A Near-Ridge Origin for Seamounts at the Southern Terminus of the Pratt-Welker Seamount Chain, Northeast Pacific Ocean

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A near-ridge origin for seamounts at the southern terminus of the Pratt-Welker Seamount Chain, northeast Pacific Ocean

Brian Cousens, Jarda Dostal, and T.S. Hamilton

Abstract: Three seamounts close to the south end of the Pratt-Welker Seamount Chain, Gulf of Alaska, have been sampled to test whether or not mantle plume-related volcanism extends south of Bowie Seamount. Lavas recovered from Oshawa, Drifters, and Graham seamounts are weathered, Mn-encrusted pillow lavas and sheet-flow fragments, commonly with glassy rims. The glasses and holocrystalline rocks are tholeiitic basalts, with light rare earth element depleted to flat primitive mantle normalized incompatible element patterns and radiogenic isotope compositions within the ranges of mid-ocean ridge and near-ridge seamount basalts from the Explorer and northern Juan de Fuca ridges. Chemically, the seamount lavas strongly resemble older, “shield-phase” tholeiitic rocks dredged from the flanks of southern Pratt-Welker seamounts, but are distinct from the younger alkaline intraplate lavas that cap Pratt-Welker edifices. The weathered, encrusted basalts were most likely erupted in a near-ridge environment, adjacent to Explorer Ridge, between 11 and 14 Ma. No evidence of plume-related activity is found in this area. Compared with northeast Pacific mid-ocean ridge and alkaline intraplate basalts, Graham seamount lavas have anomalously high 206Pb/204Pb, which does not appear to be a function of sea-floor alteration, magma contamination, or mixing between previously identified mantle components. All near-ridge seamounts in the northeast Pacific exhibit isotopic heterogeneity that does not correlate with major or trace element composition, suggesting that the mantle sources of all near-ridge seamounts have been variably depleted by prior, but recent melting events. Résumé : Trois monts sous-marins localisés près de l’extrémité méridionalede la chaîne des monts sous-marins Pratt- Welker, dans le golfe d’Alaska, ont été échantillonnés pour déterminer si oui ou non le volcanisme associé à un panache mantellique s’étend au sud du mont sous-marin Bowie. Les échantillons recueillis sur les monts sous-marins Oshawa, Drifters, et Graham sont altérés, ils sont formés de laves en coussins enrobées d’une croûte manganifère et de fragments de coulées d’épanchement, présentant fréquemment des bordures vitrifiées. Les verres et les roches holocristallines ont la composition des basaltes tholéiitiques, avec des diagrammes de terres rares légères appauvries, jusqu’à montrer des spectres d’éléments incompatibles plats normalisés au manteau primitif, et dont les compositions d’isotopes radiogéniques varient dans la fourchette des valeurs trouvées pour les basaltes des monts sous-marins de la crête médio-océanique ou à la bordure des dorsales Explorer et Juan de Fuca nord. Du point de vue chimique, les laves des monts sous-marins ressemblent fortement aux roches tholéiitiques plus anciennes, de «phase-bouclier», qui ont été draguées à même les flancs des monts sous-marins Pratt-Welker sud, mais elles diffèrent des laves alcalines intraplaque plus jeunes qui coiffent les monticules Pratt-Welker. Les basaltes altérées et encroûtés ont été déposés le plus probablement par des éruptions à proximité d’une crête adjacente à la dorsale Explorer, ilya11à14Ma.Rien ne laisse présager dans cette région une activité qui aurait pu être associée à un panache. Les laves du mont sous- marin Graham possèdent un rapport 206Pb/204Pb anormalement élevé relativement à celui de la crête médio-océanique du Pacifique Nord-Est et des basaltes alcalins intraplaque, d’autre part ce rapport isotopique ne semble pas être le résultat d’une altération du plancher marin, d’une contamination magmatique, ou du mélange de composants mantelliques déjà connus. Tous les monts sous-marins localisés à proximité d’une crête dans le Pacifique Nord-Est affichent une hétérogénéité isotopique, sans corrélation avec la composition des éléments majeurs et en traces, ce qui suggère que les sources mantelliques de tous les monts sous-marins à proximité des crêtes ont été appauvris de manière variable par des événements magmatiques antérieurs à leur mise en place, mais récents. [Traduit par la Rédaction] Cousens et al. 1031

Received June 15, 1998. Accepted January 9, 1999. Introduction B. Cousens.1 Ottawa–Carleton Geoscience Centre, Geochemical and geochronological studies of seamounts Department of Earth Sciences, Carleton University, 1125 in the Gulf of Alaska, northeast Pacific Ocean, have shown Colonel By Drive, Ottawa, ON K1S 5B6, Canada. that these volcanic edifices commonly have multiple eruptive J. Dostal. Department of Geology, St. Mary’s University, histories with distinct mantle source compositions (Engel et Halifax, NS B3H 3C3, Canada. al. 1965; Church and Tatsumoto 1975; Turner et al. 1980; T.S. Hamilton. Department of Geology, Wichita State Cousens et al. 1984; Hegner and Tatsumoto 1985; Dalrym- University, Wichita, KS 67260, U.S.A. ple et al. 1987; Desonie and Duncan 1990; Cousens 1996a; 1Corresponding author (e-mail: [email protected]). Keller et al. 1997). Many, if not all, of these seamounts are

