Isotopic geochemistry of Fernando de Noronha

Earth and Planeta~, Science Letters. 85 (19871 129-144

129

Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands I21

Isotopic geochemistry of Fernando de Noronha David C. Gerlach

1 . . J o h n C . S t o r m e r , Jr. 2 a n d P a u l A . M u e l l e r

3

J Department of Earth Sciences, Unit,er.$t(r' of Leeds. Leeds LS29JT ¢England) ' Department of Geology and Geoph)'sics. Rice Unwersl(v, P.O. Box 1892, ttoi~ton. 7".1(77251 (U.S.A.) ~ Department of Geolog~. Unwerstty of I"h>rida. Gainest~llle, FL 32611 ( U, S. A,)

Received December 31, 1986; revised version received June 10, 1987 Volcanic and hypabyssal rocks ranging in age from 12 to 3 Ma from the Fernando de Noronha archipelago in the western equatorial Atlantic Ocean can generally be divided into two age-compositional groups that have variable and distinct isotopic compositions. Predominantly older alkali basalts and trachytes are generally characterized by more radiogenic Sr-isotopic (87Sr/X6Sr 0.70457-0.70485) compositions and less radiogenic Nd-isotopic (~4SNd/la4 Nd = (I.51271-0.51281 ) and Pb-isotopic ( 206Pb/2oa Pb = 19.132-19.2821 compositions relative to the generally younger, more alkaline Si-undersaturated rocks which include nephelinites, ankaratrites, and melilitites (87Sr/86Sr 0.70365-0.70418, 143N d / l ~ Nd = 0.51277-0.51290, 206Pb/204 Pb = 19.317-19.565). These variations suggest the influence of at least two separate components in the source(s) of both series. One component is characterized by high Rb/Sr and low ix, possibly derived from delaminated subcontinental lithosphere, whereas the other has high /z and low Rb/Sr similar to the source of St. Helena lavas. A third component is suggested by correlated compositions in the latest alkaline, Si-undersaturated lavas, and this component may be derived from depleted mantle. These isotopic variations in conjunction with the generally increasing degree of alkalinity with time are consistent v,ith the temporal depletion of a Iow-~, high Rb/Sr component and increasing contributions from a high-,u component in the sources of the volanic rocks of Fernando de Noronha. =

=

I. I n t r o d u c t i o n S i g n i f i c a n t e v i d e n c e for i s o t o p i c h e t e r o g e n e i t y in the m a n t l e is d e r i v e d f r o m the s t u d y o f o c e a n i s l a n d lavas. T h i s h e t e r o g e n e i t y c a n be o b s e r v e d at scales r a n g i n g f r o m i n d i v i d u a l islands [ 1 - 5 , 1 1 ] to m u c h l a r g e r a r e a s [6-9], a n d as m a n y as five d i s t i n c t m a n t l e e n d - m e m b e r s o r c o m p o n e n t s are i n d i c a t e d [10]. D e t a i l e d s t u d i e s o f i n d i v i d u a l v o l c a n i c c e n t e r s o r islands are n e c e s s a r y in o r d e r to e s t a b l i s h the e x t e n t o f i s o t o p i c h e t e r o g e n e i t y t h a t exists in s m a l l v o l u m e s o f the m a n t l e . W e p r e s e n t h e r e the results o f a d e t a i l e d i s o t o p i c s t u d y o f v o l c a n i c r o c k s f r o m the F e r n a n d o d e N o r o n h a a r c h i p e l a g o in the w e s t e r n e q u a t o r i a l A t l a n t i c O c ean. By a n a l y z i n g s a m p l e s w i t h g e o l o g i c c o n t r o l , we are a b l e to d i s c u s s t e m p o r a l v a r i a t i o n s in isot o p i c c o m p o s i t i o n in a d d i t i o n to i d e n t i f y i n g * Now at: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington. D.C. 20015, U.S.A. 0012-821x/87/$03.50

ci; 1987 Elsevier Science Publishers B.V.

p o t e n t i a l d i s t i n c t i v e m a n t l e e n d - m e m b e r s or c o m p o n e n t s w h i c h w e r e i n v o l v e d in their genesis. S a m p l e s a n a l y z e d in this s t u d y w e r e c o l l e c t e d by o n e of us (J.C.S.) d u r i n g a visit to the island of F e r n a n d o d e N o r o n h a in A u g u s t , 1973. T h e s a m ples w e r e c o l l e c t e d f r o m all s t r a t i g r a p h i c units b a s e d o n the g e o l o g i c m a p o f d e A l m e i d a [12] a n d several s a m p l e s w e r e c o l l e c t e d f r o m s o m e o f the s a m e localities as s a m p l e d by C o r d a n i [14] for K/Ar dating. 1.1. PretJious w o r k

D e A l m e i d a [12] m a p p e d the g e o l o g y of the island o f F e r n a n d o d e N o r o n h a a n d a s s o c i a t e d islets, e s t a b l i s h e d the s t r a t i g r a p h i c r e l a t i o n s h i p s o f the e x t r u s i v e a n d i n t r u s i v e rocks, a n d p r e s e n t e d r e p r e s e n t a t i v e m a j o r e l e m e n t analyses. G u n n a n d W a t k i n s [13] also p r e s e n t e d c h e m i c a l d a t a for r o c k s f r o m this island g r o u p . C o r d a n i [14] rep o r t e d a n u m b e r of K / A r age d e t e r m i n a t i o n s ; t h o s e w h i c h c o r r e s p o n d to o u r s a m p l e s are listed in T a b l e 1.

130 TABLE 1 K/At ages of Fernando de Noronha samples analyzed in this study as reported by ('ordani [14] Sample No. (this study)

Equivalent sample of Cordani [14]

Rock type

Quixaba Formation 18 74 20

UC-FN-18 UC-FN-24 UC-FN-20

melilitite ankaratrite melilitite

1.81 ± 0.13 3.33 + 0.11 6.64 + {).20

WR WR WR

U C - F N - 8 0 7 . 809 UC-FN-17,17A

alkali basalt

8.13 + 0.36, 21.9 ± 0.8 9.38+0.94. 9.49±0.33

WR WR

MN-FN-593.617 UC-FN-I MN-FN-61M UC-FN-6 FA-FN-265 UC-FN-8 UC-FN-5 UC-FN-I 0

phonolite phonolite phonolite

S~o Jose l:ormatton 36 Remedios Formation 29 99 1{)3 72 5

10

essexite trachyte alkali basalt

Age (Ma)

8.02 _+{).24, 9.11 + 0 . 2 7 9.31 +_0.28 9.30 + 0.28 9.31 +0.28 9.19 ± 0.28 9.67 ± 0.29 10.76 _+0.32 12.32 + 0.37

a

WR. F F WR WR WR WR WR WR

Whole-rock (WR) or feldspar separates (F).

