53 research outputs found
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.Comment: 43 pages, 4 color plates, 2 figure
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 from 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 those 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 re-equilibration 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
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.Comment: To appear in "Mercury: The View after MESSENGER" edited by Solomon,
Nittler & Anderson (www.cambridge.org/9781107154452). This version is free to
view and download for personal use only. Not for re-distribution, re-sale or
use in derivative works. 37 pages, 5 figure
Mineral Processing by Short Circuits in Protoplanetary Disks
Meteoritic chondrules were formed in the early solar system by brief heating
of silicate dust to melting temperatures. Some highly refractory grains (Type B
calcium-aluminum-rich inclusions, CAIs) also show signs of transient heating. A
similar process may occur in other protoplanetary disks, as evidenced by
observations of spectra characteristic of crystalline silicates. One possible
environment for this process is the turbulent magnetohydrodynamic flow thought
to drive accretion in these disks. Such flows generally form thin current
sheets, which are sites of magnetic reconnection, and dissipate the magnetic
fields amplified by a disk dynamo. We suggest that it is possible to heat
precursor grains for chondrules and other high-temperature minerals in current
sheets that have been concentrated by our recently described short-circuit
instability. We extend our work on this process by including the effects of
radiative cooling, taking into account the temperature dependence of the
opacity; and by examining current sheet geometry in three-dimensional, global
models of magnetorotational instability. We find that temperatures above 1600 K
can be reached for favorable parameters that match the ideal global models.
This mechanism could provide an efficient means of tapping the gravitational
potential energy of the protoplanetary disk to heat grains strongly enough to
form high-temperature minerals. The volume-filling nature of turbulent magnetic
reconnection is compatible with constraints from chondrule-matrix
complementarity, chondrule-chondrule complementarity, the occurrence of igneous
rims, and compound chondrules. The same short-circuit mechanism may perform
other high-temperature mineral processing in protoplanetary disks such as the
production of crystalline silicates and CAIs.Comment: 6 pages, 3 figures, ApJL published versio
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.Comment: 23 pages, 15 figures, 5 table
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