Nishimura, Susumu

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RIGHT:URL:CITATION:AUTHOR(S):ISSUE DATE:TITLE:Variation in the Content of Uraniumand Thorium across IgneousContactsNishimura, SusumuNishimura, Susumu. Variation in the Content of Uranium and Thorium across IgneousContacts. Memoirs of the College of Science, University of Kyoto. Series B 1965, 31(4): 227-2381965-02-27http://hdl.handle.net/2433/258289AilE"IoiRs oiv [vnT.", CeliLE'GE oF SeimtsTcis, Utstivis'Rsl'ry oi" KyoTo, SERIEs B,   Vol. XXXI, INTo.4 G="oi,oGy and Mi.N}iRAi,oGy, Article 3, 1965Variation in the Content of Uranium and Thorium               across Igneous ContactsBy              Susumu NISHIMURAGeological and Sy,Eineralogical Institute, University of l<yoto              (Received Nov. 13, 1964)                               Abstract   Recently, variations in the content of uranium and thorittm were traced acrossigneous contacts of the two intrusions of Tanakami and Koya with practically con-tinuous samples by the use of a cornbined technique involving an alpha-scintillationcounter and a radioscope. The results of this determination show that the types ofthe distribtition of alpha-radioactivity across igneous contacts found by a radioscopedepend mostly on the content of uranium. On the other hand, the thorium contentvaries marl<edly from sample to sample, but the running averages of it near thecontact are nearly constant and equal to that of the imier part of the intrusive body.                              Introdiiction   As reportecl bef'ore, '} 2) 3) `} the profile curves showing the alpha-raclioac-tlvity distribution across igneous contacts were found to be classifiable intofive types4} which are presuniably corresponding to the conditions underwhich the contact phenomena took place. It has been a naatter of great concernto cleterinine to which eleinents these raclioactivity distributions are due.   HArL"u])A and the writer offered a methocl for determining tlae content ofLii-aniuin cftnd thoiritim in rocks by ineans of a conibined nieasureinent with analpha-scintillation counter and a i-adioscope with a Lauritsen element. 5) By thismethod uranium and thorium content across igneous contacts have been obtained.L)28 Susuinu NiiE i!'[}iuR.tuAnalytical procedure   The procedure of these measurements using the alpha-scintiliation counterand the radioscope with a Lauritsen element was t"uliy clescribed in the otherpaper.5} Hence, in this paper it will be suf'ficient to give the formulae witha briet' explanation. The uranium and thorium content, U and Th in ppm areglven as     Th = -tLO.7 s - l.21 i•i. -•-•••••••••••t••••••••-•t.....•••.....••••••.(1)     U = 29.9S - 1.e8 Th -+••••••+••+••i•••••••-••••••••••••••••••••-(2)where s is the shifting rate of the r-aclioscope and 'lz the counting rate ofalpha-scintillation cotmter.Samples   Along two of the typical traverses across igneous contacts which hadbeen proved experimentally to be remarkably different in radioactixritydistribution, 3N} sampling was made. So far as collditions of exposure permit,the samples were taken as nearly as possible at a regular interval of 10 cmover the whole range of the sampling. The Iocations of these traverses areKoya, Kyoto Pref. and Shishitobi•(Tanakami), Shiga Pref. In order to clarifythe distribution of elements semi-quantitative spectrographical analyses hadbeen triecl. '`) The variation of alpha-radioactivity observed by the radioscopeand betatl'adioactivity by the low-backgrouncl gas-flow cotmter were alsoslaown in the previous papers.3) 4}   The variations in the grain-size of the essential minerais, i-n the quantitiesof quartz, orthoclase and plagioclase by semFniodal analyses and in thecontent of the halo inclucled in biotite are shown in Fig. 1.ResuKs   The variations in the alpha-radioactivity obtained by theA•vo dif'ferent methods with practically continuous samplesand Koya were shown in Figs. 2-1 ancl 2-2, respectively.   Using the values of the alpha-radioactivity thus obtained,of' uranium and thorium have found by the Eqs. (1) and (2),ln Fig. 3-1 and Fig. 3-2.above-tpentionedfrorn Shishitobitheancld3stributions are shown" Soa g+sto . g en:Fo g o D = Q " _ N ca _ o+t$Fig.'