Strontium isotopes in weathering profiles, deep-sea sediments, and sedimentary rocks

Geochtmicaet CosmochimicaActa, 1060, Vol. 33, pp. 1621 to 1552. PergamonPrean. Printed in NorthernIreland

E. JULIUS DAS~H* Department of Geology and Geophysics, Yale University, New Haven Connecticut 96520 (Received 20 Jamuaq

1989; accepted in revised fm

8 July 1969)

Al&&--Modifications in the rubidium-strontium-strontium isotope system during IOWtemperature, geologio processes have been determined for major lithic units. Rubidiuxnstrontium ratios in basalt and granite increase markedly during weathering, but strontium isotopic composition is not altered significantly in many pro6lea. These relations may be useful in distinguishing active and relict weathering profiles. Deposition of aluminosilicate detritus iu the marine environment may result in a further increase of rubidium-strontium ratios for the detritus, owing to rubidium fixation. Strontium in even the finest fraction of &nninosilicates deposited in the ocean, however, apparently does not equilibrate with marine strontium, even after prolonged oontaet with sea water and interstitial marine water. Thus the isotopio composition of stronti~ in deep-sea sediments may be used as au indicator of geologic provenance. Diagemsis of aluminosilicates does not generally result in equilibration of strontium isotopes, except possibly after the sediment has been deeply buried. This lack of strontium isotope equilibration during low-temperature processes places constraints on the dating of sediments and sedimentary rooks by the rubidium-strontium method.

INTRODUCTION THIS study began with an investigation of strontium isotopes in deep-sea sediments in an attempt to define the present distribution of strontium isotope composition as a function of location in the oceans. The results led to a study of the source8 of variation as a result of weathering and erosion, and the role of diagenesis in modifying the record during lithification. Concentrations of rubidium and strontium in major geologic units are compiled in Table 1. The naturally occurring isotopes of rubidium and strontium are in the mass range 84-88 and are stable except for Rb *‘. Strontium isotopes produced in nuclear reactions (Sr89, SF) have wide ~st~butions and are useful for short-term earth science problems. Natural ~actionation of isotopes at thii high mass level has not been observed. Variations in strontium isotope ratios result strictly from the beta decay of Rbs7 to Sr *‘. Thus the abundance of Srs7 in a given closed system is a function of the initial amount of SP in the system, the Rb/Sr ratio, the age of the system, and the decay constant of Rbs7 (A = 1.47 x lo-rl yr-l). The abundance of SF commonly is given with reference to Sr 86, the non-ra~ogenic strontium isotope with the most similar abundance. * Present address: Department of Geophysics University, Canberra, A.C.T., Australia. 1521

and Geochemistry,

Australian

National

Table 1. Aluundance of rubidium and strontium in major geologic I’havc

lib, (Pm --

Sr, @I’m)

_.-._.

units

lCb/Sr

~..__._-.

I&eneousrocks: Ultrabasio: Alpine-type Stratiform sheets Basaltic Granit,ic: High calcium Low calcium Sedimentary rocks: Smdstone Sll& Carbonate Deep-ses sediments:

c&y Carbonate Water: Sea mater Fresh water

0.4Ul 3”’

7(l)

0.06

4tY” 465

0.06 0.065

110 170

440 100

0.25 l-70

60 140 3

20 300 610

3.0 0.47 @006

170(")

0.41 0.005

30

?O@) 10 0.12(” 0.0011(4’

2000 316, 0.06”’

0.015 O-018

Source of data: (1) &CUBBEB and ~~WTHY (1966). (2) This paper (Table 8). (3) BOLTER et d. (1964). (4) 9BhRxAR et az. (1963). (6) Avorsge fromthis paper(Table 8), andTumm (1964). (6) ODUM (I%'%). (7) T~~KIILN (1988). All other rubidium data from HO~T~~AN (1957); dl other strontiwn data from TURISKLW and Kuxz (1966).

Strontium isotope dating and tracer studies of materials involved in lowtemperature geologic processes have been less common than those of high-temperature processes, but several important types of studies have been made. One of the most promising applications is to the whole-sample dating of time of deposition of sediments and sedimentary rocks, particularly shales (e.g. BOFIWER and COMPSTON, 1967; BOFING~E~~et al., 1968). The Sra7/Srss ratio has been used extensively by FAURE and co-workers to determine the sources and history of marine and fresh-water bodies such as Hudson Bay (FAUXIG et al., 19671, Lake Huron (FAUXE et al., 1967) and Lake Vanda, Antarctica (JONESand FAURE, 1967). HABT and TILTON (1966)reported on the isotopic chemistry of lead and strontium in water and segment from Lake Superior. From these studies it is clear that strontium isotope composition of the water is not the same as that of the assoeiated sediment, WIC~AK (1948) suggested that oceanic strontium should reflect the isotopic composition of the crust at any given time and that the SrE7/Srs6ratio ofsee w&ter would increase through time. Marine limestones expectably would record this monotonic increase. More recent studies (GAEIT, 1955; Grsaxz~a and SHUKOLYUKOV, 1957; HEDGE and WALTHALL, 1963), have shown that this idea does not result in a workable dating technique. Because the residence time of strontium in the se& (about a million years) is long compared to the mixing time for the oceans (about a thousand years), dissolved sea w&er strontium ha.s a uniform Srs7~Sre6ra&io of about 0.7’09 (CAST, 1955; COXPSTON and PIDQEON, 1962; HEDGE and WALTHALL, 1963; FAUEE etaE.,

Strontium

isotopesin weatheringprofilt~and deep-seasediments

1623

1968; l&knvm~ and BEISER, 1968). Differences in the Sr87~Sr8E ratio from the present value of carbonate and other minerals precipitated from sea water in the past thus are of considerable interest in studying the development of the crust through time. Recently, PETERMANet al. (1967) have shown, from analyses of carbonate fossils, that not only has there been an overall increase in the Srs7/Srss ratio through Phanerozoic time, but also that there have been temporal fluctuations of the ratio. Cretaceous seas contained strontium that was less radiogenio than that of preceding or succeeding intervals of time. Thus the source of strontium supplied to the ocean should be known as precisely as possible. Based on abundance studies of strontium and calcium, TUREKIAX (196~) concluded that most of the strontium in the sea is derived from carbonate rocks on the continents; hence, marine strontium may be recycled and variations in the Srs7/Sr86ratio of sea water with time will be subdued, unless a marked change in the weathering regime occurs. Such a change is indicated by the data of PETERMAN et al. (1967). Attempts to determine SrE7/Sr8*ratios of Precambrian sea water from carbonate rocks have only partly been successful. In addition to the paucity of unmetamorphosed carbonate rocks with which to work, the major obstacles are the fixing of time of deposition of the precipitate, and the uncertainty that the precipitate formed in a well-mixed, marine environment. Absence of diagnostic fossils in Precambrian rocks contributes to both of these major problems. To evaluate more fully the strontium isotope data obtained from studies such as the dating of shales by the rubi~um~trontium method and the strontium isotopio composition of paleoseawater, it is necessary to know more about the behaviour of rubidium, strontium and strontium isotope composition during the low-temperature geochemical cycle, particularly during weathering, sedimentation and diagenesis. ANALYTICAL A. Detevwhut&m the 8ediments:

TECHNIQUES

of carbonate abundance &7adeep-sea sedimenta;

removal of carbonate

from

Carbonate determinations for most of the deep-sea, sediments studied in this report werre available from fbnelyses previously made at Yale University. The values were obtained by EDTA titration, modified after a method given by TURXKIAN (1966). For this report, carbon&e was d&mnin~ only on the size-fraction&ed samples of dmp.sea sediment described in Table II. The results were obtained by motor-given, EDTA titmtion, with the endpoint monitored by ~bfilter spectrophotometer. The results were calibrated by comprsrison with s, standard dolomite. Because deep-sea carbonate conteins a large amount of isotopically uniform strontium, it must be removed from the sediment prior to isotopic analyses of the aluminosili~te fraction. There are several methods by which carbonate can be removed, but many of the processes effect also other minerals which may be present. The non-carbonate mineral in deep-sea sediments most likely to be attacked is chlorite. The procedure used in this study wa modified after that described by JACKSON(1956), in which carbonate is dissolved from the saplea with a solution of acetic acid buffered &t a pH of about 50 with sodunn acetate. The method did not appear to alter significantly the non-carbonate phases of the sediment.

Most of the samples ofdeep semsediment usedin thisstudywereanalyzed for clrtyminer&em by BISCAYE f 1964@. Of the clay mineral data reported, only the date of Table 11 were deter_ mined 8s a result of this work. The procedures followed essentially those described by Br$oAm

1524

E. fr;LICS DASCEI

(1964a, 1965). Carbonate-free samples slurried onto glass slides, were X-rayed with a Sieman‘s X-ray diffraction unit before and after treatment with ethylene glycol, utilizing copper Iiradiation and a nickel filter. Relative amounts of the principal clay minerals were estimated t 85’ applying weighting factors to the respective clay-mineral peak areas traocd on the diffractedgrams. The factors used were as follows: montmorillonite-the 17 A peak area (glycolatt*J tracing) ; illite-four times the 10 A peak area (glycolated tracing); kaolinite and chloritetwice the 78 peak area, which was divided between the two clay minerals by IJTSCAYLG'S(19641,) method. The peak areas were obtained by averaging multiple me~~elnents made with a polar planimeter. Although these clay mineral techniques commonly are used, individual mineral abundances may be in error for a variety of reasons, and must be regarded as semiquantitative. The samples were treated identically, however, and t,he general trends of abundance probably are reliable. The homogeneous, uppermost sections of cores V12-72 and A153-144 (several hundred grams each) were disaggregated initially by stirring and ultrasonic vibration in deionized water. The samples were cleaned of soluble salts by dispersing the sediment in large amounts of deionized water, followed by filtering. This process was repeated until the filtered supernato was free of detectable chloride ion. Salts must be removed completely not only because they may induce flocculation of the particles during the size-separation process but also because sea-water strontium may distort the isotopic composition of the silicate rosidue. About 40 1. of deionized water was used in cleaning each sample. Several size fractions of diameters greater than 74 p were separated by wet sieving. The material less than 74 ,U in diameter was size fraotionated by settling and centrifuging in deionized water, following essentially the procedure described by JACKSON (1956). Clay from V1272 is very fine grained, and a considerable amount of material smaller than 068 ii in effective settling diameter was separated; sediment from A153-144 is ~o&~er-~ain~, and it was not possible to obtain significant amounts of material in the less-than-O.2 ~1fraction. The fractions were dried and weighed to determine if detectable amounts of sediment had been lost as a result of cleaning and size fractionation. The processes caused an approximate 5 per cent weight loss in both samples. Although some fine clay undoubtedly was lost in the cleaning, a large part of the weight loss may be explained by the solution of marine salts from the samples. ,4s an additional test for loss of aluminosilicates, a large amount of filtered supernate was condensed by evaporation and analyzed for aluminum by atomic absorption. The salt residue obtained from the evaporated supernate also was X-rayed to determine if diffractable al~osiliea~s had passed the filter. All tests indicated that no detectable olny was lost during the cleaning processes. The whole samples and the size fractions were homogenized and ground in a mortar and pestle, and carbonate was removed from aliquots of each sample. Weighing of selected, carbonate-poor samples before and after the washing and buffered acid treatment indicates that less than 3 per cent of clay was lost from the samples by acid leaching. D. B&w&nation

of concenkztions of rubidium and strontium

Most of the rubidium and strontium determinations reported herein wore made by X-rag fluorescence, utilizing a General Electric X-ray unit equipped with a molybdenum target tube and a topaz analyzing crystal. The general procedure folfowed that described by FAIRBAIRN (1966). E. Separation of rubidium and atrolztium foformaas spectromet~

Rubidium and strontium were separated from the samples by standard techniques of dissolution and ion exchange. The chemical steps were made in a “clean” laboratory, with filtered air maintained at a pressure higher than that outside the laboratory. Glassware and teflon used in the clean laboratory were washed, cleaned in warm, 6-normal nitric acid, and rinsed with quartzdistilled water and acetone prior to each use. Commercial brands of hydrochloric and hydrofluoric acid were redistilled in the clean laboratory with stills constructed from fused quartz and teAon materials. Tap water was titered, deionized, distilled in a commeacial apparatus, and redistilled in fused quartz glassware. Commercial brands of double vycor-glass distilled perchloric and sulfuric acid also were used.