1.2. Tectonic setting T h e island of F e r n a n d o de N o r o n h a is a p p r o x i m a t e l y 4 by 10 k m a n d is centered at 32o25 ' W a n d 3 ° 5 1 ' S , 345 k m from the n o r t h e a s t coast of Brazil. The F e r n a n d o de N o r o n h a a r c h i p e l a g o includes this m a i n island and several smaller islands, the largest of which is llha Rata. F e r n a n d o de N o r o n h a rises from a base 4000 m deep and is the e a s t e r n m o s t of a w e s t - t r e n d i n g chain of s e a m o u n t s intersecting the c o n t i n e n t a l m a r g i n near Fortaleza, Brazil. Phonolitic and trachytic intrusive rocks near F o r t a l e z a range in age from 27 to 34 M a [15] a n d m a y be an earlier expression of F e r n a n d o de N o r o n h a m a g m a t i s m a c c o r d i n g to tracking of the F e r n a n d o de N o r o n h a " h o t s p o t " by D u n c a n [16] a n d M o r g a n [17]. 1.3. Geology and stratigraphy T h e oldest e x p o s e d rocks of the F e r n a n d o de N o r o n h a a r c h i p e l a g o consist of d e e p l y weathered p h o n o l i t i c a n d trachytic pyroclastic deposits of the R e m e d i o s F o r m a t i o n . G i v e n the altered n a t u r e of these units, we d i d not include samples for isotopic analysis. T h e pyroclastics were i n t r u d e d by n u m e r o u s l a m p r o p h y r e dikes a n d rare dikes of alkali basalt and t r a c h y t e ranging in age from 9 to 12.3 M a [14]. Large p h o n o l i t i c d o m e s are the y o u n g e s t rocks of the R e m e d i o s F o r m a t i o n ( T a b l e

1) a n d form p r o m i n e n t spires in several localities on the m a i n island; m a n y islets are erosional r e m n a n t s of p h o n o l i t i c domes. De A l m e i d a [12] described b a s a n i t e s on the islet of S'~o Jose a n d o t h e r islets near the northeast end of the main island as the youngest volcanic rocks in the archip e l a g o and assigned them to a s e p a r a t e formation, the Sao Jose F o r m a t i o n . K / A r age d a t e s by C o r d a n i [14], however, suggest that these samples, which are m o r e akin to alkali basalts, are cont e m p o r a n e o u s with the later p h o n o l i t e s of the R e m e d i o s F o r m a t i o n ( T a b l e 1). A f t e r an eruptive hiatus that lasted a p p r o x i m a t e l y 6 Ma, ankaratrites, melilitites, and nephelinites of the Q u i x a b a F o r m a t i o n were erupted. The Q u i x a b a lavas b l a n k e t much of the m a i n island a n d llha R a t a (Fig. 1). Sequences up to 100 m in thickness of a l t e r n a t i n g pyroclastics a n d lava flows are evidence that e r u p t i o n s were p a r t i a l l y explosive. F o r the last 1 - 2 Ma, the F e r n a n d o de N o r o n h a massif has been subjected to extensive erosion resulting in the d e v e l o p m e n t of m a n y islets a n d the f o r m a t i o n of p r o m i n e n t wave-cut cliffs.

1.4. Petrography Petrographic descriptions, results of microp r o b e analyses, a n d a detailed discussion of the m i n e r a l o g y of volcanic rocks from F e r n a n d o de

131 REE fractions were also run on triple filaments [19]. For Pb-isotopic analyses, separate splits of sample powders were dissolved in H F + HBr, and Pb was separated on anion exchange columns [22] based on the technique of Strelow and Toerian [23]. Abundances of U and Pb were determined by isotope dilution using a spike enriched in 2°2Pb, 233U, and 2~6U. Lead was loaded in H3PO 4 and Si gel on single Re filaments, and U in HzO on Ta side filaments of a triple filament assembly as for Nd. Total blanks for Rb ( - 5 ng), Sr ( - 5 ng), Sm ( - 0 . 2 ng), Nd ( - 0 . 3 ng), U ( - 0 . 1 ng) and Pb ( - 0.8-1.2 ng) were considered negligible.

32o12,j'W

KEY

ILHAR ~ m

Seo Jose Fm

V•

O u , x l b j Fm Remed,ol Fm

i-

~

__~

~

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szke4,btss~t, ~ m ~ t ~ t Q t . leo~r~es, trachytes

~

ql)

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0~, ~ , ~

32o28'W I

~b

DENORONHA

32o26" I

i

A

Fig. 1. Simplified geologic map of Fernando de Noronha and

associated islands, adapted from de Almeida [12]. 1000-m bathymetric contours are shown in inset location map. Noronha will be presented elsewhere. Most samples analyzed in this study are aphyric to sparsely porphyritic and display no evidence of alteration. Ankaratrites. nephelinites, and melilites contain phenocrysts of olivine and rare apatite, and melilite and nepheline in the groundmass. Ankaratrites are distinguished by minor amounts of mica in the groundmass. Alkali basalts contain clinopyroxene phenocrysts in addition to olivine phenocrysts.

