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Pb isotopes in Ascension Island rocks: oceanic origin for the gabbroic to granitic plutonic xenoliths
Earth and Planetary Science Letters, 62 (1983) 273-282 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
273
[5]
Pb isotopes in Ascension Island rocks: oceanic origin for the gabbroic to granitic plutonic xenoliths D. Weis F.N.R.S. Charg~ de Recherche, Laboratoires Associ~s de G~ologie- Pktrologie, Universitb Libre de Bruxelles, 50 A venue F.D. Roosevelt, B 1050 Brussels (Belgium)
Received June 30, 1982 Revised version accepted November 15, 1982
The Pb isotopic compositions and U and Pb concentrations of the lava series (alkali basalt to comendite) and of their plutonic xenoliths (gabbro to alkaline granite) of Ascension Island are reported. The data are used to evaluate the source of the xenoliths which formed two differentiation suites: the acidic and intermediate xenoliths together with most of the lavas on the one hand, and the gabbroic xenoliths and a basaltic tuff on the other hand. The Pb isotopic compositions imply a mantle origin for the source magmas of the xenoliths and confirm the possibility of generating granitic rocks in an oceanic environment by fractional crystallization of a mantle-derived magma whose geochemical and isotopic characteristics are comparable to the source magmas of oceanic island basalts.
1. Introduction
In the oceanic environment, lavas of basic-tointermediate composition are preponderant, while acidic lavas are not very abundant. Although plutonic rocks in general are rare and, gabbroic types predominate over acidic ones. Most of the islands where acidic plutonic rocks have been found have been shown to be connected with a neighbouring continent on the basis of geophysical, petrological or isotopic data (Rockall Bank [1], Madagascar [2], Falkland Islands [3], Seychelles Islands [4]). Some of the acidic plutonic rocks resulted from the differentiation of a mantle-derived m a g m a which was subsequently contaminated by continental material [1,4]. Only the rocks of the Kerguelen Islands seem to have an origin in the oceanic mantle with no continental contribution [5,6]. The occurrence of acidic plutonic xenoliths in the lavas and tufts of Ascension Island [7] is exceptional. In this paper, Pb isotopic compositions have been analysed both in the lavas and in their plutonic xenoliths to define 0012-821X/83/t~000-0000/$03.00
the origin of the latter and their relations with the lavas. Pb isotopic compositions of Ascension lavas have already been reported by Gast et al. [8] in 1964 in their important work showing both local and regional heterogeneities in the earth mantle, as did the Pb isotope analyses of Sun [9] on young volcanic rocks from oceanic islands, mid-ocean ridges and island arcs. This latter author reanalysed four lavas already analysed by Gast et al. [8] to allow a direct comparison with other oceanic island basalt results. Ascension Island is located in the South Atlantic Ocean (7°57'S, 14°22'W), about 90 km to the west of the Mid-Oceanic Ridge and 50 km to the north of the Ascension fracture zone which shifts the ridge 230 km to the east. There are no defined relations between these major tectonic structures and Ascension Island [10]. The island has an area of 93 km 2 and a triangular shape with its biggest dimensions of 11.5 km north-south and 14 km east-west. It forms the upper part of a volcanic cone emerging from 3000 m above the ocean bot-
~3 1983 Elsevier Scientific Publishing Company
274 tom, whose summit, the "Green Mountain", reaches 859 m. Darwin [11] gave the first description of the geology while Daly [7] made an exhaustive description to which all subsequent studies [12] refer. Except for some calcareous sand banks and guano, one finds only young volcanic rocks (maximum a g e - 1.5 m.y. [13] but most of the lava flows are much younger [14]). The island stays at the level of magnetic anomaly number 6, corresponding to 7 m.y. [15]. The summits are formed by trachytic domes contrasting strongly in landscape form with basaltic flows and cinder cones which cover 85% of the island surface. The relations between the different volcanic rocks are very complicated, as both the basaltic and trachytic types have erupted at various times during the history of Ascension Island. The lavas are often quartz a n d / o r hypersthene normative and constitute a mildly alkaline (or transitional to tholeiitic) series. Petrographically, the lavas show a wide range of variations corresponding to the classical differentiation suite of oceanic island alkali basalt [14,16]; the most important members of the suite are olivine-rich basalts, basalts, hawaiites, mugearites, trachytes and comendites. Ascension Island is characterized by the presence of numerous plutonic xenoliths already mentioned by Darwin [l l] and Daly [7], comprising olivine gabbros, diorites, monzodiorites and granites [16]. These xenoliths are devoid of oriented structure or typical metamorphic minerals. Their igneous origin is demonstrated by petrographical characters such as plagioclase zonation, hypersolvus feldspar, empty acicular apatites, etc. The olivine gabbro cumulates show very comparable petrographical characters. The intermediate rocks and the granites can be divided into hyperand sub-solvus types depending on the presence of one, or two, feldspars. The monzodiorites contain hornblende. Sodic ferromagnesian minerals (aegirine and riebeckite-arfvedsonite) characterize all the granites, which may also contain the rare dalyite or vlasovite which are K- and Na-zirconium silicates [16,17]. In some acidic xenoliths traces of local partial melting are observed as thin zones of clear glass.
2. Analytical techniques An aliquot of sample powder (100-150 mg) was dissolved in a small teflon bomb with l : l HFHC104 or H F - H N O 3 mixtures. Lead was separated with anion exchange columns in a HBr-HC1 medium [18,19] (for detailed description of the procedure, see Weis [20]). Lead is loaded onto single rhenium filaments using the silica g e l / H 3 P O 4 method [21]. Total lead blanks were in the range of 2.5-3 ng and negligible compared to the sample lead. The NBS 981 lead standard [22] was routinely analysed as monitor for mass fractionation correction on the Varian MAT 260 mass spectrometer. During the study, the silica gel batch gave - 0 . 1 2 + 0.04% mass fractionation per atomic mass unit difference. The uncertainties (2o) are assumed to be better than 0.1% for the 206Pb/2o4 Pb and Z°Tpb/2°4pb ratios and better than 0.15% for the 2o8Pb/204 Pb ratios. Pb and U concentrations were determined by the isotope dilution technique and were measured successively on single rhenium filaments with a Varian MAT TH5 mass spectrometer. The precision on these concentrations is - 1%.
