Journal of Petrology Volume 42 Number 2 Pages 277-319 2001
© Oxford University Press 2001
Petrogenesis of Olivine-phyric Basalts from the Aphanasey Nikitin Rise: Evidence for Contamination by Cratonic Lower Continental Crust
1VERNADSKY INSTITUTE OF GEOCHEMISTRY AND ANALYTICAL CHEMISTRY, KOSYGIN ST. 19, 117975, MOSCOW, RUSSIA
2DÉPARTEMENT DES SCIENCES DE LA TERRE ET DE L’ENVIRONNEMENT, CP 160/02, UNIVERSITÉ LIBRE DE BRUXELLES, AVENUE F.D.ROOSEVELT, 50, B-1050 BRUSSELS, BELGIUM
3UNIVERSITÄT GÖTTINGEN, GOLDSCHMIDTSTRASSE 1, 37077 GÖTTINGEN, GERMANY
4INSTITUTE OF PRECAMBRIAN GEOLOGY AND GEOCHRONOLOGY, MAKAROV EMB. 2, 119034, ST PETERSBURG, RUSSIA
Received October 19, 1998; Revised typescript accepted May 5, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL AND TECTONIC...ANALYTICAL TECHNIQUESRESULTSDISCUSSIONMAJOR CONCLUSIONSAPPENDIXREFERENCES |
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In this work we investigate the olivine-phyric basalt suite of the Aphanasey Nikitin Rise, an intraplate volcanic structure formed during the Late Cretaceous in the Indian Ocean. The parental melt of the basalt suite has a hypersthene-normative tholeiitic composition with low H2O content (0·3–0·5 wt %) and high SiO2/Al2O3 (3·5). The basalt suite is characterized by Nb, Ta, Th and U depletion, and uniquely low 206Pb/204Pb and 143Nd/144Nd among the Cretaceous tholeiitic basalts of the Indian Ocean. Our modelling demonstrates that fractional crystallization of depleted mantle-derived melt and lower continental crust assimilation is a suitable model for the genesis of the parental magma of this suite. The continental crustal material involved is characterized by long-term Rb, U and Th depletion and probably remained isolated for >109 years in cratonic Gondwanan lithosphere. On a broader scale, two geochemical groups can be distinguished among tholeiites formed in the Indian Ocean basin during the period 115–75 Ma, from the Aphanasey Nikitin Rise, the southern Kerguelen and Naturaliste plateaux and the Broken Ridge. Both groups have a compositional range from hypersthene-normative basalt to basaltic andesite and are characterized by Nb–Ta depletion, extremely low
Nd t (-2 to -13) and high 207Pb/204Pb (15·525–15·750). The first group with high La/Th (15–19) is characterized by low 206Pb/204Pb (16·9–17·2) and 87Sr/86Sr (up to 0·706), whereas the second group with low La/Th (5–9) has higher 206Pb/204Pb (17·7–18·1) and 87Sr/86Sr (up to 0·713). The tholeiite composition is likely to be controlled by contamination of the parental tholeiitic melts by continental crust derived from cratonic Gondwanan lithosphere. This conclusion, combined with other evidence for ancient crustal material in the Indian and Atlantic Oceans, indicates the impact of continental crust in the oceanic lithosphere. KEY WORDS: Aphanasey Nikitin Rise; tholeiites; granulite; Gondwana; Dupal isotope anomaly; Indian Ocean
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC...ANALYTICAL TECHNIQUESRESULTSDISCUSSIONMAJOR CONCLUSIONSAPPENDIXREFERENCES |
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Several tholeiitic basalt suites from large Cretaceous volcanic structures in the Indian Ocean, such as at Ocean Drilling Program (ODP) Site 738 on the Kerguelen Plateau, Dredge 8 and 10 on Broken Ridge (Sites D8, M-D8, M-D10), Deep Sea Drilling Project (DSDP) Site 264 and Dredge 55-12 on the Naturaliste Plateau and the olivine-phyric basalt suite of the Aphanasey Nikitin Rise (Fig. 1a), exhibit anomalous geochemical features (e.g. Alibert, 1991
; Mahoney et al., 1995, 1996; Sushchevskaya et al., 1996). These oceanic basalts have high 87Sr/86Sr, 207Pb/204Pb, 208Pb/204Pb, La/Nb and La/Ta, and low 143Nd/144Nd relative to the Dupal ocean-island basalts (OIB) and mid-ocean ridge basalts (MORB) (Mahoney et al., 1995). These volcanic structures were emplaced in the Indian Ocean basin during the period 115–75 Ma (e.g. Storey et al., 1989, 1992; Matveenkov et al., 1991; Salters et al., 1992; Schlich & Wise, 1992; Whitechurch et al., 1992; Mahoney et al., 1995). Detailed knowledge of their geochemical characteristics allows us to better constrain the magmatic and tectonic processes operating during formation of oceanic lithosphere.
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Storey et al. (1992)
, Mahoney et al. (1995, 1996) and Sushchevskaya et al. (1996) have concluded that continental lithosphere was involved in the origin of the anomalous tholeiites of the Kerguelen Plateau, Broken Ridge, the Naturaliste Plateau and Aphanasey Nikitin Rise. Sushchevskaya et al. (1996) proposed that Gondwanan granulites or lithospheric mantle were involved in mantle melting during formation of the Aphanasey Nikitin Rise. Mahoney et al. (1996) also concluded that the continental lithosphere was involved in the petrogenesis of the Aphanasey Nikitin Rise suites. Storey et al. (1992) and Mahoney et al. (1995) proposed the contamination of Kerguelen and Naturaliste Plateau magmas by material (e.g. continental crust) derived from the Gondwanan continental lithosphere. However, two important questions remain unresolved: (1) what part of the continental lithosphere was involved: upper or lower crust or mantle? (2) How does the continental lithosphere participate in magma genesis?
Shallow-level incorporation of continental lithosphere may occur either in the plume head or in plume-derived magmas (Storey et al., 1989, 1992; Mahoney et al., 1995). Existing geophysical and geological studies suggest the possibility of the presence of such stretched fragments of continental lithosphere in the Kerguelen Plateau or Broken Ridge (Udintsev & Koreneva, 1982; Udintsev et al., 1990; Recq et al., 1994; Operto & Charvis, 1995, 1996; Frey et al., 2000). The geochemistry of some ultramafic xenoliths from the Kerguelen Archipelago and the discovery of garnet-bearing gneisses in ODP Site 1137 also suggest the involvement of pieces of Gondwanan continental lithosphere in the Indian Ocean lithosphere (Hassler & Shimizu, 1998; Mattielli et al., 1999; Frey et al., 2000). Another proposed mechanism is the relatively deep-level incorporation of continental lithospheric material into the mantle by convective flow (e.g. McKenzie & O’Nions, 1983, 1995).
This study is a geochemical investigation of the shield-stage magmatism of Aphanasey Nikitin Rise based on new (Borisova, 1997) and previously published results (Almukhamedov et al., 1993; Mahoney et al., 1996; Sushchevskaya et al., 1996; Borisova et al., 1997). The main goal is to identify the composition, age and source of the continental lithosphere material involved in the genesis of the tholeiitic olivine-phyric suite of the Aphanasey Nikitin Rise. Another goal is to determine the mechanism of the involvement of the material in the petrogenesis of similar Cretaceous tholeiites of the eastern Indian Ocean.
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GEOLOGICAL AND TECTONIC BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC...ANALYTICAL TECHNIQUESRESULTSDISCUSSIONMAJOR CONCLUSIONSAPPENDIXREFERENCES |
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Aphanasey Nikitin Rise is situated in the central basin of the Indian Ocean 750 km to the southeast of Sri Lanka (Fig. 1a). The Russian research vessel Vityaz discovered the rise in 1959 (Bogorov & Bezrukov, 1961
). The rise is 100 km wide and 250 km long and is elongated in a north–south direction with numerous volcanic cones, including the Aphanasey Nikitin Seamount (e.g. Matveenkov et al., 1991). This seamount rises to a minimum water depth of 1549 m at 3°01'S, 83°05'E. Results of several scientific expeditions have shown that the Aphanasey Nikitin Rise is of magmatic origin (e.g. Sborshikov et al., 1991). Gravimetric and bathymetric data suggest that the Aphanasey Nikitin Rise was emplaced close to an oceanic spreading axis, perhaps at the junction of the Central Indian Ocean spreading axis and the Indira transform fault (e.g. Paul et al., 1990). The age of nannofossils in sediments overlying the volcanic basement and in carbonate material filling cavities in the basalts is Late Cretaceous, and geophysical studies constrain the age to be between 75 and 90 Ma (e.g. Paul et al., 1990; Matveenkov et al., 1991). Recent palaeotectonic data seem to relate the formation of the Aphanasey Nikitin Rise to the magmatic activity of (1) the Crozet hotspot, 
1100 km east of the Marion hotspot [reconstruction by Curray & Munasinghe (1991)], (2) a long-dead hotspot now located under the eastern part of the Conrad Rise (53°24'S, 48°24'E) east of Lena Seamount (Müller et al., 1993), or (3) the Marion hotspot (Kent et al., 1992).
Magmatic stages and suites
The R.S.S. Charles Darwin (CD28) recovered several samples of trachybasalts from the Aphanasey Nikitin Seamount (Mahoney et al., 1996). More detailed investigations of the northern part of Aphanasey Nikitin Rise were carried out during the twentieth voyage of the Russian research ship Academician Mstislav Keldysh in 1990 using the submersible Mir, which allowed in situ submarine observation and sampling. About 50 samples of lava were obtained from the northern part of the rise at depth intervals between 5050 and 1570 m (Fig. 1b). Two main stages of magmatic activity are responsible for the formation of the rise: (1) a shield stage during which the basement of the Aphanasey Nikitin Rise was formed; (2) a stratovolcanic stage, which was responsible for the formation of Aphanasey Nikitin Seamount (Kashintsev, 1993; Borisova, 1997).
