hf-w chronometry and inner solar system accretion rates

HF-W CHRONOMETRYANDINNER SOLAR SYSTEMACCRETION RATES

ALEXN. HALLIDAYInstitute for Isotope Geology and Mineral Resources, Department of Earth Sciences,

ETH Zentrum, NO C61, CH-8049 Zürich, Switzerland

Received: 26 November 1999; Accepted: 9 March 2000

Abstract. Models for the mechanisms of accretion of the terrestrial planets are re-examined usingthe experimental technique of high-precision isotope ratio mass spectrometry of tungsten (W). Thedecay of 182Hf to 182W (via 182Ta) provides a new kind of radiometric chronometer of planetformation processes. Hafnium and W, the parent and daughter trace elements, are highly refractory;however, Hf is lithophile and strongly partitioned into the silicate portion of a planet, whereas Wis moderately siderophile and preferentially partitioned into a coexisting metallic phase. More than90% of terrestrial W has gone into the Earth’s core during its formation. The residual silicate portion,the Earth’s primitive mantle, has a Hf/W ratio in the range 10 ! 40, an order of magnitude higherthan chondritic ("1.3). Tungsten isotopic data for the Earth and the Moon suggest that we can date amajor event of planet formation: The Moon formed about 50 Myrs after the start of the solar system,providing strong support for the Giant Impact Theory of lunar origin. Recent simulations of thisevent imply that the Earth was probably only half formed at the time. From this we can deduce theplanetary accretion rate. Tungsten isotope data for Mars provide evidence of a much shorter accretioninterval, perhaps as little as 10 Myrs, but the rates for the Earth over the same time interval couldhave been comparable. The large W isotopic heterogeneities on Mars could only have been producedwithin the first 30 Myrs of the solar system. Large-scale mixing, e.g. from convective overturn, as isthought to drive the Earth’s plates, must be absent from Mars.

Limitations of the method such as 1) cosmogenic 182Ta effects on lunar samples, 2) incom-plete mixing of debris to cause W isotope heterogeneity on the Moon, and 3) initial 182Hf/180Hfheterogeneities of the early solar system are critically discussed.

1. Introduction

Over the past 50 years a wide variety of models have been proposed for the mecha-nisms of accretion of the terrestrial planets. These differ specifically in terms ofthe rates of accretion and the timing of metal-silicate separation. We can nowre-examine these models and evaluate their validity because of a new isotopictechnique, 182Hf-182W. Although the potential of the Hf-W chronometer has beendiscussed for several years (Harper et al., 1991), it has only been developed reallysuccessfully over the past 5 years, thanks to new techniques in high precisionisotope ratio mass spectrometry (Völkening et al., 1991; Halliday et al., 1995;1998; Lee and Halliday, 1995a).For a comprehensive review of Hf-W the reader is referred to Halliday and Lee

(1999). Some of the early ideas on the modeling and interpretation of Hf-W dataare to be found in Halliday et al. (1996), Harper and Jacobsen (1996) and Jakobsenand Harper (1996). The latest Hf-W acccretionary models are explained in Halliday(2000) and Halliday et al. (2000). The basic principles of Hf-W chronometry areas follows. 182Hf decays via 182Ta to 182W with a half-life of 9 Myrs. Hafnium and

Space Science Reviews 92: 355–370, 2000.© 2000 Kluwer Academic Publishers. Printed in the Netherlands.

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Figure 1. Schematic illustration of the essential features of Hf-W chronometry.

tungsten, the parent and daughter trace elements, should be in chondritic propor-tions ("1 : 1) in most early solar system objects because both are highly refractory.Hafnium is lithophile and is partitioned strongly into the silicate portion of a planet,whereas tungsten is moderately siderophile and should partition preferentially intoa coexisting metallic phase. As a consequence the parent to daughter ratio (Hf/W)is greatly perturbed by core formation. So for example, the chondritic Hf/W ratio("1.3) of the total Earth has been internally fractionated by core formation becausemore than 90% of the Earth’s W has gone into its core. The residual silicate portionhas a Hf/W ratio in the range 10 ! 40 (Newsom et al., 1996), an order of mag-nitude higher than chondritic. This residual silicate portion is represented todayby the Earth’s crust and mantle and is commonly referred to as the bulk silicateEarth (BSE) or primitive mantle. If the fractionation of Hf from W caused by coreformation takes place during the lifetime of 182Hf, excess 182W relative to otherisotopes of W should develop in the silicate portion of a planet as a consequence ofenhanced Hf/W (Fig. 1). Conversely, early solar system metals should be deficientin 182W relative to chondritic atomic abundances if they segregated without furtherequilibration, before 182Hf decayed. Therefore the magnitude of the W isotopiceffects are broadly speaking an indication of the rates of accretion of planetesimalsand planets, together with the timing of core formation.