2. Analytical methods Sample powders were dissolved in H F + H N O 3, redissolved after drying down in HCI, and checked for precipitate after centrifuging. Fractions containing Rb, Sr, and L R E E were separated on cation exchange columns and abundances determined by isotope dilution using spikes enriched in 87Rb, 84Sr, 149Sm, and 145Nd [18,19]. Unspiked L R E E fractions were processed to obtain Nd on columns described by Richard et al. [20] based on the technique of Cerrai and Testa [21]. Strontium was loaded in H3PO 4 on single Ta filaments, and Nd was loaded in HzO on Ta side filaments of a triple filament with a Re center filament. Spiked

3. Results Fernando de Noronha lavas are intermediate with respect to their Sr-, Nd-, and Pb-isotopic compositions when compared to lavas from other ocean islands (Figs. 2, 3). Initial 87Sr/S~'Sr ratios of the Fernando de Noronha suite range from 0.70365 to 0.70485, and the samples can be divided into two groups on the basis of a gap (0.70418-0.70457). The group with less radiogenic ~TSr/~6Sr (0.70365-0.70418) consists of highly alkaline, Si-undersaturated lavas, except for sample 36, an alkali basalt of the S'ao Jose Formation (Table 2). The second group with more radiogenic 87Sr/86Sr (0.70457-0.70485) consists mainly of less alkaline lavas ranging from alkali basalts to trachytes of the Remedios Formation, with the exception of sample 76, a basanite. Neodymium(14~Nd/144Nd = 0.512773-0.512897) and Pb-isotopic (2°6pb/2°4Pb = 19.317.-19.565) compositions of the higihly alkaline, Si-undersaturated lavas overlap with those of alkali basalts and trachytes (14~Nd/]44Nd = 0.512712-0.512811, 2°6pb/2°4Pb = 19.132-19.425). Most samples analyzed are characterized by relatively low R b / S r ratios such that age corrections are insignificant. However, STSr/86Sr ratios measured in phonolites range from 0.703890 + 8 to 0.734987 + 40, and define a Rb-Sr isochron of age 9.1 _+ 0.2 Ma. The initial ratio (0.70384) is within the range observed in the other highly alkaline, Si-undersaturated lavas. The Rb-Sr isochron age is comparable to those reported by Cordani [14] based on K / A r results (Table 1). If the phonolites are all approximately contemporaneous, these results verify that crystal-liquid sep-

132 "FABLE 2 Sr and N d isotopic c o m p o s i t i o n s and selected trace element a b u n d a n c e s in s a m p l e s from F e r n a n d o de N o r o n h a Sample

Rock type

Rb

Sr

SVSr/S6Sr "

SVSr/S6Sro

Sm

Quixaba Formation 33 ankaratrite 31 ankaratrite 18 melilitite 106 ankaratrite 74 ankaratrite 98 ankaratrite 2(1 melilitite

52.6 50.2 57.2 40.4 33.2 43.6 56.2

960 1038 1438 985 1118 1222 163,1

0.70378 +- 2 0.70383+--2 0.70388 +_ 1 0.70386 _+ 1 0.70395 + 1 0.70387 ± 1 0.70397 + 1

0.70377 0.711382 0.70388 0.70386 0.70395 0.70386 0.70396

11.1 12.9 13.5 12.4 13.2 15.5 16.2

S~o Jose Formation 36 alkali b a s a l t

26.5

769

0.70390+ 1

0.70389

314 337 258 218 229 204 163

3.4 25.4 35.8 120 332 762 1652

0.73499 4- 4 0.70873 + 2 0.70685 + 1 0.70455 +- 1 0.70404 + 1 0.70397 + 1 0.70389+-- 1

177 65.2

705 1162

95.7 45.7 122 142

Nd

143Nd/laaN d ~' •

~:N d

58.8 69.1 73.9 66.1 72.0 85.2 91.4

0.512897-i-. 15 0.512851 +- 18 0.512822_+ 16 0.512851 + 11 0.512797 +- 10 0.512817 + 16 0.512812+-- 16

5.1 4.1 3.6 4.1 3.1 3.5 3.4

9.71

48.8

0.512871 +- 16

4.6

0.70365 0.70410 0.70385 0.70378 0.70386 0.70385

0.57 2.71 1.13 1.67 1.63 4.39 12.3

5.44 23.9 11.1 15.4 14.1 32.7 78.1)

0.512838 + 16 0.512845 + 18 0.512848 +- 16 0.512829 * 16 0 . 5 1 2 8 3 7 ± 14 0.512830 +- 20 0.512840 ± 15

3.9 4.0 4.1 3.7 3.9 3.8 3.9

0.70389 + 1 0.70399 +_ 1

0.70379 0.70397

2.07 7.78

16.0 45.2

0.512849 ± 15 0.512849+-- 15

4.1 4.1

1635 859 1741 1350

0.70406 +_ 1 0.70410 _*_1 0.70380+_ 1 0.70399 +- 1

0.70404 0.70407 0.70377 0.70394

10.4 10.4 12.3 10.7

62.4 55.1 73.9 65.2

0.512877+- 8 0.512836 z 17 0.512896 ++-20 0.512828+-- 18

4.7 3.9 5.0 3.7

75.0

1777

0.70420 + 1

0.70418

12.0

71.6

0.512798 _+ 15

3.1

27.2 178 206 178 141 22.0 65.1 31.7

1597 921 750 457 1084 1549 830 567

0.70386 + 1 0.70467 +- 1 0.70474+-- 1 0.70475 + 1 0.70491 + 1 0.70466 5:1 0.70461 + 1 0.70467 + 1

0.70386 0.70458 0.70462 0.70458 0.70485 0.70465 0.70457 0.70464

18.9 4.58 3.10 4.46 13.3 15.4 13.5 8.48

108 38.7 26.5 37.9 80.4 85.9 70.1 39.6

0.512831 + 0.512780+ 0.512786 + (1.512754 + 0.512712 + 0.512773_+. 0.512811 + 0.512811 +

3.8 2.8 2.9 2.3 1.4 2.6 3.4 3.4

No.

