' .; . ' ·' I "- ... •' Reprinted from the Proceedings LBL-754 of the 4th International Symposium c;f on Geochemical Expl6'ration. London, April 1972

FISSION-TRACK RADIOGRAPHY OF URANIUM AND THORlU M IN RADIOACTIVE MINERALS

Harold A. Wollenberg

September 1971

"

Prepared for the U.S. Atomic Energy Commission under Contract W -7405-ENG-48

For Reference

Not to be taken from this room DISCLAIMER

This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain coiTect information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any waiTanty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, orservice by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of Califomia. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Govemment or any agency thereof or the Regents of the University of California. 'I ' 543.53:546.79:552.121

Fission-track radiography of uranium and thorium in radioactive minerals

Harold A. Wollenberg, M.s.

Lawrence Berkeley Laboratory, Universi~y of California, Berkel~y, California, U.S.A.

Synopsis are at least an order of magnitude less than those of steen­ strupine. The distribution and abundance of radioelements were also investigated in monazite, thorite and pigmentary The fission-track method is a quick, relatively simple and material of the lujavrites. inexpensive technique for the determination of the location Fission-track examinations of thin sections of mineralized and abundance of uranium and, in some cases, thorium fault-zone breccia from east Greenland indicated that U is in thiri or polished sections of rocks. Thermal neutrons strongly associated with limonitic material in thin fiuoritized induce fission in 235U, and 232Th arid 23SU fission with veinlets, and is occasionally found concentrated near the fast-neutron bombardment. Therefore, the sections with edges of reddish opaque grains. By combination of the appropriate track detectors are exposed, first, to thermal results of fission-track studies and an electron-microprobe neutrons to induce only U to fission, and then to fast analysis of the minerals, the abundance of U and Th may neutrons for fission of U plus Th. The detectors are etched be correlated with that of other clements. to reveal the damaged areas (tracks) caused by passage of massively charged fission fragments. High-quality musco­ vite mica is the preferable track detector for minerals with of the location of radioactive U contents greater than 10-15 ppm-mainly because tracks Deter~ination in mica are easy to recognize. Polycarbonate plastic (Lexan minerals in thin sections of rock has been or Makrofol) is preferred as a track detector for low contents accomplished for many years by alpha-track I of U and Th, because this plastic contains essentially no autoradiography. Briefly described, a-particles inherent U; therefore, it has no background track density. radiating from U- and Th-bearing zones near the Thorium is determined successfully if the Th/U ratio in surface of a thin section are registered on an the mineral is sufficiently large. Relative errors (from counting statistics) in Th are less than 25% if Th/U is overlying photographic emulsion, which, when greater than 3; for ratios less than 3 the erroro increase developed, locates sites of radioactivity and rapidly and exceed 40-50% if Th/U is less than I. permits quantitative determination of their ()(­ The method was applied to the study of U and Th in 2 3 9 particle activity. • • A drawback of the techni­ granitic rocks of the Sierra Nevada batholith, California, and in strongly contrasting mineralized terrains in south que is that even minerals with relatively high and east Greenland. The intense' U and Th mineralization radioactivity (several hundred to a few thousand of south Greenland is associated with lujavrites, peralkaline ppm uranium) require exposure times of the rocks wherein the mineral steenstrupine contributes the order of ten days to yield a statistically significant most to whole-rock radioactivity. Mean values of Th and number of countable ()(-tracks. Further, in a U in steenstrupine are 23 000 and 6200 ppm, respectively. Adjacent grains show less radioelement variation than is collection of ()(-tracks it is very difficult to dif:· seen in the granitic rocks. Lujavrites of lower radioactivity ferentiate between those from the decay series of are characterized by eudialyte, whose U and Th contents Th and those from the U-decay series, and it is

