Precipitation Of Carbonate Fluorapatite Research Articles

Reconstructing the depositional environment and diagenetic modification of global phosphate deposits through integration of uranium and strontium isotopes

The geochemistry of phosphate rocks can provide valuable information on their depositional environment and thus the redox condition of global oceans through time. Here we examine trace metal concentrations and uranium (δ238U, δ234U) and strontium (87Sr/86Sr) isotope variations of marine sedimentary phosphate rocks and the phosphate-bearing carbonate fluorapatite (CFA) mineral phase, originating from Precambrian to mid-Miocene aged major global phosphate deposits. We find elevated concentrations of several trace elements (Al, V, Cr, Cd, U, Mn, Co, Cu, As, and Rb) in the CFA mineral phase of young phosphate rocks (Miocene to Late Cretaceous) relative to those of older (Devonian to Precambrian) rocks. The δ238U of phosphate rocks of Mid-Miocene to Permian age range from −0.311‰ to 0.070‰, exhibiting a positive fractionation relative to modern seawater (−0.38‰). This is similar to the isotope fractionation reported for carbonate and shale sediments, likely resulting from the reduction of uranium in porewaters during CFA precipitation. Cambrian to Precambrian phosphate rocks have lower δ238U of −0.583‰ to −0.363‰, indicating different depositional redox conditions likely resulting from seafloor anoxia and/or diagenetic modification. The 87Sr/86Sr ratios of phosphate rocks of Cretaceous to Mid-Miocene age generally follow the secular 87Sr/86Sr seawater curve. Phosphate rocks with 87Sr/86Sr that deviate from this curve have characteristic trace metal trends, such as lower Sr/Ca and Sr concentrations, suggesting later diagenetic modification. Older phosphate rocks of Precambrian age are systematically more radiogenic than the expected secular Sr seawater composition at the time of deposition, possibly due to the greater influence of terrestrial input in peritidal zones and/or more pervasive diagenetic modification. Overall, our study reveals connections between U and Sr isotope variations for reconstructing the depositional and diagenetic conditions of global phosphate rock formation through Earth history and the transition to an oxic ocean following the Paleozoic Oxygenation Event.

Electron microscopy study on the formation of ferromanganese crusts, western Pacific Magellan Seamounts

Variations in mineralogy and chemical composition, layer structures, redox states of Fe and Mn, and microbial diversity are closely linked to the biogeochemical process when a ferromanganese crust layer forms. A sample collected from the Magellan Seamount (OSM11), in the western Pacific, was characterized in five well-defined layers, top to bottom (L1–5). Fe-vernadite occurs in all layers, compared to detritus quartz, feldspar, goethite, and hematite in L1 and L3, and carbonate fluorapatite (CFA) in L4–5. The relatively high concentrations of Ca and P in L4–5, and Fe, Co, and Si in L1 and L3 correspond to the mineralogical variations in the crust layer. Disappearance of elongated voids along the convex growth line is likely due to void filling precipitation of CFA in L4–5, resulting in the void reduction (31.6 to 6.0%). The oxidation states of Fe in Fe-vernadite measured by electron energy loss spectroscopy (EELS) ranges from 36 to 63% of Fe3+/Fetot, and a layer where CFA appeared (L4) contains a more reductive form of Fe (Fe3+/Fetot = 36–48%). Presence of Fe- (coxC) and Mn-oxidizing gene (cumA), particularly displaying a strong PCR band of coxC in L2–3, indicates a dominant oxidizing condition. Direct evidence of microbial activity in Fe - and Mn-oxide precipitation with various redox conditions was identified in the focused ion beam-sectioned microfossil. Particularly, a spectrum image displaying a more reducing form of Fe around the voids previously occupied by microorganisms, strongly supports that microorganisms play an important role in the redox reaction in the growth of a crust.

