Maitrayee Bose

AN INVESTIGATION INTO THE ORIGIN OF Fe-RICH PRESOLAR SILICATES IN ACFER 094

Presolar silicate and oxide grains from primitive meteorites are recognized as “stardust” on the basis of their extremely anomalous O isotopic compositions. We report data on 48 O-anomalous grains that were identified in grain size separates of the ungrouped carbonaceous chondrite Acfer 094. A majority of these grains exhibit high 17 O/ 16 O isotopic ratios along with solar to sub-solar 18 O/ 16 O ratios and may have originated in low-mass stars with close-to-solar metallicity. Four silicate grains that contain 18 O enrichments were also measured for their Si isotopes. A comparison of their O and Si isotopic compositions with model predictions indicates that these 18 O-rich grains may have formed in supernova ejecta. Four of the O-anomalous grains are oxides while the remaining 44 are silicates, based on elemental compositions determined by Auger spectroscopy. The presolar oxides include a TiO2 grain and a grain with spinel stoichiometry. The silicate grains largely exhibit ferromagnesian compositions, although a few grains also contain small amounts of Ca and/or Al. Stoichiometric silicates were further classified as either olivine-like or pyroxene-like, and in this study pyroxene-like grains are more abundant than olivinelike ones. The majority of silicates contain more Fe than Mg, including a few grains with Fe-rich end-member compositions. Spectroscopic observations indicate the presence of Mg-rich silicates in the atmospheres of stars and the interstellar medium. Mg-rich minerals such as forsterite and enstatite form by equilibrium condensation in stellar environments. However, non-equilibrium condensation can result in higher Fe contents and the occurrence of such processes in the outflows of stars may account for the Fe-rich grains. Alternatively, secondary processes may play a role in producing the Fe enrichments observed in the presolar silicate grains identified in the matrix of Acfer 094.

Circumstellar Fe Oxide from the Acfer 094 Carbonaceous Chondrite

We report the discovery of a unique Fe- and O-bearing circumstellar grain from the Acfer 094 ungrouped carbonaceous chondrite. The grain has a close-to-solar 17O/16O ratio and an 18O/16O ratio that is 1.34 times the solar value. Iron isotopic compositions show depletions of 100‰-200‰ in both 54Fe and 57Fe, relative to 56Fe. No evidence of excess 60Ni from the decay of extinct 60Fe was observed. Auger elemental spectra show that the grain is compositionally similar to wustite (FeO), but may contain a small amount of Mg in addition to Fe and O. The solid solution series magnesiowustite, (Mg,Fe)O, is predicted to form under nonequilibrium conditions in oxygen-rich asymptotic giant branch (AGB) stars with low mass-loss rates, and Fe-rich magnesiowustite has been proposed as the carrier of the 19.5 μm feature observed in the spectra of certain low-mass-loss AGB stars that show little silicate emission. Although the isotopic data are ambiguous, these observations argue in favor of an AGB source for this Fe oxide grain.

Rapid cooling and cold storage in a silicic magma reservoir recorded in individual crystals

Taupo Volcanic Zone magma spent more than 90% of its life deep and crystalline before rapid shallow accumulation and eruption. Quick eruption after a long bake Minerals such as zircon can record the storage conditions of magma before volcanic eruption. Rubin et al. combined traditional 238U-230Th dating with lithium concentration profiles in seven zircons from the Taupo supervolcanic complex in New Zealand to determine magma storage conditions. The zircons spent more than 90% of their lifetime in an uneruptible, mostly crystalline, and deep magmatic reservoir. The zircons were eventually transported to hotter, shallower, and eruptible magma bodies, where they spent only decades to hundreds of years before eruption. The result suggests a two-stage model for magmatic systems with large thermal variations. Science, this issue p. 1154 Silicic volcanic eruptions pose considerable hazards, yet the processes leading to these eruptions remain poorly known. A missing link is knowledge of the thermal history of magma feeding such eruptions, which largely controls crystallinity and therefore eruptability. We have determined the thermal history of individual zircon crystals from an eruption of the Taupo Volcanic Zone, New Zealand. Results show that although zircons resided in the magmatic system for 103 to 105 years, they experienced temperatures >650° to 750°C for only years to centuries. This implies near-solidus long-term crystal storage, punctuated by rapid heating and cooling. Reconciling these data with existing models of magma storage requires considering multiple small intrusions and multiple spatial scales, and our approach can help to quantify heat input to and output from magma reservoirs.

