Diffusion of helium isotopes in silicate glasses and minerals : implications for petrogenesis and geochronology

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April 1989Helium mobility in geologic materials is a fundamental constraint on the petrogenetic origins of helium isotopic variability and on the application of radiogenic and cosmogenic helium geochronology. 3He and 4He volume diffusivities determined at 25-600°C in basaltic glasses by incremental-heating and powder storage experiments (using a diffusion model incorporating grain size and shape information to obtain high precision) are three to four orders of magnitude greater than for common cations. Diffusion in tholeiitic glass can be described by an Arrhenius relation with activation energy = 16.85±.13 Kcal/mole and log Do = -2.37±.06, although low temperature data are better described by a distribution of activation energies model . The best estimate for D at 0°C in tholeiitic glass is 5±2 x 10-16 cm2/s, an order of magnitude higher than the results of Kurz and Jenkins (1981) but lower than suggested by Jambon, Weber and Begemann (1985). Measurements in an alkali basalt show that helium diffusion is composition dependent (Ea = 14.4±.5 Kcal/mole; log Do = 3.24±.2), and roughly five times faster than in tholeiites at seafloor temperatures. The corresponding timescales for 50% helium loss or exchange with seawater (1 cm spheres) are about one million years for mid-ocean-ridge- basalts, and about 100,000 years in seamount alkali basalts. Radiogenic 4He diffusion has a higher activation energy (27±2 Kcal/mole; log Do = +2.4±1.0) than inherited (magmatic) helium, suggesting very low mobility (D = 3xl0-19 cm2/s at 0°C; factor of 5 uncertainty) and that U+Th/4He geochronology of fresh seafloor basalt glasses is unlikely to be hampered by helium loss. Measured isotopic diffusivity ratios, D3He/D4He, are not composition dependent, average 1.08±.02, and vary slightly with temperature, consistent with an activation energy difference of 60±20 cal/mole. This result differs from the inverse-square-root of mass prediction of 1.15, and may be explained by quantization of helium vibrational energies. These results suggest preferential loss of 3He will be minimal at low temperature (D3He/D4He = 1.02± .03 at 0°C). Therefore, alteration of magmatic 3He/4He ratios in basaltic glasses on the seafloor will occur only by helium exchange with seawater, and be important only for samples with low helium contents (<10-8 ccSTP/g), such as those found in island arc environments. Extrapolating the glass results to magmatic temperatures yields diffusivities similar to melt values, and suggests D3He/D4He approaches 1.15 at these and higher temperatures. Helium diffusivities in olivine and pyroxene at magmatic and mantle temperatures (900-1400°C) are higher than for cations, (E = 100±5 Kcal/ mole, log Do = +5.1±.7; and 70±10 Kcal/mole, log Do = +2.1±1.2, respectively), but are still too low to transport or homogenize helium in the mantle or even in magma chambers. However, diffusion equilibrates melts and mantle minerals within decades, and interaction with wall-rocks may be enhanced for helium in comparison to other isotopic tracers because of its greater mobility. Rapid exchange of helium within xenoliths and with their host magmas set limits on origin depths and transport times for xenoliths which exhibit helium isotopic disequilibrium between minerals, or between the magma and the xenolith. Phenocrysts equilibrate helium too rapidly to exhibit zoned isotopic compositions, and are likely to retain magmatic helium quantitatively in rapidly cooled volcanic extrusives. The 100-fold higher He diffusivity in pyroxene than olivine at 1000°C allows diffusive loss effects to be evaluated in more slowly cooled rocks, when cogenetic minerals can be measured. Diffusivities of cosmic- ray produced 3He in surface exposed rocks are several orders of magnitude higher than for inherited helium. However, activation energies for olivine and quartz, 25±4 Kcal/mole (log Do = 3.7±.8) and 25.2±.9 Kcal/mole (log Do = +.2±.4) respectively, still suggest low diffusivities at surface temperatures of approximately 10-22 and 10-20 cm2/s. Equations for simultaneous helium production and diffusive loss allow model ages for surface exposure to be corrected for helium loss, and demonstrate that cosmogenic 3He geochronology will not be limited by helium loss for timescales of approximately 1 million years in quartz and 10 million years or more in olivine. The measurements also suggest that radiogenic 4He produced by U and Th decay may be a useful dating method in quartz. Application of the diffusion measurements demonstrates that part of the wide range of 3He/4He ratios (.01 to 9 Ra) of a suite of dredged basalts and andesites from the Woodlark Basin, (western Pacific) reflects post-eruptive helium addition, from seawater in glasses with low He contents and from U and Th decay in mafic mineral separates. In unaltered samples, 3He/4He ratios for tholeiites from the Woodlark Spreading Center are 8-9 Ra, similar to mid-ocean-ridges, but distinctly different than the ratio of 6.9±.2 Ra observed in Kavachi submarine volcano basaltic andesites. Helium isotopic systematics in cogenetic pyroxenes and olivines from these samples demonstrate that this is a magmatic signature, and not the result of preferential 3He loss by diffusion. Coupled Sr and He isotopic systematics in these and other samples from the region suggest the sub-arc mantle has been enriched in radiogenic helium supplied by subducted Pacific lithosphere.Thanks to the Chemistry Department, and the WHOI Education Office for providing financial support and a nice place to work. Parts of this research was funded by NSF grants OCEBS-16082, EAR86-l06ll, and OCE87-16970

