373
The CanadianMfuuralogist
Vol. 34, pp. 373-387(1996)
CARBON
OF REGYCLED
CONSEOUENCES
IN CARBONATITES
DANIEL S. BARKER
Sciences,
Cl100,TheUniversiEof Temsat AustiaAwtin, Texas78712,U.S.A.
Departnuntof Geological
ABSTRACT
Formerly, carbonatiteswere thought to be resultsof the mantle'spurging itself of "juvenile" carbon.However, evidence
has beenaccumulatingthat carbonfrom the crust is recycleddeepinto the mantle by subduction.This non-juvenilecarbon
may nourishcarbonatiticmagmas.The processby which carbonin the mantlemay eventuallybe incorporatedilrto carbonatite
in the crust is probably a circuitous one, involving transportin a variety of phasesbefore accumulationin diversesourcesof
carbonatiticmagna. With time, thesesourceshavebecomemore varied and more radiogenic.During its migration, carbonate
liquid actsas an efficient sequesteringagentin the mantle,gatheringa distinctiveretinue of traceelements,especiallySr, Nb'
Sa, the ligbt rare-earthelements,Pb, Th, andU. Elementsthat are$tronglydepletedin carbonatitesareSi, A1,Mg, Cr, andNi.
Partition ioefficients for carbonateliquid/mineralsyreld calculatedenrichmentsthat do not agreewitl thoseobservedunless
therearerepeatedeprsodesofinteraction betweencarbonateliquid andprogressivelyemichedmantle.Carbonate-richliquid can
only surviveits upwardpassageif the rock throughwhich it flows has alreadylost most of is capacityto reactwitl carbonate
liquid. Such"path clearing"requiresthe abortedascentof precursorbarchesof carbonatiticmagma l-Iltramaficxenolithsfrom
many localities have trace-elementand isotopic enrichmentand depletion pattems atfibuted to metasomaticreactionsof
xenoliths indicate repeatedinvasion of the
lithosphericmantle with carbonate-richliquids. These carbonate-metasomatized
lithosphereby carbon-bearingliquids or fluids. This in turn implies that the rarity of carbonatiticmagoa in the uppercrust is
causedby barriersto its ascen! not by a shortageof sourcematerial.
Kewords: carbonatite,Iithosphericmantle,metasomatism,carboncycle.
SovnuanB
On consid6raitcourammentle magmatismecarbonatitiquecommemanifestationd'une purgationdu marteau en carbone
"juv6ni1e".Toutefois, il devient de plus en plus dvident que le carboned'origine crustale est recycl6 dans le manteaupar
subduction.Ce carboned'origine non juv6nile peut r6-apparaltresousforme de magmacarbonatitique.Ir cheminementdu
carbonerecycl6en manifestationcarbonatitiquedansla cro0teseraitde toutedvidencetortueux,impliquantun tranfertdansune
vari6t6de phasesmindralesavantI'incorporationdansun magmacarbonatitique.Avec le temps,les sourcessont devenuesplus
vari6eset plus radiog6niques.Au cours de sa migration, le magmacarbonat6agit commesolventimportant dansle manteau,
et accumuleainsi une suite distinctive d'6ldmentstraces,particulidrementSr, Nb, Ba les terresrares l6gbres,Pb, Th et UPar contre,les 6l6mentsSi, Al, Mg, Cr et Ni sont fortsment appauvrisdansles carbonatites.l,es coefficientsde part4geentre
liquide carbonatitiqueet min6raux semblentpr6dire des enrichissementsqui ne concordentpas avec les enrichissements
observ6s,d moins qu'il y ait eu des 6pisodesr6p,6t€sd'interactionentre liquide carbonatdet manteauprogressivementenrichi.
Un liquide carbonatdne pourrait subsisterau cours de samont6eque si les rochesle long desparois desfilons nourriciersont
perdu leur capacitede r6agir avec ce liquide. Une telle pr6parationdes chenauxnourriciers implique la pr6sencede venues
prdcocesavort6esde magma carbonatitique.Des xdnolithesultramafiquesi plusieurs endroits timoignen! par un sch6ma
d'enrichissementset d'appauwissementen 6l6mentstraces et en isotopes,de r6actions m6tasomatiquesdans le manteau
desx6nolithesindiqueraitI'invasionr6p6t6ede la lithosphdre
lithosph6riqueimpliquantun liquide carbonatd.La m6tasomatose
par des venuesde liquide ou de fluide contenantdu carbone.La raretd de manifestationscarbonatitiquesdans la cro0te
supdrieureseraitduesurtoutauxentravesd lamobilitl du magmaau cou$ de sonascension,plut6t qu'i unep6nuriedemat6riau
appropri6d la source'
(Traduit par la Redaction)
cycle du carbone.