Fig. 1. Map of the southern terminus of the Pratt-Welker Seamount Chain (from Seeman 1982), showing the track and proposed present-day location of the Pratt-Welker plume (Turner et al. 1980). ᭹, dredge locations. Contour interval = 500 m. The inset is a map of the northeast Pacific ocean floor, showing spreading centres (double lines) and major seamounts (black).

composed of tholeiitic basalts that originally formed imme- et al. 1987). In support of this, a tomographic study of the diately adjacent to a spreading ridge (termed near-ridge mantle in the eastern Gulf of Alaska was interpreted to de- seamounts or NRS), and were then rafted away to the north- tect a mantle plume near the southeast end of the chain west on their oceanic crust substrate. Some of these (Nataf and VanDecar 1993). Lithospheric flexure studies in seamounts were capped by geochemically distinctive, alka- the area of Fig. 1 also support a plume origin for the alkaline line basalts during one or more rejuvenated, intraplate lavas capping the southeastern Pratt-Welker seamounts stage(s) of volcanism that occurred several million years af- (Harris and Chapman 1989, 1991). Alternatively, the alka- ter initial seamount formation. The origin of the later, alkaline lavas are proposed to be produced by melting of meta- line lavas is uncertain. somatized mantle unrelated to plume activity (Hegner and In the case of the Pratt-Welker Seamount Chain, the alka- Tatsumoto 1989). In this scenario, the tholeiitic seamount line intraplate phase has been ascribed to a mantle plume edifices are constructed over zones of weakness in young (Silver et al. 1974; Turner et al. 1980; Lambeck et al. 1984). crust adjacent to the spreading centre (Fornari et al. 1987). Geochronological work has shown that there is a general These zones of weakness are somehow imparted to the ag- younging in age of the alkaline lavas from northwest to ing, thickening lithospheric mantle such that the seamounts southeast along the chain, consistent with a plume origin can act as conduits for later low-degree, alkaline partial (Turner et al. 1980). However, further geochronological de- melts of the asthenosphere as they move away from the terminations show that one plume is unlikely to be the spreading centre. This concept is supported by the presence source of all of the alkaline lavas, although a short-lived of Pratt-Welker-like alkaline lavas in northeast Pacific oce- plume (a plumelet?) could be the source of alkaline lavas on anic rift settings at the Tuzo Wilson seamounts and the West seamounts at the south end of the chain (Fig. 1) (Dalrymple Valley segment of the Juan de Fuca Ridge (Cousens et al.

