Extensive, water-rich magma reservoir beneath southern Montserrat

South Soufriere Hills and Soufriere Hills volcanoes are two km apart at the southern end of the island of Montserrat, West Indies. Their magmas are distinct geochemically, despite these volcanoes having been active contemporaneously at 131-129 ka. We use the water content of pyroxenes and melt inclusion data to reconstruct the bulk water contents of magmas and their depth of storage prior to eruption. Pyroxenes contain up to 281 ppm H2O, with significant variability between crystals and from core to rim in individual crystals. The Al content of the enstatites from Soufriere Hills Volcano (SHV) is used to constrain melt-pyroxene partitioning for H2O. The SHV enstatite cores record melt water contents of 6-9 wt%. Pyroxene and melt inclusion water concentration pairs from South Soufriere Hills basalts independently constrain pyroxene-melt partitioning of water and produces a comparable range in melt water concentrations. Melt inclusions recorded in plagioclase and in pyroxene contain up to 6.3 wt% H2O. When combined with realistic melt CO2 contents, the depth of magma storage for both volcanoes ranges from 5 to 16 km. The data are consistent with a vertically protracted crystal mush in the upper crust beneath the southern part of Montserrat which contains heterogeneous bodies of eruptible magma. The high water contents of the magmas suggest that they contain a high proportion of exsolved fluids, which has implications for the rheology of the mush and timescales for mush reorganisation prior to eruption. A depletion in water in the outer 50-100 microns of a subset of pyroxenes from pumices from a Vulcanian explosion at Soufriere Hills in 2003 is consistent with diffusive loss of hydrogen during magma ascent over 5-13 hours. These timescales are similar to the mean time periods between explosions in 1997 and in 2003, raising the possibility that the driving force for this repetitive explosive behaviour lies not in the shallow system, but in the deeper parts of a vertically protracted crustal magma storage system.ME acknowledges NERC grant NE/I016694/1 and NERC ion probe facility grant IMF429/0511 and Cees-Jan de Hoog, who provided invaluable assistance with SIMS measurements and calibration. MC acknowledges an Alexander Von Humboldt fellowship.This is the author accepted manuscript. The final version is available from Elsevier via https://doi.org/10.1016/j.lithos.2016.02.02

CO2 content beneath northern Iceland and the variability of mantle carbon

Primitive basalt melt inclusions from Borgarhraun, northern Iceland, display large correlated variations in CO2 and non-volatile incompatible trace elements (ITEs) such as Nb, Th, Rb, and Ba. The average CO2/ITE ratios of the Borgarhraun melt inclusion population are precisely determined (e.g., CO2/Nb = 391 ± 16; 2M, n = 161). These data, along with published data on five other populations of undegassed MORB glasses and melt inclusions, demonstrate that upper mantle CO2/Ba and CO2/Rb are nearly homogenous, while CO2/Nb and CO2/Th are broadly correlated with long-term indices of mantle heterogeneity reflected in Nd isotopes (143Nd/144Nd) in five out of the six regions of the upper mantle examined thus far. Our results suggest that heterogeneous carbon contents of the upper mantle are long-lived features, and that average carbon abundances of the mantle sources of Atlantic mid-ocean ridge basalts (MORB) are higher by a factor of two than those of Pacific MORB. This observation is correlated with a similar distinction in water contents (Michael, 1995) and trace elements characteristic of subduction fluids (Ba, Rb; Arevalo and McDonough, 2010). We suggest that the upper mantle beneath the younger Atlantic Ocean basin contains components of hydrated and carbonated subduction-modified mantle from prior episodes of Iapetus subduction that were entrained and mixed into the upper mantle during opening of the Atlantic Ocean basin.Maclennan is supported by Natural Environment Research Council grant NE/M000427/1. This research was supported by the Carnegie Institution of Washington and is a contribution to the Deep Carbon Observatory

Magma decompression rates during explosive eruptions of Kīlauea volcano, Hawai'i, recorded by melt embayments

The decompression rate of magma as it ascends during volcanic eruptions is an important but poorly constrained parameter that controls many of the processes that influence eruptive behaviour. In this study we quantify decompression rates for basaltic magmas using volatile diffusion in olivine-hosted melt tubes (embayments) for three contrasting eruptions of Kīlauea volcano, Hawai'i. Incomplete exsolution of H₂O, CO₂ and S from the embayment melts during eruptive ascent creates diffusion profiles that can be measured using microanalytical techniques, and then modelled to infer the average decompression rate. We obtain average rates of ~0.05-0.45 MPa s-1 for eruptions ranging from Hawaiian style fountains to basaltic subplinian, with the more intense eruptions having higher rates. The ascent timescales for these magmas vary from around ~5 to ~36 minutes from depths of ~2 to ~4 km respectively. Decompression-exsolution models based on the embayment data also allow for an estimate of the mass fraction of pre-existing exsolved volatiles within the magma body. In the eruptions studied this varies from 0.1-3.2 wt%, but does not appear to be the key control on eruptive intensity. Our results do not support a direct link between the concentration of pre-eruptive volatiles and eruptive intensity; rather they suggest that for these eruptions decompression rates are proportional to independent estimates of mass discharge rate. Although the intensity of eruptions is defined by the discharge rate, based on the currently available dataset of embayment analyses it does not appear to scale linearly with average decompression rate. This study demonstrates the utility of the embayment method for providing quantitative constraints on magma ascent during explosive basaltic eruptions

