ALBERT

Description: Abundant volcanic activity occurs in the back‐arc region of the northern Tofua island arc where the Northeast Lau Spreading Center (NELSC) propagates southwards into older crust causing the formation of numerous seamounts at the propagating rift tip. An off‐axis volcanic diagonal ridge (DR) occurs at the eastern flank of the NELSC, linking the large rear‐arc volcano Niuatahi with the NELSC. New geochemical data from the NELSC, the southern propagator seamounts, and DR reveal that the NELSC lavas are tholeiitic basalts whereas the rear‐arc volcanoes typically erupt lavas with boninitic composition. The sharp geochemical boundary probably reflects the viscosity contrast between off‐axis hydrous harzburgitic mantle and dry fertile mantle beneath the NELSC. The new data do not indicate an inflow of Samoa plume mantle into the NELSC, confirming previously published He isotope data. The NELSC magmas form by mixing of an enriched and a depleted Indian Ocean‐type upper mantle end‐member implying a highly heterogeneous upper mantle composition in this area. Most NELSC lavas are little affected by a slab component implying that melting is adiabatic beneath the spreading center. The DR lavas show the influence of a component from the subducted Louisville Seamount Chain, which was previously thought to be restricted to the nearby arc volcanoes Niuatoputapu and Tafahi. This signature is rarely detected along the NELSC implying little mixing of melts from the low‐viscosity hydrous portion of the mantle wedge beneath the rear‐arc volcanoes into the melting region of the dry mantle beneath the NELSC.

Description: Plain Language Summary: Volcanic activity is abundant at subduction zones and the chemical analysis of the erupted rocks allows to determine the material transport in the Earth's mantle. The Northeast Lau Spreading Center (NELSC) forms by extension and volcanism behind the northern Tofua island arc. Several large volcanic structures occur east of the NELSC and the lavas of these off‐axis volcanoes are chemically and isotopically distinct implying little mixing with the magmas of the NELSC. The differences suggest decompression melting of relatively dry mantle beneath the NELSC whereas the off‐axis volcanoes reflect melting of water‐rich mantle affected by fluids from the subducting Pacific Plate. The sharp geochemical boundary between the NELSC and off‐axis volcanoes is probably due to a large viscosity contrast between hydrous harzburgitic mantle and dry fertile mantle. Element and isotope ratios indicate that the NELSC magmas form by mixing of enriched and depleted portions of the upper mantle, and we do not find evidence for inflow of the Samoa deep mantle plume from the north. Some of the off‐axis lavas contain a component from a volcanic chain that was subducted some 4 million years ago and that was previously only known in two volcanoes of the Tofua island arc.

Description: Key Points: Variably enriched mantle sources melt beneath the Northeast Lau Spreading Center (NELSC) but there is no evidence for Samoa mantle plume inflow. Relatively dry fertile mantle beneath NELSC causes sharp geochemical boundary with hydrous harzburgitic North Tonga mantle wedge. Subducted Louisville Seamount Chain material affects rear‐arc volcanism.

Description: Bundesministerium für Bildung und Forschung (BMBF) http://dx.doi.org/10.13039/501100002347

Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659

Description: We compare simulations, including the Illustris simulations, to observations of C iv and C ii absorption at z  = 2–4. These are the C iv column density distribution function in the column density range 10 12 –10 15 cm –2 , the C iv equivalent width distribution at 0.1–2 Å, and the covering fractions and equivalent widths of C iv 1548 Å and C ii 1337 Å around damped Lyman α systems (DLAs). In the context of the feedback models that we investigate, all C iv observations favour the use of more energetic wind models, which are better able to enrich the gas surrounding haloes. We propose two ways to achieve this: an increased wind velocity and an increase in wind thermal energy. However, even our most energetic wind models do not produce enough absorbers with C iv equivalent width 〉0.6 Å, which in our simulations are associated with the most massive haloes. All simulations are in reasonable agreement with the C ii covering fraction and equivalent widths around damped Lyman α absorbers, although there is a moderate deficit in one bin 10–100 kpc from the DLA. Finally, we show that the C iv in our simulations is predominantly photoionized.

Description: U–Th–Ra isotope analyses of whole rocks and mineral separates were conducted in order to perform isochron dating of three morphologically young lavas from Tatun volcano, northern Taiwan (from Mt Cising, the Shamao dome and the Huangzuei volcano). The data do not yield tight U–Th isochrons, indicating open-system magmatic processes. However, crystallization ages of two samples can be constrained: namely, less than about 1370 years for the Shamao dome, based on 226 Ra– 230 Th disequilibrium in magnetite, and less than approximately 70 ka (but potentially Holocene) for a Huangzuei flow, based on 238 U– 230 Th disequilibrium in plagioclase. Discordant Ar–Ar, 238 U– 230 Th and 226 Ra– 230 Th ages are best explained by young lavas having inherited some crystals from older lithologies (crystal mushes or rocks), and indicate that the above ages represent maxima. Our study provides the first evidence of effusive volcanism at the Tatun Volcano Group in Late Holocene times. All separates from the Shamao dome and Huangzuei volcano are in 234 U– 238 U equilibrium. Minerals in the Mt Cising sample are in 234 U– 238 U disequilibrium, despite the 234 U– 238 U equilibrium of the whole rock. We interpret this as uptake of a hydrothermally altered, old crystal cargo into fresh melt prior to eruption. A different dating approach will thus be required to constrain the eruption age of Mt Cising. Supplementary material: Ar–Ar plateaus from Mt Cising and the Shamao dome, reproduced from Lee (1996), are available at www.geolsoc.org.uk/SUP18817

