differentiation thermal evolution - deeps · 2013-10-06 · meteorites, which are fragments ......

Differentiation&

Thermal Evolution

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Meteorite Classification: Iron Meteorites

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Meteorite ClassificationBasic Types of Meteorites:

- Stony (93% of falls)- Irons (6% of falls)- Stony-Irons (1% of falls)

Stony Iron Stony-Iron

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Classification

- Undifferentiated (chondrites) (stony meteorites; ~85% of falls; chemically similar to Sun)

- Differentiated (achondrites) (stony, iron, or stony-iron meteorites; ~15% of falls; have experienced igneous “processing”)

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‘Primitive’ meteorites are stony meteorites and can be chondrites or special types of achondrites; the primitive meteorites are chemically similar to the Sun.

Classification

- Undifferentiated (chondrites) (stony meteorites; ~85% of falls; chemically similar to Sun)

- Differentiated (achondrites) (stony, iron, or stony-iron meteorites; ~15% of falls; have experienced igneous “processing”)

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Iron MeteoritesIron meteorites are largely composed of iron and nickel metal, thus they are believed to represent the cores of differentiated asteroids.

Though rare compared to stony meteorites, irons were fairly common in meteorite collections:

- they are resistant to weathering- they are more resistant to ablation during passage through the atmosphere- they were often easier to find because they look very distinct (stony meteorites, on the other hand, can often look like typical terrestrial rocks)

The Willamette meteorite, housed at the American Museum of Natural History, is the largest one to ever be found in North America. Transported to Oregon by glacial floods?

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Stony-IronsStony-Iron meteorites are subdivided into 2 major types:

- Pallasites: composed of Fe-Ni metal and silicate (mostly olivine)- Mesosiderites: composed of Fe-Ni metal and silicate (‘basaltic’;olivine, pyroxene)

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Stony-IronsStony-Iron meteorites are subdivided into 2 major types:

- Pallasites: composed of Fe-Ni metal and silicate (mostly olivine)- Mesosiderites: composed of Fe-Ni metal and silicate (‘basaltic’;olivine, pyroxene)

Because pallasites consist of metal and olivine, they are believed to represent core-mantle components in asteroids/planetesimals.

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Pallasites

The traditional view of pallasite formation is that represent differentiated bodies (olivine mantle and metallic core).

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Pallasites


Impacts then broke up the parent body to create iron meteorites (cores) and stony-iron meteorites like the pallasites (core-mantle mixtures).

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Pallasites


But measurements suggest iron meteorites have very different cooling rates than the pallasites, so maybe they are NOT from the same parent body.


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Pallasites


But measurements suggest iron meteorites have very different cooling rates than the pallasites, so maybe they are NOT from the same parent body.


Alternative hypothesis:

The solid cores were separated from the remaining (metal) melt and mantle by large impacts. The pallasites could come from many different bodies with different cooling histories.

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Mesosiderites

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Mesosiderites are composed of roughly equal portions of metal and silicate.

But the silicates are ‘basaltic’, suggesting mixing of deep metals with crustal silicates.

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Differentiation: Starting Material

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PERSPECTIVEdoi:10.1038/nature10901

Evidence against a chondritic EarthIan H. Campbell1 & Hugh St C. O’Neill1

The 142Nd/144Nd ratio of the Earth is greater than the solar ratio as inferred from chondritic meteorites, which challengesa fundamental assumption of modern geochemistry—that the composition of the silicate Earth is ‘chondritic’, meaningthat it has refractory element ratios identical to those found in chondrites. The popular explanation for this and otherparadoxes of mantle geochemistry, a hidden layer deep in the mantle enriched in incompatible elements, is inconsistentwith the heat flux carried by mantle plumes. Either the matter from which the Earth formed was not chondritic, or theEarth has lost matter by collisional erosion in the later stages of planet formation.

