Seamount Lavas Research Articles

A geochemical study of off-axis seamount lavas at the Valu Fa Ridge: Constraints on magma genesis and slab contributions in the southern Tonga subduction zone

New geochemical data are presented for fresh lavas from off-axis seamounts east and west of the Valu Fa Ridge and from seamounts south of the propagating rift tip of the Valu Fa Ridge. Most seamount lavas are basaltic and thus more primitive than the mostly andesitic to dacitic lavas from the adjacent Valu Fa Ridge. Slight differences in fluid-mobile element ratios and in Sr isotope ratios suggest small-scale heterogeneities of the slab component in this region with an apparently higher sedimentary input into the off-axis seamount lavas than into the island arc magmas. Off-axis volcanoes near back-arc basin spreading centres thus resemble off-axis seamounts adjacent to mid-oceanic ridges which also show more extreme compositions than the well-homogenized mid-ocean ridge basalts. The geochemical variations support a diapiric ascent of the hydrated or partially molten mantle leading to separate magma supply systems for the different volcanic structures. The mantle wedge between the Valu Fa Ridge and the island arc appears to be uniformly depleted in fluid-immobile incompatible elements compared to average depleted mid-ocean ridge basalts whereas lavas from seamounts west of the Valu Fa Ridge show less depletion in agreement with a depletion event of the whole mantle wedge in the backarc melting zone. Seamount lavas south of the Valu Fa Ridge propagating rift resemble the Valu Fa lavas indicating an increased magma production caused by the influx of water into the mantle.

Melting of enriched mantle beneath Pitcairn seamounts: Unusual U–Th–Ra systematics provide insights into melt extraction processes

U-series systematics as well as Sr isotopes were measured on young seamount lavas from the Pitcairn hotspot collected during the Polynaut cruise. The combined U-series and Sr isotope data reveal typical mixing relationships between two endmembers. One typical ‘plume’ endmember with radiogenic 87Sr/ 86Sr and relatively low 230Th/ 238U and a ‘lithosphere’ endmember with less radiogenic 87Sr/ 86Sr and relatively larger 230Th/ 238U. Remarkably, all the lavas, except for a few arguably older samples, are characterized by 226Ra deficits relative to 230Th. On the basis of water content and trace element systematics, we argue that this is due to melting in the presence of phlogopite, which is only stable at lithosphere temperatures. A melting model including the diffusive exchange of elements among phlogopite, garnet and melt is used to constrain melting conditions of the lithosphere. These unusual 226Ra– 230Th signatures can be explained by relatively slow melting rates at low matrix porosity. Our model also demonstrates that the effective partitioning behavior is dependent on the melting rate. A simple thermal model for lithosphere heating and melting is in good agreement with predicted melting rates.

Shallow origin for South Atlantic Dupal Anomaly from lower continental crust: Geochemical evidence from the Mid-Atlantic Ridge at 26°S

We measured trace element concentrations and Pb isotope compositions of fresh volcanic glass samples from the Mid-Atlantic Ridge at 26°S, and from nearby off-axis seamounts. The samples have previously been studied for major elements and Sr–Nd–He isotopes. All samples are depleted MORB, and include some of the most incompatible trace element depleted lavas yet reported from the Atlantic. The seamount lavas are more depleted in highly incompatible elements than the axial lavas, but have high Sr, Pb and Eu concentrations, relative to REE of similar incompatibility. The lavas with the highest Sr/Nd, Pb/Ce and Eu/Eu⁎ have the highest 3He/4He (up to 11.0 R/RA) ratios and the lowest incompatible trace element concentrations. They also have the highest 87Sr/86Sr (up to 0.7036) and 208Pb/204Pb for a given 206Pb/204Pb ratio, which are characteristics of lavas from the Dupal Anomaly in the South Atlantic, and of many EM-1 type intraplate lavas generally.Our data place constraints on the origin of the Dupal Anomaly. The enrichments in Sr, Pb and Eu, together with the low Ca/Al ratios of the seamount lavas indicate that their mantle source consists of material that at one time contained plagioclase, and must therefore have resided at crustal pressures. We argue that the trace element and isotopic compositions of the seamount lavas are best explained by derivation from a mantle source contaminated with lower continental crust, which was introduced into the upper mantle during continental rifting and breakup in the South Atlantic. Our results support previous suggestions that the Dupal Anomaly in the South Atlantic has a relatively recent, shallow origin in lower continental crust and continental lithospheric mantle, rather than in recycled material supplied from the deeper mantle by plumes.Plate reconstructions place the Parana–Etendeka flood basalt province over the central part of the Dupal Anomaly at the time of rifting of South America and Africa at 134 Ma. The flood basalts which have undergone the least crustal-level contamination also have extreme Dupal compositions. We speculate that delamination of dense lower continental crust during continental rifting causes flood basalt magmatism, whilst variably polluting the upper oceanic mantle with continental material.

