RE:-"D1CONTl DELL." SOCIETA ITALlANA Dl M1NERALOGIA E PETROLOG1A, 1985, Vol. "0, pp. 17-24

Radiometric datation of sediments

PAUL PASTEELSLaboratorium voor Geochronologie (CHROl, Vrije Universiteit Brussel, Pleinlaan 2, 8-10.50 Brusse!

ABSTRACT. - Sedimems are poor radioactive docks.Only in exceptional cases will sedimentation orearly diagenesis erase the isotopic memory of earlierevents. Diagenesis at low temperature may disturbthe ismopk equilibrium of the scdiments. Con·sequently, only in special cases can we allempt todale seJiments with radioactive methods. Materialssuitable for datation indude whole-rock shales,and day minerals occurring as isolated flakes orpellets. Rh-Sr and K·Ar methods can be utilized onwhole rocks and day.size fractions. Glauoonite is avaluable geochronometer (Rh-Sr and especially K.Arl,if the samples arc selected on the basis of theirhigh K content and absence of burial and weathering.

Key words: radiometric geochronology, sediments,glauconite, Rh-Sr, K·Ar.

RUSSUNTO. - I sedimenti non sono eccellentiorologi radioaltivi. Solo in casi eccez.ionali la sedi­mentazione 0 la diagenesi ptecocc: cancellano il ri·cordo isotopioo degli eventi precedenti. La diagenesia bassa temperatura puo disturbare !'equilibrio iso­topico dei sedimenti. Da tulih db consegue che Ingeocronologia radiometriea di sedimenti e fattibilesolo in casi eccezionali. Sedimenti argillosi e mine­rali atgillosi (sia in lamelle isolate che in pellets}possono essete utili per datazioni radiometrkhe. Imetodi Rb-Sr e K-Ar possono essere utilizzati siasu roccia tmale che suUe frazioni argillose di sedi­menti. La glauoonite e un utile geocronometro(Rh-Sr e, soprattutlo, K-Ar), purehe vengano ocellicampioni rkchi in K e privi di altetazioni.

Parole chiave: geocronologia radiometrica, sedi·menti, glauoonite, Rh-Sr, K·Ar.

The earliest attempts to date sedimentswith the geochronologic methods based onradioactivity date back from the time when

these methods were still in infancy. Naturalradioactivity and its possible applicationsbecame indeed a subject of interest for earthscientists very soon after its discovery. Forexample, the observation of the high radiumcontent of deep·sea sediments was made veryfew years after this new element wasdiscovered OOLY, 1908). But only years afterdid this observation lead to the developmentof a new method of dating sediments.

There are two types of application ofradiometric dating methods to sedimentarymaterials: those based on short-live! radio­isotopes, either intermediate members of thenatural decay chains, or cosmogenic nudideson the one hand, and those based on long.lived radionuclides on the other hand. Theobservation of a high radium content indeep-sea sediments is in relation withthe first type of application, which shallhowever not be considered further here.From methodological and fundamental pointsof view, dating Quaternary or older depositsare indeed very different problems and onlythe latter question will be the subject of thepresent short review.

If we restrict thus our attention torelatively old sediments and sedimentaryrocks, we note that in this field as in generalthe progresses of geochronology based onradioactivity were very slow in the beginning.Before world war 11, there is little to bementioned in this domain, except the attemptto date the Swedish Kolm by the V·Pbmethod (NI ER, 1939).

18 P. FA-STEELS

From this carly beginning it became ap­parent that dating sediments with theradioactive methods is no simple task.Contrary to the case of high temperaturesystems, tne constitution of which cor­responds to resetting to zero and thestarting at zero of the geochronomerer, se·diments are poor radioactive clocks. Only inexceptional cases will sedimentation or earlydiagenesis erase the isotopic memory ofearlier events. Sedimenrs undergo moreoverdiagenetic changes at low temperature, whichmay disturb their isotopic equilibrium.

The consequences of this acc that only inselected cases can we attempt 10 date se­diments with the radioactive methods andthat in many instances some degree ofimprecision must be tolerated.

Since the early fifties, the potassium-argonand the rubidium-strontium methods becameapplicable to sedimentary materials and arestill the most widely used in this case(lNGHRAM et al., 1950; GENTNER et al.,1953; WASSERBURG et al., 1956; LIPsaN,1956, 1958; CORMIER et al., 1956).

Materials suitable for datation includewhole-rock shales, clay minerals presentingthemselves as isolated flakes, or in pelletsas in the case of glauconite (or glaucony tofollow the recommendation of OOIN andDoOSON, 1982), in different types of se­diments.