2. Accretion and Core Formation

This chronometer goes right to the heart of the fundamental question of the physicalmechanisms bywhichmetal and silicate form distinct reservoirs. This is problematicbecause the metallic iron core of the Earth is difficult to reconcile with the oxidizedstate of the BSE. In some early models (Ringwood, 1966) it was considered that theFe metal in the Earth’s core had to have been formed by reduction of Fe in silicatesand oxides. This required a large CO atmosphere. The early condensation of metal

HF-W CHRONOMETRY AND INNER SOLAR SYSTEM ACCRETION RATES 357

from a nebula gas offered a mechanism around this. (Eucken, 1944) proposed sucha heterogeneous accretion model in which early-condensed metal formed a core tothe Earth around which silicate accreted after condensation at lower temperatures.In fact Fe metal condenses at a lower temperature than some refractory silicates(Larimer and Anders, 1970) but nevertheless a series of models involving pro-gressive heterogeneous accretion at successively lower condensation temperatureswere successfully developed for the Earth (e. g. Turekian and Clark, 1969). Thesemodels produced a zoned Earth with an early metallic core surrounded by silicate,without the need for massive reduction of iron on the Earth itself.The time-scales that were proposed for core formation and accretion in all

of the above models were short. All models that assume accretion as a gaseousproto-planet that was subsequently stripped of its volatiles (Cameron, 1978) pro-duced accretionary time-scales of < 106 years. Hanks and Anderson (1969) andTurekian and Clark (1969) also assumed fast accretion. However, Safronov (1954)argued that the formation of the planets would be dominated by collisions betweenplanetesimals and raised the possibility that most accretion was more protracted.Wetherill produced detailed timescales and based on the principle that runawayplanetary embryos would collide and cause further planetary growth (Wetherill,1980). Whereas models that are based on accretion with runaway growth as thefinal step complete the process within 106 years (Lin and Papaloizou, 1985) the lastcollisional stages of accretion extend major growth over periods of up to 108 years.(Wetherill, 1986) predicted that the Earth would accrete to about half its presentmass in roughly 10 Myrs but that growth would continue as a result of majorcollisions until about 100 Myrs after the start of the solar system.The timing of core formation in the history of a planet has also been a matter of

considerable debate. Many early physical models (e. g. Solomon, 1979) consideredcore formation on Earth to have developed after 107 yrs. However, more recentphysical models (Shaw, 1978; Stevenson, 1981; Sasaki and Nakasawa, 1986) haveincorporated core formation as an integral part of the density-driven differentiationof a planet heated rapidly by radioactive decay and accretional energy. Short-livednuclides can have played a negligible role in heating bodies over timescales of107 yrs or more. However, heating from accretion may start to build up very fastand persist such that melting and core formation are likely throughout much ofthe first 108 yrs, particularly as the accreting inner solar system started hotter thanpreviously considered (Boss, 1990).

3. Tungsten Isotope Data

The above issues represent the scientific arena into which Hf-W chronometry canbe brought to bear. Clearly some of the above models are critically dependent on theissue of timing. By independently determining when metal-silicate fractionationtook place in early solar system bodies we can deduce which of the above modelsare correct.

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Figure 2. Tungsten isoto-pic compositions forwholerock carbonaceous chon-drites and early segregatedmetals in ordinary chon-drites and iron meteorites,expressed in ! unitsas deviations fromthe terrestrial value (seetext). The W iso- topemodel age for the timedifferences between sepa-ration of metal from a pri-mitive chondritic reservoiris shown at the top of thefigure. Data from Lee andHalliday (1995b; 1996), Ja-cobsen and Harper (1996)and Horan et al. (1998).

