JOURNAL OF GEOPHYSICAL RESEARCH, VOL. tOO,NO. Btt, PAGES 22,261-22,282, NOVEMBER t0, 1995
Geochemistry and geochronologyof ancient southeastIndian
and southwest
Pacific
seafloor
D. G. Pyle andD. M. Christie
Collegeof OceanicandAtmosphericSciences,OregonStateUniversity,Corvallis
J. J. Mahoney
Departmentof GeologyandGeophysics,
WoodsHole Oceanographic
Institution,WoodsHole
Massachusetts
R. A. Duncan
Collegeof OceanicandAtmosphericSciences,OregonStateUniversity,Corvallis
Abstract. Within the Australian-AntarcticDiscordance(AAD), a boundaryexistsbetween
isotopicallydefined"Pacific-type"and"Indian-type"mid-oceanridgebasalt(MORB) erupted
alongthe SoutheastIndianRidge (SEIR). This boundaryhasmigratedwestwardbeneaththe
easternmost
AAD spreadingsegmentat a minimumrate of 25 mm/yr since4 Ma; however,its
long-termhistoryremainsa matterof speculation.
To determineif Pacific-typeuppermantlehas
migratedwestwardbeneaththe easternIndianOceanbasinasAustraliaandAntarcticadriftedapart
duringthe last 70 m.y., we presentnew Sr-Nd-Pbisotopedata,combinedwith traceelementand
40Ar-39Arradiometricage determinations,
for samplesfrom Legs28 and29 of the Deep Sea
Drilling Project(DSDP). Basalticbasementat theseDSDP sitesprovidesa recordof their upper
mantlesourcecompositionand showsregionalvariationsconsistentwith uppermantleflow in
this region.East of the SouthTasmanRise, all DSDP basaltshave 87Sr/86Sr
(0.7025-0.7029)
and206Pb/2O4Pb
(18.80-19.48) ratiostypical of Pacific-typeMORB indicatingthat Pacific-type
uppermantleexistedeastof theAustralian-Antarctic
continental
marginandbeneaththe Tasman
Seaduringthe early stagesof seafloorspreadingin thisregion. Basaltsfrom DSDP siteswestof
the AAD have high 87Sr/86Sr
(0.7030-0.7035), low 206Pb/204Pb
(17.99-18.10) and traceelement
characteristics
typicalof presentday Indian-typeSEIR MORB. Betweenthesetwo regions,
DSDP basaltsrecoveredalongthe westernmarginof the SouthTasmanRise haveisotopic
characteristics
that are, in onecaseconsistentwith an Indian-typeMORB source(Site 280A) and,
in the secondcase,transitionalbetweenPacific-typeandIndian-typemantlesources.The
occurrence
of seafloorbasaltswith transitionalor Indian-typeisotopiccharacteristics
well to the
eastof thepresentIndian-PacificMORB isotopicboundarywithinthe AAD stronglyimplies
that Pacific-typeuppermantlehasmigratedwestwardinto the regionsincethe SouthTasman
Riseseparated
from Antarcticacirca40 Ma.
propagation episodes, from both east and west, suggest
converging asthenosphericflow towardsthe AAD [Vogt and
The Australian-Antarctic Discordance (AAD) lies in the
Johnson, 1975; Vogt et al., 1984; Phipps-Morgan et al.,
center of the Southern Ocean between Australia and Antarctica
1988]. In addition,the AAD is centeredon an arc-shapeddepth
(Figure 1). The AAD hasbeenlong recognizedas an intriguing anomalythat stretchesacrossthe entire SouthernOcean basin,
section of the global spreadingsystem becauseof its deep cutting obliquely acrossthe easternboundingfracturezone of
axial bathymetry (4-5 km), rough (and in places chaotic) the AAD (Figure 2). This configuration suggeststhat mantle
topography, low gravity signal, high upper mantle seismic dynamicsresponsiblefor the depth anomaly have existed at
wave velocities, and intermittent asymmetric spreading least since continental rifting began at 96 Ma and possibly
[Weisseland Hayes, 1971, 1972, 1974;Andersonet al., 1980; since 300 Ma [Veevers, 1982; Mutter et al., 1985]. Vogt et al.
Forsythet al., 1987;Marks et al., 1990;Sempdrdet al., 1991; [1984] concluded that the present, transform-dominated
Palmer et al., 1993; West et al., 1994]. Multiple ridge morphologyof the AAD has developedonly since 25 Ma.
Introduction
1Nowat Department
of Geology
andGeophysics,
Woods
Hole
Oceanographic
Institution,WoodsHole, Massachusetts.
Copyright1995 by the AmericanGeophysicalUnion.
Paper number 95JB01424.
0148-0227/9
5/95JB-01424505.00
More recently, Marks et al. [1990, 1991] showedthat this
depthanomalyhasmigratedwestwardat -15 mrn/yrfor the last
20 m.y. while remainingcenteredon the northwardmigrating
SoutheastIndian Ridge (SEIR).
Klein et al. [1988] recognizedthat a uniquegeochemical
boundarybetweenIndian Oceanand Pacific Oceanmid-ocean
ridge basalt (referredto as Indian-type and Pacific-type
throughoutthe text), as definedby Sr, Nd, and Pb isotope
22,261
22,262
PYLE
ET AL.'
SOUTHEAST
INDIAN-SOUTHWEST
PACIFIC
SEAFLOOR
PYLE ET AL.:
SOUTHEAST
INDIAN-SOUTHWEST
126øE
128øE
!
PACIFIC
SEAFLOOR
22,263
130øE
!;:if:':'
depth
anomaly
trace
...........................................................
t..........
,::::{!
..........................................
•......................................................
45os
5b
!
'::' if
46øS
I
I
I
isotopic !
I
boundary
Indian.type
'zoae'12a-•.....•
I : ?
3........
}• 'l'"""'•7J•?•
]
I•.......
'
.......................................................
}-"•......
• •••
Indian-•pe
'
:.
!*
••:•:•x,•
.....
•?::
......
ii•::i!i::iiiiiiiiii•::"
•
.:?::::•?:•
......
hypothetical
trace
.:?..............
•.... of•grafing......
•48ø8
.:•:::?::
........ isotopic
bounda•
":•::•?:
!
' 'N zoneA
i
N. .........
:.......................................................
i 51øS
Indian Ocean
PacificOcean
upper
mantle
upper
m•.antle
:
i
i
i
I
i
i
I
-/oo
km I
Figure 2. Schematic
summaryof the SEIR tectonics
withinthe easternAAD andwesternzoneA. Solidcircles
areMW8801 dredgesitesthat haveIndian-typeMORB isotopic characteristics,
open symbolsare thosethat
have Pacific-typeMORB characteristics,and gray symbols are sampleswith transitional compositional
characteristicsrecoverednear the present position of the isotopic boundaryalong-axis. The single open
triangleis a R/V Vemacruise33 dredgelocationsampledandanalyzedby Kleinet al. [1988]. Off-axis samples
from zone A (MW07 and MW20) analyzedin this studyare designatedby open circles with a cross. Small
circlesindicateR/V Moana Wavecruise8801 dredgelocations without isotopic data. The easternboundaryof
theAAD is markedby a largefracturezoneat -127ø-128øE.Eastof the AAD, the SEIR is referredto as zone A
following WeisselandHayes[1971]. Rift propagationat 127.5øEiS shownby pseudo-fault
offsetsof the
magnetic anomalies (dash-dottedline). Migration of the isotopic boundaryand depth anomaly are
schematicallyrepresented
by shadedlines. Depth anomalytracewithin the AAD taken from Marks et al.
[1990].The isotopicboundaryalong-axisis presentlylocatednearthe 126øEfracturezonewithinthe AAD.
Of equal, or even greater, significance has been the
variations[Duprd and Allbgre, 1983; Hart, 1984; Hamelin and
Allbgre, 1985; Hamelin et al., 1986], presentlyexists beneath recognition that the isotopic boundaryseparatingIndian-type
the AAD. More detailedsamplingof AAD spreadingsegments and Pacific-type MORB mantle has not remained stationary
showed that the isotopic boundary presently lies near a [Pyle et al., 1992]. MORB from the present axis of the
transform fault located at-126 ø E and that the transition
zone
easternmost AAD spreading segment have Pacific-type, or
betweenupper mantle sourcesis <40 km along axis (Figures 2 transitional, isotopic characteristics, whereas off-axis
and 3) [Pyle et al., 1992]. Crossing the boundaryfrom east to
samples from 3-4 Ma seafloor have Indian-type signatures.
west, 206pb/204pband 208pb/204pbabruptly decreaseand This requiresthat Pacific-type upper mantle has migrated -100
87Sr/86Sr increase as SEIR MORB with Pacific-type km westwardin the last 4 m.y. at a rate of -25 mm/yr (Figure
characteristics progressively exhibit more Indian-type 2). Dredge samplingwithin and eastof the AAD is too limited
characteristics(Figure 3); more gradualdecreasesare observed to distinguish whether this migration is simply a local
for 207pb/2O4pband 143Nd/144Nd.The sharpnessof the perturbation of an isotopic boundarythat has always existed
isotopic transition is remarkable considering this boundary beneath the AAD or whether it represents a long-term,
separates two ocean-basin-scale upper mantle isotopic
westwardmigration of Pacific MORB mantle, perhaps since
domains.
continental rifting south of Tasmania opened a path for
22,264
PYLE ET AL.'
0.7038
SOUTHEAST
AAD (zoneB)iiiiliiiill
INDIAN-SOUTHWEST
zoneA
0.7036.....
ISOTOPE
0.7034-
BOUNDARY
.........
0.7032-
........
off-axis•
0.7030-
ß
ß ß•:'••study
r.--h
0.7028
...........
0.7026
•
,
,
,
•
.......
recovereddirectly east of the AAD (Table 1). DSDP Legs 28
...........
...........
:::::::::::
18.6
.........
18.2
":;Q this
study
- ß•
.........
..........
..........
..........
..........
..........
..........
..........
..........
18.0
17.8
v:::.
..........
..........
..........
..........
..........
.....
......
,
,
,
,
•
,
,
,
•
,:.:.........
,
,
,
,
,
,
,
,
•
,
,
,
,
,
,
,
and shallow
intrusive
basaltic
..........
38.3--
:11iiiiiii}
,•.,,off-axisC)
t8..
iiiiiiiill
-7,sstdy
38.0
37.9
ßß
3,.8
37.7 '-
.' I"'•"'I'"I
-400
extrusive
from Antarctica.
..........
-500
recovered
material from 10-70 Ma seafloor producedin a variety of
tectonic settingsbetween the KerguelenPlateau (-100 ø E) and
the Balleny basin (-170 ø E) (Figure 1). The locations of these
DSDP sites on the periphery of the Southern Ocean between
Australiaand Antarcticaallow the position of Indian-type and
Pacific-type upper mantle through time to be determined, as
well as the contribution of various hotspot mantle sources
beneath this region as seafloor spreading progressed.