Mots-cl6s:carbonatite,manteaulithosph6rique,mdtasomatose,
374
TIIE CANADIAN
REcycl,n{coF CARBON
INTorHEMavrE
MINERALOGIST
present in the mantle as the CO]- component in
magma;as COr, CO, and CHo componentsin a fluid
phase,as the solid phasesgraphite,diamond,and SiC
(Mathez et al. 1995), in intergranular films of
amorphouscarbonaceousmaterial, and in tetrahedral
sites of silicates and oxides. Wang et al. (1994)
provided a brief review of most of thesealtematives.
Phaserelationsin carbonate-bearing
peridotitehave
beeninvestigatedmost recently by Green& Wallace
(1988),Falloon& Green(1989,1990),Canil & Scarfe
(1990),WyTheet al. (1990),Ryabchikovet al. (1991),
Thibault et al. (1.992),Edgar (1993), and Sweeney
(1994). Dolomitic liquid coexists with peridotite at
20 to 50 kbar and930 to 1200'C (Sweeney1994).As
discussed
by Ryabchikovet al. (1991)"the solidusfor
apatite-bearingcarbonatedlherzolite + H2Ois closeto
the range of stablecontinentalgeothermsat 30 kbar.
Consequently,small yslumss of carbonate-richmelt
may form at many placesand times in subcontinental
mantle. However, liquid dominated by carbonate
differs from silicate liquid in im.portant ways in
addition to composition. The essential component,
COr2-,does not exist at all times and places in the
source rock and may be formed by oxidation of
elemental carbon, carbon monoxide, or metfiane.
Generation of carbonate-richliquid, in contrast to
silicateliquid, requiresa rangeof oxygenfugacity that
may not everywherebe attainedin the manfle (Dalton
& Wood 1995).Furthermoreo
carbonate-richliquid can
react with silicate wallrock in such a mannerthat tne
liquid decomposes
by evolutionof CO, asa component
of a fluid phase;seebelow, under 'Halting the rise of
carbonatitemagma".As a result of thesecomplexities,
recycled carbonbetweenits subductionand its return
to the crustmay go throughseveralphases.
Evidencefor retum of carbon to the mantle from
the crust comesfrom very high-pressure(coesite-and
diamond-bearing) assemblagesin metasedimentary
rocks, from experimentsat pressures,temperatures,
and compositionsappropriateto subductingcarbonatebearing oceanic crust, and from mass-balanceand
isotopic sonstraints.Schreyer (1995) and Harley &
Carswell (1995) reviewed the occurrencesof very
high-pressure metasedimentarycarbonate rocks,
exhumedfrom depths of 100 to 130 km. yaxley &
Green (1994) showedexperimentallythat calcite and
dolomite can survive in subductingoceaniccrust as
refractory phasesin eclogite, even in the presenceof
silicate melt. Staudigel et al. (L989) calculatedthat
subduction of altered oceanic crust containing
carbonateveins is an efftcient way to return carbonto
the mantle. Varekampet aI. (1,95D found thar gases
releasedat volcanic arcshave higher valuesof CPHe
than do those releasedalong mid-oceanridges, and
concludedthat more that SAVoof CO, exhaledby arc
volcanoeswasrecycledfrom subductedcrust.Thang&
Zlraidler(1993)usedthe ratio CFHe in gasesreleasedat
mid-oceanridges to estimatethe flux of carbonfrom
the mantle, and found that this rate could provide all
of the carbonin the Earth'satrnosphere,hydrosphere,
andcrustin 3 b.y. Suchrapid degassingsuggestsreflux
of carboninto the mantle.Their modelingindicatesthat
the total amountof carbonin the crust has decreased
since hecambrian time, a conclusionalso reachedbv
Kostervan Groos(1988).
These independentlines of evidenceshow tlat a
substantialproportion of the carbon in carbonatites
could be recycled.A suggestionthat'Juvenile" carbon
be incorporatedin the definition of carbonatite(Barker
1989) is thereforeunduly restrictive. The aim of this
Colposmoxs oF CARBoNAmE
paperis to examinethe consequences
of recycled,as
confrastedto juvenile, carbon for the generationand
Table 1 comparestle averagecalciocarbonatiteof
distribution of carbonatites.After considerationof the Woolley& Kempe(1989)with estimated
compositions
mantlephasesthat containcarbon,the compositionsof of pyrolite mantle (McDonough & Sun 1995) and
carbonate-richliquids andtheir metasomatizingeffects continental crust (Wedepohl 1995). The relative
are summarized, followed by distribution of enrichmentsand depletionsare given in tabular form,
carbonatitesthrough spaceand geological time, and rather than the familiar spidergrams,so that a more
finally, the factorsthat inhibit the ascentof carbonatire complete list of elementscan be presentedwithout
magmasandthus causetheir rarity in the crust.
assuminga parricular order of incompatibility. The
comparison is weakenedby uncertaintiesin the
CansoN-BEAR[.Ic
PHesesn{ THEMANTr-E
carbonatiteaverage;for example,the estimatesfor Sm,
Eu, and Dy are based on only two analyseseach.
