Age Constraints on Crustal Recycling to the Mantle Beneath the Southern Chile Ridge: He-Pb-Sr-Nd Isotope Systematics

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL 104. NO. B3, PAGES 5097-5114, MARCH 10, 1999

Age constraints on crustal recycling to the mantle beneath the southern Chile Ridge: He-Pb-Sr-Nd isotope systematics

Marnie E. Sturm and Emily M. Klein Division of Earth and Ocean Sciences, Nicholas School orthe Environment Duke University. Durham, North Carolina

David W. Graham College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis

Jill Karsten School of Ocean and Earth Science and Technology, University of Hawaii at Manca

Abstract. Basalts from the four southernmost segments of the subducting Chile Ridge (numbered 1-4 stepping away from the trench) display large variations in Sr, Nd, Pb, and He isotope and trace element compositions. Klein and Karsten [1995] showed that segments I and 3 display clear trace element evidence for recycled material in their source (e.g.. low CefPb). The uniformly mid- ocean ridge basalt (MORB)-like 3He/4He and modest variations in Pb, Sr. and Nd isotopes of segment 1 (nearest the trench) suggest recent (<20 Ma) introduction of a contaminant into its source, consistent with recycling of material from the adjacent subduction zone. In contrast, segment 3 Ia- vas display a dramatic southward increase in enrichment, extending to highly radiogenic Pb and Sr isotopic compositions (e.g., 206Pb/204Pb = 19.5) and the lowest 3He/4He yet measured in MORB (3.5RA). The segment 3 variations are most readily explained by ancient (-2 Ga) recycling of terrigenous sediment and altered cnist, but we cannot rule out more recent recycling of material derived from a distant continental source. The similarity in isotopic signatures of segment 4 lavas to Indian Ocean MORB extends the Dupal anomaly to the Chile Ridge. Like Indian Ocean MORB, the segment 4 isotopic variations are consistent with contamination by anciently recycled pelagic sediment and altered crust and require a complex history involving at least three stages of evolution and possibly a more recent enrichment event. Southern Chile Ridge MORB reflect the extensive degree of heterogeneity that is introduced into the depleted upper mantle by diverse processes associated with recycling. These heterogeneities occur on a scale of 50-100 kin, corresponding to transform- and propagating-rift-bounded segmentation, and attest to the presence of distinct chemical domains in the mantle often bounded by surficial tectonic features that maintain their integrity on the scale sampled by melting.

1- Introduction may uLtimately become entrained in the upper or lower mantle [e.g., Hofmann and White, 1980, 1982; Chase, 1981; White, It has been rccognized for more than three decades that the suboceanic mantle is chemically and isotopicallyheteroge- 1985; Ic Roex etal., 1989; Plank and Langmuir, 1998].In- neous on a range of scales. Various origins for distinct types deed, lava compositions from some hot spots, such as the of heterogeneities in the upper mantle have been proposed, Azores, the Society Islands, and Gough Island, show strong such as entrainment and dispersal of plume or "primitive" evidence of recycled material in their source [e.g., White and mantle material or recycling of crustal or lithosptieric mate- Hofmann, 1982; Weaver, l991a,b; Chauve! et al.,1992; rial at subduction zones [e.g., Armstrong, 1968; Schilling ci Devey ci al., 1990; Dupwy ci al., 1987]. al., 1985; .41!ègre cc at., 1984; A!!ègre and Turcotte, 1985; Ithas been notedthat some mid-oceanridgebasalts Hofmann and White, 1982; Ringwood, 1982; Zindler a al., (MORB), far removed from hotspot influences, also display geochemicalcharacteristics 19g2;McKenzieand O'Nions, 1983;Rehkanzper and of recycled materialsintheir Hofmann, 1997]. Studies of processes occurring at subduction source [e.g., Mahoney et al., 1992]. Rehkamper and Hofmann zones show that while somc of the subducting lithosphere is [19971, for example, suggested that the distinctive trace ele- immediateLy partitioned to the continent in arc magmas or by ment and isotopic signatures of some Indian Ocean MORB re- accretion, a substantial portion is returned to the mantle and suit from large-scale mixing of anciently subducted oceanic crust and a small amount of pelagic sediment into the depLeted upper mantle. Furthermore, Niu and Eatiza 1)997] argued that the trace element signatures of eastern Pacific seamounts result Copyright 1999 by the American Geophysical Union. Crom mixing of subducted oceanic crust into the depLeted upper Paper numberl998JB900lO7. mantle with subsequent upwelling, melting, and dispersal of 0148-0227/9911998J89001 07$09.O0 the enriched material as mobile rnetasomatic fluids. The corn-

5097 5098 STORM El' AL.: He-Pb-Sr-Nd ISOTOPES IN SOUTHERN Cl-tILE RIDGE LAVAS mon theme in these and other studies isthat the observed chemical enrichments originate through ancient recycling, and therefore the age and location of these recycling events remain obscure. In a recent study of MORB from the southern Chile Ridge, Klein and Karsten [1995] reported trace element evidence for contamination of the mantle by recycled sediment and altered oceanic crust (Figures 1 and 2). The southern Chile Ridge provides an interesting location to study recycling because it is one of the few modern examples of spreading center subduction c-) (Figure 1) [Cande and Leslie, 19861. Previous geophysical and theoretical work on the subduction of spreading centers suggests that the downgoing lithosphere may develop a slab window [Dickinson and Snyder, 1979; Thorkelson, 1996] or may break up in the subduction zone [van den Buekel, 1990] thus providing a potential pathway or mechanism for contaminat- ing the adjacent suboceanic mantle with slab-derived material. Drawing upon these ideas, Klein and Karsten [1995] raised the possibilitythatthetrace elementsignatureof recycling among some of the Chile Ridge lavas may have been introduced into the subridge mantle recently at the adjacent subdue- Figure 2.Nh/U versus Ce/Pb for Chile Ridge glasses after Klein and Karsten [1995] (symbolsasinFigure 1). Also tion zone. shown are fields for mid-ocean ridge basalts (MORB) and While trace element studies may provide evidence for crustal most ocean island basalts (OIB) [Sun and McDonough, 1989; recycling of material to the MORB source and potentially dis- Hofinann c/ al., 1986, HoJinann, 1988; Hickey ci al., 1986], tinguish one type of contaminant from another, they cannot marine sediments [Ben Ot/unan etal.,989; Plank and Lang- constrain the age of the contaminant and therefore whether the muir, 1993; Plank and Ludden, 1992], arc lavas Il-f ickey ci al., recycled chemical signature of the Chile Ridge MORS is re- 1986], altered oceanic crust (estimate modified from F-fart and lated to recent recycling from the adjacent subduction zone. Staudigel, [1989]Chauvel cial.,[1992]Plank,[1993] Radiogenic isotope studies of these lavas, however, can pro-Szaudige! etal., [1995]) and average upper continental crust vide constraints on both the timing of the contamination (cross labeled CC; Taylor and M'cLennan, [1985]). Lavas from event and the origin of the recycled materials. In this study we segments I and 3 extend from the MORB field toward compositions commonly associated with arc lavas, suggesting contamination of the mantle source with a small amount of sediment and altered crust [Klein and Karsien, 1995].

combine constraints from He, Pb, Sr. and Nd isotope systematics with those of trace elements to explore whether the recycled signature in some Chile Ridge lavas may be related to a recent recycling event at the adjacent subduction zone.

2. Geologic Setting and Background The Chile Ridge separates the Nazca and Antarctic plates and is currently being subducted beneath the South American plate near 47'S at a rate of 9 cmlyr [Cande and Leslie, 1986]. The southern Chile Ridge isan intermediaterate spreading center (-60 mm/yr full rate) that is generally char- acterized by a broad (10-20 km wide), deep (3400-4300 m) rift valley [Klein and Karsten, 1995; Cande and Leslie, 1986]. In January 1993 a dredging, wax coring, and mapping program was conducted along the four southernmost segments of the southern Chile Ridge, hounded by the Chile Margin Triple 82°W 800 780 760 Junction in the south and the Chiloe Fracture Zone in the Figure1-Tectonic elements of the southern Chile Ridge north, in order to investigate geochemical variations along near its intersection with the Chile Trench. Ridge segments the ridge axis as a function of distance from the trench (Figure are labeled 1-4 with increasing distance from the trench 1).For convenience,these first-order ridge segmentsare (barbed vertical line). Fracture zones (PZ) are shown as dashed numbered 1-4, with increasing distance from the trench. De- lines. Propagating rift (PR) is shown. Dredging and wax cor- tailed descriptions of the morphology of each segment are ing sites along each segment are as follows: segment I (squares), segment 2 (triangles), segment 3 (circles), segment provided elsewhere [Karsten a al.. 1996; Sherman a al.. 4A (crosses), segment 4B (diamonds), off-axis sites (small 1997]. Volcanic rock and glass fragments were recovered at solid circles). Solid symbols along the ridge indicate samples 52 sites;all sites were located in the axial valley, with the analyzed for helium isotope composition.Inset shows re- exceptionof threesiteswhichsampled twonear-ridge gional location of the study area (box): East Pacific Rise seamounts (stations 24 and 55) and a short elevated off-axis (EPR); Pacific-Antarctic ridge (PAR). ridge east of segment I (station 12). STURM El' AL.: He-Pb-Sr-Nd ISOTOPES IN SOW] tERN CHILE RIDGE LAVAS 5099

