Journal of Petrology

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Journal of Petrology Volume 42 Number 6 Pages 1197-1218 2001
© Oxford University Press 2001

Osmium Isotopic Evidence for Crust–Mantle Interaction in the Genesis of Continental Intraplate Basalts from the Newer Volcanics Province, Southeastern Australia

J. S. McBRIDE^1,*, D. D. LAMBERT¹, I. A. NICHOLLS¹ and R. C. PRICE²

¹VICTORIAN INSTITUTE OF EARTH AND PLANETARY SCIENCES, DEPARTMENT OF EARTH SCIENCES, MONASH UNIVERSITY, MELBOURNE, VIC. 3168, AUSTRALIA
²DEPARTMENT OF EARTH SCIENCES, WAIKATO UNIVERSITY, HAMILTON, NEW ZEALAND

Received March 8, 2000; Revised typescript accepted October 20, 2000

	ABSTRACT

TOP ABSTRACT INTRODUCTION BACKGROUND TO THE NEWER... SAMPLE SELECTION AND DESCRIPTION GEOCHEMISTRY DISCUSSION CONCLUSIONS APPENDIX 1 REFERENCES

 
Basalts from the Quaternary Newer Volcanics Province in southeastern Australia have a large diversity in their chemical and Nd, Sr and Os isotopic compositions. Plains series olivine tholeiites and Cones series nepheline hawaiites have distinctive isotopic compositions and are clearly not related to each other by a simple genetic process. The Cones series nepheline hawaiites have trace element abundances and Nd (_Nd = +4·1 and +3·8), Pb (²⁰⁶Pb/²⁰⁴Pb = 18·62 and 18·70) and Os (_Os = +6 and +7) isotopic compositions that fall within the range of ocean-island basalts (OIB). In contrast, the Plains series olivine tholeiites have lower _Nd (+0·9 to +2·3), more radiogenic Sr (⁸⁷Sr/⁸⁶Sr = 0·7045–0·7053), and considerably higher _Os (+42 to +250). The relatively low Os concentrations observed in the Plains series olivine tholeiites [Os = 12–45 parts per trillion (ppt)] compared with the more ‘primitive’ Cones series nepheline hawaiites (Os = 160 and 250 ppt), render them more susceptible to contamination processes that can obscure their primary mantle signatures. Trace element trends and isotopic modelling suggest that the unusual geochemical signatures observed within the Plains series olivine tholeiites are the result of assimilation of continental crust possessing variable isotopic signatures. However, the Cones series ne-hawaiites have trace element and isotopic compositions consistent with their derivation from melting of either a mantle plume (OIB-type source) or ‘veined’ sub-continental lithospheric mantle.

KEY WORDS: crust; intraplate basalts; mantle; Newer Volcanics Province; Re–Os

	INTRODUCTION

TOP ABSTRACT INTRODUCTION BACKGROUND TO THE NEWER... SAMPLE SELECTION AND DESCRIPTION GEOCHEMISTRY DISCUSSION CONCLUSIONS APPENDIX 1 REFERENCES

 
The origin of the enriched geochemical signatures observed within many continental intraplate basalts has been the source of considerable controversy. Many workers believe that these signatures are the result of a sub-continental lithospheric mantle source (e.g. Allègre et al., 1982; Turner & Hawkesworth, 1995; Molzahn et al., 1996; Stewart & Rogers, 1996), whereas others (e.g. Hawkesworth & Vollmer, 1979; Carlson et al., 1981; Brandon et al., 1993; Chesley & Ruiz, 1998) believe that they reflect crustal contamination of asthenospheric mantle- or plume-derived magmas en route to the surface. The Re–Os isotopic system is an excellent tool for distinguishing between these two distinctive enrichment processes, as unlike the more commonly used Sr–Nd–Pb isotopic systems, there is a strong Re–Os isotopic contrast between the sub-continental lithospheric mantle and the crust. Os behaves compatibly during melting, whereas Re behaves slightly incompatibly, resulting in high Re/Os in melts and the crust and low Re/Os within the mantle. Over time this results in mantle and crustal reservoirs with highly distinctive Os isotopic compositions. For example, the sub-continental lithospheric mantle generally has unradiogenic Os isotopic compositions (_Os < 0; Walker et al., 1989; Pearson et al., 1995a, 1995b; McBride et al., 1996), whereas the continental crust has highly radiogenic Os isotopic compositions (_Os = +330 to +22 000; Walker et al., 1989, 1991; Esser & Turekian, 1993; Frick et al., 1996; Asmerom & Walker, 1998; Frick, 1998). Thus, if a melt is contaminated by old crustally derived material it should have an unusually radiogenic Os isotopic signature. In contrast, melts derived from the sub-continental lithospheric mantle should have unradiogenic Os isotopic compositions.

The Newer Volcanics Province in southeastern Australia provides an excellent opportunity to study the relative influences of sub-continental lithospheric mantle and/or crust on the genesis of a young (<5 Ma) continental intraplate basaltic province. An important advantage of studying such a young volcanic province is that isotopic ratios require no age correction, eliminating a potential source of uncertainty. In addition, the sub-continental lithospheric mantle beneath this region is relatively well characterized as a result of a number of geochemical and Os isotopic studies of peridotite (e.g. McBride et al., 1996; Handler et al., 1997) and pyroxenite (McBride, 1998) mantle xenoliths carried within the Newer Volcanics Province basalts. However, the age of the continental crust in southeastern Australia is controversial and estimates range from younger than Cambrian (e.g. Gray, 1990; Gray, 1997; Anderson et al., 1998) to Proterozoic (e.g. Cas, 1983) in age. Clearly, the age of the continental crust is an important control on the Os isotopic composition, as Proterozoic crust will have significantly more radiogenic Os isotopic signatures than Palaeozoic crust (e.g. Johnson et al., 1996).

	BACKGROUND TO THE NEWER VOLCANICS PROVINCE

TOP ABSTRACT INTRODUCTION BACKGROUND TO THE NEWER... SAMPLE SELECTION AND DESCRIPTION GEOCHEMISTRY DISCUSSION CONCLUSIONS APPENDIX 1 REFERENCES

 
The Newer Volcanics Province reflects the latest expression of volcanism in southeastern Australia, which has been the site of almost continuous volcanism for the last 190 my. The Newer Volcanics Province covers an area of >15 000 km² (Fig. 1; Price et al., 1988) and dominates the surface geology of much of western and central Victoria. The Newer Volcanics Province has been subdivided into two series, the Plains and Cones, on the basis of geomorphology and composition (Price et al., 1997).

View larger version (50K): [in this window] [in a new window]   Fig. 1. Location map of Victoria, showing the regional extent of the Newer Volcanics Province (shaded area). x, Plains series ol-tholeiites locations. Dashed line represents the inferred position of the Mortlake Discontinuity (MD; Nicholls et al., 1993; Price et al., 1997).

 

The Plains basalts form a thin basaltic veneer over much of western Victoria (generally <60 m), with individual basaltic flows at most tens of kilometres in length and generally <10 m in thickness (Price et al., 1997). Basalts of the Plains basalt sub-province are predominantly tholeiitic to transitional in composition, with moderately to strongly alkalic basalts making up only 2% of the total erupted volume (Price et al., 1988). The most prominent volcanic features in Victoria are the numerous cones, maars and tuff rings that make up the Cones series of the Newer Volcanics Province. The Cones basalts are predominantly alkalic in composition, ranging from alkali olivine basalts to nepheline mugearites. Many of the Cones basalt localities contain abundant mantle and lower-crustal xenoliths, and these have been the subject of numerous petrological and geochemical studies (e.g. Frey & Green, 1974; Irving, 1974a, 1974b; Ellis, 1976; Nickel & Green, 1984; Griffin & O’Reilly, 1987; Greig, 1995). K–Ar dating (McDougall et al., 1966; Wellman & McDougall, 1974) and the field relationships of Plains and Cones series basalts indicate that the Plains series basalts (volumetric peak 2–3 Ma) are generally older than the more prominent Cones series basalts (peak activity 5–25 ka).

