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 trachyte and rhyolites are peralkaline (ac-normative; see Tables l, 3). Thus, the rhyolites are comendites, a term that Bouvetøya (official Norwegian speiling) is located 50 km is used in what follows. west of the South-West Indian Ridge (SWIR) in the The The present account is based on material from the Bouvet Triple Junction (BTJ) area of the South Atlantic Polarsirkel expedition. We present (l) major and trace (Fig. l) and consists entirely of young ( < 1.4 Ma) volcanic element composition of the various rock types, (2) rocks. Bouvetøya is 95% covered by permanent ice and no microprobe data on the minerals and trace element data detailed geological map can be made. Geologic and on plagioclase separates, and (3) isotope ratios of O, Sr, petrologicfe atures have been described by several authors. Nd, and Pb. Furthermore, we (4) discuss the results of Among these are the contributionsof Verwoerd et al. (1976) and Imsland et al. ( 1977), who described both the geology and petrology, of Prestvik(1 982a), who reported on trace element geochemical features, and of le Roex & Erlank
( 1982), who discussed the petrologic evolution of the -53' volcanic suite of the island. O'Nions & Pankhurst (1974), O'Nions et al. (1977), and Sun (1980) have published some \ )MAR / data on Sr, Nd, and Pb isotopes. All these contributionswere Shona •. / /� SWIR . 54' " / "- based on samples collected during short visits to the island /'/ ...... "'/ w/(:V/ o (which is of difficult access) from the late 1920s to 1966.
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 Geochemical features basalts' I and IT, (for hawaiites ), and 'transitional icelandite' Analytical methods (for benmoreite), but these authors recognized the peralka line nature of the rhyolites and u sed 'comendite'. Le Roex & Major elements and the trace elements Ba, Rb, Sr, Y, Nb, Erlank (1982) used the terms 'accumulated hawaiite', Cr, Ni, V, Cu, Zn, and Ga were analysed by XRF at 'hawaiite', 'mugearite', 'benmoreite' and 'rhyolite'. 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, Previous petrologic models and associated Ta, and Th on most samples and plagioclase separates were problems analysed by INAA and Zr by XRF at the University of Both lmsland et al. (1977) and le Roex & Erlank (1982) Oregon, Eugene, Oregon, USA. On samples B 55, B 57, B concluded that the rock types of the Bouvetøya suite were 58, and B 59, Se, REE, Hf, Ta, Th, and U were deterrnined related by crystal fractionation and/or accumulation pro by INAA at Imperial College - Reactor Centre, London, cesses. However, both these studies recognized 'problems' UK. Rare earth elements, Se, Hf, Ta, and Th of sample B 31 at variousmodel stages; these will be comrnentedon below. were analysed by INAA at the University of Texas at El lmsland et al. (1977) divided the basic rocks into two Paso, USA. Mineral analyses were obtained at NTNU/ groups on the basis of their K20 content and concluded SINTEF, Trondheim, Norway using a JEOL microprobe. that these two groups were not related by simple Oxygen isotopes were measured at the University of South fractionation, mostly because the model required a more Carolina, Columbia, South Carolina, USA, and some sodic plagioclase than observed in abundant megacrysts. strontium isotopes at University of Rochester, Rochester, They concluded that the benmoreites (trachytic icelan New York, USA, while the rest of the Sr isotopes as well as dites) formed by open-system fractionation from evolved the Nd and Pb isotopes were analysed at the University of hawaiites (transitional basalt Il) and that further fractiona North Carolina, Chapell Hill, North Carolina, USA. tion resulted in comendite. On the other hand, le Roex & Chemical data of fresh whole rocks are presented in Erlank (1982) suggested that porphyritic hawaiites repre Table l. However, before we discuss the general data we sented 'regular Bouvetøya hawaiite' with as much as 40% will comment on the fact that in this study, the N a20 accumulation of plagioclase (34-37%) and clinopyroxene contents of all rock types are consistentlyhigher than those (3-6% ). The only problem with this interpretation was an found in the previous studies (Table 2). Given the inferred Ba inconsistency (le Roex & Erlank 1982, p. 333). considerable significance of sodium concentrations in the These authors rejected a model in which comendite model calculations, this aspect of our data has been (rhyolite) was formed from benmoreite, both because it thoroughly studied. Our Na20 calibration is good for the required a fractionating plagioclase of An 47 (benmoreite 0.03-4.38 wt% calibration range (r = 0.999 16). However, plagioclase is An 33) and because there was a 'Ba and Sr the intermediate and silicic rocks from Bouvetøya are high problem', such that none of the minerals present as in Na20. As a check of calibration at higher values, we phenocrysts in benmoreite could remove enough barium analysed the international standard STM-1 and an intemal (and strontium) to obtain the lower levels of these elements standard TMS as unknowns and got 9.15 wt%, which, observed in comendite. Instead, they suggested that relatively, is 2.3% higher than the recomrnended value comendite was formed directly from mugearite, a very 8.94 for STM-1 (Govindaraju 1989), and 9.52 wt% for long fractionating step ( 69% ), in which a plagioclase of An TMS, which, relatively, is 2.0% higher than our preferred 47 was required. However, this is in conflict with the value of 9.33 wt%. Even though both of these results are on composition of modal plagioclase in mugearite (most the high side of recomrnended or used values, we feel that calcic plagioclase in mugearite is An 45; le Roex & Erlank these are acceptable data. (1982), Table Ill, p. 321). The fractionation model of le W e have no satisfactory explanation for this interla- z o � Table l. Chemical composition of fresh volcanic rocks from Bouvet Island. Major element data normalized to 100%, volatiles are added. "' � Cl B 51 B 55 B 56 B 57 B 58 B 59 B 62 B 70 B 79 B 28 B 67 B 16 B 39 B44 B 83 gJt"' o Si02 50.79 51.05 50.70 50.68 50.98 51.02 50.49 50.40 51.64 61.14 61.86 68.43 70.38 71.30 70.74 Ti0 3.63 3.68 2.80 3.85 3.89 3.52 3.52 3.35 3.41 1.16 1.14 0.31 0.32 0.23 0.24 til@ 2 � Ah03 14.18 14.26 16.62 14.05 14.16 14.47 14.60 15.29 14.14 14.72 14.68 14.64 13.41 13.35 13.67 ..., 12.05 11.70 11.77 ll.l5 Fe0101 12.81 12.55 10.09 12.43 12.39 8.76 8.56 4.35 4.25 3.63 3.54 tila MnO 0.20 0.20 0.16 0.20 0.19 0.19 0.19 0.18 0.21 0.24 0.20 0.13 0.12 0.08 0.08 til � MgO 3.88 4.03 4.44 4.22 4.04 4.49 4.64 4.78 3.72 0.95 0.88 0.00 0.00 0.00 0.00 � Ca O 8.18 8.12 10.09 8.65 8.60 8.99 9.16 9.48 7.85 4.01 3.89 1.58 0.79 0.60 0.60 ::1 _, Na20 4.23 4.01 3.67 3.90 4.04 3.89 3.89 3.75 4.38 5.66 5.44 6.03 6.05 5.96 6.28 'O 1.44 1.36 1.39 1.18 1.18 1.12 � K20 1.43 0.97 1.54 2.97 2.97 4.50 4.93 4.83 4.92 te 'O P20s 0.67 0.67 0.44 0.65 0.67 0.55 0.56 0.51 0.72 0.38 0.37 0.03 0.02 0.01 0.02 � H20+ 0.25 *0.91 0.21 *1.99 *1.69 *0.87 0.38 0.46 0.58 0.49 0.41 1.21 0.23 0.10 0.13 H20- 0.33 0.31 0.26 0.21 0.41 1.10 0.79 0.38 0.15 0.17 0.14 c� 0.04 0.32 0.28 Total 100.59 100.91 100.50 101.98 101.70 100.87 100.64 100.68 100.99 101.62 101.51 101.87 100.65 100.26 100.36 * =L.O.I.
