arXiv subjects

Denton S. Ebel

Publications and source records attributed to Denton S. Ebel.

At least 19 recordsLinked to original sources

arXiv2024

Abundance, Sizes, and Major Element Compositions of Components in CR and LL Chondrites: Formation from Single Reservoirs

Abundances, apparent sizes, and individual chemical compositions of chondrules, refractory inclusions, other objects and surrounding matrix have been determined for Semarkona (LL3.00) and Renazzo (CR2) using consistent methods and criteria on x-ray element intensity maps. These represent the non-carbonaceous (NC, Semarkona) and carbonaceous chondrite (CC, Renazzo) superclans of chondrite types. We compare object and matrix abundances with similar data for CM, CO, K, and CV chondrites. We assess, pixel-by-pixel, the major element abundance in each object and in the entire matrix. We determine the abundance of "metallic chondrules" in LL chondrites. Chondrules with high Mg/Si and low Fe/Si and matrix carrying opposing ratios complement each other to make whole rocks with near-solar major element ratios in Renazzo. Similar Mg/Si and Fe/Si chondrule-matrix relationships are seen in Semarkona, which is within 11% of solar Mg/Si but significantly Fe-depleted. These results provide a robust constraint in support of single-reservoir models for chondrule formation and accretion, ruling out whole classes of astrophysical models and constraining processes of chondrite component formation and accretion into chondrite parent bodies.

astro-ph.EP↗

arXiv2024

Spinel-Bearing Spherules Condensed from the Chicxulub Impact-Vapor Plume

Formation of the giant Chicxulub crater off Mexico's Yucatan Peninsula coincided with deposition of the global Ir-rich Cretaceous-Tertiary (K-T) stratigraphic boundary layer at ca. 65 Ma. The boundary is marked most sharply by abundant spherules containing unaltered grains of magnesioferrite spinel. Here we predict for the first time the sequential condensation of solids and liquids from the plume of vaporized rock expected from oblique K-T impacts. We predict highly oxidizing plumes that condense silicate liquid droplets bearing spinel grains whose compositions closely match those marking the actual boundary. Systematic global variations in spinel composition are consistent with higher condensation temperatures for spinels found at Atlantic and European sites than for those in the Pacific.

physics.geo-ph↗

arXiv2023

Absolute Doubly Differential Angular Sputtering Yields for 20 keV Kr+ on Polycrystalline Cu

We have measured the absolute doubly differential angular sputtering yield for 20 keV Kr+ impacting a polycrystalline Cu slab at an incidence angle of θi = 45° relative to the surface normal. Sputtered Cu atoms were captured using collectors mounted on a half dome above the sample, and the sputtering distribution was measured as a function of the sputtering polar, θs, and azimuthal, phi, angles. Absolute results of the sputtering yield were determined from the mass gain of each collector, the ion dose, and the solid angle subtended, after irradiation to a total fluence of ~ 1 x 10^18 ions/cm^2. Our approach overcomes shortcomings of commonly used methods that only provide relative yields as a function of θs in the incidence plane (defined by the ion velocity and the surface normal). Our experimental results display an azimuthal variation that increases with increasing θs and is clearly discrepant with simulations using binary collision theory. We attribute the observed azimuthal anisotropy to ion-induced formation of micro- and nano-scale surface features that suppress the sputtering yield through shadowing and redeposition effects, neither of which are accounted for in the simulations. Our experimental results demonstrate the importance of doubly differential angular sputtering studies to probe ion sputtering processes at a fundamental level and to explore the effect of ion-beam-generated surface roughness.

