Continental mantle lithosphere, and shallow level enrichment processes in the Earth's mantle

Earth and Planetary Science Letters, 96 (1990) 256-268 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

256

[MK]

Continental mantle lithosphere, and shallow level enrichment processes in the Earth's mantle C.J. Hawkesworth, P.D. Kempton, N.W. Rogers, R.M. Ellam and P.W. van Calsteren Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA (U.K.) Received May 2, 1989; revised version accepted September 27, 1989 The continental mantle lithosphere may be a significant reservoir for incompatible elements, and it remains a key natural laboratory in which to investigate element fractionation processes in the upper mantle. Major, trace element and isotope data on mantle xenoliths, lamproites and kimberlites, and on selected continental flood basalts are integrated to develop a model for the continental mantle lithosphere. It is argued that the composition of such mantle lithosphere, and hence its density, thickness and capacity to generate basalt, varies with age. Arcbaean mantle lithosphere is characterised by relatively low FeO abundances, which are attributed to komatiite extraction, and thus it is intrinsically less dense than the surrounding asthenosphere. In contrast the post-Archaean mantle lithosphere may be compositionally similar to that sampled recently as spinel peridotite inclusions in alkali basalts. It is therefore sufficiently fertile to contribute in the generation of continental flood basalts, and dense enough to be more readily delaminated and incorporated into the asthenosphere source regions of ocean basalts. Combining the available data on mantle xenoliths and continental flood basalts suggests that the continental mantle lithosphere contains less than 10% of the K, and 3.5% of the Sr and Nd in the crust/mantle system. Many continental mafic rocks have distinctive isotope ratios with low trod, variable Csr and often low 2°rpb/2°4pb. In particular the combination of slightly elevated 875r//S6 Sr at low 2°rpb/2°4pb is increasingly regarded as a feature of the continental mantle lithosphere. Elemental data on lamproites, kimberlites, melilitites and oceanic basalts demonstrate that potassic phase(s) strongly influence(s) U / P b fractionation in the upper mantle. Locally, as in the western U.S., there is evidence for amphibole control, but in most cases the potassic phase appears to be phlogopite, consistent with the negative R b / S r - U / P b arrays inferred from Sr- and Pb-isotopes. The significance of such arguments is that both amphibole and phlogopite are restricted to relatively shallow levels ( < 250 kin), and thus they have a key role in distinguishing shallow from deep level enrichment processes in the Earth's mantle.

1. Introduction

Two goals of contemporary geochemistry are: (1) to evaluate whether the continental mantle lithosphere is a significant repository for trace elements and hence radiogenic isotopes in the context of crust-mantle evolution models, and (2) to distinguish shallow from deep level chemical heterogeneities in the upper mantle. They are linked because much of the continental mantle lithosphere has remained isolated from the convecting mantle for long periods of geological time [1,2], and so it provides a unique natural laboratory for the study of the long-term effects of mantle trace element fractionation processes on radiogenic isotopes. This contribution evaluates the bulk composition of the continental mantle lithosphere by de0012-821X/90/$03.50

© 1990 Elsevier Science Publishers B.V.

vdoping a model which can accommodate major, trace dement and isotope data from both xenoliths and continental magmatic rocks ranging in composition from kimberlites and lamproites to flood basalts. It is argued that there are critical compositional and density differences between Archaean and post-Archaean mantle lithosphere. These differences affect the potential of the lithosphere to act as a source for basaltic magmas, and its potential for delamination and subsequent incorporation into the source regions of ocean island basalts (OIB). There is also increasing evidence that distinctive R b / S r and U / P b fractionations are effected by potassic phases, typically phlogopite and more locally amphibole, in the upper mantle. Since such phases occur at rdativdy shallow depths (< 250 km), the associated trace element patterns may be used to identify shallow

CONTINENTAL

MANTLE

LITHOSPHERE,

AND

SHALLOW

LEVEL

ENRICHMENT

level fractionation processes in both the oceanic and continental upper mantle.

