Correlations between chemical and age domains in monazite, and metamorphic reactions involving major pelitic phases: an integration of ID-TIMS and SHRIMP geochronology with Y–Th–U X-ray mapping

Chemical mapping and in situ U–Th–Pb analyses reveal a link between age domains and zones of relative yttrium (Y) depletion or enrichment within monazite crystals and are correlated with metamorphic reactions involving garnet. Conventional, small-fraction isotope dilution thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) techniques were utilized to measure U–Th–Pb isotopic compositions in metamorphic monazite from pelitic rocks of the southern Canadian Cordillera. Monazite ID-TIMS data from individual samples commonly demonstrate a 2 to 25 Ma range in U-Pb ages. This is difficult to reconcile using conventional regression techniques due to complexities such as excess 206Pb or bulk mixing of discrete age domains. Backscattered electron (BSE) imaging and X-ray elemental mapping for Y, Th, and U revealed complex internal zonation within many of the monazite crystals and served as a guide for detailed, in situ (∼30 μm) isotopic analysis using the SHRIMP. The Y maps generally provided the clearest indication of growth or recrystallization domains and were critical for targeting SHRIMP analysis because these relationships were not always clear in BSE, U, and Th images. Moreover, the Y domains consistently correlated with distinct age domains, with up to three or more in some crystals. These data clearly illustrate the cause of age dispersion within the analyzed monazite grains and demonstrate the significance of multiple age domains in metamorphic monazite that may be irreconcilable or misinterpreted when using conventional dating techniques such as ID-TIMS. studies have investigated the interaction between accessory minerals such as monazite and major pelitic phases throughout a metamorphic event. Researchers have begun to focus on the partitioning of Y between monazite and garnet because it is highly compatible in both phases. Due to its greater volume, garnet exerts considerable control over the Y budget available during metamorphism in pelitic rocks. Consequently, this is reflected in the production and consumption of monazite, as it is sensitive to the availability of Y, and is preserved as internal zones of relative Y enrichment or depletion. Thus, precise ages of contrasting Y domains within monazite provided by in situ ion probe analysis may be correlated with metamorphic reactions involving garnet and assigned to points along the P–T path.