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Mineralogy And-Rare Earth Geochemistry of Apatite And

Mineralogy And-Rare Earth Geochemistry of Apatite And

American Mineralogist, Volume 60, pages 607-620, 1975

Mineralogy_and-RareEarth Geochemistryof and Xenotimefrom the GloserheiaGranite ,Froland, SouthernNorway

Rrmen Aur-r Mineralogisk-Geologisk Museum, Sarsgt. I, Oslo 5, Norway

Abstract

The Gloserheia granite pegmatite, situated in the Precambrian Kongsberg-Bamble forma- tion, contains eight different zones (core, intermediate zones I-V, wall zone, and border zone). Apatite occurs in the core and in zones I and llI. has been found included in apatite from all three zones, and in other parageneseswithin these zones. The apatite close to xenotime inclusions is depleted in REE. Zoningof xenotime is characterizedby an enrrchment of the light REE relative to the heavy REE, from the center towards the rim of the cryslal. Xenotime included in apatite from the outer zones show lessrelative enrichment in the light REE than do thosein the inner zonesof the pegmatite.No suchtrend in REE fractiona- tion is shown by xenotimes from other parageneses.The rare earth included in apatiteare consideredto have formed by exsolution,possibly accompanied by metasomatism. Fractionation in the apatite-xenotimesystem was causedby processesbearing a simple rela- tion to the crystallization of the zones where theseminerals occur. ln contrast, the REE frac- tionation in the other xenotimesprobably has a more complex relation to the sequenceof zones in the pegmatite.

Introduction termediate zone I, where it is found as roughly equidimensionaleuhedral-to-subhedral The Gloserheia granite pegmatite is situated in the crystalsup to a sizeof one meter. Commonly well Precambrian Kongsberg-Bambleformation, in Fro- developedforms include a prism, pyramid (occasionally land commune approximately 9 km N of Arendal two sets),and pinacoid, (8"43',36'E, 59.32'20' N). basal and crystals are sometimesdoubly terminated.The color is dominantly medium to The pegmatite has been briefly described by dark green, but sometimesgrayish to reddish. Andersen (1926, l93l) and the geology of the Arendal-Gloserheia ar ea by Bugge (1943). According In the core, apatite is much lesscommon, medium to green to Bugge the rocks are of both granulite and dark in color, and generally smaller and bet- ter amphibolite facies. of rocks in developedthan in intermediate zone I. The the Arendal area has yielded agesfrom 850 to 1300 forms are the same. It is closely associatedwith m.y.; ages for the pegmatitesare from 875 to 1030 microcline perthite and sporadically with lM m.y. (Broch, 1963). muscovite. The pegmatite is V-shaped and exhibits in general a Apatite occurs in intermediate zone III as infre- well developed and regular zoning. The following quent yellowish-greensubhedral to anhedral crystals zones (Cameron et al, 1949) can be distinguished: from a few mm to several cm in length. Very rarely, core, intermediatezones I-V, wall zone, and border colorless euhedral crystals, several mm in size, are zone. Table I shows the composition of the found embeddedin calcite(all later discussionrefers, different zones. however, to the first mentioned type). This study concerns apatite and xenotime from In intermedi ate zone I, veins of colorless apatite- different zones in the pegmatite, with special some with calcite, small amounts of hematite, and emphasison rare earth geochemistryand the genesis more rarely mm-sized xenotime crystals-cut the of REE-bearing inclusionsin apatite. earlier apatite crystals, these veins presumably representinga late stageformation. It is not known if Apatite these "veins" extend outside the apatite. Apatite is among the most abundant minor All apatite from the Gloserheia pegmatite contains minerals in the pegmatite(Table l), especiallyin in- variable amounts of inclusions.Dissolution of 6.088 607 R, AMLI

Tlsr-e l Mineral Compositionof the Different Zones in the revealed euhedral-to-anhedraland short-to-long GloserheiaPegmatite prismaticxenotimes (Fig. l, I-IV). Whenviewed with Core Lt. tut. ht' Int. ht. YaU 3older microscope(X 65), xenotimeappears zoae I ?one II zone III zore Iv zone v a binocular or light yellow. '-rn o2-1n ro@ lo ctr 5-8 n O 1-1.5n ro cD colorless

il Studieswith the universalstage show that all long Q1Etz lAUMU Ulcrocline u AAMA ?fagloclase uuMAil u prismatic xenotimesare oriented with their c axis Biotlte AAAl L/r ^/r r(?) 1(?) r(?) r(?) (as is true of long 1[ T parallel to the apatite c axis T T T T I T prismatic crystals). Some inclusions Apatite T T T Euenlte T (plagioclase?) haveaxes of elongationparallel to the Calclte T ?Fite T T SaFetlte T T apatitec axisbut showinclined extinction. Randomly T T T T orientedequidimensional inclusions (both quartzand alsooccur. The long prismaticxenotimes T T xenotime) Chlorlte occur throughout the crystalsbut tend to be con- Ka6olrte centrated along planes or lines (Fig. l, Vl). Urulnite frorite T Equidimensionalinclusions (mostly xenotime)also T not always Heoatite T tend to definesuch surfaces,which are [: {aln nlneral planar. Long prismatic and equidimensionalinclu- A: Accessoly Meral T: Tlace ninerd sions frequently occur together in highly varying amounts in such distributions(Fig. 1, VI). The kg of apatite from intermediatezone I in 6N HCI apatite shows a weakly undulating extinction adja- yieldedl.l5 wt percentinsoluble inclusions identified cent to these planes/lines.The featuresdescribed as xenotime,quartz, plagioclase (?), , rutile, above are observedin apatitesin all zones' pyrite, and geothite.Their habit variesfrom euhedral Xenotime also occursas euhedrallong prismatic to anhedraland from short prismatic to extremely brown crystals,up to one cm long, on the facesof long-prismatic(Fig. l, I). Examinationof apatitethin apatitesfrom the coreand intermediatezone I. These sectionswith electronmicroprobe (semi-quantitative) havetheir c axesparallel or subparallelto the apatite and optical microscoperevealed, as additionalinclu- prism facebut are otherwiserandomly orientated. In sions,calcite (?) in apatitefrom the coreand from in- intermediate zone I, xenotime is also found as termediatezones I and III, and thorite in apatitefrom euhedrallong prismaticmm-sized brown crystalsin intermediatezone III. calcite"veins" cutting apatite. Similar inclusionsin apatiteshave been reported In the same zone, colorlesseuhedral mm-sized by severalinvestigators. Taborszky (1962) has found xenotimesoccur rarely bs the latestmineral formed zircon,hornblende, and micaoriented mostly parallel (growing on quartz, muscovite,and rutile) in the to the apatitec axis in the granitic to gabbroicrocks replacementof microcline. of the Odenwald province, West Germany. Xenotime has been found in only a few samples McKeown and Klemic (1957)observed monazite, from intermediatezone III. Two different associa- bastnaesite,and hematite in apatite from the tions are recognized;xenotime-euxenite-tourmaline- magnetitedeposits at Mineville,New York; Pigorini rutile-muscovite and xenotime-euxenite-calcite- and Veniale (1968) found monazite, xenotime, muscovite. Muscovitization of the plagioclaseis plagioclase(?), and (the long prismaticinclu- much more pronouncedin the latter paragenesis.The sions oriented parallel to the apatite c axis) in the xenotimefrom both paragenesesoccurs as up to ca 2 granitic to granodioritic rocks of the Val Sessera X 6 mm euhedralcrystals with brown color. province,Italy. Microprobeanalysis of anhedralinclusions (up to - Fracturesin the Gloserheiaapatites tend to besub- 100pm across)in the xenotimefrom the tourmaline parallel to prism or base in accordancewith the associationshowed Ca, Si, and Ti as the main ele- reported parting for apatite (Tr6ger, 1959).Long- ments. The inclusionsare thereforeassumed to be prismaticcrystals crossing a in the apatite sphene. showedno lateraldisplacement even on the pm scale. The colorlessxenotimes have <.rp' = 1.713+ 0.004 andthe yellow xenotimes <.rtq6 ) 1.713 + 0'004. Xenotime Monazite Xenotime is the most abundantmineral found in the apatitefrom all zones.Examination by scanning Monazite has been found only as inclusionsin electronmicroscope and polarizingmicroscope has apatite. It has been observedin matrix only 1n RARE EARTH OF APATITE AND XENOTIME 609