Precise W isotope data are now available for a wide range of early solar systemobjects including carbonaceous (Lee and Halliday, 1995a; 1996; see also Fig. 2),ordinary (Lee and Halliday, 1996; 2000) and enstatite chondrites (Lee and Halliday,1998), iron meteorites (Lee and Halliday, 1995a; 1996; Harper and Jacobsen, 1996;Horan et al., 1998), eucrites (Lee and Halliday, 1997), martian meteorites (Lee andHalliday, 1997) and lunar samples (Lee et al., 1997), together with a few terrestrialsamples (Lee and Halliday, 1995a; 1996; Lee et al., 1997).Lee and Halliday (1995a; 1996; 1997; 2000) showed that the abundance of

182Hf at the start of the solar system was relatively high (182Hf/180Hf > 10!4),which means the W isotopic effects produced are relatively large. The fact that it isreasonably safe to assume we know the Hf/W ratio and W isotopic composition ofbulk planets by using data for chondrites, makes this chronometer particularly use-ful for studying planetary growth. Furthermore, Hf/W fractionation mainly occursin response to partial melting and core formation, processes that should developearly in a rapidly heating body. Finally, the half-life is long compared with 26Al,53Mn and 107Pd, rendering this chronometer close to ideal for studying the longtimescales of planetary accretion proposed by Safronov and Wetherill.


Figure 3. Tungsten isotopic compositions for whole rock eucrites and carbonaceous chondrites, ex-pressed in ! units as deviations from the terrestrial value (see text). Data from Lee and Halliday(1995b; 1996; 1997).

Tungsten isotopic compositions are commonly expressed (Fig. 1) relative to thedata for the terrestrial NIST W standard in parts per 104 deviation as follows:

! 182W = {[(182W/184W)sample/(182W/184W)standard] ! 1} # 104 . (1)

The standardW is representative of the silicate Earth that wewalk on. It has identicalisotopic composition to theW in drill bits and everywhere onEarth. It turns out that itis also identical to the value found in carbonaceous chondrites (or the average solarsystem), a very important discovery discussed below. For the purposes of this paperthen one can think of !182W=0 as being the average for the solar system. A positivevalue would indicate that the body had a higher Hf/W than average solar system(= 1) early in the history of the solar system. A negative value would indicate lowHf/W early in the solar system (Fig. 1). At the time of writing W isotopic data havebeen published for over 80meteorites and lunar samples. W isotopic measurementson early metals have now demonstrated that the predicted 182W deficiency is almostubiquitous (Lee and Halliday, 1995a; 1996; Harper and Jacobsen, 1996; Horan etal., 1998; Fig. 2). Early metals are deficient by about 4 !182W units (Fig. 2).This is entirely consistent with them representing early-formed metallic segre-

gations with low Hf/W such as the cores of asteroids. The W isotopic data can beused to calculate a model time span for the formation of these metals. This turnsout to be within "10 million years (Fig 2).An important test is to demonstrate the converse, that is that early bodies with

high Hf/W have radiogenic W (high !182W), consistent with decay of former 182Hf.The eucrites are a group of early silicate-rich meteorites thought to be derivedfrom the Asteroid 4 Vesta. Highly radiogenic W is found in the eucrites (Fig. 3)confirming the basic Hf-W theory (Lee and Halliday, 1997). The data indicate thatAsteroid 4 Vesta formed and differentiated into a silicate portion and a metallic corewithin 10 million years of the start of the solar system. Iron meteorites generallycome from small (20–500 km) parent bodies Wasson (1985) and Asteroid 4 Vestais 525 km in average diameter. So from these studies of meteorites it is clear thatbodies of at least few hundred km in size had formed within the first 10 millionyears of the start of the Solar System.

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Figure 4. Tungsten isotopic compositionsfor whole rock carbonaceous chondritesand terrestrial samples, expressed in !

units as deviations from the terrestrialvalue (see text). Data from Lee and Hal-liday (1995b; 1996) and Lee et al. (1997).

