Ninth Annual V. M. Goldschmidt Conference 7306.pdf

CALCIUM-ALUMINUM-RICH INCLUSIONS IN CHONDRITE METEORITES: THE CASE FOR LOCAL PRODUCTION IN THE SOLAR NEBULA. G. J. MacPherson, Department of Mineral Sciences, U.S. National Museum of Natural History, Smithsonian Institution, Washington DC 20560-0119, USA (glenn@ glennm.si.edu).

Chondrite meteorites are 4.5-Ga aggregates con- chondrites had been analyzed for oxygen isotopes taining diverse kinds of preplanetary grains and dust, (mainly because only those from Allende and similar and retain direct physical clues to high and low tem- chondrites were large enough to analyze by conven- perature processes in the preplanetary solar nebula as tional techniques). Recent advances in ion microprobe well as accretionary and alteration processes on early techniques allowed in situ analyses of individual min- asteroidal bodies. The relative abundances of non- eral grains within small (<500 µm) CAIs in thin sec- volatile elements in chondrite bulk compositions ap- tions of ordinary chondrites. The data show that these proximate those in the solar photosphere [1]. The CAIs are similar to CAIs in carbonaceous chondrites in properties of chondrites and their components anchor terms of oxygen isotope composition [6], even though models of the earliest solar system. the chondrules from the two chondrite types lie on Chondrules (igneous olivine- and low-calcium py- different mixing lines. These early results appear in- roxene-rich spherules) and the less abundant calcium- consistent with nebula-wide production of CAIs, be- aluminum-rich inclusions (CAIs) are varieties of chon- cause of the voluminous meteoritic data showing that dritic grains that occur in most (except CI) subtypes of the asteroid belt (and by implication the nebula) was chondrites. Both formed by very high temperature isotopically quite heterogeneous in terms of oxygen processes in the solar nebula. The submillimeter to [7]. The data may be more consistent a new model [8] centimeter-sized CAIs hold special importance in un- in which all CAIs formed in a restricted and isotopi- derstanding the earliest solar system: they are the old- cally homogeneous region of the solar nebula, near the est objects formed in the solar system, and they par- infant Sun, then were later distributed more globally tially preserve isotopic signatures of the pre-solar dust via bipolar outflow to the different chondrite accretion from which they formed. They are the principal carri- regions. Supporting evidence comes from CAIs in ers of evidence for the short-lived radionuclide (t½~0.7 enstatite chondrites, whose mineralogy suggests they ma) 26Al in the solar nebula, a possible heat source for did not form in the enstatite chondrite formation region early planetary melting. They are also unusually en- of the nebula [9]. One consequence of the local CAI riched in 16O. Until the fall of the large Allende car- production model is that CAIs and chondrules may bonaceous chondrite in 1969, few people had ever have formed in different places by different mecha- heard of CAIs and fewer still had studied them. Brian nisms. The model of [8] also proposes local produc- Mason - to whom this paper is respectfully dedicated - tion of 26Al as a way of explaining the ³5x higher ini- was one of the first, publishing a series of papers [e.g. tial ratio of 26Al/27Al in CAIs relative to chondrules. 2-4] on the geochemistry of the large and abundant An anomalously 26Al-enriched CAI formation region Allende CAIs that helped establish their importance to would mean the CAI/chondrule isotopic difference solar nebula models. cannot be interpreted as resulting from a 2-3 million CAIs were originally interpreted to be the products year age difference [10]. However, correlations of of gas-solid (equilibrium) condensation from a nebula 26Al with 41Ca [11] in CAI imply 26Al was not locally that was uniformly hot at least out to the asteroid belt produced. A chronologic interpretation of the Al-Mg [e.g. 5]. But the preservation of pre-solar dust and the isotopic data remains plausible if controversial. existence of widespread isotopic heterogeneity among References: [1] Anders E. and Grevasse N. (1989) the constituent grains of chondrites – including CAIs – GCA, 53, 197–214. [2] Martin P.M. and Mason B. make a uniformly hot nebula unlikely. Yet CAIs un- (1974) Nature, 249, 333–334. [3] Mason B. and Mar- deniably formed at high temperatures, so where and tin P. M. (1974) EPSL, 22, 141–144. [4] Mason B. and how did they originate? New oxygen isotopic evi- Taylor S. R. (1982) Smithsonian Contrib. Earth Sci., dence suggests an intriguing possibility. Carbona- 25, 30 pp. [5] Grossman L. (1972) GCA, 36, 597–619. ceous, ordinary, and enstatite chondrites differ from [6] McKeegan K. D. et al. (1998) Science, 280, 414– one another in their bulk oxygen isotope compositions, 418. [7] Clayton R. N. (1993) Ann. Rev. Earth Sci., and these compositions cannot be related to one an- 21, 115–149. [8] Shu F. H. et al. (1996) Science, 271, other by processes that result in simple mass- 1545–1552. [9] Guan Y. et al. (1999) LPSC XXX. [10] dependent isotopic fractionation. Multiple isotope MacPherson G. J. et al. (1995) Meteoritics, 30, 365– reservoirs are required. Chondrules have oxygen iso- 386. [11] Sahijpal S. et al. (1998) Nature, 391, 559. tope compositions that reflect those of their host chon- drites but, until recently only CAIs from carbonaceous