Major element compositions and petrographic characteristics allowed Almukhamedov et al. (1993) to distinguish three igneous suites. The olivine-phyric basalt suite composes a volcanic cone formed during the early shield stage (Kashintsev, 1993; Borisova, 1997). Borisova et al. (1997) demonstrated that the initial composition of the parental melt for the olivine-phyric basalt suite is tholeiitic (Macdonald & Katsura, 1964), hypersthene-normative, with high MgO (10 wt %) and low H2O (<0·5 wt %), rather than subalkalic (Almukhamedov et al., 1993; Sushchevskaya et al., 1996). The major element variations of the basalt suite are mainly due to intensive secondary alteration involving loss of as much as 80 relative % of MgO (Borisova et al., 1997). Most of the sampled portion of the rise basement is composed of tholeiitic plagioclase-phyric basalt suite (Almukhamedov et al., 1993). The plagioclase-phyric basalts formed during the shield stage of the Aphanasey Nikitin Rise (Kashintsev, 1993; Borisova, 1997). The third suite of subalkalic trachybasalt–trachyte from the Aphanasey Nikitin Seamount was formed during the late stratovolcanic stage (Almukhamedov et al., 1993; Mahoney et al., 1996; Sushchevskaya et al., 1996; Borisova, 1997).
Lavas from the Aphanasey Nikitin Rise cover a wide range of isotopic compositions that include the lowest 206Pb/204Pb and 143Nd/144Nd values yet found among oceanic islands or spreading centres world-wide (Mahoney et al., 1996). Continental lithospheric material, characterized by radiogenic Sr and nonradiogenic Pb and Nd isotopic compositions and depleted in Th, U, Ta and Nb was involved to varying degrees in the melting process, as noted by Mahoney et al. (1996) and Sushchevskaya et al. (1996). However, those workers did not specify what part of the continental lithosphere was involved—upper or lower crust or mantle—or how the continental lithosphere participates in magma genesis.
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ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC...ANALYTICAL TECHNIQUESRESULTSDISCUSSIONMAJOR CONCLUSIONSAPPENDIXREFERENCES |
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To remove surface contamination before powdering, small chips were handpicked from the bulk samples to avoid vugs, veinlets and altered zones. Major and some trace element abundances in the powders were analysed by X-ray fluorescence (XRF) spectrometry, using Philips PW-1480 (Göttingen University, Germany) (Hartmann, 1994
) and PW-1600 spectrometers (GEOMAR, Germany) (Bernarz & Schmincke, 1994). Rare earth element (REE), Th, Ta, Hf and Pb contents were determined at Göttingen University using a VG Plasmaquad PQ2 inductively coupled plasma mass spectrometer in pulse-counting, peak-hopping mode [see Ionov et al. (1992) for details]. Some Ta, Th and Hf concentrations were analysed by neutron activation at the Geological Institute, Moscow, after the method of Lyapunov et al. (1980) (Tables 1, and 4a and b; see below).
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Minerals and glasses (Table 2a) were analysed at Moscow State University, using an energy-dispersive Link System linked to a Cam Scan electron microscope, operating at 15 kV accelerating voltage and 50 nA beam current, and at the Vernadsky Institute using a CAMEBAX MicroBeam electron microprobe operating at 15 kV accelerating voltage and 30 nA beam current. The spectra were accumulated during 1 min. The beam diameter was 1–2 mm for minerals and 10 mm for glasses. Reference minerals and glasses (Lavrentev et al., 1974; Jarosevich et al., 1980) were used as standards for major elements. The accuracy of major element determinations is better than ±1% of the total value. Trace elements and water contents in melt inclusions (Table 2b) were analysed using an IMS-4F ion microprobe at the Institute of Microelectronics (Yaroslavl’, Russia). All ion microprobe analyses were carried out as described by Sobolev & Shimizu (1993). H2O contents were measured following the procedures of Sobolev & Chaussidon (1996). The accuracy of H2O determination is better than ±10% of the total value. The reproducibility of the analyses was ±2–5% relative. The accuracy of the measurement estimated from reproducibility of the standards is better than 20% relative for most trace elements (e.g. Sobolev, 1996; Batanova et al., 1998).
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For isotopic analyses, small chips of plagioclase-phyric basalts (M1/33-4, M2/29-3, M2/30-7, M2/30-9, M2/30-12a) were hand-picked and cleaned in HNO3 in closed quartz glass on a hot plate for 15 min to remove surface contamination before powdering in an agate mortar. To remove weathering products, splits of powders were leached in hot concentrated nitric acid for 8 h. The leachates were centrifuged and the insoluble residues were washed three times with four-times-distilled water.
After drying, the powders were weighed, spiked with mixed spikes of 146Nd–149Sm and 84Sr–85Rb, and decomposed for 2–5 days in a HF + HNO3 mixture at 120°C. Rb, Sr, Sm and Nd separation was carried out according to a two-stage ion-exchange and extraction chromatographic method (Richard et al., 1976). All the measurements were performed in static mode on a Finnigan MAT-261 mass spectrometer equipped with eight collectors, at the Laboratory of Isotopic Geochronology and Geochemistry of the Institute of Precambrian Geology and Geochronology, St Petersburg (Table 5, see below). 143Nd/144Nd was normalized within runs to 148Nd/144Nd = 0·241570 and then adjusted relative to a 143Nd/144Nd value of 0·511860 for the La Jolla Nd. Sr isotopic compositions were normalized within runs to 88Sr/86Sr = 8·37521. The value of the Sr isotope standard SRM-987 during this work was 87Sr/86Sr = 0·71024 (six runs). Errors (2
) for 147Sm/144Nd and 143Nd/144Nd were 0·3% and 0·000015, and for 87Rb/86Sr and 87Sr/86Sr were 0·5% and 0·000025 according to results of multiple analyses of standards (external reproducibility).
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The separate splits of the acid-leached powders were spiked with 235U–208Pb and decomposed. Pb and U were separated by anion exchange in HBr media following Manhès et al. (1978)
. Isotopic analysis of lead was carried out with the aid of a silica gel emitter. Average values for BCR-1 (n = 6) measured during the course of these analyses were 206Pb/204Pb = 18·815, 207Pb/204Pb = 15·638, 208Pb/204Pb = 38·732. Estimated 2 errors, based on between-run precision of the standard, are 0·03% per a.m.u.; within-run precision for individual Pb analyses was 0·006–0·008%. Errors estimations (2) for 238U/204Pb were <1%. The raw U–Pb data were processed using the PBDAT program by Ludwig (1991a). Regression lines were calculated for 95% confidence level using the ISOPLOT program by Ludwig (1991b). The reproducibility of determined concentrations, calculated on the basis of multiple analyses of the BCR-1 standard, was 1% for Pb, and 0·5% for Rb, Sr, Sm, Nd and U. Total blank levels during analytical work did not exceed 0·5 ng for Pb, 0·05 ng for U, 0·01 ng for Sm, 0·05 ng for Nd, 0·05 ng for Rb and 0·2 ng for Sr. |
RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC...ANALYTICAL TECHNIQUESRESULTSDISCUSSIONMAJOR CONCLUSIONSAPPENDIXREFERENCES |
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Petrography and mineralogy of olivine-phyric basalts
The Aphanasey Nikitin Rise olivine-phyric basalts were sampled from depths of 5041–4594 m (Fig. 1b). The basalts contain 10–15% microphenocrysts, primarily euhedral prismatic olivine (0·5–1·0 mm), and octahedral spinel (0·2–0·3 mm). The matrix consists of glass with variable amounts of olivine, plagioclase and clinopyroxene microphenocrysts. The olivine-phyric basalts are characterized by variable degrees of secondary alteration. Some samples (e.g. M1/34-4, -5) contain fresh olivine, whereas in others (M1/34-1, -2, -3, -6, -7) olivine crystals are replaced by iddingsite (Borisova et al., 1997
). The olivine-phyric basalts have similar matrix clinopyroxene compositions (Wo10–43En31–51Fs26–39) and similar proportions of mineral phases suggesting a similar parental melt composition and degree of differentiation (Borisova et al., 1997).
The following types of primary inclusions in olivine phenocrysts have been identified in samples M1/34-4 and M1/34-5: (1) glassy and partially crystallized melt inclusions of 0·01–0·04 mm size; (2) crystalline inclusions of spinel 0·03–0·05 mm in size; (3) multi-phase inclusions represented by glass with spinel (Borisova et al., 1997). The compositions of olivines and spinels are typical of MORB (mid-ocean ridge basalt) phenocrysts (Borisova et al., 1997). The forsterite content of the olivine ranges from 86·2 to 87·9 mol %; CaO and NiO contents vary from 0·21 to 0·33 wt % and from 0·19 to 0·34 wt %, respectively. No zonation was observed in the olivine crystals (variation of composition from the core to rim is <0·5 mol % Fo). The chromian spinel of the phenocrysts and crystalline inclusions differs from those of typical hotspot tholeiitic basalts and is characterized by the following average composition (Borisova et al., 1997): TiO2 = 0·93 wt %; Mg/(Mg + Fe2+) = 0·63; Cr/(Cr + Al) = 0·53; Fe3+/(Fe3+ + Al + Cr) = 0·1. For comparison, host olivine and spinel inclusions of Hawaiian basalts are characterized by higher NiO (>0·35 wt %) for olivine; and TiO2 (>1 wt %), Cr/(Cr + Al) (>0·6) for spinel, respectively (e.g. Sobolev & Nikogosian, 1994).
