Radioactivity of the Moon, Planets, and Meteorites · RADIOACTIVITY OF THE MOON, PLANETS, AND...

Radioactivity of the Moon, Planets, and Meteorites

Yu. A. Surkov and G. A. FedoseyevV. I. Vernadskiy Institute of Geochemistry

and Analytical ChemistryAcademy of Sciences,

Moscow, U.S.S.R.

This paper reviews and summarizes analytical data for the content of natural radioactiveelements in meteorites, eruptive terrestrial rocks, and also, in lunar samples returnedby Apollo missions and the Luna series of automatic stations.

The K-U systematics of samples analyzed in the laboratory are combined with datafor orbital gamma-ray measurements for Mars (Mars 5) and with the results of directgamma-ray measurements of the surface of Venus by the Venera 8 lander.

Using information about the radioactivity of solar system bodies and evaluations ofthe content of K, U, and Th in the terrestrial planets, we examine certain aspects of theevolution of material in the protoplanetary gas-dust cloud and then in the planets of thesolar system.

As we know, radioactive elements play aspecial role in the history of the solar sys-tem. The study of their abundance in variouscelestial bodies is a key to the development ofour concepts of the solar system. However,the small number of thoroughly studiedextraterrestrial objects creates certain diffi-culties in an attempt at quantitative estima-tion of the abundance of radioactive elementsin the solar system.

In recent years the study of this problemhas been influenced by a stream of new in-formation on the celestial bodies, particularlythe Moon.

Measurements of the gamma radiation ofthe Moon were performed by the Luna 10automatic spacecraft in 1966 and first estab-lished that lunar rocks are similar to ter-restrial basalts in their content of naturalradioactive elements (refs. 1 and 2).

Many further investigations of lunar speci-mens returned by automatic and mannedspacecraft have shown that for natural radio-active elements and a number of other as-

pects of their chemical compositions, lunarrocks from the mare regions are similar toterrestrial tholeiitic basalts (and to stony me-teorites—the Ca achondrites), while the lu-nar continents are similar to anorthositicrocks (refs. 3 through 8).

In 1972, the Venera 8 automatic space-craft produced the first data on the naturalradioactivity of Venus, and allowed certainconclusions about the nature of Venusianrocks and their formation (refs. 9 and 10).

In 1974, the Mars 4 and Mars 5 automaticspacecraft first measured the intensity andspectral composition of the gamma radiationof Mars (ref. 11).

Combination of all this information on theradioactivity of solar system bodies expandsour ability to understand certain aspects ofthe evolution of material both in the proto-planetary gas-dust cloud and in the planetsof the solar system.

The history of the radioactive elements isclosely related to the history of all chemicalelements. However, as we know, radioactive

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202 COSMOCHEMISTRY OF THE MOON AND PLANETS

elements play a special role in the evolutionof the material of the solar system. In thiswork we study the cosmochemical aspect ofthe history of the natural radioactive ele-ments from their formation to the present.

We can imagine that during the earlieststage of formation of the solar nebula radio-active elements were homogeneously dis-tributed in the gas cloud whose densitydecreased with increasing distance from itscenter to its periphery. The entire subsequentevolution of solar system material and, in par-ticular, the history of the natural radioactiveelements can be divided into two main peri-ods: the preplanetary stage when the mate-rial was in a gas-dust state and the planetarystage when the material was concentratedinto the large bodies of the solar system.

At the present time, the radioactivity ofplanetary material is due primarily to U, Th,and K. The mean content of these elementsin the Earth is either estimated on the basisof the current heat flux from the interior ofthe planet (ref. 12) or is assumed equal tothe mean content of these substances in chon-dritic meteorites (ref. 13). The radioactivityof the planets in the distant past was sig-nificantly higher, due not only to the highercontent of these elements at that time, butalso to the decay of more short-lived elementswith half-lives on the order of 106 to 108 yr.These elements were made during nuclearsynthesis and later became part of the youngbodies of the solar system.

Finally, it is possible that during the pre-planetary stage and the early stage of forma-tion of the planets, super-heavy transuraniumelements existed (Z —106-116, N —284),since fission tracks of these elements havebeen found in some meteorites and lunar rockspecimens (ref. 14).

In addition to the natural radioactivity,the surface layers of all celestial bodies havea radioactivity that results from interactionwith cosmic rays. Most cosmogenic isotopesare short lived (of the longer lived, Na22

and AP8 are most common) and are re-tained only in extremely small quantities.Cosmogenic isotopes were mostly witnessesof, while the natural isotopes were~ partici-

pants in, the formation of the solar systemmaterial during the various stages of itsexistence (ref. 15).

Natural Radioactive Elementson the Earth

We know that in the Earth's crust U, Th,and K are quite unevenly distributed. Theyare present in the greatest quantities in acidrocks, in lesser quantities in intermediaterocks, still less in basic rocks, and, finally, inextremely small quantities in ultrabasicrocks. Table 1 presents the mean contentsof natural radioactive elements in the mostcommon rocks of the Earth (ref. 16). Dataon individual rocks have also been collectedin the work of Yermolayev and Sobornov(ref. 17), as well as that of Smyslov (ref.18). Given a general concept of the processesleading to the formation of these rocks, wecan imagine the behavior of natural radio-active elements and their role in the evolu-tion of our planet.

Figure 1 is a K-U plot showing data forthe most representative terrestrial rocks.This sort of systematization using log-logplots is widely used in geochemical and cos-mochemical studies. At the present time, dataon the natural radioactivity of basic and ul-trabasic rocks of the Earth have been signifi-cantly supplemented and refined, primarily asa result of intensive investigations of thecrustal rocks of the rift zones of the oceans.In particular, there is great interest in speci-mens of rock collected from the centraloceanic ridges where the rock complexes areformed in a single process of differentiationof the primitive material of the mantle. Thecentral ridges are of interest for study ofthe composition of mantle material, the pro-cess of formation of the basalt magma, andthe formation of the most primitive crust,which can serve as a prototype of the primi-tive crust formed in the early stage of devel-opment of the Earth, the Moon, and the otherterrestrial planets. These data on deep crustalrocks have also been considered in the com-position of this diagram.

