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Petrogenesis of the volcanic suite of Bouvetøya (Bouvet Island), South Atlantic
TORE PRESTVIK, STEVEN GOLDBERG & GORDON G. GOLES
Prestvik, T., Goldberg, S. & Goles, G. G. Petrogenesis of the volcanic suite of Bouvetøya (Bouvet Island), South Atlantic. Norsk
Geologisk Tidsskrift, Vol. 79, pp. 205-218. Oslo 1999. ISSN 0029-196X.
Whole rock and mineral compositions of volcanic rocks collected during the Norwegian Polarsirkel expedition (1978n9) to the volcanic istand of Bovetøya (close to the Bouvet Triple Junction) are discussed and compared with previously published data from the island. The rock types, hawaiite, benmoreite, and peralkaline trachyte and rhyolite (comendite) are related to each other by crystal fractionation processes. The trace element and radiogenic isotope signatures displayed by the Bouvetøya rocks are !hose of a moderately enriched oceanic island suite. On several isotope plots Bouvetøya rocks fall on or close to mixing lines between the euriched EM-l and HIMU mantle components. Mixing between depleted morb mantle (DMM) and euriched components is not likely. Thus, Bouvetøya displays a typical plume signature.
T. Prestvik, Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, N-7491,
Trondheim, Norway; S. Goldberg, Department of Geology, University of North Carolina, Chapel Hill, NC 27599-3315, USA.
Present address: New Brunswick Laboratory, US Department of Energy, 9800 S. Cass Avenue, Argonne, IL 60439, USA;
G. G. Goles, Department of Geological Sciences, University of Oregon, Eugene, OR 97403-1272, USA.
Introduction Bouvetøya (official Norwegian speiling) is located 50 km west of the South-West Indian Ridge (SWIR) in the The Bouvet Triple Junction (BTJ) area of the South Atlantic (Fig. l) and consists entirely of young ( < 1 .4 Ma) volcanic rocks. Bouvetøya is 95% covered by permanent ice and no detailed geological map can be made. Geologic and petrologic features have been described by several authors. Among these are the contributions ofVerwoerd et al. ( 1976) and Imsland et al. ( 1 977), who described both the geology and petrology, of Prestvik ( 1982a), who reported on trace element geochemical features, and of le Roex & Erlank ( 1982), who discussed the petrologic evolution of the volcanic suite of the island. O'Nions & Pankhurst ( 1974), O'Nions et al. ( 1977), and Sun ( 1980) have published some data on Sr, Nd, and Pb isotopes. All these contributions were based on samples collected during short visits to the island (which is of difficult access) from the late 1920s to 1966.
Geologic features of the island were described in considerable detail by Prestvik & Winsnes ( 198 1 ), who participated in the Norwegian Polarsirkel expedition during the Antarctic summer of 1978179. During this expedition 15 different locations on the istand were sampled, resulting in 83 samples with good geological control. According to the IUGS classification based on the non-genetic total alkalissilica (TAS) diagram, the rocks plot in the fields St. S2, S3, T and R (Le Bas et al. 1986). The series is sodic (Na20 -2 � K20) including a wide variety of hawaiites (grading into mugearite) together with benmoreites and rhyolites. In addition, we have two samples of altered mugearite (B 3 1 & B 34) and one fresh sample of trachyte (B 1 6). Trachyte (q <20) has not been reported from Bouvetøya previously. The whole series is oversaturated (q-normative), and the
trachyte and rhyolites are peralkaline (ac-normative; see Tables l, 3). Thus, the rhyolites are comendites, a term that is used in what follows.
The present account is based on material from the Polarsirkel expedition. We present (l) major and trace element composition of the various rock types, (2) microprobe data on the minerals and trace element data on plagioclase separates, and (3) isotope ratios of O, Sr, Nd, and Pb. Furthermore, we (4) discuss the results of
-53'
. 54'
-55'
\ MAR / Shona •. / ) /� SWIR
\ R1dge \. " / "-/'
"'/ (:V/ o /
...... w/ 11.. <oo�/ Bouvetøya
�� /
"�/ 7-· (Spi6SS
__ c_::_�?-f.!--J) Ridge
--MR
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Fig. I. Schematic configuration of the Bouvet Triple Junction (BTJ), with the location of Bouvetøya. The sketch is based on data from Simonov et al. ( 1996), Ligi et al. (1997) and Mitchell & Livermore (1998). MAR = Mid-Atlantic Ridge, AAR = American-Antarctic Ridge, SWIR =South-West Indian Ridge. The black arrow indicates that the Spiess Ridge is propagating northwestwards. Also, the location of enriched MAR basalts at 54°S, commonly referred to as the 'Shona-Ridge anomaly' (Moreira et al. 1995), is indicated by the stippled pattem of the ridge. The geographic Iocation and plate configuration of the BTJ are shown in the lower, right-hand corner.
206 T. Prestvik et al.
hydrothermal alteration, (5) present a revised petrologic model, and (6) discuss the petrogenesis of the suite.
The petrographic features of the major rock types ( except trachyte) have been well described in several of the previous studies (op. cit.). The newly discovered trachyte (B 16) mainly consists of a brown glass in which very few microphenocrysts of feldspar ( oligoclase to anorthoclase) and almost pure hedenbergite are disseminated together with crystallites of feldspar and titanomagnetite. Samples of crystalline comendite from a dome at Kapp Valdivia in the north of the island contain alkali feldspar, pure hedenbergite and fayalite, arfvedsonitic amphibole, and ilmenite.
In the study oflmsland et al. ( 1977) in particular, the rock types are described under different names; 'transitional basalts ' I and IT, (for hawaiites ), and 'transitional icelandite' (for benmoreite), but these authors recognized the peralkaline nature of the rhyolites and u sed 'comendite' . Le Roex & Erlank ( 1982) used the terms 'accumulated hawaiite' , 'hawaiite' , 'mugearite' , 'benmoreite' and 'rhyolite' .
Previous petrologic models and associated problems Both lmsland et al. ( 1977) and le Roex & Erlank ( 1 982) concluded that the rock types of the Bouvetøya suite were related by crystal fractionation and/or accumulation processes. However, both these studies recognized 'problems' at various model stages; these will be comrnented on below.
lmsland et al. ( 1 977) divided the basic rocks into two groups on the basis of their K20 content and concluded that these two groups were not related by simple fractionation, mostly because the model required a more sodic plagioclase than observed in abundant megacrysts. They concluded that the benmoreites (trachytic icelandites) formed by open-system fractionation from evolved hawaiites (transitional basalt Il) and that further fractionation resulted in comendite. On the other hand, le Roex & Erlank ( 1982) suggested that porphyritic hawaiites represented 'regular Bouvetøya hawaiite' with as much as 40% accumulation of plagioclase (34-37%) and clinopyroxene (3-6% ). The only problem with this interpretation was an inferred Ba inconsistency (le Roex & Erlank 1982, p. 333). These authors rejected a model in which comendite (rhyolite) was formed from benmoreite, both because it required a fractionating plagioclase of An 47 (benmoreite plagioclase is An 33) and because there was a 'Ba and Sr problem' , such that none of the minerals present as phenocrysts in benmoreite could remove enough barium (and strontium) to obtain the lower levels of these elements observed in comendite. Instead, they suggested that comendite was formed directly from mugearite, a very long fractionating ste p ( 69% ), in which a plagioclase of An 47 was required. However, this is in conflict with the composition of modal plagioclase in mugearite (most calcic plagioclase in mugearite is An 45 ; le Roex & Erlank ( 1982), Table Ill, p. 321) . The fractionation model of le
NORSK GEOLOGISK TIDSSKRIFT 79 (1999)
Roex & Erlank ( 1982) can be sumrnarized as follows: (l) hawaiite � mugearite � benmoreite, (2) mugearite � rhyolite, i .e. benmoreite is not an intermediate composition in the formation of rhyolite.
