A Nd and Sr isotopic study of the Trinity peridotite; implications for mantle evolution

Earth and Planetary Science Letters, 68 (1984) 361-378

361

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

[61

A Nd and Sr isotopic study of the Trinity peridotite; implications for mantle evolution S.B. Jacobsen *, J.E. Quick ** and G.J. Wasserburg The Lunatic Asylum of the Charles Arms Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 (U.S.A.)

Received September 16, 1983 Revised version received February 27, 1984

Field evidence indicates that the Trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center. A Sm-Nd mineral isochron for a plagioclase lherzolite yields an age, T = 427 _+ 32 Ma and initial {Nd + 10.4 :l: 0.4 which is distinctly higher than that expected for typical depleted mantle at this time. This age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzolite P-T field. This time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The Sm-Nd model age of the plagioclase lherzolite total rock is TcNduR= 3.4 AE. This suggests that the Trinity peridotite was derived from a mantle that was depleted rather early in earth history. The peridotite contains many generations of pyroxenite dikes and some microgabbro dikes. We report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite. A microgabbro dike yields a Sm-Nd mineral isochron age of T = 435 _+21 Ma and eNd = +6.7 :[: 0.3. A pyroxenite dike yields an initial lENd +7.3 4- 0.4, The initial ~Nd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite--demonstrating that they are not genetically related. Rb-Sr data do not give any coherent pattern. However, some bounds can be put on initial Sr values of ~sr ~< - 2 1 for the plagioclase lherzolite and Csr < - 8.7 for the microgabbro dike. It is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone. Major and trace elements and Sm-Nd isotopic data indicate a multiple stage history for the Trinity peridotite; a small melt fraction was extracted from an undepleted source - 3.4 AE or more ago to produce the proto-lherzolite; a large fraction of melt ( - 12 to 23%) was extracted from the proto-lherzolite to produce the present rock; the lherzolite was then crosscut by dikes from average depleted mantle - 0.44 AE ago. The data are compatible with the depleted mantle source being formed very early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid-ocean ridge basalts as well as the oceanic lithosphere which they intrude. =

=

1. Introduction

more reservoirs with distinctive isotopic character-

N d , Sr a n d P b i s o t o p i c s t u d i e s o f b a s a l t s h a v e given definite evidence for a chemical and isotopically h e t e r o g e n e o u s m a n t l e c o m p o s e d of two or

istics. It s e e m s c l e a r t h a t t h e s e h e t e r o g e n e i t i e s c a n yield important information about the chemical and dynamic evolution of the earth's mantle and c r u s t , as w e l l as s o m e a s p e c t s o f m a n t l e s t r u c t u r e . In o r d e r to establish a f r a m e w o r k for this dis-

Division Contribution No. 3824 (427). * Department of Geological Sciences, Harvard University, Cambridge, MA 02138, U.S.A. ** U.S. Geological Survey, APO, New York 09697, U.S.A. 0012-821x/84/$03.00

© 1984 Elsevier Science Publishers B.V.

c u s s i o n , w e d e f i n e s o m e b a s i c t e r m s . T h e undep l e t e d mantle ( o f t e n a l s o c a l l e d p r i m i t i v e ) is t h e mantle that was formed just after the separation of the m a n t l e and the core. Early work on S m / N d

362 isotope systematics indicated that continental flood basalts and Archean crustal rocks have (NO values close to zero, which is compatible with an ultimate origin from an undepleted mantle [1]. Although, recent revisions of the model parameters for Sm-Nd evolution of the bulk earth do not support this view for many Archean crustal rocks [2]; continental flood basalts still cluster around ENd 0 and may be derived from a primitive reservoir or from appropriate mixtures of crust and mantle that may approximate the undepleted mantle. The depleted mantle is the source material for mid-ocean ridge basalts (MORB) which at present have ~Nd = q- 10 and relatively low concentrations of incompatible elements compared to those of continental or ocean island basalts of similar major element composition. Many investigators agree that the depleted mantle forms the upper mantle and was the main source of incompatible elements that comprise the continental crust, however, this is still one of the most actively debated topics in geosciences today. Various geochemical and geophysical data suggest that the bulk mantle in the source regions of basalts is a lherzolite [65]; however, a variety of opinions exist as to how enriched this rock is in CaO, A1203, REE elements and other elements of petrogenetic importance. Direct information on the source regions of basalt may thus be obtained by studying the three principal modes of occurrence of lherzolite: (1) Alpine peridotites and ophiolites, (2) nodules in kimberlite pipes, and (3) nodules in alkali basalts. Lherzolite samples display a range in composition and may represent potential source material (fertile lherzolite) for basalts or may be residual material (sterile lherzolite) formed by removal of a basaltic component during partial melting. To show that a lherzolite is fertile and may represent the source material of a certain class of basalts, it is necessary to show that it has both the appropriate bulk and trace element chemistry in addition to displaying the proper patterns of radiogenic isotope ratios. Most recent studies of mantle samples have concentrated on nodules in alkali basalts and kimberlites mainly because of the highly altered nature of most Alpine peridotites. However, study of a well-preserved Alpine peridotite has the ad=

vantage over nodule studies in that field relations between the various lithologies can be well preserved, and this may aid in interpreting the processes that affected the peridotites since the time when it was derived from a primitive mantle. The Trinity peridotite contains plagioclase lherzolite that is virtually unaltered and that has a bulk composition similar to that estimated for the upper depleted mantle by Maaloe and Aoki [3] on the basis of spinel lherzolite nodules in alkali basalts. We have therefore carried out a Nd and Sr isotopic study together with major elements on a few very fresh samples of the Trinity peridotite to investigate how it relates to the chemistry and evolution of the earth's upper mantle.

