Gmehimica n Cosmcchimica ACIU Vol. 48. pp. 1965-1972 0 Perpnmon Rar Ltd. 1984. Printed in U.S.A.

00167037/84/53.00 + .30

Pb and Nd isotopic study of two Archean komatiitic flows from Alexo, Ontario

BERNARD DuPR~*, CATHERINE CHAUVEL and NICHOLAS T. ARNDT

Max-Planck-Institut fuer Chemie, Abteilung Geochemie, Saarstrasse 23, 6500 Mainz, Federal Republic of Germany

(Received March 15, 1984; accepted in revisedform July 3. 1984)

Abstract-Isotopic analysis of two Archean komatiitic flows from Alexo. Ontario, gives a PbPb isochron aae of 2690 f 15 Ma and a Sm-Nd isochron aae of 2752 f 87 Ma. These axes agree well with U-Pb zircon ~1~

ages from underlying and overlying vokani& The variations in element &ti& that define the isochrons were not produced during crystallization of the lavas. The spread in U/Pb was caused by submarine alteration soon after eruption, and the spread in Sm/Nd resulted from (a) difkrences in the composition of the residue of melting, and (b) contamination of the upper komatiite flow through thermal erosion of the lower flow.

The “‘Sm/“‘Nd ratio of uncontaminated komatiite is 0.25 which reflects the depleted nature of its mantle source. The Th/U ratio of about 3.4 is probably also representative of depkted mantle. The initial cNd of +2.44 f 0.5 I indicates that the mantle depktion took place long before magma formation.

INTRODUCTION

ANALYSIS OF Pb isotopic compositions can potentially provide valuable information about the age and the origin of m&c rocks. The utility of the method has already been demonstrated through the analysis of modem volcanics, but to date, with some exceptions (e.g. SINHA, 1972; VIDAL et al., 1980; MANHSS et al., 1980; TILTON, 1983; BREVART et al., 1984) little work has been done with ancient volcanics. Especially suit- able samples for this type of analysis are komatiites, which are produced by high degrees of melting of the mantle and appear to be largely restricted to the Ar- chean. Their analysis should provide information about the nature of the Archean mantle and, when combined with results from modem rocks, should throw light on the evolution of the mantle.

The Nd isotopic method has also been used to study Archean volcanics, but mainly as a geochronological tool. The problem with this approach is that, in order to construct an isochron, samples as diverse as basalt and granites, or volcanic rocks from several widely separated greenstone belts (e.g. MCCULLOCH and COMPSTON, 198 1; HAMILTON et al., 1977), have been analyzed together. This approach is valid only if the samples are cogenetic, a hazardous assumption in the case of some analyzed suites. The initial isotopic com- position of the rocks, as well as the age obtained from such isochrons, is questionable. If samples from single flows are analyzed, this problem is avoided. When the age of the flows is known from an independent method, initial ratios can be determined directly and unam- biguously.

In this study, we analyzed the F% and Nd isotopes in two komatiitic flows from the Alexo area, Ontario. The petrology and the type of alteration BIT well known from several previous studies (BARNES et al., 1983;

* Present acldrw I.P.G. Univer& Paris VI et VII, 4 place Jussieu, 75230 Paris-Cedex 05, France.

ARNDT, 1984) and the age of the flows is closely con- strained by U-Pb analysis of zircons from felsic volcanic rocks which underlie and overlie the komatiitic se- quence.

GEOLOGICAL SEiTINC

The Alexo area is in the Archcan Abitibi greenstone belt, Ontario, Canada. The regional geology has been described by NALDRETT and MASON (1968) and GOODWIN (1979). At Alexo, the volcanic series consist largely of komatiites and basalts and lesser amounts of more felsic vokanics intruded by a large &ram&c-m&c sill. The age is known from U- Pb zircon analysis: rhyolites from south of Timmins and fiom the Munm-Lake Abitibi area, which are cons&red to underlk the komatiite tlows, have been dated at 2725 It 2 Ma and 27 10 f 2 Ma respe&vely (NUNES and JENSEN, 1980; NUNES and PYKE. 1980). A rhyolite from a higher formation in the Timmins area, which ii interpreted to overlie the komatiites, yields an aae of 2703 f 2 Ma (NUN~ZS and PYKE. 19801

