Journal of Petrology Advance Access published online on January 22, 2009
Journal of Petrology, doi:10.1093/petrology/egn082
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Sr–Nd–Pb–He–O Isotope and Geochemical Constraints on the Genesis of Cenozoic Magmas from the West Antarctic Rift
1Dipartimento di Scienze della Terra, Università di Pisa, 56126 Pisa, Italy; Istituto di Geoscienze e Georisorse (IGG-CNR), Via Moruzzi 1, 56124 Pisa, Italy
2Dipartimento di Scienze della Terra, Università di Pisa, 56126 Pisa, Italy
3Istituto di Geoscienze e Georisorse (IGG-CNR), 56124 Pisa, Italy
4School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9Pl, UK
Received May 21, 2008; Revised typescript accepted December 24, 2008
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONTHE WEST ANTARCTIC RIFT...WHOLE-ROCK DATAOLIVINE DATADISCUSSIONCONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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The West Antarctic Rift System (WARS) represents one of the major active continental extension zones on Earth. The Ross Sea coast in northern Victoria Land (NVL) is littered with alkaline rift-related igneous products (Middle Eocene–Present). This study characterizes the nature of the magma source involved in the rift process through geochemical–isotopic investigation of Cenozoic basalts from NVL, and provides important constraints for the reconstruction of the tectono-magmatic evolution of the Ross Sea region–WARS. The chemical compositions of the basalts (Miocene–Present) display major and trace element characteristics typical of ocean island basalts (OIB), with strong enrichment in the most incompatible elements. Whole-rock isotopic compositions are in the range 0·7028–0·7034 for 87Sr/86Sr, 0·5129–0·5130 for 143Nd/144Nd (
Nd(t) 
4·8–6·7), 19·3–19·7 for 206Pb/204Pb, 15·4–15·6 for 207Pb/204Pb and 38·7–39·3 for 208Pb/204Pb, suggesting a HIMU-like (high U/Pb) signature of the mantle source. Determinations of 3He/4He on crushed olivine yielded values between 5·7 and 7·2 times the atmospheric ratio, similar to the lithospheric mantle and in the range of mid-ocean ridge basalts (MORB). The 
18Ool of olivine separates varies from 4·92 to 5·53
and is positively correlated with Fo content. Integration of our geochemical and isotope data with available geological, geophysical and geochronological data has led to the following reconstruction. The differences in the oxygen isotope values principally reflect the involvement of a heterogeneous mantle source and/or the assimilation of variable amounts of hydrothermally altered crustal rocks from the volcanic edifices. The 3He/4He data allow us to exclude a plume-driven model to explain the continuing rifting process. Based on the evidence of metasomatic processes, we propose a model to generate the mantle source(s) of the Cenozoic basaltic melts of the NVL. This is sublithospheric mantle metasomatized during an amagmatic extensional event that affected the WARS in the Late Cretaceous. During Eocene–Oligocene times, mantle flow warmed the mantle at the edge of the thick Antarctic lithosphere, and the reactivation of old translithospheric discontinuities promoted mantle melting and the rise of magmas as plutons and dyke swarms. From the Late Miocene to Present, the continuing craton-directed mantle flow led to normal faulting of the rift shoulder, which favoured the rise of magmas to build up large volcanic edifices. KEY WORDS: Antarctica; geochemistry; Sr–Nd–Pb–He–O isotopes; lithospheric mantle; metasomatism
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONTHE WEST ANTARCTIC RIFT...WHOLE-ROCK DATAOLIVINE DATADISCUSSIONCONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Studies on the evolution of continental rift systems and rifted continental margins have led to the development of many hypotheses about their origin. A classic element of controversy is whether rift zones are produced by actively upwelling mantle splitting the continent, possibly along pre-weakened zones, or whether the mantle rises passively as the continents are pulled apart during lithospheric stretching. A major line of evidence used to solve this question is commonly provided by the geochemical and isotopic compositions of rift-related igneous products. This line of investigation is framed in the overall discussion of the origin of alkaline lavas in intraplate settings, either continental or oceanic.
In the past 20 years, a wealth of new geological, geophysical, geochemical and geochronological data has shown that the West Antarctic Rift System (WARS) is one of the most important intracontinental rifts on Earth. The Middle Eocene to Present alkaline magmatism of the WARS has been interpreted in terms of different types of rift activity. Several papers have invoked the occurrence of an active plume centred below either Marie Byrd Land or Victoria Land–Mt. Erebus (LeMasurier & Rex, 1989
; Behrendt et al., 1991; Hole & LeMasurier, 1994; LeMasurier & Landis, 1996; Storey et al., 1999). This hypothesis is based on: (1) the similar geochemical characteristics of basalts from the WARS and ocean island basalts (OIB) associated with long-lived hotspot tracks; (2) the occurrence of an uplifted dome in Marie Byrd Land accompanied by horst–graben sub-ice topography; (3) the modest Cenozoic extension with respect to the large volume of observed Cenozoic magmatism; (4) the inferred lack of tectonic events coeval within rifting and volcanism in West Antarctica; (5) the high heat flow in the Ross Sea area. On the basis of the geochemical characteristics of the magmatic rocks, this model has been progressively extended to various parts of the rift.
