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JOURNAL OF GEODYNAMICS5, 49-78 (1986)

49

CONSTRAINTS O N PROCESSES AFFECTING THE ORIGIN OF OCEANIC (:RUST: GEOCHEMICAL EVIDENCE FROM THE 0-35 M. Y. AGE BASALTS, BETWEEN 30°N A N D 40°N, MAR.

G A JENNER *l and J HERTOGEN 2

i Max-Planck Instttut .fur Chemw, Mamz, F R G 2 Umversttett Leuven, Celesttjnenlaan 200C, B3030 Heverlee, Belgtum (Recezved December 10, 1984, accepted June 17, 1985)

ABSTRACT Jenner, G A and Hertogen, J, 1986 Constraints on processes affecting the ongm of oceamc crust geochemical evidence from the 0-35 m y age basalts, between 30°N and 40°N, MAR Journal of Geodynamws, 5 49-78 DSDP Leg 82 sampled the oceamc crust, at 9 sites, west of the MAR between 33°N and 38°N, i e, from the Azores "mantle plume" to south of the Hayes Fracture Zone A specific aim of this DSDP Leg was to determine if the origin, and spatial and temporal evolution of oceamc mantle heterogeneity m the vicinity was related m a predictable manner to the "mantle plume" Our approach m evaluating this queshon has been to select samples on the bas~s of trace element cbaractertstlcs and age, for radiogenic ~sotope determinations Isotopic and trace element heterogeneity occur both within and between s~tes Whde there ts a crude correlation between trace element and ~sotoplc characteristics, ~t is difficult to establish a detailed set of guidelines which would describe either trace element-isotope or isotope-isotope interrelationships This difficulty is hlghhghted by the occurrence of basalts in one hole (H558) with trace element characteristics ranging from enriched to depleted, which can have identical or different isotopic characteristics The origin, and spatial and temporal &stnbutlon of the heterogeneity cannot be reconcded with any simple models A comphcated scenario, mvolwng mantle plums or blobs, recent local heterogeneity and complex mixing models, Is reqmred

INTRODUCTION

The existence of chemical and isotopic heterogeneity in the oceamc mantle has been well estabhshed (Wood, 1979, Sun et al, 1979, Sun, 1980, White, 1985) However, the origin and dlstrlbuUon of the heterogeneity *Present adress Dept of Earth Sciences, Memorial Umverslty of Newfoundland, St John's, Nfld, Canada A1B 3X5 0264-3707/86/$3 00

© 1986 Geophysical Press Ltd

50

JENNER AND HERTOGEN

remains a subject of some debate (Davies, 1981, 1984, White, 1985, Hawkesworth et al, 1984, Wood, 1979, Allegre et al, 1984) A particular point of contention is whether the heterogeneity is related to a more or less random distribution of distract sources-the veined mantle and/or plum pudding models (Wood, 1979, Hawkesworth et al, 1984, Davies, 1984)-or ff the large part of the heterogeneity is related to the presence of mantle plumes (Schilling et al, 1983, White, 1985) The origin of the petrologic and geochemical variations observed m midocean ridge basalts (MORB), between 29°N and 73"N, has been discussed by Schllhng (1975), Schilling et al (1983), White and Schilling (1978) and White et al (1976) The model these authors propose involves mixing between a LILE-depleted mantle source and mantle plumes located under the Jan Mayen, Iceland and Azores volcanic platforms Some major features of their model can be summarized as follows 1) the LILE-depleted source (where LILE refers to large ion hthophde elements) is the source of "normal" MORB (N-MORB), 2) the plume sources are characterized by higher abundances of K, Rb, Cs, Sr, Ba, LREE (light rare earth elements), volatlles and have higher (more radlogemc) Sr and Pb isotopic compositions (87Sr/S6Sr, 2°6pb/2°4Pb, 2°7pb/2°4pb, 2°8pb/z°4pb) and lower (less radaogemc) Nd lSOStOplC composltlon (143Nd/~44Nd), relatwe to the source for N-MORB, 3) the N-MORB and plume sources have evolved separately over periods of time greater than 109 years with significantly different parent/daughter ratios (Rb/Sr, U/Pb, Th/Pb, Sm/Nd), 4) the nature of mixing of the end-member sources depends on a number of factors, including spreading rate and plume flux (the rate of flux can vary considerably and also be episodic), and 5) MORB with chemistry transitional between N-MORB and plume basalts (LaN/SmN > 0 7 and < 1 8) reflect the chemical gradient and mixing between LILE-depleted and plume sources It is also important to note that Schilling et al (1983) summarize geophysical and morphological features along the MAR, many of which they relate to the plume hypothesis Much of the work done on Atlantic Ocean basalts has been restricted to present day (zero age) basalts However, two Legs of the DSDP have sampled older oceanic crust in the vicinity of the Azores plume, with an aim of trying to determine what the temporal and spatial effects of the plume have been DSDP Leg 37 (Aumento, Melson et al, 1977) studied the evolunon of the crust (3-14m y ) perpendicular to the MAR at 36~'N-37JN (the FAMOUS area) (see Fig 1) DSDP Leg 82 (Leg 82 Scientific Party, 1982, Bougault and Cande, 1985) sampled the oceanic crust at 9 sites between 30°N and 40 °, in crust ranging from 14 to 37 m y old (Fig 1 ) An important feature of Leg 82 was that shipboard chemical determination of key trace elements was used to select the areas of most interest