RADIOACTIVITY OF THE MOON, PLANETS, AND METEORITES 203

Table 1.—Content of Natural Radioactive Elements in Various Types ofTerrestrial Rock

RockClasses

Acid

Intermediate

Basic

Ultrabasic

granites, granodiorities(quartz porphyries,rhyolites, dacities)

diorites (andesites)

gabbros (basalts),anorthosites

dunite, peridotite,harzburgite (picrite)

Contents

K.10*. gr/t

3.41-6

2.31-4

0.80.05-3.0

0.030.001-04

of Radioactive

U, g/t

3.51-20

1.81-3

0.50.03-4

0.0030.001-0.8

Elements

Th, g/t

186-40

76-30

3.00.05-10

0.0050.001-1.0

Petrographic and petrochemical study ofspecimens of oceanic rocks allowed Dmitriyevand Udintsev (ref. 19) to analyze the natureof the genetic relationship between basaltsand ultrabasalts which are divided into twomain groups of rocks: Iherzolites and harz-burgites. They classified Iherzolites as uppermantle material and harzburgites as the resi-due following production of the primitivebasalts. These opinions are apparently con-firmed by the K-U systematics.

Figure 1 shows that the mean values of theK/U ratio for Iherzolite nodules of theoceanic areas occupy a narrow band withinlimits of (2-3) X 103; i.e., fall significantlybelow the K/U ratios for the sequence oftholeiite basalts (K/U ~ 7 X 103) and themain mass of alkaline basalts (K/U = (1-2)X 104). It is natural to propose that the in-crease of the K/U ratio in these rocks shouldbe accompanied by a decrease in this ratio inother samples that are related to them bygenetical processes. Obviously, the harz-burgites are such rocks. Thus, in analyzingthe data on the K/U ratios our attention isdrawn by the branching of the K/U ratio for'Iherzolites into two series that are signifi-

cantly different from each other. The upperseries begins with the oceanic tholeiite basaltsand is characterized by a higher K/U ratio,while the lower series is represented by theharzburgites and has lower K/U ratios.

Among the rocks and formations of thecontinental crust, local reductions of theK/U ratio are not uncommon and are, as arule, related to increased uranium concen-trations that arise during the geochemicalcycle for uranium. Among the oceanic for-mations, the group of rocks with a decreasedK/U ratio is of particular interest.

In this sense, the second series, outlinedby the data on harzburgites and serpentinizedIherzolites and located below the continuationof the band of moderate K/U ratio values(which are strictly characteristic of the Iher-zolites of the mantle), is itself somewhatsupplementary to the series of oceanic tho-leiite basalts.

The decreased K/U ratios in the harz-burgite rocks of the rift zones are due to auranium content that is quite high for ultra-basic rocks. Based on a number of factors,we can assume that such high uranium con-tents result from secondary enrichment that


UJ

O(J

U CONTENT, g/t

Figure 1.—Potassium-uranium system of magmatic rocks o/ the earth.

occurred when these rocks were reworked bythe products of degassing of the mantle. Inthis case it is important that the rocks en-riched by uranium were directly related tothe mantle. In the rift zones there could havebeen no influences of the continental crust,and the variation in K/U ratios resulted onlyfrom differentiation in mantle material andits products.

Thus, using the proposal first set forth byWakita et al. (ref. 20) that Iherzolite nodules

are material from the upper mantle andkeeping in mind the distribution of K-U con-tents presented in figure 1, we can assumethat in the primitive mantle material the po-tassium content is not over 300 g/t, and theuranium content is not over 0.1 g/t.

In estimating the mean contents of U, Th,and K for the Earth as a whole, one ordi-narily starts with the following assumptions:

1. The mean heat flux at the surface ofthe Earth is 1.5 X lO"6 cal/cm2 s.


2. The ratio of radioactive elements in therocks of the upper mantle is true for the en-tire Earth.

3. The limits of the potassium content inthe Earth are estimated on the basis of K40,which is derived from the content of daughterAr40 in the atmosphere of the planet (lowerlimit), and the established minimum possiblevalue (50 percent) of fractional release ofAr40 (upper limit) (refs. 21 and 22).

Assuming that the heat flux is in equilib-rium with heat formation and consideringthat most of the heat is generated by the de-cay of uranium and thorium, MacDonald(ref. 12) obtained a mean U content for theEarth of 0.031 g/t. Similar estimates usingthermodynamic calculations have been madeby others; particularly, Tera et al. (ref. 23)obtained a mean value (assuming that thereare no radioactive elements in the core ofthe Earth) of U = 0.035 g/t, Th = 0.14 g/t,with Th/U = 4.

As concerns estimates of the mean contentof potassium for the Earth, the value of 100g/t, obtained from a K/U ratio of — 3 X 103,seems to us preferable to the value of ^350g/t calculated from the generally acceptedratio K/U —' 10*. Furthermore, the value of100 g/t corresponds better to the range ofpossible contents of potassium of 90 to 170g/t, estimated from data on daughter Ar40

(ref. 22).As we will see later, the mean contents of

U and Th in the Earth occupy an interme-diate position between the correspondingquantities characteristic of ordinary chon-drites and of basaltic achondrites, while po-tassium is several times less than in thechondrites and achondrites.

Natural Radioactive Elementson the Moon

At the present time, material has alreadybeen acquired and analyzed for eight regionsof the Moon: three maria (Mare Fecundita-tis, Mare Tranquillitatis, and Oceanus Pro-cellarum) ; three continental areas (aroundthe crater Apollonius-C and the Cayley-

Descartes and Taurus-Littrow mountain sys-tems) ; and two intermediate regions (aroundFra Mauro Crater and the Hadley-Apennineformations). It is of interest to systematizeand summarize the data produced.

Table 2 presents data on the mean con-tents of K, U, and Th, as well as their ratios(K/U and Th/U) for 400 specimens of crys-talline rocks, breccias, and fine fractions ofregolith from all eight regions studied.

A comparison of the abundance of theradioactive elements in the specimens of ig-neous rock, breccia, and fine regolith frac-tions shows a rather wide range in K, U,and Th contents for the various collection ofspecimens returned by the following mis-sions: Apollo 11, 12, 14, 15, 16, and 17 andLuna 16 and 20. The main mass of specimensstudied has the following limits of radioac-tive element content: for potassium, 0.03 to0.7 percent; for uranium, 0.1 to 5 g/t; forthorium, 0.4 to 18 g/t. The K/U ratio variesfrom 1 X 103 to 5 X 103. However, for in-dividual specimens, the data produced gosignificantly beyond these limits. For exam-ple, one of the breccia specimens collected byApollo 12 (12013) is rich in a high silica-feldspar differentiate and contains 10.7 g/turanium and 34.3 g/t thorium. These valuesare even higher than the U and Th contentsin most terrestrial granites which, charac-teristically, have the highest content of theradioactive elements.