He, Ne and Ar isotopic composition of Fe-Mn crusts from the western and central Pacific Ocean and implications for their genesis

The noble gas nuclide abundances and isotopic ratios of the upmost layer of Fe-Mn crusts from the western and central Pacific Ocean have been determined. The results indicate that the He and Ar nuclide abundances and isotopic ratios can be classified into two types: low 3He/4He type and high 3He/4He type. The low 3He/4He type is characterized by high 4He abundances of 191×10−9 cm3·STP·g−1 on average, with variable 4He, 20Ne and 40Ar abundances in the range (42.8−421)×10−9 cm3·STP·g−1, (5.40−141)×10−9 cm3·STP·g−1, and (773−10976)×10−9 cm3·STP·g−1, respectively. The high 3He/4He samples are characterized by low 4He abundances of 11.7×10−9 cm3·STP·g−1 on average, with 4He, 20Ne and 40Ar abundances in the range of (7.57−17.4)×10−9 cm3·STP·g−1, (10.4−25.5)×10−1 cm3·STP·g−1 and (5354−9050)×10−9 cm3·STP·g−1, respectively. The low 3He/4He samples have 3He/4He ratios (with R/RA ratios of 2.04−2.92) which are lower than those of MORB (R/R A=8±1) and 40Ar/36Ar ratios (447−543) which are higher than those of air (295.5). The high 3He/4He samples have 3He/4He ratios (with R/R A ratios of 10.4−12.0) slightly higher than those of MORB (R/R A=8±1) and 40Ar/36Ar ratios (293−299) very similar to those of air (295.5). The Ne isotopic ratios (20Ne/22Ne and 21Ne/22Ne ratios of 10.3−10.9 and 0.02774−0.03039, respectively) and the 38Ar/36Ar ratios (0.1886−0.1963) have narrow ranges which are very similar to those of air (the 20Ne/22Ne, 21Ne/22Ne, 38Ar/36Ar ratios of 9.80, 0.029 and 0.187, respectively), and cannot be differentiated into different groups. The noble gas nuclide abundances and isotopic ratios, together with their regional variability, suggest that the noble gases in the Fe-Mn crusts originate primarily from the lower mantle. The low 3He/4He type and high 3He/4He type samples have noble gas characteristics similar to those of HIMU (High U/Pb Mantle)-and EM (Enriched Mantle)-type mantle material, respectively. The low 3He/4He type samples with HIMU-type noble gas isotopic ratios occur in the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain and the Mid-Pacific Seamounts whereas the high 3He/4He type samples with EM-type noble gas isotopic ratios occur in the Line Island Chain. This difference in noble gas characteristics of these crust types implies that the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain, and the Mid-Pacific Seamounts originated from HIMU-type lower mantle material whereas the Line Island Chain originated from EM-type lower mantle material. This finding is consistent with variations in the Pb-isotope and trace element signatures in the seamount lavas. Differences in the mantle source may therefore be responsible for variations in the noble gas abundances and isotopic ratios in the Fe-Mn crusts. Mantle degassing appears to be the principal factor controlling noble gas isotopic abundances in Fe-Mn crusts. Decay of radioactive isotopes has a negligible influence on the nuclide abundances and isotopic ratios of noble gases in these crusts on the timescale of their formation.

Formation of island arc dacite magmas by extreme crystal fractionation: An example from Brothers Seamount, Kermadec island arc (SW Pacific)

Dacitic to rhyolitic glasses from Brothers Seamount in the southern Kermadec island arc lie on very tight major element trends and have formed by fractional crystallization from a basaltic magma rather than by partial melting of amphibolitic lower crustal rocks. The radiogenic isotope and highly incompatible element ratios of the dacites resemble the composition of basalts from other volcanoes in the same segment of the island arc which probably represent analogues to the basaltic parental magma of the Brothers Seamount lavas. Glass compositions show that the magmas did not degas extensively but retained much of their volatile inventory with very high Cl (4000 to 7000 ppm) but low water contents (∼2 wt.%). The Cl / K of 0.25 is constant in the Brothers Seamount lavas and probably reflects the composition of a sedimentary slab component in the subarc mantle below the volcano. The Brothers Seamount Cl / K reflects a sedimentary fluid rather than a melt and is lower than the Cl / K observed in Valu Fa Ridge lavas showing an influence of subducted altered basaltic crust, probably due to the higher K contents of sediments than in basalts.

