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Journal of Petrology Volume 42 Number 7 Pages 1387-1400 2001
© Oxford University Press 2001
Sr, Nd, Pb and O Isotopes of Minettes from Schirmacher Oasis, East Antarctica: a Case of Mantle Metasomatism involving Subducted Continental Material
1INSTITUTE OF GEOLOGY AND MINERALOGY, UNIVERSITY OF ERLANGEN–NÜRNBERG, SCHLOSSGARTEN 5, D-91054 ERLANGEN, GERMANY
2INSTITUTE OF MINERALOGY, UNIVERSITY OF MÜNSTER, CORRENSSTR. 24, D-48149 MÜNSTER, GERMANY
3INSTITUTE OF ISOTOPE GEOLOGY AND MINERAL RESOURCES, ETH ZURICH, NO C61, CH-8092 ZÜRICH, SWITZERLAND
Received October 7, 1999; Revised typescript accepted November 15, 2000
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ABSTRACT |
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Numerous minette dykes intersect the Precambrian crystalline basement of Schirmacher Oasis, East Antarctica. This study presents new Sr, Nd, Pb and O isotope data for 11 minette samples from four different dykes. The samples are characterized by relatively high 87Sr/86Sr (0·7077–0·7134), 207Pb/204Pb (15·45–15·55) and 208Pb/204Pb (37·8–39·8), combined with low 143Nd/144Nd (
Nd = -6·5 to -25·1) and variable 206Pb/204Pb (16·8–18·1). The 
18O values are high, ranging from +6·5 to +9·5
SMOW. Rb/Sr whole-rock–biotite isochrons suggest an age of 
455 Ma for emplacement of the minette dykes. The major and compatible element geochemistry of the minettes indicates derivation of the magmas from a mantle source. The enriched isotopic and trace element signatures of the dykes cannot be due to contamination of the ascending magmas by continental crust. Rather, the geochemical characteristics of the minettes are most reasonably explained by partial melting of a lithospheric mantle source that was enriched by metasomatic fluids derived from recycled continental crust. If mantle enrichment took place just before dyke emplacement, the isotopic systematics of the minettes must be inherited directly from the metasomatic agents, and this would indicate derivation of the fluids from recycled lower continental crust. KEY WORDS: Antarctica; isotopes; mantle metasomatism; minettes, lithospheric mantle
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSAMPLESANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Minette magmas are generally thought to represent small-degree melts that form at depths of
50–150 km within the lithospheric mantle. The deep origin of such lamprophyric magmas is indicated by entrained mantle xenoliths of spinel peridotite or garnet peridotite composition, and the geochemistry of the magmas (e.g. Rogers et al., 1982
; Stille et al., 1989; Wyman & Kerrich, 1993; Carlson & Irving, 1994; Carmichael et al., 1996). Minettes typically display intermediate to basic compositions with high contents of MgO, Cr and Ni, and high mg-number. Additionally, however, minettes are also characterized by high concentrations of large ion lithophile elements (LILE), particularly Ba, Sr and Rb, and they are enriched in light rare earth elements (LREE). Thus minettes display high abundances of both compatible and highly incompatible trace elements. This requires the involvement of at least two distinct source components for the generation of minette magmas: (1) a peridotitic mantle reservoir and (2) a component enriched in LILE and LREE. It is conceivable that the enrichment of incompatible trace elements results from the contamination of the magmas with crustal materials during dyke emplacement. Alternatively, a number of studies have indicated that minettes are derived from enriched lithospheric reservoirs. It has been suggested that such reservoirs are formed by the interaction of refractory peridotitic material with a metasomatic component released from subducted continental material in a collision or subduction zone setting (e.g. Nelson, 1992; Stern & Hanson, 1992; Carmichael et al., 1996; Becker et al., 1999).
This study presents new Sr, Nd, Pb and O isotope compositions for 11 representative minette samples from the Schirmacher Oasis, East Antarctica. The new results are used to constrain the age of the lamprophyre dykes and the origin of the incompatible element-enriched signatures of the minettes. These minette samples are of particular interest in this respect, because a previous trace element study indicated that the mantle source of the magmas may have been metasomatically modified by fluids or melts released from subducted sediments (Hoch & Tobschall, 1998).
