Crust-mantle evolution in the vicinity of the Bouvet Triple Junction - A synthesis

90 S. Air. J. Antarct. Res., Vol. 17, No. 2, 1987

Crust-mantle evolution in the vicinity of the Bouvet Triple Junction - A synthesis

Dt'fuiled petrologic ami geochemical srudies o.flam.\· from the l'icinity of the Boltl'CI rriple fwu:tion int!icarc tluu the /a)'(ls show a 1vide nmge in compositimwl dijferemimion extending f'rom ,Hg-rich picrite busalts to Fe-Ti rich ferrobasa!ts.

Furtltennore. in addition /o normal geochemica!!_r 'depleted' mid-ocean ridge bawllt (MORB}, JiCochemically 'enriched' /v!ORB occurs in abundance rhrouglwur the region bm is absent to the east of /5o F..

Tectonic location (e.x. Jracrurc zone or ridfie axis} is shown to exert rm imporram comrol 011 the degree of low pres.\w"C fractional crvstul!izalion experienced by the !avas prior to Cl"!lplion, hW has filt/c cffecl on degree of" f.:COC!II'Inical 'enrichmew' or source region composition.

An inte[.:rated model is proposed whereby the mantle source region of 'enriched" favas from t!ze Southwest Indian and American-Antarctic Rid~.:es is inraded by hm• l"o{ume partial melts related to the upH·e/ling Brmret 11!1111/le plume. The mantle source region of 'rnriched" MORB jimn the southern klid-Atlanlic !~idge has heen enriched hy a similar process bm related to the upll"d!ing of the r:eochemicaffy distinct ·sJwna' mantle> plume.

Noukeurige petrografiese en geochemiese swdies \"(1/1 lawa ttit dir omgell"ing Fan die Bom•ct-driepmuamtsluiting dui aan dui die !awa 'n bree spektrum ^\'(/11 Ferskille in samestelling dek wat strek nm Mg-1:\"ke pikrietbasafttot Fe- Ti-ryke fermbasa/t.

Benewens normale geochemies "Ferarmde" midde-

o~etwnrifbasaft (MORB) kom f.ieochemies 'venykrc" /\.10RB in oorl'ioed tilvursdeur die f.iebied l'OOr, maar is 00.1 l'l!n /5°0 a{ll"l!\ig.

Daar is gerind dar die tekwniese ligging (br. by die breuksone of die nj.as) in hoi; mate die graad l"an jhtksionele kriswflisasie by /ae druk. lWWrrUut die /m1·a l"oor uitbarsling hlootges/d i.\, beheer, maar vfegs "n geringe uirwerking het op die graad wm f4COc/temiese '1•enykinr:· oj die same.\telfinf4 l"olgens die ourspronr:!febied.

'n Samel'allende model li"Ortl w;orgcstel waan·o{gens die manteloorspronggcbied nm ·1·errrkre' fawa uit die Suidwes- Indiesr en Amerika-Amarktika sprcisones binnegedri11g is deur klein J•o/umes gedee!rclik gesmelte mareriaal IIY/1 deur die opl\"dlende Bmll·et-manlelpluim l'eroorsaak is. Die nwme{oonpron.t[gelned \"an ·~·enl'kle" ,HORB \'{Ill die suidelike Mid-Atlrmtie.1·e rif is de11r "n .\Oortgl'frke proYeS l"r:rryk, nwar i.1 aan die opweUing \"1111 die geochemie.\

kenmrrkeiJ(/e 'Sfwna ·-montclpluim gekoppel.

Introduction

Thi~ paper ~ummarize~ the re~ulb from a ~eric~ of coordinated \tmlie~ of the igneous roch of the Southern Ocean ba~in a~ pan of the Southern Ocean Litho~phcrc

Proiect (SOLP). Our work ha~ been primarily mncern.:d 11ith uce<1ni(" ba~alt~ erupted along the mid-ocean ridg<.:~ and

A.P. le Roex Department of Geology University of Cape Town, Rondebosch 7700

now forming, beneath a thin sediment cover, the floor of the ocean basins. Mid-ocean ridge basalts (MORB} have been extensively studiCd in most of the major oceanic region~

throughout the world. The Southern Ocean covers a large but previously little ~tudied portion of the earth's volcanic crust. Over the pa~t ten years that simation has subl-.tantially changed. Cooperative programmes with several in~t!tutions

<1nd several countries, plus the developing capability to dredge deep water ocean bottom samples u~ing local ships.

have allowed ~ystematic sampling of the ocean ridge systems surrounding southern Africa. This report deals with the composition and petrology of the ridge basalts. the relations between baoalt composition and tectonic setting and the influence of 'hot ~pots' (or nmntle plumes) on the nature of oceanic b<1sa\ts.

Tectonic Setting

The Bouvet triple junction. at ~54.5~5. l⁰W, marks the bifurcation of the Mid-Atlantic Ridge (MAR) into the Southwest Indian Ridge (SWIR), which trends in a NE direction toward~ the Central Indian Ocean triple junction, and the American-Antarctic Ridge (AAR) which trends in a SW direction towards the Scoti<t Arc located ilt ~24°\V (Fig.

1}. The SWIR and the AAR are both extremely >low

~preading ridge systems with half sprc<1ding rates of O.R6 and 0.90 cm yr '. respectively, while the southern end of the MAR has a moderate ~preading rate of 1.6 cm yr ' (Sc\ater eta!. 1976. Lawver& Dick 19R3).

The AAR and the western end of the SWIR are characterized by short ridge segment~ offset by long. deep transform faults, the mo~t prominent being the Bullard.

Conrad and Vulcan on the AAR, and the Bouvct. lslas Orcadas and Shakil tran~eeting the western end of the S\VIR. In contra~t. the southern end of the MAR ha>. no major offsets within the region sampled between the triple junction and the Agulhas fracture zone at -47"S.

Lawver et al. ( 1982} have noted that the Bouvet triple junction is currentlv m a ridge-transform-tran~form

configuration, and to maintain thi~ geometry it k, necessary that the triple junction episodically jumps back up the MAR. Such a northward jump requires the elongation of the SWIR and AAR with the initiation of new ~preading center~

adjacent to the triple junction. The most recent extcn~ion of the SWIR is the Spie~s Ridge (Fig. l ). Further dct<1ib on the

geophysic~ of the region can be found in Sc\ater et al. (1976}.

