Hydrothermal activity and magma genesis along a propagating back-arc basin: Valu Fa Ridge (southern Lau Basin)

Valu Fa Ridge is an intraoceanic back-arc spreading center located at the southern prolongation of the Lau basin. Bathymetric observations as well as detailed sampling have been carried out along the spreading axis in order to trace hydrothermal and volcanic activity and to study magma generation processes. The survey shows that widespread lava flows from recent volcanic eruptions covered most of the Vai Lili hydrothermal vent field; only diffuse low-temperature discharge and the formation of thin layers of siliceous precipitates have been observed. Evidence of present-day hydrothermal activity at the Hine Hina site is indicated by a thermal anomaly in the overlying water column. Our studies did not reveal any signs of hydrothermal activity either above the seismically imaged magma chamber at 22°25′S or across the southern rift fault zone (22°51′S). Lavas recovered along the Valu Fa Ridge range from basaltic andesites to rhyolites with SiO2 contents higher than reported from any other intraoceanic back-arc basin. On the basis of the highly variable degrees of crystal fractionation along axis, the development of small disconnected magma bodies is suggested. In addition, the geochemical character of the volcanic rocks shows that the transition zone from oceanic spreading to propagating rifting is located south of the Hine Hina vent field in the vicinity of 22°35′S

Melt generation beneath Arctic Ridges: Implications from Ule

We present new 238U-230Th-226Ra-210Pb, 235U-231Pa, and Nd, Sr, Hf, and Pb isotope data for the slow- to ultraslow-spreading Mohns, Knipovich, and Gakkel Ridges. Combined with previous work, our data from the Arctic Ridges cover the full range of axial depths from the deep northernmost Gakkel Ridge shallowing upwards to the Knipovich, Mohns, and Kolbeinsey Ridges north of Iceland. Age-constrained samples from the Mohns and Knipovich Ridges have (230Th/238U) activity ratios ranging from 1.165 to 1.30 and 1.101 to 1.225, respectively. The high 230Th excesses of Kolbeinsey, Mohns, and Knipovich mid-ocean ridge basalts (MORB) are erupted from ridges producing relatively thin (Mohns, Knipovich) to thick (Kolbeinsey) oceanic crust with evidence for sources ranging from mostly peridotite (Kolbeinsey) to eclogite-rich mantle (Mohns, Knipovich). Age-constrained lavas from 85ºE on the Gakkel Ridge, on the other hand, overlie little to no crust and range from small (~5%) 230Th excesses to small 238U excesses (~5%). The strong negative correlation between (230Th/238U) values vs. axial ridge depth among Arctic ridge basalts is controlled not only by solidus depth influence on 238U-230Th disequilibria, but also by variations in mantle source lithology and depth to the base of the lithosphere, which is expected to vary at ultra-slow spreading ridges. Small 231Pa excesses (65% excess) in age constrained basalts support the presence of eclogite in the mantle source for this region. Conversely, the ultraslow-spreading Gakkel Ridge basalts are homogeneous, with Sr, Nd, and Hf radiogenic isotopic signatures indicative of a long time-averaged depleted mantle source. The Gakkel samples have minimum (226 Ra/230Th) ratios ranging from 3.07 to 3.65 ± 3%, which lie along and extend the global negative correlation between 226Ra and 230Th excesses observed in MORB. The new 230Th-226Ra data support a model for global MORB production in which deep melts record interaction with shallower materials. This scenario requires either mixing with shallow-derived melts, or melt-rock reaction with shallower rocks in the lithosphere or crust

Submarine back-arc lava with arc signature : Fonualei Spreading Center, northeast Lau Basin, Tonga

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): B08S07, doi:10.1029/2007JB005451.We present major, volatile, and trace elements for quenched glasses from the Fonualei Spreading Center, a nascent spreading system situated very close to the Tofua Volcanic Arc (20 km at the closest), in the northeast Lau Basin. The glasses are basalts and basaltic andesites and are inferred to have originated from a relatively hot and depleted mantle wedge. The Fonualei Spreading Center shows island arc basalt (IAB) affinities, indistinguishable from the Tofua Arc. Within the Fonualei Spreading Center no geochemical trends can be seen with depth to the slab and/or distance to the arc, despite a difference in depth to the slab of >50 km. Therefore we infer that all the subduction-related magmatism is captured by the back arc as the adjacent arc is shut off. There is a sharp contrast between the main spreading area of the Fonualei Spreading Center (FSC) and its northernmost termination, the Mangatolu Triple Junction (MTJ). The MTJ samples are characteristic back-arc basin basalts (BABB). We propose that the MTJ and FSC have different mantle sources, reflecting different mantle origins and/or different melting processes. We also document a decrease in mantle depletion from the south of the FSC to the MTJ, which is the opposite to what has been documented for the rest of the Lau Basin where depletion generally increases from south to north. We attribute this reverse trend to the influx of less depleted mantle through the tear between the Australian and the Pacific plates, at the northern boundary of the Lau Basin.NSK acknowledges the support of an A.E. Ringwood Scholarship from the RSES

Young volcanism and related hydrothermal activity at 5°S on the slow-spreading southern Mid-Atlantic Ridge