Together, the geochemical and geochronological data
presentedbelow provide limits to the regional distribution and
migrationof Pacific-typeand Indian-type uppermantle during
the developmentof the SouthernOceanas Australiaseparated
18.8
18.4
SEAFLOOR
the AAD, are fundamental lingering questions. What seems
most apparentis the basic observation that both the isotopic
boundaryand depth anomaly have been moving westwardbut
possiblynot at the same rate.
To investigate the long-term history of the isotopic
boundary and the regional implications for Pacific mantle
migration into the SouthernOcean basin, we selectedsamples
from 10 Deep Sea Drilling Project (DSDP) sites surrounding
this basin and two additional off-axis SEIR samples(-- 2 Ma)
and 29
..........
PACIFIC
-300
-200
Iiiiiiiiii
..... l."l'"•."t'"
-100
0
100
200
300
400
500
KM
Figure 3.
Along-axis profiles of SEIR MORB isotopic
composition plotted as distance (in kilometers) from the
eastern bounding fracturezone of the AAD (heavy solid line).
Open symbols represent Pacific-type compositions, solid
symbols represent Indian-type compositions, and gray
symbols represent transitional compositions recovered near
the presentisotopic boundary within the AAD. These isotopic
data are taken from Klein et al. [1988], Pyle et al. [ 1992], and
this study (open circles with cross). Vertical solid lines
designateaxial discontinuities;rift propagation in zone A and
transform faults in the AAD. Note the distinct isotopic
differences between the axis and off-axis samples within the
easternmostAAD spreadingsegment.A similar contrast is not
observedbetween axis and off-axis samples from zone A. The
Analytical Methods
Glasses from five DSDP sites (265, 266, 267, 278, 282),
whole rock samplesfrom all ten DSDP sites,and dredgedbasalt
glass from off-axis locations east of the AAD (MW07 and
MW20) were analyzedfor major andtraceelementsby electron
microprobe,X ray fluorescence(XRF), and inductivelycoupled
plasma mass spectrometry (ICP-MS) (Tables 2 and 3).
Petrographicdescriptionsof the core samplescan be obtained
from the DSDP initial reports volumes for Legs 28 and 29
[Hayes et al., 1975; Kennett et al., 1974]. All whole rock
samples were trimmed to remove exterior surfaces, and sawn
surfaceswere ground to remove saw blade contamination, thus
obtaining the freshest possible material. This material was
crushedin a jaw crusher with ceramic plates, washed in
deionized water, dried, and hand-picked to remove alteration
veins and vug material. Whole rock powderswere produced
with a tungsten carbide shatter box at Washington State
University. Glass samples were hand crushedin a ceramic
mortar, sieved to -0.5
mm size fraction, washed and hand-
picked to remove alterationand phenocrystphases.
The major elementconcentrations
of all glass sampleswere
determinedusing a CamecaSX-50, four-spectrometerelectron
microprobeat OregonStateUniversity (OSU). Eachreported
analysis is the averageof five spot analysesthat have been
normalizedto the glass standardBASL (Smithsonian standard
present location of the isotopic boundaryand width of the
VG-A99) run after every five samples. Whole rock samples
transition zone along-axis is shown by a wide gray line which
were analyzedfor major and trace elements (Ni, Cr, Sc, V, Ba,
terminatesagainsta transformfault at 126øE.
Rb, Sr, Zr, Y, Nb, Ga, Cu, Zr) by XRF at Washington State
University following standard methods [Knaack et al., 1991 ].
Additionaltraceelementanalyseswerecompletedby ICP-MS
shallowmantle outflow from a shrinkingPacificbasin [e.g., using a Fisons PQ2+ PlasmaQuadat OSU. XRF and ICP-MS
Alvarez, 1982; 1990]. If the isotopic boundaryhas only analyses of whole rock samples are of splits from the same
recentlyarrivedbeneaththe AAD, thenthe presentassociation samplepowder. As these data are the first reportedICP-MS
of the isotopic boundary with the AAD is coincidental. The analysesof geological samples from the OSU facility, we
origin and long-term stability of the isotopic boundary,as compare the ICP-MS results to other analytical methods
well as its relationshipto the depthanomaly,the presentAAD (Tables 2 and 3). Splits of four glass samplespreviously
transformboundaries,and the mantledynamicsresponsible
for analyzed by direct current plasma (DCP) and instrumental
PYLE ET AL.: SOUTHEAST INDIAN-SOUTHWEST
PACIFIC SEAFLOOR
22,265
Table 1. DSDP Sitesand Dredge LocationsSampledfor This Study
DSDP
Site
Leg 28
Leg 29
MW8801
Latitude.
S
Longitude.
E
Magnetic
Anomaly
-Age,*
Ma
Core,
m
Recovery,
%
264
34ø 58.13'
112ø 2.68'
-
98
180
55
265
266
267
274
53 ø 32.45'
56 ø 24.13'
59 ø 15.74'
68 ø 59.81'
109 ø 32.45'
110 ø 6.70'
104 ø 29.30'
173 ø 25.64'
5b
6
15
13
15
23
38
36
17
14
14
6
20
14
16
34
56ø 33.42'
160ø 4.29'
12
33
11
50
279A
280A
282
283
278
51 ø 20.14'
48 ø 57.44'
42 ø 14.76'
43 ø 54.60'
162 ø 38.10'
147 ø 14.08'
143 ø 29.18'
154 ø 16.96'
31
23
47
55
69
5
5
15
4
70
100
50
43
20-01
07-01
49ø 31.20'
49ø 13.20'
127ø 35.40'
127ø 39.60'
1
2
1
2
dredge
dredge
* Magnetic anomalyage basedon the Kent and Gradstein[1986] timescale. Where no magneticanomaliesare apparent,age
estimatesare thoseof the DSDP Leg Scientific Party.
neutron activation analysis (INAA) were analyzed by ICP-MS
along with the DSDP samples. The ICP-MS data are
consistently within analytical uncertainty of the INAA and
DCP methods.
The XRF
and ICP-MS
trace element
results
generally agree, although ICP results are systematically high
for Ba and low for Cu, Zn, Sr, and Y. Consistency between
analytical methodsis particularly poor for elements at very
low concentration.
In such cases, the ICP-MS
method is
assumedto providebetterresultsbasedon analytical precision
and greater control on matching standardabundancesto the
sample concentration range. Isotope dilution abundancesof
glasssamplesfor Sr, Rb, Nd, and Sm are alwayslower than the
correspondingICP-MS result; this may be causedby sample
inhomogeneity in small sample sizes (50-100 mg);
incomplete samplerecovery after Pb column chemistry prior
to Nd, Sm, Rb, and Sr spiking;and/or subtlematrix differences
betweenstandardsand samplesin the ICP-MS analyses. These
discrepancieshave no effect on our conclusions and efforts to
determinetheir causeare in progress.Furthermore,Pb analyses
by ICP-MS were rejectedbecausesuitable standardswere not
available and subtle variations
in Pb concentration
are below
our perceived analytical uncertainties.
For ICP-MS analysis,samplesolutions were preparedfrom
-60-80 mg splits of glassand whole rock powderdissolvedin
tightly capped,15-mL Savillex teflon beakerswith -800 gL of
a (1:3) HF:HNO3 acid mixture heated at -80øC overnight.
Upon dissolution, beakers were uncappedand samples were
driedon a hot plate to drive off HF. Following dry-down,the
powderswere takenup oncein 6 N HC1 and redried. This cycle
was repeated twice using 4 N HNO3 to break down
fluorosilicate precipitates. The final dried powder was
dissolvedin 10 mL of 2 N HNO3, from which a further 1:5
dilution in 1% HNO3 was preparedfor introduction into the
ICP-MS
instrument.
Instrument
drift
was monitored
and
correctedwith a multiple internal standardsolution of Be, In,
andBi addedto eachsampleto attaina run concentrationof 20
ppb for each element. Unknown element concentrations were
determined using regression curves based on dissolved rock
standardsfrom the U.S. Geological Survey (BIR-1, BHVO-1,
BCR-1, W-l; recommendedvalues from Govindaraju[1989])
processedalong with the samples. The sample-standard
concentrationrange was matchedby using multiple standards
and dissolving 50, 75, and 100 mg of some individual
standards.This method provides tightly constrainedstandard
regression curves, allows evaluation of matrix effects and an
independent assessment of "recommended" standard values
(e.g., BIR-1) [see Jochum et al., 1994].
Sr, Nd, andPb isotope ratio and parent-daughter
isotopedilution analyseswere performedon fresh glassesfrom DSDP
sites265, 266, 267, 278, and 282 and two off-axis dredges
recovereddirectly east of the AAD (Table 4). Core samples
withoutglasswere subjectedto a sequentialleaching procedure
to removealteration[Mahoney, 1987]. This leaching method
differs from conventional single-step warm or hot acid
techniquesby being considerablymore intense. A coarsely
powdered sample (200-800 mg) is leached in 4N to 6N
ultrapureHC1 and agitatedultrasonicallyfor 20 min. The acid
(cloudedfrom the dissolution and suspensionof alteration
material)is removedfrom the remainingsolid with a pipette,
new acidis added,andthe powderis again agitatedand stirred.
This proceduredissolvesor separatesfine particulatematerial
(i.e., micaceousalteration minerals)andis repeateduntil the
acid remains clear. At this point, the final solid is removed,
rinsedwith ultrapurewateranddried,leaving typically 10% to
40% of the original volume, dependingon the extent of
alteration. For tholeiiticbasaltsamples,the leachedpowderis
usuallycomposedlargely of well-crystallizedplagioclaseand
clinopyroxenewith very little alteredmaterial [Mahoney,
1987].
In orderto moreaccuratelyestablishbasement
agesfor sites
with poorly constrainedmagnetic anomaly age estimates,
several whole rock samples were selected for 40Ar_39Ar
incremental
heatingagedeterminations
(Table5 andFigure4).
Rock chips 0.5-1 mm in size were irradiatedfor 6-8 hours in
the core of the OSU TRIGA reactor and conversion
of 39K to
39Ar by neutron capturewas monitoredwith hornblende
standardMmhb-1 (520.4_+1.7 Ma) [Samson and Alexander,
1987].
Five to eight incremental heating steps were
conducted,dependingon the sample potassiumcontent and
expectedradiogenic40Ar. The isotopic compositionof Ar
releasedat eachstep wasmeasured
using an AEI-MS10S mass
spectrometerat OSU.
