LL Parent Body Research Articles

Carbide-metal assemblages in a sample returned from asteroid 25143 Itokawa: Evidence for methane-rich fluids during metamorphism

We found that the particle RA-QD02-0115 returned by the Hayabusa spacecraft from near-Earth asteroid 25143 Itokawa contains the iron carbide haxonite (Fe21.9-22.7Co0.2-0.3Ni0.2-0.8)C6 and several Fe,Ni alloys, including multi-domain tetrataenite and spinodally decomposed taenite. Ellipsoidal to nearly spherical voids occur throughout the particle and suggest the presence of a fluid phase during textural and chemical equilibration of the host rock within the parent asteroid of 25143 Itokawa. The calculated solubility of carbon in Fe,Ni metal indicates that the carbide formed at temperatures larger than 600 °C during thermal metamorphism of the LL-chondritic mineral assemblage. Haxonite formed metastably with respect to graphite and cohenite, probably due to its high degree of lattice match with neighboring taenite, a low cooling rate at peak metamorphic temperatures, and the hindered nucleation of graphite. Thermodynamic equilibrium calculations indicate that the fluid present was dry (H2O-poor) and dominated by methane. The reactive fluid most plausibly had an atomic H/C ratio of 4–5 and was derived from the reduction of macromolecular, insoluble organic matter (IOM) that initially co-accreted with water ice. The initial presence of water is a necessary assumption to provide sufficient hydrogen for the formation of methane from hydrolyzed IOM. Metallic iron was in turn partially oxidized and incorporated into the ferromagnesian silicates during the high-temperature stage of metamorphism. An exemplary bulk reaction from unequilibrated material on the left to an equilibrated assemblage on the right may be written as:330 CH0.8O0.2(IOM) + 500 H2O(ice/g) + 681 Fe(in alloy) + 566 FeSiO3(in Opx) → 300 CH4(g) + 32 H2(g) + 5 Fe23C6(in Hx) + 566 Fe2SiO4(in Ol)(Opx = orthopyroxene, Hx = haxonite, Ol = olivine, g = fluid species).The best estimate of the fluid/rock ratio in the region of the LL parent body where RA-QD02-0115 formed is about 3 × 10−3 and corresponds to an initial ice/rock ratio of about 7 × 10−3 (both by mass).

Young asteroid mixing revealed in ordinary chondrites: The case of NWA 5764, a polymict LL breccia with L clasts

AbstractPolymict chondritic breccias—rocks composed of fragments originating from different chondritic parent bodies—are of particular interest because they give insights into the mixing of asteroids in the main asteroid belt (occurrence, encounter velocity, transfer time). We describe Northwest Africa (NWA) 5764, a brecciated LL6 chondrite that contains a >16 cm3 L4 clast. The L clast was incorporated in the breccia through a nondestructive, low‐velocity impact. Identical cosmic‐ray exposure ages of the L clast and the LL host (36.6 ± 5.8 Myr), suggest a short transfer time of the L meteoroid to the LL parent body of 0.1 ± 8.1 Myr, if that meteoroid was no larger than a few meters. NWA 5764 (together with St. Mesmin, Dimmitt, and Glanerbrug) shows that effective mixing is possible between ordinary chondrite parent bodies. In NWA 5764 this mixing occurred after the peak of thermal metamorphism on the LL parent body, i.e., at least several tens of Myr after the formation of the solar system. The U,Th‐He ages of the L clast and LL host, identical at about 2.9 Ga, might date the final assembly of the breccia, indicating relatively young mixing in the main asteroid belt as previously evidenced in St. Mesmin.