physics.app-ph↗

arXiv2023

Djerfisherite: Nebular Source of Refractory Potassium

Djerfisherite is an important carrier of potassium in highly reduced enstatite chondrites, where it occurs in sub-round metal-sulfide nodules. These nodules were once free-floating objects in the protoplanetary nebula. Here, we analyze existing and new data to derive an equation of state (EOS) for djerfisherites of K_{6}(Cu,Fe,Ni)^{B} (Fe,Ni,Cu)^{C}_{24} S_{26}Cl structural formula. We use this EOS to calculate the thermal stability of djerfisherite coexisting in equilibrium with a cooling vapor of solar composition enriched in a dust analogous to anhydrous, chondritic interplanetary dust (C-IDP). We find that condensed mineral assemblages closely match those found in enstatite chondrites, with djerfisherite condensing above 1000 K in C-IDP dust enriched systems. Results may have implications for the volatile budgets of terrestrial planets, and the incorporation of K into early-formed, highly reduced, planetary cores. Previous work links enstatite chondrites to the planet Mercury, where the surface has a terrestrial K/Th ratio, high S/Si ratio, and very low FeO content. Mercury's accretion history may yield insights into Earth's.

astro-ph.EP↗

arXiv2023

Laser Scanning Confocal Microscopy of Comet Material in Aerogel

We report non-destructive 3-dimensional imaging and analysis techniques for material returned by the Stardust cometary collector. Our technique utilizes 3-dimensional laser scanning confocal microscopy (3D LSCM) to image whole Stardust tracks, in situ, with attainable resolutions <90 nm/pixel edge. LSCM images illustrate track morphology and fragmentation history; image segmentation techniques provide quantifiable volumetric and dynamic measurements. We present a method for multipart image acquisition and registration in 3-D. Additionally, we present a 3D deconvolution method for aerogel, using a theoretically calculated point spread function for first-order corrections of optical aberrations induced by light diffraction and refractive index mismatches. LSCM is a benchtop technique and is an excellent alternative to synchrotron x-ray computed microtomography for optically transparent media. Our technique, developed over the past 2 years, is a non-invasive, rapid technique for fine-scale imaging of high value returned samples from the Stardust mission, as well as various other samples from the geosciences.

astro-ph.EP↗

arXiv2023

Equilibrium Condensation from Chondritic Porous IDP Enriched Vapor: Implications for Mercury and Enstatite Chondrite Origins

The origin of Mercury's anomalous core and low FeO surface mineralogy are outstanding questions in planetary science. Mercury's composition may result from cosmochemical controls on the precursor solids that accreted to form Mercury. High temperatures and enrichment in solid condensates are likely conditions near the midplane of the inner solar protoplanetary disk. Silicate liquids similar to the liquids quenched in ferromagnesian chondrules are thermodynamically stable in oxygen-rich systems that are highly enriched in dust of CI-chondrite composition. In contrast, the solids surviving into the orbit of Mercury's accretion zone were probably similar to highly unequilibrated, anhydrous, interstellar organic- and presolar grain-bearing chondritic, porous interplanetary dust particles (C-IDPs). Chemical systems enriched in an assumed C-IDP composition dust produce condensates (solid + liquid assemblages in equilibrium with vapor) with super-chondritic atomic Fe/Si ratios at high temperatures, approaching 50% of that estimated for bulk Mercury. Sulfur behaves as a refractory element, but at lower temperatures, in these chemical systems. Stable minerals are FeO-poor, and include CaS and MgS, species found in enstatite chondrites. Disk gradients in volatile compositions of planetary and asteroidal precursors can explain Mercury's anomalous composition, as well as enstatite chondrite and aubrite parent body compositions. This model predicts high sulfur content, and very low FeO content of Mercury's surface rocks.

