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Deep mantle plume osmium isotope signature from West Greenland Tertiary picrites
Earth and Planetary Science Letters 175 (2000) 105^118 www.elsevier.com/locate/epsl
Deep mantle plume osmium isotope signature from West Greenland Tertiary picrites Bruce F. Schaefer *, Ian J. Parkinson, Chris J. Hawkesworth Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK Received 24 June 1999; received in revised form 29 October 1999; accepted 17 November 1999
Abstract Picrites from Nuussuaq Peninsula and Qeqertarssuaq (Disko) Island, West Greenland, preserve trace element and isotopic signatures reflecting the composition of the Icelandic plume head. Os isotope ratios are low (187 Os/ 188 Os i = 0.1272^0.1371) in terms of global plume related magmatism, and this is coupled with anomalously high Os abundances and radiogenic 143 Nd/144 Nd i isotopes. Crustally contaminated basalts within the West Greenland sequences possess Os and Nd isotopic signatures consistent with mixing between initial plume head compositions and two discrete types of West Greenland continental crust. One crustal component is of a local sedimentary origin, and the other is typical of ancient felsic crust. The low Os isotopic signatures of the picritic units are considered to be those of the initial mantle plume. The fact that such low Os isotope ratios occur in sequences with high 3 He/4 He, and the nonsystematic variation in Os isotopes with indices of fractionation/accumulation and Pb isotopes, argue against mixing with depleted MORB mantle or ancient subcontinental lithospheric mantle. The high Os concentrations in the picrites are attributed to high degrees of partial melting ( s 25%) of mantle containing no residual sulphide. This is consistent with models for plume heads in which anomalous mantle temperatures initiate melting at high pressures generating large degrees of partial melting. Unradiogenic Os and radiogenic Nd components in plume-related CFB magmatism may preserve contributions from a reservoir which is sampled only occasionally in young oceanic basalts. Such a reservoir shares Os isotopic features with the primitive upper mantle (PUM), which may also be manifest in the `Kea' component of Hawaiian magmatism. Therefore, portions of the West Greenland continental flood basalt province arguably represent the first direct sampling of unmodified plume head material derived from the lower mantle or lower portions of the upper mantle. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: Re/Os; isotope ratios; West Greenland; Icelandic Plume; magma contamination; £ood basalts
1. Introduction Magmatism associated with mantle plumes potentially o¡ers insights into the composition and
* Corresponding author. Tel.: +44-1908-858551; Fax: +44-1908-655151; E-mail: [email protected]
thermal state of deeper portions of the Earth's mantle [1^4]. A large isotope database exists for ocean island basalts (OIB) which are characterised by a range in isotope ratios, much of which is explained by di¡erent components of recycled crust and mantle lithosphere [5]. The Re^Os isotope system has recently been utilised in trying to understand both the source components of OIB [6^8] and the e¡ects of melt percolation [9], be-
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cause of its unique geochemical characteristics. The chalcophile and siderophile nature of Re and Os combined with the compatible behaviour of Os during mantle melting set this isotope system apart from conventional lithophile isotope systems and provides a unique insight into mantle plume processes. Os isotopes are also potentially a sensitive indicator of crustal input because all crustal materials have elevated Re/Os ratios and hence with time high 187 Os/188 Os ratios [6^8,10]. Furthermore the compatible behaviour of Os during mantle melting ensures that Os isotopes provide a robust signature of ancient mantle depletion events [11,12]. Although OIB have been well studied [1^8] they tend to provide information about the composition of the plume tail, and in the case of low-£ux plumes they may only record shallow level processes. Mantle plume heads (or `starting plumes') are anomalously hot and consequently record information regarding deep mantle sources. Starting plume magmatism is very rarely preserved in the oceanic realm [1,3,4] and the best exposures of magmatism associated with plume initiation are found in continental £ood basalt (CFB) provinces. Unfortunately many CFB lavas are crustally contaminated and/or they contain signi¢cant contributions from the subcontinental lithospheric mantle (SCLM) [13^15], making inferences about deep mantle plume compositions di¤cult. This contribution presents new Re^Os isotope data for a suite of picritic and primitive CFB from West Greenland, which were previously characterised geochemically by Lightfoot et al. [16]. These lavas represent early magmatism associated with the initiation of the Iceland plume and the thick succession of picrites highlights the high degrees of melting and the greater depths of origin of the associated mantle plume. High Os concentrations in these picritic lavas means they are relatively insensitive to crustal contamination and therefore represent the best samples of deep mantle plume material so far analysed. 1.1. Background Flood basalt magmatism in the North Atlantic province began at V60.5^62.5 Ma [16^20], when
voluminous basaltic and picritic magmatism took place synchronously in contiguous portions of the Scottish Tertiary Volcanic Province, Northern Ireland, Southeast and West Greenland, the Faroe Islands and the margins of Ba¤n Island [17,18] (Fig. 1). Local foci of magmatism may in part be attributed to variations in the thickness of continental lithosphere [18], and the apparent synchronicity of magmatism over a large area (V2000 km lateral continuity [17]) is consistent with derivation from anomalously hot asthenospheric material in an impinging plume head [17,19]. This study focuses on the basal picrites of West Greenland for the following reasons: 1. the lowermost volcanic rocks are amongst the earliest magmas erupted in the North Atlantic province, 2. picrites amenable to Os isotope analysis form a large proportion of the eruptive units [16], 3. crustal contamination of the West Greenland picrites is well documented and constrained by a comprehensive trace element and isotopic dataset [16], 4. recent studies of additional isotopic systems (such as Sr, Nd, Pb and He) on the same series of lavas indicate a dominant plume component within the sequence [19,20]. Samples selected for this study are from the collection of Lightfoot et al. [16], in Qeqertarssuaq Island and the Nuussuaq Peninsula of West Greenland (Fig. 1). Picritic basalts and high-Mg basalt lithologies comprise V1 km of the eruptive sequence. The basal £ood basalt stratigraphy is often complicated by rapid lateral facies changes, including subaerial magmas, pillow basalts and hyaloclastite breccias [19]. This stratigraphy comprises the basal Va|«gat Formation, dominated by picrites, basalts and hyaloclastites, overlain by the slightly more evolved tholeiitic basalts of the Maligaªt Formation [21,22]. We have focused on picrites and basalts from the Va|«gat Formation from Qeqertarssuaq and the Nuussuaq Peninsula, and the stratigraphic details of the samples analysed can be found in Lightfoot et al. [16]. Ar^Ar geochronology on Va|«gat Formation
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samples from the Nuussuaq Peninsula consistently produces ages of 61.3^60.9 Ma [19]. Graham et al. [19] suggest that the complete eruptive history of the Va|«gat Formation on Nuussuaq Peninsula spans a period of less than V1 Myr, clearly indicative of short duration voluminous £ood basalt magmatism. Lightfoot et al. [16] calculated parental magma compositions of V15 wt% MgO, although Graham et al. [19] favour parental magmas containing s 19 wt% MgO. Even the more conservative estimate [16] is indicative of exceptionally anomalous magnesian parental magma compositions, even in the context of CFB. Melts therefore originated from anomalously thermally perturbed asthenosphere of V1540^1600³C at 60^90 km depth [16,19]. Crustal contamination is extremely variable within the more evolved basalts, and xenoliths of country rock, graphite and metamorphosed coal horizons are observed at a number of localities. Locally, exceptionally reducing conditions due to the presence of free carbon in the sedimentary sequences have resulted in the precipitation of native iron in the basaltic £ows. 2. Methodology Samples for Os isotope analysis were selected to
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characterise initial plume compositions and to span the range of potential crustal end members for contamination trends previously recognised within the sequence [16]. This involved four principle groups of samples; (1) picrites from the Nuussuaq Peninsula (8 samples), (2) picrites from Qeqertarssuaq (6 samples), (3) basalts from the Nuussuaq Peninsula (3 samples), and (4) contaminated Va|«gat Formation lavas from Qeqertarssuaq (referred to as Asuªk-Kuªga¨nguaq; 3 samples). Additionally, a comagmatic intrusion of the Serfat Sill was analysed, as was a graphite enclave from within the picritic Hammersdal Dyke. Samples from within each stratigraphic unit were selected primarily on the basis of their prospective Os contents, and a cut-o¡ value of approximately 200 ppm Ni was found to correspond to values of V0.25 ppb Os in these lavas (Fig. 2). 0.5^1.5 g of powder was spiked and digested in inverse aqua regia by Carius tube dissolution followed by solvent extraction, as described in detail elsewhere [23,24]. Re separation after removal of Os involved further solvent extraction and ¢nally an anion column pass [24]. All samples were run on a Finnigan MAT-261 mass spectrometer in N-TIMS mode at the Open University [12,24]. Table 1 summarises Re and Os concentrations and Os isotopic data for the West Greenland lavas.
Fig. 1. Location of Nuussuaq Peninsula and Qeqertarssuaq Island with respect to distribution of CFB magmatism in the North Atlantic province at V60 Ma. Adapted from [17].
