47279-G8
Quantifying Sediment Dispersal Rates in a Transcontinental Drainage Using Zircon U/Pb: (U-Th/He) Chronometry, Amazon River
Sediment fluxes, particularly those of high-quality quartzofeldspathic sands, are critical to understanding the petroleum reservoir potential in deltaic systems at the terminus of transcontinental drainages. The proposed research aims to quantify long-term sediment dispersal rates using U-Pb: (U-Th)/He double-dating of detrital zircons (Reiners et al., 2005) in the Amazon River where the Andes provide a continuous source of zero-age, first-cycle volcanic zircon grains that can be readily identified with the double-dating technique. The project seeks to use first-cycle volcanic grains to quantify downstream aging of the detrital zircon population in bedload samples from throughout the Amazon River basin, providing a direct measure of transport time from source to sink.
We have collaborated with Drew Coleman (UNC – Chapel Hill) and his graduate student, Russell Mapes in selecting three roughly equidistant sample localities spanning from the headwaters to the delta. I have recruited a talented undergraduate, Geidy Baldeon, to undertake the data collection part of this project. Baldeon and supervisor Melanie Michalak have prepared three ~200-grain mounts for initial U-Pb age characterization by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).
We have decided to place the U-Pb dating portion of this project on hold until installation and calibration of our new NSF-sponsored LA-ICP-MS facility in early 2009. The facility comprises: (1) a Photon Machines Analyte 193.H laser ablation system, which provides improved energy homogeneity, spot size range, and coupling compared to our existing laser system; and (2) an Element XR ICP-MS, which features an order of magnitude better sensitivity than existing the Element system. The decision to delay U-Pb data collection will result in improved age precision for smaller spot sizes and increased analytical throughput. Further, this "downtime" has allowed for software development projects to provide rapid U-Pb data reduction (see description below).
ACS-PRF funds sponsored substantial development and calibration work in the UC Santa Cruz (U-Th)/He Thermochronology Laboratory, which will be heavily utilized following LA-ICP-MS dating of Amazonian detrital zircons. Our principal achievements over the performance period were (1) final calibration and commissioning of a fully automated helium extraction line and (2) development and calibration of acid vapor zircon digestion and 229Th-233U isotope dilution protocols at UC Santa Cruz. ACS-PRF funds were used for the acquisition of two new Parr acid digestion vessel. These vessels increased our stock to 5, permitting digestion of ~45 single grain aliquots at a time. Further ACS-PRF funds were critical for supporting acquisition of miscellaneous vacuum and laser optical hardware for our helium extraction line. The laboratory is now fully operational and ready to generate large detrital (U-Th)/He grain age datasets following U-Pb geochronologic work. Increased capacity will permit rapid analytical throughput critical for statistically robust sampling of detrital populations.
PRINCIPAL ACHIEVEMENTS:
Sample Selection: University of North Carolina collaborator, Drew Coleman, travelled to UC Santa Cruz in Spring 2008 for a research meeting aimed at sample selection for subsequent, U-Pb: (U-Th)/He and present a brown bag seminar on his Amazonian detrital zircon efforts. Coleman and graduate student Russell Mapes have collected detrital zircon U-Pb LA-ICP-MS data on these samples under the aegis of the ACS-PRF program. Although these grains cannot be used for the double-dating method—(U-Th)/He is best conducted on whole, unmodified crystals—their dataset has helped guide sample selection criteria for the samples that will be analyzed for this study. polished grain mounting techniques and large laser ablation pits make them unsuitable for U-Pb: (U-Th)/He dating (the method requires analysis of whole unmodified crystals). However, extant data helped guide sample selection. Hourigan and undergraduate Geidy Baldeon are currently working on three roughly equidistant samples from near the headwaters, the middle reaches and delta regions. (Figure)
Student Training: Graduate Student student Researcher Melanie Michalak was paid 1 month of 100% GSR salary for her work in the Helium Thermchronology Laboratory. Her principal duties were: (1) to develop and calibrate our isotope dilution and acid vapor digestion protocols for zircon; and (2) to train and supervise and undergraduate underrepresented minority researcher Geidy Baldeon who assisted with mineral selection and mount preparation.
Method Development: Assistant Professor Jeremy Hourigan committed 1 month of summer research time to: (1) coding a LabVIEW-based software package for rapid U-Th-Pb laser ablation inductively coupled plasma mass spectrometer LA-ICP-MS data reduction, (2) final commissioning and calibration of the fully automated UC Santa Cruz helium extraction line.