3. Results and discussion Only the Pb isotopic compositions of the Ascension Island samples will be considered in this paper; detailed petrographical, mineralogical and geochemical data (trace elements and Sr isotopic composition) will be published elsewhere. 2°8pb/2°4pb, 2°7pb/2°4pb and 2°6pb/2°4pb ratios, together with U and Pb concentrations, for nine lava and fifteen plutonic xenolith samples are given in Table 1. Pb isotopic compositions for all samples are given on plots of 2°8pb/2°4pb and 2°7pb/2°4pb versus 2°6pb/Z°4pb ratios in Fig. l a and b and compared to the compositions of young oceanic basaltic rocks (both oceanic island and mid-ocean ridge basalts) in Fig. 2a and b. Pb and U concentrations in Ascension Island basaltic lavas are similar to those found in alkaline basalts ( 3 . 0 < P b < 5 . 3 ppm; 0 . 8 < U < 1 . 7 ppm [23-25]). The trachytes and acidic lavas (pantellerites-comendites) are much richer in these ele-
trachyte pantellerite comendite recent basaltic flow recent basaltic flow
Asc Asc Asc Asc Asc
granite
quartz-monzodiorite
18.439 + 0.064 18.321 +0.014 18.317 + 0.007 18.744 + 0.066 19.298 _+0.062 19.678 + 0.008 19.714__+0.016 19.308 _+0.032 19.221 + 0.030 19.467 +0.009 19.165 + 0.004 19.748 _+0.042 19.698 _+0.009 19.725 4- 0.017 19.468 + 0.010 19.480_+0.008 19.496 -+ 0.012
19.403 -+ 0.028 19.656 + 0.012 19.639 + 0.010 18.500 -+ 0.005 18.498 4- 0.018 18.502 + 0.005 19.722 -+ 0.008 19.444 -+ 0.011 19.494-+0.009 19.318 -+0.010 19.180_+0.015 19.201 _+0.006
2°6pb/2°4pb
15.579 + 0.059 15.621 +0.014 15.606 + 0.009 15.638 _+_0.074 15.631 + 0.063 15.613 + 0.008 15.632_+0.017 15.618 + 0.031 15.652 + 0.031 15.607 _ + 0 . 0 1 1 15.602 _+0.005 15.658 _+0.037 15.627 _+0.009 15.639 _+0.023 15.605 _+0.011 15.601 +0.009 15.610 -+ 0.012
15.630 _+0.026 15.631 -+ 0.012 15.642 -+ 0.010 15.551 + 0.005 15.554 -+ 0.026 15.558 + 0.007 15.630 -+ 0.008 15.548 -+ 0.011 15.618-+0.009 15.601 +0.008 15.631 +0.012 15.628-+0.005
2°7pb/2°4pb
hypersolvus rocks. All Pb ratios corrected for mass fractionation ( - 1.24 + 0.44% per a.m.u.).
Asc l i b a Asc 17343 Asc 17318P a
Asc 19 a Asc 23 a ASC 24 Asc 31 a Asc 41 Asc 43
Xenoliths Asc 25 Asc 26 Asc 51 Asc 59 Asc Ib I Asc 18
gabbro
basalt olivine basalt hawaiite basaltic tuff
Lavas Asc 15150 Asc 63 Asc 64 Asc 77
la 74 38 8 10
Petrographic type
Sample
Lead isotopic compositions and U and Pb concentrations in Ascension lavas and xenoliths
TABLE 1
38.252 + 0.140 38.272_+0.035 38.261 + 0.024 38.580 + 0.184 38.937 + 0.160 39.190 + 0.021 39.253_+0.038 38.978 + 0.081 38.946 + 0.093 39.019+0.030 38.787 + 0.011 39.379 -+ 0.097 39.254 _+0.026 39.320 _+0.046 39.033 _+0.029 39.024-+0.022 39.070 -+ 0.029
38.964 -+ 0.066 39.232 -+ 0.049 39.246 -+ 0.025 38.093 4-_0.015 38.110 + 0.062 38.115 -+ 0.030 39.228 -+ 0.020 38.992 + 0.032 39.097+0.021 38.840-+0.022 38.633-+0.030 38.634_+0.014
2°spb/2°4Pb
8.62 1.52 6.15
4.77 3.79 2.23 3.45 3.77 2.03
2.58 0.28 0.22 0.13 1.67 7.81
3.72 5.07 3.75 1.46 1.65 1.58
0.89 1.79 1.81 0.69
U (ppm)
8.49 3.36 2.93
5.17 5.08 5.31 4.49 3.66 4.18
1.25 4.47 3.01 0.55 1.55 2.94
6.69 8.60 5.32 3.48 6.46 6.98
2.50 3.10 3.45 39.95
Pb (ppm)
65.64 29.25 135.82
59.49 48.04 27.14 49.31 67.20 31.60
130.10 3.95 4.60 15.05 69.43 172.87
36.17 38.05 45.63 27.12 16.34 14.32
23.16 20.76 34.10 1.40
238U/2°4pb
276
to the right of the geochron as most of the other oceanic island basalts. The data can be divided into two groups on the basis o f 2°6pb/2°4 Pb ratios. The gabbroic xenoliths and a basaltic tuff have values of - 1 8 . 5 , whereas the acidic and intermediate xenoliths, together with all the lavas including the rhyolites have values above - 19.2.
ments, indicating a progressive enrichment during the differentiation. In a U-Pb concentration diagram (Fig. 3) all data but one (sample 10) plot on a straight line passing nearly through the origin with the exception of Asc 77. In the 207Pb/204 Pb versus 2o6Pb/204 Pb diagram (Fig. 2a), all the samples, xenoliths included, plot
208Pb 204Pb 395
Ca)
39 7
" "
38.5
mean 2#'error
f
207 P 20Z+Pb
(b)
157
156
15.5 mean 217"error 15.4 I 18.5
I 19
l 195
206 Pb 20/, Pb
Fig. 1 . 2 ° 8 p b / 2 ° 4 p b - 2 ° 6 p b / 2 ° 4 p b (a) a n d 2 ° 7 p b / 2 ° 4 p b - 2 ° 6 p b / 2 ° 4 p b (b) plot of new data and of the four samples of O a s t et al. [8], r e a n a l y s e d b y Sun ([9]; samples 2716, 2740, 2765, 2775). O p e n s y m b o l s represent lavas and closed symbols, xenoliths. C) = b a s a l t ( olivine basalt), -e-= basaltic tuff, ,7 = hawaiite, zx = trachyte, [] = rhyolite, [] = comendite, • = g a b b r o (-Oolivine gabbro), • = quartzm o n z o d i o r i t e , • = granite.
277
corresponding values range from 19.165 to 19.748, 15.601 to 15.658 and 38.633 to 39.379, respectively. The 2°8pb/2°4pb versus 2°6pb/2°apb diagram
For the first group, the 2°6pb/2°4pb ratios range from 18.317 to 18.744,'the 2°7pb/2°4pb ratios from 15.579 to 15.638 and the 2°Spb/2°4pb ratios from 38.252 to 38.580. For the second group, the
208Pb 204Pb
(a) i
CANARY ~
ROSS
HELENA
FERNANDOU / ~ / Z OENORO 2/ A oRES
KERGU~
iOUGH T R I N I D A D / ~ REUN
=x ,/
" ASCENSION
39 ~BOUVET
TOAIS~OVoEu RYJ'~'/ / ¢ ~ (
+
39
~ //
0.1% -+0,2 %
• Xenohfhs • Lavas(Sun,1980) --/_.
207Pb (b) 20/+Pb
/GEOCHRON
EASTER /ISLAND / fqFERNANDO A~ORES / REUNION I IDENORONHA/ROSS /DISCOVERY GOUGH \ / BOUle/ / TABLEM0 U ~ CANARY / ASCENS,ON /
15.715.6 /
/
//
/~
/ 15./,
HE~TNA
I hi13 IAIN 7_~IL-~V
~~.Z...~/ ~
~""--~._
~
"" CAPE VERDE
~ ISLAND
HAWAII PRIBILOFF MORB
15.3 1"/
/
+ +_0.I °/o
I
I
I
18
19
20
I
206Pb 21 20/.Ph
Fig. 2. Summary of the PB isotopic compositions of young volcanic rocks from oceanic islands, mid-ocean ridges and island arcs [9,26] and comparison with the Ascension data ( × = lavas, • = xenoliths).
278
Uppm
74
.10 5150
,
,
l
,
0
I
2
4
6
8
Pbppm
Fig. 3. U-Pb (ppm) diagram for the lavas. Symbols as in Fig. 1.
