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An experimental study of Nb and Ta partitioning between Ti-rich minerals and silicate liquids at high pressure and temperature
An experimental study of Nb and Ta partitioningbetween T&richminerals and silicate liquids at high pressure and te~~~~~ T. R. GREENand N. f. PFARSON Schoolof Earth Sciences, Macquarie University, North Rydc, N.S.W. 2 113,Awtralia
Abstract-Experimental action of partition coefficients(Df for Nb and Ta betweeu Ti-rich minerals (sphenc, rutile, ilmenite and Ti-magnetite) and coexisting silicate liquids of basaltic and&e, andesite and trachyte composition, indicate that these elemcms partition strongly into sphenc and rutile, and moderately into ilmenite and Ti-magnetite. The D values for spheric increase with decreasing temperature and with increasing SQ content of the coexisting liquid, but appear unchangd by pmwtre between 7.5 and 16 kb, and oxygen fbgacity betweeu ma&netite-wustiteand haematite-magnetite butlers. Also, 4nJ4, varies from 0.3-0.4 for sphenc, increasinS to 0.6-0.7 for rutile, 0.7-0.8 for ilmenite and 0.8 for Ti-magnetite. Thus ~aation involving these minerals may cause an increase in the Nbfra ratio of any derivative liquid; thii will be most noticeable whem sphene is involved.
GE~UL COHERENCY benvan the minor dement Ti, and the trace elements Nb and Ta is well established.This has receivedspeciaIattention in igneous rocks of convergentplate region&where these elemems are chamc@&ically low in relative abundance, comparedwith igneous rock suites from other tectonic settings(GILL,1981;PEARCE,X982).A better undies
procedu~anouttinediaG~Nand~(1985a),~d ~~~~~~~T~2.5~%~~~~ to each cotnposition, in each run. Oxygen fugachy (x02) in routine expariments cormsponds allay to the map
dte-Mte but&r(-EGG and PERSON, 198Sa), butom:experiment was conducted in which the sample cap sules (Ag-Pd) were packed in a ~~ti~~~~ (HM buffer)mlxenclosedInanou~gold~Thiswas~n for I’k hoursonly to ensureprezrvationof the HMbu&
(confirmed at theconcWon of theexperiment).
of the reason for such elemental distri-
butions may be obtained from knowiedgeof the IUb, Ta partitioningrelationsbetweenTi-ric&mineralsand coexisting silicate liquids, and the need for such data has been pointed out (CLACWE and FRFX, 1982). An
toallow glass in-
earlier experimental investigation (MczCLw_uM and C&IARRI’T&1978) d~rnen~ Nb ~tio~ be-
tween a numberof phases,inclu~o8 rutileand ilmertite, and basalticliquid at I atmosphereandfC& = MI X 10-“.s. The present exptvimemal study sets out to determine Nb and Ta partition coefficients Tiimi~ ’ ) for sphene, nttile, ilmettite and TiWt4b.T‘ mwwtite crptabing from 3 differentbulk compositions @a&tic andesite,adze and trachyte).In the case of sphene, the effectof pressure,temperatureand fG2 on the partitionrelation&tipsis examined. In addition, this workprovidesanalyticaldata on extensiive Nb and Ta substitution in sphene and rut&, which arecomparedwith naturalNb and Ta-richspheneand n&e. The lattermineralsare recordediiom two very di&rent host ro&s--kimberlites (e.g. I&KXXR’~Y, 1983) and pegmatites(e.g. PAUL etal., 1981;CLMu& etal., 1981); bothoccurrences appeslr to result from late cry&dlhtion from trace element and volatile-rich fluids or liquids.
The results are for synthesis experiments only, but are considemd to reptwent a close approach to equilibrium because ~~~~~~~~~timm~~~~. tioning experiments involving the rare earth elements (partition&t betweeu sphene, clinopymxene and liquid), where reversals and wiable-time synthe&s runs gave support for ~~~~ ntations (GRFJEN and &ARSON, 198&t, 19S6a).
~JWWTIZ
55
(1978) showed that Henry’s law was obeyed for
T. H. Green and N. J. Pearson
56 TABLE
1.
Rock compositions used in the experimental study. 1Ole.Z the Nb,Ta bearing glass was added to each rock composition. In addition 10, 7.5 and 7.5vt.X of sphere canrponents wre added to the basaltic andesite, andeaire and trschyte, respectivefy. *Additional oxides present are 5.1wt.X. Nb20s and 4.9wt.Z. Ta*05.
SfO2
llasalttc andesite
Andesite
54 ‘09
61.19
Ttachyte
65.42
*
glass
Sphene coluponent
57.1
TiO2
0.56
0.68
0.30
‘l2’3
17.76
17.10
16.46
38 .O
9 282
5.37
4.26
MI?0
0.19
0.13
0. IO
3.7
W
4.02
2.98
0.14
CaO
11.14
7.28
1.32
6.1
NE+
1.78
3.67
6.23
4.9
K20
0.56
1.39
5.71
4.9
P205
0 .OE
0.21
0.06
26.7
3.4
F
NbGontEats of 0.3- i .3
s
28.6
17.0
Fe0
i
Nb,Ta
of
wt.%
whi&
is close
to the Nb
jn Table3, T&&j&i content compositionsofmxistiog gl;hsses conten+ of the ghses were caicui8ted
in the itmen& anelyad in this work (0.8-I.7 wt.% Nb).
methad odined REsaT§
Anaiysesof Nb, Ta-bearingspbexte,n&e, itmenite and Ti-magm%ite are given in Table 2, together with
Rua No. 1044 ooap. M P&b) 7.5 T(*C> 1GiJO r(hrr) 6 n 11 EW** sphcna SfO* 29.37 ( .25) TiO 32.40 f .48) Cr203 2.59 (.lt) Alli PJ3 +ino
1.11 -
3 NsO
26.60 0.16 I 0.07
g& **;o:0
4.96 1.99 98 . 0.34
WTa Run No. Z$ n*ct t(hra) n Phue 9102 :“% cr203 PCs3
cao
Nb/Ta
1044 A 7.5 1000 6 12 sphanc 30.62( .a1 1 31.15(.74) 3.10( .34) 1 .OO( .06)
L.04( .05) D.O8( .Oi)
c.39) c.02)
26.59( 0.23t.04) .46) 0.09(.01) 0.11(.02)
25:72( .26) 0.32( .02) O.Zlf.01) 2.60( 5.82( 99 0:36
f.36) f.11)
2.31(.12) 5.55( .23) tzix 0.36
1046 A 7.3 950 S 6
1046
*pb&IWZ 30.8X 1 .OS) 30.34(t.07) 3.59 (*SO)
OPirrnc 29.28( .78) 31 .W( .SO) 2.54f .2?1
1.13
1044 T 7.5 1000 6 12 tIphc*e 29,20( .24) 32.24( .25f 2.17(&s)
(.Of)
C.02)
(*OS)
0.23 c.06) 25.36(1.01) 0.12 (.Oif 0.12 f-02) 2.87 c.20) 5.48 (.301 lw.11 0.45
7T5 950 8 6
in GREEN and PFiAFtSN
necwsarytxcwse of the mobility of a&&s -under iow vahleain the each of the mineraisare reportedin Table-4.