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SAMPLES |
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The Schirmacher Oasis is located near the Princess Astrid Coast, Central Queen Maud Land, East Antarctica at 70°44'–70°47'S, 11°25'–11°55'E (Fig. 1). This ice-free area of
35 km2 extends nearly parallel to the east–west-trending coastline and is situated approximately halfway between the coastal ice shelf and the main mountain range of Queen Maud Land. The crystalline basement of the Schirmacher Oasis forms part of the East Antarctic craton (Sengupta, 1991; Paech & Stackebrandt, 1995). The geology is dominated by six major rock sequences, which consist mainly of different gneiss varieties (Fig. 1). Numerous dykes of lamprophyre, basalt, dolerite, pegmatite and aplite intruded into this poly-metamorphic complex.
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Eleven minettes from four dykes were analysed in the present study. The samples were collected in 1983–1984 by H. Kämpf and U. Wand (now at GeoForschungsZentrum Potsdam) during the 29th Soviet Scientific Expedition to Queen Maud Land, East Antarctica. Sample locations are shown in Fig. 1. The minette dykes trend predominantly ENE–WSW, with a dip of 50–85° NNW or 40–70° SSE, and they are between 0·5 and 3 m wide. The studied samples are invariably porphyritic with panidiomorphic texture, containing mafic megacrysts (biotite > amphibole and/or pyroxene) in a feldspar groundmass (K-feldspar > plagioclase). Detailed petrographic investigations indicate that all samples are fairly fresh, with only minor signs of greenschist alteration (Hoch, 1997).
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ANALYTICAL METHODS |
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For the analyses of whole-rock samples, 100–200 mg of powder were digested with HF–HNO3–HClO4 and HCl. Pure mineral separates of biotite (100 mg) were handpicked under a binocular microscope from a split of coarsely crushed whole-rock powder. Standard chromatographic techniques were applied for the separation of Rb–Sr, Sm–Nd and U–Pb from the rock samples. Total chemistry blanks were <2·0 ng for Rb, <300 pg for Sr, <300 pg for Nd, <30 pg for Sm, <50 pg for Pb, and <16 pg for U, and are thus insignificant.
Radiogenic isotope compositions were measured at the Mineralogisch–Petrographisches Institut, Universität München (Sr, Nd), and the Institut für Geowissenschaften und Lithosphärenforschung, Universität Gießen (Pb), by thermal ionization mass spectrometry (TIMS) using a Finnigan MAT 261 instrument. The concentrations of the elements Rb, Sr, Nd, Sm, U and Pb were determined by isotope dilution-TIMS. Fractionation corrections, isotopic standard values and the external precision of the measurements are reported in the captions of Tables 2–4 (below). The oxygen isotope analyses were performed on whole-rock samples following extraction of oxygen using purified fluorine and subsequent conversion into CO2. The measurements were made on a PRISM I mass spectrometer (VG Instruments) at the Mineralogisch–Petrographisches Institut, Universität Bonn. All results are reported relative to Standard Mean Ocean Water (SMOW) in the common -notation. The overall reproducibility of 18O values averaged ±0·1 (1
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RESULTS |
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TOPABSTRACTINTRODUCTIONSAMPLESANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Major and trace elements
The major and trace element geochemistry of the Schirmacher minettes has been discussed in detail in a previous publication (Hoch & Tobschall, 1998
) and only a brief summary is given here. The minettes are characterized by an intermediate to basic composition with mg-number ranging from 57 to 74, and MgO contents of 5·1–11·6% (Table 1). In most cases, the compositional differences among samples of the same dyke are significantly smaller than the overall range in chemical compositions (Table 1). Plots of SiO2, Al2O3, Cr and Ni vs mg-number indicate that fractional crystallization of olivine and pyroxene occurred during magma evolution (Hoch & Tobschall, 1998). Not all minette samples follow a single liquid line of descent and this suggests that the dykes are derived from different parental magmas. Further characteristics of the samples are the high contents of volatiles and alkali elements, as well as the high abundances of Ni and Cr, particularly in the least evolved samples of each dyke (Table 1).
All minettes are characterized by high concentrations of LILE, particularly Rb, Ba, Th, K and LREE (Table 1, Fig. 2). The incompatible trace element abundances range from 10 (Ti, Y) to 1000 (Ba) times primitive mantle values. Positive spikes in the trace element patterns are particularly apparent for Ba and Rb (and, in many samples, Pb), whereas Nb and Ti display negative anomalies (Fig. 2). As a result of these systematics, the minettes are characterized by low Nb/U (10·5 ± 5·2) and Nd/Pb (3·7 ± 2·5) ratios.
