NdSr isotope and REE geochemistry of alkali basalts from the Massif Central, France

OOlfP.7037/84/53.00 + .oO

Vol.48. PP. 93-l IO 0 Pcrgamon Press Ltd. 1984. Printed in U.S.A.

Nd-Sr isotope and REE geochemistry of alkali basalts from the Massif Central, France CATHERINECHAUVEL*and BOR-MING JAHN Institut de Geologic, Cent. CNRS, Universiti de Rennes I, Avenue du G&k-al Leclerc, 35042 Rennes Cedex, France (Received March 4, 1983; accepted in revisedform October 4, 1983) Abstract-Nd and Sr isotopic compositions as well as trace element concentrations have been determined on a suite of alkali basalts from the Massif Central, in France. Samples show a typical enrichment in incompatible elements. In particular, the REE patterns exhibit a strong fractionation characterized by a (La/Yb), ratio of about 20. The Yb,., content is about IO times chondrite. The 143Nd/l44Nd ratios exhibit a range from 0.512775 to 0.512989, values quite comparable to those from oceanic island basalts. The 87Sr/86Sr ratios vary between 0.70338 and 0.70458 and are anti-correlated with the Nd isotopic ratio. The isotopic and the trace element (in particular REE) data have been used in order to quantitatively model the genesis of the alkali basal& Among the several types of models tested here, the most likely one appears to be the model of mantle metasomatism. A semiquantitative approach shows that the source of alkali basalts from the Massif Central was metasomatixed prior to melting. In such a model, the basalts could be produced by rather high degrees of partial melting (such as 10 or 15%) of the metasomatically enriched mantle. INTRODUCI’ION BASALTS, kimberlites, komatiites, etc. are generally believed to be derived by partial melting of mantle materials. Geochemical study of these rocks can, in principle, enhance our understanding of the chemical composition and structure of their mantle source(s). Modem terrestrial basalts can be conveniently separated into tholeiitic and alkali basalt families. Included in the tholeiitic family are those occurring on oceanic ridges and floors (several types of MORB, see SCHILLING, 1975) oceanic islands, island arcs, and on continents (continental flood basalt.+ They are much more abundant than the alkali basalt family, which includes alkali basalts, basanites, nephelinites, kimberlites, lamprophyres, etc. Alkali basalts (sensu luto) occur in a more limited variety oftectonic settings than tholeiitic basal& and normally in the interior parts of oceanic and continental plates, and are thought to be related to mantle plume activity (MORGAN, 1972; GIROT et al., 1978). Petrogenetic problems of alkali basahs have been widely discussed in recent years, both from geochemical/isotopic and experimental/petrologic points of view. Models for alkaline rock gene& can be extremely various, i.e. some of them involve very small degrees of partial melting (GAST, 1968; KAY and GAST, 1973) whereas others recommend large degrees of melting (SUN and J-fANSON,1975). The same discrepancy exists concerning the characteristics of the mantle source, i.e. chondritic source or LREE-enriched source (SUN and HANSON 1975; FREY d al., 1978), some authors favoring a mantle metasomatism in order to account for the enrichment in highly incompatible elements

* Present address Max-Plan&Institut fur Chemie, Saamtn& 23,650O Mains, Federal Republic of Germany. 93

(LLOYD and BAILEY, 1975; CARTER et al., 1978; BOETKHER et al., 1979; MENZIES and MURTHY, 198Oa, b; WASS and ROGERS,1980). In this report, we present Nd-Sr isotopic and trace element (particularly REE) data on alkali basalts (s.1.) from the Massif Central Franc&, one of the most important alkali basalt provinces in Europe, with these purposes: (1) to characterize the isotopic composition of these rocks and hence their mantle source(s). These data provide a temporal constraint to the “enrichment events”, if any; (2) to use the observed REE distribution patterns, combined with isotopic data to deduce a reasonable REE abundance in their source rocks, and hence to propose a petrogenetic model; (3) to examine all possible nondifferentiation processes (e.g., crustal contamination, magma mixing alteration, etc.) by the use of all chemical and isotopic data; (4) to test and comment on the previously proposed models on alkali basalt genesis. Geologicalsetting and sampling localities Tertiary and Quaternary volcanic activities in Eurone can be manifested in two principal stylesz (I) the limited cal* alkaline volcanism within the Alpine chains. and (21 the widespread alkaline volcanism around the Alpine c&s (WIMMENAUER,1974). The alkaline volcani&s are best known in the Massif Central, the Rhine Grabcn and the Bohemian MassifAU volcanic liquids have penetrated through basement rocks of Hercynian age. In Massif Central (Fii I), there are many individual volcanic provinces with their own ChaMaistics (JUNO and BROU~SE,1962; MAURYand VARET,1980). Since our studied samples comes tiom only five provinces: the CantaL the Mont Dons, the Aubrac, the Chaine de-s Ruys and the Velay (the IastisnotshownandistotheeastofFii l),wewihonly outline some mlevant geologic features for these live provinces as follows:

C. Chauvel and B.-M. Jahn

94

N

9 CWNE

DES PUYS LERMONT- FERRAND

dXHAUDESAWJES

AWIAC

FIG. 1. Distribution of volcanic rocks in a part of the Massif Central. From north to south, five volcanic provinces or centers are shown: Chaine des Puys, Mont Don, Cezallier, Cantal and Aubrac. No samples were collected from the C&Bier. A petidotite nodule sample studied is from the Velay province, about 50 km E. of the Cantal and is outside of this map. Sampling localities are shown by numbers in boxes.

