The oxygen-isotope composition of chondrules and isolated forsterite and olivine grains from the Tagish Lake carbonaceous chondrite

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Abstract

The oxygen-isotope compositions (obtained by laser fluorination) of hand-picked separates of isolated forsterite, isolated olivine and chondrules from the Tagish Lake carbonaceous chondrite describe a line (δ17O = 0.95 * δ18O  3.24; R2 = 0.99) similar to the trend known for chondrules from other carbonaceous chondrites. The isolated forsterite grains (Fo99.6–99.8; δ18O = −7.2‰ to −5.5‰; δ17O = −9.6‰ to −8.2‰) are more 16O-rich than the isolated olivine grains (Fo39.6–86.8; δ18O = 3.1‰ to 5.1‰; δ17O = −0.3‰ to 2.2‰), and have chemical and isotopic characteristics typical of refractory forsterite. Chondrules contain olivine (Fo97.2–99.8) with oxygen-isotope compositions (δ18O = −5.2‰ to 5.9‰; δ17O = −8.1‰ to 1.2‰) that overlap those of isolated forsterite and isolated olivine. An inverse relationship exists between the Δ17O values and Fo contents of Tagish Lake isolated forsterite and chondrules; the chondrules likely underwent greater exchange with 16O-poor nebular gases than the forsterite. The oxygen-isotope compositions of the isolated olivine grains describe a trend with a steeper slope (1.1 ± 0.1, R2 = 0.94) than the carbonaceous chondrite anhydrous mineral line (CCAMslope = 0.95). The isolated olivine may have crystallized from an evolving melt that exchanged with 16O-poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite. The primordial components of the Tagish Lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups, especially CMs. Its alteration history has its closest affinities to CI carbonaceous chondrites.

Introduction

The Tagish Lake meteorite is the most carbon-rich chondrite known to date (Grady et al., 2002). It is composed of chondrules, isolated forsterite, isolated (more Fe-rich) olivine crystals, and rare Calcium Aluminum Inclusions (CAIs) contained in a matrix dominated by phyllosilicates, carbonates and magnetite (Zolensky et al., 2002, Simon and Grossman, 2003, Izawa et al., in press). The Tagish Lake meteorite is currently classified as ‘C2-ungrouped’ but its further designation continues to be debated.

Whole-rock δ18O and δ17O values of the Tagish Lake meteorite range from 16.9‰ to 23.5‰, and 8.5‰ to 12.2‰, respectively (Brown et al., 2000, Clayton and Mayeda, 2001, Hildebrand et al., 2006). These compositions are enriched in 18O and 17O compared to CM chondrites and are similar to those of CI chondrites (Clayton and Mayeda, 1999). Results for Tagish Lake phyllosilicate oxygen- and hydrogen-isotope compositions (Russell et al., 2008) are also similar to those of CI meteorites, suggesting that water of similar composition and temperature was involved in the aqueous alteration of both. The Tagish Lake meteorite whole-rock values plot slightly below the Terrestrial Fractionation Line (TFL) (Hildebrand et al., 2006), which describes the oxygen-isotope compositions of terrestrial and lunar materials (Matsuhisa et al., 1978). Such compositions likely arise from substantial oxidation and aqueous alteration of the Tagish Lake meteorite’s primitive parent body – processes that destroyed much of the early mineralogy and replaced it with phyllosilicate, carbonate, magnetite and sulfide minerals (Zolensky et al., 2002).

Whole-rock oxygen isotopic data provide a weighted average of all processes that have affected a meteorite, including variations in source, differentiation, aqueous alteration and thermal metamorphism of the parent body. The carbonaceous chondrites generally exhibit the widest range of oxygen isotopic compositions of any meteorite group (Clayton and Mayeda, 1999). Oxygen-isotope values of CAIs, isolated olivine, isolated forsterite and chondrules provide evidence for mass-independent fractionation of early solar system materials. Until now, few data have been reported for anhydrous mineral fractions from the Tagish Lake meteorite; yet such data are needed to better understand the origin of the early formed phases.