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Rb–Sr isochron ages
The 87Rb/86Sr ratios of minette samples from the same dyke display only very limited variation. To obtain precise age information, it was thus necessary to analyse handpicked biotite mineral separates from each dyke (Table 2). Isochron ages were then calculated for the dykes, by combining the biotite data with the whole-rock results for samples from the same dyke (Tables 2 and 3). Identical formation ages of 445·5 ± 9·8 Ma and 445·8 ± 4·5 Ma were obtained for Dykes 1 and 2, respectively, whereas Dyke 3 was dated at 728 ± 13 Ma (Table 2).
There exists only a very limited number of previous geochronological studies on rocks of the Schirmacher Oasis and lithostratigrapic correlations with other metamorphic complexes in Queen Maud Land are still of uncertain significance (Paech & Stackebrandt, 1995). Grew & Manton (1983) reported U–Pb ages for allanites and zircons from gneisses of the Schirmacher Hills with upper and lower concordia intercept ages of 1500 Ma and 630 Ma, respectively. The 1500 Ma age was interpreted to represent a primary deformation event under high-grade (granulite-facies) conditions. The lower-intercept age probably marks the subsequent re-equilibration of the rocks under amphibolite-facies conditions. Ravich & Soloviev (1966) documented maximum K/Ar ages of 845–830 Ma for mafic granulites from the Schirmacher Oasis. Most ages for rocks and minerals from the Schirmacher Oasis, however, are younger at 700–500 Ma (Ravich & Krylov, 1964; Grew & Manton, 1983; Kämpf & Stackebrandt, 1985), probably as a result of the pervasive reactivation of this crustal segment during the Pan-African orogeny (Bormann et al., 1995; Paech & Stackebrandt, 1995).
The basement rocks of the Schirmacher Oasis are intersected by numerous lamprophyre, basalt, pegmatite and aplite dykes, and few of these intrusions have been the subject of geochronological investigations. Conventional K/Ar dating of whole-rock samples shows two age-groups for the basalts—Palaeozoic and Mesozoic (Kaiser & Wand, 1985; Wand et al., 1988). The only data existing for pegmatites from this region are Pb/Pb model ages of between 865 and 600 Ma for K-feldspars (Bielicki et al., 1991). Field observations indicate that the lamprophyres from the Schirmacher Oasis are older than the basalts, but younger than the pegmatites (Paech & Stackebrandt, 1995). Dayal & Hussain (1997) obtained Rb/Sr whole-rock–mineral isochron ages of 455 ± 12 Ma and 458 ± 6 Ma for two lamprophyre dykes from the Schirmacher Oasis.
It is noteworthy that the latter results are very similar to the 446–445 Ma ages that were obtained for the minettes of Dykes 1 and 2 in the present study. The 730 Ma age that was obtained for Dyke 3, however, is difficult to reconcile with the regional geological evolution. Given the existence of numerous metamorphic ages in this area that are younger than 700–500 Ma, any dyke older than 700 Ma should show clear signs of metamorphic overprint. All minette samples, including those obtained from Dyke 3, show only minor signs of metamorphic alteration under greenschist-facies conditions (Hoch, 1997; Hoch & Tobschall, 1998). The dykes are furthermore undeformed and they strike almost parallel with a steep dip. Thus it is highly unlikely that Dyke 3 was overprinted by the Pan-African orogeny, whereas Dykes 1 and 2 were unaffected by this event. This leads to the interpretation that the Rb/Sr age obtained for Dyke 3 is probably disturbed. The reason for the erroneously old age is unclear at present, but it may be due to the analysis of xenocrystic biotite, which was not in equilibrium with the host magma at the time of dyke emplacement. The good agreement in the ages of Dykes 1 and 2 with the previously published age data for lamprophyres from the Schirmacher Hills at 445 Ma indicates that all of the minette dykes analysed in the present study were probably emplaced at approximately the same time. For this reason, it is assumed in the following discussion that Dykes 3 and 4 were also formed at 445 Ma. It is important to note in this respect, however, that none of the conclusions that are drawn with respect to the geochemical evolution of the minette source are critically dependent upon this assumption. They would be equally valid for older eruption ages of between 445 and 750 Ma.
