Abstract
A number of distinct methodologies are available for determining the oxygen isotope composition of minerals and rocks, these include laser-assisted fluorination, secondary ion mass spectrometry (SIMS) and UV laser ablation. In this review we focus on laser-assisted fluorination, which currently achieves the highest levels of precision available for oxygen isotope analysis. In particular, we examine how results using this method have furthered our understanding of early-formed differentiated meteorites. Due to its rapid reaction times and low blank levels, laser-assisted fluorination has now largely superseded the conventional externally-heated Ni “bomb” technique for bulk analysis. Unlike UV laser ablation and SIMS analysis, laser-assisted fluorination is not capable of focused spot analysis. While laser fluorination is now a mature technology, further analytical improvements are possible via refinements to the construction of sample chambers, clean-up lines and the use of ultra-high resolution mass spectrometers.High-precision oxygen isotope analysis has proved to be a particularly powerful technique for investigating the formation and evolution of early-formed differentiated asteroids and has provided unique insights into the interrelationships between various groups of achondrites. A clear example of this is seen in samples that lie close to the terrestrial fractionation line (TFL). Based on the data from conventional oxygen isotope analysis, it was suggested that the main-group pallasites, the howardite eucrite diogenite suite (HEDs) and mesosiderites could all be derived from a single common parent body. However, high precision analysis demonstrates that main-group pallasites have a Δ17O composition that is fully resolvable from that of the HEDs and mesosiderites, indicating the involvement of at least two parent bodies. The range of Δ17O values exhibited by an achondrite group provides a useful means of assessing the extent to which their parent body underwent melting and isotopic homogenization. Oxygen isotope analysis can also highlight relationships between ungrouped achondrites and the more well-populated groups. A clear example of this is the proposed link between the evolved GRA 06128/9 meteorites and the brachinites.The evidence from oxygen isotopes, in conjunction with that from other techniques, indicates that we have samples from approximately 110 asteroidal parent bodies (∼60 irons, ∼35 achondrites and stony-iron, and ∼15 chondrites) in our global meteorite collection. However, compared to the likely size of the original protoplanetary asteroid population, this is an extremely low value. In addition, almost all of the differentiated samples (achondrites, stony-iron and irons) are derived from parent bodies that were highly disrupted early in their evolution.High-precision oxygen isotope analysis of achondrites provides some important insights into the origin of mass-independent variation in the early Solar System. In particular, the evidence from various primitive achondrite groups indicates that both the slope 1 (Y&R) and CCAM lines are of primordial significance. Δ17O differences between water ice and silicate-rich solids were probably the initial source of the slope 1 anomaly. These phases most likely acquired their isotopic composition as a result of UV photo-dissociation of CO that took place either in the early solar nebula or precursor giant molecular cloud. Such small-scale isotopic heterogeneities were propagated into larger-sized bodies, such as asteroids and planets, as a result of early Solar System processes, including dehydration, aqueous alteration, melting and collisional interactions.There is increasing evidence that chondritic parent bodies accreted relatively late compared to achondritic asteroids. This may account for the fact that apart from a few notable exceptions’ such as the aubrite-enstatite chondrite association, known chondrite groups could not have been the parents to the main achondrite groups.
Highlights
We look in detail at what information high precision oxygen isotope studies can provide concerning the origin of achondritic meteorites
We need to address the question of just how useful contemporary meteorites and asteroids are as indicators of the composition and structure of the first generation planetesimals that populated the Solar System prior to the accretion of the terrestrial planets? Relevant to this final question are the proposals that: (i) migration of the giant planets played a critical role in controlling the present structure of the asteroid belt (Walsh et al, 2011); (ii) that existing differentiated asteroids, such as 4 Vesta, are secondary bodies (Consolmagno et al, 2015), and (iii) that early fragmentation of planetesimals resulted in differential loss of mantle materials (Bell et al, 1989; Burbine et al, 1996; Jacobson et al, 2016)
Similar conclusions concerning the importance of accretion time on the extent of melting in early-formed planetesimals were reached by Sahijpal et al (2007) and they suggest that such modelling studies are consistent with accretion of chondritic parent bodies more than 2–3 Myr after CAIs, i.e. significantly later than the accretion ages of the iron meteorite parent bodies derived from Hf-W dating studies (Kleine et al, 2009; Kruijer et al, 2014)
Summary
It will come as little surprise to learn that oxygen is a uniquely important element. Oxygen is a highly reactive non-metal that readily forms compounds with other elements. Credit for the discovery of oxygen is controversial, being split three ways between the Swedish apothecary Carl Scheele, the English chemist Joseph Priestley and the French chemist Antoine Lavoisier; the latter generally considered to be the founder of modern chemistry (Lane, 2002). More important than its high relative abundance is the fact that oxygen is a major mineral-forming element. Even when the core is included to derive a total Bulk Earth composition, oxygen at 32.4 wt.% remains the most abundant element, just ahead of iron at 28.2 wt.% (Allègre et al, 1995). While its Solar System and Bulk Earth abundances may be important, perhaps the most critical feature of oxygen is the fact that it readily combines with hydrogen to form water. A significant proportion of the water in the solar nebula was inherited from the parent molecular cloud (Cleeves et al, 2014)
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