(I) The Cantal and the Mont Dote are two hqe stmtovolcanoes and have complex structural developments as well aspetroBnphiccltarecEaistiQ(GoERDEHEllVfiDEandMERCKXL,1971). It has been shown that there exist two trends

of magmatic evolution: an undematurated trend rangiag in composition Iium basanite to pbonolite and a saturated trend From alkali basalt to rhyolite (JUNG and BROWSZ, 1962; VAT~N-PWC~NON.1968). The eruptions in the Cantal commenced in Upper Miocene (~10 M.y.), reached a ctdmination in the period of 7.5 to 4 My. and aasad in Upper Pliocene time. By contrast, the volcanic activities in Mont Dare started a little later in the Pliocene, reached tbe culmination between 4 to I M.y. and terminated only 0.2 My. ago (BELLONef al., 1972a,b; BAUBRONand DEMANGE,1974; BAUBRONand CANTAOREL,unpub. data). (2) The Aubrac, the Velay and the Chaine des Puys belong to a fissure type volcanism. Volcanic activity in the Aubrac is concentrated in the period between 8 to 6 M.y. (BAUBRON

and DEMANGE,unpub. data), and in the Velay, from IO M.y. to the end of Pliocene (g 1 M.y.), which is then followed by the eruptions of the Chaine des Puys during the period of 50,000 yn B.P. to 4,000 yn B.P. (CANTAGRELand MERGOIL, 1970; BROWSE e( al., 1969; CAMUS et al.. 1975; BELLON, 1976). The vokanic rocks in the Massif Central can be conveniently grouped into(I) less differentiated or undiffhmntiated basanitealkali basalt, hawaiite, and (2) more. diffbrentiated rocks including, in the order of increasing diKerentiation index (D.I.), mqIeatite, benmoreite, trachyte, phonolite and rhyolite (MAURY and BROUSSE,1978). The two groups of individual rock types are thought to be related mainly by fmetional crystallization (MAURY, 1976, MAURY et al., 1980; VILLEMANTet a/., 1980), althoutth Sr isotopic data sugBtst a role of crustal contamination in some rhyolite petm8enesis (CAklUS and VIALETTE,1969; STEJTLERand ALL~GRE, 1979).

Massif Central basalts Sampling localities are shown in Fig. 1. Except for the spine1 Ihetzolite inclusion from the Velay, all samples are from massive flows with minimal apparentalterationproducts and least porosity (or vesicularity).

Analyticalprocedures Major eiements. Except for Na and Mg, these were determined by the XRF method using a Siemens spectrometer with a Philips generator. Total iron is reported as Fe20,. Na*O and MgO were determined by the atomic absorption method. Analytical uncertainties were estimated at less than 3% except for P205 at about 510%. Trace elements. Rare earth element (REE) concentrations were determined by isotope dilution after JAHN etal.(198Oa). The uncertainties were estimated at 3% for 4 Lu and Gd, and less than 2% for the rest of REE. Other trace elements wen determined by XRF on powder pellets packed by mowiol (product of Hcechst Company, W. Germany). Analytical uncertainties were between 3 to 5%for concentrations 3 20 ppm, and 10%for those ~20 ppm. Nd and Sr isotopic compositions. Isotopic analyses were performed following JAHN et al. (198Ob), except for slight modifications of chemical procedures. RESULTS (A) Major element composition Except for a few differentiated intermediate rocks, all basic rocks show a relatively uniform composition (Table 1). They have SiOr = 45.9 f 2.2( 1a)%, MgO = 8.18 & 2.2% and high contents in alkali oxides (NarO 3-Q%, K20 ranging from 0.9 to 2.5% with an average of 1.62%), in TiOr (2 to 3%) and Pro, (0.5 to 1.3%).These chemical parameters are typical of alkali basalts and related rocks. Normative Ne is present in almost all cases. Using the diagram of NarO + KsO as a function of SiOr, the basic rocks from the Massif Central fall in the fields of alkali basalts, basanites and nephelinites (Fig. 2). No tholeiitic basahs are found. Their mg values (100 X Mg/(Mg + Fe++)) range from 55 to 71. Apparently most rocks (mg < 68) have been derived by a certain degree of fractional crystahization from more primitive magma(s).

(B) Rare earth elements REE data are presented in Table 2. Except for one sample from Carnal (No. 50 13), all basaltic rocks (s. 1.) including basanites, alkali basalts and hawaiites) show highly fractionated REE patterns (Fig. 3), with chondrite-normalized (La/M), = 12 to 22. Most patterns are subparallel and have no or very slight positive Eu anomahtes (Fig 3). The heavy REE (HREE) contents are rather uniform, with (Lu)~ = 8 to 12, but the light REE (LREE) contents show a little greater variation, with (La)N = 100 to 250. There is no clear positive correlation between (La/YblN a measure of the degree of REE fractionation, and normative ne values as ob served in oceanic alkali basahs of the Honolulu volcanic series of Oahu (CLAGUE and FREY, 1982). In fact, in some cases, such as in the Chaine des Puys

