Initial ^2^6Al/^2^7Al in carbonaceous-chondrite chondrules: too little ^2^6Al to melt asteroids (original) (raw)

Early core formation in asteroids and late accretion of chondrite parent bodies: Evidence from 182Hf-182W in CAIs, metal-rich chondrites, and iron meteorites

Geochimica et Cosmochimica Acta, 2005

The 182 Hf-182 W isotopic systematics of Ca-Al-rich inclusions (CAIs), metal-rich chondrites, and iron meteorites were investigated to constrain the relative timing of accretion of their parent asteroids. A regression of the Hf-W data for two bulk CAIs, various fragments of a single CAI, and carbonaceous chondrites constrains the 182 Hf/ 180 Hf and W at the time of CAI formation to (1.07 Ϯ 0.10) ϫ 10 Ϫ4 and Ϫ3.47 Ϯ 0.20, respectively. All magmatic iron meteorites examined here have initial W values that are similar to or slightly lower than the initial value of CAIs. These low W values may in part reflect 182 W-burnout caused by the prolonged cosmic ray exposure of iron meteorites, but this effect is estimated to be less than ϳ0.3 units for an exposure age of 600 Ma. The W isotope data, after correction for cosmic ray induced effects, indicate that core formation in the parent asteroids of the magmatic iron meteorites occurred less than ϳ1.5 Myr after formation of CAIs. The nonmagmatic IAB-IIICD irons and the metal-rich CB chondrites have more radiogenic W isotope compositions, indicating formation several Myr after the oldest metal cores had segregated in some asteroids.

From protoplanetary dust to asteroidal heating: a mineralogical study of the CO3 chondrites

2020

Carbonaceous chondrites are among the most primitive extra-terrestrial materials available for study. These meteorites provide a detailed record of the geological processes and events that have shaped our solar system over the last 4.5 billion years. "Ornans-like" carbonaceous chondrite meteorites, also referred to as CO3 chondrites, comprise pristine, primitive mineralogy that has undergone no or minimal aqueous alteration. CO3 chondrites are also known to contain up to 3.5% carbon in the form of insoluble and soluble organic matter, graphite and carbonates. The CO3 chondrites form a suite of samples that have experienced increasing degrees of thermal metamorphism, from weakly heated CO3.0s such as Colony and DOM 08006, to strongly meta-morphosed CO3.8s such as Isna. Detailed studies of this suite of CO3 chondrites enables firstly a determination of the most primitive and earliest formed aggregates of crystalline, amorphous and organic solids, and their textural relations...

Shock melting of ordinary chondrite powders and implications for asteroidal regoliths

Meteoritics & Planetary Science, 2005

A series of 59 impacts in the laboratory reduced a coherent 460 g piece of the L6 ordinary chondrite ALH 85017 to a coarse-grained "regolith." We then subjected the 125-250 µm fines from this sample to reverberation shock stresses of 14.5-67 GPa in order to delineate the melting behavior of porous, unconsolidated, chondritic asteroid surfaces during meteorite impact.

Aluminum-26 in calcium-aluminum-rich inclusions and chondrules from unequilibrated ordinary chondrites

Meteoritics & Planetary Science, 2001

In order to investigate the distribution of 26A1 in chondrites, we measured aluminummagnesium systematics in four calcium-aluminum-rich inclusions (CAIs) and eleven aluminum-rich chondrules from unequilibrated ordinary chondrites (UOCs). All four CAIs were found to contain radiogenic 26Mg (26Mg*) from the decay of 26A1. The inferred initial 26Al/27Al ratios for these objects ((26Al/27Al)o = 5 x 10-5) are indistinguishable from the (26APAl),, ratios found in most CAIs from carbonaceous chondrites. These observations, together with the similarities in mineralogy and oxygen isotopic compositions of the two sets of CAIs, imply that CAIs in UOCs and carbonaceous chondrites formed by similar processes from similar (or the same) isotopic reservoirs, or perhaps in a single location in the solar system. We also found 26Mg* in two of eleven aluminum-rich chondrules. The (26Al/27Al)o ratio inferred for both of these chondrules is-1 x 10-5, clearly distinct from most CAIs but consistent with the values found in chondrules from type 3.0-3.1 UOCs and for aluminumrich chondrules from lightly metamorphosed carbonaceous chondrites (-0.5 x 10-5 to-2 x 10-5). The consistency of the (26A1/27Al)o ratios for CAIs and chondrules in primitive chondrites, independent of meteorite class, implies broad-scale nebular homogeneity with respect to 26A1 and indicates that the differences in initial ratios can be interpreted in terms of formation time. A timeline based on 26A1 indicates that chondrules began to form 1 to 2 Ma after most CAIs formed, that accretion of meteorite parent bodies was essentially complete by 4 Ma after CAIs, and that metamorphism was essentially over in type 4 chondrite parent bodies by 5 to 6 Ma after CAIs formed. Type 6 chondrites apparently did not cool until more than 7 Ma after CAIs formed. This timeline is consistent with 26A1 as a principal heat source for melting and metamorphism.