Compositions, geochemistry, and shock histories of recrystallized LL chondrites

To examine compositional changes associated with high degrees of apparent thermal metamorphism among the LL chondrites, we have examined seven LL chondrites originally classified as being petrographic type 7. For comparison, we also analyzed the L6/7 chondrite Y-790124. We found that A-880933 is actually an LL4–6 genomict breccia and Y-790124 is best described as an L6 (S3) chondrite. The remaining six chondrites (EET 92013, Uden, Y-74160, Y-790144, Y-791067, Y-82067) are clearly of LL provenance, and each experienced temperatures high enough for them to have been recrystallized. In four of these samples (EET 92013, Uden, Y-74160, Y-790144) we find elemental patterns suggesting Fe(Ni)–FeS mobilization. Others (Y-791067, Y-82067) have compositions identical to average equilibrated LL chondrites. From our compositional data, we infer that EET 92013, Uden, Y-74160, Y-790144 experienced very low degrees of partial melting prior to recrystallization, but Y-791067 and Y-82067 experienced isochemical solid state recrystallization. The heat source responsible for the high degrees of thermal alteration of these meteorites is limited to either the decay of now extinct radionuclides (26Al) or impact-related heating. To evaluate the nature of the heat source, we use 40Ar–39Ar literature data and petrographic examinations to infer the cooling history and shock history of these chondrites. We find that heating due to impact is the most likely heat source for the heating of the recrystallized chondrites. The potential impacts occurred well after the initial stages of LL chondrite thermal metamorphism, but still early in the LL parent body’s history, probably ∼4.2–4.3Ga ago. These rocks experienced mild shock histories following their recrystallization.

Multiple melting in a four‐layered barred‐olivine chondrule with compositionally heterogeneous glass from LL3.0 Semarkona

AbstractChondrule K7p from LL3.0 Semarkona consists of four nested barred‐olivine (BO) chondrules. The innermost BO chondrule (chondrule 1) formed by complete melting of an olivine‐rich dustball. After formation, the chondrule was incorporated into another olivine‐rich dustball. A second heating event caused this second dustball to melt; the mesostasis and some of the olivine in chondrule 1 were probably also melted at this time, but the chondrule 1 structure remained largely intact. At this stage, the object was an enveloping compound BO chondrule. This two‐step process of melting and dustball enshrouding repeated two more times. The different proportions of olivine and glass in chondrules 1–4 suggest that the individual precursor dustballs differed in the amounts of chondrule fragments they contained and the mineral proportions in those fragments. The final dustball (which ultimately formed chondrule 4) was somewhat more ferroan; after melting, crystallizing, and quenching, chondrule 4 contained olivine and glass with higher FeO and MnO contents than those of the earlier formed chondrules. Subsequent aqueous alteration on the LL parent body transformed the abundant metal blebs and stringers at the chondrule surface into carbide, iron oxide, and minor Ni‐rich metal. Portions of the mesostasis underwent dissolution, producing holes and adjacent blades of more resistant material. Much of the glass in the chondrule remained isotropic, even after minor hydration and leaching. The sharp, moderately lobate boundary between the extensively altered mesostasis and the isotropic glass represents the reaction front beyond which there was little or no glass dissolution.

The 40Ar/39Ar dating technique applied to planetary sciences and terrestrial impacts

