GEOPHYSICAL
RESEARCH
LETTERS,
VOL.!9, NO. 8, PAGES
761-764,
APRIL24, 1992
METALLIFEROUS SEDIMENTSAND THE SCAVENGINGRESIDENCETIME OF Nd
NEAR HYDROTHERMAL VENTS
Alex
N.Halliday
1,Jon
P.Davidson
2,Peter
Holden
2,Robert
M.Owen
i and
Annette
M.Olivarez
3
Abstract:.The isotopiccompositionof Nd is uniformin
Piepgras
andJacobsen,
!988]. Thisis because
particulate
metalliferous
sediments
formedat distances
varyingfrom
>1000kmto within10km of theEastPacificRise(EPR)
scavenging
[RuhlinandOwen, 1986; Klinkhammeretal.,
!983; OwenandOlivarez,1988; OlivarezandOwen, 1989;
Germanetal., 1990] resultsin shortresidencetimes
palaeoridge.
These
dataindicate
thathydrothermal
vent
fluids,
despite
havingconcentrations
morethan500times
compared
withthemixingtimesof theocean.Goldsteinand
Jacobsen
[1987]utilizedtheNd andSr isotopic
compositions
ofriverwaters
andtheoceans
to showthatthepresent
greater,
havenoeffectontheNdisotopic
composition
of
seawater.
Thisimplicatesefficientscavenging
of
hydrothermal
Nd byparticulates,
resulting
in extremely
short hydrothermal
flux of Nd to theoceansmustbesmall(<5%),
residence
times(< 1 year)closeto the hydrothermal
vents.
Therefore
Nd isotopicstudiesof ancientseawater
and obtained a residence time for Nd in seawater of 7 I00
years[GoldsteinandJacobsen,
1988]. Here we showthatthe
isotopic
composition
of Nd in metalliferous
sediments
from
neartheEastPacificRiseis totallyunaffected
by
hydrothermal
inputs.Thisis because
of theimportance
of
scavenging
whichpreventshydrothermal
Nd frommixing
with seawateronanythingotherthana localscalecloseto
thevents.AssuchNd isotopic
compositions
cannotbeused
toinferancient
hydrothermal
fluxeswithoutindependent
constraints
ontheextentof scavenging.
precipitates,
particularly
metalliferous
sediments,
cannot
be
used
to delimitthe magnitudeof pasthydrothermal
circulation
withoutindependentconstraintson local
scavenging
rates(or residence
times)relativeto thoseof
modem oceans.
Introduction
Estimates
of the magnitudeof hydrothermalfluxesin
ancient
oceans
havebeenbasedin partontheNd isotopic
compositions
of chemicalsediments
suchasbandediron
formations
[Jacobsen
andPimental-Klose,
1988;Derryand
Samplingandanalyticalapproaches
Themetalliferous
sediments
in DSDP corefromLeg92
Jacobsen,
1988]. It has been observedthat the initial end of
such
earlymetalliferous
sediments
is relativelyhigh. This
haslead to the conclusion that Nd derived from oceanic crust
wasmoreimportantin thepastbecauseof higher
hydrothermal
fluxes. Modem metalliferoussedimentsare
Fe-rich
pelagicdeposits
formedby precipitation
from
hydrothermal
plumesemanatingfrom ventsnearocean
ridges
suchastheEastPacificRise. Theyformoneof the
largest
repositories
of REE-richparticulates
ontheocean
floorandaredeposited
asthehydrothermal
plumeis wafted
awayfromtheridgeby oceancurrents.In a recentisotopic
study
thevariableinfluenceof oceanridge-derived
hydrothermal
Pb on suchmetalliferoussedimentwas
demons•ated
[Barrett
etal.,1987].Whiteetal.[1986]also
showed
thattherewasaMORB-like
component
ofHf in
(site598), locatedto thewestof the EastPacificRise (19øS)
representa completerecordof hydrothermalsedimentation
at
theflankof theEastPacificRiseoverthelast16 Myrs.They
areidealfor studyingbecauseof the absenceof continentderiveddetritusandlackof organicdiageneticremobilization
of Fe andMn [Ruhlin andOwen, 1986]. The lack of
diagenetic
modification
is indicatedby porewaterMn
analyses
andcomparison
of downcoreMn/Fe ratioswith
thoseof modemhydrothermal
precipitates
thatoriginated
from EPR ventfluids. Ruhlin and Owen [ 1986] showedthat
the REE concentration and ZREE/Fe ratio at this site
decreasewith increasingdepthdowncoreandtherefore
increasingageof thesesediments.This can be relatedto
proximityof theridge;the youngestsedimentswere formed
1139km fromthepalaeoridge
whereastheoldest(deepest)
sediments
wereformedonly9 km fromthepalaeoridge.