The low K content of MORB, combinedwith alteration,
contributes to near-atmospheric 40Ar/36Ar ratios and,
consequently,
largeuncertainties
in somecalculatedages. The
reliability of a crystallization age is determinedfrom the
relative concordance
betweena plateauand isochron age
calculatedfor eachsample.Plateauage estimatesare basedon
22,266
PYLE ET AL.: SOUTHEAST
INDIAN-SOUTHWEST
PACIFIC SEAFLOOR
Table 2. Major andTraceElementData for DSDP Legs28 and29 BasaltandSEIR DredgeBasaltSamples
DSDPLegs28 and 29
site
core
interval
split
analysis*
264
15
Cl2
wr
XRF
ICPMS
265
17-01 18-01
61-63 41-49
glass
wr
Probe
XRF
ICPMS
ICPMS
266
23-01 23-01
pc 5 76-82
glass
wr
Probe XRF
ICPMS
ICPMS
267
07-01 07-01
78-82 61-64
glass
wr
Probe XRF
ICPMS
ICPMS
274
44-02
45-02
88-93 110-114
wr
wr
XRF
XRF
ICPMS
ICPMS
278
35-02
35-03
64-67 112-117
glass
wr
Probe
XRF
ICPMS
ICPMS
279A
280A
13-02
48-54
23-02
106-110
wr
wr
ICPMS
ICPMS
wt%
SiO2
TiO2
A1203
FeOt
55.08
1.53
17.32
9.62
49.51
1.63
16.67
7.87
50.07
1.58
17.14
7.50
50.32
2.18
13.92
10.92
51.05
2.28
14.77
11.62
50.95
1.31
14.62
8.98
51.88
1.32
15.80
8.73
51.11
1.71
17.82
9.17
52.00
2.01
17.05
9.64
49.89
1.01
15.44
8.23
50.43
0.91
18.19
7.24
MgO
5.22
8.36
9.40
7.18
6.37
7.86
7.87
5.17
6.05
8.73
7.96
CaO
6.88
Na20
K20
P205
3.00
1.17
0.19
MnO
Total
ppm
Sc
0.11
100.11
0.12
10.51
3.27
0.60
0.30
0.14
11.12
3.45
0.64
0.25
0.19
10.35
2.96
0.25
0.26
0.20
10.40
3.11
0.35
0.27
0.17
0.15
12.39
12.31
2.63
0.13
0.13
0.15
11.00
2.95
0.31
0.12
3.35
0.12
0.15
0.11
7.87
3.40
1.27
0.47
0.15
13.39
0.13
13.29
2.31
0.10
0.10
2.46
0.18
0.08
98.86
101.29
98.52
100.42
99.16
101.45
99.75
99.88
99.35
100.87
50.95
1.42
15.66
9.49
48.68
0.90
19.04
8.50
0.14
8.36
11.50
3.01
0.22
0.16
0.10
10.47
7.65
3.13
0.07
0.06
100.91
98.59
V
Cr
Ni
Cu
Zn
Rb
27.9
215
116
19
16
91
20.2
33.3
232
325
187
50
81
9.2
29.2
186
303
268
54
80
7.2
42.7
372
285
118
54
109
4.2
38.8
336
201
90
56
111
4.5
42.9
292
296
79
70
75
1.5
41.4
260
259
78
77
75
4.8
46.8
361
222
96
97
95
1.2
38.4
267
207
62
73
102
23.3
40.9
252
430
140
100
70
1.8
33.3
197
317
115
76
55
3.1
42.0
316
178
69
59
73
3.9
36.8
213
293
159
122
66
1.2
Sr
205
297
257
150
129
130
126
128
235
125
125
160
90
Y
Zr
Nb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
25
120
7.2
0.10
307
19.47
43.52
5.13
20.78
4.81
1.53
5.09
0.81
29
139
14.4
0.11
104
9.91
24.14
3.24
14.69
3.96
1.41
4.37
0.75
25
112
13.2
0.44
97
8.60
20.49
2.77
12.80
3.41
1.23
3.92
0.67
50
165
7.8
0.06
45
7.50
22.13
3.44
17.58
5.50
1.73
6.48
1.20
49
149
8.0
0.09
47
7.34
20.80
3.27
17.34
5.42
1.64
6.04
1.14
27
77
3.4
0.02
16
3.34
10.41
1.66
9.26
3.15
1.15
3.76
0.73
29
81
3.8
0.19
12
3.16
9.74
1.57
8.71
2.95
1.06
3.50
0.65
31
103
6.9
0.05
19
4.54
13.32
2.06
10.84
3.56
1.25
4.25
0.78
43
148
27.9
0.67
99
21.40
43.15
5.27
22.48
5.42
1.76
6.00
1.03
21
62
3.7
0.02
15
3.05
8.70
1.39
6.99
2.26
0.85
2.71
0.51
20
59
3.6
0.27
8
2.52
7.56
1.16
6.30
2.07
0.80
2.59
0.49
23
80
12.5
0.06
65
7.17
17.54
2.37
11.75
3.22
1.20
3.74
0.67
18
45
1.5
0.22
20
1.23
4.19
0.69
4.22
1.58
0.69
2.17
0.43
Dy
4.79
4.63
4.04
7.78
7.49
4.56
4.29
5.21
6.46
Ho
Er
Tm
Yb
Lu
Hf
Ta
Th
U
Pb
0.91
2.51
0.36
2.10
0.30
3.25
0.90
2.74
0.42
2.58
0.39
1.91
1.12
3.25
0.50
3.10
0.45
2.58
1.38
4.05
0.62
3.68
0.55
3.80
0.18
0.07
0.39
0.23
1.67
0.53
3.72
0.42
0.93
2.75
0.41
2.55
0.38
2.86
0.91
1.03
0.31
0.706
0.83
2.49
0.37
2.22
0.34
2.41
1.34
0.35
1.68
4.94
0.78
4.79
0.71
4.16
0.48
0.64
0.17
0.695
1.60
4.71
0.74
4.53
0.67
3.88
0.46
0.17
0.96
2.98
0.45
2.76
0.41
2.03
0.19
0.16
0.08
0.404
3.31
0.69
2.12
0.33
2.02
0.31
1.48
0.18
0.16
0.08
0.253
3.06
4.18
2.98
0.64
2.02
0.30
1.81
0.28
1.35
0.88
2.64
0.41
2.43
0.38
2.23
0.65
2.03
0.33
2.16
0.33
1.25
0.18
0.07
0.71
0.23
0.15
0.04
SampledesignationincludesDSDP site (264), coresection(15), andinterval(cm in core;CC, corecatcher;pc,basaltpiece).
?Major
element
determined
onwhole
rock(wr)splits
byXRFandglass
splits
byelectron
microprobe
analyses.
consecutiveincremental heating steps that overlap within
analytical uncertainties (i.e., form a "plateau") and
Results
cumulatively include at least 50% of the total 39Ar released
Age of Volcanism
from the sample. The sample age is further evaluatedon
Site 264 is located on the southern Naturaliste
Plateau
isotope correlation diagrams (i.e., "isochrons") which have
the addedadvantageof independentage and initial 40Ar/36Ar [Ford, 1975], a shallow bathymetric feature believed to be
estimates. A near-atmospheric
initial 40Ar/36Arinterceptfor
the isochron (i.e., 295.5) and an acceptablemeasureof
goodnessof fit for the isochron to the concordantstep
compositions
are evidencefor reliableisochronage estimates.
A more thorough discussionof 40Ar-•9Ar methodology
[Duncan, 1991] andits generalapplicationto alteredvolcanic
rocks can be found elsewhere [Dalrymple et al., 1981;
McDougalland Harrison,1988].
either a small remnant
of continental
crust or a flood
basalt
plateau related to the Kerguelen hotspot [Mahoney et al.,
1995; Colwell et al., 1994]. Samplesfrom this site are part of
a basaltic andesireto rhyolite volcaniclastic sequence,lying
beneath
Cenomanian-Santonian
chalks.
A basaltic
andesire
clast from this sequenceproducedan apparent age spectrum
with a plateaubetween 99 and 102 Ma (mean age 100.6 + 1.2
Ma) and a concordantisochron age of 99.6 + 1.2 Ma (Figure
PYLE ET AL.: SOUTHEAST INDIAN-SOUTHWEST
PACIFIC SEAFLOOR
22,267
Table 2. (continued)
Southeast
IndianRidgeDredgeSamples
283
282
20-02
80-85
glass
Probe
ICPMS
20-01
107-113
wr
XRF
ICPMS
18-01
135-138
wr
XRF
ICPMS
zone A
AAD
07-01
20-01
17-26 17-26
off-axis off-axis
axis axis
glass glass glass glass
Probe
XRF Probe DCP
ICPMS
ICPMS
ICPMS
INAA
23-01
axis
glass
Probe
ICPMS
23-01
axis
DCP
INAA
26-01
axis
glass
Probe
ICPMS
area
26-01
axis
glass
DCP
INAA
27-71 27-71
axis
axis
glass glass
Probe DCP
ICPMS
sample
setting
split
analysis
ICPMS
wt%
48.65
1.48
16.27
8.83
0.14
8.13
11.95
3.11
0.O9
0.13
98.78
50.18
1.50
17.83
9.18
49.20
1.85
17.75
10.20
50.14
1.89
14.17
10.18
0.16
0.21
5.86
8.18
11.75
3.51
0.19
0.12
100.29
6.67
3.56
0.76
0.19
50.87
2.03
14.53
10.41
50.62
1.36
15.49
8.44
0.17
0.17
0.15
7.32
7.25
8.37
11.64
2.52
0.09
0.14
10.61
2.79
0.11
0.22
51.34
1.12
16.26
9.18
11.54
98.26
98.99
35.4
207
266
152
53
70
4.2
162
30
101
3.1
0.24
12
3.13
10.88
1.83
10.00
3.24
1.21
3.82
0.70
46.2
319
261
59
60
92
12.4
177
24
117
7.4
0.25
34
5.23
14.76
2.13
10.79
3.20
1.21
3.62
0.67
38.7
344
85
64
88
22.0
98
41
133
37.4
342
218
103
57
91
15.0
110
42
145
6
3.85
12.80
7
4.82
12.50
15.90
4.35
1.45
12.90
5.08
1.57
1.10
1.15
4.66
4.23
4.86
0.98
2.90
0.45
2.73
0.40
2.29
0.88
2.63
0.40
2.58
0.38
2.87
0.15
0.05
0.41
0.18
1.01
3.06
0.49
2.88
0.43
2.22
0.08
0.07
0.02
52.39
1.20
16.24
7.56
0.13
0.17
0.15
MnO
7.71
7.88
8.10
MgO
10.86
2.93
0.03
0.15
98.57
51.1
1.4
15.55
8.52
10.95
2.87
0.10
0.15
99.08
11.10
2.94
0.07
0.18
99.72
SiO2
TiO2
AI20 3
FeOt
CaO
2.95
0.10
0.17
98.76
Na20
K20
P205
99.96
Total
ppm
4.70
0.63
3.40
0.23
0.08
4.76
0.76
3.59
0.26
0.07
38.4
292
427
105
76
69
0.6
118
27
79
1.2
0.01
3
1.91
8.05
1.41
8.91
3.09
1.19
3.87
0.77
35.8
252
360
103
74
70
105
30
90
3
1.90
9.20
7.80
2.94
1.14
0.75
37.4
228
380
157
88
74
1.4
103
23
61
2.4
0.02
17
2.06
6.89
1.10
6.51
2.39
0.95
3.05
0.60
34.0
193
332
140
83
75
109
25
73
14
2.40
7.23
5.92
2.39
0.93
0.61
4.03
3.11
0.42
2.14
0.12
0.07
0.11
0.431
0.90
2.58
0.41
2.53
0.38
1.79
0.15
0.14
0.04
0.275
36.0
283
348
119
56
78
0.9
112
32
96
2.1
0.02
9
3.06
11.06
1.84
10.57
3.73
1.34
4.38
0.87
33.0
250
300
117
55
70
112
33
100
7
3.31
11.19
9.88
3.74
1.29
0.95
5.66
0.53
3.35
0.52
2.77
0.15
0.14
0.05
32.0
230
325
129
57.9
61
143
26.4
89
10.3
3.53
10.8
8.86
3
1.09
0.71
4.45
1.20
2.57
0.40
1.82
0.14
0.15
0.11
34.4
260
371
129
58
64
1.0
142
26
84
2.4
0.02
12
3.13
10.46
1.69
9.41
3.05
1.15
3.69
0.69
3.28
0.51
2.77
0.23
0.10
0.35
0.96
2.87
0.45
2.66
0.40
2.16
0.16
0.14
0.05
0.414
Sc
V
Cr
Ni
Cu
Zn
Rb
Sr
Y
Zr
Nb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
2.77
0.41
2.07
0.1
0.1
0.29
Ho
Er
Tm
Yb
Lu
Hf
Ta
Th
U
Pb*
Trace elementanalysesby ICP-MS or combinedDCP/INAA methods:Sc, V, Cr, Ni, Cu, Zn, Sr, Y, Zr, Ba by DCP and L-a,Ce, Nd, Sm, Eu,
Tb, Yb, Lu, Hf, Ta, Th, U by INAA (Pyle, 1994). All Pb analysesby isotopedilutionthermalionizationmassspectrometry.