The Sam&s analyzed for thi‘s study come Fro& two iows (Fig. I). The first is a 16 m thick ofivine spin&x-textured komatiife flow which contains the following units (from top to bottom): a flow top bra~$ various otivine spin&x texti layers, and a basal divine cumulate. This unit erupted as a 30% MgO lava and differentiated through fractionation only of olivine to produce the present layered structure (BARNES ef (II., 1983; ARNDT, 1984). The Bow was sampled from the upper breccia through the spinifex layer to the basal cumulate along two sections about 20 m apart (Fig. 1). The second flow, a less magnesian clinopyroxene spit&x unit, directly undediea the tirst. From this Bow. samples of the clinommxene spinifex layer and of an olivine .&m&e layer & &ted (Fii 1.).

Regional metamorphic gmde in the area is very low, of prehnite-pumpellyite facies, and the two flows are vety fresh by Archcan standards. Most samples are only partially hy- drate& olivine and glass are incompletely replaced by chlorite or serpentine, but the pymxene remains fresh. When extrrme, as in the flow top breccia (sampk M667), the alteration in- volves some changes in Mg and Ca contents, but in all other samples it is isochemical except for the addition of water (ARNDT, 1984). In restricted par&? of the flows, other types of alteration have been rwogni&. The ti of these. which is characterized by patches of pentiandite in amygduks, has been recognized in only two samples (M662 and M663). Minor amounts of chakophyk elements and sulphur have been added to these rocks. The second type of alteration a5zcts the flow top breccia (M667) and consists of a large addition of sulphur

1965

1966 B. Duprc, C. Chauvel and N. T. Amdt

Sectlor and

Simplified Mop of the

Alexo Flows

1667) Sample locotior

X.. fault

Olivine spinifex flow

Olivine spinifex

Olivine cumulate

Aphanitic

Pyroxene UJ\ Pyroxene

spinifex

~ ~

IOVO

splnlfex

flow Oiivine cumulate

_ -l__--i/ I

1 ? , 4 , ,8m

FIG. 1. Simplified map and section of the Alcxo flows, Ontario.

without any change in chakophyle element concentrations. The sttlphur occurs as very fine pyrite which is disseminated throughout fragments in the bmccia. The third type of al- teration is Ca-e or mdingitisation. In samples M656. M657 and M658 from the center of the ohvine spinifex Bow and sampks AX 13 and AX14 from the clinopyroxene spinifcx Sow, secondary pyroxene and in some cases garnet replace the earlier formed hydrous alteration products. The chcmicaIc&ctsofthisahcrationamanincmascinCaand a loss of Si and aII mobile L.I.L. elements (ARNDT, 1984). ThcanaIysisofsampIcsshowingdi&entdcgmesofaheration, Iiom ahnoat ftwh to compkxly altered, allowal an evahtation of the e&ct of aheration on the Pb and Nd isotopic systems.

ANALYTICAL PROCEDURES

For Pb analysis, samples were crushed into 0.1 g fmgments. Pieces without any wcathcrcd surfaces were se&ted and ck.aned with distilled water. They were then leached twice with hot HCI for an hour. After drying, about 1 g of the rock was weighed and spiked with x%. Further chemical pro- adurcs fohow the method described by MANH&S et af. ( 1978). Aaaiysca were pdmcd using a muhicolkctor Finnigan MAT 261 mass spectrometer at M.P.I. in Mainx (TODT et al., 1983). The raw data were corrected for mass discrimi- nation. The discrimination factor was determined by mca- sumtncnts on the NBS stadad 981; it amounts to 1.0014 f 0.00025 pa mass unit. Blanks anaIyscd during the COUI-W of this study were aIways below 500 pg. Analytical techniques for Nd isotope system have been de&bed elsewhere (WHITE and PATCHETT, 1984; CHAUVEL el a!.. in prep.).

