Excess 26Mg Research Articles

Formation of orange hibonite, as inferred from some Allende inclusions

Abstract— We studied three fluffy Type A refractory inclusions from Allende that contain orange hibonite. The melilite in the present samples is very Al‐rich, averaging Åk6, Åk14, and Åk12 in the three samples studied. Hibonite in two inclusions, unlike that in Murchison, has low rare earth element abundances of <10 × CI; in the other inclusion, the hibonite, melilite and perovskite have Group II‐like patterns. The hibonite and melilite in all three inclusions studied have excess 26Mg consistent with (26Al/27Al)I = 5 × 10−5. Much of the hibonite and some of the spinel in these inclusions is corroded. These phases are found enclosed in melilite, but based on bulk compositions and phase equilibria, hibonite should not be an early‐crystallizing phase in these inclusions. We conclude that the hibonite and probably some of the spinel is relic. Reversely zoned melilite, rounded spinel and isotopically heavy Mg in the inclusions probably reflect reheating events that involved melting and evaporation. Alteration of the gehlenitic melilite gave rise to some rare phases, including corundum and nearly pure CaTs pyroxene.Studies have shown that blue hibonite contains Ti3+ while orange hibonite does not (Ihinger and Stolper, 1986; Beckett et al., 1988). Orange hibonite formed either under oxidizing conditions (such as at oxygen fugacities at least seven orders of magnitude greater than that of a solar gas at 1700 K), or under conditions reducing enough (e.g., solar) that it contained Ti3+, which was later oxidized in situ. Although V and Ce oxides are volatile at the temperature and range of oxygen fugacities at which orange hibonite is known to be stable, we find that (a) the hibonite is V‐rich (∼1 wt% V2O3) and (b) there are no negative Ce anomalies in Allende hibonite. This indicates that the hibonite did not form by condensation under oxidizing conditions. In addition, there are slight excesses of Ti + Si cations relative to Mg + Fe cations (up to 0.1 of 0.8 cations per 19 oxygen anions), probably reflecting the original presence of Ti3+. The results of this study strongly support the suggestion (Ihinger and Stolper, 1986) that Allende hibonite originally formed under reducing conditions and was later oxidized. Oxygen fugacities within ∼2–3 orders of magnitude of that of a solar gas are implied; otherwise, strong Ce and V depletions would be observed.

A short duration of chondrule formation in the solar nebula: evidence from 26Al in Semarkona ferromagnesian chondrules

The 26Al– 26Mg systems of five ferromagnesian chondrules from the least metamorphosed ordinary chondrite Semarkona (LL3.0) were studied using a secondary ion mass spectrometer. Their glass or plagioclase portions contain excesses of 26Mg, and in two chondrules the 26Mg excesses are well correlated with 27Al/ 24Mg, which demonstrates the in-situ decay of 26Al. The initial 26Al/ 27Al ratios in these chondrules obtained from the slope of isochrons show a narrow range of between 6 × 10 −6 and 9 × 10 −6, indicating their short formation duration of less than 1 My. If the solar nebula was initially homogeneous in Al isotopes, the chondrule formation ages are ∼2 My younger than those of CAIs. Our results based on the study of the least metamorphosed UOC are consistent with the previous studies on Al-rich chondrules that the chondrule formation started at least 2 My after CAIs. Alternatively, the older records before 2 My were erased by chondrule recycling process. It further suggests that the young apparent ages (3 to >5 My after CAIs) for chondrules in type 3.4 UOCs are due to the disturbance of the 26Al– 26Mg system, possibly during parent body metamorphism. The result is not consistent with the extended nebular time scale of >5 My and the chondrule formation by planetary processes. The Ni isotopic analysis of the FeO-rich olivines in a type II chondrule in Semarkona did not show any detectable excess 60Ni in spite of their high Fe/Ni ratios. The upper limit of the initial 60Fe/ 56Fe ratio of the solar system was estimated to be 3.4 × 10 −7, which is consistent with the previous estimate (0.2–1.9 × 10 −7) from eucrites. This result confirms that the 60Ni excess previously observed from CAIs was not due to the decay of the short-lived nuclide 60Fe, but a Ni isotopic anomaly of nucleosynthetic origin.

Refractory inclusions from the ungrouped carbonaceous chondrites MacAlpine Hills 87300 and 88107