Strontium isotopes in weathering proflles and deep-sea sediments

1525

The samples were dissolved by w arming in HClO~ and HF. After evaporating to drynessie, a small amount of Sr%B tracer was added to the sample to monitor the yields from the ohemioal procedure; rubidium and strontium spikes also were added to a few samples which were analyzed by isotope dilution. The residue was dissolved in HCl; alkali perchlorates, including most of the rubidium, were precipitated by cooling the HCl solution in an ice bath. Strontium was obtsined from the supernate by organio-resin ion exchange. Rubidium and strontium were purified further with an inorganic ion-exchange resin, zirconium phosphate. The ion exchange columns were constructed from fused quartz materials. The purified samples of rubidium and strontium were stored in teflon planohettes. A “blank” from the olean laboratory procedure contained less than 0.1 yg strontium; because the study involved materials with relatively large amounts of strontium, this level of contamination is not significant. Strontium isotope ratios and isotope dilution analyses were made with a 15 cm mdius-ofcurvature Nier-type mass spectrometer utilizing both single and double rhenium-filament ion sources. The samples were mounted on pre-cleaned filaments as sulfates. Ion beams were produced by thermal ionization and were measured with a Cary Vibrating Reed Electrometer. Most of the data were collected with a recorder equipped with an expanded scale. Multiple scans were made across the strontium mass range, and the Srs7/Srs6 ratio was calculated by averaging about 15 to 25 ratio measurements made aoross the tracing. To correct for mass fractionation in the spectrometer resulting from unequal evaporation of the strontium isotopes, a fractionation factor was applied to the measured Srs7/Srs6 ratio. The fractionation oorrect,ion, a normalization of the Sre7/Srseratio to a Srse/Sr” ratio of O-1194,is based on the assumption that the fractionation is a function only of the differences in mass of the isotopes involved in the correction. The overall uncertainty for the isotope ratios reported in this study (&O*OOl) is based primarily on 17 mo~~ements of an ~~rlaborato~ strontium standard (Elmer and Amend SrCO,; lot No. 492327). The average measurement, O-7075 with a standard deviation of &0.0006, is within the range of values reported from other laboratories. STRONTIUM

ISOTOPES IN WEA;THEZXNG PROFILES

Weathering of “primary” crustal rock such as basalt, granite and high-grade metamorphic rock is the low-temperature process potentially capable of imprinting the strontium isotope ratio and rubidium and strontium composition on future sedimentary deposits. Weathering of sediments and sedimentary rocks brings about less pronounced chemical changes because the materials have undergone at least one earlier stage of weathering. Difficulty in untangling the physical and chemical processes makes the study of weathering profiles of previously processed materials less i~ormative for the strontium isotope system. Analyses of weathering profiles have shown the general trends of variation of the major elements potassium and calcium. To a first approximation, one would expect rubidium to behave like potassium and strontium like calcium. Calcium is lost significantly in incipiently weathered rocks and continues to be lost during later stages of weathering, Potassium commonly is enriched relative to the original rock in early stages of attack although it also is lost from the residue during more intense weathering (e.g. GOLDICH, 1938; SHORT, 1961; HARRISS and ADAMS, 1966). The few studies on rubidium and strontium in weathered rocks show that these elements have the expected analogous behavior to potassium and calcium (DASCEetal., 1966b; BOTTINO and RJLLAQAR,1968). The major changes in elemental abundances result in part from the d~erential alteration of minerals containing different amounts of the various elements. Much of the calcium and strontium in

1526

X. SuLrus

DASCH

either basalt or high-calcium granitic rocks is held within plagioclase feldspar, which contains little rubidium or potassium. Plagioclase decomposes more readily than other principal minerals in these rocks, resulting commonly in the marked loss of strontium and calcium. Because of the preferential alteration of strontium-rich, rubidium-poor minerals in many crust al rocks during weathering, the Rb/Sr ratio in progressively weathered rock increases compared to the RbfSr ratio of the fresh rock. This relative enrichment of rubidium over strontium is evident in all of the profiles described in this report as well as those studied by BOTTINO and PULLMAR (1968). Before proceeding to a discussion of the observed relations involving strontium isotopes in weathering, the constraints in the study of such a complex process need to be reviewed: 1. Pedogenic processes operative in upper sections of weathering profiles must be distinguished from the overall weathering result. In this study, emphasis was placed on analysis of fresh rock and moderately to highly weathered rock, and not on zones of accumulation within well developed soil profiles. 2. Contamination of profiles, especially relict profiles, is difficult to detect or evaluate. A first approach is to sample residual profiles that appear to have formed in place. 3. Time and duration of RbjSr and Srs7/Srss ratio modification due to weathering must be considered.’ Rubidium-strontium-strontium isotope relations obviously will vary according to age of profile formation. Thus, data obtained from weathering profiles must be evaluated with respect to and especially age of weathe~ng, The sol-forming processes, contamination following studies of weathering profiles are grouped on the basis of field relations and probable histories. A. Active weathering profcles Do&&e; Sieeeping Giant State Park, Cm~ecticut. At sleeping Giant State Park, Connecticut, a small intrusion of Triassic dolerite is exposed within Triassic arkosic strata. The dolerite is fresh except for narrow selveges which pamllel coohng joints in the rock. A fresh sample from the middle of a jointed block of doleritewas collected, along with the weatheredselvege obtained by sawing a layer approximately 2 mm in width parallel to the jointed surface. The Rb/Sr ratio of the weathered rind shows a significant increase from the Rb/Sr ratio of the fresh rook, but the Srsr/Sr**ratios are indistinguishablewithin the analytical uncertainty (Table 2). These data indioste a lack of detectable, weathering-inducedvariation, as well as tack of cont&min&tionfrom an isotopically distinct source. Grude; Kowlom, Eo~g Kong. Deep weatheringof the tropical Hong Kong Granite is well known from the study by RUXTON and BBBIZY (1957). The medium to coarse-gmined, light gray to pink granite is weathered in many places to ELdepth of about 60 m. The weathering Table 2. Rubidium-strontium data for weatheredTriassic dolerite, SleepingGiant St&e Park, Connecticut Sample

Rubidium

Fresh dolerite Weathered dolerite * Ratio nomeiised

to Sr”/SF

(ppn)

18 28 = O-1194.

Strontium 162 149

(ppm)

RbfSr 0.1 0.2

O-7069 O-7060

Stronti~

Table 3. Ru~i~~~tronti~

data for Hong Kong Cranite, Kowloon, Hong Kong

SCUllpI

Description

Fresh Hong Kong &mite Weathered Hong Kong Granite

Fresh, whole sample of comaegrained pink granite Weathered, iron-eteined whole sample; oomiderable amount of quartz, biotite, and potash feldspar, in clay matrix

Highlyweathered HongKong crsnite

1527

isotopes in weatheringprofiles and deep-sea sediments

Highlyweathered, wholesample of cl&y;veryminorclmounts of freshminerals preserved

* Ratio normalized to Sr*5~Sr*e =

@l

Rubidium (PPu9

Strontium @Pm)

Rb/Sr

#re?/Srss*

176

290

0.6

0.7163

210

106

1.1

@7169

300

I56

I.9

0.7161

194

grades from incipiently attacked granite, through olayey material which retaius its textural identity and minor amounts of unaltered minerals, to a relatively pure, textureless kaolin&e. On the basis of field relations, the granite is believed to have formed during the Cretaceous Period (RUXTON,1900). The data for the Hong Kong samples analyzed in this study are listed in Table 3. The fresh, slightly weathered, and highly weathered samples were taken from a single hand sample, and traces of original granitic texture are visible in the most weathered rind of the sample. Hence, the weathered rock very likely was identical to the fresh granite prior to weathering. As in the Connecticutdolerite, the Rb/Sr ratio increasesmarkedly in the progressivelymore weathered zones, and Sre’fSrss ratios in the three samples oannot be distinguished. Vatied terra& of ~~a~~~h~ ro&, tGZ,and 8&; ~~~~ Brook:=@a&=&?&New ~~a~~~$. In the previous examples of weathering, analyses were made on directly related materials. An alternative approach to the study of strontium isotope mention resuiting from recent weathering is to analyze a suite of materials from a single natural setting. A small forested watershed near West Thorton, New Hampshire, meets this condition. The watershed, part of the Hubbard Brook Experimental Forest of the U.S. Forest Service, has been studied in detail for several years, and reports on various aspects of the eoologyand hydrology have been published (e.g. LIKENSet al., 1987). JOHNSONet al. (1908) have recently reported a detailed analysis of chemical weatheringin the Hubbard Brook area. The area is well suited for such a study in that it has . . . “( 1) a watertight basement, (2) no through going streams, (3) mature soil colloids, and (4) a climax vegetational regime” (p. 632). The following description of the area is summarized from the paper by JOHNSOXet al. (1968). The bedrock of the area is the Littleton Formation, a medium-to coarse-grained silhmanite gneiss, probabfy of lower Paleozoic age; it is composed of quartz, plagioclase, biotite, and a small amount of slate. Since the most reoent glacial erosion, a weathering zone of about 25-25 cm in thickness has developed on the gneiss. Overlying about 85 per cent of the area is a till which averages about l-4 m in thickness; in many parts of the area, the till has been weathered extensively, and soil zones are well developed. The matrix of the till has a chemical composition that is almost identical to that of the weatheredLittleton Formation, and probably formed from it. Boulders within the till, however, are composed of Kinsman quartz monzonite, which crops out less than a mile from the study area in the direction from whioh the last glacier advanced. Where exposed, the Kinsman quartz monzonite also has been weathered. Thus the principal materials available for weatheringin the relatively closed area are Littleton gneiss, Kinsman quartz monzonite, and till. To relate the chemistry of runoff to the substrate over and through which the water passes, JOHNSON et al. (1968) collected and analyzed representative samples of each of the prinoipal fresh and weathered rocks and sediments of the region. Small, representativesplits from these large samples were obtained for analysis of rubidium, strontium, and strontium isotope composition. A sample of runoff, taken at a period of low flow, also was obtained for strontium isotope analysis. The results are listed in Table 4. The Srs7/Sr*s ratios of the fresh materials available for weathering from Littleton and

1528

E. JULIUS DASCH TabIe

4. Rubidium-a~rout~urn

Samplf3

--

Littleton Formation

Kinsman Formation

Till

Rubbard Brook water sample; period of low flow

data for Hubbard Brook Watershed, Xew Hampshire

SampIe description and treatment Unweathered, untraatod whole samplo of sillimrtnite gneiss Weathered, untreated wholo f@mplc Weathered whole sample; carbonate-free Unweathered, nntreatod whole sample of quartz monttonite Weathered, untrcatcd wholn Scmplc ~~rcathcFcdwhole sample; i carbon&s-free “Frouh” 3nntrcratcd whole sample of sediment Weathered, untreated, Aa soil horizon whole sample Weathered A, soil horizon; i carbonate-free Strontium extraated from untreated water sample

I

Rubidium (PPml

Strontium @pm)

lZb/Sr

Sr”7,&.li”*

168

122

l-3

0+7293

114

104

1.1

@7318

127

113

1.1

0.7329

152

100

1.0

0.7254

289

54

3.4

0‘7310

285

82

35

0=7334

186

110

1-7

0.7331

108

106

I.0

0.7321

123

95

l-3

w7294 0.7182

* Ratio normalized to Sr8c/Sres = 0.1194.

