Donald D. Hickmott

Matrix effects in the detection of Pb and Ba in soils using laser-induced breakdown spectroscopy

With the use of laser-induced breakdown spectroscopy (LIBS), the effects of chemical speciation and matrix composition on Pb and Ba measurements have been investigated by using sand and soil matrices. A cylindrical lens was used to focus the laser pulses on the samples because it yielded higher measurement precision than a spherical lens for the experimental conditions used here. The detection limits for Pb and Ba spiked in a sand matrix were 17 and 76 ppm (w/w), respectively. In spiked soil, the detection limits were 57 and 42 ppm (w/w) for Pb and Ba, respectively. Measurement precision for five replicate measurements was typically 10% RSD or less. Two factors were found to influence emissions from Pb and Ba present in sand and soil matrices as crystalline compounds: (1) compound speciation, where Ba emission intensities varied in the order carbonate > oxide > sulfate > chloride > nitrate, and where Pb emission intensities varied in the order oxide > carbonate > chloride > sulfate > nitrate; and (2) the composition of the bulk sample matrix. Emissions from Ba(II) correlated inversely with the plasma electron density, which in turn was dependent upon the percent sand in a sand/soil mixture. The analytical results obtained here show that a field-screening instrument based on LIBS would be useful for the initial screening of soils contaminated with Pb and Ba.

P-T paths from garnet zoning: A new technique for deciphering tectonic processes in crystalline terranes

A procedure to calculate quantitative rock pressure-temperature ( P-T ) paths based on chemical zoning profiles in garnet, combined with mineral chemistry from the other minerals in the assemblage, has been applied to samples from diverse tectonic settings with the result that the calculated P-T path is shown to be a sensitive monitor of tectonic processes. Terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the “prograde” P-T path and cooling during decompression in the “retrograde” path. In nappe terranes where hot rocks are emplaced over cool rocks, the upper plate shows cooling during decompression, whereas the lower plate shows heating during compression. In nappe terranes where cool rocks are emplaced over hot, the lower plate shows cooling during compression. Calculation of P-T paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes.

Trace-element zoning in a metamorphic garnet

Trace-element zoning has been measured in an amphibolite garnet from the Tauern window, Austria, by using an ion microprobe. Humps in the zoning profiles of Na, Sc, V, Y, and the heavy rare-earth elements mark a period of open-system behavior. These humps correspond to a part of the major-element zoning profile that is interpreted as a P-T reversal. The source of the mass excesses of these elements remains ambiguous: they were derived either on a thin-section scale by the breakdown of trace-element-enriched refractory minerals or externally from unusual trace-element-enriched fluids. P-T paths determined from garnet zoning may require modification if open-system behavior is important during garnet growth.

Metamorphic consequences of thrust emplacement, Fall Mountain, New Hampshire

Metamorphic pressure-temperature (P-T) paths from the upper and lower plates of the Fall Mountain nappe, southwest New Hampshire, reveal different thermal histories in the two structural levels. Upper-plate rocks experienced early low-P, high-T (contact?) metamorphism to peak P-T conditions of 700-750 °C, 3-4.5 kbar. These P-T conditions were followed by loading to approximately 5-6 kbar, and then by nearly isobaric cooling to approximately 500 °C, 4.5 kbar. Upper-plate rocks then experienced minor heating with unloading and finally cooling. The lower-plate rocks also experienced early low-P, high-T (contact?) metamorphism but only to P-T conditions of 480-510 °C, 2-3.5 kbar. Nappe emplacement resulted in nearly isothermal loading to 500 °C, 5-6 kbar, which was followed by either heating or cooling, depending on the proximity of the sample to the upper plate. The early pressures recorded in both the upper- and lower-plate rocks suggest a depth of approximately 10 km for origination of the nappe; the post-nappe pressures (5-6 kbar) imply a total tectonic thickness of approximately 20 km or a doubling of the overburden thickness. Diffusion calculations suggest that the emplacement of the nappe took less than approximately 10 m.y., and U/Pb chronology on zircon suggests that it occurred between 400 and 410 Ma. 4O Ar/ 39 Ar cooling ages on micas indicate that the terrane was cooled to approximately 300 °C by 340 Ma.

Trace element zoning in garnet from the Kwoiek Area, British Columbia: disequilibrium partitioning during garnet growth?