The
concentration
of REEsin particulates
mightthenbe expected
reflect
thelocalctntinental
crustal
provenanco
oftherare
to increaseasa resultof scavenging
from seawater,in
earth
elements
(REEs),
rather
thanhydrothermal
componentsproportionto the distancetravelledby the hydrothermal
[Piepgras
etal.,1979;
Piepgras
andWasserburg,
1980,1985; plumefromtheridge[RuhlinandOwen,1986]. A plotof
seawater.
In contrast,
theisotopic
composition
of Nd in
seawater
hasbeenshownto beregionally
variable
andto
Goldstein
andO'Nions,
1981;Palmer
andElderfield,
1985;
rare earth element concentrations versus distance from
pa!aeoridge
axisdisplayeda breakof slopeat the ageat
whichthesediments
passedthroughthepalaeolysocline
1Department
ofGeological
Sciences,University
ofMichigan,
suggesting
thattheconcentrations
werealsoaffectedby post2Departrnent
ofEarth
&Space
Sciences,
University
of
depositional
scavenging
at the seawater/sediment
interface
California
atLosAngeles,
[Ruh!inandOwen, 1986]. The variationsin REE
3Department
ofEarth
Sciences,
University
ofNotre
Dame
concentration could then also reflect a decrease in burial rate
with distancefrom the ridge.
Six representative
samplesfrom DSDP Leg 92, site598,
coveringtherangein agesand palcodistance(9-1085 km)
Copyright
1992
bytheAmerican
Geophysical
Union.
Paper
number
92GL00393
0094-8534/92/92GL_00393503.00
wereselected
for Nd isotopic
analysis.Detaileddescriptions
of the samplescan be foundelsewhere[Ruhlin and Owen,
761
762
Hallidayetal.' Metalliferous
sediments
1986].Thesediments
arecomposed
almostentirely
of
thebulksediment
(Figurela). Similarlythebulksediment
mixturesof carbonatesandmetalliferous
(hydr)oxides,
the
has
thesame
orslightly
higher
143Nd/r44Nd
than
the
latterrepresenting
theoverwhelmingly
dominant
repository
ofNd. Attemptsweremadeto isolatethesecomponents
utilizingsequential
leachingexperiments
asderailed
in Table
residue.
Theresidues
(withrelatively
unradiogenic
Nd)
1. In thefirst experimentthebulksediments
wereanalyzed
withno preparation.In the secondexperiment
thesamples
wereleachedwith hydrochloricacid,andthe leachateand
residueanalyzed. The hydrochloricacidleachshould
dissolve
carbonates,
butis alsoexpected
to dissolve
a large
amountof themetalliferousoxides.For thethirdexperiment
two samples(oldestandyoungest)wereleachedsuccessively
with water, aceticacid and hydrochloricacidto matchthe
preparation
usedby Barrettet al. [ 1987]for Pbisotopic
studies of sediments from the same site. The water is
couldrepresent
a smallcomponent
of atmospheric
particulates.
