Pb isotope study of black-smokers and basalts from Pito Seamount site (Easter microplate)

Chemical Geology 155 Ž1999. 45–63

Pb isotope study of black-smokers and basalts from Pito Seamount site žEaster microplate / Chrystele ` Verati a

a,)

, Joel ¨ Lancelot

a,1

, Roger Hekinian ´

b,2

Laboratoire de Geochimie Isotopique, UMR 5567 CNRS-UniÕersite´ Montpellier II, ISTEEM, cc 066, place E. Bataillon, 34095 ´ Montpellier cedex 05, France b IFREMER Centre de Brest, DRO r GM-BP70-29200 Plouzane, ´ France Received 1 August 1997; accepted 15 May 1998

Abstract Previous studies of hydrothermal deposits on active sediment-starved spreading centers have shown that the lead isotope compositions of the oceanic crust are homogenized by the circulation of high temperature fluids. This averaging effect is confirmed by the homogeneity of the sulphide isotopic values at numerous sites in the Pacific ocean. Our study was undertaken at an active site located on a seamount ŽPito Seamount. formed at the tip of a propagator on the northeast boundary of the Easter microplate near 23819X S. Lead isotope analyses, combined with a mineral paragenesis study, were performed on hydrothermal phases of black-smokers as well as on the adjacent basalts. The field defined by hydrothermal samples departs unexpectedly from the pillow lava field. Furthermore, the hydrothermal deposits display unexpected heterogeneous isotopic values with respect to 207 Pbr204 Pb and 208 Pbr204 Pb ratios. Such a large lead isotope variability has not been encountered elsewhere on the Easter microplate. Hydrothermal sample values plot between an evolved EPR basalt and sediment andror seawater, suggesting that the lead was not derived from a homogeneous source such as end-member hydrothermal fluids generated in a homogeneous reaction zone in the oceanic crust. The origin of high 207 Pbr204 Pb and 208 Pbr204 Pb ratios were tested for various mixing models. We made two major assumptions. Ž1. It is likely that hydrothermal fluids percolated through very heterogeneous volcanic sequences formed of both enriched and depleted MORBs. This suggests cyclic magmatism and a minor involvement of enriched mantle components with time, as the last magmatic events gave N-MORB-type magmas. Ž2. Near the seafloor, it is likely that other sources of radiogenic Pb are sediments andror Mn-crusts in the underlying basaltic sequences that subsequently reacted with hydrothermal fluids. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Pb isotope; Hydrothermal activity; Hot spot; Pito; Easter microplate

1. Introduction ) Corresponding author. Fax: q33-4671-43700; E-mail: [email protected]. Present address: UMR Geosciences ´ AZUR, Universite´ de Nice Sophia Antipolis, Parc Valrose, 06 108 Nice Cedex 2, France. Tel.: q33-4492-6589; Fax: q33-4492076816; E-mail: [email protected] 1 E-mail: [email protected]. 2 E-mail: [email protected].

The study of lead isotopes is important for the understanding of seafloor hydrothermal processes and the source of metals in hydrothermal deposits. It can provide constraints on whether lead in hydrothermal deposits was derived from leaching of the oceanic crust during high temperature alteration, or from

0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 1 4 0 - 5

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seawater. The lead isotope composition of geochemical reservoirs involved during oceanic hydrothermal processes at sediment-free ridges is well constrained and has very distinct signatures. Lead isotope values for deep ocean water, deduced from Mn nodules ŽAbouchami and Goldstein, 1995; Von Blanckenburg et al., 1996; Ling et al., 1997., are distinctively radiogenic. The lead isotope composition of the oceanic crust was determined from values for pillow-lava near the spreading axis ŽTatsumoto, 1978; Ito et al., 1987; White et al., 1987; Hanan and Schilling, 1989; Bach et al., 1994; Dosso et al., 1993.. The lead content of these reservoirs is well known: 2 ppt for deep ocean water ŽVon Damm, 1990. and an average of 0.5 ppm for mid-ocean ridge basalts ŽMORB. ŽHofmann, 1988.. Earlier studies show that lead in hydrothermal chimneys from sediment-free ridges originated from oceanic crust ŽVidal and Clauer, 1981; Hegner and Tatsumoto, 1987; Le Huray et al., 1988; Verati et al., 1994; Fouquet and Marcoux, 1995.; the contribution of the lead from seawater is not considered significant, although the high-temperature vent fluid endmember can have Pb isotopic compositions more radiogenic than MORB ŽGodfrey et al., 1994.. All these studies also indicate that the lead isotopic composition of sulphides is more homogeneous than for adjacent volcanic rocks suggesting that lead isotopes have been homogenized during hydrothermal circulations. Lead isotopic investigations of the Easter microplate region are of prime importance, since previous studies demonstrated a lead isotope anomaly of the mantle source beneath the entire microplate, caused by binary mixing process between a radiogenic and MORB sources ŽSchilling et al., 1985; White et al., 1987; Hanan and Schilling, 1989.. A lead isotope study of hydrothermal and magmatic products at the hydrothermal site from the Pito Seamount located near the tip of the East rift segment Ž23819X S–111838X W. should provide more insight into the nature of fluid interaction with basaltic rocks. It is at this site that are found the most enriched lavas encountered on the microplate coexisting with N-MORB-type lavas ŽHekinian et al., ´ 1996.. TIMS Pb isotope analyses of hydrothermal minerals in chimney fragments and associated basalts were performed to determine the source of hy-

