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Journal of Petrology | Volume 41 | Number 3 | Pages 411-430 | 2000
© Oxford University Press 2000
Enriched End-member of Primitive MORB Melts: Petrology and Geochemistry of Glasses from Macquarie Island (SW Pacific)
1SCHOOL OF EARTH SCIENCES AND CENTRE FOR ORE DEPOSIT RESEARCH, UNIVERSITY OF TASMANIA, GPO BOX 252-79, HOBART, TAS. 7001, AUSTRALIA
2TASMANIAN GEOLOGICAL SURVEY, PO BOX 56, ROSNY PARK, TAS. 7018, AUSTRALIA
3DEPARTMENT OF GEOLOGY, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
4RESEARCH SCHOOL OF EARTH SCIENCES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
Received October 1, 1998; Revised typescript accepted September 2, 1999
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONTECTONIC SETTING OF MACQUARIE...REGIONAL GEOLOGY OF MACQUARIE...PETROGRAPHY AND MINERALOGYANALYTICAL METHODSCHEMICAL COMPOSITION OF GLASSESDISCUSSIONREFERENCES |
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Macquarie Island is an exposure above sea-level of part of the crest of the Macquarie Ridge. The ridge marks the Australia–Pacific plate boundary south of New Zealand, where the plate boundary has evolved progressively since Eocene times from an oceanic spreading system into a system of long transform faults linked by short spreading segments, and currently into a right-lateral strike-slip plate boundary. The rocks of Macquarie Island were formed during spreading at this plate boundary in Miocene times, and include intrusive rocks (mantle and cumulate peridotites, gabbros, sheeted dolerite dyke complexes), volcanic rocks (N- to E-MORB pillow lavas, picrites, breccias, hyaloclastites), and associated sediments. A set of Macquarie Island basaltic glasses has been analysed by electron microprobe for major elements, S, Cl and F; by Fourier transform infrared spectroscopy for H2O; by laser ablation–inductively coupled plasma mass spectrometry for trace elements; and by secondary ion mass spectrometry for Sr, Nd and Pb isotopes. An outstanding compositional feature of the data set (47·4–51·1 wt % SiO2, 5·65–8·75 wt % MgO) is the broad range of K2O (0·1–1·8 wt %) and the strong positive covariation of K2O with other incompatible minor and trace elements (e.g. TiO2 0·97–2·1%; Na2O 2·4–4·3%; P2O5 0·08–0·7%; H2O 0·25–1·5%; La 4·3–46·6 ppm). The extent of enrichment in incompatible elements in glasses correlates positively with isotopic ratios of Sr (87Sr/86Sr = 0·70255–0·70275) and Pb (206Pb/204Pb = 18·951–19·493; 207Pb/204Pb = 15·528–15·589; 208Pb/204Pb = 38·523–38·979), and negatively with Nd (143Nd/144Nd = 0·51310–0·51304). Macquarie Island basaltic glasses are divided into two compositional groups according to their mg-number–K2O relationships. Near-primitive basaltic glasses (Group I) have the highest mg-number (63–69), and high Al2O3 and CaO contents at a given K2O content, and carry microphenocrysts of primitive olivine (Fo86–89·5). Their bulk compositions are used to calculate primary melt compositions in equilibrium with the most magnesian Macquarie Island olivines (Fo90·5). Fractionated, Group II, basaltic glasses are saturated with olivine + plagioclase ± clinopyroxene, and have lower mg-number (57–67), and relatively low Al2O3 and CaO contents. Group I glasses define a seriate variation within the compositional spectrum of MORB, and extend the compositional range from N-MORB compositions to enriched compositions that represent a new primitive enriched MORB end-member. Compared with N-MORB, this new end-member is characterized by relatively low contents of MgO, FeO, SiO2 and CaO, coupled with high contents of Al2O3, TiO2, Na2O, P2O5, K2O and incompatible trace elements, and has the most radiogenic Sr and Pb regional isotope composition. These unusual melt compositions could have been generated by low-degree partial melting of an enriched mantle peridotite source, and were erupted without significant mixing with common N-MORB magmas. The mantle in the Macquarie Island region must have been enriched and heterogeneous on a very fine scale. We suggest that the mantle enrichment implicated in this study is more likely to be a regional signature that is shared by the Balleny Islands magmatism than directly related to the hypothetical Balleny plume itself.
KEY WORDS: mid-ocean ridge basalts; Macquarie Island; glass; petrology; geochemistry
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONTECTONIC SETTING OF MACQUARIE...REGIONAL GEOLOGY OF MACQUARIE...PETROGRAPHY AND MINERALOGYANALYTICAL METHODSCHEMICAL COMPOSITION OF GLASSESDISCUSSIONREFERENCES |
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Geochemically enriched types of mid-ocean ridge basalts (MORB), variously referred to as transitional (T-), plume (P-) or enriched (E-) type MORB, contrast with the more abundant normal N-type MORB in having elevated concentrations of K2O and other highly incompatible elements. These geochemically ‘anomalous’ MORB types occur either within distinct areas of ridge axis magmatism [e.g. FAMOUS and AMAR valleys, 14–16°N, 43°N of Mid-Atlantic Ridge (MAR), Oceanographer fracture zone, 12–13°N, 18–19°S, the Siqueiros fracture zone of the East Pacific Rise (EPR), Galapagos spreading centre, Southern Explorer Ridge, 7°E and 12–15°E of the SW Indian Ridge, Australian–Antarctic Discordance (Langmuir et al., 1977
; Shibata et al., 1979; Schilling et al., 1982, 1983; le Roex et al., 1983, 1992; Bougault et al., 1988; Hekinian et al., 1989; Michael et al., 1989; Natland, 1989; Klein et al., 1991; Frey et al., 1993; Niu et al., 1996, 1999)] or compose off-axis seamounts randomly distributed within oceanic plates in the vicinity of spreading centres (Batiza & Vanko, 1984; Zindler et al., 1984; Allan et al., 1994; Niu & Batiza, 1997). Their formation is commonly attributed to the influence of near-ridge plumes or hotspots, or alternatively to the presence of heterogeneities (blobs or veins of enriched material) in a depleted peridotite mantle.