PROCESSES AFFECTING THE ORIGIN OF OCEANIC CRUST

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7, Zr/Nb > 12 and (La/Sm)N < 0 8, while E-MORB have H f / T a < 7 , Z r / N b < 1 2 and ( L a / S m ) N > I The enriched or depleted nature of the basalts is determined by normahzmg their incompatible element abundances to those reported in model prlmmve or undepleted mantle composmons (cf Sun, 1980, Wood, 1979, Bnqueu et al, 1984) The nomenclature adopted in this paper is slmdar to that used by Wood et al (1979), Sun et al (1979), Schllhng et al (1983) and many others Note that we do not include m our definmons rad~ogemc isotope composmons (cf Wood, 1979) We have made a further subdwlslon of the Leg 82 N-MORB (see Table 2) based on (La/Sm)N ratios ( < 0 5, 0 5-06, > 0 6) This dwlslon is arbltrary and may not be appropriate for N-MORB in other areas, however, it ts useful in dlustratmg features of the Leg 82 basalts Nd ~sotoplc compositions shown in Table 1 and Figures 2, 10 and 11 are mltml values, calculated usmg the criteria given in Table 1 (see also Hawkesworth and van Calsteren, 1983) To faclhtate discussion and minimize the effects caused by dlfferentml evolution of N-MORB and less depleted sources, epsllon notation (eNd~T~) is often used m this paper

PROCESSES AFFECTING THE ORIGIN OF OCEANIC CRUST

53

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PROCESSES AFFECTING THE ORIGIN OF OCEANIC CRUST

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Ce/Sm Fig 5 eNd(T) versus Ce/Sm for Leg 82 basalts Sohd hne dlustrates mix between sample 47 (taken to represent depleted- or normal-MORB) and an Azores basalt (Falal 33) (taken to represent the Azores Islands) Data sources for the Azores basalt are from White and Hofmann (1982) and White et al (1979) Ticks mdtcate percentage of N-MORB end member present (10% intervals) The concentrations of elements m Falal 33 used in the mixing calculation are half those reported m White et al (1979) This corrects (roughly) for the d]fference in degree of partial melting between tholentes (Azores Ridge) and alkah basalts (Azores Islands) (of Wh]te et al, 1979) Symbols same as Fig 3( + open circle = H557)

7) the HFS element ratios covary with the REE ratios in the manner one would predlct Note that Hf/Ta and La/Ta ratios exhibit somewhat larger variability then those involving the REE and T1, for example the sample from H558 in Group-C, Table 2 Not illustrated In Table 2 is that the different N-MORB groups within a Hole appear to have a stratlgraphlc relaUonsh]p, where the group with the most depleted ratios overhes the less depleted group For example, m Holes 556 and 562 (possibly 561) Group-A N-MORB overlie Group-B NMORB A possible explanaUon of this relationship could involve penod]c tapping of melts from an upwelhng mantle dlaplr Such a situation seems hkely to occur during eruption of MORB (cf Duncan and Green, 1980, Langmulr et al, 1977) JOG 5/1 5