347 y 348 HAROLD A. WOLLENBERG also possible that there is not secular equilibrium hanced by etching the detector with the appro­ in the U-decay series. Therefore, unless the Th/U priate reagent: 40-48% hydrofluoric acid (HF) ratios of the radioactive minerals are known for mica or glass, a strong base (6 N KOH or through independent measurements or are as­ NaOH) for plastic. One can then view the tracks sumed, ex-particle autoradiography does not or damage pits easily with an ordinary micro­ furnish independent determinations of U and Th. scope. The fission-track method, developed in the The formation of tracks depends upon the mid-1960s by Fleischer, Price and Walker,7 specific ionization, dJ/dx,8 which determines the permits determination of the location and indivi­ number of ions formed along the path of the r • dual abundances of uranium and, under fav­ fission fragment. dJ/dx is a function of the ourable conditions, thorium, in thin or polished effective charge of the fragment, the velocity of rock sections after short exposures (generally a the fragment, the speed of light, the ionization few minutes) to reactor neutrons. Over the past energy of the outer electrons of the detecting five years several workers have applied the material, the mass of the electron and a constant method to study the uranium contents, ranging which depends on the detecting material. usually from tens to hundreds of ppm, in thin and For our purposes, fragments from the fission polished sections containing accessory and rock­ of U or Th, with atomic mass numbers ranging forming minerals-see, for example, Kleeman approximately from 90 to 140, and with average and Lovering13 and Wollenberg and Smith.19 energies of 85-90 MeV, register rapidly in mus­ Uranium_ contents as low as a few tens of ppb covite mica, plastic or glass. With proper etching have been measured in finely powdered pre­ conditions,. tracks from the spontaneous fission of parations of whole-rock ultramafic samples.6 238U have been observed in several rock-forming Much less has been accomplished in the study of and accessory minerals. ·· the occurrence of thorium, because of limitations If uranium-free detectors are used, the lower of the method, which are discussed later. limits of detection depend mainly on the magni­ Fission-track techniques and their physical tude of the integrated neutron flux (regulated bases are described. Applications of these tech­ in most cases by the duration of the exposure). For niques are illustrated by a discussion of the distri­ example, Fisher6 used an integrated flux of bution of uranium and thorium in contrasting 1016 nfcm2 to measure U concentrations as low intrusive occurrences. It is intended to illustrate as a few ppb. The uranium content and geologic the usefulness of the fission-track method to an age of natural mica detectors combine to furnish understanding of the geochemistry of radioele­ a background track density which, essentially, ments m mineralizations of possible future governs their detection limit-----,-l-5 ppm with 13 2 economic importance. an integrated flu;. of "'10 nfcm • Therefore, such detectors are applicable to neutron-induced The fission-track method fission of U and Th in concentrations from several tens to thousands of ppm. Significant in the application of the fission phenomena to earth sciences are the facts that Computations ( 1) 232Th and 238U fission readily under irradia­ tion by fast neutrons; (2) 235U fissions most The ·equation which governs the density of fission readily under thermal-neutron (energy < 0·025 tracks in a detector placed directly on the flat eV) irradiation; and (3) 238U fissions spontan­ surface of material containing U, and irradiated eously (2 32Th, 235 U and 234U also fission with neutrons, is spontaneously, but at much slower rates than p =~aEn (1) 238 238 2 U). The energy threshold for fission of U and where p is the track density, tracksfcm , ~ is. the 232 2 Th is 0·9 MeV; therefore, in rock samples integrated neutron flux, nfcm , a is the neutron­ thermal-neutron irradiation causes only 235U to induced fission cross-section (,...., 580 barns for fission. The spontaneous fission of 238U, though thermal neutron fission of 235U; 1 barn =10-24 2 necessary for fission-track age determinations of a cm ), E is the efficiency of detection (0·5 in the mineral, also yields a background of fission tracks present case) and n is the number of 235U nucleii in natural materials used as track detectors. per square centimetre of thin-section surface , A fission track is formed when a massively within the effective range of fission fragments. charged fragment, emanating from a fissioned For fragments from U and Th fission the effective nucleus, enters the structure of a detector (gener­ range in most silicate and phosphate minerals IS 2 ally a good insulating solid such as mica, ,...., 3 mgfcm • plastic or glass), causing damage by . charge ·Thus repulsion. The resulting damaged area is en- n =(3 CafA) x I0-3 (2) FISSION-TRACK RADIOGRAPHY OF URANIUM AND THORIUM 349

where Cis the concentration of radioelement, g/g, different materials. This discussion is limited to a is Avogadro's number and A is the atomic mass the two most commonly used in geologic appli­ number of the radioelement.* cations: lexan polycarbonate plastic (Lexan or In practice it is not necessary to use the above Makrofol) and muscovite mica; they are com­ equations to determine element concentrations if pared briefly in Table l . Tracks in muscovite are standards are included in the neutron exposure. long hexagons if etched for a short time, or The equations are most useful in estimating the develop into diamond shapes as etching progresses neutron flux required for an irradiation. (Fig. l (a)). Tracks appear in Lexan (Fig. l (b)) as dark points (from fission fragments entering the Procedures detector perpendicular to the surface) or as dark The basic procedure to obtain fission tracks from uranium and/or thorium is quite simple. Solid­ state track detectors, usually mica or plastic, are mounted on a series of samples and standards, 0 0 which are then exposed to reactor neutrons. On .<:.:' Table 1 Comparison of detector materials

