Radioactive disequilibria and apparent ages of secondary uranium minerals from Sweden

Radioactive disequilibria and apparent ages of secondary uranium minerals from Sweden RUNO LOFVENDAHL & ELlS HOLM

HTI-IOS

L6fvendahl, R. & Holm, E. 1981 07 15: Radioactive disequilibria and apl~rent ages of secondary uranium minerals from Sweden. Litho,~, Vol. 14, pp. 189-201, Oslo. ISSN 0024-4937. Thorium and uranium nuclides have been measured using high-resolution alpha spectrometry in order to examine the potential of secondary u~nium minerals for disequilibrium dating purposes. Mine~.'alages are proportional to the distance below T~hebedrock surface. Minerals coated on the bedrock surface are usually post4glacial wi~h api:atent ages ~0elow 10 000 years. Minerals coi|ected from cracks and fissures 3-100 mm below the bedrock surface give apparent ages from 14 000 years to equilibrium ages of more than 300 000 years. Samples from drill
R. L6fvendahl, Department of Geolog~, Univer,ity of Stock.~olm, Box 6801, !13 86 Stockholm, Sweden: E. Holm, Department of Radiation Physics, University of Lund, Lasorettet, 221 85 Lund. Sweden: 14th October 1980.

The occurrence and geographical distribution of secondary uranium minerals in Sweden (Lffvendahl, in press) constitute the basis of this study. Examination of the degree of radioactive equilibrium and consequently the apparent ages of these minerals is the purpose of this. paper. Factors of importance for mineral precipitation and dissolution will be introduced and discussed. Decay of the radioactive parent nuclide 23~;Uto daughters with varying half-lives makes age determination of minerals and material with uranium possible. Here the daughter nuclides 234U and 2S°Th have been examined. Since very little work concerning the degree of radioactive disequilibrium of different types of secondary uranium mineral has been done, this examination should be regarded as a reconnaissance

(1964) pointed out a method of dating secondary uranium minerals similar to the conventional dating by U-Pb by use of the Concordia diagram, with examples taken from France. Sakanoue & Komura 1965 (in Sakanoue et al. 1967) have dated secondary uranium minerals from Toki, Japan. Richard et al. (1975) examined equilibriadisequilibria in French uranium minerals using low-resolution alpha spectrometry. Lively et al. (1979) have examined disequilibriam in the carnotite ore at Yeelirrie, Western Australia.

Experimental procedure

survey.

Methods used fo~" sampling, isolation ~nd identification of minerals are described in L6fvendahl (in press). The sampled minerals were carefully separated from foreign material under a binocular microscope.

Previous work

Quantitative separation of uranium and thorium

As early as in 1926 Khlopin pointed out the possibility of dating secondary radioactive minerals. Rosholt (1959) pioneered the practical use of uranium disequilibrium dating of nc,n-mmine material. He worked out a methodology and classified different types of di~quilibria. However, dating of secondary uranium minerals has been rare. Soviet workers, in particular Chalov and Cherdyntsev, apparently m'e pioneers in this particular t~eld (Cherdyntsev 1971). Aii~gre 13- Lithos 3/81

A number of samples were cl~osen for quantitative analyses, with the intention of obtaining a representative selection of all types of samples and sar~ple lcealities. About 0.2 mg of the sample, us'tally containing 50% U and <20 ppm Th, is dissolved in cone. aqua regia with the addition of some 30% hydrogen peroxide and a ~ U - n ~ l ' h tracer with known activity ratio. After equilibration, the solution i,~evaporated to near dryness and cone. hydrochloric: acid i~, ad,Jed ~itd evaporated three times. The sample is dissolved in 25 ml 9 M hydrochlo~c acid. An anion-exchange colunm, precor,ditioned with 9 M hydrochloric acid is used. Thorat'm passes thrcugh the column from this solution, while uraaium and iron (if

LITHOS 14 (19~1)

190 R. Lrfvendahl & E.. Holm Table 1. Constants used in this study (from Steiger & Jiiger 1977 avd Faure 1977). Nuclide

Half-life years

Decay constant years -~

2asU *asU 234U ~,~OTh

4.468 x 109 0.7038 x I(P 2.48 x lip 7.52 x 104

1.55125 x 10 -I° 9.84~5 x 10 -1° 2.794 x 10 -'j 9.217 x 10-°

Table 2. Alpha particle energie:; a;ad intensities. Nuclide

Panicle energy, MeV

Particle intensity, %

zasU

4.195

77 23 93.2*

4.147

r-~."U ~4U z~zU Zarl'h ~'3~i'h ~'~'STh

4.597--4.323 4.773 4.722 5.324 5.267 4.011 3.952 4.684 4.617 5.424 5.342

72 28 68 32 77 23 76 24 71 29

Alpha spectrometry The samples are counted for 1000-2000 minutes with surface barrier detectors (300 mmt sensitive area) connected to a multichannel analyser. The energy resolution of the system is 30-35 keV (Full Width at Half Maximum) for 5.16 MeV alpha particles from t~Pu.

Evaluation of spectra The alpha energy spectra are evaluated by the use of energy tables in Chanda & Deal (1970), Heath (1976-77) and Lederer et al. (1967). Energy levels for each nuclide taken from Chanda & Deal (19'70) are found in Table 2. The activity (A)= cAN, where c is the counting efficiency, N the number of atoms of the nuclide in question and ~, its decay constant. The estimated error of calculated activity_ ratios (234U/2a'~lJ, 23°Th/234U and ~3°Th/23su) is obtained by summation of errors of several independent variables 2

L.dkai]

where F is the calculated error of A (calculated activity ratio) and f~=+lF~ are the errors of ai (measured activity).

* Include,, all1peaks in this energy inteJrval.

Computation o f ages presenl) a~e sorbed onto it. Iron is eluted with 10 ml of a 1 M nitric acid - 9 3 % methanol mixtvre. Finally, uranium is eluted with 50 ml 7.2 M nitric acid. The chenfical yield of uranium is 30-60%. "l'he tho.,'ium effluent and washings are evaporated to near dryness a~d a few drops cone. nitric acid are added and evaporated ~hree times. A mixture consisting of 20 ml of a 7 M nitric acid - 30% methanol mixture is added to the sample. An anion--exct, ange celumn identical with the one described above, preconditioned with 10 ml ot" a 7 M nitric acid - 30% methanol mixture is used. The. solution is passed through the column. Thorium ~s eluted with 25 ml of a 9 M hydrochloric acid solutio,J. The chemical yield of thorium is 40-70%.

Electrodeposition For the electrodeposition, the samples are treated by a procedure modified from Talvitie ( ! ~ 2 ) . The eluate is evaporated to near dryness. A few drops of cone. nitric acid are added to the thoriam sampV,e and evaporated to near dryness. The residue is dissolved in 0.5 mi cone. sulphuric acid and hedged until white fumes appem. The solution is diluted with 5 ml of distilled water and nc:utralized with ammonia to pH 3, using tl'::;mol blue as an i~dicator. After the addition of 5 ml 1% sulphuri¢ acid the pH is readjusted. The solution is tra~asferred to an electrodeposition vial. The elements are electr,xleposited on stainless steel disc~; at a currer~t of 1.0 A for one hour.

