『Abstract
Microprobe monazite dating has been increasingly used to constrain
the timing of deformation and metamorphism because of the potential
to date very small monazite domains (down to 5μm or less) in structural
and petrologic context. This paper presents an analytical strategy,
presentation format, and error considerations for microprobe monazite
dating. The strategy involves high-resolution compositional mapping
to delineate compositional domains within monazite crystals. Then
for each compositional domain, a series of Th, U and Pb analyses
are made, and a single date and error are calculated. The number
of analyses in each domain is determined by the desired statistical
precision of the date. Results from several monazite grains are
typically combined and, along with textural relationships, are
used to build an argument that the dates constrain the age of
a deformation or metamorphic event. The total error involves three
components: short-term random error (dominated by counting statistical
uncertainty), short-term systematic error (uncertainty in background
correction, conductive coating variation, and calibration), and
long-term systematic error (uncertainty in standard composition,
mass absorption factors, decay constants, etc.). In homogeneous
compositional domains, short-term random errors (2σ) of less than
10 m.y. can be obtained from five to ten analyses. However, short-term
systematic error, mainly background estimation uncertainty, would
typically result in a doubling of the magnitude of random error.
Microprobe dates are presented as a single Gaussian probability
distribution for each domain, along with representative compositional
maps. It is recommended that a consistency standard be analyzed
during each analytical session and the results be reported along
with those from the unknown. This proposed strategy and format
are compatible with those of other geochronological techniques;
they incorporate analytical limitations associated with trace,
as opposed to major element, microprobe analysis, and will allow
better comparisons to be made between labs and between different
geochronological techniques.
Keywords: Monazite; U-Pb dating; Electron microprobe; Trace element
analysis』
1. Introduction
2. Background
2.1. Major vs. trace element analysis: distinct approaches
for distinct applications
2.2. Top-down vs. bottom-up approaches
3. Proposed analytical strategy
4. Uncertainty considerations
5. Presenting and integrating results
6. Discussion and conclusions
Acknowledgments
References