Spatial Resolution Sensitivity Accuracy Ultrachron Home
The Ultrachron project is an NSF-sponsored, collaborative effort between Cameca, France and the University of Massachusetts. This project involves comprehensive redesign, development, and optimization of the SX electron microprobe specifically for making major improvements in EPMA geochronology and trace element micro- analysis. Specific goals involve improvements in spatial resolution and sensitivity (analytical precision), with critical attention to accuracy and a better understanding of the sources of error in trace element EPMA.
The overall goal is to test the limits of microprobe monazite geochronology and to firmly establish this complimentary technique within the spectrum of geochronologic investigation. Many of the improvements are exportable to existing microprobe laboratories or will be available on new instruments. The hardware and software will benefit all aspects of trace element analysis by electron microprobe. In addition, the optimized microprobe at UMass is available to anyone for the investigation of specialized geochronological problems or other high spatial resolution trace element applications.
The Ultrachron project involves considerable hardware redesign to optimize both electron and X-Ray optics and also involves software development specifically targeted to trace element analysis and geochronology. This project also encompasses technique refinement as well as the development and evaluation of synthetic and natural standards for the analysis of monazite and other minerals of potential geochronologic use.
The Ultrachron project is an effort to create a more specialized analytical instrument which will expand the capabilities of electron probe micro-analysis (EPMA) beyond traditional performance “limitations”. EPMA instruments have historically been constructed to achieve rapid,efficient, in-situ chemical microanalyses of major and minor elements from a broad array of solid materials.
The Ultrachron project represents a major philosophical departure in that every aspect of the instrument has been re-evaluated in order to achieve specific goals, sacrificing the versatility of traditional EPMA where necessary. In this case, the development has three major goals: 1) Improved spatial resolution; 2) Improved precision; and 3) Improved accuracy. These are extraordinary challenges given that enhanced spatial resolution typically requires lower scattering volumes produced at lower voltage and smaller beam diameter produced at lower current, and that these effects are counter to enhancement of precision (count rates) produced with higher beam current and voltage.
This development takes advantage of the small electron scattering volumes in high Z accessory phases such that improvements in beam diameter make a substantial difference in the spatial resolution of the compositional analysis. Higher brightness sources and improved column optics do make a difference under these circumstances.
This development has also sought to improve precision by both developing higher current density and improving X-Ray counting efficiency. The achievement of extremely high current density presents new issues, particularly with internal space charge effects and sample damage potentially affecting accuracy.
The Ultrachron project was initiated primarily to explore the limitations and applications of trace element EPMA as related to geochronology. The inspiration for this lies in the observation that polygenetic monazite is common in multiply tectonized rocks, and that the spatial resolution required for a detailed analysis of these materials is at the micrometer scale. Accessory phase populations often cover a range in size, down to well below five micrometers. This is important as these small grains in many cases are either the only population present in some rocks, or represent mineral growth episodes distint from larger grains.
Almost all monazite is compositionally zoned, down to the micrometer scale, and these compositional domains are ofter related to specific ages in polygenetic grains. The electron probe offers the only access to both the composition and age at this spatial resolution.
EPMA geochonology compliments other geochronologic techniques (ID-TIMS, Ion probe, LA-ICPMS) by offering a method to evaluate zoning at exceptionally high spatial resolution. This technique also offers a way to correlate compositions of constituent phases in-situ, allowing quantitative assessment of reactions (and related microstructures) that result in accessory phase growth and/or breakdown, essentially allowing reaction dating in many cases.
Sources of discordance or mixed ages in isotopic analyses can be better understood, and a better overall picture of P-T-t-D evolution is realized by combining information from all these techniques.
