7.2 Crustal contamination

Many continental igneous rocks have enriched chemical and isotopic signatures similar to those discussed in the previous section. However, the critical question is whether these signatures were inherited from the mantle or the crust. In principle, isotopic methods represent an ideal tool for solving this problem, since they are not upset by the crystal fractionation processes which affect most magmas during ascent and emplacement. However, the high degrees of enrichment which can occur in plume or lithospheric mantle sources may generate isotopic signatures similar to those of the crust. Hence it has been argued (e.g. Thirlwall and Jones, 1983; Hawkesworth et al., 1984) that mantle and crustal sources cannot be distinguished simply on the basis of ‘isotopic discriminant diagrams’ in which each component has a unique field. Instead, crustal or mantle contributions to magmatism must be recognised by observing the products of processes such as magma mixing or crustal assimilation.

Philosophically, one can examine contamination processes in two ways: using a predictive model (e.g. DePaolo, 1981a) or an inversion technique (e.g. Mantovani and Hawkesworth, 1990). In the former, we set conditions and then examine consequences. In the latter, we examine products and attempt to reconstruct the original conditions. The predictive model is well suited to two-component mixing processes, such as progressive contamination of a single magma batch by wall-rock assimilation. Some examples of such models will be examined below, followed by an examination of crustal melting processes pertinent to crustal contamination models. However, volcanic lava piles often involve multi-component mixing. These processes are more difficult to examine using predictive models, because of the plethora of possible mixing scenarios. Therefore, it is more effective to model such suites using the inversion approach, bearing in mind the predictive models already developed for single magma batches. This approach will be illustrated using the classic British Tertiary Igneous Province as a case study.

7.2.1 Two-component mixing models

In its simplest form, contamination of mantle-derived magma by the continental crust can be regarded as a process of two-component mixing. However, magma)crust mixing processes usually have more than one degree of freedom (such as the compositions and proportions of mixed components). Therefore, to evaluate mixing relations adequately, it is usually necessary to apply two or more measured variables to the problem. These variables are usually isotope ratios, elemental ratios, and elemental abundances. In the context of isotope geology, it is logical to begin by examining the behaviour of isotopic tracers as a function of the elemental concentration of the same element. Therefore, we will begin by studying initial ⁸⁷Sr/⁸⁶Sr ratios as a function of Sr concentration.

Mixing of components with different isotopic and elemental compositions yields a hyperbolic curve on a diagram of initial ⁸⁷Sr/⁸⁶Sr against Sr concentration (Fig 7.8a). Ideally, initial Sr isotope ratios should be plotted against ⁸⁶Sr abundance, since the total concentration of strontium is slightly perturbed by variations in ⁸⁷Sr. This would apply to old rocks, for which a large age correction is necessary to obtain the initial ratio. However, the decay constant of Rb is so low that ⁸⁸Sr makes up the bulk of strontium in most rocks. Therefore ⁸⁶Sr abundance can be approximated by total Sr without introducing significant errors. (This is not so for Pb in old rocks, where radiogenic Pb can easily swamp the non-radiogenic component).

Fig. 7.8. Schematic illustration of two-component mixing on plots of Sr isotope ratio against (a) Sr concentration, and (b) 1/Sr. C = crustal end-member; M = mantle-derived end-member.

A bivariate diagram for two ratios with common denominators must yield linear mixing lines. Therefore the hyperbolic mixing curve of Fig. 7.8a can be transformed into a straight line (Fig. 7.8b) by plotting initial ⁸⁷Sr/⁸⁶Sr against 1/Sr (approximating 1/⁸⁶Sr). Briquet and Lancelot (1979) used this format to examine contamination and fractionation processes in a ‘selective contamination’ model (Fig. 7.9), which envisages two-component mixing between a primary basic magma and a hypothetical Sr-rich extract from the crust. Following the contamination process, plagioclase fractionation may cause the Sr content of the magma to fall as it evolves to dacitic and then rhyolitic compositions (Fig. 7.9a). If these contamination and fractionation steps were repeated sequentially, then they would create the effect seen in Fig. 7.9b. If the steps become very small, the result is simultaneous fractionation and contamination (Fig. 7.9c). However, Briquet and Lancelot’s ‘selective’ model is probably not the most realistic for magma contamination, since Nd isotope evidence suggests that most contamination is by crustal melts (e.g. Thirlwall and Jones, 1983).

