Strontium isotope and alkali element abundances in ultramafic rocks

Gcochimica et Cosmochimica Acta, 1966, Vol. 30, pp. 1243 to 1259. Pergamon Press Ltd. Printed in Northern Ireland

Strontium isotope and alkali element abundances in ~tramafic rocks ALAN Department

I%. ST~EBER~

and V. RAM*& ~%~TRTHY~

of Earth Sciences, University of Cdif~rni~, San Diego, La Jolla California (Received

28 December

1965)

Abstract-The concentrations of the trace elements Na, K, Rb and Sr and the isotopic composition of Sr have been measured in a suite of ultramafic rocks, including alpine-type intrusions, inclusions in basalts and kimberlite pipes, zones from stratiform sheets, and a mica peridotite. From these data and those available in the literature the following conclusions can be drawn. Alpine-type ultramafic material appears to be residual in nature and can be neither the source material for the derivation of basalts nor the refractory residue of modern basalts. Alpine-type ultramafic intrusions appear to have no relationship with ultramafic zones in stratiform sheets and were probably derived from the upper mantle. A genetic relationship exists between hasalts and their ultramafic inclusions, but it is extremely doubtful that this inclusion material could give rise to basalts by partial fusion. There is a possible genetic relationship between b&salts and ult’ramafio inclusions in kimberlite pipes, and this ultramnfic material is a poten+& source for the derivation of basalts. Ultramafic inclusions in basalts are probably not fragments of an alpine-type ultramafic zone in the mantle. An attempt has been made to synthesize the data and interpretations of this study by way of speculations on the role of ultramafic rocks in the differentiation history of the earth. INTRODUCTION EARTH scientists have primarily

studied the accessible and observable crust, which constitutes Iess than one per cent of the mass of the earth. Important aspects of the composition and state of the earth’s interior remain speculative. Certain rock types found in the crust are believed to ha.ve been derived from the earth’s mantle. At present the nearest approach to a consensus is that the dominant material of the mantle is an ultramafic rock of some type. Other rock types such as basalt are thought to be partial or, possibly, total melts of primary mantle material and have been studied quite extensively in an effort to place limits on the composition of the mantle. Reliable geochemical data on ultramafic rocks themselves are scarce, and this fact, when combined with their obvious significance, emphasizes the need for geochemical studies such as the present one. The purpose of this investigation is to determine certain geochemical properties of the upper mantle, insofar as it is represented by ultramafic rocks, and to examine t#heinterrelationships of the various types of ultramafic rocks, as well as their relationships with basalts. Thirty samples of the various modes of occurrence, distributed t’hroughout the world, were selected for analysis (Fig. 1). These samples include 13 ~_____ * Present Washington, t Present Minneapolis,

address: Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, D.C. address: Department of Geology and Geophysics, University of Minnesota, Minn. 1243

1244

A. M. STUEBER and V. _&MA MUIWHY

alpine-type intrusions, 8 inclusions in continental and oceanic basal&, 4 inclusions in kimberlite pipes, 4 zones from stratiform sheets, and one mica peridotite. The strontium isotopic composition and the elemental abundances of potassium, rubidium, and strontium were determined by mass apectrometric methods. Sodium was determined by non-destructive neutron activation analysis (STUEBERand GOLES,1966). In addition, two basalts were analysed for their strontium isotopic compositions.

Fig. 1.

EXPERLMENTAL PROCXDURES B-use of the very low concentration levels of K, Rb and Sr in ultram&~ rocks, extrame caze must be exemised in chemieaJproctoring in order to attaiu a ~~~3ntly low level of cont~at~on. We give below a somewha.td&ailed description of the proeed~ used. ED’ and EC1 magenta of high purity were preparedby passing IXF and EKXgases through ~u&~z-doub~~distilled water. Quartz, vycor and Tefion ware were employed wherever possible, and aiI ~tio~ and evaporations were aarried out in T&on and glass eneloauresunder a positive press- of dry, fIlter& nitrogen. The sample was broken into small pieces a& several fresh pieces were sepwated, cleaned with di&lled water, dried and then errzsheda+udground into a fine powder in an ~~d-ele~n%d steel mortar. An aliquot of approxiinstsfy 2 g of & given sample was aoourat&y weighed into a ph&mun dish. The Behmpie was then decomposed in 30 ml EF end 5 ml EC%& by heating in & covered Teflon hood, and wan evaporsterdt6 drym?as. %fthe entire sample had not been deoomposed, more EF was added, but t@ was not gezx&ly II-. The residuer8m&ning after decomposition was dissolved in 30 ml of 0 N HCX and t&en to dryness. T&e chlorides were m-dissolved in 6 N HCI and transferredquantitatively to a 100 ml volumetric flask, then diMed