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With Os total procedural blank (TPB) levels of typically 2 pg or better, blank contributions were typically less than 1% on 1 g sample dissolutions. However, particularly low abundance samples, such as those from Asuªk-Kuªga¨nguaq, were subject to blank corrections up to a maximum of 8% (sample 7650). TPBs were run as every eighth sample during the course of this study, and the average 187 Os/188 Os of TPBs = 0.291 and it remained constant, allowing a high degree of accuracy in the 187 Os/188 Os blank correction. TPBs for Re were more variable, ranging from V7 to 13 pg. A combination of low Re levels and the small amounts of sample available for analysis required greater blank corrections. These were typically 2.4^17%, with a small number of samples (4) ranging up to 25%. Since TPB were run as part of each batch of dissolutions, the appropriate blank correction for each batch was applied. Blank corrections of picritic Re concentrations generally produced minor changes ( 6 1.5%) in age corrected 187 Os/188 Os ratios, with the exception of one sample which has an exceptionally high Re/Os ratio that appears to re£ect late stage alteration processes (7860). The Os isotope data have been combined with the trace element and isotope data of Lightfoot et al. [16] to constrain the origin of the West Greenland lavas. Additionally, He isotope data for the Va|«gat Formation from the Nuussuaq Peninsula [19] is similar to that in basalts of similar age from northeast Greenland [25] and Iceland [26]. Although variable, these data suggest that high He isotopes are an intrinsic feature of the Iceland plume.
lisation. Whether Os is present as sulphide inclusions within olivine crystals, or occupies discrete positions within the olivine lattice has been discussed by Hart and Ravizza [27]. However, Fig. 2a gives some indication that Os is more likely to be distributed in inclusions within olivine since picrites from Nuussuaq have lower Os concentra-
3. Results 3.1. Re and Os distribution The measured Os concentrations in the Va|«gat Formation rocks range from 0.040 to 3.71 ppb, and Re concentrations from 0.031 to 0.42 ppb. The picrites exhibit strong positive correlations between Mg#, Ni and Os contents (Fig. 2; Mg# = Mg/(Mg+Fe2 )), consistent with Os abundances having been controlled by olivine crystal-
Fig. 2. Distribution of Os concentration as a function of (a) Mg# (Mg# = Mg/(Mg+Fe2 ), (b) Ni. Asuªk-Kuªga¨nguaq are the crustally contaminated Va|«gat Formation lavas from Qeqertarssuaq, and Nuussuaq and Qeqertarssuaq are picrites from Nuussuaq Peninsula and Qeqertarssuaq Island, respectively, Serfat is an unmineralised gabbrodolerite sill on the northern shore of Nuussuaq.
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Table 1 Re and Os concentration and Os isotopic data for the West Greenland £ood basalts and associated graphite enclave Sample
Unit
MgO (wt%)
Ni Cr Yb (ppm) (ppm) (ppm)
Contaminated Va|«gat Formation, Asuk 7650 Kuganguaq 11.13 38 7716 Kuganguaq 11.51 24 7717 Kuganguaq 12.57 147 Nuussuaq basalts 7859 B3 7.97 41 7819 B4 14.71 450 7849 B4 10.33 241 Nuussuaq picrites 7837 P1 10.15 217 7829 P2 11.62 259 7860 P3 12.47 105 7822 P4 26.47 1170 7854 P4 24.12 1160 7855 P4 16.81 605 7902 P4 13.03 372 7851 P5 16.41 586 7818 P5 24.36 1080 Qeqertarssuaq Vaigat picrites 7633 Naujanguit 15.14 531 7640 Naujanguit 26.30 1190 7649 Ordlingassoq 16.83 612 7712 Naujanguit 18.77 769 7718 Naujanguit 15.44 534 7719 Naujanguit 29.04 1300 Intrusions 7842 Serfat picrite 29.76 1410 Hammersdal Dyke Graphite HDG
Re (ppb)
Os (ppb)
Re/Os
and Kuganguaq 1090 0.25 0.419 0.0651 6.44 1168 0.29 0.0305 0.0400 0.76 962 0.27 0.0641 0.200 0.32
187
Re/188 Os
187
Os/188 Osa
187
Os/188 Osb QOsc
31.7 3.69 1.55
0.30012 71 0.15590 380 0.13288 12
0.26840 0.15221 0.13133
111 20 3.23
4.41 1.26 6.82
0.14263 68 0.13446 9 0.15029 12
0.13822 0.13319 0.14347
8.7 4.7 13 5.4 7.8 327 5.2 2.8 1.2 6.5 2.8 4.0
857 935 528
0.32 0.23 0.29
0.0452 0.0495 0.91 0.287 1.098 0.26 0.239 0.169 1.41
481 598 1156 2515 1866 1509 856 956 1494
0.28 0.27 0.25 0.18 0.18 0.24 0.25 0.21 0.16
0.497 0.564 0.754 0.241 0.0865 0.157 0.272 0.184 0.199
0.409 0.393 0.100 1.724 1.772 1.301 0.664 1.166 1.899
1.21 1.44 7.53 0.14 0.05 0.12 0.41 0.16 0.10
5.86 6.94 36.3 0.673 0.235 0.580 1.97 0.762 0.505
0.13985 21 0.14404 17 0.12897 24 0.13449 10 0.13098 8 0.12926 7 0.13735 12 0.13146 10 0.13279 8
0.13399 0.13709 0.09269 0.13382 0.13075 0.12868 0.13537 0.13070 0.13228
1048 1715 1103 1379 1007 1770
0.28 0.17 0.22 0.26 0.21 0.11
0.187 0.156 0.269 0.409 0.230 0.0673
1.359 3.101 1.594 2.331 1.181 3.373
0.14 0.05 0.17 0.18 0.19 0.02
0.664 0.243 0.814 0.845 0.938 0.096
0.12785 9 0.12887 12 0.13178 29 0.13203 10 0.13023 12 0.13070 20
0.12718 0.12862 0.13097 0.13118 0.12929 0.13061
0.01 1.1 3.0 3.2 1.7 2.7
2530
0.11
0.354
3.711
0.10
0.460
0.13512 18
0.13463
5.9
0.359
0.345
0.96
4.79
0.40346 843
0.39867
214
MgO, Ni, Cr and Yb data from [16]. a Present day 187 Os/188 Os. Errors are 2c within run precision and equal to the least units cited. b Age corrected 187 Os/188 Os ratios at 60 Ma or 63 Ma for HDG using V = 1.666U10311 yr31 [47]. c Age corrected QOs values. QOs is the percentage di¡erence between the age corrected 187 Os/188 Os ratio and the age corrected 187 Os/188 Os ratio of a chondritic mantle where the present day chondritic mantle has 187 Os/188 Os = 0.12757 and 187 Re/ 188 Os = 0.3972 [11].
tions at similar Mg# than those from Qeqertarssuaq, where both sequences contain accumulated olivine. It appears that sulphide and/or PGE alloy inclusions within olivine are the most plausible host for Os. Assuming a primary melt composition of 15 wt% MgO [16] gives a primary Os concentration of 1.34^1.60 ppb and Re concentration of 0.19^ 0.27 ppb (Fig. 2 and Table 1). These absolute concentrations of Os at a given MgO for the West Greenland picrites are typically higher than for picrites of similar MgO content from ocean islands [29,30] and many CFB provinces
[13] (Fig. 3). Only picritic basalts from Siberia [31] and komatiitic lavas from Munro Township [32] and Gorgona Island [33] have similar Os concentrations for given MgO contents. There is no reason to believe that the source region had anomalously high Os contents since the slightly suprachondritic Os isotope ratios of these lavas would imply correspondingly elevated Re contents in the source, which is inconsistent with the low observed Re concentrations of the lavas (see below). A more likely explanation of the high Os concentrations in the lavas is their high degrees of partial melting. In such cases the melts
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Fig. 3. Os concentration as a function of MgO for major plume related provinces. Note how the oceanic setting trends £atten at higher MgO contents with respect to the continental provinces. Symbols as for Fig. 2. Data from [6,13,29^ 33,40^42,45,48].
become sulphur undersaturated and therefore extract their full inventory of platinum group elements (PGE) from the mantle due to the absence of residual sulphide in the melting residue. The possibility of Os (and Ir and Ru) contents being bu¡ered by metallic or alloy phases in the source region after exhaustion of sulphide during melting is unlikely to apply in West Greenland in view of the observed high Os contents. Indeed, the high PGE contents of the Disko Island picrites has been noted previously [34] and we suggest that the elevated Os contents in the picrites are a result of sulphur undersaturation. The di¡erence in Os concentrations between the Qeqertarssuaq and Nuussuaq picrites at MgO s 20% therefore re£ects inclusion of PGE alloys or sulphides during olivine accumulation. For the inclusions to be sulphides suggests that the lavas had become sulphur-saturated at the shallow pressures of crystallisation, by contrast to their sulphur undersaturation in the melt generation region. The lack of di¡erence between the Cu contents of the Qeqertarssuaq and Nuussuaq picrites [16] is one argument that the inclusions are PGE alloys rather than sulphides. Variations in Re concentrations are broadly
analogous to those of the HREE (see also Hauri and Hart [35]). The picritic sequences on both Nuussuaq and Qeqertarssuaq have positive correlations between Yb and Re abundances, although the more evolved basaltic lithologies analysed preserve weak negative correlations. In order to understand how Re and Os distributions can be used to understand aspects of the genesis of the West Greenland picrites, plots of Os versus Re/Os and Re versus Yb are shown in Fig. 4. One of the features of the West Greenland data is their high Os abundances coupled with low Re/Os ratios, a characteristic shared only with the Siberian picrites. Although komatiites have high Os contents they also have high Re contents resulting in moderate Re/Os ratios. On a plot of Re versus Yb (Fig. 4b), MORB and komatiites plot with good positive correlations but with the komatiites displaced to higher Re contents. Since Re is strongly compatible in sulphide and mildly compatible in garnet, whereas Yb is incompatible in sulphide and compatible in garnet, these data can be interpreted as melting with residual sulphide but no residual garnet for MORB (see also [36]). This would imply that melting with neither residual garnet nor sulphide is appropriate for komatiites. By contrast OIB have low Re contents which are uncorrelated with low but restricted Yb contents, consistent with melting in the presence of residual garnet, a feature also indicated by their fractionated REE patterns. The OIB ¢eld in Fig. 4b is de¢ned by poorly correlated subhorizontal arrays. The West Greenland data di¡er from these suites in that they have low Re and Yb concentrations but preserve a strong positive correlation between the two elements (Fig. 4b) coupled with high Os contents. They might therefore re£ect melting with residual garnet that retained both Re and Yb, but with no residual sulphide. Signi¢cantly, West Greenland picrites contain higher [Dy/Yb]N at low [La/Yb]N (data from [16]) with respect to MORB, consistent with a more pronounced garnet melting signature and larger degrees of partial melting. Hence, the REE data suggest that both picritic suites have garnet melting signatures which have been diluted by large amounts of melt subsequently generated in the
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Fig. 4. (a) Os versus Re/Os and (b) Re versus Yb for plumerelated ocean island (OIB) and continental lavas (CFB), komatiites and MORB. Symbols as for Fig. 2. MORB Yb data were calculated from Tb concentrations from [49] assuming a chondritic Tb/Yb ratio.