astro-ph.EP↗

arXiv2023

The Origin of Chondrules: Constraints from Matrix-Chondrule Complementarity

One of the major unresolved problems in cosmochemistry is the origin of chondrules, once molten, spherical silicate droplets with diameters of 0.2 to 2 mm. Chondrules are an essential component of primitive meteorites and perhaps of all early solar system materials including the terrestrial planets. Numerous hypotheses have been proposed for their origin. Many carbonaceous chondrites are composed of about equal amounts of chondrules and fine-grained matrix. Recent data confirm that matrix in carbonaceous chondrites has high Si/Mg and Fe/Mg ratios when compared to bulk carbonaceous chondrites with solar abundance ratios. Chondrules have the opposite signature, low Si/Mg and Fe/Mg ratios. In some carbonaceous chondrites chondrules have low Al/Ti ratios, matrix has the opposite signature and the bulk is chondritic. It is shown in detail that these complementary relationships cannot have evolved on the parent asteroid(s) of carbonaceous chondrites. They reflect preaccretionary processes. Both chondrules and matrix must have formed from a single, solar-like reservoir. Consequences of complementarity for chondrule formation models are discussed. An independent origin and/or random mixing of chondrules and matrix can be excluded. Hence, complementarity is a strong constraint for all astrophysical-cosmochemical models of chondrule formation. Although chondrules and matrix formed from a single reservoir, the chondrule-matrix system was open to the addition of oxygen and other gaseous components.

astro-ph.EP↗

arXiv2023

Condensation in Dust-Enriched Systems

Full chemical equilibrium calculations of the sequence of condensation of the elements from cosmic gases made by total vaporization of dust-enriched systems were performed to investigate the oxidation state of the resulting condensates. Computations included 23 elements and 374 gas species over a range of -3=log10(total P) to -6 bar and for enrichments to 1000x in dust of C1 chondritic composition relative to a system of solar composition. Because liquids are stable condensates in these systems, the MELTS non-ideal solution model for silicate liquids was used. Condensation at logP=-3 bar and dust enrichments of 100x, 500x and 1000x occurs at oxygen fugacities of IW-3.1, IW-1.7 and IW-1.2, respectively, and, at the temperature of cessation of direct condensation of olivine from the vapor, yields X(fayalite) of 0.019, 0.088 and 0.164, respectively. Silicate liquid is a stable condensate at dust enrichments >~12.5x at logP=-3. At 1000x, the Na and K oxide contents of the last liquid reach 10.1 and 1.3 wt%, respectively, at logP=-3 bar. At logP=-3 bar, iron sulfide liquids are stable condensates at dust enrichments at least as low as 500x, and the predicted distribution of Fe between metal, silicate and sulfide at 1310K and a dust enrichment of 560x matches that found in H chondrites, and at 1330K and 675x matches that of L chondrites prior to metal loss. With some exceptions, many chondrule glass compositions fall along bulk composition trajectories for liquids in equilibrium with cosmic gases at logP=-3 bar and dust enrichments between 600x and 1000x. If these chondrules formed by secondary melting of mixtures of condensates that formed at different T, nebular regions with characteristics such as these would have been necessary to prevent loss of Na by evaporation and FeO by reduction from the liquid precursors, assuming that liquids and gas were hot for enough time to have equilibrated.

astro-ph.EP↗

arXiv2023

Experimental Investigation of Condensation Predictions for Dust-Enriched Systems

Condensation models describe the equilibrium distribution of elements between coexisting mineral solid solutions, silicate liquid, and vapor in a closed chemical system, vapor phase always present, using equations of state of the phases involved at a fixed total P (< 1 bar) and temperature T. The VAPORS code uses a CaO-MgO-Al$_2$O$_3$-SiO$_2$ liquid model at T above the stability field of olivine and the MELTS algorithm at lower T. Quenched high-T crystal + liquid assemblages are preserved in meteorites as Type B Ca-, Al-rich inclusions (CAIs) and olivine-rich ferromagnesian chondrules. Experimental tests of compositional regions may clarify the nature of the phases present, the phase boundaries, and the partition of trace elements among these phases. Twenty-three Pt-loop equilibrium experiments in seven phase fields on twelve bulk compositions at specific T and dust enrichment factors tested the predicted stability fields of forsteritic olivine (Mg$_2$SiO$_4$), enstatite (MgSiO$_3$), Cr-bearing spinel (MgAl$_2$O$_4$), perovskite (CaTiO$_3$), melilite (Ca$_2$Al$_2$SiO$_7$ - Ca$_2$Mg$_2$Si$_2$O$_7$) and/or grossite (CaAl4O7) crystallizing from liquid. Experimental results for forsterite, enstatite, and grossite are in very good agreement with predictions, both in chemistry and phase abundances. On the other hand, the stability of spinel with olivine, and stability of perovskite and gehlenite are quite different from predictions. Perovskite is absent in all experiments. Even at low oxygen fugacity (IW-3.4), the most TiO$_2$-rich experiments do not crystallize Al-, Ti-bearing calcic pyroxene. The stability of spinel and olivine together is limited to a smaller phase field than is predicted. The melilite stability field is much larger than predicted, indicating a deficiency of current liquid or melilite activity models. In that respect, these experiments contribute to improving the data for calibrating thermodynamic models including MELTS.