	SAMPLE SELECTION AND DESCRIPTION

TOP ABSTRACT INTRODUCTION BACKGROUND TO THE NEWER... SAMPLE SELECTION AND DESCRIPTION GEOCHEMISTRY DISCUSSION CONCLUSIONS APPENDIX 1 REFERENCES

 
Six ol-tholeiites from the Plains series and two ne-hawaiites from the Cones series were selected for this study. The six ol-tholeiites are from various localities in western Victoria (Fig. 1) and represent samples from several different Sr isotopic domains identified by Price et al. (1997). The two ne-hawaiites are from the xenolith-rich Anakies (middle cone) and Mt Porndon volcanic centres (Fig. 1). More general descriptions of the petrography of the Newer Volcanics Province basalts have been given by Ellis (1976), Irving & Green (1976), Frey et al. (1978) and Price et al. (1997). All samples were analysed for major elements by X-ray fluorescence (XRF); trace elements by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS); Rb–Sr, Sm–Nd and U–Pb isotopic data by thermal ionization mass spectrometry (TIMS); and Re–Os isotopic data by negative-TIMS (N-TIMS). All analytical techniques are outlined within the Appendix.

Plains series ol-tholeiites
Olivine is a ubiquitous phenocryst phase in all of the tholeiites studied; however, one of these samples (101) also contains phenocrysts of plagioclase. Olivine phenocrysts are generally euhedral and in several samples exhibit minor iddingsitization. The groundmass is generally medium grained (doleritic) and consists of plagioclase, clinopyroxene, olivine, opaques and a brown–black mesostasis. In general the samples are very fresh; however, there is evidence for late-stage groundwater infiltration in several samples (e.g. 160) in which the vesicles have been filled by carbonate and/or zeolite.

Cones series ne-hawaiites
The alkali basalts are considerably finer grained than the tholeiitic samples studied, with a dominant glassy groundmass. Both AB-1 and PB-1 are highly vesicular and have abundant small phenocrysts of olivine (0·1 mm), within a cryptocrystalline groundmass of plagioclase, clinopyroxene, olivine and glass. Sample AB-1 contains both mantle and crustal xenocrystic material, including large embayed clinopyroxene and olivine megacrysts, large angular andesine grains and quartz–plagioclase aggregates. The andesine and quartz–plagioclase xenocrysts show no evidence of interaction with the magma and most probably represent portions of a large exposed granitic body through which the Anakies basalts erupted. Sample PB-1 also contains some embayed olivine and clinopyroxene megacrysts and these are thought to be disaggregated mantle xenolith material, similar to the larger peridotite xenoliths hosted by this basalt. To minimize potential contamination by xenolithic material, the samples were coarsely crushed and then hand-picked under a binocular microscope before crushing to a fine powder.

	GEOCHEMISTRY

TOP ABSTRACT INTRODUCTION BACKGROUND TO THE NEWER... SAMPLE SELECTION AND DESCRIPTION GEOCHEMISTRY DISCUSSION CONCLUSIONS APPENDIX 1 REFERENCES

 
Major and trace elements
Typical major element and trace element characteristics of the Plains series ol-tholeiites have previously been discussed by Price et al. (1997) as part of a more extensive study. Price et al. (1997) showed that there is a continuum of alkalic to tholeiitic compositions within the Plains basalts, with diffuse positive correlations between Al₂O₃ and SiO₂ and negative correlations between FeO^*, MgO, K₂O and CaO. All of the ol-tholeiites studied have relatively low mg-number ranging from 66 to 64 (Table 1).

View this table: [in this window] [in a new window]   Table 1: Major element data and CIPW normative compositions for NVP basalts

 

The ne-hawaiites studied from Mt Porndon and the Anakies are lower in SiO₂ (46 and 48 wt %) and higher in TiO₂ (3 wt %) than the tholeiitic basalts (Table 1). All of the basalts studied have smooth light rare earth element (LREE)-enriched patterns, similar to those found in other studies of intraplate basalts from the Newer Volcanics Province (e.g. Price et al., 1997) and localities worldwide (Thompson et al., 1983; Huang et al., 1997). The ol-tholeiites and ne-hawaiites have similar heavy REE abundances at 8x to 11x chondrites; however, the ne-hawaiites (AB-1 and PB-1) are considerably more LREE enriched with LREE abundances of 200x chondrite compared with 65x to 100x chondrite for the ol-tholeiites (Table 2, Fig. 2).

View this table: [in this window] [in a new window]   Table 2: Trace element data for the NVP basalts

 

View larger version (36K): [in this window] [in a new window]   Fig. 2. Primitive mantle normalized [normalizing values from McDonough & Sun (1995)] multi-element diagrams for basalts from the Newer Volcanics Province. (a) and (b) Plains series ol tholeiites; (c) Cones series ne-hawaiites. The sequence of the trace elements is from McDonough (1990). The shaded field represents a comprehensive compilation of data from the literature showing a complete range of compositions for basalts from the Newer Volcanics Province (McDonough et al., 1985; Price et al., 1997).

 

All ol-tholeiites studied have very similar relatively smooth incompatible element enriched patterns (Fig. 2). The two ne-hawaiites are more enriched than the ol-tholeiites. The ne-hawaiites also have negative Zr, U and Th anomalies. In general, the Newer Volcanics Province basalts have trace element patterns and abundances similar to those observed in ocean-island basalts (OIB; Fig. 2).

Leaching experiments
On the basis of a series of leaching experiments and a study of a basalt weathering profile, Price et al. (1991) concluded that the Sr-isotope systematics of western Victorian basalts are in general only slightly altered by late-stage groundwater infiltration [see also McDonough et al. (1985) and Price et al. (1997)]. None the less, all samples used in our study were pretreated for Sr isotopic analysis by acid leaching. Powders were leached in 2N HCl at 120°C for 16 h. An aliquot of one sample (242b) was also leached in cold 1·25N HCl and ultrasonicated for 10 min. Leached residues were thoroughly rinsed with MQ H2O before processing. Data for sample 101 confirm the general conclusion that leached samples do not have substantially different Sr isotopic compositions from the leached residues (Table 3). The leachates generally have slightly higher ⁸⁷Sr/⁸⁶Sr isotopic compositions than the leached residues, although one sample (242) shows a more significant difference between leachate and residue. Petrographic examination of the samples indicates that the differences could be in each case related to amount of carbonate. Leachates from samples 160 and 242 show the highest Sr isotopic ratios relative to the leached residues and these two samples contain significant amounts of carbonate.

View this table: [in this window] [in a new window]   Table 3: Sr and Os-isotope leaching experiments

 

Leaching experiments were carried out on sample 242 to investigate whether the Os-isotope systematics was also affected. Tests were made by leaching the sample powder in 4N HCl and ultrasonicating for 10–15 min. The leached residue has an Os isotopic composition (¹⁸⁷Os/¹⁸⁸Os = 0·1787 ± 19), within error of the unleached analysis (242: ¹⁸⁷Os/¹⁸⁸Os = 0·18096 ± 52). However, the leached sample contains slightly lower Os concentrations (37 ppt vs 44 ppt, where ppt means parts per trillion) and significantly lower Re concentrations (80 ppt vs 185 ppt). Osmium leachates proved to be difficult to analyse and have correspondingly large errors with ¹⁸⁷Os/¹⁸⁸Os = 0·172 ± 56 to 0·201 ± 46, but are within error of both the leached and unleached analyses. The above leaching experiments indicate that the Os isotopic signatures have not been significantly altered by the introduction of late-stage carbonate material. This is also supported by the lack of correlation between Os isotopic signature and the amount of carbonate. For example, sample 242 has the most carbonate of any of the samples studied except 160, but has the lowest Os isotopic composition. However, the change in Re concentration observed between the unleached (185 ppt) and leached (80 ppt) sample 242 does indicate that the presence of late-stage carbonate material can increase the Re concentration and hence Re/Os, and that for samples significantly older than the Newer Volcanics Province basalts this may increase the Os isotopic composition.