Strontium

isotope

and alkali element

abundances

in ultrama%

rocks

1445

to 100 ml with qua~z-double-distilled water. The solution was allowed to stand overnight, and then a 25-ml aliquot, to be used for isotope dilution analyses, was pipetted out into a small beaker. The remaining 75ml solution, to be used for strontium isotopic analysis, was transferred to a large quartz beaker. K, Rb and Sr spikes were then added to the 25.ml aliquot, and the spiked and unspiked fractions were evaporated to dryness in separate glass hoods. A few millilitres of 2 N HCl were added to these dry residues, and each was carefully loaded on a cation exchange column (Dowex 5OW-X8 resin, 100-200 mesh, 30 cm in length) which had been pre-calibrated for elution of K, Rb and Sr. The sample fractions on both columns were eluted with 2 N HCl. Two 30 ml fractions were collected in vycor crucibles from the column to which the spiked aliquot had been added, one fraction containing the K and Rb and the other fraction containing the Sr. One 30 ml fraction, containing the Sr, was collected from the column to which the unspiked sample aliquot had been added. All three fractions were evaporated to dryness in separate glass hoods. A few drops of HClO, and a few drops of HNO, were added to each fraction in order to oxidize any organic matter that might have been released from the ion exchange cohunn. After each fraction was once again evaporated to dryness, a few drops of 2 N HCI were added to each in order to convert the final residue to chlorides. After final evaporat~ion each crucible was covered with parafllm and stored for mass spectromotric analysis. The ion exchange columns were cleaned and prepared for another sample by passing 50 ml 6 K HCl, 50 ml distilled water and 50 ml 2 N HCl through each. The extent of laboratory contamination was determined by a series of blanks run at intervals throughout the period of this investigation. These blanks were processed in exactly the same way as the samples with the total amount of reagents being the same as that for processing a 2 g sample. The results of the blank determinations indicate the following levels of contamination: Ii = 0.5 pg, Rb = 0.003 pg and Sr = 0.1 [lg. These levels of contamination are considered negligible for the samples under discussion. Mass spectrometric analyses were made with a 12-in. radius, 60” sector, solid-source mass spectrometer. A multiple rhenium filament ion soume was employed with an accelerating voltage of 55 kV. The mass spectra were scanned by varying the magnetic field st,rength. The ion beam current was amplified with an electron multiplier and measured with a vibrating reed electrometer and a strip chart recorder. The sample was loaded as a chloride on a rhenium filament which had been pre-baked at high temperatures. For Sr-isotopic composition analyses, the sample was loaded on one side filament of a double rhenium filament hat. The center filament was used as an ionizing filament. Normal operating conditions employed a sample filament current of about 1*5A and an ionizing filament current of about 2.2A. In the case of isotope dilution analyses, K and Rb spectra could be obtained from a single filament load, since the filament current required for K emission was about l*OA and that required for Rb emission was about l-2A. For single-filament isotopedilution analysis for Sr the filament current required for steady emission was approximately 2+4A. An average analysis for Sr-isotopic composition consisted of from 12 to 15 repeated scans of mass spectra. In all the Sr-isotopic analyse s, the presence of isobaric cont&mination at mass 87 was checked by scanning the spectrum at mass 85 for the presence of Rb85. As a general rule Rb presented no problem, but, if present, the sample was baked for some time at a filament current just below that required for Sr emission. An average isotope dilut,ion analysis for K, Rb or Sr consisted of approximately 10 repeated scans of mass speotra. An attempt was made to maintain sufficient signal intensity such that the mass peaks occupied nearly the full width of the chart paper, in order to minimize relative errors in reading peak heights. The chart obtained from a Sr-isotopic composition run was analyzed by drawing baselines and measuring peak heights to the nearest 0.01 cm. The Srs7/Srs6 ratios were calculated and the arithmetic mean values for the 12 to 15 repeated scans of the mass spectra were determined. An electron multiplier discrimination oorrection (square root of mass ratio) was applied to the average SrE7/Srs6and Srs8/Srs6 ratios. It is now well known that significant mass fractionation occurs during Sr isotopic analysis, probably due to fractionation from the filament during sample emission. The measured Srss/Sra” ratio was therefore normalized to an arbitrary value of 8.375, and half of the required correction was then applied to the measured Srs7/SrE6ratio, in an attempt

1246

A. M. STUEBER and

V. RAXA M~RTEY

to correct for the ffectionation effects, The standard deviation from the mean for the average SrsT/Srss ratio provided sn indicf&on of the q&&y of each isotopio analysis. The precision ox reproducibility of the strontium isotopio analysee was determined from duplicate enalyees of four individual samples and from repeat analyses of the M.I.T. Sr standard, Eimer and Amend reagent &CO,. The results of these analyses, given in Table 1, indicate that the corrected (Srsr/Srss)ratio can be measured accurately to 1 part in 700 at the 95 per cent confidence level. The reproducibility of a single strontium isotopic anctlysisis assumed to be rto.001. Table 1. Repeat rtnalysisof samples Saple

Date

(Sra7/Srss) mess.

(SrSe/W@)mess.

PCNA-5-l

4 Oct. 1963 16 Jan 1965 24 Oct. 1963 6 Nov. 1904 19 April 1963 18 Oct. 1963 22 Jan. 1065 26 March 1965

0.7084 07097 0.7113 O-7085 0.7139 0.7082 0.7061 0.7029

8.347 8.361 8.404 8.316 8,458 8.301 8.439 8.345

AppM-1 AppM-5-2 A-l

Repeat a&y&

16 June 1964 30 March 1964 22 Feb. 1965 2 March 1965 11 March 1965 19 Ms.rch 1965

(8@7/W) corr. 0.7096

0.7103 0.7101 0.7109 0.7104 0.7113 o-7034 0.7041

of Eiwwr and Amend reagent &CO, standard 0.7084 0.7108 0.7095 0.7076 0.7119 0.7069

8.391 8.447 8.406 8.346 8.463 8.368

0.7077 0.7078 0.7082 0*7088 0.7083 0.7077

Isotope dilution analyses ~s8 subject to certain errors in addition to those which apply to the strontium isotopio snslyses. An error of at least 3 or 4 per cent must be assigned to the analyses because it we not possible to make correctionsfor isotopefkotionation during mass spactrometry. It is impossible to assign rsd&rite precision to the isotope dilution analyses of this study since no repeat analyses were performed. The accuracy of the measurements cannot be tested until similar analyses are made on ultrsmafk rocks by an independent experimental technique.

RESULTS The results of isotope dilution anslyses for potassium, rubidium and strontium and neutron-activation analyses for sodium in 30 u&ram& rocks and one basdt are presented in Table 2, along with the K/Rb and Rb/Sr ratios. The results of the strontium isotopic analyses of 30 x&rams& rocks and 2 basalts are presented in Table 3. The measured Srs7/SP and SP/SP ratios are tabulated along with the Sr87/SP ratios corrected for mass fractionation. DIEXW3sION The general systematios of the isotopio relationships in the Rb-Sr system have Y (1969), COMPSTON etal.(1960) aad been desoribed in det&l by GOXPWrON and by LANPEXRIIet aE. (1964) and the reader is referred to these works. In this study, we have used the Rb*7 decay oonstant Of 1.47 x lo-r1 yr-1 (FLYNNand GLENDENIN, 1969). For present&ion and discussion of the data obtained in this study, we have

1347

Strontium isotope and alkali element abundances in ultramafic rocks Table 2. ~oncent~atious of Na, K, Rb and Sr in ultramafic rocks determined by isotope-dilution and neutron-activation analysis SF

Na

@Pm)

