Basaltic explosive volcanism: Constraints from deposits and models

Basaltic pyroclastic volcanism takes place over a range of scales and styles, from weak discrete Strombolian explosions (～10²–10³ kg s^?1) to Plinian eruptions of moderate intensity (10⁷–10⁸ kg s^?1). Recent well-documented historical eruptions from Etna, Kīlauea and Stromboli typify this diversity. Etna is Europe's largest and most voluminously productive volcano with an extraordinary level and diversity of Strombolian to subplinian activity since 1990. Kīlauea, the reference volcano for Hawaiian fountaining, has four recent eruptions with high fountaining (>400 m) activity in 1959, 1960, 1969 (–1974) and 1983–1986 (–2008); other summit (1971, 1974, 1982) and flank eruptions have been characterized by low fountaining activity. Stromboli is the type location for mildly explosive Strombolian eruptions, and from 1999 to 2008 these persisted at a rate of ca. 9 per hour, briefly interrupted in 2003 and 2007 by vigorous paroxysmal eruptions. Several properties of basaltic pyroclastic deposits described here, such as bed geometry, grain size, clast morphology and vesicularity, and crystal content are keys to understand the dynamics of the parent eruptions.The lack of clear correlations between eruption rate and style, as well as observed rapid fluctuations in eruptive behavior, point to the likelihood of eruption style being moderated by differences in the fluid dynamics of magma and gas ascent and the mechanism by which the erupting magma fragments. In all cases, the erupting magma consists of a mixture of melt and gaseous bubbles. The depth and rate of degassing, melt rheology, bubble rise and coalescence rates, and extent of syn-eruptive microlite growth define complex feedbacks that permit reversible shifts between fragmentation mechanisms and in eruption style and intensity. However, many basaltic explosive eruptions end after an irreversible shift to open-system outgassing and microlite crystallization in melt within the conduit.Clearer understanding of the factors promoting this diversity of basaltic pyroclastic eruptions is of fundamental importance in order to improve understanding of the range of behaviors of these volcanoes and assess hazards of future explosive events at basaltic volcanoes. The three volcanoes used for this review are the sites of large and growing volcano-tourism operations and there is a public need both for better knowledge of the volcanoes’ behavior and improved forecasting of the likely course of future eruptions.     

Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism

In situ oxygen isotopic measurements of primary and secondary minerals in Type C CAIs from the Allende CV3 chondrite reveal that the pattern of relative enrichments and depletions of ¹⁶O in the primary minerals within each individual CAI are similar to the patterns observed in Types A and B CAIs from the same meteorite. Spinel is consistently the most ¹⁶O-rich (Δ¹⁷O = −25‰ to −15‰), followed by Al,Ti-dioside (Δ¹⁷O = −20‰ to −5‰) and anorthite (Δ¹⁷O = −15‰ to 0‰). Melilite is the most ¹⁶O-depleted primary mineral (Δ¹⁷O = −5‰ to −3‰). We conclude that the original melting event that formed Type C CAIs occurred in a ¹⁶O-rich (Δ¹⁷O  −20‰) nebular gas and they subsequently experienced oxygen isotopic exchange in a ¹⁶O-poor reservoir. At least three of these (ABC, TS26F1 and 93) experienced remelting at the time and place where chondrules were forming, trapping and partially assimilating ¹⁶O-poor chondrule fragments. The observation that the pyroxene is ¹⁶O-rich relative to the feldspar, even though the feldspar preceded it in the igneous crystallization sequence, disproves the class of CAI isotopic exchange models in which partial melting of a ¹⁶O-rich solid in a ¹⁶O-poor gas is followed by slow crystallization in that gas. For the typical (not associated with chondrule materials) Type C CAIs as well for as the Types A and B CAIs, the exchange that produced internal isotopic heterogeneity within each CAI must have occurred largely in the solid state. The secondary phases grossular, monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace, but in one case (CAI 160) grossular is ¹⁶O-enriched (Δ¹⁷O = −10‰ to −6‰) relative to melilite (Δ¹⁷O = −5‰ to −3‰), meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration. This isotopic exchange in melilite and anorthite likely occurred on the CV parent asteroid, possibly during fluid-assisted thermal metamorphism.     

