Book Review: RiMG v. 68
Reviews in Mineralogy and Geochemistry, Volume 68, Oxygen in the Solar System edited by Glenn J. MacPherson et al, The Mineralogical Society of America, Washington D.C., 2008, 598 pp. US $40 (ISBN 978-0-939950-80-5).
For many years, the study of the fractionation of oxygen isotopes was limited to measurements of 16O/18O ratios, the rare 17O being neglected. But when Bob Clayton and Tosh Mayeda in 1973 first measured 17O, in attempting to study the condensation temperatures of the refractory inclusions or CAIs in Allende, they opened a box that might have daunted Pandora. Plots of the isotopes on a three isotope diagram (the now-famous Clayton-Mayeda diagram) revealed unexpected complexities.
Samples from the Earth, Moon and eucrites indeed behaved as expected, showing linear temperature-controlled fractionations related to the masses of the isotopes. But data from other meteorites occupied separate areas, showing that they had distinct initial abundances of the three isotopes. Most spectacular of all were the data from the CAIs that fell along a line that was unrelated to mass and at first glance seemed due to an enrichment or spike in pure 16O. One immediate benefit for meteoriticists was to remove the concept of a common parent (CI ?) from which the wide variety of meteorites might be derived.
Oxygen, as every beginning student in geochemistry is reminded, is the most abundant element in the crust and mantle of the Earth. The present volume is a tribute to how far this study has come in the past 30 years. But the book covers a much wider spectrum than oxygen isotopes. It contains something for everyone. There are separate chapters on oxygen in the interstellar medium, the Sun, the solar nebula, asteroids, comets, interplanetary dust particles, every variety of meteorite, discussions of redox conditions in planets and it also deals with redox conditions throughout the inner solar system. Editing such multiauthor volumes (over 60 in this instance) has been compared to herding cats; the present volume results from the heroic efforts of Glenn MacPherson and his team.
Following an historical survey by Bob Clayton, Bob Criss and Jim Farquhar give a very useful survey of the abundance, notation and fractionation of the light stable isotopes, although they note that “these processes are more diverse than anticipated only ten years ago”. The origin of oxygen through nucleosynthesis in stars is reviewed by Brad Meyer and co-workers, who note the useful information on such sources that is contained in pre-solar grains in meteorites.
The question of oxygen in the interstellar medium (ISM) is next discussed by Adam Jensen and colleagues. The gas phase abundances are well-known but those of the dust phase are opaque, although perhaps located mainly in silicates. Andrew Davis and another international team look at the problem of oxygen in the Sun, noting that the current value (468 ppm) is significantly lower than earlier estimates. Producing such "state of the art" summaries is never without hazard. Just as this book appeared, the vexed question of the oxygen isotopic composition of the Sun may have been resolved. Long disputed between 16O-poor (ANU) and 16O-rich (French) claimants, analysis of the Genesis samples of the solar wind at UCLA by Kevin McKeegan and associates shows that the Sun, like the CAIs, appears to be rich in 16O.
Larry Grossman and co-workers discuss redox conditions in the early solar nebula, based on extensive experimental work, concluding that “ perhaps …. the solar nebula inherited..heterogeneities …. from the parent interstellar gas cloud”. A long review of oxygen isotopes in the components of chondrites by Hisayoshi Yurimoto and an international group of experts provides a very useful summary, noting that “ efficient large scale mixing “ seems to be required to assemble 16O-rich and 16O-poor components “from widely separated regions in the nebula”.
The vexing question of oxygen isotope variations in the solar nebula that are independent of mass is addressed by Ed Young and a team of Japanese and US colleagues, who note that the isotopic composition of the Sun is a crucial parameter needed for further progress , although chemical processes in the nebula, as first suggested by Mark Thiemens, look increasingly attractive. Even the giant planets and their satellites do not escape attention, as discussed by Michael Wong and associates; a field that shows much promise for interpreting giant planet and satellite formation scenarios, but one that requires much more data. Scott Sandford and others look at the oxygen record in comets and interplanetary dust, noting that comets contain “mixtures of both volatile and refractory bearing materials” and have perhaps sampled “the entire radial extent of the solar nebula”.
Tom Burbine and colleagues and separately Ian Franchi review oxygen in asteroids that indicate the great complexity of the topic and the difficulties in interpreting reflectance spectra due to space weathering. 'Duck' Mittlefehldt and coworkers
note that the oxygen isotopic composition of planets appears to be decoupled “from other compositional characteristics” of planets, food for thought indeed. Next Mike Zolensky and colleagues assess the record of low temperature aqueous alteration in meteorites, something that seems to have happened for up to 15 myr after Tzero in their parental asteroids.
Jim Farquhar and Dave Johnson examine the record of changes in the oxygen cycle in the hydrosphere, geosphere and biosphere of the Earth through geological time that includes some reference to John Valley's work. Meeni Wadhwa, in a discussion of redox conditions on small bodies, notes that they vary by six orders of magnitude among the differentiated meteorite parent bodies, a fruitful field for research in trying to understand early conditions in the early nebula while Bob Criss discusses the complexities introduced by the upward migration of 18O within the outer regions of the Earth.
The oxidation state of the Earth’s mantle is reviewed at length, but it is a subject fraught with difficulty. Chris Herd in a discussion of basalts as probes of planetary redox state notes the pitfalls as summarised by Cin-Ty Lee in “Haiku” style:
“Fugacity has no memory
It has no past
Only what it sees last”
A penultimate chapter by Steve Mackwell discusses the rheological consequences of the redox state of planetary mantles. One learns that although oxygen fugacity varies latterly and vertically in the mantle by factors approaching 104, they may result "an order of magnitude variation in viscosity” while variations “in water fugacity and lithology may be more significant”. Sober words indeed for modellers of mantle dynamics. The volume concludes with a useful "brief tutorial" on meteorites for the uninitiated.
The cause of these spectacular variations among the isotopes of oxygen has long been controversial. Bob Clayton long suggested that they were due to a spike of pure 16O, with this highly stable nucleus (8p, 8n) probably derived from a nucleosynthetic source. But the absence of other predicted isotopic variations in elements such as Mg or Si ruled out such an origin. . Attention has now focussed on chemical processes within the nebula, following prescient suggestions 20 years ago by Mark Thiemens (UC San Diego). Curiously he does not appear as an contributor to this volume, nor is his significant contribution, although referenced, much highlighted. But it is now generally appreciated that the useful and dramatic variations among the three oxygen isotopes arise from late-stage processes within the inner solar nebula.
As is evident from reading this book, replete with enigmatic data and unexplained phenomena, there is no shortage of problems in this expanding field. The identity of oxygen isotope signatures between the Earth and the Moon remains as one such curiosity As it seems unlikely that the Moon-forming impactor had an initial oxygen signature identical to that of the Earth, Dave Stevenson and co-workers (Cal Tech) have suggested that this identity results from equilibration in the vapor cloud following the collision, something that opens yet another compartment in Pandora’s box.
Stuart Ross Taylor
Australian National University.