RBBULTS

(a) Lead Results am reported in Table 1 and Fii 2 and 3. With the exception of samples M667 and M663 (discussed below), all the rocks from the olivine spin&x flow define a very040d~Pbiaochronwithanageof2690+ 1SMa.A laqss spmad is observed in the isotopic valuts, i.e. IobF%Ppb range from 15.7 up to 30 (Fig. 2). Such a spread in %Pb/ mPb leads to variations of the p2 value (23*U/x”Pb) from 4 to 32 (Table I). In the -Pb/-Pb vs. 206Pb/mPb plot (Fig. 3), the data also define a good regression line. It is possible,

using these data, to cakuiatc the Th/U ratio for the whole series by using the following equation:

232-I-h (e v=S(e

A23fw _ 1)

238U h23*r _ 1) (1)

where T (age of the series) is obtained from the 20’Pb/‘MPb vs. 2wpb/“Pb diagram, S is the slope of the mgre&on line in the ‘Pb/sc’Pb vs. aMpblmPb diagram, X236 = 0.155 125 x lo-’ y-l and x232 = 0.049475 X IO-’ y-l, The calcuiatcd ThJIJ ratio is 3.4 It 0.02.

The Th/U ratio of each sample can also be calculated if the initial Pb isotopic composition of the series is known. Magmatic sulphidcs Born the Alexo flow, which have been analyzed by DUPR~ and ARNDT (in pmp.), can be taken to represent the initial ratio of the series (‘wPb/sc?b = 13.6 17, 207Pb/204pb = 14.566, “Pb/s”‘Pb = 33.282) and the Th/U of each sample can be calculated using the following equation:

where R and S refer to vahms for the rock and the sulphide mspectivcly. Tile cakt&edratiosamgiveninTabk I.The values vary from 2.99 to 4. I 1. This spread is mainly created bythedatacaku&taIforthckastradiogenicsampks,since anysmaUvari&oninxaI%$a%ratioimpIksalalgCwWiauon of the slope and the&ore of the Th/U ratio. However, the mean vaIue of all the Th/U ratios is compuabk to the ratio calculated using the mgmssion line (3.4 f 0.02).

The komatiitic basahs from the cfinoppxene spimjxflow lie slightly out&k ofthe Pb-Pb isochron d&nai by the rocks horn the other IIow (lower right inset in F 2) whereas they am inclisti~ in the %/x% vs. s&b Ir@tPbdirrgram.

(b) Ne&wGunt.TheNddataalaodehneanisochronwith an age of2752 f 87 Ma (Tabk 2 and Pig. 4). Ratios of ‘%m/ “‘Nd vary from 0.22 to 0.26 in between the two Bows and from 0.24 to 0.26 within the divine spinifucflow. The large error on the calculated age stems from the narrow spread of Sm/Nd ratios. An isochron drawn only through data points from the olivine spin&f&/low gives the same age within error.

Komatiitic flows from Alexo 1967

Table 1: Pb results for Alex0

*06Pb 207Pb 20*Pb 238o 23ZTb

Sample %+ mG+ w+ [ 1 204Pb # FbS % AlteretionD Depth=

U626 26.375 16.846 45.367 U626C 22.220 16.113 40.950 M651 21.439 15.949 40.512

U652 20.051 15.710 39.371 n653 20.280 15.726 39.495 U654 15.970 14.976 35.920 U654* 16.981 15.115 36.575 U656 22.563 16.213 41.225 H657 29.995 17.565 48.690 M658 20.122 15.700 39.390 M661 18.834 15.530 38.287 M663 16.329 15.132 35.569 U664 19.038 15.493 38.179 I4665 15.718 14.920 35.504 H666 16.770 15.110 35.848 M667 14.219 14.717 33.786 K668 16.351 15.047 36.060 Ax12 28.959 17.350 48.270 Ax12* 29.871 17.489 48.482 Ax13 30.771 17.561 50.453 Ax14 32.477 17.730 49.996 AX25 15.869 14.963 35.465

0.79 24.64 0.60 16.61 0.81 15.11 0.85 12.42

12.87 4.54

1.71 6.50 0.82 17.28 1.03 31.63 0.63 12.56 0.9R 10.07 1.44 1.71 10.47

4.06 1.27 6.09 5.16 1.66 5.28

2.22

3.45 3.25 3.37 3.45 3.41 4.11 3.59 3.24 3.43 3.43 3.51

3.30 3.88 2.99

3.73

Ryd . 350 Ca-•. 700 Hyd . 140 Hyd. 210 Hyd. 350 Hyd. 440 Hyd . 440 ca-!a. 600 Cc-m. 720 Hyd. 830 Hyd . 1200 NiS 130 Hyd. 210 Ca-IX. 555 Hyd. 0 PeS -100 Hyd. 600

*: analyses of separate chips from the same sample.

