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Geochemical data can tell us a greatdeal about processes in Earth’s man-tle, but obtaining that data and inter-

preting it are far from straightforward. Ifconfirmation of that point were needed itcomes from papers by Lassiter and Hauri1,and Putirka2, both of which involve analysisof Hawaiian lavas but which come to ratherdifferent conclusions about their origins.

Lassiter and Hauri present evidence fromosmium isotopes to support the idea3,4 thatthe Hawaiian mantle plume — like otherplumes, a hot and buoyant jet-like structurethat melts as it approaches Earth’s surface —is an ancient slab of subducted ocean floor.But they go further and claim that the originalvariations in isotopic composition withdepth in the ocean floor have survived mantlestirring and can now be seen as geographical-ly distributed trends among the Hawaiianlavas. This is remarkable, because it impliesthat there was little distortion of the slabdespite a billion years of mantle convection.

The magma from melting plumes underplaces such as Hawaii is known as oceanisland basalt (OIB), and is chemically andisotopically distinct from the mid-oceanridge basalt (MORB) of the ocean floorfound at divergent plate boundaries. Their

production requires long-lived and geomet-rically distinct source regions (Fig. 1). On thebasis of studies with lead isotopes (Box 1), itwas proposed3,4 that OIB is produced bymelting slabs of old MORB that have beenrecycled by subduction, the process by whichocean floor material is dragged down intothe mantle at ocean trenches (not to be con-fused with the production of island arc mag-mas above subduction zones at convergentplate boundaries). In this slab-recyclingmodel, the ocean floor material that haspassed down through the subduction zone isstirred through the hot convecting mantleover hundreds of millions of years, and isthen brought back to the surface in a plumeto provide a source for OIB.

One snag in this scheme is that chemicaldata for OIB appeared to give a very differentpicture of how the mantle must work. Forexample, the trace-element ratios cerium/lead and niobium/uranium are greatlymodified by ocean-floor alteration and arcprocesses but not by melting. So, accordingto the slab-recycling model, Ce/Pb and Nb/Uratios should also be relatively heteroge-neous in OIB. In the 1980s, it was discovered5

that, except under unusual circumstances6,they are surprisingly uniform. Paradoxically,most erupted OIB are isotopically more het-erogeneous than MORB6,7, but are chemical-ly less variable (although this could be due todifferences in the way the magma reaches thesurface). Average OIB should also be verydifferent in Nb/U and Ce/Pb from averageMORB, but in fact they are identical5. Howone generates isotopic heterogeneity in OIBwithout affecting these trace element ratiosbecame a dilemma.

For all that Hawaii is Earth’s largest active

intra-plate volcanic centre, and a wonderfulnatural laboratory for Earth scientists, untilrecently Hawaiian magmas did not figureprominently in the slab-recycling models.Their high 3He/4He ratios8 do not readily fitsuch models, although other geochemicalfeatures do so nicely. Oxygen isotope data9,10

provide evidence that the Hawaiian lavasincorporated components that had interact-ed with seawater, and assimilation of theoceanic lithosphere through which they haderupted was considered a likely explana-tion10. Some of the lavas also have unusualchemical compositions consistent with theincorporation of crystal-rich componentsfrom deep oceanic crust11, and this againcould be interpreted to result from assimila-tion of oceanic lithosphere. Both featuresshould vary with depth in the oceanic lithos-phere.

The alternative, as Lassiter and Hauri1

propose, is that these features in fact variedwith depth in the original ocean floor thatwas subducted to produce the mantle plumeitself. They provide Os data for gabbroicxenoliths exhumed from the deep oceaniclithosphere under Hawaii and show that theyare too rich in 187Os to be a suitable sourcecomponent for the lavas. So they argue thatthe Os and O isotope variations come frommelting recycled slabs of ancient ocean floorand associated sediments. At present thegeochemical variations are distributed asparallel geographical trends among theHawaiian lavas with a scale of several kilo-metres. Lassiter and Hauri contend thatthese variations span horizontal dimensionsroughly equivalent to the depth dimensionsin the original ocean floor as a result of theocean floor being brought up to the surface‘end on’ in the plume.