95

and the Aubrac, a negative correlation appears to exist and suggest the effect of fractional crystallization within the series. It has been shown that large Eu anomalies are absent in alkaline volcanic rocks (e.g. KAY and GAST, 1973; SUN and HANSON, 1975; SHIMIZU and ARCULUS, 1975). However, precise isotope dilution data often show slight positive (2-5%) Eu anomalies, as observed in the present study. Dtferentiated rocks. Two samples of relatively differentiated rocks were analyzed for REE: i.e. 5303, a phonolite, and 5308, a trachyte. These two rocks show REE patterns quite di&rent from the more matic rocks (Fig. 3c, d): (1) greater RE fractionation ((La/YblN = 25.1 for the phonolite and =24.3 for the trachyte); (2) the pattern shape is different: lower enrichment in MREE and concave shape of the HREE for the phonelite. (5303), higher enrichment in LREE and concavity of the pattern on the M-HREE’s level for the trachyte (5308). No Eu anomaly can be seen for these two samples. (C) Other trace elements Trace elements data are presented in Table 2. Samples show high concentrations in incompatible elements, e.g. Rb N 50 ppm, Sr = 800 ppm, Ba N 600 ppm, Nb = 100 ppm. This feature is typical for alkali basalts as already pointed out by GAST ( 1968) among others. The concentration in each of the incompatible elements varies greatly from sample to sample (up to several orders of magnitude); in particular, the phonolites and trachyte (5303, 5306 and 5308) show very distinctive contents in Sr, Rb, Ba, Zr and V, which can be explained by fractional crystallization. However, the contents in trace elements am also not constant within more primitive lavas. For example, if one compares the incompatible elements content of the basanites, the concentrations vary by a factor of 2 for Ba, 2.6 for Nb and 1.7 for Sr, showing a large heterogeneity between the lavas. In the same way, the compatible elements (Co, Cr and Ni) show a rather large scatter even if the general tendency is to have lower abundances when MgO decreases. Using two element plots (Fig. 4), we can examine the relative enrichment of the trace elements by comparison to the mantle. The mantle ratios have been calculated using values proposed by SUN and NESBI-IT ( 1977), SUN et al. (1979) and SUN (1982). By combining a number of diagrams, a relative enrichment order for the trace elements can be defined: V -c Y < TiOz < Zr < P205 -c Sr < Rb = Ba c Nb. This is quite comparable to the order established for alkaline rocks by DAWSON (1967), KABLE et al. (1975) and JAHN et al. (I 979) among others. Furthermore, if one assumes that the mantle has roughly 2 times chondritic abundances in trace elements (except for Pros), it is possible to evaluate the absolute enrichment of the elements and compare it with the REE enrichment as already proposed by SUN ef ul. ( 1979). Such an example is shown in Fig. 12 for the sample 5012. This sample,

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FIG. 2. (Na20 + K20) vs Si02 plot for volcanic rocks from the Massif Central (in solid symbols). Lamprophyres from the Spanish Peaks, Colorado, are shown for comparison. Demarcations (from top to the bottom) between rock tmes are drawn according to MACDONALD and KATSURA (1964). STRONG (1972) and SAGGERSON and-WILLIAMS (1964). which is the most primitive lava of the series, shows a very fractionated pattern (Fig. 12) in which the nonREE elements behave consistently with the REE elements. The strong enrichment in LREE, Rb, Nb, Ba etc. relative to the HREE suggest that the mantle source was enriched in a comparable manner and/or that the melting process which created such an enrichment affected the non-REE elements as well as the REE elements. (0) Sr and Nd isotopic compositions

87Sr/86Sr ratios vary from 0.7034 to 0.7046 (Table 3); the variation (ASr) is far beyond the analytical errors and is not affected by the small age corrections for initial 87Sr/86Sr ratios (the maximum adjustments for samples 5 125 and 50 11 are <0.00008 and the adjustment for the rest of the samples is smaller than analytical errors). Since the isotopic difference (ASr) appears real, it may suggest either that the source regions were isotopically heterogeneous to some extent or that alteration or crustal contamination changed the isotopic ratios. Note that the range of 87Sr/86Sr ratios of the alkali basalts from the Massif Central is comparable to those of ocean island basalts and many continental alkali basalts (PETERMANand HEDGE, 197 1; O’NIONS and PANKHURST, 1974; SUN and JAHN, 1975; BROOKSet al., 1976a; WHITE et al., 1976; HOFMANNand HART, 1978; etc.). Except for samples 5011 and 5 125, all rocks have 87Rb/86Sr ratios between 0.11 and 0.22, a range of values typical of alkali basalts and related rocks (see BROOKS et al., 1976b for reference). Assuming 0.7045 for the 87Sr/86Sr ratio of the upper

mantle uniform reservoir or of the bulk earth (DEPAOLO and WASSERBURG. 1976a; O’NIONS et al., 1977), model ages obtained range from about 0 to 1250 My in the future (Table 3). Nd isotopic data are displayed as a histogram in Fig. 5. No distinction can be made among the samples from different volcanic centers. The +&,values (see the definition oft value in the footnote of Table 3) range from +2.6 to +6.7, with a mean at about +5.0. These. values are also comparable to those of ocean island basalts (O’NIONS et al.. 1977; DEPAOLO and WASSERBURG, 1976a,b; WHITE and HOFMANN, 1978; MENZIESand MURTHY, 1980a) and some continental alkali basalts (NORRY et al., 1980; ALL~?GREef al., I98 1). The positive tNdvalues reflect a time-integrated LREE depleted source pattern obviously in disagreement with the enriched LREE pattern measured on the lavas. This contradiction leads to future model ages for the Nd system. In the tNd vs. es, diagram, data plot within both the oceanic island basalts field (OIB) and continental basahs field and in their upper left part (Fig. 6). (E) Spine1 lherzolite nodule (No 5702)

A few spine1 lbetzobte nodule samples were collected from the alkali basalts of Velay. Since nodules may bear important information regarding the genetic relationship with alkali basalts as well as the chemical and isotopic characteristics of the underlaying mantle, a preliminary analysis was done on a single specimen No 5702 (see Tables 1,2, 3). Petrographically, it consists of olivine, orthopyroxene, clinopyroxene, spine1 and opaques.