The compositions of CAIs, isolated forsterite, isolated olivine and chondrules typically plot along a trend of δ17O versus δ18O values known as the Carbonaceous Chondrite Anhydrous Mineral (CCAM) line, which was defined by Clayton et al. (1977) using data for physically separated minerals in CAIs from Allende (CV3). The range of oxygen-isotope compositions observed for chondrules and isolated forsterite and olivine crystals in carbonaceous chondrites is generally smaller than that known for CAIs, although large variations in chondrules are not unknown (e.g., −75‰ δ18O and δ17O values for a chondrule in the Acfer 214 meteorite; Kobayashi et al., 2003). Variations in the oxygen-isotope composition of chondrules and CAIs along the CCAM line are interpreted to result from exchange between 16O-rich solids and 16O-poor gases (Clayton et al., 1977, Clayton et al., 1983, McSween, 1977, McSween, 1985, Clayton and Mayeda, 1984, Clayton and Mayeda, 1999, Clayton, 1993, Rowe et al., 1994, Yu et al., 1995, Leshin et al., 1997, Young and Russell, 1998, Jones et al., 2004, Krot et al., 2005a, Krot et al., 2005b). The processes responsible for formation of the 16O-rich and 16O-poor reservoirs continue to be discussed (see Thiemens, 2006, for a review of the nature and causes of mass-independent fractionation in this system). Self-shielding of CO has been proposed as a mechanism by which an equally proportional distribution of heavy oxygen isotopologues is made available for chemical reactions during bombardment by ultraviolet radiation (Clayton, 2002, Yurimoto and Kuramoto, 2004, Lyons and Young, 2005). These reactions should produce 16O-poor gas relative to the 16O-rich Sun (McKeegan et al., 2009) and solids (e.g., CAIs) formed from the same reservoir.

Here we report the first comprehensive suite of data for the oxygen isotopic and associated chemical variations of isolated forsterite, isolated olivine and chondrules from the Tagish Lake meteorite – with three main purposes (1) to determine if the isolated forsterite is enriched in 16O relative to other Tagish Lake isolated olivine grains and to confirm its identity as refractory forsterite; (2) to describe the variation in oxygen isotopic and chemical compositions among the isolated forsterite, chondrules (including forsterite-rich varieties) and isolated olivine – and to explore its broader significance; (3) to contribute to the discussion on the classification of the Tagish Lake meteorite.

Section snippets

Methods

Hundreds of grams of Tagish Lake material were recovered from the frozen lake surface during expeditions mounted in 2000 by Mr. Jim Brooks, the University of Calgary and the University of Western Ontario (Hildebrand et al., 2006). Meteorite fragments that accumulated in ice-melt pockets provided naturally disaggregated samples. Some of this material was sieved into >2.38 mm, 2.38–1 mm and <1 mm size-fractions, followed by hand-picking of chondrules or olivine grains. This processing yielded 657 mg

Isolated forsterite

Tagish Lake isolated forsterite grains are single crystals or crystal fragments that typically have an irregular but rounded shape, and are several hundred microns in diameter (Fig. 1). Micro-FTIR spectra of the forsterite-rich isolated grains contain three principal Si–O bands at 837, 884 and 985 cm−1, characteristic of the olivine structure (Farmer, 1974). EPMA was performed on mounted grains, which are representative fractions of the samples consumed during oxygen-isotope analyses. On

Origin of isolated forsterite

A bimodal distribution in chemical composition exists between the Tagish Lake isolated forsterite (Fo99.6–99.8) and the isolated olivine (Fo39.6–86.8), with forsterite having a very narrow range in Fo content compared to olivine. Simon and Grossman (2003) were the first to report such results for the Tagish Lake carbonaceous chondrite, though their larger dataset includes some isolated olivine grains with compositions of Fo80–99. Bimodal patterns in olivine composition are known for other

Conclusions

  • 1.

    Chondrules, isolated forsterite and isolated olivine grains from the Tagish Lake meteorite have oxygen-isotope compositions defined by: δ17O = 0.95 * δ18O  3.24 (R2 = 0.99).

  • 2.

    The isolated grains from the Tagish Lake carbonaceous chondrite analyzed here can be divided into two main groups: isolated forsterite (Fo99.6–99.8) and isolated olivine (Fo39.6–86.8). The oxygen-isotope compositions of isolated forsterite (δ18O = −7.2‰ to −6.1‰; δ17O = −9.6‰ to −8.2‰) are significantly more 16O-rich than those of

Acknowledgments

We thank Peter Brown and Alan Hildebrand for providing samples of the Tagish Lake meteorite. We thank John Valley for the UWG-2 garnet standard, and Zach Sharp for samples of the Lausanne and Gee Whiz quartz standards. The laboratory staff at LSIS (Kim Law and Li Huang) provided technical support. We are very grateful for the substantive suggestions and advice of Drs. Sara Russell, Ian Lyon and two anonymous reviewers. Their efforts to help us improve the manuscript are very much appreciated.

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    Present address: Institute of Meteoritics, MSC03 2050, 1 University of New Mexico, Albuquerque, NM 87131, USA.

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    Present address: MSD469, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.

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