Ash, RD

Bland, PA

Russell, SS

Young, ED

Spiral - Imperial College Digital Repository

OXYGEN ISOTOPES IN CV CARBONACEOUS CHONDRITES:  THE SIGNIFICANCE OF THE CCAMLINE.   E. D. Young1, R. D. Ash1, S. S. Russell2,  and P. A. Bland2, 1Department of Earth Sciences, University ofOxford, Parks Road, Oxford OX1 3PR, UK (ed.young@earth.ox.ac.uk), 2Department of Mineralogy, Natural His-tory Museum, Cromwell Road, London SW7 5BD, UK.Introduction:   Young and Russell [1] proposedthat primordial O comprising condensed materials inthe early solar system had isotopic compositions repre-sented by a line with a slope of 1.00 on a plot of  δ17Oagainst δ18O. This line deviates from the carbonaceouschondrite (CC) anhydrous mineral line, or CCAM,defined by conventional mineral separate analyses ofCC components [2].  Which of these two lines, theslope-1.00 or the CCAM, is primordial has been sub-ject for debate.Data:  We have measured 17O/16O and 18O/16O in ahighly-altered type B CAI from the Grosnaja CV3 car-bonaceous chondrite using UV laser ablation. Regres-sion of all of these data yields a line identical to theCCAM in three-isotope space while omission of theanalyses with the highest ∆17O produces a steeper line(slopes of 0.94 vs. 0.97, respectively).There are now ~170 high-precision UV laser abla-tion analyses of 17O/16O and 18O/16O with a spatialresolution of approximately 80 µm, representing 4CAIs and 9 chondrules from three CV3 meteorites.The spatially resolved data exhibit greater dispersion athigh ∆17O than do analyses of mineral separates de-spite comparable precision between the two methods.The two data sets are similar at low ∆17O. It is the sig-nificance of this dispersion at high ∆17O defined by thelaser ablation data that is at issue. The laser ablation data (see Figure) define a spreadin δ18O of nearly 10 per mil along a mass fractionationline with a ∆17O of about –2.7 per mil (the Allendemass fractionation line, or AMF [3]). In many casesthe material with the highest δ18O on the AMF showsobvious signs of alteration, including growth of secon-dary minerals and Fe and alkali metasomatism [4]. Inother cases cryptic alteration is revealed by a novel X-ray microdiffraction method. Presence of such crypticalteration complicates interpretations of oxygen iso-tope ratio data.Discussion:   On the basis of the laser ablation datawe conclude that the CCAM can not be the loci ofprimordial oxygen in three-isotope space. If theCCAM were primordial it would require: (1) that thereis a correlation between the presence of alteration andprimordial oxygen isotope ratios, and (2) that thedominant mechanisms of mass fractionation causedδ17O and δ18O to decrease.  We regard the first impli-cation of a primordial CCAM as problematical becausewe are hard pressed to envision a process whereby themost altered materials would preserve the most primi-tive O-isotopic compositions. The second implicationof a primordial CCAM is inconsistent with the fact thatboth processes likely to have caused mass fractiona-tion, volatilization and fluid flow within a parent body,would have caused mass-dependent increases, ratherthan decreases, in δ17O and δ18O. Both objections areovercome if the slope-1.00 line represents primordialoxygen isotope ratios.References: [1] Young E. D. and Russell S. S.(1998) Science, 282, 452–455. [2] Clayton et al. (1977)EPSL, 34, 209–224. [3] Young E. D. et al. (1999) Sci-ence, 286, 1331–1335. [4] Ash R. D. and Young E. D.(2000) LPSC XXXI.63rd Annual Meteoritical Society Meeting 5258.pdf

Oxygen isotopes in CV carbonaceous chondrites – the significance of the CCAM line.

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