Fig. 7.9. Schematic modelling of selective Sr contamination and fractionation of magmas on plots of Sr isotope ratio against 1/Sr. a) contamination followed by fractionation; b) sequential contamination and fractionation events; c) simultaneous contamination and fractionation, followed by pure fractionation. After Briquet and Lancelot (1979).

Crustal melting and assimilation is an endothermic process. If the magma is on or below the liquidus then it can only obtain heat to power melting by itself undergoing fractional crystallisation. Hence, we may expect these two processes to be coupled into a mechanism which DePaolo (1981a) termed ‘assimilation fractional crystallisation’ (AFC). In this model, the effect of fractionation on the mixing trajectory will depend on the relative importance of assimilation and fractional crystallisation, and also on the crystal)liquid bulk distribution coefficient (D) pertaining at the time.

To illustrate these effects, Fig. 7.10 shows calculated mixing lines for various D_Sr values at increasing mass fractions of assimilate (M_a) relative to initial magma (M_m), for a fixed proportion of assimilation relative to crystallisation (M_a/M_c). (A smaller amount of fractionation relative to assimilation will cause less deviation from the simple mixing line, whereas a larger relative amount of fractionation will cause more deviation). When plagioclase joins the crystallising assemblage, this will have a very dramatic effect on D_Sr values, changing strontium from an incompatible element (D_Sr << 1) to a compatible element (D_Sr > 1) in the crystallising material. This may cause a magma to follow the bold dashed curve in Fig. 7.10 during its evolution.

Fig. 7.10. Plot of Sr isotope ratio against concentration to show the effect of different solid/liquid bulk distribution coefficients (D^Sr) during the process of assimilation–fractional crystallisation (AFC) by a basic magma. For discussion, see text. After DePaolo (1981a).

The Sr versus Nd isotope diagram provides a useful tool for assessing crustal contamination models. DePaolo and Wasserburg (1979a) showed that simple two-component mixing on this diagram gives rise to hyperbolae whose trajectories depend on the relative Sr/Nd concentration ratio in the two end-members (Fig. 7.11). For the special case were the Sr/Nd ratio is the same in both end-members, the mixing line is straight. When the mantle-derived component (M) has a higher Sr/Nd ratio, Nd compositions are more readily affected by contamination than Sr, yielding a concave upwards curve (K>1). This is the normal situation when the mantle-derived component is more basic than the crustal end-member (whose Sr content has been lowered by plagioclase fractionation in its previous history). However, contamination by very plagioclase-rich crust could yield a convex-upward curve (K<1).

Fig. 7.11. Schematic illustration of two-component mixing on a plot of , Nd versus Sr. M and C are mantle-derived and crustal end-members. K = Sr/Nd ratio in mantle-derived relative to crustal end-member. Normally K is between 2 and 10. After DePaolo and Wasserburg (1979a).

7.2.2 Melting in natural and experimental systems

The isotopic composition of a mantle-derived magma undergoing crustal contamination may be fairly predictable, but the composition of possible crustal melts is much more poorly constrained. Therefore a number of studies have been conducted on melting processes, both in the laboratory and in ‘natural laboratories’ in the field. Most of these studies have involved the melting of granitoid rocks, since this is believed to be the most important component of rock available for melting in the continental crust. A few of these studies will be reviewed here.

One of the first modern studies of this problem was made by Maury and Bizouard (1974) on partially melted biotite gneiss xenoliths in a basanitic melt from southern France. One of the most important findings was that more than one initial melt composition was present (represented by quenched glasses). The two principal melt compositions were a colourless rhyolitic glass resulting from melting on quartz–feldspar grain boundaries and a brown latite glass resulting from melting on biotite–feldspar grain boundaries. Most subsequent studies have confirmed these findings, but since the colourless and brown glasses tend to mix if the melting interval is prolonged, the ‘starting compositions’ have often not been found in sufficient quantity for geochemical analysis. For example, two studies on melting of the Sierra Nevada granite batholith at Rattlesnake Gulch by a trachyandesite plug (Kaczor et al., 1988; Tommasini and Davies, 1997) both identified pale brown and dark brown melt glasses. However, it was not possible to separate the initial colourless and brown glasses for geochemical analysis.

