Olivine oxygen isotopes

The first and the simplest type is component exchange between phases without growth or dissolution of either phase. Examples include oxygen isotope exchange between two minerals, and Fe -Mg exchange between olivine and garnet or between olivine and melt. This is essentially a diffusion problem. 

Example. Multiple in-situ measurements of oxygen isotopic compositions in a single olivine grain give the following values (in per mil) ... 

Oxygen isotope signatures of Vesuvio rocks show wide variations (S180 —1-7.0 to +10.0) and are negatively correlated with MgO (Ayuso et al. 1998). Helium isotope studies on clinopyroxene and olivine from historical lavas gave values of R/Ra 2.2 to 2.7, close to ratios found in the fumar-oles of Campanian volcanoes (Tedesco et al. 1990 Graham et al. 1993). 

Pallasites are stony irons composed of roughly equal amounts of silicate (dominantly olivine), metal, and troilite (Figure 25). There are three separate pallasite types that are distinguished by differences in silicate mineralogy and composition (Table 6), metal, and oxygen isotopic... 

Krot A. N., Aleon J., and McKeegan K. D. (2002) Mineralogy, petrography and oxygen-isotopic compositions of Ca, Al-rich inclusions and amoeboid olivine aggregates in the CR carbonaceous chondrites. In Lunar Planet. Sci. XXXIII, 1412. The Lunar and Planetary Institute, Houston (CD-ROM). 

During metamorphism and recrystallization, oxygen isotopes are redistributed among mineral phases, according to the mass-dependent equilibrium fractionations corresponding to the peak metamorphic temperature. The measured mineral-pair fractionations (usually for major minerals olivine, pyroxene, and feldspar) can then be used for metamorphic thermometry, yielding temperatures of 600 °C for an L4 chondrite, and 850 50 °C for several type-5 and type-6 chondrites (Clayton et al., 1991). Isotopic equilibration, even in type-6 chondrites, involves oxygen atom transport only over distances of a few millimeters (Olsen et al., 1981). 

The brachinites are a small group of olivine-rich residues, with oxygen isotopic compositions overlapping those of the HED achondrites, although no genetic association has been proposed (Clayton and Mayeda, 1996). 

Figure 15 Oxygen-isotopic compositions of individual minerals in (a) aluminum-rich chondrules from ordinary chondrites (Russell et at, 2000) and (h) CR carhonaceous chondrites (Krot et ah, 2002h). (c) Oxygen-isotopic compositions of individual minerals in the CAI-hearing chondmle 17 from Acfer 094 (data from Krot et al, 2003a) (Ahhreviations chd, chondmle cpx, clinopyroxene gl, glass hih, hihonite nph, nepheline ol, olivine opx, orthopyroxene pi, plagioclase px, pyroxene sp, spinel).

CIl chondrites also contain a small fraction of isolated olivine and pyroxene grains up to 400 p,m in size with chemical, oxygen isotopic compositions, and rounded inclusions of metallic Fe,Ni and trapped melt indicating that they were derived from chondrules (Leshin et al, 1997). Brearley and Jones (1998) estimate the abundance of olivine and pyroxene at <1 vol.%, but X-ray diffraction studies by Bland et al. (2002) indicate 7 wt.% olivine. The dilference probably reflects a high abundance of olivine crystallites embedded in phyllosilicates (P. A. Bland, private communication). CIl chondrites also contain rare refractory grains with oxygen isotopic compositions comparable to those in CAIs (see Scott and Krot, 2001). 

Jones R. H., Saxton J. M., Lyon I. C., and Turner G. (2000b) Oxygen isotopes in chondrule olivine and isolated olivine grains. Meteorit. Planet. Sci. 35, 849-857. 

Leshin L. A., Rubin A. E., and McKeegan K. D. (1997) The oxygen isotopic composition of olivine and pyroxene from Cl chondrites. Geochim. Cosmochim. Acta 61, 835-845. 

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