Ba 234 250 156 213 224 213 220 171 264 542 554 701 312 85 74 Rb 22 21 14 23 22 19 20 19 25 56 52 90 105 114 112 Sr 431 424 453 421 415 416 428 445 436 316 312 89 22 5 6 y 42 42 30 41 41 38 38 34 44 69 74 88 108 122 109 'lJ: 301 299 205 291 295 260 257 242 315 619 625 981 1143 1279 1279 Nb 50 48 34 49 49 42 41 40 54 99 99 143 158 168 169 La 27.6 36.3 17.2 34.6 30.8 28.3 22.5 23.0 31.2 54.4 57.2 75.4 89.7 108.8 122.4 Ce 71.8 75.2 47.0 69.8 63.0 54.7 57.7 59.2 79.1 138 139 182 228 229 238 Nd 72.0 42.5 33.0 35.8 36.6 31.4 39.0 47.0 52.0 84 80 120 125 137 Sm 8.6 9.8 5.7 9.2 9.3 8.4 7.5 7.5 8.7 13.5 13.2 16.7 19.7 23.4 24.6 Eu 3.22 3.1 2.3 3.0 2.9 2.6 2.72 2.66 3.38 4.56 4.52 3.47 2.98 2.26 2.35 Tb 1.61 1.4 1.18 1.3 1.1 1.3 1.33 1.4 1.66 2.50 2.52 3.12 3.85 4.08 3.98 Yb 4.21 2.7 3.12 2.4 2.5 2.4 3.72 3.26 3.81 7.03 7.2 9.43 11.5 11.8 12.6 L u 0.55 0.41 0.39 0.43 0.44 0.37 0.48 0.43 0.54 1.00 1.02 1.14 1.58 1.63 1.71 Se 24.8 24.2 27.1 27.1 23.5 12.9 12.5 2.43 0.92 0.24 0.25 Cr 5 9 28 14 12 16 15 36 7 o 4 2 o 3 3 Co 36.7 41 32.9 48 45 37 37.5 36.1 33.6 3.92 4.27 0.19 0.31 0.44 o Ni o o 14 8 12 7 6 10 o 2 o 9 12 12 15 V 294 306 259 311 331 311 317 301 295 11 16 7 o 4 o � Cu 54 13 32 39 35 32 36 10 24 9 o 7 o l 3 2 Zn 148 140 99 128 130 121 122 11 144 216 164 183 195 237 233 ;:s Ga 30 23 22 19 26 23 24 25 27 31 29 35 34 37 42 Fi' Hf 6.93 8.2 4.60 6.6 6.8 5.9 5.86 5.38 7.02 13.5 13.8 20.2 26.6 27.9 28.8 (")� Ta 2.83 2.8 1.86 2.8 2.8 2.4 2.30 2.28 3.03 5.28 5.38 7.19 8.93 9.14 9.32 Th 3.23 <2.4 2.25 <2.1 2.4 1.7 3.03 2.46 3.76 7.68 7.86 12.5 15.5 16.0 16.7 � u l.O 0.8 0.8 0.67 t::l;j <::) ;::: Y!Nb 0.84 0.88 0.88 0.84 0.84 0.90 0.93 0.85 0.81 0.70 0.75 0.62 0.68 0.73 0.64 -.::: (1:1 'ZJ:!Nb 6.02 6.23 6.03 5.94 6.02 6.19 6.27 6.05 5.83 6.25 6.31 6.86 7.23 7.61 7.57 Th/U <2.4 <2.63 3.00 2.54 � $:l Q 1.36 2.82 1.59 2.87 3.08 2.68 1.62 1.58 1.79 6.70 8.89 13.07 17.19 18.84 17.07 A c 0.68 5.20 5.00 5.02 N o -..l 208 T. Prestvik et al. NORSK GEOLOGISK TIDSSKRIFT 79 (1999)
Table 2. Measured Na20 values for corresponding K2 0 contents. Before calculating the averages, all data were norrnalized to 100% volatile free and with total iron as FeO.
Hawaiites Benmoreite Comendite
(l) (2) (3) (l) (2) (3) (l) (2) (3)
Na20 3.96 3.66 3.55 5.55 4.98 4.77 6.10 5.93 5.36 K20 1.30 1.32 1.31 2.97 2.97 3.01 4.89 4.83 4.88 No. of samples 7 5 3 2 3 5 3 3 2
(l) This study; XRF-l ithium 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).
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.
B10 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 MnO 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 Cu 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) Volcanic rocks, Bouvetøya 209
200 ��--��--T�--��-� l l l l 800
• a) ... X b)
• 600 � 100 X 4oo Æ X • 200 X X
o 1.0 X 15 d) - - 0.8 • • o 10 0.6 LD Q) oN u. • 0.4 a..
5 • ... 0.2 - • - o ��--��--��--� 0.0 5 .--.-1-.-1-.-1-.-1-.-1-,--.--,� 0.4 e) XX X f) 4 - X - 0.3 xle X lf X N X X 3 • " o o • 0.2 c: X ••• • l= -X :::;! 2 X X • • 0.1 1 •
.