Å}t1 The variations in  quartz, orthoclase  halo.o o s  t s o " E > = Q K co -- co the andn o =o'-+'n o --'`'["s' g or i o gl+x$fgrain-size, theplagioclase, and= n oo $o Q"" gc`x' ga :y_ o eou!IOSjDOOrelative amountin the abundanceof ofm = m = H o m  <   pe  :'  sc  =-  o   =   -'  x   tt  =   o   o   o   p --- r-yco o II O c-..Ooth "va gro si I= fig' :+ 9S m-  pt  =   Q   F:J;  rr  o   ej  :'  :   s   sc  o   e   o   vi  Vs Lt t" Lett ve "= o H = -• b o _ oo oe s a an-" e-_ o sN op oo v s'= o r = th o pcoon x Qa!' -re_ o sor o  R ro<•xogE() ! g st pZ' " ct, : "' e pa >Fig.2-1 The variation in the alpha-radioactivity obtained  radioscope (the upper figure) and by the scint/illation   (the lower' figure). (Shishitobi)by thecounterVariation in thedivlrniB;.o0.5ocpm20Content of Uranium   Igneous Contacts      KOYAand Thorium acrosshorntelso   5graniteIOm231        10         o                      0 5 IOm            hornfels grqnite    Fig. 2-2 The variation in thealpha-radioactivity obtained by the radioscope           Cthe upper figure) and by the scintillation cottnter (the lower           figure). (Koya)                         Discussion of the results   The variation of' the alpha-radioactivity across Shishitobi contact observedby the radioscope was classified as the first type.i) 4) The difference betweenthe alpha-!-adioactix•'ity by the radioscope and that by the alpha-scintillationcounter is accounted t-or by the different sensitivities to iiranit"n and thoriuinof the two apparatuses.   As shown in the Fig. 2-1, the variation of alpha-radioactivity by theaipha-scintillation counter (Iower) has many points ot lil<eness to that bythe radioscope (upper). So it is also to the variation of uraniuni contentobtained by the combined measurement (Fig. 3-l). As a general tendency, theconcentration of uranium clecreases as the distance from the contact bounclary232ppmo20Susun)u iN/Isl-{T}[URA    SHISHITOBIhornfel$ Tho    5graniteso rn          o                       0 5 iOm             hornfeSs                         granite    Fig. 3-1 The distribLitions of uranium (the upper figure) and thorium (the          lower figure). (ShishitobDincreases, whi]e thoriuin behaves in the reverse inanner, shox•ving a siiglatillcrease towards the center of the intrusive inass.   The variation of the alpha-radioactivity across Koya contact observedby the yaclioscope was classified as the third type.i} 'i) The sinijlarity in theprofile curves of the alpha-activity across Koya by the radioscope and by thescintillation counter is iess than that in the case of Shishitobi. The tolerable           Variation in the Content of Ut'anium and Thorium across 233                            Igneous Contacts        ppm K0YA        2o          10         00 5 tOm            hornfels granite        ppm          IO          o                       0 5 IOm             hornfels granite    Fig• 3--2 [1"he distributions of uranium (the upper figure)and thorium (the          lower figure). (Koya)cuantities of Ltraniunll were found in hor'nfels, 1)ut tlitoriuni scarcel>r was foundthere. [["he thoriuin content varies niore rnarl<edly than the uranitnn one,showing a tendency of increasing against the contact piane. This differencecan be attributed to the different. geochemical character of the uranium andtlaoriLim.   rlL"he uraniuin is an easily Ieachable elenient, nanieiy, cluring the niagniaticcycle, uraniuin as well as thoriuni were in the tetra-valent state, and thecrystallization of the both elenients goes parallel owing to 'the ciose siniilaritiesin ionic radius, but at a very late niagniatic stage, a change took place whj.chcaused the uraniuna to be oxidjzed into a hexa-valent one, easjly movable withthe hydrothermal solu'tion, ]eaving the thoritim behind to crystallize. Theo' bservecl results may be well explainecl by tliis relation.    The variations of Th/U across these contacts revealed the similar tendencyof an increase with increasing distcance from the countr>r rock (Fig. 4).234. Susumu ,NtT.s}-IIslup.!                               