]nitial trace element results showed that a large number of U, Th, and Pb abundances analyzed by ICP-MS and reported lavas from the southern Chile Ridge are unlike the majority of elsewhere [Klein and Karsten, 1995; F. M. Klein et al., manu- MORE or most ocean island basalts (0113) sampled previously script in preparation, 1999] and He contents determined both [e.g., Klein and Karsten, 1995]. Specifically, samples from by crushing and fusion of the remaining powders. As a whole, segments 1 and 3 extend well outside the MORE field toward the Chile Ridge lavas are unusually diverse in isotopic com- low Nb/U, Ce/Pb, and Rb/Cs and high ThfLa, compositions position,encompassing much of thevariabilityamong commonly associated with arc lavas (Figure 2). In addition, MORB worldwide, In addition, each ridge segment exhibits lavas from segment 4 extend toward low Ce/Pb and high unique isotopic systematics. Tb/La but at MORD-like values of Nb/U (Figure 2). Major elementsystematicssuggestthatboth normalMORB 4.1.Segment 1 (N'MORB) (JQTi c 0.15) and enriched MORB (EMORB) (ICTl'i >0.15) collected along all four segments have experienced Although segments 1 and 3 display similar trace element variable extents of low-pressure fractionation but have been systematics (Figure 2), the two segments have markedly dif- generated by relatively uniform extents and pressures of melt- ferent isotopic characteristics. Despite their wide variability ing [Sherman et al.,1997]. The absence of systematics in in trace element compositions extending from NMORB Ce/Pb melting parameters with distance from the trench suggests (26) in the norihtoarc lava-like Ce/Pb (13) nearest the that mantle temperature and spreading rate (and not spreading trench, segment I lavas have a restricted range in Sr (8'Sr/87Sr ridge subduction) exert primary control on the southern Chile 0.7027), Nd ('4'Nd/'"Nd-' O.5L310), and Pb isotope com- Ridge thermal regime [Sherman etal.. 1997]. position(e.g.,206Pb/20'Pb=18.2-18.4;Figure 3).Further- more, they exhibit uniform and MORB-like 3HePHe (7.94- 8.03 RA is atmospheric ratio of I.39X10'; Figure 3b). Methods Compared to MORE worldwide, segment I lavas generally lie Pb, Sr. and Nd isotope ratios were determined on 21 basalt in the region of overlap between Atlantic-Pacific MORB and glass samples (Table 1). hand-picked under a binocular micro- Indian MORE, i.e., they have slightly elevated 208Pb/204Pb scope to avoid alteration and phenocrysts. To remove possi- and 2olPb/2MPb for a given 2°'Pb/204Pb (Figure 4). ble surface contamination, samples were mildly leached for 10 mm. in weak HNO3 and rinsed ultrasonically in deionized 4.2, Segment 3 water. Sr and Nd analyses were performed on -250 mg of sample following standard cation exchange techniques [Hart and Segment 3 lavas form a dramatic north-to-southspatial Brooks,1977; Richard ci at,1976]. Pb isotopeanalyses gradient in isotope compositions (Figure 3). They range from were performed on a separate aliquot of -250 mg of sample, NMORB compositions in the north (e.g., 206Pb/204Pb =18.3, using a chemical separation technique patterned after that of (8?Sr/B7Sr =0.7025, 3Fie/4He =7.84 RA, Ce/Pb=23.8) to highly Manhes et at [1978].All Sr, Nd, and Pb isotope measure- radiogenic and enriched values in the south (e.g.. 20Pb/204Pb ments were performed on a Va-SECTOR 54 mass spectrometer =19.5, (Srf87Sr =0.70407, Ce/Pb=13.5) and notably extend at the University of North Carolina at Chapel Hill. A few to the lowest HePHe yet measured in MORE (3.5 RA). Art samples were spiked with a mixture of enriched 50Nd, '49Sm, off-axis seamount (D55-5) located near the southern end of 87Rb, and 4Sr for isotope dilution determinations of Nd, Sm, segment 3 displays an isotopically extreme version of the Rb, and Sr; these results agree well with inductively coupled segment 3 enrichment (Table I). Compared to oceanic basalts p'asma mass spectrometry (ICP-MS) determinations [K!ein sampled elsewhere, segment 3 lavas show some Pb-Sr-Nd iso- and Karsten, 1995; E. M. Klein et al., manuscript in prepara- topic affinities with lavas from the Society Islands [Devey ci tion, 1998]. Blanks and standard measurements are reported al., 1990; Clzauve! a al., 1992; Pa!acz and Saunders, 1986; in the Table I footnotes. Staudacher and A1!ègre, 1989], although they extend to more Helium isotope ratios were determined on a subset of 1 4 radiogenicPb isotopevaluesand haveunusuallylow samples (Table 1; sample locations shown in Figure 1). Large '43Nd/"4Nd for a givenSr/Sr compared to Society lavas glass chunks (-50 mg) were hand-picked under a binocular [I-tart, 1986] (Figure 4a). The closest known hotspots to the microscope and were ultrasonically cleaned with ethanol and study area, manifest in the islands of Juan Fernandez and San deionized water. 150-250 rug of each sample were loaded into Felix located more than 1000 km north, possess Pb isotope a sta.inJess steel canister and crushed in vacuum using a sole- compositions similar to the more radiogenic basalts of seg- noid-actuatedpistonmade of magneticstainlesssteel. ment 3 but, in contrast, have high 3He/4Hc (up to 18 RA [Far- Crushed powders of 10 samples remaining from these analy- Icy et al.. 1993]), a signature which is absent along the scs were sieved to <100 .tm and melted in a resistively heated southern Chile Ridge. furnace. All measurements were performed in the laboratory of J. Lupton at the National Oceanic and Atmospheric Adrnini- 4.1 Segment 2 stration (NOAA) Pacific Marine Laboratory in Newport, Ore- gon, on a double collector mass spectrometer designed for The limited sampling of segment 2 displays a north-to- high-precision helium analyses as described by Graham ci al. south trend from modestly higher 87Sr/7Sr and 206Pb/2°4Pb [1990]. Blanks were run before every analysis and were neg- (e.g., S1Sr/$Sr =0.7030) and lower '43Nd/"Ndand 3HeeHe ligible (see Table 1). (e.g., 3HetHe =6.4 RA) in the north to values more typical of NMORB in the south (e.g.. SSr/WSr =0.7025,3Fle/41-le =7.5 Results RA). The relatively more enriched compositions in the north resembLe the enriched chemicalcompositionof southern He, Pb, Sr, and Nd isotope compositions of southern Chile segment 3 lavas (Figure 3) and may indicate some contamina- Ridge glasses are presented in Table I and Figures 3 and 4; tion of northern segment 2 subridge mantle with a source also included in Table I for convenience are Rb, Sr, Sm, Nd, similar to that of southern segment 3. As a whole, the seg- Table 1. Isotopic and Elemental Data for Chile Ridge Glasses and Local Surface Sediment Segment 1 Material glass 45'54.60'Lat., 'S Long.. 'W Depth, m 3218 PbPPh 20'Pbftb 'PbrPb 37.968 'SrrSr dl4Nd/HNd (RJRA) 'H&Ue 117Sr Pb 0.341 Th U Rb 13.01 Nd Sm [He] ((icmCrushed SWIg) 6.41 Melted D20-1Dl5-3D14-9Dl0-5 I glass 46'02.43'45'57.19'45'57.l9' 75'53.80'7556.50'75'56.06'75'57.06' 330432983248 18.39818.270l8.I6518268 15.55315.50015.51915527 38.17937.87037.912 0.7027190.7026710.702668 0.513113 0.5131490.513119 7.988.037.94 124133123 0.7670.3320.619 0.4970.132 0.1310.0330.091 4.452.891.02 9.338.02 3.062.814.48 5.466.53 2.115.76 1)35-2D34-1033-3 2 glassglass 45'50.01'45'46.88'45'42.55' 76'45.75'76'46.79'76'52.12' 411637303993 17.84317.930 8.044 15.49015.531 5,490 37.50l37.68938.078 0.70248l0.7026740.703002 0.5132050.513162 7.466.39 23l 8866 0.4360.3150.305 0.0780.i0.236 17 0.0230.043 0.322.621.11 8.368.576.90 2,643.152.54 0.422.78 043-1042-4041-1 3 glass 45'38.77'45'38.3l' 77'53.35'7750.76' 271829092874 19.48919.103 15.67515.68815.642 39.43139.55639.099 0.7040720.704164 0.5127090.512669 3.5! 255292 1.3131.266 1.4411.223 0.3790.341 12.3413.65 12.5614.32 3.353.86 0.48 3.30 D53-2D51-!047-1 3 glass 44'32.33'44'47.45'45'l4.54'45'30.35' 7818.91'78'16.56'7806.48'77'57.76' 352335633236 18.30718,78719.282l9.380 15.54115,58015.662 38.03238.63239.286 0.7024870.7030480.7036920.703750 0.512720 0.5131460.513043 7.846.604.393.79 214275103107 0.3480.5220.8781.085 0.1750.4151.0061.282 0.0490.1080.2630.330 10.769.021.353.43 10.2911.468.009.02 2.743.012.993.16 14.273.830.214.18 5.970.435.650.28 D61-1059-7D58-4 4A glass 44'27.81'44'45.99'44'57.10' 8210.82'82t0.48'8156.20' 406027033798 1813318.40118.213 15.49615.64915.560 37.71338.44638.133 0.7025540.7031870.702519 0.5131880.5130070.513153 7.628.41 1849985 0.3220.3010.286 0.1660.5550.192 0.0610.1320.044 4.121.311.17 10.267.427.50 3.672.522.80 7.982.38 0.711.18 D65-1D63-5062-2 4B484A glass 43'26.24'43'52.38'44'11.68' 82'34.63'8225.64'82'14.09 361538533900 18.04017.95318.029 15.48015.52415.487 37.98038.3l437.639 0.7029660.7035520.702452 0.513218 0.512900 7.978.35 139ISO85 0.2741.054 639 0.6190.0841.132 0.1500.2450.020 11.360.446.00 9.348.716.93 2.832.352.59 10.170.38 7.21 D66-2D66-4d12-8 48 wholerock 45'55.l9' glass 43t6.63' 75'54.89'82'38.44' 29l93634 18.28818.04017.987 15.58815.46015.545 38.17937.59038.238 0.7039140.7024270.702996 0.513011 150100155 9.3560.3540.755 8.1340.2840.738 2.1850.0720.167 71.656.301.67 18.2010.449.65 4.243.622.81 z small off-axis ridge24-155-5 east ofChile segment Ridge I glasses(012-8), (segmentsand a surface 1-4) mud analyzed sample In fromthis study segment and 1(024-I).shownseamount in Depth, Figure determinedI are given. byAlso 3.5 included glassmud 45'07,83'45'25.90' 78'40.71'76'0l.36 19801799 18.67419.970 15.59515.710 40.00038.581 0.704548 are analyses of samples from a seamount just west of ihe southern portion of segment 3 (D55-5). a or 12 kHz echo-sounding. is the average for dredge on bottom and off bottom. Isotope 240308 14.7802.257 3.840 3.590 1.6401.211 24.4312.34 25.2818.01 4.256.12 ofreferencedrequiringJolladata 0.06%/amu), areand internallyAmescorrection to the standards,Duplicate following normalized agree Pb respectively.tovalues withinisotopeto 0.11940 for 0.005% NBS-981;measurements For for one of corrected batch16.937, ofwere Sr15.491 values). isotopemade and onMeasured analyses, 36.721 separate for higheranalytical dissolutions NBS-987 uncertainties of values two samples were (2a/'IN) obtained, (averages for Sr andrequiring Nd isotope SrP8Sr, and 0.72190 for 'Nd/'44Nd. Replicate standard analyses averaged 0.710243 for 87Sr/56Sr for NBS-987 and 0.511855 and 0.512136 for '43NdP44Nd for Pbf°tb, 2°7PbP°"Pb, and 201PbP°tb, respectively (replicate standard analyses yield a mean mass fractionation correction factor an interlaboraioty bias correction of -0.01% (duplicate measurementsratios are from generally mns not c ±0.000023 and <±0.000015, respectively. Pb data are the La Uncertaintiescorrected:was20Pb/'°4Pb, unavailable, a 207Pb/'°4Pb,subset in 3HeI4He in ofwhich ten andrepresent samplescase iotPbPolPb, a chemically was the 2aalso respectively. quadrarure measuredsimilar sample sum Maximumfor helium of from the blank in-runtheconcentration same values ion countingdredge for by Sr. melting.was Nd,uncertainties usedand Blanks Pb (D 104. onwere the014-8. 3He 033-I. beam, 1)51-3. plus uncertainties 053-I). AU heliumdue values were are 0.8, 0.2. and 0.7 ng. respectively. Helium was measured on the sample indicated except whererun before sufficient and glass after each sample, and samples were run only when blank variabilities were below 10%. are reported) and agree to 0.02-0.08%/amu. 0.02-0.119W amu and 0.04-0.1 l%/amu for to blk variability and reproducibility of air standards, and are always measured by in vacuum cmshing and have been blank Shermandissolutions,CornellD61-1<±0.06. University etal.are Helium is 8.40 [1997).±1-3% RAconcentrationsin the forand Ndlaboratory 8.42 and RA Sm were and ofand W. determined of ±1-6% While020-I forareand byother 7.98 at peak Lamont-Doherty elements.RA height and 7.96 comparison Complete RA; Earth averages major to Observatory, calibrated and are trace reported. volumes elementfollowing Trace of data airsample element standards.for representative preparation (ppm) Analaytical analyses samples from each ridge atuncertainty Duke University: in 'liewere peak reproducibilityperformed height is by<1%. inductivelyfor DuplicateICP-MS coupled analyses, helium plasma isotope as judged mass measurements spectrometry by replicate of (ICP-MS) at segment are available from Klein and Karsten (1995] and STURM El' AL.: He-Pb-Sr-Nd ISOTOPES IN SOUTHERN CHILE RIDGE LAVAS 5101