1985, 1995; Allan et al. 1993) which show that a component pillow fragment at a depth of ~1075 m from the northwest in the upper mantle can melt to produce an alkaline basalt side of the seamount. without the need for an active mantle plume. Drifters Seamount (local name only) is a northwest–south- Are the alkaline volcanic rocks on Pratt-Welker sea- east-oriented, dumbbell-shaped edifice located northeast of mounts associated with a mantle plume or not? There is a Oshawa Seamount. Dredge D6 sampled the steep south flank 360 km long gap between recently (Late Pleistocene to Ho- of the northern edifice at a water depth of ~2250 m and re- locene) active seamounts at the southeast end of the chain, covered 11 basalt blocks, of which eight are pillow frag- namely Bowie and Tuzo Wilson seamounts, both of which ments with glassy rims. The glassy rims were partially to have erupted alkaline basalts of similar composition (Herzer completely covered with thin (up to a few millimetres) 1971; Cousens et al. 1985; Cousens 1988). If one mantle palagonite, limonite, or manganese oxide rinds. plume was responsible for activity at both seamounts, then it Graham Seamount, an elongate edifice located close to the is likely that there would be evidence for alkaline volcanic east flank of Bowie Seamount, was dredged along its steep activity in the area between these two seamounts. Alterna- north face at site D4 at a water depth of ~1500 m. Most of tively, if volcanism at the Tuzo Wilson seamounts is not the basalts have glassy pillow rinds with palagonite and (oc- plume related, as concluded by Allan et al. (1993), then no casionally centimetre-thick) manganese oxide coatings. recent volcanic activity between the Bowie and Tuzo Wilson Crystalline basalt blocks are weathered with Fe–Mn staining areas would be expected. This test of plume versus non- along fractures. plume activity assumes that a single plume (or closely spaced series of plumelets) will be in a fairly constant state of magma production, and that these magmas will use any and all conduits (e.g., old seamount plumbing systems) through the lithosphere to reach the sea floor. The Canary Is- Analytical techniques lands is a credible example of plume activity that can pro- Glassy pillow rims up to 1 cm thick were broken off, and duce simultaneous volcanic activity on several islands spread fresh glass chips were hand-picked under a binocular micro- over several hundred kilometres of sea floor (Hoernle and scope. For most of the glassy samples, enough material was Schmincke 1993). The test also assumes that random, nonplume-related activity does not routinely occur on the available for major element, trace element, and isotopic sea floor. analyses. Major elements in glasses and minerals were determined by electron microprobe, and trace elements by acid- In this paper we report the geochemistry of basalts dissolution inductively coupled plasma – mass spectrometry dredged from three seamounts that fall in the critical area (ICP–MS) at the Geological Survey of Canada, Ottawa. Ad- between Bowie and Tuzo Wilson seamounts, over the pro- ditional major element analyses of basalt glass by electron posed present location of the speculative Pratt-Welker plume microprobe were performed at Dalhousie University, Hali- (Turner et al. 1980). Their apparent ages, based on degree of fax. For holocrystalline samples, or for samples where insuf- weathering of the basalts, and their lava chemistries are all ficient glass was available, whole-rock major and trace consistent with an older near-ridge origin, and no evidence element contents were determined by X-ray fluorescence for recent intraplate alkaline volcanism is found. spectrometry at St. Mary’s University, Halifax. The precision for major elements is generally less than 1%, and less than 5% for minor and trace elements (except for Cs, Ta, Nb, and Geologic setting and sample descriptions Th where precisions are 15–25%). Samples from which high MnO or loss-on-ignition values were obtained, due to inclu- Three seamounts southeast of Bowie Seamount were sion of Mn oxides or sea-floor weathering, were removed dredged in 1987 as part of marine geophysical and geologi- from the data set. Pb-, Sr-, and Nd-isotope ratios in acid- cal research cruise PGC-PAR-87-10 of the Geological Sur- washed (hot 2N HCl), hand-crushed chips were measured at vey of Canada (Fig. 1) (Chapman et al. 1987). Dredging was the University of California, Santa Barbara, and Carleton performed along cliffs and steep gullies in the shoulders and University, Ottawa (techniques described in Cousens 1996b). flanks of the seamounts, avoiding the glaciomarine mud – The average ratios measured for SRM981 were glacial diamict covered pinnacles. The seamounts are con- 206Pb/204Pb = 16.927 ± 0.007, 207Pb/204Pb = 15.480 ± 0.009, structed on oceanic crust ranging in age from 11–12 Ma for and 208Pb/204Pb = 36.664 ± 0.029 (1 sd), based on six runs Drifters and Oshawa seamounts to ~14 Ma for Graham between November 1991 and February 1992. The fraction- Seamount (Wilson 1988). ation correction (based on the values of Todt et al. 1984) is Oshawa Seamount is a conical edifice from which two +0.03%/amu. Sr-isotope ratios are normalized to 86Sr/88Sr = dredge hauls are available, D5 from the 1987 cruise and 0.11940 to correct for fractionation. Six repeat analyses of D76-10-29 from the University of British Columbia sample SRM987 yielded an average 87Sr/86Sr of 0.710252 ± 18. Nd- archive (from R.L. Chase). D5 sampled the northeast side of isotope ratios are normalized to 146Nd/144Nd = 0.72190. Six the seamount at a water depth of ~1550 m, recovering pri- runs of the La Jolla standard yielded an average 143Nd/144Nd marily pillow basalt fragments with minor sheet-flow frag- of 0.511876 ± 19. The basalts are young enough that no cor- ments and pyroclastic material. Of nine basalt fragments rections for posteruptive radiogenic ingrowth are required. available for analysis, seven included glassy selvages with Representative analyses from each dredge haul are listed in millimetre-thick manganese oxide coatings. One sample Table 1, and the complete data set is available from the first from D76-10-29 was included in this study, a mildly altered author.

