Formation of rare earth phosphate minerals in 2.45-Ga paleosol

Abstract

In order to understand whether Ce in rare earth (RE) phosphate minerals can be used as a proxy for the estimation of atmospheric oxygen evolution in the Precambrian, we examined the formation mechanisms of RE phosphate minerals in 2.45-Ga paleosol (ancient soil formed by weathering), Pronto, Canada. The RE phosphate minerals were observed and analyzed by scanning electron microscopy with energy-dispersive X-ray spectroscopy and electron back-scattered diffraction analysis, and by transmission electron microscopy. The textures and chemistries of RE phosphate minerals have revealed that monazite-[light RE elements (REE)] and xenotime-(Y) at the rims of apatite and pores are secondary products. Monazite-(light REE) and xenotime-(Y) were found to have been formed by either of two mechanisms: the direct formation during metasomatism occurring after weathering or the formation of rhabdophane containing light REE and Y during weathering and its subsequent decomposition into monazite-(light REE) and xenotime-(Y) during metasomatism. If the latter was the actual reaction pathway, the Ce contents in the secondary monazite suggest a very low atmospheric oxygen level at ~ 2.45 Ga.