+: errore on the q eesured ratios ere: ='f',b/2%b end 207Pb/204Pb - 0.05%; 208Pb/204Pb - 0.102.

t: concentrations in ppb. Thcee numbers are not very reliable because of the very strong leaching of the chips during sample preparation (see DuprC and Echeverrie, 1984).

s: These erc calculated vel”cs.

u2?&-[ ER- zj,[eA238T-1]

where R and S refer to values for the rock and the sulfide, as in equation (2) (Bee text).

6: Hyd: hydration; Ca-m: Ce-Wetaeometism; NIS: addition of Ni-sulfide; FeS: addition of iron-eulfide.

x: distance in cm from the chill at the base of breccia layer. Negative number refers te location above the chill. Sample M662 is et e depth of 25 cm.

The initial tNd VdUe cakukd from the isochron using FLETCHER and ROSMAN’S method (1982) is +2.4 + 0.5, rc- tkcting a time-integrated LREEdepleted source region in the mantle. This feature is consistent with the measured “‘Sm/ “‘Nd ratios which are typical of LREEdepleted patterns.

22 26

~%‘bflo~Pb

FIG. 2. 207Pb/2WPb vs. ‘06Pb/aPb diagram. Most of the data points from the olivine spinifex flow define an isochron with an ap of 2690 f 15 Ma. Upper lefk inset shows the position of the two samples (A4663 and M667) from the same Bow whose isotopic composition has been disturbed by sed- imentary sulfides. The lower right inset shows how samples from the clinopyroxene flow deviate from the isochron.

DISCW33lON

The age given by the Pb isotopic data, 2690 + 15 Ma, is in excellent agreement with the zircon ages from underlying and overlying formations, 2725 ?2Ma(or2710+2Ma)and2703f2Ma,respec- tively. This result demonstrates the suitability of this method for dating volcanic rocks of this age. The Nd

10 U 18 22 26 30

FIG. 3. 2ocPb/2acPb vs. 206Pb/WPb for the sampks from the olivine spinifex flow. The regession line comsponds to a ‘?h/?J ratio of 3.4 + 0.02. Same symbols as in Fig. 2.

1968 8. Duprk, C. Chauvel and N. T. Amdt

Table 2: Srn-Nd data for Alex0

14’5, 14’Nd Ti Al203

Sample Nd(ppm) Sm(ppm) =z# %-d+ Nd\ T,(Gah z, 7.r

H654 0.978 0.391 0.2427 0.513586*23 +2.47 3.11 I37 0.439

H656 1.972 0.783 0.2402 0.513548+19 +2.tifl 3.16 138 0.444

n657 2 .?28 1.074 0.23Rl 0.513511*17 +2.f,, 3.18 123 n.,86

U662 i .34O 0.569 0.2567 n.513862f25 +2.99 3.OR 161 0.463

“662” 1.336 0.567 O.2568 n.513867*30 c3.05 3.09 141 11.463

M663 1.378 “.5R5 0.2567 0.513853*22 +2.f32 3.06 1<3 0.510

H663* i ,376 0.585 0.2569 0.513866+43 +3 .I30 3.08 153 0.510

PI666 1.477 0.626 0.2561 0.513842t32 l 2.7‘ 3.05 139 0.463

“668 2.135 0.84” 0.2380 0.513522f3h +2 .I36 3.23 122 n.398

Ax14 3.553 1.322 0.2250 0.51328M21 c2.65 3.42

Ax25 1.904 0.689 0.2189 ~.513174f21 t2.71 3.63

8: error is 0.2%. The concentration measurements were made of liquid aliquats.

+: ratios normalized to 146Nd/144Nd-0.7219. Reported errors are 2am. Measured

value for La Jolla is 143Nd/144Nd.0.511847~21 (20~0~).