3         2         l         0            hornfel6 granite         3         2         l         o                     0 5 IOm        horntels                                   granige     Fig. 4 The variations of ThlU across Shishitobi contact("Lhe upper figure)           and Koya contact (the lower figure)            The factors controlling the distribution of uraRium                      and thorium in granitic bodies    In the previous paper it was int'erred t'roin the viewpoint of radioactivitythat the distribution of volatile n}atter in .ftR intrudeci bocly m.ay be associated);5/I.itiiaitiLeielllffa;i'giicCoeuiiliryteilXS.fsr.atUre anci piessttre between gianitic me}t (or-    It was reported 'D that there was iio reinaka'ble difference jn the clistributionof K`Ocontent in intrLisi.ves of' the dift'erent types ciassifjed by the alpha-radio-actixJity. Thjs fact suggests that the conditions during the cyystaliization ofpotash-feldspay have litt}e diffeyence in any granitic bodies. Besides, as theTh/U 'K0YA'            Varlation in the Content of IJranium and Thorium across L)35                             Igneous Contactsuraniuni nnoxres as a volati]e niatter under ]arge clif2ierences in teinperatttreand. pressure between gra]iitic body and counti"y rocks, it can be depos'ited inquantities witliin thin iayer of the contact, but this tenclency is less reniarkablewith thorium, as has been n'ientionecl.   NTear the co'ntact ii:t shallower depth, the teniperature ot' the intrytsivebody falls abruptiy, g'iving rise a sharp grcftdient of ten'iperature. Accordingly,tlRe vapour tension is so decreased that the diffusion of vo'latile inattertroni inner part to"Jards the contact zone takes p]ace brisk]y. On the conti-ary,"iith a sniail difference of teipperature and pressure x•vhich is conceivable necftrthe contact of great depth, uraiiiuni n]ay• b3 deposQted eciually over theintrusive bcdy. 'IL"hi's concept is supported by the variat'ion ot graill-size acrosstlie igneous contact of a granitic body.   Diffusion of uraniuin inay be con'ipared with the dit'fusion ot' po]lution inair through the grid. The poHLition in air at a high ve]ocity dift-uses in astate ot' turbulence, but ln air at a iow velocity, it cloes so jn a steaclymacnner. In tlie case of type III,4) the uraniuni diffuses through the jntersticesof the ci'ystals of quartz ancl fe]clspar and so on in a coRdition of turbulence,but iR the case of types ]I,IIII and ISI,'f) it does so in a steady manner.    A two dhnensional case may be assumecl, i. e., the part oti v})o wasintrLisive body and that of xff<umo co'untry rock. The cooling laistory anciconditions of ai'i int/i'usive bcdy weye described in 1/1'}e previous papei-. '" Irfencein this paper, the CHffusion prob]eni of uraniuin only was treated.    rl"lie diffusion of tu'anitim along the x-a>.(is wi]l be described as theecluat1on,      8s -= ,a.(i,(x)ggl ••• •• • .. ..,. (s)where c is tlie concentration of the uranjuin, L` the thne, .x the horizontalco-ordiBate, caBd f(c) the Iioyizontal dift'usion coefficient. Fjrstly, we supposethe st'trface of int.rusive body i=o to be impenetrabie or almost impenetrableto the diffusion of radioactive eleinents.                                       n    X,Xie shali take the chemical g'radient -gC to be equa] to the temperatui-e                                       oxgradient at till]e, t.    In the steady state, rc(v) may be constant, tl3en we get l'rom Eq. (5),      g2f - ,,<.) ZiiSl,- •••••••••••i••••••••••••-•••••••••••••••-i•i•••••••-••-••ii••••••• (6)    In the turbulent clift'usion, ig(t') also ]nay depend on the ten)peraturegi'adient. In this case, we shall take the foiiowing bounclar>,' conditions:           2-C -L 0 • c --A co ••••••"••••••""•••••••'•••••••••••••••••'''''•'• (7)      ti ( c)-           OX x_>o ' .z'->h236 Susumu Ni•Is}II.MuRAwhere h is the width of the turbulent region; the zone of o:f xs{h is theturbulent part ancl the part of c l) lt As jn a steacly state.   