astro-ph.EP↗

arXiv2023

Abundance, Major Element Composition and Size of Components and Matrix in CV, CO and Acfer 094 Chondrites

The relative abundances and chemical compositions of the macroscopic components or "inclusions" (chondrules and refractory inclusions) and fine-grained mineral matrix in chondritic meteorites provide constraints on astrophysical theories of inclusion formation and chondrite accretion. We present new techniques for analysis of low count per pixel Si, Mg, Ca, Al, Ti and Fe x-ray intensity maps of rock sections, and apply them to large areas of CO and CV chondrites, and the ungrouped Acfer 094 chondrite. For many thousands of manually segmented and type-identified inclusions, we are able to assess, pixel-by-pixel, the major element content of each inclusion. We quantify the total fraction of those elements accounted for by various types of inclusion and matrix. Among CO chondrites, both matrix and inclusion Mg to Si ratios approach the solar (and bulk CO) ratio with increasing petrologic grade, but Si remains enriched in inclusions relative to matrix. The oxidized CV chondrites with higher matrix-inclusion ratios exhibit more severe aqueous alteration (oxidation), and their excess matrix accounts for their higher porosity relative to reduced CV chondrites. Porosity could accommodate an original ice component of matrix as the direct cause of local alteration of oxidized CV chondrites. We confirm that major element abundances among inclusions differ greatly, across a wide range of CO and CV chondrites. These abundances in all cases add up to near-chondritic (solar) bulk abundance ratios in these chondrites, despite wide variations in matrix-inclusion ratios and inclusion sizes: chondrite components are complementary. This "complementarity" provides a robust meteoritic constraint for astrophysical disk models.

astro-ph.EP↗

arXiv2023

Thermochemical Stability of Low-Iron, Manganese-Enriched Olivine in Astrophysical Environments

Low-iron, manganese-enriched (LIME) olivine grains are found in cometary samples returned by the Stardust mission to comet 81P/Wild 2. Similar grains are found in primitive meteoritic clasts and unequilibrated meteorite matrix. LIME olivine is thermodynamically stable in a vapor of solar composition at high temperature at total pressures of a millibar to a microbar, but enrichment of solar composition vapor in a dust of chondritic composition causes the FeO/MnO ratio of olivine to increase. The compositions of LIME olivines in primitive materials indicate oxygen fugacities close to that of a very reducing vapor of solar composition. The compositional zoning of LIME olivines in amoeboid olivine aggregates is consistent with equilibration with nebular vapor in the stability field of olivine, without reequilibration at lower temperatures. A similar history is likely for LIME olivines found in comet samples and in interplanetary dust particles. LIME olivine is not likely to persist in nebular conditions in which silicate liquids are stable.