A purely laboratory-based melting study was made by Hammouda et al. (1996) on a synthetic mixture of plagioclase and phlogopite (i.e. Mg biotite). Preferential melting of the phlogopite was observed above 1200 ^oC. Since the phlogopite had been doped with radiogenic Sr to simulate the effects of Rb decay in an old granite, the melt glasses were much more radiogenic than the bulk rock. This experiment could simulate the melting behaviour of a tonalitic crustal rock, suggesting that partial melting of such material could cause ‘selective contamination’ of mafic magmas with radiogenic Sr. However, it was previously suggested by Thompson et al. (1982) that the small amounts of fusible granitic rock in a crustal section would be more important in promoting crustal contamination of basaltic magmas than the relatively refractory tonalite component.

A combined field-based and laboratory-based melting study of granitic rocks was carried out by Knesel and Davidson (1999). The field component involved melting of the Sierra Nevada granite in the vicinity of a Pleistocene-age olivine basalt at Tungsten Hills, whereas the laboratory-based component involved melting relatively large 45 g cubes of a 1200-Myr old Precambrian granite. Sr isotope results from the latter study (Fig. 7.12) show a progressive evolution in the composition of brown and colourless melt glasses as the melting temperature was increased (for a fixed 24-hour duration). The isotope ratios of the two melts were initially very distinct but evolved towards the whole-rock composition of the source. In contrast, the Sr abundances of the two melts started relatively close to the whole-rock value, but evolved away from it, reflecting increasing Sr enrichment as melting progressed. This was attributed to the evolution of the restite towards a strontium-free quartz residue.

Fig. 7.12. Plot of Sr isotope ratio against concentration, showing the effect of increased melting temperature (1100–1150–1250 ^oC) on colourless and brown melt glasses, generated in laboratory experiments on a Precambrian granite. After Knesel and Davidson (1999).

A somewhat different picture was obtained when the bulk composition of the experimental glass was calculated, based on the composition and abundance of the two components. When the Sr isotope ratio of the bulk glass was plotted against melt fraction, a monotonic decrease in (initial) Sr isotope ratio was observed as a function of melt fraction (Fig. 7.13a). Such a pattern was also observed in the field-based data from Tungsten Hills and Rattlesnake Gulch (Knesel and Davidson, 1999; Tommasini and Davies, 1997), shown in Fig. 7.13 (parts b and c).

Fig. 7.13. Plot of initial Sr isotope composition of bulk melts as a function of melt fraction in three studies of granitic melting described in the text. After Knesel and Davidson (1999).

The laboratory experiments described above were all performed at atmospheric pressure, and hence under anhydrous conditions which do not accurately represent crustal melting in the deep crust. Therefore, Knesel and Davidson (2002) repeated the experiments on the same granite sample at a confining pressure of 600 MPa (6 kbar), equivalent to a depth of about 20 km in the Earth’s crust. These conditions permitted runs of longer duration (up to 2 months), and also allowed the melt products to be ‘extracted’ from the source into a vacant pore space created by a bed of industrial diamonds at one end of the sample charge. The sample itself consisted of finely crushed granite (75 – 100 microns) that was intended to preserve the mineralogical proportions of the original rock. The only water in the system was derived from the hydrous minerals of the source rock.

