Chondrules In Unequilibrated Ordinary Chondrites Research Articles

Barred olivine chondrules in ordinary chondrites: Constraints on chondrule formation

AbstractIn general, barred olivine (BO) chondrules formed from completely melted precursors. Among BO chondrules in unequilibrated ordinary chondrites, there are significant positive correlations among chondrule diameter, bar thickness, and rim thickness. In the nebula, smaller BO precursor droplets cooled faster than larger droplets (due to their higher surface area/volume ratios) and grew thinner bars and rims. There is a bimodal distribution in the olivine FeO content in BO chondrules, with a hiatus between 11 and 19 wt% FeO. The ratio of (FeO rich)/(FeO poor) BO chondrules decreases from 12.0 in H to 1.6 in L to 1.3 in LL. This is the opposite of the case for porphyritic chondrules: the mean (FeO rich)/(FeO poor) modal ratio increases from 0.8 in H to 1.8 in L to 2.8 in LL. During H chondrite agglomeration, most precursor dustballs were small with low bulk FeO/(FeO + MgO) ratios and moderately high melting temperatures. The energy available for chondrule melting from flash heating was relatively low, capable of completely melting many ferroan dusty precursors (to form FeO‐rich BO chondrules), but incapable of completely melting many magnesian dusty precursors (to form FeO‐poor BO chondrules). When L and LL chondrites agglomerated somewhat later, significant proportions of precursor dustballs were relatively large and had moderately high bulk FeO/(FeO + MgO) ratios. The energy available from flash heating was higher, capable of completely melting higher proportions of magnesian dusty precursors to form FeO‐poor BO chondrules. These differences may have resulted from an increase in the amplitude of lightning discharges in the nebula caused by enhanced charge separation.

High precision 26Al-26Mg chronology of chondrules in unequilibrated ordinary chondrites: Evidence for restricted formation ages

Chondrules in ordinary chondrites are considered to form in high density environments, likely related to the evolution of protoplanets and large planetesimals. In order to determine the timing of their formation at high time resolution (≤0.1 Ma), we conducted high precision Al-Mg chronology of 17 porphyritic chondrules from 6 different unequilibrated ordinary chondrites (UOCs) of low petrologic subtypes (3.00–3.05). Detailed petrology, mineralogy, and oxygen isotope ratios of individual chondrules were also obtained that include 10 additional chondrules without Al-Mg ages. Seventeen chondrules for Al-Mg chronology consist of 14 chondrules with plagioclase (An1-An87) and three chondrules with Na-rich glassy mesostasis, all of which have high 27Al/24Mg ratios (30–3000). The inferred initial (26Al/27Al)0 ratios range between (6.5 ± 0.6) × 10–6 to (9.5 ± 1.0) × 10–6, corresponding to chondrule formation ages of 1.74 ± 0.12/0.11 Ma to 2.13 ± 0.09 Ma after CAIs, which have a canonical (26Al/27Al)0 ratio of 5.25 × 10–5. Six albite-bearing chondrules (An<30) show a much more restricted ages range, spanning between 2.00 ± 0.11/0.10 Ma and 2.07 ± 0.11/0.10 Ma. Including 14 anorthite-bearing chondrules studied previously, chondrules in ordinary chondrites have a restricted range of formation ages from 1.8 Ma to 2.2. Ma after CAIs.Based on the newly acquired oxygen isotope data and previous high precision studies, chondrules in L and LL chondrites do not show systematic difference in their δ18O and δ17O signatures. Majority of plagioclase-bearing chondrules studied for Al-Mg chronology show similar oxygen isotope ratios to those of glass-bearing chondrules. There is no obvious difference in Al-Mg ages of chondrules between L and LL chondrites. Thus, chondrules in L and LL chondrites would have formed in common environments and processes, though they accreted to two separate parent bodies by 2.2 Ma after CAIs, which timing is consistent with the proposed thermal model for ordinary chondrite parent bodies. Onset of chondrule formation at 1.8 Ma after CAIs may be caused by the delay of Jupiter formation or the formation of protoplanets in ordinary chondrite chondrule forming regions if chondrules formed by large scale disk shock or impact jetting of protoplanets. Alternatively, early formed chondrules would not be preserved before 1.8 Ma if chondrules formed by the impacts of molten planetesimals.

