Ppm Sr Research Articles

Quantitative analysis of sea-bed sediments from Eastern Black Sea by EDXRF spectrometry

Sea-bed sediments from Eastern Black Sea have been analyzed for their Fe, Ba and Sr contents using the methods of standard addition and energy dispersive X-ray fluorescence (EDXRF). Samples were prepared from powder sifted by a 200 mesh sieve. An annular 100 mCi241Am radioactive source emitting 59.543 keV photons was used for excitation and a Ge(Li) detector, which has a 190 eV FWHM for 5.9 keV photons was used for intensity measurements. 2048 channels of the MCA are employed for 20 or 30 min. Dried and ground sea-bed sediments are found to contain 2.95–7.50% Fe, 0.032–0.268% Ba and about 73–273 ppm Sr. These results agree with those obtained via atomic absorption methods.

A metallographic study of porosity and fracture behavior in relation to the tensile properties in 319.2 end chill castings

A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture surface analysis (phase identification), respectively. The results showed that the local solidification time(viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching. The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al5FeSi, α- Al15(Fe,Mn)3Si2, and Al2Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties to DAS allows for the use of the latter as an indicator of the expected properties of the alloy.

Paleozoic Mg calcite preserved: Implications for the Carboniferous ocean

Modern Mg calcite echinoderm skeletons are thought to lose Mg so rapidly that no pre-Pleistocene ossicles retain their original Mg concentrations; however, Carboniferous echinoderm skeletons from the Imo Formation of Arkansas and the Holder Formation of New Mexico are reported herein that are still composed of Mg calcite (7.8 to 14.5 mol% MgCO 3 ). Their Sr concentration is also high (620 to 3710 ppm). The pore system in these echinoderm skeletons is filled with ferroan calcite or siderite that has not grown in optical continuity with the skeleton. The discovery of Carboniferous Mg calcite confirms an interpretation already made from their diagenetic replacements. Marine Mg calcite has been associated with proposed icehouse conditions, and although the Carboniferous echinoderm skeletons are compatible with this as they formed during Gondwana glaciation, they are too few to be of significance. Also they are organic secretions and so can override secular mineral trends based on inorganic precipitates. The Carboniferous echinoderm skeletons, produced in tropical waters, have a concentration of Mg similar to values for modern echinoderms. The simplest explanation for this similarity is that both precipitated by similar processes from oceans of similar composition. This does not support predictions, based on mass-balance modeling, that the Mg/Ca ratio of the Carboniferous ocean was half that of today9s. The Sr content of Carboniferous echinoderms is less than that for modern echinoderms, leading to a prediction of 6 ppm Sr concentration for the Carboniferous ocean. Further examples of Mg calcite are required to substantiate these interpretations.

Lead isotopic compositions of Neogene volcanic rocks from the Aegean extensional area

Lead isotope composition has been determined for a diverse suite of Cenozoic igneous rocks in the central and north Aegean Sea. These include voluminous Miocene shoshonitic and calc-alkali volcanic rocks and correlative plutonic phases; middle Miocene adakitic (high Mg) andesite and associated rhyolite; and middle Miocene to Quaternary ne-normative basalt, trachyte and K-rich andesite. The trace-element contents of most of the studied rocks suggests that Pb-isotopic composition is not a result of upper-crustal contamination, but rather is a feature of the mantle source for K-rich rocks. This mantle source is represented by primitive mafic to intermediate volcanic rocks with 87Sr86Sr ≈ 0.7095, high 207Pb204Pb and 208Pb204Pb, >1000 ppm Sr and Ba, and >35 ppm Pb. The high 207Pb204Pb requires a metasomatic origin that lowered U/Pb and increased Th/Pb, followed by a second-stage residence time of >1 Ga. This K-rich magma was generated as a result of late Tertiary extension of the Aegean. In the South Aegean Arc, there is more evidence for crustal contamination, either from present crust or from subduction of Nile-derived sediment.

Developing a Raman Spectral Method for Measuring the Strontium and Calcium Concentrations of Fish Otoliths

Surface-enhanced Raman scattering (SERS) spectra of Ca2+ and Sr2+ complexes of four dyes (O-cresolphthalein, PAN black, murexide, and eriochrome-black T(EBT) are reported. It has been demonstrated that SERS spectra of the murexide complex of Ca2+ can be used for the accurate determination of the cation in solutions at pH 12–8. Similarly, the SERS spectra of the EBT complex of Sr2+ can yield accurate determination of the cation concentration in solutions at pH 12–8. The SERS technique has linear resolutions of Sr2+ and Ca2+ concentrations down to 219 and 20 ppb, respectively. The chemistry of the otoliths of Hyperoglyphe antarctica (Centrolophidae: Teleostei) and Beryx splendens (Berycidae: Teleostei) proved to be similar. Using the SERS murexide complex technique for Ca and the SERS EBT complex for Sr, concentrations were 39.33% weight Ca and 272.7 ppm Sr for H. antarctica and 44.93% weight Ca and 273.133 ppm Sr for B. splendens.