2. Continental magmatism Great care must be exercised in using continental basaltic magmatism as a probe of the continental mantle lithosphere. In particular, the effects of crustal contamination must be stripped off, but also rift-related magmatism, as along the Cameroon Line and the East African Rift, is characterised by basalts which exhibit many major, trace dement and isotope similarities with oceanic basalts [3,4]. Thus, they appear to have been derived from upwelling asthenosphere which is also present beneath oceanic areas and to have undergone little interaction with either the crustal or mantle portions of the continental lithosphere. Lamproites and kimberlites, by contrast, are small volume, high-Mg magmatic rocks characterised by high concentrations of incompatible elements, and a wide range of radiogenic isotope ratios (~Nd = + 1 to -- 26, CS, = -- 19 to + 226, see Fig. 1, and 2°6pb/2°4pb = 16.02-19.27). The age of their source regions is best inferred from Pb isotopes, and particularly for Group II kimberlites and lamproites, these range from 2.5 to 1.0 Ga [5,6]. Since similar ages are often observed in the overlying crust, it is argued that these old mantle source regions are within the continental lithosphere. The idea that the continental crust and the underlying mantle lithosphere may yield similar ages is also a feature of the continental flood basalt (CFB) literature (see below), and in many areas it has been a key argument in support of enriched isotopes being derived from shallow levels within the mantle lithosphere [7], rather than from deep seated heterogeneities perhaps associated with subducted oceanic crust (e.g. [8]). For lamproites and kimberlites a mantle lithosphere source is uncontenfious: they are small in volume, they represent small degrees of'partial melting and they appear to have been derived from source regions which are both depleted in major elements and enriched in incompatible elements, something which is a feature of many xenoliths from the continental lithosphere [9-11]. Continental flood basalts, however, are large in volume, they appear to have been derived from picritic parental magmas [12], and their derivation

257

PROCESSES

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Fig. 1. Initial Nd- and Sr-isotope variations in mid-ocean ridge basalts, continental flood basalts, and lamproites and kimberlites [5,6,13,15-18,22,23,44-46,53]. The fields enclose samples which appear not to have been contaminated significantly en route through the continental crust.

from mantle lithosphere has been contentious both on thermal grounds and from arguments which claim that the lithospheric mantle is too depleted in major elements to generate much basalt. The CFB provinces of the Columbia River, the Deccan, Karoo, Parana and Antarctica have been the subject of numerous detailed studies [13-20] which have sought to distinguish the effects of crustal contamination from mantle enrichment. Thus there is increasing confidence in the estimated isotope and trace element ratios derived from mantle source regions. In many areas, crustal contamination affects CSr more than ~Nd, and so in practice uncontaminated ~Nd values are better constrained than uncontaminated ~Sr" Nonetheless it is striking that the "uncontaminated" CFB plot in a relatively restricted field on an ~NO--~Sr diagram (Fig. 1). In detail, two subsets may be recognised within the CFB data and these appear to be linked to whether a particular province is within or marginal to a stable cratonic area. The Columbia River and Deccan basalts were erupted on cratonic margins, with the Deccan apparently closely re-

258

C.J. HAWKESWORTH

lated to the Reunion hotspot [21]. Although four or five flows within the Deccan exhibit geochemical features which have been interpreted as a mantle lithosphere signature [16], most uncontaminated basalts from both the Columbia River and the Deccan have isotope signatures consistent with derivation from sub-lithospheric, OIB-type mantle reservoirs (Fig. 1). By contrast, the majority of CFBs generated within the Gondwana supercontinent, namely the Karoo, Parana, TransAntarctic and Tasmanian provinces, are characterised by negative end and positive eSr. These provinces can be further subdivided into two groups which have contrasting minor and trace element (e.g. high and low Ti) and radiogenic isotope characteristics, but both groups exhibit end = -- 3 to --7 in apparently uncontaminated basalts, suggesting derivation from distinct mantle reservoirs. Pb- and Nd-isotope variations imply source ages of 1-2 Ga [22,23], similar to those in the overlying crust, and the trace element patterns are sufficiently different from those in OIB (e.g. relatively low Nb and Ta [17,18,24]) to require a different source. Several studies have used these and other arguments to conclude that at least the minor and trace elements of these Gondwana CFBs were largely derived from the continental mantle lithosphere [17-20,22-24]. The next step is

0.5136

ET AL.

to see if this conclusion is consistent with the available results on mantle xenoliths. 3. Mantle xenoliths

3.1. Radiogenic isotopes Sr-, Nd- and Pb-isotope analyses on separated mineral phases and xenolith bulk rock samples have established that the lithospheric mantle supports a wide range of isotope ratios (e.g. [25-29]). Some end and CSr data are presented in Fig. 2, but the picture is complicated because most spinel peridotite data are on separated clinopyroxenes, whereas the garnet peridotite results are on whole rock samples. The latter may show some evidence of interaction with the host kimberlite, but this will tend to displace the isotope values to lower c sr and higher end. Thus observed values may possibly be less extreme than true representatives of the continental mantle lithosphere. Overall, the end and esr data reflect the young age of most lithosphere sampled as spinel peridotites, and the lengthy and complex history of most peridotite xenoliths from older terrains. In southern Africa, for example, Archaean ages are only preserved in mineral inclusions in diamonds [30]. None of the peridotites yet analysed from southern Africa have experienced closed system

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Fig. 2. Present-day Nd- and Sr-isotope variations in xenoliths of spinel and garnet peridotite. The spinel peridotites (open circles) are largely from alkali basalt hosts erupted through comparatively young crustal provinces, and most of the isotope analyses are on separated phases. In contrast, the garnet peridotite (filled circles) are largely from kimberlite pipes in Archaean and Proterozoic basement, and many of the analyses are on whole rock samples. Data from many sources including [11,25-29, and references therein].

