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decreases steadily with increasing supersatu-ration, produced using different values of temperature or pressure, and remains finite atthe ‘spinodal’ point where the mother phasebecomes thermodynamically unstable. In con-trast to phase transitions that originate from ametastable phase, transitions from an unstablemother phase (‘spinodal decomposition’6)occur spontaneously, bypassing the need forcritical nuclei to form. Theories that approxi-mate interfacial thermodynamics by neglect-ing density or concentration fluctuationspredict a sharp transition from nucleation tospinodal decomposition and a diverging criti-cal nucleus at the spinodal6. Scaling argu-ments, in contrast, predict a smooth transitionfrom metastability to instability8. In agreementwith recent experiments on concentrated protein solutions9, Pan et al.1 find that the size of the critical nucleus decreases smoothly with supersaturation and remains finite at thespinodal. They see no evidence of a divergingnucleus size.

An important open question is the geom-etry of the critical nucleus. Computer simula-tions10 indicate that, at large supersaturations,nuclei send out offshoots to become ramified,fractal objects. But the definition of the criticalnucleus size used by Pan et al. in their simula-tions assumes a compact object, and it wouldbe interesting to account for the shape of thecritical nucleus in future calculations.

Although a qualitative picture of the transi-tion from nucleation to spinodal decomposi-tion exists7, this has yet to result in a predictivetheory of nucleation at large supersaturations.Because of the ambiguity associated with the very notion of a critical nucleus underthese conditions7, such a theory will probablyrequire kinetic arguments (based on the ratesof growth and decay of embryonic nuclei)11,rather than thermodynamic arguments basedon the free-energy cost of forming a criticalnucleus. The work of Pan et al. providesmicroscopic insight on the smallest building-block of a new phase, an important ingredientfor future theoretical descriptions. ■

Pablo G. Debenedetti is in the Department ofChemical Engineering, Princeton University,Princeton, New Jersey 08544, USA.e-mail: pdebene@princeton.edu

1. Pan, A. C., Rappl, T. J., Chandler, D. & Balsara, N. P. J. Phys. Chem. B 110, 3692–3696 (2006).

2. Debenedetti, P. G. Metastable Liquids: Concepts andPrinciples (Princeton Univ. Press, 1996).

3. Lefebvre, A. A., Lee, J. H., Balsara, N. P. & Hammouda, B. J. Chem. Phys. 116, 4777–4781 (2002).

4. Lefebvre, A. A., Lee, J. H., Balsara, N. P. & Vaidyanathan, C.J. Chem. Phys. 117, 9063–9073 (2002).

5. Stanley, H. E. Introduction to Phase Transitions and CriticalPhenomena (Oxford Univ. Press, 1971).

6. Cahn, J. W. & Hilliard, J. E. J. Chem. Phys. 31, 688–699 (1959).7. Wood, S. M. & Wang, Z.-G. J. Chem. Phys. 116, 2289–2300

(2002).8. Binder, K. Physica 140A, 35–43 (1986).9. Shah, M., Galkin, O. & Vekilov, P. G. J. Chem. Phys. 121,

7505–7512 (2004).10. Heerman, D. W. & Klein, W. Phys. Rev. Lett. 50, 1062–1065

(1983).11. Ruckenstein, E. & Dijkaev, Y. S. J. Coll. Interf. Sci. 118, 51–72

(2005).

The economic value of the even-toed ungulates of the camelidfamily known as llamas(pictured) has been principallyfounded on their use as packanimals and as a source of hide and wool in their nativeSouth America. That llamaantibodies are suited tomeasuring levels of caffeine indrinks had been overlooked, anomission now corrected by RuthC. Ladenson and colleagues(Anal. Chem. doi:10.1021/ac058044j; 2006).

Although caffeine-specificantibodies for use inimmunoassays are availablecommercially, they becomeirreversibly denatured at hightemperatures. But camelidsproduce certain antibodies thatconsist exclusively of heat-resistant chains of amino acids.Such antibodies would be idealfor assessing the caffeinecontent of both hot and cold beverages.

Over a period of ten weeks, the authors immunized fivecamelids — three llamas andtwo camels — with caffeinelinked to an immune stimulantcalled keyhole limpethaemocyanin. Theysubsequently identified andisolated antibody fragments thatbind only to caffeine in bloodsamples from the immunizedanimals, and produced one ofthese as a soluble protein.

The chosen antibodypreparation retained almost allof its activity after incubation at

temperatures up to 90 �C,whereas that of comparablepreparations derived from micedropped away sharply above 70 �C. Swilled through regularand decaffeinated coffee andcaffeinated soft drinks, in eachcase it registered caffeine levelsin good agreement withestablished values.