Microbial release of potassium from K-bearing minerals by thermophilic fungus Aspergillus fumigatus

A strain of thermophilic fungus Aspergillus fumigatus was cultured with K-bearing minerals to determine if microbe-mineral interactions enhance the release of mineralic potassium. Experiments were carried out in two settings, one with the mineral grains and the fungal cells in direct contact, and the other employing a membrane (pore size 0.22 μm) to separate the two. Measurements over a period of 30 days showed that, irrespective of the experimental setup, the concentration of free K in the culture was drastically higher than those in any of the control experiments where no living organism was present. Moreover, the occurrence of mineral-cell physical contact enhanced potassium release by an additional factor of 3 to 4 in comparison to the separation experiments. For contact experiments, Electron Probe Microanalysis revealed the formation of mycelium-mineral aggregates, and Atomic Force Microscopy imaging further indicated the possible ingestion of mineral particles by the fungus cells. Contrasting to what was observed and expected in control experiments, the potassium solubilization rate showed a positive dependence upon pH when fungi and minerals were mixed directly, and exhibited no correlations with solution acidity if cell-rock contact was restrained. These results appear to suggest that A. fumigatus promoted potassium release by means of at least three likely routes, one through the complexation of soluble organic ligands, another appealing to the immobile biopolymers such as the insoluble components of secretion, and the third related to the mechanical forces in association with the direct physical contact between cells and mineral particles.     

Fast kimberlite ascent rates estimated from hydrogen diffusion profiles in xenolithic mantle olivines from southern Africa

Fourier transform infrared spectrometry (FTIR) analyses of olivines from peridotite xenoliths found in southern African kimberlites indicate 0 to 80 ppm H₂O concentrations. OH absorbance profiles across olivine grains show homogeneous H contents from core to edge for most samples. In one sample the olivines are H-free, while another has olivines characterized by lower H contents at the grain edges compared to the cores, indicating H loss during transport of the xenolith to the surface. Flat or near-flat H profiles place severe constraints on the duration of H loss from olivine grains, with implications for kimberlite magma ascent rates. Diffusion equations were used to estimate times of H loss of about 4 h for the sample with heterogeneous olivine H contents. Resulting kimberlite ascent rates are calculated to be 5-37 m s⁻¹ minimum, although these estimates are highly dependent on volatile contents and degassing behavior of the host kimberlite magma. Xenolithic olivines from alkali basalts generally have lower H contents and more pronounced H diffusion profiles than do those from kimberlites. This difference is likely caused by higher magma temperatures and lower ascent rates of alkali basalts compared to kimberlites.     

A depleted, not ideally chondritic bulk Earth: The explosive-volcanic basalt loss hypothesis

It has long been customary to assume that in the bulk composition of the Earth, all refractory-lithophile elements (including major oxides Al₂O₃ and CaO, all of the REE, and the heat-producing elements Th and U) occur in chondritic, bulk solar system, proportion to one another. Recently, however, Nd-isotopic studies (most notably Boyet M. and Carlson R. W. (2006) A new geochemical model for the Earth’s mantle inferred from ¹⁴⁶Sm-¹⁴²Nd systematics. Earth Planet. Sci. Lett.250, 254-268) have suggested that at least the outer portion of the planet features a Nd/Sm ratio depleted to ∼0.93 times the chondritic ratio. The primary reaction to this type of evidence has been to invoke a “hidden” reservoir of enriched matter, sequestered into the deepest mantle as a consequence of primordial differentiation. I propose a hypothesis that potentially explains the evidence for Nd/Sm depletion in a very different way. Among the handful of major types of differentiated asteroidal meteorites, two (ureilites and aubrites) are ultramafic restites so consistently devoid of plagioclase that meteoriticists were once mystified as to how all the complementary plagioclase-rich matter (basalt) was lost. The explanation appears to be basalt loss by graphite-fueled explosive volcanism on roughly 100-km sized planetesimals; with the dispersiveness of the process dramatically enhanced, relative to terrestrial experience, because the pyroclastic gases expand into vacuous space (Wilson L. and Keil K. (1991) Consequences of explosive eruptions on small Solar System bodies: the case of the missing basalts on the aubrite parent body. Earth Planet. Sci. Lett.104, 505-512). By analogy with lunar pyroclastic products, the typical size of pyroclastic melt/glass droplets under these circumstances will be roughly 0.1 mm. Once separated from an asteroidal or planetesimal gravitational field, droplets of this size will generally spiral toward the Sun, rather than reaccrete, because drag forces such the Poynting-Robertson effect quickly modify their orbits (the semimajor axis, in a typical scenario, is reduced by several hundred km during the first trip around the Sun). Assuming a similar process occurred on many of the Earth’s precursor planetesimals while they were still roughly 100 km in diameter, the net effect would be a depleted composition for the final Earth. I have modeled the process of trace-element depletion in the planetesimal mantles, assuming the partial melting was nonmodal and either batch or dynamic in terms of the melt-removal style. Assuming the process is moderately efficient, typical final-Earth Nd/Sm ratios are 0.93-0.96 times chondritic. Depletion is enhanced by a relatively low assumed residual porosity in batch-melting scenarios, but dampened by a relatively high value for “continuous” residue porosity in dynamic melting scenarios. Pigeonite in the source matter has a dampening effect on depletion. There are important side effects to the Nd/Sm depletion. The heat-producing elements, Th, U and K, might be severely depleted. The Eu/Eu^∗ ratio of the planet is unlikely to remain precisely chondritic. One of the most inevitable side effects, depletion of the Al/Ca ratio, is consistent with an otherwise puzzling aspect of the composition of the upper mantle. A perfectly undepleted composition for the bulk Earth is dubious.     