Origin of depleted components in basalt related to the Hawaiian hot spot: Evidence from isotopic and incompatible element ratios

The radiogenic isotopic ratios of Sr, Nd, Hf, and Pb in basaltic lavas associated with major hot spots, such as Hawaii, document the geochemical heterogeneity of their mantle source. What processes created such heterogeneity? For Hawaiian lavas there has been extensive discussion of geochemically enriched source components, but relatively little attention has been given to the origin of depleted source components, that is, components with the lowest 87Sr/86Sr and highest 143Nd/144Nd and 176Hf/177Hf. The surprisingly important role of a depleted component in the source of the incompatible element‐enriched, rejuvenated‐stage Hawaiian lavas is well known. A depleted component also contributed significantly to the ∼76–81 Ma lavas erupted at Detroit Seamount in the Emperor Seamount Chain. In both cases, major involvement of MORB‐related depleted asthenosphere or lithosphere has been proposed. Detroit Seamount and rejuvenated‐stage lavas, however, have important isotopic differences from most Pacific MORB. Specifically, they define trends to relatively unradiogenic Pb isotope ratios, and most Emperor Seamount lavas define a steep trend of 176Hf/177Hf versus 143Nd/144Nd. In addition, lavas from Detroit Seamount and recent rejuvenated‐stage lavas have relatively high Ba/Th, a characteristic of lavas associated with the Hawaiian hot spot. It is possible that a depleted component, intrinsic to the hot spot, has contributed to these young and old lavas related to the Hawaiian hot spot. The persistence of such a component over 80 Myr is consistent with a long‐lived source, i.e., a plume.

Petrogenesis of lavas from Detroit Seamount: Geochemical differences between Emperor Chain and Hawaiian volcanoes

The Hawaiian Ridge and Emperor Seamount Chain define a hot spot track that provides an 80 Myr record of Hawaiian magmatism. Detroit Seamount (∼76 to 81 Ma) is one of the oldest Emperor Seamounts. Volcanic rocks forming this seamount have been cored by the Ocean Drilling Program at six locations. Only tholeiitic basalt occurs at Site 884 on the eastern flank and only alkalic basalt, probably postshield lavas, occurs at Sites 883 and 1204 on the summit plateau. However, at Site 1203 the basement core (453 m penetration) includes four thick flows of pahoehoe alkalic basalt underlying ∼300 m of volcaniclastic rocks interbedded with submarine erupted tholeiitic basalt. The geochemical characteristics of these alkalic lavas indicate that phlogopite was important in their petrogenesis; they may represent preshield stage volcanism. The surprising upward transition from subaerial to submarine eruptives implies rapid subsidence of the volcano, which can be explained by the inferred near‐ridge axis setting of the seamount at ∼80 Ma. A near‐ridge axis setting with thin lithosphere is also consistent with a shallow depth of melt segregation for Detroit Seamount magmas relative to Hawaiian magmas, and the significant role for plagioclase fractionation as the Detroit Seamount magmas evolved in the crust. An important long‐term trend along the hot spot track is that 87Sr/86Sr decreases in lavas erupted from ∼40 to 80 Ma. Tholeiitic basalt at Site 884 on Detroit Seamount is the extreme and overlaps with the 87Sr/86Sr‐143Nd/144Nd field of Pacific mid‐ocean ridge basalts (MORB). Complementary evidence for a depleted component in Detroit Seamount lavas is that relative to Hawaiian basalt, Detroit Seamount lavas have lower abundances of incompatible elements at a given MgO content. These lavas, especially from Sites 883 and 884, trend to extremely unradiogenic Pb isotopic ratios which are unlike MORB erupted at the East Pacific Rise. A component with relatively low 87Sr/86Sr and 206Pb/204Pb is required. Lavas erupted from a spreading center in the Garrett transform fault, 13°28′S on the East Pacific Rise, have this characteristic. A plausible hypothesis is mixing of a plume‐related component with a component similar to that expressed in lavas from the Garrett transform fault. However, basaltic glasses from Detroit Seamount also have relatively high Ba/Th, a distinctive characteristic of Hawaiian lavas. We argue that all Detroit Seamount lavas, including those from Site 884, are related to the Hawaiian hot spot. Rejuvenated stage Hawaiian lavas also have high Ba/Th and define a trend to low 87Sr/86Sr and 206Pb/204Pb. We speculate that rejuvenated stage lavas and Detroit Seamount lavas sample a depleted mantle component, intrinsic to the plume, over the past 80 Myr.

The Cocos and Carnegie Aseismic Ridges: a Trace Element Record of Long-term Plume–Spreading Center Interaction

The aseismic Cocos and Carnegie Ridges, two prominent bathymetric features in the eastern Pacific, record ∼20 Myr of interaction between the Galapagos hotspot and the adjacent Galapagos Spreading Center. Trace element data determined by inductively coupled plasma-mass spectrometry in >90 dredged seamount lavas are used to estimate melt generation conditions and mantle source compositions along the ridges. Lavas from seamount provinces on the Cocos Ridge are alkalic and more enriched in incompatible trace elements than any in the Galapagos archipelago today. The seamount lavas are effectively modeled as small degree melts of a Galapagos plume source. Their eruption immediately follows the failure of a rift zone at each seamount province's location. Thus the anomalously young alkalic lavas of the Cocos Ridge, including Cocos Island, are probably caused by post-abandonment volcanism following either a ridge jump or rift failure, and not the direct activity of the Galapagos plume. The seamounts have plume-like signatures because they tap underlying mantle previously infused with Galapagos plume material. Whereas plume heterogeneities appear to be long-lived, tectonic rearrangements of the ridge plate boundary may be the dominant factor in controlling regional eruptive behavior and compositional variations.

