Trace element and Sr and Nd isotope geochemistry of peridotite xenoliths from the Eifel (West Germany) and their bearing on the evolution of the subcontinental lithosphere

Earth and Planetary Scwnce Letters, 80 (1986) 281-298

281

Elsevaer Soence Pubhshers B V, Amsterdam - Pnnted in The Netherlands

[1]

Trace element and Sr and Nd isotope geochemistry of peridotite xenoliths from the Eifel (West Germany) and their bearing on the evolution of the subcontinental lithosphere H.-G. Stosch 1,2 and G.W. Lugmair 2 Mmeraloglsch-Petrographtsches Insntut der Unwersltat zu Koln, Zulptcher Str 49, 5000 Koln 1 (F R G) 2 Scripps Instttutton of Oceanography, A -012, Umverstty of Cahforma at San Diego, La Jolla, CA 92093 (U S A )

Received June 20, 1986, rewsed versmn accepted August 18, 1986 Pendottte xenohths from the Elfel can be dwlded into incompatible element-depleted and -enriched members The depleted group Is restncted to dry lherzohtes whereas the enriched group encompasses dry harzburgltes, dry webstente and amplubole and/or phlogoplte-beanng pendoutes Isotoplcally the depleted group is very diverse with :43Nd/:4aNd ranging from - 0 51302 to 0 51355 and S7Sr/S6Sr from - 0 7041 to 0 7019, thus occupying a field larger than expected for oceamc-type subcontmental mantle These xenohths are derived from a mantle whtch appears to have &verged from a bulk-earth Nd and Sr isotopic evolution path - 2 Ga ago as a consequence of partial melttag The combination of lugh :43Nd/t'~Nd with htgh S7Sr/a6Sr m some members of the depleted-xenohths state is hkely to be the result of mclp~ent reacUon with mcompaUble element-ennched flmds m the mantle In the enriched group such reactmns have proceeded further and erased any pre-ennchment isotope memory resulting m a smaller lsotoptc dwers~ty(:43Nd/~44Nd - 0 51256-0 51273, 87Sr//86Sr ~ 0 7044-0 7032) An evaluatton of Sm-Hf and Yb-Hf relatlonslups suggests that the amplubole-beanng lherzohtes and harzburgttes acquired their lugh ennchment of hght rare earth elements by fltad lnffltratton Into prewously depleted pendoute rather than by stl~cate melt-reduced metasomatlsm Upper mantle composed of such metasomatmed pendotltes does not represent a potential source for the basamtes and nephehmtes from the Eafel The :sotop:c and chermcal diversity of the subcontmental hthosphenc part of the mantle may result from :t having remained :solated from the convectmg mantle for t~mes> 1 Ga

1. I n t r o d u c t i o n

T h e Elfel (West G e r m a n y ) ts part of the R h e m s h Shield which has b e e n u n d e r g o i n g uplift since at least the n u d - T e r t i a r y [1]. C o n t e m p o r a n e o u s with a n d p r o b a b l y linked to the uphft, volcanic acUvaty developed, starting i n the central part of the Elfel. V o l c a m s m m the western part is characterized b y slhca u n d e r s a t u r a t e d basaltic lavas (basamtes, n e p h e h m t e s , leucltites [2-4] whereas chfferentlated types like p h o n o h t e s a n d trachytes are p r o m i n e n t a m o n g the volcamcs m the East Eifel [2,5]. Several of these v o l c a m c centers have sampled xenoliths f r o m the u p p e r m a n t l e a n d / o r the whole crust. M a n t l e x e n o h t h s are restricted to alkahc basaltic rocks a n d thus are c o n c e n t r a t e d m t h e West Elfel whereas crustal x e n o h t h s are more frequently f o u n d in the more differentiated lavas of the East Elfel. I n this p a p e r we provide chemical a n d Isotope data o n m a n t l e spinel perldotites from two 0012-821X/86/$03 50

© 1986 Elsevier Science Pubhshers B V

Q u a t e r n a r y M a a r volcanoes i n the West Elfel, 1.e. the w e l l - k n o w n Drelser Wether a n d from Meerfelder M a a r which is situated - 15 k m south of Dreiser Wether. This work is a n extension of o u r earlier investigations [6,7]. O u r study was a i m e d at identifying the c o m p o s i t i o n of the u p p e r m a n t l e i n tlus area a n d at better u n d e r s t a n d i n g the processes of i n c o m p a t i b l e e l e m e n t depletion a n d e n r i c h m e n t which have led to its m o d i f i c a t i o n over geologic time. X e n o h t h s rapidly e a r n e d to the earth's surface m these Q u a t e r n a r y m a g m a s represent essentially u n m o d i f i e d p r e s e n t - d a y u p p e r m a n t l e a n d are n o t affected b y the processes that usually m o d i f y tectonically emplaced u p p e r m a n t l e sections, e.g. retrogression a n d alteration. 2. S a m p l e s

The spmel peridotites mvesUgated b y us b e l o n g to group I as defined i n [8]. T h e y can be grouped

282 TABLE 1 Modal composltmns of xenohths

Ib/5 D45 D58 D42 IIa/40 Df120 Ia/157 D8

a

% ol

% opx

% cpx

% spinel

% amph

% phlog

Mg/(Mg+Y'Fe)

% CaO

Rock name [11]

73 9 69 5 68 6 80 9 25 8 75 69 2 73

15 8 19 7 21 5 11 8 22 8 12 7 00 00

92 93 88 64 51 4 10 2 16 6 18 5

11 15 11 09 < 05 16 < 05 < 05

00 00 00 00 00 05 12 8 85

00 00 00 00 00 00 11 00

0 912 0 908 0 913 0 918 0 908 0 916 0 895 0 891

1 99 2 09 1 94 1 28 10 40 2 24 5 10 5 30

lherzohte lherzohte lherzohte harzburglte webstente lherzohte wehrhte 6 wehdlte b

a Apaute - 03% b Green chrome dlopslde-beanng Modes were calculated from major element chenucal composltaons of bulk rocks and rmnerals except Df120 and D8 winch were calculated from trace element compositions of bulk and minerals

into (i) a n h y d r o u s , consisting of ohvlne (ol), ort h o p y r o x e n e (opx), c l m o p y r o x e n e (cpx) a n d spinel o n l y (samples I b / 5 , D45, D58, D42, I I a / 4 0 ) , a n d (n) hydrous, whlch a d d m o n a l l y b e a r a m p h i b o l e ( a m p h ) or p h l o g o p l t e (phlog) (samples Df120, I a / 1 5 7 , DS). T h e h y d r o u s p e n d o t i t e s have t a b u l a r r e c r y s t a l h z e d textures following the c o m m o n l y a d o p t e d n o m e n c l a t u r e [9,10]. T h e d r y rocks are best classified as " c o a r s e " or " p r o t o g r a n u l a r " alt h o u g h t r a n s i t i o n a l textures with o n l y parUal rec r y s t a l h z a t l o n are also present. M o d a l c o m p o s i tions of the x e n o h t h s as c a l c u l a t e d f r o m b u l k rock a n d m i n e r a l chemical c o m p o s m o n s are r e p o r t e d in T a b l e 1. In a d d m o n to these p e n d o t l t e s two b a s a n i t e s have b e e n analyzed. O n e ( I a / 1 5 7 - b a s ) is a n n d a r o u n d wehrhte I a / 1 5 7 . T h e second s a m p l e ((J2-1-bas.) comes from a q u a r r y n e a r the vdlage of U d e r s d o r f a b o u t half-way b e t w e e n Drelser Welher and Meerfelder Maar.