This conclusion raises several questions.How can slabs be so well preserved after a bil-lion years of cycling through the mantle? The3He/4He ratios and scale of the Hawaiianplume imply that it is derived from the lower

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NATURE | VOL 399 | 24 JUNE 1999 | www.nature.com 733

Mantle geochemistry

Unmixing Hawaiian cocktailsAlex N. Halliday

OIB MORB

Subduction,volcanism

Continent

Oce

anic

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sp

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Low187Os/188Os

High Re/Os in the subducting slab produces high 187Os/188Osover ~109 years

Rising plume fromlower mantle

Mantle

Core Long-lived radioactiveisotopes such as 238Uand 187Re producechanges in the isotopiccompositions of theirdaughter elements (Pband Os) in the mantle.The higher the parent-to-daughter ratio (Re/Os, forexample), the greater thechange in the isotopiccomposition of thedaughter element in agiven time. Theseparent/daughter ratiosare altered by processessuch as melting and low-

temperature interactionwith water. The Re/Osratio is also affected byassimilation of sulphidesand earlier crystallizedmaterials. Subductedslabs of altered basaltand surface sedimentstherefore have U/Pb andRe/Os values that differfrom those in the mantlethat produced theoriginal MORB material.So over (say) a billionyears the result is Pband Os isotopiccompositions that differ

from those in modernMORB (Fig. 1). The Pbisotopic compositions ofOIB are highly variableand consistent with time-integrated high U/Pb,confirming the basicmodel that OIB is derivedfrom old recycledslabs3,4. It is only recentlythat the Re/Ostechnique, as used byLassiter and Hauri1, hasbeen refined to theextent that it could beused to test the earlierresults. A. N. H.

Box 1: Isotopic tracing of slab material

Figure 1 Production of mid-ocean ridge basalt(MORB), and of ocean island basalt (OIB) suchas that which forms Hawaii, as seen in the slab-recycling model. Subduction of oceaniclithosphere continuously transports rocks thatare produced near the Earth’s surface, inparticular MORB and associated sediments,back into the deep mantle. These introducedheterogeneities are stirred and distorted byconvection in the hot mantle. Eventually theycan become buoyant and rise as plumes thatmelt to produce OIB. The time taken for thisrecycling process is thought to be typically abouta billion years. By the time the plume melts toproduce OIB it has ‘aged’ isotopically and hashigher 187Os/188Os than the surrounding mantle.

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mantle, so why do these heterogeneities notget smeared out instead of coming back intothe uppermost mantle with roughly thesame dimensions as originally formed onthe ocean floor? Furthermore, why do theincompatible trace-element ratios of Hawai-ian magmas not look more like those ofaltered oceanic basalt? If the Hawaiian mag-mas are re-melting well-preserved ancientocean floor in a plume, why are the traceelements not highly variable as in modernaltered oceanic crust? Some of these moremobile elements may be partially removed inthe subduction zone, but there should besome trace of these processes in the Nb/Uand Ce/Pb ratios for example.

Some of these issues are addressed byPutirka2, who uses trace-element composi-tions of lavas combined with melting modelsto ‘retrieve’ the original composition of themantle beneath Hawaii. He argues that thedegree of variability in mineralogy andchemical composition in the source is in factvery small. Subducted slabs should be highlyheterogeneous in major and trace elements,and in mineralogy, not just because they havebeen altered by seawater but because theupper portion of the slab is metamorphosedbasaltic rocks with high levels of sodium, cal-cium and aluminium (very distinct from themore magnesium-rich and Al-depleted peri-dotite that makes up the bulk of the mantle).The exact mineralogy would depend on thepressure, but rocks called garnet pyroxeniteand eclogite are the most likely types. Themineralogy also affects the partitioning ofcertain trace elements. Furthermore, even ifthe mineralogy remains the same, the degreeto which key trace elements partitionbetween melt and silicate minerals is pres-sure dependent.

Putirka constructs models of partition-

ing between peridotite and melt as a functionof temperature and pressure (depth). Thesodium/titanium ratio is particularly sensi-tive to pressure during melting, and fromsuch data Putirka argues that the Hawaiianmantle does not vary greatly in major-element composition. If recycled ocean floorwas involved, a reasonably efficient homoge-nization of any such chemical heterogeneitymust have occurred. Furthermore, Putirkaclaims that the isotopic heterogeneities, suchas those used by Lassiter and Hauri1, arerelated to the depth of melting of the mantleand are not simply lateral variations in aplume.

Putirka’s arguments run counter to thoseof Hauri12, who demonstrated correlationsbetween the isotopic and major-elementcompositions of Hawaiian lavas that he

interpreted to reflect the incorporation of upto 20% of silica-rich ‘dacitic’ melts fromrecycled oceanic crust (now converted togarnet pyroxenite). Putirka’s arguments arebased more on trace elements and their par-titioning, and he admits that not all of thevariability in Hawaii can be explained by ahomogeneous source. Nonetheless, he dis-putes Hauri’s claim that a garnet pyroxenite(recycled slab) component is so important.