In this study, we address several of the problems connected with the models proposed in the two mentioned studies. Since our radiogenic isotope data (see below) are consistent with a model in which the various rock types at Bouvetøya are closely related, we present a refined crystal fractionation model, based on new mineral and chemical data.
/
Geochemical features Analytical methods
Major elements and the trace elements Ba, Rb, Sr, Y, Nb, Cr, Ni, V, Cu, Zn, and Ga were analysed by XRF at Washington State University, Pullman, Washington, USA. Loss on ignition and volatiles on all samples as well as Co (XRF) on samples B 55, B 57, B 58, and B 59 were measured at NTNU, Trondheim, Norway. Scandium, Co, REE, Hf, Ta, and Th on most samples and plagioclase separates were analysed by INAA and Zr by XRF at the University of Oregon, Eugene, Oregon, USA. On samples B 55, B 57, B 58, and B 59, Se, REE, Hf, Ta, Th, and U were deterrnined by INAA at Imperial College - Reactor Centre, London, UK. Rare earth elements, Se, Hf, Ta, and Th of sample B 3 1 were analysed by INAA at the University of Texas at El Paso, USA. Mineral analyses were obtained at NTNU/ SINTEF, Trondheim, Norway using a JEOL microprobe. Oxygen isotopes were measured at the University of South Carolina, Columbia, South Carolina, USA, and some strontium isotopes at University of Rochester, Rochester, New York, USA, while the rest of the Sr isotopes as well as the Nd and Pb isotopes were analysed at the University of North Carolina, Chapell Hill, North Carolina, USA.
Chemical data of fresh whole rocks are presented in
Table l. However, before we discuss the general data we will comment on the fact that in this study, the N a20 contents of all rock types are consistently higher than those found in the previous studies (Table 2). Given the considerable significance of sodium concentrations in the model calculations, this aspect of our data has been thoroughly studied. Our Na20 calibration is good for the 0.03-4.38 wt% calibration range (r = 0.999 16). However, the intermediate and silicic rocks from Bouvetøya are high in Na20. As a check of calibration at higher values, we analysed the international standard STM- 1 and an intemal standard TMS as unknowns and got 9.15 wt%, which, relatively, is 2.3% higher than the recomrnended value 8.94 for STM- 1 (Govindaraju 1989), and 9.52 wt% for TMS, which, relatively, is 2.0% higher than our preferred value of 9.33 wt%. Even though both of these results are on the high side of recomrnended or used values, we feel that these are acceptable data.
W e have no satisfactory explanation for this interla-
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208 T. Prestvik et al. NORSK GEOLOGISK TIDSSKRIFT 79 (1999)
Table 2. Measured Na20 values for corresponding K20 contents. Before calculating the averages, all data were norrnalized to 100% volatile free and with total iron as FeO.
Hawaiites
(l) (2) (3)
Na20 3.96 3.66 3.55 K20 1.30 1.32 1.31 No. of samples 7 5 3
(l) This study; XRF - lithium tetraborate:rock powder=2:1. (2) Imsland et al. (1977); atomic absorption spectrometry. (3) le Roex & Erlank (1982); XRF - data from Verwoerd et al. (1976).
(l)
5.55 2.97 2
Benmoreite
(2) (3)
4.98 4.77 2.97 3.01 3 5
(l)
6.10 4.89 3
Comendite
(2)
5.93 4.83 3
(3)
5.36 4.88 2
boratory discrepancy. However, we think it is partly studies. Thus, the altered rocks we have studied (volatiles related to alteration (see below) and perhaps to a dilution >2%) are all significantly lower in sodium than their fresh effect through using strongly porphyritic samples in some equivalents (Fig. 3e, Table 3) . It is therefore possible that
Table 3. Altered basaltic and interrnediate rocks. Major element data norrnalized to 100%, volatiles are added.
B 10 B 19 B 20 B 22 B 23 B 24 B 31 B 34 B 49 B 14
SiOz 48.09 48.36 49.98 45.34 50.72 49.81 52.44 52.46 61.95 64.77 TiOz 4.10 4.09 3.77 3.47 3.54 2.03 3.38 2.21 1.12 2.04 Alz03 15.59 14.05 14.03 18.35 15.14 18.53 14.66 15.06 14.69 14.16 FeOtot 12.80 14.31 12.94 10.95 11.99 8.33 12.54 13.09 8.76 8.50 Mn O 0.28 0.25 0.23 0.21 0.14 0.13 0.23 0.33 0.23 O.ll MgO 3.33 4.41 5.ll 3.96 5.76 5.81 3.29 3.05 0.81 2.94 Ca O 11.24 10.68 9.94 12.75 8.43 11.87 7.44 7.86 3.82 3.57 Na20 3.22 1.98 2.42 2.08 3.19 2.71 3.62 4.08 5.21 0.98 KzO 0.76 0.95 0.98 2.36 0.53 0.50 1.50 0.97 3.04 2.54 P205 0.59 0.92 0.59 0.52 0.55 0.28 0.90 0.89 0.35 0.39 HzO+ 2.36 4.71 5.05 5.10 1.97 0.99 *5.34 1.82 2.86 2.97 HzO- 0.88 0.25 0.47 0.42 5.55 1.98 3.91 0.78 0.76 COz 3.86 5.10 4.70 6.75 0.76 1.00 2.58 0.12 2.28
107.10 ll0.06 110.21 ll2.26 108.27 103.97 105.34 108.31 103.74 106.01 * = L.O.I.
Ba 165 159 160 661 208 ll8 299 371 559 209 Rb ll 6 14 36 3 4 23 12 58 52 Sr 551 510 407 750 500 524 436 550 309 107 y 42 43 41 34 37 20 53 58 71 55 Zr 285 260 277 251 262 153 366 425 620 476 Nb 47 46 46 39 41 22.6 61 68 101 73
La 27.4 30.5 25.5 21.5 25.4 13.1 39.5 42.1 58.8 37.0 Ce 63.4 65.6 60.8 49 55.4 32.7 79.0 105 137 90.5 Nd 42 38.8 35.9 31.5 42 26 47.7 64 75 48.5 Sm 8.79 9.2 7.92 7.07 8.1 4.7 11.3 11.4 14.2 9.9 Eu 2.77 3.23 2.68 2.33 2.52 1.6 3.01 3.94 4.34 2.76 Tb 1.38 1.38 1.23 1.06 1.32 0.67 1.74 2.1 2.50 1.67 Yb 3.44 3.75 3.5 2.8 3.16 1.9 4.20 4.68 6.97 5.1 L u 0.53 0.44 0.43 0.40 0.46 0.25 0.64 0.69 0.98 0.72
Se 27.7 24.9 25.8 25.2 27.1 21.9 22.6 15.1 12.4 14.7 Cr 27 7 13 31 28 107 18 4 4 6 Co 30.7 38.6 34.5 35.2 35.7 32.1 29.8 14.3 3.52 16.6 Ni 5 o 2 14 15 49 o o 5 8 V 363 316 322 301 293 195 269 88 13 147 C u 25 14 38 38 48 53 6 26 2 12 Zn 145 153 123 ll3 122 70 156 171 2ll 118 Ga 27 22 20 27 22 21 23 26 30 21
Hf 6.02 5.36 6.13 4.95 5.49 3.21 8.36 9.20 13.6 10.7 Ta 2.45 2.27 2.38 2.26 2.18 1.24 3.49 3.46 5.14 3.60 Tb 2.58 2.64 2.77 2.06 2.47 1.38 4.25 4.98 7.93 5.64
Y/Nb 0.89 0.93 0.89 0.87 0.90 0.88 0.87 0.85 0.70 0.75 Zr/Nb 6.06 5.65 6.02 6.44 6.39 6.77 6.00 6.25 6.14 6.52
8180 + +1.9 +2.8 +1.9 +6.0 +6.2 +7.2 +5.8 +5.1
Q 3.14 7.90 7.15 5.98 1.60 7.20 3.61 9.79 36.61 Ne 2.75
NORSK GEOLOGISK TIDSSKRIFf 79 (1999)
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Volcanic rocks, Bouvetøya
l l l l
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Fig. 2. Variation diagrams of Nb, Ba, FeO, P205, Ti02 and MnO vs. Ta. Filled circles represent fresh rocks; crosses represent hydrotherrnally altered samples (Tables l, 3).
most of the discrepancies might be due to the use of altered and/or porphyritic samples in the previous studies.