Description: Petrological study of eruptive units in two locations along the Galápagos Spreading Center provides insight into how the rate of magma supply affects mid-ocean ridge magmatic systems. Study areas with lower magma supply (95°W) and higher magma supply (92°W) have similar spreading rates (53 and 55 mm a –1 ), but differ by 30% in the time-averaged rate of magma supply (0·3 x 10 6 and 0·4 x 10 6 m 3 km –1 a –1 ) as a result of varying proximity to the Galápagos hotspot. We use major and trace element compositions of glass and whole-rock samples, chemistry of mineral phases, and petrography to characterize parental magma variability, fractional crystallization and magma mixing in crustal reservoirs, and timescales of magmatic recharge relative to eruption. At the low magma supply study area, eruptible magma appears to be present only intermittently within the crust; magma recharge is probably infrequent, occurring with a periodicity of several hundred to one thousand years. The shallowest magma body in the crust is thought to be at ~3 km below the seafloor, and lavas are restricted to a relatively limited compositional range (6·2–9·1 wt % MgO). Magmatic evolution at this location is probably dominated by processes occurring within a crystal-rich mush, with limited subsequent residence in melt-dominated magma reservoirs. Eruptions here appear to be closely coupled to magmatic recharge events; lower MgO lavas have compositional trends controlled by mixing of low- and high-MgO magmas from compositionally distinct parents, and commonly contain both normally and reversely zoned crystals. In contrast, at the high magma supply study area, where a seismically imaged melt lens is located ~1·7 km below the seafloor, fractional crystallization within a melt-rich magma reservoir results in a larger range in major element compositions of the erupted magmas (2·7–8·2 wt % MgO) with less variation in trace element concentrations or ratios. Temperatures within the melt lens over the last several hundred years have varied by at least 100°C (1070–1170°C); cooling rates within the melt lens are estimated to be greater than 0·5°C per year. Relatively low-MgO lavas have over-enrichments in Cl that are best explained by assimilation of brine associated with hydrothermal circulation within the overlying crust. Between magmatic recharge events, resident magma fractionates and feeds one or more low-volume fissure eruptions. Small bodies of magma may become isolated from the larger magmatic system in the crust, allowing more extreme degrees of fractionation, locally reaching basaltic andesite. This study demonstrates that persistent melt lenses at intermediate rates of magma supply need not be ‘steady state’. The variations in magma composition among eruptive episodes at each location allow us to assess the temporal variability in magma reservoir properties at ridge segments along the Galápagos Spreading Center, in the context of regional variations in magma supply.

Description: We investigate the variation of the ratio of the equivalent widths of the Fe ii 2600 line to the Mg ii 2796, 2803 doublet as a function of redshift in a large sample of absorption lines drawn from the Johns Hopkins University - Sloan Digital Sky Survey Absorption Line Catalog. We find that despite large scatter, the observed ratio shows a trend where the equivalent width ratio $\mathcal {R}\equiv W_{\rm Fe\,\small {II}}/W_{\rm Mg\,\small {II}}$ decreases monotonically with increasing redshift z over the range 0.55 ≤  z  ≤ 1.90. Selecting the subset of absorbers where the signal-to-noise ratio of the Mg ii equivalent width $W_{\rm Mg\,\small {II}}\ {\rm is} \ge 3$ and modelling the equivalent width ratio distribution as a Gaussian, we find that the mean of the Gaussian distribution varies as $\mathcal {R}\propto (-0.045\pm 0.005)z$ . We discuss various possible reasons for the trend. A monotonic trend in the Fe/Mg abundance ratio is predicted by a simple model where the abundances of Mg and Fe in the absorbing clouds are assumed to be the result of supernova (SN) ejecta and where the cosmic evolution in the SNIa and core-collapse SN rates is related to the cosmic star formation rate. If the trend in $\mathcal {R}$ reflects the evolution in the abundances, then it is consistent with the predictions of the simple model.

Description: We analyse the mass, temperature, metal enrichment, and OVI abundance of the circumgalactic medium (CGM) around z ~ 0.2 galaxies of mass 10 9 M 〈 M * 〈 10 11.5 M in the Illustris simulation. Among star-forming galaxies, the mass, temperature, and metallicity of the CGM increase with stellar mass, driving an increase in the OVI column-density profile of ~0.5 dex with each 0.5 dex increase in stellar mass. Observed OVI column-density profiles exhibit a weaker mass dependence than predicted; the simulated OVI abundance profiles are consistent with those observed for star-forming galaxies of mass M * = 10 10.5–11.5 M , but underpredict the observed OVI abundances by 0.8 dex for lower mass galaxies. We suggest that this discrepancy may be alleviated with additional heating of the abundant cool gas in low-mass haloes, or with increased numerical resolution capturing turbulent/conductive mixing layers between CGM phases. Quenched galaxies of mass M * = 10 10.5–11.5 M are found to have 0.3–0.8 dex lower OVI column-density profiles than star-forming galaxies of the same mass, in qualitative agreement with the observed OVI abundance bimodality. This offset is driven by AGN feedback, which quenches galaxies by heating the CGM and ejecting significant amounts of gas from the halo. Finally, we find that the inclusion of the central-galaxy's radiation field may enhance the photoionization of the CGM within ~50 kpc, further increasing the predicted OVI abundance around star-forming galaxies.