(Plw Alpine-type WPI-3 WPI-4.1 WPI.& AusPCNA-5-l PCNA-9 AppM-1 AppX.2 AppM-B-2 Gr-I Car-1 Med.5-2 Soen-4-2

IO-1 Ant- I EPI-2-l EPI.2.2 PI-l-1 Ear-I Mex-I PCNA-6 Af-11 Af-7 Af-20 Af.21 WPI-6-1

PCC-I-I PCC-1-2 A-l &&n-l-l Mon.2

ultramu,JTc

K/Rb

Rh/Sr

0.025

rocks

Dun Mtn. New Zealand dunite Papnfi dun&e Shikoku Jap&n dnnite Kalgoorlie Australia serpentinite Tulameen British Columbia &mite Cantwell Alaska dunite Mt. Albert Quebec peddotite Ad&!-Webster North Carolinn dunito B&t’s Cove Newfoundland serpentinite Siorarysoit Greenland dunite Tinaquillo Venezuela peridotite Konya Turkey dunite Almklovdalen Norway dunite

Kergwlen Islands pcridotite inclusion Ross Island Antarctica dunite inclusion Gal~pagae Islands peridotito inclusion Galapazos Isiands bltsdt Hawaiian Islands 1801 pwidotite inclusion Iispfe.nst& Austria, peridotite inclusion Chihuahua Mexico peridot&e inclusion Ludlow California peridotite inclusion Monduli Tanganyika pcridotito inch&on Wesselton Pipe South Africra poridotite inclusion Bultfontein Pipe South Africa peridotite inclusion Visser Pipe Tanganyika eclogitc inclusion Ksklmai New Zealand garnet pcridotite

Muskox NWT Canltda serpentinite Muakox NWT Canada pyroxenite Kola Peninsula Russia pyroxenite Stiiiwrtter ultram&o zone Highwood Mtns, Montane peridotite

* Wet Chemical analysts

by K. Aoki.

97 I”6 81 104

23.9 61.2 -o-i.,0 119

0.111 0.302 0,099

4.39 6.28 8.10

215 203 344

0.281

6.89

4 Ia

0.041

10.2 3.32 5.52

204 “68 375

0.013 O.T)fli 0.029

0.048 O*OE

90 117 430

26-5 19.3 59.3

O-130 0.072 0,158

81

16-O

0.077

2.99

1%

O-026

236 643 25.8 36.9 44.2

1.036 2.42 0.093 0.140 0.131

8.36 14.7 3.89 4.28 9.89

22s 266 277 263 337

0.124 O-165 0.024 0.033 0.013

540

190

Oa?O

10.5

4.52

0,039

380

216

I.15

132

1%

O-087

123 9460*

0.382 55-Q

35.1 435

349 161

0.010 0.135

115

0*048

1250 290 1200 75 119

4540 24400* 3840 380 630 2090

31.1 152 17.9

0.271

5.69

0.413

1:i.o

368

0.027

0.398

13-3

196

0.030

470

727

3.67

33.2

198

0~007

670

641

448

72.4

143

O-062

S5O

483

2.03

58.6

238

0.035

716

7.32

46.7

95

0,161

32.7

512

0.053

172

1.32

317

o-030

353 348

O-022 0.028

102

0.079

9900 3050

9x3

1.73

160

1330

7.75

3620

1100

347

3330 760

396 133

1540

14500

1.12 0.382 142

5.85 114 60.5 13.36 1784

A. M.

1248

&KJEBEB

and V.

Table 3. Isotnpic e~~it~on

I%AMA

of ~tronti~

MURK

in ultrama& rooka

Rb/Sr _-.

-., .

Alpine-type t&wnq%c rocks WPI-3 WPI-4-l WPI-5 AUSPCNA-5-I PC!NA-9 AppM- 1 AppM-2 AppM-5-2 Gr-1 Gas-1 Mad-52 Scan-P-2

Dun. Mtn. New Zealand dunite Papua. dunite Shikoku Japan dnnite KtalgoorlieAustralitl srxpenttite Tulameen British Columbia dun&% Cantwell A.la&azdun&e I%. Albert Quebec peridotite Add&Webster North Carolina dunite B&t& Cove Newfoundland serpentinite Siormuit Greenland dunite Tinsquillo Vmezuela peridotite Kenya Turkey dun&a ~lovd~l%n Norway dunite

0*7008 0.7070 0.7099

8.320 8,356 8.46L

0,7OBl 07078 B-7063

O.OdB 0.048

0~7151 0.7097 0.7084 Q-7085 0*7085 0.7113

8+373 8.361 8.347 8.336 8,316 8‘404

0.7152 0.7103 Q.7096 0.7101 0.7109 ft.7101

OQ41 0‘013

07126 0.7082 0.7139 0.7313 0.7075 0.7276 0.7076

8.304 8.301 8.458 8.427 8.361 8.385 8.369

0.7156 0.7113 0.7104 0.7290 0.7084 O-7272 0‘7078

0.026 O”124

0.7107

8,440

0*7080

OQ3Q

0.7054

8,365

0*7058

cW87

0.7041 0.7016

8.385 8.330

0.7036 0.7035

WI0 0.135

0.7071 0.7021

8.390 8.299

0.7064 07054

0*7055

8.346

0.7067

0.048

O-7064

8.421

0.7045

6027

07048

8.343

0.7062

0.030

0.7057

8.401

0.7046

0.107

0.7093

8.443

0.7064

0.062

0.7052

8.383

0.7049

0.035

o-7049

8.381

0.7046

0.161

0.7108

8.433

0*7083

0.053

0.012

0,031 0~029

0.165 0,024 0,033 0.013

w~t~~~~~ ~~~~~~~~ 10-l Ant-1 EPI-2-l EPI-2-2 PI-l-l PI-l-2 Eur-1 Mex-l PCNA-6 Af-11 Af-7 Af-20 Af-21 WPI-6-l

Kerguelen Islmds p0~doti~ in&&on Ross Island Antarctica dunits imluaion Galapagos Islands peridotite in&&on Galapagos Islmds basalt Hawaiian Islauds 1801 paridotits iuchmion Kawaiiau Uaml.s 1801 basalt Kapfenstein Austria peridot&a inclusion ~~u~ua &&z&o peridotit~ iuolusion Ludlow ~&l~o~& paridotite iuohzsion Monduli Tanganyika paridotite inclusion Walton Pipe South Africa peridotits in&&on Bultfonteiu Pipe South Africa pexidotite iucl. Vissm Pips Tangmiyiks eclogite inclusion Kakanui New Zealand Garust Paridotite

Strontium isotope and alkali element abundances in ultramafic rocks Table 3

1249

cod (Srs8/Srs6) meas.