Inhomogeneities of unknown size, composition and origin in the mantle source region are usually invoked to explain the correlation between trace element and radiogenic isotope characteristics reported in many MORB suites [so-called ‘coupled MORB’ of Niu et al. (1996)]. Alternatively, MORB suites with limited isotopic variations but more extensive incompatible element ratio variations have been attributed to mixing of primary melts formed by different extents of melting of an isotopically homogeneous source [e.g. FAMOUS (Langmuir et al., 1977; Frey et al., 1993); EPR at 21°N (Dupre et al., 1981; Macdougall & Lugmair, 1986)]. Another type of chemical ‘coupling’ between major elements (e.g. MgO) and incompatible trace element ratios was also recorded for some MORB lavas (e.g. Frey et al., 1993; Allan et al., 1994; Niu et al., 1999) and interpreted in terms of two-component mixing (Niu et al., 1999), although this phenomenon has not yet been fully evaluated.
If erupted MORB were primary or near-primary magmas, it might be possible to use the coupling between major and trace element contents and isotopic ratios to distinguish the potential roles in MORB magma genesis of source heterogeneities vs partial melting of a homogeneous source. However, popular notions of melt generation, segregation and extraction (e.g. McKenzie & Bickle, 1988; Natland, 1989) predict that discrete primary melts are extremely rare or even absent in the oceanic lithosphere. Rather, it has been suggested that the generally uniform erupted N-MORB magmas are produced by extensive polybaric mixing and fractionation of diverse melt fractions formed within a melting column. Therefore, source heterogeneities and the extents of mantle melting processes are generally obscured within the relatively long-lived and robust magmatic systems located beneath oceanic spreading centres. Recent studies of melt inclusions trapped within MORB phenocrysts of primitive composition have added to our knowledge of the geochemistry of primary melts with identification of contrasting ultra-depleted to ultra-enriched trace element compositions (e.g. Sobolev & Shimizu, 1993, 1994; Shimizu, 1994; Kamenetsky, 1996; Tsamerian & Sobolev, 1996; Kamenetsky & Crawford, 1998; Kamenetsky et al., 1998). However, the post-entrapment crystallization and re-equilibration of melt inclusions, combined with our current inability to perform radiogenic isotope analyses on individual melt inclusions, place significant limitations on any reconstruction of the major element and isotopic compositions of primary melts.
In this paper we present the results of petrological and geochemical studies of hyaloclastites and pillow-rim glasses sampled from Macquarie Island, an exposure above sea-level of the Macquarie Ridge system. The results indicate that some of the glasses represent primitive, near-primary melts, which span a large geochemical range from N-MORB to highly enriched varieties. The fact that their major element, radiogenic isotope and trace element geochemical characteristics are all well correlated represents a new contribution to the continuing debate on the nature of MORB primary and parental melts, source heterogeneity, parameters and effects of melting.
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TECTONIC SETTING OF MACQUARIE ISLAND |
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TOPABSTRACTINTRODUCTIONTECTONIC SETTING OF MACQUARIE...REGIONAL GEOLOGY OF MACQUARIE...PETROGRAPHY AND MINERALOGYANALYTICAL METHODSCHEMICAL COMPOSITION OF GLASSESDISCUSSIONREFERENCES |
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Macquarie Island (54°30'S, 158°56'E) and a few nearby islets constitute the only exposure above sea-level of the Macquarie Ridge, an elongate arcuate bathymetric high mostly lying at <2000 m water depth, which extends for 1200 km SSW from New Zealand (47°S) to about 57°S (Fig. 1a). Rocks dredged from the Macquarie Ridge include tholeiitic MORB-like basalt, dolerite, olivine gabbro, troctolite and serpentinized harzburgite (Watkins & Gunn, 1971
; Ovenshine et al., 1974; Schilling & Ridley, 1974; Matveyenkov & Baranov, 1981; Mortimer, 1995) and resemble those from Macquarie Island.
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The Macquarie Ridge, together with an adjacent and parallel trench system, marks the boundary between the Australia and Pacific plates, and was formed from ocean crust in the last 10 My by transcurrent to compressional relative motion (transpression) resulting from a mismatch in the rates of Australia–Antarctic and Pacific–Antarctic sea-floor spreading (Johnson & Molnar, 1972
; Ruff et al., 1989).
The ridge is seismically very active [average return period 1 year for magnitude >6·2, magnitude 7·8 recorded in 1943 (Jones & McCue, 1988)]. The seismicity is shallow and probably limited to the crust (Ruff et al., 1989), indicating that subduction is not occurring at present. Earthquake fault plane solutions for the Macquarie segment suggest sub-horizontal, east–west oriented principal stress axes, and probably dextral strike-slip movement along fault planes parallel to the ridge (Jones & McCue, 1988). This implies counter-clockwise rotation of the Pacific plate, relative to the Australian plate, and right lateral strike-slip movement along the Macquarie Ridge.
The ridge is divided into four discrete segments (Hayes & Talwani, 1972; Massell et al., 2000) by marked breaks in its continuity and strike near 51°S, 53°S and 56°S. Along the Macquarie segment (53°S to 56°S, including the Macquarie Island region), the shallow depth and flat-topped morphology of the ridge suggest that much of the Macquarie segment might have been at sea-level in the past (Griffin & Varne, 1980; Massell et al., 2000). The ridge is here paralleled by the Macquarie Trench, which lies immediately to its east, mostly at 5000–6000 m water depth.
Gravity studies suggest that about 400 km NNE of Macquarie Island the ocean crust of the Australian plate thickens from 
7 km, west of the Macquarie Ridge, to 14 km just east of the ridge (Williamson & Johnson, 1974). This is consistent with 65 km of crustal shortening by transpression (Williamson, 1988).
West of the Macquarie segment, the crust of the South Tasman Basin slopes gently down to 4000 m. This crust belongs to the Australia plate, and between the Balleny Fracture Zone to the west and the southern part of the Macquarie Ridge to the east (Fig. 1a), is assigned to the Balleny spreading corridor (Cande et al., 1998) formed by spreading at the Southeast Indian Ridge that began in Cretaceous times, and has continued until the present. To the east, between the ridge and the continental Campbell Plateau margin (Fig. 1), lies the Emerald Basin (4000 m), on the Pacific plate.