astro-ph.EP↗

arXiv2023

Condensation of Rocky Material in Astrophysical Environments

Volatility-dependent fractionation of the rock-forming elements at high temperatures is an early, widespread process during formation of the earliest solids in protoplanetary disks. Equilibrium condensation calculations allow prediction of the identities and compositions of mineral and liquid phases coexisting with gas under presumed bulk chemical, pressure and temperature conditions. A graphical survey of such results is presented for systems of solar and non-solar bulk composition. Chemical equilibrium was approached to varying degrees in the local regions where meteoritic chondrules, Ca-Al-rich inclusions, matrix and other components formed. Early, repeated vapor-solid cycling and homogenization, followed by hierarchical accretion in dust-rich regions, is hypothesized for meteoritic inclusions. Disequilibrium chemical effects appear to have been common at all temperatures, but increasingly so in less refractory meteoritic components. Work is needed to better model high-temperature solid solutions, indicators of these processes.

astro-ph.EP↗

arXiv2023

Trace Element Partitioning between CAI-Type Melts and Grossite, Melilite, Hibonite, and Olivine

We determined the mineral-melt partition coefficients (Di's) and the compositional and/or temperature dependency between grossite, melilite, hibonite, olivine and Ca-, Al-inclusion (CAI)-type liquids for a number of light (LE), high field strength (HFSE), large ion lithophile (LILE), and rare earth (REE) elements including Li, Be, B, Sr, Zr, Nb, Ba, La, Ce, Eu, Dy, Ho, Yb, Hf, Ta, Th. A series of isothermal crystallization experiments was conducted at 5 kbar pressure and IW+1 in graphite capsules. The starting compositions were selected based on the calculated and experimentally confirmed phase relations during condensation in CI dust-enriched systems (Ebel and Grossman, 2000; Ebel, 2006; Ustunisik et al., 2014). Partition coefficients between melt and gehlenite, hibonite, and grossite show that the trace element budget of igneous CAIs is controlled by these three major Al-bearing phases in addition to pyroxene. In general, LE, LILE, REE, and HFSE partition coefficients (by mass) decrease in the order of Di(Gehlenite-Melt) > Di(Hibonite-Melt) > Di(Grossite-Melt). Results suggest that Di(Gehlenite-Melt) vary by a factor of 2-3 in different melt compositions at the same T (~1500 C). Increased melt Al and Ca, relative to earlier work, increases the compatibility of Di(Gehlenite-Melt), and also the compatibility of Di(Hibonite-Melt), especially for La and Ce. Olivine partitioning experiments confirm that olivine contribution to the trace element budget of CAIs is small due to the low Di(Olivine-Melt) at a range of temperatures while D-Eu, Yb(Olivine-Melt) are sensitive to changes in T and oxygen fugacity. The development of a predictive model for partitioning in CAI-type systems would require more experimental data and the use of analytical instruments capable of obtaining single phase analyses for crystals < 5 micron.

astro-ph.EP↗

arXiv2023

Vapor-Melt Exchange -- Constraints on Chondrite Formation Conditions and Processes

The bulk volatile contents of chondritic meteorites provide clues to their origins. Matrix and chondrules carry differing abundances of moderately volatile elements, with chondrules carrying a refractory signature. At the high temperatures of chondrule formation and the low pressures of the solar nebula, many elements, including Na and Fe, should have been volatile. Yet the evidence is that even at peak temperatures, at or near the liquidus, Na and Fe (as FeO and Fe-metal) were present in about their current abundances in molten chondrules. This seems to require very high solid densities during chondrule formation to prevent significant evaporation. Evaporation should also be accompanied by isotopic mass fractionation. Evidence from a wide range of isotopic systems indicates only slight isotopic mass fractionations of moderately vola-tile elements, further supporting high solid densities. However, olivine-rich, FeO-poor chondrules commonly have pyroxene-dominated outer zones that have been interpreted as the prod-ucts of late condensation of SiO2 into chondrule melts. Late condensation of more refractory SiO2 is inconsistent with the apparent abundances of more volatile Na, FeO and Fe-metal in many chondrules. Despite significant recent experimental work bearing on this problem, the conditions under which chondrules behaved as open systems remain enigmatic.