2. Geologic setting of the Trinity peridotite The Trinity peridotite is exposed over an area of about 1600 km 2 in the eastern Klamath Mountains of northern California (Fig. 1). Regional mapping [4-6] and geophysical data [7,8] demonstrated that the Trinity peridotite is an easterly dipping sheet that is sandwiched between the rocks of the underlying Central Metamorphic Belt and the overlying eastern Klamath Belt. Lindsley-Griffin [9] assigned the Trinity peridotite to the ultramafic portion of an Ordovician ophiolite on the basis of field mapping. However, in an intensive field and petrographic study, Quick [10,11,16] showed that the Trinity peridotite is composed of a greater diversity of ultramafic lithologies than are commonly found in ophiolites. Harzburgite and lherzolite are the most abundant lithologies comprising 60-70% of the peridotite. The remaining rocks are 10-15% plagioclase lherzolite, 15-20% dunite, and 1-2% clinopyroxene-rich dikes that range in composition from clinopyroxenite to websterite to gabbro. Furthermore, in contrast with other ophiolites, the gabbros associated with the Trinity represent a long and complex igneous history. Lindsley-Griffin [9] identified gabbro deposited on the Trinity peridotite and a younger gabbro that intrudes both the older gabbro and peridotite. Goullaud [12] and Quick [11] identified other discrete gabbro plutons that intrude the peridotite. The sheeted dike corn-

363

OREGON CALIFORNIA

l_ , 50km J ~ []

,,:.:,~

[]

I 0

COVER ] GABBRO L. Pz SEDIMENTS I DUNITE HB.DIORITE [ ] UNDIFFERENTIATED PERIDOTITE

2km J

SAMPLE LOCATION

Fig. 1. Geologic map (after Quick [16]) of approximately 150 kmz near the northeast margin of the Trinity peridotite showing distribution of cover (Quaternary sediments), lower Paleozoic sediments, hornblende diorite, gabbro, dunite and undifferentiated peridotite (harzburgite, lherzolite, plagioclase lherzolite and small dunite bodies). Sample locations for the samples used in this study are shown on the map. Plagioclase lherzolites 1 and 2 are from locations 1 and 2, respectively, on the map. The clinopyroxene dike (sample 3) is from location 3. The microgabbro (sample 4) dike is from a very large peridotite boulder at location 4. The source of this boulder is location 5 on the map.

p l e x a b o v e the T r i n i t y p e r i d o t i t e is p o o r l y develo p e d a n d the entire mafic section is only a b o u t 2 k m thick [9] or a b o u t a third of the thickness of the oceanic crust. T h e c o n t a c t b e t w e e n the mafic a n d u l t r a m a f i c sections is n o t e x p o s e d a n d is, therefore, n o t known. R a t h e r t h a n pelagic cherts or shales, the T r i n i t y " o p h i o l i t e " is c a p p e d b y i m m a t u r e v o l c a n o g e n i c graywackes a n d m u d s t o n e i n t e r b e d d e d with mafic volcanic rocks a n d massive s e d i m e n t a r y breccias [9]. A l t h o u g h the T r i n i t y p e r i d o t i t e a p p e a r s to have reached the b a s e of the

crust in an oceanic setting, the details of the rock associations a n d structures of the T r i n i t y are thus a t y p i c a l of classic ophiolites. O n e p o s s i b i l i t y is t h a t it m a y have b e e n w e l d e d to the lower p a r t of the oceanic l i t h o s p h e r e at a b a c k - a r c s p r e a d i n g c e n t e r in the vicinity of an island arc. Such an e n v i r o n m e n t c o u l d a c c o u n t for the coarse clastic a n d volcanogenic s e d i m e n t s that overlie the T r i n ity " o p h i o l i t e " , a n d volcanic, volcanic-clastic a n d intrusive rocks with calcalkaline affinities are f o u n d t h r o u g h o u t the K l a m a t h M o u n t a i n s [13-15].

364

3. Samples Samples were collected during the summer of 1978 from an area mapped in detail by Quick [11,16-18]. Although most of the Trinity peridotite is pervasively serpentinized, some outcrops show less than 5% serpentinization. Plagioclase lherzolite samples, showing no signs of serpentinization or weathering, were collected from outcrops of relatively unserpentinized peridotite. Samples of microgabbro and clinopyroxene-rich dikes were collected from somewhat more serpentinized outcrops, but show little evidence of alteration. Samples 1, 2 and 4 correspond to samples 9W20, 8W67 and 8W100 of Quick [11], respectively. Plagioclase lherzolite samples 1 and 2, are porphyroclastic intergrowths [19] of large (0.5-8 mm) olivine, orthorpyroxene and clinopyroxene grains set in a matrix of smaller olivine, orthopyroxene, clinopyroxene, spinel, plagioclase and amphibole. Clinopyroxene occurs in isolated clusters. Small pyroxene, amphibole and plagioclase grains Occurs as interstitial grains with cuspate contacts suggesting that they formed by crystallization of trapped melt following minor anatexis [11]. Plagioclase also forms rims on embayed spinel grains. Intragranular deformation features such as polygonalization of grains and development of preferred orientations of minerals suggest plastic deformation and re-crystallization. The rocks as a whole show less than 5% alteration to serpentine and associated secondary minerals. Pyroxenes are essentially pristine and olivine is only incipiently altered to serpentine, which is confined to narrow fractures within and along edges of olivine grains. Plagioclase is about 10-20% altered to clinozoisite. Sample 3 is a 6 cm thick clinopyroxene-rich dike. Interstices between large (1-3 cm) clinopyroxene grains are filled by orthopyroxene, olivine, spinel and plagioclase. The texture appears to be igneous but is overprinted by a recrystallization that polygonallized grains. The modal mineralogy is extremely variable in outcrop and locally the rock ranges in composition from clinopyroxenite, to wherlite (olivine + clinopyroxene) to websterite (orthopyroxene + clinopyroxene). Large, unaltered primary pyroxene grains could be handpicked although alteration to serpentine, talc and tremolite

is present and more extensive than in samples 1 and 2. Sample 4 was collected from a swarm of microgabbro dikes. These dikes are hypidiomorphic granular to granoblastic intergrowths of subequal amounts of plagioclase, clinopyroxene and orthopyroxene, and trace amounts of magmatic amphibole. The sample was extremely "fresh", showing less than 1% alteration. At the time these samples were collected, the microgabbroic dikes were thought to be unrelated to the gabbro plutons that also intrude the Trinity. Subsequent field work by one of us (J.E.Q.) during the summer of 1981 found that similar swarms of microgabbro dikes were abundant at a gabbro-peridotite contact near Castle Lake, ten miles to the southeast, suggesting that the two types of gabbro are cogenetic.