(Fig. lb) shows a similar grouping. There is no correlation between 238U/2°4pb and 2°6pb/2°4pb, or between 235u/Z°4pb and 2°Tpb/Z°4pb either for the lavas or for the xenoliths thus providing no chronological evidence for discrete mantle fractionation events. Furthermore, there is not significant correlation between isotopic composition of the lavas and petrography of a given sample. Gast et al. [8] showed a positive correlation between the isotopic composition and the SiO 2 content of the lavas; however, this is based on only one trachyte which has more radiogenic Pb ratios than the basaltic rocks. This correlation is no more apparent for the lavas analyzed in this work indeed, the acidic lavas have lead ratios less radiogenic than those of the trachytes and comparable or slightly more radiogenic than those of the basalts (Fig. la,b). So, no general trend can be deduced and the only observation which can be given for the moment is that the lavas show a small spread in Pb isotopic compositions in spite of the wide range of chemical compositions. The variations observed are comparable to those observed in other oceanic island basalts ([9,26]; Fig. 2a,b). Comparison between the isotopic compositions of the plutonic xenoliths and of the lavas can be used to constrain the origin of the xenoliths (Fig. la,b). The granitic xenoliths have Pb isotopic ratios identical within analytical error to those of the acidic lavas (comendites and pantellerites), whereas the subsolvus quartz-monzodioritic xenoliths have Pb ratios in close agreement with those of the
trachytes. On the other hand, the gabbroic xenoliths have 2°6pb/2°4pb and 2°sPb/2°4pb ratios significantly lower than those of either the basalts or the hawaiites, but close to the ratios of the basaltic tuff Asc 77. Although the xenoliths must have been heated by the magmas, it is evident that they have not suffered Pb isotopic homogeneization with the enclosing lavas. Indeed, lavas and xenoliths sampled at the same outcrop do not have the same Pb isotopic compositions. For example, granitic xenoliths 17318P and Ib 1 were sampled at the same locality (" Five Mile Post") as the trachyte Ia. Similarly, xenoliths of different petrographic type, namely gabbros 25 and 26 and all quartz monzodiorites which were sampled at the same locality ("Middleton's Peak") do not have the same Pb isotopic composition. This observation is corroborated by the fact that gabbroic xenoliths from different localities which are very similar petrographically, have comparable isotopic compositions. It appears that the close similarity of Pb isotopic compositions between the lavas and the acidic and intermediate plutonic xenoliths reflects a primary characteristic of their parental magma. The observed differences in isotopic composition between the gabbroic xenoliths and the basaltic lavas could a priori result either from an age difference or from source regions with different U / P b ratios. The first hypothesis is implausible because the required age differences vary between 430 and 130 m.y. and cannot have any geological significance. Thus, a distinct origin for the gabbroic xenoliths and the basaltic lavas is indicated. This is confirmed by the disposition of data in a modified concordia diagram ([27]; measured Pb isotopic ratios corrected by the Pb isotope ratios of the Canyon Diablo troilite [28]), in which the quartz monzodiorite and granite xenoliths as well as most of the lavas (from basalts to rhyolites) plot on one straight discordia curve while the gabbro xenoliths and the basaltic tuff Asc 77 plot on another curve (Fig. 4). The lower intercepts of the discordia curves with Concordia pass within analytical error through the origin (i.e., the present time) and through upper intercepts of 4.3 b.y. and 4.5 b.y., respectively, i.e., in the range of noted values for other oceanic islands (4.2-4.6 b.y. [25]).
279
206 Pb m 238 U 0.7 7 -0-
77
0.6 G
63 0.5 5
15150
0.4
24
4
173/+3 /+3 -," la 7/* A-"
0,3 3
23-~j 0.2 2
"26
--[- mean
/.I
2 cr error
-~- mean 2~" error
0.1 1
~
4.4 5bx.
~8 &'_-_ 17318 P f s9
I I0
I 20
tO0
200
I
I
I
30
/*0
50 500
300
400
207 Pb = 235 U
Fig. 4. Modified concordia diagram [27]. Symbols as in Fig. 1. Two differentiation suites are observed: the quartz monzodiorite and granite xenoliths as well as most of the lavas (from basalts to rhyolites through trachytes) which plot on one straight discordia chord form one suite while the gabbro xenoliths and the basaltic tuff Asc 77 plot on another chord and form the second suite.
This distribution along two straight lines reflects the existence of two differentiation suites, implying the presence of at least two different magma source regions in the upper mantle for the Ascension Island rocks. This is a very important observation in view of chemical mantle dynamics because it confirms the existence of local heterogeneities in the oceanic mantle in addition to the regional heterogeneities [29,30]. Three main hypotheses have been expressed a priori for the origin of the plutonic xenoliths [31]. They are discussed later in the light of the Pb isotope data. (1) The plutonic xenoliths (gabbros to alkaline granites) represent the plutonic equivalents of the lavas (basalts to comendites) and thus have an
oceanic origin [32,33]. In this hypothesis, the acidic rocks have crystallized from the residual granitic liquid which resulted from differentiation of the mantle-derived parental magma. (2) The plutonic xenoliths were sampled from an ancient Precambrian basement underlying the volcanic cone [7]. When Daly suggested this origin, the floor of the Atlantic Ocean was commonly regarded as continental in character resulting from the breaking down of an ancient continent. (3) The acidic plutonic xenoliths result from the slow crystallization of hyperalkaline magmas emplaced during a magmatic episode contemporaneous with the uplift phase and beginning of break-up of the Gondwana supercontinent [31]. The peralkaline xenoliths of Ascension Island would then be analogous and represent the equa-
280 torial correspondents of the peralkaline granites of the Rockall Bank [1] or of the Nigerian Younger granites [34]. Two arguments might be used in support to the third hypothesis. Firstly, the mineralogical and chemical characteristics [16] of some of the acidic plutonic xenoliths are comparable to those of the Nigerian peralkaline granites [34]. Secondly, on a pre-drift reconstruction of the Atlantic Ocean, Ascension Island is located exactly where the Guinea Gulf formed in a southward prolongation of the linear trend shown by the Nigerian Younger granites. The Pb isotopic data are impossible to reconcile with Daly's hypothesis unless secondary mixing is involved; however, this does not seem to be the case here because there is no correlation either between Pb isotopic ratios and Pb concentrations or between Pb and Sr isotopic ratios (D. Demaiffe, personal communication). In this case they would be much more radiogenic and plot on an isochron whose slope corresponds to the basement age, particularly since no lead isotopic homogeneization with the lavas has occurred. Similarly, the Pb isotope data are not compatible with the third hypothesis because some early Pb isotopic ratios on the Nigerian Younger granites [35,36] are more radiogenic than the Ascension xenoliths. The similarity of Pb isotopic compositions between the lavas and plutonic xenoliths demonstrates their common source and corroborates the hypothesis of a plutonic or subvolcanic equivalence for the xenoliths. This view was already put forward by Tilley [32] in 1950, in view of the chemical similarities between these rocks. The origin of the Ascension rocks can also be discussed in terms of chemical mantle dynamics but since this is not the scope of this paper, only some general remarks will be given. The gabbroic xenoliths and the basaltic tuff constituting the first group have Pb isotopic ratios close to those of mid-ocean ridge basalts (MORB) and could have crystallized from a m a g m a whose source has similar isotopic characteristics as those of MORB. The second group, more radiogenic and with an alkaline affinity could have crystallized from a m a g m a whose source is comparable to those of other oceanic island basalts (OIB), i.e., deeper in the mantle [37].