Abstract— MacAlpine Hills (MAC) 87300 and 88107 are two unusual carbonaceous chondrites that are intermediate in chemical composition between the CO3 and CM2 meteorite groups. Calcium‐aluminum‐rich inclusions (CAIs) from these two meteorites are mostly spinel‐pyroxene and melilite‐rich (Type A) varieties. Spinel‐pyroxene inclusions have either a banded or nodular texture, with aluminous diopside rimming Fe‐poor spinel. Melilite‐rich inclusions (Åk4–42) are irregular in shape and contain minor spinel (FeO <1 wt%), perovskite and, more rarely, hibonite. The CAIs in MAC 88107 and 87300 are similar in primary mineralogy to CAIs from low petrologic grade CO3 meteorites but differ in that they commonly contain phyllosilicates. The two meteorites also differ somewhat from each other: melilite is more abundant and slightly more Al‐rich in inclusions from MAC 88107 than in those from MAC 87300, and phyllosilicate is more abundant and Mg‐poor in MAC 87300 CAIs relative to that in MAC 88107. These differences suggest that the two meteorites are not paired. The CAI sizes and the abundance of melilite‐rich CAIs in MAC 88107 and 87300 suggests a genetic relationship to CO3 meteorites, but the CAIs in both have suffered a greater degree of aqueous alteration than is observed in CO meteorites. Aluminum‐rich melilite in CAIs from both meteorites generally contains excess 26Mg, presumably from the in situ decay of 26Al. Although well‐defined isochrons are not observed, the 26Mg excesses are consistent with initial 26Al/27Al ratios of approximately 3–5 times 10−5. An unusual hibonite‐bearing inclusion is isotopically heterogeneous, with two large and abutting hibonite crystals showing significant differences in their degrees of mass‐dependent fractionation of 25Mg/24Mg. The two crystals also show differences in their inferred initial 26Al/27Al ratios, 1 × 10−5 vs. ≤3 × 10−6.

Low‐Density Graphite Grains and Mixing in Type II Supernovae

Primitive meteorites contain presolar grains that originated in stellar outflows and supernova ejecta. Low-density graphite grains from the Murchison carbonaceous meteorite were analyzed for the isotopic compositions of C, N, O, Mg, Si, K, Ca, and Ti by ion microprobe mass spectrometry. The grains are characterized by a large range of 12C/13C ratios (from 3.6 to 7200 compared to the solar ratio of 89), excesses in 15N (15N/14N up to 10 times solar) and 18O (18O/16O up to 185 times solar), large inferred 26Al/27Al ratios (from 26Mg excesses) ranging up to 0.15, a large range in Si isotopic ratios (from 50% deficits in 29Si and 30Si relative to 28Si up to more than 120% excesses), large excesses in 41K and 44Ca from the prior presence of now-extinct 41Ca (T?=105 yr) and 44Ti (T?=59 yr), respectively, and excesses in 42Ca, 43Ca relative to 40Ca, and 49Ti, and 50Ti relative to 48Ti. Several of these isotopic signatures indicate a supernova origin. In particular, the initial presence of 44Ti and excesses of 28Si as well as the size of the inferred 41Ca/40Ca ratios are proof that the carrier grains formed in supernova ejecta. We explored the possibility that the low-density graphite grains originated from C-rich ejecta of Type II supernovae. In such stars 44Ti and 28Si are produced in the inner layers and the presence of these two isotopes in carbonaceous grains is evidence for extensive mixing of different supernova layers in the explosion. We performed mixing calculations of different layers of the SN models by Woosley & Weaver under the imposed boundary condition that C?O and compare the resulting isotopic ratios with the isotopic ratios measured in the meteoritic grains. The mixing model can explain the observed 12C/13C, 16O/18O, 30Si/28Si, and 44Ti and 41Ca fairly well as long as jets of material from the Si-rich zone, carrying 44Ti and pure 28Si, are assumed to penetrate the O-rich zone and are ejected into and mixed with the C-rich layers, where carbonaceous grains can form, without overwhelming these layers with the massive amounts of oxygen. Problems with the model are that it produces not enough 15N and consistently yields lower 29Si/30Si ratios than those in the grains. Furthermore, large excesses in 42Ca and 50Ti found in several grains, which can be attributed to neutron capture, in the model are obtained only in layers with O>C. It remains to be seen whether adjustment of cross sections and/or multidimensional SN models can overcome some of these problems. It also remains to be seen whether Type Ia supernovae, which have been proposed as a source of SN grains in meteorites, can provide a better explanation. The fact that essentially all supernova grains identified so far are diamond, graphite, SiC of Type X and Si3N4, and only one oxide grain with a supernova signature has been found remains a puzzle.

A Thermal Model for the Differentiation of Asteroid 4 Vesta, Based on Radiogenic Heating

A finite element code has been developed to model the thermal history of Asteroid 4 Vesta. This is the first attempt to model the thermal history of a differentiated asteroid through core and crust formation and subsequent cooling until geochemical closure is attained. The results of the simulation are consistent with chronological measurements and other constraints provided by cumulate and noncumulate eucrites believed to have been derived from Vesta. The work solves two major problems with the hypothesis of heating by decay of26Al, an extinct radionuclide, postulated to be a plausible heat source in the early Solar System. First, the model demonstrates that it is possible to keep the mantle of Vesta hot for ∼100 Ma, thereby explaining the observed difference in ages between cumulate and noncumulate eucrites. Second, the simulation offers a possible explanation of why detectable excesses of26Mg (the decay product of26Al) are not observed in noncumulate eucrites. The simulation draws a model chronology of Vesta and predicts times (relative to CAI formation) for accretion at 2.85 Myr, core formation at 4.58 Myr, crust formation at 6.58 Myr, and geochemical closure at ∼100 Myr for a H-chondrite asteroidal bulk composition. Decay of60Fe is found to cause no perceptible difference in the thermal history of Vesta, even when sequestered into a central core. Although chondritic xenoliths have not been described in HED igneous lithologies, the thermal model suggests the possibility that a veneer of unmelted near-surface material should remain.