Datation of shales wilh the whole-rockruhidium_strontium method

Inasmuch as exchange of strontium isotopeswith sea water or sediment water shortlyafter deposition actuaIJy occurs, the Rh-Srmethod is in principle applicable to shales.Argon being a noble gas, such equilibrationis unexpected for its isotopes, so that at­tempts to fix the sedimentation time withthe K-Ar method using whole-rock sampleswould make little sense.

Among the main contributions to thesubject of dating shales, we may mention:CoMPSTON and PIDGEON (1962), WHITNEY·and HURLEY (1964), CoMPSTON et a1. (1956),PETERMAN (1966), CHAUDURI and BROOKINS(1970), CLAUER (1973, 1976), PERRY andTVREKIAN (1974), SPEARS (1974), CHAV­OHURI (1976).

The basic condition (besides that of closed­system) for the application of the Rb·Srisochron method is the availability of severalsamples with different rubidium to strontiumratios which all contained a strontium withthe same isotopic composition at the initial,i.e. sedimentation time. It is important toobserve that both conditions are to besatisfied simultaneously, since some factorswhich may cause the elemental ratio to vary,may cause the isoropic ratio ro vary also. Inaddition, the repartition, a straight line ofexperimental poims, does not prove that bothconditions above are satisfied since themixture in variable proportion!i of twocomponents with different Rb/Sr and81Sr/WSr ratios also results in a straight linerepartition of experimental points, which iswithout chronological significance in thatcase. The investigation of weathering profilesand recent sediments shows an almostcomplete lack of strontium isoropic equili­bration during we:nhering and oceanic orestuarine sedimentation (DAscH, 1969;BISCAYE and DASCH, 1971; BISCAIE, 1972).I t has been shown however by CLAUER etal. (1976) that isotopic exchange actuallyoccurs, at least in some cases, but aftersedimentation and progressively. A rerentreview of this problem is made by CLAUER(1982). Isotopic equilibration shortly aftersedimentation is only one of the differentsituations which are actually observed. Therate of this equilibration depends on mi­neralogy and grain size, among other factors.Detritic days may fail to have reachedisotopic equilibration with sea water, se­diment water or other components of thesediment, more than 50 Ma after deposition.

The proposal of CoRDANl et al. (1978)should also be considered with some caution.They argue that some degree of isotopichomogeneisation can be achieved by me­chanical mixing of detritic, more or lessweathered, diagenetic, more or less equili­brated components, inasmuch as similardepositional conditions ptevail over a widearea. If isotopic equilibration is attained bysuch a process, some degree of chemicalhomogeneisation is likely to occur also. Thecase becomes impossible to solve by theRb-Sr method because of tOO similar Rb/Srratios. Alternatively, a colJection of sampleswith Rb-Sr ratios which differ widely will

RADIOMETRIC DATATION OF SEDIM.ENTS 19

generally present It wide range of mine­ralogical compositions. Carbonate-rich sam­ples with low Rb-Sr ratio will tend to havelower initial 8TSrr6Sr ratios than silicate­rich ones.

Several cases have been reponed wherediagenesis (or «weak metamorphism ») isconsidered the cause of isotopic rehomogenei­sation (BoFINGER et al., 1968, 1970; COR­llANl et al., 1978; PERRY and TUREKIAN,1974; BONHOMME et al., 1982, etc.). A «trueisochron» or It not too scatteted «errochron»will then give the age of the diagenesis, ora good approximation of it.

We shall not consider here the possible«disturbing factors» which may cause theexperimental points to be widely scatteredon the isochron diagram. In this case, it isindeed obvious that no age can be derivedfrom the data. The important point is thepossible existence of «false isochrons»yielding spurious age indications, or trueisochrons, giving the age of another eventthan that at which the attempt at datationis directed (in casu, the sedimentation time).

This second possibility is not to be ruledout a priori owing to the possibility of aslightly positive or (less likely) negativeinitial slope for the isochron, or to latediagenetic evolution. From this may result,even in the general case, a variable error,which may amount to several lV1a at toseveral tens of Ma. Thus, for some ap­plications, as for example the calibration ofthe time-scale for the Phanerozoic, whole­rock Rb-Sr data on shales should be usedonly with great caution. For the deciphetingof unmetamorphic Precambrian terraneshowever they may reveal themselves veryuseful.

