Karen S. Harpp

Mantle plume helium in submarine basalts from the galapagos platform.

Helium-3/helium-4 ratios in submarine basalt glasses from the Galapagos Archipelago range up to 23 times the atmospheric ratio in the west and southwest. These results indicate the presence of a relatively undegassed mantle plume at the Gal�pagos hot spot and place Gal�pagos alongside Hawaii, Iceland, and Samoa as the only localities known to have such high helium-3/helium-4 ratios. Lower ratios across the rest of the Gal�pagos Archipelago reflect systematic variations in the degree of dilution of the plume by entrainment of depleted material from the asthenosphere. These spatial variations reveal the dynamics of the underlying mantle plume and its interaction with the nearby Gal�pagos Spreading Center.

Submarine Fernandina: Magmatism at the leading edge of the Galápagos hot spot

New multibeam and side-scan sonar surveys of Fernandina volcano and the geochemistry of lavas provide clues to the structural and magmatic development of Galapagos volcanoes. Submarine Fernandina has three well-developed rift zones, whereas the subaerial edifice has circumferential fissures associated with a large summit caldera and diffuse radial fissures on the lower slopes. Rift zone development is controlled by changes in deviatoric stresses with increasing distance from the caldera. Large lava flows are present on the gently sloping and deep seafloor west of Fernandina. Fernandinas submarine lavas are petrographically more diverse than the subaerial suite and include picrites. Most submarine glasses are similar in composition to aphyric subaerially erupted lavas, however. These rocks are termed the “normal” series and are believed to result from cooling and crystallization in the subcaldera magma system, which buffers the magmas both thermally and chemically. These normal-series magmas are extruded laterally through the flanks of the volcano, where they scavenge and disaggregate olivine-gabbro mush to produce picritic lavas. A suite of lavas recovered from the terminus of the SW submarine rift and terraces to the south comprises evolved basalts and icelandites with MgO = 3.1 to 5.0 wt.%. This “evolved series” is believed to form by fractional crystallization at 3 to 5 kb, involving extensive crystallization of clinopyroxene and titanomagnetite in addition to plagioclase. “High-K” lavas were recovered from the southwest rift and are attributed to hybridization between normal-series basalt and evolved-series magma. The geochemical and structural findings are used to develop an evolutionary model for the construction of the Galapagos Platform and better understand the petrogenesis of the erupted lavas. The earliest stage is represented by the deep-water lava flows, which over time construct a broad submarine platform. The deep-water lavas originate from the subcaldera plumbing system of the adjacent volcano. After construction of the platform, eruptions focus to a point source, building an island with rift zones extending away from the adjacent, buttressing volcanoes. Most rift zone magmas intrude laterally from the subcaldera magma chamber, although a few evolve by crystallization in the upper mantle and deep crust.

Wolf-Darwin lineament and plume-ridge interaction in northern Galapagos

(1) The Wolf-Darwin Lineament (WDL), located in the northwestern sector of the Galapagos Archipelago, lies between the focus of the Galapagos hot spot and the Galapagos Spreading Center. Consequently, most researchers have attributed its origin to the interaction between the plume and the adjacent ridge. We propose that the WDL is caused only partially by the plume-ridge interaction, and instead that it is primarily the result of tensional stresses emanating from the inside corner of the transform fault at 91� W. An additional factor that amplifies the tension in this region is the oblique orientation of the major transform fault with respect to the Nazca plates spreading direction. This setting creates a transtensional zone whereby strain is partitioned into strike-slip motion along the transform and extension throughout the inside corner of the ridge-transform system. The area under tension is magmatic owing to the overlapping effects of the ridge and the Galapagos plume. The extensional model predicts no age-progressive volcanism, which is supported by observed age relationships. The WDL volcanoes define two distinct chemical groups: lavas erupted south of Wolf Island have compositions similar to those produced along the GSC west of 93� W, while those from the northern WDL resemble GSC lavas from the segment directly north of the lineament. This geographic distribution implies that the WDL is supplied by the same type of plume-affected mantle as the segment of the GSC that produced the lithosphere underlying the volcanoes. The observed WDL geochemical gradients are consistent with the extension model; the region under tension simply taps hybrid products of mixing at the margins of the subridge convection system and the periphery of the plume. Essentially, the stress field around the transform fault, normally not observable in a typical midocean ridge setting, is illuminated by the presence of melt from the adjacent hot spot. Components: 8821 words, 8 figures. Index Terms: 1025 Geochemistry: Composition of the mantle; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; 3035 Marine Geology and Geophysics: Midocean ridge processes.