FIc. I'Inclusionsinapatite.I: Scanningelectronmicroscope(SEM)photo(X400)showinggeneraltypesofinclusions.II:SEMphoto(x 2000) showing a short prismatic xenotime. III and lV: SEM photo ( X 3500; X 3000) showing xenotimes with staircase-liketerminations. V: Photo (x 500) showing needlesofxenotimes oriented parallel to apatite c axis. VI: Photo (x 55) showing part ofa curved face or line defined by parallel-oriented xenotimes.

material from the quartz core, where it occurs as Analytical Methods colorlessanhedral grains (up to -100 pm) in contact with xenotime.It is alsopresent, in amounts(l per- The analyseshave beencarried out using an AnL- cent,in the materialdissolved out of the apatitefrom rvx microprobe with a 52.5" takeoff angle, housed intermediatezoneI. Hereit occursas anhedral yellow in the Central Institute for Industrial Research,Oslo. grains up to t mm across, which are easily The analytical methods and the empirical correction distinguishedfrom the xenotimesby their yellow factors used in this study were those described by fluorescenceunder short wave light (not Amli and Griffin (1975). filteredthrough cobalt-glass).The identificationwas The error bars in the figures (except Fig. 2) corre- confirmedby X-ray powderpattern, chemistry, and spond to the relative standard deviations quoted bv optics. Amli and Griffin (1975\. 610 R. AMLI

contains0.71 percent YrOr, comparedwith a YzOs content ranginggenerally between 0.09 and 0.l2 wt o ^^^ percentfor apatiteadjacent to xenotimes.Deviations rC I from this patternreflect excitation of xenotimeinclu- spectrum e2 sionsby fluorescencefrom the continuous and/or by characteristicfluorescence, as in the case E 1 of analysesno. l0 and no. 23 inTable 2. owqY lrom inclusions I Apotite is probably accom- O Apolitc cocxisling wilh REE-rich The lower content inclusions (numbcr rcfcrs to paniedby a loweringin the contentof the restof the toblc 2 ) REE, but this is only weakly indicated by the analyticalresults, because of the poor precisionof I., theseanalyses. I Zoning is a common feature in all types of xenotimes.Figure 3 showsthe zonalvariation in Y, Yb, and Gd in a xenotime(68-13, see Tables 2 and 3) edgc ol qpqtile . & 0.4mm edgc of opotitc (growth fdce) (cryslql locc) includedin apatitefrom intermediatezone III' Y and Frc. 2. Yttrium concentrationprofile (,1 c axisin axialplane) in Yb are enrichedin the central parts of the crystal apatite69-l from the quartz core. Note the low Y content for while Gd is depleted.Figure 4 shows the core- apatiteadjacent to rare-earth-richinclusions. Anomalously high REE distribution in the rim and an in- profileprobably arise from interferenceby normalized and low Y valuesin the profile in xenotimeand quartzrespectively. A low yttrium contentcould also termediate position, along the shown bedue to the analysisbeing taken near to a xenotimethat hasbeen Figure3. As can be seen,there is an increase(if most removedduring the cutting.Error barson a high and low Y value weight is placed on the elementsthat have the best indicateanalytical precision. On the "growth face" apatiteis in analyticalprecision) in the ratio of the lighter to the contact with microcline. Simultaneous(?)growth of the two heavierREE from the core to the rim in the crystal. probably of mineralshas resulted in an irregularsurface composed Pr, and an oscillatorycombination of apatiteprism and pyramidforms, as Sinceno data havebeen obtained for La, Ce, havebeen seen on largeapatite crystals. Nd, it is not known if theseelements show the same relative enrichmentin the rim. Cursory studiesof other xenotimes(whether included in apatiteor not) AnalyticalResults indicatethe presenceof the sametype of zoningas Analyses of xenotimes,, and shownby xen. 68-13.Quite anothertype of zoning, from the Gloserheiagranite pegmatite are presented however,has been found for xen.29-2m(see Table 3 in Table 2, andTable 3 givesa key to the analysesin and Fig. 5). Figure6 showsthat thiszoning probably Table 2. reflectsfirst a decreasein the ratio of light to heavy In additionto theelements in Table2, Na, Mg, Fe, rareearths and then an increase,so that the REE dis- Mn, As, Sr, U, and Th have been looked for in tribution when crystallizationceased was nearly the severalrandomly selectedsamples of each mineral; sameas when it started. theyare present only in verysmall amounts ((0.1 wt The REE distributionsfor xenotimesincluded in percent) except for Na, which comprisesapprox- an apatitefrom the qvartzcore are shownin Figure imately0.12 wt percentin apatiteshaving abundant 7. Becausethe curves are smootherthan the error amountsof rare earth inclusions.Ca contentsare bars indicate they should be, and becauseall the (0.05 wt percent for xenotimesand monazitesin- analyseswere done in the sameday, the analytical cludedin apatite. precisionis consideredto be much betterthan is in- The poor precisionfor the REE in the apatiles dicated.The ratio of the light to the heavyrare earths analyzedadjacent to REE-rich inclusionsprecludes in the xenotimeinclusions increases from the center detailed discussionof REE distribution for these of the apatitetowards the rim. The only pronounced apatites,since any variations are consideredto be deviation is the depletion in Nd and Sm in the within the analyticalerror. However,Figure 2 shows xenotimelocated at or very closeto the rim of the that the apatiteis zoned(asymmetrically?) with respect apatitecrystal. The trend seemsto be unaffectedby to yttrium and that the yttrium content is definitely the fact that xen. 5 and xen. 3 occur in contactwith lower for apatite close to the xenotimeinclusions, monazitegrains. along roughly the sameprofile. Apatite 85 (anal. no. Figure 8 showsREE distributionpatterns for ran- 8, Table2), whichhas almost no REE-richinclusions, domly selectedxenotimes included in apatite from RARE EARTH GEOCHEMISTRY OF APATITE AND XENOTIME 6II