Other studies favour a passive model in which the source for all the HIMU volcanism in the SW Pacific can be related to a ‘fossil’ plume head fixed at the base of the lithosphere prior to the mid-Cretaceous break-up of New Zealand from Antarctica (Rocholl et al., 1995; Hart et al., 1997; Panter et al., 2000).
Alternative interpretations to both plume-driven and passive rifting have been proposed by Rocchi et al. (2002, 2003, 2005), which suggest that magma genesis and emplacement is due to the reactivation of pre-existing NW–SE translithospheric faults (Salvini et al., 1997), which promoted local decompression melting of an enriched mantle previously veined during a Late Cretaceous amagmatic extensional rift phase. This hypothesis was developed by integrating structural, geochronological and geochemical observations such as the following: (1) the dominance of Cenozoic geodynamics by intraplate right-lateral strike-slip tectonics, which induced a significant oblique component to the rifting process; (2) the geometric link of the rift-related magmatism to the interplay between NW–SE Palaeozoic faults (linked with the major transform faults in the Southern Ocean) and north–south late Cretaceous extensional faults; (3) the absence of doming on the western coast of the Ross Sea, and rather, the presence of a linear surface uplift dissected by transverse fault systems in northern Victoria Land (NVL); (4) the occurrence of the main extensional episode during the Late Cretaceous, and the beginning of the main denudation–uplift event in the Middle Eocene, coeval with the onset of magmatic activity.
Another model, proposed by Finn et al. (2005), suggests that swift detachment and sinking of Palaeozoic–Mesozoic (500–100 Ma) subducted slabs in the Late Cretaceous induced instabilities along the former Gondwana margin that triggered lateral and vertical flow of warm Pacific mantle. The interaction between the warm mantle and the metasomatized subcontinental lithosphere of the SW Pacific magmatic province would have concentrated magmatism along zones of weakness. In particular, Panter et al. (2006) have suggested that the mantle sources of the alkaline magmatism represent continental lithosphere that hosted amphibole- or phlogopite-rich veins formed by plume- and/or subduction-related metasomatism between 500 and 100 Ma.
This study characterizes the mantle source(s) involved in the WARS igneous province through geochemical and isotopic studies of Cenozoic basalts from NVL, and focuses on the reconstruction of the links between the WARS–Southern Ocean evolution and the development of the source-melt system of the western Ross Sea area.
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THE WEST ANTARCTIC RIFT SYSTEM |
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TOPABSTRACTINTRODUCTIONTHE WEST ANTARCTIC RIFT...WHOLE-ROCK DATAOLIVINE DATADISCUSSIONCONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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The WARS (Fig. 1) is bordered on the west by a high-standing rift flank, the Transantarctic Mountains (TAM). Geophysical data indicate a crustal thickness of the order of 40 km beneath the TAM, which rapidly decreases to 25 km beneath the coast of the Ross Sea and reaches a minimum of about 18 km in the Victoria Land Basin (Della Vedova et al., 1997
).
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Since the Middle Eocene, the WARS was affected by widespread alkaline igneous activity in northern Vicoria Land, mainly developed at the intersection of NW–SE and north–south extensional faults (Salvini et al., 1997
). Plutons and dyke swarms, collectively named the Meander Intrusive Group (Müller et al., 1991; Tonarini et al., 1997), and volcanic products of the McMurdo Volcanic Group (MMVG; Harrington, 1958) are exposed in the coastal area of Victoria Land. Plutons crop out in a 200 km long section of the western rift shoulder (between the Campbell and Leap Year Faults), and are usually associated with strong positive magnetic anomalies (Müller et al., 1991). Their age is between 48 and 23 Ma (Müller et al., 1991; Tonarini et al., 1997). Dykes occur over a widespread area and crosscut both Palaeozoic basement and Cenozoic igneous complexes over a time span of 48–34 Ma (Tonarini et al., 1997; Rocchi et al., 2002). Volcanic edifices that can be recognized within the MMVG are basaltic shield volcanoes, stratovolcanoes with dominant trachytic and phonolitic compositions, and small, isolated basaltic scoria cones and lava flows. Plutonic and volcanic activities represent, respectively, the earliest (45–23 Ma) and the latest (19–0 Ma) (Armienti & Baroni, 1999) records of regionally extensive rift-related magmatic activity. |
WHOLE-ROCK DATA |
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TOPABSTRACTINTRODUCTIONTHE WEST ANTARCTIC RIFT...WHOLE-ROCK DATAOLIVINE DATADISCUSSIONCONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Petrography and trace element geochemistry
Samples were collected on the Ross Sea coast, between the Tucker and Priestley glaciers and on Coulman Island, from monogenetic scoria cones where small lava flows often occurred at the end of the explosive activity (Fig. 1).