An Example of Mixing-zone Dolomite, Middle Eocene Avon Park Formation, Floridan Aquifer System

ABSTRACT A late-formed dolomite cement in a core of the Middle Eocene Avon Park Formation, peninsular Florida, provides an example of dolomite cement from a mixing zone and illustrates how dolomite textural alteration and stabilization can occur at earth-surface conditions. The Avon Park Formation is a pervasively dolomitized peritidal platform carbonate 400 m thick in the Floridan aquifer system. Typical Avon Park dolomite is inclusion-rich, fine-grained (< 40 mm), noncathodoluminescent, highly porous (average, 20%), and formed during the Eocene by normal to hypersaline seawater (18O = + 3.7 PDB; 13C = + 2.0; 87Sr/86Sr = 0.70778; Sr = 167 ppm). In a 20 m interval in a core from southwest Florida, inclusion-free, cathodoluminescent dolomite overgrows the early-formed noncathodoluminescent marine dolomite. The cathodoluminescent dolomite cement profoundly alters the texture of Avon Park dolomite from typical Cenozoic-like porous, poorly crystalline dolomite to hard, dense, low-porosity, highly crystalline Paleozoic-like dolomite. The dolomite cement is not a replacement of limestone but an overgrowth of early-formed marine dolomite and pore-occluding cement. From analyses of samples containing both the early marine dolomite and the late dolomite cement, quantitative estimates of pure cathodoluminescent dolomite cement are: 13C = -0.5 and 18O = +1.7; 87Sr/86Sr = 0.7085; Sr = 225 ppm. The stable-isotope compositions are intermediate between Avon Park marine dolomite and dolomite predicted to be in equilibrium with Floridan aquifer freshwater (estimated 18O = 0 to + 0.5, P B), indicating the dolomite precipitated from waters intermediate in composition between seawater and freshwater. Due to the rise in seawater 87Sr/86Sr since the Middle Eocene, the 87Sr/86Sr composition of the cathodoluminescent dolomite cement demonstrates that it must have formed after the Middle Miocene (incorporating radiogenic Sr from seawater or rocks of Middle Miocene age or younger). A water sample was taken from this same core from an interval in the Avon Park Formation in the dilute part of the modern mixing zone, approximately 60 m above the first occurrence of altered dolomite. Fluids from this interval have 3400 ppm TDS and are a mixture of 94% Floridan aquifer freshwater and 6% normal seawater. These pore fluids are Sr-rich (24 ppm) and SO4-rich (755 ppm) due to extensive Avon Park gypsum dissolution and have a 87Sr/86Sr composition (0.7078) in equilibrium with Avon Park host rock. The salinity of the fluid that precipitated the cathodoluminescent dolomite cement is quantitatively calculated with a fluid-fluid mixing model using (1) the Sr concentration (24 ppm) and 87Sr/86Sr composition (0.7078) of the von Park pore fluid sample from the core, (2) the Sr concentration (8 ppm) and 87Sr/86Sr compositions of Modern (0.7092) and Late Miocene (0.7089) seawater, and (3) the 87Sr/86Sr composition (0.7085) of the cathodoluminescent dolomite cement. These calculations indicate that the dolomite cement precipitated from a mixed marine-meteoric fluid with salinity approximately 75% seawater. This same fluid would be both calcite supersaturated and contain about 12 ppm Sr. Such a fluid could precipitate dolomite cement with 225 ppm Sr using a reasonable Kd = 0.05; this accounts for the higher Sr concentrations in the mixing-zone dolomite cement (225 ppm) relative to Avon Park marne dolomite (167 ppm). This study demonstrates that: (1) dolomite precipitated from a 75% seawater mixing-zone fluid that was both calcite saturated and sulfate-rich, and (2) dramatic textural maturation and stabilization in dolomite can occur in the near surface environment, without elevated temperature and burial conditions.