�-...__J__._ ___,_L __ ��--�-- "----��... 0 1 3 5 7 9 1 3 5 7 9 Ta Ta
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 They are high in Ti02 (3.35-3 .89 wt%) and total FeO and/or porphyritic samples in the previous studies. (11.15-12.81 wt%), and low in MgO (3.72-4.78wt%) and CaO (7.85-9.48 wt%). We have only used such samples in our model calculations. General characteristics In Figs. 2 & 3 we have plotted selected major and trace Both this and the former studies show that the K20 elements in variation diagrams versus Ta, which was contents of hawaiites vary considerably: from 0.75 to 1.65 apparently incompatible throughout the development of wt% (lmsland et al. 1977), 0.72 to 1.31 wt% (le Roex & the suite and also rather immobile during alteration. Three Erlank 1982) and from 0.97 to 1.54 in this study. The low different pattem styles are observed: (a) Purely incompa values are almost always associated with high Al203 tible elements (vs. the observed minerals) such as Nb (Fig. (> 15%) and CaO (> 10%) contents, sometimes with 'high' 2a) vary almost linearly with Ta throughout the series. This MgO (samples B 24 & B 70, this study) and relatively low type of pattem is observed for Zr, Hf, Y, La, Lu, K20, and values of several other elements. These are the effects of Rb, while Ba increases linearly up to the trachyte stage (B plagioclase (and sometimes olivine) accumulation as 16) after which it drops considerably to and among the revealed by the porphyritic nature of many hawaiite comendites (Fig. 2b). (b) FeO (Fig 2c), P205 (Fig. 2d), samples. The sparsely porphyritic or aphyric Bouvet Ti02 (Fig. 2e), MnO (Fig. 2f), and Se (Fig. 3f) increase hawaiites, which are thought to be doser to melt com from the most primitive rocks and reach a maximum positions, seem to varyin K20 in the range 1.12-1.54 wt%. before decreasing in the intermediate and silicic rocks. 210 T. Prestvik et al. NORSK GEOLOGISK TIDSSKRIFT 79 (1999)
6 l 120 .. 5 a) • • 100 • 4 80 .c o�N 3 60 0:: X 2 40 •• X •• 1 e It X X 20 X X X o o 15 800 IX X c) d) -
XX 600 X X X 10 • � - �l l.... o • . , .. X ro :s: 400 Cf) () 5 - 200 • • - ...... o o 7 l 30 • J e) • .. f) 6 - .• • .'� • X X X 5 - - 20 o ('\1 • u ro 4 ' X - X e · X Cf) z xx 3 10 X X 2 l! • l L _]_ _j_ _l_j_ 1 � o 1 3 5 7 9 1 3 5 7 9 Ta 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 accumulated plagioclase, B 31 and B 34 are altered increases linearly to mugearite (B 31 & B 34), indicating mugearites, and B 49 is altered benmoreite. With the that apatite fractionation must have been rather insignif exception of B 23 and B 24, the altered samples were icant up to this point, but from then on it decreases in more collected from a hydrothermally altered sequence, which is evolved compositions. Ti02 reaches its maximum some exposed in the cliffs behind the newly formed platform what earlier than FeO; we can also see (Fig. 2c, f) that Nyrøysa (<100 m a.s.l.) on the west coast of Bouvetøya. manganese removal is delayed compared to iron, a feature Samples B 23 and B 24 were collected at the site of the compatible with the increased Mn/Fe ratio of the various active fumaroles in the north of the island (see Prestvik & ferromagnesian minerals we have analysed in benmoreite Winsnes 1981 for details). and comendite (Table 5). (c) CaO (Fig. 3c), MgO and Sr Chemically, the alteration is characterized by large, but (Fig. 3d) decrease almost linearly throughout the series. variable H20+ and C02 contents in most samples. The secondary minerals include calcite, celadonite, chlorite, various clay minerals, pyrite, and quartz. Alteration Elements such as Nb (Fig. 2a), Hf, Zr, Y, the REE, and Nine samples are listed in Table 3, all of which are altered Se (Fig. 3f) were rather immobile during the alteration by hydrothermal processes. Of the se, samples B 10, B 19, (assuming that Ta was also immobile). It is mentioned B 20, B 22, and B 23 seem to represent 'normal' to evolved above that sodium is low in the altered samples (Fig. 3e). Bouvetøya type hawaiite, B 24 represents a hawaiite with With the exception of one sample (B 22), K20 and Rb have NORSK GEOLOGISK TIDSSKRIFI' 79 (1999) Volcanic rocks, Bouvetøya 211
Table 4. Trace element composition of plagioclase separates (in ppm). and +6.2%o, respectively, and + 7.2o/oo in B 34. The
B24 B 56 B 49 significantly higher values of B 23 and B 24 may be Hawaiite Hawaiite Benmore. related to their location in the fumarole area, while the samples with the lowest 6180 are from the hyaloclastite Fe(%) 0.49 0.44 0.38 Se 0.44 0.30 0.14 section. Apparently, the fumarolic water has other isotopic Cr 1.66 0.97 ND signatures than the fluids that altered the lower hyaloclas 1 Co 1.35 1.23 ND tite formation of the island. Average 6 80 for five fresh Na(%) 2.34 1.97 5.30 Ba 57 36 397 hawaiites is +5.5%o. Comparing our data on altered rocks La 1.46 1.16 6.5 with the data on Rb, Sr, Na20 and K20 published by Ce 2.72 2.31 9.00 O'Nions & Pankhurst (1974) and O'Nions et al. (1977) on Nd 1.13 ND 3.7 Sm 0.20 0.175 0.646 hawaiite and benmoreite from Bouvetøya, we can con Eu 0.528 0.50 5.78 clude that some of the samples analysed by these authors Th ND ND 0.061 represent heavily altered material. However, this alteration Yb ND ND ND does not appear to have affected the 87 Sr/86Sr and Lu ND ND 0.014 14 144 Hf 0.051 0.067 ND 3Nd/ Nd ratios. 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. Mineral compositions Several hundred mineral grains have been analysed by electron microprobe. Some olivine, pyroxene and amphi been leached during hydrothermal alteration (Fig. 3a, b). bole data are summarized in Table 5 and Fig. 4. With a few exceptions Ba, P205, Ti02, and FeO appear to The feldspars show large compositional variation. In the have been less mobile. CaO, Sr (Fig. 3c, d) and MnO (Fig. hawaiites they range from An 83 to An 45 with An 60-75 2t) show enrichment in most altered hawaiites. One sample as the most common composition. It is difficult, however, (a hawaiite sill; B 22, the most heavily altered of all to evaluate the equilibriumplagioclase composition of the samples in terms of total volatiles) is especially notable hawaiites, but in the sparsely porphyritic samples An 55- because it is the only sample considerably enriched in 62 are common compositions. Plagioclase phenocrysts of K20, Rb, Ba, Sr and Ca. In this sample, all phenocrysts and the benmoreites are typically around An 30, and microlites most groundmass plagioclase are completely replaced by grade into the field of anorthoclase. The feldspar of the sericite + calcite ± quartz, and other phases have been trachyte (B 16) is anorthoclase, whereas the comendites transformed into a mixture of clay minerals. Vesicles have have a very Ca-poor feldspar; the most potassium-rich been filled with calcite, chlorite, various clay minerals and varieties are close to the alkali-feldspar minimum melt sometimes quartz. composition, i.e. Or 35. Oxygen isotope data (Table 7) show low 6180 values in Plagioclase separates have been analysed for some the heavily altered samples, as low as + 1.9o/oo in B 22 and trace elements (Table 4). The small concentrations of Fe, B 19. However, in samples B 23 and B 24, 6180 is +6.0%o Se, Cr, and Co show that very clean separates were
Table 5. Average composition of olivine, clinopyroxene, and amphibole.
