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Niu, Y.L. and Batiza, R (1993) 'Chemical variation trends at fast and slow spreading mid-ocean ridges.',
Journal of geophysical research : solid earth., 98 (B5). pp. 7887-7902.
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Niu, Y., and R. Batiza (1993), Chemical variation trends at fast and slow spreading mid-ocean ridges, Journal of
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JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 98, NO. B$, PAGES 7887-7902, MAY 10, 1993
ChemicalVariation Trends at Fast and Slow SpreadingMid-Ocean Ridges
YAOI.INGNIU1ANDRODEYBATIZA
Departmentof Geologyand Geophysics,
Universityof Hawaii at Manoa, Honolulu
We examinedan expanded
globaldatasetof mid-ocean
ridgebasalt(MORB) majordementanalyses.
In agreement
with previousresults,we showthatslowspreading
ridgestendto havemoreprimitive(high
Mg/Fe)lavas
thanfastspreading
ridges.
Fractionation-corrected
values
ofNa(8
) andCa($)/AI(8
) (indices
of
theextent
of melting)
andSi(s)/Fe(s
) (anindexof thepressure
of melting)
donotvarysystematically
with
spreadingrate. Assuminga mantlethat is generallyhomogeneous
in major elements,we concludethat
averagemanfietemperature
in the regionof meltingbelowmid-oceanridgesis independent
of spreading
rate. Usingdatafor 32 bestsampledridgesegments
of variablelength,we showthatthe so-calledglobal
and local trendsof chemicalvariation(Klein and Langmuir,1989) are systematically
distributedwith
spreading
rate. The globaltrend(positivecorrelation
betweenextentof meltingand meltingpressure)
occursat fast spreading
ridges(> 60 mm/yr),whilethe localtrend(negativecorrelation
betweenextentof
melting and melting pressure)occursat slow spreadingridges (< 50 mm/yr). This distributionis
independent
of geographic
lengthscale. Amongthe 32 ridgesystems
we examined,the dopesof the two
trendson chemicaldiagramsshowsomevariability,but no regularpauern,suchas fanning. The global
trendis well-explained
by differences
in averagemantletemperature
occurringat severallengthscales
withinmantlerisingpassivelyin response
to plateseparation.We proposethatthe localtrendarisesfrom
processes
occurringin buoyantdiapirsundergoing
meltingandmelt-solidreequilibratiou.Severallinesof
geophysicaland geologicalevidencepoint to the importanceof buoyant,three-dimensional
manfie
upwellingbeneathslow spreadingridges. Petrologicmodelingpresentedhere is consistentwith this
hypothesis,as is the existenceof the local trend at seamountson the flanks of the East Pacific Rise.
INTRODUC'FION
Spreadingrate is an important variable associatedwith
many significantdifferencesalongthe globalmid-oceanridge
system. For example, morphologicdifferencesare largely
controlled by spreading rate [e.g., Macdonald,
1982;
Franchteauand Ballard, 1983], with slow spreadingridges
typically having deep axial rift valleys that are much less
pronouncedor are absent at fast spreadingrates. Related
topographicroughness
on the flanks of mid-oceanridgesalso
dependson spreadingrate [Malinverno and Pockalny, 1990;
Malinverno, 1991;Hayesand Kane, 1991;Small and Sandwell,
In this study, we examinean expandedglobal data set of
mid-oceanridge basalt(MORB) major elementanalyses. In
agreementwith previousstudies,we find that fast spreading
ridges have, on average,more fractionated,lower Mg/Fe
MORB lavasthanslow spreadingridges. We thenusedatafor
32 well-sampledportions of the global ridge system to
evaluate chemical variation trends as a function of plate
spreadingrate. Klein and Langmuir [1987] showed that
globally,
MORBshows
aninverse
correlation
between
Fe(8)and
Na(8), wherethesubscript
denotes
valuescorrected
for shallow
fractionation
to an MgO contentof 8.0 wt %. Theyinterpreted
meltingmodeland
1989]. Along-axisgravity [Lin et al., 1990;Lin and Phipps this in termsof a polybaricdecompression
showed
that
deep
(high
Fe(8)),
extensive
(low
Na(8))melting
Morgan, 1992] showsimportantchanges
with spreading
rate,
interpretedto reflect fundamentaldifferencesin the style of would be expectedfrom the pooling of polybaric melts
mantle upwelling, with dominantlytwo-dimensional,plate- producedfrom a longmantlecolumnwith high initial melting
drivenupwellingbelow fast spreadingridgesandmore three- temperature.In contrast,a shortmantlemeltingcoluntowith
wouldbe expectedto produce
dimensional
buoyantlydriven,diapiricupwellingbeneathslow low initial meltingtemperature
pooled
meltsreflecting
shallower
(lowFe(8))andlessextensive
spreading
ridges[ParmentierandPhippsMorgan,1990].
averaged
MORB analyses
In addition to topographicand geophysicaldifferences, (highNa(8))melting. Regionally
a trendof negative
correlation
between
Fe(8
) andNa(8
)
petrologicdifferencesexist betweenfast and slow spreading exhibit
ridges [Morel and Hekinian, 1980; Natland, 1980; Flower, calledthe globaltrendby Klein andLangmuir[1989].
However,somemid-oceanridgesegments,
suchas the mid1980; Batiza, 1991; Sinton and Detrick, 1992] with slow
spreadingridges typically having more primitive (higher Atlanticridgeat 26øS[Batizaet al., 1988],exhibitan opposite
) andNa(8) showa positive,
notinverse
correlation.
Mg/Fe) lavas and fast spreading ridges having more trend:Fe(8
(lowFe(8)),
extensive
(lowNa(8))melting
ison
fractionareal
ones. Relationships
betweenisotopicvariability Thatis,shallow
and spreadingrate [Cohen and O'Nions, 1982; Hamelin and a trendwithdeep(highFe(8)),lessextensive
(highNa(8))
All•gre, 1985;Batiza, 1984] are more controversial[Ito et al., melting. This trend of chemicalvariationwas termedthe local
trendby Klein and Langmuir [1989]. The global and local
1987; Holnessand Richter, 1989].
trendsare shownin Figure 1.
In this study,we find thatthe so-calledglobaltrendis found
at fast (> 60 mm/yr) spreadingridges and that the so-called
1Now
atLamont-Doherty
Earth
Observatory,
Palisades,
NewYork. local trend is found at slow (< 50 mm/yr) spreadingridges.
Transitionalridges(50 - 60 mm/yr) may exhibiteithertrendof
Copyright1993 by the AmericanGeophysical
Union.
chemicalvariation. This finding is independent
of the ridge
length and thus, the terms "global" and "local" are not
Paper number 93JB00149.
