ALBERT

Description: Volatile elements play an important role in many aspects of the physicochemical architecture of sub-volcanic plumbing systems, from the liquid line of descent to the dynamics of magma storage and eruption. However, it remains difficult to constrain the behaviour of magmatic volatiles on short timescales before eruption using established petrological techniques (e.g. melt inclusions); specifically, in the final days to months of magma storage. This study presents a detailed model of pre-eruptive volatile behaviour in the Campi Flegrei system (Italy), through combined analyses of apatite crystals and glass. The deposits of eight eruptions were examined, covering the full spectrum of melt compositions, eruptive styles and periods of activity at Campi Flegrei in the past 15 kyr. Measured apatite compositions are compared with thermodynamic models that predict the evolution of the crystal compositions during different fractional crystallization scenarios, including (1) volatile-undersaturated conditions, (2) H2O-saturated conditions and (3) varying P–T conditions. The compositions of clinopyroxene-hosted and biotite-hosted apatite inclusions are consistent with crystallization under volatile-undersaturated conditions that persisted until late in magmatic evolution. Apatite microphenocrysts show significantly more compositional diversity, interpreted to reflect a mixed cargo of crystals derived from volatile-undersaturated melts at depth and melts that have undergone cooling and degassing in discrete shallow-crustal magma bodies. Apatite microphenocrysts from lavas show some re-equilibration during cooling at the surface. Clinopyroxene-hosted melt inclusions within the samples typically contain 2–4 wt % H2O, indicating that they have been reset during temporary magma storage at 1–3 km depth, similar to the depth of sill emplacement during recent seismic crises at Campi Flegrei. Comparable apatite compositional trends are identified in each explosive eruption analysed, regardless of volume, composition or eruption timing. However, apatites from the different epochs of activity appear to indicate subtle changes in the H2O content of the parental melt feeding the Campi Flegrei system over time. This study demonstrates the potential utility of integrated apatite and glass analysis for investigating pre-eruptive volatile behaviour in apatite-bearing magmas.

Description: Xenorhabdus bovienii strain jolietti (XBJ) is a Gram-negative bacterium that interacts with several organisms as a part of its life cycle. It is a beneficial symbiont of nematodes, a potent pathogen of a wide range of soil-dwelling insects and also has the ability to kill soil- and insect-associated microbes. Entomopathogenic Steinernema nematodes vector XBJ into insects, releasing the bacteria into the insect body cavity. There, XBJ produce a variety of insecticidal toxins and antimicrobials. XBJ's genome also encodes two separate Type Six Secretion Systems (T6SSs), structures that allow bacteria to inject specific proteins directly into other cells, but their roles in the XBJ life cycle are mostly unknown. To probe the function of these T6SSs, we generated mutant strains lacking the key structural protein Hcp from each T6SS and assessed phenotypes related to different parts of XBJ's life cycle. Here we demonstrate that one of the T6SSs is more highly expressed in in vitro growth conditions and has antibacterial activity against other Xenorhabdus strains, and that the two T6SSs have a redundant role in biofilm formation.

Description: While different microalgae tend to be associated with different bacteria, it remains unclear whether such specific associations are beneficial for the microalgae. We assessed the impact of bacterial isolates, derived from various marine benthic diatoms, on the growth of several strains belonging to the Cylindrotheca closterium diatom species complex. We first tested the effect of 35 different bacterial isolates on the growth of a single C. closterium strain, and then evaluated the impact of 8 of these isolates on the growth of 6 C. closterium strains and 1 Cylindrotheca fusiformis strain. Surprisingly, most interactions were neutral to antagonistic. The interactions were highly specific, with diatom growth in the presence of specific bacteria differing between Cylindrotheca strains and species, and closely related bacteria eliciting contrasting diatom growth responses. These differences could be related to the origin of the bacterial isolates, as only isolates from foreign diatom hosts significantly reduced diatom growth, implying coadaptation between different Cylindrotheca strains and their associated bacteria. Interestingly, the antagonistic effect of a Marinobacter strain was alleviated by the presence of a microbial inoculum that was native to the diatom host, suggesting that coadapted bacteria might also benefit their host indirectly by preventing the establishment of harmful bacteria.

Description: The Upper Zone of the Rustenburg Layered Suite of the Bushveld Complex contains the world’s largest Fe–Ti–V ± P deposit and formed from the last major injection of magma into the chamber. Quantitative textural analysis of Upper Zone rocks was undertaken to constrain the processes operating during mush formation and solidification, focussing on horizons with the greatest density contrast to isolate the effects of gravitational loading. We examined three magnetitite layers, together with their underlying and overlying anorthosites. The similarity of microstructures in anorthosites above and below the dense magnetitite layers suggests that the rocks were not affected by viscous compaction driven by gravitational loading. The magnetitite cumulate layers formed by crystal accumulation from a mobile crystal slurry dominated by the Fe-rich conjugate of an unmixed immiscible liquid. We suggest a new mechanism of crystal nucleation in deforming crystal-rich systems, driven by undercooling caused by cavitation as grains slide past each other during simple shear. We propose that the super-solidus deformation recorded in these rocks was caused by prolonged regional subsidence of the magma chamber at Upper Zone times.

Description: The physicochemical characteristics of sub-volcanic magma storage regions have important implications for magma system dynamics and pre-eruptive behaviour. The architecture of magma storage regions located directly above high buoyancy flux mantle plumes (such as Kīlauea, Hawai’i and Fernandina, Galápagos) are relatively well understood. However, far fewer constraints exist on the nature of magma storage beneath ocean island volcanoes that are distal to the main zone of mantle upwelling or above low buoyancy flux plumes, despite these systems representing a substantial proportion of ocean island volcanism globally. To address this, we present a detailed petrological study of Isla Floreana in the Galápagos Archipelago, which lies at the periphery of the upwelling mantle plume and is thus characterized by an extremely low flux of magma into the lithosphere. Detailed in situ major and trace element analyses of crystal phases within exhumed cumulate xenoliths, lavas and scoria deposits indicate that the erupted crystal cargo is dominated by disaggregated crystal-rich material (i.e. mush or wall rock). Trace element disequilibria between cumulus phases and erupted melts, as well as trace element zoning within the xenolithic clinopyroxenes, reveal that reactive porous flow (previously identified beneath mid-ocean ridges) is an important process of melt transport within crystal-rich magma storage regions. In addition, application of three petrological barometers reveals that the Floreana mush zones are located in the upper mantle, at a depth of 23·7 ± 5·1 km. Our barometric results are compared with recent studies of high melt flux volcanoes in the western Galápagos, and other ocean island volcanoes worldwide, and demonstrate that the flux of magma from the underlying mantle source represents a first-order control on the depth and physical characteristics of magma storage.