Sr, Nd and Pb isotopic compositions
The 87Sr/86Sr ratios of the Schirmacher minettes range from 0·7077 to 0·7134 with initial (87Sr/86Sr)i values of 0·7062 to 0·7102 (Table 3, Fig. 3). The Sr isotopes are thus markedly more radiogenic than would be expected for a magma derived from a ’normal’ depleted upper-mantle source. Similar results are obtained for the Nd isotope compositions, with 143Nd/144Nd varying between 0·51135 and 0·51231 (Nd = -6·5 to -25·1) and initial 143Nd/144Nd values of 0·5110–0·5119 (Nd(t) = -3 to -20; Table 4, Fig. 3). The isotopic compositions of the Schirmacher minettes plot significantly below the ’mantle array’ defined by mid-oceanic ridge basalts (MORB) and ocean island basalts (OIB) in a diagram of 143Nd/144Nd vs 87Sr/86Sr. In this respect, the minettes are similar to many ultrapotassic rocks (e.g. lamproites, kersantites, kimberlites) from other localities worldwide (Fig. 3).
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In diagrams of 207Pb/204Pb and 208Pb/204Pb vs 206Pb/204Pb, the minette samples plot above and to the left of the MORB array (Fig. 4a and b, and Table 5) and above the Northern Hemisphere Reference Line (NHRL) of Hart (1984). Thus the samples are characterized by positive 
7/4 and 8/4 values as defined by Hart (1984). With one exception, the present-day isotopic compositions of the samples also plot to the left of and above the 4·55 Ga geochron in Fig. 4a. It is furthermore noteworthy that the present-day and the initial Pb isotope ratios define a linear array in Fig. 4a, whereas more scatter is apparent in Fig. 4b. In both instances, however, the initial isotope ratios of the samples (which were calculated using the measured U–Th/Pb ratios of the rocks) define a tighter data cluster than the measured present-day values. This is significant, because it indicates that the U–Th/Pb systematics of the minettes was probably not significantly perturbed since dyke emplacement, for example, by recent loss of U. Clearly, minor effects of alteration cannot be ruled out at present and such effects may be responsible for the differences of initial 206Pb/204Pb ratios among the samples from Dyke 1 (Table 5, Fig. 4). In the case of Dykes 2 and 3, however, each pair of samples displays almost identical 206Pb/204Pb ratios.
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In summary, the data shown in Fig. 4 are indicative of a complicated multi-stage history for the Pb isotopic evolution of the minettes: (1) the high 7/4 values suggest an ancient evolution of the Pb isotopes in a high-µ environment (µ = 238U/204Pb); (2) the relatively low 206Pb/204Pb ratios, on the other hand, record a more recent evolution in a low-µ environment; (3) the scatter of the data in a plot of 208Pb/204Pb vs 206Pb/204Pb (Fig. 4b) is suggestive of variable time-integrated Th/U ratios.
Oxygen isotopes
The whole-rock 18O values of the Schirmacher minettes range between +6·5 and +9·5 (Table 6). Without exception, the minettes are thus characterized by 18O significantly higher than inferred for the upper mantle (18O = 5·5–6). The observation that all minette samples are fresh, with only minor petrographic signs of alteration or overprinting by metamorphism (Hoch, 1997; Hoch & Tobschall, 1998), argues against the suggestion that the high 18O values are primarily the result of secondary alteration. Rather, the O-isotope signatures are thought to be a primary feature of the magmas. Further support for the latter interpretation is provided by the coherent initial (87Sr/86Sr)i ratios calculated for the minette samples of the individual dykes. If the minette samples had been strongly affected by alteration processes in the past, the calculated (87Sr/86Sr)i values would be expected to show more scatter than the present-day Sr isotopic compositions. Figure 3, however, indicates that this is not the case.
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DISCUSSION |
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Evidence for crustal components in the Schirmacher minettes
The minettes from Schirmacher Oasis have a number of geochemical characteristics (e.g. high MgO, Ni and Cr abundances, mg-number of up to 74; Table 1) that indicate that the magmas were derived from a mantle reservoir (Hoch & Tobschall, 1998
). Like other lamprophyric magmas the minettes, however, also display isotopic and trace element signatures that are not reconcilable with derivation of the magmas from a primitive or depleted peridotitic mantle source alone.
The Schirmacher minettes are characterized throughout by high initial 87Sr/86Sr and 207Pb/204Pb and low initial 143Nd/144Nd (Figs 3 and 4), and these results place important constraints on the time-integrated parent–daughter ratios of the isotopic systems. The data are indicative of high Rb/Sr coupled with low Sm/Nd and require an ancient high-µ environment for the development of the positive 7/4 values. Such signatures are diagnostic for the involvement of a component derived from the continental crust in the formation of the minette magmas. This is also in accord with the old Nd-model ages of the samples, which vary between 1·5 and 2·5 Ga (Table 4). Given that the Sm/Nd ratios of the minettes are probably similar to or higher than the respective values of the magma sources, the old model ages should record trace element fractionation events that occurred before dyke emplacement. The model ages thus indicate that the Sm–Nd systematics of the minettes is dominated by material that experienced a long time-integrated evolution in a low Sm/Nd environment, and this is likely to be continental crust. The involvement of a crustal component in magma genesis is also suggested by the oxygen isotopes, because the 18O data for the lamprophyres (18O 7–10) are significantly higher than depleted upper-mantle values (18O = 5·5–6).