L<lwvcr e!al. (19t\2} and Lawvcr & Dick (1983).

Analytical

During. the cour~e of thi~ ~tudy 520 bulk rock l-.ample~ have hc.:n ;maly~ed fnr lO major and 14 trace element> by X-ray

tluore~cence (XRF) technicjue~. Jn <H.lclition. over 3000

S. Afr. T. Nav. Antarkt., Deel17, No. 2, 1987

major element mineral analyses of phenocryst.

microphenocryst and xenolith pha~es. and -600 quench basaltic glass major element analyses ha\'e been completed

u~ing electron microprobe analy~is.

Rare earth elements were analysed using instrumental neutron actiYation analy~is techniques (Ila & Frey 1984) at the Massachusetts Institute of Technology. In all 36 ~am pies were analysed for their rare earth demcllt contents. Sr and Nd isotopic analy~es were conducted on 37 samples using the mass ~pectromctcr facilities of Prof. S.R. Hart at the Massachusetts Institute of Technology. These methods have been de~cribed by llart & Brooks (1977) and Zindler et al.

(IY7lJ).

Mineralogy and Geochemistry of Southern

·Ocean la vas

The majority of samples dredged from the Southern Occun ridge ~ystcms comprise fragments of pillow basalt with selvag.es of quench basaltic gla~~- In most samples, textures range from aphyric through microporphyritic to sparsely and moderately plagioclase phyric. Highly plagioclase phyric la vas arc particularly common <ilong the American-Antarctic Ridge and <~t ~cattcrcd localities along the Southwest Indian Ridge and .<,outhern Mid-Atlantic Ridge. A single dredge haul from the southern Mid-Atlantic Ridge is characterized by the occurrence of abundant highly olivine phyric (picrite)

ha~a1t.

Olivinc and plagioclasc are ubiquitous mineral

con~tituents in the sample~ studied, while Cr-spincl microphenocrysts are common in the more primitive lavas.

Clinopyroxene occurs as an important phenocryst phase in the majority of fracture zone basalis and in many of the ridge axis samples from the western end of the Southwest Indian Ridge and from the southern end of the Mid-Athmtic

91

Ridge. Highly evolved ferrobasalts from the Spie~s Ridge

~cgment of the Southwest Indian Ridge contain titanomagnctite micruphenocryst~. while titanomagnetite and ilmenitc are common matrix phases in the hypabyssal rocks. Plagioclase in particular shows complex zoning

pal!em~ (normal, reverse and oscillatory zoning) and commonly contain~ melt inclubions. Olivine and clinopyroxene phenocry~t~ tend to show more uniform and moderate degrees of normal zoning.

Analysed samples from the ridge sy~tems are all basaltic in composition and include both extrusive (basalt) and intrusive (diahase) varietie~. With few exceptions the hasalts are olivinc normative tholeiites; a few of the more evolved

lava~ contain normative quartz and some lava~ are slightly nephe!inc normative (up to 1.2 <;( Ne, with Fe,OJFeO = 0.15). Whole rock composition~ of aphyric lava~ range from primitive basalt with 8 to 10 per cent MgO and high Mg#

ranging f10m 65 to 70 (Mg# = IOOMg/Mg

+

Fe'· in atomic percentages), to highly evolved ferrobasalts with high iron contents (!Otal iron expressed as FeO = 10 to 14 S""i-) and TiO, (2 - 4 'Ji:) contents and low Mg# (35 - 45).

Incompatible minor elements sttch as P and K correlate with degree of differentiation (e.g. Mg#) and high absolute abundances of Al,O, and MgO correlate respectively with modal ph1gioclase (AI,Q, reache~ 24 '?-i.- in the most plagioclase phyric basalts) and oli\'ine (MgO reaches 16 q in the olivine phyric picrite basalis from the southern MAR). Representative whole rock analy~e~ are given in Table I.

Ferromagnesian trace elements have a wide range m concentration in aphyric Southern Ocenn basalis (e.g. Ni = 15 - 250 ppm; Cr = 5 - 600 ppm) und show a general negative correlation with Mg#. Absolute incompatible trace element abundances generally correlate positively with degree of differentiation and are also exuemcly varied (e.g.

BOUVET TRIPLE JUNCTION

50

55

7

DREDGE SITES

A 11- 107/6 ISLAS ORCADAS VULCAN S

•

~ V

•

0

•

55

fig. Sk<:tch map of the ilom"Cl tripk jun<:tion an;:a in the Southern Ocean 'tmwi11g tocalitin of individual dn:d~c hauh <md (l\Tratt 'ample coverage.

SiO, TiO, AJ,Q, Fco•

M nO M gO CnO Nn;O K:O

r,o,

LOI Total Nb Zc y Rb Ba

s,

Co Cc Ni V Zn

c"

_Se

La

NO Cc Sm E"

Tb Yb

L"

ZrtNb YtNb Zr!Y Ti.-Zr

La;srn, Mg#

31--1 MiP R T 5 l. 14

2.72 13.96

12.~2 0.~1

4.7J

!i.72 4.42 0.85 0.-18 0.81 100.26 272

29 51 16.3 171 256 39 22 23 227 120 62 31

9A 1.8 5.3 60 43.9

Table I

Major and trace element analyses of selected basalts from th~ Southern Ocean. FeO* =all iron as FcO; Mg# =atomic Mg*IOOI(Mg + Fe'') with Fe,OJFeO = 0.15; AP = aphyric, 1\IiP = microporphyritic: SP =sparsely phyric; PP= highly plaginclase phyric; OP =high!)' nih·ine phyric; Fz = frac·

lure mne: R = ridge axb; ;..', T aud P refer to type of MORI!. Data from le Roe:~. et al. ( 1983, \985, 1987).