Massif Central basalts

Nodule 5702 has a LREEdepleted REE Pattern with HREE = 1.5 x chondritic abundance (Fig. 3e). It has c&O) = -20.6(*‘Sr/‘?lr = 0.70305 f 5) and cNd(O)= +6.7(‘43Nd/‘uNd = 0.512989 + 36). Although the Sr isotopic composition is significantly lower than those of alkali basal& the Nd isotopic composition is comparable to the highest ratio observed in alkali basalts (N” 5011 of Camal, Table 3). DISCUSSION

(A) Importance of crystal fractionation Crystal fractionation is an important phenomenon in the Massif Central series as shown by a considerable number of studies, e.g. MERVOYER e2 al. (1973), MAURY (1976), MAURY et al. ( 1980), VILLEMANTet al. (1980). The three differentiated rocks (trachyte: 5308 and phonolites: 5303 and 5306) have low concentrations of Pz05, V, Sr and TiOz and high Rb and Zr contents compared to the more primitive rocks (Tables 1 and 2). The REE patterns of the trachyte and one phonolite are also different from those of the more basaltic rocks: they show a rather strong downward concavity in the heavy REE (Fig. 3c, d). These features can plausibly be interpreted as a consequence of the fractionation of apatite (-2%), plagioclase (MAURY et al., 1980; VILLEMANTet al., 1980) and FeTi oxides as shown by the drastic decrease of the Til Zr ratio in those lavas compared to the more primitive ones (Table 2). Concerning the less evolved liquids such as the basanites, the alkali basalts and the hawaiites, the effects of crystal fractionation cannot be totally ignored but they are thought to be minor and to have had no important consequence for the REE because only olivine and clinopyroxene were involved in the first steps of fractionation (see VILLEMANTet al., 1980). The only effects are a slight increase in incompatible element contents without any changes in the element ratios such as (La/Yb)N, (Ti/Zr), (Rb/Sr), etc. (B) Assessment of contamination by the continental crust Since the Massif Central basalts occur in a continental environment, the examination of a possible crustal contamination is of the first priority. It can be checked by plotting an isotopic ratio versus the element content as proposed by VOLLMER(1976) and LANG MUIR etal.(1978). Unfortunately, a hyperbolic trend is only observed when the end-members are constant and when no other processes (such as fractional crystallization) interferes (DEPAOLO, 198 1). In both 87Sr/ 86Sr vs. Sr and 143Nd/ 144Nd vs. Nd diagrams, data only define a cloud which can be interpreted by the superposition Of fractional crystallization effects. Crustal contamination effects can be better observed when 87Sr/86Sr (or 143Nd/ 144Nd) are plotted against MgO content. Mgo can indeed be considered as an inveise measure of differentiation. In this diagram (fig.

99

7a), a rough negative slope is observed for Sr isotopes. The correlation is very bad and mainly controlled by the two more differentiated rocks. In the same way, those two samples control a slightly positive correlation in the 143Nd/l44Nd vs. MgO diagram (Fig. 7b). These two samples are, then, thought to be slightly contaminated by the continental crust: the contamination probably took place in the magma chamber. No quantitative approach for the percentage of contamination is possible because of the lack of isotopic data for the basement. Crustal contamination of the more primitive lavas can, of course, not be totally rejected. However, the more mafic lavas exhibit a large range of both Nd and Sr isotopic compositions, which does not correlate with any parameter (SiOz, MgG, D.I., (La/yb)~, (Ti/Zr), etc.). This range is thought to be due to heterogeneities in the mantle source rather than to crustal contamination. Such heterogeneity is not greater than those already observed for oceanic islands (e.g. WHITE and HOFMANN, 1982). Furthermore, the lavas which have been analyzed sample a rather large mantle source (or several mantle sources) corresponding to a geographical surface of about 4000 km*. In addition, the rocks range in age from about 10 M.y. to prehistoric times. This amount of time is probably long enough to allow the mantle source(s) to change in terms of isotopic characteristics. In the following discussion, we will discuss only the genesis of the more primitive liquids (basanites and alkali basal@. We will ignore the more differentiated lavas, assuming that their trace-element and isotopic characteristics are partly controlled by fractional crystallization and crustal contamination, and therefore cannot be integrated in any model of genesis directly from the mantle.

(C) Petrogenetic models as proposed by geochemists for alkali basalts

Numerous petrogenetic models for alkali basalts and related rocks have been proposed by petrologists and geochemists. In the past decade, genetic hypothesis for alkali basalts (sensu lato) as proposed by geochemists can be categorized as follows: l-Low percentage (~3%) melting of a chondritic type upper mantle (CAST, 1968; KAY and CAST, 1973). 2-Higher percentage (5- 15%) melting of a non-chondritic mantle which was resulted from: (a) melt contamination (GREEN, 197 1; SUN and HANSON,1975; GREEN and LIEBERMAN,1976; FREY et al.. 1978). (b) fluid phase metasomatism (LLOYDand BAILEY, 1975; FREY et al., 1978; BOE~CHER et al., 1979; MENZIESand MURTHY, 1980a,b; WASS and ROGERS, 1980). 3-Mixing of liquids derived from isotopically distinct mantle sources.

8’06 I’51

9’89

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9’05

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6’SP 9’6Qt

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95’5 IC'L lL'Z

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MassifCentral basahs

101

The major&y Of alkali bar&s have positive @,,,Values, as in the case of Massif Central (DEPAOLO and WASSEREURG,1976a,b; O’NIONS et al., 1977; WHITE and HoP~~ANN, 1978; MENZIESand MURTHY, 198Oa). This observation indicates that their source regions have had a time-integrated depletion in LREE. Consequently, melting of chondritic mantle (Harris 1, above) or melting of mantle source- regions which have been enriched in LREE for a long period of time (> 1 AE), is not considered as a likely process for alkali basalt genesis. In the following we shall use the combined constraints from Nd isotope and REE geochemistry to test the validity of hypothesis 2 and 3 listed above.