Olivine Olivine Cpx Cpx Amphib. Amphib. Benmore. Comendite Benmore. Comendite Comendite Cations
Si02 33.39 30.44 Si02 50.54 48.19 51.12 Si 7.7 FeO 51.31 66.94 Ti02 0.63 0.07 0.21 Al 0.4 MnO 1.62 2.47 Ah03 1.21 0.18 0.24 Ti 0.2 MgO 13.77 0.08 Fe O 17.16 28.98 35.93 Mg 0.1 Ca O 0.42 0.44 MnO 0.64 0.92 0.72 Fe 4.5 Total 100.51 100.36 MgO 10.00 0.19 0.07 Mn O.O Ca O 20.08 19.75 1.9 Ca 0.3 Na20 0.31 0.20 7.69 Na 2.2 KzO 1.27 K0.2 Total 100.57 98.48 99.15 16.0 Molecular ratios: Fo 31.7 0.2 Wo 42.4 46.4 Fa 66.2 96.2 En 29.4 0.6 Tf 2.1 3.6 Fs 28.2 53.0 D(Mn) 7.4 24.7 D(Mn) 2.9 9.2 7.2 D(Ca) O.l 0.6 D(Ca) 5.2 25.0 2. 4 D(Fe) 5.9 16.7 D(Fe) 2.0 7.3 9.0 MnO/FeO 0.032 0.037 MnO/FeO 0.037 0.032 0.020 212 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 B. Hawaiite B 59 100.00% B79 obs B79 est -Olivine Fo 70 0.71% Si 51.640 51.882 -Olivine Fo 70 1.35% Si 51.640 52.679 -Clinopyroxene Fs 17 8.84% Ti 3.410 3.431 -C linopyroxene Fs 17 6.98% Ti 3.410 3.418 -Plagioclase An 56 13.42% Al 14.140 14.126 -Plagioclase An 65 12.67% Al 14.140 13.951 -Ilmenite 1.72% Fe 12.390 12.495 -Ilmenite 1.82% Fe 12.390 12.704 -Titanomagnetite 0.46% Mn 0.210 0.202 =Hawaiite B 79 77.18% Mn 0.210 0.202 =Hawaiite B 79 74.85% Mg 3.720 3.741 Mg 3.720 3.722 Ca 7.850 7.873 Ca 7.850 7.847 Na 4.380 4.359 Na 4.380 4.416 K 1.540 1.523 K 1.540 1.525 E�= 0.073 p 0.720 0.737 E�= 1.216 p 0.720 0.721
(Il) From hawaiite to benmoreite (B 79-B 28) (Ill) From benmoreite to trachyte (B 28-B 16) Hawaiite B 79 100.00% B28 obs B28 est Benmoreite B 28 100.00% Bl6 obs BJ6 est -Olivine Fo 64 3.57% Si 61.140 61.133 -Olivine Fo 32 3.29% Si 68.410 68.491 -Clinopyrox Fs 19 15.69% Ti 1.160 1.159 -C linopyroxene Fs 28 4.24% Ti 0.310 0.307 -Plagioclase An 42 24.17% Al 14.720 14.713 -Plagioclase An 31 24.13% Al 14.640 13.978 -Apatite 1.27% Fe 8.760 8.760 -Titanomagnetite 4.57% Fe 4.350 4.544 -Ilmenite 2.91% Mn 0.240 0.232 -Apati te 0.84% Mg 0.020 0.020 -Titanomagnetite 5.13% Mg 0.950 0.958 =T rachyte B 16 61.93% Ca 1.580 1.609 =B enmoreite B 28 47.26% Ca 4.010 4.023 Na 6.030 5.999 Na 5.660 5.731 K 4.500 4.469 K 2.970 2.998 E�= 0.006 p 0.380 0.363 E�= 0.486 p 0.030 0.030
(IV) From trachyte to comendite (B16-B 39) (V) From benmoreite to comendite (B 28-B39) Trachyte B 16 100.00% B39 obs B39 est B28 100.00% B39 obs B39 est -Clinopyroxene Fs 47 3.81% Si 70.400 70.404 -Olivine Fo 32 3.55% Si 70.380 70.376 -Plagioclase An JO 8.72% Ti 0.320 0.291 -Clinopyroxene Fs 33 5.10% Ti 0.320 0.311 -Feldspar Or�b�2 17.34% Al 13.410 13.575 -Plagioclase An 31 30.68% Al 13.410 13.430 -Titanomagnetite 0.43% Fe 4.250 4.254 -Apatite 0.86% Fe 4.250 4.252 = Rhyolite B 39 69.70% Mn 0.120 0.164 -Titanomagnetite 4.64% Mg 0.001 0.002 Ca 0.790 0.779 =R hyolite B 39 55.35% Ca 0.790 0.796 Na 6.050 5.630 Na 6.050 6.006 K 4.930 4.904 K 4.930 4.977 E�= 0.207 p 0.020 0.043 E�= 0.005 p 0.020 0.012 obtained. Especially interesting is the large Ba content of phenocrysts of benmoreite are around Fo 30, whereas pure benmoreite plagioclase (397 ppm) compared to 35 and fayalite (Fo 0.2, Table 5) is found in several comendite 56 ppm in plagioclase of hawaiites. This contrast shows samples. plaglliquid that the distribution coefficient of Ba (D8a ) is Hedenbergite and fayalite have not been reported from increasing from about 0.2 to 0.72 as An contents (and T Bouvetøya previously. In both of these minerals the of equilibrium) decrease. The benmoreite plagioclase MnO content is relatively large (Table 5). Thus, MnO (An 33) is considerably richer in the REE than are contents range to 2.59 wt% in olivine of comendite accumulated bytownites (An 71-75) in hawaiites (see sample B 83. below). Imsland et al. (1977) reported rare blue-green alkali The clinopyroxene data are shown in Fig. 4, and the amphibole in some comendite samples. We present averages of representative pyroxenes and olivines of the microprobe data of such amphiboles from a fresh intermediate and silicic rocks are presented in Table 5. holocrystalline comendite (Table 5). This is arfvedsonite Pyroxene of benmoreite is Fe-rich augite, and in trachyte according to the classification of Leake (1978). and comendite almost pure hedenbergite is present. Because most hawaiite pyroxenes are irregularly zoned, Rare earth elements (REE).- In the petrologic models of it is difficultto estimate a reliable 'average' composition. It Imsland et al. (1977) and le Roex & Erlank (1982), as is, however, important to recognize the compositional well as in our own models (Table 6), plagioclase variation, because that sheds light on the magma's pre fractionation is important. Extensive plagioclase fractio eruption his tory. nation is usually thought to deplete the residual melt in Olivine varies from Fo 87 to Fo 45 in the hawaiites, but Eu because this element readily replaces Ca (and Sr) in compositions around Fo 70 seem to be most common. the feldspar lattice. These may represent equilibrium olivine, because a range Plagioclase separates of hawaiite and benmoreite all of Fo 75--65 would be expected for the actual whole-rock show a significant positive Eu anomaly (Fig. 5) where Eu Mg numbers (Roeder & Emslie 1970). The most magne is enriched about 10 times compared to the neighbour sium-rich olivine that occurs in the hawaiites is, however, elements in hawaiite and about 30 times in benmoreite. As petrogenetically important because it shows the existence can be seen from Fig. 5, benmoreite does not display of more primitive magmas at deeper levels. The olivine negative Eu anomalies despite the approximately 24% NORSK GEOLOGISK TIDSSKRIFT 79 (1999) Volcanic rocks, Bouvetøya 213
Table 7. Isotope data for Bouvetøya rocks.