General characteristics
Both this and the former studies show that the K20 contents of hawaiites vary considerably: from 0.75 to 1 .65 wt% (lmsland et al. 1977), 0.72 to 1 . 3 1 wt% (le Roex & Erlank 1982) and from 0.97 to 1 .54 in this study. The low values are almost always associated with high Al203 (> 15%) and CaO (> 10%) contents, sometimes with 'high' MgO (samples B 24 & B 70, this study) and relatively low values of several other elements. These are the effects of plagioclase (and sometimes olivine) accumulation as revealed by the porphyritic nature of many hawaiite samples. The sparsely porphyritic or aphyric Bouvet hawaiites, which are thought to be doser to melt compositions, seem to vary in K20 in the range 1 . 12-1 .54 wt%.
They are high in Ti02 (3.35-3 .89 wt%) and total FeO (1 1 . 15-12. 8 1 wt%), and low in MgO (3.72-4.78 wt%) and CaO (7.85-9.48 wt%). We have only used such samples in our model calculations.
In Figs. 2 & 3 we have plotted selected major and trace elements in variation diagrams versus Ta, which was apparently incompatible throughout the development of the suite and also rather immobile during alteration. Three different pattem styles are observed: (a) Purely incompatible elements (vs. the observed minerals) such as Nb (Fig. 2a) vary almost linearly with Ta throughout the series. This type of pattem is observed for Zr, Hf, Y, La, Lu, K20, and Rb, while Ba increases linearly up to the trachyte stage (B 16) after which it drops considerably to and among the comendites (Fig. 2b). (b) FeO (Fig 2c), P205 (Fig. 2d), Ti02 (Fig. 2e ), MnO (Fig. 2f), and Se (Fig. 3f) increase from the most primitive rocks and reach a maximum before decreasing in the intermediate and silicic rocks.
210 T. Prestvik et al.
6
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NORSK GEOLOGISK TIDSSKRIFT 79 (1999)
.. 120
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- 200 • -
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Ta Fig. 3. Variation diagram for K20, Rb, CaO, Sr, Na20 and Se vs. Ta. Symbols as in Fig. 2.
Note, for example, the pattern of P205 contents, which increases linearly to mugearite (B 3 1 & B 34), indicating that apatite fractionation must have been rather insignificant up to this point, but from then on it decreases in more evolved compositions. Ti02 reaches its maximum somewhat earlier than FeO; we can also see (Fig. 2c, f) that manganese removal is delayed compared to iron, a feature compatible with the increased Mn/Fe ratio of the various ferromagnesian minerals we have analysed in benmoreite and comendite (Table 5). (c) CaO (Fig. 3c), MgO and Sr (Fig. 3d) decrease almost linearly throughout the series.
Alteration
Nine samples are listed in Table 3, all of which are altered by hydrothermal processes. Of the se, samples B 10, B 19, B 20, B 22, and B 23 seem to represent 'normal' to evolved Bouvetøya type hawaiite, B 24 represents a hawaiite with
accumulated plagioclase, B 3 1 and B 34 are altered mugearites, and B 49 is altered benmoreite. With the exception of B 23 and B 24, the altered samples were collected from a hydrothermally altered sequence, which is exposed in the cliffs behind the newly formed platform Nyrøysa (< 1 00 m a.s.l.) on the west coast of Bouvetøya. Samples B 23 and B 24 were collected at the site of the active fumaroles in the north of the island (see Prestvik & Winsnes 198 1 for details) .
Chemically, the alteration is characterized by large, but variable H20+ and C02 contents in most samples. The secondary minerals include calcite, celadonite, chlorite, various clay minerals, pyrite, and quartz.
Elements such as Nb (Fig. 2a), Hf, Zr, Y, the REE, and Se (Fig. 3f) were rather immobile during the alteration (assuming that Ta was also immobile). It is mentioned above that sodium is low in the altered samples (Fig. 3e). With the exception of one sample (B 22), K20 and Rb have
NORSK GEOLOGISK TIDSSKRIFI' 79 (1999)
Table 4. Trace element composition of plagioclase separates (in ppm).
B 24 B 56 B 49 Hawaiite Hawaii te Benmore.
Fe(%) 0.49 0.44 0.38 Se 0.44 0.30 0.14 Cr 1.66 0.97 ND Co 1.35 1.23 ND Na(%) 2.34 1.97 5.30 Ba 57 36 397 La 1.46 1.16 6.5 Ce 2.72 2.31 9.00 Nd 1.13 ND 3.7 Sm 0.20 0.175 0.646 Eu 0.528 0.50 5.78 Th ND ND 0.061 Yb ND ND ND Lu ND ND 0.014 Hf 0.051 0.067 ND Ta ND ND ND Th 0.027 ND 0.055 D(Ba) plag!melt 0.28 0.18 0.72
ND= not detected. Detection limit for Yb and Ta = 0.08 and 0.03 resp.
been leached during hydrothermal alteration (Fig. 3a, b ) . With a few exceptions Ba, P205, Ti02, and FeO appear to have been less mobile. CaO, Sr (Fig. 3c, d) and MnO (Fig. 2t) show enrichment in most altered hawaiites. One sample (a hawaiite sill; B 22, the most heavily altered of all samples in terms of total volatiles) is especially notable because it is the only sample considerably enriched in K20, Rb, Ba, Sr and Ca. In this sample, all phenocrysts and most groundmass plagioclase are completely replaced by sericite + calcite ± quartz, and other phases have been transformed into a mixture of clay minerals. Vesicles have been filled with calcite, chlorite, various clay minerals and sometimes quartz.
Oxygen isotope data (Table 7) show low 6180 values in the heavily altered samples, as low as + 1 .9o/oo in B 22 and B 1 9. However, in samples B 23 and B 24, 6180 is +6.0%o
Table 5. Average composition of olivine, clinopyroxene, and amphibole.
Olivine Olivine Benmore. Comendite
Si02 33.39 30.44 Si02 FeO 51.31 66.94 Ti02 Mn O 1.62 2.47 Ah03 MgO 13.77 0.08 Fe O Ca O 0.42 0.44 Mn O Total 100.51 100.36 MgO
Ca O Na20 KzO Total
Molecular ratios: Fo 31.7 0.2 Wo Fa 66.2 96.2 En Tf 2.1 3.6 Fs
D(Mn) 7.4 24.7 D(Mn) D(Ca) O.l 0.6 D(Ca) D(Fe) 5.9 16.7 D(Fe)
MnO/FeO 0.032 0.037 MnO/FeO
Volcanic rocks, Bouvetøya 21 1
and +6.2%o, respectively, and + 7.2o/oo in B 34. The significantly higher values of B 23 and B 24 may be related to their location in the fumarole area, while the samples with the lowest 6180 are from the hyaloclastite section. Apparently, the fumarolic water has other isotopic signatures than the fluids that altered the lower hyaloclastite formation of the island. Average 6180 for five fresh hawaiites is +5.5%o. Comparing our data on altered rocks with the data on Rb, Sr, Na20 and K20 published by O'Nions & Pankhurst ( 1974) and O'Nions et al. ( 1977) on hawaiite and benmoreite from Bouvetøya, we can conclude that some of the samples analysed by these authors represent heavily altered material. However, this alteration does not appear to have affected the 87 Sr/86Sr and 143Nd/144Nd ratios.