Trace element zoning in garnets from two contact-metamorphosed rocks from the Kwoiek area, British Columbia (Hollister 1969a), was measured with an ion microprobe. Zoning profiles have three distinct parts with chemical breaks defined by co-variation of major and trace elements. Important features of the trace element zoning profiles are: (1) roughly ‘bell-shaped’ zoning profiles for Y and the HREEs, (2) an abrupt increase in Ti at a point midway through each garnet with inflections in the zoning profiles of other elements (Li, Na, Cr, V, Y, Zr, and the HREE), and (3) irregular Cr and V profiles. Unlike Mn zoning, the zoning profiles of most other trace elements cannot be easily modeled using simple Rayleigh fractionation models. Ti activity in the two samples is buffered by phase relations with ilmenite. Garnets from a continuously heated contact metamorphic environment should display continuous Ti zoning profiles if equilibrium was maintained and provided the Ti buffering assemblage did not change during garnet growth. The irregular Ti profiles suggest disequilibrium behavior. Several elements (Cr, V) may indicate breakdown of a phase enriched in trace elements during metamorphism. The source for the mass excess of these elements is probably the refractory cores of ilmenite grains. Either differing matrix transport rates of trace lements or interface kinetic controlled segregation could explain the unusual trace element behavior at the element inflection point. The preferred explanation involves segregation of elements at the interface of the garnet that were trapped during episodes of rapid garnet growth.

Trace-element partition coefficients for ilmenite, orthopyroxene and pyrrhotite in rhyolite determined by micro-PIXE analysis

Abstract Ilmenite is a common accessory mineral in igneous and metamorphic rocks, yet few data are available on its partitioning behavior. We present in situ proton-induced X-ray emission (PIXE) microanalytical data for ilmenite (Ilm), orthopyroxene (Opx), pyrrhotite (Po) and coexisting silicic glass in a rhyolite lava from Clear Lake, California. The rhyolite contains grains of Ilm (Xilm=0.96–1.00) associated with Opx and Plag phenocrysts. Po occurs as inclusions in Ilm and Opx, and as rare single grains. Ilmenite concentrates Nb (D = 51–71), Ta (64–85), Zn (10–11), Mo (> 5) and Zr (1.0–1.4). Opx concentrates Ni (D=∼ 11–25) and Zn (14–16). Po concentrates Mo (D=∼ 161), As (2–3), Se (∼ 36–44), Pb (2), Co (∼ 999), Cu (∼ 459–502) and Ni (∼ 1842) relative to silicic melt, but these coefficients may reflect liquid/liquid rather than solid/liquid partitioning. Ilmenite is the dominant oxide in Clear Lake volcanic rocks, occurring in silicic to intermediate lavas and metasedimentary xenoliths. Silicic volcanic rocks of the area formed by interaction of mafic magmas with deep crustal rocks. The dominance of ilmenite over magnetite reflects the bulk composition and low fO2 of a metasedimentary source component. The relative abundance of ilmenite in such systems and its high DNb- and DTa-values suggest that it will strongly influence Nb and Ta concentrations during partial melting of crustal rocks and during crystal fractionation. Similarly Po will exert an important influence on the distribution of chalcophile and siderophile elements. Ilm and Po must be considered in quantitative fractionation models involving HFSE and metals, even if present in trace abundances.

A Porous Metal−Organic Replica of α-PbO2 for Capture of Nerve Agent Surrogate

A novel metal-organic replica of α-PbO(2) exhibits high capacity for capture of nerve agent surrogate.

Metasomatism in a subduction complex: Constraints from microanalysis of trace elements in minerals from garnet amphibolite from the Catalina Schist

Trace element abundances and zoning were measured in minerals from a metasomatized garnet-amphibolite block from the Catalina Schist, using both ion and proton microprobes. Zoisite strongly concentrates light rare earth elements (REEs), Sr, Y, and Pb; amphibole concentrates Ni and Zn; garnet concentrates Y and heavy REEs; and titanite concentrates Nb. Major and trace elements in the garnets are zoned. Garnets in the core of the block display an overgrowth enriched in Mn, Y, and heavy REE, on a Y- and heavy-REE-poor core. Zirconium values remain relatively constant. The element enrichments in the garnet overgrowth suggest mobility of REEs at either a hand-sample or regional scale at subduction-zone pressure-temperature conditions. Metamorphic fluids may selectively transport heavy REEs relative to some high field strength elements in some convergent-margin settings. The distribution of Sr and Pb within subduction zones may reflect the dehydration and melting behavior of epidote-group minerals.