18 87 86 144 Sample 8 0** Sr/ Sr 143Nd/ Nd 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 Table 3). ND= not detennined. plagioclase fractionation that is indicated from the 1.3), and therefore further fractionation may be very petrologic models (Table 6; model ll). Intuitively, and as efficient in developing negative Eu anomalies in the was stated by Dickey et al. (1977), one should expect a more silicic melts. It is thus logical that trachyte (B 16), considerable effect on the Eu content of the derived melt which is somewhat less silicic than the comendite, displays when Eu-enriched plagioclase is removed. For the an REE pattern which is parallel with that of comendite, Bouvetøya suite, therefore, two questions arise: (i) Is it but has smaller REE abundances and a less pronounced possible to fractionate a feldspar with a pronounced negative Eu anomaly. 2 positive Eu anomaly without developing a negative It was mentioned above that Eu + is expected to have anomaly in the derived melt, and/or (ii) Is the petrologic geochernical behaviour similar to Sr. However, Eu occurs model principally wrong, perhaps because plagioclase fractionation was not that important? The answers appear 1000 .------, 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 2 100 aggregate distribution coefficient (for Eu3+ and Eu + 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 crystal � andesine/melt 10 lized from; i.e. u is greater than unity (about
La Ce Nd Sm Eu Tb Yb Lu
En Fs Fig. 5. Chondrite-normalized REE (rare earth elements) patterns of average ha (Fo) (Fa) waiite, benmoreite and comendite, plotted together with patterns of plagioclase separates from hawaiites (An71 & An 75) and benmoreite (AN 33). Values for Fig. 4. Clinopyroxene plotted in the En-Fs-Di-He quadrilateral. For simplicity, HREE of basic plagioclases are stipulated on the basis of data obtained for basic olivine compositions (Fo-Pa) are plotted in the same diagram, along the En-Fs plagioclase from Iceland (Prestvik l 985) which was analysed in the same join (filled circles). experiment. 214 T. Prestvik et al. NORSK GEOLOGISK TIDSSKRIFT 79 (1999)
Peter 1. øy
- :li:: (.) o 0::
10 • Bouvetøya * Bouvet ridge
Ba Th Nb La Sr p Zr Ti y Yb Rb K Ta 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, Antarc tica (Prestvik et al. 1990). in both the 2+ and the 3+ states. Thus, the aggregate technique (Bryan et al. 1969) to evaluate the possible distribution coefficient for Eu depends on the prevailing relationships between the observed rock and mineral oxidation states. Distribution coefficients are also tem compositions. Before modelling, all data were normal perature dependent (Blundy & Wood 1991); they increase ized to 100% volatile-free, and total iron was expressed with decreasing temperature. If we apply distribution as FeO. In models involving silicic rocks (models IV and 2 coefficients between plagioclase and melt for Eu + and V), Mg was omitted in model IV, because the poor Eu3+ similar to those for s�+ and REE3+, respectively, we analytical accuracy of whole rock analyses with can estimate the proportion of the two Eu ions in the melt. MgO :::; 0.10% made error estimates meaningless. Mn If we distributethe observed aggregate DEu of 0.2 between was omitted from models IV and V. Our models are bytownite plagioclase and hawaiite with DEuZ = l and presented in Table 6. 2 + DEuJ = 0.03, we find that the Eu3+Æu + ratio in the melt W e made the calculations in several steps. As + is ca. 4.7; in other words, Eu is rather oxidized. Choosing a described above, there is considerable chemical variation 2 higher coefficient f or Eu + (Dsllagioclaselmelt range within the hawaiite group, and we wanted to include this between l and 3) would correspond to an even more variation in our modelling. We chose sample B 59 oxidized melt. Making use of the same procedure for (K20 = 1.18) as representative of the least evolved andesinelbenmoreite, using aggregate DEu = 1.3 (as hawaiites and B 79 (K20 = 1.54 and transitional to observed), DEuJ = 0.05, and DEuZ = 2.55 gives a Eu3+/ mugearite) as the most evolved. Both these groups are 2 + + Eu + ratio of 0.85, i.e. considerably more reduced than in almost non-porphyritic. Models lA and lB (Table 6) the basic melts. These results are in general agreement represent variation within the group of hawaiites. Models with the conclusion drawn by Prestvik (1982a) on a Il and Ill show the evolution from evolved hawaiite (B qualitative basis. If we compare with the experimental data 79) to benmoreite (B 28; K20 = 2.97) and furthermore to of Weill & Drake (1973), we find that f 02 for bytownite/ trachyte (B 16; K20 = 4.50). The evolution of comendite 8 hawaiite at l100°C is about 10-5 Pa and about 10- ·5 Pa (B 39; K20 = 4.93) is presented in two models, model IV for andesinelbenmoreite at 1000°C. For comparison, le from trachyte and model V directly from benmoreite. Roex & Erlank (1982) estimatedf02 for hawaiite at l00°C With one exception (lb), we present models with low to be about 10-6·0 Pa on the basis of coexisting Ti residuals CE� <0.5), but we also rejected models with magnetite and ilmenite. 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 New petrologic model � trachyte � comendite (models la, Il, Ill & IV), see W e used a common least squares mass balance mixing discussion below. NORSK GEOLOGISK TIDSSKRIFT 79 (1999) Volcanic rocks, Bouvetøya 215
suggested that such phyric hawaiites represented regular 15, 9 t- l HIMU Bouvetøya hawaiite with as much as 40% accumulation of
15,8 plagioclase (34-37%) and clinopyroxene (3-6%). As .c mentioned above, the only problem with this interpretation a.. Nl. 15,7 o was an inferred Ba inconsistency (le Roex & Erlank 1982,
.o p. 333). However, the very small Ba concentrations ,._a.. 15,6 -$ o obtained by us on hawaiite plagioclase separates (Table N 4) explain and eliminate this problem. 15,5 r- - EM-1 Transitional basalts I of Imsland et al. (1977) correspond well to 'accumulated' hawaiite of le Roex & Erlank 15,4 DMM (1982). This rock type is represented by samples such as B l 24 and B 56 (the hawaiite samples from which we 17 18 1 9 20 21 22 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 HIMU 40 hawaiites from Bouvetøya range from 36 to 55 (not o 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 (1982), it would require that as much as 15% cpx + ol was added to hawaiite melt (B 59) in addition to plagioclase. EM-1 Unless significant resorption occurred, this seems to be unlikely, based on observed hawaiite modal composition. 38 DMM W e favour an interpretation in which more primitive hawaiitic magmas (than those represented by the aphyric l l 17 18 21 22 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 composi 0,705 - EM-1 - tional change also within the group of sparsely phyric or non-phyric hawaiites. Low Mg#s and generally small i> concentrations of compatible elements such as Ni, Co, Se, ::: 0, 704 ,._(/) and Cr in these hawaiites (Tab le l) strongly indicate a CD .� history in which olivine and clinopyroxene ( + spinel?) fractionated from the more primitive magmas. Pre 0,703 HIMU fractionation of plagioclase is not documented by negative Eu anomalies in REE pattems of the basic rocks, because DMM plagioclase has such small Eu concentrations (Tab le 4). o,702 ..."__.___._....____..._�_._-...._I_.___.,_...L...-J' 17 18 19 20 21 22 However, the strong, negative anomaly of strontium in the spidergram of the Bouvetøya hawaiite (Fig. 6) indicates that parental magmas at depth also crystallized plagio Fig. 7. Radiogenic isotope plots of Bouvetøya rocks. Filled circles - hawaiite; clase, because distribution coefficients of Sr are large open circles - evolved rocks; stars - Ph data from Sun (1980). Locations of DMM (depleted morb mantle), EM-l (enriched mantle l) and HlMU (hlgh enough (1-3; Henderson 1982) effectively to remove this 2 8 11;11 = 3 Uf041>b) fields are taken from Hofmann (1997). element during plagioclase fractionation.
Ne w models Discussion We present two fractionation models (Table 6, lA & lB) to explain compositional variation in the hawaiites. Both Former models suggest ca. 25% fractionation of ol + cpx + We agree with Imsland et al. (1977) that the irregularly plag + ilm + mt, all of which occur in hawaiite. The zoned and commonly corroded basic plagioclase mega difference in the two models is the plagioclase composi crysts in the hawaiites are not real phenocrysts. On the tion (An 56 in lA; An 65 in lB). However, the basis of mixing calculations, le Roex & Erlank (1982) unacceptably high residual in model lB makes that model 216 T. Prestvik et al. NORSK GEOLOGISK TIDSSKRIFT 79 (1999) unrealistic. Even though modal plagioclase in many was formed directly from mugearite, a very long fractio hawaiites falls in the range An 60 - 75, we did not nating step (69%) in which a plagioclase of An 47 was consider models using plagioclase more calcic than An 65. required. 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 The barium (and strontium) problem observed in hawaiite. Thus, IA is our preferred model for this stage. The amount of fractionation is consistent with Prestvik (1982a) reported Ba contents of three comendites the observed incompatible element variation. (183-216-245 ppm) from Bouvetøya. Similarly, le Roex In model Il, benmoreite is derived from evolved & Erlank (1982) reported 180 and 298 ppm Ba in two hawaiite by slightly more than 50% fractionation. This is comendites. In this work, we obtained 312 ppm Ba in about 10-15% less than reported by both Imsland et al. comendite sample B 39 (obsidian). However, our crystal (1977) and le Roex & Erlank (1982). However, the parent line comendites are much more depleted in Ba (and Sr), hawaiites used in the models of these authors were less with only 80 ppm Ba. Apparently, the 'barium problem' evolved than thosewe have used. The model uses minerals would have been even larger if these samples bad been and compositions that are typical in evolved hawaiite, used in the models. From the whole rock data we have except that the plagioclase is slightly more sodic than is calculated the value of DBa and Dsr for the fractionating common in hawaiite. Since this step is so large, we tind it feldspar of models IV and V (the comendite models). In reasonable that the average fractionating plagioclase is model IV (formation of comendite from trachyte) we get intermediate between parent and daughter compositions. DBa = 3.8 and Dsr = 5.7. The fractionating feldspar in this Olivine and clinopyroxene compositions are both fairly model is about 2/3 alkali feldsparand 113 anorthoclase (An magnesium-rich, but that can be explained by postulating 10). The estimated D's are well within the possible ranges that these minerals fractionated early in the sequence. This for both elements (Henderson 1982; Blundy & Wood model has very low residuals. Model Ill shows bow 1991). For model V (formation of comendite directly from trachyte can be formed by ca. 38% fractionation from benmoreite) the values of DBa and Dsr for fractionating benmoreite. Only benmoreite mineral compositions were feldspar are 2.8 and 4.2, respectively. For plagioclase (An used in this model, which has a somewhat high residual. 31) this is a plausible value, but the DBa value is much This is mainly due to misfits in the Al and Fe contents larger than that found by us for this plagioclase when we (both ca. 4.5% relative). We still believe this is an analysed the mineral separate (DBa = 0.72). We thus agree acceptable model for the derivation of trachyte, not least with le Roex & Erlank (1982) that comendite did not form because trachyteis intimately related to benmoreite in the by fractionation directly from benmoreite. We disagree, field. The trachyte is relatively enriched in incompatible however, that the comendite was formed by fractionation elements such as Ba, Rb, Y, il and Nb compared to directly from mugearite. Despite the Na discrepancy of the benmoreite, but strontium is considerably lower (89 vs. ca. comendite derived by fractionation of trachyte, we think 315 ppm). This requires bulk Dsr and DBa of ca. 3.6 and that model IV provides a plausible explanation for the way 0.5, respectively. With the actual proportion of plagioclase in which Bouvetøya comendites are derived. The very high a in the fractionating assemblage, this gives D�� g of 5.4, Ba content of trachyte - and much lower content in which is reasonable for plagioclase An 31 (Nash & comendite - explains bow the benmoreite magmas Crecraft 1985), and Dfl:g = 0.72, which is identical to fractionate through a trachyte stage and shows that there calculated values from the analysed plagioclase separate is a shift from plagioclase to alkali feldspar as a from benmoreite (Table 4). fractionating phase around the trachyte stage. Furthermore, In models IV and V (Table 6), we have sought to the ferromagnesian minerals are much more evolved in deterrnine bow further fractionation from trachyte (IV) or trachyte than benmoreite. Fractionation of these minerals benmoreite (V) can yield comendite. In model IV, there is not only results in comenditic composition, but also a considerable deficiency in Na content (4.49% relative) of explains the trace element data, such as the Ba and Sr the derived comendite. In this model, we used mineral concentration. We have no valid explanation for the Na compositions from the trachyte, including alkali feldspar. discrepancy seen in model IV, but the model is, of course, In order to avoid the Na deficiency, a much less sodic and 'vulnerable', owing to the fact that we have only one unrealistic feldspar composition was attempted, which in sample of trachyte. turn created problems with other elements. In model V, We have also tried to model (not tabulated) the mineral compositions of phenocrysts in benmoreite were relationship between comendite B 39 (glass) and crystal used, and the fit is almost perfect for all elements. line comendite. This is possible by approximately 7% However, le Roex & Erlank (1982) rejected their model fractionation of alkali feldspar and minor amounts (ca. 2%) in which comendite could be formed from benmoreite, of o l + cpx + mt if Dfeldsparfor both Sr and Ba is about 19. because the model required a fractionating plagioclase of This is within the published range for Sr, but higher than An 47 (An 31 in our model V), and because there was a 'Ba the value we have seen published for Ba (up to 12.9; and Sr problem'. Instead, they suggested that comendite Henderson (1982)). NORSK GEOLOGISK TIDSSKRIFT 79 (1999) Volcanic rocks, Bouvetøya 217
Mantle signature al. (1996) indicated that the anomalously enriched segment of the southem MAR east of the Shona seamount (the On the basis of Y - Ti - Zr ratios Prestvik ( 1982b) 'Shona ridge-anomaly', Fig. l)- only about 600 km from characterized the basic rocks from Bouvetøya as 'within Bouvetøya - might represent a separateShona plume. 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 Conclusions them as identical to P(lume)-type MORB of the American Antarctic Ridge (AAR) and South-West Indian Ridge, Our study supports previously published models which which he suggested had formed by partial melting of concluded that the main rock types of Bouvetøya are depleted astenosphere that had been veined by enriched interrelatedby fractional crystallization. However, we find melt (2-4%) in a previous metting event. Our new data that the best evolutionary path of the suite was as follows: show that the geochemical signatures displayed by the hawaiite � evolved hawaiite � benmoreite � trachyte � Bouvetøya suite (Fig. 6) are those of a 'mi ldly enriched' comendite. ocean island type (Thompson et al. 1984), with typical Primitive megacrysts of olivine, clinopyroxene and high normalized abundances of the HFS elements Nb and plagioclase in hawaiite represent phenocrysts of more Ta. The average YINb and Zr/Nb ratios of Bouvetøya primitive (parental) magmas that crystallized at depth, hawaiites, which are 0.86 and 6.06, respectively (Table 1), indicating the presence of an open-system magma cham also attest to their enriched nature. ber(s) in which variously evolved melts and crystal Although there is a considerable amount of Sr and Nd assemblages were able to mix. isotope data for the ridge axes of the BTJ area (e.g. Dickey Similarity in trace element and isotope ratios indicate et al. 1977; le Roex et al. 1983; 1985; 1987), very few such that the Bouvetøya suite, and the much more primitive data are published from Bouvetøya (O'Nions & Pankhurst basalts dredged from the Bouvet Ridge east of the island, 1974; O'Nions et al. 1977), and the only knownPb isotope originated from a common, enriched mantle source. The data from Bouvetøya are three determinations of Sun radiogenic isotope data show that this source is complex, (1980). Here we report new Sr and Nd (as well as O) consisting of several mantle components. isotopic data on 11 samples from Bouvetøya and Pb isotope data on 5 samples (Table 7), all collected during Acknowledgements. - We express our thanks to the Norsk Polarinstitutt for giving T.P. the opportunity to participate in the Polarsirkel expedition to Bouvetøya in the Polarsirkel expedition. In Fig. 7 we have plotted some 1978/79. A grant fromThe Norwegian Research Council and several contributions of the new data together with the previously published from the Department of Geology and Mineral Resources Engineering, NTNU Bouvetøya data (O'Nions & Pankhurst 1974; O'Nions et supported the analytical work. We also extend our thanks to Drs Elizabeth E. Anthony and Asish R. Basu for analytical assistance and to DrsPall lrnsland,Sven al. 1977; Sun 1980). The Bouvetøya rocks fall on, or very Maaløe, Calvin G. Barnes and one anonymous journal reviewer for their dose to, mixing lines between EM-l and HIMU basalts in constructive comments on the manuscript. 207 04 06 04 04 both PbP Pb vs? PbP Pb, 20 8 PbP Pb 206 04 87 86 206 04 Manuscript received February 1999 vs. PbP Pb and Sr/ Sr vs. PbP Pb plots (Fi,f,; 7) and very dose to such mixing trends in 143Nd/1 Nd 06 04 143 144 87 86 vs? PbP Pb and Nd/ Nd vs. Sr/ Sr diagrams (not shown). However, the combined isotope data (e.g. the References 87 86 ?06 P04 Sr/ Sr vs Pb Pb plot) are not compatible with Blundy, J. D. & Wood, B. J. 1991: Crystal-chemical controls on the partitioning of mixing between a depleted mantle source (DMM in Fig. 7) Sr and Ba between plagioclase feldspar, silicate melts, and hydrothermal and enriched sources such as EM-l (and EM-2) or HIMU solutions. Geochimica et CostrUJchimicaActa 55, 193-209. Bryan, W. B., Finger, L. W. & Chayes, F. 1969: Estimating proportions in sources. This implies that the model of le Roex (1987) has petrographic mixing equations by least-squares approximation. Science 163, to be modified. 