O148-0227/93/93JB-00149505.00
particularly
apt. Evenso,we retaintheprevious
usageof these
7887
NIU AND BATIZA: MID•
7888
5.0
I
I
I
RIDGE BASALT CHEMICAL TRENDS VERSUS SPREADING RATE
I
I
RESULTS
OxideTrendsWithSpreadingRate
Plotsof the globalMORB data againstspreadingrate show
much scatterbut statisticallysignificanttrendsfor mostof the
major and minor elements,confirmingpreviousf'mdingsthat
lavasfrom fast spreadingridgesare typically more fractionated
than thoseeruptedat slow spreadingridges. With increasing
spreadingrate, TiO2, FeO, Na20, and P205 increase,while
AI20 3, MgO, andCaO decrease.The exceptionsare SiO2 and
K20. SiO2 may showno trendbecauseof analyticalproblems
discussedby Klein and Langrnuir [1989]. K20, on the other
hand, showsa significantcorrelationwith spreadingrate, but it
is inconsistentwith a fractionation.This is becauseK is a very
incompatibleelement and is very sensitiveto mantle source
heteroheneity; higher K20 contents in MORBs at slow
spreadingrate obviouslyresultfrom hot spoteffect suchas at
the North Atlantic ridge. Table 2 gives the slopes,intercepts,
and correlationcoefficientsof theselinear trends,and Figure2
shows representative examples. When the analyses are
4.0
3.0
2.0
1.0
0.9
0.6
corrected for shallow fractionation (see notes to Table 2 and
Niu [1992] for correction procedure), these correlations
3
4
5
6
7
8
9 virtually disappear. As shownin Table 2 and Figure2, after
correction, the slopes are reduced by at least an order of
magnitude;plots of chemicalvariationsagainstspreadingrate
Fig. 1. The globaland local trendsof chemicalvariation[Klein and are essentiallyfiat (with much scatter).
Assuming a homogeneousmantle source and a column
Langmuir,1989]are shownhereon Na(8)andCa<.)/
8 A1<)
8 versus
Si(8)/Fe(8
). Thesubscript
denotes
valuesof theOXlae
corrected
for meltingmodelsimilarto thatof Klein andLangmuir[1987], we
shallow fractionationto a MgO content of 8.0 wt % [Klein and
Langmuir, 1987]. FollowingNiu and Batiza [1991a], we use both next plot indicesof the extentof melting and depthof melting
0.5
Si(8)/Fe(8)
8 e()
8
Na(s)andCa(•8 A1()I• asindicesof theextentof meltingand.Si()/F
against
spreading
rate(Figure3). Si(8)/Fe(8
) is a verysensitive
as an index of the pressureof melting. Arrowson eachdiagrampoint indicatorof the pressure
of melting[Niu and Batiza, 1991a] and
to
high
pressure
and
higher
extent
ofmelting.
Thedata
points
arefromNa(s)and
Ca(s)/AI(8
) aregood
indices
of theextent
ofpartial
our new globaldata setof MORB. The heavyline showsthe so-call•
melting of pooled column melts [Klein and Langmuir, 1987;
globaltrend,and the thin solidlines with arrowsshow(schematically)
Niu and Batiza, 1991a]. As shownin Figure 3, plots of these
examplesof local trends.
quantitiesagainstspreadingrate exhibitmuchscarer. There is
a suggestionthat the scatter is more pronouncedat slow
spreadingrates. This, as will be discussedlater, is associated
termsin this paper. We considerseveralpossibleoriginsfor with the so-called local trend at slow spreading ridges.
the local trend and conclude that it is due to fundamental
Nevertheless, there is no evidence for systematic trends in
differences in the dynamics of mantle upwelling and melt eitherdepthof meltingor extentof meltingwith differencesin
segregation
beneathslow and fast spreadingridges.
spreading rate. This is an interesting finding because it
confirmsthat averagemantletemperature,an importantcontrol
Ti-IE DATA SEr
on melting below ridges [McKenzie, 1984; Klein and
To the data setof Brodholt and Batiza [1989], we have added Langtnuir, 1987; McKenzie and Bickle, 1988], is variable,but
the data of Sinton et al. [1991] for the southernEast Pacific essentiallyindependent
of spreadingrate [Klein and Langmuir,
Rise (EPR), Thompsonet al. [1989] andHekinianet al. [1989] 1987].
for the northern EPR, Klein et al. [1991] and Dosso et al.
[1988] for the South-EastIndianridge,Humler and Whitechurh ChemicalVariationTrendsVersusSpreadingRate
Next, we searchedthe global data set for spreading-rate
[1988] for the Central Indian ridge, Michael et al. [1989] for
the Explorerridge, and Bougaultet al. [1988] for the northern dependencein MORB chemicalvariationtrends. As shownby
Mid-Atlantic Ridge(MAR). We alsousethe updatedversionof Klein and Langrnuir [1987], regionally averagedMORB data
Melson and O'Hearn's [1986] Smithsonian Volcanic Glass exhibit correlations among axial depth, inferred extent of
depthof melting.Unaveraged
rawdata
Project (SIVGP) data base (W. G. Melson and T. O'Hearn, meltingandinferred
personalcommunication,1991). We use samplegroupmeans exhibit similar trends [Brodholt and Batiza, 1989], such that
areformedby higher
for our analysisand include both glass and whole rock data. deepermelts(highFe(8),low Si(8)/Fe(8))
Using correctionfactorsof Batiza and Niu [1992], we adjust extents
of melting(lowNa(8) andhighCa(8)/AI(8)).Similarly,
Lamont-DohertyEarth Observatoryand University of Hawaii shallower
melts(lowFe(8),highSi(8)/Fe(8))
areformed
bylower
glass probe analyses to conform with the Smithsonian extentsof melting(higherNa(8)). Thisbehavior,
observed
analyses.Data sourcesof MORB analysesaregivenin Table 1. globally, was called the global trend [Klein and Langmuir,
Spreadingrates are from NUVEL 1 [DeMets et al., 1990] 1989]. The oppositetrend,with a positivecorrelationbetween
exceptfor the Explorerridge [Botrosand Johnson,1988], Juan Fe(8
) andNa(8), wasnotedbyBatizaet al. [1988]andBrodholt
de Fuca [Wilson, 1988] and Gorda ridge [Davis and Clague, and Batiza [1989] and called the local trend by Klein and
1987].
Langmuir [1989]. Here, we try to determine whether the
NIU AND BATIZA: MID-OCEAN RIDGE BASALTCHEMICAL TRENDSVERSUSSPREAD•G RATE
TABLE
Ridge
Group
Means
7889
1. Data Set
M•O>Swt
%
DataSources
NorthEPR
SouthEPR
553
263
531
243
1,4,6,11,23,24,25,26,30,35,42,43,54,55,56
1,12,45,51,55
Gorda
JuandeFuca
Explorer
Galapagos
Easter,EastRidge
Easter,West Ridge
28
184
87
172
15
40
28
181
83
130
15
40
1,16
1,19,31,32
1,15,41
1,2,13,14,21,22,46,47
1,49
1,49
NorthMAR
SouthMAR
CaymanRise
704
89
26
Carlsberg
712
97
26
10
1,7,8,9,29,34,40,43,44,48,50,52
1,5,17,28,39,56
1,5 3
1
Red Sea
19
18
1
America-Antarctic
SW Indian
SE Indian
CentralIndian
46
39
63
33
46
39
63
33
1,3 8
1,20,36,37
1,3,18,33
1,27
Total
2387
2279
10
1, Smithsonian;
2, Anderson
et al. [1975];3, Anderson
etal. [1980];4, BatizaandNiu [1992];5, Batizaet al. [1988];6, Benderet al.
[1984];7, Bougault
andHekinian
[1974];8,Bougault
etal. [1988];9, BryanandMoore[1977];10,Bryan[1979];11,Byersetal. [1986];
12,Campsie
et al. [1984];13,Christie
andSinton[1986];14,Clagueet al. [1981];15,Cousens
et al. [1984];16,DavisandClague
[1987];17,Dickeyet al. [1977];18,Dossoet al. [1988];19,Eabyet al. [1984];20,EngelandFisher[1975];21, Fisket al. [1982];22,
Perfitetal. [1983];23, HawkinsandMelchior[1980];24,Hekinianand Walker[1987];25, Hekinianet al. [1985];26, Hekinianet al.