Mineral compositions Several hundred mineral grains have been analysed by electron microprobe. Some olivine, pyroxene and amphibole data are summarized in Table 5 and Fig. 4.
The feldspars show large compositional variation. In the hawaiites they range from An 83 to An 45 with An 60-75 as the most common composition. It is difficult, however, to evaluate the equilibrium plagioclase composition of the hawaiites, but in the sparsely porphyritic samples An 55-62 are common compositions. Plagioclase phenocrysts of the benmoreites are typically around An 30, and microlites grade into the field of anorthoclase. The feldspar of the trachyte (B 16) is anorthoclase, whereas the comendites have a very Ca-poor feldspar; the most potassium-rich varieties are close to the alkali-feldspar minimum melt composition, i.e. Or 35.
Plagioclase separates have been analysed for some trace elements (Table 4). The small concentrations of Fe, Se, Cr, and Co show that very clean separates were
Cpx Cpx Amphib. Amphib. Benmore. Comendite Comendite Cations
50.54 48.19 51.12 Si 7.7 0.63 0.07 0.21 Al 0.4 1.21 0.18 0.24 Ti 0.2
17.16 28.98 35.93 Mg 0.1 0.64 0.92 0.72 Fe 4.5
10.00 0.19 0.07 Mn O.O 20.08 19.75 1.9 Ca 0.3
0.31 0.20 7.69 Na 2.2 1.27 K 0.2
100.57 98.48 99.15 16.0
42.4 46.4 29.4 0.6 28.2 53.0
2.9 9.2 7.2 5.2 25.0 2.4 2.0 7.3 9.0
0.037 0.032 0.020
2 12 T. Prestvik et al. NORSK GEOLOGISK TIDSSKRIFT 79 (1999)
Table 6. Petrogenetic models of various rock types from the Bouvet Island. A least-squares approximation method (Bryan et al. 1969) was used.
(I) Fractionation within hawaiites (B 59-B 79) A. Hawaiite B 59 100.00% B79 obs B79 est
-Olivine Fo 70 0.71% Si 51.640 51.882 -Clinopyroxene Fs 17 8.84% Ti 3.410 3.431 -Plagioclase An 56 13.42% Al 14.140 14.126 -Ilmenite 1.72% Fe 12.390 12.495 -Titanomagnetite 0.46% Mn 0.210 0.202 =Hawaiite B 79 74.85% Mg 3.720 3.741
Ca 7.850 7.873 Na 4.380 4.359 K 1.540 1.523
E�= 0.073 p 0.720 0.737
(Il) From hawaiite to benmoreite (B 79-B 28) Hawaiite B 79 100.00% B28 obs B28 est
-Olivine Fo 64 3.57% Si 61.140 61.133 -Clinopyrox Fs 19 15.69% Ti 1.160 1.159 -Plagioclase An 42 24.17% Al 14.720 14.713 -Apatite 1.27% Fe 8.760 8.760 -Ilmenite 2.91% Mn 0.240 0.232 -Titanomagnetite 5.13% Mg 0.950 0.958 =Benmoreite B 28 47.26% Ca 4.010 4.023
Na 5.660 5.731 K 2.970 2.998
E�= 0.006 p 0.380 0.363
(IV) From trachyte to comendite (B 16-B 39) Trachyte B 16 100.00% B39 obs B39 est
-Clinopyroxene Fs 47 3.81% Si 70.400 70.404 -Plagioclase An JO 8.72% Ti 0.320 0.291 -Feldspar Or�b�2 17.34% Al 13.410 13.575 -Titanomagnetite 0.43% Fe 4.250 4.254 = Rhyolite B 39 69.70% Mn 0.120 0.164
Ca 0.790 0.779 Na 6.050 5.630 K 4.930 4.904
E�= 0.207 p 0.020 0.043
obtained. Especially interesting is the large Ba content of benmoreite plagioclase (397 ppm) compared to 35 and 56 ppm in plagioclase of hawaiites. This contrast shows that the distribution coefficient of Ba (D8a
plaglliquid) is increasing from about 0.2 to 0.72 as An contents (and T of equilibrium) decrease. The benmoreite plagioclase (An 33) is considerably richer in the REE than are accumulated bytownites (An 71-75) in hawaiites (see
below). The clinopyroxene data are shown in Fig. 4, and the
averages of representative pyroxenes and olivines of the intermediate and silicic rocks are presented in Table 5 . Pyroxene of benmoreite is Fe-rich augite, and in trachyte and comendite almost pure hedenbergite is present. Because most hawaiite pyroxenes are irregularly zoned, it is difficult to estimate a reliable 'average' composition. It is, however, important to recognize the compositional variation, because that sheds light on the magma' s preeruption his tory.
Olivine varies from Fo 87 to Fo 45 in the hawaiites, but compositions around Fo 70 seem to be most common. These may represent equilibrium olivine, because a range of Fo 75--65 would be expected for the actual whole-rock Mg numbers (Roeder & Emslie 1970). The most magnesium-rich olivine that occurs in the hawaiites is, however, petrogenetically important because it shows the existence of more primitive magmas at deeper levels. The olivine
B. Hawaiite B 59 100.00% B79 obs B79 est -Olivine Fo 70 1.35% Si 51.640 52.679 -Clinopyroxene Fs 17 6.98% Ti 3.410 3.418 -Plagioclase An 65 12.67% Al 14.140 13.951 -Ilmenite 1.82% Fe 12.390 12.704 =Hawaiite B 79 77.18% Mn 0.210 0.202
Mg 3.720 3.722 Ca 7.850 7.847 Na 4.380 4.416 K 1.540 1.525
E�= 1.216 p 0.720 0.721
(Ill) From benmoreite to trachyte (B 28-B 16) Benmoreite B 28 100.00% Bl6 obs BJ6 est
-Olivine Fo 32 3.29% Si 68.410 68.491 -Clinopyroxene Fs 28 4.24% Ti 0.310 0.307 -Plagioclase An 31 24.13% Al 14.640 13.978 -Titanomagnetite 4.57% Fe 4.350 4.544 -Apati te 0.84% Mg 0.020 0.020 =Trachyte B 16 61.93% Ca 1.580 1.609
Na 6.030 5.999 K 4.500 4.469
E�= 0.486 p 0.030 0.030
(V) From benmoreite to comendite (B 28-B39) B28 100.00% B39 obs B39 est
-Olivine Fo 32 3.55% Si 70.380 70.376 -Clinopyroxene Fs 33 5.10% Ti 0.320 0.311 -Plagioclase An 31 30.68% Al 13.410 13.430 -Apati te 0.86% Fe 4.250 4.252 -Titanomagnetite 4.64% Mg 0.001 0.002 =Rhyolite B 39 55.35% Ca 0.790 0.796
Na 6.050 6.006 K 4.930 4.977
E�= 0.005 p 0.020 0.012
phenocrysts of benmoreite are around Fo 30, whereas pure fayalite (Fo 0.2, Table 5) is found in several comendite samples.