K·Ar dating 011 whole-rock samples andsize fractionll of shales

K-Ar me<lsurements on whole-rock andsize fracti0ns, the mineralogical compositionof which, being determined by X-rays, mayhelp the interpretation of the Rh-Sr dates,or provide information on the source of thedetritic components or the diagenetic evo­lution of the sediment (HOFMANN et al.,1974; PERRY, 1974). In the same rock com­ponents may be. found which keep recordof their pre-depositional history and others

recording the last diagenetic event. ThoughRb·Sr systems may react differently, theseobservations tend to support the idea thatthe determination of an age which approxi­mates that of sedimentation, by the Rb-Srmethod, may in some cases result from akind of averaging of the ages of pre- andpost-depositional events.

Fine fractions

The Rb-Sr and K-Ar methods may beapplied to the clay-size fraction of thesediment. In such cases the detritic com­ponents or at least a large patt of them maybe eliminated. Moreover the mineralogicinvestigation of the clay helps greatly to theinterpretation. Reviews of this questionhave been published recently by the maincontributors to the development of thismethod (BONHOMME, 1982; CLAUER, 1982).They report cases were early diagenesis couldbe dated on low temperature iIlite. However,in otber instan,es detritic 2M iUite pre­dominates and yields the approximate ageof the source. In othet sediments or se­dimentary rocks the presence of 2M illitereflects an anchimetamorphic evolution andthe only event which can be dated is acertain stage of this evolution. Finally, thepresence of kaolinite denotes alteration byweathering which helps to explain the scatterof experimental points on an isochrondiagram.

Very clearly, the «ages of sedimentation»(or close to sedimentation) which can beobtained this way are much more preciseand established on 6rmer ground than whenwhole-rocks are chosen as material for da-.tation, in the absence of mineralogical study.The variety of situations observed also showsthat there is no miracle to be expectedwhich would allow the precise dating ofsediments when no attempt is made at theelimination of the detritic components andwhen the composition of the clay fraction(which may be detritic, early or late dia­genetic) remains unknown.

Glaucony

The nature 0/ glaucony or glauconiteIn the present case, it seems more im­

portant to define as precisely as possible .the

20 P. FA-STEELS

material which is actually submitted toanalysis when geochronologislS decide totl date glauconite,. than to give a precis«:definition of a mineral species, or a group ofminerals. The use of the term «glaurony"with the same definidon as in OOIN andDoOSON (1982) and (UUER (1982) is quiteconvenient for Ihis pu~. The \\'ordglaucony will thus be used as a fades nalm:in order to avoid confusion wilh a peculiarmineralogical composition.

Glaurony presents itself in the pelletalform in sediments in most instances andwhat is thus analyzed are these pellets, whichmay be plurimineral, in some instances .Ill

least. However, in most cases the essential(or only) constituent of the glaucony pelletsis a member of the family of «, glauconiticminerals », an cvolmive series starling froma smectitic protoglauconite and ending withthe K-rich illitic member (mineral glauconiteor glauconitic mica), which can be defined:IS an iron illite polymorph. The intermediateterms are: mixed.layered minerals. Besides thecommon pelIetal form, glauconitic mineralsmay be observed as flakes dispersed withina sa:limCnl or as a coating on pebbles orhard·grounds.

FOrmQlion 01 glQuconilic minerQiJ

The question of glauconite formation hasbeen much disputed in the past. From anumber of rather recent studies, a few pointswould appear which can be summarizedbriefly (EHLMANN et aI., 1963; GIRESSE andOOIN, 1973; GIRESSE, 19U; LAMBOY,1968, 1975; OOlN, 1973, 1975; OOtN andMATTER, 1981).

Q • Glauconitization, i.e. the formationof glauconitic minerals, does not require aspecific precursor, as for example detriticclays or micas, but a suitable microenviron­ment or substrate. This microenvironment is<le semi<on6ned I): ion exchange with sea­water is possible but Eh and pH conditionsmay be slightly different from that of seawater. The ideal size of this substrate de­pends its porosity. Coproliths. foraminiferalskeletons, detrital mica Rakes are amongthe most commonly observed substrates,evolving inro ~Iauconite grains the shape ofwhich, at early stage of development, isindiClltive of the nature: of the initial sub-

strate. At stage, the size of the initial grainmay increase and its morphology be altered.The original clay minerals, mica, carbonatesbeing then largely or totally eliminated, theglauconite pellet itself becomes the substratefor further glauconitization.

b - Within the substrate, the smectiticend-member appears, Sttmingly generated byionic chemical precipitation. Simultaneously,the pre-existing minerals (clay, mica, car­bonates, but also quartz, feldspar ... ) are«replaced., Le. start to disappeat. Thisoccurs at variable rates, depending on theinitial mineral. Carbonates, for example, aremore easily replaced than silicates or quartz.

c - By potassium uptake the smectiticend-member progressively evolves towardsthe illitic end.member. A further eliminationof the substrate takes place. In someinstance the elimination occurs mainly at thisstage.

d - Individual glaucony pellets mayconsist out of a mixture of low-K and high-Kglauconitic minerals (VELDE, 1976).

e - This evolution comes to an end whenion exchange with sea water is no longerpossible, because of burial. Alternatively, amature: glauconite is generated before burial,and the evolution cannot proceed further.Glauconite level!: are: indeed often associ~ted

with lowered sedimentation rates or uncon­formities.