Mg diffusion in anorthite: implications for the formation of early solar system planetesimals

We have measured the self diffusion coefficients of Mg, Ca, and Sr in anorthitic plagioclase in order to assess the potential of Mg isotopic heterogeneities in early solar system planetesimals to survive thermal metamorphism. Diffusion couples were constructed from polished single crystals of natural anorthite and synthetic, isotopically enriched anorthite glass. Couples were annealed at atmospheric pressure and 1200–1400°C and isotopic concentration profiles were measured with an ion microprobe. The results show that Mg diffusion in anorthite is surprisingly fast, with D Mg being over 2 orders of magnitude greater than D Sr. This indicates that the diffusion coefficient of Mg in anorthite cannot be approximated with that for Sr. Mg diffusion in the c-direction is also slightly faster than in the b-direction, while Ca and Sr diffusion appear to be isotropic. The results provide important constraints on the thermochronological history of anorthite-bearing mineral assemblages that preserve radiogenic 26Mg excesses. In a planetesimal heated by the decay of 26Al, the temperature at any point depends on the planetesimal size, time of formation, thermal conductivity, and depth within the planetesimal. Given sufficient heating, 26Mg heterogeneities produced by the in-situ decay of 26Al in Ca-, Al-rich inclusions (CAIs) and chondrules will be erased by diffusive equilibration. Using the self diffusion coefficient for Mg in anorthite measured in this study, we show that the common occurrence of 26Mg excesses in these inclusions requires that they must be stored in small (≲15 km) bodies or the outermost rims of larger bodies for the first 1–2 million years of the solar system's history. For early formed bodies larger than 15 km, most of the mass will have been heated sufficiently for any radiogenic 26Mg to have been diffusively homogenized in the Mg-rich planetary environment.

Origin of hibonite‐pyroxene spherules found in carbonaceous chondrites

Abstract— We have studied both of the known glass‐free, hibonite‐pyroxene spherules: MYSM3, from Murray (CM2), and Y17–6, from Yamato 791717 (CO3). They consist of hibonite plates (∼2 wt% TiOtot2) enclosed in Al‐rich pyroxene that has such high amounts of CaTs (CaAl2SiO6) component, up to ∼80 mol%, that it must have crystallized metastably. Within the pyroxene, abundances of MgO and SiO2 are strongly correlated with each other and are anticorrelated with those of Al2O3, reflecting an anticorrelation between the diopside and CaTs components of the pyroxene. In contrast with previous results for Type B fassaite, however, we do not observe an anticorrelation between MgO and TiOtot2, possibly reflecting different relative distribution coefficients for Ti3+ and Ti4+ in the aluminous pyroxene of the spherules from those found for fassaite in Type B inclusions. Previously described hibonite‐silicate spherules have 26Mg deficits but the present samples do not. Furthermore, the pyroxene in Y17‐6 has excess 26Mg, while the hibonite it encloses does not, indicating that the two phases either had different initial 26Al/27Al ratios or different initial 26Mg/24Mg ratios. The Ti isotopic compositions of the present samples are highly unusual: δ50Ti = 103.4 ± 5.2%o in MYSM3 and ‐61.4 ± 4.1%0 in Y17‐6, which are among the largest 50Ti anomalies reported for any refractory inclusion. The textures suggest that hibonite crystallized first; but based on the calculated bulk compositions of both spherules, it is not the liquidus phase in either sample, which suggests that the hibonite in both samples is relict. The presence of ragged hibonite grains in MYSM3 and rounded hibonite grains in Y17‐6 and a lack of isotopic equilibrium between pyroxene and hibonite support this conclusion. The spherules crystallized from liquid droplets that probably formed as a result of the melting of solid precursor grains that included hibonite. The heating events were too short and/or not hot enough to melt all the hibonite. The droplets cooled quickly enough that CaTs‐rich pyroxene crystallized instead of anorthite. Based on the observed differences in isotopic composition, it is unlikely that the precursors of the present samples formed in the same reservoir as each other or as the previously described hibonite‐silicate spherules, providing further evidence of the isotopic heterogeneity of the early solar nebula.

