Abstract— Sample preparation is always a critical step in the study of micrometer‐sized astromaterials available for study in the laboratory, whether their subsequent analysis is by electron microscopy or secondary ion mass spectrometry. A focused beam of gallium ions has been used to prepare electron transparent sections from an interplanetary dust particle (IDP), as part of an integrated analysis protocol to maximize the mineralogical, elemental, isotopic, and spectroscopic information extracted from one individual particle. In addition, focused ion beam (FIB) techniques have been employed to extract cometary residue preserved on the rims and walls of microcraters in 1100 series aluminum foils that were wrapped around the sample tray assembly on the Stardust cometary sample collector. Non‐ideal surface geometries and inconveniently located regions of interest required creative solutions. These include support pillar construction and relocation of a significant portion of sample to access a region of interest. Serial sectioning, in a manner similar to ultramicrotomy, is a significant development and further demonstrates the unique capabilities of focused ion beam microscopy for sample preparation of astromaterials.
ABSTRACT Mixing of meteoric and marine water and diagenetic reactions within sediments of the modern (< 10,000 yr old) Fraser River Delta have been studied using sediment cores and pore-water compositions from 15 cored holes. Pore waters in the Fraser River delta sediments appear to be connate waters, and the concentration of major ions (Cl-, Mg2+, Na+ reflects the degree of mixing of meteoric water and marine water that occurred at the sediment-water interface as the sediments were deposited. Deviations from simple mixing of meteoric and marine water are observed for Ca2+, Mn2+, Fe2+, SO42-, and HCO3-, suggesting diagenetic modification of the pore water since sediment deposition. Although Na+ concentrations appear to be controlled by simple mixing rather than by diagenetic reactions, examination of reactions between clay minerals and water suggests that pore waters are in cation exchan e equilibrium with smectite. Some chemical modification of the pore waters has occurred subsequent to sediment deposition, but there is no chemical evidence either for extensive meteoric-water flushing of the sediments or for tidal influxes of marine water into the sediments. Framboidal pyrite and calcite are the only diagenetic minerals observed in the sediments. Calcite concretions formed at, or near, the sediment-water interface in distributary channels. Calcite in carbonate concretions is depleted in 13C, relative to PDB, because of oxidation near the surface of 13C-depleted methane formed at greater depths by methanogenesis. Oxidation of methane caused elevated HCO-3 concentrations that reacted with Ca2+ supplied by tidal influxes of marine water to promote calcite precipitation in this localized environment.
Fayalite grains in chondrules in the oxidized, aqueously altered CV3 chondrite Mokoia have large excesses of radiogenic chromium-53. These excesses indicate the in situ decay of short-lived manganese-53 (half-life = 3.7 million years) and define an initial53Mn/55Mn ratio of 2.32 (±0.18) × 10–6. This ratio is comparable to values for carbonates in CI and CM chondrites and for several classes of differentiated meteorites. Mokoia fayalites formed 7 to 16 million years after Allende calcium-aluminum–rich inclusions, during hydrothermal activity on a geologically active asteroid after chondritic components had ceased forming in the solar nebula.
The 2175 angstrom extinction feature is the strongest (visible-ultraviolet) spectral signature of dust in the interstellar medium. Forty years after its discovery, the origin of the feature and the nature of the carrier(s) remain controversial. Using a transmission electron microscope, we detected a 5.7-electron volt (2175 angstrom) feature in interstellar grains embedded within interplanetary dust particles (IDPs). The carriers are organic carbon and amorphous silicates that are abundant in IDPs and in the interstellar medium. These multiple carriers may explain the enigmatic invariant central wavelength and variable bandwidth of the astronomical 2175 angstrom feature.
We measured the 26Al-26Mg isotope systematics of a approximately 5-micrometer refractory particle, Coki, returned from comet 81P/Wild 2 in order to relate the time scales of formation of cometary inclusions to their meteoritic counterparts. The data show no evidence of radiogenic 26Mg and define an upper limit to the abundance of 26Al at the time of particle formation: 26Al/27Al < 1 x 10(-5). The absence of 26Al indicates that Coki formed >1.7 million years after the oldest solids in the solar system, calcium- and aluminum-rich inclusions (CAIs). The data suggest that high-temperature inner solar system material formed, was subsequently transferred to the Kuiper Belt, and was incorporated into comets several million years after CAI formation.
Mass transport of SiO{sub 2} may be evaluated by determining the volume Md SiO{sub 2} solubility of the fluid expelled during burial and compaction of sediments. Petrographic observations indicate that dissolution of detrital quartz and chert is pervasive in the Albian Harmon Member (Fort St. John Group). This suggests a prolonged period of pore-fluid undersaturation with respect to SiO,. The abundance of detrital chert and the early preservation of K-feldspar suggest that the SiO{sub 2} may best be represented by chalcedony saturation. Using existing porosity reduction, burial history, and SiO{sub 2} (chalcedony) solubility data a minimum volume (one pore volume) of expelled fluid is calculated to maintain 10.3g of SiO{sub 2} per cubic meter of Harmon Member sediment that may be used in adjacent formations as cement. At a shale/sandstone ratio of 10, to produce 1% by volume of quartz cement in a cubic meter of sandstone would require 100 pore volumes of fluid. These calculations indicate that considerable fluid flux is required for significant mass transport of SiO{sub 2}. Consideration of burial history indicates that most SiO{sub 2} export occurred between 200 and 1,000 m of burial in less than 10 m.y. Fluid flow rates of approximately 1 mm/yrmore » therefore were required to export significant quantities of SiO{sub 2}, and presumably other mobile elements, from compacting Harmon Member sediments.« less