Also distinguished by their high values ofK/U ratio (around 7 X 103) are specimensof anorthosite (62236) and basalt (63335)from the Apollo 16 collection, as well asspecimens 15415 (Apollo 15) and 62275(Apollo 16), which have K/U ratios of over104. On the other hand, such specimens ofcrystalline rock as 62295 (Apollo 16) and10062 (Apollo 11) have K/U ratio of < 103.Our attention is drawn by the similarity ofK/U ratios (but decreased contents of potas-sium (200 g/t), uranium (0.07 g/t), andthorium (0.28 g/t), as compared with themain mass of lunar specimens) to the groupof breccias from the collection of Apollo 15and Apollo 16 (15418, 60135, 62237, 64435,67455, and 69955).


Table 2.—Content of Radioactive

Element

K e7t

U ™/fi e/1-

Th eVt

Th/U

K/TI

LunarSample

Crystallinerocks

«!ml

Crystallinerocks

Soil

Crystallinerocks

<3nil

Crystallinerocks

Crystallinerocks

*Snil

MareTran-

quillitatisApollo 11

1460230-2515

1330940-1600

1 1 nn

0.480.18-0.78

0.500.29-0.63

n *\

2.40.86-3.54

2.21.4-2.8

2 q

4.63.4-5.5

4.43.8-5.0

A K

3000820-4650

27602280-3420

99nn

OceanusProcellarum

Apollo 12

500260-670

23101040-4360

22001700-3100

0.240.21-0.31

2.10.6-3.46

1.470.72-2.35

0.910.77-1.2

8.12.5-13.6

5.93.52-8.8

3.61.9^4.0

4.03.2-4.2

3.93.3-4.9

23401760-4290

14701260-1740

16601240-2360

MareFecundatatis

Luna 16

1500

01 n

n i

0 1

11^

1 I

Q 0

3 7

4.700

3640


Elements in Lunar Sam/pies

Fra MauroApollo 14

3030880-4150

43701100-4780

45203570-5300

2.140.6-3.2

3.341.0-4.5

3.693.1-4.0

8.442.24-21.1

12.74.4-15.6

13.612.0-14.4

3.83.5-4.2

3.633.3-3.9

3.73.6-3.9

14001280-1560

1330830-1760

12501120-1390

Hadley-ApenninesApollo 15

400120-500

189086-4780

1410750-1860

0.140.003-0.16

1.180.04-3.14

1.00.42-1.69

0.490.028-0.6

4.530.12-12.3

3.741.73-6.34

3.632.9-4.3

3.783.0-4.2

3.793.4-4.5

30002220-4.104

17001470-2150

15201230-2050

Apollonius-CLune 20

13701000-1740

71

KA1

0.70.5-0.9

n 7

n 90

2.551.7-3.4

1 9

1 n

3.63.4-3.8

1 7

3 Af;

19701930-2000

1 ft9ft

999ft

Cayley-DescartesApollo 16

117090-4000

100050-2000

940425-1230

0.760.016-3.0

0.570.03-1.00

0.560.18-0.74

2.840.1-9.4

2.10.1-4.85

2.080.6-2.74

3.582.82-4.84

3.733.33-4.86

3.752.81-4.45

2070740-7200

20701270-3470

17301300-2900

Taurus-LitthrowApollo 17

1610410-3000

oofin

800600-1330

1.020.11-2.0

n fi

0.30.15-1.0

3.960.31-8.0

o no

1.250.3-2.4

3.462.50-4.0

3 QfT

3.32.0-3.92

24101290-418

25001750-4200


10" :

•52t-"

I03-

1-17 (T. )VH6 ( K . . B . )

"VM2 ( K.)H5(K. )

K = Crystalline Rocks8 = BrecciaT = Soil

^i r-T ' ' ' ' ,i

I03 102 10*

U CONTENT, g/t

Figure 2.—Potassium-uranium system of lunar specimens.

10"

Analysis shows that regions of the Moonsuch as Mare Tranquillitatis, Mare Fecundi-tatis, and the Fra Mauro region are charac-terized within each region by similarpotassium-uranium contents and K/U ratiosfor specimens of various types (crystallinerocks, breccias, fine regolith fractions). Thisfact apparently indicates that the formationof the regolith in these regions was basicallylocal in nature. At the same time, the in-creased content of radioactive elements inthe regolith of the Oceanus Procellarumand the Hadley-Apennine formation in com-parison with the original rocks of theseregions is related to horizontal movement ofthe substance over the surface of the Moon.

In particular, the lunar soil and breccias inthese regions are a mixture of local basaltand a foreign component with high K, TJ,and Th content. This material could not havebeen anorthosite, which has low contents ofK, U, and Th and would act as a dilutingagent to decrease the K-U content in com-parison with those for the soil and breccia.The most probable component is the materialcalled KREEP (UTh), which is character-ized by a high content of natural radioactiveelements, rare earths, and phosphorous.

Figure 2 presents the analytical results ofall known lunar specimens on a K-U plot.As we can see from the figure, most lunarspecimens from the regions studied can be


differentiated by their contents of potassium,uranium, and K/U ratio into a number ofareas with more or less clearly definedboundaries. These areas are partially over-lapping and form the links in several moreor less clearly pronounced chains that con-nect the zone of low content of radioactiveelements with several higher zones that havediffering K/U ratios.

First, we see a long chain with K/Uratios of (1-3) X 103. The lowest link in thischain is formed by the mare basalts ofApollo 15, which have the lowest content ofnatural radioactive elements. The highestlink is formed by the KREEP (UTh) ma-terial of the Apollo 14' collection from theFra Mauro region. In the middle portion ofthe chain are the links that characterize theregolith from various regions and the linksthat correspond to crystalline rocks from theTaurus-Littrow, Cayley-Descartes, and Apen-nine Mountain regions. The same values ofK and U content were found by analysis ofthe individual fragments of crystalline rockfrom the Luna 20 core sample.