Extremely high He isotope ratios in MORB-source mantle from the proto-Iceland plume

The high <sup>3</sup>He/<sup>4</sup>He ratio of volcanic rocks thought to be derived from mantle plumes is taken as evidence for the existence of a mantle reservoir that has remained largely undegassed since the Earth's accretion. The helium isotope composition of this reservoir places constraints on the origin of volatiles within the Earth and on the evolution and structure of the Earth's mantle. Here we show that olivine phenocrysts in picritic basalts presumably derived from the proto-Iceland plume at Baffin Island, Canada, have the highest magmatic <sup>3</sup>He/<sup>4</sup>He ratios yet recorded. A strong correlation between <sup>3</sup>He/<sup>4</sup>He and <sup>87</sup>Sr/<sup>86</sup>Sr, <sup>143</sup>Nd/<sup>144</sup>Nd and trace element ratios demonstrate that the <sup>3</sup>He-rich end-member is present in basalts that are derived from large-volume melts of depleted upper-mantle rocks. This reservoir is consistent with the recharging of depleted upper-mantle rocks by small volumes of primordial volatile-rich lower-mantle material at a thermal boundary layer between convectively isolated reservoirs. The highest <sup>3</sup>He/<sup>4</sup>He basalts from Hawaii and Iceland plot on the observed mixing trend. This indicates that a <sup>3</sup>He-recharged depleted mantle (HRDM) reservoir may be the principal source of high <sup>3</sup>He/<sup>4</sup>He in mantle plumes, and may explain why the helium concentration of the 'plume' component in ocean island basalts is lower than that predicted for a two-layer, steady-state model of mantle structure

Evidence for the return of subducted continental crust

Author Posting. © Nature Publishing Group, 2007. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 448 (2007): 684-687, doi:10.1038/nature06048.Substantial quantities of terrigenous sediments are known to enter the mantle at subduction zones, but little is known about their fate in the mantle. Subducted sediment may be entrained in buoyantly upwelling plumes and returned to the earth’s surface at hotspots, but the proportion of recycled sediment in the mantle is small and clear examples of recycled sediment in hotspot lavas are rare. We report here remarkably enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures (up to 0.720830 and 0.512285, respectively) in Samoan lavas from three dredge locations on the underwater flanks of Savai’i island, Western Samoa. The submarine Savai’i lavas represent the most extreme 87Sr/86Sr isotope compositions reported for ocean island basalts (OIBs) to date. The data are consistent with the presence of a recycled sediment component (with a composition similar to upper continental crust, or UCC) in the Samoan mantle. Trace element data show similar affinities with UCC—including exceptionally low Ce/Pb and Nb/U ratios—that complement the enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures. The geochemical evidence from the new Samoan lavas radically redefines the composition of the EM2 (enriched mantle 2) mantle endmember, and points to the presence of an ancient recycled UCC component in the Samoan plume

Mantle Pb paradoxes : the sulfide solution

Author Posting. © Springer, 2006. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 152 (2006): 295-308, doi:10.1007/s00410-006-0108-1.There is growing evidence that the budget of Pb in mantle peridotites is largely contained in sulfide, and that Pb partitions strongly into sulfide relative to silicate melt. In addition, there is evidence to suggest that diffusion rates of Pb in sulfide (solid or melt) are very fast. Given the possibility that sulfide melt ‘wets’ sub-solidus mantle silicates, and has very low viscosity, the implications for Pb behavior during mantle melting are profound. There is only sparse experimental data relating to Pb partitioning between sulfide and silicate, and no data on Pb diffusion rates in sulfides. A full understanding of Pb behavior in sulfide may hold the key to several long-standing and important Pb paradoxes and enigmas. The classical Pb isotope paradox arises from the fact that all known mantle reservoirs lie to the right of the Geochron, with no consensus as to the identity of the “balancing” reservoir. We propose that long-term segregation of sulfide (containing Pb) to the core may resolve this paradox. Another Pb paradox arises from the fact that the Ce/Pb ratio of both OIB and MORB is greater than bulk earth, and constant at a value of 25. The constancy of this “canonical ratio” implies similar partition coefficients for Ce and Pb during magmatic processes (Hofmann et al. 1986), whereas most experimental studies show that Pb is more incompatible in silicates than Ce. Retention of Pb in residual mantle sulfide during melting has the potential to bring the bulk partitioning of Ce into equality with Pb if the sulfide melt/silicate melt partition coefficient for Pb has a value of ~ 14. Modeling shows that the Ce/Pb (or Nd/Pb) of such melts will still accurately reflect that of the source, thus enforcing the paradox that OIB and MORB mantles have markedly higher Ce/Pb (and Nd/Pb) than the bulk silicate earth. This implies large deficiencies of Pb in the mantle sources for these basalts. Sulfide may play other important roles during magmagenesis: 1). advective/diffusive sulfide networks may form potent metasomatic agents (in both introducing and obliterating Pb isotopic heterogeneities in the mantle); 2). silicate melt networks may easily exchange Pb with ambient mantle sulfides (by diffusion or assimilation), thus ‘sampling’ Pb in isotopically heterogeneous mantle domains differently from the silicate-controlled isotope tracer systems (Sr, Nd, Hf), with an apparent ‘de-coupling’ of these systems.Our intemperance should not be blamed on the support we gratefully acknowledge from NSF: EAR- 0125917 to SRH and OCE-0118198 to GAG