was
.26f .13)
1045 SA 16 1000 6 10 S,ph8nC 29.82( .29> 32.20< .54) 3.20( .19> l&5? -
.oat
t.09)
27.12(.28) 0.15(.02) -
26.54(.49) O.ZO( ,031 O.ll(.Ol) O.OP( ,011
0.27 0.26 -
4.47(.42> 1.97<.12) m 0:39
4.12l.35) 2.11f.12) 99 . 0.46
11.43 9.54 XK 0.11
1056 Ia4 A 4*2 1000 1.25 15 sphana 28.83( .39) 30.23c.33) 2.11( .13) 0.23(,03) 1.91( .I&)
1056 RM A 4_+2 1000 1.25 3 haxeaoiiaenite 0.31 (.OSf 40.40(1.181 1.71 <.OSl 0.11 c.011 44.61(1.81)
sphmna 28.#( .17) 30.7Of .52) 2.05( .091
Qagcet1tt O.!Sf .Of) 18,42(.31) 1.29( -04)
2.51(.161 0.14(.03) 26.37c.40) 2.59( .13f 6.&.W 0.35
: rut t le 73.77 C.39) 0.27 1.02 ((.04) .09) 2.56
LO56 RH SA 422 IO00 1.25 11
1.16(.04)
1045 A 16 1000
O‘95~.04)
1056 KM SA 4t2 1000 L .25 17
O.IO( .03) 25.69(.58) 0.34f -02) 0.221.04> 2.aoi .09j f.6H.16) ?xTT 0.42
1045 A 16 1000 6 10 siphcne 29&2( .64) 32.24( .38) 3.18(.15)
60”.;;I
3;;
3:01(:12) 0.24( .02f
O-36( .05)
O.bO
0.18(
.03)
2.95( .16) 6.131.29) RE 0:41
1006 SA 7.5 355 8 IO SplWiU 28.89 (.ZO) 30.11 (.Sfo 3.32 (.09f
1046 7% 950 a 3
Ilmitc 0.20 52.50 0.35
(.MI) t.741 (.021
1.09 0.12
(.03) C.01)
1.25 -
C.04)
39m 0.30
<.27) C.02)
t.05) C.03)
25.64 0.40 0.22
c.37) C.03) t.021
26.52 0.15 I -
(925) C.07)
0.33 5.40
1.101 (.04)
(c.36 .62)
5.05 2.68 ~lbb.ol 0.45
(.26) f.13)
6.61 2.85 x?K 0.37
t.63) c.17)
4.09 0.22
C.21) (.Ol)
0.05 2.42 2.37 96-i3 0.87
(&I) c.23) t.19)
_
1045 f 16 1000 6 14 aphent 29.65 c.36) 32.31 C.29) 2.45 (*ii)
_
1056 RI4 T 422 1000 1.25 L1 SP?WW 31.89 i .76f 29.16
1056 1M 4T* 1000 1.29 3 Iutile
0.94 1113 (.I31 f.oW ion 1.03 1044 7> 1000 6 8 pyroxracl
49.77 1.69 3.83
<.%) f.32) f.99)
c.32)
7 “83 0.16 13.64 22.50 0.21
( .34) c.02) c.23) (.35) C.03)
11.92 (.4&I 16.10(1.17> li)D;b
” 99Tb3
59.9Sfi.07) 0.98( I .09) 11.02
D .b?
Mineral-meltNb, Ta partitioning
57
Sphene analyses in Table 2 clearly show that it readily a~cornrn~~ Nb and Ta, favouring Ta over Mt for all ~rn~itio~ and ex~rn~~ conditions (L&J&~ varies from 0.3G-Q.44). The total Nb205 f Taz05 content is in the range 6-9 wt.% which is less than the highest content of these elements recorded in natural sphenes from pegmatitesand kimberlites(PAUL et al., 1981; HAGGEIZTY, 1983), but is ~~~~~tly higher than the 1.3 wt.% recorded by WOLFF(1984) in a sphene from a phcmolite. Table 4 shows that id generally increase with SiOz, shown conby comparison of the results for basaltic andesite and andesite compositions. However the D variation with commotion is less clear when the values obtained for the andesite and trachyte are corner, and it is possible that the high alkali content of the trachyte, compared with the andesite, at%ctsthe D vdws, in the opposite fashion to SiG and continns the suggestion of LARSEN (t979) and DRKSXLERet al., ( 1983)that D values decrease with increasing alkalinity The results in Table 4 also show that increase with decreasing temperature ( T), but are not si~ifi~~y affected by pressure (p) in the range from 75 16 kb, when D values far similar compositions at the same temperature are compared. Similarly, D vaiuesare unaffected byf O2variation between that co~~o~~ to the HM and -MW buffers at 1ooO*C,with variable pressure. (The precise pressure in the HM buffered run is uncertain, due to a technical problem in tbis particular run giving friction between the piston and bridge assembly in viewof the separately demonstrated lack of e&t of P on D, this is not a serious probI~m.)
Rutile was identified from a run on the tmchyte composition (HM buffer) and a run on the andesite ~rn~tion (-MW buffer), and shows very different compositionwith respectto Nb, Ta and Fe caption but has similarI&.&& (0.60,0.66f (see Table2). Natural niobian n&s also exhibit widely offing compositions,overpay the Nb, Ta and Fe contents obtained in the syn~eti~ rut&s (CERN~ et al.,1981; t%AGGERTY,1983). However,Nb-Tarutilesfromkimberlites also contain ~~~fi~nt Cr203, and in this respect are unlike the synthetic exampies.B%!!$@@’ for rutiiefromthe and~te and trachyteare~n~ly the same (Table4), despitecompositional,P andfQ differences.The rutile in the runs on the trachyticcornposition occurredas acicularcrystalswhich were impossibleto analyzeBe.eof glassinte~e~n~. Hence the analyseshave been correctedfor glasscontamination (5-20%),assumingzero SiQ content in the rutile.The rutiie from the andeaite occurred as larger rod-like tly a&c&d by glassinterference( Irrystals,only 241, and these werecorrectedalso. The am” vaiues obtained for the andesitic and tmchytic composi-
58
T. H. Green and N. J. Peamn TABLE 4.
Run
Partition ccefficients phase8 and cosx~stin~ Table 2.
cmp.
P
T
NO.
1044
1045
1046
BA A T BA A 2 h
T 1056 tar
FiA h T
Sphene %. 10.8 15.4 15.7 10.6 15.9 14.0 15.9 19.6 18.3 11.5 17.0 17.7
1000 1000 1000 1000 ioO0 1000 950
3.5 5.3 5.4 4.6 5.9 6.1 5.7
7.5 7.5 -4 4 -4
950 950 1000 1000 1000
7.6 1.6 4.8 5.6 6.1
Nb and melt.