drothermal lead and basaltic rocks at the Pito Seamount site, and to measure the degree of isotopic homogenization during hydrothermal alteration.

2. Geological setting The Easter microplate is about 400 = 400 km, surrounded by the Nazca and Pacific plates and is delimited to the north and the south by complex transform fault zones referred to as the Pito and Orongo fracture zones, respectively ŽHey et al., 1985; Francheteau et al., 1988; Naar and Hey, 1991.. The Easter microplate is located in a region where the fastest seafloor spreading is occurring. Its eastern and western boundaries are active spreading centres called respectively the East rift and the West rift ŽFig. 1.. These active spreading centres propagate towards north and south, respectively. The Easter microplate rotates 158rmy in an anti-clockwise direction causing rapid evolution of its rifted boundaries ŽSchilling et al., 1985; Searle et al., 1989; Naar and Hey, 1991.. The microplate started to form about 4.5 my ago, when the East rift propagated north ŽNaar and Hey, 1991.. The hydrothermal site of the Pito Seamount is centered on 23819X S–111838X W and it is located north of the East rift propagator, on the eastern boundary of the Easter microplate where full-spreading rates are approximately 50–60 kmrmy ŽFig. 1.. Active black smokers were discovered in november 1993 at a 2243 m depth during Nautile submersible dives near the summit of the seamount ŽFrancheteau et al., 1994; Naar et al., submitted.. The various ridge segments in the vicinity of the Pito Seamount are associated with a sediment starved spreading axis.

3. Laboratory techniques and analytical procedures Basalt showing no sign of seafloor weathering and sulphide chimney fragments were crudely crushed in a tungsten-carbide morta. Hydrothermal minerals and glassy rims of basalts Ž1–2 mm thick.

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Fig. 1. Location of the Pito Seamount hydrothermal site on the Easter microplate and bathymetric map of the site Žmodified after Francheteau et al., 1988.. Double lines correspond to the inferred spreading axis, single lines are transform faults; scarps bounding the rift zone are indicated. NZ represents the present neovolcanic zone ŽPito Seamount may be serving as a focal point for a new seafloor spreading axis to develop ŽNaar et al., submitted..

were hand-picked under a binocular. Aphyric basalts, minerals and glassy rims were repeatedly cleaned ultrasonically in tridistilled water for a few hours prior to grinding to ensure the removal of seawater salts or actual common lead. For lead isotope analysis, 0.5–1 g of basaltic material was dissolved by an HF–HNO 3 –HCl0 4 acid mixture at 1008C, and 5–10

mg of hydrothermal minerals was dissolved by an HCl–HNO 3 mixture. Lead was separated in a 100 ml AG 1-X8 anionic resin exchange column using procedures described by Manhes ` Ž1982.. Lead isotopic ratios were measured in static mode with a VG Sector mass spectrometer, and the results were corrected for mass fractionation by ; 0.13% amuy1 ,

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based on replicate measurements of NBS 981 Ž n s 10; Verati, 1997.. The external precision of the measurements inferred from a large number of data on NBS981 is less than "0.02% amuy1 . Procedural total blanks averaged ; 120 pg. Scanning Electron Microscope ŽSEM. with a coupled Energy-Dispersive Spectrometric ŽEDS. analyser was used to investigate mineral paragenesis in the black-smoker chimney samples and the degree of the alteration of the basalts. These observations were made on handpicked minerals as well as on polished thin sections.

