Showing papers on "Silicate minerals published in 1997"

Liquid Immiscibility in the Join NaAlSiO4—NaAlSi3O8—CaCO3 at 1 GPa: Implications for Crustal Carbonatites

Abstract: The synthetic system Na_2O–CaO–Al_2O_3–SiO_2–CO_2 has been widely used as a model to show possible relationships among alkalic silicate magmas, calciocarbonatites, and natrocarbonatites. The determined immiscibility between silicate- and carbonate-rich liquids has been strongly advocated to explain the formation of natural carbonatite magmas. Phase fields intersected at 1.0 GPa by the composition joins NaAlSiO_3O_8–CaCO_3 (Ab–CC, published) and NaAlSiO_4(Ne)_(90)Ab_(10)–CC (new), along with measured immiscible liquid compositions, provide pseudoternary phase relationships for the composition triangles Ab–CC–Na_2CO_3(NC) and Ne_(90)Ab_(10)–CC–NC. Interpolation between these, and extrapolation within the CO_2-saturated tetrahedron Al_2O_3–SiO_2–CaO–Na_2O, provides pseudoquaternary phase relationships defining the volume for the miscibility gap and the surface for the silicate–carbonate liquidus field boundary. The miscibility gap extends between 10 and 70 wt % CaCO_3 on the triangle Ne–Ab–CC at 1.0 GPa; it does not extend to the Na_2O-free side of the tetrahedron. The liquidus minerals in equilibrium with both silicate- and carbonate-rich consolute liquids are nepheline, plagioclase, melitite, and wollastonite; with increasing Si/Al the liquidus for calcite reaches the miscibility gap. We use these phase relationships to: (1) illustrate possible paths of crystallization of initial CO_2-bearing silicate haplomagmas, (2) place limits on the compositions of immiscible carbonatite magmas which can be derived from silicate parent magmas, and (3) illustrate paths of crystallization of carbonatite magmas. Cooling silicate–CO_2 liquids may reach the miscibility gap, or the silicate–calcite liquidus field boundary, or terminate at a eutectic precipitating silicates and giving off CO_2. Silicate–CO_2 liquids can exsolve liquids ranging from CaCO_3–rich to alkalic carbonate compositions. There is no basis in phase relationships for the occurrence of calciocarbonatite magmas with ∼99 wt % CaCO_3; carbonate liquids derived by immiscibility from a silicate–CO_2 parent (at crustal pressures) contain a maximum of 80 wt % CaCO_3. There are two relevant paths for a silicate liquid which exsolves carbonate-rich liquid (along with silicate mineral precipitates): (1) the assemblage is joined by calcite, or (2) the assemblage persists without carbonate precipitation until all silicate liquid is used up. The phase diagrams indicate that high-temperature immiscible carbonate-rich liquids must be physically separated from parent silicate liquid before they can precipitate carbonate-rich mineral assemblages. Path (1) then corresponds to the silicate–calcite liquidus field boundary, and a stage is reached where the carbonate–rich liquids will precipitate large amounts of calcite and fractionate toward alkali carbonates (not necessarily matching natrocarbonatite compositions). In path (2) the high-temperature immiscible carbonate liquid precipitates only silicates through a temperature interval until it reaches the silicate–carbonate liquidus field boundary, where it may precipitate calcite or nyerereite or gregoryite. Sovites are readily explained as cumulates, with residual alkali-rich melts causing fenitization. We can see no way in phase diagrams for vapor loss to remove alkalis and change immiscible natrocarbonatite liquids to CaCO_3–rich liquids; adjustments to vapor loss would be made not by change in liquid composition but by precipitation of calcite and silicate minerals. The processes illustrated in this model system are applicable to a wide range of magmatic conditions, and they complement and facilitate interpretation of phase relationships in the single paths represented by each whole- rock phase euilibrium study.

