Gary R. Huss

The matrices of unequilibrated ordinary chondrites: Implications for the origin and history of chondrites

Abstract The matrices of sixteen unequilibrated ordinary chondrites (all witnessed falls) were studied microscopically in transmitted and reflected light and analyzed by electron microprobe. Selected specimens were also studied by scanning electron microscopy. These studies indicate that the fine-grained, opaque, silicate matrix of type 3 unequilibrated chondrites is compositionally, mineralogically and texturally distinct from the chondrules and chondrule fragments and may be the low temperature condensate proposed by Larimer and Anders (1967, 1970). Examination of the matrices of unequilibrated chondrites also shows that each meteorite has been metamorphosed, with the alteration ranging in intensity from quite mild, where the matrix has been only slightly altered, to a more severe metamorphism that has completely recrystallized the opaque matrix. Most of the metamorphic changes in the matrix occurred without significant effects on the compositions or textures of the chondrules. The metamorphic alteration probably resulted from a combination of processes including thermal metamorphism and the passage of shock waves. The present appearance of each unequilibrated chondrite is a result of the particular temperature and pressure conditions under which it and its components formed, plus the subsequent metamorphic alteration it experienced.

The Initial Abundance of 60Fe in the Solar System

60Fe, which decays to radiogenic 60Ni (60Ni*), is a now extinct radionuclide. 60Fe is produced only in stars and thus provides a constraint on the stellar contribution to solar system radionuclides. Its short half-life [t1/2 = 1.49 × 106 yr (1.49 Myr)] makes it a potential chronometer for the early solar system. We found clear evidence for 60Ni* in troilite (FeS) grains from the Bishunpur and Krymka chondrites, two of the least metamorphosed (LL3.1) ordinary chondrites. The weighted means of inferred initial 60Fe/56Fe ratios [(60Fe/56Fe)0] for the troilites are (1.08 ± 0.23) × 10-7 and (1.73 ± 0.53) × 10-7 for Bishunpur and Krymka, respectively. We compare our data with upper limits established previously on (60Fe/56Fe)0 for a chondrule in an unequilibrated ordinary chondrite, Semarkona, and for troilites in a relatively metamorphosed chondrite, Ste. Marguerite, taking into account their 26Al-26Mg ages. The 60Fe and 26Al chronometers can be combined to produce a consistent chronology for Ca-Al-rich inclusions, which are thought to be the earliest solar system solids, chondrules, troilites, and Ste. Marguerite. The initial 60Fe/56Fe for the solar system is inferred from this chronology to have been 2.8 × 10-7 to 4 × 10-7. This is at or below the low end of predictions for a supernova source.

Extreme ^(26)Mg and ^(17)O enrichments in an Orgueil corundum: Identification of a presolar oxide grain

A corundum (Al_2O_3) grain from the Orgueil meteorite is greatly enriched in ^(17)O and ^(26)Mg^*. The measured ^(16)O/^(17)O is 1028 ± 11 compared to ^(16)O/^(17)O)_☉ = 2610. This is the largest ^(17)O excess so far observed in any meteoritic material. The ^(26)Mg excess (^(26)Mg^*) is most plausibly due to in situ decay of ^(26)Al. The inferred ^(26)Al/^(27)Al)_0 ratio of 8.9 x 10^(-4) is ~ 18 times larger than the 5 x 10^(-5) value commonly observed in refractory inclusions formed m the solar system. The large ^(17)O excess and high ^(26)Mg^*/^(27)Al ratio unambiguously identify this corundum as a presolar oxide grain. Enrichments in ^(17)O and ^(26)Al are characteristic of H-burning and point to red giant or AGB stars as likely sources.

Circumstellar Hibonite and Corundum and Nucleosynthesis in Asymptotic Giant Branch Stars

We report the discovery of two hibonite grains (CaAl_(12)O_(19)) whose isotopic compositions show that they formed in the winds of red giant and asymptotic giant branch (AGB) stars. While hibonite is the second major phase (after corundum, Al_2O_3) expected to condense from stellar ejecta with C/O < 1, it has not previously been found. One circumstellar hibonite grain is highly enriched in ^(17)O and slightly depleted in ^(18)O relative to the solar composition and has large excesses in ^(26)Mg and ^(41)K, decay products of ^(26)Al and ^(41)Ca. The inferred initial values (^(26)Al/^(27)Al)0 ≈ 5 × 10^(-3) and (^(41)Ca/^(40)Ca)0 ≈ 1.5 × 10^(-4) of this grain are consistent with models of nucleosynthesis in an AGB star. The other hibonite is enriched in ^(17)O, strongly depleted in ^(18)O, shows no evidence of ^(41)Ca and formed with (^(26)Al/^(27)Al)0 ≈ 2 × 10^(-2). The low ^(18)O/^(16)O and very high (^(26)Al/^(27)Al)_0 may indicate substantial proton exposure during cool bottom processing in a low-mass parent star. The low upper limit on ^(41)Ca/^(40)Ca (≤ 3.2 × 10^(-5)) implies that little or no He-shell material had been dredged into the envelope when this grain formed. We also report isotopic compositions for 12 new circumstellar corundum grains. The compositions of 11 of these grains are consistent with current models for red giant and AGB stars. One corundum grain has extremely high ^(17)O/^(16)O and near-solar ^(18)O/^(16)O and may have formed in a star that was initially enriched in ^(17)O and ^(18)O.