Remedzos Forrnatzon 87 phonolite 99 phonolite 32 phonolite 103 phonolite 29 phonolite 90 phonolite 54 phon. tephrite 101 syenite inclusion 72 essexite 58 phon. nephelinite 71 basanite 93 tephrite 104 basanite 79 nepheline basanite 96 cpx + hb inclusion 5 trachyte 19 trachyte g0 trachyte 84 mugearite 76 basanite 10 alkali basalt 25 alkali basalt

16 13 17 15 16 13 20 19

a N o r m a l i z e d to SeSr/SaSr = 0.1194 a n d to E&A s t a n d a r d STSr/S6Sr = 0.70800. Errors represent in-run precision (i.e., 20 of the mean) and c o r r e s p o n d to least significant digits. Initial Sr-isotope ratios (SVSr/a6Sr0) calculated are based on ages of s a m p l e s e s t i m a t e d from T a b l e 1. ~' N o r m a l i z e d to l ~ N d / l ' ~ N d = 0.7219 and to BCR-I s t a n d a r d 1 4 3 N d / t ' U N d = 0.51263. Errors reported as above.

aration processes which produced the range of R b / S r ratios in the phonolites were sufficiently rapid to enhance development of an isochron. 4. Discussion

4.1. Correlation of magmatic series and isotopic compositions The distinctions between the alkaline (higher

SVSr/S6Sr, lower range of 143Nd/144Nd) and the more alkaline, Si-undersaturated (lower 87Sr/86Sr, higher range of 143Nd/t'~Nd) lavas of Fernando de Noronha are roughly comparable to distinctions or trends observed among Hawaiian tholeiitic, alkalic, and post-erosional nephelinitic lavas [1.-3, 24-26]• The temporal trend to more alkaline and Si-undersaturated lavas on Fernando de Noronha is also similar to late stages of Hawaiian

133

FERNANDO DE NORONHA

.5132-

M 0 R B

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hllwiiltell to trachytell alklltbmsatta

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.5128"

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[ .7030

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I .7040 87

'

Sr/

l .7050 86

I .7060

I .7070

Sr

Fig. 2. 1 4 3 N d / l ~ N d vs. 87Sr/86Sr in selected ocean islands [1-4,6,7,11,24-26.31,32,37-40,43,45].

TABLE 3 Pb isotopic compositions of samples from Fernando de Noronha Sample

Rock type

U

Pb

2o6Pb/204 Pb

207Pb/204 Pb

20s Pb/2o,, Pb

No.

Quixaba Formation 33 31 106 74 98

ankaratrite ankaratrite ankaratrite ankaratrite ankaratrite

1.89 2.05 2.49 2.34 2.82

2.90 2.61 2.64 4.63 2.83

19.317 19.354 19.445 19.470 19.473

15.599 15.623 15.647 15.648 15.636

39.077 39.230 39.448 39.493 39.414

1.85

3.09

19.425

15.626

39.290

7.70 11.99 5.20 4.91 4.89 2.38

19.565 19.559 19.565 19.553 19.280 19.145 19.507 19.199 19.202 19.132

15.642 15.657 15.652 15.663 15.595 15.571 15.683 15.620 15.625 15.569

39.357 39.450 39.466 39.481 39.187 39.054 39.602 39.139 39.144 38.940

S~o Jose Formation 36

alkali basalt

Remedies Formation 54 72 104 79 80 84 76 10 25

phon. tephrite essexite basanite nepheline basanite trachyte mugearite basanite alkali basalt alkali basalt

4.19 5.45 2.56 2.17 2.21 0.99

-

Pb-isotope analyses are normalized for mass discrimination based on analyses of NBS SRM 981 and reproducibility is estimated to be better than 0.05% a.m.u. Duplicate isotope dilution analyses of U and Pb in sample 10 were performed with different spike and sample dissolutions.

134

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207 P b / 2 0 4 p b

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15.4 !

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Fig. 3. 2°8pb/2°4pb and 2°7pb/2(~pb vs. 2°6pb/2~u~Pb in selected cv,:ean islands [2-6,11,24,31,33,37,38,41,42]. The Northern Hemisphere Reference Line (NHRL) is that of Hart [9].

volcanism, although de Almeida [12] suggested that the early alkali basalt dikes appeared to be contemporaneous with various alkaline, lamprophyric intrusive rocks. It is notable, however, that the youngest rocks of Fernando de Noronha (e.g., phonolites of the Remedios Formation and

ankaratrites, nephelinites, and melilitites of the Quixaba Formation) are more alkaline, more Siundersaturated, and are characterized by less radiogenic Sr-isotopic and slightly more radiogenic Nd-isotopic compositions than the earlier lavas and hypabyssal rocks of the Remedios Formation•

135

4.2. lsotopic and trace element systematics of Quixaba lavas Ankaratrites and melilitites of the Quixaba Formation display a limited range in their isotopic compositions (Figs. 2, 4), and are nearly homogeneous in their major element compositions (Stormer et al., unpublished data). The Sr-, Ndo, and Pb-isotopic compositions of the Quixaba lavas are correlated; the sample with the most radiogenic Sr-, and Pb-isotopic composition is characterized by the least radiogenic Nd-isotopic composition, and vice versa. These variations are not likely to be the result of high-level crustal interactions. The mafic and grossly similar major element compositions ( M g # =62-69) and elevated Sr abundances relative to most samples analyzed from the most likely contaminant, the older Remedios Formation, suggest that assimilation-fractional crystallization models are not reasonable. Consequently, these variations in isotopic compositions and trace element abundances in the Quixaba iavas should have implications for their source characteristics and petrogenesis.

Abundances of most incompatible trace elements (e.g., Ba, U, Nb, La, Zr) in the Quixaba lavas are intercorrelated, and some abundance ratios such as Z r / N b are constant (Fig. 5). Other abundance ratios such as Sr/Nd (14.3-19.5) and N d / S m (5.27-5.65) vary, and are correlated with abundances of Sr (960-1641 ppm), Nd (58.8-91.4 ppm), and Sm (11.1-16.2 ppm). Other ratios such as Ba/Nb (Fig. 5) and Rb/Sr also vary but are anticorrelated with 875r/S6Sr and 2°6pb/2(~Pb ratios (Fig. 6) and with abundances of Ba and Nb, and Rb and Sr, respectively. The isotopic and trace element variations within the Quixaba lavas are consistent with two geochemically distinct components in their genesis. The close association of the Quixaba lavas in space and time may not support a significant amount of lateral or vertical separation of distinct source components; the implied scale of geochemical source heterogeneity would then be on the order of a few kilometers or less. In subsequent discussion, we evaluate two models which may explain the small-scale source heterogeneity im-

.7060

/

JTr,stan

I Marauesas

.7050

I~

/

Fernando de N o r o n h a

8rSr/aSSr .7040

rerce,ra ( A z o r e s )

f"