[1989];27,HumlerandWhitechurch
[1988];28,Humphris
etal. [1985];29,Jakobsson
etal. [1978];30,Juteau
et al. [1980];31,Karsten
[1988];32, Karstenet al. [1990];33, Kleinet al. [1991];34, Langmuir
et al. [1977];35, Langmuir
et al. [1986];36, Le Roexet al.
[1982];37,Le Roexet al. [1983];38,LeRoexetal. [1985];39,LeRoexet al. [1987];40,MelsonandO'Hearn[1986];41,Michaelet al.
[1989];42,Mooreet al. [1977];43,Morel[1979];44,Neurnanm
andSchilling
[1984];45,Renard
et al. [1985];46,Schilling
et al.
[1976];47, Schilling
et al. [1982];48, Schilling
et al. [1983];49, Schilling
et al. [1985];50, Sigurdsson
[1981];51, Sintonet al.
[1991];52, Stakes
et al. [1984];53, Thompson
et al. [1980];54, Thompson
et al. [1989];55, Tighe[1988](EPRsynthesis);
56,
unpublisheddata of Batiza and Niu.
TABLE
2. Global Correlations
Raw Data
Slope
Slope
Intercept
R
7.45x10
'4
3.84x10
'3
-7.14x10
'3
8.87x10
'3
50.65
1.39
15.30
9.98
2.63x10
'2
3.73x10
'•
3.12x10
-•
2.51x10
-•
Si(8)
Ti(8
)
AI(8
)
Fe(8
)
-2.18x10
'3
6.96x10
-4
-2.60x10
-3
-2.04x10
-3
50.52
1.31
15.63
9.70
1.45x10
-1
1.42x10
-•
1.87x10
'•
1.04x10
'•
CaO
Na20
K20
P205
-4.17x10
'3
2.40x10
'3
-4.74x10
'4
5.22x10
'4
11.70
2.51
0.21
0.14
1.79x10
'•
2.61x10
-•
1.33x10
'•
2.84x10
'•
Ca(s)
Na(8
)
K($)
P(8)
Si(8)/Fe(8
)
1.04x10
-3
8.81x10
-4
-8.89x10
'4
2.11x10
'4
5.85x10
'4
11.99
2.45
0.20
0.13
5.27
7.09x10
'2
1.19x10
-•
3.11x10
-•
1.64x10
-•
5.12x10
-2
Ca($}/AI•$}2.00x10
'4
0.77
1.63x10
-1
-6.43x10'3
7.65
R
for Fractionation*
SiO
2
TiO2
A1203
FeO
MgO
Intercept
Corrected
2.55x10-•
-
-
-
* The correction
procedure
we used[Niu, 1992]is similarto theonebyKlein andLangmuir[1987]withfollowingdifferences:
We
usea generalpolynomial
equation
that allowsto correcteveryoxideandto useall the samples
with MgO > 5.0 wt %. This general
equation
accounts
for thecurvature
of variation
trendsin theregionof MgO= 9-7 wt %. We corrected
all oxidesto MgO = 8.0 wt %. The
corrected
oxidevaluessumto 100•_1%,
andplotsof corrected
oxidevaluesagainst
MgO showessentially
zeroslope,suggesting
thatour
correlation
procedure
doesnotintroduce
anyartifacts.We alsoapplya regionalcorrection
to datafromeachmajorgeographic
areaas
listedin Table 1 (seeNiu [1992]for correction
coefficients).Valuesof R > 0.081 are significant
at 99% confidence
level (F testfor N =
2387:
99%confidence
atF = 6.63).Si(8)/Fe(8),
Ca(a)/AI(8),
andNa(8
) values
areforthetrends
shown
inFigure
3.
distribution
of the globalandlocal trendsis systematic
with
spreadingrate.
segment,its length,spreading
rate,numberof chemicalgroup
meansandsampledensity.The lengthsof theseridgeportions
To doso,we selectthe32 bestsampled
portions
of theridge vary greatly (Table 3), and all have transformsor nontransform
systemfrom the globaldatasetandexaminethe datafrom each discontinuities
or largeoverlapping
spreading
centers(OSCs)
areain detail. Thesearelistedin orderof increasing
spreading [Macdonaldet al., 1988]as endpoints.Samplingdensityalso
ratein Table3, whichalsogivesthe endpoints
of eachridge varies greatly with the poorest-sampled
ridge having ~ 0.7
7890
NIU AND BATIZA:
MID•
RIDGE BASALT CI-IEMICAL TRENDS VERSUS SPREADING RATE
Rawdata
Corrected
for
fractionation
14
;lh
'..I.;' . _
.
12
10
8
6
i
i
ß
i
I
I
I
Ti(8)
Ti02
4
ß
2
ß
" ' •' "
•''
'
ß
ß
ß
ß
ß
0
0
0
40
80
120
160
I
0
40
80
120
160
Full Spreading Rate (mm/yr)
Fig. 2. Examplesof the correlation
betweenmajorelementvariationand full spreading
rate. Thesecorrelations
are
consistent
withincreasing
degrees
of low pressure
fractionation
withincreasing
spreading
rate,as shownby the absence
of
trendsin plotsof fractionation
corrected
data. SeeTable2 for dataonothermajorelements.
samples
- 10km'1. Ideally,onewouldchoose
ridgesegmentsglobal trend whereas slow spreadingridges (< 50 mm/yr)
of comparablelength and sampling density; however, the
present sample distribution makes this impossible. Another
consequence
of incompletesamplingis that our groupof 32
best sampledareasdoesnot includeany portionof the Indian
Ocean ridge system (Figure 4). Figure 1 shows plots of
exhibit the local trend. Furthermore, this relationship is
apparentlyindependentof geographiclength scale as Table 3
has ridge segmentsof highly variablelength. For example,at
slow spreadingrates,both Narrowgate(34.2 km long) and the
entireMid-AtlanticRidgesouthof the KaneFractureZone(885
Na(8)andCa(8)/AI(8)(indicesof extentof melting)against km long) exhibit very clear local trends. At fast spreading
Si(8)JFe(8
) (anindexof pressure
of melting)for ourentiredata rates,both small segmentsand the entire northernand southern
set. Figure 5 shows the same two plots for each of the 32 EPR exhibit the globaltrend,as shownpreviouslyby Niu and
Batiza [1991a] and Klein and Langmuir [1989]. The
individualridge portionsof Table 3.
Most of the 32 ridge segments
showclearlineartrendswith distribution of these chemical variation trends is thus
slopesindicativeof either the globalor local trend. However, apparentlycontrolledby spreadingrate and not spatialscale,
somesegmentsshow less clear trends,either intrinsicallyor as impliedby the termsglobaland local.