T he paradigm that underpins much of modern geochemistry isthat the integrated chemical composition of the whole Earthshould be that of the Sun, except for depletion in volatile ele-

ments, according to their volatility under the conditions of the solarnebula. Similar solar-related compositions are found in ‘chondritic’meteorites, which are fragments of small rocky bodies that escapedthe usual course of planetary differentiation into a metallic core, silicatemantle and crust. The composition of a chondritic meteorite is thereforepresumed to reflect its entire parent body. Although the solar composi-tion can be determined from spectroscopic measurements of the solarphotosphere, measurement is not possible or imprecise for many ele-ments, is model-dependent and does not give information on the iso-topic make-up of the elements1. Instead, a more complete picture of thesolar composition is inferred from chemical analyses of chondrites. Thecompositions of the chondrites vary, with at least 27 parent bodiessampled2, reflecting local differences in the solar-nebula-like densityor proportions of gas to solids, or different accretion processes. Thevarious chondrite compositions are distinguished by enrichment ordepletion of refractory elements, ratio of lithophile to siderophile ele-ments (for example, Mg/Fe), oxidation state, oxygen isotopic composi-tions, and their patterns of depletion of the volatile elements. Noexamples with volatile-element enrichment are known, except for aslight enrichment in a few of the least volatile of these elements in thehighly reduced enstatite chondrites. Yet all chondrites share one dis-tinctive compositional feature: their refractory lithophile elements(RLEs) are present in the same ratio relative to each other and to thesolar composition. The RLEs are defined by two properties: they arerefractory, because they condense from a gas of solar composition attemperatures higher than the main constituents of rocky planets, themagnesium silicates and iron metal; and they are lithophile, becausethey do not enter metal or sulphide phases, either in chondrites or intothe metallic cores formed during planetary differentiation. There are 28RLEs that are stable or have long half-lives; they include Ca and Alamong the major elements, the entire suite of rare earth elements(REEs), and the radiogenic heat-producing elements U and Th.

The constant RLE ratio rule is ever challenged on several fronts: byexceptions due to terrestrial weathering or ad hoc effects on parentbodies such as impact brecciation, incipient melting or aqueous altera-tion; by the scale of heterogeneity in chondrites relative to the smallsample sizes available for analysis; by improvements in the precision ofchemical analysis; and by the increasing numbers of chondritic meteoritesavailable for analysis. It is therefore difficult to quantify the precision towhich the rule holds, but variations from the solar ratios reflecting whole-body chemistry that are larger than a few per cent are exceptional. The

REE pattern in the RLE-rich CV chondrite Allende is perhaps the largestwell-attested deviation3. New techniques of isotopic analysis are revealingsmall anomalies in the isotopic make-up of heavy elements in bulksamples of chondrites, ascribed to less than perfect homogenization ofdifferent nucleosynthetic components in the solar nebula, such as Ti(ref. 4), Ni (ref. 5), Ba (refs 6 and 7) and Mo (ref. 8). This evidencechallenges the conceptual basis behind the constant RLE ratio rule, butas yet at no more than the few-per-cent level already accepted. Forexample, although the range in Lu/Hf and Sm/Nd in unequilibratedcarbonaceous, ordinary, and enstatite chondrites is as much as 7.9%and 3.5% respectively, the average Lu/Hf and Sm/Nd values for these threemain classes of chondrites agree within 1% and 0.3% respectively9.

Although most geochemists long ago abandoned the notion that theEarth’s composition mimics any particular type of chondrite10, the ideathat the Earth has solar ratios of RLEs has persisted, providing the fun-damental reference frame against which trace element and radiogenic iso-topic ratios are compared. This reference frame has usually, if confusingly,been termed ‘chondritic’ rather than ‘solar’ in the literature, because of thehistory of fine-tuning the presumed solar composition to the compositionsof chondrites. Emphasis has been placed on the CI chondrites, which matchthe solar composition within uncertainty for many elements irrespective oftheir chemical properties, except for the most volatile elements; but CIs arerare, and useful analyses come from just three falls, Orgueil, Ivuna andAlais11, and mostly from Orgueil1. The implicit assumption is that the bulkchemical composition of rocky bodies is established at the earliest stages inthe planet-building process, of which the chondrite parent bodies arerelicts. In this view, the subsequent stages by which these small chondriticbodies collide and merge to form planetary embryos and ultimately Earth-sized planets, while resulting in extensive differentiation associated withmelting, does not affect the integrated whole-planet compositions: these, itis assumed, remain ‘chondritic’.