.7030

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plied by geochemical variations in the Quixaba lavas. The trace element and isotopic variations of the Quixaba lavas may be consistent with variable melting of a mixed source consisting of a depleted upper mantle veined by material which is not in isotopic equilibrium with the host. Assuming that the melilitites (samples 18 and 20) represent the

smallest degree of partial melting in the suite, the hypothetical vein material would have relatively radiogenic St-isotopic compositions and be the least refractory component in the mixed source. Although this model would explain the apparent mixing trends in isotopic compositions and trace element abundance ratios in the Quixaba lavas (Fig. 6), it also requires that the veins have a low Rb/Sr ratio relative to the host matrix. However, the low Rb/Sr ratio may be only an apparent characteristic if a Rb-bearing phase such as phlogopite, in wich Rb is compatible, is residual during melting. Phlogopite in metasomatized xenoliths (of. [27]) is characterized by high Rb/Sr ratios (# 1.5) and clinopyroxene by low Rb/Sr ratios (~< 0.01). The bulk St-isotopic composition and Rb/Sr ratio of a veined or metasomatized source would largely depend on the proportions of phlogopite and clinopyroxene both in the residue and entering the melt, since the other major phases, olivine and orthopyroxene, are one to two orders of magnitude lower in Rb and Sr [27]. The XTSr/S~'Sr and Rb/Sr ratios of a phlogopite-veined source will be intermediate relative to the same ratios in the clinopyroxene and the phlogopite. Melts produced from such a source will also have intermediate ratios Rb/Sr and SVSr/X6Sr relative to those in phlogopite and clinopyroxene. This is based on two assumptions, that (1) at least part of the initial phlogopite is residual and not in isotopic equilibrium with the host matrix, and (2) that for phlogopite, Dsr << 1 and DRh >> 1, and for clinopyroxene, both Dsr and DRb << 1. Melts generated by varying degrees of partial melting or by a series of successive melt extraction episodes may display correlated ~7Sr/~6 Sr and Rb/Sr ratios. As shown in Fig.7, the Quixaba lavas display a trend which is transverse to any mixing curves between clinopyroxene and phlogopite and this indicates that this trend may be largely controlled by varying amounts of residual K-richterite in a metasomatized source. For isotopic disequilibrium to be maintained between hydrous veins and host matrix, melt extraction processes need to have been rapid since the presence of melt facilitates equilibration of veins and host material [28], and the metasomatism of the veined source may have occurred in recent geologic time. A variation of the above model is that multiple metasomatic events may have produced two types

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grees of partial melting may explain the variations in the Quixaba lavas given appropriate geochemical characteristics of the two types of veins. Ankaratrite sample 33, with the highest Rb/Sr and lowest 8 7 S r / 8 6 S r ratios, may have been generated by partial melting of a veined source with residual K-richterite. Melilitite sample 20, with the lowest Rb/Sr ratio and highest 8 7 S r / 8 6 S r ratios and highest abundances of incompatible trace elements, may have been generated by relatively smaller degrees of partial melting of a veined source with both residual phlogopite and clinopyroxene, to account for an intermediate composition with respect to clinopyroxene and phlogopite, as suggested in Fig. 7. Mixing during ascent of extreme magmas, represented by samples 20 and 33, may explain the variations in the Quixaba lavas. An alternative model that explains the apparent mixing trends (Fig. 6) is a variable or progressive contamination of geochemically distinct magma during ascent through the upper mantle and lithosphere. Considering extreme samples in the Quixaba lavas, if sample 33 (ankaratrite) is as-

138 sumed to be the least modified, and sample 20 (melilitite) the most modified, the effects of this contamination process include increased abundances of Sr and incompatible trace elements, an increase in 87Sr/86Sr and Z°6pb/l°4pb ratios, and lower 143Nd/144Nd ratios. Additional effects include changes in trace element abundance ratios, e.g., decreasing S m / N d (0.1896 to 0.1769), B a / L a (12.6 to 9.0), and B a / N b (10.3 to 7,3), whereas other ratios (e.g., N b / U , Z r / N b ) are constant (Figs. 5, 6). If contamination occurred through the addition of small-degree partial melts from wallrocks, melts from a depleted MORB-source mantle wallrock would have lower B a / L a , B a / N b , 2o6Pb/204 Pb, and 8VSr/86Sr ratios relative to sample 20. The alternative case may be more geologically reasonable, i.e., if sample 33 represents the most contaminated or modified magma and sample 20 the least modified, the effects listed are reversed. The Quixaba data trends may be explained as a result of contamination of magma (represented by sample 20) by re-equilibration with, or addition of small-degree partial melts from a depleted MORB source with less radiogenic Sr and Pb, and this is consistent with the variation in S m / N d ratios. However, B a / N b , B a / L a , and R b / S r ratios in melts from depleted wallrock may be lower than those in sample 20 unless MORB-source melts added were produced by very small degrees of partial melting, assuming bulk DBa < ONb, Dt~,~< DI,~, and ORb < Ds,, or were removed from residue which included residual phases with high Ko's for Sr, Nb, and REE. The most problematic characteristics of the Quixaba lavas include inverse correlations of R b / S r , B a / N b , and B a / L a with St, Ba, Nb, and La abundances, and with 8VSr/86Sr and 2°6Pb/ 2°4pb ratios. The above models may explain these variations, although a model involving metasomatized sources seems more viable, based on our present inferences regarding mantle phase assemblages and abundances and relative incompatibility of trace elements in the mantle. In any case, the range of isotopic compositions of the Quixaba lavas corresponds to a minimum contrast required between a matrix and metasomatic veins, between two metasomatic veins, or between enriched magma generated at depth and wallrock contaminant. The two distinct components required to explain variations in the Quixaba lavas