Obviously, formal placement of severallinks characterizing specimens from widelyvaried parts of the Moon in a single chaindoes not mean that they belong to a singlesequence or related series of rock formed inthe process of magmatic differentiation.Nevertheless, the great similarity in distri-bution within the very same chain of speci-mens from different collections such as thoseof Apollo 12, 15, and 16 allows us to speakof general trends manifested in the originand evolution of the material of theseregions.

The second chain, not so clearly expressedas the first and having K/U ratios in therange (2-4) X 103, can be traced for linksthat include specimens from the mareregions of the Moon, particularly from theMare Tranquillitatis. We can support thatthis chain also has its origin in the zonewhere we find the links that characterize thelow-potassium basalts of the OceanusProcellorum (Apollo 12) and the Hadleyregion (Apollo 15), as well as basalt 10003from Mare Tranquillitatis (Apollo 11).

Apparently, this very same chain should in-clude the link from the Apollo 17 series,which includes regolith specimens from MareSerenitatis, particularly since in their con-tent of both major and rare-earth elementsthese specimens are similar to the regolithspecimens from the Mare 'Tranquillitatis.

Comparison of these two chains and theirindividual links leads to the conclusion thatthe rock specimens characteristic of moun-tain regions lie basically in the range ofK/U ratios that are lower when comparedwith the range of K/U ratios characteristicof mare basalts. This enrichment of igneousrocks with uranium relative to potassium, asshown by Taylor (ref. 24), might result fromfractional crystallization of melts becausethe physiochemical properties of these ele-ments are different, particularly in theirionic radii and charges. According to thecalculations of the author, the content ofpotassium in the continental crust relativeto the mean content in the entire Moon is32 percent, whereas for uranium this figureis 54 percent and for thorium 56 percent.

The third chain presented in figure 2 isless obvious and statistically weaker. It in-cludes a number of specimens of the Apollo17 series (coarse-grained basalts 70135 and79155, regolith specimens 74220 and 75081,and breccia 76255, which is distinguished bycomparatively high contents of potassiumand uranium with K/U ~> 4700) ; the Apollo16 series of breccias collected at one station(67035, 67115, and 67915) ; the Apollo 11series (specimens 10056 and 10058) ; as wellas regolith and basalt fragment specimensfrom the Luna 16 series. In any case, thepresence of a whole group of specimens dis-tributed in the diagram along the area ofelevated K/U ratios gives us reason to con-sider this chain as well.

These chains can be represented as ap-proximating regression lines with differentslopes relative to the straight lines for K/U= const, and probably intersecting the valuesof potassium-uranium content correspondingto the material of the lunar mantle. Ob-viously, this material should be poorer inradioactive elements than the lunar surface


rocks, since otherwise the radiogenic heatreleased by the Moon should have main-tained it in the melted state.

As we know, zone melting of the substanceof the mantle is accompanied by accumula-tion of a number of elements, includinglithophilic K, U, and Th in the upper zone ofthe melt, and the corresponding impoverish-ment of the mantle in the zone of formationof the melt. As noted above, the presence inthe collection of lunar specimens of a groupof breccias of deep origin with extremely lowcontents of radioactive elements (area out-lined by dashed line on fig. 2) allows us toaffirm more reliably the existence of deepmagmatic zones poor in K, U, and Th.

In order to define the initial matter of theMoon, we must establish the genetic affinityof rocks formed in a single process of mantledifferentiation. This could be confirmed bythe complementary nature of their chemicalcompositions and by data on the distributionof trace elements in them.

The difference in the approximatelystraight slopes that characterize various"families" of lunar specimens in a K-U plotapparently indicates the existence of comple-mentary parts of these families that wereformed as a result of magmatic differentia-tion in an early stage of lunar development(ref. 25).

Certainly, for the Moon, the picture ofdifferentiation and the genetic relationshipof rocks formed in a single process is not asclear as for the Earth. This is explained bythe fact that the process of differentiation onthe Moon was probably not as intensive ason the Earth. However, it is interesting thatthe primary undiff erentiated material of boththe Earth and Moon apparently had ap-proximately the same ratio of natural radio-active elements K/U ^ 2.8 X 103 (ref. 26).

Obviously the order of magnitude differ-ence in K/U ratios between the Moon andEarth, as used in a number of papers aboutsimilar systematics of the data, has nodefinite meaning because it results fromcomparing only secondary, differentiatedrocks and not from considering the averagecomposition of the entire planet.

We can estimate the mean content of U,Th, and K on the Moon as 0.07, 0.25, and 180grams per ton of lunar material, respectively.This is accomplished by keeping in mind theratios K/U ~ 2.8 X 10s and Th/U - 3.6 andthe mean heat flux values of 30-40 erg/cm2s(determined by a radio-astronomy method)(ref. 27) or 28-31 erg/cm2s (measured di-rectly on the Moon by Apollo 15 and 17)(ref. 28).

It should be noted that these contents ofnatural radioactive elements for the primitivematerial of the internal zones of the Moon,which have been subjected to radiogenicheating and chemical differentiation, areclose to the contents produced by Wanke etal. (ref. 29) in calculating the chemicalcomposition of their two-component modelof the Moon (U = 0.086, Th •= 0.32, and K= 250 g/t). Finally, these data for the Moonare also close to the mean values of U, Th,and K contents in achondrites-data that willbe considered below.

With these data for the mean content ofthe natural radioactive elements on the Moonand data on the mean content of lunarspecimens (see table 2) that characterize thelunar crust, we can make a few statementsconcerning the thickness of the lunar crust.

For example, if we assume that at thepresent time practically all of the uraniumhas been concentrated and rather evenlydistributed in the lunar crust with a meancontent of ^ 0.5 g/t, then its thicknessshould be 60 to 70 km. If uranium has re-mained partially in the mantle, the crustshould be thinner; if its content in the crustis uneven and decreases with depth, thecrust should be thicker.

Comparing the content of radioactive ele-ments on the Moon and the Earth, we notethe following. The mean content of radio-active elements in the Moon is twofold higherthan in the Earth, while the mean contentof radioactive elements in the lunar crust islower than on the Earth (e.g., for uranium-' 0.5 g/t and —' 2 g/t, respectively). Thismeans that the processes of differentiation ofmatter on the Moon have not been as inten-sive as on the Earth, and as a result the en-


richment of radioactive elements in the lunarcrust has been at least an order of magnitudelower than on the Earth. This might also bethe reason why we do not observe Moonrocks that are highly enriched with potas-sium, as is the case for certain igneousterrestrial rocks that have K/U ratios of-104.

