86Sr in rocks from the Mid-Indian Oceanic Ridge

EARTH AND PLANETARY SCIENCE LETTERS 18 (1973) 223-228. NORTH-HOLLANDPUBLISHINGCOMPANY

[]

K, R b , Sr A N D 8 7 S r / 8 6 S r I N R O C K S F R O M T H E M I D - I N D I A N O C E A N I C R I D G E * K.V. SUBBARAO ** Institute of Marine Sciences, University of Alaska, College, Alaska 99701, U.S.A. and Carl E. HEDGE U.S. GeologicalSurvey, Denver, Colorado 80225, U.S.A. Received 9 August 1972 Revised version received 3 January 1973

The 87Sr/86Sr ratios of Mid-Indian Oceanic Ridge (MIOR) basalts are nearly identical (0.7032 to 0.7035), with the exception of one more highly radiogenic sample (0.7043). These values are consistently higher than the strontium isotopic ratios of the ridge basalts from Atlantic and Pacific Oceans, suggesting that the source of the MIOR basalts was depleted in alkalies more recently and/or to a lesser degree than the basalts from other oceans.

1. Introduction 20*

In this paper, we present concentrations of K, Rb, Sr and 87Sr/aaSr ratios for the dredge and cored samples from Mid-Indian Oceanic Ridge (MIOR), which lies as an inverted "Y" in the center of the Indian Ocean. The analyzed samples are mainly recovered from the central part of the MIOR (fig. 1). A few samples (published analyses) from the islands of R6union and Rodriguez are also discussed in order to show a regional distribution of rocks in the Indian Ocean from different tectonic environments. The location, depth, topography and brief petrographic description of the samples we have analyzed from the MIOR are given in table 1. Earlier investigations on the MIOR are confined to petrologic, bathymetric, magnetic and seismic reflection studies [ 1 - 5 ] . It has been shown that most of the rocks recovered from MIOR are mainly low-K tholeiites [1], whereas gabbro, anorthosite, lherzolite and pyroxenite are exposed in the fracture zones [2, 5]. Cann [6] reported spilites from Carlsberg Ridge. Engel et al. [ 1] record the near absence of sedi_, *Institute of Marine Sciences Contribution No. 176. **Present address: Department of Earth Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2.

I O° 0o

I O*

2 0°

50 °

40 •

Fig. 1. Localities of samples from Mid-Indian Oceanic Ridge. Stippled areas represent oceanic ridges and associated rises shallower than 4 000 m. (Base is from a portion of the Physiographic Diagram of the Indian Ocean by B.C. Heezen and M. Tharp, copyright c 1964 by B.C. Heezen and published by the Geological Society of America. Reproduced with permission.)

224

K. V. Subbarao, C.E. Hedge, K, Rb, Sr and 87Sr/86Sr in rocks from the Mid-Indian Oceanic Ridge

,~.

~ .

,. = ~ " ~

o ~ ~

"~

~

~

~

~

0

~

0

"~ ~

~..~

~ o

.~

0

~ o~

~ ,..,

=

~~'~ ~ ~ •~ .~ ~ ,~ =

~ o o .

oa

o'~

o~

r,~

~

~°~

o

¢xl

o

o

e'q

~

0

o

"~ •~

0

b~

0

,~

< ~-8

~ ~o

~ ~.~

.I= "~

.=

~

.~

t~ 0

eL.~ eel

o~

""~

o

o~

~8

o

o

~1~

¢~.11 ~ -

o~

e-

~.~

.~o

~o o

,,~

:n

=

•~

0

0

0

"t3 o .=

~.~ ~a

0

eL

o~

~

o_

o

~ eL

© ,.o n~ 0

0

~rm

o

I

o

o

K. V. Subbarao, C.E. Hedge, K, Rb, Sr and 87Sr/S6Sr in rocks from the Mid-Indian Oceanic Ridge

~d 0

~.~

~

~"~

,~ ~ . ~

~

~'-" ~

~ o ~ _ ~

~ ~ ~ ~ ~.~.~ ~

0

,..~

~--

~

~

~ ~ .~

~ ~

~ o

~,

= ~

.~

_

..~ =

"0

~1~ ~'~

0 c~ 0

~o ~

°°

2

0

~

oJ

0

e

~z ~ I

o

I

o

0

tt3

225

226

K. V. Subbarao, C.E. Hedge, K, Rb, Sr and 87Sr/86Sr in rocks from the Mid-Indian Oceanic Ridge

ment-filled depressions with heat flow values higher than that of the oceanic average, and the presence of only volcanic shards in cores taken nearby by the Monsoon Expedition, and present these as evidence in support of abundant recent volcanism.