The trace element results obtained for the Schirmacher minettes provide further support for this interpretation. The lamprophyres are characterized by extremely high incompatible trace element abundances, and these enrichments cannot be accounted for by partial melting of a ’normal’ peridotitic upper mantle. Furthermore, the trace element patterns display particularly high abundances for elements such as Rb, Ba, Pb and the LREE, combined with negative Nb and Ti anomalies (Fig. 2). It is notable that these signatures are similar, in general, to the patterns of many continental crust materials (e.g. Taylor & McLennan, 1985). The minettes furthermore have low Nb/U and Nd/Pb. With respect to these critical diagnostic ratios, the minettes thus differ significantly from oceanic basalts (which have Nb/U = 47 ± 10 and Nd/Pb = 24 ± 5) but are very similar to continental crust, which is characterized by Nb/U 10 and Nd/Pb 5 (Hofmann et al., 1986).
Taken together, these results clearly indicate that the geochemistry of the minettes records the involvement of a crustal component in magma genesis and there are two obvious interpretations of this observation. (1) It is conceivable that the enriched trace element and isotopic signatures of the Schirmacher minettes were produced by the contamination of depleted, mantle-derived magmas with material derived from the upper and/or lower continental crust during dyke emplacement. (2) A number of previous studies have suggested that lamprophyres are produced by partial melting of an enriched mantle reservoir characterized by high LILE and LREE concentrations. The occurrences of many calc-alkaline lamprophyres are associated with continental collision events, whereas others are related to arc environments (e.g. Foley et al., 1987). In both cases continental material (either bulk crust or sediments) is recycled back into the mantle. Metasomatic agents derived from such materials are expected to be enriched in incompatible elements and could account for the formation of enriched lithospheric mantle reservoirs. The isotope and trace element data obtained for the Schirmacher minettes are used in the following to constrain the origin of the crustal signatures detected in the magmas.
Crustal contamination of magmas or contamination of the mantle source?
The minettes have significantly higher trace element abundances than average upper continental crust, with respect to most of the incompatible trace elements plotted in Fig. 2. In comparison with abundance estimates for the lower continental crust, the lamprophyres from the Schirmacher Oasis display incompatible element concentrations that are higher by a factor of 100 for some elements (e.g. Ba, Rb; Fig. 2). This clearly indicates that bulk contamination of the depleted magmas with upper- or lower-crustal material cannot account for the enriched trace element signatures of the samples.
It is also possible that the minette magmas were contaminated by highly enriched partial melts derived from crustal material, as a result of heating of the wall rocks during dyke emplacement, and the incorporation of such melts could account for the high incompatible trace element abundances. The unradiogenic initial 206Pb/204Pb isotope ratios of some of the minettes, however, are not consistent with the assimilation of large amounts of upper-crustal material. Compared with the upper mantle, upper continental crust is generally characterized by high 206Pb/204Pb, combined with high 87Sr/86Sr and low 143Nd/144Nd. Therefore, contamination of a mantle-derived magma by upper crust should generate mixing curves that display increasing 206Pb/204Pb correlated with increasing 87Sr/86Sr and decreasing 143Nd/144Nd. The initial isotopic ratios of the minettes define trends with different systematics, however (Fig. 5). A comparison of the initial isotope data of the minettes at 445 Ma with the isotopic compositions of possible mantle endmembers, such as depleted mantle (DM) or primitive mantle (BSE; bulk silicate Earth), furthermore indicates that the crustal contaminant would need to display low 206Pb/204Pb (<16·5) coupled with high 87Sr/86Sr and low 143Nd/144Nd (Fig. 5). This clearly argues against the contamination of the magmas by upper-crustal material. Xenoliths from the lower crust are often characterized by unradiogenic 206Pb/204Pb ratios, coupled with high 87Sr/86Sr and low 143Nd/144Nd (e.g. Rudnick & Goldstein, 1990). Despite the low average trace element concentrations of the lower crust (Fig. 2), assimilation of enriched lower-crustal melts could thus conceivably explain the isotopic correlations of the minette dataset. However, simple mass balance calculations indicate that this would require the minette magmas to assimilate >10–50% of melt generated by low-degree partial melting (<5%) of the lower crust. Such lower-crustal melts would need to be extracted from a source volume that is significantly larger than the total volume of the erupted magmas. It is considered to be unlikely that the emplacement of the volumetrically small minette dykes is capable of initiating widespread melting of the lower crust. This conclusion is supported by the high magma ascent and cooling rates inferred for lamprophyres (Spera, 1984; Esperanca & Holloway, 1987). More complex interaction processes, such as disequilibrium melting of enriched lower-crustal phases (Becker et al., 1999) are also conceivable, and it is difficult to rule out such mechanisms at present. Even such contamination processes would be expected to generate increases in 87Sr/86Sr that correlate with increasing 18O/16O and SiO2, and decreasing MgO. Such systematic correlations are not detected in the data for minette samples that are derived from the same dyke, and hence the same parental magma (Tables 1, 4 and 6). In summary, this indicates that the addition of a metasomatic fluid or melt derived from subducted continental materials to the lithospheric mantle source of the lamprophyres provides the most straightforward explanation for the trace element and isotope composition of the Schirmacher minettes.