3. Analytical procedures Bulk rock p o w d e r p r e p a r a t i o n a n d m i n e r a l sepa r a t i o n have b e e n p e r f o r m e d using m e t h o d s outl i n e d previously [12]. T r a c e elements have b e e n d e t e r m i n e d b y i n s t r u m e n t a l ( I N A A ) or r a d i o c h e m i c a l ( R N A A ) n e u t r o n activation analysis (Tables 2 a n d 3) T h e errors q u o t e d are 2o of c o u n t ing staUstics T o t a l errors for La, Sm, Sc, Cr a n d C o are e s t i m a t e d to be < + 5 % F o r those trace elements where the 20 errors of c o u n t i n g statistics exceed + 5 % the errors given in T a b l e s 2 a n d 3 c a n b e c o n s i d e r e d reahstic estimates of the total

errors T h e m a s s s p e c t r o m e t r i c analyses (e.g. m e a s u r e m e n t o f N d a n d Sr isotopic c o m p o s i t i o n s a n d R b , Sr, Sm a n d N d concentraUons, T a b l e 4) have b e e n p e r f o r m e d using e s t a b h s h e d p r o c e d u r e s (e g [131)

4. Results 4 1 Trace elements Bulk rock trace element d a t a are listed m T a b l e 2. C h o n d n t e - n o r m a h z e d rare e a r t h element ( R E E ) c o n c e n t r a U o n s are p l o t t e d in F i g 1. T h e two wehrhtes ( I a / 1 5 7 , D8) have the highest R E E a b u n d a n c e s a n d tugh light over heavy R E E enr i c h m e n t s A m p h l b o l e - b e a n n g lherzohte, Dr120, a n d d r y spinel harzburglte, D42, are also strongly l i g h t / h e a v y R E E enriched Interestingly, all three a m p h l b o l e - b e a n n g rocks have ( L a / C e ) c n < 1 ( c n = c h o n d r l t e normahzed). I n contrast, the three d r y lherzohtes ( I b / 5 , D58, D45) have ( L a / Y b ) ¢ n < 1 , b u t one of them, D45, has a s o m e w h a t p e c u h a r relaUve e n r i c h m e n t of the hghtest R E E with ( L a / N d ) ~ n - 1 . 5 D r y harzburgite, D42, has b y far the lowest a n d the wehrhtes a n d o h w n e websterlte, I I a / 4 0 , have the highest h e a v y R E E contents Nevertheless, the Varlabihty in h e a v y R E E concentraUons a m o n g all p e r l d o t l t e s is only a factor of - 6, m o r e t h a n o n e o r d e r of m a g m t u d e smaller t h a n for the hght R E E (a factor of - 1 0 0 ) This v a r i a t i o n in R E E p a t terns is typical for similar suites worldwide, 1.e h y d r o u s p e n d o U t e s a n d d r y harzburgltes are slgm f l c a n t l y hght R E E - e n n c h e d , d r y lherzohtes are

283

+l ~ +l 0

+l

+1

+1

¢'q

~q

t"q

~, o~ ~, so ~, ~ ~ o ~, o~ ~,

~o ~,

+1

~

+1 ~

~

I

0

g +1

+l ~

0

I0

+lO

I~

+l o

I

I

I

I

I

I

I

0 ~,

I

e'q

0 I

I

o

I

r

~

+l

I

o~,°~,~°

~,

0

o

Io

I~

8

~z

e~

[-,

Io

I~

I~

I ~ + 1 ~ +1

~ +l

"~

284 ,

i

i

i

,

,

i

,

i

i

i

REE ABUNDANCES IN SPINEL PERIDOTITE XENOLITHS FROM THE IdESTEIFEL/ Id 6ERMANY

20 10 II

k.J

"

2

02

Into] .

La

O. 100

.

.

Ce

Q095

0090

0.08:

.

Pr Nd

Sm Eu

T

Ho

TIn Yb Lu

Fig 1 Chondnte-normahzed REE patterns of bulk pendotlte xenohths from Meerfelder Maar (Dr120) and Drezser Weiher (all others) plotted against lomc radii (m nanometer) for szxfold coordmatzon [14] For normahzatlon the " r e c o m m e n d e d c h o n d n t e values" of Boynton [15] were used Filled squares = hydrous xenohths, open squares = dry xenohths

hght REE-depleted relative to the heavy REE (see review of Frey 1984 [16] and references cited therein). Trace element abundances in minerals are given in Table 3 and REE patterns are plotted in Fig. 2. Since chnopyroxene (and, if present, amphibole) are usually the main careers of the REE in spinel perldotltes it is not surprising that in most cases the REE patterns of these minerals closely reflect those of the bulk rock A notable exception is harzburglte D42 where the h g h t / h e a v y REE ratio in the bulk is markedly higher than in the chnopyroxene, thus requiring an additional repository for hght REE Such an additional carrier phase for La is also required for lherzollte I b / 5 whose chnopyroxene, judged from ItS modal abundance, can only account for about half of the bulk rock La. The presence of various types of contaminants is well documented for other suites of mantle xenohths (e.g. Baja Cahfornla [18], the southwestern U.S A. [19,20] or central Mongoha [12]) and does not require detailed discussion here. The REE patterns of amphibohtes (not plotted) and chnopyroxenes from the hydrous rocks are very similar. Like the bulk rocks all these minerals have

REE patterns with maxima at Ce Moreover, whereas Ia/157-amph and Ia/157-cpx are bagher in light REE than D8-amph and D8-cpx by a factor of almost two, both chnopyroxenes and both amphlbohtes have about identical heavy REE abundances (Table 3). Two fractions of phlogopxte from wehrhte Ia/157 have been separated. One fraction was only washed in H 2 0 after hand-picking whereas the second fraction was washed in 2n HC1 for - 10 minutes. The acid-leached separate does not contain measurable quantities of REE, whereas appreciable quantities of light REE were measured in the H20-treated fraction. We believe that this difference IS due to the presence in the unetched phlogoplte of a small amount ( < 0 5%) of apatlte dissolved during acid leaching Sc abundances are highest in chnopyroxene-rlch perldotites which is not surprising because in dry peridotites chnopyroxene has by far the highest Sc concentrations. Amphibole has also high Sc concentrations, but due to its low modal abundance is unimportant for the mass balance except for wehrlites Ia/157 and D8 (Table 1). Phlogoplte is negligible as a carried of Sc as well Its Sc content is lower than that of chnopyroxene by about one

REE ABUNDANCES IN PERIDOTITE CPX's AND A BASAN/TEFROM THE WESTE/FEL/WGERMANY

"Ia/157-~

200 - ~s 100 F~

50

lan57

D2tf0

~ 2o .,~

lO

lla/40

5

os8

cO

042

__

In#)/

L'a

0100 I

C'e

08

0095 I

P'r'Nd Stuff.

0090

r'bDyNlo

0081 I

TmY'bLu

Fig 2 Chondnte-normaJlzedREE patternsof chnopyroxenes from pendotztes and basarate n m around xenohth I a / 1 5 7 Filled squares = chnopyroxenes from hydrous pendotites, open squares = chnopyroxenes from dry pendotztes, filled dzamond = basamte

285

o

~o 0

-H

I

I

I

o

I

+1

o

o

o

o

+~+~ 0

+,

I

~

o

+1

~+1

~+1

~+1

O 0

~ 0

O 0

~ +~

~. +~

~ +~

8 o

s ?,~ ?,s $ ~ $

0

o

o

00

~o o

o

o

o

o 0

~o +l

o

Zo~o +1

~,S~ ~ o

o

+1 o

+1

r,S~S,

o r,,i

,-..-i

0

S, ~ S, a $ o

0

~n

~ ~o

0

0

+1

0

k,3 I

I

I0

~

~

°

~

s

+'

+1 0

0

~

$ - 0 S, o~ ~, 0o g

0

g~

~t, ~$ o~,~,~?,

~J

0

I

o~,g~,

0

g

~

o

8

0

-H

~

I

~

10o

I

I V

,n

100

I

I

I

I

o~ ~r

I

~

+l

~o I

0

$ 0: S, o ~

0

t

I

c~

0

~

+1~

+1 0

§

~0

$

I

~

+1

~+1

i

0

~

+1

~

~

Ng

.

~o o

286

order of magnitude and about twice as lugh as that m ohvane. In oliwnes and orthopyroxenes Hf and Ta are below the detecuon lmaits of INAA ( - 0.2 and 0.1 ppm, respectively). Hf abundances are lugh in chnopyroxenes from both wehrlites ( - 2 9 and 4.4 ppm for Ia/157 and D8) and much lower (< 1 ppm) in chnopyroxenes from all other peridotltes, independent of their REE patterns and concentrations. Appreoable amounts of Ta were detected in Ia/157-phlog and D8-amph (Table 3) This findmg suggests that hydrous minerals may be potentially important repositories for thts high-fieldstrength element in the mantle and has to be taken into conslderatmn in models deahng with global Ta partmomng Finally, Ba and Cs can easdy be accommodated into the phlogop~te structure and, indeed, have been detected only in this nuneral. (For concentratmns in clean ollvane, ortho- and chnopyroxene see [20].) 4 2