New constraints from 26Al-26Mg chronology of anorthite bearing chondrules in unequilibrated ordinary chondrites

26Al-26Mg ages were determined for 14 anorthite-bearing chondrules from five different unequilibrated ordinary chondrites (UOCs) with low petrologic subtypes (3.00–3.05). In addition, oxygen three isotopes of these chondrules were also measured. The selected chondrules are highly depleted in alkali elements, and anorthite is the only mesostasis phase, though they show a range of mafic mineral compositions (Mg# 76–97 mole%) that are representative of chondrules in UOCs. The mean Δ17O values in these chondrules range from −0.44 ± 0.23‰ to 0.49 ± 0.15‰, in good agreement with previous studies of plagioclase-bearing chondrules from UOCs. Anorthite in all chondrules exhibit resolvable excess 26Mg (>1.0 ± 0.4‰). Their inferred (27Al/26Al)0 range from (6.3 ± 0.7) × 10−6 to (8.9 ± 0.3) × 10−6 corresponding to a timescale for chondrule formation of 1.8 ± 0.04 Ma to 2.16 ± 0.12/0.11 Ma after CAIs using a canonical (27Al/26Al)0 value of 5.25 × 10−5. The ages from six chondrules in LL chondrites are restricted to between 1.8 Ma and 1.9 Ma, whereas eight chondrules in L chondrites show ages from 1.8 Ma to 2.2 Ma, including three chondrules at ∼2.0 Ma and two chondrules at ∼2.15 Ma.The inferred chondrule formation ages from this study are at the peak of those previously determined for UOC chondrules, though with much shorter durations. This is potentially due to the time difference between formation of anorthite-bearing chondrules and typical UOC chondrules with alkali-rich compositions. Alternatively, younger chondrules ages in previous studies could have been the result of disturbance to the Al-Mg system in glassy mesostasis even at the low degree of thermal metamorphism in the parent bodies. Nevertheless, the high precision ages from this study (with uncertainties from 0.04 Ma to 0.15 Ma) indicate that there was potentially more than one chondrule forming event represented in the studied population. Considering data from LL chondrites only, the restricted duration (≤0.1 Ma) of chondrule formation ages suggests an origin in high density environments that subsequently lead to parent body formation. However, the unusually low alkali contents of the studied chondrules compared to common alkali-rich chondrules could also represent earlier chondrule formation events under relatively lower dust densities in the disk. Major chondrule forming events for UOCs might have postdated or concurrent with the younger anorthite-bearing chondrule formation at 2.15 Ma after CAIs, which are very close to the timing of accretion of ordinary chondrite parent bodies that are expected from thermal evolution of ordinary chondrite parent bodies.

In situ60Fe-60Ni systematics of chondrules from unequilibrated ordinary chondrites

The initial 60Fe/56Fe of chondrules from unequilibrated ordinary chondrites (UOCs) can potentially constrain the stellar source of short-lived radionuclides and develop the 60Fe-60Ni (t1/2=2.6Ma) system for early solar system chronology. However, progress with the 60Fe-60Ni system has been hindered by discrepancies between initial ratios inferred from bulk and in situ Fe-Ni analyses. Telus et al. (2016) show that discrepancies between these different techniques stem from late-stage open-system Fe-Ni mobilization. Here, we report in situ analyses of the Fe-Ni isotopic composition of ferromagnesian silicates in chondrules from UOCs using the ion microprobe. Of the 24 chondrules analyzed for this study, a few chondrules have resolved excesses in 60Ni of up to 70‰; however, the correlations with Fe/Ni are weak. Although complications from Fe-Ni redistribution make it difficult to interpret the data, we show that the initial 60Fe/56Fe for UOC chondrules is between 5×10−8 and 3.0×10−7. This is consistent with a late supernova source for 60Fe, but self-enrichment of the molecular cloud is another possible mechanism for incorporating 60Fe in the solar system. Discrepancies between bulk and in situ analyses remain, but likely stem from late-stage open-system Fe-Ni mobilization.