Howeverthesevariations
aretrivialandtheNd
isotopic
compositions
areeffectively
uniform,
withnosign
ofa greater
MORBcontribution
in oldersamples
deposited
closerto thepaleoridge
(Figurelb). TheNd isotopic
compositions
aremoreuniformandlower (eNd= -5.2 to
-4.4)thantherangeforPacificauthigenic
sediments
reported
byPiepgras
etal.[1979](eNd=-4.4 to-0.2). Thehigher
valuesreportedby Piepgraset al. werefor metalliferous
sediments
andhydrothermal
crusts.Comparable
datawere
obtainedO'Nions et al. [1978] and Goldsteinand O'Nions
[19811.
The uniformNd isotopiccompositions
cannotreflectlate
expectedto dissolveonly highlysolublephases(suchas
NaCI), whereasmost carbonateswill be solublein acetic
acid. The HC1 acidis expectedto dissolvethemain
alteration
of metalliferous
oxides
in thesediment
pile.First,
thereisnoevidence
of REEenrichment
withdepth,
asmight
metalliferous(hydr)oxidephases.The smallamountof
residuein all theseexperiments(dissolvedin hydrofluoric,
perchloricandnitric acids)is expectedto be a mixtureof
minorsilicatesandorganiccompounds.
beexpected
if theNd isotopiccompositions
wereduetolate
adsorption
of REEsfromdownward
penetrating
fluids.The
Nd isotopiccompositions
of metalliferous
sediments
The Nd isotopiccompositions
of eachof theleachates
andresiduesarepresented
in Table 1. Metalliferous
oxidedominated
Nd isotopiccompositions
areto beexpected
from
thebulk sediments
andall the hydrochloric
acid!eachates.
These
leachates
have
thesame,
orhigher
143Nd/144Nd
than
ZREEsdecreases
dramatically
withdepth,
ageandproximity
to thepalcoridge[RuhlinandOwen, 1986]. Second,the
Fe/Mn ratiosandorganiccarbonconcentrations,
whichare
verysensitive
to diagenesis,
areclearlywell preserved
[OlivarezandOwen,1989]. Third,theoldestsamples
have
the lowest REE concentration and should have sufferedthe
most extensive alteration. Yet these have the most
pronounced
positiveEu anomaliesrelativeto seawater
[HOgdahl
et al., 1968]indicatingthepreservation
of a
significant
hydrothermal
component
[Ruh!inandOwen,
TABLE1. 143Nd/144Nd
ratios
ofDSDPsediments
fromsite598
Samplenumber
1
2
3
4
5
6
DSDP number
1/1-46/48
1/3-88/90
3.89
802
229
0.512380-2_8
0.512429+7
0.512365+13
2/4-28/30
8.74
3/13-!8/20
12.3
4/6-19/21
!4.6
5/6-58/60
15.9
450
95
0.512426_+9
-0.512377+13
227
70
0.512409+8
0.512451+8
--
85
66
0.512418+7
0.512435+7
0.512421+23
9
54
0.5123905-_6
0.512402•4
--
Age (Ma)
0.47
Distancefrompaleoridge(km) 1085
Nd ppm
155
(1) Bulk sediment
0.512373+7
(2) Hydrochloricacid leach
(2) Residue
(3) Water leach
0.512389+8
0.512384+11
0.512377+15
(3) Acetic acid leach
0.512379_+9
(3) Hydrochloricacidleach
(3) Residue
0.512411+12
0.512372.•.12
........
........
........
0.512387-+9
0.512390•_10
0.512360•_12
All dataobtained
attheRadiogenic
Isotope
Geochemistry
Laboratory
attheUniversity
of Michigan,
usingstandard
ionexchange
separation
techniques
anda VG Sector
moulticollector
massspectrometer,
asdescribed
elsewhere
(Hal!iday
et al., 1989).%e
average
value
ofI43Nd/144Nd
fortheLaJolla
Ndstandard
was
0.511855+8
(2c•
mean,
n=18).