4. Analytical procedures and data representation The abundances of the primary minerals, average mineral compositions and calculated bulk composition of the samples were determined by automated electron microprobe point counting [11,20]. The mineral compositions represent averages from 2400 to 6500 randomly selected points and should therefore be reliable. For each sample, several pieces (total - 1 0 grams) of unweathered material were obtained by splitting large samples weighing several kilograms and then gently crushed in a stainless mortar. Each sample was split into two parts: one was saved for mineral separation and the other was taken for total rock sample without further crushing and dissolved in HC104 and HF. Clinopyroxene and plagioclase mineral separates were first concentrated with a Franz magnetic separator from samples 1 and 4. Each of these fractions were then purified by handpicking under a microscope. Only very clear grains free of visible alterations were used. The separates were then rinsed in distilled water. Clinopyroxene and plagioclase mineral separates from sample 1 were essentially 100% pure separates. For sample 4 we obtained a clinopyroxene separate consisting of - 90% clinopyroxene, - 5% plagioclase and - 5% orthopyroxene. The plagioclase separate of sample

365

4 was essentially 100% pure. The cleaned mineral separates were dissolved without further crushing in H F and HC104. Separation of Sm and Nd follows the procedure of a Eugster et al. [21] with the slight modifications reported by DePaolo and Wasserburg [1] and Papanastassiou et al. [22]. The procedures for RbSr are those reported by Papanastassiou and Wasserburg [23]. The total procedural blank was less than 15 pg for Nd and 100 pg for Sr. The isotopic compositions were measured using the procedures reported in detail by Wasserburg et al. [24]. Wasserburg et al. [24] changed to oxygen isotope composition used to reduce the raw data for Nd isotopes. As a consequence, all 143Nd/ 144Nd ratios published by this laboratory prior to that paper should be corrected by +0.000011 to be consistent with our presently reported values. The data are discussed in terms of the c and f notation introduced by DePaolo and Wasserburg [1]. The end and fSm/Nd values are calculated relative to the average chondritic evolution (CHUR) of the 147Sm-143Nd geochronometer. For a rock with a crystallization age T and an initial 143Nd/144Nd value INd(T) this initial Nd value may be given as: ~Nd =

Nd

1

IcuuR(T)

×

104

where IC~UR(T)= INCdHUR(0)-- ( a 4 7 S m / 144~n~0~.,,+CHUR (e xSmT -- 1). Here INdcHUR(0) = 0.511847, (147Sm/l~Nd)°HU R =0.1967 and Xsm = 0.00654 AE -1 [2]. The C H U R values for Sm-Nd are believed to directly reflect the bulk earth SmN d isotopic evolution. The chemical enrichment factor for 147Sm/~'~Nd relative to that in C H U R is defined as: 147Sm/144Nd S m / N d ~ /147~

_144~

~m/

- 1

1~,0

r~a/CHU g

Similarly, for the Rb-Sr system, the reference values used to approximate a bulk earth reservoir ( U R ) are: (87Sr/S6Sr)°HUR = 0.7045 and (87Rb/86Sr)°HU R = 0.0827 [25] and ~kRb " ~ - 0 . 0 1 4 2 AE-1. Values for esr and fRb/Sr are defined in an analogous manner to that for Sm-Nd. Note that the bulk earth values for Rb-Sr depend both on

the choice of the C H U R values for Sm-Nd and the systematics of Nd and Sr isotopes in young basalts. They are therefore not as widely accepted as those for Sm-Nd although the value used here is a good estimate.

5. Results

5.1. Major and trace element abundances The major and trace element data are given in Tables 1 and 3 and shown in Figs. 2 and 3. Plagioclase lherzolite samples 1 and 2 together with other data on the Trinity metamorphic peridotites [11] are shown in an Mg/Si vs. AI/Si diagram (Fig. 2). Mantle peridotites consists largely of five major components: MgO-FeO-SiO2-A1203CaO. However, since both the Ca/A1 and F e / M g ratios are relatively constant in mantle peridotites, the most abundant major elements in mantle peridotites may be treated with a Mg/Si vs. A1/Si diagram. The Trinity metamorphic peridotites (except for the harzburgite) plots on a strikingly linear array in such a diagram as shown in Fig. 2. Also shown are data on spinel lherzolite nodules in basalts and some well-documented Alpine lherzolite compositions. All these data plot on broad linear trends with a negative slope in this diagram. A group of spinel lherzolite nodules have been called "primitive nodules" by Jagoutz et al. [26] and are also shown. Increasing degree of depletion of a basaltic component in the peridotites shifts their composition to the left of the primitive group of nodules. As shown, even the most fertile Trinity plagioclase lherzolite (1-WR) is substantially depleted in a basaltic component compared to the "primitive" group. However, it is similar in major element composition to average spinel lherzolite nodules as shown in Table 2. Most of the other lherzolite samples from the Trinity complex are, however, even more depleted in basaltic components. The abundances of some major and trace elements in the Trinity plagioclase lherzolite are shown in Fig. 3 normalized to undepleted mantle. They are compared to those of three other well-studied, relatively fertile, Alpine peridotites and the patterns of the "primitive" ultramafic

TABLE l Bulk c o m p o s i t i o n , a b u n d a n c e s a n d a v e r a g e c o m p o s i t i o n s of p r i m a r y m i n e r a l s in plagioclase lherzolite s a m p l e s 1 a n d 2 a n d microgabbro sample 4 Plag

Hbl

O1

En

Di

Sp

WR"

8

2400

Sample 1 Points pb

185 2.75

5 3.25

1693

456

3.35

53

3.25

3.27

4.31 0.4

3.29

wt.%

6.5

0.2

71.8

18.8

2.2

100.0

40.90

56.12

52.55

0.11

44.08

0.12 2.94

0.27 3.27

0.11 28.22

0.03 2.94

49.65 0.06

0.75 32.63 0.97

1.32 17.33 21.79

38.42 14.08 -

0.34 42.27 1.90

0.12 6.30 . -

0.09 2.83 0.34

0.22 18.30 -

0.13 7.79 0.09 0.00 0.23

SiO 2

45.35

45.52

TiO 2

0.01

1.69

A1203

33.67

10.96

Cr203 MgO

0.03

1.82 18.68

CaO

18.16

12.08

MnO FeO Na20

0.16 1.27

0.03 3.58 2.34

K 2° NiO

0.02 -

0.02 -

0.14 8.98 . 0.32

Sum

98.67

96.72

100.05

99.95

99.79

99.46

99.78

111 2.75

5 3.25

1937 3.34

451 3.24

172 3.27

25 4.30

2701 3.30

3.4

0.2

72.6

16.4

6.3

1.2

100.0

45.84

45.18

40.74 0.03 50.56

54.31 0.07 3.29 0.96 33.14

52.32 0.15 3.46 1.29 18.93

0.13 0.15 30.13 37.89 14.45

43.38 0.02 2.29 0.72 43.40

0.14 8.80

2.00 0.10 5.50

20.15 0.10 2.77

0.14 17.42

2.24 0.13 7.66

-

.

. -

-

Sample 2 Points p b w t . %

SiO 2 TiO 2 A1203

33.81

0.80 12.40

MgO CaO

18.32

1.34 18.86 12.46

MnO FeO NazO

0.15 1.51

0.05 3.27 3.22

K20 NiO

0.01 -

-

Sum

99.64

97.58

2119 2.75

14 3.15

Wt.%

28.6

0.2

SiO 2 TiO 2 A1203 Cr203 MgO CaO MnO FeO Na20 K 20 Sum

45.73 0.04 34.42 0.01 18.60 0.30 0.91 0.02 100.03

45.97 1.26 10.69 0.15 15.84 12.19 0.08 9.03 1.99 0.03 97.23

Cr203

-

. 0.34 100.61

.