Stellar Sapphires: The Properties and Origins of Presolar Al2O3in Meteorites

Thirty-seven isotopically highly anomalous presolar Al2O3 grains and one presolar MgAl2O4 grain from a separate of the Tieschitz H3.6 ordinary chondrite were identified out of 17,000 isotopically normal refractory oxide grains by an automatic 16O/18O low mass resolution ion-imaging mapping technique in the ion microprobe. Eight additional presolar Al2O3 grains were found by high mass resolution ion probe measurements of all three stable O isotopes in individual grains, including several that would have been missed by the ion-imaging search. Forty-five of the grains were analyzed for their 16O/17O and 16O/18O ratios. Twenty-four grains were also analyzed for Al-Mg and 17 of them have large excesses of 26Mg, attributable to the radioactive decay of 26Al. The highly anomalous isotopic composition of the grains is evidence for their presolar, stellar origin. The 46 oxide grains of this study together with 42 previously identified presolar grains were divided into four groups. These groups most likely comprise grains from distinct types of stellar sources. Group 1 grains have 17O excesses and moderate 18O depletions, relative to solar, and many of them exhibit 26Mg excesses as well. Group 2 grains have 17O excesses, large 18O depletions, and high inferred 26Al/27Al ratios. Group 3 grains have solar or higher 16O/17O and 16O/18O ratios. Group 4 grains have 17O and 18O enrichments. One Al2O3 grain of this study, T54, has an 16O/17O ratio of 71, lower than any previously observed, and 16O/18O much greater than the solar value. The O-isotopic compositions of Group 1 and Group 3 grains are consistent with an origin in O-rich red giant stars, which have undergone the first dredge-up. The range of O-isotopic ratios of these groups requires multiple stellar sources of different masses and initial isotopic compositions and is well explained by a combination of Galactic chemical evolution and first dredge-up models. The inferred 26Al/27Al ratios of many of these grains indicate that they formed in thermally pulsing asymptotic branch (TP-AGB) stars that had undergone the third dredge-up. Group 2 grains probably formed in low-mass AGB stars as well, and their substantial 18O depletions are the likely result of extra mixing (cool bottom processing). The origin of the 18O enrichments in Group 4 grains is unknown, but it might be due to initial compositional differences of the stellar sources or to unusual third dredge-up in low-mass AGB stars. The highly 17O-enriched grain T54 could have formed in an AGB star undergoing hot bottom burning or in a massive star in the Of-WN phase. O-rich circumstellar dust seems to be underrepresented in meteorites, relative to C-rich. Explanations include the possibility that most O-rich stardust grains are silicates and have been destroyed either in the laboratory or in nature and the possibility that presolar Al2O3 has a finer grain size distribution than SiC and graphite.

Ion microprobe studies of Efremovka CAIs: II. Potassium isotope composition and 41Ca in the early Solar System

The potassium isotope composition of refractory phases in Efremovka CAIs has been measured by an ion microprobe. Extensive studies of terrestrial standards were carried out to ensure that the experimental procedures adopted for potassium isotope studies provide accurate 41K/ 39K ratios of the analysed phases. Excess 41K is found to be present in refractory phases with high Ca/K ratios (>3 × 10 5) in all the four Efremovka CAIs analysed in this study. The excess 41K in these phases correlates well with their 40Ca/ 39K ratios. Several possible causes for this excess have been considered and it is concluded that in situ decay of 41Ca can best explain our observations. The results obtained in this work substantiate the conclusion drawn from our initial study (Srinivasan et al., 1994) and confirm the presence of the short-lived nuclide 41Ca (τ ~ 0.15 Ma) in the early Solar System with an initial 41Ca/ 40Ca value of (1.41 ±0.14) × 10 −8 at the time of formation of the Efremovka CAIs. We have considered several processes that may lead to the presence of 41Ca in the early Solar System. These include, production by energetic particles from an active early Sun, low energy particle induced reactions in a molecular cloud complex of which the proto-solar cloud was a part and freshly synthesized material from suitable stellar source(s). The last alternative turns out to be the best one from plausibility considerations. The Efremovka CAIs with excess 41K also have excess 26Mg that can be attributed to the decay of the short-lived nuclide 26Al(τ ~ 1 Ma) within these objects. The presence of both 26Al and 41Ca in Efremovka CAIs is used to infer the most probable stellar site(s) for the synthesis of these nuclides in a self consistent manner. Our observations coupled with predicted stellar production rates suggest an asymptotic giant branch (AGB) star to be a plausible source. However, we cannot completely rule out a supernova or a Wolf-Rayet star as being responsible for the synthesis and subsequent injection of these nuclides to the solar nebula. In spite of this inability to pinpoint the exact stellar source, our result constrains the time interval between the injection of freshly synthesized 41Ca and 26Al to the solar nebula and the formation of first Solar System solids (CAIs) to less than a million year.

The distribution of aluminum‐26 in the early Solar System—A reappraisal

Abstract— A compilation of over 1500 Mg‐isotopic analyses of Al‐rich material from primitive solar system matter (meteorites) shows clearly that 26Al existed live in the early Solar System. Excesses of 26Mg observed in refractory inclusions are not the result of mixing of “fossil” interstellar 26Mg with normal solar system Mg. Some material was present that contained little or no 26Al, but it was a minor component of solar system matter in the region where CV3 and CO3 carbonaceous chondrites accreted and probably was a minor component in the accretion regions of CM chondrites as well. Data for other chondrite groups are too scanty to make similar statements. The implied long individual nebular histories of CAIs and the apparent gap of one or more million years between the start of CAI formation and the start of chondrule formation require the action of some nebular mechanism that prevented the CAIs from drifting into the Sun. Deciding whether 26Al was or was not the agent of heating that caused melting in the achondrite parent bodies hinges less on its widespread abundance in the nebula than it does on the timing of planetesimal accretion relative to the formation of the CAIs.

