Blair F. Jones

Isotope studies of dolomite formation under sedimentary conditions

Measurements of stable isotope abundances of the carbonate portion of the sediment in Deep Springs Lake, California, indicate the presence of at least three phases: a magnesian calcite, a primary sedimentary dolomite, and a detrital dolomite. The former two have isotopic compositions consistent with precipitation at isotopic equilibrium from waters of the lake area. The measured isotopic fractionation factor between sedimentary dolomite and its interstitial water is 1.0351, which is outside the range possible for calcite-water. This indicates that the dolomite has formed by direct crystallization from solution and not from a caloite precursor without further isotope exchange. Isotopic and X-ray evidence does not support the contention of Petersonet al. (1966) that Deep Springs Lake dolomite crystals grow by means of a calcite-like surface layer.

Transmission electron microscopy of subsolidus oxidation and weathering of olivine

Olivine crystals in basaltic andesites which crop out in the Abert Rim, south-central Oregon have been studied by high-resolution and analytical transmission electron microscopy. The observations reveal three distinct assemblages of alteration products that seem to correspond to three episodes of olivine oxidation. The olivine crystals contain rare, dense arrays of coherently intergrown Ti-free magnetite and inclusions of a phase inferred to be amorphous silica. We interpret this first assemblage to be the product of an early subsolidus oxidation event in the lava. The second olivine alteration assemblage contains complex ordered intergrowths on (001) of forsterite-rich olivine and laihunite (distorted olivine structure with Fe3+ charge balanced by vacancies). Based on experimental results for laihunite synthesis (Kondoh et al. 1985), these intergrowths probably formed by olivine oxidation between 400 and 800°C. The third episode of alteration involves the destruction of olivine by low-temperature hydrothermal alteration and weathering. Elongate etch-pits and channels in the margins of fresh olivine crystals contain semi-oriented bands of smectite. Olivine weathers to smectite and hematite, and subsequently to arrays of oriented hematite crystals. The textures resemble those reported by Eggleton (1984) and Smith et al. (1987). We find no evidence for a metastable phase intermediate between olivine and smectite (“M” — Eggleton 1984). The presence of laihunite exerts a strong control on the geometry of olivine weathering. Single laihunite layers and laihunite-forsteritic olivine intergrowths increase the resistance of crystals to weathering. Preferential development of channels between laihunite layers occurs where growth of laihunite produced compositional variations in olivine, rather than where coherency-strain is associated with laihunite-olivine interfaces.

Evidence That the ZNT3 Protein Controls the Total Amount of Elemental Zinc in Synaptic Vesicles

The ZNT3 protein decorates the presynaptic vesicles of central neurons harboring vesicular zinc, and deletion of this protein removes staining for zinc. However, it has been unclear whether only histochemically reactive zinc is lacking or if, indeed, total elemental zinc is missing from neurons lacking the Slc30a3 gene, which encodes the ZNT3 protein. The limitations of conventional histochemical procedures have contributed to this enigma. However, a novel technique, microprobe synchrotron X-ray fluorescence, reveals that the normal 2- to 3-fold elevation of zinc concentration normally present in the hippocampal mossy fibers is absent in Slc30a3 knockout (ZNT3) mice. Thus, the ZNT3 protein evidently controls not only the “stainability” but also the actual mass of zinc in mossy-fiber synaptic vesicles. This work thus confirms the metal-transporting role of the ZNT3 protein in the brain.

An aem-tem study of weathering and diagenesis, Abert Lake, Oregon: I. Weathering reactions in the volcanics

Abstract Abert Lake in south-central Oregon provides a site suitable for the study of sequential weathering and diagenetic events. In this first of two papers, transmission electron microscopy was used to characterize the igneous mineralogy, subsolidus alteration assemblage, and the structural and chemical aspects of silicate weathering reactions that occur in the volcanic rocks (basalts, basaltic andesites, and dacitic/ rhyolitic extrusive and pyroclastics) that outcrop around the lake. Olivine and pyroxene replacement occurred topotactically, whereas feldspar and glass alteration produced randomly oriented smectite in channels and cavities. The tetrahedral, octahedral, and interlayer compositions of the weathering products, largely dioctahedral smectites, varied with primary mineral composition, rock type, and as the result of addition of elements released from adjacent reaction sites. Weathering of the highly evolved, Fe-rich Jug Mountain complex at the north end of the lake produced a homogeneous smectite assemblage that contrasts with the heterogeneous smectite assemblage replacing the volcanics along the eastern margin of the lake. The variability within and between the smectite assemblages highlights the microenvironmental diversity, fluctuating redox conditions, and variable solution chemistry associated with mineral weathering reactions in the surficial environment. Late-stage exhalative and aqueous alteration of the volcanics redistributed many components and formed a variety of alkali and alkali-earth carbonate, chloride, sulfate, and fluoride minerals in vugs and cracks. Overall, substantial Mg, Si, Na, Ca, and K are released by weathering reactions that include the almost complete destruction of the Mg-smectite that initially replaced olivine. The leaching of these elements from the volcanics provides an important source of these constituents in the lake water. The nature of subsequent diagenetic reactions resulting from the interaction between the materials transported to the lake and the solution will be described in part II ( Banfield et al., 1991).