In the case of initial equilibrium between the nuclides 234U and 23sU (i.e. the initial activity ratio 2a41J/2asu is unity), the age of the mineral can be found by the following equation (eqs. 2-4 from Faure I977:290-292): S =: 1 - e x p ( - hot)

(2)

Here S is the calculated 23°Th/234Uactivity ratio; go = decay constant for 23°Th; t = age 0t' mineral. In the second case, when an approximate apparent age, the aa age, of a mineral is known from eq. 2 and a 23~U/23sU activity ratio, R, ~ 1 has been measured, an initial 234U/23sU activity ratio, Ro, can be estimated:

R = 1 +(Ro-1) exp(-h4t)

(3)

where R is the measured '!34U/23su activity ratio;

Ro is the activity ratio of za4U/23sU at t = 0 (time of formation); h~ = decay constant for 234U. With this approximate Ro value known, a more precise apparent age is found:

Radioactive disequilibria

LITFIOS 14 (1981)

21+

191

approached with the help of e:ls. 3 and 4. The estimated error in age (lot) is obtained by use of the Gaussian error formula for several independent variables. With the methods and equations used, coverage of the time-span 10 000 to 300 000 years is possible. Age determination in the time interval 1-10 000 years is not dependable. Imperfection of electrodeposition with low energy tailing in the spectra gives scattered impulses increasing the background. In particular a small 2a°Th activity can be multiplied by low-energy tailing of ruTh. Aa ages below 10 000 years must consequently be regarded as maximum ages.

22+

20+

16+

151

Sample description This description is restricted to those factors which are regarded as essential for a discu.';sion of the contritions of secondary uranium mineral formation. These factor s include kind of bedrock in the sample area, types of secondary uranium mineral phase(s) present, primary radioactive mineral(s), if present, sample altitude, relation of sample site to the highest postglacial shore-level, and apl~¢oximate time elapsed since the rise of the sample locality above sea-level. For an approximate location, indexing of the ~wedish topographical map system is indicated directly after the sample locality, i.e. ~qtenshuvud, 2E 4a. Consecutive numbering (1-22) is used in Fig. 1 and in the following description to further facilitate sample location. Sample numbering is explained in L6fvendahl (in press). The following abbreviations are used: B - bedrock from which the sample originates, SM - secondary uranium mineral phase(s), PM - primary radioactive mineral phase{s), SA sample altitud¢, AS(BS) - sample locality situated above the highest shore-level (sample locality situated below the higaest shore level), and T - approximate time elapsed since the ris ~.of the sample site above sea-level (a.s.l.). !. Stenshuvud, 2E 4a, samples 790126 and 790171, the first one collected from dumps, the second from an outcrop. B fissured granite, SM - uranophane in the first sample, boltwoodite in the second, PM - aUanite (Jarl yon Feilitzen, pers. comm. 1978), SA - 10 m a.s.l, for the fn,st, unknown for the second, BS, T - the fn'st site rose above sea-level more than 10 000 years BP.

Fig. I. Sampling localities. For numbering and description of samples l to 22, see text.

T = I - e ~ p ( - 7~ot)+ ~o(7~o- ~4) -~ (Ro -- I) [exp(- 7,4t)- exp(- Xot)]

(4)

where T is the calculated za~rh/2asu activity ratio. With further iteration steps, the precise Ro value and precise apparent age, pa age, can be

2. H~ts, ()xnevalla, 6C 2a, sample P 567, supplied by Eric Welin, Stockholm. The sample was collected from mine dumps of a feldspar quarr!r, and consists of syngenetic uraninite in pegmatite. The uraninite has been dated by Welin & Blomqvist (1964), and Was supposed to be in radioactive equilibrium. 3. Viissings6, 6B 3e~ sample 790093. B -. pegmatite lenses and bodies in amphibolite gneiss, SM - hydrogen-autunite as coatings on the bedrock surface and in microfissures down to a few mm into the bedrock surface, PM - not known, SA - 8-10 m. a. s. !., BS, T - rose above sea-level about 4 000 years BP. The locality is si~,uated at the' sea-shore. 4. Vattentornet, 7H 0a. Fo'ar samples. 78:113, 78:117, 78:119, and 790172A. The last sample was collected at tile bedrock

1'92

R. Li~fvendahl & E. Holm

LITHOS 14 (1981)

surface, the others are drill-core material. B - quartzite, SM sample 78:113 with coffit~ite was collected 14 m below the surface, 78:117 with ~ippeite and thucholite 10 m below the surface, 78:119 w~th uranophane 7 m below the surface and 790172A witt. l~,olt#oodite as fissure coatings a few c m below the bedrock su~ace, PM - 'thucholite' in cracks and fissures and uranium adsorbed onto iron-titanium oxides in palaeoplacers, SA - a b o u t 29 m a. s. I., BS, T - rose above sea-level about 10 000 years BP.

fissures a few c m below the surface (7385:223) and cuprosHodowskite from cracks (lhe others), PM - urac~inite in the T o s s , s e n sample, not known in :he othe[s (Mike Wilson, Lule[t, pets. comm. 1979), SA - 580 m a.s.l. (Stlvbacktjarn) and 800-810 m (the others), AS.

5. L6gm'berget, 7G 0-1j. Seven samples, 7287:798, 7287:799, 790084A, 790085A, 78:053B, 78:t26, and 78:127, the first four collected from outcrops, the remaining three from drill-cores. B - quartzite, SM - sample 7287:798 a mixture of boltwoodite and uranophane collected a few cm below the bedrock surface just like the two samples 7287:79.) and 790084A consisting of boltwoodite. Sample 790085A with boltwoodite coated the bedrock surface. Sample 78:053B with thucholite was collected 43.5 m below the surface, 78:126 with beta-uranophane 15.5 m below the surface and 78:~27 with zippeite 4 m below the surface, PM - 'thucholite' in cracks and fissmes, SA - 2 - 5 m above the Baltic, BS, T - the bedrock rose above sea-level (the Baltic) about 2 000 years BP.

14. L t t n g t j ~ n , 20E 9b. Three samples, 7483:6011-II, 7483:624, and 790063. B - boulders with granite (the first two samples) and a metabasite dyke cutting granite (the last one), SM uranophane in all three safe,pies, PM - black uranium oxides, SA - 520 m a.s.l., AS.