Volcanism on the Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71°N): age, source, and relationship to the Iceland and putative Jan Mayen plumes

The Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71°N) is a topographically anomalous shallow area with scattered volcanic peaks extending between the island of Jan Mayen and East Greenland and straddling the northern segment of the mid-Atlantic Kolbeinsey Ridge axis. Basalts dredged from the Eggvin Bank range from variably depleted, tholeiitic, near-axis lavas to enriched, transitional-to-alkaline, off-axis seamount lavas. In terms of normalised incompatible element patterns, the most depleted, near-axis tholeiite is similar to neighbouring Kolbeinsey Ridge basalts, whereas the off-axis, transitional-to-alkaline lavas are similar to other alkaline basalts occurring close to the Eggvin Bank region, e.g., those of Jan Mayen. 40Ar/39Ar step heating data indicate that the off-axis seamount lavas are coeval with other alkaline lavas erupted in the Central Norwegian-Greeland Sea at ca. 0.6–0.7 Ma. In contrast, the Eggvin near-axis tholeiites are <0.1 Ma. Volcanic peaks west and north of Jan Mayen show no indication of a systematic age progression. Therefore, the Jan Mayen hot spot hypothesis is not supported by the available radiometric age data. Sr, Nd, and Pb isotope compositions of near-axis and off-axis Eggvin Bank lavas are distinct, implying differences in their mantle sources. Isotope ratios of the off-axis basalts (87Sr/86Sr=0.70344–0.70352, 143Nd/144Nd=0.51283–0.51288, 206Pb/204Pb=18.82–18.85) resemble those of neighbouring alkali basalt occurrences, however, isotope ratios of the near-axis tholeiites correspond to lavas erupting in the south-eastern volcanic zone of Iceland, e.g., at Vestmannaeyjar. The near-axis tholeiites are generated by an unusual source with highly radiogenic Pb (206Pb/204Pb=18.95) together with relatively radiogenic Nd (143Nd/144Nd=0.51295) and low-radiogenic Sr (87Sr/86Sr=0.70314), respectively, representing an unique composition in the mantle north of central Iceland. The overlap in isotope compositions between Eggvin Bank near-axis tholeiites and south-east Iceland alkaline lavas could be an indication that the Eggvin Bank tholeiite source was derived from the Iceland plume and that it was emplaced in the upper mantle by the original Iceland plume head during the Early Tertiary as suggested by Trønnes et al. [Trønnes, T., Planke, S., Sundvoll, B., Imsland, P., 1999. Recent volcanic rocks from Jan Mayen: low degree melt fractions of enriched north-east Atlantic mantle. J. Geophys. Res. 104, 7153–7167]. Isotope and trace element data indicate an abrupt change in source composition along the Kolbeinsey Ridge axis at latitude ca. 70.6°N, apparently reflecting a boundary between two chemically distinct mantle domains with limited interaction. Based on Pb versus Pb isotope diagrams, no dispersion of enriched material is observed adjacent to the hypothetical Jan Mayen/Jan Mayen Platform plume, neither to the north-east along the Southern Mohns Ridge nor to the south along the Central Kolbeinsey Ridge.

Basalts from the Efate Island Group, central section of the Vanuatu arc, SW Pacific: geochemistry and petrogenesis

Basalts of the Efate Island Group (<0.7 Ma) occur in the central part of the Vanuatu island arc between the ‘normal’ southern section of the arc and the central part of the arc affected by collision with the Eocene D'Entrecasteaux Ridge. These basalts form small cones constructed upon a volcaniclastic apron principally composed of trachydacitic breccias. Temporal and compositional features allow identification of three basalt groups. Subtle compositional differences exist between two of these suites, whereas the third shows pronounced enrichment in K and Rb, with few differences for other elements. These variations are taken to record slight compositional variations in the ambient mantle wedge composition, with the high-K, high-Rb suite deriving from localised zones relatively enriched in phlogopite. Compared with other Vanuatu arc basalts, the Efate basalts show appreciable enrichment in Sr and P 2O 5. The Pb isotopic characteristics of the Efate basalts are commensurate with the transitional position of this island within the Vanuatu arc, between the southern arc lavas and those erupted in the region affected by D'Entrecasteaux Ridge collision. However, 87Sr/ 86Sr values (0.70387–0.70424) are slightly higher and 143Nd/ 144Nd values slightly lower than predicted by the spatial change of these values along the arc. These features are taken to indicate derivation of the Efate basalts from a local, unusual mantle source. Two contrasting models are considered to account for the unusual features of the source mantle of these basalts. Model 1 suggests that the high Sr and P 2O 5 characteristics of the peridotite source of the Efate basalts were produced by subduction of an intraplate volcano similar to ORSTOM seamount, currently subducting immediately northwest of Efaté. Recent studies have demonstrated massive P 2O 5 and Sr enrichments in highly altered formerly glassy seamount lavas and hyaloclastites. Model 2 proposes that the ambient strongly depleted mantle wedge has been locally modified by carbonatite metasomatism. A well-defined tear or gap in the subducted slab beneath the Efate sector of the Vanuatu arc may have provided appropriate conditions for production of an ephemeral carbonatite partial melt from asthenosphere convecting into the hole in the slab. This melt invaded the mantle wedge, stamping enrichment of Sr and P 2O 5 on a mantle wedge peridotite that had already been metasomatised by fluids derived from sediments on the subducting plate.