Mössbauer spectroscopic studies on the iron forms of deep-sea sediments

Abstract: Mossbauer spectroscopy was applied to characterize the valence states Fe(II) and Fe(III) in sedimentary minerals from a core of the Peru Basin. The procedure in unraveling this information includes temperature-dependent measurements from 275 K to very low temperature (300 mK) in zero–field and also at 4.2 K in an applied field (up to 6.2 T) and by mathematical procedures (least-squares fits and spectral simulations) in order to resolve individual spectral components. The depth distribution of the amount of Fe(II) is about 11% of the total Fe to a depth of 19 cm with a subsequent steep increase (within 3 cm) to about 37%, after which it remains constant to the lower end of the sediment core (at about 40 cm). The steep increase of the amount of Fe(II) defines a redox boundary which coincides with the position where the tan/green color transition of the sediment occurs. The isomer shifts and quadrupole splittings of Fe(II) and Fe(III) in the sediment are consistent with hexacoordination by oxygen or hydroxide ligands as in oxide and silicate minerals. Goethite and traces of hematite are observed only above the redox boundary, with a linear gradient extending from about 20% of the total Fe close to the sediment surface to about zero at the redox boundary. The superparamagnetic relaxation behavior allows to estimate the order of magnitude for the size of the largest goethite and hematite particles within the particle-site distribution, e.g. ∼170 A and ∼50 A, respectively. The composition of the sediment spectra recorded at 300 mK in zero-field, apart from the contributions due to goethite and hematite, resembles that of the sheet silicates smectite, illite and chlorite, which have been identified as major constituents of the sediment in the <2 μm fraction by X-ray diffraction. The specific “ferromagnetic” type of magnetic ordering in the sediment, as detected at 4.2 K in an applied field, also resembles that observed in sheet silicates and indicates that both Fe(II) and Fe(III) are involved in magnetic ordering. This “ferromagnetic” behavior is probably due to the double-exchange mechanism known from other mixed-valence Fe(II)–Fe(III) systems. A significant part of the clay-mineral iron is redox sensitive. It is proposed that the color change of the sediment at the redox boundary from tan to green is related to the increase of Fe(II)–Fe(III) pairs in the layer silicates, because of the intervalence electron transfer bands which are caused by such pairs.

Alkalinity Buildup During Silicate Weathering Under a Snow Cover

Abstract: Results from the study of experimental plots at Hubbard Brook, New Hampshire for the years 1987, 1988, 1993, 1994, 1995, and 1996 show that the water draining from under a plot planted with pine trees exhibits its highest alkalinity during the year at about the time of spring snowmelt. This high alkalinity is believed to be due to buildup during the winter under a snow cover. The soil solutions are protected from acidic precipitation by the snow, and the natural process of the reaction of organic acids and carbonic acid with minerals and exchange complexes to form dissolved HCO3 − (and organic anions) proceeds with an increase in alkalinity through the winter. When the snow melts the acidic meltwater mixes with, neutralizes and displaces the water previously occupying the soil interstices. This leads to a decided drop in alkalinity of the drainage water. The alkalinity buildup under the pine plot was found to be two to ten times greater than under a similar plot containing no higher plants. This strongly emphasizes the important role of plants, in their ability to produce organic acids and high levels of CO2, in accelerating the weathering of silicate minerals.

Quantitative analysis of major elements in silicate minerals and glasses by micro-PIXE

Abstract: The Guelph micro-PIXE facility has been modified to accommodate a second Si(Li) X-ray detector which records the spectrum due to light major elements (11 ≤ Z ≤ 20) with no deleterious effects from scattered 3 MeV protons. Spectra have been recorded from 30 well-characterized materials, including a broad range of silicate minerals and both natural and synthetic glasses. Sodium is mobile in some of the glasses, but not in the studied mineral lattices. The mean value of the instrumental constant H for each of the elements Mg, Al, and Si in these materials is systematically 6–8% lower than the H -value measured for the pure metals. Normalization factors are derived which permit the matrix corrections requisite for trace-element measurements in silicates to be based upon pure metal standards for Mg, Al and Si, supplemented by well-established, silicate mineral standards for the elements Na, K and Ca. Rigorous comparisons of electron microprobe and micro-PIXE analyses for the entire, 30-sample suite demonstrate the ability of micro-PIXE to produce accurate analysis for the light major elements in silicates.

