Petrochronology Based on REE-Minerals: Monazite, Allanite, Xenotime, Apatite

Abstract

### REE-minerals Monazite, xenotime, and allanite are REE1-minerals sensu stricto because lanthanides (La…Lu) and yttrium are critical constituents in them. Apatite does not require REE, but because it contains substantial REE in many rocks, it is included in this review. All four minerals also host unusually high radionuclide concentrations, notably Th and U, forming the basis of their utility as geochronometers. This quartet of accessory minerals is playing an increasingly important role in petrochronology because they provide ways to link robust spot ages to petrogenetic ( P–T ) conditions so can lend petrogenetic context to chronology based on other minerals. Part I of this review assembles the basic requisites prior to integrative petrochronologic analysis. Individual characteristics of the four REE-minerals are addressed first, i.e., their crystal chemistry and stability relations. Thermobarometers and trace element geochemistry used for tracing petrogenesis are discussed next, and finally their chronology is summarized. Part II presents case studies to highlight the specific strengths of REE-minerals used to resolve the dynamics of a broad range of processes, from diagenetic to magmatic conditions. Finally, a brief section at the end outlines a few of the current challenges and promising perspectives for future work. To introduce the four REE-minerals in style, let us recall the origins of their names. The three phosphates have well respected Greek grandparents, and allanite has solid Scottish roots, yet of all four of them show idiosyncracies in etymology or type material. Apatite had long puzzled naturalists, as it shows great chemical and physical variability and can resemble other minerals. Once properly identified, Abraham Gottlieb Werner named it apatite. His reasoning referred to the Greek root ἀπατὰω and giving the precise Latin translation: decipio . Taken literally, both mean “I deceive” or “I mislead”, which sounds like an apt confession from this mineral for having fooled …