Isotopes

Our isotope data are hsted in Table 4 and are plotted in Fig. 3 m a 143Nd/144Nd vs. 87Sr/86Sr variation diagram. Bulk pendotlte xenoliths frequently contain contaminants lSOtopically different from carefully cleaned perldotite minerals (for detaded discussxon see [12] or [20]). In order to avoid the contribution from such externally derived phases we have preferentmUy analyzed acidleached chnopyroxenes only. Owing to the absence of severe Nd and Sr isotopic dxsequdlbnum between coexastmg chno- and orthopyroxene, amptubole and phlogoplte (see Table 4 and section 5.3) the uncontaminated bulk rock Nd and Sr isotopic composltmns wall be almost identical to the clmopyroxene values. The new results on mantle xenollths complement and extend our previous data base for Drelser Wedaer clmopyroxenes [6] which are also included m Fig 3. For companson the average of the "mantle array" as constructed from various hterature sources lS plotted. The data for chnopyroxenes from dry lherzohtes have tughly vanable 1sotopic composmons and plot between cjuv - + 8 and + 19 and 87Sr/S6Sr - 0.7019-0.7041. The data for Ib/K1, Ib/8, D1 (from [6]) and D58 fall in the field of rind-ocean ridge basalts (MORB), only 87Sr/86Sr of Ib/K1 1s slightly lower than any such basalt Ib/5-cpx, the sample with the lowest con-

n

i

i

i

i

i

$r AND Nd ISOTOPIC COhtPOSITION OF

PERIDOTITE EPX's AND BASALTIC VOLCANICS FROM THE h/ESTEIFEL / W 6ERMANY 05~6

2O

0

18 [ [~ cox s from dry therzohfes

0 5134

\ [] ",,

.~

05132

"~

05130

~IIm cpx s from hydrous perldohfes

16

~

14

[71 cpx s from dry horzburglfes ~[¢C] cpx from dry webstertte

12 klJ"~ "17

\\x\

10

rtl

\x,

x

~rJ

"-/ 2../

\ average of mantle array J

\

\

\\

8

Westetfe! volcamcs

6

\N

05128 NN

4

Xx

~

" 2

05126

0 I

I

I

I

I

I

x

i

zo2o

o 7030 o zo4o o zo5o 8zSr/SaSr Fig 3 143Nd/144Nd vs 87SrffSr diagram for chnopyroxenes from pendotlte xenohths and basanltes from the West Eifel Error bars are only given where they exceed the size of symbol m the plot The date are from ttus paper (for clinopyroxenes from Ib/5, D45, D58, D42, IIa/40, Dfl20, Ia/157, D8 and basanltes Ia/157 and U2-1) and [6] for chnopyroxenes from Ib/K1, Ib/8, D1, Ib/3, Ia/236 and Ia/249) The "West Elfel volcamcs" held was constructed from data of Kramers et al [211

centraUon of light REE has by far the htghest CJuv which sigmflcantly exceeds values measured for oceamc basalts The broadly negative covariatlon of Nd and Sr isotopes m mantle rocks indicates that 87Sr/a6Sr of Ib/5-cpx and D45-cpx is much higher than expected from their Nd isotopic compositions, thus calling for a comphcated petrogenetxc history It is tempting to correlate the pecuhar L a / N d enrlchrnent of D45-cpx (Fig. 2) with the process or event which also led to its tugh 87Sr/S6Sr. Similarly, Ib/5-cpx appears to have htgher La than esUmated from extrapolation of its E u / N d fracUonatlon (Fig. 2) and hence may have experienced some secondary enrlchrnent in large ton hthoptnle elements subsequent to an early partial melting episode. Chnopyroxenes separated from hydrous pendotites Dfl20, Ia/157 and D8 and from dry harzburgate D42 plot at Cjuv values between - 0 and + 4. The data for these and our previously

287

+1 -FI -H -H +l +1 -k -H +1 -H +1

+I~+I

e, V

v

V

V V V V 0

"

z z

o b-,

b V V V V V V V

~

z

2+~~+~7+~ z

8 ~

z

r~

m

z

~

,~

"~ Z

+ 1 ~

+1 +1 4-f +1 +1 +1 +1 -H +1 +1 +l

II

2

8 ,.0 ~o

z~ 0

~

m

e~

<

2~

~V

=

288 analysed samples plot, with the exception of D42, shghtly to the left of the "mantle array", a feature frequently observed for metasomatmed xenoliths from basaltic rocks associated with mtracontinental rifting and uphfting [22]. A sample of host basanite around nodule Ia/157 has higher 87Sr/ 86Sr and 143Nd/144Nd than the constituent minerals of the peridotlte and plots well wltlun the "mantle array" and wlthm the field of other West Eifel volcanlcs (Fig. 3). The Nd and Sr isotopic variation of the hydrous xenohths is subparallel to the "mantle array". For the dry hght REE-ennched rocks a major variation, although defined by three samples only (IIa/40, Ib/3, D42), is noted for 87Srffrsr which may relate to the secondary process which also caused the Sr isotopic increase of dry lherzohtes Ib/5 and D45. An interesting feature of our data for the Effel perldotites is the small Nd and Sr xsotoplc variatmn displayed by the xenoliths with relative hght over heavy REE enrichment as opposed to the large variab~hty shown by the light REE-depleted lherzohtes. At a first-order approxamation this suggests that 143Nd/144Nd and 87Sr/86Sr of the depleted lherzohtes evolved for considerable geologtc ume in response to their present Sm/Nd and Rb/Sr ratms; in contrast, the measured Rb/Sr and S m / N d ratios of the ennched pendotites mtght have been established only recently All dry peridotltes from Drelser Weiher equilibrated at around 1150°C [23-25]. This is explamed easiest in terms of these rocks coming from mantle lithologies being spatially closely associated m wtuch case the whole isotopic vanatmn shown m Fig. 3 must be found in a small vertical section of the mantle. 5. Discussion

5.1. Depleted nodules We define tlus group as that having chnopyroxenes wtth (La/Yb)c n < 1, thus restricting it to the dry lherzolites. From a major and compatibletrace-element point of view this group may be regarded as a series rangmg from prirmtlve mantle to rocks having expertenced moderate degrees of partial melting [7]. The incompatible light REE, however, only partaally fit such a simple model. The majonty of the dry lherzolites (and their chnopyroxenes) do not show the patterns of slg-

nlficant light over heavy REE depletion expected for partial melting residues. This requires caution m interpreting the Sm-Nd and also the Rb-Sr xsotope data m terms of a single-stage evolution. If a single-stage model were applicable, reformation on the age of the partml melting episode which produced depleted lherzohtes out of pnrmtwe mantle should have been recorded by both isotopic systems. These single-stage model ages are given m Table 5 Sm-Nd ages range from 1.1 to 2.9 Ga and Rb-Sr ages from 0.5 to 2.2 Ga. Only for one sample (Ib/K1) is there good agreement between both ISOtOpic systems with model ages of - 2 . 1 and - 2 . 2 Ga. This sample (bulk rock as well as chnopyroxene [7]) has a hght over heavy REE depleted pattern luke normal ocean ridge basalts. It gtves no re&cation of modlficauon (enrichment) subsequent to an ancient depletmn. Therefore the posslbthty cannot be &scounted that 2.1-2.2 Ga is the age of major depletion for parts of the mantle underlying the Elfel Model ages around 2 Ga are frequently encountered in Nd isotope studies of mantle spinel pendoUtes from diverse geographic locations [12,20,26]. In contrast, the REE pattern of lherzohte D45 shows enrichment of La over Ce or Nd, probably secondary m origin (Fig. 2) The same holds true, though less obviously, for Ib/5-cpx and, possibly, for D58-cpx (Fig 2) as well as the bulk of D1 [27]. We have suggested previously that a two-stage model fits the Sm-Nd Isotope data of the four dry pendotltes analyzed earher [6]. From a Sm-Nd isochron plot the last of these episodes, a hght REE depletion (partial melting) for lherzohtes combined with relative hght REE enrichment (equilibration with a fluid of hquld) for a harzburglte was xnferred to have occurred -0.5-0.6 Ga ago. However, when comblmng the old with our new data a -0.5-0.6 Ga event is no longer supported. All Sm-Nd data are plotted in Fig. 4 xn an isotope evolutmn diagram. Clear lsochron relationships are observed neither for all rocks nor for the group of dry pendotltes alone. The data points for dry peridotltes, except lherzohte D58 and harzburglte Ib/3, scatter along the suppled line the slope of which corresponds to an age of - 0.8 Ga. An £bulkearth of -- +6 at - 0 8 Ga ago is indicated by the line, thus clearly requiring a multistage evolution A two-stage model would still be consistent with the Nd isotopic data Nev-