Zero isotopic Qge 01 gltJuconiteIt is quite evident from the description

above of the processes of glauconite for­mation that an «isotopic memory ~ of the:;ilicate precursor may be kept by theglallcony pellet a~ long as a maturation stageis not reached where this ptecursor is totatlyeliminated. The question has been recentlyreviewed by ODIN and DonsoN (1982).Several c:tses are discussed by them, showingconvincingly that immature: pellets withahalll 1 % K may yield apparent K-Arages up to 100 Ma higher than the time ofsedimentation. Whenever glauconite suitesare found at the same locality or in the sameformation, showing different stages of ma­turation, the «excess argon. sharply decreaseswith increasing K content. At about 5 % K(6 % KzO) the isotopic memory of the

RADIOMETRIC DATATION OF SEDIMENTS 2\

precursor seems to have been erased. Whenthe substrate consists of carbonate debris,low-K glauconites may yield ages dose tosedimentation, An «isotopic memory» of theprecursor may however be present in caseswhere X-ray diffractometry fails to show thepresence of other minerals than glauconiteminerals in the pellets.

These K-Ar data on « immature» glauco­nite have been recently complemented by aRb-Sr investigation of the same samples. Theapparent ages found by Rb-Sr are similaror slightly older than the corresponding K-Arages. In other words, glauconies carryingexcess radiogenic argon also carry excessradiogenic strontium and this in similar orslightly higher proportions (KEPPENS et al.,1984). It has also been observed (KEPPENSand Q'NEtL, 1984) that in suites of samplesof the same age, high-K glauconies havehigher gltiQ values than low-K glauconies.The latter case would correspond to incom­plete equilibration with sea-water oxygen.

Other cases than those selected by OOINand DODSON (1982) may be found in theliterature which are evidently to be inter­prcted in terms of argon or strontium excessprescnt in low-K glauconite (e.g. OWENS andSOHL, 1973; PASTEELS and KEPPENS,1981). In the second mentioned paper, asuite of glauconies from the Cretaceous yieldconcordant Rb-Sr and K-Ar ages except inone case where the Rb-Sr age is higher. Thisparticular sample has a lower K content(5.2 '10) than all the others. From its strati­graphic position and other data allowing thecalibration of this part of the time-scale, itwould appear that the 'K-Ar ages are (at leastapproximately) correct, the Rb-Sr ages some10 Ma too high.

Influence of hurial and heating on glall'conite ages

Few systematic investigations have beenundertaken on the influence of diagenesissensu-stricto (anadiagenesis) or deep dia­genesis (anchimetamorphism) on glauconyK-Ar ages (FREY et al., 1973; CoNARD er al.,1982; FISCHER et al., 1984). In the HelveticAlps, a high K glaucony starts to lose radio.­genic argon before the stilpnomelane iso~rad

is reached (FREY et al., 1973). Manypublished data, namely on samples from

bore-holes, dearly show that burial diagenesiscauses argon loss from glaucony, but preciseinformation on the diagenetic conditions inwhich this occurs are generally lacking.

The problem has also been considered fromthe experimental point of view, Glauconiesof different K contents were subjected tohydrothermal treatments during periods oftime up to three months. The mica.typeglauconite loses from 4 to 8 '10 of its argonar 320° C, but K·poorer, mixed layeredglauconitic minerals are less retentive in thesame conditions (OOlN et al., 1977; OOIN,\982).

Corresponding Rb-Sr data are still muchmore scarse and inconclusive. It has beenargued, on the basis of the alses wherea comparison is available, that Rb·Sr agesmight be less influenced than K-Ar ages byburial diagenesis (KEpPENS and PASTEELS,1982) but this point needs confirmation.

11lflucnce of weathering on glauconite ages

The KAr and Rb-Sr investigation of thesame in-situ and reworked, weathered glau.conies in Normandy tends to show that,upon weathering, glauconies release morecasily mdiogenic strontium than radiogenicargon (OOIN and HUNZIKER, 1974; CLAUER,1976).