Hydrogen and oxygen isotope compositions of eclogites from the Dabie Mountains and geodynamic implications

Abstract: Heterogeneous δ18O values as low as - 2.6‰ to+7.0% are observed for ultrahigh pressure eclogites from the Dabie Mountains in East China. Oxygen isotope equilibrium has been approached between the eclogite minerals, suggesting that the rocks would have acquired the unusual δ18O values prior to ultrahigh pressure metamorphism by interaction with18O-depleted fluid. δD values of hydroxyl-bearing are between — 51% and - 83‰, precluding the possibility of paleoseawater involvement. The only likely fluid is ancient meteoric water that exchanged oxygen isotopes with the eclogite precursor (a kind of basaltic rocks) formerly resident on the continental crust. This suggests a crustal recycling process in the suture zone of late subduction. Because silicate minerals undergo rapid oxygen isotope exchange at mantle pressures, preservation of the isotopic signature of meteoric water in the eclogites indicates limited crust-mantle interaction and thus a short residence time (<20 Ma) when the plate containing the eclogite precursor was subducted to mantle depths. The agreement in oxygen isotope temperatures for different mineral pairs suggests a rapid cooling and ascent process for the eclogites subsequent to their formation at mantle depths.

On the genesis of sedimentary apatite and phosphate-rich sediments.

Abstract: Igneous rocks consist essentially of silicate minerals which are unstable at the earth’s surface where they gradually undergo hydrolysis. When the constitutive chemical elements of these silicate minerals pass from the crystal state to that of ions in water, the relative affinities among the elements change, some of them moving away from each other, others becoming attracted towards each other. This concept of “convergence and divergence of elements” was precious to Millot (1964), who considered it to be one of the keys to understanding the organization and evolution of the geologic materials at the surface of the earth. Some elements in solution have a mainly inorganic geochemical behavior and are quickly incorporated into clay minerals. Other elements are involved preferentially in biological activity and are associated with organisms and organic matter before becoming constituents of authigenic minerals in sediments. Apatite is one of these authigenic minerals which, because of its relatively low solubility, is commonly preserved in its primary state. As a consequence, it has a far greater potential to reflect the original geochemical characteristics of its sedimentary environment than common biogenic minerals such as carbonates and sulfates.

Primary and secondary features of analcimes formed in carbonate-zeolite ocelli of alkaline basalts (Mecsek Mts., Hungary): textures, chemical and oxygen isotope compositions.

Abstract: Analcime is common in magmatic rocks of the alkaline basalt-phonotephrite-phonolite suite of the Mecsek Mts., Hungary. Besides the occurrence of xenomorphic groundmass analcime, wedge-shaped crystals between feldspars, and products of feldspar and nepheline alteration, analcime also occurs in calcite ocelli formed in basaltic dikes. Microscopic textures of these ocelli are characteristic of rapidly crystallized carbonate melt, suggesting that the ocelli are droplets of carbonate-rich melt separated from the silicate magma by liquid immiscibility. This carbonate would have crystallized below magmatic temperatures, and above hydrothermal ones. Thus, the analcime might be regarded as transitional between primary magmatic (P type) and hydrothermal (H type) analcimes. SEM studies reveal that the analcimes of the ocelli form euhedral crystals whose surfaces are smooth without signs of the porous texture characteristic for products of leucite alteration. Chemical compositions determined by electron microprobe are close to the theoretical NaAlSi2O6·H2O formula, with minor Ca substitution. XSi values (0.642 to 0.686) fall between the ranges of analcimes considered to be of primary magmatic and hydrothermal origins, whereas the low Fe contents indicate relationships with H type ones. Oxygen isotope compositions of silicate minerals in lavas and dikes and calcite ocelli in dikes have been determined in order to investigate the preservation of magmatic compositions, and the effect of low-temperature isotope exchange. The most positive δ18O values among the studied mineral separates were found in the analcimes (17.5 to 19.0‰). Based on comparisons with oxygen isotope compositions of calcites of the ocelli (13.0 to 13.3‰) and amphiboles (7.4‰) of their host rock, this 18O-enrichment could be a result of retrograde oxygen isotope exchange with magmatic fluids at decreasing temperatures. Effects of low-temperature isotope exchange appear also in the amphiboles, biotites and feldspars of the Mecsek series, resulting in increasingly more positive δ18O ranges (5.1 to 7.4‰, 7.2 to 7.4‰, and 7.6 to 15.0‰, respectively) as a function of sensitivity to retrograde isotope exchange. Primary magmatic compositions have been preserved in pyroxenes (6.0 to 6.5‰), indicating generation of basaltic melt by low degree partial melting of mantle peridotite.