4. Sample descriptions

also form tubular structures, 3–5 mm in diameter ŽFig. 2B. which are similar in morphology to structures that are considered to have originated by biocatalysis processes associated with filamentous bacteria ŽJuniper and Fouquet, 1988.. 4.1.2. Fe-sulphides Numerous iron-sulphides display different framboidal textures. Subspherical iron sulphides formed of micrometric crystals constitute the first type ŽFig. 2C.. The second type is spherical Fe-sulphides without well-defined crystals ŽFig. 2D.. Observations on polished sections of the latter type show radial structures where Fe-sulphides precipitate around a core of silica 10 mm in diameter.

4.1. Sulphides Hydrothermal samples of fragments from the external part of sulphide chimneys collected at Pito Seamount site Žsamples PI0710a and PI0710b. were examined for mineralogy. Scanning Electron Microscopy ŽSEM. observations performed on handpicked materials and polished sections revealed a common mineral paragenesis of euhedral to subhedral pyrite and sphalerite, chalcopyrite, marcasite. observed elsewhere ŽFouquet et al., 1988; Bendel et al., 1993; Krasnov et al., 1995; Duckworth et al., 1995.. Chalcopyrite inclusions are common in euhedral sphalerites Ž10 to 50%., as well as sphalerite inclusions in euhedral pyrites Ž1 to 10%.. Euhedral pyrite, sphalerite and chalcopyrite define typical high-temperature mineral assemblage from the most internal part of chimney fragments. In the external parts of these fragments, low-temperature paragenesis consists of marcasite, sphalerite and sulphates. Marcasite is often associated with melnikovite, and sphalerite does not display any chalcopyrite inclusions. In addition, a large amount of other zinc and iron crystalline phases have also precipitated with these low-temperature assemblages. 4.1.1. Zn-sulphides The SEM images show an interlocking, open network of zinc-sulphide crystals covering a subspherical collomorph of iron sulphides which are probably marcasite andror melnikovite minerals ŽFig. 2A.. The interface between this mat and the iron sulphides appears to be avoid. Zinc-sulphides

4.1.3. Other phases On the outer part of irregularly shaped chimney fragments, we observed a red thin layer. SEM observations, DRIFTS and HPLC measurements provide iron and elemental sulphur as the major components ŽVerati et al., 1998.. Numerous bacterial imprints are present in this layer ŽFig. 2E.. The layer is occasionally associated with jarosite. Based on these observations, it is likely that mineral precipitation depends only on a combination of two parameters: Ž1. temperature gradients within the chimney Ž2. and biocatalysis processes. 4.2. Basalts Basalts associated with hydrothermal sulphide deposits consist of fresh pillow lavas from the upper part of the north flank of the seamount Ž23818.6X – 23819.7X S; 111840X –111838X W.. The geochemistry of these basalts indicated that they are enriched Žhighest values of Na 8 Ž2.9–3.4%., high Mg No. Ž0.6–0.7.. and typical of N-MORB basalts ŽHekinian et al., ´ 1996..

5. Pb isotope compositions Lead isotope data on black-smoker samples have a range of 206 Pbr204 Pb s 18.448–18.520; 207 Pbr204 Pb s 15.482–15.583; 208 Pbr204 Pb s 37.784–38.208 ŽTable 1.. These data are more variable than for other oceanic sulphides samples from a given site

C. Verati et al.r Chemical Geology 155 (1999) 45–63

Fig. 2.

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Fig. 2.

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Fig. 2. Scanning Electron Microprobe photographs Žsecondary electron mode. of hydrothermal phases in sulphide deposits at Pito Seamount site in the Easter microplate. ŽA. Zn-sulphides interlocking, open crystals network covering sub-spherical collomorphic iron sulphides. ŽB. Zn-sulphides forming a network of hollow filaments. Diameter of filamentss 3–5 mm. ŽC. Subspherical Fe-sulphide aggregates consisting of micrometric crystals. Crystal are up to 5 mm across. ŽD. Fe-sulphides, without crystalline outlines, displaying framboid-like forms. ŽE. General view of the external layer containing numerous bacterial imprints that are 1–2 mm in length. Coarse grained white crystals located at the corners of the photograph correspond to jarosite crystals.

ŽVidal and Clauer, 1981; Hegner and Tatsumoto, 1987; Le Huray et al., 1988; Verati et al., 1994; Fouquet and Marcoux, 1995.. The Pb content of hydrothermal samples range from 70 to 1356 ppm. The euhedral pyrites are the most radiogenic, although there is no obvious correlation between Pb isotopic values and mineralogy. Pb isotope values for sulphide deposits are thus independent of formation temperature of the sulphide minerals as well as biocatalysis processes. Highly radiogenic 207 Pbr204 Pb and 208 Pbr204 Pb values represent a new important feature. These values for the Pito Seamount site define an isotopic field distinct from the others fields for active chimneys along the East Pacific Rise ŽFig. 6.. Lead isotope data from basalts give the following variations: 206 Pbr204 Pb s 18.296–18.484; 207 Pbr204

Pb s 15.467–15.545; 208 Pbr204 Pb s 37.707–38.011 ŽTable 1.. The lead isotope composition of glassy rims of basalts are slightly more radiogenic than that of the whole rocks. Even if the 207 Pbr204 Pb and 208 Pbr204 Pb ratios are not as great as the hydrothermal samples, they are generally higher than values for South East Pacific Rise ŽSEPR. 20–228S basalts.