(Sr6’/Srs6)

meas.

(Sr~‘/Sr~6)

Ultrama&

zones ,i.u,stratiform

Rb/Sr

COIT.

sheets

PCC-l-1

Muskox NWT Canada serpentinized dunito

0.7911

8.475

0.7864

1.33

PCC-l-2

Muskox NWT Canada pyroxenite Kale Peninsula Russia pyroxenite

0.7097 0.7061 0.7029

8.365 8.439 8.345

0~7101 0.7034 0.7041

0.030 0.042 0.023

0.7077 0.7053

s.409 8.352

0.7063 0.7062

0.048 0.079

A-l Mon-

1- 1

Stillwater Complex ultramafie zone Highwood Mtns. Montana pcridotite

Blon-2

* Normalized

to (Srs8/Srs6) = 8.375.

chosen to divide the ultramafic rocks into the following groups, based on their mode of occurrence. I. Alpine-type ultramafic intrusions. These are the dunites, peridotites and pyroxenites which occur in folded geosynclinal sediments of erogenic belts. 2. Ultramafie inclusions in basalts and kimberlite pipes. 3. Ultramafie zones from stratiform sheets. This group contains dunites, peridotites and pyroxenites which occur as massive, banded la,yers in the lower levels of stratified basic intrusions in the crust. Following a discussion of the data pertaining to the above three groups, we present a discussion of the nature of the source material of basalt and its relationship to the ultramafic material represented by the above three groups of samples, in the context of the present data. Finally, we present a speculative model on the role of ultramafic rocks in the differentiation history of the earth. This model is consistent with the present isotopic and elemental abundance data as well as other geophysical and geochemical requirements. We wish to emphasize that this model is by no means unique but is primarily meant to provide an operational framework from which to proceed with further geochemieal investigations to check its validity.

The Sr*7/Sr86ratios of alpine-type ultramafic rocks (Table 3) are generally higher than the SP/Srs6 ratios of ultramafic zones from stratiform sheets determined in this study and the Srs7/SrE6ratios of basalts measured by other investigators (FAURE and HURLEY,

1963;

HEDGE and WALTHALL, 1963;

LESSING and CATANZARO, 1964;

The data for alpine-type intrusions and basalts are compared by means of histograms in Fig. 2. With the exception of two samples which apparently did not remain closed systems, the Newfoundland serpentinite (AppM-5) and the Greenland dunite f&--l), the Rb/Sr ratios of the alpine-type ultrama& intrusions se~ent~ite (AppM-5) contains a large are uniformly low. The Newfoundland amount of talc, and the sample was obtained near the sheared contact with the GAST et al. 1964).

1280

A. M. Smm

and V. Raarca Muzmi~

country rock. The Greenland dunite (Gr-1) was subject to granulite facies tonal rne~rno~~ with the development of a metamorphic aureole, and ~~ogopi~ is seen in thin section. From the Rb/Sr and S+fSr*e ratios given in TabIe 3 growth Iines can be constructed for the alpine-type intrusions, and these are shown in Fig. 3. With the exception of the two samples which are suspected of not having remained closed systems, these strontium growth lines do not pass &rough a Sr’J’/Sre6 value as low as that for the primordia1 earth (O~SSSS),even when projected back to 4.5 b.y. ago. This indicates that the Sr in alpine-type ultramafic intrusions

OCEANIC BASALTS

CONTINENTAL BASALTS

ULTRAMAFIC

0705

0.710

0,715

0,720

0725

lNCLUSl0N.S

0.730

sre7/sr=

Fig.

2.

had at least a two-stage growth history and must have been a part of a system or systems with higher Rb/Sr ratios than the present ones at some earlier time in th,eir histories. The p~ib~ty that the alive-to u~~arna~c systems have been con~m~ated in the sialic crust in some way must be considered. Thy-section ex~nation of the samples has revealed no obvious eon~mination effects. The alpine-type ~trarna~~ material examin ed here has been intruded throughout the world into sialic material of highly variable composition, characterized by a wide range of Rb[Sr ratios. It appears unlikely that sialic contamin&ion in such diverse Rb/Sr environments could have taken place in such a way as to produce the uniformly low Rb/Sr ratios of the alpine-type ultramafic intrusions. The concentrations of the alkali elements Na, K and Rb are extremely low in the dpin8-tyPe i&llSiOnS. The strontimn isotope data, however, indicate that the intrusive material was part of a r&tively more Rb-rich (and by imp&&ion generally more alkali&h) system OP systems earlier in its history. Thus the alpine-type rdtramafic material appears to be deplete in alkalis relative to Sr and therefore

Strontium isotope and alkali element abundances in ultramafic rocks

1251

may be residual in nature. The Srs7/Srs6 ratios suggest that the alpine-type intrusions are not reIated to the ultramafic zones of normal stratiform sheets nor to basalts. On this basis Bow&s suggestion (1928; 1949) that ultramafic intrusions are the result of crystal accumulation from mafic magmas followed by mechanical intrusion is unlikely. An alternative suggestion of HESS (1938; 1955) and others, that the i

,0732

“7/sr

Sr

66

DEVELOPMENT

pg=J ALPINE-TYPE

ULTRAMAFIC

ROCKS 0 724

60

50

4.0

Fig. 3. Srs7/Srse development

30 Tsme (by)

20

for alpine-type

IO

0

ultramafic rocks.