4), somewhatyoungerthan southernKerguelen Plateaubasalts
(109-118 Ma) [Leclaire et al., 1987; Whitechurchet al., 1992]
and somewhat older than Broken Ridge basalts (88-89 Ma)
[Duncan, 1991].
Sites265, 266, and 267 are locatedon progressivelyolder
seafloorof the AntarcticPlate alonga transectroughly normal
to the SEIR between 105ø and 110ø E (Figure 1). Seafloor
magnetic anomalies indicates ocean crustal ages of 14 Ma
(A5), 25 Ma (A7), and34 Ma (A13), respectively[Hayesand
Frakes, 1975; Vogt et al., 1984]. Layer 2 oceanic crust was
penetrated at Sites 267 and 265, but at Site 266, intermixed
basalt and sediment were recovered, suggesting magmas
eruptedinto soft sediment, possibly slightly away from the
spreadingaxis [Ford, 1975]. Magnetic anomaly patterns
indicate seafloor production occurredat Site 267 during the
earlieststagesof rapid seafloor spreadingin this region. The
plateau and isochron ages are concordant(23.4 + 0.8 Ma and
23.3 + 3.1 Ma, respectively; Figure 4), but younger than the
age estimate from magnetic anomaly identification. The age
of the lowermost sediment at this site is mid-Oligocene [Hayes
and Frakes, 1975], or slightly older than the basalt age
determined in this study. The discrepancy between the
magnetostratigraphic and biostratigraphic age estimates was
noted by Hayes and Frakes [1975], who suggestedthat sills
formed significantly off-axis at this location. Our radiometric
dataare consistent with sill emplacementabout 10 m.y. after
crustal formation. In the absence of suitable samples for
further 40Ar-39Ar work on these cores, sites 267, 266, and 265
22,268
PYLE ET AL.: SOUTHEAST INDIAN-SOUTHWEST
PACIFIC SEAFLOOR
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PYLE ET AL.: SOUTHEAST
INDIAN-SOUTHWEST
PACIFIC SEAFLOOR
22,269
Table 5. 4øAr-39Ar
PlateauandIsochron
AgeEstimates
for BasalticSamples
FromDSDPLegs28 and29,
Southeast Indian Ocean and Southwest Pacific Ocean
Sample
Type
Plateau
Age,
Steps
Ma
264-15-cc
267-07-01
274-44-02
278-35-03
clast
basement
breccia
basement
279A-13-02
280A-23-02
282-20-01
basement
intrusive
basement
100.6
23.4
68.3
25.8
39Ar*
Isochron
Age
N
Ma
+ 1.2
+ 0.8
+ 2.0
_+2.9
3
5
4
5
55
100
49
60
14.7 + 1.6
64.2 + 3.3
60.7 + 1.5
5
5
5
84
86
89
97.0
23.3
67.0
17.0
+
_
_
+
4øAr/36Ar
Sums
Intercept
(No2)
0.5
3.1
0.7
9.7
325.8 + 54.8
296.9 _ 7.9
299.8 _ 6.7
299.8 _+11.8
0.37
2.13
0.01
4.58
13.9 + 1.0
69.5 + 2.2
59.2 + 2.2
296.9 _ 0.8
294.6 _ 2.2
293.2 + 16.8
0.38
0.12
0.11
* Percent of total.
are assumedto provide a record of the upper mantle in this
region at progressivelyyoungerages,from -23 Ma to 14 Ma.
East of the AAD, seafloor ages at DSDP sites range from
-13 Ma to 70 Ma. The oldest seafloor in the region is
associatedwith seafloor spreadingin the Tasman Basin (Site
283; A31,-69 Ma), but equally old volcanism is indicatedby
the 40Ar-39Arsystematics
of basaltsfrom Sites 280A and 282
along the western margin of Tasmania and the South Tasman
Rise (Figure 4). Massive, pillowed, and brecciated basalts
erroneouslylow, due to a suspectedleak in the extraction line)
and an isochronage of 67.0 + 0.7 Ma (Figure 4a). These ages
are considerablyolder than the magneticanomaly age estimate
for seafloorbeneaththis site (A13, --36 Ma),but younger than
the oldestrecognizedanomaliesin this region (A34, --84 Ma)
[Mayes et al., 1990]. The measuredage at Site 274 is very
similar to that at sites 282 and 280A, located at roughly the
conjugate position of ocean basin opening, confirming that
rifting, possibly associated with seafloor spreading, began at
were recovered
about 65 Ma.
Site 280A
at Site
encountered
282
below
late Eocene
three massive
sediments
basalt units below
and
mid-
Eocene sediments [Ovenshine et al., 1974].
Plateau and
isochronages for Site 280A basalts(64.2 + 3.3 and 69.5 + 2.2
Ma) overlap, or slightly predate,thoseof Site 282 (60.7 + 1.5
Major
Element
Variations
With the exception of the basaltic andesite from the
NaturalistePlateau(Site 264), the DSDP Leg 28 and 29 basalts
and59.2 + 2.2 Ma). The 40Ar-39Aragesof volcanismat sites have compositionstypical of basaltserupted in an ocean basin
280A and 282 are older than the paleontological estimate of
setting. Major element variations of these DSDP basalts are
middle to late Eocene age for sediments recovered above the
complicatedby alteration in whole rock samples, and the wide
basalts [Kennett et al., 1974]. The near-atmosphericinitial
geographical distribution of sample sites precludes a direct
40Ar-39Arvalues(294.6 + 2.2 and 293.2 + 16.8) obtainedfrom
petrogenetic relationship
between these
samples.
the isochrons indicate that trapped (excess) argon is not a
Nevertheless, the major elements provide a context in which
significantfactor in the measuredage of these basalts. Given
to interpret the trace element data, so we offer some general
that no paleontological control is reportedfor the sediments observationsfor perspective(Figure 5).
immediatelysurrounding
the basalts,we believe the 40Ar-39Ar
West of the AAD, unaltered glasses from Sites 267, 266,
resultsrepresentthe best estimate for the age of volcanism at
and 265 decreasein CaO/A1203 from 0.85 to 0.60 with
these near-continental margin sites.
decreasing age; a large variation for a limited section of
At Site 279A on the Macquarie Ridge, concordant plateau spreadingridge which, together with variations in TiO2 and
(14.7 + 1.6 Ma) and isochron(13.9 + 1.0 Ma) ages of oceanic FeOt (total Fe as FeO) at high MgO contents, must reflect
crust (Figure 4) are slightly older than seafloor exposedfarther significant changes in degree of melting and/or source
south on MacquarieIsland (11.5-9.7 Ma) [Duncanand Varne, composition beneath the SEIR. East of the AAD, a simple
1988], consistent with the formation of seafloor at both sites
mid-oceanspreadingorigin for all the DSDP samples is less
along a common spreadingcenter to the south. Recognized clear, and consequently,major element variations may partly
anomalies to the east in the Emerald Basin are A12 and A13
reflect differences in tectonic settings. Basalts from the
(-34 Ma), where Oligocene sediment directly overlies
continentalmargin of Tasmania(282), the SouthTasmanRise
basement [Ovenshine et al., 1974]. The poorly constrained (280A), and the Tasman Sea (283) are relatively primitive
25.8 + 2.9 Ma plateau age and isochron age (17.0 + 9.7 Ma)
(10.5 to 8.0% MgO)and display a large range in CaO/A1203
for Site 278 basalt are consistent with the overlying mid(0.38 to 0.88). The low CaO/A1203 values most likely reflect
Oligocene sediments.
pervasive alteration and variable destructionof plagioclase
The volcanic section at Balleny basin Site 274 is a
and pyroxene. Less altered samplesare generally within the
brecciateddeposit (mostly likely a slump feature) located on
compositional range of present SEIR basalts dredgedeast of
southwest Pacific seafloor older than A13 (36-39 Ma) [Ford,
the AAD [Klein et al., 1991; Pyle, 1994].
1975]. Drilling here sampledtwo geochemicallydistinct rock
compositions. Low-temperature,hydrothermal alteration of
Trace Element and Isotopic Variations
the lower part of this core has producedsomewhat alkalic
For the western DSDP Sites 267, 266 and 265, the rare earth
whole rock compositions (i.e., celadonite-rich tuff), inviting
comparisonto nearby Balleny Island basalts. A fresher upper element patterns of the basalts change from light rare earth
zone yielded MORB-like compositions. A holocrystalline
element(LREE) depletedto LREE-enrichedwith decreasingage
basalt clast recovered from this upper zone produced an (Figure 6). There is also a general correlation between highly
apparentplateau at 68.3 + 2.0 Ma (ignoring step two, which is
incompatible trace element concentration and isotopic
22,270
PYLE ET AL.'