2. Experimental details The dredge samples were sawed to remove the marginal altered portions and all secondary fillings within the cracks. Petrographic study was carried out for any possible mineral alterations. Small slabs were cut, wiped with 1 N HC1, washed for about an hour in hot double-distilled water and then were crushed. The potassium concentration was measured by atomic absorption spectrometry. The concentration of Sr was determined by X-ray fluorescence using basalts analyzed by isotope dilution as standards. The Rb concentration was measured by isotope dilution techniques and the Sr isotopic composition was measured on the USGS (U.S. Geological Survey, Denver, Colorado) 6"-radius, 60 ° sector mass spectrometers using triple rhenium filament mode of ionization. All strontium isotope measurements were made on a mass spectrometer with data acquisition through an integrating digital voltmeter and paper tape system (Stacey et al. [7]). 87Sr/86Sr ratios were normalized to correspond to 86Sr/88Sr of 0.1194. The 875r/86Sr ratio of the E and A SrCO3analyzed on the USGS mass spectrometer is 0.7080. The standard deviation for a single 875r/86Sr analysis is -+ 0.0002 at 95% confidence level. Analytical uncertainties for concentrations are as follows: K -+ 3%, Rb + 2%, Sr -+ 5%.

3. Results and discussion The characteristic concentrations of K, Rb and Sr in ocean-ridge basalts are now well documented (see Hart [8] for a recent summary). These rocks are low in K and Rb (0.116% K and 1.I ppm Rb; Hart's averages), contain about 135 ppm Sr and have high K/Rb and low Rb/Sr ratios. The samples from the MIOR reported here (table 2) have K, Rb and Sr concentrations within the ranges of samples studied from other ridges, but K and Rb are higher than Hart's average values for the Mid-Atlantic Ridge and the East-Pacific Rises.

The 87Sr/86Sr ratios of basalts dredged from the oceanic ridges are distinctive in that they are the lowest values found in any recent volcanic rocks. Hart [8] reported values of 0.7028 and 0.7025 for composites of samples from the Atlantic and Pacific respectively. Hedge and Peterman [9] and Subbarao [10] found a similar average of 0.7026 (0.70220.7031) for 12,samples from the Gordo and:Juan de Fuca Rises, and 0.7028 (0.7025-0.7035) for 8 samples from the East Pacific and Chile Rises, respectively. The 878r/86Sr ratios, found in this study, for samples from the MIOR are higher than those cited above from the Atlantic and Pacific. Five of our MIOR basalts have nearly identical 87Sr/86Srratios of from 0.7032 to 0.7035. The sixth gives an unusually high value, for an ocean-ridge basalt, of 0.7043. This sample is fresh with a low H20 + content of 0.55%. Hart [8] reported potassium and rubidium concentrations of 1146 ppm and 0.99 ppm respectively for a sample from the Carlsberg Ridge (see fig. 1). Cann [6] also obtained concentrations of K, Rb and Sr for another sample from this ridge (table 2). These meager data suggest that the Carlsberg Ridge basalts may have lower concentrations of K and Rb than basalts from the central part of the MIOR. The K/Rb ratios of 750 to 1000 are normal for basalts of this chemical composition, but since Rb tends to be higher than the average ocean-ridge basalts, the Rb/Sr ratios are a little higher than average. Nonetheless, only the sample with the highest Rb content (IO-35) has a high enough Rb/Sr ratio to have produced its 87Sr/86Srfrom a primordial 87Sr/ 86Sr of 0.699, even during all of the 4.5 by of the earth's history. This deficiency of the Rb, relative to Sr, necessary to have produced the observed 875r/86Sr ratios is another feature which appears to be characteristic of ocean-ridge basalts in general, and such deficiency was a major factor leading to Gast's [11] proposal that the sources of ocean-ridge basalts has been depleted in many large cations (including Rb) by previous volcanism. Peterman and Hedge [ 12] expanded on Gast's depletion model and noted that differing 875r/86Sr ratios could result either from different times of depletion of the respective sources or from different degrees of depletion. Various degrees of depletion should be reflected by variations in such chemical parameters as Rb/Sr and K20/K20 + Na20, whereas different times of depletion would be reflected only in the 87Sr/86Sr ratios. The K and Rb

227

K. V. Subbarao, C.E. Hedge, K, Rb, Sr and 87Sr/86Sr in rocks from the Mid-Indian Oceanic Ridge

Table 2 Analytical data. Sample No.

