Forsterite‐bearing type B refractory inclusions from CV3 chondrites: From aggregates to volatilized melt droplets

Abstract– Detailed petrologic and oxygen isotopic analysis of six forsterite‐bearing Type B calcium‐aluminum‐rich inclusions (FoBs) from CV3 chondrites indicates that they formed by varying degrees of melting of primitive precursor material that resembled amoeboid olivine aggregates. A continuous evolutionary sequence exists between those objects that experienced only slight partial melting or sintering through objects that underwent prolonged melting episodes. In most cases, melting was accompanied by surface evaporative loss of magnesium and silicon. This loss resulted in outer margins that are very different in composition from the cores, so much so that in some cases, the mantles contain mineral assemblages that are petrologically incompatible with those in the cores. The precursor objects for these FoBs had a range of bulk compositions and must therefore have formed under varying conditions if they condensed from a solar composition gas. Five of the six objects show small degrees of mass‐dependent oxygen isotopic fractionation in pyroxene, spinel, and olivine, consistent with the inferred melt evaporation, but there are no consistent differences among the three phases. Forsterite, spinel, and pyroxene are ¹⁶O‐rich with Δ¹⁷O ～ ?24‰ in all FoBs. Melilite and anorthite show a range of Δ¹⁷O from ?17‰ to ?1‰.     

Oxygen isotopic zoning of reversely zoned melilite crystals in a fluffy type A Ca‐Al‐rich inclusions from the Vigarano meteorite

Abstract– The oxygen isotopic microdistributions within melilite measured using in situ secondary ion mass spectrometry correspond to the chemical zoning profiles in single melilite crystals of a fluffy type A Ca‐Al‐rich inclusions (CAIs) of reduced CV3 Vigarano meteorite. The melilite crystals show chemical reverse zoning within an individual single crystal from the åkermanite‐rich core to the åkermanite‐poor rim. The composition changes continuously with the crystal growth. The zoning structures suggest that the melilite grew in a hot nebular gas by condensation with decreasing pressure. The oxygen isotopic composition of melilite also changes continuously from ¹⁶O‐poor to ¹⁶O‐rich with the crystal growth. These observations suggest that the melilite condensation proceeded with change consistent with an astrophysical setting around the inner edge of a protoplanetary disk where both ¹⁶O‐rich solar coronal gas and ¹⁶O‐poor dense protoplanetary disk gas could coexist. Fluffy type A CAIs could have been formed around the inner edge of the protoplanetary disk surrounding the early sun.     

The White Angel: A unique wollastonite‐bearing,mass‐fractionated refractory inclusion from the Leoville CV3 carbonaceous chondrite

Abstract— We report the study of an unusual compact type A refractory inclusion, named the White Angel, from the Leoville CV3 meteorite. The petrologic, mineral chemical, isotopic, and trace‐element signatures of this once‐molten Ca‐Al‐rich inclusion (CAI), which contains large, equant wollastonite crystals, indicate a short multistage history that occurred very early, before substantial decay of ²⁶Al. Magnesium in the inclusion is isotopically heavy, with F_Mg reaching 18‰/amu, in the range of fractionated and with unidentified nuclear effects (FUN) inclusions. However, the absence of any nuclear anomalies in Ca and Ti and an inferred ²⁶Al/²⁷Al ratio of (5.5 ± 0.9) × 10^?5 indicate that the White Angel belongs to the F inclusions. Silicon and oxygen are also mass fractionated in favor of the heavy isotopes, but to a lesser extent. The O isotopes show a range in ¹⁶O excesses. On an O three‐isotope plot, data points lie on a line parallel and to the right of the carbonaceous chondrite anhydrous mineral mixing line, with wollastonite being the most ¹⁶O‐rich phase. The chondrite‐normalized rare earth and trace‐element pattern of the whole inclusion is the complement of an ultrarefractory pattern indicating that precursor phases of the CAI must have condensed in an Al‐, heavy rare earth element (HREE)‐depleted reservoir. Melting of those precursor phases in an ¹⁶O‐rich environment and evaporation led to mass‐dependent isotopic fractionation of Mg, Si, and O. Partial isotopic exchange with a reservoir containing unfractionated Mg took place at a later stage but before any measurable decay of ²⁶Al. Some minerals (melilite and perovskite) in the White Angel equilibrated oxygen isotopes with a relatively ¹⁶O‐poor reservoir that was also mass‐fractionated toward the heavy isotopes, different from that with which the normal or FUN inclusions interacted.     