Natural Radioactive Elementsin Meteorites

The peculiarities of the composition ofmeteorites agree with their formation as aresult of condensation and fractionation ofthe matter of the solar nebula as it cooled.This process has been thermodynamicallystudied in detail (refs. 30, 31, and 32). Theearly condensate was rich in elements (Al,Ca, Ti) that formed refactory compounds,after which the silicates and iron condensed,followed by the condensation of the morevolatile elements that enter into the com-position of the low-temperature phases. Thefractionation of solar nebula matter led tothe formation of carbonaceous, ordinary(bronzite, hypersthene, amphoterite), andenstatic chondrites and achondrites, includ-ing basaltic achondrites (eucrites andhowardites). After their formation, somemeteorites were subjected to secondary heat-ing and recrystallization.

In the process of condensation of meteoriticmatter, fractionation of the radioactive ele-ments U, Th, and K obviously occurred.

The first two of these elements are rela-tively refractory and apparently condensedin one of the early stages of cooling. Theearly condensate was doubtless poor involatile compounds, including K20. The con-temporary distribution of the radioactiveelements in various groups of meteorites isprobably the result of these processes andoccurred in the early existence of the solarsystem.

Table 3 and figure 3 show data on thedistribution of K, U, and Th in stonymeteorites. Naturally, the greatest quantityof published data on the content of radio-

active elements in the meteorites relates tothe most representative and numerouschondrite group. For the achondrite group,statistically significant radioactivity data(for comparison of individual classes ofmeteorites), are available mostly for eucrites,which amount to 45 percent, and howardites,which amount to 27 percent of all knownachondrites.

What are the distinguishing features ofthe K-U and U-Th systems of the meteorites?As regards the chondrite group, there isparticular interest in data on carbonaceouschondrites whose material is generallythrought to be closest to the protoplanetarymatter of which the planets and asteroidswere formed. More precisely, the carbona-ceous chondrites are divided into three types(Cl, C2, C3) ; what we have said relates totype Cl chondrites, whose chemical composi-tion has atomic ratios and amounts ofvolatile substances that are most similar tothe chemical composition of the Sun. Inparticular, types C2 and C3 carbonaceouschondrites are poorer in volatile elementsthan type Cl. The content of potassiumrelative to silicon in Cl, C2, and C3 carbona-ceous chondrites is 3200, 2100, and 1700atoms/106 atoms of Si, respectively; i.e., C2and C3 are poorer in potassium than Cl by0.63 and 0.52 times, respectively (ref. 30).

The mean values of the K/U ratio for allthree types of carbonaceous chondrites differsignificantly: ^ 4.3 X 10" for Cl; ^ 3 X 104

for C2; and ^ 2.3 X 104 for C3. Unfortu-nately, the insufficiency of statistical ma-terial, particularly for type Cl and C3 car-bonaceous chondrites, and the possibility oftheir contamination on Earth, forces us toapproach with caution estimates of the meanvalues of K, U, and Th contents characteristicof any given class of meteorites.

Nevertheless, the content of potassium inordinary chondrites is greater than in car-bonaceous chondrites, although they arerather similar to type Cl carbonaceouschondrites in K/U ratio. As we can see fromfigure 3, the separate fields that characterizeindividual classes of ordinary chondritesoverlap each other significantly and fall


Table 3.—Content of Radioactive Elements in Stony Meteorites

Cho

ndri

tes

1

(/>

11<

Meteorites

Carbonaceous Cl

Carbonaceous C2

Carbonaceous C3

Amphoterite

Hypersthene

Bronzite

Enstatite

Eucrites

Howardites

K, g/t

510380-580

415130-1100

500100-1400

840700-1240

900640-1660

850500-1580

850500-1740

360150-650

250170-400

u, io-3 g/t

11.08.7-14.0

12.610.8-15

1814-24

16.510.5-25.5

168.6-25

13.69.0-24.5

106.0-16

9315.9-200

5223-89

ThlO-'g/t

3929-59

4438-61

8557-120

4543-50

4238-49

4036-42

3429-42

35053-590

16763-312

Th/U

3.52.8-4.2

3.53.1-4.3

4.44.0-5.0

3.63.0-3.9

3.42.5-3.9

3.42.9-3.8

3.72.6-7.0

3.82.9-6.1

3.12.6-3.45

K/U, 10*

4.83.9-6.7

3.01.5-4.1

2.31.6-2.8

5.23.3-8.1

5.33.5-11.6

6.54.4-9.1

8.44.4-11.2

0.40.37-0.76

0.40.34-0.48

within a rather limited area of potassium-uranium contents (K from 600 to 1000 g/t,U from 0.1 to 0.25 g/t). The range ofuranium contents for enstatite chondrites isshifted somewhat into the area of lower Uconcentrations (from 0.06 to 0.16 g/t).

Achondrites of various classes have ratherwide variations in potassium and uraniumcontents. Although due to the small numberof such chondrites as aubrites, ureilites,angrites, nakhlites, and shergottites, it isdifficult to draw any conclusions concerninggenetic relationships; most of the calcium-rich achondrites—eucrites and howardites—show a number of common characteristics.This conclusion can be made on the basis ofboth structural peculiarities and mineralogi-cal composition. As concerns radioactive ele-

ments, we can state that howardites andeucrites have practically identical meanvalues of K/U ratio (—4 X IO3), but differin absolute uranium and potassium contents.Whereas in figure 3 the field defined by thehowardites Frankfurt and Kapoeta lies nearthe point with coordinates K = 200 g/t andU = 0.05 g/t, the area of eucrites extends tovalues of 0.2 g/t uranium and 700 g/t potas-sium. This point of the diagram correspondsto the eucrite Stannern, the basaltic achon-drite richest in radioactive elements, andagrees with the conclusion of Ahrens (ref.33) that Stannern is possibly an end productof the differentiation of achondritic matter.