Natural muscovite Polycarbonate plastic

Very durable Surface scratches easily

Tracks easy to recognize, Tracks perpendicular to readily distinguished from surface appear as small scratches dots, easily confused with sera tches

Etching time, -!- 4 h Etching time, 10- 15 min

Track size regulated by Surface removed readily varying etching time as etching progresses; without danger of not possible to enlarge appreciable surface loss tracks after 10- 15 min

Uranium content plus Essentially no geologic age yield background tracks, background tracks from detection limit spontaneous and induced governed by neutron fission . This governs exposure detection limit (1- 5 ppm U )

Print of rock surface forms after etching 14

Fig. 1 Fission-track etch pits in muscovite (a) (top) and completion of the exposure, the detectors are Lexan jJolycarbonate (b) (bottom). Track densify, 5 removed and etched in the appropriate reagents. 2·25x 10 tfcm 2 • Mica was etched in 48% HF for ~ 1 h, The resulting tracks are then viewed and counted the Lexan in 6 N KOH at 60° C for 15 min with the aid of a microscope. lines, the apparent lengths of which depend on the entry angle of the fragment and its starting Choice of detectors position in the sample. Lexan's principal advan­ In choosing a fission-track detector one must tage is that it has essentially no background weigh the advantages and disadvantages of uranium content, whereas even the highest­ quality mica shows a background of tracks from 238 *The ratio of 238U to 235U, 139, is essentially constant in nature. spontaneous and induced fission of its U con- 350 HAROLD A. WOLLENBE RG

'-<: tent. When examining minerals with U contents placed as close as possible to the core of the > I 0 to 15 ppm, detector background is not a reactor, and is encased by at least a 3-mm thick­ problem because track densities from the sample ness of cadmium to permit irradiation by as are considerably greater than background. If a 'pure' a flux of fas t neutrons as possible. In the low content of U is expected in the sample, it may flu x estimate for a thorium irradiation one must be preferred to use a Lexan detector or expose include the large contribution to the observed a mica detector without the sample to obtain the track density from 238 U. effective background value. The estimated integrated neutron flux is then Cfl = p/E (crThnTh +crunu) (3) Standards where p is the desired track density, crTh and a:u are the fast neutron (neutron energy ;;:; 3 M eV ) To furnish track-density- element-content cali­ fi ssion cross-sections for 232 Th ( ;;:; 0· 3 barn) and brations, and to serve as neutron flux monitors, 238 U ( ;;:; 1·1 barn), respectively, and n Th and nu glass standards containing known amounts of U are the estimated concentrations of Th and 23 8U and Th are irradiated with the samples. The nucleii . neutron flux may be depressed within a stack of thin-section-detector sandwiches ; therefore, stan­ Etching dard glasses inserted at the centre and ends of the stack yield close approximations of the true Etching time is varied to achieve an optimum calibration factors for each thin section. Soda track size for a given track density. This is mos t disilicate (Na 2 0 .2Si0 2 ) is convenient as the important if tracks are counted from photo­ standard glass medium, primarily because of its graphs of the detectors. In this respect mica has low m elting temperature ( ;;:; 1100 °C). The range an advantage over Lexan in that track size in of fi ssion fragments in glass is roughly the same as mica can be controlled by HF etching time ; 3 2 in most minerals (approximately 3 x I0- g/cm ) . very little of the mica's surfa ce is removed, even after etching for several hours. After I 0- 15 min Irradiation the strong base etchant used for Lexan rapidly removes its surface, causing a continuous de­ Exposure, processing and observation conditions pletion of tracks with time- first the shallower for different uranium concentrations are sum­ tracks, then those of greater penetration. m arized in Table 2. The choice of irradiation conditions depends on the element to be deter­ Observation mined and its expected concentration. Easily observable track densities range from 104 to 106 T o determine the loca tion of the sites of radio­ 2 tracksfcm • To determine an appropriate thermal­ elements the detector with the developed tracks n eutron flux for a uranium irradiation, desired can be replaced on the thin section, or matching

T able 2 E xposure) processing and observation conditions for various evenly distributed uranium concentrations

U content, Neutron exposure, Track density, Etching time, Magnificati on Tracks /fi eld ppm nfcm 2 X 1Q I3 tfcm 2 h*