6. 0dsm~l, 8B 0d. T w o samples, 790097 arid 79fj098. B pegmatite lenses in amphibolite gneiss, SM -- boltwoodite as surface co,tings in both samples, PM - not known, SA - 8 m a.s.l., BS, T - rose above sea-level about 3 500 years BP. The sampling locality is situated at the sea-shore. 7. Tidaberg, 9H 2a, sample 790121. B - coarse granite, SM pho~rhuranylite collected from fissures 2-3 mm below the surface.. PM - not known, SA - 4,3 m a.s.l., BS, T - rose above sea-level about 8 000 years BP. 8. H~i-serud, 9C 9a. T w o samples, Da 15/69 and 790101. B albitit,zed schist, samples collected a few cm below the surface, SM - uranophane in both samples, PM - finely disserainated pitchblende, SA - 175 m a.s.l., AS. 9. K,ngs~ra, IIH ib, sample 790104. B - g r a n i t e , S M phosphuranylite from fissures a few mm below the surface, PM - not known, SA - 40 m a.s.l., BS, T - rose abe,re sea-level about 5 000 years BP. 10. Stripa, I IH 4c. T w o samples, 790180 and 790184, collected from the walls of S~ripa iron mine. Green-yellow secondaries were formed on the mine walls within a month (Welin 195:3). The mineral is liebigite in both samples, primary uranium oxides occur in connection with the iron mineralization. The samples are used as reference samples, as they are bel:~eved to be formed in late 1956 or early 1957 (collected by Jarl yon Feilitzen in the s u m m e r of 1957). II. S6derboda, 131 li. Two samples, 790106 and 790111). i:~ gneissic granite, SM - boltwoodite in both samples. Yellow coatings of 'uranochra' are known from this site since the 1880s (Svenonius 1887), the area was examined in 1946 by Ihe Boliden Company (David Malmqvist, Uppsala and Frans E Wickman, pers. comm. 1977). PM - not known, SA - 6.-~', :rn a.s.l., BS, T - rose above sea-level about 1 000 years BP. 12. Storsj6 area (S61vbac~t.j~irt~, I:]D 3d, T o s s , s e n 18D 6e ~nd Kroktj~irnsvallen 18D 7e). Three samples, 7385:223 (SiSIvbackzj~irn), 790118 (Tcss~sen), and 7588:529 (Kroklj~irnsvallea), all collected from boulders, iq - mylonite (7385:223) a.nd impure qua.rtzite (the others). SM - ur~anophane collected from

13. Lilljuthatten, 20E 9a, s ~ t p l e 799060. B - fissured granite, SM - uranophane, PM - black uranh~m oxides, SA - 750 m a.s.l., AS.

15. Bergskiir, 23L 0a. T w o samples, 790028 and 790030. B pegmatite lenses and b~odie~; in metagneiss, SM - uranophane in the first sample, phosphltranylite in the latter, PM - uraninite and monazite (Wel~n 1965), S A - 5-10 m a.s.!., B~;, T rose above sea-level 5 0 ~ 1 0 0 0 years BP.

16. Bj6rklund, 241 3e. Saraple 7373:639, collected from an outcropping granite wi~.h epigenetic uran~nite mineralizations in fissures. The uraninite material has been used for U-Pb dating (Adamek & Wiilson 1977), and was assumed to be in radioactive equilibrium. 17. V~istra Rebraur area, 251 7-8b. T w o samples, 7280:303 and 7289:613, collected from boulders, the former originating from Tjuorrevaratj, the latter from Viistra Rebraur. B - breccia in an acid metavolcanic rock (7280:303) and coarse granite (7289:613). SM - uranophaJae in cracks and fissures a few c m below the boulder surface. PM - uraninite (Bo Gustafsson, Lule?~, pers. comm. 1979), :']A - 500 m a.s.i., AS. 18. Pleutaj~kk, 26H Of. Six samples, 7373:62 !, 77:116A, 790004, 790005, 790006, and 790129. Samples 790004 and 790129 were coil,ected from local boulders, the others from outcrops, all material collected from fissures a few cm below the bedroc~ or boulder s u r f a c e B - acid metavolcanic rock, SM sample 77:116A consists of wtlsendorfite, 790004--06 are uranophane samples and 790129 a mixture of uranophane and kasolite, PM - epigenetic uraninite minel-alization (Bo Gustafsson, pers. c o m m . 1979). Sample 7373:621 consists of pure uraninite collected from a fissure. The materi~d has been used for dating (Adamek & Wilson 1977). The 22°Ra/~gU activity ratio is app~roximately unity (Mike Wilson, pers. comm. 1980), which means tha! the sample is in or near re,dioactive equilibrium. SA - 450 m a.s.l., AS. 19. Harrej~kk, 26H ld. Sample 7373:630 from a local boulder of monzonitic bedrock, co~sisting of uraninite collected from a fissure. The material has been used for dating (Adamek & Wilson 1977), the nuclides. 22eRa and ~3sU are in approximate equilibrium (Mike Wilson, pers. c o m m . 1980). 20. Lulep Manak, 271 9d. Four samples, 77:100A-B, 77:'.01AB, 790048A-B, and 790114. all originating from bouldeL~s. B biotite-rich pegmatite, SM --sample 77: lt]~) with wtlsendorfite (A) and beta-uranophane (13), 77:1(.q with wfSlsendorfite (A) and uranophane (B), 790@18 with beta-uranophane (A) and wtlsendorfite (B) and 7901 !.4 finally a metamict phase, probably wtlsendorfite. PM - t~Jranin~i6 (Bo Gustafsson, pers. c o m m . 19799, SA - 470 m a.s I., AS. 21. Kebnats, 281 6d. Two samples, 790045 and '790046 from

L I T H O S 14 (1981) local boulders. B - impure qu~wtzite, SM - uranophane in both samples, PM - not k n o w n , SA - 470 m a.s.l., AS. 22. Haukivaara, 29J 6g--h. Sample 790127 from drill-core material, coilecteci about 100 m below the bedrock surface (supplied by Jail yon Feilitzen). B - quartz-banded iron ore, SM meta-autunite filling cavities, PM - not known.

Results The examined material originates from 22 localities spread all over Sweden (Fig. 1). It can be divided into three groups: (A) Minerals occurring as coatings directly on the bedrock surface. (B) Minerals from cracks and fissures 3-100 mm below the bedrock surface. (C) Minerals from drill-cores more than one metre below the bedrock surface. Unfortunately we have not acquired any samples from the depth interval 0.1 to 1 metre. Samples belonging to these three groups are found in Table 3. In addition, two samples are known to have formed recently (type D) and are listed at the end of the table. The samples listed in Table 3 are numbered within each group from south to north. The aa ages of the minerals are calculated using eq. 2. In Table 3 no age error is indicated. This error is estimated to be within 5-15% when the R-value is approximately unity, the older the sample the larger the percentage error. When the R-value is larger than unity, the aa age is too large, when it is smaller, the aa age is too small. For further discussion and evaluation of aa ages, see (e) and "Fable 4. Aa ages below 10 000 ye.~rs are rounded off to the nem'est 100 years, aa ages between 10000 and 100 000 years to 1000 ~,ears and those above 100 000 years to 5 000 years. Four samples supposed to be in radioactive equilibrium are listed in Table 5.