Another importantresult from Table 3 and Figure 5 is that
becauseof low samplingdensity. In order to objectively
assess
how closelythe datafromeachareamatchthe globalor the slopes of the regressionlines for the global and local
as can be seenon Figure
local trend,we developeda methodfor assigning
a numerical trendsare not constant.Furthermore,
scoreto each segment. The scoresare derivedfrom (1) the 7, the individualregressionlines do not exhibit a systematic
slopeandsignificance
(F test)of theregression
lines(positive pattern(such as fanning)on the plots. One possibilityis that
or negativevaluesof the correlationcoefficientR); (2) The thisvariabilityin slopeis due to uncertaintyin the slopeof the
value and significance(t test) of the correlationcoefficients; regression lines (data scatter); however, we find no
and (3) samplingdensity. By this method,areaswith very relationshipbetweenthe correlationcoefficientsand the slope
clear trends (little scatter and many samples)receive the of the regressionlines. Thus it is more likely that this slope
highestnumericalvalues(seeTable3, negativescorefor local variation is real and reflects characteristics of the natural
trend and positive scoresfor global trend) and areas with processesthat result in the chemical systematicsof both fast
scatteredtrendsand/orfewer samplesreceive low numerical and slow spreadingridges. Interestingly,there appearsto be
scores. Details of the scoringprocedureare given in the more clusteringof global trendsthan of local trends.
appendix.
DISCUSSION
Table3 andFigure6 summarize
theresultsandshowclearly
thatMORB chemicalsystematics
(localversusglobaltrend)are
If we assume that the mantle is approximately
relatedto spreading
rate. Fastridges(> 60 mm/yr)exhibitthe homogeneous,at least for major elements,then the resultsof
NIU AND BATIZA: MID-OCEAN •
BASALT CHEMICAL TRENDS VERSUS SPREADING RATE
9.0
/
.....
.
:
ß
6.0 • I
..),,.f..{
/
.,!.
ß
includes regionally averaged data for many slow spreading
ridges,it seemslikely that passiveupwelling of mantle is also
importantat slow spreadingridges.
There exists strong evidence from crustal thickness and
MORB chemicaldata [Klein and Langrnuir, 1987], theoretical
considerations [McKenzie, 1984], and models of MORB
melting [Klein and Langmuir, 1987; McKenzie and Bickle,
1988; Niu and Batiza, 1991a] that the global trendof MORB is
due principally to differencesin averagemantle temperature.
Hot mantle rising adiabatically intersects the solidus at a
deeperlevel than cooler mantle and continueddecompression
melting with matrix compactionshould result in pooled melts
3.0
1.0
ßI
I
I
.J
0.9
oil
ß
0.7
0.6
0.5
5.0
t I
ø
ß
ß
I
I
:-.,.
z
I
...
'
2.0
1.o
0
4o
8o
120
Full Spreading Rate (mm/yr)
with chemical
signatures
of highpressure
(highFe(8)andlow
Si(8)/Fe(8))andhighextentsof melting(low Na(8) andhigh
Ca(s)/AI(8)).
Regional
differences
in manfietemperature
should
give rise to ensemblesof data which exhibit the global trend.
Manfie temperaturedifferencesof about 200øC are capableof
producingthe global array [Klein and Langmuir, 1987]. On a
smaller geographic length scale, differences in mantle
temperaturecan give rise to the same global systematicsat a
more local or regional level, as shown by Niu and Batiza
[1991a].
Thus this explanation of global trend is fully
consistentwith both petrological evidence and models of
mantle flow beneathfast spreadingridges[e.g., Lin and Phipps
Morgan, 1992].
The global trend is well-explained by adiabatic
decompression
melting of a mantle column in which polybaric
melts are efficiently separatedfrom their residuesand pooledin
a reservoirat low pressure[Klein and Langmuir, 1987;Niu and
Batiza, 1991a]. However, in rising diapirs,separationof melt
from residue may be less efficient. Whereas rapid melt
segregation in a melting column of global trend preserves
0.8
4.0
7891
160
high-pressure
signatures
(low Si(8)/Fe(8))at highextentsof
melting [Klein and Langmuir, 1987], extensivemelts of the
localtrendhavelow-pressure
signatures
(highSi(8)/Fe(8)).
One
possibleexplanationis that slow melt segregationin diapirs
Fig.3. Variations
of Si(8)/Fe(8),
Na($),andCa(s)/Al($
) asa function allows matrix-melt reaction during ascent, such that high
of spreadingrate. Note that there appearsto be greaterdispersionat
pressuresignatures
are only preservedin meltsproducedby low
slow spreading rates but that values are essentially constant (and
extents of melting shortly after initiation of a diapiric
scattered)with spreadingrate.
instability.
During ascent of a mantle diapir undergoing melting,
Figure 3 indicate that average mantle temperature is
inefficient melt segregationwould allow melts to reequilibrate
independentof spreadingrate. However, chemical variation with the solidmatrix. If thesereequilibratedmeltswere tapped
trendsinvolving pressureand extentsof melting do appearto by dikes [Nicolas, 1986; Sleep, 1988] at variouspressures,an
be relatedto spreading
rate(Table3 andFigures5 and6). These ensembleof melts resemblingthe local trend would result.
trends, the global and local chemical variation trends, appear Deep meltswouldbe producedby smallextentsof meltingsoon
to be independentof geographiclength scale along axis, and after initiationof the diapir. If thesemelts are tappedby dikes,
the slopes of the trends are variable (Figure 7). These they will have chemical signaturesof high pressureand low
observationssuggeststrongly that the dynamics of mantle extentsof melting. With continueddiapiric ascent,melting
upwelling, thought to differ fundamentallybetween fast and and melt-solid reequilibration, melts would have chemical
slow spreadingridges [e.g., Parmentier and Phipps Morgan, signaturesof successivelylower pressureand more extensive
1990; Lin and Phipps Morgan, 1992; Scott and Stevenson, melting. In this scenario, the chemical signature of melts
1989], may play a role in controllingthe distributionof the dependscritically on their ascentand segregation
history.
global and local chemical trendsin MORB. We hypothesize
While this somewhatspeculativeidea is difficult to prove,
that the global trend results from melting under conditions there are many independentlines of evidencesuggestingthat
dominatedby passive,plate driven mantle upwelling that is diapirs are important at slow spreadingridges. Gravity and
broadly two-dimensionalin character [e.g., Phipps Morgan,
topographydata [Lin and Phipps Morgan, 1992; Lin et al.,
1987]. In contrast, the local trend would be the result of
1990], studiesof mid-oceanridge segmentation[Parmentier
melting that occurs in rising mantle dominatedby buoyant, and PhippsMorgan, 1990; Whiteheadet al., 1984;Schoutenet
al., 1985; Crane, 1985], and theoreticalstudiesof mantle flow
three-dimensional upwelling [e.g., Sotin and Parmentier,
1989; Scott and Stevenson,1989]. Such buoyantinstabilities [Scott and Stevenson,1989; Sotin and Parmentier, 1989] all
could be embeddedwithin a mean upward plate-driven flow
point to the possibleimportanceof diapirs at slow spreading
[Scott and Stevenson,1989]. In fact, since the global trend ridges. Furthermore, geologic mapping in ophiolites has
7892
NIU AND BATIZA: MI])•N
RIDGEBASALTCHEMICALTRENDSVERSUSSPREAD•G RATE
i
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!
!
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!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
i
!
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i
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o o
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NIU ANDBATIZA: MID-OCEANRIDGEBASALTCttEMICALTRENDSVERSUSSPRF.
ADINGRATE
7893
\\
60 ø
15-19
14
30"
N
11,12
22-2E
0o
29-
.'.'.'52
30"
S
60 ø
0ø
30 ø
60 ø
90 ø
120 ø
150øE
180 ø
150øW
120 ø
90 ø
60 ø
30 ø
0ø
Fig. 4. Map of the mid-oceanridgesystem.The 32 bestsampledareasselectedfor this studyare shownwith numberskeyed
to Table 3. Note the absenceof well-sampledareain the Indian Ocean.
revealeddiapiric structurein mantle rocks [Rabinowiczet al.,
1987; Nicolas, 1986]. These independentlines of evidence
indicate that the notion of diapirs at slow spreadingridges
deservesseriousconsideration. Assumingascentrates of the
order of plate motionratesand slow melt segregation,chemical
reactionbetweenmelt and solid is difficult to avoid [Kelemenet
al., 1992; Qin, 1992]. Below, we develop this hypothesis
further and offer two additionallines of supportingevidence.