Rare earth elements (REE) and yttrium in stream waters, stream sediments, and Fe-Mn oxyhydroxides: Fractionation, speciation, and controls over REE + Y patterns in the surface environment

We have collected ∼500 stream waters and associated bed-load sediments over an ∼400 km² region of Eastern Canada and analyzed these samples for Fe, Mn, and the rare earth elements (REE + Y). In addition to analyzing the stream sediments by total digestion (multi-acid dissolution with metaborate fusion), we also leached the sediments with 0.25 M hydroxylamine hydrochloride (in 0.05 M HCl), to determine the REE + Y associated with amorphous Fe- and Mn-oxyhydroxide phases. We are thus able to partition the REE into “dissolved” (<0.45 μm), labile (hydroxylamine) and detrital sediment fractions to investigate REE fractionation, and in particular, with respect to the development of Ce and Eu anomalies in oxygenated surface environments. Surface waters are typically LREE depleted ([La/Sm]_NASC ranges from 0.16 to 5.84, average = 0.604, n = 410; where the REE are normalized to the North America Shale Composite), have strongly negative Ce anomalies ([Ce/Ce^∗]_NASC ranges from 0.02 to 1.25, average = 0.277, n = 354), and commonly have positive Eu anomalies ([Eu/Eu^∗]_NASC ranges from 0.295 to 1.77, average = 0.764, n = 84). In contrast, the total sediment have flatter REE + Y patterns relative to NASC ([La/Sm]_NASC ranges from 0.352 to 1.12, average = 0.778, n = 451) and are slightly middle REE enriched ([Gd/Yb]_NASC ranges from 0.55 to 3.75, average = 1.42). Most total sediments have negative Ce and Eu anomalies ([Ce/Ce^∗]_NASC ranges from 0.097 to 2.12, average = 0.799 and [Eu/Eu^∗]_NASC ranges from 0.39 to 1.43, average = 0.802). The partial extraction sediments are commonly less LREE depleted than the total sediments ([La/Sm]_NASC ranges from 0.24 to 3.31, average = 0.901, n = 4537), more MREE enriched ([Gd/Yb]_NASC ranges from 0.765 to 6.28, average = 1.97) and Ce and Eu anomalies (negative and positive) are more pronounced.The partial extraction recovered, on average ∼20% of the Fe in the total sediment, ∼80% of the Mn, and 21-29% of the REEs (Ce = 19% and Y = 32%). Comparison between REEs in water, partial extraction and total sediment analyses indicates that REEs + Y in the stream sediments have two primary sources, the host lithologies (i.e., mechanical dispersion) and hydromorphically transported (the labile fraction). Furthermore, Eu appears to be more mobile than the other REE, whereas Ce is preferentially removed from solution and accumulates in the stream sediments in a less labile form than the other REEs + Y. Despite poor statistical correlations between the REEs + Y and Mn in either the total sediment or partial extractions, based on apparent distribution coefficients and the pH of the stream waters, we suggest that either sediment organic matter and/or possibly δ-MnO₂/FeOOH are likely the predominant sinks for Ce, and to a lesser extent the other REE, in the stream sediments.     