Planetary accretionThe ‘chondritic’ hypothesis for the Earth’s composition is a survivorfrom times when the understanding of how terrestrial planets formwas quite different. Current models can be traced back to Safronov,whose monograph on the subject was only translated into English in1974 (ref. 12). Our present understanding is that planetary accretionproceeds through several stages of ever increasing average size. Initially,runaway growth is from kilometre-sized bodies (approximately the sizeof parent bodies of chondrite meteorites) to form planetesimal oligarchsabout a thousand kilometres in diameter. This is the approximate size ofthe differentiated bodies that are thought to be parental to the achondritemeteorites, like Vesta (diameter 500 km). The achondrite parent bodies

1Research School of Earth Science, Australian National University, Canberra, Australian Capital Territory 0200, Australia.

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Macmillan Publishers Limited. All rights reserved©2012

[Nature, 2012]

Moderately volatile, lithophile elements (alkalis, halogens, boron) are depleted in bulk silicate Earth (BSE).

of the planet-building process, the significance of this age informationremains ambiguous.

Likewise, the interpretation of other early-Earth geochronometers is alsoaffected if the Earth has a non-chondritic composition. For example, short-lived 182Hf (half-life 9 Myr) decays to 182W, providing a chronometer withwhich to constrain the duration of core formation, because W is amoderately siderophile element that partitioned incompletely into thecore. Evaluating the time significance of this chronometer for the Earthdepends on knowing the W/Hf ratio in the BSE56. The W content of theBSE has been estimated by noting that W/Th (or W/Ba) ratios remainconstant in igneous processes, hence W/Hf 5 (W/Th) 3 (Th/Hf).Because Hf and Th (or Ba) are both RLEs, it is then assumed that theirratio is the same as in chondrites. But the non-chondritic Earth model ofref. 13 predicts that Th/Hf would be only around 70% of the chondriticratio; for a simple two-stage model of core formation, this would increasethe calculated time from about 30 to about 35 Myr.

Alternative hypothesesThere are two classes of explanation for the Earth not being chondritic.Most simply, the compositions of chondrites may not reflect that of the

Solar System precisely enough to deduce detailed element ratios for RLE.It needs to be remembered, however, that the average Sm/Nd ratio of thethree main classes of chondrites agrees to within 0.3%.

Alternatively, the Earth could have been assembled from initiallychondritic material that was then modified during the subsequent stagesof the planet-building process by collisional erosion13,57,58. Current esti-mates of the Earth’s Fe/Mg ratio are consistent with about 10% of itssilicate part having been lost by this mechanism relative to its Fe-richmetallic core. The meteorite record attests to the differentiation of smallrocky bodies into metal and silicate being inevitably associated withpartial melting and hence also the formation of an incompatible-element-enriched crust. If material from these crusts were preferentiallylost during the collisions, it would deplete the Earth systematically inincompatible elements according to their incompatibility (Fig. 3). Oneweakness of the hypothesis is that it implies the loss of incompatible-element-enriched material to space that no class of meteorite hassampled. However, no meteorites sample the moderately volatile ele-ments missing from all chondrites (apart from the CIs) and from theachondrites and terrestrial planets. Did the gravitation field of the Sun orJupiter capture this missing material?

The pattern of depletion caused by preferential collisional erosion isgeochemical rather than cosmochemical, and its effect on the composi-tion of the Earth’s mantle is essentially the same as that which subse-quently occurred throughout the Earth’s history by crust formation.Detecting the effects of collisional erosion therefore depends on obser-vations that can sum all the reservoirs in the BSE to see whether they addup to chondritic ratios of RLEs. The Sm/Nd ratio provides the mostcompelling evidence, but once the idea of a non-chondritic Earth isallowed, the resolution of other so-called geochemical paradoxesbecomes achievable.

Geochemists are fascinated by the many paradoxes of the Earth’s man-tle, which are summarized in Table 1. All of these paradoxes are predicatedon geochemistry’s most fundamental paradigm; that the Earth was pro-duced by the accretion of meteorites with the same ratios of RLEs as inchondrites. Most of these paradoxes disappear if this assumption isrelaxed, but one existing paradox becomes worse: the low value of theEarth’s Urey ratio. The Urey ratio is the Earth’s radiogenic heat produc-tion divided by its surface heat flux, which—under the assumption of BSEU, Th and K concentrations given by the chondritic hypothesis—is about0.5. The difference must be accounted for by secular cooling. Collisionalerosion lowers the heat-producing element content of the BSE by up to

