RARE EARTH ELEMENT DISTRIBUTIONS IN CARBONATE FLUORAPATITE (CFA): INSIGHTS FROM SPATIALLY RESOLVED U-PB AND TRACE ELEMENT ANALYSES OF THE PHOSPHORIA ROCK COMPLEX, USA

Phosphorites form from the confluence of unique oceanographic conditions and therefore their geochemistry can be useful for reconstructing the composition and redox state of ancient oceans. Paleoenvironmental proxies in phosphorites are typically from whole-rock analyses; how much of these signals represent “primary” depositional conditions vs. subsequent diagenetic to late alteration is poorly known. The Phosphoria Rock Complex (PRC) in the western USA has been intensely investigated for its rich stratigraphic record of environmental conditions during phosphorite deposition. Here, we performed LA-ICPMS split-stream (U-Pb + trace element) analyses on ten phosphorite samples from three PRC exposures that exhibit varying forms of carbonate fluorapatite (CFA), including numerous phosphatic fossils (fish bones/teeth, brachiopods, conodonts), coated grains, and peloids.

The results from these analyses show several distinct signals. Some population of each sample lies along an “errorchron” with a ~250–245 Ma U-Pb date, which is younger than the published stratigraphic date for the PRC (≥260 Ma). This offset suggests “early” diagenetic disturbances to U-Pb systematics over the entire PRC. Other analyses clearly plot off these errorchrons due to “late” post-crystallization U and/or Pb gain. Those analyses with strongly disturbed U-Pb age spectra typically include biogenic apatite – particularly fish, conodonts, and brachiopods – although other trace elements like rare earth elements (REEs) are similar between these fossils and other non-biogenic apatites in the same stratigraphic layers. There are few systematic REE differences between apatites within a single sample and the REE patterns are indicative of an oxygenated seawater source. The combined fossil and sedimentary apatite geochemistry suggest recrystallization lasted for millions of years after deposition, leading to young U-Pb dates, but in an overall closed system with respect to REEs. These combined U-Pb + trace-element measurements are thus effective for understanding phosphorite deposition, diagenesis, and subsequent alteration in the PRC, with potential applications to other phosphorite deposits globally.