The 40Ar/39Ar technique is a powerful geochronological method derived from the K/Ar technique that can help to unravel the evolution of the solar system. The 40Ar/39Ar system can not only record the timing of volcanic and metamorphic processes on asteroids and planets, it finds domain of predilection in dating impact events throughout the solar system. The 40Ar/39Ar method is a robust analytical technique when the events to be dated are well understood and data are not over interpreted. The power of the 40Ar/39Ar technique resides in the ability to check the validity of age data internally by statistical means and multiple lines of evidence, and hence to evaluate when Ar age data are unreliable. Yet, too many ‘ages’ reported in the literature are still based on over-interpretation of perturbed age spectra. This review is by no means exhaustive and is centred on the most recent applications of the 40Ar/39Ar technique applied to planetary material, not the history of the planetary bodies themselves, or a historical review of the development of the argon dating technique. This paper presents selected case-study examples on terrestrial impact structures, the Moon and meteorites with ages recalculated using the latest decay constants. Currently, only 21 terrestrial impact structures are precisely and accurately dated, and the only proven age concordance is between the Chixculub impact and the Cretaceous/Paleogene mass extinction. 40Ar/39Ar dating of volcanic events on the Moon suggests that volcanism was concentrated between 3.8 and 3.1 Ga. The study of lunar volcanism would also benefit from dating of volcanic spherules for which only few data are available. Rigorous filtering of the 40Ar/39Ar age database of lunar melt breccias yielded concordant ages with high precision for two major basins of the Moon, but more precise age data would be needed to further test and validate the Late Heavy Bombardment (LHB) hypothesis. 40Ar/39Ar dating of lunar impact spherules shows an increase of ages <400 Ma suggesting a recent increase in the impact flux. The impact history of the LL parent body (bodies?) has yet to be well constrained but may mimic the LHB observed on the Moon, which would indicate that the LL parent body was quite large. Basaltic meteorites (HEDs) show an 40Ar/39Ar age range between 4.1 and 3.4 Ga, suggesting a diffuse LHB event; however, the spread of apparent ages may be a data-interpretation artefact, and the LHB parent body (bodies?) might have experienced a bombardment closer to the duration of the LHB age range than expected. Martian meteorites contain clues on Mars atmospheric and mantle argon compositions.

Pb–Pb dating constraints on the accretion and cooling history of chondrites

We have analyzed the Pb isotopic compositions of whole-rocks and various components (CAIs, chondrules, and/or mineral separates) of two carbonaceous chondrites, Allende (CV3) and Murchison (CM2), and nine ordinary chondrites, Sainte Marguerite (H4), Nadiabondi and Forest City (H5), Kernouvé (H6), Bjurböle (L/LL4), Elenovka and Ausson (L5), Tuxtuac (LL5), and Saint-Séverin (LL6) by MC-ICP-MS. Three CAI fractions from Allende define an isochron with an age of 4568.1 ± 9.4 Ma (MSWD = 0.08) and plot on the same isochron as fragments of the Efremovka inclusion E60 analyzed by Amelin et al. [Amelin, Y., Krot, A. N., Hutcheon, I. D., and Ulyanov, A. A. (2002a). Lead isotopic ages of chondrules and calcium–aluminum-rich inclusions. Science 297, 1679–1683]. When these two groups of samples are combined, the isochron yields an age of 4568.5 ± 0.5 (MSWD = 0.90), which is our best estimate of the age of the Solar System. Chondrules and pyroxene–olivine fractions from the ordinary chondrites yield ages that reflect the blocking of Pb isotope equilibration with the nebular gas. The combination of these ages with the corresponding metamorphic phosphate ages provides constraints on the thermal history of the different chondrite parent bodies. Among the H chondrites, Sainte Marguerite cooled to below ∼1100 K within a few My at 4565 Ma and to ∼800 K at 4563 Ma. Nadiabondi appears to have experienced a slightly more protracted cooling history with the corresponding interval lasting from 4559 to 4556 Ma. The data from Forest City and Kernouvé show evidence of late-stage perturbation with resulting U/Pb fractionation. Likewise, Pb isotopes in Tuxtuac (LL5) record a cooling history lasting from ∼4555 to 4544 Ma, which may indicate that the cooling history for the LL parent body was more prolonged than for the H parent body. We suggest a thermal evolution model for the growth of the planetary bodies based on the release of radiogenic heat from 26Al and 60Fe. This model incorporates the accretion rate, which determines the time at which the radiogenic heat becomes efficiently trapped, and the terminal size of the parent body, which controls its overall thermal inertia. The parent bodies of carbonaceous chondrites, which show little indication of metamorphic transformation, collect cooler nebular material at a relatively late stage. Small asteroids of ∼10–50 km radius accreting within 1–3 My could be the parent bodies of H and LL chondrites. The parent body of the L chondrites is likely to be a larger asteroid ( r > 100 km) or possibly the product of collisions of smaller planetary bodies.

