A deep mantle source for high 3 He/ 4 He ocean island basalts (OIB) inferred from Pacific near-ridge seamount lavas

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L20316, doi: /2009gl040560, 2009 A deep mantle source for high 4 He ocean island basalts (OIB) inferred from Pacific near-ridge seamount lavas D. Hahm, 1,2 P. R. Castillo, 1 and D. R. Hilton 1 Received 14 August 2009; revised 23 September 2009; accepted 28 September 2009; published 30 October [1] One of the most contentious issues in the debate on the origin of volcanic island and seamount chains is the significance of high 4 He ratios at such locations. The contemporary hotspot hypothesis calls for the high 4 He signature to be derived from a distinct source reservoir that lies deep in the mantle. The competing plate stress hypothesis claims that extreme isotopic signals, such as the high 4 He, come from dispersed crustal lithologies in the upper mantle. Here, we show that lavas from the East Pacific Rise the ridge axis and near-ridge seamounts, which have radiogenic isotope compositions overlapping with other Pacific OIB, do not have high 4 He ratios. This suggests that high 4 He is not associated with dispersed, heterogeneous lithologies embedded in the upper mantle. We conclude that the mantle source of high 4 He OIB is unique to volcanic island and seamount chains and likely resides at depth in the mantle. Citation: Hahm, D., P. R. Castillo, and D. R. Hilton (2009), A deep mantle source for high 4 He ocean island basalts (OIB) inferred from Pacific nearridge seamount lavas, Geophys. Res. Lett., 36, L20316, doi: /2009gl Introduction [2] The origin of ocean island basalts (OIB) that form the bulk of volcanic islands and seamount chains remains controversial. One of the earliest models for generating OIB is the hotspot or plume hypothesis, whereby OIB are melts from mantle plumes upwelling from deep regions of the mantle [Morgan, 1971]. Helium isotope ratios ( 4 He) greater than that observed along spreading ridges (8 ± 1 R A [Graham, 2002], where R A = 4 He of air) were first recognized in Hawaiian OIB, and this led to the proposal that high 4 He ratios could be used to trace the surface expression of deep mantle plumes [Craig and Lupton, 1976]. Subsequent investigations showed that other volcanic chains also had high 4 He characteristics [Kurz et al., 1982]. When combined with other isotope systems, apparent He-Sr-Nd-Pb co-variations exist which have been interpreted as representing binary mixtures between recycled source components and a common OIB component characterized by high 4 He ratios [Hart et al., 1992; Farley and Craig, 1992; Graham et al., 1996]. [3] Although the plume hypothesis has remained popular for almost four decades, there are difficulties in reconciling the need for a deep mantle storage reservoir for high 4 He 1 Geosciences Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA. 2 Now at Korea Polar Research Institute, Incheon, South Korea. Copyright 2009 by the American Geophysical Union /09/2009GL and many geophysical models of mantle convection that find it impossible to maintain an isolated lower mantle over timescales of the Earth (see review by van Keken et al. [2007]). The principal alternative explanation is the plate stress hypothesis [Foulger, 2002], which calls for the origin of OIB through lithospheric tectonic processes (i.e., top-bottom) instead of by upwelling deep mantle plumes (i.e., bottomup). Further, it claims there is no need for distinctive mantle reservoirs as the isotopic compositions of both OIB and MORB, including 4 He ratios, can be explained through sampling upon melting and averaging (SUMA) of the compositionally heterogeneous upper mantle [Meibom and Anderson, 2004]. In this scenario, the upper mantle consists of a random mixture of small to moderate scale (1 100 km) enriched, crustal lithologies embedded throughout a depleted mantle matrix; such enriched lithologies are injected into the upper mantle through plate subduction [e.g., Anderson, 1998; Meibom and Anderson, 2004]. [4] In this study, we evaluate the mantle source of high 4 He OIB in the Pacific Ocean through a combined Sr- Nd-Pb-He isotope analysis of OIB-like lavas from seamounts near to, and along, the spreading axis of the East Pacific Rise (EPR; Figure 1). This approach stems from the fact that the chemical and Sr, Nd and Pb isotopic composition of these OIB-like lavas can be seen to