4. The Chondritic W Isotopic Composition of the Non-Chondritic BulkSilicate Earth

TheHf/W of the BSE is>10. There should therefore be about a percent-level excessin 182W/184W if the Earth’s core formed at the same time as many iron meteorites.One of the most surprising discoveries of Hf-W chronometry is that the W inthe silicate Earth (the same W as we use in everyday life) is identical in isotopiccomposition to that found in carbonaceous chondrites, to within <70 ppm (Fig. 4).This lack of a resolvable difference between the silicate Earth and chondrites,

(if you like, the solar system standard for average Hf-W systematics) is a funda-mental observation that places important constraints on the early history of theEarth, discussed below. Note that the W isotopic composition of the bulk silicateEarth (BSE) is, by definition, zero because this is the composition of laboratorystandards to which all other data are compared. So, somewhat confusingly, thebig issue to explain is why the chondritic W isotopic composition is zero. Put innon-technical terms, how does the BSE have chondritic W if its Hf/W ratio isso strongly non-chondritic (Fig. 5)? The obvious explanation for the differencebetween eucrites and the silicate Earth is that the fractionation of Hf from W tookplace relatively late in the (early) history of the Earth. Such a fractionation wouldoccur as a consequence of core formation. So either the Earth’s core formed lateafter nearly all the 182Hf had decayed ("50Myrs), or the accretion of the Earthwas protracted. As described above there is no good reason to expect that coreformation should develop late on Earth.Moreover there is good evidence that a protracted history of accretion is a better

explanation than delayed core formation. In particular the W isotopic variations asa function of size of the planetary body reinforce this viewpoint. Iron meteoritesgenerally come from small (20–500 km) parent bodies (Wasson, 1985). They havethe least radiogenic W yet measured (Lee and Halliday, 1995a, 1996; Harper andJacobsen, 1996; Horan et al., 1998). A corresponding silicate reservoir of the parentbodywould be expected to have !W " 50 ! 100 if it had a Hf/W like eucrites, theMoon or the silicate Earth. Data for eucrites, thought to be derived from the Asteroid


Figure 5. Schematic showingthe fractionaton of Hf from Win the early Earth but the lackof resolvable W isotopic effect.

4Vesta (525 km diameter) are also highly radiogenic (!W"+30) (Lee and Halliday,1997). Martian meteorites range from chondritic to slightly radiogenic (!W =0 to+ 3) (Lee and Halliday, 1997). The Earth, the largest of these bodies has !W =0, like carbonaceous chondrites (Lee and Halliday, 1995a, 1996). So with these scantdata there is a pattern that the smaller the differentiated body the greater the excessof 182W in the silicate portion. This is not expected if core formation does not occuruntil a critical stage of planetary development. Indeed Pb isotopic data forMartianmeteorites provide confirmation that Mars segregated its core rapidly (Chen andWasserburg, 1986). However, the W isotopic effects are as expected if larger plan-ets simply take longer to accrete than smaller planets, which in turn take longer toaccrete than planetesimals the size of asteroids. So protracted accretion with con-tinuous core formation seems amore likely explanation than delayed core formation.

5. Protracted Accretion with Continuous Core Formation

Wetherill’s model (1986) corresponds roughly to an exponentially decreasing rateof growth with time. So:

ft = 1! e!"·#t , (2)

where ft is the fractional mass of the Earth,#t is the time since the start of the solarsystem and " is the time constant for accretion. In order to explain the chondritic Wisotopic composition of the BSE in terms of protracted accretion with continuouscore formation one has continuously dampen the radiogenic W in the BSE by accre-tion of more material with on average chondritic W (Fig. 6). As the Earth accretes

Figure 6. Schematic showingthe principles of continuouscore formation (after Hallidayet al., 2000).

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Figure 7. (A) The fractionalmass of the Earth as a func-tion of time using an expo-nentially decreasing continu-ous growth and various valuesfor the accretionary mean life(Myrs), as indicated from ratessimilar to those implied bythe model of Wetherill (1986)(shaded region) to values abouteight times slower. (B) TheW isotopic effects in the sil-icate Earth (calculated with aHf/W of 15) predicted withcontinuous core formation atthe accretionary rates shown inA. The stippled bar indicatesthe uncertainty in the level ofagreement between the W iso-topic compositions of the bulksilicate Earth (BSE) and car-bonaceous chondrites (Lee andHalliday, 1996). From Halli-day (2000), with permission.