Table 1. Major element, trace element, and isotopic data. DR-302 (gl) DR-306 (gl) GR-101 (wr) GR-104 (gl) GR-110 (wr) OS-10-29 (gl) OS-208 (gl) OS-213 (gl) Major elements (wt.%) SiO2 49.50 50.58 49.89 48.80 49.63 49.68 49.46 49.75 TiO2 1.59 1.65 1.58 1.70 1.60 2.04 1.88 2.10 Al2O3 14.62 14.71 18.50 15.29 17.83 14.10 14.22 13.84 FeOt 10.40 9.72 7.20 10.35 7.71 11.28 10.95 11.64 MnO 0.20 0.17 0.13 0.18 0.14 0.14 nd 0.07 MgO 7.48 7.30 3.33 7.45 3.11 6.84 6.87 6.48 CaO 12.13 12.28 13.54 12.11 13.00 11.50 11.54 11.30 Na2O 3.26 3.04 3.47 3.56 3.36 3.44 3.15 3.35 K2O 0.16 0.11 0.24 0.23 0.27 0.24 0.22 0.24 P2O5 nd 0.23 0.31 0.15 0.22 0.33 0.29 0.36 LOI nd nd 0.49 nd 1.00 nd nd 0.20 Total 99.34 99.62 98.68 99.82 97.87 99.69 98.58 99.33 Mg# 0.59 0.60 0.48 0.59 0.44 0.55 0.56 0.53 Trace elements (ppm) (XRF) Cr nd 310 290 nd 298 195 164 88 Ni nd 64 11 nd 14 28 53 44 V nd 310 274 nd 283 330 268 316 Zn nd 89 94 nd 88 89 99 111 Rb nd 9 6 3 nd 8 11 5 Ba nd 75 65 nd 38 92 49 44 Sr nd 179 224 nd 221 210 186 208 Nb nd 5 7 nd 6 8 8 8 Zr nd 105 99 92 117 137 123 140 Ynd33322534372735 Trace elements (ppm) (ICP) Cs 0.04 0.10 0.04 0.11 nd 0.09 0.05 0.08 Ta 0.4 0.4 0.7 0.4 nd 0.6 0.4 0.2 Nb 2.7 3.1 5.5 5.1 nd 6.6 6.0 6.7 Hf 2.3 2.5 2.3 2.2 nd 3.2 3.1 3.1 Th 0.18 0.19 0.38 0.33 nd 0.44 0.37 0.68 U 0.23 0.37 0.64 0.48 nd 0.27 0.26 0.48 La 4.1 4.7 6.0 4.7 nd 7.0 6.1 7.3 Ce 13 14 15 13 nd 19 18 21 Pr 2.0 2.2 2.3 2.0 nd 2.9 2.7 3.0 Nd 11 12 12 10 nd 15 13 15 Sm 3.5 3.7 3.7 3.2 nd 4.7 4.1 4.4 Eu 1.4 1.5 1.5 1.3 nd 1.8 1.6 1.7 Gd 4.7 5.1 4.7 4.0 nd 5.9 5.5 5.7 Tb 0.81 0.87 0.82 0.71 nd 1.00 0.92 0.98 Dy 5.2 5.4 5.2 4.4 nd 6.5 5.5 6.1 Ho 1.1 1.2 1.1 0.9 nd 1.4 1.2 1.3 Er 3.1 3.3 3.2 2.8 nd 3.8 3.3 3.6 Tm 0.49 0.53 0.51 0.45 nd 0.60 0.52 0.57 Yb 3.1 3.3 3.1 2.7 nd 3.6 3.1 3.5 Lu 0.46 0.50 0.47 0.41 nd 0.56 0.47 0.48 La/Smcn 0.64 0.70 0.89 0.81 nd 0.82 0.82 0.90 Isotope results 87Sr/86Sr 0.702596 nd 0.702397 0.702442 nd 0.702624 0.702552 0.702589 143Nd/144Nd 0.513195 nd 0.513168 0.513148 nd 0.513122 0.513058 0.513088 208Pb/204Pb 37.791 nd 38.037 38.169 nd 38.068 38.060 38.038 207Pb/204Pb 15.443 nd 15.448 15.498 nd 15.492 15.480 15.477 206Pb/204Pb 18.524 nd 18.977 18.892 nd 18.821 18.803 18.807 Notes: DR, Drifters Seamount; GR, Graham Seamount; OS, Oshawa Seamount; gl, glass; wr, whole rock; XRF, X-ray fluorescence; ICP, inductively coupled plasma – mass spectrometry; cn, chondrite normalized; LOI, loss on ignition; nd, no data. Mg# = Mg/(Mg + Fe2+), where Fe2+ = 0.9Fetotal. See text and Cousens (1996b) for details on analytical methods and standards.