Numbers
inparentheses
denote
separate
experiments
(seetext). Forthedetailed
leaching
experiment
(3) onsamples
! and6, thefollowing
procedure
was
adopted.
Thepowdered
sample
wasslurried
inpuredeionized
H20 andleftforc_.18hours,
warmed
for 1/2hourandfiltered.
Thetiltrate(containing
water-soluble
components
suchassalt),wascentrifuged
andanyprecipitate
whichhadcircumvented
the
filtration
process
was
discarded.
It was
then
evaporated
andtaken
upin! mlof2.5NHC1,
andanalyzed
for143Nd/144N
d
following
standard
ionexchange
separation.
Theresidue
wasrinsed
fromthefilterpaperintoa beaker,
evaporated
todryness,
and !0% distilledacetic acid added. This shouldhave resultedin the dissolutionof most carbonate. The leachateandresidue
wereagainseparated
byfiltration
andtheleachate
analyzed.
Theresidue
wasevaporated
to dryness,
thenleached
with10%
distilled
HC1
for2 hours.
Leachate
and
residue
were
separated
asbefore
and
theHCl-soluble
component
analyzed.
Followi
ng
evaporation,
totaldissolution
of theremaining
residue
wasachieved
using
HF,HNO3 andHC10
4. Thisfraction
wast•e•
analyzed.
Hallidayet al.' Metalliferoussediments
BulkSediment I-I Leachate I
"1
I,,
I
I,,, I
Residue
I
I
I
1:3
0.51245 -
O
'
:
O
•!1
7 63
[Michardet al, 1983;Piepgras
andWasserburg,
1985;
MichardandA!bar•de,1986;HinkleyandTatsumoto,
1987],
thevariabilityreflectingdilutionwith seawater,
and
interaction
andscavenging
enrouteto thesamplingsite.
Giventheextremelylow concentrations
of Nd in seawa'ter
(<10
-5ppm)
[HOgdahl,
1968],
compared
withconcentrations
500 timeshigherin projectedendmember
EPRventfluids
0.51240 -
[Hinkley andTatsumoto,1987], the ratio of vent fluid to
|
"0 0.51235 Z
I
I
I
I
I
I
i
b
Z
ß.-
seawater needs to be < 0.001, even at a distance as close as 9
km from thepa!eoridge,unlessNd is beingvery efficiently
removedby scavengingcloseto the vents.
A roughindependent
estimateof the amountof ventfluid
relativeto seawaterin thevertical sectionoverlyingthe
pa!eosediment
surfacecanbe obtainedfrom Mn/N:dratiosin
the sediments. The Mn in the metalliferous
0.5130
sediment is
derivedwhollyfrom thehydrothermal
component
sincethe
-
Mnenrichment
intheplume
relative
toseawater
is106. The
isotopicdatapresented
hereindicatethatthe Nd is derived
entirelyfrom seawater.The ratio of Nd in seawaterto Mn in
hydrothermal
vent
fluids
isroughly
7x!0-5[German
etal.,
1990;Von Dammet al., 1985]. The sedimentdeposited9
0.5125 -
'
I
18.7 --
I
I
I
II
!
r-i
r-i
o
B
o
,
0
4
8
12
16
Age (Ma)
Fig.1. Variationin isotopiccompositions
of metalliferous
sediments
fromDSDPLeg92, Site598,locatedtothewest
oftheEastPacificRise(19øS),asa functionof age.a)
143Nd/144Nd
ofbulksediments,
metalliferous
oxide
km from the ridge has a Mn/Nd ratio of 5400 [Ruhlin and
Owen,1986]. ignoringdifferentialscavenging,
theproduct
of thesetwo figures(0.4) gives an order of magnitudevalue
for thehydrothermalfluid/seawaterratio at thisdistance.
