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45112-G8
U-Pb Age and Hf Isotope Composition of Detrital Zircons from Strata of the Ouachita Orogenic Belt, Arkansas, Oklahoma, and Texas
This research aims to constrain the source of allochthonous slope and rise sediments as well as flysch from the Ouachita Mountains in Arkansas and Oklahoma and the Marathon Uplift in southwest Texas. The rocks from these two locations provide a nearly complete stratigraphic record from Cambrian to Late Carboniferous. Within this record, a continent wide shift in sediment provenance around 450 Ma is recorded in both the Ouachita Mountains and Marathon Uplift (Patchett et al., 1999, Science, V283, p. 671-673), and this change in provenance is characterized by a sharp shift in Nd isotope composition (Gleason et al., 1995, GSA Bulletin, 107, 1192-1210). Although Nd isotopes can place general constraints on average sediment source age, specific source regions (e.g., Proterozoic vs. Archean craton) as well as relative mixing proportions of sediment derived from variable age sources cannot be resolved by Nd isotopes alone. This research proposes to use zircon, which is an extremely durable mineral in sedimentary environments, to better understand sediment sources and their relative contributions to the Ouachita embayment. The use of detrital zircons to solve this problem is attractive because they retain a record of source age as well as source lithology (e.g., evolved versus juvenile crust based on Hf isotope compositions) enabling better correlations between ultimate sediment sources and sinks as well as better constraints on sediment mixing prior to deposition in the Ouachita and Marathon regions.
The primary efforts of myself, graduate student Barry Shaulis, and undergraduate student Darshan Ghandi in the first 12 months of this research has been in the procurement, characterization, and processing of roughly 20 representative samples spanning the Cambrian to Mississippian and development of laser ablation mass spectrometry methods at UH. As of writing this statement, we are now analyzing our samples and breaking down the data. Based on the forthcoming results, we will target other samples in our collection to be processed and analyzed this Fall as well as select samples for Hf isotope analyses. We are also currently writing a manuscript based on the laser-ablation mass spectrometry methods we have developed at the University of Houston. It is important to note that NONE of this research and development would have been possible without this grant that was made possible by generous contributors to the American Chemical Society and Petroleum Research Fund.
The 20 selected samples had relatively high zircon yield and my students were easily able to pick >200 grains from each. Through the picking process, an assessment of grain shape and size indicates that sediments with pre-Taconic depositional ages (> ~450 Ma) contain extremely well polished and rounded zircons, whereas younger sediments have grain shapes ranging from euhedral to well rounded. We are correlating mineral roundness with our measured ages in an effort to correlate age to degree of transport/recycling. It is important to note that we are also taking into consideration the U and Th content of these rounded grains so that radiation damage can be assessed in terms of abrasion characteristics. We are also conducting modal analyses of the entire heavy mineral fractions in order to gain further information of sediment source characteristics. One sample of note is a sample of the Ordovician Blakely sandstone that contains only titanite (often euhedral) as well as corundum+quartz as its heavy mineral fraction. This is an exciting find that may point to a contribution from a southern sediment source, possibly derived from an Ordovician (?) high-grade metamorphic terrane. The titanite will be dated by thermal ionization mass spectrometry at the University of Wisconsin-Madison and we will attempt to correlate these ages with high-grade metamorphic terranes of known age from Laurentia and Gondwana.
Development of the laser-ablation mass spectrometry methods at the University of Houston is one of the most important and time-consuming steps of this investigation. We have spent most of the winter through summer of 2007 analyzing standard and unknown zircons and have made improvements to the methods throughout. A very large hurdle has been developing a robust method for common lead corrections, an important analytical detail for accurate age determinations. We have made significant advancements but we will continue to improve on this method for years to come. The results of our development work indicate that we are able to measure the age of individual zircons with excellent precision and accuracy, which approaches that of expensive and time-intensive ionprobe techniques. Through repeated analyses of zircon standards of known age, we are able to measure U-Th/Pb ages of single zircons (individual analyses) to a precision of 1-7% (2σ; typically 1-3% for pre-Mesozoic grains), mainly depending on age. This precision is very well suited for work on detrital zircons and this method is now being used on the samples from the Ouachita and Marathon mountains described above.