Major and trace element chemistry of olivine-phyric basalts
Melt inclusions
To determine the melt compositions and temperatures of crystallization of the olivine-phyric basalts, an experimental study on partially crystallized melt inclusions in olivine phenocrysts was carried out using a high-temperature heating stage in an atmosphere of high-purity He under visual control (Sobolev & Slutzky, 1984). The compositions of quenched homogenized inclusions and glassy inclusions [data from Borisova et al. (1997)] are listed in Table 2a. To calculate distribution coefficients (D) for Fe2+/Mg between host olivine and melt inclusions we used a melt Fe2+/Fe3+ value obtained from the average Fe2+/Fe3+ in spinel inclusions using the equation given by Maurel & Maurel (1982). The D between host olivine and quenched homogenized inclusions was 0·29–0·31, close to that measured experimentally for dry and H2O-bearing systems (0·30 ± 0·03, Roeder & Emslie, 1970), whereas the calculated D between the host olivine and glassy inclusions was found to range from 0·09 to 0·14. These values imply that olivine precipitated on the inclusion wall during cooling (Roedder, 1984) with re-equilibration of Fe2+ and Mg between the melt inclusions and the host olivine after trapping (Gurenko et al., 1991; Danyushevsky, 1998). The calculation of melt composition in equilibrium with the host olivine was carried out by modelling the reverse fractional crystallization (Sobolev & Shimizu, 1993; Danyushevsky, 1998) and diffusion (Danyushevsky et al., 1991). In these calculations, we used the olivine–melt equilibrium equation of Ford et al. (1983), and Fe2+/Fe3+ values were estimated according to the magnetite–wüstite (WM) oxygen buffer. The recalculated melt compositions are also listed in Table 2a.
Parental magma composition and crystallization conditions
The Aphanasey Nikitin Rise olivine-phyric basalts are characterized by a wide range of MgO (1·55–8·65 wt %), K2O (0·98–1·85 wt %) and P2O5 contents (0·31–1·05 wt %) (Table 1). However, all of the basalts have similar values of Al2O3/FeO*, Al2O3/TiO2 and Na2O/TiO2 (Al2O3/FeO* = 1·7–1·9; Al2O3/TiO2 = 11·2–11·6 and Na2O/TiO2 = 1·6–1·8; total Fe presented as FeO*). Moreover, similar matrix clinopyroxene compositions and mineral phase proportions suggest similar parental melt compositions and degree of differentiation (Borisova et al., 1997). Therefore, the variations in the major element compositions of the basalts (Fig. 2) are due largely to secondary alteration. Alteration led to a major loss of MgO, minor loss of CaO, MnO and SiO2, and a gain of K2O and P2O5, whereas Al2O3, TiO2, FeO* and Na2O remained immobile (Fig. 3) (Borisova et al., 1997). The loss of MgO, SiO2 and MnO was controlled by olivine alteration below 150°C (Borisova et al., 1997) [the temperature below which seawater is undersaturated with Mg-rich minerals; Snow & Dick (1995)]. In addition, there is a rough negative correlation between MgO content in basalts and weight loss on ignition (Fig. 2).
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The composition of homogenized inclusions, and the calculated compositions of the glassy melt inclusions and olivine-phyric basalts are hypersthene-normative (Borisova et al., 1997). The fact that the compositions of these inclusions are close to that of the fresher basalt (M1/34-5) and that these inclusions are in equilibrium with the olivine phenocrysts suggests that the basalts did not accumulate olivine. The average composition of the homogenized inclusions and calculated composition of the glassy melt inclusions can be considered as the parental magma composition of the olivine-phyric suite (Fig. 2, Table 2a).
Taking into consideration the accuracy of the melt inclusion homogenization method (±20°C, Sobolev & Danuyshevky, 1994), a value of 1240°C is considered as the liquidus temperature of the most magnesian olivine phenocrysts (Borisova et al., 1997). Redox conditions were estimated with the spinel–melt equilibrium equations (Ariskin & Nikolaev, 1996) from the compositions of the homogenized inclusions, average spinel composition and the value of the homogenization temperature. The data suggest that the parental melt crystallized under log (fO2) from -9·1 to -8·4, which corresponds to the MW buffer.
Specific features of the parental melt
To explore possible differentiation trends of the olivine-phyric parental magma, we modelled fractional crystallization using the COMAGMAT (Ariskin et al., 1993) and the MELTS (Ghiorso & Sack, 1995) programs at pressures of 0·001 and 3 kbar and fO2 corresponding to the MW buffer (Fig. 2). To compare the melt compositions with typical MORB, we also modelled differentiation trends for two main types of primary MORB melts, TOR-1 and TOR-2, at pressures of 0·001 and 4 kbar and fO2 corresponding to the MW buffer using the COMAGMAT program. TOR-1 and TOR-2 composition are obtained from composition of abyssal glasses from Mid-Atlantic Ridge and Indian Ocean Ridges (e.g. Dmitriev et al., 1985; Sushchevskaya et al., 1994). The calculated differentiation trends of the parental melt for the olivine-phyric basalt suite demonstrate high SiO2, K2O and P2O5 and low CaO concentrations in melts formed during possible fractionation of the parental melt. The trend for the parental melt for the olivine-phyric basalts also exhibits higher SiO2, FeO*, TiO2, K2O and P2O5, lower Al2O3 concentrations, and high SiO2/Al2O3 (3·5), all of which are unusual for MORB melts. High SiO2/Al2O3 can result from low-pressure conditions during primary melt generation (Sobolev & Shimizu, 1993). However, such MORB magmas would be characterized by extreme depletion in incompatible elements. Therefore, high SiO2/Al2O3 in the olivine-phyric suite parental melt suggests a specific composition of the melt source or the effect of the contamination process rather than unusual conditions of melt generation.
H2O content of parental melt
Measured H2O contents in two glassy melt inclusions are 0·31 and 0·55 wt % (Table 2b). Post-entrapment crystallization increases the water content of the melt inclusion. We used the Rayleigh fractional crystallization equation to calculate primary water contents before olivine fractionation (Table 2b). Calculated H2O contents for the melt are 0·26 and 0·48 wt %, which are characteristic of E-MORB (Sobolev & Chaussidon, 1996). The obtained values, however, could be slightly underestimated (by about 1% relative) because of possible diffusional loss of water from the inclusions (Qin et al., 1992).
Influence of secondary alteration processes on trace element concentrations
The similar compositions and proportions of mineral phases in the Aphanasey Nikitin Rise olivine-phyric basalts imply similar initial compositions of the basalts. We can investigate the influence of secondary alteration processes on the trace element abundances by comparing more altered basalts with the freshest basalt (M1/34-5). If Zr, Hf, Nb, Ta and Th are considered as immobile, trace elements that show more variable abundances were affected by secondary alteration (Fig. 3). Figure 3 indicates that during the alteration of the olivine-phyric suite REE (except Ce), Th and Sc were immobile, U, Ba, Rb and Zn were gained, and Co, Cr, V, Pb and Ce were lost. Because the loss of Mg is likely to be controlled by olivine alteration (Borisova et al., 1997), the loss of Co and Cr seems to be controlled by the same process. The gain of U, Ba, Rb and Zn is explained by seawater–rock interaction (Jochum & Verma, 1996), whereas loss of Pb could be controlled by either bottom seawater–rock interaction (Jochum & Verma, 1996) or a leaching process established for hydrothermal alteration (e.g. Chauvel et al., 1995). As a result, the process of secondary alteration of olivine-phyric basalts seems to be a combination of seawater–rock interaction and hydrothermal alteration.
Trace element composition of the parental melt
The parental melt composition of the olivine-phyric suite can be estimated using the composition of melt inclusions. To estimate the trace element composition of the melt in equilibrium with the host olivine, we calculated the concentrations for the glassy inclusions using the Rayleigh fractional crystallization equation (Table 2b). The calculated melt inclusion compositions are characterized by (La/Sm)n = 3·1–3·2, (La/Yb)n = 6·1–6·2 and La/Nb = 1·6–1·9, ratios that overlap with those of the basalts [normalized to the primitive mantle composition of Sun & McDonough (1989)]. The mantle-normalized trace element patterns of melt inclusions and host olivine-phyric basalts are characterized by similar degree of light REE (LREE) enrichment and Nb depletion (Fig. 4).
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The REE patterns of Aphanasey Nikitin Rise basalts are characterized by (La/Sm)n = 2·7–2·9 and (La/Yb)n = 5·8–6·6, typical of P-type MORB of the Southwest Indian Ridge (le Roex et al., 1983). Nb/Ta (16–22) and Zr/Hf (44–47) are within the range of those of OIB (e.g. Green, 1995). High La/Ta (29–34) and La/Nb (1·5–2·1) are not typical of MORB or OIB [La/Ta = 13·4–18·9, La/Nb = 0·76–1·07; Sun & McDonough (1989)]. La/Ta and La/Nb of Aphanasey Nikitin Rise olivine-phyric basalts overlap with those of the anomalous basalts from ODP Site 738 on the Kerguelen Plateau, and DSDP Site 264 on the Naturaliste Plateau (Table 3; Mahoney et al., 1995). However, the La/Th values of Aphanasey Nikitin Rise olivine-phyric basalts (La/Th = 15·5–18·8) are much higher than those of the tholeiitic suites from the Kerguelen and Naturaliste plateaux and Broken Ridge (La/Th = 4·8–9·1; Storey et al., 1992; Mahoney et al., 1995).
