Nathan R. Miller

Iodide in CTAB prevents gold nanorod formation.

The gold nanocrystal seed-mediated approach using cetyltrimethylammonium bromide (CTAB) as a stabilizing surfactant is commonly used to make large quantities of monodisperse gold nanorods. This method, however, has been at times difficult to reproduce in different laboratories. We recently showed [Smith, D. K.; Korgel, B. A. Langmuir 2008, 24, 644-649] that a very low concentration impurity in CTAB obtained from some suppliers prevents nanorod growth but were not able to identify the impurity. Here, we report that the impurity is iodide. Inductively coupled plasma mass spectroscopy (ICP-MS) revealed that iodide concentrations vary in CTAB from different suppliers, from less than 2.75 ppm up to 840 ppm. When CTAB with iodide concentrations greater than 50 ppm is used, nanorods do not form and the product consists entirely of spherical nanocrystals. Iodide slows the reduction of Au(III) to Au0. Iodide adsorption on Au {111} surfaces inhibits nanorod growth.

A Late Neoproterozoic (∼630 Ma) high-magnesium andesite suite from southern Israel: implications for the consolidation of Gondwanaland

Abstract The East African Orogen formed as a result of collision between portions of East and West Gondwanaland as the Mozambique Ocean closed in Late Neoproterozoic time, but it is not known exactly when. We use distinctive chemical and isotopic composition of deformed ‘schistose’ dykes in southern Israel to argue that this collision occurred after about 630 Ma, when the dykes were emplaced. These magmas had compositions of basaltic andesites and andesites but had high Mg# (100 Mg/Mg+Fe; 55–70 ppm), Ni (70–240 ppm), and Cr (100–400 ppm) indicating that the most primitive samples were in equilibrium with mantle peridotite; evolved samples suffered modest fractionation. The schistose dykes are a medium-K, calc-alkaline suite, strongly enriched in light rare earth elements and depleted in heavy rare earth elements. They are high-magnesium andesites and are similar to low-Ca type 2 boninites; similar magmas today only form over active subduction zones. The schistose dykes have non-radiogenic initial 87 Sr/ 86 Sr (0.7026–0.7033) and radiogenic 143 Nd/ 144 Nd, with ϵ Nd (630 Ma) of +2.7 to +4.9 and T DM =0.77–0.94 Ga. Hf isotopic compositions ( ϵ Hf (630 Ma)=+6.8 to +8.8) confirm the juvenile nature of these magmas. The dykes are closely related to a nearby quartz diorite, although it is not clear whether the dykes represent magma that fed into a magma body now filled with the quartz diorite, or issued from it. The generation of high-Mg andesite magma at 630 Ma involved reactive porous flow of a slab-derived melt through the mantle, requiring an active subduction zone, and strongly suggesting that young, hot seafloor – perhaps the spreading ridge of the Mozambique Ocean – was subducted. This indicates that collision between components of E. and W. Gondwana to form the East African Orogen must have occurred more recently than 630 Ma.

From Volcanic Winter to Snowball Earth: An Alternative Explanation for Neoproterozoic Biosphere Stress

The ~450 million years of Neoproterozoic time (1000–542 Ma) was a remarkable episode of change in the Earth system and the biosphere. Here we develop and explore the hypothesis that explosive volcanism was at least partly responsible for Neoproterozoic climate change, synopsized as the “Volcanic winter to snowball Earth” (VW2SE) hypothesis. We review how climate cools as a result of sulfuric acid aerosols injected into the stratosphere by violent volcanic eruptions. A protracted increase in explosive volcanism could disrupt Earth’s radiative balance by continuously injecting sulfur aerosols into the stratosphere, causing cooling that could lead to glaciation. This mechanism would be especially effective when acting in concert with other agents for cooling. We show that the global Neoproterozoic magmatic flux was intense, so that explosive volcanism episodicly had a major effect on climate. Neoproterozoic volcanism and glacial activity happened about the same times in the Cryogenian and Ediacaran periods with no glaciation and reduced igneous activity in the Tonian Period. Glaciation followed soon after igneous activity increased as the supercontinent Rodinia broke apart, suggesting a causal relationship. The tectonic setting of climate-controlling explosive volcanism changed systematically over the Neoproterozoic supercontinent cycle, from extension-related early to arc-related late. Marinoan (~635 Ma) glaciation in particular corresponds to a peak time of subduction-related igneous activity in the Arabian-Nubian Shield and the East African Orogen. Isotopic chemostratigraphies are generally consistent with VW2SE hypothesis. These observations cumulatively support the VW2SE hypothesis as a viable explanation for what solid Earth processes caused Neoproterozoic climate oscillations.