TenI-B2. ElectronMicroprobe Analysis of Xenotimes,Monazites, and Apatitesfrom GloserheiaGranite pegmatite

atrom. atom. atom, alon. alon. aton. aton. Xfenent wt.y'" prop. wt.% prop, wt.% prop. wt,% prop. wt.fi prop, wt.% !rop. prop. na r' na na na 1.12 0.8541 na na o.36 o.o12l o.+a 0,0154 A,21 0.0071 0.0019 nd. O.23 0. OO7? O.3'l o,ol04 34.34 o.9a77 0.9846 14.91 34.52 O.9929 14,62 o,9961 4a.94 3.0000 35,t8 O,9923 34.37 o. 9896 Ca0 nd nd nd nd 55.1'1 5.1111 nd nd Y^O- 46.35 o.a1a3 47.21 o.eJ59 46.25 0.A361 42,4e 4,7674 0.09 0.0041 46.15 0,8'tt3 44.O3 o.7964 la^0- nd nd nd nd nd nd nd Ce nd ZO3 nd nd nd 0,04 O.OO12 nd trd PY nd zaj nd nd nd nd nd nd Nd^0_ W w nd O.22 0.0026 O.17 0, 0020 sn^0- o.21 o.oo24 0,28 o,oo12 0.26 0,oolo a,74 0,0087 o.04 0.0012 O.49 0.0056 0,49 o, oo57 Eu^0- nd nd nd o.15 O,OO41 nd O.11 O,OO15 O.OT o, oooS Gd^0- 2,01 o,o?34 1.69 0.0187 2.50 0,0262 +,72 o.4532 0.05 0.0015 1,91 0.0211 2,69 o,o1o3 o,56 o. 006 o.4O I o. oo44 o,49 O.OO54 1,O5 0,0117 nd o,42 0,0046 o.5B 0.0064 Dv^0- 0.0557 3,a2 0.0410 5.4t O.0594 7,O2 o.0768 0,10 0,OO27 3.e4 0.0410 4,91 o.0540 Ho^0- 1.20 o,o1 '1.27 lo o.87 0.oog2 1.11 O.O1I 9 1.23 o,o1J1 nd 1.01 0.0107 o,o't37 Er^0- 3.67 o.at92 3.60 o,0376 3.+3 0,0166 3.47 a,a17o nd 1.78 O,o193 4.05 o.o412 tm^0- 0.006l o.62 o"0064 o.67 0.OO?1 0.61 0.0065 nd 0,68 O,OO7O O.?O o, oo?l Yb^0- o.o391 4.O4 0.0410 3,1A O.0129 2,93 0.0104 nd 4,25 O,O429 4:36 O;0452 lu^0- 4,55 0.o056 A.r9 0.0060 0,41 O,OO42 0.0019 nd 0.86 0,0086 o.BT o. o0B9 1'31 Sw 98.74 ,#; 98,46 ,,7 98.1B 99,11 98.89 Struct. (REE). ^^^- aFxn\ - t.vty) '.--.1 (mE)r.oz+e (mE)r.oro (ca, REE) (mE)o,ggr, (mE)r.orqr ,4034 5 .1 22c trror./1.6696 (P,si)1.s6es (P,si)i.oooo \- '"r/1.oooo \r 'v!/l.o0oo (?,si)r.oooo (P,s1)r.ooo0 ro, s54t

12 13 aton, aton. atom. atom, Elenent wL.y', prop. atom, aton, a tom. wt.fu prop. wt.ft prop. vrt./" pro!. wt.y'" prop, prop. wt.% prop, F 2,92 o.7931 na 2 '96 0, 8076 na na si0^ 4.49 o.4425 o'41 0 .0119 o. 60 o,051 6 o,44 0.0148 A.17 O.OO5B O,26 O.OO89 0.28 O,0096 40.6'7 2,9575 34,25 o. 9861 44,36 2,94e4 14,69 O,9852 34.A1 O,gg42 34.29 O.9911 34.r, o.9904 Ca0 .47 54 5,01 47 nd 54.76 5,0622 Y^O- 4.71 o,a12o 46,71 o,a452 o.67 0 , ol09 4r.72 O.A162 46.3O O.B3'14 47.40 0.8611 4.1.55 0.a616 nd nd nd nd nd nd nd 0.10 o, 0015 nd o.12 o. o016 nd nd. nd nd Pr nd nd nd "0, nd nd nd nd Nd^0_ 4,12 o, 0016 o.1B 0.0054 0.09 O,OO11 nd nd Sm^0- o. 05 0.0014 o,rB 0.0044 0,07 0,0021 0.20 o,oo23 o.2O O.OO23 0..16 O,oo19 o..16 o.oo1q Eu^0- nd nd nd 0.06 0. oo07 nd Gd^o- 0,10 0.0028 2,O5 o.0231 0.11 0,0010 1,4A 0.0164 1.76 0,0197 1,46 0.0166 1.47 0.0.166 Tb^o- nd o.52 0.0058 o.o? o.oo18 A.37 O.Oo41 O.44 0.0049 O.11 O.OOJ? O.75 O,OOr9 Dv^O- 0.11 0.0029 4.24 o.o464 o.14 o.0ol9 3,A2 O,0413 4.?8 O.0466 3.6A O.O4O5 3.74 O.O41O Ho^0- nd o.94 0.0101 nd o.99 0.0106 1.01 0.0109 0,95 0.0101 1,02 o.O1.t1 Er^0- 0. 04 0.0010 o.0360 nd 3,81 0.0404 1.90 0.0414 3.8O O,O4O8 Tln^O- 1,18 O.O4O5 nd o.47 o. oo49 nd o,67 0.0070 0.66 0.0069 0,61 0.0065 0.61 o.0066 tb 2o1 0,08 o. 0020 4.24 o,0439 0.09 0.0024 4,66 O.0477 4.56 O.0469 +.46 O.0464 4.44 0,0461 lu^O- nd o. oo8, nd o.95 0.0097 0.61 0.0062 0.59 o.0061 0,62 0,0064 1,2j 1.24 Sw qBJi tB.e' 97.91 94.76 ffi (..,*")l.oe (*")., rs (*r)o.sseo ::::".. .orff 1.",*r)r..,.,r, \'-" t 1 .O174 (m)r.or:g (Rrx) (t,rt)r.oooo (?,si)r.oooo (P,si)4 (p,st).-^^ r .orlr o^n^ ,..-oo (P,si)r.oooo (P,Si)1.OOOo - '0. o.79J1 B076 differentzones in the pegmatite.Also shown is the towardsthe core is analogousto that observedfrom REE distributionfor a xenotimepartly embedded in the centerto the rim of a singleapatite crystal and the prism faceof an apatitefrorn intermediatezone I. shownin Figure7. Xenotime84 alsofits this seheme, This pattern of increasingrelative enrichment in the as can be more easily secn from Figure 9, Frorn lighter REE from the outer zone of the pegmatite Figures7 and 8 it is furthermoreseen that the degree otz R. AMLI