overlap those of other OIB which also possess high 4 He [e.g., Class and Goldstein, 2005] (Figure 2). Moreover, as we report here, they have similar Sr and Nd isotopic signatures as the picritic lavas with the highest yet reported 4 He ratios (up to 50 R A from Baffin Island and West Greenland [Graham et al., 1998; Starkey et al., 2009]), which are associated with the Iceland hotspot. We argue that if the high 4 He OIB signature in the Pacific (e.g., from the Hawaii and Galapagos hotspots) indeed originates through random sampling of dispersed lithologies in the upper mantle (i.e., the SUMA hypothesis), then EPR segments and nearby seamounts producing OIBlike lavas by the same mechanism acting on the same heterogeneous upper mantle should also have high 4 He ratios. 2. Geologic Background and Samples [5] There are numerous small volcanic seamounts erupted close to active and fossil spreading centers in the Pacific Ocean [Batiza, 1982]. They range in composition from tholeiitic basalts, similar to MORB, to alkalic basalts and akin to some OIB in that they are enriched in highly- to mildly-incompatible trace elements and have moderately high 87 Sr/ 86 Sr and 206 Pb/ 204 Pb but low 143 Nd/ 144 Nd ratios. Moreover, some of these seamounts form volcanic chains, though these are much shorter ( km) and volumetrically smaller (<1000 km 3 ) than those forming major volcanic L of5

2 Figure 1. Map showing the locations of samples. Green circles and purple triangles represent samples from the ridge axis and seamounts, respectively. Gray contours show the 3000 m water depth. chains such as the Hawaiian-Emperor Seamount chain [Batiza, 1982]. It is generally agreed that seamount basalts result from smaller degrees and volumes of melting of the same mantle source as MORB, and bypass the mixing and homogenizing processes that generally occurs directly beneath ocean spreading centers: thus, their OIB-like signature is derived from small, easily fusible enriched components embedded in a depleted peridotite matrix [e.g., Batiza and Vanko, 1984; Zindler et al., 1984]. In this respect, the generation of alkalic seamount lavas is similar to the origin mentioned above for OIB by the SUMA hypothesis, i.e., both origins rely on the presence of dispersed, variably enriched lithologies in the upper mantle. [6] For this study, we analyzed the 4 He ratios and He concentrations of fresh glasses of 6 OIB-like basalts and one andesite from seamounts located near segments of the EPR these were selected specifically to be distal from any prominent linear volcanic chains [Castillo et al., 2000; Niu and Batiza, 1997] (Figure 1). Thus, in contrast to the near-ridge seamounts located near a number of prominent volcanic chains in the southern Atlantic Ocean [Graham et al., 1996], their origin(s) cannot be ascribed to the influence of long-lived, possibly deeply-sourced mantle plumes, but to small melting anomalies likely located in the uppermost mantle. In addition, a total of 5 ridge basalts (3 compositionally normal and enriched (N- and E-) MORB from the adjacent segment of the EPR, and 2 chemically and isotopically Hawaiian MORB from a nearby EPR segments) were analyzed to test whether some of the enriched lithologies that form OIB-like MORB can indeed be sampled by normal spreading center magmatism [Niu and Batiza, 1997]. Glasses were crushed to sample only vesicle-sited (magmatic) volatiles [Kurz, 1986]. We also report the U and Th (and Pb) concentrations as well as Sr, Nd and Pb isotopic ratios of these samples to constrain both the 4 He ratios at the time of eruption and the radiogenic isotope characteristics of the mantle source, respectively. 3. Results [7] Analytical results are presented in Table 1 and illustrated in Figures 2 and 3. The EPR and seamount samples show moderate, but generally overlapping ranges of Sr, Nd and Pb isotope values (Figure 2). As a whole, they have relatively lower 87 Sr/ 86 Sr, higher 143 Nd/ 144 Nd, and higher 206 Pb/ 204 Pb ratios than the Bulk Silicate Earth (BSE). The radiogenic isotope values of our sample suite overlap with Figure 2. (a) Nd-Sr and (b) Nd-Pb isotope relationships between samples analyzed in this study (same symbols as in Figure 1) and some oceanic lavas. Fields for OIB from Hawaii (red), Iceland (blue), and Galapagos (green) are from the GEOROC database; MORB from the East Pacific Rise (EPR) (black) is from ready-to-use datasets at PetDB; OIB for West Greenland (yellow) in Figure 2a is from Starkey et al. [2009]. No Pb isotopes are available for West Greenland lavas. The composition of the bulk silicate Earth (BSE) [Zindler and Hart, 1986] is represented by gray rectangles. 2of5

