Within continental lithosphere, widespread seismic evidence suggests a sharp discontinuous downward decrease in seismic velocity at 60–160 km depth. This midlithospheric discontinuity (MLD) may be ...due to anisotropy, melt, hydration, and/or mantle metasomatism. We survey global seismologic observations of the MLD, including observed depths, velocity contrasts, gradients, and locales across multiple seismic techniques. The MLD is primarily found in regions of thick continental lithosphere and is a decrease in seismic shear velocity (2–7% over 10–20 km) at 60–160 km depth, the majority of observations clustering at 80–100 km. Of xenoliths in online databases, 25% of amphibole‐bearing xenoliths, 90% of phlogopite‐bearing xenoliths, and none of carbonate‐bearing xenoliths were formed at pressures associated with these depth (2–5 GPa). We used Perple_X modeling to evaluate the elastic moduli and densities of multiple petrologies to test if the MLD is a layer of crystallized melt. The fractional addition of 5–10% phlogopite, 10–15% carbonate, or 45–100% pyroxenite produce a 2–7% velocity decrease. We postulate this layer of crystallized melt would originate at active margins of continents and crystallize in place as the lithosphere cools. The concentration of mildly incompatible elements (Y, Ho, Er, Yb, and Lu) in xenoliths near the MLD is consistent with higher degrees of melting. Thus, we postulate that the MLD is the seismological signature of a chemical interface related to the paleointersection of a volatile‐rich solidus and progressively cooling lithosphere. Furthermore, the MLD may represent a remnant chemical tracer of the lithosphere‐asthenosphere boundary (LAB) from when the lithosphere was active and young.
Key Points:
MLD is found at consistent depths below cratons
Xenoliths from this horizon contain hydrous minerals
Hydrous minerals are trapped when melt cools
Abstract Many volcanoes erupt compositionally homogeneous magmas over timescales ranging from decades to millennia. This monotonous activity is thought to reflect a high degree of chemical ...homogeneity in their magmatic systems, leading to predictable eruptive behaviour. We combine petrological analyses of erupted crystals with new thermodynamic models to characterise the diversity of melts in magmatic systems beneath monotonous shield volcanoes in the Galápagos Archipelago (Wolf and Fernandina). In contrast with the uniform basaltic magmas erupted at the surface over long timescales, we find that the sub-volcanic systems contain extreme heterogeneity, with melts extending to rhyolitic compositions. Evolved melts are in low abundance and large volumes of basalt flushing through the crust from depth overprint their chemical signatures. This process will only maintain monotonous activity while the volume of melt entering the crust is high, raising the possibility of transitions to more silicic activity given a decrease in the crustal melt flux.
We present new helium and neon measurements in dredged basaltic glasses from the Western Galápagos, with an emphasis on the submarine flanks of Fernandina volcano, but including the adjacent flanks ...of Darwin, Ecuador, Wolf, and Roca Redonda. The samples from the submarine flanks of Fernandina volcano have the least radiogenic helium and neon;
3He/
4He ratios vary from 18 to 29 times atmospheric (Ra), which spans the range previously observed at subaerial Fernandina. Samples from north of Fernandina have
3He/
4He ratios closer to Mid-ocean ridge basalt (MORB) values (7.6 to 11.8 Ra), and are similar to subaerial lava flows from Darwin, Ecuador, Wolf, and Roca Redonda volcanoes. On a three-isotope neon diagram, the new submarine Fernandina data define a line that is closer to “solar” than data from Hawaii and Iceland, and therefore are among the least radiogenic, most primitive, neon isotopic compositions found on Earth. In contrast, the northern dredges have neon isotopic compositions similar to MORB. This sharp isotopic contrast between Fernandina and the adjacent volcanoes, whose summit calderas are only 35 to 40 km apart, is consistent with the hypothesis that Fernandina lies over the hotspot center. Measured
3He/
22Ne ratios, coupled with helium and neon isotopic systematics show that
3He/
22Ne values are extremely low in the Fernandina samples compared to the northern dredges. Coupled crushing and melting experiments show that vesicles often have lower
20Ne/
22Ne and higher
3He/
22Ne, suggesting that vesicle formation is not necessarily in solubility equilibrium, and that vesicles are preferentially affected by atmospheric contamination (as compared to host glass). There is a crude correlation between Mg# and total helium content in the glasses, suggesting that fractionation/degassing in a shallow magma reservoir is a primary control on noble gas contents. Due to similarities in volcanic plumbing between the Galápagos volcanoes, and in major and trace elements, the large
3He/
22Ne variations (extrapolated to solar neon) between Fernandina (1.5) and the northern dredges (15) may be related to mantle source characteristics rather than the effects of recent degassing or melting. Simple closed-system evolution models show that the unradiogenic neon isotopic compositions require preservation since the first few hundred million years of Earth history, which is consistent with undegassed material in the lower mantle.