The magnetic anomaly system of the Balleny spreading corridor has been traced northwards to the east-trending bathymetric features of the Nella Dan rift and its eastern continuation, the Resolution Ridge system (Cande et al., 1998). The location of these bathymetric structures corresponds to the angular magnetic unconformity between the magnetic anomalies of the Tasman Sea and the younger anomalies of the oceanic crust of the Balleny spreading corridor, generated at the Southeast Indian Ridge. The now extinct Tasman spreading centre (Fig. 1a) can be easily identified on gravity anomaly maps [see fig. 2 of Sutherland (1995)].
Weissel et al. (1977) identified magnetic anomaly 7 (25 Ma) to the west of Macquarie Island, and Williamson (1988) argued that this anomaly was traceable eastward, across Macquarie Island to the Macquarie Trench. This would imply that the ocean crust exposed on Macquarie Island was formed at the Southeast Indian Ridge by late Oligocene spreading, and remains part of the Australia plate, and that the Macquarie Trench marks the plate boundary.
However, in the vicinity of Macquarie Island, the Macquarie Ridge has a double crest with a central valley, and the island is an exposure of the eastern ridge crest (Summerhayes, 1969; Massell et al., 2000). According to Massell et al. (2000), the active transcurrent plate boundary in the Macquarie segment coincides with a zone of multiple ridge-parallel lineaments confined to the central valley, rather than with the Macquarie Trench, as Williamson (1988) supposed. Macquarie Island is therefore now located on the Pacific plate.
Moreover, new gravity and swath data acquired for the oceanic crust south of New Zealand support a new explanation for the tectonic evolution of the region in general and for the Macquarie Island region in particular.
The Resolution Ridge and the western margin of the Campbell Plateau plus the eastern boundary of the Emerald Basin (Fig. 1a) are conjugate margins, formed in Eocene times by the rifting of continental and older oceanic crust when the Australia–Pacific boundary developed into a spreading ridge south of New Zealand (Sutherland, 1995).
Fracture zones and tectonic spreading fabric within this triangle of new oceanic crust record the evolution since Eocene times of the Australia–Pacific boundary from a spreading system into a system of long transform faults linked by short spreading segments, and now into a right lateral strike-slip plate boundary (Sutherland, 1995; Wood et al., 1996; Lamarche et al., 1997; Massell et al., 2000). Plate reconstruction models show the Australia–Pacific relative pole of rotation migrating southeast, away from the New Zealand landmass, during late Eocene–late Miocene times, and remaining close to the plate boundary south of New Zealand, its closeness implying slow spreading at the plate boundary. It then migrated southwest, even nearer the plate boundary, during Plio-Pleistocene time (Walcott, 1978; Stock & Molnar, 1982; Sutherland, 1995).
Lamarche et al. (1997) estimated that spreading and oceanic crust creation may have continued at the Australia–Pacific plate boundary immediately south of New Zealand until 12–16 My ago, and their model for the evolution of the plate boundary geometry shows it still active as a spreading plate boundary in the Macquarie Island region at 11 Ma (their fig. 6C).
Varne et al. (2000) have used the new swath data of Massell et al. (2000) for the Macquarie Island region, coupled with data from Deep Sea Drilling Project (DSDP) Site 278, magnetic anomaly data of Weissel et al. (1977) in the Emerald Basin, and calculated spreading rates of Lamarche et al. (1997) for conjugate crust, to calculate that oceanic crust of the Macquarie Island region was still being created 9–14 My ago at the Australia–Pacific plate boundary. This is in agreement with radiometric and palaeontological ages for rocks of the island (see the next section).
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REGIONAL GEOLOGY OF MACQUARIE ISLAND |
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TOPABSTRACTINTRODUCTIONTECTONIC SETTING OF MACQUARIE...REGIONAL GEOLOGY OF MACQUARIE...PETROGRAPHY AND MINERALOGYANALYTICAL METHODSCHEMICAL COMPOSITION OF GLASSESDISCUSSIONREFERENCES |
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Macquarie Island,
37 km long and between 3 and 5 km wide (Fig. 1b), is elongate in a NNE direction along the Macquarie Ridge. In most parts a narrow shore platform rises steeply to a plateau 250–300 m above sea-level, with a maximum elevation of 433 m. Dating of raised beach deposits on the plateau suggests an uplift rate of 0·8 mm/year, implying that the island has been emergent for only 0·6–0·7 My (Adamson et al., 1996). Seismic activity and the presence of numerous active fault scarps indicate continuing tectonism.
The earliest geological work on Macquarie Island, by L. R. Blake (1911–1914), was published by Mawson (1943), but the island was first recognized as an uplifted fragment of ocean crust by Varne et al. (1969). Subsequent work includes that of Varne & Rubenach (1972), Griffin & Varne (1980), Griffin (1982), Christodoulou et al. (1984), Lees (1987), Duncan & Varne (1988) and Varne (1989), and the island has recently been systematically mapped at 1:10 000 scale by the Tasmanian Geological Survey (Goscombe & Everard, 1998, 1999).
Faulting, on all scales with a spacing down to a few metres, pervades the island (Fig. 1b). The greatest tectonic uplift has occurred in the northern quarter of the island, exhuming a tilted middle- to lower-crustal section of sheeted dolerite dykes, which pass downward with increasing frequency of gabbro screens to massive gabbro and then into a cumulate sequence of layered olivine gabbro, troctolite, minor plagioclase wehrlite, wehrlite and dunite and finally to residual mantle harzburgite (Griffin & Varne, 1980; Basylev & Kamenetsky, 1998). This section is everywhere disrupted by numerous faults (although reasonably continuous along the northwestern coast), and gabbros, cumulates and harzburgites are all intruded by dolerite dykes. Fault-bounded blocks and thrust slices of extrusives (mostly pillow basalt) are also present, most notably at the northeastern extremity of the island.
The southern three-quarters of the island (Fig. 1b) consists mainly of basaltic pillow lavas with subordinate massive to tabular basalt flows, hyaloclastite, rare picrite and minor pelagic sedimentary rocks (mostly red mudstone, with rare sandstone and conglomerate). The basalts are highly porphyritic (>50% plagioclase ± olivine ± clinopyroxene) to aphyric and include some rare hornblende-bearing types. Calcareous oozes occur between pillows, and are widely distributed although not abundant.