include (1) material characterized by relatively less radiogenic Sr- and Pb-isotopic compositions and relatively lower abundances of incompatible trace elements possibly derived from depleted MORBsource type mantle, and (2) a component with more radiogenic Sr and Pb, less radiogenic Nd, and possibly higher abundances of Sr, REE, and high field strength trace elements (e.g., Nb) relative to Rb and Ba. 4.3. Isotopic and trace element characteristics oj Remedios and S~o Jose lavas Most lavas and intrusive rocks of the Remedios Formation evolved principally be fractional crystallization (Stormer et al., in preparation), rendering their trace element abundances and abundance ratios less diagnostic in interpreting source characteristics. Thus, the trace element and isotopic characteristics of only the most marie samples (basanites 71, 76, 79, and alkali basalts 10, 25, 36) will be used to constrain magma sources tapped during an earlier period in the evolution of Fernando de Noronha. The Sr-, Nd-, and Pb-isotopic compositions of samples 36 (S~o Jose alkali basalt) and 79 (Remedios basanite) are within the range of the Quixaba lavas and other alkaline, Si-undersaturated Remedios volcanics (Tables 2 and 3, Fig. 4). Sample 76 (basanite) is similar in St- and Nd-isotopic composition to Remedios alkali basalts (samples 10 and 25) but has more radiogenic Pb. It is the alkali basalts (10, 25) with associated evolved lavas (5, 19, 80, 84) which have the least radiogenic Pb-isotopic compositions of the Fernando de Noronha suite, and thus reflect a third, isotopically distinct component. The Remedios alkali basalt-to-trachyte group is distinct and isolated in most projections of isotopic composition space (Figs. 2, 4), and no intermediate samples are evident which link this group with the highly alkaline, more Si-undersaturated rocks of the Remedios Formation. Thus, sources of Remedios alkali basalts may have been separated from those of more alkaline Remedios iavas, or at least there is no significant evidence of mixing of magmas after separation from their respective sources despite close spatial proximity. In contrast to sources of Remedios alkali basalts and trachytes, those of Remedios nephelinitic to phonolitic lavas and S:~o Jose alkali basalt are

139 similar in isotopic composition to sources of the later Quixaba lavas. No single group or sublinear trend is evident in Sr- and Nd-isotopic compositions of the Remedios nephelinites, basanites, tephrites, essexites, and phonolites. The scatter in the data for these rocks suggests the derivation of their parental magmas from multiple sources with isotopic compositions overlapping the range displayed by the later Quixaba lavas (Fig. 2). Relative differences in the trace element characteristics of the sources may be inferred from trace element abundance ratios in the lavas. For example, Remedios alkali basalts (10, 25) are characterized by generally higher Z r / N b (6.5-6.7), L a / N b (0.91-0.95), and C e / N b (2.0) ratios compared to Sao Jose alkali basalt, Remedios basanites, and Quixaba lavas ( Z r / N b = 3.5-4.5, L a / N b = 0 . 6 7 - 0 . 9 4 , C e / N b = 1.4-1.8). These abundance ratios are not easily changed except at very small degrees of partial melting or by means of a residual phase which has a high K d for Nb. The alkali basalts and basanites display N b / U ratios (24-38) which are lower than those of the Quixaba lavas (40-48). Hofmann et al. [29] suggest that Nb and U in the mantle are similar in their degree of incompatibility based on similar N b / U ratios (average 47 + 9) among MORB and ocean island basaltic samples. If the contrasts in certain trace element abundance ratios (e.g., B a / N b , Z r / N b , S m / N d ) between the various rock groups are due to varying degrees of partial melting of their respective sources, then the Remedios alkali basalts were generated by relatively larger degrees of partial melting than the alkaline Remedios and Quixaba lavas. If so, N b / U ratios may also vary to some extent during melting. If not, however, lower N b / U ratios in the alkali basalts and basanites compared to those in the Quixaba lavas may suggest that sources of the alkali basalts and basanites are enriched in U, lower in Nb, or contain a residual phase with Dub > 1, in contrast to sources of the Quixaba iavas. 5. M a n t l e s o u r c e s

5.1. Identification In general, extreme isotopic compositions in various oceanic basahs have been interpreted to indicate the presence of various isotopically ex-

treme mantle components (e.g., H I M U , EMI, EMIl [10]). Most interpretations of oceanic basalts from single localities, areas, or islands, which show isotopic variations suggestive of mixing, assume that the mixing was a relatively recent event. The trends of data arrays are thus interpreted as indicating present-day isotopic compositions of extreme mantle end-members or components. If, however, mixing of source end-members preceded magma genesis by a significant amount of time, isotopic compositions of two distinct mixed sources, each a result of mixing of the two distinct end-members, will have intermediate p a r e n t / daughter ratios relative to each of the original, more extreme end-members. Intermediate mixed sources will evolve along radiogenic growth curves which may be different than those followed by the original extreme end-members. Thus, a two-component mixing array defined by a series of isotopically heterogeneous lavas may not extrapolate to the present-day isotopic characteristics of the original extreme end-members, but can only constrain those of the intermediate mixed sources. Nevertheless, extrapolation of data trends aid in speculation on the numbers, isotopic compositions, and origins of distinct extant mantle components. Processes such as metasomatism or deep crustal recycling via subduction have been proposed as means by which distinct mantle components were created, and these processes may have been continuous throughout geologic time. The present-day isotopic compositions of mantle components depend on the time-integrated details of these processes in addition to their p a r e n t / daughter ratios. It is therefore likely that mantle heterogeneities will display a continuum of isotopic compositions, rather than a few extreme compositions. Even though the isotopic data base for OIB and MORB has expanded in recent years, a sampling problem may exist, and this has led to the tentative identification of a few end-members with extreme fixed isotopic compositions and the working hypothesis that OIB and MORB with intermediate isotopic characteristics are derived from sources created by recent mixing of two or more of a limited number of distinct mantle endmembers. Based on previous discussion, three isotopically distinct end-members or components are required in the sources of Fernando de Noronha volcanic