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Geochemical evolution of the mantle source of the minettes
In the following discussion, the isotope and trace element data for the minettes are used to place constraints on the composition of the mantle source of the magmas and the materials from which the metasomatic agents were derived. To this end, we explore the implications of two evolution models.
Model 1: two-component mixing
In diagrams of Nd vs Sr isotopes and 206Pb/204Pb vs 207Pb/204Pb (Fig. 5a and b) it would appear that the isotopic compositions of the Schirmacher minettes can be readily explained by a simple two-component mixing model involving a component characterized by extremely low 206Pb/204Pb (low-6/4 component, Table 7) and a ’depleted’ endmember with high 143Nd/144Nd, low 87Sr/86Sr and more radiogenic 206Pb/204Pb. In the simplest case, the depleted endmember may represent the composition of the ambient unmetasomatized mantle lithosphere.
If two-component mixing is to explain the observed range of magma compositions, the minette data place firm constraints on the composition of the mantle endmember. The 207Pb/204Pb ratios of the DM and most mantle compositions intermediate between DM and BSE are much lower than the 207Pb/204Pb data of the minettes (Table 7, Fig. 5b). Mantle compositions similar to BSE, however, provide a suitable endmember to explain the Sr, Nd and Pb isotope systematics of the minettes by mixing with a low-6/4 component. For such a two-component mixing model to explain the spread of the minette data in diagrams of 87Sr/86Sr and 143Nd/144Nd vs 206Pb/204Pb, the low-6/4 crustal endmember is required to have very variable Sr/Pb and Nd/Pb ratios, whereas Sr/Nd would need to be relatively constant (Table 7). Such systematics is required, to generate both concave and convex mixing hyperbolas in Sr–206Pb/204Pb and Nd–206Pb/204Pb isotope space (Fig. 5c and d). Importantly, such a scenario is feasible only if the formation of the metasomatized mantle source of the minettes was closely associated in time with the emplacement of the dykes at 445 Ma. We arrive at this conclusion because a time difference of several 100 my between source enrichment and magmatism would be associated with significant in situ radiogenic decay. Therefore, the variable trace element ratios of the low-6/4 endmember would translate into variable isotopic signatures, leading to the formation of sources with distinct isotopic compositions.
An obvious advantage of Model 1 is that it provides the most straightforward explanation for the almost linear trends of the minette data in Fig. 5a and b. The merits of the present model can be further evaluated by comparing the trace element ratios of the samples with those predicted by the mixing model (Fig. 6). Because of the heterogeneous trace element composition of the low-6/4 material (Table 7), the model would predict that some minettes (particularly sample Oa3) should be characterized by Sr/Pb and Nd/Pb ratios of >500 and >50, respectively (Fig. 6). Clearly, both trace element ratios can be altered during magma genesis, such that the magma compositions may not faithfully record the actual source values. The variability in Sr/Pb and Nd/Pb observed for the minettes is, however, much lower than would be predicted by the mixing hyperbolas (Fig. 6). This indicates that the present model probably does not provide a good characterization of the geochemistry of the Schirmacher minette source.