Petrography and mineral chemistry nant mineral phase, occurring as abundant lath-shaped, subhedral to anhedral microphenocrysts, microlites, and rare Oshawa Seamount phenocrysts. Phenocrysts are zoned, ranging from An70 to Pillow lavas have fresh glassy rims up to 1 cm thick that An81, and microphenocrysts and microlites have An contents are microvesicular; vesicles (<7% by volume) have thin between 75 and 70. Subhedral microphenocrysts of olivine opaline linings. The interior of the pillows are aphyric to have compositions typically between Fo83 and Fo85. scarcely phyric, with samples containing at most a few per- Microphenocrysts of augitic clinopyroxene and Cr-spinel are cent phenocrysts and numerous vesicles. The interiors are rather rare. Compared with the other two sites, the Graham composed of microphenocrysts of mainly subhedral plagio- Seamount lavas contain significantly higher amounts of clase set in fine-grained matrix with a significant amount of plagioclase and are also less vesicular than the other sites. glass and plagioclase microlites. Euhedral to subhedral This probably reflects lower volatile contents in the magma, microphenocrysts in glassy rims have compositions compa- as a high content of H2O suppresses plagioclase crystalliza- rable to those from the interior. Plagioclase crystals reach up tion in the mid-ocean ridge basalts (MORB) (Michael and to 1.5 mm in size, although they are typically <0.5 mm. Chase 1987). Microphenocrysts and microlites have compositions ranging typically between An60 and An72. Rarely, euhedral to Geochemistry subhedral plagioclase phenocrysts reach up to about 4 mm in size. These phenocrysts or megacrysts are zoned with The seamount basalts show only minor compositional cores having ~An79–75. All plagioclase crystals are usually variations within each dredge haul, but lavas from different euhedral and do not show significant marginal reactions with seamounts are geochemically distinct. All of the basalts plot surrounding groundmass. in the tholeiitic field in a plot of total alkalis versus SiO2 Olivine is very rare and occurs as microphenocrysts in (MacDonald and Katsura 1964; Irvine and Baragar 1971). glassy margins and in crystalline interiors. The crystals are Note that these basaltic rocks have undergone some sea- subhedral to anhedral. Olivine also forms glomerocrystic floor weathering, and thus K2O (also Rb and U) abundances clusters with plagioclase. Its composition ranges from Fo79 may be suspect. The basalts are slightly to moderately frac- 2+ to Fo84. Most olivine crystals analyzed have compositions in tionated, having Mg numbers ((Mg# = Mg/(Mg + Fe ), equilibrium with host glass (see Leybourne and Van Wag- where Fe2+ = 0.9Fetotal) between 0.67 and 0.44. Whereas oner 1991). Rare clinopyroxene occurs as subhedral to basalts from the Drifters Seamount have roughly constant anhedral microphenocrysts or in glomerocrysts with major element compositions, basalts from Graham and plagioclase microphenocrysts. Clinopyroxene is augite to Oshawa seamounts follow crystal fractionation trends seen endiopside in composition. Chrome-spinel forms minute in- in Explorer Ridge basalts (Michael et al. 1989; Cousens clusions in olivine or occurs as small isolated grains. 1996a). Olivine and plagioclase are the primary fractionating phases, consistent with extensive shallow-level crystal- Drifters Seamount lization. With the exception of one basalt from Graham Quench glass rims on pillow basalt fragments are up to Seamount with an Mg# of 0.67, Ni contents are uniformly about 1 cm thick. The glass contains small proportions of less than 65 ppm, consistent with early crystallization of ol- microphenocrysts of euhedral to subhedral plagioclase ivine. Sr concentrations do not increase appreciably with de- (An67–68) and olivine (Fo85). Both glass rims and the interi- creasing Mg#, indicative of plagioclase fractionation. Cr and ors contain numerous vesicles. The pillow interiors are V contents are constant at 200–350 and 250–400 ppm, re- aphyric or slightly porphyritic, with microphenocrysts spectively, as Mg# decreases from 0.67 to 0.44, indicating mainly of plagioclase that rarely reach up to 1.5 mm. The that chromite and titanomagnetite are not important fraction- large size crystals are subhedral with corroded margins, sug- ating phases. Clinopyroxene may be a minor fractionating gesting they are not in equilibrium with the host magma. phase, based on roughly constant CaO/Al2O3 ratios and V The groundmass contains glass and microlites of plagio- contents in the basalts. clase. Plagioclase does not show much variation in composi- Incompatible elements such as Ba, Zr, and Hf and the rare tion (typically An65–68). Rare olivine microphenocrysts both earth elements generally increase in abundance with decreas- in glass rims and crystalline pillow interiors occur as iso- ing Mg#, with some scatter. Zr/Nb is higher and La/Sm is lated subhedral to euhedral crystals or as glomerocrysts usu- lower in Drifters basalts compared with Oshawa and Graham ally with plagioclase. Olivine ranges in composition from lavas, such that Drifters basalts are classified as normal Fo83 to Fo85. Most olivine crystals appears to be in equilib- MORB (N-MORB), whereas Oshawa and Graham basalts rium with glass (see Leybourne and Van Wagoner 1991). are transitional MORB (T-MORB) (Michael et al. 1989). Some samples also display fine-grained hyalo-ophitic to Normalized to primitive mantle (Fig. 2), incompatible ele- subophitic texture with plagioclase, clinopyroxene, glass and ment patterns for Oshawa and Graham seamounts are flat, rare opaque minerals. showing only slight depletions in Ba through La relative to the middle rare earth elements, but Drifters basalts are Graham Seamount strongly depleted in La, Nb, K, Th, Rb, and Ba. Glassy rims on pillow fragments are up to about 1 cm Isotopically, the three seamounts are distinct. In a plot of thick and contain lath-shaped euhedral to subhedral 143Nd/144Nd versus 87Sr/86Sr (Fig. 3A), basalts from Graham microphenocrysts of plagioclase and subordinate amounts of Seamount have the lowest 87Sr/86Sr, whereas lavas from euhedral olivine (Fo83–85) and clinopyroxene. The pillow in- Drifters and Oshawa seamounts overlap. Oshawa glasses teriors have a hyalo-ophitic texture. Plagioclase is the domi- have slightly lower 143Nd/144Nd than Graham and Drifters

Fig. 2. Primitive mantle (Sun and McDonough 1989) normalized incompatible element patterns for typical lavas from Oshawa, Drifters, and Graham seamounts, compared with oceanic basalts from the northeast Pacific (Cousens et al. 1984, 1985; Dalrymple et al. 1987; Cousens 1988; Leybourne and Van Wagoner 1991; Smith et al. 1994). NRS, near-ridge seamounts.

4 DR-302 20249 # GR-104 gl

100 6 DR-308 gl n OS-10-29 wr

✠ GR-101 wr l OS-213 gl

Pratt-Welker,Tuzo Wilson Pratt-Welker, Alkaline Basalts Explorer NRS n

imitive Mantle imitive l l n nl n nl n 10 l n n l nl l n nl ✠ n ✠ ✠ ✠ 64✠ 6✠ nl n n l ✠ ✠ 46 46✠ 6 4 46✠ 6 l nl l n nl #46 # # 4✠ # 46✠ 4✠ 46✠ l nl n # # 6✠ 64✠ 46✠ 46✠ l ln 46# # # # # 4 64✠ 46✠ n 4 # # # # # # ✠ 6 6 # 4 #✠ Sample / Pr Heck, Heckle, 64 Vance NRS 46