Soutilwc;t Indian RhJgc (All-107-) Southern Mid-Atlantic Ridge (AG32-) Amcrican-Antarctic Ridge (V5-)

36-11 pp FZ N 50.06

1.36 16.97 7.87 (), 15 6.83 12.25 3.29 0.27 0.14 1.00 100.19

Y3 2.0 30

4.7

00 15~

41 269 100 247 77 69

))

4.2 11.9 9.5 3.09 116 0.77 2.84 0.44

"

¹⁵ _3.1

88 0.83 63.7

5(!-27 pp

R N 48.71

0.76 18.29 8.91 0.14 8.83 12.22 2.33 0.05 0.06 0.32 100.65

44

NO ND 25 NO 76 50 1R7 176 27 55 91 41 1.5 5.4

·L2 I .73 0.74 0.45 2.81 0.44

>22

>12 l.R 104

0.53 66.7

57-2 AP R T 50.29

1.87 15.0{) 9.99 0.11\

H.50 ]0.79 2.53 0.37 0.22 0.46 100.20 129

IO.H 34

6.8 70 200 49 321 166 275

"

65 40 8.9 21.6 14.1 4.55

1.69 0.95 3.52 0.47 I !.H

3.1 3.8 87

1.19 63.3

57-Hi AP

R N 49.97

1.45 15.61 9.85 0.\R R.78 11.14 2.8J 0.16 0.13 0.64 100.7-1

"

ND 311

0 0

11.3 114

53 362 185 257 S2 76 40 3.5 11.3 9.5 3.37

1.27 0.84 3.42 0.48

>38

>13 30 9R

0.63 64.3

58-12 OP

R p 48.04

\.57 16.21 IU.:D 0.17 9.53 8.43 3.22 0.74 0.40 1.70 IOU. 54 141

22

"

^4.8

112 505 53 286 2U4 166 88 49 31 17.-1 37.6 18.9 4.11 1.46 0.69 2.38 0.35 68 1.1 6.3 64

2.58 64.7

66-16 AP FZ T

<1-H.74

!.HO 15.12 9.83 0.16 IUI 10.84 2.50 0.41 0.17 2.29 100.(]7

BR 9.5

"

^8.9

29 188 5fl 509 226 259 122 R4 35 5.9 14.9 10.6 3.07

Ll6 (1.(!5 1.09 0.32 9.3 2.5 3.6 123 1.17

62.8 3-75

AP FZ N 49.44

I .93 14.62 11.26 0.18 7.01 10.86

2.84 0.28 0.20 1.02 99.64 123

6.0 44

53 36 116 49 112 95 331 lOO 52 41

21 74 94 28

55.7

3-36 OP FZ N 47.65

1.85 12.50 11.73 0.19 13.62 8.89 2.69 0.24 0.22 0.86 100.4-1

1"

_5.6

41 5.8 47 102 68 994 524 318 99 46

37

21 7.3 2.9 94

70.2

7-36 Sp FZ N 50.23

1.45 15.18 10.12 0.18 7.30 11.53 2.45 0.39 0.20 0.96 99,93 111

5.5 36

7.8 56 117 45 271 99 259 RR 59 39

20 6.6 31 78 59.2

6-2 pp R N 50.36

1.45 14.61 10.33 0.19 7.71 11.57

2.56 0.19 0.14 0.37 99.41-:

94 4.9 32

J.Y 46 116 49 197 788 288 85 63 43

19.1 6.5 2.9 92 611.2

9-1 MP

R T 50.58

1.79 14.14 11.01 0.20 6.67 10.88 2.81 0.38 0.20 0.52 99.18

m.

9.9 34

6.6 811 176 48 52

38 310 95 57

44

12.3 3.4 36 88

55.1 10-1

AP R T 50.53

1.60 14.95 10.13 0.19 7.20 11.60 2.94 IU9 ll.lll ll.S9 100.62 108

7.3- 31

4.7 68 183 46 202 67 277 81 64 42

14.8 4.3 3.5 89 59.0

25-1

SP

R N 50.39

1.78 16.59 9.45 0.18 6.56 11.18 3.28 0.27 0.21 0.87 100.76 1-10

1.8 41

3.1

<2.3

!50 48 209 77 267 R4 47 35 48 18. 1 14.2 4.68 1.61 1.03 3.9\) 0.60 78 21 3.4 76

0.63 58.4

30-60

SP

R N 50.20

1.RO 15.69 9.48 ll.l8 8.33 10.47 3.15 11.3::;

0.23 11,1:\8 100.71' 140

R.2 3K

4.7 46 193 49

" 0

·'-'-

!66 245 R7 51 37 7.4 22.6 15.1 -1.6H 1.59 O.S8 3.53 0.53 17.1

4.6 3. 7 7R

0.96 fl-U)

38-15 MP FZ N 50.5S

1.55 16.30 t)_l()

O.IR 7.34 10.99 3.17 0.29 0.2U 0.82 100.52 116

4]

35 3.0 11.8 148

42 306 1110 236 H2 54 35 5.0 17.5 12.6 3.81 1.38 1.!2 3.25 0.-:19 27

8.1 .l3 811

0.80 62.0

27-34 AP

R T 50.42

1.7S 15.86 8.56 0.17 7.22 11.09 3.33 0.71 0.2H 0.75 100.17 137

15.6 2l)

7.4 113 337

270 44 87 235 74 55 36 11.1 30.1 18.0 4.42 1.59 0.76 2.73 0.40 88

1.9 4.7 78

1.53 63.1

38-13 OP FZ T 50.35

1.27 17.45 8.!!5 0.14 7.62 I I 13 2.79 0.35 0.13 1.79 101.87

76 6.1 20

7.6 78 218 46 317 131 171 97 67

"

^6.2

1<1-.4 9.2 2.93 1.11 0.66

!.SO 0.28 12.5

3.3 J.H 1011

1.29 63.5

33-66 OP FZ p 49.08

2.04 16.06 10.18 0.22 8.13 8.66 3.24 LOb' 0.44 1.43 100.56 170

27 24 10.5 173 562

,_

·o 258 116 188 98 53 27 20.8 46.5 22.8 5.25 1.77 0.81 2.U7 0.30 6.3 09 7 1 72

2.42 6Ul

~

~

~

~

I ~

~

~

-~ z 9

_N

~ ~

S. Air. T. Nav. Antarkt., Deel17, No. 2, 1987

Zr = 60-360ppm;Nb= 1-47ppm; Ba = 2-300ppm; La

= 1.5 - 31 ppm). There are no obvious systematic differences in trace element abundance ranges between lavas from the different ridge systems, although there are some differences in the mutual variations in some diagnostic incompatible trace element ratios between lavas from the

~outhern MAR and those from the SWIR and AAR (see later discussion).