44. Segment 4 Seg.1 , a 7, . S.3 S.2 30 ,t .. It.,' o.../ö .'" : MORB Segment 4 lavas as a whole have affinities with the low 25 206Pb/204Pb and elevated 207Pb/20tband 201Pb/204Pb for a 4' given 2°6Pb/'°4Pb often found in Indian Ocean MORB and ow 21) of the Dupal anomaly [e.g., Mahoney et aL,1992; Hart. Seg.4B Seg.4A 1984]. and they trend toward compositions associated with EM 1-type localities(e.g.. Cough Island and Walvis Ridge; ,jyt Figure 4). On the basis of Pb-Sr-Nd isotopes, as well as major S elements [Sherman et aL, 1997], segment 4 forms two geo- Ira...... - chemical provinces: a southern (4A) and northern (4B) group,

6 hounded by a southward propagatingrift (Figures I and 3)

S [Tebbens ef at., 1997]. Segment 4A lavas extend from a depleted end-member toward moderate 20tb/20tb (to 18.4) and very high 207Pb/204Pb (to 15.65; Figure 4) and show isotopic affinitieswith lavas from Gough Island (Figure 4) [Sun, c ' y\., ' 1980]. In contrast, segment 4B lavas extend from a depleted end-member likethatof segment 4A buttoward low I. 206Pb/2°4Pb (to 17.95) and elevated 207Pb/204Pb (to 15.54; Fig- -- ure 4) and have isotopic affinities with lavas from Walvis ...... Ridge [Richardson et at., 1982]. Both segments 4A and 4B -s.., -: have typical MORB I-1eI'I-Ie, and no clear along-axissys- tematics are apparent for either group (Figure 3). The presence of MORB with Dupal anomaly characteristics 1565 tHan. 1984] along segment 4 (43°S-45°S) offers some sup- port for the controversial idea that the Dupal anomaly is in- deed globe encircling in extent [Castillo, 1988]. While lavas with Dupal characteristics occur sporadically outside of 30- 60'S (Shirey et at, 1987; Graham a at, 1988; Rotten ci at, 1994], previous work has shown it to be most prevalent in this latitudinal band. In our sample suite the Dupal character- 0 istics occur only in samples from segment 4, and previous sampling of the northern Chile Ridge at 37°S shows no evi- 0 703 : dence of this anomaly [Bach et al.,1996].Therefore, al- 0.703 though segment 4 provides evidence for the occurrence of the

0 702 Dupal anomaly in the subridge mantle of the eastern Pacific, sampling to date suggests that the anomaly is not widespread.

f A 'C \ . 5. Discussion '-S Each of the four transform-bounded segments along the southern Chile Ridge is compositionally uniquc. Thcsc spa- tial systemalics suggest that the mantle in this region is het-

.4 erogeneous with respect to lie, Pb, Sr. and Nd isotopic com- 0512 position and, by inference, to time-integrated U/He, U/Pb, N PR N N Th/Pb, Rb/Sr. and Sm/Nd ratios, on a scale at least as small as C that of the ridge segments ('50-l0O km; Figure 1). Further- a more, within each ridge segment, there exists a range of compositions which demonstrate an even flner scale of source Latitude ('S) heterogeneity. Figure 3. Latiwde versus (a) Cc/Pb, (b) 3He/41-Ie (Ri RA), (c) 5.1.Segment1 El5/65, 206Pb/204Pb, (d) '°7Pb/20Pb, (e) and (1) '43Ndt'Nd for Klein and Karsten [1995] showed that segment I lavas dis- Chile Ridge glasses (symbols as in Figure 1). Locations ofplay trace element signatures which suggest incorporation in the trench, fracture zones, and propagating rifts (PR) relative to the ridge segments are noted. Fields shown are for MORB their source of recycled, continentally derived material. The [Hofitwnnetal.,1986; Hofmann, 1988; Sun and heavily sedimented nature of this ridge segment due to its lo- McDonough, 1989; Lupton and Craig, 1975]. Analytical un- cation immediately adjacent to the trench raises the possibil- certainties (±2c) that exceed symbol size are shown at right. ity that the observed recycled trace element signature (e.g., low Ce/Pb, Nb/U; Figure 2) may result from incorporation of ment 2 lavas lie in the region of overlap between Atlantic- ocean floor sediment during magma residence or upon erup- Pacific MORS and Indian Ocean MORB, although they extend tion. In order to evaluate the feasibility of contamination via to lower 206Pb/204Pb (to 17.8) and slightly elevated 87Sr/57Sr, assimilation we have measured the isotopic and trace element