0.03 .

0.38

-

.

-

100.31

0.09 0.00 0.25

99.90

99.55

100.18

1553 3.35 25.6

2810 3.30 45.6

-

6496 3.13 100.0

55.17 0.16 1.80 0.08 27.09 1.93 0.27 13.88 0.01

52.74 0.38 2.43 0.22 15.27 21.83 0.16 6.24 0.22

-

51.37 0.23 11.46 0.12 13.93 15.81 0.14 6.50 0.37

Sample 4 c Points p b

-

100.39

-

-

99.49

Plag = plagioclase, H b l = h o r n b l e n d e , Ol = olivine, En = enstatite, Di = diopside, Sp = spinel, W R = w h o l e rock. a W h o l e rock c o m p o s i t i o n calculated f r o m m i n e r a l c o m p o s i t i o n s a n d a b u n d a n c e s . b Density estimated from mineral compositions. c N i O b e l o w d e t e c t i o n limit in all phases.

0.01

99.94

367 3.0

IM'g" Si-'AI FRACT'IONATIONj . . . .• PERIDOTITE . . NODULES 1.6

PERIDOTITES ,5.TRINITYPERIOOTITE o CHONDRITICMETEORITES

&ALPINE LHERZOLITE

1.4

'~...~3 J~ - e ' ~

~

,.*-., i,i la,..l _.J 13._ u_l

-

~ . l.C"

TERRESTRIALi IFRACTIONATIONI -

TINAQUILLO y !~'o~r ~ pyROL'11:'l~'"-/ypX~....o/~ND ' EPLETED q/ MANTLE

I

i

i

l.JJ Z

PL-LHERZOLITE _ / 21WR

i

2.0

z

1.0 0.8 ~ 0.6 0.4 0.2

o ,.-..,,

0.1

°'F

N

F

~

0

~

/-E

L

06~ 0.02

0.04

Y 0.06

I FRACTIONATIONI

0.04

I TRENDS I

0.08

0.10

0.12

(AI/Si)wr

hn Z

Fig. 2. Mg/Si vs. AI/Si diagram for chondritic meteorites and terrestrial peridotites. Data for peridotite nodules are from Jagoutz et al. [26] and chondritic meteorite data are from Larimer [32]. Trinity peridotite data are from Table 1 and Quick [11]. The pyrolite composition is from Ringwood [65]. Also shown are data for the Alpine peridotites Beni Bouchera [39], Kalskaret [66], Lizard [67], Lanzo [68] and Tinaquillo [69].

nodules discussed earlier. Note that even the "primitive" group of lherzolite nodules is somewhat depleted in highly incompatible elements showing that these do not represent truly unfractionated mantle material. The Trinity plagioclase lherzolite shows a strong depletion in the in-

~

o.o8I o.o61

0.02

Z

0.01 O.OOB 0.006

Co 2.40

I AI 2.22

I

Ti 0.128

I Sm 0.41

Nd 1.26

Sr 22

Rb 0.63

K 260

Fig. 3. Abundances of various refractory lithophile elements and Rb and K normalized to the undepleted mantle values. The undepleted mantle values for Sm, Nd, Sr, Rb and K are those of Jacobsen and Wasserburg [31]. The undepleted mantle Ca and AI values are from Table 2 and Ti from a chondritic (Ca/Ti)w t ratio ( = 18.8) [32]. The data for primitive ultramafic nodules are from Jagoutz et al. [26]. In addition to the Trinity plagioclase lherzolite data (this work) the figure shows data for the Alpine peridotites Beni Bouchera [39], Lanzo [38,68] and Tinaquillo [69,70].

TABLE 2 Comparison of the major components in the Trinity plagioclase lherzolite with that of average spinel lherzolite nodules and model mantle compositions

Trinity plagioclase-lherzolite a: 1 2 Average spinel lherzolite b Undeleted mantle c Depleted mantle d

SiO2

A1203

MgO

CaO

FeO

44.08 43.38 44.20 45.23 44.70

2.94 2.29 2.05 4.19 3.88

42.27 43.40 42.21 38.39 38.87

1.90 2.24 1.92 3.36 3.26

7.79 7.66 8.29 7.82 7.78

" Data from Table 1. b Average of 384 spinel lherzolites [3]. c Derived on the basis of the Mg-Si-Al systematics shown in Fig. 2, a chondritic ( C a / A l ) , t ratio ( = 1,08) and ( M g O / M g O + FeO), t = 0.83 (average from five "primitive" ultramafic nodules from Jagoutz et al. [26]). These values are essentially those of Jagoutz et al. [26]. The minor differences reflect the fact that we adjusted the Ca/A1 ratio to be exactly chondritic and a slight difference in the reference lines drawn for terrestrial and chondritic fractionation trends in the Mg-Si-A1 diagram. d The depleted mantle is calculated by extracting the continental crust out of the upper mantle using the undepleted mantle values given above and the continental crust composition of Taylor and McLennan [64].

Weight b

0.098 0.129 0.593

1.30

1.35 1.24 1.06 1.04

0.590 0.160 0.321

0.436

0.165 0.154 0.005

Rb(ppm)

a Reported errors are 20 of the mean. b Weight of the dissolved sample.

4-WK# 1 4-WR#2 4-CPX 4-PLAG

111. Microgabbro

3-WR

IL Pyroxenite

1-CPX 1-PLAG 2-WR

L Plagioclase lherzolite

Sample

Rb-Sr and Sm-Nd analytical results a

TABLE 3

106 14.5 160

3.31

13.4 101 0.381

Sr(ppm)

1.01 1.01 1.80 0.088

0.326

1.23 0.052 0.025

Sm(ppm)

1.73 1.73 2.96 0.37

0.588

2.11 0.237 0.068

Nd(ppm)

0.0161 0.0319 0.0058

0.381

0.0356 0.0044 0.0379

87Rb//S6Sr

0.70386+ 2 0.70357 ± 3 0.70373 ± 4

0.70529± 5

0.70249± 4 0.70327 ± 15

87Sr//86Sr

0.3536 0.3543 0.3684 0.1438

0.3349

0.3539 0.1327 0.2219

147Sm/144 Nd

0.512633+24 0.512634+27 0.512690 ± 28 0.512039 ± 25

0.512613±22

0.512863 ± 30 0.512179±36 0.512415 + 61

143Nd/la4Nd

+ 15.36 + 0.47 +15.38±0.53 + 16.47 + 0.55 + 3.75 ± 0.49

+ 14.97 ± 0.43

+19.85 ± 0.58 + 6.49 ± 0.70 +11.10±1.19

eNd(0)