SOUTHEAST
•
PACIFIC
m
-l•O•/-Pvr9•
J¾O•,/J•9g
(elM)o•V
i
INDIAN-SOUTHWEST
(elM)o•V
(el/q)
!
i
'PS/O•/'PVr9
•
.pS/O•/.PVr9œ
i
i
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'PS/O•/'PVr9
•
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SEAFLOOR
PYLE ET AL.:
SOUTHEAST
INDIAN-SOUTHWEST
0.90
PACIFIC
SEAFLOOR
22,271
87Sr/86Srvaluesapproachingthose of Balleny Island lavas
[Hart, 1988; Lanyon et al., 1993], whereasthe holocrystalline
basalt clast (-67.0 Ma)is comparableto presentday Pacifictype SEIR MORB (Figures 7 and 8). The MacquarieRidge
basalt(DSDP 279A) is very similar to basaltsfrom Macquarie
Island[Griffin and Varne, 1980; Griffin, 1982; Lanyon et al.,
1993], indicating that the Balleny hotspot may have
influencedthis part of the Pacific-Antarcticspreadingridge
0.80
0.70
0.60
0.50
274
•
since at least -20
Ma.
This Balleny hotspotinfluencein not, however,apparentat
Site 278 in the Emeraldbasin (-23 m.y.). The EmeraldBasin
and Balleny Basin (DSDP 274-44) basalts have subparallel,
flat to slightly depletedLREE patterns, convex-upcurvature
betweenLa and Sm (Figure 6), suggestingthat they were
derivedfrom a depletedMORB mantlesourcesimilarto present
0.40
day MORB eastof the AAD. These basalts also fall within or
near the Nd-Sr, Nd-Pb, and Sr-Pb isotopic fields (Figure 7)
definedby Pacific type MORB lavas from the SEIR (i.e., zone
.50
A), but with somewhat
•higher
2OSpb/2O4pb
and2O7pb/2O4pb
values (Figure 8).
For seafloor basalts surroundingthe South Tasman Rise
(Sites 280A, 282, 283), trace element data indicate a mantle
sourcecomparableto, or more incompatible-element-depleted
than, the MORB source beneath the Balleny and Emerald
Basins (Figure 6). The weakly depleted rare earth element
pattern of the Site 282 basalt resemblesthose of SEIR basalts
from east of the AAD (Lan/Smn <1, Smn/Ybn>l, and Cen/Ybn
-1, wheren denoteschondritenormalized)[cf. Schilling and
Ridley, 1974]. By contrast, Site 280A basalt is strongly
11.0
10.0
9.0
LREE-depleted
(Lan/Smn=0.49,Cen/Yb'n=0.50)relative to the
8.0
other DSDP samples but not as depleted in elements more
incompatible than La as are some Pacific-type SEIR MORB
26•
7.0
280A
east of the AAD.
6.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
Site 280A
basalt has low concentrations
of
elementsmore compatible than La and a distinctive, positive
Sr anomaly. The Site 283 basalt is typical of Pacific-type
compositionsin 87Sr/86Srand 2O6pb/2O4pb
but has slightly
higher2OSpb/2O4pb
and2O7Pb/2O4pb.
Site 282 basaltfrom the
MgO
Figure 5. Selected major element variations of DSDP Leg 28
and Leg 29 samples. Glass samples are designatedby open
symbols and whole rock samplesby solid symbols (triangles
MW8801 basalt glasses;squares,Pacific-type DSDP samples;
circles, Indian-type or transitional DSDP samples). Sample
pairs from individual DSDP cores are connected by lines.
western margin of Tasmania lie outside the SEIR Pacific-type
isotopicfieldsdue to relatively low 2O6pb/2O4pb.
In addition,
the high 87Sr/86Sr,2OSpb/2O4pb,
and 2O7pb/2O4Pb
of Site 280A
basalt, when compared to Pacific-type samples of similar
2O6Pb/2O4pb
contents,indicatesan Indian-typemantlesource.
Taken together, the South Tasman Rise basalts are
Compositional
fieldsfor AAD andzone A SEIR MORB glasses isotopically intermediate to SEIR Indian-type MORB from
are indicated (upper field, zone A: lower field, AAD). Glass westof the AAD and SEIR Pacific-typeMORB from east of the
samplesshow trendsconsistent with SEIR MORB glass data AAD.
from the AAD and zone A.
The Naturaliste Plateau Site (DSDP 264) was included in this
studybecauseof its possible relationship to early rifting of
composition(i.e., 87Sr/86Srand 2OSpb/2O4pb
increasewith
LREE enrichment;Figu?es7 and 8) indicating changesin
mantle source composition;not simply variations in melting
of a homogeneousmantle sourcebeneath this section of the
SEIR.
For DSDP basalts east of the AAD,
trace element and
isotopic variations require a variety of mantle sources. The
Balleny Basin tuff (DSDP 274-45) and MacquarieRidge basalt
(DSDP 279A) have LREE-enrichedpatternsand broad, convexup primitive-mantle-normalized patterns (Figure 6). Such
patternsare thought to indicate a HIMU hotspot component in
their source[Weaver, 1991]; most likely relatedto the Balleny
hotspot [Lanyon et al., 1993; Crawford et al., 1994]. The
Balleny Basin tuff (sample 274-45) has 2O6pb/2O4Pb
and
the easternAustralian-Antarctic
continentalmargin. The 4OAr39Ar age determinationssuggest that volcanism on the
NaturalistePlateauis related in time and spaceto Kerguelen
hotspot volcanism, but compositional contrasts between
NaturalistePlateaulavas and recentKerguelenhotspotproducts
indicate very different mantle sources and/or petrogenesis
(Figures 6-8). The strong depletion of Nb and Ta in the Site
264 sample indicates a subcontinental-lithosphere source
[Thompsonet al., 1983], as do high 87Sr/86Sr,2OSpb/2O4pb,
and2O7pb/2O4pb
andlow 143Nd/144Nd
isotopicsignatures
that
are well outside the range of oceanic basalts [Hawkesworthet
al., 1990; Peng et al., 1994]. At Site 264, the Kerguelen
hotspot signature is either overwhelmedby contamination or
the plume's thermal effects were greater than any material
contribution [e.g., Storey et al., 1992; Mahoney et al., 1995].
22,272
PYLE ET AL.'
SOUTHEAST
INDIAN-SOUTHWEST
PACIFIC
SEAFLOOR
100
lOO
:.
264
-110 ø E
"-.,.,
!'"
,
ß
265
•.•
266
'-.•
*:• • :'•:i•:•11•:::.:•'!i•!
•
'
267
IndianOcean
::::•ii ::::::::..:
,
•::
AADIndia-type
....
thisstudy
lOO
South Tasman Rise:.
off-axis, Zone A
Zone A
/•
/•
/•,
• •o•
•o•
SEIR
Pacific-type
•is study
/
Tasman
Sea
SEIR Pacific-type
...........
•,
MW07
...........
'•';:,•..
...........
%, ....ß....•...:.,-.,..•,.•,....•,•:.•:,•..,•
.,,,,.......• ß,,
283 •:::::::-"::"'•'•
...............
:'•'•:"•
.........
':'•:"'
.......
_
•
:
• :
280A
most'depleted'SEIR
100
lOO
:
MacquafieIslandOphiolite
/
basaltr•ge
-Southwest
Pacific
:74_44
10
......
lO
I
•a • T•;:La
Pr P Zr SmTi Y Lu La Ce Pr Nd PmSmEu Gd Tb Dy Ho Er Tm Yb Lu
I
Rb Th
Nb K
Ce Sr
Nd Hf
Eu Dy Yb
Figure 6. Primitive mantlenormalizationdiagramsand chondritenormalizedrare earth elementdiagramsfor
DSDP Leg 28 and Leg 29 samples. Normalizationvaluesfor primitive mantle are from SunandMcDonough
[1989] and rare earth element normalizationvaluesare those of Boynton [1984] for "average"ordinary
chondrites.The compositional
rangeof MacquarieIslandOphiolitebasalts,AAD Indian-typeMORB (MW231, MW26-1, andMW27-71) andzoneA Pacific-typeMORB (MW07-01, MW20-01) are shownby shadedfields
for comparison.The mostdepleted,primitiveSEIR Pacific-typeglasssampleanalyzedin this study(MW1726; seeTable3) is designated
by a dottedline pattern.All datacollectedby ICP-MS duringthisstudy.Ta values
for whole rock analysesare assumed
equivalentto Nb on primitive-mantle-normalized
diagrams(i.e., 264,
274, 279A, 280A, 283, 282); normalizedTa values for basaltic glass (i.e., 265, 266, 267, 278, MW07,
MW20) are representative
of resultsreportedin Table 2. UnusuallyhighRb abundances
(relative to Ba andTh)
andK concentrations
(e.g., 283) are artifactsof alterationin somewholerock samples.
Discussion
type plume mantle [e.g., Hart, 1988; Weis et al., 1992, 1993],
Balleny-type plume mantle (HIMU-type; terminology of
Continentalrifting and openingof an oceanbasinrequires Zindler and Hart [1986]) [e.g., Lanyon et al., 1993], Pacific-
flow of suboceanic
asthenosphere
into a region previously type depletedMORB mantle,andIndian-typedepletedMORB
occupiedby subcontinental
lithosphere.Oceanicuppermantle mantle. Alvarez[1982,1990]originallyproposed
thatupper
mustupwell from beneaththe zone of rifting and/ormigrate mantleflow, in thiscasefrom a shrinkingPacificOceanbasin,
laterally as the oceanbasin developedbetween Australia and mustbe channeledbetween,or obstructedby, the roots of
Antarctica.Whetherupwellingor lateral asthenospheric
flow continentsthat reachhundreds
of kilometersinto the upper
dominatesthis processcan be assessedqualitatively by mantle [Jordan, 1975]. Excess volcanism associated with
determining the regional distribution of geochemically hotspotsandlargeoceanicplateausmay also act to promote
distinct upper mantle sourcesthrough time as Australiaand
Antarcticadrifted apart. In this region, four distinct mantle
uppermantleflow. Severalshallowbathymetricfeaturesin the
Southeast Indian and Southwest Pacific Oceans are related
sourcescan be identifiedand tracedisotopically: Kerguelen- either to continentalremnants(e.g., South TasmanRise and
PYLE ET AL.: SOUTHEAST INDIAN-SOUTHWEST
PACIFIC SEAFLOOR
22,273
NaturalistePlateau)or to oceanicflood basalt provinces(e.g.,
KerguelenPlateau, Broken Ridge) [Storey et al., 1988, 1989;
Mahoney et al., 1995]. Thus reconstructing the relative
positions of these continental masses and hotspot-related
features through time is important for understandingtheir
influence on upper mantle flow during the opening of the
type upper mantle has been migrating westwardat 25 mm/yr,
the minimum allowed by our observations within the AAD
[Pyle et al., 1992], the position of the isotopic boundary
roughly coincides with the South Tasman Rise at
approximately 50 Ma.