Controls on ferromanganese crust composition and reconnaissance resource potential, Ninetyeast Ridge, Indian Ocean

A reconnaissance survey of Fe–Mn crusts from the 5000km long (~31°S to 10°N) Ninetyeast Ridge (NER) in the Indian Ocean shows their widespread occurrence along the ridge as well as with water depth on the ridge flanks. The crusts are hydrogenetic based in growth rates and discrimination plots. Twenty samples from 12 crusts from 9 locations along the ridge were analyzed for chemical and mineralogical compositions, growth rates, and statistical relationships (Q-mode factor analysis, correlation coefficients) were calculated. The crusts collected are relatively thin (maximum 40mm), and those analyzed varied from 4mm to 32mm. However, crusts as thick as 80mm can be expected to occur based on the age of rocks that comprise the NER and the growth rates calculated here. Growth rates of the crusts increase to the north along the NER and with water depth. The increase to the north resulted from an increased supply of Mn from the oxygen minimum zone (OMZ) to depths below the OMZ combined with an increased supply of Fe at depth from the dissolution of biogenic carbonate and from deep-sourced hydrothermal Fe. These increased supplies of Fe increased growth rates of the deeper-water crusts along the entire NER. Because of the huge terrigenous (rivers, eolian, pyroclastic) and hydrothermal (three spreading centers) inputs to the Indian Ocean, and the history of primary productivity, Fe–Mn crust compositions vary from those analyzed from open-ocean locations in the Pacific.The sources of detrital material in the crusts change along the NER and reflect, from north to south, the decreasing influence of the Ganga River system and volcanic arcs located to the east, with increasing influence of sediment derived from Australia to the south. In addition, weathering of NER basalt likely contributed to the aluminosilicate fraction of the crusts. The southernmost sample has a relatively large detrital component compared to other southern NER crust samples, which was probably derived predominantly from weathering of local volcanic outcrops.Fe–Mn crusts from a dredge haul at 3412m water depth, 2°S latitude, are pervasively phosphatized along with the substrate rocks (site D7). Phosphatization took place through replacement of carbonate, preferential replacement of Fe oxyhydroxide relative to Mn oxide in the crusts, preferential replacement of silica-rich phases relative to Al-rich phases in the crusts, and precipitation of carbonate fluorapatite in pore space. The preferentially replaced silica may have been Si adsorbed on the Fe oxyhydroxide. The enrichment of Ni, Zn, and Cu in the phosphatized crust reflects preferential adsorption into the tunnel structure of todorokite. The rare earth element plus yttrium (REY) patterns indicate a lower oxidation potential during phosphatization of the NER crusts compared to Pacific phosphatized crusts. NER phosphatization occurred in a deeper-water environment than typical for phosphatization of Pacific crusts, occurred post-middle Miocene, a younger age than phosphatization the Pacific crusts, and had in part a different set of chemical changes produced by the phosphatization than did the Pacific crusts.The southern third of NER has Fe–Mn crusts with the highest Co (0.91%), Ni (0.43%), ΣREY (0.33%), Cu (0.22%), Te (146ppm), Pt (1.5ppm), Ru (52ppb), and Rh (99ppb) contents. These are among the highest Pt, Ru, and Rh concentrations measured in marine Fe–Mn deposits. Because of these high metal concentrations, exploration is warranted for the southern sector of the NER, especially at shallower-water sites where the platinum group elements (PGE) and Co are likely to be even more enriched.

Probing atomic scale transformation of fossil dental enamel using Fourier transform infrared and nuclear magnetic resonance spectroscopy: A case study from the Tugen Hills (Rift Gregory, Kenya)

A series of fossil tooth enamel samples was investigated by Fourier transform infrared (FTIR) spectroscopy, 13C and 19F magic-angle spinning nuclear magnetic resonance (MAS NMR) and scanning electron microscopy (SEM). Tooth remains were collected in Mio-Pliocene deposits of the Tugen Hills in Kenya. Significant transformations were observed in fossil enamel as a function of increasing fluorine content (up to 2.8wt.%). FTIR spectroscopy revealed a shift of the ν1 PO4 stretching band to higher frequency. The ν2 CO3 vibrational band showed a decrease in the intensity of the primary B-type carbonate signal, which was replaced by a specific band at 864cm−1. This last band was ascribed to a specific carbonate environment in which the carbonate group is closely associated to a fluoride ion. The occurrence of this carbonate defect was consistently attested by the observation of two different fluoride signals in the 19F NMR spectra. One main signal, at ∼−100ppm, is related to structural F ions in the apatite channel and the other, at −88ppm, corresponds to the composite defect. These spectroscopic observations can be understood as resulting from the mixture of two phases: biogenic hydroxylapatite (bioapatite) and secondary fluorapatite. SEM observations of the most altered sample confirmed the extensive replacement of the bioapatite by fluorapatite, resulting from the dissolution of the primary bioapatite followed by the precipitation of carbonate-fluorapatite. The ν2 CO3 IR bands can be efficiently used to monitor the extent of this type of bioapatite transformation during fossilization.