Hedenbergite and fayalite have not been reported from Bouvetøya previously. In both of these minerals the MnO content is relatively large (Table 5) . Thus, MnO contents range to 2.59 wt% in olivine of comendite sample B 83.
Imsland et al. (1977) reported rare blue-green alkali amphibole in some comendite samples. We present microprobe data of such amphiboles from a fresh holocrystalline comendite (Table 5) . This is arfvedsonite according to the classification of Leake (1978).
Rare earth elements (REE).- In the petrologic models of Imsland et al. (1977) and le Roex & Erlank ( 1982), as well as in our own models (Table 6), plagioclase fractionation is important. Extensive plagioclase fractionation is usually thought to deplete the residual melt in Eu because this element readily replaces Ca (and Sr) in the feldspar lattice.
Plagioclase separates of hawaiite and benmoreite all show a significant positive Eu anomaly (Fig. 5) where Eu is enriched about 10 times compared to the neighbour elements in hawaiite and about 30 times in benmoreite. As can be seen from Fig. 5 , benmoreite does not display negative Eu anomalies despite the approximately 24%
NORSK GEOLOGISK TIDSSKRIFT 79 (1999) Volcanic rocks, Bouvetøya 213
Table 7. Isotope data for Bouvetøya rocks.
Sample 8 180** 87Sr/86Sr 143Nd/144Nd 206PbP04Pb 207Pb/204Pb 208PbP04Pb
Hawaiite B 56 +5.8 0.703626 0.512895 ND ND ND Hawaiite B 59 ND 0.703695 0.512961 ND ND ND Hawaiite B 62 +5.4 0.703629 0.512919 ND ND ND Hawaiite B 79 +6.0 0.703575 0.512891 19.105 15.671 38.836 Benmor. B 28 +6.5 0.703609 0.512887 ND ND ND Trachyte B 16 +7.7 0.703738 0.512962 19.535 15.641 39.138 Comendite B 39 +7.7 0.703754 0.512852 19.606 15.636 39.165 Comendite B 44 +7.4 0.703615 0.512900 ND ND ND *Hawaiite B JO +4.2 0.703675 0.512883 ND ND ND *Hawaiite B 22 +1.9 0.703739 0.512850 19.420 15.630 39.053 *Mugear. B 34 +7.2 0.703524 0.512888 ND ND ND *Benmor. B 49 +5.8 0.703665 0.512845 19.687 15.723 39.434
* Altered samples. ** Additional 8 180 detenninations on two fresh hawaiites are +5.0 and +5.5, and from + 1.9 to +6.2 in 5 altered hawaiites (see also Tab le 3). ND = not detennined.
plagioclase fractionation that is indicated from the petrologic models (Table 6; model ll). Intuitively, and as was stated by Dickey et al. (1977), one should expect a considerable effect on the Eu content of the derived melt when Eu-enriched plagioclase is removed. For the Bouvetøya suite, therefore, two questions arise: (i) Is it possible to fractionate a feldspar with a pronounced positive Eu anomaly without developing a negative anomaly in the derived melt, and/or (ii) Is the petrologic model principally wrong, perhaps because plagioclase fractionation was not that important? The answers appear to be very simple: the plots in Fig. 5 also show another important feature; the bytownites of hawaiite have very small concentrations of REE compared to those of the whole rock compositions. Even the apparently large Eu values are significantly lower than in the host hawaiites, i.e. the plot is a visual way of showing that Eu has a small aggregate distribution coefficient (for Eu3+ and Eu
2+ combined) between basic plagioclase and a mafic melt, a fact that has been known for many years. A similar relationship exists between the plagioclase (An 33) and the host benmoreite (Fig. 5) except that in this case the Eu content is larger than in the melt the plagioclase crystallized from; i.e. �u
andesine/melt is greater than unity (about
En (Fo)
Fs (Fa)
Fig. 4. Clinopyroxene plotted in the En-Fs-Di-He quadrilateral. For simplicity, olivine compositions (Fo-Pa) are plotted in the same diagram, along the En-Fs join (filled circles).
1.3), and therefore further fractionation may be very efficient in developing negative Eu anomalies in the more silicic melts. It is thus logical that trachyte (B 16), which is somewhat less silicic than the comendite, displays an REE pattern which is parallel with that of comendite, but has smaller REE abundances and a less pronounced negative Eu anomaly.
It was mentioned above that Eu2+ is expected to have
geochernical behaviour similar to Sr. However, Eu occurs
1000.---------------------------------------,
100
10
La Ce Nd Sm Eu Tb Yb Lu
Fig. 5. Chondrite-normalized REE (rare earth elements) patterns of average hawaiite, benmoreite and comendite, plotted together with patterns of plagioclase separates from hawaiites (An71 & An 75) and benmoreite (AN 33). Values for HREE of basic plagioclases are stipulated on the basis of data obtained for basic plagioclase from Iceland (Prestvik l 985) which was analysed in the same experiment.
214 T. Prestvik et al. NORSK GEOLOGISK TIDSSKRIFT 79 (1999)
Peter 1 . øy
-:li:: (.) o 0::
1 0 • Bouvetøya * Bouvet ridge
Ba Th Rb K
Nb La Ta
Sr p Zr Ti y Yb Ce Nd Sm Hf Tb
Fig. 6. Chondrite-nonnalized minor and trace element diagram (spider diagram,Thompson et al. (1984)) of average Bouvetøya hawaiite (large filled circles). For comparison, and at fixed (Yb)N = 10, are plotted pattems of Bouvet Ridge basaalt (Dickey et al. 1977) and alkali basalt from Peter I Øy, Bellinghausen Sea, Antarctica (Prestvik et al. 1990).
in both the 2+ and the 3+ states. Thus, the aggregate distribution coefficient for Eu depends on the prevailing oxidation states. Distribution coefficients are also temperature dependent (Blundy & Wood 1991) ; they increase with decreasing temperature. If we apply distribution coefficients between plagioclase and melt for Eu
2+ and
Eu3+ similar to those for s�+ and REE3+, respectively, we can estimate the proportion of the two Eu ions in the melt. If we distribute the observed aggregate DEu of 0.2 between bytownite plagioclase and hawaiite with DEuZ
+ = l and
DEuJ+
= 0.03, we find that the Eu3+Æu2+ ratio in the melt
is ca. 4.7; in other words, Eu is rather oxidized. Choosing a higher coefficient for Eu2+ (Dsllagioclaselmelt range between l and 3) would correspond to an even more oxidized melt. Making use of the same procedure for andesinelbenmoreite, using aggregate DEu = 1 .3 (as observed), DEuJ
+ = 0.05, and DEuZ
+ = 2.55 gives a Eu3+/
Eu2+ ratio of 0.85, i.e. considerably more reduced than in the basic melts. These results are in general agreement with the conclusion drawn by Prestvik ( 1982a) on a qualitative basis. If we compare with the experimental data of Weill & Drake ( 1973), we find that f 02 for bytownite/ hawaiite at l 100°C is about 10-5 Pa and about 10-8·5 Pa for andesinelbenmoreite at 1000°C. For comparison, le Roex & Erlank ( 1982) estimatedf02 for hawaiite at l00°C to be about 10-6·0 Pa on the basis of coexisting Timagnetite and ilmenite.
New petrologic model W e used a common least squares mass balance mixing
technique (Bryan et al. 1969) to evaluate the possible relationships between the observed rock and mineral compositions. Before modelling, all data were normalized to 1 00% volatile-free, and total iron was expressed as FeO. In models involving silicic rocks (models IV and V), Mg was omitted in model IV, because the poor analytical accuracy of whole rock analyses with MgO :::; 0. 10% made error estimates meaningless. Mn was omitted from models IV and V. Our models are presented in Table 6.