50 2 1·2 X 104 3- 4 x36 ~5 00 4 500 6 X 10 2- 3 x 80 ~ 4 5 0 5000 6 X 105 1- 2 x270 ~ 400

*Etching of high-quality muscovite mi ca in 40- 48% HF at room temperature.

track density is combined with the estimated U areas of sample and detector can be photographed concentrations of the samples in equations 1 and separately and the pictures compared . The image 2. The samples are exposed in the thermal column of the rock surface on Lexan, observed by of the reactor (a position outside the core, Kleeman and Lovering 13 after intense neutron surrounded by graphite to exclude fast neutrons), irradiations, aids in rematching plastic detectors where the thermal/fas t neutrons ratio is highes t to thin sections following etching. Otherwise, (preferably ~ 1000). small fiducial marks on mica or plastic detectors Conversely, a fast-neutron irradiation is used and thin sections may be matched, or obvious for thorium. The sample-detector package is track-density patterns matched with their mineral FISSION- TRACK RADIOGRAPHY OF URANIUM AND THORIUM 35 1

grain contributors. Associated track densities can be counted in the microscope, from photographs, 13 or, if contrast and spacing permit, by machine. In the case of a thermal-neutron exposure for U, 12 sample and standard track densities are compared directly to give the uranium content. Errors are from track-counting statistics. 10

Thorium determination

The use of fast neutrons permits a determination of Th content if the Th/U ratio is great enough. Since practically all Th-bearing minerals also contain some U, it is necessary, first, to determine U by a thermal-neutron exposure. Then the sample, with a new detector and U and Th stan­ dards, is exposed to fast neutrons (energies > 1 MeV, usually 3-4 M eV ). The resulting track density is from 238U and 232Th, but, knowing the U content, and having simultaneously ex­ posed a U standard, the U contribution is determined. The contribution from Th is then the difference between the total track density and the contribution from U. The main difficulty with this method is that the relative errors due to counting statistics are rather large; the cross-section ratio for fast 20 30 40 50 50 70 BO 90 neutrons favours U over Th by a factor of "'3. Relative Error in Th {0 /o) The error in the thorium determination comes Fig. 2 Th/ U ratio versus relative error in Th for typical from the accumulation of counting statistics in exposure and counting conditions measurements of U and Th standards, and from measurements of the samples' track densities from rocks of the Sierra Nevada batholith, 19 the writer fast- and thermal-neutron exposures . Fig. 2 determined the locations and abundances of relates Th/U ratios and relative errors in Th for uranium and thorium in rock and grain-mount typical exposure and counting conditions of thin sections. Photomicrographs of fission tracks accessory-mineral samples. Relative errors are from uranium registered in mica, superimposed < 25 % if ThfU is greater than "'3; for ratios < 3 on thin sections, are given in Fig. 3. the errors increase rapidly, and they are con­ In the mica-rich Sierran samples U is associated sidered unacceptably high ( > 40%) for ThfV mainly with biotite, occurring near the edges of < 1. grains or along dislocations within the grains. In other Sierran granitics U and Th are found Results and discussion primarily within the non-magnetic accessory minerals. In 11 of 13 samples, representing rocks The fission-track method has been applied to the typical of the batholith's various plutons (whole­ study of the distribution of radioelements in calc­ rock averages: U, 3·9; Th, 14·0 ppm), sphene, alkaline granitic rocks of the Sierra Nevada averaging 250 ppm U, contributes from 9 to batholith, California; in peralkaline rocks of the 38% of the rocks' uranium (F. C. W . Dodge, U.S. Ilimaussaq intrusion, south Greenland; and in a Geological Survey, private communication); the mineralized fault zone in east Greenland. A brief abundance of sphene does not exceed 0·6 % . review of these studies serves to demonstrate (Sphene makes up less than 0·004% of the other the usefulness of the method in discerning the two samples : thus, it is not a significant contri­ geochemistry of radioactive mineral assemblages. butor to their whole-rock U.) Zircon, though less prevalent than sphene, is an important con­ Radioactive minerals of the Sierra Nevada tributor to whole-rock uranium. ' batholith Only five of the samples contain appreciable thorium in sphene. Although thorium has not As part of a comprehensive investigation of the been investigated extensively in grain mounts of distribution of radioelements in the granitic other Sierran accessory minerals, examination 352 HAROLD A . WOLLENBERG

··"(_ grains of allanite from a high-radioactivity granodiorite contain 8900 and 7200 ppm Th; U is, respectively, 23 7 and 217 ppm. In many instances uranium and thorium vary by factors of two to four between grains of the same acces­ sory mineral within a single thin section.

