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Relict Mantle From Earth's Birth


http://www.geo.umass.edu/petrology/PetSem/Graham-Nature-2010-RelictMantle.pdf

higher variability in real spoken speech. This result depended crucially on the cochlear scale chosen for estimating entropy: because it is quasi-logarithmic, it gives greater weight to frequencies in the ‘formant range’ — that is, the locus of most vowel structure, which, contrary to what is often assumed, changes considerably over time and therefore has greater potential information. Consonants can also contain rapid change, but typically in higher frequencies, which receive less weight. The results call into question the canonical view of speech as a sequence of consonants and vowels, and open up avenues to new statistical descriptions that might better describe its fluid nature. Approaches involving information theory have offered several insights into the signal-level sensory codes of natural images and sounds7,8. These approaches work from the bottom up to identify sets of features that most compactly describe a class of signals, but are as yet insufficient to capture the highly nonlinear regularities in speech. Ultimately, getting to the bottom of speech intelligibility will require understanding not only the dimensions of variability that underlie the discriminability of speech sounds, but other informative aspects of the speech signal, such as its harmonic patterns and tonal variation, which vary from individual to individual. A further complication is that intelligibility can be adjusted dynamically to compensate for sources of noise. Understanding the basis of speech intelligibility would have obvious benefits for speechrelated technologies. Today’s hearing aids already contain quite sophisticated processing, but still break down in noisy environments. Today’s computers understand speech in the same way that they play chess, by brute force, and not as proficiently as humans. According to legend, a test of an early speech-recognition system goes something like this: Computer: “Please state your four-digit personal identification number.” User: “three four five six.” Computer: “I did not understand. Please repeat.” User: “three … four … five … six.” Computer: “I did not understand. Please repeat.” User: “Three … Four … Five … Six.” Computer: “I did not understand. Please repeat.” User: “THREE … FOUR … FIVE … SIX.” Computer: “I did not understand. Please repeat.” At which point the test user slams down the phone in frustration, and the scientists retreat back to their labs. The speaker actually did something perfectly reasonable in the face of miscommunication: spoke more clearly. Of course, when anger and frustration come into it, the message is completely lost on the computer. The irony is that what makes speech more intelligible for a human listener actually makes matters worse for the computer. Some day, perhaps, computers will be able to listen to us and not only get the words but also catch our drift. And some day, perhaps, we will be able to listen to our grandchildren, or even a foreign language, using hearing aids that work more like ‘ear glasses’ that bring clarity to a noisy world. ■ Michael S. Lewicki is in the Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, Ohio 44106-7071, USA. e-mail: michael.lewicki@case.edu GEOCHEMISTRY Relict mantle from Earth’s birth David Graham Geochemical evidence for the existence of the mother of all mantle-source reservoirs for volcanism has come to light. The new results have provocative implications for our understanding of Earth’s interior. Earth is a differentiated planet. Its primordial building blocks of iron-rich metal, oxides, silicate minerals, and volatile elements and compounds have been transformed over geological time into the modern-day structure of core, mantle, crust, ocean and atmosphere. Chemical and physical processes such as mantle convection, tectonic-plate recycling and magma generation through partial melting should have scrambled, if not obliterated, any coherent geochemical signature of the primordial material. Even if a vestige of such material remained, it seems unlikely that it would be found in any samples from Earth’s surface or the shallow subsurface that are available to geologists. Yet that is what new evidence suggests. On the basis of a trace-element and isotopic study, Jackson et al.1 (page 853 of this issue) propose that lavas from Baffin Island, Canada (Fig. 1), were derived from a deep-Earth reservoir that has remained isolated since the earliest days of planetary accretion some 4.5 billion years ago. Their work relies on combined analyses of the isotopes of helium, lead, neodymium and hafnium. Collectively, the results are the first of their kind for