As shown in Fig. 3, alkali basahs (s. 1.) are highly enriched in LREE relative to HREE, with (La/yb)~ = 10 to 25. During partial melting, incom~tib~e elements, such as Rb, Sr, or REE, cannot be infinitely enriched; the enrichment is limited to a factor of l/ D for zero degree of melting (F = 0), where D represents bulk distribution coefficient. The following cases are now considered

0

5:

(11 Direct melting of mantle peridc?ites having LRTE-depleted patterns. This is the simplest model in consideration. Assuming that the source peridotites were fractionated from chondritic type mantle more than billion years ago (taking 1.5 AE for simplicity) as for MORB sources, the enrichment factor, S(Sm/ Nd), for the source would be +O. 135 in order to produce the present day average eNdvalue of +5. This f(Sm/Nd) value corresponds roughly to (La/Sm)N and (La/YblN ratios equal to 0.5, granting a MORB-like pattern with flat HREE. This type of REE pattern is similar to the spine1 lherzolite REE pattern (Fig. 3e). Starting with a peridotitic source of (LafyblN = 0.5 and (Yb),, = 2, it is possible to calculate the limit of REE enrichment (represented by (YblN values) and the degree of REE f~o~~o~ frepresented by (La/ Y~)N ratios). Using a set of distribution coefficients (KD) as summarized in JAHN et al. ( 198Oa) or HANSON (1980), Fig. 8 shows a few representative trends of liquid evolution assuming various phase assemblages It is clear that the liquid evolution trends are mainly controlled by the garnet contents in residual solids. The envelope of the theoretical limiting values, as represented by the arrow heads, lies entirely outside of available data points (Fig. 8), precluding such a simple model for the Massif Central rocks genesis. By using different K. values compiled by ARTH and HANSON (197% the position of the arrows is strongly shifted to the upper left of the diagram but with ybh, values lower than 5 and are therefore farther away from the data points. When other KB valuesare used, the arrows move in the diagram but are also outside of the data point field. Evidently, the highly fractionated patterns

C. Chauvel and B.-M.

102

I

I

I

I

I

I

I

Jahn

I

Cantal

$200_ .E ElOO= 5 . 60x g 40a

2010:

10 k

l-l

1 Ce

I

1

I

I

I

I

I

SmEu Gd

I

I

Lu

Er

I

I I I. SmEu Gd

I

La Ce

II

Nd

I

400

Mont

III

Chaine

I Dy

I

I I Yb Lu

I

Er

I

I

I

des Puys

10 7 s =

l_&OO -

g

20

$00

-

5 ’ x

10

560 \ 1

_

g

6 4

Lace

Nd

‘3nEuGd

Gy

Er

Lo Ce

YbLu

SmEu Gd

Dy

Er

YbLu

Velay

z3 8:

Nd

:/-o-----~Yza,

z :: .6 z CK I I La Ce

I

I I I

I

I

I I

Nd

Sm Eu Gd

Dy

Er

YbLu

FIG. 3. REE distribution patterns of volcanic rocks from: (a) Cantal, (b) Aubrac, (c) Mont Dare, (d) Chaine des Puys, (e) Velay-a spine1 peridotite nodule. Chondritic values used here are those Of NAKAMURA ( 1974).

Massif Central basalts

103

Rbtppm)

O

IV 0

8

11

SO

1 lal

P205%

1.0

50

100

150

1

0

I,,, 0

,1,,

La

sb

Ce ppm

os06OL-

02

' 0

I

FIG. 4. Two-element plots. The mantle’s ratios are representedfor comparison (see text for data sources), These diagrams allow us to determine the enrichment order among trace elements. Symbols as follows: circlesz Cant& squareazAubrac; triangles: Chaine des Puys; hexagons Mont Dore. Solid symbofs are for the basanites, half-solid symhois for alkali basalts, open symbols for hawaiites and mugearites, open symbols with dots for more differentiated rocks (phonolites and trachytes).

= 10 to 30 require two or multi-stage history for their petrogenesis. That is, it is not possible to produce alkali basalts by melting of peridotite sources with LRE depletion and such as the spinet lherzolite analyzed here. In other words, LREE en-

with (I_a/Yl&

richment p~~~sses prior to melting episodes seem to be a prerequisite, and the enrichment events must be shortly before the melting episodes. (2) Is melt contamination model ~hy~hesis .?a) possible? SUN and HANSON (1975) were the first to

104

C. Chauvel and B.-M. Jahn Table

3

Sample

:

Sr

and

Nd

isotope

comuositions

of

the

volcanic

rocks

87Rb

87Sr

147Sm

143Nd

86Sr

86Sr

144Nd

144Nd

from

the

Haaslf

Central

P

P

t oNd

UR

CHUR

CANTAL 5008

0.154

0.70378

f7

-10.2

0.1055

0.512982

f41

5009 5010 5011

0.179

0.70338

f5

-15.9

0.1133

0.512943

f34

0.133 0.497

0.70381 0.70350

f4 f6

- 9.8 -14.2

0.1022 0.1072

0.512857 0.512988

5012 5013

0.142 0.141

0.70353 0.70393

f0 f5

-13.8 - 8.1

0.1054 0.1318

0.162

0.70375

l6

-10.6

0.1096

0.70379

f5

-10.1

5126

0.628 0.137

0.70365

f4

-12.1

5127

0.142

0.70376

*5

-10.5

5301

0.109 0.210

0.70435 0.70378

l9 f8

5302 5303

0.172 0.980

0.70381 0.70397

AUBRAC 5124 5125

MONT DORE 5256

-

590

na

f36

+6.6 +5.9 +4.2

- 583 Ma - 358 Ma

*36

+6.7

- 612

Ma

-1064 Xa - 172 Ha

0.512916

l45

0.512833

f36

+5.3 t3.7

469 - 471

Ha "a

-1253 - 747

Ha na

0.512908

l35

+5.2

- 492

p(a

-

na

0.1142

0.512950

*23

+6.0

- 605

"a

-1207

PIa

0.1239

0.512886

l31

+4.7

- 544 "a

- 954

na Ma

- 761 Ha - 863 Ha

708

-

92 ne

0.1167

0.512878

f37

+4.6

- 477

na

- 482

0.1186

*37 f33

+5.4 +5.0

Ma

0.1114

0.512919 0.512902

-

f7 ti

-10.2 - 9.8

- 413 na - 574 na

-

7.5

0.0858

0.512826

f35

+3.6

-

0.70411 0.70387

l7 l4

-

5.5 8.9

0.1131 0.1084

0.512966 0.512912

*35 *25

+6.4 +5.2

- 614 - 483

na Ma

0.175 0.179

0.70388

f5

-

8.8

0.70366

f4

-11.9

0.1155

0.512891

f39

t4.0

- 495

Ha

0.180

0.70403

f5

-

5132

0.186 0.153

0.70373 0.70422

*7 f7

-10.9 - 4.0

0.1185 0.1080

+3.8 +4.4

5133

0.198

0.70406

l6

-

7.0

+2.a

- 405 - 415 - 270

Ha Ma "a

5308

1.225

0.70458

*6

+

I.1

na

- 299 Ma - 312 tla + 5 na

VELAY 5702

0.062

0.70305

l5

-20.6

Ha

+3970

0.218

5304 5307

0.163

-

0.02

574

- 492

tla

26Ha

-

42Ma

- 210 na - 586 Ha

CH.PUYS 5128 5129 5130 5131

Definitions Nd

Of ES=,

48aiIIllt 0.70814 dard

TNd and CHUR

ratlo6 were normlized

isotope

standard

ENd,

salt

since

86Sr/88Sr f 4 (2ti)

-

T;:

are

against

04/04/1978 yielded 0.1194. 87srlbbSr

respectively.

6.8

All

of

found

0.512838

*32

0.1100

0.512871 0.512788

f27 f29

0.0876

0.512775

f25

+2.6

-

0.2133

0.512989

*36

+6.7

+2668

in

and

DePaolo

146Ndi144Nd

- 0.7219. 20 0.511143 f 13 and the E 6 A

143Ndi144Nd NBS-SRM 987

values

reported

Uasserburg.

herein

have

not

191

- 498

Ma

- 646 - 357

Ua na

- 528

Pla

na

1976.

determinations of Johnson Matthey Nd203 (2Cm). Sr isotopic ratios “ere normalized standard were 0.71028 l 4 (2#m) and been

further

adjusted

to

any

other

sta”-

values.

present convincing geochemical arguments for LREE enriched mantle materials as the sources of alkali basalts and related rocks. They envisaged that an early liquid derived by small degrees of melting of a chondritic mantle would be undersaturated (with normative ne) and highly enriched in LRER relative to HREE. This early liquid may intrude the surrounding or overlying mantle as veinlets, dikes or bands, and produces a contaminated mantle. A mass balance calculation reveals that in order to produce a source with ti = +5, the mixing proportion

MassIf

UC 0

for the early undersaturated liquid (Liq I; tNd = 0) to the “sterile” upper mantle peridotite(s) (Cr.&, = -I 10) is about 10% to 90% (schematically illustrated in Fig. 9). The resultant secondary region source would have a REE pattern of (La/Sm)N d 1.0, but (L~/YTJ)~ r, 1.2. Liquid I here was conveniently assumed to be derived by 10% melting. However, it could be assumed regardless of the degree of melting (e.g. 1% or less; or 20% or more) and the resultant secondary source pattern would remain essentially unchanged. This is due to the constrained Nd isotopic composition of the secondary source which controls the mixing proportions of the Liq I and the “sterile” mantle.

Central

+16 +I2 +8 +4

home des hys

Monthre

umveloy

ENd

-

*-L -81

, -30

I -20

I 1 0

L -10

--. I +I0

Esr

FIG. 5. Histogram of Nd isotopic data for rocks from the

Massif Central.

FOG.6. cNdvs tti diagram for modem volcanic rocks. Note that the data of Massif Central fall in the area of ocean island basalts. Black circles: Massif central alkali basalts: Lozenge: nodule.

105

Massif Central basahs

143Nd/144Nd,!IBi1I1111~~~, 87W86Sr rA

0.5126-

I/

MgO%

Upper Hontle

Lace FIG. 7. (a) 87Sr/86Sr vs MgG plot. (b) 143Nd/144Nd vs MgG plot. Note that as MgG value decmase (i.e. rocks become more differentiated), Nd isotopic ratio seems to become smaller but such a trend is not very apparent. A similar but opposite trend is more apparent in (a). Symbol designations as in Fig. 4.

With this secondnty sowxx charactwized by ti = +5, a remelting would produce a liquid (liq II) whose REE pattern will in no way match the observed ones (Fig. 9). In Fig. 10 are shown four trends of liquid

evolution deriving from this secondary source. It is clear that the 1% melting envelope does not pass through the bulk of the data points, and the 2% envelope is totally outside the data field. As it is doubtful that liquids of less than 2% melting can be efficiently removed from source rocks, it is concluded that the melt contamination model as proposed by SUN and

&d =*VI

1 1 1 I Nd SmEuGd Dy

1 Er

1 1 YbLu

FIG.9. REE patterns representing successive stages of melting-mixing (contamination)-remelting. 4 represents liquid produced by 109bpartial melting of a chondritic source (residue: 01; Gpx; Cpx; Gt = 65; 15; 10; IO), and 4, represents liquid produced by 1% partial melting of a contaminated secondary source (residue: 01; Gpx; Cpx; Gt = 65; 20; IO; 5). This secondary source result of mixing of liquid I and upper mantle. The mixing proportions depend on the tNd value of the mixture (here tNd = +5) and are such as upper mantle solid represents ~90% of the mixture and liquid I = 10%.