Cleat-Filling Calcite in Illinois Basin Coals: Trace-Element Evidence for Meteoric Fluid Migration in a Coal Basin

The microdistribution of Mn (786 to 9480 ppm), Sr (<5 to 461 ppm), and Fe (52 to 16,700 ppm) was determined for cleat-filling calcites in the Herrin, Springfield, and Colchester Coals, using focussed synchrotron radiation. Compositional zonation in the calcites reflects a multistage history of precipitation from fluids with differing trace-element characteristics. In nearly all of the samples, a high Fe () calcite is the latest composition to form, replacing earlier stages with varying, but generally lower Fe contents. Partitioning data for Sr suggest that a typical calcite with 200 ppm Sr formed from a fluid with a Sr/Ca of about 0.005. This Sr/Ca is ~4 times lower than seawater, and ~12 times lower than the average for present day Illinois Basin saline formation waters. Formation of cleat-filling calcite is best explained by precipitation from fluids similar to present day dilute (shallow) formation waters having a meteoric source.

Criteria for the retention of original seawater [formula omitted] in ancient shelf limestones

A suite of 40 Pennsylvanian and Permian shelf limestones from Kansas and Oklahoma has been analyzed for major and trace elements and for C, O and Sr isotopes. The chemistry and C and O isotope ratios were compared to the difference between the measured 87 Sr 86 Sr and original seawater ratio. Concentrations of Mn, Sr and Fe varied most systematically with the change in the seawater Sr ratio. All samples containing < 300 ppm Mn yielded correct ratios. All samples with > 700 ppm Mn yielded altered ratios. Fe and Sr contents were also found to have a less well-defined correlation with the retention of the original Sr ratio. All limestones with > 900 ppm Sr and 20 of 22 samples with a Sr Mn > 2.0 yielded correct ratios. The geochemical guidelines were applied to the chemical and isotopic results from 76 shelf limestones, mostly of Pennsylvanian age, from Texas and Oklahoma. More than 92% of the samples that contain < 300 ppm Mn, or with a Sr Mn > 2.0 yielded correct seawater ratios. Every sample with < 3000 ppm Fe and a Sr Mn > 2.0 retained the original seawater ratio.

Dolomitization of lower Permian platform facies, Wichita Formation, north platform, Midland basin, Texas

Three specific occurrences, or types, of dolostone are recognized in regionally widespread platform dolostones in the Wichita Formation (Permian, Lower Leonardian) in the northern Midland basin of Texas: (1) non-porous dolostones composing supratidal-intertidal facies: rocks deposited and inherently overprinted by peritidal diagenetic features in this environment, (2) porous dolostones composing subtidal-intertidal facies: rocks deposited originally in subtidal environments and subsequently exposed and overprinted diagenetically on intertidal flats; and (3) porous subtidal dolostones: rocks deposited originally in subtidal environments and which were never exposed syndepositionally to peritidal diagenetic overprinting. Fine crystalline (5–40 μm) dolomites in all three dolostone types have mean δ18O composition of 0.6‰ PDB, but are enriched in δ18O by as much as 3.3‰ relative to estimated lower Leonardian sea water. These δ18O-enriched, fine crystalline dolomites are preserved mainly in the supratidal-intertidal dolostones, whereas more coarsely crystalline (> 40 μm, mean =78 μm) dolomites occur in the subtidal and subtidal-intertidal dolostones. Coarsely crystalline dolomites have δ18O compositions that are slightly lighter (mean=−0.25‰) than the fine crystalline dolomites, but are still enriched by a mean of 1.6‰ relative to Leonardian sea water. All dolomites are near-stoichiometric, with87Sr/86Sr within the range of values of lower Permian sea water. Coarse crystalline dolomites are more depleted in Sr, Mn, and Fe than fine crystalline dolomites although, regardless of crystal size, all dolomites contain less than 125 ppm Sr and less than 100 ppm Mn.

Geochemistry of orbicular diorite from the Grenville Front zone, eastern Labrador

AbstractOrbicules in diorite from the Grenville Front zone of eastern Labrador consist of biotite- and/or hornblende-studded, dioritic cores enclosed by fine-grained shell structures alternately enriched and depleted in biotite. The orbicules occur in a mesocratic, quartz-bearing matrix. Epidote of inferred magmatic origin occurs in all parts of the rock. Plagioclase in the matrix is relatively sodic, and biotite more ferroan than in the orbicules, suggesting that the matrix material has the most evolved composition, and crystallized last.The diorite is unusually aluminous (orbicules: 24.9-27.4 wt.% Al2O3; matrix: 22.4-23.6% Al2O3) and calcic (orbicules: 7.0-8.4 wt.% CaO; matrix: 6.0-6.9% CaO); it shows a positive Eu anomaly, and has elevated Sr concentrations (1800-2500 ppm Sr), demonstrating that, compositionally, it resembles a plagioclase cumulate. Mass-balance calculations suggest that the orbicule cores had a crystal/melt ratio of ≤5. This accounts for the extreme fractionation of the rock (e.g., in orbicules, Zr &lt;5 ppm). Compared with fractional crystallization patterns, variation diagrams show counter-trends (e.g. the siliceous matrix contains elevated TiO2) or scatter for several components, suggesting that the crystal/melt ratio governed some of the geochemical characteristics of the diorite.The presence of coarse mafic clots containing primary epidote, biotite and/or hornblende testify to an elevated water content in the orbieule cores. The shell magma apparently formed as a result of the interaction of supercooled orbicule core fluids with the matrix magma, and tended to serve as a reservoir for alkalis and Fe. Alkalis and Ca diffused in opposite directions, possibly as a result of a temperature gradient at the orbicule/matrix interface. This, however, requires decoupling of the thermodiffusional behaviour of alkalis and femic components in hydrated intermediate magma, which contrasts with documented Soret diffusion in mafic systems.The solidification of the shell magma prior, to the orbicule cores and matrix is attributed to dewatering, consistent with the fine grain size of the shell structures. Except where remobilized core material has disrupted the shells, the cores crystallized in isolation from the matrix, which fractionated toward a more evolved composition.