Rock type

K%

Rb

Sr

K20

(ppm)

(ppm)

K20 + Na20

Rb/Sr

STSr/S6Sr Reference a

0.083

358

0.374

0.7079

IO-8

Serpentinized peridotite

0.041

1.16

IO-10

Porphyritic alkali basalt Glassy olivine tholeiite

0.182

2.44

144

0.064

748

0.017

0.7033

0.191

2.39

179

0.059

797

0.013

0.7043

Tholeiite (glassy)

0.141

1.38

110

0.056

1022

0.012

0.7032

10-14 IO-33

3.1

K/Rb

10-35

Tholeiite

0.257

3.37

136

0.095

763

0.024

0.7033

IO-37

Tholeiite

0.232

2.36

142

0.081

985

0.016

0.7032

10-38

Tholeiite

0.232

2.51

137

0.082

926

0.018

0.7035

5111.7 b

Tholeiite

0.1146 c

0.993 c

5111.2 b

Tholeiite

0.135 e

1.0e

144 e

Mid-Atlantic composite tholeiite

0.126

1.16

East Pacific composite tboleiite

0.106

1.06

0.048 d

1154

0.051

1350

0.007

140

1080

0.008

0.7028

Hart [8]

131

1040

0.008

0.7025

Hart [8]

Hart [81 Cann [6]

a This work, unless otherwise designated. b Carlsberg Ridge. c Concentrations by isotope dilution. d K and Na concentrations from Cann [6]. e Concentrations by X-ray fluorescence. contents of these MIOR, although slightly higher than the averages of other ridge tholeiites, are within the ranges found in other localities. The S7Sr/86Sr ratios are outside the ranges thus far reported for other ridge tholeiites and, therefore, the higher SVSr/S6Sr ratios of the MIOR basalts are seemingly due primarily to a later time of depletion of K and Rb in the mantle source. The depletion of the source of MIOR basalt, may have occurred several hundred million years later than the depletion of the source of the basalts from the East Pacific Rises. A similar interpretation was given by Peterman et al. [13] for even higher STSr/S6Sr ratios from the Troodos Massif, which is believed to be a Mesozoic ocean-plate remnant. McDougall and Compston [ 14] found STSr/86Sr ratios of 0.7038 to 0.7044 and 0.7037 to 0.7038 for a variety o f rocks from R4union and Rodriguez Is-

lands respectively. Hamilton [ 15 ] also reported s 7Sr / 86Sr ratios of from 0.7046 to 0.7053 for rocks from R4union (all published data adjusted to an E and A value of 0.7080). These islands thus appear to have higher STSr/S6Sr ratios than the Mid-Indian Oceanic Ridge basalts. We therefore have a situation similar to that found in the Pacific and Atlantic Ocean basins where the island basalts are also consistently higher in STSr/S6Sr than the ridge basalts. In all three oceans the sources of the ridge basalts apparently have undergone, in the geologic past, a depletion of Rb relative to Sr, whereas sources of the island basalts either were not depleted or were depleted but to a lesser degree. We have also analyzed a serpentinized peridotite ( I O - 8 , table 2). This sample is extremely poor in K and Sr. Its STSr/S6Sr ratio is very high - probably the result o f the interaction with sea water (STSr/S6Sr = 0.7090) during serpentinization.