Mobility of iron and nickel at low temperatures: Implications for 60Fe–60Ni systematics of chondrules from unequilibrated ordinary chondrites

The Fe and Ni isotopic composition of ferromagnesian silicates in chondrules from unequilibrated ordinary chondrites (UOCs) have been used to estimate the initial abundance of the short-lived radionuclide, 60Fe, in the early Solar System. However, these estimates vary widely, and there are systematic discrepancies in initial 60Fe/56Fe ratios inferred from in situ and bulk analyses of chondrules. A possible explanation is that the Fe–Ni isotope system in UOC chondrules has not remained closed (a necessary condition for isotopic dating), and Fe and Ni have been redistributed since the chondrules formed. In order to evaluate this, we collected high-spatial-resolution X-ray fluorescence (XRF) maps of UOC chondrules to better understand the distribution and mobility of Fe and Ni at the low metamorphic temperatures of these chondrites. We used synchrotron X-ray-fluorescence microscopy to map the distribution of Fe, Ni and other elements in portions of 71 chondrules from 8 UOCs (types 3.00–3.2). The synchrotron XRF maps show clear enrichment of Fe and/or Ni in fractures ranging down to micrometer scale in chondrules from all UOCs analyzed for this study regardless of petrologic type and regardless of whether fall or find, indicating that there was significant exchange of Fe and Ni between chondrules and matrix and that the Fe–Ni system was not closed. Sixty percent of chondrules in Semarkona (LL3.00) have Fe and Ni enrichment along fractures, while 80–100% of chondrules analyzed from the other UOCs show these enrichments. Mobilization was likely a result of fluid transport of Fe and Ni during aqueous alteration on the parent body and/or during terrestrial weathering. In situ and bulk Fe–Ni analyses that incorporate extraneous Fe and Ni from chondrule fractures will result in lowering the inferred initial 60Fe/56Fe ratios.

Nonporphyritic chondrules and chondrule fragments in enstatite chondrites: Insights into their origin and secondary processing

AbstractSixteen nonporphyritic chondrules and chondrule fragments were studied in polished thin and thick sections in two enstatite chondrites (ECs): twelve objects from unequilibratedEH3 Sahara 97158 and four objects from equilibratedEH4 Indarch. Bulk major element analyses, obtained with electron microprobe analysis (EMPA) and analytical scanning electron microscopy (ASEM), as well as bulk lithophile trace element analyses, determined by laser ablation inductively coupled plasma–mass spectrometry (LA‐ICP‐MS), show that volatile components (K2O + Na2O versus Al2O3) scatter roughly around theCIline, indicating equilibration with the chondritic reservoir. All lithophile trace element abundances in the chondrules from Sahara 97158 and Indarch are within the range of previous analyses of nonporphyritic chondrules in unequilibrated ordinary chondrites (UOCs). The unfractionated (solar‐like) Yb/Ce ratio of the studied objects and the mostly unfractionated refractory lithophile trace element (RLTE) abundance patterns indicate an origin by direct condensation. However, the objects possess subchondritic CaO/Al2O3ratios; superchondritic (Sahara 97158) and subchondritic (Indarch) Yb/Sc ratios; and chondritic‐normalized deficits in Nb, Ti, V, and Mn relative toRLTEs. This suggests a unique nebular process for the origin of theseECs, involving elemental fractionation of the solar gas by the removal of oldhamite, niningerite, and/or another phase prior to chondrule condensation. A layered chondrule in Sahara 97158 is strongly depleted in Nb in the core compared to the rim, suggesting that the solar gas was heterogeneous on the time scales of chondrule formation. Late stage metasomatic events produced the compositional diversity of the studied objects by addition of moderately volatile and volatile elements. In the equilibrated Indarch chondrules, this late process has been further disturbed, possibly by a postaccretional process (diffusion?) that preferentially mobilized Rb with respect to Cs in the studied objects.