Experimental studies show that dolomite and Although dolomitic magma can be primary @ailey
magnesitecan be stablein the deepmantle @alton &
1993a, Sweeney L994), the averagesfor magnesioWood 1993b, Biellmann et al. 1993. Fiedfen et al. carbonatiteand ferrocarbonatitereportedby Woolley
1993).Carbonateminerals,however,axerare in ultra- & Kempe (1989) are basedon fewer analysesthan for
mafic mantle-derived xenoliths (Berg 1986, Smith calciocarbonatite,so the lafter were taken as represen1979, 1987). Decompressionduring ascentprobably tative of carbonate-richmelts. Furthermore, many
destroyscarbonatesin most xenoliths(Canil 1990), Mg- and Fe-rich carbonatites appear to be of
but some relatively coarse-grainedcarbonateshave secondary,
subsolidus,
origin (Barker1993).
survived (Ionov et al. 1993). Carbon can also be
Partition coefftcientsfor carbonateliquid yersrr the
375
RECYCI.EDCARBON IN CARBONATITES
TABLE 2. E'PERIMB{?AL PARTMOAI COEFFIOEN1S
(CARBONAIE LIQUID/'IUATBRAL)
TABLE 1. CARM}NAITT& ENRICHMBNTS AND DBPI.ETIoI.IS
@b@S!l,brcEtez
st
Ti
AI
Mn
Mg
Ca
Na
P
cl
s
B9
S!
G
Oo
Nt
C!
7a
Rb
Sr
Y
7t
Nb
Ba
IA
Ce
Nd
50
Bo
Dy
Yb
Ib
Th
U
stottelolilmat
0.(b
0J5
oa
038
3.85
0.05
t39
0.81
EJ9
102.
&16
116
41.1
30.4
333
0.43
0.98
o.ffi
0.10
0.@
0J0
3,42
36
n.6
t:t.6
1830
62
93E
1007
76
253
m
6v
4A
cud
004
o.2,
0.o7
0J5
opt
cp!
50g
50g
>7b
ulg
0.4b
0.39
7J s
0.7f
E38
0.6 b
03b
ojg
>250b
3.6s
2,4b
orjg
>ng
Na
503
1.69
10.9
r0.0
0,44
0.&
0.10
0.4
0,32
0.96
2,N
0.lt
2t9
4.96
0.91
63,4
l.os
Jtm c
50g
l9S
33a
23c
t25f.
339
333a
590b
lxs
6s
14b
lES
>338
5.69
5a
33f
3Jg
0Ja
03b
0.7E
5a
>758
3e
Llt
3'4B
l9a
025b
lr5 8
4a
l25b
>lm8
lma
217a
333a
&,b
>209
l67a
2130c
2lNe
143f
!)3t
>50b
>nE
6c
Glld
43e
11.1I
459
5.9s
Ag
Ag
..
n3
^l
a5
30.0
835
2,fi
3.?8
612
5.12
Fl
ol
509
0J0
9.r2
0.@
0.10
7-50d
509
to
17a
6b
lma
llb
mg
ug
19
a c@44t o92),25r6a,1Wq b Swqet€t4L(1992)'tb@'1mg
strc
c B,@ & Wsrr@(1991),15!6@,1f0qq d Rlrbddtw a ar (194)' m ltu' 950o(l
f Kl@ d ai (195), 4-22 rba' 1060-llrc
e t@ d ar.(191), 55iba, llfq
g sv@y a 4t (19t, 1&46tS6, 1100-D5fC lvii4l stEbols ol olieh&oF ctiopyo@, .pr cliroly@@ gn gEGa,adE@ohiboto,
lht Dhloplile.
1 Arslsc @ldocsb@8d& of w6[ry A Koop6 0989).
2 SIX@reBoth - Pyrohe of McDom!! & Su (195, p. 238).
3 Cdffil
6od of we&'pohl (1995,p. 1220).
minerals encounteredin peridotite have been experiby Brenan& Watson(1991),Green
mentallymeasured
et al.
et al. (1,992),Ryabchikovet al. (1993),Sweeney
(1992,1995),Joneset al. (1995),andKlemmeet al.
(1995).Dataon partition coefficientsfrom experiments
at 15 to 55 kbar and 950 to 1250'C are summarizedin
Table 2. Table 3 includestheseresultsand atlemptsto
fill out the datasetfor Rb in olivine andorthopyroxene,
Ba in olivine, and Ce in phlogopite, using mineraU
mineralpartition coefficientsfor phasesin equilibrium
liquiil
with basaltic liquid and the equation Dtrbonate
mineral A =
Dmineml B/mineml A ; pmbonate
K
the predicted emichmentsare maxima. One singlestage equilibration of average lherzolite with IVo
carbonateliquid will not producethe large
enrichments,and repeatedfertilization of therzolite
through multiple encounterswith carbonatefiquid is
needed.
Although carbonateliquid is efficient in scavenging
sometraceelements,it is eclectic,and
andsequestering
somecarbonatitesshowlittle enrichmentin thesesame
liquiilmineml B. The
mineraUmineralpartition coefficients are all taken
from data in Green (1994) for internal consistency.