64

JENNER AND HERTOGEN

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Ce/Sm Fig 6 ~Nd(r) versus (_e,'Sm for samples from Leg 82 H558, Leg 37 H332 and H335 and the FAMOUS area Sohd hne dlustrates mixing between a representahve N-MORB basalt and Azores Island basalt (Fatal 33) Concentratmns used for AZORES end-member are those discussed for Fig 5 Concentratmns m the N-MORB basalt are (m ppm) Ce-7 Sm-2 8, Nd-7 9 Data for Leg 37 basalts from O'Nmns et al (1977), Cohen and O'Nmns (1982), and Schflhng et al (1977) Data for the FAMOUS area are from White (1979) (corrected for 143Nd/144Nd m BCR = 0 51264) Symbols sohd squares-H558, FAMOUS-half-filled square, H332 and H335 (Leg 37 ~ half-filled triangle and half-filled circle, respectively Tick marks m 10% intervals

DISCUSSION

Tests jor bmary mtxmg

As outhned In the mtroduct]on, Schilling (1975), Schdlmg et al (1977, 1983) and White and Schilling (1978) have developed a series of hypotheses which relate variations in chemical and isotopic characteristics of oceanic basalts to mixing between plumes and the depleted source of MORB The slmphst situation to envisage would be one m which, for any given area or state, only bmary source mixing has occured In this situation one would predict a straight forward relat!onshlp between incompatible element ratios and isotopic c o m p o s m o n (Langmulr et al, 1977, 1978) Slmdarly, it should also be possible to test mixing relatlonshlps on ratlo-raUo plots mvolvmg exclusively Incompatible elements

65

PROCESSES AFFECTING THE ORIGIN OF OCEANIC CRUST 21

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Fig 7 2°6pb/2°4pb versus Ce/Sm Stippled field is a schematic illustration of the trend found m Atlant]c Ocean basalts (based on data from Dupre et al, 1981, Wh]te et al, 1979, White, unpubl data)

LEG 82

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F]g 8 Chondnte normahzed REE plot for samples selected to be representatxve of the variation m H558, Leg 82 Normahzang values used are those reported m Taylor and Gorton (1977)

66

JENNER AND HERTOGEN

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147Sm/144Nd Fig 9 S m - N d ]sochron diagram for samples from H558 Leg82 Imtlal 14~Nd,144Nd ratio versus I47Sm,~anNd ratios (calculated from Sm and Nd concentrations) have been plotted Age given m the figure ts relahve to T = 0 Ipresent)

To test the posslblhty that binary mixing 1s responsible for the incompatible element variation observed in the Leg82 basalt we used a La/Ce-Tb/Yb ratio plot We selected these elements and ratios for the following reasons 1) we had an extensive data base for these elements, 2) the elements involved are thought to be free from alteration effects (cf Hertogen et al, 1985), 3) for the degrees of partial melting (10-20%) mvolved m the formaUon of ohvme tholentes-tholentes (Jacques and Green, 1980), both rat]os should reflect source characteristics A calculation of bulk distribution coefficients for Ce and Yb in a mantle source with 65% ohvlne, 25% orthopyroxene and 10%chnopyroxene gwes D = 0 0 1 for Ce and D = 0 0 3 for Yb (partition coeffioents from Frey et al, 1978) Gwen the uncertainty m the source composition and proportion of phases entering the melt (see Jacques and Green, 1980), it seems a reasonable assumption that the ratios chosen reflect source characteristics, 4) the ratio pa~rs were sufficiently d]fferent so as to mmlm]ze the hkehhood of arufioally good mixing fits The results are shown in F]gure 3 and its compamon plot, Figure 4 (Langmmr et al, 1978) There IS considerable scatter about the calculated mixing hne Low pressure fractional crystallization should not effect these

PROCESSES AFFECTING THE ORIGIN OF OCEANIC CRUST

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AZORES

Fig 10 Schematic Illustration of the vanatlon m eNd(T), for Leg 82 basalts, through time and distance (from the present day location of the Azores mantle plume) Leg 82 sites are shown on the top of the dmgram for 35 m y old crust and on the bottom for 17 m y old crust HFZ, OFZ and PFZ refer to the Hayes, Oceanographer and Plco fracture zones, respectively (see Ftgure 1) An ldeahzed binary mixing field between N-MORB and AZORES mantle sources is outlined by the sohd hnes Questmn marks (9) on the lines for 0, 17 and 35 m y indicate these hnes are schematic, may not be related to the present Azores and possibly should not be extended through fracture zones See text for more detmled discussion