205Pb/'04Pb andw?Pb/2NPbcompared to segments I and 3 and composition of a nearby surface sediment (D24-l) and used it thus extend into the Indian Ocean MORB field (Figure 4). as the assimilant in a simple assimilation-fractional crystal- 5102 STURM El' AL.: He-Pb-Sr-Nd ISOTOPES IN SOUTHERN CHILE RIDGE LkVAS

05332

I 05130 I / El ,'IF?SF _____' I 3 0.7050-c (ocigh) 05127 7-..--,' / I 2: ¼ en / ./ 07040 / 0.5115 Seg. 40/ 44 1mb.. .M0RB 0.7030 05122 Se& 0.7030 07040 07050 U 71X%1 SrISr It-PAC MORB 2 '4" 18RA

Sokfly Mthclja 15.65 LU ¼) Sc4A AItPAC MOSS Seg. 4B 1560

MORe. Se 6 Se2 Gough / - -. -'I I Tcathtia 15.50 soc4 RocarI Srni 15.45

ATL-MCMOSS

39.5

0.5 130 'C 7 39.0 3 .3 SegA 05128 38.5 2: "N en NN' -I -.3. 38.0 N f 05125 INSowly' ,_ Goug) )/-.____I 37.5

37.0 17.5 18.0 18.5 19.0 19.5 20.0 15 18.0 18.5 19.0 19.5 20 0 206PbI204Pb Pb/Pb Figure 4.(a) t7SrI16Sr versus '43NdJ"4Nd and 20°Pb/204Pb versus (b) 3Hef'He (R/ RA) and (c) '43Nd/"4Nd, (d) "Sr/16Sr, (e) 207Pb/2MPb and (1) 201Pb/204Pb for Chile Ridge glasses (symbols as in Figure 1). Also shown are fields for Atlantic-Pacific MORB and Indian MORB, Gough Island, Walvis Ridge, Juan Fernandez Islands (IF), San Felix Islands (SF) and Society Islands (dashed lines). Data sources include Sun [1980], Gerlach eal. [1986], Devey etal. (1990]. Richardson a al. [1982], Hofinaun et al. [1986], Hoftnann [1988], Hart and Zin- dler 11989], Sun and McDonough [1989], Sraudacher and Al!ègre [1989], Chauvel et at. [1992], and Farley et al., [1993]. Analytica] uncertainties (±2a) that exceed symbol size are shown. lization (AFC) model [DePaolo, 1981; Reagan ci al., 1987]. (e.g., Rb/Cs versus Cs suggests -10% crystallization with Results of the AFC calculations over a range of rates of as- R,IR=0.O1, while Nb/U versus Nb suggests -70% crystalliza- similation to crystallization (R,IR=O.0l to 0.9) fail to repro- tion with R,/RC =0.9; not shown).Similarly,assimiLation duce the segmenL 1isotopic trends (e.g., Figure 5a) and in (with fractional crystallization) of bulk marine sediment is terms of trace elements form trends which, although they re- also unable to reproduce the segmentItrends (Figure 5a). produce the segment 1 lavas on individual plots,require in- These findings are in agreement with the conclusion of Klein consistent parameters depending on the elements involved and Karsten [1995], based solely on trace element arguments, STURM El' AL: He-Ph-Sr-Nd ISOTOPES IN SOUTHERN CI-ULE RIDGE LAVAS 5103 that contamination by bulk marine sediment does not repro- ing the modeling approach of Klein and Karsten [1995], we duce the geochemical trends observed along segment 1. have explored a simple contamination model involving sub- While synvolcanic assimilation does not explain the geo- duction of sediment (see Figure 5 caption) and altered oceanic chemical trends observed along segment 1, the trace element crust, mixing of this recycled material with depleted mantle to signatures of segment I lavas strongly suggest contamina- form localized heterogeneities,and subsequent mixingof tion by recycled materials (Figures 2 and 5a and 5h). Follow- melts from the contaminated mantle with MORB melts. These model results reproduce both the trace element and isotopic trends observed along segment 1(least well reproduced is the

19 57Srt7Sr trend, which may require a higher MORB 87Sr/87Sr de- Averagebulk pleted end-member; Figure Sc). This suggeststhat only a marine sediment small amount (-2%) of a recycled contaminant (consisting of MS.509k; Ra/Rc-0.9 88 -20% sediment and -80% altered crust) mixed into the de- -Local pleted mantle is required to explain the segment Itrace ele- Sediment ment and Pb, Sr, and Nd isotope trends. 24405 - S Segment 186- I lavas have relatively high3He/41-le values (-8 2 RA). The timing of the recycling event for segment 1 can be roughly constrained through the 3He/4He systematics because this ratio can decrease over relatively short timescales due to 8,4 / the decay of U and Th. The high 'HetHe ratios of segment 1, Contaminated \\Ra/Rc=00I Mantte Melt therefore, can be used to estimate the amount of time which -.5 may have passed since recycling without producing a signifi- 8.2 - Segi , cant reduction in 3HetHe. This calculation requires knowledge of the U/He ratio of the source which we estimate from the U a MORB and [He] contents of the most enriched segmentI sample 18 0 I0 20 30 40 50 60

Nb/U Figure5. Evaluation of two contamination models for segmentIglasses(squares). (a) Nb/U versus 2ooPb/20tPb; Ce/Ph versus (b) 206Pb/204Pb and (c)7Sr/86Sr. In Figure 5a, AFC trends using local sediment (D24-l; dashed lines) or bulk b marine sediment (MS Ben Othman e: al., 19891 dotted line) as assimilant with variable rates of assimilation to crystalli- 19 5 zation (RJR =0.01 and 0.9; labeled); percentages of crystallization are shown. Note that lines for RJRç =0.01 and 0.9 overlap for MS case and 50% crystallizationis shown for Seg3 each RJR . Solid bold lines represent mixing (percent indi- '9 cated) between a 10% MORB melt (oblique ruled oval) and a Altered 10% melt derived from a mantle contaminated with 2% re- Nazca Plate Ocean Sedment Crust cently subducted contaminant (consisting of 20% sediment and 80% altered oceanic crust; oval labeled CM). For recycling model calculations, Nb, U, Ce, Pb. Sr, 87Sr/86Sr. and 18.5 206Pb/2Pb in mantle source are 0.269, 0.005, 0.775, 0.031, 12, 0.70248, and 18.15, respectively [Sun and McDonough. 1989; Siolper and Newman. 1994; Hart and Zindler, 1989; Seg I MORB Hart and Staudigel, 19891; in subducted sediment they are 10, 1.6, 73, 20. 200, 0.71002, and 18,55, respectively [Unruh 0.7036 and Tajsu,nofo, 1976; Ben Othman et al.,1989] (and local sediment D24.-l); in altered crust they are 1.22, 0.275, 6.19, 0.23, 168, 0.70400 and 18.6. respectively [Staudigel el al., 07034 1995; Chauvel et al., 1992; I-Ian and Zindler, 1989; Dasch, 19811. Note that a marine sediment of intermediate composi-

2 07032 (ion compared to the global range [Ben Othman et al., 1989; Plank and Langnuir, 1993; Plank and Ludden, 1992] and consistent with measurements of local sediments was used in 07030 these calculations because full trace element analyses are not available for the sediment section being subducted in this region [Plank and Langmuir, 19981. Also shown in Figure 5b 0.7028 arc fields for Nazca plate sediments and altered oceanic crust [Unruh and Tatsumoto,1976;Dasch,1981; Sunand 0.7026 McDonough, 1989; Stolper and Newman, 1994]. Bulk distribution coefficients for Nb, U. Ce, Pb and Sr for melting calculations and AFC models are 0.0085, 0.008, 0.011. 0.015, and 0.015, respectively [Green, 1994; McCulloch and Gam- 0 0 '5 20 25 30 ble, 1991; Sun and McDonough, 1989; Stolper and New,nan, 19941. Analytical uncertainties (±2a) that exceed symbol size Ce/Pb are shown. 5104 STtJRM FT AL.: He-Pb-Sr-Nd ISO WPES IN SOUTHERN CUILE RIDGE LAVAS

(D20- 1), adjusted for melting and degassing [afterFancya at,1992]. Sample D20-1 has a total[I-Ic] content (crushed plus melted) of 7.57 picm3 STP/g and a U content of 0.13 ppm (Table I). If we assume that these measured abundances result from at least 10% partial melting[Sherman ci at,19971 and from -90% degassing during upwelling, residence, and erup- tion, the contaminated mantle source would have a U content of -001 ppm and a ti/He ratio of -1700, a value consistent with the estimate ofHart and Zindler[1989]. Given this U/He ratio, a mm ratio of 3.8, and an assumed initial 'He/He ratio of 8 RA, a significant reduction of the 'HeI4He ratio (to <7.94 Foundering of slab material RA) would occur in -20 Myr. A more conservative estimate of b 50% degassing yields a reduction of the 3HePFle in only -2 Myr. An extreme value of 99% degassing would produce a significant reduction in 'He/He in -200 Myr, hut this would imply a mantle [He] near 75 jscm3/g, even higher than the highest values found in mid-ocean ridge popping rocksI&irda and Graham,1990]. We infer that relatively short timeseales (106 to 10' years) are involved for introduction of recycled components to the subridge mantle, suggesting that the un- Communication between subarc mantle and subz-ldge mantle through a slat, window usual enriched sigiiature in the segment I lavas may derive from contamination from the adjacent subduction zone. The fact that the adjacent subduction zone is a plausible source of recycled material for segment 1raises the important question about how these contaminants have been introduced into the subridge mantle. There are several possible mecha- nisms which may produce this result, and we can only specu- late on which may occur here. First, as a young, hot, buoyant ridge approaches a trench, resistance to subduction may cause the subducting lithosphereto break up[van den Bucket, 1990], and as a result, portions of the slab (or slab-derived components) may founder or be sheared into the suboceanic Melt focusing along a slab window mantle underlying segment I(Figure Ga). Second, continued spreading at the ridge following subduction may occur with- Figure 6.Schematic diagrams of a subducting spreading out magrnatic accretion and may lead to the development of a center that show possible modes of contamination of the