Trace element abundances and magnesium, calcium, and titanium isotopic compositions of grossite-containing inclusions from the carbonaceous chondrite Acfer 182

The carbonaceous chondrite Acfer 182 contains Ca,Al-rich inclusions (CAIs) that differ from most CAIs in other meteorites. Many of them contain grossite (CaAl 4O 7), whose modal abundance exceeds 30 vol % in most of these CAIs (“grossite-rich inclusions”). Similar inclusions have been found only in the CH chondrite ALH85085 and the CR chondrite Acfer 059-El Djouf 001. Ion microprobe analyses of trace elements were made on nineteen CAIs from Acfer 182 and two CAIs from Acfer 059-El Djouf 001, of AlMg isotopes on eighteen Acfer 182 and the two Acfer 059-El Djouf 001 inclusions, and of calcium isotopes on ten Acfer 182 inclusions (on six of them also of titanium isotopes) and one Acfer 059-El Djouf 001 inclusion. Trace element and isotopic signatures of grossite-containing inclusions from Acfer 182 resemble those of some inclusions from ALH85085. Volatility-fractionated trace element abundance patterns, ranging from ultrarefractory-depleted (similar to Group II pattern) to volatile-enhanced, are predominant (17 CAIs, including both inclusions from Acfer 059-El Djouf 001). Two inclusions have ultrarefractory patterns, one has a Group III pattern, and one inclusion has a Group III-related pattern similar to patterns found in HAL-type inclusions. Only one CAI from Acfer 182, but both inclusions from Acfer 059-El Djouf 001, have nonlinear excesses of 26Mg corresponding to the initial 26Al 27Al ratio of ∼ 5 × 10 −5. None of the twenty inclusions has a significant intrinsic isotopic mass fractionation of Mg. Titanium is normal in all analyzed CAIs and only small 48Ca excesses (4.3 and 3.8‰) are present in two inclusions. Isotopes of both Ca and Ti show no significant intrinsic mass fractionations. Grossite-containing inclusions from Acfer 182 occur in different petrographic contexts. In many cases petrographic characteristics are correlated with trace element abundances. Almost all inclusions studied in this work have a condensation history but no evidence for extensive evaporation processes is present. Many of the CAIs must have formed by direct gas-solid condensation, while some inclusions with clearly identifiable igneous textures formed from a refractory melt. Grossite, whose formation was the subject of controversy in the past, probably condensed from a reservoir with a higher-than-chondritic Ca/Al-ratio and depleted in the highly refractory trace elements, or crystallized from a Ca,Al-rich melt. The two ultrarefractory inclusions from Acfer 182 have extremely high refractory trace element abundances (sometimes exceeding 10 5 × CI). The lack of isotopic fractionation indicates a formation at very high temperatures by condensation rather than evaporation. The lack of 26Mg excesses in almost all inclusions from Acfer 182 is best explained by heterogeneous distribution of 26Al in the primitive solar nebula. Grossite-rich inclusions having volatility-fractionated ultrarefractory-depleted trace element abundance patterns and lacking 26Mg excesses could be the complement of platy hibonites (PLACs) from Murchison with Group III or ultrarefractory patterns and likewise no 26Mg excesses.

Petrologic and AlMg isotopic clues to the accretion of two refractory inclusions onto the Leoville parent body: One was hot, the other wasn't

Two type B refractory inclusions found within several centimeters of each other in the foliated Leoville meteorite differ greatly in their styles of deformation, textures, and Mg-Al isotopic characteristics. One is a subspheroidal type B1 with textures similar to those observed previously in many type B1s from other CV3 meteorites, and whose deformation (fracturing, some granulation) has been almost entirely brittle in nature. Its isotopic composition shows 26Mg 24Mg correlated with 27Al 24Mg which, although disturbed, indicates a lower limit for initial 26Al 27Al of ~4.9 × 10 −5. The other inclusion is a highly elongate ( L W ~6 ) object whose long axis lies in the plane of foliation of the Leoville meteorite. It has a predominantly metamorphic texture, but relict islands of an earlier and more “normal” type B precursor are preserved in its core. The isotopic compositions of the two textural variants in this inclusion are different, anorthite in the metamorphic region mostly showing little or no excess 26Mg [( 26Al 27Al ) 0 < ~ 1.9 × 10 −6 ] that would be attributable to in situ decay of 26Al whereas anorthite in the relict islands shows well-resolved excess 26Mg that gives a lower limit for initial 26Al 27Al of ~4.6 × 10 −5. The effects of shock metamorphism on the Leoville meteorite are clearly seen in both inclusions, including brittle deformation and local shock melting, but are not responsible for the major textural differences between the two inclusions. The most likely origin for the deformed and recrystallized features of the elongate inclusion is that it accreted onto the Leoville parent body while still very hot, and deformed into its present shape at the instant it impacted. If the resetting of the Al-Mg system occurred during this event, it must have taken place at least ~3.3 million years after formation of the precursor inclusion. The possibility that other deformed Leoville components accreted while hot should not be discounted.