Mantle/crust partition coef!cient

U

La

SmHf

Nd

Th

Lu

Ca Al

10–4 110–3 10–2 10–10.4

0.5

0.6

0.7

0.8

0.9

1.0

Ba

Nor

mal

ized

dep

letio

n

Figure 3 | Depletion of some RLEs in the BSE by preferential collisionalerosion of early-formed basaltic crust during accretion of planetesimals andplanetary embryos. The figure is based on the model of ref. 13, assuming threeconstraints: (1) loss of silicate (crusts plus mantles) relative to metallic cores is10%; (2) the most incompatible RLEs (here represented by Ba) are depleted to50% of their chondritic abundance; and (3) Sm/Nd is 6% above the chondriticratio. The partition coefficients during crust formation are from ref. 67 for theproduction of oceanic crust.

Pb

0.001

0.01

0.1

1

400 600 800 1,000 1,200

CVEH

50% condensation temperature (K)

10

100

1,000

0.5 1 1.5 2 2.5 3

Zn/Mg

Cd

Cl

InRb

B

Li

MnNa K

I

ZnCs

Br

F

?

Chondrites

BSE

Zn/B

rB

SE/

Mg

Zn/Mg

Figure 2 | The pattern of volatile element depletion in the BSE for lithophileelements compared to CV carbonaceous chondrites and EH enstatitechondrites. CV carbonaceous chondrites are the most volatile-depleted of thechondrites and EH enstatite chondrites are a class of chondrites sometimesconsidered to have affinities to the Earth because of their stable isotope ratios.Elements are normalized to CI abundances and Mg. The calculated 50%condensation temperatures of elements for the solar nebula are from ref. 10.Some elements in the BSE may have been additionally depleted by core formation(for example, Zn, In and Pb), in which case their depletions due to volatility alonewill be overestimated. The chondrites form smooth depletion trends withcalculated condensation temperatures, but the BSE is not only more depleted inmoderately volatile elements than any known chondrite, but the pattern ofdepletion is qualitatively different, probably owing to post-nebula volatile lossunder more oxidizing conditions13. This is shown, for example, by the BSE Zn/Brratios (inset), which are about an order of magnitude greater than that found inany class of unmetamorphosed chondrites (black circles). The lack of any clearvolatility trend in the BSE means that it is not at present possible to constrain howmuch Pb, if any, was partitioned into the Earth’s core, making the interpretationof Pb isotopic systematics in terms of Earth accretion and core formationuncertain. Meteorite abundances are from ref. 66, and BSE abundances from ref.13, which was derived under the ‘chondritic assumption’. A non-chondritic BSEwould have lower abundances of B, K, Rb, Cs, Cl, Br and I but not Mn, Na, F, Znor In (ref. 12), increasing the discrepancy with the chondritic trends.

RESEARCH PERSPECTIVE

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142Nd/144Nd ratio of terrestrial mantle is higher than chondritic values.

- Earth is non-chondritic

or

- Earth experienced early fractionation event w.r.t. Nd

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Differentiation: Starting Material

9half13, which halves the already low Urey ratio, implying unlikely coolingrates extrapolated over geological time. Perhaps the Earth is currently in aphase of abnormally fast ocean crust formation and subduction59.

It is apparent that the only reliable way of determining the composi-tion of the Earth is by sampling the Earth itself. As argued in this study,the heads of mantle plumes entrain primitive lower mantle. By studyingbasalts produced by melting this material, especially Archaean basaltsassociated with komatiites, provided they are not affected by crustalcontamination, we are sampling basalts derived from the Earth’s earliestand most primitive mantle. It may also be possible to obtain the inte-grated BSE composition of two of the RLEs most susceptible to col-lisional erosion—Th and U—from the geoneutrino flux60. It wouldthen be possible to see whether the ratios of these elements with otherRLEs, such as Ca and Al, were indeed within the range found in chon-dritic meteorites or that predicted by collisional erosion.

1. Lodders, K., Palme, H. & Gail, H.-P. in Landolt-Bornstein New Series (ed. Trumper, J.E.) Vol. VI/4B, Ch. 4.4 560–630 (Springer, 2009).

2. Meibom, A. & Clark, B. E. Evidence for the insignificance of ordinary chondriticmaterial in the asteroid belt. Meteorit. Planet. Sci. 34, 7–24 (1999).