Isotope and trace element variations in lavas from Raivavae and Rapa, Cook–Austral islands: constraints on the nature of HIMU- and EM-mantle and the origin of mid-plate volcanism in French Polynesia

We have examined the major and trace element and Sr–Nd–Hf–Pb–Os isotope variations in two suites of lavas from the islands of Raivavae and Rapa from the Cook–Austral chain to better constrain the nature and origin of the source(s) of these lavas. Combined with data from other Cook–Austral islands and seamounts, the Raivavae and Rapa samples reveal the presence of three distinct components involved in the genesis of these lavas, a HIMU component, a depleted (or DM) component, and an “enriched” (EM) component. Northern Cook–Australs lavas (those north of the Austral Fracture Zone (AFZ)) falling along the “Macdonald” pseudo-age progression reflect mixing of the HIMU and DM components, whereas lavas south of the AFZ reflect mixing of the DM and EM components.Strong depletions in potassium and other fluid soluble elements in HIMU lavas suggest that the HIMU source contains ancient recycled (dehydrated) oceanic crust. The EM lavas have systematically less radiogenic Os-isotopes than the HIMU lavas. Therefore, the enriched or EM component cannot be generated through the addition of pelagic or terrigenous sediment to the HIMU source. Instead, the EM component in the southern Cook–Australs may represent recycled subduction-modified sub-arc mantle. High 208Pb/204Pb values in the DM component relative to East Pacific Rise mid-ocean ridge basalts (MORB) preclude derivation of this component from assimilated lithospheric mantle. The Sr–Nd–Pb isotope characteristics of the DM component resemble Pacific alkalic seamount lavas, and may derive from easily fusible components (e.g., pyroxenite veins) dispersed in the Pacific upper mantle.The Raivavae and Rapa lavas define a Hf–Nd isotope correlation that is considerably steeper than the mantle Hf–Nd isotope array. The unusually low εHf values (for a given εNd value) characteristic of the HIMU source are consistent with the presence of recycled oceanic crust. However, the presence of low εHf values in the Rapa EM lavas reveals that anomalously low εHf values are not restricted to the HIMU source. Regardless of the ultimate origin of the Raivavae and Rapa lavas, the comparatively low εHf values in these lavas indicate that multiple low-εHf components are present in the mantle. The addition of these components may help resolve the apparent terrestrial Hf–Nd mass-imbalance with respect to estimates of the bulk earth Nd- and Hf-isotope composition.Islands and seamounts from the Cook–Austral chain do not follow a clear age progression as predicted by the standard plume model for the origin of ocean island basalts (OIB). Previous models for the origin of the Cook–Austral chain have appealed to the existence of multiple small plumes. However, these models are implausible because they do not explain how several deep-seated plumes become aligned with each other in the direction of plate motion. We propose that volcanism along the Cook–Austral chain (and perhaps in much of French Polynesia) is generated through self-perpetuating melting anomalies that preferentially sample enriched components in a marble-cake mantle. These melting anomalies preferentially form or strengthen where lithospheric boundary conditions are favorable, such as at pre-existing transform faults or where the lithosphere has been thinned by a previous period of volcanism.

Comparison between magmatic activity and gold mineralization at Conical Seamount and Lihir Island, Papua New Guinea

Summary Grab samples from the submarine Conical Seamount, located about 10 km south of the giant Ladolam gold deposit, Lihir Island, reveal the highest gold concentrations yet reported from the modern seafloor. Lavas from Conical Seamount are characterized by high K2O contents, high K2O=Na2O ratios, and high Ce=Yb ratios, which are typical of high-K igneous rocks from oceanic (island) arc-settings. The primitive character of the rocks from Conical Seamount implies a magmatic evolution related to a single eruptive phase, which contrasts with the more evolved rocks forming Lihir Island. Geochemical as well as mineral chemical data suggest that the melts from both Conical Seamount and Lihir Island originate from the same magma source. In common with the samples from Lihir Island, elevated oxygen fugacities of 0.7–2.5 log units above the FMQ buffer are recorded from the Conical Seamount lavas. There are distinct differences between the mineralization styles at Conical Seamount and at the Ladolam gold deposit, Lihir Island. While early-stage pyritic stockwork mineralization at Conical Seamount is hosted by clay-silica altered basaltic rocks

Melt inclusions as indicators of parental magma diversity on the northern East Pacific Rise