alpine type ultramafic material has been derived from a hypothetical peridotite substratum, seems more likely, especially in view of seismic evidence for the nature of upper mantle material. Ultramajc

inclusions

in basalts and kinzberlite pipes

rat,ios of ultramafic inclusions from continental basal&, oceanic The SP/SP basalts, and kimberlite pipes (Table 3) are in general agreement with the range of Sr~7jEP ratios in basal@ as measured by other investigators (Fig. 2). This general agreement is particularly confirmed in this study in two cases where the strontium isotope ratios were measured in host basalts and their ultramafic inclusions from the Galapagos Islands and Efawaii (Table 3). No distinction can be made between ultramafic inclusions from continental basalts, oceanic basalts and kimberlite pipes on the basis of Sr87/Srs6 ratios. This isotopic evidence is in agreement with the conclusions of NIXON et al. (1963) and others, that ultramafic inclusions in kimberlite pipes are analogous to those in basalts and that both types are derived from a source of worldwide distribution, presumably the mantle. The agreement between the Srs7/Srss ratios of basalts and their ultramafic inclusion indicates a genetic relationship between them. The existence of such a

1262

A. M.

t%TJEBEB

and V. Rau

Mmtm

genetic relationship, however, does not permit a distinction to be made between any of the three following possibilities: the inclusions are crystal accumulations from the basalts ; the inclusions are the primary mantle material which gives rise to the bssalts by partial fusion; or, the ultramafic inolusions are the refractory residue of the basalts. Ross et caE.(19&t), after an extensive study of ultramafic inclusions in basalts and alpine-type ultramafic intrusions, concluded that there is a remarkable similarity in all essential relationships between the ultramafic materials of these two modes of occurrence, which is suggestive of a genetio relationship. They postulated that both have been derived from the earth’s ‘“peridot& shell”, the intrusions having been brought up from this “shell” by orogenio processes and the inclusiarwjby eruptive processes. The results of this study are not consistent with such a genetic relationship. The Srs7/Srsaratios of alpine-type ultramafic intrusions are higher than the corresponding ratios for ultramafic inclusions, and the alkali element concentration levels of the alpine-type intrusions are considerably lower than the corresponding levels of ultrama~c delusion. The differences in the alkali element concentration levels may be due to differenees in mineral composition of the samples, although such an effect does not appear to be likely, For example, the Antarctica dunite inclusion (Ant-I), which is composed entirely of olivine, has alkali element concentrations distinctly higher than those of the alpine-type dun&es. It is conceivable that the ultramafie inclusions in basalts originally had higher Srs7/Srssratios, somewhat similar to alpine-type material, but that the present ~~rnent between the S~?~Srs6 ratios of baaalts and their ultramafic inclusions has been caused by isotopic equilibration of strontium between- the inclusions and the relatively strontium-rich basalt8 magmas .

It is commonly accepted by ~trolog~ts that basic layered intrusions result from fractional crystallization of an initially basaltictmagma. The ultramafic zones of these intrusions should therefore have initial Sr&7/Sra6 ratios similar to those of basalts. A sample of peridotits from the ultramafic zone of the Stillwater Complex, Montana has a present Sr87/Srs6ratio of 04’063. Assuming an age of 2700 m.y. for this intrusion, the calculated initial Sr87~S~ ratio is 0*?029, A pyroxenite from the Mt. Nittis drill hole of the Monchegora pluton, Kola Peninsula, Russia has a Rb/Sr ratio as well as an isotopic oomposition of Sr very similar to those found in basalts. This pyroxenite shows e&&lent textural evidence for crystal settling and in the light of the present observations we conclude that the Monchegora pluton is a Iayered basic intrusion. Guumo et ad. (1962) however, have concluded that the ~trama~e rock samples ffom the Mt. Nit& drill hole are xenoliths of the mantle. A serpentinized dunite and a elinopyroxenite from the Mnskox Intrusion, located in the Coppermine River area of the District of MacKenzie, Canada, were analyzed in the present study. The high Srs7/SrsSand Rb/Sr ratios in the serpentinized dunite (Table 3) are due to the presence of approximately 3% aooessory biotite in this rock. The pyroxenite has a low Rb/Sr ratio and its ~~7~Sr~~tio can be assumed to represent an upper limit to the initial Srs7fSrmratio of the entire intrusion. Under this assumption, the age of crystallization for the s~rpentinite is 1360 m.y. as a lower limit and can be compared to the K/Ar age of 1X55m.y. (Sti, 1962).

Strontium isotope and alkali element abundances in ultramafic rooks

1253

The present Sr isotopic data for the few ultramafic samples from stratiform sheets are consistent with the idea that ultramafic zones in stratiform sheets result from fractiona. crystallization and crystal settling from a basaltic magma. Xource material of basalt Certain types of ultramafic rock which have been analyzed in the present study, namely, alpine-type ultramafic intrusions, ultramafic inclusions in basalts, and ultramafic inclusions in kimberlite pipes have at various times been proposed as either the source material which gives rise to basalts through partial fusion or as the residue left behind after the extraction of basalt. The Srs7/Srs6 ratios of alpine-type intrusions are higher than the Sr87/Srs6 ratios of basalts. This isotopic data provides a definitive evidence to the fact that the alpine-type ultramafic material cannot be the source material which gives rise to basalts by partial melting. Furthermore, the alkali element concentrations of this material are so low as to preclude the possibility of producing the concentration levels found in basalts under any realistic assumptions with regard to chemical fractionation during the partial melting process. A similar conclusion has been reached by IIAMILTON and MOUNTJOY (1965). The nature of the SrS7/Srs6 growth lines of the alpine-type ultramafic intrusions indicates that the intrusive material was part of a relatively more Rb-rich (and by implication generally more alkali-rich) system or systems earlier in its history. Thus alpine-type ultramafic rock gives every indication of being residual in nature. If the higher Sr87/Srs6 ratios of the alpine-type intrusions were the result of an increase in the Rb/Sr ratio after removal of basalt, the Srs7,Srss growth lines must intersect the basalt development region within 4.5 b.y. Figure 3 shows that this is not the case and therefore we conclude that the alpine-type material cannot be the parental residue of basalts. The strontium isotopic evidence for the existence of a genetic relationship between basalts and ultramafic inclusions in basalts does not indicate whether these inclusions are basaltic parent material, basaltic residue, or cumulate in origin, The alkali element concentrations of the ultramafic inclusions, however, are so low as to make it extremely doubtful that this material could give rise to basalts by partial fusion, as concluded by OXBURGH (1964). If the determinations of the uranium concentrations of ultramafic inclusions by TILTON and REED (1963) are correct and representative, the total radioactive heat production of this type of ultramafic material would be so low as to preclude its forming a major portion of the mantle, on the basis of heat-flow considerations. It seems, therefore, that the ultramafic inclusions in basalts are either fragments of basaltic residue from the mantle or cumulate in origin, and cannot be the source material for production of basalts. Four inclusions from kimberlite pipes, three garnet peridotites and one garnet pyroxenite which might be called an eclogite, were analyzed in this study. The Sr87/Sr86ratios obtained for the inclusions in kimberlite pipes are within the general range of Srs7]Srs6 ratios in basalts, permitting a genetic relationship between these inclusions and basalts. The alkali-element concentrations of the kimberlite inclusions are distinctly higher than those of the ultramafic inclusions in basalts, and sufficiently high to lead to basaltic concentration levels upon their partial fusion. The data obtained in this study are therefore consistent with the hypothesis of the derivation