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The important features of the Aphanasey Nikitin Rise olivine-phyric basalts patterns are Ta, Nb, Th and U depletion, and Ba and Pb enrichment relative to REE, and Rb depletion relative to Ba. It should be noted that the established effect of the basalt secondary alteration is the variable gain of U and slight loss of Pb; therefore the observed relative U depletion and Pb enrichment cannot be due to the alteration. Nb and Ta depletion and Pb enrichment of olivine-phyric basalts are similar to those of oceanic tholeiites at Sites 738 and 264, Broken Ridge (D8, M-D10) and some continental tholeiites, such as Bunbury and Rajmahal Traps (Frey et al., 1996; Kent et al., 1997) (Fig. 5). However, the Aphanasey Nikitin Rise olivine-phyric basalts differ from these oceanic and continental tholeiites by relative Th and U depletions.
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Major and trace element chemistry of the plagioclase-phyric basalts
The Aphanasey Nikitin Rise plagioclase-phyric basalts were sampled from shallower depths of 3894–4362 m (M1/33, M2/29, M2/30; Fig 1b) in comparison with those of olivine-phyric basalts. The plagioclase-phyric basalts contain 5–30% phenocrysts, which are euhedral plagioclase (5–20%) and augite crystals 1–5 mm in size. Phenocrysts are represented by plagioclase (An42–90 with FeO = 0·2–1·5 wt %) and augite (En28–50Fs9–39Wo33–42, Al2O3 = 1–5 wt %, TiO2 = 0·2–1·2 wt % and Na2O = 0·2–0·3 wt %) (Borisova, 1997). The basalt matrix consists of altered glass with variable amounts of plagioclase, augite and titanomagnetite crystals of 0·1–0·5 mm in size.
The major element compositions of the plagioclase-phyric basalts are characterized by a narrow range of MgO (3·8–5·6 wt %) and SiO2 (48·1–50·4 wt %) (Table 4a). The variations of CaO and Al2O3 (CaO = 9·22–12·25 wt % and Al2O3 = 14·8–18·6 wt %) seem to be due to plagioclase accumulation and variable amounts of augite. The compositions of the plagioclase-phyric basalts correspond to hypersthene-normative tholeiites (Borisova, 1997). The basalt mineral phases composition and whole-rock major element contents overlap with those of MORB rather than those of OIB (Borisova, 1997).
The REE patterns of the fresher basalts have (La/Sm)n = 0·7–1·7 and (La/Yb)n = 1·2–2·6, typical of T-type MORB (Fig. 4). The basalt compositions show a small variation of Zr/Y (3–3·6), whereas La/Nb, Zr/Nb and La/Th ratios have larger variations (0·7–1·9, 11–28 and 9·7–15·2, respectively), indicating different degrees of Nb and Th depletion in the magmas (Borisova, 1997). Zr/Hf (30–47) ratios are similar to those of MORB and OIB magmas (e.g. Green, 1995). The fresher basalts have high Ba, K and Pb and low Nb, Ta and Th contents, resembling the olivine-phyric basalt patterns.
Major and trace element chemistry of the trachybasalts
The Aphanasey Nikitin Seamount trachybasalts and trachyte were sampled from shallow depths of 2687–3394 m (M1/36, M2/33; Fig. 1b). Trachybasalts contain 10–30% phenocrysts, which are euhedral plagioclase (5–20%), olivine, augite and magnesian ilmenite (± titanomagnetite) 1·5–6 mm in size. Phenocrysts are represented by plagioclase (An43–60Or3–7), augite (En39–43Fs11–16Wo43–45, Al2O3 = 2–5 wt %, TiO2 = 0·5–2·5 wt % and Na2O = 0·3–0·7 wt %), magnesian ilmenite (MgO = 6·7–8·7 wt %) and magnesian titanomagnetite (MgO = 5·5–6·2 wt %). Contemporaneous crystallization of plagioclase, olivine, augite and ilmenite (titanomagnetite) is indicated by the presence of olivine, augite, and ilmenite (titanomagnetite) inclusions in plagioclase (Borisova, 1997). Olivine crystals and matrix glass are replaced by iddingsite. The trachybasalt samples contain secondary apatite (M1/36-7) or calcite (M2/33-3, M2/33-6). Amygdaloidal lava samples AFN-1 and AFN-2 described by Mahoney et al. (1996) are similar to trachybasalts M2/33-3 and M2/33-6 obtained from depths of 2687–2799 m.
The Aphanasey Nikitin Seamount trachybasalt compositions exhibit low MgO (0·82–2·34 wt %) and high P2O5 contents, ranging from 0·54 to 5·29 wt % (Table 4b). The highest P2O5 content, 5·29 wt % in sample M1/36-7, is due to accumulation of secondary apatite (Borisova, 1997). Low MgO and high P2O5 contents in the other trachybasalts are due to the replacement of olivine and matrix glass by iddingsite, which led to a loss of MgO and gain of P2O5 (Borisova, 1997). Using the MgO partition coefficient between ilmenite and basaltic melt (Agata, 1998), the trachybasaltic melt composition is established to have MgO = 5·0–6·5 wt %. The variation in the Fe2O3, TiO2, Al2O3 and CaO contents of the trachybasalts [Fe2O3 = 5·2–10·4 wt %, TiO2 = 1·6–3·1 wt %, Al2O3 = 15·4–19·7 wt % and CaO* = 1·7–10·7 wt %; CaO* is recalculated on a volatile-free and secondary apatite-free basis (Borisova, 1997)] seems to result from ilmenite, titanomagnetite and clinopyroxene crystallization and plagioclase accumulation.
The apatite accumulation in trachybasalt sample M1/36-7 results in REE enrichment and a negative Ce anomaly, which is absent in the other trachybasalts (Borisova, 1997). Our data (Table 4b) and those of Mahoney et al. (1996) show that the REE patterns of the freshest trachybasalts are characterized by (La/Sm)n = 2·8–5·6 and (La/Yb)n = 12–16. The trachybasalts exhibit Eu, Sr and Ti depletions relative to REE, which are due to plagioclase, ilmenite or titanomagnetite crystallization. Because the Zr partition coefficient between clinopyroxene and basaltic melt is lower than that for Sm and Nd (e.g. Hart & Dunn, 1993), Zr enrichment relative to REE in trachybasalts results from clinopyroxene fractionation (Borisova, 1997). Nb/Ta (14–21) and Zr/Hf (36–50) are similar to those of MORB and OIB magmas (e.g. Green, 1995). However, high La/Ta (15·0–17·7), La/Nb (0·9–1·3) and La/Th (3·8–8·2) are not usual for OIB [average La/Ta = 13·7, La/Nb = 0·77 and La/Th = 9·3; Sun & McDonough, (1989)]. Samples AFN-1 and AFN-2 show the strongest Nb and Th depletion relative to La, and the highest La/Nb (1·25–1·45) and La/Th (9·2–12·3) among trachybasalts (Mahoney et al., 1996). The Aphanasey Nikitin Seamount trachybasalts also possess high Ba, K and Pb and variably low Nb, Ta, Th and U contents, broadly resembling the olivine-phyric basalt patterns; many show larger peaks at Pb relative to REE than the olivine-phyric basalts. However, the latter exhibit higher La/Ta, La/Nb and La/Th ratios and lower La/Sm.
Isotope chemistry of Aphanasey Nikitin Rise lavas
Corrected for an age of 80 Ma, Sr and Nd isotopic compositions of the leached olivine-phyric basalts of Aphanasey Nikitin Rise lie within a narrow range: 87Sr/86Srt = 0·705777–0·705961 and 143Nd/144Ndt = 0·512161–0·512192 (Table 5). However, Pb isotope ratios exhibit larger variations: 206Pb/204Pbt = 16·850–17·237, 207Pb/204Pbt = 15·529–15·611 and present-day 208Pb/204Pb = 37·397–37·803. The leached olivine-phyric basalts are characterized by low 238U/204Pb and 147Sm/144Nd ratios (2·6–7·8 and 0·14–0·16, respectively) and have high age-corrected 
7/4 and present-day 8/4 ratios (20–25 and 128–140, respectively), high even compared with most basalts of the Dupal anomaly (Hart, 1984). Moreover, the leached olivine-phyric basalts have low age-corrected 143Nd/144Nd and 206Pb/204Pb similar to but lower than those proposed for the EM-1 end-member (Zindler & Hart, 1986). The basalts of the olivine-phyric suite are characterized by lower 206Pb/204Pb relative to their 143Nd/144Nd compared with the tholeiitic basalts of the Naturaliste and Kerguelen plateaux and Broken Ridge (Fig. 6). Compared with basalts from Sites 738 and 264 on the Kerguelen and Naturaliste plateaux (Table 3; Mahoney et al., 1995), the olivine-phyric basalts of Aphanasey Nikitin Rise have the lowest 87Sr/86Sr relative to their 143Nd/144Nd.
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Trachybasalts AFN-1 and AFN-2 show the lowest Nd and 206Pb/204Pb and highest 87Sr/86Sr (Nd t = -7·6 to -8; 206Pb/204Pbt = 16·772–16·801; 87Sr/86Srt = 0·70641–0·70662), resembling those of plagioclase phenocrysts of the M2/33-3 trachybasalt, with Nd t = -7·0, 206Pb/204Pbt = 16·781 and 87Sr/86Srt = 0·70667. According to Mahoney et al. (1996) and Sushchevskaya et al. (1996) (Table 5), both the olivine-phyric basalts and the trachybasalts are remarkable in possessing very low 206Pb/204Pbt and 143Nd/144Ndt, which lie beyond those of the EM-1 end-member (Fig. 6). In comparison with the olivine-phyric basalts (Fig. 6) the trachybasalts (Table 5) exhibit higher 87Sr/86Srt (0·705653–0·706670) relative to 143Nd/144Ndt (0·512117–0·512353) and lower 207Pb/204Pbt (15·452–15·550) relative to 206Pb/204Pbt (16·781–17·695). Age-corrected isotopic compositions of the Aphanasey Nikitin Seamount trachybasalts show broad linear trends on Sr–Nd–Pb diagrams (Fig. 6).
From our new data here and the data of Sushchevskaya et al. (1996), the age-corrected Pb–Nd–Sr isotopic compositions of leached plagioclase-phyric basalts exhibit a narrow range of 206Pb/204Pbt (17·832–18·382), 207Pb/204Pbt (15·577–15·625), 208Pb/204Pb (37·810–38·413) and 143Nd/144Ndt (0·512571–0·512817). The plagioclase-phyric basalts show the highest 206Pb/204Pbt and 143Nd/144Ndt, and lowest 87Sr/86Srt (0·703678–0·704585) ratios (Table 5, Fig. 6) among the Aphanasey Nikitin Rise basalts. These values differ from those of Crozet Archipelago, Réunion and Marion hotspot islands (Kent et al., 1992; Mahoney et al., 1996), but overlap the range of the Ninetyeast Ridge ODP Site 758 basalt composition (Weis & Frey, 1991) (Fig. 6). The isotopic data do not support the Crozet or Marion plume model for the source of the Aphanasey Nikitin Rise.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC...ANALYTICAL TECHNIQUESRESULTSDISCUSSIONMAJOR CONCLUSIONSAPPENDIXREFERENCES |
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Source composition of the Aphanasey Nikitin Rise olivine-phyric suite
As concluded by Mahoney et al. (1996)
and Sushchevskaya et al. (1996) for the Aphanasey Nikitin Rise tholeiitic basalts and trachybasalts, the low age-corrected 143Nd/144Nd and 206Pb/204Pb, and the Ta and Nb depletion and Pb and Ba enrichment of the mantle-normalized patterns suggest the involvement of continental lithosphere. Below we discuss in detail the contribution of possible sources, and the role of assimilation–fractional crystallization and contamination of the mantle source in the origin of the Aphanasey Nikitin Rise tholeiitic olivine-phyric suite.
Continental mantle lithosphere as a source
Delaminated and convectively recycled continental mantle lithosphere could provide a source component for oceanic island basalts (McKenzie & O’Nions, 1983). However, the very low incompatible element concentrations and the lack of Nb depletion relative to REE of Proterozoic and Phanerozoic continental lithospheric mantle xenoliths (McDonough, 1990) (Fig. 7) imply that a large part of mantle lithosphere does not possess the necessary composition to be a source component for the Aphanasey Nikitin Rise olivine-phyric basalts.
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As stated above, both olivine-phyric basalts and trachybasalts show Nb and Ta depletion relative to K and REE, Ba enrichment, and low 143Nd/144Ndt. Similar trace element patterns (Fig. 7) and isotopic compositions are found in some highly alkalic lavas, such as the Miocene minettes of northwest Colorado and the Gaussberg leucitites of Antarctica, interpreted by many researchers to originate in metasomatized continental mantle lithosphere as low-degree melts (e.g. Nelson et al., 1986; Thompson et al., 1990). However, the estimated parental melt of the Aphanasey Nikitin Rise olivine-phyric suite has a hypersthene-normative tholeiitic composition with high SiO2/Al2O3 (3·5), low H2O content and liquidus phase compositions resembling those of MORB and continental tholeiites (e.g. Macdougall, 1988). These features suggest a relatively high degree partial melt of the mantle, which would probably be generated in the asthenosphere or plume-type mantle rather than in the relatively cold lithospheric mantle. The low water content in the olivine-phyric suite parental melt is typical of E-type MORB, and excludes melting of a wet lithospheric mantle. Asthenosphere or plume mantle, rather than the mantle part of the continental lithosphere, has been suggested as the main source for continental tholeiites, which are argued to be variably contaminated by crustal material (e.g. Dupuy & Dostal, 1984; Macdougall, 1988; Arndt & Christensen, 1992; Peng et al., 1994). Therefore, the geochemical features of the olivine-phyric basalts may result from contamination of depleted mantle-derived or plume-derived melt by continental crust.
The subalkalic and alkalic suites of the Aphanasey Nikitin and Conrad Rises, including trachybasalts, trachytes, leucite-bearing basanites and tephrites (Borisova et al., 1996), show high REE patterns and are characterized by relatively high alkalinity. These characteristics suggest that the alkalic melts are likely to originate in the subcontinental mantle lithosphere as low-degree partial melts (Borisova et al., 1996; Borisova, 1997).
Sediments as a contaminant
The addition of a sedimentary component to the mantle source can produce relative Nb and Ta depletion (e.g. Ben Othman et al., 1989; Barling et al., 1994), which are the specific characteristics of the Aphanasey Nikitin Rise olivine-phyric basalts. Figure 8 shows variations of the Ce/U, Ce/Th and (Ce/Sm)n ratios of some modern oceanic sediments, average OIB and MORB, and Aphanasey Nikitin Rise olivine-phyric basalts. Pelagic, terrigenous and biogenic sediments are more enriched in Th and U relative to REE than OIB and MORB, and Aphanasey Nikitin Rise olivine-phyric basalts. The global subducting sediment (GLOSS) composition estimated by Plank & Langmuir (1998) also demonstrates Th and U enrichment relative to REE [Ce/Th = 8·3, Ce/U = 34, (Ce/Sm)n = 2·5]. Therefore, the addition of chemically unmodified sediments to the mantle source cannot explain the chemistry of the olivine-phyric basalts.
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Sedimentary material in the oceanic basalt sources could result from recycling via subduction of oceanic lithosphere (e.g. Hofmann & White, 1982; Weaver, 1991). The fact that many pelagic sediments have low 238U/204Pb ratios (2–6) (e.g. Ben Othman et al., 1989) suggests that ancient recycled sediment might be characterized by low 206Pb/204Pb compared with modern mantle values (e.g. Barling et al., 1994). However, Chauvel et al. (1995) and Miller et al. (1994) demonstrated that dehydration of whole oceanic crust and mantle leads to Pb depletion and a significant increase in the U/Pb of subducted oceanic lithosphere. Therefore, ancient subducted pelagic sediments would be unlikely to change the bulk composition of old subducted oceanic lithosphere towards the lower 206Pb/204Pb ratios observed in the Aphanasey Nikitin Rise olivine-phyric basalts (Fig. 6).
Continental crust as a contaminant
The observed anomalous geochemical features may have alternatively resulted from addition of a component that underwent an ancient depletion in U relative to Pb, an effect typically attributed to granulite facies metamorphism (e.g. Taylor & McLennan, 1985; Kay & Kay, 1986; Rudnick & Presper, 1990). Figure 9 shows the average composition of Archaean Lewisian granulitic gneisses (Weaver & Tarney, 1981; Taylor & McLennan, 1985) and the mean and median compositions for granulite xenoliths (Rudnick & Presper, 1990). Many continental granulite rocks are depleted in Nb, Ta, Th and U relative to LREE. Mantle-normalized patterns of flood basalts shown by many investigators to be contaminated by lower continental crust also typically show depletions in Nb, Ta and Th (e.g. Thompson et al., 1983). As a result, contamination of a mantle source or parental melt by granulite facies material may explain these anomalous features of the Aphanasey Nikitin Rise olivine-phyric basalts.
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Constraints on the petrogenesis of the olivine-phyric suite
Most previous investigators interpreted the Aphanasey Nikitin Rise magmatism as resulting from the magmatic activity of a hotspot close to a spreading axis (e.g. Paul et al., 1990; Curray & Munasinghe, 1991; Matveenkov et al., 1991; Müller et al., 1993). Therefore, involvement of depleted mantle- or plume-derived melts is likely to have been an important process, in particular for the petrogenesis of the early shield-stage tholeiitic olivine-phyric and plagioclase-phyric suites. The tholeiitic plagioclase-phyric suite, which represents most of the sampled part of the Aphanasey Nikitin Rise basement, is the plausible candidate for largely plume-derived magmas (Mahoney et al., 1996). Conversely, liquidus mineral and major element composition of plagioclase-phyric basalts are consistent with melting of depleted mantle as the mantle source (Borisova, 1997). Variable Nb, Ta and Th depletions in plagioclase-phyric basalts imply involvement of ancient continental material similar to Archaean and some post-Archaean granulite facies crust (Borisova, 1997).
The composition of the liquidus assemblage for the spinel and olivine of the olivine-phyric basalts is typical of MORB and differs significantly from those of plume-derived melts, supporting depleted mantle as a mantle source for the basalt suite. We evaluated the possibility that the parental melt of the Aphanasey Nikitin Rise olivine-phyric suite was generated by contamination of depleted mantle-derived melts by continental crust. For the calculation we began with an initial melt composition equal to average MORB (Sun & McDonough, 1989). To quantify the composition of the continental material, we used the assimilation–fractional crystallization equations of DePaolo (1981) for trace element concentrations and isotopic ratios (see Appendix, Table A1a). The melt inclusions and basalts of the Aphanasey Nikitin Rise olivine-phyric suite show Ti/Eu typical of E-MORB and no Sr or Eu depletion relative to REE, thereby excluding fractionation of Ti-bearing phases and plagioclase. Because olivine and spinel are the only liquidus minerals of the parental melts, the bulk solid–liquid partition coefficients (D) for the incompatible elements between the fractionating crystalline phases and the magmas are much lower than unity. For D << 1, it is possible to estimate the composition of the wallrock being assimilated. Assuming the relative mass of magma remaining, F (F = Mm/Mm°, where Mm is the mass of magma and Mm° is the initial mass of magma), ranges from 0·1 to 0·95, it is possible to estimate trace element ratios in the wallrock. If the initial composition of the magmas is equal to that of average N-, T-, or E-type MORB, the calculated ratios of (La/Sm)n (5–13), La/Nb (3–51), La/Ta (38–127), La/Th (15–21), Ce/Pb (12–14), Th/U (6–13), Nb/Th (0·4–6) and Nb/U (5–34) are in the range for Archaean Lewisian granulites, some Indian granulites and granulite facies xenoliths, and estimated average continental crustal composition. These values differ from those of typical Proterozoic and Phanerozoic continental mantle lithospheric xenoliths (see Fig. 10, and Table A1a).
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For the case where D << 1, the value of Ca/Cm° is in the range 10–100 (DePaolo, 1981), where Ca is the concentration of the element in the assimilated wallrock and Cm° is the initial element concentration in the magma (see Table A1b and c). The maximum volume of wallrock that could be assimilated by an igneous body is <10% (e.g. Bowen, 1928); however, the percentage is a function of the difference in temperature between the wallrock and igneous body (DePaolo, 1981). We used Ma/Mm < 0·3, where Ma is the rate of assimilation (mass/unit time). Using the age-corrected isotopic compositions of the leached olivine-phyric basalts as the composition of the contaminated magmas, it is possible to estimate the isotopic composition of the assimilated wallrock. Assuming that an initial, uncontaminated magma had the average isotopic composition of the plagioclase-phyric basalts and those of the Indian MORB, corresponding to depleted mantle-derived melts, the calculated isotopic composition of the wallrock is 206Pb/204Pb = 16·838–17·235, 207Pb/204Pb = 15·524–15·615, 208Pb/204Pb = 37·385–37·803, 87Sr/86Sr = 0·7058–0·7070 and 143Nd/144Nd = 0·51215–0·51219. The isotope ratios are typical of Archaean and some post-Archaean granulite facies terranes (e.g. Rudnick & Goldstein, 1990) and resemble those of granulite facies xenoliths of Lesotho (Rudnick, 1992).
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Mahoney et al. (1996)
suggested that old continental crustal material could be introduced into a plume or depleted mantle source before Aphanasey Nikitin Rise magmatism. It is possible to estimate the isotopic composition of the continental component using the mixing equations of Langmuir et al. (1978) and assuming that the ‘contaminated’ mantle source has the age-corrected isotopic composition of the olivine-phyric basalts (see Table A1d). For the calculation we assume an initially uncontaminated mantle source to have the isotopic composition of the plagioclase-phyric basalts and those of the Indian MORB, trace element composition of primitive mantle (Sun & McDonough, 1989), and trace element concentrations of average continental crust (Rudnick & Fountain, 1995). Assuming the fraction of the continental component in the ‘contaminated’ mantle ranges from 0·1 to 0·3, the calculated isotopic composition of this component is 206Pb/204Pb = 16·60–16·98, 207Pb/204Pb = 15·46–15·50, 87Sr/86Sr = 0·7063–0·7074 and 143Nd/144Nd = 0·5117–0·5121 (see Table A1d). The isotopic ratios are typical of Archaean and some post-Archaean granulite facies terranes (e.g. Rudnick & Goldstein, 1990) and resemble those of granulite facies xenoliths of Lesotho (Rudnick, 1992).
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Because high ratios of SiO2/Al2O3 are characteristic of continental crust [SiO2/Al2O3 = 3·2–4·3 for the average lower, middle and upper continental crust (Rudnick & Fountain, 1995
); SiO2/Al2O3 = 3·3–7·5 for the Indian granulites studied by Weaver (1980)], the high SiO2/Al2O3 (3·5) in the parental melt of the Aphanasey Nikitin Rise olivine-phyric basalt suite probably represents contamination by crustal material rather than the involvement of low-degree melts from continental mantle lithosphere. Thus, petrological and geochemical data on the Aphanasey Nikitin Rise olivine-phyric suite support both models of assimilation and fractional crystallization of depleted mantle-derived melts and contamination of depleted mantle by continental material similar to Archaean and some post-Archaean granulite facies rocks.
Estimation of model age of the continental contaminant
The Pb and Nd isotopic compositions of the Aphanasey Nikitin Rise olivine-phyric basalts can be used to estimate the age of the continental contaminant. Model Pb ages may reflect a ‘rejuvenation’ process as established by Rudnick (1992) for lower continental crustal rocks. Rudnick (1992) and Rudnick & Goldstein (1990) demonstrated that, in general, the Pb isotopic compositions of granulite xenoliths are more radiogenic than those of Precambrian granulites. As argued by Rudnick (1992), such changes in granulite Pb-isotopic composition to more radiogenic values and ‘rejuvenation’ of xenolith ages result from the interaction of granulitic crust with mantle-derived magmas or plume mantle. Therefore, the relatively unradiogenic Pb-isotope ratios that characterize the continental crust contaminant of the Aphanasey Nikitin Rise magmas require a long period of preservation without addition of mantle material. To estimate the minimum age of the continental component we assumed that (1) the isotopic compositions of the Aphanasey Nikitin Rise olivine-phyric basalts reflect the isotopic compositions of the crustal contaminant, and (2) uranium was completely lost during granulitization of the source (U/Pb = 0). Such assumptions are reasonable for estimating minimum ages. For example, the Pb isotopic composition of olivine-phyric basalts results from contamination of depleted mantle-derived melts or mantle source by crustal components. In this case, Pb isotopic composition of the assumed continental component would move to the left and/or up on the 206Pb/204Pb–207Pb/204Pb isotope diagram (Fig. 6, Table A1b–d) relative to the composition of the olivine-phyric basalts, thus resulting in an increase in the model age. A similar shift in Pb isotope ratios would occur if uranium was only partially lost during granulitization.
According to the two-stage Stacey & Kramers (1975) model, these estimates give ages of 1030–1190 Ma (model 238U/204Pb = 9·9–10·1) (Table A2); these model ages are in accordance with the suggestion of Rudnick (1992) that the granulite xenoliths of Lesotho [which show similar Sr–Nd–Pb isotopic compositions (Fig. 6)] represent residual lower-crust material that was unchanged for >109 years. As the model of two-stage evolution is one of the most appropriate for continental crust evolution, the range 1030–1190 Ma is plausible as a minimum value of age for the continental component.
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The measured 147Sm/144Nd of the Aphanasey Nikitin Rise olivine-phyric basalts (147Sm/144Nd = 0·14–0·16) differs from that of OIB and MORB (147Sm/144Nd
0·2) and resembles that of average continental material (147Sm/144Nd = 0·12) (e.g. Milisenda et al., 1994). We can estimate the mean storage age of the continental component based on the two-stage Nd model proposed by Milisenda et al. (1994), assuming the second stage began at 80 Ma. Taking as the continental component 147Sm/144Nd = 0·12, and for the N-MORB mantle 143Nd/144Nd = 0·513151 (Goldstein et al., 1984) and 147Sm/144Nd = 0·24, and the measured 143Nd/144Nd and 147Sm/144Nd of the olivine-phyric basalts, the estimated ages range from 1140 to 1180 Ma (Table A2). These values are similar to those obtained from the Pb model; therefore both isotopic systems require the storage age to be >109 years. No material remains in the oceanic mechanical boundary layer or in convecting mantle boundary layers for such a long period (e.g. McKenzie & O’Nions, 1995). Moreover, Pb model ages suggest that the old continental crustal contaminant was isolated from the mantle for >109 years and could have been stored as a fragment of a craton. This fragment could be involved in Aphanasey Nikitin Rise basalt petrogenesis either in the mantle before melting or as assimilated wallrock.
Petrogenesis of anomalous tholeiitic suites of the Indian Ocean
Anomalous oceanic and continental tholeiites demonstrate a range of compositions reflecting involvement of continental crust in their petrogenesis. Figure 11 illustrates SiO2 and key trace element ratios of anomalous Indian Ocean tholeiitic suites from ODP Site 738 on the Kerguelen Plateau, Broken Ridge (Sites D8, M-D8, M-D10), DSDP Site 264 on the Naturaliste Plateau, and the Aphanasey Nikitin Rise olivine-phyric suite formed during the period 115–75 Ma. As a group, the anomalous oceanic tholeiites exhibit a very broad trend of increasing La/Nb and La/Ta with increasing (La/Sm)n and low Ce/Pb, suggesting increasing Nb and Ta depletion and Pb enrichment with increasing LREE enrichment in the magmas (Fig. 11). Figures 6 and 12 illustrate the correlation between the Sr–Nd–Pb isotopic composition and La/Nb and La/Th characteristics of the anomalous oceanic and continental tholeiites. The ratios of 143Nd/144Nd vs La/Nb form a broad trend from MORB and plume-derived melts, such as Ninetyeast Ridge (Frey et al., 1991; Weis & Frey, 1991), and plagioclase-phyric basalts from the Aphanasey Nikitin Rise to the composition of the most anomalous tholeiites, characterized by high 87Sr/86Sr, low 143Nd/144Nd, and Nb and Ta depletion relative to LREE (Fig. 12b and c). The tholeiites from the Kerguelen and Naturaliste plateaux and the Broken Ridge, and Bunbury and Rajmahal Traps basalts [MgO = 4–8·1 wt %, Na2O + K2O = 2·3–5·0, and high SiO2/Al2O3 = 3·0–3·6 (Mahoney et al., 1995; Frey et al., 1996; Kent et al., 1997)] display a trend of increasing SiO2 contents from 48·5 to 54·5 wt %, accompanied by increasing silica saturation up to basaltic andesite. The anomalous oceanic tholeiites show low 143Nd/144Nd, high 207Pb/204Pb and variably high 87Sr/86Sr and 208Pb/204Pb resembling those of crustally contaminated continental tholeiites, such as the Bunbury basalts, Rajmahal basalts and basaltic andesites (Figs 6 and 12), and southwestern Madagascar basalts (Mahoney et al., 1991). The extreme low-Nd compositions of the anomalous oceanic tholeiites reflect low time-integrated Sm/Nd ratios characteristic of ancient continental crust. Because most continental crust is characterized by relative Nb–Ta depletion, Pb enrichment, high SiO2 content and high SiO2/Al2O3 ratios (e.g. Weaver, 1980; Weaver & Tarney, 1981; Taylor & McLennan, 1985; Rudnick, 1995; Rudnick & Fountain, 1995; Wedepohl, 1995), the trends seem to result from the increasing amount of involvement of the continental crustal material. Such continental material possesses high La/Nb and La/Ta and low Ce/Pb ratios overlapping with those of Lewisian Archaean granulites, some Indian granulites (Fig. 10) and Archaean tonalites–trondhjemites–granodiorites (e.g. Rudnick, 1995).
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Variable Th depletion relative to LREE in these oceanic and continental tholeiites seems to originate from different contaminant compositions. Diagrams of 206Pb/204Pb vs 87Sr/86Sr and 87Sr/86Sr vs La/Th create two groups (Fig. 12a and c), which can also be seen on an Sr–Nd–Pb isotopic diagram (Fig. 6). The Aphanasey Nikitin Rise olivine-phyric basalts have higher La/Th than the anomalous tholeiites from the Kerguelen and Naturaliste plateaux and Broken Ridge, reflecting variable Th depletion relative to LREE in the anomalous oceanic magmas (Fig. 11c). Variable La/Th is characteristic of continental tholeiites contaminated by continental crust, such as the British Tertiary province basalts (e.g. Thompson et al., 1983), the Gosselin-type Bunbury basalts (Frey et al., 1996), Rajmahal Traps quartz-normative basalts and basaltic andesites (Mahoney et al., 1983; Storey et al., 1992; Kent et al., 1997) (Fig. 11), and southwestern Madagascar basalts (Mahoney et al., 1991). Lewisian Archaean granulite facies rocks exhibit high Th depletion relative to LREE, whereas granulite facies xenoliths, some Indian granulites and average Archaean and post-Archaean granulite facies terranes show variable Th depletion (Fig. 10). According to estimates of average continental crust composition by Rudnick & Fountain (1995), the average lower continental crust is characterized by lower Th concentrations and higher La/Th than the average middle, upper and whole continental crust compositions (Hofmann, 1988; Wedepohl, 1995) (Fig. 10). We have concluded above that the material contaminating the olivine-phyric suite of the Aphanasey Nikitin Rise was lower continental crust, characterized by low time-integrated Rb/Sr, U/Pb, Th/Pb and Th/U. The second groups, represented by tholeiitic basalts of Sites 738 and 264 on the Kerguelen and Naturaliste plateaux, and from Broken Ridge, exhibit Th enrichment relative to LREE and more radiogenic Sr and Pb isotopic compositions (Fig. 6). Moderate 206Pb/204Pb and relative high 208Pb/204Pb, 207Pb/204Pb and 87Sr/86Sr reflect high time-integrated Rb/Sr, U/Pb, Th/U and Th/Pb ratios, characteristic of middle or upper continental crust.
Storey et al. (1992) and Mahoney et al. (1995) suggested that SiO2-rich lavas of the Kerguelen and Naturaliste plateaux with high 87Sr/86Srt represent contamination by continental crust derived from the Gondwanan continental margins. Indeed, anomalous tholeiites at Site 264 on the Naturaliste Plateau show Sr–Nd–Pb isotopic compositions similar to those of the Gosselin-type Bunbury basalts of western Australia, also argued to be contaminated by a similar crustal component (Storey et al., 1992; Frey et al., 1996). Crustal xenoliths of the Gaussberg volcanic province of Antarctica exhibit radiogenic Pb isotopic compositions [206Pb/204Pb = 18·083; 207Pb/204Pb = 15·721 and 208Pb/204Pb = 41·384, Nelson et al. (1986)] similar to or more extreme than those of the basalts from the Kerguelen and Naturaliste plateaux (Fig. 6). Compositions of the anomalous oceanic tholeiites from Kerguelen, Naturaliste plateaux and Broken Ridge overlap with those of continental tholeiites of Rajmahal traps (116 Ma) and Bunbury provinces (130–123 Ma), (Figs 6, 11 and 12), which pre-date the break-up of Gondwana (Mahoney et al., 1983; Frey et al., 1996). The data strongly support the model of involvement of Gondwanan continental margin crust in the anomalous oceanic tholeiite petrogenesis. According to Early Cretaceous plate tectonic reconstructions (e.g. Müller et al., 1993), the sites of the most anomalous tholeiites from the Kerguelen and Naturaliste plateaux were located close to post-Gondwanan continental margins. In contrast, the Aphanasey Nikitin Rise and Broken Ridge were not formed close to the continental margins, according to Late Cretaceous plate reconstructions (Müller et al., 1993).
Several workers have suggested that the anomalous oceanic tholeiites could be generated by entrainment of isotopically distinct, non-plume material into plumes, or the interaction of plume-derived magmas with extraneous material at relatively shallow depths (e.g. Storey et al., 1989, 1992; Mahoney et al., 1995, 1996; Douglass & Schilling, 1999). The anomalous low 238U/204Pb oceanic material could be stranded continental lithosphere, thermally eroded from post-Gondwana continents (e.g. Mahoney et al., 1992, 1995; Storey et al., 1992). Other investigators have suggested that material with low 143Nd/144Nd and 238U/204Pb could be situated in the deep mantle as a plume component (e.g. Weis et al., 1993, 1998; McKenzie & O’Nions, 1995; Mahoney et al., 1996).
Most of the evidence for the presence of continental crust in the lithosphere of the Indian Ocean supports the model of continental crust assimilation in the petrogenesis of anomalous tholeiitic melts rather than the contamination of their mantle source. From the investigations by Hassler & Shimizu (1998) and Mattielli et al. (1999), it is established that the Os–Sr–Nd–Pb compositions of some Kerguelen Archipelago ultramafic xenoliths reflect incorporation of fragments of Gondwanan lithosphere into the northern Kerguelen Plateau lithosphere. In the central part of the plateau, radiogenic helium isotope ratios in basalts from Heard Island lavas (Hilton et al., 1995) and the Sr–Nd–Pb isotopic composition of a trachyte from Heard Island (Barling et al., 1994) also support shallow-level contamination of oceanic magmas by continental lithosphere. Grégoire et al. (1994) reported abundant mafic granulite xenoliths in Kerguelen Archipelago lavas. Granulites require high temperatures and pressures for their formation, corresponding to depths of 20–45 km (Grégoire et al., 1994), and are therefore normally found only in continental settings where the thick crust provides the conditions required for the development of granulite-facies mineralogy (e.g. Storetvedt, 1997). However, there is no evidence for old continental crust from available Sr–Nd–Pb–Os isotopic compositions of the Northern Kerguelen granulite xenoliths (Mattielli et al., 1996; Hassler, 1999). Results of geophysical and morphological investigations of Broken Ridge performed during the voyage of the Russian research ship Georgiy Maksimov in 1979, coupled with results of palynological investigation of DSDP (Kemp & Harris, 1975), support a continental source for microflora from part of the Broken and Ninetyeast Ridges (Udintsev & Koreneva, 1982). According to those studies, the pollen and spores from Sites 214 and 254 are typical of continental floras for Southern Hemisphere rather than floras of the oceanic islands. Occurrence of such flora in these sites requires a very near source of the continental crust; therefore a fragment of Gondwanan crust could be involved in parts of the Ninetyeast and Broken Ridges (Udintsev & Koreneva, 1982; Udintsev et al. 1990). However, available Sr–Nd–Pb isotopic data on the Ninetyeast Ridge lavas do not support the hypothesis of continental crust involvement (e.g. Weis & Frey, 1991; Weis et al., 1991; Frey & Weis, 1995). Existing geophysical and geological studies suggest the possibility of the presence of stretched fragments of continental lithosphere in the Kerguelen Plateau (Recq et al., 1994; Operto & Charvis, 1995, 1996). According to geophysical data, the structure of the southern domain of the Kerguelen Plateau is similar to that of modern volcanic continental margins (Schaming & Rotstein, 1990). Discovery of garnet-bearing gneisses in a conglomerate interbedded with basalt at a Leg 183 drillsite on Elan Bank is the first unequivocal evidence of continental crust in the Kerguelen Plateau (Frey et al., 2000). The data suggest that contamination by continental margin crust is the most plausible hypothesis for the origin of the anomalous oceanic tholeiites from the southern Kerguelen (Site 738) and Naturaliste plateaux. Therefore, we conclude that the broad trends in Figs 6, 11 and 12 for the anomalous oceanic tholeiites formed in the Indian Ocean during the period 115–75 Ma represent the effect of the melt contamination by continental crust. The crustal material could have been isolated for >109 years within a craton of, and later as a fragment of Gondwanan lithosphere or post-Gondwanan continental margin.
Several investigators of the Atlantic Ocean also have documented occurrences of ancient crust close to modern spreading ridges. Pilot et al. (1998) described Palaeozoic and Proterozoic zircons in gabbros drilled from the Mid-Atlantic Ridge. Those workers emphasized that these zircons could not have resided in a shallow region of the upper convecting mantle for more than 1 my without totally losing their radiogenic lead, but could have been stored in cold detached slices of continental lithosphere. Also, Precambrian gneiss has been dredged at the Mid-Atlantic Ridge at 26°N, a region believed to be outside the influence of ice-rafted debris (Belyatsky et al., 1997). Bonatti et al. (1996) showed that ancient sedimentary rocks also occur close to the Mid-Atlantic Ridge, and argued that the sedimentary sequence was deposited in a palaeo-Romanche continent–continent transform fault during the opening of the Atlantic Ocean with subsequent ridge jumping and transform migration (Bonatti & Crane, 1982; Kepezhinskas & Dmitiev, 1992). In this case, beneath the transform fracture there may be non-drifting segments where older material could remain for a long period (Bonatti et al., 1996). Therefore, most of the evidence for ancient crustal material in oceanic environments suggests a general mechanism for involvement of continental crust in the Indian and Atlantic oceanic lithosphere.
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MAJOR CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC...ANALYTICAL TECHNIQUESRESULTSDISCUSSIONMAJOR CONCLUSIONSAPPENDIXREFERENCES |
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The results of this investigation imply the involvement of lower continental crust in the petrogenesis of the olivine-phyric basalt suite of the Aphanasey Nikitin Rise formed in the Late Cretaceous Indian Ocean. The parental magma of the basalt suite is a hypersthene-normative tholeiitic composition with MgO (9·4 wt %), low H2O contents (0·3–0·5 wt %), and crystallized with liquidus olivine and spinel and under redox conditions (WM buffer) typical of MORB. The high TiO2, K2O and P2O5 contents and high SiO2/Al2O3 of this parental melt, however, differ from those of MORB. The olivine-phyric basalt suite is characterized by Nb, Ta, Th and U depletion and low 206Pb/204Pb and 143Nd/144Nd more extreme than any MORB or OIB. Our modelling demonstrates that fractional crystallization of depleted mantle-derived melt and lower continental crust assimilation is a suitable model for the generation of the parental magma of the olivine-phyric basalt suite. Pb and Nd model ages suggest that the lower continental crust material was isolated for >109 years, probably in cratonic Gondwanan lithosphere.
We distinguish two groups of trace-element and Sr–Nd–Pb isotopic compositions among anomalous tholeiites of Aphanasey Nikitin Rise, the Kerguelen and Naturaliste plateaux and Broken Ridge, which all formed in the Indian Ocean basin in the period 115–75 Ma. Melt compositions of both groups range from hypersthene-normative basalt to basaltic andesite and are characterized by Nb and Ta depletion and extremely low 143Nd/144Nd. The first group, defined by the olivine-phyric basalt suite of the Aphanasey Nikitin Rise, is characterized by Th depletion and unradiogenic Pb isotopic composition. The second group, defined by tholeiitic suites at Sites 738 and 264 on the Kerguelen and Naturaliste plateaux and sites on the Broken Ridge (D8, M-D8, M-D10), demonstrates Th enrichment and more radiogenic Pb and Sr isotopic composition. These compositions overlap with those of continental tholeiites from the Rajmahal traps and Bunbury province, which pre-date the break-up of Gondwana. The compositional range for this second group of oceanic tholeiites can be explained by shallow-level contamination of the parental melts by middle or upper continental crust isolated for >109 years within a craton of, and later as a fragment of, Gondwanan lithosphere or the post-Gondwanan continental margin. This conclusion, combined with other evidence for the presence of ancient crust material in oceanic environments, suggests a general mechanism for involvement of continental crust in the oceanic lithosphere.
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APPENDIX |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL AND TECTONIC...ANALYTICAL TECHNIQUESRESULTSDISCUSSIONMAJOR CONCLUSIONSAPPENDIXREFERENCES |
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Estimation of the composition of wallrock for parental melt of the Aphanasey Nikitin Rise olivine-phyric suite based on wallrock assimilation and fractional crystallization
To estimate the composition of the wallrock assimilated by the parental melt of the Aphanasey Nikitin Rise olivine-phyric suite, we used the assimilation–fractional crystallization (AFC) model equations of DePaolo (1981)
for D << 1: Cm/Cm° = F-1{1 -(Ca/Cm°)[r/(r -1)]}, where Cm is the trace element concentration in the contaminated magma, Cm° is the initial element concentration in the magma and Ca is the elemental concentration in the wallrock; F = Mm/Mm°, where Mm is the mass of magma and Mm° is the initial mass of magma; and r = Ma/Mc, where Ma is the assimilation rate (mass/unit time) and Mc is rate at which fractionating phases are being effectively separated from the magma. The value of Ca can be calculated from the equation Ca = {1 -(Cm/Cm°)F[(r -1)/r]}Cm°. Assuming F ranges from 0·1 to 0·95 and various Cm°, different elemental ratios in the wallrock can be calculated. The ratios depend not on r but on Cm°. Trace element concentrations in the contaminated magma (Cm) are estimated as the average composition of the freshest basalt (M1/34-5) and from melt inclusions (Tables 1 and 2b). Trace element concentrations in the initial magma (Cm°) are those of average N-type, T-type and E-type MORB, where N-type and E-type MORB are from Sun & McDonough (1989). T-type MORB is calculated as [(CiN-type MORB + CiE-type MORB)/2], where the compositions of N- and E-type MORB are from Sun & McDonough (1989).
To estimate the isotopic composition of the wallrock assimilated by the parental melt of the Aphanasey Nikitin Rise olivine-phyric suite, we used the AFC model equations of DePaolo (1981) for D << 1: (Em - Em°)/(Ea - Em°) = [1 + Cm°Mm/(CaMa)], where Em is the isotopic ratio in the contaminated magma, Em° is the initial isotopic ratio in the magma and Ea is the isotopic ratio in the wallrock, Mm is the mass of magma and Ma is the assimilation rate (mass/unit time). Ea can be calculated from the equation Ea = Em° + (Em - Em°)[1 + Cm°Mm/(CaMa)]. Assuming Ma/Mm and Ca/Cm° ratios are in the range 0·1–0·3 and 10–100, respectively (DePaolo, 1981), the isotopic ratio in the wallrock can be estimated. Isotopic compositions in the assimilated magma (Em) are the age-corrected isotopic compositions of leached olivine-phyric basalts (Table 5). Isotopic compositions of the initial magma (Em°) are the average composition of the Aphanasey Nikitin Rise plagioclase-phyric basalts and Indian MORB (Mahoney et al., 1992) taken to be depleted mantle-derived melts (206Pb/204Pbt = 18·0, 207Pb/204Pbt = 15·6, 208Pb/204Pb = 38·0, 87Sr/86Srt = 0·704, 143Nd/144Ndt = 0·51269; and 206Pb/204Pb = 18·0, 207Pb/204Pb = 15·5, 208Pb/204Pb = 38·0, 87Sr/86Sr = 0·703, 143Nd/144Nd = 0·51310, respectively). Ca/Cm° ratios in the range 1–10 have been used for estimation of Sr isotopic composition.
Estimation of the isotopic composition of a continental component in the ‘contaminated’ mantle source of the Aphanasey Nikitin Rise olivine-phyric suite
To estimate the isotopic composition of the continental component involved in the ‘contaminated’ mantle source, we used the equation of Langmuir et al. (1978): (87Sr/86Sr)A = (87Sr/86Sr)M(1 + {[Sr]B/[Sr]A}(1 - f)/f) - (87Sr/86Sr)B{[Sr]B/[Sr]A}(1 - f)/f, where A, M and B are the continental component, ‘contaminated’ mantle and uncontaminated mantle, respectively, and f is the fraction of continental component. We used the average isotopic composition of the olivine-phyric suite as the composition of the ‘contaminated’ mantle source (M), the trace element composition of primitive mantle (Sun & McDonough, 1989), the average isotopic composition of plagioclase-phyric basalts as uncontaminated mantle (B), and the trace element concentrations in average continental crust material (Rudnick & Fountain, 1995) for the continental component composition (A). Assuming that the fraction of the continental component (f) is in the range 0·1–0·3 and Pb, Sr and Nd concentrations in the continental material are in the range 4–20 ppm, 280–350 ppm and 11–26 ppm, respectively (Rudnick & Fountain, 1995), it is possible to estimate the isotopic composition of the continental component (A).
Estimation of the model age of the continental component for the Aphanasey Nikitin Rise olivine-phyric suite
Pb model ages and values of model 238U/204Pb for the continental end-member are based on the age-corrected Pb-isotopic composition of leached basalts and the two-stage model of Stacey & Kramers (1975). Nd model ages are based on the isotopic composition of present-day 143Nd/144Nd ratios and 147Sm/144Nd ratios of leached basalts using the two-stage model of Milisenda et al. (1994) assuming for continental material 147Sm/144Nd = 0·12 (Milisenda et al., 1994) and for the N-MORB source 143Nd/144Nd = 0·513151 (Goldstein et al., 1984) and 147Sm/144Nd = 0·24.
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ACKNOWLEDGEMENTS |
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The first author dedicates this work to the memory of her sister Ekaterina Yu. Borisova and mother Elena G. Borisova. We thank F. A. Frey, R. W. Kent, J. J. Mahoney, J. Scoates, A. V. Sobolev, G. B. Udintsev, D. Weis, M. Wilson and G. Wörner for reviews and very important comments and suggestions that significantly improved the manuscript, and J. Barling, S. Ingle and N. Mattielli for their help in discussion and paper preparation for publication. The first author thanks G. Wörner for opportunity to work on this manuscript in Göttingen University (Germany). We also thank K. Simon and G. Hartmann for help in conducting the ICP-MS and XRF analyses in Göttingen University. This work would not have been possible without support of a grant from Universitätsbund der Georg August Universität Göttingen. The analytical studies using SIMS and TIMS methods were supported by a grant from the Russian Scientific Foundation (96-0565569). The first author is carrying out post-doctorate studies in the Université Libre de Bruxelles with a grant of the Communauté Française de Belgique–Direction de la Recherche Scientifique–Actions de Recherche Concertées.
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FOOTNOTES |
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*Corresponding author. Present address: Vernadsky Institute of Geochemistry and Analytical Chemistry, Kosygin st. 19, 117975, Moscow, Russia. E-mail: aborisso{at}ulb.ac.be and anastassiafr{at}yahoo.fr
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