Enhanced zinc consumption causes memory deficits and increased brain levels of zinc

Zinc deficiency has been shown to impair cognitive functioning, but little work has been done on the effects of elevated zinc. This research examined the effect on memory of raising Sprague-Dawley rats on enhanced levels of zinc (10 ppm ZnCO3; 0.153 mM) in the drinking water for periods of 3 or 9 months, both pre- and postnatally. Controls were raised on lab water. Memory was tested in a series of Morris Water Maze (MWM) experiments, and zinc-treated rats were found to have impairments in both reference and working memory. They were significantly slower to find a stationary platform and showed greater thigmotaxicity, a measure of anxiety. On a working memory task, where the platform was moved each day, zinc-treated animals had longer latencies over both trials and days, swam further from the platform, and showed greater thigmotaxicity. On trials using an Atlantis platform, which remained in one place but was lowered on probe trials, the zinc-treated animals had significantly fewer platform crossings, spent less time in the target quadrant, and did not swim as close to the platform position. They had significantly greater latency on nonprobe trials. Microprobe synchrotron X-ray fluorescence (microSXRF) confirmed that brain zinc levels were increased by adding ZnCO3 to the drinking water. These data show that long-term dietary administration of zinc can lead to impairments in cognitive function.

Sedimentology and geochemistry of carbonates from lacustrine sequences in the Madrid Basin, central Spain

Abstract Lacustrine and alluvial carbonate facies have been investigated in Middle Miocene successions of the western side of the Madrid Basin in order to evaluate paleoenvironments in which carbonates formed. Carbonate facies are varied and include: (1) calcrete and dolocrete; (2) pond deposits; (3) lake margin dolostone; (4) mudflat carbonate; and (5) open-lake carbonate facies. The dominant mineralogy of these is dolomite and/or low-Mg calcite. No high-Mg calcite or aragonite have been detected in any sample. δ 18 O- and δ 13 C-values range from −8.20 to −1.80% PDB and −10.25 to −0.70% PBD, respectively. More negative δ 18 O- and δ 13 C-values correspond to predominantly calcite calcretes and to carbonate deposited in ponds at the foot of arkosic alluvium. Higher δ 18 O-values are from both lacustrine carbonate and dolocrete. This latter lithofacies has strong geochemical similarities to dolostones deposited in a lake margin environment. Mudflat carbonate, deposited on shallow platforms subject to lake water oscillation, shows great heterogeneity in both stable isotope value and trace-element content. The mineralogy of these carbonates is dominated by calcite and the limestones contain molds of gypsum. Occurrence of calcitized dolomite textures in these facies suggests the influence of fresher water during expanding lacustrine cycles or further interaction with less saline groundwater. Trace-element contents are considered to be potential indicators of the different carbonate facies types, thus aiding the paleoenvironmental interpretation. However, discrimination among carbonate facies on the basis of trace-element contents appears to be dependent on the statistical method utilized for treatment of data. More information is needed to ascertain their use as paleoenvironmental indicators.

Geochemistry of great Salt Lake, Utah II: Pleistocene-Holocene evolution

Sedimentologic and biostratigraphic evidence is used to develop a geochemical model for Great Salt Lake, Utah, extending back some 30,000 yrs. B.P. Hydrologie conditions as defined by the water budget equation are characterized by a lake initially at a low, saline stage, rising by about 17,000 yrs. B.P. to fresh water basin-full conditions (Bonneville level) and then, after about 15,000 yrs. B.P., dropping rapidly to a saline stage again, as exemplified by the present situation. Inflow composition has changed through time in response to the hydrologie history. During fresh-water periods high discharge inflow is dominated by calcium bicarbonate-type river waters; during saline stages, low discharge, NaCl-rich hydrothermal springs are significant solute sources. This evolution in lake composition to NaCl domination is illustrated by the massive mirabilite deposition, free of halite, following the rapid drawdown until about 8,000 years ago, while historic droughts have yielded principally halite. Hydrologic history can be combined with inferred inflow composition to derive concentration curves with time for each major solute in the lake. Calcium concentrations before the drawdown were controlled by calcite solubility, and afterwards by aragonite. Significant amounts of solutes are removed from the lake by diffusion into the sediments. Na+, Cl− and SO42− are also involved in salt precipitation. By including pore fluid data, a surprisingly good fit has been obtained between solute input over the time period considered and the amounts actually found in lake brines, pore fluids, salt beds and sediments. Excess amounts are present for calcium, carbonate and silica, indicating detrital input.