W e made the calculations in several steps. As described above, there is considerable chemical variation within the hawaiite group, and we wanted to include this variation in our modelling. We chose sample B 59 (K20 = 1 . 1 8) as representative of the least evolved hawaiites and B 79 (K20 = 1 .54 and transitional to mugearite) as the most evolved. Both these groups are almost non-porphyritic. Models lA and lB (Table 6) represent variation within the group of hawaiites. Models Il and Ill show the evolution from evolved hawaiite (B 79) to benmoreite (B 28; K20 = 2.97) and furthermore to trachyte (B 16; K20 = 4.50). The evolution of comendite (B 39; K20 = 4.93) is presented in two models, model IV from trachyte and model V directly from benmoreite. With one exception (lb), we present models with low residuals CE� <0.5), but we also rejected models with low residuals if the relative discrepancy for Na and K was > 1 .5%.
Our preferred sequence of evolution of the Bouvet suite is as follows: hawaiite � evolved hawaiite � benmoreite � trachyte � comendite (models la, Il, Ill & IV), see discussion below.
NORSK GEOLOGISK TIDSSKRIFT 79 (1999)
1 5, 9 t-
1 5, 8 .c
l. a.. N 1 5, 7
.o ,._a.. 1 5, 6 o N
1 5, 5 r-EM-1
i>
1 5,4 D M M
1 7
40
EM-1
38 DMM
1 7
0, 705 - EM-1
1 8
::: 0, 704 ,._(/) CD
0, 703
1 8
o
-$
1 9
'
o
l l
. �
l H I M U
-
l 20 21 22
H I M U
2 1 22
-
H I M U
DMM o, 702 ..."__.___._....____..._�_._-...._I_.___.,_...L...-J'
1 7 1 8 1 9 20 21 22
Fig. 7. Radiogenic isotope plots of Bouvetøya rocks. Filled circles - hawaiite; open circles - evolved rocks; stars - Ph data from Sun (1980). Locations of DMM (depleted morb mantle), EM-l (enriched mantle l) and HlMU (hlgh 11; 11 = 238Uf041>b) fields are taken from Hofmann (1997).
Discussion
Former models
We agree with Imsland et al. ( 1977) that the irregularly zoned and commonly corroded basic plagioclase megacrysts in the hawaiites are not real phenocrysts. On the basis of mixing calculations, le Roex & Erlank ( 1982)
Volcanic rocks, Bouvetøya 215
suggested that such phyric hawaiites represented regular Bouvetøya hawaiite with as much as 40% accumulation of plagioclase (34-37%) and clinopyroxene (3-6% ). As mentioned above, the only problem with this interpretation was an inferred Ba inconsistency (le Roex & Erlank 1 982, p. 333). However, the very small Ba concentrations obtained by us on hawaiite plagioclase separates (Table 4) explain and eliminate this problem.
Transitional basalts I of Imsland et al. ( 1977) correspond well to 'accumulated' hawaiite of le Roex & Erlank ( 1982) . This rock type is represented by samples such as B 24 and B 56 (the hawaiite samples from which we separated plagioclase). However, the strongly phyric hawaiites not only have An-rich plagioclase, bot also tend to have the most primitive clinopyroxene and olivine, i.e. a mineral assemblage representing more primitive melts. Our data show that Mg numbers (Mg#s) of the hawaiites from Bouvetøya range from 36 to 55 (not tabulated) . The largest Mg#s are found in the phyric hawaiites. If these rocks were formed by accumulation of pheno/xenocrysts, as suggested by le Roex & Erlank ( 1 982), it would require that as much as 15% cpx + ol was added to hawaiite melt (B 59) in addition to plagioclase. Unless significant resorption occurred, this seems to be unlikely, based on observed hawaiite modal composition. W e favour an interpretation in which more primitive hawaiitic magmas (than those represented by the aphyric hawaiites) were present and fractionated at depth, and that only porphyritic varieties are represented in our sample collection. Sometimes, also more evolved hawaiite melts accumulated phenocrysts (especially plagioclase) from their basic precursors and brought these to the surface.
There is a tendency, however, for a gradual compositional change also within the group of sparsely phyric or non-phyric hawaiites. Low Mg#s and generally small concentrations of compatible elements such as Ni, Co, Se, and Cr in these hawaiites (Tab le l) strongly indicate a history in which olivine and clinopyroxene ( + spinel?) fractionated from the more primitive magmas. Prefractionation of plagioclase is not documented by negative Eu anomalies in REE pattems of the basic rocks, because plagioclase has such small Eu concentrations (Tab le 4 ) . However, the strong, negative anomaly of strontium in the spidergram of the Bouvetøya hawaiite (Fig. 6) indicates that parental magmas at depth also crystallized plagioclase, because distribution coefficients of Sr are large enough ( 1-3; Henderson 1982) effectively to remove this element during plagioclase fractionation.
New models
We present two fractionation models (Table 6, lA & lB) to explain compositional variation in the hawaiites. Both suggest ca. 25% fractionation of ol + cpx + plag + ilm + mt, all of which occur in hawaiite. The difference in the two models is the plagioclase composition (An 56 in lA; An 65 in lB) . However, the unacceptably high residual in model lB makes that model
216 T. Prestvik et al.
unrealistic. Even though modal plagioclase in many hawaiites falls in the range An 60 - 75, we did not consider models using plagioclase more calcic than An 65 . As was discussed above, the true equilibrium plagioclase composition may well be lower; and we think the plagioclase (An 56) used in model IA is a realistic choice. The composition of olivine and clinopyroxene is as observed in hawaiite. Thus, IA is our preferred model for this stage. The amount of fractionation is consistent with the observed incompatible element variation.
In model Il, benmoreite is derived from evolved hawaiite by slightly more than 50% fractionation. This is about 10-15% less than reported by both Imsland et al. ( 1977) and le Roex & Erlank ( 1982). However, the parent hawaiites used in the models of these authors were less evolved than those we have used. The model uses minerals and compositions that are typical in evolved hawaiite, except that the plagioclase is slightly more sodic than is common in hawaiite. Since this step is so large, we tind it reasonable that the average fractionating plagioclase is intermediate between parent and daughter compositions. Olivine and clinopyroxene compositions are both fairly magnesium-rich, but that can be explained by postulating that these minerals fractionated early in the sequence. This model has very low residuals. Model Ill shows bow trachyte can be formed by ca. 38% fractionation from benmoreite. Only benmoreite mineral compositions were used in this model, which has a somewhat high residual. This is mainly due to misfits in the Al and Fe contents (both ca. 4.5% relative). We still believe this is an acceptable model for the derivation of trachyte, not least because trachyte is intimately related to benmoreite in the field. The trachyte is relatively enriched in incompatible elements such as Ba, Rb, Y, il and Nb compared to benmoreite, but strontium is considerably lower (89 vs. ca. 3 1 5 ppm). This requires bulk Dsr and DBa of ca. 3.6 and 0.5, respectively. With the actual proportion of plagioclase in the fractionating assemblage, this gives D��ag of 5 .4, which is reasonable for plagioclase An 31 (Nash & Crecraft 1985), and Dfl:g = 0.72, which is identical to calculated values from the analysed plagioclase separate from benmoreite (Table 4).