Stardust Investigation into the CR Chondrite Grove Mountain 021710

We report the presolar grain inventory of the CR chondrite Grove Mountain 021710. A total of 35 C-anomalous grains (∼236 ppm) and 112 O-anomalous grains (∼189 ppm) were identified in situ using NanoSIMS ion imaging. Of 35 C-anomalous grains, 28 were determined to be SiC grains by Auger spectroscopy. Seven of the SiC grains were subsequently measured for N and Si isotopes, allowing classification as one nova grain, one Y grain, one Z grain, and four mainstream grains. Eighty-nine out of 112 O-anomalous grains belong to Group 1, indicating origins in low-to-intermediate-mass red giant and asymptotic giant branch stars. Twenty-one are Group 4 grains and have origins in supernovae. Auger spectroscopic elemental measurements of 35 O-anomalous grains show that 33 of them are ferromagnesian silicates. They have higher Mg/(Mg+Fe) ratios than those reported in other meteorites, suggesting a lower degree of alteration in the nebula and/or asteroid parent bodies. Only two oxide grains were identified, with stoichiometric compositions of MgAl2O4 and SiO2, respectively. The presolar silicate/oxide ratio of GRV 021710 is comparable with those of the CR3 chondrites (QUE 99177 and MET 00426) and primitive interplanetary dust particles. In order to search for presolar sulfides, the meteorite was also mapped for S isotopes. However, no presolar sulfides were found, suggesting a maximum abundance of 2 ppm. The scarcity of presolar sulfides may be due to their much faster sputtering rate by cosmic rays compared to silicates.

Stardust material in the paired enstatite chondrites: SAH 97096 and SAH 97159

Stardust grains, more commonly referred to as presolar grains, are solid condensates of stars that are studied in terrestrial laboratories with a variety of analytical techniques. Here we report on submicrometer silicate, oxide and carbonaceous stardust grains identified in the paired enstatite chondrites SAH 97096 and SAH 97159. A majority of the grains with O isotopic anomalies exhibit O excesses and probably originated in the dusty envelopes of low-mass AGB or RG stars. One grain is highly Orich and has a normal Si isotopic composition; based on its O and Si isotopic composition, an origin in a nova is most likely. However, another scenario that may explain this grain’s O isotopic composition is a binary star system consisting of an evolved or mainstream star accreting material from its nova companion. Elemental characterization of the O-anomalous grains shows the presence of eleven magnesian silicate grains with or without Fe and three Fe-oxide grains; none of the grains contain Ca or Al. Carbon-anomalous grains have C/C ratios from 19–78; most are probably SiC. The abundances of the Oand C-anomalous grains are 98±34 and 51±13 ppm, respectively, which is much higher than previously observed in other enstatite chondrites, and close to that of some carbonaceous chondrites.

Isotopic and elemental compositions of stardust and protosolar dust grains in primitive meteorites

of the Dissertation Acknowledgements List of Figures........................................................................................x List of Tables.......................................................................................xii

Single-Cell View of Carbon and Nitrogen Acquisition in the Mixotrophic Alga Prymnesium parvum (Haptophyta) Inferred From Stable Isotope Tracers and NanoSIMS