Comparing changes in fuel loading, tree regeneration, and forest structure in once- and twice-burned mixed-conifer forests with a before-after-control-impact case study in the Bob Marshall Wilderness

Wildfires drive landscape character in the seasonally dry mixed-conifer forests of western North America. Forested landscapes in this region are a mosaic of overlapping burn perimeters, which span a wide gradient of severity and burn age. The goal of this study was to compare the effects of single and repeat wildfires on fuel loading and forest structure and composition. Our study site spans the east and west sides of the South Fork of Flathead River in the Bob Marshall Wilderness. The east side of the river burned in 2000 in the Helen Creek Fire. The west side of the river burned in 2003 in the Little Salmon Complex. Data was collected in 2011. In 2013, the east side of the river burned again, but the west side of the river did not burn a second time. In 2015, plots on both the east and west side of the river were resampled. Between 2011 and 2015, mean coarse woody debris load (\u3e7.6 cm diameter) in twice-burned plots decreased by 23%, while once-burned plots increased by 76%. Total mean fine woody debris (\u3c7.6 cm diameter) decreased by 30% in twice-burned plots and increased by 80% in once-burned plots. These changes in woody debris are the net outcome of inputs from standing dead trees that fell between 2011 and 2015 (including branch fall) and outputs from combustion and decomposition. For both once- and twice-burned plots, the density of live trees changed very little between measurements, but the density of dead trees significantly decreased. The density of dead western larch saplings and seedlings tended to be greater on twice-burned plots. The once- or twice-burned variable has a strong effect on surface fuels and tree regeneration, but a weak effect on forest structure and composition. The results of this study suggest that shorter fire return intervals lead to lower surface fuel load and more fire-tolerant forest structure and composition

Diffusion of helium isotopes in silicate glasses and minerals : implications for petrogenesis and geochronology

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1989.Includes bibliographical references (leaves 194-199).by Thomas W. Trull.Ph.D

Introduction to Special Section: SAZ Project

Oceanographic processes in the subantarctic region contribute crucially to the phys. and biogeochemical aspects of the global climate system. To explore and quantify these contributions, the Antarctic Cooperative Research Center organized the SAZ Project, a multidisciplinary, multiship study carried out south of Australia in the austral summer of 1997-1998. We present an overview of the SAZ Project and some of its major results

Temporal changes in size distributions of the Southern Ocean diatom Fragilariopsis kerguelensis through high-throughput microscopy of sediment trap samples