In models IV and V (Table 6), we have sought to deterrnine bow further fractionation from trachyte (IV) or benmoreite (V) can yield comendite. In model IV, there is a considerable deficiency in Na content (4.49% relative) of the derived comendite. In this model, we used mineral compositions from the trachyte, including alkali feldspar. In order to avoid the Na deficiency, a much less sodic and unrealistic feldspar composition was attempted, which in turn created problems with other elements. In model V, mineral compositions of phenocrysts in benmoreite were used, and the fit is almost perfect for all elements. However, le Roex & Erlank ( 1982) rejected their model in which comendite could be formed from benmoreite, because the model required a fractionating plagioclase of An 47 (An 3 1 in our model V), and because there was a 'Ba and Sr problem' . Instead, they suggested that comendite
NORSK GEOLOGISK TIDSSKRIFT 79 (1999)
was formed directly from mugearite, a very long fractionating step (69%) in which a plagioclase of An 47 was required.
The barium (and strontium) problem
Prestvik ( 1 982a) reported Ba contents of three comendites (1 83-216-245 ppm) from Bouvetøya. Similarly, le Roex & Erlank ( 1982) reported 180 and 298 ppm Ba in two comendites. In this work, we obtained 3 1 2 ppm Ba in comendite sample B 39 (obsidian). However, our crystalline comendites are much more depleted in Ba (and Sr), with only 80 ppm Ba. Apparently, the 'barium problem' would have been even larger if these samples bad been used in the models. From the whole rock data we have calculated the value of DBa and Dsr for the fractionating feldspar of models IV and V (the comendite models). In model IV (formation of comendite from trachyte) we get DBa = 3.8 and Dsr = 5 .7. The fractionating feldspar in this model is about 2/3 alkali feldspar and 113 anorthoclase (An 10) . The estimated D's are well within the possible ranges for both elements (Henderson 1982; Blundy & Wood 1991) . For model V (formation of comendite directly from benmoreite) the values of DBa and Dsr for fractionating feldspar are 2.8 and 4.2, respectively. For plagioclase (An 3 1 ) this is a plausible value, but the DBa value is much larger than that found by us for this plagioclase when we analysed the mineral separate (DBa = 0.72). We thus agree with le Roex & Erlank ( 1982) that comendite did not form by fractionation directly from benmoreite. W e disagree, however, that the comendite was formed by fractionation directly from mugearite. Despite the Na discrepancy of the comendite derived by fractionation of trachyte, we think that model IV provides a plausible explanation for the way in which Bouvetøya comendites are deri ved. The very high Ba content of trachyte - and much lower content in comendite - explains bow the benmoreite magmas fractionate through a trachyte stage and shows that there is a shift from plagioclase to alkali feldspar as a
fractionating phase around the trachyte stage. Furthermore, the ferromagnesian minerals are much more evolved in trachyte than benmoreite. Fractionation of these minerals not only results in comenditic composition, but also explains the trace element data, such as the Ba and Sr concentration. We have no valid explanation for the Na discrepancy seen in model IV, but the model is, of course, 'vulnerable' , owing to the fact that we have only one sample of trachyte.
We have also tried to model (not tabulated) the relationship between comendite B 39 (glass) and crystalline comendite. This is possible by approximately 7% fractionation of alkali feldspar and min or amounts (ca. 2%) of o l + cpx + mt if Dfeldspar for both Sr and Ba is about 19 . This is within the published range for Sr, but higher than the value we have seen published for Ba (up to 12.9; Henderson (1982)).
NORSK GEOLOGISK TIDSSKRIFT 79 (1999)
Mantle signature
On the basis of Y - Ti - Zr ratios Prestvik ( 1982b) characterized the basic rocks from Bouvetøya as 'within plate' type (i.e. influenced by a mantle plume in more modem terms). Based on trace element ratios such as Y/Nb, Zr/Nb, (La/Yb)N, le Roex ( 1987) characterized them as identical to P(lume )-type MORB of the AmericanAntarctic Ridge (AAR) and South-West Indian Ridge, which he suggested had formed by partial melting of depleted astenosphere that had been veined by enriched melt (2-4%) in a previous metting event. Our new data show that the geochemical signatures displayed by the Bouvetøya suite (Fig. 6) are those of a 'mildly enriched' ocean island type (Thompson et al. 1984), with typical high normalized abundances of the HFS elements Nb and Ta. The average Y INb and Zr/Nb ratios of Bouvetøya hawaiites, which are 0.86 and 6.06, respectively (Table 1), also attest to their enriched nature.
Although there is a considerable amount of Sr and Nd isotope data for the ridge axes of the BTJ area (e.g. Dickey et al. 1977; le Roex et al. 1983; 1985; 1987), very few such data are published from Bouvetøya (O'Nions & Pankhurst 1974; O'Nions et al. 1977), and the only known Pb isotope data from Bouvetøya are three determinations of Sun (1980). Here we report new Sr and Nd (as well as O) isotopic data on 11 samples from Bouvetøya and Pb isotope data on 5 samples (Table 7), all collected during the Polarsirkel expedition. In Fig. 7 we have plotted some of the new data together with the previously published Bouvetøya data (O'Nions & Pankhurst 1974; O'Nions et al. 1977; Sun 1980). The Bouvetøya rocks fall on, or very dose to, mixing lines between EM-l and HIMU basalts in both 207 PbP04 Pb vs?06 PbP04 Pb, 20 8 PbP04 Pb vs.206 PbP04 Pb and 87 Sr/86 Sr vs. 206 PbP04 Pb plots (Fi,f,; 7) and very dose to such mixing trends in 143Nd/1 Nd vs?06 PbP04 Pb and 143Nd/144Nd vs.87 Sr/86 Sr diagrams (not shown). However, the combined isotope data (e.g. the 87 Sr/86 Sr vs?06 PbP04 Pb plot) are not compatible with mixing between a depleted mantle source (DMM in Fig. 7) and enriched sources such as EM-l (and EM-2) or HIMU sources. This implies that the model of le Roex (1987) has to be modified.
Enrichment was probably integral to the plume. HIMU signatures are primarily seen from elevated U/Pb ratios because of loss of Pb (Vidal l992). Unfortunately, we lack elementa! Pb data on the Bouvetøya rocks, but typical HIMU signatures are apparent also from the Ce/Rb and Bal Nb ratios of hawaiites (average 3.13 and 4.78, respectively, of 9 samples).
Dredged samples from the South-West Indian Ridge immediately east of Bouvet (Fig. l) are isotopically almost identical to the Bouvetøya rocks and apparently related to the Bouvet plume as well. However, the regional influence of the Bouvet plume is not well understood. The possible presence of more than one plume in the BTJ area has been discussed by several authors (i.e. le Roex 1987). Thus, Moreira et al. ( 1995), Douglas et al. ( 1995) and Simonov et
Volcanic rocks, Bouvetøya 217
al. ( 1996) indicated that the anomalously enriched segment of the southem MAR east of the Shona seamount (the 'Shona ridge-anomaly' , Fig. l)- only about 600 km from Bouvetøya - might represent a separate Shona plume.
Conclusions
Our study supports previously published models which concluded that the main rock types of Bouvetøya are interrelated by fractional crystallization. However, we find that the best evolutionary path of the suite was as follows: hawaiite � evolved hawaiite � benmoreite � trachyte � comendite.
Primitive megacrysts of olivine, clinopyroxene and plagioclase in hawaiite represent phenocrysts of more primitive (parental) magmas that crystallized at depth, indicating the presence of an open-system magma chamber(s) in which variously evolved melts and crystal assemblages were able to mix.
Similarity in trace element and isotope ratios indicate that the Bouvetøya suite, and the much more primitive basalts dredged from the Bouvet Ridge east of the island, originated from a common, enriched mantle source. The radiogenic isotope data show that this source is complex, consisting of several mantle components.