Visualizing such heterogeneities and consequent variations in the oceanic mantle is an up-to-date problem, subject of a lot of debates as shown by the numerous papers recently published on both isotopic and trace element geochemistry (e.g. [30,38-40]). It is interesting to notice that acidic lavas have not yet been recovered at this time by either dredging or drilling the oceanic sea floor while they were found on islands as for instance Iceland [41]. In that view, Ascension Island appears comparable to Iceland also by its position near the Mid-Atlantic Ridge which could explain the very close association of rocks whose geochemical characteristics are comparable to those of M O R B and of OIB. Unfortunately, Ascension Island is a very small island and it is not possible to define regular variations in both isotopic ratios and trace elements as observed for Iceland [42,43]. The possibility of generating acidic rocks either plutonic or volcanic from the upper mantle in oceanic environment is important in relation with mantle heterogeneity; indeed, if these observations on recent rocks can be extrapolated back in time, they could indicate, that remelting of these acidic rocks included in a recycled oceanic crust can be a supplementary source for heterogeneity in the upper mantle. Numerous recent models (e.g. [30,39]) indicate that O1B could derive from the mixing of typical oceanic mantle (i.e., mantle source of MORB) and recycled oceanic crust in diapirs coming from deeper in the mantle.
4. Conclusions
The plutonic xenoliths of Ascension Island represent the plutonic equivalents of the lavas, and have an oceanic origin. The xenoliths have sampled two differentiation suites, namely the acidic and intermediate xenoliths together with most of the basaltic to rhyolitic lavas on the one hand, and the gabbroic xenoliths and a basaltic tuff on the other hand. This indicates the presence of at least two magma source regions in the upper mantle under Ascension Island and confirms the existence of local heterogeneities on the scale of magnitude of separate magma chambers in addition to regional heterogeneities of the oceanic mantle.
281 T h e o c e a n i c o r i g i n of the g r a n i t e x e n o l i t h s of A s c e n s i o n I s l a n d is t h u s d i r e c t l y a n a l o g o u s w i t h the origin of the s y e n i t i c - t o - g r a n i t i c a l k a l i n e rocks o f the R a l l i e r d u B a t y p e n i n s u l a of the K e r g u e l e n I s l a n d s [5,6]. M o r e o v e r , the A s c e n s i o n acidic p l u t o n i c x e n o l i t h s c o n f i r m the p o s s i b i l i t y of g e n e r a t i n g g r a n i t i c r o c k s b y d i f f e r e n t i a t i o n [44] of m a n t l e - d e r i v e d m a g m a w h o s e g e o c h e m i c a l a n d isot o p i c c h a r a c t e r i s t i c s are e n t i r e l y c o m p a r a b l e w i t h t h o s e of the s o u r c e m a g m a s o f o c e a n i c island basalts.
Acknowledgements Prof. J. H o n n o r e z of M i a m i U n i v e r s i t y carefully s a m p l e d a n d s e l e c t e d all s a m p l e s u s e d in this P b i s o t o p e study. T h e a u t h o r t h a n k s h i m for p r o v i d i n g these s a m p l e s as well as s o m e i n f o r m a t i o n a b o u t A s c e n s i o n Island. Prof. S. M o o r b a t h (Oxf o r d U n i v e r s i t y ) a n d Dr. D. D e m a i f f e h a v e r e a d the first d r a f t of this p a p e r a n d are well t h a n k e d for their h e l p f u l a d v i c e a n d r e m a r k s . Prof. C.J. A l l r g r e is t h a n k e d for his h e l p f u l c o m m e n t s . S. C a u ~ t g a v e useful i n f o r m a t i o n a b o u t the p e t r o l o g y o f the s a m p l e s . Mrs. C r o m p s is t h a n k e d for d r a w ing the i l l u s t r a t i o n s . T h e a u t h o r is C h a r g 6 de rec h e r c h e of the F . N . R . S . ( N a t i o n a l F u n d of Scientific R e s e a r c h ) w h i c h s u p p o r t e d this study.
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6 L. Dosso and R. Murthy, A N d isotopic study of the Kerguelen islands: inferences on enriched oceanic mantle sources, Earth Planet. Sci. Lett. 48 (1980) 268-276. 7 R.A. Daly, The geology of Ascension Island, Proc. Am. Acad. Arts Sci. 60 (1925) 3-80. 8 P.W. Gast, G.R. Tilton and C.E. Hedge, Isotopic composition of Pb and Sr from Ascension and Gough Islands, Science 145 (1964) 1181-1185. 9 S.S. Sun, Lead isotopes study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs, Philos. Trans. R. Soc. London, Ser. A, 297 (1980) 409-45. 10 Tj.H. van Andel and T.C. Moore, Magnetic anomalies and seafloor spreading rates in the northern South Atlantic, Nature 226 (1970) 328-330. 11 C. Darwin, Geological Observations on the Volcanic Islands and Parts of South America Visited during the Voyage of H.M.S. "Beagle" (Smith Elder & Co., London, 1876) 647 PP. 12 F.B. Atkins, P.E. Baker, J.D. Bell and D.G.W. Smith, Oxford expedition to Ascension Island, Nature 204 (1964) 722-724. 13 .I.D. Bell, F.B. Atkins, P.E. Baker and D.G.W. Smith, Notes on the petrology and age of Ascension Island, South Atlantic, Trans. Am. Geophys. Union 53 (1972) 168 (abstract). 14 P.E. Baker, Islands of the South Atlantic, in: The Ocean Basins and Margins, 1, A.E.M. Nairn and F.G. Stehli, eds. (Plenum Press, London, 1973) 505 pp. 15 TjH. van Andel, D.K. Rea, R.P. von Herzen and H. Haskins, Ascension fracture zone, Ascension Island, and the Mid-Atlantic Ridge, Geol. Soc. Am. Bull. 84 (1973) 1527-1546. 16 S. Cau~t, Les enclaves plutoniques de File de l'Ascension: prtrologie et grochimie, M~moire de licence, Universit6 Libre de Bruxelles (1979) 96 pp. 17 R. van Tassel, Dalyite, a new potassium-zirconium silicate from Ascension Island, Atlantic, Mineral. Mag. 29 (1972) 850-857. 18 V.M. Oversby, Lead isotopic systematics and ages of Archean acid intrusives in the Kalgoorlie-Norseman area, Western Australia, Geochim. Cosmochim. Acta 39 (1975) 1107-1125. 19 G. Manh~s, J.F. Minster and C.J. All~gre, Comparative uranium-thorium-lead and rubidium-strontium study of the Saint-Srverin amphoterite: consequences for early solar system chronology, Earth Planet. Sci. Len. 39 (1978) 14-24. 20 D. Weis, Lead isotopic composition in whole rocks: methodology, Bull. Soc. Chim. Belg. 90 (1981) 1127-1140. 21 A.E. Cameron, D.H. Smith and R.L. Walker, Mass spectrometry of nanogram-size samples of lead, Anal. Chem. 41 (1969) 525-526. 22 E3. Catanzaro, T.J. Murphy, W.R. Shields and E.L. Garner, Absolute isotopic abundance ratios of common, equal atom, and radiogenic lead isotope standards, J. Res. U.S. Natl. Bur. Stand. 72A (1968) 261-267. 23 M. Tatsumoto, Isotopic composition of lead in volcanic rocks from Hawaii, lwo-Jima and Japan, J. Geophys. Res. 71 (1966) 1721-1733.