First, we discuss some illustrative mass balance calculations
indicating that melts of the local trend could be related by a
melting-crystallizationreaction that plausibly could occur in
rising diapirs undergoingmelting and melt reequilibration.
Second,we show that zero-agelavas of seamountson the
flanksof the EastPacificRiseexhibitboththe globalandlocal
chemicalvariationtrends. This is consistentwith the diapir
hypothesisas diapiric buoyancyinstabilitiesmay be expected
to developat the edgesof the broadupwellingregionbelowthe
EPR axis and flanks [PhippsMorgan, 1987;Niu and Batiza,
1991a]. Similarly,at transitionalridges(50-60 mm/yr) where
both the global and local trends may occur, diapiric and
widespreadpassiveupwellingmay both occur. In this casethe
petrologic signatureof erupted lavas may be dominatedby
matrix would have enhanced opportunity to interact
chemically,even as melting continued. We envisiona process
similar to that studiedby Kelemen [1990] and Kelemenet al.
[1990] except that the processis polybaric. For this reason,
the resultsof Kelernen [1990] and Kelemen et al. [1990] at 0.5
GPa only cannot be rigorously applied. Instead, we explore
possiblepetrologic processesin diapirs with some extremely
simple but illustrative mass balance calculations. With these
calculationswe attempt to directly constrainthe minerals that
may be involved in producing melts of the local trend of
chemical
variation.
A fundamentaldifficulty in attemptingto interpretthe cause
of the local trend is that, by definition, this trend is composed
of sampleswith identicalMgO = 8.0 wt %. Even thoughthe
local trend probably is the result of dynamic physical and
chemicalprocesses
which lead to variationsin MgO (as well as
other major element abundance),our present definition and
view of the trend are an artificial, constant-MgOsnapshot
only. Whethermeltsformingthe local trendarerelatedto each
other directly throughsomepetrologicprocesssuchas melting
or crystallization, or related only indirectly (for example,
sequential products of a petrologic process), it is most
either trend.
improbablethat they coexistas constantMgO melts.
In order to circumventthis problemof constantMgO, while
Model Calculations
at the same time trying to explicitly constrain the major
Relatively little is known about the physico-chemical silicatephasesthat might be involved in the local trend, we use
processesoccurring in mantle diapirs undergoingmelting simple stoichiometric least squares modeling of the type
[e.g., Ribe, 1983; Cawthorn, 1975]. Under appropriate describedby Bryan et al. [1969] andBryan [1986]. Figure 8
conditions,material in diapirs shouldmelt just as passively showsseveralwell-definedtrendsfrom slow spreadingridges
upwelling mantle does. However, melt extractionby matrix which we use in our illustrativemodel. We implicitly assume
compaction,a processthoughtto be generallyimportantfor thatthe magmasof the local trendarerelatedto oneanotherby
upwelling mantle [McKenzie, 1984], could be retardedbecause some petrologic processes(melting or crystallization, for
both melt and residuerise together. In this case,the melt and example) involving major silicate phases,and we attempt to
3.8
3.6
3.2
ß
ß
•
3.0
t
I
,
I
-
.
..•0.65*
ß
,.••eoet ß ß ß
u•.5', '
i
!
6.0
i
6.5
.
.0
3.5
4.0
4.5
5.0
.S
4.5
5.0
5.5
6.0
6.5
4.5
7.0
S.O
S.S
5.0
5.5
6.0
6.5
4.5
5.0
5.5
6.0
6.0
:2.4.
"øt.'i.._%
•:
o.g
•
_/
..%_
ß
ß **MB'*%
0.7[
4
.
,
5
.
,
6
7
Si(o)/Fe(o)
5.0
6.0
4.0
Si(o)/Fe(o)
Si(o)/Fe(o)
lO
9
5.5
Si(o)/Fe(o)
11
12
C•2.8
Z 2.4
0.8
ß
d
ß
0.7
4.8
5.2
5.6
6.0
6.4
4.4
4.8
14
5.2
5.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
16
15
3.(]
2.8
2.6
oo
2.6
•.2
*-0
ß ß
e©
ß
ß
ße
2.4
ß
'
•:
o.g
0.76
m
0.8
0.72
'
'
I
I
I
.
,
***
el
o.•.0 '5•, ' 5:4 ' 5•s
Si(8)/Fe(8)
4
5
6
Si(o)/Fe(o)
7
4.6
4.8
5.0
Si(8)/Fe(8)
Fig. 5. Chemicalsystematics
of the 32 denselysampledridgesor ridgeportionsshownon the sametwo diagrams
of Figure
1. The numeralsreferto the ridgeID numberlistedin Table 3. Note the greatvariabilityin the correlationand the slopes
(signandmagnitude)of the regression
lines. Table3 givescorrelation
coefficients
andvaluesof statisticalsignificanL
5.2
17
18
19
20
ß
el
e
4.6
21
22
ß
4.8
5.0
5.2
4
5
6
23
3.2
7
24
.......
3.3
.....
2.6
.
'
:.,,/r...: :
: : :
:
:
:
:
l
CI• •.?
O.IK)
ø
- o6
0.76
- 0.7,
ß
ß
ot5 - 5:o - 5:5 :
Si(.)/Fe(.)
6.0
5.2
5.6
6.0
ø' 5 5.o 5.5 ,.o ,.5 ø'1, ' 5:, ' ,:o ' ,:4
Si(8)/Fe(8)
Si(8)/Fe(8)
26
25
Si(8)/Fe(8)
28
27
2J
2.1
t ßß ß
'l -. '-.:.'..-'
2."
-.•-o.85,.
l , 'I , I ,
•
0.6' i,
o,ot .
0.8(
o."f
' '"'0
70'
.......
ß 4.0
4.5
5.0
:'
5.5
6.0
0.7!
4
0.7:,
5
6
7
8
L8
30
29
5.2
5.6
6.0
.5
5.0
31
5.5
6.0
5:8
6.2
32
2.7
2.5
eel
2.3
,
I
,
I
ß
ß
,
--•0•4
ß lee
'"•o.82
'
ei
ß ß
ß
ß
C.)o.8o
ß
ee
0'7•.1 ' 5:3 ' 515 ' 5.7 4.8 5.0 5.2 5.4 5.6 4.6 5.0 5.4 5.8 6.2 5.0
Si(8)/Fe(8)
Si(8)/Fe(8)
Fig. 5. (continued)
Si(8)/Fe(8)
5:4
Si(8)/Fe(8)
7896
NIU AND BATIZA: MID•
•
lS•,
130,
Local
giving non-uniquesolutions,our result appear to be very
32,•$$.49
•5s.60
robust. We have tested hundreds of models using many
combinations
of mineralend membersbut only a few yield selfconsistent results with good fits (low residuals).
t50.89
•29,
•2s,
Trend
RIDGE BASALT CHEMICAL TRENDS VERSUS SPREAD•G RATE
•49.60
H0.42
•27,
•06.99
Representative
resultsare shownin Table 5. Note that the
poorestfits are for Ti and K, whichis not surprisingsincewe
did not includemineral endmemberscontainingeither element
(thoughwe couldaddTi to pyroxene).Oneinteresting
resultis
that plagioclaseis apparentlynot neededto describethe a-b
path for any locality as all solutionswith plagioclasegive
very high residuals(> 2.0 or so), with especiallypoor fits for
Ca and AI. This is not surprising because melting in
plagioclasestability field may be quite restricted[Nicolas,
1986]. Table 5 showsthat in general, the path from a to b
(Figure 9) seemsto involveboth meltingand crystallization.