289

ertheless, at present we do not wish to assign too much slgmficance to such a - 0 . 8 Ga age, given the lack of support from ages of crustal events m central Europe. Among the trace elements Nd, Sm, Sr and Rb the latter is by far the most incompatible one for an essentially dry mantle assemblage. Therefore, upon partial melting of the mantle it wall quanUtauvely enter the melt, provided that no K-bearing rmnerals remain m the sohd residue. For a mantle segment having experienced a depletion (partial melting) event, the 87Sr//86Sr of the residual dry mantle, in contrast to 143Nd//144Nd, should be essentially frozen subsequent to this event if melt extractmn was complete. Another partial melting event, therefore, cannot further fractlonate R b / S r . Hence, the memory of lnmal melting is not erased as long as lsotoplcally different Sr was not rmxed into the system via eqmhbratmn with llqmds or fluids. In contrast, multiple depletmn events can be recorded by the Sm-Nd system. Since any mantle melting results m an increase of the S m / N d ratm in the residue, single-stage model ages as those given m Table 5 are expected to be lower for Sm-Nd than for Rb-Sr which m fact is not the case. This indicates that the evoluUon of the dry

TABLE 5 Sm-Nd and Rb-Sr model ages for anhydrous lherzohtes

Ib/K1 ~ Ib/8 ~ D1 a Ib/5 D45 D58

Sm-Nd

Rb-Sr

2 09 2 90 1 45 1 10 1 63 3 20

2 21 1 79 1 66 1 43 0 53 1 93

Ga Ga Ga Ga Ga Ga

Ga Ga Ga Ga Ga Ga

a 147Sm//la4Nd ' CJuv' and 87Sr//g6Sr from [6], 87Rb//S6Sr assumed as 0 0005, 87Sr//S6Sr values from [6] increased by 0 0001 for mass spectrometer bias correction of L J-MS1 to L J-MS2 Sm-Nd ages calculated vath )~(147Sm): 6 54×10 -12, 147Sm// 144Nd = 0 1966 (bulk earth) and 143Nd/1'~Nd = 0 512636 (bulk earth) Rb-Sr ages calculated with A(a7Rb) = 1 42 × 10 -11, R b / S r = 0 03 (bulk earth) a n d 87Sr//g6sr = 0 7047 (bulk earth) Note that these ages were calculated from data for chnopyroxenes However, since orthopyroxene, ohvane and spinel carry only neghgjble hght REE concentratmns [17] S m / N d ratios of bulk lherzohtes wdl be almost identical to those of chnopyroxenes Sxmdar arguments apply to the R b / S r ages Mmerals other than chnopyroxene are neghglble for the Sr mass balance and Rb concentratmns are close to 0 m all minerals from these dry lherzohtes Hence, R b / S r rataos of chnopyroxenes should be close to those of the bulk pendotltes (surface contaminants excluded)

05136

05134

,

i

i

i

i

I

i

i

,

Sm-Nd ISOTOPIC SYSTEMATICS OF CLINOP Y R O X E N E S FROM SPINEL PERIDOTITES

20

/0

18

/

[ E I F E L / 5 E R M A N Y]

/

0

/

16

/

14

/ 05132

0

0

12

/ 1o

O/ /0

05130

8

- o86a - f - - /

/

0 dry pendohtes

/ /

05128 •

OO • 0

• j

0/ /0

~

• "hydrous"pendohtes

6 4 2

-ltbbulk

earth

05126

0 I

I

008

I

I

I

0 12

016

I

i

I

020

I

024

I

I

028

I

I

032

147Sm/144Nd Fig 4 Sm-Nd isotope evolution dtagram for chnopyroxenes from perldotlte xenohths Note that the S m / N d ratios of the bulk perldot~tes are dominated by chnopyroxene and amphtbole and therefore should be very close to those of cllnopyroxenes

290

lherzohtes from the Effel cannot satisfactorily be described by episodes of partial melting of mltrolly pnnutwe mantle (wxth bulk earth S m / N d and Rb/Sr ratios) Episodes of chenucal and isotopic modlficatmn, superimposed upon depletion events, may have to be considered here. Such modlficatmn caused by flmds or hqmds with generally much htgher 87Sr/86Sr, Nd/Sm, Rb/Sr, and lower 143Nd/X44Nd than those of the depleted lherzohtes, m principle, can lead to a decrease but also to an increase of model ages as well as decouple both isotope systems. In partxcular, the 87Sr/86Sr ratms of clmopyroxenes from D45 and Ib/5 which are much higher than expected from thmr t43Nd/ ~44Nd are suggestive of modfflcatmn by an enrichment event. If true, we m.~ghthope to see exndence for this modlflcatmn not only in the isotope record, Le. in elevated 87Sr/86Sr, but in elevated Sr concentrations of chnopyroxenes as well. In the inset of Fig. 5 we have plotted Nd vs. Sr concentratmns for chnopyroxenes from dry pendotltes from the Effel volcanlcs (this study) as well as from central Mongoha [12] Apparently, both data sets form one single strongly correlated trend if only the lherzohtes are considered A strong correlatmn between Sr and Nd was noted prexa-

J

ppm Nd

ously by McDonough and McCuUoch for spinel pendotlte xenohths from southern Austraha [28]. In general, the chnopyroxenes from the Mongolian state have higher Sr and Nd concentrations than those from the Eifel rocks. This is a consequence of the near-pnmltwe nature of the majority of spinel lherzohtes from the Tanat Depression, Mongoha, whereas refractory perldotltes are dominant m the states from the Effel. Unfortunately, we do not have Sr concentratxon data for chnopyroxenes from the most fertde lherzohtes from Drelser Welher, Le. Ib/8 and Ib/K1 from our earher work [6] The Sr-Nd covanatlon in Fig 5 may be regarded as a depletion trend resulting from variable degrees of partml melting Sr/Nd decreases from - 1 8 at 100 ppm Sr (a value typical for chnopyroxenes from prlrmtlve mantle xenohths from Mongoha [12]) to - 13 at 25 ppm Sr (typical for chnopyroxenes from moderately depleted lherzolites from Drelser Weflaer, Effel). Importantly, chnopyroxene data for D45 and Ib/5 (the chnopyroxenes which have high 87Sr/86Sr relative to their ~43Nd/~44Nd rauos and may have experienced a late enrichment of at least La and Ce) also follow this trend. Thus, decoupled behavmur of Nd and Sr is not in evidence here If their elevated Sr ~sotoplc compos~txons are the

i

i

1

i

i

Sr vs Nd concenfraf/ons /n chnopyroxenes from

30

•

spinel p e r t d o h f e xenohfhs n,1 20 0,.

cpx's from dry and hydrous perldohfes (Eifel / Germany)

i

•

40

60

8o

1oo

pp~Ndl. . . . . . . .

cpx's from dry and hydrous perldohfes (Tarlaf Oepresston /

6~ [

#prosr 12o

~11

J o/

~

i J

o% /

/

10 /

.~o D c9~-~

~/io

//~ f/~,°,

r'l 0

0

20

[

burg#eand /

Io,~px'~omth~zo,t.~ ,", . . . . . / 40

60

80

r'l 0

i

o

Ftg

i

~oo

i

I

200

i

I

300

L

i

I

400 ppm Sr

5 Nd vs Sr concentraUon dmgram for chnopyroxenes from pendoUte xenohths from the Elfel (this study) and central M o n g o h a [12] The reset is an expanmon of the low Sr-low N d scale for chnopyroxenes from dry perldotltes

291 consequence of a secondary modification, then chemical exchange must have completely removed any concentration grachents by re-equilibration, but more or less retained the indlxqdual isotopic signatures. Although we cannot constrain the scale of the isotopic heterogeneity in the dry spinel perldotlte mantle we consider such a mechamsm unhkely. The former presence in D45 and I b / 5 of a small amount of a Rb-rich phase hke phlogopite - - w h i c h was lost during a younger (maybe - 0 8 Gag) melting event--is suggested as an alternative. For parts of the lowermost crust under the Elfel we have previously estimated an igneous age of - 1.5 Ga [29]. Apparently, from the depleted peridotltes there is no evidence for such an event which could be interpreted in terms of crust-mantle differentiation Only the most primitive spinel lherzolite from Drelser Welher ( I b / 8 , [6,7]) could have followed a hypothetical single-stage evolution path hke the - 1 . 5 Ga old lower crustal granuhtes. We conclude that either (i) lower crust and upper mantle are not related by this differentiation episode, or (n) fluid and hqmd activity in the mantle as well as in the lowermost crust has led to enrichment in incompatible elements, thus partly erasing the isotope memory. 5 2 Enriched nodules

This group is defined by showing light REE enrichment in their clmopyroxenes, i.e. (La/Sm)c n and (La/Yb)~ n > 1. It comprises mlneraloglcally very diverse perldotites: hydrous harzburgites and lherzohtes, dry harzburgites, dry websterite and hydrous wehrlites. Different processes are 1Lkely to have been involved m their petrogenesls. For harzburgltes and lherzohtes the two-component model of Frey and Green [30] is apphcable according to which a partial melting event controis the major element composition and a subsequent enrichment e v e n t - - n o w frequently called mantle metasomatlsm--injects incompatible elements into these rocks without markedly affecting their major element composition The formation of amphibole in rocks hke Dfl20 may be linked to this event also. The petrogenesis of wehrlites D8 and I a / 1 5 7 and websterite IIa/40, however, is not easily interpreted in terms of melting residues of prlrmtlve mantle which should move from the lherzolite into the harzburglte field