C: cNd values calculared using the U-Pb zircon age.

x: model age CT.) calculated using (‘47sm/144Nd)o”R-0.1966, (143Nd/‘44Nd)cHuR-

0.512638 and h’47.0.00654 h-l.

age of 2752 f 87 Ma also agrees, within error, with the zircon data.

Nd system

The fact that a Nd isochron was obtained at all is surprising. The Sm-Nd isochron is partly defined by differences in Sm/Nd between the two flows. The cli- nopyroxene spinifexfrow is less ma!ic than the olivine spinifex flow. It probably formed by a lower degree of partial melting and could have left some pyroxene in the residue, a situation that would have produced a lower Sm/Nd ratio in the melt. On the other hand, a spread in Sm/Nd is also observed within the olivine spin$zxflow itself. It would be normally accepted that the composition of the lava within the flow would not have changed during eruption and that the variation in composition now seen in this layered unit, resulted form post-eruptive fractional crystallization. The only mineral to have crystaIIii in this flow was olivine (ARNDT, 1984). This being the case, the Sm/Nd ratios should be identical for all the samples from this flow. In fact, measured “‘Sm/“‘Nd ratios vary from 0.238

am- 021 022 023 OZL 0.25

‘L7S,/1L4Nd

FIG. 4. Nd isochron diagram. All analyzed samples from Alex0 fall on the isochron with an age T = 2752 + 87 Ma (2~) (MSWD = 0.391). The initial eNd value is 2.4 + 0.5. Quares refer to samples from the olivine spinifex flow and circles to those from the pyroxene spinifex flow.

to 0.257, a 7% variation which far exceeds the analytical error (0.2%). It might be thought that the alteration could have created this spread of Sm/Nd ratios. How- ever, the severe alteration that has a&cted some sam- ples (e.g. M667) appears to have produced no change in Sm/Nd ratio as discussed further below, and the three samples with high Sm/Nd (M662, M663, M666) show no evidence of any particular type of alteration that would distinguish them from other samples in the flow.

On the other hand, Sm/Nd ratios correlate with ratios of demonstrably immobile elements such as Ti/ Zr and Al,OJZr (ARNDT, 1984, and Fig. S), suggesting that the control is a magmatic one. The implication is that the composition within the oiivine spint@xjlow was not uniform. Evidence to support this suggestion can be found in the spatial distribution of the samples through the flow, in that all three samples with high Sm/Nd ratio are located in one small area at the flow top (Fig. 1). Being at the top, these samples were the first to crystallize and therefore represent the com-

FIG. 5. Ti/Zr and A&OJZr vs. ‘47Sm/1uNd diagram for samples from the olivine spinifex flow. A rough comlation can be observed in both plots.

Komatiitic flows from Alex0 1969

position of the initially erupted iava. Samples deeper in the 5ow represent the composition of lava that flowed through the unit at a later stage in the eruption. It is possible that the composition of the lava changed with time: the question is what could have caused this change? Two alternatives exist: the ikst is that there were! changes at the site of melting, such as variation in the proportion of pyroxene (or garnet) left in the mantle residue. It is difficult to imagine, however, that magmas with different compositions could ascend sep- arately through the mantle and then erupt through the same fissure without being mixed and homogenized.

The second alternative is that the lava in the lower part of the 5ow was contaminated with a material with lower Sm/Nd. If the amount of contamination had increased during the course of eruption, then the earlier lava (that is, the lava preserved in the 5ow top) would be less contaminated than the later lava. Thermal era- sion by the komatiite lava is a process by which this contamination could have occurred. According to HUPPERT et al. (1984), hot turbulent komatiite lava can melt and digest the rocks over which it 50~s. At Alexo, the clinopyroxene spinifex jlow underlies the olivine spinifexjlow. The clinopyroxene spinifex lava has lower Sm/Nd than the komatiite, and if this were mixed with a lava with a composition like that of M666, hybrid lavas with compositions like M668 would be produced.