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

The 26Al– 26Mg 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 26Al/ 27Al ratio, ( 26Al/ 27Al) 0. Well-resolved 26Mg excesses (δ 26Mg) from the in situ decay of the short-lived nuclide 26Al ( 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 ( 26Al/ 27Al) 0 values ranging from ∼3 × 10 −6 to ∼1.5 × 10 −5. Data for seven additional chondrules from three CV and two CR chondrites show evidence suggestive of the presence of 26Al but do not yield well defined values for ( 26Al/ 27Al) 0, while the remaining chondrules do not contain excess radiogenic 26Mg and yield corresponding upper limits of (11–2) × 10 −6 for ( 26Al/ 27Al) 0. The observed range of ( 26Al/ 27Al) 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 ( 26Al/ 27Al) 0 values in PRCs from CCs and the degree of thermal metamorphism experienced by the host meteorites. High and low values of ( 26Al/ 27Al) 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 ( 26Al/ 27Al) 0 values in PRCs from CCs, relative to most CAIs, are consistent with a model in which 26Al 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 ( 26Al/ 27Al) 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.

26Al records in chondrules from unequilibrated ordinary chondrites: II. Duration of chondrule formation and parent body thermal metamorphism

Fossil records of the now-extinct short-lived nuclide 26Al were analyzed in twelve chondrules from four unequilibrated ordinary chondrites (UOCs), Semarkona (3.0), Bishunpur and Y-791324 (both 3.1) and ALHA76004 (3.3), belonging to the LL group to infer their time of formation. The present data along with those reported by us previously firmly establish a short duration of less than a million years for the major episode of UOC chondrule formation. Nebular processes responsible for chondrule formation started ~ 1 Ma after formation of CAIs and effectively stopped within another one and a half million years in the inner region of the asteroidal belt. Modeling of thermal evolution of chondritic parent bodies suggest that the longer duration of UOC chondrule formation inferred in some of the earlier studies could be attributed to thermal metamorphism experienced by some chondrules in UOC parent bodies. The duration of formation of UOC chondrules is significantly shorter than that for carbonaceous chondrite chondrules.

Glass‐rich chondrules in ordinary chondrites

Abstract There are two types of glass‐rich chondrules in unequilibrated ordinary chondrites (OC): (1) porphyritic chondrules containing 55–85 vol% glass or microcrystalline mesostasis and (2) nonporphyritic chondrules, containing 90–99 vol% glass. These two types are similar in mineralogy and bulk composition to previously described Al‐rich chondrules in OC. In addition to Si‐, Al‐ and Na‐rich glass or Ca‐Al‐rich microcrystalline mesostasis, glass‐rich chondrules contain dendritic and skeletal crystals of olivine, Al2O3‐rich low‐Ca pyroxene and fassaite. Some chondrules contain relict grains of forsterite ± Mg‐Al spinel. We suggest that glass‐rich chondrules were formed early in nebular history by melting fine‐grained precursor materials rich in refractory (Ca, Al, Ti) and moderately volatile (Na, K) components (possibly related to Ca‐Al‐rich inclusions) admixed with coarse relict forsterite and spinel grains derived from previously disrupted type‐I chondrules.

The rock-forming elements in Halley's dust

Fossil records of the now-extinct short-lived nuclide 26Al were analyzed in twelve chondrules from four unequilibrated ordinary chondrites (UOCs), Semarkona (3.0), Bishunpur and Y-791324 (both 3.1) and ALHA76004 (3.3), belonging to the LL group to infer their time of formation. The present data along with those reported by us previously firmly establish a short duration of less than a million years for the major episode of UOC chondrule formation. Nebular processes responsible for chondrule formation started ~ 1 Ma after formation of CAIs and effectively stopped within another one and a half million years in the inner region of the asteroidal belt. Modeling of thermal evolution of chondritic parent bodies suggest that the longer duration of UOC chondrule formation inferred in some of the earlier studies could be attributed to thermal metamorphism experienced by some chondrules in UOC parent bodies. The duration of formation of UOC chondrules is significantly shorter than that for carbonaceous chondrite chondrules.