"slab window" within a subduction zone[Dickinson and Sny- segment 1 mantle. (a) incorporation of slab-derived material der,1979;Thorkelson,1996;Ramos and Kay,1992]. In this into the melting regime beneath segment 1 as a result of case, materiat derived from the subdueting lithosphere or from foundering alter slab breakup as postulated byvatt den flue/ce! the metasomatized mantle wedge may become entrained in [1990]. (b) A plan view (left) of development of a "slab window"[Thorketson,1996] and (right) a cross-sectional view lateral mantle flow feeding upwelling under the segment I ridge axis (Figure Gb), possibly carried by shallow, westward which illustrate communication that may occur between meta- asthenospheric flow which has been postulated for this re- somatized subarc mantle and the mantle beneath segmenLI. Large gray arrows (in Figures 6a and 6b) represent shallow as- [Parmentier and Oliver,1979].Third, the subducting gion thenospheric mantle flow postulated for this region[Parmen- spreading center may act to focus rising melt (which has been tier and Oliver,1979]. (c) A slab window occurs and focuses contaminated with slab components) back toward the trench, melt upward toward the trench as melt rises tofill the void in a tunnel formed on the seaward side of the subdueting litho- formed by continued spreading. This melt incorporates recy- sphere (Figure Ge). This latter mechanism may be most plau- cled materials (perhaps in the form of contaminated fluids or sible because the most enriched samples along segment 1 lie melts) as it migrates upward along ridges and transform faults closest to the trench. tn any case, the mechanism must trans- on the trenchward side of the slab and ultimately contami- port materials from the subarc mantle or the subducting litho- nates the segment I mantte source. sphere to the suboceanic mantle on timescales of tens of mil- lions of years given the helium isotope arguments presented above. Hilton etal.,1992;Graham ci aL,1993]. In the case of ocean islands, low ratios may result from pre-eruptive or post- 5.2. Segment 3 eruptive degassing and subsequent 4He ingrowth, rather than low 3HeT4He in the mantle source[Condomines ci at.,1983; One of the most striking aspects of the chemical varia- Graham ci al., 1988; Hilton ci al., 1995; Zindler and Hart, tions along segment 3 is the low helium isotope ratios (3.5 1986;Hart and Zindler;1989;Seaudacher and Allègre, 1989]. RA compared to MORE of 7-9 RAIKurz and Jenkins,1981; For segment 3, however, the strong correlation of 'HetHc Graham ci al.,1992b]. Low 'lie/lie ratios such as these, al- with all other radiogenic isotoperatios measured and with though not previously observed in MORE, have been found latitude, as well as the "zero age" of the lavas (rendering any in some ocean islands and in island arc lavas and continental post-eruptive ingrowth of 4He insignificant),suggests that hasalts [e.g.,Gro.ha'n etal.,1988;Hilton and Craig,1989; the low 'HeI4He of segment 3, like the radiogenic Pb, Sr and STURM El' AL.: He-Pb-Sr-Nd ISOTOPES EN SOUTHERN CHILE RIDGE LAVAS 5105

Nd isotope ratios, is an inherent characteristic of the enriched lands[e.g.,Chauvel et al.,1992; Hé,nond et at.,1994; end-member in the segment 3 mantle source. White, 1985; White and Hofinann, 1982] (Figure 4). Klein and Karsten [1995] noted the low Ce/Pb, Nb/U, and Modeling of the segment 3 variations in a manner similar Rb/Cs of segment 3 lavas and showed that like segment I,in- to that developed for the Society Islands [e.g., Chauve! et al., corporation of a small amount of subducted sediment and al- 1992; Devey et al.,1990; Staudacher and At!ègre, 1989] tered oceanic crust into the depleted MORB source can explain shows that the observed segment 3 trends can be reproduced their unique trace element signatures (Figure 2). The isotope by recycling a small amount of altered oceanic crust plus ter- variations can be used to further examine the feasibility of ngenous sediment (e.g., -8-15% contaminant consisting of this model and to place constraints on the age and origin of -3% terrigenous sediment and -97% altered crust) to the man- the contamination event. The isotopic characteristics of the tle at -.2 Ga [e.g., White and Hofmann 1982; Chauvel et at., enriched end-member in segment 3 lavas,particularly the 1992; Weaver, l99la. l991b; Klein and Karsten, 1995] (Fig- high 207Pb/204Pb, suggest that the contaminant is old: because ures 8 and 9). In contrast, mixtures of ancient pelagic sedi- of the relatively short half-life of 235U, 207Pb/204Pb ratios were ment (in amounts >1%) with altered crust (Figure 8 and 9) and largely determined early in Earth's history [e.g., Ben Othman young mixtures of either pelagic or terrigenous sediment with et at., 1989; White, 1993]. Therefore high 207Pb/204Pb of the altered oceanic crust (Figure 8) fail to reproduce the segment 3 segment 3 enriched lavas must have been produced during a trends. These models assume bulk addition of recycled com- period of U/Pb enrichment early in Earth's history. The components, rather than selective contamination as a result of bined isotopes and the trace element systematics suggest two processing within the subduction zone, because our under- end-member scenarios, which differ in where radiogenic in- standing of how the composition of material returned to the growth occurred and when the contaminant was introduced mantle may differ from what is subducted is insufficient[cf. into the mantle. At one extreme, the segment 3 signature may Stolper and Newman, 1994; Plank and Langmuir, 1998].Re- result from contamination by ancientlysubducted altered cent work on volatiles on these samples, for example, sug- ocean crust (with high U/Pb) [Chauvet et al., 19921 and sedi- gests that a hydrous siliceous melt may be the phase in which ment (with low Ce/Pb) [Chauvel et at., 1992; Ben Othman et recycled components were transferred to the segment 3 mantle al., 1989] followed by ingrowth during residence in the man- source [Sherman, 1998]. tle. At the other extreme, the segment 3 signature may result The broadly defined age constraint for the segment 3 man- from contamination by more recently recycled material de- tle contaminant (-2 ± 0.5 Ga) derives from the Pb isotopes rived from an old continental source [White and Dupré, 1986; (Figure 8a). Helium isotopes provide little constraint on the Ben Othman et al., 1989; Devey et al., 1990; Hanan and Gm- timing of ancient recycling events because the 3HetHe ratio ham, 1996]. changes rapidly over these timescales and because the U/He The latter hypothesis (recent recycling of old continental ratio of the mantle is poorly known [e.g., Hart and Zindler, material) can be evaluated by examining the Pb isotopic 1989]. Nonetheless, model results using estimates of the compositions of nearby materialsavailablefor recycling. mantle U/He ratio through time (Table 2) [Hart and Zindler; The isotopic compositions of sediments from Deep Sea Drill- 1989] and using U/He values that vary over 3 orders of magni- ing Project (DSDP) Hole 319 on the Nazca plate [Unruh and tude to bracket the uncertainty are broadly consistent with the Tatsumoto, 1976] and along segment I (D24-l; Table 1) do -2 (± 0.5) Ga timing of the segment 3 recycling event ob- not have sufficiently radiogenic Pb isotope ratios to act as an tained through the Pb isotope modeling (Figure 8b). appropriate contaminantfor segment 3 lavas(Figure 7). In the context of our preferred model of ancient recycling Similarly, materials which may be eroding into the southern for segment 3 the fact that the contaminant has retained its and central Chile Trench, including basement material from chemical integrity implies that the segment 3 contaminant southern and central South America, and various lithologies experienced isolation from convective stirring in the mantle including Andean volcanic rocks and cerrigenous sediments for most of its history. The regular along-axis variation and also fail to provide a source with sufficiently radiogenic Pb strongly binary nature of the trends formed by segment 3 Ia- isotopes (Figure 7) [Kay et al., 1996, Aitcheson er al., 1995; vas suggest that the contaminant is discrete and relatively Tilton and Barriero, 1980; Barreiro and Clark, 1984; Allègre homogeneous on a scale sampled by melting. The heteroge- et at.,1996;J.Tarney, personal communication.1998]. neityunderlying the southernmostportionof segment3 This finding does not preclude incorporation of material from therefore appears to have a history which began more than a a more distant source and subduction zone which is recycling billion years ago, with subduction of a minor amount of tern- sediments containing a large proportion of old continental genous sediment and altered oceanic crust; thereafter,itre- material [e.g., White et al.,1985; White and Dupré. 1986] mained isolated from mantle convection, perhaps stored as a (Figure 7 inset) or, alternatively, from the southern South raft of material in a boundary layer at depth [e.g., McKenzie American lower crust (of unknown but presumably not "an-and O'Nions, 1983; McCuttoch, 1993; Hanan and Graham, cient" composition) by a process such as tectonic erosion of 19961. In the relatively recent past this small plum of mate- the basement by the subducting plate [e.g., Cloos, 1993]. rial probably rose diapinically through the upper mantle and Thus, while we cannot rule out a scenario involving recent re- emerged beneath the southern portion of segment 3. Distur- cycling of old cratonic material, the lack of a nearby "smok- bance of the boundary layer in which this blob was originally ing gun" of appropriate composition renders this less likely stored may have been initiated by pulsation from the nearby than the alternate scenario in which the segment 3 chemical subducted plate [Larsen el at., 1996]. variations are derived from an ancient recycling event (with ingrowth of radiogenic signatures occurring in the mantle). 5.3.Segment 4 This latter explanation is also commonly invoked to explain the similar isotopic and trace element variations in ocean is- Segment 4 lavas differ from segment Iand 3 lavas in both lands with recycled signatures (EMil) such as the Society Is- trace elements and isotopes (Figure 3). In particular, the iso- 5106 STURM El' AL.: He-Pb-Sr-Nd ISOTOPES IN SOUThERN CHILE RIDGE LAVAS