As early rifting betweenAustralia and Antarcticaprogressed
Southern Ocean basin.
from west to east, Indian-type MORB mantle most likely
flowed into the region from the west, as suggested by the
recoveryof Indian-type basalts dredgedfrom -50 Ma seafloor
Regional
Plate Motions
north of the SEIR [Lanyon et al., 1995] well east of the present
Continental rifting and seafloor spreading between position of the isotope boundary. The dispersal of plume
Australiaand Antarcticabegan at 110 to 90 Ma, propagating mantle associated with a voluminous plume head during the
from west to east, following a period of continental extension early stagesof Kerguelen hotspotactivity likely facilitated an
(amountingto >300 km) that began at -160 Ma [Powell et al.,
eastward flow of Indian-type upper mantle as the two
1988]. Roughly contemporaneous
with continental rifting, continents separated. Farther to the east, the South Tasman
the earliest manifestations of the Kerguelen hotspot are Rise remained an obstruction to the westward migration of
recorded by flood basalt volcanism in western Australia Pacific-type upper mantle until the transitionfrom slow to fast
(Bunbury Basalts, 105-130 Ma) and northeastern India spreadingcirca 42 Ma (A18). This transition,which coincided
(RajmahalTraps, 116-117 Ma) [Baksi et al., 1987; Morgan, with rifting of the KerguelenPlateau from Broken Ridge and
1981; Mahoney et al., 1983; Duncan and Richards, 1991; the final rifting stage of the South Tasman Rise, most likely
Pringle et al., 1994]. Seafloor spreadingbetween Australia marks the beginning of upper mantle flow from the Pacific
and Antarcticabeganat -96 Ma and progressedin two phases into the Indian Ocean basin.
[Candeand Mutter, 1982; Veevers,1987; Royer and Sandwell,
In contrastto the Kerguelen-SEIR system, hotspot activity
1989]. Seafloor magnetic lineations record an early slow in the southwest Pacific appears to have been much less
spreadingphase (-9 mrn/yr, full rate) from 84 Ma (anomaly voluminousand not closely associatedwith spreading. Three
34) through 43 Ma (anomaly 18) [Cande and Mutter, 1982],
separate, age-progressive volcanic lineaments have been
duringwhich time the eastern part of Broken Ridge and the documentedin this region; one along the eastern Australian
northeastern Kerguelen Plateau were constructed by a continentand two others,the Tasmantidand Balleny seamount
Kerguelenhotspot centered on or near the SEIR [Cande and chains, that originate in the Tasman Sea [McDougall and
Mutter, 1982; Leclaire et al., 1987; Schlich and Wise, 1989;
Duncan, 1988; Johnson, 1989; Eggins et al., 1991]. The
Duncan, 1991]. At 43 Ma, spreadingabruptlyincreasedto >60 Balleny hotspot has produceda discontinuousvolcanic chain
mrn/yr(full rate) and has graduallyincreasedto -74 mrn/yr at that has been traced from the Balleny Islands back to -36 Ma
present [Vogt eta!., 1984; Royer and Sandwell, 1989]. The [Duncan, 1981; Duncan and McDougall, 1989] where it was
SEIR eventually separatedBroken Ridge from Kerguelen located beneath the east Tasman Plateau (i.e., a small
Plateau at 43 Ma (A18) to 35 Ma (A13), establishing the bathymetricfeature east of the South TasmanRise) [Lanyon et
presentconfigurationof the SEIR spreadingcenter(Figure 9) al., 1993]. Of these eastern hotspots, only the Balleny
[Lawveret al., 1992; Royerand Sandwell, 1989]. Throughout hotspot has encountereda mid-ocean spreadingcenter as it
this period, the Kerguelen hotspot was very active and is moved from the Australian Plate to the Antarctic Plate between
likely to have had a significant compositional influence on 10 and 20 M a.
the upper mantle throughoutthe region.
East of the AAD, spreadingin the Tasman Sea from 84 Ma
Distribution
of Mantle
Sources
(A34) to 55 Ma (A24) and the inceptionof spreadingalong the
Pacific-Antarctic Ridge at -84 Ma (A34) [Veevers, 1984;
The distribution of Indian-type Pacific-type and hotspot-
Hayesand Ringis, 1973; WeisselandHayes, 1977; Mayes et
al., 1990] coincided with early slow spreading between
type mantle is recordedmost effectively by the isotopic
composition of the DSDP basalts. Basaltic glasses from the
Australia and Antarctica.
The Tasman Sea and the Pacificthree sites near -110øE, west of the AAD, are isotopically
Antarctic spreading systemsmay have been continuous until
identical to those of present-day AAD basalts [Klein et al.,
-55 Ma (A24), after which TasmanSea spreadingceasedand 1988; Pyle et al., 1992] and Indian-type MORB from the SEIR
transformmotion along the MacquarieRidge progressively in general [Cohen et al., 1980; Michard et al., 1986; Dosso et
offset the two systems [Weissel et al., 1977; Stock and al., 1988], especiallyin having low 206pb/204pbrelative to
Molnar, 1982, 1987].
Since Miocene time, northward nearby Pacific-type MORB.
In general, 208pb/204pb,
migrationof the SEIR away from Antarcticahas produceda 207Pb/204pb, and 87Sr/86Sr increase and 206pb/204pb and
seriesof short spreadingsegmentsoffsetby a seriesof large
transformfaultsthat connectthe Pacific-Antarctic
Ridgeto the
easternSEIR. Rifting of a small continental fragment, the
SouthTasmanRise, markedthe final separationof Australia
fromAntarcticabetween42 Ma (A18) and36 Ma (A13) [Royer
and Sandwell, 1989; Lawver et al., 1992 ]; radiometricdata
reportedhere indicate that this separationbegan at about 65
Ma.
The South Tasman Rise remained
an obstruction
to
circum-Antarcticdeepwater circulationuntil late Oligocene
time (-30 Ma) [Kennett et al., 1974]; however, when it ceased
to act as a barrier to uppermantleflow is unknown. If Pacific-
143Nd/144Nd
decreasewith decreasing
age of eruptionat -110 ø
E, suggesting that a Kerguelen-type contaminant becomes
more abundant in the MORB source as seafloor spreading
progressed.That is, the youngestdrill site (Site 265) exhibits
the most Kerguelen-like contaminated MORB composition.
This is particularly noteworthy becausethe most Kerguelenlike MORB signature might be expected for the oldest site
(Site 267) if an initial Kerguelenplume head with a radius of
-1000 km [e.g., Griffiths and Campbell, 1990; Coffin and
Eldholm, 1993; White and McKenzie, 1989] progressively
grew and dispersedbeneath this region of the SEIR during the
22,274
PYLE ET AL.:
SOUTHEAST
INDIAN-SOUTHWEST
iEPR
•!.i...11•
[MORB
:i!
...................
.{..:.......,.[,,..,,,,
o".
ß
west
of
AAD
Aoff-axis
dredges,
Zone
n
-+12
-+11
0.51320-•
/ii k: .......
•:•
ii:i•.
:-•
[
•"%
Indian
}
SEAFLOOR
decreasing age instead strongly suggests that Kerguelen
hotspot material was not present (or at least less abundant)
beneath this section of the SEIR until the spreading rate
increased after-43 Ma. Faster spreading rates may have
enhancedor even initiated the eastwardmigration of material
derived from the Kerguelen plume. Moreover, this limited
samplingof SEIR MORB at -110øE throughtime is consistent
with the Kerguelen-type signal being carried eastwardwithin
early stages of seafloor spreading. Eruption of Keruelen
influencedMORB compositionsmight be expectedinitially if
Kerguelenmaterial upwelledbetweenthe westernAustralian
and Antarcticcontinentsduringthe slow spreadingphaseof SE
Indian Oceandevelopment. Furthermore,the spreadingridge
environment could concivably dilute this hotspot signature
with time, resulting in progressively less hotspot-like
compositions.
The increasein a Kerguelen-likesignaturewith
0.51325
PACIFIC
OSouth
Tasman
Rise
[] eastof SouthTasmanRise
:; •*%,
-+10
:(•Indian-type
SEIR,
AnD
-
+9
-
0.51295
27•X
Ba!ie•v •
•,,•.
--
+6
--
+5
•x St.Paul
0.51290
::
0.51285
+7
rosterdam
aikalicbasalt
'
' '
0.7020
I ....
', ....
0.7025
,
0.7030
+4
I ' ' ' ' ', ....
0.7035
0.7040
0.7045
87Sr/86Sr
15•
0.51325
+12
EPR
oO
1•'1I
.........
:.':
...........
MORB
0.51320
i
0.51315
/" '5, .......
'%',,,,,,,
:!,,'... ,...
ß
" 'n
'" "53
0.51310
_ +8•
0.51305
/••i[267::O•
•:•"'"•"0'-"'•
274
Q•
0.51300
0.51295
0.51290
X ....
0.51285
>•
•
I
•/
W
17.5
18.0
18.5
+7
-
+6
-
+5
aikali• has_air
I
17.0
-
19.0
+4
20.0
19.5
206Pb/204Pb
0.7050
0.7045
•
0.7040
r•
0.7035
'• t•265
t Tasmanian
X•ooOOø'%.
....ooo
..........
%
IS' alkalic
basalt
0.7030
ß,,,,
'.
0.7025
•(•.••'•:'A• ....................
,•.•..•
.............
.•;^.......
? •4 •
•
._,:•.;282 •
•. ,/"
Ba!ieny
,,.,•2..8....3...y:.
, ):....,.,
•
......
........
,.,..,/:"'EPR•
171.0I
.....................
ZoneA
.,•s. MORB
.......
•..........
•.
, ................... PAR
4.
[
17 5
I
I
18.0
I
I
18.5
206Pb/204Pb
I
i
19.0
I
i
19.5
I
20.0
PYLE ET AL.:
SOUTHEAST
INDIAN-SOUTHWEST
•'
•.c•'
15.7
•
•
•
•I
15.5
SEAFLOOR
Amsterdam
22,275
I
2Zglele
St.
Paul
Is.
n-Heard
•-)
•-•15.6•
I
•
PACIFIC
/•A•....
•///•••
' . ] •'•8• 279•
••,•
•'
•
.......... •
•,
.
,'•
•
•_0••••
I
•!eny I
............
-•a•anian
Is.II
alk•icbasalt
265• ' ' •/•
-
• .........
15.4 Indian
...........
'-'•"
•'"•'•'•=
::•274
•:•?:'Zone
A
..............................
EPR
rosterdam
St. Paul
39.0 -
Is.
•
•
,.• 38.5••
38.0
J
//• • J
.::•
[-1279AJ
Tasmaman
• • 28•'•
alkalicbasalt
Hear ,_,.•.•.
iit 27••.•/:::::"
S
IS'Ei•,/•:/•}•
•}
•..•?/Zone
A
lnd•n .,,,•f•5
.......
....
•
,,,• 267•
37.5
17.0
17.5
18.0
18.5
19.0
19.5
20.0
206Pb/204Pb
Figure 8. Pb-Pb isotopic variations for Leg 28 and 29 DSDP basement samples (fields and referencesas in
Figure 7; symbolsas in Figure 5). The NorthernHemisphereReferenceLine (NHRL) is an averagePb-Pb trend
through oceanicbasalt compositionsfrom the northernhemisphere;A 8/4 and A 7/4 •e calculatedrelative to
this line [Hart, 1984].
the ambient
Indian MORB
mantle as Australia
and Antarctica
separated,ratherthan advectedby excessplumematerial. This
may indicate that the continental roots of Australia and
Antarctica obstructedeastwardflow of early Kerguelen plume
head material prior to rifting.