Textures vary from sub-aphyric to weakly porphyritic. Olivine is generally the only phenocryst phase, thus representing the first liquidus phase, although some of the samples show also minor amounts of augitic clinopyroxene phenocrysts and sporadic olivine and orthopyroxene xenocrysts. Olivine phenocrysts are usually euhedral with polygonal habit and frequently contain subhedral to euhedral spinel inclusions and rare multiphase melt inclusions (Orlando et al., 1997). Microphenocrysts are represented by unzoned clinopyroxene and euhedral olivine. The groundmasses vary from microcrystalline to cryptocrystalline and are composed of clinopyroxene, opaque minerals, plagioclase and sometimes olivine. Interstitial nepheline often occurs in the most crystalline varieties. Apatite is a common accessory phase in all the studied rocks. Olivine and orthopyroxene xenocrysts are usually rounded in shape, and their size is usually larger than that of phenocrysts. Olivine xenocrysts always display kink banding whereas orthopyroxene xenocrysts are mantled by coronas of clinopyroxene and sometimes olivine.
Major and trace elements analyses are reported in Table 1. The selected lavas are basanites and basalts, with the exception of one sample (SAX20), which is an olivine nephelinite. In the total alkalis vs silica classification diagram, they plot within the alkaline field (Fig. 2). The SiO2 content is in the range of 41–46 wt %, with K2O/Na2O values from 0·29 to 0·44 revealing a distinct sodic affinity (Fig. 2). The overall set of samples has 59·3 < Mg-number < 71·8 [Mg-number = 100 Mg/(Mg + Fe2+)]. Cr contents range from 360 to 713 ppm. According to their CIPW norms, all the rocks are ne-normative, except for one, which is hy-normative (sample MB65b; Table 1). Variations in incompatible element ratios are very small; for example, La/Nb (< 0·80) and Ba/Nb do not vary with magnesium content (MgO 8–12 wt %). The chondrite-normalized distribution of the rare earth elements (REE) in the lavas (Fig. 3) shows similar patterns for all the samples, with significant light REE (LREE) enrichment (LaN 120–180) with respect to chondritic values. The Eu anomaly is absent or slightly positive (Eu/Eu* 0·95–1·06) without evidence of cumulus plagioclase. All the patterns are very similar, with the exception of the olivine nephelinite (SAX20), which is extremely enriched in REE (LaN 530). The multi-element patterns normalized to the Primordial Mantle composition (Fig. 4) reveal high ratios of Nb–Ta to large ion lithophile elements (LILE; Ce, Rb, K, Ba, Sr, Eu) and high primitive mantle-normalized La/K. The negative anomalies of K and Pb are prominent and occur in all samples. Sample SAX20 is the most enriched in incompatible elements and, consequently, shows the most prominent negative K anomaly.
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Sr, Nd and Pb isotope ratios
Analysed samples from NVL have relatively low 87Sr/86Sr, moderately high 143Nd/144Nd and high 206Pb/204Pb isotopic values (Table 2). Isotopic compositions are in the range of 0·7028–0·7034 for 87Sr/86Sr, 0·51288–0·51298 for 143Nd/144Nd (corresponding to
Nd(t = 0) 4·8–6·7), 19·3–19·7 for 206Pb/204Pb, 15·4–15·6 for 207Pb/204Pb and 38·7–39·3 for 208Pb/204Pb. In Figs 5 and 6 the data are compared with the mantle end-members of Zindler & Hart (1986) and with other OIB occurrences according to their geographical location. Figure 5 shows the Nd(t = 0)–87Sr/86Sr correlation diagram. The samples plot in the depleted quadrant of the diagram and display a negative correlation. The 206Pb/204Pb isotope ratios are fairly similar when compared with the relatively variable 87Sr/86Sr and 143Nd/144Nd values (Fig. 6a and b). The most radiogenic Nd isotope composition is displayed by sample SAX20.
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OLIVINE DATA |
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TOPABSTRACTINTRODUCTIONTHE WEST ANTARCTIC RIFT...WHOLE-ROCK DATAOLIVINE DATADISCUSSIONCONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Chemistry
Petrographic and geochemical data combined with published experimental work (Orlando et al., 1997
, 2000; Perinelli et al., 2006) for the studied lavas suggests that they are relatively primitive. For this reason the study of olivine phenocrysts is particularly suited to give geochemical and thermal information about the mantle source region.
The olivine grains are 0·5–2·0 mm in size and have a pale green body colour. Their forsterite contents are in the range of Fo92–66 (Table 3), indicating different degrees of evolution of the melt from which they crystallized. Zoning is weak or absent. Ni and Cr are above the detection limit, as expected in Mg-rich olivine; chromium usually occurs in minute exsolved lamellae of chromite. Olivines with CaO contents lower than 0·10 wt % and with very high Fo (90–80 mol %) (Fig. 7) overlap the composition of olivine from peridotite xenoliths from Mt. Melbourne (Zipfel & Wörner, 1992). On the basis of more than 145 electron microprobe analyses, most of the olivine compositions (MgO and CaO) are in equilibrium with those of the whole-rocks (Roeder & Emslie, 1970; Jurewicz & Watson, 1988); some of the olivine from samples SAX20 and MB33 is clearly derived from mantle xenoliths, which show pronounced kink-banding and irregular shapes typical of disequilibrium crystals. Microthermometric experiments on primary melt inclusions hosted by olivine (samples MB33, MP34, MP24) gave homogenization temperatures of 1260–1280°C (Orlando et al., 1997). The very low crystal content and the Mg-rich composition of the olivines suggest that their host magmas are close to their liquidus. Consequently, these T values are probably similar to the potential temperature of the mantle source of these melts (Green & Falloon, 2005).