282 24 V.M. Oversby and P.W. Gast, Isotopic composition of lead from oceanic islands, J. Geophys. Res. 75 (1970) 2097-2114. 25 J.R. Lancelot, Les syst6mes U-Pb, chronombtres et traceurs de l'rvolution des roches terrestres, Dr. Thesis, Universit~ de Paris VII (1975) 280 pp. (unpublished). 26 M. Tatsumoto, Isotopic evidence of lead in oceanic basalts and its implication to mantle evolution, Earth Planet. Sci. Lett. 38 (1978) 63-87. 27 T.J. Ulrych, Oceanic basalts lead: a new interpretation and an independent age for the earth, Science 158 (1967) 252-256. 28 M. Tatsumoto, R.J. Knight and C.J. All~gre, Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206, Science 180 (1973) 1279-1283. 29 R.K. O'Nions and R.J. Pankhurst, Petrogenetic significance of isotope and trace element variations in volcanic rocks from the Mid-Atlantic, J. Petrol. 15 (1974) 603-634. 30 C.J. All~gre, O. Brrvart, B. Dupr6 and J.F. Minster, Isotopic and chemical effects produced in a continuously differentiating convecting Earth mantle, Philos. Trans. R. Soc. London, Ser. A, 297 (1980) 447-477. 31 J. Honnorez, unpublished report (University of Miami, 1976) 42 pp. 32 C.E. Tilley, Some aspects of magmatic evolution, Q. J. Geol. Soc. London 106 (1950) 37-61. 33 R.E. Roedder and D.S. Combs, Immiscibility in granitic melts, indicated by fluid inclusions in ejected granitic blocks from Ascension Island, J. Petrol. 8 (1967) 417-451. 34 P. Bowden and D.C. Turner, Peralkaline and associated ring complexes in the Nigeria-Niger Province, West Africa, in: The Alkaline Rocks, H. Sarensen, ed. (Wiley-lnterscience, New York, N.Y., 1974) 330-351.
35 R.R.E. Jacobson, N.J. SneUing and J.F. Truswell, Age determinations in the geology of Nigeria, with special references to the older and younger granites, Overseas Geol. Miner. Res. 9 (1963) 168-182. 36 A. Tugarinov, Age absolu et particularit~ g~n~tiques des granites du Nigeria et du Cameroun septentional, Proc. Symp. Granites of West Africa (1968) 119-22. 27 P.W. Gast, Trace element fractionation and the origin of tholeiitic and alkaline magma types, Geochim. Cosmochim. Acta 32 (1968) 1057-1086. 38 A.W. Hofmann and S.R. Hart, An assessment of local and regional isotopic equilibrium in the mantle, Earth Planet. Sci. Lett. 38 (1978) 44-62. 39 A.W. Hofmann and W.M. White, The role of subducted oceanic crust in mantle evolution, Carnegie Inst. Washington Yearb. 79 (1980) 477-483. 40 C.J. All~gre, Chemical geodynamics, Tectonophysics 81 (1982) 109-132. 41 J. Thorarinsson, Hekla and Katla--the share of acid and intermediate lava and Tephra in the volcanic products through the geological history of Iceland, in: Iceland and Mid-Ocean Ridges, BjOrnsson, ed. (Reykjavik, 1967) 190-199. 42 R.K. O'Nions and R.J. Pankhurst, Secular variations in the Sr isotopic composition of Icelandic volcanic rocks, Earth Planet. Sci. Lett. 21 (1973) 13-21. 43 J.G. Schilling, Iceland mantle plume: geochemical study of the Reykjanes Ridge, Nature 242 (1973) 565-571. 44 N.L. Bowen, The Evolution of Igneous Rocks (University of Princeton Press, Princeton, N.J., 1928) 334 pp.
273
[5]
Pb isotopes in Ascension Island rocks: oceanic origin for the gabbroic to granitic plutonic xenoliths D. Weis F.N.R.S. Charg~ de Recherche, Laboratoires Associ~s de G~ologie- Pktrologie, Universitb Libre de Bruxelles, 50 A venue F.D. Roosevelt, B 1050 Brussels (Belgium)
Received June 30, 1982 Revised version accepted November 15, 1982
The Pb isotopic compositions and U and Pb concentrations of the lava series (alkali basalt to comendite) and of their plutonic xenoliths (gabbro to alkaline granite) of Ascension Island are reported. The data are used to evaluate the source of the xenoliths which formed two differentiation suites: the acidic and intermediate xenoliths together with most of the lavas on the one hand, and the gabbroic xenoliths and a basaltic tuff on the other hand. The Pb isotopic compositions imply a mantle origin for the source magmas of the xenoliths and confirm the possibility of generating granitic rocks in an oceanic environment by fractional crystallization of a mantle-derived magma whose geochemical and isotopic characteristics are comparable to the source magmas of oceanic island basalts.
1. Introduction
In the oceanic environment, lavas of basic-tointermediate composition are preponderant, while acidic lavas are not very abundant. Although plutonic rocks in general are rare and, gabbroic types predominate over acidic ones. Most of the islands where acidic plutonic rocks have been found have been shown to be connected with a neighbouring continent on the basis of geophysical, petrological or isotopic data (Rockall Bank [1], Madagascar [2], Falkland Islands [3], Seychelles Islands [4]). Some of the acidic plutonic rocks resulted from the differentiation of a mantle-derived m a g m a which was subsequently contaminated by continental material [1,4]. Only the rocks of the Kerguelen Islands seem to have an origin in the oceanic mantle with no continental contribution [5,6]. The occurrence of acidic plutonic xenoliths in the lavas and tufts of Ascension Island [7] is exceptional. In this paper, Pb isotopic compositions have been analysed both in the lavas and in their plutonic xenoliths to define 0012-821X/83/t~000-0000/$03.00
the origin of the latter and their relations with the lavas. Pb isotopic compositions of Ascension lavas have already been reported by Gast et al. [8] in 1964 in their important work showing both local and regional heterogeneities in the earth mantle, as did the Pb isotope analyses of Sun [9] on young volcanic rocks from oceanic islands, mid-ocean ridges and island arcs. This latter author reanalysed four lavas already analysed by Gast et al. [8] to allow a direct comparison with other oceanic island basalt results. Ascension Island is located in the South Atlantic Ocean (7°57'S, 14°22'W), about 90 km to the west of the Mid-Oceanic Ridge and 50 km to the north of the Ascension fracture zone which shifts the ridge 230 km to the east. There are no defined relations between these major tectonic structures and Ascension Island [10]. The island has an area of 93 km 2 and a triangular shape with its biggest dimensions of 11.5 km north-south and 14 km east-west. It forms the upper part of a volcanic cone emerging from 3000 m above the ocean bot-
~3 1983 Elsevier Scientific Publishing Company
274 tom, whose summit, the "Green Mountain", reaches 859 m. Darwin [11] gave the first description of the geology while Daly [7] made an exhaustive description to which all subsequent studies [12] refer. Except for some calcareous sand banks and guano, one finds only young volcanic rocks (maximum a g e - 1.5 m.y. [13] but most of the lava flows are much younger [14]). The island stays at the level of magnetic anomaly number 6, corresponding to 7 m.y. [15]. The summits are formed by trachytic domes contrasting strongly in landscape form with basaltic flows and cinder cones which cover 85% of the island surface. The relations between the different volcanic rocks are very complicated, as both the basaltic and trachytic types have erupted at various times during the history of Ascension Island. The lavas are often quartz a n d / o r hypersthene normative and constitute a mildly alkaline (or transitional to tholeiitic) series. Petrographically, the lavas show a wide range of variations corresponding to the classical differentiation suite of oceanic island alkali basalt [14,16]; the most important members of the suite are olivine-rich basalts, basalts, hawaiites, mugearites, trachytes and comendites. Ascension Island is characterized by the presence of numerous plutonic xenoliths already mentioned by Darwin [l l] and Daly [7], comprising olivine gabbros, diorites, monzodiorites and granites [16]. These xenoliths are devoid of oriented structure or typical metamorphic minerals. Their igneous origin is demonstrated by petrographical characters such as plagioclase zonation, hypersolvus feldspar, empty acicular apatites, etc. The olivine gabbro cumulates show very comparable petrographical characters. The intermediate rocks and the granites can be divided into hyperand sub-solvus types depending on the presence of one, or two, feldspars. The monzodiorites contain hornblende. Sodic ferromagnesian minerals (aegirine and riebeckite-arfvedsonite) characterize all the granites, which may also contain the rare dalyite or vlasovite which are K- and Na-zirconium silicates [16,17]. In some acidic xenoliths traces of local partial melting are observed as thin zones of clear glass.