Kinsman formations and “fresh” till average about 0729. The weathered connterparts of the Littleton and Kinsman formations have Srs7fSrsa ratios which are about equal; they are slightly higher than the fresh counterparts in each ease, however, and twerage about O-732. To determine if carbonate affects si~ifioantly the Srs~/Srssratios of the weathered samples, carbonate was removed, and strontium isotope ratios were measured from the residues. For the Littleton and Kinsman weathered samples, the Sr87/Sr86ratios of the carbonate-freematerials are slightly more radiogenic than the respective whole samples. The till, which had been processed prior to its weathering in the Hubbard Brook area, shows a slight decrease from the “fresh” sediment to the weathered zone; carbonate removal from the more highly weathered sample resulted in a more marked decrease in the Sr*r/Srssratios. The weathered sample was collected from the As soil horizon within the till, and the lower Sr87/Sr8s ratios may result from concentration of strontium with lower Sr87/Sr8a ratio, released from fresh minerals in the till or adjacent rock, and incorporated within pedogenio clay of the soil horizon. Snrfaoe runoff contains the least radiogenic strontium found in the area, about 0.718. A sample of rainwater has a ratio of about 0.710. It cannot at present be established whether the low runoff value results from preferential leaching of minerals with low radiogenic strontium concentration, or if it is due to the oombined effect of aerosol strontium of low radiogenic condent and weathered, more radiogenic strontium. It is evident from the studies of active pro&s that SrB7fSrs6 ratios of “primary” erustal do not undergo a drastic chauge as a result of weathering. For the Connecticut dolerite and the Hong Kong Granite there is no distinct modification. In the New Hampshire watershed the weathered materials are slightly more radiogenic than associated fresh rock, but the differenoes are not great. Owing to the low sampling density, the results for the watershed are not unequivocal. B. Relict (9) weathering

pro$Ees

A weathering profile can be designated with certainty as relict only if overlain by rock or sediment which has not been weathered to the same extent as the underlying material, if it has been deformed structurally, or if it is found within a climatic regime not severe enough to have produced it. weathering profiles exposed at the present surface of erosion may or may not have formed within the climatic regime where found, although, as in the case of the Hong Kong Granite, the lateritic weathering profiles of tropical regions probably are modern.

Strontium isotopes in weatheringprofiles and deep-sea sediments Table 6, Rubid~~~tronti~ Sample Fresh basalt Weathered basalt Highly weathered basalt

1529

data for weathered basalt, Halifax County, North Carolina Description

Medium-grained, dark gray dolerite Heavily iron-stained, dark red, kaolinitio material Iron-stained. orange-red keolinitia material

Rubidium @pm) * 78 248 80

strontium (PPm) * 88 28 7.9

RbfSr

Srs7/Srsst

0.9 9.0

0.7056 0’7138

10.0

w7139

* Rubidium and strontium anslysea by isotope dilution. f Ratios normalized to Srss/Srss = +lfB4.

Because Ibiza-strontium ratios increase in weathered rock relative to fresh rock, Sr*‘/Sr= ratios in an&nt, weathered debris may be expected to be higher than in recently weathered debris when compared to the unweathered rook. This effect should be especially marked in basalt, which shows significantlyhigher rubidium-strontium ratios in the weathered material compared to the originalrock. The strontium isotope variation in a weatheringprofile thus may indicate whether or not the proflle is relict. Basalt spheroid; North Carolina. Basaltic rocks in highly weathered regions commonly exhibit rounded, fresh aore stones, surroundedby oonoentriczones of progressivelyweathered rock. In some rooks, the effect apparently is caused by weathering along cooling joints within the rock; weathering may continue inwardly until no fresh basalt or only a small amount of fresh basalt remains. A sequenceof fresh and weatheredbasdt from such an outcrop was collected from a dolerite intrusion in Halifax County, North Carolina. The medium-grained, fresh dolerite is surrounded by iron-stained kaolin&e. The age of the intrusion is not known. Analytical data from the samples, Table 5, indicate a marked increase from fresh to most weatheredsamplesnot only of Rb/Sr ratios but also of Sr87/Srs6ratios. Based on the resultsfrom active profiles, weatheringby itself probably is not oapable of producingsuch distinct changes in the Srs7/Srssratios of the fresh and weathered basalt. Hence the weatheringprobably is relict, unless the higher ratios within the weatheredrock result from contaminationby strontium from a different source. Elberton Grakte; Elbertvn, Georgia. The age of weatheringof the deeply altered rocks of the southeastern Piedmont of the U.S. has not been established unequivocally. Many workers feel that the widespread,lateritic weatheringtook place during a more tropical climate than the present regime; some believe that the major weathering may have taken place in the early Tertiary (e.g. SANII, 1956). The Elberton granitic pluton crops out in the Piedmont of northeastern Georgia and is deeply weathered in several stages to a p~domin~tly kaolinitic material. The fresh rock is a medic-rained, light gray biotite ~an~iori~. It contains about 32 per cent quartz, 24 per cent oligoclsse, and 12 per cent biotite, along with mangetite, sphene, and epidote as trace minerals (HARRIS5and .&DAMS, 1966). Zircons from the pluton have been dated at about 450 m.y. (GRUNJSNFELDER and SILVER,1958). Samples of fresh and successively weathered granite were obtained for analysis (Table 6). Similar to previously describedprofiles,rubidium is slightly more concentratedin the moderately weathered rock but is less abundant in the more highly weathered rock relative to the whole, fresh rock; strontium decreases progressively in the more weathered samples. The Rb/Sr ratio thus shows a progressiveincreasewith increasingintensity of weathering. On the assumption that zirconium is relatively immobile during the weathering process, Zr/Rb and Zr/Sr ratios were obtained, to determine better the relative losses of rubidium and strontium. These data are shown graphically in Fig. 1. The Zr/Rb and Zr/Sr curves show that the inorease in the Rb/Sr ratio resultsprincipally from the sharp decreasein strontium abmdance in the weathered rocks. The isotopic data are plotted in Fig. 1. For reference, a 450 m-y. (GRINDER and Smvn~, 1958) isoehron has been drawn through the fresh-rock point. As in the weathered 5

I<. ,Jri~rus DAscrf

1530 Table

6. Rubidium-strontium

Description*

Sample* B-6 B-5 B-4 B-3 B-2 B-l

data for Elberton

Granite

Weathering

W

pm

127

84

171

102

I.7

0.7194

200

270

0.7

0.7167

203

249

0.8

0.7158

216

248

0.9

0.7142

180

290

0.6

0.7130

B-6

A,

100r-7 200

x

ADAMS

Zr x

Sr, wm

iO~_$OO

300

2-

(1966).

Zr

Rb s;

s; 4

2-7~5 4 x

Y

.5r --T 1.0 -71.5 x

Y

a-5 e-4 B-3 8-2 B-I

.720

B-I

.700~m

IFRESH

ROCK)

_~ L ___~_J

-’ 2

3

4

5

Jp sr=

Fig.

1. Elberton

Gr&te

Georgk

Strontium.i(rpm)

* Sample numbers and modified deaoriptions from H.UBIES and t Rubidium end strontium values obtained by isotope dilution. $ Ratios normalized to Sraa/Srss = @1194.

Pro!ile Lhcrlption

Elberton,

Rubidium @pm) t

Red saprolitc; predominantly iron oxide, kaolinito, illitc, chlorite, and quartz Buff saprolite; predominantly kaolinite, illite, chlorite, quartz, altered K-feldspar Highly weathered rock; plagioclase and biotite highly altered Highly weathered rock; plagioclase and biotite highly altered and iron stained Slightly weathered rock; plagioclase and biotite altered, K-feldspar unaltered Fresh, medium-grained, biotite granodiorite

-N9.

Profile,

Weathering

Protie,

Elberton,

Georgia.

1631

Strontium isotopes in weathering profiles end deep-sea sediments

dolerite from North Carolina, there is an overall positive correlation between degree of weathering, Rb/Sr ratio, and Srsr/Sr~ ratio. Slightly to moderately weathered rock shows increases in both Rb/Sr and Sr*r/Srssratios, and the data plot reasonably close to the isochron. Points representing the most highly weathered material, however, sbow marked departure from the isochron. An isochron constructed from fresh rock and highly weathered rock thus would give a younger age than the true age of the fresh rock. Somewhat similar data were obtained from biotite (and feldspar and whole-rock samples by analogy) from the Morton Gneiss of Minnesota by GOLDICH snd CAST (1966). Boulder Creek Qranodiorite; Flqptafl Mountain, Boulder, Colorado. The relatively complete, pre-Pennsylvanian weathering profile in the Boulder Creek Granodiorite at Flagstaff Mountain was studied extensively by WAELST~OM (1948) and later by Plier and ADAMS (1962). The pluton, which has been dated at about 1700 m.y. (PETER&U et al., 1908) is weathered through a “bleached” rock stage to a deep brownish “red rock” (WAHLSTROM, 1948). Fountain &kose (Pennsylvanian) overlies the profile, which is about 25 m thick at Flagstaff Mountain; the arkose formed at least in part from the weathered granodiorite. The weathering took place prior to the Pennsylvanian time represented by Fountain strata; WAHLSTROM (1948) believed that the weathering mainly occurred immediately prior to the deposition of the arkose. The topmost soil zone (“A” horizon) is missing from the section (WAECLSTROY,1948). Selected samples from the study by PLILER and ADAMS (1962) were obtained for rubidiumstrontium analysis and the resulting data are shown in Table 7. Whereas the fresh rock and Table 7. Rubidium-strontium

SfUllpk NO. 11 2 3 4 6I

al 7. 8

I

10 13

data for Pre-Pennsylvanian weathering profile, Flagstaff Mountain, Boulder, Colorado

Description

“Red rook”* brownish-red, highly weathered granodiorita-about 30 ft in tbiokness “Bleaohed rook”** , gray, moderately weathered grenodiorite-about SO-60 ft in thiokness Fresh rook; pinkish-gray, OOBSBBgrained granodiorite

very light

Rubidium

Strontium

(PPm)

(PPm)

260 260 270 290 280 222 267 208 196

1670 1370 770 290 360 400 212 370 480

170

640

Rb/Sr O-7141 O-7138 O-7162 0.7240 O-7273 O-7308 O-7240 O-7281 O-3

O-7179

l Rook deaription from WAEUETILO~~ (1948). t Ratio normalized to Srs*/Sraa = O-1194.

lower section of the profile show rubidium-strontium-strontium isotope relations similar to previously described profiles, data for the upper section indicates added complexities. Whereas strontium is lost progressively from the fresh rock through the bleached rock, its abundance shows a reverse trend in the uppermost section and is progressively more concentrated in the more weathered horizons; strontium is about 2.5 times more concentrated in the topmost horizon than in the fresh granodiorite. The Sra7/Srssratios form a trend in the upper section that also is reverse from the trends described in previous profiles; relative to fresh rock, the ratios increase in the “bleached” zone but decrease in the zones containing the most concentrated strontium. The Sr*r/Sr*s ratio of the upper zone (0.714) is distinctly lower than the whole, fresh-rock ratio (0.718). It appears either that less radiogenic strontium was concentrated in the upper zone as a result of 8O~-fO~~g processes within the profile, or that strontium from a different source was added selectively to the upper section of the profile.