140

and hypabyssal rocks. These include: (1) a component with more radiogenic Sr (87Sr/86Sr/> 0.7050) and less radiogenic Nd (143Nd/l~Nd~<0.5127) and Pb (2°rPb/2°4pb~< 19.1), (2) a component with less radiogenic Sr ( 8 7 8 r / 8 6 S r ~ < 0.7039) and more radiogenic Pb (2°6pb/2°4pb/> 19.6), and (3) a component with less radiogenic Sr (~7Sr/"rSr ~< 0.7036) and Pb (2°rpb/2°nPb~< 19.3) and more radiogenic Nd ( 1 4 3 N d / l ~ N d >/0.51290). Source component (1) above may be similar to that suggested for extreme samples from Kerguelen (Fig. 4). The isotopic characteristics of Kerguelen volcanics with the highest SVSr/SrSr and lowest 143Nd/l~Nd ratios have been attributed to.an enriched component, EMIl [10], which may have originated by reinjection of oceanic crust, continental crust, or delaminated subcontinental lithosphere into the mantle [10,30,31]. Sources of the alkali basalts and trachytes of Fernando de Noronha may contain an EMIl component. However, all Fernando de Noronha lavas analyzed lack significant enrichment in 2°sPb as observed in samples from Kerguelen and Walvis Ridge (Fig. 8). If data for Fernando de Noronha samples in Fig. 8 reflect mixing of at least two distinct components, these components have had similar time-averaged T h / U ratios, each of which were lower than those of enriched components in Kerguelen and Walvis Ridge magma sources. This indicates that a spectrum of enriched mantle components exists in addition to those indicated by extrapolating the ranges of isotopic compositions of Kerguelen, Samoa and Walvis Ridge lavas. Source end-member (2) may represent a component with high U / P b and low R b / S r such as that postulated to dominate St. Helena, Tubuaii, and Mangaia magma sources [6,32,33]. The origin of this type of component, termed HIMU, may be linked to a deep-mantle metasomatic process or recycling of old altered oceanic crust initially enriched in U by addition from seawater during hydrothermal alteration [34] and subsequently depleted in Rb and Ba relative to high field strength trace elements (HFSE) and Sr. However, if discrete units of recycled oceanic crust with enhanced U / P b ratios are very old (several 100 Ma), in situ growth of radiogenic Pb would likely result in A7/4 values in this component which are significantly higher than A7/4 values observed in lavas from HIMU ocean islands. Nevertheless, a HIMU

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!43 N d / / 1 4 4 N d Fig, 8. Variations in A7/4 and A8/4 (cf. [9]) vs. 14:~Nd/l~Nd in selected ocean islands, t l = Hawaiian Islands [2,11,241, U = Ua Pou, Marquesas Islands 144], S = Samoa [32,45l, M = Mangaia [32], T = Tubuaii [6], TC = Tristan da Cunha [31], K = Kerguelen [38], W = Walvis Ridge [37], P = Parana [36], and C V = Cape Verdes (Gerlach, unpublished data). The acronyms " H I M U " , ""EM .... EMI", " E M i l " , and '" DM" indicate possible compositions of enriched (EMI. EMIl). high-y (HIMU), and depleted mantle (DM) components as suggested by Zindler and Hart [10].

end-member for the sources of Fernando de Noronha lavas may be implied by the more radiogenic Pb-isotopic compositions of some of the Quixaba lavas and Remedios basanites (Figs. 4, 7). It has been suggested that a HIMU source of St. Helena lavas contains excess Nb, resulting in low B a / N b and L a / N b ratios in lavas relative to those of most other Atlantic ocean islands [35]. This may be consistent with relatively lower B a / N b ratios and Z r / N b ratios in the Quixaba lavas relative to the Remedios alkali basahs as discussed earlier. The two major groups of Fernando de Noronha rocks, the alkali basalt-trachyte group and the highly alkaline, Si-undersaturated series rocks of the Remedios and Quixaba Formations, may have been derived from mixed sources consisting of

141

different proportions of a HIMU-dominated endmember and an enriched EM-type end-member. The steep slope of the Pb data (Fig. 3) and the variation in A7/4 in samples with less radiogenic Nd (Fig. 8) suggests that the HIMU-dominated component may have been relatively restricted in Pb-isotopic composition, whereas an EM component may have been variable in A7/4 or more than one EM-type component contributed to the magma sources of Fernando de Noronha lavas with low 143Nd/144Nd and high 87Sr/86Sr ratios. This range in A7/4 must be an "old" feature because > 97% of the available 235U has been depleted for the last 0.5-1.0 Ga. Source component (3) may have some affinity with depleted upper mantle and is indicated as a possible component to explain the variation in Quixaba lavas (e.g., sample 33) towards less radiogenic Sr- and Pb-isotopic compositions (section 4.2, Figs. 4, 6). The isotopic variation in the Quixaba lavas is consistent with their derivation from sources consisting of variable amounts of a HIMU-dominated component and a depleted mantle component, both of which may reside in veins created by metasomatism and autometasomatism, respectively.

5.2. Origin of mantle source components The isotopic compositions of the Remedios alkali basalts and trachytes may be linked to an enriched component which has experienced a long-term depletion of U relative to Rb. This type of component has been suggested to be derived from subcontinental lithosphere [10,30,31]. With regard to Fernando de Noronha, this component may consist of a portion of Brazilian subcontinental lithosphere which was delaminated during initial rifting of this portion of the Atlantic Ocean approximately 120 Ma ago. This mechanism was proposed by Hawkesworth et al. [36] to explain a high 87Sr/86Sr, high A7/4, low 2°~pb/2°apb ("Dupal", [9]) component apparent in certain South Atlantic islands. If subcontinental lithosphere was present in the sources of Remedios alkali basahs and trachytes, lower Ba/Nb and higher Z r / N b and L a / N b ratios in these rocks relative to other Fernando de Noronha lavas may indicate that this component was depleted in incompatible elements (e.g., Ba, U, K, Nb). This is further suggested by the relatively smooth, chon-

drite-normalized trace element patterns of Remedios alkali basalts (Fig. 9). In contrast, Quixaba lavas display a slight enrichment in Nb and a marked deficiency in K (Fig. 9). Basanite 76, which has a similar Sr- and Nd-isotopic composition though more radiogenic Pb relative to Remedios alkali basalts, also has a K deficiency (Fig. 9). Thus, all Fernando de Noronha lavas with relatively radiogenic Pb (2°6pb/2°4pb >/19.3), regardless of their Sr- and Nd-isotopic compositions, are characterized by a K deficiency. If these lavas with more radiogenic Pb-isotopic compositions were derived from a source with a significant HIMU component, as discussed earlier, this component may be at least locally linked with a trace element signature of excess Nb and deficient K. Excess Nb is suggested to be a feature of lavas from Atlantic ocean islands such as St. Helena and Ascension which have relatively radiogenic Pb-isotopic compositions, although only St. Helena lavas display any