Results of this experiment are shown in Fig. 7.14 for three different melting temperatures in the rabge 850–950 ^oC. The surprising thing about these results is that, unlike the previously reported field and laboratory experiments, the initial melt was less radiogenic than the source, although it eventually reached Sr isotope equilibrium with the whole-rock composition. In contrast, an experiment at 1000 ^oC (not shown) gave results similar to the previous experiments, with an initial melt more radiogenic than the whole-rock. The unradiogenic Sr composition of the low temperature melts was attributed to the melting of plagioclase in a reaction involving the dehydration of a small amount of muscovite in the sample. In contrast, biotite breakdown was the most important reaction above 950 ^oC. These results are interesting because they suggest that crustal contaminants are not necessarily enriched in radiogenic Sr relative to the source rock. However, the crushing of the original sample may have created artificial mineral contacts that do not accurately represent the original rock. Therefore, more experiments are clearly needed to test the behaviour of this material and other source compositions under similar melting conditions.

Fig. 7.14. Plot of Sr isotope ratio of granitic melts generated in piston-cylinder experiments over different time intervals and at different temperatures. (Pressure = 6 kbar). Inset shows experimental duration on a log scale. After Knesel and Davidson (2002).

The general conclusion from all of these melting experiments is that the isotopic composition of a crustal melt evolves slowly towards the bulk composition of the rock as melting progresses. However, since the temperature and duration of melting are different for any given crustal contamination event, it is not possible to make general predictions about the extent of disequilibrium melting to be expected in crustal contaminants. Each case must be investigated in its own context.

7.2.3 Inversion modelling of magma suites

The above modelling has considered the evolution of single magma batches during melting, assimilation and/or fractionation processes. However, a suite of analysed lavas may represent magma batches that reached different stages of differentiation (and hence had different trace element contents) before contamination. Just as different bivariate plots can be used to model progressive contamination of a single magma, the same variety of plots can be used to examine the evolution of magma suites. The Tertiary volcanic province of NW Scotland represents a good natural ‘laboratory’ in which to examine some of these processes for two main reasons. Firstly, magma)crust interaction was relatively intense, due to the volatile-poor nature of the magmas. This prevented them from punching through the crust quickly. Secondly, isotopic contrasts between mantle and crustal end-members are well developed, because old lithospheric mantle had been melted away from under the Tertiary volcanic centres by earlier magmatism.

An example of the co-variation of Sr isotope ratio with Sr concentration is provided by Tertiary basic-to-intermediate lavas from the Isle of Skye, NW Scotland (termed the Skye Main Lava Series). Moorbath and Thompson (1980) found a weak negative correlation between Sr isotope ratio and concentration in this suite, forming a hyperbolic trend (Fig. 7.15). However, any individual mixing line between a hypothetical mantle-derived precursor and the estimated crustal component has a slope perpendicular to the observed trend. Such a trajectory is displayed by a small suite of low-potassium (low-K) basalts in Fig. 7.15.

Fig. 7.15. Plot of initial Sr isotope ratio against concentration for Tertiary lavas from Skye, NW Scotland. Skye Main Lava Series: ( ! ) = basalt; ( > ) = hawaiite; ( € ) = mugearite)benmoreite. Other lavas: ( " ) = silica-oversaturated intermediates; ( Ë ) = Low-K basalts. After Moorbath and Thompson (1980).

To explain the main data set, Moorbath and Thompson proposed that crystal fractionation had occurred in the upper mantle to yield a series of magmas with variable Sr contents. These were then subjected to similar degrees of contamination with radiogenic crustal Sr, so that those with high Sr contents were less affected than those with low Sr contents; yielding a hyperbolic pattern for the suite as a whole. The scatter in the data probably results from somewhat variable degrees of contamination in different magma batches.

Thirlwall and Jones (1983) made Nd isotope determinations on the same suite of Skye lavas. The data are shown (Fig. 7.16) on a plot of Nd isotope ratio against 1/concentration. Most of the basalts define an approximately linear array (equivalent to a hyperbola on a plot of ¹⁴³Nd/¹⁴⁴Nd ratio against Nd concentration). However, this linear array does not have the trajectory expected for two-component mixing (steep vectors in Fig. 7.16). Instead, it is attributed to contamination of a magma series with variable Nd contents, in which the most ‘primitive’ magmas, with lowest Nd contents, show the greatest effects of contamination. On the other hand, a few basalts, tohether with silica-rich intermediate lavas, show the effects of an AFC process, in which Nd contents rise rapidly as contamination progresses (Fig. 7.16).

