EARLY-DIAGENETIC REE-PHOSPHATE MINERALS (FLORENCITE, GORCEIXITE, CRANDALLITE, AND XENOTIME) IN MARINE SANDSTONES - A MAJOR SINK FOR OCEANICPHOSPHORUS

Early-diagenetic REE-phosphate minerals are widespread, although minor (<0.005-0.14 wt percent), constituents of Archaean to Cretaceous sand stones from Australian sedimentary basins. Authigenic florencite ((REE )Al-3(PO4)(2)(OH)(6)), crandallite (CaAl3(PO4)2(OH)(6)), and gorceixit e (BaAl3(PO4)(2)(OH)(6)) occur as clusters of minute crystals (<0.1-10 mu m in width) within pockets of clay matrix lining detrital quartz s urfaces and less commonly within altered mica and feldspar grains. Rar e florencite crystals form along the surface of detrital monazite grai ns, and in one locality, crandallite and gorceixite crystals line cavi ties left after grain dissolution. The aluminophosphate minerals commo nly display compositional zoning, with major cation interchange betwee n REE, Sr2+, Ca2+, and Ba2+, and minor anion interchange between PO43- , SO42-, and CO32-. Xenotime (YPO4), which contains minor amounts of T h, U, and REE, forms pyramidal, dentate overgrowths (<1-20 mu m) on de trital zircon grains and locally partly surrounds detrital quartz surf aces. Authigenic apatite is generally less abundant than diagenetic al uminophosphate and occurs as minute, pore-filling and grain-lining cry stals. The authigenic phosphate minerals precipitated shortly after bu rial within the zone of sulfate reduction and methanogenesis. The phos phates probably formed from P and REE released into marine sediment po re waters following bacterial decomposition of organic matter and redu ction of hydrous iron oxides, as well as from partially dissolved and altered detrital minerals (monazite, clay matrix, feldspar, and mica). For the aluminophosphates, the critical factor controlling their prec ipitation site is the availability of Al (and to a lesser extent adsor bed REE and P), whereas xenotime crystals mostly Precipitate on isomor phous substrates (for example, detrital zircon grains). Given the wide spread occurrence of early-diagenetic aluminophosphates and xenotime i n Australian marine-deposited sandstones, it seems surprising that the y have not been noticed elsewhere. However, their minute crystal size (<0.1-10 mu m) and low concentration (generally <0.05 wt percent) prob ably hindered their previous identification by conventional microscopi c techniques, while their marked insolubility at low temperatures rend ered them inert to sequential solvent extraction geochemistry. Accordi ngly, these minerals represent a previously unrecognized marine sink f or reactive P. Estimates from this study show that the global flux of oceanic P removed through the precipitation of the REE-phosphates (con servatively 7.56 x 10(10) moles yr(-1)) may be of equal importance to other major sinks (for example, carbonate fluorapatite, organic-P). If so, then the total reactive P output is probably considerably greater than previous estimates and, for the oceanic P cycle to remain In a s teady state, implies a decrease in the output of other sinks or an inc rease In the P input, A higher output of reactive P will also reduce e stimates of the oceanic residence time of P (from this study, between 11-20 ka), As P is crucial for sustaining marine life, such changes in the P budget of the oceans may affect estimates of the biomass of mar ine organisms the biotic fixation of CO2, and therefore predictions of global climatic change.