The differences in 14’Sm/tuNd between samples from the two flows result either from differences in melting conditions, or differences in source compo- sitions. In the latter case, the samples would not nec- essarily have the same initial Nd isotopic ratio. This possibility can be tested if we use the zircon data to constrain the age of the rocks, and then calculate the ENd Value for each sample at 2.7 Ga. The average of all the values (Table 2) is +2.8 f 0.2 and there is no significant correlation between cW (2.7 Ga) and “‘Sm/ lWNd. This proves that the difference of initial ratios between the two 5ows was extremely small, if it ever existed. This conclusion is corroborated by the agree- ment between the zircon age (or the Pb-Pb age) and the age given by the Nd isochron. In the same way, the initial ratio deduced from the Nd isochron is similar to the average of the initial ratios of the rocks calculated using the zircon age.

Pb system

Samples from the olivine spinifex flow (except M663 and M667 which will be discussed below) define an isochron (2690 f 15 Ma) but the reason for the spread in U/Pb ratios is not obvious. As in the Sm/Nd case, fractional crystallization cannot be the explanation because olivine does not change the U/Pb ratio. Nor can the phenomenon that creates the spread in Sm/ Nd be responsible. The samples from the top of the 50~ with high Sm/Nd ratios (M662, and M663 and M666) do not have appreciably lower U/Pb than sam- ples from the rest of the flow. Furthermore, although

Sm/Nd ratios are constant within each part of the flow, UJPb ratios vary from 4 to 20 in the 5ow top samples and from 6 to 32 in the rest of the flow.

Further consideration of the nature of the variation is necessary. If ThJU ratios are plotted against U/W ratios (Fig. 6), it can be seen that the ThJU remains constant while the UJPb varies from 4 to 32. The variations in UJPb are due, therefore, to changes in the Pb contents of the samples. A possible cause could be variations in the amount of magmatic sulphide, but this explanation is ruled out by the lack of con- comitant changes in the concentrations of chalcophyle elements: those samples with lowest 206pbfKwPb ratios do not have anomalously hi concentrations of Cu, Zn or Pt (BROG~~ANN et al., in prep.).

The remaining explanation is that the spread in UJPb ratios is the result of a hydropic alteration comparable to that which occurs at present day mid- ocean ridges. It has been shown that at the ridge, very intense sea-water circulation occurs and leads, in some cases, to a removal of Pb, Zn and Cu from the oceanic crust (CYAMEX !WENRFK TEAM, 1979; VIDAL and CLAUER, 1981; BREVART~~ af., 1981). These elements are then redeposited from black smokers and form sulphide deposits. This process could also have oc- curred in Archean, as is suggested by the contempo- raneous deposition of exhalative sulphides and erup tion of komatiite in the Dundonald area near Alexo (MUIR and COMBA, 1979). This mechanism could lead to an increase of the h value in the altered part of the crust, and be responsible for the spread in U/Pb. On the other hand, TATSUMOTO (1978) and MICWARD et al. (1983) show that hydrothermal alteration can add up to several percent of IJZ (originally in the sea-water) to submarine lavas. Addition of uranium to the Alexo komatiites cannot explain the 400% change in UJPb ratios but might be the cause of the slight scatter of ThJU ratios (Fig. 6).

If the release of the Pb from the lavas by sea-water is the phenomenon which created the variations in U/Pb, then the age obtained by the PbPb method integrates the alteration effects. The time difference between the age of crystallization and the Pb-Pb age, then, corresponds to the mean time of hydrothermal alteration. In the present case, the Nd age is not precise enough to calculate this amount of time, but it can be calculated using the zircon ages of NUNES and PYKE ( 1980) and NUNES and JENSEN ( 1980). The

*u 0 10 20 30

23su,/20+,,

FIG. 6. “‘qh/‘Mv VS. 2?J/mPh diagram, The ThJU ratio appears to be constant when U/l% varies from 4 to 32.

1970 B. Dupti, C. Chauvel and N. T. Amdt

mean time of hydrothermal activity is then 0 to 35 Ma.

Although the Pb isotopes define a good isochron (2690 + 15 Ma) a number of points do not fall on the regression line. In most cases, their position is easily explained by secondary processes. The two sam- ples from the oiivine spinl#kxfrow that plot above the isochron (M663 and M667, upper left inset in Fig. 2) have been shown by ARNDT ( 1984) to have significantly higher sulphur contents than all other samples in the flow. Both samples come from the uppermost part of the flow and presumably gained sulphur during in- teraction with sea-water. They also have high contents of chalcophyle elements, particularly of Pb. The likely source of the added components is sedimentary sul- phide, which contain lead with higher *07Pb/2a4Pb than the komatiite (DUPRB and ARNDT, in prep.).