The 65-70 Ma basaltsfrom the Tasman Sea and the Balleny
basin have isotopic characteristics and trace element
similarities to Pacific-type MORB from east of the AAD,
indicating that Pacific-type mantle existed adjacentto the
easternmargin of Gondwanabefore Australia and Antarctica
rifted. The compositional effects of the Balleny hotspot are
evident in basalts from the MacquarieRidge (Site 279A) and
Figure7. (opposite)
TheœNd-87Sr/86Sr,
œNd-206Pb/204pb,
and87Sr/86Sr-206Pb/204pb
variations
for Leg28
and29 DSDP samples.Indian-typeMORB is characteristically
higherin 87Sr/86Sr,lowerin 143Nd/144Nd,
and
higherin 208pb/204pb,
207pb/204pb
contentat a given206pb/204Pb
valuerelativeto PacificandAtlantic
MORB [e.g.,DupreandAllegre,1983;Hart, 1984;HamelinandAllggre,1985;Hamelinet al., 1986; Michard
et al., 1986;Priceet al., 1986;Dossoet al., 1988;Kleinet al., 1988;Mahoneyet al., 1989, 1992]. Only
PacificMORB from the EPR is represented
by the Pacific field shown (i.e., excludesGorda,Juande Fuca, and
Galapagos
spreading
centers).PacificOceanMORB samplesrecovered
closestto the regionof this studyare
represented
by thefield for Pacific-Antarctic
Ridge(PAR) [FergusonandKlein, 1993]. A compositionalfield
for SEIR MORB westof 110øE(dottedfield) is shownfor comparison
to AAD andDSDP data. Pacific-type(dark
shadedfield labeledzoneA) andIndian-type(lightshadedfield labeledAAD) MORB fields from vicinity of the
AAD are shown.The doublearrowsbetweenthesefieldsmarka mixingtrenddefinedby SEIR samplesdredged
near the present-dayisotopicboundary(dredgesMW05 and MW06, seeFigures2 and 3). Note that the Site 282
basaltcompositionlies within the mixing array definedby presentday SEIR MORB lavas. Arrows with dotted
pattern show the general trend in SEIR compositionsthat appearto have hotspot contaminants in their source
(i.e., Kerguelen-likeor Amsterdam-St.
Paul-like). Compositional fields for hotspots surroundingthis region
are KerguelenIsland [Weis et al., 1989; Gautieret al., 1990; Storey et al., 1988]; HeardIsland [Barling and
Goldstein, 1990]; St. Paul and AmsterdamIslands(W. White, personal communication, 1994); Balleny Island
[Hart, 1988; Lanyon et al., 1993]; Tasmaninnalkalic basalt (13-46 Ma) [Ewart et al., 1988].
22,276
PYLE ET AL.: SOUTHEAST
INDIAN-SOUTHWEST
PACIFIC SEAFLOOR
120 ø
90 ø
90 ø
267
"
.-KP
.........
'......
:...............
:.....
i......
t•.......
'......
".................................
60 ø
60 ø
60 ø
60 ø
30 MA
120 ø
90 ø
90 ø
90øE
WB
60 ø
60 ø
'TS
,:
,
,
,
,
,
,
,
60 ø
60 ø
50 MA
Figure 9. Tectonic reconstructionsof the SoutheastIndian Ocean and SouthwestPacific Oceanat 50 Ma and
30 Ma [Lawver et al., 1992]. These reconstructions
show the locationof the DSDP sitesrelative to continental
boundaries
andoceanic
plateaufeatures
(shaded
areas)priorto andfollowingthe transitionfromslowto fast
spreading
in theregion.At -40 Ma, spreading
ratesincreased
asKerguelen
andBrokenRidgePlateaus
rifted
andthefinal separation
of the SouthTasmanRisefromthe Antarcticcontinentoccurred.KP, Kerguelen
Plateau;BR, BrokenRidge;NP, Naturaliste
Plateau;90E, NinetyeastRidge;STR, SouthTasmanRise; TS,
TasmanSea;LHR, LordHoweRise;B Is.,BallenyIsland;MR, Macquarie
Ridge.
PYLE ET AL.-
SOUTHEAST
INDIAN-SOUTHWEST
Balleny Basin (sample 274-45)' however, the older Balleny
Basinsample(274-44) and the EmeraldBasinbasalt(Site 278)
PACIFIC
SEAFLOOR
22,277
idea of westwardmigration of Pacific-type mantle into the
region [Alvarez, 1982, 1990].
The low 206pb/204pb
compositionof Indian-typeMORB is
Volcanism surroundingthe South Tasman Rise is of critical to our interpretation of mantle sourcefor the DSDP
particularinterest becauseit appearsto be associatedwith samples [Mahoney et al., 1989]. A distinction between
early rifting of this feature. Lanyon et al. [1993] suggested Indian-type and Pacific-type compositionsin DSDP samplesis
show no such influence.
that the east Tasman Plateaumay actually be a volcanic feature
more difficult
producedby excessivevolcanism of the Balleny hotspot,
chosen a few key incompatible elements such as Ba, Th, La,
and Zr to determinemantle sourcesignatures. These elements
are fairly resistant to alteration and are available for both
modernSEIR MORB andthe DSDP samples. Unfortunately, Ta
rather than a remnant continental fragment like the South
Tasman Rise, based on the compositional similarities of
TasmanianTertiary alkalic basaltsand Balleny hotspot lavas.
Furthermore, continental-lithosphericmaterial might be
expectedto have contaminatedseafloorbasalts eruptedclose
to the continental South Tasman Rise, in a similar fashion to
thatobserved
at the Naturaliste
Plateau(site 264). The highly
incompatible-element-depleted,
typical MORB signaturesof
the SouthTasmanRise samplessuggestthat contamination
by
continental and/or hotspot material has been insignificant
along this continental margin region. Similarly depleted
to demonstrate
with
and Nb can not be used because
trace elements.
Ta values
for
We have
whole
rock
samaplesare contaminated(i.e., tungsten carbideshatter-box)
andNb data are lacking for SEIR MORB. This leads to a heavy
relianceon Ba, which may be suspectin older samplesowing
to contamination
from seawater alteration.
Nevertheless,
coherencebetweenhigh Ba in the DSDP samplesand high Th
and La, elements which are both much less susceptible to
alteration, suggests Ba abundances are in large part
basalts have been recovered from ODP sites 765 and 766 in the
representativeof magma composition(Figure 10).
Indian-type SEIR MORB are enriched in Ba and Th
Argo Basin and in the Red Sea, suggesting that such
compositionsmay be commonly associatedwith early phases concenration relative to Pacific-type SEIR when plotted
against less incompatible compatible elementssuchas La or
of seafloorspreading[Luddenand Dionne, 1992; Elssen et al.,
Zr. The DSDP sample data show no such contrast in trace
1989; Schilling et al., 1992].
Whether DSDP basalts from the South Tasman Rise are
elements.In fact, all of the DSDP basalts,including those east
classified as Pacific-type or Indian-type depends,to some of the South Tasman Rise, have trace element characteristics
degree, on the criteria used to define these upper mantle which overlap the Indian-type SEIR MORB range.
:reservoirs. The Site 283 basalt (Tasman Sea) is well within the Furthermore, the boundary between Indian-type and Pacificisotopic range of other Pacific-type MORB samples from the type mantle is apparent from highly incompatible element
region, but for sites 280A and 282, the isotopic designationof ratios for presentday SEIR MORB, which covary with isotopic
the basalt source is less clearly defined. Although the Site composition (Figure 11). A similar coupling between trace
280A sampleis high in 206pb/204pbrelative to Indian-type element compositionand isotopic ratios is not apparentin the
MORB, its higher 208Pb/204pb,
2ø7pb/204pb,and 87Sr/86Sr DSDP sample data; the DSDP samples can be divided on the
and206pb/204pb
but have Ba/LaandBa/K20
valuesplace it outsidethe entire Pacific OceanMORB field, in basisof 87Sr/86Sr
the range of "enriched" Indian MORB and ocean island ratios which are more like Indian-type compositions. A more
compositions. We regard these characteristicsas indicating an detailed examination of the SEIR MORB trace element data is
Indian-type mantle sourcefor Site 280A basalt. Characterizing forthcoming. For our present purposes, distinctions between
the sourceof Site 282 basalt as Pacific-type or Indian-type is Indian-typeand Pacific-typeMORB traceelementsarelimited
more subjective because its 206pb/204pb, 208pb/204pb, to large ion lithophile elements (LILE) more incompatible
207pb/204pb,
and87Sr/86Sr
areall within the greaterPacific than La. Direct comparison of DSDP trace element data with
MORB field. However, its 206pb/204pbis low relative to that of SEIR MORB may be problematic given the variety of
present-dayPacific-type MORB from the SEIR and plots in a analyticalmethodsutilized and a mix of both glass and whole
region intermediate to the Indian- and Pacific-type MORB rock analyses. Some analytical bias may exist between ICPfields definedby SEIR samplescollectedwithin and east of the MS and INAA/DCP analyses (cf., SEIR MORB glasses) (see
AAD [Pyle et al., 1992]. Similar intermediatecompositionare Figure 11). Also, alteration of older seafloor samples make Ba
displayed by samples collected near the present isotopic and K20 values suspectfor whole rock analyses. At present,
these limitations requirethat DSDP samples be identified as
mixing between Pacific-type and Indian-type mantle (double Indian-or Pacific-type MORB based soley on isotopic
arrows;Figures7 and 8). Age corrections would lower the Pb characteristics.
The presenceof Indian-type trace element signaturesin all
isotopicratios, moving the Site 282 data point sample farther
from Pacific-type SEIR compositions. SEIR Pacific-type the DSDP samplescan be explained in either of two ways. (1)
MORB is relatively restricted in composition, low in The trace elementratios actuallyrecord physical differencesin
87Sr/86Sr, 208pb/204pb, and 207pb/204pb for a given melting processes, rather than compositional differencesin
206pb/2O4pb
when comparedto the overallPacificMORB field. the mantle source.To fractionate highly incompatible element
We conclude that the Site 282 basalt source is transitional
ratios like Ba/La, Th/La, or Ba/K20, much lower degreesof
between Indian-type and Pacific-type MORB mantle based on mantle melting are requiredfor DSDP basalts relative to
the restrictedisotopic range representedby the presentPacific- Pacific-type SEIR MORB assuming the Pacific-type mantle
type SEIR MORB. The intermediatenatureof both Site 282 and trace element composition has remained unchangedin this
280A isotopic compositionssuggeststhat their mantle source region; or (2) Pacific-type SEIR mantle has evolved
may have existed near an earlier isotopic boundary between chemically since emplacement of DSDP basalts east of the
Indian-type and Pacific-type mantle or that the boundary was AAD. For example, the relative LIIF, enrichment(i.e., higher
Ba, K20, Th, Ta) of ancientPacific-typeMORB has apparently
less well defined during the early stages of continental
separation.In either case, these data are consistent with the been removed by melting processesand/or modern Pacific-
22,278
PYLE
ET AL.'