Paleoceanographic conditions following the end-Permian mass extinction recorded by giant ooids (Moyang, South China)

Early Triassic oceans were characterized by deposition of a number of “anachronistic facies”, including microbialites, seafloor carbonate cement fans, and giant ooids. Giant ooids were particularly prevalent in Lower Triassic sections across South China and exhibit unusual features that may provide insights into marine environmental conditions following the end-Permian mass extinction. The section at Moyang (Guizhou Province) contains abundant giant ooids ranging in size between 2 and 6mm (maximum 12mm) and exhibiting various cortical structures, including regular, deformed, compound, regenerated and “domed”. Preservation of ooid cortical structure is generally good as indicated by petrographic observations, and trace element and carbon isotope analyses suggest that diagenesis occurred in a closed diagenetic system. All ooids exhibit fine concentric laminae, frequently alternating between light-colored coarsely crystalline and dark-colored finely crystalline layers probably reflecting variation in organic content or original mineralogy. Under scanning electron microscope, biomineralized filaments or biofilms and tiny carbonate fluorapatite (CFA) crystals are commonly found in the finely crystalline layers. We infer that the precipitation of CFA was related to adsorption of P via microbial activity on the surfaces of ooids following episodic incursions of deep waters rich in CO2, H2S and phosphate into shallow-marine environments. Giant ooid precipitation may have been promoted in shallow ramp settings during these events by increased watermass agitation and supersaturation with respect to CaCO3, as well as reduced carbonate removal rates through biotic skeletal formation. Spatio-temporal distribution data reveal that giant ooids were widespread in the Tethyan region during the Early Triassic, and that they were most abundant immediately after the end-Permian crisis and disappeared gradually as metazoans repopulated marine environments.

Miocene phosphorites from the Murray Ridge, northwestern Arabian Sea

Phosphorites from the Murray Ridge, NW Arabian Sea comprise nodules, bioclasts, and bone fragments. The nodules are made up of a homogeneous, light-colored phosphate nucleus consisting of Rivulariacean filamentous cyanobacteria and a thin dark-grey colored phosphate cortex showing abundant microbial filaments and microborings. The bioclasts comprise of ∼ 14–14.5 Ma old planktonic foraminifers, accepted as the time of deposition. Spherical to ovoid-shaped apatite microparticles resembling fossil bacteria are distinct components in the bioclasts. Bone fragments exhibit apatite fillings. The nodules and bone fragments consist entirely of carbonate fluorapatite (CFA) with low Al, K, and Th concentrations suggesting absence of continental detritus. Shale-normalized REE patterns of the samples support a seawater-derived composition. The highly uniform initial ɛ Nd values of − 4.8 to − 5.1 are interpreted as the seawater value at the onset of phosphatization ∼ 14 Ma ago. In contrast, 87Sr/ 86Sr ratios show a large range of 0.709055 to 0.709124 corresponding to unusually young stratigraphic ages of ∼ 1 to 3 Ma. The data are interpreted as evidence for post-depositional Sr exchange of the recrystallizing phosphorites with fluids isotopically not much different from modern seawater. It is concluded that the phosphorites formed under oxic, shallow-water conditions where microbial populations assimilated phosphorus primarily from seawater and mediated precipitation of CFA during early diagenesis at the sediment–water interface on different substrates.

Early diagenetic redistribution and burial of phosphorus in the sediments of the southwestern East Sea (Japan Sea)

