Shigenori Ogihara

Geochemical characteristics of phosphorite and carbonate nodules from the Miocene Funakawa Formation, western margin of the Yokote Basin, northeast Japan

Abstract Phosphorite nodules were discovered in diatomite and diatomaceous mudstones of the Late Miocene Funakawa Formation in Dewa Height, on the western margin of the Yokote Basin, northeast Japan. They occur as small nodules, cemented burrows, and small trace fossils in diatom-rich sediment that was deposited just above the contact between underlying siliceous shales. The underlying shales are essentially devoid of fossils and the overlying bioturbated, diatomaceous mudstones are fossil-rich. The underlying shale is rich in manganese (Mn) calcite nodules, and the overlying mudstone is rich in magnesian (Mg) calcite nodules. Some phosphorite nodules exhibit a zonal structure caused by two phases of Mg-calcite cementation with or without Mn-calcite within the pore space of the nodule. The phosphorus (P) content of these nodules exceeded 25 wt% in P2O5. The principle phosphate mineral composing the nodules is fluor(F)–carbonate–apatite crystals, which are a few μm in length and cocoon-shaped. Their aggregate includes a small amount of silica, aluminum, and water (Si, Al and H2O, respectively). The rare earth element (REE) composition of the phosphorite nodules is characterized by a positive europium (Eu) anomaly and no cerium (Ce) anomaly. The biomarker distribution of the phosphorite nodules is characterized by low (CPI), low carbon number n-alkane, and high hopenes, ββ-type hopane and isoprenoid ketone. Phosphorite nodules originate from the phosphate contained in diatoms. The mineralization occurs in the bacterial sulphate-reduction zone, and bacteria appear to have played an important role in the concentration of P and precipitation of apatite.

Unusual distribution of hydrocarbons in a hydrothermally altered phosphorite nodule from Kusu Basin, northern Kyushu, Japan

Abstract Lacustrine phosphorite nodules occur in diatomaceous mudstone from the Kusu Basin, northern Kyushu, Japan. Most of these phosphorite nodules have been hydrothermally altered. Features of the extractable hydrocarbons in an altered nodule are n -alkanes with an even-carbon-number predominance and unusual suites of dimethyalkanes and alkylcyclopentanes. Maturity indicators based on hopanes show that this non-altered sample is much more mature than a hydrothermally altered one. The hydrocarbons in the altered phosphorite were produced during the alteration process and dimethylalkanes and alkylcyclopentanes can be regarded as specific biological markers for hydrothermal environments.

COMPOSITION OF CLINOPTILOLITE FORMED FROM VOLCANIC GLASS DURING BURIAL DIAGENESIS

The composition of clinoptilolite formed from volcanic glass was investigated from cuttings collected from the offshore MITI-Somaoki borehole, 52 km east of Soma City, Japan. Unaltered volcanic glass and clinoptilolite-replaced glass were found coexisting in the same silicic tuff at 1000 m depth, but unaltered volcanic glass and clinoptilolite did not coexist in individual glass shards, and each material analyzed in this study was pure. The SiO2 and Al2O3 contents of the pure clinoptilolite and the pure, unaltered glass were 69.32 and 13.05 wt. %, and 70.03 and 11.54 wt. %, respectively. The SiO2 content was almost the same and the Al2O3 content was greater in the clinoptilolite-replaced glass compared with the unaltered volcanic glass. As a result, the SiO2/Al2O3 ratio in the clinoptilolite-replaced glass (SiO2/ A12O3 = 5.31) was slightly lower than that of the unaltered volcanic glass (SiO2/Al2O3 = 6.07). The extra-framework cation composition of the clinoptilolite-replaced glass was three times richer in K (K2O = 7.84 wt. %) than the unaltered volcanic glass (K2O = 2.59 wt. %). The cation selectivity of clinoptilolite, which shows a preference for K to Ca and Na, played an important role in the concentration of K in clinoptilolite during clinoptilolite formation from volcanic glass, although the initial pore waters and the volcanic glass were not especially relative K.Clinoptilolite-replaced glass in the calcite-cemented tuff at 1100 m depth was not enriched in Ca, suggesting that the exchangable-cation composition of clinoptilolite did not change by exposure to Ca-rich solutions at this depth. Apparently, the exchangable-cation composition of clinoptilolite, once formed, is relatively stable and difficult to change, even under conditions in which calcite is precipitating on clinoptilolite crystals.

Ba-bearing clinoptilolite from ODP Leg 127, Site 795, Japan Sea

Ba-bearing clinoptilolite (with >2 wt. % BaO) occurs in Neogene marine siliceous sediments from ODP Leg 127 Site 795 in the Japan Sea. Ba-bearing clinoptilolite is formed by reaction of volcanic glass and Ba-rich pore water during burial diagenesis.