1 Rb Ba Th Nb K La Ce Pr NdSM Zr Hf Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu glasses. All of the seamount basalts fall within the northeast Discussion Pacific mantle array (Cousens 1996a), and most fall within the range of other tholeiitic basalts (near-ridge seamounts) Origin of the seamounts from the southern Pratt-Welker seamounts. In a plot of If a significant volume of the volcanism on Graham, 207Pb/204Pb versus 206Pb/204Pb (Fig. 3B), the three seamounts 206 204 Drifters, and Oshawa seamounts was related to melting of also fall into discrete groups based on Pb/ Pb, but there the proposed Pratt-Welker mantle plume (Fig. 1), then is complete overlap in 207Pb/204Pb and near-complete over- 208 204 young, fresh alkaline basalts similar to those dredged from lap in Pb/ Pb (see Table 1). All of the seamount glasses Bowie Seamount should have been recovered. Although plot below the Northern Hemisphere Reference Line most of the pillow fragments retain glassy margins, the (NHRL, Hart 1984), as do most oceanic basalts from the dredged basalts are weathered and are commonly rimmed by northeast Pacific. The basalts from Graham Seamount, with 87 86 manganese oxides up to 1.5 cm in thickness. This degree of the lowest Sr/ Sr of the three edifices, have the highest weathering is never seen in young alkaline basalts from 206Pb/204Pb, whereas Drifters lavas have high 87Sr/86Sr and 206 204 Bowie Seamount, but is consistent with an age similar to low Pb/ Pb. Thus no seamount consistently defines a that of the underlying oceanic crust (11–14 Ma, Wilson “depleted” or “enriched” mantle source: in Fig. 3A, Graham 1988). Compositionally, the basalts from Graham, Drifters, lavas define a time-integrated low-Rb/Sr and high-Sm/Nd and Oshawa seamounts are normal to transitional MORB source and Oshawa basalts a high-Rb/Sr and low-Sm/Nd rather than the incompatible-element-enriched basalts typical source, whereas Fig. 3B indicates that Graham lavas have a of young alkaline lavas that cap Pratt-Welker seamounts, and high time-integrated U/Pb source and Drifters lavas have the strongly resemble the older, tholeiitic phase of several south- lowest U/Pb source. The near-vertical arrays formed by ern Pratt-Welker edifices (Turner et al. 1980; Hegner and Drifters and Oshawa samples in Fig. 3B could be interpreted Tatsumoto 1989). Vigorous mantle plumes such as Hawaii as secondary isochrons, but considering the implied age of produce both tholeiitic and alkaline basalts, but in the case ~4 Ga it is more likely that these arrays are mixing lines as a of the Pratt-Welker seamounts all the late-phase basalts in- result of source heterogeneity. terpreted to be associated with a plume are alkaline in com- No positive correlation between Pb, Sr, or Nd isotopic position (Forbes and Hoskin 1969; Forbes et al. 1969; composition and incompatible element enrichment, exempli- Dalrymple et al. 1987; Cousens 1988). Isotopically, basalts fied by La/Sm or Nb/Zr, is evident in basalts from the three from Graham, Drifters, and Oshawa seamounts commonly seamounts (Fig. 4). Although the seamounts span much of plot in the region of overlap between northeast Pacific the isotopic range of northeast Pacific MORB, La/Sm is vir- MORB and alkaline basalts, as do other Pratt-Welker NRS tually constant. This decoupling of incompatible element ra- lavas (Fig. 3). No evidence of recent volcanic activity exists tios from isotope ratios is not common in basalts from the between Bowie and the Tuzo Wilson seamounts, and we northern Juan de Fuca Ridge and Explorer Ridge or in north- conclude that Graham, Drifters, and Oshawa seamounts east Pacific seamounts (Cousens 1996a). were constructed in a near-ridge setting between 11 and

Fig. 3. 87Sr/86Sr vs. 143Nd/144Nd (A), 207Pb/204Pb vs. 206Pb/204Pb (B), and 143Nd/144Nd vs. 206Pb/204Pb (C) in basalts from Oshawa, Drifters, and Graham seamounts, compared with mid-ocean ridge, near-ridge seamount (NRS), and alkaline intraplate lavas from the northeast Pacific (Cousens et al. 1985, 1995; Hegner and Tatsumoto 1987, 1989; White et al. 1987; Cousens 1988; Rhodes et al. 1990; Allan et al. 1993). NHRL, Northern Hemisphere Reference Line (Hart 1984). Insets compare seamount basalts from this study (᭹) with all northeast Pacific basalts (NEP, grey field), ocean-island basalts (OIB), and mantle end members (lined boxes; from Zindler and Hart 1986).

14 Ma ago. If a Pratt-Welker plume does exist and is located south of Bowie Seamount, then plume activity is not evident in the study area. Sampling of the pinnacles of the three seamounts is required to confirm this conclusion, although the pinnacles are blanketed by glacial deposits and dredge sampling of the underlying volcanic rocks may prove impos- sible.