Hafnium isotopic variations in volcanic rocks from the Caribbean Large Igneous Province and Galápagos hot spot tracks

[1] We report Hf isotope compositions of 79 lavas that record the early (∼5–95 Ma) history of the Galapagos plume volcanism. These include lavas from the Caribbean Large Igneous Province (CLIP; ∼95–70 Ma), the accreted Galapagos paleo-hot spot track terranes (54–65 Ma) of Costa Rica (Quepos, Osa and Burica igneous complexes), and the Galapagos hot spot tracks (<20 Ma) located on the Pacific seafloor (Cocos, Carnegie, Malpelo, and Coiba Ridges and associated seamounts). These samples have previously been well characterized in terms of major and trace elements, Sr-Nd-Pb isotopes and Ar/Ar ages. As a result of the relative immobility of the high field strength and rare earth elements during syn- and post-emplacement hydrothermal activity and low-temperature alteration, combined Lu-Hf and Sm-Nd isotope systematics, when used in conjunction with Pb isotopes, provide a particular powerful tool, for evaluating the source compositions of ancient and submarine lavas. The combined Nd-Hf isotope data suggest that three of the isotopically distinct source components found today in the Galapagos Islands (the Floreana-like southern component, the Fernandina-like central component, and the depleted Genovesa-like eastern component) were present in the CLIP already by 95–70 Ma. The fourth Pinta-like northern component is first recorded at about 83–85 Ma by volcanism taking place during the transition from the plume head/CLIP to plume tail stage and has then been present in the hot spot track continuously thereafter. The identification of the unique northern and southern Galapagos Plume Hf-Nd-Pb isotope source signatures within the CLIP and the oldest hot spot track lavas provides direct evidence that the CLIP represents the plume head stage of the Galapagos hot spot. Hafnium isotopes are consistent with the possibility that two types of sediment components may have contributed to the Hf, Nd and Pb isotope compositions of the Galapagos plume lavas. One component, characterized by Δ207Pb/204Pb ≈ 0 and high positive ΔeHf has an isotope signature indicative of relatively recently recycled pelagic sediment, a signature typical of the southern Galapagos island Floreana. The other component has an EM like isotopic composition resembling modern seafloor sediments with positive Δ207Pb/204Pb and lower ΔeHf, a signature typical of the northern Galapagos island Pinta.

Northern Galapagos Province: Hotspot-induced, Near-ridge Volcanism at Genovesa Island.

Genovesa Island is a small volcano located between the Galapagos hotspot and the Galapagos spreading center. Several observations suggest that Genovesa may not have originated as a direct product of the Galapagos plume, but instead by anomalous activation of the upper mantle by the plume. Genovesas lavas exhibit remarkably homogeneous, depleted compositions with no detectable plume contribution; the volcanos trace element contents indicate a deeper origin than either pristine spreading center lavas or most lavas from the Galapagos Archipelago. Genovesa is virtually indistinguishable from the Lamont Seamounts (near the East Pacific Rise) in composition, volume, height, and distance from the ridge; and Genovesa formed close to its current near-ridge location, more recently than previously assumed (age younger than 350 ka). Numerous similar volcanoes populate the northern perimeter of the Galapagos Archipelago. We propose that the northern volcanic province is the result of the serendipitous combination of excess temperatures, weak lithosphere, and regional stresses from interaction between the plume and the ridge, yielding volcanism where none would be observed otherwise. The Galapagos system may define an eruptive process at hotspots, distinct from the Hawaiian model, in which plume-related volcanism can be regionally diffuse, coeval, and compositionally variable. Such a mechanism has profound implications for our understanding of plume-ridge interactions, as well as for island ages and adaptive radiation in the Galapagos.

Crustal growth by magmatic overplating in the Galápagos

The isotopic compositions of xenoliths hosted in lavas from Floreana Island indicate that they formed from magmas unlike those at present-day Floreana. Instead, the xenoliths are geochemically more similar to magmas now erupting from Sierra Negra and Cerro Azul volcanoes, at the leading edge of the Galapagos hotspot. This is the first evidence for compositional evolution at a Galapagos volcano and indicates increasing contributions from an iso topically enriched source with time as the volcano is carried away from the focus of the hot-spot. Clinopyroxenes in many of the xenoliths exhibit positive anomalies of Sr and Eu, which are attributed to the breakdown of plagioclase. The growth of clinopyroxene at the expense of plagio clase results from compression as the crust cools. Compression is caused by growth mostly from above, as shallow intrusions and lavas load the middle and upper oceanic crust.

Ocean mixing and ice-sheet control of seawater 234U/238U during the last deglaciation.

