A CaAl4O7-bearing refractory spherule from Murchison: Evidence for very high-temperature melting in the solar nebula

We studied 20 R chondrites and found 126 Ca,Al-rich objects (101 CAIs, 19 Al-rich chondrules, and 6 spinel-rich fragments). Based on mineralogical characterization and analysis by SEM and electron microprobe, the inclusions can be grouped into six different types: (1) simple concentric spinel-rich inclusions (42), (2) fassaite-rich spherules, (3) complex spinel-rich CAIs (53), (4) complex diopside-rich inclusions, (5) Al-rich chondrules, and (6) Al-rich (spinel-rich) fragments. The simple concentric and complex spinel-rich CAIs have abundant spinel and, based on the presence or absence of different major phases (fassaite, hibonite, Na,Al-(Cl

Section: Similar Cais Within Other Groups Of Chondritesmentioning

confidence: 99%

Ca,Al‐rich inclusions in Rumuruti (R) chondrites

Rout¹,

Bischoff²

2008

“…Probably the most common hibonite-bearing inclusions are the silica-poor, spinel-hibonite spherules found in CM2 chondrites (e.g., Macdougall, 1981;MacPherson et al, 1983;Simon et al, 1994). Much less common are spherules in which hibonite laths are enclosed in a silicate.…”

Section: Introductionmentioning

confidence: 99%

Origin of hibonite‐pyroxene spherules found in carbonaceous chondrites

Simon

Davis

Grossman

et al. 1998

Abstract-We have studied both of the known glass-free, hibonite-pyroxene spherules: MYSM3, from Murray (CM2), and Y 17-6, from Yamato 791717 (C03). They consist of hibonite plates (-2 wt% TiOT' ) enclosed in Al-rich pyroxene that has such high amounts of CaTs (CaAI,SiO,) component, up to -80 mol%, that it must have crystallized metastably. Within the pyroxene, abundances of MgO and Si02 are strongly correlated with each other and are anticorrelated with those of AlzO,, 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 TiOyt, 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 Y 17-6 has excess 26Mg, while the hibonite it encloses does not, indicating that the two phases either had different initial 26A@7A1 ratios or different initial 26Mg/24Mg ratios. The Ti isotopic compositions of the present samples are highly unusual: 650Ti = 103.4 2 5.2%0 in MYSM3 and -61.4 2 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.

“…An example of the expected composition of the refractory residue, for -3x refractory enrichment, can be determined from the data of Hashimoto (1983) as -86% Al2O3, 2% Ti02 and 12% CaO and would probably consist of Cadialuminate (grossite CaA1204), Ca-hexa-aluminate (hibonite CaA112019), traces of CaTiO3 (perovskite) and perhaps glass as these are the phases produced by evaporation experiments (Floss et al, 1996;Ireland et al, 1992) and have been observed in very Al-rich CAIs of many meteorites (Weber and Bischoff, 1994;Simon et al, 1994a).…”

Section: Composition Of the Initial Refractory Residuementioning

confidence: 99%

“…They are valuable probes of the extreme conditions in the earliest solar nebula, when gas and dust had not yet mixed to the point of isotopic uniformity in their formation region (Wasserburg and et al, 1986) and Leoville (Davis and MacPherson, 1994), the CV meteorite Ningqiang (Lin and Kimura, 1998), the CM meteorites Murray , Cold Bokkeveld (Greenwood et al, 1994) and Murchson (MacPherson et al, 1983;Ireland, 1988;Simon et al, 1994a), the CK meteorites (Keller, 1992;Weisberg et al, 1996), some CH and CR Acfer meteorites with grossite-bearing CAIs (Weber and Bischoff, 1994;Weber et al, 1995) and even in E chondrites (Fagan et al, 1999;Guan et al, 1999).…”

Section: Introductionmentioning

confidence: 99%

The formation of rims on calcium‐aluminum‐rich inclusions: Step I—Flash heating

Wark

Boynton

2001

Abstract-Wark-Lovering rims of six calcium-aluminum-rich inclusions (CAIs) representing the main CAI types and groups in Allende, Efremovka and Vigarano were microsurgically separated and analysed by neutron activation analysis (NAA). All the rims have similar -4x enrichments, relative to the interiors, of highly refractory lithophile and siderophile elements. The NAA results are confirmed by ion microprobe and scanning electron microscope (SEM) analyses of rim perovskites and rim metal grains. Less refractory Eu, Yb, V, Sr, Ca and Ni are less enriched in the rims. The refractory element patterns in the rims parallel the patterns in the outer parts of the CAIs. In particular, the rims on type B1 CAIs have the igneously fractionated rare earth element (REE) pattern of the melilite mantle below the rim and not the REE pattern of the bulk CAI, proving that the refractory elements in the rims were derived from the outer mantle and were not condensates onto the CAIs. The refractory elements were enriched in an A1203-rich residue <50pm thick after the most volatile -80% of the outermost 200 y m of each CAI had been volatilized, including much Mg, Si and Ca.Some volatilization occurred below the rim, and created refractory partial melts that crystallized hibonite and gehlenitic melilite. The required "flash heating" probably exceeded 2000 "C, but for only a few seconds, in order to melt only the outer CAI and to unselectively volatilize slow-diffusing 0 isotopes which show no mass fractionation in the rim. The volatilization did, however, produce "heavy" mass-fractionated Mg in rims. In some CAIs this was later obscured when "normal" Mg diffused in from accreted olivine grains at relatively high temperature (not the lower temperature meteorite metamorphism) and created the -50 y m set of monomineralic rim layers of pyroxene, melilite and spinel.