Ca-,Al-rich inclusions in the unique chondrite ALH85085: Petrology, chemistry and isotopic compositions

The unique chondrite ALH85085 contains Ca-,Al-rich inclusions (CAIs) that differ from CAIs found in other types of meteorites. The ALH85085 inclusions are smaller (5–80 μm) and more refractory than their counterparts in carbonaceous chondrites. We have performed a comprehensive study of these inclusions: forty-two for petrography and mineralogy, fifteen for bulk major and minor element chemical composition, six for Mg-Al isotopic systematics, ten for Ca isotopes, nine for Ti isotopes and six for trace element abundances. In addition, oxygen-isotopic compositions were determined in minerals from a single inclusion (CAI 125). The mineralogy of the CAIs is dominated by oxide phases. About half of the inclusions contain either hibonite (hib) or spinel (sp), commonly associated with melilite (mel) or perovskite (pv); the others contain calcium dialuminate (CaA2), rarely found in CAIs from other meteorites. Most CAIs are spherical in shape and many exhibit igneous textures indicating crystallization from a melt. Textures in others are reminiscent of recrystallization due to thermal processing. Many of the CAIs are coated with a double layer of gehlenitic melilite and diopside or a single layer of diopside. The CAIs analyzed for Ca and Ti have normal isotopic compositions except for two that have nonlinear 48Ca excesses of 7.4 and 11.7‰, respectively. Only three CAIs show 26Mg excesses, two of them have ( 26Al 27Al ) o ~ 5 × 10 −5 , the third has 2 × 10 −6 . There is no obvious correlation between Al-Mg systematics and any other properties of the inclusions. All minerals (sp, pv, hib, CaA2), including melilite, in CAI 125 have 16O excesses of ~5%, indicating that ALH85085 inclusions never experienced the thermal event that caused the oxygen isotopic equilibration of melilite in Allende CAIs. The ALH85085 inclusions are fractionated in their refractory element abundances: Most of them have very high Al/Ca and Ti/Ca ratios, four CAIs have volatility fractionated (refractory-depleted) Group II-related trace element patterns, one a Group III pattern with a slightly ultrarefractory trace element signature, and one a rather fiat pattern with depleted Eu. There is no correlation between mineralogy, major element chemistry and trace element abundances. It is further shown that the high-temperature geochemical behavior of ultrarefractory trace elements is decoupled from that of the major elements Ca and Ti (Ti is correlated with the relatively volatile elements Mb and Yb) implying that perovskite is of only minor importance as carrier of ultrarefractories. ALH85085 inclusions with refractory-depleted trace element abundances and no 26Mg excesses could be the complement to platy hibonites (PLACs) with Group III trace element patterns from the Murchison meteorite.

A petrologic and ion microprobe study of a Vigarano Type B refractory inclusion: Evolution by multiple stages of alteration and melting

USNM 1623-8 is a Type B Vigarano inclusion, ~6 mm in diameter, consisting of melilite (Åk 12–72), fassaite ( 17–23% A1 2O 3, 5–17% TiO 2), anorthite, and spinel. Secondary alteration is negligible and consists of diopside and sparse nepheline. Melilite is normally zoned and contains up to 0.35 wt% Na 2O, higher than generally seen in melilite from other refractory inclusions; a strong correlation between åkermanite content and Na 2O in 1623-8 melilite suggests that the Na was present during igneous crystallization rather than introduced later by diffusion. Anorthite in 1623-8 is coarse-grained, but commonly has an irregular border with neighboring melilite crystals; many spinel grains enclosed within anorthite have selvages of melilite around them which, in places, are continuous with melilite exterior to the anorthite. The anorthite appears to be replacing melilite. Ion microprobe analyses of anorthite in 1623-8 show that there is no detectable excess 26Mg in nine anorthite crystals that have 27Al 24Mg as high as 970, implying an upper limit of 26Al 27Al 0 < 8× 10 −7 ; one crystal was found that has excess 26Mg consistent with 26Al 27Al 0 = ~3 × 10 −6 . Melilite in the outer portion of the inclusion 27Al 24Mg = 2.7−11.5) contains excess 26Mg that, considered together with spinel and pyroxene, defines an isochron consistent with the canonical early solar system value of 26Al 27Al 0 = ~4.5 × 10 −5 . Melilite near an anorthite ( 27Al 24Mg =1.6−5.9 ) has δ 26Mg values in excess of those predicted from 26Al 27Al 0 = ~4.5 × 10 −5 , presumably caused by redistribution of radiogenic 26Mg from adjacent anorthite. The high Na 2O inferred to have been present in the melt suggests that the precursor solids were Na-bearing, such as, for example, refractory material having an Allende-like overprint of feldspathoid-bearing secondary alteration. The melting event must have been brief in order that not all of the Na was vaporized during melting. The isotopic composition of the anorthite relative to melilite indicates a subsequent, much later (after the decay of most or all 26A1) episode of partial melting or solidstate recrystallization that did not affect the Al-rich melilite but reset most of the anorthite and some Mg-rich melilite near anorthite. Because the inclusion is both rimmed and (slightly) altered, this late resetting event appears to have been a nebular one. An assumption of homogeneously distributed 26A1 in the regions of the nebula where the inclusion formed and was later modified implies a several millionyear interval between condensation of the precursors and final melting of 1623-8.

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 (FMg = 3.6 ± 1.8‰/amu). In contrast, spinels from the spinel‐rich regions adjacent to the fine‐grained border have normal Mg isotopic composition (FMg = 0.1 ± 1.5‰/amu). The cores of large calcic (An90,99) plagioclase have no excess 26Mg, corresponding to 26Mg*/ 27Al &lt; 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.