39Ar- 40Ar evidence for early impact events on the LL parent body

We determined 39Ar- 40Ar ages of eight LL chondrites, and one igneous inclusion from an LL chondrite, with the object of understanding the thermal history of the LL-chondrite parent body. The meteorites in this study have a range of petrographic types from LL3.3 to LL6, and shock stages from S1 to S4. These meteorites reveal a range of K-Ar ages from ≥3.66 to ≥4.50 Ga, and peak ages from ≥3.74 to ≥4.55 Ga. Significantly, three of the eight chondrites (LL4, 5, 6) have K-Ar ages of ∼4.27 Ga. One of these (MIL99301) preserves an 39Ar- 40Ar age of 4.23 ± 0.03 Ga from low-temperature extractions, and an older age of 4.52 ± 0.08 Ga from the highest temperature extractions. In addition, an igneous-textured impact melt DOM85505,22 has a peak 39Ar- 40Ar age of ≥4.27 Ga. We interpret these results as evidence for impact events that occurred at about 4.27 Ga on the LL parent body that produced local impact melts, reset the 39Ar- 40Ar ages of some meteorites, and exhumed (or interred) others, resulting in a range of cooling ages. The somewhat younger peak age of 3.74 Ga from GRO95658 (LL3.3) suggests an additional impact event close to timing of impact-reset ages of some other ordinary chondrites between 3.6–3.8 Ga. The results from MIL99301 suggest that some apparently unshocked (S1) chondrites may have substantially reset 39Ar- 40Ar ages. A previous petrographic investigation of MIL99301 suggested that reheating to temperatures less than or equal to type 4 petrographic conditions (600°C) caused fractures in olivine to anneal, resulting in a low apparent shock stage of S1 (unshocked). The 39Ar- 40Ar age spectrum of MIL99301 is consistent with this interpretation. Older ages from high-T extractions may date an earlier impact event at 4.52 ± 0.08 Ga, whereas younger ages from lower-T extractions date a later impact event at 4.23 ± 0.03 Ga that may have caused annealing of feldspar and olivine.

Oxygen isotopes in R-chondrite magnetite and olivine: links between R chondrites and ordinary chondrites

Ion-microprobe studies yield Δ 17O (=δ 17O − 0.52 · δ 18O) values in magnetite from the Rumurti chondrite (RC) PCA91241 (which is paired with PCA91002) of +3.1 to +3.9‰, slightly higher than that in O from whole-rock R samples. Despite Δ 17O values in whole-rock RCs that are much (by ca. 1.6‰) higher than in whole-rock LL chondrites, the Δ 17O in R magnetite is much lower (by ca. 2‰) than the values (+4 to +7‰) from LL3 Semarkona and Ngawi (Choi et al., 1998). The δ 18O values in PCA magnetite (−15 to −10‰) are the lowest known in meteorites, well below the range in Semarkona (−4 to +9‰). On a δ 17O–δ 18O diagram both magnetite data sets form linear arrays with slopes of ca. 0.7, indicating mixing of O from different isotopic reservoirs; the slopes and intercepts of the two arrays are similar enough to permit them to be segments of a single array. This suggests that, in RCs and LL chondrites, magnetite formed from the same raw materials by the same processes, probably by aqueous alteration of metal in an asteroidal setting. We observed Δ 17O values in olivines and pyroxene from RCs ranging from −1.2 to +2.9‰ and δ 18O from +1.4 to +9.1‰. These compositions scatter in the same general range observed in chondrules from ordinary chondrites. The similarity in the O-isotopic composition of minerals that preserve a record of formation in the solar nebula supports a model in which RCs formed from nebular components similar to those in H chondrites, but with a matrix/chondrule ratio several times higher in the RCs, and with more extensive aqueous alteration in the RCs than in known H chondrites. We postulate that the matrix in R chondrites has Δ 17O higher than whole-rock values. We suggest that the original Δ 17O value of H 2O in the RC body was similar to that incorporated into the ordinary chondrites, previously estimated by Choi et al. (1998) to be ca. +7‰ in the LL parent body.