Diagenesis of the Upper Permian Jiantianba Reef, West Hubei, China

ABSTRACT The Late Permian Jiantianba reef complex is up to 250 m thick, 4 km wide and 50 km long. It is well exposed at Jiantianba, where the reef overlies black siliceous basinal limestones and is generally covered by algal-rich carbonates and, locally, by Triassic rocks. Multiple generations of cement are recognizable in the reef complex. Early cements in the complex include, in order of formation, fibrous calcite in multiple layers, isopachous fine-textured dolomite, radiaxial calcite, and bladed calcite, all of which are interpreted to be of marine origin. Broad calcite fans are rare and are also considered to be of marine origin. Later cements include, in order of deposition, coarsely crystalline dolomite, zoned calcite, and blocky calcite. These appear to be results of diagenesis in envi onments from shallow burial to burial over 3000 m deep. In addition to the two generations of dolomite cementation, two phases of replacement dolomitization can be differentiated, one during shallow burial and the other following deep burial cementation. Fluid inclusions suggest moderately high temperatures of burial during precipitation of the coarse dolomite and blocky calcite cements, as indicated by homogenization temperatures (170 to 200°C) of two-phase fluid inclusions. Melting temperatures of frozen primary fluid inclusions (-38 to -20°C) indicate that parent solutions of the coarse dolomite and blocky calcite were brine waters. Comparisons of magnesium and strontium data from the blocky calcite cement with data from modern marine aragonite suggest that precursors of the fibrous cement (with 1439 to 2439 ppm Mg and 121 to 135 ppm Sr) and radiaxial cement (with 1293 ppm Mg and 26 ppm Sr) were Mg-calcite or low Mg-calcite. Similarly, elevated Sr values in the blocky calcite indicate it did not form in meteoric water. Oxygen and carbon stable isotope compositions (13C = + 1.8 to + 4.19 PDB; 18O = -5.37 to -8.76 PDB) of cements and dolomites of the reef complex reflect influences of both marine water and elevated diagenetic temperatures.

Proterozoic mantle under Quesnellia: variably reset Rb–Sr mineral isochrons in ultramafic nodules carried up in Cenozoic volcanic vents of the southern Omineca Belt

Handpicked, acid-washed diopside, orthopyroxene, and olivine separates from 14 lherzolite nodules from four localities in southern British Columbia have been analyzed for 87Rb/86Sr and 87Sr/86Sr. The host lavas, of late Cenozoic to Pleistocene age, are nepheline-normative alkali basalt and basanite with Sr and Nd isotopic ratios (0.7025–0.7029; 0.51310–0.51326) close to those of mid-ocean ridge basalt. The present-day Sr isotopic compositions of nodule diopsides (9–239 ppm Sr) are 0.7021–0.7041, similar to those of the host rocks. Orthopyroxene and olivine, however, have 87Rb/86Sr ratios of 0.1 to &gt; 1.0 and much more radiogenic Sr (87Sr/86Sr commonly 0.708–0.720), and in several cases the three minerals form linear arrays in 87Rb/86Sr versus 87Sr/86Sr plots. If interpreted as mineral isochrons, these give dates from 99 to 1989 Ma; many cluster around 440–770 Ma. Four nodules from two localities (West Kettle River and Big Timothy Mountain) give dates of 1201–1989 Ma. Maximum dates for the other localities (Jacques Lake and Lassie Lake) are 600–650 Ma.87Rb/86Sr ratios for acid leaches of crushed and sieved nodule material are generally &lt; 0.706, and 87Sr/86Sr ratios for the host rocks are &lt; 0.703, so radiogenic interstitial material or host contamination cannot explain the high ratios observed in the low-Sr minerals. The acid leaches lie both above and below mineral isochrons, virtually never exactly on the isochrons, indicating that secondary material, not cogenetic with the nodules, is present along grain boundaries. Discordant mineral points lie as expected for partially reset old isochrons, with olivine most easily reequilibrated by Sr isotopic exchange with less radiogenic minerals, orthopyroxene next in changeability, and clinopyroxene least modified.The nodules cannot be cognate or related in any way to recent magma genesis. The radiogenic Sr in low-Sr minerals is due to in situ decay of Rb and thus the nodules are Proterozoic upper mantle, some of an age similar to the 1.7–2.3 Ga granitic basement that tectonically underlies southwestern Quesnellia in the same region containing the nodule-bearing vents. The 440–770 Ma isochrons are speculatively correlated with Late Proterozoic to early Paleozoic craton-margin-forming rift event(s). Pre-Phanerozoic mantle lithosphere extends as far west as the Okanagan Valley in southern British Columbia.