Certain incompatible trace element ratios are diagnostic of mid-ocean ridge basalt types (Sun et al. 1979) and ratios such as Zr/Nb, Y/Nb, chondrite normalized La/Sm (La/Sm~) and Zr!Y are particularly applicable for characterizing basa!ts

93

from this region of the Southern Ocean (le Roex et al. !983, 1985). For example, plots of Zr versus Nb (Fig. 2a) or chondritc norm<tlized REE abundances (Fig. 2b) serve to distinguish three types of lave which occur within the region, viz. N-, T- and P-type MORB. Gcochemically depleted N- type MORB is the most common variety of MORB. occurs in varied abundance throughout the region, and has the following incompatible trace element characteristics (Table 2); high Zr/Nb (17- 102) and Y/Nb (5- 34) ratios, low Zr/Y (2- 4) ratios and LREE depletion (La/Sm ... "" 0.5- 1.0).

Geochemically enriched plume or P-type MORB is the least abundant variety with low Zr/Nb (5.8 ~ 6.8) and Y/Nb

Fig. 2. Variations in Zr-Nb (a) am! chondritc normalized rare earth t:lemenn, (b) in Southern Oce.an MORB. A chondritic ZriNb ratio of 16 h shown for reference in (a). N-type MORB is geochemically "depleted". P-type MORB is geochcmically ·enriched" and T-type MORE is 'tr;msitional' between the two. Chemical ch<~racteri>tics of the three varietic> of MORE arc summHrized in Table 2.

94

(0.9 - 1.2) ratios. high Zr/Y ratios (6 - R) and strong chondrite normali~.:ed LREE enrichment (Lu/Sm~ = 2.1 - 2.6). T-type MORB is geochemica!ly tr;msitional \~ith

relatively low Zr/Nb (8.0- 16) and Y/Nb (1.3- 5.0) ratios, intermediate Zr/Y ratim (3 - 7) and slight chondrite normalized LREE enrichment (La/Sm~ = 1.1- 2.0).

S. Afr. J. Antarct. Res., Vol. 17, No. 2, 1987 Although a large number of samples have been grouped into the depleted MORB category, there i~ a ~ignificant

range in degree of depletion reflected by these lavas. Figure 3 depicts a number of histograms illustrating variations in Zr!Nb ratio (a sensitiYe indicator of enrichmcnt/depktion in MORB) in lavas from the three ridge ~ysterns, and it h Table 2

Summary of diagnostic trace element ratios in basalis from the southern Mid-t\tlantic Ridge and the Southwest Indian and American- Anhlrctic Ridges lie Roex et a/1983. 1985. 1987). Data from Bouvct from le Roe~ & Erlank (1982).

Southern M.A.R.

N-typc T-typc

Zr/Nb 16-38 8.0-16

Y/Nh 5.2-15.7 2.1-5.0

ZriY 2.0-3.6 2.9-4.0

Ti/Zr 73-117 75-1011

ZriBa l.!l-4.2 !A-Lil

'"Sri"'Sr 0.70290- 0.70339-

0.70356 0.7036~

"'Ndi'"Nd 0.51303- 0.51282-

0.51286 0.51289

'O'Nion~ & Pankhurst ( 1974)

"O'Nions er t!/. ( 1977)

28

.. ₂₀ 24 0. ..

E 16

Ill 0

0 ¹² z 8

4 0

28

Southwest Indian Ridge (0-11"E)

N-typc 17-78

~-6-23

u:--1.2 65-!25 1.7>33.6 0.70246-

0.70297 0.51302-

0.51312

24 American-Antarctlc Ridge

.. ₂₀

.!

"-

E 16

Ill 0

0 ¹² z 8

16 20 24 28 32 Zr/Nb

S.W.LR_ and A.A.R_ Bouvct

28

.. 24 ₂₀ 0. ..

E 16

Ill 0

0 ¹² z 8

4 0

28

.. 24 ₂₀ 0. ..

E t6

0 Ill

12 z

0

8 4

0

T-type 7.7-!.:'-5

LJ-4.3 3. !-7.!

60-127 0.9-5.0 0.70291- 0.70370 0.51301-

0.51284

P-type 5.S-6.8 0. 9-!.2 6.1-7.9 M-Ill l.0-1 .3 0.70356- 0.70364 ll.51295-

0.5!286

6.4 ll.Y 7.5 78 1.2 0.70365-

0.7()376"' 0.51282-

0.512.S5"

Southwest Indian Ridge (15-25"E)

Southern Mid-Atlantic Ridge

Fig. 3. !!i>tograms of Zr'Nb ratio {~~ !11Ci1\llfC nf ~ourc~ r~gion depletion ~nrichment) in Southern Occnn MORB \bowing varied

abundance~ of "-J. htippkd). T- hhmlctl) and 1'-tvpc ( cw"-lwtcl1cd) :>!ORB on th~ diiTcrcnt ridge ~y,tcm,_

S. Air. T. Nav. Antarkt., Deel17, No. 2,1987

5r---~---~---~---~--,

4

1

Amerlcan-Antarctlc Ridge

A A

A

A Fracture zones 4 Ridge eegmente

0

3~0---~---~50~---6~0~---7~0----J

5r---~--~----~Mg~~~--~~--~--~---.

4

1

Southern Mld-Atlantlc Ridge

A Fracture zones 4 Ridge eegmente

0^3~0

____ ._ __ _. ________

~5~0---6~0---~70~--~

5r---~--~----~M~g~~--~~--~--~---.

4

1

Southweet Indian Ridge a aA

A

• • A

A •

•

A

. ^,... _,.. . ..

A Fracture Zonea 4 Ridge Segmente

• Spleee Ridge

03~o---~~---~s~o

____ ._ ___

6•o---~70~--~

Mg#

95

f·tg. -1. Variation in TiO· "llh w;pecl to \lg# ( 1011\lg .\l_g + FL"

Sn1Hhern ()~~an.

in <Homic percentage) in ridge axi~ and fraetur~ ;one ha'>alt' frulll the

evident thilt not only do Zr;Nh ratiO<, i.n N-type :O.IOH.B r<mgc from .<.lightly greater than chondritic (i.e. ~16) lO over 100, hut tho;:re appear~ to be a ~Y..,tcmatic difference in

degree nf depletion ~hown by N-typc MORB from the different ridge~ (and therefore by implication their ~ource region~). N-type MORB to the ea~t of lYE i~ the mo~t

96

depleted with Zr!Nb ratios generally greater than 40, while N-type MORB from the southern MAR and the AAR is the least depleted with the majority of basalts having Zr/Nb

ratio~ in the range 16 - 25, i.e. only slightly greater than chondritic. This observation is borne out by other trace element ratios of the lavas.