CONTINENTAL MANTLE LITHOSPHERE, AND SHALLOW LEVEL ENRICHMENT PROCESSES

evolution, and although their major element depletion is arguably an Archaean feature (see below), the oldest model Nd ages tend to be Proterozoic. In addition, many of the peridotites analysed from the Kimberley area experienced phlogopite and K-richterite metasomatism in the Cretaceous [28,29], and while such processes have implications for the present isotopic signature of the mantle lithosphere, their effect has been to obscure the real antiquity of the lithosphere in this area. 3.2. Major elements There are a number of persuasive geophysical arguments to suggest that the sub-cratonic lithospheric mantle is compositionally distinct from the convecting upper mantle. Jordan (e.g. [1]) has repeatedly argued that S-wave seismic velocity anomalies persist beneath cratonic areas to depths of at least 300 km. Richter [2] suggested that in order for diamonds to have formed within the lithosphere during the Archaean, it must have stabilised by a mechanism other than conductive cooling in order to have attained the necessary thickness. Following Jordan, he concluded that the continental mantle lithosphere must have been compositionally distinct in order to remain thermally isolated, and proposed a major change in the mechanism of lithosphere stabilisation roughly coincident with the Archaean-Proterozoic boundary. The only direct evidence for the major element composition of the continental mantle is from the study of xenoliths. This approach was adopted by Maaloe and Aoki [31] in a search for a bulk mantle composition, and they recognised that the mantle represented by garnet peridotites, particularly those from southern African kimberlites, is compositionally distinct from that of the spinel peridotite facies. Harte [10] also appreciated the special status of these samples, and sought to identify lithospheric, as distinct from asthenospheric, material on the basis of petrography and P - T estimates. Such "lithospheric" xenoliths were designated NLB-type (referring to localities in Northern Lesotho and Bultfontein) to distinguish them from both the higher P-T sheared peridotites of possibly asthenospheric origin, and the metasomatically enriched xenoliths characterised by hydrous and other more exotic accessory

259

phases. It is these NLB-type garnet peridotites that are considered to be most representative of lithospheric mantle beneath the Archaean cratons. We have investigated major dement variations in 1300 lherzolite and harzburgite analyses, many of them from a data base kindly provided by C. Herzberg [32], in order to re-evaluate the compositional variations and their origins. For the purposes of this discussion only the xenofith material is considered because its continental setting is readily established. However, there is an inherent bias in the data set in that the spinel peridotite xenoliths (n = 564) are largely from alkali basalts erupted through relatively young crust, whereas the garnet peridotites from kimberlite pipes (n = 352) tend to be in Archaean and Proterozoic basement. Table 1 indicates the different peridotite types and summarises their average compositions, and Fig. 3 illustrates the Fe-Mg trends. The general consensus is that the major element abundances are largely controlled by melt extraction processes [7,9,11,33]. Spinel peridotites range in composition from so-called fertile samples, closely comparable with model primitive mantle [34], to the more depleted varieties from which a basaltic melt has been extracted. In contrast, the garnet peridotites tend to be more restricted in their compositional range, with higher MgO and SiO2 than the spinel peridotites (Table 1). However, even more striking is the difference in the abundances of the "basaltic" major elements, with the garnet peridotites being highly depleted (CaO= 1.1% and FeO = 7.3%, at MgO = 43.9%) relative to common spinel peridotites (CaO = 2.1% and FeO = 8.4%, at MgO = 42.3%). This is even more marked if just the garnet peridotites from Cretaceous kimberlites (i.e. NLB) are considered (CaO = 1.0%, FeO = 7.0%, at MgO = 44.0%), and the garnet harzburgites from these suites include some of the most depleted mantle compositions known (CaO= 0.34%, FeO = 5.8% at MgO = 48.7%). The most significant difference between the garnet and spinel peridotites is illustrated by the FeO-MgO variations in Fig. 3. This clearly shows that whereas both groups define subhorizontal trends at roughly constant FeO, the garnet peridotites are characterised by significantly lower average FeO, regardless of their MgO content. Since the spinel peridotite group contains com-

260 TABLE 1

C.J, HAWKESWORTH

ET AL.