terrestrial volcanic rocks. The essence of the argument for the survival of primordial material is threefold. First, there is the occurrence of the highest ever measured 3 He/4 He ratios, in 60-million- to 62-millionyear-old volcanic rocks from Baffin Island and West Greenland2 . The ratios are more than five times higher than values commonly observed along mid-ocean ridges that tap the shallow upper mantle. The very high 3 He/4 He signifies an elevated ratio of primordial to radiogenic noble gas — a trait that geochemists often regard as supporting the existence of a reservoir deep within Earth that is the ultimate source for volcanic hotspots such as Hawaii and Iceland3 . Second, some of the Baffin and West Greenland lavas have lead-isotope compositions that lie on, or very close to, a 4.5-billion-yearold geochron. The geochron is a line in the 207Pb/204Pb–206Pb/204Pb diagram describing all possible parent/daughter (uranium/lead) ratios corresponding to Earth’s age. Third, the lavas have neodymium isotope compositions (143Nd/144Nd) consistent with derivation from a mantle source comprised of primordial silicates. On the basis of the landmark discovery of positive terrestrial 142Nd anomalies4 , this mantle source has a samarium/neodymium ratio that is about 5% larger than the ratio in chondrites. (Chondrites are stony meteorites that formed during the earliest stages of Solar System development and were never melted after their formation. They are thought to closely resemble the building blocks involved in Earth’s formation.) Because 142Nd was produced by the extinct radioactive decay of 146Sm (half-life of 103 million years), the excess of 142Nd relative to chondrites in all terrestrial samples seems to require the formation, early in Earth’s history, of an ‘early depleted reservoir’ with a high Sm/Nd ratio as a residue of partial melting. Negative 142Nd anomalies have not been found in terrestrial rocks, so this early depleted reservoir represents the ultimate ancestor of all other mantlesource reservoirs for volcanism. These three fundamentally coherent isotopic characteristics of helium, lead and neodymium have now been found together for the first time. Notably, they occur in contemporaneous primitive lavas from Baffin Island and West Greenland that erupted during the opening of the North Atlantic. The results are evidence that a 1. Stilp, C. e. & Kluender, K. r. Proc. Natl Acad. Sci. USA107, 12387–12392 (2010). 2. Cole, r., Yan, Y., Mak, B., Fanty, M. & Bailey, t. Proc. IEEE Int. Conf. Acoustics Speech Signal Processing Vol. 2, 853–856 (1996). 3. Kewley-Port, D., Burkle, t. Z. & Lee, J. H. J. Acoust. Soc. Am. 122, 2365–2375 (2007). 4. Liberman, a. M., Harris, K. S., Hoffman, H. S. & Griffith, B. C. J. Exp. Psychol. 54, 358–368 (1957). 5. Jenkins, J. J., Strange, W. & edman, t. r. Percept. Psychophys. 34, 441–450 (1983). 6. Lee, J. H. & Kewley-Port, D. J. Acoust. Soc. Am. 125, 1153–1163 (2009). 7. olshausen, B. a. & Field, D. J. Nature 381, 607–609 (1996). 8. Smith, e. C. & Lewicki, M. S. Nature 439, 978–982 (2006). 822 NEWS & VIEWS NATURE|Vol 466|12 August 2010 © 2010 Macmillan Publishers Limited. All rights reserved deep source, and one that has been effectively isolated for all geological time, fuelled the mantle plume that gave rise to volcanic activity on Baffin Island and Greenland, and that is now responsible for volcanism in Iceland. The results support the well-established idea that Earth’s mantle is heterogeneous, although the origin and survival of this heterogeneity have long been debated. It is also well established that the depth structure of mantle viscosity, the strain rate associated with mantle convection, and the thermal history of Earth all act in preserving and destroying such heterogeneity5 . Until now, geochemists have had to devise geodynamic arguments, not all of them mutually exclusive, to reconcile 3 He/4 He variations in mantle-derived rocks with isotopic variations in other elements such as lead and neodymium. Those arguments include enhanced migration of helium into previously melted and degassed rock as material is stretched and folded during mantle convection6 ; freezing of some magma within the upper mantle beneath mid-ocean ridges that is later recirculated to the deep mantle, where it contributes to oceanisland (hotspot) volcanism7 ; and isolation of the mantle sources for ocean-island volcanism away from sites of melting near Earth’s surface during the relatively recent geological past8 . However, evolutionary models for helium isotopes based on mass balance, and the range of possible geological histories for the formation of Earth’s crust, make it clear that high 3 He/4 He ratios are best explained by isolation of ancient mantle regions, rather than by continuous generation