Fig. 7.16. Plot of initial Nd isotope ratio (, Nd), against reciprocal Nd content in Skye lavas. Symbols as in Fig. 7.15. Arrows show the effects of contamination by magma mixing and by AFC. After Thirlwall and Jones (1983).

Thirlwall and Jones confirmed this interpretation (Fig. 7.17) using a plot of Nd isotope ratio against the major-element differentiation index FeO/(FeO+MgO). They showed that the ‘F/M’ ratios of the Skye lavas must have been generated by fractionation at the base of the crust, since they were too high in most of the rocks to have been in equilibrium with mantle olivines. It follows that the strong correlation of , Nd with F/M must be the result of a subsequent process, i.e. contamination in the crust. The most primitive basalts (lowest F/M) were the most contaminated, since their lower Nd contents rendered them more sensitive to contamination. Again, the linear array in this diagram does not correspond to a two-component mixing line. Crustal contaminants have low Fe and Mg concentrations, so that they do not affect the F/M ratio of the contaminated magma. Hence, sub-vertical mixing vectors are generated in Fig. 7.17. The formation of the array of lava compositions at an oblique angle to these vectors can be ascribed to a regular and predictable contamination mechanism affecting a suite of related differentiates.

Fig. 7.17. Plot of , Nd against ‘F/M’ ratio for Tertiary lavas from Skye, showing more intense crustal contamination in more magnesian basalts. Symbols as in Fig. 7.15. After Thirlwall and Jones (1983).

Huppert and Sparks (1985) attributed the type of contamination process seen in the Skye lavas to thermal erosion of wall rocks by turbulently flowing magma during its ascent through the crust. The more magnesian magmas were hotter and less viscous, therefore enhancing the turbulent flow of these magmas. This prevented the formation of a chilled margin by continually bringing fresh, hot magma into contact with the conduit walls, and thereby allowing more wall rock erosion. Huppert and Sparks imagined this process occurring in dykes, but a more probable site for such wall-rock assimilation may be sill complexes in the crust, where the longer magma residence time would allow more opportunity for contamination.

An example of the possible effects of turbulent wall rock assimilation was described by Kille et al. (1986) from the Hebridean island of Mull, where inclined intrusive sheets are intruded into metasedimentary units of the Moine series. Large embayments were seen in the more fusible units of the sedimentary sequence, suggesting ‘excavation’ by turbulently flowing magma. Subsequently, additional evidence was found from the Mull lava pile for wall rock assimilation during turbulent magma ascent (Kerr et al., 1995). This study showed that lavas of the Mull Plateau Group (MPG) had similar patterns of , Nd against elemental concentration to those previously seen in Skye. Hence, it appears that this process is of quite widespread occurrence, prompting Kerr et al. to coin the expression ‘Assimilation during turbulent ascent’ (ATA).

Isochron diagrams are a particular example of a bivariate plot involving isotope ratios and trace element ratios, and may therefore be useful for studying crustal contamination processes. For old rock suites, initial isotope ratios are plotted on a pseudo-isochron diagram. Because the denominator on both axes is the same, two-component mixing must give rise to products which lie on a straight line between the end-members. However, a magma suite may again generate a data array which does not project to the mixing end-members. The Tertiary lavas from Skye provide a good example of this problem also.

Thirlwall and Jones (1983) found a linear array of , Nd versus Sm/Nd ratios in Skye basalts (Fig. 7.18). They interpreted this array as a mixing line between a mantle-derived magma with constant Sm/Nd ratio and a partial melt of intermediate (tonalitic) Lewisian gneiss. The projection of the mixing line onto the Lewisian isochron then indicates an , Nd value (at 60 Myr) of ca. !15. However, Dickin et al. (1984) argued that the basalt array was not a single mixing line, but was generated by a series of obliquely angled mixing lines involving mantle-derived magmas with different Sm/Nd ratios. These trajectories converge on a crustal end-member with , Nd of ca. !40, corresponding to Lewisian granitic (acid) gneiss. This controversy may be resolvable by Ce isotope evidence (section 9.4.3), but it serves to reiterate the importance of distinguishing between individual mixing lines and magma evolution trends on all plots where contamination models are considered.

