The Galápagos and Hawai'i archipelagos are formed by mantle plumes originating at the large low shear velocity province (LLSVP) boundary. We report new high‐precision Pb, Sr, Nd, and Hf isotopic ...analyses on 83 Galápagos samples and compare them with those of Hawai'i. The data confirm that like Hawai'i, Galápagos is a bilaterally asymmetric plume whose compositional boundary trends NW‐SE. On their northeast sides, the plumes share a common source, Pacific lower mantle, whose intermediate isotopic signature may be common to many plumes. The Hawaiian and Galápagos plumes' southwestern sides are anchored in the Pacific LLSVP and are compositionally distinct; in Hawai'i, Loa trend lavas reflect contributions from the EM1 mantle end‐member, whereas in Galápagos, HIMU is dominant, suggesting that the Pacific LLSVP is compositionally heterogeneous and includes different types of recycled material. Furthermore, the surficial expression of a bilaterally asymmetric plume is strongly influenced by its tectonic setting: (a) Thick Hawaiian lithosphere supports a volcano evolution process, including rejuvenated volcanism, whereas the thin Galápagos lithosphere inhibits Hawai'i‐style rejuvenated‐stage eruptive activity, instead causing extended, widespread volcanism; (b) the proximity of the Galápagos to a mid‐ocean ridge causes entrainment of the depleted upper mantle, overwhelming depleted material intrinsic to the plume and affecting volcanoes' magmatic architecture; and (c) the geometric relationship between the LLSVP boundary and plate motion influences geochemical patterns at the surface. Thus, despite striking differences in surficial expression of the Galápagos and Hawai'i plumes, they share a common generation mechanism, supplied by the Pacific LLSVP and the lower mantle.
Plain Language Summary
The Galápagos and Hawaiian Islands are formed by mantle plumes, which provide an opportunity to document the compositional structure of the lower mantle. The Hawai'i plume ascends from the core–mantle boundary along the interface between two major mantle reservoirs, the Pacific large low shear velocity province (LLSVP), and the surrounding lower mantle. The LLSVP is a region at the core–mantle boundary that seismic velocities suggest is denser and hotter than the rest of the mantle. The Galápagos is also located along the LLSVP‐lower mantle interface, in the eastern Pacific. Prior to this study, the Galápagos plume was thought to be quite different from the one supplying Hawai'i, primarily because the distribution of geochemical compositions across the islands forms two parallel volcanic chains in Hawai'i, a pattern not observed in the Galápagos. Our new geochemical data from 83 lavas across the Galápagos indicate that the Galápagos plume also exhibits bilateral compositional asymmetry, but it is expressed as two broad zones that cross the archipelago instead of parallel chains. We propose that the Hawai'i and Galápagos plumes are generated by the same mechanism, consisting of parallel filaments of compositionally distinct material rising from the boundary of the LLSVP with the lower mantle.
Key Points
The Galápagos and Hawai'i plumes are bilaterally asymmetric in composition, sourced at the LLSVP‐lower mantle interface
The Galápagos and Hawai'i plumes share a common mantle source, the Pacific lower mantle
The Pacific large low shear velocity province is heterogeneous, with a variety of ancient recycled material (HIMU‐ and EM1‐type)
The older eastern Galápagos are different in almost every way from the historically active western Galápagos volcanoes. Geochemical, geologic, and geophysical data support the hypothesis that the ...differences are not evolutionary, but rather the eastern volcanoes grew in a different tectonic environment than the younger volcanoes. The western Galápagos volcanoes have steep upper slopes and are topped by large calderas, whereas none of the older islands has a caldera, an observation that is supported by recent gravity measurements. Most of the western volcanoes erupt evolved basalts with an exceedingly small range of Mg#, Lan/Smn, and Smn/Ybn. This is attributed to homogenization in a crustal-scale magmatic mush column, which is maintained in a thermochemical steady state, owing to high magma supply directly over the Galápagos mantle plume. In contrast, the eastern volcanoes erupt relatively primitive magmas, with a large range in Mg#, Lan/Smn, and Smn/Ybn. These differences are attributed to isolated, ephemeral magmatic plumbing systems supplied by smaller magmatic fluxes throughout their histories. Consequently, each batch of magma follows an independent course of evolution, owing to the low volume of supersolidus material beneath these volcanoes. The magmatic flux to Galápagos volcanoes negatively correlates to the distance to the Galápagos Spreading Center (GSC). When the ridge was close to the plume, most of the plume-derived magma was directed to the ridge. Currently, the active volcanoes are much farther from the GSC, thus most of the plume-derived magma erupts on the Nazca Plate and can be focused beneath the large young shields. We define an intermediate sub-province comprising Rabida, Santiago, and Pinzon volcanoes, which were most active about 1 Ma. They have all erupted dacites, rhyolites, and trachytes, similar to the dying stage of the western volcanoes, indicating that there was a relatively large volume of mush beneath them. The paradigm established by the evolution of Hawaiian volcanoes as they are carried away from the hotspot does not apply to most archipelagos.