Acknowledgements. - We express our thanks to the Norsk Polarinstitutt for giving T.P. the opportunity to participate in the Polarsirkel expedition to Bouvetøya in
1978/79. A grant from The Norwegian Research Council and several contributions from the Department of Geology and Mineral Resources Engineering, NTNU supported the analytical work. We also extend our thanks to Drs Elizabeth E. Anthony and Asish R. Basu for analytical assistance and to Drs Pall lrnsland, Sven Maaløe, Calvin G. Barnes and one anonymous journal reviewer for their constructive comments on the manuscript.
Manuscript received February 1999
References
Blundy, J. D. & Wood, B. J. 1991: Crystal-chemical controls on the partitioning of Sr and Ba between plagioclase feldspar, silicate melts, and hydrothermal
solutions. Geochimica et CostrUJchimica Acta 55, 193-209. Bryan, W. B., Finger, L. W. & Chayes, F. 1969: Estimating proportions in
petrographic mixing equations by least-squares approximation. Science 163, 926-927.
Dickey, J. S., Frey, F. A., Hart, S. R. & Watson, E. B. 1977: Geochemistry and petrology of dredged basalts from the Bouvet triple junction, South Atlantic. Geochimica et Cosmochimica Acta 41, 1105-1118.
Douglas, J., Schilling, J.-G. & Kingsley, R. H. 1995: Influence of the Discovery and Shona mantle plumes on the southem Mid-Atlantic Ridge: Rare earth evidence. Geophysical Research Letters 22, 2893-2896.
Govindaraju, K. (ed.) 1989: Analytical standards of minerals, ores and rocks. Geostandards Newsletter, vol. XIII, special issue.
Hart, S. R. 1984: A large-scale isotope anomaly in the southem hemisphere mantle. Nature 309, 753-757.
Henderson, P. 1982: lnorganic Geochemistry, 353 pp. Pergamon Press, London. Hofmann, A. W. 1997: Mantle geochemistry: the message from oceanic
volcanism. Nature 385, 219-229.
Imsland, P., Larsen, J. G., Prestvik, T. & Sigmond, E. M. 1977: The geology and petrology of Bouvetøya, South Atlantic Ocean. Lithos JO, 213-234.
Leake, B. E. 1978: Nomenclature of amphiboles. American Mineralogist 63, 1023-1052.
Le Bas, M. J., Le Maitre, R. W., Streckeisen, A. & Zanettin, B. 1986: A classification of volcanic rocks based on the total alkali-silica diagram. Journal
of Petrology 27, 745-750.
218 T. Prestvik et al.
le Roex, A. P. 1987: Source regions of mid-ocean ridge basalts: evidence for enrichment processes. In Menzies, M. A. & Hawkesworth, C .1. (eds.): Mantle Metasomiltism, 389-422. Academic Press, London.
le Roex, A. P. & Erlank, A. J. 1982: Quantitative evaluation of fractional crystallization in Bouvet Island lavas. Journal of Vo/canology and Geothermal Research, 13, 309-338.
le Roex, A. P., Dick, H. J. B., Erlank, A. J., Reid, A.M., Frey, F. A. & Hart, S. R. 1983: Geochemistry, mineralogy and petrogenesis of lavas erupted along the southwest Indian Ridge between the Bouvet triple junction and Il degrees east. Journal of Petrology 24, 267-318.
le Roex, A. P., Dick, H. J. B., Reid, A.M., Frey, F. A., Erlank, A. J. & Hart, S. R. 1985: Petrology and geochemistry of basalts from the American-Antarctic Ridge, Southem Ocean: implications for the westward infiuence of the Bouvet mantle plume. Contributions to Mineralogy and Petrology 90, 367-380.
le Roex, A. P., Dick, H. J. B., Gulen, L., Reid, A.M. & Erlank, A. J. 1987: Local and regional heterogeneity in MORB from the Mid-Atlantic Ridge between 54.5 os and 51°S: Evidence for geochemical enrichment. Geochimica et Cosmochimica Acta 51, 541-555.
Ligi, M., Bonatti, E., Bortoluzzi, G., Carrara, G., Fabretti, P., Penitenti, D., Gilod, D., Peyve, A. A., Skolotnev, S. & Turko, N. 1997: Death and transfiguration of a triple junction in the South Atlantic. Science 276, 243-245.
Mitchell, N. C. & Livermore, R. A. 1998: The present configuration of the Bouvet triple junction. Geology 26, 267-270.
Moreira, M., Staudacher, T., Sarda, P., Schilling, J.-G. & Allegre, C. J. 1995: A primitive plume neon component in MORB: the Shona ridge-anomaly, South Atlantic (51-52°5). Earth and Planetary Science Letters 133, 367-377.
Nash, W. P. & Crecraft, H. R. 1985: Partition coefficients for trace elements in silicic magmas. Geochimica et Cosmochimica Acta 49, 2309-2322.
O'Nions, R. K. & Pankhurst, R. J. 1974: Petrogenetic significance of isotope and trace element variations in volcanic rocks from the Mid-Atlantic. Journal of Petrology 15, 603-634.
O'Nions, R. K., Hamilton, P. J. & Evensen, N. M. 1977: Variations in 143Ndi1""Nd and 87Sr/86Sr ratios in oceanic basalts. Earth and Planetary Science Letters 34, 13-22.
NORSK GEOLOGISK TIDSSKRIFf 79 (1999)
Prestvik, T. 1982a: Trace element geochemistry of volcanic rocks from Bouvetøya, South Atlantic. In Craddock, C. (ed.): Antarctic Geoscience,
771-774. The University of Wisconsin Press, Madison. Prestvik, T. 1982b: Basic volcanic rocks and tectonic setting. A discussion of the
Zr-Ti-Y discrimination diagram and its suitability for classification purposes. Lithos 15, 241-247.
Prestvik, T. 1985: Petrology of Quatemary volcanic rocks from Oræfi, south-east Iceland. Report 21, Geological Institute, NI'H, Trondheim, 81 pp.
Prestvik, T. & Winsnes, T. 1981: Geology of Bouvetøya, South Atlantic. Norsk
Polarinstitutt Skrifter 175, 41--{;9. Prestvik, T., Barnes, C. G., Sundvoll, B. & Duncan, R. A. 1990: Petrology of Peter
I Øy (Peter I Island), West Antarctica. Journal of Volcanology and Geothermal
Research 44, 315-338. Roeder, P. & Emslie, R. F. 1970: Olivine-liquid equilibrium. Contribution to
Mineralogy and Petrology 29, 275-289. Simonov, V. A., Peyve, A. A., Kolobov, V. Y., Milosnov, A. A. & Kovyazin, S. V.
1996: Magmatic and hydrothermal processes in the Bouvet Triple Junction Region (South Atlantic). Terra Nova 8, 415-424.
Sun, S.-S. 1980: Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs. Philosophical Transactions of the Royal
Society of London, series A 297, 409-445. Thompson, R. N., Morrison, M. A., Hendry, G. L. & Parry, S. J. 1984: An
assessment of the relative roles of crust and mantle in magma genesis: an elementa! approach. Philosophical Transactions of the Royal Society of
London, series A 310, 549-590. Verwoerd, W. J., Erlank, A. J. & Kable, E. J. D. 1976: Geology and geochemistry
of Bouvet Island. In Gonzales Ferran, O. (ed.): Andean and Antarctic
Volcanology Problems, 203--237. lA VCEI, Naples. Vida!, P. 1992: Mantle: More HIMU in the future? Geochimica et Cosmochimica
Acta 56, 4295-4299. Weill, D. F. & Drake, M. J. 1973: Europium anomaly in plagioclase feldspar:
experimental results and semiquantitative model. Science I 80, l 059-1060.