Bulk partition-coefficients were calculated for
carbonateliquid equilibratedwith lherzolte containing
60VooiJvrne,20Voorthopyroxene,lQVoclinopyroxene,
'7 garnet, amFhrbole, l%ophlogopite,with the
Vo
2/p
and
assumptionthat all these phasespersist in the solid
assemblage.
enrichments
For Sr, Nb, Ba and Ce, the ooobserved"
(average carbonatite/pyrolite,Table 1) are at least
20 times those predicted fot 1.Vomelting in Table 3.
Becauseaccessoryphaseswere ignored in the calculations in Table 3, the bulk partition-coefficientsfor
therzolite/carbonateliquid are minimum values. and
TABLE 3. CATCUI,ATED PARTMON-COEMCIENTS
FOR LIIERZOLITSCANBONATE LIQUID
ol
K
T1
Rb
s!
Y
k
Nb
Ea
Ce
(L(D
il)s
0.m
O(D
oJb
oM
uE
0.m
O@
opr
qs
0.o2
0.13
0.004
0.$
0.16
0.t
0.01
(uF
onj
0s2
o.fit
x
0"'213J20.U.4
0.M
0d!,
0.42
0Jr,
0.x
om
M
o,t
L4i:t
o29
0J
0J
(L17
0.01
0.01
0M
o09
ottt
00J
0,22
0!2
8rt
aep
!fi1
Bs&Dl
Pedldedz
33
0.10
9
4!
0(5
o04
o$
u,
I
0n
0.05
0.6
0.19
0.16
0.c]
0.05
0.0J
r7
l7
5
6
6
17
11
7% gM"
lM' cliryl|o,@,
1 Fd&@oliE
odaitrirg ffi oltvfu.A% @t1@,
h li@o[ib,
liqdd/@!ad@
2qo Nntihta,l%
2 C@r€dad@ in @t(@
frlogqitt
trd vabs h bold&e @
oloilor.d 16 1% @b@
liSdd esdlihalrd eith lbralt6.
sipwt
of eqsioeolat dda &od Toils q o$6s re €1618&d 6 dr€qlbgd in fte m
gt89dd,a8D @@ol&
Mi@tsynbols:
ololivb, ogrorhopy@Arolilopy@,
pltl phlogopite.
376
TIIE CANADIAN
MINERALOGIST
ETIT2
{14
0.700
EMl
G'
0.705
v)
o.704
0.703
o/
0.702
0.701
12
Q.5
17
1o1s-;</
,rav;zP\e
l.o/
rlee 1lo!fiHo
,L.*,
. 4-,53e
E_1165
B
v)
F
o
E-61
?il
1g7E^s119
'\p
{
^
E-v
/
83
a.:
2ooG's
tr
I
^p
)n/
-.i
r s
..J
-.'
2605-q
9:2680
14
G-'1087
1.900 DMM
cf
\szz
16
18
23
HIMU
2A
22
e/Pby2.ot6
ftc. 1. Minimum reported initial 875r/865ryg1573minimula initial 26PbP&1Pbfor
carbonatites,comparedwith the mantle-sourceend-membercomponentsof Zindler
& Hart (1986)andHart (1988).DMM: depletedMORB-sourcemantle,HIMU: high
U/Pb mantle,EMl: enrichedmantle 1. EM2: enrichedmantle2. Bulk-earthevolution
curveof Kwon et al. (1989,Fi9. 14.8)is labeledin Ga.Numbersbesidedatapointsare
ages(Ma). Data for Figures1 and 2 are in the Appendix.
elements(for example,Nb is not enrichedat Mountain carbonatite, and 53 source ref'erences.Neodymium
Pass, California or in many African and Canadiax isotopic dataarenot includedbecauseof the difftculty
carbonatites:
Bell & Dawson1995,Paslicket al. 1995). in ascertaining,from many of the publications, the
The large scat0erin isotopic ratios as well as in trace- normalizationusedfor initial ratios and eN6.
elementconcenfrationsreflectsthe variationin sources
If some carbon in carbonatitesis non-juvenile, its
from which the carbonate-rich liquid drew its recycled nature should be shown by the 13Clr2C
components.
yerszs isotopic ratio. This ratio, as expressedby 613C/oo
Figure I, a ptot ofinitial 87SrF6Sr
[email protected],
relates carbonatitesto the mantle-sowce relative to PDB, rangesfrom -8 to -2 in carbonatites
end membersof zindler & Hafi (1986) and Han (Deines 1989), from -2 to +4 in marine sedimentary
(1988).Thesesourcesarebestviewedas abstractions carbonates,and from -30 to -10 in organic (reduced)
that refer to conditions,ratherthan as discretevolumes carbon@aure1986).SamFlesof diamondgenerally
of mantle. Tilton & Bell (1994) showed rhar range from -8 to -2, but those in eclogite are from
carbonatitesyoungerthan200 Ma fall in this plot in an -34 to +4 (Pearsonet al. 1994);gaphite in peridotite
array between EM1 and HIMU end members,and and eclogiterangesfrom -20.3 to -2.8 (Pearsonet al.