Reliability conditions To get reliable results, the following conditions must be satisfie:!: (a) Homogeneity of sample. (b) The time i~lterval during which the mineral forms must be negligibly sho~rt in relation to its age. (c) The initial 23°Th/U-nuclide activity ratio must be known. (d) The mineral must constitute a closed sys~Iem

Radioactive disequilibria

1~c,3

with respect to uranium and its intermediate daughters down to thorium. (e) The Ro value must be known. In the following we discuss each o f these conditions in turn. (a)Homogeneity of sample. The sample homogeneity has been checked by examining different portions of the same sample. The resuits can be found in Table 3 (duplicate analyses marked with an asterisk). All ratios are recorded as activity ratios. At perfect equilibrium the ratios are unity within the 23su disintegration chain The theoretica~ ~2ssu/2asu activity ratio is 0.046. In Table 3 this ratio has been excluded in a number of cases, as low energy tailing of the 234U-peak interfere with 235U, giving too high value; ot."the latter. The duplicates in Table 3 show that the analyses, with one e,,ception, are reproducible within the error lim~s. The exception is the sample of uranophan¢ from Hotagen (7483:601), where the examined material is a mixture from two different fissures in the same boulder. (b) The formation period of the mineral in relation to its age. This is a critical point and difficult to check with any certainty. It has been show~ in mine shafts, for example in the Stripa iron mine, that secondary uranium minerals can be formed within a month or two (Welin 1958). Material from Stripa, supposed to be formed early in 1957, was found to be almost devoid of thorium, indicating a very low age (Table 3, the last two samples), and in agre~.'ment with Welin's statement. (c) The in#ial 23°Th/234Uand 23°Th/238Uactivity ratios must be known. This conditior~ is satisfied if thorium is initially absent in the mineral. The initial absence of 2a°Th is difficu',t to dete~rmine directly, as this nuclide is formed continuously be decay of 23sU. However, it is poss.ible to solve this problem indirectly by examir~ing 232Th. If this isotope is missing in the sample, it ~ieevls likely that also 23°Th was missing at the time of formation. All examined samples, with the exception of the w~lsendorfites from Lulep Manak (77:100A, 77:10 [ A and 790048 B) and a ur~ nin~te sample from Hfis, 0xnevalla (P 567), give no trace of a 2:32Thpeak in their spectra. Uraninite is the source of uranium in many of the secondary uranium minerals in this study (see description of samples). Uraninite usually contains some thorium, from some tens of ppm to several percent (Frondel 1958). Rich et al.

LITHOS 14 (1981)

194 R. La~,endaM & E. Holm Table 3. Activity ratios and -'~aages of examined minerals.

Aa age years

Locality

Samr ~e No.

Mineral

Sample type

R value

mUImU

S v',due

Viissings6 Lfgarberget Odsm~l Odsm~! Kungs~ra S6derboda Sfderboda Bergskiir Bergsk~

790093 790085A 790097 7c~)098 790104 790106 790110 790028 790030

H yd rogen-aut. Uranophane Boltwoodite Boltwood[te Phosphuranylite Boltwoodite Boltwoodite Uranophane Phosphuranylite

A A A A A A A A A

1.03 + 0.02 1.00 + 0.03 1.06 4- 0.01 1.03 + 0.02 0.96 + 0.02 1.03 + 0.02 1.11 + 0.01 0.93 4- 0.02 1.014- 0.01

0.0,6 + 0.003 0 . 0 ~ + 0.004 0.043 +_0.002 0.047 + 0.002

0 . 1 6 _ 0.ti)i 0.03 4- 0.;)04 0.03 _4:.0.:3C4 0.02 4- 0.003 0.114-0.01 0.02 + 0.003 0.03 + 0.004 0.05 + 0.01 0.03 + 0.005

19000 3 300 3 3OO 2 200 13000 2 200 3 3OO 5 600 3 300

Stenshuvud Stenshuvud Vattentornet L6garberget L6garberget L6garberget Tidaberg H~serud Hfirserud S61vbackti/irn Tosslisen Toss[isen* Kroktj~irnsvallen L ~mgtjiirp L ~ingtj~irn* L[ngtj~irn L~ngtj~irr~ Lilljuthat~en Viistra Rebraur V~istra Rebraur Pieutaj~lkk Pieutaj~kk* Pleutajlikk Pieutaj~kk* Pieutaj~tkk Pleutaj~ikk Pieutaj,~kk Lulep Manak Lulep Manak* Lulep Manak Lulep Manak* Lulep Manak Lulep Manak Lulep Manak Lulep Manak Lulep Mar, ak Kebnats Kebnats

"/90126 790171 790172A 7287:798 7287:799 790084A 790121 Da i 5/69 790101 7385:223 790118 790118 7588:529 7483:6011 7483:60111 7483:624 790063 790060 7280:303 7289:613 77:115A 77:i 16A 790004 790004 790005 79 9 b ~ 790129 77:100A 77:i00A 77:100B 77:i00B 77:101A 77:101B 790048A 790048B 790114 790045 790046

Uranophane Boltwooclite Boltwoodite Beta-uranophane Uranop., boltw. Boltwooclite Phosphuranylite U ranophane Uranophane Uranophane Cuprosklodows. Cuprosklodows. Cuprosklodows. U ranophane Uranophane Uranophane Uranophane Uranophane Uranophane Uranophane Wblsendorfite W61sendorfite U ranophane U t anophane Uranophane Uranophane ~ Uranop., kasol. W61sendorfite W61sendorfite Uranophane Uranophane Wflsendorfite B eta-uranophane Beta-uranophane W61sendorfite 9 Uranophane Uranophane

3 B q 3 3 3 3 B B B B B B B B B B B B B B B B B B ~ B B B B 0 B B E B B E B B

0.99 + 0.01 0.99 + 0 0 1 1.09 + 0.03 1.004- 0.01 1.07 + 0, 01 1.16_+ 0.02 i .04 4- 0.01 1.04 + 0.0 I 1.03 + 0.01 1.04 + 0.01 0.99 4. 0.01 1.04 + 0.01 0.99+ 0.01 1.06 + 0.01 1.01 + 0.02 1.01 + 0.02 1.01 + 0.02 1.06 + 0.01 1.16 4. 0.02 0.99 + 0.01 1.01 + 0.01 0.99 + 0.01 0.94 + 0.0 I 0.88 + 0.02 1.01 + 0.02 1.05 + 0.02 1.02 + 0.02 0.95 + 0.02 0.91 _+ 0.01 1.2 i +_0.02 1.18 + 0.01 i .01 + 0.01 0.96 _+0.03 0.91 + 0.01 1.00 + 0.01 1.00 4. 0.00 1.03 __ 0.01 1.01 _+O.01