spinel peridotite facies. Both suites represent high degrees of partial melting (V25%), but on average the Qeqertarssuaq picrites are higher degree partial melts. Hence, a likely scenario for the West Greenland picrites is that they re£ect melting which was initiated in the garnet ¢eld, and continued in the spinel ¢eld, and because of their high degrees of partial melting they became sulphur undersaturated. The distribution of Os, Re
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and Yb abundances (Fig. 4) in these rocks is likely to be restricted to large degree melts where melting is initiated in the garnet facies and sulphide is eliminated from the residue. This is possible during initiation of plume melting. Although the West Greenland picrites contain Re and Yb concentrations similar to those in oceanic intraplate basalts (Fig. 4b), they have higher Os and the linear relationships observed in Fig. 4 di¡er from the subhorizontal Re and Yb arrays of the small degree melt basalts that de¢ne the oceanic ¢eld. Assuming a primary melt composition of 15 wt% MgO [16] gives a primary Os concentration of 1.34^1.60 ppb and Re = 0.187^0.27 ppb. Using these values and assuming that Re and Os are present in chondritic proportions, the West Greenland picrites suggest bulk partition coe¤cients after 25% partial melting of 2.3^2.8 and 1.1^1.6 for Os and Re, respectively. These values are slightly lower for Os and higher for Re than partition coe¤cients used in the literature [8,28,36]. In summary, the elevated Os concentrations in the Va|«gat Formation picrites are consistent with these lavas being high degree sulphur-undersaturated partial melts. Elevated Os concentrations in the Qeqertarssuaq lavas with MgO s 16%, relative to those from Nuussuaq Peninsula are attributed to the presence of PGE alloys, or possibly sulphides, as inclusions in the accumulated olivine. The low Re, Yb and Re/Os ratios of the lavas support models of high degrees of partial melting with residual garnet during the early stages of melting, and no residual sulphide. 3.2. Os isotopes of uncontaminated Va|«gat Formation picrites The initial Os isotopes of Va|«gat picrites are generally suprachondritic, but lower than those in most global plume related magmatic rocks (187 Os/188 Os i = 0.1278^0.1388, compared with V0.131^0.15 in typical OIB [6^8,41,42]) (Figs. 5 and 6; Table 1). One Nuussuaq picrite (7860) has an anomalously high Re/Os ratio with low measured 187 Os/188 Os (0.12897), producing improbably low age corrected 187 Os/188 Os (Table 1), and it is disregarded from following discussion. Post em-
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Fig. 5. Global distribution of Os vs QOs i Note the high concentrations and low QOs i values of the West Greenland and Karoo Picrites. Q notation is plotted in preference to initial ratios in order to emphasise the temporal evolution of di¡erent source terrains with respect to chondrite.
placement mobilisation of Re may have been responsible for its high Re/Os ratio. The Os isotope ratios in plume related magmatism are either those of the plume source, or they are due to interaction with relatively shallow level crustal or mantle reservoirs. The West Greenland picrites tend to have slightly suprachondritic Os isotope ratios, and potentially Os may have been incorporated from the SCLM and/or depleted MORB-type mantle (DMM). Fig. 6 illustrates that the West Greenland picrites are not mixtures of DMM and SCLM, since the Nd^Os array lies to higher Os ratios than either of these reservoirs. Further, mixing of DMM with continental crust produces arrays which rapidly decrease in Nd isotopes with little change in Os isotopes, and hence do not intersect the West Greenland array. Hence it appears that the Os and Nd characteristics of West Greenland picrites are truly those of the plume head, and not a combination of other reservoirs. Hence, we have modelled the least radiogenic end member in the West Greenland Nd^Os picrite
array (Fig. 6) as representative of a primary plume component. Sample 7633 is the most depleted in terms of Nd and Os isotopes and it has MgO = 15.14wt%, appropriate for a primary melt (Table 1). Fig. 6 presents a series of mixing curves for Nd and Os isotopes between sample 7633 and possible SCLM and crustal components. Low Os isotope ratios (QOs = 321 to 313) have been reported from xenoliths from Tertiary Dykes in East Greenland [37], and the presence of such highly depleted ancient SCLM beneath East Greenland [37,38] suggests it is reasonable to assume that there is similar lithospheric material beneath West Greenland. In detail, the choice of the depleted SCLM end member has little e¡ect on the calculated mixing curves. Rather, since sample 7633 has the lowest 187 Os/188 Os i within the picrite suite, mixing trends with bulk SCLM (curve 1 on Fig. 6a) or partial melts of the SCLM (curve 2) are unable to account for the Nd^Os isotope array within the Va|«gat picrites. Alternatively, the variation of Nd and Os iso-
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Fig. 6. Comparison of mixing curves between primary Va|«gat picritic liquid with SCLM and crustal contaminants. DMM and SCLM are plotted at 60 Ma. (a) QOs i vs ONd i . (b) As for (a), but with expanded QOs i axis to illustrate mixing trends for crustally contaminated basalts of the Va|«gat Formation. Symbols as in Fig. 2. Curves 1 and 2 are mixtures with bulk SCLM and a partial melt of SCLM, respectively, and curve 3 = mixing with graphitic sediment. Curves labelled with concentration values are mixing arrays to average continental crust of varying Os concentration, as indicated. Tick marks are at 10% intervals. End member parameters: Average continental crust; Os = as indicated on curves, 187 Os/ 188 Os 60 = 2(Q = 1448), Nd = 20 ppm, 143 Nd/144 Nd 60 = 0.51098 (O = 331.4); Bulk SCLM (curve 1); Os = 9.2 ppb, 187 Os/ 188 Os 60 = 0.11026 (Q = 313.3), Nd = 1 ppm, 143 Nd/144 Nd 60 = 0.5118 (O = 315.1); SCLM melt (curve 2); Os = 0.9 ppb, 187 Os/188 Os 60 = 0.11026 (Q = 313.3), Nd = 10 ppm, 143 Nd/ 144 Nd 60 = 0.5118 (O = 315.1); Graphitic Continental Crust (curve 3); Os = 0.15 ppb, 187 Os/188 Os 60 = 2(Q = 1448), Nd = 10 ppm, 143 Nd/144 Nd 60 = 0.51098 (O = 331.4). Va|«gat picrite composition as per sample 7633 on Table 1. Picrite Nd data from [16]. Data for Siberia from [31], East Greenland [40], and Samoa and the Cook-Austral islands from [6].
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topes in the West Greenland picrites might be accounted for by interaction with continental crust. However, this is unlikely since, (1) both cumulates and fractionates span the complete range of Os isotopic values, and so there is no systematic change in isotopes with indices of differentiation. (2) There is no covariation in, for example Pb and Os isotopes, and (3) the exceptionally high 3 He/4 He isotopes of some Va|«gat Formation olivines [19] precludes signi¢cant interaction with crustal material during olivine crystallisation. Further, detailed trace element and lithophile isotope studies have previously established the lack of signi¢cant crustal contamination in the generation of the Va|«gat picrites [16,20]. Additionally, calculated mixing trends with estimates of bulk continental crust do not intersect the observed Nd-Os isotope array (Fig. 6). It is possible to construct very speci¢c crustal mixing curves through the picritic array (e.g., curve 3 on Fig. 6), but such curves require high Os contaminants and such curves are appropriate only for the basalts, which are demonstrably contaminated (see [16] and discussion below). Hence the Os isotopes are consistent with the conclusions of Lightfoot et al. [16], who observed little variation in crustal indicators (such as La/Sm and Nd isotopes) in the picritic rocks. We conclude that the range of Os isotopes in the picrites re£ects source heterogeneity of the mantle plume. 3.3. Os isotopes of contaminated Va|«gat Formation basalts In contrast to the picrites, many basalts of the Va|«gat Formation preserve clear evidence of crustal contamination in terms of major and trace elements, and lithophile isotope ratios. Lightfoot et al. [16] identi¢ed contamination of Va|«gat Formation lavas with material from two distinct crustal sources, one containing elevated La/Sm and Th/Nb ratios (trend 1; ¢gure 6a of [16]) and another containing elevated La/Sm ratios but with low Th/Nb and slightly higher 143 Nd/144 Nd (trend 2 of [16]). These trends were attributed to mixing with sedimentary and granodioritic end members, respectively [16]. A sample from each trend was selected for Os isotope analysis.