Rutile
%b 7.5 7.5 7.5 16 16 16 7.5
tions in this wo* in sigkkantly
CD) for silicate
higher than the
*Nb
26.5
29.8
Ta bet*+en Abbrevfatione
Ti-rich as
Ilmenite ‘Ta
for
ugnctlte
%b
Dtb
2.3
2.7
4.6
6.6
DNb
‘Ts
0.7
0.8
44.0
44-7
DISCU!BION
The chcmiutl coherence of Nb, Ta and Ti in rut& ~~~~~~~~~=.~~~ in Tabk 4, and fkzionution ofonly aaxuory amounts thCtWOStUdkS. ofthcseminaots~baveainnifinnt~otlthe Nb, Ta ~b~ti~n and Nb/Ta ratio in dcrivativc magmas. However the much kxvcr Nb, Ta content, iwcr D vahss and Ma close to 1 for ilmenite and llmeniteoccun&intbcnmontbeba8alticaadesite titaIlomagn~~tetbatth#:pbnarwiubavca oompoaitionat 7.5 kb, 9H)*C,pad d siglesserelFa% on Nb, Ta abundawc and Nb/Ta b&wniiicantIymore N&OS (1.13 wt.%)and TazOs (0.94 iour. In oontrast, Yhc Ii&$&* ratio of 0.3 to 0.4 for ~.%)~~2)~~~~~~~ spheac dcmonstratcs that it may fmctionatc Nb and kimbulite (HACXXRTY, 1983). e vahw arc Ta,andsocw8echaqpsintbeNb/Taratioofderivlow (compucd with nttik and spbcnc) and ilmcnitc ativc magma& as su##m@d by WOUF ( 1984) for does not s&on&y fractiaorte Nb and To (D&Jr, sphene cryMMng ftem pbonoiites from Tencrife. = 0.84) (Tabk 4). The w VaIweof 2.3 is sigWo~s~~t~~~~a~~y~ ~~~y~~a~~.8~~M~ektmnc@vitythimTaitmayatiain2vabna&atcp LUM and ~MREITE (1978) which, as in the caec of (Nb”, Nd*) more tzadiIy than Ta Th& cot&i account ~~C,~y~~~~~ for~~~~~o~~~~ di&rcnas behvecn the two studies (viz. baa&k anTa. For ex&m$k, Nbfi may be kss oompatibie than dcsiteat95O”C~withbantt8t l&00-113Vc). Iw+ for &ubstitutioninto spbe5c and rut&‘ gnretuns, FOrf~C0D&iOn&UXrUp&&totbeHMbttfk, ~~~~~~~t~~ a “Arnold” was obtain& in the run on tbc ~~3+,a~~~~a~o~~~~~ and&c compo&ion. This accommodates Nb and Ta tutilc. However, in the ptwcnt cxperiment~, aim&r monmadiiythanilmc&ef~8tmuchkwerfO~ Nb/Taand~ocwTaiforsphtwn~(sceTabks2and4,coiquiqtbcdatafixtbc2ilittg for/G conditions coto the HM and mcnitcs),butissimiktothkQmcaitcinthatitdocs MW bufk conditiot~ at IOOOYI.Thus+ for this fol not strongly &action&&Nb, Ts (BP&&~ = 0.69). faoet, which encompancd theI@ deduced for the Tcnerife pbono&es, the e&ct propowl by Wolff is not evident (Wow, 1984). ( 1978X(26-30 compared with 16), probabIydue to
~~rn~~~~~~~
59
Mineral-meltNh, Ta partitioning
be considerably less if a T&rich magnetite co-precipitates. EWART(1979)tabulates intermediate-sihcicrocks with up to 0.4 vol.% sphene or up to 2 vol.% Fe-Ti oxides as phenocrysts. Similarly, intermediate to evolved rocks of transitional alkaline or alkaline intraplate volcanic series frequentty contain sphere and/or Fe-Ti oxides as phenocryst phases (RTTONand HUGHES,1977; CORNEN and MAURY,1980, POSTALet ai., 1982, WIRER ez al., 1983;WOLFF,1984)with the proportion of sphene involved possibly reaching 1.5 wt.%, but more commonly in the range of 0.5-l wt.%. Model calculations summarized in Table 5 use geochemical data presented by DOSTALet al. (1982) for a subalkaline (with some transitional alkaline afhnities)series.These data include Nb, Ta contents of hasalts, andesites, and dacites and rhyolites which were suggestedto be genetically linked via crystal fractionation. The observed Nb/Ta ratio varies from 15-17 in the dacites where sphene was recorded as a phenocryst phase to 20 in the low SiO, rhyodacites. Variable involvement of sphene (0.5, 1, 1.5 wt.%) in a fractionation scheme is shown in Table 5 to change the Nb/Ta ratio to 19,20 and 21 respectively, from an initial ratio of 17.5. Thus with up to a maximum possible amount of sphene fractionation in relatively evolved magma suites, a possible20% change in the Nb/Ta ratio may occur. In the case of crustal melting, two calculations are given corresponding to pressures of 5 and 12 kb (Table 5), based on a maftc source composition and modal proportions from HELz (1976) and HELLMANand GREEN( 1979) and assuming an initial concentration of 3.1, 0.2 15 ppm Nb, Ta, respectively, with Nb/Ta = 14.4 (m~elling on typical N-type mid-ocean ridge basalt-WOOD, 1979). This shows that silicic melts with Nb/Ta of 16.8 and 16.7 may be derived at 5 and 12 kb, respectively. The final model calculation adopts the eclogite melting hypothesis for derivation of intermediates&ic
MANand GREEN( 1979)suggeskd that residual sphene and/or wile may occur under hydrous melting conditions in the deep crust Thus for intermediate to silicic magmas, Nb/Ta may not be a reliable indicator of possible genetic links. However, for mantle compositions and melting conditions producing the more common and voluminous basalt compositions it is unlikely that a residual ‘&rich accessory phase will remain in the source region, (GREENand &+&SON, 1986b). Thus the Nb/Ta ratio of basahs may be expected to reflect the Nb/Ta ratio of their source region. This argument does not apply to magmas such as kimberlites and highly undersaturated nephelinites which represent fluid-present and very low degrees of melting of the mantle, such that a Ti-rich phase may remain in the source region (GREENand PEARSON,1986b). In Table 5 some relatively simple models of crystal fractionation and partial melting am presented, incorporating Ti-rich phases, and examining the possible effect of these phases on Nb/Ta in derivative magmas, using the &,,r* values determined in the present study. The proportion of Ti-rich phase invokd in the modeliing is constrained by (a) observed abundance of Tirich phenoctyst phases in some magma suites and (b) TiOr content of silicate liquids saturated in a Ti-rich phase (GREENand PEARSON,1986b). In convergent plate boundary suites EWART(1979) noted that sphene occutred as an accessory phenocryst phase in high-SiQ andesites-rhyolites in high-K suites from Western U.S.A. (both Eastern and Western Belts), Western South America and Mediterranean provinces. The TiOr+aturation curves for intermediate-silicic liquids from GREENand PEARSON(1986b) indicate that highSi& andesites to da&es, typically with 0.60.8 wt.% Tic&, may fractionate and produce rhyohtes with 0.2-0.4 wt.% TiOz at temperatures of 850-95O”C, via separation of a Ti-rich phase (amongst other phases) such as sphene. The change in TiOz content represents a maximum of 1.2wt.%sphene fractionation, but may
1.
cry*tr1
fractionation
Hypothetical dacttc: Fractionation Pbues fractimatcd:
0.6-0.9X
product:rhyolirc
252 pl~~foel88e. 1.5x epilene
5% cli~~~pyroxeae,
DNb Ta wed: 0.05, . Famultant Nb/Ta of 2.
Partial (a)
rltiag
0.05: IiquSd
of ufic
0.4, -
DNb,Ta used:
u
&ova.
Nb/Ta of
0.4;
18.9,
m”tve:
Cru8ral depths 5kb: Pbarm fractionated: LZkb: Phmes fractienrtcd:
Resultant
T+, 35ppr Nbb, Zppm Ta, NblTa : 0.1-0.2X Ti02 2% Ii-m.gnetite
0.4.
0.7.
20.1, 3.1.
0.3,
2.7;
a 16.8 (5kb)
0.3;
0.6;
and either
6.7,
0.5,
1.0,
17.6 respectively
21.3 with 0.5. 1.0, 1.5% aphane, respectively.
O.ZlSppa
762 amphibole, 74% mphlbole.
plus 2.3,
liquid
5% hornblende.
17.5
0.3.
Nb.Ts respactivcly;
3.2% pl~loclasc, 5.2%
Nb/Ta .
1% ilwmita,
,j.met, 21clinopyrox,,ne,
0.3
for
Ilmite
and gmmt,
46.5% gamer,
0.8% eplwnc 0.8% sphcne
m,pactivcly.
and 16.7 (12kb).
(b) Mantle depths 30kb: Phaes fractionated: 30X cllnopyroxew. DNb Ta aa rbovc. plus 26.5, 44 far turflc. * Pasultent Nbh of liquid - 16.6
14.4.
0.6% nstilc
60
T.