Although most of the data points of the clinopy- roxene spinifexjlow fall, within error, on the isochron, one sample (AX14) plots well below the line (lower right inset in Fig. 2) and a regression line through the data points of this Sow define a younger age of 2200 Ma. The significance of this age is not understood. These samples appear to have been affected by a later alteration or metamorphic event (as seems to have occurred in Munro Township, 50 km east; BREVART et al., 1984), but is is not clear why samples from the clinopyroxene spinifexfrow should have been affected and not those from the olivine spin$exJow.

Eflects of alteration

The approach taken in this study allows the effects of alteration on the Sm-Nd and U-Th-Pb systems to be evaluated. Unambiguous conclusions are obtained for the following reasons: (a) the majority of samples come from a single flow in which the only fractionating phase was olivine; (b) within this flow, the alteration varies considerably both in type and severity. By com- paring samples with similar bulk composition but al- tered in different ways, the effects of alteration on Sm- Nd and U-Th-Pb systems can easily be recognized.

Considerable debate persists about the immobility of REE during alteration and metamorphism. Various authors have sugeestcd on the basis of studies of altered modem submarine volcanics and Archean greenstones, that LREE are mobile (LUDDEN and THOMPSON, 1979). The present study shows that, under conditions of the Alexo flow, neither the Sm/Nd ratio nor the Nd isotopic composition is affected. For example, Ca- metasomatism which results in very large changes in some elemental ratios (e.g. CaO/AlrOr from 0.88 in the relatively unaltered sample M654 to 1.36 in the metasomatized sample M657; Rb/Zr from 0.17 to 0.04; Na20/A1203 from 0.006 to 0.003) has had no effect on Sm/Nd ratios. In the same way, neither hydration nor sulfide addition had any noticeable effect (e.g. M663 has the same Sm/Nd ratio as M666).

In the case of the lead system, Th/U ratios arc rel- atively constant and appear to have been little affected

by alteration. In contrast, the variation in U/Pb ratios has been controlled by alteration, and sulfide addition completely disturbed the zo7Pb/za(Pb of samples M663 and M667.

It must be stressed, however, that the Alexo flows have been subjected only to prehnite-pumpellyite facies metamorphism and are quite fresh by Archean stan- dards. The conclusions reached here do not necessarily apply to the more highly metamorphosed and car- bonated volcanics commonly found in other Archean UCZIS.

Implications for the Archean mantle

The geochemical and isotopic compositions of the Alexo komatiites (‘47Sm/‘UNd - 0.24; c&T) = +2.4) indicate that they came from a depleted mantle source. The Th/U ratio of 3.4 f 0.02 is much lower than the 4.2 value assumed for the Bulk Earth (DUPR~ et al.. 1984) and can be considered to be characteristic of a depleted mantle.

When Th/U and Sm/Nd ratios for komatiites from four different areas are plotted, a negative correlation with a slope of about - 13 is obtained (Fig. 7). The mantle process that created the depletion recorded by the increase of Sm/Nd ratio apparently also affected Th/U by preferentially depleting the more incompat- ible Th. In addition, the degree of depletion of both Sm/Nd and Th/U seems to increase with decreasing age (Fig. 7).

The composition of MORB, when compared with that of the komatiites (Fig. 7), provides information about the source of modem oceanic basal& MORB have the same Nd and Pb isotopic compositions as the Gorgona komatiites (DUPRE and ECHEVERRIA, 1984), but plot on the low Sm/Nd side of the corre- lation trend. Their lower Sm/Nd probably resulted from a process such as pyroxene fractionation during the melting event, and their source would have had a higher Sm/Nd ratio, comparable to that of the ko- matiites.

L5

25

FIG. 7. Th/U vs. Sm/Nd plot. Komatiites with different ages &fine a rough negative trend. The dope (- f 3) sum that U/Th and Sm/Nd ratios are Monated topether during the depletion process. Data are from this work BRE~AW ef al. (1984), TILTON (1983), DUPRB el al. (1984). AirKEN and ECHEVERRIA (1984), COND~MINES et al. (198 1) and JocHUM er al. (1983).