It is evident from the discussion of the results from weathering profiles that EF/srss ratios are either unmodified in fresh and weathered rock, or are slightly higher in the weathered residue from old silicic rocks. Whether the ratios increase as a result of weathering depends on age, Rb/Sr ratios and differential reactivity

of the minerals in the particular rock studied. Although the S&7/&8” ratio increase may be significant in ancient, partially weathered granitie rock, owing to preferen. tial alteration of low IZb/Sr-ratio minerals, the more general result of the erosion of primary crustal rocks may be a near-uniformity of isotopic composition in fresh and weathered counterparts. Because of the increase of Rb/Sr ratios on weathering, however, Sr8’/SrS6 ratios in relict, weathered material may be distinctly higher than in the fresh, whole rock. The isotopic variation may be a useful criterion for distinguishing active and relict profiles, provided that there has been no contamination of the profile with strontium from a different source. Relict weathering profiles are less subject to erosion and transport than active weathering profiles. Moreover, active erosion may yield detritus that has undergone little chemical alteration. Thus it is concluded that weathering and erosion, though resulting in an overall increase of Rb/Sr ratios in the residual weathered material, do not modify significantly the strontium isotopic composition of the material which comprises detrital sediments and sedimentary rocks, and which ultimately may be deposited in the deep sea. Tn view of the relatively slight strontium isotopic modification in fresh and weathered rocks, it is of interest to evaluate the role of water in modifying the strontium isotope composition of rocks during weathering. As pointed out, water from Hubbard Brook, New Hampshire contains strontium that is much less radiogenie than strontium in either fresh or weathered rocks in the area. The value for the Hubbard Brook water, about 0*718, is within the range of values reported for fresh-water mollusks collected from the North American Precambrian Shield by FAURE and HURLEY (1963). Although FAURE and HURLEY (1963) found an overall positive correlation between Srs7/Srs6 ratios in the mollusks and the age of bedrock on which the mollusks were collected, the ratios, like the ratio for Hubbard Brook water, are low relative to Sr87/Sr*6 ratios in associated rock and sediment. Fresh water from other areas also contains strontium that is comparatively non-radiogenie (HART and TILTON, 1966; FAURE et al., 1967). In Lake Vanda, however, JONES and FAURE (1967) found agreement between Srs7/Sr86 ratios of water and of associated sediment. This result, in light of the weathering data reported herein, may be more reasonable than the other, relatively less radiogenic values reported for fresh-water strontium, and raises the question of the interpret~tio1~ of strontium isotope composition in fresh waters. Composition of fresh waters heretofore has been explained mainly in terms of the solution of old silicic rocks, mafic volcanic rocks, and marine precipitates, chiefly carbonate. In the Hubbard Brook area, however, there are no readily available sources of marine carbonate or basalt, and the fresh-water strontium may be influenced isotopically by another source. A possible source of strontium of low Srs’/Sr 86 ratio is from rainfall and aerosols. A sample of rainwater from the Hubbard Brook area gave a strontium isotope ratio of 0.710, similar to sea water. This strontium combined with strontium from rock weathering could give the observed strontium isotope ratio for the runoff. STRONTIUM ISOTOPES IN DEEP-SEA SEDIMENTS Recent studies of stable, radiogenic isotopes and of clay mineralogy have provided significant information about the provenance of deep-sea clay. CEOW and

Strontium isotopes in weatheringprofiles and deep-sea,sediments

1633

PATTERSON(1962) measured PbBog/Pbso7ratios in lead leached with hydrochloric acid from deep-sea sediments and found regional variations within the major ocean basins. They concluded that the lead is removed biologically from overlying water before complete mixing of the water occurs, implying that soluble lead supplied to the oceans from the major drainage basin retains its isotopic identity in the marine realm. HURLEYet al. (1963) obtained potassium-argon “dates” from deepsea sediments of the North Atlantic in the range of from 200 to 400 million years; these figures are compatible with the old ages of rocks in adjacent continental areas. The 80 m.y. number obtained for a Pacific deep-sea sample reflects the overall younger age for rocks that rim the Pacific basin. These results imply that detritus is transported from the continents with only minor modification of at least some of the istopic abundances inherited from source areas. The clay-mineral distribution maps of BISCAYE(1965) indicate the dominance of continental detritus in the clay components of Atlantic deep-sea sediments. However, whereas the isotopic data indicate geologic provenance, the clay-mineral data reflect more closely a weathering provenance. BISCAYE’Swork shows that continental weathering, rather than authigenic mineral formation, is the principal process controlling the clay mineralogy of recent Atlantic deep-sea sediments. A map of BISCAYE’Smost sensitive weathering-provenance parameter, the kaolinite/ chlorite ratio, reflects the latitudinal control of continental weathering on the composition of deep-sea clay: chlorite is destroyed, and kaolinite commonly is developed in the intensely weathered rocks of tropical regions; chlorite is more typically preserved under the higher latitude conditions of chemical weathering which do not generally result in kaolinite formation. The isotopic and clay-mineral studies are compatible with the premise that if authigenic minerals are forming in the regions represented by the studies, their presence is masked by debris from the continents, and that extensive reactivity between sea water and detritus, with respect to the parameters that were measured, is not a process by which the sediments acquire their major chemical aspects. However, a relatively large degree of reactivity between sea water and silicates recently has been suggested by GARRELS(1965), MACKENZIEand GARRELS(1966), and SILL&N(1967), among others. As an important control of sea-water chemistry, particularly regarding the balance of silica and bicarbonate, these authors have invoked the formation of significant amounts of silicate minerals in the sea. Strontium is almost a major constituent of sea water (8 mg/l). Because it is isotopically uniform in the ocean, it is a useful tracer in evaluating further the effects of geologic provenance and sea-water reactivity on deep-sea clay. A. Strontium isotope variations as indicators of geologic provenance for deep-sea sediments Deep-sea sediments consist principally of clay and carbonate, although phases such as detrital rock. fragments, siliceous ooze, and zeolites may be important locally. Marine carbonates contain at least 2000 ppm of strontium with a uniform Sre7/EW6ratio identical to the SF/Sr*6ratio of sea water. To determine the Sr*‘/Sr*6 ratio of the clay fraction, carbonate was removed from the sediments prior to isotopic analysis so that variations of the strontium isotopic composition of the

1534

E.

Table 8. Location,

clay mineralogy,

JULIUS

I)ASCH

and r~bidi~-strontium sediments

data for Stlantic,

core-top --1-

water Core SP3-33 SP3-38 SPlO-4 SPll-2 AISZ-84 A163-143 A163-144 Alb3-156 A166-4 Alb7-9 Also-76 Also-87 Also-118 AISl-4 AlSl-10 AlSb-31 V4-8 v4-13 V4-32 v1!2-7 VlZ-10 Vl2-18 V12-23 V12-53 V12-72 Vl2-73 VI4-27 v14-49 VIP-51 v14-60 V14-87 v14.94 v14.117 V14-128 v14.135 v14-141 v14-143 v14-149 Vlb-9 Vlb-130 VlS-177 V16-16 V16-19 V16-38 Vl6-44 V16-46 V16-53 V16-201 v17-65 V17-88 V17-121 V17-123 V17-124 V17-161 v17-196 VI?.196 V17-198 V17-199 VIS-I61

Latitude 47”ll’N 62°06’N 64°bO+‘N 62”17’N 44’2l’N 32O41’N 33”OS’N 27OlS’N 34’49’N 44’0’N O0°46’S 12*08’s 23”36’S 03’32’N 26’24‘N 24’27’N 37’13.5’N 36’36’N 35’03’N 11°12’N 02”22.8’8 2S041.7’S 35’40.2’8 40~54’s 06’37*6’S 05064’S 41ObO’S 54’18’s 56’37’5 bb”08’S 23’32.5’9 08’5O’S 18’48’N 32’27’N 35’42’N 39O38.5’N 41’39’N 35’57’N 09”4@3’N 57061’S l5O28’N 25’15’N 19O04’N 22059’S 30°b8’S 32’08’5 36”bO’S 02’01’N 40033’S 67~01~5’5 43Ob8’S 45”22’W 48’34’s 23’16’N 60’42.3’N 60’44’N b5’49’N 63’42’N 38’25’5

Longitude 1102b’W 20°21’W 02’31*6’W 1503O’W 30’16’W b0°b4’W 48’08’W 34OO7’W 7P41’W 38’13’W 26”02’W 37”03’W 41033’W 45’82’W 61’66’W 91’16’W 33’08+2’W 18’34.b’W x 1°37’W 36’01’W 34’48.9’W 34’29.6’W 3603&3’W 20023’W 10*39.6’E 09”53.l’E 64’2O’W 39’2l’W 34’48’W 04°67’w 54’42’E 67’37’F 39031G 29O46’E 18040’E 11’49’E 05OOO’E 0703O’W 79O3FbW 60°@93’W 64O56’W 62’31’W 53*46.b’w 06‘346’W 0b026’E lO”45’E 2l’lFb’E 37OO9’W 75°10’W 74”29’W 52O09’W 4F058’W 36’04’W 26’43’W 46”Ol~lW 67Qbotw b2”27’W bb”36’W 47Obb’W

depth (m) 4610 3730 3182 2195 2750 5210 4850 5300 3100 4025 3812 2834 146

3702 1655 4389 2232 5444 4020 4022 3797 2107 3064 l28 238 3186 3957 5996 1372 1931 4027 3460 2439 1206 971 3477 4165 3225 1931 4925 6134 4821 1500 4276 3636 4064 6784 6269 6172 3007 2818 3326 143 5214

C&O, (%) 22.6 76.2 44.6 12.1 83.3 24.7 59-6 62.4 2&Q 65.3 92-4 3.9 47.2 20.0 3.5 ad. 89.5 34.5 80.5 36.7 22.8 8b.2 0.6 8pO 1.5 3*0 7.8 1.0 0.9 9.1 12.4 23.7 39.9 66.4 11.3 35.8 48.3 23.5 13.8 I.2 4Q.3 3.5 13.1 74.6 @8 87.0 58.8 337 2.7 25.8 2.4 2.0 2.3 398 0.9 18.8 28.7 I.6 1-o

Clay mineralogy I K C ix 6 11 24 25 6 4 16 16 10 8 24 3 11 28 11 n.d. 14 6 9 33 5 16 20 24 30 4 80 0 24 16 SO 65 65 58 27 16 6 18 61 24 21 10 2s 42 61 35 7 9 56 7 30 36 27 29 24 20 10 37

72 10 12 70 9 10 as 10 10 67 8 10 76 8 10 70 11 15 62 10 12 6% 22 7 67 10 13 72 7 13 38 34 4 20 76 1 28 87 4 6016 6 63 16 10 n.d. n.d. ad. 6612 9 73 14 7 6815 8 32 30 5 88 5 2 6412 8 89 7 4 57 6 13 29 39 2 47 47 2 14 2 4 4b 2 53 44 4 23 56 6 22 4 4 2 3 0 532 21 10 4 1524 3 50 12 11 6016 8 76 8 10 67 15 10 19 12 8 62 4 20 62 22 5 69 10 11 45 20 6 46 6 6 32 4 3 55 6 5 89 2 2 6124 6 21 3 20 67 2 24 43 6 22 45 4 1.5 49 4 20 41 27 3 47 3 26 60 6 14 78 3 12 (SAND) 46 4 14

Rb (ppm) 135 75 30 39 75 101 88 ad. 83 95 22 52 45 87 116 107 <20 90 121 92 6l n.d. 6S 99 77 71 75 <20 45 (20 <20 <20 28 62 132 113 134 96 22 44 83 101 92 90 93 n.d. 119 124 22 51 53 57 b7 68 87 48 47 63 48

Sr (ppm)

Rb/Sr

Srs’/Sr@

I.5 0.7 0.4 0.3 0.4 1.1 0.9

0.7348 0.7208 0.7249 0.7157 @7226 0.7276 0+7232 @7209 0.7228 0.7215 0.7213 0.7312 0.7134 0*7093 0+7317 0.7200 w70.57 O-7152 0.7 167 0.7196 @7189 0.7076 0.7083 ~7099 @7429 0.7394 e7073 0.7068 0.7062 ~7099 0.7046 @708Q @7072 0.7054 0.7203 0.7171 0.7169 @7098 0.7064 0*7080 @7208 0+7262 0+242 @I153 0.7126 0.7197 p7216 0.7224 0.7044 0.7072 0.7071) 0.7088 0.7112 @7206 0.7156 0.7211 0.7177 *7x73 @7064

90

102 72 13S 176 Q3 110 ad. 108 226 18 92 131 141 73 87 194 07 104 S2 410 n.d. 263 158

46 42 213 212 146 17 238 60 185 142 112 90 96 90 106 326 8fi 86 80 101 118 ad. 121 113 211 181 148 102 118 115 462 312 323 571 I62

0.8 0.4 1.2 0.6 0.3 0.6 1.6 1.3 CO.1 0.9 1.2 1.0 0.2 e3 0.6 1.7 1.7 0.4
Sre7J8fe Rubidium and strontium oonoent~~io~ and Srersl&3fl ratios det%rmin& on ~~~~~~ fraction; ratios normalized to Sr*‘/Sr8* = @1194; o&y mineralogy (weighted X-ray pet&-area pemsntsges)determined on (2~ frection by BlScAm (1964, 1966): X = montmorillonite, f = illite, K = ksolinite, C = ohlorite; ad. = not determined.