Chronological constraints on the thermal evolution of ordinary chondrite parent bodies from the 53Mn-53Cr system

Geochimica et Cosmochimica Acta, 2021

The 53 Mn-53 Cr isotope systematics in ordinary chondrites constrains the accretion and thermal history of their parent bodies. Mineralogical observations and olivine-spinel geothermometry suggest that chromite in ordinary chondrites formed during prograde thermal metamorphism with the amount of chromite increasing with petrologic grades in type 3 to type 6 ordinary chondrites. Assuming a chondritic evolution of the respective parent bodies, 53 Cr/ 52 Cr model ages for chromite range from 3:99 þ0:93 À0:79 to 11:1 þ6:0 À2:8 Ma after the formation of calcium-aluminium-rich inclusions (CAIs). Chromite and silicate-metal-sulphide isochrons define an age range from 2:78 þ0:55 À0:50 to 15:4 þ2:4 À1:6 Ma. Both chromite model ages and isochron ages correlate with the petrological grade of the samples, which is consistent with an onion-shell structure of the chondrite parent bodies. The study shows that unlike the isochron ages, which are prone to impact-related disturbances or partial re-equilibration during cooling from high temperatures, the chromite model ages are not easily affected by thermal metamorphism or later events and yield robust mineral growth ages. The results are consistent with a homogenous distribution of 53 Mn and an initial canonical 53 Mn/ 55 Mn = 6.28 Â 10 À6. The estimated closure temperatures for the Mn-Cr system in chromites range from~760°C for type 6 to~540-620°C for type 3 ordinary chondrites. The high closure temperatures estimated for type 3 and type 6 ordinary chondrites imply that the chromite ages correspond to the peak metamorphic temperature reached during the thermal history of the chondrite parent bodies. The oldest chromite model age obtained for type 3 samples along with the established Al-Mg chondrule formation ages constrain the accretion of the parent bodies to >2.1 Ma after CAI formation, implying that planetesimal accretion immediately followed chondrule formation.

Did the carbonaceous chondrites evolve in the crustal regions of partially differentiated asteroids?

Journal of Geophysical Research, 2011

Carbonaceous chondrites are considered to be the most primitive meteorites that contain some of the earliest formed solar system phases, e.g., Ca-Al-rich inclusions and chondrules. The parent bodies of the carbonaceous chondrites are traditionally known to have experienced aqueous alteration. However, the recent paleomagnetic records of the CV chondrite, Allende, indicate remnant magnetism that suggests that these chondrites possibly evolved on the crustal region of a partially differentiated asteroid having a convective molten iron core. We present results based on our comprehensive numerical simulations of the scenario involving melting and planetary differentiation of asteroids. The possibility of a sizable carbonaceous chondritic crust on these differentiated bodies is explored. The simulations indicate that it could be possible to explain the paleomagnetic records of the chondrites by the dynamo-generated magnetic field from the convective molten iron core. Apart from the slow linear accretion scenario, we envisage a scenario involving two episodes of accretion of the asteroids. An early rapid accretion of the asteroids in the initial stage, perhaps within the initial ∼2 Myr, would produce a sizable molten iron core subsequent to differentiation to produce the required magnetic fields. This would be followed by a slow accretion of consolidated chondritic crust, perhaps over several million years. There is also a possibility of the disrupted expulsion of the chondritic crust by the internal pressures generated by gases released during hydration/dehydration reactions.