H. Greenand N. J. Pearson
mw from subducted oceanic crust (GREEN and rutile is recorded in minersd assemblages inteqmtcd RINGWOOD,1968). The Nb, Ta values from WOOD as upper mantie in origin, associated with hypothetical (1979) which were nsed for the crustal melting model mystic events related to the ~~tion’of magmas were also adopted for tbe oceanic crust, converted to belongingto the alkaline suite (e.g. alkali olivine basahs eclogite. Gill’s particular eclogite melting model for to melilitites) (JONESet al., 1982;WAS and ROGERS, generation of dacite (GILL, 1974) was followed. This 1980). involves only 0.6 wt.% rutile, and the calculated Nb/ DNb, I& values for pyroxene and garnet are not Ta of the derived dacite is 16.6. well constrained, but appear to be low (typically, Thus relatively small amounts of sphene and/or ru- D%-’ am taken lo be 0. l-see PETARCE and tile involved in melting m at either crnstal or NORRY, 1979; no public data are av~b~e for mantle depUxqmay tly alter the Nb/Ta ratio+ B%x@“‘@t 1.The present work can only indicate semiand the derived melts need not have NbfTa reelecting ~~nti~tive values of &I$*-* of KO.3.There is the maftc source undergoing partial melting. no Sutton of f~o~tion of Nb, Ta from each other by either clinopyroxene or garnet. Thus it is unlikely that involvement of reIatively abundant silicate phases in the generation of alkaline suite magmas could (a) Basaltic suites-fiationation und mantle melt- account for variation in Nb/Ta, and the role of a Tiing. Experimental and trace element studies on basalts rich phase in the source region is supported further (cf: indicate that thokiites are produced by relatively large CLAGUEand FREY,1982). degrees of melting of the mantie souroe region, while (b) Interrn~i~t~sii~~~~ suites: fr~io~tio~ and alkahba!Bl~tbrougb crustal melting. Ewart and co-workers @WART,1982; ilitite are derived by EWART et al., 1977, 1986) pubis Nb, Ta data on melting (GREENand a number of widely separated, evolved, alkahne suites 1978). WOOD(1978) from Eastern Australia. The total range in Nb/Ta for all suites is from 8 to 35, and there is no advent correlation with host rock type, although one suite (Glass House Mountains) shows HbiTa of 11-f.2 in trachyte, Incas to 35 in comendite. However no for the degree of melting envisaged for these rocks. potential Nb/Ta ftionating phases such as sphene klso,Woo~(1978)~~b~~~~tobe
Mineral-melt Nb, Ta partitioning
the derivation of these compositions (GREEN and PEARSON,
1986b). Hence Nb, Ta depletion in these magmas suggests a source region depleted in these elements (GREEN, 1972). CONCLUSIONS Sphene and rutile readily accommodate Nb and Ta, and show high D vahres for these elements. D@$‘@‘@ exhibits similar variation with composition and temperature to that established for other trace elements, such as the rare earth elements (WATSONand GRIZIZN, 1981; GREENand PEARSON,1983, 198&b). However no sign&ant effect of pressure orfO1 was observed on eti. The Nb/Ta ratio for sphene, rutile and Ti-magnetite is significantly lower than for their coexisting liquid. Thus if these phases are involved in magma generation or &actionatio~ an increase in Nb/Ta may be expected, as documented in some evolved suites where there is evidence for sphene crystallization (Tenerife phonolites-WorJF, 1984). Modelled NbJfa variation for up to 1.S wt.% of sphene fractionation shows a 20% incmase in Nb/Ta. This is much less than the 250% increase observed in the Tenerife suite. However L&J &, for sphene/liquid obtained experimentally ranges from 0.30-0.44, which is very close to the value of 0.46 obtained by WOLFF(1984). At least part of the difhince in increase in Nb/Ta between the natural and modelled processes could be due to the model&l process using higher temperature D values, for less evolved and less silicic liquids compared with the Tenerife situation, where higher amounts of sphene may also fractionate. In spite of these biting factors, the very large increase in Nb/Ta in the Tenerife suite cannot be explained adequately by the present resuhs. Apart from this suite, there is, so far, little evidence for a consistent pattern of variation in Nb/Ta in igneous rock suites, but the partition data recorded here suggest that determination of such variations may be a useful petrogenetic indicator for the role of Ti-rich phases. COnvermly, differing Nb/Ta ratios for igneous rocks in a given province may not be reliable indicators of different sources, if sphene or r-utile(or to a lesser degree, T&magnetite and ilmenite) were involved in the derivation of the rock suite. Acknowk&emenrs-This msearch has been supported by grants from the Austmhaa Research Grants Scheme and Mat’quarieUniversity. Drs. A. E. Ewart and J. A. Wolffkindly ctiticaIly read the mannscript, and Drs. J. Jones, J. Longhi and I. HdcIuum provided constructive journal reviewswhich also ettcouragedsign&cant improvement of the final product. Editorial handling: E. B. Watson
REPERRNCES BF.NCE A. E. and ALBEEA. L. (1968) Empirical correction lbctomfarthedectrmlmicmamd &iSOft5lbttSandOxidcs. 1. f&X 761382-403. CERNY P., PAUL B. J., H~wlfioRNE F. C. and &U’MAN R. (1981) A niobiax~rut&&so&& columbite intergrowth
61
lkom the Huron claim pegmatite, southeastern Manitoba. Can. Minerat. 19,541-548.
CLAGUE
D. A. and FREYF. A. (1982) Petrology and trace element geochemistry of the Honolulu Volcanics CkItu: implications for the oceanic ma& below Hawaii.J. Pet&. 23,447-504. G. and MALJRY R. c. ( 1980)petrotogyoftbe ~Okanic
CORNEN
island of Am&on, Gulf of Guinea. Marine Geol. 36,253267. Dosr~L J., DUPLJY C. and VENTUREUG. (1982)Geocbemistry of volcanic rocks from the Monte Arci (West Sardinia, Italy). C&m. Geol. 35,247-264. DREXLERJ. W., BORNH~~~T. J. and NOBLED. C. (1983) Trace-element sanidineklass distribution coefficients for pemlkahne silicicrocksa$ their implications to pemlkahne petrogenesis.Lithos 16,265-27 1. EWARTA. E. (1979)A mviewof the mineralogy and chemistry of the Tertiary-Recent dacitic, latitic, rhyolitic and related salic volcanic rocks. In Trondhjemites,Dacitesand Related Rocks (ed. F. BARKER), pp. 13-21. Elsevier, Amsterdam. EWARTA. E. (1982) Petrogeneaisof the Tertiary anorogenic volcanic series of Soutbem Queemland, Australia, in the light of trace element geochemistry and 0, Sr and Pb isotopes. J. Petrol.23, 344-382. EWARTA. E., OVERSBY V. M. and MATEENA. (1977) Petrology and isotope geochemistryof Tertiary lavas from the northern hank of the Tweed volcano, southern Queen&& J. Petrol. la, 73-l 13. EWARTA. E., CEWPELL B. W. and LEm R. W. ( 1986) Aspectsof minemlogy and chemi&y of intermedk~cic Cainozoic volcanic rocks of Eastern Austraha. Part I. Introduction and geochemistry.Aust. J. Earth Sci. 32,359382.
J. G. and HUGHESD. J. ( 1977)Petrochemistry of the volcanic rocks of the isIand of Ptincipe, Gulf of Guinea.
FTITON
Con&. Mineral. Petrol.64257-272.
F. A., GREEND. H. and ROYS. D. (1978) Integrated models of basalt petrogenesis: a study of quartz tholeiites to olivine melilitites from south eastern Austmlia utilixing geochemicaI and experimental petrological data. J. Petrol.
FREY
19,463-513.
GILL J. B. (1974) Role of unde~rust oceanic crust in the genesis of a Fijian talc-alkaline suite. Contri& Miners Petrol.43.29-45.
J. B. (1981) Orogenic Andesites and PIate Tectonics. Springer-Verla&399 p. GREEN D. H. ( 1972) Malpnatic activity as the major process in the chemical evolution of the earth’s crust and mantle. In The UpperMantle (ed. A. R. RUSEMA);Tectonophysics 13.47-71. GREEND. H, and RINGWOOD A. E. (1967) The genesis of basaltic magmas. Contrib.Mineral. Petrol. 15, 103-190. GREEN T. H. and PEARSON N. J. (1983) Effectof pressure on rare earth element partition coefficients in common magmas. Nature 305,414-416. GREEN T. H. and PEARSON N. J. (1985a) Rare earth element ~tioni~ between clinopyroxene and s&ate liquid at moderate to high pressure. Contrib. Mineral. Petrol. 91, 24-36. GREENT. H. and PEARSON N. J. (1985b) Experimental determination of REE partition coefficientsbetween amphibole and basaltic to aadesitic liquids at high pressure. Gco. GILL
chim. Cosmochim. Acta 49, 1465-1468.