We have measured Ni, Ca, and Mn in olivine phenocrysts from volcanoes in the Galápagos Archipelago to infer the mantle source lithologies. Results show that peridotite is the dominant source ...lithology for Fernandina, Floreana, Genovesa, Wolf Island, and Darwin Island. These volcanoes largely characterize the PLUME, WD, FLO, and DUM Nd, Sr, and Pb isotopic endmembers of Harpp and White (2001). Volcan Wolf, Alcedo, Marchena, and Cerro Azul, also produced from the melting of peridotite sources, have isotopic compositions that can be defined by mixing of the four isotopic endmembers. Our analysis suggests that peridotite was present in the sources of the volcanoes covered in this study and therefore is the dominant source lithology of the Galápagos plume. Pyroxenite melting is generally focused in two isotopically distinct domains: Roca Redonda, Volcan Ecuador, and Sierra Negra in the enriched western part of the archipelago and Santiago, Santa Cruz, and Santa Fe in the depleted east. One implication of this finding is that the Western and Eastern Pyroxenite Domains represent two separate bodies of recycled crust within the Galápagos mantle plume. Furthermore, both isotopically enriched and depleted domains of the archipelago were generated from mixtures of peridotite and pyroxenite. This suggests that there is no relationship between the source lithology of the Galápagos plume and its isotopic characteristics. The identification of peridotite‐source melting in volcanoes with isotopic characteristics that have been attributed to recycled crust points to the importance of mixing in OIB genesis, consistent with studies in the Canary Islands.
Key Points
The primary source litology of the Galapagos plume is peridotite
Two pyroxenite domains in the archipelago are isotopically distinct
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.
The evolution of the Galápagos mantle plume Soderman, Caroline R; Shorttle, Oliver; Gazel, Esteban ...
Science advances,
2023-Mar-10, 2023-03-10, 20230310, Letnik:
9, Številka:
10
Journal Article
Recenzirano
Odprti dostop
The lavas associated with mantle plumes may sample domains throughout Earth's mantle and probe its dynamics. However, plume studies are often only able to take snapshots in time, usually of the most ...recent plume activity, leaving the chemical and geodynamic evolution of major convective upwellings in Earth's mantle poorly constrained. Here, we report the geodynamically key information of how the lithology and density of a plume change from plume head phase to tail. We use iron stable isotopes and thermodynamic modeling to show that the Galápagos plume has contained small, nearly constant, amounts of dense recycled crust over its 90-million-year history. Despite a temporal evolution in the amount of recycled crust-derived melt in Galápagos-related lavas, we show that this can be explained by plume cooling alone, without associated changes in the plume's mantle source; results are also consistent with a plume rooted in a lower mantle low-velocity zone also sampling primordial components.
Using recent advancements in high-performance computing data assimilation to combine satellite InSAR data with numerical models, the prolonged unrest of the Sierra Negra volcano in the Galápagos was ...tracked to provide a fortuitous, but successful, forecast 5 months in advance of the 26 June 2018 eruption. Subsequent numerical simulations reveal that the evolution of the stress state in the host rock surrounding the Sierra Negra magma system likely controlled eruption timing. While changes in magma reservoir pressure remained modest (<15 MPa), modeled widespread Mohr-Coulomb failure is coincident with the timing of the 26 June 2018 moment magnitude 5.4 earthquake and subsequent eruption. Coulomb stress transfer models suggest that the faulting event triggered the 2018 eruption by encouraging tensile failure along the northern portion of the caldera. These findings provide a critical framework for understanding Sierra Negra's eruption cycles and evaluating the potential and timing of future eruptions.
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.
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.
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.
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