The axial plumbing system is recognized as the site of much of the chemical processing that takes place at mid-ocean ridges. However, the extent of that processing and the characteristics of the original parental components are difficult to characterize using the lavas that erupted at the surface alone. To better understand the relative roles of different magmatic processes in the formation of axial and off-axis magma systems, we have studied plagioclase and olivine-hosted melt inclusions from two axial segments, 10°30′N and 11°20′N, and four seamounts between 5°47′N and 9°17′N of the northern East Pacific Rise (EPR). These data are a potential source of information on the characteristics of precursor magmas. By comparing the compositions of the inclusions with those of the host lava suite, one can discern the nature of the intervening processes. The melt inclusions found in these axial and seamount lavas were rehomogenized and analyzed by electron and ion microprobe for major and trace elements. The lava groups and their inclusions overlap and contain very similar N-MORB compatible and incompatible element concentrations, with the exception of one N-MORB host lava which contains both N- and E-MORB inclusions. Inclusion compositions are co-linear with and generally fall at the primitive end of the host lava suites. Inclusion diversity, i.e. the chemical variation in melt inclusion compositions within a single sample, decreases with decreasing MgO, probably representing contemporaneous mixing and fractionation. Seamount lavas differ from axial lavas in that they are more crystal-rich and they contain a greater number of inclusions that are generally more primitive. In addition, they exhibit a larger compositional range in both the incompatible major and trace elements. Axial and seamount lavas have comparable crystal assemblages and chemistry, melt inclusion diversity and range of compositions. All these characteristics suggest that axial and seamount magmas are formed in the same mantle-melting regime. We believe that axial and seamount magmas are created from a single parent magma array derived by the melting of a relatively homogeneous source. The parent magmas undergo an initial stage of fractionation and mixing, then, the axial and seamount magma paths diverge. Axial lavas continue through the axial magma chamber (AMC), becoming more fractionated and having more phenocrysts removed (possibly in the upper melt lens). Seamount magmas travel to their volcanoes without further significant fractionation and crystal separation. The East Pacific Rise differs from slow- and moderate-spreading rate ridges (AMAR, Juan de Fuca (JdF), Gorda and SEIR) in its lack of phyric lavas, scarcity of inclusions, lack of chemical variation between melt inclusions from the same crystal, lack of overall compositional variability and absence of E-MORB inclusions in the majority of the N-MORB host lavas, suggesting that EPR must have a fundamentally different axial plumbing system characterized by more extensive fractionation and mixing in the upper mantle and lowermost crust.

President Jackson Seamounts, northern Gorda Ridge: Tectonomagmatic relationship between on‐ and off‐axis volcanism

Basaltic lavas and hyaloclastites dredged from the President Jackson Seamounts, a 65‐km‐long, linear volcanic chain starting near latitude 42°20′N west of the northern Gorda Ridge, are normal mid‐ocean ridge basalt (MORB), with compositions ranging from highly depleted (0.07% K2O, 0.54 La/SmN) to moderately enriched in incompatible elements (0.24% K2O, 0.86 La/SmN). The seamount lavas are similar in many respects to those erupted at the adjacent ridge but have some important geochemical differences. Seamount lavas are concentrated toward primitive compositions with higher MgO contents (to 9.3%, Mg number 69.5) than most of the nearby ridge lavas. They also have systematically lower TiO2 and FeO and higher CaO, Na2O, and Sr at comparable MgO content than adjacent ridge basalts. Seamount basalts contain phenocrysts in equilibrium with the melt and lack compositionally diverse glass inclusions and compositional zoning common in phenociysts of most ridge basalt. Minor and trace element variability at a given MgO content indicates complex petrogenetic processes. Radiogenic isotopic ratios indicate somewhat less depleted sources than for basalt from the adjacent ridge axis. The higher Pb isotopic ratios of some lavas trend toward enriched MORB. The seamounts typically have multiple, nested calderas or pit craters, stepping downward toward the ridge axis, indicating formation in the active, near‐ridge, extensional environment. The predominantly primitive nature of the lavas suggests that they pass through crustal magma reservoirs, presumably underlying the calderas, very rapidly. The lack of evidence for magma mixing suggests that batches of magma are delivered to the seamounts episodically and either solidify or are drained into ridge‐parallel faults before the next batch arrives. In contrast, lavas from the ridge axis show evidence for magma mixing involving more fractionated melts that show evidence for clinopyroxene fractionation. Despite a lack of seismic evidence for magma chambers under slow spreading centers, continuous melt zones must be present under the Gorda Ridge axis to give the ubiquitous imprint of magma mixing.