Spherulites as in-situ recorders of thermal history in lava flows

Spherulites in rhyolitic obsidian provide a record of the thermal history of their host lava during the interval of spherulite growth. We use trace element concentration profiles across spherulites and into the obsidian host from Yellowstone National Park (USA) to reconstruct the conditions that existed during spherulite formation. The measured transects reveal three behaviors: expulsion of the most diffusively mobile elements from spherulites with no concentration gradients in the surrounding glass (type 1); enrichment of slower-diffusing elements around spherulites, with concentration gradients extending outward into the glass (type 2); and complete entrapment of the slowest-diffusing elements by the spherulite (type 3). We compare the concentration profiles, measured by laser ablation–inductively coupled plasma–mass spectrometry and Fourier transform infrared spectroscopy, to the output of a spherulite growth model that incorporates known diffusion parameters, the temperature interval of spherulite growth, the cooling rate of the lava, and data on the temporal evolution of spherulite radius. Our results constrain spherulite nucleation to the temperature interval 700–550 °C and spherulite growth to 700–400 °C in a portion of lava that cooled at 10 –5.2 ± 0.3 °C s –1 , which matches an independent experimental estimate of 10 –5.3 °C s –1 measured using differential scanning calorimetry. Maximum spherulite growth rates at nucleation are on the order of 1 μm hr –1 and are inferred to decrease exponentially with time. Hence, spherulites may serve as valuable in-situ recorders of the thermal history of lava flows.

Chapter 21 The Tambien Group, Northern Ethiopia (Tigre)

Abstract The Tambien Group of northern Ethiopia (Tigre), with probable correlatives in Eritrea, is a 2–3-km-thick siliciclastic–carbonate succession that was deposited in an intra-oceanic arc platform setting within the southern Arabian–Nubian Shield (ANS) area (southern extension of the Nakfa Terrane) of the Mozambique Ocean. Its deposition occurred prior to ocean closure between converging fragments of East and West Gondwana and concomitant structural emergence of the East African Orogen (EAO). The Tambien Group is well exposed and best studied in the Mai Kenetal and Negash synclinoria, where litho- and chemostratigraphy (including δ13Ccarb, 87Sr/86Sr) provide the basis for a composite reference section. Two glaciogenic intervals have been suggested from exposures within the Didikama and Matheos Formation in the Negash Synclinorium. No reliable palaeomagnetic data exist to constrain the palaeolatitude of Tambien Group deposition and the southern ANS, but palaeogeographic reconstructions and evaporite pseudomorphs in lower carbonate units (Didikama Formation) imply low to intermediate latitudes (<45°). Integration of available geochronological information (regional magmatism and detrital zircon) suggests c. 775–660 Ma as a plausible window constraining deposition of the prospective glacial intervals. The Tambien Group appears to preserve a coherent chemostratigraphic framework that can be effectively subdivided according to shifts in δ13Ccarb polarity [polarity intervals A (+), B (–), C (+), D (–)]. Slates underlying and interstratified with polarity interval A carbonate preserve evidence of extreme chemical weathering that lessened prior to deposition of polarity interval B carbonate. Tambien Group carbonate units have sedimentological characteristics consistent with both shallow and deeper marine depositional settings. The lower prospective glacial interval lacks diagnostic sedimentological evidence of synglacial deposition, but is overlain by negative δ13C carbonate (polarity interval B) with sedimentological characteristics consistent with well-documented cap-carbonate successions. The upper prospective glacial interval in the Negash Synclinorium (Matheos Diamictite) best exhibits characteristics consistent with glaciogenic deposition (matrix-supported polymictic clasts, possible dropstones, possible bullet-nosed and striated clasts). In contrast to pericratonic rift margin settings that are common for Cryogenian glaciogenic deposits, palaeogeographic reconstructions for the 775–660 Ma timeframe place northern Ethiopia within an intra-oceanic setting that was likely far removed from cratonic hinterlands. More work on Tambien Group sedimentology, geochronology and palaeogeography is required to better evaluate the extent and timing of glacial conditions associated with the prospective glaciogenic intervals. Supplementary material: Supplementary Table 21.1 of Tambien Group geochronological age constraints is available at http://www.geolsoc.org.uk/SUP18462.

Spatiotemporal Stability of Trace and Minor Elemental Signatures in Early Larval Shell of the Northern Quahog (Hard Clam) Mercenaria mercenaria

ABSTRACT The potential of trace and minor elements within biominerals to track the larval dispersal of bivalves was investigated by examining elemental composition in early larval shell of the northern quahog (hard clam) Mercenaria mercenaria. Larvae were cultured in three shellfish hatcheries using the adjacent estuarine waters of the southern Delmarva Peninsula in Virginia. Spatial distinction (∼1–50 km) and temporal stability (triweekly) of elemental concentrations was assessed using inductively coupled plasma mass spectrometry. Seventeen minor and trace elements were present at detectable levels in all shell samples: Ca, Mg, Ti, Co, Ni, Zn, Se, Rb, Al, V, Cr, Mn, Cu, Sr, Ba, Pb, and U. Discriminant function analyses using metal-to-Ca ratios as independent variables assigned hard clams to their hatchery of origin correctly, with 100% success. The ratio Cr:Ca proved to be the most effective discriminator, explaining 78.1% of among-group variance. Elemental concentrations within early larval shell also differed temporally. Discriminant function analysis classified individual spawning events with 100% success, with Al:Ca explaining the bulk of among-group variance (81.4%). Despite temporal variability of elements within larval shell, it was possible to resolve elemental signals spatially among hatcheries regardless of spawning date. These results demonstrate for the first time that the chemical composition of hard clam larval shell records spatial elemental signatures with the potential to trace the environment of natal origin as well as subsequent dispersal trajectories of this economically important species.