OO

369

compatible elements. The depletion is much more pronounced than that seen in the "primitive nodules" and the other Alpine peridotites. We conclude that both for major and trace elements the Trinity plagioclase lherzolite is substantially more depleted in a basaltic component than the so-called "primitive nodules" in "alkali basalts" and also more depleted than some lherzolites from other Alpine peridotites. The bulk composition of the microgabbro dike (Table 1) is similar to that estimated by Quick [11] for a primary partial melt from the peridotite. However, the very low concentrations of Na, K and the very high fSm/Nd value of + 0.80 appear to be inconsistent with this being a primary melt. This rather suggests that the microgabbro dikes represent cumulates left behind by melts that escaped from the peridotites. 5.2. N d and Sr isotopes

The Sm-Nd and Rb-Sr isotopic results are given in Tables 3 and 4 and shown in Figs. 4 and 5. Plagioclase and clinopyroxene mineral separates of plagioclase lherzolite sample 1 were analyzed for Sm and Nd isotopes. The results define a two-point isochron giving an age of T = 472 _+ 32 Ma and an initial Nd isotopic composition of end = + 10.4 0.4. The whole rock sample of the plagioclase lherzolite sample 2 yields a model age TcN~dvR= 3.4

AE. This sample plots close to the mineral isochron for sample 1 and using the age of 472 Ma it has an initial end = + 9.6 :g 1.2. An internal isochron (plagioclase-clinopyroxene-whole rock) for the microgabbro gives an age of T = 435 +" 21 Ma and an initial Nd isotopic composition of end = + 6.7 T-0.3. AS the bulk of the Nd is in clinopyroxene and plagioclase, this is essentially a two-point isochron. The third phase in this rock is orthopyroxene which has an S m / N d ratio very close to the clinopyroxene and much lower Nd concentration. This phase was therefore not analyzed for its isotopic composition of Nd. The pyroxenite dike crosscuts the metamorphic peridotite and is therefore younger. For the purpose of calculating initial values it has been assigned a nominal age of 435 Ma which is the Sm-Nd age obtained for the microgabbro dike. It plots close to the microgabbro isochron and has a calculated initial value of ~Na = +7.3 + 0.4. This initial Nd value is indistinguishable from that of the micrograbbro. The Rb-Sr isotopic data on the same samples are shown in Fig. 5. In contrast to the Sm-Nd data, the Rb-Sr data are rather scattered and very little age information can be extracted from these data alone. The minor serpentinization described earlier has severely disturbed the Rb-Sr systematics of these samples while the Sm-Nd systematics appear essentially undisturbed. The lowest

TABLE 4 R b - S r a n d S m - N d initial e-values a n d m o d e l ages Sample

frtb/sr

L Plagioclase Iherzolite ( T 1-CPX 1-PLAG 2-WR

1L Pyroxenite 3-WR

- 0.570 - 0.947 - 0.542

ESr = 472 + 32 M a ) - 21.1 5:0.6 - 13.2 +_ 2.2

TuSk( A E )

fSm/Nd

eNO

Tc~u a (A E )

1.78 + 0.03 1.91 + 0.25

+ 0.7992 - 0.3254 + 0.1281

+ 10.4 + 0.6 + 10.4 + 0.7 + 9.6 + 1.2

0.985 + 0.031 - 0.795 + 0.085 3.41 + 0.39

- 15.0 +_0.7

0.186 + 0.015

+ 0.7026

+ 7.3 + 0.4

0.845 + 0.026

= 435 + 21 Ma) - 0.805 - 3.2 5:_0.3 - 0.614 - 8.7 + 0.4 - 0.930 - 4.2 5:0.6

0.674 + 0.022 1.28 5:0.05 0.702 5:0.037

+ + + -

+ + + +

( T = 435 M a ) a + 3.61

IlL Microgabbro ( T 4-WR# 1 4-WR# 2 4-CPX 4-PLAG

0.7977 0.8012 0.8729 0.2689

6.6 + 6.6 + 6.9 + 6.7 ±

0.5 0.5 0.6 0.5

0.764 + 0.762 + 0.748 + - 0.556 +

0.025 0.027 0.028 0.071

a T h e p y r o x e n i t e is geologically y o u n g e r than the plagioclase lherzoUte a n d plots close to the m i c r o g a b b r o S m - N d isochron. It has therefore b e e n assigned a n o m i n a l age of 435 Ma.

370 0

.

5

0"51321

1

3

4

~

+

I ~,~RB~XRENIIIE

°-"'°r

I

o

3

0

/ TrNdii.=~.4~E

LS""

4

_...4

..

SL

t 0

. 0.1

I/

5

1

1

'

8

0.2

~

'

0.3

I

0

0.4

147Sm/144Nd Fig. 4. Sm-Nd evolution diagram for the Trinity peridotite samples. The dashed lines represent the values for average chondrites (CHUR) of Jacobsen and Wasserburg [2].

87Sr/86Sr ratio of 0.70249 was measured on a plagioclase separate from plagioclase lherzolite sample 1 and provides an upper limit for the 87Sr/86Sr ratio of the lherzolite 472 Ma ago. This initial Sr value is typical of a highly depleted source and consistent with the very high ~Nd value

~.o

ZFRINiTYPERIDOTITE

07o~

"-450 Ma

0 m 4-PL

• T/

•450 MO

0703 ~

II

'~I- PL 0,702 '

0.02 '0.04

-20

A PL-LHERZOLTE -30 • PYROXENITE • GABBRO I I I 0,06 I 0.~)8' I ~0.37 0.38 0.39

87Rb/e6Sr

Fig. 5. Rb-Sr evolution diagram for the Trinity peridotite samples. The dashed lines represent the bulk Earth values (UR) of DePaolo and Wasserburg [25]. A reference line with a slope of 450 Ma and that goes through the pyroxenite data point is shown.