Mg and Si isotopic fractionation patterns in types B1 and B2 CAIs: Implications for formation under different nebular conditions

Magnesium and silicon isotopic profiles across melilite grains in two type B1 and two type B2 calcium‐aluminum‐rich inclusions (CAIs) reveal differing but constant enrichments in heavy isotopes everywhere except ≤1000 μm from the CAI margins. There is no close correlation in the B1s or the B2s between isotopic composition and åkermanite content of the melilite, a measure of progressive igneous crystallization, yet such a correlation might be expected in a type B2: without a melilite mantle (as in B1s) to seal the interior off and prevent further evaporation, the melt would have maintained communication with the external gas. These observations indicate a model in which B1s and B2s solidified under differing conditions. The B2s solidified under lower hydrogen pressures ( ≤ 10^?4 – 10^?5 bars) than did B1s ( > 10^?4 bars), so surface volatilization was slower in the B2s and internal chemical and isotopic equilibrium was maintained over the interval of melilite crystallization. The outermost zones of the CAIs (≤1000 μm from the edge) are not consistently enriched in heavy isotopes relative to the interiors, as might be expected from diffusion‐limited surface evaporation of the melt. In all cases, the magnesium in the CAI margins is lighter than in the interiors. In one case, silicon in the margin also is lighter, but locally in some CAIs, it is isotopically heavier near the surface. If melt evaporation played a role in the formation of these outer zones, a later event in many cases caused isotopic re‐equilibration with an external and isotopically near‐normal reservoir.     

The role of exchange reactions in oxygen isotope fractionation during CAI and chondrule formation

Abstract— The role of oxygen isotope exchange during evaporation and condensation of silicate melt is quantitatively evaluated. Silicate dusts instantaneously heated above liquidus temperature are assumed to cool in gas and experience partial evaporation and subsequent recondensation. The results show that isotopic exchange effectively suppresses mass‐dependent O‐isotope fractionation even if the degree of evaporation is large, which is the fundamental difference from the case without isotopic exchange. The final composition of silicate melt strongly depends on the initial abundance of oxygen in the ambient gas relative to that in silicate dust, but not on the cooling rate of the system. The model was applied to O‐isotope evolution of silicate melts in isotopically distinct gas of the protoplanetary disk. It was found that deviation from a straight mixing line toward the δ¹⁸O‐rich side on the three‐oxygen isotope diagram is inevitable when mass‐dependent fractionation and isotopic exchange take place simultaneously; the degree of deviation depends on the abundance of oxygen in an ambient gas and isotopic exchange efficiency. The model is applied to explain O‐isotopic compositions of igneous CAIs and chondrules.     

Oxygen isotopic composition of the solar nebula gas inferred from high‐precision isotope imaging of melilite crystals in an Allende CAI

Abstract– High‐precision isotope imaging analyses of reversely zoned melilite crystals in the gehlenitic mantle of Type A CAI ON01 of the Allende carbonaceous chondrite reveal that there are four types of oxygen isotopic distributions within melilite single crystals: (1) uniform depletion of ¹⁶O (δ¹⁸O ≈ ?10‰), (2) uniform enrichment of ¹⁶O (δ¹⁸O ≈ ?40‰), (3) variations in isotopic composition from ¹⁶O‐poor core to ¹⁶O‐rich rim (δ¹⁸O ≈ ?10‰ to ?30‰, ?20‰ to ?45‰, and ?10‰ to ?35‰) with decreasing åkermanite content, and (4) ¹⁶O‐poor composition (δ¹⁸O ≥ ?10‰) along the crystal rim. Hibonite, spinel, and perovskite grains are ¹⁶O‐rich (δ¹⁸O ≈ ?45‰), and adjoin ¹⁶O‐poor melilites. Gas‐solid or gas‐melt isotope exchange in the nebula is inconsistent with both the distinct oxygen isotopic compositions among the minerals and the reverse zoning of melilite. Fluid‐rock interaction on the parent body resulted in ¹⁶O‐poor compositions of limited areas near holes, cracks, or secondary phases, such as anorthite or grossular. We conclude that reversely zoned melilites mostly preserve the primary oxygen isotopic composition of either ¹⁶O‐enriched or ¹⁶O‐depleted gas from which they were condensed. The correlation between oxygen isotopic composition and åkermanite content may indicate that oxygen isotopes of the solar nebula gas changed from ¹⁶O‐poor to ¹⁶O‐rich during melilite crystal growth. We suggest that the radial excursions of the inner edge of the protoplanetary disk gas simultaneously resulted in both the reverse zoning and oxygen isotopic variation of melilite, due to mixing of ¹⁶O‐poor disk gas and ¹⁶O‐rich coronal gas. Gas condensates aggregated to form the gehlenite mantle of the Type A CAI ON01.     