SOUTHEAST
INDIAN-SOUTHWEST
PACIFIC
SEAFLOOR
typeSEIRMORB is derivedfrom a moreLILE-depleted
mantle
sourcewhich has migratedinto the region over time. It is also
250
LC
Ba/La-
•
200-
12.5
PM
Ba/La
-10.2
J ••OIB
a ~ 9.5
150-
Mantle
••©
50-
-ß
•
ß ..:...,....:...,,:.:.:.::,:.:..a
>:---,,:.:.:.
.......
..
,
,
2
4
'":
.......
i ocO
o,,
,,,
....
6
i
,,,
8
o
, .....
t ....
,
10
i,, ,, i,, ,,, ....
12
14
16
t ,,
18
20
, ,
22
1.8
1.6
i
LC
ThlLa ~. 14
PM
Th/La ~ 0.12
1.4
0//
OIB
1.2
1.0
0.8
/ / /
N-MORB
•
0.6
0.4
0.2
2
4
6
8
10
12
14
16
18
20
22
AAD.
La (•r m)
14ø
l
120 -'l-
OIB
LC
ß
Ba/Zr - 1.25
[ Ba/Zr5.83
100
-[- /
/
/
.
ß []
PM
60
40
[/
0
/
•_
O
25
50
ß
75
__ •
100
125
•..................................
N-MORB
........:.:,,x
............
'.......
150
175
Ba/Zr- 0 08
200
225
275
250
Zr (ppm)
140
OIB
Ba/K20~ 240
120
100
'.
LC
Ba/K20 - 290
• Ba/K20
-230
•
O * O
40
......
s-.'.'-':
........
N-MORB
Ba/K20
~ 88
I
0.10
0.20
0.30
The resultsof this study suggestthat it is unlikely that the
isotopeboundaryhas remainedwithin the AAD throughoutthe
opening of the SoutheastIndian OceanBasin becausebasalts
from the western margin of the South Tasman Rise have
transitional or Indian-type isotopic characteristics. The
recoveryof Indian-type MORB from 38-45 Ma seafloor north
and east of the AAD strengthenthis conclusion[Lanyon et al.,
1995]. All thesedata supportthe hypothesis that the isotopic
boundary was located east of the AAD in the past. Similar
reasoningcan be used to infer that the depth anomaly and the
isotopeboundaryare decoupled. The depth anomaly intersects
the Australian continent near 140ø E [Veevers, 1987], far to
the west of the South Tasman Rise. If the depth anomaly and
the isotopeboundaryresultedfrom the sameprocess,then the
South Tasman Rise basalts should have unambiguousPacifictype MORB isotopic signatures.
The most direct geochemical evidence for mantle flow
outside the AAD is the apparent eastward migration of
Kerguelen hotspot components beneath the SEIR after
spreadingrates increasedbetween Australia and Antarctica.
The oldest MORB sample from -110 ø E (DSDP Site 267) is
depletedin composition,very similar to ambientIndian-type
MORB from the AAD. Apparently, the Kerguelenplume head
60
0.00
easternmostspreadingsegment within the AAD [Pyle et al.,
1992]. Multiple ridge propagation events and the westward
migration of the depth anomaly are also likely surface
expressionsof uppermantle flow towardthe AAD [Vogt and
Johnson, 1973; Weissel and Hayes, 1974; Forsyth et al.,
1987; Klein et al., 1988; Marks et al., 1990, 1991]. On the
basis of presentdata, the isotopic boundarymay relate to the
upper mantle dynamicsof the region in one of three ways: (1)
it may have alwaysexistedbeneaththe AAD (as definedby the
bounding fracture zones) since seafloor spreading began,
although its position is susceptibleto small-scale, east-west
perturbations; (2) it may be associated with the depth
anomaly, in which case small-scalefluctuationsin mantle flow
causing short-term variations in the rate of westward
migration; or (3) it may have migrated westwardindependent
of the depth anomaly and only recently arrived beneath the
Th/La
~0.05
..........
0
Flow
The strongest evidence for westward migration of the
Indian-PacificMORB isotopic boundaryremainsthe isotopic
differencebetweenon- and off-axis MORB samples from the
N-MORB
La (ppm)
20
possiblethat the northwardmigrationof the spreadingsystem
continuallypositionsthe SEIR axis over parts of the upper
that have alreadyexperiencedmelting,contributingto a longterm depletionof the Pacific-typeMORB sourcein this region.
0.40
0.50
0.60
0.70
t
0ø80
K20 (wt. %)
Figure 10.
(opposite) Incompatible element variation
diagrams(Ba-La, Th-La, Ba-Zr, and Ba-K20) of DSDP samples
relative to SEIR MORB glasses from the AAD and zone A.
Variations in Ba, La, Th, and Zr are selectedhere becausethey
are fairly resistantto alterationand are well representedin SEIR
MORB andDSDP sampleanalyses.Whole rock Ba abundances
appearunaffected
by alterationandvaryconsistently
with fresh
basaltic glass data. SEIR MORB glassesdefinetwo distinct
groups(Pacific-typesmall open circles; Indian-typesmall
solid circles);however,similar differencesare not apparentfor
the DSDP trace element data. Ratios for average lower crust
(LC), oceanisland basalt (OIB), primitive mantle (PM) and
MORB are taken from Sun and McDonough [1989] and Taylor
and McLennan [1985].
PYLE ET AL.' SOUTHEAST INDIAN-SOUTHWEST PACIFIC SEAFLOOR
22,279
boundarybetween Indian-type and Pacific-type MORB mantle
has migrated westwardin the last 4 m.y. On the basis of the
westwarddisplacementof the AAD depth anomaly, multiple
rift propagationepisodes,and the small-scaledisplacementof
the isotopicboundary,mantle flow from the SouthwestPacific
into the southeastIndian Ocean basin is entirely reasonable,
and the isotopic data presentedhere suggestthat it is highly
[] 279A
probable. The present sample distribution is insufficient to
unequivocally trace large-scale flow of Pacific-type upper
280A
[] 278
mantle or to describe the development of the isotopic
zO
[] 274-44 274-45
[- boundary through time.
However, several important
constraintson upper mantle flow in this regioncan be stated.
øz•oo •o o o Pacific-type
1. Pacific-typemantle appearsto have been presenton the
19.50easternmarginof Gondwanaprior to seafloorspreadingin the
18.50
19.00
18
2820
16-
264 •
12
ß
265 ß
ß
Indian-type
•
8
I
S
266
[]283
267 ß0
ß
ß•ß•
50
•
18.00
Tasman Sea and Southwest Pacific Ocean, at least since -70 Ma
206pb/204pb
300
279A []
o 280A
264 .•
0.7134
•'-
250
(A34). This observation is consistent with, but not proof of,
the postulatedmigration of Pacific mantle into the southeast
IndianOceanbasinsincerifting of the SouthTasmanRiseat
-40
200
278274-44
150
ßO266
i o •ß ß0267
O265
ß
100
o o•
ø 3 282
[] 274-45
oo
%oo[--1
283
Ma.
2. Volcanism at DSDP Sites 280A and 282, along the
western margin of Tasmania, appearsto be related to early
rifting of the SouthTasman Rise -60-70 Ma. Basalts from
thesesitesare compositionally similar to presentday basalts
from the vicinity of the AAD and are transitional between
Pacific-type and Indian-type MORB in their isotopic
signatures. The eruption of lavas with "mixed" Indian-type
and Pacific-type MORB characteristicsfar to the east of the
present isotopic boundary further supports the notion of
Pacific-typemantlemigrationwestwardafter the SouthTasman
0.70220.70240.70260.70280.70300.70320.70340.70360.7038Rise rifted. Thesedataalso suggestthat Indian-typemantle
87Sr/86Sr
Figure 11.
initiallymigrated
eastward
as theAustralian
andAntarctic
Ba/La-206pb/204pb
and Ba/K20-87Sr/86Srplots
of SEIR MORB and DSDP data (symbols as in Figure 10). In
present-daySEIR MORB glasses,the boundarybetweenIndiantype andPacific-type SEIR MORB is most apparentin Sr and
Pb isotopes and is observedwith some incompatible element
continentsprogressivelyrifted from west to east.
3. Isotopic and trace element data show a progressive
"enrichment" trend in old SEIR lavas (-25
Ma to 15 Ma)
between 100øE and 110øE, suggesting that the dispersion of
Kerguelen-relatedmaterialwas initiatedand/orpromotedby an
increase
in spreadingrate. Migration of Kerguelen hotspotratios (e.g., Ba/La andBa/K20). Mixing trendbetweenthese
two MORB reservoirsnear the presentboundary(shown by related contaminants beneath the SEIR appears to be the
of uppermantleflow relatedto continentalrifting
doublearrow) is identifiedby transitional trace element and consequence
rather
than
a
direct result of excessplume input.
isotopic signatures.Isotopically, DSDP samplescan be
categorized
as Indian-type,Pacific-type,andintermediate
(e.g.,
280A and 282),
but their trace element signatures are
Acknowledgments. We appreciate the time and efforts of K.
Spencer,Z. Peng,andG. Waggonerin the SOESTisotopelab. The ICPMS analyseswere completedwith the aid of A. Ungerer and the expert
inconclusive.Analytical bias is illustratedby points with
lines. Ba/La is systematicallylowerin DCP-INAAdata(small attention
of L. Hoganhelpedin the40Ar-39Aranalyses.
We thankL.
open andsolid circles)relativeto the ICP-MS results(line Gahagan and L. Lawyer for furnishing made-to-order plate
showsdisplacement)
for the samesampleglasses.The low reconstructionsfor this region of the SouthernOcean and B. West for
Ba/La ratiosof someIndian-typeSEIR MORB is primarily due help in editing the final reconstructionfigures. R. Lanyon and A.
to INAA
La uncertainties
at low concentrations.
These
Crawford providedunpublisheddata with some welcome insightinto
analytical problems serve to highlight the difficulty in volcanismsurroundingthe SouthTasman Rise. Thoughtfulsuggestions
distinguishingsubtle trace element distinctions between by F. Frey, E. Klein, and two anonymousreviewersgreatlyimprovedthe
content and presentationof this manuscript. The Ocean Drilling
Indian-type
andPacific-type
MORB in thisregion.
did not initially spreadbeneath this region, suggestingthat
the dispersion of Kerguelen componentsmay be more a
consequenceof upper mantle flow due to continental
separation, rather than mantle flow inducedby the plume
ProgramEastCoastRepositoryprovidedsamplesfor this study.Curation
of SoutheastIndian Ridge samplesusedin this studyare provided for
under grant OCE91-02881 to Oregon StateUniversity. This research
was conductedwith fundsprovidedby NSF grantsOCE90-00595and
OCE92-17186
to D. Christie.
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