This study was carried out in order to understand the early diagenetic redistribution of phosphorus and relevant mass balance in the sediments of the East Sea. In two cruises during May 1993 and October 1995, 11 box cores were collected in the southwestern part of the East Sea. Dissolved phosphorus and iron were analyzed in the porewater from the cores. Sediment samples were analyzed for solid-phase P species and solid-phase Fe oxyhydroxide by sequential extraction. Phosphorus speciation results show that organic P is the major chemical form of phosphorus in young sediments within the upper 50 cm of sediment. However, the authigenic fraction of total P increases with depth, indicating the precipitation of carbonate fluorapatite (CFA) in the sediments. The authigenic CFA (Ca 5(PO 4) 2.6(CO 3) 0.4F) was formed and buried at rates of 11–110 μmol cm −2 kyr −1. The main source of dissolved phosphorus for the precipitation of CFA is organic P. Dissolved phosphorus, released from the decomposition of organic P, diffuses upward to return to bottom water, or is sorbed to iron oxides in the oxidized sediments. As sedimentation proceeds, the iron oxide-bound P is released in the reduced layer and enters the dissolved phase, which contributes P to the formation of CFA in addition to that contributed by the organic P. The burial flux of reactive P (iron oxide-bound P+authigenic P+organic P) is 0.09–0.53 g P m −2 yr −1 that accounts for 18–58% of the reactive P arriving at the sediment/water interface. The burial flux of reactive P is high in the upper and lower continental margin sediment. The burial flux of reactive P in the Ulleung Basin sediment is less than those in the continental margin sites by a factor of 6, indicating that the reactive P burial flux is mainly dependent on sedimentation rate.

Halide systematics in comparison with nutrient distributions in sites 1033B and 1034B, Saanich Inlet: ODP Leg 169S

Abstract The entire suite of halogens was measured in the pore fluids of Hole 1033B and 1034B from Saanich Inlet: ODP Leg 169S. The fast sedimentation rates and large amount of organic carbon burial coupled with anoxia of the overlying waters promotes an advanced stage of diagenesis within the sediment column. Chloride interstitial water profiles suggest salinity variations within the waters of Saanich Inlet. Concentration profiles for iodide and bromide support the argument that they are produced through the degradation of organic matter. Although the concentration increases in I− and Br− indicate that these halides are not regenerated in similar proportions to marine organic matter, it appears that iodide and bromide are regenerated to similar degrees within the sediment column and in similar proportions to the sediment halide concentrations. Fluoride porewater values show a complicated pattern, most likely caused by secondary reactions involving complexation with Mg2+, carbonate fluorapatite precipitation, carbonate mineral diagenesis, and/or uptake into alumino-silicate minerals.

Microbial infestation: a pathway of fluorine enrichment in bone apatite fragments (Negev phosphorites, Israel)

Backscattered electron (BSE) imaging and electron probe-coupled analysis of bone debris in the Negev phosphorites (Israel) show that F concentrations vary considerably in these components, and that these variations are driven by microbial activity. The bone debris are microbially tunneled, commonly with a micritic carbonate fluorapatite (CFA) phase filling the tunnels and gradually replacing the bone matrix. As a rule, the micritic CFA within the microbial tunnels is significantly much more F-enriched (F/P 2 O 5 —0.106–0.125) than the bone matrix (F/P 2 O 5 —0.083–0.104). Similar results are obtained in bones at final stages of microbial infestation and micrite phosphate replacement. The infested-replacing envelope invading the bone debris is much F enriched (enrichment factor of about 1.27 on average) than the surviving bone that escaped boring. These results are explained by redeposition as CFA of the microbially bored and dissolved hydroxyapatite, presumably fostered by microbial F transfer from porewaters to the sites of CFA precipitation in bone microborings. This pathway of F-enrichment driven by microbial activity may also explain the enigmatic differential U-enrichment previously observed in Negev phosphorite bones.

Early diagenetic processes in recent sediments of the Gulf of St-Lawrence: phosphorus, carbon and iron burial rates

Selective extraction procedures were used to quantify different forms of solid-phase phosphorus, carbon and iron in marine sediments, and to evaluate the impact of authigenic formation of mineral forms such as carbonate fluorapatite (CFA), calcium carbonate (CaCO 3) and pyrite (FeS 2) on major elemental cycles during early diagenesis. Detrital P and Fe phases were successfully used as indicators of the constancy or variability of detrital inputs to several sedimentary environments from the deep channels of the Gulf of St-Lawrence. In cores characterized by near steady state influx rates, solid-phase P, C and Fe data and sediment burial rates indicate that CFA, CaCO 3 and probably FeS 2 are currently forming in the sediments of the Gulf. However, high concentrations and/or formation of CaCO 3 in marine sediments appear to inhibit the formation of authigenic CFA. On the other hand, the formation of FeS 2 does not influence authigenic CFA precipitation. In the deep troughs of the Gulf of St-Lawrence, total P burial rates range from ≈ 50 to 500 mgP/m 2/yr. Truly authigenic precipitation of CFA, when observed, may represent up to ≈ 25% of the total burial rate of P. Bioturbation of sub-surface sediments reduces the potential for authigenic precipitation of CFA and CaCO 3, thus affecting immobilization reactions that have a strong impact on the global oceanic cycles of C and P. The spatial heterogeneity of diagenetic reactions precludes the establishment of an accurate quantification of P removal on the scale of a continental shelf such as the Gulf of St-Lawrence.