Komatiitic flows from Alex0 1971

The tNd value of the Alex0 komatiites is +2.4 * 0.5 indicating that their mantle source was depleted long before eruption. If the Sm/Nd ratio of the komatiite is taken to represent that of the source, the mean time of the. depletion can be cstimattxi fram the model age of the komatiites. The calculated value using all sam- ples is 3.2 Ga. However, as explained above, samples from the lower part of the olivine spinifcx flow and from the clinop~oxene spinifex flow show evidence of Sm/Nd ftionation, and their composition prob ably is not representative of their mantle source. Sam- ples Erom the upper part of the olivine spinifex flow (M662, M663 and M666) do not show this effect. The model age calculated for these samples is 3.07 + 0.02 Ga. The source of these komatiites, therefore, would have separated from a chondritic mantle 400 m.y. before the time of eruption.

The depleted nature of mantle sources is usually explained by a previous extraction of highly incom- patible elements from a chondritic reservoir. As several studies (O’NIONS ef al., 1979; DEPAOLO, 1980; PATCHETT t-f a[., 198 1; AL~RE et al., 1983abf have already suggested, depletion of the mantle results from the formation of the continental crust. The model age calculated for the mantle source of the Alex0 komatiites suggests, then, that Archean crust was differentiated prior to the formation of the Abitibi lavas and has a mean age of 3.1 Ga.

Although the two flows sampled at Alexo have sim- ilar ENd values, this is not the case for komatiites in other parts of the Abitibi belt. in Newton, eNd values range from + 1.5 up to +4 (CA’ITELL et ai., 1984). In Munro, if an age of 2.7 Ga is assumed, calculated CNd values vary from Sf.0 to +2.5 (ZINDLER, 1982). Al- though all samples appear to have come from depleted material, the source from which they were derived was clearly not uniform, but contained significant heter- ogeneities.

CONCLUSIONS

Analysis of Pb and Nd isotopes in two komatiitic flows at Alex0 has yielded valuable isolation about (a) the magmatic processes that affected the lavas dur- ing their eruption, (b) effects of subsequent alteration and (c) the nature of their mantle source. In order to interpret these data, a sound knowledge of the geology of the lava flows was necess8fy, including precise sam- ple locations and an understanding of the various types of alteration affecting the samples. The following con- clusions were reacha

(1) At Alexo, the Sm/Nd ratio was unaffected by alteration: these elements can be considered immobile under conditions of low-grade metamorphism and hy- drothermal alteration.

(2) Isochrons were obtained for both Nd and Pb systems. The Pb age of 2690 f 15 Ma and the Nd age of2752 + 87 Ma are in good agmement with zircon dating of the volcanic succession. In each case, how- ever, the present-day spread in isotopic ratios that de-

fines the isochrons results from pmcesses not directly related to the crystallization of the flows. in the case of Pb, the spread of U/W was probably caused by hydrothermal alteration immediately after eruption; in the Nd system, the spread was created by diffmnces in the conditions of melting, and by contamination of the oiivine spinifx jiow.

(3) The mantle source of the Alexo flows had a Th/ U of 3.4 t 0.02 and an +d of 2.4 f 0.5. The Nd model age caicula@d from the more depleted samples is 3.1 Ga. The mantle somce, therefore, was LREE-depleted and probably differentiated from a chondxitic n!servoir 400 Ma before eruption of the komatiites. The Th/U c&dated for the source (3.4 f 0.02) is then char- acteristic of depleted mantle at 2.7 Ga.

(4) Comparisons with isotopic data from other Abitibi komatiites show that their source was highly heterogeneous at 2.7 Ga.

Acknowledgments-We are very grateful to Bill White, Al Hofmann and Jon Patchett, who made useful comments of the early draft of this manumipt, and to C$. R. Tilton and R. Carison who reviewed this oaw. We would aiso like to thank Kayc Patchm for her ef%ent typiw This work was supported by the Deutsche Forschungsgemeinschafk, Project no Kr so/l 1-I.

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