ratios, and, as expected, screening the samples for fract]onaUon (based on Mg number) did not improve the fit It is eas]er to evaluate the mixing hypotheszs using the c o m p a m o n plot of La/Ce versus Yb/Ce (Fig. 4) The data should form a straight hne. The sohd hne illustrates mixing between the end-members used in Figure 3 There is still considerable scatter about this hne The dotted hne lS a linear regress]on line calculated for the entire data set and has a correlation coefficient, r 2, of 0 87 The relatwely good fit of the whole population to a hnear fit could be interpreted as quahtatwe support for the binary mixing hypothesis (cf White and Sch~lhng, 1978) However, closer inspection of the data suggests that the scatter reflects mixmg between a number of different source combinations For example, the dashed hnes illustrate separate, possible mixing trends in Holes 561 and 558 In presenting the results (Table 2) we noted that there was an overall correlation between Nd isotopic c o m p o s m o n and trace element charactensUcs To more carefully evaluate th]s relationship we plotted eNd(T) versus Ce/Sm Choice of Ce/Sm as the ratio used was determined by our less

68

JENNER AND HERTOGEN

OM Y 143Nd/144Nd I

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Fig 11 Imtlal 14~Nd/t44Nd composlttons of Leg 82 basalts shown m comparison to possible mantle sources Present day values for the chondntlc umform reservotr (CHUR) were taken to be 143Nd/J44Nd=051264 and J47Sm/taaNd=01966 (Hawkesworth and van Calsteren 1983) Most depleted N-MORB from Machado et al (1982) and Azores-Falal from White and Hofmann (1982) Value for Mean N-MORB is taken from Jacobsen and Wasserburg (1979) The lower hmlt for the MORB-reservolr is based on White and Patchett (1984), Morns and Hart (1984) and section (m) of the &scussion (this paper) 147Sm/~Nd values used m calculating the reference hnes are 0 24 for most depleted and mean N-MORB, 0 2 for lower hmlt N-MORB, and 0 13 for Azores-Fatal (values selected are based on Jacobsen and Wasserburg, 1979, White et a l , 1979 and this study and are not necessarily those measured on a gwen sample)

extensive data base for 10 out of the 23 samples analysed for Nd isotopic composition and by analytical uncertamues Nonetheless, this raUo of incompatible elements should reflect source varlaUons reasonable well As illustrated m Figure 5, the data do not fit a binary mixing curve This is best dlustrated if the data from Holes 561 and 558 are considered (open squares and dosed squares). Again, it seems more hkely that at least 3 different sets of binary mixing combinations would be necessary to explain the data Rare earth and Nd isotopic data from the FAMOUS area, H332 and H335, Leg 37 and H558, Leg 82 are shown m Figure 6 These sample sites

PROCESSES AFFECTING THE ORIGIN OF OCEANIC CRUST

69

may represent tapping along a mantle flowhne between the Azores mantle plume and a depleted source (see Fig 1, Schilling et al, 1977, Bougault and Cande, 1985, Bougault et al., 1985) REE variations in H335, H332, and FAMOUS area basalts have been interpreted to reflect increasing contnbutlons from the Azores mantle plume, from 0% at 335 to 45% at FAMOUS (Schilhng et al., 1977) In contrast to this suggestion, we would suggest the following 1) based on Nd ISOtOpic composmon the plume component in H335, H332 and FAMOUS basalts can be considered to be constant at (for this specific model) 8%, 2) REE data are not always a rehable indicator of the amount of plume component, 3) the isotopic data suggest that H558 would have the highest plume component Therefore, if this is due to the Azores mantle plume the effect has decreased, then stayed constant, rather than increased through time We can also quahtatwely evaluate the binary mixing hypothesis using the Ce/Sm data (this study) and the Pb isotopic data reported in this study and Dupre et al (1985) (see Fig 7) These results demonstrate clearly that for a given isotopic composlUon that there are a wide range of trace element composmons possible and conversely that a wide range of isotopic composmons can be found for any gwen lncompaUble element ratio. Vartatton m Hole 558