Mantle sources in beneath the northern Juan de Fuca Ridge – Explorer Ridge area Regional variations in MORB and near-ridge seamount chemistry can be used to map large-scale upper mantle “provinces” in the northeast Pacific and determine the temporal and spatial scale over which mantle of one bulk composition is displaced by mantle of a different composition. It has been observed that the upper mantle beneath the north- ernmost part of the Juan de Fuca Ridge and the Explorer Ridge system includes an important enriched component that variably contributes to the chemistry of modern MORB lavas which is not present in the mantle beneath the southern Juan de Fuca Ridge (Michael et al. 1989; Karsten et al. 1990; Smith et al. 1994; Cousens 1996a). The trace element and isotopic character of this enriched component, which tends towards a HIMU-like (high U/Pb; Zindler and Hart 1986) composition, is extremely similar to the alkaline intraplate lavas of the Tuzo Wilson and Pratt-Welker seamounts (Cousens 1996a). The basalts from Graham and Oshawa seamounts have trace element and isotopic compositions similar to other near-ridge seamount lavas that origi- nated in the area of Explorer Ridge, including the enriched- tholeiitic edifices of the Dellwood Seamounts and the Pratt- Welker chain (Figs. 2, 3B) (Cousens et al. 1984; Hegner and Tatsumoto 1989). Drifters seamount glasses are clearly more depleted in incompatible elements and have distinctly lower 206Pb/204Pb, indicating that these basalts have a smaller contribution from the enriched component. The three seamounts of this study show that upper mantle with an enriched component was present beneath the northern Juan de Fuca Ridge – Explorer ridge system 11–14 Ma and, if the southern Pratt-Welker edifices are included, was present at least as far back as 20 Ma. Although the basalts from Oshawa, Graham, and Drifters source of Graham basalts may have included a third mantle seamounts plot within the northeast Pacific isotopic arrays in component not found in the sources of other northeast Pa- Figs. 3A and 3B, the seamount data are not consistent with a cific oceanic basalts, having both high Pb and high Nd iso- two-component mixing model involving depleted mantle tope ratios, but no such mantle end member has been (DM) and a HIMU-like component (Cousens 1996a). Bas- identified in oceanic basalts (Zindler and Hart 1986). Alter- alts from Graham Seamount, with the lowest 87Sr/86Sr and natively, a secondary process has either added Pb with a highest 143Nd/144Nd, do not have the lowest 206Pb/204Pb, as higher 206Pb/204Pb or lowered 87Sr/86Sr and raised the model of Cousens (1996a) would predict (Fig. 3C). The 143Nd/144Nd in Graham basalts. Incorporation of manganese

87 86 143 144 Fig. 4. Sr/ Sr vs. La/Smcn (A), Nd/ Nd vs. La/Smcn (B), precision Pb data, but there is no evidence of anomalously 206 204 and Pb/ Pb vs. La/Smcn (C) in basalts from Oshawa, high U contents in Graham basalts (Table 1). We find no ex- Drifters, and Graham seamounts compared with oceanic basalts planation for the high 206Pb/204Pb in Graham basalts, and from the northeast Pacific (from compilation by Cousens 1996a). further sampling of the seamount is required to further eluci- Dellwood, southeast Dellwood Knoll; NRS, near-ridge date the petrogenesis of these lavas. seamounts; PW–TW, Pratt-Welker and Tuzo Wilson seamounts; Furthermore, as a group, northeast Pacific oceanic basalts PW, Pratt-Welker seamounts. have isotopic compositions that correlate well with incompatible element enrichment, as indicated by the chondrite- normalized La/Sm (La/Smcn) (Fig. 4) (Cousens et al. 1995; Cousens 1996a). However, Graham, Oshawa, and Drifters lavas have roughly constant La/Smcn over a significant range in 87Sr/86Sr, 143Nd/144Nd, and 206Pb/204Pb. Only Graham Seamount falls in the northeast Pacific array in Fig. 4, whereas Drifters and Oshawa plot to the right of it at higher 87 86 206 204 Sr/ Sr (in a plot of La/Smcn vs. Pb/ Pb it is the Gra- ham basalts that plot outside the northeast Pacific array). The most anomalous near-ridge seamount basalt from the northeast Pacific, having an 87Sr/86Sr of 0.70279 at a La/Smcn of ~1 (Cousens et al. 1984), is from the Southeast Dellwood Knoll, located east of the Dellwood Seamounts in Fig. 1 (Bertrand 1972; Riddihough et al. 1980). Further in- spection of the diagram indicates that lavas from each of the near-ridge seamount groups, the Pratt-Welker, Heck–Heckle, and Oshawa–Drifters–Graham, have constant La/Smcn but variable isotope ratios. Why are trace element and isotopic signatures coupled in northeast Pacific MORB and alkaline intraplate lavas, but decoupled in near-ridge seamount lavas? A possible explanation for the “anomalous” high 87Sr/86Sr at their La/Smcn in basalts from Drifters and Oshawa might include a variable contribution of Sr from seawater, although the picked glasses are fresh (especially when compared with Graham Seamount basalt samples which have lower 87Sr/86Sr) and were acid washed prior to analysis. This would not explain the significant variation in Nd- and Pb- isotope ratios, which are not effected by short-term (e.g., <30 Ma), low-temperature sea-floor alteration (Cheng et al. 1987). The rare earth element patterns of Drifters and Oshawa seamount basalts also show no evidence of modifi- cation by sea-floor weathering (e.g., negative Ce anomalies, Ludden and Thompson 1979). Alternatively, as a seamount magma migrates upward through the crust, it may assimilate hydrothermally altered oceanic crust which would boost the 87Sr/86Sr of the magma (e.g., Eiler et al. 1996) without significantly effecting its La/Sm. This process could effect any intraplate magma, including the alkaline lavas of the Tuzo Wilson and Pratt- Welker seamounts which exhibit a 0.0002–0.0003 variation 87 86 oxides into small cracks in the basalts might increase Pb- in Sr/ Sr at a given La/Smcn (Fig. 4). The altered crust isotope ratios in the analyzed samples, but these oxides would need an initial 143Nd/144Nd < 0.51300 and should also add radiogenic Sr and nonradiogenic Nd to the 206Pb/204Pb > 19.0 to be a viable contaminant, however, and samples (e.g., Goldstein and O’Nions 1981). Sea-floor few MORB from the Explorer and Juan de Fuca ridges have weathering in relatively young crust should only increase Sr- those characteristics. isotope ratios, but should not effect Nd or Pb (Cheng et al. A third possibility is that the near-ridge seamounts are 1987; Perfit et al. 1994). One way to boost the 206Pb/204Pb sampling enriched mantle enclaves (“plums” or veins) that without modifying 207Pb/204Pb and 208Pb/204Pb (or Sr- and have already been depleted of most of their fusible compo- Nd-isotope ratios) would be to add U to the system either nents by an earlier melting event. This earlier melting event just prior to melting in the mantle or after the basalts were would strip the enriched mantle enclave of incompatible ele- erupted on the sea floor. If so, the 238U/204Pb would have to ments, thereby lowering La/Sm but leaving isotope ratios be increased to ~200 in order for the rocks to evolve from unaffected in the residue. The degree to which the La/Sm “initial” 206Pb/204Pb of ~18.4 (lower than Drifters basalts) to would be lowered depends on the degree of melting experi- the observed values of 18.85 in only 14 Ma. We lack high- enced during the earlier melt event, and what the residual