Of the achondrites poor in calcium, ourattention is drawn primarily by the group ofdiogenites shown in the graph. Schnetzler


/ /^ /- // / /' , Ordinary / /

Enstatite /i Jba* Chondrites / /Chondrites r»^(ft»*.J>^ / / /

/ / ''/ V «... .X /

Basaltic .'Achondrites

/

/

/ //

//

/

HyperstheneBronziteAmphoterite

Enstatite

O Carbonaceous C1• Carbonaceous C2• Carbonaceous C3

• Eucrites

• Howardites

0 Diogenites

\0'3 10 -2 10'U CONTENT, g/t

Figure 3.—Potassium-uranium system of rocky meteorites.

and Philpotts (ref. 34) believed that thediogenites are a residual product produced bythe melting of calcium-rich achondrites outof chondrite matter. On the other hand,based on their similarity to hypersthenechondrites, the opinion has been stated (ref.35) that diogenites were formed from iron-poor chondrites as a result of intensivemetamorphism. As follows from figure 3,radioactivity data do not contradict this idea.Actually, some diogenites (Johnston andShalka) are quite poor in potassium (byapproximately one order of magnitude) incomparison with the chondrites and at thesame time have K/U ratios similar to the Caachondrites (<— 4 X 103), but significantlylower K and U contents.

In contrast to K/U ratios, the Th/U ratio

for these classes of meteorites is less subjectto fluctuations, although the actual contentof Th and U in the stony meteorites is dis-tributed over broad limits. For example, themain mass of chondrites falls within 0.03 to0.06 g/t and 0.016 g/t for Th and U contents.Most eucrites for which thorium and ura-nium were determined are concentrated inthe area of relatively high contents: Th =/- 0.4 g/t and U - 0.1 g/t.

At the same time, howardites (Frankfurt,Kapoeta) occupy an intermediate area andeven fall adjacent (Binda) to the MooreCounty and Serra de Mage eucrites in thelower area of Th and U contents, correspond-ing to the chondrite group of materials. Stillpoorer in U and Th are the diogenites; inparticular, the Ellemeet diogenite has the


Mean K and U contents for the Moon, planets,and meteoritesContents of K and U in surface rocks ofVenus and Mir* (according 10 Venera Sand Mart 5 data)

•• Mean K and U conienu in rocks of the Earth'scrust

Figure 4.—Potassium-Uranium system of bodies ofthe solar system.

radioactive elements in the solar system,using the previously discussed data on theEarth, Moon, and meteorites, and supple-menting them with some estimates on thecontent of U, Th, and K in the planets.Assuming that the relationship of U and Thchanges little or remains completely un-changed within at least 4 AU, we will studyonly the content and ratio of U and K.

Figure 4 presents the mean contents ofradioactive elements on the Moon andplanets and in meteorites, as well as the meancontent in rocks of the crust of the Earth,Venus, and Mars.

What considerations can be used to esti-mate the mean contents of U, Th, and K onthe planets? Obviously, the determiningfactors will be placement in the solar system,size and density, and current internal struc-ture. Let us consider these characteristics.

lowest radioactivity, with U = 0.0015 g/tand Th = 0.0038 g/t.

Nevertheless, as can be seen from table 3,the mean Th/U ratios for meteorites of vari-ous classes and types fall in a relativelynarrow band of 3.6 ± 0.5, which agrees wellwith the value of 3.8 ± 0.3 calculated in-directly for the solar system by Fowler andHoyle (ref. 36).

Natural Radioactive Elementsin the Solar System

Whereas the fractionation of volatile ele-ments and compounds between the terrestrialplanets and the giant planets is beyonddoubt and results from the variation in tem-peratures at various distances from the Sun,the question of fractionation dependent onheliocentric distance within the limits of theterrestrial planets remains open. However,an impression of this fractionation can begained by analysis of both the various con-densation models of planetary formationfrom the protoplanetary cloud and indirectdata on the structure of the planets.

We will study the abundance of natural

VENUS

According to these considerations, Venus,probably more than the other planets, issimilar to the Earth. It is currently knownthat the content of volatile components(particularly C02, N2, 02, etc.) on Venus isclose to that of Earth, and only the differencein temperatures on the surface of the planetsdetermines the quantity of these substancesin the atmosphere or in the bonded state inthe rocks of the planets. The presence onVenus of an atmosphere consisting of com-ponents which, as on the Earth, can bereleased only in the process of degassing ofmelted crustal material indicates that Venusshould be a differentiated planet.

Finally, the contents of U, Th, and K inthe Venusian rock measured by the Venera8 spacecraft amount to 2.2, 6.5, and 4 X 104

g/t, respectively (point V-8 on fig. 4), andindicate the existence of igneous rock on thesurface of Venus. Even if we assume thatthese initial data may not be representativeof the distribution of radioactive elementsthroughout the crust of the planet, this point,in our opinion, is quite indicative when wecompare it with the K/U data for the other


planets of the solar system. It is not difficultto see that these contents correspond to thoseof high-potassium acidic rocks on Earth.

Vinogradov et al. (ref. 37) studied possibleprocesses leading to the enrichment of thesurface Venusian rock with natural radio-active elements. One reason for the stabilityof high-potassium material in particularmight be related to the acid-alkaline proper-ties of melts flowing on the surface of theplanet. Korzhinskiy (ref. 38) and Yakovlevet al. (ref. 39) showed that as the acidity ofa melt increases the resistance to evapora-tion of the alkaline components of such meltsalso increases. This, apparently, is the reasonfor the high concentration of potassium inthe Venusian rocks.

Thus, everything indicates that Venus isa differentiated planet. The process of differ-entiation on Venus apparently occurred quiteintensively, as indicated by the high con-tents of U, Th, and K in the Venusian rockas measured by Venera 8.

If we also keep in mind the high tempera-tures on the surface of the planet (ref. 40),the mountainous relief (ref. 41), and thedense atmosphere consisting of gases liber-ated as a result of volcanic activity (refs.37 and 42), we can imagine that the processof differentiation is still continuing, the in-terior of the planet is still quite hot, and theheat flux at the present time is obviouslyhigher than that of Earth.

Since with identical initial data (andparticularly identical contents of radioactiveelements) Venus should have a slightly lowerheat flux, it is reasonable to assume thatVenus has a slightly higher content ofuranium than the Earth's 0.035 g/t. TheK/U ratio for Venus might be somewhatlower than for Earth.

MARS

Quite a bit is now also known about Mars.Its mass and moment of inertia indicate asignificant increase in density with depthand the possible existence of a dense core.This is also indicated by the weak dipole

magnetic field of Mars (ref. 43). However,everything that we now know concerningthe surface of Mars, based on the photo-graphs of Mariner 9 (volcanism, tectonicphenomena, high mountain regions, etc.),indicates the possible existence of a Martiancrust consisting of igneous rocks (ref. 44).Finally, recent measurements of the gammaradiation of Mars by Mars 5 (ref. 11) in-dicate a comparatively high content ofnatural radioactive elements in the surfacerock of Mars in comparison with the possiblemean content of these elements on Mars. (Infig. 4, the point corresponding to the K-Ucontent in the Martian rock is representedby the index M-5.)