the newly admixedmaterial with totally equilibrates with, and dilutes the radiogenicW in the BSEbefore re-partitioning between silicate and metal and segregating toform additional core material (Fig. 6). So the average age of the core is the same asthat of the Earth. Although Wetherill’s model was slower than previously consid-ered, it still yields an excess of 182W in the silicate Earth if modelled as a smoothlydecreasing rate (Fig. 7). With slower accretion the BSE would have W that wasradiogenic in its early history but this would be eliminated later. The mean life ($e)is the inverse of the time constant for accretion (") and corresponds to the timetaken to achieve "63% growth. It can be seen that a mean life ($e) of > 20 Myrs isrequired to effectively eliminate the W isotopic excess in the present BSE (Fig. 7)(Halliday et al., 1996). This result is very robust. Delaying the onset of core for-mation during accretion by 20 million years say does not change the effect becauseit is the timing and mass balance of the last stages that are most critical to theW isotopic effect produced (Halliday, 2000). So the W data provide very strongsupport for the models of protracted planetary growth proposed by Safronov andWetherill. Conversely rapid accretion with the early completion of core formation,as in some heterogeneous accretion models, is inconsistent with W isotope data.While the Earth may have started melting and segregating metal at the same timeas represented by asteroidal cores, the history was clearly far more protracted.


Figure 8. Tungsten isotopiccompositions for lunar samplesexpressed in ! units as devia-tions from the terrestrial value(see text). Data from Lee et al.(1997).

6. Giant Impacts and the Origin of the Moon

Tungsten isotopic data for the Moon allow us to test some of these models morefully. Although other theories used to be prevalent (e. g. Urey, 1966), the Moon isnow generally thought to represent an early sample of the silicate Earth and / orthe material that impacted its surface in a major collision with another planet(Benz and Cameron, 1990). Despite geochemical similarities between the Moonand the Earth, most simulations generate the Moon from the silicate portion of theimpacting planets, rather than from the Earth itself. I refer to this extinct impactingMoon-forming parent planet by the name “Theia”, the mother of Selene, the GreekGoddess of the Moon (Halliday, 2000).The Moon has reservoirs with even higher Hf/W than that of the BSE (Palme

and Wänke, 1975) and yields both chondritic and radiogenic W (Lee et al., 1997)(Fig. 8). Although, other explanations are possible, the most likely explanation forthese results is that the Moon was affected to varying degrees by late growth of182W as a result of fractionation of Hf/W to extreme values in the lunar interior.This yields a model age of 4.52 ! 4.50Gyrs for the Moon, roughly 50millionyears after the start of the solar system, lending strong support to late-stage giantimpact models of lunar origin.

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Figure 9. Isotopic effects ofslow accretion followed by theaddition of an impactor with aproto-Earth to impactor massratio of 7 : 3. (A) The meanlife for Earth’s accretion variesto produce a spectrum of so-lutions with a total mass ofEarth after the Giant Impactbetween 65% and 90% of thecurrent Earth mass. (B) TheW isotopic effects in the BSEthat follow from the accre-tion curves in (A) assumingthe BSE Hf/W stays close to15 (present day). The excess182W is eliminated with all ofthese solutions. From Halliday(2000), with permission.

Recent dynamic simulations for the formation of the Moon via a giant impact(Cameron and Canup, 1998) result in a larger relative mass of Theia than previouslyconsidered (Cameron and Benz, 1991). Cameron and Canup (1998) accomplishthis by modeling the Earth with a mass only half that of its present value. If theEarth had only half-formed at the time of the impact ("4.51 Ga) its accretion ratemust indeed have been significantly slower ($ "50Myrs) than previously recog-nized, at least up until the time of formation of the Moon. The critical parameter inthe Cameron and Canup simulation is the mass of the proto-Earth relative to thatof Theia, which needs to be in the proportion 7 : 3. This is very different fromthe “traditional” ratio of about 7 : 1, used in previous simulations. Furthermore, asignificant amount of material may be added subsequently. Figure 9 shows a rangeof possibilities for keeping the mass ratio at 7 : 3 and having further accretion afterthe impact. Six such mass accumulation curves with a total mass of proto-Earth andTheia of 0.65, 0.70, 0.75, 0.80, 0.85 and 0.90 Earth masses are shown in Fig. 9.With this smaller proto-Earth it can be seen that a wide range of scenarios yieldchondritic W at the present if the Giant Impact takes place at 50Myrs (Fig. 9b). Sothe W isotope data for the Earth make sense if the Moon formed in a Giant Impactabout 50Myrs after the start of the solar system and the Earth was only about half