those carbonatitesthat are older than 2.0 Ga plot in a 1994), and SiC is -29 to -22 Q{athez et al. 1995).
tight cluster that indicates a much more depleted CO2-rich fluid inclusions in olivine from mid-ocean
source. Figwe I of this paper adds carbonatitesof ridge and oceanicisland basaltsare-15 to -5 (Mattey
intermediate ages, and shows trends of increasing et al. 1994).
diversity and more highly radiogenic sources(with
The observedrangein 6l3C of carbonatitesdoesnot
respect to Sr and Pb) through time. Phalaborwa overlap values of either marine carbonateor organic
(2060 Ma) and Fen (539 Ma) are strongly anomalous carbon.Taken at face value. this would indicate that
data-pointson this trend; they indicate that radiogenic recycled carbon does not contribute to carbonatites,
anddepletedsourceswerelocally availablein Archean unlessit is the appropriatemixture of marinecarbonate
andin Paleozoictime, respectively.
andorganiccarbon.Deines(1989,p.3n) haspointed
The compilation of data on which Figures I and 2 out tlat because carbon is a major element in
are basedis an Appendix to this paper.This appendix carbonatitesbut a trace elementin the mantle,it must
lists minimum reported ages,87SrF6Srinitial values, be extracted from large source-volunes and thus
ar.d 206Pb[2MPbvalues for 88 occurrences of homogenized,convergingon a relatively nanow ftlnge
RECYCLEDCARBON IN CARBONATITES
of 613Cbetweenthose of sedimentarycarbonateand
reducedcarbon.
Fractionation data ue sparse at the pertinent
temperaturesand pressures,but Matley et al. (1990)
havemeasuredcarbonisotopefractionationat 1200to
1400'C and 5 to 30 kbar; for CO2vapor equilibrated
with either silicate or carbonateliquid, the vapor has
and this fractionation
the higher 613Cby 2 to 2.4Voo,
appearsto be independentof pressureand temperature
over the ranges investigated. Between immiscible
silicate and carbonateliquids, there is apparentlyno
fractionationof carbonisotopes.Other experimentsat
mantleconditionsarelacking,but fractionationfactors
at 600'C (Faure1986,p. 500) shouldprovidemaximum values for fractionation at higher temperatures.
CO2 vapor equilibrated with methane,diamond, and
calcite at 600oChas the higher (more positive) 613C
values by 9, 7, alLd 2.5Voo,respectively; graphite
equilibrated with calcite and with diamond has the
lower values by 5 and 0.5%o,respectively.These
estimatesallow little changein 6l3C valuesby single$tageprocesses,but multiple stepsin the progression
from subductedcrust to solidified carbonatitemay
'Ihe
613Cdatathus
accountfor the observedvariations.
in indicating a
enrichments
support trace-element
multistageprocessto generatecarbonatiticmagma.
Macua as a
CansoNA.Tmc
MsresoMAuznlc Acn,ru
Many ultramafic xenolithsfrom lithosphericmantle
show textures and mineral assemblagesthat indicate
modification of the original rock before the xenoliths
were entrainedin the magmathat broughtthem to the
surface.New minerals include clinopyroxene,spinel,
phlogopite, amphibole and, rately, apatite. Some of
these xenoliths are enriched in light rare-earth
elements,Ba, Rb, Sr, Pb, U, andTh, and arerelatively
depletedin Ti, Nb, Ta, Zr, andHf (Meen et al. 1989,
Dautiaet aI. I992,Ionovet aI. l993,Hutiet al. 1993,
Rudnick et al. L993). These changesare generally
attributed to interaction of lithospheric mantle with
a carbonatitic magma. Roden el aI. (1994) alo'd
Sweeney et al. (1995) summarizedthe criteria for
carbonatemetasomatismof mantle sources.Silicale
liquids and hydrousor COlrich fluids are considered
less likely to produce the characteristicenrichments
and depletionsbecausethey cannotdissolvesufficient
amounts of the transportedelements (Schneider &
Eggler 1986,Watson et al. 1990) and have too large
dihedralwetting anglesto permeateultramafic rock of
the mantle(Watson& Brenan1987).Carbonateliquid,
on the otler hand, has liquid/mineral partition
coefftcients,solidus relations,and physical properties
that make it a likely agent of some mantle metasomatism(Green& Wallace1988,Meen et al. 1989,
Watson et al. 1990, Grwn et al. 1992, Ryabchikov
et al. 1993,Baker& Wyllie I992,Sweeneyet al. 1992,
377
1995, Brenan & Watson 1991, Minarik & Watson
1995).
On comparingnormal lherzolite of the lithospheric
mantle with that moffied by carbonatite-induced
metasomatism,the latter has higher valuesfor CalAl,
Na/Ca,CalSc,Ba/Rb,Ba./Nb,MlTa, SrlT4 LalTa, and
Sr/Sm"lower valuesfor TilSr, Ti/Eu, atdZrlY (Green
et a\.1992,Rudnicket al. 1993,Sweeneyet al. 1995),
and lower IIflSm @upuy et al. 1992).WhetherLalNb
increasesor decreasesis uncertain(Ionov et al. 1993,
Rudnickera/. 1993).Hauri et al. (1993)alsodescribed
metasomatizedperidotite xenoliths from Samoa and
the Austral Islands in which Pb is more highly
radiogenic than Sr, and Sr more so than Nd. They
attribute the metasomatismto carbonate-richmelts
from recycledcrust.