228

K. V. Subbarao, C.E. Hedge, K, Rb, Sr and 87Sr/86Sr #1 rocks from the Mid-Indian Oceanic Ridge

4. Conclusions

References

87Sr/86Sr ratios of MIOR basalts appear to be higher than those found in ridge tholeiites from other oceans. Potassium and rubidium contents also tend to be higher, on the average, but are within the ranges found along other ridges. The source material of the MIOR basalts has no doubt been depleted in potassium and rubidium, as have the source of other oceanridge basalts, but probably to a somewhat lesser degree and at a later time. Strontium isotopic differences between MIOR tholeiites and alkali island basalts (R~union and Rodriguez Islands) argue in favor of the generation of the former from a depleted mantle, reflected in terms of Rb/Sr ratios lower than the least depleted mantle which provides the source for the island basalts. Crustal-mantle processes would be vital in maintaining different depletion zones in the mantle. Although some sort of convection is undoubtedly operative, it does not appear to be an effective homogenizer, since isotopic differences still exist in the mantle.

[1] C.G. Engel, R.L. Fisher and A.E.J. Engel, Igneous rocks of the Indian Ocean floor, Science 150 (1965) 605. [2] C.G. Engel and R.L. Fisher, Lherzolite, anorthosite, gabbro, and basalt dredged from the Mid-Indian Ocean Ridge, Science 166 (1969) 1136. [3] R. Hekinian, Rocks from the Mid-OceanicRidge in the Indian Ocean, Deep-Sea Res. 15 (1968) 195. [4] R.L. Fisher, J.G. Sclater and D.P. McKenzie, Evolution of the Central Indian Ridge, Western Indian Ocean, Geol. Soc. Am. Bull. 82 (1971) 553. [5] R.Hekinian, Gabbro and pyroxenite from a deep-sea core in the Indian Ocean, Marine Geol. 9 (1970) 287. [6] J.R. Cann, Spilites from the Carlsberg Ridge, Indian Ocean, J. Petrol. 10 (1969) 1. [7] J.S. Stacey, E.E. Wilson, Z.E. Peterman and R. Terrazas, Digital recording of mass spectra in geologic studies, I, Can. J. Earth Sci. 8 (1971) 371. [8] S.R. Hart, K, Rb, Cs, Sr and Ba contents and Sr isotope ratios of ocean floor basalts, Phil. Trans. Roy. Soc. Lond. A 268 (1971) 573. [9] C.E. Hedge and Z.E. Peterman, The strontium isotopic composition of basalts from the Gordo and Juan de Fuca Rises, Northeastern Pacific Ocean, Contr. Mineral. Petrol. 27 (1970) 114. [10] K.V. Subbarao, The strontium isotopic composition of basalts from the East Pacific and Chile Rises and abyssal hills in the Eastern Pacific Ocean, Contr. Mineral. Petrol. (in press). [ 11 ] P.W. Gast, Trace element fractionation and the origin of tholeiitic and alkaline magma types, Geochim. Cosmochim. Acta 32 (1968) 1057. [ 12] Z.E. Peterman and C.E. Hedge, Related strontium isotopic and chemical variations in oceanic basalts, Geol. Soc. Am. Bull. 82 (1971) 493. [13] Z.E. Peterman, R.G. Coleman and R.A. Hildreth, 87Sr/ 86Sr in mafic rocks of the Troodos Massif, Cyprus, U.S. Geol. Survey Prof. Paper 750-D (1971) D157. [14] I. McDougall and W. Compston, Strontium isotope composition and potassium-rubidium ratios in some rocks from R~union and Rodriguez, Indian Ocean, Nature 207 (1965) 252. [ 15] E.I. Hamilton, Isotopic composition of strontium in a variety of rocks from R~union Island, Nature 207 (1965) 1188.

Acknowledgments We are grateful to many individuals for assistance in this study. D.W. Hood, University of Alaska, encouraged the study and provided funds. G.S. Clark, A.C. Turnock and H.D.B. Wilson provided laboratory facilities at the University of Manitoba in the early stages of this work. K. Ramlal, also of the University of Manitoba, helped with the atomic absorption spectrometry. W.R. Riedel, Scripps Institution of Oceanography of the University of California, M. Ewing and R. Hekinian, Lamont-Doherty Geological Observatory of Columbia University, provided dredge and core samples, respectively. The work at that Observatory was supported by ONR contracts N 0 0 0 1 4 - 6 7 - A - 0 1 0 8 004 and NSF Grant 1969 OB. R. Hekinian furnished three thin sections for study. Z.E. Peterman and M. Tatsumoto, U.S. Geological Survey, reviewed the manuscript and provided helpful discussions; W.T. Henderson, R.A. Hildreth and W.P. Doering, also with the U.S. Geological Survey, provided technical assistance.