T. H. and PEARSON N. J. (1986a) Rare earth element partitioning between sphene and coexisting silicate liquid at high pressure and temperature. Chem. Geol. 55, IOS119. GREENT. H. and PEARX?NN. J. (1986b) Ti-rich accessory phase saturation in hydrous ma&felsic compositions at high P, T. Chem. Geol. !34,185-201. GREENT. H. and FUNGW~OD A. E. (1%8) Genesis of the cabaMine igneous rock suite. Contrib. Midas. Pet&. 18, 105-162. GREEN
62
T. H. Green and N. J. Pearson
HAGGERTYS. E. (1983) The mineral chemistry of new titanates Born the Jagemfontein kimberlite, South Africa: imphcations for metasomatism in the upper mantle. Geochim. Cosmochim. Acta 47, 1833-1854. HANSON G. N. and LANGMUIR C. H. (1978) Modelling of major elements in mantle-melt systems using trace element approaches. Geochim. Cosmochim. Aaa 42,725-74 1. HELLMANP. L. and GREEN T. H. (1979) The role of sphene as an accessory phase in the high-pressure partial melting of hydrous malic compositions. Earth P/and. Sci. Lett. 42, 191-201. HELZ R. T. (1976) Phase relations of basalts in their melting ranges at Pufi = 5 kb. Part II. Melt compositions. J. Petrol. 17, 139-193. HENDERSONP. (1982) Inorganic Geochemistry. Pergamon Press, 353 p. JONESA. P., SMITHJ. V. and DAWSONJ. B. (1982) Mantle metasomatism in 14 veined peridotites from Bultfontein Mine, South Africa. J. Geol. 90,435-453. LARSEN L. M. (1979) Distribution of REE and other trace elements between phenocr@s and pemlkahne undersaturated maemas, exempli6ed by rocks of the Gatdar igneous provina, south Gmenland. Lifhos 12,303-3 15. MCCALLUMI. S. and CHARETTEM. P. (1978) Zr and Nb partition coe3kients: imphcations for the genesis of mare basal&, KREEP and sea floor basal& Geochim. Cosmochim. Acta 45859-869. NICHOLLSI. A. and RINGWOODA. E. (1973) Effect of water on divine stabi& in thokiites and the production of silicasaturated magmas in the island-arc environment. J. Geol. 81,285-300. PAULB. J., CERN? P., CHAPMANRandHWMo~J.(l981) Niobian titanite from the Huron claim pegmatite, southeastern Manitoba Can. Mineral. 19,549-552. F’EARCE J. A. (1982) Trace ekment characteristics of lavas from destructive late margins. In Andesites: Orogenic An-
desites and Reiafed Rocks (ed. R. S. THORPE), pp. 52% 548. J. Wiley and Sons Ltd. PEARCEJ. A. and NORRY M. J. (1979) Petrogenetic implications of Ti, Zr, Y, and Nb variations in volcanic rocks. Contrib. Mineral. Petrol. 69, 33-47. SAUNDERSA. D., TARNEY J. and WEAVER S. D. (1980) Transverse geochemical variations across the Antarctic Peninsula: implications for the genesis of talc-alkaline magmas. Earth Planet. Sci’. Lett. 46,344-360. SMEDLEYP. L. (1986) The relationship between c&-alkaline volcanism and within-plate continental rift volcanism: evidence from the Scottish Palaeozoic lavas. Earrh Plane!. Sci. Leaf. 76, 113-128. SMITHA. L. (1970) Sphene, perovskite and coexisting Fe-Ti oxide minerals. Amer. Mineral. 55, 264-269. WASSS. Y. and RCGERSN. W. (1980) Mantle metasomatismprecursor to continental alkaline volcanism. Geochim. Cosmochim. Aaa 44, 181 l-1823. WATSONE. B. and GREEN T. H. (198 1) Apatite/liquid partition coefficients for the rare earth elements and strontium. Earth Planet, Sci. Lett. 56,405-42 1. WOLFFJ. A. ( 1984) Variation in Nb/Ta during dilTere.ntiation of phonolitic magma, Tenerife, Canary Islands. Geochim. Cosmochim. Acta 48, 1345- 1348. WOOD D. A. (1978)Major and trace element variations in the Tertiary Lavas of eastern Iceland and their significance with respect to the Iceland gecchemical anomaly. J. Petrol. 19.393-436. WOOD D. A. (1979) A variably veined suboceanic upper mantle-genetic signi8cance for mid-ocean ridge basalts from geochemical evidence. Geol. 7,499-503. WCIRNERG., BEUSENJ.-M., DUCHATEAUN., GIJBELSR. and S~HMINCKE H.-U. (1983) Trace element abundances and mineral/melt distribution coefficients in phonolites from the Laacher See.Volcano (Germany). Contrib. Mineral. Petrol. 84, 152-173.
Abstract-Experimental action of partition coefficients(Df for Nb and Ta betweeu Ti-rich minerals (sphenc, rutile, ilmenite and Ti-magnetite) and coexisting silicate liquids of basaltic and&e, andesite and trachyte composition, indicate that these elemcms partition strongly into sphenc and rutile, and moderately into ilmenite and Ti-magnetite. The D values for spheric increase with decreasing temperature and with increasing SQ content of the coexisting liquid, but appear unchangd by pmwtre between 7.5 and 16 kb, and oxygen fbgacity betweeu ma&netite-wustiteand haematite-magnetite butlers. Also, 4nJ4, varies from 0.3-0.4 for sphenc, increasinS to 0.6-0.7 for rutile, 0.7-0.8 for ilmenite and 0.8 for Ti-magnetite. Thus ~aation involving these minerals may cause an increase in the Nbfra ratio of any derivative liquid; thii will be most noticeable whem sphene is involved.
GE~UL COHERENCY benvan the minor dement Ti, and the trace elements Nb and Ta is well established.This has receivedspeciaIattention in igneous rocks of convergentplate region&where these elemems are chamc@&ically low in relative abundance, comparedwith igneous rock suites from other tectonic settings(GILL,1981;PEARCE,X982).A better undies
procedu~anouttinediaG~Nand~(1985a),~d ~~~~~~~T~2.5~%~~~~ to each cotnposition, in each run. Oxygen fugachy (x02) in routine expariments cormsponds allay to the map
dte-Mte but&r(-EGG and PERSON, 198Sa), butom:experiment was conducted in which the sample cap sules (Ag-Pd) were packed in a ~~ti~~~~ (HM buffer)mlxenclosedInanou~gold~Thiswas~n for I’k hoursonly to ensureprezrvationof the HMbu&
(confirmed at theconcWon of theexperiment).
of the reason for such elemental distri-
butions may be obtained from knowiedgeof the IUb, Ta partitioningrelationsbetweenTi-ric&mineralsand coexisting silicate liquids, and the need for such data has been pointed out (CLACWE and FRFX, 1982). An
toallow glass in-
earlier experimental investigation (MczCLw_uM and C&IARRI’T&1978) d~rnen~ Nb ~tio~ be-
tween a numberof phases,inclu~o8 rutileand ilmertite, and basalticliquid at I atmosphereandfC& = MI X 10-“.s. The present exptvimemal study sets out to determine Nb and Ta partition coefficients Tiimi~ ’ ) for sphene, nttile, ilmettite and TiWt4b.T‘ mwwtite crptabing from 3 differentbulk compositions @a&tic andesite,adze and trachyte).In the case of sphene, the effectof pressure,temperatureand fG2 on the partitionrelation&tipsis examined. In addition, this workprovidesanalyticaldata on extensiive Nb and Ta substitution in sphene and rut&, which arecomparedwith naturalNb and Ta-richspheneand n&e. The lattermineralsare recordediiom two very di&rent host ro&s--kimberlites (e.g. I&KXXR’~Y, 1983) and pegmatites(e.g. PAUL etal., 1981;CLMu& etal., 1981); bothoccurrences appeslr to result from late cry&dlhtion from trace element and volatile-rich fluids or liquids.
The results are for synthesis experiments only, but are considemd to reptwent a close approach to equilibrium because ~~~~~~~~~timm~~~~. tioning experiments involving the rare earth elements (partition&t betweeu sphene, clinopymxene and liquid), where reversals and wiable-time synthe&s runs gave support for ~~~~ ntations (GRFJEN and &ARSON, 198&t, 19S6a).