The peridotites with relative hght over heavy REE ennchment show much smaller scatter in the N d vs. Sr isotope diagram (Fig 3) than do the depleted xenohths. The small variation in Nd isotopic composition together with the lack of correlation between S m / N d and 143Nd/144Nd (Fig. 4) suggests that the process which led to formation of the enriched suite, or incompatible element enrichment of a previously depleted suite, occurred at about the same time (i.e. within a couple of 108 years) and rather recently. Otherwise at least a rough positive correlation in the S m / N d isotope evohitlon diagram (Fig 4) would have to be expected, similar to that observed for the depleted lherzohtes. If, at the time of formation, the Sr and N d isotopic compositions of the enriched xenohth suite ever were within the mantle array some 200 Ma might have elapsed since that event. Considerable time between formation and transport to the earth's surface within the host basamte is required on phase petrologic grounds" These perldotltes have metamorphic textures. The lack of chemical zonatlon of imnerals argues for complete re-equtllbratlon of dry xenohths at - 1100-1200°C and of hydrous xenohths at - 9 0 0 - 1 0 5 0 ° C [23,24], thus ruling out the posslblhty that light REE enrichment could have been the result of their introduction by reaction with the host basamte For the hydrous xenohths equilibration with a fluid or hquld highly enriched in the light over heavy REE is also indicated by the REE patterns of their chnopyroxenes (Fig. 2). These typically show an inflection in the hght REE region leading to ( L a / D e ) c n < 1, the significance of which has been discussed previously [7]. In contrast, simple incorporation of REE from a basamtic to lomberlitlc liquid into pertdotltes with relative light over heavy REE depletion (like those of I b / 5 or D58) would result in light REE-enriched patterns slnular to that of Ia/157-host basanlte. In principle, the inflected REE patterns could result from (1) a metasomatic process (i.e. reaction of a fluid or a hquld, highly enriched in incompatible elements, with perldotite depleted in such elements), or (u) a magmatlc process (the xenohths being cumulates of an ultramafic magma, highly enriched m large ion hthophile elements). On the basis of REE patterns these origins cannot be readily distingmshed. However, if published c p x / l l q m d partition coeffioents [31] are apphcable, extremely

292 high relative hght REE enrichment in the parent hquid would be reqmred for a cumulative origin, similar to that found for klmberlites; this appears unlikely. In the plot of Sr vs Nd concentrations (Fig. 5) chnopyroxenes from hydrous xenohths show a large scatter. S r / N d ratios of clmopyroxenes and amphiboles from the Elfel as well as central Mongolia [12] are both higher and lower than S r / N d in clmopyroxenes from the hght REE-depleted dry lherzolites. Also, cllnopyroxenes from the hght REE-enrlched dry harzburedtes and websterlte from the Elfel are sigmficantly displaced from the correlation hne of chnopyroxenes from the dry lherzohtes (inset in Fig. 5). These features are suggestive of some decouphng of Sr and Nd during mantle enrichment processes To gain some insight into the nature of mantle enrichment processes we may look at elements which allow us to distinguish between element transport m silicate melt or a fluid phase Among the major elements T1 seems to behave rather immobile towards transport by H20-CO 2 fluids in the subcrustal hthosphere and therefore should not become markedly enriched in perldotltes altered by such fluid infiltration [32]. On the other hand, the light REE are very soluble In silicate melts as well as m CO2-rlch vapour [33]. In amphibolltes from metasomatlzed lherzohtes and harzburgltes from Drelser Weiher TIO2 contents rarely exceed 0 5%. The T102 contents of chnopyroxenes from such rocks are below -0.25% and are lndistingmshable from values for cllnopyroxenes from unmetasomatlzed lherzohtes [23]. This contrasts with the lugh light REE concentrations m chnopyroxenes from metasomauzed lherzolites and harzburgltes as compared to chnopyroxenes from dry lherzohtes. These features suggest that the enriched nodule suite from the Eifel acquired its high hght REE concentrations by a flmd-mduced metasomatlc process rather than by reacUon of prewously incompatible element-depleted perldotite with a sdlcate hquld If correct for T1, Zr and Hf might be expected to behave immobile towards transport by H20CO 2 fluids also Both elements are present in very low concentrations in mantle rocks only [27] and m these their abundance trends are only poorly known Zr and, by analogy, Hf are incompatible for clinopyroxenes (and other mantle minerals)

and thus wall readily enter mantle partial melts [34] The geochemical behavlour of both elements in oceanic and continental basalts is well documented. Empirically it has been found that for prlnutlve MORB chondrlte-normahzed Zr and Hf approach the same values as chondrlte-normahzed Sm [35,36] Therefore, m extended rare earth diagrams Zr and Hf can be plotted next to Sm. Since MORB are partial melts of mantle perldotite Sm, Zr and Hf must be nearly equally incompatible for minerals from mantle perldotlte. In Fig. 6 we have plotted Hf against Sm and Yb concentrations in chnopyroxenes from spinel perldotite xenohths from the Elfel, the southwestern U.S.A. and central Mongolia. In anhydrous perldotites clmopyroxenes are the main earners of Hf and Sm. In hydrous perldotltes amphiboles may be important hosts also and should have similar S m / H f ratios hke the coexisting chnopyroxenes (see data for minerals from wehrlite D8 in Table 3). Therefore, the data for chnopyroxenes wall be representative of the Sm-Hf ratio of bulk perldotltes also The Sm-Hf fields for mid-ocean ridge and Island arc basalts, for basamtes and nephehnltes from the Eafel as well as a line corresponding to a chondntlc S m / H f ratio are plotted for comparison in Fig. 6. The basalt data plot in a broad band along the chondrltlC (or bulk-earth) fractxonatIon line, thus indicating that the basalt formation processes do not severely fractlonate Sm and Hf. The Sm-Hf data for chnopyroxenes from dry spinel lherzohtes and harzburgates plot close to and in their majority shghtly below the chondrltlC hne and overlap with the low concentratlon end of the field for oceanic basalts. This Sm-Hf covarlatlon suggests that dry chnopyroxene-nch spinel lherzohtes upon partial melting could produce oceamc basalts whereas chnopyroxene-poor lherzohtes and maybe some harzburgltes might represent residues left after such basaltic melt extraction Apart from petrologtc arguments such a view is also supported by MORB-type Nd and Sr isotopic signatures of dry spinel lherzohtes worldwide. Chnopyroxenes from hydrous lherzohtes and harzburgltes from the Eifel have umformly low Hf concentrations (< 1 ppm) but very variable Sm concentrations ( - 0 . 7 - 7 . 5 ppm) and thus, with increasing Sm concentration, are increasingly &splaced from the chondrltlC fracttonatIon hne. To

293 I

1

I

!

I

'

'

'~

I

I"1 cpx's from dry /herzohtes/Etfel I~ cpx's from dry horzburgffes / Etfel • cpx's from hydrous pertdohfes/Effe/ cpx's from dry webstemfes/Effe/ RI cpx's from wehrhtes / Effel

ppmHf

i

bosonttes,

nephehm fes --~,~/~,~ ... [Elfel]//X~

5

typ,calanalyhca/

,,"" ,'-f

,,","RI [,' J

error

I

I

I

1

0 cpx's from dry Iherzohfes/ Tartar Depresston [Mongoho] • cpx from phlogopffe Iherzohte/ Tarlot Depression [Mongoho] 0 cpx's from dr), Iherzohtes/ southwestern USA

"~

+typJm/analyhcal erl'or

[]

4

.""

;~oceon

,'" ~ /

,,," ~oo/~/,/

/"

r,dge

.,slond~c

,,, OC

j

/

.

