5 New Frontiers in Geologic Microanalysis

Monday, 25 March 2002
8:00 AM - 12:00 PM

INSIGHTS INTO THE CHEMICAL NATURE AND ISOTOPIC BEHAVIOUR OF METAMORPHIC MONAZITE; INTEGRATION OF ID-TIMS AND SHRIMP GEOCHRONOLOGY WITH BSE AND Y-TH-U X-RAY MAPPING TECHNIQUES

Paper number: 5-4
Program: THEME
Time: 10:55 AM – 11:15 AM

Authors

Abstract

In this contribution we address the complexities of monazite isotopic systematics and provide evidence that links yttrium (Y) zoning in monazite to metamorphic reactions involved in the pre-, syn- and post-garnet growth and breakdown phases. Small-fraction conventional isotope-dilution (ID-TIMS) and Sensitive High Resolution Ion Microprobe (SHRIMP) techniques were utilised to analyse U-Th-Pb in metamorphic monazite from pelitic and leucogranitic rocks collected in the southeastern Canadian Cordillera. The ID-TIMS data commonly demonstrated a significant range in U-Pb ages (e.g. 2 to 25 Ma) within a population of single-grain monazite analyses. This problem was difficult to resolve using conventional regression techniques due to additional isotopic complexities such as excess 206Pb. Consequently, in situ analyses (ca. 30 µm diameter) were carried out using the SHRIMP II. Prior to the SHRIMP analyses, the internal morphology of the monazites were imaged using back-scattered electron (BSE) imaging and X-ray elemental mapping techniques for Y, Th, and U. Both methods exposed complex internal zoning in many of the monazites (e.g. blocky and fir-tree sector zoning, oscillatory zoning). However, the most revealing images were provided by X-ray maps of sectioned monazite; specifically, the Y-maps generally provided the best images of different growth/recrystallization domains within monazite, and thus, were most useful for targeting SHRIMP spot analyses. More importantly, the Y-maps that clearly displayed these relationships (not always clear in the BSE images) consistently revealed distinct age domains. Moreover, selected crystals sometimes contained up to three age domains that correlated strongly with Y concentration, but not necessarily with U or Th-zones.

Previous studies have established that garnet is a major Y-sink during prograde metamorphism in pelitic rocks. Our data strongly support previous interpretations that contend if monazite grew before the appearance or during the breakdown of garnet it will be enriched in Y, whereas monazite that grew after garnet will be Y-depleted. Thus, the relative concentration of Y in the imaged zones and the ages attached to them may be used as a proxy for dating the metamorphic reactions involving garnet, and can be used to establish absolute points in P-T-t space.