The May 2005 eruption of Fernandina volcano, Galápagos, occurred along circumferential fissures parallel to the caldera rim and fed lava flows down the steep southwestern slope of the volcano for ...several weeks. This was the first circumferential dike intrusion ever observed by both InSAR and GPS measurements and thus provides an opportunity to determine the subsurface geometry of these enigmatic structures that are common on Galápagos volcanoes but are rare elsewhere. Pre- and post- eruption ground deformation between 2002 and 2006 can be modeled by the inflation of two separate magma reservoirs beneath the caldera: a shallow sill at ~1 km depth and a deeper point-source at ~5 km depth, and we infer that this system also existed at the time of the 2005 eruption. The co-eruption deformation is dominated by uplift near the 2005 eruptive fissures, superimposed on a broad subsidence centered on the caldera. Modeling of the co-eruption deformation was performed by including various combinations of planar dislocations to simulate the 2005 circumferential dike intrusion. We found that a single planar dike could not match both the InSAR and GPS data. Our best-fit model includes three planar dikes connected along hinge lines to simulate a curved concave shell that is steeply dipping (~45–60°) toward the caldera at the surface and more gently dipping (~12–14°) at depth where it connects to the horizontal sub-caldera sill. The shallow sill is underlain by the deep point source. The geometry of this modeled magmatic system is consistent with the petrology of Fernandina lavas, which suggest that circumferential eruptions tap the shallowest parts of the system, whereas radial eruptions are fed from deeper levels. The recent history of eruptions at Fernandina is also consistent with the idea that circumferential and radial intrusions are sometimes in a stress-feedback relationship and alternate in time with one another.
In July and August of 2006 and May of 2010, Tungurahua volcano, Ecuador, produced pyroclastic flow-forming eruptions, representing increased explosivity compared to the Strombolian events that ...characterized its behavior since its renewal in 1999. Volatiles (H
2
O, CO
2
, S, Cl) and major elements were analyzed in 35 melt inclusions hosted in olivine and pyroxene phenocrysts in tephra from both events to reconstruct the pre-eruptive magmatic conditions and mechanisms that led to these more explosive episodes. Melt inclusion composition paired with host phenocryst zonation indicate mixing of two distinct magmas: a volatile-rich (∼4.0 wt% H
2
O and ∼1,800 ppm S) basaltic andesite containing olivine phenocrysts and a degassed (∼1.0 wt% H
2
O and 100–500 ppm S) andesite with plagioclase and pyroxene phenocrysts that contain andesitic to dacitic melt inclusions. We attribute the lower volatile concentrations in the evolved melt inclusions to degassing that occurred during residence in a shallow reservoir, where fractional crystallization led to the production of dacitic melt. Our melt inclusion data confirm the hypothesis made on the basis of phenocryst zoning profiles (J Volcanol Geotherm Res 199:69–84, 2011) that the intrusion of a volatile-rich basaltic andesite into a more evolved chamber and subsequent mixing led to explosive eruption in 2006. Melt inclusions from the 2006 and 2010 eruptive products have comparable volatile and major element compositions. High H
2
O concentrations in melt inclusions from 2010 olivine indicate little diffusive loss from the melt inclusions following mixing with the degassed andesitic reservoir, which requires that the 2010 eruption be the result of a new recharge event and not remobilization of the 2006 hybrid.
Isotopic and trace element analyses of basalts dredged from across the Galápagos Platform confirm the previously established east facing horseshoe pattern of depleted geochemical signatures at the ...center of the archipelago and more enriched signatures along the periphery. Statistical analysis of the isotopic data indicates that geochemical variations in the Galápagos cannot be explained by mixing between only the plume and the depleted asthenosphere. Instead, four isotopically distinct end‐members must be interacting to account for the subtleties of the Sr, Nd, Pb, and He isotopic data. Three of the components are geographically restricted: one in the south, one in the central region, and one in the north. These three plume components then mix with the fourth component, depleted mantle, which is indistinguishable from the MORB source. The central component resembles the high 3He/4He mantle reservoir that may be common to many plumes and has variously been called PHEM, FOZO, and C by others. Whereas this mantle reservoir appears to make a minor contribution to the composition of most hot spot systems, it may constitute the main body of the Galápagos plume. Geographic distribution of the end‐members suggests that the plume is centered near Fernandina ∼92°N but may be significantly diluted by depleted mantle even near the main conduit. The geochemically distinct end‐members trace an eastward mantle flow, manifested as decreasing contributions of the plume in the direction of plate motion. The plume may be tilted by shear in the asthenosphere from plate motion and ambient mantle flow. As it is bent, the plume thermally entrains surrounding upper mantle, resulting in the horseshoe‐shaped distribution of depleted and enriched material. The end‐members may also outline a deep, strong lateral flow of mantle toward the Galápagos Spreading Center, supplying plume material to the ridge system. Overall, our results suggest that the Galápagos hot spot is both compositionally and dynamically complex owing to its tectonic setting adjacent to a mid‐ocean ridge.