3. Pourmand, A., Dauphas, N. & Ireland, T. J. A novel extraction chromatography andMC-ICP-MS technique for rapid analysis of REE, Sc and Y: revising CI-chondriteand Post-Archean Australian Shale (PAAS) abundances. Chem. Geol. http://dx.doi.org/10.1016/j.chemgeo.2011.08.011(in the press).

4. Trinquier, A. et al. Origin of nucleosynthetic isotope heterogeneity in the solarprotoplanetary disk. Science 324, 374–376 (2009).

5. Regelous, M., Elliott, T. & Coath, C. D. Nickel isotope heterogeneity in the early SolarSystem. Earth Planet. Sci. Lett. 272, 330–338 (2008).

6. Andreasen, R. & Sharma, M. Mixing and homogenization in the early solar system:clues for Sr, Ba, Nd, and Sr isotopes in meteorites. Astrophys. J. 665, 874–883(2007).

7. Carlson, R. W., Boyet, M. & Horan, M. Chondritic barium, neodymium, andsamarium isotopic hetrogeneity and early Earth differentiation. Science 316,1175–1178 (2007).

8. Burkhardt, C. et al. Molybdenum isotope anomalies in meteorites: constraints onsolar nebula and origin of the Earth. Earth Planet. Sci. Lett. 312, 390–400 (2011).

9. Bouvier, A., Vervoort, J. D. & Patchett, P. J. The Lu–Hf and Sm–Nd isotopiccomposition of CHUR: constraints from unequilibrated chondrites andimplications for the bulk composition of terrestrial planets. Earth Planet. Sci. Lett.273, 48–57 (2008).

10. Drake, M. J. & Righter, K. Determining the composition of the Earth. Nature 416,39–44 (2002).

11. Lodders, K. Solar system abundances and condensation temperatures of theelements. Astrophys. J. 591, 1220–1247 (2003).

12. Safronov, V. S. Evolution of the Protoplanetary Cloud and Formation of the Earth andthe Planets (Israel Program for Scientific Translations, 1972).

13. O’Neill, H., St C.., &. Palme, H. Collisional erosion and the non-chondriticcomposition of the terrestrial planets. Phil. Trans. R. Soc. A 366, 4205–4238(2008).This paper discusses how the compositions of terrestrial planets may changeduring the later stages of their accretion.

14. Halliday, A. N. & Porcelli, D. In search of lost planets—the paleocosmochemistry ofthe inner solar system. Earth Planet. Sci. Lett. 192, 545–559 (2001).

15. Boyet, M. & Carlson, R. W. 142Nd evidence for early (.4.53 Ga) globaldifferentiation of the silicate Earth. Science 309, 576–581 (2005).

Here the 142Nd/144Nd for chondritic meteorites is shown to be 20 parts permillion less than that of most terrestrial rocks.

16. Boyet,M.& Carlson, R.W.Anewgeochemicalmodel for the Earth’s mantle inferredfrom 146Sm–142Nd systematics. Earth Planet. Sci. Lett. 250, 254–268 (2006).This paper discusses the implications of excess 142Nd/144Nd in terrestrialsamples in terms of a non-chondritic mantle and a hidden reservoir.

17. Kinoshita, N. et al. Geocosmochronometer 146Sm: a revised half-life value. Mineral.Mag. (Goldschmidt Conf. Abstr.) 75, 1191 (2011).

18. Qin, L.-P., Carlson, R. W., Alexander, C. M. & O’D.. Correlated nucleosyntheticisotopic variability in Cr, Sr, Ba, Sm, Nd and Hf in Murchison and QUE 97008.Geochim. Cosmochim. Acta 75, 7806–7828 (2011).

19. Patchett, P. J., Vervoort, J. D., Soderlund, U. & Salters, V. J. M. Lu–Hf and Sm–Ndisotopic systematics in chondrites and their constraints on the Lu–Hf properties ofthe Earth. Earth Planet. Sci. Lett. 222, 29–41 (2004).

20. Andreasen, R. Sharma, M. Subbarao, K. V. & Viladkar, S. G. Where on Earth is theenriched Hadean reservoir? Earth Planet. Sci. Lett. 266, 14–28 10.1016/j.epsl.2007.10.009 (2008).