Origin of plagioclase-olivine inclusions in carbonaceous chondrites

Plagioclase-Olivine Inclusions (POIs) are an abundant group of chondrule-like objects with igneous textures found in carbonaceous chondrites. POIs consist of plagioclase, olivine, pyroxene, and spinel, and cover a wide range of compositions between Type C Ca-Al-rich Inclusions (CAIs) and ferromagnesian chondrules. POIs are distinguished from CAIs by the absence of melilite, lack of refractory siderophile-rich opaque assemblages, more sodic plagioclase, and abundance of olivine and aluminousenstatite. Rare accessory minerals including armalcolite, zirconolite, rutile, and sapphirine are found in several POIs. The petrographie and chemical properties of POIs indicate that they are not condensates or evaporative residues but formed by melting or partial melting of pre-existing solids. Seven of fourteen POIs contain isotopically fractionated Mg, and despite their textures these POIs are not isotopically homogeneous. Spinel is the major carrier of fractionated Mg in six POIs. The magnitude of Mg isotopic fractionation ( F Mg ) of spinel ranges from −8 to +11%./amu among the POIs but F Mg is always either positive or negative within an individual POL Within a single inclusion, F Mg of coexisting spinel is not constant but varies by up to 7%./amu. Isotopic fractionation of Mg in silicates is less common and always positive ( F Mg > 0). Pyroxenes in two POIs and plagioclase in a third contain isotopically heavy Mg. Radiogenic 26Mg∗ is rare in POIs as only two inclusions show evidence of excess 26Mg with 26Mg∗/ 27Al ~ 4 × 10 −6. The preservation of isotopic heterogeneity in objects whose igneous textures suggest crystallization from a homogeneous melt implies that melting was incomplete, allowing survival of relict phases. A comparison of the essential characteristics of POIs and CAIs suggests that the major processes leading to formation of POIs—including condensation, dust/gas fractionation, aggregation of chemically and isotopically disparate materials, and partial melting—are common to most CAIs and chondrules. We present a scenario for the formation of these objects and conclude that the homogeneity of the final assemblage—CAI, POI, or chondrule—is primarily a reflection of the thermal history rather than the nature of precursor materials.

Evidence from the Semarkona ordinary chondrite for 26A1 heating of small planets

The radioactive decay of 26Al, to form 26Mg, may have constituted an important heat source for melting of small planetary bodies in the early Solar System. Although large excesses of radiogenic 26Mg, arising from in situ decay of 26Al (half life ~7.2 x 105 yr), have been found in ten carbonaceous and ordinary chondritic meteorites1,2, the distribution of 26Al in the solar nebula and its importance as a heat source remain to be determined. All previous observations of excess 26Mg are confined to Al-rich minerals in refractory, Ca,Al-rich inclusions (CAIs). An assessment of the distribution of 26Al in the Solar System from these data is difficult because CAIs are only a minor constituent of chondritic meteorites associated with high-temperature nebular processes, and because the initial 26Al/27Al ratio inferred from the CAI data is not constant3 but varies from the commonly measured value of ∼ 5 x 10−5 to values <1 x 10−7. Here we report the first observation of radiogenic 26Mg in non-refractory meteoritic material, a plagio-clase-bearing, olivine-pyroxene clast chondrule in the Semarkona ordinary chondrite. The inferred initial abundance of 26Al (26Al/27Al = (7.7±2.1)xl0−6) is sufficient to produce incipient melting in well insulated bodies of chondritic composition. We conclude that planetary accretion and differentiation must have begun on a timescale comparable to the half life of 26Al and that, even if widespread melting did not occur, 26Al heating played a significant role in thermal metamorphism on small planets.

Correlated morphological, chemical, and isotopic characteristics of hibonites from the Murchison carbonaceous chondrite