Pearsonet al. (1994) have shown that gaphite in
xenoliths of peridotite, pyroxenite, and eclogite grew
within its P-T stability field (i.e., not metastably)and
below solidusconditions.The carbonhas613Cvalues
that arethe sameas thoseof CO2in fluid inclusionsin
olivine phenocrystsin mid-ocean-ridgeand oceanicisland basalts,and thereforeare deducedby Pearson
et al. to have comefrom below the lithosphere.These
graphite-bearingxenoliths are rare, and appearto be
derived only from lithosphereunder Archeancratons.
Pearsonet al. (1994) concludedthat the samplesof
graphite-bearingperidotite are not subductedoceanic
lithosphere (becausethey lack the trace-element
depletionsexpectedin oceanic lithospheric mantle),
and that the carbonwas not expelledfrom subducting
oceanic crust (becauseno carbon-bearingxenoliths
havebeenfound that werederivedfrom mantlewedges
overlying subduction zones). Mclnnes & Cameron
(1994), however, have described carbonate-bearing
ultramafic xenolithsfrom sucha wedge,in PapuaNew
Guinea.
The flow of magmain lithosphericmantleis much
more likely to be controlledby fractures(Spera1987,
Wilshire & Kirby 1989,Nielson & Wilshire 1993),
rather than by percolationthrough a porous medium,
except within a few tens of centimetersof fractures.
Metasomatismof the conduitwalls protectslater liquid
that flows t}rough the samefracturefrom reactingwith
the wallrock, so that later fiquid can travel farther
beforebeing stalledby reaction(Meen 1987).
The wide geographicdispersalof thesexenolithsof
altered rock suggeststhat carbonate-richliquid has
beenmoreconnnonin the uppermantlethanthe sparse
distribution of carbonatitesin the upper crust would
suggest.According to the testimonyof thesesamples,
carbonatitic magma, ascendingthrough lithospheric
mantleocommonlyis trappedin fracturesbeforeit can
invade oceanicor continentalcrust. These constitute
veined sourceslike thoseinvoked by Foley (1992) to
yield ultrapotassicmagmas.By analogy, carbonatitic
magmacouldbe producedby melting of carbonate-rich
wallrocks.
veins and their variably metasomatized
378
TIIE CANADIAN
MINERALOGIST
Calronetrre M.qcuerrsvrTln oucs
crust (Table 1), and isotope ratios involving these
elements in carbonatitesare therefore relatively
insensitiveto crustalcontamination;the isotopic ratios
Throughgeologicaltime, the isotoperatios 87SrF6Sr must reflect those of mantle sources.The general
aad 2 FhPMPb in carbonatitebecome increasinelv changethrough time in strontium, neodymium, and
diverseandmoreradiogenicGigs. 1, 2), aspointed6ut lead isotopic ratios of carbonatitestoward more
by Nelson et al. (1988), and Tilton & Bell (1994). radiogenicvaluesis well explainedby storageof these
Carbonatitesare enrichedin Sr, Nd" and Pb relative to incompatibleelementsin the lithosphere(Meen 1987,
T[\e
AND SpACE
o.7a7
0.706
at
a
g
0.705
E
Eg
tr
!.
v)
\o
€
l
(t)
F
€
o.704
E
E
#
E
0.703
EE
E
EoqF
E
EE
4.702
s
4="
qr
@ E
E
0.701
300 600 900 1200 1500 18002100 2400 270a
age, Ma
21
b
EI
!!
20
19
,D
n
I
*
O.
IGI
E
18
.fo
17
f
E
15
14
t3
0
3 0 0 6 0 0 9 0 0 1 2 0 01 5 0 0 1 8 0 02 1 A 0 2 4 A 02 7 A 0
age,Ma
FIc. 2. (a) Initial 8751165rversus agefor 84 occurrencesofcarbonatites.Straight line is
bulk-earthevolutionline from Bell & Blenkinsop(1989,Fig. 12.1).@)2t6PbPMPb
versusagefor 33 occurrencesof carbonatite.
379
RECYCLEDCARBON IN CARBONATITES
Wyllie er al. L990, Ringwood et al. 1992). T\e
divenity of ratios at a given age reflects varying
contributionsfrom large volumes of sourceto small
volumesof magma.
Carbonatite magmatism has occurred repeatedly
over long time-spansin some regions. Examples
include eastemCanadaat 1900, 1100,66G-560,and
110 Ma (Bell & Blenkinsop1989, Woolley 1989),
West Greenland at 2650, 1300-1100, 600, and
176-169 Ma (Larsen & Rex L992), East Africa at
1040,75M80, 120-100,and 40-O Ma (van Straaten
1989),and southernAfrica at 2047,1750,750, 500,
200, 140-100,and 85-63 Ma (Harmer1985,Woolley
L989,Zregler1992).