In a number of its characteristics, Marsoccupies a middle position between the Earthand the Moon. At the present time it is notas active as the Earth, but not as cold as theMoon. It can be assumed that the heat fluxof Mars is now intermediate between thevalues for the Moon and the Earth, i.e., ^ 40erg/cm2s.

Based on analysis of the internal structureof Mars, Binder and Davis (ref. 45) in-dicated that the composition of the Martianmantle is similar to that of the Earth; i.e.,the content of olivine and fosterite shouldamount to 65 to 80 percent. The mass of thecore of Mars should be ^ 10 percent of themass of the entire planet, and the radius ofthe core is approximately 1250 km.

Mars was formed in an area of the cloudwhere the temperature was <-' 500 K (i.e.,significantly lower than the condensationtemperature of potassium and the othervolatile elements and compounds). As such,we should expect that Mars has retainedthese volatile substances, though perhaps notto the same extent .as the chondrites whoseaccretion occurred in the temperature range350 to 500K (ref. 46), but obviously morethan Earth.

Therefore, based on these considerationswe can assume that the mean content of Uon. Mars is no less than on Earth and thatMars has a K/U ratio of — 10*. The value ofK/U for Mars is intermediate between thecorresponding values for Earth (K/U >— 3


X 103) and the chondrites (K/U = (2.4-8.4) X 104), which are determined by theirposition at 1.5 AU from the Sun.

MERCURY

There is somewhat less information avail-able concerning Mercury. The presence ofregions broken by craters and of lower,smooth regions similar to the continents andmare of the Moon (ref. 47) may indicatevolcanic activity in the past and the possibleexistence of a crust at present. The presenceof a weak magnetic field, as shown byMariner 10 (ref. 48), probably indicates theexistence of at least a partially melted core.Thus, we can assume that Mercury is also adifferentiated planet. Mercury apparently isricher in refractory elements than the otherplanets and has little or no volatile elements,including potassium. For Mercury we canassume very approximately a K content of< 1 g/t and a U content of — 0.04 g/t.

All these peculiarities in the abundance ofthe radioactive elements on the Moon,planets, and meteorites fit into our generalconcept of the evolution of the matter in thesolar system (refs. 49, 50, and 51).

According to these concepts, the proto-planetary matter, of which all bodies in thesolar system were later formed, took onsignificant heterogeneity in the course of itschemical evolution, as is reflected in thecomposition of the Moon, planets, andmeteorites. This compositional heterogeneityof the protoplanetary mass was determinedprimarily by the relationship between volatileand refractory elements and their com-pounds.

During the evolution of protoplanetarymatter, the volatile elements and compoundsmoved to the periphery of the cloud (in thearea of lower temperatures), while therefractory elements moved closer to the Sun(in the area of higher temperatures). Thisprocess of differentiation in the preplanetaryperiod was naturally reflected in the abun-dance of radioactive elements (particularlytheir relationships) in the solar system. Forexample, as the distance changes from

Mercury (/-- 0.4 AU) to the asteroid belt(^2-4 AU), the K/U ratio changes by al-most five orders of magnitude.

Thus, the various bodies of the solar sys-tem that formed later inherited thosequantities and relationships of radioactiveelements which existed in the formationregion of each body.

The formation of the planetary Earth-Moon system is somewhat more complex. Inspite of the identical nature of the pre-planetary matter in the zone which fed thesetwo bodies, differences in their condensationconditions (pressure, temperature) haveresulted in differences in their chemicalcompositions. According to condensationmodels, the metallic core of the Earth wasable to form before the beginning of thecondensation of low-temperature magnesiumsilicates. Due to this, the relative shortage ofiron on the Moon may be related to the factthat during its accretion a significant portionconsisted of high-temperature condensatesenriched in refractory elements (includingU and Th).

On the other hand, the mechanism of inter-action of the solar wind with particles of theforming swarm must have played a signifi-cant role in the shortage of volatile elementson the Earth and the Moon. According tothe model developed by Ruskol (ref. 52), theinfluence of the solar wind should have beenparticularly strong for the Moon during thefinal stage of accretion of its surface layers.During this period, at the periphery of thecluster around the Earth the transparencywas sufficiently great that the atoms of thevolatile elements, released by evaporation ofmatter during the processes of collision ofsolid particles and bodies, were partiallyremoved from the swarm under the influenceof the solar wind.

After the formation of the planets, furtherevolution occurred. As we know, during thecourse of differentiation of the matter of theplanets, the crust was enriched with naturalradioactive elements, particularly potassium.

Whereas in the lunar rocks the differentia-tion was relatively slight, Earth, Venus, andMars underwent a long process of differ-


entiation during their thermal history, which 23.is indicated particularly by the high K/Uratio of the surface magmatic rock in com- „.parison to the assumed relationship of mean 25.contents of these radioactive elements in theplanets. 26.

References 27-

1. VINOGRADOV, A. P., Yu. A. SuRKOv, G. M. CHER- 28.NOV, F. F. KIENOZOV, AND G. B. NAZARKINA,

Kosmich. Issled., Vol. 4, 1966, p. 874.2. VINOGRADOV, A. P., Yu. A. SURKOV, G. M. CHER- 29.

Nov, F. F. KIRNOZOV, AND G. B. NAZARKINA,Kosmich. Issled., Vol. 5, 1967, p. 871.

3. LSPET, Science, Vol. 165, 1969, p. 1211.4. LSPET, Science, Vol. 167, 1970, p. 1325.5. LSPET, Science, Vol. 173, 1971, p. 681. 30.6. LSPET, Science, Vol. 175, 1972, p. 363.7. LSPET, Science, Vol. 179, 1973, p. 23. 31.8. LSPET, Science, Vol. 182, 1973, p. 659.9. VINOGRADOV, A. P., Yu. A. SURKOV, F. F. KIR- 32.

NOZOV, AND V. N. GLAZOV, Dokl. AN SSSR,Vol. 208, 1972, p. 723. 33.