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Model 2: different sources with distinct isotopic compositions
This alternative model for the evolution of the Schirmacher minette source invokes the formation of several metasomatic sources within the mantle that are characterized by distinct isotopic signatures. In the following, we focus on the three dykes with the most extreme compositions in Sr–Nd–Pb isotope space. These three dykes correspond to the following compositions (Table 7, Fig. 5): (1) a low-206Pb/204Pb composition, with an ’enriched’ geochemical signature characterized by high time-integrated Rb/Sr and low Sm/Nd (Low-6/4; Dyke 1); (2) an ’enriched’ high-206Pb/204Pb composition, displaying high time-integrated Rb/Sr and low Sm/Nd (Enr. high-6/4, Dyke 4); (3) a ’depleted’ high-206Pb/204Pb composition characterized by low time-integrated Rb/Sr and high Sm/Nd (Depl. high-6/4, Dyke 3). The identification of each dyke with a unique composition does not imply that these compositions provide a characterization of the metasomatic components that were responsible for mantle enrichment. The metasomatic agents may have displayed more extreme characteristics, but they were sampled only in diluted form or as mixtures by the minette magmas. Each of the three dykes, however, appears to be dominated by a different metasomatic component. The fourth dyke may thus represent an intermediate composition because it is a more ’balanced’ mixture of the endmember components.
An advantage of Model 2 is that it does not require the existence of endmembers with ’extreme’ trace element compositions (as in Model 1) that produce highly curved mixing hyperbolas in isotope space to account for the isotopic diversity of the Schirmacher minettes (Figs 5 and 6, Table 7). The Sr/Pb and Nd/Pb ratios of the Schirmacher minettes display values of about 20–100 and 2–10, respectively, and such values are typical for most (continental and oceanic) crustal rocks. This indicates that the metasomatic sources of the Schirmacher minettes probably have similar characteristics and are unlikely to be characterized by the extreme trace element compositions that were inferred for the low-6/4 endmember of Model 1.
Compared with Model 1, the present scenario also places far fewer constraints on the timing of mantle source enrichment. The enrichment event may have occurred just before or significantly before dyke emplacement at 445 Ma. In the first case, the isotopic compositions of the metasomatic sources would have to be inherited directly from the metasomatic agents, which in turn were derived from the subducted material. The low-6/4 composition is most reasonably derived from the continental crust, because it is characterized by high 87Sr/86Sr and 207Pb/204Pb, and low 143Nd/144Nd. Such enriched isotopic signatures coupled with unradiogenic 206Pb/204Pb are rare in general, but they are a common characteristic of the lower continental crust (e.g. Rudnick & Goldstein, 1990). Incidentally, such isotopic signatures have been reported for granulite-facies rocks from Enderby Land, East Antarctica (DePaolo et al., 1982). This suggests formation of the low-6/4 material by metasomatic processes involving fluids derived from lower-crustal rocks or sediments, upon subduction into the mantle. Upper-crustal material, which is typically characterized by radiogenic 206Pb/204Pb, coupled with high 87Sr/86Sr and unradiogenic 143Nd/144Nd, may dominate the enriched high-206Pb/204Pb composition. The depleted high-6/4 material could either represent unmetasomatized lithospheric mantle with a BSE-like composition, or be related to a metasomatic component derived from subducted oceanic crust.
It is conceivable, however, that the enrichment of the lithospheric mantle took place several hundred million years or more before dyke emplacement, for example, in the tectonic setting of an active continental margin during the Proterozoic. In this case, a simple model age can be calculated for the formation of the low-6/4 component if it is assumed to have formed solely by retardation of in situ production of 206Pb in low-U/Pb metasomatic materials that were derived from fluids expelled from recycled crustal rocks. This model age is obtained by estimating how long radiogenic ingrowth of 206Pb must have been retarded in a low-µ environment, to account for the low 206Pb/204Pb ratios of the minettes at the time of dyke emplacement. The initial Pb isotope ratios of the metasomatic material at the time of mantle enrichment are calculated by using a two-stage Stacey–Kramers evolution (Stacey & Kramers, 1975), because this is appropriate for material ultimately derived from the upper continental crust. The samples N9 and N10 (from Dyke 1) have the most unradiogenic 206Pb signatures of the present database, with (206Pb/204Pb)i ratios of 16·5–16·6 (Table 5). To generate 206Pb/204Pb isotope ratios <16·55 at 445 Ma, the enrichment event would need to have taken place at 1·3 Ga, given a metasomatic mantle source that displayed no further radiogenic ingrowth of 206Pb as a result of a µ value of zero. For a more realistic µ value of >2, the enrichment process would need to have occurred >1·5 Ga ago. If the low-206Pb/204Pb signatures of the minettes are thus to be explained solely by retarded in situ decay of 206Pb in a low-U/Pb metasomatic source, this source would need to have been preserved in the mantle for a time period of 1 by.