The relationship between incompatible element ratios and isotopic composmon (particularly Pb) in Leg 82 basalt is a complex one Nowhere is this more clearly indicated than with the results from Hole 558 The variation in REE patterns in H558 is illustrated in Figure 8 The samples range (excluding 33-3, probably cumulate) from LREE-depleted with YbN concentrations of 13 (28-3) to LREE-enrlched with YbN concentrations between 9 and 12 Nd and Pb composmon is not necessarily related to this variation in any systematic manner. As pointed out by Dupre et al (1985) and illustrated in Figure7, samples with nearly identical trace element characteristics can have significantly different Pb isotopic compositions. Samples 27-3 and 28-3 (E-MORB and N-MORB, respectwely) have virtually identical Nd isotopic compositions, but show slight differences in Pb isotopic composition (Table 1) Sample 33-3 has a Nd isotopic composition identical to that of 27-3 and 28-3, but has an even more radiogenic Pb isotopic composlUon than 27-3 The most coherent group of samples is that of 31-2, 32-1 and 33-2 These samples have very similar trace element patterns and isotopic compositions The variation in incompatible element ratios (parUcularly Hf/Ta and La/Ce), crossing REE patterns, similarity m Mg-numbers, and inconsistent trace element-lsotop~c and ISOtOpic-isotopic variations preclude any simple

70

JENNER AND HERTOGEN

relationship between these samples, e g by binary mtxmg and/or dynamic melting (Langmmr et al, 1977) While recogmmng that the relationship between trace elements and ISOtOpic c o m p o s m o n is a complex one, we would have nonetheless expected that the H558 basalts would be enriched m incompatible elements This expectation is based primarily on the low Nd and radiogenic Pb composmon It is supported by the observations m most of the basalts from H558 and oceanic basalts in general Given th~s expectation, sample 28-3 (with N-MORB trace element characteristics) Is anomalous, perhaps the most anomalous sample m the whole study The large variation m Hf/Ta (3 5 vs 11 3), Ce/Sm (5 3 vs 3 4) (columns 13 and 15, Table 1) or Zr/Nb (8 4 vs 27 3, Sachtleben et al, 1986) ratios between 27-3 and 28-3, In samples with virtually identical isotopic compositions, is of considerable slgmficance As has been shown by Frey et al (1978), Pearce and Norry (1979) and Le Roex et al (1983), these ratios are good m&cators of source composmons since ~t ~s extremely ddficult to fract~onate the elements from each other during partial melting events For example, using the range of &stnbuUon coefficients for Zr and Nb (Pearce and Norry, 1979) (remembering that Z r = S m = H f and N b = T a = C e , Bougault, 1980), and assuming sources with different proportions of garnet, chnopyroxene and amphlbole, m ad&tlon to ohwne and orthopyroxene, the ratio of bulk distribution coefficients for these two elements (Dzr/D,~b) gives a range of 0 9-1 4 Muttlstage melting episodes over a short, recent period of time could perhaps produce the observed fractlonatlon (Bougault et al, 1985) An alternative hypothesis ~s that during a complex partml melting event of a volatile-bearing s o u r c e ( H 2 0 + C 0 2 ) larger than expected varmtlons m trace elements are produced by vapour (gas)- or flmd-sohd partmonlng (see Wylhe, 1984 for a &scusslon of hquld, vapour and fired definmons) There is httle data with which to evaluate these processes However, we know that CO2-nch magmas (klmberhtes, nephelemtes, alkah basalts) are anomalously enriched in incompatible elements (Wood, 1979, Muramatsu, 1983) and furthermore that it is &fficult, if not impossible, to account for the &fferences between ocean floor and ocean island basalts unless some process more effecuve at discriminating between incompatible elements then silicate melt-sohd partitioning occurs (Hanson, 1977, Frey et al, 1978) What we envisage is a sltuatmn in which near the boundary (low temperature regmn) of a melting zone a small degree of hqmd + vapour Is created The vapour effecavely concentrates the mcompaable elements and because ~t can exist at sub-sohdus temperatures, in contrast to the melt (hqmd) (cf Wylhe, 1984), could migrate from the Immediate area, elther as a result of cooling and/or preferentaal mobility Thus, we would see this phase as causing the fractmnatmn in the trace elements from an enriched source, leaving a mantle source with a depleted trace element pattern but