astro-ph.EP↗

arXiv2023

Condensation Calculations in Planetary Science and Cosmochemistry

Cool a piece of the Sun to 1000 K at one millibar pressure to yield a mineral assemblage consistent with those found in the most primitive meteorites. This is an equilibrium or fractional condensation experiment simulated by calculations using equations of state for hundreds of gaseous molecules, condensed mineral solids, and silicate liquids, the products of a century of experimental measurements and theoretical studies. Such calculations have revolutionized our understanding of the chemistry of the cosmos. The mid-20th Century realization that meteorites are fossil records of the early Solar System made chemistry central to understanding planetary origins. Thus "condensation", the distribution of elements and isotopes between vapor and condensed solids and/or liquids at or approaching chemical equilibrium, deeply informs discussion of how meteor/comet compositions bear on planets. Condensation calculations have been applied to disks around young stars, to the mineral "rain" of mineral grains expected to form in cool dwarf star atmospheres, to the expanding envelopes of giant stars, to the vapor plumes that form in planetary impacts, and to the chemically and isotopically distinct "shells" computed and observed to exist in supernovae. As with all sophisticated calculations, there are inherent caveats, subtleties, and computational difficulties. Local chemistry has yet to be consistently integrated into dynamical astrophysical simulations so that effects like the blocking of radiation by grains, absorption and reemission of light by grains, and buffering of heat by grain evaporation/condensation feed back into the physics at each node of a gridded calculation over time. A deeper integration of thermochemistry with physical models makes the prospect of a general protoplanetary disk model as hopeful now as a general circulation model for global climate was in the early 1970's.

astro-ph.EP↗

arXiv2018

Fine-grained Material Associated with a Large Sulfide returned from Comet 81P/Wild 2

In a consortium analysis of a large particle captured from the coma of comet 81P/Wild 2 by the Stardust spacecraft, we report the discovery of a field of fine-grained material (FGM) in contact with a large sulfide particle. The FGM was partially located in an embayment in the sulfide, so appears to have been largely protected from damage during hypervelocity capture in aerogel. Some of the FGM particles are indistinguishable in their characteristics from common components of chondritic-porous interplanetary dust particles (CP-IDPs), including glass with embedded metals and sulfides (GEMS) and equilibrated aggregates (EAs). The sulfide exhibits surprising Ni-rich lamellae, which may indicate that this particle experienced a long-duration heating event after its formation but before incorporation into Wild 2. We discuss the relationship of the FGM to the sulfide, to other Wild 2 particles and to the history of the Solar nebula.

astro-ph.EP↗

arXiv2018

Dust concentration and chondrule formation

Meteoritical and astrophysical models of planet formation make contradictory predictions for dust concentration factors in chondrule forming regions of the solar nebula. Meteoritical and cosmochemical models strongly suggest that chondrules, a key component of the meteoritical record, formed in regions with solids-to-gas mass ratios orders of magnitude above background. However, models of dust grain dynamics in protoplanetary disks struggle to surpass factors of a few outside of very brief windows in the lifetime of the dust grains. Worse, those models do not predict significant concentration factors for dust grains the size of chondrule precursors. We briefly develop the difficulty in concentrating dust particles in the context of nebular chondrule formation and show that the disagreement is sufficiently stark that cosmochemists should explore ideas that might revise the concentration factor requirements downwards.

astro-ph.EP↗

arXiv2017

The Elusive Origin of Mercury

The MESSENGER mission sought to discover what physical processes determined Mercury's high metal to silicate ratio. Instead, the mission has discovered multiple anomalous characteristics about our innermost planet. The lack of FeO and the reduced oxidation state of Mercury's crust and mantle are more extreme than nearly all other known materials in the solar system. In contrast, moderately volatile elements are present in abundances comparable to the other terrestrial planets. No single process during Mercury's formation is able to explain all of these observations. Here, we review the current ideas for the origin of Mercury's unique features. Gaps in understanding the innermost regions of the solar nebula limit testing different hypotheses. Even so, all proposed models are incomplete and need further development in order to unravel Mercury's remaining secrets.

astro-ph.EP↗