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Thursday, April 17, 2014

The combination of the NanoSIMS-50L and the 7f-Geo at the Caltech Microanalysis Center provides powerful diverse capabilities in trace and isotope analysis, depth profiling, ion imaging and geochronology, with low detection limits, high spatial resolution, and/or high precisions. The following are some of the research projects that have been carried out at the center since the NanoSIMS-50L and the 7f-Geo were accepted in April and October 2007, respectively.

Cosmochemistry

Analysis of C and N in GENESIS targets:  Due to the excellent vacuum on our relatively new instruments, and our ability to quickly and freely iterate between new cleaning procedures and analytical sessions, C and N from solar wind have been first observed in GENESIS samples.  This work is being led by Don Burnett (a Professor of Geochemistry at Caltech).

Deuterium in meteoritic organic matter:  Primitive meteorites contain abundant, diverse assemblages of organic compounds. These compounds are both evidence for the chemical environments of the early solar system and potentially influenced the emergence of life on Earth and other terrestrial planets and moons.  We are using the nanoSIMS and ims-7f geo to study the distributions of Deuterium, Hydrogen, Carbon and Nitrogen in organic matter from diverse primitive meteorites, and the textural relationships between that organic matter and other constituents of these meteorites. This work is being led by Laurent Remusat (Postdoctoral Fellow at Caltech).

Search for oxygen isotope anomalies in Enstatite Chondrites:  Whole rocks, chondrules and matrix from the same groups of chondrites, in general, have the same or very similar oxygen isotopic compositions. Among three major clans of chondrites, enstatite chondrites have the most homogeneous oxygen isotopic compositions. Study of possible oxygen isotopic heterogeneity of enstatite chondrites in microscale has been carried out on the 7f-geo. This work is led by Byeon-Gak Choi (Seoul National University) and Jon Wasson (UCLA).

Short-lived radionuclides in the early solar system:  The first several million years of solar-system history were a busy time — this is the period when the proto-planetary disk accreted and differentiated to form the planets, and when those planets underwent their initial differentiation to form cores and crusts. Important chronological information of events during this period can be obtained by studying short-lived radionuclides (e.g., 26Al, 53Mn, 60Fe, and 36Cl) in primitive meteorites. This work is being led by Yunbin Guan (Staff Scientist and CMC Laboratory Manager).

Trace-element zonation in CAI’s:  Primitive meteorites contain Calcium-Aluminum rich inclusions (CAI’s) that appear to be products of condensation and/or residues of sublimation during the early, hot phase of solar system evolution. Each of these objects is in some sense unique, and each provides a distinctive record of the early solar system’s evolution. We have been studying the thermal history of these objects by using the nanoSIMS to determine the extent which diffusion has erased sharp (µm-scale) chemical boundaries between their fine-grained constituents.  This work is being led by Don Burnett (Professor of Geochemistry at Caltech).

Climate change

Experimental studies of trace-element partitioning between water and carbonates:  Carbonate minerals are the most commonly studied archives of information about past climate change. Among the most important information stored in these minerals comes from the abundances of trace elements (Sr, Ba, U, etc.), which reflect both the composition of the ocean at the time of mineral growth and the conditions of that growth (including, but not restricted to temperature). To-date, most work on these materials makes use of empirical calibrations of the temperature-dependent partitioning of elements between carbonates and seawater. We have undertaken an experimental study of the relative roles of temperature, growth rate and solution chemistry on partitioning between inorganic carbonates and their parent solutions; the results of this work will let us reconstruct improved records of past climate. The nanoSIMS is a critical enabling technology for this work because it allows us to measure carbonate mineral composition at the µm length scales of its growth banding. The lead scientists for this work are Jess Adkins (Professor of Geochemistry at Caltech) and Rinat Gabitov (Postdoctoral Scholar at Caltech).

Experimental studies of trace-element partitioning between cultured corals and seawater:  It is well established that biogenic carbonate minerals, such as corals or mollusk shells, differ systematically in composition from inorganic minerals grown under the same conditions; these differences are called vital effects and presumably reflect the mechanisms of biomineralization. We are interrogating vital effects on trace element partitioning between seawater and deep-sea corals by measuring the compositions of corals cultured in the laboratory under known and controlled conditions. This work complements the experiments on inorganic carbonates described above, and is similarly dependent on the use of the nanoSIMS to measure trace elements in solids on µm scales. The lead scientists for this work are Jess Adkins (Professor at Caltech) and Alex Gagnon (Graduate Student at Caltech).

Trace-element variations within deep-sea corals:  The large body of existing paleoclimatological work on the trace element contents of carbonate minerals is largely based on bulk measurements or relatively crude in-situ measurements at scales of several 10’s of µm. Biomineralization generally produces coherent layers or other compositional domains with characteristic dimensions much smaller than these analyses. We are using both the nanoSIMS and the ims-7f geo to determine trace-element compositions of natural deep-sea corals at scales down to fractions of a µm. This work is revealing the chemistry of individual biomineralization centers, in some cases down to the scales of individual cells.  This work is being led by Jess Adkins (Professor at Caltech) and Rinat Gabitov (Postdoctoral Scholar at Caltech).