PROCESSES AFFECTING THE ORIGIN OF OCEANIC CRUST

71

with the low eNd and high Pb isotopic signatures. If this phase altered the source area from which other H558 basalts were derived it may offer an explanatmn for the large range of v a n a t m n in 2°Spb/2°4pb and 2°6pb/2°4pb, with only small v a n a t m n m 2°Tpb/2°4Pb, seen in the H558 basalts (Dupre et al, 1985). A S m - N d evolution plot for the H558 data is shown m Figure 9 It is difficult to interpret the sigmficance of the hnear array shown. If the array is interpreted as a mantle lsochron, then the regression line indicates an inltml ratio of 0.51271 and an age of the dlfferentmtlon event of 225 m y before present (or 190 m. y. before eruptmn). An alternative interpretation would be that it is a mixing array with no age slgmficance As discussed above simple mixing cannot relate the samples defining this array It should also be pointed out that if the analytical error is taken Into account the array is essentially defined by two dmtmct groups Nonetheless, there are two interesting features of this array 1) if there is any age significance, then this locality joins a number of others (LeRoex et al, 1983 and references therein) in the Atlantic in defining an "event" (break up of Pangea) at about 200 m. y , 2) eruption of recent basalts, with apparent age signatures, poses serious problems for geochronologlsts (cf Chauvel et al, 1985)

Spattal and temporal dlstrtbutton of the heterogenezty If the dlstnbutlon of heterogeneity over the last 35 m y m the region between 30°N and 40°N was related to a fixed p o s m o n of the Azores mantle plume, then one would expect to find a predictable variation in time and space of the heterogeneity An attempt to portray this is shown m Figure 10 Some comments are necessary before we discuss this figure 1) a range in Nd isotopic composition for the Azores is indicated. As previously noted in this discussion the variation in Leg 82 basalts cannot be described by binary mixing, furthermore results from studies by Hawkesworth et al (1979), Dupre et al (1982) and White (1985) mdmate the Azores are heterogeneous themselves This figure attempts to take this into account, 2) similarly, a range m Nd isotopic c o m p o s m o n is indicated for the N - M O R B source. The choice of the lower hmit for this field is somewhat subjective. White (1985) extends his present day MORB-group to include values down to eNd = 3 (see Fig 2a), however, histograms of the Nd isotopic composition in recent MORB (cf. Morris and Hart, 1983) show that the mean value for M O R B hes m the range of eNd = 9-10 An average eNd(a-) of 10 IS also indicated for the N - M O R B of this study (excluding H558) (Table 2). A limiting value of eNd = 10 would exclude almost all overlap between island arc and oceanic volcamcs Essentially what this amounts to is excluding all transitional

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MORB and defining an N-MORB-reservom which has httte or no overlap with fields for other oceanic basalts Support for this restriction of the MORB-reservolr to higher Nd values (and lower 87Sr/86Sr) is also provided by the degradation of the Pb-Pb and Pb-Sr trends in oceamc basalts in general (White, 1985), and apparen mixing trend shown by the Leg82 basalts (see Fig 2c) towards the St Helena-reservoir (cf White, 1985) It is clear from Figure 10 that the mantle source for basalts south of the Hayes Fracture Zone has been characterized by high eNd values over the period between 17 and 35 m y Extrapolation of the Sr lsotoplc results from Schllhng et al (1983) suggests this conclusion could be extended to include recently erupted basalts Pb isotopic results shown in Figures 2b and 7, and Dupre et al (1985), similarly indicate that the Pb isotopic c o m p o s m o n over the last 35 m y has remained relatively unradIogemc over the last 35 m y (2°6Pb/2°4pb< 18 5) These results are compatible with the trace element studies on the zero age ridge (Bougault and Treull, 1980) and imply that the H F Z has acted as a "boundary" over the last 35 m y (cf Langmulr and Bender, 1984) Basalts erupted over the last 35 m y in the area north of the H F Z and within the vlomty of the O F Z appear to have had access to a source area characterized by low eNd and radlogemc Pb and Sr isotopic compositions (see Fig 7 and Schllhng et al (1983)) For example the present day eNd values predicted from the negative relation between Nd and Sr (White, 1985) would range from 4-8 The o n g m of this isotopic variation appears to have been the New England hot spot and is not attributable to the Azores mantle plume (Duncan, 1983, Cande et al, 1983) White and Schflhng (1978) had previously suggested that one possible origin of the heterogeneity they observed m this area might be a separate mantle plume The origin of the heterogeneity north of the O F Z is somewhat problematical If the Azores mantle plume was responsible for the low eNd values m H558, then it is not clear why H556 (presumably closer) was not affected to a similar or larger extent This problem is further compounded by the fact that although the Nd isotopic and trace element characteristics found in H556 are N-MORB like, the same cannot be stud of the Pb ~sotopes (Table 2, Fig 7, Wood, 1979) It certainly seems unhkely that the same sources present at 35 m y are presently being tapped today along the MAR, m the area of this study This conclusion is supported by the observation that the Azores, while heterogeneous, are known to tap Whlte's (1985) Society- and MORB-reservolrs, whereas we noted earlier that the charactenstlcs exhlblted by H556 and H558 trend towards his St Helenareservoir Thus, it would appear that if the Azores are responsible for any of the variations observed these are restricted to those in crust less than 17m y old and north of the O F Z We have prewously noted that the