Isotope geochemistry
A regional discontinuity has been defined on the basis of the Sr isotopic compositions of Newer Volcanics Province basalts on either side of a north–south line passing through the town of Mortlake (Price et al., 1988, 1997; Nicholls et al., 1993). Basalts to the east of the ‘Mortlake Discontinuity’ (Fig. 1) generally exhibit more radiogenic Sr isotopic compositions than those to the west (Price et al., 1988, 1977; Nicholls et al., 1993). The Plains series ol-tholeiites analysed for this study are all from east of the ‘Mortlake Discontinuity’ (Price et al., 1997) and have leached Sr isotopic compositions ranging from ⁸⁷Sr/⁸⁶Sr 0·70446 to 0·70531 (Table 3), within the range of ⁸⁷Sr/⁸⁶Sr isotopic compositions observed for the eastern Plains series ol-tholeiites (Price et al., 1997). The Plains series ol-tholeiites also have Nd isotopic compositions (_Nd = +0·89 to +3·03; Table 4) that fall within the Nd isotopic range observed for the Cones series ne-hawaiites (McDonough et al., 1985). The two Cones series ne-hawaiites have relatively unradiogenic Nd isotopic compositions (_Nd = +3·80 to +4·07), close to the least radiogenic Nd isotopic compositions observed for other Cones series ne-hawaiites (e.g. McDonough et al., 1985).

View this table: [in this window] [in a new window]   Table 4: Sm–Nd, Pb and Re–Os isotopic data for basalts from the Newer Volcanics Province

 

Figure 3 plots leached ⁸⁷Sr/⁸⁶Sr vs _Nd for basalts from the Newer Volcanics Province and shows the shift of the Plains basalts to more radiogenic ⁸⁷Sr/⁸⁶Sr isotopic compositions relative to the Cones series ne-hawaiites. Figure 4 shows a plot of ¹⁴⁷Sm/¹⁴⁴Nd vs ¹⁴³Nd/¹⁴⁴Nd for the Newer Volcanics Province basalts. The Cones series ne-hawaiites have less radiogenic Nd isotopic compositions than the Plains series ol-tholeiites, despite having lower ¹⁴⁷Sm/¹⁴⁴Nd ratios.

View larger version (66K): [in this window] [in a new window]   Fig. 3. 87Sr/86Sr vs Nd for Plains series ol-tholeiites from the Newer Volcanics Province. , ol-tholeiites from this study; , Newer Volcanics Province Cones series ne hawaiites from the literature (McDonough et al., 1985). Fields for Columbia River flood basalts [Chesley & Ruiz (1998) and references therein] and the SE Australian sub-continental lithospheric mantle (SCLM; McDonough & McCulloch, 1987; McBride et al., 1996; McBride, 1998) shown for comparison. BE, Bulk silicate Earth (Goldstein et al., 1984; DePaolo, 1988). Dashed parallel lines represent the mantle array [position from Rollinson (1993)].

 

View larger version (48K): [in this window] [in a new window]   Fig. 4. 147Sm/144Nd vs 143Nd/144Nd for basalts from the Newer Volcanics Province. Field for SE Australian sub-continental lithospheric mantle (SCLM; McDonough et al., 1987; McBride et al., 1996; McBride, 1998) shown for comparison. DM, depleted mantle (Michard et al., 1985; Taylor & McLennan, 1985).

 

Pb isotopic data for the Newer Volcanics Province basalts are presented in Table 4 and Fig. 5. All of the Newer Volcanics Province basalts analysed fall well to the right of the geochron (Faure, 1986), suggesting that they have evolved from sources that have experienced a long-term history of elevated U/Pb ratios. The five Plains series ol-tholeiites and two Cones series ne-hawaiites analysed have ²⁰⁶Pb/²⁰⁴Pb values ranging from 18·57 to 18·84, falling within the range observed for many OIB, continental flood basalt provinces (e.g. Zindler & Hart, 1986; Carlson, 1991) and other Newer Volcanics Province basalts [²⁰⁶Pb/²⁰⁴Pb 18·4–18.9; C. M. Gray, unpublished data, see fig. 10 of Price et al. (1997)]. However, all of the Newer Volcanics Province basalts have elevated ²⁰⁷Pb/²⁰⁴Pb at a given ²⁰⁶Pb/²⁰⁴Pb (see Fig. 5), plotting close to Pb isotopic compositions observed for EMII basalts, basalts of some continental flood basalt provinces and portions of the continental crust [Rollinson (1993) and references therein].

View larger version (37K): [in this window] [in a new window]   Fig. 5. (a) 207Pb/204Pb vs 206Pb/204Pb for Plains and Cones series ne-hawaiites from the Newer Volcanics Province. Fields for lower continental crust, upper continental crust, mid-ocean ridge basalts (MORB), continental flood basalts (CFB) and the geochron are plotted for comparison (Zindler & Hart, 1986; Carlson & Hart, 1988; Hooper & Hawkesworth, 1993; Rollinson, 1993; Kent, 1995). (b) 208Pb/204Pb vs 206Pb/204Pb for Plains and Cones series ne-hawaiites from the Newer Volcanics Province. The field for MORB is shown for comparison (Zindler & Hart, 1986; Rollinson, 1993).

 

Re–Os isotopic data for ol-tholeiites and ne-hawaiites from the Newer Volcanics Province are reported in Table 4. The Plains series ol-tholeiites have extremely low Os abundances (12–45 ppt) and relatively low Re abundances, ranging from 69 to 242 ppt. The Cones series ne-hawaiites have significantly higher Os abundances (160–251 ppt), but their Re abundances (185–228 ppt) fall within the upper end of the range observed for the Plains series ol-tholeiites. Because of their variable Re and Os abundances, the Plains series ol-tholeiites exhibit variable ¹⁸⁷Re/¹⁸⁸Os ratios, ranging from 10·92 to 80·24, considerably higher than those observed within the Cones series ne-hawaiites (¹⁸⁷Re/¹⁸⁸Os = 3·55–6·88). A plot of Os abundance vs Re/Os (Fig. 6) shows that all of the Newer Volcanics Province basalts fall well within the field for mantle-derived melts.

View larger version (54K): [in this window] [in a new window]   Fig. 6. Os (ppb) vs Re/Os for the Newer Volcanics Province basalts. Field for sub-continental lithospheric mantle and mantle melts plotted for comparison (Walker et al., 1988, 1989, 1991; Martin, 1991; Reisberg et al., 1991; Pegram & Allègre, 1992; Reisberg et al., 1993; Carlson & Irving, 1994; Martin et al., 1994; Roy-Barman & Allègre, 1994, 1995; Marcantonio et al., 1995; Pearson et al., 1995a, 1995b; Bennett et al., 1996; Brandon et al., 1996; Hauri, 1996; Hauri et al., 1996; Kumar et al., 1996; McBride et al., 1996; Meisel et al., 1996; Roy-Barman et al., 1996; Widom & Shirey, 1996; Handler et al., 1997; Saal et al., 1998; McBride, 1998). CM, chondritic mantle abundances (Morgan, 1986).

 

Os isotopic ratios vary widely within the Newer Volcanics Province basaltic suite, ranging from ¹⁸⁷Os/¹⁸⁸Os of 0·1342 to 0·4456. The Cones series ne-hawaiites have slightly radiogenic Os isotopic compositions (_Os = +6 to +8), similar to those observed within many OIB (Fig. 7). In contrast, the Plains series ol-tholeiites have more radiogenic Os isotopic compositions, ranging from _Os of +42 to +250, considerably more radiogenic than those observed within uncontaminated oceanic basalts (>0·03 ppb; Fig. 7). However, they are only slightly more radiogenic than other intraplate basalts from the Columbia River region (Chesley & Ruiz, 1998) and northwestern USA (Hart et al., 1997).