Some aspects of the life cycle of the Southern Ocean diatom Fragilariopsis kerguelensis have been investigated previously, but many of its details have not been surveyed in nature. We investigated material from a two-year sediment trap time series by high-throughput imaging and image analysis, looking for morphometric signals of life cycle stages. Valve length distributions appeared close to unimodal but positively (right-) skewed. Size cohorts resulting from synchronized sexual reproduction events were not clearly distinguishable. Nevertheless, based on changes in valve length distributions, we found three general seasonal phases. These corresponded to periods of proliferation (with higher proportions of smaller cells during late spring/early summer), cessation of growth (relative loss of smaller cells during late summer/early autumn), and overwintering (little change in size distributions, with an increased proportion of large cells). We discuss possible causes of these signals, and their relevance to growth, sexual activity and adaption to environmental conditions, such as grazing pressures and the need for an overwintering strategy.This work was supported by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the priority programme 1158 ‘Antarctic Research with comparative investigations in Arctic ice areas’ under grant nr. BE4316/4-1, KA1655/3-1; and by an outgoing scholarship, as well as travel expenses, granted by the Helmholtz Graduate School for Polar and Marine Research (POLMAR). Part of this work was supported by the Australian Government’s Australian Antarctic Science Grant Program under project number 4078, and Macquarie University (A. RigualHernández and L. Armand)

Interprofessional Learning Readiness: Health Policy Summit

Purpose: Interprofessional Education (IPE) and Health Policy are important components in health professional curricula. Students from business, communication sciences and disorders, dietetics, occupational therapy, nursing, and social work participated in an innovative IPE event working in an IPE group to apply discipline specific knowledge and propose solutions to the Medicaid Expansion gap in Virginia. Students presented their final proposals to legislators while advocating for issues important to their discipline. Methodology/Results: This study used the Readiness for Interprofessional Learning Scale (RIPLS) to examine student Teamwork and Collaboration, Professional Identity, and Roles and Responsibilities following participation in a Health Policy Summit. The results revealed a difference at baseline between health professions students and business students (N= 260) in their perception of teamwork and collaboration between groups. The themes of the question items found to be significant within the scale pre- and posttest were student perception of learning with other health-care/professional students, shared learning to help students understand their limitations, and welcoming opportunities to work with IPE students. Conclusion: This data indicates that there remains an opportunity to promote student perceptions of their abilities to participate in teamwork, collaborate significantly, and to understand the scope of their discipline specific knowledge and contributions to a team

Thorium-234 as a tracer of spatial, temporal and vertical variability in particle flux in the North Pacific

Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 56 (2009):1143-1167, doi:10.1016/j.dsr.2009.04.001.An extensive 234Th data set was collected at two sites in the North Pacific: ALOHA, an oligotrophic site near Hawaii, and K2, a mesotrophic HNLC site in the NW Pacific as part of the VERTIGO (VERtical Transport in the Global Ocean) study. Total 234Th:238U activity ratios near 1.0 indicated low particle fluxes at ALOHA, while 234Th:238U ~0.6 in the euphotic zone at K2 indicated higher particle export. However, spatial variability was large at both sites- even greater than seasonal variability as reported in prior studies. This variability in space and time confounds the use of single profiles of 234Th for sediment trap calibration purposes. At K2, there was a decrease in export flux and increase in 234Th activities over time associated with the declining phase of a summer diatom bloom, which required the use of non-steady state models for flux predictions. This variability in space and time confounds the use of single profiles of 234Th for sediment trap calibration purposes. High vertical resolution profiles show narrow layers (20-30 m) of excess 234Th below the deep chlorophyll maximum at K2 associated with particle remineralization resulting in a decrease in flux at depth that may be missed with standard sampling for 234Th and/or with sediment traps. Also, the application of 234Th as POC flux tracer relies on accurate sampling of particulate POC/234Th ratios and here the ratio is similar on sinking particles and mid-sized particles collected by in-situ filtration (>10-50 μm at ALOHA and >5–350 μm at K2). To further address variability in particle fluxes at K2, a simple model of the drawdown of 234Th and nutrients is used to demonstrate that while coupled during export, their ratios in the water column will vary with time and depth after export. Overall these 234Th data provide a detailed view into particle flux and remineralization in the North Pacific over time and space scales that are varying over days to weeks, and 10’s to 100’s km at a resolution that is difficult to obtain with other methods.Funding for VERTIGO in the US was provided primarily by research grants from the US National Science Foundation Programs in Chemical and Biological Oceanography with additional support by the US Department of Energy (DAS). For TWT, support came from the Australian Cooperative Research Centres program