3 Figure 3. Relationships between 4 He ratios and 4 He abundance. Except R75-2, which is extremely degassed and air-contaminated (low [ 4 He] and He/Ne ratio; Table 1), all our samples fall within the canonical MORB-range. In contrast, samples from Hawaiian-Emperor seamounts, forming a linear volcanic chain, show typically high 4 He OIB characteristics [Keller et al., 2004]. those of geochemically depleted EPR MORB. More importantly, they also overlap with the depleted end-members of OIB that typically have the highest (15 R A ) 4 He ratios, such as those from Galapagos and Hawaii in the Pacific and from Iceland and West Greenland in the Atlantic [e.g., Class and Goldstein, 2005]. [8] The 4 He ratios and 4 He concentration ([ 4 He]) of seamount basalts range from R A and cm 3 STP g 1, respectively (Figure 3). Andesite sample R75-2 has an exceptionally low [ 4 He] of cm 3 STP g 1 and air-normalized He/Ne ratio of 6, consistent with extreme degassing followed by air-contamination [Hilton et al., 1993]. Hence it is excluded from further consideration. The 4 He and [ 4 He] of ridge axis basalts range from R A and cm 3 STP g 1, respectively. Thus, none of the seamount and EPR OIB-like basalts have 4 He higher than the canonical MORB value of8±1r A [Graham, 2002], consistent with previous 4 He work on seamounts in the area [Graham et al., 1987, 1988]. 4. Discussion and Conclusions [9] Almost all our ridge and seamount samples are mildly to moderately alkalic in composition, and hence contain relatively high concentrations of U, Th and Pb. The decay of U and Th produces 4 He which leads to a lowering of the 4 He ratio with time. Therefore, prior to considering the implications of the He results for the SUMA hypothesis, it is important to consider the effect of the addition of radiogenic He on the 4 He values of those samples which may not be zero-age. [10] The amount of radiogenic He ( 4 He*) produced in time T (for T < 10 7 years) is given by the following equation [Graham et al., 1987]: 4 He* ¼ 2: ½UŠð4:35 þ Th=UÞTcm 3 STP g 1 Table 1. Isotopic Results of Helium, Sr, Nd, and Pb of the Basalts From the East Pacific Pb (ppm) Th (ppm) 87 Sr/ 86 Sr 143 Nd/ 144 Nd 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb (ppm) U 4 He [He] C (R C /R A ) a (10 9 cm 3 STP g 1 ) 4 He (R/R A ) a X Depth (m) Location ( N, W) Sample Rock Type Near-Ridge Seamounts R121-3 Tholeiitic bas , ± R109-5 Alkalic bas , ± R94-2 Tholeiitic bas , ± R79-2 Alkalic bas , ± R83-3E Alkalic bas , ± R78-6E Alkalic bas , ± R75-2 Alk. andesite 10.69, ± Ridge Axis R120-1 N-MORB 14.59, ± CH6-1 T-MORB 13.65, ± CH19-3 N-MORB 12.37, ± PH90-2 Hawaiian 11.37, ± PH108-1 Hawaiian 11.34, ± a Measured 4 He ratios (R/R A) are corrected for the addition of atmospheric helium using the equation RC/RA = {{(R/RA)X} 1}/{X 1}, where RC is the corrected 4 He value, RA is the air 4 He value ( ), and X is the air-normalized He/Ne ratio (column 6). Measured helium concentrations are also corrected for the effects of atmospheric He to [He]C using [He]C = [He](X 1)/X - see Hilton [1996] for details. Not determined values are shown as dashes. 3of5

4 where T is in Myr, Th/U is the atomic ratio, and [U] is the uranium concentration in ppm. Using the above equation, we calculate that the 4 He ratio of all basalts (except R79-2) collected either on axis or close to the axis ( year-old [Niu and Batiza, 1997]) changes very little after eruption (i.e., they remain within the MORB range). On the other hand, one sample (R79-2, years old [Niu and Batiza, 1997]), due to its high [U] and low [ 4 He], shows a substantial change in 4 He after eruption (from 24 to 8 R A ). However, such a change of 4 He in R79-2 can only occur under the very unlikely condition of complete transfer of 4 He* from the glass matrix (where it is produced) to the vesicle phase. Graham et al. [1987] predicted that the fraction of 4 He* transferred to vesicles from the glass matrix falls in the range 1 10% with the assumption of 1% vesicularity and mm vesicle radius. If we apply the maximum 10% transfer rate, the calculated 4 He at the time of eruption still falls within the MORB-range. Thus, the entire sample suite seamounts and on-axis lavas - were indeed erupted with 4 He ratios that fall within the canonical MORBrange. [11] Our results show that Pacific OIB-like seamount lavas [see also Graham et al., 1987, 1988] do not have the high 4 He signature of OIB. The Hawaiian-type MORB and E-MORB in our study area also have MORB-like 4 He ratios and this observation stands in marked contrast to the high 4 He ratios that characterize E-MORB along ridge segments near linear volcanic chains, e.g., Galapagos Spreading Center near the Galapagos Islands [Graham et al., 1993] and the southern EPR near the Easter Island chain [Poreda et al., 1993]. These observations are clearly inconsistent with the predictions of the SUMA hypothesis, which claims that the mantle source of the high 4 He signature and the enriched radiogenic isotope characteristics is subducted crustal lithologies embedded within a depleted matrix. Our conclusion is unsurprising because the bulk of oceanic crust that is recycled back to the mantle is depleted tholeiitic basalt which has been degassed twice during formation at the ridge axis and during subduction via the trench [Staudacher and Allegre, 1988]. Additionally, seawater alteration usually destroys the main carriers of mantle He, glass and olivine, as the crust transits from ridge to trench. Moreover, the oceanic crust is enriched in U + Th relative to its depleted mantle source. Such a combination of low 4 He and high U favors the decrease of 4 He with time. Thus, subducted oceanic crust cannot be the source material of high 4 He in the mantle. [12] An alternative explanation is that subducted volcanic chains and/or oceanic plateaux could act as a potential high 4 He source lithology although not voluminous they consist of high 4 He OIB-like alkalic material [Keller et al., 2004]. However, they also suffer He loss during formation and have even higher U and Th contents than normal, tholeiitic oceanic crust. Moreover, subduction of large volcanic chains, and especially giant oceanic plateaux, would lead to break-up at the trench [e.g., Lonsdale, 1988] which, in turn, would enhance the degassing process. In short, the typical recycled material comprising the ubiquitous subducted lithologies in the upper mantle, as called for by the SUMA hypothesis, typically have low, not high, 4 He ratios and cannot be the source of high 4 He OIB. [13] Depleted residues of mantle melting are another possible source of high 4 He ratios in the upper mantle [e.g., Albarede, 2008]. This suggestion is based upon the assumption of relatively compatible behavior of He with respect to its parent isotopes (U and Th) and, in turn, implies that the OIB source reservoir has a lower 3 He abundance that its MORB counterpart. Whereas the relative behavior of He and (U + Th) during mantle melting remains a contentious issue, there is increasing evidence that lower 3 He contents of some OIB relative to MORB is not a reflection of source characteristics but instead is related to degassing processes [Hilton et al., 2000; Gonnermann and Mukhopadhyay, 2007]. In any case, the high diffusivity of He at mantle temperatures makes it highly unlikely that dispersed, small-scale heterogeneities in the upper mantle could maintain 4 He ratios different from those of the peridotitic upper mantle [Hart et al., 2008]. [14] If the source of 4 He ratios >9 R A of OIB and linear volcanic chains is not enriched lithologies in the upper mantle, and prior work has ruled out the sub-continental mantle [Day et al., 2005], then our results imply that the source of the high 4 He OIB must lie deeper in the mantle. However, a number of studies suggest that an undegassed, primitive mantle is not such a source [cf. Craig and Lupton, 1976; Kurz et al., 1982; Farley and Craig, 1992; Graham, 2002] based on the observations that high 4 He OIB have Nd, Sr, Pb isotopic characteristics that overlap with those of MORB (Figure 2). In light of these results, we favor a depleted yet less-degassed reservoir as the source of high 4 He OIB. Its likely location is the deep (lower) mantle which is less degassed than the MORB reservoir through isolation [Class and Goldstein, 2005] or limited invasion by recycled crust [Gonnermann and Mukhopadhyay, 2009]. 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Hilton, Geosciences Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA , USA. D. Hahm, Korea Polar Research Institute, Incheon , South Korea. (hahm@kopri.re.kr) 5of5