If mantle enrichment occurred significantly before dyke emplacement, the isotopic tracing of the sources of the metasomatic agents is rendered difficult, if not impossible, because the trace element fractionation processes that occur during the formation of the metasomatic fluids and/or the metasomatized mantle sources are followed by significant radiogenic ingrowth. Thus, both the in situ radiogenic ingrowth that occurs in the minette sources and the inherited isotopic fingerprint of the metasomatic agent would play a role in defining the isotopic compositions of the minette sources at 445 Ma. The trade-off of ancient mantle enrichment, however, is the requirement that the metasomatized lithosphere is not permitted to melt for a time period of up to 1 by, to maintain its budget of incompatible elements. Such a scenario is conceivable, but it may be unrealistic given that the Schirmacher Oasis is not situated on a stable cratonic platform, but in a mobile belt that underwent multiple tectonic events before 445 Ma.
Ultimately, the 1·5 Ga model age is also applicable if the low-6/4 signatures were inherited from lower-crustal rocks during metasomatism that occurred just before the emplacement of the minette dykes. In this case, the model age, however, would ’date’ crustal differentiation and the formation of a lower-crustal reservoir characterized by a low µ value. Furthermore, it should be noted that either scenario is compatible with the high 18O values of the samples. The ultimate origin of the heavy oxygen isotope signatures remains unclear, but it is likely that the high 18O values were inherited directly from the subducted rocks or sediments.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONSAMPLESANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Schirmacher minettes are characterized by high mg-number as well as high MgO, Ni and Cr abundances. This clearly indicates derivation of the magmas from a mantle source. The oxygen isotope signatures of the samples (
18O 7–10), however, are not reconcilable with the derivation of the magmas from a normal peridotitic mantle reservoir. This conclusion is further supported by the highly enriched trace element patterns and the Sr–Nd–Pb isotope systematics of samples. The geochemical characteristics of the minettes are most reasonably explained by partial melting of a lithospheric mantle source that was enriched by metasomatic components derived from recycled continental crust.
Some samples display initial 206Pb/204Pb ratios of 16·5 at 445 Ma, coupled with comparatively high 207Pb/204Pb. This indicates the involvement of an ancient crustal component that experienced a more recent evolution a low-U/Pb environment. This could be an inherited signature of old lower-crustal material that was subducted during the Pan-African orogenic event. It is also conceivable that the mantle enrichment event pre-dated the emplacement of the minette dykes by several hundred million years. In this case, source tracing is rendered difficult, because both the in situ radiogenic ingrowth that occurs in the metasomatic material and its inherited isotopic fingerprint play a role in defining the isotopic composition of the minettes at the time of dyke emplacement. Which of these two factors played the dominant role will primarily be a function of the unknown timing of mantle source enrichment.
The variability of the isotopic compositions for the Schirmacher minettes is most reasonably explained by the derivation of the magmas from metasomatic sources characterized by both variable isotope and trace element compositions. Such diversity can be produced if the enriched mantle sources are formed by the addition of metasomatic fluids derived from different materials, such as upper and lower crust or pelagic and detrital sediments. Alternatively, the diversity of compositions may be related to the trace element fractionation processes that can occur during the formation of metasomatic fluids and metasomatized mantle sources. If the isotopic diversity of the Schirmacher minettes was produced solely by the recycling of upper continental crustal material, followed by trace element fractionation and variable radiogenic ingrowth, however, this would require the mantle enrichment process to have occurred >1 by before dyke emplacement. Such a scenario may be unrealistic, given that the Schirmacher Oasis is situated in a mobile belt that experienced multiple tectonic events in the past.
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ACKNOWLEDGEMENTS |
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We thank U. Haack (Universität Gießen), S. Hoernes (Universität Bonn) and H. Köhler (Universität München) for the opportunity to use their mass spectrometry laboratories, and B. Hofmann, J. Schneider and M. Brauns for vital help with the analytical work. M.H. is grateful to U. Haack for helpful discussions, and to H. Kämpf (GeoForschungsZentrum Potsdam) for providing the minette samples. This paper benefited greatly from an informal manuscript review by K. Mezger and formal reviews by H. Becker and T. Reischmann.
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FOOTNOTES |
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*Corresponding author. Telephone: 0049-9131-852-2660. Fax: 0049-9131-852-9294. E-mail: mhoch{at}geol.uni-erlangen.de
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