6. Local and regional scale compositional variations A major feature of the Pb isotope data is that the field for hydrothermal samples does not overlap the field for associated basalt ŽFig. 3.. The Pb isotope composition of sulphides would be expected to reflect the mean value of the Pb isotope composition

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Table 1 Lead isotope compositions of hydrothermal samples and basalts from the Pito Seamount site in the Easter microplate Depth Žm.

Sample numbers

Hydrothermal samples PI07.10a

PI07.10b

Basaltic rocks 2456 2457 2457 2457 2392 2364 2364 2328 2243 2268

PS.07r01 PS.07r02 PS.07r03 PS.07r03 PS.07r04 PS.07r05 PS.07r05 PS.07r06 PS.07r08 PS.07r11

Type

206

Pbr204 Pb

207

Pbr204 Pb

208

Pbr204 Pb

Pyrite Sphalerite Sphalerite mat Framboidal pyrite Jarosite Bacterial layer Whole layer Pyrite Duplicate of pyrite Cu-sulphates

18,520 ("2) 18,448 ("11) 18,497 ("4) 18,486 ("3) 18,475 ("2) 18,478 ("4) 18,468 ("1) 18,516 ("4) 18,492 ("2) 18,497 ("18)

15,583 ("1) 15,482 ("8) 15,555 ("4) 15,542 ("3) 15,523 ("2) 15,533 ("3) 15,521 ("1) 15,574 ("3) 15,552 ("2) 15,562 ("16)

38,208 ("5) 37,784 ("21) 38,067 ("11) 37,991 ("8) 38,012 ("5) 37,989 ("8) 37,943 ("3) 38,098 ("7) 38,059 ("4) 38,122 ("39)

whole rock glassy rim glassy rim whole rock glassy rim glassy rim whole rock glassy rim glassy rim whole rock

18,374 ("7) 18,341 ("11) 18,386 ("6) 18,296 ("7) 18,484 ("11) 18,468 ("7) 18,331 ("20) 18,435 ("9) 18,432 ("22) 18,398 ("46)

15,500 ("7) 15,516 ("9) 15,508 ("5) 15,467 ("6) 15,492 ("9) 15,545 ("6) 15,472 ("17) 15,517 ("7) 15,479 ("18) 15,503 ("39)

37,810 ("17) 37,885 ("23) 37,878 ("12) 37,707 ("14) 37,920 ("21) 38,011 ("15) 37,761 ("42) 37,921 ("18) 37,813 ("45) 37,870 ("93)

Pb Žppm. 125 287 1180 236 386 342 1356 70 96

All Pb data were corrected for mass fractionation, and analytical error Ž2 s . is reported for each ratio. Lead contents of the hydrothermal samples were determined by isotope dilution method. Whole layer represents a bacterial layer strongly associated with underlying minerals Žsulphides, jarosite..

of the associated volcanics ŽVerati et al., 1994; Fouquet and Marcoux, 1995.. At a local scale, high variability of Pb isotopic values for Pito Seamount hydrothermal deposits with respect to that of their nearby volcanics suggests the leaching of compositionally different rock units andror a possible contribution of seawater lead. The lack of overlap of sulphide values with those for adjacent basalts could indicate that the hydrothermal reaction zone for the fluids that generated the sulphide chimneys and the source regions from the current basaltic lavas are different. Taking into consideration the experimental errors on the Pb isotope

measurements, which also does not include sample variability, then the glass and whole rocks basalt fields almost overlap, except for three samples of glassy rims ŽFig. 3.. The fact that glass samples display more radiogenic values than whole rocks is not easily explained ŽFig. 3.. This radiogenic difference could be due to a more extensive alteration of the basaltic rims by seawater. Previous studies of altered hyaloclastite crusts of basaltic rocks from the Gorda ridge indicated that the Pb isotopic values of the altered and unaltered basalts from the same vent field are very similar ŽZierenberg et al., 1995.. SEM observations of the glassy rims of the more radio-

Fig. 3. Lead isotope diagrams showing the distribution of hydrothermal samples and basalts from the Pito Seamount in the Easter microplate. The pelagic sediment field ŽBarrett and Friedrichsen, 1982., the Pacific seawater field ŽVon Blanckenburg et al., 1996., the EPR 20–228S ŽWhite et al., 1987; Hanan and Schilling, 1989. and the Easter microplate basalts from the West and East rifts, and Easter Island basalts ŽHanan and Schilling, 1989. are also plotted. North Hemisphere Reference Line ŽNHRL. defined by the MORB and OIB collinear trend is from Hart Ž1984..