Identification and implications of off-axis lava flows around the East Pacific Rise

[1] Off-axis eruptions at ocean ridges provide critical information with respect to underlying crustal plumbing and mantle melting systems. A detailed study of basaltic glass samples around the East Pacific Rise from 12°00′ to 12°30′N provides geological evidence for the existence of off-axis eruptions with a distinctive chemical composition. This composition has not been found along the axis of the EPR from 8° to 14°N except at a ridge-transform intersection but has been recovered in numerous locations that were farther than 1 km off axis. These off-axis normal mid-ocean ridge basalts, or OA-NMORB, are distinguished by low Na2O, Sr, and Al2O3 and are unusually depleted in incompatible elements. Moderately enriched off-axis transitional mid-ocean ridge basalts (OA-TMORB) with the same compositional tendencies can also be identified. Comparison of EPR axis lavas and the OA type suggests that they come from the same range of (unmelted) mantle source compositions but that the source of the OA magmas was depleted in incompatible trace elements by removal of a small- degree partial melt. This would be consistent with the OA type as an EPR pooled melt that is missing the low-degree melt fraction from deep in the melting regime, which provides a reasonable physical model for their formation. In this case, off-axis magmas do not represent the same range of chemical variation as magmas delivered to the axial magma system. The OA-NMORB are similar to depleted lavas from near-EPR seamounts. Other seamount lavas with depleted trace elements have TMORB-like major elements, and may be classified as OA-TMORB. The similarity between seamount lavas and the lavas erupted off axis close to the EPR suggests that the two are manifestations of the same phenomenon. We suggest that seamount-type volcanism effectively starts within 1–2 km of the axis. This is within the range where lavas derived from the axial plumbing system may also erupt. Therefore there is a narrow zone where young lava flows from the axial plumbing system and from the off-axis systems may overlap. Lavas erupted off axis may ultimately cover 20% of the seafloor around the EPR, which is substantially more than previous estimates that were based primarily on morphological studies.

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

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.

Phenocryst and melt inclusion chemistry of near-axis seamounts, Valu Fa Ridge, Lau Basin: insight into mantle wedge melting and the addition of subduction components

Phenocryst assemblages, and mineral and melt inclusion compositions of magmas erupted at near-axis seamounts on either side of Valu Fa Ridge provide a hitherto unprecedented insight into the complexity of magma generation in this back-arc basin tectonic setting. Two fundamentally different primitive primary melt compositions are identified based on melt inclusion compositions, olivine phenocryst chemistry, and the early co-crystallisation of either magnesian clinopyroxene (Mg# to 93) or magnesian orthopyroxene (Mg# to 93.5) with magnesian olivine (to Fo 94) and Cr-rich spinel (Cr# = 0.78–0.87). One magma type is a H 2O-rich (∼ 2.5 wt%), high-CaO (∼ 14 wt%), low-Al 2O 3 (∼ 8 wt%) magnesian basalt, variants of which occur in both the eastern and western seamounts. The other is a low-Ca boninite-like magma that only occurs as a component of the western seamount magmas. Large and systematic variations in incompatible trace-element compositions of melt inclusions trapped in primitive olivine phenocrysts, reflect an integration of diverse but geochemically related melt fractions to produce the magmas at each seamount. Trace-element systematics require the variable addition of a LILE-, Pb-, and Cl-rich component to the mantle wedge source with increased influence toward the Tofua arc. This component, as invoked in most models of arc magma genesis, is likely to be a supercritical aqueous fluid released by dehydrating subducting ocean crust beneath the volcanic arc front. We propose that southward propagation of the back-arc basin spreading center mantle provided heat necessary to generate both magmatic suites by decompression melting of refractory hydrated sub-arc lithosphere, probably veined by clinopyroxene-rich dykes in the case of the high-CaO magma series. These near-ridge seamount lavas are very similar to those drilled at ODP Site 839 in the Lau Basin, and we suggest that the Site 839 basalts, as well as other Lau Basin seamount arc-like magmas, were produced from sub-arc lithosphere during southward propagation of the Eastern Lau Spreading Center ∼ 2–3 Ma.

Correlated He and Sr isotope ratios in South Atlantic near-ridge seamounts and implications for mantle dynamics