View larger version (22K): [in this window] [in a new window]   Fig. 7. Os concentration vs Os for the Newer Volcanics Province basalts. Source for continental flood basalts is Chesley & Ruiz (1998). Field of ocean-island basalts shown for comparison (Martin, 1991; Pegram & Allègre, 1992; Hauri & Hart, 1993; Reisberg et al., 1993; Martin et al., 1994; Marcantonio et al., 1995; Roy-Barman & Allègre, 1995). The vertical line represents the artificial 0·03 ppb cut-off, which is often used by researchers (e.g. Marcantonio et al., 1995) to exclude potentially contaminated OIB; the horizontal line is Os = 0.

 

A diffuse positive trend is observed between ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os (Fig. 8) with the basalts with the most radiogenic Os isotopic compositions also generally having the highest ¹⁸⁷Re/¹⁸⁸Os. However, there is clearly no simple ‘isochron’ relationship between the different Newer Volcanics Province basalts.

View larger version (52K): [in this window] [in a new window]   Fig. 8. 187Re/188Os vs 187Os/188Os for Plains and Cones series ne-hawaiites from the Newer Volcanics Province. Field of literature intraplate basalts shown for comparison (Hart et al., 1997; Chesley & Ruiz, 1998).

 

	DISCUSSION

TOP ABSTRACT INTRODUCTION BACKGROUND TO THE NEWER... SAMPLE SELECTION AND DESCRIPTION GEOCHEMISTRY DISCUSSION CONCLUSIONS APPENDIX 1 REFERENCES

 
Petrogenesis of the Newer Volcanics Province basalts
The origin and petrogenesis of the Newer Volcanics Province basalts have been the subject of a number of geochemical and isotopic studies (e.g. Irving & Green, 1976; Frey et al., 1978; McDonough et al., 1985; Price et al., 1997). Major and trace element studies undertaken by Frey et al. (1978) and Irving & Green (1976) showed that it is possible to generate the wide variety of basaltic magma types observed within the Newer Volcanics Province by variable melting of an incompatible element enriched pyrolite source. However, because of the marked variability of Sr, Nd and Os isotopic compositions within the Newer Volcanics Province, the Cones and Plains series ol-tholeiites are clearly not related to a single source by a simple genetic process.

Price et al. (1997) recognized a number of distinct Sr isotopic domains within Plains series ol-tholeiites throughout Victoria and suggested that this required a variable lithospheric mantle source influence. The Cones series ne-hawaiites also show some degree of isotopic heterogeneity, with significant Sr, Nd and Pb isotopic differences from east to west across the Newer Volcanics Province (McDonough et al., 1985). The regional and local Sr, Nd and Pb isotopic heterogeneities observed within the Newer Volcanics Province basalts led Price et al. (1997) and McDonough et al. (1985) to propose petrogenetic models whereby the Plains and Cones series ne-hawaiites are the products of variable mixing between melts from plume (OIB) and lithospheric mantle sources. The isotopic heterogeneity observed within primary magmas, particularly those represented by the mantle xenolith-bearing Cones series ne-hawaiites, requires a heterogeneous mantle source. However, the relative influence of plume and lithospheric mantle sources in the genesis of the two Newer Volcanics Province basaltic series remains unclear. The petrogenetic models proposed by Price et al. (1997) and McDonough et al. (1985), although in general agreement regarding source end-members, are fundamentally different with respect to the influence of each end-member (plume vs the sub-continental lithospheric mantle) on the two Newer Volcanics Province series. Price et al. (1997) envisaged that the Plains series basalts are dominated by lithospheric mantle sources and the Cones series basalts by OIB-type sources, whereas McDonough et al. (1985) proposed the opposite relationship.

The influence of crustal contamination on isotopic compositions observed within the Newer Volcanics Province is generally thought to be relatively minor (e.g. McDonough et al., 1985; Price et al., 1997). However, particularly in the case of the Plains series ol-tholeiites, the possibility of contamination of the magmas by material not observed at the surface within the Lachlan Fold Belt (LFB) crust cannot be dismissed.

In the following sections Re–Os isotopic data, in conjunction with more widely used Sr, Nd and Pb isotopic data, will be used to evaluate petrogenetic models for the origin of the Newer Volcanics Province basalts. Three main end-member models will be discussed: (1) a plume-dominated or asthenospheric mantle-dominated source; (2) a lithospheric mantle source; or (3) contamination of mantle-derived melts by either the continental crust or radiogenic portions of the sub-continental lithospheric mantle (e.g. pyroxenite layers).

Primary and/or primitive magmas?
The identification of primary vs more evolved magmas is an important part of studying the petrogenesis of a particular suite of basaltic magmas, as primitive magmas will provide more direct information on the nature of the mantle source whereas the more evolved magmas may have had more opportunity to interact with the lithosphere en route to the surface, possibly obscuring the original mantle source signature. Primitive or at least primary magmas are generally recognized on the basis of their highly mafic geochemistry and the presence of mantle xenoliths. On this basis the two Cones series ne-hawaiites (AB-1 and PB-1) represent primitive magmas as they both have high Ni concentrations (>320 ppm) and contain mantle xenoliths, suggesting that they have travelled rapidly through the lithosphere and therefore have probably not been modified by secondary crystal fractionation and assimilation processes. The Anakies ne-hawaiite (AB-1) also contains granite inclusions, which show no evidence of reaction with the melt, again indicating that this basalt has probably not been significantly modified by assimilation processes. However, the analysed Plains series ol-tholeiites, and indeed the Plains series ol-tholeiites in general (Price et al., 1997), have undergone olivine fractionation (<10% in most cases). The Plains series ol-tholeiites generally do not contain mantle xenoliths and also have lower Ni abundances (213 ppm) and mg-numbers (<66), and higher SiO₂ abundances than the Cones series ne-hawaiites (Tables 1 and 2; Price et al., 1997). Therefore, if crustal contamination rather than source isotopic variability plays a significant role in the petrogenesis of the Newer Volcanics Province basalts, we would expect to observe radiogenic Os isotopic compositions for these more evolved basalts, rather than for the more primitive Cones series ne-hawaiites.

A plume and/or asthenospheric mantle source?
The Cenozoic volcanism within southeastern Australia is considered by some workers to be the result of the interaction of one or more mantle plumes with the southeastern Australian lithosphere (e.g. Wellman & McDougall, 1974; Sutherland, 1981; Wellman, 1983) or alternatively continental rifting processes (e.g. Karner & Weissel, 1984; Price et al., 1997). Therefore, the possible influence of a plume (OIB) or an asthenospheric (DMM) mantle source on the formation of the Newer Volcanics Province basalts needs to be investigated. Trace element abundances and multi-element patterns for the Newer Volcanics Province (Fig. 2) are similar to those observed for many OIB, leading some workers to suggest that there has been a significant influence of an OIB-type source (e.g. McDonough et al., 1985). In addition, all of the Newer Volcanics Province Cones series basalts have Sr and Nd isotopic compositions that fall within the ‘mantle array’ defined by oceanic lavas worldwide. However, the Plains series ol-tholeiites trend off to the right of the mantle array with more radiogenic ⁸⁷Sr/⁸⁶Sr isotopic compositions, similar to some Columbia River flood basalts (Fig. 3). Fig. 9 shows _Nd vs _Os for the Cones and Plains series ol-tholeiites analysed for this study. The two Cones series ne-hawaiites fall within the OIB range, but the Plains series ol-tholeiites have significantly more radiogenic Os isotopic compositions (_Os = +42 to +250) than most OIB (_Os < +25).

View larger version (40K): [in this window] [in a new window]   Fig. 9. Os vs Nd for the Newer Volcanics Province basalts. Fields for ocean-island basalts (OIB), sub-continental lithospheric mantle (SCLM; peridotite xenoliths), and the position of depleted MORB mantle (DMM) are shown for comparison (refer to Fig. 6 for data sources). Columbia River flood basalts (Chesley & Ruiz, 1998) and general trend for the continental crust (e.g. Esser & Turekian, 1993) are also shown for comparison. Vertical and horizontal lines represent chondritic values for Nd and Os (see Table 4 caption for parameters and definitions).