Average peridotite compositions

SiO 2 TiO 2 A1203 FeO MnO MgO CaO Na 20 K 20 CaO/A1203

SiO 2 TiO 2 A1203 FeO MnO MgO CaO Na20 K 20 CaO/A1203 The garnet Harte [10], isotopes at lithosphere

Avg. garnet (n = 422) lherzolite xenoliths

Garnet (n = 276) lherzolite K kimberlite

N L B (n = 180)

Garnet (n = 70) lherzolite P kimberlite

Garnet (n = 70) lher+zolite basalt host

45.8 5:1.62 0.125:0.11 1.45 5:0.84 7.31 5:1.22 0.11 5:0.03 43.94 5:0.74 1.13 5:0.94 0.13 5:0.12 0.11 5:0.17

46.29 5:1.57 0.11 + 0.20 1.29 5:0.71 7.00 5:1.13 0.10 5:0.03 44.04-t- 2.51 0.99 5:0.83 0.16 + 0.12 0.13 5:0.18

46.24 5:1.55 0.11 5:0.18 1.28 5:0.79 6.92 5:1.15 0.10 5:0.03 44.215:2.66 0.96 5:0.83 0.17 + 0.11 0.12 5:0.18

45.45 5:1.26 0.16 5:0.11 1.76 5:1.07 7.62 + 0.91 0.12 5:0.03 43.64 5:2.71 1.19 5:0.88 0.04 5:0.08 0.13 5:0.19

44.24 + 0.99 0.15 5:0.10 1.74 5:1.07 8.20 + 1.36 0.13 + 0.02 43.86 5:3.56 1.61 5:1.18 0.14 5:0.11 0.05 + 0.05

0.78

0+77

0.75

0.68

0.93

Spinel (n = 564) lherzolite xenoliths

Spinel (n = 179) lberzolite massif

Garnet (n = 90) lherzolite massif

Garnet (n = 39) lherzolite oceanic

44.63 + 1.19 0.08 5:0.09 2.23 + 1.10 8.44 + 1.11 0.115:0.05 42.3 5:2.96 2.08 + 1.16 0.195:0.15 0.05 5:0.10

44.79 + 2.09 0.13 5:0.11 2.42 5:1.37 8.75 + 1.50 0.135:0.05 41.27 5:3.49 2.40 + 1.37 0.20+0.21 0.05 5:0.10

44.84 5:1.36 0.12 5:0.12 2.98 5:1.43 8.27 5:0.96 0.135:0.07 40.40 5:3.81 3.11 5:1.69 0.23+0.23 0.03 5:0.05

44.99 5:1.09 0.12 5:0.07 3.47 5:0.96 8.49 5:0.80 0.14+0.01 39.79 5:2.79 2.78 5:1.17 0.32+0.14 0.05 5:0.04

0.93

0.99

1.04

0.8

lherzolites are subdivided into those from Cretaceous (K) kimberlites, which roughly correspond to the N L B Group of and from the Proterozoic Premier Pipe (P) in S. Africa. The Premier xenoliths had low initial Sr and high initial N d the time of emplacement and are interpreted as an example of the convecting upper mantle incorporated into the during the Proterozoic [60,61]. The errors are + 1 standard deviation.

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% MgO Fig. 3. Bulk rock FeO and M g O contents in (A) spinel lherzolites, (B) garnet lherzolites in Cretaceous kimberlites, (C) garnet lherzolites in Proterozoic kimberlites, and (D) garnet lherzolites in basaltic host rocks. Data from a combined Herzberg [32]-O.U. data base. The dashed lines mark primitive mantle with FeO = 7.8% and MgO = 38.3% [34].

CONTINENTAL MANTLE LITHOSPHERE, AND SHALLOW LEVEL ENRICHMENT

positions ranging from fertile to highly basalt-depleted varieties, these diagrams demonstrate that the low-FeO garnet peridotite xenoliths cannot be related to a fertile mantle composition by the extraction of a basaltic melt. The FeO contents of primitive basalts are simply too low [12] to reduce the FeO abundance in the residue sufficiently to produce the low-FeO garnet peridotite compositions. Furthermore, the variation of MgO at constant FeO within the garnet peridotite group cannot be related to the development of the low-FeO characteristics as there is no trend towards a more fertile (i.e. FeO, CaO, A1203-rich ) composition within this trend. Models for the generation of mantle lithosphere tend to regard it either as residual after melt extraction [33], or as the crystallisation products from an early magma ocean [35]. Herzberg et al. [32] recently developed an ingenious model to explain particularly the low-FeO peridotites by majorite fractionation from molten peridotite of chondritic composition. However, they started from the premise that the trend defined by the low-FeO peridotites was related to the process(°s) responsible for the low FeO abundances, an assumption which we have queried above. In detail, experimental determination of majorite composi-