Possible neaihy sources of recycled material (local marine sediment and southern/central South American lithologies)

Amazon River sediments (basement and Andes) Eastern Corditlera, Bolivia

Central Andean Southern S. Am. volcanics, metapelite basement southcni Pent S. Altiplano northern Seg. i basement Altiplano Mud I" '90 95 basement Sediments eroded from Archean basement. w1.northern South America

Xenoliths from Possible distant Precordilleran (old continental) source haseipent. Argenliia of recycled material 37 17 18 19 20

Figure 7.Evaluation of nearby and distant (inset) contaminants for segment 3 lavas (circles): 206Pb/204Pb versus 2ttPb/204Ph for Chile Ridge glasses (symbols as in Figure 1). Also shown are fields for local sediment dredged from the segment I ridge axis (D24- 1; horizontal ruLed field) and Nazca plate sediment from DSDP Mole 319 (horizontaL ruled field) [Unruh and Tatswnoto, 1976]. Fields for possible nearby sources of recycled material derived from southern and central South American Iithologies include Amazon River sediments eroded from basement and Andes volcanics[Al!ègre et al.,1996]; central Andean volcanics from southern Pen[Barreiro and Clark.1984]; xenoliths from Precordilleran basement in Argentina[Kay et al.,1996]; basement material (orthogneiss, schist) beneath the southern and northern Altiplano and the Eastern Cordillera centered in Bo- livia[Aitcheson et al.,1995; Tilion and Barriero,1980]; and southern South American metapelitic basement material (samples provided by J. Tarney). Neither local sediment (horizontal ruled fieLds) nor any known South American lithologies (open fields) that may be currently subducted at the Chile Trench forms a suitable end- member for the segment 3 basalt array (solid circles). Inset shows possible distant source of recycled material derived from northern South American lithologies for segment 3 lavas; field shown for sediments sampled in front of the Antilles arc [White et al., 1985] which eroded from Archean Guiana basement in northern South America [White and Dupré, 1986]. We cannot rule out possible incorporation of sediment from a distal old continental source that was subducted at a distant subduction zone and convectively conveyed to the mantle beneath segment 3.

topic variations reflect an affinity with Indian Ocean and Du- In a recent study of isotope and trace element data for In- pal compositions (Figure 4). The similarity to Indian Ocean dian Ocean MORB.Rehkamper and ffof;nann[1997] con- MORB compositions raises the possibilitythat the unique cLuded that mixing a small amount of pelagic sediment and al- geochemical signatures of segment 4 lavas may result from tered oceanic crust into an Atlantic-Pacific-type depleted man- processes similar to those that produced the distinctive Indian tIe may explain the high 207Pbf204Pb b and 20tPb/204Pb ob- Ocean MORB signathre.Previous studies of Indian Ocean served in Indian Ocean MORB. They confirmed the findings MORB have suggested that their particular isotopic character- of Weaver [1991b] that the low ji of pelagic sediment [Ben istics may bc due to an ancient, basin-wide, contamination Oth.'nan et al.,1989]resultsinsubstantiallyretarded in- event resulting from delamination of continental Lithosphere growth of 206Pb/204Pb relative to uncontaminated mantle with possibly related to the breakup of Gondwanaland leg.. Ma- incorporation of only a few percent ancient pelagic sediment, honey er aL,1992;Hart,1984]. Others, however, have called a model which has also been invoked to explain EMI-type upon contamination via subduction of sediment with altered sources [e.g.,Weauer,199lb;White and Hofinann,1982; oceanic crust followed by homogenizationinthemantle Chauvel etal.,1992]. Since the segment 4 lavas share some [e.g.,1-Jamelin etal.,1986;Price et al.,1986;le Roex etal., of the observed Indian Ocean (and EMI OW) trace element 1989; 1wet al.,1987;Michard et al.,1986;Rehkamper and (e.g., low Nb/U, high Ba/Nb) and isotopic characteristics, we Hofinann.1997]. examined a similar scenario for the development of the two S'WRM ET AL.: lie-Pb-Sr-Nd ISOTOPES IN SOUThERN CHILE RIDGE LAVAS 5107

segment 4 signatures. These results suggest that the segment pected from the relatively low 206Pb/204Pb in the segment 4A 4B systematics may be consistent with a simple contamina- lavas (Figure lOa inset and lOd). This factor of 5 variation in tion model consisting of three stages of isotope evolution U/Pb (0.07 to 0.44) is unlikely to be due to variations in the and mixing of MORB melt with melts from mantle contami- extent of melting or crystal fractionation and suggests that a nated at least a billion years ago with pelagic sediment and al- relatively recent enrichment event, perhaps mctasomatic in tered oceanic crust (e.g., 10% contaminant consisting of 3- origin, occurred in the mantle source but has not yet been 7% ancient pelagic sediment and 97-93% ancientaltered manifest in the Pb isotopes. The high, but more homogenous crust, Figure 10 and Table 3). The higher 206Pb/204Pb (and U/Pb (0.20 to 0.23) of the segment 4B lavas coupled with lower 81Sr/41Sr) of the segment 4A lavas can be explained by a their low 206Pb/204Pb (Figure lOa inset and lOd) suggests that somewhat greater proportion of altered oceanic crust in the the enrichment event also affected the segment 4B mantle recycled component compared to segment 4B lavas (Figure source. Furthermore, the MORB-like 3He/4He ratios (7.97 to 10). Alternative scenarios such as recent recycling of sedi- 8.41 RA) of both segments 4A and 4B coupled with their high ment and altered oceanic crust[Ben Oth,nan et al.,19891, an- U abundances (up to 0.25 ppm; Table 1) suggest that this en- cient recycling of pelagic sediment alone, or a two-stage Pb richment event occurred so recently that it has not yet affected isotopeevolution model (insteadof a three-stagemodel the helium isotopes[e.g.,Hart and Zindler,19891. Recent [White,1993]), fall to reproduce the segment 4 trends. work on volatiles in these samples also suggest that the seg- The trace element variations highlight another level of ment 4 mantle source has been metasornatized by an aqueous complexity in the history of the segment 4 source. Notably, fluid[Sherman,1998]. the most isotopically enriched segment 4A samples have ex- Although the helium isotope compositions are similar for tremely high U/Pb (up to 0.44), while low U/Pb would be cx- both segment Iand segment 4 (Figure 3), itis important to emphasize thatthe segment 4 inter-isotopictrends,as a whole, differ significantly from those of segment I(Figure 15.90 4). In the case of segment I, all the isotopic and trace element evidence points toward a simple explanation of recent recycling of locally subducting sediment and altered oceanic crust.

2.5Ga

Model B: Figure 8.20PbI204Pb versus (a) 27Pb/204Pb and (b) 3He/4He Terrigenous Sediment ," (R/ RA) for segment 3 lavas (circles). Modeling results show + Altered Ocean Crust,' that Pb isotopes (Figure 8a) require the timing of the recycling event for segment 3 to be -2 (±0.5) Ga. 3HetHe (R/ R4) "2 Ga, (Figure 8b), modeled using U/He values in Table 2[Hart and Zindler,1989], are also consistent with the -2 Ga time since " ..w/ Seg.3 recycling (see text). lop inset shows two-stage model for in- Model A: growth of 3HeJ4He and 206Pb/204Pb for primitive mantle (PM) Pelagic Sediment from -4.4 Ga to 2.0 Ga (stage 1) and for altered oceanic crust 15.60 + Altered Ocean Crust ')tage1 (AC), terrigenous sediment S) and pelagic sediment (PS) from 2.0 Ga to present; model parameters are given in Table 2. At the beginning of the second stage, altered oceanic crust 2.5 Ga with sediment is formed and subducted resulting in reduction Stage 2 of 3He/1Hc. Note that the "two-stage" terminology 1 Ga MI RB is some- III 4444 44 what misleading because the crustal component reflects a mul- MORB tistage evolution that is not considered in this model. Solid b circles show the present composition of aged pelagic and ter- Seg. 3 DM. rigenous sediment and altered oceaniccrust. Bottom inset Stage2 Mixmg. shows detail of top insetfor mixing systematics(dashed ', lines) for producing contaminated mantle (model B) shown in Figure 8b. The contaminant (open circle labeled 3%) is a mixture of 3% temgenous sediment and 97% altered oceanic crust. AC The contaminated mantle (shaded oval labeled 8%) is produced by mixing 8% of the terrigenous sediment-altered oceanic crust mixture with the depleted mantle (oblique ruled oval). - Using these mixing proportions, different compositions of - I Ga contaminated mantle result from variations in the time since I IGa the beginning of stage 2 and are shown in (Figures 8a and 8b) .5 Ga with aging of recycled material for 1, 1.5,2, or 2.5 Ga (insets 2Ga 1.5 Ga show ingrowth and mixing for 2 Ga example; thus the shaded oval in bottom inset is equivalent to the shaded oval labeled 2 2.5Ga 2Ga j Ga in (Figure 8b). The calculated trends (models A and B) in - Model A: Model B: 2.5 Ga - (Figures 8a and 8b) show mixing between a 10% melt derived Pelagic Sediment Terrigenous Sediment from mantle contaminated 1-2.5 Ga (labeled shaded ovals) and + Altered pecan .C!u.1. + Altered Ocean Crust 2 a 10% MORB melt (oblique ruled oval). Bulk distribution co- 175 8 18.5 9 19.5 20 efficients for Pb and He for melting calculations are 0.015 and 0.008, respectively[Stolper and Newman,1994;Marty and Pb/Pb Lussiez,1993]. 5108 STURM El' AL.: He-Pb-Sr-Nd ISOTOPES IN SOUTHERN CHILE RIDGE LAVAS

a MORE MORE b

A-B Terrigenous eff 3 Sediment + A Altered Crust Bulk Sediment Pelagic + Sediment Altered Crust + Altered Crust C-D A..