Radioactive minerals of the Ilimaussaq intrusion

The radioelement and rare-element contents of the strongly differentiated Ilimaussaq alkaline 1 10 11 15 18 20 intrusion • • · · · have been intensively studied. The intrusion (age, ca 1000 m.y.) con­ sists of an early augite syenite followed by a sequence of peralkaline nephelene syenites. 17 Lujavritic zones contain relatively high concen­ trations ofU, Th, Zr, Nb and rare-earth elements. The highest radioelement concentrations occur in rocks of the Kvanefjeld plateau, near the intrusion's northwest border, where lujavrites are in contact with sheared cap-rock lavas. In this area steenstrupine* is the most abundant radio­ active mineral; whole-rock uranium ranges from a few hundred to 800 ppm, and thorium from 200 to 2000 ppm. In the southern part of the intrusion lujavrites, rich in eudialytet and con­ taining appreciable Zr, have U and Th in the range of 30-60 ppm. Also examined by the fission­ track technique were monazite, thorite and uranium-rich pigmentary material in lujavrite of the Kvanefjeld plateau.

Steenstrupine

Steenstrupine is characterized by a broad range in the amount of U and Th occurring, even from grain to grain, in a single thin section. This is exemplified in samples from a borehole drilled in lujavrites of the Kvanefjeld plateau. In a single thin section of rather homogeneous-appearing brown steenstrupine U ranges from 2700 to 7000 ppm and Th from 13 700 to 25 700- approximately factor-of-two differences between proximate grains. There is close agreement, however, between the overall average Th/U (3· 1) in steenstrupine of the thin sections of the borehole with a Th/U ratio of 2· 7 in con·es­ ponding whole-rock samples. This substantiates Fig. 3 Fission tracks in detecting mica superimposed on the conclusion from observation of the thin uranium-bearing minerals of Sierra Nevada granodiorites: sections that in this lujavrite the abundance of (a) tracks associated with U, rimming the end of a grain of steenstrupine essentially controls whole-rock uran­ biotite and at discontinuities within the biotite; ( b) tracks associated with U ( 120 ppm) in a grain of sphene; (c) ium and thorium. The ratios of whole-rock and tracks from U (160 ppm) in sphene, encompassed by steenstrupine Th and U contents indicate that hornblende this mineral makes up to 2-4% of the rock. of thin-section radiographs indicates that it is *Na 2Ce(Mn,Ta,Fe)H z((Si,P)0 4)3· concentrated in monazite and allanite. Two t (Na,Ca,Fe)6Zr(OH,Cl) (Si309)2. . '

FISSION-TRACK RADIOGRAPHY OF URANIUM AND THORIUM 353

The distribution of U and Th within a large steenstrupine grain from a pegmatite vein is illustrated graphically in Fig. 4. The traverse shows a rather uniform distribution of uranium, whereas thorium varies by a factor of two to three. The error bars associated with the thorium points are from the counting statistics.

5 Y. Th

u 0.5

Fig. 4 D istribution of U and Th in jJro.file across a large steenstrupine grain from a pegmatite

Other steenstrupine grains are distinguished by their marked zoning, which is exemplified in reddish steenstrupine (described by Buchwald 3 and S0rensen ) . H ere the centres of the grains appear somewhat isotropic under crossed nicol prisms, wh ereas the outer areas are anisotropic (Fig. 5). Buchwald and S0rensen noted a marked difference in

There are large differences in the uranium content Pigmentary material of eudialyte in different rock types. Uranium, averaging 209 ppm, appears ra ther evenly Pigmentary material closely associated with distributed in unaltered eudialyte from a bore­ arfvedsonite laths in lujavrite of the Kvanefjeld hole (Fig. 7) drilled in 'green' (aegirine-arfved­ plateau (Fig. 8) contains up to 2·5 % U, with sonite) lujavrite in the southern portion of the little or no thorium. (This material is similar to Ilimaussaq intrusion. This contras ts with con­ that described by H ansen 1 1 as occurring in radio­ siderably lower U (50- 100 ppm) in eudialyte active veins near the Ilimaussaq intrusion.) This closely associated with steenstrupine in a borehole strong contribution solely from uranium, super­ on the K vanefjeld plateau. The latter eudialyte imposed on those of Th and U in steenstrupine, has a range of U similar to the 60 ppm reported may account for the relatively low whole-rock 354 HAROLD A. WOLLENBERG ,..,._ 10 STEENSTRUPINE

u.ranium (Arith.Mean 6160ppml

Thorium

Fig. 6 Frequen~y distributions of U, Th and Thf U in steenstrupine : lined areas represent steenstrupine from a borehole on the Kvanif.jeld plateau; unlined areas represent reddish and pegmatitic steenstrupine