1254

A. M. STUEBERand V. RAMA MURTHY

The eclogite inclusion might be of basalts by partial melting of garnet peridotite. interpreted as a pod of what was once liquid basaltic material which never left the source region, as suggested by RINGWOOD (1962). Thus, the present isotopic and elemental abundance data seem to support the idea of possible derivation of basalt from the partial fusion of garnet peridotite material, but not from either the alpine-type ultramafic material or from the ultramafic inclusions found in basal&. Speculations on the role of dttramafic rocks in the diferentiation

history of the earth

Experimental evidence which has a bearing on the nature, modes of genesis, and geologic significance of various types of ultrama~c rocks has been presented in this study. An attempt will now be made to synthesize the possible interpretations of this data through speculations on the role of ultramafic rocks in the di~erentiation history of the earth. The alpine-type ultramafic intrusions appear to be residua1 in nature. Many geophysicists (e.g. BULLARD, 1954; BULLEN, 1956) have called upon dunite as the most likely material to constitute at least the outer part of the mantle beneath the Mohorovicic (M) discontinuity. This suggests that the alpine-type ultramafic intrusions have beeninjectedinto the crust from a residual ultramafic layer immediately beneath the N discontinuity at the base of the continental crust, similar to the residual dunite and peridotite layer postulated by RINCWOOD (1962). It was shown above that alpine-type ultramafic material can be neither the source material nor the parental residue of basalts, at least not of the post-Precambria,n basalts to which strontium isotope analyses have genesally been limited. The relativety high Sra7/Sr8” ratios in the alpine-type u~tramafic material require a multiple or at least a two stage evolution. Models of two sta,ge evolution for the crust-mantle system have recently been proposed by PATTERSON and TATSUIIIOTO(1964) and by BIRCI~ (1965). BIRCH suggested that the earth accreted about 5000 m.y. ago and that heating by radioactivity and tidal friction resulted in the formation of a liquid iron core after about 500 m.y. Core formation was accompanied by conversion of gravitational energy to heat, with resultant fractional melting of the mantle, concentrating the radioactive and lithophile elements in the upper mantle. Formation of stable continental crust became possible after this upward concentration and decay of radioactive elements, beginning approximately 3500 m.y. ago. Crustal ~fferentiation has left the subcontinental mantle, but not the suboceanic mantle, impoverished in radioactive and lithophile elements. We suggest that during the iI~termediate stage between core formation and crust formation, a stage in which the upper mantle was enriched in the radioactive and lithophile elements, the Sr87/Srs6 ratios in what is now the alpine-type ultramafic residual layer grew to nearly their present values. The subsequent crustal differentiation produced the residual ultramafic systems which are now found as alpine-type intrusions. In order to produce the observed Sr87/Sr86 growth during the 1000 m.y. intermediate stage, a Rb/Sr ratio of approximately 0.17 is required in the upper mantle. This is not an improbable Rb/Sr ratio, especially in view of current estimates of this ratio for the sialie crust. The suggested two-stage growth of the Sti7/Srs6 ratios in alpine-type ultramafic material is shown schematically in Fig. 4. Most early geologic studies of alpine-type ultramafic intrusions made very little

Strontium isotope and alkali element abundances in ultramafic rocks

1255

mention of any field relationsl~ips with gabbroic rocks. Thus HESS (1938) stressed the idea that ultramafic intrusions containing little or no gabbroic material are typical of erogenic belts. More recent work has tended to emphasize a close association between gabbro and ultramafic rock in alpine-type intrusions. For example, GUILD (1947) described large amounts of gabbro intimately mixed with ultramafic rock in eastern Cuba; GONZALES et al. (1957) reported alternating belts of gabbro and ultramafic rocks in the Philippines; GREEN (1961) described interlayered and transitional gabbro and peridotite in the Papuan ultramafic belt; and THAYER (1963) Core Formotion

Major Crustal Formotlon

SP pi

53

I

I

40

35

!

1

2~3 lime

I I.0

1

Iby)

Fig, 4. Hypothetical Sr8’/Srssdevelopment in alpine-type ultramafic material, mapped extensive gabbro with the ultramafic rock of the Canyon Mountain Complex in Oregon. SMITH (1958), in his study of the Bay of Islands Complex, suggested a gradational series between alpine-type peridotite-gabbro intrusions and stratiform sheets, and that the different modes of occurrence need not reflect different primary magmas. He further suggested that differences in cooling, emplacement history and post-emplacement deformation have caused the variations in the relative abundance of gabbroic and ultramafic rocks found in the different intrusions. TWAYER (1960) made a comparative study of many of the physical features of alpine-type and truly stratiform complexes and concluded that the alpine-type complexes were intruded as differentiated crystal mushes, the differentiation having taken place somewhere deep in the crust or perhaps in the mantle, and that gabbro and peridotite were mixed during emplacement. It is our opinion that this differentiation may have taken place during the major phase of the crustal differentiation of the upper mantle in BIRCH’S evolutionary scheme, and that such a differentiation history would be revealed by the Srs7/Srae ratios of the gabbros associated with the alpine-type ultramafic intrusions. Under the present hypothesis the ultramafic rock-gabbro contact is taken to represent the upper-mantle-lower-crust boundary. The strontium isotopic study of ultramafic inclusions in basalts has disclosed a genetic relationship between the inclusions and basalts. On the basis of their low alkali element concentrations, it is our opinion that the ultrama~c inclusions represent 5