~JWWTIZ
55
(1978) showed that Henry’s law was obeyed for
T. H. Green and N. J. Pearson
56 TABLE
1.
Rock compositions used in the experimental study. 1Ole.Z the Nb,Ta bearing glass was added to each rock composition. In addition 10, 7.5 and 7.5vt.X of sphere canrponents wre added to the basaltic andesite, andeaire and trschyte, respectivefy. *Additional oxides present are 5.1wt.X. Nb20s and 4.9wt.Z. Ta*05.
SfO2
llasalttc andesite
Andesite
54 ‘09
61.19
Ttachyte
65.42
*
glass
Sphene coluponent
57.1
TiO2
0.56
0.68
0.30
‘l2’3
17.76
17.10
16.46
38 .O
9 282
5.37
4.26
MI?0
0.19
0.13
0. IO
3.7
W
4.02
2.98
0.14
CaO
11.14
7.28
1.32
6.1
NE+
1.78
3.67
6.23
4.9
K20
0.56
1.39
5.71
4.9
P205
0 .OE
0.21
0.06
26.7
3.4
F
NbGontEats of 0.3- i .3
s
28.6
17.0
Fe0
i
Nb,Ta
of
wt.%
whi&
is close
to the Nb
jn Table3, T&&j&i content compositionsofmxistiog gl;hsses conten+ of the ghses were caicui8ted
in the itmen& anelyad in this work (0.8-I.7 wt.% Nb).
methad odined REsaT§
Anaiysesof Nb, Ta-bearingspbexte,n&e, itmenite and Ti-magm%ite are given in Table 2, together with
Rua No. 1044 ooap. M P&b) 7.5 T(*C> 1GiJO r(hrr) 6 n 11 EW** sphcna SfO* 29.37 ( .25) TiO 32.40 f .48) Cr203 2.59 (.lt) Alli PJ3 +ino
1.11 -
3 NsO
26.60 0.16 I 0.07
g& **;o:0
4.96 1.99 98 . 0.34
WTa Run No. Z$ n*ct t(hra) n Phue 9102 :“% cr203 PCs3
cao
Nb/Ta
1044 A 7.5 1000 6 12 sphanc 30.62( .a1 1 31.15(.74) 3.10( .34) 1 .OO( .06)
L.04( .05) D.O8( .Oi)
c.39) c.02)
26.59( 0.23t.04) .46) 0.09(.01) 0.11(.02)
25:72( .26) 0.32( .02) O.Zlf.01) 2.60( 5.82( 99 0:36
f.36) f.11)
2.31(.12) 5.55( .23) tzix 0.36
1046 A 7.3 950 S 6
1046
*pb&IWZ 30.8X 1 .OS) 30.34(t.07) 3.59 (*SO)
OPirrnc 29.28( .78) 31 .W( .SO) 2.54f .2?1
1.13
1044 T 7.5 1000 6 12 tIphc*e 29,20( .24) 32.24( .25f 2.17(&s)
(.Of)
C.02)
(*OS)
0.23 c.06) 25.36(1.01) 0.12 (.Oif 0.12 f-02) 2.87 c.20) 5.48 (.301 lw.11 0.45
7T5 950 8 6
in GREEN and PFiAFtSN
necwsarytxcwse of the mobility of a&&s -under iow vahleain the each of the mineraisare reportedin Table-4.
was
.26f .13)
1045 SA 16 1000 6 10 S,ph8nC 29.82( .29> 32.20< .54) 3.20( .19> l&5? -
.oat
t.09)
27.12(.28) 0.15(.02) -
26.54(.49) O.ZO( ,031 O.ll(.Ol) O.OP( ,011
0.27 0.26 -
4.47(.42> 1.97<.12) m 0:39
4.12l.35) 2.11f.12) 99 . 0.46
11.43 9.54 XK 0.11
1056 Ia4 A 4*2 1000 1.25 15 sphana 28.83( .39) 30.23c.33) 2.11( .13) 0.23(,03) 1.91( .I&)
1056 RM A 4_+2 1000 1.25 3 haxeaoiiaenite 0.31 (.OSf 40.40(1.181 1.71 <.OSl 0.11 c.011 44.61(1.81)
sphmna 28.#( .17) 30.7Of .52) 2.05( .091
Qagcet1tt O.!Sf .Of) 18,42(.31) 1.29( -04)
2.51(.161 0.14(.03) 26.37c.40) 2.59( .13f 6.&.W 0.35
: rut t le 73.77 C.39) 0.27 1.02 ((.04) .09) 2.56
LO56 RH SA 422 IO00 1.25 11
1.16(.04)
1045 A 16 1000
O‘95~.04)
1056 KM SA 4t2 1000 L .25 17
O.IO( .03) 25.69(.58) 0.34f -02) 0.221.04> 2.aoi .09j f.6H.16) ?xTT 0.42
1045 A 16 1000 6 10 siphcne 29&2( .64) 32.24( .38) 3.18(.15)
60”.;;I
3;;
3:01(:12) 0.24( .02f
O-36( .05)
O.bO
0.18(
.03)
2.95( .16) 6.131.29) RE 0:41
1006 SA 7.5 355 8 IO SplWiU 28.89 (.ZO) 30.11 (.Sfo 3.32 (.09f
1046 7% 950 a 3
Ilmitc 0.20 52.50 0.35
(.MI) t.741 (.021
1.09 0.12
(.03) C.01)
1.25 -
C.04)
39m 0.30
<.27) C.02)
t.05) C.03)
25.64 0.40 0.22
c.37) C.03) t.021
26.52 0.15 I -
(925) C.07)
0.33 5.40
1.101 (.04)
(c.36 .62)
5.05 2.68 ~lbb.ol 0.45
(.26) f.13)
6.61 2.85 x?K 0.37
t.63) c.17)
4.09 0.22
C.21) (.Ol)
0.05 2.42 2.37 96-i3 0.87
(&I) c.23) t.19)
_
1045 f 16 1000 6 14 aphent 29.65 c.36) 32.31 C.29) 2.45 (*ii)
_
1056 RI4 T 422 1000 1.25 L1 SP?WW 31.89 i .76f 29.16
1056 1M 4T* 1000 1.29 3 Iutile
0.94 1113 (.I31 f.oW ion 1.03 1044 7> 1000 6 8 pyroxracl
49.77 1.69 3.83
<.%) f.32) f.99)
c.32)
7 “83 0.16 13.64 22.50 0.21
( .34) c.02) c.23) (.35) C.03)
11.92 (.4&I 16.10(1.17> li)D;b
” 99Tb3
59.9Sfi.07) 0.98( I .09) 11.02
D .b?
Mineral-meltNb, Ta partitioning
57
Sphene analyses in Table 2 clearly show that it readily a~cornrn~~ Nb and Ta, favouring Ta over Mt for all ~rn~itio~ and ex~rn~~ conditions (L&J&~ varies from 0.3G-Q.44). The total Nb205 f Taz05 content is in the range 6-9 wt.% which is less than the highest content of these elements recorded in natural sphenes from pegmatitesand kimberlites(PAUL et al., 1981; HAGGEIZTY, 1983), but is ~~~~~tly higher than the 1.3 wt.% recorded by WOLFF(1984) in a sphene from a phcmolite. Table 4 shows that id generally increase with SiOz, shown conby comparison of the results for basaltic andesite and andesite compositions. However the D variation with commotion is less clear when the values obtained for the andesite and trachyte are corner, and it is possible that the high alkali content of the trachyte, compared with the andesite, at%ctsthe D vdws, in the opposite fashion to SiG and continns the suggestion of LARSEN (t979) and DRKSXLERet al., ( 1983)that D values decrease with increasing alkalinity The results in Table 4 also show that increase with decreasing temperature ( T), but are not si~ifi~~y affected by pressure (p) in the range from 75 16 kb, when D values far similar compositions at the same temperature are compared. Similarly, D vaiuesare unaffected byf O2variation between that co~~o~~ to the HM and -MW buffers at 1ooO*C,with variable pressure. (The precise pressure in the HM buffered run is uncertain, due to a technical problem in tbis particular run giving friction between the piston and bridge assembly in viewof the separately demonstrated lack of e&t of P on D, this is not a serious probI~m.)