Pyroxenecomponentsare taken into liquid a while olivine
solidifies;together,thesetwo effectsproduceliquid b. In all
four cases,the original liquid a representsabouthalf of the
eventualliquid b, andthe ratio of pyroxenecomponents
(added
to melt) to olivine component(removedfrom the melt) is 3.64.0. Sucha trendis thusa net meltingtrendcharacterized
by a
•26,
•25,
97.86
•24,
95.•
••s,
9s.•
•22,
•21,
89.92
66.26
18, 59.•
••7,
16, 58.68
s9.o8
.....:.:•
15,58.29
14,55.00
zs, 5s.98
..:::..:...•
::[]
•12,
5234
9,30.70
7, 26.20
4, 20.94
:.'.._.•
......
.....
3,17.67
x, t rso
-3.0
-2.0
Global
•........
Trend
"'•":E
mineral-melt reactionof the generalform:
•
-1.0
liquida + pyroxene<=•liquidb + olivine
0.0
1.0
2.0
3.0
Score
and is similar to the reactions studied theoretically and
experimentally
by Kelemen[1990] andKelemenet al. [1990]
at 0.5 GPa. Under conditionsof ascentto lower pressure,this
Fig. 6. Bar diagramsto showthe relationships
betweenchemical
systematics
andthe full spreading
rate. Eachbar refersto a specific reaction would proceedto the right [e.g., O'Hara, 1965;
ridge segmentlisted in Table 3 and the first numbergivesits ID Stolper, 1980; and Elthon and Scarfe, 1984], obviously
number. The value after the commais the full spreadingrate. This favoringthe a to b path over the alternativeb to a path.
figureclearlyshowsthatfastspreading
ridgesexhibittheglobaltrend
This result seems reasonable and is consistent with
and slow spreadingridgesthe local trend. Thereis alsoa transition
processes
expectedin a buoyantsolid-liquiddiapirundergoing
(50-60 mm/yr) whereeithertrendis possible.
4.0
determinethe phasesand their proportionsusingmole percent
least squaresmassbalancecalculations.
For simplicity, in Figure 9 we omit data points and show
only the regressionlines for each of the local trends. We
essentiallyask what assemblagesof silicate phasescould be
involvedin producingthe trendfrom a to b (or b to a)? To get
around the problem of constant M gO, we use a two step
approach. First, in the stoichiometricmodeling, we combine
Fe and Mg to form one divalentcation(FM). This allowsus to
obtain a first-ordersolutionwhile, for the moment,neglecting
the constant MEO problem. Using FM also allows us to
temporarilydisregardthe complexityof Fe/Mg partitioningin
natural silicatephasesand the effectsof pressure,temperature
and compositionon the partitioningbehavioras we seek only
a generalizedsolution. But having done the initial modeling
with FM, we return to considerhow much MgO may actually
changealongthe a-b or b-a pathsof Figure9.
With the cation FM, we make olivine (FM2SiO4) and
pyroxene (FM2Si206: both orthopyroxeneand Mg-Fe
3.0
z
2.0
1.0
0.9
0.8
0.7
0.6
componentsof clinopyroxene). In this simple stoichiometric
modeling,we also considerendmemberdiopside-hedenbergite,
0'52 3 4 5 6 7 8 9
jadeire, Ca-tschermaks,and albite-anorthite. Table 4 shows
the liquid compositionsat points a and b of each of the local
trendsof Figures8 and9. We thenusea least-squares
methodto
linesof Figure5 plotted
together
onNa(s)
determine the stoichiometricendmemberphaseswhich can, Fig.7. All theregression
andCa($)/AI($)versusSi($)/Fe($)diagrams.
Theheavyline(for
whenaddedto liquida, produceliquid b (or vice versa).
reference)is the globaltrend from Figure4. Note that the slopesof
While least-squaresmixing modelsof this type, especially both the local and global trendsvary and that no simplepattern(such
with few componentsand many phases, are notorious for as a fanningpattern)exists.
Si(8)/Fe(8)
N1UANDBATIZA:MID•
RIDGEBASALT
CHEMICAL
TRENDS
VERSUS
SPREADING
RATE
7897
physical-chemical
processes
in rising diapirsundergoing
melting,thisvariabilityin thedatais not surprising.
CR
Kinzler and Grove [1992] recently proposedpolybaric
fractionalcrystallization
as a possibleorigin for the local
SKFZ
3
trend. We feel that fractional crystallizationis inevitable
26øS
within risingmantlediapirs,but suchfractionation
should
occuronlyduringthelateststages
of diapirism
whenmelting
Na(8)
2
ceasesdue to slow ascentand higherheatloss. Mineralogic
featuresof MORB lavas from slow spreadingridges suggest
Fe(8)
6
8
10
12
14
16
4
that polybaricfractionationmay play a role in their
petrogenesis[Bryanet al., 1981;flusteret al., 1989].
However,we suggest
thatpolybaric
fractionation
shouldnotbe
the majorcauseof the localtrend;a rigorous
modelshould
explainthechemical
relationships
among
parameters
of Fe, Si,
Ca, AI, andNa asshownabove,notjustFe andSi versusNa. A
rigorous
testamongexistingmodelsrequires
muchadditional
studyof possible
processes
in diapirs,pluscarefulpetrologic
modeling
of bothmeltingresidue(peridotite)
andindividual
lava suites that exhibit the local trend.
Na(8)
2 .•
---SKFZ
Evidence From Near-EPR Seamounts
I , ,, , , , , •,•', , , St(8)
The near-axis flanks of the EPR have abundant seamounts
composed
dominantly
of MORB [Batizaeta/., 1990]. They
47
48
49
50
51
52
53
occur as individual seamountsand also as chainsparallel to
eitherrelativeor absolute
platemotion. Figure10 showsthat
Fig.8. Plots
ofNa(s
) versus
Fe(s
) andNa(S
) versus
Si(s
) showing
the theseamount
arrayis verysimilarto theglobalarray(defined
globaltrendof KleinandLangmuir
[1989](thickline)andseveral
best
using
axial
samples
only) (Figure1) and appearsto be a
definedlocaltrends(crosstrends)recognized
at slowspreading
midoceanridgesegments
[KleinandLangmuir,
1989;Batizaet al., 1988].
CR,Cayman
Rise;26øS,MAR at 26øS;SKFZ,MAR south
of theKane
FractureZone;and RP, ReykjanesPeninsula.
CR
SKFZ
3
additionalmelting. Note that in Table 5 the combined
compositions
of the pyroxenesenteringthe meltsare unlike
Na(8)
any naturalpyroxenesin that Jd, andparticularlyCt, are too
2
high. This is, in fact, expected,as bothorthopyroxene
and
clinopyroxene
are residualphasesduringmantlemeltingto
26øS
(9)
produceMORB [Dicket al., 1984]. Continuous
meltingwill
tend to depleteJd and Ct components
in residualpyroxenes
because
bothare incompatible
duringmeltingandlessstablein
pyroxenes
aspressure
is reduced.Thuspyroxenes
in the solid
will graduallybe depletedin thesecomponents,
whilethemelt
in a risingdiapirwill be enrichedin JdandCt components.