261

PROCESSES

tions co-existing with an ultramafic liquid reveal CaO/A1203 ratios (0.12-0.24 [36]) that are significantly different from the chrondritic values commonly observed in both spinel and garnetbeating peridotites (see Table 1). Thus separation of majorite is unlikely to have had a significant role in the development of the low-Fe compositions, and certainly the suggested 50-70% fractionation is difficult to reconcile with the observed CaO/A1203 ratios. Moreover, based on the high SiO2 contents in experimental majorite (54-55%) it is predicted that majorite fractionation would result in low S i O 2 contents in the low FeO peridotites, while in practice the converse is true (Table 1). Thus our preferred interpretation of the combination of low FeO, with relatively high MgO and SiO2, and unfractionated CaO/A1203 in the garnet peridotites is that they primarily reflect melt extraction. However, with the exception of komatiite, there are few primary melt compositions that have high enough FeO contents to account for this extreme depletion. Since low-FeO garnet peridotites appear to be restricted to kimberlites that penetrate old cratonic areas [10] and komatiite generation was an essentially Archaean phenomenon, we conclude that the depletion occurred during the Archaean and that

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MgO Fig. 4. Variations of normative density [38] with M g O content for bulk rock lherzolite xenoliths. Open diamonds = spinel peridotites; filled diamonds = garnet peridotites. The mean density of the spinel peridotites is 3.335 +0.018 and that for the garnet peridotites is 3.305:t=0.017, and for this n u m b e r of samples these populations are statistically different at > 99.9~ probability. The correlation coefficient for density vs. M g O is 0.62 ( R 2 = 0.39) for the spinel peridotites, and 0.46 (R 2 = 0.21) for the garnet peridotites, indicating that MgO is not the main factor controlling the observed density variations.

262

the mantle lithosphere represented by such garnet peridotites is Archaean in age. This conclusion is supported by recent determinations of Os isotopes on NLB-type xenoliths which reflect Re depletion prior to 2.0 Ga [37]. The major element differences between spinel and garnet peridotites have wider implications, both because the former are more capable of generating basalt, and because they result in significant differences in density. This effect is illustrated in Fig. 4 in which MgO is plotted against normative density, calculated according to the method described by Jordan [38]. Spinel peridotite xenoliths have densities of 3.33-3.34 Mg m -3 regardless of their MgO content or their degree of basalt depletion. This is because the iron content largely controls peridotite density and extraction of basalt does not significantly reduce the FeO content in the residue (Fig. 3). In contrast, the low FeO content of the garnet peridotite xenoliths ( - 6.5%) gives them an intrinsically lower density by an average of - 1% ( - 0.03 Mg m -3) which is equivalent to a temperature difference of 2 0 0 - 4 0 0 ° C in a homogeneous mantle [1]: i.e. lithosphere composed of low-Fe garnet peridotite can be 2 0 0 - 4 0 0 ° C cooler than the surrounding asthenosphere and still be relatively buoyant. It appears that this was the property that allowed the stabilisation of lithospheric mantle in the Archaean with an internal temperature gradient much as it is today, but thick enough to allow diamond to crystallise at depth within the lithosphere ( > 150 km) [2,39]. The extra buoyancy results from the extreme depletion of the lithospheric mantle which in turn was controlled by komatiite extraction. However, as komatiite extraction was restricted largely to the Archaean, it follows that Proterozoic and later lithosphere stabilised by an alternative mechanism. Furthermore, the predominance of basaltic volcanism during the Proterozoic and up to the present day means that post-Archaean residual mantle would take on the higher FeO and lower SiO 2 characteristics of spinel peridotites. Today, lithosphere develops by conductive cooling of the uppermost mantle, and this appears to apply to young continental areas as well as to the ocean basins [40], resulting in a total thickness of < 120 km, insufficient for diamond stabilisation. However, Richter [2] has argued that by about the end

C.J. H A W K E SW O RT H ET AL.

of the Archaean or possibly a little later, the Earth had already cooled sufficiently for lithosphere to form by conductive cooling and that this switch, from a compositionally controlled to a thermally driven process, is responsible for the marked change in tectonic style across the Archaean-Proterozoic boundary. It is interesting to consider the interaction of composition and density on the nature of the post-Archaean lithosphere. Figs. 3 and 4 indicate that both fertile and basalt-depleted peridotite xenoliths have similar FeO contents and hence similar densities. There is thus no a priori reason for post-Archaean lithosphere to be significantly depleted in basaltic constituents. Indeed there is some evidence that modern oceanic lithosphere as seen in ophiolites becomes more fertile in the deeper sections of the mantle sequence [41], and if it is assumed that post-Archaean lithosphere stabilised in a similar manner then it is not unreasonable to suggest that portions could also have a fertile composition. It is therefore argued that in terms of major dements, lithosphere stabilised during the Proterozoic was similar to that sampled by modern-day spinel peridotites. Furthermore, because it stabilised primarily by conductive cooling, it was less thick than the Archaean lithosphere and also more susceptible to delamination, particularly when involved in regional tectonic events. This susceptibility may be further increased by chemical enrichment processes since the Fe enrichment associated with melt migration as seen in composite spinel xenoliths results, at least locally, in a density increase of up to 2%. Finally, its more fertile composition makes it a more acceptable source for basaltic magmas such as the CFB than the Archaean lithospheric mantle. At present there is a paucity of data on xenoliths from magmatism which penetrates Proterozoic terrains to test this model further. However, in its support, we re-emphasise the rarity of komatiites and the dominance of basaltic magmatism during the Proterozoic and Phanerozoic, the rarity of diamonds and their associated indicator minerals from Proterozoic terrains except where that terrain involves reworked Archaean [5,6], and the observation that the source ages inferred from Nd- and Pb-isotopes in CFB are 1-2 Ga rather than Archaean [20,22,23].