Rutile was identified from a run on the tmchyte composition (HM buffer) and a run on the andesite ~rn~tion (-MW buffer), and shows very different compositionwith respectto Nb, Ta and Fe caption but has similarI&.&& (0.60,0.66f (see Table2). Natural niobian n&s also exhibit widely offing compositions,overpay the Nb, Ta and Fe contents obtained in the syn~eti~ rut&s (CERN~ et al.,1981; t%AGGERTY,1983). However,Nb-Tarutilesfromkimberlites also contain ~~~fi~nt Cr203, and in this respect are unlike the synthetic exampies.B%!!$@@’ for rutiiefromthe and~te and trachyteare~n~ly the same (Table4), despitecompositional,P andfQ differences.The rutile in the runs on the trachyticcornposition occurredas acicularcrystalswhich were impossibleto analyzeBe.eof glassinte~e~n~. Hence the analyseshave been correctedfor glasscontamination (5-20%),assumingzero SiQ content in the rutile.The rutiie from the andeaite occurred as larger rod-like tly a&c&d by glassinterference( Irrystals,only 241, and these werecorrectedalso. The am” vaiues obtained for the andesitic and tmchytic composi-
58
T. H. Green and N. J. Peamn TABLE 4.
Run
Partition ccefficients phase8 and cosx~stin~ Table 2.
cmp.
P
T
NO.
1044
1045
1046
BA A T BA A 2 h
T 1056 tar
FiA h T
Sphene %. 10.8 15.4 15.7 10.6 15.9 14.0 15.9 19.6 18.3 11.5 17.0 17.7
1000 1000 1000 1000 ioO0 1000 950
3.5 5.3 5.4 4.6 5.9 6.1 5.7
7.5 7.5 -4 4 -4
950 950 1000 1000 1000
7.6 1.6 4.8 5.6 6.1
Nb and melt.
Rutile
%b 7.5 7.5 7.5 16 16 16 7.5
tions in this wo* in sigkkantly
CD) for silicate
higher than the
*Nb
26.5
29.8
Ta bet*+en Abbrevfatione
Ti-rich as
Ilmenite ‘Ta
for
ugnctlte
%b
Dtb
2.3
2.7
4.6
6.6
DNb
‘Ts
0.7
0.8
44.0
44-7
DISCU!BION
The chcmiutl coherence of Nb, Ta and Ti in rut& ~~~~~~~~~=.~~~ in Tabk 4, and fkzionution ofonly aaxuory amounts thCtWOStUdkS. ofthcseminaots~baveainnifinnt~otlthe Nb, Ta ~b~ti~n and Nb/Ta ratio in dcrivativc magmas. However the much kxvcr Nb, Ta content, iwcr D vahss and Ma close to 1 for ilmenite and llmeniteoccun&intbcnmontbeba8alticaadesite titaIlomagn~~tetbatth#:pbnarwiubavca oompoaitionat 7.5 kb, 9H)*C,pad d siglesserelFa% on Nb, Ta abundawc and Nb/Ta b&wniiicantIymore N&OS (1.13 wt.%)and TazOs (0.94 iour. In oontrast, Yhc Ii&$&* ratio of 0.3 to 0.4 for ~.%)~~2)~~~~~~~ spheac dcmonstratcs that it may fmctionatc Nb and kimbulite (HACXXRTY, 1983). e vahw arc Ta,andsocw8echaqpsintbeNb/Taratioofderivlow (compucd with nttik and spbcnc) and ilmcnitc ativc magma& as su##m@d by WOUF ( 1984) for does not s&on&y fractiaorte Nb and To (D&Jr, sphene cryMMng ftem pbonoiites from Tencrife. = 0.84) (Tabk 4). The w VaIweof 2.3 is sigWo~s~~t~~~~a~~y~ ~~~y~~a~~.8~~M~ektmnc@vitythimTaitmayatiain2vabna&atcp LUM and ~MREITE (1978) which, as in the caec of (Nb”, Nd*) more tzadiIy than Ta Th& cot&i account ~~C,~y~~~~~ for~~~~~o~~~~ di&rcnas behvecn the two studies (viz. baa&k anTa. For ex&m$k, Nbfi may be kss oompatibie than dcsiteat95O”C~withbantt8t l&00-113Vc). Iw+ for &ubstitutioninto spbe5c and rut&‘ gnretuns, FOrf~C0D&iOn&UXrUp&&totbeHMbttfk, ~~~~~~~t~~ a “Arnold” was obtain& in the run on tbc ~~3+,a~~~~a~o~~~~~ and&c compo&ion. This accommodates Nb and Ta tutilc. However, in the ptwcnt cxperiment~, aim&r monmadiiythanilmc&ef~8tmuchkwerfO~ Nb/Taand~ocwTaiforsphtwn~(sceTabks2and4,coiquiqtbcdatafixtbc2ilittg for/G conditions coto the HM and mcnitcs),butissimiktothkQmcaitcinthatitdocs MW bufk conditiot~ at IOOOYI.Thus+ for this fol not strongly &action&&Nb, Ts (BP&&~ = 0.69). faoet, which encompancd theI@ deduced for the Tcnerife pbono&es, the e&ct propowl by Wolff is not evident (Wow, 1984). ( 1978X(26-30 compared with 16), probabIydue to
~~rn~~~~~~~
59
Mineral-meltNh, Ta partitioning
be considerably less if a T&rich magnetite co-precipitates. EWART(1979)tabulates intermediate-sihcicrocks with up to 0.4 vol.% sphene or up to 2 vol.% Fe-Ti oxides as phenocrysts. Similarly, intermediate to evolved rocks of transitional alkaline or alkaline intraplate volcanic series frequentty contain sphere and/or Fe-Ti oxides as phenocryst phases (RTTONand HUGHES,1977; CORNEN and MAURY,1980, POSTALet ai., 1982, WIRER ez al., 1983;WOLFF,1984)with the proportion of sphene involved possibly reaching 1.5 wt.%, but more commonly in the range of 0.5-l wt.%. Model calculations summarized in Table 5 use geochemical data presented by DOSTALet al. (1982) for a subalkaline (with some transitional alkaline afhnities)series.These data include Nb, Ta contents of hasalts, andesites, and dacites and rhyolites which were suggestedto be genetically linked via crystal fractionation. The observed Nb/Ta ratio varies from 15-17 in the dacites where sphene was recorded as a phenocryst phase to 20 in the low SiO, rhyodacites. Variable involvement of sphene (0.5, 1, 1.5 wt.%) in a fractionation scheme is shown in Table 5 to change the Nb/Ta ratio to 19,20 and 21 respectively, from an initial ratio of 17.5. Thus with up to a maximum possible amount of sphene fractionation in relatively evolved magma suites, a possible20% change in the Nb/Ta ratio may occur. In the case of crustal melting, two calculations are given corresponding to pressures of 5 and 12 kb (Table 5), based on a maftc source composition and modal proportions from HELz (1976) and HELLMANand GREEN( 1979) and assuming an initial concentration of 3.1, 0.2 15 ppm Nb, Ta, respectively, with Nb/Ta = 14.4 (m~elling on typical N-type mid-ocean ridge basalt-WOOD, 1979). This shows that silicic melts with Nb/Ta of 16.8 and 16.7 may be derived at 5 and 12 kb, respectively. The final model calculation adopts the eclogite melting hypothesis for derivation of intermediates&ic
MANand GREEN( 1979)suggeskd that residual sphene and/or wile may occur under hydrous melting conditions in the deep crust Thus for intermediate to silicic magmas, Nb/Ta may not be a reliable indicator of possible genetic links. However, for mantle compositions and melting conditions producing the more common and voluminous basalt compositions it is unlikely that a residual ‘&rich accessory phase will remain in the source region, (GREENand &+&SON, 1986b). Thus the Nb/Ta ratio of basahs may be expected to reflect the Nb/Ta ratio of their source region. This argument does not apply to magmas such as kimberlites and highly undersaturated nephelinites which represent fluid-present and very low degrees of melting of the mantle, such that a Ti-rich phase may remain in the source region (GREENand PEARSON,1986b). In Table 5 some relatively simple models of crystal fractionation and partial melting am presented, incorporating Ti-rich phases, and examining the possible effect of these phases on Nb/Ta in derivative magmas, using the &,,r* values determined in the present study. The proportion of Ti-rich phase invokd in the modeliing is constrained by (a) observed abundance of Tirich phenoctyst phases in some magma suites and (b) TiOr content of silicate liquids saturated in a Ti-rich phase (GREENand PEARSON,1986b). In convergent plate boundary suites EWART(1979) noted that sphene occutred as an accessory phenocryst phase in high-SiQ andesites-rhyolites in high-K suites from Western U.S.A. (both Eastern and Western Belts), Western South America and Mediterranean provinces. The TiOr+aturation curves for intermediate-silicic liquids from GREENand PEARSON(1986b) indicate that highSi& andesites to da&es, typically with 0.60.8 wt.% Tic&, may fractionate and produce rhyohtes with 0.2-0.4 wt.% TiOz at temperatures of 850-95O”C, via separation of a Ti-rich phase (amongst other phases) such as sphene. The change in TiOz content represents a maximum of 1.2wt.%sphene fractionation, but may
1.