The effect of volcanic activity on submarine hydrothermal systems has been well documented along fast- and intermediate-spreading centers but not from slow-spreading ridges. Indeed, volcanic eruptions are expected to be rare on slow-spreading axes. Here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated, apparently magmatically robust segment center on the slow-spreading southern Mid-Atlantic Ridge near 5°S. Three high-temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase-separated, vapor-type fluids. Exit temperatures at one of the fields reach up to 407°C, at conditions of the critical point of seawater, the highest temperatures ever recorded from the seafloor. Fluid and vent field characteristics show a large variability between the vent fields, a variation that is not expected within such a limited area. We conclude from mineralogical investigations of hydrothermal precipitates that vent-fluid compositions have evolved recently from relatively oxidizing to more reducing conditions, a shift that could also be related to renewed magmatic activity in the area. Current high exit temperatures, reducing conditions, low silica contents, and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source, probably related to a young volcanic eruption event nearby, in which basaltic magma is actively crystallizing. This is the first reported evidence for direct magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge

Electron microprobe characterization of ash layers in sediments from the central Bransfield basin (Antarctic Peninsula): evidence for at least two volcanic sources

Bransfield Strait, a narrow active rift with three submarine basins, separates the South Shetland Islands from northern Antarctic Peninsula. Volcanism in Bransfield Strait commenced prior to 0.75 Ma and continues, with recent subaerial eruptions at Deception, Bridgeman and Penguin islands, submarine hydrothermal activity and numerous young basaltic seamounts located along the rift axis. Gravity cores were collected from five locations within the central Bransfield basin. Diatomaceous mud interbedded with terrigenous detritus and discrete ash layers up to 10 cm thick are the commonest sediment types in all the cores. The major element compositions of glass shards within the ash layers are, apart from the uppermost layer, compositionally similar to pyroclastic units preserved on Deception Island, a young (< 0.75 Ma) active stratovolcano. The uppermost ash layer cannot be closely matched compositionally to any known source in the Antarctic-Scotia Sea-southem South America region. Its presence indicates that a volcanic centre other than Deception Island contributed ash to the Bransfield basin. Based on the shallow stratigraphical position of the compositionally distinctive ash layer, only a few decimetres beneath the seafloor, its source volcano was probably active in historical times (< few hundred years)

Explosive activity of the Reunion Island volcanoes through the past 260,000 years as recorded in deep-sea sediments

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Structure and morphology of submarine volcanism in the hotspot region around Réunion Island, western Indian Ocean

The form of the Deccan-Maldives-Mascarene-Réunion hotspot trace suggests that it has, at least in part, been strongly controlled by crustal structures, especially fracture zones. This makes it difficult to assess the present-day or past location of the hotspot, and thus complicates the interpretation of African plate motion reconstruction. We present here results of a cruise to the Réunion area of which the aims were: (a) to determine the extent of present-day volcanism associated with the Réunion hotspot in the region; and (b) to examine the rôle of pre-existing oceanic crustal structures in controlling the location of present-day volcanism. Additionally, we examined the morphology and geology of the important extinct spreading centre southwest of Réunion abandoned when spreading jumped to separate Seychelles from India during the Deccan flood basalt episode some 60–65 Ma ago. The extensive bathymetrie, seismic and geological investigation shows that significant present-day hotspot volcanism is confined to the Piton de la Fournaise edifice on Réunion Island itself. Apparently, the location of recent Réunion volcanism has not been controlled by a crustal fracture and the major fracture zones on both sides of the island are not acting as magma conduits. For plate motion reconstruction and plume flux calculation purposes, Piton de la Fournaise must be taken as the present location of the Réunion hotspot. Accretion at the extinct spreading centre progressively ceased at the time of anomaly A27 (63 Ma), and was associated with marked propagation of the rift tips

Resolving mantle components in oceanic lavas from segment E2 of the East Scotia back-arc ridge, South Sandwich Islands

The East Scotia Ridge, situated in the South Atlantic, is the back-arc spreading centre to the intra-oceanic South Sandwich arc. Samples from the ridge show a wide diversity in erupted magma compositions. Segment E2, in the northern part of the ridge, has an axial topographic high, which contrasts with the rift-like topography common to most of the ridge. Lava compositions in the segment have been modelled by mixing of magmas derived from normal mid-ocean ridge basalt (N-MORB)-like mantle, a mantle plume component similar in composition to that sampled by Bouvet Island and mantle modified by addition of components from the subducting slab. The ‘Bouvet’-like plume signature has higher 87Sr/86Sr, 206Pb/204Pb, Nb/Yb, and lower 143Nd/144Nd and 4He/3He, than the local upper mantle. It can be traced geochemically from the Bouvet Island hot spot to segment E2, via the South American-Antarctic Ridge, which connects the Bouvet triple junction to the South Sandwich subduction system. Four samples dredged from segment E2 have 4He/3He ratios of 85 000–90 200 (8.5–8.0 R/RA, where) R/RA is the 4He/3He ratio normalized to air) and three wax core samples taken from the segment axis have values of 104 300, 101 560 and 176 620 (6.9, 7.1 and 4.1 R/RA). These latter data are similar to values from the South American-Antarctic Ridge which have no discernable plume input. Whilst the dredge samples have a measurably lower 4He/3He ratio than the South American-Antarctic Ridge and samples from the segment axis, these He isotope data contrast with a dominant plume signature recorded by other petrogenetic tracers. This is interpreted to be due to re-melting of an entrained plume component, with an inherent low He concentration, incorporated into the E2 mantle. Helium depletion from the plume component can be seen to be a consequence of mantle processing and does not imply shallow-level degassing prior to entrainment within the upper-mantle-melting zone. As a consequence, He is characterized in the back-arc by values more similar to the upper mantle, whereas lithophile tracers are more influenced by the plume component