Oxygen isotopic heterogeneities, their petrological correlations, and implications for melt origins of chondrules in unequilibrated ordinary chondrites

Ten whole chondrules separated from the Dhajala (H3, 4), Hallingeberg (L3), and Semarkona (LL3) chondrites were individually analyzed for bulk element composition by instrumental neutron activation with half of each chondrule subsequently sacrificed for oxygen isotopic analysis and half retained for petrographic and electron microprobe analysis. On a three-isotope plot (δ 17O vs. δ 18O), the chondrules neither cluster near their respective chondrite hosts nor in the vicinities of previously recognized chondrite group averages. Instead, they define a trend resolvable into mixing and fractionation components but dominated by mixing in a manner similar to that previously observed for clasts from the LL3 chondrite ALHA76004. Covariations of chondrule isotopic mixing and fractionation parameters with petrological parameters were sought by two-variable linear least-squares regression analyses. However, the only two isotopic/petrological correlations significant at the 95% confidence level were δ 17O vs. total bulk Fe ( r = −0.68) and mixing parameter, m 18, vs. bulk weight ratio (CaO + Al 2O 3)/MgO ( r = +0.67). Other correlations of apparent statistical significance were found by treating the chondrules as separate porphyritic (3 porphyritic olivine-pyroxene, 1 porphyritic olivine, 1 barred olivine) and non-porphyritic (4 radial pyroxene, 1 granular pyroxene/cryptocrystalline) textural subgroups. The reliability of the trends, based on so few samples, is not clear but the results at least indicate that possible existence of distinct isotopic/petrological subgroups of chondrules should be further investigated. Absence of certain isotopic/petrological trends expected as condensation effects argues against direct nebular condensation as the dominant process of chondrule formation. Instead, a model involving melting of heterogeneous solids, followed by various degrees of liquid/gas exchange, is favored. In any case, chondrule oxygen isotopic evolution was dominated by two-component mixing; fractional vaporization was, at most, a second-order effect. In addition to chondrules, parent bodies of unequilibrated ordinary chondrites must have also incorporated a 16O-rich component which might have been fine-grained “matrix”.

Fine, nickel-poor Fe-Ni grains in the olivine of unequilibrated ordinary chondrites

Fine (≲2 μm), Ni-poor (≲ 10 mg/g) Fe-Ni grains are common inclusions in the olivine in porphyritic chondrules in unequilibrated ordinary chondrites. The olivine grains appear to be relicts that survived chondrule formation without melting. The most common occurrence of this “dusty” metal is in the core of olivine grains having clear Fe-poor rims and surrounded either by small euhedral clear olivine grains zoned with FeO increasing toward the border of the grains or by large elongated Fe-poor orthopyroxenes oriented parallel to the chondrule surface and enclosing small round olivine grains. Various amounts of Ca, Al-rich glass are always present. The dusty metal is occasionally found in the rims of olivine grains either isolated in the matrix or included in chondrules. A rare occurrence is as bands in highly deformed olivines. This dusty metal appears to be the product of in situ reduction of FeO from the host olivine. Among the possible reductants H 2 or carbonaceous matter (CH 2) n seem the most likely. Hydrogen may have been implanted by solar-wind or solar-flare irradiation, but this requires that dissipation of nebular gas occurred before the end of the chondrule formation process. Carbonaceous matter may have been implanted by shock. Less likely reductants are nebular CO or C dissolved in the olivine lattice. The large relict olivine grains may be nebular condensates or, more likely, fragments broken off earlier generations of chondrules.

A study of chondrule rims and chondrule irradiation records in unequilibrated ordinary chondrites

In order to investigate the possibility that chondrules may have had an independent existence in space, we have searched for unusual nuclear track densities in chondrules and studied the compositions of chondrule rims on chondrules from thirteen unequilibrated ordinary chondrites. Our search for unusual radiation features has been negative. Observed track densities can be explained in terms of cosmic ray exposure ages of the respective meteorites. Fine-grained rims that surround chondrules in unequilibrated ordinary chondrites are heterogeneous in composition consisting of varying proportions of iron sulfide and a poorly characterized silicate phase. The latter phase or phases are roughly chondritic in composition. Fine-grained rims of the kind seen in primitive type 3 ordinary chondrites are absent in higher petrographie grades; more crystalline, coarse-grained and lacy sulfide rims, however, are observed. Our observations can be explained by chondrules having had an independent existence in space during which they acquired rims either by condensation on their surfaces or by accretion of fine particles. However, accumulation of rims while chondrules resided on a meteorite parent body cannot be ruled out at this time. In any case, we do not propose that the chondrules themselves formed by condensation. Absence of a track record of space exposure of chondrules could be due to shielding by matter in space if, for example, chondrules were present in space in clouds made of dust, gas and/or chondrules.