Differentiation of planetesimals and the thermal consequences of melt migration

Abstract– We model the heating of a primordial planetesimal by decay of the short‐lived radionuclides ²⁶Al and ⁶⁰Fe to determine (1) the time scale on which melting will occur, (2) the minimum size of a body that will produce silicate melt and differentiate, (3) the migration rate of molten material within the interior, and (4) the thermal consequences of the transport of ²⁶Al in partial melt. Our models incorporate results from previous studies of planetary differentiation and are constrained by petrologic (i.e., grain‐size distributions), isotopic (e.g., ²⁰⁷Pb‐²⁰⁶Pb and ¹⁸²Hf‐¹⁸²W ages), and mineralogical properties of differentiated achondrites. We show that formation of a basaltic crust via melt percolation was limited by the formation time of the body, matrix grain size, and viscosity of the melt. We show that low viscosity (<1 Pa · s) silicate melt can buoyantly migrate on a time scale comparable to the mean life of ²⁶Al. The equilibrium partitioning of Al into silicate partial melt and the migration of that melt acts to dampen internal temperatures. However, subsequent heating from the decay of ⁶⁰Fe generated melt fractions in excess of 50%, thus completing differentiation for bodies that accreted within 2 Myr of CAI formation (i.e., the onset of isotopic decay). Migration and concentration of ²⁶Al into a crust results in remelting of that crust for accretion times less than 2 Myr and for bodies >100 km in size. Differentiation would be most likely for planetesimals larger than 20 km in diameter that accreted within approximately 2.7 Myr of CAI formation.     

Mg isotopic heterogeneity,Al‐Mg isochrons,and canonical 26Al/27Al in the early solar system

There is variability in the Mg isotopic composition that is a reflection of the widespread heterogeneity in the isotopic composition of the elements in the solar system at approximately 100 ppm. Measurements on a single calcium‐aluminum‐rich inclusion (CAI) gave a good correlation of ²⁶Mg/²⁴Mg with ²⁷Al/²⁴Mg, yielding an isochron corresponding to an initial (²⁶Al/²⁷Al)_o = (5.27 ± 0.18) × 10^?5 and an initial (²⁶Mg/²⁴Mg)_o = ?0.127 ± 0.032‰ relative to the standard. This isochron is parallel to that obtained by Jacobsen et al. (2008) , but is distinctively offset. This demonstrates that there are different initial Mg isotopic compositions in different samples with the same ²⁶Al/²⁷Al. No inference about uniformity/heterogeneity of ²⁶Al/²⁷Al on a macro scale can be based on the initial (²⁶Mg/²⁴Mg)_o values. Different values of ²⁶Al/²⁷Al for samples representing the same point in time would prove heterogeneity of ²⁶Al/²⁷Al. The important issue is whether the bulk solar inventory of ²⁶Al/²⁷Al was approximately 5 × 10^?5 at some point in the early solar system. We discuss ultra refractory phases of solar type oxygen isotope composition with ²⁶Al/²⁷Al from approximately 5 × 10^?5 to below 0.2 × 10^?5. We argue that the real issues are: intrinsic heterogeneity in the parent cloud; mechanism and timing for the later production of ¹⁶O‐poor material; and the relationship to earlier formed ¹⁶O‐rich material in the disk. ²⁶Al‐free refractories can be produced at a later time by late infall, if there is an adequate heat source, or from original heterogeneities in the placental molecular cloud from which the solar system formed.     