Geobiology

In-situ study of microbial communities:  Single-celled organisms are the agents of geochemical processes of global significance, including the rise of atmospheric oxygen early in Earth history and the budget of CH4 in sediments on the modern sea floor (a potentially key resource of fossil fuels). However, it is difficult to study the activity of these organisms in their natural environments, particularly because some of their most important effects result from interactions among several members of complex microbial communities.  We are using the nanoSIMS to image the chemical and isotopic compositions of individual cells in communities of microbes grown in cultures that mimic natural environments but contain ‘labeled’ compounds to illuminate biogeochemical pathways (e.g., 13C-labeled CO2 to identify cells that fix inorganic carbon). This work is being led by Victoria Orphan (Professor of Geobiology at Caltech).

In-situ study of microbial sulfur cycles:  Sulfate is a major constituent of seawater, and sulfate reduction one of the most common and important microbial processes.  However, many of the organisms that perform sulfate reduction cannot live in oxidized environments and so must live in sediments, isolated from the oxygen-rich marine water column. Therefore, the tops of marine sedimentary columns contain dynamic gradients in sulfate and oxygen and finely layered microbial communities.  We are studying these complex environments by using the nanoSIMS and ims-7f geo to document µm-to-mm scale variations in the concentration and isotopic composition of sulfur dissolved in the pore waters of microbial mats (sediments in which microbial communities make up a large fraction of the mass). This work is led by Victoria Orphan (Professor at Caltech) and David Fike (Postdoctoral Fellow at Caltech).

Volcanic and Magmatic Processes

Thermal history of Hawaiian lavas:  The Hawaiian islands are the tips of some of the most dramatic volcanic features on earth — the island of Hawaii itself, measured from the floor of the Pacific ocean, is as tall as Mt. Everest and many times greater in volume. The interiors of such large volcanoes are poorly understood, and have been sampled in long, continuous section only once — by the Hawaiian Scientific Drilling Project core (a project that was co-led by Caltech Professor Ed Stolper). We used the nanoSIMS, in concert with instruments in the GPS division’s analytical facility to determine the compositional zonation of olivine crystals recovered from this core. These data constrain the thermal histories of the magmas that host these crystals, and thus the structure of the interior of the island of Hawaii. This project was led by Ed Stolper (Professor at Caltech) and Steven Chemtob (Graduate Student at Caltech).

Exploration of µm-scale melt inclusions: Igneous minerals commonly contain small inclusions or droplets of trapped melt. These melt inclusions often escape the processes of crystallization and degassing that modify the compositions of erupted magmas, and so they can provide a record of relatively primitive magmatic compositions and processes. Such records are useful for reconstructing the composition of the mantle and the mechanisms by which it melts. The vast majority of melt inclusions are of-order 1 µm in size — too small to be analyzed by conventional means. Therefore, little is known about them. We have been using the nanoSIMS to explore the major and minor element chemistry of these µm-scale inclusions and their chemical interactions with the minerals in which they are trapped. We believe this will lead to an improved understanding of the processes by which melt inclusions form, and thus an improved record of Earth’s most primitive magmas. This work is being led by John Eiler, Ed Stolper (Professors at Caltech) and Frances Mansfield (Graduate Student at Caltech). 

H diffusion in hydrous glasses:  Explosive volcanic eruptions are fueled by degassing of water dissolved in magmas. The details of how water exsolves from magma, collects to form vapor bubbles, and ultimately vents to the atmosphere dictate whether degassing will be gentle or violent. One of the simplest but most important steps in this process is diffusion of hydrogen through melt into growing vapor bubbles. We are using the ims-7f geo to characterize rates of H diffusion through synthetic glasses. The results of these experiments will be used to model natural magmatic degassing processes. This work is being led by Ed Stolper (Professor at Caltech) and Sally Newman (Staff Scientist at Caltech).

Chlorine in Earth’s mantle:  Chlorine is a trace constituent of the Earth as a whole, but over the course of geological history it has been strongly concentrated into the oceans and marine sediments. Reconstructing the rate at which this has occurred, and the counter-balancing processes by which Cl is returned to the earth’s interior by subduction, will provide insights into the origin and growth of the oceans. We are using the ims-7f geo to reconstruct the Cl contents of primitive terrestrial magmas by measuring Cl concentrations of melt inclusions trapped in igneous minerals. This work will provide insights into the distribution of Cl in the mantle today. It is being led by Magali Bonifacie and Laurent Remusat (both Postdoctoral Fellows at Caltech).

History of the Earth’s crust

Hadean zircons:  No rocks remain from the first half billion years of Earth history; in fact, the known rock record from this epoch is more complete for the Moon and Mars than it is for the Earth. This is because weathering, erosion and dynamic tectonic processes destroy or transform rocks in the earth’s crust faster than on these other planets. However, the mineral zircon, a common constituent of crustal rocks, is exceptionally resistant to these processes. In at least one place in Australia, zircon as old as 4.4 Ga (100 million years after the formation of the earth) are preserved. The grains are small and compositionally complex, so it is key that they be analyzed at the smallest possible scales to recover a maximum of information from them (e.g., their temperatures of growth and the nature of the magmas from which they grew). We are using the nanoSIMS to document the trace-element compositions of these ancient zircons at scales down to fractions of a µm. This work is being led by John Eiler (Professor at Caltech) and Amy Hofmann (Graduate Student at Caltech).

