CHEMISTRY AND MICROPROBE GEOCHRONOLOGY OF S-BEARING MONAZITE IN XENOLITHS IN THE CORTLANDT COMPLEX, NY: AN EXAMPLE OF MONAZITE IN A UHT ENVIRONMENT

Portions of the Cortlandt mafic complex, New York, contain numerous xenoliths of original sulfidic, graphitic pelitic schist, now converted to a partially melted, alkali- and silica-depleted restitic and oxidized assemblage of UHT minerals (sillimanite, spinel, sapphirine, magnetite, ilmeno-hematite, corundum, garnet, hypersthene +/-cordierite) that has been called "emery". Textures and mineral reactions in emery record T in excess of 1100 °C. Some emery samples contain monazite grains that have survived their thermal excursion from pre-intrusion, Taconic Orogeny upper amphibolite-facies conditions (600 °C, 5-6 kbar), through the UHT conditions (>1000 °C) and subsequent cooling, and have apparently preserved the chemistry (relative Th, U, Pb contents) of their original regional metamorphic formation so that they yield sensible geochronologic information upon microprobe analysis. However, these monazites are unusual chemically in that they contain significant concentrations of sulfur - ca. 1 wt% S or 2 wt% SO₂. All Cortlandt Complex emeries are strongly oxidized (ca. log fO₂=FMQ >+ 2) so the speciation is likely to be sulfate (no sulfide occurs in any of the emeries). Other monazite components are Ce (0.38 cations/4 oxygens), La (0.15), Nd (0.16), Th (0.02), Sm (0.03) and Ca (0.11), with very low concentrations of Si, Gd and Dy . CaO is unusually high in these monazites (>2.5 wt%) and appears to be split between three substitutions in the monazite structure: Ca₃(PO₄)₂, brabantite (Ca_.5Th_.5PO₄) and anhydrite (CaSO₄). The presence of significant sulfur has important implications for microprobe U-Th-Pb geochronology because the S Ka peak is close enough to Pb Ma to interfere with background measurements at typical background offsets for Pb (ca. 1000 sine-theta units), resulting in overcorrection of background and low values for Pb. Calculation of monazite age for this S-bearing monazite using standard analysis resulted in a calculated age hundreds of Ma too young. Adjustment of Pb background offsets yielded analyses with higher Pb and credible calculated ages. It is therefore advisable to ensure that monazites from unusually high-T and oxidizing environments are S-free before performing standard U-Th-Pb analyses for microprobe geochronology.