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Diamonds Deliver the Dirt Dinosaurs Usually Found in Younger Sediments

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Diamonds Deliver the Dirt Dinosaurs Usually Found in Younger Sediments NEWS AND VIEWS RESUME------ - - complexes such as [Mlll(bipy-h] rather may be regarded as 'expanded' ele­ Early bird than [M0 (bipyh). The niceties of this ments, in which the effective size of the How a routine re-examination of a ambiguity are of direct relevance, as the alkali metal atom has been increased - modest fossil reptile turned into a complex cation [Ru(bipyh)2+ plays a instead of finding the electron in one of search for the closest relatives of birds vital role in many chemical photoconver­ the valence orbitals of the metal ion it is is recounted by A. R. Milner and S. E. sion systems. The critical excited state found slightly out in the orbitals of the Evans in Palaeontology (34, 503-513; [Ru(bipy)3] 2+* is best described10 as directly bonded ligands. D 1991). Fragmentary material of charge-separated [Ru III(bipy h(bipy-) ]2 + •. Lisboasaurus estesi from Guimarota in Portugal, about 160 million years old, The new compound described by Lehn Edwin C. Constable is in the Cambridge originally described as from a lizard. and co-workers. provides a structural Centre for Molecular Recognition, University was model for this photoexcited state as well Chemical Laboratory, University of Cam­ Further study by R. Estes revealed the as challenging our preconceptions about bridge, Lensfield Road, Cambridge CB2 presence of socketed teeth, more the ionic state. These new compounds 1EW, UK. characteristic of archosaurs (crocodiles, MANTLE GEOCHEMISTRY ________________ dinosaurs and birds). Milner and Evans now show that Lisboasaurus most closely resembles the troodontids, a group of advanced, birdlike theropod Diamonds deliver the dirt dinosaurs usually found in younger sediments. More tentatively, some J. J. Gurney aspects of the dentition place the fossil within the Avialae, a clade that includes As nearly impenetrable and inert cap­ at low temperatures, so that mantle Archaeopteryx and all other birds. sules, diamonds carry within them tiny heterogeneity or crustal recycling are Archaeopteryx dates to about 150 mineral inclusions trapped as they grew. again two counterpoised interpretations. million years ago, so the existence of On page 649 of this issue 1, Eldridge et The new data of Eldridge et al. show other, more-or-less contemporaneous al. use ion microprobe analyses of the that sulphides associated with the same birdlike reptiles seems reasonable, say sulphur and lead isotope ratios in sul­ eclogitic diamond class have sulphur the authors. phide inclusions to learn something of isotope-ratio variations well beyond the geochemistry of the mantle rocks in those expected in the generally accepted On the scent which they formed. In particular, their primary mantle. The cumulative evi­ THE nose, twitching in the wind, will results emphasize that some diamonds dence that three stable isotopes C, S and catch a fugitive fragrance on a receptor in the mucus contain sediments subducted, with 0 show signs of pronounced fractiona­ in a cilium, embedded layer of the olfactory epithelium. The from the Earth's tion in eclogitic minerals is far more oceanic lithosphere, receptor (one of many) is linked to a persuasive than any of these results in surface. G-protein, which in turn control an ion From the mineralogy of diamond in­ isolation. All these variations can be channel activated by cyclic AMP. clusions, it has already become estab­ readily achieved at low temperatures. Efficient capture of the molecule that lished that diamonds come in two classes Geochemists faced with a choice be­ represents the odour evidently depends - peridotitic and eclogitic - reflecting tween radically reassessing the range in on the mucus matrix, and this, it turns the mantle source rocks in which they isotope ratios for these elements within out, contains an abundant protein called formed2 . It is also becoming apparent3 the primary mantle and accepting a recy­ olfactomedin (D. A. Snyder et al. that peridotitic diamonds are derived cled crustal origin for diamond eclogites, Biochemistry30, 9143-9153; 1991). from primary mantle carbon with a lim­ must surely now opt for the latter. The polypeptide chain has a relative ited range in 13C/12C ratio and that these An additional piece of the jigsaw, molecular mass of 57,000 and exists as diamonds may be Archaean in age (over previously missing, comes with the lead­ a disulphide-linked dimer, heftily 2,500 million years old)4 • Conver­ isotope evidence in the new work, which glycosylated. There are loads of this sely, there is considerable evidence for indicates that the recycled component material in specialized cells, in which it the formation of younger eclogitic dia­ includes continental sediments. If eclo­ is presumably made, as well as in the monds with widely variable carbon iso­ gite is recycled oceanic lithosphere, olfactory epithelium. But just what it is tope ratios interpreted to have origin­ some sediments must also accompany doing remains to be seen . ated in recycled (subducted) crusts. the subducted crustal volcanic material Flash and burn Unfortunately these observations are down to regions of high temperature and THERE'S no smoke without lightning, or not conclusive. The equivocal nature of pressure. A missing component that is, not where large forest fires are the evidence permits several schools of volumetrically, even more significant re­ concerned. Don Latham (J. geophys. thought, amongst them one which holds mains to be defined: where is the exten­ Res. 96, 17,151-17,157; 1991) that eclogitic diamonds too are formed sive oceanic peridotite that must have describes a fire, started deliberately for from primary carbon, but in an isotopi­ been the major component of the down­ forest management one dry summer cally inhomogeneous mantle6 • Eclogite going slab? afternoon in Ontario, that consumed - broadly the high-pressure equivalent This slab, essentially the oceanic 460 hectares in 3 hours. Within half an of basalt or gabbro - would be a lithosphere at the time, would consist of hour of the start, a plume of smoke had product of partial melting of the mantle surface and near-surface sediments and risen 6-7 kilometres into the sky which in this model. The carbon isotopic heter­ igneous components, plus the underlying was drawn out into an anvil shape by ogeneity would have to be largely inher­ volumetrically dominant depleted mantle gentle wind shear. Latham recorded 37 ited from the source, as high­ peridotite from which the oceanic crust cloud-to-ground lightning strikes, most temperature fractionation processes do formed by partial melting. According to of them composed of positive charge. In not seem capable of creating it. A para­ known ages of eclogitic diamonds, these thunderstorms, such strikes are not only llel wide variation in oxygen isotope subduction events occurred in the rare but also dangerous, because they have the highest currents. But Latham ratios occurs in the silicate minerals in Proterozoic8. The older Archaean ages suspects the plume resembles more 7 and mantle carbon signatures have been eclogite rocks entrained in kimberlite those emitted from volcanoes, which (extinct volcanic pipe) in which di­ found in garnets from garnet carry net positive charge and have also 4 amonds are found. Oxygen isotope frac­ harzburgites - rocks so depleted in been seen to cause lightning. tionation can be strongly developed only their major elements that they have the NATURE· VOL 353 · 17 OCTOBER 1991 601 © 1991 Nature Publishing Group.
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