The Midcontinent Rift System (MRS) is an example of a mafic-dominated continental rift where silicic magmatism is locally significant. The best-preserved example of this is on the NE limb of the rift ...along the North Shore of Lake Superior where rhyolites comprise a large percentage (up to 25%; Green, J.C., Fitz, T.J., III, 1993. Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent rift plateau volcanics, Minnesota: petrographic and field recognition. J. Volcanol. Geotherm. Res. 54, 177–196) of lava flows and where hypabyssal granophyric intrusive complexes are common. In this paper, we report U–Pb zircon ages, Nd isotopic compositions, and major and trace-element data for seven granophyric complexes of the MRS exposed in NE Minnesota.
U–Pb zircon ages for the granophyres define two different age groups: an older group with ages from 1109 to 1106
Ma; and a younger group with ages from 1099 to 1095
Ma. These ages coincide with the “early” and “main” magmatic stages of Midcontinent rift evolution suggested by Miller and Vervoort Miller, J.D., Jr., Vervoort, J.D., 1996. The latent magmatic stage of the Midcontinent rift: a period of magmatic underplating and melting of the lower crust. Inst. Lake Superior Geol., 42nd Ann. Mtg., Proceedings, vol. 42, pp. 33–35 and Miller and Severson Miller, J.D., Jr., Severson, M.J., 2002. Geology of the Duluth Complex. In: Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E. (Eds.), Geology and Mineral Potential of the Duluth Complex and Related Rocks of Northeastern Minnesota. Minnesota Geological Survey Report of Investigations, vol. 58, pp. 106–143. Although the two groups of granophyres have similar major and trace-element compositions, they have different Nd isotopic compositions. The older, or “early stage”, granophyres have more radiogenic Nd isotopic compositions (
ɛ
Nd(
i)
=
−3.7 to −0.5) whereas the younger, or “main stage”, granophyres have more crustal, unradiogenic Nd isotopic compositions (
ɛ
Nd(
i)
=
−7.6 to −3.1). The age correlative Nd isotopic signatures of the granophyres are broadly consistent with the ages and isotopic compositions of the rhyolites within the North Shore Volcanic Group (NSVG) and illustrate the episodic nature of the Midcontinent rift evolution.
The early stage (1109–1106
Ma) of MRS magmatism is characterized by mafic mantle-derived magmas with minor amounts of silicic magmas. The evolved magmas were likely derived by partial melting of either earlier formed rift related rocks or older crust with near chondritic Nd isotopic composition. This was followed by a period of relative quiescence lasting about 5
million years from which no significant MRS magmatism has been preserved. The main stage of MRS magmatism resumes at ca. 1100
Ma with voluminous basaltic volcanism and mafic intrusions. The silicic magmas produced during this stage are more abundant and distinctly more crustal in character than during the early magmatic stage magmas. We suggest that these silicic magmas have been derived from partial melts of more evolved crustal sources, perhaps at higher levels in the crust.
New multibeam and side‐scan sonar surveys of Fernandina volcano and the geochemistry of lavas provide clues to the structural and magmatic development of Galápagos 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. Fernandina's 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 Galápagos 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.
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.
Sierra Negra volcano began erupting on 22 October 2005, after a repose of 26 years. A plume of ash and steam more than 13 km high accompanied the initial phase of the eruption and was quickly ...followed by a ~2-km-long curtain of lava fountains. The eruptive fissure opened inside the north rim of the caldera, on the opposite side of the caldera from an active fault system that experienced an m
b
4.6 earthquake and ~84 cm of uplift on 16 April 2005. The main products of the eruption were an `a`a flow that ponded in the caldera and clastigenic lavas that flowed down the north flank. The `a`a flow grew in an unusual way. Once it had established most of its aerial extent, the interior of the flow was fed via a perched lava pond, causing inflation of the `a`a. This pressurized fluid interior then fed pahoehoe breakouts along the margins of the flow, many of which were subsequently overridden by `a`a, as the crust slowly spread from the center of the pond and tumbled over the pahoehoe. The curtain of lava fountains coalesced with time, and by day 4, only one vent was erupting. The effusion rate slowed from day 7 until the eruption’s end two days later on 30 October. Although the caldera floor had inflated by ~5 m since 1992, and the rate of inflation had accelerated since 2003, there was no transient deformation in the hours or days before the eruption. During the 8 days of the eruption, GPS and InSAR data show that the caldera floor deflated ~5 m, and the volcano contracted horizontally ~6 m. The total eruptive volume is estimated as being ~150×10
6
m
3
. The opening-phase tephra is more evolved than the eruptive products that followed. The compositional variation of tephra and lava sampled over the course of the eruption is attributed to eruption from a zoned sill that lies 2.1 km beneath the caldera floor.
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