Oxygen isotopic and chemical zoning of melilite crystals in a type A Ca‐Al‐rich inclusion of Efremovka CV3 chondrite

Abstract– Different oxygen isotopic reservoirs have been recognized in the early solar system. Fluffy type A Ca‐Al‐rich inclusions (CAIs) are believed to be direct condensates from a solar nebular gas, and therefore, have acquired oxygen from the solar nebula. Oxygen isotopic and chemical compositions of melilite crystals in a type A CAI from Efremovka CV3 chondrite were measured to reveal the temporal variation in oxygen isotopic composition of surrounding nebular gas during CAI formation. The CAI is constructed of two domains, each of which has a core‐mantle structure. Reversely zoned melilite crystals were observed in both domains. Melilite crystals in one domain have a homogeneous ¹⁶O‐poor composition on the carbonaceous chondrite anhydrous mineral (CCAM) line of δ¹⁸O = 5–10‰, which suggests that the domain was formed in a ¹⁶O‐poor oxygen isotope reservoir of the solar nebula. In contrast, melilite crystals in the other domain have continuous variations in oxygen isotopic composition from ¹⁶O‐rich (δ¹⁸O = ?40‰) to ¹⁶O‐poor (δ¹⁸O = 0‰) along the CCAM line. The oxygen isotopic composition tends to be more ¹⁶O‐rich toward the domain rim, which suggests that the domain was formed in a variable oxygen isotope reservoir of the solar nebula. Each domain of the type A CAI has grown in distinct oxygen isotope reservoir of the solar nebula. After the domain formation, domains were accumulated together in the solar nebula to form a type A CAI.     

Chemical and isotopic constraints on the formation and crystallization of SA-1, a basaltic Allende plagioclase-olivine inclusion

Abstract— SA-1, an unusual basaltic plagioclase-olivine inclusion (POI) in Allende, has concentric textural and mineralogic zones, a fine-grained, 100μm outer border and a coarse-grained interior with subophitic texture. Fassaite, diopside and olivine from the exterior border and interior of SA-1 have uniform intrinsic mass fractionation with isotopically heavy Mg (F_Mg = 3.6 ± 1.8‰/amu). In contrast, spinels from the spinel-rich regions adjacent to the fine-grained border have normal Mg isotopic composition (F_Mg = 0.1 ± 1.5‰/amu). The cores of large calcic (An_90,99) plagioclase have no excess ²⁶Mg, corresponding to ²⁶Mg*/ ²⁷Al < 3.7 × 10^?6. The Mg isotopic heterogeneity in SA-1 requires initial cooling rates of spinel-rich regions adjacent to the fine-grained border to be greater than ～75 °C/hr. In contrast, the subophitic texture of the interior suggests cooling rates of 5–20 °C/ hr. The minerals in SA-1 exhibit a wide range of REE abundances. Lanthanum concentrations vary from 1 × chondritic (ch) in early crystallizing diopside to 100 × ch in late crystallizing fassaite. Nepheline has 18–20 × ch LREE and 11–25 × ch HREE and iron-rich mesostasis is highly enriched in the REE with 270–400 × ch LREE and 230–280 × ch HREE. The complementary REE patterns of clinopyroxene and plagioclase and the enrichment of incompatible trace elements in the mesostasis and late crystallizing phases is consistent with closed system crystallization. The REE data for nepheline and the iron-rich mesostasis indicate these phases are in equilibrium and that nepheline crystallized from a melt. Influx of alkalies, minor Fe and halogens must have occurred during the last stages of crystallization or the inclusion must have been partially molten during Na influx as both anorthite (An₉₉) and nepheline are present in this inclusion. The preservation of isotopic heterogeneity in an inclusion that crystallized from a melt implies that melting was incomplete, allowing for survival of the relict spinels. The major and trace element abundances in SA-1 are inconsistent with formation as a mixture of nebular materials and suggest that SA-1 contains a chemically fractionated component produced by igneous differentiation.     