Figure 10. The calculated Wisotopic effects in the bulk sili-cate impactor showing the pre-dicted magnitude of the initialand present day average W iso-topic anomaly on the Moonthat follows with a 30 and50Myr age. The accretionarymean lives are calculated to bevery slow in order to gener-ate a chondritic W isotopic ini-tial ratio. The W isotopic com-position of the lunar mantle(Moon minus crust and core)is calculated assuming a Hf/W"30, a little higher than thebulk silicate Moon. The cal-culated W isotopic composi-tion of the average lunar man-tle provides an excellent matchof the observed range of datafor lunar rocks (Fig. 8) if theMoon formed at "50Myrs. Incontrast if the Moon formedat "30Myrs it would be ex-tremely radiogenic and notmatch lunar compositions.

formed at the time. Other scenarios are also possible, as explained inHalliday (2000)and Halliday et al. (2000). Indeed whenever the Earth is affected by a giant impactstyle collision the radiogenic W in the silicate portion is eliminated. So for examplethe addition of a body with a mass of 2% of the Earth 100Myrs after the start ofthe solar system will eliminate small W isotopic effects in the BSE (Halliday etal., 2000). Conversely if the last major impact was earlier than 50Myrs after thestart of the solar system one has to invoke considerable post-impact accretion toeliminate the excess. However, an earlier age for the Moon is difficult to reconcilewith the magnitude of the W isotopic effect in the Moon (Fig. 10).

7. Differences Between the Accretion for Earth and Mars

Martianmeteorites provide uswith samples of igneous rocks formed bymelting theinterior ofMars over the past billion years or so. In this case the rocks all formed longafter 182Hf became extinct, so that the W isotopic variations have nothing to dowiththemeasured Hf/W.TheHf/W ratio of the martianmantle is about 3, i. e. a factor of 5smaller than found for the Earth (Lee and Halliday, 1997). Yet martian samplesinclude some samples with radiogenic W, quite unlike the silicate Earth (Fig. 11).Given the low Hf/W ratio of the martian mantle this implicates the effects of very

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Figure 11. Tungsten isotopiccompositions for martianmeteorites and carbonaceouschondrites, expressed in !

units as deviations from theterrestrial value (see text). Datafrom Lee and Halliday (1995b;1996; 1997).

early core formation on Mars (Fig. 12). Evidently the accretion and differentiationof Mars was probably complete within about 15Myrs (Lee and Halliday, 1997).The accretion rates deduced for the Earth would build a body the size of Mars

(1/8th the mass of the Earth) in about 10 million years. This is in good agreementwith theWdata for martianmeteorites. The main difference between the Earth andMars is not the rate of accretion but the fact that Earth carried on accreting for longer.Unlike the Earth, Mars appears to have suffered no major giant impacts about20 million years after the start of the solar system of the kind that formed ourmoon.There are other striking differences between Earth and Mars highlighted by

the W isotope data. Mars clearly differentiated very early and this explains the Wisotopic anomalies. However, it is also striking that such an anomaly has survivedat all as a feature of the martian mantle. In the Earth 4.5 billion years of convection

Figure 12. Schematic showingthe small fractionaton of Hffrom W in the early mars butthe discovery of a resolvable Wisotopic effect.

has stirred the mantle sufficiently vigorously that all trace of such early hetero-


geneity has been eliminated. The earliest isotopic heterogeneities sourced in theconvecting mantle of the Earth are less than 2 billion years in age. In contrast themartian mantle has heterogeneities that can only have been produced in the first30 million years of the solar system. So mantle mixing must be far less effective onMars. Large-scale convective overturn such as drives the Earth’s plates and makescontinents move must be absent from Mars. Indeed the W data are difficult toreconcile with any kind of long-term, plate tectonic activity on Mars.

