Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..369h&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 369
Other
11
Allende, Carbonaceous Chondrites, Interstellar Dust, Isotopes, O, Mg, Murchison, Orgueil
Scientific paper
In a study of Mg isotopes in oxide grains from an Orgueil SiC-spinel-rich residue, [1] reported a corundum grain with ^26Mg*/^27Al = 8.9 x 10^-4, a value ~18 times greater than the canonical 5 x 10^-5 value characteristic of refractory phases formed in the solar nebula. Comparable ratios had previously been found only in carbon-rich interstellar materials, SiC and graphite, [2] leading [1] to suggest that Orgueil corundum B is a pre-solar oxide grain. Subsequently, [3] discovered Murchison corundum 83-5 with a sirnilar ^26Mg*/^27Al of 8.7 x 10^-4; the very unusual oxygen isotope composition (delta^17O = 1072 +- 59 per mil, delta^18O = -244 per mil) led [3] to conclude 83-5 is an interstellar oxide grain. The Panurge ion probe was used to determine ^170/^160 and ^180/^160 ratios in 27 Orgueil oxide grains--16 corundum, 2 hibonite, and 9 spinel--and in 6 Allende spinels. Orgueil corundum B has an extreme ^17O excess (delta^17O = 1394 +- 178 per mil (2sigma(mean)) and a hint of an ^18O depletion (delta^18O = -65 +- 64 per mil) (Fig. 1). The extraordinary enrichments in ^26Mg* and ^17O identify Orgueil B as an interstellar oxide grain. Orgueil B and Murchison 83-5 have remarkably similar O- and Mg-isotope compositions. Red giant stars are enriched in ^17O with ^17O/^18O >~ 1 [4], suggesting these stars are a likely source of the interstellar corundum. Production of ^26Al during H-burning in AGB stars also appears to account for the ^26Mg* excess [5,6]. Condensation of corundum in the circumstellar envelope must occur before dredge up of processed material from the stellar interior decreases ^17O/^16O and creates a C-rich atmosphere. The oxygen isotope compositions of the remaining oxide grains fall into three groups (Fig. 1). All but six corundums and one Orgueil spinel exhibit ^16O excesses and lie along the ^16O-mixing line with compositions similar to those of corundum and spinel from Murchison LS, LU, and CFO(sub)c [7]. Data from Allende spinels cluster about a mean value of delta^17O = -45.1 +- 2.1 per mil and delta^18O = -41.2 +- 2.2 per mil, very similar to the composition of Allende CAI spinel [8]. The Orgueil spinel and hibonite are slightly less ^16O-rich with delta^18O values between -18 per mil and -42 per mil. One Orgueil spinel and four corundums have normal O-isotope compositions with delta^17O ~ delta^18O ~ 0. The only other refractory oxide grains with similar composition are a group of Cr-rich Murchison spinel [9]. Two Orgueil corundums are enriched in the heavier O-isotopes and have compositions falling along the terrestrial fractionation line (delta^17O ~ 42 per mil, delta^18O ~ 75 per mil). This is the heaviest oxygen isotopic composition reported in meteoritic materials. If this composition is derived from isotopically normal oxygen by mass-dependent fractionation, the magnitude of fractionation is similar to that measured in the Allende FUN inclusion HAL [10]. References: [1] Huss G. R. et al. (1992) LPS XXIII, 563. [2] Zinner E. K. et al. (1991) Nature, 349, 51. [3] Nittler L. R. et al. (1993) LPS XXIV, 1087. [4] Lambert D. L. (1988) In Evolution of Peculiar Red Giant Stars, 101. [5] Forestini M. et al. (1991) Astron. Astrophys., 252, 597. [6] Wasserburg G. J. et al. (1993) LPS XXIV, 1487. [7] Virag S. A. et al. (1991) GCA, 55, 2045. [8] Clayton R. N. et al. (1977) EPSL, 34, 209. [9] Grossman L. et al. (1988) LPS XIX, 435-436. [10] Lee T. et al. (1980) GRL, 7, 498- 496. Division Contribution 5273(815); NASA NAGW-3297. Fig. 1, which appears here in the hard copy, shows oxygen isotopic composition of refractory oxide grains from Orgueil and Allende. The insert is an expanded view of the region inside the open box, near delta^17O ~ delta^18O ~ 0. The solid lines in the insert represent the terrestrial fractionation line (slope = 1/2) and the ^16O-mixing line (slope = 1). Ratios are shown as the per mil deviation from SMOW; errors are +-2 sigma(sub)mean.
Fahey Albert J.
Huss Gary R.
Hutcheon Ian D.
Wasserburg Gerald J.
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