We now reconsider
possiblechangesin MgO andFeO. To
(7)
Fe(8)
6
8
10
12
14
16
calculate
theMgO andFeOchanges,
wedispense
withtheuseof
FM. Usingthe phaseproportions
from Table 5, we partition
FeO andMgO amongthe solidsilicatephases
(seeTable4 note
for details)and calculateliquid b as a combination
of liquid a
andthe phasesof Table 5. The FeO andMgO valuesobtained
are givenin Table 4. MgO changesonly slightlyalongour
calculated
patha - b. It is importantto notethatbecause
of the
MgO = 8.0 conditionof thelocaltrendlinesin Figures8 and9
47
48
49
50
51
52
53
and becausethe calculationsindicatea small changeof MgO,
the calculateda - b pathscannotbe identicalto thea - b paths Fig. 9. Sameas Figure8 butwiththedatapointsof thelocaltrends
omitted. Regression
lines are bestfit linesfor all elements(notjust
onesshown).Thepointslabeleda (solidsquare)andb (emptysquare)
aretheliquidswe consider
in Tables4 and5 whichdefinethea-b path
we attemptto model. The thin linesbetweensolidsquaresare the
resultsof ourtwo-stepmodelingprocedure
(seeTables4 and5 andtext
positions.Thus, while the calculateda- b pathsare not for details). Notethatthe thinlines(model)andthe thicklines(data)
generallywell matched.Themodelwe presentgivesa goodfit to
identicalto the slowspreading
dataarraysin Figure9, theyare are
all elements,
notjustNa, Fe, andSi. Alsoshownasthinlightlinesfor
veryclose.Indeed,sincetheslopesof actuallocaltrendsvary comparison
arethelocaltrenddatacorrected
toMgO= 7.0 and9.0 wt %
(Figure7), thereis evidentlysomevariabilityin the processes as labeled(RP only, othersare similar). Note that thesedatahave
leadingto this trend. Given the potentialcomplexityof similarslopesto the MgO = 8.0 wt % localtrend.
of the local trends.
To assess the difference in the calculated
versusobservedpaths,we plot the calculated
endpoints
(b) on
Figure9. In addition,we plotlocaltrendscorrected
to MgO =
7.0 and MgO = 9.0, which fall close to the MgO = 8.0
7898
NIU AND BATIZA: MID•
RIDGE BASALT CHE••
TRENDSVERSUSSPRF_ADING
RATE
TABLE 4. Melt Compositions
for EachLocalTrend(Figures8 and9)
RP-a
RP-b
CR-a
CR-b
SK-a
SK-b
26øS-a
26øS-b
WeightPercentat MgO = 896
Si(8)
Ti(8)
AI(8)
Fe(8
)
Ca(8
)
Na(8)
K(8)
P(8)
48.00
2.48
13.56
14.37
10.12
2.41
0.25
0.24
49.38
1.06
15.60
10.13
13.15
1.89
0.10
0.07
50.75
1.67
16.14
8.91
10.11
3.45
0.23
0.18
51.75
1.46
16.79
7.68
10.85
3.20
0.28
0.18
49.80
1.85
15.30
10.73
10.55
2.89
0.23
0.19
50.69
1.28
16.25
8.78
11.88
2.61
0.20
0.13
50.69
1.68
15.48
9.65
11.48
2.70
0.09
0.16
51.62
1.13
15.70
8.31
12.41
2.52
0.06
0.11
MgO*
9.55
8.74
8.76
9.58
FeO*
8.01
6.71
7.38
6.59
Recalculated
Si
Ti
A1
FMt
to Cation Mole Percent
45.35
1.77
15.10
46.19
0.75
17.19
46.82
1.16
17.55
47.32
1.00
18.10
46.33
1.30
16.77
46.82
0.89
17.70
46.93
1.17
16.90
47.69
0.78
17.09
22.63
19.08
17.88
16.78
19.44
17.80
18.52
17.44
Ca
Na
K
10.25
4.42
0.30
13.18
3.44
0.12
10.00
6.19
0.27
10.63
5.70
0.33
10.52
5.22
0.27
11.76
4.69
0.23
11.39
4.87
0.11
12.29
4.54
0.07
P
0.19
0.06
0.14
0.14
0.15
0.11
0.12
0.09
RP, ReykjanesPeninsula;CR, CaymanRise;SK, southof The KaneFractureZone;and 26øS,26øSareaMAR.
*MgOandFeOcompositions
forb werecalculated
explicitly
todetermine
theirchange
along
thea-bpath.Inthiscalculation,
other
elementsagreewith thosein Table 5. We usedthe calculatedphaseproportions
from Table 5 andperidotitcmodalabundances
fromDick
[1989]. We partitionEn-FsbetweenOpx andCpx in Table 5 in the ratio 63:37,with Opx free of Di andHd components.We alsoused
Fo90.3,
OpxwithMg# = 91.3andCpxwithMg# = 94.6from
Dick[1989].
Asexplained
inthetext,weconsider
these
mass
balance
models
illustrative, not definitive.
t FMisthecombined
cation
mole
percent
ofFeOandMgO(= 8.0wt%).
TABLE 5. Resultsof theLeastSquares
ModelCalculations
a
Di-Hd
Jd
Ct
En-Fs
Ol
SQ'D
RP-b
CR-b
SK-b
26øS-b
0.4884
0.1569
0.0518
0.1704
0.2913
-0.1569
0.6816
0.0599
0.0597
0.0929
0.2220
-0.1200
0.6667
0.0841
0.0488
0.1060
0.2104
-0.1167
0.5414
0.1249
0.0767
0.1204
0.2816
-0.1467
0.0142
0.0654
RP-b
Calc
Si
Ti
A1
FM
Ca
Na
K
46.18
0.86
17.19
19.08
13.19
3.45
0.15
Obser
46.19
0.75
17.19
19.08
13.18
3.44
0.12
0.0032
CR-b
Diff
Calc
-0.01
0.11
0.00
0.00
0.01
0.01
0.03
47.32
0.79
18.10
16.78
10.64
5.71
0.18
Obser
47.32
1.00
18.10
16.78
10.63
5.70
0.33
0.0220
SK-b
Diff
Calc
0.00
-0.21
0.00
0.00
0.01
0.01
-0.15
46.82
0.87
17.70
17.80
11.77
4.70
0.18
Obser
46.82
0.89
17.70
17.80
11.76
4.69
0.23
26øS-b
Diff
Calc
0.00
-0.02
0.00
0.00
0.01
0.01
-0.05
47.68
0.63
17.09
17.44
12.30
4.55
0.06
Obser
47.69
0.78
17.09
17.44
12.29
4.54
0.07
Diff
-0.01
-0.15
0.00
0.00
0.01
0.01
-0.01
Di-Hd, diopside-hedenbergite;
Jd,jadeitc;Ct, Ca-tschermaks;
En-Fs,enstatite-ferrosilite;
O1,olivine;SQ'D, sumof residualssquared
(,Y.
X2).Calc,calculated;
Obser,
observed;
andDiff,difference.