Some xenoliths of lithospheric mantle show the
effects of metasomatismthat occuned long before
the xenolithswere entrainedin the magmathat carried
them to the surface (Green & Wallace 1988, Hauri
et al. 1993, Ionov e/ al. L993, Rudnick et al. 1993,
Smitl 1979. Thibault et al. 1992\. A few suites of
xenocrysts and ulframafic xenoliths have provided
documentationof multiple episodesin which carbon
was introduced. Richardson et al. (1993) have
identified threeepisodesof carboninflux and diamond
generationbeneaththe Premiermine; one was from a
strongly enriched harzburgitic source at 3200 Ma,
a secondwas from a mildly enrichedlhemolitic source
ar 1,930Ma andthe third wasfrom a stronglydepleted
eclogiticsourceat 1150Ma. Boyd et al. (1992)studied
samplesof "coated'odiamond from Afric4 Siberia,
and Australia. The diamond coats are overgtowths
precipitatedfrom fluid that was faidy homogeneous
in
terms of carbonand nitrogen isotoperatios, and with
87SrF6Sr
ratiosof 0.7038to 0.7052"similarto sources
of oceanic-islandbasalts.The core of thesesamplesof
coateddiamondis older (to an unknown degree)and
isotopically much more variable than the coats.
Rudnick et al. (1993)inferred, from a study of mantle
xenoliths from northern Tanz.ania,two episodesof
carbonate metasomatism,one with less Ca and
producinga different Nd isotopic ratio than the other.
The significanceof such recurrenceswas anticipated
by Wyllie (1978,p. 442):"Re.peated
magmaticactivity
indicatesthat H2OandCO2mustbe replenished,either
from deeper sorrces or by recycling via subducted
oceaniccrust".
The recurrence of carbonatite magmatism in a
specificregionmay not be due solely to replenishment
of magma sources, however. Some carbonatite
intrusive bodies are localized within linear grabens
outboardof cratons,and especiallyat the intersections
of faults (e.9.,Bailey 1993b,White et aI. 1995).T\is
distribution suggeststhat reactivation of fractures as
conduits,in addition to replenishmentof sources,is a
cause of repeated and episodic emplacementof
carbonatitein the crust.
Mantle xenolithswith metasomaticeffectsatfibuted
to carbonatitic magma have been described from
oceanicislands(e.9.,Havi et al. L993,Scbianoet aI.
L994),andcarbonatitesreportedin oceaniccrustarein
the Canary and Cape Verde Islands (Ir Bas 1984,
Davies & Mendes 1991, Kogarko 1993) and on
Trindade (Cornen et al. 1990). Furthermore,
carbonatite associated with deep-water marine
sedimentsoccurs under the Semail ophiolite in the
United Arab Emirates (Woolley et al. L99l). T\ese
occurences suggestthat carbonatitescan occlu on
and within oceanic crust, but are usually buried or
destoyed by subduction.
An old view" that carbonatitesand their associated
alkali-rich rocks arehallmarksof continentalrifting, is
being replacedby awarenessthat theserocks are not
restricted to a single tectonic regime @arker 1969,
Woolley 1989,Bailey 1993b,Hall et al. 1995).T\ey
occur in oceanic and continental crust, and have
formed in compressionalfold belts and stable
cratonsas well as in regionsof crustal extension.The
widespread occulrence of xenoliths of carbonatemetasomatizedlithospheric mantle suggeststhat,
pending availability of heat for melting and an easy
magmatismis waiting to
pathto the surface,oocarbonate
be unleashed"(Bailey 1993a).
Harrnqc rfi RJsBoF
Cennouermc Mecrua
In additionto the factorsthat can stopthe rise of any
magma(heatloss,increaseof solidustemperaturewith
decreasein pressure,decreasein density and increase
in shengthof wallrock), carbonatiticmagmacan also
be halted by reaction with wallrock @alton & Wood
1993a)to form calcium and magnesiumsilicatesplus
CO2as a componentof a fluid phase(Iable 4) andby
less-oxidizing conditions that reduce carbonate to
TABLE 4. SOMB PI.AI'SIBLS REACNONS
INVOLVING CARBOT.IAIES AND MANrI.E StrJCATBS
aM$tQ
CaMg(CO3). +
dolffi
+ 4othopyry@
a
F
2[i4gdsp4
2olffie
r
C0MgSiro6
+ clitroFt@
3CaMg(@!)?
3doloElte
+
r
.
-
2Mg1StO.
2oh.lF
t
+
4C8@i
2IldE@3
2@gmite
2rugsio3
r
+ 2o@tM@
!
-
zlu&sio4+
+
2olivioo
Cl)2
1f,lid'
2Ca@3
2@lcite
+
2[rlgSiO.
+ 26tlFpyl@
a
Csidg(Corz+
&lodle
r
Caltl$i.Ou
clhoplm@
E
-
2ldtco!