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Sample 7650, a high Th/Nb (trend 1 of [16]) basalt, preserves the highest 187 Os/188 Os i ratio (0.26835) of all the Va|«gat Formation rocks analysed, consistent with contamination by an ancient sedimentary component. Tertiary sediments on Qeqertarssuaq are dominated by Archaean Nd and Sr isotopic signatures [39]. Simple mixing with bulk continental crust of isotopic compositions appropriate for West Greenland is unable to account for the Nd and Os ratios of 7650, however it does lie on a mixing trend with an inclusion of graphitic sediment in the Hammersdal Dyke (Fig. 6b). Since coal measures are a feature of the Tertiary sequence that underlies the West Greenland lavas, this crustal end member is appropriate for trend 1 basalts. Approximately 40% bulk mixing with graphitic sediment is required to produce the observed Nd and Os ratios in sample 7650, a ¢gure similar to that estimated by Lightfoot et al. [16] for the same sample from trace element and Nd isotope variations. Conversely, the high La/Sm, low Th/Nb contaminated basalt (trend 2 of [16]) has an initial Os isotope ratio similar to the uncontaminated picrites, despite a signi¢cant shift to lower Nd isotope values (Fig. 6). This is consistent with V10% mixing with Archaean felsic continental crust (Fig. 6b), but we note that the Os concentration in most crustal rocks is so low as to have little in£uence on the calculated amounts of mixing (Fig. 6b). Whereas the Nd^Os isotope shift is consistent with an ancient granodioritic component [16], the Os isotopic systematics do not allow effective discrimination between alternative ancient low Os abundance crustal sources. 3.4. Summary of results In summary, the osmium isotope ratios of the Va|«gat Formation rocks record both primary plume head signatures in the picrites, and crustal contamination processes in the more evolved basalts. The primitive picrites in West Greenland show little variation in Nd isotopes, but they range from unusually low 187 Os/188 Os i values of V0.1272 up to the lower end of typical plume related Os isotope signatures (V0.1371). It is possible to construct crustal mixing curves through
the picritic Nd^Os isotope array using speci¢c contaminants with high Os contents. However trace element and Sr, Nd, Pb isotope studies [16], coupled with the lack of systematic variation in Os isotopes with major and trace element contents, and the presence of demonstrably non-crustal He isotopes in the same sequence [19], argue strongly against signi¢cant crustal interaction. Hence the observed Nd^Os array is interpreted to re£ect that of the impinging plume head. Further, the large volume of picritic magmatism in West Greenland (V1 km thick [14] over V55 000 km2 [20] (Fig. 1)) indicates large degrees and high temperatures of melting from a deep mantle source [16,19,20]. Such conditions are favourable for rapid extrusion and minimal lithospheric contamination of the picrites. Hence the Va|«gat Formation picrites of West Greenland faithfully record primary isotopic signatures of the impinging plume head, and invite comparison with other plume related lavas. 4. West Greenland plume lavas in a global context As noted previously, a striking feature of the basal West Greenland CFB province is its elevated Os concentrations and coupled low Os and high Nd isotope ratios in comparison with magmas from active mantle plumes. East Greenland lavas of the same age also have low Os isotope signatures [40], indicating these are an inherent feature of the plume head associated with the modern Iceland hot spot. A low Os isotopic component is by no means unique in global hot spot related magmatism, in that similar components have been seen in magmas from the Karoo [13], Azores [41], Canary Islands [42], Samoa [6] and Hawaii [29,30]. A number of explanations have been o¡ered for these components, and many involve interaction with lithospheric source material (e.g., [13,41,42]). However, the low 187 Os/188 Os ratios of the West and East Greenland CFBs [40] appear to be a feature of the upwelling plume, and not due to signi¢cant contributions from the lithospheric mantle, particularly given the extremely low Os isotope ratios of East
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Greenland lithospheric xenoliths [37]). Thus it appears that large mantle plumes may sample deepseated reservoirs which are themselves only slightly suprachondritic for Re/Os. The key feature of the West Greenland picritic suites is the combination of slightly suprachondritic Os isotopes with elevated Nd isotopes. This feature is surprising because it is distinct from DMM which has coupled high Nd isotopes and subchondritic Os isotopes, indicating an ancient melting event and time integrated high Sm/Nd and low Re/Os ratios. The West Greenland data suggest a source with chondritic to slightly suprachondritic timeintegrated Re/Os ratios but with high Sm/Nd ratios; inconsistent with a single ancient melt depletion event which would explain the high Nd isotopes but not the Os isotope ratios. In fact the primary melt composition end of the West Greenland Nd^Os isotope array is very similar to recent estimates of the putative PUM ([43]), and also to the `Kea' component of the Hawaiian plume [44]. Taken together, these data indicate the presence of a reservoir in the Earth with slightly suprachondritic Os isotopes and radiogenic Nd isotopes. Its occurrence in the two high buoyancy£ux plumes (i.e., Hawaii and Iceland), coupled with their elevated 3 He/4 He ratios provides evidence that this reservoir may be located in the lower mantle. Alternatively, decoupling of He isotopes from Os and Nd might allow domination of the He budget by a small mass contribution from the lower mantle, while Os and Nd are dominated by a true PUM (i.e., in the lower upper mantle) component. In either case, such characteristics are observed during plume initiation and/or by the hottest, deepest plumes, suggesting they are features of a speci¢c reservoir not sampled by smaller, shallower plumes, such as beneath the Azores [41,42]. Distinguishing near chondritic components from mixing with unradiogenic lithosphere reservoirs solely on the basis of Os isotopes is problematic. However recognition of plume-derived low 187 Os/188 Os may o¡er an alternative to elaborate lithosphere interaction models. This is particularly pertinent in view of the ambiguity surrounding the Os isotope characteristics of lithospheric sources. Most SCLM^mantle plume
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mixing models invoke ancient depleted peridotite end members, whereas only the Karoo and Greenland lavas were demonstrably erupted through such mantle lithosphere [13,38,39]. While mixing with subchondritic lithospheric mantle is appropriate for some plume related provinces, West Greenland o¡ers an example of large volume, high temperature, mantle plume-derived magmatism impinging beneath ancient continental lithosphere and preserving primary plume isotopic compositions. Although there are limited Os isotopic data for the present day Iceland plume at least one analysis [45] has a value (0.1286) similar to the West Greenland picrites. The identi¢cation of a depleted source di¡erent from MORB-type mantle in modern day Icelandic lavas [46] suggests that the Icelandic plume is tapping a reservoir similar to that identi¢ed here for the West Greenland picrites. The Qeqertassuaq picrites [16,20] plot within Iceland and Kolbeinsey Pb isotope data ¢elds of Thirlwall [46]. The four picrites that have Pb isotope ratios similar to the modern Iceland plume data have 187 Os/188 Os of 0.1272^ 0.1310 and ONd of V8 further indicating that slightly suprachondritic Os isotope and high Nd isotope ratios characterise the source sampled by the Iceland plume. 5. Conclusions The picrites of West Greenland o¡er a rare opportunity to evaluate the nature of primary plume isotopic compositions in a voluminous CFB province. They are characterised by high Os abundances, and relatively low Os isotopes (187 Os/ 188 Os i = 0.1272^0.1371) for plume-related magmas and high Nd isotopes. By contrast, Os isotopic data for the more evolved West Greenland magmas support crustal contamination models independently established from trace element and lithophile isotope variations [16]. These crustally contaminated magmas involved mixing with two distinctive crustal end members; one dominated by graphitic Tertiary sediments derived largely from Archaean crust, and a second typical of felsic continental crust. Both suites of contaminated
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lavas have low Cu and Ni contents, consistent with sulphide formation within the crust [16]. The rapid extrusion rate and large volumes of high temperature picritic magmatism indicate that partial melting was of unusually hot and deep seated source rocks [16,19,20]. It is likely that the Va|«gat Formation picrites were located away from the central axis of the plume at V60 Ma, suggesting that the plume head was itself extremely hot, and able to maintain such anomalously high temperatures while dispersed beneath the continental lithosphere. Despite impingement beneath ancient continental lithospheric mantle, modelling of the Nd-Os isotope data indicates that the plume head picrites record minimal interaction with lithospheric sources. Simple partial melting calculations yield bulk partition coe¤cients of 2.3^2.8 for Os and 1.1^1.6 and for Re during melt generation of the West Greenland picrites. These values indicate melting with no residual sulphide, but some residual garnet during the initial stages of partial melting, a scenario arguably unique to high degrees of partial melting in high buoyancy mantle plumes. The distinctive isotopic characteristic of the West Greenland picrites is their slightly suprachondritic Os isotopes coupled with high Nd isotopes. These features have also been identi¢ed in some Hawaiian lavas [44], and the Os isotope composition of the West Greenland picrites is similar to putative PUM [43]. We argue that the Os and Nd (and He) isotope characteristics are of a reservoir which is only tapped during plume initiation and/or by high buoyancy-£ux plumes such as Hawaii and Iceland, and which is located in the lower mantle or lower portions of the upper mantle. Acknowledgements Peter Lightfoot is thanked for access to his exhaustive sample collection and constructive comments on an earlier version of the manuscript. Kent Brooks is thanked for access to a pre-print containing the East Greenland data. Jessica Bartlett provided several initial analyses on the picrites. Eric Hauri, Laurie Reisberg and Graham
Pearson provided stimulating reviews. Peter Van Calsteren is thanked for maintaining the mass spec, and Anthony Cohen for guidance in laboratory matters. I.J.P. is funded by a NERC/ODP fellowship, and isotopic research at the Open University is funded by NERC.