21. Caro, G., Bourdon, B., Birck, J.-L. & Moorbath, S. High-precision 142Nd/144Ndmeasurements in terrestrial rocks: constraints on the early differentiation of theEarth’s mantle. Geochim. Cosmochim. Acta 70, 164–191 (2006).

22. Labrosse, S., Herlund, J. W. & Coltice, N. A crystallizing dense magma ocean at thebase of the Earth’s mantle. Nature 450, 866–869 (2007).

23. Davies, G. F. Mantle plume, mantle convection andhotspot chemistry. Earth Planet.Sci. Lett. 99, 94–109 (1990).

24. Loper, D. & Lay, T. The core-mantle boundary region. J. Geophys. Res. 100,6397–6420 (1995).

25. Korenaga, J. A method to estimate the composition of the bulk silicate Earth in thepresence of a hidden geochemical reservoir. Geochim. Cosmochim. Acta 73,6952–6964 (2009).

26. Bourdon, B., Touboul, M., Caro, G. & Kleine, T. Early differentiation of the Earth andthe Moon. Phil. Trans. R. Soc. A 366, 4105–4128 (2008).

27. Nyquist, L. E. et al. 146Sm–142Nd formation interval for the lunar mantle. Geochim.Cosmochim. Acta 59 (13), 2817–2837 (1995).

28. Boyet, M. & Carlson, R. W. A highly depletedmoon or a nonmagma ocean origin forthe lunar crust? Earth Planet. Sci. Lett. 262, 505–516 (2007).

29. Brandon, A. D. et al. Re-evaluating 142Nd/144Nd in lunar mare basalts withimplications for the early evolution and bulk Sm/Nd of the Moon. Geochim.Cosmochim. Acta 73, 6421–6445 (2009).

30. Caro, G. & Bourdon, B. Non-chondritic Sm/Nd ratio in the terrestrial planets:consequences for thegeochemical evolution of themantle-crust system.Geochim.Cosmochim. Acta 74, 3333–3349 (2010).

31. Tonks, W. B. & Melosh, H. J. Magma ocean formation due to giant impacts.J. Geophys. Res. 98, 5319–5333 (1993).

32. Campbell, I. H. The identification of old mantle plumes. Geol. Soc. Am. Spec. Publ.352, 5–21 (2001).

33. Tolstikhin, I. N., Kramers, J.D. & Hofmann, A. W. A chemical Earthmodelwith wholemantle convection: the importance of a core-mantle boundary layer (D99) and itsearly formation. Chem. Geol. 226, 79–99 (2006).

34. Labrosse, S., Hernlund, J. W.& Coltice,N. A crystallizing densemagma ocean at thebase of the Earth’s mantle. Nature 450, 866–869 (2007).

35. Campbell, I. H. & Griffiths, R. W. The changing nature of mantle hotspots throughtime: implications for the geochemical evolution of the mantle. J. Geol. 100,497–523 (1992).This paper uses the melting products of mantle plumes in conjunction withknowledge of their dynamics to determine the chemical structure of the mantleand shows how it has evolved with time.

36. Davies, G. F. Ocean bathymetry and mantle convection. 1. Large-scale flow andhotspots. J. Geophys. Res. 93, 10467–10480 (1988).

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Table 1 | The geochemical paradoxes of the mantleParadox Chondritic solution Non-chondritic solution

The 142Nd/144Nd ratio of chondritic meteorites is206 5 parts per million less than that of rocks ofterrestrial mantle origin.

A low-Sm/Nd hidden reservoir became isolated from theconvecting mantle within 10Myr of the Earth’s formation15,61.

The Sm/Nd ratio of the primitive Earth was about6% above the chondritic value13.

Earth’s oldest rocks show evidence of being derivedfrom a mantle with positive eNd and eHf before theformation of the first preserved continental crust.

Extensive continental crust formed before the first preservedcontinental crust and was recycled through the mantle62 orthere is a hidden basaltic low-Sm/Nd reservoir15.

The Sm/Nd ratio of the primitive Earth was about6% above the chondritic value13.

The Ar concentration in the mantle is about halfthe value predicted from the chondritic model63.

Only half of the mantle is degassed63. The collisional erosion hypothesis13 predicts a Kcontent of the mantle appreciably below thatexpected from the chondritic model.Alternatively, the Earth is not chondritic16.