Alltheelements
arecation
molepercent.
combinationof both the global and local trends. Very clear beneaththe EPR appearsto becomelessvigorousoff axis such
local trends are found at individual seamounts, as shown in that at 40-50 km away from the EPR axis the upwellingis
Figure 11. Thus the local trend is not confined to slow feeble. In suchan off-axis mantlesetting,diapiricinstabilities
spreadingridges but also is found from zero-age lavas of wouldhaveadequatetime to nucleateandrise,providingMORB
melts for some off-axis
active volcanoes.
While this
seamountsnear fast spreadingridges.
As shown by Niu and Batiza [1991a], mantle upwelling suggestion is speculative, the existence of diapiric
NIU AND BATIZA: MID-OCFAN RIDGE BASALT CttEMICAL TRENDS VERSUS SPREADINGRATE
2. We confirm that there appear to be no systematic
differencesbetweenslow and fast spreadingridgesin the depth
or extent of partial melting; thus mantle temperatureseemsto
be independentof spreadingrate.
3. The so-called local and global trends of MORB
systematicsare a function of spreadingrate. The global
systematicsare found at fast spreadingridge segmentsand the
local systematicsare found at slow spreadingridges. Either
trend can be found in transitionalridges spreadingat 50-60
mm/yr.
4. The so-calledlocal and global trendsare not apparently
sensitiveto geographiclength scale.
5. The global and local trendsdisplaya rangeof slopeson
chemicaldiagrams,but no consistentor regularbehavior,such
as a fanning pattern.
6. The local trend is not confinedto slow spreadingridges
4.0
3.5
3.0
2.5
0.9
but also occurs at near-EPR
0.5
4
7899
t
I
5
n
I• O•oi
6
IO i
7
8
Si(8)/Fe(8)
Fig. 10. Data for seamounts
near East Pacific Rise (8" to 15"N) from
Batiza and Vanko [1984], Allan et al. [1989], Batiza et al. [1989,
1990], and unpublisheddata. REL points are from membersof small
seamountchainsparallel to relativemotion;ABS for chainsparallel to
the absoluteplate motion. For reference,we show the global trend
seamounts.
7. Strong geophysicaland field evidence (in ophiolites)
points to the importanceof diapirs at slow spreadingridges.
We show that the local trendis characteristic
of slow spreading
ridgesandproposethat this trendcouldresultfrom processes
in
rising diapirsundergoingmelting and melt reequilibration.
8. Transitional ridges spreading at 50-60 mm/yr may
display either the global or local trend of chemicalvariation.
We interpret this to indicate that both two-dimensional,
passive upwelling and buoyant, three-dimensional(diapiric)
mantle upwelling occur and that the petrologic signaturesof
lavaseruptedmay be dominatedby eitherprocess.
APPENDIX
Numerical values(scores)are derivedto objectivelyassess
[Klein amt Langmuir, 1989], a representative
local trendand datafor
how well the observed trends match idealized local and/or
the northem EPR from Niu amt Batiza [1991a]. Note that the seamount
data array stronglyresemblesthe õ1obalaxial MORB array (Figure l).
global trend.
4.0
instabilitiesbelow the flanksof the EPR is very plausible. If
so, then the existence of the local trend at near-EPR seamounts
is consistentwith the other lines of evidence indicating that
the local trend could originateby solid-meltreactionin rising
diapirs undergoingmelting.
Diapirs at SlowSpreadingRidges
Slow spreadingridge segmentsin the Atlantic [e.g., Kuo
and Forsyth,1991;Linet al., 1990] are typicallycharacterized
by central along-axis highs and negative residual gravity
anomalies. These anomaliesmay be due to thick crust, a
thicker mantlecolumnof low-densityresidualmantle,and/or
higher-temperaturemantle. All these possibilities are
consistentwith focusedbuoyantmantle upwelling with more
melting in the centerof the segment.Whetherthis representsa
singlelarge diapir or numerousindividualdiapirsin the center
of the segment is not known. Nevertheless, given the
numerousindependentlines of evidence favoring diapirs at
slow spreadingridges, plus the two additionalpetrological
lines of evidencepresentedabove,we suggesta diapir model
for the originof the local trendof chemicalvariation.
CONCLUSIONS
3.5
Illll"
I lB
3.0
"
o
© N-2-1I
o N-2-2I
ß AP-5 I
ß AC-4 I
2.5
-%
ß N-5
i
I
,
I
O*o .= ="
o
0.6
4
5Si(8)/Fg
(8) 7
8
1. With an expandedglobal data set, we confirm that slow
spreadingridgesgenerallyappearto have more primitive lavas Fig. 11. Seamountdatafrom Batiza and Vanko [1984] andBatiza et al.
than fast spreadingridges.
[1990] showinglocal trendsfor individualseamounts.
7900
N1U AND BATIZA: MID-OCEAN RIDGE BASALT CHEMICAL TRENDSVERSUSSP•ING
RCa/A
1andRNaarethecorrelation
coefficients
of Ca(s)/AI(8
)
versus
Si(s)]Fe(s
) and
Na(8
) versus
Si(s)/Fe(s
),respectively;
R95•
is the critical value of the correlation coefficient
at 95%
confidence level (t test); In our test, if the R is significantat
95% confidencelevel, the slopesof the regressionlines are
alsosignificantat that level (F test).
Local trend
Good (- 1.0):
Med (-0.8):
Poor (-0.6):
Global
RCa/A1
> 0,RNa
< 0,Rca/^l
andIRNa
I> R95•
RCa/A•
> 0,RNa
< 0,RCa/A•
or IRN,I> R95•1'
RcatA1
> 0,RNa
< 0,RcatA1
andIRNa
I_<R95•
trend
Good (1.0):
Med (0.8):
Poor (0.6):
RCaiA
1<0,RNa
> 0, IRc,/dI and
RNa
> R95%
RCa/A1
< 0,RNa
> 0, [Rca/A
• I RNa
> R95%
RCa/A1
< 0,RNa
> 0, ]Rca/A
1]and
RNa
<R95%,
Transitional
Local
med(-0.4):RCa/A1
> 0,RNa
> 0,Larger
R> R95•'
or RCa/A1
< 0,RN.< 0, ILarget
RI> R95•
Local
poor
(-0.2):RCa/A1
> 0,RNa
> 0,Larger
R< R95•'
or RCa/A1
< 0,RN,< 0, ILarget
RI< R95•
Global
med(0.4):RCa/A
1< 0,RNa
< 0, ILarget
RI> R95•'
or RCa/A
1 > 0,RNa
> 0,Larger
R> R95•
Global
poor(0.2):RCa/A1
< 0,RNa
< 0, ILarget
RI< R95•'
or RCa/A
1 >_0,RNa
> 0,Larger
R< R95%
RATE
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We thankR. Hey andJ. Karstenfor help at various
variationsin normalmid-oceanridgebasaltfrom 22ø-25øN: Midstagesof the work and J. Sinton for discussions.We also thank J. Allan,
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J. Brophy, R. Hekinian, W. Bach, and K. Harpp for very helpful com11,815-11,836, 1981.
mentson an early versionof the paper. E. Klein, P. Michael, and J. Lin
Byers, C. D., M. O. Garcia, and D. W. Muenow, Volatiles in basaltic
providevery helpfulreviews.This work was supported
by NSF (OCEglassesfrom the EastPacificRise at 21øN: Implicationsfor MORB
89-96191, and OCE-90-00193) and the Office of Naval Research. The
sourcesand submarinelava flow morphology,Earth Planet. Sci.
final revisionis supportedby a L-DEO PostdoctoralFellowshipto YN.
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(ReceivedApril 3, 1992;
revised November 30, 1992;
acceptedJanuary15, 1993.)