The lack of potassium‐isotopic fractionation in Bishunpur chondrules

Abstract— In a search for evidence of evaporation during chondrule formation, the mesostases of 11 Bishunpur chondrules and melt inclusions in olivine phenocrysts in 7 of them have been analyzed for their alkali element abundances and K‐isotopic compositions. Except for six points, all areas of the chondrules that were analyzed had δ⁴¹K compositions that were normal within error (typically ±3%, 2s?). The six “anomalous” points are probably all artifacts. Experiments have shown that free evaporation of K leads to large ⁴¹K enrichments in the evaporation residues, consistent with Rayleigh fractionation. Under Rayleigh conditions, a 3% enrichment in δ⁴¹K is produced by ～12% loss of K. The range of L‐chondrite‐normalized K/Al ratios (a measure of the K‐elemental fractionation) in the areas analyzed vary by almost three orders of magnitude. If all chondrules started out with L‐chondrite‐like K abundances and the K loss occurred via Rayleigh fractionation, the most K‐depleted chondrules would have had compositions of up to δ⁴¹K ? 200%. Clearly, K fractionation did not occur by evaporation under Rayleigh conditions. Yet experiments and modeling indicate that K should have been lost during chondrule formation under currently accepted formation conditions (peak temperature, cooling rate, etc.). Invoking precursors with variable alkali abundances to produce the range of K/Al fractionation in chondrules does not explain the K‐isotopic data because any K that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation. If K loss and isotopic fractionation was inevitable during chondrule formation, the absence of K‐isotopic fractionation in Bishunpur chondrules requires that they exchanged K with an isotopically normal reservoir during or after formation. There is evidence for alkali exchange between chondrules and rim‐matrix in all unequilibrated ordinary chondrites. However, melt inclusions can have alkali abundances that are much lower than the mesostases of the host chondrules, which suggests that they at least remained closed since formation. If it is correct that some or all melt inclusions remained closed since formation, the absence of K‐isotopic fractionation in them requires that the K‐isotopic exchange took place during chondrule formation, which would probably require gas‐chondrule exchange. Potassium evaporated from fine‐grained dust and chondrules during chondrule formation may have produced sufficient K‐vapor pressure for gas‐chondrule isotopic exchange to be complete on the timescales of chondrule formation. Alternatively, our understanding of chondrule formation conditions based on synthesis experiments needs some reevaluation.     

Oxygen isotope and 26Al‐26Mg systematics of aluminum‐rich chondrules from unequilibrated enstatite chondrites

Abstract— Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum‐rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum‐rich chondrules and two plagioclase fragments measured for ²⁶Al‐²⁶Mg systematics, only one aluminum‐rich chondrule contains excess ²⁶Mg from the in situ decay of ²⁶Al; the inferred initial ratio (²⁶Al/²⁷Al)_o = (6.8 ± 2.4) × 10^?6 is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites. The oxygen isotopic compositions of five aluminum‐rich chondrules and one plagioclase fragment define a line of slope ?0.6 ± 0.1 on a three‐oxygen‐isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more ¹⁶O‐rich compositions with a range in δ¹⁸O of about ?12‰. Based on their oxygen isotopic compositions, aluminum‐rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium‐aluminum‐rich inclusions (CAIs). Relative to their counterparts from unequilibrated ordinary chondrites, aluminum‐rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess ²⁶Mg from the in situ decay of ²⁶Al, probably resulting from higher degrees of equilibration and isotopic exchange during post‐crystallization metamorphism. However, the presence of ²⁶Al‐bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that ²⁶Al was at least approximately homogeneously distributed across the chondrite‐forming region.     

Conditions in the protoplanetary disk as seen by the type B CAIs

Abstract— Type B coarse‐grained calcium‐aluminum‐rich inclusions (CAIs) are the oldest known materials to have formed in the solar system and are a unique source of information regarding conditions and processes in the protoplanetary disk around the young sun. Recent experimental results on the crystallization and evaporation of type B‐like silicate melts allow us to place the following constraints on the conditions in the protoplanetary disk during the formation of type B CAIs. 1) Once type B CAIs precursors have been condensed from a solar composition gas, they were reheated at 1250–1450 °C, as is indicated by their igneous texture. 2) The melilite mantles characteristic of type B1 CAIs could be formed by crystallization of magnesium‐ and silicon‐depleted melt in the outer part of the partially molten droplets. Such depletion can arise when evaporation is fast compared to chemical diffusion in the melt. This requires the pressure of the surrounding solar composition gas to be at least 10⁻⁴ bars during the initial crystallization of melilite mantle. Type B2 CAIs with uniform distribution of melilite are expected to form at pressures less than 10⁻⁵ bars. 3) Evaporation calculations are used to place bounds on the thermal history of the type B CAIs. Observed compositional zoning in melilite suggests that the temperatures in the protoplanetary disk where the type B CAIs resided after crystallization could not have exceeded ˜1000 °C for more than a few tens of thousands of years. A recent calculation of the physical conditions associated with nebular shocks produced transient temperatures and gas pressures very much like what we find is required to melt reasonable CAI precursors and evaporate these sufficiently quickly to make a type B1 CAI.     