•

I:

-, ppm Sm

i

1

05 10 15 ~0 25 ppm Yb

Fig 6 Hf vs Sm and H f vs Yb concentrations in chnopyroxenes from pendotlte xenohths from the Fafel (Drelser Wether, Meerfelder Maar, Table 3 and unpublished results of H G S ), the southwestern U S A (San Carlos, Kalbourne Hole, unpublished results of H G S ), and central Mongolia (Tanat Depression [12]) These data have been collected over sgveral years and are of variable precision In the Sm-Hf diagram the field for ocean ridge basalts has been constructed from hterature sources [37-43] omlttang data for highly fractaonated rocks with very tugh Sm and H f concentrations N o attempt was made to dlstangmsh between ocean ridge and island arc basalts since there is almost complete overlap The field for basamtes and nephehmtes from the Eafel is only partly displayed and extends to values of up to - 13 p p m S m / 6 p p m Hf and - 10 p p m S m / 7 5 p p m H f (unpubhshed results of H G S ) The chondntlc fractaonatlon hne corresponds to a S m / H f ratio of 1 45 and was derived from chondntlc L u / H f = 0 240 [44] and chondntlc S m / L u = 6 06 [15]

account for tins &splacement either at given H f contents the Sm concentraUons in the chnopyroxenes could be anomalously high or at " n o r m a l " Sm contents their H f values could be too low F r o m an evaluation of the Y b - H f relaUons in these cllnopyroxenes (Fig 6) the first of these alternatives appears to be the correct one. For mantle minerals Yb is much less incompatible than Sm and is not considered to be markedly affected by metasomatlc processes. In general, Yb concentratxons are lowest m chnopyroxenes from harzburgltes and Inghest chnopyroxenes from p r i m m v e lherzolites. H f concentratxons vary fairly sympathetically with Yb and no differences are observed between chnopyroxenes from lherzohtes and harzburgites with hght over heavy R E E enrichment or depletion Tins variation is likely to

be the consequence of different degrees of partial melting in the mantle and has not been obscured by later mantle metasomausm. Hence, for the ampbabole-beanng and hght over heavy REE-enrtched lherzohtes and harzburgltes the Sm concentratxons of their clinopyroxenes are anomalously Ingh Tins seems to rule out the posslbihty that these pendotltes gained their high light R E E concentraUons by a reaction between prewously anhydrous, hght REE-depleted pendotxte and a basaluc melt, gwen the fact that such melts would have S m / H f around the chondntlc ratio. It is more hkely that this type of metasomauc alteratxon occurred by reaction of previously anhydrous lherzohte and h a r z b u r ~ t e with a fluid enriched m large ion hthopinle d e m e n t s Indeed, there is some evidence that in nature H f is rather immobile

294 towards transport in H20-dormnated fluids [45]. Judged from their high hght over heavy REE enrichment the hydrous lherzolites and harzburgltes could be potential sources of basanites and nephehnltes from the West Elf el. However, this idea is discounted by the different isotopic composition of hydrous nodules and basaltic rocks (Fig. 3) and even more by the Sm-Hf relationships (Fig. 6) Most hydrous lherzohtes and harzburgltes have S m / H f ratios > 8, the basaltic rocks from the West Elfel ratios of - 1 . 2 - 2 . 4 (unpubhshed results by H.G.S ). Therefore, such hydrous pendotites could at best only contribute components to the formation of basanltes and nephellnltes but not be their only source. The lack of un- or only shghtly fractionated basaltic rocks with high S m / H f in the Eifel and elsewhere also suggests that these hydrous pendotltes form only minor hthologtes of the upper mantle. Chnopyroxenes from wehrhtes have high Hf and Sm concentrations fairly close to the chondntic ratio and low Yb concentrations (Fig 6). This indicates an origin of the wehrhtes involving a slhcate melt Crystalhzatlon of these rocks in the mantle from magmas with high Cr, NI and light REE concentrations would be possible. The N d and Sr isotopic relations rule out the West Elfel volcamcs to be parental to these wehrhtes (Fig. 3). A wall-rock origin (reaction of perldotlte with basanlte hquId at the rims of a conduit or magma chamber m the mantle) which prewously has been suggested for other suites of mantle pendotltes [46,47] is a reasonable and more hkely alternatwe. The chnopyroxene from webstente I I a / 4 0 has Sm-Hf-Yb relationships and a REE pattern indistinguishable from those of chnopyroxenes from anhydrous harzburgltes. This webstente may have acquired its high modal pyroxene content by a process of mechanical enrichment in a harzburglte or cllnopyroxene-poor lherzohte portion of the mantle, similar as proposed for chnopyroxene-rlch spinel lherzohtes from Mongolia [12,48]

5 3 Small-scale tsotoptc equthbrtum / dtsequthbrtum and tts lmphcattons for the recent evolution of the subcontmental hthosphere Mineral to nuneral isotopic equillbrla/disequlhbna, in pnnclple, have implications for the most recent evolution of the pendotites and the

mantle part from which they derive. Cllno- and orthopyroxene from dry lherzolite D58 are m Nd isotopic equilibrium (Table 4, Fig 7). This IS not surprising in view of the high equlhbration temperature of this rock ( - 1150°C) since it has been shown previously that orthopyroxene-chnopyroxene Nd isotopic equlhbnum can be mamtalned at temperatures at least as low as - 1000°C in a dry mantle [12,19] In addition, D58-cpx and -opx are in Sr isotopic eqmhbnum as well Although such Sr behavlour is expected for pyroxenes with equlhbrated Nd isotopes [19], it has been found previously that orthopyroxenes can have higher 87Sr/ 865r than coextstlng chnopyroxenes in spite of their eqmhbrated 143Nd/144Nd [12,19]. This has been speculated to be due to diffusive contamination of orthopyroxene with a phase hawng both, high Sr content and high 87Sr/a6Sr or simply to contarmnatlon during chenucal separation of Sr in the laboratory. Other possibilities would include impurities in orthopyroxene such as silicate glass associated with fired inclusions which escaped recognition during handpicklng The only time constraints for D58-opx and -cpx are derived from the Sm-Nd isotope system Owing to the much higher S m / N d ratio of orthocompared to the coexisting clinopyroxene both minerals cannot have been frozen with respect to N d isotopic exchange for times longer than - 30 Ma, measurable differences in I43Nd/144Nd between the two rmnerals would have developed otherwise This suggests that the equilibration temperature calculated for D58 on the basis of major [24] or trace element [25] partitiomng between coexisting minerals relates to the recent rather than a fossil thermal state of the hthospheric part of the mantle under the Eifel. Nd isotopic equlhbnum is also observed for coexisting mmerals from wehrhtes Ia/157 and D8 The S m / N d ratios of chnopyroxene, amphibole and phlogoplte are low, thus rendering 143Nd/ 144Nd rather insensitive towards recording the closure time of minerals with respect to Nd isotopic exchange. For Ia/157-phlog and -cpx this time resolution is as poor as - 200 Ma. Owing to the high R b / S r ratio ot~ phlogopite the Rb-Sr system prowdes a more sensitive tool to disclose the recent evolution of wehrhte I a / 1 5 7 and sxrmlar perldotltes The measured chfferences In 87Sr/86Sr between phlogoplte and amphibole or

295 i

05134

i

ISOTOPIC RELATIONSHIPS BE TWEEN COEXISTIN6

opx

[~ cpx

MINERA L S 05132

o 704

,

II,

•

,

• phlog amph •

L.

05130

0 703 ~,.t.,3 oa

cpx I ~ opx

0 702

!

0

'

' o 4"3 a~b/a6Sr

05128

phlog

amph l ~ cpx cpx "remph

[] D58 OD8 • Ia/157

05126 I

0

'

010

'

i

020

I

030

i

I

040

t4~la/7*4Nd

Fig 7 Sm-Nd and Rb-Sr isotopic composmonof coexistingrmnerals from Drelser Welher pendotltes chnopyroxene could have been produced by in situ decay of 87Rb in phlogopite within - 2 Ma if all minerals behaved as closed systems towards Sr exchange This age by far predates the age of eruption of the mantle xenohth bearing Dreiser Weiher tufts which is < 0.1 Ma [49]. Hence, the following possibilities remain to explmn the isotopic disequihbnum. (1) Phlogopite formed metasomatically after chnopyroxene and amphibole crystallization from a phase with 87Sr/86Sr higher than that of the other minerals (11) Phlogoplte was affected by post-eruption alteration and contalmnation, for instance with percolating groundwaters with high 87Sr/86Sr ( 0.73 if derived from Devoman sediments [29]). (ni) The Sr isotopic composition was modified by selective reaction with a melt or fluid at depth; this imght have happened during ascent of the xenohth to the earth's surface by the host basamte or at an earher stage in the mantle by a magma unrelated to the host basamte. (iv) The isotopic chsequihbnum related to cooling of the mantle segment from which wehrhte L a / 1 5 7 derives down to a temperature where Sr isotopic exchange between coexisting minerals becomes neghglble.