Nrow, we shali f'incl the basic solutioB of the equation (5). X'Vhen aninstaittaneous unitrorm distribution in a granitic body is observecl. at tlae momentt ::= o, 'th3s solution sa'tis'fies the initial condition:      c(.tr, t) - cx•••••••••••••••••+•••••••••••••••••••••••••••-•••+•••••••t-••••••••- (8)             t->o   In accordance with the siiRilarity theory 'for the turbulence through theparcftllei screen developecl by LAc])"Ell (1950), tlie coef'ficient of/ rc(z]) has thel'orm,     tc(`x) = ei,• (x) •••••••••••••••••i••••••••-••••••••••i••••••••••-••--+••:•••••••••••••• (9)where 0iJ' is called the unidimensional eneirgy spectrun] tensor and shown int}le 11ext figul"e.     eFig. 5From this conceptioll, the variations in thethe homogeneous medium, 'in the case ofcase of the turbti]ent diffusion, inay roughiy                                     g                                    E                                    i                                     co  Fig. 6 The variation in radioactive O=        element in the homogeneous 8        medium        1: Steady diffusion        2: Steady diffusion with a mo-          derate       temperature giradieiit3: Turbulent diffusion with a severe temperature gradientThe variation of eij.r-directioncontent of radioactivethe steady ciiffusion be shown in the nexttowardselenaen.tsand infigure.contact x illthe           Variation in the Content of Uranium and Thorium across 237                            Igneous Contacts   The radioactive elements in rocks were founcl generally as tlae 'impuritiesin or arouncl crystals, so that tlaese heterogeneous areas are apt to causeirregularity in clistribution, especially in the latter case. This description ofthe diffusion of utraniuni inay be Iinked very closely i,vith the resul.t of themeasurement, as the first appro>cimation.   A part of the xJ•ariations of the uraniuna content across igneous contactsinay be explained in this way. On the other hancl, the variation of thoriuincontent seemcl to suggest the absence of sucli diffusion as consjdered 'in 'thecase of uran'iuin in liexa-va]ent forni.                               Conclusion  1) The types of the alpha-raclioactivity distribution aci'-oss contact found bya radioscope depend niostly on the content of uranium, showing less relationwith that of thoritim.  2) The variation of thoriuin content is naore conspicuous than that oful-anium across jgneous contacts.  3) The variation of TIzfU across the two contacts showed a similair tendencyof lncrease with the increasing clistance from tlae country rock.  4) Uranium is more leachable than thorium.  5) At a very late magmatic stage, tiranium is oxidized from tetira- tohexa-valent compound and carried about with the hyclrothernaal solution,leaving the thorium behind to crystallize.  6) The variation observed 'in alpha-radioactivity by the raclioscope is wel'1explained by the diffusion of ui'anitnn in a steady state, btit in the case o'ftype l[ , the turbulent di'ffusion niust be taken htto consideration.                           Acknewledgements    The wiriter is greatly indebted to the 1<Snd guidance by 1'rof. Z. I-IA'1"'cr.l)Athrovtg'hottt the present study. Lcastly this 'investigation hcfts been part]ysupported by the financial aid of the Scientific Research Expenditure of theAilinistrv of Ed.llcation.                               References1) I/IAFruDA, Z. and S. NTism"wRA, Mem. Coll. Sci., Univ. Kyoto, B. 23, pp. 285-295       (1956)2) H,x'rvD.t, Z. and S. ISTism"iuRA, iY/[em. Coll. Sci., Univ. Kyot.o, B. 25, pp. II5-123238 StlStlMU NIsl•{TMURA3)4)5)6)     (1958)Nism.uuRA, S., Mem. Coll. Sci., Univ. Kyoto, B.NismxuRA, S., .ix'Iem. Coll. Sci., Univ. I<yoto, B.H.x'ruDA, Z. and S.Nig.y=tuR.x, in this volume, pp.L.-Nufi-r,R, .I ., Joui'. Aero. Sci., 17, p. 277 (1950)28, pp•28, pp•21.5-226255 -264 (1961)L) 6 5 -2 8 ti ( 1 9 6 1)(1964)

Variation in the Content of Uranium and Thorium across Igneous Contacts

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Variation in the Content of Uranium and Thorium across Igneous Contacts

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