Geochemistry of hydrothermally altered rocks at the Horne Mine, Noranda, Quebec

Low K rhyolites of the Archean Blake River Group that host the Horne volcanogenic massive sulfide deposit are altered to quartz-sericite-chlorite-albite (quartz-sericite zone) and chlorite + or - quartz (chlorite zone) assemblages. The quartz-sericite zone accounts for approximately 90 percent of the 5 km 3 of the altered hanging-wall and footwall rocks. Chlorite zone alteration is limited to portions of the footwall and flanks of the Cu-rich H orebodies. Clusters of gold-bearing chlorite-quartz veinlets crosscut large volumes of the quartz-sericite zone rocks.Alteration has added an average of 13 wt percent SiO 2 , 2.3 wt percent K 2 O, 430 ppm Ba, and 46 ppm Rb to the system and leached 3.6 wt percent Na 2 O, 2.2 wt percent CaO, and 41 ppm Sr. Middle and light REE were increasingly mobile from Yb to La; they were leached from the chlorite zone rocks and added to quartz-sericite zone rocks. Chlorite was more Fe rich (Fe/Fe + Mg = 0.65-0.85) in the chlorite zone rocks than in other altered lithologies (Fe/Fe + Mg = 0.2-0.5).Sulfur isotope values for massive ores are in the range delta 34 S = -2.1 to 1.9 per mil. Mineral separates from altered rocks yielded delta 18 O values of 10.2 to 12.1 per mil for quartz, 7.2 to 9.5 per mil for calcite, 2.2 to 3.0 per mil for chlorite, and 0.1 to 1.0 per mil for magnetite. Oxygen and sulfur isotope fractionation between coexisting minerals suggests formation from fluids having delta 18 O = 3 to 4 per mil at temperatures between 275 degrees to 380 degrees C. Oxygen isotope compositions of host rocks are delta 18 O = 8 + or - 1 per mil for freshest rhyolite, delta 18 O = 6.6 to 11.6 per mil for quartz-sericite zone rocks, and delta 18 O = 3.9 to 4.4 per mil for chlorite zone rocks. These measured values compare closely to estimated whole-rock delta 18 O values based on mass balance considerations using normative mineralogies.Alteration of wall rocks encompassing the massive sulfide orebodies is characterized by intense sericitization, a high degree of silicification, and a marked positive delta 18 O anomaly. Calculations based on SiO 2 solubilities and probable K contents of the hydrothermal fluids suggest a water-rock mass ratio of 50 to 300 during the mineralization event.

Effects of a chlorhexidine‐fluoride‐strontium rinsing program on caries, gingivitis and some salivary bacteria among Finnish school children

In order to find out if it is possible to prevent caries and gingivitis by periodical use of chlorhexidine-fluoride mouthrinses with or without strontium, and to find out what effects they have on salivary mutans streptococci and lactobacilli counts, a total of 243 schoolchildren aged 11 yr with high DMFS scores were randomly divided into four groups. One group (C) served as a basic control. Subjects in the second group (CXF) rinsed their mouths twice a day every third week with a rinsing solution containing 0.05% chlorhexidine gluconate and 0.04% NaF. In the third group (CXFS) the rinsing solution contained 500 ppm Sr during the first and second year and 15 ppm during the last 6 months, in addition to chlorhexidine and fluoride. In the fourth group (CX) the solution contained only 0.05% chlorhexidine gluconate. All the rinsing solutions had pH 5.8 buffered with succinic acid-NaOH buffer. After 2 yr and 9 months, the mean DMFS (SD) increments in the C, CXF, CXFS, and CX groups were 3.8 (5.7), 2.5 (3.2), 3.5 (4.8), and 3.4 (5.5), respectively. The percentage of subjects with bleeding gingival units had decreased from initial to final values as follows: C, 81-38; CXF, 88-42; CXFS, 89-56; CX, 89-37. The number of lactobacilli and mutans streptococci in saliva remained virtually unchanged throughout the study. For caries increment and gingival bleeding, the differences between groups were not statistically significant. The chlorhexidine-fluoride combination tended to prevent caries, but the effect on gingival bleeding and salivary counts of mutans streptococci and lactobacilli was negligible.