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genic samples at the Pito Seamount site show plagioclase and pyroxene minerals surrounded by halos of glass microlites ŽFig. 4.. The matrix including the unaltered minerals with halos is slightly enriched in Fe when compared to glassy margin suggesting seawater alteration process. Although this matrix is compositionally very homogeneous ŽSi, Al, Ca, Fe, Mg, ´Ti, ´Na, ´K., a small amount of micrometric spherical sulphides were detected. These sulphides consist of chalcopyrite and are located preferentially within the matrix where seawater is likely to percolate throughout. The origin of these sulphides is believed to be hydrothermal, and may explain the higher Pb isotope ratios of glassy rims compared to the basalt, since the sulphides from black-smokers are more radiogenic than the associated basalts at the Pito Seamount. When the nearest EPR Ž20–228S. and Easter microplate basalts are compared at a regional scale ŽWhite et al., 1987; Hanan and Schilling, 1989., the 206 Pbr204 Pb ratios of sulphides and basalts from the Pito Seamount span the less radiogenic part of the East rift field in the Easter microplate ŽFigs. 3 and 5.. There is decoupling between the Pb isotope composition of the Pito basalts and the sulphides. Some Pito Seamount basalts plot in the field defined by typical N-MORB located further north on the South EPR. The 207 Pbr204 Pb and 208 Pbr204 Pb ratios of the hydrothermal deposits are clearly more radiogenic than those of a typical SEPR Ž20–228S. MORB and fall on a trend towards a sedimentary or a seawater isotopic composition ŽFigs. 3 and 5.. Ratios for associated glassy basalts likewise trend toward more radiogenic values ŽFig. 3.. Compared with other high-temperature hydrothermal samples from the Pacific ocean, sulphides from the Pito Seamount define a distinct field ŽFig. 6.. This field is much more radiogenic than that of the SEPR at 21826X S which consists essentially of noncontaminated N-MORB basalts, but comparable to EPR basalts between latitude 138N–18830X S ŽFouquet and Marcoux, 1995., although it has a different shape ŽFig. 6.. The Pito Seamount lead isotope field

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is also distinct from the hydrothermal deposits found on sedimented ridges. The high 207 Pbr204 Pb and moderately high 208 Pbr204 Pb values of the Pito hydrothermal samples are not easily explained considering the sediment-starved character of the seamount on which they formed.

7. Discussion Basalts from the East rift show a systematic westward decrease in Pb isotope ratios with distance from Easter and Sala y Gomez islands ŽHanan and Schilling, 1989.. Based on their location on the microplate, Pito Seamount hydrothermal deposit samples would be expected to correspond to the least radiogenic ratios, but they have strongly more radiogenic 207 Pbr204 Pb and 208 Pbr204 Pb ratios for a given 206 Pbr204 Pb ratio when compared to hot spot basalts from Easter Island ŽFig. 3.. Two hypothesis based on mixing between four different end-members ŽTable 2 and Eq. Ž1. in Appendix A. may explain Pb isotope enrichment in Pito Seamount samples ŽFig. 7.. Ža. The depleted mantle is defined by hydrothermal samples from the East Pacific Rise at 21826X S ŽFouquet and Marcoux, 1995., which is the closest active site to the Pito Seamount, since sulphides commonly represent a reliable average of underlying oceanic crust. Žb. The enriched source is represented by the mantle source responsible for the Easter Island hot spot magmatism. The mixing trend between these two components explains the largest variation of 206 Pbr204 Pb values of the hydrothermal samples from the Pito Seamount, whereas the anomalously high 207 Pbr204 Pb and 208 Pbr204 Pb ratios must originate from other sources ŽFig. 7.. Hence, the contribution of two additional sources is proposed to explain such high Pb isotope values: these are Žc. a pelagic sediment and Žd. a seawater. Seawater and sediment sources are used in the following for hypothesis to explain the highly radiogenic Pb isotope values at Pito Seamount.