The age of the Roter Kamm impact crater,Namibia: Constraints from 40Ar-39Ar,K-Ar,Rb-Sr,fission track,and 10Be-26Al studies

Abstract The well-preserved 2.5 km diameter Roter Kamm impact crater is located in the Namib desert in Namibia. The impact has occurred in Precambrian granitic and granodioritic orthogneisses of the 1200–900 Ma old Namaqualand Metamorphic Complex which were partly covered by Gariep metasediments; the granites are invaded by quartz veins and quartz-feldspar-pegmatites. Previous geological field evidence suggested a crater age of about 5–10 Ma. In order to constrain this age, we selected a set of basement rocks (granites, granodiorites) exposed at the crater rim and studied the Rb-Sr, K-Ar, ⁴⁰Ar-³⁹Ar, and ¹⁰Be-²⁶Al isotopic systems as well as apatite fission track ages of these samples. The Rb-Sr isotopic systematics confirm the derivation of these samples from the Namaqualand basement (age about 1.29 Ga), which underwent Damaran orogenesis at about 650 Ma. No basement rocks with Rb-Sr ages younger than about 410 Ma were identified. The K-Ar ages of pseudotachylite and melt breccia samples show that these samples are dominated by incompletely degassed fragments of basement rocks, with some retaining their original metamorphic ages of about 470 Ma. The apatite fission track ages range from 20–28 Ma, which may be interpreted as an extension of the 25 Ma Burdigalian peneplanation event, or as incomplete resetting of the apatite fission tracks during the impact event. The ¹⁰Be and ²⁶Al exposure age of a quartz sample isolated from a quartz-pegmatite was found to be 150 ka; it is likely that the exposure of the sample began after material covering it had been removed by erosion 150 ka ago. Two glassy fractions extracted from a rim granite were dated by ⁴⁰Ar-³⁹Ar analysis. One sample gives practically a plateau age of 3.7 ± 0.3 Ma, while the other gives a minimum age of 3.6 Ma. The best available age estimate for the Roter Kamm crater is therefore 3.7 ± 0.3 Ma.     

Laboratory experiments bearing on the origin and evolution of olivine‐rich chondrules

Abstract– Evaporation rates of K₂O, Na₂O, and FeO from chondrule‐like liquids and the associated potassium isotopic fractionation of the evaporation residues were measured to help understand the processes and conditions that affected the chemical and isotopic compositions of olivine‐rich type IA and type IIA chondrules from Semarkona. Both types of chondrules show evidence of having been significantly or totally molten. However, these chondrules do not have large or systematic potassium isotopic fractionation of the sort found in the laboratory evaporation experiments. The experimental results reported here provide new data regarding the evaporation kinetics of sodium and potassium from a chondrule‐like melt and the potassium isotopic fractionation of evaporation residues run under various conditions ranging from high vacuum to pressures of one bar of H₂+CO₂, or H₂, or helium. The lack of systematic isotopic fractionation of potassium in the type IIA and type IA chondrules compared with what is found in the vacuum and one‐bar evaporation residues is interpreted as indicating that they evolved in a partially closed system where the residence time of the surrounding gas was sufficiently long for it to have become saturated in the evaporating species and for isotopic equilibration between the gas and the melt. A diffusion couple experiment juxtaposing chondrule‐like melts with different potassium concentrations showed that the diffusivity of potassium is sufficiently fast at liquidus temperatures (D_K > 2 × 10^?4cm² s^?1 at 1650 °C) that diffusion‐limited evaporation cannot explain why, despite their having been molten, the type IIA and type IA chondrules show no systematic potassium isotopic fractionation.     

