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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 1021 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 Can. J. Earth Sci. 36: 1021–1031 (1999) © 1999 NRC Canada 1022 Can. J. Earth Sci. Vol. 36, 1999 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, alka- line 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. © 1999 NRC Canada Cousens et al. 1023 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.
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