A. M. STUEBERand Y. RAMA MLTRTEY

12ciG

fragments of basaltic residue from the mantle rather than fragments of the parent material which gives rise to basalts by partial fusion. The possibility that the inclusions are crystal accumulations from basaltic magmas cannot be exciuded, although a large body of evidence would seem to favor a xenohthic rather than a cumulate origin (e.g. WILSHIRE and BINNS, 1961). The world-wide occurrence and similar mineral composition of ultramafic inclusions in basalts suggests that they constitute a portion of the mantle beneath both the continents and oceans, provided they are not cumulate in origin. According to BIRCH (1965), the average concentrations of radioactive elements in the upper mantle are 0.05 ppm U and 500 ppm K beneath the continents and 0.1-0.2 ppm U and 1000-2000 ppm K beneath the oceans. On this basis ultramafic inclusions in basal&, with approximately 0.006 ppm U

Primary garnet Fig.

6.

Mantle Moterlai (?)

pefidotite Pyrolite (7)

Proposed relationshipsof ultramafk rocks to present mantle-crust structure.

(TILTON and REED, 1963) and 100 ppm K (this study), cannot be volumetrically important in the upper mantle, at lea.stunder the oceans. We suggest that they form a small transition zone between the alpine-type ultramafic residue and the primary mantle material beneath the continents, and a small residual layer which grades downward into primary mantle material beneath the oceanic crust. The zone of basaltic ultramafic inclusion material is therefore probably continuous, or nearly so, throughout the upper mantle. The data obtained in this study are consistent with the hypothesis of the derivation of basalts by partial melting of garnet peridotite, such as that found as inclusions in continental kimberlite pipes. It is assumed that such material forms the primary mantle material beneath both the continents and oceans, although it is entirely possible that there is no available sample of the primary mantle material beneath the oceans. It is perhaps better to designate the primary mantle material as some mixture of basalt plus residual dunite, such as RWGWOOD’S pyrolite (19621, although this mater% would be very similar to garnet peridotite. The primary mantle material, be it garnet peridotite or pyrolite, would be found in the upper mantle immediately beneath the basaltic ultramafic inclusion zone, and the material is here suggested as the source of basalt magmas. The proposed relationships of the various types of ultramafic rocks to the present crust-mantle structure are schematically presented in Big. 6. We feel that these relationships, along with the assumed terrestrial differentiation history, can account for the observed SPfSrs6 ratios of alpine-type ultramafic rocks and their residual nature, the common association of gabbro with the alpine-type ultramafic intrusions, the observed genetic relationship between basalts and their ul~rama~c inclusions,

Strontium isotope and alkali element abundances in ultramaflc rocks

1257

and the observed differences between

the nature of alpiile-type ultramafi~ material and ultramafic inclusions in basalts, In addition, the proposed model is consistent with certain important geophysical, geoehemical, and petrologic observations. &IACDONALD(1964) stated that heat flow and gravity measurements imply that the structure of continents extends to great depths, on the order of 500 km. The average equality of gravity and heat flow between the continents and ocean basins argues for dominant vertical segregation of material in continent formation. Although the average heat flow in continental areas equals that from the ocean basins, the thick sialic continental crust contains a much larger amount of the heat-producing radioactive elements than the thin basaltic oceanic crust. In order to account for the observed equality of heat flow at the surface, there must be a zone in the upper mantle beneath the Continental crust which is extremely depleted in radioactive elements. This zone would coincide with the alpine-type ultramafic residual layer under the present hypothesis. In order to explain certain features of the lead isotope evolution in the crust of the earth, PATTERSON and TATSUM~TO (1964) proposed a two-stage evolutionary model for the earth, in which continental segments were primarily formed in the interval 2500-3500 m.y. ago. An important part of their model is a thick residual layer underlying the continental crust, seriously depleted in U and Pb, which served to isolate the crust from the remainder of the mantle. Thus, the alpine-type ultramafic residual layer underneath the continental crust would also be consistent with the requirements of the lead-isotope evidence. The experimental work of BOYD and ENGLAND (1963) indicated that the amount of Al~O~~hichenterstheo~hop~oxene lattice is sympathetically related toin~reasing pressure, up to 19.7 kb. At higher pressures the amount of A&O, in orthop~oxene decreases, but garnet appears as a stable phase. O’HARA and XERCY (1963) and others have shown that enstatite from garnet peridotite inclusions in kimberlite pipes usually contains less Al,O, than enstatite from peridotite inclusions in basalts, which implies that garnet peridotite inclusions have been derived from greater depths in the mantle, consistent with the relationships proposed here. The thickness of the various ultramafic layers of the mantle are undefined in the present hypothesis, although in our opinion the alpine-type ultramafic residual zone is volumetrically far more important than the basaltic ultramafic inclusion zone. Perha.ps the transition to primary mantle material contributes to the seismic low-velocity zone, as suggested by others (RI~GWOOD, 1962; OXBUROH, 1961), thus establisl~ing approximate values for the depth of this transition. As a consequence of the deep structure of continents and dominant vertical segregation of material in continent formation, the suboceanic upper mantle should be enriched in lithophile elements relative to the subcontinental mantle. This implies that volcanic material derived from the suboceanic mantle should be more radiogenic than that derived from the mantle under the continents, which is not found to be the case for strontium. The relatively non-radiogenie oceanic basnlts must therefore be derived from that part of the mantle beneath the zone in which the lithophile elements are concentrated, to be consistent with the proposed relationships. wish to acknowledge our thanks to the followingfor providingsome of the ultramaficrock samples: M. H. BATTEY,K. CONDIE,A. E. J. ENQEL, D. 3%GREEN,W. H. GROSS, Acknowledgement-We