St~nti~

isotopes in weathering prof%s and deep-sea sediments

0

15

30

43

SO

_

=.

I05

Fig. 2. Sr87/Srsa ratios

from the detritsl, aluminosilic&e sediments.

fraotion

of core-top

aluminosilicate fraction would not be masked or subdued. Most of the samples are from the tops, or very nesr the tops of cores raised by the Lament Geological Observatory. The strontium isotope and other data for the core-top sediments are listed in Table 8; the Sr*‘~SrE6ratios also B;replotted on Fig. 2. It is obvious that these Holocene sediments have not equilibrated their strontium with that dissolved in se8 water or in associated marine carbonate, although 8 few of the samples have ratios which are indistinguishable analyticrtlly from the ratio for marine strontium (DASCH et al., 1966a).The SP/Sr** ratios range from 0,704to 0,743, a variation of more than 6 per cent. These wide variations from the value for marine strontium indicate the strong control of geologic provenance on the aluminosilicate components, rather than authigenic mineral formation, or extensive rertctivity of the detritus with sea water or marine csrbonate. Broad, area1 differences in rubidium and strontium concentration also occur in the clay (Table 9). The lowest Sr*7/ss ratios approach the low values (O-702to 0,704)reported from oceanict volcanic rooks (FAUREand HUXLEY, 1963; GAST ebal,, 1964).The highest v&es are similar to numbers obtained from sediment derived from the ancient

1536 Table

E. JULIUS DASCH 9. Average

rubidium

and strontium Average

SOUW? Aversge deep-seaeediment (basedon 61 samples) Argentinebasin sediment (basedon 65 samples) Mid-Atlanticridge sediment from the regionof about 30’N (basedon 24 samples)

Precambrian

Rb

abundtlnco (PPm)

for carbonate-free,

Eange of Rb

deep-sea sediments

Rango of Sr abundance (DDN

abutldanoo @Pm)

AxWage I
G9.G

.i- 140

155.6:

17~-152

0.64

66.9

i--l 15

172.3

Z-367

0.51

87.6

x&140

134.0

5@-315

o-70

surrounding Lake Superior and Hudson Bay (HART and and BEISER, 1968). The strontium isotope ratios of the deep-sea clays apparently result principally from the age, Rb/Sr ratios, and probably to a lesser extent the degree of weathering of source rocks which contributed the detritus. A contoured map of the data (Fig. 2) thus outlines broad regions of similar provenance. Regions from which the most isotopic data are available are described briefly in the following paragraphs. Western North Atlantic OGectn. Deep-sea sediments from the western North Atlantic Ocean are enriched significantly in radiogenic strontium; the average Sra’/Sr a6 ratio for these detrital sediments is about 0.72. Mainly on the basis of the distribution of mixed-layer clay minerals, BISCAYE (1965) argued that the Mid-Atlantic Ridge is an effective topographic barrier to the transport of detritus between the western and eastern North Atlantic. BISCAYE believes that the immediate source of Holocene sediment in the western North Atlantic is the continental shelf and slope of North America. A second source, indicated by southward bending contours in the maps of “higher-latitude” minerals, is the southward flowing, North Atlantic Deep water, which apparently transports material into the northern part of the North American Basin. Windblown sediment does not appear to form a significant amount of the deep-sea assemblage in this region. The principal source of detritus in this region thus is Mesozoic and Cenozoic sediment from the continental margin or the adjacent continental deposits, with perhaps minor components from the short North American rivers which drain into the North Atlantic. The weathered and transported, or processed, material expectably would retain the general isotopic aspect of the original rock, as indicated by the weathering studies. The lead-isotope data of CHOW and PATTERSON (1962) show a similar enrichment of radiogenic lead in deep-sea sediments of the western North Atlantic compared to other parts of the Atlantic Ocean. Argentine Basin. Recent deep-sea sediments of the Argentine Basin are depleted in radiogenic strontium with respect to other sediments of the Atlantic Basin. The Srs7/Sra6 ratios from sediments of the Basin are almost all distinguishably less than the value for marine strontium, indicating a significant component of non-radiogenic strontium from young, non-radiogenic rocks or from mafic volcanic rocks derived presumably from the low Rb/Sr environment of the upper mantle. The average ratio for the basinal top sediment is about 0.707. The origin of the thick sequence of almost carbonate-free sediment comprising TILTON,

1966;

terrain

concentrations

MURTHY

Stro~ti~

isotopes in weathering profilm and deep-sea mdimenta

1637

the Argentine Basin is uncertain. Rates of clay aocumul~tion are the highest known for the Atlantic Ocean (6 g~om8/1000 yr; TUREKIAXand STUNER, 1964) and thus require a high flux of detritus. BISCAYE’Sclay-mineral distribution data led him to believe that the sediment may have come from the Antarctic Ocean to the south, across the Scotia Ridge, or moved generally with Antarctic Bottom Water, The clay-mineral composition of sediment from near the mouth of the Rio de la Plats is sufficiently different from that of the Argentine Basin, that Biscaye believed the Basin sediment was not derived principally from this river system. Whatever the source of the Recent sediment in the Basin, a significant supply of strontium with Sr*7/Sr86ratios lower than sea water is required to derive the ratios plotted on Fig. 2. Samples from near the Chilean coast of the Pacific also have low ratios, suggesting a young or low-radioge~c-strontium provenance in rocks of the nearby Andes Mountains. This topographically high province probably contributes also to sediments of the Argentine Basin, perhaps through strong oceanic currents. Work by BISCAYEand DASCH (1968) indicates that deep ocean current transport is compatible with the isotopic and mineralogic data for the Argentine Basin, and that sediments from the major river feeding into the Southern Atlantic, the Rio de la Plats, are markedly more radiogenic than the basinal sediment. Mid-Atla&k Ridge. Strontium isotope ratios from sediments of the northern part of the Ed-Atlantic Ridge show two distinct populations. Most of the values are enriched in radiogenic strontium similar to the Western North Atlantic province (avera~ng about 0.72). One of the Sr87~Sr86ratios, however, is quite low (0.7057; Table 8), indicating a volcanic provenance. AIthough BISCAYE’Sdata show that continentally derived clay is the dominant aluminosilicate phase, even along the crest of the Mid-Atlantic Ridge, HATHAWAY and SACHS(1965) and SIEVERand KASTNER(1967) point out that ponded basins along the flanks of the Ridge, as well as the thin cover of sediment covering higher topographic parts of the Ridge, contain sediment which was derived locally from the submarine weathering of ultrabasic and basaltic rocks of the Ridge. These conclusions are based on the mineralogy of rock and mineral fragments in the coarse fraction of dredge hauls and piston cores, and the presence, within coarse and fine fractions alike, of ~stin~tive, locally derived minerals such as sepiolite, clinoptilolite-heulan~te, phlogopite, Mg-rich chlorite, we-crystall~ed montmori~onite and antigorite, among others. The strontium isotope composition of Holocene, carbonate-free sediments from along the Ridge province support the ideas of both continent-derived and locally-derived detritus, and offers a more quantitative method for the evaluation of each. Emt Pacijic Basin. Although only two samples of deep-sea sediment from the east Pacific were analyzed in this study (Table lo), it appears likely that carbonatefree sediment of the basin as a whole is characterized by strontium isotopic composition approximately equal to, or less radiogenic than that of marine strontium, and is less variable than deep-sea sediment from the Atlantic. The overall provenance for the sediment in the east Pacific certainly is younger and more mafia than that for Atlantic detritus. The principal factors which control the strontium isotope oomposition are the youthful, oceanic-volcanic source rooks, the probably

1538

E. Jur,rvs Table 10. Rubidium-strontium

DASCH

dnta for East Pacific deep-sea sediments tvtttcr

Core

EM 8-l

(m-65 cm) Ris 114 p (614-844 cm)

lhtitude

Longitude

28”59’K

I1703O’W

21”33’N

134%~

Rubidium

Strontium

@Pm)

(PPW

Rb/Sr

3660

85

280

0.3

O~iO68

5040

50

0.7

0.7092

depth (m)

7.5

w-/we*

significant percentage of authigenic phases which incorporate marine strontium, and the relative lack of detritus from older sources. More detailed strontium isotope work within the basin will help to distinguish between these sources but the variations likely will be more subtle than those observed from the Atlantic. Location Near major Rioerg. Segment from near the mouths of major rivers, before extensive mixing can take place in the deep sea, expectably will reflect closer the local or regional provenance than more seaward sediments. This effect is apparent for sediments from the Amazon and Congo rivers. Although the Amazon contributes about a fifth of the total volume discharge of world rivers (GIBBS, 1967), its sus~nded load is light. BISCAYE { 1965) argued for a “headwater,” Andean provenance for much of the Amazon detritus on the basis of the composition of deep-sea sediments off the north Brazilian coast which were low in “tropical” suites of clay minerals. Smaller streams draining coastal mountains in eastern Brazil carry the more “tropical” clay assemblage found in deep-sea sediments off that coast. GIBBS (1967) analyzed the sediment of the Amazon itself and confirmed Biscay’s hypothesis. GIBBS believes that topo~aphi~ relief within the Amazon drainage area exerts the strongest control not only on the suspended materials, but also the dissolved salts of the river. Strontium isotope ratios from marine sediments in the region of Amazon debouchment are explainable in terms of the conclusions of BISCAYXIand GIBBS. Debris, especially andesite or more basic volcanic debris, contributed from Amazon head-water likely contains strontium not sig~fica~tly enriched in radiogenic SF. Deep-sea sediments near the mouth of the Amazon, as well as sediment from a region north of the river (MURTHYand BEISER, 1968), have Sra7/Sr*@values of about 0.710. Outward from the mouth in all directions, however, mixing of Amazon clay with the ambient, more radiogenic detritus, results in higher ratios. In support of BISCASE’S argument for more “tropical” provenance for some of the shorter streams, Also-87 contains comparatively radiogenic strontium (07312), and may result from local provenance of the ancient rocks within the Guiana shield, by way of the San Francisco River. On the basis of strontium isotope composition and clay mineralogy, a similar “headward” provenance is suggested for detritus from the Congo River. Strontium isotope ratios offshore from the mouth of the Congo contain the most radiogenic strontium found in deep-sea sediments (0.7394 and 6.7429). The Congo drains in part some of the oldest known rocks, and although it traverses a large region of much younger rock and sediment, it appears that a significant part of the detritus may result from the more ancient provenance.