0.046 + 0.001 0.045 + 0.001 0.050 + 0.004

0.50 + 0.03 0.12+ 0.01 0.50+ 0.03 1.06 + 0.07 0.30 + 0.02 0.55 + 0.04 0.59+ 0.04 0.12+ 0.01 0.19+0.01 0.50 + 0.03 0.18 + 0.01 0.19+0.01 0.18+0.01 0.31 _ 0.03 0.52 + 0.03 0.18 + 0.02 0.23 + 0.02 0.60 + 0.04 0.50 + 0.03 0.71 _+0.05 2.64+0.16 2.74 + 0.20 0.81 + 0.05 0.73 + 0.05 0.85 + 0.06 0.89 + 0.06 0.24 + 0.01 1.32 + 0.08 1.22 + 0.07 0.78 + 0.05 0.76 + 0.05 0.86 + 0.06 0.89_+ 0.06 0.98 + 0.06 0.98 X 0.06 1.68+0.10 1.30_+ 0.08 0.53 + 0.03

75000 14000 75OOO Equilibr. 40OOO 86O0O 97 000 14000 23 000 75000 22OO0 23 000 22000 40O00 80000 220O0 28 000 99000 75 000 135 000 :> 1 >1 180 000 140 000 205 000 240000 30000 >1 >1 165 000 155000 215000 240 000 E¢~uilibr. Equilibr. >1 >1 8O 000

Vattentornet Vattentornet Vattentornet L6garberget l.dgarberget 1.6garberget H aukiv aara

78:i 13 78:117 78: 119 78:053B 78: ! 26 78:127 790 i 27

Coffinite Zippeite Uranophane 'Thucl3olite' Beta-uranophane Zippeite Meta-autunite

C C C C C C C

1.16_+ 0.02 I. 18 __ 0.03 1.08 + 0.01 0.99 4. 0.04 1.06 4. 0.0 I 1.29 4. 0.02 0.99 + 0.01

0.90 ± 1.12+ 0.8 ! _ 0.89+ 1.01 + 0.91 + 0.98 +

250 000 >i 180 000 240 000 Equilibr. 260 000 Eqt~ilibr.

Stripa Stripa

7~0180 7~0184

Liebigite Liebigite

D D

0.98 + 0.01 1.10 4- 0.02

*Duplicate analyst:s.

0.047 + 0.046 + 0.044 + 0.O44 +

0.003 0.002 0.0O4 0.O04

0.048 + 0.002 0.049_+ 0.002 0.047 + 0.002 0.044 + 0.OO2

0.046 + 0.001 0.049 + 0.002 0.047 + 0.003 0.049 + 0.003 0.047 + 0.O03

0.050 + 0.003 0.047 + 0.002 0.O46 + 0.OO2 0.O45 .+_0.002 0.045 + 0.002 0.048 + 0.003 0.046_+ 0.003 0.049 + 0.0~2 0.042 4- 0.001 0.046 _+0.0O3 0.043 + 0.003 0.047 + 0.002 0.049_+ 0.001 0.048 _+0.001 0.045 _ 0.002 0.043 _+0.002

0.046 _ 0.002

0.050 + 0.002 0.050 + 0.003 0.050 4. 0.002 0.047 __ 0.002

0.66 0.07 0 a5 009 0.06 0.06 0.06

0.008 + 0.¢~)2 0.013 _+0.0.34

<8OO < 1 400

Radioactive disequilibria

LITHOS 14 (1961) Table 4. Aa ages and pa ages. (Aa ages calculated using eq. 2, pa ages using eq. 3 and 4.) Sample

Measured CalcuR value lated Ro value

Aa age years

Pa age years

790118 77:100B 2 77:100B s

1.0359 1.2092 1.1821

22 745 162 800 156 1(10

22 445 + 1 880 I 151 500+ 20 600a 147 000+ 17 700a

1.03821 1.3200a 1.2747a

One iteration step; 2 Duplicate analyses; "Three iteration steps.

(1977) state that hydrothermal uraninites contain little thorium. The examined uraninites in this study, except the sample from H~s, Oxnevalla, contain negligible amounts of thorium, since no trace of a 2~2Th peak can be found in their spectra. In the Oxnevalla sample a ~32Th peak has been identified. It appears that uraninites formed hydrothermally are poor in thorium, while uraninites in pegmatites may be thoriumrich. Weathering of the uraninites causes uranium to oxidize from tetravalent to hexavalent form (if not already hexavalent), form a uranyl complex and appear in a weathering crust on uraninite or be transported away. Thorium in this process remains inert (Cherdyntsev 1971; Pere~man 1977) if conditions are not extremely acid (Landa 1980), and will be enriched in the residual phase. Consequently precipitated secondary uranium minerals formed at ambient temperatures and pressures seem to be devoid of thorium. (d) The mineral system has been closed jbr U-Th since the time oJformation. A prerequisite is that microscopically fresh and uncorroded

material is used. As discussed above, under natural conditions it is preferentially uranium that goes into solution while thorium remair~s. In the case of a secondary uranium miner~d in radioactive equilibrium, dissolution and removal of a proportion of the uranium will result in an S-value larger thaa unity. However, in recently formed minerals with S-values just above zero, almost all uranium must be removed before this value exceeds unity. That is, it is only when a large proportion of the uranium is lost that open system behaviour can be detected. Table 3 shows that uranium has been removed from w61sendorfite samples collected at Pleutaj[ikk (77:116A) and Lulep Manak (77:100A). Sample 790114 (probably a metamict w61se~dorfite) from Lulep Manak is also impoverished in uranium, as the S-value is larger than unity. Another indication of the sa~e process is an R-value smaller than unity in sample 77:100A, indicating a fractionation of the uranium isotopes. This phenomenon is discussed under (e). (e) The Ro value must be known. This value shows large variation accor~iing to the literature (Cherdyntsev 1971; Osmond & Cowart 1976) As a rule it is larger than one in secondary uranium occurrences and in water and less than one in detrital sites. The reasons for 234U being preferentially mobilized compared to ~sU are complex. By decay of 23sU, the alpha recoil makes the daughter nuclide occupy a foreign position in the crystal, where it is more loosely bound. The effect of radiation increases metamictization of radiogenic minerals with time, decreases their cohesive forces and facilitates their breaking up and dissolution. Fleischer & Raabe (1978) have demonstrated that a recoil nuclei can be ejected out of a solid grain and that regions hit by intense alpha-emission are damaged. These regions can

Table 5. Uraninite samples in supposed equilibrium. Locality

Sample No.

Collector

R value

S value

H~s, Oxnevalla H ~ , Oxnevalla* H~is, Oxnevalla* Bj6r~und Pleutaj~kk Harrej~lkk Harrej~ikk* H arrej,$kk*

P 567 P 567 P 567 7373:639 7373:621 7373:630 7373:630 7373:630

Welin & Blomqvist Welin & Blomqvist Welin & Blomqvist Adamek & Wilson Adamek & Wi.~son Adal. ek & Wilson Adamek & Wilson Adamek & Wilson

0.99_+ 0.03 0.99+_ 0.01 0.~_+ 0.01 1.00 + 0.01 1.01 + 0.01 0.99+ 0.02 0.99 + 0.01 1.00+ 0.01

0.96+ 0.07 0.98 + 0.06 1.08 + 0.06 : .05 + 0.06 1.03 + 0.08 1.00+ 0.06 : .04 + 0.07 1.01 ± 0.06

* Replicate analyses.