 

If all of the Newer Volcanics Province basalts are formed from plume or asthenospheric mantle sources, with no interaction with the lithosphere en route to the surface, the radiogenic Os isotopic compositions observed within some samples must be a primary feature of the mantle source. Figure 10 shows the results of mixing Pb and Os from average oceanic crust and oceanic crust + 10% pelagic sediment with a plume mantle source, with the Newer Volcanics Province basalts plotted for comparison. To obtain the highly radiogenic Os isotopic compositions observed within some Newer Volcanics Province basalts, >60% of either contaminant mix is required (Fig. 10). It is difficult to envisage the physical processes that would be required to mix such a large amount of relatively cold subducted slab with a plume or convecting mantle source. In addition, the highly radiogenic Os isotopic values observed within the Newer Volcanics Province Plains series ol-tholeiites have not been observed within any oceanic basalts, suggesting that they reflect their continental setting, and are therefore likely to involve a component not found within the convecting mantle.

View larger version (16K): [in this window] [in a new window]   Fig. 10. 206Pb/204Pb vs 187Os/188Os showing calculated mixing trends involving plume-type mantle (Pb = 0·05 ppm, 206Pb/204Pb = 19, Os = 3000 ppt, 187Os/188Os = 0·13) and 1·8 Ga oceanic crust (Pb = 0·25 ppm, 206Pb/204Pb = 23·5, Os = 100 ppt, 187Os/188Os = 5) and 90% 1·8 Ga oceanic crust + 10% pelagic sediment (Pb = 1·73 ppm, 206Pb/204Pb = 17·59, Os = 110 ppt, 187Os/188Os = 4·18). Large filled circle indicates plume-type mantle. Modelling after Hauri et al. (1996). HIMU and EMI shown for reference. Tick marks and numbers on mixing lines represent weight fraction added of each contaminant.

 

Newer Volcanics Province Pb isotopic data also preclude this mechanism for generating the radiogenic Os isotopic compositions, as there is no trend toward either low or high ²⁰⁶Pb/²⁰⁴Pb, which would be expected if large amounts of either oceanic crust or pelagic sediment were introduced into a plume or asthenospheric mantle source by subduction processes (Fig. 10). Therefore, although trace element and Os isotopic data are consistent with a plume source for the Cones series ne-hawaiites, the Plains series ol-tholeiites cannot be derived simply from a similar source by higher degrees of partial melting. In addition, the diffuse relationship observed between ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd (Fig. 4) indicates that there is no simple genetic relationship within the Plains series ol-tholeiites. Alternatively, the Plains series ol tholeiites may be derived from a mantle source similar to that of the Cones series ne-hawaiites, but have undergone contamination with radiogenic Os during their ascent through the lithospheric mantle and crust.

A sub-continental lithospheric mantle source?
Could the Newer Volcanics Province basalts have been derived by melting of a heterogeneous sub-continental lithospheric mantle source? Recent studies of the sub-continental lithospheric mantle (e.g. Walker et al., 1989; Pearson et al., 1995a, 1995b; McBride et al., 1996; Handler et al., 1997) have shown that this reservoir generally evolves over time with low Re/Os ratios to sub-chondritic Os isotopic compositions (_Os < +1). The Plains series ol-tholeiites have significantly more radiogenic Os isotopic compositions (_Os = +42 to +250) than would be derived from either a sub-continental lithospheric mantle source (_Os < +1) or a sub-continental lithospheric mantle or plume source mixture (_Os < +25). However, slightly more radiogenic Os isotopic compositions have been observed within some portions of the sub-continental lithospheric mantle (e.g. _Os = +10, McBride et al., 1996) and sub-arc lithospheric mantle (e.g. _Os +40, Brandon et al., 1996; McInnes et al., 1999). However, these _Os values (40) are still considerably lower than those observed within the Plains series ol-tholeiites (_Os = +42 to +250). In addition, the paucity of mantle peridotite xenoliths that exhibit radiogenic Os isotopic compositions suggest that they do not form a significant portion of the sub-continental lithospheric mantle and therefore are unlikely to be a major source reservoir for the relatively abundant Newer Volcanics Province basaltic magmatism.

Al-augite suite xenoliths, which are accepted as representing cumulates from basaltic magmas within the sub-continental lithospheric mantle (e.g. Griffin et al., 1988; McDonough et al., 1991), may be another potential source for magmas with high ¹⁸⁷Os/¹⁸⁸Os ratios, as they also have radiogenic Os isotopic compositions (_Os = +4 to +327; McBride, 1998). However, to produce the highly radiogenic Os isotopic compositions of the Plains basalts, >90% of the source material would need to be garnet pyroxenite layers, rather than unradiogenic mantle peridotite, because of the large disparity in Os abundances between the two end-members (mantle peridotite 3 ppb vs pyroxenite <0·1 ppb). Thermodynamic modelling using the MELTS program (Ghiorso & Sack, 1995) indicates that melting of these garnet pyroxenite layers at pressures of 20 kbar (McBride, 1998) is unlikely to produce melts with major element compositions, particularly TiO₂, CaO, Na₂O and K₂O, similar to the Newer Volcanics Province ol-tholeiites. Therefore, the garnet pyroxenites probably do not represent suitable sources for the Newer Volcanics Province basalts. Moreover, the Al-augite suite xenoliths themselves are considered to represent cumulates of basaltic magmas traversing the lithospheric mantle, some of which are potentially related to the Newer Volcanics Province magmatism (e.g. Irving, 1974a). However, interaction of mantle melts with radiogenic pyroxenite material within the sub-continental lithospheric mantle may have contributed to the overall isotopic variation observed within the Newer Volcanics Province basalts.

However, what about the Cones series basalts? A number of workers (e.g. Foley, 1992; Carlson et al., 1996; Foley et al., 1998) have proposed that melting of enriched hydrous veins within the sub-continental lithospheric mantle and subsequent reactions of these melts with mantle wallrock is the best mechanism to explain the geochemical characteristics of many alkaline melts (e.g. lamproites, kimberlites, some nephelinites). Less than 20% veined material [assuming 50 ppt Os and _Os = +100; median southeastern Australian pyroxenite xenoliths from McBride (1998)] would be required to be added to average sub-continental mantle peridotite [assuming 3 ppb Os and _Os = -2; average southeastern Australian mantle peridotite from McBride et al. (1996)] to generate the slightly radiogenic Os isotopic compositions observed in the Cones series ne-hawaiites (_Os = +6 to +7). Therefore, the Cones series ne-hawaiites may have been generated by melting of mantle plume material (OIB-type) or alternatively veined sub-continental lithospheric mantle.

Another complicating factor is that the low Os concentrations (12–45 ppt Os) of the Plains series ol-tholeiites renders them susceptible to crustal contamination and assimilation–fractional crystallization (AFC) processes. As a result of the extreme contrast between mantle (e.g. _Os < +25; Walker et al., 1989, 1991; Martin, 1991) and crustal (e.g. _Os > +1000; Esser & Turekian, 1993; Yin et al., 1996) Os isotopic signatures, crustal contamination of mantle melts can obscure primary mantle signatures. Thus, if the Plains series ol-tholeiites have undergone crustal contamination en route to the surface, evidence of their primary mantle Os isotopic composition will be obscured. Therefore, it is difficult to definitively determine whether the Plains series ol-tholeiites were initially derived from asthenospheric, plume or sub-continental lithospheric mantle sources.

