Correlation of EMP chemical ages and IMP U-Pb isotopic ages: issues of spatial resolution including nature and orientation of age domain boundaries
Abstract
Monazite (mnz) geochronology is rapidly becoming the technique of choice for unraveling local and regional polyphase thermotectonic histories. High retention for radiogenic Pb makes monazite ideal for various in-situ radiometric dating methods including techniques such as electron microprobe (EMP) and ion microprobe (IMP, especially SHRIMP). Recent studies have shown the validity of total Th-U-Pb chemical ages (EMP) by reproducing U-Pb isotopic ages (IMP) within a homogeneous age domain, and in many cases with the same statistical resolution of error. The EMP has the advantage of spatial resolution, allowing a larger number of analyses to be performed on a single grain. Elemental mapping (Y, Th, U, Ca, and Pb) and heavy element distribution (through BSE images)of individual grains are used in combination for determining placement of point analyses or traverse-lines, identifying chemical domains, and constraining reactions associated with monazite growth. Polyphase monazite has been documented in metapelitic rocks metamorphosed from greenschist through granulite facies conditions. Given its high retention for radiogenic Pb, a single monazite grain may contain several age domains, reflecting its complex growth and recrystallization history, but which may or may not correlate with single-element chemical domains. IMP ablation pits and EMP spots that overlap age domain boundaries yield "mixed ages," thereby affecting the accuracy of geochronologic, microstructural, and tectonic interpretations. Isotopic age population determination (IMP) is typically performed using a U-Pb concordia plot of both concordant and discordant data. Zircon analyses falling below concordia are considered to reflect Pb loss during a younger event, from which a chord or tie-line is commonly drawn to infer the timing of Pb loss. However, because monazite cannot incorporate Pb in its crystal structure, all radiogenic Pb is expelled during recrystallization and typically, therefore, it cannot yield discordant ages in the traditional sense. New chemical-dating age-domain identification techniques have been developed from EMP traverse-line analyses that use a variety of plots to better locate nearly vertical to shallow dipping age-domain boundaries. One prominent example on which these models have been tested is a 100-micron mnz grain from the Tobacco Root Mountains, Montana (TRMR-2). It contains a low-Th older core (ca. 2.85 Ga), a higher-Th mantle domain of about 2.45 Ga, and a low-Th rim of 1.78 Ga. This grain has 6 IMP spots that range in age from 1880 Ma (near-rim) to 2785 Ma (core). Only two IMP pits fall totally within a single chemical and age zone delineated by EMP analyses or compositional maps (the medial age zone): 2451 (±4) and 2432 (±10) Ma. Domains were determined using traverse plots that depicted age plateaus and mixing lines between separate plateaus. Additionally, Th/U plots and linear probability plots were used to determine which EMP analyses yielded "mixed ages." These techniques yielded ages for this sample that are more precise and accurate and helped to explain the apparently concordant "mixed ages" obtained by IMP. Application of these techniques to younger monazites (500-350 Ma) has yielded similar results.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2006
- Bibcode:
- 2006AGUSM.U41B..06T
- Keywords:
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- 3620 Mineral and crystal chemistry (1042);
- 1115 Radioisotope geochronology;
- 1125 Chemical and biological geochronology