The extrusive sequence is intensely faulted. Narrow basaltic dykes that cut the extrusive rocks were intruded at a high angle to bedding. The volcanic rocks were first tilted around axes that were near-horizontal and parallel to dyke–bedding plane intersections, causing variations in dip, and later rotated about vertical axes, causing variations in strike (Varne & Rubenach, 1972). Palaeomagnetic data (Williamson, 1979) suggest that clockwise rotation of kilometre-sized fault blocks has occurred.
An uplifted fault block composed of sheeted dolerite dykes, with rare gabbro screens, occurs along the southeast coast. An occurrence of sheeted dolerite dykes along the central west coast lacks gabbro screens and in places passes, both laterally and upward, into extrusive sequences.
There is general agreement that Macquarie Island is built of rocks that were formed at an oceanic spreading centre (Varne et al., 1969; Varne & Rubenach, 1972; Griffin & Varne, 1980; Christodoulou et al., 1984; Goscombe & Everard, 1998, 1999; Varne et al., 2000) but the timing and location of the spreading was disputed. From the central third of the island, a massive basalt flow has yielded a well-defined 40Ar/39Ar plateau age of 11·5 ± 0·3 Ma, and a hornblende separate from a massive flow or near-surface intrusion yielded a K–Ar age of 11·5 ± 0·3 Ma, both interpreted as original crystallization ages (Duncan & Varne, 1988). A basalt from the northernmost headland of the island (Fig. 1b) yielded a well-defined 40Ar/39Ar plateau age of 9·7 ± 0·3 Ma, which Duncan & Varne (1988) again interpreted as its original crystallization age. Nearby, poorly preserved coccoliths from oozes associated with pillow lavas were thought by Quilty et al. (1973) to be of early or middle Miocene age (11–24 Ma), and Foraminifera from oozes suggest a late Miocene (5–11 Ma) or younger age (Varne et al., 1969). The radiometric and palaeontological ages are therefore broadly in keeping with the range of 9–14 Ma for the timing of the generation of oceanic crust in the Macquarie Island region independently calculated by modelling of the tectonic history (Varne et al., 2000).
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PETROGRAPHY AND MINERALOGY |
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TOPABSTRACTINTRODUCTIONTECTONIC SETTING OF MACQUARIE...REGIONAL GEOLOGY OF MACQUARIE...PETROGRAPHY AND MINERALOGYANALYTICAL METHODSCHEMICAL COMPOSITION OF GLASSESDISCUSSIONREFERENCES |
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Glasses occur both as quenched rims of basaltic pillow lavas and, more commonly, as fragments in hyaloclastite breccias. The latter usually consist of subangular to angular or rounded glass lapilli, glassy and crystalline pillow fragments, up to 3 cm (
40% of breccias), and may carry olivine and/or plagioclase crystals, in a matrix of smaller glassy debris cemented by a greenish grey aggregate possibly after shattered and dispersed glass. Carbonates, zeolites and smectite–chlorite intergrowths are commonly present in interstices and as tiny veins in the hyaloclastites. In thin section the glasses are transparent and dark to light brown in colour, and may contain scattered opaque devitrified zones rich in crystallites. Microphenocrysts of olivine (20–300 µm) with trapped spinel and glass inclusions are widespread but make up <5 vol. %. In some glasses octahedra of reddish brown spinel and laths of plagioclase are also present, although no clinopyroxene crystals were observed. Microphenocrysts of olivine and plagioclase are skeletal to euhedral and show no signs of resorption or textural disequilibrium. Large vesicles filled with aggregates of secondary minerals and surrounded by alteration haloes are common within many of the glasses. Alteration seems to have begun along cooling contraction fractures in glasses, and expanded away from these, resulting in polygonal areas of fresh glass set in a matrix of alteration products.
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONTECTONIC SETTING OF MACQUARIE...REGIONAL GEOLOGY OF MACQUARIE...PETROGRAPHY AND MINERALOGYANALYTICAL METHODSCHEMICAL COMPOSITION OF GLASSESDISCUSSIONREFERENCES |
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A total of 56 glasses from Macquarie Island and two glasses from Macquarie Ridge were crushed and fresh chips were hand-picked and analysed for major elements, S, Cl and F, using a Cameca SX50 electron microprobe (University of Tasmania) and basaltic glass VG-2 as an internal standard. Special care was taken to avoid altered areas within the glasses and the vicinity of microphenocrysts where compositional heterogeneities are more likely to occur. Each reported glass composition is the average of at least 15 analyses. Glasses have very homogeneous compositions (low standard deviation values), and the analytical errors in determination of each of the major element oxides are as follows (in rel. %): 0·7–0·9 (SiO2, Al2O3); 1·5–2·2 (FeO, MgO, CaO); 2·7–5 (TiO2, Na2O, K2O); 12 (P2O5).
Fourier transform infrared (FTIR) spectroscopy measurements of H2O in glasses were obtained using a Bruker IFS66 FTIR spectrometer at the University of Tasmania. Details of the technique and calibration have been given by Danyushevsky et al. (1993).
Trace elements in glasses were analysed by LA-ICPMS (laser ablation–inductively coupled plasma mass spectrometry) at the Research School of Earth Sciences, Australian National University, Canberra, using an ArF (193 nm) EXCIMER laser, a Fisons PQ2 STE ICPMS instrument, and custom-built sample introduction system (Eggins et al., 1998). Analyses were performed using a spot size of 200 µm and a laser pulse rate of five repetitions per second. The total analysis time for each sample was 130 s, comprising a 60 s background and 70 s analysis with laser on. Sixty replicate measurements were made for each isotope. Standard glass NIST 612 was used for calibration. Data reduction was performed using background subtracted count rates and the methods outlined by Longerich et al. (1996). 43Ca was employed as the internal standard isotope, based on CaO concentrations measured by electron probe. Analytical precision of the measurements is in the range 1–5% for most elements.
Isotope analyses of Sr, Nd and Pb in six glasses were performed at the Research School of Earth Sciences, Australian National University, Canberra, using a Finnigan MAT261 mass spectrometer operated in static multi-collector mode. Details of sample preparation and analytical technique have been reported by Lanyon (1994) and Woodhead (1996).
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CHEMICAL COMPOSITION OF GLASSES |
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TOPABSTRACTINTRODUCTIONTECTONIC SETTING OF MACQUARIE...REGIONAL GEOLOGY OF MACQUARIE...PETROGRAPHY AND MINERALOGYANALYTICAL METHODSCHEMICAL COMPOSITION OF GLASSESDISCUSSIONREFERENCES |
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Major and trace element compositions
All the analysed Macquarie Island glasses are basaltic in composition, with 47·4–51·1 wt % SiO2 and 5·65–8·75 wt % MgO (Table 1), and include both saturated (hypersthene-normative) and undersaturated (nepheline-normative) compositions in about equal proportions. An outstanding feature of the glasses is their large range of major element abundances, in particular K2O (0·12–1·76 wt %), Na2O (2·37–4·24 wt %) and TiO2 (0·97–2·10 wt %), and moderately to highly incompatible element abundances [e.g. Rb (3·2–59·5 ppm), La (4·3–46·6 ppm), Eu (0·92–2·0 ppm)].
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The glasses have been subdivided into two distinct compositional groups (Table 1, Fig. 2) using the relationship between their mg-number [Mg/(Mg + Fe2+), where Fe2+ = 0·9Fetotal) and contents of highly incompatible elements. The latter are best expressed by K2O/TiO2 or La/Sm as these ratios are not strongly affected by crystal fractionation of ol + plag, which mostly increases the absolute abundances of incompatible elements. In the description and following discussion we use La/Sm as an index of degree of enrichment, though other ratios such as Rb/K, Zr/Ti or Nb/Zr could be used equally well. Eighteen Group I glasses have the highest mg-number (63·2–69·1) at a given K2O content (K2O/TiO2, La/Sm) and their compositions form a regular trend with a negative slope in Fig. 2. Thirty-eight Group II glass compositions are scattered below this trend, and have relatively lower mg-number (57·1–67·2) at similar K2O abundances.
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Group I or near-primitive glasses, defined as having the highest mg-number (the lowest FeO at a given MgO), also have the highest Sr/Nd, Ba/Rb and Sc/Yb values at a given La/Sm value, as well as the lowest Zr/Sm and Hf/Eu and heavy rare earth element (HREE) and Y abundances (Fig. 3, Table 1). They also carry phenocrysts and microphenocrysts of relatively primitive olivine Fo85·8–89·5 and Cr-spinel [mean cr-number, Cr/(Cr + Al), is 35].
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In contrast, Group II or fractionated glasses carry more evolved olivine crystals in most samples (Fo83·5–88·3), spinel grains with higher cr-number (mean 42), and plagioclase (An76–88). Fractionation of olivine, plagioclase and clinopyroxene in the genesis of the Group II glasses is reflected in their lower mg-number Al, Ca, Sc, Sr and Ba, and higher SiO2, FeO, Hf (Zr) and HREE abundances (Figs 2–4). It should be noted that the enriched members (La/Sm > 7) of Group I glasses do not have counterparts among fractionated compositions.
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Group I glasses show a seriate variation within the compositional spectrum of MORB, and extend the compositional range from N-MORB compositions to enriched compositions that seem to represent a new primitive enriched MORB end-member. Their range in MgO abundances from low (down to 5·9 wt %) to high (up to 8·8 wt %), found, respectively, in the most enriched and the least enriched Group I Macquarie glasses (Fig. 4), appears to be typical of many other E- to N-MORB suites (le Roex et al., 1983, 1992; Batiza & Vanko, 1984; Bougault et al., 1988; Allan et al., 1993, 1994; Niu & Batiza, 1997; Niu et al., 1999). Although the MgO contents of enriched Group I glasses are too low to allow the interpretation of these glasses as primary mantle-derived melts, they have strikingly low FeO contents (Table 1, Fig. 4). Indeed, addition of only 5–8% equilibrium olivine would bring these compositions into equilibrium with mantle olivine Fo90·5 (see below). However, the Group I glasses are still among the least fractionated MORB melts known, because this correction for olivine fractionation is relatively small and because neither plagioclase nor clinopyroxene were on their liquidus (Figs 3 and 4).
Significant variations in incompatible major, trace and volatile element concentrations and ratios are observed in both Group I and II glasses (Figs 2–5). Particularly apparent is the wide range in concentration of the most incompatible trace elements (Fig. 5; Table 1: e.g. Rb/Yb 1·1–27·9, La/Yb 1·5–21·9). Although the degree of enrichment in many Group I glasses is exceptional, even for E-MORB, it is, however, similar to that observed in many alkali ocean island basalts (OIB). In particular, it bears strong similarities to the regional OIB from Balleny and Scott islands (Fig. 5a). The geochemical patterns of Macquarie Island glasses on PM- (primitive mantle) normalized diagrams (Fig. 5) share the following features of the regional OIB signature: a negative K anomaly, greater abundances of Ta and Nb relative to the adjacent incompatible trace elements, and a general decrease in the abundances of elements more incompatible than Ta and Nb. These features, which are characteristic of HIMU ocean island basalts (Weaver, 1991; Woodhead, 1996), have also been reported from many E-MORB suites (Langmuir et al., 1977; Wood, 1979; le Roex et al., 1983, 1992; Bougault et al., 1988; Niu & Batiza, 1997). The most significant difference, however, is the more pronounced depletion of HREE in the OIB relatively to even the most enriched Macquarie Island MORB glasses.
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The glasses appear to have preserved magmatic compositions remarkably well, as suggested by the excellent correlation of all incompatible elements (Fig. 5). This phenomenon extends to low abundance and/or highly mobile elements (e.g. Cs, Be, Fig. 6), not usually analysed for, and even to volatile elements. For example, H2O (0·25–1·49 wt %) and Cl (0·01–0·14 wt %) contents are positively correlated with other highly incompatible elements and the degree of enrichment (Fig. 6). There is a tendency for H2O and Cl to be slightly higher in the Group II compared with the Group I glasses, which is a likely result of crystal fractionation.
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Glass isotope compositions
Radiogenic isotope data for four Group I and two Group II Macquarie glasses are presented in Table 2 and Fig. 7. Sr, Nd and Pb isotope values for these glasses correlate well with their major and trace element compositions (Fig. 7b): enriched glasses have the most radiogenic Pb and Sr ratios.
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The isotopic compositions of the studied glasses show a much greater affinity with Pacific MORB than with Indian Ocean MORB, particularly in their generally higher 206Pb/204Pb values (Fig. 7a). Their isotopic compositions indicate the presence of a distinctive regional mantle isotopic signature characterized by high Nd and low Sr isotopic ratios, similar to MORB, and high 206Pb/204Pb values intermediate between those of MORB and HIMU OIB and indicative of time-integrated elevated U/Pb. Although the isotopic compositions of the Macquarie glasses generally overlap with Pacific MORB, particularly in Pb–Pb space, they trend towards a HIMU-like isotopic component similar to the Balleny plume (Fig. 7a, and Hart, 1988; Lanyon et al., 1993; Lanyon, 1994).
Olivine–glass relationships and primary melt compositions
The presence of olivine phenocrysts and microphenocrysts in the Macquarie Island glasses suggests that at least some of the olivine grains have crystallized from the liquid that transported them to the surface and then quenched to form a glass. However, the possibility that some or all of the olivine crystals are xenocrysts should also be considered. The genetic relationship between the olivine crystals and their host glass can be investigated using the compositions of melt inclusions trapped in olivine, and calculated olivine–melt equilibria.
Most olivine crystals within glasses contain melt inclusions (up to 100 µm), which represent small portions of melt trapped during crystallization. Optically, melt inclusions have the appearance of glass, and their composition is depleted in MgO (Table 3) and enriched in all other elements. These features are explained by the post-entrapment crystallization of olivine onto the walls of the inclusions. However, the ratios of elements that are incompatible in olivine (e.g. K2O/TiO2, Na2O/TiO2, CaO/Al2O3) remain unaltered. Comparison of these incompatible element ratios in melt inclusions with their values in the host glass (Table 3) shows that olivine phenocrysts crystallized from a liquid identical to the host glass in terms of incompatible element ratios.
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A comparison of olivine compositions with the calculated compositions of olivine in equilibrium with their host glasses is shown in Fig. 8. The most evolved (Fe-rich) crystals in each sample are in equilibrium with the host glass, as is indicated by Kd Ol–meltMg–Fe2+ = 0·27–0·30. The presence of more Fo-rich olivines, whose compositions are not in equilibrium with their host glasses, suggests their crystallization from more primitive magmas. This implies that none of the Macquarie Island glasses can represent a primary melt in terms of mg-number. The most primitive olivine compositions found in the Group I glasses range from Fo88·2 in high-K glasses to Fo89·5 in low-K glasses. These olivine Fo values match closely the compositions of the most primitive olivines found in the Macquarie Island low-K picrites (Fo90·5), cumulate peridotites (Fo90·5) and restite peridotites (Fo91·5) (Basylev & Kamenetsky, 1998), and also abyssal mantle peridotites worldwide (Fo91, Dick, 1989). Therefore, the compositions of the most Fo-rich olivine within the Macquarie glasses can be used in association with the Group I glass compositions to estimate the composition of near-primary melt with which the olivine would be in equilibrium.
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To calculate the compositions of Macquarie Island near-primary melts from the compositions of Group I glasses we assumed that the effects of spinel and plagioclase (where they are present in glasses) crystallization were insignificant. The measured compositions of the Group I glasses were corrected for olivine-only crystallization by adding olivine component in the amount required to maintain an olivine–melt equilibrium (Ford et al., 1983) until calculated equilibrium with the most primitive Macquarie Island olivine (Fo90·5) was achieved. The degree of iron oxidation in melts (Fe2+/Fe3+ = 6·7) was calculated using the mean Fe2+/Fe3+ of Cr-spinel and the empirical model for spinel–melt equilibria (Maurel & Maurel, 1982). The results obtained for representative Group I glasses from calculating their ‘primary’ melt compositions and crystallization temperatures are presented in Table 4. The amount of added olivine does not exceed 5–8 wt %, and it could be even smaller, and thus MgO of primary melts could be lower, if we overestimated the Fo content of liquidus olivine in the enriched primary magmas and it is less than Fo90·5.
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This discussion tacitly assumes that the compositions of glasses as measured by electron probe represent the bulk compositions of the erupted melts. However, the glasses carry microphenocrysts of olivine that apparently crystallized, at least in part, where they are now found. Therefore, the analysed glass compositions do not represent original melt compositions because these have been modified by olivine extraction. The olivine microphenocrysts make up 5 vol. % (equivalent to 6 wt %) or less of the samples. This amount is roughly similar to the amount of olivine added during the correction procedure (5–8 wt %). It is possible, therefore, that the actual magma compositions fell between the analysed glass compositions and the calculated ‘primary’ melt compositions of Table 4.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONTECTONIC SETTING OF MACQUARIE...REGIONAL GEOLOGY OF MACQUARIE...PETROGRAPHY AND MINERALOGYANALYTICAL METHODSCHEMICAL COMPOSITION OF GLASSESDISCUSSIONREFERENCES |
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Evidence for a primitive enriched member in the MORB family
The major and trace element diversity demonstrated by the Macquarie Island glasses has allowed two compositional groups to be distinguished. These near-primitive (Group I) and fractionated (Group II) melts are genetically related via the process of olivine ± plagioclase ± clinopyroxene crystallization. To a certain extent, the geochemical dispersion recognized in the primitive Group I glasses has been inherited by the more evolved Group II glasses, although the most enriched compositional varieties (La/Sm > 7) are found exclusively among Group I glasses. Taking into account the large number of samples studied, it is therefore tempting to assign genetic significance to the fact that the enriched primitive compositions lack counterparts amongst the fractionated glasses. However, this can be readily explained by the fact that the elevated H2O contents of enriched melts up to 1·5 wt % (Table 1, Fig. 6) cause a significant delay in plagioclase crystallization (Housh & Luhr, 1991
). In addition, the original enriched magmas were probably erupted without suffering extensive mixing and fractionation, probably as a result of their being the first melt fractions within an immature magmatic system, i.e. before a magma chamber(s) developed. This is generally consistent with the observations that the sites of ephemeral to minor E-MORB magmatism coincide with the settings of off-axis seamounts (Batiza & Vanko, 1984; Zindler et al., 1984; Allan et al., 1994; Niu & Batiza, 1997), extremely slow spreading ridges above regions of diffuse mantle upwelling (le Roex et al., 1992), or tectonically complex regions that are transitional between transform faults and ridge systems, along which diffuse, locally concentrated extension is occurring (Allan et al., 1993).
Alkali-enriched lavas associated with typical depleted MORB are being increasingly frequently reported (e.g. le Roex et al., 1983, 1992; Batiza & Vanko, 1984; Bougault et al., 1988; Allan et al., 1993, 1994; Niu & Batiza, 1997), and the Macquarie Island enriched glass compositions have many features in common with other E-MORB suites, including their HIMU affinities. However, the Macquarie Island Group I glasses are unique in that they have:
- the greatest enrichment in K2O and highly incompatible trace elements (Figs 2 and 5a), Al2O3 and CaO (Fig. 4), and depletion in SiO2 and FeO (Fig. 4) at a given mg-number (MgO);
- the most primitive (near-primary) compositions, modified by olivine crystallization only, at a given degree of enrichment (Figs 2 and 8);
- strong bivariate correlations between major, trace and volatile element abundances and isotope ratios (Figs 2–7);
- a well-defined compositional transition from enriched compositions to compositions typical of low-K MORB (Figs 2 and 4).
Despite the general similarity in incompatible trace element compositions between enriched Macquarie glasses and HIMU oceanic intraplate basalts, and regional Balleny plume lavas in particular (Fig. 5a), notable striking differences in key major element and HREE contents (Table 1, Fig. 5a) and discrepancy in isotope composition (Fig. 7a) between the enriched Macquarie Island MORB and Balleny plume OIB make it difficult to assign the magmatism to an OIB mantle source. In other words, Macquarie Island melts are not the melting products of Balleny plume. Moreover, the presence of sheeted dykes, the relatively low calculated temperatures of primary Macquarie Island magmas (Table 4) and typical MORB values of spinel cr-number (26–56, compared with >54 in OIB spinel) are in disagreement with the idea of hotspot oceanic magmatism. Therefore, we argue that the magma type represented by Macquarie Island Group I enriched glasses defines a new primitive end-member within the known compositional spectrum of MORB. It is characterized by the lowest abundances of MgO, FeO, SiO2 and CaO coupled with the highest concentrations of Al2O3, TiO2, Na2O, P2O5, K2O and incompatible trace elements, and has the most radiogenic Sr and 206Pb/204Pb regional isotope composition.
This is not to disregard the possibility that the upper-mantle source might have intermixed with material from the Balleny plume. An important tectonic feature of the region is a postulated mantle plume trace, extending southward from eastern Tasmania (40–45 Ma) and marked by a chain of seamounts, passing west of the Macquarie Ridge, and ending in the active volcanoes of the Balleny Islands north of the Ross Sea (Lanyon et al., 1993). Magmas resulting from this plume activity differ from typical ocean floor basalts in having a HIMU-like signature (Fig. 7a) and OIB major and trace element compositions (Table 1, Fig. 5a).
Primitive glass trends: mixing vs melting
Group I primitive glasses show remarkably well correlated variations in the concentrations of the major, trace and volatile elements and their ratios (Figs 2–6). The consistent seriate compositional trends from the enriched compositions to those typical of N-MORB argue for a certain linkage in the genesis of these diverse compositions. Two mechanisms, namely melt mixing and fractional melting, might account for the observed covariations.
Mixing between enriched and depleted ‘end-member’ melts, generated from different mantle lithologies or even within different source regions, is the most simple explanation for the linear covariations of elements of similar incompatibility. However, the binary mixing of the two geochemical ‘end-members’ defined by our data is inconsistent with the trends observed (Figs 3 and 9). On the other hand, multistage mixing or contamination of an initial dominant volume of enriched melt with very minor and nearly constant amounts of a depleted end-member component could produce non-linear geochemical trends with little dispersion along them. In both variants of mixing the presence of a depleted melt in the mantle at all stages of melting or in the magmatic system is essential. This is a keystone of the so-called ‘melting-induced two-component mixing’ model (Niu et al., 1996; Niu & Batiza, 1997), which requires the melting of highly depleted mantle to be triggered by an interaction with, and solidus temperature reduction by, highly enriched melts. As enriched melts cannot be generated from the same depleted mantle source, a heterogeneous mantle becomes a vital component of the above model.
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Dynamic (critical, continuous or near-fractional) partial melting, during which both pressure and source composition change (Langmuir et al., 1977; Maaloe, 1982; Eggins, 1992), produces virtually the same trends of melt composition as does fractional melting of a heterogeneous mantle followed by melt mixing and accumulation (Natland, 1989). According to Langmuir et al. (1977), variations in trace element abundances and their ratios can result from different degrees of partial melting of a rising mantle source that is undergoing continuous but incomplete removal of melt as melting proceeds. In an extension of this model, Plank & Langmuir (1992) suggested that a homogeneous mantle source can produce magmas that are mixtures of numerous instantaneous melt fractions produced at various points throughout the melting regime. This interpretation can be successfully applied to the primitive Macquarie glasses, although in this case the integration of discrete melt packets either had a minimal effect or did not occur at all.
The major element systematics of the Group I Macquarie glasses are consistent with continuous variations in the extent of melting, as defined by the global major element systematics of MORB (Klein & Langmuir, 1987; Langmuir et al., 1992) and isobaric experimental batch melts (e.g. Jaques & Green, 1980; Fujii & Scarfe, 1985; Kinzler & Grove, 1992a, 1992b). For example, if the La/Sm value is taken as a measure of the extent of melting (F), then for Macquarie Island the transition from low- to higher-F melts is characterized by increasing SiO2, FeO, MgO and CaO, and decreasing TiO2, Al2O3, Na2O, K2O and P2O5 (Figs 2 and 4, Table 4). Using the Macquarie Island data, the ‘seemingly fortuitous relationship between incompatible element enrichment and MgO’ (Plank & Langmuir, 1992) and between radiogenic isotopes and MgO (Fig. 7b) could be accounted for by positive correlation between the degree of melting and the MgO content of the partial melts. The fact that the Group I glasses retain their original major and trace element signatures also implies that melts may become mobile and segregate from their mantle source regions at very small volume percent, and their rapid migration towards the surface (probably via channel flow) can prevent re-equilibration at lower pressures.
Plank & Langmuir (1992) explained the local trends and global variations of fractionation-corrected Na2O and Ce abundances in MORB as resulting from end-member mixing between high-degree and low-degree melts derived from a homogeneous depleted source (Fig. 10). In this diagram the Group I Macquarie glasses form a steep trend which extends from the so-called ‘melting baseline’ (Plank & Langmuir, 1992) towards a composition moderately enriched in Na2O and highly enriched in Ce. This extreme enrichment in Ce and other highly incompatible elements (e.g. by a factor of 20 in terms of Rb) cannot be explained even invoking the lowest possible extents of melting (<1%) of depleted mantle (Ce 0·7 ppm, Plank & Langmuir, 1992). Therefore, a source that was originally more enriched than N-MORB but progressively depleting as a result of melting towards the composition of a typical N-MORB source can be suggested for Macquarie Island melts.
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A role for mantle heterogeneities in the origin of primary melts
The radiogenic isotope diversity of MORB is usually ascribed to chemical and isotopic heterogeneities in the mantle. These heterogeneities are thought to be represented by enriched material, which in many cases is interpreted to originate by partial melting of complex mantle including deep mantle plume-derived enriched mantle and enriched upper-mantle asthenosphere. The range of Pb isotope compositions in Macquarie Island glasses as well as the range of incompatible elements are therefore suggestive of geochemical heterogeneities in the mantle source region. The scale of mantle heterogeneities must be very small to account for distinct compositions of lavas within such a small area as Macquarie Island. However, this simple model cannot explain all of the compositional features of Macquarie Island melts and requires further development.
The rate of re-equilibration of isotopic and chemical heterogeneities with the rest of the mantle is a function of many factors [see review by Zindler & Hart (1986)], most of which are hard to determine quantitatively. The most important factors are the size of the heterogeneity and cation diffusion rates. The latter depend on mantle phase composition, diffusion direction, temperature, and the presence or absence of fluid or melt phases in the system. Small-scale heterogeneities may survive re-equilibration and thus be represented in partial melts either in fluid- or melt-free mantle over a long time period, or if they were introduced into the mantle just before melting. In the latter case, the melting can be readily attributed to formation of enriched mantle domains.
Correlations between isotopes, and major and incompatible trace element characteristics in the Macquarie Island glasses (Fig. 7b) imply not only that these chemical factors are coupled in the source, but that the elements and isotope ratios fractionate synchronously during melting. This, combined with the improbability of a simple two end-member melt mixing, supports a conclusion that the mantle source was homogeneous on a large scale, but heterogeneous on a very small, probably grain-size, scale. From observations of changes in the relative incompatibility of elements that usually do not fractionate from each other over short intervals of melting (Fig. 9), we conclude that the mineral assemblage of the mantle source also experienced melting-related modifications. The most pronounced changes in geochemical behaviour of certain elements (e.g. Ba/Rb, Th/U, Nb/U) occur at low degrees of partial melting (La/Sm 6, Fig. 9), which is coincident with the HREE starting to fractionate (Fig. 3, La/Sm vs Yb). On the other hand, the abrupt changes in other element ratios (e.g. Rb/Cs, Ce/Pb, Sr/Nd, Zr/Sm, Nb/Ta, H2O/Ce) seem to happen at higher melt fractions (La/Sm 4–5). This points to the melting-out or breakdown of phases in the mantle source, so the relative incompatibility of many trace elements changes. Unfortunately, our limited isotope data are not sufficient to trace the origin of documented bends in element ratio trends (Figs 3 and 9).
In conclusion, rocks with isotope, and major and trace element compositions lying well within the compositional range of Macquarie Island glasses were recovered from DSDP Sites 279A (Macquarie Ridge north of Macquarie Island) and 274 (Balleny Basin southeast of the Balleny Islands; Pyle et al., 1995). This supports our assumption that the mantle heterogeneity implicated in this study is not related directly to Balleny plume mantle or magmas, but is more likely to be a regional signature.
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ACKNOWLEDGEMENTS |
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We are indebted to B. J. Griffin, M. J. Rubenach and B. Goscombe for assembling excellent collections of Macquarie Island rocks, which have been used during this study. J.L.E. especially thanks Ben Goscombe for collaboration in the field, and A. V. Brown (Tasmania Director of Mines) for arranging fieldwork on Macquarie Island. We are grateful to Maya Kamenetsky, Simon Stephens, Wieslaw Jablonski, Les Kinsley and Graham Rowbottom for their assistance with sample preparation and various analytical studies. Thanks go also to Leonid Danyushevsky for allowing us to use his computer program PETROLOG and introducing V.S.K. to his technique of H2O analyses. The manuscript has benefited from discussions with many people, in particular S.-L. Chung, G. Davidson, L. V. Dmitriev, M. Gasparon, D. H. Green, A. Gurenko, A. W. Hofmann, C. H. Langmuir, S. Meffre, M. Portnyagin, J. E. Snow, A. V. Sobolev, N. M. Sushchevskaya and G. M. Yaxley. We thank Y. Niu and E. M. Klein for helpful reviews. This work is supported by an Australian Research Council (ARC) Large Grant to A.J.C. and R.V., an ARC Small Grant and an Antarctic Science Advisory Committee Grant to V.S.K., an ARC Research Fellowship Grant to V.S.K., and funding from RSES, the Australian National University and the Australian Research Council’s Research Centres Program.
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FOOTNOTES |
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*Corresponding author. Fax: +61 3 62232547. E-mail: Dima.Kamenetsky{at}utas.edu.au
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TOPABSTRACTINTRODUCTIONTECTONIC SETTING OF MACQUARIE...REGIONAL GEOLOGY OF MACQUARIE...PETROGRAPHY AND MINERALOGYANALYTICAL METHODSCHEMICAL COMPOSITION OF GLASSESDISCUSSIONREFERENCES |
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