Carbon isotopic composition and lattice-bound carbonate of Peru-Chile margin phosphorites

New light-stable carbonate-carbon isotope and lattice-bound CO 2 data from Quaternary Peru-Chile margin phosphatic nodules, crusts and pelletal grains, and from associated dolomicritic concretions, are presented, which provide constraints on the timing and mechanisms of growth of these phases in organic carbon-rich sediments. Comparison of δ 13C values from carbonate fluorapatite (CFA) nodules and pelletal grains (−4.8 to 0.0‰ and −2.9 to +1.0‰ PDB, respectively) with pore-water total dissolved δ 13C values from these sediments suggests early authigenic CFA precipitation from pore waters within a few centimeters of the sediment-water interface in association with suboxic to perhaps anoxic microbial degradation of organic matter. In contrast, the dolomicritic cores of nodules recovered from about 12°S display both strongly negative to positive δ 13C values (−10.8 to +6.1‰) characteristic of formation deeper in the sediments in association with methanogenic and perhaps sulfate reduction microbial processes. The amount of structural carbonate in CFA suggests that carbonate substitution generally increases as δ 13C in CFA decreases, a probable consequence of increasing carbonate and accompanying charge-balancing substitutions in the CFA lattice in response to increasing pore-water carbonate ion concentrations with depth below the sediment-water interface. In one buried upward-growing nodule, decreasing CFA δ 13C and increasing structural CO 2 also correspond to decreasing CFA growth rates. These data suggest that in addition to other constraints such as pore-water phosphorus and fluoride availability, the lower limit of CFA precipitation in suboxic to anoxic sediments may be controlled by lattice poisoning due to excessive dissolved carbonate ion concentrations. In organic-rich Peru-Chile margin sediments this depth threshold appears to be at approximately 5–10 cm below the sediment-water interface where maximum CFA CO 2 contents of about 6 Wt.% occur; in less organic-rich settings, greater depths of precipitation of CFA may be anticipated. Below this relatively shallow depth of CFA precipitation on the Peru shelf, high pore-water alkalinity and associated elevated total dissolved carbon and carbonate ion concentration apparently favor the precipitation of authigenic carbonates.

Early diagenesis of organic matter in Peru continental margin sediments: Phosphorite precipitation

Pore water chemistry (total dissolved CO 2, NH 4, NO 3, NO 2, PO 4, Si(OH) 4, Ca, Mg, Fe, Mn, SO 4, H 2S and F, and titration alkalinity), solid phase chemistry (C org, P org, C TOT, N TOT, F, Si OPAL and S II), and sediment characteristics (porosity, dry bulk density and formation factors) were determined on a centimeter-scale spacing in the upper 20–40 cm of sediments under intense upwelling areas on the Peru continental shelf. These data demonstrate that carbonate fluorapatite (CFA) is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content (up to 20% C org), very high porosity (> 0.96 ml cm −3) and very low dry bulk density (< 0.1 g cm −3). Dissolved phosphate concentrations at the sediment-water interface range from 20 to 100 μ M, orders of magnitude higher than bottom-water concentrations, and much higher than predicted from regeneration of organic matter. The mechanism of this interfacial phosphate release is unclear, but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments. Fluoride is incorporated into CFA by diffusion from the overlying seawater, and carbonate ions are provided from pore-water alkalinity. Magnesium concentrations in this reaction zone are not significantly different from those of seawater, suggesting that magnesium depletion is not a necessary prerequisite for CFA precipitation. The environment of precipitation is interface-linked rather than driven by organic diagenesis of phosphorus deeper in the sediment. Most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region, but almost all show the precipitation signature near the interface. This interface-linked early diagenetic porewater environment for the precipitation of CFA explains many of the geochemical characteristics of phosphorites and provides a “testable” model to compare the modern phosphogenic analog with ancient phosphorite deposits. Two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites.