Petrology and geochemistry of Marion and Prince Edward Islands, Southern Ocean: Magma chamber processes and source region characteristics

Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

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Petrology and geochemistry of Marion and Prince Edward Islands, Southern Ocean: Magma chamber processes and source region characteristics A.P. le Roex a,⁎, L. Chevallier b, W.J. Verwoerd b, R. Barends a a b

Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa Department of Earth Sciences, University of Stellenbosch, Stellenbosch, 7600, South Africa

a r t i c l e

i n f o

Article history: Received 30 September 2011 Accepted 18 January 2012 Available online 30 January 2012 Keywords: Marion and Prince Edward Islands Geochemistry Petrogenesis Southern Ocean Basalt

a b s t r a c t New bulk rock geochemical data on an extensive suite of samples from the Marion and Prince Edward Islands, located in the Southern Ocean at ~37°45′E, 46°55′S, show highly coherent major and trace element variations. Rock types are dominated by alkali basalt, with lesser hawaiite and minor mugearite and benmoreite. Prince Edward Island has in addition rare trachytes exposed on the plateau, and xenoliths of comenditic rhyolite pumice were found in a basalt flow of the 1980 eruption on Marion Island. Lavas from Prince Edward Island differ from those erupted on Marion Island by having subtly distinct incompatible trace element ratios (e.g. Zr/Nb = 9.1 vs 6.7; Ba/Nb = 7.8 vs 6.7; Ce/Pb= 31 vs 36), and lower 87Sr/86Sr (0.70302) and higher 143Nd/144Nd (0.51300) isotope ratios than found on Marion Island (0.70336–0.70349; 0.51292–0.51295). The two islands are indistinguishable in terms of Pb isotope ratios (206Pb/204Pb = 18.61–18.75, 207Pb/204Pb= 15.54–15.56). Quantitative modelling suggests that the range in rock types found on each island can be ascribed to extensive low pressure crystal fractionation of phenocryst phases. For example, hawaiite can be related to the more primitive alkali basalts by some 24% fractional crystallisation of olivine, clinopyroxene, plagioclase and minor FeTi-oxides, whereas the more evolved mugearites and benmoreites require, respectively a further 36% and 24% fractional crystallisation to account for their felsic compositions. The trachytes of Prince Edward Island appear to represent the residual 28% magma following extensive crustal fractionation of dominantly clinopyroxene, plagioclase and FeTi-oxide with minor apatite. None of the sampled lavas have compositions characteristic of true primary magmas, but the least evolved, with Mg# ~63, appear to have experienced as much as 18% olivine fractionation from a primary magma with ~15 wt.% MgO. The rhyolite pumice is distinct in composition from Bouvet rhyolite or South Sandwich Island pumice and may belong to an earlier, submarine eruption at or near Marion Island. The enriched incompatible element ratios (Zr/Nb= 6–9; La/Ybn = 10.6; Y/Nb = 0.73) and radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd isotope ratios show that the lavas erupted on Marion and Prince Edward Islands resulted from melting an enriched mantle source, consistent with derivation from an upwelling deep mantle plume. Pb isotope ratios (Δ8/4Pb ~ 33) confirm the absence of any DUPAL signature in the source region of these magmas. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The study of lavas erupted on ocean islands (commonly referred to as OIB) has been fundamental to our understanding of petrogenetic processes giving rise to mafic magmas and their differentiates (e.g. Ludden, 1978; White et al., 1979; Oskarsson et al., 1982; Feigenson, 1984; Meyer et al., 1985; Albarède and Tamagnan, 1988; Chen et al., 1990; Frey et al., 1990; Clague and Moore, 1991; Furman et al., 1991; Thy, 1991; Eggins, 1992; Chen, 1993; Baker et al., 1996). The absence of continental crust and associated potential contamination has also allowed such mafic magma compositions to be used as ‘windows’ to the upper mantle, and thereby have provided considerable ⁎ Corresponding author. Tel.: + 27 21 650 2711; fax: + 27 21 650 2710. E-mail address: [email protected] (A.P. le Roex). 0377-0273/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2012.01.009

understanding of mantle melting processes and insights into upper mantle heterogeneity (e.g. Schilling, 1973a; White et al., 1976; Hawkesworth et al., 1979; Zindler et al., 1979; Clague and Frey, 1982; Kurz et al., 1987; Hauri, 1996; Hofmann and Jochum, 1996; Widom et al., 1997; Lassiter and Hauri, 1998; Moreira et al., 1999; Gaffney et al., 2004; Ren et al., 2005). Such studies have illustrated the distinct compositional characteristics of OIB compared to magmas erupted along midocean ridge spreading centres, and have led to the recognition of distinct source regions of these two contrasting mantle derived melts, and to the generally accepted concept of mantle plumes giving rise to ocean island magmatism that taps deep (possibly recycled), upwelling, mantle domains (e.g. Schilling, 1973b; Schilling et al., 1977; Zindler et al., 1979; Bougalt and Treuil, 1980; Schilling et al., 1982; White and Hofmann, 1982; Hofmann, 1984, 1988; Eiler et al., 1996; Hofmann, 1997; Cannat et al., 1999).

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Most detailed studies of ocean island basalts have focussed on the more well known islands such as Hawaii (e.g. Feigenson, 1984; Chen et al., 1990; Clague and Moore, 1991; Chen, 1993; Baker et al., 1996), Reunion (e.g.Ludden, 1978; Albarède and Tamagnan, 1988), Iceland (e.g. Zindler et al., 1979; Oskarsson et al., 1982; Thy, 1991) and the Azores (e.g. Hawkesworth et al., 1979; White et al., 1979; Mungall and Martin, 1995; Widom et al., 1997; Moreira et al., 1999). In the South Atlantic and Southern Ocean, detailed petrogenetic studies have been limited to Tristan da Cunha (le Roex et al., 1990), Gough Island (le Roex, 1985), St Helena (Weaver et al., 1987), Bouvet (although relatively little outcrop occurs) (Imsland et al., 1977; Prestvik and Winsnes, 1981; le Roex and Erlank, 1982; Prestvik, 1982; Prestvik et al., 1999) and Kerguelen (e.g. Gautier et al., 1990; Doucet et al., 2002; Frey et al., 2002). In contrast, Marion and Prince Edward Islands, located at ~37° 45′E 46° 55′S, just east of the Southwest Indian Ridge, have been little studied in terms of modern geochemistry, although Chevallier (1986) has described the tectonic evolution of the islands and the edifice on which they are built. Early field mapping and a description of the petrology of the lavas were undertaken by Verwoerd (1971) and Verwoerd et al. (1981) who recognised an older grey lava series and a younger black lava series of alkali basalts and trachybasalts, and interpreted the islands as twin peaks of a single volcano. Verwoerd (1987) later subdivided the suite of lavas on textural grounds into Types I through IV, and argued that these subdivisions had evolutionary significance, but did not quantitatively develop a petrogenetic model. An early trace element study by Kable et al. (1971) reported slight but distinct differences between selected incompatible trace element ratios (Zr/Nb, K/Ba, Ba/ Zr) of the lavas from the two islands. These authors showed, for example, that the lavas on Marion Island had slightly lower Zr/Nb ratio (6.0 to 7.2) compared to Prince Edward Island lavas (8.1 to 8.6). With the exception of higher U/K ratios in the younger black lavas, little difference was observed in incompatible element ratios between lavas of the older and younger series on each island. The inherently different source compositions of magmas giving rise to the two islands as reflected in their distinct trace element ratios were later confirmed by the few published Sr, Nd and Pb isotope analyses (Verwoerd, 1987; Mahoney et al., 1992). Whereas Verwoerd originally recognised two main lava series occurring on both islands, on the basis of K–Ar dating of stratigraphic sections on Marion Island, McDougall et al. (2001) recognised eight periods of subaerial effusive activity at approximately 450, 350, 240, 170, 110, 85, 50 and b10 ka. In this study we describe the petrology, mineral chemistry and bulk rock geochemistry of a new and extensive suite of samples taken primarily from Marion Island, but with a few from Prince Edward Island. Petrogenetic models for the low pressure compositional evolution of the lavas are developed, and the compositions of the most magnesium-rich lavas are used to place constraints on the composition of the underlying mantle source region(s). 2. Geological setting and sampling Marion and Prince Edward islands are two shield volcanoes constructed on a − 200 m submarine plateau located some 300 km southeast of the Southwest Indian Ridge (SWIR), on the western edge of a fracture zone that cuts the SWIR at ~40°E (Fig. 1A and B). The aseismic plateau extends to the east and west of the 40°E fracture zone and is unsampled, except for the Funk Seamount (Fig. 1B) located on the northwesternmost extremity which has yielded basalt of similar composition to that found on Marion and Prince Edward Islands (Reid and le Roex, 1988). Fig. 1C and D shows the subaerial topography of the two islands. Verwoerd (1971) recognised on both island edifices two main periods of activity, the Pleistocene and Holocene (dated by McDougall, 1971; McDougall et al., 2001), separated by the last glacial event (50,000–15,000 y) (Fig. 2). In the absence of geochronological data on

most of the Holocene lavas, these have merely been grouped together as old, intermediate and young on the basis of superposition and the amount of vegetation on them. The most recent volcanic event recorded on the islands was a fissure eruption accompanied by several flows of limited areal extent that occurred on Marion Island in 1980 (Verwoerd et al., 1981). The volcanism on both islands has been of strombolian character, including ‘aa’ and ‘pahoehoe’ flows, tunnels and levees. Effusive activity (Hawaiian type) with spatter cones and ramparts is less common. Surtseyan tuff cones occur at or near sea level and remnants of hyaloclastite of similar origin are interbedded with Pleistocene lava and fluvioglacial deposits (Verwoerd and Chevallier, 1987). Marion volcano is composed of two coalescent volcanic shields not easily distinguishable on morphological grounds, but evidenced structurally by two sets of radial fractures along which all the lateral Holocene strombolian eruptions took place (Chevallier, 1986), and stratigraphically by two distinct Pleistocene series: the main (eastern) shield series and the western shield series (Fig. 2). Tectonics and edifice dynamics of Marion Island are characterised by large radial grabens that resulted from landslides, and the absence of caldera collapse at the top of each centre. Regional tectonics has played a major role in the structural evolution of both Marion and Prince Edward volcanoes (Chevallier, 1986). The N20°E fracture zone along which the two volcanoes are located, and an associated E–W tectonic direction parallel to the ridge axis, controlled the shape of the submarine plateau and down-faulting on both islands. Such faulting is responsible for the disappearance below sea-level of four-fifths of the Prince Edward edifice. Earlier studies (e.g.Verwoerd, 1971, 1987) have shown that both islands are composed predominantly of alkali basalt and less abundant trachybasalt, with more evolved compositions being very rare. To fully establish the compositional variations shown by the Marion and Prince Edward Islands, and to better understand the compositional evolution of the lavas and their mantle source region(s), an intensive sampling programme of the islands has been conducted. Sample sites were selected on the basis of the revised geological map of the islands, taking cognisance of the structural evolution of Marion Island as discussed by Chevallier (1986). In total over 200 new samples have been collected, primarily from Marion Island (East and West Shield series) but also from Prince Edward Island, which form the basis of this study. Sample localities are shown in Fig. 2. Exact coordinates are not available as the samples were collected prior to access to a hand-held GPS. Fig. 3 shows the Marion and Prince Edward Island lava compositions plotted on a total alkali-silica diagram, and the predominance of alkali basalt and hawaiite (trachybasalt) is clear. To date only a handful of samples have been found with more evolved compositions, which follow a mugearite–benmoreite–trachyte–rhyolite(?) evolutionary trend. Several white rhyolite pumice fragments, up to 5 cm in diameter, were found as angular xenoliths in the basalt of the 1980 eruption on Marion. No other evidence of silicic volcanism has been found on either Marion or Prince Edward Islands; the most evolved exposed lava is trachyte found on the plateau of Prince Edward Island. The possibility that the pumice xenoliths are of extraneous origin must therefore be considered. 3. Petrography The samples analysed in this study are all remarkably fresh, showing little evidence of alteration under the microscope. Where present, alteration is limited to minor Fe-hydroxide staining along fractures crosscutting olivine phenocrysts. The vast majority of lavas sampled from both islands are alkali basalts which range in texture from aphyric through mildly porphyritic to coarsely porphyritic. In the porphyritic varieties, olivine (1–8 mm) and clinopyroxene (1–10 mm) dominate, although in a few cases (LC-21, -67, -93, -94, -160) large (1–12 mm) plagioclase phenocrysts are predominant (~20–30%). In the latter,

3

Port Elizabeth

Mo za m Rid biqu e ge

Africa

Mad agas c Ridg ar e

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

35

A

3

B 35 46 o

3

3

13

3000

3

2500

Funk Seamount Agulhas Plateau

40 0

4.

3 4.5

30

250

5

40

3

00

Prince Edward Is. 4.5

3

45

Del Cano Rise

Funk Smt

2 3

Marion & Prince Edward Islands

45

Crozet Islands

47 o

4.5

4.5

25

W

Marion Is.

30

35

3500

40

45

50

37o

W

E

39o

E

C

4 km

D Ri

r ve

46o 37’S 600

MARION

PRINCE EDWARD

ISLAND

ISLAND

1000 800 600

o

46 55S

400

300 200 100

46o 38’S

400 300

2 km

200 100

37o 54’E 37o 40’E

38o 00’E

37o 50’E

Fig. 1. A. Location of Marion and Prince Edward Islands relative to the Southwest Indian Ridge; contours in thousand metres. Plate motion parallels fracture zone traces. B. Bathymetric structure surrounding the two islands and Funk Seamount to the northeast. C and D. Sub-aerial topography of Marion Island and Prince Edward Island. Note scale difference.

Holocene lavas

Marion Island

1980 eruption

0

215

Western shield Eastern shield

Moraine 58

211 212

170-172

Cave Bay

Volcanic Cones Holocene Series

46o38’

222

Pleistocene Series McNish Bay

164

210

Kaalkoppie

Legend

57

221

190

46,47

37,38,39

Snow & debris

218

49-53 67,160 1,4,5,9 163

43,44

Prince Edward Is.

61

209

184 62

W

148

37o54’

37o56’

37o58’

63

59

207

42

South Cape

55

193

46o55’

149

206

111

135

91

192 112

201

25-27 25-28 30,31,98 30, 31

93,94 96

85 82

98

87

120

200 48

92

194

18 15,16

113

199

108 103

198 128

21-23

115,117

79

Km 0

W

2

4

37o40’

2

van Zinderen Bakker Peak 672 m

46o 37’

Approx. boundary between eastern and western Pleistocene shields

154

Pleistocene lavas

Km

Postulated shield centres

37o50’

E

Fig. 2. Simplified geological maps of Marion and Prince Edward Islands, showing sample locations. Modified from Verwoerd (1987).

38o00’

E

14

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

Marion Is. Prince Edward Is.

14

tephriphonolite

12

Na2O + K2O(wt%)

Table 1 Replicate analyses of BHVO-2 (n = 3) showing% relative standard deviation as indication of precision. Tpb = total procedural blank. All data in ppm.

phonolite

trachyte

phonotephrite

10

rhyolite

Benmoreite

8

tephrite (ol<10%) basanite (ol>10%)

6

Mugearite Hawaiite basaltic andesite

4 picrobasalt

2

40

andesite

dacite

basalt

50

60

70

SiO2 (wt%) Fig. 3. Total alkali–silica diagram showing compositions of Marion and Prince Edward Island lavas.

plagioclase occurs as glomerocrysts or single large crystals, together with minor olivine and augitic clinopyroxene. A number of the more coarsely porphyritic lavas (LC-15, -16, -31, -43, -87, -103, -128, -164, -199, -211) can be classified as ankaramites, with total phenocrysts making up some 30–40% of the rock, with augitic clinopyroxene dominating over olivine in the phenocryst assemblage. FeTi-oxides occur disseminated through the matrix, and rarely as equant microphenocrysts. The hawaiites are largely aphyric to mildly porphyritic in texture, with plagioclase dominating when rare phenocrysts are present. The groundmass comprises plagioclase laths (dominant) together with lesser olivine, augitic clinopyroxene and disseminated FeTi-oxides. Biotite and amphibole are rare, and when present constitute less than 1% of the rock. The trachyte and the single benmoreite samples are aphyric with a fluidal texture, and are composed predominantly of sodic plagioclase microlites in an aphanitic matrix. Rare augitic clinopyroxene and FeTi-oxides occur as microphenocrysts (b0.5 mm) in both rock types. The single rhyolite is an aphyric pumice with an alkali feldspar dominant groundmass having a trachytic texture. 4. Analytical A subset of 90 samples was selected for major and trace element analysis by XRF using the standard Norrish technique as applied at the University of Stellenbosch (Scheepers, 1995). A further subset of 41 samples was selected for trace element analysis by ICP-MS. These were done at the University of Cape Town, using a Thermo Scientific X-Series 2 quadrupole ICP-MS and the following analytical procedure: 50 mg of − 300 mesh sample powder were dissolved in a 3:1 concentrated HF/HNO3 acid mixture in sealed Savilex beakers on a hotplate for 48 h, followed by evaporation to dryness and two treatments of 2 ml concentrated HNO3. The final dried product was then taken up in a 5% HNO3 solution containing 10 ppb Re, Rh, In and Bi as internal standards. Standardisation was against artificial multi-element standards. Long-term replicate analyses of BHVO-2 give overall procedural errors of better than 3% relative. Analysis of BHVO-2 and a procedural blank done during the course of this study are reported in Table 1. Minerals were analysed at the University of Cape Town using a Cameca Camebax electron microprobe. Analyses were undertaken using a 40 nA beam with an accelerating voltage of 15 kV. The incident electron beam was focussed to a diameter of 5 μm. Ten second

Li Sc V Cr Co Ni Cu Zn Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

BHVO-2

BHVO-2

Geo-Rem

n=3

4.8 32 317 280 45 119 127 103 9.11 396 26 169 18.1 131 15.2 37.5 5.35 24.5 6.07 2.07 6.24 0.92 5.31 0.98 2.54 0.33 2.00 0.27 4.36 1.14 1.60 1.22 0.40

4.27 27.5 313 272 42.9 114 98.7 117 8.64 401 23.4 163 16.4 128 14.6 36.4 5.08 23.9 6.01 1.95 5.98 0.87 5.07 0.93 2.41 0.32 1.93 0.28 4.24 0.99 1.65 1.17 0.40

%rsd

tpb

0.86 2.42 1.29 1.21 1.70 1.19 1.36 2.59 2.48 1.37 0.40 1.24 1.28 1.56 1.09 1.32 1.11 2.45 1.52 0.54 1.67 0.87 1.29 0.63 1.61 2.53 0.75 4.35 1.71 2.39 2.97 1.21 4.27

0.03 0.04 0.01 0.02 0.01 0.25 0.74 0.97 0.1 0.07 0 0.090 0 0.11 0 0 0 0 0 0 0.001 0 0 0 0 0 0 0 0 0.04 0.03 0.020 0

counting times on peak and background position were used for all elements. Standardisation was against a mixture of natural and synthetic microprobe standards, and data were reduced using the PAP data reduction method, as described by Pouchou and Pichoir (1984). Sr, Nd and Pb isotope analyses were performed using a NuPlasma HR MC-ICP-MS in the African Earth Observatory Network, EarthLAB Facility in the Department of Geological Sciences at the University of Cape Town. Following HF:HNO3 digestion of approximately 50 mg of sample material, Sr, Nd and Pb fractions were isolated using Sr.Spec, TRU.Spec and Ln.Spec resins (Eichrom) according to routines adapted from standard ion-exchange methods (Metz and Mahood, 1991; Pin et al., 1994; Pin and Zalduegui, 1997). Existing concentration data were used to obtain final solutions of 3 ml of 200 ppb Sr in 0.2% HNO3, 1.5 ml of 50 ppb Nd in 2% HNO3 and 1.5 ml 50 ppb Pb in 2% HNO3 solutions. A single batch of HNO3 was used for each isotope system. For Sr isotope analysis, solutions were aspirated into the plasma via a microcyclonic spraychamber, whereas for Nd isotope analysis a Nu Instruments DSN-100 desolvating nebuliser was used. Measured isotope analyses were corrected for background signals (including 84 Kr and 86Kr) using an on-peak background measurement of the 0.2% HNO3 solution. Instrumental mass fractionation was corrected using the exponential law and a fractionation factor based on the measured 86Sr/ 88Sr ratio and the accepted value of 0.1194. The 87Rb contribution on the 87 amu signal was calculated and subtracted using this fractionation factor, the exponential law, the measured 85 Rb signal and a 85Rb/ 87Rb ratio of 2.5926. NIST SRM987 was analysed as bracketing standard yielding external, measured 2σ reproducibility of 0.000021 (n = 4) on an average 87Sr/ 86Sr ratio of 0.710255. All 87Sr/ 86Sr data were normalised to 0.710255, the in-

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

house long-term average, which agrees with published results. The average 84Sr/ 86Sr ratio was 0.05643 ± 0.00012, also in agreement with published values. All Nd isotope measurements were corrected for Sm and Ce interferences and for instrumental mass fractionation, using the exponential law and a 146Nd/144Nd value of 0.7219. For further details of the analytical techniques see Will et al. (2010). JNdi-1 was used as bracketing standard, and all Nd isotope data presented are referenced to this standard, using a 144Nd/143Nd ratio of 0.512115 (Tanaka et al., 2000). A measured external 2σ reproducibility of 0.000010 (n= 5) was obtained on an average 144Nd/ 143Nd ratio of 0.512106. Pb isotopes were analysed on 50 ppb 2% HNO3 solutions doped with (NIST SRM997) Tl to give a ±10:1 Pb:Tl ratio with injection via a Nu Instruments DSN-100 desolvating nebuliser. NIST SRM981 was used as reference standard, with normalising values of 36.7219 ( 208Pb/ 204Pb), 15.4963 ( 207Pb/ 204Pb), and 16.9405 ( 206Pb/ 204Pb). All Pb isotope data were corrected for Hg interference and instrumental mass fractionation using the exponential law and a 205Tl/203Tl value of 2.3889. The long term averages obtained for BHVO-2 (n= 8) were 38.228 ± 0.044 (208Pb/204Pb), 15.534 ± 0.016 (207Pb/204Pb) and 18.633 ± 0.066 (206Pb/204Pb). By way of comparison, Weiss et al. (2006) cite values of 38.237 ± 0.018, 15.533 ± 0.009 and 18.647 ± 0.024, respectively, for BHVO-2.

15

~Fo61 to ~ Fo50. A single olivine analysis in benmoreite LC-23 gives a composition of Fo38. 5.2. Clinopyroxene Selected clinopyroxene analyses are reported in Table 2. The vast majority of pyroxenes in the alkali basalts and hawaiites are titanaugite in composition (Wo47En42Fs11 to Wo42En38Fs20), but some matrix phases extend to ferroaugite as iron-rich as Wo23En31Fs46. Minor elements are highly variable with TiO2 ranging from 0.7 to 11.8 wt.% and Al2O3 from 0.7 to 13.0 wt.%. Unusually for alkaline magmas, pigeonite (Wo3.3En71.8Fs24.9) has been found in the 1980 hawaiite lava flow (LC-13). The matrix pyroxene in benmoreite LC-23 is ferroaugite in composition (Wo42–21En34–37Fs24–41). 5.3. Plagioclase

5. Mineral chemistry

Selected plagioclase analyses are reported in Table 3. Compositions range systematically from about An84 in phenocrysts in the more primitive alkali basalts to ~An56 in the matrix plagioclase. The latter compositions are also characteristic of matrix plagioclase in the aphyric hawaiite magmas. The benmoreite has more sodic plagioclase (An43), whereas the feldspar compositions in the trachytes extend towards anorthoclase (An28Ab64Or8). Feldspar compositions in the rhyolite pumice range from alkali feldspar (An5Ab61Or34) to anorthoclase (An12Ab70Or18).

5.1. Olivine

5.4. FeTi-oxides

Selected olivine analyses are reported in Table 2. Phenocryst and microphenocryst compositions range from ~Fo82 (ankaramitic lavas; e.g. LC-103) down to ~Fo71 in the alkali basalt samples, with matrix olivine compositions extending down to Fo52 (LC-171). Olivine occurs only as matrix phase in the hawaiites with compositions ranging from

FeTi-oxides occur in the Marion and Prince Edward islands as both titanomagnetite and ilmenite (either as separate phases or in exsolution relationship). Compositions are quite variable (Table 3) and plot systematically along the ulvospinel–magnetite and ilmenite–hematite solid solution joins in the TiO2–FeO-compositional triangle. A single

Table 2 Selected olivine and clinopyroxene analyses from Marion Island lavas. p = phenocryst; mp = microphenocryst; m = microlite. Olivine

SiO2 TiO2 Al2O3 FeO MnO MgO CaO NiO Total Fo

LC-103

LC-111

LC-171

LC-13

LC-108

LC-82

LC-117

LC-117

LC-186

LC-23

p

p

p

mp

p

mp

m

m

m

m

39.40 nd 0.05 17.15 0.17 43.36 0.30 0.12 100.55 81.6

38.88 nd 0.34 18.57 0.27 42.17 0.21 0.16 100.6 79.8

38.84 0.01 0.03 20.20 0.20 40.82 0.20 0.15 100.42 78.0

36.35 0.09 1.43 20.99 0.29 40.3 0.36 0.12 99.9 77.0

37.77 nd 0.22 21.3 0.31 40.00 0.26 nd 99.86 76.7

37.37 nd 0.12 25.31 0.34 36.39 0.27 nd 99.80 71.6

35.61 0.07 0.05 33.28 0.66 29.22 0.31 0.02 99.23 60.5

34.99 0.14 0.05 36.55 0.71 27.2 0.30 nd 99.94 56.5

34.53 0.05 0.03 40.08 0.98 24.11 0.33 nd 100.14 51.1

32.88 0.06 nd 48.53 1.46 17.24 0.24 nd 100.41 38.1

Clinopyroxene

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO N a2O Total Wo En Fs

LC-97

LC-97

LC-111

LC-13

LC-13

LC-82

LC-91

LC-23

LC-23

LC-188

p

m

p

m

m

m

m

m

m

m

48.38 1.85 5.85 0.46 6.74 0.13 14.09 22.10 0.38 99.98 47.0 41.7 11.3

44.96 3.90 7.40 0.23 9.43 0.16 12.36 21.35 0.41 100.20 46.4 37.4 16.2

44.00 3.81 8.44 0.24 9.07 0.17 12.25 21.49 0.37 99.84 47.0 37.3 15.7

38.03 0.30 2.91 nd 21.08 0.28 34.60 2.22 0.11 99.53 3.3 71.8 24.9

48.57 1.84 5.07 nd 8.64 0.17 14.04 20.65 0.32 99.30 43.9 41.5 14.6

47.75 2.44 4.08 nd 10.50 0.22 13.69 20.44 0.67 99.83 42.7 39.8 17.5

44.25 0.71 0.76 nd 26.92 0.98 14.27 11.36 0.29 99.54 23.3 30.6 46.1

50.10 1.49 1.86 nd 13.68 0.64 11.51 20.00 0.52 99.92 42.4 33.9 23.7

50.96 1.46 2.11 nd 11.43 0.10 13.22 20.75 0.51 100.87 42.9 38.1 19.0

49.05 1.45 5.18 0.65 6.39 0.10 14.76 21.82 0.37 99.77 46.0 43.3 10.7

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Table 3 Selected feldspar and FeTi-oxide analyses from Marion Island lavas. Mgt = magnetite; ilm = ilmenite; Ulv = ulvospinel; Hem = hematite. p = phenocryst; mp = microphenocryst; m = microlite. Feldspar

SiO2 Al2O3 FeO MgO CaO Na2O K2O Total An Ab Or

LC-97

LC-97

LC-93

LC-67

LC-154

LC-117

LC-47

LC-171

LC-23

LC-186

p

m

p

p

mp

m

m

m

m

m

49.60 31.22 0.55 0.09 15.64 2.22 0.11 99.43 79.0 20.3 0.7

51.44 30.92 0.71 0.12 13.51 3.29 0.23 100.22 68.4 30.2 1.4

51.71 30.66 0.79 0.08 12.31 3.80 0.33 99.68 62.8 35.2 2.0

62.01 23.48 0.36 0.01 5.63 7.10 1.28 99.91 28.1 64.3 7.6

55.16 28.68 0.55 0.04 11.35 4.46 0.33 100.58 57.3 40.7 2.0

48.52 32.31 0.52 0.07 16.15 1.82 0.11 99.49 82.5 16.8 0.7

51.27 29.99 0.88 0.09 13.44 3.28 0.25 99.20 68.3 30.2 1.5

48.18 32.45 0.46 0.06 16.45 1.65 0.07 99.32 84.3 15.3 0.4

57.99 25.55 0.82 0.05 8.47 5.79 0.58 99.22 43.2 53.5 3.3

55.24 28.10 0.81 0.06 11.16 4.54 0.37 100.30 56.3 41.5 2.2

LC-186

LC-186

LC-108

LC-108

LC-120

LC-120

LC-103

LC-103

LC-154

LC-23

mgt

ilm

mgt

ilm

mgt

ilm

mgt

ilm

mgt

mgt

0.08 19.96 1.68 0.02 72.63 0.67 1.70 0.00 96.74 37.52 39.03 100.65 0.51 0.49

0.05 50.50 0.12 0.03 45.45 0.81 2.69 0.04 99.69 39.92 6.14 100.30

0.06 17.82 5.15 0.05 66.66 0.37 5.76 0.00 95.87 29.48 41.32 100.01 0.46 0.54

0.24 48.82 1.00 0.00 43.89 0.63 2.87 0.39 97.84 38.06 6.48 98.49

0.08 18.99 2.49 0.06 73.17 0.55 2.32 0.00 97.66 35.85 41.47 101.82 0.48 0.52

0.04 47.19 0.04 0.00 45.64 4.61 0.31 0.00 97.83 37.51 9.03 98.74

0.00 29.60 2.03 1.26 61.32 0.56 3.27 0.00 98.04 49.26 13.40 99.38 0.82 0.18

0.08 51.68 0.09 0.04 43.14 0.68 4.03 0.32 100.06 38.37 5.30 100.59

0.11 23.97 4.55 0.57 64.84 0.50 3.35 0.07 97.96 41.79 25.61 100.52 0.65 0.35

0.06 19.10 1.27 0.00 75.60 0.72 0.70 0.00 97.45 38.02 41.77 101.64 0.48 0.52

FeTi-oxides

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Total FeOc Fe2O3c Total Mgt Ulv Ilm Hem

0.94 0.06

0.94 0.06

Cr-spinel has been found in one of the ankaramitic lavas from Prince Edward Island (WJE21). The coexistence of titanomagnetite and ilmenite allows an estimate to be made of the prevailing temperature and fO2 conditions during crystallisation of the Marion and Prince Edward Island lavas. Calculated fO2 and temperatures of equilibration indicate that FeTioxide crystallisation occurred at or slightly above the QFM buffer; temperatures range from 1100 to 810 °C at fO2 of 10 − 10 to 10 − 14.5 atm, using either the Buddington and Lindsley (1964) or the Powell and Powell (1977) formulations. 6. Bulk rock geochemistry Representative major and trace element analyses of lavas from Marion and Prince Edward Islands are reported in Tables 4 and 5. The full dataset is provided in an electronic supplement. Despite the samples being remarkably fresh, as evidenced by low LOI values (b1%) and absence of secondary minerals in thin section, the data have been normalised to 100% on a volatile free basis before plotting on the various diagrams shown. 6.1. Major elements Major element variations in lavas from both Marion and Prince Edward are systematic, but there are no meaningful differences in major element composition between the Pleistocene and Holocene series, nor between the lavas forming the East and West Pleistocene shields. Consequently, in discussing the compositional variations, no distinctions are made between the different series. The vast majority of lavas from both islands are alkali basalt and hawaiite in terms of a

0.91 0.09

0.95 0.05

total alkali-silica classification (Fig. 3), with a single sample from Marion (LC-58) falling within the mugearite field, and one (LC-23) in the benmoreite field. Two samples from Prince Edward (LC-46, -47) can be classified as trachyte and a single sample found as a xenolith within a lava on Marion Island (LC-149) plots within the rhyolite field (Fig. 3). Fig. 4 shows variation of selected major elements with respect to MgO. It is clear that systematic trends are described, with lavas from Marion and Prince Edward largely overlapping in composition. MgO varies from ~14 wt.% down to 6 wt.% in the basaltic lavas and drops away to b1% in the trachytes and rhyolite. Mg-numbers (atomic Mg/ Mg + Fe2 +, Fe2O3/FeO= 0.15) shows a corresponding range from 0.73 to 0.06. As MgO decreases, SiO2, CaO and Fe2O3 remain relatively constant down to ~6 wt.% MgO, after which CaO and Fe2O3 both drop dramatically, and SiO2 increases. Al2O3 correlates negatively with MgO, and TiO2 and K2O first increase with decreasing MgO and then at ~5% MgO drop (TiO2) or rise (K2O) dramatically. A few plagioclase-rich samples (e.g. LC-21, - 93, -160) are clearly displaced from the trends involving Al2O3 and CaO towards the composition of plagioclase. A second group of samples (LC-1, -22, -17, -148 from Marion and LC-37, -38, -44 from Prince Edward) shows strong enrichment in TiO2 (>4 wt.%) at about 5 wt.% MgO (Fig. 4). The most MgO-rich samples (LC-15, -16, -31, -103, -128, -199, -211) are the most coarsely porphyritic with large olivine and clinopyroxene phenocrysts. The composition of the rhyolite pumice xenoliths is consistent with the major element trends shown by the Marion Island lavas. 6.2. Trace elements Lavas from the Pleistocene and Holocene series from each island show no systematic differences in their trace element characteristics

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

and in the following descriptions no distinction is made between the different series. Trace elements vary systematically across the range of rock types, with compatible elements like Ni (395 − b2 ppm), Cr (880− b2 ppm) and Sc (34 −2 ppm) decreasing systematically with MgO (not shown). Incompatible trace elements define tightly defined positive correlations with Zr (Fig. 5) and extend over significant abundance ranges (e.g. Nb= 24–97 ppm; Ba 40–755 ppm; Th =1.7 to 13 ppm; Zr 170–960 ppm). The rhyolite pumice is displaced from these trends. It is clear from Fig. 5 that Prince Edward Island samples define distinct correlations amongst many of the incompatible trace elements that are readily distinguished from Marion Island samples. The former have slightly lower Nb, Th, Rb, Ba and Ce at a given Zr concentration compared to Marion Island lavas. This difference is reflected in slight differences in certain incompatible element ratios (Table 6), for example Zr/Nb (6.7±0.3 Marion; 9.1 ±0.3 Prince Edward), Ba/Nb (7.8 ±0.1 Marion; 6.7 ±0.4 Prince Edward) and Ba/Th (113 ±5 Marion; 100 ±4 Prince Edward), whereas many others are identical (e.g. La/Nbn =0.82 ±0.02; Y/Nb = 0.73 ± 0.06; Zr/Hf= 45.7 ± 3; Nb/U = 47.3 ±1.8). Compared to Gough Island lavas, Marion and Prince Edward lavas are not as enriched in the most highly incompatible elements (e.g. Nb, Th, Ba) relative to Zr (Fig. 5), and incompatible element ratios allow distinction between Marion and Prince Edward Island lavas and other South Atlantic OIB (Table 6). In terms of its trace element composition, the rhyolite pumice found as a xenolith in the 1980 eruption can be clearly distinguished from the rhyolite found on Bouvet Island (the latter has very much higher Zr and Y; Zr > 1000 ppm, Y > 90 ppm; le Roex and Erlank, 1982) and from those characteristic of the South Sandwich Islands (LeMasurier and Thomson, 1990), which show strong relative depletion in Nb and Ta, consistent with their island arc setting. Consequently, although drift pumice has been documented on the beaches of Marion Island, with purported origin from the vicinity of the South Sandwich Islands (Frick and Kent, 1984), the rhyolite pumice xenoliths described here are unlikely to be extraneously derived, at least from either of these two potential sources. Chondrite normalised rare earth element (REE) patterns are near identical for the mafic lavas from the two islands (La/Smn = 2.58 ± 0.3; La/Ybn = 10.6 ± 1.4) and are strongly subparallel, increasing in absolute abundances equally across the REE mass range from alkali basalt through to benmoreite (Fig. 6). Trachyte LC-46 from Prince Edward Island has a slightly different, concave, pattern (Fig. 6) and no Eu anomaly, whereas the rhyolite from Marion has a modest negative Eu anomaly (Eu/Eu* = 0.48), and similar HREE abundances to the benmoreite lavas. Trace element patterns are equally subparallel on a primitive mantle normalised diagram (not shown), with only the mugearite and benmoreite showing relative depletion in Sr and Ti and the trachyte from Prince Edward showing strong relative depletion in Sr, P and Ti. The rhyolite from Marion Island shows strong relative depletion in Ba, Sr, Zr, Hf and Ti. 6.3. Sr, Nd and Pb isotopes Sr and Nd isotope ratios are unremarkable (Table 7) and match the few previously published data for the two islands (Mahoney et al., 1992). Prince Edward Island has slightly lower 87Sr/ 86Sr (0.7030200) compared to Marion Island (0.70336–0.70349) and higher 143Nd/ 144Nd (0.51300 versus 0.51292–0.51295), indicating derivation of the lavas from a slightly less enriched source (Fig. 7). There are no significant differences in Sr and Nd isotope composition between the various stratigraphic sequences identified — i.e. between the Pleistocene and Holocene series, or between the Eastern and Western Shields on Marion Island (Table 7). Compared to Bouvet Island, the Marion and Prince Edward Islands are isotopically less enriched (Fig. 7), with Prince Edward being similar in isotope composition to Ascension Island, the least enriched of the South Atlantic–Southern Ocean islands. The spatially associated

17

Funk Seamount lavas are isotopically similar to Prince Edward Island lavas (Fig. 7). Crozet Island lavas are isotopically more enriched than Marion Island, as are E-type MORB dredged to the northeast from 40°E on the SWIR (Fig. 7). E-type MORB from the SWIR to the west and north of Marion and Prince Edward Islands extend from isotopically depleted compositions to compositions similar to those found on the two islands (Fig. 7). Pb isotope ratios (206Pb/204Pb = 18.61–18.75, 207Pb/ 204Pb = 15.54– 15.56; Table 7) show no significant difference between the two islands. Both islands plot slightly above the Northern Hemisphere Reference Line (Fig. 7), and are distinct in their isotope ratios from Bouvet Island (to the southwest) and Crozet (to the east). They plot within the field of MORB from the Southwest Indian Ridge. 7. Petrogenesis The coherent compositional variation shown by the Marion and Prince Edward Island lavas suggest that they formed by relatively simple petrogenetic processes. The simplicity of the compositional variations, shown by lavas from both islands, including uniform isotope and incompatible element ratios, is perhaps the most notable feature of the lavas. 7.1. Fractional crystallisation–crystal accumulation Major and trace element variations shown by lavas erupted on both Marion Island and Prince Edward Island are coherent and qualitatively consistent with compositional control by crystal fractionation processes (e.g. Figs. 3 and 4). Fig. 8 shows that the coarsely porphyritic olivine plus clinopyroxene-rich lavas are displaced from the compositional field of aphyric lavas directly towards a mixture of olivine (~40%) and clinopyroxene (~60%) suggesting that these lavas are partial cumulates of these two phases. Likewise, although less pronounced, the coarsely plagioclase-phyric lavas are displaced towards the composition of plagioclase, indicative of partial plagioclase accumulation. Quantitative major and trace element modelling, using least squares approximation techniques, application of trace element partitioning equations (Gast, 1968, Shaw, 1970) and the results of the major element models, confirms these qualitative observations (Tables 8 and 9). The compositions of the most clinopyroxene plus olivine-rich (ankaramitic) lavas (e.g. LC-199) are consistent with up to ~33% accumulation of olivine (Fo78) plus clinopyroxene (Wo50En41Fs9), and in some cases minor plagioclase and FeTioxides, in the proportions 39:44:16:1. The highly plagioclase-phyric lavas can be shown to have experienced up to 20% plagioclase only accumulation (not shown). The change in composition from alkali basalt through hawaiite to mugearite and benmoreite on Marion Island can be quantitatively attributed to crystal fractionation of major phenocryst phases, olivine, clinopyroxene, plagioclase, FeTi-oxides, together with minor apatite in the most evolved lavas. Table 8 shows some examples of successful least squares approximation modelling of major element variations, and it is clear that the compositional changes across the range alkali basalt through hawaiite, mugearite to benmoreite (shown for the Pleistocene lavas as example, but can be shown also for the Holocene lavas) are consistent with crystal fractionation of the observed low pressure phenocryst phases. The lavas are not able to all be related along a single liquid-line-of-descent, which would be an unrealistic expectation. The examples shown in Table 8 are chosen rather for the quality of the model solutions while spanning the range of rock types. Compositional variation within the alkali basalts can be accounted for by some 20% crystal fractionation of olivine, clinopyroxene and plagioclase, whereas a further 24% fractionation drives the compositions to hawaiite. Mugearite is produced after a further 36% crystal fractionation, with FeTi-oxides becoming a major fractionating

18 Table 4 Selected major and trace element analyses of Marion Island lavas. Mg# = atomic Mg/Mg + Fe2 + with Fe2O3/FeO = 0.15. A = aphyric; MP = moderately porphyritic; P = porphyritic; PP = plagioclase phyric. Reported compositions of samples LC23, LC31, LC51, LC58 and LC160 differ slightly from those reported in Verwoerd (1987) due to re-averaging and re-analysis. LC-25

LC-27

LC-31

LC-164

LC-117

LC-58

Pleistocene: Eastern Shield

LC-21

LC-50

LC-51

LC-52

LC-93

LC-96

LC-103

LC-160

LC-4

LC-5

LC-23

Pleistocene: Western Shield

Alkali basalt

Alkali basalt

Alkali basalt

Alkali basalt

Hawaiite

Mugearite

Alkali basalt

Alkali basalt

Alkali basalt

Alkali basalt

Alkali basalt

Alkali basalt

Alkali basalt

Alkali basalt

Hawaiite

Mugearite

Benmoreite

Texture

A

A

P

P

A

A

PP

MP

MP

MP

PP

MP

P

PP

A

A

A

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI H2OTotal Mg# Cr V Zn Cu Ni Sc Co Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

46.25 2.90 14.55 12.18 0.15 8.92 10.34 3.35 0.49 0.36 − 0.43 0.18 99.24 62.2 271 231 4 60.1 140 22.3 61.4 14.5 462 22.1 226 29.7 238 23.4 53.6 6.43 29.1 6.1 2.02 5.99 0.82 4.77 0.86 2.18 0.29 1.72 0.23 4.58 1.84 1.85 2.13 0.62

47.40 3.11 15.71 12.16 0.16 6.12 10.15 3.74 0.73 0.45 − 0.55 0.11 99.30 53.1 72.8 311 132 33.1 57.5 21.3 50.1 21.9 623 27.2 261 37.8 285 30.1 68.1 8.28 35.9 7.79 2.54 7.36 1.01 5.83 1.04 2.70 0.34 2.13 0.29 5.92 2.27 1.92 2.91 0.83

46.70 2.56 13.08 12.11 0.16 11.05 10.24 3.08 0.24 0.30 − 0.38 0.25 99.49 67.2 420 239 84.5 87.5 233 24.7 64.8 9.4 409 20.5 185 24 184 18.8 43.6 5.27 24.4 5.42 1.82 5.52 0.76 4.57 0.82 2.06 0.26 1.59 0.22

45.39 2.78 13.92 12.34 0.17 10.13 10.85 3.14 0.98 0.46 − 0.05 0.12 100.22 64.9 409 294 116 37.1 193 28.5 69.2 16.0 578 22.3 214 30.5 227 24.6 54.8 6.75 29.0 6.24 2.11 6.04 0.84 4.74 0.86 2.18 0.28 1.75 0.24 4.74 1.86 1.65 2.29 0.54

45.64 4.24 16.08 14.59 0.19 5.33 8.93 4.18 1.31 0.55 − 0.80 0.14 100.36 45.1 14.3 281 98.9 38.4 26.1 19.7 43.3 18.2 534 27.2 266 40.4 321 29.7 68.4 8.05 36.1 7.62 2.45 7.36 1.01 5.93 1.06 2.69 0.35 2.11 0.29 5.49 2.37 2.51 2.58 0.77

50.76 2.47 17.22 11.69 0.18 4.18 6.76 5.05 1.60 0.82 − 0.58 0.17 100.33 44.5 23.7 82.1 86.7 11.4 10.1 10.3 21.1 31.2 682 31.3 388 57.9 489 48.2 106 12 51.8 9.77 2.98 8.91 1.16 6.72 1.21 3.07 0.4 2.5 0.35 7.48 3.18 3.45 4.43 1.3

46.43 3.30 18.22 11.95 0.15 4.94 11.31 3.17 0.54 0.36 − 0.31 0.08 100.14 48.1 42.2 341 117 26.6 42.4 20.3 53.3 17.4 715 22.0 206 31.3 244 24.8 55.1 6.73 29.1 6.32 2.10 5.91 0.82 4.72 0.83 2.21 0.28 1.76 0.24 4.73 1.98 1.75 2.34 0.67

47.02 3.55 16.46 13.48 0.16 4.96 9.74 3.63 0.77 0.44 − 0.78 0.10 99.53 45.2 22.0 355 139 13.9 30.1 19.6 55.5 20.8 617 26.5 253 36.9 281 28.7 64.7 7.84 34.2 7.54 2.46 7.16 0.98 5.75 1.02 2.65 0.35 2.15 0.29 5.80 2.26 1.97 2.87 0.80

45.90 3.16 14.67 13.54 0.16 8.95 10.16 3.18 0.48 0.33 − 0.80 0.08 99.85 59.8 267 332 122 54.9 155 23.1 66.1 14.6 514 22.5 201 29.0 219 23.1 52.2 6.52 28.9 6.49 2.05 6.05 0.83 4.81 0.87 2.26 0.29 1.81 0.25 4.79 1.85 1.54 2.16 0.64

46.70 3.56 16.63 13.65 0.17 5.12 9.99 3.48 0.72 0.43 − 0.74 0.12 99.84 45.8 27.1 310 94.8 37.6 33.2 19.2 53.5 15.2 483 23.5 242 34.8 276 26.8 60.5 7.1 31.9 6.61 2.16 6.4 0.89 5.23 0.94 2.38 0.31 1.9 0.25 4.93 2.08 2.16 2.54 0.64

46.00 3.31 18.72 11.82 0.15 4.69 11.56 3.31 1.00 0.43 − 0.48 0.07 100.57 47.1 71.3 296 75.9 58.3 48.8 20 37.7 15.1 548 20.1 205 30.4 243 23.2 53.5 6.24 27.8 5.75 1.9 5.56 0.74 4.43 0.8 2.01 0.27 1.61 0.22 4.26 1.7 1.99 2.16 0.65

45.03 3.08 15.94 13.12 0.16 8.53 10.97 3.03 0.97 0.42 − 0.56 0.22 100.92 59.4 207 325 121 38.4 142 22.8 71.4 16.4 603 21.5 196 29.5 238 23.7 52.9 6.53 28.8 6.31 2.00 5.78 0.79 4.63 0.82 2.17 0.27 1.71 0.23 4.55 1.92 1.55 2.29 0.67

45.36 2.41 11.20 12.62 0.17 14.30 11.23 2.68 0.72 0.38 − 0.57 0.22 100.71 71.8 767 241 77.3 107 279 27.8 66.4 11.1 421 17.5 184 25.8 213 21 47.2 5.57 25.3 5.2 1.63 5.00 0.66 3.91 0.69 1.73 0.23 1.35 0.18 3.85 1.54 1.55 1.84 0.54

46.54 3.64 17.78 12.60 0.17 4.32 10.64 3.80 1.21 0.54 − 0.39 0.17 101.03 43.5 9.06 380 129 5.99 26.8 22.5 49.5 21.4 704 25.7 250 37.4 283 28.8 64.4 7.83 33.3 7.13 2.44 6.91 0.96 5.47 0.98 2.52 0.33 2.03 0.29 5.53 2.19 1.88 2.81 0.70

48.90 3.05 15.84 12.68 0.19 4.24 7.59 4.89 1.53 0.75 − 0.50 0.07 99.24 42.9 0.07 184 149

50.42 2.91 15.47 12.13 0.20 4.06 7.38 4.76 1.38 0.79 − 0.19 0.14 99.45 43.0 0.094 135.2 159

10.8 42.1 31.7 750 36.4 355 54.0 430 44.5 98.4 11.8 50.3 10.8 3.39 9.96 1.36 7.82 1.39 3.62 0.47 2.91 0.40 7.82 3.23 2.86 4.18 1.18

11.3 43.3 32.0 716 40.3 393 57.5 432 46.5 104 12.6 54.5 11.7 3.69 11.0 1.49 8.61 1.54 4.04 0.52 3.24 0.44 8.55 3.5 2.89 4.34 1.22

55.59 1.49 17.02 9.47 0.21 2.48 5.09 5.78 2.18 0.68 − 0.48 0.11 99.63 37.1 16.4 16.1 129 3.62 7.55 10.3 22.1 41 592 46.5 521 76.3 629 59.3 132 15.3 66.6 13.1 4.13 12.2 1.65 9.8 1.76 4.63 0.59 3.77 0.51 10.5 4.18 4.29 5.57 1.66

1.46 1.88 1.72 0.48

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

Rock type

Table 4

LC-61

LC-85

LC-87

LC-128

LC-199

LC-211

LC-108

Holocene: Old

LC-62

LC-63

LC- 111

LC-206

LC-59

LC-91

Holocene: Intermediate

LC-82

LC-112

LC-201

LC-207

LC-149

Holocene: Young

Unknown

Alkali basalt

Alkali basalt

Alkali basalt

Alkali basalt

Alkali basalt

Hawaiite

Alkali basalt

Alkali basalt

Alkali basalt

Alkali basalt

Hawaiite

Hawaiite

Alkali basalt

Alkali basalt

Alkali basalt

Hawaiite

Rhyolite

A

A

P

P

P

P

MP

A

A

MP

MP

MP

A

A

A

A

A

A

46.50 3.20 15.36 12.91 0.18 7.18 10.12 3.39 1.16 0.51 − 0.79 0.14 99.86 55.6 224 269 89.3 62.2 90.5 23.7 54.9 19 539 24.6 268 39.2 311 30.5 68.7 7.91 35.2 7.15 2.29 6.82 0.93 5.44 0.97 2.48 0.32 1.94 0.27 5.47 2.31 2.55 2.9 0.84

46.10 3.21 16.33 13.17 0.18 5.82 10.37 3.53 0.93 0.45 − 0.73 0.12 99.50 49.9 62.2 335 133 29.5 46.2 22.4 45.0 17.4 581 26.2 222 33.8 257 26.7 60.3 7.49 33.3 7.46 2.32 6.85 0.93 5.55 0.99 2.59 0.34 2.08 0.28 5.22 1.95 1.75 2.54 0.76

47.10 2.56 14.10 12.45 0.17 10.08 10.52 2.57 0.62 0.33 − 0.74 0.18 99.94 64.5 451 258 115 82.17 211 26.1 54.4 12.5 449 21.4 182 25.8 188 21.0 48.5 5.98 26.5 6.00 1.91 5.69 0.78 4.63 0.82 2.14 0.27 1.69 0.23 4.44 1.51 1.44 1.93 0.57

46.09 2.17 10.35 12.93 0.18 15.29 10.67 2.39 0.68 0.30 − 0.85 0.09 100.29 72.7 736 239 107 50.7 450 27.5 73.0 12.5 439 17.3 152 24.3 202 19.1 42.2 5.29 22.8 5.01 1.62 4.70 0.66 3.71 0.67 1.70 0.22 1.35 0.19 3.58 1.34 1.25 1.88 0.46

46.30 2.37 12.41 12.64 0.17 12.27 10.52 2.69 0.69 0.32 − 0.79 0.14 99.73 68.6 502 240 83.5 66.8 262 27.2 71.8 11.9 400 17.9 169 23.6 196 18.2 42.2 5.08 23.3 4.95 1.61 4.92 0.67 4 0.7 1.82 0.23 1.43 0.19 3.62 1.47 1.56 1.7 0.5

46.27 2.28 13.05 11.67 0.16 11.54 11.51 2.50 0.71 0.32 − 0.46 0.08 99.63 69.0 546 258 105 40.9 248 30.1 56.9 14.6 531 19.5 177 26.4 196 20.5 47.0 5.81 24.8 5.40 1.85 5.26 0.74 4.18 0.75 1.92 0.25 1.53 0.21 4.10 1.50 1.41 1.92 0.49

47.65 3.60 15.78 12.97 0.19 5.20 9.08 3.88 1.52 0.68 − 0.56 0.12 100.09 47.4 37.2 293 102 37.2 35 19.3 50 23.7 590 29.1 323 50.6 403 39.7 87.7 9.91 43.3 8.52 2.58 7.88 1.06 6.17 1.1 2.79 0.37 2.2 0.31 6.31 2.97 2.92 3.62 1.01

46.80 2.60 13.92 12.54 0.17 9.49 10.11 2.81 0.80 0.48 0.08 0.33 100.13 63.0 362 265 115 28.5 175 24.1 53.5 16.3 490 21.8 193 29.9 237 24.7 54.1 6.59 28.8 6.32 1.98 5.84 0.79 4.62 0.83 2.17 0.28 1.77 0.24 4.58 1.71 1.71 2.48 0.71

46.16 3.10 15.70 12.91 0.18 7.24 9.95 3.19 1.01 0.50 − 0.45 0.30 99.80 55.8 228 284 124 36.1 100 22.7 46.9 17.9 589 25.7 253 35.0 274 29.5 65.90 8.08 35.6 7.80 2.39 6.92 0.94 5.47 0.97 2.56 0.33 2.07 0.28 5.81 2.04 2.00 2.86 0.84

46.08 2.84 13.37 13.08 0.17 9.61 10.64 2.98 0.89 0.39 − 0.68 0.07 99.45 62.3 427 292 120 67.0 190 28.2 55.9 17.5 531 23.6 213 31.0 236 24.6 54.8 6.84 29.7 6.64 2.13 6.24 0.88 4.99 0.91 2.30 0.30 1.87 0.26 4.91 1.76 1.73 2.44 0.61

46.50 3.53 15.40 13.57 0.18 6.13 9.61 3.36 1.15 0.51 − 0.70 0.06 99.30 50.4 66.4 294 100 56.1 63.1 22.7 54.5 20.1 530 25.6 270 39.6 303 30.5 70.3 8.07 36 7.23 2.3 6.99 0.93 5.5 0.99 2.53 0.34 2.04 0.28 5.5 2.43 4.28 2.8 0.83

45.99 3.84 16.01 14.13 0.19 5.17 9.16 3.63 1.34 0.58 − 0.80 0.18 99.44 45.1 14.2 355 149 2.63 31.1 17.8 43.2 25.8 670 29.5 286 44.8 359 35.1 77.9 9.44 41.2 8.92 2.72 7.91 1.08 6.24 1.12 2.92 0.37 2.35 0.32 6.41 2.58 2.36 3.48 1.02

46.41 3.77 16.54 14.23 0.19 5.28 8.90 3.89 1.39 0.59 − 0.88 0.13 100.30 45.5 15.9 348 150 2.89 28.6 17.6 42.9 27.2 683 28.9 286 47.3 390 36.5 79.9 9.52 41.2 8.68 2.71 7.76 1.07 6.12 1.10 2.87 0.36 2.31 0.32 6.36 2.67 2.39 3.73 1.06

46.07 3.64 15.97 13.62 0.06 5.72 9.71 3.54 1.27 0.56 − 0.85 0.10 99.41 48.6 28.9 306 142

47.75 3.39 14.00 13.27 0.18 5.39 9.64 3.47 1.22 0.51 − 0.79 0.12 98.02 47.7 27.9 294 140

47.68 3.66 14.55 13.71 0.17 5.77 9.72 3.54 0.88 0.46 − 0.67 0.10 99.56 48.6 30.8 295 138

47.51 3.80 15.20 13.99 0.19 5.08 9.06 3.82 1.36 0.59 − 0.86 0.02 99.75 44.9 10.0 309 148

33.5 20.4 40.3 23.1 656 27.0 252 40.6 332 31.8 70.6 8.65 37.6 8.19 2.64 7.59 1.05 5.97 1.06 2.74 0.35 2.18 0.30 6.26 2.40 2.26 2.96 0.87

46.5 20.5 38.7 22.9 581 28.2 247 39.9 287 31.8 70.4 8.62 37.3 8.05 2.62 7.58 1.07 6.17 1.09 2.84 0.37 2.26 0.32 6.18 2.34 2.20 3.00 0.88

36.4 22.8 40.1 16.7 488 27.0 204 32.2 237 24.8 56.8 7.15 32.3 7.53 2.48 7.38 1.03 6.01 1.05 2.73 0.35 2.13 0.30 5.52 1.91 1.77 2.30 0.70

20.0 19.0 37.7 25.2 592 30.6 282 45.2 340 36.0 80.2 9.77 41.9 8.98 2.84 8.30 1.16 6.58 1.19 3.08 0.40 2.51 0.34 6.93 2.66 2.44 3.37 0.99

69.37 0.15 16.05 1.65 0.01 0.05 0.65 6.42 3.97 0.01 0.53 0.27 99.12 6.1 0.64 0.79 40.5 5.11 0.64 1.64 2.51 71.7 110 33.9 85.6 105 182 94.0 181.7 18.2 64.0 11.1 1.57 8.93 1.33 7.52 1.29 3.23 0.42 2.42 0.29 2.91 6.13 4.28 9.46 1.97

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

Alkali basalt

19

20

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

Table 5 Selected major and trace element analyses of Prince Edward Island lavas. Mg# = atomic Mg/Mg + Fe2 + with Fe2O3/FeO= 0.15. A = aphyric; MP = moderately porphyritic; P = porphyritic. Reported compositions of samples LC42 and LC46 differ slightly from those reported in Verwoerd (1987) due to re-averaging and re-analysis. LC-37

LC-39

LC-46

Pleistocene

LC-43

LC-44

LC-42

Holocene

Rock type

Alkali basalt

Alkali basalt

Trachyte

Alkali basalt

Alkali basalt

Hawaiite

Texture

A

A

A

P

MP

A

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI H2OTotal Mg# Cr V Zn Cu Ni Sc Co Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

46.29 3.87 15.53 12.64 0.16 5.81 9.77 3.71 0.87 0.63 − 0.76 0.16 98.72 50.8 51.6 271 106 42.4 49.5 21 43.7 17.5 595 30.6 327 42.4 278 32.1 74.7 8.82 40.9 8.8 2.77 8.54 1.14 6.73 1.18 2.93 0.38 2.28 0.31 6.52 2.46 1.97 2.64 0.53

47.20 3.97 15.26 13.06 0.16 5.53 9.76 3.86 0.78 0.60 − 0.76 0.14 99.57 48.8 56.6 329 150

61.98 0.49 17.59 6.14 0.16 0.82 1.95 6.65 3.62 0.15 − 0.13 0.09 99.51 23.2 21.6 b 4.7 156 2.87 5.75 2.86 17.2 60.4 429 40.4 966 96.6 755 67.9 149 16.3 66 11.8 3.31 9.5 1.36 8.4 1.52 4.14 0.59 3.95 0.55 19.3 5.51 5.09 7.79 2.41

46.03 3.55 13.02 13.42 0.16 10.37 9.31 3.42 0.63 0.55 − 0.55 0.11 100.08 63.5 268 239 93 39.6 177 22 72.7 14 508 26.2 289 29.7 190 24.5 61.3 7.83 36.5 7.73 2.47 7.46 1.01 5.87 1.02 2.52 0.32 1.9 0.25 5.77 1.89 1.69 1.92 0.64

45.66 4.24 15.40 13.12 0.15 6.50 10.35 3.36 0.60 0.50 − 0.25 0.25 99.92 52.7 134 315 144 4.79 69.6 22.3 64.8 15.5 752 28.8 311 32.4 203 25.7 62.7 8.28 37.6 8.66 2.81 8.22 1.12 6.37 1.10 2.82 0.35 2.15 0.28 6.74 2.07 1.56 2.12 0.63

47.26 3.72 15.67 13.11 0.16 5.48 8.71 4.22 1.02 0.71 − 0.48 0.18 99.79 48.5 34.2 232 104 33.1 38.3 18.1 50.8 20.5 675 32.7 364 42.5 283 34.7 81.5 10 46.4 8.95 3.13 9.57 1.26 7.28 1.25 3.07 0.4 2.29 0.31 7.23 2.55 2.06 2.84 0.87

52.0 20.8 47.3 17.8 685 34.0 313 34.9 234 28.9 68.4 8.84 40.6 9.67 3.14 9.43 1.29 7.39 1.30 3.34 0.42 2.57 0.35 7.07 2.09 1.62 2.39 0.69

phase, and a further ~ 26% fractional crystallisation dominated by clinopyroxene, plagioclase and FeTi-oxides drives the residual composition to benmoreite. On Prince Edward Island, the rare trachytes represent the residual ~ 28% magma after extensive crystallisation of clinopyroxene, plagioclase, FeTi-oxides and apatite from a hawaiite magma in the approximate proportions 39:41:19:1. Trace element variations amongst the lavas are consistent with the major element models presented in Table 8 and provide independent support for the role of crystal fractionation as the primary cause of the compositional variation of the lavas found on Marion and Prince Edward Islands (Table 9). The trace element modelling shown in Table 9 was undertaken assuming fractional crystallisation (consistent with the zoned nature of phenocryst phases), using partition coefficients from the literature (Nagasawa, 1970; Fujimaki, 1986; Mahood and Stimac, 1990; Späth et al., 2000).

The genetic relationship (if any) of the single rhyolitic pumice LC149 to the rest of the Marion Island suite is difficult to determine. It has been argued above that in terms of trace element composition, it is unlike the silicic products found on either Bouvet Island or the general area of the South Sandwich Islands. It may thus well represent fragments of an earlier, unexposed pumice eruption on or near Marion Island. Major element least squares modelling produces an excellent fit relating it to the benmoreite sample LC-23 via some 70% fractionation of dominantly alkali feldspar (anorthoclase), clinopyroxene, FeTioxides and apatite. To account for the very low Zr, zircon would also be required as a fractionating phase. However, the majority of trace element differences are not readily reconciled with the major element model, and in particular the most highly incompatible elements (Rb, Th, etc.) are significantly overpredicted (by a factor of 2 to 4), even with apatite and zircon contributing to the fractionating assemblage. The origin of the xenoliths of rhyolite pumice remains thus ambiguous. 7.2. Partial melting The majority of lavas found on Marion and Prince Edward Island are either evolved in composition relative to primary magma compositions, or are enriched in MgO, with high Mg#, as a consequence of crystal accumulation processes. Very few of the MgO-rich lavas can be considered to be liquid compositions not partially enriched in cumulate olivine and/or clinopyroxene. To allow evaluation of partial melting processes and source region characteristics, it is necessary to consider only compositions that could represent liquid compositions. The most MgO-rich ‘parental’ magma compositions identified which, based on petrography, could represent ‘liquids’ are LC-25, LC-62 and LC-111 (Table 4). These lavas have between 8.9 and 9.6 wt.% MgO, Mg# = 62–63, and are aphyric to very mildly porphyritic. The low Mg# suggests that they too are not primary magmas, but all three appear to be saturated only in olivine. To determine a possible primary magma composition for the Marion Island lavas, we have used the Herzberg and Asimow (2008) approach and their Excel programme PRIMELT2. This calculation requires that the magma composition used is saturated only in olivine, and adds back equilibrium olivine until a magma composition is obtained that is in chemical equilibrium with mantle peridotite (or pyroxenite). If the average of the above three compositions (LC-25, LC-62, LC-111) is used as the best estimate of the parental Marion Island magma, the Herzberg and Asimow (2008) model suggests that ~18% olivine fractionation occurred prior to eruption of these lavas, and the primary magma, whether formed by equilibrium (batch) partial melting or by accumulated fractional melting, had some 15.7 wt.% MgO, with Mg#= 72.7. Pyroxenite does not appear to be a viable source material, as the CaO content of the estimated primary magma is too low (Herzberg and Asimow, 2008). The calculated primary Marion Island magma, using this approach, is given in Table 10. It is clear from Fig. 9 that this best estimate of a primary magma has a composition that falls just outside the partial melting grid of Herzberg and Asimow (2008) in terms of FeO*–MgO variations, but within the grid for SiO2 versus MgO. Possible explanations for this discrepancy in FeO*–MgO is the uncertainty in knowing the correct Fe2O3/FeO ratio of the melt, and/or underestimation of the amount of olivine fractionation that has occurred. Accepting this best estimate at face value, the calculated liquidus temperature would have been ~1360 °C (following Herzberg and Asimow, 2008), with pressure of melting close to 3.5 GPa — i.e. at some 105 km depth and below the likely depth of oceanic lithosphere in this region (80 Ma crust, Martin and Hartnady, 1986). 8. Source region characteristics Incompatible trace element and isotope ratios show little difference between the different stratigraphic successions erupted, respectively, on Marion and Prince Edward Islands, but show subtle differences

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

21

5

Marion Is. Prince Edward Is.

4

TiO2 (wt%)

SiO2 (wt%)

70

60

3

2

50 1

40

0

5

10

15

0

20

0

5

MgO (wt%)

15

20

15

20

15

20

20

20

18

15

Fe 2O3 (wt%)

Al2O3 (wt%)

10

MgO (wt%)

16

14

10

5 12

10

0

5

10

15

0

20

0

5

MgO (wt%)

10

MgO (wt%)

15

10

K2O (wt%)

CaO (wt%)

4

3

2

5 1

0

0

5

10

15

20

MgO (wt%)

0

0

5

10

MgO (wt%)

Fig. 4. Selected major element variation diagrams showing compositional variation in Marion and Prince Edward Island lavas. Note concordant position of rhyolite pumice LC-149.

between the two islands (Tables 6 and 7; Fig. 5), implying slight compositional differences in their respective source regions. Assuming no significant fractionation during melting, the mantle source giving rise to Marion Island has slightly lower Zr/Nb (6.7 ± 0.3), and Ce/Pb (~31) and higher Ba/Nb (~7.8 ± 0.1) compared to Prince Edward Island lavas (Zr/Nb = 9.1 ± 0.3; Ce/Pb= ~37.5; Ba/Nb = 6.7 ± 0.1). In general, for a given Zr content, Prince Edward Island lavas are less enriched in Nb, Th, Rb, Ba and Ce than Marion Island lavas (Fig. 5). These differences are reflected in the slightly lower 87Sr/86Sr (0.70302) and higher 143Nd/ 144 Nd (0.51300) isotope ratios shown by Prince Edward Island lavas compared to Marion Island lavas (~0.70341, ~0.51294; Table 7). The spatially associated Funk Seamount lavas are not dissimilar in terms of their incompatible trace element ratios (Table 6) and have isotope ratios similar to Prince Edward Island lavas (Fig. 7). Taken together, the incompatible element ratios of Marion and Prince Edward Island lavas, and by implication their mantle source regions, are unremarkable

compared to typical OIB the world over, and with few exceptions are not significantly different to other Southern Ocean OIB (Table 6). In this latter regard, Marion and Prince Edward lavas show stronger LREE enrichment than Bouvet or Kerguelen, but are not as LREE enriched as Gough or St Helena (Table 6). They have Ba/Nb ratios similar to most OIB the world over, but are lower than Gough or Kerguelen and E-MORB erupted at 40°E on the SWIR (Table 6). A noteworthy feature of the Marion and Prince Edward Island alkali basalts is that they all show a slight but variable depletion in K (and Th) relative to Nb and La, elements of similar incompatibility, i.e. they have K/Nb and Th/Nb ratios less than primitive mantle values. Where amphibole is not a fractionating phase (as is the case for alkali basalts on Marion and Prince Edward), relative depletion in K in alkaline basalts is commonly attributed to melting in the presence of residual amphibole (e.g. Class and Goldstein, 1997; Späth et al., 2000, 2001). This explanation is not supported in the case of alkali basalts from Marion

22

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

10 100

LC-149

LC-149

8

60 40 Marion Is. Prince Edward Is.

20 0

0

200

400

600

800

Gou gh I s.

Th (ppm)

Nb (ppm)

80 6

4

2

0

1000

0

200

Zr (ppm)

400

600

800

1000

800

1000

Zr (ppm) 800

80 LC-149

600

Ba (ppm)

Rb (ppm)

60

40

400

LC-149

200

20

0

0

200

400

600

800

0

1000

0

200

Zr (ppm)

400

600

Zr (ppm) 20

200 LC-149

.2

15

150

= Hf

46

Hf (ppm)

Ce (ppm)

/

Zr

100

5

50

0

10

0

200

400

600

800

1000

Zr (ppm)

0

LC-149

0

200

400

600

800

1000

Zr (ppm)

Fig. 5. Selected trace element variation diagrams showing compositional variation in Marion and Prince Edward Island lavas. Field of Gough Island lavas (le Roex, 1985) shown for comparison. Note discordant position of rhyolite pumice LC-149.

and Prince Edward Islands, since they show a similar depletion in Th (i.e. Th/Nbn b1), an element not fractionated by amphibole (LaTourrette et al., 1995 — check..). The slight depletion in K (and Th, Ba and Rb) relative to Nb and La is consequently attributed to being a compositional feature of the mantle source, rather than a fractionation effect resulting from residual mineralogy. The calculated trace element pattern of the source region (assuming 5% partial melting of a garnet lherzolite source) is illustrated in Fig. 10, where the slight and unusual relative depletion in elements more incompatible than Nb, and particularly Th and K, is evident. The calculated source, assuming 5% melting, required to give rise to the Marion and Prince Edward Island magmas is slightly enriched in incompatible elements by a factor of 1–2 over primitive mantle. For a garnet lherzolite source, HREE values are less than primitive mantle (×0.5), and decrease to a factor of 0.3 if a spinel lherzolite source is assumed.

Marion and Prince Edward Islands have been postulated to represent the present-day surface expression of a deep seated upwelling mantle plume (e.g. Duncan, 1981). The paleo-track of the African plate over this mantle plume is represented by the southward extending Madagascar Ridge, prior to northward drift of the Southwest Indian Ridge that placed the Marion plume beneath the Antarctic plate at 80– 60 Ma (Morgan, 1981). Relatively few samples are available from the Madagascar Ridge (Erlank and Reid, 1974), but it is noteworthy that the lavas erupted on the SWIR at 40°E (the approximate location of where the Marion and Prince Edward plume track crosses the SWIR) bear no compositional similarity to those erupted on Marion or Prince Edward Islands (le Roex et al., 1989; Mahoney et al., 1992). In contrast, E-MORB erupted along the SWIR to the west of the present day location of Marion and Prince Edward Islands show clear compositional mixing relationships between N-MORB and lavas from Marion and Prince

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

23

Table 6 Selected incompatible element ratios in Marion and Prince Edward Island lavas, and from some South Atlantic islands, and E-MORB from the adjacent Southwest Indian Ridge. EMORB is from the Southwest Indian Ridge in vicinity of Marion and Prince Edward Islands, whereas E-MORB 40°E is from the SWIR segment at 40°E. Data sources: le Roex (1985), le Roex and Erlank (1982), Reid and le Roex (1988), le Roex et al. (1989). http://www.georoc.mpch-mainz.gwdg.de/georoc/. Marion

Prince Edward

Pleistocene Eastern Shield Zr/Nb Ba/Nb Y/Nb La/Smn La/Ybn Ce/Pb Nb/U Zr/Hf Nb/Ta La/Nbn

7.08 ± 0.47 7.84 ± 0.37 0.71 ± 0.10 2.58 ± 0.32 10.4 ± 1.8 29.8 ± 4.4 49.5 ± 4.4 47.8 ± 3.1 16.8 ± 0.74 0.82 ± 0.03 Funk Seamount

Zr/Nb Ba/Nb Y/Nb La/Smn La/Ybn Ce/Pb Nb/U Zr/Hf Nb/Ta La/Nbn

6.38 6.53 0.62 2.16 16.1 41.4 56.8 44.4 18.5 0.87

Holocene Western Shield 6.80 ± 0.17 7.86 ± 0.27 0.69 ± 0.04 2.57 ± 0.16 10.3 ± 0.64 32.1 ± 2.9 47.6 ± 3.3 45.8 ± 2.6 16.6 ± 0.9 0.82 ± 0.02 Kerguelen 8.51 11.0 1.32 2.38 8.35 26.1 56.8 42.8 15.7 0.85

Old 6.71 ± 0.34 7.83 ± 0.41 0.72 ± 0.09 2.52 ± 0.27 10.2 ± 1.5 31.3 ± 3.3 48.5 ± 3.6 45.1 ± 3.9 17.2 ± 0.7 0.82 ± 0.01 Bouvet 6.58 5.51 0.89 2.32 6.75 26.0 46.1 44.0 17.0 0.69

Edward Islands; a feature that has been attributed to source mixing between the upwelling Marion and Prince Edward plume and the surrounding ambient, geochemically depleted, asthenospheric mantle (le Roex et al., 1989; Mahoney et al., 1992). The new compositional data for Marion and Prince Edward Islands confirm this relationship, and

Intermediate 6.63 ± 0.42 7.88 ± 0.24 0.69 ± 0.06 2.55 ± 0.14 10.4 ± 0.65 29.9 ± 6.6 45.2 ± 3.7 44.6 ± 2.4 17.3 ± 0.5 0.83 ± 0.03 Gough 6.33 15.00 0.56 3.43 17.1 24.3 49.9 47.3 17.8 1.1

Pleistocene

Holocene

8.90 ± 1.15 7.03 ± 0.69 0.70 ± 0.28 2.67 ± 0.93 10.2 ± 2.1 36.4 ± 6.6 45.2 ± 7.2 48.2 ± 3.4 17.2 ± 0.43 0.79 ± 0.07

9.29 ± 0.64 6.44 ± 0.20 0.85 ± 0.07 2.16 ± 0.31 9.57 ± 1.17 38.6 ± 2.1 48.9 ± 2.5 48.8 ± 2.4 16.0 ± 0.6 0.84 ± 0.02

Young 6.24 ± 0.07 7.57 ± 0.43 0.72 ± 0.08 2.44 ± 0.21 9.79 ± 1.0 32.1 ± 0.6 46.0 ± 0.6 39.5 ± 1.7 17.0 ± 0.09 0.82 ± 0.01 St. Helena 4.51 6.29 0.48 3.46 15.9 24.7 48.5 49.8 18.8 0.77

E-MORB SWIR

E-MORB 40°E

9.33 6.13 2.89 2.98 2.44 24.1 43.9 43.0 – 0.98

11.0 14.7 2.95 1.39 1.65 17.6 39.6 42.6 – 1.13

are taken to indicate that the incompatible trace element abundances, and Sr, Nd and Pb isotope ratios in the mafic lavas are reflective of the composition of the upwelling Marion and Prince Edward mantle plume. It is noteworthy that the new Pb-isotope data, plotting as they do close to or on the NHRL, confirm the absence of any DUPAL signature (e.g. Hart, 1984) in the Marion and Prince Edward plume, i.e. the plume lies to the south of the regional DUPAL anomaly as identified by Hart (1984). 9. Conclusions

1000

Sample/Chondrite

Prince Edward Is.

100

10

1

alkali basalt hawaiite trachyte

La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

1000

Sample/Chondrite

Marion Is.

100

10 alkali basalt hawaiite benmoreite rhyolite

1

La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 6. Chondrite normalised rare earth element diagrams showing REE variation in Marion and Prince Edward Island lavas. Normalising values from Sun et al. (Sun and McDonough, 1989).

Compositional variation amongst the lavas of Marion and Prince Edward Islands is relatively simple and coherent, with lava types ranging from alkali basalt through hawaiite to mugearite and benmoreite (and possibly rhyolite) on Marion Island, and on Prince Edward Island through to trachyte. Alkali basalt is by far the dominant rock type exposed. The most MgO-rich lavas are partially cumulate enriched in olivine and clinopyroxene and quantitative least squares modelling suggest that the full range in rock types found on both islands can be accounted for by fractional crystallisation of observed phenocryst phases. Hawaiites are the product of approximately 24% crystallisation of olivine, clinopyroxene and plagioclase from a parental alkali basalt magma, whereas rare mugearites require a further ~36% fractional crystallisation of hawaiite. Fractionating phases include olivine, clinopyroxene, plagioclase and FeTi-oxides. The highly evolved benmoreite magma represents a further ~26% fractionation of the same phases without olivine (i.e. represents the final 37% liquid after extensive fractionation in sub-crustal magma chambers). On Prince Edward Island, the trachytes have formed by some 78% fractional crystallisation of a parental hawaiite magma, with fractionating phases being clinopyroxene, plagioclase, FeTi-oxides and apatite. Most primitive lavas sampled on Marion and Prince Edward (Mg# = 0.63) are probably not primary magmas, but may have experienced as much as 18% olivine fractionation prior to eruption. The composition of the primary magmas giving rise to the Marion and Prince Edward Islands (SiO2 = 45 wt.%; MgO = 15.7 wt.%) is

24

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

Table 7 Sr, Nd and Pb isotopes of selected Marion and Prince Edward Island samples. Errors reported are 2 sigma internal. 143

Marion Island Pleistocene: Eastern Shield LC-25 LC-164 Pleistocene: Western Shield LC-51 LC-96 Holocene: Old LC-61 LC-87 Holocene: Intermediate LC-62 LC-111 Holocene: Young LC-82 LC-201 Prince Edward Island Pleistocene LC-37 Holocene LC-44

A

0.5132

Nd/144Nd

± 2s

87

Sr/86Sr

±2s

206

0.512945 0.512944

6 6

0.703431 0.703358

13 14

0.512936 0.512939

6 6

0.703386 0.703432

0.512926 0.512936

4 5

0.512926 0.512944

± 2s

207

18.7598 18.7112

8 7

15.5573 15.5527

8 7

38.5916 38.5460

21 19

15 10

18.7137 18.7167

9 8

15.5582 15.5565

9 12

38.5733 38.5656

26 40

0.703421 0.703402

14 12

18.6946 18.7422

6 7

15.5547 15.5584

6 7

38.5715 38.6087

20 20

7 6

0.703457 0.703378

12 11

18.6882 18.7465

8 8

15.5555 15.5591

9 8

38.5671 38.5991

26 25

0.512922 0.512948

4 7

0.703492 0.703358

12 12

18.6599 18.6103

7 8

15.5582 15.5567

6 8

38.5538 38.4681

16 25

0.513019

5

0.703004

12

18.7130

5

15.5371

5

38.4237

16

0.513002

5

0.703020

15

18.5747

4

15.5349

5

38.3095

20

SWIR MORB

Pb/204Pb

Marion Prince Edward Funk Seamount

Ascension

0.5130

Pb/204Pb

208

± 2s

Pb/204Pb

±2s

cpx

20

Publ. data

plag

St Helena Bouvet

SWIR 40oE

15

CaO (wt%)

Gough

EM-II

0.5124 EM-I

0.5122 0.702

0.703

0.704

B

10

5

0.705 87

accum.

oliv + cpx

.

Tristan da Cunha

Inaccessible

um

0.5126

cc

ga

143

0.5128

pla

Nd/ 144Nd

Crozet

HIMU

0.706

0.707

0.708 oliv

Sr/ 86Sr

0

0

10

20

41

30

40

MgO (wt%) RL

NH

Gough

St Helena Vema

39

Al2O3 (wt%)

SWIR

37

m. ccu

Bouvet & Ascension

S. Atlantic MORB

38

30 ga

Pb/ 204Pb

Crozet

pla

208

Porphyritic Moderately porphyritic Aphyric

plag

40

20

oli

v+

36

17

18

19 206

20

cp xa

cc

10

um

.

21

Pb/ 204Pb

cpx oliv

Fig. 7. (A) Sr–Nd isotope compositions of Marion and Prince Edward Island lavas. Shown for comparison are compositional fields for Bouvet, St Helena, Gough, Tristan da Cunha, Crozet and Ascension islands and Southwest Indian Ridge. (B) 208Pb/204Pb versus 206Pb/204Pb in Marion and Prince Edward Island lavas. NHRL = Northern Hemisphere Reference Line. Compositional fields for Bouvet, St Helena, Gough, Crozet, Tristan da Cunha and Ascension islands are shown for comparison. Source of data: www.Detdb.org; www.georoc.mpch-mainz.gwdg.de/georoc/.

0

0

10

20

30

40

MgO (wt%) Fig. 8. CaO and Al2O3 versus MgO in Marion Island lavas, together with compositions of clinopyroxene, plagioclase and olivine. Compositional trajectories for olivine plus clinopyroxene and plagioclase accumulation are shown.

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

25

Table 8 Least squares approximation models depicting crystal fractionation or crystal accumulation in Marion and Prince Edward Island lavas. SiO2

TiO2

Al2O3

FeO

MnO

MgO

CaO

Na2O

K2O

Crystal fractionation models: Marion Island Coarsely porphyritic alkali basalt (accumulation model: LC-199 (porphyritic alkali basalt) = LC-61 (alkali basalt) + 0.1291 Oliv + 0.1473 Cpx + 0.0522 Plag + 0.0059 FeTi-oxide) LC-61 parent 46.50 3.20 15.36 12.91 0.18 7.18 10.12 3.39 1.16 LC-199 daughter 46.30 2.37 12.41 12.64 0.17 12.27 10.52 2.69 0.69 LC-199 calculated 46.23 2.44 12.40 12.56 0.16 12.25 10.49 2.60 0.78 Residuals 0.07 − 0.07 0.02 0.08 0.01 0.01 0.03 0.09 − 0.09 Sum of squares of diff. 0.034; F = 0.6656 Hawaiite (fractionation model: LC-27 (hawaiite) = LC-25 (alkali basalt) − 0.0778 Oliv − 0.0693 Cpx − 0.0742 Plag − 0.0156 FeTi-oxide) LC-27 daughter 47.40 3.11 15.71 12.16 0.16 6.12 10.15 3.74 0.73 LC-25 parent 46.25 2.90 14.55 12.18 0.15 8.92 10.34 3.35 0.49 LC-25 calculated 46.30 2.90 14.61 12.25 0.15 8.96 10.39 3.29 0.57 Residuals − 0.06 0.01 − 0.06 − 0.08 0.00 − 0.03 − 0.04 0.06 − 0.08 Sum of squares of diff. 0.026; F = 0.7631 Mugearite (fractionation model: LC-58 (mugearite) = LC117 (hawaiite) − 0.0192 Oliv − 0.1104 Cpx − 0.1393 Plag − 0.0923 FeTi-oxide) LC-58 daughter 50.76 2.47 17.22 11.69 0.18 4.18 6.76 5.05 1.60 LC-117 parent 45.64 4.24 16.08 14.59 0.19 5.33 8.93 4.18 1.31 LC-117 calculated 45.65 4.25 16.06 14.56 0.18 5.29 8.87 4.07 1.06 Residuals − 0.01 − 0.01 0.02 0.02 0.02 0.04 0.06 0.11 0.25 Sum of squares of diff. 0.080; F = 0.6461 Benmoreite (fractionation model: LC-23 (benmoreite) = LC5 (mugearite) − 0.0054 Oliv − 0.1128 Cpx − 0.0665 Plag − 0.0648 FeTi-oxide) LC-23 daughter 55.59 1.49 17.02 9.47 0.21 2.48 5.09 5.78 2.18 LC-5 parent 50.42 2.91 15.47 12.13 0.20 4.06 7.38 4.76 1.38 LC-5 calculated 50.41 2.90 15.49 12.13 0.20 4.07 7.39 4.68 1.63 Residuals 0.01 0.02 − 0.02 0.00 0.00 − 0.01 − 0.01 0.07 − 0.25 Sum of squares of diff. 0.071; F = 0.7459 Crystal fractionation models: Prince Edward Island Trachyte (fractionation model: LC-46 (trachyte) = LC-37 (hawaiite) − 0.0176 Oliv − 0.2795 Cpx − 0.2865 Plag − 0.1333 FeTi-Oxide − 0.0100 apatite) LC-46 daughter 61.98 0.49 17.59 6.14 0.16 0.82 1.95 6.65 LC-37 parent 46.29 3.87 15.53 12.64 0.16 5.81 9.77 3.71 LC-37 calculated 46.26 3.88 15.51 12.60 0.17 5.81 9.76 3.75 Residuals 0.03 − 0.01 0.02 0.04 − 0.01 0.00 0.01 − 0.03 Sum of squares of diff. 0.075; F = 0.2831

consistent with having formed at pressures of 3.5 GPa (~ 110 km) and a temperature of 1360 °C, through melting of garnet lherzolite. Sr and Nd isotope ratios, coupled with incompatible trace element ratios, indicate derivation from a time-average depleted mantle, which has experienced one or more episodes of source enrichment. The enriched source is attributed to an upwelling mantle plume located beneath this region, which, in terms of isotope ratios ( 87Sr/ 86Sr =

3.62 0.87 1.14 − 0.27

0.7030–0.7034; 143Nd/144Nd = 0.51293–0.5130) and incompatible trace element ratios (Ba/Nb= 8; Zr/Nb = 8; La/Smn = 2.5), is rather ‘normal’ with respect to most OIB and inferred plume compositions, and lacks any DUPAL signature (Δ8/4Pb ~ 33), unlike lavas characteristic of the ridge axis further to the north. Supplementary materials related to this article can be found online at doi:10.1016/j.jvolgeores.2012.01.009.

Table 9 Trace element fractionation models using major element least squares results given in Table 8, and partition coefficients from Fujimaki (1986), Mahood and Stimac (1990), Nagasawa (1970) and Späth et al. (2000). Sc

Ni

Rb

Ba

Sr

Zr

Th

Nb

La

Ce

Nd

Sm

Crystal fractionation models: Marion Island Coarsely porphyritic alkali basalt (accumulation model: LC-199 (porphyritic alkali basalt) = LC-61 (alkali basalt) + 0.1291 Oliv + 0.1473 Cpx + 0.0522 Plag + 0.0059 FeTi-oxide) LC-61 parent 24 91 19 311 539 268 2.9 40 31 69 35 7.2 LC-199 daughter 27 262 12 196 400 169 1.7 23 18 42 23 5.0 LC-199 calculated 22 269 13 208 442 181 1.9 27 20 46 24 4.8 Hawaiite (fractionation model: LC-27 (hawaiite) = LC-25 (alkali basalt) − 0.0778 Oliv − 0..0693 Cpx − 0.0742 Plag − 0.0156 FeTi- oxide) LC-25 parent 22 140 15 238 462 226 2.1 30 23 54 29 6.1 LC-27 daughter 21 58 22 285 623 261 2.9 38 30 68 36 7.8 LC-27 calculated 20 53 19 307 545 287 2.8 38 31 70 38 7.9 Mugearite (fractionation model: LC-58 (mugearite) = LC117 (hawaiite) − 0.0192 Oliv − 0.1104 Cpx − 0.1393 Plag − 0.0923 FeTi-oxide) LC-117 parent 20 26 18 321 623 266 3 40 30 68 36 8 LC-58 daughter 10 10 31 489 682 388 4 58 48 106 52 10 LC-58 calculated 11 3 28 403 681 397 4 62 45 104 54 11 Benmoreite (fractionation model: LC-23 (benmoreite) = LC5 (mugearite) − 0.0054 Oliv − 0.1128 Cpx − 0.0665 Plag − 0.0648 FeTi-oxide) LC-5 parent 11 – 32 432 716 393 4.3 58 47 104 55 12 LC-23 daughter 10 – 41 629 592 521 5.6 76 59 132 67 13 LC-23 calculated 8 – 43 571 598 506 5.8 69 62 138 72 15 Crystal fractionation models: Prince Edward Island Trachyte (fractionation model: LC-46 (trachyte) = LC-37 (hawaiite) − 0.0176 Oliv − 0.2795 Cpx − 0.2865 LC-37 parent 21 50 18 278 595 327 LC-46 daughter 3 6 60 755 429 966 LC-46 calculated 6 4 61 747 451 943

Plag − 0.1333 FeTi-Oxide − 0.01 Ap) 3 42 32 8 97 68 9 93 68

75 149 150

41 66 67

9 16 17

26

A.P. le Roex et al. / Journal of Volcanology and Geothermal Research 223-224 (2012) 11–28

Sample/Primitive mantle

Table 10 Calculated primary melt composition of the Marion Island lavas, using PRIMELT2 of Herzberg and Asimow (2008). Calculated primary melt SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total

45.89 2.37 11.82 1.18 10.46 0.17 15.71 8.81 2.58 0.62 0.35 100.00

7GPa 6

oliv fractionation path

F=0.2

F=0.1

5

FeO (wt%)

F=0.3

7 Garnet Peridotite Solidus

F=0.4

4

4 F=0.5 L+Ol

2

10 1

L+Ol+Opx

3

8 2 Spinel Peridotite Solidus

5

10

15

20

25

30

MgO (wt%) 1

SiO2 (wt%)

55 Spinel Peridotite Solidus

50

0.2

L+Ol+Opx

0.1

1 2

2

L+Ol F=0.4

4

45

3 4 Garnet Peridotite 5 Solidus

0

5

F=0.05

1

Gt lherzolite source

Rb Ba Th K Ta Nb La Ce Pb Pr Sr Nd Zr SmEu Ti Gd Tb Dy Ho Er Tm Yb Lu

Fig. 10. Primitive mantle normalised composition of an estimated Marion Island primary magma, together with calculated source composition assuming 5% equilibrium melting. Melting parameters: garnet lherzolite source: ol:opx:cpx:gt = 0.57:0.23:0.15:0.05 melting in proportions 0.05:0.05:0.50:0.40, spinel lherzolite source: ol:opx:cpx:sp = 0.57:0.23:0.15:0.05 melting in proportions 0.22:0.22:0.54:0.04. Partition coefficients from Späth et al. (2001).

14

0

10

Sp lherzolite source

This research was funded through the South African National Antarctic Programme (LC and WJV) and the South African National Research Foundation (AlR). Petrus le Roux is thanked for his assistance in acquiring the isotope data. Constructive comments from Bill White and Alison Koleszar are greatly appreciated.

6

Marion primary magma

0.1

Acknowledgements

12

100

10

15

20

F=0.3 F=0.2

6

7GPa

25

30

MgO (wt%) Fig. 9. Partial melting grid following approach by Herzberg and Asimow (2008) calculated using their PRIMELT2 spreadsheet. Composition of aphyric, moderately phyric and porphyritic Marion Island lavas is shown, together with a calculated olivine fractionation path extending from the assumed parental magma composition (average of LC-25, -62, -111).

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