Fig. 7.18. Sm)Nd pseudo-isochron diagram for Tertiary lavas from Skye, showing proposed contamination vectors in comparison to the array of contaminated basalts. Symbols as in Fig. 7.15. After Dickin et al. (1984).

Since the Sr and Nd isotope compositions of a contaminated lava suite may be a complex function of Sr and Nd concentrations, depending on the differentiation history of the suite, these factors must be borne in mind when interpreting the Nd versus Sr isotope diagram for a magma suite. For the data from Skye and Mull (Fig. 7.16), most samples define an array with a negative slope, implying coupled behaviour of Sr and Nd in the mixing process. However, it has been shown that contamination effects for each isotope system (⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd) are individually controlled by the Sr and Nd concentrations of the differentiating magmas. Furthermore, these trace elements often do not behave coherently during magma differentiation, since Nd is always incompatible, whereas Sr becomes a compatible element once plagioclase crystallisation begins. Therefore, we should expect some scatter in the Sr–Nd isotope correlation in order to reflect these complexities.

Fig. 7.19. Plot of Nd versus Sr isotope ratio for Skye ( ! ) and Mull ( " ) lavas, showing a trend towards local crustal units: LG = Lewisian granulite-facies gneiss; LA = Lewisian amphibolite-facies gneiss; 7H = Archean pegmatite sheet. After Kerr et al. (1995).

A further problem with the interpretation of Skye and Mull data on the Sr–Nd isotope diagram is that the correlation line of Tertiary lavas trends half way between the fields for Rb-depleted granulite-facies and Rb-rich amphibolite facies Lewisian gneisses, argued to represent the lower and upper parts respectively of the present-day crust under Skye. However, the relative contributions of these crustal components can be resolved using Pb isotope data, as shown below.

Pb isotopes are a powerful tool in studies of mantle and crustal evolution, because the three different radiogenic isotopes are generated from parents with a wide span of half-lives, two of which are a common element. By using ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios in conjunction, it is not only possible to measure the importance of crustal contamination, but also the age of the crustal component. On the other hand, by using ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios in conjunction, it is sometimes possible to locate the depth of the crustal contaminant, since the crust may develop a stratified signature of these isotopes in response to high-grade metamorphism. Both of these possibilities are illustrated by the Tertiary magmatism of Skye and Mull.

Moorbath and Welke (1969) found that both acid and basic Tertiary igneous rocks from Skye lay on a strong linear array on the ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb diagram, with a slope age of ca. 3 Byr. They interpreted the linear array as a mixing line between radiogenic mantle-derived Pb and very unradiogenic Archean (Lewisian) crustal Pb. Dickin (1981) repeated this study with more modern techniques and found a mixing line with a slope-age of 2920 " 70 Myr (Fig. 7.20a), the same as the Sm)Nd age of the Lewisian complex (see section 4.1.3). By plotting ²⁰⁸Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb ratios (Fig. 7.20b), it was possible to resolve three components in the Skye Tertiary igneous rocks. The lavas are interpreted as mantle-derived magmas that had suffered strong contamination in the granulite-facies lower crust, whereas gabbros of the Cuillins layered complex are attributed to contamination in amphibolite-facies upper crust. Finally, the Skye granites are attributed to differentiated basic magmas that suffered contamination in the lower crust, followed by further differentiation and contamination in the upper crust.

In this model, the crustal end-members were based on average compositions of gneisses from NW Scotland, supported by evidence from crustal xenoliths carried up in a Tertiary intrusion from Skye. The lower crustal rocks were depleted in both U and Th relative to Pb during the 2.7 Byr-old Scourian granulite-facies metamorphism, while the present-day upper crust contains rocks that were depleted in U but not Th (relative to Pb) in the Archean middle crust. The original upper crust, enriched in U and Th relative to Pb, has largely been removed by erosion.