obtained on the same sample. As shown, the whole rock of sample 2 and the plagioclase from sample 1 both give TS~ ages of about 1.8 AE. However, using the Sm-Nd age of T = 4 7 2 _ 32 Ma, the plagioclase yields an initial Sr value of 0.70246 while the whole rock of sample 2 yields an initial Sr of 0.70302. Since these two samples had the same initial Nd value, this suggests that the higher STSr/86Sr value for sample 2 is due to the relatively minor serpentinization observed in this sample. Therefore, the TS~ model ages calculated do not have much significance for the mantle history of the metamorphic peridotite. We conclude that the metamorphic peridotite has an initial Sr value 0.70246. The microgabbro data give a higher value for 87Sr/86Sr in the plagioclase than in clinopyroxene and the whole rock point lies above the plagioclase-clinopyroxene tieline showing that there must be another phase with a higher 87Sr/86Sr ratio that we did not analyze; orthopyroxene probably does not have sufficient Sr and it is most likely due to alteration products of plagioclase. For an age of 435 Ma the calculated 87Sr/86Sr values of the clinopyroxene, plagioclase and total rock are 0.70337, 0.70369 and 0.70376, respectively. They are all similar and distinctly higher than the initial Sr value obtained for the metamorphic peridotite, suggesting that these samples also had primary differences in initial Sr. However, it is well known that only minor alteration may have rather severe effects on the Sr isotopic composition so we cannot prove that these samples had a distinctly different initial Sr isotopic composition. The clinopyroxenite has a much higher R b / S r ratio than any of the other samples. This is probably not an intrinsic feature of any of the primary minerals in this rock, but rather due to the presence of alteration products as serpentine, talc and tremolite. The observed alteration products did not form in the mantle and must have formed sometime during or after the crustal emplacement Trinity ultramafic sheet. Considering the range in initial Sr shown by the other samples, the "age" of the alteration should lie in the time period between 522 Ma ago and 346 Ma ago. Using the Sm-Nd age of 435 _+ 21 M a of the micrograbbro yields an initial Sr value of 0.7029 + 1. A reference isochron

371

of 450 Ma is also shown in Fig. 5 for the clinopyroxenite data point. The data show that the initial Sr value of the plagioclase lherzolite is Csr ~< -21.1 _+ 0.6 and for the microgabbro it is Csr - 8 . 7 +_ 0.4. As discussed earlier, although these samples were both modified by later alteration, the fairly large difference obtained on relatively unaltered mineral separates hints that this may reflect a primary difference in the isotopic composition of Sr in these samples.

6. Age A number of ages that relate both to the igneous/metamorphic and emplacement history of the Trinity ultramafic sheet have been published. U-Pb dating on zircons from quartz diorite and trondhjemite belonging to the Trinity complex was reported in an abstract by Mattinson and Hopson [27]. The U-Pb data yields concordia intercept ages of 460 Ma for these rocks. It is, however, not clear what the relationship is between these rocks and the mafic-ultramafic rocks. Lanphere et al. [28] obtained K-At ages of 418 _+ 17 Ma and 439 +_ 18 Ma for gabbros that intrude the metamorphic peridotites. The K-Ar ages clearly represent minimum ages for the gabbros and the metamorphic peridotite in the central part of the Trinity sheet. The final emplacement of the Trinity peridotite over the Central Metamorphic Belt is inferred to have been about 380 Ma ago based on R b / S r ages for samples from the central metamorphic belt in the vicinity of the thrustzone below the Trinity peridotite [28]. The Sm-Nd mineral isochron age of 472 _+ 32 Ma is the first age information obtained directly on the metamorphic peridotite and it is consistent with previous data that provided a lower limit to its age. It is, however, not entirely clear what event the Sm-Nd age for the peridotite represents. Quick [11] interpreted the Trinity peridotite in terms of a model involving rise of the peridotite in the convecting part of the upper mantle (perhaps as a diapir). At shallow depths, plagioclase appears to have formed at the expense of spinel in the plagioclase lherzolite suggesting that these rocks were previously equilibrated in the spinel lherzolite

field ( P > 8-10 kbar). Cores of large pyroxene grains in the plagioclase lherzolite preserve high equilibration temperatures (>/1150 o C) that are consistent with field and textural evidence for small amounts of partial melting of those rocks. Apparently, during decompression, the more A1rich parts of the peridotite crossed their solidus as spinel lherzolites at depths on the order of ~< 30 km; subsequent crystallization of trapped melt produced a plagioclase lherzolite, and removal of partial melt may have formed the harzburgite and lherzolite. During and after its rise, the solid Trinity peridotite mass was invaded by multiple pulses of gabbroic melts that were derived from greater depths in the mantle and that reacted with the Trinity peridotite and formed clinopyroxene-rich dikes, dunite bodies, and gabbro plutons and dikes [11,16]. In light of these petrologic observations, it is reasonable to infer that the Sm-Nd age of the peridotite represents the time of crystallization of the trapped melt in the plagioclase lherzolite P - T field as both the crystallization of new minerals and the high diffusivities of Nd and Sm in the presence of a melt phase would facilitate rapid Sm-Nd isotopic equilibration between clinopyroxene and plagiodase. It is, however, possible that the Sm-Nd mineral isochron age represent some later time in the cooling history of the Trinity peridotite. This possibility cannot be avaluated without a precise knowledge of the pressure-temperature-time history of the Trinity ultramafic sheet and values of the diffusivities of Nd and Sm in plagioclase and pyroxene. The Sm-Nd mineral isochron age obtained on one of the microgabbro dikes yield an age of 435 _+ 21 Ma which is barely within error of the age of the peridotite. The similarity of this with K-Ar ages obtained on hornblende gabbros and the U-Pb ages from zircons all suggest that the many complex igneous and metamorphic events recorded in the Trinity ultramafic sheet occurred over a relatively short time interval from - 4 8 0 to - 4 2 0 Ma ago. The detailed absolute chronology of the many events recognized by Quick [11] cannot be established with available data. However, the somewhat younger ages obtained for the gabbros are reasonably interpreted as representing the time when melts were rising from some part of the convecting

372

mantle into the lherzolitic portion of the oceanic lithosphere that had already cooled to a point where plastic deformation had essentially ceased.