HANSON( 1975) (hyp. 2a) is quantitatively not plausible for the petrogenesis of alkali basal& This melt contamination model can also be considered when both the first melt and the “sterile” man-

30

20 k,

10 0

2

L

6

8

10

12

lL

16

18

10

Yb, 0 "

2

L

6

8

x)

12

1L

16

l8

20

Yb,

FIG. 8. (La/Yb),,. vs YbN diagram. Also are shown trends of liquid evolution by melting of mantle periodotites with ditlbrent residual phase assemblages. The starting composition Of the SOUI'CC is CharacteriZCd by fNd = +5 and (LaTybl, = 0.5. Note that the field of theoretical liquid composition (below the dashed line which corresponds to F - 0) lies entirely outside the data field. Data sources: 0: this works, Ma&if Central; AZ SUN and HANSON, 1975; 0: KAY and GAST, 1973; 0: JAHN et al., 1979; 0: TERAKADO, 1980.

Ftc. 10. (La/yblN vs YbNdiagram. Liquid trends of melting of a mixed source with (L.a/YblN = I.2 and YbN = 2 (see Fig 9 for the REE pattern) are shown. Residual mineralogy is assumed as follows:

Curve Curve Curve Curve

1 2 3 4

Olivine

GPX

CPX

Gt

10

10

:: 65 65

:; 20 20

10 12 14

: 1

The envelopes of 1% and 2% melting do not intersect the bulk of the data points. Symbol designations as in Fig 8.

106

f

Chauvel and

tie are assumed to have the same Nd isotopic composition (cNd= +5) than the lava.% In such a case. the theoretical liquid composition field covers the data points in specific conditions: (i) the first liquid has to be produced by a very low degree of partial melting ( I%), (ii) the mixing propo~ions between the iiq I and the solid source must be of the order of 20~/80% and (iii) the second liquid must also be produced by a very low degree of partial melting. Although this melt contamination model could be acceptable as long as (La/ Yb),.+and Ybn, are concerned, the REE patterns calculated in such a way do not fit with the patterns measured for the samples. The theoretical patterns exhibit a strong upward concavity for both light and heavy REE (more pronounced than in Fig. 9) when the lava patterns present a slight downward concavity. Such a difference in the shape of the REE pattern, together with the very high proportion of first melt required in this mode1 leave us unsatisfied. In addition, this model requires an efficient extraction of a very small degree of partial melting (< 1W) as in the previous case, and for this reason we do not favor it. 13) Mixing of isotopically distinct liquids. Based on Nd isotopic data, WASSERBURGand DEPAOLOf 1979) have presented a model of chemical structure of the mantle. In this model, alkali basaits are seen to be derived by mixing of liquids derived from the lower mantle (L.M.; tNd = 0) and the upper oceanic mantle (O.M.; eNd = +12). However. if the constraint of tNd = +5 is maintained, mass baIance calculations show that (La/Yb)f; ratios of the mixtures will not be greater than 3 as shown in Fig. 11. Aithough the REE pattern of the liquid from L.M. is not known. it would not matter what type of REE pattern used in the calculation. Figure 1I illustrates an extreme case where the liquid I of cNd= 0 is highly enriched in LREE. In any case, the calculated liquid with cNd = -+_5has a REE pattern sufficiently different from that of alkali basalts of Massif Central or elsewhere. Thus, the mixing model is considered not adequate here. (E) Fluid phase metasomatism The concept of mantle me~~matism by fluid phases has recently been accepted by many workers since the earliest publication of LLOYD and BAILEY (1975). There are many lines of evidence in favor of metasomatism including petrographic and experimental studies (e.g. LLOYD and BAILEY, 1975; BOETTCHERet ai., 1979; among others) and geochemical and isotopic studies (e.g. F~OETTCHER and O’MEIL, 1980; MENZIES and MURTHY, 1980a,b). Metasomatized mantle domains are generally manifested by the presence of hydrous minerals such as phlogopire and amphiboles (BROWN et al.,1980). However, the origin of these fluids is still problematic, but isotopic study suggests a deep origin (BOEITCHERand O’NEIL, 1980). The REE distribution coefficients (KoREE)between fluid (HzO) and solid phases (crystals) are known to be very small ($1 .O) in low pressure (i.e. crustal) con-

B.-M.

Jahn

1 I I Lo Ce

I Nd

I

I

I

Sm Eu cd

I

I

Dy

Er

I

I

,

Yb Lu

FIG. 11. Hy~thet~~l REE pattern produced by mixing of liquids derived fromtwo isotopica&distinctmantle domains (Lo = 0 and eNd= + 10) as proposed by DEPAOLOand WAS-

SERBURG(1979). it is seen that the mixed liquid with fNd = +5 constrained will not have a REE pattern similar to that of alkali basal&

ditions (e.g. CULLERSet ai., 1973). However, recent experimental results indicate that the KDRE values increase drastically at high pressures (MYSEN, 1979, 198 1). During equilibration between peridot&?-vapor at high pressures, the REE are preferentiaUy partitioned to the fluid phase and the &=a values for LREE are greater than for HREE. Consequently, the REE distributions in the vapor (fluid) phase relative to $amet peridotites are highly enriched in LREE. Water in equilibrium with garnet peridotites at high pressures (>40 Kb) might then be enriched in REE and probably other incom~tibIe elements (K, Rb, P, Ti, Sr, etc.) as well. If this fiuid were to migrate upwards to shallower levels of the upper mantle and react with depleted mantle, then the “excessive” REE would be deposited in the earlier depleted mantle peridotites. These peridotites are thus metasomatized and are enriched in total REE (in particuiar in LREE) as we11as in other in~m~tible elements. Subsequent partial meiting of the metasomatized mantle c&d produce alkali bklsalts or other alkali-rich magmas, such as kimbetiites and lamprophyres, etc. (MENZIES and MURTHY, 198Oa,b; FREY el al., 1978; WASS and ROGERS, 1980). Because of the isotopic constraints, the metasomating fluid must ultimately derive from depleted mantle peridot&es(cNd= +5). Otkwise, mass balance calculations show that the metasomatized source cannot be very enriched in LREE campared to the depleted mantle.