Strontium isotopes in weatheringproflea and deep-sea sediments

B. Strontium

isotope

ratios

as indicators

of silicate-sea-water

1539

interaction

In many models for control of sea-water chemistry, a significant amount of silicate (about 7 per cent of the detrital load of streams) forms in the marine environment. A suggested generalized reaction indicates the construction (also variously called “back reaction,” “back titration,” and “reverse weathering”) from “X-ray amorphous aluminosilicate,” silica, bicarbonate and “cations,” of “cation-aluminosilioate,” carbon dioxide and water (MACKENZIEand GARRELS, 1966). If present in marine sediments, the low abundance of this extensively altered material may not be detectable in studies of clay mineralogy such as that by BISCAYE. These altered sediments should be discernible, however, by isotopic studies of size-fractionated marine sediment. The amount of authigenesis or chemical modification can be expected to be greater with decreasing size of the reacting grains and a consequently greater surface area. If strontium in sea water is exchanging with lattice strontium or is being incorporated in newly formed clay minerals, the fine fractions of deep-sea clays should show the greatest degree of modification. The deep-sea cores analyzed for this experiment (VU-72, offshore from the mouth of the Congo River; A153-144, North American Basin) were chosen from areas of the Atlantic where the Srs7/fW6 ratio from the unsized, aluminosilicate fraction of core-top sediment was known to be distinctly higher than the Sr*‘/Sra6 ratio in sea water. Core A153-144 is from a region where recently deposited clay has Sr87/Sr*6ratios of about 0.72 and the V12-72 core was raised from an area where the clay fractions exhibit the highest ratios found in the deep sea, about O-74. The cores were chosen also so that materials of disparate sedimentary features, such as grain size, amount of carbonate, and especially clay mineralogy could be oompared. Core-top clay from A153-144, which is light, reddish-brown in color, is medium-grained and contains about 60 per cent carbonate; core-top sediment from V12-72 is a medium grey, fine-grained clay with less than 2 per cent carbonate. Clay from Al53-144 is dominantly illitic, but with a significant amount of chlorite. Clay from V12-72 has roughly equal amounts of kaolinite and montmorillonite, less ill&e, and only a trace of chlorite (Table 11). Sample preparation for the two cores is described in the Appendix. The two cores are distinct in most of their sedimentary features. Over 70 per cent of the carbonate-rich A153-144 sediment is contained within the 20-74 ,U size fraction. This fraction, however, consists of more than 64 per cent carbonate, mainly foraminifera tests. The aluminosilicate component of the sediment is more uniformly distributed over the size ranges studied, though most of it also is within the 20-74 ,urange. Part of the clay of the 20-74 p fraction may have been enclosed within the tests and therefore would not be size-fractionated. The carbonate-poor sediment of V12-72 is very fine grained; the bulk of the material is in the 0.2-2.0 p size fraction. The sediment is better sorted than that of A153-144. Concentrations of rubidium and strontium in the samples, before and after removal of carbonate, are recorded in Table 11. Practically all of the strontium loss during carbonate removal is the result of removal of strontium associated with calcium carbonate particles. The strontium concentration associated with the carbonate phases increases from about 850 ppm in the coarser-fraction

Table 11. Clay Effcotive diameter of particles COP3

Viz-72 (6”S, 11’Ef

(P) Unsized Sample <@08 @O&P2 @z-z~o 2.0-20 20-74 74-149 149-250

Unsized Sample (0.2 0.2-2-o A1632.0-20 144 20-74 (33’N, 74-149 48OW) 149-250 250-420

mineralogy

and

r~~b~dium-~tront~u~~ data sediments

Weight per oent WhOl0 Carbonate S&ml1110 (%,

for

size

Sr

Rb/Sr

xb

Sr

deep-xcn --

Jtb and Sr abundance (ppm) _____-aluminosilioato whole sample fraction Rb

fractionated,

Rb/Sr

Clay mineralogy r&tive per aont* M I K C

SF/W”f Aluminosilicate fraction

77

46

I.7

34

19

44

2

0.743

66 73 83 89 70 87 78

20 30 48 47 46 34 40

3.3 P4 I.7 1-S 1.5 2.6 2.0

59 42 44 25 45 48 45

10 31 5 62 1% 38 34 39 15 38 16 33 23 29

0 1 2 2 3 3 3

0.730 0.739 *740 6.749 0.734 0.746 0.140

8

18

0’723

&9 l!P4 35.3 24.9 lo.5 5+9 I.1

2.6 2.1 1‘5 I.4 1.5 l-3 1‘6

67 63 80 82 60 76 74

60 56 70 74 65 67 68

lO@O

59.6

38

795

98 110

0.9

1.0 4.7 16.1 70.6 3.3 1.9 1.7

4.2 12.7 62.3 64.1 85.9 84.4 82.4

110 110 44 25 <6 <6 <5

157 316 770 867 732 703 700

126 63 141 126 102 124 89 120 34 83 24 38 37 39

2.4 1.1 0.8 0.7 0.4 0.6 1.0

67

8

9 73 5 13 15 6% 6 15 9 65 5 21 13 60 6 22 indeterminate indeterminete indeterminate

* Clay mineralogy (weighted peak-ares percentages for aluminosilicate components): illite; K, kaolin&e; C, chlorite. t Sra7[Sres rcstiosnormlalizod to SP/Sr~~ = O-1194.

-

0.743 0.733 0,727 0+722 0.717 0.717 0.72 1

i+f, montmoriilonite;

I,

carbonate to almost 2600 ppm in the carbonste of the finest size fraction of A153144, similar to results reported by TUREKIAN (1964). The rubidium concentrations are not seriously affected by the carbonate removal process. The RbfSr rstio increases markedly with decreasing grain size in the carbonate-free, clay-sized fractions of both cores. In V12-72, the increase in the Rb/Sr ratio results from decreasing amounts of strontium in the finer particles; the ratio increase in Al53144 is caused both by greater amounts of rubidium and smaller amounts of strontium with the finer fractions. Size separation of the sediments resulted in a partial fractionation of the clay minerals. Montmorillonite tends to settle more slowly than other principal clay types and is concentrated in the finer-sized frFractionsof V12-72. Less chlorite occurs in the finer frictions of Al53-144 than in the coarser frections. With decreasing grain size, illite, the most common clay in deep-sea sediments, decreases in abundance in Vf2-72 and increases irregularly in A153-144. Kaolinite, abundant only in V12-72, has an irregular distribution with greatest concentration in the 0.08-2 p fraction. In V12-72, aluminosilicate components show decreasing Srs7/Srsa ratios with decreasing grain size. Sediments of A153-144 show the opposite correlation-the finer clay is more radiogenic. The Sr87/Sr8aratios in all of the samples are considerably larger than the Srs7/Srss ratio of dissolved sea water strontium; the sample value closest to marine strontium, 0.717 for the 74-250 ,Hfraction of A153-144, is higher by more than 2 per cent. Thus strontium isotopic variations appear to be largely independent of the size of the clay particles and there is no detectable trend toward equilibrium of strontium in clay and strontium dissolved in sea water with decreasing particle size.

Strontium isotopes in weathering protIes and deep-sea sediments

1641

The strontium isotopic variations correlate well, however, with clay mineralogy in the less then 20 p size fractions. The correlations are most apparent for ill&, which correlates positively with the Sre7/SrB6ratio in both cores, and for montmorillonite, which correlates negatively with the ratio in the montmorillonite-rich sediment of V12-72. These trends are reasonable: illite, the principal rubidiumbearing clay mineral in the sediments, expectably has the highest Rb/Sr ratio, and, if derived from old rocks, the highest Srs7/Srss ratio; montmorillonite has by far the greatest ion-exchange capacity of the major clay minerals, and would be expected to equilibrate more readily its exchangeable cations whether on land or in the sea. It is possible that montmorillonite, if formed in the ocean by alteration of basaltic material, might retain basaltic strontium, and hence a low Sra7/SrB6ratio. The ratio could be as low as about 0.704 (GASTet al., 1964, and papers cited therein). However, PUSHKARand PETERSON(1967), have shown that phillipsite from the Pacific, which probably formed in the sea, by alteration of oceanic volcanic rock, has a strontium isotopic composition identical, within analytical uncertainty, to that of sea water. Chlorite abundance may not correlate directly with strontium isotope composition. Chlorite is relatively abundant in A153-144, and appears to correlate positively with the Sr*‘/Sr*e ratio; the isotopic trend, however, more likely is a result of an increase in illite, which forms the bulk of each of the size fractions. Kaolinite forms roughly a third of each fraction of Vl2-72 but its presence probably does not affect the strontium isotope values. The large effect of carbonate on whole-sediment Srs7/Srs6 ratios is shown by Table 11. The Sra7/Srssratio for untreated sediment from A153-144 (0*710), more than half of which is carbonate, is indistinguishable from that of dissolved sea-water strontium. Although sediment of V12-72 contains less than 2 per cent carbonate, the untreated material has a Sra7/Srseratio (O-727) which is much closer to the seawater value than that of its carbonate-free analogue (0.743). It is evident that even the finest aluminosilicates of the cores studied have not equilibrated their strontium with marine strontium, although the finer material ma.y have taken many years to settle through thousands of meters of sea water. Lowering of the Srs7/Sr86 ratios from a high, whole-clay value, with decreasing size, in V12-72 may be interpreted as a trend toward equilibration, but the variation appears to be more reasonably a function of proportion of different clay minerals. Negative correlation of strontium isotopic composition and grain size in A153-144 supports this conclusion. SILL&N(1967), in a discussion of silicate-sea-water equilibrium, stated that “ ..* the fine-grained material, especially the clay fraction (finer than 2 ,u) reacts at appreciable speed.” If equilibration of strontium in clay minerals with strontium in sea water is used as a criterion, this does not seem to be the case, for clay much finer than 2 p (less than 800 angstroms for the finest sediment of V12-72) has not reacted detectably with sea water. These results can be generalized, however, only for reactions involving the alteration of chemical components of detritus. The experiments are not sensitive to the formation of truly authigenic minerals such as phillipsite, possibly some montmorillonite, glauconite, or sepiolite in other parts of the oceans.

