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

Abstract

Research Article| June 01, 2016 Partial resetting of the U-Th-Pb systems in experimentally altered monazite: Nanoscale evidence of incomplete replacement Alexis Grand’Homme; Alexis Grand’Homme 1ISTerre (Institut des Sciences de la Terre), University Grenoble Alpes, 38058 Grenoble Cedex 9, France Search for other works by this author on: GSW Google Scholar Emilie Janots; Emilie Janots * 1ISTerre (Institut des Sciences de la Terre), University Grenoble Alpes, 38058 Grenoble Cedex 9, France *E-mails: janotse@ujf-grenoble.fr; anne.magali.seydoux@univ-st-etienne.fr. Search for other works by this author on: GSW Google Scholar Anne-Magali Seydoux-Guillaume; Anne-Magali Seydoux-Guillaume * 2Laboratoire Magmas et Volcans LMV (UMR 6524 CNRS-UBP-UJM1 IRD), 42023 Saint-Etienne, France *E-mails: janotse@ujf-grenoble.fr; anne.magali.seydoux@univ-st-etienne.fr. Search for other works by this author on: GSW Google Scholar Damien Guillaume; Damien Guillaume 2Laboratoire Magmas et Volcans LMV (UMR 6524 CNRS-UBP-UJM1 IRD), 42023 Saint-Etienne, France Search for other works by this author on: GSW Google Scholar Valérie Bosse; Valérie Bosse 3Laboratoire Magmas et Volcans LMV (UMR 6524 CNRS-UBP-UJM-IRD), 63038 Clermont-Ferrand, France Search for other works by this author on: GSW Google Scholar Valérie Magnin Valérie Magnin 1ISTerre (Institut des Sciences de la Terre), University Grenoble Alpes, 38058 Grenoble Cedex 9, France Search for other works by this author on: GSW Google Scholar Geology (2016) 44 (6): 431–434. https://doi.org/10.1130/G37770.1 Article history received: 08 Feb 2016 rev-recd: 12 Apr 2016 accepted: 12 Apr 2016 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alexis Grand’Homme, Emilie Janots, Anne-Magali Seydoux-Guillaume, Damien Guillaume, Valérie Bosse, Valérie Magnin; Partial resetting of the U-Th-Pb systems in experimentally altered monazite: Nanoscale evidence of incomplete replacement. Geology 2016;; 44 (6): 431–434. doi: https://doi.org/10.1130/G37770.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Alteration experiments on natural monazite crystals (Manangotry standard, Madagascar) under alkali conditions at 300, 400, 500 and 600 °C and 200 MPa were conducted to clarify mechanisms behind incomplete resetting of U-Th-Pb geochronological systems in monazite replaced by dissolution and precipitation processes. Above 400 °C, experimental products show typical replacement textures: a compositionally distinct monazite rim, referred as altered rim, surrounds the primary monazite (Mnz1). Isotopic and electron microprobe U-Th-Pb in situ dating of the altered rim yields intermediate ages between pristine monazite (555 Ma) and complete experimental resetting (0 Ma). Lead is systematically detected in altered rims, with concentration decreasing from 400 °C to 600 °C. The origin of incomplete resetting is elucidated with transmission electron microscope images that reveal an incomplete replacement of Mnz1 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. Because no structural Pb or Pb nanoinclusions were observed, Pb in the altered rim is attributed to the Mnz1 component. Partial resetting of U-Th-Pb systems depends on the nanomixture of different Mnz1 proportions in the analyzed volume, and explains the higher rejuvenation at 600 °C than at lower temperatures. Although microanalytical techniques have the spatial resolution to date micrometer-sized rims, they are unable to resolve a nanoscale mixture of pristine and secondary monazite that could occur in altered rims formed by fluid-driven replacement, especially at low temperatures. Porosity and/or inclusions and complex age scattering in zoned monazite are significant markers that can indicate a possible nano-sized partial replacement. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.