Experimental evidence for the preservation of U-Pb isotope ratios in mantle-recycled crustal zircon grains

Zircon of crustal origin found in mantle-derived rocks is of great interest because of the information it may provide about crust recycling and mantle dynamics. Consideration of this requires understanding of how mantle temperatures, notably higher than zircon crystallization temperatures, affected the recycled zircon grains, particularly their isotopic clocks. Since Pb2+ diffuses faster than U4+ and Th+4, it is generally believed that recycled zircon grains lose all radiogenic Pb after a few million years, thus limiting the time range over which they can be detected. Nonetheless, this might not be the case for zircon included in mantle minerals with low Pb2+ diffusivity and partitioning such as olivine and orthopyroxene because these may act as zircon sealants. Annealing experiments with natural zircon embedded in cristobalite (an effective zircon sealant) show that zircon grains do not lose Pb to their surroundings, although they may lose some Pb to molten inclusions. Diffusion tends to homogenize the Pb concentration in each grain changing the U-Pb and Th-Pb isotope ratios proportionally to the initial 206Pb, 207Pb and 208Pb concentration gradients (no gradient-no change) but in most cases the original age is still recognizable. It seems, therefore, that recycled crustal zircon grains can be detected, and even accurately dated, no matter how long they have dwelled in the mantle.This paper has been financed by the Spanish Grants CGL2013-40785-P and CGL2017-84469-P

Quantifying garnet-melt trace element partitioning using lattice-strain theory: New crystal-chemical and thermodynamic constraints

Many geochemical models of major igneous differentiation events on the Earth, the Moon, and Mars invoke the presence of garnet or its high-pressure majoritic equivalent as a residual phase, based on its ability to fractionate critical trace element pairs (Lu/Hf, U/Th, heavy REE/light REE). As a result, quantitative descriptions of mid-ocean ridge and hot spot magmatism, and lunar, martian, and terrestrial magma oceans require knowledge of garnet-melt partition coefficients over a wide range of conditions. In this contribution, we present new crystal-chemical and thermodynamic constraints on the partitioning of rare earth elements (REE), Y and Sc between garnet and anhydrous silicate melt as a function of pressure (P), temperature (T), and composition (X). Our approach is based on the interpretation of experimentally determined values of partition coefficients D using lattice-strain theory. In this and a companion paper (Draper and van Westrenen this issue) we derive new predictive equations for the ideal ionic radius of the dodecahedral garnet X-site,

Eruption style at Kīlauea Volcano in Hawaiʻi linked to primary melt composition

Explosive eruptions at basaltic volcanoes have been linked to gas segregation from magmas at shallow depths in the crust. The composition of primary melts formed at greater depths is thought to have little influence on eruptive style. Primary melts formed at ocean island basaltic volcanoes are probably geochemically diverse because they are often associated with melting of a heterogeneous plume source in the mantle. This heterogeneous primary melt composition, and particularly the content of volatile gases, will profoundly influence magma buoyancy, storage and eruption style. Here we analyse the geochemistry of a suite of melt inclusions from 25 historical eruptions at the ocean island volcano of K¯ılauea, Hawai’i, over the past 600 years.We find that more explosive styles of eruption at K¯ılauea Volcano are associated statistically with more geochemically enriched primary melts that have higher volatile concentrations. These enriched melts ascend faster and retain their primary nature, undergoing little interaction with the magma reservoir at the volcano’s summit. We conclude that the eruption style and magma-supply rate at K¯ılauea are fundamentally linked to the geochemistry of the primary melts formed deep below the volcano. Magmas might therefore be predisposed towards explosivity right at the point of formation in their mantle source region

Unexpected large eruptions from buoyant magma bodies within viscoelastic crust

Large volume effusive eruptions with relatively minor observed precursory signals are at odds with widely used models to interpret volcano deformation. Here we propose a new modelling framework that resolves this discrepancy by accounting for magma buoyancy, viscoelastic crustal properties, and sustained magma channels. At low magma accumulation rates, the stability of deep magma bodies is governed by the magma-host rock density contrast and the magma body thickness. During eruptions, inelastic processes including magma mush erosion and thermal effects, can form a sustained channel that supports magma flow, driven by the pressure difference between the magma body and surface vents. At failure onset, it may be difficult to forecast the final eruption volume; pressure in a magma body may drop well below the lithostatic load, create under-pressure and initiate a caldera collapse, despite only modest precursors