The persistence of lead from past gasoline emissions and mining drainage in a large riparian system: Evidence from lead isotopes in the Sacramento River, California

Lead concentrations and isotope ratios measured in river water colloids and streambed sediment samples along 426 km of the Sacramento River, California reveal that the influence of lead from the historical mining of massive sulfide deposits in the West Shasta Cu-mining district (at the headwaters of the Sacramento River) is confined to a 60 km stretch of river immediately downstream of that mining region, whereas inputs from past leaded gasoline emissions and historical hydraulic Au-mining in the Sierra Nevadan foothills are the dominant lead sources in the remaining 370 km of the river. Binary mixing calculations suggest that more than 50% of the lead in the Sacramento River outside of the region of influence of the West Shasta Cu-mining district is derived from past depositions of leaded gasoline emissions. This predominance is the first direct documentation of the geographic extent of gasoline lead persistence throughout a large riparian system (>160,000 km²) and corroborates previous observations based on samples taken at the mouth of the Sacramento River. In addition, new analyses of sediment samples from the hydraulic gold mines of the Sierra Nevada foothills confirm the present-day fluxes into the Sacramento River of contaminant metals derived from historical hydraulic Au-mining that occurred during the latter half of the 19th and early part of the 20th centuries. These fluxes occur predominantly during periods of elevated river discharge associated with heavy winter precipitation in northern California. In the broadest context, the study demonstrates the potential for altered precipitation patterns resulting from climate change to affect the mobility and transport of soil-bound contaminants in the surface environment.     

Calcite precipitation instability under laminar, open-channel flow

We present a 2D numerical model for the growth of calcite from supersaturated aqueous solutions under laminar, open-channel flow conditions. The model couples solution chemistry, precipitation at solution/calcite interfaces, hydrodynamics, diffusion and degassing. The model output is compared with experimental results obtained using an oversaturated calcite solution produced by mixing CaCl₂ and Na₂CO₃. The precipitation rate is observed to increase when the supersaturated solution flows over an obstruction, leading to a growth instability that causes the formation of terraces. At relatively high flow rates, the most important mechanism for this behaviour seems to be hydrodynamic advection of dissolved species either towards or away from the calcite surface, depending on location relative to the obstruction, which deforms the concentration gradients. At lower flow rates, steepening of diffusion gradients around protrusions becomes important. Enhanced degassing over the obstruction due to shallowing and pressure drop is not important on small scales. Diffusion controlled transport close to the calcite surface can lead to a fingering-type growth instability, which generates porous textures. Our results are consistent with existing diffusive boundary layer theory, but for flow over non-smooth surfaces, simple calcite precipitation models that include empirical correlations between fluid flow rate and calcite precipitation rate are inaccurate.     

Trace Element Partitioning and Accessory Phase Saturation during H2O-Saturated Melting of Basalt with Implications for Subduction Zone Chemical Fluxes

Experimental phase equilibrium and trace element partitioningdata are reported for H₂O-saturated mid-ocean ridge basalt at2·5 GPa, 750–900°C and oxygen fugacities atthe nickel–nickel oxide buffer. Garnet, omphacite andrutile are present at all temperatures. Amphibole and epidotedisappear as residual phases above 800°C; allanite appearsabove 750°C. The Na–Al-rich silicate glass presentin all run products is likely to have quenched from a supercriticalliquid. Trace element analyses of glasses demonstrate the importantcontrol exerted by residual minerals on liquid chemistry. Inaddition to garnet, which controls heavy rare earth elements(HREE) and Sc, and rutile, which controls Ti, Nb and Ta, allanitebuffers the light REE (LREE; La–Sm) contents of liquidsto relatively low levels and preferentially holds back Th relativeto U. In agreement with previous experimental and metamorphicstudies we propose that residual allanite plays a key role inselectively retaining trace elements in the slab during subduction.Experimental data and analyses of allanite-bearing volcanicrocks are used to derive a model for allanite solubility inliquids as a function of pressure, temperature, anhydrous liquidcomposition and LREE content. The large temperature dependenceof allanite solubility is very similar to that previously determinedfor monazite. Our model, fitted to 48 datapoints, retrievesLREE solubility (in ppm) to within a factor of 1· 40over a pressure range of 0–4 GPa, temperature range of700–1200°C and for liquids with anhydrous SiO₂ contentsof 50–84 wt %. This uncertainty in LREE content is equivalentto a temperature uncertainty of only ± 27°C at 1000K, indicating the potential of allanite as a geothermometer.Silicic liquids from either basaltic or sedimentary protolithswill be saturated in allanite except for Ca-poor protolithsor at very high temperatures. For conventional subduction geothermsthe low solubility of LREE (+ Th) in liquids raises questionsabout the mechanism of LREE + Th transport from slab to wedge.It is suggested either that, locally, temperatures experiencedby the slab are high enough to eliminate allanite in the residueor that substantial volumes of H₂O-rich fluids must pass throughthe mantle wedge prior to melting. The solubility of accessoryphases in fluids derived from subducted rocks can provide importantconstraints on subduction zone thermal structure. KEY WORDS: subduction; experimental petrology; allanite; solubility; supercritical liquid; eclogite