PROCESSES AFFECTING THE ORIGIN OF OCEANIC CRUST

73

o n g m of the chemical variation along the hypothetical 0-17 m y mixing hne, perpen&cular to the MAR at 36°N-37°N, is not clearly related to the Azores mantle plume

Summary and tmphcattons A precise method of illustrating the variation m Nd isotopic composition of Leg 82 basalts is shown in Figure 11 The crude correlation between Nd isotopic compos~tlon and trace element characteristics ~s well illustrated in this figure; with the N - M O R B basalts (see trace element defimtlons of rock types on the figure) plotting above the lower hmlt of the MORB field and E-MORB (or mixed source basalts) below The exception to this "rule" is also shown, 1 e trace element defined H558 N - M O R B are lndlstmgmshable from E-MORB. What is not illustrated on this figure, but shows up clearly on Figures 5 and 6, is that the most trace elemented depleted N - M O R B do not have the most depleted Nd ~sotop~c compositions That a complicated relationship exists between trace element characteristics and radlogemc isotopes is most clearly dlustrated by the plot of Ce/Sm versus Pb isotopic composluon (Fig 7, see also Fig 3, Dupre et al, 1985). The lack of a regional trace element/isotope correlation combined with the problems m explaining the & s t n b u t m n m Ume and space of the heterogeneity (see above) suggests that a number of locahzed mixing events are occurring When there is a correlation between trace elements and ~sotopes within a locahzed area, for example H561, then the heterogeneity is time-integrated very old Similar results have been reported by Langmulr and Bender (1984), LeRoex et al (1983), Davies (1984) and references thereto These finding would tend to support the concept of mantle "plums" or "blobs" (Allegre et al, 1984, Davies, 1981, 1984) While much of the heterogeneity m Leg 82 basalts ~s time-integrated very old and may be related to mantle "plums", there is ewdence that another process is operative Consider H558, our conventional wisdom leads us to expect an enriched source (m terms of both isotopic composition and trace element ratios) The alternative argument that there was a trace element depleted source with enriched isotopic characteristics, that was then enriched in trace element is a possible but less likely proposltmn Either starting eon&tmns requires a mechanism to create trace element fract~onatlons, of an order inconsistent with known d~strlbuUon coefficients On a larger scale, the origin of the trace element vanatlon observed between H332, H335 and F A M O U S basalts, all at an apparently constant Nd isotopic composition (see Fig 6) must also be explained It IS also difficult to reconcile the trace element and Pb isotopic composlt~on of H556 basalts

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(see Fig 7), the first lndmating a depleted source, while the other indicates an enriched component (cf Wood, 1979) In these cases where there is severe trace element fractionation, not accompanied by isotopic change, we envisage recent events in which vapour-sohd or fired-solid partitioning must have occurred The rote these processes play in generating long-lived heterogeneity depends on the scale of mantle convection If as Davies (1984) has suggested heterogenelties on the scale of tens of kilometers can survive for times in excess of 109 years, then they may play a role Some indication of the nature and perhaps the scale these types of metasomatlc processes play can be found by comparing ocean island and island arc volcamcs Isotopic studies of island arc volcamcs by White and Patchett (1984) and Morris and Hart (1984) have stressed the similarity of their composition to that found in ocean island basalts Many models of the origin of mantle heterogeneity draw on this similarity to suggest that an important component of the source of oceanic island basalts i~ ancient subducted oceamc crust (including sediment) (White, 1985, Davies, 1984, Allegre et al, 1984, and references therein) The trace element enrichment patterns for the two types of volcanxcs are notably different (Sun, 1980, Brlqueu et al, 1984), particularly with respect to the depletion in Nb and Ta relative to Th and La in island arc volcanics If this depletion in Nb in island arc volcanlcs is an inherited source feature and not due to stabilization of a selective Nb-enriched phase (later recycled) (cf Brlqueu et al, 1984, Hole et al, 1984), then, as Hole et al (1984) pointed out, some other process must be called on to explain the absence of depleted Nb signatures in ocean island basalts derived in part from subducted oceamc crust