Al‐Mg systematics of CAIs,POI, and ferromagnesian chondrules from Ningqiang

Abstract— We have made aluminum‐magnesium isotopic measurements on 4 melilite‐bearing calcium‐aluminum‐rich inclusions (CAIs), 1 plagioclase‐olivine inclusion (POI), and 2 ferromagnesian chondrules from the Ningqiang carbonaceous chondrite. All of the CAIs measured contain clear evidence for radiogenic ²⁶Mg* from the decay of ²⁶Al ( = 1.05 Ma). Although the low Al/Mg ratios of the melilites introduce large uncertainties, the inferred initial ²⁶Al/²⁷Al ratios for the CAIs are generally consistent with the value of 5 times 10^?5. There is clear evidence of ²⁶Al* in one POI and two chondrules, but with considerable uncertainties in the value of (²⁶Al/²⁷Al)₀. The (²⁶Al/²⁷Al)₀ ratios for the POI and the chondrules are 0.3–0.6 times 10^?5, roughly an order of magnitude lower than the canonical value. Ningqiang shows very little evidence of metamorphism as a bulk object and the (²⁶Al/²⁷Al)₀ ratios in its refractory inclusions and chondrules are consistent with those found in other unmetamorphosed chondrites of several different classes. Our observations and those of other workers support the view that ²⁶Al was widely and approximately homogeneously distributed throughout the condensed matter of the solar system. The difference in (²⁶Al/²⁷Al)₀ between CAIs and less refractory materials seems reasonably interpreted in terms of a ～2 million year delay between the formation of CAIs and the onset of formation of less refractory objects. The POI shows clear differences in ²⁵Mg/²⁴Mg between its constituent spinels and olivine, which confirms that they are partially reprocessed material from different sources that were rapidly quenched.     

Accretionary mixing of a eucrite impactor and the regolith of the L chondrite parent body

Northwest Africa (NWA) 869 is the largest sample of chondritic regolith breccia, making it an ideal source for research on accretionary processes and primordial chemical mixing. One such process can be seen in detail through the first identification of a eucrite impactor clast in an L chondrite breccia. The ~7 mm diameter clast has oxygen isotope compositions (Δ¹⁷O = ?0.240, ?0.258‰) and pigeonite and augite compositions typical for eucrites, but with high areal abundance of silica (9.5%) and ilmenite (1.5%). The rim around the clast is a mixture of breccia and igneous phases, the latter due to either impactor‐triggered melting or later metamorphism. The rim has an oxygen isotope composition falling on a mixing line between known eucrite and L chondrite compositions (Δ¹⁷O = 0.326‰) and, coincidentally, on the Mars fractionation line. Pyroxene grains from the melt component in the rim have compositions that fall on a mixing line between the average eucrite pyroxene composition and equilibrated L chondrite composition. The margins of chondritic olivine crystal clasts in the rim are enriched in Fe as a result of diffusion from the Fe‐rich melt and suggest cooling on the scale of hours. The textures and chemical mixing observed provide evidence for an unconsolidated L chondrite target material, differing from the current state of NWA 869 material. The heterogeneity of oxygen isotope and chemical signatures at this small length scale serve as a cautionary note when extrapolating from small volumes of materials to deduce planetesimal source characteristics.     

Oxygen and Al‐Mg isotopic compositions of grossite‐bearing refractory inclusions from CO3 chondrites