19.5

LITHOS 14 (I~I)

196 R. LSfvenarahl & E. Holm ,1.

Table 6. Measured R values in different media. Medium

R values interval

Soviet rivers U-minerals, hypersene zone Oxidized uranium ores Drilled wells SW Finland Surface waters Underground waters

1.06-1.46 0.62-1.39 0.59-1.41 0.97-2.40 0.9-2.5 0.5-12.5

1.30

~--7-"

~

"'

m

'

,

, ' - -

t

~

~

,

m

R value mean

No. of samples

Reference

1.23 !.01 0.96

19 "I4 28 44

C h , = r d y n t s e v (1971) C h e r d y n t s e v (1971) R o s h o l t et el. (1963)

1.64 -

t

-

-

Mass data Mass data

• 12 ~

1,7 o

,

Asikainen & Kahlos (1979) O s m o n d & Cowart (1976) O s m o n d & Cowart (1976)

~

,

""

÷10

1.20 x X ~

D% .z

100~=100

~

-

2;,o ,;o ,;o

~

do

Ro=O.,S

,,

0;0 1;oo ,,'oo

ko

Fig. 2. Approach to equilibrium for different Ro values. I

o

so

~oo

~so

200

ka

Fig. 3. Percentage deviatit,n in age for different Ro values. !

ka = i 000 years, R = 2a'U/e3~U activity ratio.

ka = 1000 years, D= percent deviation of approximateapparent age (aa age) from precise apparent age (pa age).

subsequently be eroded by water with mobilizati~.~n of the daughter nuclide. These processes would be of importance in secondary uranyl mineral coatings with high specific surface area. Further factors have been proposed but not proved (Dooley et al. 1%6). Kobashi et al. (1979) have shown that thorium acts in the same way as uranium, i.e. the daughter nuclMe (22aTh) is naore easily mobilized compared to the parent

tion with an assumed activity ratio of unity (eq. 2). Fig. 2 shows the approach of different arbitrary Ro values to an R value of uniity with increased age by use of eq. 3. Fig. 3 demonstrates the deviation in percent of calculated aa ages (eq. 2) from pa ages (eq. 4) for a number of arbitrary but reasonable Ro values. Thiis figure shows that the deviation of the aa ages is moderate below 50000 years, but increases rapidly above 100 000 years. To check the approximate error in our aa ages (Table 3) of samples with R values larger than one, a few samples, those believed to satisfy conditions (a) to (e) best, have been iterated using eqs. 3 and 4 (Table 4). As a cl',eck, a few uraninite samples used for U.:Pb dating (Welin & Blomqvist 1964; Adamek & Wilsoq 1977) have been e~amined. Their R values are close to unity and they also show radioactive equilibrium between z:~°Th and 2a4U (Table 5).

(,':~2Th).

R values have been treated in several papers. The range of their variation in nature can be found in Table 6. Chenlyntsev (1971 ) has pointed out that this value is more stable approaching equilibrium it, surface ~,aters compared ~o grou "~d waters, where it may fluctuate violemly This conclusion i s supported by later data (Osntond & Cowart 1976, Figs. 2-3). Determination of the Ro value with an iterative method is used here t{~ demonstrate the hereditary bias in a straightfo.3rward aa age determina-

Radioactive disequilibria 197

LITHOS i4 (t~t) N

i

i

, [ !

!

i

,

;

i

,

,

,

,

I--I ='-r'-T

|

,

i

|

.

|

!

,

|

|

,

w

,

Ik

,

B

C

I

~0

100

150

200

250

300

ko

Fig. 4. Ae ages of samples belonging to the three different groups. 1 ka = 1 000 years, N = number of samples. (A) Minerals coated on the bedrock surface; (B) Minerals from cracks and fissures 3-100 mm below the bedrock surface; (C) Minerals from drill-cores more than 1 metre below the b¢drock surface. Not shown: Group B - three samples in radioactive equilibrium, six samples with R value > 1 (uranium lost by leaching). Group C - two samples in radioactive equilibrium, one sample with R value > 1.

Table 7. Measured R values, secondary uranium minerals, this work. Sample type

Surface Cracks, fissures Drill-cores

R values

0.93-1. I 1 0.88-1.21 0.99-1.29

R value mean

No. of samples

1.02 1.02 1.11

9 38* 7

* Includes 6 duplicate, analyses.

Geochemical discussion The basis for our discussion will be the vertical distance of samples from the bedrock surface. Three groups b~sed on this coacept were introduced on p. 193. The S vaktes for the three groups show that the numerical age is proportional to the depth of the ,;ample from the surface. However, there is sorae overlap in age in samples from groups B and C (Fig. 4). Evidently during the last millenni~ precipitation of secondary uranium minerals usually took place very superficially, more or le,,;s at the surface. From 3 mm downwards we hay e no indication of

II

8

6 /, 2

0.85 0190 0.95

1.00 1.05 i.10 1.15 1.20 1.25 130 R

Fig. 5. R values for the three different groups. R = n'UF2~U activity ratio, N - namber of samples. (A) Minerals coated on the bedrock surface; (B) Minerals from cracks and fissures 3-100 mm below the bedrock surface; (C) Minerals from drillcores more than ! metre below the bedrock surface.

secondary uranium mineral formation during postglacial times, except in an active mine. On the other hand removal of uranium is indicated from a number of samples from groups B and C. The overlap in age between groups A and B might be accidental, as the ages for group A samples older than 10 000 years are uncertain, as is evident from the following discussion. Thus, samples collected as coatings on the bedrock surface (Fig. 4a) have been formed recently. Minerals from cracks and fissures just below the bedrock surface (Fig. 4b) are older than the surface samples and give ages from 14000 years to equilibrium, i.e. more than 300 000 years. Some minerals in this group have lost uranium by leaching, resulting in an S value larger than one. The drill-core samples are rather old, with a minimum Df 180 000 years (Fig. 4c). The R values also exhibit differences between the three group.,; (Fig. 5a-c), summarized in Table 7. Although the number of samples of groups A and C are small, an increase of R values with depth is indicated. Our results ac-

198 R. L6fvendahl & E. Holm cordingly seem to be consistent with Cherdyntsev's theory, that is, the R values are larger in ground waters compared to surface waters or, expressed differently, the activity ratio is larger at depth compared to the surface. The majority of secondary uranium mineralizations are found in close connection with 'primary uranium minerals'. In some cases the secondaries are found as weathering crusts directly on the ~urface of primary black oxides, i.e. LfingtjSrn, Lilljuthatten and Pleutaj[ikk, or in spatial relation to a 'primary' mineralization, i.e. Vattentornet, L6garberget, Toss,sen and Lulep Manak. In some cases no 'primary uranium mineral' is known in the neighbourhood of the secondaries (e.g. V~issings6, Odsm~i and S6derboda), while in other cases there exists primary uranium minerals in the neighbourhood (e.g. Stenshuvud and Bergsk~ir). However, these seem unlikely as the uranium source since few, if any, descriptions of natural leakage of uranium from minerals like monazite or aHanite exist. The source and precipitation mechanism for some secondary uranium occurrences, in particular coatings on the bedrock surface, are unknown. These matters are the subject of an ongoing program on mineral formation and stability.