Stable Isotopes in Allan Hills 84001: an Ion Microprobe Study

Abstract: The temperature of formation of carbonates in ALH 84001 provide a test of the hypothesis that they contain features produced by ancient martian life [1], and contributes to our understanding of the global budgets of martian volatiles. We have determined the micrometer-scale distribution of O and C isotope ratios in carbonate concretions, enstatite, and SiO_2 in ALH 84001 by ion microprobe as a means of constraining the range of possible temperatures of fluid-rock interaction [2]. The principal results are: (1) Carbonate δ^(18)O is 9.5-20.6‰, SMOW, significantly expanding the range from bulk analysis. The magnesite rim is higher in δ^(18)O than Ca-rich (0.07 ≤ X_(ca) ≤ 0.13) cores. (2) Variations in δ^(18)O occur over length scales as small as 50 µm. (3) Interiors of fractured orthopyroxene that hosts carbonate are homogeneous to within ±1‰. (4) Secondary SiO_2 has a δ^(18)O of 20.4‰. These data are inconsistent with mutual O isotope equilibrium among carbonate and silicate minerals and indicate either fine-scale mineral-mineral equilibrium at a range of temperatures ≤ 300°C or a failure to attain mineral-mineral equilibrium. Mineralogical evidence for disequilibrium supports the latter interpretation. Our δ^(18)O values for carbonates have been confirmed by two studies [3], and extended to compositions of carbonate not found in our sample.

Studies of progressive leaching in single mineral pb/pb dating

Abstract: Silicate mineral Pb Pb dating (PbSL) using progressive leaching steps was recently developed. Although the method represents a breakthrough in dating many silicate minerals, it is not fully understood on an atomic scale. It is not clear whether the leaching process is coupled with selective removal of elements from the crystal lattice or whether it involves major dissolution-redeposition along the reaction front. Further investigation was carried out on a single crystal of gem quality titanite from Otter Lake, Canada. PIXE mapping was performed to study the behaviour of U, Th and Pb during the stepwise leaching in comparison to the main elements in this mineral. (See Fig. 1.) Raster scans of 3 MeV protons were made using the true elemental imaging system (Dynamic Analysis) of the NAC Van de Graaff nuclear microprobe. PIXE elemental distribution maps, together with other techniques, revealed that the increased Pb isotope spread obtained during PbSL (enabling single mineral isochrons to be calculated) is the result of two competing and interacting processes, namely (i) a very effective surface dependent hydrolysis of metal cations, and (ii) a release rate delimiting, volume controlled adsorption of HFS-elements within the leached layer, which gets more effective as leaching progresses.

Semimicroanalysis of silicate minerals by means of xrf thin layer method. determination of selected chromatic elements : v, cr, mn, fe, co

Abstract: 50 mg of a powdered silicate mineral is spread on a substrate (filter paper glued on to glass or teflon by means. of two-sided adhesive tape), dissolved in a concentrated hydrofluoric acid and dried under an infrared heater; a sample is then protected with a Mylar film. Apparatus parameters: a sequence wavedispersive X-ray spectrometer; an X-ray tube with a Mo anode; 50 kV; 40 rnA; Kalines; LiF 200; flow counter; vacuum measurements; rotation of the sample; 100 scounting time for V, Cr, Mn, Co and 40 s or 100 s for Fe, according to the concentration; synthetic standards for calibration. Statistical parameters: detection limits for 50 mg samples: V (3-6 ppm), Cr (3-6 ppm), Mn (25 ppm), Fe (4-5 ppm), Co (2-7 ppm).

Crystallographic Image Processing on Minerals

Abstract: High resolution electron microscopy (HREM) combined with crystallographic image processing (CIP) is a powerful technique for structure determination of crystals too small to be studied by common techniques and for studying defects and interfaces. This technique is especially useful in mineralogy since low grade metamorphic rocks and clay-rich sediments are composed of fine grain crystals; and since the crystal structures of minerals are very complicated and not well ordered. Here we will present some examples on the application of HREM and CIP to silicate minerals.