Test B 2, Continued

16 1B 19 21 aton. aron. aton. aton. atom. at on. atom. prop. wt,% Efenent wt.y'" prop. wt .y'" prop. wt ,/" plop. wt./" ProP. wt ./" prop. wt.y'" ProP' 1.0371 tr' na na na na na na t.B9 si02 o.32 0.0109 o.3+ 0.0117 a.22 0.0076 o.2a 0.0096 0.1B 0.0065 0 .68 O,0272 na .25 0. 34.15 0.9924 34.2a o.9904 34.49 a.9937 2A.66 a.912A 42.41 3.OOOO PzA5 3+.19 0.9891 14 9BB' '4175 Cao nd nd nd nd nd nd 55'52 5 Iza3 47.88 O,8706 +7.71 0. 8554 47.13 O.4512 46.46 0. B4l8 41.78 o,7925 0.96 0.0204 O.1O 0.0048 LazAj nd nd nd nd nd 11.05 a.1633 nd CeZA3 nd nd nd nd nd 12 19 0.4725 nd PT2o3 nd nd nd nd nd 5.71 O.Oet4 nd Nd20l nd nd nd nd 4.32 0.ool9 17.73 o.253A O.06 0.0018 Sm20l 0.11 0.0011 0.20 0.0024 o .23 O.OO27 o,23 A.AO27 A.61 O.OO71 2.61 0,036A nd EurO:) nd nd nd nd 0.11 A.AO13 nd nd Gd2ol 1.21 A.4139 1 .39 4.4151 1.63 0.OlBl 1.77 O.02OO 2.69 a.o3a4 o.75 0.0099 0.06 0.0018 Tb2a3 a.2, 0.oo28 o.34 0.0018 a.+2 0.0047 a.42 0.0047 0,57 0.0061 nd nd DyzaS 3.60 o.o191 3.58 o.4191 1.91 O.0427 4.19 0.0451 4.BO a.ar26 a.27 0.0014 nd HorO., 0.84 0'0091 1 .05 0.0',114 1.A2 0.0111 1.OO 0.0108 1,2+ O.0134 0.16 0.0020 nd Erza3 1.64 4,4191 J .AA O.0415 3.61 0.4385 1.56 O.A+1+ 4.11 0.0446 O,07 O.O0OB nd 1^2aj 0,58 0,0061 o.65 0.0069 0.60 o. 0064 0.68 0.0072 o.71 0.0075 nd nd tbzol 4,41 o.0459 4.+S 0.0466 4.26 O.O4+1 +.49 0.0467 4.14 0.0487 0.10 0.0012 0,06 0.0015 I'D2A3 0.58 0.0060 a.66 0.0068 o.55 0.0056 o.60 o.0062 0.90 0.0092 nd nd 1.67

Sm 97.63 9A.12 99,27 1OO.% loo,o?

Struct. (RXE). (RxE)1.s39e (mE)r.ozlt (Rxn)r.oz9e (mE)r.orz5 (run)r.o+el (ca'Rxx)5.ozT4 form, ro "",. (P,si)r.ooOO (P,Si)r.oooo (?,si)t.oooo (?,si)1.oooo (?,sl)3.ooo0 \!,v!/1.OOOO",i'"'=' '*'"-/1.0000 _F 1 ,0377

2',1

ato m. aton. atom. aron . at on. at on. aton. prop. wr.tu pro p. prop. wt .y'. prop. flt./" Element prop. wL,'rt prop. !rop. na 1,0297 na Fna 3.BA 1.0190 na na o.oo57 na si02 a.25 0'0087 na a.22 0.0075 0.51 0 .0206 o.9794 .9a l.oo0o 1+.ae a.99+1 41.62 I.OO0O PzOS 33.6e a.9911 +1.79 l.oo0o 34.18 a.9925 2B.51 41 5.0416 ,5.64 5.a76o 55.39 5.4j25 nd nd 5r.65 CaO nd 0.0051 o.87 0. 0187 O.1 O o .0048 +1.96 o.7815 o,12 \za3 43.12 o.7971 o.2a 0.0129 43.47 A.1877 0.1402 nd nd nd LaZA3 nd nd nd 9.18 a.+567 nd nd nd cuzoT nd nd nd 30,71 o. oB59 nd nd nd Pr2O3 nd nd nd 5.42 a.aoz+ 20.43 a.3o15 0.06 0.OO1B o. 21 0. oo26 0.oB Nd2o3 a,15 O.Oo44 0.10 0.0010 o. 4e 0.0059 .59 0.0501 nd o.71 0.oo85 nd Sn2O3 0.61 O.oa73 nd 0 .87 0.0102 1 nd o.24 0.0027 nd 0.09 o.o01o nd O.12 0.0014 nd Eu2ol 0018 0.81 0.0111 0.05 0.0015 1.59 A.O4A2 o. 07 0. Gd20l 2.67 0.0708 o.05 0,0015 2.a6 o.a323 nd 0.70 0.oo78 nd rb2o3 a.55 o.0062 nd O,55 0. 0062 nd 0.0026 nd 5.41 0.0587 nd nyzal 4.77 o.o51+ nd 4.66 0.0511 0.20 nd 1.25 A.A131 nd HoZo3 1.23 a.4135 nd 1.18 0.0128 nd nd 3.81 O.O4O5 nd o .0436 nd l.BB 0.0415 nd Er 2O3 3 .99 nd a.65 0.0068 nd hzaT o .75 0. oo8l nd O.65 0.0069 nd 0 .0018 o.09 O.OO11 nd 3.73 a.o1a3 0 .07 \b2o1 +.65 O.O491 o.o9 0.0021 4,31 0.0448 nd a.62 0.oa63 nd Lu2a3 0.85 o.oo89 nd a.a2 0.0084 nd 1.64 1.62 1A1.42 99.99 99.91 Sm 97.56 98.45 (mE)r.orlz (Ca,REx)r.out., (c",i.nE) (mx)r.oo9z (REx)r.o6Bo (ca,RxE)5.0517 Struct. \ru!/. ^r/, olzo 5. (r,)1/1.OOOO (?,si)r.oooo (?,si)r.oooo form. (p,sij;";;;o (P,si):.oooo (?,s1)r.oooo (P,si) r .oooo -F ' '1.0190 1 .4297 1 .106'7

and P + Si = 3 far apatltes Atom. prop. are normalized to P + Sl = 1 for xenotimes anal monazites wv; V{1ld va1ue. nd: Not detected. na: Not analyzed. RARE EARTH GEOCHEMISTRY OF APATITE AND XENOTIME 613