Fig. 7.20. Plot of initial Pb isotope ratios for Tertiary igneous rocks from Skye, showing evidence for three-component mixing. ( € ) = lava series; ( Q ) = granites; ( + ) = low-K basalts; ( Ë ) = layered gabbros. Mid Atlantic Ridge approximates the local mantle composition. Modified after Thompson (1982).

The combination of Pb with Sr isotope evidence allows additional constraints to be applied to the evolution of contaminated magma suites. For example, the shallow slope of the Sr–Pb isotope correlation line in Fig. 7.21 is consistent with contamination of the Skye and Mull lavas by granulite-facies lower crust. However, it must be remembered that (unlike the Pb–Pb isotope diagram) mixing lines on the Sr–Pb isotope diagram can be strongly hyperbolic. In fact, the lavas were probably contaminated by the most felsic components in the granulite facies basement, which were more Rb-enriched than the bulk lower crust. Felsic minor intrusions were argued by Thompson et al. (1982) to be the most likely fraction in the crust to melt, leading to a kind of ‘selective contamination’ mechanism due to melts of fusible rock types. One such rock (sample ‘7H’) is shown in Fig. 7.19, where it is found to explain the Sr–Nd correlation line in the lavas.

A contrasting type of behaviour was seen in a distinct magma type from SW Mull, which was important in the very early eruptive history of the complex (Morrison et al., 1985). These lavas of the so-called Staffa Magma Type lie far off the Sr–Pb correlation line formed by the later lavas (Fig. 7.21), and were therefore attributed to a two-stage contamination process. This began at the base of the crust, as seen in the other lavas, but was followed by a residence period in the uppermost crust, where the magmas were contaminated by supracrustal meta-sediments of the Moine series. Thus, as more isotope tracers have become available, the application of multiple tracers to magma suites has allowed more complex magma evolution histories in the crust to be modelled and understood.

Fig. 7.21. Plot of initial Sr versus Pb isotope ratios in lava suites from Mull, showing the distinct contamination histories of the Mull Plateau Group (MPG) and the early erupting Staffa Magma Type ( ! ). After Morrison et al. (1985).

7.2.4 Lithospheric mantle contamination

Isotopic tracers have been widely used to monitor crustal contamination of continental magmas during their ascent, and to some extent this process can now be quantified. In contrast, the relative importance of lithospheric and asthenospheric mantle sources continues to be a matter of debate. Thermal constraints (McKenzie and Bickle, 1988) suggest that the high melting rates necessary to erupt flood basalt provinces can only be satisfied by melting of mantle plumes. On the other hand, lithospheric extension will cause small volume melting of metasomatised lithosphere, generating mafic potassic magmas which may have extreme isotopic compositions. Several workers recognised that these processes might act together, leading to contamination of asthenospheric magmas by the mantle lithosphere, as well as by the overlying crust. Good examples of this process come from the Mesozoic flood basalt province of Gondwana (Southern Africa, South America and Antarctica).

In the Karoo Province of Southern Africa, picrite basalts from Nuanetsi define a Sm–Nd pseudo-isochron with an apparent age of 1 Byr. This was initially interpreted as a mantle isochron dating the age of a magma source in the lithosphere (Ellam and Cox, 1989). However, re-examination of the data (Ellam and Cox, 1991) suggested that the array was a mixing line, formed by contamination of picritic magmas (from a depleted plume source) by mafic potassic magmas (lamproites) from a lithospheric source (Fig. 7.22). Trace-element mixing calculations suggested that the lamproite component made up about 20% of the mixed magma, but dominated its Nd and Pb isotope signature. On the other hand, osmium contents of the two end-members were about equal (section 8.4.4). A similar model may explain Nd–Os isotope data for the Stillwater Complex, Montana (Lambert et al., 1994).

Fig. 7.22. Sm/Nd pseudo isochron diagram showing a linear array of Nuanetsi picrite compositions ( ! ), interpreted as a mixing line between a primitive picritic magma and a hypothetical lamproite magma. Open symbols are oceanic volcanics. After Elam and Cox (1991).

