T25 Microprobe Monazite Geochronology: New Developments and Applications

Sunday, 27 October 2002
8:00 AM - 12:00 PM

INSIGHTS INTO THE CHEMICAL AND ISOTOPIC COMPLEXITIES OF METAMORPHIC MONAZITE; INTEGRATION OF IDTIMS AND SHRIMP GEOCHRONOLOGY WITH BSE AND Y-TH-U X-RAY MAPPING TECHNIQUES

Paper number: T25-8
Program: Technical Sessions
Time: 9:45 AM – 10:00 AM

Authors

Abstract

Chemical mapping and in situ age dating reveal the link between age domains and zones of yttrium (Y) depletion/enrichment within monazite that can be correlated to metamorphic reactions involving garnet. Small-fraction isotope dilution (IDTIMS) and Sensitive High Resolution Ion Microprobe (SHRIMP) techniques were used to analyze U-Th-Pb in metamorphic monazite from Mesozoic pelitic rocks of the southeastern Canadian Cordillera. The IDTIMS data commonly demonstrated a range in U-Pb ages (2 to 25 My) within a population of single-grain monazite. This range was difficult or impossible to reconcile using conventional regression techniques due to additional isotopic complexities such as excess 206Pb or bulk mixing of discrete age domains. Consequently, in situ analyses (~30 µm diameter) were carried out using the GSC SHRIMP II. Prior to SHRIMP analysis, the internal morphology of the monazites was imaged using back-scattered electron (BSE) imaging and X-ray elemental mapping for Y, Th, and U. This revealed complex zoning in many of the monazites. The Y-maps generally provided the best indication of growth/recrystallization domains, and were critical for targeting SHRIMP analyses because these relationships were not always clear in BSE, U, and Th images. Moreover, the Y-maps consistently correlated with distinct age domains, up to three or more in some crystals. These data clearly illustrate the cause of age dispersion within the analyzed monazites, and demonstrate the ubiquity of multiple age domains in metamorphic monazite that may be irreconcilable or misinterpreted when using conventional dating techniques (e.g. IDTIMS).

Recent studies have investigated the interaction between accessory (e.g. monazite) and major pelitic phases throughout a metamorphic event, and more specifically the partitioning of Y between these phases. They have established that garnet exerts considerable control over the Y-budget available during metamorphism in pelitic rocks. Production and consumption of monazite is sensitive to the availability of Y, and is reflected internally in preserved Y-zones; data from this study appear to support these interpretations. Thus, precise ages of Y-domains within monazite provided by in situ SHRIMP analyses may be correlated with metamorphic reactions involving garnet, and assigned to points along the P-T path.