7. Discussion

7.1. lnitial values and model ages The most significant result of the isotopic data is the fact that the plagioclase lherzolite has a much higher initial eNd value than the pyroxenite and microgabbro dikes. The plagioclase lherzolite value of eNd = + 10.4 _+ 0.4 is distinctly higher than that inferred for the depleted oceanic mantle at this time ( + 7.6) from the slightly older Bay of Islands ophiolite [29]. However, the dikes have initial eNd = +6.7 and + 7.3 fairly similar to that expected for the average depleted mantle at this time. Our data clearly shows that the microgabbro and the pyroxenite are not cumulates from partial melts derived from the protolith of the surrounding metamorphic peridotite. Rather, these melts that formed the microgabbro and pyroxenite must have been derived from a distinctly different portion of the depleted mantle. The difference in initial end values implies that the mantle source region for these dikes must have been separate from the mantle protolith of the plagioclase lherzolite for at least - 370 Ma (assuming A f s m / N d ~< 0.4). We suggest the following model to explain the available geologic, chemical and isotopic data. The peridotite most likely recrystallized in the plagioclase lherzolite field at or in the close vicinity of a spreading center as this is about the only setting where the crust is thin enough and the mantle sufficiently hot for this mineral facies. As discussed earlier, the peridotite must represent a protolith which lost a large fraction of melt (10-15%) at some stage and this seems most likely to have occurred at a ridge. The resulting residual peridotite was then added to the lower part of the oceanic lithosphere. The extracted melt product may possibly be represented in the overlying section of pillow-basalts and sheeted dikes. The peridotite was later intruded by melts with eNd -- + 7 more typical of the oceanic mantle at that time and the cumulates left behind from these younger

melts are now found as pyroxenites, gabbros and dunites in the metamorphic peridotite. As discussed earlier, the metamorphic peridotite has eNd = + 10.4 and es, ~< - 2 1 which is within the present-day MORB field on the Nd-Sr mantle array. In contrast, the micrograbbro has eNd = + 6.7 _+ 0.4 and es, ~< - 9 which puts it possibly substantially to the right of the Nd-Sr mantle array. This is typical of most island arc magmas; but, as discussed earlier, we cannot prove that these represent the primary initial Sr values. However, as the overlying sediments, volcanics and some of the associated intrusive rocks show clear island arc affinities, it is plausible that these cumulates were left behind after island arc magmas that rose through the oceanic lithosphere in the vicinity of a subduction zone. We now turn to the significance of the high Tc~dR model age of - 3.4 AE for the metamorphic peridotite. Fig. 6 shows the initial eNd values of the plagioclase lherzolite and microgabbro compared to an inferred evolution band for the depleted mantle in an end VS. age diagram. Also shown is the initial eNd value of the slightly older Bay of Islands ophiolite. The simplest interpretation is that the measured fSm/Ndvalue of +0.12 for the plagioclase lherzolite was the same prior to the melting and recrystallization - 4 7 0 Ma ago. Then, as shown in Fig. 6, the protolith of the I

16

i

I

I

I

I

I

I

14 Ik

IO ~

0

I

I

I

I

I

I

I

I

I

I

I

I

• BAY OF iSLANDS OPHIOLITE PL- L HERZOLITE

4 G'S'BBRO~

0

I

e TRINITY PERIDOTITE

12

2

I

DEPLETED MANTLE Nd EVOLUTION

~ ~

i i i i i 1.0

i

2.0

5.0

4.0

AGE (/E) Fig. 6. eNd-age diagram showing inferred depleted mantle evolution through time (shaded area). The initial eNd values of the Trinity plagioclase lherzolite and the microgabbro dikes are plotted in this diagram. Also shown is the initial eNd value of the Bay of Islands ophiolite [29].

373 Trinity plagioclase lherzolite was either fractionated from an unfractionated mantle reservoir at 3.4 AE ago, or from the depleted mantle - 2.4 AE ago. In general, however, successive melting events of the same lherzolite material is expected to lead to an increase of the fSm/Nd value during each melting event since it is well established that Nd is more enriched in the melt than Sm relative to source material. Thus, we would expect that fsm/N~ < 0.12 prior to 470 Ma ago and TgduR -- 3.4 AE must be a minimum age for the time since the peridotite was fractionated from CHUR. If fsm/Na 0.09, then the model age of the source of the plagioclase lherzolite is equal to the age of the earth. Thus 0.09 ~
7.2. Mantle chemical and isotopic variations We now turn to discussing the relationship of Alpine peridotites to proposed depleted and undepleted mantle compositions on the basis of major, trace element and isotopic data. To evaluate which peridotites are fertile and which are sterile with respect to major elements, it is necessary to have an estimate of where the undepleted mantle plots in the Mg-Si-A1 diagram (Fig. 2). This diagram compares the Trinity peridotite with other Alpine peridotites and spinel-peridotite nodules from alkali basalts. Comparison with data on chondritic meteorites may be used as a guide for estimating the undepleted mantle composition. Larimer [32] discussed Mg-Si-A1 variations of chondritic meteorites and Fig. 2 shows for comparison the average values for the various chondrite classes. We first note that chondrites do not have a unique value and that they display two distinct trends [32]. Larimer [32] has discussed the origin of these trends; the trend defined by CI, CM and CV meteorites probably only reflects an enrichment in CAI inclusions. The other correlation defined by the E, H, L, LL and CI chondrites is less well understood and is opposite to the trend defined by mantle peridotites. The intersection of the M g / S i vs. A1/Si correlation line for terrestrial peridotites and the trend through E, H, L, LL and CI chondrites is taken as an estimate of the undepleted mantle composition. The group of primitive nodules plot close to this composition. As noted by Jagoutz et al. [26], these nodules have close to chondritic ratios of the more compatible refractory lithophile elements but show substantial depletions in the highly incompatible refractory LIL elements. Thus, these nodules must have lost a small melt fraction. A striking feature of this plot is that all peridotites lie on a remarkably straight line. Such linear trends extend to most other major and minor elements as shown by Maaloe and Aoki [3]. This can be explained by either two-component mixing (liquid and residual solid), or as a trend of the residual solid left after extracting a liquid of almost constant composition. We prefer the latter interpretation. The data array for the Trinity lherzolites show

374 an even more linear trend than all the data for other peridotites. Thus it appears that local trends may be more regular than the worldwide field defined in this diagram. However, other occurrences have not been investigated in the same detail as the Trinity peridotite. A melt extraction trend has been calculated and is shown in this diagram for the solid residue. The composition of the primary melt is that estimated by Quick [11] for the Trinity peridotite [(Mg/Si)w t --0.334, (A1/Si)w t = 0.358]. This composition is similar to those produced experimentally at low P from lherzolite [33-35]. The initial composition of the solid used is the undepleted mantle composition shown in Fig. 2. [(Mg/Si)w t = 1.1 and (A1/Si)w t = 0.106]. The Trinity plagioclase lherzolite samples, if interpreted as residues left after a single stage of melting, must have had from - 13% to 22% melt extracted from them. All the peridotites shown are in the range from 0 to 3070 melt extracted on this diagram. The group labelled primitive nodules and Lizard appears to represent the only peridotites that have had less than - 570 melt extracted. Table 2 compares the major element composition of the Trinity plagioclase lherzolite with that of average spinel lherzolite nodules, and depleted and undepleted model mantle compositions. Although the Trinity plagioclase lherzolite is similar to average spinel lherzolite nodules, it is distinctly more depleted in basaltic components than both depleted and undepleted model mantle compositions. The inference is that even the most fertile plagioclase lherzolite in the Trinity peridotite is residual matter left after a large degree of partial melting. The difference in major elements between depleted mantle formed by continent extraction and undepleted mantle is trivial as shown in Table 2. However, continent extraction from the upper mantle leads to rather larger differences in the highly incompatible trace elements and this is the basis for using the Rb-Sr and Sm-Nd isotopic systems as tracers for crust and mantle evolution [30,31]. We conclude that the major element composition of the Trinity lherzolites display a trend which is controlled by the generation of primary basaltic magmas (rather than by continent extraction). The composition of the depleted mantle and the unde-