Tnrr-e3. Key to Analysesin Table2

Anal , Mineral, no , Smple Renarks

1 B1-n EuhedTal xenotine (m- Analysfs retresents averaEe slzed), associated'with of-i6-T; ;;5i;-i;;;";;;-;F- tourmaline, rutile, anirreguluinlionogenity) euenite ed nuscorite for each eleme4t on a cross in int. zone lIL Sphene section near the center of ______r!_!9}!g_gE__rtglgtl9!!:-__ !!g_sryg!el.______-_____ 2-+ 6a-11, ^ Core, intermediate and Oriented with the c-dis xen. r-J ram respectively of a I)aralleI to the apatlte subhedral xenotime c_dis. Shows positlve (-5OxeO,,a), included in .t-*iur.o". a anhedTal apatite fron ____-_int,_zone III. 5 6a43, ap. ap"iil"-.a;""";;-;;------;;;;;;;;;-;;;;;;i-;;;;;--- xen. 68-13. in a region ru15r fr6n -___!!9_=9!9!rsg-i49lgEi9!:_____ 6 P3/68-1 2 Euhedral xenotime Analyses are from m area (!xi m) associated of approx. 5x5A , asswed with nuscovite euenite to be-fron near the center ild calcite in int. va urrs rr,y-uda. ______29!9 _I_rI: ______7 A+ Xuhedral xenotine (ro- Average of B anafyses (,trn1i & slzed) ^fron the prism Griffin, 1e7s) . Ea;h ardfysis face of a apatlte from are from the sane are. oi -lll_llll l_ _iiititalli?lfialitiiliil 885 nuhedral dark green Arlthnetic mea of three apatite fron lnt. analyses of graing taken out zone 7. of approx. 1/4 cm) apatite. The iietita |.e v6?v-f5v' ____l!9_199t9t9:-__-______9 69-52 n, Xuhedral xenotime Oriented with c-ryis xen. 4 incl-ded in apatlte from parallel to Ehe apatite inl. z,one I. c_axis. 4.27y'. apatite is ____!B!lr?e!9q_!r9s_grsly!19.____ 10 69-52 m, Apatite adjacent to Cowts fTom several spots ----_-_ep.-4-______69:9?_s._r9rr:_4:______!L24__!r_gg-lbs_srys_tai.______11 P5/69-11-1 xuiedral xenotime (m- The malysis is asswed to sized) fron a fractwe be fron ned the center of an apatite with newly the crystal md are from forned apatite, plus an mel of approx. 5x5A. calclte md henatite. ______Int:_zone f. 12-18 q/2e-2 n rur"arli-;;;;;;-?;:----il-;;;;;;;-;;;;;;-;------Lo. 1-1 sized) assoclated ,;ith cross (-tl+"t/i *) quartz, muscovite md near the""6tion center of the rutlle in altered parts crystal, No. 1 is at the of .int. zone I microcline.center and no. T at the rin of the crystal, The dlstance between the aalvsed ____ps+l!e_9rg_Iggcblx_sggel:_-__ 19 69-1, Subhedral xenotime Xlongation is perpendiculil xen, 5 (-1OOx2OOr) included in to t;e apatlte c_dis. a eu.hedral apatlte Direction of the xenotine (t,4*t.g cm)-fron the c-dis has not been deter- quartz core. (t_11 the mined because of inter_ following are fron the ference from wderlylns sme apatite). monaziie. The xenoiin; is located in the centex of the apatlte md approx. 8000,r fron the prisn face, taal- ysis is fron the center of the xenotime. O,16y'" ap. iE ______:5:ir_ I yt: s_::y!t_ i L1:_ glJg1Jp_j!:

of enrichmentin light rare earthsfrom intermediate lightto heavyREE for thesexenotimes (except 68-12) zoneIII to the coreof the pegmatiteis much greater relative to those includedin apatitefrom the same than from the core to the rim of the apatitecrystal zones. from the quartzcore. Consideringthe REE distributionpatterns ob- Discussion tained for the xenotimesnot includedin apatite(see This discussionwill consider(a) origin of inclu- Fig. 9), it is obviousthat the regularfractionation sionsof rareearth minerals in apatite,(b) rareearth trendseen in the xenotimesincluded in apatiteis ab- fractionation in the apatite-xenotime-monazitesys- sent.Other deviations are the decreasein the absolute tem, and (c) rare earth fractionationfor xenotimes amount of the lighter REE and the lower ratio of not includedin apatite. 614 R, AMLI

TlsI-r 3, Continued

.Anal, Mineral, no, Sample paragenesis etc. Renarks

20 69-1, Anhed.Tal nonazite Analysis is from applox' mon.5 (-100x1O0,1) in contact the sile spot. 1.39h with 59-1, xen. 5. apatite has been sub- ;i--'--e;:1-,*------;;;;;;;-;;;;;;;;;;-a:;-iiffiil;tli';ili;il-iiiti'- ap.5 xen./mon.5. approx. lSAfron.the -----I9!:1 99!:-]!9199]91:------22 69-1, Subhedral xenotime Oriented with c-axis xen. 4 (-2ox2001)' paxal1e1 to the apatite c-axis. Distance from aPatite Prisn face is 25OAt\. 0.61% apatite is - subtracted fron the ;t-'- -'z;:;-,'------;;;;;;-;;;;;;;-;;- 3l*tiii';;-;;;;;i-;il;-- ap' 4 69-1' xen, 4' approx, 1),1/ iron lne xen. inclusion. 24 69-1, Ar*red.ra1 xenotine oriented wi-th c-axis *.en. 3 (ry20x30rr). parallel to the aPatite c-axis, Di8tance fron aDatite lrism-0.49F face i.s loOOra . apatlte is subtracted fTom the -319lYgi9:---- 25 69-1, Anhedral monazite Analysis is from applox. mOn,7 ('1ox30A) in contact the sme spot. 1.9ry viitfr e9-t, xenr 3. apatlte subtracted fTon ------lhg-enclxgl9r------26 69-1, Apatite adiacent to Cowtlng on several spots ------li:-l------ll:l:-l:::1::::-::------iEllzil;-llfiiiiE'lli------69-1 ' Anhedral xenotine C-axis is not paralfel to xen. 2 (. lox8oA) . a.patite c-axis. located very close to the apatite prisn face, probably less tha 5oA (the wcertaintY is rel-ated to renoval of apatite drfing cutting and polishing). Ana1ysis is ffom center of the grain in an area of approx. 5x5p., 0.14F apatite cubtracted fron malysis. 69-1 , Apatj.te adJacent to Couhted at sevefal spots ap, 2 69-1, xen, 2, approx. lSAfrom the xen.