Extinct 10Be in Type A Calcium-aluminum-rich inclusions from CV chondrites

We have found clear evidence of live 10Be in five normal Type A Calcium-aluminum-rich inclusions (CAIs), one normal Type B CAI, and one FUN Type A CAI, all from CV3 chondrites. The (10Be/9Be)0 ratios range from ∼0.36 × 10–3 to ∼0.77 × 10–3 and are similar to those found by previous workers. The (10Be/9Be)0 ratios do not correlate in a temporal fashion with (26Al/27Al)0, suggesting that 10Be and 26Al were produced by different mechanisms. An examination of possible sources for the short-lived radionuclides indicates that production of 10Be was almost certainly by particle irradiation, possibly within the solar system, and was probably accompanied by significant production of 41Ca and 53Mn. In contrast, all of the 60Fe, most of the 26Al, and some of the 53Mn were probably produced in stars and were imported into the solar system within presolar dust grains.

Aluminum-Magnesium and Oxygen Isotope Study of Relict Ca-Al-rich Inclusions in Chondrules

Relict Ca-Al-rich inclusions (CAIs) in chondrules crystallized before their host chondrules and were subsequently partly melted together with chondrule precursors during chondrule formation. Like most CAIs, relict CAIs are 16O enriched (Δ17O -9‰). Hibonite in a relict CAI from the ungrouped carbonaceous chondrite Adelaide has a large excess of radiogenic 26Mg (26Mg*) from the decay of 26Al, corresponding to an initial 26Al/27Al ratio [(26Al/27Al)I] of (3.7 ± 0.5) × 10-5; in contrast, melilite in this CAI and plagioclase in the host chondrule show no evidence for 26Mg* [(26Al/27Al)I of <5 × 10-6]. Grossite in a relict CAI from the CH carbonaceous chondrite PAT 91546 has little 26Mg*, corresponding to a (26Al/27Al)I of (1.7 ± 1.3) × 10-6. Three other relict CAIs and their host chondrules from the ungrouped carbonaceous chondrite Acfer 094, CH chondrite Acfer 182, and H3.4 ordinary chondrite Sharps do not have detectable 26Mg* [(26Al/27Al)I < 1 × 10-5, <(4-6) × 10-6, and <1.3 × 10-5, respectively]. Isotopic data combined with mineralogical observations suggest that relict CAIs formed in an 16O-rich gaseous reservoir before their host chondrules, which originated in an 16O-poor gas. The Adelaide CAI was incorporated into its host chondrule after 26Al had mostly decayed, at least 2 Myr after the CAI formed, and this event reset 26Al-26Mg systematics.

60 Fe AND 26 Al IN CHONDRULES FROM UNEQUILIBRATED CHONDRITES: IMPLICATIONS FOR EARLY SOLAR SYSTEM PROCESSES

The presence of about a dozen short-lived nuclides in the early solar system, including 60Fe and 26Al, has been established from isotopic studies of meteorite samples. An accurate estimation of solar system initial abundance of 60Fe, a distinct product of stellar nucleosynthesis, is important to infer the stellar source of this nuclide. Previous studies in this regard suffered from the lack of exact knowledge of the time of formation of the analyzed meteorite samples. We present here results obtained from the first combined study of 60Fe and 26Al records in early solar system objects to remove this ambiguity. Chondrules from unequilibrated ordinary chondrites belonging to low petrologic grades were analyzed for their Fe-Ni and Al-Mg isotope systematics. The Al-Mg isotope data provide the time of formation of the analyzed chondrules relative to the first solar system solids, the Ca-Al-rich inclusions. The inferred initial 60Fe/56Fe values of four chondrules, combined with their time of formation based on Al-Mg isotope data, yielded a weighted mean value of (6.3 ± 2) × 10–7 for solar system initial 60Fe/56Fe. This argues for a high-mass supernova as the source of 60Fe along with 26Al and several other short-lived nuclides present in the early solar system.

Heterogeneous phase reactions of Martian volatiles with putative regolith minerals

SummaryThe chemical reactivity of several minerals thought to be present in Martian fines is tested with respect to gases known in the Martian atmosphere. In these experiments, liquid water is excluded from the system, environmental temperatures are maintained below 0°, and the solar illumination spectrum is stimulated in the visible and UV using a Xenon arc lamp. Reactions are detected by mass spectrometric analysis of the gas phase over solid samples. No reacions were detected for Mars nominal gas over sulfates, nitrates, chloride, nontronite clay, or magnetite. Oxidation was not observed for basaltic glass, nontronite, and magnetite. However, experiments incorporating SO2 gas - an expected product of volcanism and intrusive volatile release - gave positive results. Displacement of CO2 by SO2 occurred in all four carbonates tested. These reactions are catalyzed by irradiation with the solar simulator. A calcium nitrate hydrate released NO2 in the presence of SO2. These results have implications for cycling of atmospheric CO2, H2O, and N2 through the regolith.