Sr and Nd isotope ratios of selected N-, T- and P-type MO RB correlate with their incompatible trace element ratios, although there is some degree of overlap between the different MORE types: N-type MORB from the SWIR and AAR have low "Sri"'Sr = 0.70246 - 0.70297, and high '"''Nd/'"Nd = 0.51302 - 0.51312 ratios; T-type MORB have intermediate "'Sri""Sr = 0.70291-0.70370, and "'Ndl'"Nd = 0.51301 - 0.51284 ratios; and P-type MORE have high ''Sri""Sr"" 0.70356-0.70364, and low '"Nd!'"Nd = 0.51295- 0.51286 ratios. The range in isotopic ratios fa!! within the typical mantle array on a Sr-Nd correlation diagram. and the samples extend from compositions typical for depleted MORB to compositions similar to those of the Bouvet Island lavas (i.e. ''Sr!"'Sr = 0.7037, "'Nd!"'Nd = 0.51284).

In terms of their Sr and Nd isotopic composition, MORB from the southern MAR are unusual in that even those with 'depleted' incompatible trace element characteristics arc clearly 'enriched' with respect to isotope ratios when compared to normal 'depleted' MORB from areas such as the Kane fracture zone (Machado et al. 1982). In this respect southern MAR basalts are similar to those from the Central Indian triple junction (Price er al. 1986).

Influence of tectonic setting on Java composition

One of the hmdamental aims of this study has been to attain an understanding of the influence of tectonic setting on the compositions of lavas erupted at contrasting tectonic environments in the Southern Ocean. Such knowledge is a prerequisite to using the compositions of oceanic basalts from diverse tectonic settings - e.g. ridge axes, fracture zones. oceanic islands, seamounts - to infer source region characteristics and ultimately the composition and evolution of the Southern Ocean mantle as a whole.

Comparison of the composition of lavas found at ridge

<Lxes and fracture zones. shows that fracture zone basalts tend to reflect a greater range in differentiation than ridge axis basalt:>. llli:> difference i:, particularly well illusuated in terms of the greater range in Mg#. greater range in incompatible elements such a~ TiO,, Zr or Y, and in the greater relative abundance of low Mg# ]avas, shown by fracture zone basalts relative to ridge basalts (le Roex &

Dick lYSL le Roex el al. 191->2, 19~0, 1987). Figure ..J. show~

a plot of TiO, versus 1\.-Ig# for basalis from ridge segments and fracture zones from the three ridge systems. and it is dear that the fracture zone basa!ts from the AAR and southern MAR tend towards higher concentrations of TiO, and lower Mg# than do the ridge axis basalis. Thi~

obwrvation i~ equally true for other incompatible clement~.

The SWJR does not show a~ clear a difference between ridge and fracture zone basalts in Figure 4, but with the exception of the Spiess Ridge basalts (sec later discu.~sion), a greater number of fracture zone basalts have high TiO, and low :..lg# than ridge basalb {l>ce Fig. 7 in le Roex e/ af. 19R3).

Quantitative modelling techniques, using lea~! squares

approximation~ and trace clement par!itioning equations

S. Afr. J. Antarct. Res., Vol. 17, No. 2, 1987

(Bryan et al. 1969, Gast 1968), have been applied to individual lava suites from ridge segments and fracture zones to determine whether the overall range in degree of differentiation shown by Southern Ocean basa!ts can be attributed to fractional crystallization of observed phenocryst phases. Results of these studies show that the range in differentiation can in general be quantitatively accounted for by simple low-pressure fractionation of observed phenocryst phases.

The more highly fractionated nature of fracture zone basalts compared to ridge axis basalts from the circum- Antarctic ridge systems and the southern MAR can be attributed to the fracture zone basalts having on average experienced greater degrees ( -40 - 76 o/c) of low pressure fractional crystallization than the ridge axis la vas (10- 30 IJc) (le Roex & Dick 198!, le Roex er al. 1983). Fractionating minerab comprise the phenocryst assemblage olivine ± plagioelase ± clinopyroxene in proportions consistent with the modal mineralogy of the lavas. A typical example of a least squares calculation and the corresponding trace element modelling is given in Table 3 where a highly evolved ferrobasalt from the Islas Orcadas fracture zone is derived from a less fractionated parental magma by -40 per cent crystallization of plagioclase (- 23 %) , dinopyroxenc ( -15 q,) and minor olivine (-I o/c ). It i~ dear that both the major and trace element variations are well satisfied by this model.

Le Roex et al. (1982) have shown that the basalts from the Spiess Ridge segment of the Southwest Indian Ridge are an exception to the above mentioned generalization that fracture zone lavas tend to have experienced greater degrees of fractional crystallization than the ridge basalts from the Southern Ocean. Lavas dredged from this ridge segment arc a\! highly fractionated ferrobasalts (Total iron as FeO = 10.3 - 14.3 <;;-: TiO, = 2.0-3.4 %; M gO = 6.0-3.5 o/c) which contrasts sharply with basalt compositions normally found on ~low spreading ridge systems. Quantitative modelling shows that the compositional variation found in the Spiess Ridge Javas can be attributed to extensive (up to 65 %) fractional crystallization of plagioclasc and clinopyroxene, in approximately equal proportions. minor olivine and, in the most evolved samples. titanomagnctite.

In studies of other ridge systems the occurrence of highly fractionated fcrrobasalts has been correlated with spreading rates as ferroba~alh are more abundant on fast spreading ridges, e.g. the East Pacific Rise (Ciague & Bunch 1976).

This is obviously not the case for the slow spreading Spiess Ridge segment where we have concluded that low rate of magma supply, relative to spreading rate, is the overriding control on the production of ferrobasalt rather than simply spreading rate (le Roex et al. !YR2). A ~imilar model hils been propus.ed to account for the more evolYed magma

compo~itions found at fracture zones relative to ridge axes (it· Rocx & Dick 19R!. le Rocx er ul. 1983). Under these

condition~ of low magma ~upply a given magma batch would undergo little replcni~hment and consequently experience more extreme fractionation than under conditions of more frequent f-Upply and mixing with new batches of primitive magma.