Water/Rock Interaction Efficiency and Seawater Dolomitization in the Eocene Avon Park Formation, Floridan Aquifer: ABSTRACT

The Floridan aquifer has often been proposed as a system of extensive meteoric carbonate diagenesis and mixing zone dolomitization. However, the dominance of marine isotope (C, O, {sup 87}Sr/{sup 86}Sr) and trace element (Sr, Fe, Mn) compositions in dolomites and limestones in the Eocene Avon Park Formation, Floridan aquifer, suggests that the very active low temperature meteoric groundwater system has, over the past 40 m.y., been an inefficient mechanism of diagenesis. {delta}{sup 18}O values of all but two replacement dolomites sampled range from +2.0 to +5.1 (PDB) with high Sr concentrations (90-325 ppm), indicating dolomitization by near-normal marine water involving no significant interaction with meteoric groundwater. The two {delta}{sup 18}O-depleted (0.0 {plus minus} 1) dolomites have low Sr concentrations ({approximately}100 ppm) suggesting limited recrystallization in meteoric water. Several dolomite samples have radiogenic {sup 87}Sr/{sup 86}Sr compositions (0.70810-0.70883 {plus minus} 2), but have heavy oxygen isotope compositions (> +2.0) and high Sr concentrations (<200 ppm) suggesting precipitation from cold Miocene age or younger seawater that circulated through the Florida platform. Most limestone stable isotope compositions cluster around marine values (({delta}{sup 18}O = {minus}1 to +1, PDB) {delta}{sup 13}C = +0.5 to +2.5) and have Eocene seawater {sup 87}Sr/{sup 86}Sr compositions (0.70775more » {plus minus} 2 to 0.70779 {plus minus} 2) with 400 to 500 ppm Sr. Isotopic compositions of limestones from the east coast of Florida are all within these ranges. Only some limestones from central Florida and the west coast contain depleted stable isotopic compositions and low Sr concentrations. The sample with the most depleted stable isotope values has a radiogenic {sup 87}Sr/{sup 86}Sr composition (0.70870 {plus minus} 2), suggesting that diagenetic meteoric water migrated through post-Miocene strata.« less

Geochronologic and isotopic evidence for involvement of pre-Pan-African crust in the Nubian shield, Egypt

Research Article| August 01, 1990 Geochronologic and isotopic evidence for involvement of pre-Pan-African crust in the Nubian shield, Egypt Mohamed Sultan; Mohamed Sultan 1Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130 Search for other works by this author on: GSW Google Scholar Kevin R. Chamberlain; Kevin R. Chamberlain 1Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130 Search for other works by this author on: GSW Google Scholar Samuel A. Bowring; Samuel A. Bowring 1Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130 Search for other works by this author on: GSW Google Scholar Raymond E. Arvidson; Raymond E. Arvidson 1Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130 Search for other works by this author on: GSW Google Scholar Hassan Abuzied; Hassan Abuzied 2Department of Geology, Faculty of Science, Cairo University, Cairo, Egypt Search for other works by this author on: GSW Google Scholar Baher El Kaliouby Baher El Kaliouby 3Department of Geology, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt Search for other works by this author on: GSW Google Scholar Geology (1990) 18 (8): 761–764. https://doi.org/10.1130/0091-7613(1990)018<0761:GAIEFI>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Mohamed Sultan, Kevin R. Chamberlain, Samuel A. Bowring, Raymond E. Arvidson, Hassan Abuzied, Baher El Kaliouby; Geochronologic and isotopic evidence for involvement of pre-Pan-African crust in the Nubian shield, Egypt. Geology 1990;; 18 (8): 761–764. doi: https://doi.org/10.1130/0091-7613(1990)018<0761:GAIEFI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Two Late Proterozoic granitic bodies from the Eastern Desert of Egypt, the ca. 578 Ma Nakhil and the ca. 595 Ma Aswan granites, provide insights into processes of crust formation in the Arabian-Nubian shield. Evidence for involvement of an older crustal component in the formation of the Nakhil granite includes (1) U/Pb zircon data that establish a crystallization age of 578 ±15 Ma and indicate the presence of inherited zircons possibly as old as 1.6 Ga; (2) an elevated model initial 87Sr/86Sr (0.7136); and (3) an elevated initial 207Pb/204Pb (15.561) relative to model mantle compositions at 578 Ma. Evidence for involvement of an older crustal component in the Aswan granite comes from the elevated initial 207Pb/204Pb (15.611). In contrast, extensive crustal contamination is not reflected in the high initial ϵNd(+5.7) for the Nakhil and the low initial 87Sr/86Sr (0.7029) for the Aswan granite. The contrasting inferences from the different isotopic systems can be explained by the high whole-rock Nd and Sr concentration for the Nakhil (87 ppm Nd) and the Aswan (173 ppm Sr) granites, respectively, that suggest that the Nd and Sr isotopic composition of the older component has been overshadowed by the more primitive material. Similar contrasts in Ph, Sr, and Nd isotopic data from the eastern and western shield margins can be interpreted in the same manner and might suggest widespread involvement of older crustal components in the formation of the Late Proterozoic Arabian-Nubian shield. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Vuggy carbonate crust formed by hydrocarbon seepage on the continental shelf of Baffin Island, northeast Canada.