10. SURKOV, Yu, A., F. F. KIRNOZOV, 0. P. SOBORNOV,G. A. FEDOSEYEV, L. N. MYASNIKOVA, B. N.KONONOV, S. S. KUROCHKIN, AND D. YE. FERT-MAN, Kosmich. Issled, Vol. II, 1973, p. 781. 34.

11. SURKOV, Yu. A. et al., Kosmich. Issled, In press,1974. 35.

12. MACDONALD, G. J. F., J. Geophys. Res., Vol. 69,1964, p. 2933. 36.

13. UREY, H. S., Proc. Natl. Acad. Sci. U.S., Vol. 41,1955, p. 127. 37.

14. BHANDARI, H. S. BHAT, D. LAI, G. RAJAGOPLAN,A. S. IHAMHANE, AND V. S. VENKATAVARADEN,Nature, Vol. 230, No. 5291, 1971. 38.

15. VERNOV, S. N., AND A. K. LAVRUKHINA. Soviet- 39.American Conference on Cosmochemistry ofthe Moon and Planets, Summary of Reports,1974, p. 129. 40.

16. VINOGRADOV, A. P., Geokhimiya, No. 7, 1962, p.555.

17. YERMOLAYEV, N. P., AND 0. P. SOBORNOV, Geokhi-miya, No. 6, 1973, p. 803. 41.

18. SMYSLOV, A. A., Trudy VSEGEI, Vol. 164, 1968.19. DMITRIYEV, L. V., AND G. B. UDINTSEV, Studies

on the Problem, of the Rift Zones of the World 42.Ocean, Collection of Works, Vol. 1, NaukaPress, Moscow, 1972.

20. WAKITA, H., H. NAGASAWA, S. UYEDA, AND H. 43.KUNO, Earth Planet. Sci. Letters, Vol. 2, 1967,p. 377.

21. HURLEY, P. M., Geochimica et Cosmochimica 44.Acta, Vol. 32, 1968, p. 273.

22. LARIMER, J. W., Geochimica et Cosmochimica 45.Acta, Vol. 35, 1971, p. 769.

TERA, F., D. A. PAPANASTASSIOU, AND G. J.WASSERBURG, Earth Planet. Sci. Letters, Vol.22, 1974, p. 1.

TAYLOR, S. P., Nature, Vol. 245, 1973, p. 203.FANALE, F. P., AND D. B. NASH, Science, Vol.

171, 1971, p. 282.SURKOV, Yu. A., G. A. FEDOSEYEV, Q. P. SOBOR-

NOV, AND L. S. TARASOV, Kosmich. Issled, Vol.II, 1973, p. 926.

TlHONOVA, T. V., AND V. S. TROITSKIY, FizikaLuny i Planet, Collection of Works, NaukaPress, Moscow, 1972, p. 178.

LANGSETH, M. G., S. P. CLARK, J. L. CHUTE, S. J.KHEIM AND A. E. WECHSTER, The Moon, Vol.4, 1973, p. 390.

WANKE, H., H. BADDENHAUSEN, G. DREIBUS, E.JAGOUTZ, H. KRUSE, H. PALME, B. SPETTELAND F. TESCHKE, Proc. Fourth Lunar ScienceConference, Geochimica et Cosmochimica Acta,Supplement 4, Vol. 2, 1973, p. 1461.

LARIMER, J. W., AND E. ANDERS, Geochimica etCosmochimica Acta, Vol. 31, 1967, p. 1239.

LARIMER, J. W., AND E. ANDERS, Geochimica etCosmochimica Acta, Vol. 34, 1970, p. 367.

ANDERS, E., Ann. Rev. Astron. Astrophys., Vol.9, 1971, p. 1.

AHRENS, L. H., International Symposium on theChemistry and Mineralogy of Meteorites andExtraterrestrial Matter, London, April 6-8,1970.

SCHNETZLER, C. C., AND J. A. PHILPOTTS, Meteor-ite Research, 1969, p. 206.

DODD, R. T., Trans. Am. Geophys. Union, Vol. 52,No. 7, 1971.

FOWLER, W., AND F. HOYLE, Ann. Phys., Vol. 10,1960, p. 280.

VINOGRADOV, A. P., Yu. A. SURKOV, AND B. M.ANDREYCHIKOV, Dokl. AN SSSR, Vol. 190,1970, p. 552.

KORZHINSKIY, D. S., Geokhimiya, No. 7, 1956.YAKOVLEV, O. I., A. I. KOSOLAPOV, A. V. KUSNET-

sov, AND M. D. NUSINOV, Vestnik MGU Ser.Geol., No. 5, 1973, p. 85.

MAROV, M. YA., V. S. AVDUYEVSKIY, M. K.ROZHDESTVENSKIY, N. F. BORODIN, AND V. V.KERZHANOVICH, Kosmich. Issled., Vol. 9, 1971,p. 570.

CAMPBELL, D. B., R. B. DYCE, R. P. INGALLS, G.H. PETTENGILL, AND 1.1. SHAPIRO, Science, Vol.175, 1972, p. 514.

VINOGRADOV, A. P., Yu. A. SURKOV, K. P. FLOP-ENSKIY, AND B. M. ANDREYCHIKOV, Dokl ANSSSR, Vol. 179, 1968, p. 37.

DOLGINOV, SH. SH., Soviet-American Conferenceon Cosmochemistry of the Moon and Planets,Summary of Reports, 1974, p. 177.

MASURSKY, H., J. Geophys. Res., Vol. 78, 1973,p. 4009.

BINDER, A., AND D. DAVIS, No more informationavailable, 1973.


46. LARIMER, J. W., Space Science Reviews, Vol. 15, 49. LEWIS, J. S., Earth Planet. Sci Letters, Vol. 15,1973, p. 103. 1972, p. 286.

47. TKASK, N. J., Trans. Am. Geophys. Union, Vol. 50. ANDERSON, D. L., Earth Planet. Sci. Letters, Vol.55, 1974, p. 340. 18, 1973, p. 301.

48. BEHANNON, K. W., R. P. LEPPING, N. F. NESS, 51. GROSSMAN, L., AND J. W. LARIMER, Rev. Geophys.K. H. SCHATTBN, AND Y. C. WHANG, Trans. and Space Phys., Vol. 12, 1974, p. 71.Am. Geophys. Union, Vol. 55, 1974, p. 340. 52. RUSKOL, YE. L., Izv. AN SSSR, Fizika Zemli,

No. 7, 1972, p. 99.

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