of high 3 He/4 He domains during Earth’s history9 . The new results1 are not without complexity. High 3 He/4 He is found in Baffin lavas with both enriched and depleted trace-element signatures, emphasizing the need for a better understanding of the potential decoupling of noble gases from other elements during mantle convection, partial melting and magma transport. Additional tests may come from measuring 3 He/4 He in mantle-derived rocks of different Figure 1 | Baffin Island — source of the lava samples isotopically analysed by Jackson and colleagues1 . ages, but it remains to be seen how successful those attempts will be in deciphering the helium-isotope evolution of the mantle. Such measurements rely on extracting noble gases contained within tiny fluid and melt inclusions trapped in crystals of the erupting lava. In older rocks, the measurements can be fraught with overprinting, mainly from the 4 He produced by radioactive decay of uranium and thorium within the rock since it was emplaced10. The large 3 He/4 He variability in the Baffin and West Greenland lavas1,2 attests to the difficulty in relating 3 He/4 He to the isotopes of lead, neodymium and hafnium in rocks that are more than a few tens of million years old. Nonetheless, the frontier is open in that endeavour, in light of which the study of Jackson et al.1 takes on increased importance — the possible survival of primitive mantle relicts needs full consideration in future models of mantle structure and evolution. ■ David Graham is at the College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA. e-mail: dgraham@coas.oregonstate.edu 1. Jackson, M. G. et al. Nature 466, 853–856 (2010). 2. Starkey, n. a. et al. Earth Planet. Sci. Lett. 277, 91–100 (2009). 3. Kurz, M. D., Jenkins, W. J. & Hart, S. r. Nature 297, 43–47 (1982). 4. Boyet, M. & Carlson, r. W. Science 309, 576–581 (2005). 5. Gurnis, M. & Davies, G. F. Geophys. Res. Lett. 13, 541–544 (1986). 6. albarède, F. Science 319, 943–945 (2008). 7. Davies, G. F. Geochem. Geophys. Geosyst. doi:10.1029/ 2009GC002801 (2010). 8. Class, C. & Goldstein, S. L. Nature 436, 1107–1112 (2005). 9. Porcelli, D. & elliott, t. Earth Planet. Sci. Lett. 269, 175–185 (2008). 10. Matsumoto, t. et al. Earth Planet. Sci. Lett. 196, 213–225 (2002). Don FranCiS NEUROSCIENCE A mine of imprinted genes eric B. Keverne Some genes exclusively express only their maternal or paternal copy. Studies of the brain extend the list of such imprinted genes by an order of magnitude, highlighting their spatial and temporal regulation. Mammals inherit one copy (allele) of each gene from their mother and another copy from their father. Yet for many genes, only one of these alleles is always expressed in a cell1 . The choice of which allele to express is random in some cell types — notably those of the olfactory and immune systems; for others, such as those of the developing placenta and brain, certain genes are ‘imprinted’2 . The hallmark of imprinted genes is that some are expressed only when inherited from the mother and others only when inherited from the father. Imprinted genes were thought to be fewer than 100 in number. But two remarkable studies published in Science by Gregg and colleagues3,4 identify 1,308 candidate imprinted genomic regions in the mouse brain, encompassing 824 annotated genes as well as the entire X chromosome. It has already been shown that, in the mouse placenta, the X chromosome is imprinted, with genes from the maternal X being exclusively expressed, thereby avoiding immunological rejection of ‘foreign’ fetal proteins that might be encoded by the paternal X chromosome2 . Many of the genes on the X chromosome are also expressed in the brain. In males (XY), the single X copy always originates from the mother, but in females (XX), either the maternal or the paternal copy of the X chromosome is inactivated early in embryonic development, and this typically occurs at random. Gregg et al.3 report preferential expression of the maternal X chromosome in two brain regions. Compared with the paternal X, the expression of the maternal X chromosome was 11% higher in glutamate-secreting neurons of the cortex, and 19% higher in the preoptic region of the basal forebrain. In general, the expression of imprinted genes is exclusively either maternal or paternal, with loss of exclusivity usually leading to expression of both alleles. This suggests that the biased gene expression described by Gregg et al.3 may be due to selection of cells expressing the maternal X chromosome, rather than imprinting. DNA replication errors increase with the number of cell divisions, which are an order of magnitude higher in the production 823 NATURE|Vol 466|12 August 2010 NEWS & VIEWS © 2010 Macmillan Publishers Limited. All rights reserved