Possiblhty (1) appears unlikely because petrographic evidence for late formation of phlogopite is lacking. Moreover, identical Nd isotopic compositions in chnopyroxene, amphibole and phlogoplte are also suggestive of co-preopltatlon of all nunerals or equlhbration after formation of hydrous minerals. Explanation (11) does not appear very likely either since the high Sr content of phlogopite (238 ppm) would tend to buffer the original 87Sr/86Sr. Process (hi) may be more likely since for the wehrhtes we favour an ongln by reaction of wall-rock with a slhcate melt in deepseated conduits or magma chambers (see section 5.2). Nevertheless, the most straightforward possibihty is (iv). In this case, the - 2 Ma age calculated above would represent a mlmmum age for the isotopic disequlhbnum since cllnopyroxene and phlogopite cannot be expected to close with respect to Sr diffusion at a defined temperature but rather over a temperature interval. From experimental Sr diffusion data in chnopyroxene [50] it has been calculated that in a slowly cooling mantle diaplr diopside will effectively close for Sr diffusion at around - 9 0 0 ° C . For I a / 1 5 7 an equilibration temperature of - 1 0 0 0 ° C has been estimated previously from Sc partitioning between ohvme and chnopyroxene [25]. Both temperatures

296

can be regarded as reasonably close Hence, we may assume that I a / 1 5 7 was indeed residing in the mantle within the closure temperature interval where Sr diffusion, at least between cllnopyroxene and amphibole, becomes neghgible, in accordance with the experimental data [50]. Such cooling could be related to diaplrism under the Elfel for which there is textural and chetmcal evidence from several pendotlte xenohths suites [51]. Alternatively, if process (111) is responsible for the Sr iSOtOpiC disequdlbrlum, metasomatic modification of phlogoplte must have occurred at a temperature low enough for cllnopyroxene and amphibole to behave as closed systems, l.e phlogoplte must close at much lower temperature than cllnopyroxene or amphibole. We note that m some granuhtes from the lowermost crust under the Elfel there is evidence for isotopic modification of amphibole through metasomatlsm whereas in the same rocks plagxoclase, chnopyroxene and garnet behaved as closed systems with respect to Nd isotopic exchange [29]

6. Summary and conclusions Mantle pendotite xenohths from the Elfel in their clinopyroxenes show a systematic dependence of REE patterns upon bulk chemistry or mineralogy. Chnopyroxenes from dry lherzohtes (bulk rock CaO > - 1.5%) are hght over heavy REE depleted or have flat REE patterns. Cllnopyroxenes from dry harzburgltes and a dry websterlte are significantly, cllnopyroxenes from amphibole- a n d / o r phlogopite-beanng xenohths are highly hght over heavy REE enriched. With respect to Nd and Sr isotopic compositions the dry harzburgltes and hydrous nodules plot witlun a relatively small field between 143Nd/ I 4 4 N d - 0.51256-0.51273 and 8 7 S r / 8 6 S r 0.7044-0 7032 Most of these data plot to the left of and, for the hydrous pendotites, parallel to the "mantle array". The dry lherzohtes are xsotopically much more diverse and overlap with the hght REE-enrlched rocks in their 87Sr/86Sr but, in general, have higher i43Nd/144Nd in excess of 0 5130. Vanable Nd and Sr model ages for these depleted lherzohtes indicate that their petrogenesis cannot be satisfactorily descnbed by one single partial melting event, possibly - 2 Ga ago. Subse-

quently (a) process(es) of shght ennchment in large 1on hthophile elements must have affected at least some dry lherzohtes whereas others nught have been involved m a second partial melting episode. For the dry harzburgltes and hydrous harzburgltes and lherzohtes this secondary enrichment event has proceeded far enough to completely modify the isotopic composition and erase any pre-ennchment memory. There is no unequivocal ewdence for involvement of the light REE-depleted lherzohtes in a lower crust formation event which, from the study of granuhte xenohths, was estimated to have occurred - 1 . 5 Ga ago [29]. Therefore, either such evidence is obscured by the effects of metasomatlc modification in the mantle or, less hkely, this section of the hthosphenc mantle was part of the convecting mantle system at that time and was not in ItS present locale. Hf-Sm-Yb abundances in chnopyroxenes suggest a formation of the hydrous harzburgjtes and lherzohtes by reaction of a fluid highly ennched in large ion hthophile elements with mantle previously depleted in such elements. Wehrhtes represent either wall-rocks metasomatlzed by reaction with slhcate liquid (preferred lnterpretataon) or are cumulates of a magma with high concentrations of incompatible elements. Nd and Sr isotopes and S m / H f fractionations suggest that the depleted lherzohtes are genetically related to oceamc basalts. Mantle matenal such hke the hydrous lherzohtes and harzburgttes does not represent a potential source for the basanltes and nephehmtes in the Elfel. Complete re-equlhbration after metasomatlc modification has chermcally homogemzed single tmneral grams and lSOtOpically homogenized coexastlng mineral species, thus requlnng that considerable time has elapsed between ennchment and transport of the xenohths to the earth's surface. The Nd isotopic smailarlty between ennched perldotltes and West Eifel basanttes suggests, however, that metasomatism and volcamsm are genetically hnked by having a common ongln. The scale of metasomatism cannot be disclosed through xenohth stuches. The lack "of mmeral chemical zonation within and of chemacal gradients across indivadual xenohths suggest that metasomatlsm is a phenomenon on a scale much larger than xenohth size.

297

Acknowledgements Some of the perldotltes were londly donated by J F r e c h e n , U m v . B o n n . H . G S. t h a n k s W . H e r r and U Herpers for access to the NAA facdltles of t h e I n s t l t U t filr K e r n c h e n u e , U n i v K 6 1 n , as well as t h e s t a f f o f t h e K F A J i i h c h w h e r e n e u t r o n lrradxauons were performed. Helpful discussions with H.A. Seek are gratefully acknowledged. This m a n u s c r i p t b e n e f i t e d f r o m t h e c o n s t r u c t i v e rewews by F.A Frey, W.F. McDonough, F Begem a n n a n d M . M e n z l e s . S p e c i a l t h a n k s t o J.J. M a h o n e y , C. M a c I s a a c a n d S R o u z e f o r h e l p during the analyucal stages of this project. This study was supported by grants from the DFG to H.A. Seek and NSF to J D. Macdougall and G . W L S I O - I G E L C o n t r i b u t i o n N o . 030.

References 1 K Fuchs, K von Gehlen, H Malzer, H Murawslo and A Semmel, Epdogue Mode and mechamsm of Rhemsh Plateau uphft, m Plateau Uphft, the Rhemsh Sh_teld--a Case History, K Fuchs et al, eds, pp 405-411, SpnngerVedag, Berlm, 1983 2 H -U Schmincke, V Lorenz and H A Seek, The Quaternary Eafel volcanic field, m Plateau Uphft, the Rhemsh Slueld - - A Case I--hstory, K Fuchs et al, eds, pp 139-151, Spnnger-Vedag, Berhn, 1983 3 H - G Huckenholz, Tertiary volcamsm of the Hochelfel area, m Plateau Uphft, the Rhemsh Shleld--a Case History, K Fuchs et al, eds, pp 121-128, Spnnger-Vedag, Berhn, 1983 4 H Mertes and H - U Schrmncke, Marie potasslc lavas of the Quaternary West Effel volcamc field, I Major and trace elements, Contnb Maneral Petrol 89, 330-345, 1985 5 A Duda and H - U Schnuncke, Quaternary basamtes, mehlmte nephehmtes and tephntes from the Laacher See area (Germany), Neues Jahrb Mineral, Abh. 132, 1-33, 1978 6 H - G Stosch, R W Carlson and G W Lugmmr, Eplsochc mantle dffferentmtmn Nd and Sr ~sotoplc evidence, Earth Planet Scs Lett 47, 263-271, 1980 7 H -G Stosch and H A Seck, Geochemistry and mineralogy of two spinel pendotlte suites from Dresser Welher, West Germany, Geoclum Cosmocbam Acta 44, 457-470, 1980 8 F A Frey and M Pnnz, Ultramafic mclusmns from San Carlos, Arizona petrologic and geochemical data beanng on their petrogenesis, Earth Planet Scl Lett 38, 129-176, 1978 9 J C Mercxer and A Nicolas, Textures and fabrics of ultramafic pendotstes as illustrated by xenohths from basalts, J Petrol 16, 454-487, 1975 10 B Harte, Rock nomenclature w~th particular relation to deformation and recrystalhzatmn textures in ohwne-bearmg xenohths, J Geol 85, 279-288, 1977