CONTINENTAL

MANTLE LITHOSPHERE, AND SHALLOW LEVEL ENRICHMENT

3.3. Minor and trace elements

Since xenoliths and continental magmatic rocks can be integrated into an internally consistent model, both should contribute to any attempt to estimate average isotope and trace element abundances in the continental mantle lithosphere. The CFBs presumably represent the largest volume of lithosphere, and although the estimates are necessarily rough, the data in Figs. 1 and 5 indicate that average ENd - 4 , which in turn suggests £Sr is in the range + 2 0 to + 3 0 and 2°6pb/2°apb-17.0-17.3. The mean of 1500 analyses of CFBs with "old" isotope signatures yields 1.3% K 2 0 and 400 ppm Sr. Since recent discussions of mantle melting processes have emphasised the role of small degree melts [42], such averages are at least 10 × those in their mantle source regions. The xenolith data are even more heterogeneous, although Erlank et al. [28], for example, calculated an average K 2 0 = 0.43% in the mantle sampled by xenoliths in the Kimberley area. However, that is unlikely to be typical of such mantle lithosphere given the low surface heat flow through Archaean terrains [43], and it is much higher than the average K 2 0 contents of both the garnet and spinel peridotites in Table 1. Since, as indicated above, the K 2 0 content in the source of most CFB is likely to be less than 0.13%, and that is both similar to the average K 2 0 in selected garnet peridotite suites (Table 1) and the upper limit of K 2 0 abundances in spinel peridotites, it is suggested as the upper limit for the K 2 0 content of the continental mantle lithosphere. Similar argu=

263

PROCESSES

ments yield a maximum value of 40 ppm for Sr, and assuming S r / N d - - - 1 8 in the upper mantle [34], Nd = 2.2 ppm. If the continental mantle lithosphere comprises - 2.2% of the silicate portion of the earth, such figures suggest it contains a maximum of 10% of the K, and 3.5% of the Sr and Nd in the c r u s t / m a n t l e system [34]. 4. Shallow level heterogeneities in the upper mantle

Isotope and trace element variations in uncontaminated basalts testify to the chemical heterogeneity of the upper mantle [44]; however, key questions remain over the spatial distribution of different mantle reservoirs. In principle, depth constraints are available if particular element fractionation patterns can be linked to phases which are only stable over a limited pressure range. It has been demonstrated above that the Gondwana CFBs have Sr- and Nd-isotope characteristics which are sufficiently similar to each other, and yet different from most oceanic basalts to require derivation from a distinct mantle reservoir (Fig. 1). However, an even more striking feature of these CFB provinces is the association o f l o w 2 ° 6 p b / / 2 ° 4 p b with moderately high 875r/86 S r (Fig. 5A) [18,22,23,45]. Low 2°6pb//2°4pb in particular appears to be a feature of many continental volcanics, and it attains its ultimate expression in lamproites which have values as low as 16.02, but with highly variable 87Sr/86Sr (0.7059-0.7204) [5,6,46]. Such unradiogenic Pb with moderately B

A

ParanaHighTi / ) D e c K ~ -~Kergueten 0.70, = Lo " ~ '~-=IW \ j.,~.Walvisand90°E ~ ~ " ~ I ridges j. ( o, o-, . .,,~,,,~--, ,.,,~,,~.~ C eo~tra/i£~ldaRin

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depleted(" ~-~J//NMAR- EPR..,~ Mantle~.--~ ~ 2oepb/~0,,pb

A A

0.7( I 17

I

19 2osPb120'l:~0p

I

211

Fig. 5. A. Initial Sr- and Pb-isotope variations in MORB, selected OIB and high-Ti continental flood basalts [22,23,45,47]. B. Present-day bulk rock Sr- and Pb-isotope variations in peridotite xenoliths from Kimberley [28,29]. P P = phlogopite peridotite; P K P = phlogopite K-richterite peridotite; open triangles = GPP, garnet phlogopite peridotite; open circles = GP, garnet peridotite.

264

C.J. HAWKESWORTH ET AL.