26Al in plagioclase-rich chondrules in carbonaceous chondrites: Evidence for an extended duration of chondrule formation

Geochimica et Cosmochimica Acta, 2009

The 26 Al-26 Mg isotope systematics in 33 petrographically and mineralogically characterized plagioclase-rich chondrules (PRCs) from 13 carbonaceous chondrites (CCs) -one ungrouped (Acfer 094), six CR, five CV, and one CO -reveal large variations in the initial 26 Al/ 27 Al ratio, ( 26 Al/ 27 Al) 0 . Well-resolved 26 Mg excesses (d 26 Mg) from the in situ decay of the short-lived nuclide 26 Al (t 1/2 $ 0.72 Ma) were found in nine chondrules, two from Acfer 094, five from the CV chondrites, Allende and Efremovka, and one each from the paired CR chondrites, EET 92147 and EET 92042, with ( 26 Al/ 27 Al) 0 values ranging from 3A^10Aˋ6to3 Â 10 À6 to 3A^10Aˋ6to1.5 Â 10 À5 . Data for seven additional chondrules from three CV and two CR chondrites show evidence suggestive of the presence of 26 Al but do not yield well defined values for ( 26 Al/ 27 Al) 0 , while the remaining chondrules do not contain excess radiogenic 26 Mg and yield corresponding upper limits of (11-2) Â 10 À6 for ( 26 Al/ 27 Al) 0 . The observed range of ( 26 Al/ 27 Al) 0 in PRCs from CCs is similar to the range seen in chondrules from unequilibrated ordinary chondrites (UOCs) of low metamorphic grade (3.0-3.4). However, unlike the UOC chondrules, there is no clear trend between the ( 26 Al/ 27 Al) 0 values in PRCs from CCs and the degree of thermal metamorphism experienced by the host meteorites. High and low values of ( 26 Al/ 27 Al) 0 are found equally in PRCs from both CCs lacking evidence for thermal metamorphism (e.g., CRs) and CCs where such evidence is abundant (e.g., CVs). The lower ( 26 Al/ 27 Al) 0 values in PRCs from CCs, relative to most CAIs, are consistent with a model in which 26 Al was distributed uniformly in the nebula when chondrule formation began, approximately a million years after the formation of the majority of CAIs. The observed range of ( 26 Al/ 27 Al) 0 values in PRCs from CCs is most plausibly explained in terms of an extended duration of $2-3 Ma for the formation of CC chondrules. This interval is in sharp contrast to most CAIs from CCs, whose formation appears to be restricted to a narrow time interval of less than 10 5 years. The active solar nebula appears to have persisted for a period approaching 4 Ma, encompassing the formation of both CAIs and chondrules present in CCs, and raising important issues related to the storage, assimilation and mixing of chondrules and CAIs in the early solar system.

Thermal Alteration of Labile Elements in Carbonaceous Chondrites

Icarus

Carbonaceous chondrite meteorites are some of the oldest Solar System planetary materials available for study. The CI group has bulk abundances of elements similar to those of the solar photosphere. Of particular interest in carbonaceous chondrite compositions are labile elements, which vaporize and mobilize efficiently during post-accretionary parent-body heating events. Thus, they can record low-temperature alteration events throughout asteroid evolution. However, the precise nature of labile-element mobilization in planetary materials is unknown. Here we characterize the thermally induced movements of the labile elements S, As, Se, Te, Cd, Sb, and Hg in carbonaceous chondrites by conducting experimental simulations of volatile-element mobilization during thermal metamorphism. This process results in appreciable loss of some elements at temperatures as low as 500 K. This work builds on previous laboratory heating experiments on primitive meteorites and shows the sensitivity of chondrite compositions to excursions in temperature. Elements such as S and Hg have the most active response to temperature across different meteorite groups. Labile element mobilization in primitive meteorites is essential for quantifying elemental fractionation that occurred on asteroids early in Solar System history. This work is relevant to maintaining a pristine sample from asteroid (101955) Bennu from the OSIRIS-REx mission and constraining the past orbital history of Bennu. Additionally, we discuss thermal effects on surface processes of near-Earth asteroids, including the thermal history of "rock comets" such as (3200) Phaethon. This work is also critical for constraining the concentrations of contaminants in vaporized water extracted from asteroid regolith as part of future in situ resource utilization for sustained robotic and human space exploration.