Vuggy carbonate crust 10 cm in thickness has been discovered associated with hydrocarbon seepage on the continental shelf of Baffin Island, northeast Canada, at a water depth of about 400 m. The carbonate crust is principally composed of high magnesian calcite (14.1 to 18.1 mol.% MgCO3) with subordinate amounts of aragonite (∼9200 ppm Sr) and calcian dolomite (51.5 to 53.2 mol.% CaCO3). Magnesian calcite occurs as microcrystalline cement, and aragonite forms botryoidal aggregates in cavities and solution pits of the crust. Dolomite occurs as isolated euhedral grains with characteristic corroded surfaces. Calcite and aragonite have the similar isotopic signatures, ranging in δ13C between -26.1 and -33.2‰ PDB and in δ18O between 4.4 and 7.8‰ PDB. Relative to calcite and aragonite, dolomite is enriched in 13C (-8.9 to -11.6‰ PDB δ13C) and depleted in 18O (-1.2 to -1.7‰ PDB δ18O). Combined petrographic and gochemical evidence suggests that dolomite was formed in the sulfate reduction zone prior to the formation of carbonate crust under the strong influence of 18O-depleted freshwater, which presumably was derived from the melting of glacial ice. Then high magnesian calcite precipitated to forma dense carbonate layer in the near surface sediments caused by anaerobic oxidation of hydrate-derived 13C-depleted methane and increased alkalinity. When the sediments on the crust were winnowed and blown away by an expulsion of methane from the sea bed or a strong bottom current, the crust would be exposed to a zone of aerobic methane oxidation at the sediment/water interface. This led to dissolution of the upper surface of the crust and subsequent precipitation of botryoidal aragonite within solution vugs and cavities.

Shoshonitic volcanism in the Northern Mariana Arc: 2. Large‐ion lithophile and rare earth element abundances: Evidence for the source of incompatible element enrichments in intraoceanic arcs

The Mariana‐Volcano‐Izu arc system extends 2400 km north to south and is an outstanding example of an intraoceanic magmatic arc. In spite of this, the system is poorly known because most of it is submarine. Volcanism is entirely submarine in the northernmost Mariana arc, between 20°40′ and 24°N. This is the Northern Seamount Province (NSP) and was the focus of a detailed marine geologic and geochemical study, with additional data drawn from adjacent arc segments to the north (Volcano arc) and south (Central Island Province (CIP) of the Mariana arc). Samples from 24 submarine volcanoes and three islands were analyzed for concentrations of K, Rb, Sr, Ba, and the rare earth elements (REE). These data show strong variations along the arc, being relatively depleted in the more mature, tholeiitic and low‐K calc‐alkaline volcanoes of the Volcano arc and the Mariana CIP, containing on average 6100 ppm K, 300 ppm Sr, 200 ppm Ba, and 6 ppm La. All of the NSP is enriched in large ion lithophile (LIL) and light rare earth elements (REE), particularly the northern half (26,000 ppm K, 700 ppm Sr, 900 ppm Ba, 47 ppm La); these lavas have strong shoshonitic affinities. These enrichments do not result from fractional crystallization of CIP‐type melts. The source responsible for these enrichments shares some features in common with Mariana CIP and Volcano arc sources: K/Rb and K/Ba in particular are similar (∼500 and ∼30, respectively). However, Ba/La, Sr/Nd, and (Ce/Yb)n change drastically, with Ba/La and Sr/Nd decreasing to mantle values with increasing LIL and LREE enrichment. The origin of the LIL and LREE enrichments in the NSP shoshonites does not result from variations in the behavior or composition of the subducted lithosphere. Melting occurs exclusively within the mantle wedge, and forward modeling of the REE patterns for all Mariana and Volcano arc lavas indicates that melt generation occurs within the stability field of spinel lherzolite, probably within the upper 40–50 km of the subarc asthenosphere. Lavas from the large volcanoes of the Mariana CIP and Volcano arc result from 10–20% melting of spinel lherzolite, followed by varying amounts of low‐P fractional crystallization. Inferences based on REE forward models that the NSP shoshonites manifest very low (1%) degrees of partial melting of LIL‐ and LREE‐enriched spinel lherzolite are inconsistent with observed similar concentrations in tholeiites and shoshonites of high field strength cations such as TiO2 and Yb. Some of this inconsistency can be explained as resulting from source or melt mixing, with the NSP shoshonites being derived from a LIL‐ and LREE‐enriched source or melt, with Ba/La and La/Yb indistinguishable from that of ocean island basalts (OIB), while Mariana CIP and Volcano arc melts are derived from a depleted mild‐ocean ridge basalt‐like mantle that has been recharged with K, Rb, Sr, and Ba by hydrous fluids. These variations are interpreted as reflecting the evolution of the subarc asthenosphere, with a depletion in time resulting from the continuous extraction of basaltic melts.