2. Analytical techniques An aliquot of sample powder (100-150 mg) was dissolved in a small teflon bomb with l : l HFHC104 or H F - H N O 3 mixtures. Lead was separated with anion exchange columns in a HBr-HC1 medium [18,19] (for detailed description of the procedure, see Weis [20]). Lead is loaded onto single rhenium filaments using the silica g e l / H 3 P O 4 method [21]. Total lead blanks were in the range of 2.5-3 ng and negligible compared to the sample lead. The NBS 981 lead standard [22] was routinely analysed as monitor for mass fractionation correction on the Varian MAT 260 mass spectrometer. During the study, the silica gel batch gave - 0 . 1 2 + 0.04% mass fractionation per atomic mass unit difference. The uncertainties (2o) are assumed to be better than 0.1% for the 206Pb/2o4 Pb and Z°Tpb/2°4pb ratios and better than 0.15% for the 2o8Pb/204 Pb ratios. Pb and U concentrations were determined by the isotope dilution technique and were measured successively on single rhenium filaments with a Varian MAT TH5 mass spectrometer. The precision on these concentrations is - 1%.
3. Results and discussion Only the Pb isotopic compositions of the Ascension Island samples will be considered in this paper; detailed petrographical, mineralogical and geochemical data (trace elements and Sr isotopic composition) will be published elsewhere. 2°8pb/2°4pb, 2°7pb/2°4pb and 2°6pb/2°4pb ratios, together with U and Pb concentrations, for nine lava and fifteen plutonic xenolith samples are given in Table 1. Pb isotopic compositions for all samples are given on plots of 2°8pb/2°4pb and 2°7pb/2°4pb versus 2°6pb/Z°4pb ratios in Fig. l a and b and compared to the compositions of young oceanic basaltic rocks (both oceanic island and mid-ocean ridge basalts) in Fig. 2a and b. Pb and U concentrations in Ascension Island basaltic lavas are similar to those found in alkaline basalts ( 3 . 0 < P b < 5 . 3 ppm; 0 . 8 < U < 1 . 7 ppm [23-25]). The trachytes and acidic lavas (pantellerites-comendites) are much richer in these ele-
trachyte pantellerite comendite recent basaltic flow recent basaltic flow
Asc Asc Asc Asc Asc
granite
quartz-monzodiorite
18.439 + 0.064 18.321 +0.014 18.317 + 0.007 18.744 + 0.066 19.298 _+0.062 19.678 + 0.008 19.714__+0.016 19.308 _+0.032 19.221 + 0.030 19.467 +0.009 19.165 + 0.004 19.748 _+0.042 19.698 _+0.009 19.725 4- 0.017 19.468 + 0.010 19.480_+0.008 19.496 -+ 0.012
19.403 -+ 0.028 19.656 + 0.012 19.639 + 0.010 18.500 -+ 0.005 18.498 4- 0.018 18.502 + 0.005 19.722 -+ 0.008 19.444 -+ 0.011 19.494-+0.009 19.318 -+0.010 19.180_+0.015 19.201 _+0.006
2°6pb/2°4pb
15.579 + 0.059 15.621 +0.014 15.606 + 0.009 15.638 _+_0.074 15.631 + 0.063 15.613 + 0.008 15.632_+0.017 15.618 + 0.031 15.652 + 0.031 15.607 _ + 0 . 0 1 1 15.602 _+0.005 15.658 _+0.037 15.627 _+0.009 15.639 _+0.023 15.605 _+0.011 15.601 +0.009 15.610 -+ 0.012
15.630 _+0.026 15.631 -+ 0.012 15.642 -+ 0.010 15.551 + 0.005 15.554 -+ 0.026 15.558 + 0.007 15.630 -+ 0.008 15.548 -+ 0.011 15.618-+0.009 15.601 +0.008 15.631 +0.012 15.628-+0.005
2°7pb/2°4pb
hypersolvus rocks. All Pb ratios corrected for mass fractionation ( - 1.24 + 0.44% per a.m.u.).
Asc l i b a Asc 17343 Asc 17318P a
Asc 19 a Asc 23 a ASC 24 Asc 31 a Asc 41 Asc 43
Xenoliths Asc 25 Asc 26 Asc 51 Asc 59 Asc Ib I Asc 18
gabbro
basalt olivine basalt hawaiite basaltic tuff
Lavas Asc 15150 Asc 63 Asc 64 Asc 77
la 74 38 8 10
Petrographic type
Sample
Lead isotopic compositions and U and Pb concentrations in Ascension lavas and xenoliths
TABLE 1
38.252 + 0.140 38.272_+0.035 38.261 + 0.024 38.580 + 0.184 38.937 + 0.160 39.190 + 0.021 39.253_+0.038 38.978 + 0.081 38.946 + 0.093 39.019+0.030 38.787 + 0.011 39.379 -+ 0.097 39.254 _+0.026 39.320 _+0.046 39.033 _+0.029 39.024-+0.022 39.070 -+ 0.029
38.964 -+ 0.066 39.232 -+ 0.049 39.246 -+ 0.025 38.093 4-_0.015 38.110 + 0.062 38.115 -+ 0.030 39.228 -+ 0.020 38.992 + 0.032 39.097+0.021 38.840-+0.022 38.633-+0.030 38.634_+0.014
2°spb/2°4Pb
8.62 1.52 6.15
4.77 3.79 2.23 3.45 3.77 2.03
2.58 0.28 0.22 0.13 1.67 7.81
3.72 5.07 3.75 1.46 1.65 1.58
0.89 1.79 1.81 0.69
U (ppm)
8.49 3.36 2.93
5.17 5.08 5.31 4.49 3.66 4.18
1.25 4.47 3.01 0.55 1.55 2.94
6.69 8.60 5.32 3.48 6.46 6.98
2.50 3.10 3.45 39.95
Pb (ppm)
65.64 29.25 135.82
59.49 48.04 27.14 49.31 67.20 31.60
130.10 3.95 4.60 15.05 69.43 172.87
36.17 38.05 45.63 27.12 16.34 14.32
23.16 20.76 34.10 1.40
238U/2°4pb
276
to the right of the geochron as most of the other oceanic island basalts. The data can be divided into two groups on the basis o f 2°6pb/2°4 Pb ratios. The gabbroic xenoliths and a basaltic tuff have values of - 1 8 . 5 , whereas the acidic and intermediate xenoliths, together with all the lavas including the rhyolites have values above - 19.2.