Origttt of Inclusionsof Rare Earth Minerals in Apatite marized by Seifert (1953). Staircase-likecrystal morphology due to simultaneousgrowth of two A simplifiedapproach to the problemof formation minerals(irr contact)has been describedby Haynes inclusions rare earth mineralsin apatiteis given of of (1959),and the forms observedon some of the by the following models: xenotimes(III and IV, Fig. l) couldhave originated I. The rare earth minerals formed before the irr such a way. On the other hand, oriented apatite that locally surroundsthem, and were "irtclusions" in mineralsate often consideredto be depositedon the growing faces. exsolution phenomena;for instancevein perthites ll. The rare earth minerals crystallized simul- (Lavesand Soldatos,1963) and orthopyroxenelamel- taneouslyand in contactwith the growingapa- lae in clinopyroxene(Bown and Gay, 1960), tite. If the xenotimes,in accordancewith model l, IIL The rard 6arth mineralsformed after the apatite formed separatelyfrom the apatite,the low content by a. exsolution,or b. exsolution* meta' of rare earthsin apatiteadjacent to xenotimeinclu- somatism. sions (seeFig. 2) cannot be explained.If, however, These three models have been consideredpartly the xenotimes formed very close to the growing becauseinclusions in minerals(with a mutual orien- apatite, and very rapidly, this could deplete the tation of the two crystal structures)can oliginate mineral-formingfluid in REE, and lower the REE from growth of the "included crystals"on the faces contentofthe apatite.According to Jahns(1953) and of the host minerall various examplesare sum- Schneiderhdhn(1961), giant crystals (such as some of RARE EARTH GEOCHEMISTRY OF APATITE AND XENOTIME

the apatites)are believedto haveformed in a volatile_ 28 rich silicate melt of very low viscosity,with rapid diffusionof componentsand formation of few cryrtal \" 26 "---- nuclei. If this is true, it seemslikely that the liquid would reequilibrate(at leaston the scalein discus_ sion)faster than the apatitecould grow and hencethe apatitewould show no depletionin the rare earths zl around the xenotimes.Furthermore, Jahns' (1953) and Schneiderhiihn's(1961) proposal that very few nuclei were formed has definitelynot beenthe case v for the xenotimes.For thesereasons it is assumed t! that modelI cannotexplain the formation of the rare v. earth mineralsincluded in apatite. O Model II is rejectedfor the obviousreason that the apatiteon LU oneside of the xenotimemust have crystal_ .J lized beforethe inclusion,and henceis not likily to be depletedin REE (as shownby Fig. 2) evenif the apatite crystallizing simultaneouslywith the a xenotimewas depleted. 08

06 J' 480

46.0

lrlr 0 Sm Tb Dy Y Er TmYbLu 12.0 lonic nadius Flc. 4. Spotanalyses in the core,intermediate region, and rim of 68-13/xenotimeI (alongthe sameprofile as shownin Fig. 3) nor- malizedto the core composition.In this and the followingcom- parison diagrams,ionic radii are for REE8+in 8-coordination (Shannonand Prewitt, 1969).

If we now considermodel IIIa, it is quiteclear that 50 rare earth minerals starting to exsolve from the apatitecould depletethe surroundingapatite in 45 REE. Exsolutioncould take placeas temperaturefell, and 4.0 be due to oversaturationof the apatite with rare earths. However, several authors (Rass, 1964; 35 Lindberg and Ingram, 1964; Young et al, 1969: 30 Youngand Munson,1966; Lyakovich, 1962; Denisov et al, 196l; and Omori and Konno, 1962)report 25 apatitesthat havetotal REE contentssimilar to, or 2.0 higher than, thosein the Gloserheiaapatites and yet contain no rare earth mineralinclusions. Of these 15 apatites,only that describedby Omori and Konno is enrichedin the heavyREE; the othersare enriched in the light REE. In view of the REE distribution Ftc. 3. Electronmicroprobe scans showing y, yb, and Gd dis- of the tribution in a profile,l_c axis for 68-13/xenotimeI includedin apatite describedby Omori and Konno, and the apatitefrom intermediatezone III. Themore irregular curves for y presenceof monazite inclusionsin the Gloserheia and Yb are due to increasedrecorder sensitivitv. pegmatite,it is assumedthat any exsolutionis not 616 R, AMLI

controlled primarily by the distribution of the rare earths.The apatiteswith no rare earth mineralinclu- sionsmay havecooled under conditions which did not promoteexsolution. For an exsolutionmodel several mechanisms, each involvingnon-stoichiometry, can be considered' According to Cruft, Ingamells, and Muysson (1965),apatite has the followingstructural positions: l(Ca,REE.. . . .),B(P,Si . . . .),C(O), D(F'CI'''') with the stoichiometricproportions: AuBgCnDr If xenotimeor monaziteexsolve from a stoichiometric apatite,elements in the A, B, and C positionsare remouedin the atomicproportions I : I :4. The result will be an apatitewith vacantsites in theA, B, and C positions.Exsolution of the other phasesidentified in the apatitewould enhancethis non-stoichiometry. The following schematicreactions can thus be set

tr A o X c F___-1_oo4_f Frc. 5. Variation for some selectedrare earth oxides (those with best analytical precision) from the core to rim in a cross sectton (-t/a X Vzmm) near the center of a xenotime (29-2m, seeTa6le 3; associated with muscovite, quartz, and rutile in replacements of microcline in intermediate zone L Error bars for Sm, Gd, and Dy equal the size of the cross.

rO 2,, trl,l : *----'-^ O) in a --_-_ a -09 I,IJ c o dos

E N o 2J - Zm xen 1 (11) o oB o -\N 2 l1Z) 08 A 3 (13) o o X L Jtl E (15) o l 5 o a 6 {16) I 7 t17l