pleted mantle must be broadly situated on this trend; however, since the continents likely have lower M g / S i (for similar A1/Si values) than primary basalt, this is probably not true in detail. However, the slope of the ENd-ESr mantle array [25,36] is largely controlled by continent extraction from the upper mantle [29,31] although recycling of oceanic and continental crusts probably also affects the details of this correlation. To summarize, we conclude that the bulk of plagioclase and spinel lherzolites from Alpine peridotites and nodules in alkali basalt represent depleted peridotitic lithosphere and are not fragments of the still fertile source regions of basalts. Generally, the large major element fractionations observed in these peridotites are due to extraction of mid-ocean ridge magmas while the incompatible element fractionations are largely controlled by continent extraction.

7.3. Comparison with previous studies on Alpine peridotites REE patterns on Alpine peridotites have been reported by Frey [37], Loubet et al. [38], Loubet and All6gre [39], Menzies [40,41], Menzies et al. [42] and Ottonello et al. [43]. These studies all demonstrate a LREE depletion in most lherzolite samples from Alpine peridotites. This indicates the residual nature of these bodies and all available REE data can be modelled assuming that the lherzolites are residual after removal of 1-5% of partial melt [38]. However, as shown in Fig. 2, major element variations indicate that more than 570 melting is required to deplete some of the peridotite massifs. Trace element patterns in both peridotite and pyroxenite veins indicate the complex history of such massifs. They normally show both trace element and field evidence of several episodes of partial melting. The complexity of the history of these massifs is also indicated by the complex Rb-Sr isotopic patterns [44,45]. The Sr isotopes certainly indicate a strong affinity with depleted mantle; however, the complexity in Rb-Sr is probably largely due to the serpentinization of these massifs. Sm-Nd isotope systematics are less likely to be affected by serpentinization as demonstrated by the coherent behavior of REE in these

375 bodies [38]. The only previous Nd isotopic study of Alpine peridotites is by Richard and Allrgre [46] who noted that eNd(0 ) varies from + 18 to + 2 in the three orogenic lherzolites Baktissero ( + 18.5), Lanzo ( + 11.5 and + 9) and Beni Bouchera ( + 13.5 and + 2.1). These bodies were all tectonically emplaced recently; - 0 . 1 - 0 . 1 3 AE ago. The TcNHduR model ages of these massifs range from - 0 . 6 to - 2.4 AE and imply that these were fractionated from the undepleted mantle in a number of distinct events. No internal isochrons were reported, however, so it is somewhat more difficult to evaluate these data. Zindler et al. [71] recently reported a Sm-Nd internal isochron for a mafic layer from the Ronda ultramafic complex. Their results yielded an initial end value of + 11 and an age of 22 + 2 Ma. Ophiolites [29,46] and orogenic lherzolites of Alpine peridotites all appear to have eNd-eSr isotopic characteristics similar to those in midocean ridge basalts [1,36]. These data and our Trinity peridotite data are therefore all broadly consistent with these massifs being derived from the suboceanic depleted mantle. Most likely they represent the lower depleted oceanic lithosphere formed by - 10-20% melt extraction at a spreading center. From the TcNduR model ages for the Trinity and previously studied Alpine peridotites, it seems clear that a timescale exceeding - 3.5 AE is necessary to form the MORB source. This is consistent with the finding of Jacobsen and Wasserburg [2,47] that the depleted MORB source existed throughout the past 3.8 AE. We may conclude, as Church and Stevens [48] and-Williams and Smyth [49] did, that Alpine peridotites or orogenic lherzolites are representative of the deeper parts of the oceanic lithosphere and are therefore partly complementary to ophiolites; certainly the pseudostratigraphy of the Trinity peridotite [9] indicates this and it has therefore even been called an ophiolite. As the Trinity and most other Alpine or orogenic peridotites most likely represents depleted oceanic lithosphere rather than the source region of basalts, they cannot be used to directly estimate the scale and distribution of isotopic and chemical heterogeneities in the convecting mantle. However, the depleted-fertile-source-region mantle must include, as an important component, depleted lithosphere which has been mixed back into

the mantle over earth history. Therefore, it is difficult to determine whether a depleted peridotite mass represents "depleted lithosphere" or an unusually depleted segment of the depleted but fertile upper mantle. Also, while orogenic peridotites are not perfect samples of the upper mantle, they may be the best samples we are likely to get.

7.4. Comparison of ultramafic nodules and Alpine peridotites In contrast to Alpine peridotites, a large volume of both Sr [50-53], Pb [54-56] and Nd [57-63] isotopic data have been published for ultramafic nodules from basalts and kimberlites. While nodules in kimberlites are mostly somewhat different both chemically and isotopically from those in Alpine peridotites, many spinel peridotite nodules from alkali basalts are quite similar. Spinel peridotite nodules from the subcontinental lithosphere [59,63] as found in alkali basalts show eNd(0 ) values ranging from + 2 to + 13 and have Tc~dR ages ranging from about 1.5 to 3.0 AE. Some of these nodules roughly follow the eNd-eSr correlation of young basalts, while several plot significantly below this correlation line. Thus the peri 7 dotitic subcontinental lithosphere seems (at least in areas of continental alkali basalt volcanism and rifting) to be largely derived from an old depleted mantle reservoir as both the isotopic variations and time scale are similar. However, on the basis of chemical and isotopic data it does not appear possible to establish whether this type of subcontinental lithosphere is different from ancient suboceanic lithosphere as inferred for the Trinity or the peridotitic part of the oceanic lithosphere formed more recently. Field evidence is also necessary to clearly identify a given Alpine peridotite or orogenic lherzolite massif as a fragment of the suboceanic rather than the subcontinental mantle.

Acknowledgements

This paper was supported by the National Aeronautics and Space Administration grant No. N A G 9-43, and the National Science Foundation

376 g r a n t No. PHY82-15500. W e t h a n k A. Z i n d l e r for a h e l p f u l r e v i e w o f this p a p e r .

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