cry*tr1
fractionation
Hypothetical dacttc: Fractionation Pbues fractimatcd:
0.6-0.9X
product:rhyolirc
252 pl~~foel88e. 1.5x epilene
5% cli~~~pyroxeae,
DNb Ta wed: 0.05, . Famultant Nb/Ta of 2.
Partial (a)
rltiag
0.05: IiquSd
of ufic
0.4, -
DNb,Ta used:
u
&ova.
Nb/Ta of
0.4;
18.9,
m”tve:
Cru8ral depths 5kb: Pbarm fractionated: LZkb: Phmes fractienrtcd:
Resultant
T+, 35ppr Nbb, Zppm Ta, NblTa : 0.1-0.2X Ti02 2% Ii-m.gnetite
0.4.
0.7.
20.1, 3.1.
0.3,
2.7;
a 16.8 (5kb)
0.3;
0.6;
and either
6.7,
0.5,
1.0,
17.6 respectively
21.3 with 0.5. 1.0, 1.5% aphane, respectively.
O.ZlSppa
762 amphibole, 74% mphlbole.
plus 2.3,
liquid
5% hornblende.
17.5
0.3.
Nb.Ts respactivcly;
3.2% pl~loclasc, 5.2%
Nb/Ta .
1% ilwmita,
,j.met, 21clinopyrox,,ne,
0.3
for
Ilmite
and gmmt,
46.5% gamer,
0.8% eplwnc 0.8% sphcne
m,pactivcly.
and 16.7 (12kb).
(b) Mantle depths 30kb: Phaes fractionated: 30X cllnopyroxew. DNb Ta aa rbovc. plus 26.5, 44 far turflc. * Pasultent Nbh of liquid - 16.6
14.4.
0.6% nstilc
60
T.
H. Greenand N. J. Pearson
mw from subducted oceanic crust (GREEN and rutile is recorded in minersd assemblages inteqmtcd RINGWOOD,1968). The Nb, Ta values from WOOD as upper mantie in origin, associated with hypothetical (1979) which were nsed for the crustal melting model mystic events related to the ~~tion’of magmas were also adopted for tbe oceanic crust, converted to belongingto the alkaline suite (e.g. alkali olivine basahs eclogite. Gill’s particular eclogite melting model for to melilitites) (JONESet al., 1982;WAS and ROGERS, generation of dacite (GILL, 1974) was followed. This 1980). involves only 0.6 wt.% rutile, and the calculated Nb/ DNb, I& values for pyroxene and garnet are not Ta of the derived dacite is 16.6. well constrained, but appear to be low (typically, Thus relatively small amounts of sphene and/or ru- D%-’ am taken lo be 0. l-see PETARCE and tile involved in melting m at either crnstal or NORRY, 1979; no public data are av~b~e for mantle depUxqmay tly alter the Nb/Ta ratio+ B%x@“‘@t 1.The present work can only indicate semiand the derived melts need not have NbfTa reelecting ~~nti~tive values of &I$*-* of KO.3.There is the maftc source undergoing partial melting. no Sutton of f~o~tion of Nb, Ta from each other by either clinopyroxene or garnet. Thus it is unlikely that involvement of reIatively abundant silicate phases in the generation of alkaline suite magmas could (a) Basaltic suites-fiationation und mantle melt- account for variation in Nb/Ta, and the role of a Tiing. Experimental and trace element studies on basalts rich phase in the source region is supported further (cf: indicate that thokiites are produced by relatively large CLAGUEand FREY,1982). degrees of melting of the mantie souroe region, while (b) Interrn~i~t~sii~~~~ suites: fr~io~tio~ and alkahba!Bl~tbrougb crustal melting. Ewart and co-workers @WART,1982; ilitite are derived by EWART et al., 1977, 1986) pubis Nb, Ta data on melting (GREENand a number of widely separated, evolved, alkahne suites 1978). WOOD(1978) from Eastern Australia. The total range in Nb/Ta for all suites is from 8 to 35, and there is no advent correlation with host rock type, although one suite (Glass House Mountains) shows HbiTa of 11-f.2 in trachyte, Incas to 35 in comendite. However no for the degree of melting envisaged for these rocks. potential Nb/Ta ftionating phases such as sphene klso,Woo~(1978)~~b~~~~tobe
Mineral-melt Nb, Ta partitioning
the derivation of these compositions (GREEN and PEARSON,
1986b). Hence Nb, Ta depletion in these magmas suggests a source region depleted in these elements (GREEN, 1972). CONCLUSIONS Sphene and rutile readily accommodate Nb and Ta, and show high D vahres for these elements. D@$‘@‘@ exhibits similar variation with composition and temperature to that established for other trace elements, such as the rare earth elements (WATSONand GRIZIZN, 1981; GREENand PEARSON,1983, 198&b). However no sign&ant effect of pressure orfO1 was observed on eti. The Nb/Ta ratio for sphene, rutile and Ti-magnetite is significantly lower than for their coexisting liquid. Thus if these phases are involved in magma generation or &actionatio~ an increase in Nb/Ta may be expected, as documented in some evolved suites where there is evidence for sphene crystallization (Tenerife phonolites-WorJF, 1984). Modelled NbJfa variation for up to 1.S wt.% of sphene fractionation shows a 20% incmase in Nb/Ta. This is much less than the 250% increase observed in the Tenerife suite. However L&J &, for sphene/liquid obtained experimentally ranges from 0.30-0.44, which is very close to the value of 0.46 obtained by WOLFF(1984). At least part of the difhince in increase in Nb/Ta between the natural and modelled processes could be due to the model&l process using higher temperature D values, for less evolved and less silicic liquids compared with the Tenerife situation, where higher amounts of sphene may also fractionate. In spite of these biting factors, the very large increase in Nb/Ta in the Tenerife suite cannot be explained adequately by the present resuhs. Apart from this suite, there is, so far, little evidence for a consistent pattern of variation in Nb/Ta in igneous rock suites, but the partition data recorded here suggest that determination of such variations may be a useful petrogenetic indicator for the role of Ti-rich phases. COnvermly, differing Nb/Ta ratios for igneous rocks in a given province may not be reliable indicators of different sources, if sphene or r-utile(or to a lesser degree, T&magnetite and ilmenite) were involved in the derivation of the rock suite. Acknowk&emenrs-This msearch has been supported by grants from the Austmhaa Research Grants Scheme and Mat’quarieUniversity. Drs. A. E. Ewart and J. A. Wolffkindly ctiticaIly read the mannscript, and Drs. J. Jones, J. Longhi and I. HdcIuum provided constructive journal reviewswhich also ettcouragedsign&cant improvement of the final product. Editorial handling: E. B. Watson
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61
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CLAGUE
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CORNEN
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FTITON
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T. H. Green and N. J. Pearson
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