Protection by F, I, Sr, and combinations against fermentation attack by Streptococcus sobrinus artificial plaque on bovine enamel.

Labial surfaces of 64 bovine incisors (8 teeth/treatment) were subjected to 1-min treatment with (1) 500 ppm Sr, (2) 0.5% I2 plus 1% KI solution, (3) F varnish treatment (Duraphat) for 24 h, or (4) combined treatments. The treated teeth were incubated under an artificial Streptococcus sobrinus plaque for 10 days. The 'oral fluid' with maleate buffer (pH 5.8) partially saturated with Ca3(PO4)2 and with or without 3.3% sucrose or sucrose plus 25 ppm F, was replaced by a mixture containing thioglycolate broth and the buffer for 4 h daily. This was done in an attempt to maintain the viability of the plaque as it was not renewed. Analysis of the Ca and inorganic P in the fluid phase taken after the 1st and the 10th day of incubation indicated that complete protection was obtained with F varnishing plus 25 ppm F in the fluid, with added sucrose. The Sr plus F treatment was more protective than F or Sr alone. The iodine treatment was slightly protective when combined with F varnishing. The results of the enamel surface and subsurface F and Sr as well as measurements of surface microhardness also indicated the highest protective effect with the double-F treatment and a marked protection provided by the Sr plus F treatment. The efficacy of the double-F treatment was partly explained by the prevention of a fall in 'plaque' pH and partly by the release of bacterial inorganic P in the extracellular fluid. The present caries model is versatile in quantification of changes in numerous parameters (14 parameters measured) involved in the caries-like process and its inhibition.

Mineralogic and isotopic constraints on the origin of strontium-rich cap rock, Tatum Dome, Mississippi, U.S.A.

The limestone portion of the salt dome cap rock at Tatum dome, Mississippi, is composed of an upper massive zone and a lower banded zone. The upper zone consists of equigranular fine-crystalline calcite with veinlets and disseminations of carbonaceous matter associated with minor amounts of detrital and authigenic quartz, sulfides and Sr minerals. The lower zone is composed of alternating light and dark calcite bands. The dark bands are composed of fine-grained and peloidal calcite, quartz, bitumen and disseminated sulfide minerals. The lighter bands consist of variable proportions of generally coarse-crystalline euhedral calcite, celestite and strontianite resulting in Sr contents of up to 30% locally. Solubility data for celestite, strontianite, calcite and anhydrite suggest that a decrease in temperature favors the replacement of Ca minerals by Sr minerals, which is consistent with the observed mineral textures and paragenesis. However, the source of cap rock Sr is difficult to determine. Anhydrite at Tatum dome contains 800 ppm Sr, but the abundance of Sr minerals in the limestone cap rock and the 87 Sr 86 Sr ratios of limestone cap rock minerals require a Sr source other than local anhydrite. Sr released by the replacement of anhydrite by calcite is capable of producing a molar ratio of celestite to calcite of only ∼ 0.001, yet locally this ratio is ∼ 3. A likely source of additional Sr is oilfield brines, such as those in central Mississippi that contain up to 3000 ppm Sr along with significant Pb and Zn. Episodic introduction of brines into the cap rock in combination with the action of sulfate-reducing bacteria probably caused the sequential production of the dark bands. The lighter bands probably reflect introduction of a later Sr-enriched fluid or evolution of the original fluid in combination with changes in the chemical parameters controlling mineral precipitation. Calcite was replaced by Sr minerals during this later Sr-rich event.