2DaSEdie
r
r
C€MgSLOg
d@y@
C€SiO3
+
+
CaMg(COJ?+
dolodb
t
C!M!(CO!)? +
+
dolffi
A@l%s
esrd@
ciMgslzoo
clhopym@
2XvlgSiO3
26tlopls@
ugzstoo
otetoe
heolE 6ffid
-
Fwstib
llqd4 nft6
to
elcib
M8O
lqlcls
ekdo, dolo@,
+
+
2ffi.
rndd'
i
4
M.
'f,dd"
r 214@3
+2DrS[eds
q Eaglestb.
380
THE CANADIAN MINERALOGIST
elementalcarbon(graphiteor diamond)or to methane
(Blundy et al. 199L,Ballhaus 1993).Both of these
changesremove dissolved COI- from the magma,
causingcrystallization.The capacityof peridotitesand
pyroxenitesto react with carbonatitemagmais high;
"... complete carbonatizationof peridotite requires
laddition ofl 7 to 20 percentCOi' (Schneider& Eggler
1986, p. 713), so that tle buffering capacity of
peridotitefor carbonationreactionsmay be only locally
reached.
Mantle redox reactions pertinent to carbonate
stabi[ty are discussedby Ballhaus (1993), Btundy
et al. (1991),Haggerty(1990),Kastr"g et al. (1993),
Luth (1993),Luth et al. (L993),and Dalton & Wood
(1995). In the upper lithospheric mantle, oxygen
fugacity is apparently controlled by Fez+ - Fe3*
reactions (Ballhaus 1993), but at greater depths,
reactions involving carbon probably become
important. Theseinclude the enstatite+ magnesite+
olivine + graphiteor diamondassemblage
(Olafsson&
Eggler 1983), which in flO) - temperaturespace
extendsfrom the quartz- fayalite - magnetitereaction
to below the magnetite- wiistite equilibrium. On the
other hand, accordingto Blundy et al. (L991),thereis
probably sufficient carbon (60 ppm C) in the upper
mantle to control oxygen fugacity to within one log
unit of the quartz- fayalite - magnetite"buffer". Luth
(1993) concluded tlat the relative stabilities of
elementalcarbonand carbonateor CO, are influenced
by varying assemblages
of silicates;it is possiblethar,
at tfie same pressure, temperature, and oxygen
fugacify, peridotites can contain carbonate plus
graphiteor diamond,andeclogitescancontaingraphite
or diamondwithout carbonate.
Theremay be portionsof the lithosphericmanflein
which oxygenfugacity is locally as low as the iron v/tistite buffer, at which CHr-HrO fluids would be
stablerelativeto CO, fluid or carbonateliquid flilatson
et al. 1990).Then, as recognizedby Taylor & Green
(1988),decreased
fugacity of oxygenwould resultiLn
increasedtemperatureof the solidusandcrystallization
or decompositionof carbonateliquid. The other
mechanismfor halting the ascentof carbonateliquid,
by reaction witl silicates, seenx more likely than
reduction,becauseolivine and orthopyroxene,the two
most abundantphasesin most lithosphericmantle,are
the dominant reactants in liberation of CO, from
carbonateliquids(Table4).
CoNclustoNs
Carbonate-richliquid shouldform at relatively low
temperaturesby partial fusion of carbonate-bearing
mantle. If all the carbonin the mantle werejuvenile,
it is likely that the rate of carbonatite magmatism
would havedeclinedafterpeakingin the Archean.The
distribution of ages of carbonatite emplacement
(Woolleyl989,Yerzeret al. 1992),however,
showsno
clear evidence of a secular decline in the rate of
carbonatitemagmatism.Unlessthe mantlels fscaming
increasinglyoxidized, so thatjuvenile reducedcarbon
is being converted to carbonate, carbonatite
magmatismmust be sustainedby carbonrecycled by
subduction.
The assumption that much of the carbon in
carbonatites is recycled, not juvenile, leads to
predictions that befter agree with observations
concemingthe small volumesof individual bodies of
carbonatite,the persistence,recurrenceandwidespread
spatialdistribution of carbonatitemetasomatismin the
mantle and magmatismin the crust, and the tendency
for the sourcesof carbonatitemagmato becomemore
diverse and more radiogenic tlrough time, through
long-term storage in varied, small, and dispersed
reservoirs. A single source of carbonatite magma
appea$ unlikely. The return migration of deeply
recycled carbon is probably slow, diffuse, and
intemrpted. Carbonatiticmagmatismis like a poorly
managedindustry, at the mercy of erratic suppliesof
raw materials,inefficient processing,wasteful storage
ofthe inventory, and fitfrrl distribution ofthe finished
product.
ACKNoWI-EDGEUTENTS
Commentsfrom R.F. Martin, R.H. Mtchell, D.
Smith, J. Wolff, and an anonymousreviewer led to
improvements,but these friends do not necessadly
endorseall interpretations.
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RECYCLEDCARBON IN CARBONATITES
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