Collaborative Research: Improving the accuracy and precision ofmonazite and allanite geochronology via ID Th-Pb ages for referencematerials
The principal limitation for obtaining high-precision, accurate standard-based 208Pb/232Th ages from monazite and allanite is the lack of appropriate, well-characterized reference materials. Because both monazite and allanite are compositionally variable and SIMS, LA-ICP-MS incur instrumental mass-dependent fractionation, it is essential to closely match standards with unknowns. This proposal seeks to determine isotope dilution (ID) Th-Pb ages for the Th-rich accessory minerals monazite and allanite. Th-Pb ages currently exist for only one reference material that is commonly used-'554'. Consequently, all standard-based geochronologic measurements require an assumption that Th-Pb and U-Pb ages are equivalent. This assumption is unnecessary and, in many cases, invalid. By obtaining high-precision ID Th-Pb ages for a suite of well-characterized, community-wide reference materials, this research will provide a means of independently calibrating Th-Pb ages for minerals that can be linked to fundamental tectonic processes.
Intellectual Merit o Eliminates the need to assume concordance among the three decay systems (235U, 238U and 232Th decay chains) by providing independently verified U-Pb and Th-Pb ages. o Improves precision on age measurements for a chronometer that can be directly linked to the metamorphic and magmatic history of a terrane o Provides well-characterized standards to the community for high-precision, standardbased geochronology. Broader impacts 1. This proposed research seeks to address the fundamental aim of the EARTHTIME initiative: "the development of the geochronological techniques necessary to produce temporal constraints with uncertainties approaching 0.1 % of the radioisotopic ages." o A website will be developed that contains the information about the reference materials and the methods for measuring the ID TIMS and ICP-MS ages. This information will be linked to the EARTHTIME website. o An educational module will be developed that can be integrated into the EARTHTIME mission for educating students about understanding geologic time. This module will demonstrate linking pressure, temperature and time histories. 2. This project supports two early career scientists-John Cottle (UCSB faculty) and Emily Peterman (Stanford Postdoctoral Fellow). In addition, Peterman is a young female scientist (Ph.D. 2009). 3. This study will train undergraduates (UCSB) in laboratory investigations and provide a component of graduate research for graduate students at UCSB and Stanford. 4. The results of the proposed research will impact the geochronology community at large: o Thermal Ionization Mass Spectrometry o Secondary Ionization Mass Spectrometry o Electron Probe MicroAnalysis o (Laser Ablation) Inductively Coupled Plasma Mass Spectrometry
Understanding the timing and duration of geologic events in the deep earth (e.g. burial and heating rocks as well as their uplift toward the surface) provide first order constraints on the processes that control the physical evolution of our planet. An important mineral that records many of these processes is monazite. It is useful because, not only is it a common mineral, but it also contains 232Th that radioactively decays to 208Pb. Precise measurement of this decay scheme can be utilized by geochronologists to place specific time constraints on geologic events. However, in order to produce dates that are accurate, the isotope ratios produced by a plasma-source mass spectrometer must be compared to a standard or known age. In the scientific community there is only one monazite standard with a well-characterized 232Th / 208Pb ratio. This project aims to rectify this by measuring Th/Pb ratios to high precision in a range of natural monazites. These samples and data will then be distributed to the geochronology / geochemistry community. In the course of this study, three new analytical techniques were developed. The first enables measurement of both isotope ratios and trace element concentrations using two linked mass spectrometers. This technique is cutting edge and provides new opportunities for geochronologic research on a variety of rocks from the deep crust to the surface. The second new technique enables collection of isotope ratios two orders of magnitude faster that with previous analytical configurations, enabling characterization of many 100's of samples in a relatively short time span. The third involves a new design modification to the mass spectrometer that was developed to increase the sensitivity of the instrument. This now widely adopted piece of technology enables measurements to be made with higher precision and/or less sample to be consumed during the measurement process. Nine potential monazite standards were identified and characterized for both their trace elements and isotope ratios. This involved a variety of measurement techniques using both electron- and ion-beam spectroscopic techniques at the University of California, Santa Barbara and at the British Geological Survey. The majority of monazites measured have isotope ratios that vary by >5% indicating that they are not ideal standards. However, at least three of the monazites have trace element concentrations and isotope ratios that vary by <2%, indicating that they are potentially useful as standards for mass spectrometric analysis. These data and findings have been published in four scientific articles. In addition, a new piece of data reduction software has been written and a new design technology has been described in detail. In the course of this study, five undergraduates were trained in the use of electron- and ion-beam instruments and two early-career scientists were provided with research support. The geochronology and earth science community will also benefit by provision of well-characterized monazite standards for use in a range of research projects.