Secondary uranium minerals and palaeogeography Several factors probably influence the formation of secondary uranium minerals, the most important being the continuous physical and, especially, chemical weathering. Other factors of importance in Sweden are glaciation-deglaciation, euslatic changes in sea-level, vertical isc,static movements at the surface of the earth, change:; in the level of ground water, changes in the composition and conditions of precipitation. The common denominator is the presence or absence of water. Development of the Weichsel (Wfirm) glaciation must have played a key r61e in the formation of secondary uranium minerals. There are different opinions as to the areal distribution of its ice-cov,~r. The earliest hypothesis supposed a more c.r less complete ice-cover over Scandinavia during the last glaciation. Recent research has indicated a more complex history, with alternating glacial regimes (stadials) and non-gla:ial regimes (interstadials) (Lundqvist 1974; van den Harnmen 1979).

LITHOS 14 (1981)

Formation of secondary uranium minerals from primm'y uranium oxides takes place through solution of uranium. The oxidized uranium is transported as the urav~yl ion (UO[ ÷) or a complex of this ion in aqueous solution. Water is essential for transpo~ of uranyl ions and consequent precipitation of secondaxy uranium minerals in cracks and fissures. Dry glacial environments, such as a glacial sheet frozen to the ice-bedrock interface, will prohibit secondary uranium mineral formation. Thus there are two different situations where formation of these minerals has been possible. (a) Melting of the ice with uncovering of the bedrock surface. In this case secondaries would form by subaerial precipitation from water solution. (b) Glacial ice with a wet ice-bedrock interface, i.e. existence of water in contact between the ice and the underlying bedrock. Urai~,~um minerals would be precipi~.ated subglacially. Subaerially the temperature might fluctuate some tens of degrees, while subglacially it would be just above the freezing-point of water at the pressure in question. The Scandinavian glacial chronology is far from certain, but some main lines can be discerned. Lundqvist (1974) and M6rner (1976) have proposed time scales for the Weichsel Glaciation in Scand~navia (for Lundqvist's time scale see Fig. 6). Although M6rv,er's scale is not chronologically fixed, there is obviously good agreement between their scales back to 22 000 years BP, and reasonable agreement back to about 40000 years BP. Between 40000 and 130 000 years BP the time scale is not resolved, and beyond 130 000 years BP (Kukla 1970) it is rather speculative. Concerning minerals occurr~m ~s coatings on the bedrock surface, only two c~ut of nine samples indicate aa ages greater than 10 000 years. The sample from Kungs~ra (Table 3) with an aa age of 13 000 years seems suspect, because it is corroded and its R value is less than unity. The age of the mineral from V~issings6, 19 000 years, must be regarded as uncertain until more analyses have been done. Minerals from cracks and fissures just below the bedrock surface (group B in Table 3) are most interesting in relation to the last glaciation. Of 38 samples, 20 fall within the time-span l0 0C~)-I00 000 years. Of these, | l samples indicate aa ages between 10 000 and 40 000 years and nine between 70 000 and 100 000 years. No mineral indicates an aa age bet.ween 40000 and

Radioactive disequ~libri,:z 199

LITHOS 14 (1981) 1000Yn, BP

NORTH SWEDEN FINLAND

SOUTH S W E D E N CONTINENT NORWAY DENMARK BRITISH ISLES

°I Aller~d

LATE VALDAI Bryansk Y.Di~sebacko-Ellesbo

C~ta al,v Gudbrandsdolen Glurmsli~v

Heng~lo

,3q''°°

Moershoofd 0. Di~sebacka-E|les~o Koruki.i|o

Periipohjola •,,~ ~oOlaciatioa ::

Fig. 6. Tentative diagram of the extension of the Weichsei Glaciation (after Lundqvist I974).

LEVEANIEMI

70 000 years. It is very interesting to note that four samples from central Sweden (790118, 7588:529, 7483:624, and 790063) indicate aa ages between 22 000 and 28 000 years. Two cuproskIodowsk;te samples from Toss~Isen and Kroktjhrnsvallen are partictdarly interesting. They occur in boulders a few km apai~ in the same type of quartzitic bedrock with similar minerals present. It seems probable that they originate from the same mineralization, a conclusion that is supported by the agreement in aa ages, 22 000 to 23 000 years. The question is what this age means. The time scales of Ltmdqvist (1974) and M6rner (1976) are consistent on this point, about 22 000 years BP was the start of the latest glacial advance, leading to the ice maximum 18 000 to 26 000 years BP. As mentioned above, two more samples from central Sweden (7483:624 and 7'4)063) give aa ages between 22 000 and 28 000 y mrs. For secondary uran;um minerals to form under a glacial sheet, we must ihave a temperate l~acial cover with water at the ice-bedrock interface. This possibility is quite realistic. I,ocal wet ice-bedrock interfaces ha,,e been postulated for the Antarctic ice sheet. I he distribution of wet and frozen ice-bedrock interfaces have been

Odderade

Chelfc:d

Amersfoort

MIKULINO

EE M

calculated (see Embleton & King 1975). The existence of both interface types have beep demonstrated in drill-cores from Greenland (Colbeck & Gow 1979) and possibly also Antarctica (Raynaud et al. 1979). Boulton (1972) has classified differe~,t glacial types, from bottom frozen to completely water-drained. There is usually an approach of temperature to the melting point of water with depth in large glacial ice sheets (Colbeck & Gow 1979: Raynaud et al. 1979). The salient point is whether the melting poinl of water is reached at the ice-bedrock interface. Nine samples collected all over Sweden give aa ai~es between 70 000 and 100 000 years. These aa ages coincide with the Eem interglacial tLunt!qvist 19'74) or predominantly interstadial conditions (Mbrner, pets. comm. 1980). As t i e chronology of tats period and beyond is specuJarive, and we do not know the exact meaning of our aa ages, it seems premature to discuss them further. The maximal extension of the sea since the last glaciation is well known in Scandinavia. The highest shore-level wvs not synchronous and water composition at these high stands may have been marine, brackish ,~r fresh. The depressed

LITHOS 140981)