The averageseawaterresidencetime of Nd wasdetermined
by GoldsteinandJacobsen
[GoldsteinandJacobsen,1988] at
7100 years. This is considerablyin excessof the scavenging
residencetime of Mn in a hydrothermalplumedeterminedby
Weiss [1977] at 50 years. However the only way for the
hydrothermal
fluid/seawater
ratio of < 0.001 calculatedusing
theNd isotopicdatato be reconciledwith thatcalculated
usingMn/Nd ratiosis for the scavenging
residencetime of
Nd to be lessthan50 years.Post~depositional
scavenging
of
Nd wouldnot affect thisconclusion,and the dataimplicate
rapidscavenging
nearthevents,effectivelyremovingthe
hydrothermal
component
of Nd suchthatthereis no
contribution
to the metalliferoussediments.Applicationof
modelsquantifyingtraceelementscavenging
in ternasof
oxideleachatesrelative to field for East Pacific Rise basalts
residencetime relativeto dispersalrate in the oceans[Weiss,
1977; Craig, 1974] indicatesthat localizedresidencetimes
for hydrothermal
Nd fromtheplumehaveto be extremely
short(< 1 year)to achieveevena factorof 2 reductionin
concentration
within !0 kms of the vents. The mostlikely
[Whiteet al., 1987].
explanation
of suchrapidscavenging
is thatNd is removed
leachates,
andleachate
residues.
b) datashownin (a)relative
tofieldfor EastPacificRisebasalts[Whiteet al., 1987].c)
Pbisotopic
dataof Barrettet al. [1987]formetalliferous
1986],
consistent
withtheiroriginaldeposition
site,closest
to
thepalcoridge.
Last,majorelementstudies
of porewaters
suggest
a minor amountof carbonatedissolution;otherwise
nosignificant
post-depositional
effects.
Theresultsfor Nd contrastwith thosefor Pb [Barrett
instantaneously
at hydrothermalvents,presumablyby
absorptionon the surfacesof the large volumesof dense
particulates
producedin andaroundtheplumes[Olivarezand
Owen, 1989; German et al., 1990].
Hydrothermal
Nd is notincorporated
to any greatextent
in oceancirculationat thepresenttime, aspredictedby
etal,1987],whichshowa clearMORBcomponent
overall
Piepgras
andWasserburg
[! 980] andGoldsteinandJacob,,;en
Leg92sites,
whichincreases
withproximity
tothe
[1987]. It is for the samereasonthat the REE patternof
seawatershowsonly localizedminor evidenceof enhanced
Eu relatingto thepositiveEu anomalyin hydrothermal,
•ent
fluids[GoldsteinandJacobsen,
1988]. Similarly,
vents
(Figurelc). Thisisnotsurprising
in viewof thefact
thatocean
floorhydrothermal
solutions
maycontain
upto
2,000
ppmPb[VidalandClauer,1981;Brevart
etal.,198!]
fluxesarebetterestimated
by massbala]'•cing
butless
than
0.01ppmNd[Hink!ey
andTatsumoto,
1987]. hydrothermal
the
Sr
rather
than
Nd
isotopic
compositions
of seawaterand
Nevertheless,
thehydrothermal
component
should
stillbe
detectable
intheNdisotopic
compositions
if theycontributecontinentalrunoff [Goldsteinand Jacobsen,1987].
aslittleas3% of theNd. The concentrations
of Nd in vent
fluids
relative
toseawater
typically
lieintherange
6- 100
Increased
hydrothermal
ridgefluxeswill probabl,y
not
changethisscenariosinceit is the hydrothermal
pa•,•dculates
764
Halliday
etal.: Metalliferous
sediments
themselvesthatarecausingthe scavenging
[Owenand
Olivarez,1988, 1989]. The greaterthecontribution
of
hy•'othermal particulatesthe moreefficientthe scavenging
andthe lesshydrothermalNd entersthe oceans.The Nd
isotopiccompositions
of metalliferoussediments
ancientand
Halliday,A.N., Mahood,G., Holden,P., Metz, J.M.,
modem are thereforean ineffectivemonitorof theintensity
of oceanfloor hydrothermalactivity. The extentto which
theNd isotopiccompositionof a metalliferous
sediment
indicatesthe importanceor otherwiseof hydrothermal
fluxes
Jacobsen,
S.B.andPimental-Klose,
M.R. EarthPlanet.$ci.