© 1999 NRC Canada Cousens et al. 1029 mineral assemblage was. If melts from these enclaves were gree of weathering, and the lava chemistry are consistent mixed with melts of depleted mantle, this would produce with a near-ridge origin 11–14 Ma for the three seamounts mixing arrays in Fig. 4 with steeper positive slopes (or per- in the area of Explorer Ridge. haps even negative slopes if La/Sm is strongly fractionated Most basalts from near-ridge seamounts formed at Ex- during melting of the enclave) than mixtures of primary plorer Ridge over the past 20 Ma are transitional between N- melts of the “fresh” enclaves and depleted mantle. and E-type MORB, indicating that an enriched component has been present in the mantle beneath Explorer Ridge over Near-ridge seamounts: Explorer Ridge versus Juan de this time period. Near-ridge seamounts formed adjacent to Fuca Ridge the Juan de Fuca Ridge are all composed of highly depleted Near-ridge seamounts serve as excellent spot-probes of N-MORB, and thus no enriched component is present in the composition of the upper mantle because they do not ap- their mantle sources. The direction of mantle flow beneath pear to tap magmas from the adjacent mid-ocean ridge the ridge may be a controlling factor in the presence or ab- (Allan et al. 1987, 1989, 1994; Fornari et al. 1988). The bas- sence of enriched components in the melting column be- alts from near-ridge seamounts originating adjacent to the neath near-ridge seamounts. All near-ridge seamounts in the Explorer Ridge area are all more incompatible element en- northeast Pacific exhibit isotopic heterogeneity that does not riched than basalts from the Heck, Heckle, and Vance near- correlate with major or trace element composition, suggest- ridge seamount chains of the Juan de Fuca Ridge ing that the mantle sources of all near-ridge seamounts have (Leybourne and Van Wagoner 1991; Smith et al. 1994; been variably depleted by prior, but recent melting events. Cousens et al. 1995), all of which lie west of their respective spreading centres (Fig. 2). Heck, Heckle, and Vance lavas are highly depleted in incompatible elements compared with Acknowledgments MORB erupted at the spreading centres adjacent to them, We sincerely appreciate the efforts of the officers and particularly in the case of the Heck and Heckle seamounts, a crew of the CSS Parizeau during cruise PGC-PAR 87B-10, feature which is not evident in near-ridge seamount lavas which was one of the last research cruises of the CSS erupted in the Explorer Ridge area between 20 Ma and the Parizeau on the west coast. Along with CNAV Endeavour, present. The distinction in basalt chemistry for the Heck, she is sorely missed. We also thank chief scientist Trevor Heckle, and Vance seamounts implies that, although en- Lewis and the rest of the science team for their assistance riched components in the upper mantle contribute to Juan de during dredging operations. We thank Conrad Gregoire and Fuca Ridge basalts, they are not tapped during melting at the John Stirling of the Geological Survey of Canada, Ottawa, adjacent seamount (Smith et al. 1994; Cousens et al. 1995). for help with ICP–MS and microprobe analyses. George This suggests that these near-ridge seamount sources have Tilton graciously provided access to laboratory space and already undergone melting beneath the ridge, thereby strip- the mass spectrometer at the University of California, Santa ping the sources of incompatible elements, and have later Barbara, during the initial phase of isotopic work in 1990. undergone a second melting event beneath the seamounts Geochemical analyses at St. Mary’s University and isotopic (note the low abundances of the heavy rare earth elements in work at the Université de Montreal and Carleton University Heck, Heckle, and Vance seamount basalts in Fig. 2). How- were supported by Natural Sciences and Engineering Re- ever, near-ridge seamount basalts erupted adjacent to Ex- search Council of Canada grants to JD and BLC. Support to plorer Ridge do contain the same enriched component found TSH was provided by A-base funding to Project 820018 in the spreading-centre lavas. This may be explained if there from the Geological Survey of Canada. is a component of mantle flow from the west in the Explorer area, but from the east beneath the Juan de Fuca Ridge. If flow is from the west, enriched components in the mantle References may be tapped by seamounts on the west side of the ridge prior to melting beneath the Explorer spreading centre. If Allan, J.F., Batiza, R., and Lonsdale, P. 1987. 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