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142 suggestion of a deficiency in K [35]. In Fernando de Noronha magma sources, low B a / N b ratios and apparent K deficiencies may result from residual hydrous metasomatic phases during partial melting. Since both the phase assemblages and the extents of melting in the respective magma sources of ocean islands may vary, isotopically distinct source components may only be associated with unique trace element characteristics locally and not in general among all ocean islands. Additional and more detailed combined trace clement and isotopic studies of ocean islands are necessary to determine if local variations in trace dement characteristics (e.g., ratios of B a / N b , etc.) are minor relative to overall ranges observed in ocean island volcanic rocks. Isotopic signatures of extreme mantle heterogeneities are apparently preserved in the mantle despite convective mixing and possible repeated episodes of melt exctraction. Oceanic crust ( + sediment) reinjected into the mantle [34] will probably be depleted in LIL elements by dehydration during subduction and repeated melt extraction in the mantle. High B a / N b , Ba/La, or L a / N b ratios expected in sedimentary components may not be preserved. Lavas from Atlantic islands such as Gough Island. which display high B a / N b . Ba/La. and L a / N b ratios relative to those of other ocean island lavas, have been interpreted as evidence of a sedimentary' component in their sources [35]. Instead, these lavas and their sources may have inherited both their trace element and isotopic characteristics from an end-member consisting of delaminated subcontinental lithosphere (of. [36]). Additional local variations in trace clement ratios may be imposed by residual phases during melting and magma genesis. If HIMU-dominated components have been continually reinjected into the mantle by subduction throughout geologic time, they should be more continually available in the magma sources of OIB and MORB. EM-type components, if derived from delaminated subcontinental lithosphere, occur by chance or possibly only during rifting. Although the isotopic data set for ocean islands is yet incomplete, there are no indications in the distribution of the data that HIMU components are more prevalent than EM components in ocean island magma sources. Based on combined trace element and isotopic considerations in this

study and others [32], we would suggest that ItlMU components may have more recognizable geochemical signatures, including radiogenic Pb and excess Nb, whereas enriched components, whether derived from recycled crust [31,34,35] or subcontinental lithosphere [36], are variable in their trace element and isotopic characteristics. 6. Summary and conclusions (1) Volcanic and hypabyssal rocks of Fernando de Noronha vary in composition with time: the earliest samples belong mainly to an alkaline magma series and the youngest lavas are more alkaline, highly Si-undersaturated rocks. The two rock series are largely distinguished by differences in isotopic composition. Alkali basalts and trachytes generally display more radiogenic Sr-isotopic compositions and less radiogenic Pb- and Nd-isotopic compositions relative to the more alkaline Si-undersaturated rocks. 12) Geochemical characteristics of the youngest lavas, the Quixaba ankaratrites and melilitites are consistent with mixing of magmas derived from two geochemically distinct sources. One source is characterized by more radiogenic Pb, possibly a IIIMU-dominated signature, whereas the other source component with less radiogenic Sr and Pb may ultimately be derived from depleted mantle. Models of variable melting of enriched metasomatic veins in a depleted host matrix, mixing of magmas derived from two types of metasomaticaily veined sources, or contamination of enriched magmas from a HIMU source by partial melts from depleted mantle may explain the variations in the Quixaba lavas but require small degrees of partial melting or control of certain trace element ratios by residual phases. (3) In addition to depleted mantle and HIMU components, the trace element and isotopic characteristics of the alkali basalt-trachyte group suggest the influence of a component which may be recycled crust. This component may have been derived from the Brazilian subcontinental lithosphere and introduced into the upper mantle during rifting of the Atlantic Ocean. The later Quixaba lavas with more radiogenic Pb- and Nd-isotopic compositions, reflect the increasing effect of a HIMU component with time. We thus infer that the crustal or lithospheric component in the

143

sources of the alkali basalts and trachytes was gradually depleted, However, in the older Remedios Formation, lavas of both series were isotopically heterogeneous and overlap in age. This indicates that the scale of mantle source heterogeneities associated with lavas of Fernando de Noronha was relatively small, on the order of only a few kilometers.

Acknowledgements The guidance, technical support, and camaraderie of P. Guise, R. Green, G. Davies, R. Cliff, and D. Rex are gratefully acknowledged. D. James performed XRF analyses at Edinburgh University. Additional thanks to Bully, Scram, and Fossil. The second author (J.C.S.) thanks A. Mello for assistance during fieldwork. This study was partially funded by N.S.F. grant GA-40565, and fieldwork was carried out while J.C.S. was a Visiting Professor at the Instituto de Geociencias, Sao Paulo, Brazil. This paper was improved with suggestions from C.-Y. Chen, S. Shirey, P. Castillo, F. Tera, and an anonymous referee. D.G. acknowledges support during this study by a postdoctoral fellowship at the University of Leeds. References 1 c.-Y. Chen and F.A. Frey, Trace element and isotopic geochemistry of lavas from Haleakala Volcano, East Maul, Hawaii: implications for the origin of Hawaiian basalts, J. Geophys. Res. 90(B10), 8743-8768, 1985. 2 P. Stille, D.M. U n r u h and M. Tatsumoto, Pb, Sr, Nd, and Hf isotopic constraints on the origin of Hawaiian basahs and evidence for a unique mantle source, Geochim. Cosmochim. Acta 50, 2303-2320, 1986. 3 H.B. West and W.P. Leeman, Isotopic evolution of lavas from Haleakala Crater, Hawaii, Earth Planet. Sci. Lett. 84, 211-225, 1987. 4 D. Weis, Pb isotopes in Ascension Island rocks: oceanic origin for the gabbroic to granitic plutonic xenoliths, Earth Planet. SCi. Lett. 62, 273-282, 1983. 5 B. Dupr& B. Lambert and C.J. All~gre, Isotopic variations within a single (x:eanic island: the Terceira case, Nature 230, 620-622, 1982. 6 P. Vidal, C. Chauvel and R. Brousse, Large mantle heterogeneity beneath French Polynesia, Nature 307, 536-538,

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