The greater ~pread in composition in fracture zone ba~alts stem~ from the fact that such basalis were originally erupted at the ridge·transform intersections, away from the central zone of magmatic activity and in a cooler heat flow regime than is found mid-way between two adjacent transform fault

S. Afr. J.Antarct. Res., Vol. 17, No. 2,1987 97

Table 3

Lca~i squares approximation and related trace element model rdating tbe average lslas Orcadas fracture zone ferrnbasalt to the mMt prirnith·c basalt from the regiun. Table reprnduced rrom le Roex et al. {1983). Distribution coeffidcnts u~cd in the trace element modelling

rrom le Roex & Dkk r1981) and le R.,ex et al. (1982).

Parental Magma

Ob.,. Calc.

SiQ, 50.7! 50.79

TtO, l..'i'J L6R

A!,O, !6.93 16.88

Feo· R.7K K.S'.l

M nO 0.16 0.18

M gO 6. [6 6.17

c,o

12.-10 12.2K

Na,O 2.79 2.72

P,Q, _{0.20 0.20}

Sum of square~ of residuals = O.OR Trace element',:

Parental tvlagma Obs.

z,

116

Nb 13

y 30

Se 235

Se 34

Go 17

Ni 72

V 247

off~el~ (Forsyth & Wilson !984). Magma chambers in these distal regions arc therefore more ~phemeral and subject to more rapid cooling (from the influence of the adjacent cold litbospheric plate) than basalt~ erupted from near the centcr of a well established ~preading ridge. This hypothesi;, i~

supported hy recent work by Christie & Sinton (l9.Sl) who note that East Pacific ferrobasalb are predominantly associated with the tips of propagating rifts (where rifting is being initiated. and magma chamber~ are ephemeral) and tend to grade to le,, differentiated compositions away from t!1c propagating tips (where spreading is well established and magm;t chamber' may have ;tttained <t steady slate).

Within this context. 1~ Roex er al. (JlJt;2) h<tVC proptJscd that the abundance of l"errobasalt on the northern end of the Spicss Ridge i~ the r~sult of ih unique tectonic setting adjacent tu the northward migrating Bouvet triple junction (Sclater Cl ul. 1976, Lawver et al. 19.'C). The Spic~s Ridge marks the site of the mo~t r~cent bre<Jkthrough of the SWIR through the coldcr African plate in re~pon~e to the northward migration of the triple junction. Thi~ ha~ n·~ulred

in a balance betwe~n magma 'upply and cooling rate that h<t\ favoured largt:" degret:"' of fractionation: the rai~ed

topography of the area may in addition furthtT facilit<tte the eruption of evolved magma~.

The cooler tlh'rmal n~gime a~\nciated with riJg<.:-lfitn\fonn

iMt:r\cction~ (fnp,~·th & Wibnn JYK.J.) can rewlt in la\·ao.

erupt<.:d at \Uch lnt.:alitit:"\ hein.~ generated b\ ~lighth· hl\\<:r

degree\ of part1al mdting than t~ pi\· a! for \rre<tdine. nJ!Le

ha~;tlt\. Thb dket. known a' tht" tran~form fault dfcct (TFE. Langmuir ,\; lknder llJX..J.J. i\ no! 'trnn(!l~ m~tlllk,ted

Mix

Diff. Variable wt. q.

O.OR Ferrobasalt 60.43

0.09 Plag (An,,) 23.00

-0.05 Cpx (') 15..:17

-0.!9 Oliv (Fo") 1.02

0.02

().()! Total 99.9!

-0.1:!

-0.07 "\Vo,,En.,Fs10

0.20

Fcrrobasalt

C'ulc Obs.

183 18.1

21 23

43 42

211 2:!4

36 ^{19 20}

"

34 30

335 337

in basalts from Southern Ocean fracture zones. llowever, some evidence for a TFE is seen in fracture zone ba~<!lts from the southern MAR in that they tend to have elevated absolute abundances of incompatible elements (e.g. TiO, and Zr) for a given Mg#. and lower Mg# for a given Ni content relative to a~sociated ridge axi~ basalt~ (le Roex el

al. 1987). Lowering the degree of melting hw, a greater influence on incompatible element abundances than on Mg#, and also ha~ a greater inlluence on Mg# than Ni in view of the high bulk Ni distribution coefficient for mantle

a~semblages.

One final aspect that need~ to he specifically addre~'cd b the question of whether fracture zone hft~alts and ridge axi~

ba~a!b systematically tap di~tinct ~ource region~.

Incompatible ekmcnt ratio' (e.g. Zr/Nb. Y!Nb. La.-Sm) and Sr and Nd isotopic ratios are not readily fractionated during partial melting or fractional cry5tallization pnlce.,~e~ and are therefore particularly u~;cful for characteri/ing source region

compo~ition (Er!ank & Kable !976. Pcarce & Nony llJ79.

llofmann & Hart llJ71\). The relative <Jbundance tlt"

geochemic;tlly enriched and depleted lava\ from fracture zone., and ridge ^•IXC\ from the !hrt:"e ridge ~y-qem~ nf the Sotltht.•rn Ocean are compared in f'igure ."\\hen: it i'> o:vident

!hit! no ~yqematic difference\ occur in the type!

or

lma t.'rupto:d at the"e two contra~ting tectonic ~t't!ing;,. :~nd b~

implication in the nature of the \OUrec 1cgiPn~ !liYing ri~e [(}

the lava ... Both ketonic L!m'ironment., .. how a 'oimilar Ubtrihution of gt.•ochemica!l) ·depleted" (i.e. Zr ~b ;.... !h.

(LaSmJ .. < I ) anU gcochemicall) ·cnnchcd" (i.e. Zr~h / 16. (La Snl)·.;.... I) ba.,alt tYP""'·

ln ~ummary. compari~on ot the geochemi~try ot ba~alb

erupted at the contra~ting tectonic s<.:ttmg~ ol tr<Klun: zone\,

e~tab!i~heJ ridge "egment:, and at the position of ren:lll ridge jumps, indicate; that tectonic ~ening can ha\e ,m important influence on the compo~ition of the erupted lava~.