Coccolithophore biodiversity controls carbonate export in the Southern Ocean

Southern Ocean waters are projected to undergo profound changes in their physical and chemical properties in the coming decades. Coccolithophore blooms in the Southern Ocean are thought to account for a major fraction of the global marine calcium carbonate (CaCO3) production and export to the deep sea. Therefore, changes in the composition and abundance of Southern Ocean coccolithophore populations are likely to alter the marine carbon cycle, with feedbacks to the rate of global climate change. However, the contribution of coccolithophores to CaCO3 export in the Southern Ocean is uncertain, particularly in the circumpolar subantarctic zone that represents about half of the areal extent of the Southern Ocean and where coccolithophores are most abundant. Here, we present measurements of annual CaCO3 flux and quantitatively partition them amongst coccolithophore species and heterotrophic calcifiers at two sites representative of a large portion of the subantarctic zone. We find that coccolithophores account for a major fraction of the annual CaCO3 export, with the highest contributions in waters with low algal biomass accumulations. Notably, our analysis reveals that although Emiliania huxleyi is an important vector for CaCO3 export to the deep sea, less abundant but larger species account for most of the annual coccolithophore CaCO3 flux. This observation contrasts with the generally accepted notion that high particulate inorganic carbon accumulations during the austral summer in the subantarctic Southern Ocean are mainly caused by E. huxleyi blooms. It appears likely that the climate-induced migration of oceanic fronts will initially result in the poleward expansion of large coccolithophore species increasing CaCO3 production. However, subantarctic coccolithophore populations will eventually diminish as acidification overwhelms those changes. Overall, our analysis emphasizes the need for species-centred studies to improve our ability to project future changes in phytoplankton communities and their influence on marine biogeochemical cycles.info:eu-repo/semantics/publishedVersio

Control of Phytoplankton Growth by Iron Supply and Irradiance in the Subantarctic Southern Ocean: Experimental Results From the SAZ Project

The influence of irradiance and Fe supply on phytoplankton processes was studied, north (47°S, 142°E) and south (54°S, 142°E) of the Subantarctic Front in austral autumn (March 1998). At both sites, resident cells exhibited nutrient stress (Fv/Fm 0 at 47°S and 9% I0 at 54°S because of MLDs of 40 (47°S) and 90 m (54°S), when these stations were occupied. The greater MLD at 54°S is reflected by tenfold higher cellular chlorophyll a levels in the resident phytoplankton. In the 47°S experiment, chlorophyll a levels increased to \u3e1 μg/L-1 only in the high-Fe treatments, regardless of irradiance levels, suggesting Fe limitation. This trend was also noted for cell abundances, silica production, and carbon fixation rates. In contrast, in the 54°S experiment there were increases in chlorophyll a (to \u3e2 μg/L-1), cell abundances, silica production, and carbon fixation only in the high-light treatments to which Fe had been added, suggesting that Fe and irradiance limit algal growth rates. Irradiance by altering algal Fe quotas is a key determinant of algal growth rate at 54°S (when silicic acid levels are nonlimiting); however, because of the integral nature of Fe/light colimitation and the restricted nature of the current data set, it was not possible to ascertain the relative contributions of Fe and irradiance to the control of phytoplankton growth. On the basis of a climatology of summer mean MLD for subantarctic (SA) waters south of Australia the 47° and 54°S sites appear to represent minimum and maximum MLDs, where Fe and Fe/irradiance, respectively, may limit/colimit algal growth. The implications for changes in the factors limiting algal growth with season in SA waters are discussed

Parametric localized modes in quadratic nonlinear photonic structures

We analyze two-color spatially localized modes formed by parametrically coupled fundamental and second-harmonic fields excited at quadratic (or chi-2) nonlinear interfaces embedded into a linear layered structure --- a quasi-one-dimensional quadratic nonlinear photonic crystal. For a periodic lattice of nonlinear interfaces, we derive an effective discrete model for the amplitudes of the fundamental and second-harmonic waves at the interfaces (the so-called discrete chi-2 equations), and find, numerically and analytically, the spatially localized solutions --- discrete gap solitons. For a single nonlinear interface in a linear superlattice, we study the properties of two-color localized modes, and describe both similarities and differences with quadratic solitons in homogeneous media.Comment: 9 pages, 8 figure