Michail I. Petaev

Primitive FeNi metal grains in CH carbonaceous chondrites formed by condensation from a gas of solar composition

Some FeNi metal grains, similar to 150 mu m in apparent diameter, in CH carbonaceous chondrites are concentrically zoned in Ni (similar to 5-10 wt%), Co (0.2-0.4 wt%), and Cr (0.3-0.8 wt%); Silicon is present at the similar to 0.1 wt% level. These observations are consistent with predicted gas-solid condensation from a gas of solar composition at temperatures of similar to 1370-1270 K and total pressure of similar to 10(-4) bar. Estimates of FeNi metal grain growth and cooling rates in this temperature range are consistent with brief and localized thermal episodes in the solar nebula. Compositionally similar FeNi metal grains have also been reported in CR and Bencubbin-like chondrites. Because FeNi metal is highly susceptible to secondary alteration (i.e., metamorphism, melting, oxidation), the observed FeNi metal condensates in CH, Bencubbin-like, and CR chondrites indicate that these meteorites experienced no thermal processing after their lithification and thus are among the most primitive meteorites in our collections.

The ZONMET thermodynamic and kinetic model of metal condensation

The ZONMET model of metal condensation is a FORTRAN computer code that calculates condensation with partial isolation-type equilibrium partitioning of the 19 most abundant elements among 203 gaseous and 488 condensed phases and growth in the nebula of a zoned metal grain by condensation from the nebular gas accompanied by diffusional redistribution of Ni, Co, and Cr. Of five input parameters of the ZONMET model (chemical composition of the system expressed as the dust/gas (D/G) ratio, nebular pressure (Ptot), isolation degree (), cooling rate (CR), and seed size), only two—the D/G ratio and the CR of the nebular source region of a zoned Fe,Ni grain—are important in determining the grain radius and Ni, Co, and Cr zoning profiles. We found no evidence for the supercooling during condensation of Fe,Ni metal that is predicted by the homogeneous nucleation theory. The model allows estimates to be made of physicochemical parameters in the CH chondrite nebular source regions. Modeling growth and simultaneous diffusional redistribution of Ni, Co, and Cr in the zoned metal grains of CH chondrites reveals that the condensation zoning profiles were substantially modified by diffusion while the grains were growing in the nebula. This means that previous estimates of the physicochemical conditions in the nebular source regions of CH and CB chondrites, based on measured zoning profiles of Ni, Co, Cr, and platinum group elements in Fe,Ni metal grains, need to be corrected. The two zoned metal grains in the PAT 91456 and NWA 470 CH chondrites studied so far require nebular source regions with different chemical compositions (D/G 1 and D/G 4, respectively) and thermal histories characterized by variable cooling rates ( CR 0.011 0.0022 T K/h and CR 0.05 0.0035 T K/h, respectively). It appears that the metal grains of the CH chondrites were formed in multiple nebular source regions or in different events within the same source region as the CB chondrite metal grains were formed. Copyright

Large Pt anomaly in the Greenland ice core points to a cataclysm at the onset of Younger Dryas

One explanation of the abrupt cooling episode known as the Younger Dryas (YD) is a cosmic impact or airburst at the YD boundary (YDB) that triggered cooling and resulted in other calamities, including the disappearance of the Clovis culture and the extinction of many large mammal species. We tested the YDB impact hypothesis by analyzing ice samples from the Greenland Ice Sheet Project 2 (GISP2) ice core across the Bølling-Allerød/YD boundary for major and trace elements. We found a large Pt anomaly at the YDB, not accompanied by a prominent Ir anomaly, with the Pt/Ir ratios at the Pt peak exceeding those in known terrestrial and extraterrestrial materials. Whereas the highly fractionated Pt/Ir ratio rules out mantle or chondritic sources of the Pt anomaly, it does not allow positive identification of the source. Circumstantial evidence such as very high, superchondritic Pt/Al ratios associated with the Pt anomaly and its timing, different from other major events recorded on the GISP2 ice core such as well-understood sulfate spikes caused by volcanic activity and the ammonium and nitrate spike due to the biomass destruction, hints for an extraterrestrial source of Pt. Such a source could have been a highly differentiated object like an Ir-poor iron meteorite that is unlikely to result in an airburst or trigger wide wildfires proposed by the YDB impact hypothesis.

Exsolution in Ferromagnesian Olivine of the Divnoe Meteorite

Ferromagnesian olivine, one of the most common minerals in the solar system, has been widely regarded as a continuous solid solution, although several thermodynamic analyses have suggested the possibility of a miscibility gap at low temperatures. Natural ferromagnesian olivine from the Divnoe meteorite contains compositionally different exsolution lamellae, providing direct evidence for the existence of a miscibility gap in iron-magnesium olivine solid solutions.