Sm Eu Gd lonic radius

in an lonic rodrus+ f'rc. l. ngn, distribution patterns of xenotimes included quartz core. Xenotime 5 is located Frc 6. REE distribution patterns (for some selectedrare earth apatite crystal (69-1) from the the crystal, xenotime 2 at the rim or oxides) in a cross section (-V4 x l, mm) near the center of a approximately in the center of others in intermediate positions (see xenotime (29-2m, see Table 3) associatedwith muscovite' quartz' vlry close to the rim, and the The analyses are normalized to the and rutile in replacements of microcline in intermediate zone I. Table 3 for further details). in the core of the apatite' Numbers in parenthesesrefer to Table 2. composition of the xenotime RARE EARTH GEOCHEMISTRY OF APATITE AND XENOTIME 6t7

up for the exsolutionof xenotime,/mcinazite from I apatite: I l.8t a 8l-m (lnl zoneltl) (l) A|BsCnDt + A 6_18s_ rcn_nrD, * A rB rD 4, o68_12(_4_) Etolchlometrlc non-8tolchlometrlc xenoilme_ r 69_13_t(_ ., _l apattte apailte monezlte ) . 29-2m no I (- " -) . 69-l,xcn (2) A6+"Bs+,Cr"*o,D, - AsBsCnDr 5(corc) + A,B,D4, q Xanotim.64 non - stolchlometrlc stolchlometrlc apailte xenotime_ apatlte monazlte

(3) AsBsarCnaa,Dl- AsltrBr,gCr2*",Dtr A,B,Da, non-stolchlometrlc non_Btolchlometrlc xenorlme_ apatlte apailte monaztre The principleswould be the sameif exsolutionof { ll qxaftz and plagioclaseis taken into account.Ob_ viously one could complicatethe picture by E o 1.0 postulatingnon-stoichiometry in the exsolvedphases c as well, but adjustmentof a high-temperaturenon_ . 09 stoichiometryin the apatiteby exsolutionat lower A E temperature o -\\ seemsto offer a satisfactoryexplanation o of the inclusions.Unfortunately the precisi,onof the rl analyticaldata does not allowdistinction among pos_ \\ sibilities(l)-(3). \I !

o 68-13,renI (lntcrmediqtczonetlt) Nd Ib DyHoY Er Tm yb Lu I}

Frc.9. REEdistributions for xenotimes t,lll"to.l"'t"""tour,t". For comparativepurposes the REE distribution"", for xenoiime5 (included in apatite 69-l from the quartz core) is r8 also shown. See Table 3 for further details on the occurrence of the different ,|? xenotimes. Lines connect the rare earth values (excluding y) for o to each xenotime. l5 o E t4 Exsolutionis assumedto take placepreferentially 6 '13 at crystaldefects (Rast, 1965). Irregular distribution ofsuch defectscould thenexplain the spatialdistribu- tion of the inclusions. A xenotime exsolving from the apatite would prefer the heavy, rather than the light, REE. The surroundingapatite would be steadily enriched in the light relative to the heavy REE, so that during its growth the xenotimewould have accessto a con- tinuouslyincreasing ratio of light to heavy REE. Sm Gd Tb Dy Hoy Er Tm yb Lu Model IIIa is thereforein accordancewith the zoning actually observedin the xenotimes(see Fig. 3). Frc. 8. REE distributionpatterns r", -J'J;::tlll,"o.o ,n The most probable explanationfor apatites from intermediate zones III, I, and from the core, nor_ formation of malized to the intermediate zone III xenotime. REE distribution the monazitesis that monazitestarted to grow after patterns for xenotime 84 (not included in apatite) are shown for the adjacentxenotime was formed. This is supported comparative purposes. by the fact that, after the xenotimewas formed as 618 R, AMLI

variablesare un- mentionedabove, the apatitewould be enrichedin be too speculative since even more the light rareearths, which arepreferentially taken up controlled. by monazite.Another fact supportingthis explana- Rare Earth Fractionation in the Apatite-Xenotime' tion is the occurrenceof monaziteonly in contact Monazite System with xenotime. xenotimes Model lIIb impliesthat the REE in the xenotime/ In the previous sectionthe zoning in the model for monazitehave been supplied by a combinationof (l) was explained in light of an exsolution is what extractionfrom the apatite and (2) supply by solu- their formation. The next questionthat arises tionsin microfissuresin the apatite.It thusconforms to the observeddepletion in rare earthsin apatitead- jacent to xenotime/ monazite. The "advantage" of postulating a metasomatic processin addition to the exsolutionprocess is that the phasespresent before and after exsolutioncould be completelystoichiometric, because transport of rim of apatite) (see Figs. 7 and 8). material in and out of the "apatite-included It is assumedthat the apatites formed successively and that minerals" systemis allowed.A metasomaticprocess from the outer towards the inner zone, before might alsoexplain why previouslyreported apatites apatite formation was completed in each zone (cf 1952, that have total contentsof rare earthssimilar to, or if began in the next inner zone Fersman, and Jahns higher than, thosein the Gloserheiaapatites do not Cameron et at, 1949 Schneiderh6hn,l96l; containinclusions of rare earthminerals. and Burnham, 1969). during The zoningin the xenotimescannot be explained If the pegmatite was an open system at least fractionation by the metasomaticprocess alone, whether this is of a the formation of the apatites, the REE in the diffusionor an infiltration type, in the termsused by could be explained by a continuous increase apatites Korzhinskij(1965). According to Korzinskij'smodel' ratio of light to heavy REE available to the proceeded, the solid solutionseries would remainclose to con- during their formation. As crystallization lighter stantin compositionin eachmetasomatic zone during the apatites would incorporate increasingly in the infiltrationmetasomatism. It alsoseems unlikely that REE, a trend which then would be reflected a diffusion metasomatism,allowing compositional xenotimes which exsolved afterwards. the same variationsfor solid solutionseries, could producethe According to Mineyev (1963), however, REE dis- large variationsin REE fractionationfrom the core fractionation can be obtained even if the of the to rim in a singlexenotime crystal while alsoproduc- tribution was constant during the formation ex- ing the smallervariations found betweenxenotimes apatites. Mineyev suggests from theoretical, frac- at the core and rim of a single apatite crystal (see perimental, and geological evidence that REE pH of a Figs.4 and 7). The zoningin the xenotimes,at least,is iionation can be caused by changes in the with F-, thereforeconsidered to be causedby a mechanism solution that carries the REE as complexes, in suchas that proposedunder model IIIa. Cl-, COB'-, POot-, e/c aspossible ligands. Changes selective Someof the inclusionsare found crossingminor the pH of this solution may cause a a selec- cracksin the apatite(Fig. t, VI), thus supporting breakdown of the REE complexes and hence phases. model IIIb. However,many of the xenotimesare tive incorporation into the solid found isolated in the apatite. If material transport According to this model the REE fractionation implies along fissureswas important for the formation of observed in the xenotimes from Gloserheia these xenotimes,the fissuresmust have healed at that the pH ofthe fluid phaseincreased as crystalliza- with somelater time. tion of the apatites proceeded.This would agree a stage The data are insufficientto decidewhether the in- the interpretation of Yermakov (1964) that clusionshave formed in accordancewith model IIla with an increase in the pH of the solutions appears temperature' or IIIb. The following discussionconsiders model below the a-B quattz transformation (1963) for IIIa and showsit to be consonantwith severalex- The mechanism proposed by Mineyev by planations for the REE fractionation found for REE fractionation is also partly supported a xenotimesincluded in apatitefrom differentzones in Khomyakov (1967), who, however, favors only the pegmatite.An attempt to explain the fractiona- "weak" pH control on this fractionation. Schilling possibility tion in accordancewith modelIIIb is consideredto and Winchester (1967)also point out the RARE EARTH GEOCHEMISTRY OF APATITE AND XENOTIME 6t9

