Abstract
Phosphorites collected from three seamounts off Japan have been analyzed for their major oxide, rare earth element (REE) composition and biomarkers. Two phosphorites, from Daiichi-Kashima seamount and Uyeda Ridge, were originated from limestone and another one from seamount "N" was altered from basalt. The only phosphate mineral investigated in the samples was carbonate-fluorapatite with excellent crystallinity. The REE patterns of the three seamount phosborites were similar to the REE pattern of sea water, suggesting that the REE pattern of phoshorite is not dependent on the type of original rock.
The biomarker distribution of seamount phosphorites were characterized by low carbon number n-alkanes with C14 and C18 dominant, low CPI (Carbon Preference Index), and the presence of MMA (monomethylalkanes). SEM observations showed that these samples consisted mostly of phosphatized filaments and with a smaller amount fraction of hexagonal apatite crystals. The mineral associations, geochemistry, textures and geological settings indicate that microbes played an important role of in the seamount phosphorite formation.