Table 1 presents other data bringing afirm confirmation to this, in the case of asuite of samples from the New Jersey­Maryland greensands. The K-Ar data aretaken from the paper of OWENS and SoHL(197.3). The Rb·Sr ages have been measuredin Brussels (VUB). The labelling of thesamples 1,2,.3 is that of the above-mentionedpublication and refers to the material usedfor K-Ar dating. The corresponding numberswith an asterisk refer to (he glaucony usedfor Rb·Sr dating which result from anothermineral separation from the same sedimentsample. Most of these greensands showevidence of alteration by weathering, asdescribed by WOLFF (1967). For lack of abetter criterion the abundance of gocthitcwas taken as an indication of the intensityof weathering. In samples .3, 5, 6 some goc·thite pseudomorphs after glauconite areobserved which were carefully removed fromthe annlyzcd materi'lt. Samples 12 and 13consist of glaucony pellets embeddt...d in a

22 P. PASTEELS

TABLE 1

N('w Jersey-Maryland greensands. Comparison 01 geological and radiometric data

st..,. Probable "'0 01 S...pl.Il_Ar (Ma) tRb_sr(lIa)

/j ( , ) ,,)..~I..nutlon ". '_u 8Rb _ Sr W••thorlni

Jp""'''' 50·56 ,"

55.7:1.8 " 3 : ".3 , ,~nugnlt1oant

'fho.no \la» " ) ),

" 5 : 1. 9 " .5:1.7 , - 5·10 ••~.rU.

-,~ " ," " · J ~ 2.0 " ,8: \.5 , - 5·10 _dueto

" ," " · 1 : 1.1 " .311.0 , -" IOOduU.

Probable "".nnJ'O....U1

Loo"or MaulNchUan " ,,' " 5 : ".1 57.5'1.1 -10-15 ->0 ~rv....lu.. "Uppor C...po.nlan " n' " · 4 : 2.0 56.): 2.5 -10·15 - 20-25 po,.....I"

C....panl..., " " ", " · .. : ..., 69.6:1.9 , -5·10 "".li"Ut••"t

{llo: percent departure (tom probable true agc.

goerhite matrix. Again, the goethite wasremoved and only fresh-looking glauconieswere analyzed. If wc compare the K-Ar andRb-Sr ages with the stratigraphic age weobserve that only samples 12 and 13 showargon loss, while a lowering of Rb·Sr agesis much more important in the same casesand also occurs for what would appear atfirst sight «slightly weathered» glauconite.These data also show that radiogenic stron­tium may be lost in cases where, apparently,no argon is lost.

Concluding remarks on lhe usefulnessof gluul'onile us 1I geochronomcter

If selected on the base of high K content,absence of burial and weathering, glauconitemay prove a valuable geochronometer withthe K-Ar and Rb·Sr methods, especially theformer.

Comparison of Rb·Sr and K-Ar datesmay help interpretation since radiogenicstrontium is more easily lost than radiogenicargon upon alteration by weathering, andin the case of immature glauconies it wouldappear that «inherited» radiogenic stron­tium may be less easily eliminated thanradiogenic argon.

A subsisting question is that of the degreeof predsion which can be obtained in themost favourable case. This problem can besolved by the analyses of many samples from

the same stratigraphic level which are re·latively young in order to evidence smalldifferences in radiometric ages. Such studyhas been carried Out on late Oligoceneglauconies by KREUZER et al. (1980). Theanalytical errors only cannot account for thescatter of the results they observe. Thereforea « principle error in the glauconite system»should be considered which is «at least ofthe order of several hundred thousand years»(ref. cit.). This combines the «zero age»uncertainty with the possible alteration ofthe parent-daughter system its formation.Another factor which may contribute tothis principle uncertainty is the principaldifference between age of formation and ageof deposition. Many glauconites are probablynOt formed where they are found buttransported over short distances before finaldeposition (it should be kept in mindhowever that fossils can be transported inthe same way too).

This «principle error» restricts the useof glauconite to samples older than the lateTertiary. It remains nonetheless among thedatable outhigenic minerals, that which isthe easiest to identify and recover fromsediments, thus probably also the mostuseful.

Acknowledgmen/S. - The author is indebted to]. OaRAoovlCH for his comments on the New .Iersey­Maryland case and to B. MAMET and J. OWENSfor making available interesting material from th"New Jersey-Maryland g[a\lconite belt.

RADIOMETRIC DATATION OF SEDIMENTS

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