The dommant difkrenee i~ the d..:grec of !niCtiorwl LTY'-tallization nperienced by the magma-., with gre<tter fractinnation being fa\'Oured at fracture zone and n.:ccnt ridge jump ~etting~. Thi~ diffcrcm:e i~ primarily due 10 the lower heat tlow regime a~~ociatcd with the latter two

S. Air. T. Nav. Antarkt.. Dee! 17, No. 2, 1987

.0 z ... ....

N

100

10

99

P-type

- - - - M R - - -

~---SWIR---11

w 20 ^{16 12 8 4 LONGITUDE 0 4 8 12 16 20 24 E}

Fig. 6. Variation in Zr.'N!l ratio with re~pcct to Jongitu<.lc along the Southwe\t Indian and American·Alllarctic Ridge>. Data for Bouvet from le Rucx & Erlnnk ( l OJI\2).

tectonic environments. Lower heat flow may also have influenced the degree of partial melting experienced by some fracture zone basalts.

Influence of mantle plumes on the evolution of the Southern Ocean mantle

It has long been postulated (e.g. Morgan 1972. Johnson ^e1 al. 1973) that Bouvet Island, located immediately to the west of the Southwc~t Indian Ridge and to the southea~t of the triple junction (Fig. I). marks the surface expression of a deep seated mantle plume or hotspot (sec Morgan (1972.

1973) and A!legre & Turcotte (19/l:i) for a discussion of mantle plumes or hotspots). ln the fir~t study of Javas from the Bouvet triple junction region Dickey et al. (1977} noted that lavas dredged from the ridge axis to the immediate east of Bouvet Island and from the Spiess Ridge ~egment of the SWIR were geochemically enriched and appeared to reflect the influence of the Bouvet hotspot. These authors noted that the plume influence did not appear to extend to the

we~t of the triple junction. One of the major objecti\e~ of our studies in the Southern Ocean has been to investigate the nature o.md particularly the geographical extent of any hotspot influence on the lava~ of the eircum·Antan:nc ridge

~ystems and sub-oceanic mantle rn this region.

The geochcmically distinct dl<lracteri~tics of hot<.pot

\'olcani~m relative to normal ridge axi~ vo!canism ha~

allowed u~ to address thts problem by ill\e~ti_!!<lting ~pecific

io,otope and incompatil1k trace clement ratim in the l•na~ of the relevant ridge ~~~tern~. Tn ;JVoid pn,sihle alteration effech the immobile incompatible trace element ratio'

Zr/~b. Y(Nh and ZfiY and the i"otopic ratim of ···;-.<d-'"'Nd

and '"Sri~Sr are emphasized. As mentioned previously, these ratios are diagnostic of source region composition and as such provide imight into the nature of the 'ub-oceanic mantle beneath these ridge systems. Hotspot lavas have low Zr/Nb (<-S}. Y/Nb (<1) and '"Ndi'"Nd (<0.51290) ratios.

and high Zr/Y (>-6) and '"Srf'"Sr (>0.7030) ratios.

Trace element and isotopic variation~ in ocean floor la vas from throughout the study area serve to distinguish three compositional groupings: N-, T- ;md P-type MORB. AI!

three lava types show ~imilar variations in terms of major clement compo~ition, but are readily di~tinguished in term~

of incompatible trace element and isotopic variations. the most important of which are given in Table 2. Figure 6 illustrates the geographic distribution of geochemtea\Jy enriched and depleted MORB along the SWIR and AAR in

term~ of the variHtion in Zr/Nb ratio of the lava~ with respect to longitude. Normal, geochemica\ly depleted MORB is the most abundant variety {see abo Fig. 3} and occurs throughout the region. while geochemically enriched T·type MORB occurs in abundance on the SWIR to the west of 1 !~E and at scattered localitie~ along the entire length of the AAR. Highly enriched P-type :O..lORB has only been recovered from the we~ tern end of the S\VIR and from a ~ingle loc<J.Iity on the AAR. Geochemically enriched 1\IORH is apparently absent to the ea~t of 15~E on the S\\lJR. In contrast to the we\tern end of the SW!R. where enriched and depleted MORB occur juxtapo~ed throughout the region and in approximately equal abundance, enriched :"1.-IORB i~ companttively ~carce (~12 r;l along the AAR {fig. 3}. Enriched and depleted \·!ORB are juxtapo;ed throughout the ~a111pled region of the \Outhern \tAR and occur !11 iipproximately equal abundance (Fig. J}

De,-,pite the difkn.'nce in abundance of enriched \lORB

100

0::: m

0 2

10

... 1 . 0

Q)

0..

E

0 UJ

S. Afr. J. Antarct. Res., Vol. 17, No. 2. 1987

---o--- ^MR

- D - SWIR

I?1@.Wl MAR-51 to ^54.5S

0.1 BaRb K Nb(La) Sr Zr ^p Ti ^y Yb Se V Fe

Fig. 7. Abundance variation~ of incompatible clcmenh in a1·erage enriched MORB from the Southwc>t Indian, Amcrican-Antarctic and

~outhcrn Mid- Atlantic Ridge~. Data arc normalized 10 average depleted MORB from the American- Antarctic Ridge (!c Roex e1 al.

1985).

along !he SWIR and AAH.. the inherent geochemical characteristics of enriched lavas from these two ridge syo.tems are indistinguishable. Figure 7 shows the average abundance patterns (normalized to average depleted MORE from the AAR. le Roex e1 al. l9H5) for a number of incompatible trace and minor elements in enriched MORE from the AAR and the SWIR. The two patterns are extremely similar, and show subtle differences (particularly with respect to Nb. Sr and P) from those typical of enriched MORB from the southern MAR (Fig. 7). On it:;, own this difference is not unequivocal, but taken in conjunction with botopic differences (see later discus~ion) is significant.