Crustal contamination?
The two Cones series ne-hawaiites analysed have trace element abundances similar to those observed for plume-derived oceanic basalts. In addition, Sr, Nd, Pb and Os isotopic data are consistent with their derivation from a convecting mantle source. The Plains series ol-tholeiites are more problematic, as they are isotopically more heterogeneous than the Cones series ne-hawaiites (Dasch & Green, 1975; McDonough et al., 1985; Ewart et al., 1988; Price et al., 1997) and, where both Sr and Nd isotopic data are available, tend to have more evolved isotopic compositions (i.e. higher ⁸⁷Sr/⁸⁶Sr values and lower _Nd values; Fig. 3). Many trace elements are fractionated from each other during the processes that form crust and therefore crustal contamination will often impart distinctive geochemical signatures upon magmas. Several trace element ratios (e.g. Ce/Pb, Nb/U) have been observed by different workers to remain uniform in both MORB and OIB basalts (Hofmann et al., 1986). However, the continental crust has much lower Ce/Pb and Nb/U ratios (4 and 10, respectively; Hofmann et al., 1986) than those of oceanic basalts (25 ± 5 and 47 ± 10; Hofmann et al., 1986), and therefore these ratios can be useful indicators of crustal contamination within continental basalts. The two Cones series ne-hawaiites have Ce/Pb (18·7 and 20·7) and Nb/U (54·5) ratios similar to OIB; however, the Plains series ol-tholeiites have much lower ratios (Ce/Pb 6·6–13·8; Nb/U 22·2–34·4) trending towards more crust-like compositions. The Plains series ol-tholeiites also have higher Ba/Nb ratios (11·0–17·3) than the Cones series ne-hawaiites (6·1–7·3), which may also reflect crustal assimilation. Magmas with a uniform source that have been variably contaminated en route through the continental crust might be expected to show strong correlations between their isotopic compositions and potential contamination indices such as Ce/Pb, Nb/U and SiO₂. Figure 11 shows plots of isotopic ratios vs Ce/Pb. Although the Newer Volcanics Province basalts do not exhibit strong correlations, the analysed samples do show general trends indicating the possible influence of a crustal component(s). Similar trends are observed in plots of isotopic ratios vs SiO₂.

View larger version (21K): [in this window] [in a new window]   Fig. 11. Ce/Pb vs Sr, Nd, Pb and Os isotopic composition for the Newer Volcanics Province basalts. MORB and OIB are shown for comparison where their fields are reasonably well defined (Sun & McDonough, 1989; Rollinson, 1993, and references therein). CC, continental crust (Taylor & McLennan, 1985, 1995; Rudnick & Taylor, 1987; Rollinson, 1993; Rudnick & Fountain, 1995; Saal et al., 1998).

 

The Cones series ne-hawaiites generally plot close to OIB–MORB with the Plains series ol-tholeiites trending towards the continental crust (Fig. 11). In a more extensive study of the Sr isotopic compositions of the Newer Volcanics Province, Price et al. (1997) also noted the presence of a general positive trend between ⁸⁷Sr/⁸⁶Sr and SiO₂, although with a large range of ⁸⁷Sr/⁸⁶Sr at any given SiO₂ abundance. As the analysed Plains series ol-tholeiites represent melts from regions throughout western Victoria, it is perhaps not surprising that they exhibit significant isotopic heterogeneity, which may reflect both the influence of a heterogeneous mantle source and different crustal materials. Studies by Price et al. (1997) have discounted the influence of the upper continental crust on the isotopic compositions of the Newer Volcanics Province basalts on the basis of both trace element and isotopic arguments. However, they did not discount the influence of variable lower-crustal contamination.

A general positive correlation is observed between 1/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os (Fig. 12) within Plains series ol-tholeiites, suggesting that mixing between two isotopically distinct end-members is a major control on ¹⁸⁷Os/¹⁸⁸Os values. A similar, although less well-defined, relationship is observed when 1/Nd is plotted against ¹⁴³Nd/¹⁴⁴Nd. These correlations are consistent with mixing between two reservoirs with the general Nd and Os isotopic characteristics of continental crust (high _Os values and low _Nd values) and asthenospheric mantle (low _Os values and high _Nd values). Two groups can be recognized within the Plains series ol-tholeiites on the basis of their Nd, Sr and Os isotopic compositions. Group 1 (samples 131 and 160) has highly radiogenic _Os values (+186 and +250), relatively radiogenic _Nd values (+2·2 and +3·0) and unradiogenic Sr isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0·70446 and 0·70473), whereas group 2 (samples 101, 196, 242 and 243) has lower _Os values (+42 to +98), lower _Nd values (+0·9 to +2·2) and more radiogenic Sr isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0·70493–0·70531) than group 1. If group 2 Plains basalts alone are considered, general trends consistent with mixing between mantle and a crustal end-member are observed in plots of ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁴Pb, ¹⁴³Nd/¹⁴⁴Nd vs ¹⁸⁷Os/¹⁸⁸Os (Fig. 13). However, group 1 Plains basalts are displaced to more ‘mantle-like’ Nd and Sr isotopic compositions, despite having significantly more radiogenic Os isotopic compositions. The ¹⁸⁷Re/¹⁸⁸Os, combined with the age and composition of continental crust, will have a strong influence on resulting Sr, Nd, Pb and Os isotopic compositions.

View larger version (12K): [in this window] [in a new window]   Fig. 12. 1/188Os (ppt)-1 vs 187Os/188Os for Newer Volcanics Province basalts.

 

View larger version (25K): [in this window] [in a new window]   Fig. 13. (a) 207Pb/204Pb and 206Pb/204Pb vs 187Os/188Os for the Newer Volcanics Province basalts. (b) 87Sr/86Sr and 143Nd/144Nd vs 187Os/188Os for Newer Volcanics Province basalts. Fields 1 and 2 refer to Plains series ol-tholeiites groups 1 and 2.

 

To assess the effects of lower or upper continental crustal contamination on the Os isotopic composition of basaltic melts passing through the southeastern Australian lithosphere, it is necessary to estimate Os elemental abundances and isotopic compositions for the requisite end-members. There have been only a limited number of Re–Os isotopic studies on lower- and upper-crustal samples; however, the continental crust has generally been observed to have relatively low Os abundances (50 ppt; e.g. Esser & Turekian, 1993; Saal et al., 1998) and, depending on the average age of the crust, extremely radiogenic Os isotopic compositions (e.g. up to _Os = +5000; Frick, 1998). A recent study by Saal et al. (1998) presented analyses of a number of lower-crustal xenoliths from the Chudleigh and McBride suites, North Queensland, which ranged from _Os = +230 to +1300, with an average of _Os = +530. The North Queensland lower-crustal xenoliths have highly variable Os concentrations, ranging from 3·2 to 1043 ppt, with median abundances of 50 ppt. To estimate a possible range of crustal Os isotopic compositions within southeastern Australia, an initial ¹⁸⁷Os/¹⁸⁸Os of 0·1271 was assumed and compositions were then modelled for evolution over different time periods using a range of ¹⁸⁷Re/¹⁸⁸Os ratios. Because of the controversy regarding the maximum age of the southeastern Australian lithosphere (e.g. Proterozoic vs early Palaeozoic, Cas, 1983; Gray, 1990), for the purposes of this modelling, continental crust of different ages (250–1500 Ma) was investigated as a potential contaminant. In support of the presence of Proterozoic lithosphere, Re–Os isotopic studies of mantle peridotite xenoliths from the Newer Volcanics Province (McBride et al., 1996; Handler et al., 1997; McBride, 1998) indicate that portions of the sub-continental lithospheric mantle beneath southeastern Australia have undergone melt depletion at least as early as the late Proterozoic [Re depletion (T_RD) model ages 1·1 Ga].

Another uncertainty in modelling the effects of crustal contamination is the difference between the Os isotopic compositions of the lower and upper continental crust. Some workers suggest that the lower crust is less radiogenic than the upper crust (e.g. Yin et al., 1996; Saal et al., 1998), whereas others believe the reverse (e.g. Johnson et al., 1996; Asmerom & Walker, 1998; Lambert et al., 1998b). It is clear that until further Os isotopic data are obtained for both lower and upper crust of different ages, modelling of crustal contamination will be relatively poorly constrained. However, available Os isotopic data for the continental crust indicate that it is a highly radiogenic reservoir, distinct from both the sub-continental lithospheric mantle and MORB and OIB sources.

AFC processes lead to more pronounced increases in the Os isotopic compositions of basaltic magmas relative to those produced by bulk mixing, as Os behaves compatibly and is retained within the fractionating crystal phases. This means that as AFC proceeds the melt is more easily contaminated by crustal material, as the relative difference between Os abundances within the melt and crust increases. If a melt derived from sub-continental lithospheric mantle rather than an OIB-type melt is modelled, a higher proportion of added crustal material is required to produce the radiogenic Os isotopic signatures observed within the Plains series ol-tholeiites.