Uranium in the deep sea The ratio of 234U to 238U in seawater underlies modern marine uranium-thorium geochronology, but it is difficult to establish the ratio precisely. Chen et al. report two 234U/238U records derived from deep-sea corals (see the Perspective by Yokoyama and Esat). The records reveal a number of important similarities to and differences from existing records of the past 30,000 years. Higher values during the most recent 10,000 years than during earlier glaciated conditions may reflect enhanced subglacial melting during deglaciation. Science, this issue p. 626; see also p. 550 The 234U/238U ratios of deep-sea corals illuminate glacially driven changes of the past 30,000 years. Seawater 234U/238U provides global-scale information about continental weathering and is vital for marine uranium-series geochronology. Existing evidence supports an increase in 234U/238U since the last glacial period, but the timing and amplitude of its variability has been poorly constrained. Here we report two seawater 234U/238U records based on well-preserved deep-sea corals from the low-latitude Atlantic and Pacific Oceans. The Atlantic 234U/238U started to increase before major sea-level rise and overshot the modern value by 3 per mil during the early deglaciation. Deglacial 234U/238U in the Pacific converged with that in the Atlantic after the abrupt resumption of Atlantic meridional overturning. We suggest that ocean mixing and early deglacial release of excess 234U from enhanced subglacial melting of the Northern Hemisphere ice sheets have driven the observed 234U/238U evolution.

Widespread Secondary Volcanism Near Northern Hawaiian Islands

Hot spot theory provides a key framework for understanding the motion of the tectonic plates, mantle convection and composition, and magma genesis. The age-progressive volcanism that constructs many chains of islands throughout the worlds ocean basins is essential to hot spot theory. In contrast, secondary volcanism, which follows the main edifice building stage of volcanism in many chains including the Hawaii, Samoa, Canary, Mauritius, and Kerguelen islands, is not predicted by hot spot theory. Hawaiian secondary volcanism occurs hundreds of kilometers away from, and more than 1 million years after, the end of the main shield volcanism, which has generated more than 99% of the volume of the volcanos mass [Macdonald et al., 1983; Ozawa et al., 2005]. Diamond Head, in Honolulu, is the first and classic example of secondary volcanism.

Hot clasts and cold blasts: thermal heterogeneity in boiling-over pyroclastic density currents

Abstract Partial thermal remanent magnetization data from clasts in pyroclastic density current (PDC) deposits provide information on the emplacement temperatures of both lithic and juvenile magmatic clasts contained in the deposits. We collected palaeomagnetic data from clasts in PDC deposits emplaced during historical eruptions of two volcanoes in Ecuador, the 2006 eruption at Tungurahua and the 1877 eruption at Cotopaxi. These eruptions were characterized by emplacement of PDCs mainly related to boiling-over activity. The deposits of these eruptions are similar and are characterized by cauliflower-textured juvenile scoria clasts up to 1 m in diameter and a diverse assemblage of lithic clasts surrounded by an unwelded ashy matrix. On the basis of progressive thermal demagnetization experiments, we infer that emplacement temperatures for most of the lithic clasts in PDC deposits are below 90 °C. In contrast, palaeomagnetic data from juvenile clasts from the same deposits provide emplacement temperatures higher than 540 °C. These data indicate the PDC were thermally heterogeneous over short length scales (decimetres) also after deposition. We hypothesize that PDCs emplaced by the boiling-over mechanism cool quickly owing to atmosphere entrainment, causing the juvenile clasts to form a rind that retains heat and that also prevents lithic clasts from appreciable heating. Several deposits on Cotopaxi, despite being morphologically similar to the PDC deposits, contain both cold lithic and juvenile clasts, which we interpret to be lahar deposits formed by PDCs travelling across glacial ice and snow. Rare deposits containing both hot lithic and hot juvenile clasts are classified as well-mixed, hot PDCs, and were erupted during a more energetic phase at Tungurahua.

Orogenic to postorogenic (1.20–1.15 Ga) magmatism in the Adirondack Lowlands and Frontenac terrane, southern Grenville Province, USA and Canada

Magmatism in the southern Grenville Province records a collisional and postcollisional history during the period 1.20–1.15 Ga in the Adirondack Lowlands (New York State, USA) and the Frontenac terrane (Ontario, Canada). The 1.20 Ga bimodal Antwerp-Rossie suite of the Adirondack Lowlands was produced by subduction in the Trans-Adirondack backarc basin. This was followed by intrusion of the 1.18 Ga alkalic to calc-alkalic Hermon granite, which may have been generated by melting of metasomatized mantle during collision of the Adirondack Lowlands and Frontenac terrane during the Shawinigan orogeny. The Hyde School gneiss plutons intruded the Adirondack Lowlands at 1.17 Ga, and Rockport granite intruded into the Adirondack Lowlands and Frontenac terrane, stitching the Black Lake shear zone, which marks the boundary between these terranes. Subsequent extensional collapse and lithospheric delamination caused voluminous anorthosite-mangerite-charnockite-granite plutonism. In the Frontenac terrane, this event is represented by the 1.18–1.15 Ga Frontenac suite, which is composed predominately of ferroan granitoids produced from melting of the lower crust by underplating mafic magmas. The Edwardsville, Honey Hill, and Beaver Creek plutons are newly recognized members of this suite in the Adirondack Lowlands. High oxygen isotope ratios of this suite in the central Frontenac terrane and western Adirondack Lowlands point to the presence of underthrust altered oceanic rocks in the lower crust. Oxygen isotopes of the Frontenac suite in both terranes preclude its derivation from mantle melts alone.