The authors’ next step is toproduce a dipstick assay,similar to that used in homepregnancy tests, for point-of-consumption use.Richard Webb

ANALYTICAL CHEMISTRY

Cause for a llama

GEOCHEMISTRY

The noble art of recyclingTakuya Matsumoto

Xenon trapped beneath Earth’s crust provides clues to how our planetevolved, but quantifying atmospheric contamination has been impossible.The latest analysis surmounts a barrier to our understanding.

Because of their scarcity, chemical stability andpresence as many distinguishable isotopes,noble gases in Earth’s mantle — the solid layerbetween its outer crust and liquid core — pro-vide constraints on the origin, structure andevolution of Earth and its atmosphere. Onpage 186 of this issue1, Holland and Ballentineuse investigations of well gases from the upper mantle to challenge an established tenetconcerning noble-gas abundance: the exis-tence of a ‘subduction barrier’ that preventsthe noble gases from recycling into the mantlethrough tectonic activity. In fact, the authorsconclude that some 80% of xenon in the mantle comes from sea water introduced byjust such a process.

The subduction barrier was described in aclassic paper2 showing that at subduction zones— where one of Earth’s tectonic plates divesunder another — the noble gases in subducting

materials are returned to the surface throughvolcanic activity, leaving the composition of themantle unaffected. The fact that noble gases of the same isotopic composition as Earth’satmosphere are found in nearly all analyses ofsamples of rock extruded from the mantle hasbeen attributed, quite reasonably, to contami-nation from the atmosphere during, or after,the eruption of magma3,4. With the exceptionof certain mantle-derived rocks that acquirefluid at subduction-zone settings5,6, the re-cycling of noble gases into the mantle reser-voirs by subduction has never been proved.

Holland and Ballentine1 analyse high-quality isotope data from well gases in NewMexico to demonstrate that xenon of atmos-pheric nature is intrinsic to the mantle. Theystudy the abundances of three primordial iso-topes of xenon — 124Xe, 126Xe, 128Xe, which arenot produced in any reaction inside Earth —

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in relation to that of a fourth primordial isotope, 130Xe, and compare the ratios obtainedwith those found in xenon in the Sun and inEarth’s atmosphere (Fig. 1a). Their results areconsistent with a xenon content of 90% atmos-pheric and 10% solar origin.

The authors address the origin of the air-likexenon component using the ratios of two fur-ther isotopes, 129Xe and 136Xe, to 130Xe. Theseare radiogenic isotopes, respectively producedby radioactive decay from a now-extinctiodine isotope (129I), and by fission of a now-extinct plutonium isotope (244Pu) and long-lived uranium-238. Non-atmospheric129Xe/130Xe and 136Xe/130Xe ratios have alreadybeen established in data from extruded mantlerocks known as mid-ocean-ridge basalts(MORBs). These higher ratios can probably be explained by assuming that significantamounts of primordial 130Xe were removedfrom the mantle source by early catastrophicdegassing. Quantifying the ‘pristine’ mantlecomposition will therefore constrain modelsof Earth’s evolution from its early stages to thedevelopment of distinct mantle reservoirs7.But the pristine composition cannot be identi-fied from MORB data alone, as the degree ofatmospheric contamination after the rockshad been extruded is unknown.

The authors1 find that, on a three-isotopediagram, 129Xe/130Xe and 136Xe/130Xe ratios ofthe New Mexico well gases form a straight linethat does not extend from the point that repre-sents those ratios in air (Fig. 1b). Instead, thedata lie on a line that extends from a point

consistent with a hybrid of atmospheric xenonand xenon of crustal origin. The point of inter-section of this line with that of the MORB datawill represent the xenon composition of thecommon source of MORBs and well gases,most probably the upper mantle.

The results from the radiogenic isotopesindicate that the air–crust hybrid component,which is likely to be added to well gasesthrough local contamination during their stor-age in the continental crust1,8, can account foronly about half of the 90% atmospheric xenoncomponent revealed by the non-radiogenicxenon isotopes. The rest of this air xenon mustbe intrinsic to the mantle reservoir, and themost likely process to introduce it into themantle source region would be subduction.

Thus it seems that the noble-gas subductionbarrier is not as effective as had been pre-sumed. The survival of only 2% of pore waterin subducting oceanic plates would be suffi-cient to account for the quantity of isotopicallyair-like noble gases found in the upper man-tle1. But can these gases be recycled to thedeeper mantle regions? Rocks known asoceanic island basalts (OIBs), which are pre-sumed to be of lower-mantle origin, yieldnoble-gas isotopic ratios that are less radi-ogenic (and thus more air-like) than MORBs.The difference is normally explained byassuming that MORBs originate from adegassed reservoir, from which the primordialgas has largely been removed to form the present atmosphere.