Oxygen isotopes in magnetite and fayalite in CV chondrites Kaba and Mokoia

Abstract— We report in situ measurements of O‐isotopic compositions of magnetite and primary and secondary olivine in the highly unequilibrated oxidized CV chondrites Kaba and Mokoia. In both meteorites, the magnetite and the secondary olivine (fayalite, Fa90–100) have O‐isotopic compositions near the terrestrial fractionation (TF) line; the mean Δ17O (= δ17O‐0.52 × δ18O) value is about −1%‰. In contrast, the compositions of nearby primary (chondrule), low‐FeO olivines (Fa1–2) are well below the TF line; Δ17O values range from −3 to −9%‰. Krot et al. (1998) summarized evidence indicating that the secondary phases in these chondrites formed by aqueous alteration in an asteroidal setting. The compositions of magnetite and fayalite in Kaba and Mokoia imply that the O‐isotopic composition of the oxidant was near or somewhat above the TF line. In Mokoia the fayalite and magnetite differ in δ18O by ∼20%‰, whereas these same materials in Kaba have virtually identical compositions. The difference between Mokoia magnetite and fayalite may indicate formation in isotopic equilibrium in a water‐rich environment at low temperatures, ∼300 K. In contrast, the similar compositions of these phases in Kaba may indicate formation of the fayalite by replacement of preexisting magnetite in dry environment, with the O coming entirely from the precursor magnetite and silica. The Δ17O of the oxidant incorporated into the CV parent body (as phyllosilicates or H2O) appears to have been much (7–8%‰) lower than that in that incorporated into the LL parent body (Choi et al, 1998), which suggests that the O‐isotopic composition of the nebular gas was spatially or temporally variable.

The Galim LL/EH polymict breccia: Evidence for impact‐induced exchange between reduced and oxidized meteoritic material

Abstract— Galim is a polymict breccia consisting of a heavily shocked (shock stage S6) LL6 chondrite, Galim (a), and an impact‐melted EH chondrite, Galim (b). Relict chondrules in Galim (b) served as nucleation sites for euhedral enstatite grains crystallizing from the impact melt. Many of the reduced phases typical of EH chondrites (e.g., Si‐bearing metallic Fe‐Ni; Ti‐bearing troilite) are absent. Galim (b) was probably shock‐melted while in contact with a more oxidized source, namely, Galim (a); during this event, Si was oxidized from the metal and Ti was oxidized from troilite. Galim (a) contains shock veins and recrystallized, unzoned olivine. The absence of evidence for reduction in Galim (a) may indicate that the amount of LL material greatly exceeded that of EH material; shock metamorphism may have taken place on the LL parent body. Shock‐induced redox reactions such as those inferred for the Galim breccia appear to be restricted mainly to asteroids because the low‐end tail of their relative‐velocity distribution permits mixing of intact disparate materials (including accretion of projectiles of different oxidation states), whereas the peak of the distribution leads to high equilibration shock pressures (allowing impact‐induced exchange between previously accreted, disequilibrated materials). Galim probably formed by a two‐stage process: (1) accretion to the LL parent body of an intact EH projectile at low relative velocities, and (2) shock metamorphism of the assemblage by the subsequent impact of another projectile at significantly higher relative velocities.

The Richfield LL3 chondrite

Abstract— Richfield is a moderately shocked (shock stage S4) LL3.7 genomict breccia find consisting mainly of light‐colored recrystallized clasts and dark clasts exhibiting significant silicate darkening; a few impact‐melt‐rock clasts and LL5 chondrite clasts also occur. The cosmic‐ray exposure age of 14.5 Ma is indistinguishable from the main exposure peak for LL chondrites (15 Ma). Although the exposure ages indicate little He loss, the gas‐retention ages indicate high gas losses that must have occurred prior to or during ejection from the LL parent body.