926-927. Enrichment was probably integral to the plume. HIMU 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. signatures are primarily seen from elevated U/Pb ratios Geochimica et Cosmochimica Acta 41, 1105-1118. because of loss of Pb (Vidal l992). Unfortunately, we lack Douglas, J., Schilling, J.-G. & Kingsley, R. H. 1995: Influence of the Discovery elementa! Pb data on the Bouvetøya rocks, but typical and Shona mantle plumes on the southem Mid-Atlantic Ridge: Rare earth evidence. Geophysical Research Letters 22, 2893-2896. HIMU signatures are apparentalso fromthe Ce/Rb and Bal Govindaraju, K. (ed.) 1989: Analytical standards of minerals, ores and rocks. Nbratios of hawaiites (average 3.13 and 4.78, respectively, Geostandards Newsletter, vol. XIII, special issue. 1984: of 9 samples). Hart, S. R. A large-scale isotope anomaly in the southem hemisphere mantle. Nature 309, 753-757. Dredged samples from the South-West Indian Ridge Henderson, P. 1982: lnorganic Geochemistry, 353 pp. Pergamon Press, London. immediately east of Bouvet (Fig. l) areisotopically almost Hofmann, A. W. 1997: Mantle geochemistry: the message from oceanic 219-229. identical to the Bouvetøya rocks and apparently related to volcanism. Nature 385, Imsland, P., Larsen, J. G., Prestvik, T. & Sigmond, E. M. 1977: The geology and the Bouvet plume as well. However, the regional influence petrology of Bouvetøya, South Atlantic Ocean. Lithos JO, 213-234. of the Bouvet plume is not well understood. The possible Leake, B. E. 1978: Nomenclature of amphiboles. American Mineralogist 63, 1023-1052. presence of more than one plume in the BTJ area has been Le Bas, M. J., Le Maitre, R. W., Streckeisen, A. & Zanettin, B. 1986: A discussed by several authors (i.e. le Roex 1987). Thus, classification of volcanic rocks based on the total alkali-silica diagram. Journal Moreira et al. (1995), Douglas et al. (1995) and Simonov et of Petrology 27, 745-750. 218 T. Prestvik et al. NORSK GEOLOGISK TIDSSKRIFf 79 (1999) le Roex, A. P. 1987: Source regions of mid-ocean ridge basalts: evidence for Prestvik, T. 1982a: Trace element geochemistry of volcanic rocks from enrichment processes. In Menzies, M. A. & Hawkesworth, C .1. (eds.): Mantle Bouvetøya, South Atlantic. In Craddock, C. (ed.): Antarctic Geoscience, Metasomiltism, 389-422. Academic Press, London. 771-774. The University of Wisconsin Press, Madison. le Roex, A. P. & Erlank, A. J. 1982: Quantitative evaluation of fractional Prestvik, T. 1982b: Basic volcanic rocks and tectonic setting. A discussion of the crystallization in Bouvet Island lavas. Journal of Vo/canology and Geothermal Zr-Ti-Y discrimination diagram and its suitability for classification purposes. Research, 13, 309-338. Lithos 15, 241-247. le Roex, A. P., Dick, H. J. B., Erlank, A. J., Reid, A.M., Frey, F. A. & Hart, S. R. Prestvik, T. 1985: Petrology of Quatemary volcanic rocks from Oræfi, south-east 1983: Geochemistry, mineralogy and petrogenesis of lavas erupted along the Iceland. Report 21, Geological Institute, NI'H, Trondheim, 81 pp. southwest IndianRidge between the Bouvet triplejunction and Il degrees east. Prestvik, T. & Winsnes, T. 1981: Geology of Bouvetøya, South Atlantic. Norsk Journal of Petrology 24, 267-318. Polarinstitutt Skrifter 175, 41--{;9. le Roex, A. P., Dick, H. J. B., Reid, A.M., Frey, F. A., Erlank, A. J. & Hart, S. R. Prestvik, T., Barnes, C. G., Sundvoll, B. & Duncan, R. A. 1990: Petrology of Peter 1985: Petrology and geochemistry of basalts from the American-Antarctic Ridge, Southem Ocean: implications for the westward infiuence of the Bouvet I Øy (Peter I Island), West Antarctica.Journal ofVolcanology and Geothermal mantle plume. Contributions to Mineralogy and Petrology 90, 367-380. Research 44, 315-338. le Roex, A. P., Dick, H. J. B., Gulen, L., Reid, A.M. & Erlank, A. J. 1987: Local Roeder, P. & Emslie, R. F. 1970: Olivine-liquid equilibrium. Contribution to and regional heterogeneity in MORB from the Mid-Atlantic Ridge between Mineralogy and Petrology 29, 275-289. 54.5 os and 51°S: Evidence for geochemical enrichment. Geochimica et Simonov, V. A., Peyve, A. A., Kolobov, V. Y., Milosnov, A. A. & Kovyazin, S. V. Cosmochimica Acta 51, 541-555. 1996: Magmatic and hydrothermal processes in the Bouvet Triple Junction Ligi, M., Bonatti, E., Bortoluzzi, G., Carrara, G., Fabretti, P., Penitenti, D., Gilod, Region (South Atlantic). Terra Nova 8, 415-424. D., Peyve, A. A., Skolotnev, S. & Turko, N. 1997: Death and transfigurationof Sun, S.-S. 1980: Lead isotopic study of young volcanic rocks from mid-ocean a triple junction in the South Atlantic. Science 276, 243-245. ridges, ocean islands and island arcs.Philosophical Transactions of the Royal Mitchell, N. C. & Livermore, R. A. 1998: The present configuration of the Bouvet Society of London, series A 297, 409-445. triplejunction. Geology 26, 267-270. Thompson, R. N., Morrison, M. A., Hendry, G. L. & Parry, S. J. 1984: An Moreira, M., Staudacher, T., Sarda, P., Schilling, J.-G. & Allegre, C. J. 1995: A assessment of the relative roles of crust and mantle in magma genesis: an primitive plume neon component in MORB: the Shona ridge-anomaly, South elementa! approach. Philosophical Transactions of the Royal Society of Atlantic (51-52°5). Earth and Planetary Science Letters 133, 367-377. London, series A 310, 549-590. Nash, W. P. & Crecraft, H. R. 1985: Partition coefficients for trace elements in Verwoerd, W. J., Erlank, A. J. & Kable, E. J. D. 1976: Geology and geochemistry silicic magmas. Geochimica et Cosmochimica Acta 49, 2309-2322. of Bouvet Island. In Gonzales Ferran, O. (ed.): Andean and Antarctic 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 Volcanology Problems, 203--237. lAVCEI, Naples. Petrology 15, 603-634. Vida!, P. 1992: Mantle: More HIMU in the future? Geochimica et Cosmochimica O'Nions, R. K., Hamilton, P. J. & Evensen, N.M. 1977: Variations in 143Ndi1""Nd Acta 56, 4295-4299. and 87Sr/86Sr ratios in oceanic basalts. Earth and PlanetaryScience Letters34, Weill, D. F. & Drake, M. J. 1973: Europium anomaly in plagioclase feldspar: 13-22. experimental results and semiquantitative model. Science I 80, l 059-1060.