Fig. 4. SEM photographs Žbackscattered electron mode. of the glassy rims of basalts at Pito Seamount site. ŽA. General view showing plagioclase and pyroxene surrounded with dark halos of glass microlites. The light colored matrix corresponds to the relatively higher Fe contents. ŽB. Enlarged view showing unaltered plagioclase and its glass halo, with a Cu–Fe sulphide of 1–2 mm in size located within the matrix Žbottom left of the photograph..

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Fig. 5. Variations of 206 Pbr204 Pb, 207 Pbr204 Pb and 208 Pbr204 Pb with respect to latitudes, along the South EPR and the East rift of the Easter microplate. The Data from the EPR and East rift are from White et al. Ž1987. and Hanan and Schilling Ž1989.. White circles correspond to the Pito Seamount hydrothermal deposits.

7.1. Hypothesis 1: inÕolÕement of seawater lead? The 207 Pbr204 Pb and 208 Pbr204 Pb ratios of hydrothermal samples extend towards the seawater isotopic domain, whereas the 206 Pbr204 Pb values show

less variability. The 206 Pbr204 Pb ratio is attributed to a binary mixing between the EPR and Easter Island basalts ŽFig. 7., with respectively 95% and 5% of contribution for the two mantle sources. Although previous studies of hydrothermal deposits at unsedi-

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Fig. 6. Lead isotope ratios plots for the hydrothermal deposits from various regions of the Pacific ocean. The white circles correspond to the hydrothermal samples from the Pito Seamount. The field for sulphides from sedimented ridges and the Juan de Fuca ridge are from Le Huray et al. Ž1988.; the North Fiji Basin field is from Verati et al. Ž1994.; the Lau basin, EPR, Galapagos and Explorer fields are from Fouquet and Marcoux Ž1995..

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Table 2 Lead isotopic end-members used for mixing calculations for the Pito Seamount samples End-member compositions

206

Pbr 204 Pb

EPR Žsulphides. Easter Island basalts ŽEI. Pacific Seawater ŽMn-crusts. Pelagic sediments 95% EPR q 5% EI Žhydrothermal samples.

18.232 19.388 18.756 18.759 18.428

207

Pbr 204 Pb

15.476 15.597 15.634 15.629 15.496

208

Pbr 204 Pb

37.705 39.030 38.745 38.629 37.929

Pb Žppm. 0.5 2.0 2E y 6 20 0.575

The East Pacific Rise ŽEPR., Easter Island, seawater and sediment end-member compositions represent average values for sulphides from X the South EPR 21826 S ŽFouquet and Marcoux, 1995., basalts from Easter Island ŽHanan and Schilling, 1989., the Mn-crusts from Pacific ocean ŽVon Blanckenburg et al., 1996. and from pelagic sediments ŽBarrett and Friedrichsen, 1982; Hamelin et al., 1990.. The Pb contents are from Hofmann Ž1988. for the basalts, from Von Damm Ž1990. for seawater, and from Loock et al. Ž1990. for sediments. The ‘end-member’ composition of hydrothermal samples from the Pito Seamount, falls on a mixing line between a depleted ŽEPR end-member type. and a more enriched Ž5%. mantle ŽEaster Island-type..

mented ridges suggest that the seawater Pb contribution to hydrothermal fluids is not significant ŽVidal and Clauer, 1981; Hegner and Tatsumoto, 1987; Le Huray et al., 1988; Verati et al., 1994; Fouquet and Marcoux, 1995., seawater sources cannot be completely ruled out. If Pb in seawater contributes to high Pb isotopic ratios in final hydrothermal fluids, it is probably due to the intensity of hydrothermal alteration of the oceanic crust. There is no evidence of secondary alteration of hydrothermal phases by cold or warm seawater after precipitation. In order to test the possible contribution of seawater, we calculate the waterrrock ratio ŽEq. Ž2. in Appendix A. required to obtain the observed Pb isotope composition for the Pito Seamount hydrothermal samples. W r R gives mean value of 7.5 = 10 4 , according to a coefficient of extraction K s 1, and taking into account that the leached basalts correspond to the end-member of 95% EPR q 5% EI ŽTable 2.. The obtained waterrrock ratio is higher than the 3.1 y 6 = 10 3 calculated for fluids venting at 218N on the