107

Massif Central basalts We thus believe that fluid phase metasomatism could create mantle domains with LRREemiched pattern and CharacteristiC pOSitbe cNdVdUeS. The ID?tasomatized mantle can be represented in an incompatibility order diagram (Fig. 12) in order to relate the RRE enrichment to the other trace elements emichment. The element concentrations in the metasomatixed mantle were calculated for the REE by mixing a depleted mantle source with an enriched fluid in the following proportions: 95%; 5%. The concentrations of the 5uid were estimated from the KD values proposed by MYSEN (1979). The non-RR elements were plotted, assuming that they have a coherent behavior with the REE. This assumption is compatible with the general agreement that the alkali and alkali-earth elements are enriched in 5uid phases (RYAJXHIKOV and B~EI-KHER, 1980; LLOYD and BAILEY, 1975). Taking metasomatized mantle as the starting point (box in Fig. 13), liquid trends formed by partial melting of this mantle can be calculated. All residual solids are assumed to contain garnet. It is seen that 10 to 20% of melting can generate liquid compositions corresponding to the majority of alkali basalt data (Fig. 13). Crystal fractionation would have little effect on our model because the fractionation of major phases, such as olivine or pyroxene, would only slightly increase the REE contents but not change the (La/Yb), ratio. Recently, HOFMANN and WHITE (1982) proposed a new model for the origin of the oceanic island basal& The genesis of these rocks is explained in terms of remelting of an old oceanic crust which is returned to the mantle during subduction. This model could be an alternative to explain alkali basalts genesis. Nevertheless, the model of recent mantle metasomatism

1

111111111111111 PbRbanluMlnLocrsrNdFsmE”w CS

To

P

Ill of I Er 2r

III Itl L”

r,

FIG. 12. Enlarged REE diagram as proposed by SUNn al. ( 1979).One sample (50 12, basanite) and a hypothetical metasomatized source arc rePresented. Values concerning the non-REE in the metasomatized source have heen intercalated from the REE pattern. Normalization values are from NAK(1974). SUNand NESBI-IT (1977). SUN ef al. (1979)

AMURA

and SUN(1982).

M-

2U-

La YFN

10 -

t

01 0

I 4 2

HETASOMATISM

’

’

6

L

’

’

8

10

’

12

’

U

’

16

’

lE

’

20

%i

FIG. 13. (La/Y@, vs YbNdiagram showing various trends of liquid evolution by melting a metasomatized mantle peridotite (box). Residual mineralogy is assumed as follows: Olivine

Curve a Curve b

65

Curve c Curve d

:: 65

GPX :: 20 20

CPX

Gt

10

5

12 13 14

3 2

I

It is seen that IO-20% melting of the metasomatized mantle peridotite could yield liquid compositions similar to the observed data. Symbol designations as in Fig. 8.

appears to explain quite well the discrepancy between the depleted isotopic signature of the basalts and their very LIL-enriched nature and so we favor it. CONCLUSIONS (I) Alkali basalts (s.1.) from the Massif Central show high levels of enrichment in REE and other incompatible elements. The REE patterns exhibit a typical LREE enrichment with (La/Yb), ratios between 10 and 25. (2) The isotopic compositions of Nd and Sr indicate a LREE and Rbdepleted source region for these rocks. The ranges of values measured on the samples are between +3 and +7 for CNdand between 0 and - 16 for tsr. They are comparable to those obtained for other continential alkali basalts as well as oceanic island basahs. (3) Calculations show that the alkali basalts cannot be derived by direct partial melting from a depleted upper mantle. Thus the analysed spinel-lherzolite nodule cannot represent the source for the alkali basalts. (4) Quantitative approaches and mass balance calculations preclude models such as (a) melting of mantle source regions with LREE enrichment over a long period of time, (b) mixing of isotopically distinct liquids, (c) contamination of a depleted upper mantle by a time-integrated enriched liquid. (5) Mantle metasomatism is here considered to be the most likely model. This model involves:

C. Chauvel and B.-M.

IOX

(a) the existence of a fluid phase in the mantle, (b) this fluid is enriched in incompatible elements, in particular in LREE. (c) the problem of its origin in the mantle is not solved here. It should have a deep mantle origin as suggested by the 60” measurements (BOETTCHER and O’NEIL, 1980) or by the He isotopic composition of oceanic island rocks (KURZ LJ~ al.. 1982). But it can also be subducted water which has been carried deep down to great depths and equilibrated with the surrounding rocks, (4 the alkali basalts could result from IO to 20% partial melting of this metasomatized mantle, (e) the fluid phase must have the same isotopic characteristics as the mantle it invades, because of mass balance constraints. Thus, as this fluid is very LREE-enriched, it has to have a short lifetime before metasomatism. must (0 for the same reasons. metasomatism occur just before (or a short time before) eruption, (g) the spinel-lherzolite nodule should, if one accepts this metasomatism model, represent a possible mantle source of alkali basalts, but before metasomatism. Acknowledgements-We

thank R. Maury, S. Blais and the petrologists from Clermont-Femmd for their assistance in the field. Chemical and mass spectrometric work were performed with the support of J. Comichet, F. Vidal, M. Lemoine and J. Mace. We thank B. Auvray and W. M. White for providing helpful comments on the early version of this manuscript. We also thank R. Mtuthy and an anonymous reviewer for their constructive criticism of this paper. Partial support for this research was received from a French ATP Geochimie grant. REFERENCES ALL~GREC. J., BREVARTO., DUPR~B. and MINSTERJ. F. (1980) Isotopic and chemical effects produced in a continously differentiating convecting earth mantle. Phil. Trans. Rqv. Sot. London A297,441-477. ALLZGRE,C. J., DUPREB., LAMBRETB. and RICHARDR. ( 198I) The s&continental versus suboceanic debate, I Lead-

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