Dempster,T.J. and Davidson,J.P. Earth Planet.Sci.
Lett. 94, 274-290 1989.
Hinkiey,T.K. andTatsumoto,
M. Jl. Geophys.
Res.,92,
11,400-11,410 1987.
Lett., 87, 29-44 1988.
Klinkhammer,
G., Elderfield,H. andHudson,
A. Nature,
305, 185-188 1983.
Michard, A., Albar•de, F., Michard, G., Minster,J.-F. and
Chafious, J.-L. Nature, 303,795-797 1983.
Michard, A. and Albar•de. Chem. Geol., 55, 51-60 1986.
Olivarez, A.M. and Owen, R.M. Geochim. Cosmochirn.
will cleaxlydependonproximityof theplumesource,the
plumevelocityandresidence
times.REE residence
timesare
a functionof scavenging,
whichis dependent
onocean
chemistry.Henceuseof Nd isotopiccompositions
of
chemicalsediments
for studyingpalcogeography
of
continental
landmassescouldyieldambiguous
resultsfor the
Acta, 53, 757-762 1989.
O'Nions,R.K., Carter,S.R., Cohen,R.S., Evensen,N.M. and
Hamilton, P.J. Nature, 273, 435-438 1978.
Owen, R.M. andOlivarez, A.M. Mar. Chem.,25, 183-196
Precambrian,sinceREE residencetimesmay havebeen
longerbecauseof greatersolubilityof Fe in theearlyoceans.
It is thereforecritical to understandthe relativeimportanceof
scavenging
in thegeologicalpast,andto document
the
isotopiccompositionof Nd in thecontinental
sources
exposedat the time in question,beforetheNd isotopic
compositions
of Precambrian
bandedironformations
in
particular,canbe utilizedto indicatethemagnitude
of
ancienthydrothermalfluxes [Jacobsen
andPimental-Klose,
1988.
Palmer,M.R. andElderfield,H. EarthPlanet.Sci.Lett.,73,
299-305
1985.
Piepgras,D.J. andJacobsen,S.B. Geochim.Cosmochirn.
Acta, 52, !373-1381 1988.
Piepgras,
D.J. andWasserburg,
G.J. EarthPlanet.Sci.Lea.,
50, 128-138 1980.
9881.
Piepgras,D.J. andWasserburg,G.J. Earth Planet.Sci.Lett.,
Acknowledgements.
This researchwas supported
from
NationalScienceFoundation,MichiganMemorialPhoenix
ProjectandTurnerFundgrantsto ANH, for isotopicstudies
of hydrothermalprocesses.ANH thanksKlausMezger,
CharlieDeWolf andGarethDaviesfor criticismof the script
Piepgras,D.J., Wasserburg,G.J., andDasch,E.J. Earth
72,341-356 1985.
Planet. Sci. Lett., 45, 223-236 1979.
Ruhlin, D.E. and Owen, R.M. Geochim. Cosmochim.Acta,
50, 393-400 1986.
Vidal, P. and Clauer, N. Earth Planet. Sci. Lett., 55, 237-246
and, or, discussion.
1981.
Von Damm, K.L., Edmond, J.M., Grant, B., Measures,C.I.,
Walden, B. and Weiss, R.F. Geochim. Cosmochim.Acta,
49, 2197-2220 1985.
Weiss, R.F. Earth Planet. Sci. Lett., 37, 257-262 1977.
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