EPR ŽChen et al., 1986.. It is difficult to explain a different behaviour of Pb at Pito Seamount during hydrothermal processes, compared to the other EPR hydrothermal sites, since the Pb contents measured in these hydrothermal deposits ŽFouquet and Marcoux, 1995. give the same compositional range. If Pb from seawater has influenced the Pb isotope composition of the black-smokers, it must be related to seawater interaction with more enriched isotopic signatures than those recorded by Mn-crusts in the whole ocean. According to the short residence time of the Pb in the oceans Ž50–100 years, Schaule and Patterson, 1981, 1983., and taking into account that Pb isotope composition of seawater could be influenced by local volcanic activity, it is possible that seawater at the Pito Seamount provides more enriched Pb isotope signatures. Similarly, for comparison, it is known that in Troodos ophiolites, a seawater component was required in addition to basaltic lead to explain the high 207 Pbr204 Pb values recorded by sulphides ŽSpooner and Gale, 1982.. However,

Fig. 7. Lead isotope mixing trends of different possible Pb sources and Pito Seamount Žwhite circles. hydrothermal deposits. The EPR X end-member basalt is calculated from EPR sulphides at 21826 S ŽFouquet and Marcoux, 1995., the Easter Island composition is deduced from basalts ŽHanan and Schilling, 1989., the sediment ratios were obtained from data for pelagic sediments ŽBarrett and Friedrichsen, 1982; Hamelin et al., 1990., and for Mn-crust that interpreted to represent seawater ŽVon Blanckenburg et al., 1996.. The mixing trends are calculated following Eqs. Ž1. and Ž2. Žsee Appendix A.. Seawater trend is graduated in WrR Žseawaterrrock. values; the sediment trend and Easter Island trend are in percent. Ž1., Ž2.: first and second hypothesis Žsee text for explanation.. ŽA. Processes responsible for high 207 Pbr204 Pb and 208 Pbr204 Pb, i.e. seawater or sediment contribution. ŽB. Process responsible for the 206 Pbr204 Pb variations, i.e. mixing process between a N-MORB-like mantle source and a OIB-like mantle source. = represent the mean values for each end-member. Pb isotope ratios of hydrothermal samples can be accounted for by mixing between evolved basalt Ž95% EPR q 5% Easter Island. and sediment andror seawater with an average value Žl. slightly different than the common average values for sediment and seawater.

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more recent study indicates that a seawater component does not have to be involved in the Troodos sulphides, showing that the Pb isotope compositions of the sulphides can be related to the existence of enriched lavas ŽLawson et al., 1992–1994.. 7.2. Hypothesis 2: Pito Seamount as a geochemical particular site in the Easter microplate enÕironment? The large Pb isotope variations of hydrothermal deposits, compared to those of the associated basalts, could be related to the leaching of a heterogeneous reaction zone underneath the Pito Seamount. The leaching of Pb from enriched and depleted rock units enhances the heterogeneity of the Pb sources. This variance could be amplified by higher spreading rates Ž50–60 kmrmy. and a higher rate of magmatism occurring at Pito Seamount site, although no correlation was established between Pb-isotopic values for hydrothermal deposits and spreading rates ŽFouquet and Marcoux, 1995.. The postulated heterogeneity of the oceanic crust may be related to the mantle source variations during partial melting. Therefore, the Pb isotope composition of Pito Seamount sulphide deposits may reveal older deeper more enriched lavas. Enriched lavas may have been generated during ancient eruptions whereas the most recent lavas exposed on the seamount surface give rise to N-MORB. Hot spot contribution to the source of the Easter microplate magmatism is indeed believed to have decreased since microplate formation ŽHekinian et al., 1995.. The enrichment shown by ´ high 207 Pbr204 Pb and 208 Pbr204 Pb ratios and slightly high 206 Pbr204 Pb can be related to a particular type of mantle source at Pito Seamount where a depleted source ŽN-MORB-type. underwent two simultaneous contaminations from an undepleted mantle and a recycled sedimentary component. Since there is no apparent sediment at the Pito Seamount hydrothermal site, the sedimentary component may instead be an enriched component in the mantle that was influenced by the Easter Island hot spot. On a mixing trend, the Pito Seamount mantle consists of 95% of a N-MORB-like source and 5% of an undepleted source containing at least 2% of recycled sediments ŽFig. 7.. Although it is difficult to favour either of these two hypothesis on the basis of the current data, we