Thermal history of the Rand Schist: The Rand Schist is a key geological unit in Southern California that appears to be a sort of ‘grease’ layer along which the proto-Pacific and North American tectnoic plates slid past one another during the period when the San Andreas fault first came into being. We are using the nanoSIMS to determine the fine-scale chemical zonation within garnet crystals that grew in the Rand Schist during its deformation. These data constrain the thermal history of the Schist, and thus the tectonic processes by which it was deformed. This work is being led by Jason Saleeby (Professor at Caltech) and Alan Chapmann (Graduate Student at Caltech).

Ultra-high-pressure metamorphic rocks:  Subduction is the process by which Earth’s tectonic plates sink into the mantle. This process is generally irreversible, and so generally we never see a plate again after it sinks (other than indirectly, through seismic imaging). However, a special group of ‘Ultra-high-pressure’ metamorphic rocks appear to be fragments of subducting plates that reached depths of 200 km or more into the Earth’s mantle and then somehow returned to the crust. These rocks provide key evidence for conditions within the Earth’s interior and the mechanisms of subduction. We used the nanoSIMS to measure the oxygen isotope compositions of ultra-high-pressure metamorphic rocks from Northeast China; these data constrain the origin and fate of water bound within deeply subducted plates. This work was led by Lingsen Zhen (a Professor who visited our lab from China).

Geochronology

206Pb-207Pb dating of trace minerals:  Geochronology — the measurement of ages of geological events — depends critically on the ability to connect measured abundances of radiogenic isotopes to rock textures that have interpretable physical meaning. This is challenging when textural elements of interest are small (e.g., thin diagenetic or weathering crusts; magmatic inclusions). We are using the nanoSIMS to develop new ways of measuring geological ages of µm-sized objects or compositional domains, based on the relative abundances of two radiogenic Pb isotopes — 206Pb and 207Pb. Our work focuses on U-rich trace minerals zircon, baddelyite, monazite and xenotime. This project is being led by Charlie Verdel (Graduate Student at Caltech).

Mineralogy

Boron-rich fibers in quartz:  Pink quartz gets its color from nm-scale fibrous inclusions of a boron-rich mineral. Because of it occurs only as small, disseminated crystals, it is difficult to study this mineral’s composition. We have been using the nanoSIMS to determine the stoichiometry of felted masses of this mineral extracted from quartz by acid leaching.  This project is being led by George Rossman (Professor at Caltech).

Phosphorous-rich nanophase crusts on hematite:  Nanophases (sub-micron-scale minerals) are increasingly recognized as important constituents of natural environments, and are of interest to mineralogy and materials science because of their exotic size-dependant properties. One example is the set of P-rich minerals that form sub-micron-scale coatings on natural hematite.  We are using the nanoSIMS to characterize the stoichiometry of such phases. Perhaps the biggest challenge presented by this material is sample preparation; we have not yet established a way of concentrating and mounting these phases so that they are sufficiently flat and thickly layered for accurate quantitative analysis.  In the coming year, we will develop sample preparation methods to address these problems. This work is led by George Rossman (Professor at Caltech).

Hydrogen analysis in water-poor materials:  Hydrogen is a trace constituent (tens to hundreds of parts per million) of many geological materials, but even these small amounts control mineral rheologies and melting points.  Existing methods of hydrogen analysis in water-poor materials are too coarse or insensitive to measure minerals that are smaller than 10’s of µm.  We recently began a project to calibrate analysis of trace H in minerals using both the ims-7f geo and the nanoSIMS. This work will establish methods for studying water distributions in small phases. It is being led by George Rossman (Professor at Caltech).

Materials Science Engineering

Characterization of nanowire solar cells:  Photovoltaic devices having a thin (µm and smaller) wire shape have advantages over thin films because they permit efficient light collection along the wire axis, but short charge-transfer distances across the radial thickness of the wire.  However, the performance of these wires depends critically on the abundances in them of trace chemical defects, especially gold. We have been measuring distributions of trace gold in silica nanowires using the nanoSIMS. This project has been particularly helpful for developing analytical capabilities in the CMC because it calls for measurements in several different modes (spots, maps and depth profiles) of a single, relatively simple object.  This work is being led by Harry Atwater (Professor at Caltech) and Morgan Putnam (Graduate Student at Caltech).

Precisely doped semiconductors:  The electrical and physical properties of semiconductors can depend sensitively on the concentrations of trace dopants, but it is frequently difficult to precisely control dopant concentrations.  We used the ims-7f geo to measure depth profiles of Zn in silicon semiconductors; these data provide quality control for experiments aimed at improving control of dopant concentration. This work is being led by Zhaoyu Zhang (Graduate Student at Caltech) and Axel Scherer (Professor at Caltech).