The distribution of the short‐lived radionuclide ²⁶Al in the early solar system remains a major topic of investigation in planetary science. Thousands of analyses are now available but grossite‐bearing Ca‐, Al‐rich inclusions (CAIs) are underrepresented in the database. Recently found grossite‐bearing inclusions in CO3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearing CAIs in the Dominion Range (DOM) 08006 (CO3.0) and DOM 08004 (CO3.1) chondrites. All minerals in DOM 08006 CAIs as well as hibonite, spinel, and pyroxene in DOM 08004 are uniformly ¹⁶O‐rich (Δ¹⁷O = ?25 to ?20‰) but grossite and melilite in DOM 08004 CAIs are not; Δ¹⁷O of grossite and melilite range from ~ ?11 to ~0‰ and from ~ ?23 up to ~0‰, respectively. Even within this small suite, in the two chondrites a bimodal distribution of the inferred initial ²⁶Al/²⁷Al ratios (²⁶Al/²⁷Al)₀ is seen, with four having (²⁶Al/²⁷Al)₀ ≤1.1 × 10^?5 and six having (²⁶Al/²⁷Al)₀ ≥3.7 × 10^?5. Five of the ²⁶Al‐rich CAIs have (²⁶Al/²⁷Al)₀ within error of 4.5 × 10^?5; these values can probably be considered indistinguishable from the “canonical” value of 5.2 × 10^?5 given the uncertainty in the relative sensitivity factor for grossite measured by secondary ion mass spectrometry. We infer that the ²⁶Al‐poor CAIs probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in the DOM 08006 CAIs, as well as spinel, hibonite, and Al‐diopside in the DOM 08004 CAIs retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where the CO grossite‐bearing CAIs originated. Oxygen isotopic heterogeneity in CAIs from DOM 08004 resulted from exchange between the initially ¹⁶O‐rich (Δ¹⁷O ~?24‰) melilite and grossite and ¹⁶O‐poor (Δ¹⁷O ~0‰) fluid during hydrothermal alteration on the CO chondrite parent body; hibonite, spinel, and Al‐diopside avoided oxygen isotopic exchange during the alteration. Grossite and melilite that underwent oxygen isotopic exchange avoided redistribution of radiogenic ²⁶Mg and preserved undisturbed internal Al‐Mg isochrons. The Δ¹⁷O of the fluid can be inferred from O‐isotopic compositions of aqueously formed fayalite and magnetite that precipitated from the fluid on the CO parent asteroid. This and previous studies suggest that O‐isotope exchange during fluid–rock interaction affected most CAIs in CO ≥3.1 chondrites.     

Petrography,classification, oxygen isotopes,noble gases,and cosmogenic records of Kamargaon (L6) meteorite: The latest fall in India

A single piece of meteorite fell on Kamargaon village in the state of Assam in India on November 13, 2015. Based on mineralogical, chemical, and oxygen isotope data, Kamargaon is classified as an L‐chondrite. Homogeneous olivine (Fa: 25 ± 0.7) and low‐Ca pyroxene (Fs: 21 ± 0.4) compositions with percent mean deviation of <2, further suggest that Kamargaon is a coarsely equilibrated, petrologic type 6 chondrite. Kamargaon is thermally metamorphosed with an estimated peak metamorphic temperature of ~800 °C as determined by two‐pyroxene thermometry. Shock metamorphism studies suggest that this meteorite include portions of different shock stages, e.g., S3 and S4 (Stöffler et al. 1991 ); however, local presence of quenched metal‐sulfide melt within shock veins/pockets suggest disequilibrium melting and relatively higher shock stage of up to S5 (Bennett and McSween 1996 ). Based on noble gas isotopes, the cosmic‐ray exposure age is estimated as 7.03 ± 1.60 Ma and nitrogen isotope composition (δ¹⁵N = 18‰) also correspond well with the L‐chondrite group. The He‐U, Th, and K‐Ar yield younger ages (170 ± 25 Ma 684 ± 93, respectively) and are discordant. A loss of He during the resetting event is implied by the lower He‐U and Th age. Elemental ratios of trapped Ar, Kr, and Xe can be explained through the presence of a normal Q noble gas component. Relatively low activity of ²⁶Al (39 dpm/kg) and the absence of ⁶⁰Co activity suggest a likely low shielding depth and envisage a small preatmospheric size of the meteoroid (<10 cm in radius). The Kr isotopic ratios (⁸²Kr/⁸⁴Kr) further argue that the meteorite was derived from a shallow depth.     

A theory of local formation of isotopic anomalies in meteorites

The consequences of a postulated collision between planets in the early solar system have been investigated. At least one of the planets has been taken with a D/H ratio similar to that of Venus (0.016) and the temperature of the collision interface (3 × 10⁶ K) triggers chain reactions in near-surface material beginning with D-D reactions. The initial composition of the reacting material is consistent with a silicate + ices surface and a hydrogen-helium-inert gas atmosphere. The reaction chain contains 284 reactions, plus reverse reactions, and 40 radioactive decay processes. When the pressure in the reacting region is sufficiently high the colliding planets are blown apart and the highly-processed material at the heart of the explosion mixes with less processed and unprocessed material from cooler parts of the system. Mixtures of materials are found to explain isotopic anomalies associated with oxygen, magnesium, neon, silicon, carbon and nitrogen. The local production of isotopic anomalies avoids the problems associated with other suggested explanations - in particular the observation of neon E, almost pure²²Ne, assumed as the product of the decay of²²Na with a half-life of 2.6 years.