IS 20 25 07(1250.70300.70350.704007045 Ce/Pb 87Sr/Sr

Terrigenous Sediment Terrigenous + Sediment Altered Crust + Itered Crust Pelagic Sediment Pelagic + Sediment Altered + Crust Altered Crust

17.5 is I$.5 19 19.5 15.55 1560 1565 15.70 I 5.75 PbPMPb 7Pb/Pb Figure 9.31-le/41-Ic (RI RA) versus (a) Ce/Pb, (b) !lSrflôSr, (c) 206Pb/2°4Pb, and (d) 20Pb/204Pb showing mixing systematics of the proposed contamination model for segment 3 glasses (solid circles; as discussed in the text and FigureS and Table 2).The calculated trends (models A, B, C. and I)) show mixing between 10% melt derived from mantle variably contaminated with anciently recycled material (shaded ovals) and normal MORE melts (oblique ruled ovals) also produced by 10% melting. Models B and D show mixing between MORB melt and melt derived from mantle contaminated 2.2 Ga ago with 8 and 15% recycled altered ocean crust with terrigenous sediment in a 97-3% mixture; Models A and C are similar to models A and B except that pelagic instead of terrigenous sediment is incorporated. Ce/Pb and 3HePHe cannot resolve pelagic versus terrigenous sediment. Bulk distribution coefficients arc from Figures 5 and S except for Nd, which is 0.005 [Stolper and New- man; 1994; Green. 1994]. Model parameters are reported in Table 2.

In the case of segment 4, however, the Pb isotopes require a recent contamination with terrigenous sediment and altered poorly constrained but clear multistage evolutionary history. oceanic crust possiblyrelated to theadjacent subduction As a result, segment 4 lavas, like many of the Indian Ocean zone. Segment 2 is the shortest ridge segment in the region MORB they resemble, require a complex history involving and does not display a distinctive type of enrichment,al- severaL stages of variable enrichments over hundreds of mil- though lavas erupted at its northern end appear to be influ- lions to billions of years and at least one enrichment event enced by material derived from a source similar to that at the that occurred relatively recently. southernmost end of segment 3 (located 50 km wcst). The radiogenic He, Pb, and Sr (and unradiogenic Nd) signature in segment 3 lavas, most pronounced in the south and diminish- 6. Conclusions ing gradually to the north, suggests a history of ancient sub- This study provides a window into the extensive degree of duction of altered oceanic crust plus terrigenous sediment heterogeneity that is introduced into the depleted upper man- which may have been recycled to the deep mantle and isolated tle by diverse processes associated with plate tectonic recy- from convective mixing for most of its history. The two geo- cling. The largest variability along the southern Chile Ridge chemical groups of segment 4 (4A and 4B), separated spa- occurs on a scale of 50-100 km. corresponding to the trans- tially by a southward propagating rift, share the isotopic sig- form- and propagating-rift-bounded segmentation. A sche- nature commonly associated with Indian Ocean MORE and matic drawing summarizing our conclusions is shown in Fig- Dupal compositions and show cvidence of a complex history ure 11. The enriched signature of segment I, strongest near involving ancient recycling and a more recent enrichment the trench and diminishing to the north, shows evidence of event which decoupled trace element and isotope ratios. Table 2. Segment 3 ModelPrimitive Mantle Parameters Aged Since 4.55 Ga NMORB Aged .Since 4.55PelagicContinental Ga Sediment Crust Tenigenous SedimentContinental Crust Altered NMORB Pelagic Sediment Terrigenous Sediment Depleted Mantle Rh 4.55 Ga 0.4 at 2.2 Ga at 2.2 Ga Aged Since 4.55 Ga at 2.2 Ga Aged Since 2.2 Ga at Present 3.0 Aged Since 2.2 Ga at Present 90 Aged Since 2.2 Ga at Present 130 at Present 0.09 CeNbThU 1.7750.7130.0240.09 6.190.240.071.22 9.5731.610 '4.33.173[0 0.2230.0050.0190.78 SmNdSrPb 0.4440.185 211.3 3.750.2311.2168 29038226 190 30206 0.83003 10 [He] cm' STP/g IlRbf8Sr 0.054 0.054 0.19 0.30 7.4E-05 0.054 l.OE-03 0.9 l.7E-03 2 6.5E-06 0.38 IcSsl4lNd Kii 0.213.88.2 0.19 3,88.2 0.143.88.2 0.173.88.2 0.193.426 0.096.34.6 0.12 105 Ce/PbU/He* [0 18,000 25 5 5 3250 27 1400 3 1790 4 25 z lletHe (RfRy 'SrrSr 0.506840.69897 9.307 103 0.510090.70151 14.38 50 0.5090!0.705280.03-90 14.38 0.509470.70915 0.03 0.50.70323 1285 2.1 0.510310.73384 0.03 0.5112]0.77262 0.03 0.5130.70248 IS 8.4 Geochemical parameters "PbP°1Pbare given thatPbPMPbPbrPb are used to model contamination of the mantle beneath segment 3. The isotopic compositions of altered MORB, pelagic sediment, and 29.48710.294 34.0015.03 34.0015.03 34.0015.0314.38 44.1624.9616.49 37.3315.2916.25 39.7515.5918.45 terilgenous sediment at 2.2 Ga 38.0015.5318.20 coupledsimilarandforwere Nd Zindier's calculated isotopes;model to 4He [1989]results radiogetucusingCanyon modelare a single-stage obtainedDiablo production.for helium (Tatsumoto over evolution isotope aElemental large ci model,at,evolutionrange 19731 abundances of starting values offor the Pb at (0.03-90 Earth. isotopes; and4.55 parent/daughter Ga3HerHc Rj. withand Aged Basaltic Hanof pelagic and ratios Achondrile Zindler's andsediment in terrigenous altered [1989] Best at timeoceanic Initialmodel sediment of [Papanastassiousubduction forcrust He at and isotopepresent sedimentvaries evolutionhasand depending minimum Wac.ce,burg,are drawnof the onpossible mantle.from proportion 1969) Staudigel 311e/'He for ofSr meteorica] isotopes;etal. considering [1995]. Moama Klein achondrite and Karsten U/He ratios noted for each stage are estimates from Han helium [e.g., Ozima et at, 1984],the hut nucleogenic production of 3Fle [Hamet et at., 1978] [1995], hart and highratioscompositionZind(er M of (HtMl.!) [[989];the isotopicis basalts fromStaudacher Sunsystems [RehkOinper and and McDonoughareThese Allègrein and agreement values Hofinann, [1989]; [1989], are withpoorly 1997]. BenHofmann the constrainedOthntan Readlisted [1988), 74E-05trace a! (seeal. elementSw/per as(1989]. text). 7.4X10'5. andand Pkink Newmanisotopic and data Langmuir[1994]. except Rehkamper [1993]; for altered Plank and MORB hiofrnannand (atLudden present) [1997], [[992]; whose Han Plank and t and 7.indler [1993], [1989], and C/wave! and Cilauvel etal. [1992]; et at, K were derived using the present-day isotope systematics of [1992]. The parent/daughter depleted mantle source 5110 STURM FT AL.: He-Pb-Sr-Nd ISOTOPES IN SOUTHERN CHILE RIDGE LAVAS

35 16.0 MixIng between pelagic ted and Stage 3: Pelagic altered crust 30 Sediment 1.5 Gad -- Sag. 4A. 15.5 Scg.4B Melts from mantle contaminated Stage 2: .' - Stage 3: Altered Ocean Crust 20 Continental - with 3-7% pcI. Crust/ ned and alt crust Is 15.0 //°L5Ga 565 - Stage 2: Depicted Mantle 555 f 04 14.5 3 Ga

5.65 03 Stage 1: Primitive Mantle 76 10 I I% IS! II05II! 0.2 12 14 16 18 20 22 206Pb'204Pb 0l

Melts from mantle cc,istaniinated with 3-7% pet sed and alt. cnat 50 0,7045 C 0.7040 40 CID Seg. 4B 0.7035 B