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B.F. Schaefer et al. / Earth and Planetary Science Letters 175 (2000) 105^118 [15] N.T. Arndt, G.K. Czamanske, J.L. Wooden, V.A. Fedorenko, Mantle and crustal contributions to continental £ood basalt volcanism, Tectonophysics 223 (1993) 39^ 52. [16] P.C. Lightfoot, C.J. Hawkesworth, K. Olshefsky, T. Green, W. Doherty, R.R. Keays, Geochemistry of tertiary tholeiites and picrites from Qeqertarssuaq (Disko Island) and Nuussuaq, West Greenland with implications for the mineral potential of comagmatic intrusions, Contrib. Mineral. Petrol. 128 (1997) 139^163. [17] A.D. Saunders, J.G. Fitton, M.J. Norry and R.W. Kent, The North Atlantic Igneous province, in: J.J. Mahoney and M.F. Co¤n (Eds.), Large Igneous Provinces, AGU Monogr. 100 (1997) 45^93. [18] J.A. Chalmers, L.M. Larsen, A.K. Pederson, Widespread Paleocene volcanism around the northern North Atlantic and Labrador Sea: evidence for a large, hot, early plume head, J. Geol. Soc. Lond. 152 (1995) 965^969. [19] D.W. Graham, L.M. Larsen, B.B. Hanan, M. Storey, A.K. Pedersen, J.E. Lupton, Helium isotope composition of the early Iceland mantle plume inferred from the Tertiary picrites of West Greenland, Earth Planet. Sci. Lett. 160 (1998) 241^255. [20] P.M. Holm, R.C.O. Gill, A.K. Pedersen, J.G. Larsen, N. Hald, T.F.D. Nielsen, M.F. Thirlwall, The Tertiary picrites of West Greenland: contributions from 'Icelandic' and other sources, Earth Planet. Sci. Lett. 115 (1993) 227^244. [21] D.B. Clark and A.K. Pedersen, Tertiary volcanic provinces of West Greenland, in: A. Escher and W.S. Watts (Eds.), The Geology of Greenland, The Geological Survey of Greenland, Copenhagen (1976) 364^385. [22] N. Hald, A.K. Pedersen, Lithostratigraphy of the Early Tertiary volcanic rocks of central West Greenland, Rapp. GrÖnl. Geol. Unders. 69 (1975) 17^24. [23] S.B. Shirey, R.J. Walker, Carius tube digestion for low blank rhenium^osmium analysis, Anal. Chem. 67/13 (1995) 2136^2141. [24] A.S. Cohen, F.G. Waters, Separation of osmium from geological materials by solvent extraction for analysis by thermal ionisation mass spectrometry, Anal. Chim. Acta. 332 (1996) 269^275. [25] B. Marty, B.G.J. Upton, R.M. Ellam, Helium isotopes in early Tertiary basalts, northeast Greenland evidence for 58 Ma plume activity in the North Atlantic-Iceland volcanic province, Geology 26 (1998) 407^410. [26] M. Condomines, K. Gronvold, P.J. Hooker, K. Muehlenbachs, R.H. O'Nions, N. Oskersson, E.R. Oxburgh, Helium, oxygen, strontium and neodymium isotopic relationships in Icelandic volcanics, Earth Planet. Sci. Lett. 66 (1983) 125^136. [27] S.R. Hart and G.E. Ravizza, Os partitioning in phases from lherzolite and basalt, in: A. Basu and S.R. Hart (Eds.), Earth Processes: Reading the Isotopic Code. Am. Geophys. Un., Geophys. Monogr. 95 (1996) 123^ 134. [28] K. Righter, M.J. Drake, Metal^silicate equilibrium in a
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Deep mantle plume osmium isotope signature from West Greenland Tertiary picrites Bruce F. Schaefer *, Ian J. Parkinson, Chris J. Hawkesworth Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK Received 24 June 1999; received in revised form 29 October 1999; accepted 17 November 1999
Abstract Picrites from Nuussuaq Peninsula and Qeqertarssuaq (Disko) Island, West Greenland, preserve trace element and isotopic signatures reflecting the composition of the Icelandic plume head. Os isotope ratios are low (187 Os/ 188 Os i = 0.1272^0.1371) in terms of global plume related magmatism, and this is coupled with anomalously high Os abundances and radiogenic 143 Nd/144 Nd i isotopes. Crustally contaminated basalts within the West Greenland sequences possess Os and Nd isotopic signatures consistent with mixing between initial plume head compositions and two discrete types of West Greenland continental crust. One crustal component is of a local sedimentary origin, and the other is typical of ancient felsic crust. The low Os isotopic signatures of the picritic units are considered to be those of the initial mantle plume. The fact that such low Os isotope ratios occur in sequences with high 3 He/4 He, and the nonsystematic variation in Os isotopes with indices of fractionation/accumulation and Pb isotopes, argue against mixing with depleted MORB mantle or ancient subcontinental lithospheric mantle. The high Os concentrations in the picrites are attributed to high degrees of partial melting ( s 25%) of mantle containing no residual sulphide. This is consistent with models for plume heads in which anomalous mantle temperatures initiate melting at high pressures generating large degrees of partial melting. Unradiogenic Os and radiogenic Nd components in plume-related CFB magmatism may preserve contributions from a reservoir which is sampled only occasionally in young oceanic basalts. Such a reservoir shares Os isotopic features with the primitive upper mantle (PUM), which may also be manifest in the `Kea' component of Hawaiian magmatism. Therefore, portions of the West Greenland continental flood basalt province arguably represent the first direct sampling of unmodified plume head material derived from the lower mantle or lower portions of the upper mantle. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: Re/Os; isotope ratios; West Greenland; Icelandic Plume; magma contamination; £ood basalts
1. Introduction Magmatism associated with mantle plumes potentially o¡ers insights into the composition and
* Corresponding author. Tel.: +44-1908-858551; Fax: +44-1908-655151; E-mail: [email protected]
thermal state of deeper portions of the Earth's mantle [1^4]. A large isotope database exists for ocean island basalts (OIB) which are characterised by a range in isotope ratios, much of which is explained by di¡erent components of recycled crust and mantle lithosphere [5]. The Re^Os isotope system has recently been utilised in trying to understand both the source components of OIB [6^8] and the e¡ects of melt percolation [9], be-
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 2 9 0 - 3
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cause of its unique geochemical characteristics. The chalcophile and siderophile nature of Re and Os combined with the compatible behaviour of Os during mantle melting set this isotope system apart from conventional lithophile isotope systems and provides a unique insight into mantle plume processes. Os isotopes are also potentially a sensitive indicator of crustal input because all crustal materials have elevated Re/Os ratios and hence with time high 187 Os/188 Os ratios [6^8,10]. Furthermore the compatible behaviour of Os during mantle melting ensures that Os isotopes provide a robust signature of ancient mantle depletion events [11,12]. Although OIB have been well studied [1^8] they tend to provide information about the composition of the plume tail, and in the case of low-£ux plumes they may only record shallow level processes. Mantle plume heads (or `starting plumes') are anomalously hot and consequently record information regarding deep mantle sources. Starting plume magmatism is very rarely preserved in the oceanic realm [1,3,4] and the best exposures of magmatism associated with plume initiation are found in continental £ood basalt (CFB) provinces. Unfortunately many CFB lavas are crustally contaminated and/or they contain signi¢cant contributions from the subcontinental lithospheric mantle (SCLM) [13^15], making inferences about deep mantle plume compositions di¤cult. This contribution presents new Re^Os isotope data for a suite of picritic and primitive CFB from West Greenland, which were previously characterised geochemically by Lightfoot et al. [16]. These lavas represent early magmatism associated with the initiation of the Iceland plume and the thick succession of picrites highlights the high degrees of melting and the greater depths of origin of the associated mantle plume. High Os concentrations in these picritic lavas means they are relatively insensitive to crustal contamination and therefore represent the best samples of deep mantle plume material so far analysed. 1.1. Background Flood basalt magmatism in the North Atlantic province began at V60.5^62.5 Ma [16^20], when
voluminous basaltic and picritic magmatism took place synchronously in contiguous portions of the Scottish Tertiary Volcanic Province, Northern Ireland, Southeast and West Greenland, the Faroe Islands and the margins of Ba¤n Island [17,18] (Fig. 1). Local foci of magmatism may in part be attributed to variations in the thickness of continental lithosphere [18], and the apparent synchronicity of magmatism over a large area (V2000 km lateral continuity [17]) is consistent with derivation from anomalously hot asthenospheric material in an impinging plume head [17,19]. This study focuses on the basal picrites of West Greenland for the following reasons: 1. the lowermost volcanic rocks are amongst the earliest magmas erupted in the North Atlantic province, 2. picrites amenable to Os isotope analysis form a large proportion of the eruptive units [16], 3. crustal contamination of the West Greenland picrites is well documented and constrained by a comprehensive trace element and isotopic dataset [16], 4. recent studies of additional isotopic systems (such as Sr, Nd, Pb and He) on the same series of lavas indicate a dominant plume component within the sequence [19,20]. Samples selected for this study are from the collection of Lightfoot et al. [16], in Qeqertarssuaq Island and the Nuussuaq Peninsula of West Greenland (Fig. 1). Picritic basalts and high-Mg basalt lithologies comprise V1 km of the eruptive sequence. The basal £ood basalt stratigraphy is often complicated by rapid lateral facies changes, including subaerial magmas, pillow basalts and hyaloclastite breccias [19]. This stratigraphy comprises the basal Va|«gat Formation, dominated by picrites, basalts and hyaloclastites, overlain by the slightly more evolved tholeiitic basalts of the Maligaªt Formation [21,22]. We have focused on picrites and basalts from the Va|«gat Formation from Qeqertarssuaq and the Nuussuaq Peninsula, and the stratigraphic details of the samples analysed can be found in Lightfoot et al. [16]. Ar^Ar geochronology on Va|«gat Formation
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samples from the Nuussuaq Peninsula consistently produces ages of 61.3^60.9 Ma [19]. Graham et al. [19] suggest that the complete eruptive history of the Va|«gat Formation on Nuussuaq Peninsula spans a period of less than V1 Myr, clearly indicative of short duration voluminous £ood basalt magmatism. Lightfoot et al. [16] calculated parental magma compositions of V15 wt% MgO, although Graham et al. [19] favour parental magmas containing s 19 wt% MgO. Even the more conservative estimate [16] is indicative of exceptionally anomalous magnesian parental magma compositions, even in the context of CFB. Melts therefore originated from anomalously thermally perturbed asthenosphere of V1540^1600³C at 60^90 km depth [16,19]. Crustal contamination is extremely variable within the more evolved basalts, and xenoliths of country rock, graphite and metamorphosed coal horizons are observed at a number of localities. Locally, exceptionally reducing conditions due to the presence of free carbon in the sedimentary sequences have resulted in the precipitation of native iron in the basaltic £ows. 2. Methodology Samples for Os isotope analysis were selected to
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characterise initial plume compositions and to span the range of potential crustal end members for contamination trends previously recognised within the sequence [16]. This involved four principle groups of samples; (1) picrites from the Nuussuaq Peninsula (8 samples), (2) picrites from Qeqertarssuaq (6 samples), (3) basalts from the Nuussuaq Peninsula (3 samples), and (4) contaminated Va|«gat Formation lavas from Qeqertarssuaq (referred to as Asuªk-Kuªga¨nguaq; 3 samples). Additionally, a comagmatic intrusion of the Serfat Sill was analysed, as was a graphite enclave from within the picritic Hammersdal Dyke. Samples from within each stratigraphic unit were selected primarily on the basis of their prospective Os contents, and a cut-o¡ value of approximately 200 ppm Ni was found to correspond to values of V0.25 ppb Os in these lavas (Fig. 2). 0.5^1.5 g of powder was spiked and digested in inverse aqua regia by Carius tube dissolution followed by solvent extraction, as described in detail elsewhere [23,24]. Re separation after removal of Os involved further solvent extraction and ¢nally an anion column pass [24]. All samples were run on a Finnigan MAT-261 mass spectrometer in N-TIMS mode at the Open University [12,24]. Table 1 summarises Re and Os concentrations and Os isotopic data for the West Greenland lavas.