can investigate the most probable one. The contribution of an enriched component could vary depending on the choice of the lead isotopic ratio. Concerning the Pito Seamount, the 207 Pbr204 Pb ratios recorded by black-smokers give larger range than 208 Pbr204 Pb ratios ŽFigs. 3 and 5–7.. A seawater contribution should give the same tendency for these two ratios, whereas old recycled sediments would provide an isotopic fractionation because of differences in the radioactive decays of U and Th isotope parents. The contribution of a particular enriched mantle source at Pito seamount at the northeast boundary of the Easter microplate is thus suggested. This enriched source must play a key role in the lead mass balance during hydrothermal process at Pito Seamount site, although seawater Pb contribution cannot to be ruled out. The Pb isotope composition of hydrothermal samples may therefore reflect significant isotopic heterogeneity of basaltic rocks. Similar to Hekinian et al., 1996 we ´ suggest that this heterogeneity is related to ancient enriched magmatic cycles. According to the isotopic results for locally formed basalts, the most recent eruptions suggest an increase of the depleted mantle component. This could be related to a readjustment of the upwelling diapirs underneath the spreading axis at 238S towards the Pito Seamount during the Easter microplate anti-clockwise rotation. Finally, another possible sources of radiogenic Pb are pelagic sediments andror Mn-crusts on older basaltic sequences that subsequently reacted with hydrothermal fluids. But, a 4 km transect along the Pito Seamount area only observed very young, unaltered basaltic flows with no sediment cover in places ŽNaar et al., submitted.. The last possible source of radiogenic Pb is old altered hydrothermal sediments. Sulphides on the seafloor are rapidly oxidized to oxyhydroxides of Fe which would have a capacity to absorb Pb from seawater. This hypothesis can be reinforced by the presence of accumulation of sedimentary hydrothermal deposits observed near the summit of the Pito Seamount near the site of active vents sites ŽNaar et al., submitted.. However, previous Pb isotope analysis performed on the secondary oxidation products from the hydrothermal chimneys did not show any later Pb seawater contamination, because probably of the very low content of Pb in ambient seawater ŽVerati et al., 1994; Fouquet and Marcoux, 1995..

C. Verati et al.r Chemical Geology 155 (1999) 45–63

8. Conclusions The Pb isotope study of basalts and sulphides from sediment-starved Pito Seamount provide unexpected results when compared to other active hydrothermal sites in the Pacific ocean. Ž1. Hydrothermal samples display abnormally high 207 Pbr204 Pb and 208 Pbr204 Pb values when compared basalts from the Easter microplate and, more specifically, Easter Island. The hydrothermally altered components of the heterogeneous upper oceanic crust are considered as the main Pb sources for hydrothermal products, although other Pb sources can account for these Pb radiogenic values. The high variance of black-smoker Pb isotope values represents a combined effects of Pb contributed from a number of different sources in the hydrothermal reaction zoneŽs. andror at the seafloor. Ž2. As there is no direct evidence for a radiogenic source based on surface flows, it is likely that the deep reaction zoneŽs. for the fluids are very different from the source regions from the current basaltic lavas. Mantle source at the Pito Seamount has evolved with time, according to the less radiogenic features of the pillow lavas which correspond to the recent magmatic cycles. The origin of such heterogeneity is probably related to the existence of enriched rocks units suggesting a past local mantle anomaly below the Pito Seamount. We have identified a depleted mantle source contaminated with 5% of the source responsible for Easter Island volcanism with at least 2% of ancient recycled sediments. Ž3. At the seafloor, another possible sources of radiogenic Pb are pelagic sediments andror Mncrusts on older basaltic sequences that subsequently reacted with hydrothermal fluids, as well as the involvement of Pb added by sorption of seawater to altered sedimentary hydrothermal deposits near the summit of Pito Seamount, near the vent sites.

Acknowledgements We are thankful to the Captain, officers, and crew of the R.V. Nadir as well as to the Nautile team for their help during the ‘Pito Cruise’. The Pito Cruise was jointly sponsored by IFREMER and INSU, and

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organised by UBO ŽUniversite´ de Bretagne Occidentale.. We are thankful to D.F. Naar, J. Francheteau ŽChief Scientists. and R. Hekinian for providing the ´ samples. We would like to thank L. Briqueu for stimulating discussions and E. Delaperriere for improving the English.

Appendix A General equation for the Pb isotope composition in a mixture Žafter Chen et al., 1986. i

Pb

ž / 204

Pb

m i

XCa204

Pb

ž / 204

Pb

i

q Ž 1 y X . C b204

a XCa204 q

s

Ž1yX .

Pb

ž / 204

Pb

b

C b204

Ž 1. a, b: end-members; m: mixture; C 204 : 204 Pb concentration in the end-member reservoir; X: weight fraction of the a end-member in the mixture; i: Pb isotopes Ž206, 207, 208..Calculation for the waterrrock ratio Žafter Chen et al., 1986. W s R

Cri Ž R ri y R fw . C wi Ž R wf y R iw .

=K

Ž 2.

C: concentration; R: isotope composition; r: for the basaltic rock; w: for seawater; i: initial; f: final; K : efficiency of extraction.

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