An important aspect of the Gondwana flood basalt province is its geochemical provinciality, which provides strong evidence for lithospheric control of magma chemistry. One example of this provinciality is the identification of high-Ti and low-Ti flood basalt provinces in Southern Africa and in the Parana basin of South America (e.g. Hawkesworth et al., 1984). If the same provinciality was found in mafic potassic rocks, this would support the lithospheric contamination model for flood basalts. Such a case was observed for the high-Ti Parana province (e.g. Hawkesworth et al., 1992), but the absence of low-Ti alkali mafic rocks caused major uncertainty about the petrogenesis of low-Ti Parana basalts (Hergt et al., 1991). This problem was solved by the discovery of low-Ti alkali mafic rocks on the flanks of the Parana basin (Gibson et al., 1996).

Comparison of the Nd isotope systematics of high-Ti and low-Ti alkali mafic rocks from the Parana showed that while the ranges of Sm/Nd ratios were identical, Nd isotope compositions were quite distinct (Fig. 7.23). This suggests that the lithospheric mantle underlying high-Ti and low-Ti provinces has distinct trace element enrichment ages, possibly reflecting the geographical extent of a Late Proterozoic crustal reworking event. Therefore, this confirms that the high-Ti and low-Ti suites reflect the provinciality of the sub-continental lithosphere.

Fig. 7.23. Nd isotope evolution diagram showing predicted evolution lines of the lithospheric sources of high-Ti ( ! ) and low-Ti ( " ) alkali mafic magmas, with ages of ca. 0.8 and 1.4 Ga respectively. Inset shows the lava data on which the evolution lines are based. After Gibson et al. (1996).

If the isotopic signatures of the most enriched alkali mafic suites are used as end-members in a mixing model, the isotopic compositions of low-Ti flood basalts can be explained by ca. 20% contamination of a primitive plume end-member in the lithospheric mantle, followed by extensive contamination in the crust (Fig. 7.24). On the other hand, the high-Ti flood basalts require ca. 50% contamination in the mantle lithosphere, but less crustal contamination. Such multi-stage models for the interaction of plumes with the mantle lithosphere are likely to be a continuing major focus in geochemical studies of continental basalts.

Fig. 7.24. Nd–Sr isotope diagram showing fields for high-Ti rocks (white) and low-Ti rocks (shaded). Parana basalts are explained by two-stage contamination of plume magmas, firstly in the sub-continental lithosphere and secondly in the crust. Mixing lines are marked in 20% increments. After Gibson et al. (1996).

7.2.5 Phenocrysts as records of magma evolution

Inversion modelling is unavoidable when attempting to reconstruct the evolution of large magma suites during emplacement through the crust. However, when this is based only on the whole-rock composition of the final products, it may overlook internal mineralogical evidence that could help to constrain contamination models. Therefore, some recent studies have investigated the internal Sr isotope heterogeneity of feldspar phenocrysts in volcanic lavas in order to reconstruct near-surface magma plumbing. Early work (e.g. Davidson and Tepley, 1997) used a micro-drilling technique, followed by conventional ion exchange chemistry, to study Sr zoning in plagioclase phenocrysts from three volcanic systems. However, the advent of MC–ICP–MS (section 2.2.2) has allowed in situ Sr isotope analysis of felspar phenocrysrts by laser ablation (Davidson et al., 2001).

Figure 7.25 shows a typical isotope profile from the study of Davidson et al. (2001), which was measured along the length of a plagioclase phenocryst from the El Chichon volcano, Mexico. The data are in good agreement with microdrill results from the same crystal (Davidson and Tepley, 1997), and clearly show two phases of magma evolution recorded by the crystal. The data are attributed to successive injections of mantle-derived magma with unradiogenic Sr into a magma chamber in the crust. Plating out of crystals on the walls is argued to protect subsequent magma injections from contamination, so that the bulk magma becomes less radiogenic with time.

Fig. 7.25. Sr isotope profile along the length of a zoned plagioclase with a clear core and a patchy outer rim zone reflecting two stages of felspar growth. The width of each data point indicates the approximate size of each ablation pit in the scan. After Davidson et al. (2001).

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