Incompatible trace element and Sr and Nd isotopic ratios of :\IORB from this region are well correlated and those from the SWJR and the AAR have gcochemical

tharacteri~tic~ which range from compm.itions similar to depleted MORB. to compo~itions indistinguishable from

tho~e of the Bouvet bland lava~ (Fig. 8) and, hy implication, th<:' Bou\·er mantle plullle (le Roex er al. ll/1·(1. lliX5). The

~imilarity and coheren~:e of compo~itional variation:;, of enriched MORE from the SWJR and AAR b taken to imply that the geochemic:d enrichment cxpcrienced by the~e lava~

i~ the ro::,ult of ~~ ~pccific proces~ related to a ,pecific ~ourL'e

and not to arbitrary enrichm~nt which ha~ occurred with time in the mantle ~ource region.. ot the''"' lava~. In contra~!.

enriched \-!ORB from the ~outh~rn end uf the \fAR have inl1en:nt g.c(lcllemical characteri~tic~ which arc di~tinct from tho'le a...,~ociatcd with cnriL'heJ lava~ from the SWIR. AAR Jnd trom the Boun~t mantk plume (Fig. :-t). f.nricheU :\!ORB !"rnm the ~outhern :\1AR exknd to more radiogenic

Sr and less radiogenic Nd bolopic compositions, relative to Zr/Nb ratio. than the SWlR and AAR ba~alt~.

Gcochemical variations within and between the different MO RB types from t!Je SWIR and AAR arc consistent with mixing between two major components; a heterogeneous.

depleted end-member (similar in composition to that characteristic of typical depleted sub-oceanic mantle or N- typc MORB) and an enriched end-member of very re~tricted compo~itwn corre~ponding to the Bouvct hot,pot (le R.oex cl al. 19H3. 1985). Although the mixing relationship~ are well

con~trained, they do not allow one to distinguish readily bet\veen magma mixing and source region mixing as being the most likely proce~s by which the mixing occurred.

Ilowcver, the lack of geochemical gradient away from the

po~tulated location of the Bouvet mantle plume (Fig. 6). tht:

extreme di~tanecs from the present plume location over which gcochemically enriched lava~ have been identified

(~1:->0ll km). and the large transform offsch ~cparating occurrence~ of enriched !\-!ORB from the plume location h<tVe led to a ~ource region rnixin)! molid being tm·(Jured (le Rocx et al. l'JH3. IYH5). The model of ~ource region mixing

i~ abo con~i,tent with thc elm.; juxtHpo,ition of enriched and dcpkted lava~ at ~<:'\'era! location~ <!long the SWIR anrJ

•\t\H.

\li\tn_!l in the ~uurec region can he modelled u'ing two

~:omponenh. The componenb u~eJ in the modelling are hdcrog:encou, depleted a'-theno~phcre (>ource region tn :'>JORBJ and an enriched compom:m equivalent m compn'-ition to P-type ~fORE am.! the Bouvet hnt~pot. The model Je ... ~:riht:'- a dcpl<:'teJ a\thcno~pherc veined by

S.Air. T. Nav.Antarkt., Deel17, No. 2, 1987

..a z ';::-

N

100

10

M/AI SWIR & AAA

• Mid-Atlantic Ridge

Gough Tristan

.5124 .5126

101

Bouvet

.5128 .5130 .5132

1 o43Nd/ ¹ -«Nd

100

FS\.\'*3 SWIR & AAA

•

• Mid-Atlantlc Ridge

' a,

'

..a

_., ,_

';::- z ¹⁰ _Gaugh

N 8

~ ^Trlstan

Bouvet

.7020 .7030 .7050 .7060

I·~ ~ \'.in.llll•ll "' Zr "\b rauo '.\lth ~d · "d -111tl S1 · Sr ratio' in 'i<lulbcrn Orc.m l;!\,1,, D.n" )or BouH'I and Tnq,m lr""l ()''\iH•Il' 1'1 ,1/. { I'!":'_, I .utd 1L R"n .'.:. l rl,mL (]'IS: I ;m cl tnr (iullg_h lr"m Le Roe>.. I I'J\))

~·nrkhed ,jhcatc magma etpnvaknt 1n low volume (2- 4 r:~)

partial rndt~ gcn.:rated v.itbin the garnet ~wt>ility field (and

a~~uciatL'd v.ith the Bou1·c:t I'Jurne). Partial mt'lting within dnm;1in' With greater. k\\cT "r no 1·ein component would

).!iH· ri~e tu P-. T- and N-type MORB. Ou<-tntitatin•

applicatit'n of mixing and part!Hl melting equation~

JLangmui-r et al. 1971-i. Shaw 1970) ~hows that •-,uch a phy~ical

mm.kl can generate the r<J.ngc nf S\\'1 R and AAR Oa.,nlh.

102

souTH pLATE

. . . . . . _{. . .}

^_

.... ^{· ...}

S. A fr. J. Antarct. Res., Vol. 17, No. 2, 1987

.AffiiCAH

...

· · ' ' '·. · ' ' ,·RE · ' ',. · .

. · ... · · '11u::sosPH . .• · ·

. . .. · ...

. . . . . . . ^{. .}

cP

'Enriched' (veined) manflfl domains.

Fig. lJ. Scho:matic diagram' >howing po~~ible model~ for the o:volution of the Southern Ocean litho'>phcre and a,theno~phcrc in the

\·icinity of tho: Bouvet triple jum:tion. In these rnodcl>. upwdling a~;odated with the Bouvct (a) atJd Shona (b) mantk plume" ha'> led to the veining o! domain\ of originnlly depleted mantle bv "natll vnlumc partial melt>. generated within the garnet 'tabili1y field. Radial a'>thcno>pheric tlow away from the ri'>ing plume; ha~ led to the latcral <k.pcr>ion of th<:!.c enriched domains beneath the Southwc~t Indian 'lfH.i American- Antarctic Ridge; (Bouvct hot~po!) and the ~outhcrn MiJ-Atlantil: RiJgc (Shona hnhpn!).

Lateral dbper~iun by aMhenu~phcric tlow of variable veined m<mtle domain~. away from the rising plume (Fig. 9a) allnv.-.., the erupton of P-type and T-type ba~alb at con~iderabk Ui~tancc~ from the plume location.

GeochemKal variation~ in southern MAR basalt~ also wnfonn to theoretical mixing relationships and this has ltd

to a ~imilar model, to that propo~ed for the SWJR and AAR. being ~uggestctl for the evolution of the mantle bcno:-ath the ~outhern end of the MAR (F1g. 9h). However.

the model differ~ in that the ~ource of the low volume partial mdts leading to the enrichment in thi' region i-, not the Bouvet mantle plunw (le R11n et al. ltJX7). Thl' i~Ptopit·