Twenty-four hibonite-dominated refractory grains from the Murchison carbonaceous chondrite have been identified from polished grain-mounts of density separates, and have been morphologically and chemically divided into three groups: (1) 14 colourless PLAty Crystal fragments (PLACs) with less than 2.7% TiO 2; (2) 3 pleochroic Blue AGgregates (BAGs) which are composed of crystal plates and fragments and have TiO 2 concentrations between 5.1% and 6.5%; (3) Six Spinel-HIBonite spherules and crystal aggregates (SHIBs), often with spinel, Fe-silicate, and clinopyroxene rims. The maximum TiO 2 concentrations of hibonite in the SHIBs ranges from 3.5 to 7.6%, but the TiO 2 concentration in one grain was heterogeneous and ranged from 0.5% to 5.0%. Hibonite in all types is stoichiometric with Ti and Mg predominantly in a coupled substitution of Mg 2+ and Ti 4+ for 2Al 3+. One grain with a rounded hibonite core does not clearly fit into the three designated groups. The hibonites have been analysed by ion microprobe mass spectrometry for their Mg and Ti isotopic compositions. Twelve of the PLACs have ( 26Al 27Al ) 0 ratios less than 1 × 10 −5 but the other two PLACs have ( 26Al 27Al ) 0 of 5.5 × 10 −5 and 7.7 × 10 −5. The excess 26Mg may reflect initial Mg isotopic heterogeneities in the source region of the PLACs, or be the result of in situ 26Al decay, in which case the distribution of 26Al was heterogeneous. Three BAGs show substantial positive mass-fractionation of Mg. One grain has excess 26Mg while the other two have 26Mg depletions. The 26Mg depletions indicate the preservation of Mg isotopic anomalies in the BAGs, but the excess 26Mg in one grain could be due to in situ 26Al decay. The six SHIBs have excess 26Mg that is correlated with 27Al 24Mg . The Mg isotopic systematics of the SHIBs are consistent with the in situ decay of 26Al with ( 26Al 27Al ) 0 of ca. 5 × 10 −5. The unclassified hibonite has a clearly-resolved deficit in 26Mg. Titanium isotopic anomalies are common in the hibonites measured here. The largest anomalies are in 50Ti with variations in 50Ti 48Ti ranging from −50 to +16%. relative to terrestrial. Smaller anomalies are present in 49Ti and 47Ti. The Ti isotopic compositions are not directly correlated with morphological type, but the PLACs show the largest variations in isotopic composition, whereas the SHIBs are generally close to terrestrial. The presence of hibonite populations with distinct morphological, chemical and Mg isotopic systematics suggests that there were several hibonite formation episodes within the early solar system. The hibonites probably formed by the melting of refractory dust aggregates during local transient thermal events.

Trace-element abundances in hibonites from the Murchison carbonaceous chondrite: Constraints on high-temperature processes in the solar nebula

Abundances of trace elements, including 14 rare earth elements (REE) and 8 other refractory trace elements (RTE) have been measured in 33 hibonite- and 3 perovskite-bearing grains from the Murchison carbonaceous chondrite. The normalized abundance patterns show a good correlation with three morphological and chemical groups of hibonites. Sixteen platy hibonite crystals (PLACs) typically have patterns which are depleted in the relatively volatile trace elements V, Nb, Ba, Eu, and Yb (Group III). Three PLACs also show large depletions in Ce; one PLAC is enriched in the ultrarefractory REE, while two other PLACs have fractionated patterns enriched in the light REE, with positive Tm anomalies. The PLACs commonly have a roll-off in the abundances of the HREE, which is consistent with solid-liquid partitioning. Three blue aggregates (BAGs) have patterns with low overall abundances and depletions in the ultrarefractory REE except Tm (Group II). Spinel-hibonite grains (SHIBs) show a variety of patterns mainly of Group II, but 4 SHIBs have relatively smooth patterns with anomalies in the volatile REE Ce, Eu, and Yb. The perovskites have patterns similar to Group II but with excesses in Eu, and Yb. The HREE roll-off in the PLACs and the igneous morphologies of the SHIBs indicate that these grains formed by the crystallization of melts. Distillation may be important for the BAGs and two of the PLACs which have large isotopic mass-fractionation effects. PLACs have the simplest trace element patterns and the largest Ti isotopic anomalies and probably were the first hibonites to form in the early solar system. The SHIBs have trace-element signatures that require multiple processing episodes. They are somewhat complementary to the PLACs in that they are enriched in the volatile elements which are depleted in the PLACs. On average, the SHIBs contain isotopically less-anomalous Ti than the PLACs. However, SHIBs contain 26Mg excesses that are consistent with the in situ decay of 26Al, whereas PLACs generally have only small, or no excesses of 26Mg despite high 27Al 24Mg . While an overall relationship exists between the morphology, chemistry, and Ti and Mg isotopic systematics of meteoritic hibonites, they do not, as yet, fit into a simple scenario for the formation of the solar system.

Physicochemical Isotope anomalies

Isotopic composition of refractory elements can be modified, by physical processes such as distillation and sputtering, in unexpected patterns. Distillation enriches the heavy isotopes in the residue and the light isotopes in the vapour. However, current models appear to be inadequate to describe the detailed mass dependence, in particular for large fractionations. Coarse- and fine-grained inclusions from the Allende meteorite exhibit correlated isotope effects in Mg both as mass-dependent fractionation and residual anomalies. This isotope pattern can be duplicated by high temperature distillation in the laboratory. A ubiquitous property of meteoritic inclusions for Mg as well as for most of the other elements, where measurements exist, is mass-dependent fractionation. In contrast, terrestrial materials such as microtektites, tektite buttons as well as lunar orange and green glass spheres have normal Mg isotopic composition. A subset of interplanetary dust particles labelled as chondritic aggregates exhibit excesses in 26Mg and deuterium anomalies. Sputtering is expected to be a dominant mechanism in the destruction of grains within interstellar dust clouds. An active proto-sun as well as the present solar-wind and solar-flare flux are of sufficient intensity to sputter significant amounts of material. Laboratory experiments in Mg show widespread isotope effects including residual 26Mg excesses and mass dependent fractionation. It is possible that the 26Mg excesses in interplanetary dust is related to sputtering by energetic solar-wind particles. The implication of the laboratory distillation and sputtering effects are discussed and contrasted with the anomalies in meteoritic inclusions and other extraterrestrial materials we have access to.