1268

A. M. STUEBERand V. RAMA MURTHY

H. II. HESS, G. V. D. KAA~E:N, G. KENNEDY, B. MASON,A. R. MOBIRNEY,E. R. W. NEILLE, D. RAUA~, C. S. Ross, C. H. SMITH,H. SOR~SEN and G. YO~EINO,and the Geological Surveys of South Africa and Tanganyika, We are indebted to Professors G. GOLES,J. A. GRANTand W. C. PHINNEYfor critical reading of the manuscript and many valuable discussions. This researchwas made possible by the financial support provided by a National ScienceFoundation Grant, NSF gp-3448 (V.R.M.) and a grant from the Sigma Xi-RESA researchfund. REFERENCES BIRCUF. (1965) Speculations on the earth’s thermal history. &II. asoeol. &c. Am. 76, 133-154. BOWEN N. L. (1928) The Evolution ofthe Igneous Rocks. Princeton University Press. BowENN.L. andTuTTLEO.F. (1949) ThesystemMgO-SiO,-H,O. &&. ffeoE.Soc.Am. 8&439-460. BOYD F. R. and ENGLANDJ. L. (1963) Some effects of pressureon phase relations in the system Y.B. Carnegie Inst. Wash. @, 121-124. MgO-Al,Os-SiO,. BULLARD E. C. (1956)The interior of the earth. In The berth as a P&net (Ed. G. P. KUIPER). Vol. 2, 57-137. Univ. of Chicago Press. BULLENK. E. (1956) Seismology and the brosd structure of the earth’s interior. Physics Chem. Earth 1,68-93. COMVSTON W. and JEFFERYP. M. (1959) Anomalous common strontium in granite. Nature 184, 1792-1793. COMPSTON W., JEFFERYP. M. and RILEY 0. II. (1960) Age of emplacement of granites. Nature 186, 702-703. FARCE G. and HURLEY P. M. (1963) The isotopic composition of strontium in oceanic and continental basalts. Application to the origin of igneous rocks. J. Petrol. 4, 31-50. FLYNNN. F. and GLENDENINL. E. (1959) The half-life and beta spectrum of Rbe’. Phys. Rev. 116, 744-748. GAST P. W., TILTONG. R. and HEDGEC. (1964) Isotopic compositionof lead and strontium from Ascension and Gough islands. Science145,1182-1185. GERL~NGE. K. S~~~OLYUROVY. A., KOLTSOVA T. V., MATVEYEVAI. I. and YAKOVLEVAS. V. (1962) Dating of mafle rocks by the K-Ar method. Geochemists 11,1055-1062. GONZALES M. L., PEOPLESJ. W., FERNANDEZ N. S. and VICTORIA V. (1957) Ultramafie and mafic rocks of the Zambales Range, Luzon, Philippines(Abstract). B&E. Geol. Sot. Am. 68, 1736. G~~EEND. H. (1961) Ultramafic breccias from the Muss valley, Eastern Papua. Geol. Mug. 98, l-25. GUILD P. W. (1947) Petrology and structure of the Moa chromite district, Oriente Province, Cuba. Trans. Am. Geophys. Union $8, 218-246. HAMILTONW. and MOUNTJOYW. (1965) Alkali content of alpine ultramafic rooks. Geochim. Cosmochim.Acta 29, 661-671. HEDGE C. E. and WALTEALLF. G. (1963) Radiogenic strontium-87 as an index of geologic processes. Sc&ce 140, 1214-1217. HESS H. H. (1938) A primary peridotite magma. Am. J. Sci. 35, 321-344. HESS H. H. (1955) Serpentines,orogeny and epeirogeny. In (Ed. POLDERVA~T,ALE) Cmt of the Ear&--a symposium. Geol.SOG.Amer. Special Paper 62, 391407. LANPEIERE M. A., WASSER~JRGG. J. and ALBEEA. L., (1964) Redistribution of strontium and rubidium isotopes duringmetamorphism, World Beater Complex, Panamint Range, California. in Isotopic and Cosmic Chemistry, 269-312. North Holland. LEAKING I?. and CATANZARO E. J. (1964) Srs7/Srssratios in Hawaiian lavas. J. Geophys. Res. 69, 1599-1601. MACDONALD G. J. F. (1964) The deep structure of continents. Scieltce143, 921-929. NIXON P. H., v. KNORRINQ0. and ROOKE J. M. (1963) Kimberlites and associatedinclusionsof Basutoland. A mineralogicaland geochemicalstudy. Am. Miner. 48, 1090-1131. O’HARA M. J. and MERCY E. L. I?. (1963) Petrology and petrogenesis of some garnetiferous peridotites. Trans. Roy. Sot. Ed&b. 65, l-64. OXBOROH. E. R. (1964) Petrologicalevidencefor the presenceof amphibole in the upper mantle and its petrogenetic and geophysical implioations. CeoE.&fag. 101, 1-19,

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PATTERSONC. and TATSUMOTOM. (1964) The significance of lead isotopes in detrital feldspar with respect to chemical differentiation within the eart.h’s mantle. Geochim. Cosmo&m. Acta 28, l-22. RINGWOODA. E. (1962) A model for the upper mantle. J. Geophys. Res. 67, 857-866. ROSS C. S., FOSTERRI. D. and MYERS A. T. (1954) Origin of dunites and of olivine-rich inclusions in basaltic rocks. Am. Miner. 39, 693-737. SMITHC. H. (1958) Bay of Islands igneous complex, Western Newfoundland. Mem. Geol. SZLTL*. Canada 290. SMITHC. H. (1962) Notes on the Muskox intrusion, Coppermine River area, District of Mackenzie Mem. Ceol. Sura. Canada paper 61-25. STUEBERA. M. and GOLESG. G. (1966) Abundances of Na, Mn, Cr, SC and Co in ultramafic rocks. To be published in Geochim. Cosmochim. Actn. THAYER T. P. (1960) Some critical differences between alpine-type and stratiform peridotitegabbro complexes. 21st Int. Geol. Congress, Copenhagen, Rept., pt. 13, 247-259. THAYER T. P. (1963) The Canyon Mountain complex, Oregon, and the alpine mafic magma stem. U.S. Geol. Surv. Prof. Paper 475-C, 82-85. TILTON G. R. and REED G. W. (1963) Radioactive heat production in eclogite and some ultramafic rocks. In Earth Science and Meteoritics, (Ed. J. GEISS and E. D. GOLDBERG), 31-43. North Holland. WILSHIRE H. G. and BINNS R. A. (1961) Basic and ultrabasic xenoliths from volcanic rocks of New South Wales. J. Petrology 2, 185-208.