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He isotope ratios
Early formed Fe–Mg phenocryst minerals (olivine and pyroxene), grown before extensive degassing and crustal contamination, may preserve the noble gas isotopic signature of the mantle source. In particular, olivine contains negligible U and Th compared with clinopyroxene and is therefore less susceptible to radiogenic 4He ingrowth. The olivine in the selected samples trapped magmatic volatiles in primary melt inclusions (Orlando et al., 1997
). We report the noble gas data in Table 3.
Values for 3He/4He from the NVL range between 5·73 ± 0·15 and 7·22 ± 0·26 Ra and 40Ar/36Ar vary between the atmospheric value of about 300 (air 295·5) and a slightly more radiogenic value of 750. To remove any influence of atmospheric helium contamination within these analyses, air contamination corrections were applied (Harrison et al., 2004), indicating negligible contamination effects (shifts in 3He/4He were typically 
0·01 Ra). It is necessary also to consider whether post-eruption radiogenic 4He ingrowth or cosmogenic 3He has affected the measured 3He/4He. It should be noted that both these processes produce nuclides that are sited in the crystal matrix. All the analyses were performed using a pressure fracturing mechanism to release volatiles from the samples (Burnard et al., 2003). Pressure fracturing crushers are designed to release volatiles from fluid inclusions and not matrix-sited volatiles. Only a single step was performed on each sample and the crushing efficiency was of the order of 10–15%. This degree of crushing is highly unlikely to release any significant amounts of matrix-sited gases (Scarsi, 2000). Moreover, crushing efficiency was approximately the same for all samples, thus allowing a comparison between samples. The helium measured can therefore be considered representative of the fluid inclusions hosted by the phenocrysts.
In Fig. 8 3He is plotted against 4He, normalized to 40Ar. Two lines are plotted that represent 7Ra and 5Ra at variable helium/argon ratios and therefore helium concentrations. The addition of excess radiogenic 4He would cause the data to deviate to the right as shown by the arrow on the diagram. All the data plot between the two modelled compositions. Figure 9 is a helium isotope vs 4He concentration diagram. This re-emphasizes that the helium isotope compositions are not controlled by the concentration of radiogenic 4He and can thus be considered representative of the magmatic source. Also, plotting helium isotope ratios against the forsterite content of the analysed olivine (or Mg-number; not shown) shows no correlation. This reveals that magma differentiation had no effect on the He mantle signature.
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O isotope ratios
Oxygen isotopic compositions of olivine are listed in Table 3. Measured oxygen isotope ratios range from 4·92 to 5·53
. Separated olivines, rather than whole-rock powders, have been analysed to avoid any possible effects of post-eruptive alteration (Muehlenbachs, 1986; Harmon & Hoefs, 1995). In addition, olivine crystallizes early in the magmatic evolution of basaltic lavas and is present as a major constituent in the mantle, therefore the effects of fractionation during crystallization or low degrees of partial melting are minimal (Taylor & Sheppard, 1986; Harmon & Hoefs, 1995). No correlation between olivine 18O and 3He/4He ratios exist. The absence of correlations between 18Ool and whole-rock SiO2, MgO or Mg-number, and Fe contents, which might be expected, could reflect the presence of cumulus olivine in the basalts. However, the positive correlation between 18Ool and Fo content (R2 = 0·63) indicates that oxygen isotope composition of olivine was not controlled by crystallization from progressively differentiating magmas (Fig. 10). An exception to this trend is represented by one of the HP–HT olivines, which shows a low 18O value (sample MB27 with 18Ool = 5·0).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONTHE WEST ANTARCTIC RIFT...WHOLE-ROCK DATAOLIVINE DATADISCUSSIONCONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Origin of oxygen isotope variability
Most mantle-derived magmas span a narrow range in
18O values [i.e. 18Ool between 5·0 and 5·4, or 18Omelt between 5·4 and 5·6 (Eiler, 2001)], which implies that recycled crustal materials are absent in the mantle source or are less than a few per cent. Analysed samples that are outside this range require specific explanations for the origin of the isotopically anomalous material in the mantle source region. The measured oxygen isotope ratios in olivine crystals from this study range from 4·92 to 5·53; thus they only slightly deviate from the unmodified mantle composition. However, the 18Ool decreasing trend with decreasing Fo content suggests a possible contribution of a low 18O contaminant at mantle depths during olivine crystallization. The only major Earth reservoir with a 18O value significantly lower than average mantle is the hydrosphere; crustal rocks can record this value as a result of high-temperature water–rock interaction. The greatest volume of low 18O rocks produced by this process is in the oceanic lithosphere, which undergoes extensive hydrothermal alteration at mid-ocean ridges (Shanks, 2001).
Two main processes occurring at mantle conditions may account for the observed low 18O values: recycling of subducted hydrothermally altered lower oceanic lithosphere (Eiler et al., 1997; Eiler, 2001) and/or assimilation of low 18O wall-rocks during the course of magma crystallization. A key aspect of all low 18O components is that they must be sampled in large mass fractions to produce measurable decreases in the 18O of basaltic lavas. Mass-balance calculations indicate that lowering the 18O average value of olivine crystallized from a primary mantle melt from 5·5 to 4·9 would require at least 10% of a contaminant with 18O 0 (i.e. the lowest portions of oceanic lithosphere sampled as a subducted component or shallow-level contaminant). The fact that one of the HP–HT olivines shows a depleted 18O value (sample MB27 with 18O = 5·0), highlights the possibility that the mafic melts originated from a heterogeneous source that had already inherited a low 18O signature. Accordingly, higher Fo olivines crystallized from the more basic magmas, which preserved their original heterogeneous 18O values, whereas lower Fo olivines crystallized from slightly more evolved magmas, which received a greater input of the low 18O component during their rise through the mantle lithosphere (Eiler et al., 1997; Garcia et al., 1998).
The possible interpretations of the contamination process are: (1) assimilation of the ultramafic portions of subducted oceanic lithosphere with low 18O values resulting from seawater–rock interaction at a mid-ocean ridge prior to subduction (Muehlenbachs, 1986; Hart et al., 1999; Panter et al., 2003); (2) assimilation of hydrothermally altered intrusive igneous rocks emplaced within the upper crust during the Middle Eocene (Dallai et al., 2001); (3) assimilation of material from hydrothermally altered volcanic edifices (Widom & Farquhar, 2003; Wang & Eiler, 2008). Because the studied basalts show no evidence of Sr–Nd–Pb isotope variation or major or trace element systematics typical of crustal contamination, the contaminant needs to be similar in composition to the primary basalts. The lack of oceanic lithosphere (vs thinned continental lithosphere) in the Ross Sea region and the nature of the mostly acidic (not mafic) hydrothermally altered intrusive rocks suggest that hydrothermally altered volcanic edifice rocks are the likely effective agents of contamination.
No plume evidence from Helium isotope data
High 3He/4He ratios (up to 50 Ra, Stuart et al., 2003) have generally been used to argue that the deeper mantle is relatively undegassed and preserves its primordial volatile composition compared with the MORB source region, which has a lower time-integrated 3He/(U + Th) (8 Ra, Graham, 2002; Harrison & Ballentine, 2005); however, some attention should be paid to shallower mantle processes (Meibom et al., 2003).
The 3He/4He ratios of the NVL basalts are 6–7 Ra and, as previously argued, can be considered as representative of the helium isotope signature of their mantle source. These data are similar to lithospheric mantle values (Gautheron & Moreira, 2002) and overlap the lower limit for some MORB data (Harrison et al., 2003). The He isotope ratios do not support the involvement of deep undegassed material (i.e. a mantle plume) in the genesis of NVL alkaline basaltic melts. In this context the helium isotope data provide an important constraint on the Ross Sea Cenozoic magmatism, for which a plume model must probably be excluded.
The mantle source of the rift-related magmas
Cenozoic basaltic rocks from northern NVL are weakly porphyritic with high Mg-number and Cr contents that indicate compositions close to primary magmas, and thus are suitable samples with which to investigate the nature of their mantle source. Moreover, olivine–melt equilibrium indicates T of about 1200–1300°C, in agreement with the homogenization temperatures found by Orlando et al. (1997) for melt inclusions hosted in olivine. To reconstruct the liquidus curve and to establish the phase relations of a near-primary magma (basanite MP24), Orlando et al. (2000) carried out several high-T, high-P experiments. Their results suggested that the basanite could have been generated by partial melting either of a spinel lherzolite (at P = 1·5–2·0 GPa and T = 1390–1490°C) or of a garnet pyroxenite (at P > 3·0 GPa and T > 1550°C).
Further insights into the relationships between the mantle source(s) and the partial melts are provided by the CaTs–Ol–SiO2 projection (Fig. 11), in which the less evolved basalts are plotted along with some relevant experimental results. The presence of compositions lying on the En–CaTs join provides strong evidence for an origin from a pyroxenitic source when we consider the evident role of this join as a thermal barrier, which is active at P > 2 GPa (Hirschmann et al., 2003). On the other hand, some ne-normative compositions, plotting just to the left of the boundary, are compatible with the experimental data of Hirschmann et al. (1998) and seem to be related to partial melting of a peridotitic source. However, the most undersaturated compositions plot on the pyroxenite melting trend suggested by the high-pressure experiments of Keshav et al. (2004), providing an unambiguous marker of the occurrence in the mantle source of pyroxenite domains. These mantle pyroxenites may represent either deep fractionates of parental melts generated on the thermal divide or the source itself of the nepheline-normative melts, or both. Possible minor fractionations of ± olivine ± pyroxene would not affect these relationships. In this context, the variable degree of silica saturation of the NVL near-primary melts can be accounted for by a two-step process. First, melts that are more enriched in Mg and less saturated in SiO2 are derived from the partial melting of a peridotitic source, producing pyroxenite domains. Second, these pyroxenite domains can themselves become the source of subsequent partial melts which are less enriched in Mg and less understurated.
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The trace element distribution patterns (Fig. 4) are dominated by high ratios of Nb and Ta to LILE and Y–heavy REE (HREE) and high La/K. Multi-element patterns normalized to the Primordial Mantle composition display the typical shape of OIB (McDonough & Sun, 1995
). Ba/Rb and La/Nb ratios display typical OIB values, ranging respectively between 10 and 16 (in contrast to Ba/Rb = 5 and 28 in average upper and lower crust, respectively) and between 0·6 and 0·7 (in contrast to La/Nb = 2·2 of continental crust), and Ba/Nb ratios are very low and constant (5–7, in contrast to crustal reservoirs for which Ba/Nb ranges between 20 and 150), indicating the lack of significant crustal contamination.
The overall enrichment of incompatible elements, as well as the Sr–Nd–Pb isotopic ratios, highlights the contribution of a HIMU-like component to the genesis of the studied samples. This feature seems to be common throughout the WARS (Hart & Kyle, 1994; Hart et al., 1997; Panter et al., 2000, 2003) with 206Pb/204Pb values varying between 19·3 and 20·1 in the western Ross Sea region, whereas in Marie Byrd Land 206Pb/204Pb values vary between 19·5 and 20·9 (Hart et al., 1997; Panter et al., 2000). Such high 206Pb/204Pb values are commonly thought to be derived from deep mantle plumes that include slab materials recycled into the deep mantle over a time period of the order of 109 a. However, it has been suggested that similarly high 206Pb/204Pb ratios in ocean island settings could also result from the enrichment in U relative to Pb of the magma source at rather shallow depths over a time interval of the order of 108 a, similar to the age of the adjacent oceanic lithosphere (Halliday et al., 1995). Furthermore, it is increasingly recognized that HIMU sources may also exist within the sub-continental lithosphere, generated by the infiltration of metasomatic fluids and melts (Stein et al., 1997; Pilet et al., 2005, 2008).
Evidence for metasomatic enrichment processes in the source of the NVL Cenozoic basalts is found in peridotite xenoliths containing amphibole veins and/or pyroxenitic portions with dominant amphibole (Coltorti et al., 2004; Perinelli et al., 2006). These amphiboles could have originated by metasomatic reaction of pre-existing clinopyroxene with a metasomatizing Na-alkaline, highly undersaturated, hydrous silicate melt (Coltorti et al., 2004). This type of veined mantle source readily explains the occurrence of K and Ti (for nephelinitic samples) negative anomalies in the trace element distribution patterns in terms of residual amphibole and/or phlogopite in the mantle source.
Knowledge of the age(s) of this(these) metasomatic event(s) is necessary to improve the geodynamic time-frame of the generation of the magma sources. A rough, yet very useful estimate of such an age can be obtained based on the assumption of a two-stage (Finn et al., 2005) or a multi-stage (Lucassen et al., 2008) mantle evolution, as suggested by petrological constraints. In this context, our modelling assumes that a Bulk Earth–FOZO mantle composition is affected by one or more (auto)metasomatic events, increasing its U/Pb ratio. In detail, the starting mantle composition lies on the evolution line of the ubiquitous FOZO-like mantle, which with a U/Pb value of 0·16 would lead to a present-day 206Pb/204Pb of 19·5. Lead isotope compositions attained at 150, 100 and 50 Ma benchmarks are assumed as further starting points to calculate the Pb isotopic evolution of metasomatically enriched domains in which the U/Pb ratio was increased to values between 0·5 and unity (a value 10% lower than the maximum observed in our samples to take into account the slightly lower incompatibility of U with respect to Pb during melting).
The modelling (Fig. 12) shows that even the highest 206Pb/204Pb values observed for Marie Byrd Land lavas (20·9: Hart et al., 1997; Panter et al., 2000) can be obtained by invoking a metasomatic event at about 150 Ma, significantly younger than that suggested by Finn et al. (2005) and Panter et al. (2006). In particular, the 206Pb/204Pb values observed for the NVL lavas (varying from FOZO-like values to 20·1) can be obtained via two different mechanisms. The first scenario yields HIMU-like Pb isotopic values by means of a moderately recent metasomatic event (100 Ma), which increased the source U/Pb ratios to values of about 0·8. The intermediate values are compatible with melting of a source affected by a more recent metasomatic event (50 Ma), leading to high U/Pb values of unity. The melts with FOZO-like 206Pb/204Pb ratios would be derived from a source that was not affected by metasomatism.
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A simpler, hence preferred, alternative scenario, takes into account the possibility that the melts were derived from variable contributions of metasomatic veins or domains and the surrounding mantle. Specifically, the enriched mantle domains created during the Late Cretaceous and isotopically matured since then underwent a process of ‘sampling upon melting and averaging’ (Meibom & Anderson, 2004
). In this scenario, the isotopic variations of the magmas are linked to slightly different scales of sampling the mantle heterogeneity during melt production and/or averaging during melt collection. The modelling results, coupled with the high U/Pb of the studied lavas, indicate that a likely age for the metasomatism affecting the source of NVL magma could be set in the Late Cretaceous, when a major event of amagmatic orthogonal extension affected the WARS. Lithospheric thinning allowed the rise of the weaker sublithospheric mantle and led to the production of very small degree partial melts, not sufficient to feed surface magmatism (amagmatic rift phase). However, these melts were essential in distributing fertile, enriched, low-melting point veins or domains through a wide zone of the Antarctic plate mantle. This mantle and its metasomatic domains started to age, becoming progressively the new lithospheric mantle underlying the WARS.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONTHE WEST ANTARCTIC RIFT...WHOLE-ROCK DATAOLIVINE DATADISCUSSIONCONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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The data presented here support a model for the origin of the Cenozoic basaltic magmas of the NVL from a sublithospheric mantle source metasomatized during an extensional event that affected the WARS in the Late Cretaceous (Fig. 13a). After metasomatism, the sublithospheric mantle started to age and was rheologically incorporated into the lithosphere. The link between the Cenozoic geodynamic evolution of the WARS region and the development of the source-melt system is illustrated in the following scenario.
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During Eocene–Oligocene times, mantle flow under the extended, thinned part of the WARS, directed towards the thick Antarctic Craton (Figure 13b), provided the heat to warm the mantle at the edge of the thick Antarctic lithosphere (Faccenna et al., 2008
). In the Middle Eocene, the increase in differential velocity across the Southern Ocean fracture zones reactivated Palaeozoic tectonic discontinuities in northern Victoria Land as intraplate dextral strike-slip fault systems (Rocchi et al., 2003, 2005; Storti et al., 2007). The activity of these lithospheric deformation belts induced local mantle decompression melting. Emplacement of the rising magmas as plutons and dyke swarms was related to the main NW–SE discontinuities (Rocchi et al., 2002).
From Late Miocene to Present, the continuing craton-directed mantle flow led to collapse of the rift shoulder and normal faulting (Faccenna et al., 2008), which favoured the rise of magmas to the surface and the construction of large volcanic edifices along north–south normal-transtensional faults arrays departing from the master NW–SE systems (Fig. 13c).
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APPENDIX: ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONTHE WEST ANTARCTIC RIFT...WHOLE-ROCK DATAOLIVINE DATADISCUSSIONCONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Whole-rock analyses
Trace elements were determined by inductively coupled plasma-mass spectrometry (ICP-MS) (Fisons PQ2 Plus®) at the Department of Earth Sciences, University of Pisa. Samples were dissolved in screw-top PFA Savillex® vessels on a hot plate at
120°C with HF–HNO3 and then analysed following the method reported by D’Orazio (1995). Analyses were performed by external calibration using the BE-N basaltic geochemical reference sample (Govindaraju, 1994). The correction procedure includes: (1) blank subtraction; (2) instrumental drift correction using Rh–Re–Bi internal standardization and repeated (every five samples) analysis of a drift monitor; (3) oxide–hydroxide interference correction. Precision, evaluated by replicate dissolutions and analyses of the in-house standard HE-1 (Mt. Etna hawaiite), was generally between 2 and 5% RDS, except for Gd (6%), Tm (7%), Pb and Sc (8%). Detection limits at 6
level are in the range of 0·002–0·02 ng/ml in the solution (corresponding to 0·002–0·02 ppm for a 1000-fold sample dilution) for all elements except for Ba, Pb and Sr (0·1–0·2 ng/ml) (D’Orazio, 1995).
Sr and Nd isotopic compositions were determined on whole-rock powders using a Finnigan MAT 262V multi-collector mass-spectrometer (dynamic state) at the IGG-CNR, Pisa. 87Sr/86Sr was corrected for mass fractionation using 86Sr/88Sr = 0·1194 and 143Nd/144Nd was corrected using 146Nd/144Nd = 0·7219. During the collection of isotopic data, 15 replicate analyses of NIST SRM 981 (SrCO3) gave average values of 0·710200 ± 0·000008 (2) and 14 analyses of La Jolla standard gave an average 143Nd/144Nd of 0·511857 ± 0·000007 (2). Nd was calculated relative to the present-day 143Nd/144Nd = 0·512638. External precision for Sr and Nd was better than 0·000010 for each measurement. Sr and Nd isotope ratios are given as measured values. No age corrections are necessary because of their young age (<18 Ma) and low Rb/Sr and Sm/Nd ratios.
Pb isotope compositions were determined using a Finnigan MAT 262V multi-collector mass-spectrometer (steady state) at the IGG-CNR, Pisa. During the collection of isotopic data, 15 replicate analyses of the NBS 981 standard gave average values of 208Pb/204Pb 37·0613 ± 0·0312 (2), 207Pb/204Pb 15·6061 ± 0·0096 (2) and 206Pb/204Pb 17·0199 ± 0·0070 (2).
Single mineral analyses
Major and trace elements analyses of olivine separates were obtained at the IGG-CNR, Florence using a JEOL JXA-8600 electron microprobe with four wavelength-dispersive spectrometers at 15 kV accelerating voltage and 10 nA beam current (focused beam of about 1 µm in diameter). The applied matrix correction was according to Bence & Albee (1968). Analytical error agrees with the data reported by Vaggelli et al. (1999).
For helium isotope analyses unaltered olivine phenocrysts (0·5–2·0 mm in size), from selected basaltic samples, were ultrasonically cleaned with de-ionized water and finally acetone. After drying, the separates were handpicked under a binocular microscope and then loaded into an online in vacuo crusher system. The crusher and neighbouring sections of the stainless steel extraction line were baked at 150°C under vacuum for 12 h to remove any adsorbed atmospheric gases prior to analysis. Helium isotope, neon abundance and argon isotope determinations were performed using an MAP 215 noble gas mass spectrometer at the Department of Earth Sciences, University of Manchester, following the procedures and the measurements protocols of Harrison et al. (1999, 2003). Gases extracted by crushing were exposed to a SAES Zr–Al getter operating at 250°C to clean and remove any active gas (i.e. N2, O2 and CO2) released during sample crushing. After purification, the gases were condensed onto an activated charcoal finger held at liquid nitrogen temperature (–196°C). The non-condensable species, including helium, neon and H2, were then exposed to another SAES Zr–Al getter, operating at room temperature to reduce any H2 prior to introduction into the mass spectrometer. A second charcoal finger (held at –196°C), adjacent to the source inlet valve, was used to minimize and stabilize the background levels of any residual argon, CO2 and H2O during the helium analysis. 20Ne was measured during the helium run. After helium analysis, the condensable gases were released from the charcoal finger and, prior to expansion into the mass spectrometer for argon analysis, were exposed to a room temperature SAES Zr–Al getter for secondary clean-up. 4He, 36Ar and 40Ar isotopes were measured using a Faraday collector whereas 3He and 20Ne were measured using an electron multiplier in digital pulse counting mode. A resolving power of 650 on the electron multiplier at 5% of the peak height allowed for the complete separation of the 3He+ beam from any interference by the H3–HD+ doublet. Noble gas abundances were calculated by peak height comparison with a known pressure of calibration gas. Analytical blanks were determined prior to crushing steps.
Oxygen isotopes ratios on olivine separates were measured at the IGG-CNR, Pisa, by conventional laser fluorination (Sharp, 1995). To measure the oxygen isotope composition of each phase, 1–1·5 mg aliquots of 0·5–2·0 mm pale green olivine separates were taken and laser fluorinated. A 25 W CO2 laser, operating at a wavelength of 10·6 µm, was used to irradiate the samples placed in a 3 mm diameter hole, within a 32-hole nickel sample plug. Pure fluorine desorbed at 290–310°C from hexafluoropotassium-nickelate salt (Asprey, 1976) and stored in a F2 reservoir, was used as reagent. The O2 produced during laser fluorination was purified from excess fluorine by means of a KCl trap held at 180°C, and the chlorine was trapped cryogenically. The gas was then transferred to a 13 Å molecular sieve-filled cold finger. Further oxygen gas purification was achieved by desorbing oxygen from the molecular sieve at –110°C, using a liquid nitrogen–ethanol mixture. In this way the NF compounds were retained on the 13 Å zeolites (Clayton & Mayeda, 1983). The gas was then analysed for oxygen isotope composition using a Finnigan Delta Plus Mass Spectrometer. On each day of analysis, 4–7 aliquots of our laboratory quartz standards were normally analysed, with an average reproducibility of ±0·14 (1). All samples were analysed in duplicate, with analytical precision of ±0·16 (1) or better. No data correction is necessary for the results, which are reported in the standard per mil () notation. During the time of the study 11 aliquots of NBS-28 quartz standard were measured, yielding an average value of 9·54 ± 0·17 (certified value is 9·60). All 18O values are relative to VSMOW (Vienna Standard Mean Ocean Water) according to the International Atomic Energy Agency.
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ACKNOWLEDGEMENTS |
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This work is dedicated to the memory of Filippo Olmi (EMPA-WDS) for his human and scientific values. The authors wish to thank M. Bertoli (wet chemistry), M. D’Orazio (ICP-MS), G. De Grandis (mineral separations), A. Dini (Pb isotopes), M. Tamponi (XRF) and S. Tonarini (Sr and Nd isotopes) for their expert assistance during laboratory work. We are grateful to Kurt Panter, Godfrey Fitton, Phil Leat and Keith Putirka for journal reviews, which led to a significant improvement of the original manuscript. This work was financially supported by Programma Nazionale di Ricerche in Antartide (PNRA).
*Corresponding author. Present address: Istituto di Geoscienze e Georisorse (IGG-CNR), Via Moruzzi 1, 56124 Pisa, Italy. Telephone: +39 050 3153410. Fax: +39 050 3869104. E-mail: nardini{at}igg.cnr.it
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