Nb/Ta and Nb/La values of both continental crustand depleted mantle lie below (Nb/Ta) and above(Nb/La) the primitive mantle values of 17.5 forNb/Ta and 0.9 for Nb/La.

Hidden reservoir enriched in Nb, Ta and Nb with super-chondritic Nb/Ta and sub-chondritic Nb/La64.

The Nb/Ta and La/Nb values of the primitivemantle lie between those of the depleted mantle(15.5 and 1.2) and the continental crust (12.5and 2.2).

4He production in oceans is less than thatpredicted from observed heat flow and abouthalf that predicted from chondritic Earth model.

4He stored in lower mantle that is separated by a boundarylayer that transmits heat but not 4He (ref. 65).

Collisional erosion model predicts the Th–Ucontent of the BSE to be about half the chondriticvalue13.

PERSPECTIVE RESEARCH

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If the bulk composition of Earth is non-chondritic, then this has major implications for our understanding of the evolution of Earth.

Although it raises many new questions, this idea could also solve some major paradoxes (see table below).

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Asteroid Differentiation

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Melting of asteroids is a high-temperature but low-pressure process.(even the largest asteroids have ‘low’ pressure in interiors)

10


10


Melting will be affected by presence/absence of water.(e.g., c-chondrites versus ordinary chondrites)

10


10



For dry conditions (ordinary chondrites), melting begins near 1223K and 1323K for silicate melts.(this is several hundred K higher than typical ‘wet’ conditions on Earth)

10


10



For dry conditions (ordinary chondrites), melting begins near 1223K and 1323K for silicate melts.(this is several hundred K higher than typical ‘wet’ conditions on Earth)

Silicate melting is likely needed for significant melt migration; thus mantle-core separation likely requires higher temperatures (>1323K).

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Crystallization of Iron Meteorites

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At high T, taenite (γ-Fe) is the stable phase.

Decreasing T = conversion to kamacite (α-Fe)

Taenite holds more Ni, and Ni diffuses into the grains.

Rate of diffusion is controlled by T, and by 500°C it effectively stops.

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12

The ‘classic’ phase diagram below is o.k. for general purposes, but the full story of cooling is much more complex and may be strongly affected by presence of elements such as P and S (see Goldstein et al., 2009).

12


13

13


14(a subgroup of the group IAB complex, Wasson andKallemeyn, 2002), IIAB, and IVB. Much of the oldercooling rate data (chemical groups IAB, IIICD, IVB)

were obtained using the kamacite bandwidth method(Short and Goldstein, 1967). The kamacite bandwidthmethod cannot be used as discussed in the previous

ARTICLE IN PRESS

IIIAB

IVA

10

100

1,000

10,000

100,000

7Ni content (wt.%)

Coo

ling

rate

(°C

/Myr

)C

oolin

g ra

te (°

C/M

yr)

1000

100

10

16 7 8 9 10 11

Bulk Ni (wt. %)

8 9 10 11

Fig. 15. Cooling rate measurements for iron meteorites in chemical groups IIIAB and IVA (Yang and Goldstein, 2006; Yang et al.,2007). Variations of a factor of 6 and 66 are observed in Groups IIIAB and IVAB, respectively. The error bar for each meteoriterepresents the 2s uncertainty range.

Table 2. Cooling rate variations in iron meteorite chemical groups.

Group Cooling rate variation (1C/Myr) Authors Method

IAB 2–3 Goldstein and Short (1967) 163–980 Rasmussen (1989) 125–70 Herpfer et al. (1994) 2

IIICD 87–480 Rasmussen (1989) 1IIAB 0.8–10 Randich and Goldstein (1978) 3IIIAB 1.0–10 Goldstein and Short (1967) 1

21–185 Rasmussen (1989) 256–338 Yang and Goldstein (2006) 2

IVA 7–90 Goldstein and Short (1967) 12–96 Rasmussen (1982) 219–3400 Rasmussen et al. (1995) 2100–6600 Yang et al. (2008a) 2

IVB 2–25 Goldstein and Short (1967) 1110–450 Rasmussen et al. (1984) 11400–17,000 Rasmussen (1989) 1

1. Kamacite bandwidth method (Short and Goldstein, 1967).2. Taenite central Ni content method (includes effects of P on phase diagram and on diffusion coefficients for Ni in taenite).3. Phosphide growth simulation.