Fig. 1. Location of Nuussuaq Peninsula and Qeqertarssuaq Island with respect to distribution of CFB magmatism in the North Atlantic province at V60 Ma. Adapted from [17].
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With Os total procedural blank (TPB) levels of typically 2 pg or better, blank contributions were typically less than 1% on 1 g sample dissolutions. However, particularly low abundance samples, such as those from Asuªk-Kuªga¨nguaq, were subject to blank corrections up to a maximum of 8% (sample 7650). TPBs were run as every eighth sample during the course of this study, and the average 187 Os/188 Os of TPBs = 0.291 and it remained constant, allowing a high degree of accuracy in the 187 Os/188 Os blank correction. TPBs for Re were more variable, ranging from V7 to 13 pg. A combination of low Re levels and the small amounts of sample available for analysis required greater blank corrections. These were typically 2.4^17%, with a small number of samples (4) ranging up to 25%. Since TPB were run as part of each batch of dissolutions, the appropriate blank correction for each batch was applied. Blank corrections of picritic Re concentrations generally produced minor changes ( 6 1.5%) in age corrected 187 Os/188 Os ratios, with the exception of one sample which has an exceptionally high Re/Os ratio that appears to re£ect late stage alteration processes (7860). The Os isotope data have been combined with the trace element and isotope data of Lightfoot et al. [16] to constrain the origin of the West Greenland lavas. Additionally, He isotope data for the Va|«gat Formation from the Nuussuaq Peninsula [19] is similar to that in basalts of similar age from northeast Greenland [25] and Iceland [26]. Although variable, these data suggest that high He isotopes are an intrinsic feature of the Iceland plume.
lisation. Whether Os is present as sulphide inclusions within olivine crystals, or occupies discrete positions within the olivine lattice has been discussed by Hart and Ravizza [27]. However, Fig. 2a gives some indication that Os is more likely to be distributed in inclusions within olivine since picrites from Nuussuaq have lower Os concentra-
3. Results 3.1. Re and Os distribution The measured Os concentrations in the Va|«gat Formation rocks range from 0.040 to 3.71 ppb, and Re concentrations from 0.031 to 0.42 ppb. The picrites exhibit strong positive correlations between Mg#, Ni and Os contents (Fig. 2; Mg# = Mg/(Mg+Fe2 )), consistent with Os abundances having been controlled by olivine crystal-
Fig. 2. Distribution of Os concentration as a function of (a) Mg# (Mg# = Mg/(Mg+Fe2 ), (b) Ni. Asuªk-Kuªga¨nguaq are the crustally contaminated Va|«gat Formation lavas from Qeqertarssuaq, and Nuussuaq and Qeqertarssuaq are picrites from Nuussuaq Peninsula and Qeqertarssuaq Island, respectively, Serfat is an unmineralised gabbrodolerite sill on the northern shore of Nuussuaq.
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Table 1 Re and Os concentration and Os isotopic data for the West Greenland £ood basalts and associated graphite enclave Sample
Unit
MgO (wt%)
Ni Cr Yb (ppm) (ppm) (ppm)
Contaminated Va|«gat Formation, Asuk 7650 Kuganguaq 11.13 38 7716 Kuganguaq 11.51 24 7717 Kuganguaq 12.57 147 Nuussuaq basalts 7859 B3 7.97 41 7819 B4 14.71 450 7849 B4 10.33 241 Nuussuaq picrites 7837 P1 10.15 217 7829 P2 11.62 259 7860 P3 12.47 105 7822 P4 26.47 1170 7854 P4 24.12 1160 7855 P4 16.81 605 7902 P4 13.03 372 7851 P5 16.41 586 7818 P5 24.36 1080 Qeqertarssuaq Vaigat picrites 7633 Naujanguit 15.14 531 7640 Naujanguit 26.30 1190 7649 Ordlingassoq 16.83 612 7712 Naujanguit 18.77 769 7718 Naujanguit 15.44 534 7719 Naujanguit 29.04 1300 Intrusions 7842 Serfat picrite 29.76 1410 Hammersdal Dyke Graphite HDG
Re (ppb)
Os (ppb)
Re/Os
and Kuganguaq 1090 0.25 0.419 0.0651 6.44 1168 0.29 0.0305 0.0400 0.76 962 0.27 0.0641 0.200 0.32
187
Re/188 Os
187
Os/188 Osa
187
Os/188 Osb QOsc
31.7 3.69 1.55
0.30012 71 0.15590 380 0.13288 12
0.26840 0.15221 0.13133
111 20 3.23
4.41 1.26 6.82
0.14263 68 0.13446 9 0.15029 12
0.13822 0.13319 0.14347
8.7 4.7 13 5.4 7.8 327 5.2 2.8 1.2 6.5 2.8 4.0
857 935 528
0.32 0.23 0.29
0.0452 0.0495 0.91 0.287 1.098 0.26 0.239 0.169 1.41
481 598 1156 2515 1866 1509 856 956 1494
0.28 0.27 0.25 0.18 0.18 0.24 0.25 0.21 0.16
0.497 0.564 0.754 0.241 0.0865 0.157 0.272 0.184 0.199
0.409 0.393 0.100 1.724 1.772 1.301 0.664 1.166 1.899
1.21 1.44 7.53 0.14 0.05 0.12 0.41 0.16 0.10
5.86 6.94 36.3 0.673 0.235 0.580 1.97 0.762 0.505
0.13985 21 0.14404 17 0.12897 24 0.13449 10 0.13098 8 0.12926 7 0.13735 12 0.13146 10 0.13279 8
0.13399 0.13709 0.09269 0.13382 0.13075 0.12868 0.13537 0.13070 0.13228
1048 1715 1103 1379 1007 1770
0.28 0.17 0.22 0.26 0.21 0.11
0.187 0.156 0.269 0.409 0.230 0.0673
1.359 3.101 1.594 2.331 1.181 3.373
0.14 0.05 0.17 0.18 0.19 0.02
0.664 0.243 0.814 0.845 0.938 0.096
0.12785 9 0.12887 12 0.13178 29 0.13203 10 0.13023 12 0.13070 20
0.12718 0.12862 0.13097 0.13118 0.12929 0.13061
0.01 1.1 3.0 3.2 1.7 2.7
2530
0.11
0.354
3.711
0.10
0.460
0.13512 18
0.13463
5.9
0.359
0.345
0.96
4.79
0.40346 843
0.39867
214
MgO, Ni, Cr and Yb data from [16]. a Present day 187 Os/188 Os. Errors are 2c within run precision and equal to the least units cited. b Age corrected 187 Os/188 Os ratios at 60 Ma or 63 Ma for HDG using V = 1.666U10311 yr31 [47]. c Age corrected QOs values. QOs is the percentage di¡erence between the age corrected 187 Os/188 Os ratio and the age corrected 187 Os/188 Os ratio of a chondritic mantle where the present day chondritic mantle has 187 Os/188 Os = 0.12757 and 187 Re/ 188 Os = 0.3972 [11].
tions at similar Mg# than those from Qeqertarssuaq, where both sequences contain accumulated olivine. It appears that sulphide and/or PGE alloy inclusions within olivine are the most plausible host for Os. Assuming a primary melt composition of 15 wt% MgO [16] gives a primary Os concentration of 1.34^1.60 ppb and Re concentration of 0.19^ 0.27 ppb (Fig. 2 and Table 1). These absolute concentrations of Os at a given MgO for the West Greenland picrites are typically higher than for picrites of similar MgO content from ocean islands [29,30] and many CFB provinces
[13] (Fig. 3). Only picritic basalts from Siberia [31] and komatiitic lavas from Munro Township [32] and Gorgona Island [33] have similar Os concentrations for given MgO contents. There is no reason to believe that the source region had anomalously high Os contents since the slightly suprachondritic Os isotope ratios of these lavas would imply correspondingly elevated Re contents in the source, which is inconsistent with the low observed Re concentrations of the lavas (see below). A more likely explanation of the high Os concentrations in the lavas is their high degrees of partial melting. In such cases the melts
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Fig. 3. Os concentration as a function of MgO for major plume related provinces. Note how the oceanic setting trends £atten at higher MgO contents with respect to the continental provinces. Symbols as for Fig. 2. Data from [6,13,29^ 33,40^42,45,48].