The histories of ordinary chondrite parent bodies: U,Th‐He age distributions

Abstract— Age patterns observed in meteorite groups reflect the different thermal or impact histories experienced by their parent bodies. To assess the number of ordinary chondrite (OC) parent bodies rare‐gas data in the Schultz and Kruse (1989) data base were used to calculate U,Th‐He gas‐retention ages. Most H‐ and LL‐chondrite ages are high; ∼81% are &gt;2.2 Ga. In contrast, most L‐chondrite ages are low; ∼69% are ≤2.2 Ga, and ∼35% are ≤0.9 Ga. The latter fraction is substantially lower than the value of 44% given by Heymann (1967). The difference is attributed to the preferential inclusion of shocked L chondrites in early studies. Broad age peaks in the H and LL groups near 3.4 Ga probably reflect thermal loss during metamorphism, but in the H distribution there is a hint of minor outgassing “events” near 1 Ga. The L/LL chondrites have chemical properties intermediate between and unresolvable from L and LL chondrites. The high ages of most L/LL chondrites are evidence against these originating on the L parent body; the L/LL age distribution is consistent with an origin on the LL parent body or on an independent body.

Composition and classification of clasts in the St. Mesmin LL chondrite breccia

Seven samples of the unique St. Mesmin meteorite have been analyzed by instrumental and radiochemical neutron activation analysis for Na, Ca, Sc, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, Se, In, Sm, Yb, Ir and Au. St. Mesmin is unique in being the only ordinary chondrite known to contain an unmelted xenolith of another ordinary chondrite. Data for two host matrix samples and three light clasts are consistent with their classification as LL chondrite material. The composition of the large dark xenolith confirms earlier evidence that it is an H chondrite; volatile abundances are consistent with it being highly shocked, petrologic type-4 material. In an olivine microporphyry, siderophile abundances are mostly about 0.13 times LL abundances, an apparent indication of metal loss during the shock melting which produced the clast. As in other regolithic chondrites, the dark host has higher contents of highly volatile elements than do the light clasts. We suggest that this results from a combination of differences in intensity of preexisting metamorphism as well as a redistribution of volatiles during regolith gardening. The H-group xenolith in St. Mesmin is a relatively recent addition to the parent body (< 1.4 Ga ago), but it is argued that this does not require regolith activity at that time. Rather the view is supported that the regolith period occurred very early in the meteorite's history (≳4.0 Ga ago) and may have been related to the growth of the parent body. The H-group fragment may be part of the projectile whose impact excavated the St. Mesmin meteoroid from the LL parent body.

The metamorphic history of LL-group ordinary chondrites

A study of feldspar grain size and microprobe analysis of coexisting orthopyroxene, olivine, diopside, and metal in twenty equilibrated LL-group chondrites and clasts defines the conditions of metamorphism, indicates the relative position of samples within the parent body, and suggests an accretionary-metamorphic history. Variation of mineral chemistry indicates a range of equilibrium temperature and parallels textural variation, providing two continuous relative temperature scales, based on mean feldspar grain size and the calcium content of orthopyroxene coexisting with diopside. Maximum equilibrium temperatures range from less than 600° up to 950°C. The total Tschermak's component in orthopyroxene indicates that maximum pressure was less than 1 kbar, and a pressure gradient correlates directly with temperature. A reduction sequence, evident from systematic variation of iron in ferromagnesian silicates and metal, correlates inversely with temperature. The maximum permissible sampling depth for the formation of surface breccias (2/10 r) also limits pressure to less than 1 kbar. The LL parent body had a minimum radius of 200 km, determined using the permissible sampling depth and published cooling rates. The data are broadly consistent with simple reheating of colder material within a completely accreted body. This model may have problems with volatile depletion, a required lack of impact sampling during metamorphism, and the timing of the event. The data are more consistent with accretion of colder material on a hot but cooling surface. Both models apparently require a recycling process during accretion to account for the reduction sequence.