4He/3He and 87Sr/86Sr ratios are highly anti-correlated for a suite of seamount glasses from both sides of the Mid-Atlantic Ridge at 26°S; the linear correlation coefficient (r2) is 0.99 for 5 localities at 3 different seamounts. The seamounts are located on crust up to 2.5 myr old, and have 4He/3He as low as 65,400 (3He/4He = 11 RA) and 87Sr/86Sr as high as 0.70350. These isotopic values are significantly lower and higher, respectively, than those for basaltic glasses recovered from 13 localities along the adjacent ridge axis, where the lowest 4He/3He ratio is 92,000 (3He/4He = 7.8 RA) and the highest 87Sr/86Sr is 0.70258. Geophysical studies and the small (1–2%) degree of helium isotope disequilibrium between vesicles and glass for three seamount lavas suggest that the seamounts formed on or near the ridge axis. Because no off-ridge hotspots are present in this area, formation of the seamounts probably involved capture by the ridge of a passive mantle heterogeneity or ‘blob’ during rift propagation and tectonic evolution of the Moore fracture zone.The HeSrNdPb isotopic results for the seamounts show a general trend toward compositions observed for the Réunion hotspot in the Indian Ocean. Collectively, the seamount and ridge axis results are somewhat enigmatic. In addition to the highly correlated He and Sr isotopes at the seamounts, a fair correlation exists between He and Nd isotopes (r2 = 0.70). In contrast, a correlation between He and Pb isotopes is absent for the seamount glasses, while an independent, positive correlation exists between 4He/3He and 206Pb/204Pb for axial lavas. Apparently, different processes are responsible for the seamount HeSrNd isotope relationships and for the nearby ridge HePb isotope relationship. If these relations are only of local significance and result from complications inherent in multi-stage mixing of more than two mantle components, then they imply that the upper mantle may contain domains with variable 4He/3He ratios, in some cases significantly lower than 80,000 (3He/4He> 9 RA). On the other hand, binary mixing adequately explains the linear HeSr isotope trend in the seamount lavas. This linear trend suggests similar 3He/86Sr ratios in the local MORB mantle source and in the source region of the low 4He/3He blob, which is most likely the lower mantle or the transition zone region. This similarity in 3He/86Sr is inconsistent with a lower mantle 3He/86Sr ratio that exceeds the upper mantle ratio by at least a factor of 50, deduced from geochemical models of mantle evolution. Consequently, rare gas models invoking a steady-state upper mantle and quasi-closed lower mantle may be inappropriate if applied at length scales on the order of ∼ 100 km, characteristic of mid-ocean ridge segments.

Mixing of magmas from enriched and depleted mantle sources in the northeast Pacific: West Valley segment, Juan de Fuca Ridge

The 50 km-long West Valley segment of the northern Juan de Fuca Ridge is a young, extension-dominated spreading centre, with volcanic activity concentrated in its southern half. A suite of basalts dredged from the West Valley floor, the adjacent Heck Seamount chain, and a small near-axis cone here named Southwest Seamount, includes a spectrum of geochemical compositions ranging from highly depleted normal (N-) MORB to enriched (E-) MORB. Heck Seamount lavas have chondrite-normalized La/Smcn∼0.3, 87Sr/86Sr=0.70235–0.70242, and 206Pb/204Pb=18.22–18.44, requiring a source which is highly depleted in trace elements both at the time of melt generation and over geologic time. The E-MORB from Southwest Seamount have La/Smcn∼1.8, 87Sr/86Sr=0.70245–0.70260, and 206Pb/204Pb=18.73–19.15, indicating a more enriched source. Basalts from the West Valley floor have chemical compositions intermediate between these two end-members. As a group, West Valley basalts from a two-component mixing array in element-element and element-isotope plots which is best explained by magma mixing. Evidence for crustal-level magma mixing in some basalts includes mineral-melt chemical and isotopic disequilibrium, but mixing of melts at depth (within the mantle) may also occur. The mantle beneath the northern Juan de Fuca Ridge is modelled as a plum-pudding, with “plums” of enriched, amphibole-bearing peridotite floating in a depleted matrix (DM). Low degrees of melting preferentially melt the “plums”, initially removing only the amphibole component and producing alkaline to transitional E-MORB. Higher degrees of melting tap both the “plums” and the depleted matrix to yield N-MORB. The subtly different isotopic compositions of the E-MORBs compared to the N-MORBs require that any enriched component in the upper mantle was derived from a depleted source. If the enriched component crystallized from fluids with a DM source, the “plums” could evolve to their more evolved isotopic composition after a period of 1.5–2.0 Ga. Alternatively, the enriched component could have formed recently from fluids with a less-depleted source than DM, such as subducted oceanic crust. A third possibility is that enriched material might be dispersed as “plums” throughout the upper mantle, transported from depth by mantle plumes.

Focused mantle upwelling beneath mid-ocean ridges: evidence from seamount formation and isostatic compensation of topography

Observational evidence for the geometry of mantle flow beneath mid-ocean ridges is often considered limited. I offer two constraints supporting interpretations of strong focusing of upwelling through the depths where MORB magmas are generated. The first is based on a modification of the model of Davis and Karsten [ 1] for the formation of near-axis seamounts by the melting of fertile mantle heterogeneities. Several types of observations indicate that most seamounts in the eastern Pacific form in a remarkably narrow range of distance from the axis, about 6–15 km. The outer limit of this range is interpreted to represent the width of the upwelling zone in the upper part of the garnet stability field (depth ∼ 80 km) where the most fertile mantle melts, while the inner limit indicates the width of most magma generation (perhaps depths of 30–50 km). The diverse and depleted chemistry of young seamount lavas is inferred to reflect originally enriched magmas generated at depth that bypass the principal zone of melting and interact with strongly depleted material at intermediate depths. A comparable and entirely independent constraint on the width of most magma generation is available from the assumption that anomalously shallow topography at ridge axes is isostatically compensated by melt at depth. The width of the topographic anomaly limits the melt zone to within 10 km from the axis, while the amplitude of the gravity anomaly, at least in some places, requires most of the melt to be deeper than 15 km. If the fraction of melt is assumed to be small ( ∼ 2%), the partial melt must extend to depths exceeding 40 km.