O G p ~

X~lilitites • •

GplKimberlites l O~ 0,"

Lamproite 0.01

10 K/Nb 100

1000

Fig. 6. MeasuredU/Pb and K/Nb ratiosin selectedlamproites, kimberlites, melilititesand oceanicbasalts. St.H. = St. Helena; T = Tristan da Cunha; G = Gough; PM = primitive mantle [34]. Data from[5,53,59, and referencestherein].

high 87Sr/86 Sr is in sharp contrast to the combination of increasing 2°rpb/2°4pb with 87Sr/86Sr widely observed both within MORB provinces and between MORB and large-scale hot spot OIB [47-49]. Empirically, therefore, it is further evidence for the involvement of a different mantle reservoir in the generation of Gondwana CFB, and this has been regarded as continental mantle lithosphere [18,20-23,45]. The cause of this low U/Pb, high Rb/Sr association is still debated, but two lines of evidence suggest that it may be related to the stabilisation of a potassic phase: (1) Hydrous mantle metasomatism is well developed in phlogopite and K-richterite bearing peridotite xenoliths, and their high Rb/Sr ratios primarily reflect the open system crystallisation of phlogopite [28]. The negative present day 87Sr/ 86Sr-2°6pb/E~pb array illustrated in Fig. 5B demonstrates that the higher Rb/Sr bulk rocks are characterised by relatively low U/Pb. (2) Measured U / P b and K / N b ratios in a diverse range of mantle derived magmas exhibit a striking negative correlation (Fig. 6), indicating that U / P b fractionation particularly in small de-

gree mantle melts is strongly influenced by a potassic phase(s). The K / N b ratios in these rocks reflect the extent to which K was preferentially enriched in mantle source regions, or retained in a residual phase during melting. Lamproites, for example, have high K / N b ratios attributed to potassic phase(s) in their source regions, and their low measured U / P b ratios are consistent with their distinctive unradiogenic Pb isotope ratios [5,6]. Locally, as in the Smoky Butte lamproites [5,50], low U / P b is accompanied by low Rb/Sr and high K / R b , suggesting amphibole control. However, in most cases the high K / N b and low U / P b is associated with high Rb/Sr, and so the potassic phase is inferred to be phlogopite. Both phlogopite and amphibole have relatively low U / P b [29,51], but the negative 87Sr/86Sr-2°6pb/Z°4pb trends (e.g. Fig. 5) indicate that phlogopite may more often influence R b / S r - U / P b fractionation. Depending on the P-T-x conditions phlogopite may melt, or remain as a residual phase [52]. Thus the U / P b - K / N b array is interpreted to be largely the result of changes in the stability of phlogopite during melting, affecting U / P b in the magma as well as K / N b and Rb/Sr. Melts with high K / N b also tend to have low U/Pb, and often high Rb/Sr, having inherited these characteristics from phlogopite-bearing source regions. Clearly, with time such trace element ratios would lead to low 2°rpb/E°4pb and high 87Sr/86Sr, and just as such isotope characteristics are a feature of phlogopitebearing xenoliths (Fig. 5B), similar trends in certain CFB are interpreted to be the result of small amounts of phlogopite in their source regions (Fig. 5A). Magmas with low K / N b , such as melilitites (Fig. 6), are believed to have been generated in the presence of residual phlogopite under CO2-rich conditions [53]. They exhibit high U / P b and low Rb/Sr, similar to the time integrated characteristics of the so-called HIMU ocean island basalts [44,54], and thus it is interesting to speculate whether the source regions of such basalts were enriched by the introduction of small volume melilitite-like melts. Melilitites are produced at high enough pressures for CO 2 to be significantly soluble in silicate melts and within the stability field of dolomite peridotite (> 25 kbar). As a direct consequence of the shape of the CO2-pres-

CONTINENTAL MANTLE LITHOSPHERE, AND SHALLOW LEVEL ENRICHMENT PROCESSES

ent peridotite solidus, such liquids can migrate to lower pressures (corresponding to depths of - 75 km), before their progress is halted and they crystallise an assemblage of pyroxene, amphibole and carbonate [55]. Such as assemblage naturally preserves low Rb/Sr and S m / N d in amphibole and pyroxene, and high U / P b in the carbonate. Thus it is possible to envisage a zone at or near the base of the continental, and even oceanic lithosphere that traps migrating silica-undersaturated melts and evolves with time towards HIMU isotope characteristics. Subsequent re-melting of this enriched zone might then give rise to the HIMU signature in OIB. It is interesting to note that HIMU OIB also have trace element ratios similar to those in melilitites. Both are characterised by low K / N b , Rb/Ba, Rb/Sr and T h / U ratios, all of which are inferred to be features of the HIMU source rather than related to basalt generation processes [54,56]. Thus there is increasing evidence that potassic phases, typically phlogopite and locally amphibole, have a major role in R b / S r - U / P b fractionation in the upper mantle. This is best demonstrated in the low U/Pb, high Rb/Sr portions of the sub-continental mantle, but the extreme U / P b ratios of melilititic liquids may yet cause a major reappraisal of Pb-isotope variations in the convecting upper mantle. The key point is that the stability limits of these potassic minerals constrain the depth at which the associated enrichment processes took place. Amphibole breaks down at pressures of 25 kbar, and although the upper stability limit of phlogopite is less well determined, it appears to be restricted to depths of less than 250 km [55]. Trace element fractionation patterns which have been influenced by phlogopite or amphibole must therefore reflect relatively shallow level enrichment processes in the upper mantle. 5. Concluding comments Available major, trace element and isotope data are consistent with a model in which the composition of continental mantle lithosphere, and hence its density, thickness and capacity to contribute in the generation of basalt, varies with age. Archaean continental mantle lithosphere is ultra-depleted after komatiite extraction, and it is thus intrin-