become sulphur undersaturated and therefore extract their full inventory of platinum group elements (PGE) from the mantle due to the absence of residual sulphide in the melting residue. The possibility of Os (and Ir and Ru) contents being bu¡ered by metallic or alloy phases in the source region after exhaustion of sulphide during melting is unlikely to apply in West Greenland in view of the observed high Os contents. Indeed, the high PGE contents of the Disko Island picrites has been noted previously [34] and we suggest that the elevated Os contents in the picrites are a result of sulphur undersaturation. The di¡erence in Os concentrations between the Qeqertarssuaq and Nuussuaq picrites at MgO s 20% therefore re£ects inclusion of PGE alloys or sulphides during olivine accumulation. For the inclusions to be sulphides suggests that the lavas had become sulphur-saturated at the shallow pressures of crystallisation, by contrast to their sulphur undersaturation in the melt generation region. The lack of di¡erence between the Cu contents of the Qeqertarssuaq and Nuussuaq picrites [16] is one argument that the inclusions are PGE alloys rather than sulphides. Variations in Re concentrations are broadly
analogous to those of the HREE (see also Hauri and Hart [35]). The picritic sequences on both Nuussuaq and Qeqertarssuaq have positive correlations between Yb and Re abundances, although the more evolved basaltic lithologies analysed preserve weak negative correlations. In order to understand how Re and Os distributions can be used to understand aspects of the genesis of the West Greenland picrites, plots of Os versus Re/Os and Re versus Yb are shown in Fig. 4. One of the features of the West Greenland data is their high Os abundances coupled with low Re/Os ratios, a characteristic shared only with the Siberian picrites. Although komatiites have high Os contents they also have high Re contents resulting in moderate Re/Os ratios. On a plot of Re versus Yb (Fig. 4b), MORB and komatiites plot with good positive correlations but with the komatiites displaced to higher Re contents. Since Re is strongly compatible in sulphide and mildly compatible in garnet, whereas Yb is incompatible in sulphide and compatible in garnet, these data can be interpreted as melting with residual sulphide but no residual garnet for MORB (see also [36]). This would imply that melting with neither residual garnet nor sulphide is appropriate for komatiites. By contrast OIB have low Re contents which are uncorrelated with low but restricted Yb contents, consistent with melting in the presence of residual garnet, a feature also indicated by their fractionated REE patterns. The OIB ¢eld in Fig. 4b is de¢ned by poorly correlated subhorizontal arrays. The West Greenland data di¡er from these suites in that they have low Re and Yb concentrations but preserve a strong positive correlation between the two elements (Fig. 4b) coupled with high Os contents. They might therefore re£ect melting with residual garnet that retained both Re and Yb, but with no residual sulphide. Signi¢cantly, West Greenland picrites contain higher [Dy/Yb]N at low [La/Yb]N (data from [16]) with respect to MORB, consistent with a more pronounced garnet melting signature and larger degrees of partial melting. Hence, the REE data suggest that both picritic suites have garnet melting signatures which have been diluted by large amounts of melt subsequently generated in the
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Fig. 4. (a) Os versus Re/Os and (b) Re versus Yb for plumerelated ocean island (OIB) and continental lavas (CFB), komatiites and MORB. Symbols as for Fig. 2. MORB Yb data were calculated from Tb concentrations from [49] assuming a chondritic Tb/Yb ratio.
spinel peridotite facies. Both suites represent high degrees of partial melting (V25%), but on average the Qeqertarssuaq picrites are higher degree partial melts. Hence, a likely scenario for the West Greenland picrites is that they re£ect melting which was initiated in the garnet ¢eld, and continued in the spinel ¢eld, and because of their high degrees of partial melting they became sulphur undersaturated. The distribution of Os, Re
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and Yb abundances (Fig. 4) in these rocks is likely to be restricted to large degree melts where melting is initiated in the garnet facies and sulphide is eliminated from the residue. This is possible during initiation of plume melting. Although the West Greenland picrites contain Re and Yb concentrations similar to those in oceanic intraplate basalts (Fig. 4b), they have higher Os and the linear relationships observed in Fig. 4 di¡er from the subhorizontal Re and Yb arrays of the small degree melt basalts that de¢ne the oceanic ¢eld. Assuming a primary melt composition of 15 wt% MgO [16] gives a primary Os concentration of 1.34^1.60 ppb and Re = 0.187^0.27 ppb. Using these values and assuming that Re and Os are present in chondritic proportions, the West Greenland picrites suggest bulk partition coe¤cients after 25% partial melting of 2.3^2.8 and 1.1^1.6 for Os and Re, respectively. These values are slightly lower for Os and higher for Re than partition coe¤cients used in the literature [8,28,36]. In summary, the elevated Os concentrations in the Va|«gat Formation picrites are consistent with these lavas being high degree sulphur-undersaturated partial melts. Elevated Os concentrations in the Qeqertarssuaq lavas with MgO s 16%, relative to those from Nuussuaq Peninsula are attributed to the presence of PGE alloys, or possibly sulphides, as inclusions in the accumulated olivine. The low Re, Yb and Re/Os ratios of the lavas support models of high degrees of partial melting with residual garnet during the early stages of melting, and no residual sulphide. 3.2. Os isotopes of uncontaminated Va|«gat Formation picrites The initial Os isotopes of Va|«gat picrites are generally suprachondritic, but lower than those in most global plume related magmatic rocks (187 Os/188 Os i = 0.1278^0.1388, compared with V0.131^0.15 in typical OIB [6^8,41,42]) (Figs. 5 and 6; Table 1). One Nuussuaq picrite (7860) has an anomalously high Re/Os ratio with low measured 187 Os/188 Os (0.12897), producing improbably low age corrected 187 Os/188 Os (Table 1), and it is disregarded from following discussion. Post em-
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Fig. 5. Global distribution of Os vs QOs i Note the high concentrations and low QOs i values of the West Greenland and Karoo Picrites. Q notation is plotted in preference to initial ratios in order to emphasise the temporal evolution of di¡erent source terrains with respect to chondrite.
placement mobilisation of Re may have been responsible for its high Re/Os ratio. The Os isotope ratios in plume related magmatism are either those of the plume source, or they are due to interaction with relatively shallow level crustal or mantle reservoirs. The West Greenland picrites tend to have slightly suprachondritic Os isotope ratios, and potentially Os may have been incorporated from the SCLM and/or depleted MORB-type mantle (DMM). Fig. 6 illustrates that the West Greenland picrites are not mixtures of DMM and SCLM, since the Nd^Os array lies to higher Os ratios than either of these reservoirs. Further, mixing of DMM with continental crust produces arrays which rapidly decrease in Nd isotopes with little change in Os isotopes, and hence do not intersect the West Greenland array. Hence it appears that the Os and Nd characteristics of West Greenland picrites are truly those of the plume head, and not a combination of other reservoirs. Hence, we have modelled the least radiogenic end member in the West Greenland Nd^Os picrite
array (Fig. 6) as representative of a primary plume component. Sample 7633 is the most depleted in terms of Nd and Os isotopes and it has MgO = 15.14wt%, appropriate for a primary melt (Table 1). Fig. 6 presents a series of mixing curves for Nd and Os isotopes between sample 7633 and possible SCLM and crustal components. Low Os isotope ratios (QOs = 321 to 313) have been reported from xenoliths from Tertiary Dykes in East Greenland [37], and the presence of such highly depleted ancient SCLM beneath East Greenland [37,38] suggests it is reasonable to assume that there is similar lithospheric material beneath West Greenland. In detail, the choice of the depleted SCLM end member has little e¡ect on the calculated mixing curves. Rather, since sample 7633 has the lowest 187 Os/188 Os i within the picrite suite, mixing trends with bulk SCLM (curve 1 on Fig. 6a) or partial melts of the SCLM (curve 2) are unable to account for the Nd^Os isotope array within the Va|«gat picrites. Alternatively, the variation of Nd and Os iso-
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Fig. 6. Comparison of mixing curves between primary Va|«gat picritic liquid with SCLM and crustal contaminants. DMM and SCLM are plotted at 60 Ma. (a) QOs i vs ONd i . (b) As for (a), but with expanded QOs i axis to illustrate mixing trends for crustally contaminated basalts of the Va|«gat Formation. Symbols as in Fig. 2. Curves 1 and 2 are mixtures with bulk SCLM and a partial melt of SCLM, respectively, and curve 3 = mixing with graphitic sediment. Curves labelled with concentration values are mixing arrays to average continental crust of varying Os concentration, as indicated. Tick marks are at 10% intervals. End member parameters: Average continental crust; Os = as indicated on curves, 187 Os/ 188 Os 60 = 2(Q = 1448), Nd = 20 ppm, 143 Nd/144 Nd 60 = 0.51098 (O = 331.4); Bulk SCLM (curve 1); Os = 9.2 ppb, 187 Os/ 188 Os 60 = 0.11026 (Q = 313.3), Nd = 1 ppm, 143 Nd/144 Nd 60 = 0.5118 (O = 315.1); SCLM melt (curve 2); Os = 0.9 ppb, 187 Os/188 Os 60 = 0.11026 (Q = 313.3), Nd = 10 ppm, 143 Nd/ 144 Nd 60 = 0.5118 (O = 315.1); Graphitic Continental Crust (curve 3); Os = 0.15 ppb, 187 Os/188 Os 60 = 2(Q = 1448), Nd = 10 ppm, 143 Nd/144 Nd 60 = 0.51098 (O = 331.4). Va|«gat picrite composition as per sample 7633 on Table 1. Picrite Nd data from [16]. Data for Siberia from [31], East Greenland [40], and Samoa and the Cook-Austral islands from [6].
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topes in the West Greenland picrites might be accounted for by interaction with continental crust. However, this is unlikely since, (1) both cumulates and fractionates span the complete range of Os isotopic values, and so there is no systematic change in isotopes with indices of differentiation. (2) There is no covariation in, for example Pb and Os isotopes, and (3) the exceptionally high 3 He/4 He isotopes of some Va|«gat Formation olivines [19] precludes signi¢cant interaction with crustal material during olivine crystallisation. Further, detailed trace element and lithophile isotope studies have previously established the lack of signi¢cant crustal contamination in the generation of the Va|«gat picrites [16,20]. Additionally, calculated mixing trends with estimates of bulk continental crust do not intersect the observed Nd-Os isotope array (Fig. 6). It is possible to construct very speci¢c crustal mixing curves through the picritic array (e.g., curve 3 on Fig. 6), but such curves require high Os contaminants and such curves are appropriate only for the basalts, which are demonstrably contaminated (see [16] and discussion below). Hence the Os isotopes are consistent with the conclusions of Lightfoot et al. [16], who observed little variation in crustal indicators (such as La/Sm and Nd isotopes) in the picritic rocks. We conclude that the range of Os isotopes in the picrites re£ects source heterogeneity of the mantle plume. 3.3. Os isotopes of contaminated Va|«gat Formation basalts In contrast to the picrites, many basalts of the Va|«gat Formation preserve clear evidence of crustal contamination in terms of major and trace elements, and lithophile isotope ratios. Lightfoot et al. [16] identi¢ed contamination of Va|«gat Formation lavas with material from two distinct crustal sources, one containing elevated La/Sm and Th/Nb ratios (trend 1; ¢gure 6a of [16]) and another containing elevated La/Sm ratios but with low Th/Nb and slightly higher 143 Nd/144 Nd (trend 2 of [16]). These trends were attributed to mixing with sedimentary and granodioritic end members, respectively [16]. A sample from each trend was selected for Os isotope analysis.
