Partial resetting of the U-Th-Pb systems in experimentally altered monazite: Nanoscale evidence of incomplete replacement

International audienceAlteration experiments on natural monazite crystals (Manangotry standard, Madagascar)under alkali conditions at 300, 400, 500 and 600 °C and 200 MPa were conducted to clarifymechanisms behind incomplete resetting of U-Th-Pb geochronological systems in monazitereplaced by dissolution and precipitation processes. Above 400 °C, experimental products showtypical replacement textures: a compositionally distinct monazite rim, referred as altered rim,surrounds the primary monazite (Mnz1). Isotopic and electron microprobe U-Th-Pb in situdating of the altered rim yields intermediate ages between pristine monazite (555 Ma) andcomplete experimental resetting (0 Ma). Lead is systematically detected in altered rims, withconcentration decreasing from 400 °C to 600 °C. The origin of incomplete resetting is elucidatedwith transmission electron microscope images that reveal an incomplete replacement ofMnz1 by a secondary monazite (Mnz2) within the altered rim. With increasing temperature,the size and volume of the Mnz2 within the altered rim become more important. Becauseno structural Pb or Pb nanoinclusions were observed, Pb in the altered rim is attributed tothe Mnz1 component. Partial resetting of U-Th-Pb systems depends on the nanomixture ofdifferent Mnz1 proportions in the analyzed volume, and explains the higher rejuvenationat 600 °C than at lower temperatures. Although microanalytical techniques have the spatialresolution to date micrometer-sized rims, they are unable to resolve a nanoscale mixture ofpristine and secondary monazite that could occur in altered rims formed by fluid-drivenreplacement, especially at low temperatures. Porosity and/or inclusions and complex agescattering in zoned monazite are significant markers that can indicate a possible nano-sizedpartial replacement

U-Pb geochronology on zircon and columbite-group minerals of the Cap de Creus pegmatites, NE Spain

Peer Reviewe

U-Pb geochronology on zircon and columbite-group minerals of the Cap de Creus pegmatites, NE Spain

Peer Reviewe