One way in which it may be possible to remove the characteristic island arc signature from the recycled oceanic crust would be to mix in a proportion of CO2-rich fluid Studies of volcanlcs where CO2 is thought to play an important role in the petrogenesls, l e, mehhtltes, carbonatltes and klmberhtes (Frey et al, 1978; Wylhe, 1980) show that these rocks are characterized by high concentrations of incompatible elements (Muramatsu, 1983, Wood, 1979, Frey et al, 1978) In part, these rocks acquire their high concentrations of elements by extremely small degrees of partial melting, however some prior enrichment may have been necessary and it is likely that CO2 has played a role in this (Frey et al, 1978) Certainly in hydrothermal systems where CO2 is an important component significant quantmes of HFS are moblhzed (Taylor et al, 1981) Hofmann and Whlte's (1982) model to explain the chemistry of ocean island basalts using subducted subducted oceanic crust requires that the .... real" source material would be ecloglte intermixed or "contaminated" with some perldotlte" According

PROCESSES AFFECTING THE ORIGIN OF OCEANIC CRUST

75

to a model recently presented by Spera (1984), mantle metasomatlsm ~s a natural consequence of non-adiabatic magma ascent and therefore it is possible that the "contaminating pendotlte" required in the Hoffman and White model contains regions of CO2 metasomatlsed mantle. The Importance CO2 plays in this type of metasomatism is witnessed by the fact that in metasomatic events related to island arcs, where H 2 0 is thought to be the dominant volatile component (Gill, 1981), the Nb anomaly is either created or survives The poor regional correlation between trace elements and isotopes, the evidence for locahzed mixing and trace element fractlonation events, and the apparent lnablhty of the Azores mantle plume to explain the origin of much of the heterogeneity observed, suggests that the mantle plume hypothesis needs rewslon A complete discussion of the alternatives is beyond the scope of this paper and we refer Interested readers to recent reviews by Allegre et al (1984), Dawes (1984) and White (1985) At present we favour the explanation offered by Davies (1984), in which the mantle is viewed as unlayered with a more or less random distribution of enriched mantle "plums" Our pnnclpal reason for favouring Dawes' model is that his arguments against mantle layering are far more persuasive then those for (cf. Allegre et al, 1984, Fisher, 1985) The solution to the question of layered versus unlayered mantle will probably not be found using geochemistry (cf White, 1985), however, there is hope that a geophysical way may be found (see Tanlmota and Anderson, 1984) CONCLUSIONS

Basalts erupted over the last 35 m y at the MAR between 30°N and 40°N have recorded evidence of chemical heterogeneity in their source regions Attempts to relate the origin of this heterogeneity to a major mantle plume, presently centered in the Azores Islands, have not been particularly successful While it is clear that long-lived ( > 10 9 years) enriched mantle sources have been tapped, It seems more likely that these were distributed as mantle plums or, perhaps, mantle blob clusters Significant, recent trace element fractionation events have "decoupled" trace element and isotope signatures in some areas These findings suggest that fluld-sohd and/or vapour-sohd partitioning (mantle metasomatlsm) must play a role in generating mantle heterogeneity At present, we prefer the model of Davies (1984) to explain the distribution of heterogeneity in the mantle However, this model should be modified to include a significant role for mantle metasomatlsm.

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ACKNOWLEDGMENTS

Early versxons of this manuscript benefitted from reviews by B J Fryer and S Swlnden Bdl White is thanked for his generous assistance, crmclsm and a reprint of his manuscript The manuscript was improved by comments from H Bougault and an anonymous referee Fmanoal support for this work came from the BelgmnNSF, Max-Planck Institute and NSERC Canada

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