Nutritional modes of unicellular eukaryotes range from pure photoautotrophy of some phytoplankton to pure heterotrophy of species typically called protozoa. Between these two extremes lies a functional continuum of nutrient and energy acquisition modes termed mixotrophy. Prymnesium parvum is an ecologically important mixotrophic haptophyte alga that can produce toxins and form ecosystem disruptive blooms that result in fish kills and changes in planktonic food web structure. We investigated carbon and nitrogen acquisition strategies of single cells of P. parvum using a combined experimental-imaging approach employing labeling of live cells with stable isotope tracers (13C and 15N) followed by measurement of cellular isotopic ratios using nanometer-scale secondary ion mass spectrometry (NanoSIMS). With this method, we were able to quantify the relative contributions of photosynthesis and heterotrophy to the nutrition of the alga. Our results suggest that P. parvum relies on predation primarily for nitrogen, while most carbon for cellular building blocks is obtained from inorganic sources. Our analysis further revealed that nitrogen assimilation can vary up to an order of magnitude among individual cells, a finding that would be difficult to determine using other methods. These results help to improve our understanding of mixotrophy across the enormous diversity of eukaryotes, one cell and one species at a time.

Response to Comment on “Rapid cooling and cold storage in a silicic magma reservoir recorded in individual crystals”

In a recent paper, we used Li concentration profiles and U-Th ages to constrain the thermal conditions of magma storage. Wilson and co-authors argue that the data instead reflect control of Li behavior by charge balance during partitioning and not by experimentally determined diffusion rates. Their arguments are based on (i) a coupled diffusion mechanism for Li, which has been postulated but has not been documented to occur, and (ii) poorly constrained zircon growth rates combined with the assumption of continuous zircon crystallization.

Carbon fixation from mineral carbonates

Photoautotrophs assimilate oxidized carbon obtained from one of two sources: dissolved or atmospheric. Despite its size, the pool of lithospheric carbonate is not known to be a direct source for autotrophy. Yet, the mechanism that euendolithic cyanobacteria use to excavate solid carbonates suggests that minerals could directly supply CO2 for autotrophy. Here, we use stable isotopes and NanoSIMS to show that the cyanobacterium Mastigocoleus testarum derives most of its carbon from the mineral it excavates, growing preferentially as an endolith when lacking dissolved CO2. Furthermore, natural endolithic communities from intertidal marine carbonate outcrops present carbon isotopic signatures consistent with mineral-sourced autotrophy. These data demonstrate a direct geomicrobial link between mineral carbonate pools and reduced organic carbon, which, given the geographical extent of carbonate outcrops, is likely of global relevance. The ancient fossil record of euendolithic cyanobacteria suggests that biological fixation of solid carbonate could have been relevant since the mid-Proterozoic.Evidence showing that carbonates can directly supply CO2 for autotrophy is lacking. Here, using stable isotope analyses and NanoSIMS imaging, the authors show that a model euendolith, Mastigocoleustestarum strain BC008, derives the majority of its biomass carbon from the mineral carbonates it excavates.

Coordinated X-ray, Ion, and Electron Microanalysis Approach Towards Understanding the Earliest-Formed Solids in the Solar System

Introduction: Calcium-Aluminum-rich Inclusions (CAIs), hosted in primitive meteorites, are the oldest solids in the Solar System (e.g., [1]). These inclusions are made of highly refractory mineral phases and they are surrounded by a multi-mineralic rim sequence, called Wark-Lovering (WL) rims [2]. The CAIs and their WL rims preserve a record of the earliest Solar System processes [3]. Each mineral layer in the WL rim sequence is typically only a few microns thick (Fig. 1), making a detailed study of their textural, chemical and isotopic characteristics challenging. Because of their fine-grained nature, the mechanisms and timescales of formation of the WL rim sequences are not well understood. We applied a coordinated analytical approach to study these rims, by using X-ray Wavelength Dispersive Spectroscopy (WDS), Energy Dispersive Spectroscopy (EDS), and Electron Back-Scatter Diffraction (EBSD) to determine their composition and microstructure, as well as NanoSIMS (Nano-Secondary Ion Mass Spectrometer) to determine their relative age of formation.