ments, indicating a progressive enrichment during the differentiation. In a U-Pb concentration diagram (Fig. 3) all data but one (sample 10) plot on a straight line passing nearly through the origin with the exception of Asc 77. In the 207Pb/204 Pb versus 2o6Pb/204 Pb diagram (Fig. 2a), all the samples, xenoliths included, plot
208Pb 204Pb 395
Ca)
39 7
" "
38.5
mean 2#'error
f
207 P 20Z+Pb
(b)
157
156
15.5 mean 217"error 15.4 I 18.5
I 19
l 195
206 Pb 20/, Pb
Fig. 1 . 2 ° 8 p b / 2 ° 4 p b - 2 ° 6 p b / 2 ° 4 p b (a) a n d 2 ° 7 p b / 2 ° 4 p b - 2 ° 6 p b / 2 ° 4 p b (b) plot of new data and of the four samples of O a s t et al. [8], r e a n a l y s e d b y Sun ([9]; samples 2716, 2740, 2765, 2775). O p e n s y m b o l s represent lavas and closed symbols, xenoliths. C) = b a s a l t ( olivine basalt), -e-= basaltic tuff, ,7 = hawaiite, zx = trachyte, [] = rhyolite, [] = comendite, • = g a b b r o (-Oolivine gabbro), • = quartzm o n z o d i o r i t e , • = granite.
277
corresponding values range from 19.165 to 19.748, 15.601 to 15.658 and 38.633 to 39.379, respectively. The 2°8pb/2°4pb versus 2°6pb/2°apb diagram
For the first group, the 2°6pb/2°4pb ratios range from 18.317 to 18.744,'the 2°7pb/2°4pb ratios from 15.579 to 15.638 and the 2°Spb/2°4pb ratios from 38.252 to 38.580. For the second group, the
208Pb 204Pb
(a) i
CANARY ~
ROSS
HELENA
FERNANDOU / ~ / Z OENORO 2/ A oRES
KERGU~
iOUGH T R I N I D A D / ~ REUN
=x ,/
" ASCENSION
39 ~BOUVET
TOAIS~OVoEu RYJ'~'/ / ¢ ~ (
+
39
~ //
0.1% -+0,2 %
• Xenohfhs • Lavas(Sun,1980) --/_.
207Pb (b) 20/+Pb
/GEOCHRON
EASTER /ISLAND / fqFERNANDO A~ORES / REUNION I IDENORONHA/ROSS /DISCOVERY GOUGH \ / BOUle/ / TABLEM0 U ~ CANARY / ASCENS,ON /
15.715.6 /
/
//
/~
/ 15./,
HE~TNA
I hi13 IAIN 7_~IL-~V
~~.Z...~/ ~
~""--~._
~
"" CAPE VERDE
~ ISLAND
HAWAII PRIBILOFF MORB
15.3 1"/
/
+ +_0.I °/o
I
I
I
18
19
20
I
206Pb 21 20/.Ph
Fig. 2. Summary of the PB isotopic compositions of young volcanic rocks from oceanic islands, mid-ocean ridges and island arcs [9,26] and comparison with the Ascension data ( × = lavas, • = xenoliths).
278
Uppm
74
.10 5150
,
,
l
,
0
I
2
4
6
8
Pbppm
Fig. 3. U-Pb (ppm) diagram for the lavas. Symbols as in Fig. 1.
(Fig. lb) shows a similar grouping. There is no correlation between 238U/2°4pb and 2°6pb/2°4pb, or between 235u/Z°4pb and 2°Tpb/Z°4pb either for the lavas or for the xenoliths thus providing no chronological evidence for discrete mantle fractionation events. Furthermore, there is not significant correlation between isotopic composition of the lavas and petrography of a given sample. Gast et al. [8] showed a positive correlation between the isotopic composition and the SiO 2 content of the lavas; however, this is based on only one trachyte which has more radiogenic Pb ratios than the basaltic rocks. This correlation is no more apparent for the lavas analyzed in this work indeed, the acidic lavas have lead ratios less radiogenic than those of the trachytes and comparable or slightly more radiogenic than those of the basalts (Fig. la,b). So, no general trend can be deduced and the only observation which can be given for the moment is that the lavas show a small spread in Pb isotopic compositions in spite of the wide range of chemical compositions. The variations observed are comparable to those observed in other oceanic island basalts ([9,26]; Fig. 2a,b). Comparison between the isotopic compositions of the plutonic xenoliths and of the lavas can be used to constrain the origin of the xenoliths (Fig. la,b). The granitic xenoliths have Pb isotopic ratios identical within analytical error to those of the acidic lavas (comendites and pantellerites), whereas the subsolvus quartz-monzodioritic xenoliths have Pb ratios in close agreement with those of the
trachytes. On the other hand, the gabbroic xenoliths have 2°6pb/2°4pb and 2°sPb/2°4pb ratios significantly lower than those of either the basalts or the hawaiites, but close to the ratios of the basaltic tuff Asc 77. Although the xenoliths must have been heated by the magmas, it is evident that they have not suffered Pb isotopic homogeneization with the enclosing lavas. Indeed, lavas and xenoliths sampled at the same outcrop do not have the same Pb isotopic compositions. For example, granitic xenoliths 17318P and Ib 1 were sampled at the same locality (" Five Mile Post") as the trachyte Ia. Similarly, xenoliths of different petrographic type, namely gabbros 25 and 26 and all quartz monzodiorites which were sampled at the same locality ("Middleton's Peak") do not have the same Pb isotopic composition. This observation is corroborated by the fact that gabbroic xenoliths from different localities which are very similar petrographically, have comparable isotopic compositions. It appears that the close similarity of Pb isotopic compositions between the lavas and the acidic and intermediate plutonic xenoliths reflects a primary characteristic of their parental magma. The observed differences in isotopic composition between the gabbroic xenoliths and the basaltic lavas could a priori result either from an age difference or from source regions with different U / P b ratios. The first hypothesis is implausible because the required age differences vary between 430 and 130 m.y. and cannot have any geological significance. Thus, a distinct origin for the gabbroic xenoliths and the basaltic lavas is indicated. This is confirmed by the disposition of data in a modified concordia diagram ([27]; measured Pb isotopic ratios corrected by the Pb isotope ratios of the Canyon Diablo troilite [28]), in which the quartz monzodiorite and granite xenoliths as well as most of the lavas (from basalts to rhyolites) plot on one straight discordia curve while the gabbro xenoliths and the basaltic tuff Asc 77 plot on another curve (Fig. 4). The lower intercepts of the discordia curves with Concordia pass within analytical error through the origin (i.e., the present time) and through upper intercepts of 4.3 b.y. and 4.5 b.y., respectively, i.e., in the range of noted values for other oceanic islands (4.2-4.6 b.y. [25]).
279
206 Pb m 238 U 0.7 7 -0-
77
0.6 G
63 0.5 5
15150
0.4
24
4
173/+3 /+3 -," la 7/* A-"
0,3 3
23-~j 0.2 2
"26
--[- mean
/.I
2 cr error
-~- mean 2~" error
0.1 1
~
4.4 5bx.