Holland and Ballentine speculate, however,

that the difference in noble-gas isotope signa-tures between the MORBs and OIBs might infact result from different degrees of regassingby subduction. Such a model requires thatnoble gases in the OIB source should be moresignificantly modified by the recycled compo-nent than are those in the MORB source, butthis is quite consistent with results from othergeochemical tracers such as neodymium,strontium and lead isotopes. Recent attemptsto determine precisely the ratios of primordialxenon isotopes in OIBs and MORBs couldalso support the idea that subduction adds anintrinsic air component to the source regionsof these rocks9–11.

Holland and Ballentine explore the details ofthis air influx by showing that the recycled airfound in the New Mexico well gases has relativenoble-gas abundance ratios similar to those ofsea water. But although sea water is undoubt-edly a ready source of air-like noble gases insubducting materials, it is unclear how the seawater signature could survive subduction,storage in the mantle or crustal reservoirs andemanation at the continental gas field, withoutchanges to its relative noble-gas abundances.Even the least-contaminated MORBs do not9

contain solar-like xenon, so the underlyingassumption that MORBs and the New Mexicosamples share the same uniform mantle reser-voir needs further clarification. There are alsomany unresolved issues about the feasibility ofdeep air recycling — for example, the presenceof a carrier in subducting materials other thanpore water that can relay air-like noble gas intothe deeper mantle regions.

Nevertheless, supported by an independentdetermination of xenon isotopic compositionin the upper mantle12, Holland and Ballentine’sconclusion1 that some air-like xenon in theNew Mexico well-gas samples is intrinsic totheir mantle source seems robust. However,extending the recycling hypothesis to othersamples, such as MORBs and OIBs, with their uncertain air contamination, will require further work. ■

Takuya Matsumoto is in the Department of Earthand Space Science, Graduate School of Science,Osaka University, Toyonaka 560-0043, Japan.e-mail: matsumoto@ess.sci.osaka-u.ac.jp

1. Holland, G. & Ballentine, C. J. Nature 441, 186–191 (2006).2. Staudacher, T. & Allègre, C. J. Earth Planet. Sci. Lett. 89,

173–183 (1988).3. Patterson, D. B., Honda, M. & McDougall, I. Geophys. Res.

Lett. 17, 705–708 (1990).4. Ballentine, C. J. & Barfod, D. N. Earth Planet. Sci. Lett. 180,

39–48 (2000).5. Matsumoto, T., Chen, Y. & Matsuda, J. Earth Planet. Sci. Lett.

185, 35–47 (2001).6. Matsumoto, T. et al. Earth Planet. Sci. Lett. 238, 130–145

(2005).7. Trieloff, M. & Kunz, J. Phys. Earth Planet. Inter.148,13–38 (2005).8. Ballentine, C. J., Sherwood Lollar, B., Marty, B. & Cassidy, M.

Nature 433, 33–38 (2005).9. Kunz, J., Staudacher, T. & Allegre, C. J. Science 280,

877–880 (1998).10. Trieloff, M., Kunz, J. & Allegre, C. J. Earth Planet. Sci. Lett.

200, 297–313 (2002).11. Trieloff, M. et al. Science 288, 1036–1038 (2000).12. Moreira, M., Kunz, J. & Allegre, C. Science 279, 1178–1181

(1998).

Figure 1 | Differing isotopic ratios of xenon. a, The ratios 124Xe/130Xe and 126Xe/130Xe in the Sun and inEarth’s atmosphere are well known (large circles). Holland and Ballentine’s data1 from gas wells in NewMexico (blue squares) fit a straight line drawn between these two points, indicating samples withvariable components from solar- and air-like sources. The fraction, f, of air 130Xe in each sample can beestimated from these data by solving a mixing equation (scale bar). Part of the required air-like xenon(red on the scale bar) cannot be accounted for by local air contamination. b, The ratios 129Xe/130Xe and136Xe/130Xe of New Mexico well gas (blue squares) and MORB rock (grey circles)9,12 plot on differentmixing lines: whereas MORB data extend from air composition, suggesting that they contain purelyatmospheric contamination, data from New Mexico well gases extend from a point consistent with pre-mixed air–crust gases, which are enriched in 136Xe from crustal uranium fission. The intersection of thetwo lines defines the isotopic ratios for the mutual source of the two samples — most probably theconvecting upper mantle.

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