265

sically less dense than the surrounding asthenosphere. This low density was a controlling factor in its original stabilisation, allowing it to remain both thick and cool enough for diamonds to form at depths below 150 km [1,2,39]. In contrast, it is argued that post-Archaean, i.e. post-komatiite, mantle lithosphere is compositionally similar to that sampled recently as spinel peridotite inclusions in alkali basalts. It stabilised by conductive cooling, and thus it does not need to be compositionally distinct from the asthenosphere to remain convectively isolated. As a result, such postArchaean lithosphere is fertile enough to contribute significantly to the generation of CFBs, and it is also dense enough to be more readily delaminated and incorporated into the asthenosphere source region of oceanic basalts. Combining the available data on mantle xenoliths and continental flood basalts with "old" isotope signatures suggests that the continental mantle lithosphere contains a maximum of 10% of the K, and 3.5% of the Sr and Nd in the crust/mantle system. Many continental mafic rocks have distinctive isotope ratios with low end, variable Csr and often low 2°6pb//2°4pb. A negative Pb-Sr-isotope array has also been identified in metasomatised mantle xenoliths [28,29], and the displacement to high 87Sr/86Sr at low 2°6pb/2°4pb is increasingly regarded as a feature of the continental mantle lithosphere [18,45]. Elemental data on lamproite, kimberlites, melilitites and oceanic basalt demonstrate that potassic phase(s) strongly influence(s) U / P b fractionation in the upper mantle. Locally, as in the western U.S.A., there is evidence for amphibole control, but in most examples the potassic phase is inferred to be phlogopite which would also result in the negative R b / S r - U / P b arrays inferred from Sr- and Pb-isotopes (Fig. 5). The significance of such arguments is that both amphibole and phlogopite are probably restricted to relatively shallow levels (< 250 kin) and thus they have a key role in distinguishing shallow from deep level enrichment processes in the Earth's mantle. With time the fractionated parent/daughter element ratios will develop isotope compositions similar to those in the components commonly used to describe isotope variations in oceanic basalts [44,57]. Components remain a convenient

266

shorthand, but their use shapes the resultant models for the upper mantle. The components must be old, for they are defined on the basis of isotope ratios, despite the evidence for enriched and depleted material of widely differing ages in the upper mantle. Moreover, "old" components must age somewhere and that encourages models in which foreign materials are recently mixed back into the convecting upper mantle, and intermediate isotope ratios are described as mixtures when there may be no evidence for mixing, and when, at least in continental areas, it can often be precluded. An alternative is to describe mantle heterogeneities in terms of enrichment and depletion processes [58]. This seeks to encourage a view of the mantle as a dynamic system in which heterogeneities may be created and destroyed within relatively short time periods as, for example, within the age of an ocean basin. How long such material survives then depends on the life span of the ocean and the efficiency of mixing. The processes described above generate considerable Rb/Sr and U / P b fractionation, with the result that significant 87Sr/86Sr and 2°6pb/2°apb variations can evolve in a few 100 Ma. The same is not true for ta3Nd/144Nd and 2°Tpb/2°4pb, and so they offer better evidence for the introduction of older material. Current interpretations of the isotope ratios in oceanic basalts are still shrouded in uncertainty. However, the view that isotope variations largely reflect deep-seated heterogeneities, often due to recycled crust and mantle lithosphere, may increasingly be countered by the notion that enrichment processes are likely to develop significant heterogeneities at shallow levels in the Earth's mantle.

Acknowledgements Our ideas on the nature of the continental mantle have evolved in discussions with many colleagues and friends, and in particular it is a pleasure to acknowledge the help and encouragement of Keith Cox, Tony Erlank, Ben Harte, Dan McKenzie, Martin Menzies and David Ormerod. Isotope Research at the Open University is supported by the NERC, and Janet Dryden typed the ever-changing manuscript.

C.J. HAWKESWORTHET AL.

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