Figure 14 presents results of AFC modelling between OIB-type melts and continental crust of different ages. For sulphide-undersaturated systems a relatively low D_Os = 7 value was assumed (Saal et al., 1998). However, if the magmas reached sulfide saturation before contamination, as suggested by Vogel & Keays (1997), a higher D_Os is more appropriate, as sulphide rather than olivine fractionation would have a larger effect on the Os concentration ([Os]) within magmas. Assuming that the sulphide melt/silicate melt K_{d Os} = 15 000 based on K_{d Ir} from Peach et al. (1990)], and up to 1% sulphide fractionation, in addition to olivine fractionation, a bulk D_Os of up to 150 is estimated. To assess the validity of the AFC modelling, all calculations were completed for a range of R values (0·2–0·9), crustal contaminants evolved with a range of ¹⁸⁷Re/¹⁸⁸Os ratios (10–250) and D_Os ranging from 20 to 150 for sulphide-saturated magmas. Os concentrations were modelled in conjunction with the Os–Nd isotopic models (Fig. 14), to place an additional constraint on the AFC models. All parameters used in the modelling are listed in the caption to Fig. 14.

View larger version (30K): [in this window] [in a new window]   Fig. 14. 143Nd/144Nd and Os (ppt) vs 187Os/188Os showing assimilation–fractional crystallization (AFC) between OIB basaltic melts and continental crust of different ages. Continental crust was evolved for different time periods assuming 143Nd/144NdDM = 0·513114, 147Sm/144NdDM = 0·222 (Michard et al., 1985), Nd = 6·54 x 10-12 a-1 (Lugmair & Marti, 1978), 187Os/188OsCHUR,t = 4·56Ga = 0·096, 187Re/188OsCHUR = 0·40076 (Walker & Morgan, 1989), Re = 1·666 x 10-11 a-1 (Smoliar et al., 1996), average lower-crustal xenolith values for 147Sm/144NdCRUST = 0·26, Nd = 15 ppm (Rudnick & Taylor, 1987; Rudnick & Fountain, 1995; Taylor & McClennan, 1995), 187Re/188OsCRUST = 25 (Saal et al., 1998), Os = 50 ppt (Saal et al., 1998). AFC modelling constraints: R [(rate of assimilation of crust)/(rate of fractional crystallization)] for all models shown above is 0·9. The melt is assumed to have an initial 143Nd/144NdMELT = 0·513, Nd = 26 ppm (Sun & McDonough, 1989; Rollinson, 1993, and references therein), 187Os/188Os = 0·128MELT and Os = 70 ppt (average tholeiite; Walker et al., 1989, 1991; Birck & Allègre, 1994; Martin et al., 1994). Olivine only fractionation DNd = 0·0066 (Arth, 1976), DOs = 7 (Saal et al., 1998). Bulk DOs calculated for olivine + sulphide fractionation assuming Kd Os silicate melt = 7, Kd Os sulphide melt = 15 000 (from Kd Ir = 15 000, Peach et al., 1990, for all models above DOs sulphide = 35 based on 0·1% sulphide fractionating in the assemblage. (a) and (b) 250 Ma evolved crust, 143Nd/144Nd = 0·51189 and 187Os/188Os = 0·230; (c) and (d) 1000 Ma evolved crust has 143Nd/144Nd = 0·50964 and 187Os/188Os = 0·541. All AFC and bulk mixing models were calculated on the basis of mixing 187Os and 188Os separately. •, starting composition. Tick marks on lines represent the percentage of crustal contamination.

 

Modelling presented in Fig. 14a and b suggests that AFC processes involving young lower crust (250 Ma) cannot generate the isotopic compositions observed within the group 1 Plains series ol-tholeiites. It is possible to form the group 2 ol-tholeiites with up to 18% AFC (Fig. 14a) with such young crust; however, [Os] constraints (Fig. 14b) indicate that such high degrees of AFC would result in much lower [Os] values (<10 ppt) than are observed in the Plains series ol-tholeiites (12–45 ppt). Changing the value of R (ratio of assimilation to fractionation) from 0·2 to 0·9 does not improve the fit of the data to the AFC trends. AFC models using high ¹⁸⁷Re/¹⁸⁸Os (250) crustal assimilants of any age form trends well above the group 2 ol-tholeiites. This indicates that although this type of crust could produce the radiogenic Os isotopic compositions of the Plains series basalts at relatively small degrees of AFC, it cannot also produce the observed Nd isotopic compositions. It is not until the low ¹⁸⁷Re/¹⁸⁸Os (25) crustal assimilant is 750 Ma or more that the AFC trends (R 0·5) begin to plot close to the group 2 ol-tholeiites. The best-fit AFC model for the group 2 ol-tholeiites is for assimilation with 1000 Ma low ¹⁸⁷Re/¹⁸⁸Os (25) continental crust with D_Os = 7 and R = 0·9, which requires <2·5% AFC to form both the observed [Os] and Nd–Os isotopic compositions (Fig. 14c and d). Group 1 ol-tholeiites are more problematic, plotting with low [Os] and more radiogenic Os isotopic compositions and requiring either a higher D_Os (= 35; Fig. 14d), or alternatively contamination with continental crust that has evolved over time with a much higher ¹⁸⁷Re/¹⁸⁸Os ratio (>100). Bulk mixing models (not shown) do not fit the data as well as AFC models and require significantly higher degrees of contamination. For example, bulk mixing between an OIB-type melt and 1000 Ma low ¹⁸⁷Re/¹⁸⁸Os continental crust requires 15% contamination in comparison with AFC, which requires <2·5% using the same parameters.

Although this modelling is not well constrained, it suggests that AFC processes in Proterozoic lower continental crust acting on OIB-type melts are a viable method of generating the highly radiogenic Os isotopic signatures observed within the Newer Volcanics Province basalts. However, the presence of Proterozoic continental crust beneath SE Australia is highly controversial. An alternative possibility that there is an older isotopically evolved component within the lithospheric mantle cannot be entirely ruled out but the currently available data best support a model whereby the isotopic variation observed within the Newer Volcanics Province basalts arises from AFC processes occurring in the crust.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION BACKGROUND TO THE NEWER... SAMPLE SELECTION AND DESCRIPTION GEOCHEMISTRY DISCUSSION CONCLUSIONS APPENDIX 1 REFERENCES

 

1. Trace element abundances and multi-element patterns for Newer Volcanics Province basalts are similar to those observed within many OIB-type basalts. The Cones series ne-hawaiites have relatively high Ni concentrations (>320 ppm) and contain abundant mantle xenoliths, indicating that they represent ‘primitive’ magmas that have travelled rapidly through the lithosphere. In contrast, the Plains series ol-tholeiites have experienced more extensive olivine fractionation, implying that they have had a greater crustal residence time, and therefore have had more opportunity to assimilate crustal material en route to the surface.
   
2. The Cones and Plains series ol-tholeiites have distinctive isotopic compositions, indicating that they are not related to each other by a simple genetic process affecting a single mantle source. In addition, the analysed Plains series ol-tholeiites also exhibit significant isotopic heterogeneity, suggesting that they may have had several different source magmas and/or experienced different degrees of contamination.
   
3. Some Plains series ol-tholeiites have extremely radiogenic Os isotopic compositions (_Os = +42 to +250), significantly higher than those of any uncontaminated oceanic basalt (< +25). In contrast, the Cones series ne-hawaiites have only slightly radiogenic Os isotopic compositions (_Os = +6 to +7), falling well within the field for OIB. Several trace element ratios (e.g. Ce/Pb, Nb/U) sensitive to the influence of crustal material show scattered correlations with different isotopic systems trending towards more crustal-like compositions within the Plains series ol-tholeiites. However, the Cones series ne-hawaiites generally have Ce/Pb, Nb/U and Nd–Sr–Pb–Os isotopic compositions similar to those found in OIB.
   