Th/U ratio (1·54) observed in some arfvedsonite Fig. 7 Unaltered eudialyte in aegirine- mjvedsonite lujav­ 15 lujavrites of the Kvanefj eld. The apparent rite showing a rather even distribution of U ( ~ 200 ppm). close association of pigmentary material with Field diameter, I · 2 mm

Table 3 Comj;arison ~/('f. - track data with radioelement contents

2 Description (J. tracks/cm /h * U , ppm Th, ppm

Steenstrupine in lujavrite, K vanefjeld area 1300- 12 000 820- 15 000 1720-40 000

Sample 18467b-l, zoned red steenstrupine: anisotropic borders 3100- 5300 2000- 2700 7700- 20 000 isotropic centres 6200- 8300 4200-7200 33 000 (I measurement)

Eudialyte in green lujavrite, southern area 100- 640 2IOt

*From Buchwald and S0rensen. 3 tMean value of borehole samples.

arfvedsonite, a mineral characterized by a pre­ Thorite dominance of Fe2+ over FeH, may indicate reducing conditions in favour of the selective Material considered likely to be thorite has been localization of uranium. observed in veinlets of the KvanefjeldY In thin .• -I ~) ' . } '- j ~• .) I · . 1 0

FISSION-TRACK RADIOGRAPHY OF URANIUM AND THORIUM 355 section (Fig. 9) it appears light yellowish-brown, and is associated with greenish cloudy monazite. The high thorium (28-55%) and uranium con­ tents (2-3%) determined by fission-track radi­ ography are consistent with those reported for thorite in reference texts. 16

Fig. 9 Thorite (28-55% Th, 2- 3% U) associated with greenish cloudy monazite in a veinlet in lujavrite. Large tracks are from a 16-h etch prior to the fast-neutron exposure. Field diameter, 1·2 mm

Comparison with alpha-track autoradiographic data

A comparison of uranium and thorium values Fig. 8 Pigmentary material ( 1·4- 2·6% U) in and between grains of aifvedsonite in lujavrite. Field diameter, with corresponding a-track densities (T able 3), 3 1·2mm reported by Buchwald and S0rensen, indicates relatively good concurrence. Low a-track densi­ Monazite ties of eudialyte from the green lujavrite men­ tioned earlier correspond to U ranging from 160 Monazite, which occurs generally in cloudy­ to 410 ppm. Considerably higher a-track den­ appearing aggregates of small elongate crystals sities, varying over nearly a factor of 10 in steen­ (Fig. 10), was examined in some drill cores and strupine of Kvanefjeld lujavrite, match broad surface samples of lujavrite from the Kvanefjeld ranges in U and Th in thin sections from the plateau. Monazite associated with the thorite boreholes. described above contains approximately 25 % Th, There is also good agreement between the U and has a high ThfU ratio. Considerably lower and Th contents and the a-track data for iso­ Th contents (1- 2%) were measured in monazite tropic and anisotropic zones in the red steen­ from medium- to coarse-grained lujavrite of the strupine shown in Fig. 5. To directly compare K vanefjeld; Th/U ratios in this monazite fall in Buchwald and S0rensen's observed a-track den­ the range 9- 14. These are similar to the ratios in sities with those calculated from fission-track thorite, and contrast with the considerably lower data the mean U and Th values for the aniso­ Th/U measured in steenstrupine in the same thin tropic and isotropic zones were entered in the sections. formula of Coppens,4 which relates radioelement 356 HAROLD A . WOLLENBERG

of whole-rock uranium generally varies with the intensity of fluoritization. From fission-track radiography (Fig. II ) it is apparent that U is closely, but not always directly, associated with the fluorite. U also occurs with yellowish to brownish limonite that lines fluorite veinlets, is intergranular between