8. Problems to Sort out and Future Work

It is already clear that Hf-W chronometry offers major advantages over other iso-topic methods when it comes to determining accretion rates in the inner solarsystem. However, it is worth considering the future of this technique carefullyand addressing some of the uncertainties and ideas that will guide further research.First, we now have evidence that the production of 182W via cosmic ray interactionscan be a problem. Masarik (1997) showed that the production of cosmogenic Wisotopes from other isotopes of W was not a significant problem in iron meteorites.Recent calculations have revealed that this is also true for other kinds of meteoritesand lunar samples. However, we have now recognized a cosmogenic effect that islikely to be significant in some lunar samples as a consequence of neutron irra-diation of 181Ta to produce 182Ta, which decays to 182W. Initially we consideredthis to be a negligible concern but more detailed calculations have revealed that amajor 182W cosmogenic effect should be present in high Ta/W lunar samples withlong exposure ages (Leya et al., 2000). Meteorites, including martian ones, arenot a problem in this respect. From our preliminary work we now know that forcertain samples, a large proportion of the 182W anomaly could be cosmogenic. Formany samples with high 182W/184W (e. g. 15555) the non-cosmogenic componentis still likely to be dominant. Furthermore, we have new data for high Ti-marebasalts that extend the range of compositions to !W > +11 (D.-C. Lee, personalcommunication). These have similar Hf-W model ages. However, detailed studiesare clearly needed to demonstrate this conclusively and resolve the cosmogenicand non-cosmogenic component. In order to do this convincingly it will, in thefuture, be necessary to separate minerals that have differing Ta/W from lunar sam-ples. Since 182Hf had become effectively extinct by the time of mare volcanismthere should be no variations in 182W/184W within mare basalts as a consequenceof 182Hf decay. However, cosmogenic 182W will change the 182W/184W in directproportion to the Ta/W ratio. We should therefore find that minerals with highTa/W ratio have higher 182W/184W if the effect is cosmogenic. A plot of 182W/184Wagainst 181Ta/184W for separated minerals should define an intercept equal to the182W/184W of the rock when it formed. Ilmenite should prove particularly useful indefining a high Ta/W endmember.Second, it is conceivable that the W isotope heterogeneity on the Moon could

be caused by incomplete mixing of debris that accreted to form the moon or in-

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complete homogenization of material that was admixed afterwards. This shouldbe tested by searching for small differences in oxygen isotopic composition. Theprecision of #17O measurements achievable on small samples is now excellentwith laser fluorination techniques. If the variable W isotopic effects (corrected forcosmogenic components) are correlated with oxygen isotopic composition thenincomplete mixing is the explanation and the age of the Moon is more poorlydefined than we have so far thought to be the case.Third, we now have sufficient sensitivity with Hf-W to make measurements

on individual mineral phases. We have begun work on internal isochron studies ofmetal-silicate mixtures such as ordinary and enstatite chondrites (Lee and Halliday,1998; 2000). With this work one can address the issues surrounding how metal andsilicate mixed in the early solar system. There is a positive correlation betweenHf/W and W isotopic composition in the three ordinary chondrites studied sofar (Lee and Halliday , 2000), intercepting the data for the carbonaceous chon-drites Murchison and Allende (Lee and Halliday, 1996). These first Hf-W internalisochrons, allow us to map precisely determined time differences onto an absolutetime scale. Furthermore the carbonaceous and ordinary chondrites appear to haveoriginated from a common chondritic reservoir in terms of Hf-W. In contrast,all silicates and whole rocks from enstatite chondrites show a clear offset to theright of the ordinary chondrite isochron (Lee and Halliday, 1998). This remarkableand unexpected discovery cannot be explained by re-equilibration on the scalerepresented by the meteorites. All whole rock enstatite chondrite measurementsdisplay clear and significant 182W deficits relative to carbonaceous chondrites andthe silicate Earth. The data are most simply explained if the enstatite chondritesrepresent mixtures of metal and silicate materials which differentiated separatelyfrom a chondritic (with respect to Hf/W) parent body at times separated by roughly10–20 Myrs. This opens up the whole issue of how metal and silicate mix in toproduce chondritic proportions of siderophile and lithophile refractory elements, aproblem that must be a priority for further study.Lastly, there is huge amount of work to be done in terms of determining the

basic chronometry of accretion, differentiation and re-mixing in the inner solar sys-tem by studying mesosiderites, pallasites, IIE irons, acapulcoites, enstatite achon-drites, aubrites, eucrites, howardites, diogenites and ureilites. We need further in-ternal isochrons for independently dated materials to tighten the absolute timecalibration of Hf-W and search for heterogeneity in the initial 182Hf/180Hf of theearly solar system. This will facilitate direct comparison with Mn-Cr (Lugmair andShukolyukov, 1998) and broaden the applicability of Hf-W.

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Address for Offprints: Institute for Isotope Geology and Mineral Resources, Dept. of Earth Sciences,ETH Zentrum, NO C61, Sonneggstrasse 5, CH-8049, Zürich, Switzerland;[email protected]

hf-w chronometry and inner solar system accretion rates

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