1542

E. JULIUSUASCH

It may be argued that “authigenic” clay occurs as surface phases on the clay minerals, and, along with very fine authigenic clay, has dissolved during the buffered acid treatment. The isotopic composition of the strontium in such phases would be identical to that dissolved from carbonate and would not be detectable by analyses of the supernate from the acid treatment. Careful weighing of the samples before and after washing and the buffered-acid treatments, however, indicate that less than 5 per cent clay was lost from the samples by solution. The data show also that clay finer than the size fractions separated must form extremely small percentages of the two core-top sediments. C. Strontium isotopes in authigenic components of deep-sea sediments Truly authigenic phases of deep-sea sediments should contain strontium that is isotopically identical to dissolved sea-water strontium. This holds true for the authigenic materials that so far have been studied: the work of PUSHKAR and PETERSON (1967) indicated a marine origin for the strontium contained in authigenie phillipsite from the Pacific; a similar origin for the strontium of manganese nodules can be demonstrated. For this study, two nodules were analyzed; one from the Scotia Sea, where sediments have Sr8’/Srs6 ratios generally less than that of sea water, and the water from the north western Atlantic, where the sediments have Srs7/Sr86 ratios higher than that of sea water. Strontium was extracted by solution of the nodules in aqua of carbonate, Rb, Sr and strontium isotope ratios were regia. Concentrations determined on the nodules. and on the sediment from which the nodules were recovered. These data are listed in Table 12. The SrS7/Srs6 ratios suggest that strontium associated with the manganese nodules is marine in origin, rather than volcanic, or from contiguous detrital sediment. These data, however, do not indicate the origin for manganese and iron of the nodules. D. Synthesis It is evident from the results presented factor controlling the strontium isotopic Table 12. Rubidium-strontium Sample Manganese nodule from northwestern North Atlantio; (33”67W; 85O47’W); untreeted Nodule aa above; oerbonate removed Sediment aasooiated with nodule; Imtmted Sediment as above; carbonate removed Manganese nodule from Scotia Sea (67054’S; 67000’W); Iln~t.ed Nodule es above; osrbonate removed Sediment asaoaieted with nodule; untreated Sediment 88 above; carbonate removed * Ratio normalized to SrB6/Srss = @I 194.

that geologic composition

provenance is the major of deep-sea clays. No

d&a for Atlantic manganese nodules

Carbonate

Rubidium

Strontium

(%)

(pp4

(PPm)

Rb/Sr

(20

310

(0.06

0.7092

<20 48

138 710

<@iii 0.07

73

340

0.21

@7099 not determined 0.7143

2

23

360

0.06

0.7091

3

20 40

245 280

0.08 0.14

61

210

0.24

0.7087 not determined 0.7079

3

40 -

-

-

srs’/srs@*

Strontium isotopes in weatheringprofiles and deep-sea Bediments

1643

detectable alteration of the strontium isotope composition of detritus takes place as a simple result of deposition in the marine environment. Actual reconstruction or authigenesis has been demonstrated isotopically only for phillipsite, manganese nodules, and some barite. Probably the most critical factors affecting Sr87/Srsaratios of carbonate-free, deep-sea deposits thus are the age and Rb/S r ratios of source rocks which contribute the debris. Size fractionation of sediments results in partial separation of clay types, and thus the effects of clay mineralogy on Sr87/Sr86ratios can be assessed: illite, the most common clay in deep-sea sediments, probably is the most important carrier of radiogenic strontium if derived from old rocks; the strontium isotope composition of montmorillonite may approach that of sea water by ion exchange; pure kaolinite contains little strontium and probably has an insignificant effect on strontium isotope ratios; and chlorite, while its role is not readily determined, is not a major constituent of deep-sea clay except at high latitudes. BEHAVIOROF STRONTIUM ISOTOPESIN ALUMINOSILICATES DURIW DLWENESIS In the previous section it was shown that contemporary deep-sea clays show variations in their strontium isotope ratios with geography and, by implication, geologic provenance. It was further shown that exposure of detrital grains to sea water for hundreds if not thousands of years does not affect strontium isotope equilibration between the phases in that differences in strontium isotopic ratios were observed across the spectrum of grain sizes. The differences could best be explained in terms of the mineralogic composition of each size fraction. Hence detritus and associated sea water commonly do not have the same strontium isotopic composition. It is evident that metamorphism of sediments ultimately can be expected to homogenize strontium isotope ratios. The question remains as to what degree homogenization of strontium isotopes takes place during the less intensive processes of change characteristic of diagenesis. This question must be settled not only for completing the history of the marine cycle of strontium but also for placing constraints on the rubidium-strontium dating of shales in the geologic record. A suite of sediments and sedimentary rocks which have undergone different forms of diagenesis has been studied to assess the role of the sedimentary cycle in modifying strontium isotope composition. A. Strontium

isotope ratios in sediments of diverse diqenetic

histories

Cretaceous Deep-sea Sediment, Northwestern North Atlantic. Unconsolidated sediment from a depth interval of 370-373 cm in Lamont core RC 5-12 (water depth = 5104 m) was treated with buffered acid to remove carbonate and was analyzed for strontium isotope composition. The sediment has been dated paleontologically as Cretaceous by LLOYDBTJRKLE (personal communication). Thus it has been in contact with interstitial or marine waters for at least 70 m.y. The SP/Sr*6 ratio of the carbonate-free clay is 0.720; correcting for radiogenic strontium which has accumulated since earliest Cretaceous time results in a minimum value of 0.717 for the ratio at time of deposition. Equilibration of the clay with marine