°07030 Seg. 4A 20 0.7025 - MOP.B

18.0 18.2 18.4 186 0.7020 07030 07040 0.7050 206pb/204Pb 87Sr/86Sr

Figure 10. 206Pb/204Pb versus (a) 207Pb/104Pb and (b) t7SrImSr and t7Sr/86Sr versus (c) Ce/Pb, (d) U/Pb, and (e) Nb/U showing mixing systematics for lavas from segment 4A (crosses) and 4B (diamonds). In Figure1 Oa, growth curves are shown for a three-stage isotopic evolution model for segment 4. The first stage (4.55-3.0 Ga; solid line) represents evolution of primitive mantle. The beginning of the second stage (at 3.0 Ga) marks the formation of the continental crust and depleted mantle (dashed lines are stage 2 growth curves for each). At the beginning of the third stage (1.5 Ga), oceanic crust is formed from the depleted mantle and hydrothermally altered, and pelagic sediment is deposited with a continental isotopic signature (dotted lines are stage 3 growth curves for each). Altered oceanic crust is subducted with pelagic sediment although their evolution is considered separately in the model, (Note that the "three-stage" terminology is somewhat misleading because the crustal component reflects a multistage evolution that is not considered here.) Recently, the altered crust and pelagic sediment mixed with depleted subridge mantle to form localized heterogeneities.During upwelling, these heterogeneities melt and the contaminated melts mix with depleted MORB melt (shown in Figure l0a inset). The calculated trends in the inset to Figure Wa and Figures lOb-lOc show mixing between a 10% MORB melt (oblique ruled oval) and a 10% melt derived from mantle contaminated with 10% recycled material (consisting of 3% (model A), 5% (model B) and 7% (Model C) pelagic sediment with 97%, 95%, and 93% altered oceanic crust, respectively). Bulk distrihution coefficients for melting calculations are from Figures 5 and 9. Elemental abundances and isotopic composition for model components during each stage are summarized in Table 3.

The approach taken in this study is based on the combined floor and about the bulk composition of pelagic and terrige- use of radiogenic isotopes and trace elements to identify plau- nous sediment, both of which are known to vary spatially and sible types and amounts of contaminants introduced into the temporally. Futhermore, a consensus has not yet emerged on mantle and their evolutionary history. While thisis a com- how the composition of material returned to the mantle differs mon approach to model recycled signatures in MORB and OIB from what is subducted. We are also only in the early stages of sources,it is underconstrained. Assumptions must be made exploring the potential effects of recycling on the major ele- about the bulk composition of altered oceanic crust based on ment composition and mineralogy of the mantle and their re- limited exposures of the upper igneous section of modern sea- sultant effects on melting parameters. Despite these uncer- Table 3. Segment 4 Model Parameters Primitive Mantle at 4.55 Ga AgedSince4.55GaDepleted Mantle at 3.0 Ga AgedSince4.550aContinental Crust at 3.0 Ga AgedSince3GaAltered NMORB at 1.5 Ga PelagicAgedSince3Ga Sediment at 1.5 Ga AgedSinceAltered l.5Ga NMORB at Present AgedSincel.5GaPelagic Sediment at Present Depleted Mantle at Present ThRb U 0.6350.020.09 0.2750.073.0 9.51.690 0.0050.0190.09 NdCeNbPb 0.7131.7750.17 1.4 6.190.233.516.7 207324 0.2490.650.030.90 rRb,vSr Sm Sr 0.444 21 0.11 3.75168 300 6 0.38 12 'Sm/Nd K 0.190.09 8.2 0.190.098.2 0.190.09 8.2 4 0.0250.21 93 9.50.6 5 0.0540.193.426 0.096.34.60.9 Nb/UCe/Pb 32104 4 1527 153 27 13 154 4725 ''Nd/Nd206PbP°4Pb'1Sr/SrPbP°Pb 0,506840.6989710.2949.307 0.508780.70097 14.4112.86 0.508780.70097 14.4112.86 0.510850.70151 15.8315.37 0.509860.71389 15.4316.00 0.512720.70226 22.6416.01 0.510750.72681 15.5417.20 0.513150.70248 15.5318.20 depletedinstead mantle of two. are Growth drawn from curves references Geochemicalfor all three in Table stages parameters 2. are shownPb/20Pb are given in Figure that are ba. used The totrace model element contamination and isotopic of thecomposition mantle beneath of primitive segment mantle, 4. Calculations continental are crust, similar to those in Table 2 except 29.487 32.52 32.52 33.27 36.46 40.08 altered oceanic crust, pelagicthree stages sediment, of evolution and were considered38.69 38.00 5112 STURN El AL.: He-Pb-Sr-Nd ISOTOPES IN SOUTHERN CHILE RIDGE LAVAS

Figure 11.Schematic drawing illustrating the varabili y in the mantle source composition and histories for segments 1, 2, 3, 4A. and 4B as discussed in the text. Ridges (double lines), trench (barbed oblique line), fracture zones (FZ; dashed lines)! and propagating rift along segment 4 (V) are indicated. A cross-sectional view of oceanic lithosphere is shown as shaded area for each segment and is subducting at the trench. Also shown are Ce/Pb and 3HetHe for each segment (symbols as in Figure 1). Mantle source composition for each segment is as required by the chemistry. Mantle beneath segment I may have been recently contaminated by recycled materials (small shaded blocks) or by contaminated melts (light shaded arrows) rising along the slab window (lightly shaded dashed lines; see Figure óc). The discrete mantle heterogeneity beneath segment 3 is most readily explained by ancient recycling of continental material to the deep mantle, with more recent upwelling through the upper mantle. Mantle components beneath segments 4A and 4B differ from one another in detail, but share a common history of multistage evolution and possibly a more recent metasomatic enrichment. tainties, the fact remains that trace elements and isotopes cur- Allègre, C. I., B. Hamelin, and B. DupM, Statistical analysis of isotopic rently provide our best way to track the evolution of distinct ratios in MORB: The mantle blob cluster model and the convective regime of the mantle. Earth Planet. Sd. Len.. 71, 71-84. 1984. portions of the mantle. Our results for the southern Chile Allègre, C. I., B. Duprë. P. Negrel. and J. Gaillardet, Sr-Nd-Pb isotope Ridge show that unique combinations of isotope and trace systematics in Amazon and Congo River systems: Constniints about element signatures can be used to constrain distinct evolu- erosion processes. C/tern. Geol. 131,93-112, 1996. tionary histories resulting from diverse recycling processes. Armstrong, R. L.. A model for the evolution of strontium and lead isotopes in adynamic earth, Rev. Geophys.. 6, pp. 175-199, 1968. Acknowledgments. This paper has benefited from many thoughtful Bach, W. J., Erzinger, L. Dosso, C. Bollinger. H. Bougault. J. Etoubleau. discussions with T. Plank, I. Karson. C. Langmuir, J. Douglass, S. L. and J. Sauerwein, Unusually large Nb-Ta depletions in North Chile Goldstein. P. Michael, C. Devey. J. Miller. S. B. Sherman, M, Stewart, Ridge basalts at 3650' to 3856'S: Major element, trace element and 0. Curewitz. We especially thank Matthew Kohn, Bill White and and isotopic data, Earth Planet. Sd. Len., 142, 223-240, 1996. an anonymous reviewer for their insightful and constructive comments. Baneiro, B. A.. and A. 11. Clark, Lead isotopic evidence for evolution- We also thank J. Lupton for providing access to the helium isotope azy changes in magma-crust interaction, central Andes, southern laboratory which is supported by the NOAA Vents Program. 'This work Peru, Earth Planet Sd. Let:.. 69, 30-42, 1984. was supported by grantsfrom theNationalScienceFoundation Ben Othman, P., W. White, and,I.Patchett, l'he geochemistry of marine (OCE9116169,0CE9504280.and0CE9618736 to B. Klein, sediments, island arc magma genesis, and crust-mantle recycling, 0CE9402506 toO. Graham, and OCE9I 16430 and 0CE9503617 to J. Earth Planet Sd. Left., 94, 1-21, 1989. Karsten). Cande S. C-, and R. B. Leslie, Late Cenozoic tectonics of the southern Chile trench. J. Geophys. Ret, 91. 471-496, 198. Castillo, F,, The Pupal anomaly as a trace of the upwelling lower man- References tle, Nature, 336, 667-670, 1988. Aitcheson. S. J., R. S. Harmon, S. Moorbath, A. SchneiderP. Soler, B. Chase, C. C.. Ocean island Pb: Two-stage histories and mantle evolution, Soria-Escalante, (3. Steele, 1. Swainbank, and C. Worner, Pb iso- Earth Plane:. Sd. Let:., 52, 277-284, 1981. topes define basement domains of the Altiplano. central Andes, Ge- Chauvel, C., A. Hofmann, and P. Vidal, HIMU-EM: The French- ology, 23. 555-558. 1995. Polynesian connection,EarthPlaner. ScL Len., 110, 99-119, 1992. Allègre, C. J., and D. L., Turcotle, Geodynamic mixing in the meso- Cloos. M., Lithospheric buoyancy and collisional orogenesis: Subduction sphere boundary layer and the origin of oceanic basalts, (Jeophys. of oceanic plateaus, continentalmargins, island arcs, spreading Res.Let:.. J2, 207-210, 1985. ridges, and scarnounts. Geot Soc. Am. Bull., 105; 715-737, 1993. STURM EF AL.: He-Pb-Sr-Nd ISOTOPES EN SOUTHERN CHILE RIDGE LAVAS 5113

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