11 A Streckelsen, Classlficat~on and nomenclature of plutomc rocks, Geol Rundsch 63, 773-786, 1974 12 H - G Stosch, G W Lugmazr and V I Kovalenko, Spinel pendotlte xenohths from the Tartar Depresslon/Mongoha, II Geochetmstry and Nd and Sr lSotoplc composltton and their lmphcatlons for the evolution of the subconunental hthosphere, Geoclum Cosmochtm Acta 50, m press, 1986 13 R W Carlson, G W Lugmarr and J D Macdougall, Columbia Raver volcamsm the quesuon of mantle heterogeneity or crustal contanunatlon, Geoclnm Cosmoclum Acta 45, 2483-2499, 1981 14 R D Shannon, Revised effective ionic radn and systematic studies of mteratonnc distances in hahdes and chalcogemdes, Acta Crystallogr, Sect A 32, 751-767, 1976 15 W V Boynton, Cosmochemlstry of the rare earth elements meteorxte studies, m Rare Earth Element geochemistry, P Henderson, ed, pp 63-114, Elsewer, Amsterdam, 1984 16 F A Frey, Rare earth element abundances m upper mantle rocks, in Rare Earth Element Geochenustry, P Henderson, ed, pp 153-203, Elsevaer, Amsterdam, 1984 17 H - G Stosch, Rare earth element partltlomng between minerals from anhydrous spinel pendotlte xenohths, Geocbam Cosmoclum Acta 46, 793-811, 1982 18 A Basu, Geochermstry of ultramafic xenohths from San Qumtm, Baja Cahforma, m Proc 2nd Int Kamberhte Conf, Vol 2, F R Boyd and H.AO Meyer, eds, pp 391-399, Amencan Geophysical Umon, 1979 19 E Jagoutz, R W Carlson and G W Lugmatr, Equihbrated Nd-uneqmhbrated Sr isotopes m mantle xenohths, Nature 286, 708-710, 1980 20 A Zmdler and E Jagoutz, Mantle cryptology, Geoctum Cosmoclum Acta, submitted, 1986 21 J D Kramers, P J Betton, R A Chff, H A Seek and T Sachtleben, Sr and Nd isotopic variations m volcamc rocks from the West Eafel and thesr slgmficance (abstract), Fortschr Maner, Beth 59, 246-247, 1981 22 M Menzles, Mantle ultramafic xenoliths in alkahne magmas evidence for mantle heterogeneity modified by magmatlc act~wty, m Continental Basalts and Mantle Xenohths, C J Hawkesworth and M J Norry, eds, pp 92-110, Sluva Pubhshmg, Nantw~ch, 1983 23 T Sachtleben, Petro]ogte ultrabassscher Auswurfhnge aus der Westelfel, 201 p p , Ph D Dissertation, Umv K61n, 1980 24 T Sachtleben and H A Seck, Chemical control of Al-solublhty m orthopyroxene and its imphcatlons on pyroxene geothermometry, Contnb Mineral Petrol 78, 157-165, 1981 25 H - G Stosch, Sc, Cr, Co and N1 partitmmng between rmnerals from spinel pendotlte xenohths, Contnb Maneral Petrol 78, 166-174, 1981 26 M Menzles, P Kempton and M Dungan, Interactmn of continental hthosphere and asthenosphenc melts below the Gerommo volcamc field, Arizona, U S A , J Petrol 26, 663-693, 1985 27 E Jagoutz, H Palme, H Baddenhansen, K Blum, M Cendales, G Drelbus, B Spettel, V Lorenz and H W~inke, The abundances of major, minor and trace elements m the earth's mantle as derived from pnrmt~ve ultramafsc nodules, Proc 10th Lunar Planet Scl Conf, pp 2031-2050

298 28 W F McDonough and M T McCulloch, Geochenucal and isotopic systemalacs of spinel lherzohtes from SE Australm (abstract), EOS 66, 1110, 1985 29 H - G Stosch and G W Lugmmr, Evolutmn of the lower continental crust granuhte facies xenohths from the Eafel, West Germany, Nature 311, 368-370, 1984 30 F A Frey and D H Green, The nuneralogy, geochemistry and ongm of lherzohte mclusmns m V~ctonan basamtes, Geocham Cosmochtm Acta 38, 1023-1059, 1974 31 A J Irving and F A Frey, Trace element abundances m megacrysts and their host basalts constraants on part~taon coefficients and megacryst genesis, Geoclum Cosmocbam Acta 48, 1201-1221, 1984 32 M E Schneider and D H Eggler, Fltnds m eqmhbnum w~th pendotlte rmnerals ~mphcataons for mantle metasomat~sm, Geoc~m Cosmochxm Acta 50, 711-724, 1986 33 R F Wendlandt and W J Hamson, Rare earth partttmmng between lmmasclble carbonate and slhcate hqmds and CO2 vapor results and lmphcatmns for the formatmn of hght rare earth-emached rocks, Contnb Maneral Petrol 69, 409-419, 1979 34 T Duma and I S McCallum, The partatmmng of Zr and Nb between dmps~de and melts m the system dmpslde-alblte-anortlute, Geocham Cosmoclum Acta 46, 623-629, 1982 35 S-S Sun, R W Nesbltt and A Ya Sharaslon, Geochemical charactenstlcs of mid-ocean ridge basalts, Earth Planet Scl Lett 44, 119-138, 1979 36 L Bnqueu, H Bouganlt and J L Joron, Quantfflcataon of Nb, Ta, T1 and V anomalies m magmas associated wgh subductaon zones petrogenettc lmphcatlons, Earth Planet Scl Lett 68, 297-308, 1984 37 C H Langmmr, J F Bender, A E Bence, G N Hanson and S R Taylor, Petrogenesls of basalts from the FAMOUS area Mad-Atlantic Radge, Earth Planet Scl Lett 36, 133-156, 1977 38 D A Wood, Major and trace element vanauons m the TerUary lavas of eastern Iceland and their slgmficance with respect to the Iceland geochermcal anomaly, J Petrol 19, 393-436, 1978 39 F A Frey, J S Dickey, Jr, G Thompson, W F Bryan and H L Dawes, Evadence for heterogeneous pnmary MORB and mantle sources, NW Indian Ocean, Contnb Mineral Petrol 74, 387-402, 1980 40 D A Clague, F A Frey, G Thompson and S Randge, Manor and trace element geochetmstry of voleamc rocks

41

42

43

44

45

46

47

48

49

50

51

dredged from the Galapagos spreading center role of crystal fractlonatlon and mantle heterogeneity, J Geophys Res 86, 9469-9482, 1981 W M Wlute and J Patchett, Hf-Nd-Sr ~sotopes and mcompatible element abundances m xsland arcs lmphcatlons for magma ongms and crust-mantle evolution, Earth Planet Scl Lett 67, 167-185, 1984 Basalttc Volcamsm Study Project, Basallac Volcamsm on the Terrestrial Planets, Chapter 1 2 7, pp 193-213, Pergamon Press, New York, N Y , 1981 M Rautenschlem, G A Jenner, J Hertogen, A W Hofmann, R Kemch, H - U Schmmcke and W M Wbate, Isotopic and trace element compos~Uon of volcamc glasses from the Akaka Canyon, Cyprus lmphcatmns for the ongm of the Troodos optuohte, Earth Planet Sc~ Lett 75, 369-383, 1985 P J Patchett, Importance of the Lu-Hf isotopic system an studies of planetary chronology and chermcal evolution, Geoctum Cosmochtm Acta 47, 81-91, 1983 A J Erlank, H S Smxth, J W Marchant, M P Cardoso and L H Ahrens, Hafmum, m Handbook of Geochemistry, Vol 11/5, K H Wedepohl, ed, Spnnger-Verlag, Berhn, 1978 A J Irving, Petrology and geochemistry of composite ultramafic xenohths m alkahc basalts and ~mphcatlons for magmat~c processes w~ttun the mantle, Am J Sel 280-A, 389-426, 1980 M F Roden, F A Frey and D M Francis, An example of consequent mantle metasomattsm In pendotate inclusions from Numvak Island, Alaska, J Petrol 25, 546-577, 1984 S Press, G Wltt, H A Seck, D Ionov and V I Kovalenko, Spinel pendotlte xenohths from the Tanat Depressmn, Mongoha, I Major element chemistry and rmneralogy of a pnwatwe mantle xenohth state, Geocham Cosmoclum Acta 50, m press, 1986 G Buchel and V Lorenz, Zum Alter des Maarvulkamsmus der Westeffel, Neues Jahrb Geol Pahlontol, Abh 163, 1-22, 1982 M Sneennger, S R Hart and N Shamlzu, Strontmm and samanum diffusion m dmpslde, Geochtm Cosmoclaxm Acta 48, 1589-1608, 1984 G Wltt, Die junge Evolutmn des Erdmantels unter der rhemlschen Vulkanprovmz, abgeleltet aus der Temperaturgeschxchte, dem Gefuge und der Geochemle perldoUtlscher Mantelxenohthe, P h D Dlssertatmn, Umv K61n, 1985