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Sample 7650, a high Th/Nb (trend 1 of [16]) basalt, preserves the highest 187 Os/188 Os i ratio (0.26835) of all the Va|«gat Formation rocks analysed, consistent with contamination by an ancient sedimentary component. Tertiary sediments on Qeqertarssuaq are dominated by Archaean Nd and Sr isotopic signatures [39]. Simple mixing with bulk continental crust of isotopic compositions appropriate for West Greenland is unable to account for the Nd and Os ratios of 7650, however it does lie on a mixing trend with an inclusion of graphitic sediment in the Hammersdal Dyke (Fig. 6b). Since coal measures are a feature of the Tertiary sequence that underlies the West Greenland lavas, this crustal end member is appropriate for trend 1 basalts. Approximately 40% bulk mixing with graphitic sediment is required to produce the observed Nd and Os ratios in sample 7650, a ¢gure similar to that estimated by Lightfoot et al. [16] for the same sample from trace element and Nd isotope variations. Conversely, the high La/Sm, low Th/Nb contaminated basalt (trend 2 of [16]) has an initial Os isotope ratio similar to the uncontaminated picrites, despite a signi¢cant shift to lower Nd isotope values (Fig. 6). This is consistent with V10% mixing with Archaean felsic continental crust (Fig. 6b), but we note that the Os concentration in most crustal rocks is so low as to have little in£uence on the calculated amounts of mixing (Fig. 6b). Whereas the Nd^Os isotope shift is consistent with an ancient granodioritic component [16], the Os isotopic systematics do not allow effective discrimination between alternative ancient low Os abundance crustal sources. 3.4. Summary of results In summary, the osmium isotope ratios of the Va|«gat Formation rocks record both primary plume head signatures in the picrites, and crustal contamination processes in the more evolved basalts. The primitive picrites in West Greenland show little variation in Nd isotopes, but they range from unusually low 187 Os/188 Os i values of V0.1272 up to the lower end of typical plume related Os isotope signatures (V0.1371). It is possible to construct crustal mixing curves through
the picritic Nd^Os isotope array using speci¢c contaminants with high Os contents. However trace element and Sr, Nd, Pb isotope studies [16], coupled with the lack of systematic variation in Os isotopes with major and trace element contents, and the presence of demonstrably non-crustal He isotopes in the same sequence [19], argue strongly against signi¢cant crustal interaction. Hence the observed Nd^Os array is interpreted to re£ect that of the impinging plume head. Further, the large volume of picritic magmatism in West Greenland (V1 km thick [14] over V55 000 km2 [20] (Fig. 1)) indicates large degrees and high temperatures of melting from a deep mantle source [16,19,20]. Such conditions are favourable for rapid extrusion and minimal lithospheric contamination of the picrites. Hence the Va|«gat Formation picrites of West Greenland faithfully record primary isotopic signatures of the impinging plume head, and invite comparison with other plume related lavas. 4. West Greenland plume lavas in a global context As noted previously, a striking feature of the basal West Greenland CFB province is its elevated Os concentrations and coupled low Os and high Nd isotope ratios in comparison with magmas from active mantle plumes. East Greenland lavas of the same age also have low Os isotope signatures [40], indicating these are an inherent feature of the plume head associated with the modern Iceland hot spot. A low Os isotopic component is by no means unique in global hot spot related magmatism, in that similar components have been seen in magmas from the Karoo [13], Azores [41], Canary Islands [42], Samoa [6] and Hawaii [29,30]. A number of explanations have been o¡ered for these components, and many involve interaction with lithospheric source material (e.g., [13,41,42]). However, the low 187 Os/188 Os ratios of the West and East Greenland CFBs [40] appear to be a feature of the upwelling plume, and not due to signi¢cant contributions from the lithospheric mantle, particularly given the extremely low Os isotope ratios of East
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Greenland lithospheric xenoliths [37]). Thus it appears that large mantle plumes may sample deepseated reservoirs which are themselves only slightly suprachondritic for Re/Os. The key feature of the West Greenland picritic suites is the combination of slightly suprachondritic Os isotopes with elevated Nd isotopes. This feature is surprising because it is distinct from DMM which has coupled high Nd isotopes and subchondritic Os isotopes, indicating an ancient melting event and time integrated high Sm/Nd and low Re/Os ratios. The West Greenland data suggest a source with chondritic to slightly suprachondritic timeintegrated Re/Os ratios but with high Sm/Nd ratios; inconsistent with a single ancient melt depletion event which would explain the high Nd isotopes but not the Os isotope ratios. In fact the primary melt composition end of the West Greenland Nd^Os isotope array is very similar to recent estimates of the putative PUM ([43]), and also to the `Kea' component of the Hawaiian plume [44]. Taken together, these data indicate the presence of a reservoir in the Earth with slightly suprachondritic Os isotopes and radiogenic Nd isotopes. Its occurrence in the two high buoyancy£ux plumes (i.e., Hawaii and Iceland), coupled with their elevated 3 He/4 He ratios provides evidence that this reservoir may be located in the lower mantle. Alternatively, decoupling of He isotopes from Os and Nd might allow domination of the He budget by a small mass contribution from the lower mantle, while Os and Nd are dominated by a true PUM (i.e., in the lower upper mantle) component. In either case, such characteristics are observed during plume initiation and/or by the hottest, deepest plumes, suggesting they are features of a speci¢c reservoir not sampled by smaller, shallower plumes, such as beneath the Azores [41,42]. Distinguishing near chondritic components from mixing with unradiogenic lithosphere reservoirs solely on the basis of Os isotopes is problematic. However recognition of plume-derived low 187 Os/188 Os may o¡er an alternative to elaborate lithosphere interaction models. This is particularly pertinent in view of the ambiguity surrounding the Os isotope characteristics of lithospheric sources. Most SCLM^mantle plume
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mixing models invoke ancient depleted peridotite end members, whereas only the Karoo and Greenland lavas were demonstrably erupted through such mantle lithosphere [13,38,39]. While mixing with subchondritic lithospheric mantle is appropriate for some plume related provinces, West Greenland o¡ers an example of large volume, high temperature, mantle plume-derived magmatism impinging beneath ancient continental lithosphere and preserving primary plume isotopic compositions. Although there are limited Os isotopic data for the present day Iceland plume at least one analysis [45] has a value (0.1286) similar to the West Greenland picrites. The identi¢cation of a depleted source di¡erent from MORB-type mantle in modern day Icelandic lavas [46] suggests that the Icelandic plume is tapping a reservoir similar to that identi¢ed here for the West Greenland picrites. The Qeqertassuaq picrites [16,20] plot within Iceland and Kolbeinsey Pb isotope data ¢elds of Thirlwall [46]. The four picrites that have Pb isotope ratios similar to the modern Iceland plume data have 187 Os/188 Os of 0.1272^ 0.1310 and ONd of V8 further indicating that slightly suprachondritic Os isotope and high Nd isotope ratios characterise the source sampled by the Iceland plume. 5. Conclusions The picrites of West Greenland o¡er a rare opportunity to evaluate the nature of primary plume isotopic compositions in a voluminous CFB province. They are characterised by high Os abundances, and relatively low Os isotopes (187 Os/ 188 Os i = 0.1272^0.1371) for plume-related magmas and high Nd isotopes. By contrast, Os isotopic data for the more evolved West Greenland magmas support crustal contamination models independently established from trace element and lithophile isotope variations [16]. These crustally contaminated magmas involved mixing with two distinctive crustal end members; one dominated by graphitic Tertiary sediments derived largely from Archaean crust, and a second typical of felsic continental crust. Both suites of contaminated
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lavas have low Cu and Ni contents, consistent with sulphide formation within the crust [16]. The rapid extrusion rate and large volumes of high temperature picritic magmatism indicate that partial melting was of unusually hot and deep seated source rocks [16,19,20]. It is likely that the Va|«gat Formation picrites were located away from the central axis of the plume at V60 Ma, suggesting that the plume head was itself extremely hot, and able to maintain such anomalously high temperatures while dispersed beneath the continental lithosphere. Despite impingement beneath ancient continental lithospheric mantle, modelling of the Nd-Os isotope data indicates that the plume head picrites record minimal interaction with lithospheric sources. Simple partial melting calculations yield bulk partition coe¤cients of 2.3^2.8 for Os and 1.1^1.6 and for Re during melt generation of the West Greenland picrites. These values indicate melting with no residual sulphide, but some residual garnet during the initial stages of partial melting, a scenario arguably unique to high degrees of partial melting in high buoyancy mantle plumes. The distinctive isotopic characteristic of the West Greenland picrites is their slightly suprachondritic Os isotopes coupled with high Nd isotopes. These features have also been identi¢ed in some Hawaiian lavas [44], and the Os isotope composition of the West Greenland picrites is similar to putative PUM [43]. We argue that the Os and Nd (and He) isotope characteristics are of a reservoir which is only tapped during plume initiation and/or by high buoyancy-£ux plumes such as Hawaii and Iceland, and which is located in the lower mantle or lower portions of the upper mantle. Acknowledgements Peter Lightfoot is thanked for access to his exhaustive sample collection and constructive comments on an earlier version of the manuscript. Kent Brooks is thanked for access to a pre-print containing the East Greenland data. Jessica Bartlett provided several initial analyses on the picrites. Eric Hauri, Laurie Reisberg and Graham
Pearson provided stimulating reviews. Peter Van Calsteren is thanked for maintaining the mass spec, and Anthony Cohen for guidance in laboratory matters. I.J.P. is funded by a NERC/ODP fellowship, and isotopic research at the Open University is funded by NERC.
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