Fig. 10 Fission tracks from U fJlus Th ( U ~ 1200 jJpm, Th ~ 12 000 ppm) in monazite (grey) with aifvedsonite ( black) in medium to coarse luJavrite. Large tracks are from a 16-h etch prior to the .fast-neutmn exposure. Field diameter, 1· 2 mm content with IZ-track density. The resulting values, 8200 and 3500 ~Z j cm 2 /h for isotropic and anisotropic zones, respectively, fall within the Fig. 11 Strong association of U (up to 4000 ppm) with reported tZ -density ranges. limonite ( light grey) and fluorite (dark and intermediate) in fluoritized fault-z one breccia, east Greenland. Field An east Greenland fault-zone occurrence diameter, 1· 2 m.m

The distribution of uranium in thin sections from fluorite grains, or exists separately fi·om the a mineralized fault zone was inves tigated by the vcinlcts. Uranium concentrations in this ma terial fission-track method. The fault forms the bound­ range from several hundred to more than 4000 ary between crystalline rocks of the Caledonian ppm. Less common associations of uranium are fold belt to the west and Palaeozoic- Mesozoic illustrated by very intense fission-track densities, sedimentary deposits of J ameson Land to the occurring with yet unidentified translucent grains, cast. The geology of the area has been described and with U enrichment at the margin of reddish in general terms by K empter. 12 The samples semi-opaque grains, possibly hematite (Fig. 12 ). from which the thin sections were prepared There is little consistency between whole-rock consist of moderately to strongly oxidized fault uramum contents and those of the mineralized breccia, mineralized with varying intensity, prin­ zones in the thin sections. In this environment cipally by fluorite and barite. M egascopic ex­ uranium is confined to thin veinlets, which are aminations, coupled with y-ray spectrometric very unevenly distributed within the fau lt-zone determinations, indicate that the concentration breccia. / _) f \) ·.J'

FISSION-TRACK RADIOGRAPHY OF URANIUM AND THORIUM 357

The fission-track results and recent whole­ rock y-ra y spectrometric measurements in the Ilimaussaq intrusion 15 indicate that the a bun­ dance of steenstrupine governs whole-rock radio­ activity in lujavrites of the Kvanefjeld area, and eudialyte controls radioactivity in agpaitic rocks in the remainder of the intrusion. In lujavrite large disparities were observed between radio­ element contents of steenstrupine grains only a few millimetres apart. Uranium vari es much less in eudialyte of the aegirine- arfvedsonite drill core from the southern portion of the intrusion. Measured uranium and thorium contents of steenstrupine agree well with ~Z-track densities observed in autoradiographs by Buchwald and S0rensen.3 In a hydrothermall y mineralized fault zone in east Greenland fission-track radiography reveals that uranium, in concentrations up to several thousand ppm, occurs closely associated with limonitic material in strongly fluoritized veinlets. Thus, the intensity of fluoritization generally controls whole-rock radioactivity. As with almost all geochemical techniques, the fission-track method is most valuable when its results are combined with chemical data obtained by other methods, e.g. electron-microprobe ex­ aminations that disclose the distributions of rare­ earth elements, Zr and Ti in accessory minerals, and reveal the chemical nature of uranium-rich pigmentary material. Microprobe data would aiel Fig. 12 Enrichment in U ( 2 500 to 3000 ppm) at margin in identifying scattered uranium-rich minerals of an opaque grain (hematite?) in .fluoritized fault-zone such as those observed in an east Greenland breccia, east Greenland. Field diameter, 0·88 mm fault ione.

Summary and conclusions Acknowledgment The fission-track method is a quick, simple and The author is grateful to the Danish Atomic inexpensive way to determine the location and Energy Commission, R esearch Establishment, abundance of uranium, and, in some cases, Ris0, for support of fi ssion-track studies while he thorium, in uncovered thin sections. The method was a visiting scientist during 1969-71. Much of can be used to determine accurately U concen­ the Greenland-orientated work reported here was trations ranging from several ppb up to several accomplished in cooperation with Leif L0vborg, per cent. The accuracy for the determination of Ris0, and Professor Henning S0rensen and John thorium is quite poor when Th/U < l , but Rose-Hansen, Institute of Petrology, Copenhagen improves to about 25% (relative error) when University. Th/U > 3. The study of radioelements in the Sierra Nevada Fission-track radiography of granitic-rock thin batholith is part of a continuing cooperative sections from the Sierra Nevada batholith indi­ effort between the Lawrence Berkeley Laboratory cates that uranium associates most strong! y with and the United States Geological Survey. Mineral biotite in rocks rich in this mica; otherwise, U concentration data and uncovered Sierran thin occurs most frequently with the accessory mineral sections were kindly provided by Paul C. Bateman sphene, and Th with monazite and allanite. Such and Franklin C. Dodge of the Survey. The sup­ associations sugges t that the uranium and thorium port of H. Wade Patterson and Alan R . Smith, balance in a rock unit may be obtained by com­ H ealth Physics Department, Lawrence Berkeley bining data from whole-rock y-ray spectrometric Laboratory, is greatly appreciated. Part of the analyses with data from fi ssion-track measure­ work reported here was clone under the auspices ments on populations of separated mineral grains. of the U.S. Atomic Energy Commission. 358 HAROLD A. WOLLENBERG

