U-Pb Dating of Apatite in Sanidinite-Facies Pyrometamorphosed Rocks from the Hatrurim Basin, Israel

Apatite, an abundant accessory mineral in igneous and metamorphic rocks, may accommodate appreciable amounts of trace elements including U, Pb and REE and is thus a useful U-Pb geochronometer and tracer of high-temperature metasomatism. Since apatite is susceptible to recrystallization, new growth and fluid-induced chemical changes, a good understating of the U-Pb system closure in apatite at variable geological conditions is required to correctly interpret apatite U-Pb ages. In this research, we delve into the behavior of apatite in marine phosphorites metamorphosed at sanidinite facies conditions to constrain U-Pb systematics in apatite under extreme high-T low-P conditions.

We sampled non-metamorphosed, metamorphosed, and hydrothermally altered marine phosphorites from the Mottled Zone pyrometamorphic complex exposures in the Hatrurim Basin, Israel. Petrographic study of the samples was followed by in -situ measurement of major (EMPA) and trace element compositions and U-Pb dating (LA-ICP-MS) of apatite.

Our results show that during the pyrometamorphic event apatite recrystallized and incorporated carbonate, silica, and sulphate into its crystal lattice. Thermally derived apatite recrystallization resulted in a submillimeter-scale Pb isotopic homogenization, inheritance of common lead of very low ²⁰⁷Pb/²⁰⁶Pb ratio from the U-rich, biogenic apatite precursor, and contemporaneous U-Pb system closure of apatite on a wider scale. The Miocene and Pliocene U-Pb ages of apatite coincide with previously known ⁴⁰Ar/³⁹Ar and K-Ar ages of the Mottled Zone pyrometamorphic event (Gur et al., 1995). Post-metamorphic interaction of high-T metamorphosed phosphorites with carbonate-, uranyl- and vanadate-rich fluids resulted in U and V enrichment of apatite, as well as redistribution of REE.

The results of our research shed new light on the timing and extent of the Mottled Zone pyrometamorphic event, contribute to understating the behavior of biogenic apatite under extreme high-T, low-P conditions, and exemplify the utilization of apatite in meta-sedimentary rocks for dating thermal events including combustion metamorphism.