~8 &'_-_ 17318 P f s9
I I0
I 20
tO0
200
I
I
I
30
/*0
50 500
300
400
207 Pb = 235 U
Fig. 4. Modified concordia diagram [27]. Symbols as in Fig. 1. Two differentiation suites are observed: the quartz monzodiorite and granite xenoliths as well as most of the lavas (from basalts to rhyolites through trachytes) which plot on one straight discordia chord form one suite while the gabbro xenoliths and the basaltic tuff Asc 77 plot on another chord and form the second suite.
This distribution along two straight lines reflects the existence of two differentiation suites, implying the presence of at least two different magma source regions in the upper mantle for the Ascension Island rocks. This is a very important observation in view of chemical mantle dynamics because it confirms the existence of local heterogeneities in the oceanic mantle in addition to the regional heterogeneities [29,30]. Three main hypotheses have been expressed a priori for the origin of the plutonic xenoliths [31]. They are discussed later in the light of the Pb isotope data. (1) The plutonic xenoliths (gabbros to alkaline granites) represent the plutonic equivalents of the lavas (basalts to comendites) and thus have an
oceanic origin [32,33]. In this hypothesis, the acidic rocks have crystallized from the residual granitic liquid which resulted from differentiation of the mantle-derived parental magma. (2) The plutonic xenoliths were sampled from an ancient Precambrian basement underlying the volcanic cone [7]. When Daly suggested this origin, the floor of the Atlantic Ocean was commonly regarded as continental in character resulting from the breaking down of an ancient continent. (3) The acidic plutonic xenoliths result from the slow crystallization of hyperalkaline magmas emplaced during a magmatic episode contemporaneous with the uplift phase and beginning of break-up of the Gondwana supercontinent [31]. The peralkaline xenoliths of Ascension Island would then be analogous and represent the equa-
280 torial correspondents of the peralkaline granites of the Rockall Bank [1] or of the Nigerian Younger granites [34]. Two arguments might be used in support to the third hypothesis. Firstly, the mineralogical and chemical characteristics [16] of some of the acidic plutonic xenoliths are comparable to those of the Nigerian peralkaline granites [34]. Secondly, on a pre-drift reconstruction of the Atlantic Ocean, Ascension Island is located exactly where the Guinea Gulf formed in a southward prolongation of the linear trend shown by the Nigerian Younger granites. The Pb isotopic data are impossible to reconcile with Daly's hypothesis unless secondary mixing is involved; however, this does not seem to be the case here because there is no correlation either between Pb isotopic ratios and Pb concentrations or between Pb and Sr isotopic ratios (D. Demaiffe, personal communication). In this case they would be much more radiogenic and plot on an isochron whose slope corresponds to the basement age, particularly since no lead isotopic homogeneization with the lavas has occurred. Similarly, the Pb isotope data are not compatible with the third hypothesis because some early Pb isotopic ratios on the Nigerian Younger granites [35,36] are more radiogenic than the Ascension xenoliths. The similarity of Pb isotopic compositions between the lavas and plutonic xenoliths demonstrates their common source and corroborates the hypothesis of a plutonic or subvolcanic equivalence for the xenoliths. This view was already put forward by Tilley [32] in 1950, in view of the chemical similarities between these rocks. The origin of the Ascension rocks can also be discussed in terms of chemical mantle dynamics but since this is not the scope of this paper, only some general remarks will be given. The gabbroic xenoliths and the basaltic tuff constituting the first group have Pb isotopic ratios close to those of mid-ocean ridge basalts (MORB) and could have crystallized from a m a g m a whose source has similar isotopic characteristics as those of MORB. The second group, more radiogenic and with an alkaline affinity could have crystallized from a m a g m a whose source is comparable to those of other oceanic island basalts (OIB), i.e., deeper in the mantle [37].
Visualizing such heterogeneities and consequent variations in the oceanic mantle is an up-to-date problem, subject of a lot of debates as shown by the numerous papers recently published on both isotopic and trace element geochemistry (e.g. [30,38-40]). It is interesting to notice that acidic lavas have not yet been recovered at this time by either dredging or drilling the oceanic sea floor while they were found on islands as for instance Iceland [41]. In that view, Ascension Island appears comparable to Iceland also by its position near the Mid-Atlantic Ridge which could explain the very close association of rocks whose geochemical characteristics are comparable to those of M O R B and of OIB. Unfortunately, Ascension Island is a very small island and it is not possible to define regular variations in both isotopic ratios and trace elements as observed for Iceland [42,43]. The possibility of generating acidic rocks either plutonic or volcanic from the upper mantle in oceanic environment is important in relation with mantle heterogeneity; indeed, if these observations on recent rocks can be extrapolated back in time, they could indicate, that remelting of these acidic rocks included in a recycled oceanic crust can be a supplementary source for heterogeneity in the upper mantle. Numerous recent models (e.g. [30,39]) indicate that O1B could derive from the mixing of typical oceanic mantle (i.e., mantle source of MORB) and recycled oceanic crust in diapirs coming from deeper in the mantle.
4. Conclusions
The plutonic xenoliths of Ascension Island represent the plutonic equivalents of the lavas, and have an oceanic origin. The xenoliths have sampled two differentiation suites, namely the acidic and intermediate xenoliths together with most of the basaltic to rhyolitic lavas on the one hand, and the gabbroic xenoliths and a basaltic tuff on the other hand. This indicates the presence of at least two magma source regions in the upper mantle under Ascension Island and confirms the existence of local heterogeneities on the scale of magnitude of separate magma chambers in addition to regional heterogeneities of the oceanic mantle.
281 T h e o c e a n i c o r i g i n of the g r a n i t e x e n o l i t h s of A s c e n s i o n I s l a n d is t h u s d i r e c t l y a n a l o g o u s w i t h the origin of the s y e n i t i c - t o - g r a n i t i c a l k a l i n e rocks o f the R a l l i e r d u B a t y p e n i n s u l a of the K e r g u e l e n I s l a n d s [5,6]. M o r e o v e r , the A s c e n s i o n acidic p l u t o n i c x e n o l i t h s c o n f i r m the p o s s i b i l i t y of g e n e r a t i n g g r a n i t i c r o c k s b y d i f f e r e n t i a t i o n [44] of m a n t l e - d e r i v e d m a g m a w h o s e g e o c h e m i c a l a n d isot o p i c c h a r a c t e r i s t i c s are e n t i r e l y c o m p a r a b l e w i t h t h o s e of the s o u r c e m a g m a s o f o c e a n i c island basalts.
Acknowledgements Prof. J. H o n n o r e z of M i a m i U n i v e r s i t y carefully s a m p l e d a n d s e l e c t e d all s a m p l e s u s e d in this P b i s o t o p e study. T h e a u t h o r t h a n k s h i m for p r o v i d i n g these s a m p l e s as well as s o m e i n f o r m a t i o n a b o u t A s c e n s i o n Island. Prof. S. M o o r b a t h (Oxf o r d U n i v e r s i t y ) a n d Dr. D. D e m a i f f e h a v e r e a d the first d r a f t of this p a p e r a n d are well t h a n k e d for their h e l p f u l a d v i c e a n d r e m a r k s . Prof. C.J. A l l r g r e is t h a n k e d for his h e l p f u l c o m m e n t s . S. C a u ~ t g a v e useful i n f o r m a t i o n a b o u t the p e t r o l o g y o f the s a m p l e s . Mrs. C r o m p s is t h a n k e d for d r a w ing the i l l u s t r a t i o n s . T h e a u t h o r is C h a r g 6 de rec h e r c h e of the F . N . R . S . ( N a t i o n a l F u n d of Scientific R e s e a r c h ) w h i c h s u p p o r t e d this study.
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