Redistribution of Pb and other volatile trace metals during eruption, devitrification, and vapor-phase crystallization of the Bandelier Tuff, New Mexico

A diverse suite of micron-scale minerals was deposited from vapor during eruption and post-emplacement crystallization of the Bandelier Tuff, New Mexico. The mineral suite is rich in sulfides, oxides, and chlorides of both common and rare metals (e.g., Fe, Pb, Bi, Cu, Ag, Re), and oxides and silicates of incompatible elements (e.g., P, Zr, Y, Nb, Ba and LREE). Minerals preserved in glassy samples grew from magmatic vapor trapped during emplacement, or from vapor migrating along contacts with more impermeable rocks; minerals observed in devitrified samples also grew from crystallization of glass and vapor liberated during this process. In devitrified samples, mafic silicate phenocrysts were partially replaced by an assemblage dominated by smectite and hematite. The syn- to post-eruptive mineral assemblage observed in upper Bandelier Tuff (UBT) samples bears striking similarity to those deposited by cooling gases near active volcanic vents. However, several differences exist: (1) the mineral suite in the UBT is disseminated throughout the unit, and formed over a broad temperature range (> 700 to < 150 °C) at higher rock:gas ratios; (2) the highly evolved composition of the UBT yielded a greater abundance of minerals rich in incompatible elements compared to sublimates from less evolved volcanoes; and (3) the UBT has suffered over 1 million years of post-emplacement exposure, which resulted in solution (or local re-precipitation in fractures) of soluble compounds such as halite, sylvite, and gypsum. Pb was enriched toward the roof of the UBT magma body due to its affinity for the melt and vapor phases relative to crystals (Bulk Dpb < 0.2). Micron-scale Pb minerals appear to have grown from vapor exsolved during eruption, as well as vapor liberated during later devitrification. Additional Pb was scavenged by smectite and hematite that probably formed during the later stages of the devitrification and cooling process. Up to ten-fold increases in Pb concentrations are seen in zones of fumarolic concentration in the UBT, however, most bulk tuff samples have Pb values that appear to preserve magmatic values, indicating only very local trace-metal redistribution. The concentration of Pb and other heavy metals in micron-scale mineral coatings in porous tuff indicates that these metals could be readily mobilized and transported by acidic groundwaters or hydrothermal fluids, and thus locally concentrated into ore-grade deposits in long-lived systems.

High-pressure/low-temperature neutron scattering of gas inclusion compounds: Progress and prospects

Alternative energy resources such as hydrogen and methane gases are becoming increasingly important for the future economy. A major challenge for using hydrogen is to develop suitable materials to store it under a variety of conditions, which requires systematic studies of the structures, stability, and kinetics of various hydrogen-storing compounds. Neutron scattering is particularly useful for these studies. We have developed high-pressure/low-temperature gas/fluid cells in conjunction with neutron diffraction and inelastic neutron scattering instruments allowing in situ and real-time examination of gas uptake/release processes. We studied the formation of methane and hydrogen clathrates, a group of inclusion compounds consisting of frameworks of hydrogen-bonded H2O molecules with gas molecules trapped inside the cages. Our results reveal that clathrate can store up to four hydrogen molecules in each of its large cages with an intermolecular H2–H2 distance of only 2.93 Å. This distance is much shorter than that in the solid/metallic hydrogen (3.78 Å), suggesting a strong densification effect of the clathrate framework on the enclosed hydrogen molecules. The framework-pressurizing effect is striking and may exist in other inclusion compounds such as metal-organic frameworks (MOFs). Owing to the enormous variety and flexibility of their frameworks, inclusion compounds may offer superior properties for storage of hydrogen and/or hydrogen-rich molecules, relative to other types of compounds. We have investigated the hydrogen storage properties of two MOFs, Cu3[Co(CN)6]2 and Cu3(BTC)2 (BTC = benzenetricarboxylate), and our preliminary results demonstrate that the developed neutron-scattering techniques are equally well suited for studying MOFs and other inclusion compounds.