References 19. WoLLENBERG, H. A. and SMITH, A. R. Radio­ geologic studies in the central part of the Sierra I . BoHsE, H . BROOKS, C. K. and KuNzENDORF, H. Nevada batholith, California. ]. geophys. Res. , 73, Field observations on the kakortokites of the Ili­ 1968, 1481 - 95. maussaq intrusion, southwest Greenland, including 20. WoLLENBERG , H. A. K uNZENDORF, H . and mapping and portable X -ray fluorescence analyses RosE-HANSEN, ]. Isotope-excited X -ray fluorescence for zirconium and niobium. RefJ. geol. Surv. Gronland, analyses for Nb, Zr, and La+Ce on outcrops in the m press. Ilimaussaq intrusion, South Greenland. Econ . Geol., 2. BowiE, S. H . U. Nuclear emulsion techniques. 66, 1971' l 048- 60. In Nuclear geolog)' FAUL, H. ed. (New York: Wiley, 1954), 48- 64. 3. BucHvVA LD , V. and S0RENSEN, H . An autoradio­ graphic examination of rocks and minerals from the Ilimaussaq batholith, South W es t Greenland. J\!Ieddr Gronland, 162, no. II , 196 1, 43 p. 4. CoPPENS, R. Sur Ia m esure de Ia radioactivite des roches par !'emulsion photographique. Bull. Soc . Jr. Minir. Cristallogr., 75, 1952, 5 7- 8. 5. FERGUSON, ]. Geology of the Ilimaussaq alkaline intrusion, South Greenland. Meddr Gronland, 172, no. 4, 1964, 83 p. 6. FISHER, D. E. Homogenized fission track deter­ mination of uranium in whole rock geologic samples. Anafyt. Chern. , 42, 1970, 414- 6. 7. FLEISCHER, R. L. PRICE, P. B. and WALKER, R. M. Tracks of charged particles in solids. Science, N. T, 149, 1965, 383- 93. 8. FLEISCHER, R . L. et al. Criterion for registration in dielectric track d etectors. Phys. Rev., 156, 1967, 353- 5. 9. HAMILTON, E. The distribution of radioactivity in the major rock forming minerals. Meddr Gronland, 162, no. 8, 1960, 41 p. 10. HAMILTON, E. The geochemistry of the northern part of the Il.imaussaq intrusion, S.W. Greenland. M eddr Gronland, 162, no. 10, 1964, 104 p. 11 . HA NSEN, ]. A study of radioactive veins con­ taining rare-earth minerals in the area surrounding the Ilimaussaq alkaline intrusion in South Greenland. Meddr Gronland, 181, no. 8, 1968, 51 p. 12 . KEMPTER, E. Die Jungpahiozoischen Sedimente von Sud Scoresby Land. Meddr Grrmland, 164, no. 1, 1961, 125 p. 13. KLEEMAN,]. D. and LovERING,]. F. Uranium dis­ tribution in rocks by fission-track registration in Lexan plastic. Science, N.Y., 156, 1967, 512- 3. 14. KLEEMA N, ]. D. and LovERING, ]. F. Lexan plastic prints: how are they formed? Radial. Eff. , 5, 1970, 21 - 6. 15. L0VBORG, L. et al. Field determination of uranium and thorium by gamma-ray spectrometry, exemplified by measurements in the Ilimaussaq a lkaline intrusion, South Greenland. Econ. Geol., 66, 1971 , 368- 84. 16. NININGER, R . D . Minerals for atomic energy ( ew York: Van ostrand, 1954), 367 p. 17. S0RENSEN, H. On the magmatic evolution of the alkaline igneous province of South Greenland. Rep. geol. Surv. Gronland no. 7, 1966, 20 p. 18 . S0RENSEN, H . HANSEN, J . and BoNDESE N, E. Preliminary account of the geology of the Kvanefj eld area of the Ilimaussaq intrusion, South Greenland. R ep. geol. Surv. Gronland no. 18, 1969, 40 ·p. II

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