4. The radiogenic Os isotopic compositions observed within the Plains series ol-tholeiites can be generated either by melting of a radiogenic source or, alternatively, contamination of the magmas en route to the surface. Contamination of the Newer Volcanics Province Plains series magmas by Proterozoic crustal material (>750 Ma) is considered to be more likely. AFC modelling between different continental crustal end-members and OIB-type melts indicates that the radiogenic Os isotopic signatures observed for the group 2 Plains series ol-tholeiites can be generated by <2·5 wt % AFC. The group 1 ol-tholeiites either require a higher D_Os or were contaminated with isotopically distinct continental crust. The Newer Volcanics Province Plains series basalts most probably represent either plume- or sub-continental lithospheric mantle-derived magmas which were contaminated within the crust, whereas the Cones series magmas ascended quickly and did not assimilate crustal material en route to the surface. The Cones series ne-hawaiites have trace element and isotopic compositions consistent with their derivation from melting of a mantle plume (OIB-type) or veined sub-continental lithospheric mantle.
   

	APPENDIX 1

TOP ABSTRACT INTRODUCTION BACKGROUND TO THE NEWER... SAMPLE SELECTION AND DESCRIPTION GEOCHEMISTRY DISCUSSION CONCLUSIONS APPENDIX 1 REFERENCES

 
Analytical techniques
For all whole-rock analyses samples were crushed to a fine powder using a ceramic jaw crusher and agate Tema^TM mill, which were cleaned with quartz–silica sand and precontaminated with a small amount of sample material between each separate sample. For samples containing mantle and crustal xenoliths (AB-1 and PB-1), samples were handpicked after ceramic jaw crushing under a binocular microscope before being crushed in an agate Tema^TM mill.

Major and trace element analyses.
Major elements were analysed on a Siemens SRS3000 X-ray fluorescence mass spectrophotometer at the University of Melbourne using a 5:1 lithium metaborate–rock powder fused bead. The technique used is a modification of that described by Haukka & Thomas (1977) and Mori et al. (1999). Matrix corrections were determined by duplicate preparation of 64 certified reference and spec-pure synthetic samples and using the Lachance–Traill algorithm (S. Reeves, personal communication, 2000). In general, relative accuracy for the major elements analysed is ±0·5%. All trace elements were determined by inductively coupled plasma–mass spectrometry (ICP-MS) on a Finnigan-MAT ELEMENT high-resolution magnetic sector instrument at Monash University. Preparation of samples for ICP-MS analysis involved digestion of 50–100 mg of sample powder by repeated fluxing with HF–HNO₃, HNO₃ and HCl. Samples were then picked up in a solution of 2% HNO₃ containing 10 ppb In as an internal standard and then diluted 6000 times. One analytical blank and four standard solutions (based on 50 mg and 100 mg of digested US Geological Survey standard BHVO-1, BIR-1 and/or BCR-1) were typically prepared with each run. BHVO-1 was used as a check standard to monitor accuracy during the analysis procedure. Precision is typically better than 5%, with accuracy 5% at the 95% confidence level.

Re–Os isotopic analyses.
Re and Os concentrations and Os isotopic compositions were determined by isotope dilution. Sample powder (5–10 g) was desilicified 2–3 times by HCl–HF–EtOH before sample-spike equilibration in a Carius tube. Sample digestion and Re–Os purification procedures are modified from those of Shirey & Walker (1995) and have been described in detail by Lambert et al. (1998a). Re and Os were analysed using a Finnigan-MAT 262 multicollector mass spectrometer using N-TIMS after Creaser et al. (1991). Analyses for Os were performed using the single electron multiplier (SEM) and for Re the Faraday collectors. Spike correction and data reduction were performed off-line. Total procedural blanks are <2 pg for Os and <6 pg for Re. The long-term mean ¹⁸⁷Os/¹⁸⁸Os = 0·17400 ± 21 for the Department of Terrestrial Magnetism (DTM; long-term average ¹⁸⁷Os/¹⁸⁸Os = 0·17429 ± 55; Shirey, 1997) Os isotopic standard (made using Johnson–Mathey (NH₄)₂OsC_l6 batch 5.56870-A in 6N HCl).

Sm–Nd, Rb–Sr isotopic analyses.
Sm–Nd–Rb–Sr concentrations and isotopic abundances were determined by isotope dilution. Sample powder (50–100 mg) was weighed and appropriate amounts of a ⁸⁷Rb, ⁸⁴Sr and a mixed ¹⁴⁹Sm–¹⁵⁰Nd enriched spikes were added. Samples were then digested as for ICP-MS trace element preparation above and then picked up in dilute HCl. After centrifuging, Rb, Sr and the REE were separated from the solution by gradient elution on standard cation exchange columns using BioRad AG50Wx8 (200–400 mesh) resin. Nd and Sm were then separated from the REE fraction using 6 ml diameter columns of 2 di-ethyl-hexyl phosphate (HDEHP) on a Kel-F support. Rb, Sr, Sm and Nd were all analysed using a Finnigan-MAT 262 multicollector mass spectrometer at La Trobe University. All elements were analysed in static mode into Faraday cup collectors. Raw ratios were corrected for mass fractionation using ¹⁴⁶Nd/¹⁴⁴Nd = 0·7219 (power law), ¹⁵²Sm/¹⁴⁷Sm = 1·78307 (linear law), and ⁸⁸Sr/⁸⁶Sr = 8·37521 (linear law). Typical blanks are <200 pg for Sr, Nd and Sm, and <500 pg for Rb. External reproducibility for the La Jolla Nd standard is ¹⁴³Nd/¹⁴⁴Nd = 0·511860 ± 20 (2 SD) and for the Sr standard SRM 987 is ⁸⁷Sr/⁸⁶Sr = 0·71025 ± 4 (2 SD). Precision for ⁸⁷Rb/⁸⁶Sr is <0·8% (2 SD) and for ¹⁴⁷Sm/¹⁴⁴Nd is <0·2% (2 SD).

Pb isotope analysis.
Approximately 100 mg of sample powder was digested in HF–HNO₃–HCl. After converting to bromide form, Pb was purified by conventional HBr–HCl anion exchange chemistry, using two passes over small (0·1 ml) beds of AG-1 X8 supported in mini-columns made from heat-shrink Teflon (e.g. Manhes et al., 1984). Samples were then split into an isotope composition (IC, 2/3 of sample) and an isotope dilution (ID) fraction; the ID fraction was spiked in the beaker with a suitable amount of ^204–207Pb double spike (e.g. Woodhead, 1995). Total blanks were <500 pg and usually <200 pg. Pb isotope analyses of IC and ID fractions were made in static mode on the Finnigan-MAT 262 at filament temperatures of 1170–1300°C. Runs of >10 ng SRM981 Pb standard show very little systematic mass fractionation throughout this temperature interval. Typically three blocks of ten 8-s integrations constitute one run, with in-run 2_mean precisions of 0·01%. The double spike run allows correction of mass fractionation to a very high degree of accuracy and reproducibility, with external precisions for well-spiked samples similar to in-run errors.

	ACKNOWLEDGEMENTS

 
This work was undertaken while J.S.M. was a recipient of a Monash University Graduate Scholarship and was supported by Australian Research Council Small Grants to D.D.L. and I.A.N. in 1995, 1996 and 1997. We thank Louise Frick, Roland Maas and Shane Reeves for help with N-TIMS, ICP-MS and XRF data acquisition. Plains basalt samples were collected by C. M. Gray and R.C.P. as part of a project funded during the period 1984–1988 by the Australian Research Council (ARC) under the large grants scheme. Many thanks go to I. Parkinson and W. Griffin, whose comments contributed significantly to improving the quality of this paper.

	FOOTNOTES

 
^*Corresponding author. Tel.: +61-3-9905-5764. Fax.: +61-3-9905-4903. E-mail: jmcbride{at}mail.earth.monash.edu.au

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TOP ABSTRACT INTRODUCTION BACKGROUND TO THE NEWER... SAMPLE SELECTION AND DESCRIPTION GEOCHEMISTRY DISCUSSION CONCLUSIONS APPENDIX 1 REFERENCES

 
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