J.I. Goldstein et al. / Chemie der Erde 69 (2009) 293–325 309

[Goldstein et al., 2009]

Previous studies have suggested the IIIAB irons may be from the same parent body as the pallasites.

However, new measurements on cooling rates in ‘cloudy zone’ and chemistry bring this into question (see Yang & Goldstein, 2006, Yang 2008).

14

To Melt, Or Not To Melt.....

15

After ~3 Myr, rate of heat generation from 26Al had dropped to ~12% and that from 60Fe had dropped to 25% of initial rates.

If chondrite parent bodies accreted 2-3 Myr (or longer?)

after CAIs then there was likely not enough energy to melt them to the level that metal would be segregated

from silicate.

(but what about the evidence we discussed that shows

some chondrule formation overlapping with CAI

formation?)

ANRV309-EA35-19 ARI 20 March 2007 17:9

Early Solar System ChronologyTo test whether the 207Pb-206Pb method and the short-lived nuclide systems are ca-pable of providing useful chronological information about chondritic componentsand early Solar System processes, we require chondritic components that formed athigh temperatures and escaped significant metamorphism and alteration or igneouslyformed meteorites that cooled relatively rapidly (in <106 years). In addition, the sam-ples must have escaped any subsequent impact heating effects that caused isotopicexchange between minerals and perturbed or reset the radiometric clocks. Because ofthe prevalence of impacts over 4 Gyr and the extensive metamorphism and alterationof chondrite parent bodies that lasted typically for 10–100 Myr, few meteorite sam-ples satisfy these criteria: chondrules and CAIs in a few type 2 and 3 chondrites, a fewfine-grained, basaltic meteorites (angrites and eucrites), rapidly cooled achondritescalled ureilites, and strongly metamorphosed meteorites called acapulcoites. Figure 5

4570 4565 4560 4555Age (Myr)

Eucrite(A-881394)

Angrite(D’Orbigny)

Ureiliteclasts

Bish

CAI (E60)

Sem

CBSem

CO

CRCV

H4 H4

H4

Acapulcoite

Angrite(LEW 86010)

53Mn-53Cr

26Al-26Mg

206Pb-207Pb

182Hf-182W

129I-129Xe

Qingzhen

CAI

Chondrules (OC)

Chondrules (CC)

Chondrules (EC)

Differentiatedmeteorites

Metamorphosedchondrites

Primitiveachondrites

Irons

Shallowater

ang.

Kaidun carb

Carbonate

Figure 5Early Solar System chronology inferred from radiometric ages of CAIs, chondrules inordinary and carbonaceous chondrites (OC, CC), metamorphosed chondrites (H4), primitiveachondrites (acapulcoite), differentiated meteorites (eucrites, ureilites, and angrites), andalteration minerals. The Mn-Cr, Al-Mg, and Pb-Pb data and diagrams are taken from Kitaet al. (2005) who list data sources. Additional data: Hf-W (Kleine et al. 2005, Markowski et al.2006); I-Xe data and Shallowater (Whitby et al. 2002, Gilmour et al. 2006); Kaidun carbonate(Hutcheon et al. 1999). Vertical arrows connect the samples used to anchor the short-livedchronometers (Mn-Cr, Al-Mg, Hf-W, and I-Xe) to the absolute ages derived from Pb-Pbdating. Dashed lines connect data from the same meteorite. 2! error bars for short-livedchronometers do not reflect additional errors in the ages of the anchors. Key: Bish, Bishunpur;Sem, Semarkona.

www.annualreviews.org • Chondrites and the Protoplanetary Disk 597

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[See Kleine et al. (2005), GCA for discussion of iron, CAI, chondrule timing]15

To Melt, Or Not To Melt.....

16

Cooling rates are important for understanding the potential sizes of the iron parent bodies.

Cooling rates would be expected to be very slow for cores of very large objects, so ‘historical’ view is that irons come from modest-sized bodies.

However, newer models suggest that iron and stony-iron parent bodies:

- accreted/melted shortly after CAI formation - formed closer to Sun (1-2 AU) - may have been much larger (1000 km?) - were disrupted by grazing impacts (with planetesimals?) - smaller fragments/bodies were then flung into the asteroid belt

16

differentiation thermal evolution - deeps · 2013-10-06 · meteorites, which are fragments ......

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