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