200 R. L6fvendahl & E. Holm land has risen gradually since the maximal glaciation, starting 13000 years BP (M6rner 1979). The rise has been strongest in easterncentral Sweden. All sampling sites in the southern third of Sweden and along the Bothnian coast are situated below the highest shore-level. The sampling sites at L6garberget, Odsm~, S6derboda and Bergskiir have risen above sealevel during the last 5 000 years. There is a strong tendency for group A minerals to be found near the coast. The only exception is the sample from Kungs~ra, but this sample is found near Lake M~ilaren, which was isolated from the Baltic about 700-800 years BP. Uranium is strongly soluble in both marine and fresh water (Rogers & Adams 1969). In the Baltic there is a positive correlation between increased salinity and uranium content (Koczy et al. 1957:94). The frequent occurrence of coatings of secondary uranium minerals on the bedrock surface in the neighbourhood of the coast can partly be caused by slight weathering in sites just risen above the sea, and the paucity of lichen and moss cover in these areas. Former coatings on bedrock surfaces situated inland are probably weathered away or hidden by vegetation and moraine. During the last decades there has been an accelerating change in the composition of precipitation over northwestern Europe. The increase in acidity (sulphate), in particular, is a problem in southwestern Scandinavia (Odin 1971). The effects following this change are very recent, but might influence the uranium/thorium ratio in secor:dary uranium minerals. In particular, minerals coated on bedrock surfaces might be affected. According to Langmuir (1978), uranophane for example is least soluble at pH values between 5 and 8 ~.ad! increasingly soluble with decreasing (or increasing) pl-i. It is a common impression that the yellow secondaries coating bedrock surfaces, for example along the Swedish east coast, have faded in the last 35 years (Frans E. Wickman~ pers. comm. 1976). This can be caused by the uranium being transported away, while immobile constituents such :~s thorium re~nain in situ. This would result in an erroneously high age. The sample from Kungs~tl'a (790104) seems suspiciously old, which is possibly also the case with one sample from Bergsk~ir (790028). Both samples also have low R values. The local copper smelter at RSnnst:~ir, i5 km SE Bergsk~ir and emission from the heavily industrialized town of V~ister~s, I0 km

W Kungskra might be local triggers for uranyl dissolution. Hence these samples may indicate a solution mechanism caused by environmental pollution.

Improved techniques for disequilibrium dating of secondary uranium minerals The reliability of the mineral ages can be greatly improved. The following steps are essential in further age determination of secondary uranium minerals. (1) Improved sampling in the field. The possibility of solving Well-defined problems means that future sampling must be more precise, particularly regarding sample depth, occurren.e of c.ther secondary minerals formed nearby, primary mineralogy and so on. Our registration of these facts is rather chide in this reco,naissance work, in many cases we have not done the sampling ourselves. (2) Thorough examination of the sampled material under a binocular microscope to check that it is :~resh and uncorroded, and accurate removal of the examined material. (3) Use of a tracer with exactly defined thorium/ uranium activity ratio (preferentially a tracer in radioactive equilibrium). The errors of age in this work are mainly due to uncertainty in the activity ratio of the tracer. (4) Measurements of the nuclides aaau, 235U, 234U, 232Th, ~3°Th, 227Th (or 23~pa) for determination of the independent ratios 23°Th/2asu and

231pa/235U. (5) At least duplicate analyses of isolated samples to have a check of their homogeneity. (6) Examination of the approach to closed system conditions in the mineral by determination of 226Ra (half-life 1622 years) and ~°Pb (half-life 22 years). Provided the prerequisites listed above are taken into consideration, the use of secondary uranium minerals for dating Quaternary events is well worth examining further. Especially under temperate to arctic climates, these minerals might approach closed system conditions. Promising fields are dating of glacial-interglacial events and examination of radioactive nuclide

LITHOS 14 (1981)

transport in cracks and fissure,~, The paucity of secondary uranium minerals and raaterial might locally be a limitation. However, other secondary minerals and material may .also be worth examining. Acknowledgements. - Chemical work and alpha spectrometry measurements have been done at the Department of Radiation Physics, University of Lund. The manuscript has benefitted from criticism by G6ran Aberg, Niclas MSrner, David Rickard and Eric Welin, all in Stockholm ~nd Jan Lundqvist, Uppsala, David Rickard -also scrutinized it linguistically. This work would not h~-ve been possible without the imagination and continuous support of Frans E. Wickman, Stockholm.

References Adamek, P. M. & Wilson, M. R. 1977: Recognition of a new uranium province from the Pro.cambrian of Sweden, 199215. Int. At. Energy Agency (IAEA-TC-25/16), Vienna. Allb,gre, C. 1964: De l'extension de la m6tkode de calcul graphique Concor~a au~ mesures d'0,ges absolus effectu~s l'alde du d6s6quilibre radioactif. Cas des min~ralisations secondaires d'uranium. C. R. Acad. Sci. Pa,is, 2.59, 40864089. Asikainen, M. & Kahlos, H. 1979: Anomalously high concentrations of uranium, radium and radon in water from drilled wells in the Helsinki region. Geochim. Cosmochim. Acta 43, 1681-1686. Boulton, G. S. 1972: The role of ~hermal r~gime in glaciat sedimentation. Inst. Br. Geogr., Spec. Publ. ,~, 1-19. Chanda, R. N. & Deal, R. A. 1970: Catalogue of semiconductor alpha-particle spectra. 159 p. U.$. At. Energy Comm. Res. Dev. Rap., NTIS, Springfield. Cherdyntsev, V. V. 1971: Uranium-234, Isr. Programme Sci. Transl., Jerusalem. Colbeck, S. C. & Gow, A. J. 1979: The margin of the Greenland ice sheet e,t lsua. J. Glacial. 24, 155-165. Dooley, J. R., Granger, H. C. & Rosholt, J. N. 1966: Uranium234 fractionation in the ~andstone type deposits of Ambrosia Lake District, i~lew Mexico. Econ. Geol. 61, 1362-1382. Embleton, C. & King, C. A. M. 1975: Glaciaf Geomorphology. Edward Arnold, London. Faure, G. 1977: Principles o f Isotope Geology. John Wiley & Sons. Fleischer, R. L. & Raabe, O. G. 1978: kecoiliog alpha-emitting nuclei. Mechanism for uranium-series d~s,.~quilibrium. Geochim. Cosmochim. ,~cta 42, 973-978. Frondel, C. 195~: Systematic mineralog7 of uranium and thorium. U.S. Geol. Surv. Bull. 1064, 400 !~p. oar, den Hammen, T. 1979: Changes in life conditions on Earth during the past million years. K. Dan. Vidensk. Selsk., Biol. Skr. 22:6. Munksgaard, KObenhavn. Heath, R. L. 1976-77: Tables of Isotopes, B 270-354 in West, R. C. (ed.), Handbook of Chemistry aw~' Physics, 57 ed. CRC Pres~, Cleveland. Khlopin, V. C. 1926: C'n the migration of rad~oelements on the earth's crust (in Russian). £)okl. Acad. Nauk SSSR, 178-180. Kobashi, A., Sara, J. & Saito, N. 1979: Radioactive disequilibrium with uranium, thorium and radium isotopes leached from euxenite. Radiocher~. Acta 26, 107-111. Koczy, F. F., Tomic, E. & Hecht, F. 1957: Zur Geochemie des Urans im Ostsecbecken. Geochim. Cosmochim. Acta 11, 86-102. Kukla, ]. 1970: Correlations between loesses and deep-sea sediments. Geol. F6re~. F6rh. 92, 148-180.

Radioactive disequilibria

201

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