trace element patterns of hydrothermal and magmatic zircons. Supersolidus experiments at 1000-1200°C in which zircon, melt and aqueous fluid are equilibrated will allow simultaneous measurement of D(melt/fluid), D(zircon/melt), and D(zircon/fluid) for all 25 trace elements. These values will be compared with D(zircon/melt) and D(zircon/fluid) values measured at the same conditions using other methods to check for internal consistency. The applicability of lattice strain theory to mineral/fluid systems will also be tested. Characterizing the trace element signatures of magmatic and hydrothermal zircons will allow for more accurate identification of hydrothermal zircons and interpretation of zircon U-Pb ages measured in-situ. Measured D(zircon/fluid) and D(zircon/melt) values can be used to estimate compositions of fluids and melts that equilibrated with zircons in subducting slabs or other HP-UHP settings. Zircon partition coefficients will enable more accurate classification of 'out of context' zircons, including Hadean detrital and xenocrystic zircons, some of which have been shown to contain diamond inclusions, and can be used to calculate the composition of the source rock (e.g., REE pattern) and to identify by analogy the type of source (crust vs. mantle, granite vs. tonalite).
Broader Impacts. This project will fund one post-doctoral researcher, one current Vanderbilt Ph.D. student, 1-2 future M.S. or Ph.D. students, and 1-2 undergraduate students. It will also fund the research of 1-2 high school students through a research internship program that the P.I. is involved in. Results will be presented by the P.I. and students at national and international meetings and in journal publications. The P.I. will continue outreach activities such as primary school presentations and publication of popular accounts of his research. He has written or contributed to three popular articles about his research in the last two years.