that REE fractionation during late stage differentia_ rence and their characteristicassociation with tion ()is controlled by REE complexing in muscovite,rutile, and calcite.Schneiderhdhn (1961) a volatile-rich environment. considerseuxenite to form during a pneumatolytic The takeup of REE by other quantitatively impor_ stage,and to be alteredduring a hydrothermalstage tant minerals in the pegmatite(euxenite and allanite with releaseof Ti. This titanium could havebeen the in intermediate zone III, and allanite in intermediate sourceof the rutile found in the pegmatite,very often zone I) would be an additional complication,if these associatedwith the xenotimesin question. crystallized simultaneously with the apatites. Because The possibilityof a pH-controlon the REE takeup of the regular trend and rather small absolute varia_ by the xenotimes(Mineyev, 1963) could also be con- tions in the observed REE distributions, and since sidered.Changes in pH might be causedby crystal- euxenite judging and allanite, from rough estimates, lizationof OH-bearingphases, such as muscovite and must contain the bulk of the total REE in the biotite, in the local environmentaround the different pegmatite, it is reasonableto infer that theseminerals xenotimes.This could happenwhether the pegmatite did not crystallize during the same period as the was an open or a closedsystem during formation of apatites. the xenotimes.This hypothesisis supportedto some The REE fractionation can also be explained if the extentby the observedchanges in REE-fractionation pegmatite was a cloied system during tire formation in the xenotimesthat formedtogether with muscovite of the apatites. Apatite, forming successivelyfrom and quartzduring replacement of microcline(Figs. 5 the outer towards the inner zone, may have preferen_ and 6). tially incorporated heavy REE and thereby enriched Other studieson REE fractionationin similar the mineral-forming fluid in the light npp. ff," mineral systemsare unsatisfactoryfor a comparison xenotimes exsolving from theseapatites would inherit with this work. In general,too little is specifiedabout this fractionation trend (Figs. 7, 8), provided the par_ the mineral parageneses,the homogeneityof the tition coefficient for REE between apatite and analyzed minerals, the location of the analyzed xenotime was more or less constant during exsolu_ materialwithin the crystals,and other important fac- tion. The mechanismof Mineyev (1963)would be in tors. accordancewith this explanation,provided there was Any conclusionson mechanismsand causesfor an increasein the pH as crystallization of the apatites the rare earth fractionations observed must be proceeded. speculative.However, it seemsprobable that frac- The fit of the REE distribution for xenotime g4 tionation in the apatite-xenotime-monazite system, as (not included in apatite) to the trend for the defined by the observed fractionation for the xenotimesincluded in apatite may indicate that any xenotimeinclusions, was causedby processesrelated differences for rare earth partition between (a) liq- only to the continuouscrystallization of the apatite- uid-apatite and apatite-xenotime and (b) liquid- bearingzones towards the center of the pegmatite. xenotime more or less balanceout. This is in accordancewith the proposedsequences of crystallizationfor similar mineral Rare Earth Fractionation of Xenotimes assemblagesin not Included in Apatites other zoned granitic pegmatites(Jahns, 1953; Schneiderhdhn,1961, etc).In contrast,the rareearth The trend in rare earth fractionation shown by fractionationin the otherxenotimes probably has a xenotimesincluded in apatite is not observedin the more complexrelation to the sequenceof thezones in other xenotimes (Fig. 9). the pegmatite. Assuming the pegmatite to be a closed system(dur_ In light of the dataobtained in this work, it seems ing the formationof thesexenotimes), the absenceoi clearthat carefulanalytical work in microscopicdo- this trend could show that the xenotimesformed at a mainscan contributesignificantly to our understand- later stageor stages(the pneumatolytic-hydrothermal ing of pegmatitegenesis. stagesof Schneiderhcihn,1961, and others) than the apatites.This may have beenduring or partly during Acknowledgments the same period when euxenite (and allanite in in_ I wishto expressmy sincerethanks to Dr. W. L. Griffin, profes- termediate zone III?) was formed and later altered, sor H. Neumann,and B. B. Jensenfor stimulatingdiscussions and advice with accompanyingvariations in the REE distribu_ during this work. The electronmicroprobe analyses were supportedby NorgesTeknisk Naturvitenskapelige Forskningsrdd tion in the residual fluid. Late formation of these (NTNF), and scanning electron microscope facilities xenotimes is also were indicated by their mode of occur_ generouslymade available by JEOL-scandinavia. 620 R. AMLI

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Int' Geol Congress'Rep' 22nd Session'Part I. Occurrence and JeHNs, R. H (1953) The genesisof pegmatites. VI. Proc. Sect' 6, Minerals and Genesis of Pegmatites' New origin of giant crystals. Am. Mineral 33' 563-598. Dehli,p.86-ll5 studies of --! AND C. W. BunNs,,rv (1969) Experimental YouNc, i. J., n*o E. L' Mur'rsor (1966) Fluor-chlor-oxy apatite pegmatite genesis.I A model for the derivation and crystalliza- and sphene from Crystal Lode pegmatite, near Eagle, Colorado. tion of granitic pegmatites.Econ Geol.64' 843-864. Am Mineral. 51, 1476-1493. factors KHovrv,qrov, A. P. (1967) Chemical and crystallochemical - A. T. Mvrns, E. L. Mur'rsoN, AND N. M. CoNxl-tN (1969) -205 in the distribution of rare earths.Geokhimiya,2, 197 llransl. and geochemistry of from Cerro de Geochem Int. 4,127-135 (1967)1. Mercado, Durango, Mexico. tl S. Geol. Sutu' Prof' Pap' 650-D' Prozesse KorzutNsrrl, D. S. (1965) Abriss der Metasomatischen Zrcsoro,T. O., ervo R. E. 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