Where does all this leave us? We knowthat subduction occurs and that ultimatelysubducted slabs must contribute to mantleheterogeneity as evident in OIB. But whichisotopic features are the product of re-melt-ing of ancient subducted components? Onemight get cynical about mantle geochem-istry because basic questions such as thesekeep recurring. Particularly with integratedchemical and isotopic studies, however,they look more tractable than they ever havebeen. We must keep going.Alex N. Halliday is in the Institute for IsotopeGeology and Mineral Resources, Department ofEarth Sciences, ETH Zentrum, NO C61,Sonneggstrasse 5, CH-8092 Zürich, Switzerland. e-mail: halliday@erdw.ethz.ch1. Lassiter, J. C. & Hauri, E. H. Earth Planet. Sci. Lett. 164,

483–496 (1998).

2. Putirka, K. J. Geophys. Res. 104, 2817–2829 (1999).

3. Hofmann, A. W. & White, W. M. Earth Planet. Sci. Lett. 57,

421–436 (1982).

4. Chase, C. G. Earth Planet. Sci. Lett. 52, 277–284 (1981).

5. Hofmann, A. W., Jochum, K. P., Seufert, M. & White, W. M.

Earth Planet. Sci. Lett. 79, 33–45 (1986).

6. Halliday, A. N. et al. Earth Planet. Sci. Lett. 133, 379–395

(1995).

7. Allègre, C. J., Schiano, P. & Lewin, P. Earth Planet. Sci. Lett. 129,

1–12 (1995).

8. Kurz, M. D., Jenkins, W. J., Hart, S. R. & Clague, D. Earth

Planet. Sci. Lett. 66, 388–406 (1983).

9. Garcia, M. O., Jorgenson, B. A., Mahoney, J. J., Ito, E. & Irving,

A. J. J. Geophys. Res. 98, 537–550 (1993).

10.Eiler, J. M., Valley, J. W. & Stolper, E. M. J. Geophys. Res. 101,

11807–11813 (1996).

11.Hofmann, A. W. & Jochum, K. P. J. Geophys. Res. 101,

11831–11839 (1996).

12.Hauri, E. H. Nature 382, 415–419 (1996).

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Cancer

DNA damage enables p73Eileen White and Carol Prives

For some weeks London tube-travellerswere treated to posters of this DavidHockney painting, “A Closer GrandCanyon, 1998”, advertising the annualsummer exhibition of the Royal Academyof Arts at Burlington House, Piccadilly, incentral London. The posters, let alone thispicture, scarcely do justice to the scale ofthe work, which is painted in oils on 96canvases, with an overall dimension ofabout 11 by 24 feet. A room at theexhibition is devoted to six such vividdepictions of the canyon.

According to Hockney, it is the spacerather than geology of the canyon he hastried to capture in this and similar studies:“the sense of the eye meandering through avast landscape”. Photography failed to doso. So pastel drawings were used as thebasis for the final works in oils, which have

come to London after being on display atthe Centre Georges Pompidou in Paris.

The summer exhibition includes art ofvarious forms, including architecturalmodels, both by academicians such asHockney and non-academicians, and runsuntil 15 August. Hockney’s Grand Canyonpastels and other drawings can be seen atAnnely Juda Fine Art, 23 Dering Street,London W1, in a separate showing whichstarts on 30 June and ends on 18September. Tim Lincoln

Geomorphology

A bigger Hockney

The p53 gene product is a criticalhuman tumour suppressor. Inresponse to cellular stresses such as

DNA damage and oxygen starvation, p53, asequence-specific transcriptional activator,induces cell-cycle arrest or programmed celldeath (apoptosis)1.

Until 1997, scientists working on p53assumed that this gene was unique. So, thediscovery of two p53-related genes — p73and p63, each of which is comprised of sever-al isoforms — was a big surprise2. As mightbe predicted, both genes encode proteinswith transactivation, DNA-binding andtetramerization domains, and they shareconsiderable homology with p53. Some iso-

forms of p63 and p73 can transactivate manyof p53’s targets (albeit to differing extents),and some forms also induce cell-cycle arrestand apoptosis. Hence the idea that certaincellular responses previously assumed to be‘p53 independent’ might, in fact, be attribut-able to these relatives of p53.

One small comfort for p53 researcherswas the possibility that only p53 can beinduced when cells are exposed to stress sig-nals such as DNA damage. Now, this assump-tion has been challenged too. Papers by Gonget al.3, Agami et al.4 and Yuan et al.5 (pages806, 809 and 814 of this issue) together pro-vide evidence that p73 is a target of the non-receptor tyrosine kinase c-Abl in response to

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