Weathering of Basaltic Tephra on the Island of Hawaii

Three basaltic tephra deposits of Kilauea were studied in order to determine the nature and amount of weathering as a function of age and climate. One of these deposits, the Pahala Ash, is a regional tephra blanket 10,000 to 17,000 yrs old that is weathering under present rainfall ranging from about 25 to 635 cm/yr. The Keanakakoi Formation and Uwekahuna Ash, which contain the other deposits investigated, are less than 10,000 yrs old, and are present only in the vicinity of Kilauea caldera, where rainfall is from 115 to 255 cm/yr. Basaltic glass in the Pahala Ash is extensively weathered, whereas that in the Keanakakoi Formation generally is little altered. The principal weathering products of glass are palagonite and clay. Montmorillonite and calcite are weathering products of the Pahala Ash in the drier areas; gibbsite has formed at rainier sites. Plagioclase, pyroxene, and olivine are weathered only in the Pahala Ash. Chemical changes on weathering of the tephra deposits vary as a function of climate. From comparative analysis of unaltered and altered solids, SiO2 and Na + K + Ca + Mg seem to be lost under heaviest rainfall in average proportions as high as 2:1; in the driest areas they have been leached in the ratio of about 1:3. In an area of intermediate rainfall, SiO2 and Na + K + Ca + Mg are lost in an average ratio of 3:2, as shown by chemical composition of both solids and interstitial waters. However, an unresolved discrepancy occurs between the rate of leaching, as calculated from solids and that calculated from solution data, when an age of 10,000 yrs is taken for the upper part of the Pahala Ash. Ion-activity products calculated from the solution analyses provide a framework for elucidation of the weathering processes and products.

Geochemical evolution of brines in the Salar of Uyuni, Bolivia

Recent analyses of brines from the Salars of Uyuni and Coipasa have been compared with published chemical data for Lakes Titicaca and Poopo in order to evaluate solute compositional trends in these remnants of two large Pleistocene lakes once connected by overflow from the northern to the southern part of the Bolivian Altiplano. From Titicaca to Poopo the water chemistry shows an increase in concentration of Cl and Na somewhat greater than the total solutes, suggesting preferential inputs of these elements or significant losses of other constitutents downstream from Lake Titicaca proper. In contrast, Ca and SO4 increase to a lesser extent than do total dissolved solids, and carbonate species are relatively constant, suggesting solute losses most readily related to the precipitation of calcite and gypsum. Between Poopo and Coipasa the proportions of Ca, SO4 and CO3 continue to decrease. At Coipasa and Uyuni, the great salars frequently evaporate to halite saturation and widespread halite crystallization is accompanied by an increased proportion of K, Mg and SO4 in residual brines. Notably high concentrations of Li and B in brines from the south end of Uyuni, near the mouth of the Rio Grande de Lipez, are clearly in excess of that expected for the solute concentration trends for inflow from Coipasa and the northern Bolivia Altiplano, as shown by comparison with Mg and K. This supports other indications that the major source of Li and B are the recent rhyolitic volcanic rocks prevalent in the drainage of the Rio Grande.

Gels Composed of Sodium-Aluminium Silicate, Lake Magadi, Kenya

Sodium-aluminum silicate gels are found in surficial deposits as thick as 5 centimeters in the Magadi area of Kenya. Chemical data indicate they are formed by the interaction of hot alkaline springwaters (67� to 82�C; pH, about 9) with alkali trachyte flows and their detritus, rather than by direct precipitation. In the process, Na2O is added from and silica is released to the saline waters of the springs. Algal mats protect the gels from erosion and act as thermal insulators. The gels are probably yearly accumulates that are washed into the lakes during floods. Crystallization of these gels in the laboratory yields analcite; this fact suggests that some analcite beds in lacustrine deposits may have formed from gels. Textural evidence indicates that cherts of rocks of the Pleistocene chert series in the Magadi area may have formed from soft sodium silicate gels. Similar gels may have acted as substrates for the accumulation and preservation of prebiological organic matter during the Precambrian.