Ion microprobe 208Th-208Pb ages from high common Pb monazite, Morefield Mine, Amelia County, Virginia: Implications for Alleghanian tectonics

Monazite [(Ce,Th)PO4] from a pegmatite in the Morefield Mine of the eastern Piedmont of central Virginia has unusually high and variable amounts of common Pb, leading to problematic interpretations of its U-Th-Pb ages and how the monazite relates to nearby granite intrusions and faults. To address these issues, we analyze a single large monazite grain from the pegmatite using electron microprobe analysis (EMPA, n = 64), laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS, n = 58), and secondary ion mass spectrometry (SIMS, n = 59). The monazite study grain exhibits compositional variations in proximity to microcracks consistent regions of secondary alteration and recrystallization. Although the compositions of these regions fit the ideal stoichiometry of monazite, they have lower Si, Th, U, and Y, and higher P, rare earth element (REE), and Ca concentrations compared to visibly unaltered portions of the grain. LA-ICP-MS and SIMS analyses demonstrate that common Pb, as proxied by 204Pb, is enriched in proximity to microcrack regions and correlates with 137Ba. SIMS 232Th-208Pb analysis from grain regions with lowest contents of common Pb (208Pb comprises >99% of Pb isotopes) yields two sets of ages: 263.5±3.0 Ma (±1σ; MSWD = 1.7; n = 11) and 234.1±3.3 Ma (±1σ; MSWD = 0.4; n = 13). Regionally, the ages are similar to the youngest Appalachian pegmatite bodies emplaced during the terminal (Alleghanian) Laurentia-Africa collision. However, the monazite ages are younger than locally surrounding intrusions. The closest intrusive in distance (∼30 km) and age is the Petersburg granite (296.33±0.11 Ma, zircon 238U-206Pb), the emplacement of which coincided with activity along the Hylas Fault. Because the fault experienced a brittle-ductile transition in the Late Permian (∼262 Ma), coeval with the older 232Th-208Pb monazite age group, we postulate that the Hylas Fault may have operated as a fluid migration system between the Petersburg granite and Morefield Mine pegmatite. The younger monazite age coincides with Triassic normal and/or sinistral faulting linked to the development and deformation of local rift basins.

The interplay between ceria particle size, reducibility, and ethanol oxidation activity of ceria-supported gold catalysts

The structure of a support material can have profound impacts on the behavior of a catalyst, altering the activity and selectivity of chemical reactions. In this article, we investigate the influence of the support materials structure on the activity of Au/CeO2 catalysts for selective oxidation of ethanol in a fixed-bed flow reactor. By doping the ceria support with Al, La, and Zr during synthesis and by altering the temperature of pretreatment in air after synthesis, ceria particles varying in size between 3 nm and 22 nm were prepared. The smaller ceria particles exhibited higher oxygen storage capacities as determined by temperature programmed reduction testing and resulted in more active catalysts for ethanol oxidation. We note a linear correlation between oxygen storage capacity and catalytic activity for ethanol oxidation.

Effects of sample cleaning and storage on the elemental composition of shark vertebrae

RATIONALE Application of vertebral chemistry in elasmobranchs has the potential to progress our understanding of individual migration patterns and population dynamics. However, the influence of handling artifacts such as sample cleaning and storage on vertebral chemistry is unclear and requires experimental investigation. METHODS Vertebrae centra from blacktip sharks (Carcharhinus limbatus) were cleaned with bleach (NaOCl) for 5 minutes (min), 1 hour (h) and 24 (h) in a cleaning experiment and stored frozen, in 70% ethanol, and 10% formalin treatments for 20 days in a storage experiment. Element concentrations (Li, Na, Mg, Mn, Cu, Zn, Sr, Ba, Pb) were quantified in the outer edges of vertebrae centra using laser ablation inductively coupled plasma mass spectrometry and the [element:Ca] molar ratios were compared among treatments and individual sharks. RESULTS Bleach cleaning significantly increased [Na:Ca] and formalin storage decreased [Na:Ca] and [Mg:Ca], but ethanol storage did not affect any [element:Ca] ratios. Vertebrae edge [Sr:Ca], [Ba:Ca] and [Mn:Ca] varied among individual sharks, potentially reflecting different environments that they had previously inhabited. CONCLUSIONS This study shows how archiving methods for vertebrae cartilage can affect primary element:Ca compositions. We demonstrate greatest element:Ca stabilities for vertebrae with limited bleach exposure that are either stored in ethanol or frozen, supporting the use of comparably archived sample sets in future elemental studies.