The origin of the Moon within a terrestrial synestia

The giant impact hypothesis remains the leading theory for lunar origin. However, current models struggle to explain the Moons composition and isotopic similarity with Earth. Here we present a new lunar origin model. High-energy, high-angular momentum giant impacts can create a post-impact structure that exceeds the corotation limit (CoRoL), which defines the hottest thermal state and angular momentum possible for a corotating body. In a typical super-CoRoL body, traditional definitions of mantle, atmosphere and disk are not appropriate, and the body forms a new type of planetary structure, named a synestia. Using simulations of cooling synestias combined with dynamic, thermodynamic and geochemical calculations, we show that satellite formation from a synestia can produce the main features of our Moon. We find that cooling drives mixing of the structure, and condensation generates moonlets that orbit within the synestia, surrounded by tens of bars of bulk silicate Earth (BSE) vapor. The moonlets and growing moon are heated by the vapor until the first major element (Si) begins to vaporize and buffer the temperature. Moonlets equilibrate with BSE vapor at the temperature of silicate vaporization and the pressure of the structure, establishing the lunar isotopic composition and pattern of moderately volatile elements. Eventually, the cooling synestia recedes within the lunar orbit, terminating the main stage of lunar accretion. Our model shifts the paradigm for lunar origin from specifying a certain impact scenario to achieving a Moon-forming synestia. Giant impacts that produce potential Moon-forming synestias were common at the end of terrestrial planet formation.

Kumdykolite, a high-temperature feldspar from an enstatite chondrite

Abstract We report the first occurrence of kumdykolite in a meteorite (Sahara 97072, EH3). This orthorhombic form of albite occurs in the core of a concentrically zoned metal-sulfide nodule. In contrast to the terrestrial kumdykolite, the meteoritic sample has a domain structure that is consistent with either orthorhombic (Pmnn) or monoclinic (P21) space groups. The two symmetries are indicated by the presence or lack, respectively, of h + k = 2n + 1 reflections in [001] selected-area electron diffraction patterns, effects that likely result from different Si-Al ordering. Pmnn kumdykolite has only one tetrahedral site for Si and Al, whereas P21 kumdykolite would have three tetrahedral sites for Si and one for Al. We propose that kumdykolite formed above 1300 K and cooled rapidly enough to preserve its unique structure. Apparently, the cooling rate varied on the scale of nanometers allowing the local development of Si-Al ordering.

Reply to Boslough: Is Greenland Pt anomaly global or local?

Besides providing additional arguments against the Pt depositing event (1) as a cause of the Younger Dryas cooling, Boslough’s letter (2) raises an important question about the scale of this event. Indeed, a localized deposition of Pt by the Cape York meteorite shower is an attractive hypothesis considered by us initially (3), but abandoned because of (i) a large difference in the Pt/Ir ratios between the Cape York iron and the Pt anomaly in the GISP2 ice core, and (ii) a long ingrowth time of the anomaly (∼20 y), significantly exceeding the expected lifetime (∼5 y) of fine dust in the atmosphere. Such a long ingrowth time is unlikely to result from a later disturbance of ice because both chemical (sulfate) and particle (volcanic ash) spikes induced by volcanic eruptions before and after the Pt anomaly are typically contained within a thin ice layer deposited over 1–2 y. Therefore, the alternative assumed by us is either an abnormally high dust suspension time in the stratosphere or multiple injections of Pt-rich materials to the atmosphere, or both. In either case, a global anomaly is expected.

First evidence for silica condensation within the solar protoplanetary disk

Significance The oldest solar system solids dated are refractory inclusions [Ca-Al–rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)], which occur in chondritic meteorites and provide records of high-temperature processes in the early solar system. An ultrarefractory CAI and the silica-phase quartz occur in an AOA from the carbonaceous chondrite Yamato-793261, indicating formation over a temperature range exceeding 650 K. The minerals have 16O-rich compositions consistent with the nebular setting associated with refractory inclusions. This AOA provides direct evidence that silica condensed from gas in a CAI/AOA-forming region in our solar system indicates that gas became Si-rich as Mg condensed and may explain the origin of silica detected from infrared spectroscopy of T Tauri and asymptotic giant branch stars. Calcium-aluminum–rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs), a refractory component of chondritic meteorites, formed in a high-temperature region of the protoplanetary disk characterized by approximately solar chemical and oxygen isotopic (Δ17O ∼ −24‰) compositions, most likely near the protosun. Here we describe a 16O-rich (Δ17O ∼ −22 ± 2‰) AOA from the carbonaceous Renazzo-type (CR) chondrite Yamato-793261 containing both (i) an ultrarefractory CAI and (ii) forsterite, low-Ca pyroxene, and silica, indicating formation by gas–solid reactions over a wide temperature range from ∼1,800 to ∼1,150 K. This AOA provides direct evidence for gas–solid condensation of silica in a CAI/AOA-forming region. In a gas of solar composition, the Mg/Si ratio exceeds 1, and, therefore, silica is not predicted to condense under equilibrium conditions, suggesting that the AOA formed in a parcel of gas with fractionated Mg/Si ratio, most likely due to condensation of forsterite grains. Thermodynamic modeling suggests that silica formed by condensation of nebular gas depleted by ∼10× in H and He that cooled at 50 K/hour at total pressure of 10−4 bar. Condensation of silica from a hot, chemically fractionated gas could explain the origin of silica identified from infrared spectroscopy of remote protostellar disks.