strontium,

with values ranging from 0.7078 for the Cretaceous

~~~~ER~A~

ef d..

1967) to 0.7093 for modern strontium, obviously has not taken place over 70 m,y. The ratio of 0.72 is similar to the value for recently deposited clays in the same general region of the sea (Fig. 2). Xubaerially exposed pleistocene reef rock.

Clay from a relatively pure marine carbonate rock (Key Largo Limestone, Quaternary), which has been exposed to meteoric waters, was analyzed, The rock has been uplifted and lithified but not subjected to elevated temperatures. Some of the aragonite has been converted to calcite and in the process strontium of marine isotopic composition has been flushed through the permeable carbonate. The sample of limestone analyzed had 2000 ppm Sr and less than 6 ppm Rb. The Sr5~~Sr~~ ratio of the clay residue is 0.712, similar to neighboring deep-sea clay values. Evidently strontium isotope exchange between clay and water had not occurred despite the partial recrystallization of the reef.

Ordovician limestone. The ultimate test of strontium equilibration between clay and marine carbonate as a result of low-temperature, geologic processes may be expected from a study of an old, lithified but unmetamorphosed carbonate rock. Such a rock will have undergone most of the processes usually included in the term “diagenesis”. Because there is a significant loss of strontium from carbonate rocks during diagenesis, clay within the limestone also will have been subjected to a flux of strontium with an isotopic composition typical of the original marine calcium carbonate deposit. A sample of almost pure limestone from the Black River Group (lower Hiddle Ordovician; about 475 m.y.) of New York State was chosen for study because of the extensive stratigraphic and paleocologic work done by WAXXER (1968) on this formation. The strata lap onto the crystalline Precambrian rocks of the Adirondack Dome. From the distribution of superjacent strata, WALKERbelieves that the Group was not buried by more than a few hundred feet of overburden during its 475 million year history. No structural disturbance of the strata other than uplift is evident, The sample chosen for study was collected at the Roaring Brook section along Black River (see WALKER, 1968, for detailed description). At this locality the carbonate sequence rests on granitic gneiss {Precambrian; about 1500 m.y.). The gneiss is overlain by about seven feet of regolith-an arkosic debris streaked with chlorite, followed by a foot of sandy shale. Overlying the reworked Precambrian detritus is about 40 feet of what WALKER regards as typical supratidal dolomite. Above the dolomite is intertidal and protected subtidal limestone (Lowville Formation) from which the sample was collected for analysis; it comes from near the base of the unit. The rook is described as a very fine-grained, dark gray limestone with thin algal laminations. In thin section its textural composition is composed of about 82 per cent fine to medium grained calcite, 12 per cent pellets, and 5 per cent dolomite rhombohedrons. The rock contains 250 ppm Sr and less than 5 ppm Rb. Solution of the carbonate with a 25 per cent acetic acid solution yielded a 3 per cent insoluble residue. The residue was size fractionated by settling and ~nt~fu~~ in deionized water. Much of the aluminosilieate residue is less than 2 ,Uin diameter (Table 13). The

Strontium isotopes in weatheringprofiles and deep-sea sediments

1546

Table 13. Rubidium-strontium data for limestone from Lowville Formation, Blaok River Group (Lower Middle Ordovician), Northeastern New York

UntrestedLimestone Very tie-grained, dark gray lime&one with thin algal lamixmtions (about 82 per oent miorite tmd apar, 12 per sent pellete, and 6 per cent dolomite rhombohedrons; 260 ppm Sr, < 20 ppm Rb. The clay-minerd v&x% refer to the peroentage of tot& clay miner&. Acid-Insoluble Residue (3 per cent of sample) Effsotive settling diemeter 01) >60 20-50 ii-20 2-6 <2

Weight per cent of m&due

Mineralogy

l&4 1192 . 23-o 23~7 26.7

* R&o normtied

I

Rook frwents decrease 1

Mainly rook fmgments, with some quartz, feldspar and sggmgatea of olay More than hdf clay; 80% illite, 20 % chlorite 78 ‘4 illite, 22 % chlorite 83 % ills, 17 % chlorite Mainly clsy; 81% ill&e, 19 % chlorite

Rbs’/ Sr*‘/ Rb Sr (ppm)(ppm) Rb/Sr SP Srss*

91

66

1.66

7.73 not determined 4-66 0-7663

03 I63 236

43 29 27

2-16 6-62 8-70

6.10 0.7806 16.88 O-8292 24+68O-8528

140

56

2-73

to Sre@/Sr*a = O-1194.

coarser fractions, especially the greater than 56 p fraction, is made up do~antly of mineral and rock fragments and aggregates of clay which were not dispersed in the acid-leach and size-separation processes. The mineral and rock fragments decrease in abundance with decreasing grain size, and clay minerals dominate the smaller size fractions. The clay fraction consists entirely of illite and chlorite, in the same abundance in each of the size fractions, about 80 per cent illite and 20 per cent chlorite. One of the reasons for choosing this locality for study was proximity to the ancient, crystalline rocks of the Adirondack Dome. The souree of &luminosilic~te detritus within the carbonate rock, however, is not certain. Outcrops of Adirondack rock probably were the closest source of sediment; according to WALKER (1968), the shoreline was not more then a few hundred yards from this particular site at the time the limestone was deposited. There were other possible sources of sediment in the Appalachian region; although they were much farther away, they probably were being eroded more actively than the low-lying Adirondack rocks. Ad~ond~~k rock, however, was the most likely source of the residue separated from the limestone described above. Analytical data for the insoluble residue are shown in Table 13. In the successively finer fractions, Rb/Sr ratios increase markedly ss a result of increasing rubidium ~oncentr&tions and decreasing strontium concentr&tions. The SPfSr** ratios of the size fractions increase with increasing Rb/Sr ratio, as is evident in the plot of Fig. 3. The best “isochron” that can be drawn through the points gives a “date” of about 360 m.y. The points, however, do not lie along a line but show considerable dispersion, This indicates that the residue either did not behave as a closed system, or that it began with a,variable strontium isotope ratio for the different fractions. The “isochron,” rather than giving a true age sctudly may be the result of mixtures of two end members; relatively coarse, Precambrian detritus weathered from the Adirondacks, and a fine-grained dominantly illitic clay assemblage. If, as is most reasonable, the detritus as a whole did not equilibrate with the marine 6

1546

IS. Jmras

DASCR

.82C

.75c

,720

____

------O~&~i~ig~

.$,*~wofer---

8

4

'2

Rbs,

____--__

16

------

20

24

28

-iif+

Fig. 3. Rb-Sr

data for size-fractionsted, acid-insoluble m&due from lime&one of Lowville Formation, Northersstern New York.

strontium of the surrounding calcium carbonate, equihbration does not take place as a result of contact, time and the prooess of diagenesis, as suggested by CHAUDHURI and FAIJRE(1967). Deep burial ultimately may produce equilibration, but the ages obtained from such sediment record a post-depositional event, or an event which essentially may be olassified as metamorphic.

8edimentary rocks Several Phanerozoic, shales whose approximate ages can be inferred geologically have been analyzed by the Rb-Sr whole-rock method (e.g. Ckxws~oa and PIDGEON, 1962; WHITNEY and HWRJAY, 1964). The narrow dispersion of points around many of the isochrons and the reasonable correspondence of the measured age in some cases with that inferred Ecom stratigraphic arguments has made this dating method of much interest. If the method were completely reliable, its greatest application would be in the dating of Precambrian strata. It is not yet clear,

Strontium isotopes in weatheringprofiles and deep-sea sediments

1647

however, whether many of the dates reflect provenance, time of deposition, or a post-depositional event. Age of deposition can be obtained only if the samples that are analyzed, at or shortly after deposition, had a uniform Srs7/Sras ratio and subsequently remained This could be attained by one of several paths listed in Table 14. closed systems. The most unequivocal process through which true age of deposition may be obtamed, of course, is the authigenic marine formation of the bulk of the dated sample (Table 14; No. 6). It has been shown in previous sections that homogenization of strontium isotopes does not take place as a result of contact of sea water with detritus (Table 14; No. l), or, except perhaps for very fine sediment, during diegenesis (Table 14; No. 2). Table 14. Mechanisms and processesby which samples of sediment and sedimentary rock may exhibit, or acquire, homogeneousstrontium isotope composition Mechanism or process

Initial Srs’/8ras ratio

Interpretation

of age data

Coologic environment

Time of deposition

Marine; near-shore or deep-sea

Time equal to or leas than sge of deposition

Marine diagenetio event may take plaae after sediment is uplifted from se* water Requires a young, isotopioslly uniform source; most general condition may be the weathering and deposition of graywacke sediments from the basalt-andesite association of eugeosyn&nal areas Arkose; oratonal sediments in general

1. Equilibration of detritus by oontaot with marine strontium 2. Diagenetic equilibration of detritus with marine strontium

Marine strontium; about @709 for modern sea water Marine strontium; about @709 for modern sea water

3. Inheritanoe of homogeneous isotope aomposition from source region

Inherited from souroe region; @702-@704 for modern, primary strontium-higher ratio for recently metamorphosed, older rook or sediment

Time of deposition

4. No low-temperature modification of either Rb/ Sr or Sr*‘/Srss ratios of detritus 6. Low-temperature modifioation of Rb/Sr ratios and, to a much lesser degree, Srsr/Srss ratios t whereby the effeot of inherited radiogenic stron. tium is minim&d 6. Authigenetio formation of bulk of sample from isotopioally uniform media 7. Complete mixing of detritus which may oonradiogenio strontium

Inherited from sourae region

Age of souroe rook

Inherited region

Maximum time of deposition

Continental or submarine weathering followed perhaps by marine deposition

Time of deposition

Applicable prinoipally to authigenio formation of minerals in sea water Effective mixing of detritus during transportation to or deposition in sedimentary basin Formation and deposition of marine carbonate with detritus

8. Mixing of detritus, whioh may oontain varying amounts of radiogenio strontium, with marine carbonate 0. Equilibration of strontium between phases as a result of metamorphism

from souroe

Applicable principally to sea water system; about 0.709 Integrated averege inherited from souroe

Time of deposition

r0gi0lL3

Effeotively a masking of “detrital” strontium by marine strontium; about Q709 Integrated, average ratio inherited from souroe region

Slope of “isochron” may be positive, eero, or even negative, depending on composition of detritus and type of mixture Time of metamorphism, or minimal age of deposition

by deep burial in an actively subsiding basin of deposition; regional metamorphism Metamorphism

I*545

E.

JULIO

L)ASCEI

The inheritance of isotopically homogenous strontium from a young, volcanic source area (Table 14; No. 3) may be a relatively general mechanism by whie.1~ sediments may satisfy this requirement. Eugeosynclinal areas of sedimentatiorl with associated volcanic terrains are common geologic features. Detritus weathered from these young rocks would be uniform isotopically, but could have different, Rb/Sr ratios. Through transportation and deposition processes, a sedimentary basin might acquire materials of different Rb/Sr ratios, thus satisfying requirements for dating by the whole-rock method. This mechanism is not restricted to the eugeosynclinal-volcanic regime. Weathering of any uniform source, such as a continen-. tal flood basalt region will work as well, although the likelihood of mixing with more varied detritus is increased. Older rooks that have been metamorphosed immediately prior to erosion and deposition as sediments also belong to this category. In this case the initial Sra7/Sr86 ratio would be higher than the primitive value listed in the table. Breakup of older rocks, primarily by physical processes may yield detritus in which the minerals are separated to some degree resulting in varying amounts of radiogenic strontium and Rb/Sr ratios in the strata at the time of deposition. If little modification of either the Rb/Sr or the strontium isotope ratio occurs except for this mineralogic separation, the resulting isochron will simply reflect the age of the source rock (Table 14; No. 4). The Lower Permian Stearns Shale of Kansas yields an apparent Devonian age (C~MJDHURI and BROOKINS, 1967); the upper Cretaeeous Pierre Shale of the western U.S. has been shown to have an apparent Permian isochron age (PETERMAN and TOURTELOT 1966), although the “isochron” may result from a earbonate-detritus mixing process (PETERMAN, personal communication). Because of this possibility, isochrons from completely unmetamorphosed sediments, prior to more refined analysis, must be interpreted as maximal ages of deposition. It is apparent that some presently undetermined degree of metamorphism will cause equilibrium of strontium isotopes within sediments and sedimentary rocks (Table 14; No. 9). Isochrons of these metasedimentary rocks, apparent examples of which are given by PETERMAN (1966) and by HILLS et al. (1968), thus record time of metamorphism and have no consistent relation to time of deposition. Additio~lally, the effect of burial of sediment on the equilibration of strontium If equilibration takes place, it is between the phases present is not understood. again a metamorphic event and the date obtained will be younger than age of deposition. How much younger is not known since it must not only be equilibrated, but this equilibration must be frozen in by rapid removal from the environment to Some of the minimal isochron ages of Precambrian sediments give an isochron. noted by BOFINGIERand COMPSTON (1967) perhaps fall within this category. Complete mixing of detritus, either with itself (Table 14; No. 7) or with a isotopically uniform phase (Table 14; No. 8) may result in strontium isotope homogenization between whole-rock samples. In addition to a uniform Sra7/Sr8” ratio near time of deposition, however, the samples also must have varying Rb/Sr It is not likely that both conditions ratios, so that an isochron can be generated. are satisfied by simple mixing of detritus, if the sources of debris have varying Mixing of detritus with marine carbonate will amounts of radiogenic strontium.

Strontium isotopes in weakheringprofiles and deep-sea sediments

1649

tend to homogenize the bulk strontium isotope composition of the samples; resulting Rb/Sr ratios, however, will be lowered, and the samples may be unsuitable for dating. In this context, it should be pointed out that the data for deep-sea clays indicate a degree of mixing, with respect to Sra7/Sr86 ratios, over broad regions of the sea. Omitting results from local areas such as the Labrador Sea, these data form a linear trend on an isochron diagram; the trend has an age value of about 500 m.y. It is clear that common geologic processes do not consistently result in strontium isotope homogenization, and therefore in sediments and sedimentary rocks which are suitable for whole-rock dating. Perhaps whole-rock age data from sediments can best be evaluated with reference to the geologic setting from which the samples were obtained. True age of deposition, for example, may most commonly be obtained from young volcanic detritus which was deposited rapidly and buried in a eugeosynclinal regime; marine sediments composed largely of authigenic materials also may fall in this category, provided that the system is not later perturbed by metamorphism. Maximal ages for deposition commonly may be derived from cratonal shales and arkosic sediments. Minimal ages of deposition may be obtained most frequently from regions of rapid sedimentation and deep burial, followed by uplift and removal from the equilibrating environment.

SUMMARY AND CONCLUSIONS Modification of the rubidium-strontium-strontium isotope system in low-temperature, geologic processes may be summarized as follows: (1) Weathering of primary crustal rocks results in a marked psrtitioning of rubidium and strontium between weathered residue and ground or surface water, and ultimately sea water. The major factor in the increase of the Rb/Sr ratio of weathered rock is loss of strontium. In contrast, the strontium isotope composition of many primary rocks is largely unaffected by weathering and erosion, although the preferential solution of rubidium-poor, strontium-rich minerals of an old granitic rock obviously will bring about strontium isotope modification between fresh and weathered counterparts. A consequence of rubidium-strontium-strontium isotope behavior during weathering is that some active and relict profiles may be distinguished. (2) Transport of weathered detritus through the ocetbn, one of the largest surficial reservoirs of isotopically uniform strontium, and deposition in the marine environment does not result in significant strontium isotopic changes in detritus. Authigenic phases such as phillipsite and manganese nodules in the sea, however, take on the expected isotopic composition of marine strontium. Although the evidence is not unequivocal, it appears that absorption of rubidium, during weathering or marine deposition, may bring about a general, further increase in the Rb/Sr ratio of weathered detritus. Because of the lack of isotopic modifictttion of detritus during transportation and deposition, useful provenance information can be obtained from strontium isotope measurements on the detritus; additionally, continental and authigenic marine components in some cases may be distinguished. (3) Isotopic equilibration of strontium between the detrital components of sediments and sedimentary rocks, or between detritus and sea water or carbonate,

1550

E. JULIUSDASCH

does not take place simply by prolonged contact or as a consequence of low-temperature diagenesis. Deep burial within an actively subsiding basin of deposition may, however, bring about strontium isotopic equilibration, and complete mixing of detritus brings about an effective homogenization of isotope ratios in whole-rock samples. From these studies, several limiting conditions can be placed on the interpretation of rubidium-strontium age data obtained from whole samples of sediment. True age of deposition may be derived only under certain geologic conditions, but the technique offers considerable promise, especially in the study of Precambrian sediments and sedimentary rocks. In larger perspective, low-temperature modifications within the rubidium-strontium-strontium isotope system have important implications to the worldwide partitioning of these species between fresh rock, weathered rock, and sea water. If the general result of weathering is lack of strontium isotope modification, sea water should become markedly more radiogenic through time, as pointed out by WICKMAN. It is clear that while sea water has become more radiogenic, the increase has been relatively slight, and that marine strontium does not directly reflect the average strontium isotopic composition of the weathered crust. The addition of nonradiogenic mantle strontium to surficial deposits and to sea water obviously exerts a strong control on the strontium isotope composition of sea water, and helps maintain the comparatively low amount of radiogenic strontium in the oceans. More striking, however, has been the constant, effective recycling of marine strontium, through the solution of previously deposited and uplifted marine carbonates and evaporites, and perhaps also by atmospheric cycling of marine strontium. Acklaoulledgnzents-KaRL K. TIJREKIAN of Yale University supervised this study and contributed to many of the conclusions. The work has benefited from suggestions and criticisms of others at Yale, particularly R. L. ARMSTRONG, R. A. BERNER and F. A. Hmns; the manuscript also has been crit.icized by P. E. BISCAYE of Columbia University, and by Z. E. PETERMAN of the U.S. Geological Survey. Financial support for construction of the mass spectrometer was obtained through grants from the ESSO Educational Foundation and the Westinghouse Educational Foundation. The work has been financed by Yale University fellowships and by NSF Grants GP-2456 and GA-1413. Deep-sea sediments were obtained from Lamont Geological Observatory through the courtesy of Maurice Ewing. Other samples were obtained from J. A. S. ADAMS, P. E. BISCAYE, Y. K. CHAU, J. R. DYMOND, R. C. HARRISS, F. A. H~Ls, D. G. JOHNSON, N. M. JOHNSON, D. C. RHOADS, B. P. RUXTON and K. R. WALIIER. Laboratory assistants during the study were B. C. BROCKETT, S. E. METCALFE, L. M. MARTZ and Mrs. P. N. TAYLOR. REFERENCES BISCAYE P. E. (1964a) Mineralogy and sedimentation of the deep-sea sediment fine fraction in the Atlantic Ocean and adjacent seas and oceans, 86 pp. Ph.D. Thesis, Yale University. BISCAYE P. E. (1984b) Distinction between kaolinite and chlorite in recent sediments by X-ray diffraction. Amer. Mineral. 48, 1281-1289. BISCAYE P. E. (1965) Mineralogy and sedimentation of Recent deep-sea clay in the Atlantic Ocean and adjacent seas and oceans. Beol. Sot. Amer. Bull. 78,803~832. BISCAYE P. E. and DASCH E. J. (1968) Source of Argentine Basin sediment, southwestern south Atlantic Ocean. CTeoE. Sot. Amer. 1968 Ann. Meeting Program, p. 28. BOFINGER V. M. and COMF-STON W. (1967) A reassessment of the age of the Hamilton Group, New York and Pennsylvania, and the role of inherited radiogenio Srs7. Qeochim. Coemochim. Acta

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